CIS-STILBENE PHOTOCHEMISTRY - SOLVENT DEPENDENCE OF THE INITIAL DYNAMICS AND QUANTUM YIELDS

Photochemical quantum yields for the formation of trans-stilbene and 4a,4b-dihydrophenanthrene from cis-stilbene have been measured in cyclohexane, hexane, acetonitrile, and methanol solvents. Degassed cis-stilbene samples at 27-degrees-C were irradiated at 280 nm, and the formation of photoproducts was monitored by absorption spectroscopy. The quantum yield for formation of dihydrophenanthrene is two to three times greater in the nonpolar solvents than in the polar ones (15-19% vs 5-8%), while a slight solvent dependence of the yield of trans-stilbene is observed (35-39% in the polar solvents vs 32-35% in the hydrocarbons). Ground-state resonance Raman spectra of cis-stilbene have also been obtained with 266-nm excitation in each of the four solvents in order to probe the initial dynamics of nuclear motion in the Franck-Condon region. The intensities suggest very slightly faster distortion of the vertically excited state along out-of-plane and hydrogen wagging and rocking coordinates in polar relative to nonpolar solvents. The implications for the dynamics of the cis-trans isomerization and ring closure reactions are discussed.