Electron transfer reactions induced by the triplet state of thiacarbocyanine dimers

Abstract

Abstract The photoinduced electron transfer between either cationic 5,5 ′ -dichloro-3,3 ′ ,9-triethylthiacarbocyanine ( 1 ) or a structurally similar anionic dye ( 2 ) and appropriate donors, e.g. ascorbic acid, and acceptors, e.g. methyl viologen, was studied by ns-laser photolysis. In aqueous solution the dyes in the ground state are present as an equilibrated mixture of dimers and monomers, whereas the triplet state is mainly populated from dimers. The triplet states of both dimers and monomers are quenched by electron donors or acceptors and the rate constant for quenching is generally 2–4 times higher for dimers than for monomers. The kinetics of triplet decay and radical formation and decay as a result of primary and secondary electron transfer were analyzed. While the one-electron reduced dimer decays due to back reactions, the one-electron oxidized dimer rapidly dissociates into the monomer and the monomeric dye radical. For the dimeric dye/donor/acceptor systems the primary photoinduced electron transfer occurs either from the donor or to the acceptor yielding the dimeric dye radicals. The one-electron reduced dimer can be efficiently oxidized by acceptors, e.g. the rate constant for reaction of the dimeric dye radical of 1 with methyl viologen (photoreductive pathway of sensitization) is 1.6×10 9 M −1 s −1 . The photooxidative pathway of sensitization is more complicated; after dissociation of the dimeric dye radical, the monomeric dye radical is reduced in a secondary electron transfer from ascorbic acid, e.g. with a rate constant of 1×10 9 M −1 s −1 for 2 , yielding the monomer. On increasing the donor concentration the photooxidative pathway of sensitization is switched to a photoreductive one.