Abstract
The photochemistry of five novel water-soluble para-substituted benzophenone derivatives is examined in water and 2-propanol using absorption, luminescence and conventional flash photolysis techniques. The data are related to their ability to photoinduce the polymerization of 2-hydroxyethyl methacrylate in aqueous media. Photopolymerization of the latter is found to occur effectively only in the absence of oxygen and the presence of a secondary or tertiary amine. In the latter case, the lower the ionization potential of the amine the more efficient is the photopolymerization. These results are consistent with the spectroscopic and flash photolysis data which show a correlation between the photoinitiation activities and the measured phosphorescence quantum yields of the initiators. On flash photolysis, the lowest excited triplet state of the initiator is abstracting an electron from the amine via an excited triplet exciplex followed by hydrogen abstraction to give the ketyl radical. This is confirmed through a detailed study on the effect of oxygen, pH and the ionization potential of the amine on transient formation. The significance in the behaviour of these initiators in contrast to the water-soluble thioxanthones is discussed.
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