Construction of\nCd(II) Metal–Organic Frameworks\nBased on a Robust Heterocyclic Phosphazene and Divergent N‑Donor\nLigands: A Comprehensive Photocatalytic Investigation on Dye Degradation

Solvothermal reactions of cyclophosphazene-based hexacarboxylate\n(H<sub>6</sub>L) and rigid N-donor ligands (phen = phenantroline,\ntrp = 2,2′:6′,2″-terpyridine, 2,2′-bpy\n= 2,2′-bipyridine, 4,4′-bpy = 4,4′-bipyridine)\nresulted in four novel Cd(II) metal–organic frameworks (MOFs),\nformulated as ([(CH<sub>3</sub>)<sub>2</sub>NH<sub>2</sub>]<sup>+</sup>)[{Cd<sub>6</sub>(phen)<sub>2</sub>(L)<sub>2</sub>(HCOO)(H<sub>2</sub>O)}]·4DMF·3H<sub>2</sub>O (<b>PCP-3</b>), {Cd<sub>2</sub>(trp)<sub>2</sub>(H<sub>2</sub>L)}]·DEF·2H<sub>2</sub>O (<b>PCP-4</b>), [{Cd<sub>3</sub>(2,2′-bpy)<sub>2</sub>(L)(DMF)(H<sub>2</sub>O)}]·2DMF (<b>PCP-5</b>), and [{Cd<sub>6</sub>(4,4′-bpy)<sub>2</sub>(L)<sub>2</sub>(H<sub>2</sub>O)<sub>9</sub>}]·5DMF·H<sub>2</sub>O (<b>PCP-6</b>). These MOFs have been successfully characterized using Fourier-transform\ninfrared spectroscopy (FTIR), single crystal and powder X-ray diffraction\n(SC and PXRD), thermal analyses (TGA), scanning electron microscopy\n(SEM), ultraviolet–visible diffuse reflectance measurements\n(UV-DRS), and solid-state photoluminescence measurements. SCXRD results\nindicated that the cyclotriphoshazene ligand in MOFs coordinates with\nCd(II) ions to give a wide variety of structures ranging from a one-dimensional\n(1D) tubular structure to three-dimensional (3D) porous frameworks. <b>PCP-3</b> shows a 3D framework with two secondary building units\n(SBUs) constructed from trinuclear Cd(II) clusters. <b>PCP-4</b> has a porous π–π stacking framework constructed\nfrom the 1D tubular channels through strong face-to-face π–π\ninteractions. <b>PCP-5</b> displays a two-dimensional (2D) layered\nframework. <b>PCP-6</b> exhibits a 3D porous framework in which\npillar 4,4-bpy ligands coordinate with Cd(II) ions. After the removal\nof guest molecules, <b>PCP-3</b>, <b>PCP-4</b>, <b>PCP-5</b>, and <b>PCP-6</b> have substantial free volumes,\nwhich are 21.04, 17.3, 17.0, and 27.3%, respectively. All PCPs demonstrated\nhigh efficiency in photocatalytically degrading four organic dyes:\nmethylene blue (MB), methyl orange (MO), rhodamine B (RhB), and reactive\norange 16 (RO16) under UVA light irradiation. A proposed photocatalytic\nmechanism, based on trapping experiments, identified the O<sub>2</sub><sup>•–</sup> and <sup>•</sup>OH as the primary\nreactive radicals involved in dye degradation. PCPs exhibited remarkable\nphotocatalytic activity, achieving up to 95% degradation efficiency\nwithin 40 to 60 min. Additionally, PCPs showed excellent reusability\nand maintained efficiency over five consecutive runs. In addition,\nthe thermal and photoluminescence characteristics of PCPs were thoroughly\nexamined.