photoinduced electron transfer reactions investigated by...
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Photoinduced electron transfer reactions investigated by ultrafast spectroscopy
Eric VautheyDépartement de chimie-physique
Université de Genève
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Outline
Introduction. Photoinduced intermolecular electron transfer reactions, remaining questions
Picosecond optical calorimetric studies: looking at the energetics
Distinguishing different types of ion pairs:- pump-pump-probe spectroscopy- transient IR absorption
Artificial photosynthesis
Excited-state dynamics of radical ions
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D* + A D.+ + A.-ET
Photoinduced electron transfer (ET)
ET operative if (Weller equation):
D + A
D*+ A
D .A-˙
+˙
D + A-˙
+˙ ΔGET
E*
e(Eox- Ered)
C
ΔGET = - E* + e[Eox(D)- Ered(A)]+C < 0
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Classical Marcus theory
1.0
0.9
0.8
0.7
0.6
0.5
0.4
k ET
/km
ax
2.52.01.51.00.50.0-0.5ΔGET/λ
normal region
barrierless region
inverted region
AD A-D+
ΔGET
λ
−ΔGET= λ
−ΔGET> λ
−ΔGET< λ
kET ∝ exp −ΔGET + λ( )2
4λkBT
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normal region
barrierless region
inverted regionln
kET
2.01.51.00.5 −ΔGET/λ
classical
v=1
v=2
AD A-D+
v=0
solvent coordinate
v=0
semi-clas.
Semi-classical Marcus theory
kET = kET0→v
v=0
∞
∑ ΔGET≠ 0→ v( ) =
ΔGET0→0 + vhυvib + λs4λskBT
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- 1984, intramolecular charge shift(Miller, Closs et al.)
- 1987, ‘intermolecular’ charge recombination(Gould, Farid et al.)
- 1985, intramolecular charge recombination(Wasielewski et al.)
- 2001, intramolecular charge separation(Mataga et al.)
- 1997, intermolecular charge shift(Guldi et al.)
Observation of the inverted region
Miller et al., J. Am. Chem. Soc. 106 (1984) 3047
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Photoinduced bimolecular CS (polar solvents)
A
A* + D (A *. D) (A .- .D.+) A.- + D.+
(A .D)
hν
+ D
DIF CS SEP
CR
D. Rehm, A. Weller, Isr. J. Chem. 8 (1970) 259
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Hypothesis:
- CS distance increases with driving force (Sutin et al., 1984)
- the product is formed in an electronic excited state (Weller)
- ...
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E
A D
A* D
A.- D.+
ΔGCS
A.- D.+*
ΔGCS*
ΔGCS ΔGCS*
1) Formation of the product in an electronic excited state ?
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2) ΔGCS dependence of the CS distance
€
kCS =2πh
V 2
4πλkBT( )1/2exp − ΔGCS +λ( )2
4λkBT
€
λs =e2
4πε012aA
+22aD
−1r
1n2−1εs
1.8
1.6
1.4
1.2
λs (
eV)
3025201510r (Å)
B. S. Brunschwig, S. Ehrenson, N. Sutin, J. Am. Chem. Soc. 106 (1984) 6858
The barrier of highly exergonic ET is predicted to decrease with distance
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S. Murata, M. Tachiya, J. Phys. Chem. 100 (1996) 4064
2) ΔGCS dependence of the CS distanceFemto08
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Usual scheme ofCS quenching in polar solvents
(Gould et al., ACR 1996)
CIP (TIP): contact (tight) ion pairs, small ΔGCS
or CT excitation of DA complex
SSIP (LIP): solvent-separated (loose) ion pairs, large ΔGCS
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Other unanswered questions:
- absence of normal region in the CR of excited donor/acceptor complexes
- structure and geometry of the reactions intermediates
- ....
