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 photoinduce d isomerization, k(iso), in a stilbene derivative in which the single bond torsion is hindered by the introduction of CH2 bridges. The results obtaine d with n-hexane and methylcyclohexane as solvents (0.2 mPa s ( eta) < 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 relat ionship k(iso) = C.(eta/mPa s)(-a) . exp( -E-A/RT). The value for a drops f rom 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 methylcyclo hexane, respectively. (C) 1999 Elsevier Science B.V. All rights reserved.