Download From metal complexes to fullerene arrays: exploring the exciting

From metal complexes to fullerene arrays: exploring the exciting
world of supramolecular photochemistry fifteen years after its
birth †
Nicola Armaroli
Istituto per la Sintesi Organica e la Fotoreattività, Laboratorio di Fotochimica,
Consiglio Nazionale delle Ricerche, via Gobetti 101, 40129 Bologna, Italy.
E-mail: [email protected]
Received 29th October 2002, Accepted 11th December 2002
First published as an Advance Article on the web 22nd January 2003
After over 15 years of extensive research work in many
laboratories worldwide, supramolecular photochemistry is
a well-established and highly recognized branch of science.
A brief retrospective view on the birth and infancy of this
research area is given and some of the latest developments
are discussed. In supramolecular photochemistry Ru(II)
and Cu(I) diimmine complexes and C60 fullerenes are some
of the most widely investigated chromophores and over
the years big efforts have been made to implement and
tune their photophysical and excited state properties, which
are briefly reviewed. Thanks to a huge amount of synthetic
and analytical research work, it has been possible to insert
or combine these organic and inorganic subunits in a
variety of fascinating supramolecular architectures. Some
results concerned with photoinduced processes occurring
in dyads, triads, catenanes, rotaxanes, dendrimers, and
protonated self-assembled architectures are briefly
illustrated. The overall picture stemming form the current
† This manuscript is the 2001 Grammaticakis-Neumann International
Prize lecture.
Nicola Armaroli was born in 1966; he is married with three
children. He graduated in chemistry in 1990 (laurea degree) and
obtained his PhD at the University of Bologna, Italy, under the
guidance of Professor Vincenzo Balzani in 1994. He carried out
post-doctoral research activity at the Center for Photochemical
Sciences (Bowling Green, Ohio, USA), the Italian National
Research Council (CNR), and the University of Bologna. In
1997 he was appointed researcher at the CNR. His scientific
interests concern the photochemistry and photophysics of
supramolecular systems, mainly focussing on those containing
coordination compounds and fullerene subunits. He was awarded
the 2001 Grammaticakis-Neumann International Prize in
Photochemistry.
Nicola Armaroli
DOI: 10.1039/b210569a
state of the art in supramolecular photochemistry is that
of a discipline gaining an increasing degree of multidisciplinarity. Interconnections with biology, physics and
information technology are being established at a very fast
pace, suggesting a bright future for this still young research
field.
1
Introduction
In April 1987 a seminal workshop gathering many prominent
chemists in different areas (photochemistry, theoretical and
preparative chemistry, catalysis) was organized by Vincenzo
Balzani in Anacapri, Italy.1 On that occasion, for the first time,
a heterogeneous group of scientists met to discuss the perspectives of a new branch of science that was emerging, which was
termed Supramolecular Photochemistry. Some months later the
Nobel Prize in Chemistry was awarded to Jean-Marie Lehn 2
(who participated in the meeting), Donald Cram 3 and Charles
Pedersen 4 for their seminal contribution to the birth of
synthetic supramolecular chemistry. The near occurrence
of these events show that in 1987, although supramolecular
chemistry was already a highly recognized and mature research
field, the encounter between supramolecular chemistry and
photochemistry was still at a very preliminary stage.
From a retrospective viewpoint, the reading of the landmark
proceedings 1 of the Anacapri meeting is quite intriguing
because some of the developments that had just been envisaged
have now been accomplished. For instance, the key concepts for
the construction of photonic molecular switches and machines,
now very popular,5,6 were outlined by Balzani, Moggi and
Scandola.7 Furthermore, it is interesting to compare the early
artificial photosynthetic systems then presented by Gust and
co-workers,8,9 with those of the last generation in which an
impressive degree of sophistication 10,11 and even operativity 12
has been achieved. In early 1991, as documented by the
fundamental monograph of Vincenzo Balzani and Franco
Scandola,13 supramolecular photochemistry had already
developed to a great extent and, since then, the related scientific
production has undergone a spectacular development.
The successful story of this discipline has been made possible
not only because of the joint efforts of photochemists and synthetic chemists belonging to many research groups worldwide,
but also thanks to timely progress in other fields of research and
technology that occurred independently. For instance, in the
last decade, femtosecond laser equipment for various types of
pulsed spectroscopy have become readily available.14 Hence the
time resolution for photochemical experiments could be
lowered from the pico- to the femtosecond time regime thus
allowing a deeper insight into photodynamic processes. On the
other hand, advances in optical and electronic technology now
permit the investigation of spectral regions which, for a long
Photochem. Photobiol. Sci., 2003, 2, 73–87
This journal is © The Royal Society of Chemistry and Owner Societies 2003
73
time, were substantially unaccessible to electronic spectroscopists. Nowadays steady state and time resolved near
infrared luminescence spectroscopy is a fast-growing research
field 15 with exciting perspectives in terms of technological
applications.16
Progress in synthetic and analytical chemistry has provided,
starting from the 1980’s, fully characterized supramolecular
architectures such as catenanes, knots, pseudorotaxanes,
rotaxanes and dendrimers 17–20 which exhibit novel photochemical and photophysical properties.20–25 The key principle
that governs the design of a photoactive supramolecular array
is relatively simple. A certain number of molecular subunits are
assembled in a suitable spatial arrangement. Light excitation
of a specific molecular fragment may modify electronic interactions among the components and give rise to phenomena
such as energy or electron transfer.
