Download Andrea Marino - Université de Rennes 1

ANNÉE 2015
THÈSE / UNIVERSITÉ DE RENNES 1
sous le sceau de l’Université Européenne de Bretagne
pour le grade de
DOCTEUR DE L’UNIVERSITÉ DE RENNES 1
Mention : Physique
Ecole doctorale (Science de la Matière)
présentée par
Andrea Marino
Préparée aux unités de recherche :
Institut de Physique de Rennes UMR CNRS 6251
Institut des Sciences Chimiques de Rennes UMR CNRS 6226
Ultrafast
investigation of
electronic and
structural dynamics
in photomagnetic
molecular solids.
Thèse soutenue à l'Université de
Rennes 1
le 16 Juillet 2015
devant le jury composé de :
Shinichiro IWAI
Professeur, Université de Tohoku / rapporteur
Laurent CARIO
Directeur de Recherche CNRS
Université de Nantes / rapporteur
Talal MALLAH
Professeur, Université de Paris-Sud / examinateur
Eric COLLET
Professeur, Université de Rennes 1
/ directeur de thèse
Marc FOURMIGUE
Directeur de Recherche CNRS,
Université de Rennes 1 / co-directeur de thèse
A mio nonno e mia nonna, continua fonte d'ispirazione,
che hanno sempre creduto in me.
Acknowledgements
During this amazing three years of intense PhD, I had the pleasure, honor and luck to strictly work
with many outstanding scientists whom strongly helped me to carry out this huge many-hands job
which is the PhD thesis here reported.
I would like to thank Région Bretagne and CNRS for the financial support.
I deeply own the success of my work to my two supervisors Prof. Eric Collet and Prof. Marc
Formigué. They accompanied me during this three years and efficiently supervised my work,
addressing me to the correct answers to the many scientific questions I had; and I had a lot... and
still have many. I cannot be more satisfied from two supervisors like them, they really gave me a
lot, and what I learned is priceless!
I want to acknowledge Prof. Shinichiro Iwai, Prof. Laurent Cario and Prof. Talal Mallah for having
invested their time to deeply examine and review the work of my PhD presented with this
manuscript. I do appreciate their commitments in a deep analysis of my work, as well as all the
remarks and discussions raised during the oral defense. I especially enjoyed the scientific debate
with all of them, where I had the pleasure to challenge myself in front of world-class scientists like
them.
I will always bring with me "pieces" of the fabulous team I joined at the IPR at the University of
Rennes1. Maciej Lorenc with whom I had the pleasure to confront and debate about the meaning of
life (and science too). It's thanks to him I learned how to handle ultrafast laser systems, his lessons
are priceless too. I own you a lot of buzka buzka. To Marco Cammarata and all his family that
strongly supported me during these years. Marco you have been the person of reference for my
scientific growth. Obtaining an approval from you meant a lot for me. To Herve Cailleau, Marina
Servol and Marylise Buron and their fruitful discussions as well as their precious teaching
respectively on statistical physics, lasers and diffraction. Thanks to Loic Toupet for having always
supported my work with the X-ray characterization of my samples, to Franck Camerel for the
project on the organic metals and to Olivier Jeannin for the preparation of (EDT-TTFI2)2(TCNQFn)
crystals. Many thanks also to Laurent Guerin for its strong moral and ethic support, nevertheless
mate of many adventures.
A special "grazie" to Sergio Di Matteo for having mostly always answered me to the many bizarre
questions on science and for being to me a fantastic therapist and counselor for this crazy period of
my life.
But I really want to thanks all these people mostly because they believed in me and made me proud
every day. All of you supported and encouraged me every day in these years, I will be always
grateful for that.
A really special and deep-heart thanks goes to Prof. Andreas Hauser. I admit to have been really
lucky to had the honor to directly work with him. Being in lab, acquiring data, and interpreting them
with Andreas, gave me so much in terms of knowledge and ability of scientifc thinking. The
experience with him was something extraordinary which will accompany me for all my career.
Huge thanks for Roman Bertoni and Wawrzyniec Kaszub for having initiated me to the ultrafast
world. Together with them and with Liya Khadeeva I had the pleasure to travel for conferences and
share many stimulating scientific discussions. They have been great colleagues and fantastic mates
making this trip being a lot of fun!
I deeply acknowledge to all the Institute of Physics of Rennes which welcomed me during my PhD.
Many thanks also to all the people with whom I collaborated. From the Institut de Chimie de la
Matière Condensée de Bordeaux Jean-François Létard, Gillaume Chastanet, Samir Matar and Cindy
Mauriac that provided amazing spin crossover crystals and great TD-DFT calculations important for
the results we obtained here. I also want to acknowledge R. Henning, A. D. DiChiara and K. Moffat
for their technical support during the measures in the BioCARS beam line at the APS synchrotron.
Matvey Fedin from the Novosibirsk State University for the project on the copper-nitroxide
breathing crystals. Pradip Chakraborty from the University of Geneva for the complementary
measurements on the reverse LIESST project. Yoichi Okimoto and Tadahiko Ishikawa from the
Tokyo Institute of Technology for having shared with me their time and knowledge in the
laboratory.
But all of this would have not been possible without the infinite support and motivation of my all
my family. My most special thanks goes to my love Alena Makhotkina, who always pushed me to
go on, always believed in me and cheered me up, helping me to overcome all the difficulties a PhD
can find during its way. Without her, nothing of this could have been possible!
Again, thousands of grazie, thank you, mercì, дякую, спасибо, dzięki, 感謝 !
with all my heart, yours
Andrea
Abstract
English
The ability to photo-switch physical/chemical properties of functional materials through photo
induced phase transition opens fascinating perspectives for driving the material towards new state
out of thermal equilibrium. However, it is fundamental to disentangle and understand all the
dynamical phenomena, otherwise hidden in statistically averaged macroscopic transformations.
Arguably, time-resolved studies represent unique approaches to access the necessary information on
the multiple degrees of freedom and elementary processes involved during the macroscopic
switching. As photo-reversible molecular switches, spin crossover (SCO) materials are of particular
interest. These photomagnetic and photochromic prototype materials undergo metastable
photoinduced phase transition between two states of different spin multiplicity, namely low-spin
(LS) and high-spin (HS).
In this PhD work it will be presented the ultrafast electronic and structural dynamics of SCO
molecular solids emphasizing the importance of using complementary probes sensitive to different
degrees of freedom. The photoinduced spin state switching concerns initially only an ultrafast, but
localized, molecular response which through strong electron-phonon coupling activates coherent
intra-molecular vibrations. An ultrafast energy transfer from the molecule to the lattice, via phononphonon coupling, allows an efficient trapping of the system in the new photoinduced state. However
in molecular solids, the excess of energy released from the absorber molecule results in a complex
multi-scale aspect involving several degrees of freedom at different time scales. In this contest, we
investigated the multi-step out-of equilibrium dynamics of a SCO system undergoing symmetry
breaking between the HS phase and the intermediate (IP) phase where a long range ordering of HS
and LS molecules results in a spin state concentration wave (SSCW), analogous to charge or spin
density waves. Combined time-resolved X-ray diffraction and optical spectroscopy studies provide
a complete overview of the out-of-equilibrium thermodynamics of the SSCW, revealing how the
two order parameters describing the system evolve in time.
Andrea Marino 2015
Andrea Marino 2015
Français
La capacité de photo-commuter le propriétés physiques / chimiques des matériaux fonctionnels
grâce à des transition de phase induites par la lumière, ouvre des perspectives fascinantes pour
diriger un matériau vers un nouvel état hors équilibre thermique. Cependant, il est fondamental de
comprendre tous les phénomènes élémentaires, habituellement cachés dans une moyenne statistique
lors des transformations à l'équilibre. Les études résolues en temps représentent une approche
unique pour accéder à l'évolution des différents degrés de liberté du système et connaitre les
processus élémentaires mis en jeu lors de la commutation macroscopique. Les matériaux a
transition de spin (SCO) sont d'un intérêt particulier car ce sont des systèmes photo-réversibles. Ces
matériaux sont aussi des prototypes photomagnétiques et photochromiques qui commutant entre
deux états de différente multiplicité de spin, nommés bas spin (LS) et haut spin (HS).
Dans ce travail de thèse, nous étudions les dynamique ultrarapides électroniques et structurales de
cette classe de solides moléculaires, en soulignant l'importance d'utiliser des sondes
complémentaires sensibles à différents degrés de liberté. Les commutation photo-induite entre états
de spin est ultra-rapide et initialement localisée à l'échelle moléculaire, où le couplage électronphonon active des vibrations cohérentes intramoléculaires. Un transfert d'énergie ultra-rapide de la
molécule au réseau, via un couplage phonon-phonon, permet de piéger efficacement le système
dans le nouvel état photo-induit. Cependant, dans les solides moléculaires, l'excès d'énergie libérée
de la molécule excitée résulte dans un aspect complexe multi-échelle impliquant plusieurs degrés de
liberté à des échelles de temps différentes. Dans ce travail de thèse, nous avons étudié la dynamique
multi-étape hors équilibre d'un système SCO présentant une brisure de symétrie entre la phase HS et
la phase intermédiaire (IP) où une mise en ordre à longue distance des états HS et LS des molécules
résulte en la formation d'une onde de concentration de spin (SSCW). La diffraction des rayons X
résolue en temps combinée avec des études de spectroscopie optique donnent une description
complète de la dynamique hors-équilibre de la SSCW hors-équilibre en mesurant l'évolution
temporelle des deux paramètres d'ordre décrivant le système.
Andrea Marino 2015
Andrea Marino 2015
TABLE OF CONTENTS
Résumé
ix
Chapter 1: Photoactive Materials
15
1.1 Controlling Physical Properties by Light
18
1.2 Time-resolved pump-probe techniques
20
1.3 Materials and Solids
24
1.4 Spin Crossover Systems
28
1.4.1 The O Ligand Field
28
1.4.2 LS ↔ HS spin crossover in FeII based systems
30
1.4.3 Photoinduced spin-state switching
32
1.4.4 Ultrafast and Out-of-Equilibrium Dynamics
35
1.5 Contest and aim of the PhD project.
Chapter 2: Ultrafast LIESST and Energy redistribution
2.1 The
spin-crossover compound
2.2 Electronic vs Structural Dynamics
40
41
43
46
2.2.1 Femtosecond Optical Pump-Probe
46
2.2.2 Time resolved XANES
50
2.3 Coherent Structural Dynamics
2.3.1 Analysis of coherent vibrational modes
53
55
2.4 Ultrafast Energy Redistribution
58
2.5 Conclusions
61
Chapter 3: LIESST vs reverse-LIESST
65
3.1 The role of Ligand-Field States
67
3.1.1 A bit of History
68
3.2 Description of the compounds
70
3.2.1 The
70
3.2.2 The
73
3.2.3 Strategy of the experiments
74
3.3 LIESST via d-d excitation
77
3.4 reverse-LIESST
80
3.4.1 A Triplet Intermediate State
80
3.4.2 A kinetic model
83
3.5 Discussions and Conclusions
88
Chapter 4: Spin State Concentration Wave
93
4.1 Molecular state Ordering and Symmetry Breaking
4.1.1 State of the art of stepped SCO transitions
4.2 Spin State Concentration Wave in [FeIIH2L2Me][PF6]2
4.2.1 Description of the
vi
crystal
95
96
100
100
4.2.2 Experimental description of SSCW
102
4.2.3 The Landau Theory of Phase Transitions applied to SSCW
106
4.2.4 The symmetry breaking order parameter 
112
4.3 Ultrafast out-of-equilibrium symmetry breaking
117
4.3.1 Optical characterization
117
4.3.2 Excitation density and non linear response
119
4.3.2 Temperature dependence of the thermal step
121
4.3.3 Time Resolved X-Ray Diffraction
124
4.4 Conclusion
Chapter 5: Conclusions and perspectives
5.1 Conclusions
128
131
134
5.1.1 Photoswitching in SCO solids
134
5.1.2 Ultrafast Dynamics of Molecular Magnet Breathing Crystals
136
5.2 Development of New Photoactive Hybrid Materials
138
5.2.1 Insulating-Metal materials with photoactive ions
138
5.2.2 Volume change as a driving force
140
Bibliography
143
Annex I : List of Abbreviations
161
Annex II : List of Publications
163
vii
viii
Résumé
Etudes ultrarapide de la dynamiques électroniques et
structurale dans des solides moléculaires
photomagnétiques
La capacité de photo-commuter les propriétés physiques/chimiques de matériaux fonctionnels
grâce à des transition de phase induites par la lumière, ouvre des perspectives fascinantes pour
commuter rapidement un matériau entre deux états ou vers un nouvel état n'existant pas à l'équilibre
thermique. Le but de ce nouveau domaine des transitions de phases photo-induites est de
comprendre comment, par irradiation lumineuse, un système peut atteindre une nouvelle phase avec
des propriétés structurales et électroniques différentes. Il est cependant fondamental de comprendre
tous les phénomènes élémentaires dynamiques pilotant ces transformations, pouvant être de nature
déterministes ou cohérente. Il est pour cela nécessaire de les étudier sur une échelle de temps ou ils
ne sont pas noyés dans une moyenne statistique comme c'est le cas à l'équilibre thermique. Les
études résolues en temps représentent une méthode unique pour accéder à l'évolution temporelle de
multiples degrés de liberté durant ces transformations hors-équilibre.
Pour aborder ces différents aspects il est nécessaire d'une part d'étudier des systèmes modèles et de
pousser les développements expérimentaux pour apprendre à appréhender les mécanismes sousjacents. Nous nous sommes donc intéressés dans un premier temps à l'étude de matériaux à
transition de spin (SCO) qui sont des composés photo-magnétiques et photo-chromiques prototypes
présentant une efficacité quantique proche de 100%. Dans ces composés les effets sont photoréversibles et, de plus, les molécules bistables répondent à différent stimuli émergeant lors de la
dynamique hors-équilibre macroscopique du matériaux: effet élastique, effet thermique...
Un autre aspect important pour la science du contrôle des matériaux concerne le développement de
nouveaux matériaux photo-actifs pouvant présenter des changements d'états de conduction,
magnétiques, ferroélectrique... Durant ce projet nous avons donc exploré différentes pistes et
identifié l'intérêt de matériaux hybrides offrant la possibilité de contrôler un sous-système
moléculaire par l'effet de la lumière sur un autre.
Résumé
1 Techniques ultra-rapides pour sonder la matière.
Pour étudier ces transformations, nous avons utilisé dans le cadre de cette thèse un panel de
techniques ultrarapides reposant toutes sur le même principe : la méthode pompe-sonde. Une
impulsion lumineuse femtoseconde, nommée pompe, excite le système à une énergie choisie. Une
seconde impulsion sonde le système à différents délais de façon à reconstruire son évolution en
fonction du temps. Ces mesures reposent sur les mesures de spectroscopie optique effectuées sur la
plateforme laser ultrarapide de l’Institut de Physique de Rennes. De telles mesures permettent de
suivre la variation d’absorption ou de réflectivité optique du composé avec une résolution
temporelle de l’ordre de la centaine de femtosecondes. Ces mesures sont particulièrement utiles car
elles peuvent déceler la création d’états excités transitoires générés durant le processus de photocommutation. Ces sondes optiques sont surtout sensibles au changement d'état électronique, mais
son couplage avec les degrés de liberté structuraux nous renseigne sur la présence d’une dynamique
structurale cohérente.
Ces études ont été complétées par des mesures de diffraction des rayons X résolues en temps
effectuées au synchrotron Advanced Photon Sources (APS, Argonne, USA). Elles permettent
d'obtenir la structure transitoire du cristal avec une résolution temporelle de 100 picosecondes. Pour
suivre des changements structuraux locaux à l’échelle de la femtoseconde, la technique du XANES
mesurée au seuil du Fer est aussi particulièrement appropriée pour étudier l'évolution de son
environnement local. Elle permet d’avoir une évidence directe de l’élongation de la liaison FeLigand qui est le processus jouant un rôle essentiel dans le piégeage de l'état photoinduit. Ces
mesures de XANES avec une résolution de l’ordre de la centaine de femtoseconde ont été réalisées
sur le laser électron-libre à rayons X à Stanford (Linac Coherent Light Source X-FEL).
2 Matériaux à transition de spin.
2.1 Effet LIESST et reverse-LIESST
La première partie de ce travail de thèse a pour objectif principal d’étudier la commutation photoinduite ultrarapide de matériaux moléculaires à transition de spin (SCO). Ces composés sont des
prototypes de systèmes moléculaires bistables possédant deux états électroniques nommés Haut
Spin (HS) et Bas Spin (LS). Ce changement d’état électronique s’accompagne de changements
structuraux principalement autour de l’ion métallique central. Nous nous sommes particulièrement
intéressés à la classe de systèmes moléculaires à base de FeII (fig. 1) présentant un état fondamental
bas spin (LS, S=0) et un état haut spin (HS, S=2) accessible à haute température, ou par irradiation
de l'état LS. Cette classe de matériaux possède la propriété de pouvoir commuter entre ces états de
spin sous l'effet de stimulation externe comme un changement de température, de pression, de
x
Andrea Marino 2015
champ magnétique. Le mécanisme de commutation de l'état LS à l'état HS sous irradiation
lumineuse est connu sous le nom de Light Induced Excited Spin State Trapping (LIESST). Le
phénomène inverse commutant l'état HS en LS est nommé reverse-LIESST.
Fig. 1 representation des SCO dans le deux different etat de spin
Nous avons étudié les dynamiques ultra-rapides des photo-commutations LIESST (HS → LS) et
reverse-LIESST (LS → HS) sur des solides à transition de spin et obtenu ainsi une compréhension
générale des processus élémentaires concernant le réarrangements électroniques et aussi les
changements structuraux. Les résultats obtenus (Fig 2) montrent que la génération de molécules HS
par photo-commutation se fait à une échelle sub-picoseconde. Dans le cas du phénomène LIESST,
une irradiation lumineuse active un changement d'état électronique par transfert de charge du métal
vers le ligand. La relaxation de cet état électronique instable autour des orbitales t2g et eg localisées
autour du fer(II) a lieu en moins de 50 fs. En parallèle de ces mesures optiques nous avons effectué
des mesures de XANES et absorption X au X-FEL LCLS. Ces mesures nous permettent d’avoir une
signature structurale du processus avec une résolution de l’ordre de la centaine de femtosecondes.
Nous avons ainsi déterminé avec précision la dynamique d’élongation de la liaison Fer-Azote: 150
± 10 fs caractéristique de la formation de structure HS. Ce temps correspond à la demi-période du
mode de respiration de la molécule, lié à l'élongation Fe-N. Cette dynamique ultra-rapide engendre
une réponse structurale cohérente avec la génération de plusieurs phonons optiques.
L’amortissement rapide de ces phonons piégeant efficacement l'état HS dans son potentiel est à
l'origine de la haute efficacité quantique de ce processus.
Le processus reverse-LIESST est différent. Nos études révèlent un comportement cinétique avec
une dynamique plus lente car l'état LS n'est atteint qu'en 40 ps. De plus un état triplet intermédiaire
est clairement identifié.
xi
Résumé
Fig. 2 Comparison processus LIESST (gauche) et processus reverse-LIESST (droite).
Nos études ont aussi mis en évidence que la réponse du matériau à une excitation lumineuse se situe
au niveau moléculaire et la dynamique femtoseconde observée en solution est similaire à celle
observée ici dans les solides moléculaires. Cependant, à la différence du comportement de
molécules en solution, les cristaux moléculaires présentent une dynamique hors équilibre complexe
où l'excitation se propage à travers différents mécanismes à une échelle macroscopique:



L'énergie déposée au niveau moléculaire est rapidement redistribué au réseau et nous avons
mis en évidence que le transfert d'énergie se fait par l'activation cohérente de modes de
réseaux à l'échelle picoseconde.
Le changement de volume de la molécule engendre des variations de volume du cristal et les
couplages élastiques entre molécules peuvent alors les commuter de façon coopérative.
Le transfert d'énergie au réseau engendre aussi une élévation de température et un
peuplement thermique de l'état HS à l'échelle de temps µs.
Nous avons pu étudier en détail le caractère multi-échelle et séquentiel de la transformation
photoinduite de matériaux moléculaires, où différents degrés de liberté sont activés sur leur échelle
de temps intrinsèque.
xii
Andrea Marino 2015
2.2 Onde de concentration de spin (SSCW)
Ces matériaux moléculaire bistables présentent aussi des phases inhabituelles, liées à la mise en
ordre à longue distance de molécules dans les états HS et LS. Cette mise en ordre spatiale ...HS-LSHS-LS... peut être décrite en terme d'onde de concentration de spin (SSCW pour spin-state
concentration wave) décrivant la modulation spatiale de γHS, la probabilité d'occuper l'état LS ou HS
et l'ordre est directement décrit par l'amplitude η de l'onde (Fig. 3). Dans le composé étudié ici,
cette onde est associée à une brisure de symétrie avec doublement de la maille cristalline. Elle est
observée par diffraction des rayons X et les nouveaux pics de Bragg liés à ce doublement de maille
ont une intensité I(hkl) ∝ η2. Pour étudier la réponse de ces ondes à une excitation laser
femtoseconde, commutant de façon sélective des molécules de l'état LS à HS, nous avons donc
réalisé des expériences de diffraction X résolues en temps sur la ligne Biocars du synchrotron APS
(Argonne, USA). Nous avons montré une dynamique hors équilibre complexe, où les deux
paramètres d'ordre (γHS la concentration moyenne de molécules HS et η l'amplitude de l'onde)
évoluent sur leurs propres échelles de temps. L'évolution temporelle des intensités des pics de
Bragg mesurant l'ordre HS-LS montrent qu'il détruit lorsque le système a le temps d'explorer
différentes configuration HS-LS, ce qui a lieu sur l'échelle de temps ms. Cette onde s'efface et se
reforme sur l'échelle de temps de 20 ms.
Fig. 3 a) et b) représentation schématique de SSCW liée à l'ordre HS-LS sur différents sites
cristallins. c) Les mesures de diffraction X résolues en temps montrant que l'ordre HS-LS disparait
sur 1 ms et se reforme en 20 ms.
xiii
Résumé
2.3 Matériaux multi-fonctionnels hybrides.
Les matériaux organiques conducteurs de basse dimensionnalité sont généralement constitués de
colonnes de cations dérivés du TTF (tétrathiafulvalène) et d’anions organiques et inorganiques qui
servent à compenser la charge. L’anion choisi pour sa charge et sa géométrie reste souvent
spectateur dans les propriétés électroniques et magnétiques du matériau qui dépendent donc
essentiellement de l’empilement et de l’état de charge des cations TTF. Nous voulions associer à
des empilements de TTF des anions photo-actifs qui, en se transformant sous lumière, vont pouvoir
commuter les propriétés électroniques et magnétiques de matériaux. Nous avons pu réaliser la
synthèse et la cristallogenèse de composés originaux basés sur des sels conducteurs de
tétrathiafulvalènes associés, par électro-cristallisation, à des contre-ions photostimulables, soit
fluorescents (dérivés de bodipy), soit photo-commutables (diaryléthènes) au sein de l’Institut de
Chimie de Rennes.
Nous avons ainsi mis en évidence une transition isolant-métal dans le composé organique
δ‑(BEDT-TTF)4[2,6-Anthracene-bis(sulfonate)]•(H2O)4. Le mécanisme à la base de cette
transition de phase est lié à une réorganisation du réseau de liaison hydrogène. Malheureusement
cette transition de phase à lieu vers 70 K, ce qui rend les expériences optiques pompe-sonde
délicates au niveau de l'environnement cryogénique. Nous avons pu aussi synthétiser des cristaux
de diaryléthène sulfonates et étudier leur photo-commutation. Si elle est bien observée en solution,
elle ne l'est pas dans les premiers cristaux obtenus sous forme de sel de tétraphénylphosphonium.
La distance entre atomes de carbone trop longue de ce cristal ne permet pas de fermeture du cycle.
L'inclusion de ces anions dans des sels conducteurs est en cours par électro-cristallisation.
L'utilisation de telles molécules est importante pour le développement de matériaux hybrides. La
variation de volume moléculaire, thermiquement stable de part la force de la liaison chimique,
ouvre la possibilité de stabiliser à la demande les phases photo-induites. Les études ultra-rapides
ayant montré l'importance des effets de volume sur la transformation macroscopique du matériau, et
le développement de matériaux hybrides à base de diaryléthènes est une piste prometteuse pour le
futur.
xiv
Chapter 1
Photoactive Materials
Andrea Marino 2015
16
Photoactive Materials
In the field of material science, the last 50 years have been characterized by the scientific trend of
continuously pushing back its frontiers toward smaller and faster scales. Striking progresses in
increasingly sophisticated technologies have led to tremendous improvements of instrumentations
and novel analytical techniques. These developments have impacted and encouraged the expansion
of new scientific explorations in many different fields, and it is nowadays possible to observe,
investigate and control phenomena at the ultrasmall and ultrafast scales.
Impressing achievements have been made in the field of nanoscience, allowing to manipulate matter
at atomic scale. For instance, it is astonishing the accuracy with which it is possible to control a
single atomic layer deposition [Joyce 1988], or to develop artificial molecular machines designed to
perform work under an appropriate external stimuli [Balzani 2000, Terao 2012]. With respect to
these achievement at ultrasmall scales, advancements on ultrafast scales should be regarded on
equal footing. Indeed, the development of ultrashort laser pulses makes it now possible to trigger
ultrafast dynamics and reaction exciting and transforming matter on the timescale of basic atomic
motions. The longing interest on the control of matter properties and applied functionalities induced
a hectic research on matter transformations, which have been usually governed by varying
macroscopic parameters such as temperature, pressure, electric or magnetic field. Theoretical
studies on thermodynamics and statistical physics helped describing the macroscopic and average
microscopic behaviors of the systems at thermal equilibrium.
However, the use of light as a control parameter has been more and more preferred in the different
fields of science. Its non-invasive and highly selective character combined with its exceptional
temporal and spatial resolution makes light an ideal external stimulus for triggering and probing
chemical reactions as well as changes in material properties. A new challenge appears now to
control materials on the ultrafast time-scale by using intense and ultrashort laser pulses. New
emerging fields such as photochemistry and photophysics makes it now possible to understand and
control properties of molecules and solids by light. These efforts were awarded of a Nobel Prize for
important advances reported in femto-chemistry in 1999 [Zewail 2000] which spurred further
interests in the investigation of elementary processes at the ultra-small and ultra-fast time scales.
The light-control of molecular transformation founds applications in various fields from
technological applications [Irie 2000] expanding towards biological systems and medicine with biocompatible chromophores for drug delivery and cancer cure for instance [Pierri 2012, Very 2012,
Wachter 2012]. Nevertheless, the opportunity to drive Photo-Induced Phase Transition (PIPT) in
solid state physics [Nasu 2004, Koshihara 2009] founds general excitement in impacting the
macroscopic state of materials with light pulses.
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Andrea Marino 2015
1.1 Controlling Physical Properties by Light
The ability to photoinduce changes of physical and chemical properties opens fascinating
perspectives for driving the material towards new states far from thermal equilibrium. Materials
with photoactive multifunctional molecules are of particular interest for future technologies as
provide different doorways to the light control of various photo-switchable functions (magnetic,
optical, conducting, ferroelectric etc. etc.). However the control of the material functionalities
requires a deep understanding of the complex processes involved in the reorganization of its atomic
and electronic constituents.
Ultrashort laser pulses make now possible to drive material changes on femtosecond time scale
(
). Due to the complexity of molecular systems, such ultrafast light-driven switches
imply complex out-of-equilibrium dynamics where many different degrees of freedom can be
involved. Furthermore, since the ultrashort excitation pulse brings the system in an excited state far
from its equilibrium in a timescale that can be shorter than atomic motions, the system
reorganization toward a new equilibrium could also imply different time scales at which the various
degrees of freedom rearrange. Thus, different subsystems of a different nature, such as electrons,
spins, phonons, molecular configurations, unit-cell deformations etc. etc. play their part with their
own intrinsic typical timescales [Cailleau 2010]. Therefore, for a given timescale only certain
degrees of freedom are involved during the transformation. The other degrees of freedom act either
as their statistical average or as frozen depending if their configurational rearrangements are faster
or slower with respect to the concerned process. In this way, different photoinduced processes
during the photoinduced change can span over different length and time scales and appear as a
sequence of distinct events.
Fig. 1.1 Typical timescales of different consecutive physical processes involved during a
dynamical photoinduced phase transition in the solid state. [Cailleau 2010].
