TRANS CIS PHOTOISOMERIZATION MECHANISM OF CARBOCYANINES - EXPERIMENTAL CHECK OF THEORETICAL-MODELS

Two different theoretical models for the cyanine photoisomerization, emphasizing, respectively, the role of the solvent effects (model a) and that of the intramolecular forces (model b), have been experimentally tested by investigating the photophysical and photochemical behaviour of two carbocyanines (DOC and DTC). The decay kinetics of the excited-state (Sl) stable isomer and of the ground-state (S0) unstable photoisomer have been measured in solvents of very different polarities, but similar and low viscosities (methanol, methylene chloride and chlorobenzene were extensively investigated, but some test measurements were also carried out in p-dioxane and in 1:1 toluene-chlorobenzene). The activation energy for the non radiative decay of the spectroscopic S(l) minimum, corresponding to a twisting around one of the polymethine bonds, is independent of solvent polarity, whereas a small but significant dependence is displayed by the activation energy and preexponential factor of the S(O) back isomerization. These results can be easily accounted for within model b, while they sharply contrast with the predictions of model a. We have also measured the quantum yields of photoisomer formation for DOC and DTC at various temperatures and have shown that, in both cases, radiationless decay occurs from the perp S(l) minimum to ground state, with a branching ratio favouring reformation of the stable (trans) isomer. The parallel temperature dependence of the fluorescence decay and photoisomer formation has provided further support to model b. Finally, the temperature dependence of the quantum yield for the cis --> trans photoisomerization of DOC has been analyzed. A very low activation energy has been found suggesting that this process is probably intrinsically barrierless.