SYNTHESIS AND SPECTROSCOPIC PROPERTIES OF NOVEL CINNAMATE DERIVATIVES OF BENZOPHENONE - PHOTOCURING ACTIVITY VERSUS PHOTODIMERIZATION

Eight novel mono and bis-para-substituted derivatives of benzophenone containing cinnamate and cinnamamide groups are synthesized and characterized. The spectroscopic properties of these new photoinitiators are examined, related to their photocuring behaviour in an epoxy acrylate resin and compared with those of benzophenone. Despite an increase in the longest wavelength absorption maximum and corresponding extinction coefficient, most of the derivatives were found to be much less active than benzophenone itself. Their lower activities were reflected in both reduced phophorescence and photolysis quantum yields. In the latter case, photoreaction was observed in both nitrogen-saturated acetonitrile and air-saturated 2-propanol. Using Fourier Transform I.R. and FT-NMR analysis, the cinnamate groups were found to undergo a primary reaction involving (2 + 2) cycloaddition via the lowest excited singlet excited state. The cycloaddition competes effectively with hydrogen atom abstraction as confirmed by the absence of any ketyl radical formation in nitrogen-saturated 2-propanol on microsecond flash photolysis. The role of the cycloaddition in controlling the photochemical activity of the benzophenone chromophore is discussed.