Cooperating Rings in cis-Stilbene Lead to an S0/S1 Conical Intersection

Photoexcited cis-stilbene decays to the ground state on the femtosecond 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 rings 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 take place. In particular, a conical intersection has been documented along photocyclization reaction coordinate to DHP which may be reached with almost no barrier from a quinoid minimum. This intersection arises from interaction between the two benzene rings. Such a crossing is suggested 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 mechanism for the decay of this well-studied molecule.