Thermal isomerization of a symmetrical carbocyanine molecule: charge transfer aspects

The thermal back isomerization of 5,5′,6,6′-tetrachloro-1,1′,3,3′-tetraethylbenzimidacarbocyanine cis photoisomers studied by nanosecond laser flash photolysis is reported in polar solvents and at different temperatures. The data show the formation of a cis isomer after laser excitation, which disappears afterwards in the microsecond timescale with first-order kinetics, forming again the trans conformer. The reaction rate constant does not depend on the solvent viscosity, but exhibits strong polarity dependence. Accordingly, the activation energy exhibits a parabolic polarity dependence, which is attributed to the charge transfer nature of the transition state. Such effect may be enhanced by stereochemical hindrance in the cis photoisomer.