Photoinduced Electron Transfer in a Donor-Acceptor Dyad Amy Ferreira August 14 th, 2007

Outline Introduction  Background  Applications Charge Transfer Quarterthiophene-Anthraquinone (T4-AQ)  Spectroscopic properties  Experimental 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–e– D +. A –. Electron transfer et E D+D+ A–A–   E DA Ground State Charge transfer (CT) state

E LUMO HOMO D A Photoinduced electron transfer D* Locally excited (LE) state et E LUMO HOMO D+D+ A–A–   Charge transfer (CT) state

Electron transfer D A D +. A –.  G ≠ = G ≠ – G 0 i  G 0 = G 0 f – G 0 i E q i f H if transition state Marcus and Sutin Biochim. Biophys. Acta 1985, 811, 265.

Electron transfer D A D +. A –. G0G0 G≠G≠ E q i f

Photoinduced electron transfer D A D +. A –. G0G0 G≠G≠ E q i f D* A h

Normal vs. Inverted region

Jablonski Diagram T4*-AQ T4-AQ CT State KfKf Absorbance K nd K et = k CS K bet = k CR E

Quarterthiophene-Anthraquinone e–e– Model Compound T4-COOH (Quarterthiophene-carboxylic acid) Test Compound T4-AQ (Quarterthiophene-Anthraquinone)

Quantum Yield

Lifetime ♦ T4-AQ ♦ T4a — Scatterer ♦ T4-AQ ♦ T4a — Scatterer

Experimental Results Solvent ( , n) Compound a (max) / nm f (max) / nm ΦfΦf τ / nsk f c × 10 9 / s -1 k nd c × 10 9 / s -1 HexaneT4-COOH ± (2.0, 1.38)T4-AQ ± TetrachloromethaneT4-COOH ± (2.2, 1.46)T4-AQ ± TolueneT4-COOH ± (2.4, 1.50)T4-AQ ± ChloroformT4-COOH ± (4.8, 1.45)T4-AQ394510———— EthylacetateT4-COOH ± (6.0, 1.37)T4-AQ391496———— TetrahydrofuranT4-COOH ± (7.5, 1.41)T4-AQ384494———— DichloromethaneT4-COOH ± (9.1, 1.44)T4-AQ400493———— AcetoneT4-COOH ± (22, 1.36)T4-AQ400496———— AcetonitrileT4-COOH ± (38, 1.34)T4-AQ400494————

T4-COOH in Toluene T4-AQ in Toluene Femtosecond Flash Photolysis Absorbance

T4-COOH in Acetonitrile T4-AQ in Acetonitrile

Femtosecond Flash Photolysis Lifetime T4-AQ in Acetonitrile T4-COOH in Toluene T4-AQ in Toluene

Experimental Results Solvent out / eV  G et (0) / eV  G bet (0) /eV Hexane  Tetrachlromethane  Toluene  Chloroform   Ethylacetate   Tetrahydrofurane   Dichloromethane   Acetone   Acetonitrile   FC: Frank Condon factor λ: Reorganization energy W: Coulombic factor ΔG (0) : Driving force ΔG s : Electrode potential correction = E 00 : 0-0 electron transition =  CS (ps) k CS (x s -1 )  CR (ps) k CR (x s -1 ) Toluene———— Chloroform Dichloromethane Acetonitrile

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G0  G ≠ > 0 E q i f

G0G0  G ≠ = 0 E q i f Normal vs. Inverted region

G0G0  G ≠ > 0 E q i f Normal vs. Inverted region

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 Optics S2000 CCD Detector Sample Cell Filter Wheel Chopper CLARK -MXR CPA nm, 1 kHz 1 mJ/pulse (7fs -1.6 ns) Probe Pump Ultrafast Systems