18
Photoactive Materials
Figure 1.1 illustrates the relevant time scales for the different degrees of freedom involved during
the photo-process. Femtosecond light pulses (with photon energy of the order of eV) excite
electrons. The resulting ultrafast change of inter-atomic forces can induce a deterministic (coherent)
and collective atomic oscillation around new equilibrium positions. In femto-chemistry where
photochemical molecular processes in solution are independent, an ultrashort light pulse can trigger
a coherent atomic wave packet for each excited molecule [Zewail 2000]. Similar coherent processes
are also found into materials. For molecular solids, the excitation may be localized at the molecular
level and the local relaxation can also manifests itself with a coherent structural reorganization
resulting in intra-molecular vibrations [van der Veen 2011, Iwamura 2011]. However, in solids the
excitation may also be delocalized. In the case of Bismuth for instance, the electronic excitation
from valence to conducting bands activates collective coherent optical phonon modes through the
modification of the inter-atomic potential [Sokolowski-Tinten 2003, Fritz 2007, Johnson 2013]. The
physics involved here behind these coherent processes is therefore described by quantum
mechanics.
On the other hand, macroscopic changes, like phase front propagation [Okimoto 2009], require a
movement of atoms or molecules over long distances, and therefore involves slower processes.
Then, elastic deformations propagate on timescales given by the ratio of the system size and the
speed of sound. On longer timescales, different sub-systems or degrees of freedom can equilibrate
and statistical physics may be applied again since transient temperature are reached and different
configurational arrangements are explored. Finally, the finite penetration depth of light in material
may cause gradients of deposited energy and thus of local temperature. The time required to
homogenize the temperature along the sample is then governed by the slow heat diffusivity which
can fall in the order of  s-ms. In addition, the thermalization of the system with the environment
is governed by the system–environment heat transfer, which in turns strongly depends and varies
with the ratio between the sample heat capacity and the heat-transfer rate between the sample
surface and the external environment. The thermal equilibration with the sample environment is
governed by stochastic dynamics which are much slower than those of elementary physical
processes which are therefore hidden in a statistical average [Cailleau 2010]. In this way, the
elementary processes are only resolved during the photo-triggered dynamics and they can be
observed only at their intrinsic timescale. It is therefore required to use ultrafast techniques to
instantaneously clock their dynamics. This is possible with the use of the so-called pump-probe
method, which will be briefly introduced in the next paragraph.
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Andrea Marino 2015
1.2 Time-resolved pump-probe techniques
The development of ultrashort laser sources leaded to a tremendous improvement in the
investigation of the elementary process at the fs
time scale. Nowadays, the pump-probe is
an appropriate technique to track in real time electronic and structural dynamics at the base of
chemical reaction and material transformations. This technique is based on the simple principle to
synchronize the dynamics initiated by a light pulse with the imaging acquisition process and change
the time delay ( fig. 1.2) between the synchronized pump and probe pulses in order to reconstruct
a real time movie.
Fig. 1.2 representation of the pump-probe method
Fig. 1.3 elucidates the pump-probe method with a simple schematic description. At first a pump
pulse excites the system inducing change of state. A successive probe pulse monitors the evolution
of the state of the system at different time delays. In this way, snapshot by snapshot, it is possible to
reconstruct a real time movie of the system evolution. With the change in the temporal delay from
the trigger pump pulse, the probe pulse monitors the system at its different states (fig. 1.3). A single
snapshot acquisition results from the average of the system evolution monitored within the duration
of the probe interaction with the system. Therefore, the shorter is the pulse the shorter is the
interaction and hence, all the slower processes are observed as frozen. This method provides crucial
information on the mechanisms and time scales of electronic and atomic changes.
Fig. 1.3 Scheme of reconstruction of ultrafast dynamical movies. Probe arrives at different time
delays and measure the state of the system.
20
Photoactive Materials
At the present stage, pump-probe techniques are commonly diffused in "home-laboratories", with
compact table-top set up, as well as in large scale facilities such as synchrotrons and free electron
lasers. The nature of both the pump and the probe can easily span from hard X-ray [Rousse 2001,
Sokolowski-Tinten 2003, Bressler 2009, Fritz 2007, Johnson 2009a,b], UV-VIS and IR [Kubicki
2012, Touceda 2012] up to THz [Perfetti 2006, Hirori 2011, Tani 2012] radiations as well as
ultrafast electron diffraction [Siwick 2003, Baum 2007, Gao 2013].
Depending on the nature of the excitation pulses, it is possible to act differently on the various subsystems and directly stimulate different kinds of dynamics. Whereas single UV-VIS excitation
pulses direct chemical reaction [Polli 2010, Pan 2014] or activate phase transitions passing from
transient excited electronic states [Kawavami 2010, Möhr-Vorobeva 2011], an excitation with a
train of multiple pulses of a well defined shape can for instance selectively induce acoustic wave
propagation [Pezeril 2009]. On the other hand, intense terahertz fields can directly act on the
resonant modulation of carriers, molecular rotations or spin precession [Kampfrath 2013]. A single
cycle THz pulse can control the polarization of a material acting on dipolar-induced lattice vibration
[Miyamoto 2013] as well as it can switch on and off coherent spin waves in antiferromagnetic
materials [Kampfrath 2011]. Resonant femtosecond excitation in IR region also makes it possible to
perform vibrational ladder climbing [Strasfeld 2007]
However, not only the pump can selectively induce different dynamics, but also the probe can
selectively investigate the different degrees of freedom, depending on the probe energy and the
probing technique. For example, X-ray fluorescence spectroscopy is sensitive to the spin dynamics
[Zhang 2014], whereas X-ray absorption spectroscopy is sensitive to structural changes and bond
elongation dynamics around the absorbing element [Bressler 2009, Cammarata 2014]. In addition,
ultrafast X-ray diffraction can determine the elementary structural dynamics and atomic motions in
both biological [Levantino 2015a,b] and material systems [Mansart 2013] as well as ultrafast
electron diffraction [Gao 2013]. With these recent advances in the field of dynamical structural
science [Collet 2010] it is now possible to track structural reorganization on the timescale of atomic
or molecular motions (i.e. the phonon frequency).
If this PhD work is not aimed to discuss or develop novel experimental techniques, the studies
reported in the following chapters make a robust use of different pump-probe measurements in the
field of material science. It is therefore appropriate to give to the reader a brief description of the
different techniques used here.
The optical pump-probe measurements have been performed in both reflectivity and transmission
geometries depending on the transparency of the sample. A detailed description of the experimental
set-up can be found in ref [Lorenc 2012]. Both pump and probe optical laser pulses are tunable in
the
range by using optical parametric amplifiers (TOPAS Light Conversion). Two
main optical experiments have been performed:
Two color pump-probe measurements, with heterodyne detection in order to track the dynamical
time traces of the photoinduced phenomena with sufficient time resolution ( 100 fs). A
synchronous phase-sensitive detection at the pump frequency ensured that only the difference of the
probe and reference signals were amplified, hence increasing the signal to noise ratio [Lorenc
2012].
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Andrea Marino 2015
White-light probe measurements, generated from a 800 nm fs pulse focused in a sapphire crystal.
The white-light was detected after sample interaction with a CCD detector (Princeton Instruments
PIXIS 100 with
detection range), giving direct access to the complete VIS
spectra of the photoinduced excited states [Kaszub 2012].
In addition, the time delay between the pump and the probe pulses is usually controlled with a
mechanical motor-stepped stage. The mobile mirror leads to a change in the optical pathway for one
of the two pulses, therefore generating a pump-probe delay on the sample which can be set from 10
fs up to  3 ns. In the out-of-equilibrium photoinduced processes investigated here, slower
processes are of importance. The table-top set up at the Institute of Physics of Rennes allowed to
overcome the temporal delay limitation of conventional mechanical delay stages with an electronic
synchronization of two regenerative amplifiers. By selecting the different single pulses from each of
the amplifiers, it is possible to control the pump-probe delay within 13 ns step (that is the spacing of
the train pulse) thus reaching up the ms time delay between the pump and the probe [Lorenc 2012].
This limitation is given by the intrinsic laser repetition rate of 1 kHz.
Part of this PhD project concerned also the use of complementary time resolved X-ray
measurements. Chapter 2 will present the ultrafast photoinduced structural dynamic of a spin
crossover system obtained with ultrafast X-ray absorption near edge structure (XANES) at the XPP
station of the LCLS X-ray Free Electron Laser (X-FEL) in Stanford [Lemke 2013]. A special timing
tool was used to control and correct the temporal delay between the fs optical laser pump and the Xray pulse of  30 fs [Harmand 2013].
In addition, chapter 4 will report a detailed structural analysis performed by picosecond time
resolved X-ray diffraction at the BioCARS beam line at the Argonne Photon Source APS (USA).
BioCARS, is a NIH-supported national user facility for time-resolved X-ray crystallography,
optimized for laser-pump X-ray-probe measurements with time resolution as short as 100 ps. The
source consists of two in-line undulators with periods of 23 and 27 mm that together provide highflux pink-beam capability at 12 keV as well as first-harmonic coverage from 6.8 to 19 keV. A highheat-load chopper reduces the average power load on downstream components, thereby preserving
the surface figure of a Kirkpatrick–Baez mirror system capable of focusing the X-ray beam to a spot
size of 100 µm horizontal by 20 µm vertical (fig. 1.4). A high-speed chopper isolates single X-ray
pulses at 1 kHz. A high-power picosecond laser system delivers pump pulses tunable over the
wavelength range 450–2000 nm [Graber 2011].
22
Photoactive Materials
Fig 1.4 Setup of the BioCARS beamline at APS Synchrotron. A mechanical chopper system is
used to isolate single X-ray pulses from the storage ring. The laser beam is oriented orthogonal to
the X-ray beam and intersects the crystal at the center of the goniometer rotation. The
chopper/shutter includes a high-heat-load chopper, which produces a 22 ms burst of X-rays and the
Julich chopper capable of isolating a single 50 ps X-ray pulse at a rate of 1 kHz [Graber 2011].
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Andrea Marino 2015
1.3 Materials and Solids
Beside natural or synthetic photo-switches, molecular crystals are promising systems composed of
many interacting elements which can give rise to collective properties as ferro-electricity [Collet
2003, Okamoto 2004, Okamoto 2010] or metal-insulator transitions for instance [Chollet 2005, Gao
2013, Kawakami 2009]. This new possibility to use light for acting on the condensed matter opens
fascinating perspectives to direct functionalities through photoinduced phase transitions. [Nasu
2004]. Therefore, molecular crystals are the perfect candidates to bridge the gap between chemistry
and solid state physics. In this systems, the absorber molecules can act as center to control the
switching of macroscopic functionalities of the material via electronic exited states.
Fig. 1.5 Feedback effect related to non linear response to external perturbation [Cailleau 2012]
In addition to the independent molecular response, cooperative interactions between constituents of
a complex system give rise to an effective field acting on each atom or molecule, leading to abrupt
phase transition and cooperative responses under the external stimuli (fig. 1.5). The strong
cooperativity exhibited by some molecular crystals drives to the emergence of remarkable photoswitchable properties. Beyond the femto-chemistry where the molecule responds independently, in
here the medium is no longer passive but better active through a positive feedback which in turns
originates self-amplification effects and non linear responses to the light excitation as reported in
fig. 1.5 [Cailleau 2012]. Understanding how new physical properties emerge in materials when a
large number of constituents interact with each other represents a basic scientific challenge, which
will be addressed mainly in chapter 4.
Fig. 1.6 Schematic view of the lattice and electronic changes accompaining the M-I phase
transition in
[Chollet 2005]
24
Photoactive Materials
A good example of photoactive molecular solid can be represented by the organic salt
TTF2 PF6. This system is a quasi-1D 3/4-band filled charge transfer organic salt which undergoes
thermal and photo induced insulator-to-metal phase transition [Chollet 2005, Gao 2013].
The electron donor
(D) molecules are stacked forming quadsi-1D columns spaced by
sheets of electron acceptor
anions (A). The high-temperature metallic phase M is characterized
by a charge delocalization over the
columns, while the electron distribution among the
molecules is equivalent along the stacking direction
as shown
(fig. 1.6 right). This inter-layer charge delocalization along the cation stacks confers a high charge
mobility characteristic of the metallic properties of the system. On the other hand, the lowtemperature is characterized by a charge orderd (CO) phase where the electrons are localized only
on one donor molecule over two
leading to dimerization and an insulating phase
(fig. 1.6 left) [Chollet 2005, Gao 2013].
A charge transfer photo-excitation in the insulating phase
leads to
an efficient ultrafast photoinduced phase transition to the metal phase accompanied by a melting of
the charge order. The strong electron-lattice interaction of the
salt is at the base
of the strong non linear response to the light excitattion as it is reported in fig. 1.7 [Chollet 2005].
Fig. 1.7
Non linear response to the pump excitation fluence [Chollet 2005]
Another example of organic charge transfer molecular solids can be represented by the TTF-CA
crystal which exhibits ionic to neutral and neutral to ionic phase transition. The system correspond
to a 1D mixed-stack charge transfer (CT) complex where the 1D chain is composed of TTF donor
(D) and CA acceptor (A) alternately stacked as in fig. 1.8. In the ionic phase, the charge transfer
from D to A is accompanied by symmetry breaking and the formation of dimerized
pairs
along the chain (fig. 1.8).
25
Andrea Marino 2015
Fig. 1.8 Schematic illustrations of neutral D0A0 chains and ionic D+A- chains and respective energy
level structure [Okamoto 2004].
The studies by the group of S. Koshihara demonstrated the possibility to generate the photoinduced
ionic-to-neutral (IN) and its reverse neutral-to-ionic (NI) phase transitions [Koshihara 1999].
Okamoto et al have shown a clear difference between their ultrafast dynamics which has been
explained by considering the ferroelectric nature in the I phase. As a matter of fact, for the IN PIPT
the resonant excitation of the CT band results in a 3 step process where a first confined 1D N
domain (that is a sequence of D0A0 pairs) is multiplied through the cooperative interactions along
the crystal and a macroscopic N phase is stabilized indicating that the charge-carrier injection
makes the neighboring I state strongly unstable. On the other hand, even if the CT excitation
promotes the NI PIPT the ionic size domains are not so large and they decay in 20 ps as they are not
structurally stabilized [Okamoto 2004]. More recently, this group detected by transient reflectivity
measurements that ionic domains are photo generated in the neutral lattice via collective chargetransfer processes within 20 fs. The photoinduced CT is also accompanied by a structural trapping
and coherent molecular vibrations are activated during the process, as observed in fig. 1.9 [Uemura
2010]. The couplings between charge and molecular degrees of freedom play important roles in
photoinduced neutral-to-ionic transition: the subsequent molecular deformations and bending is
associated with the change of molecular ionicity and the charge redistributions in molecules
modulated by molecular motions.
26
Photoactive Materials
Fig. 1.9 [Uemura 2010] (a) Oscillatory component of reflectivity associated with dimeric
molecular displacements (b) and intra-molecular modes. (c & d)
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Andrea Marino 2015
1.4 Spin Crossover Systems
Robust efforts are being made in chemical engineering in order to realize at the level of a material
what has been achieved with molecules in femto-chemistry. Directing the macroscopic
functionalities by controlling the molecular switching of the constituents would enable tremendous
development in technology applications. In this contest, spin-crossover (SCO) complexes are
photoactive prototypes showing reversible bistability between two states of different spin
multiplicity. These materials present a suitable combination of both molecular and solid state
aspects, as SCO molecules have a large impact on the physical properties of the parent solid
material (magnetic moment, color, dielectric constant and electrical resistance). SCO solids are
hence composed of photoactive multifunctional molecules which can give rise to macroscopic
collective and cooperative effects, providing new doorways to the light control of different photoswitchable functions. The switching of an absorbing molecule can therefore act as center to control
the macroscopic functionalities of the material via electronic exited states. In this way, SCO
systems become perfect candidates to bridge the gap between molecular chemistry and solid state
physics [Gütlich&Goodwin 2004, Halcrow 2013].
Despite the still scarce ability to predict the spin-crossover behavior in the solid state and therefore
to perform a fine chemical engineering, SCO systems have been recently addressed toward
promising practical applications such as molecular electronics, data storage, display devices, nonlinear optics and photomagnetism [Létard 2004].
1.4.1 The
Ligand Field
The majority of SCO systems are octahedrally coordinated complexes based on d-block transition
metals, most commonly with
electronic configuration [Gütlich&Goodwin 2004]. In the
octahedral geometry, the influence of the ligands (namely the ligand field) removes the 5-fold
degeneracy of the metal-like d-orbitals into two sub-sets of orbitals: three non-bonding t2g orbitals
of lower energy, and two eg antibonding orbitals at higher energy [Ballhausen 1962]. The splitting
between the t2g and eg orbitals is commonly referred to as the ligand field strength and is
represented in terms of 10Dq (fig. 1.10).
Fig. 1.10 Representation of the SCO octahedron and degeneracy removing of the 3d orbitals.
28
Photoactive Materials
In the ground state, when the ligand field splitting 10Dq is higher than the Coulomb repulsion, the
energetically favorable configuration is found with the maximum number of paired electrons, which
is commonly known as the low-spin (LS) configuration. Otherwise, when the 10Dq is smaller than
the Coulomb repulsions, the electrons will populate the five d-orbitals according to the Hund's rule
with the maximum number of unpaired electrons. The latter is referred to as the high-spin (HS)
configuration [Hauser 2004a].
The spin-crossover phenomenon consists in a reversible rearrangement of electrons around the
metal ion between these two states of different spin multiplicity under an external perturbation. The
different electronic population of the d-orbitals between the HS and LS states strongly influence the
color, magnetic moment of the complex as well as the molecular structure. In fact, for
systems (in
symmetry) the HS configuration presupposes that at least one electron populates the
anti-bonding eg orbitals, resulting in a greater metal-ligand distance for the HS state. Figure 1.11
schematically represents the potential energy curves for the HS state and the quantum mechanical
LS ground state. The minimum of the less bonding HS potential (corresponding to the equilibrium
position of the HS state) is therefore shifted to larger the metal-ligand bond lengths with respect to
the LS state.
At thermal equilibrium, entropy and volume effects play an important role. When the energy
difference
between the potential energy curves of the two spin configurations is comparable
with the thermal energy
, both HS and LS states become thermally accessible (fig. 1.11). Then,
an entropy driven thermal conversion may occur between the LS state (stable at low temperature)
and the HS state (stable at high temperature). More in general, the relative stability of the low-spin
(LS) and high-spin (HS) states may be balanced by external parameters such as temperature,
pressure, or light.
Fig. 1.11 Potential energy wells describing the LS quantum mechanical ground state and the high
entropic HS state thermally accessible if
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Andrea Marino 2015
1.4.2 LS
HS spin crossover in FeII based systems
Among this class, iron(II)-based systems present as outsider shell a 3d6 electronic configuration.
The Fe atom is commonly coordinated by six ligands trough six surrounding nitrogen in a nearly
octahedral symmetry
(fig. 1.12). These molecules exhibit a spin crossover between a singlet
diamagnetic LS state 1A1
and a quintet paramagnetic HS state 5T2
[Gütlich&Goodwin 2004]. Figure 1.12 represent the two possible electronic distributions on the 3d
orbitals of the central Fe ion. In the LS configuration all the six electrons are paired in the lower
lying t2g orbitals and the total spin momentum is equal to
. The HS state presents four
unpaired electrons, two of which occupying the upper lying anti-bonding eg orbitals and resulting in
a total spin momentum
. Because of the change in relative occupancies of the t2g and eg
orbitals, the main structural consequence of the spin crossover results in an average bond elongation
between the Fe atom and the six coordinating N atoms of the ligands. Typically,
LS =2.0
and
for iron(II) SCO systems (fig. 1.12).
HS =2.2
Fig. 1.12 Structural and electronic representation of the LS and HS state
In a general theoretical description, an arbitrary phase transition is commonly associated with the
change of the thermodynamical potential
in function of the independent variables
where the system prefers the state which minimizes the related energy [Dimitriev 2010].
The thermal equilibrium between LS and HS species is given by the difference G of their Gibbs
free energy :



where 
and 
describe a set of N molecules. The entropy change has
several origin: one part is due to the electronic degeneracy (spin, orbital):
g HS

g BS
In a first approximation, considering both the HS and LS state in a perfect
symmetry, the HS
5
( T2) state is 15-fold degenerate, resulting from the orbital triplet and spin quintet degeneracy. On
30
Photoactive Materials
the other hand, since in the singlet LS (1A1) state all the t2g orbitals are fully occupied, its
degeneracy is equal to 1. Then, the molecular entropy for the HS and LS states is:
and
other contribution to the entropy change include vibration terms (as the frequency of the phonon
change with the electronic state). Both effects results in a higher entropy for the HS state and

,
corresponds to the temperature where:
H
S
However in the solid state the interaction between the molecules constituting the crystal can induce
various degrees of cooperativity and long- and short- range neighboring interactions have to be
considered.

The SCO is commonly reported as the evolution of the HS molecular fraction (XHS) and it can be
monitored using various techniques. From the most common measurements, Mössbauer
spectroscopy is able to identify the separate contributions of the HS and LS states. SQUID
techniques measure directly the magnetic susceptibility which is strongly related to the molecular
spin-state. On the other hand, X-ray diffraction can have access to the HS fraction by measuring the
average
bond length characteristic of the t2g/eg occupancy [Gütlich&Goodwin 2004].
Furthermore, since the spin crossover between the HS and LS electronic configuration has a strong
impact on the color of the system, optical spectroscopy is also a good candidate to follow the
evolution of XHS. However a more detailed description of the different spin transition curves (as
well as for the techniques to obtain XHS) will be given in Chapter 4, where it will be discussed the
aspect of photoinduced phase transition associated to a symmetry breaking.
For the moment, let us concentrate on a simpler description of the SCO phenomenon. The thermal
behavior of the HS fraction (XHS) can be described in terms of an Ising-like model [Bousseksou
1992, Boukheddaden 2007]. Considering the effective Hamiltonian of the system:
where
is the Ising variable which describes the two spin configuration of the ith molecule with
eigenvalues
for the HS configuration, and
for the LS. The first term in eq. 1.1 correspond to
the effective one-site Hamiltonian expressed in terms of a field
which correspond to the Gibbs
free energy difference per molecule between the fully HS and LS crystal:
where is the enthalpy difference between the HS and LS states, and the entropic term is instead
governed by
the degeneracy ratio of the HS and LS states. On the other hand, the
second term of the Hamiltonian in eq. 1.1, expresses the cooperative interaction of the system in
terms of a coupling constant between the first neighbors.
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Andrea Marino 2015
Fig. 1.13 Up Schematic (h, T) phase diagram for a cooperative (a) and a non-cooperative (b) SCO
system. Bottom corresponding thermal evolution of the HS fraction XHS respectively showing a
first order transition and smooth spin crossover.
The thermal SCO behaviors is therefore determined by the crossing of the isobaric oblique line with
the
horizontal line in the
diagram, as it is represented in fig. 1.13. At the crossing the
transition temperature is defined
and
. Both the HS and LS states are equally populated
is independent on the degree of cooperativity . On the other hand, the
critical temperature is given in the mean-field approximation
taking into account the
coupling interaction between the other n neighboring molecules. If the SCO line crosses the
above , then a smooth spin crossover occurs (fig. 1.13b). Otherwise if the crossing occurs below
an abrupt 1st-order transition will take place (fig. 1.13a). Despite this simple model does not
consider elastic interactions, it well expresses the cooperativity between the molecules and nicely
describes the SCO behavior of a solid.
1.4.3 Photoinduced spin-state switching
The main scientific attraction for the SCO systems dwells in their bidirectional photo-switchable
properties mainly resulting in photo-chromic and photo-magnetic responses. Light has been
demonstrated to be an efficient control parameter, which selectively directs a reversible spin
conversion between the HS and LS states. In fact, by irradiating the LS state into specific absorption
32
Photoactive Materials
bands it is possible to quantitatively convert the LS into a metastable HS state or vice versa [Hauser
1986].
Fig. 1.14 Schematic representation of a generic SCO phase transition with LIESST at low temperature.
Blue line representing the thermal crossover with hysteresis loop characteristic of a cooperative system. Red
arrow representing the LIESST at 10 K. Green line reports the thermal recover from the PIHS state. Purple
arrow represent the reverse-LIESST from the PIHS state toward the LS ground state.
The magnetic susceptibility M of a SCO crystal (obtained with SQUID measurements) gives direct
information on its spin state. More especially, since for FeII systems the LS state is diamagnetic
(S=0) and the HS state is paramagnetic (S=2), the fraction of molecules in the HS state is directly
related to the MT product. Figure 1.14 reports an example of a typical thermal transition curve of a
SCO solid system showing strong cooperativity, underlined by the first order transition and its
thermal hysteresis loop. At low temperature, continuous wave cw laser excitation at the appropriate
wavelength can photo-induce the LS
HS conversion. The photoexcited molecules will remain
trapped in the HS state if the temperature is sufficiently low, so that the energy barrier between the
HS and LS potential minima is not thermally overcome. This metastable light-induced population
of HS state has been named Light Induced Excited Spin State Trapping (LIESST) [Decurtins 1984].
Since then, robust literature have built up on the investigation of this photoinduced phenomenon
[Gütlich&Goodwin 2004, Halcrow 2013]. However, once the irradiation is stopped, the system
recovers the thermal equilibrium in the LS ground state. The relaxation process from the metastable
photoinduced HS (PIHS) state strongly varies with the temperature and hence its lifetime can span
from ms to several days [Hauser 2004b]. The red arrow
in fig. 1.14 represent the LIESST
phenomenon at 10 K. At this temperature the PIHS is long lived and magnetic measurements can
demonstrate the effective population of the HS state by the strong increase of the MT product. This
process has been demonstrated to be fully photo-reversible for some systems. In fact, with a
33
Andrea Marino 2015
different appropriate wavelength
it is possible to switch back the HS molecules to the ground
LS state. This phenomenon is known as the reverse-LIESST [Hauser 1986].
At sufficiently low temperature, conventional SQUID measurements can easily prove the
occurrence of the photo-switching. The green curve in fig. 1.14 reports the heating process
(typically 3 K/min) after LIESST. The HS state is still detectable up to a pseudo-critical temperature
TLIESST above which the HS
LS recover is too fast to allow conventional techniques to monitor
the LIESST process [Létard 1998, Létard 1999]. In addition to SQUID measurements, other
conventional techniques can be coupled to light irradiation for characterizing the long-lived
photoinduced HS states, such as Mossbauer spectroscopy, optical, IR and Raman spectroscopies, Xray diffraction etc. etc. [Halcrow2013]. When the relaxation process from the photoinduced HS
state to the LS state is slow enough, these techniques can also provide information on the
cooperativity of the process and on the macroscopic dynamics. The LIESST and reverse-LIESST
phenomena have been deeply investigated under cw and ns laser irradiation well below the pseudocritical LIESST temperature TLIESST, focusing the attention mostly to the HS
LS relaxation
processes [Hauser 2004b]. However, conventional techniques such as SQUID are too slow for
detecting the elementary processes at the origin of the switching which are then measured only in
their statistical average. Therefore, these studies did not provide a view of the mechanism and the
time scales involved in the LS
HS photoswitching. It is evident then, the necessity of ultrafast
studies in order to obtain a complete understanding of the elementary processes of the LIESST and
reverse-LIESST mechanisms.
34
Photoactive Materials
1.4.4 Ultrafast and Out-of-Equilibrium Dynamics
The first ultrafast time-resolved investigations of the LIESST mechanism were performed for
isolated SCO molecules in solution. The major developments by the groups of Chergui, McCusker,
McGarvey, Hendrickson, Mathies, Schoenlein and others, tried to unveil the complete mechanism
of the intersystem crossing (ISC) mechanism revealing each necessary step to reach the
photoinduced HS state and trying to identify the LS to HS pathway [McCusker 1993]. Timeresolved fluorescence up-conversion evidenced an ultrafast ISC between the photoexited 1MLCT
and the 3MLCT identified as an intermediate state. The latter in turn, seemed to undergo ISC with a
triplet ligand-field state, thus ending up with the 1MLCT 3MLCT 3T HS sequence [Gawelda
2007]. In addition to optical pump-probe techniques, the new generation of X-ray free electron
lasers, opened the doorways to a more complete view of the elementary structural processes
involved in the photo-induced mechanisms. Femtosecond time-resolved XANES (X-ray absorption
near edge structure) gave a comprehensive description of the subtle coupling between the change of
electronic state and the structural reorganization highlighting the direct 3MLCT decay into the HS
state, thus bypassing the ligand field triplet state [Bressler 2009].
