Reaction Mechanism of Monoacyl- and Bisacylphosphine Oxide Photoinitiators Studied by 31 P-, 13 C-, and 1 H-CIDNP and ESR

31 P-NMR-CIDNP (CIDNP = chemically induced dynamic nuclear polarization) spectroscopy was applied for the first time to investigate the formation and reaction of phosphorus-centered radicals obtained from phosphorus-containing photoinitiators. 13 C-NMR- and 1 H-NMR-CIDNP and ESR spectroscopies were used as complementary experimental techniques for the elucidation of the photochemistry of these compounds. The large hyperfine coupling constants of the 31 P-nucleus results in a violation of Kaptein's rules, which is the only observation of this kind in 13 C-NMR-CIDNP spectra reported so far. Interpretation of the CIDNP spectra, using a modification of Kaptein's rules for the 13 C-NMR- and 1 H-NMR-CIDNP, consistently shows that all compounds investigated undergo a photoinduced cleavage of the carbonyl−phosphinoyl bond from a triplet state. The fate of the primary radicals is discussed, and it is unambiguously shown by trapping experiments that the novel bisacylphosphine oxide photoinitiators give four radicals in a stepwise process.