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Charge and Energy Transfer Processes in functional material for photocatalysis
investigated with real-time spectroscopies
M. Gazzetto, M. Nazari, A. Rondi, E.Rohwer, T. Feurer, A. Cannizzo
Institute of Applied Physics, University of Bern
We present our progress in the comprehension and modeling of long-range energy transfer (EnT) and charge transfer (CT) processes in different photoactive systems, such as a H2evolving supramolecular photocatalyst for artificial photosynthesis, a novel family of fluorescent carbon-based nanodots and hyper-polarizable chromophores.
Our setup is a versatile time-resolved femtosecond spectrometer that can operate as a transient absorption (TA) or a stimulated Raman setup (FSR) to explore dynamics of processes
with timescale ranging from 10s fs to 300 ps. It covers DUV-to-Vis spectral region.
Timescale of fundamental processes
10-3
10-8
10-9
milli
10-10
10-11
10-12
nano
10-13
Setup features
10-14
10-15
pico
Radiative
decay
s
femto
Vibrational
motion
Rotational motion
Vibrational relaxation
(polyatomic)
fundamental
physical
Solvent relaxation
Collision time
Proton transfer
in liquids
chemical
Photochemical isomerization
Photo-ionization
Photo-dissociation
Torsional dynamics
in DNA
Vision (isomerization)
Protein internal motions
Photosynthesis
biological
(Energy & Charge
Transfer)
Key example: H2-evolving supramolecular



catalyst§
TA scheme
Supramolecular chemistry applied to artificial photosynthesys is a promising strategy to develop
photocatalyzers based on a «lego» approach. Ultrafast spectroscopy permits to unravel the route of the
excite electron as the charge migrates within the photochemical molecular device pointing out criticalities as
coupling, losses, non-radiative channels and branching.
• Aim of the experiments was to reveal structural
information important for photocatalytic activity
through Raman spectroscopy.
• FSR study reveals structural changes in the 200500 cm−1 vibrational bands, pointing to a change
in the coordination structure of Pd-4d  Pd-3d
coordination in the reaction center.
• The fit of FSR curves obtained produce time
constants of 0.9±0.2 ps and 400±129 ps.
• DUV probe studies focus in studying the time
constants for CT mechanism in the active center
and relay, to identify the right model. Calculation
and analysis are still on-going, but lead to a new
CT model.
τ < 1 ps
τ ≈ 5 ps
Probe (t = Δt)
splitted for referencing
τ ≈ 1ps
τ ≈ 310 ps
CMOS
Detection
Sample

FSR scheme
BRIDGE
LIGHT
HARVESTER
CATALYTIC
CENTER
OLD MODEL
LIGHT
HARVESTER
BRIDGE
CATALYTIC
CENTER
Pump (t = 0)
NEW MODEL
Background (pre-zero spectrum subtracted) and baseline corrected FSR spectra.
Raman pump: ∆λ=1.5 nm, λ=581 nm, actinic pump : λ= 530 nm , Spectral
resolution <20 cm-1.
Sample
Probe and Raman(t = Δt)
splitted for referencing
CMOS
Detection
Carbon nanodots*
0.0
TA_SVD_200 fs
TA_SVD_1 ps
TA_SVD_10 ps
TA_SVD_20 ps
TA_SVD_200 ps
340
H
O
C
O
H
-30
H
O
-20
35
H
B
O
A
H
-10
H
N
40
OH
330
•
Harder than diamond
•
Not toxic
•
Wide gap material
•
•
Intrinsic catalytic properties
•
Cheap production, suitable to replace semiconductor
•
nanoparticles
O
O
320
•
•
O
O
1-10 nm
310
wavelength [nm]
OH
C
300
O
O
290
OH
OH
-3
Remarkable optical properties and applications:
Novel family of nanomaterials with a β-C3N4 structure :
O
-0.5
-1.0x10


O
TA signal [OD]
Pump (t = 0)
Two proposed mechanism for charge transfer:
DUV measurements
Pump : λ= 520 nm
0.5

