Interpenetration Control in Metal-Organic Polyrotaxanes via Solvent Composition and Ligand Steric-Modulation: Supramolecular Isomerism and Luminescence Detection toward Fe3+ Ions.

Abstract

Three fluorescent Zn coordaintion polymers (CPs) have been synthesized from the reactions of Zn(NO₃)₂ ⋅ 6H₂O, benzene-1,4-dicarboxylic acid (1,4-H₂bdc), and angular carbazole-derived bispyridyl ligands (Cz-3,6-bpy or Cz-Pr-3,6-bpy). CPs 1-3 all adopt similar two-dimensional (2D) ring-and-rod layer structures, described as topologically 4-connected 2 ⋅ 6⁵ nets where the Zn(II) centers act as 4-connected nodes. CPs 1 and 2 are a pair of solvent-mediated supramolecular isomers where the former shows a two-fold interlocked 2D →2D polyrotaxane-like entangled net and the latter reveals a four-fold interpenetrated 2D →3D polyrotaxane entanglement. CP 3 is structure analgue of 1. The formation of 1-3 implies the realization of interpenetration control and supramolecular isomerism in polyrotaxanes by fine-tuning the solvent compositions and ligand steric-modulation. CPs 1-3 all show high thermal stability approaching 400 °C and good chemical stability in various solvents at room temperature for 24 h. Further, CPs 1-3 all are excellent turn-off sensors to recognize Fe³⁺ in water with high selectivity and sensitivity, great anti-inference ability, and low limit of detection (LOD). The sensing mechanisms are found to be competitive absorption of excitation wavelength energy and the occurrence of framework-Fe³⁺ interactions.