Fig. 1.15 Scheme of the ultrafast LIESST pathway across potential energy curves [Cannizzo 2010].
The picture in fig. 1.15 corresponds to the schematic scenario of the ultrafast LIESST present at the
beginning of this PhD project. The photo-excitation promoted the LS 1MLCT transition which
triggered the ISC cascade toward the HS state. The ISC between the two 1MLCT and 3MLCT
manifolds have been set to occur in less than 30 fs, whereas the 3MLCT relaxes in 130 fs into a
high vibrational exited the HS state, time constant corresponding to the half period of the
oscillation. A non radiative vibrational cooling has been observed to bring the system to the bottom
of the HS potential in a timescale between
[Cannizzo 2010]. The system would then
recover the LS ground state in 650 ps. This time constant at room temperature strongly differs from
35
Andrea Marino 2015
the one measured for SCO in solids around 10 K [Hauser 2004b] and underline the importance of
time-resolved techniques for tracking such short-lived states.
A more recent study of LIESST in solution pointed out a coherent activation of vibrational modes
under LIESST phenomenon in the Fe(bpy)32+ compound, which was interpreted as the activation of
a coherent wave-packet for all the switched molecules resulting from ligand bending [Consani
2009].
Fig. 1.16 Coherent wave-packets observed in Fe(bpy)32+ after LIESST [Consani 2009].
Concerning the ultrafast studies at the solid state, at the beginning of this project only few works
were published on iron(III)-based SCO systems [Lorenc 2009, Lorenc 2012, Bertoni 2012, Collet
2012] where the nature of electronic states involved are slightly different
. However, the initial photoswitching process in these solids correspond just to molecular
phenomena where the dynamics are similar to the one reported for molecules in solution (fig. 1.17):
the LMCT state is induced by light excitation, which relaxes, possibly through intermediate states,
to the HS state with
. The dissipation of energy between the photoexcited molecule and
the lattice is associated with a vibrational cooling (VC) falling in the ps range.
36
Photoactive Materials
Fig. 1.17 LIESST dynamics for FeIII SCO complexes [Bertoni 2012]
However, the photoinduced switching of a SCO crystals is complex and involves several
consecutive steps. The excitation via ultrashort laser pulses perturbs the system as it deposits energy
on the lattice within a timescale much shorter than thermodynamical equilibration. The system is
carried far from its thermal equilibrium and several processes take place, from a local (molecule) to
a macroscopic (crystal) change involving several degrees of freedom. Femtosecond photoexcitation
in SCO solids induces a multi-step out-of-equilibrium dynamics evidenced on FeIII-based SCO
crystals [Lorenc 2009, Lorenc 2012, Collet 2012, Kaszub 2013]. The sequence of events reported in
these works is schematically represented in fig. 1.18 where typical time-resolved optical and X-ray
data present a three steps dynamic before the system recovers the ground state:
1. fs Molecular Switching
2. ns Elastic Expansion
3. s Thermal Population
4. ms Recover
As explained above, at first the laser excitation locally switches a small fraction of molecules
from LS to HS state within 200 fs. This process, commonly referred to as photoswitching step, is
confined at the molecular scale and shows a linear response to the excitation density: one photon
convert one molecule. The elementary process of the photo-induced spin-switch corresponds to an
electronic response localized around the absorbing molecule [Bertoni 2012]. At this stage, X-ray
diffraction shows that the crystal volume remains unchanged underling the local nature of this step
[Lorenc 2012].
37
Andrea Marino 2015
Fig 1.18 Multi-step out-of-equilibrium dynamic of FeIII SCO crystal probed via a) optical
spectroscopy b) X-ray diffraction [Lorenc 2012].
1. Molecular Switching: local trapping and linear response.
2. Elastic Step: propagation of elastic interaction and self amplification.
3. Thermal Step: heat diffusion and non-linear thermal population.
4. Ground State Recover: Thermalization with the N2 cryostream.
c) Schematic representation of the LIESST 3-steps process before the ground state recover.
blue and red respectively represent the molecules and the lattice in the LS and HS states.
38
Photoactive Materials
The photoexcited HS molecules, of higher volume, generate internal constrains as schematically
depicted in fig 1.19. The local heating together with this internal pressure lead to a lattice expansion
via propagation of elastic interaction as reported in fig. 1.18b, observed here at ns timescale, where
optical spectroscopy monitors an increase of the HS fraction
. This pressure-driven process is
known as elastic step and it is limited by the elastic strain propagation with the typical timescale of
the speed of sound in solids ns. In analogy with a chemical substitution of in an homogeneous
matrix [Hauser 2004b], the bigger HS molecules exercise an internal (or negative) pressure which
stabilizes the HS potential and therefore favors the LS
HS switching. Furthermore, as the
schematic (P,T) phase diagram in fig. 1.19 elucidates, the volume expansion will shift the value of
the crystal's internal pressure to lower values. At the ns timescale elastic equilibrium has already
occurred, and therefore the crystal reaches a new transient pressure
where the HS fraction
equilibrates to a new transient value at
. It was shown that in FeIII SCO systems a
typical molecular switching of
could easily correspond to a local internal pressure of
Kaszub 2013].
Fig. 1.19 Representation of elastic step due to molecular swelling and internal negative pressure.
[Kaszub 2013]
Finally, at the s timescale the thermal step occurs: the heat deposited by the laser excitation
propagates resulting in a macroscopic crystal heating. The temperature increase T promotes a
further thermal population of the HS state
. Then, the temperature of the crystal decreases by
heat exchange with the cryostat and the LS ground state is recovered in the ms time scale. The
thermal step will be discussed in more details in chapter 4.
39
Andrea Marino 2015
1.5 Contest and aim of the PhD project.
The development of photoactive materials is important for new technological application. However,
the mechanisms involved in the photoswitching process are poorly understood. The multi-scale
aspects observed in molecular materials makes this analysis even more difficult.
As prototypes photo-reversible molecular switches, SCO materials are of particular interest. Until
the beginning of this PhD the question about the possibility to generate ultrafast LIESST in FeII
solids was still open. Can the ultrafast LIESST mechanism observed in solution be generalized for
any SCO system? Does the mechanism observed in solution differ in the solid state? Which are the
important parameters governing the photoswitching? How the different degrees of freedom react
and in which timescale? Do LIESST and reverse-LIESST involve similar mechanism? How other
degrees of freedom related to symmetry and ordering play their role? Is it then possible to explore
new routes for developing new photoactive materials?
Ultrafast studies such as this PhD work aim therefore to open new perspectives in the development
of new hybrid-material making use of the current understanding of the elementary physical
processes.
Chapter 2 will present the first femtosecond investigation of LIESST in an iron(II)-based spincrossover crystal. A detailed description of the femtosecond spin-state photoswitching will be give
based on the experimental results and a comparison with the literature. The LIESST phenomenon is
observed to be accompanied by a coherent structural dynamics were a rapid energy transfer to the
lattice traps the system in the photoinduced HS state.
On the other hand, Chapter 3 will present the first investigation of the comparison of the LIESST
(induced with a ligand field 1T1 excitation) and reverse-LIESST mechanisms. In the latter process,
an intermediate triplet state has been clearly identified with a lifetime corresponding to tens of ps.
However, if the reverse-LIESST follows the Born-Oppenheimer approximation, the LIESST
process shows a strong coupling between the electronic and atomic wavefunctions.
Chapter 4 will present a structural investigation of the response of a spin-state concentration wave
(SSCW) to light excitation. The evolution of the order parameters describing the spin concentration
and the molecular HS-LS ordering was followed in real time performing complementary timeresolved X-ray diffraction and optical spectroscopy.
Finally Chapter 5 will present the perspectives and conclusions of this work.
40
Chapter 2
Ultrafast LIESST
and
Energy Redistribution
Part of this work was published in:
A. Marino, M. Servol, R. Bertoni, M. Lorenc, C. Mauriac, J.-F. Létard, E. Collet
Femtosecond optical pump-probe reflectivity studies of spin-state photo-switching in the
spincrossover molecular crystals [Fe(PM-AzA)2(NCS)2]
Polyhedron 66, 123-128 (2013)
Andrea Marino 2015
42
Ultrafast LIESST and Energy Redistribution
Photoinduced phase transitions open fascinating perspectives for controlling with light the physical
properties of materials and especially molecule-based magnets. A well-known example is the LightInduced Excited Spin State Trapping (LIESST) phenomenon undergone by numerous spincrossover (SCO) compounds [Gütlich&Goodwin 2004, Halcrow 2013]. Weak continuous wave
(cw) laser irradiation at low temperature is known as an efficient way for controlling SCO materials
by light. By choosing the appropriate excitation wavelength, it is possible to selectively populate the
high spin (HS) state (LIESST) or low spin (LS) state (reverse-LIESST) which are long-lived at low
temperature. Despite many reports on the relaxation mechanisms after cw or nanosecond laser
excitation, little has been known about the molecular transformation dynamics. Ultrafast studies of
the photo-switching process performed on single molecule in solution have mainly shown that LS
HS LIESST occurs on the sub-picosecond time scale [Chergui 2012].
At the solid state, the ultrafast LIESST dynamics was studied only for iron(III)-based SCO
compounds using femtosecond optical spectroscopy [Moisan 2008, Lorenc 2009, Lorenc 2012,
Bertoni 2012]. Part of this project was the first report on the LIESST dynamics in iron(II)-based
SCO solids [Marino 2013], followed by other more recent works reporting on the photoswitching
dynamics of the
SCO material, investigated by femtosecond optical and X-ray
absorption spectroscopies [Cammarata 2014]. However, in the solid state it is not always possible to
perform transient absorption spectroscopy for studying photoinduced dynamics. In fact, the large
optical density of some SCO materials (as for example the
crystal
presented in this chapter) imposes the use of reflectivity techniques [Marino 2013].
The first part of this chapter will present the
compound and demonstrate
the good correlation between the change of optical reflectivity (OR) and the spin-crossover
observed at thermal equilibrium. Then, a second part will focus on time-resolved optical reflectivity
and X-ray absorption studies to discuss the LIESST dynamics in the solid state. Since the LIESST
process has a pronounced local molecular aspect in the picosecond time scale, the ultrafast
dynamics of the
crystals is later compared with the dynamics of the same
molecule diluted in a passive
matrix. This trick opened the perspectives
to a better understanding of the energy exchange between the photo-switched molecule and its
crystalline environment.
43
Andrea Marino 2015
2.1 The
spin-crossover compound
Among the class of iron(II) SCO solids, the
single crystal was adopted in
order to perform ultrafast investigations aimed to understand the elementary dynamics of the LS
HS photoswitching at the solid state. The
(cis-bis(thiocyanato)bis[(N-2'pyridylmethylene)-4-(phenylazo)aniline]) molecular system is represented in fig. 2.1. It crystallizes
in the monoclinic space group P21/c with one molecule as asymmetric unit and not coplanar phenyl
rings [Guionneau 1999]. The dark colored crystals of typical
size presents a
characteristic parallelepiped shape, with smooth clean and large surfaces [Marino 2013].
Fig. 2.1 Molecular structure of the [Fe(PM-AzA)2(NCS)2] compound.
The
crystals undergo a smooth isostructural spin crossover centered at
, typical of non cooperative systems [Létard 1999]. As usual for SCO compounds, a
structural reorganization is strongly coupled to the spin-conversion. The main structural
deformation concerns the
octahedron with a change in
bond length as well as a robust
distortion of the
angles. As a matter of fact, the consequence of the promotion of two
non-bonding t2g electron into two anti-bonding eg orbitals during the change of electronic
configuration from LS
to HS
leads to an overall increase of the
bond
lengths from
to
. Moreover, the HS state of the present
compound is strongly distorted with the
angles far from 180° [Guionneau 1999].
Several techniques sensitive to different parameters can be used as good probes to follow the spin
state switching. Fig. 2.2 reports the nice correlation of the spin crossover monitored with SQUID
measurements, X-ray absorption near edge structure (XANES), and optical reflectivity (OR).
SQUID magnetic measurements in fig. 2.2a indicate, through the evolution of the
product
( being the magnetic susceptibility), the gradual spin state changes from the mainly HS phase
above 270 K to a mainly LS phase below 130 K. At 10 K cw laser irradiation centered at 830 nm
induces a partial LS HS conversion long lived up to
[Létard 1999]. The cause
of a non complete photoinduced spin state switch derives from the crystal thickness and the high
absorption coefficient which limit the penetration depth of the excitation wavelength. In any case,
SQUID measurements are too slow for investigating the photoinduced dynamics and therefore the
fraction of crystal converted by light on the time scale of elementary processes which typically falls
in the sub-picosecond range. Nevertheless, this limitation is overcome with the use of ultrashort
optical and X-ray pulses which can now routinely achieve 100 fs (or less) time resolution.
44
Ultrafast LIESST and Energy Redistribution
X-ray absorption spectroscopy (XAS) at the FeII K-edge has been demonstrated to be a sensitive
probe to the
bond lengths for very similar FeII compounds [Bressler 2009, Cammarata
2015]. Therefore the change of XANES signal at 7125 eV in fig. 2.2 (which will be discussed later)
mainly results from the
elongation and constitutes a clear fingerprint of the formation of
the HS structure.
Fig. 2.2 Thermal spin crossover of the [Fe(PM-AzA)2(NCS)2] monitored with the evolution of the
(Up) magnetic susceptibility via SQUID measured, (Middle) X-ray absorption spectroscopy at the
Fe-K edge 7125 eV, (Bottom) total optical reflectivity OR. Transition temperature
.
The increase of
and OR is related to the LIESST phenomenon under cw excitation observed at
low temperature.
Furthermore, since the SCO phenomenon strongly influences the color of the material, the spin state
change can also be monitored with optical measurements. Fig. 2.2 reports the overall reflectivity
change with temperature, and this matches the variation of HS fraction obtained with a more direct
techniques. In the case of study, the crystals appear fully dark and the large optical density on the
visible range (VIS) does not allow transmission measurements [Marino 2013]. On the other hand
crystals become more transparent in the near-infrared region (NIR) where it was possible to perform
transient absorption spectroscopy for a crystal thickness lower than 50 m. In such a way, the
optical pump-probe experiments were configured in NIR-transmission and VIS-reflection geometry
with quasi-collinear configuration. The temporal evolution of Optical Density (OD) and Optical
Reflectivity (OR) were respectively obtained from the relative change of transmitted and reflected
probe signal. Due to the high sensitivity of the surface roughness, during the OR measurements the
quality of the large faces reflectivity was maintained under cycling between LS and HS states.
45
Andrea Marino 2015
2.2 Electronic vs Structural Dynamics
This chapter aims to a clear understanding of the elementary process at the base of the ultrafast
LIESST mechanism. It is consolidated that the spin crossover implies a strong structural
reorganization but just recently the strong electron-structural correlation has been found to play a
key role in the trapping of the HS state [van Veenendaal 2010, Cammarata 2015]. In this context,
the use of complementary probes, sensitive to different degrees of freedom, strongly helps a
complete investigation of both ultrafast electronic and structural dynamics. Optical probes are
mostly sensitive to the outsider electronic states and hence to the electronic configuration of the Fe
atom. On the other hand, the atomic spatial resolution of X-rays probes, made them one of the most
suitable tools for structural investigations. More in the specific, XANES measurements at the Fe Kedge are strongly sensitive to Fe first coordination shell [Briois 1995] and therefore to the average
bond elongation of SCO solids, a characteristic fingerprint of the formation of the HS
structure. For all the following pump-probe experiments reported in this chapter, the pump
excitation wavelength was set in the MLCT band at around  850 nm, which has been demonstrated
to efficiently induce LIESST with the typical pump fluence of 2.5 µJ/mm2.
2.2.1 Femtosecond Optical Pump-Probe Studies
The thermal SCO behavior in the
crystal has been firstly optically
characterized. Fig. 2.3 reports the thermal change of the optical reflectivity (OR) in the spectral
region of interest (630 nm - 750 nm) for the pump-probe experiments. An isosbestic point at around
690 nm denotes two distinct regions which can be identified as the optical fingerprint of the spin
conversion [Marino 2013]. Below 690 nm the optical reflectivity increases during the LS
HS
switching, whereas it decreases above 690 nm.
Fig. 2.3 Change of [Fe(PM-AzA)2(NCS)2] optical reflectivity spectra acquired at different
temperatures. In red (──) and dark blue (──) respectively the OR spectra of the fully HS and LS
states.
46
Ultrafast LIESST and Energy Redistribution
Broad-band white-light spectroscopy is an efficient technique to detect the photoinduced change in
the whole VIS spectrum at the same time. Fig. 2.4 compares the optical fingerprint under thermal
activation (from fig. 2.3) with the OR change observed by time-resolved white-light spectroscopy.
In order to have a direct comparison with the time-resolved measurements, the plot in fig. 2.4a is
obtained from the ratio between the OR of the fully LS state at 100 K and the OR of the HS state at
270 K. The OR of the photoinduced state measured 10 ps after laser excitation (fig. 2.4b)
reproduces the optical signature of the LS
HS conversion observed at thermal equilibrium (fig.
2.4a), giving a direct proof of the occurrence of the ultrafast photoswitching. However, whereas the
OR obtained after 10 ps is a clear fingerprint of the LS HS photoconversion, the white-light OR
measured within the first 50 fs shows different optical properties from the LS and HS states. Such a
transient variation of the OR in the sub-picosecond timescale corresponds to the OR spectrum of the
one or several intermediate states involved in the intersystem crossing (ISC) toward the HS
potential.
Fig. 2.4 a) Optical fingerprint of the LS HS crossover reported in form of ratio between the HS
and LS spectra respectively recorded at 270 K and 100 K b) Time-resolved 850 nm pump white
light probe. Photoinduced HS state spectrum at 10 ps after laser excitation (red solid line ──).
Intermediate states (INT) spectrum (green solid line ──) recorded as the time zero envelope for
each wavelength taking into account the wavelength chirp of the probe. The white light bunch pulse
has a group velocity dispersion of approximately 2.6 ps due to an optical path dispersion in the
sapphire crystal
47
Andrea Marino 2015
On the other hand, dynamical time traces recorded at selected probe wavelength enable to track in
real time the ultrafast photoswitching dynamics. For the following experiments, two
monochromatic ultrashort laser pulses of the duration of 40 fs each one, lead to an overall
instrumental response function (IRF) in the order of 80 fs. The time traces obtained with the probe
set at 640 nm, 690 nm and 720 nm, and reported in fig. 2.5, clearly reproduce the optical
fingerprints characteristic of the LS → HS switching, with an ultrafast OR increase at 640 nm and
decrease at 720 nm. Furthermore, time resolved analysis around the isosbestic point (690 nm),
where LS and HS states contribute equally, allows an isolated observation of the dynamics of the
intermediate state(s) involved during the spin-state photo-switching. Such intermediate states (INT),
as the initially photoexcited 1MLCT state (t2g5eg0L1), are responsible for the transient reflectivity
peak around
. An exponential fit convoluted with a Gaussian temporal IRF of 80 fs, pointed
out that both increase and decrease of OR at respectively 640 nm and 720 nm are equivalent to a
stepwise function, as well as the fit the Gaussian shape transient peak observed at 690 nm indicates
that the INT state(s) decays toward the HS state within less than 50 fs. This is a clear indication that
the HS state is reached in less than the experimental time resolution (since the peak is found of
Gaussian shape) which in turns prevented from an accurate determination of the lifetime of the
1
MLCT state and of the ISC time constant. The depopulation of the 1MLCT state is too fast to be
observed and therefore the first probed dynamics correspond to the population of HS state by the
depopulation of the intermediate (INT) states. Hence, the arrival on the HS potential can only be
estimated to be less than 50 fs [Marino 2013]. Furthermore the temporal limitation of the
experiments impedes an identification of the INT states involved in the ultrafast LIESST. These
results are in good agreement with previous works reported for iron(II) compounds in solution
[Gawelda 2007, Cannizzo 2010].
Fig. 2.5 Two-color pump-probe measurements recording the LIESST dynamical time traces of the
[Fe(PM-AzA)2(NCS)2] single crystal upon excitation in the MLCT bands at 850 nm. The change of
OR for different probe wavelengths respects the optical fingerprint of the LS HS switching.
48
Ultrafast LIESST and Energy Redistribution
Besides, longer records of the time traces present a slower dynamical relaxation towards a plateau
after the first OR stepped-like change. Fig 2.6 reports the dynamics recorded at 660 nm and 635 nm
up to 8 ps after pump excitation. In this case, an exponential fit exploits a time constant in the order
of 1-2 ps varying on the different probe wavelengths. This process corresponds to a non radiative
vibrational relaxation inside the HS potential known as vibrational cooling (VC). Since the energy
difference between the LS and HS states is in the order of the thermal energy ( 20 meV), a huge
excess of energy is deposited on the molecule by the absorbed photon ( 1.46 eV). At the subpicosecond timescale, the energy has yet no time to dissipate and after 100 fs most of the absorbed
energy is still located on the molecule. The HS state, populated via ISC, is therefore reached on a
highly vibrational exited state and its relaxation toward the bottom of the HS potential occurs in a
time range of 1-2 ps. This phenomenon has been demonstrated to be wavelength sensitive due to
spectral narrowing which is a well-known marker of vibrational cooling [Juban 2005, Smeigh 2008,
Bertoni 2012]. Compared to the cooling constant reported for iron(II) molecules in solution (1 – 10
ps) [Gawelda 2007], the vibrational cooling in solids is faster. The dissipation of the photodeposited energy is strongly related to the ability of the absorber molecules to couple its vibrational
modes with the environment. At solid state inter-molecular coupling and lattice vibrations may open
additional and more efficient channels.
These data present a good matching with the actual literature on SCO molecules in solution [Juban
2006, Cannizzo 2010] as well as for other transition metal-based SCO solids [Juban 2005, Bertoni
2012]. This underlines that the ultrafast LIESST process in SCO solids is strictly confined to a
molecular response where the elementary electronic process answers to light irradiation
independently to the environment.
Fig. 2.6 Two-color pump-probe measurements recording the LIESST dynamical time traces of the
[Fe(PM-AzA)2(NCS)2] single crystal upon excitation in the MLCT band at 850 nm. Both probe
wavelengths exhibit a step rise in less than 50 fs and a slower vibrational cooling toward a plateau
reached in  1-2 ps.
49
Andrea Marino 2015
2.2.2 Time resolved XANES
So far, optical pump-probe experiments enabled to observe the electronic elementary process of the
LIESST mechanism. Since a strong structural reorganization accompanies the spin crossover, a
detailed study of the ultrafast structural modifications involved is needed.
With the development of new ultrashort X-ray laser pulses with X-ray free electron lasers (XFEL),
the structural dynamics can be tracked with the accuracy of 100 fs. For this project measurements of
the X-ray Absorption Near Edge Structure (XANES) were performed. Fig. 2.7 reports the XANES
spectrum of a similar complex
for the fully LS and HS states [Cammarata
2014]. In the HS state the Fe K-edge present a shift to lower energies with respect to the LS
absorption edge. The difference between these two spectra (plotted as XANES in fig. 2.7) presents
an important XANES change for the energy of 7125 eV sensitive to the molecular spin state
change. More in particular, the main contribution at the FeII K-edge comes from its local
coordination geometry, that is the six surrounding nitrogen [Briois 1995]. This implies that the
difference in the XANES absorption is proportional to the change in
distance, ergo the FeII
K-edge XANES is a good probe to observe the average
bond length change.
Fig. 2.7 Bottom XANES spectra for the [Fe(phen)2(NCS)2] in the HS state
as well as in the LS
state
. Up ΔXANES: difference between the HS and LS XANES spectra reported on bottom. The
7125 eV photon energy has been selected as sensitive probe to the change of the molecular spin
state [Cammarata 2014].
50
Ultrafast LIESST and Energy Redistribution
Time-resolved X-ray absorption spectroscopy (XAS) was performed at the XPP station of the
LCLS X-FEL (X-Ray Free Electron Laser) in Stanford. For this experiment  50 fs laser pulses (at
850 nm) triggered the above mentioned LIESST via MLCT process. The change in XANES was
recorded with  30 fs X-ray probe pulses centered at of 7125 eV, the most sensitive energy to the
spin state change (see fig. 2.7). Figure 2.8 reports the time course of the XANES signal. The
increase of XANES after hundreds of fs mainly results from Fe-N elongation and sets a clear
identification of the characteristic fingerprint of the HS structure formation.
The experimental points in fig. 2.8 were fitted by convolving a Gaussian temporal instrument
response function (IRF) with an exponential rise
. The 110 (10) fs FWHM IRF, obtained with
a timing tool designed to synchronize the optical and the X-ray laser pulses [Harmand 2013],
allowed an accurate determination of
. Due to the high probe sensitivity to the
elongation, this time constant can be easily attributed to the time it take to the molecule to
stretch the
bond after photoexcitation. Furthermore, it is important to highlight that the 
160 fs for the
bond elongation is of the order of the half period of the
stretching
vibrational mode, the so-called breathing mode [Cammarata 2014].
Fig. 2.8 Ultrafast LS
HS photoswitching probed with the change of XANES at the Fe K-edge
after laser irradiation at
for the [Fe(PM-AzA)2(NCS)2]. The
increase of XANES signal results from the
bond elongation characteristic of the HS
structure formation.
These findings are of fundamental importance. They open to a broad vision of the elementary
processes involved during the photoswitching mechanisms. More in particular for this project, the fs
optical reflectivity gives access to the understanding of the electronic mechanisms, whereas
structural probes such as X-ray lead to the complementary observation of the structural dynamics.
51
Andrea Marino 2015
The combination of optical and X-ray pump-probe spectroscopy determined the ISC in within tens
of fs, while the
bond elongation is measured in  160 fs with time-resolved XANES. These
results are in good agreement to those reported for other FeII molecules both in solution [Bressler
2009], as well as in solid [Cammarata 2015].
Thus far, these results can be summarized in a schematic representation reported in fig. 2.9. The
1
optical laser pump at 850 nm promotes the LS
MLCT spin allowed transition. Once in the
excited state the system undergoes an ultrafast ISC which occurs in 50 fs and possibly through
several intermediate (INT) states. This time constant is obtained from optical measurements, which
again are sensitive to the outsider electronic configuration. Therefore it underlines that the
electronic HS configuration is reached in less than 50 fs. However, it is only after 160 fs that the
molecule reaches the HS structural configuration with the characteristic
bond elongation.
Fig. 2.9 Schematic summary of the ultrafast LIESST mechanism activated via LS 1MLCT laser
excitation. Decay of the excited 1MLCT state in less than 50 fs where possible intermediate (INT)
states are involved in the intersystem crossing to the HS state. The HS electronic configuration
implies a shift of the reaction coordinates to higher
bond lengths. This elongation is
measured to occur in  160 fs. The HS potential is reached in a vibrational excited state. The non
radiative vibrational cooling inside the HS potential to its bottom occurs in  1.5 ps
The ISC occurs in less than 50 fs and the HS potential is reached in a vibrational excited states.
However at this stage, the systems is in a HS electronic configuration shrunk in a LS structure.
Then, the electrons in the anti-bonding eg orbitals lead to a robust
elongation reaching the
HS geometrical configuration in within  160 fs. Furthermore, the system relaxes to the bottom of
the HS potential in 1-2 ps via a non radiative vibrational cooling.
These first conclusion of the LIESST mechanism in the solid state raise additional questions: What
happens to the energy released on the molecule by the absorbed photon? Where does the energy go?
How is it redistributed inside the crystal?
52
Ultrafast LIESST and Energy Redistribution
2.3 Coherent Structural Dynamics
A detailed analysis was carried on around the isosbestic point (690 nm), allowing an isolated
observation of the dynamics of the INT states involved in the spin-state switching. In the ultrafast
OR changes recorded at 680 nm, 690 nm and 700 nm and reported in fig. 2.10, it is possible to
identify the transient peak characteristic of the short living 1MLCT exited state
and its
ultrafast ISC toward the HS state, followed by two main oscillating components. As a matter of
fact, the OR time traces in fig. 2.10 exhibit oscillation of the reflected signal deriving from a
coherent activation of molecular vibration modes. Due to the really ultrafast time scale of the LS
HS photoswitching
, all the following processes are triggered simultaneously resulting in
coherent structural dynamics. Practically, since the excitation pulse is  50 fs all the absorbing
molecules undergoes the same photoswitching process, where the phase of the process of two
molecules can be delayed of maximum the excitation pulse duration. Moreover, the displacive and
ultrafast nature of the LS
HS structural change (measured to occur in  160(20) fs) selectively
induces intra-molecular vibrational modes such as the
stretching. In this way, the oscillation
observed in fig. 2.10 are a clear signature of the coherent molecular vibrations which accompanies
the ultrafast LIESST phenomenon. At a first look it is possible to identify two main oscillating
components in fig. 2.10. During the first two ps a faster oscillation is observed with a period of
approximately 350 fs. A second component gives the impression to appear delayed with respect to
the first one. In fact, the OR starts to oscillate with a much higher amplitude at around 2.5 ps with a
period of  1 ps, and it is observed to vanish at 8 ps.
