Phosphate-Induced Electron Injection Drives Selective Fluorescence Enhancement in a Pyrazinoquinoxaline-Based Zr-MOF

The selective detection of phosphate ions (PO43–) remains a challenge in fluorescence sensing. This work reveals the underlying mechanism of a unique fluorescence enhancement response in a Zr-MOF constructed with a pyrazinoquinoxaline-based tetracarboxylate linker. The MOF exhibits high selectivity for PO43– over other anions, with a strong binding affinity (K = 1.02 × 105 M–1). Combined experimental and theoretical studies demonstrate that PO43– preferentially adsorbs at the Zr6O4(OH)4 clusters via strong hydrogen bonding. Crucially, density functional theory calculations indicate that PO43– acts as an efficient electron donor, injecting electrons into the ligand’s antibonding orbitals. This injection narrows the HOMO–LUMO gap from 0.913 to 0.312 eV, thereby facilitating charge transfer and enhancing radiative transition. The fluorescence enhancement efficiency correlates with the phosphate protonation state (PO43– > HPO42– > H2PO4–), and both noncoordinative adsorption and direct coordination to Zr(IV) yield similar electronic effects. This study provides an electronic-structure basis for the rational design of MOF-based sensors.