THE PHOTOLYSIS OF ACYLPHOSPHINE OXIDES .4. INVESTIGATIONS WITH DI-NORMAL-BUTYL-2,6-DICHLOROBENZOYL PHOSPHINE OXIDE AND DICYCLOHEXYL-2,4,6-TRIMETHYLBENZOYL PHOSPHINE OXIDE

Di-n-butyl-2,6-dichlorobenzoyl phosphine oxide (DCBPO) and dicyclohexyl-2,4,6-trimethylbenzoyl phosphine oxide (TMCPO) undergo alpha scission on irradiation with UV light: phi-(alpha) = 0.3 (DCBPO) and phi-(alpha) = 0.2 (TMCPO). Picosecond and nanosecond flash photolysis studies, in conjunction with electron spin resonance (ESR) and optical absorption detection, provide evidence for the formation of phosphinoyl radicals R2P = O (R = n-butyl or cyclohexyl respectively). From the results, it is inferred that excited singlet states deactivate by two routes: alpha scission and intersystem crossing to the triplet manifold (phi-(T) = 0.5 (DCBPO) and phi-(T) = 0.7 (TMCPO)). The triplets deactivate essentially by intramoleclar hydrogen abstraction resulting, to a small extent, in the formation of olefins: phi-(1-butene) = 0.04 (DCBPO) and phi-(cyclohexene) = 0.03 (TMCPO). Enolization with subsequent reketonization is the main triplet deactivation route in the case of TMCPO.