Triplet states of cyanine dyes and reactions of electron transfer with their participation

Abstract

The radiationless decay processes of the excited singlet states of thiacarbocyanines are considered. It was revealed that the quantum yield (Φ ST) of intersystem crossing is low (Φ ST ?? 0.1) for certain unsubstituted (in the meso position) thiacarbocyanines. The triplet level for unsubstituted thiacarbocyanines can be populated efficiently owing to triplet—triplet energy transfer. The values of Φ ST are increased markedly for 9-CH 3O- and 9-NH 2-thiacarbocyanines where T—T absorption is observed on direct flash excitation. Using triplet—triplet energy transfer it was established that cis—trans isomerization for certain unsubstituted thiacarbocyanines occurs via a singlet excited state. For unsubstituted thiacarbocyanines the main pathways of the singlet excited states decay are cis—trans isomerization and internal conversion. The main processes of the triplet state decay are radiationless deactivation, quenching by impurities ( e.g. traces of oxygen), triplet—triplet annihilation and self-quenching. The values of rate constants were determined for certain thiacarbocyanines. Mixed triplet—triplet annihilation was established on sensitized excitation of thiacarbocyanines in the presence of anthracene (sensitizer). Triplet states of thiacarbocyanines play a major role in redox reactions. Experimental evidence favours the extreme participation of triplet states in electron transfer processes. Efficiency of triplet state quenching by certain electron donors (ascorbic acid, β-naphthylamine) and acceptors ( p-benzoquinone, methylviologen) was considered. In some cases quenching is accompanied by the production of ion-radicals. The oxidative and reductive mechanisms of sensitization are considered and the values of rate constants for some elementary steps with thiacarbocyanines triplet states involved are determined.