Photophysical and photochemical studies of novel thioxanthone-functionalized methacrylates

Five novel thioxanthone (TX) based monomeric photoinitiators (PIs) for free radical polymerization of acrylates were synthesized from reactions of tert-butyl α-bromomethacrylate with 2-hydroxy thioxanthone (TX1) and 1,4-dihydroxy thioxanthone (TX3); and by cleavage of their respective tert-butyl ester groups with trifluoro acetic acid (TX2 and TX4); and by reaction of the acid chloride derivative of TX2 with 2-hydroxy-1-[4-(2-hydroxyethoxy)phenyl]-2-methyl-1-propanone (Irgacure 2959) (TX5). This last monomeric PI possesses two side-chain photoinitiating groups. The photoinitiator TX1 has also been copolymerized with N,N-dimethylaminoethyl methacrylate (DMAEM) to give a polymeric photoinitiator. All these monomeric PIs absorb in the near UV-visible region (390-420 nm; absorption red-shift ~10-40 nm) with comparable  values to TX. Their photoinitiating abilities of acrylate polymerizations are studied under LED exposure at 385 and 405 nm using real-time FTIR and under mercury lamp using photo-DSC. The performances of TX1 and TX2 are similar to isopropyl thioxanthone, and TX used as reference photoinitiators. The reactivities of TX3 and TX4 are lower, due to their difunctional character, which however makes it possible for them to simultaneously serve as crosslinking agents. The successful photoinitiating by TX5 under visible light proves that TX5 contains both photoinitiator and photosensitizer groups. The photochemical mechanisms are studied by electron spin resonance, steady state photolysis, laser flash photolysis experiments and cyclic voltammetry techniques.