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Two types of energy gap law for CR
From T. Asahi and N. Mataga, J. Phys. Chem. 95 (1991) 1956
CT excitation (CIP, TIP)
ET quenching (SSIP, LIP)
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Picosecond optical calorimetric studies: looking at the energetics
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€
C = −e2
4πε0εsd
Energetics
D + A
D*+ A
D .A-˙
+˙
D + A-˙
+˙ ΔGCS
E*
e(Eox- Ered)
C
d d
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sample
grating formation
(four-wave-mixing, real-time holography)
2 formalisms: 1) holography 2) nonlinear optics
θ
Picosecond optical calorimetry using transient gratingFemto08
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modulation axis
light intensity
product concentration
reactant concentration
ΛFemto08
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Spatial modulation of the optical properties of the sample
€
˜ n x( ) = ˜ n 0 +Δ ˜ n cos 2πΛ
x
modulation amplitude
€
˜ n average value
Transient gratingFemto08
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amplitude grating
Δn
ΔAΔnphase grating
population changes
Δnppopulation changesoptical Kerr effect
ΔnKdensity changes
Δnd
changesthermal
ΔndvΔnd
TΔnds
volumeexpansion electrostriction
ΔnKeΔnK
n
nuclear electronic
non resonant interactions
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Probing the grating
θB
probe pulse
€
sinθB =λs2Λ
Bragg angle:
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diffracted intensity
€
η =Idif
Is
∝Δ ˜ n 2
≈ c1Δn2 + c2ΔA2
Applications of the transient grating technique:
- population dynamics
- ultrafast calorimetry
- transient dichroism
- …
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€
Idif∝Δnd2
23
ΔA = Δnp = 0
Picosecond calorimetry
Measurement of Δnd only: non-resonant probing
€
Δnd t( ) = Ci R t − t'( )−∞
t
∫ ⋅fi t'( )dt'i∑
€
R t( ) =1− cos 2πτ ac
t
exp −αacvst( )
Ci α Qi : amount of heat deposited upon process i
R(t) : response of the sample to instantaneous heat deposition
€
f t( ) = exp −t τ r( )fi(t) : time evolution of the temperature change
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I dif (
a.u.
)
1086420time delay/τac
24
- shape of the time profile: dynamics of the heat releasing process
- amplitude of the signal: amount of heat deposited€
τ ac =Λvs
=λpu
2 sin(θ / 2)vs
τr = τac/4 τr = τac τr =4τac
Picosecond calorimetryFemto08
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ON
N
25
Picosecond calorimetry
Photoinduced CS between benzophenone (BP) and DABCO
VR
ISCultrafast heat release (0.5 eV)
CSslow heat release (? eV)
very slow heat release (µs)
A DE(eV)
0
1
2
3
A + D
1A* + D3A* + D
A·¯ + D·+
J. Phys. Chem. 99 (1995) 8652
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translation stage
frequencyconverter
LASER
detectorsample
Experimental setup
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27
Picosecond calorimetry
VR
ISC CS
E(eV)
0
1
2
3
A + D
1A* + D3A* + D
A·¯ + D·+
600
500
400
300
200
100
0
I dif (
a.u.
)
3000200010000
time delay (ps)
300
200
100
I dif (
a.u.
)
0.05 M
0.3 M
0.05 M0.3 M
[D] Is/If1.901.93
ΔGCS=-0.86 eVC=-0.28 eV
The ions are in close contactJ. Phys. Chem. 99 (1995) 8652
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Excited-state dynamics of radical ions
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E
A D
A* D
A.- D.+
ΔGCS
A.- D.+*
ΔGCS*
ΔGCS ΔGCS*
Formation of the product in an electronic excited state ?
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Solution: detection of excited radical ions
Problems:
- Essentially nothing is known on the excited-state properties of radical ions
- Only a few fluorescing radical ions known (many artefacts)
- Absorption spectrum of excited radical ions unknown
O
O
O
O
F
F
F
F
F
F
N
N
SS
SSOCH3
H3CO OCH3
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N
N
N
NH2
Photophysics of radical ions in liquid
Formation of radical ions:
- Photochemical (bimolecular electron transfer): perylene cation and anion
- Chemical (salts of radical ions): Wurster’s Blue and Wurster’s Red
- Electrochemical: perylene cation and anion
WB WR Pe·+ Pe·¯
For all ions investigated so far: very short excited state lifetime (≤ 5 ps)
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Photophysics of radical ions in liquid
Formation of radical ions:
- Photochemical (bimolecular electron transfer): perylene cation and anion
- Chemical (salts of radical ions): Wurster’s Blue and Wurster’s Red
- Electrochemical: perylene cation and anion
For all ions investigated so far: very short excited state lifetime (≤ 5 ps)
1.0
0.5
0.0
-0.5
-1.0
Δ A (
a.u.
