Construction of Cd(II) Metal–Organic Frameworks Based on a Robust Heterocyclic Phosphazene and Divergent N-Donor Ligands: A Comprehensive Photocatalytic Investigation on Dye Degradation

Solvothermal reactions of cyclophosphazene-based hexacarboxylate (H₆L) and rigid N-donor ligands (phen = phenantroline, trp = 2,2′:6′,2″-terpyridine, 2,2′-bpy = 2,2′-bipyridine, 4,4′-bpy = 4,4′-bipyridine) resulted in four novel Cd(II) metal–organic frameworks (MOFs), formulated as ([(CH₃)₂NH₂]⁺)[{Cd₆(phen)₂(L)₂(HCOO)(H₂O)}]·4DMF·3H₂O (PCP-3), {Cd₂(trp)₂(H₂L)}]·DEF·2H₂O (PCP-4), [{Cd₃(2,2′-bpy)₂(L)(DMF)(H₂O)}]·2DMF (PCP-5), and [{Cd₆(4,4′-bpy)₂(L)₂(H₂O)₉}]·5DMF·H₂O (PCP-6). These MOFs have been successfully characterized using Fourier-transform infrared spectroscopy (FTIR), single crystal and powder X-ray diffraction (SC and PXRD), thermal analyses (TGA), scanning electron microscopy (SEM), ultraviolet–visible diffuse reflectance measurements (UV-DRS), and solid-state photoluminescence measurements. SCXRD results indicated that the cyclotriphoshazene ligand in MOFs coordinates with Cd(II) ions to give a wide variety of structures ranging from a one-dimensional (1D) tubular structure to three-dimensional (3D) porous frameworks. PCP-3 shows a 3D framework with two secondary building units (SBUs) constructed from trinuclear Cd(II) clusters. PCP-4 has a porous π–π stacking framework constructed from the 1D tubular channels through strong face-to-face π–π interactions. PCP-5 displays a two-dimensional (2D) layered framework. PCP-6 exhibits a 3D porous framework in which pillar 4,4-bpy ligands coordinate with Cd(II) ions. After the removal of guest molecules, PCP-3, PCP-4, PCP-5, and PCP-6 have substantial free volumes, which are 21.04, 17.3, 17.0, and 27.3%, respectively. All PCPs demonstrated high efficiency in photocatalytically degrading four organic dyes: methylene blue (MB), methyl orange (MO), rhodamine B (RhB), and reactive orange 16 (RO16) under UVA light irradiation. A proposed photocatalytic mechanism, based on trapping experiments, identified the O₂^•– and ^•OH as the primary reactive radicals involved in dye degradation. PCPs exhibited remarkable photocatalytic activity, achieving up to 95% degradation efficiency within 40 to 60 min. Additionally, PCPs showed excellent reusability and maintained efficiency over five consecutive runs. In addition, the thermal and photoluminescence characteristics of PCPs were thoroughly examined.