Extended Conjugation Tuning Carbon Nitride for Non‐sacrificial H2O2 Photosynthesis and Hypoxic Tumor Therapy**.

Artificial photocatalysis offers a clean approach for producing H2O2. However, the poor selectivity and activity of H2O2 production hamper traditional industrial applications and emerging photodynamic therapy (PDT)/chemodynamic therapy (CDT). Herein, we report a C5N2 photocatalyst with a conjugated C=N linkage for selective and efficient non‐sacrificial H2O2 production in both normoxic and hypoxic systems. The strengthened delocalization of π‐electrons by linkers in C5N2 downshifted the band position, thermodynamically eliminating side H2 evolution reaction and kinetically promoting water oxidation. As a result, C5N2 had a competitive solar‐to‐chemical conversion efficiency of 0.55 % in overall H2O2 production and exhibited by far the highest activity under hypoxic conditions (698 μM h−1). C5N2 was further applied to hypoxic PDT/CDT with outstanding performance in apparent cancer cell death and synchronous bioimaging. The study sheds light on the photosynthesis of H2O2 by carbon nitrides for health applications. [ABSTRACT FROM AUTHOR]