)
806040200
time delay (ps)
500 nm 580 nm 594 nm
τ1 ~ 4 ps (IC) τ2 ~ 15 ps (cooling)
-15
-10
-5
0
ΔA x
103
750700650600550500450
wavelength (nm)
3 ps 13 ps 5 ps 20 ps 7ps 30 ps
50 ps -steady state
electrochemically produced Pe·¯ (ACN + 0.1 M Bu4NCLO4)
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Photophysics of Wurster’s Blue
-30
-20
-10
0
10
20
ΔA x
103
151050time delay (ps)
τ1= 0.3 psτ2= 0.5 psτ3= 4.2 ps
-30
-20
-10
0
10
20
ΔA x
103
700650600550500wavelength (nm)
0.0 ps 3.0 ps 0.1 ps 4.0 ps 0.2 ps 10.0 ps 0.4 ps 20.0 ps 2.0 ps
NN
A
700600500400300
wavelength (nm)
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Photophysics of Wurster’s Blue
Fast decay components independent of solvent (polarity, viscosity)Slow decay component slower in non H-bonding solvent
hν
D0,v=0
D1,v>0
D0,v>0
fast
slow
Why is internal conversion so fast ?- a very small D1-D0 gap cannot be invoked (ΔE (D1-D0)=1.95 eV)- presence of a conical intersection ?
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Photophysics of Wurster’s Blue
A.C. Albrecht, JACS 1955
A clue: How can a molecule with a 500 fs excited-state lifetime exhibit significant fluorescence?
fluor
esce
nce
(a.u
.)
800750700650
wavelength (nm)
MeOH:EtOH λexc=600nm 140K 95K 110K 92K 105K 90K 100K 88K 98K 85K
rct coord.
hv
barrierin
tens
ity (a
.u.)
800700600500
wavelength (nm)
MeOH:EtOH (1:1) at 85K
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At the present stage, the observation of excited radical ions upon highly exergonic CS is highly difficult.
The photophysics of radical ions has to be better understood.
Interesting for the astrochemists
Nature 391 (1998) 259
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At the present stage, the observation of excited radical ions upon highly exergonic CS is highly difficult.
The photophysics of radical ions has to be better understood.
Interesting for the astrochemists
Can we obtain new information on ion pair structure from the excited state dynamics of ions?
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Distinguishing different types of ion pairs
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Usual scheme of ET quenching in polar solvents
CIP: contact ion pairs, small ΔGCS
or CT excitation of DA complex
SSIP: solvent-separated ion pairs, large ΔGCS
Gould et al., Acc. Chem. Res. 29 (1996) 522
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How to distinguish these ion pairs?
1) ion pairs and free ions have essentially the same UV-vis absorption spectrum
3) Look at other properties, which might be sensitive to ion pairing J. Phys. Chem. 110 (2006) 7547
2) Time-resolved resonance Raman spectroscopy:
- no information on singulet systems
- apart from very few exceptions, triplet geminate ion pairs and free ions have essentially the same resonance Raman spectrum (above 500 cm-1).
J. Phys. Chem. 96 (1992) 7356
J. Am. Chem. Soc. 116 (1994) 9182
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A + D
A+ D*
A·¯ D·+ A·¯ + D·+pump 1
A·¯···D·+
Pump-pump-probe experiment
A·¯ D·+*
pump 2 probe
D·+*
early time long time
J. Phys. Chem. A 110 (2006) 7547
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Q :CN
NC
Pe DCB DCE
CN
CN
Pump-pump-probe experiment
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Pump-pump-probe experiment
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Pump-pump-probe experiments (GSR of Pe.+) -Δ
A (a
.u.)
121086420Δt23 (ps)
Δt12= 60 ps Δt12= 1 ns
Pe/DCB
- ΔA
(a.u
.)
