Absorption, fluorescence, and fluorescence excitation spectra have bee
n studied for a number of anionic, cationic, and cationic-anionic poly
methine dyes in low-polar and nonpolar solvents, as well as in binary
mixtures of solvents differing in polarity. For most of the dyes studi
ed, fluorescent aggregates have been found to form. Their broad fluore
scence bands are located in the long-wave region with respect to those
of the initial dyes. The quantum yield of the aggregate fluorescence
is normally higher than that of the initial dyes. Fluorescence excitat
ion spectra of some cationic-anionic dyes in nonpolar solvents disagre
e with their absorption spectra because of contact and solvent-separat
ed ion pairs simultaneously present in the solution.