Sulfur-hinged L-shaped ligand-based Cd(ii)–organic framework: a fluorescent tool for targeting environmental nitroaromatics

Abstract

We present the synthesis and detailed structural characterization of a cadmium-based metal–organic framework (Cd-MOF) with the formula [Cd(L^4-Py)(nipa)(H₂O)]_n, where L^4-Py stands for N,N′-(thiobis(4,1-phenylene))diisonicotinamide and anionic nipa²⁻ represents 5-nitroisophthalate. This Cd-MOF has been investigated for its potential application as a fluorescent probe, demonstrating highly selective recognition of nitroaromatic compounds (NACs). The Cd-MOF was synthesized using ligand L^4-Py, co-ligand H₂nipa, and cadmium iodide. Single crystal X-ray diffraction analysis revealed that the Cd-MOF forms a one-dimensional (1D) polymeric structure and is finally adapted into a three-dimensional (3D) supramolecular network through various C–H---π interactions. The 3D framework features channels with cross-sectional dimensions of 17.31 × 20.31 Ų, aligned along the crystallographic b-axis, and accommodates coordinated water molecules. Remarkably, the Cd-MOF exhibits high fluorescence stability in aqueous solutions and is capable of selectively detecting 4-nitrophenol (4-NP) and 4-nitrotoluene (4-NT). The detection is characterized by high quenching constants and low detection limits, specifically 3.09 × 10⁴ M⁻¹ and 0.166 μM for 4-NP, and 3.09 × 10⁴ M⁻¹ and 0.184 μM for 4-NT. We argued that the fluorescence quenching of Cd-MOF in the presence of NACs is driven by competitive absorption and a synergistic interaction between the amidic functionalities and hinged ‘S’ atom in the ligand framework of Cd-MOF and NACs. This powerful synergy enables quick detection of NACs through triggered fluorescence quenching response.