Tracking intermediates of photocatalytic hydrogen peroxide generation from a heptazine-based covalent organic polymer in water

Abstract

Metal-free organic photocatalysts with tailorable molecular structures that possess light-harvesting capability, tunable band gaps, accessible active sites and preorganized pathways for charge carrier migration are ideal candidates for generating H₂O₂ from O₂. To design such photocatalysts, it is imperative to formulate a structure–activity relationship through a detailed mechanistic understanding of photoexcited charge carrier generation, reactivity and dynamics. Inspired by the polymeric carbon nitride (PCN) backbone which is well known to stabilize anion radicals, here we have synthesized a new covalent organic polymer, RedCN, for photocatalytic H₂O₂ generation by integrating analogous heptazine-hydrazine moieties with 2,4,6-triformylphloroglucinol (Tp) via a β-keto-enamine linkage. RedCN exhibited remarkable activity for H₂O₂ generation in an aqueous medium with numbers reaching up to 1460 μmol per g per hour while maintaining high activity even in seawater. Broadband femtosecond transient absorption spectroscopy provides the first optical signature of the heptazine-Tp anion radical that decays reactively within ∼70 ps in the presence of O₂. Electron paramagnetic resonance (EPR) spectroscopy revealed the presence of both photogenerated radicals delocalized on the RedCN framework as well as the O₂ reduced superoxide intermediate. We subsequently used the H₂O₂ photoreaction scheme to carry out controlled oxidation of organic sulfides to sulfoxides. Our work therefore paves the way for rational design of heptazine-based photocatalysts for effective H₂O₂ production from O₂ while demonstrating promise for coupled photoredox catalysis in an aqueous medium under ambient conditions.