Photoinduced Electron Transfer in a Donor-Acceptor Dyad
Amy Ferreira
August 14th, 2007
Outline
IntroductionBackgroundApplications
Charge Transfer Quarterthiophene-Anthraquinone (T4-AQ)
Spectroscopic propertiesExperimental results
Future goals
Background and Applications Thiophenes
Have potential in photovoltaic cells, light emitting diodes, and thin film transistors
Photosynthesis Photosynthesis occurs with no back electron
transfer, while it still occurs in most man-made systems.
Charge transfer Charge transfer through peptide bonds Long range charge transfer
D A
e–
D+. A–.
Electron transfer
et
E
D+ A–
E
D A
Ground State Charge transfer (CT) state
E LUMO
HOMO
DA
Photoinduced electron transfer
D*
Locally excited (LE) state
et
E LUMO
HOMO
D+
A–
Charge transfer (CT) state
Electron transfer
D A
D+. A–.
G≠ = G≠ – G0i
G0 = G0f – G0
i
E
q
i
f
Hif
transitionstate
Marcus and Sutin Biochim. Biophys. Acta 1985, 811, 265.
Tk
GR
h
TkFCk
B
Bifet 4
expexp2
202H
Electron transfer
D A
D+. A–. G0
G≠
E
q
if
WEEFGAADD
000
/
0
/
0 E DA
DA
R
ezzW
0
2
4
1
Photoinduced electron transfer
D A
D+. A–.
G0
G≠
E
q
i
f
D* A
h
4
20 G
G
k et
-1.4 -1.2 -1.0 -0.8 -0.6
G
normal region
invertedregion
Normal vs. Inverted region
kT
GTk DAet exp
2
Jablonski DiagramT4*-AQ
T4-AQ
CT State
KfAbsorbance Knd
Ket = kCS
Kbet = kCR
E
Quarterthiophene-Anthraquinone
e–
Model Compound
T4-COOH
(Quarterthiophene-carboxylic acid)
Test Compound
T4-AQ
(Quarterthiophene-Anthraquinone)
Quantum Yield
A
Srf
101
)(
)0(
)0(
)0(
101
101 S
S A
Af
f
Lifetime
♦ T4-AQ
♦ T4a— Scatterer
♦ T4-AQ
♦ T4a— Scatterer
f
fk
fndk
1
Experimental ResultsSolvent (, n) Compound a
(max) / nm f(max) / nm Φf τ / ns kf
c × 109 / s-1 knd c × 109 / s-1
Hexane T4-COOH 392 487 0.21 0.529 ± 0.041 0.40 1.5
(2.0, 1.38) T4-AQ 391 470 0.26 0.527 ± 0.014 0.50 1.4
Tetrachloromethane T4-COOH 401 500 0.17 0.631 ± 0.053 0.27 1.3
(2.2, 1.46) T4-AQ 394 496 0.18 0.596 ± 0.015 0.31 1.4
Toluene T4-COOH 394 506 0.18 0.522 ± 0.036 0.35 1.6
(2.4, 1.50) T4-AQ 402 501 0.11 0.785 ± 0.016 0.14 1.1
Chloroform T4-COOH 404 505 0.12 0.474 ± 0.023 0.26 1.9
(4.8, 1.45) T4-AQ 394 510 — — — —
Ethylacetate T4-COOH 400 500 0.17 0.504 ± 0.037 0.33 1.7
(6.0, 1.37) T4-AQ 391 496 — — — —
Tetrahydrofuran T4-COOH 406 501 0.18 0.5581 ± 0.043 0.32 1.5
(7.5, 1.41) T4-AQ 384 494 — — — —
Dichloromethane T4-COOH 395 513 0.14 0.540 ± 0.041 0.26 1.6
(9.1, 1.44) T4-AQ 400 493 — — — —
Acetone T4-COOH 387 499 0.14 0.581 ± 0.001 0.24 1.5
(22, 1.36) T4-AQ 400 496 — — — —
Acetonitrile T4-COOH 386 498 0.056 0.631 ± 0.053 0.089 1.5
(38, 1.34) T4-AQ 400 494 — — — —
400 600 800 1000 1200
0.00
0.05
-0.01
0.00
0.01
0.02
0.03
A
Wavelength, nm
0 5 27 97 202 402 802 1422 (ps)
T4-COOH in Toluene T4-AQ in Toluene
400 600 800 1000 1200
-0.01
0.00
0.01
0.02
0.03
0.04
-0.005
0.000
0.005
0.010
0.015 0 5 27 97 202 402 802 1422 (ps)
A
Wavelength, nm
Femtosecond Flash Photolysis Absorbance
Femtosecond Flash Photolysis Absorbance
400 600 800 1000 1200
0.0
0.1
0.2
0.0
0.1
0.2
0 0.4 1 2 5 20 100 200 500 1000 1450 (ps)
