The singlet-state photophysics and photochemistry of polyenes: Applicationof the twin-state model and of the phase-change theorem

The A(2(1)A(g)) state of polyenes has been shown by Kohler and co-workers t o be of central importance for the understanding of polyene photophysics an d photochemistry. The twin-state model is used to provide a physical explan ation for the well-known frequency exaltation of the a(g) symmetric stretch mode frequency upon excitation of the molecule from the ground X(1(1)A(g)) state to the A(2(1)A(g),) state and for the increased stabilization of the planar form. The smaller members of the polyene series, ethylene and butad iene, are nonfluorescent, while higher members are. It is shown that the di rect (singlet) photochemistry of all polyenes can be largely accounted for by assuming that these two lowest lying A(g) states are connected by a coni cal intersection. The nature of the products and the stereochemical charact eristics of the photoreactions can be rationalized using the phase-change t heorem of Longuet-Higgins (Longuet-Higgins, H. C. Proc. R. Sec. London A 19 75, 344, 147). A general procedure for locating the conical intersections a nd their associated products is suggested.