SUDDEN POLARIZATION IN INTERACTING MODEL PI-SYSTEMS - AN EXACT STUDY

The twisted excited state of polyenes has attracted much theoretical and experimental attention due to the postulate that it exhibits sudden polarization for twist angle θ= 90°. We examine this phenomenon in long chain polyenes using correlated model Hamiltonians. While the Hubbard model is employed to study this process as the correlations are gradually turned on, the Pariser-Parr-Pople (PPP) Hamiltonian is employed for a realistic study of long chain polyenes. Exact polarizabilities of the ground and excited states of polyenes with upto ten carbon atoms are calculated within the Hubbard and PPP models as a function of twist angle. We find that the dipole allowed excited states exhibit sudden polarization in all the cases. Besides, the 3 1Ag state in the covalent subspace which is a two-photon state also exhibits this phenomenon as it becomes degenerate with the dipole allowed excited state. It is suggested that photochemical processes involving zwitterionic intermediates should be possible through two-photon excitation to this covalent excited state. © 1990.