Aggregation of cation-anionic and related polymethine dyes

摘要

Absorption, fluorescence, and fluorescence excitation spectra were studied for a number ofcation-anionic and related anionic polymethine dyes in weakly polarand nonpolarsolvents, as well asin binary mixtures of solvents of different polarity. For some dyes, aggregation is observed in toluene oracetonitrile-toluene mixtures with low amounts of acetonitrile, which is revealed as appearance of new absorptionbands and/or broadening of the initial bands of a monomeric dye. Solvent mixtures butyronitrilehexanewith low butyronitrile content were found to greatly stimulate the formation of dye aggregates formost of the dyes studied. The absorption spectra of the aggregates are often blue-shifted with respectto the corresponding absorption spectra of parent monomeric dyes and/or represent broad continuumslocated both in the blue and red regions. For one of the cation-anionic dyes studied, which consists of3,3′-diethylthiamonomethinecyanine cation and trimethinebenzoxanine anion, fluorescent aggregates wereobserved; their broad fluorescence band is located in the long-wavelength region. For this dye, gradual transitionfrom nonfluorescent aggregates to fluorescent ones and then to monomeric ion pairs and dissociatedions was observed in butyronitrile-hexane mixtures with growing butyronitrile content.