PHOTOISOMERIZATION DYNAMICS OF 3,3'-DIETHYL OXACARBOCYANINE - INTRAMOLECULAR AND SOLVENT VISCOSITY EFFECTS

The dynamics of photoisomerization and of ground-state back isomerization of 3,3'-diethyl oxacarbocyanine (DOC) in linear alcohols are analyzed. Intramolecular activation energies are obtained and compared with those of the corresponding dicarbocyanine (DODCI) in an attempt at establishing the consequence of the polymethine chain lengthening on the isomerization dynamics. The activation energy for the photoisomerization of DOC is slightly lower than that of DODCI, whereas the reverse is found for the ground-state back isomerization. The coupling of the internal twisting coordinate with solvent drag is analyzed within the framework of the hydrodynamic Kramers model. Although this approach does not provide a fully satisfactory description of the experimental points, it has been taken as a reference model: the parameters obtained from the data fitting have been assigned a semi-quantitative significance and a number of possible reasons of breakdown of the hydrodynamic Kramers description have been considered and qualitatively discussed.