Fig. 2.10 Dynamical OR time traces recorded at 680 nm, 690 nm and 700 nm, showing in phase
oscillations of the reflectivity signal.
53
Andrea Marino 2015
2.3.1 Analysis of coherent vibrational modes
For a more detailed analysis, the oscillating components of fig 2.10 were extracted from the data fits
and plotted in fig. 2.11. An accurate time dependent fast Fourier transform (t-FFT) analysis was
performed highlighting the presence of three different modes at around
,
and
. A shorter scan (performed with higher time resolution  50 fs) reveals the presence of the
activation of two modes (
and
) which are observed just in the 0-2 ps range (fig.
2.11a). Whereas, a third mode
only appears after  2.5 ps. In the frame of a
collaboration with S. Matar (ICMCB Bordeaux) DFT calculations were performed for better
understanding the molecular vibration modes. The first two modes respectively corresponds to an
average
stretching and a ligand torsion, while the mode at
is identified to be a
mixing of a lattice mode associated with inter-molecular vibration of the
ligands with the
surrounding molecules. Moreover, the t-FFT indicates that the mode at
appears around at
 200-300 fs after the maximum amplitude of the first activated mode
. However, since
the delay between the two vibrations can be comparable with the accuracy of the t-FFT analysis, it
may slightly affect the retardation of the vibrations, and therefore influence our interpretation. On
the other hand, the longer scan fig. 2.11b discerns the spectral weight transfer from the high
frequency to lower frequency ones. It is important to notice how the amplitude of the vibration at
, which is maximum at 4 ps, gradually decreases until vanishing at 8 ps. This is a clear
evidence of the dumping and dephasing of the vibrational modes. With the passing of time,
stochastic processes can interpose to the coherent dynamics. In this way, the dephasing of the
photoswitched molecules causes the passage from a coherent oscillatory dynamics into an
incoherent exponential kinetic regime. This dumping process has been observed also for SCO
molecules in solution [Consani 2009] and, it has been located to play a key role in the metastable
trapping of the HS state avoiding the system to fall back into the LS ground state [van Veenendaal
2010].
Fig. 2.11 Oscillating component of the OR at 690 nm with their respective time dependent FFT
analysis (bottom) showing the sequential activation of different coherent vibrational modes. On the
left a high resolution scan ( 80 fs) extracting the first intra-molecular modes which activates
slower vibrational modes via phonon-phonon coupling and energy transfer in  2 ps
54
Ultrafast LIESST and Energy Redistribution
The t-FFT analysis of the oscillatory components in fig. 2.11 points out a clear sub-sequential
activation of the different coherent vibrational modes. After the arrival on the HS potential, the
population of the antibonding eg states leads to an increase of the
bond lengths which has
been measured to occur in 160 fs. Again, this time constant (extrapolated by time-resolved XANES
fig. 2.8) corresponds to the half period of the
stretching mode in the HS state. In agreement,
the first OR oscillations appear only after this time (fig. 2.11). Hence, the displacive nature of the
LIESST process triggers the
stretching-like mode which in turns seems to be strongly
coupled to other vibrational modes. As a matter of fact, the other observed modes are not directly
activated by the laser pulse, but they are rather excited toward a phonon-phonon coupling with the
first selected mode. This implies that the
stretching-like mode (which is the main reaction
coordinate driving the system towards the HS potential), strongly affects other (bending) modes
which are activated later.
It is clear from fig. 2.11 that the inter-molecular mode
located on the ligands is of larger
amplitude with respect to the other two intra-molecular modes. Furthermore, by recording several
time traces at different probe wavelengths (fig. 2.12), it was possible to identify a spectral region in
which the oscillations were observed. Figure 2.12 points out that the oscillation are observed just
for a probe wavelength in between 670 nm and 710 nm. All these findings are due to the probe
sensitivity, as light absorption is associated with optical transition between different molecular
orbitals. The analysis of the molecular orbital energy diagram obtained by DFT calculations in
collaboration with S. Matar at ICMCB (fig. 2.13) indicates that in the HS state, at the probe of 690
nm (or closer), the photon absorption (1.8 eV) corresponds to an electronic transition from a t2g-like
orbital of the HS state to its first corresponding 5MLCT state.
Fig. 2.12 Two-color pump-probe measurements recording the LIESST dynamical time traces of
the [Fe(PM-AzA)2(NCS)2] single crystal. Identification of the spectral range which is sensitive to
the coherent vibrational modes oscillations. 670 nm and 710 nm
55
Andrea Marino 2015
The wavefunction distribution for the , and 5MLCT orbitals deduced from DFT calculations is
shown in fig. 2.13. The density for the -like molecular orbitals is mostly located on the central Fe
atom, whereas the distribution on the eg-like orbitals points toward the six surrounding nitrogens, as
common for an anti-bonding orbital. On the other hand, the wavefunction of the first lowest
unoccupied molecular orbital (LUMO) of the HS state (corresponding to the MLCT orbitals) is
located on one of the
ligands (fig. 2.13). In this way, the absorption of the probe
wavelength, resulting from transition between such energetic levels, can be modified by molecular
vibration through the variation of atomic wavefunction overlap as the electronic structure
distribution depends on the molecular structure. The activation of such a molecular vibrational
mode will therefore results in a change of optical properties, translated in the OR oscillations.
Fig. 2.13 TD-DFT calculation of the NTO molecular orbitals of the
,
and 5MLCT electronic
levels of the hole and particle, corresponding to transitions from t2g-like to L-like orbitals as shown
here for the LS state (left) and at 1650 nm from t2g-like to eg-like orbitals for HS state only (right).
The probe wavelength promotes a
electron in the MLCT orbital were the electron density is
located on one of the two
ligand.
Such phenomena have also been noticed in a similar complex
where depending
on the probe wavelengths (sensitive to different orbitals) the oscillations corresponded either to the
stretching mode or to the
bending mode [Bertoni 2015a]. However, in the
present case DFT calculation associated the modes at
and
to a mix of
stretching and bending as well. Even though the laser around 90 nm probed the LUMO located on
the PM-AzA ligand (fig. 2.13) the appearance of OR oscillation corresponding also to Fe-N
stretching-like modes around is due to the low symmetry nature of the
molecule. In fact, for the other compounds (like
[Bertoni 2015a] as well as
56
Ultrafast LIESST and Energy Redistribution
[Consani 2009]) with highly symmetric molecules a selected electron transition probes
selectively the mode associated to vibration of the electronic distribution.
However, the low symmetry of the system in analysis let much more degrees of freedom entering
into play. It was not possible to identify a pure
stretching mode where the six
bonds
expand in phase, as the six
bonds are not symmetry equivalent. All the modes showing such
stretching character are also mixed with bending of the
angles.
All these results are in agreement with previous reported works for similar SCO molecules, on
which it has been already observed that the ultrafast photo activation of LIESST process is strongly
accompanied by specifically selected vibrational modes both in solution [Consani 2009] and in
solid state [Cammarata 2015], associated with the
elongation mode. This global picture for
describing the photophysics of LIESST is schematically represented in fig. 2.9. An ultrafast ISC
from the MLCT state (τ < 50 fs) occurs and the less bonding HS potential is rapidly reached
defining a new equilibrium of
bond length. Intermediate states serving as mediators appear
in the process but are difficult to identify here. The new equilibrium position in the HS state is
therefore reached only after the
elongation within ∼160 fs. In this way, the HS potential is
reached with excess kinetic energy and the molecule oscillates in the potential as
bond
breathes. Because of the low symmetry of the system, this single mode picture is too simple. It may
be pertinent for high symmetry systems for which the six
bonds are symmetry equivalent.
But in the present low symmetry compound, this potential energy curve description should be
replaced by potential energy surfaces with several degrees of freedom corresponding to the main
molecular modes involved during the LS HS conversion. Of course, the present physical picture
is a simple molecular view of the process. But this photoswitching occurs in a solid and there are
some important information to be obtained from the activation of the lattice mode at 33 cm-1. This
will be discussed in the next part.
57
Andrea Marino 2015
2.4 Ultrafast Energy Redistribution
In order to clarify the nature of the vibrational modes observed in fig. 2.11 the same experiments
were performed on diluted crystal. The
was diluted in a neutral zinc matrix
with concentration of iron(II)
. The Zn host matrix of higher
volume with respect to the Fe atom stabilizes the HS state, therefore shifting the crossover at lower
temperatures [Spiering 1982, Kohlhaas 1997, Jeftic 1997]. Fig. 2.14a reports the thermal transition
curves for the pure Fe-based crystal
and for the diluted crystal
, as well as the
calculated absorption spectra of the pure Fe compound in the LS and HS state and for the pure Zn
matrix. The zinc is optically silent in all the visible (VIS) as well as in the infra-red (IR) regions.
Moreover, the Zn does not present any spin crossover as it is of 3d10 electronic configuration and
therefore, it should not undergo a structural modification. In this way the already low cooperativity
of the pure system is further decreased if not totally absent. This allowed an isolated examination of
the response of the single absorber molecule. Indeed a dilution of
means that a Fe-based
molecule is in average isolated in a box surrounded by
Zn molecules. Therefore, in
average the Fe-based molecules are not in contact each other.
Fig. 2.14 a) Thermal spin-crossover transition curves for the pure
crystal
(▼) and for the 10 % diluted
crystal (●). b) TD-DFT calculated
absorbance spectra for the
in the LS (──) and HS (──)state and for the
(──) compounds.
58
Ultrafast LIESST and Energy Redistribution
Figure 2.15 reports the comparison of the dynamical time traces for the pure and diluted crystal
recorded with the same experimental conditions:
and
as well
2
as same pump fluence of 2.5 µJ/mm . In the really first picosecond after the electronic peak (not
shown in fig. 2.15 in order to have a better zoom on the zone of interest), both samples show a step
response with fast OR oscillations more evident in the pure compound, and less pronounced in the
diluted one. The lower oscillation amplitude in the diluted crystal may be due to a smaller Fe-based
molecular density. However, the higher noise for this record forbids an accurate extrapolation of the
high frequency modes. Nevertheless, it is possible to state that the low frequency oscillations are
clearly absent in the dilute crystal. Indeed if present, their amplitude should be much higher than the
noise and therefore easily detected.
Fig. 2.15 Comparison of the coherent structural dynamics accompanying the ultrafast LIESST
(
and
) for the the pure
crystal (──)
and for the 10 % diluted
crystal (──) and fitted oscillatory
components (──).
The comparison of the coherent ultrafast structural dynamics between the pure and diluted samples
presented in fig. 2.15, gives the confirmation that the low frequency mode at
effectively
corresponds to a lattice vibration. The optical probe enables only the observation of Fe-based
molecule. Therefore, the non-observation of this mode in the dilute compound clearly indicates that
it is the surrounding lattice which vibrates at a frequency of
and not the absorber Fe
molecule. In fact, since the neutral Zn matrix is silent to the probe wavelength (as shown from the
59
Andrea Marino 2015
absorbance reported in fig. 2.14b), the oscillation of the Zn molecules surrounding the absorber Fe
molecules cannot be detected by OR.
A further confirmation is given by ultrafast transient absorption spectroscopy in the IR region on
diluted crystals. Fig. 2.16 reports a 2D plot of the optical density (OD) time evolution for a probe
range which varies from 1150 nm up to 1600 nm. It clearly points out the typical signature of the
vibrational cooling (VC) in the HS potential [Bertoni 2012]. A band narrowing appears around
1450 nm (estimated by DFT calculation to correspond to the ligand field t2g
eg gap of the HS
state) from which the assignment of the VC time constant is given around 2 ps. Fig. 2.16a
demonstrate that the VC is observed in both pure and diluted system to occur within the same time
in  2 ps. Moreover, fig. 2.11 highlighted the activation of the
mode to be delayed of 2 ps
after the photo-excitation. These findings definitively are the first clear evidence of the ultrafast
energy transfer from the absorber molecule to the crystal lattice.
Fig. 2.16 a) Comparison of transient absorption time traces at 1300 nm for the pure
AzA2NCS2 crystal (red) and for the 10 % diluted Zn0.9Fe0.1
2NCS2 crystal (blue). The
VC is the same in both crystals b) 2D near-IR transient absorption spectroscopy for the
diluted crystal showing a band narrowing around 1450 nm where it
is expected to be the
gap of the HS state.
60
Ultrafast LIESST and Energy Redistribution
2.5 Conclusions
This chapter was a first attempt to study ultrafast spin-state photo-switching by using optical pumpprobe reflectivity spectroscopy on solids. The results reported here demonstrate that detailed
analysis makes it possible to obtained important information with high accuracy on the mechanisms
involved during the process. First of all, the first report on the ultrafast photoswitching in Fe II SCO
solids in the solid state brings key information on the ultrafast dynamics, driven by the coherent
structural dynamics which accompanies the LIESST phenomenon. The use of complementary
probes (Optics and X-ray) allowed the determination of temporal evolution of the different
electronic and structural degrees of freedom. The structural dynamics are identified to occur in
several sequential steps. The ultrafast displacive nature of the LS
HS switching triggers the
bond elongation within 160 fs. This structural reorganization in turn selectively induces
coherent intra-molecular vibrations which results in OR or in OD changes.
This findings are in good agreement with previous studies. The first oscillations observed for the
SCO in solution were associated to a wave-packet dispersion without a clear
identification of the molecular modes [Consani 2009]. The coherent vibration mode observed
around 124 cm-1 in this system was initially attributed to a ligand mode. But recent calculation
[Sousa 2013] underlined that the 124 cm-1 mode in this system corresponds to the Fe-N breathing
mode, which is the main reaction coordinate. It is only recently that the coherent activation of the
breathing mode during LIESST was demonstrated in the
crystals and that in
addition different vibrational modes are sequentially involved during the process. The vibrational
transfer was also identified as a robust phonon-phonon coupling between different intra-molecular
modes: breathing first and bending after [Bertoni 2015]. Moreover, for the latter system of nearly
Oh symmetry, probing a well define electronic state with a well localized charge distribution
corresponded to the observation of the associated vibrational mode. On the contrary, the low
symmetry of the system here reported makes impossible to distinguish a pure
stretching
from the
bending which have been observed to progress together.
In the present chapter, the comparison of the ultrafast LIESST dynamics between a pure crystal
with a dilute one where SCO Fe molecules are almost isolated between passive Zn ones, allowed to
highlight the energy transfer from the absorber SCO molecule to its surrounding lattice. It is this
energy transfer which is responsible for lattice heating and the consecutive lattice expansion
observed on ns timescale and the thermal population of the HS state observed on µs timescale. The
non radiative vibrational cooling and the damping of the breathing mode were already placed as the
escape hatch for the excess of energy with which the HS state is populated [Cannizzo 2010, Bertoni
2012] and that is one of the reason why the LIESST effect is so efficient [van Veneendaal 2010].
Fig. 2.17 summarizes the sequential steps after the photo excitation. The excitation light pulse
locally photoswitches the absorbing iron molecules in the crystal. As it undergoes LIESST, it
reaches the HS potential with an excess of energy in the order of 1-2 eV. The population of the
antibonding eg orbitals leads to a reorganization of the
octahedron which in turns triggers a
coherent activation of intra-molecular vibrations. These high frequency modes (
and
) activate only once the
distance elongates, that is after 160 fs. Such coherent
oscillations in the HS potential are present up to around 2 ps, which corresponds to the timescale of
61
Andrea Marino 2015
the vibrational relaxation of the HS state. Therefore, the observation of the low frequency mode at
(delayed appearing at  2 ps in fig. 2.11 ) corresponds to the sequential activation lattice
mode. DFT calculation show that such low frequency modes are associated with large molecular
motions and these involve mainly ligand torsion. With these findings it is possible to conclude that
the iron-ion rapidly equilibrates with the environment, transferring to the lattice the energy released
by the laser pulse in  2 ps. This transfer is evidenced in the pure Fe crystal with the appearance of
ligand-like inter-molecular modes which induces the neighboring molecules to vibrate as well. This
is clear when in an optically silent Zn matrix this vibration is not observed, a confirmation that such
vibrations are located on the environment surrounding the absorber molecule. Since the lattice
modes are coherently activated, it is possible to tentatively consider that the mechanism involved in
the process is a coupling between the coherent intra-molecular breathing (and/or bending) modes
and the lattice modes. Indeed, phonon-phonon coupling is known as an efficient way to transfer
energy between different vibration modes.
Fig. 2.17 Schematic representation of the energy transfer from the absorber Fe-ion to the crystal
lattice. A first laser excitation release an excess of energy of  1.5 eV on the Fe-ion. Due to the
electron-phonon coupling, the ultrafast
bond elongation triggers coherent structural intramolecular vibrations. A strong phonon-phonon coupling transfer the excess of energy from the
molecule to the crystal lattice via phonon-phonon coupling.
62
Ultrafast LIESST and Energy Redistribution
The identification of relevant molecular coordinates accompanying photoinduced phenomena in
molecular solids and the coupling of the different modes is very important for understanding the
pathway from the initial to the final photoinduced state. This is also very well illustrated in other
types of systems showing neutral-ionic [Uemura 2010] or Mott [Kawakami 2009] photoinduced
transition. The results presented here underline the selective roles of the phonon involved during
LIESST: breathing drives the system towards the HS potential, bending helps the molecule relaxing
in its HS structure and the transfer of energy to lattice modes allows for an efficient vibrational
cooling.
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Andrea Marino 2015
64
Chapter 3
LIESST vs reverse-LIESST:
Dynamics vs Kinetics
Part of this work was published in:
A. Marino, P. Chakraborty, M. Servol, M. Lorenc, E. Collet, and Andreas Hauser
The Role of Ligand-Field States in the Ultrafast Photophysical Cycle of the Prototypical Iron(II)
Spin-Crossover Compound [Fe(ptz)6](BF4)2
Angew. Chem. Int. Ed. 53, 3863 –3867 (2014)
Andrea Marino 2015
66
LIESST vs reverse-LIESST
3.1 The role of Ligand-Field States
The previous chapter reported the first studies on the ultrafast dynamics of the LS
HS LIESST
in iron(II) spin-crossover (SCO) crystals upon metal-to-ligand charge-transfer (MLCT) excitations
[Marino 2013]. Conversely, the project reported here, result of a fruitful collaboration with
Professor Andreas Hauser from the University of Geneva, who is deeply acknowledged, was born
with the purpose to study and understand the ultrafast photoswitching dynamics of the reverseLIESST process. Furthermore, the possibility to drive the spin-state switching with an excitation
process different from the metal-to-ligand charge-transfer (MLCT), raised also questions on the role
of the ligand field (LF) states and how they are implied in the photophysics of SCO compounds.
As already discussed in this PhD thesis, the ultrafast studies conducted so far on SCO complexes
(and in particular on iron(II/III)-based complexes) have mainly reported on the dynamical process
involved in the LIESST upon MLCT excitation. The major developments obtained by the groups of
Chergui, McCusker, Schoenlein and others, stated that the double intersystem crossing (ISC) from
the initially excited low-lying 1MLCT state toward the HS state proceeds possibly via the 3MLCT
state, thus bypassing the lower-lying singlet (1T1,1T2) and triplet (3T1,3T2) LF states [Bressler 2009,
Cannizzo 2010, Huse 2011].
However, the ligand-field (LF) states in transition-metal photophysics of d6 systems such as iron(II)
complexes have nevertheless a crucial role. They are by no means innocent and it is proved with the
occurrence of LIESST upon irradiation into the spin-allowed as well as the spin-forbidden LF
absorption bands of the LS molecules of the SCO compound
, which has no lowenergy MLCT states. [Hauser 1991, Hauser 1999, Hauser 2004b]
Hitherto, the literature that refers to the experimental investigations of LIESST and reverse-LIESST
via LF excitation only concerns the use of cw and ns lasers and mainly focused on the analysis of
the relaxation mechanism from both photoinduced spin-states.
The studies presented hereafter are the first ones dealing with the ultrafast photoswitching
dynamics after LF excitations, that is via electronic d-d transition for LIESST and reverse-LIESST
mechanisms. In order to insert this project in a clear context, and to set down the question so far
unsolved, the chapter begins with a small summary over the last 30 years of intense studies after the
discovery of the LIESST process. A general description follows presenting the samples under
investigation, together with the strategy adopted for the pump-probe experiments. The third and the
fourth sections will be devoted to the ultrafast dynamics of LIESST and reverse-LIESST
respectively. Finally, the chapter will conclude with a detailed discussion and comparison between
the processes observed here and the ones in the present literature.
3.1.1 A bit of History
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Andrea Marino 2015
The first report of photo-induced phenomena in spin-crossover (SCO) complexes dates back to
1982, when McGarvey and Lawthers observed that at room temperature the equilibrium between
the spin states of iron(II) SCO complexes in solution can be perturbed by ns pulsed laser irradiation
into the spin- and parity-allowed 1MLCT absorption bands of the low-spin (LS) species. However at
room temperature these light-induced high-spin (HS) states are transient states with typically submicrosecond lifetimes [McGarvey 1982].
This result was followed by the discovery in 1984 of the Light-Induced Excited Spin-State Trapping
(LIESST) phenomenon in the solid state. Decurtins et al. reported that at low temperatures (20 K),
irradiating the LS ground state into the above mentioned 1MLCT bands as well as into the LF
bands, makes possible to efficiently populate the HS state as a long-lived metastable state
[Decurtins 1984]. This process was demonstrated as being reversible in 1986 when Hauser showed
that the irradiation of photoinduced metastable HS state at low temperature switches back the SCO
crystal to the LS ground state. This HS-to-LS process is referred to as the reverse-LIESST
mechanism [Hauser 1986].
Fig 3.1 Scheme of the ground and excited states fron an iron(II) spin-scrossover system. The
straight arrows represent the vertical transition doe to photon absorption. The curly arrows
tentatively proposed the pathway for the LIESST and reverse-LIESST mechanism, without
necessarily reflecting the exact pathway of the process. [Hauser 2004b]
Ever since, the studies carried on this phenomena mostly dealt with the HS 5T2
68
LS 1A1
LIESST vs reverse-LIESST
relaxation dynamics in solution [Brady 2004], as well as in the solid state, where
cooperative effects are of interest [Hauser 2004b].
The scheme in figure 3.1 represents a schematic description of potential energy curves related to the
different electronic structures of iron(II) SCO compounds proposed by Hauser in 1986 [Hauser
1986, Hauser 1991] and improved later [Hauser 2004b]. It summarizes the LIESST and reverseLIESST mechanisms activated upon MLCT and LF excitations, tentatively proposing the pathways
of the photo-processes involved in the ISC between several intermediate (INT) states. The tracking
of the pathway across the potential curves of the different electronic states represented by the curly
arrows, were based only on energetic calculations and hypothetical assignment of the INT states
involved, without any measured time constant or accurate evidence of the processes. Furthermore,
in order to explain the lower efficiency of the reverse-LIESST process, the concept of a branching
ratio at the stage of the lower lying triplet 3T1 state was proposed to be 4:1 favoring the decay into
the HS potential.
With the advent of ultrashort laser techniques, ultrafast pump–probe spectroscopy including
structural probes on solution of iron(II) SCO [Gawelda 2007, Smeigh 2008, Bressler 2009] started
to assign the time constants to the various elementary processes involved in the photophysics of
SCO complexes.
Despite all the efforts for correctly stating each step in the mechanism, the general scheme of the
ultrafast processes of LIESST and reverse LIESST (represented in fig. 3.1) was still poorly
understood. Although the time constants and the nature of the intermediate states (INT) are defined
for the ISC relaxation processes starting from the excited 1MLCT state [Cannizzo 2010], the role of
the LF states and the dynamics of the reverse HS-to-LS process was not investigated so far.
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Andrea Marino 2015
3.2 Description of the compounds
Given that the Metal-to-Ligand Charge Transfer (MLCT) optical absorption bands are usually of
several orders of magnitude more intense than the typical bands for the Ligand-Field (LF)
transitions, spin-crossover systems with low-lying MLCT states (e.g. SCO complexes with pyridyl
type ligands) may show broad and intense absorption bands in the visible range. In particular their
LF bands are usually submerged in these MLCT bands which usually dominate the visible
spectrum, and it is thus difficult to discriminate their electronic transitions.
In order to study the role of the ligand-field states in the photo-physical phenomena of photoinduced spin-crossover, systems with low-lying MLCT bands should be avoided. Instead, systems
for which the visible region presents mainly LF absorption bands, and the MLCT are pushed in the
UV region at higher energy are more appropriate. This is the case for the two spin-crossover
systems investigated here and presented below.
3.2.1 The
Since the discovery of LIESST phenomenon in the
compound [Decurtins 1984], this material has been the subject of intense studies. The iron center is
coordinated by a nearly regular octahedral ligand (fig. 3.2). The complex crystallizes in the
rhombohedral space group
resulting in well-shaped hexagonal plates [Franke 1982, Hauser
1986].
Fig 3.2 Schematic representation of the
cation forming a 2D polymeric network.
Figure 3.3 reports the thermal behavior of
. The neat crystal shows a 1st order spin
state transition with a hysteresis loop of 7 K
accompanied by a
crystallographic phase transition between the HS and LS phases [Hauser 2004b]. However, the
crystallographic phase transition can be suppressed by a rapid cooling, in which case the thermal
spin conversion remains abrupt but fully reversible with a transition temperature of
.
This Fe complex can also be diluted in a isostructural matrix of passive Zn complex, resulting in a
non cooperative dilute crystal
presenting a continuous spin-crossover, as
70
LIESST vs reverse-LIESST
reported in fig. 3.3 (left) with black circles. As a matter of fact, metal hosts with larger volumes
than the average of the HS and the LS volume of the pure iron(II) compound such as Zn, stabilize
the HS state by inducing a so-called negative pressure [Kohlhaas 1997, Jeftic 1997, Spiering 1982].
Consequently, the thermal spin crossover is shifted toward lower temperature as the Fe dilution
decreases (left of fig. 3.3).
Fig. 3.3 Left Thermal transition curves for the pure
crystals showing an
hysteresis loop and for the diluted
(x = 1%) crystals showing a smoother
spin-conversion. Right Single crystal absorption spectra for the diluted
(x
= 1%) in the region of the MLCT transitions between 10 and 295 K.
The intense MLCT bands were conveniently studied in the Fe diluted
crystals. The corresponding absorption spectrum (right of fig. 3.3) is strongly temperature
dependent. The broad and intense low-temperature band with the absorption maximum at
approximatively
, can be readily assigned to the 1MLCT transition of the
LS species. A decrease of absorption is detected upon heating as the thermal population of HS state
sets in. The 1MLCT band loses intensities as temperature increases. At room temperature it is
entirely replaced by the weaker 5MLCT band of the HS species centered at slightly higher energy
. Such a strong difference in the MLCT absorption coefficients for the
LS and HS states is due to the smaller metal-ligand bond length in the LS state which results in a
better overlap between metal- and ligand-centered orbitals with respect to the HS state with longer
bond lengths.
Figure 3.4 shows optical microscope images of the neat
crystal in the LS
and HS
states with their respective absorption spectra. The spin conversion is accompanied
with a dramatic change of color which turns from completely transparent at room temperature, to
deep purple in the LS state (fig 3.4a and 3.4b). This change of color is due to the appearance of the
ligand-field absorption bands characteristic of the LS state. Since the MLCT bands are placed in the
UV region, the visible spectrum is dominated by the LF transitions. The HS state has only one spinallowed absorption band which corresponds to the 5T2 5E transition. Since this band is located in
the near-infrared region centered at 12000 cm-1 (830 nm), it results into the characteristic
transparency of the HS state.
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Andrea Marino 2015
In the LS phase, this band completely disappears (fig. 3.3c). Two new absorption band appears at
18400 cm-1 (540 nm) and 26650 cm-1 (375 nm) corresponding respectively to the spin allowed 1A1
3
T1 and 1A1 3T2 ligand-field transitions [Decurtins 1985a, Hauser 1991].
