Photophysical and Photochemical Studies of Novel Thioxanthone-Functionalized Methacrylates through LED Excitation

Five novel thioxanthone (TX)-functionalized methacrylates as copolymerizable photoinitiators (CopPIs) for polymerization of acrylates are synthesized from reactions of tert-butyl alpha-bromomethacrylate with 2-hydroxy thioxanthone (TX1) and 1,4-dihydroxy thioxanthone (TX3), cleavage of their respective tert-butyl ester groups with trifluoroacetic acid (TX2, TX4), and by reaction of the acid chloride derivative of TX2 with Irgacure 2959 (TX5). These CopPIs absorb in the near UV-visible region (390-420 nm; absorption redshift approximate to 10-40 nm, epsilon values comparable to TX). Their performances are studied under LED exposure at 385 and 405 nm using real-time FTIR. The reactivities of TX1 and TX2 are similar to isopropyl thioxanthone, used as reference; TX3 and TX4 have lower reactivities, due to their difunctionality, which however makes them good crosslinking agents. TX5 is the first monomeric PI possessing two different type side-chain photoinitiating groups. The migration stability of two of these CopPIs is shown in the cured polyacrylates. The photochemical mechanisms are studied by electron spin resonance, steady state photolysis, laser flash photolysis, and cyclic voltammetry.