Bipyridine-Functionalized Cd(II) Coordination Frameworks Based on Robust Cyclophosphazenes: Structural Engineering toward Enhanced Photocatalytic Activity

Three novel Cd(II)-based coordination polymers were constructed using the 2,2′-bipyridine (bpy) ligand, alongside cyclotriphosphazene-functionalized multicarboxylates (L1, L2, and L3), which were carefully optimized to yield a 2D layered architectures (PCP-11 and PCP-13) and a 1D polymeric chain structure (PCP-12). The coordination polymers were synthesized via solvothermal methods and thoroughly characterized using FT-IR, powder and single-crystal X-ray diffraction (PXRD and SCXRD), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), and UV–vis diffuse reflectance spectroscopy (UV-DRS). Remarkably, all PCPs demonstrated outstanding photocatalytic efficiency in the degradation of various organic dyes, namely, methylene blue (MB), methyl orange (MO), and rhodamine B (RhB), under UVA irradiation, achieving degradation rates of up to 90%. Among them, PCP-12 exhibited the highest performance, which is attributed to its binaphthoxy-substituted ligand, lower band gap, and the strong π···π stacking interactions between the 1D chains. UV-DRS analysis was employed to estimate band gap energies, which are critical for evaluating photocatalytic potential. Furthermore, the role of reactive oxygen species (ROS) in the dye degradation process was elucidated, highlighting the dominant contribution of superoxide radicals (O₂•⁻) across all dyes. Durability tests through repeated photocatalytic cycles showed that PCP-12 retained the highest efficiency with minimal decline, underscoring its superior reusability. Electrochemical analyses, including cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS), revealed efficient charge transfer dynamics in the PCPs, with PCP-12 exhibiting the lowest charge transfer resistance and optimal band edge positions, contributing to its superior photocatalytic performance. This study highlights the potential of PCPs (Phosphazene-based Coordination Polymers) as effective photocatalysts for environmental remediation, emphasizing the importance of chemical composition and structural properties in optimizing photocatalytic performance.