Electronic excitation energies, three-state intersections, and photodissociation mechanisms of benzaldehyde and acetophenone

We report a theoretical study on the electronically excited states and the mechanisms of photodissociation of C6H5CHO and C6H5COCH3. For both molecules, we find an S-1/T-2/T-1 three-state intersection region, which allows for an efficient S-1 -> T-1 intersystem crossing via the T-2 state that acts as a relay. Consequently, T-1 reactions become the major radical photodissociation channels. According to the computed energy profiles, T-1 photodissociation mainly yields phenyl and formyl radicals in the case of benzaldehyde, and benzoyl and methyl radicals in the case of acetophenone, with different C-C bonds being cleaved preferentially. The computational results agree well with the available experimental data. (C) 2012 Elsevier B.V. All rights reserved.