A Multifunctional Anionic Cyclotriphosphazene-Based MOF Featuring a Heptanuclear Zn(II) Cluster for Selective Chemical Sensing and Efficient Photocatalytic Activity

The anionic Zn(II)-based MOF, {[bisopibH₂][Zn₇(μ₉-L)₂(μ₃-OH)₂(μ-OH)₂(H₂O)₂]·6DMF}ₙ (PCP-18), was successfully synthesized and structurally characterized using a cyclotriphosphazene-functionalized hexacarboxylate linker (H₆L) in combination with a protonated imidazole-based ligand (bisopibH₂⁺) serving as a charge-balancing cation. Structural analysis revealed that PCP-18 is a robust three-dimensional framework constructed from two discrete {Zn₃O₈}(μ₂-OH)(μ₃-OH) secondary building units (SBUs), which are bridged by the hexacarboxylate linkers to form a highly interconnected framework. PCP-18 displays remarkable fluorescence sensing efficiency toward hydroxylated nitroaromatic analytes, including 2-nitrophenol (2-NP), 4-nitrophenol (4-NP), 2,4-dinitrophenol (DNP), and 1,3,5-trinitrophenol (TNP), with the ability to detect DNP at concentrations as low as 5.06 × 10⁻⁵ M, maintaining selectivity even under competitive conditions. Additionally, it demonstrates high sensitivity and selectivity for Fe³⁺ ions, with a detection limit reaching 5.58 × 10⁻⁵ M. Its semiconductive nature and anionic framework enable efficient photocatalytic degradation of cationic methylene blue (MB) with degradation efficiencies exceeding 90% under light irradiation, attributed to electrostatic interactions, ion charge effects, and a hole-mediated mechanism. The reusability of MOF in photocatalytic applications underscores its stability and practical value. This study highlights PCP-18 as a versatile material for addressing critical environmental challenges, including pollutant degradation and hazardous compound detection, emphasizing its innovative and impactful contribution to MOF-based technologies for sustainable environmental remediation.