<p>The fundamental transitions of metal-based photocatalysts from ground to excited states are typically metal-to-ligand or ligand-to-metal charge transfer, which require high-energy visible-light absorption. Herein, we describe a noncanonical base-to-ligand charge transfer (BLCT) between cheap inorganic base phosphates and a robust iridium-based polypyridyl photocatalyst. The phosphate and photocatalyst form an electron donor-acceptor complex to generate a valuable phosphate radical that can activate various inert C–H/Si–H bonds. The currently popular view is that the reductive quenching of excited photocatalysts is dominated by Br⁻/Cl⁻ when Br⁻/Cl⁻ and phosphate are both present in a photocatalytic system. However, this BLCT model, which is verified by in-depth theoretical calculations and experimental findings, reveals an alternative pathway and is capable of harvesting lower-energy light. These mechanistic insights will provide practical guidance for the photocatalytic activation of inert chemical bonds mediated by low-energy light.</p>