Wavelength-Dependent Photochemistry of Oxime Ester Photoinitiators

The design of efficient radical photoinitiating systems requires a systematic and detailed evaluation of their photochemical characteristics. Correlating absorbance and the corresponding electronic transitions of a photoinitiator is critical for understanding its photoinduced reaction pathways. In the current contribution, we provide an in-depth investigation into the photochemistry and photophysics of two oxime ester derivatives (O-benzoyl-α-oxooxime, OXE01, and O-acetyloxime, OXE02), known for their excellent performance in pigmented formulations. In particular, we shed light on their wavelength-dependent photopolymerization properties. We utilized a combination of UV–vis spectroscopy, density functional theory (DFT) calculations, photochemically induced dynamic nuclear polarization spectroscopy (photo-CIDNP), and pulsed-laser polymerization with a wavelength-tunable laser with subsequent size exclusion chromatography coupled to high-resolution electrospray ionization mass spectrometry (PLP-SEC-ESI-MS) for obtaining detailed insights. Both photoinitiators have high molar extinction coefficients (ε) of greater than 1.75 × 10⁴ L mol^–1 cm^–1 at close to 330 nm, with the n−π* and π–π* transitions, relevant for cleavage of the N–O bond, at approximately 335 nm according to DFT calculations. We have probed the wavelength-dependent initiation behavior of both OXE01 and OXE02 in the presence of methyl methacrylate (MMA) via PLP with a wavelength-tunable laser between 285 and 435 nm at constant photon counts. Surprisingly, the highest conversions of MMA were found at a wavelength of 405 nm, even though the molar extinction coefficients of the photoinitiators are low (ε₄₀₅ of 45 and 2 L mol^–1 cm^–1 for OXE01 and OXE02, respectively) compared with shorter wavelengths. Accordingly, the absorption spectrum of a photoinitiator is not a straightforward guide for selecting the most efficient excitation wavelength.