Extended Conjugation Tuning Carbon Nitride for Non‐sacrificial H ₂ O ₂ Photosynthesis and Hypoxic Tumor Therapy**

Artificial photocatalysis offers a clean approach for producing H₂ O₂ . However, the poor selectivity and activity of H₂ O₂ production hamper traditional industrial applications and emerging photodynamic therapy (PDT)/chemodynamic therapy (CDT). Herein, we report a C₅ N₂ photocatalyst with a conjugated C=N linkage for selective and efficient non-sacrificial H₂ O₂ production in both normoxic and hypoxic systems. The strengthened delocalization of π-electrons by linkers in C₅ N₂ downshifted the band position, thermodynamically eliminating side H₂ evolution reaction and kinetically promoting water oxidation. As a result, C₅ N₂ had a competitive solar-to-chemical conversion efficiency of 0.55 % in overall H₂ O₂ production and exhibited by far the highest activity under hypoxic conditions (698 μM h⁻¹ ). C₅ N₂ 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 H₂ O₂ by carbon nitrides for health applications.