An annelated thioxanthone as a new Type II initiator

5-Thia-naphthacen-12-one (TX-Np) was synthesized as a potential photoinitiator for radical polymerization. Polymerization studies revealed that TX-Np acts as a photoinitiator for radical polymerization of acrylic type monomers in the presence of N-methyldiethanolamine (MDEA). The phosphorescence spectrum measured at 77 K in ethanol showed an emission band at λmax = 505 nm corresponding to the approximate triplet energy of ca. 237 kJ/mol.The phosphorescence lifetime was found to be 197 ms corresponding to a π–π* nature of the lowest triplet state. The quantum yield for fluorescence emission (ϕf) in benzene was found to be 0.045. Spectroscopic and polymerization studies revealed that photoinitiation occurs through the α-aminoalkyl radical, similar to thioxanthone-based photoinitiators. Since MDEA is required for TX-Np to act as an initiator for acrylate polymerization, it is suggested that the TX-Np reacts with MDEA to give α-aminoalkyl radicals which initiate polymerization.