The “phantom” photochemical singlet state of stilbene and its diphenylpolyene relative

Recently the conception on the cis-trans photoisomerization of stilbene has emerged that this process is governed by a higher excited ¹A_g state which exhibits a minimum at the perpendicular conformation crossing the lowest excited ¹B_u state upon bond rotation. An evaluation of the potential surfaces governing the photoisomerization process by a PPP SCF CI method revealed that there exists indeed such a photochemically active ¹A_g state which in the planar molecule lies about 1 eV above the lowest absorption band and involves the excitations of two electrons from the SCF ground state. Extensions of the calculations to diphenylpolyenes demonstrate that this ¹A_g state is related to the forbidden low-lying doubly excited ¹A_g state observed earlier in these molecules.