Solvent influence on absorption and fluorescence spectra of merocyanine dyes: a theoretical and experimental study

The solvaton-CS INDO model, previously successfully used to describe the solvatochromic properties of merocyanines, has been extended to the study of the solvent influence on the fluorescence spectra (fluorosolvatochromism) of these dyes. A ketocyanine (M1) and a stilbazolium betaine (M2) were chosen as representatives of positively and negatively solvatochromic behaviours, respectively. The gap of experimental knowledge concerning the emission properties of M2 was filled by a spectrofluorometric analysis in a set of solvents covering a large range of the E-T(30) scale. Solvato- and fluorosolvatochromism were described by calculating the S-0 (eq.) --> S-1 (Franck-Condon) and S-1 (eq.) --> S-0 (Franck-Condon) transition energies as a function of a polarity factor related to the static dielectric constant of the solvent, and ranging from 0 to 1. The absorbing S-0 (eq.) and emitting S-1 (eq.) units (solute molecule + solvent cage) were approximated using the S-0 and S-1 geometries of the unsolvated molecule and the respective charge distributions fitted to the current value of k(epsilon). The calculation results fully confirm that S-0 and S-1 states of merocyanines can be viewed as a mixture of a neutral and a zwitterionic structure whose composition is controlled by the solvent polarity. The plots of the calculated spectral data (absorption and emission maxima and corresponding Stokes shifts) vs k(epsilon) are in fairly good agreement with those of the experimental data over almost the entire range of the normalized E-T(N) values, thus showing that specific solvent interactions are at least partly simulated within the solvaton-CS INDO scheme. The methodological prerequisites for a correct prediction of solvatochromic shifts are recalled with reference to previous conflicting theoretical interpretations.