Fig. 3.4 a) Optical microscope immage of the
at 20 K in the LS state b) b) and
at 293 K in the HS state. c) Single crystal absorption spectra of
at 20 K (──), and
after reverse-LIESST with irradiation at 830 nm (──). d) Single crystal absorption spectra at 20 K
after irradiation at 514 nm (──) (matching the HS spectra at 293 K) [Hauser 1986, Hauser 2004]
The
is well known for its photo-chromic and photo-magnetic responses.
Irradiating with cw or ns lasers the crystal at 20 K (dark blue solid line in fig. 3.4c) into the spinallowed 1T1 and 1T2 LF bands as well as into the spin forbidden bands of the LS species, efficiently
leads to a complete LS
HS conversion [Hauser 1986]. The absorption spectrum recorded after
LIESST corresponds indeed to the one reported in fig. 3.4d, with only one weak LF band, and by
matching with the spectrum of the full HS state at 293 K proves the occurrence of the spin-state
switching.
At 20 K, the HS state is long living, and the LS state can be photo-induced back upon the 5T2 5E
excitation. However the LS spectrum acquired after subsequent irradiation at 830 nm (light blue in
fig. 3.4c) indicates that the photo-induced HS
LS conversion is not fully complete. Indeed, the
3
reverse-LIESST process competes with the LIESST induced by the spin forbidden 1A1
T1 LF
5
5
transition of the LS state, which overlaps with the T2
E bands [Hauser 1991]. This compound is
therefore well suited for femtosecond optical pump-probe studies because the optical fingerprints of
the LS and HS states are clearly different in a broad spectral range.
72
LIESST vs reverse-LIESST
3.2.2 The
The
(fig. 3.5a) is a 2D
coordination polymer analogue of the
described above [Bronisz 2005]. As for the
previous crystal, the color (fig. 3.5b,c) [Chong 2011] is characteristic of the spin-state change due to
the appearance of the LF absorption bands of the LS state (fig. 3.6 right).
Fig. 3.5 a) Representation of the
2D polymeric network [Bronisz 2005]
b) Top view of the
hexagonal prism in the LS state with the characteristic
purple color and c) in the colorless HS state [Chong 2011] d) Thermal spin transition curves for
pure and diluted crystals
at different concentration of iron(II).
[Chakraborty 2012a]
The neat
crystal displays an abrupt spin transition accompanied by a thermal
hysteresis loop of 13 K (fig. 3.5d in red) [Bronisz 2005, Kusz 2011]. Regarding the diluted systems
, systems with high Fe concentration still show cooperative effects. On
the other hand, for the most dilute ones the transition temperature T1/2 shifts to lower values
[Chakraborty 2012a, Chakraborty 2014]. A really high dilution in Zn complex matrix favors the HS
down to temperatures where T1/2 (HS-to-LS conversion temperature) and TTIESST (relaxation
temperature of the photo-induced and trapped HS state) meet and therefore the thermal spin
crossover becomes incomplete (fig. 3.5d) [Paradis 2012, Paradis 2013].
Figure 3.6 reports the absorption spectra of the
complexes, showing the characteristic
1
5
MLCT and MLCT bands (3.6 left) and the much weaker spin allowed LF transitions (3.6 right).
The left image corresponds to spectra recorded with an highly diluted system (
) where the
73
Andrea Marino 2015
transition is not complete. Indeed, at 10 K the characteristic 1MLCT band of the LS state is not as
intense as after the irradiation of the HS molecules into their LF band, that is after reverse-LIESST.
Fig. 3.6 Absoprtion spectra of the
Single crystal Left for
in
the region of the the MLCT bands. The blue solid line represent the absorption spectra after
irradiation at 830 nm at 10 K, that is when all the crystal completely switch into the Ls state. Right
for
in the region of the ligand field d-d transitions. In green and blue solid line the
absorption spectra at 10 K after irradiation respectively at 532 nm and 830 nm [Chakraborty 2012a].
The left image corresponds to spectra recorded with a highly diluted crystal (
) where the
1
spin-crossover is not complete. Indeed, at 10 K the intensity of the characteristic MLCT band
correspond to the absorption of the 20 % of molecules LS state. It is only after irradiating into the
LF band of the HS state (reverse-LIESST) that the crystal switches completely to the LS state, as
demonstrated with the tremendous increase of absorption in the 1MLCT band (left of fig. 3.6). On
the other hand, high diluted systems do not allow to experimentally detect the much weaker LF
bands. For that, the picture presented on the right of fig. 3.6 reports the spectra acquired on a crystal
with higher dilution (
), which undergoes complete spin-crossover.
3.2.3 Strategy of the experiments
The results presented above indicate that the light-induced population of the HS state is
qualitatively efficient under both LF and MLCT excitations. On the other hand, the reverse-LIESST
is generally much less efficient. Based on previous studies on the
system, the
values of the overall quantum efficiency of LIESST and reverse-LIESST were estimated at 10 K to
be of 0.8 and 0.1 respectively [Hauser 1986]. The high quantum efficiency of LIESST is typical for
all the SCO complexes. Indeed it may just vary slightly from compound to compound and/or as a
function of temperature, but overall it remains the same. Conversely, the quantum efficiency for
reverse-LIESST is strongly temperature and compound dependent. As a matter of fact, the
photoinduced HS
LS efficiency for
compounds rapidly drops above 130 K, reaching
74
LIESST vs reverse-LIESST
zero value already at 160 K, whereas, for the
detectable up to 270 K [Krivokapic 2010].
complexes the reverse-LIESST is still
In such a way, the optimal conditions for ultrafast pump–probe experiments have to be wisely
assessed: The experiments were performed on diluted mixed crystals
in order to shift the spin-crossover to lower temperature as reported in fig. 3.7. This favored
the HS state at lower temperature, where the quantum efficiency of reverse-LIESST is higher. At
the same time, such dilution kept a sufficient concentration of iron(II) ions able to photo-switch.
Fig. 3.7 Spin-crossover curves for the
(▼), and for the dilute
in the supercooled high-temperature phase
crystal with
(●) [Marino 2014]
The temperature of 125 K was specifically chosen for three main reasons: the HS fraction is
approximately 85 %; the reverse-LIESST quantum efficiency is still reasonably high, and the
recovery from the photoinduced LS state to the stable HS state occurs within approximately 0.3 ms.
The latter allowed the use of the full repetition rate of 1 kHz of our laser system for a better signal
averaging.
Other technical difficulties to detect the ultrafast reverse-LIESST process, derive from the
5
absorption cross section of the 5T2
E
transition, which is low. This, together
with the already weak quantum efficiency of the reverse-LIESST, leads to only a small fraction of
light-induced LS population. Fortunately, the intense 1MLCT band makes possible to detect a small
fraction of photoinduced LS population. Thus the low absorption cross section at the excitation
wavelength is compensated by the high sensitivity to the light-induced population of the LS state at
the probe wavelength. The pump fluence was set to 5 J focused down to 200 m diameter spot
size avoiding sample damage.
Figure 3.8 summarizes the pump and probe experimental procedures comparing the spectrum of the
on the left, with the corresponding electronic transition on the right. The reverse75
Andrea Marino 2015
LIESST was triggered with the excitation wavelength set at 830 nm in the middle of the
5
1
T2 5E absorption band. The 1A1
MLCT
transition was used to sensitively
record the photo-induced LS population. The probe wavelength was set at 300 nm on the tail of the
1
MLCT band where the absorption coefficient is high for the LS species, whereas the HS state is
totally transparent. Monitoring the 1MLCT band in reverse-LIESST should then allow the
determination of the rate of arrival in the LS state.
At 125 K, the LS molecules are present in the concentration of 15%. In this way, the same
experimental conditions have been maintained for the LIESST phenomenon. The photoinduced
LS
HS process has been activated though d-d excitation, pumping the LS state into its spin
1
allowed ligand field state 1A1
T1
at 530 nm. In this case, the 300 nm probe
monitors the LS state bleaching to the benefit of the HS population.
Fig. 3.8 Left Absorption spectra of the
pure crystal at 10 K (──), after
irradiation at 530 nm (──), and at 290 K (──) with the assignement of the spin-allowed and spinforbidden figand field d-d trans ition in the Ls and HS states. The temperature dependent
absorption spectra in the region of the MLCT transitions (right axis) correspond to the diluted
crystal with
. Right Scheme of the energetic levels of the
different electronic states with the assignement of the photoinduced electronic vertical transitions
for the LIESST (green arrow), reverse-LIESST (red arrow) and the probe wich monitors the
LS 1MLCT transition (dashed violet arrow).
In comparison with the spectrum of
systems (fig. 3.3), the MLCT bands of
-based crystals presents a red shift (fig. 3.6). Therefore, a probe set at 300 nm would
also be absorbed from the HS state, as its 5MLCT band is extended until 320 nm. For the
experiments performed on the
crystals, the probe was set at 335 nm at
1
the tail of the MLCT band. However, the pump laser parameters were kept unchanged at 830 nm,
always promoting the 5T2 5E transition.
The following ultrafast pump-probe experiments were performed with a temporal instrumental
response function of  150 fs.
76
LIESST vs reverse-LIESST
3.3 LIESST via d-d excitation
The LIESST phenomenon via d-d excitation was studied on the
. The
process was triggered by an 100 fs laser pulse centered at 530 nm, which induced the spin allowed
1
A1 1T1 ligand-field transition. A second 100 fs probe pulse, at 300 nm, recorded the ultrafast
dynamics.
Contrariwise the LIESST upon MLCT activation, which promotes the t2g electrons into a non
bonding ligand-like orbital
, LIESST upon d-d excitation concerns the
promotion of one electron from a non bonding t2g orbital of the LS state directly into an antibonding
metal-like eg orbital
.
Figure 3.9 shows the optical time-trace at 300 nm of the LIESST recorded after the LF 1A1 1T1
pump excitation. The data collected show the same typical features of a "standard" LIESST via
MLCT presented in the previous chapter: an ultrafast transient peak with a second slower
exponential decay.
Fig. 3.9 Ultrafast transient absorption profile of the LIESST upon d-d excitation (830 nm)
monitored at 300 nm (33330 cm-1) (green circle) together with a doble exponential fit (blue line)
Inset decomposition of the different contributions to the transient signal. [Marino 2014]
The transient signal at time zero decays with a time constant
shorter than the instrumental
response function (IRF) of the setup. Indeed it displays a Gaussian shape due to the pump-probe
pulses cross correlation profile. The peak is thus attributed to the absorption from the excited singlet
1
T1 state and/or from the intermediate (INT) states involved during the intersystem crossing (ISC)
toward the HS potential.
77
Andrea Marino 2015
The decrease of OD after this electronic peak, indicates that the crystal becomes less absorbing than
in the initial LS state. Such bleaching must be attributed to the depletion of the LS state from its
equilibrium value of 15% at 125 K, which is the most strongly absorbing species at the probe
wavelength of 300 nm.
However the transient OD increase, resulting from the combination of the excited 1T1 state and
other possible INT states, exceptionally shows an higher absorption with respect to the LS state. In
fact, in addition to the MLCT absorption form the INT states, the electronic configuration of the
excited singlet 1T1
state opens the perspective to a somehow electronic transition from a
lower lying ligand-like orbital into its t2g available orbital. Such ligand-to-metal charge transfer
(LMCT) absorptions bands are also observed to be intense in the LS state of FeIII compounds with
electronic configuration [Bertoni 2015b,c].
Fig. 3.10 Schematic representation of LIESST pathway after pump excitation at 530 nm (green
arrow) and probed at 300 nm (purple arrow) 1) ISC decays of the 1T1 excited state toward a non
defined mix of INT states (dashed dark grey arrow) 2) Vibrational cooling inside the HS potential in
1.2 ps (black oscilating arrow).
78
LIESST vs reverse-LIESST
In fig. 3.9 a second slower process is observed. Once the signal arrive at negative OD changes, an
exponential decay with a time constant
leads to a still more negative signal, which
persists for the duration of the experiment. Likewise the previous chapter, this decay can be
assigned to the non-radiative vibrational cooling inside the HS potential.
The LIESST phenomenon upon d-d ligand field excitation can be summarize in the scheme
presented in fig. 3.10.
1
The pump excitation promotes the 1A1
T1 transition, leading to a change of electronic
configuration from
to
. Once in the singlet exited 1T1 state, the system undergoes an
ultrafast ISC within the experimental temporal resolution. Therefore, it was not possible to clearly
detect and indicate which are the intermediate states involved in the LS HS spin conversion.
The photoinduced HS state is reached in a vibrational exited state. It takes about 1.2 ps for the
system to relax inside the HS potential, redistributing the energy via phonon-phonon coupling,
likewise the phenomena reported in the previous chapter. The photo-induced HS state decays with
the time constant of approximately 0.3 ms of the HS LS relaxation at 125 K.
This study demonstrated that the LIESST process via LF transition occurs on a timescale similar to
the one observed upon the MLCT excitation [Marino 2013, Cammarata 2014]. Again, this ISC is
mainly limited by the intrinsic elongation of the ligand around the Fe ion, which falls in the 150 fs
timescale as discussed in chapter 2. This Fe-N elongation was directly measured by femtosecond
time-resolved XANES in [Fe(PM-AZA)2(NCS)2] [chapter 2] as well as in [Fe(phen)2(NCS)2]
[Cammarata 2014].
79
Andrea Marino 2015
3.4 reverse-LIESST
3.4.1 A Triplet Intermediate State
Fig. 3.11 reports the very first ultrafast investigation of the reverse-LIESST mechanism. The OD
evolution during the first 30 ps plotted in fig. 3.11a shows an intense transient peak appearing
immediately after the laser excitation. This OD peak decays with a time constant of
into an OD value which appears to be constant in this range. This initial decrease may be at first
associated with the creation of LS states. Indeed if one considers only fig. 3.11a, the reverseLIESST could appear equivalent to the LIESST dynamics where all the electronic processes occur
in the sub-ps time scale as the vibrational cooling is also in the order of 1-2 ps.
Fig. 3.11 Ultrafast transient absorption profiles for the
at 125 K for
and
80
crystal recorded
LIESST vs reverse-LIESST
However, longer time scans as the one shown in fig. 3.11b demonstrates that this is not the case.
The value reached few ps after the decay corresponds instead to a minimum from which the OD
increases again with a second time constant
. The plateau reached at the end of the
rise does not decay within the whole range of the ultrafast setup (up to 1 ns). It thus corresponds to
the absorption from the light-induced LS (1A1) state, which in turn decays with the above mentioned
recovering time constant of 0.3 ms to the HS stable state.
Fig. 3.11b shows a two step process after laser excitation. The 39 ps rising time to the plateau
corresponds therefore to the building up of the population of the LS state. On the other hand, the
minimum in the transient signal reached in 1.7 ps, corresponds to the population of an intermediate
state. Based on energetic and geometric considerations [Ordejon 2008], the intermediate state of
lower energy compared to the quintet exited 5E state can only be the triplet 3T1 LF state, pointing
1
without any doubt to the sequence of ultrafast reverse-LIESST: 5T2 5E 3T1
A1 .
However the time-traces reported in fig. 3.11 were unexpected. Indeed at the probe wavelength of
300 nm, the LS state was supposed to be the only absorbing species with really high extinction
coefficient. The data were therefore expected to show only one exponential OD increase
corresponding to the population of the LS state. Furthermore, since at 300 nm the vertical 1MLCT
transition from the LS (1A1) state has the highest absorption coefficient, one would expect that the
OD change corresponding to the creation of LS state would be the most intense one. This, again, is
not what the results exhibit. The OD of the plateau corresponding to the LS sate absorption is
clearly inferior with respect to the transient peak, and it does not mark a significant difference from
the value reached at the minima. This can be explained taking in consideration the quantum
efficiency of reverse-LIESST mechanism which is quite low. All the photoexcited molecules
contribute to the 5E absorption peak, whereas only the ones populating the LS state contribute to the
absorption increase with 39 ps time constant. Therefore a weak amplitude of the OD after 39 ps
indicates that the light-induced LS population is not very large.
Regardless, all this processes and the correct assignment of every step to its respective LF state can
be clarified with the aid of a schematic picture: fig. 3.12 elucidates the different steps of the
ultrafast reverse-LIESST mechanism, reporting the sequence of the different processes involved.
The pump excitation of the initial HS state at 830 nm triggers the reverse-LIESST phenomenon
induced through the 5T2 5E ligand field transition. The promotion of one of the
electrons of
5
the HS ( T2) state into one of the antibonding
metal-like orbitals leads the change of electronic
configuration from the HS state
to the first quintet excited state 5E
. The HS state
5
is transparent to the probe wavelength at 300 nm, since the MLCT transition from the 5T2 state is
expected to be around 42000
(240 nm) (see fig. 3.3). However as the quintet excited state 5E
is higher in energy with respect to the HS state, its 5MLCT transition occurs at lower energy (see fig
3.12a and fig. 3.3), that is at around 30000
, and is thus monitored with the probe wavelength.
The transient peak observed at very short times thus corresponds to the absorption from the 5E state.
It decays either via internal conversion back into the HS state (grey arrow in fig 3.12b) or to the
triplet 3T1 intermediate state via ISC (dashed green arrow), with an apparent global decay time
constant of
resulting from the combination of vibrational relaxation, internal
conversion, and intersystem crossing.
81
Andrea Marino 2015
Fig 3.12 Schematic representation of the reverse-LIESST mechanism probed at
(purple arrows). 1) HS 5E excitation at 830 nm. 2) The system is in the 5E state which
decays either to the HS ground state via IC (curly grey arrow) or to the 3T1 via ISC (dashed green
arrow). 3) The 3T1 undergoes ISC both with the HS state (curly grey arrow) and with the LS state.
Only the latter process will be monitored by the probe wavelenght at 300 nm (purple arrow) as
the photoinduced LS population.
Only the occurrence of an intersystem crossing can explain the transient minimum absorption
experimentally observed. Therefore, it corresponds to the 3MLCT absorption from the 3T1 (purple
arrow in fig 3.12b), which is spin-allowed and has an absorption coefficient that should be
intermediate between those of the spin-allowed 5MLCT and 1MLCT bands from their respective LF
states.
The scheme in fig. 3.12c reports the different options of the decay from the triplet intermediate 3T1
state. The 3T1 state can either undergo ISC with the HS state (curly grey arrow), or with the LS state
(dashed green arrow). Quantum mechanical calculations suggested that the intersystem crossing is
more likely towards the HS state [Ordejon 2008, Sousa 2013]. Therefore, most of the photo-exited
molecules relax back to the HS state during the cascade from the INT states before reaching the LS
state, and just a fraction will result in the light-induced LS population which gives rise to the small
OD increase reached at the plateau in
.
In contrast with the LIESST process, here the INT states are well determined with their own clear
optical signature. Furthermore, the ISC cascade from the INT exited states occurs from their
thermally relaxed vibrational state. In fact, the time constants of the two processes
and
3
1
(particularly for the case of the T1
A1 transition), are much longer than a typical oscillation
period in the potential ( 300 fs). Therefore the observed intermediate state is vibrationally cooled
and structurally relaxed before undergoing intersystem crossing toward the HS or LS states.
82
LIESST vs reverse-LIESST
3.4.2 A Kinetic model
The fact that the quantum efficiency is not very high and drops off at higher temperatures, indicates
that during the reverse-LIESST process there is a temperature dependent competition between
different relaxation pathways. Therefore accurate studies on the temperature dependence of the
ultrafast reverse-LIESST mechanism were needed.
The limiting factor for the
based crystals is the HS
LS quantum efficiency, which is
already really weak and drops rapidly to zero above 135 K [Marino 2014]. For this reason, the
compound is suitable to perform ultrafast reverse-LIESST at different
initial temperatures. In fact, its reverse efficiency remains still significantly high up to about 200 K
[Krivokapic 2010].
The first picoseconds of the HS LS photoswitching process in the
based compounds are
reported in fig. 3.13. After the pump laser excitation, the transient peak associated to the absorption
from the quintet excited 5E state presents a Gaussian profile. As for the previous cases of LIESST,
this is a clear indication that the decay form the initial exited state occurs within the experimental
temporal resolution. Here, conversely to the above discussed case of the
complex, the quintet exited 5E state undergoes a much faster ISC toward the intermediate exited
triplet state 3T1.
Fig. 3.13 Ultrafast transient absorption profiles for the
at 135 K for
and
.
crystal recorded
Besides, the building up of the LS state population remains in the order of tens of ps in agreement
with the previous report on the
[Marino 2014]. Figure 3.14 reports the timetraces of the reverse-LIESST recorded at different temperatures on the
.
83
Andrea Marino 2015
All tracks shows the same two step-process already typical of the reverse-LIESST mechanism: a
first transient peak (better seen in fig. 3.13) decaying into a transient minimum from which a slow
exponential rise finally leads to a plateau which persist up to the ms timescale.
Fig 3.14 Ultrafast transient absorption profiles for the
at different temperatures for
and
crystal recorded
.
Since it is now clear that the OD change at the plateau undoubtedly corresponds to the absorption of
the photo-switched LS molecules, its amplitude gives a quantitative indication of the amount of the
final photo-induced LS population. The curves plotted in fig. 3.14 clearly show the tendency of the
OD change (at the plateau) to decrease with the increase of temperature. Therefore, it points out that
the higher the temperature, the lower the number of photoexcited molecules reaches the LS state.
These results perfectly match with the thermal behavior of the quantum efficiency for the reverseLIESST, previously reported upon cw and ns laser excitation [Krivokapic 2010].
However, the time-constants
(corresponding to the time needed to reach the LS state) manifest
an unexpected temperature dependence. At 135 K, when the quantum efficiency is maximal (in the
considered experimental temperature range of fig. 3.14), the LS state is populated within
. Whereas at 265 K, despite the quantum efficiency is much lower, the LS state is reached
faster with
.
The photo-induced population of the LS state clearly results only from the decay via ISC from the
intermediate triplet 3T1 state. Therefore, if to consider that the 3T1 state can only decay into the LS
1
potential, then the 3T1
A1 ISC rate should be inversely proportional to the time-constant
. Thus, when
gets shorter with the temperature increase, the ISC rate toward the LS state
should also increase, and so the photo-induced population of the LS state. Again, this logic is faulty.
The observed data in fig. 3.14 indicate instead that by increasing the temperature the LS is less and
less populated but also faster reached. In the experimental range of temperature (135 K - 265 K) the
HS fraction does not vary considerably and therefore the probed variation of the LS fraction XLS
can be considered directly related to the reverse-LIESST quantum efficiency. Fig. 3.15 reports the
extracted percentage of photoswitched LS molecules XLS versus the temperature as well as the LS
84
LIESST vs reverse-LIESST
population time . It is clear from fig. 3.15 that both the reverse-LIESST quantum efficiency and
decrease with the increase of temperature indicating that the ultrafast population of the triplet 3T1
undergoes a competitive decay between two different pathways: 3T1 LS and 3T1 HS.
Fig. 3.15 Plot of the photoinduced variation of the LS spin fraction (XLS) and time constant of the
LS population (2) versus temperature.
This phenomena can be easily explained with the use of chemical kinetics taking into account all
the pathways from the exited triplet 3T1 state. Let us now consider simple kinetic equations,
considering as the starting specie the 3T1 state, and all its possible decays. Fig. 3.16 schematically
represents the excited triplet 3T1 state, which can undergoes ISC either with the HS state with a
rate constant, or with the LS state with a
rate constant. However, only the latter process will
result in the exponential build up of the OD change at the plateau in fig. 3.14, as the HS state is
optically silent at the probe wavelength. In addition, both the LS and HS states can undergo
tunneling process and populate each other. The LS
HS tunneling would result with
rate,
whereas the HS
LS with
rate.
By using the rate constant law, it is possible to determine the temporal evolution of the
depopulation of the triplet intermediate state (a fraction x3T1 of molecules are populating this state)
by solving the following differential equation:
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Andrea Marino 2015
That is:
when integrated:
Where
is the population of the 3T1 state at a general time and
the
3
3
initial population of the T1 state before it decays. Therefore the time-evolution of the T1
population can be written in the form of an exponential decay:
Fig 3.16 Kinetic representation of the 3T1 decay. ISC toward the HS and LS states respectively
with kinetic rate constants of
and . The LS and HS states can undergo tunnelling process
between thier potential energy curves with
for the LS HS tunneling and
for viceversa.
From eq. 3.1 it is possible to extract the time constant of the decay, which is:
In the same way the population of the LS state can be predicted with the following differential
equation:
86
LIESST vs reverse-LIESST
However, since the tunneling process is definitively slower with respect to the ISC process (and
thus less probable), the
and
rate constants can be neglected:
Since now the expression of the 3T1 state in function of time is known from eq. 3.1:
then the integration:
results in:
From this equation, it is possible to observe that the time constant of the exponential population of
the LS state
exactly corresponds to the lifetime of the exponential decay of the
3
T1 state:
This simple model match with the above reported data. The increase of temperature strongly shifts
the 3T1
LS : 3T1 HS branching ratio toward the HS ground state, favoring its recover and
resulting therefore in a lower population of the LS state. Furthermore, the strong increase of the
rate results to shorten the lifetime of the triplet intermediate 3T1 state and therefore to quicken the
photo-induced LS population.
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Andrea Marino 2015
3.5 Discussions and Conclusions
In conclusion, the results reported here are the first clear-cut evidence of the ultrafast LIESST and
reverse-LIESST dynamics upon d-d ligand-field excitation [Marino 2014]. Whereas the LIESST
processes are driven by ultrafast structural dynamics, the reverse-LIESST is governed by kinetics of
slower nature. These findings definitively point out a remarkable difference between the physics of
the two complementary photo-induced phenomena. Besides, it is also possible to advance a
complete explanation for the substantial difference of their respective quantum efficiencies.
For a detailed overview on the photophysics of LIESST and reverse-LIESST mechanisms, the two
processes are schematically summarized in fig. 3.17. The former process has being demonstrated to
follows the same ultrafast dynamics as for irradiation into the MLCT states (fig. 3.17a): the d-d
excitation triggers an intersystem crossing (ISC) cascade which occurs within the experimental
temporal resolution ( 50 fs), and therefore the speed of the LIESST process is mainly limited by
the intrinsic structural dynamic of the metal-ligand bond elongation ( 150 fs). Again, the HS
potential is reached in a highly vibrational excited state. The dissipation of the excess of energy
occurs via phonon-phonon coupling and results in a non-radiative vibrational cooling inside its
potential. On the other hand, the double intersystem crossing of the reverse-LIESST process is
sequential and its time scale is limited by the electronic lifetime of the different intermediate states
involved (5E and 3T1) (fig. 3.17b). Hence, it is governed by kinetics.
Fig 3.17 a) LIESST activation upon LF excitation. Ultrafast ISC trought INT mixed mediators to a
vibrational exited HS state in less than  50 fs. Concomitant
bond elongation in  150 fs.
VC in 1.2 ps toward the bottom of the HS potential. b) reverse-LIESST upon HS 5E excitation.
The 5E undergoes ISC in 1.7 ps toward the 3T1 INT state which in turns populates the LS state in 40
ps. Both the INT states in the reverse-LIESST process, undergoes ISC from a vibrationally relaxed
state.
88
LIESST vs reverse-LIESST
Even though the LIESST dynamics upon LF excitation follows a diverse pathway though different
INT states with respect to the MLCT activation, the nature of the ISC remains ultrafast. The HS
electronic configuration is still reached in less than 50 fs. Then the population of the antibonding
orbitals strongly induces the bond elongation moving the system towards the HS potential, with a
characteristic HS structure formation in 150 (20) fs. The time scale of such ultrafast structural
reorganization has been established to correspond to the half period of the
vibrational
mode in the HS state [Cammarata 2015]. In this way, the ISC cascade clearly occurs in less than the
inter-atomic vibrational period. This fact denotes that the INT states have no time to vibrationally
relax inside their potential before undergoing the ISC. It is then demonstrated that the electronic and
the nuclear wave functions are strongly coupled during the process leading to a breakdown of the
Born–Oppenheimer approximation. Indeed, the electronic and structural degrees of freedom cannot
be separated and the triplet state(s) cannot be identified as true intermediate states. They only serve
as dynamically mixed mediators into the electronic function as the system evolves (as it will be
explained in more details below). However, recently ultrafast X-ray fluorescence spectroscopy
measurements (highly sensitive to the metal-ion spin multiplicity) have observed the signature of an
unquenched INT triplet 3T state during the LIESST activation from the 1MLCT manifold [Zhang
2014], thus correcting their previous assumption of bypassing the LF states [Cannizzo 2010]. Even
so, a clear identification between the two competing triplet intermediate 3T1 and 3T2 states was not
possible.
