High-pressure study of the effect of viscosity on fluorescence and photoisomerization of trans-stilbene

Abstract

The fluorescence quantum yield phi/sub f/ and the initial photoisomerization rates for trans-stilbene in several deoxygenated solvents were obtained as a function of solvent viscosity at room temperature by using pressures to 11 kbar to alter solvent viscosity. It was observed that in solvents of the same viscosity, phi/sub f/ is generally larger in hydrocarbons than in hydroxylic solvents. Fluorescence lifetimes at 1 atm were measured with a picosecond laser and streak camera allowing calculation of the radiative deactivation rate of the excited singlet state in several solvents. At high viscosities in paraffin oil and glycerol phi/sub f/ approaches a limiting value associated with the complete inhibition of molecular twist. Analysis of the data gives the rates of intersystem crossing (isc) and of viscosity-dependent deactivation via twist of the planar excited singlet. These parameters indicate that in nonviscous solvents such as n-hexane at room temperature, only about 2% of the planar excited singlet states undergo isc. The isc yield in glycerol at room temperature is calculated to be 21% implying significant isomerization via the triplet state in viscous solvents. 4 figures, 2 tables.