20151050Δt23 (ps)
Δt12= 60 psΔt12= 1 ns
Pe/DCE
1.1 ps (0.5) + 6.2 ps (0.5)
3 ps
3 ps (0.5) + 10 ps (0.5)
J. Phys. Chem. A 110 (2006) 7547
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GSR dynamics as a function of the ‘age’ of the ion
monophasicGSR dynamics
biphasic GSRdynamics
J. Phys. Chem. A 110 (2006) 7547
long timeearly time
Transition from biphasic to monophasic GSR on the 400 ps timescale
3 ps3 ps
6 ps
2 ps
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How to distinguish these ion pairs?
1) ion pairs and free ions have essentially the same UV-vis absorption spectrum
4) Time-resolved IR spectroscopy J. Phys. Chem. A 110 (2006) 13676
Ang. Chem. in print
3) Look at other properties, which might be sensitive to ion pairing J. Phys. Chem. 110 (2006) 7547
2) Time-resolved resonance Raman spectroscopy:
- no information on singulet systems
- apart from very few exceptions, triplet geminate ion pairs and free ions have essentially the same resonance Raman spectrum (above 500 cm-1).
J. Phys. Chem. 96 (1992) 7356
J. Am. Chem. Soc. 116 (1994) 9182
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Time-resolved IR spectroscopy(with E. Nibbering, MBI, Berlin)
J. Phys. Chem. A 110 (2006) 13676
Pe DCB
CN
CN
In acetonitrile (ACN): dissociation into free ions Φion= 30%
In dichloromethane (DCM): no free ions (CR only)
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J. Phys. Chem. A 110 (2006) 13676
Pe DCB
CN
CN
1.0
0.5
0.0
ΔA
(nor
m.,
a.u.
)
2130212021102100209020802070Wavenumber (cm -1)
ACN DCM
Pe + DCB (700 ps)
CN stretch region
In acetonitrile (ACN): dissociation into free ions Φion= 30%
In dichloromethane (DCM): no free ions (CR only)
Time-resolved IR spectroscopy
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Pe TCNE
CNNC
NC CN
τCRΔGCRΔGCS
-0.74 eV-2.17 eV 1.6 ns1)
1) N. Mataga et al., J. Phys. Chem. 90 (1986) 3380, Chem. Phys. 127 (1988) 239
D A
D A
D* A
D·+A·-
CS
CR
hν
The Pe-TCNE system
D A
D·+A·-
CRhν
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Time-resolved IR spectroscopy
MPe TCNE
CNNC
NC CN
Can one use the sensitivity of C-N stretch to differentiate the stronglycoupled and weakly coupled ion pairs found with Pe/TCNE ?
0.6
0.4
0.2
0.0
ΔA
(mO
D)2200218021602140
Wavenumber (cm -1)
-100 ps 3 ps 7 ps 10 ps 20 ps 30 ps 50 ps 70 ps 100 ps
0.2 M TCNE
Ang. Chem. in print
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Concentration dependence
MPe TCNE
CNNC
NC CN
0.6
0.4
0.2
0.0
ΔA
(mO
D)
2200218021602140
Wavenumber (cm -1)
-100 ps 3 ps 7 ps 10 ps 20 ps 30 ps 50 ps 70 ps 100 ps
0.2 M TCNE
2.0
1.5
1.0
0.5
0.0
ΔA
(mO
D)
2200218021602140
Wavenumber (cm -1)
-100 ps 1 ps 3 ps 5 ps 7 ps 10 ps 20 ps 30 ps 70 ps
0.9 M TCNE
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Concentration dependence
MPe TCNE
CNNC
NC CN
1.0
0.9
0.8
0.7
0.6
0.5
0.4
ΔA
(mO
D)
2155215021452140Wavenumber (cm-1)
1 ps 5 ps 10 ps 30 ps 50 ps 70 ps 100 ps 120 ps 150 ps
0.05 M TCNE
0.6
0.4
0.2
0.0
ΔA
(mO
D)
2200218021602140
Wavenumber (cm -1)
-100 ps 3 ps 7 ps 10 ps 20 ps 30 ps 50 ps 70 ps 100 ps
0.2 M TCNE
2.0
1.5
1.0
0.5
0.0
ΔA
(mO
D)
2200218021602140
Wavenumber (cm -1)
-100 ps 1 ps 3 ps 5 ps 7 ps 10 ps 20 ps 30 ps 70 ps
0.9 M TCNE
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Two IR spectral components
1) Broad and short lived (dominant at high [TCNE])
2) Narrow and long lived (dominant at low [TCNE])
Strongly coupled ion pairs
Weakly coupled ion pairs
1.0
0.8
0.6
0.4
0.2
0.0
Δ A
(a.u
.)
