INFLUENCE OF INERT COUNTERIONS ON THE PHOTOCHEMISTRY OF SOME CATIONICPOLYMETHINE DYES

The effects of ''inert'' counterions on some photochemical processes i nvolving cationic polymethine dyes have been studied in ion pairs form ed in low-polar media. In particular, the counterion effects on the ba ck isomerization kinetics of photoisomers of benzimidacyanine and indo tricarbocyanine dyes and on the triplet state decay of the benzimidacy anine dyes have been investigated by flash photolysis. The increase in the rate constant and decrease in the activation energy of back isome rization of benzimidacyanines are observed when dye-counterion ion pai rs are formed in low-polar media, whereas the increase in the activati on energy is observed for indocyanines under the same conditions. For benzimidacarbocyanine with the I- counterion, ion pair formation gives rise to an increase in the decay rate constant of its triplet state d ue to interaction of the dye cation with the I- heavy atom; meanwhile, there is no appreciable rise in S-1 --> T intersystem crossing. The e nhancement of the S-1 --> S-0 internal conversion in ion pairs has bee n revealed for all benzimidacyanines with the I- counterion.