Potential-energy surfaces for ultrafast photochemistry Static and dynamic aspects

Abstract

The first singlet excited states (S₁) which control the ultrafast (i.e. sub-picosecond) photochemistry of 2-cis-penta-2,4-dieniminium cation (2-cis-C₅H₆NH₂⁺), all-trans-hexa-1,3,5-triene (all-trans-HT) and cyclohexa-1,3-diene (CHD) have been investigated using abinitio MCSCF and multireference MP2 theories. The structure of the corresponding potential energy surfaces (PESs) has been characterized by computing novel unconstrained and symmetry-constrained minimum-energy paths (MEP) starting from Franck–Condon and S₂/S₁ conical intersection points on S₁. Furthermore, analytical frequency computations have been used to produce quantitative information on the surface curvature.

We show that the S₁ energy surface is characterized by two domains, region I and region II. Region I controls the initial acceleration of the excited state molecule. In contrast, region II is a low-lying region of S₁ and controls the evolution towards fully efficient decay to the ground state. The energy surface structure indicates that the double-bond isomerization of 2-cis-C₅H₆NH₂⁺ and all-trans-HT and the ring-opening of CHD are prototypes of three classes of barrierless reactions characterized by a different excited state dynamics. In 2-cis-C₅H₆NH₂⁺ and, more loosely, in all-trans-HT the initial relaxation results in the production of a totally symmetric S₁ transient. The following triggering of the S₁→S₀ decay requires energy redistribution along a symmetry-breaking (torsional) mode leading to an S₁/S₀ conical intersection (CI). In contrast, the shape of region I of CHD indicates that an almost direct (i.e. impulsive) motion towards an asymmetric S₁/S₀ CI occurs upon initial relaxation. Previously reported and novel semi-classical trajectory computations and the available experimental evidence seem to support these conclusions.