Unfortunately the theory regarding the ultrafast mechanism of the ISC is still at an early stage and
the basic mechanisms behind the ISC are yet not well understood. In literature two main theoretical
approaches can be identified. The first is based on ab-initio electronic structure calculations (DFT
coupled to multifunctional CASPT2 approach) [Odrejón 2008, Suaud 2009, Sousa 2013], while the
second is built on a model Hamiltonian describing the interactions of phonon and electron degrees
of freedom [van Veenendaal 2010, Chang 2010].
The ab-initio approach, combining density functional theory (DFT) and multiconfigurational
wavefunction-based (CASSCF/CASPT2) methods, aims to describe the ISC rates starting from the
calculation of the equilibrium geometries, relative energies and spin-orbit coupling for all ground
and excited states [Sousa 2013]. However, the many assumption and approximations taken by the
authors, led the theoretical results far from the experimental observations.
Firstly, the proposed ISC pathways is based on the possibility of an energetic and geometrical
conical intersections between the potential curves of different electronic states [Odrejón 2008,
Suaud 2009]. Such description implies several variations of the structural reaction coordinates
during the ISC cascade. It was experimentally proven that it is not the case (see previous chapter).
Secondly, the ISC rates, and therefore their time constants, are based on spin-orbit coupling and
vibrational terms of the electronic states, which are calculated at their equilibrium geometries. In
addition, it was also assumed that all ISC processes follow the Fermi's Golden Rule within the
Born-Oppenheimer approximation [Sousa 2013], that is, that vibrational relaxation in each state is
faster than its lifetime and the electronic and vibrational wavefunctions can be separated.
These assumptions are obviously not valid in the case of LIESST. The experimental data clearly
point out that there is no time for the INT states to relax before undergoing the intersystem crossing
towards the HS state. Therefore, calculating the ISC rates from the equilibrium potentials can lead
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Andrea Marino 2015
the theoretical values to substantially differ from the experiments. It is really important to stress that
since the electronic "equilibrium" inside the potential wells is not reached before several ps (time
scale of the vibrational cooling in the HS potential), the INT state(s) cannot be truly defined. They
do participate to the spin-flip process, but just as mixed wavefunctions which help the dynamical
process to reach the HS state configuration (fig. 3.17a). Otherwise, this assumptions can be correct
for the 3T1 state, and for the quintet excited state 5E of
complexes, when populated
during reverse-LIESST (fig. 3.17b). In this case, their lifetime is longer or comparable with the
vibrational cooling and therefore, definitively much longer than a typical
oscillation
( 300 fs). Therefore, it is essential to highlight that the ISC involved in the reverse-LIESST
process, occurs from vibrationally cooled and structurally relaxed INT states. This is a clear
evidence of the diversity of the physics with respect to the LIESST. In this way the photoinduced
HS LS pathway undoubtedly correspond to: 5T2 5E 3T1 1A1. The INT states are rigorously
defined, and their decay is expected to follow the semi-classical behavior of a non-adiabatic multiphonon process occurring between two well-defined Born–Oppenheimer states [Buhks 1980], as it
is also found for the HS LS relaxation itself. However, the theoretical proposed time scale for the
3
T1 1A1 ISC [Sousa 2013] does not match the above experimental observations.
Contrary to ab-initio calculation, the Hamiltonian-based model considers the coupling mechanism
between the first photoexcited state and its dephasing into a general phonon state. This semiempirical approach explains the ultrafast ISC cascading through the electron-phonon self energy
difference between two different states, again considered at the equilibrium, ergo leading to
tremendous deviations from the experimental observations [Chang 2010]. However this approach
well describe the ultrafast trapping of a metastable HS state. The key point is to consider the phonon
dumping which dissipate the excess of energy through phonon-phonon coupling. Therefore the
intra-molecular energy redistribution stabilizes the photoinduced HS state, otherwise the system
could fall back into the LS ground state [van Veenendaal 2010].
Hank back the general picture in fig 3.17, and analyzing the aspect of the coherent dynamics treated
in the previous chapter, it is possible to highlight another marking difference in the photo-physical
processes of LIESST and reverse-LIESST.
Despite the previous chapter demonstrated that the ultrafast displacive nature of the LIESST
process leads to a selection of coherent vibrational modes, both in solution [Consani 2009] and in
solid state [Cammarata 2015], the data here reported in fig. 3.9 on the LIESST of the
do not show any coherent dynamics. Indeed in the latter case, the probe wavelength (300 nm) was
set to only monitor the LS state bleaching, whereas the HS state was silent. It was not possible to
experimentally observe any activated coherent vibrational mode. Still, this fact does not prove the
absence of coherent phenomena during the ultrafast LIESST via d-d activation.
Conversely, the reverse-LIESST has been demonstrated occurring in a much longer timescale
[Marino 2014]. In this case, the phase matching is lost between the individual molecular events, and
the coherent structural vibrations are not observed during the process. These results also underline
that the mechanism is not driven by the dynamics of the structural trapping of the electronic state, as
it is the case for LIESST, but mainly by the lifetime proper of the electronic exited states.
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LIESST vs reverse-LIESST
In order to give an explanation for the lower quantum efficiency of the reverse-LIESST process let
us compare the intensities of the plateau of the reverse-LIESST in the
(fig. 3.11) and
in the
(fig. 3.14) crystals. Considering that the experimental conditions were kept
identical, a higher number of molecules switch from the HS state to the LS state for the
crystals, in agreement with the overall higher quantum efficiency for the latter
crystal. This can suggest that the major loss of photo-excited molecules occurs at the stage of the 5E
state. It can be presumed that since the quintet excited state 5E of the
crystals has a
longer life-time, this may favor an internal conversion back to the HS state. Contrariwise, the faster
ISC of the
remove the opportunity to fall back to the ground state. In the latter case,
likewise the ultrafast ISC of the LIESST process, it favors the photoswitching quantum efficiency,
moving the direction of the process toward the LS state. Therefore, it is possible to generalize that
the longer the lifetime of an intermediate states, the higher the probability to undergo different
pathways and thus lower the quantum efficiency of the process.
However in a global contest, these achievements are of general importance for the photophysics of
transition-metal compounds. For instance chromium(III) [Juban 2005, Schrauben 2010] or
ruthenium(II) [Sun 2015] complexes are being used in photovoltaic devices [Grätzel 2001, Grätzel
2005], photocatalyst [Pan 2014], or in cancer phototherapy [Very 2012, Higgins 2012, Howerton
2012, Wachter 2012] and they do need a full understanding of the elementary photo-processes. It is
interesting to note indeed that in RuII complexes the corresponding 3T1 state has recently been
located as an intermediate state in the ultrafast quenching of the 3MLCT luminescence, having itself
a lifetime of 450 ps [Sun 2013].
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Andrea Marino 2015
92
Chapter 4
Spin State Concentration Wave
Part of this work was published in:
A. Marino, M. Buron-Le Cointe,M. Lorenc, L. Toupet, R. Henning, A. D. DiChiara, K. Moffat, N.
Bréfuel and E. Collet
Out-of-equilibrium dynamics of photoexcited spin-state concentration waves
Faraday Discuss. 177, 363–379 (2015)
Andrea Marino 2015
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Spin State Concentration Wave
4.1 Molecular state Ordering and Symmetry Breaking
The
previous chapters tackled how different degrees of freedom are involved in the local
molecular trapping of LIESST and reverse-LIESST mechanisms. These studies underlined the
complexity of the pathway across the different electronic intermediate states involved in the spin
state conversion, and the ultrafast energy redistribution in the lattice via electron-phonon and
phonon-phonon coupling. These phenomena were in all cases confined at the molecular level.
In spin-crossover (SCO) solids, the crystal packing of molecules can result in short- or long-range
interactions leading to other types of concerted transformations which appear in relation with their
bistable nature and are related to ordering phenomena of HS and LS states and/or structural
symmetry breaking. The concept of broken symmetry, also associated with ordering phenomena, is
very important in physics and in materials science as it is at the origin of the emergence of physical
properties, such as ferromagnetism or ferroelectricity for example. An important aspect is not only
the structural atomic or molecular order, but also the ordering of electronic states, which is
manifested by the appearance of regular patterns in charge-density waves (CDW) [Möhr-Vorobeva
2011, Torchinsky 2013, Porer 2014], spin-density waves (SDW) [Klemme 1995, Kim 2012] or
superconductors [Wu Nature 2011, Laliberté 2011], for instance.
Generally, it is possible to modify or to destroy the order via external parameters, such as
temperature or pressure, in order to drive macroscopic functionalities related to ferromagnetism,
ferroelectricity or conductivity for example. More recently, light was also used as a new type of
external parameter able to tune order on the ultrafast timescale. [Collet 2003, Gao 2013, Beaud
2014] Mixing, symmetry breaking, multi-stability and photoinduced phenomena open new
perspectives for directing complex ordering and physical properties in technologies and material
science.
In the case of SCO solids, several cases of long-range ordering of molecules in HS and LS states
were experimentally reported [Boinnard 1994, Yamada 2003, Yamada 2006, Weber 2008]. In
analogy with CDW or SDW, such spin-state ordering was then described as a spin-state
concentration wave (SSCW) [Collet 2012a, Marino 2015]. Furthermore, the SCO molecular
bistability with regard to LIESST and reverse-LIESST, opens the opportunity to destroy or create
such SSCW with light. It was recently reported that cw laser irradiation can erase a SSCW via
LIESST thanks to the complete and selective photo-switching from LS to HS states [Collet 2012a].
However, the response of such SSCW to ultra-short pulsed lasers was never investigated.
Nevertheless, it is now well established that ultra-short laser pulses can induce a complex out-ofequilibrium and multi-step dynamical switching in SCO solids [Lorenc2012], able to completely
switch a crystal from the LS state to the HS state in thermal hysteresis[Bonhommeau 2005, Cobo
2008].
The intent of this chapter is to investigate the out-of-equilibrium dynamical response of the SSCW
to femtosecond laser excitation. Time-resolved x-ray diffraction was combined with ultrafast optical
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Andrea Marino 2015
spectroscopy to probe how the two different types of order parameters (XHS, the average HS
fraction and  related to the spin-state order) evolve in time.
After a brief recall of the present literature on stepped SCO transitions, the second part of this
chapter will introduce how the molecular ordering of LS and HS states can be described in terms of
spin-state concentration wave (SSCW) and how the order parameters describing such wave can be
extracted from experimental measurements.
At first the SCO system in analysis [FeIIH2L2Me][PF6]2 will be presented. Further experimental
studies will report a more complete and detailed characterization of the thermal behavior of the
SSCW appearing in the intermediate phase of the system. These results will be discussed in the
general frame of the Landau theory of phase transitions in order to formally describe the symmetry
breaking phenomenon involved when the SSCW forms.
Regarding the research topics developed at the Institute de Physique de Rennes, on the out-ofequilibrium dynamics of SCO materials initiated by a fs laser excitation, the questions of the
response of SSCW to fs laser excitation appear. It is important to understand this dynamics and to
evidence during which step, i.e. photoinduced, elastic, thermal or other, the SSCW disappears (or
not). How do the independent molecular sites go back to symmetrical equivalence? What is the
temporal evolution of the two order parameters XHS and ? Are they coupled somehow?
The third and last part of this chapter will report on such investigation. Combined time resolved Xray diffraction and optical spectroscopy measurements enabled to characterize and reveal the outof-equilibrium dynamics of the SSCW.
4.1.1 State of the art of stepped SCO transitions
The majority of SCO solids undergoes isostructural and single step conversions between the HS
high temperature phase and the LS low temperature phase [Gütlich&Goodwin 2004, Halcrow
2013]. There are few systems showing structural symmetry breaking during the thermal SCO [see
for example Létard 1997, Watanabe 2013] or photoinduced spin conversion [see for example
Bréfuel 2009, Bréfuel 2010]. Such kind of symmetry breaking is of structural nature and results
from molecular distortions which lead to a loss of symmetry operators. It is well known that weak
interactions between molecules constituting the SCO crystal can lead to continuous conversions,
whereas stronger coupling can drive first order phase transitions with hysteresis [Boukheddaden
2000a,b, Buron 2012]. But the competition between short- and long-range interactions can also give
rise to stepped transitions or partial conversion. More precisely, it is the competition between
ferroelastic-like and antiferroelastic-like interactions which is responsible for the appearance of
intermediate phases, where LS and HS molecules are spatially ordered [Nishino 2003, Bréfuel
2009, Bréfuel 2010, Boukheddaden 2007]. In literature, different types of stepped conversions are
reported. The two steps transition can come from the molecular multi-stability itself, as it is the case
for binuclear systems: the relative stability of the LS-LS, LS-HS and HS-HS states can be balanced
by temperature or light [Real 1992, Létard 1999, Ksenofontov 2004, Trzop 2007]. There are also a
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Spin State Concentration Wave
few systems for which the asymmetric unit in the crystal packing comprises two symmetry
independent molecular sites (therefore with different ligand fields): the global two-step response of
such crystals is then the sum of the single response of the two independent sub-lattices undergoing
their own SCO at two different temperatures [Hinek 1996, Weber 2008]. These examples are
therefore not associated with the symmetry breaking due to long-range ordering of molecules in HS
and LS states. However among mononuclear complexes, few examples report an intermediate phase
resulting from a symmetry breaking due to such a long- or short-range ordering of molecular spinstate (HS-LS) [Yamada 2008, Nihei 2010, Griffin 2011] with the appearance of different HS
concentration over the crystalline sites.
Figure 4.1 shows some cases of SCO crystals undergoing stepped transitions. In addition to the HS
and LS phases an Intermediate Phase (IP) occurs, where a symmetry breaking results from spatially
ordered HS and LS states. Different steps with HS fraction close to 1/2, 3/4, 1/3 or even irrational
have been reported [Chernyshov 2003, Murnaghan 2014, Bonnet 2008, Collet 2012a]. Fig. 4.1a
reports an example of a two-step SCO associated with two first order transitions, observed in the
system [Chernyshov 2003]. In the IP, a symmetry breaking occurs with the
appearance of two independent molecular sites per unit cell (almost completely HS or LS). The
bond length was used as a marker to measure the HS fraction XHS, which can also be
estimated on each molecular site. The bottom of fig. 4.1a shows that on site1 in the IP phase of this
compound
corresponds almost to the molecular HS state, while on site2
corresponds almost to the LS state. Other systems such as
(fig. 4.1b) (which
will be discussed in details later in this chapter) show a continuous high temperature step and a
first-order low temperature step. The IP phase is accompanied by a partial spin-state ordering
between the crystalline sites: the probability to find the molecules either in one or another spin-state
is not close to 0 or 1 [Bréfuel 2009]. Figures 4.1c and 4.1d report other exotic stepped transition of
SCO complexes, were the IP phase manifests at respectively XHS equal to 3/4 and 1/3, with their
corresponding schematic HS-LS patterns [Murnaghan 2014, Bonnet 2008].
Fig. 4.1 Thermal transition curves of SCO complexes undergoing two stepped transition with an
intermediate phase appearing at different HS fraction and respective crystal packing representation
a)
with complete ordering [Chernyshov 2003] b)
with partial ordering [Bréfuel 2009] c)
[Murnaghan 2014] d)
@
[Bonnet 2008]
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Andrea Marino 2015
These HS-LS ordering phenomena were recently discussed on the basis of the universal Landau
theory of phase transitions [Chernyshov 2004]. On this basis, the spin-state ordering can be simply
described as resulting from the appearance of a spin-state concentration wave (SSCW) [Collet
2012a]. According to the wave description, the value of the HS fraction XHS(r) at the position r in
the crystal lattice can be expressed as follow:
where q is the wave vector, XHS the global average HS fraction over the molecular sites in the
crystal, and  is the amplitude of the wave measuring the difference in HS fraction between the two
sub-lattice sites
.
Fig 4.2 Representation of the spin-state concentration wave as wave modulation of the
fraction along the crystalline sites.
The three phases can be then described with the use of the two parameters XHS and which define
the SSCW in the crystal. Table 4.1 reports an example of complete spin-state ordering which helps
to elucidate the formation and description of the SSCW in the IP phase reported in fig. 4.2. In the
high symmetry phase, the crystal sites present no difference in the HS or LS phases. All molecules
are either in the HS state (therefore
) or in the LS state (therefore
). At the IP, the
average HS fraction over all the crystal is
, for example. However, the molecular ordering
forces one site to be in the HS state with
, and the neighboring site in the LS state
. Then the amplitude of the wave 2 is equal to 1 as it measures the difference in the
HS fraction between the two sites.
Table 4.1 Schematic description of the SSCW in the three HS, IP and LS phases with the use of the
parameters XHS and 
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Spin State Concentration Wave
Literature reports several examples of SCO showing 3D long-range [see the references in fig. 4.1
for example] as well as 1D short-range [Neville 2008] molecular spin-state ordering. These works
focused mostly on the structural characterization of the different phases at the thermal equilibrium.
Structural investigations performed at low temperature after LIESST from the LS phase, also
pointed out an atypical two-step relaxation from the photoinduced-HS (PIHS) to the LS phase with
an ordered IP phase appearing during the thermal relaxation from the complete PIHS state to LS
state [Pillet 2012, Murnaghan 2014].
So far only a single work reported on the effect of light excitation on SSCW, demonstrating an
effective photo-erasing of the spin-state concentration wave under cw irradiation [Collet 2012a].
However, nothing was yet known on the response of such SSCW to femtosecond light excitation.
For this purpose, the
compound (fig. 4.1b) has been chosen for several reasons:
it undergoes a continuous transition from IP to HS phase (which may limit sample damage); the IP
phase is stable over a large temperature range (which will allow to study the temperature
dependence of its response); finally, the LIESST is very efficient as the crystal can be completely
switched to a PIHS phase with light excitation at around 530 nm [Bréfuel 2009].
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Andrea Marino 2015
4.2 Spin State Concentration Wave in [FeIIH2L2Me][PF6]2
4.2.1 Description of the
crystal
The cation of the SCO
molecular
crystal is schematically represented in fig. 4.3 where
the H2L2Me denotes the acyclic hexadentate N6 Schiff
base bis[N- (2-methylimidazol-4-yl ) methylidene-3amonipropyl] ethylenediamine. SQUID analysis,
which monitors the thermal crossover via magnetic
susceptibility sensitive to the HS fraction (fig. 4.4),
indicates that the complex undergoes two-step
transition. In the high temperature phase above 250 K
all molecules are in the HS state. Below 250 K the
compound exhibits a continuous transition to a
pseudo-plateau in which a new Intermediate Phase
(IP) appears in the 90 K - 142 K region with
approximately 50% of HS molecules. A further 1st
order transition (with hysteresis loop of 6 K) lead the
IP phase to the low temperature phase where all
molecules are in the LS state. Quantitative LIESST,
under cw laser irradiation centered at 532 nm,
promotes LS molecules into a new photo-induced HS
phase (PIHS) [Bréfuel 2009].
Fig 4.3 Chemical representation of the
[FeIIH2L2Me]2+ cation. The iron ion is
octahedrally coordinated with the 6
nitrogen of the acyclic hexadentate N6
Shiff base, and balanced with two negative
PF6 anion not represented here.
Fig 4.4 Thermal evolution of magnetic susceptibility for
crystal. In blue
cooling and in red heating processes. Two-step transition with a pseudo-plateau in the
. The black arrow shows the LIESST at 15 K which qualitative switches LS molecules in the
PIHS state. In green is reported the two-step relaxation of the PIHS toward the LS ground state
[Bréfuel 2009].
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Spin State Concentration Wave
Accurate structural analyses were performed on the different phases of the complex, by determining
the crystal structure of the HS, LS, IP and PIHS phases respectively at 250 K, 80 K, 110 K and 15
K after irradiation [Bréfuel 2009]. It emerged that the
crystal undergoes
multiple symmetry breaking between all present phases. In the highest symmetric HS phase with
space group P22121, and a reference crystal unit cell
as shown in fig. 4.5, the asymmetric
unit is made of one half
cation located on 2 fold symmetry axis and one
anion. On the
other hand, both LS and PIHS phases present a structural symmetry breaking leading the crystal
unit cell to
and
respectively also with space groups P22121. As a result of the cell
parameter multiplicity, some 2 fold axis are lost in the LS, IP and PIHS phases, leading to small
structural distortion (molecular titling). There are 2 symmetry independent cations in these phases.
(see fig. 4.5).
Fig 4.5 Representation of the
crystal packing projected along the b axis, and
corresponding diffraction pattern for the a) high symmetry HS phase and b) low symmetry
partially ordered IP phase [Bréfuel 2009]. With lattice vectors a vertical and c horizontal.
X-ray diffraction analysis revealed that the origin of the IP phase results from a spin-state ordering
among the FeII sites with the appearance of HS-LS-LS-HS sequence along the crystal axis c, as
already schematically represented in the bottom of fig. 4.1.b. In the high symmetry HS phase the
two molecules in the unit cell are symmetry equivalent by a 21 screw axis, with the consequence of
having only one independent molecular site (fig. 4.5a). With respect to the high symmetry HS
phase, the symmetry breaking occurring in the IP phase corresponds to a cell doubling along the c
axis. This is characterized by the appearance of additional Bragg reflections observed comparing
the X-ray diffraction patterns of the HS and IP phases in the right side of fig. 4.5. Therefore, in the
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Andrea Marino 2015
IP phase there are four molecules per unit cell, but only two symmetry independent sites which will
be named from now on site1 and site2. Figure 4.5 highlights that while in the high symmetry HS
phase site1 and site2 are equivalent by the translation c, the symmetry breaking in the IP phase
results in a change of the translation symmetry
.
In the IP phase, the measurements of the
bond lengths on the two new crystallographic
independent molecular sites i, indicate that the average
distance on site1 is shorter
compared to the usual
bond length corresponding to the molecular HS state. On the
other hand the
on site2 is longer compared to the
bond length corresponding
to the molecular LS state. Such observation suggests the presence of a partial ordering between two
non equivalent sites with the molecules on site1 being mostly in the HS state, and on site2 being
mostly in the LS state. This partial spin-state ordering corresponds to the long-range HS-LS-LS-HS
regular pattern in the IP phase.
4.2.2 Experimental description of SSCW.
The SCO phenomenon, i.e. the conversion of molecules from LS to HS states and vice versa, can be
followed through the evolution of the fraction of HS molecules XHS:
where NHS is the number of molecules in the HS state among the N molecules of the crystal. In the
first chapter it was discussed that XHS can be measured via different techniques such as SQUID,
Mossbauer spectroscopy, optics or X-ray. The thermal evolution of XHS is reported in fig. 4.6
showing, indeed, the matching between the values measured thought the average
bonds
(blue circles), and via magnetic susceptibility (black line).
In a first approximation, the magnetic susceptibility is weighted by the fraction of molecules in LS
state (with magnetic susceptibility
) and HS state (with magnetic susceptibility
) and
therefore give direct access to the average spin-state fraction of the molecules:
It comes out that:
In the same way, XHS can be also derived by optical measurements weighting the optical density
(OD) at a given (T,P) phase over the OD difference of the HS and LS phases:
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Spin State Concentration Wave
Furthermore in SCO systems the average
bond length, characteristic of the molecular
spin-state, is a good marker for the HS fraction XHS as it weights the contribution of the HS and LS
molecules in the observed value:
where the two molecular spin-states of the
compound correspond to:
and
This is also true for the independent molecular site1 and site2 in the IP phase with respectively local
HS fractions
and
:
Fig 4.6 Evolution of the HS fraction (right axis) deduced from magnetic measurements (solid line)
and X-ray diffraction (blue circles) thought the average <Fe-N> bond length (left axis). Red
triangle shows the <Fe1-N> bond length on site1 and blue triangle measure the <Fe2-N> on site2.
At the IP phase site1 is found to be prevalently in the HS state, whereas site2 prevalently in the LS
state. Therefore, the two site are no longer symmetry equivalent and a cell doubling must occur.
Figure 4.6 shows the thermal evolution of
and
, respectively represented in red
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Andrea Marino 2015
and blue triangles. In the IP phase below 142 K,
and
differ from the average
Fe-N bond length, indicating a partial spin-state ordering between the two independent
sites. In this way, the four molecules in the doubled cell consists of two LS et two HS, resulting in
the HS-LS-LS-HS pattern (see left of fig. 4.5b). Conversely, above 142 K,
and
turn back equal as the high symmetry phase is reached, i.e. the two sites become symmetry
equivalent
and the spin-state ordering disappears.
Such a spin-state ordering can be interpreted with a wave-like model representing the probability to
find a HS molecule at a given site. The formulation of the spin-state concentration wave (SSCW)
therefore corresponds to a spatial modulation of the HS fraction
along the crystalline sites:
Three parameters describe the wave.
corresponds to the modulation wave vector, which
defines the periodicity of the SSCW in the IP phase.
is the average HS fraction over all the
crystal and varies in temperature according to fig. 4.6. It thus corresponds to the totally symmetric
order parameter (OP) as it does not depend on the position r in the crystal.  measures the
amplitude of the wave, and it is then related to the difference of HS fraction on the two independent
sub-lattices sites:
Nevertheless,represents the order parameter associated to the symmetry breaking since it is
directly related to the degree of ordering in the IP phase.
The SSCW is a powerful tool able to simply describe the spin-state ordering of SCO solids with a
limited number of parameters. Figure 4.7 elucidates the formation of the SSCW. A schematic
representation of the spin-state modulation at thermal equilibrium is reported for the three phases of
the system. Their respective schematic crystal packing with several molecules in the unit cell, is
also represented.
For the HS or LS phases as well, the two sub-lattice sites are symmetry equivalent
,
which means there is no preferential HS or LS population between the sites. Since in this phases the
amplitude of the wave is
, the SSCW does not appear. There is no modulation as
is the
same over all the crystalline sites. (fig. 4.7a and 4.7c)
On the other hand, the IP phase is characterized by the appearance of a HS-LS-LS-HS regular
pattern. This molecular spin-state ordering highlight that the sub-lattice sites 1 and 2 become
symmetry independent. X-ray data at 130 K measured
and
for the two independent sites in the IP phase. This allowed an estimation of the HS
fraction per site:
which determined the values for the two order parameters describing the modulation-wave:
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Spin State Concentration Wave
and
The spin-state concentration wave at 130 K (represented in fig. 4.7b) is therefore defined by:
Fig. 4.7 Representation of the SSCW in the three HS, IP and LS phases, with their corresponding
schematic unit cell (black square). Red and blue respectively represents the mainly HS and mainly
LS crystalline sites, as well as for their probability represented with the SSCW.
With the use of a SSCW description of the systems, is therefore possible to determine and predict
the HS fraction at each crystalline site. For example, since in the IP phase the new periodicity is
, defined by the wavevector q:
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Andrea Marino 2015
These expressions underline how the HS fraction at a given site r,
equal to , from the overall HS fraction of the crystal
value.
, differs with a deviation
4.2.3 The Landau Theory of Phase Transitions applied to SSCW
The concept of spin-state concentration waves in SCO crystals, resulting from a symmetry breaking
can be described in the generalized framework of Landau theory of phase transitions. [Landau 1937,
Landau 1980, Landau 2008] In his heritage Landau treated transitions of different solid
modifications between crystals with different symmetries associated to the disappearance or
appearance of some elements of symmetry.
During a phase transition while the state of the system (the HS fraction) can change continuously or
discontinuously, the symmetry elements are either present or absent; no intermediate case is
possible. At any moment of the transition the symmetry of the system is therefore well defined. A
very small perturbation around the phase transition can modify the symmetry and induce drastic
changes of the physical properties of the material.
First of all, Landau's approach consists in describing a crystal using a spatial distribution probability
where x, y and z are the space coordinates of the crystal. The most generic function that
can be considered is a density
which describes the properties and the mean density at each
point of the crystal, and more importantly it determines the crystal symmetry: i.e. the group of
symmetry operators, under which the density
remains unchanged in different equivalent
coordinates. This defines also the symmetry group G of the crystal.
Let us now consider as an example a crystal which undergoes a phase transition between two phases
of different symmetry. In the high symmetry phase the crystal is described by the density
and the symmetry operators defined by the space group G0. Then,
should be considered as totally
symmetric as it is invariant under all the symmetry elements of G0. In this way
when (x,y,z) and (x',y',z') are equivalent coordinates by the symmetry operators of the
space group G0. More in particular
is invariant under the translation symmetry operators T of the
crystal:
,
a, b and c being the lattice vectors and m, n, p integers.
For example, it is invariant under the translation along one of the crystal lattice parameters, as
represented in fig. 4.9b:
.
