Photochemical Radical Pathways of a Methyl-Substituted Thioxanthone Disulfide Photoinitiator Enabling Oxygen-Tolerant Photopolymerization and In Situ Gold Nanocomposites

Abstract

The photophysical and photochemical performance of a visible-light-active disulfide-based photoinitiator, 4,4 '-disulfanediylbis(1-methyl-9H-thioxanthen-9-one) [CH3-TX-S-]2, was investigated and photopolymerization of mono and diacrylate and simultaneous metal nanoparticle generation were reported. The photoinitiator exhibits strong absorption in the visible region with a molar absorptivity of epsilon = 14,450 L & centerdot;mol-1 & centerdot;cm-1 at 380 nm, enabling efficient excitation under light irradiation. Steady-state fluorescence measurements revealed a low fluorescence quantum yield (Phi f = 0.018), indicating efficient intersystem crossing, which is further supported by phosphorescence emission recorded at 77 K with a lifetime of tau p = 63 ms. The triplet energy level was determined as E T = 264 kJ.mol-1, confirming the suitability of the excited triplet state for radical generation and photochemical reduction processes. Real-time FTIR study demonstrated rapid and efficient photopolymerization of diacrylate formulations, with double-bond conversion percentage values reaching 60 within 600 s. Cyclic voltammetry measurements revealed an irreversible oxidation process (E p,a approximate to 1.76 V) and, together with photophysical data, indicated a strongly reducing excited state, supporting its role in photoinduced electron transfer processes. In parallel, the photogenerated thiyl radicals act as strong photoreductants, enabling the in situ reduction of Au3+ ions to metallic Au0 and the formation of gold nanoparticles during the photopolymerization process.