Electronic structure and spectra of finite linear polyenes

A brief review of the current status of our knowledge of the excited electronic states of finite linear polyene chains is presented. The main conclusion of recent studies is that the neutral molecular species with an even number of carbons greater than or equal to six have a state of A_g symmetry as their lowest excited singlet state. Theoretical studies lead to the conclusion that the proper description of this excited state requires inclusion of electronic configurations involving two or more excitations. The low energy of this state is due to extensive electron correlation in these molecules. This excited A_g state gives rise to transitions that are electric dipole forbidden from the ground state which has the same symmetry. The ordinary absorption spectrum is dominated by the very strong transition to the B_u singlet state, a state well described as a one-electron excitation corresponding to the band gap for the infinite chain. This lack of intensity for the lowest transition and the good agreement obtained between predicted and observed spectra led to the long held, but erroneous, conclusion that the states of these linear chains were well described by simple one-electron theories.