In these terms, indeed, nothing seems to be particularly
original if one considers that intermolecular photoinduced
energy and electron transfer have been studied for decades
in bimolecular reaction schemes where two chemical species
can freely diffuse to each other. However, linking together
different molecular moieties in a supramolecular ensemble
and promoting intramolecular or, more precisely, intercomponent processes yields important consequences and opens new
research perspectives.13
From the fundamental point of view this approach has
allowed the study of the dependence of photoinduced
energy or electron transfer as a function of the partners distance or of the nature of interposed chemical bonds (e.g.
covalent vs. hydrogen).26–28 Very importantly, the control of
intercomponent distance allowed the verification of the predictions of the Marcus theory on electron transfer, as far as the
“odd” behaviour in the so-called inverted region is concerned.29
Photoinduced processes on supramolecular arrays can also
bring about effects that can in principle be exploited for
practical purposes. Funnelling of excitation energy to a specific
component,30 molecular rearrangements,31 creation of longlived charge separated states,26 and reversible switching on-off
of luminescence following an external input (e.g. chemical or
electrochemical) 32 are probably the most investigated. Thus
it is not surprising that supramolecular photochemistry has
progressively enlarged the spectrum of interest towards
light-powered molecular machines,33 molecular switches 5 and
sensors,34 and molecular electronics,35 thus reaching the crossroad between chemistry, biology, and information technology.36
A sharp definition of supramolecular array is not straightforward, when based on the nature of the chemical bonds
between molecular subunits. According to some authors,
intermolecular forces gathering a supramolecular entity should
not be of covalent nature. Nevertheless it is now widely
accepted that, when supramolecular systems are studied from
the viewpoint of effects caused by external stimuli such as
light, a useful approach can be based on the degree of intercomponent electronic interaction. This concept, developed by
Balzani and Scandola,37 is simply illustrated in Fig. 1 and helps
us to define the domain of supramolecular photochemistry.
A simple two-component ensemble A–B is a “supermolecule” when a light excitation input is able to generate a localized
electronic excited state on a specific subunit. Likewise, when
light irradiation stimulates intercomponent electron transfer,
A–B is a supermolecule if the positive and negative charge are
substantially localized on a specific moiety. By contrast, when
excitation does not result in localized excited states or charge
distributions, A–B has to be considered a “large molecule”. A
further relevant rule defining a supramolecular array is that an
intercomponent electronic interaction (though not very strong)
must be present to some extent, in order to get some new
peculiar ensemble properties.
The first consequence of the above concepts is that the
chemical linkage between A and B can be of any sort as long as
74
Photochem. Photobiol. Sci., 2003, 2, 73–87
Fig. 1 Schematic representation addressing the difference between
supermolecules and large molecules according to the photochemical
approach.
it provides a “well calibrated” electronic relationship between
the molecular subunits able to grant, at the same time, the
maintenance of the specific molecular properties (e.g. excited
state and redox parameters) and the onset of new features,
characteristic of the multicomponent array (e.g. photoinduced
processes). Hence, although someone may find it difficult
to include covalently linked systems in the family of supramolecular arrays, they can be strictly considered as such for the
photochemical vocabulary, once they fulfil the above illustrated
electronic requirements.
The scope of this paper, of course, cannot be that of presenting the state the of the art in supramolecular photochemistry.
Rather, some selected hot topics of this now huge research field
are briefly illustrated, mainly focussing on those we have
devoted our attention to. An updated and extensive reference
section will hopefully act as a springboard for further reading
along different directions.
2 Ru(II)-polypyridine complexes : a relentless
contribution to the development of photochemical
sciences
Starting from the late 1960’s Ru() complexes of the polypyridine family have attracted a great deal of attention from the
photochemical community.38 Most of these complexes combine
remarkable features like: (i) ease of preparation; (ii) reversible
electrochemical behaviour; (iii) light absorption in the visible
spectral region; (iv) long-lived electronically excited states; (v)
intense luminescence from the lowest metal-to-ligand-chargetransfer triplet excited state (3MLCT).39 All these characteristics
make them attractive for the study of fundamental processes
such as photoinduced energy- and electron-transfer under diffusional (bimolecular) conditions or within multicomponent
(supramolecular) arrays.
In the late 1980’s the field was developed to a great extent and
the photochemical and photophysical properties of a huge
number of Ru()-polypyridine complexes had been elucidated.40
Further impetus for this research was given by advances in
supramolecular synthetic chemistry, which allowed insertion
of [Ru(bpy)3]2⫹-type and related motifs into fascinating
architectures,19,41,42 in which they play the role of chromophores,
sensitizers, or electron relays 23,43–47
After over 30 years, studies relating to Ru()-polypyridine
complexes are still at the forefront of photochemical research.38
These compounds have been assembled in large light harvesting
dendrimers 22 where intramolecular energy transfer processes
can be controlled by a thorough assembly of the dendrimer
shells.21,23 The use of Ru() complexes as sensitizers for nanocrystalline wide band-gap semiconductors is well documented,
with interesting fallout from this research in solar energy
conversion technology.48,49 In the very active area of artificial
photosynthetic mimics,10 Ru() supramolecular arrays play a
prominent role.50,51 The same applies to the hot topic of molecular wires were Ru() complexes may serve as energy donor
units for Os() analogues, interconnected by covalently linked
spacers of different length and chemical nature.28,52–54 Photoinduced energy transfer mediated by hydrogen bonding and
involving Ru() complexes as energy donors has also been
discussed.55,56 Interestingly, [Ru(bpy)3]2⫹-type complexes have
also been employed as active components in prototypic light
powered molecular-level machines,57 showing their potential in
this expanding area of research.33
Improvement of the photophysical performances of
photoactive Ru() complexes (viz. luminescence quantum
yield and excited state lifetime) is another challenging task.
A strategy leading to a prolongation of the excited state lifetime
of [Ru(bpy)3]2⫹-type complexes was suggested by Ford and
Rodgers a decade ago.58 By linking this inorganic chromophore
to an organic fragment with long lived and low-lying 3ππ* level,
such as pyrene, a thermal equilibration between this level and
the 3MLCT emitting state centred on the metal complexed
moiety is obtained. If 3ππ* is slightly lower lying than 3MLCT
and the energy gap is within 500 cm⫺1 a dramatic increase in the
3
MLCT lifetime of the Ru chromophore can be observed.59,60
The extent of lifetime prolongation is related to the number
of the organic “reservoir” units, as shown for the series of
compounds in Fig. 2.61
Fig. 2 Excited state lifetime prolongation for a series of [Ru(bpy)3]2⫹type complexes with an increasing number of pyrene units appended in
deoxygenated CH3CN solution.
