Electronic Structure and Spectral-Fluorescent Properties of Thiopyrylo-4-Tricarbocyanine Laser Dyes

We found that the long-wavelength absorption band of the laser dye IR 1061 and its analogue with an unsubstituted polymethine chain is strongly broadened and decreases in intensity in polar solvents, while the fluorescence band remains narrow and does not change in a wide range of solvent polarities. Based on the quantum-chemical calculations performed for these dyes by the ab initio DFT/B3LYP/6-31G(d,p) method and TDDFT and taking into account the polarity of the medium, we demonstrated by the PCM method that the reason for this difference is the weakening of solvation in the fluorescent state as compared to the ground state due to the greater equalization of the charge in the former dye than in the latter one. An increase in the alternation of bond orders in the polymethine chain in the fluorescent state was found, which causes an increase in vibronic interactions in the radiative transition compared to the absorptive one. Spectral effects caused by changes in the angle of rotation of phenyl groups in the thiopyrylium cycle upon excitation have been analyzed.