Polarization of cis-trans photoisomerization funnels, size and suddenness

Size and suddenness of the polarization effect in the lowest lying excited states (S1, S2) 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 (ψ21) and the open shell (ψ1ψ −1) configurations (ψ1 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) S1 and S2 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 S1 and S2 states on either side of the perpendicular configuration, at the points where avoided crossings of the two closely lying S1 andS2 potential surfaces occur.