The [Ru(bpy)3]2⫹ motif is stereogenic and this implies that
polynuclear complexes are a mixture of isomers.45,62 An obvious
choice to avoid this problem is the [Ru(tpy)2]2⫹ motif (tpy =
2,2⬘:6⬘,2⬙-terpyridine) which, unfortunately, is characterized by
a short-lived and virtually non luminescent 3MLCT excited
state.63 An increase in the luminescence yield and lifetime was
obtained by attaching electron accepting substituents to the
tpy ligand (Fig. 3), or by making heteroleptic compounds with
different substituents on the chelating unit.64 Remarkable
results have been obtained also in dinuclear complexes where
[Ru(tpy)2]2⫹ centers are interconnected by spacers able to
delocalize the ligand electronic charge in the excited state; both
thiophenediyl 65 and butadyinylene 66 spacers have been used
(Fig. 3).
More recently two different approaches have been proposed
to enhance the luminescence performances of [Ru(tpy)2]2⫹-type
compounds. Both are aimed at lowering the 3MLCT excited
state energy, in order to limit undesired thermal population of
upper lying short-lived metal-centred levels that deactivate via
non-radiative paths.63,64 First it has been suggested to replace
the central pyridine of one tpy ligand with a triazine (tz) unit,
thus obtaining a heteroleptic [Ru(tz)(tpy)]2⫹ complex.67 Then it
was proposed to attach a coplanar pyrimidine residue to one
Fig. 3 Excited state lifetime prologation of [Ru(tpy)2]2⫹ and some
related complexes in oxygen free CH3CN solution.
central tpy ligand; in this way a respectable lifetime of 200 ns is
measured (Fig. 3).68
Notably the [Ru(tpy)2]2⫹ motif has been used to assemble
supramolecular triads containing porphyrin terminals, with the
aim of obtaining long distance charge separation along the
triad.69 However, it was shown that energy transfer may successfully compete over electron transfer, as illustrated in Fig. 4.70–72
Fig. 4 Stepwise photoinduced energy transfer processes (E1, E2, E3) in
a supramolecular triad assembled around a [Ru(tpy)2]2⫹ core upon
excitation of the free base porphyrin moiety (butyronitrile solution).
The final triple–triplet back energy transfer step (E3) is observed only in
a rigid matrix at 77 K.
The replacement of the central [Ru(tpy)2]2⫹ core with an
[Ir(tpy)2]3⫹ motif, was made possible thanks to recent advances
in the synthesis of Ir() complexes.73 [Ir(tpy)2]3⫹ is a stronger
oxidant than [Ru(tpy)2]2⫹, and its lowest lying electronic level
(long-lived and highly luminescent) is located at about 2.5 eV,
i.e. much higher that the 3MLCT state of [Ru(tpy)2]2⫹ (1.9 eV).74
This tends to promote electron-transfer over energy transfer,75
the latter being the prevalent quenching process in porphyrin
triads assembled around the Ru() complex.72 Switching
between the two photoinduced processes is observed upon
excitation of different chromophores within the triad.76
3
Cu(I)-phenanthroline complexes
3.1 Mononuclear Cu(I)-phenanthrolines: recent advances in the
optimisation and rationalization of their photophysical properties
The ground and excited state electronic properties of Cu()bisphenanthroline complexes ([Cu(NN)2]⫹) have been the
object of intense investigations throughout 25 years.24,77,78
A key feature that distinguishes them from the hexacoordinated octahedral Ru()-polypyridines is the lower coordination
number (4), which leads to a more or less distorted tetrahedral
geometry (vide infra). The less demanding coordination
environment of Cu()-phenanthrolines allows extended structural distortions in the ground and excited states, thus affording
Photochem. Photobiol. Sci., 2003, 2, 73–87
75
a fine tuning of the photophysical and electrochemical
properties.24
Similar to Ru()-polypyridine complexes, [Cu(NN)2]⫹
systems exhibit relatively weak absorption features in the VIS
spectal region attributed to metal-to-ligand-charge-transfer
(MLCT) electronic transitions.24,77,78 The MLCT absorption
bands cover a wide spectral range (380–700 nm), as a result of
an envelope of at least three different electronic transitions.79
The corresponding spectral intensities are strictly related to the
symmetry of the complex that, in turn, is affected by the distortion from the tetrahedral geometry. Such distortion is largely
dictated by the position and the chemical nature of the
substituents on the chelating ligand.24 For instance, complexes
of 2,9-arylphenanthroline are characterized by π-stacking
interactions between the aryl groups of one ligand and the
phenanthroline moiety of the other ligand, which brings about
a strongly distorted ground state tetrahedral geometry (D2
symmetry). This explains the very different MLCT absorption
profile that characterizes complexes of 2,9-arylphenanthroline
ligands compared to those of the 2,9-alkylphenanthroline
type.24
The parent compound of the [Cu(NN)2]⫹ family i.e. [Cu(phen)2]⫹ (phen = 1,10-phenanthroline) is not luminescent in
solution, although it proved to be a weak emitter in the solid
state.80 Instead, Cu() complexes of substituted phenanthrolines
may exhibit respectable luminescence efficiency in fluid media,
which is attributed to the deactivation of two MLCT excited
states in thermal equilibrium, i.e. a singlet (1MLCT) and a
triplet (3MLCT).79 The energy gap between these states is about
1500–2000 cm⫺1 and, at room temperature, the population of
the lower lying 3MLCT level largely exceeds that of 1MLCT,
though the minority 1MLCT excited molecules are responsible
for most of the observed room temperature luminescence.
Interestingly, as for ground state electronic properties (e.g.
absorption spectra), excited state characteristics (e.g. emission
spectra and excited state lifetimes) also strongly depend on the
substitution pattern of the phenanthroline ligands.24 The effect
on the luminescence intensity and lifetime are attributed to a
flattening distortion occurring in the MLCT excited state, since
the metal centre changes its formal oxidation state from Cu()
to Cu(), thus assuming a more flattened coordination
geometry.81 In this ⬙open⬙ structure a fifth coordination site is
made available for the newly formed d9 ion,82 that can be
attacked by nucleophilic species such as solvent molecules and
counterions, leading to pentacoordinated excited complexes
(exciplexes), that deactivate via non-emissive deactivation
paths.24,83
The effect of the size of substituents on the excited state
lifetime of the Cu() complexes is nicely illustrated for the
[Cu(NN)2]⫹ complexes of the ligands displayed in Fig. 5.