A
Wavelength, nm
T4-COOH in Acetonitrile
400 600 800 1000 1200
0.00
0.02
0.04
0.06
0.00
0.02
0.04
0 0.4 1 2 5 500 1450 (ps)
A
Wavelength, nm
T4-AQ in Acetonitrile
Femtosecond Flash Photolysis Lifetime
-2 0 2 4 6 8 10
-1.0
-0.5
0.0
0.5
1.0
= 1.2 ps
= 1.3 ps
807 1180 420 nm
= 0.7 ps
A
Time, ps
T4-AQ in Acetonitrile
0 500 1000 1500
0.00
0.01
1= 7.2 ps
2=122 ps
3=429 ps
815 nm
A
Time, ps
0 500 1000 1500
0.000
0.005
1= 24 ps
2= 217 ps
3= 538 ps
875 nm
A
Time, psT4-COOH in Toluene
T4-AQ in Toluene
Experimental ResultsSolvent out / eV Get
(0) / eV Gbet
(0) /eV
Hexane 0.0614429 1.24599 4.04599
Tetrachlromethane 0.0441769 1.00292 3.80292
Toluene 0.0540527 0.854122 3.654122
Chloroform 0.526537 0.187432 2.612568
Ethylacetate 0.710502 0.398511 2.401489
Tetrahydrofurane 0.724188 0.564164 2.235836
Dichloromethane 0.717626 0.678385 2.121615
Acetone 0.961557 0.996453 1.803547
Acetonitrile 1.03181 1.09567 1.70433
WGEEFG SAADD 00/0
/00 E
Tk
GR
h
TkFCk
B
Bifet 4
expexp2
202H
111
2
1
2
1
4 20
2
nrrr
e
ADinoutin
FC: Frank Condon factor λ: Reorganization energy
W: Coulombic factor ΔG(0): Driving force
ΔGs: Electrode potential correction =
E00: 0-0 electron transition =
AADD rr 111111
4
1
0
hc
CS (ps) kCS (x 1011 s-1) CR (ps) kCR (x 1011 s-1)
Toluene — — — —
Chloroform 4.7 2.1 73 0.14
Dichloromethane 1.2 8.3 15 0.67
Acetonitrile 0.7 14 1.3 7.7
Experimental Results
Conclusions Solvent Polarity
Charge transfer properties have a strong dependence on the polarity of the solvent
Charge Recombination Although the driving forces were large, the rates of
charge recombination were 2-20 times slower than that of photoinduced charge separation
Inverted Marcus Region The decrease in the electron-transfer rate constants
with the increase in the driving forces for the three solvents suggests that the charge recombination processes occur in the inverted Marcus regions for the particular media
Future goals Long range charge transfer
In proteins, efficient charge transfer cannot occur over 1.5nm, but we aim to prepare a system that mediates charge transfer over several nanometers
Currently we are preparing the redox species to affix to the sides of non-native α-L-amino acids that will mediate 3 types of charge transfer: Tunneling, electron hopping, hole hopping
Acknowledgements Wei Xia Valentine Vullev Duoduo Bao Jiandi Wan Radiation Lab at Notre Dame Chak Him Chow Vullev’s lab group
Normal vs. Inverted region
G0
G≠ > 0
E
q
i
f
4
20 G
G
kT
GTk DAet exp
2
G0
G≠ = 0
E
q
i
f
Normal vs. Inverted region
4
20 G
G
kT
GTk DAet exp
2
G0
G≠ > 0
E
q
i
f
4
20 G
G
Normal vs. Inverted region
kT
GTk DAet exp
2
Fluorometer Scheme
Scheme: an example of a spectrofluorometer for lifetime and fluorescence measurements
Arc Lamp
Excitation Monochromator
Sample Curvet
Emission Monochromator
Diode Laser
Flash Photolysis
To PC
Optical Delay Rail
Frequency Doubler
Ocean OpticsS2000 CCD Detector
SampleCell
Filter Wheel
Chopper
CLARK-MXR
CPA-2010
775 nm, 1 kHz1 mJ/pulse
(7fs -1.6 ns)
Probe
Pump
Ultrafast Systems
