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 (10methylisoalloxazine: MeFl) and intermolecular back electron transfer (BET) from MeFl to D⁺ in benzonitrile (PhCN) has been investigated in light of the Marcus theory of ET. The rate constants of intermolecular photoinduced ET (k_ct) from D to the singlet excited state (¹MeFl) and the triplet excited state (³MeFl) were determined by fluorescence quenching and enhanced decay rates of triplet-triplet (T-T) absorption by the presence of D, respectively. The k_ct 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 MeFr to D⁺n PhCN. In contrast to the case of k_ct, the driving force dependence of K_bets a pronounced decrease towards the highly exothermic region. The reorganization energy (A) of intermolecular BET is determined to be 0.68 eV by applying the Marcus equation in the inverted region, where the /chel value decreases with increasing the BET driving force. The slowest BET was observed for BET from MeFP" to N,N-dimethylaniline radical cation (DMA^.+) with the K_bet value of 3.5xl0⁶M^-1s^-1, which is 1600 times smaller than the diffusion rate constant in PhCN (5.6×M^-1S ^-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 F1 moiety to form the charge-separated (CS) state (DMA^,+-F1^.-) that has an extremely long lifetime (2.1 ms) in PhCN at 298 K.