By increasing the size or ramification of the 2,9-alkyl
substituents a respectable prolongation of excited state lifetimes
is achieved (from 90 to 400 ns in CH2Cl2). Further additions
of substituents on other positions leads to a remarkably long
lifetime of 920 ns,84 comparable to that of the very popular
[Ru(bpy)3]2⫹ in CH3CN.40
The trend of luminescence intensity as a function of
temperature is not conventional for [Cu(NN)2]⫹ complexes
since usually weaker and red-shifted luminescence is observed
with decreasing temperature.85 This is interpreted with the
above mentioned two-level scheme; at lower temperatures
the 1MLCT–3MLCT thermal equilibrium is shifted towards the
lower lying poorly emitting triplet.85 This issue has been
addressed in more detail recently, when it was shown that the
“odd” intensity vs. temperature trend is obeyed for all
[Cu(NN)2]⫹ complexes in fluid matrices (above 150 K), but not
in rigid media (below 120 K).86 In a frozen glass, complexes
of ligands with long alkyl chains exhibit an opposite trend,
i.e. blue-shifting and an increase in intensity, Fig. 6.
Again, the different behaviour of alkyl vs. aryl substituted
76
Photochem. Photobiol. Sci., 2003, 2, 73–87
Fig. 5 Selected 2,9-dialkylphenanthrolines with increasing length or
ramification of the alkyl substituents. The lifetimes values (CH2Cl2
oxygen-free solution) correspond to those of the corresponding
homoleptic [Cu(NN)2]⫹ complexes. Dramatic lifetime prolongation is
obtained by adding further substituents in the 3,4,7,8-phenanthroline
positions.
phenanthroline complexes was attributed to geometric rather
than electronic factors. Most importantly, these studies have
defined more accurate criteria for the design of highly luminescent [Cu(NN)2]⫹ complexes both in fluid and in rigid media.86
Finally, it is important to note that some heteroleptic Cu()phenanthroline complexes exhibit extremely enhanced photophysical performances 87,88 with a lifetime in fluid solution as
long as 16 µs.88 Also in that case the observed behaviour is
mainly ascribed to steric factors that affect the coordination
geometry and then the electronic properties. Although the
insertion of heteroleptic coordination compounds in a supramolecular array can be a very challenging task, it is conceivable
that these results can stimulate new research for taking
advantage of these new highly performing chromophores.
3.2
Supramolecular arrays with Cu(I)-phenanthroline subunits
The high affinity of Cu() for phenanthroline ligands makes
the preparation of mononuclear [Cu(NN)2]⫹ complexes
relatively easy. The development of sophisticated synthetic
strategies, which take advantage of this affinity, has afforded a
number of fascinating molecular architectures like catenanes,
rotaxanes, and knots, as originally developed by Sauvage,
Dietrich-Buchecker and coworkers.17
Notably, the [Cu(NN)2]⫹ coordination motif proved to be
very fruitful not only for supramolecular synthetic chemistry
but also for supramolecular photochemistry.24,89,90 The Cu()catenate reported in Fig. 7 exhibits absorption and luminescence
properties which are typical for [Cu(NN)2]⫹ complexes (see
above).91
It is possible, within the same ligand frame, to substitute
Cu() with a variety of different metal ions (Li(), Ag(), Co(),
Ni(), Zn(), Pd(), Cd()) thus obtaining a fine tuning of the
ground and excited state properties of the corresponding
complexes. Remarkably, all the catenates of the metal ions
listed above are luminescent and their emission bands are
spread over the whole visible spectral region.91
This prompted the study of families of polynuclear catenates
and knots containing one or two [M(NN)2]n⫹ centres, where the
Mn⫹ ion could be varied (see above).92–96 The different metal
complexed moieties exhibit rather different photophysical
and redox properties. Thus, once assembled within a supramolecular architecture, they may allow a large variety of inter-
Fig. 6 Temperature dependence of the luminescence spectra of two [Cu(NN)2]⫹ complexes in CH2Cl2–MeOH 1 : 1 (v/v); the specific ligands are
indicated, λexc is on the maximum of the corresponding MLCT absorption bands. In the fluid domain (up to 170 K) emission intensity decrease and
spectral red-shifting is observed by lowering temperature in both cases. By contrast when the solvent matrix becomes rigid (around 120 K), the two
compounds behave differently. For the 2,9-dialkylphenanthroline complex (right hand-side) a complete reversal of the previous trend is observed,
with intensity recovery and blue shift. At 96 K a very strong luminescence band is recorded.
dinuclear coordination compounds RuM which are made of an
octahedral ([Ru(tpy)2]2⫹-type) and a tetrahedral ([M(NN)2]n⫹type) moiety. The direction of photoinduced energy and/or
electron transfer is reversed by substituting Cu() with Ag(),
Zn() or by simply leaving the tetrahedral coordination centre
free.96
Photoinduced processes, also able to trigger motions at
the molecular level,97,98 have been observed in a number of
supramolecular Cu()-phenanthroline arrays. This topic has
been covered by a recent review where more details can be
found.24
Fig. 7 A [2]-catenate where two 30-membered rings with a phenanthroline unit are interlocked around a Cu() ion. The line connecting
the oxygen atoms represents –(CH2)2– groups.
component photoinduced processes. Particularly interesting is
the possibility of tuning the direction of such processes by
means of a thorough choice or combination of the [M(NN)2]n⫹
subunits.24 This concept is illustrated in Fig. 8 for a family of
4 Acid–base supramolecular photochemistry:
switches, machines and self-assembled structures
As briefly outlined above, 1,10-phenanthroline and its derivatives are very popular among supramolecular chemists and
photochemists thanks to their capability of chelating metal ions
and generating sophisticated supramolecular architectures by
self assembly.17 However a no less important feature of these
ligands is their ability to bind protons, which brings about some
interesting chemical and photochemical results.
