Lighting Up upon Phosphate Binding: Supramolecular Assembly of Ir(III) Complexes for Luminescent Sensing and Molecular Logic Gate

Inorganic phosphates and polyphosphates are vital to biological function, and their dysregulation is linked to metabolic and renal disorders, driving the need for selective, sensitive, and reversible luminescent probes for their detection. Here, we report three bis-heteroleptic Ir(III) complexes (IC1–IC3) bearing imidazole-functionalized ancillary ligands that show selective “switch-on” emission toward phosphate (H2PO4–) and pyrophosphate (HP2O73–) over competing anions, driven by C–H···O hydrogen bonding between imidazole donors and phosphate oxygens. Variable-temperature studies (80–300 K) reveal long-lived phosphorescence (τ = 1.4 μs at 80 K) governed by restricted intramolecular motion and a large singlet–triplet gap. Emission analysis shows mixed 3MLCT character for IC1/IC2, while nitro-substituted IC3 displays combined 3MLCT/3LLCT character. The interaction of IC1 with H2PO4– gives a 1:2 binding stoichiometry (K = 8.0 × 105 M–1) and detection limits of 35.4 nM and 16.3 nM for H2PO4– and HP2O73–, respectively. NMR and DLS studies indicate that phosphate binding triggers hydrogen-bond-mediated supramolecular aggregation, rigidifying the excited state and enhancing emission, while Zn2+ reversibly quenches this response, enabling switch-on/switch-off behavior applicable to molecular logic gate. Phosphate binding extends IC1’s excited-state lifetime from 91.68 to 357.66 ns. These Ir(III) complexes thus emerge as a tunable platform for selective, reversible phosphate sensing.