Pathways of electron transfer photosensitized by thiacyanine dimers

Pathways of electron transfer reaction between p-nitroacetophenone (p-NAP) and ascorbic acid (AA) photosensitized by dimers of 3,3'-disulfopropyl-5,5'-dichlorothiacyanine triethylammonium (TC) and 3,3'-disulfopropyl-5,5'-dichloro-9,11-[ββ-dimethyltrimethylene]thiadicarbocyanine triethylammonium (TDC) are considered. In aqueous solution the dyes are present as an equilibrated mixture of monomers (M(-)) and dimers (M(2)(2-)). In contrast to monomers, the dimers of TC are characterized by a noticeable yield of intersystem crossing, whereas for TDC the triplet-triplet absorption of both monomers and dimers is easily observed upon ns-laser pulse. In the presence of p-NAP and AA the triplet state of the dimers of both dyes is mostly quenched by p-NAP yielding the radical pair [M(2)(-)˙p-NAP(-)˙] with subsequent dissociation of M(2)(-)˙ into M(-) and M˙ followed by one-electron reduction of M˙ by AA. These steps constitute a pathway of photosensitization by the dimers. For TDC an additional pathway of photosensitization was found to occur. The primary step consists of electron transfer in the excited singlet state of the dimer resulting in the formation of the radicals M˙ and M(2-)˙. The next steps involve one-electron reduction of M˙ by AA and one-electron oxidation of M(2-)˙ by p-NAP which results in the formation of M(-) followed by dimerization.