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

The A(2¹A_g) state of polyenes has been shown by Kohler and co-workers to be of central importance for the understanding of polyene photophysics and photochemistry. The twin-state model is used to provide a physical explanation for the well-known frequency exaltation of the a_g symmetric stretch mode frequency upon excitation of the molecule from the ground X(1¹A_g) state to the A(2₁A_g) state and for the increased stabilization of the planar form. The smaller members of the polyene series, ethylene and butadiene, are nonfluorescent, while higher members are. It is shown that the direct (singlet) photochemistry of all polyenes can be largely accounted for by assuming that these two lowest lying A_g states are connected by a conical intersection. The nature of the products and the stereochemical characteristics of the photoreactions can be rationalized using the phase-change theorem of Longuet-Higgins (Longuet-Higgins, H. C. Proc. R. Soc. London A 1975, 344, 147). A general procedure for locating the conical intersections and their associated products is suggested.