4.1
Fig. 8 [2]-catenates RuM and schematic representation illustrating the
control of the direction of photoinduced processes by changing the
metal ion in the [M(NN)2]n⫹ moiety. e⫺ Indicates electron transfer, E
denotes energy transfer.
Tuning of excited state energies: proton molecular switches
In CH2Cl2 solution, 1,10-phenanthrolines undergo protonation
reactions that can be monitored by UV-VIS absorption and
luminescence spectroscopy.99 This is exemplified in Fig. 9 where
the changes in the absorption and luminescence properties of a
solution of a 2,9-dianisyl-1,10-phenanthroline ligand upon
addition of increasing amount of acid are reported.100
The presence of well defined isosbestic and isoemissive
points, suggests that a single chemical reaction occurs, i.e. the
protonation of the phenanthroline unit. Importantly, upon
protonation, the energy of the lowest singlet phenanthroline
level is decreased from ≈3.1 to ≈2.2 eV, as shown by the remarkable red-shift of the fluorescence band.
This prompted us to design a supramolecular dyad (OPV–
Phen) in which a strongly fluorescent moiety (oligophenylenevinylene, OPV), is attached to an anisylphenanthroline unit
(Phen).100 The luminescent level of OPV is intermediate in
energy relative to the levels of Phen and PhenⴢH⫹. The results
obtained with OPV–Phen upon reversible additions of acid and
base are illustrated in Fig. 10.
Photochem. Photobiol. Sci., 2003, 2, 73–87
77
Fig. 9 Changes in the absorption (top) and fluorescence spectra
(bottom, λexc = 359 nm, isosbestic point) of a 2,9-dianisyl-1,10phenanthroline (R = –C12H25) in CH2Cl2 solution at room temperature.
Fig. 10 On/off switching of the luminescence in OPV–Phen as a
consequence of the inversion of photoinduced energy transfer
direction.
The direction of photoinduced energy transfer is addressed
at wish by means of the reversible protonation–deprotonation
reaction of the Phen receptor, because the fluorescent energy
levels of Phen and PhenⴢH⫹ are put below/above the luminescent level of OPV, which acts act as an energy acceptor
(in OPV–Phen) or donor (in OPV–PhenⴢH⫹). The process is
conveniently signalled by the on/off switching of the very
intense OPV fluorescence. Thus a simple method of controlling
the widely exploited luminescence of OPV’s is also suggested.100
Many examples of molecular switches and sensors that take
advantage of changes in proton concentration can be found in
current literature.101–104
4.2 Unexpected supramolecular architectures by protonation:
an example of molecular machine
When two or more phenanthroline subunits are integrated in a
single array, fascinating and somehow unexpected supra78
Photochem. Photobiol. Sci., 2003, 2, 73–87
molecular architectures can be generated following acidification. Again, these processes can be conveniently monitored by
UV-VIS absorption and luminescence spectroscopy.91,93,105,106
The [2]-catenand in Fig. 11 is obtained upon decomplexation
of the Cu() [2]-catenate shown in Fig. 7. Acidification of
solutions of the [2]-catenand does not bring about protonation
of single phen units but, instead, cooperative protonation
of the two phenanthroline ligands occurs. This is demonstrated
by 1H NMR and UV-VIS absorption and luminescence
spectroscopy.91
Practically, a structure resembling that of the metal complexed catenates discussed above is obtained, as also suggested
by strong similarities in the absorption spectra. The driving
force for the formation of the catenate-type structure is the
favourable π-stacking interactions between the aryl groups of
one macrocycle and the phenanthroline moiety sitting on the
other subunit. The [2]-catenand exhibits a much stronger basic
force relative to that of a single isolated macrocyclic unit, since
10 times less acid is necessary to drive the protonation reaction
to the end in the supramolecular ensemble.91
Notably, the [2]-catenand here described is a very special case
of a multicomponent array (dyad) in which the two macrocyclic
moieties are not kept together by chemical interactions
(removed upon decomplexation of the parent Cu() complex)
but by a purely physical linkage. The supramolecular nature
of this dyad, somehow “hidden” in the basic form, is fully
disclosed in the presence of acid, when a cooperative effect
(protonation) is evidenced.
A decade after the publication of this work, we can
emphasize a feature that was not recognized at that time. The
reversible protonation–deprotonation reactions of the [2]catenand triggers the closure/opening of the system. Under
basic conditions the phenanthroline moieties are far apart for
steric reasons (open form) while in acidic environment they are
entwined around the proton (closed form). Absorption and
luminescence spectroscopy allow easy detection of either
species (Fig. 12). This is a typical example of what is now
indicated as a molecular machine driven by chemical input,
a concept not fully developed by the chemical community in
the early 1990’s.6
Other phenanthroline arrays can be assembled together by
protonation and form double stranded structures, as observed
for the bisphenanthroline ligand reported in Fig. 13.106 The first
protonation step leads to the formation of a self-assembled
structure (helical or face-to-face). Further protonation destroys
the structure since electrostatic repulsive forces prevail. This
case interestingly compares to that of Fig. 11, where electrostatic repulsion are not able to open the catenate structure in
the second reaction step. All these processes are monitored
by 1H NMR and UV-Vis absorption and luminescence
spectroscopy.106
Cooperative protonation of two or three phenanthroline units
has been also described in cage-type arrays.105 On the contrary,
in a large 56-membered macrocycle containing two phenanthroline moieties two independent protonation reactions have
been monitored.107 Protonation of phenanthroline-containing
polyamine macrocycles may also be exploited for sensing
purposes.108
5
Fullerene multicomponent systems
During the 1990’s a brand new family of molecules, i.e. fullerene C60 109 and its derivatives,110,111 was made available. The
fortuitous contemporary growth of two apparently independent research lines, namely synthetic fullerene chemistry
and supramolecular photochemistry, has been reciprocally
beneficial and contributed to boost activity in both fields. C60
and its derivatives exhibit a number of very attractive photochemical and photophysical properties 112 such as absorption
throughout the UV-Vis region,113–116 fluorescence,114–117
Fig. 11 Cooperative protonation reactions of phenanthroline subunits leading to catenate-type structures in a [2]-catenand made of two 30membered rings, CH2Cl2 solution. The process is conveniently monitored by UV-Vis absorption and luminescence spectroscopy. The line connecting
the oxygen atoms represents –(CH2)2– groups.
