As a result of mathematical analysis, using the method of moments, of the electronic spectra of symmetrical cyanine dyes based on indolenine, the following observations were noted. (1) Loss of mirror symmetry of the absorption and fluorescence bands caused by narrowing of the latter. These effects increase with increase of the solvent nucleophilicity and with lengthening of the polymethine chain of the dyes. (2) Decrease of solvatofluorochromic effects in comparison with solvatochromic effects; this is especially apparent in the change of the bandwidth;. (3) An excellent correlation between the mean frequency of the fluorescence band and the solvent refractive index function (Bayliss) (this correlation is absent in absorption). On the basis of the quantum-mechanical calculations of the cationic dyes, the relationships discovered were attributed to the weakening of specific electrostatic interactions (nucleophilic salvation) in the excited state of the dyes due to a decrease of charge values on their cations upon excitation. © 1989.
Ishchenko, A. A., Svidro, V. A., & Derevyanko, N. A. (1989). Solvatofluorochromy of cationic cyanine dyes. Dyes and Pigments, 10(2), 85–96. https://doi.org/10.1016/0143-7208(89)85001-6