Synergistic enhancement of photocatalytic sulfide oxidation in porphyrin-based binary systems

In this work, the synergistic effects in the photocatalytic oxidation of methyl phenyl sulfide (MPS) by metal-free/metalloporphyrins as photocatalysts and [Ru(bpy)₃]²⁺ (RuBPY) as a photosensitizer were systematically investigated. Metal-free porphyrins are capable of oxidizing MPS via a singlet oxygen (¹O₂) pathway, despite their excited-state reduction potentials being lower than the oxidation potential of MPS. In contrast, metalloporphyrins containing Fe, Co, Cu, Mn, or Ni suppress ¹O₂ generation but exhibit higher excited-state reduction potentials than the oxidation potential of MPS. In binary systems (RuBPY : porphyrin = 25 : 5 μM), these metalloporphyrins promote MPS oxidation more effectively than their metal-free counterparts. Mechanistic studies reveal that upon excitation, electrons are transferred from the excited oxidation potential of metalloporphyrins to the excited reduction potential of RuBPY. This enables the excited reduction potential of metalloporphyrins to oxidize MPS to MPS˙⁺, while the excited oxidation potential of RuBPY reduces O₂ to superoxide (O₂˙^-). Thus, the porphyrin-based binary systems enhance photocatalytic MPS oxidation through two distinct pathways: with metal-free porphyrins by facilitating O₂˙^- generation, and more notably, with metalloporphyrins, by promoting both O₂˙^- and MPS˙⁺ formation. This study provides insights into the rational design of homogeneous dual-component systems for photocatalytic organic oxidation, particularly those based on porphyrin-derived molecular photocatalysts.