Influence of solvent viscosity on the photoisomerization of a novel trans-stilbene derivative with hindered single bond torsion

Abstract

Fluorescence life-time measurements were performed at elevated pressure and temperature to examine the viscosity dependence of the rate of photoinduced isomerization, k iso, in a stilbene derivative in which the single bond torsion is hindered by the introduction of CH 2 bridges. The results obtained with n-hexane and methylcyclohexane as solvents (0.2 mPa s< η<2 mPa s) could not be fitted satisfactorily on the basis of Kramers' expression. A good global fit was, however, achieved when applying the empirical relationship k iso= C·( η/mPa s) −a·exp(− E A/ RT). The value for a drops from about 0.30 in n-hexane to about 0.15 in methylcyclohexane thus pointing to solvent specific contributions to the friction. The derived activation energies E A are about 12.8 and 12.5 kJ mol −1 in n-hexane and methylcyclohexane, respectively.