PHOTOCHEMISTRY AND PHOTOINITIATOR PROPERTIES OF NOVEL 1-CHLORO-SUBSTITUTED THIOXANTHONES .1. INFLUENCE OF 4-ACYLOXY SUBSTITUTION

The photoinduced polymerisation activities of five novel 1-chloro-4-ac yloxy substituted thioxanthones have been determined and compared to t hat of the 4-hydroxy and 4-n-propoxy derivatives in different monomers and prepolymers using RTIR and photocalorimetry methods. Absorption, fluorescence and phosphorescence analysis and photoreduction studies h ave also been undertaken on the compounds and the data inter-related t o their photopolymerisation activities, Fluorescence and phosphorescen ce analysis indicates a high rate of intersystem crossing to the tripl et state. The latter is essentially pi pi in nature but strongly mixe d with a close lying n pi as indicated by the phosphorescence lifetim e data. Compared with the 1-chloro-4-hydroxy derivative used as a comp arative model all 4-acyloxy derivatives exhibit much higher photoiniti ation activity. The results are also consistent with a marked enhancem ent in photoreduction quantum yields, especially in the presence of an amine cosynergist. The enhanced photoactivity of the 4-acyloxy deriva tives is associated with reduced fluorescence quantum yields and conse quent increased n pi activity of the triplet state through the electr on withdrawing effect of the acyl group. High photoconversion during p olymerisation is observed in the presence of oxygen indicating the pre dominance of photodehalogenation which is enhanced by the presence of a tertiary amine cosynergist. Using RTIR higher degrees of conversion are observed under visible light irradiation than under UV exposure. ( C) 1997 Elsevier Science S.A.