Fig. 12 Interconversion between the open and closed form of a [2]catenane upon reversible addition of acid and base. The two forms can
be monitored by absorption and luminescence spectroscopy. The
wavelength maximum of the emission band is reported.
diagnostic transient absorption features,116 singlet oxygen
sensitization capability,118–120 electron 121 and energy 122 accepting character. On the other hand functionalized fullerenes have
been successfully integrated in a variety of multicomponent
supramolecular arrays.10,123–126 Thus it is not surprising that the
study of the photochemical and photophysical properties of
supramolecular systems containing C60 fullerenes is a very
active area of research 10,122,127–129 with important fallouts in
terms of fundamental knowledge 130 and practical applications.131 Some of the most actively investigated classes of
fullerene supramolecular arrays will now be briefly illustrated.
5.1 Fullerene hybrids with Ru(II), Re(I), Cu(I) complexes and
metal porphyrins
The relatively long-lived metal-to-ligand-charge-transfer
(MLCT) excited states characterizing complexes of Ru(),40
Re(),132 and Cu() 24 with 2,2⬘-bipyridine or 1,10-phenanthroline ligands have been widely exploited in the design of
supramolecular molecular architectures featuring photoinduced energy- and electron-transfer processes (see
above).24,46,47,54 The MLCT excited states of these metal
complexes have a marked reducing character that, in principle,
make them ideal partners for C60 fullerene oxidants in the
construction of donor–acceptor arrays for photoinduced
electron transfer.
The popular [Ru(bpy)3]2⫹ chromophore has been coupled
with fullerene subunits in supramolecular architectures and
electron transfer has been observed.129 However photophysical
investigations on the systems reported in Fig. 14 have recently
shown that electron transfer in fullerene–[Ru(bpy)3]2⫹ hybrids is
rapidly followed by fast and quantitative charge recombination
to the low-lying fullerene triplet; the same applies to a Re()
analogue.133 This suggests that these hybrid systems are
probably not suited for the generation of long-lived and highly
exoergonic charge separated states.
In a rotaxane made of a Cu()-bisphenanthroline core
([Cu(NN)2]⫹) and C60 terminal units the typical excited state
properties of each moiety are strongly quenched, namely
MLCT emission of the core, C60 fluorescence, and the C60
triplet absorption. Also, the singlet oxygen sensitization, typical
of both (separated) subunits, is dramatically reduced. All these
findings are a consequence of the fact that a low-energy chargeseparated state is made available in the multicomponent
rotaxane. Excitation of the central inorganic chromophore
C60) electron transfer. Importantly, this
causes direct (Cu()
[Cu(NN)2]⫹ energy transfer step
process is preceded by a C60
when the light input is addressed to the fullerene chromophores
(Fig. 15).134
([Cu(NN)2]⫹ complexes are stronger excited state reductants
than [Ru(bpy)3]2⫹ compounds 24 thus, in principle, they can be
more promising candidates for the construction of photochemical devices for charge separation, since not suffering
from the wasting sink effect of the fullerene triplet mentioned
above.133
The first report on fullerene–porphyrin arrays dates back to
late 1994, when Gust and co-workers demonstrated that in a
zinc porphyrin–fullerene array photoinduced electron transfer
from the inorganic to the organic moiety occurs.135 This
prompted a huge amount of synthetic work aimed at the
construction of increasingly sophisticated arrays containing
fullerene and porphyrin moieties. Mainly covalently linked
arrays have been prepared,10,136,137 however, arrays relying on
weaker interactions are increasingly popular.126,138,139 Many of
these systems have been designed in order to get artificial
models featuring the fundamental acts of natural photoPhotochem. Photobiol. Sci., 2003, 2, 73–87
79
Fig. 13 Spontaneous assembly of a bisphenanthroline molecule in CH2Cl2 upon addition of trifluoroacetic acid (up to 2 equivalents). The double
stranded structure (helicoidal or face-to-face) is destroyed when larger amounts of acid are added, owing to effect of electrostatic repulsive forces.
Fig. 14 Hybrid dyads containing a C60 fullerene moiety and a Ru() or
Re() complex.
synthetic systems, namely light harvesting and charge
separation.128 To this end impressive successes have been
obtained, as already pointed out.10
An interesting aspect of the chemistry of fullerenes and
porphyrins is that they are spontaneously attracted to each
other, as a result of electronic donor–acceptor interactions.
This can be observed both in the solid state 140 and in
solution.141 For instance, regardless of solvent polarity, the
triad depicted in Fig. 16 adopts a conformation in which one
carbon sphere is tangential to the porphyrin plane, as derived
by NMR investigations.142 This spontaneous attraction can also
be monitored photochemically since ground state charge
transfer absorption bands (CT) are recorded, which is not the
case for reference solutions containing the three molecular
subunits unlinked. Quite remarkably, the CT states are
luminescent in the near infrared region (λmax = 890 nm) and
exhibit a lifetime of 720 ps.142 Photoinduced charge separation
in the triad of Fig. 16 is also observed upon selective excitation
of either chromophore. The photochemistry and photo80
Photochem. Photobiol. Sci., 2003, 2, 73–87
Fig. 15 Stepwise photoinduced energy and electron transfer processes
(E and e⫺, respectively) in a rotaxane containing a [Cu(NN)2]⫹ core and
two fullerene stoppers, following excitation of the organic moieties.
physics of face-to-face porphyrin-fullerene arrays is a still
rather unexplored field.
An extensive and updated treatment on the photophysics of
fullerene arrays with metal complexes and porphyrins can be
found in recent review papers.128,129
5.2
Fullerodendrimers: C60 inside and outside
Dendrimer chemistry is an extremely active area of research.
