Polarization of cis-trans photoisomerization funnels, size and suddenness

Abstract

Size and suddenness of the polarization effect in the lowest lying excited states (S 1, S 2) of twisted polyenes were theoretically investigated at the INDO level of approximation.The simple 3 × 3 CI scheme of Salem and Rowland was first reconsidered and the effects of more extended CI's (selected with references to the basic three-configuration wavefunctions) were then analysed by using MO's optimized for both the closed shell (ψ 2 1) and the open shell (ψ 1ψ −1) configurations (ψ 1 and ψ −1 being the two almost degenerate frontier orbitals). Our results indicate that: (i) introduction of not very large, but properly selected, CI is generally advisable in order that “artifacts” inherent in the adopted monoconfigurational SCF techniques (and/or symmetry constraints) be avoided, (ii) in strongly dissymmetric systems (e.g. terminally-twisted polyenes) S 1 and S 2 states are characterized by broad well-shaped dipole moment peaks, whose maxima range from ≈3 D (in dienes) to an upper limit of ≈6 D (in larger polyenes), (iii) in slightly dissymmetric systems (e.g. “centrally” twisted polyenes) (at least) two sharp dipole moment maxima (not excessively high in value) are likely to be present in S 1 and S 2 states on either side of the perpendicular configuration, at the points where avoided crossings of the two closely lying S 1 andS 2 potential surfaces occur.