22102200219021802170216021502140Wavenumber (cm-1)
1 2
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Comparing LE and CT excitations
500400300 1000900800700600500
0.0 M 0.2 M 0.6 M 1.0 M
Abso
rban
ce
Wavelength (nm)
x40
LE
2.0
1.5
1.0
0.5
0.0
ΔA
(mO
D)
2200218021602140
Wavenumber (cm -1)
-100 ps 1 ps 3 ps 5 ps 7 ps 10 ps 20 ps 30 ps 70 ps
0.9 M TCNE
Strongly+weakly coupled IP
CT
1.5
1.0
0.5
0.0
ΔA
(mO
D)
2200218021602140
Wavenumber (cm -1)
-100 ps 2 ps 3 ps 5 ps 7 ps 10 ps 12 ps 15 ps 30 ps
CT excitation
Stronglycoupled IP
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Structure of the ion pairs ?
Polarisation anisotropy measurements upon 400 nm excitation
0.10
0.05
0.00Pola
rizat
ion
Aniso
tropy
0.1 1 10 100Time Delay (ps)
0.9 M TCNE in ACN
raw smoothed best fit
2150 cm-1
r = 0.1±0.03 on both IR transitions
N
N N
N
ΔA
(mO
D)
2200218021602140
Wavenumber (cm -1)
0.9 M TCNE
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Structure of the ion pairs ?
Strongly coupled IP: anisotropy r = 0.1 on both CN bands
=> probably sandwich-like coplanar geometry
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Structure of the weakly coupled ion pairs ?
Solvent-separated ?
All our measurements suggest two ions in contact but poorly oriented.
In bimolecular ET theories, the reactants are always considered as spheres
Distance is the only coordinate modulating the coupling.
Mutual orientation should be considered.
Tight and loose ion pairs is more judicious than
contact and solvent-separated ion pairs
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Artificial photosynthesis
with Stefan Matile (Geneva)
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NN
+H3N
H2NO O
O
O
O
R2
R1
CF3COO-
O
OO
OO
OO
O
NDI
O
NDI
O
NDI
O
NDI
O
NDI
O
NDI
O
NDI
O
NDI
O
Octachromophoric systems
NDI
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A
600500400
wavelength (nm)
fluor
esce
nce
800700600500400
wavelength (nm)
NDI chromophores
NN
+H3N
H2NO O
O
O
O
HN
NH
NN
+H3N
H2NO O
O
O
O
HN
Cl
NN
+H3N
H2NO O
O
O
O
O
O
RY B
F. Würthner et al., Chem. Eur. J. 8 (2002) 4742
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O
NN
O
H
H
H
H
NN N
O
O
O
ON
O
H
H
OO
H
H
+H3N
+H3N
N NO
O
O
O
NH
R
R
R
R
R =
Self-assembly in lipid bilayers
BB
BB
BB
BB
4
EYPC-LUVegg yolk phosphatidylcholinelarge unilamellar vesicle
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Photoinduced transmembrane pH gradient
EDTA
HPTS
O
O
SO3-
Q HPTS
OH-O3S
-O3S SO3-
0
2 105
4 105
6 105
8 105
1 106
350 400 450Wavelength [nm]
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
6.8 7 7.2 7.4 7.6pH
I 462 /
I 405
A B
cps
a)
d)c)b)
e)
irradiation at 635 nm a) 0 s, e) 600 s
Science 313 (2006) 84
Q
EDTA
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Photoinduced transmembrane pH gradient
Schulten et al. (1998) PNAS 95, 5935
photosynthetic apparatus of purple bacteria
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EDTA
0
2 105
4 105
6 105
8 105
1 106
350 400 450Wavelength [nm]
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
6.8 7 7.2 7.4 7.6pH
I 462 /
I 405
A B
cps
a)
d)c)b)
e)
irradiation at 635 nm a) 0 s, e) 600 s
Science 313 (2006) 84
*CS
-+
Q
EDTA
EDTA+
Q.-QH.*
+
EDTA+
QH-
CS
QH2
O
O
SO3-
Q HPTS
OH-O3S
-O3S SO3-
Photoinduced transmembrane pH gradientFemto08
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Blue systems
Time-resolved fluorescence
NN
O
O
O
O
HN
NH
NH
H2N
+H3N
B1
B2
B8
8.1 ns
5.7 ps63 ps av.: 2 ns
7.1 ps
51 ps … ps
av.: 0.2 nsinte
nsity
(a.u
.)
