Lanthanide contraction induced coordination-mode transition of phosphotungstate-based complexes for visible-light-driven photocatalytic degradation of ciprofloxacin

Polyoxometalate (POM) based coordination assemblies offer tunable platforms for modulating structural connectivity and photoinduced charge transfer, yet controlled switching of POM between coordinating linkers and non-coordinating counter-anions within a single system remains challenging. Herein, 14 POM-based lanthanide complexes (La-Lu, excluding Pm) were constructed using tetraethyl methylenediphosphonate as an auxiliary ligand. Single-crystal X-ray diffraction reveals a structural transition driven by lanthanide contraction: in complexes 1-4 (La-Nd), the POM coordinates directly to Ln³⁺ centers and bridges two [LnL₄]³⁺ units, whereas in complexes 5-14 (Sm-Lu), it acts as a counter-anion to discrete [LnL₄]³⁺ cations. This switching in coordination mode correlates with distinctly different optical and photocatalytic properties: complexes 1-4 exhibit narrower band gaps (2.94-2.95 eV) and visible-light activity, while complexes 5-14 show wider band gaps (3.22-3.27 eV) and respond mainly to UV light. Complex 1 achieves 94.68% ciprofloxacin degradation within 120 min under visible light (k = 0.02413 min^-1), with in situ XPS, EIS, band-structure analysis and  in situ EPR collectively supporting photoinduced charge redistribution, improved interfacial charge transport, and generation of •O₂^- and •OH. Meanwhile, complex 6 exhibits a Eu³⁺ centered luminescence lifetime of 3.34 ms and a quantum yield of 29.94%, suggesting radiative relaxation may compete with photocatalytic charge transfer. Collectively, these results support a correlation among lanthanide-contraction-induced coordination-mode transition, POM coordination behavior, and photocatalytic performance, providing a strategy for designing POM-based crystalline functional materials.