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-acyloxy substituted thioxanthones have been determined and compared to that 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 have also been undertaken on the compounds and the data inter-related to their photopolymerisation activities, Fluorescence and phosphorescence analysis indicates a high rate of intersystem crossing to the triplet state. The latter is essentially pi pi* in nature but strongly mixed with a close lying n pi* as indicated by the phosphorescence lifetime data. Compared with the 1-chloro-4-hydroxy derivative used as a comparative model all 4-acyloxy derivatives exhibit much higher photoinitiation activity. The results are also consistent with a marked enhancement in photoreduction quantum yields, especially in the presence of an amine cosynergist. The enhanced photoactivity of the 4-acyloxy derivatives is associated with reduced fluorescence quantum yields and consequent increased n pi* activity of the triplet state through the electron withdrawing effect of the acyl group. High photoconversion during polymerisation is observed in the presence of oxygen indicating the predominance 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.