Trace amounts of mercaptans with key roles in forming an efficient three-component photoinitiation system for holography

Investigating the photoinitiation mechanisms will help to find new directions for high-efficiency photoinitiation systems (PS). This article aimed to explore other possible roles of mercaptans in a PS where mercaptans were widely regarded as chain transfer agents. A bis-chalcone dye (BDEA) and a biimidazole (HABI) were employed as photosensitizer and co-initiator, and 2-mercaptobenzoxazole (MBO) was chosen as a representative of mercaptans. Trace amounts of mercaptans have been shown a negligible chain transfer effect in previous research studies, but in this work, they were found a miraculous ability to significantly promote the diffraction efficiency. Besides, the diffraction efficiency showed an interesting trend of first increasing and then decreasing with the increase of MBO content. The diffraction behavior was subsequently confirmed to be closely correlated with photoinitiation efficiency by monomer polymerization kinetics. Therefore, we made many efforts to detect the photoinitiation mechanism of PS and the roles of mercaptans. Combining the thermodynamic feasibility analysis with the spin trapping of free radicals, the presence of mercaptans could make the PS evolve a double-cycle photoreaction, which was considered to be an important contribution to the obvious promotion of photoinitiation efficiency. Meanwhile, considering the results of the fluorescence and absorption spectra, the static quenching between BDEA and MBO was presumed to be the factor of the decline in photoinitiation efficiency. Finally, the optimized photopolymers exhibited good performance in recording and reproducing reflection holograms.