PHOTOCHEMISTRY OF AN UNSYMMETRICAL POLYMETHINE-CYANINE DYE - SOLUTE SOLVENT INTERACTIONS AND RELAXATION DYNAMICS OF LDS-751

The photophysical and photochemical properties of the LDS 751 or styryl 8 molecule have been investigated in various solvents. Our results indicate that the photostability of this laser dye is mostly due to an efficient S1 --> S0 non-radiative internal conversion process which is faster than the radiative process. This radiationless relaxation is governed by a viscosity-dependent process, tentatively assigned to the free rotor effect between the polymethinic chain and the end groups due to the single bond character of the dominant benzenoid structure. Moreover, intersystem crossing to the triplet state and trans-cis photoisomerization are totally inefficient. The shape (relatively small molar extinction coefficient and large bandwidth) and the blue shift of the absorption spectrum in polar solvents show that this unsymmetrical polymethine-cyanine dye is highly polar in the ground state with the cationic positive charge mainly localized on the benzothiazole nitrogen and inducing a large orientational polarization of the surrounding solvent molecules. The absorption transition involves a much less polar Franck-Condon excited state and a charge transfer from the dimethylanilino nucleus to the benzothiazole nucleus. The shape, of the fluorescence spectrum, similar to that of symmetrical polymethine-cyanines and almost independent of solvent polarity, confirms the unpolar character of the first singlet excited state. The sub-picosecond analysis of the time-dependent fluorescence Stokes shift thus probes the relaxation of solvent molecules freed from solute-solvent interactions after the Franck-Condon absorption transition.