Wavelength-Dependent, Large-Amplitude Photoinitiating Reactivity within a Carbazole-Coumarin Fused Oxime Esters Series

In this current contribution, we provide a detailed investigation into the photochemistry and the free radical photoinitiating reactivity of LED light-sensitive photoinitiators (PIs). This series was designed on the basis of a judicious association of a carbazole-coumarin fused subunit and an O-acyl-alpha-oxooxime branch, which integrates an N-O photocleavable bond. Within this molecular framework, several substitution changes affecting specifically two distinctive sites of the oxime group have been proposed to rationalize some relevant structure-reactivity relationships. We show that the photobleaching rates of the oxime esters (OXEs) are clearly influenced by an ethyl-to-isopropyl substitution effect on the oxime methine carbon whereas the photoinitiating efficiency is mainly driven by a O-benzyl-to-O-acetyl substitution change. Of particular interest, we show that the photoinitiating efficiencies of these OXEs largely depart from their respective absorption spectra in such manner that their photopolymerization performance can be amplified by more than 2 orders of magnitude between 365 and 425 nm LED irradiation. This effect clearly outperforms the photoinitiating efficiency of the commercially available Irgacure OXE-02 oxime ester used as a reference. In the proposed mechanism that accounts for this original wavelength-dependent photopolymerization property, we highlighted the role of an imine-based transient species whose reactivity toward the acrylate monomer can be phototriggered promoting thereby an alternative competing reaction sequence.