When a phase transition leads the system to a phase of lower symmetry defined by a space group G,
which is a sub-group of the high symmetry phase group G0, Landau postulated that the density of
the lower symmetry phase can be decomposed in a sum of two components:
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Spin State Concentration Wave
where
is the density of the high symmetry phase (space group G0) and
corresponds to a
perturbation of lower symmetry (G G0), e.g. augmentation and depletion of density between the
neighboring sites as in fig. 4.9c. Since the sum of two functions has the same symmetry as the less
symmetric term, a symmetry breaking occurs between
(high symmetry phase) and
(low
symmetry phase). This can be easily observed considering the different symmetry elements of the
space groups G0 and G of the two phases, and in particular the lost of a translation symmetry
operator as in fig. 4.9.
Fig. 4.9 Schematic representation of a generic density function for a) a low symmetry phase ,
b) high symmetry phase , and c) asimmetric perturbation
of lower symmetry.
Given that the space group G of the low symmetry phase is a sub-group of G0, all the symmetry
elements of G are also elements of G0, but not vice versa. This is the case for the translation
symmetry operator
. , and
are all invariant under this translation symmetry , which
is the new translation operator of the low symmetry phase:
as well as
.
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Andrea Marino 2015
Conversely this is not true for the symmetry element c which is an element of G0 but not of G.
Indeed, in the present case
is antisymmetric with respect to c:
,
and therefore:
.
This example illustrates how the loss of one of the symmetry elements (corresponding here to a cell
doubling) in the low symmetry phase modifies the spatial distribution of the density describing the
high symmetry phase.
The amplitude of the deviation from the high symmetry density , which caused the symmetry
breaking, can be measured by the order parameter in terms of density variation on the two sites:
Consequently, any density
in the low symmetry phase can be written as a totally symmetric
term
plus an antisymmetric term
, with a given amplitude  In this way represents
the order parameter associated to the symmetry breaking and it measures the deviation of the low
symmetry phase from the high symmetry phase:
(4.1)
Therefore, when the amplitude  is equal to zero the two neighboring sites do not show any
difference in their density distribution
. It corresponds therefore to the high
symmetry phase where both sites are symmetry equivalent (
G).
This general approach can be easily extended to the present case of spin-state concentration wave in
SCO crystals where the analogy is evident with the general formulation of the Landau theory (eq.
4.1). The density XHS(r) describing the HS fraction over the different lattice sites is composed of
two terms with different symmetry:
, the overall average HS fraction is the totally symmetric order parameter since it remains equal
all over the crystalline sites. It is invariant under all the symmetry elements of the HS space group
of the crystal and therefore, it is analogous to 0. Especially, it remains unchanged under the
translation symmetry of the HS phase c. On the other hand, the term
represents the
perturbation which lowers the symmetry in the IP phase. The wave vector
, defines a
new periodicity
and therefore, the space group G of the low symmetry IP phase is a subgroup of the high symmetry group G0 of the HS phase. More in particular
in
the low symmetry IP phase. Again, defines the amplitude of the perturbation and it is directly
related to the degree of ordering in the IP phase. Hence, corresponds to the symmetry breaking
order parameter (OP).
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Spin State Concentration Wave
The evolution of the system is generally associated with the change of the thermodynamical
potential
, for example, with an order parameter x characterizing the state of the system for
which
is minimum . In the case of conventional single-step SCO crystals without symmetry
breaking, x is a totally symmetrical OP related to the fraction of molecules in the HS state XHS. It is
a scalar, which does not describe symmetry breaking and it transforms as the totally symmetric
irreducible representation of the crystal's space group. In this case, the spin conversion is similar to
a liquid-vapor transition where the symmetry of the two phases remains the same although their
physical properties (especially the density ) change. The evolution of the state of the system,
characterized by x, is given by the variation of the minima of the free energy as shown in fig. 4.10.
Fig. 4.10 Free energy in function of the totally symmetric OP at different temperatures,
representing the spin transition between the HS phase
to the LS phase
Figure 4.10 depicts the free energy landscape  at different temperatures
in
function of the order parameter
. Therefore the HS fraction is given
by
. In the example reported, at high temperatures (T1) there is only one stable
point at
corresponding to the HS phase. Lowering the temperature (T2) two minima appears
corresponding to one stable and one metastable point. A further decrease of temperature (T3) leads
the system to a new stable point located at
witch correspond to the LS state.
In the vicinity of the transition point, the Gibbs free energy can be developed in series of the OP x.
For revealing the main features associated with the phase transition, a simple expression of the
thermodynamical potential is given in text books [Tolédano 1996] by:
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Andrea Marino 2015
In this simple description, the coefficients
and
are linearly dependent on temperature and
pressure. Then, the equilibrium conditions of the system must satisfy:
Fig. 4.11 reports the solutions of the above equation as a function of the coefficient a1 (and
therefore of the temperature) as a planar section of the free energy
at a given . Hence,
two different regimes are defined for
and
. When
the free energy
has a
single minimum at any temperature. The variation of
would correspond to a smooth spincrossover rather than a spin-transition, as for example the case of the SCO
crystal reported in chapter 2. A different behavior appears when
. In this case eq. 4.3 has
three solutions: two minima and one maximum. The two minima would then correspond to a stable
and metastable point as for the intermediate curve in fig. 4.a. When
the two minima overlap,
and lead to an abrupt first-order spin-transition as the case for the fast cooling of the pure
system reported in chapter 3. Otherwise the spinodal shape of the free energy with the
two minima represent a spin-transition accompanied by hysteresis loop.
Fig. 4.11 Solutions of eq. 4.3 plotted as transition curves for an isostructural transformation The
upper half-plane corresponds to the HS phase, whereas the lower half-plane to the LS phase. a
smooth crossover from HS to LS b)
abrupt first-order transition. c)
firstorder transition with hysteresis where the stability limits are shown as circles. [Chernyshov 2004]
However this model describes the case of a standard isostructural spin transition where the space
group of the physically different phases remains unchanged. In the actual case of study, the
crystals present a symmetry breaking at the IP phase. Therefore the model
should include the free energy dependence on the structural transition which is represented by the
symmetry breaking order parameter . In this way, the free energy can be developed in power of 
and x in the form:
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Spin State Concentration Wave
Since under the change of symmetry the energy of the system must remain the same, it imposes that
. Therefore the expression in eq. 4.4 only includes only even terms of he
crossed terms define the coupling of the spin and structural transformations respectively represented
by the order parameters x and 
The equilibrium conditions are then satisfied when:
The evident solution with
above.
correspond to the standard isostructural spin transition discussed
The solutions to the eq. 4.5 and 4.6, together with all the possible transition scenarios are reported
in the phase diagram in the
plane (fig. 4.12). The IP phase appears in a closed area between
the upper HS and lower LS phases in the phenomenological and coefficient space (fig. 4.12).
Fig. 4.12 Generic P,T phase diagram for a two-step spin transition making clear also the
temperature and pressure dependence of the a1 and a2 coefficients. Several isobaric spin-crossover
thermal transition are reported with the dashed-dotted vertical lines and discussed below in the
text. [Chernyshov 2004]
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Andrea Marino 2015
In this phase diagram, the grey shadowed areas represent the coexistence of two or more phases
(dark grey area). A line crossing this area would then correspond to an hysteresis loop, of which
width is defined by the thickness of the crossed grey area. The vertical cuts in fig. 4.12 represent the
various isobaric regimes for the different kind of spin-crossover:
1. The HS LS crossover follow a smooth gradual isostructural conversion (2nd order-like).
2. The isobar crosses the IP phase area. The sequence HS
IP
LS undergoes two gradual 2nd
order structural spin transitions.
3. This spin-transition is as described in point 2 with the difference of a 1st order phase transition
between the IP and LS phases.
4. In this case, both HS IP and IP LS phase transitions are of the 1st order with hysteresis.
5. The isobaric line crosses a triple point (D) where the three phases coexist.
6. This cut corresponds to the conventional 1st order phase transition with hysteresis.
From this phenomenological description of the different kind of spin conversions, it is evident that
the case of study
correspond to the 3rd regime where the HS
IP phase
st
transition is gradual and the IP LS is of the 1 order.
4.2.4 the symmetry breaking order parameter 
As the SSCW is defined by two order parameters
and , it will be possible to study its out-ofequilibrium dynamics through the time dependence of these parameters, firstly characterizing their
thermal behavior and then monitoring the time evolution by means of ultrafast pump-probe
techniques.
As indicated before, the totally symmetric OP
can be obtained through a variety of techniques
such as magnetic, X-rays diffraction and optical measurements. This is not the case for the
symmetry breaking OP . Only diffraction techniques (x-ray, neutron, ...) sensitive to
intermolecular order, are able to distinguish the HS fraction at each molecular site, and then
measure the degree of spin-state ordering, i.e. the amplitude of the SSCW.
Given the general formula for the structure factor of a generic unit cell:
where the sum is over all the atoms in the unit cell at position
form factor . Here, the unit cell of the IP phase (with lattice parameter
independent molecular sites shifted by a vector
as in fig. 4.13.
112
with atomic
) contains two
Spin State Concentration Wave
Fig 4.13 Simplified unit cell of the IP phase made of two independent sites around
and
.
Only the two symmetry independent SCO molecules on the two sites are shown for clarity. Red
color correspond to HS state while blue color correspond to LS state. Above 142 K and below 90
K,
and
are equivalent by a translation , but not in the IP phase where the spin-state
ordering leads to a cell doubling, i.e.
.
Hereafter site1 and site2 are respectively represented with the reduced coordinates
and in a first approximation
of the IP phase unit cell. Therefore, the structure
factor at the IP phase can be decomposed in the sum of the contribution of each independent
molecular site:
(4.2)
where the sum over j in
is over all the atoms on sites 1 and 2.
Fig. 4.13bis Average structure of site1 and site2. In blue and red are respectively represented the
HS and LS structures. On site1 the main contribution is given by the HS molecules, therefore the
HS red structure is much dense the transparent LS representation. On the other hand, site2 is
mainly LS and hence the bleu LS structure result more dense in this schematic picture.
Due to the partial spin-state ordering, the molecules on site1 can be either in the HS state, with a
probability
and corresponding molecular structure factor
, or in the LS state with a
probability
and structure factor
(fig. 4.13bis). Then the structure factor of site1is:
(4.3)
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Andrea Marino 2015
In the same way, the molecules on site2 can be in the HS state with probability
factor
, or in the LS state with probability
and structure factor
and structure
. Then:
Remembering that the structure factor of the atoms in site2 have to be calculated in the position
, that is shifted by one half of the corresponding unit cell parameter of the IP
phase (fig 4.13):
The first term in the summation correspond exactly to the structure factor of the HS(LS) molecules
calculated in
, namely site1. Furthermore
for
. Then the structure factor of the HS(LS) molecules calculated in the position results:
.
The total average structure factor of site2 can be then weighted by the HS and LS molecular
fraction:
(4.4)
Inserting eq. 4.3 and eq. 4.4 in the former eq. 4.1 it gives:
Since the SSCW has an amplitude 2around the average value
and
and
, it comes out that:
,
.
Then:
which finally results in:
In other words, in a first approximation, the Bragg peaks indexed
are characteristic of
the symmetry breaking and their intensity is proportional to the square of the order parameter
measuring the amplitude of the wave:
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Spin State Concentration Wave
In this way
are the reflection intensities characterizing the spin-state ordering
between the independent molecular sites 1 and 2, and then, allow a direct measure of the symmetry
breaking OP . The intensities of these peaks are equal to zero just when = 0, i.e. when the HS
fraction on sites 1 and 2 are equal (
) and the SSCW is not present.
In a first approximation, it is possible to consider the molecular structure factors
and
being
temperature independent. Meanwhile, the OP
does depend on the temperature, as it is shown
in fig. 4.7 by the thermal evolution of the average bond lengths on the two sites:
and
. This turns in affect that the intensities of the Bragg reflections corresponding to the
symmetry breaking are also temperature dependent
.
Fig. 4.14a shows the evolution of such intensity, which vanishes at 142 K and characterizes the
temperature above which the SSCW is erased and the high symmetry HS phase is reached. The
continuous evolution around 142 K is characteristic of a 2nd order (or weakly 1st order) phase
transition, in agreement with the continuous evolution of XHS around the high temperature step.
Around 90 K the SSCW discontinuously disappears as all the molecular sites completely switches
to the LS state, and this results from the first order nature of the low temperature step.
Fig 4.14 a) Thermal evolution of the Bragg reflection intensities indexed
which are
related to the symmetry breaking. b) Value of scaled from the Bragg intesities, knowing that at
130 K the SSCW amplitude has been measured being
. The solid lines are only a
guide to the eyes.
Fig. 4.14b reports the thermal evolution of the OP
which can be easily extracted from the
intensities of the above mentioned Bragg intensities (Fig. 4.13a) :
These data show that the order rapidly saturates below 130 K as remains constant. This is also in
agreement with fig. 4.7 which shows an approximately constant difference between
and
of
below 130 K (although the average HS fraction decreases), whereas the bond
length difference also vanishes at 142 K. Given that the X-ray structure refinement at 130 K
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Andrea Marino 2015
estimates the amplitude of the wave to be close to 0.5 as discussed above, the value of 2 reported
in fig. 4.14b was scaled in conformity with such experimental observations.
The thermal evolution of the spin-state concentration wave around the IP-HS transition can be
summarized as depicted in fig. 4.15. The SSCW describe the modulation of the HS fraction on the
crystalline sites around an average value
which varies with temperature according to fig 4.7.
Between 90 K and 130 K the amplitude of the wave reaches his maximum around
with a
clear mark of the Bragg intensities
reported on the right of fig. 4.15a. On the other
hand, the intensities of such Bragg peaks are weaker around 140 K (fig. 4.15b), indicating that the
amplitude of the SSCW gradually vanishes and the system is less and less ordered. In fact the
temperature increase lowers the free energy barrier and therefore favors mixing. At 142 K (fig.
4.15c), although a conspicuous number of LS molecules is still present in the crystal (
),
the disappearance of the Bragg reflection related to the symmetry breaking gives a proof that the
two sites are symmetry equivalent, hence
. In such a case the SSCW disappears and the
high symmetry phase is reached.
Fig 4.15 Sheme of the thermal evolution of the SSCW (left) deduced from the evolution of the Xray diffraction pattern (right). In this narrow temperature range XHS 0.5, whereas 2 changes
from 0.5 (130 K) to 0 (142 K). The appearance of additional brag peaks in the diffraction pattern
of 130 K for odd l values shows a cell doubling along the crystal axis c due to the appearance of
the spin-state concentration wave among molecular sites 1 and 2.
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Spin State Concentration Wave
4.3 Ultrafast out-of-equilibrium symmetry breaking
The out-of-equilibrium dynamic of the spin-state concentration wave (SSCW) was studied by
means of two complementary time-resolved techniques in order to obtain a complete view of the
photoinduced process at the electronic and atomic scales. Femtosecond optical pump-probe
absorption spectroscopy performed in the laboratory at the University of Rennes 1 enabled tracking
the evolution of the totally symmetric order parameter
. The optical set up was suitable to study
the crystal response to different laser excitation densities as well as at different initial temperatures
on a time scale spanning from fs to ms. On the other hand, picosecond time-resolved X-ray
diffraction, performed in the BioCARS beamline at the APS synchrotron in the Argonne National
Laboratories in Chicago, monitored the time evolution of the symmetry breaking OP as well as
the lattice parameters, and the
bond lengths at both independent site
4.3.1 Optical characterization
Fig. 4.16 shows the optical density (OD) change in the visible range as the spin-crossover occurs
from the IP phase to the HS phase. The characteristic Metal-to-Ligand Charge-Transfer (MLCT)
band of the LS state appears around 500 nm upon cooling. The system was photoexcited in the tail
of this band at 530 nm to ensure a larger light penetration depth and an efficient photo-switching
from the LS to the HS states as observed at 10 K in previous studies [Bréfuel 2009].
A global OD increase in the visible range is observed upon cooling when the molecular HS
LS
conversion occurs. The OD change at 610 nm was chosen as sensitive probe to monitor the
evolution of XHS after fs laser excitation. All the ultrafast optical pump-probe measurements were
performed on
single crystals with typical dimensions of
. The probe was focused down to a 50 m diameter spot size,
whereas the pump was focused on a 300 m spot size in order to maximize the spatial overlap
between the two pulses and to ensure an homogeneous crystal excitation.
Fig. 4.16 Temperature dependence of optical density (OD) measured on a signle crystal, revealing
the MLCT band of the LS state at around 500 nm. For time-resolved studies the pump was set at
530 nm and the probe at 610 nm.
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Andrea Marino 2015
The sub-picosecond spin-state photoswitching of LS
cations induced by
femtosecond laser excitation (reported in fig. 4.17), presents the typical ultrafast nature of LIESST
mechanism investigated in the previous chapters. Again, this process consists in a merely molecular
response where the environment is not involved. The transient peak, appearing immediately after
laser excitation, is associated to the MLCT Frank-Condon excitation which decays via ultrafast
intersystem crossing (ISC) toward the HS potential in less than the experimental time resolution
. The formation of photoinduced HS molecules is proven in fig. 4.17. The characteristic
decrease of OD at the plateau, reached after hundreds of femtosecond, is a clear fingerprint of the
ultrafast LS-to-HS spin-switching.
Fig. 4.17 Ultrafast dynamical OD trace obtained by two-color pump (530 nm) - probe (610 nm)
experiment on the ps time-scale.
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Spin State Concentration Wave
4.3.2 Excitation density and non linear response
The use of two synchronized amplifiers, peculiar characteristic of the ultrafast set-up at IPR, allows
bypassing the temporal limitation of a standard motor-stepped delay stage (which in optimal case
can reach up 2-3 ns delay between the pump and the probe) [Lorenc 2012] In such way, the time
evolution of the HS fraction can be monitored up to the ms time scale (this limitation is given by the
laser repetition rate being of 1 kHz).
Fig 4.18 Monitoring the time evolution of XHS with two-color pump (530 nm) - probe (610 nm) at
142 K (upper part) and 135 K (lower part) for different pump fluences.
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Andrea Marino 2015
For the analysis of the crystal response in function of the pump fluence, the experiments were
performed at different excitation densities in the 0.1–1.4 mJ range focused in a 300 m spot which
corresponds to excitation fluencies in the 1–16 J/mm2 range. The temperature was set first at 135
K, where the spin-state ordering generates the SSCW with maximum amplitude, and after at 142 K
corresponding to the transition point between the IP and high symmetry phases where the SSCW
disappears. However, the optical responses of the crystal to fs laser excitation at both 135 K and
142 K (fig. 4.18) show a 3-step process, similar to the one already reported for other single-step
SCO crystals. [Lorenc 2009, Lorenc 2012, Kaszub 2013, Collet 2012b].
The absorption of light at the molecular level, referred to as photoswitching step, locally switches a
small fraction
of molecules from the LS to the HS state. Since a low excitation density was
used, a small fraction of LS molecules was photo-switched to the HS state. Otherwise the use of
higher excitation density resulted in sample damage. From the OD change shown in fig 4.17, it is
estimated that only
of the molecules switch from the LS to the HS state at the ps timescale.
The fraction of HS molecules increases anew up to
after tens of ns as the elastic step occurs.
In the introduction it was discussed that this process is driven by an internal pressure, which leads to
a lattice expansion and further promotes the LS HS conversion.
Finally, the laser energy, which is converted to heat, induces a macroscopic crystal warming and the
HS state is thermally populated at the s time scale. This further HS fraction increases
corresponds to the so-called thermal step.
Fig 4.19 Evolution of photoinduced XHSh and thermal XHSTh step at 130 K and 142 K versus the
pump laser excitation density.
120
Spin State Concentration Wave
Figure 4.19 reports the crystal response of the photo-induced and thermal steps for different
excitation energies. On the photo-induced step,
clearly shows a linear photo-switching with
the fluence of the pump laser pulse. This linear response was also reported for cooperative FeIII
SCO material [Bertoni 2015] and underlined the local nature of this step. Indeed, it indicates that at
the ps time scale only the absorber molecules undergo the spin-state change.
On the other hand, the thermal step
exhibits a non-linear response with a threshold effect. It is
evident from fig. 4.19 that above a critical point the thermal switching changes slope. Moreover, the
threshold critical point is observed to be dependent from the initial temperature. As a matter of fact,
at 135 K (where the SSCW is present) the slope changes above
whereas at 142 K this point
is lowered at
.
4.3.2 Temperature dependence of the thermal step
In order to clarify the origin of this effect, the photo-induced response of the
crystals was studied at different initial temperature with a fixed pump fluence corresponding to
per pulse.
Fig. 4.20 Temporal evolution of XHS for a fixed excitation density of 0.7 µJ at different initial
temperatures.
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Andrea Marino 2015
The data reported in fig. 4.20 indicate that the HS population at the thermal step
around
, maximal around
and almost nil at already 180 K.
is moderate
In previous works, the thermal step have been described as a result of a macroscopic temperature
jump
of the crystal which favors the HS state of higher entropy [Lorenc 2012]. Indeed, the pump
energy
which drives the LS-to-HS switching, is much higher than the potential
energy difference between the two molecular states
. This excess of
energy rapidly dissipates through molecular and lattice vibrational modes (see chapter 2) and it
results in a macroscopic crystal heating. After this global heating, the crystal reaches a transient
temperature
, and the HS fraction equilibrates at the s time scale to a new transient
value at
(fig. 4.21).
However, the difference in the HS population of the thermal switching at different starting
temperature of the experiments depends on the thermal slope of the spin conversion curve and in a
first approximation:
Fig 4.21 elucidates the thermal step at 135 K and 142 K, graphically showing how the thermal
population
strongly depends to the slope of the thermal crossover.
The closest is the initial temperature to T1/2 as the highest is the number of photoswitched
molecules at the thermal step.
After all, the temperature jump
is proportional to the amount of absorbed energy
which in
turn depends on the number of absorbing molecules present in the crystal. Since the optical density
at 530 nm is around 4 times higher for the LS state (LS fraction:
) than for
the HS state (HS fraction:
), the temperature jump can be approximated as:
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Spin State Concentration Wave
Then it is possible to fit the experimental temperature evolution of the HS thermal population
from the temperature dependence of
provided in fig. 4.6:
This rough model reported in fig. 4.22 reproduces well the experimental data showing the
maximum of thermal population at around 150 K, where the slope of the thermal evolution of
is maximum. It also explain the difference of the threshold between 135 K and 142 K.
Starting at 135 K means that the system need an higher photoinduced temperature jump (hence
higher excitation density) in order to reach a new transient equilibrium where the HS fraction is
much higher than at the initial temperature. On the other hand, since 142 K is closest to the
transition temperature, the higher crossover slope enable to the system to reach an higher
photoinduced HS value with a smaller temperature jump.
Fig 4.22 Evolution of the thermal step with temperature. In red circles the experimental points,
while the model corresponding to eq. 4.8 is represented by te small black dots and a solid black
line serves as a guide for eyes.
As a matter of fact, fig. 4.22 indicates that the thermal step is minimum at around 135 K for an
excitation pump energy of
, where the HS population is negligible and smaller than 1%.
However, both data at 135 K for different pump energies (fig. 4.18) and for different temperatures
(fig. 4.20) indicate that thermal effects are small for a pump energy of
at 135 K, where the
SSCW is formed. From these results it is possible then to opportunely set the experimental
conditions needed for the time-resolved X-ray diffraction measurements in order to study the
response of the symmetry breaking OP of the SSCW. 
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4.3.3 Time Resolved X-Ray Diffraction
Time-resolved diffraction studies were performed at the BioCARS beamline at the Advanced
Photon Source in the Argonne National Laboratory, (Chicago, USA) where individual X-ray pulses
were selected by a fast chopper. The X-ray diffraction data were collected at 15 keV with a MARCCD detector at different time delays
between the laser pump and the X-ray probe. Partial data
collection were performed in order to measure the temporal evolution of the lattice parameters and
diffracted intensity. At each time delay, 60 frames each with a 10 s exposure were collected, at 1°
steps of the diffractometer axis.
The optical pump laser beam was similar to the one used for optical measurements in terms of
pump wavelength
laser spot size
and pump fluence
,
apart for its pulse duration, which was of the order of tens of ps. However, the time-resolution of
the X-ray diffraction experiments were limited by the X-ray pulse duration dependent on the
electron bunch's length which is around 100 ps.
Fig. 4.23 reports the comparison of the time evolution of the totally symmetric OP XHS (obtained by
fs optical spectroscopy) with the unit cell parameter a, quite sensitive to the thermal spin-state
conversion [Marino2015]. It also reports the intensity of two selected Bragg reflections related with
the cell-doubling and the appearance of the of the spin-state concentration wave in the IP phase.
The optical measurements (fig. 4.23a and fig. 4.18), underline the negligible thermal population of
HS state which is less than 1% for the selected experimental conditions: 135 K and
.
Time-resolved X-ray diffraction revealed in fig. 4.23b a multi-step expansion of the lattice
parameter a, in agreement with the optical data. At the photo-switching stage there is no change of
the lattice parameter. The first expansion appears after a few ns, corresponding to the so-called
elastic step [Lorenc 2012, Collet 2012c]. At this stage, the larger volume of the photo-switched HS
molecules generates an internal pressure which leads to a lattice expansion and so of the lattice
parameter a. A second and larger expansion is observed with a maximum
after laser
excitation, when other molecules thermally populate the HS state. These data correlate quite well
with the optical data presented in Fig. 4.23a and fig 4.18, as well as with previous reports on other
single-step spin-crossover materials [Lorenc 2009, Lorenc 2012]. Nevertheless, the new feature of
the present work is the symmetry change between the HS and the IP phases associated with the
molecular spin-state ordering.
The temporal evolution of the symmetry breaking order parameter , and ergo the formation and
light-induced erasing of the SSCW, can be probed by following the time course of the characteristic
Bragg reflections indexed
in the doubled cell, as shown in fig. 4.23c and 4.23d. On
the photo-switching step, the intensity of these Bragg reflections is weak because the HS conversion
and the resulting structural reorganization are small. On the other hand, at the ns stage (elastic step),
the intensity of some peaks decreases, e.g. for the
peak (fig. 4.22c), but also increases: e.g.
for the
peak (fig. 4.22d). This indicates a strong structural reorganization associated with
the lattice expansion.
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Spin State Concentration Wave
Fig 4.23 Time dependence of XHS obtained by optical studies (a), of the lattice parameters a (b),
and of the intensity of Bragg peaks characterizing the presence of the SSCW (c and d). Data are
shown on a log scale for positive time delays.
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Andrea Marino 2015
Just on longer timescales, the intensity of all the
Bragg peaks, related to the
symmetry breaking, monotonically decreases and approaches 0 within 1.5 ms. This is the direct
proof that the amplitude of the SSCW vanishes and that the disordered high symmetry phase forms.
Fig. 4.24 shows the evolution of the diffracted intensity in the (3 k l) reciprocal plane 6 ns before
and 1.5 ms after laser excitation. The decrease of intensity down to 20%, observed in fig. 4.24 for
the (3 k 3) Bragg reflections, is similar for all the Bragg peaks indexed
.
Fig 4.24 (left) Diffracted intensity in the reciprocal (3 k l) plane indexed in the doubled cell,
showing the decrease of (3 k 2p + 1) Bragg peaks before (-6 ns) and 1.5 ms after laser excitation.
(right) Slice of the diffracted intensity along (3 k 3).
The time resolved X-ray set up at the BioCARS beamline enabled tracking the out-of-equilibrium
dynamic on longer time delays. Figure 4.25 reports the crystal recovering to the initial state prior to
photo-excitation. The Bragg intensities, as well as the lattice parameters, are observed to recover
their initial value within 15 ms.
Fig 4.25 Recovery of the lattice parameters a, and of the intensity of the (2 5 5) Bragg peaks
characterizing the presence of the SSCW
126
Spin State Concentration Wave
From the crystal structures obtained by time-resolved X-ray diffraction, we could also extract the
evolution of the order parameters XHS from the time evolution of the
and
bond
lengths and  from their difference, in a similar way to that explained above for characterizing the
SSCW at thermal equilibrium. Data shown in fig. 4.26 indicate that XHS does not change
significantly during the out-of-equilibrium dynamics (in agreement with the optical data), whereas
 decreases by a factor
around 1 ms in agreement with the decrease down to
of
I
Bragg peaks.
Fig 4.26 Evolution of the order parameters XHS and  obtained by time-resolved X-ray
Diffraction. Lines are guides for the eyes.
