4-Aminothiophenol Photodimerization Without Plasmons

The photodimerization of 4-aminothiophenol (PATP) into 4,4’-dimercaptobenzene (DMAB) has been extensively utilized as a paradigm reaction to probe the role of surface plasmons in nanoparticle-mediated light-driven processes. Over the past 25 years, a lively debate about the reaction mechanism involved several research groups. Plasmon-mediated generation of energetic (hot) electrons and holes has been invoked as the main driving-force, although plasmonic heating has recently gained attention. However, either model and their combinations are not sufficient to explain some experimental observations and appear, in some cases, contradictory. No matter the differences, there is a general, firm consensus about the presence of plasmonic nanoparticles (Ag or Au), which has always been considered mandatory for triggering the photoconversion. Here I report the first observation of the PATP-to-DMAB photoreaction in the absence of any plasmonic mediators. In particular, neither plasmonic heating nor charge transfer from hot carriers were exploited. The reaction was observed to occur with different kinetics on plasmon-free TiO2 nanoparticles, as well as on self-standing droplets. Confocal microRaman spectroscopy enabled to investigate the reaction progress in different plasmon-free contexts, either aerobic or anaerobic, suggesting a new interpretation of the photodimerization process, based on direct laser-induced activation of singlet oxygen species. These results provide new insights in light-driven redox processes, elucidating the role of sample morphology, light and oxygen.