| Abstract
| - Photoexcited cis-stilbene decays to theground state on the femtosecond time scale, so fast thatfluorescenceis not generally observed and vibrational relaxation is incomplete.Decay can be accompanied by cis-transisomerization about the double bond linking the two benzene rings andby electronic rearrangement to yield4a,4b-dihydrophenanthrene (DHP). MMVB calculations suggestthat there are several geometries (in additionto the perpendicular minimum) at which efficient decay can take place.In particular, a conical intersectionhas been documented along photocyclization reaction coordinate to DHPwhich may be reached with almostno barrier from a quinoid minimum. This intersection arises frominteraction between the two benzene rings.Such a crossing is suggested in the recent experimental work ofYoshihara et al. (Petek, H.; Yoshihara, K.;Fujiwara, Y.; Lin, Z.; Penn, J. H.; Frederick, J. H. J. Phys.Chem.1990, 94, 7539−7543) and must formpartof a new mechanism for the decay of this well-studiedmolecule.
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