Substituent Effects in Monoacylphosphine Oxide Photoinitiators: Efficiency and Performance

Photoinitiators are crucial components in the polymerization processes of lightcurable materials, with-TPO (diphenyl(2,4,6-trimethylbenzoyl)phosphinoxide) being a widely used example. Given the limited number of alternative photoinitiators suitable for the wavelength range in which TPO is employed, this study will investigate the potential of new variants of monoacylphosphine oxide (MAPO) photoinitiators. For this purpose, a series of structures were synthesized in which the phosphorus was modified with different substituents. Subsequently, these structures were analyzed with regard to their absorptive behavior and reactivity. Three derivatives, featuring different alkyl substituents, were selected for detailed investigation. In this detailed study, the addition of emerging phosphorus radicals to double bonds and quantum yields were also considered. It was demonstrated that introducing sterically demanding tert-butyl groups in 2,4,6-trimethylbenzoyl-phenyl(tert-butyl)phosphine oxide (2) and 2,4,6-trimethylbenzoyl-di-tert-butylphosphine oxide (3) results in absorptions shifted to longer wavelengths. Additionally, using 2,4,6-trimethylbenzoyl-cyclohexylphenylphosphine oxide (1), it was shown that replacing the phenyl ring of TPO with a cyclohexyl ring does not significantly alter the absorption behavior, but that alkyl substituents reduce the reactivity of the corresponding P-centered radicals.