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

Fluorescence life-time measurements were performed at elevated pressure and temperature to examine the viscosity dependence of the rate of photoinduced isomerization, kiso, in a stilbene derivative in which the single bond torsion is hindered by the introduction of CH2 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 Kramersʹ expression. A good global fit was, however, achieved when applying the empirical relationship kiso=C·(η/mPa s)−a·exp(−EA/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 EA are about 12.8 and 12.5 kJ mol−1 in n-hexane and methylcyclohexane, respectively.