An annelated thioxanthone as a new Type II initiator

Abstract

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 lambda (max) = 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 pi-pi* nature of the lowest triplet state. The quantum yield for fluorescence emission (phi(f)) in benzene was found to be 0.045. Spectroscopic and polymerization studies revealed that photoinitiation occurs through the (alpha-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 alpha-aminoalkyl radicals which initiate polymerization. (c) 2008 Elsevier B.V. All rights reserved.