This is attested by the impressive number of review and
highlight articles which have appeared in the last couple of
years, in which encouraging perspectives for several practical
applications are also clearly outlined.19,20,23,25,143–152
The three-dimensional tree-type structure of dendrimers may
allow the isolation of internal parts from the external environment, with substantial changes in their chemical and physical
solvents such as acetonitrile or THF is obtained and an
enhanced wrapping effect is evidenced relative to apolar
toluene.157 Further proof of the protective effect exerted on the
fullerene center is the reduction of the yield of singlet oxygen
sensitization in solution for the dendrimer of Fig. 17, compared
to unsubstituted fulleropyrrolidine or smaller dendrimers.157
As already mentioned, C60 fullerene is not only an excellent
electron acceptor but also an outstanding excitation energy
acceptor, thanks to its low-lying electronic energy levels.122 This
makes it attractive for the design of light harvesting dendritic
supramolecular arrays as proposed recently.159 In the oligophenylenevinylene (OPV) fullerene array depicted in Fig. 18 the
Fig. 16 Preferred conformation of a fullerene–porphyrin triad, as a
consequence of attractive donor–acceptor electronic interactions
between the two chromophores.
properties.151,152 UV-Vis absorption and emission spectroscopy
is a powerful tool for investigating encapsulation effects in
solution because specific changes in the microenvironment of
chromophoric dendrimer cores can be monitored by observing
changes in the intensity, shape, or position of absorption and
luminescence bands.23 The same applies to electrochemical
techniques when electroactive fragments are located in the
interior of a dendrimer.144 Several synthetic strategies have been
devised to build up and characterize large dendrimers with
C60 fullerene subunits as central, intermediate, or peripheral
units.150,153,154 All these species are termed fullerodendrimers.125
Fullerene inside. Despite the fact that C60 is a photoactive
molecule, it has only recently been employed as a central core in
dendritic structures with polyaryl ether branches.155–158 By
increasing the size of the dendrimer external shell a reduction in
the rate of bimolecular energy and electron transfer processes
between the central, increasingly protected fullerene unit and
some external molecules was observed.155 The wrapping of the
C60 core by the external dendrons has also been monitored by
1
H NMR and UV-Vis spectroscopy and attributed to electronic
donor–acceptor interactions.156 When triethyleneglycol chains
are attached as terminal units (Fig. 17) solubilization in polar
Fig. 18 Photoinduced energy transfer in an OPV–fullerene array.
R indicates –C12H25 groups.
OPV–C60 energy transfer process is likely to be followed by
charge separation in polar solvents.159 Work is underway to
elucidate in detail the pattern of photoinduced processes in this
fullerodendrimer.
Fullerene outside. Nierengarten et al. have prepared three
fascinating dendrimers in which a [Cu(NN)2]⫹-type core is
surrounded by 4, 8, or 16 C60 terminal units.115 In Fig. 19 the
smaller representative of the series is shown.
It has been shown that upon excitation of the [Cu(NN)2]⫹
MLCT absorption bands, no MLCT luminescence is detected,
thanks to an energy transfer quenching process to the peripheral C60 subunits (Fig.18).115 In the two largest dendrimers,
the [Cu(NN)2]⫹ core is buried in a sort of C60-made “black box”
since it is hardly or not accessible to external molecules,
electrons, and even photons. The much higher absorption coefficient displayed by the (many) C60 units relative to the (single)
[Cu(NN)2]⫹ core, practically prevents excitation of the core,
relative to the footballene fragments.115
The study of the photochemical properties of fullerodendrimers is still at an early stage, therefore fast and remarkable
developments can be expected shortly.
5.3
Fig. 17 Fullerodendrimer with triethyleneglycol chains allowing
solubilization of the carbon sphere in a variety of solvents. The large
dendritic structure is able to provide some protection of the core
towards the external environment.
Fullerene–conjugated oligomer dyads
A few years ago it was demonstrated that supramolecular
systems in which an oligophenylenevinylene unit is covalently
linked to a fulleropyrrolidine moiety (OPV–C60, Fig. 20) can be
successfully employed in the construction of photovoltaic
Photochem. Photobiol. Sci., 2003, 2, 73–87
81
Fig. 19 A [Cu(NN)2]⫹ complex with four fullerene moieties appended,
which represents the smallest representative of a series of dendrimers
with up to 16 external carbon spheres. R indicates –C8H17 groups.
Fig. 21 Energy-level diagram for the oligophenylenevinylene–
fulleropyrrolidine array (OPV–C60) reported on top. The reported
localized excited levels (full lines) corresponds to the lowest singlet state
of the OPV and the lowest singlet and triplet states centred on the C60
moiety. Dashed lines represent the charge separated state (OPV⫹–C60⫺)
in solvents of different polarity (PhMe, toluene; PhCN, benzonitrile).
Dotted arrows indicate electron transfer steps, EnT stands for energy
transfer, i.s.c. for intersystem crossing. Stepwise photoinduced energy
and electron transfer steps, following excitation of the OPV moiety, can
occur only in polar PhCN.
Fig. 20 Oligophenylenevinylene–fulleropyrrolidine arrays (OPV–C60)
used as active materials in photovoltaic devices.
devices.160,161 Thus, light irradiation of a thin film of these
C60 electron
materials is able to trigger photoinduced OPV
transfer.
This so called “molecular” approach to photovoltaic devices,
in which the donor and the acceptor are chemically linked, is
a viable alternative to the classical approach 162 in which the
photoactive material is a blend of fullerene and poly(p-phenylenevinylene).163 Furthermore the study of the electronic and
photophysical properties of the supramolecular array may
allow structure–activity relationships to be obtained and devise
strategies for the implementation of the device performances.
