Photochemistry of 2-acetoxy and 2-acryloxythioxanthone and copolymers with methylmethacrylate: A conventional and laser flash photolysis study

Abstract

The photochemistries of the monomer 2-acryloxythioxanthone and its copolymers with methyl methacrylate have been examined and compared with that of a model 2-acetoxythioxanthone using both conventional microsecond and nanosecond laser flash photolysis in a variety of solvent systems. On microsecond flash photolysis in tetrahydrofuran, transient absorption spectra are produced with absorption maxima at 300 and 420 nm (assigned to the ketyl radical) together with a weak, broad absorption tail extending to longer wavelengths due to the radical-anion. Transient absorbances are reduced in non-hydrogen atom donating acetonitrile apart from that of the copolymer due to intramolecular hydrogen atom abstraction. Transient intensities follow the order monomer > model > 8% > 21% copolymer. Intramolecular self-quenching is believed to be an important process here with the effect of a tertiary amine (diethylethanolamine) being variable. On nanosecond laser flash photolysis in acetonitrile, transient absorption maxima are produced at 330, 440 and 630 nm and assigned to a triplet-triplet absorption and follow the order monomer > model > copolymer (21%). The triplet is strongly reduced in the copolymer due to intramolecular self-quenching. The presence of a tertiary amine (diethylethanolamine) enhances the triplet absorptions at 330 and 630 nm for the monomer and model thioxanthones due to triplet exciplex formation but is quenched by the addition of methyl methacrylate monomer. The effect is more pronounced for the model than for the monomer but again strong quenching is observed for the copolymer (21%). At 440 nm the transient absorption is significantly increased for both the monomer and model because of electron transfer within the exciplex to give the radical-anion. In the case of the copolymer, only a slight increase in the transient absorption in this region is observed because intramolecular coiling of the polymer chain sterically impairs exciplex formation with the tertiary amine. Concentration studies on transient decays show that whilst intermolecular interactions are important for the model, intramolecular self-quenching is important to some degree for the monomer. On the other hand, in the case of the copolymer (21%), the rate constant is observed to decrease with increasing concentration of copolymer in solution because of cage effects enhancing the triplet lifetime. The effect of various tertiary amines on transient decays for the model show a linear correlation between the first-order quenching rate constants and the ionization potential of the amine, confirming the importance of electron transfer via triplet exciplex formation. The importance of these results in photo-initiated polymerization is discussed.