Synthesis of novel one-component photoinitiators based on thioxanthone derivatives

Thioxanthone derivatives are synthesized as one-component photoinitiators via a simple condensation reaction between thiosalicylic acids and heterocyclic aromatic hydrocarbons in a concentrated sulfuric acid medium. TX-C (Thioxanthrone–carbazole), TX-N-I (Thioxanthrone–N-methylindole) and TX-I (Thioxanthrone–indole) are characterized by ¹HNMR (nuclear magnetic resonance), FTIR (fourier transform infrared spectroscopy), ultraviolet–visible spectrophotometry, fluorescence spectroscopy, elemental analysis and TG (thermal gravimetric analysis). TX-N-I and TX-I both possess an absorption characteristic similar to the parent thioxanthone with a maximum at 397nm (ε = 3240 L mol⁻¹ cm⁻¹) and 395 nm (ε = 1920 L mol⁻¹ cm⁻¹). The introduction of heterocyclic structures makes the maximum absorption peak red-shifted by 10 nm compared to TX. Its capabilities to act as initiator for the photopolymerization of PEGDA (polyethylene glycol diacrylate) and TMPTA (trimethylolpropane triacrylate) in the absence and presence of TEOA (triethanolamine) media are also examined. TX-N-I acts as an efficient initiator in the UV curable ink, whose light curing time is only 12 s.