A Mechanistic Study of Photoinduced Electron Transfer from Triplet Erythrosin to Various Quinones Using Time Resolved Absorption and ESR-CIDEP Measurements

Abstract

Photoinduced electron transfer reactions from the triplet erythrosin (Ery 2− ) dianion to various quinones (1,4-benzoquinone, 2,5-di-t-butyl-1,4-benzoquinone, duroquinone, 2,5-dichloro-1,4-benzoquinone, chloranil, bromanil, 2,3-dichloro-5,6-dicyano-1,4-benzoquinone and 9,10-anthraquinone) and other organic acceptors like 7,7,8,8-tetracyanoquinodimethane, 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane and with tetracyano-1,4-benzoquinone (cyanil), one of the strongest oxidizing agent reported in literature ( ∘ = 0.90 V vs. SCE), were studied by laser flash photolysis in acetonitrile/water mixtures at room temperature. Quenching rate constants are obtained from Stern-Volmer plots. The measured bimolecular quenching rate constants are close to the diffusion controlled rates. Excitation of a contact radical-pair or a triplet exciplex as an intermediate in the photoinduced electron transfer reaction in a slightly polar solvent (1-propanol) is confirmed by the observation of the net absorptive chemically induced dynamic electron polarization (CIDEP) spectra. The unusual net-absorptive CIDEP spectrum is explained by spin-orbit coupling interactions due to presence of four iodine atoms in the structure of erythrosin (heavy atom effect). The dependence of the electron transfer rate constants, et , on the driving force, Δ et is slightly parabolic and indicates the Marcus Inverted Region.