IS THE NONRADIATIVE DECAY OF S-1 CIS-STILBENE DUE TO THE DIHYDROPHENANTHRENE ISOMERIZATION CHANNEL - SUGGESTIVE EVIDENCE FROM PHOTOPHYSICAL MEASUREMENTS ON 1,2-DIPHENYLCYCLOALKENES

Photophysical properties of cis-stilbene and several homologues are investigated in supersonic expansions as free molecules and in large Ar clusters. Assignment of sharp vibrational structure in the spectra of 1,2-diphenylcyclobutene to long progressions in phenyl twisting and bending vibrations shows that in the S1 state cis-stilbene phenyl twist angles are more planar and the phenyl-ethylene bend angles are more bent than in the S0 state. The spectra do not show a large change in the ethylene torsional angle. On the basis of the assignment of the 1,2-diphenylcyclobutene spectra and the similarity between the nonradiative decay of cis-stilbene and 1,2-diphenylcyclopentene, a new mechanism is proposed for the nonradiative decay of the S1 state of cis-stilbene by both the cis-trans and photocyclization to dihydrophenanthrene isomerization channels. It is proposed that the initial motion of a wave packet excited to the S1 surface of cis-stilbene is along the reaction coordinate for photocyclization to dihydrophenanthrene rather than along the cis-trans isomerization coordinate. © 1990 American Chemical Society.