Qualitative Potential Energy Surfaces. 3. Stereoselection Rules for Spin Inversion in Triplet Photochemical Reactions

The LCFC approach, with inclusion of spin-orbit coupling parts in the effective one-electron Hamiltonian operator, has been used to formulate mechanisms of spin inversion in triplet photoreactions and stereoselection rules for radiationless decay of triplet complexes to singlet ground state products. It is suggested that π π* triplet photoreaction complexes can be classified according to the number and direction of atomic orbital rotations required to maximize intermolecular spin-orbit coupling and simultaneously maintain intermolecular bonding. Three classes of triplet photoreaction complexes can be distinguished: (a) Photoaromatic reaction complexes, e.g., [2πs + 2πs], can undergo efficient radiationless decay to ground product if an endomolecular disrotation (ED) accompanied by a simultaneous translational motion is performed, (b) Photoantiaromatic reaction complexes, e.g., [4πs + 2πs]. can undergo radiationless decay to ground product if an endomolecular conrotation (EC) accompanied by a simultaneous translational motion is performed. It is argued that monorotational and pyramidalization mechanisms can compete with the ED and the EC mechanisms. The relative importance of the spin inversion mechanism depends on the reaction polarity, (c) Photononaromatic reaction complexes can undergo radiationless decay to ground product if an orthogonal AO pair is generated at the reaction union site.