Three-Motif Molecular Junction with Long-Lived Charge Carriers for Efficient Overall Photosynthesis of H₂O₂

Photosynthesis of hydrogen peroxide (H₂O₂) from air and water has emerged as one of promising alternative strategies to the conventional anthraquinone process. Nevertheless, its practical development is impeded by limited charge separation efficiency and rapid charge-carrier recombination. In this study, a covalently connected molecular junction is synthesized via sequential Schiff and Knoevenagel polymerization reactions for visible-light-driven and sacrificial-agent-free H₂O₂ synthesis. The molecular junction effectively promotes charge separation and enhances photocatalytic efficiency. Femtosecond transient absorption (fs-TAS) spectra reveals that construction of the three-motif molecular junction dramatically extends carrier lifetimes up to 12 ns, which is about 100 times longer than the two-motif junction. As expected, the three-motif molecular junction (TAS₄) exhibits an impressive photocatalytic H₂O₂ production rate of 4302 µmol g^-1 h^-1 under AM 1.5G irradiation without any sacrificial agent in air atmosphere, which is 2.4 and 2 times higher than that of the two-motif junctions (TA and TS). Density functional theory (DFT) calculations and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) confirm that H₂O₂ production on three-motif molecular junction TAS₄ occurs via a stepwise one-electron oxygen reduction reaction (ORR). This work demonstrates the potential of molecular junction for efficient solar-driven H₂O₂ production.