Probe range: 270 nm to 750 nm.
Pump range: 270 nm to 350 nm, 400 nm,
520 nm to 700 nm.
Raman tunable range: 520 nm to 700
nm.
Single shot Detection approach:
• Reducing duration of measurements
with low accumulation times.
• Minimize sample photodegradation.
• Avoid drifts
in experimental
conditions and pulse spectra.
Refencing beams strategy:
• Correct
intensity and spectral
fluctuations of pump, probe and
Raman beam.
• Rejection of outliers.
Signal/Noise: 103 single shot.
Time resolution <40 fs
“Tunable” visible fluorescence.
Emission sensitive to perturbations such as presence
of metal cations.
Photo-excited CDs are efficient photo-activated
acceptors or donors of electrons or protons.
Suitable for bio-imaging as nanosensor and marker.
µOD
µOD
30
25
-60
200-340 cm-1
420-550 cm-1
20
TA study is essential to understand the mechanism of photoluminescence in nano-CDs to design the
photo-chemical activity, specificity and selectivity.
TA study included a the polarization variation between pump and probe and varying pump wavelength
at 266 nm, 335 nm, 350 nm, 400 nm and 500 nm. Here I present the more significant result.
345-420 cm-1
1200-1275 cm-1
1310-1360 cm-1
1400-1480 cm-1
-70
-80
15
0.1
1
10
100
-90
0.1
1000
1
t / ps
10
100
1000
t / ps
Unraveling the photocycle
Kinetic FSR traces are shown together with respective fit curves (solid lines) from a global biexponential fit.
Fluorescence of
acqueous solution of
CD, excited at the
indicated wavelength.
§ In collaboration with Dietzek,’s group Friederich Schiller University of Jena.
TA signal [OD]
•
6
Pump and Probe at magic angle
polarization
TA_200fs
TA_400fs
TA_999fs
TA_1998fs
TA_4675fs
TA_9670fs
TA_42637fs
'TA_1.0258e+05fs'
'TA_2.0248e+05fs'
4
2
-4
450
-8x10
400
1
TA measurements in DMF
Pump : λ= 400 nm
500
550
600
5
10
15
20
25
30
50
100
150
200
-3
-2
-4
-6
-8
TA_magicangle
TA_parallel
TA_orthogonal
0
500
550
Time delay [ps]
600
650
266 nm excitation
At 266 nm core
electrons are excited in
the conduction band,
with isotropic ensemble
energy transfer, losing
information on the
rotational dynamics.
1.0
TA_orthogonal
TA_parallel
TA_magicangle
0.5
0.0
-12x10
400 nm excitation
450
Kinetic traces at 550 nm
(266 nm excitation)
Kinetic traces at 550 nm
(400 nm excitation)
-3
Proposed
Photocycle
-5
3
1.5x10
-2
400
0
2
-3
0
0
-3
5
450
0
500
Pump at 400 nm: rotational depolarization with 60 ps time constant observed (faster than
rotational diffusion of ≈6 ns).
Pump at 266 nm: no rotational depolarization observed.
-8x10
10
400
•
-6
15x10
350
4
-10
TA_175fs
TA_225fs
TA_524fs
TA_1124fs
TA_5370fs
TA_20355fs
TA_42333fs
TA_82293fs
Absorption
-10
550
Time delay (ps)
-4
-3
Time dependent
measurement for
Photocatalytic Hydrogen
Generation
8
TA signal [OD]
Homoleptic complexes have recently proved to be
natural H2-generation photocatalyzers in acid
environment:
• Study of charge distribution on the units.
• Study of reaction path to understand intermediate
step in H2 formation.
600
0
TA signal [OD]
Metal complexes with non-innocent ligands, leading
to attractive optical and structural properties to
investigate:
• Hyperpolarizability with dual-emission and higher
state emission.
• Strong coupling with local field, opening to THz
applications.
• Extreme structural rigidiy, leading to symmetric
higly delocalized states.
650
OD
Several dithiolene complexes show an uncommonly strong hyper-polarizability, which can be easily
modulated with an external parameter as pH, temperature or a laser pulse. This property originates
from a high delocalization of the frontier electrons along with charge-localized excited states, that
can be optically populated with visible light.
Homoleptic dithiolene present promising photocatalytic properties in H2 generation, thus studying
CT dynamics is fundamental to design and synthetize optimized systems.
Wavelength (nm)
Transient Absorption Time-resolved plot at 400 nm
TA signal [OD]
Homoleptic dithiolene
#
complexes
50
100
150
time delay [ps]
200
0
50
100
150
time delay [ps]
200
400 nm excitation
At 400 nm there
are localized
intraband
transitions, with
energy/charge
surface
diffusion.
650
Wavelength (nm)
# In collaboration with Deplano’s group, University of Cagliari.
* In collaboration with L. Sciortino, F. Messina, University of Palermo