Photochemistry and photoinitiator properties of 2-substituted anthraquinones: 2. Photopolymerization and flash photolysis

Abstract

The photoinduced polymerization activities of fifteen 2-substituted anthraquinones have been determined in different monomers and prepolymers using real-time Fourier-transform infra-red spectroscopy and photocalorimetry. The relative order in photoinitiator efficiency is found to be highly dependent upon the method used, the nature of the light source, amine co-synergist and monomer being crucial factors. Oxygen quenching in all cases indicates that the triplet state is the active precursor. Without an amine co-synergist, absolute quantum-yield measurements show that anthraquinones with electron-withdrawing groups are more active than those which are electron-donating. However, in the presence of a tertiary amine and under polychromatic illumination, the effects are different. In the latter case amido derivatives with long-wavelength charge-transfer bands and mixed nπ ∗ ππ ∗ triplet states tend to be more reactive. Fluorescence and phosphorescence analyses indicate a high rate of intersystem crossing to the triplet state. The relative positions of the lowest excited singlet ππ ∗ and second excited nπ ∗ triplet states play an important role in determining their photoactivities, as shown in paper 1. Semiquinone radical and radical-anion intermediates are observed using microsecond flash photolysis and the data interrelated to their spectroscopic and photopolymerization activities. The haloanthraquinones are shown to undergo an additional mechanism involving dehalogenation.