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

Photoinduced electron transfer reactions from the triplet erythrosin(Ery2−)dianion to various quinones (1,4-benzoquinone, 2,5-di-t-butyl-1,4-benzo-quinone, duroquinone, 2,5-dichloro-1,4-benzoquinone, chloranil, bromanil, 2,3-dichloro-5,6-dicyano-1,4-benzoquinone and 9,10-anthraquinone) and other or-ganic acceptors like 7,7,8,8-tetracyanoquinodimethane, 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane and with tetracyano-1,4-benzoquinone (cyanil), oneof the strongest oxidizing agent reported in literature (𝐸∘=0.90Vvs. SCE), werestudied by laser flash photolysis in acetonitrile/water mixtures at room temper-ature. Quenching rate constants are obtained from Stern-Volmer plots. The mea-sured bimolecular quenching rate constants are close to the diffusion controlledrates. Excitationof a contact radical-pairor a triplet exciplex asanintermediate inthephotoinducedelectrontransferreactioninaslightlypolarsolvent(1-propanol)is confirmed by the observation of the net absorptivechemically induced dynamicelectron polarization (CIDEP) spectra. The unusual net–absorptive CIDEP spec-trum is explained by spin-orbit coupling interactions due to presence of four io-dine atoms in the structure of erythrosin (heavy atom effect). The dependenceof the electron transfer rate constants,𝑘et,onthedrivingforce,Δ𝐺etis slightlyparabolic and indicates the Marcus Inverted Region.