Photochemistry and photopolymerization activities of novel phenylthiobenzophenone and diphenylthiophene photoinitiators

Abstract

The photoinduced polymerization activities of seven novel phenylthio substituted benzophenones have been determined and compared to that of benzophenone itself in different monomers and prepolymers using real time infra-red (RTi.r.) and pencil hardness methods. Four novel dibenzopthiophene compounds have also been include in the investigation to examine the influence of bridging the sulfur atom between the benzophenone chromophore and phenyl groups via an alicyclic bridge. Absorption, fluorescence and phosphorescence analysis, as well as photoreduction/photolysis studies have been undertaken on the compounds and the data is inter-related to their photopolymerization activities. Fluorescence and phosphorescence analysis indicates a high rate of intersystem crossing to the triplet state. The latter is essentially a mixed excited nπ/ππ* state in nature for the phenylthio derivatives as indicated by the phosphorescence lifetime data and quantum yields of emission. In comparison the dibenzopthiophenes retained much of their nπ* character coupled with a rigid molecular structure giving rise to very high phosphorescence quantum yields. Compared with benzophenone all the phenylthio derivatives exhibit higher activity using commercial pencil hardness curing. Strong synergism is also observed with a tertiary amine compared to the activities of the dibenzothiophenes. This is also generally reflected in the RTi.r. data. The dibenzothiophenes exhibit lower activities than the phenylthio derivatives, but remained somewhat greater than that of benzophenone. The enhanced photoactivities of the phenylthio derivatives are associated with their higher extinction coefficients and longer wavelength absorptions in the near ultra-violet region above 300 nm. Steric effects in the case of the dibenzopthiophenes are responsible for their poor synergism with tertiary amines. Their photoreduction quantum yields in the presence of an amine match exactly their photopolymerization effects determined by RTi.r. Microsecond flash photolysis showed weak ketyl radical formation in the region 500-600 nm due to hydrogen atom abstraction by the excited triplet state of the benzophenone chromophore in both structures. Significant transient absorption in region 300-400 nm is suggestive of side chain scission giving rise to additional aryl radical species. © 1997 Elsevier Science Ltd.