Abstract:

The temperature dependence of fluorescence emission and photoisomerization of five symmetric carbocyanines was studied in a group of primary n-alcohols from 0 to 70°C. The trans-cis isomerization on the excited state potential surface was studied by steady-state fluorescence emission and flash photolysis. The back isomerization rate constant on the ground-state potential surface from the photoisomer to the normal form was also studied by flash photolysis. In all cases fluorescence quantum yields were found to diminish with temperature and to increase with viscosity (η), while isomerization quantum yields showed the opposite behavior. All deactivation rate constants of the excited singlet state were calculated, and activation energies for the processes of excited and ground states were obtained. The solvent does not affect radiative and internal conversion rate constants of the excited singlet state. The only effect of the solvent on the isomerization rates (kiso) of both processes is through η. Results indicate that the dependence of kiso on η is kiso = f(η) exp(-E0/RT), where E0 was found to be solvent independent. Fits f(η) by the Kramers equation (Physica 1940, 7, 284) and by an activation-volume-based model were performed. Kramers's equation provides a good fit of the data for the carbocyanines with the smallest E0, but for other cases it shows systematic deviations already reported for other carbocyanines, which are discussed on the basis of the hypothesis involved in the model. The activation-volume-based model fits very well the data in all cases. A comparison is made between excited- and ground-state isomerizations. The influence of structural properties of the dyes on the photophysical parameters is also discussed. © 1994 American Chemical Society.