Synthesis and spectroscopic properties of novel cinnamate derivatives of thioxanthone: photocuring activity versus photodimerization

Abstract

Two novel 2-substituted derivatives of thioxanthone-containing cinnamate (ethyl ester of 2-propenoic acid) and cinnamamide ( N, N-dimethylacrylamide) groups are synthesized and characterized. The spectroscopic properties of these two new photoinitiators are examined and related to their photocuring behaviour in an epoxy acrylate resin and compared with those of commercial 2-isopropylthioxanthone. Despite an increase in the longest wavelength absorption maximum and corresponding extinction coefficient, most of the derivatives showed little improvement in photocuring activity when compared with 2-isopropylthioxanthone. Their lower activities were reflected in reduced fluorescence quantum yields and reduced phosphorescence quantum yields. Photolysis quantum yields of the cinnamate esters, however, were found to be increased compared with those of 2-isopropylthioxanthone. In this case photoreaction was observed in both nitrogen-saturated and oxygen-saturated 2-propanol. Using FTi.r. and FTn.m.r. analysis, the cinnamate groups were found to undergo a primary reaction involving (2 + 2) cycloaddition via the lowest excited singlet excited state. The cycloaddition reaction is found to compete effectively with hydrogen atom abstraction. This is confirmed on microsecond flash photolysis by the absence of any ketyl radical formation in nitrogen-saturated 2-propanol. The role of the cycloaddition reaction in controlling the photochemical activity of the thioxanthone chromophore is discussed.