COOPERATING RINGS IN CIS-STILBENE LEAD TO AN S-0 S-1 CONICAL INTERSECTION/

Photoexcited cis-stilbene decays to the ground state on the femtosecon d time scale, so fast that fluorescence is not generally observed and vibrational relaxation is incomplete. Decay can be accompanied by cis- trans isomerization about the double bond linking the two benzene ring s and by electronic rearrangement to yield 4a,4b-dihydrophenanthrene ( DHP). MMVB calculations suggest that there are several geometries (in addition to the perpendicular minimum) at which efficient decay can ta ke place. In particular, a conical intersection has been documented al ong photocyclization reaction coordinate to DHP which may be reached w ith almost no barrier from a quinoid minimum. This intersection arises from interaction between the two benzene rings. Such a crossing is su ggested in the recent experimental work of Yoshihara et al. (Petek, Il .; Yoshihara, K.; Fujiwara, Y.; Lin, Z.; Penn, J. H.; Frederick, J. H. J. Phys. Chem. 1990, 94, 7539-7543) and must form part of a new mecha nism for the decay of this well-studied molecule.