Thioxanthone Photoinitiators with Heterocyclic Extended Chromophores
The high molar absorptivity of thioxanthone in the near-UV spectral region makes this chromophore attractive as a photoinitiator for free-radical polymerization. Recent advances in blue LEDs have made these low-cost, highly energy-efficient light sources available for use in photo-curing tools, which generates demand for blue-light-absorbing photoinitiators. In this chapter we review recent progress to shift the absorbance of thioxanthone into the visible spectral region using heterocyclic extension with thiophene, benzothiophene, indole, triazole and others. First, the photophysical properties of these blue-light-absorbing chromophores are discussed followed by examining their photoreactivity. Finally, the efficacies of these heterocyclic extended thioxanthones as photoinitiators for free-radical polymerization of acrylates are evaluated. The polymerization efficacies are compared to photoinitiation with the popular camphorquinone/amine system. The improved performance of these new thioxanthone derivatives compared to camphorquinone is mostly caused by the orders of magnitude higher molar absorptivity of the heterocyclic extended thioxanthones.