Evidence for a novel bisacylphosphine oxide photoreaction from TRIR, TREPR and DFT studies

Magnetic field effects on the photolysis of homogeneous solutions containing (2,4,6-trimethylbenzoyl)diphenylphoshine oxide, MAPO, and bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide, BAPO, were studied using time-resolved infrared spectroscopy. The two molecules display distinctly different field dependences in conflict with established photochemistry. Time-resolved EPR was employed to examine the photochemistry in detail, resulting in the detection of previously unobserved radical species when BAPO was photoexcited in alcoholic solvents. Plausible reaction mechanisms were used to suggest candidate species that may be responsible for the new EPR signals. DFT calculations were then used to evaluate the likelihood of formation of these species and to estimate their hyperfine coupling constants for comparison with the recorded spectral data. The most likely identities of the new species are a two-coordinate phosphorus radical anion for the species with an observed hyperfine coupling of 2.9 mT and a four coordinate phosphorus centred radical for the species with the large 49.8 mT coupling.