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

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%.