Neutral and Charged Biradicals, Zwitterions, Funnels in S1, and Proton Translocation: Their Role in Photochemistry, Photophysics, and Vision

Abstract

A knowledge of the geometries at which excited molecules return to the electronic ground state (S 0) is essential for the understanding of the structures of photoproducts. Particularly good candidates are geometries corresponding to local minima on the S 1 (lowest excited singlet) and T 1(lowest triplet) surfaces, as well as S 0–S 1 conical intersections (funnels). Given sufficient effort, such geometries can nowadays be found numerically for small enough molecules. Still, it is interesting to ask whether more approximate, but also more general, statements can be made concerning the geometries at which the S 0 and S 1 surfaces closely approach each other. Since many of these are biradicaloid geometries, it is logical to examine the properties of biradicals and related species at some length. After reviewing the two‐electron two‐orbital model for molecules at biradicaloid geometries, we formulate the conditions under which the S 0 and S 1 surfaces touch. The results obtained for the simple model are supported by ab initio large‐scale configuration interaction (CI) calculations for the twisting of ethylene in the polarizing field of a positive charge and for the twisting of charged double bonds and π‐donor‐to‐π‐acceptor single bonds, and by similar calculations for “push‐pull” perturbed cyclobutadienes, some of which are predicted to have nearly degenerate S 0, S 1, and T 1 states. The likely consequences of these results for the detailed description of the mechanisms of cis‐trans isomerization, the formation of t wisted i nternal c harge‐t ransfer (TICT) states, proton translocation, and possibly of the initial step in vision, as well as for the understanding of the regiospecificity of singlet photocycloaddition, are summarized.