The emission of α,ω-diphenylpolyenes: A model involving several molecular structures

Available photophysical evidence for the emission of α,ω-diphenylpolyenes is shown to be consistent with a previously reported model [J. Catalan, J.L.G. de Paz, J. Chem. Phys. 124 (2006) 034306] involving two electronically excited molecular structures of 1Bu and Cs symmetry, respectively. The 1Bu structure is produced by direct light absorption from the all-trans form of the α,ω-diphenylpolyene in the ground state and its emission exhibits mirror symmetry with respect to the absorption of the compound. On the other hand, the Cs structure is generated from the 1Bu structure of the α,ω-diphenylpolyene by rotation about a C-C single bond in the polyene chain, its emission being red-shifted with respect to the previous one and exhibiting markedly decreased vibrational structure. At room temperature, both emissions give the excitation spectrum, which are ascribed to the first absorption band for the compound. It is shown that some polyenes may exist in more than one structure of Cs symmetry in the excited electronic state with lower energy than that of the 1Bu state, from which the Cs structures are produced. Hence, more than one electronic structure may be involved in the deactivation processes of the 1Bu state, which is initially populated upon photo-excitation of the polyene molecule in the ground electronic state