PHOTOCHEMISTRY AND PHOTOPOLYMERIZATION ACTIVITIES OF NOVEL ALKYLTHIOBENZOPHENONE PHOTOINITIATORS

The photoinduced polymerisation activities of five novel alkylthio sub stituted benzophenones have been determined and compared to that of th e commercial 4-methylthio derivative and benzophenone itself in differ ent monomers and prepolymers using RTIR and pencil harness methods. Ab sorption, fluorescence and phosphorescence analysis, as well as photor eduction/photolysis studies have been undertaken on the compounds and the data are interrelated to their photopolymerisation activities. Flu orescence and phosphorescence analysis indicates a high rate of inters ystem crossing to the triplet state. The latter is essentially a mixed excited n pi/pi pi state in nature, as indicated by the phosphoresce nce lifetime data and quantum yields of emission. Compared with benzop henone all the alkylthio derivatives exhibit higher activity using com mercial pencil hardness curing. This is also reflected in the RTIR dat a apart from the sulphoxide derivative, which displays a lower activit y than benzophenone. The differences in activity are accounted for by differences in oxygen quenching effects during the two curing studies. The enhanced photoactivities of the thio derivatives are associated w ith their higher extinction coefficients and longer wavelength absorpt ions in the near UV region above 300 nm. Microsecond flash photolysis showed weak ketyl radical formation in the region 500-600 nm due to hy drogen atom abstraction by the excited triplet state of the benzopheno ne chromophore. Significant transient absorption in the region 300-400 nm is suggestive of side chain scission of the phenyl-S bond while th e sulphoxide group is also prone to scission of the alkyl-S bond. (C) 1998 Elsevier Science Ltd. All rights reserved.