Electronic structure and spectral-fluorescent properties of laser dyes thiopyrylo-4-tricarbocyanines

It was 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 practically does not change in a wide range of solvent polarities. Based on the quantum-chemical calculations of these dyes by the ab initio DFT/B3LYP/6-31G (d,p) and TDDFT methods, taking into account the polarity of the medium by the PCM method, it is shown 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 first than in the latter. An increase in the alternation of bond orders in the polymethine chain in the fluorescent state was found, which causes an increase of vibronic interactions in the radiative transition as compared to the absorptive one. Spectral effects caused by a change in the angle of rotation of phenyl groups in the thiopyrylium cycle upon excitation have been analyzed.