ULTRAFAST STUDIES OF SOLVENT EFFECTS IN THE ISOMERIZATION OF CIS-STILBENE

The ultrafast dynamic of cis-stilbene in solution are studied using a femtosecond pump-probe laser system. Excited-state lifetimes in n-hexane, tetradecane, methanol, 1,4-dioxane, and cyclohexane are examined following excitation at 312 nm. In addition, spectra of excited cis-stilbene within 500 fs of excitation are recorded using an optical multichannel analyzer (OMA). The lifetimes observed in n-hexane and tetradecane are in excellent agreement with literature values and correlate well with solvent bulk viscosity. The measured lifetimes in methanol, 1,4-dioxane, and cyclohexane do not correlate with solvent bulk viscosity. The lifetime in methanol is 0.50 +/- 0.03 ps, only slightly longer than the gas-phase value of 0.32 +/- 0.02 ps (Greene, B. I.; Farrow, R. C. J. Chem. Phys. 1983, 78, 3336-3338). The lifetime in 1,4-dioxane is similar to that of an alcohol of the same bulk viscosity. The lifetime in cyclohexane is much longer than lifetimes in linear alkanes of comparable viscosity. An examination of several probe wavelengths between 490 and 690 nm in several solvents reveals little or no wavelength dependence of the lifetime with the possible exception of cyclohexane. The cis-stilbene lifetime data in methanol and 1,4-dioxane suggests that microscopic solvent-solute interactions based on polarity are present and are larger than the bulk viscosity dependencies. The relatively slow cis- to trans-stilbene isomerization rate in cyclohexane may indicate a steric hindrance to isomerization.