Spin-selectivity in photochemistry: A tool for organic synthesis

The photochemistry of carbonyl substrates is largely dependent on the spin multiplicity of the excited state. Singlet an triplet excited carbonyls can differ strongly in chemo-, regio- and stereoselectivity. In bimolecular Paterno-Buchi reactions, both excited singlet and triplets states give rise to oxetane formation, albeit with different selectivities and completely different activation parameters. In unimolecular photocyclizations (Norrish-Yang reaction) the triplet state dominates the reaction. For triplet to singlet intersystem crossing at the biradical level, spin-orbit coupling optimizing geometries are crucial. These geometries are different from classical closed-shell interactions and thus give rise to unusual product stereochemistry as well as unusual concentration and temperature dependences.