Triplet Exciton-Enhanced Photosynthesis of Hydrogen Peroxide Enabled by Topologically Tuned Covalent Organic Frameworks.

Abstract

Artificial photosynthesis technology can utilize water, oxygen, and solar energy to produce hydrogen peroxide (H₂O₂), an environmentally friendly oxidant and a clean fuel. However, H₂O₂ photosynthesis mainly follows photogenerated electrons/holes pathway, which suffers from high thermodynamic barriers and competing reactions. Triplet excitons can spontaneously convert O₂ into singlet oxygen (¹O₂) intermediate and bypass these challenges, but demonstrating its effects on photocatalysis is still scarce. Here, this study designs twist pyrimidine-based covalent organic frameworks with excellent triplet exciton production using a topological tuning strategy. The twist configuration modulates the molecular orbital overlap between singlet and triplet states and achieves a 1.8 × 10⁷ enhancement in the intersystem crossing rate, obtaining excitation of triplet excitons and the generation of ¹O₂, rather than exciting photogenerated electrons and holes. A novel triplet exciton-¹O₂ H₂O₂ photosynthesis pathway is achieved and demonstrates a 38.6% reduction in the generation barrier compared to typical redox pathway, obtaining record activity with rates of 10.80 mmol g^-1 h^-1 in an O₂ atmosphere and 7.82 mmol g^-1 h^-1 in air, without the need for a sacrificial agent. The solar-to-chemical conversion efficiency is 1.25%.