Natural-sunlight-harvesting benzothiadiazole-based molecular photocatalyst for H2O2 production: a recyclable homogeneous biphasic system

Abstract

Photocatalytic hydrogen peroxide (H₂O₂) synthesis offers a sustainable route for solar-to-chemical energy conversion to address the increasing global energy demand. However, developing an efficient and sustainable photocatalytic system for H₂O₂ synthesis remains a critical bottleneck. Herein, we report a unique toluene–water two-phase system for highly efficient H₂O₂ synthesis, employing a benzothiadiazole-based small organic photocatalyst, Py–BT–Th. Py–BT–Th shows a high H₂O₂ production rate of 107.18 mmol g⁻¹ h⁻¹, with a turnover frequency of ∼32 h⁻¹, under natural sunlight irradiation in the toluene–water biphasic system, which uses triethylamine as a sacrificial agent. The biphasic system promotes H₂O₂ mass transport, leading to rapid kinetics across the toluene–water interface. Experimental studies reveal that the superior photocatalytic performance of the catalyst Py–BT–Th is attributed to its donor–acceptor-type molecular architecture, which promotes efficient exciton separation, high molar absorptivity (ε = 12.27 × 10³ M⁻¹ cm⁻¹), long exciton lifetime (12.9 ns) and broad light-harvesting capacity. Scavenger and in situ EPR spectroscopic studies confirm that Py–BT–Th produces H₂O₂ by following a dual mechanistic route: (1) indirect two-step one-electron ORR via an electron transfer pathway and (2) singlet-oxygen generation via an energy transfer pathway. In both the routes, ˙O₂⁻ is the key intermediate that ultimately gets converted into hydrogen peroxide, in association with the oxidation of triethylamine by photogenerated holes. Overall, this work disentangles the dilemma associated with small organic photocatalysts for H₂O₂ synthesis through a recyclable toluene–water biphasic system.