6050403020100
time delay (ps)
B1
B2
B8
Φfl
0.32
0.05
0.01
Efficient self-quenching!
J. Phys. Chem. B, 112 (2008) 8912
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-40
-30
-20
-10
0
10
20
ΔA x
103
700650600550500450
wavelength (nm)
0.5 ps 5 ps 20 ps 70 ps 700 ps 1.4 ns
Blue systems
NN
O
O
O
O
HN
NH
NH
H2N
+H3N
B1
B2
B8
Transient absorption
-60
-40
-20
0
20
40
60
Δ A
x 10
3
700650600550500450
wavelength (nm)
0.3 ps 1.2 ps 4 ps 10 ps 20 ps 40 ps
B1·-
B1* + D B1.- + D.+B1*
-15
-10
-5
0
5
10
15
Δ A x
103
700650600550500450
wavelength (nm)
0.3ps 1ps 3ps 6ps 14ps 30ps 50ps 700ps B8
CSS
B8
LES
hν FLIC
CSS
LESCSS
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CS state dynamics
CS state longer-lived in B8 !
Charge hopping?
Blue systems
ΔA
(a.u
.)
250200150100500
time delay (ps)
D (25 ps) O (60 ps)B2 (25 ps)B8 (60 ps)
J. Phys. Chem. B, 112 (2008) 8912
NN
O
O
O
O
HN
NH
NH
H2N
+H3N
B1
B2
B8B8
LES
hν FLIC
CSS
Transient absorptionFemto08
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€
r =ΔA|| −ΔA⊥ΔA|| + 2ΔA⊥
polarisation anisotropy(ground state bleach)
evidence of charge hopping Blue systems
NN
O
O
O
O
HN
NH
NH
H2N
+H3N
B1
B2
B8reorientation of the S0-LES transition dipole in 8 ps - too fast for rotational diffusion (200 ps)- charge hopping
0.4
0.3
0.2
0.1
0.0
-0.1
anis
otro
py
140120100806040200
time delay (ps)
~8 ps
200 ps
~50 ps
B1 B8
Transient absorptionFemto08
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15
10
5
0
-5
-10Δ
A·10
3750700650600550500450
wavelength (nm)
2.2ps 2.5ps 3ps 5ps 10ps 20ps 30ps 50ps 80ps 100ps 150ps 200ps 300ps 500ps 1000ps 1900ps
B84
B84 in vesicles (transient absorption)
16
14
12
10
86
4
2
0
ΔA·
103
1 10 100 1000time delay (ps)
tau1 = 5 ps (49%) tau2 = 25 ps (40%) tau3 = 400 ps (11%)
CSS
Long-lived component (400 ps) in vesicles!
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Possible improvements ?
NN
O
O
O
ONH
O
OH2N
+H3N
O
O
Y
Y
Y
Y
Y
Y
Y
Y
Y
Increase E-gap for CR
1) Slowing down charge recombination
2) Increasing charge mobility
R
R
R
R
R
R
R
R
R
R
R
NN
O
O
O
ONH
O
OH2N
+H3N
HN
Cl
Change the ‘rod’Increase E-gap for CR
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- Investigate the photophysics on gold surface
Perspectives
- Other chromophores with OPE
- Other NDI chromophores
- Effect of excitation wavelength
- Excitation energy migration
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Acknowledgements
Pierre-Alain MullerAna MorandeiraOlivier NicoletStéphane PagèsAlexandre FürstenbergOmar MohammedBernhard LangNatalie BanerjiJakob GriljGuillaume DuvanelOksana Kel
Collaboration:Stefan Matile (Geneva)Erik Nibbering (Berlin)Anatoly Burshtein (Weizmann)Anatoly Ivanov (Volgograd)
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