Substituted stilbene-based oxime esters used as highly reactive wavelength-dependent photoinitiators for LED photopolymerization

Abstract

Six novel oxime carboxylate photoinitiators were designed on the basis of a π-conjugated (E)-1-(4-styrylphenyl)-1-one subunit. Different substituent groups were introduced into three distinctive sites of the chromophore to rationalize their related electronic and structural effects on free-radical photoinitiating reactivities. The electronic properties, photodissociation mechanism, and structure–reactivity relationships were investigated by UV–visible spectroscopy, DFT calculations, real-time ¹H NMR, electron spin resonance spectroscopy, and photo-DSC. We demonstrate that the structural change affecting the photoinitiating properties corresponds to a para-substitution effect on the stilbene subunit using an electron-acceptor group. Such a structural change leads to a 10-fold enhancement in initiation efficiency (Φ_i) upon visible irradiation of light-emitting diodes. The photoinitiating performances of these new derivatives are strikingly sensitive to the excitation wavelengths, leading to an impressive enhancement of Φ_i by more than three orders of magnitude from 365 nm to 405 nm irradiation. This wavelength-dependent reactivity previously observed for commercially available oxime esters can be generalized in our series and has high potential photocuring applications in low-power visible LEDs.