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Andrea Marino 2015
4.4 Conclusion
Combined time-resolved optical spectroscopy and X-ray diffraction studies provide a complete
overview of the out-of-equilibrium dynamics of the spin-state concentration wave. During the three
steps associated with the out-of-equilibrium dynamics in the solid state, there are a few differences
compared to the response of single-step spin-crossover materials.
On the photoswitching step, only a small fraction
of molecules are photo-switched.
This amount is too small to erase the wave. At 135 K the initial fraction of HS molecules on site2 is
, and light cannot bring this value up to
in order to make it equal to the fraction on
site1 and thus erase the wave. The number of photons (and the corresponding energy) required in a
fs pulse would simply destroy the sample. This is however possible in some materials to completely
photoswitch LS molecules to HS state with cw light at low temperature, since the photoinduced HS
state is long-lived. With cw lasers, the energy used is of the order of 10 mW/mm2 and the laser
heating is limited to few K. With our femtosecond laser experiment the 10µJ/mm2 at 1kHz
repetition rate the energy corresponds to 10 mW/mm2. But since the energy is deposited within less
than a picosecond, the instantaneous energy is of the order of 106W/mm2.
Fig 4.27 Thermal expansion of the lattice parameter a.
Nevertheless, even with low excitation densities some molecules switch from the LS to the HS
state. This is enough to induce the lattice expansion driven from an internal pressure which relaxes
in tens of ns after the photo-excitation. Then at the s time scale, as the crystal warms up, the
temperature increases and the HS/LS equilibrium is shifted. An additional fraction of molecules
reaches the HS state. But again, this fraction of thermally populated HS molecules is very small
with
. This means that during this process the temperature increase is small and that
the system remains on the vicinity of the plateau where the IP is stable. It is possible to estimate the
photo-induced temperature jump
of the crystal from the comparison of the photo-induced
128
Spin State Concentration Wave
expansion of the lattice parameter a (fig. 4.23b), with the thermal expansion coefficient of
reported in fig. 4.27. The experimental pump fluence of 0.7 J induces therefore a
temperature jump
in the order of 5–7 K. In this way, the thermal equilibrium of the crystal is
carried from the initial 135 K to a transient temperature close to 142 K within hundreds of s. In
this temperature range the average HS fraction XHS remains almost constant (fig. 4.6), but the final
transient temperature at 142 K correspond to the transition point where the SSCW is erased (fig.
4.26). As a matter of fact, the robust decrease of the peak intensities
observed in fig.
4.23 and fig. 4.24, clearly denotes that the SSCW almost vanishes within 1.5 ms, that is after the
crystal stabilizes at the new photoinduced transient temperature. These intensities drop down to
20% of their initial value and according to eq. 4.8 it corresponds to a decrease of the symmetry
breaking order parameter  by a factor larger than 2. This is in agreement with the value of 
extracted from the bond length difference between sites 1 and 2 and reported in fig. 4.26.
By using higher excitation energy it was possible to completely erase the wave as the superstructure
peaks characteristic of the SSCW disappear within one ms. However it was not possible to perform
complete time-resolved studies because of the sample damage.
From these experimental results, it is possible to conclude that it takes time to destroy the longrange order of the IP phase and reach the high symmetry HS phase. Erasing the SSCW means that
each molecular site needs to reach the same new equilibrium value of XHS:
. The
typical timescale for the system to explore different HS/LS configurations is observed to correspond
to the timescale needed for the system to reach the transient thermal equilibrium, and it falls in the
–
range. Finally, as the crystal cools down by heat exchange with the external cryostat,
the initial 135 K temperature is regained. The lattice parameters relaxes to their initial values, as
observed on single step SCO compounds [Collet 2012b], and the SSCW reappears around 15 ms.
Fig. 4.28 schematically summarizes the full photo-cycle of the SSCW during the out-of-equilibrium
dynamics triggered by a fs laser pulse. Nevertheless, it is important to clarify that the HS state
generated here with fs laser excitation starting from the IP phase, undoubtedly corresponds to the
structure of the high symmetry HS phase with lattice parameters (a b c). This is clearly identified
from the crystallographic pattern reported in fig. 4.24. This transient photoinduced state therefore
differs from the PIHS generated at 15 K from the LS phase, for which another type of symmetry
breaking with a tripled unit cell (a b 3c) occurs. This is not observed here by time-resolved X-ray.
This is another illustration that in such systems, different false ground state compete and that
external parameters (T, P, light) balance the relative stability of these states.
In conclusion, the present results point out that, in addition to provide molecular movies, timeresolved diffraction is a powerful tool for investigating symmetry breaking aspects of photoinduced
phase transitions and out-of-equilibrium thermodynamics. This study revealed that the photoswitching step alone cannot erase the SSCW by promoting 50% of the molecules from LS to HS.
This requires an excitation density well above the sample damage. It is only during the transient
thermal equilibrium, reached with a moderate temperature jump, that the spin state concentration
wave can be erased as the HS/LS configurations equilibrate on the different crystalline sites. It is
then the photo-induced temperature jump that leads the IP-HS phase transition by driving the
independent molecular sites to self-equilibrate their HS-LS fraction.
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Andrea Marino 2015
Fig 4.28 Summary of the SSCW photo-cycle. The ordering between sites is destroyed by heating
effects and in  1 ms the SSCW disappear. The system thermalize with the cryostat and it recovers
the initial state in around 10-15 ms after irradiation.
More interestingly, the time course of the two order parameters X HS and  are different. This is due
to the fact that both parameters evolve with different intrinsic timescales. The change of X HS results
from the relative shift of HS/LS potential free energy together with the temperature increase. Fig.
4.20 indicates that the totally symmetric OP XHS reacts to the photo-excitation independently of the
initial temperature. Its out-of-equilibrium dynamic is independent from the phase where it is photoexcited. The transient equilibrium occurs within 100 µs and is characterized by the maximum of
in fig. 4.23a, which is slightly anticipated with respect to the maximum lattice expansion due
to laser induced-warming. Indeed, the crystal structural reorganization always follows the electronic
switching. However, it takes more time to equilibrate the HS fraction on all the molecular sites as
some thermal diffusion may also exist within the sample because of the finite penetration depth of
light. It is the time required to macroscopically equilibrate the temperature on the crystal that limits
the erasing of the SSCW. Fig. 4.23a clearly reveals that the intensities
start to vanish
at the time when
is maximum, and they reach their minimum when the HS fraction
equilibrates back. Eventually, it takes even more time to close the photo-cycle and reform the wave
(15 ms) (fig. 4.28).
Time-resolved techniques therefore open the opportunity to watch how different degrees of freedom
evolve in time, contrary to what can be done at thermal equilibrium where such dynamics are
hidden in a statistical average. Similar investigations may be developed in a near future to other
types of materials associated with ordering phenomena and bistability, such as CDW or insulatingto-metal materials.
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Chapter 5
Conclusions and perspectives
Andrea Marino 2015
132
Conclusions and perspectives
It is of fundamental importance for further applications of light-active materials to entangle and
understand all the phenomena hidden in the overall macroscopic photo-induced transformations.
The findings obtained within this PhD project are aimed to extend the knowledge in the field of
materials control science, possibly opening new doorways and ideas for further developments and
design of novel photo-active materials. Time-resolved studies are the way to access the necessary
information on the multiple degrees of freedom and elementary processes involved during the
macroscopic switching. A clear understanding of the photoswitching pathway together with all the
accessible intermediate electronic and structural states is both demanding and of vital importance
for shaping further research in this field. In this contest, this PhD work pointed out the variety and
complexity of the multi-scale processes around photoinduced phenomena in molecular solids. These
results highlighted that several degrees of freedom evolve on their intrinsic timescale during the
macroscopic photoinduced transformation of molecular solids.
The experiments reported in the previous pages, focused on the particular case of SCO molecular
crystals. Again these bistable solid systems are perfect photoswitchable prototype materials for
studying and investigating the photoinduced elementary processes. SCO solid systems show a large
variety of phase transitions as well as different changes in their physico-chemical properties
(magnetic and dielectric susceptibility, color, volume ...). Nevertheless, their main photo-active
interests dwell in the high selectiveness of the photoswitching and on the fact that the macroscopic
response of the crystal concerns initially only an ultrafast, but localized, molecular response. This
peculiar characteristic opens new striking perspectives to control material properties with a first
selective localized switching which in turns can act on the macroscopic system via strong and
cooperative interactions and feedback effects. Not less important development of photoactive
materials, it is the possibility to switch back the system and therefore efficiently switch from a
phase to another and vice versa.
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Andrea Marino 2015
5.1 Conclusions
5.1.1 Photoswitching in SCO solids
The second chapter "ultrafast LIESST and energy transfer" aimed to determine the temporal
evolution of the different electronic and structural degrees of freedom during LIESST upon MLCT
excitation. This project was the first attempt to investigate the ultrafast LIESST dynamics in
iron(II)-based SCO crystals and was carried out in parallel with other similar studies [Cammarata
2014, Bertoni 2015]. The use of complementary optical and X-ray probes allowed to highlight and
entangle the strong coupling between the electronic and structural rearrangements. As well as for
molecules in solution [Bressler 2009], the electronic LIESST reorganization on the
and
orbitals induces an ultrafast
bond elongation which takes place in half of its vibrational
period. In addition to the present literature, the comparison of the ultrafast LIESST dynamics
between a pure and diluted SCO crystals allowed to entangle the ultrafast dynamics related to the
molecular switching from the structural dynamics concerning its environment. It has been
evidenced that the coherent phenomena observed in the sub-picosecond timescale are localized at
the molecular level, whereas the slower oscillations observed for the pure Fe compound and not for
the diluted one have been identified to correspond to an energy transfer from the absorber SCO
molecular to its surrounding environment through phonon-phonon coupling.
New in literature was the comparison of the ultrafast LIESST and reverse-LIESST dynamics. This
first attempt of LIESST upon d-d ligand field excitation clearly demonstrated that the LIESST
phenomenon occurs with the same dynamical processes as for irradiation into the MLCT bands. So
far, the results reported on the LIESST dynamics (upon MCLT excitation both in solid and in
solution) showed the same ultrafast switching mechanism for all the different FeII SCO compounds.
In addition, the results reported here point out that the SCO photoswitching mechanism of LIESST
does not depend on the different excitation processes. Similar observations were also reported for
FeIII SCO materials [Bertoni 2015]. Therefore the model which explain the efficient photoinduced
HS trapping [van Veenendaal 2010] could be generalized to the photoswitching LIESST-like
dynamics of transition-metal organic compounds. Indeed, even if the symmetry of the various
complexes can be different, the first ultrafast switching only concerns the electron redistribution on
the 3d orbitals and their closely related structural reorganization of the first iron coordination shell.
Of course, depending on the symmetry of the systems and on mass of the ligand, the observed
coherent structural dynamics involving mainly
breathing and ligand torsion have different
frequencies. But again the overall process can be stated to follow an universal picture:
1. Electronic spin state switching via ultrafast intersystem crossing.
2.
bond elongation in half period of its vibrational mode moving the system
to the HS potential.
3. Activation of coherent structural dynamics via electron-phonon coupling.
4. Energy redistribution and damping via phonon-phonon coupling.
5. Vibrational cooling via energy transfer to the surrounding.
134
Conclusions and perspectives
However, the most striking result presented in chapter 3 concern the reverse-LIESST dynamic. The
data reported in this PhD undoubtedly underline the huge differences in the mechanism between the
LIESST and its reverse process. While the timescale of the LIESST is governed by the intrinsic
structural dynamic of the
bond elongation, the double intersystem crossing of the reverseLIESST is sequential with a timescale between
orders of magnitude longer. The reverseLIESST time scale is therefore limited by the electronic life time of the different intermediate states
involved. Hence, it is not driven by a coherent dynamical process, but rather by stochastic kinetics
where the different photoswitched molecules independently undergoes the various ISC. Therefore
the phase is lost between the individual molecular events and the ultrafast coherent structural
dynamics observed for the LIESST are here hidden in their statistical dynamical average.
Furthermore, from this work it also possible to draw an explanation on the different quantum
efficiencies regarding the two complementary mechanisms. It has been validated that an efficient
ultrafast "phonon" damping related to the energy dispersion ensure an efficient trapping of the
photoinduced state [Van Veenendaal 2010, Cammarata 2015]. This is the case of the LIESST where
the coherent intra- and inter-molecular dynamics are quickly damped thought phonon-phonon
coupling with the environment (see chapter 2). Such an ultrafast energy transfer forbid to the system
to fall back in the ground state. On the other hand, the reverse-LIESST is not properly "ultrafast".
The "long" life time of the intermediate states allows a branch decay into different directions, thus
lowering the overall photo-switching efficiency. The comparison of the dynamics for the
and
with their overall quantum efficiency further confirms this
hypothesis. In fact, the
system reports a slower ISC at the level of the 5E 3T1 with a
respective lower HS LS efficiency. Conversely, the
exhibit an ultrafast 5E 3T1
decay which strongly enhances the reverse-LIESST efficiency.
Chapter 2 and 3 confirmed that the first steps of the photoinduced processes of LIESST and reverseLIESSST are localized at the molecular scale. However, there are many other degrees of freedom
such as volume expansion and thermal effects which take place on longer time scales. In this
contest, the possibility to investigate such out-of-equilibrium dynamics over a long range of time
delays allowed to present in chapter 4 the temporal evolution of the various individual degrees of
freedom at their own time scale. In the case of study, it emerged that the two order parameters
and , describing the formation of spin-state concentration waves in the IP phase, behave
independently and with their intrinsic time scales. While the average HS fraction
increase on 3
steps (photoinduced, elastic, thermal), reaching its maximum at the s time scale, the symmetry
breaking OP responds significantly to the light stimuli only on the ms time scale where thermal
effects are of importance. The results presented in chapter 4 highlight that it is not the increase of
which is responsible to reestablish the equilibrium population between the two independent
sites and therefore erase the spin state concentration wave. On the contrary, it is the homogenization
of HS population over the different lattice sites which modifies the order and this thermally
activated exploration of the different HS-LS configurations occurs on the ms timescale. It is
therefore the thermal jump which allows the configurational rearrangements to destroy the site
ordering and erase the SSCW. It could be of interest also to extend the methodology used here to
investigate the different degrees of freedom involved during the more or less complex out-ofequilibrium in other classes of materials. The combined use of probes sensitive to different degrees
135
Andrea Marino 2015
of freedom is a must for a deep understanding of photoinduced phenomena from molecular to
material scales.
These findings are of more general importance, as they can be extended to similar complexes such
as Ru-based systems with light-activated functions, not only committed to technological
applications [Grätzel 2001, Grätzel 2005, Pan 2014], but also to similar systems already widely
used in biological and medical applications [Higgins 2012, Howerton 2012, Wachter 2012].
5.1.2 Ultrafast Dynamics of Molecular Magnet Breathing Crystals
Besides SCO materials, other kinds of promising molecular crystals have been investigated in the
frame of this PhD project. The
(
hexafluoroacetylacetonate,
nitronyl
nitroxide) belongs to the family of copper-nitroxide-based thermo- and photoswitchable molecular
magnets. Contrary to SCO where the spin state change is related only to the spin state of the
transition metal ion, in these systems the spin on the Cu is coupled to the spin of each nitroxideligands. These systems undergo magneto-structural rearrangements between a weakly and a
strongly exchange-coupled states of spin triads nitroxide–copper(II)–nitroxide, namely WS and SS
states respectively. The left of fig. 5.1 schematically reports the structural and electronic changes
between the two SS and WS spin state. Due to the exchange coupling J in the spin triads, each
vibronic level is split into 8 spin sublevels: two doublets D, d and one quartet Q (inset fig. 5.1). As
it is the case for SCO, these two molecular states have different entropy and the WS state is favored
at high temperature. At low temperature the copper-nitroxide spins are coupled by a strongly
antiferromagnetic exchange interaction (SS state) leading to the population of only the lowest
doublet state
(
). On the other hand, at high temperature the spin coupling becomes
weak (WS state) and all spin multiplets are thermally accessible leading to a weak ferromagnetic
exchange coupling [Fedin 2008, Fedin 2012, Drozdyuk 2013].
Fig. 5.1 Left Structure and key properties of the SS and WS states of the spin triads. Right Transiet
absorption spectroscopy of
nanocrystals embed in PVC.
. Inset Photoswitching scheme and proposed pathway in terms of potential energy curves
(left) and in terms of the spin sub levels, two doublets (d, D) and one quartet (Q) [Kaszub 2014].
136
Conclusions and perspectives
Crystals like
are also known as "breathing crystals" due to their important structural
change between the two magnetic states. Moreover, despite the difference in the nature of the
switching they present some similarities with SCO systems as for example the ability to undergo
LIESST-like phenomena. However, the mechanism and the dynamics of photoswitching in
breathing crystals was not yet investigated. In the frame of a new collaboration with the group of M.
Fedin (Novosibirsk, Russia) time-resolved transient spectroscopy studies were performed.
The SS → WS was efficiently induced with laser excitation at 675 nm promoting a metal to ligand
charge transfer
. The two-color pump–probe measurements reported on the right of fig.
5.1 present a step-like bleaching at the probe wavelength of 500 nm, clearly proving the evidence of
the ultrafast switching. However, the photoinduced broad spectral change measured gave a clear
fingerprint of the SS → WS photoswitching fully demonstrating its occurrence [Kaszub 2014]. The
surprisingly ultrafast arrival on the photoinduced WS state has been estimated to be less than 50 fs
with no signature of any intermediate states. On the other hand, after the step-like bleaching, a
secondary slower decrease of OD toward a plateau with 1.5 ps time constant was attributed to a
vibrational cooling process. From these findings, the
breathing crystal seems to
follow similar ultrafast dynamics to the LIESST in SCO complexes, as represented in the inset of
fig. 5.1. In addition, the OD change is linearly dependent on the pump fluence underlying the local
nature of the trapping of the photoexcited state. However, the electronic process presents some
differences. After the laser excitation of the SS state into the exited
state the decay toward the
WS state is most likely direct without passing through intermediate states. In fact, since in the WS
state J is weakly ferromagnetic, at the experimental temperature (90 K) all the three spin multiplets
are thermally accessible. Therefore, the proposed decay from the spin allowed
excited state
does not require any intermediate spin state as it can directly convert into one of
the excited doublets
of the WS state, which in turn can vibrationally decay into the
lower quartet
state. On the other hand, the
would then require an intersystem
crossing and therefore hypothetically proceed through an intermediate state, which could thus slow
down the process. Therefore, in contrast to what is already reported for SCO systems, the existence
of low-lying exited spin levels
in breathing crystals makes the spin state switching
partially allowed and an overall SS WS switching ultrafast.
Since the photo-switching in such molecular solids involves important volume changes, there is an
opportunity to investigate the photo-response of such materials on longer time scales where
macroscopic volume expansion should occur, as discussed in chapter 1 and 4 on SCO solids. The
photo-switching may induce a cooperative response driven by elastic coupling, which is not yet
detectable at the ps time scale. Such investigation are currently underway.
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Andrea Marino 2015
5.2 Development of New Photoactive Hybrid Materials
The field of photoinduced phase transition is getting broader and broader as the research for new
photo-active systems is an important target for technological applications. Besides the well-known
photo-active spin-crossover materials, other types of functional materials can also be identified for
interesting new routes for future developments. In this contest, the understanding of physical
mechanisms driving and stabilizing the photoinduced phase transition obtained within this PhD
work can be of advantage. Interesting possibilities appear now in developing a novel class of hybrid
materials, where the interaction of the different complexes can lead to striking new light-driven
properties. It is therefore of interest to develop hybrid materials made of different molecular subsystems associated with different function, one of them being photo-active. The assembly of
molecules into stable and non-covalently joined aggregates is a promising approach to obtain new
functional molecular materials, especially in view of the tuning of their physical properties by light.
However, the achievement to the final application aim can be complex and difficult both on the
chemical side, since the synthesis of the material is already challenging, as well as on the physical
point of view, where the materials' photo-response may not be of specific interest for driving
photoinduced phenomena.
5.2.1 Insulating-Metal materials with photoactive ions
In the frame of this PhD work and the design of new photoactive materials, a first attempt was made
on a novel organic metal
elaborated at the ISCR in the group of M. Fourmigué [Camerel 2013]. The aim was to develop a
material undergoing metal-insulator (M-I) phase transition triggered by a counter-ion with
photoactive properties. In this contest, organic conductors based on cation radical salts such as
BEDT-TTF (fig. 5.2, up left) are known to be highly sensitive to small modifications of their
structure. Therefore, there is an opportunity to control the M-I phase transition by driving structural
modification on the photoactive anion layer, as the
(fig. 5.2, bottom
left) is well known to exhibit photoactive properties such as fluorescence.
Fig. 5.2 Left Chemical picture of the cation radical salts organic conductor BEDT-TTF (top) and
counter-ion
Center Band structure for the metallic phase (top)
and insulating phase (bottom) Right thermal evolution of the white light optical reflectivity (OR)
138
Conclusions and perspectives
Fig. 5.2 reports the band gap opening characteristic of the metal to insulating phase transition
respectively stable at high and low temperatures. A subtle modification of the structural
organization of the anion layers, resulting from the switching of hydrogen-bonding interactions,
drives the macroscopic M-I transition proved by the striking changes in the transport properties of
the whole salt (fig.5.3).
However, technical difficulties make this system not properly suitable for photo-control and
especially for ultrafast studies. This material did not show a clear M-I phase transition in the range
of temperature available for the experiments (that is above 80 K), as reported in fig. 5.2 with
optical reflectivity measurements. Upon cooling ( 1 K/ min) the system did not undergo the
expected M-I transition. However, it is observed that at 80 K a latent transition took place in  4
hours in order to reach the insulating phase, whereas in the warming up a standard I-M transition
appears with a hysteresis loop characteristic of high cooperativity around 110 K.
Fig. 5.3 Temperature dependence of the resistivity
in
of
showing the hysteretic behavious of the M-I transition
between warming (──) and cooling (──). Inset evolution of the
giving an
evaluation of the hysteresis width in the 61-91 K range
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Andrea Marino 2015
5.2.2 Volume change as a driving force
The development of photoinduced phase transition in materials is facing two important issues.
Firstly all the physical properties cannot be directly controlled by light excitation. It is therefore
important to find a photoactive degrees of freedom which can act as a driving force to drive the
transformation of the material. On the other hand, most of the photoinduced phase transitions are
transient in nature and recover thermal equilibrium within µs-ms. Some systems show a stable
thermally reversible excited state only in the bistable regime inside thermal hysteresis [Cobo 2008]
But it is crucial for technological applications to have both phases stable after the photo excitation
over a broad temperature range.
One of the few example of thermally stable photoswitches is represented by the case of diarylethene
derivatives. They compose a family of photo-chromic compounds exhibiting striking photoactive
properties such as efficient photoinduced reversibility (irradiating with two different wavelengths),
thermal irreversibility and fatigue resistant. All these properties make them perfect candidates for
applications in photonic devices as optical memories, switches and photo-actuators [Irie 2000, Irie
2014]. Apart showing striking photochromic response, the photoinduced ring opening/closing (fig.
5.4) is also accompanied with a strongly structural reorganization leading to an important volume
change.
Combined optical pump-probe and femtosecond electron diffraction have demonstrated the ultrafast
nature of the ring closing after femtosecond laser irradiation [Jean-Ruel 2011, Jean-Ruel 2013]. The
formation of an open-ring excited state following photo excitation in time constant of
approximately 200 fs have been placed as intermediate state before undergoing the important
molecular reorganization which led to ring closed form within 5 ps [Jean-Ruel 2011, Jean-Ruel
2013]. Compared to the molecular volume change in SCO material occurring on sub ps time-scale
but driving macroscopic volume expansion on ns timescale, there is an opportunity to use
diarylethene derivatives to drive volume change.
Fig. 5.4 Chemical picture Schematic of the ring-closing/opening reaction of the diarylethene
derivative 1,2-bis(2,4-dimethyl-5-phenyl-3-thienyl) perfluorocyclopentene [Jean-Ruel 2011,].
140
Conclusions and perspectives
It is well known that volume change is often an important trigger of phase transition, and especially
of insulating-metal transition. The change of inter-molecular distances can modify the transfer
integral, the band structure and consequently induce structural instabilities which can lead to
symmetry breaking. Well-known examples are Mott insulator, charge ordered insulator... [Uemura
2010, Kawakami 2010]. SCO solids are also well known to be strongly coupled by the volume
change. Therefore, the use of spin crossover with diarylethene building blocks is of specific interest
since these show a high photo-response with large changes of molecular volumes. In addition, SCO
materials are also very sensitive to volume change and pressure effects, since elastic molecular
coupling is the driving force for cooperative transformations. In this way, diarylethene could be
used as a photoactive counter-ion where their photoinduced volume change could then drive the
SCO. Furthermore, since some diarylethene derivatives are stable and thermally irreversible after
irradiation, this would in the end finally stabilize both the HS and LS states over a long range of
temperatures. Nevertheless, since in this way the spin crossover would be induced by the absorber
diarylethene volume change and not induced via electronic excited state the quantum efficiency for
a LIESST-like and reverse-LIESST like should be the same. Diarylethene would then be efficient
switches able to reversibly control a fully stable spin crossover at high temperatures.
In conclusion, this PhD work underlined the importance of fundamental research committed to the
understanding of elementary physical processes around photoinduced phenomena. Taking
advantage of these recent results could then tremendously enhance the developments of new
photoactive materials aimed to technological and biomedical applications.
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Andrea Marino 2015
142
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Annex I
LIST OF ABBREVIATIONS
A
Acceptor
CDW
Charge-Density Wave
CO
Charge Order
CT
Charge Transfer
CW
Continuous Wave
D
Donor
FWHM
Full Width Half Maximum
HS
High Spin
I
Ionic
IN
Ionic-to-Neutral
INT
Intermediate electronic state
IP
Intermediate Phase
IR
Infrared
IRF
Instrumental Response Function
ISC
Intersystem Crossing
LIESST
Light Induced Exited Spin State Trapping
LF
Ligand Field
LMCT
Ligand-to-Metal Charge Transfer
161
162
LS
Low Spin
MLCT
Metal-to-Ligand Charge Transfer
N
Neutral
NI
Neutral-to-Ionic
NIR
Near Infrared
OD
Optical Density
OR
Optical Reflectivity
PIHS
Photo-Induced High Spin
PIPT
Photo-Induced Phase Transition
SCO
Spin Crossover
SDW
Spin-Density Wave
SSCW
Spin-State Concentration Wave
VIS
Visible
VC
Vibrational Cooling
XANES
X-ray Absorption Near Edge Structure
XAS
X-ray Absorption Spectroscopy
X-FEL
X-ray Free Electron Laser
Annex II
LIST OF PUBLICATIONS
1
Marino, M. Buron-Le Cointe, M. Lorenc, R. Henning, A. D. DiChiara, K. Moffat, N. Bréfuel, E.
Collet. "Out-of-equilibrium dynamics of photoexcited spin-state concentration waves " Faraday
Discussion 177, 2015 [Link]
2
A. Marino, P. Chakraborty, M. Servol, M. Lorenc, E. Collet, and A. Hauser "The Role of LigandField States in the Ultrafast Photophysical Cycle of the Prototypical Iron(II) Spin-Crossover
Compound [Fe(ptz)6](BF4)2" Angew. Chem. Int. Ed. 53, 3863 –3867, 2014 [Link]
3
Marino, M. Servol, R. Bertoni, M. Lorenc, C. Mauriac, J.F. Létard, E. Collet "Femtosecond
optical pump–probe reflectivity studies of spin-state photo-switching in the spin-crossover
molecular crystals [Fe(PM-AzA)2](NCS)2"
Polyhedron 66, 123–128, 2013 [Link]
4
W. Kaszub, A. Marino, M. Lorenc, E. Collet, E. G. Bagryanskaya, E. V. Tretyakov, V. I.
Ovcharenko, M. V. Fedin "Ultrafast photoswitching in copper-nitroxide based molecular magnet"
Angew. Chem. Int. Ed. 53, 2014 [Link]
5
F. Camerel, G. Le Helloco, T. Guizouarn, O. Jeannin, M. Fourmigué, A. Frąckowiak, I. Olejniczak,
R. Swietlik, A. Marino, E. Collet, L. Toupet, P. Auban-Senzier, and E. Canadell "Correlation
between Metal−Insulator Transition and Hydrogen-Bonding Network in the Organic Metal δ(BEDT-TTF)4[2,6-Anthracene-bis(sulfonate)]·(H2O)4" Cryst. Growth Des. 13, 5135−5145, 2013
[Link]
163