PHOTOCHEMISTRY AND PHOTOPOLYMERIZATION ACTIVITIES OF NOVEL PHENYLTHIOBENZOPHENONE AND DIPHENYLTHIOPHENE PHOTOINITIATORS

The photoinduced polymerization activities of seven novel phenylthio s ubstituted benzophenones have been determined and compared to that of benzophenone itself in different monomers and prepolymers using real t ime infra-red (RTi.r.) and pencil hardness methods. Four novel dibenzo pthiophene compounds have also been include in the investigation to ex amine the influence of bridging the sulfur atom between the benzopheno ne chromophore and phenyl groups via an alicyclic bridge. Absorption, fluorescence and phosphorescence analysis, as well as photoreduction/p hotolysis studies have been undertaken on the compounds and the data i s inter-related to their photopolymerization activities. Fluorescence and phosphorescence analysis indicates a high rate of intersystem cros sing to the triplet state. The latter is essentially a mixed excited n pi/pi pi() state in nature for the phenylthio derivatives as indicat ed by the phosphorescence lifetime data and quantum yields of emission . In comparison the dibenzopthiophenes retained much of their n pi() character coupled with a rigid molecular structure giving rise to very high phosphorescence quantum yields. Compared with benzophenone all t he phenylthio derivatives exhibit higher activity using commercial pen cil hardness curing. Strong synergism is also observed with a tertiary amine compared to the activities of the dibenzothiophenes. This is al so generally reflected in the RTi.r. data. The dibenzothiophenes exhib it lower activities than the phenylthio derivatives, but remained some what greater than that of benzophenone. The enhanced photoactivities o f the phenylthio derivatives are associated with their higher extincti on coefficients and longer wavelength absorptions in the near ultra-vi olet region above 300 nm. Steric effects in the case of the dibenzopth iophenes are responsible for their poor synergism with tertiary amines . Their photoreduction quantum yields in the presence of an amine matc h exactly their photopolymerization effects determined by RTi.r. Micro second flash photolysis showed weak ketyl radical formation in the reg ion 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. (C) 1997 Els evier Science Ltd.