Inter- and Intramolecular Photoinduced Electron Transfer of Flavin Derivatives with Extremely Small Reorganization Energies

Photoinduced electron transfer (ET) of a series of aromatic electron donors (D) to the singlet or triplet excited state of a flavin analogue (10-methylisoalloxazine: MeFl) and intermolecular back electron transfer (BET) from MeFl(center dot-) to D center dot+ in benzonitrile (PhCN) has been investigated in light of the Marcus theory of ET. The rate constants of intermolecular photoinduced ET (k(et)) from D to the singlet excited state ((1)MeFl*) and the triplet excited state ((3)MeFl*) were determined by fluorescence quenching and enhanced decay rates of triplet triplet (T-T) absorption by the presence of D, respectively. The ket values increase with an increase in the ET driving force to reach the diffusion-limit value that remains constant with a further increase in the ET driving force. Nanosecond laser flash photolysis was performed to determine the rate constants of intermolecular BET (k(bet)) from MeFl(center dot-) to D center dot+ in PhCN. In contrast to the case of k(et) the driving force dependence of kbet shows a pronounced decrease towards the highly exothermic region. The reorganization energy (2) of intermolecular BET is determined to be 0.68 eV by applying the Marcus equation in the inverted region, where the kbet value decreases with increasing the BET driving force. The slowest BET was observed for BET from MeFl(center dot-) to N,N-dimethylaniline radical cation (DMA(center dot+)) with the k(bet), value of 3.5 x 10(6)M(-1) which is 1600 times smaller than the diffusion rate constant in PhCN (5.6 x s(-1)). Then, DMA was linked to the 10-position of isoalloxazine to synthesize a DMA flavin linked dyad (10-[4'-(N,N-dimethylamino)phenyl] isoalloxazine: DMA-Fl). Photoexcitation of DMA-Fl results in photoinduced ET from the DMA moiety to the singlet excited state of Fl moiety to form the charge-separated (CS) state (DMA(center dot+) -Fl(center dot-)) that has an extremely long lifetime (2.1 ms) in PhCN at 298 K.