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Inter‐ and Intramolecular Photoinduced Electron Transfer of Flavin Derivatives with Extremely Small Reorganization Energies
Author(s) -
Murakami Motonobu,
Ohkubo Kei,
Fukuzumi Shunichi
Publication year - 2010
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200903236
Subject(s) - chemistry , intermolecular force , marcus theory , photochemistry , reaction rate constant , flash photolysis , electron transfer , quenching (fluorescence) , singlet state , excited state , intramolecular force , atomic physics , fluorescence , stereochemistry , kinetics , molecule , organic chemistry , physics , quantum mechanics
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 .− to D .+ 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 k et 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 .− to D .+ in PhCN. In contrast to the case of k et , the driving force dependence of k bet shows a pronounced decrease towards the highly exothermic region. The reorganization energy ( λ ) of intermolecular BET is determined to be 0.68 eV by applying the Marcus equation in the inverted region, where the k bet value decreases with increasing the BET driving force. The slowest BET was observed for BET from MeFl .− to N , N ‐dimethylaniline radical cation (DMA .+ ) with the k bet value of 3.5×10 6 M −1 s −1 , which is 1600 times smaller than the diffusion rate constant in PhCN (5.6×10 9 M −1 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 .+ –Fl .− ) that has an extremely long lifetime (2.1 ms) in PhCN at 298 K.