Synthesis of carbazole–chalcone bis-oxime esters (CCBOEs) as blue light photoinitiators of polymerization

Abstract

In this work, a series of sixteen asymmetrically substituted bis-oxime esters, CCBOEs, bearing different substituents on the oxime ester sides were designed as Type I photoinitiators for photopolymerization experiments done with blue light (405, 450 and 470 nm) under low light intensities (use of LEDs as the light sources). These natural product-inspired compounds showed a strong absorption in the visible range and exhibited a fast cleavage of the oxime ester groups upon photoexcitation, enabling complete oxime ester consumption to be achieved within 2 s. By using the above-mentioned light sources, three CCBOEs (namely CCBOE1, CCBOE4, and CCBOE5) bearing alkyl substituents on the oxime ester sides exhibited high acrylate monomer conversions but also good thermal polymerization abilities, evidencing their dual activation modes. By means of electron spin resonance (ESR) experiments, the formation of free radicals was demonstrated and led to the proposal of a plausible initiation mechanism. The hypothesis of homolytic cleavage of the N–O bond was also supported by the detection of the CO₂ absorption peak by Fourier Transform Infrared spectroscopy (FT-IR). The initiating mechanism was also investigated by theoretical calculations, enabling the excited state involved in the cleavage process to be determined. Finally, high-resolution 3D patterns were successfully fabricated using the CCBOE5/TMPTA system through direct laser writing (DLW).