TEMPERATURE-DEPENDENCE OF FLUORESCENCE AND PHOTOISOMERIZATION IN SYMMETRICAL CARBOCYANINES - INFLUENCE OF MEDIUM VISCOSITY AND MOLECULAR-STRUCTURE

The temperature dependence of fluorescence emission and photoisomeriza tion of five symmetric carbocyanines was studied in a group of primary n-alcohols from 0 to 70-degrees-C. The trans-cis isomerization on the excited state potential surface was studied by steady-state fluoresce nce emission and flash photolysis. The back isomerization rate constan t on the ground-state potential surface from the photoisomer to the no rmal form was also studied by flash photolysis. In all cases fluoresce nce quantum yields were found to diminish with temperature and to incr ease with viscosity (eta), while isomerization quantum yields showed t he opposite behavior. All deactivation rate constants of the excited s inglet state were calculated, and activation energies for the processe s of excited and ground states were obtained. The solvent does not aff ect radiative and internal conversion rate constants of the excited si nglet state. The only effect of the solvent on the isomerization rates (k(iso)) of both processes is through eta. Results indicate that the dependence of k(iso) on eta is k(iso) = f(eta)exp(-E0/RT), where E0 wa s found to be solvent independent. Fits of f(eta) by the Kramers equat ion (Physica 1940, 7, 284) and by an activation-volume-based model wer e performed. Kramers's equation provides a good fit of the data for th e carbocyanines with the smallest E0, but for other cases it shows sys tematic deviations already reported for other carbocyanines, which are discussed on the basis of the hypothesis involved in the model. The a ctivation-volume-based model fits very well the data in all cases. A c omparison is made between excited- and ground-state isomerizations. Th e influence of structural properties of the dyes on the photophysical parameters is also discussed.