The effects of "inert" counterions on some photochemical processes involving cationic polymethine dyes have been studied in ion pairs formed in low-polar media. In particular, the counterion effects on the back isomerization kinetics of photoisomers of benzimidacyanine and indotricarbocyanine dyes and on the triplet state decay of the benzimidacyanine dyes have been investigated by flash photolysis. The increase in the rate constant and decrease in the activation energy of back isomerization of benzimidacyanines are observed when dye-counterion ion pairs are formed in low-polar media, whereas the increase in the activation energy is observed for indocyanines under the same conditions. For benzimidacarbocyanine with the Icounterion, ion pair formation gives rise to an increase in the decay rate constant of its triplet state due to interaction of the dye cation with the 1" heavy atom; meanwhile, there is no appreciable rise in Si 1-T intersystem crossing. The enhancement of the Si w*-So internal conversion in ion pairs has been revealed for all benzimidacyanines with the I" counterion.
Spectral and fluorescent properties of ketocyanine dyes (polyenic bis-ω,ω′-aminoketones) and cation-anionic polymethine dyes of various structures were studied. The symmetric ketocyanines were shown to have a long-wavelength absorption band bathochromically shifted in comparison with that of the asymmetric ketocyanines with the same total length of the polyenic chain. The nonlinear ketocyanines exhibit the additional short-wavelength band in their absorption spectra, which can be more intense than the longwavelength band. The absorption spectra of ion pairs of cation-anionic dyes with overlapping cation and anion bands contain a new intense short-wavelength band inactive in fluorescence excitation. These spectral peculiarities are explained on the basis of chromophore interaction model. It has also been shown that theT1levels of ketocyanine chromophores do not essentially interact with each other in a ketocyanine molecule in nonpolar solvents; in polar solvents this interaction becomes appreciable due to lowering the potential barrier for conjugation.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.