DYNAMIC SOLVATION EFFECTS ON THE CIS-TRANS ISOMERIZATION REACTION - PHOTOISOMERIZATION OF MEROCYANINE-540 IN POLAR-SOLVENTS

The fluorescence quantum yield, PHI(f), of merocyanine 540 in n-alkyl alcohol and n-alkanenitrile solvents was measured as a function of temperature and solvent shear viscosity at ambient pressure. The Strickler-Berg equation was used to calculate the radiative rate, k(r), from the absorption and fluorescence spectra. The values of PHI(f)and k(r) were used to obtain the rate of photoisomerization, k(iso). This rate is consistent with the Smoluchowski limit of the Kramers theory for activated barrier crossing. The intrinsic barrier height, which is taken to be equal to the difference between the single-solvent activation energy and the viscosity activation energy, is lower in alcohols than in nitriles. The lower barrier can be partly rationalized in terms of static solvation effects which stabilize the twisted state of the dye. Because of the dye's zwitterionic resonance structures, hydrogen bonding plays a key role in stabilizing the twisted state in alcohols. The intrinsic barrier depends weakly on the solvent for the nitriles, but it decreases dramatically in going from lower to higher alcohols. The behavior of the intrinsic barrier in these solvents is attributed to dynamical solvation effects which couple the solvent polarization to the intramolecular motion.