Detailed investigations on the OPV–C60 systems reported in
Fig. 20 161 and on other similar arrays 164–167 have shown that
C60 Förster-type energy transfer occurs in
ultrafast OPV
C60 electron transfer may
solution. In principle direct OPV
also take place but it is highly exergonic and is located in the
Marcus inverted region, thus it cannot compete with the energy
transfer process.165,167 Therefore electron transfer may only be
originated from the lowest singlet excited state of the fullerene
moiety and the OPV fragment simply act as an antenna
unit.159,167 This might be the intimate pattern of photoinduced
processes also in solid-state devices, even though it has been
suggested that the photovoltaic effect is likely to be the consequence of “material” rather than “molecular” processes.168
Electron transfer from the fullerene singlet in OPV–C60 arrays
suffers from competition of internal deactivation and can be
promoted by solvent polarity, which can conveniently lower the
energy of the OPV⫹–C60⫺ charge separated state. All the above
effects are illustrated in the energy diagram of Fig. 21 which
concerns an OPV–C60 array recently investigated.167
More sophisticated OPV–C60 arrays have been recently
prepared. In the system depicted in Fig. 22 the fullerene is
provided with both an energy (OPV) and an electron donor
(pyrazoline) unit.167 Detailed studies of the dependence of
82
Photochem. Photobiol. Sci., 2003, 2, 73–87
Fig. 22 Oligophenylenevinylene–fullerene–pyrazoline triad where the
fullerene unit acts as energy or electron acceptor for the OPV and the
pyrazoline moiety, respectively. Switching of photoinduced processes
can be obtained by operating on different parameters, namely
excitation wavelength, proton concentration, solvent polarity, and
temperature.
photoinduced processes on solvent polarity, addition of acid,
and temperature reveal that this compound can be considered a
fullerene-based molecular switch. The switchable parameters
are photoinduced processes, namely OPV–C60 energy transfer
C60 electron transfer.167
and pyrazoline
The incorporation of this triad in photovoltaic devices results
in very low light to current efficiency since charge separation
involving the fullerene moiety and the pyrazoline N atom is not
able to contribute to the photocurrent and, instead, the pyrazoline unit can act as an electron trap. Nevertheless the design
principle of multicomponent arrays featuring an antenna unit
(like OPV) and a charge separation module (like pyrazolineC60) is very appealing for the construction of devices for charge
separation and light energy conversion. Further work may be
expected along this direction
Two new approaches to the construction of organic donor–
acceptor arrays containing fullerenes have been recently
proposed. The first one is based on the so-called double cable
concept.169 Fullerene units are grafted to conjugated polymeric
backbones so as to obtain intrinsically bipolar double cable
polymers in which the negative charge carriers (fullerenes)
are spatially close to each other and covalently linked to the
positive charge carrier (polymeric backbone). In this way the
effective donor–acceptor interfacial area is maximized and
positive effects on device efficiency and duration are expected.169
The second approach is termed “supramolecular” and is
aimed at creating morphological organization in the active layer
of photovoltaic cells via spontaneous supramolecular organization.170 Recently, photophysical investigation on adducts
between a methanofullerene and an OPV molecule, both provided with self-complementary 2-ureido-4[1H]-pyrimidinone
units have been carried out. In low polarity solvents, the
self-association constant is very high and the supramolecular
C60 singlet energy transfer, with no
adduct exhibits OPV
evidence of electron transfer even in polar solvents.171
Finally, it’s worth pointing out that not only OPV–C60 arrays
but also oligophenyleneethynylene–C60 172 and oligothiophene–
C60 168,173–175 systems are intensively investigated in order to elucidate their photophysical properties and test them for photovoltaic applications. Photocurrents have been generated in
devices including gold electrodes modified with fullerene-linked
oligothiophenes.176
6
Conclusions
Supramolecular photochemistry has gained a great deal of
attention from the scientific community since its very beginning, that can be dated back to second half of the 1980’s. Over
the years, research activity has developed along a number of
routes. Some of them can be outlined by examining the classes
of compounds discussed in this article, i.e. Ru()-polypyridines,
Cu()- and proton-phenanthrolines and fullerenes. Advances in
synthetic and analytical chemistry have allowed extensive
modifications to the pristine motifs [Ru(bpy)3]2⫹, [Ru(tpy)2]2⫹,
[Cu(phen)2]⫹, and C60. Accordingly, fine tuning of their photophysical properties may be achieved and, at the same time,
insertion of these subunits into a variety of multicomponent
arrays, such as dyads, triads, catenanes, rotaxanes, dendrimers,
etc., can be accomplished. Such systems, when suitably
designed in terms of spatial arrangement and electronic/
photophysical properties, may behave as molecular machines,33
switches,5 wires,54 sensors,34 antennas,30 charge separation
modules,10 logic gates 35 and so on. Some examples have been
described here. Nearly all these results, which are among the
most relevant advancements in chemical sciences in recent
years, have been obtained in homogeneous solution and, at the
present stage, are essentially of academic interest. This important research activity is certainly expected to yield further
important results. At the same time, however, the knowledge
acquired at the molecular level in bulk solution needs to be
transferred into technological applications. This most likely
means shifting to solid and/or heterogeneous environments,
although we cannot exclude that “liquid” (wet) devices may find
some practical use.177 At present, indeed, the know-how
acquired by supramolecular photochemistry in solution has
been successfully applied in heterogeneous systems for solar
energy conversion schemes.12,49,178 Analogous transfer to the
solid state environment is expected to be more difficult. For
instance it seems that the patterns of photoinduced processes in
oligothiophene–C60 arrays may be rather different in solution
or in thin films embedded in photovoltaic devices.168
Perspectives for further developments can be easily envisaged, also taking into account the increasing availability of new
molecules such as open cage 179,180 and endohedral fullerenes,181
just to remain within the classes of compounds discussed
here. The still rather unexplored field of single molecule
manipulation and spectroscopy is also quite promising for
photochemical sciences.182,183 Then it is not difficult to imagine
a leading role for thoroughly designed supramolecular arrays,
addressed by light input, in the rapidly expanding field of
nanotechnology.
Supramolecular photochemistry has been a fundamental
research field for many years but this character is changing
very quickly. Fruitful interactions with applied research and
technology are frequently established. Thus we may anticipate
that supramolecular photochemistry is doomed to acquire
the character of a truly interdisciplinary research enterprise in
a not too distant future.
Acknowledgements
I want to express my gratitude to my colleagues of the photochemistry group at ISOF/CNR Institute: Francesco Barigelletti, Lucia Flamigni and Sandra Monti for their support and
many stimulating discussions. Special thanks are due to
Jean-Francois Nierengarten, Jean-Pierre Sauvage, Francois
Diederich, Mike Ward, Ed Constable, Fernando Langa and
many people from their research teams for our fruitful
collaborations. This paper is dedicated to Professor Vincenzo
Balzani, the father of supramolecular photochemistry.
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