Dihydroxyanthraquinone derivatives: natural dyes as blue-light-sensitive versatile photoinitiators of photopolymerization
Four dihydroxyanthraquinone derivatives [i.e. 1,2-dihydroxyanthraquinone (12-DHAQ), 1,4-dihydroxyanthraquinone (14-DHAQ), 1,5-dihydroxyanthraquinone (15-DHAQ), and 1,8-dihydroxyanthraquinone (18-DHAQ)], when combined with various additives (e.g. iodonium salt, tertiary amine, N-vinylcarbazole, phenacyl bromide, and 4-cyanopentanoic acid dithiobenzoate), are investigated as photoinitiating systems for free radical photopolymerization [e.g. cross-linked free radical photopolymerization of multifunctional monomers or reversible addition–fragmentation chain transfer (RAFT) photopolymerization of monofunctional monomers] and cationic photopolymerization. 14-DHAQ, 15-DHAQ and 18-DHAQ exhibit good solubility in solvent (acetonitrile) and monomers (methacrylate and epoxide) and demonstrate absorption maxima in the blue light wavelength range, which makes them potential candidates to work under the irradiation of a household blue LED bulb. Among all the investigated dihydroxyanthraquinone derivative-based photoinitiating systems, 18-DHAQ based systems exhibit the highest photoinitiating ability for both free radical and cationic photopolymerization while 12-DHAQ based systems are inefficient. It illustrates that the positions of hydroxyl substituents in the anthraquinone molecule play a significant role in the photoinitiating ability of dihydroxyanthraquinone derivatives. The photochemical mechanisms are investigated by fluorescence, laser flash photolysis, steady state photolysis, and electron spin resonance spin trapping techniques, and the results are in agreement with the relevant photopolymerization efficiency.