Excited State and Photodissociation Pathway of an Oxime Ester as a Radical Photoinitiator

Abstract Oxime esters have emerged as promising alternatives to conventional photoinitiators due to their high efficiency, strong absorption, and reduced regulatory concerns. However, their fundamental photochemical behavior remains insufficiently understood. This study investigates the photophysical and photochemical mechanisms of a carbazole-based oxime ester photoinitiator, OXE03, with a focus on its excited states and their role in radical generation. A combination of experimental techniques─including UV–visible spectroscopy, ultra-high-performance liquid chromatography, fluorescence and phosphorescence spectroscopy, laser flash photolysis, and real-time Fourier transform infrared spectroscopy photopolymerization monitoring─along with time-dependent density-functional theory calculations, was employed to characterize OXE03 and compare it to OXE02 and conventional phosphine oxide photoinitiators. Results show that OXE03 exhibits very high absorption coefficients and superior photopolymerization performance at 365 nm, outperforming reference photoinitiators in terms of polymerization rate, conversion, and reduced inhibition time. The quantum yield of photolysis (Φph = 0.53) and radical formation (Φrad = 0.90) confirms its high efficiency, consistent with the generation of two radicals per molecule. Excited-state investigations reveal that OXE03 possesses both singlet (S1) and triplet (T1) excited states, but mechanistic studies using triplet–triplet energy transfer demonstrate that radical formation originates mainly from the singlet excited state. Transient absorption spectroscopy and quenching experiments support this conclusion, while theoretical calculations confirm that N–O bond cleavage is thermodynamically much more favorable from S1 than from T1. Analysis of radicals by liquid chromatography–mass spectrometry identifies methyl and iminyl radicals as primary photoproducts. The methyl radical is responsible for initiating polymerization, whereas the iminyl radical mainly participates in termination processes.