Thioxanthone photoreduction

Free and polymer-bound 2-benzyloxy-thioxanthone exhibit similar flash photolysis behaviour and the same photoreduction quantum yield in the presence of 2-(A, AT-diethylamino) ethanol. This clearly shows that the polymeric nature does not appear to affect photophysical properties of the thioxanthone moiety. The photoinitiated polymerization of MMA in benzene solution, using BOTX and poly(StX-co-St) in combination with 2-(MA -dieffiylamino) ethanol, indicates that the polymer-bound chromophore seems to operate in the same way and with similar efficiency as the free photoinitiator, at least in conditions of dilute chromophore concentration. 

The synergistic behaviour of the above systems has been explained in terms of both triplet-triplet energy transfer and photoreduction of the thioxanthone derivative by the amino group of the moipholino moiety [157-161]. 

Several studies were carried on the relationship between the chemical structures of ketones and of amines and the rates and efficiencies of photoreductions. It was observed, for instance, that Norrish I reaction of aromatic ketones is affeeted by electron donating and electron withdrawing substituents of the aromatie portions of the ketones. That, of course, is a result of stabilization or destabilization of the partial eharges in the transition state by the substituents. The rates of eleetron transfer reaetions in photo reduction of various ketones, like benzophenone or thioxanthones and their derivatives by some amines can be found in the literature. Some examples are listed in Table 2.5. 

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