On the photolysis of acylphosphine oxides: 1. Laser flash photolysis studies with 2,4,6-trimethylbenzoyldiphenylphosphine oxide

Abstract

2,4,6-trimethylbenzoyldiphenylphosphine oxide (TMDPO) was irradiated in dilute solutions of benzene, methanol and dichloromethane with 20 ns flashes of 347 nm light. By optical absorption measurements, a transient spectrum with a maximum at about 330 nm detected at the end of the flash was ascribed to the diphenyl phosphonyl radical ( Ph) 2 P ̇ =0 . This radical is formed by the reaction. Evidence was obtained for fragment radicals being formed, to some extent at least, from triplet states. Both the singlet and the triplet lifetimes were estimated as τ < 1 ns. The singlet energy was estimated as 288 kJ mol −1. In the absence of radical scavengers ( Ph) 2 P ̇ =0 radicals undergo a self reaction (2k 2≈5 × 10 9 l mol −1 s −1). Rather high rate constants (in l mol −1 s −1) were found for reactions of ( Ph) 2 P ̇ =0 radicals with oletinic compounds: 6.0 × 10 7 (styrene), 6.0 × 10 7 (methyl methacrylate), 2.3 × 10 7 (methacrylate), 1.8 × 10 7 (acrylonitrile), 5.0 × 10 6 (t-butyl vinyl ether), 2.0 × 10 6 (vinyl acetate). The quantum yield of radical formation is ø(-TMDPO) = 0.5 to 0.7. These results clearly show why TMDPO is highly appropriate to be applied as an initiator for photocuring of coatings of various compositions.