Chemical. CAS 492-22-8. Formula C13H8OS. MW 212.27. Thioxanthone is a type II photoinitiator. The high molar absorptivity of thioxanthone in the near-UV spectral region makes this chromophore attractive as a photoinitiator for free-radical polymerization. Thioxanthone-based photoinitiators show some of the best light absorption properties in the near-UV spectral region and are therefore widely used. Spectral data Absorption maximum = 380nm. Also used as building block and intermediate for synthesis.