Effects of conjugated systems on UV-visible light-sensitive D-π-A type sulfonium salt photoacid generators

A series of D-π-A type sulfonium salt photoacid generators with different π-conjugated structures, such as triphenyl, phenylstilbene, styryl-biphenyl, and stilbene, were designed to determine the effect of molecular structures on the photochemical and photophysical properties. The mechanisms of photochemical generation of H+ were studied by UV-Vis spectroscopy, theoretical calculations, and fluorescence spectroscopy. It is found that the frontier orbits determine the absorption, the molar extinction coefficients, and the quantum yields of photoacid generation. Triphenyl systems connected with sulfonium are beneficial to increase the quantum yields of acid generation. The photoreactivity of four sulfonium salts was further evaluated through the polymerizations of various epoxide monomers at different irradiation wavelengths (365–425 nm) by using the real-time infrared spectroscopy with light-emitting diodes. The high quantum yields for acid generation (Φ H + = ~0.32 to 0.58) and the high molar extinction coefficients (e = ~23500 L∙mol−1∙cm−1 to 31000 L∙mol−1∙cm−1) of the sulfonium salts lead to high conversion rates (over 50%–80%). Hence, these photoinitiators exhibit potential for the photocuring applications.