Series of Fluorescent Coordination Polymers Exhibiting Structural Evolution and Optimized Selective Ion Detection

A series of Zn(II)-H₂L (H₂L = 3,3′-(9,9-diethyl-9H-fluorene-2,7-diyl)dibenzoic acid) fluorescent coordination polymers were synthesized using a mixed-ligand approach through solvothermal methods, demonstrating a structural transition from three-dimensional (3D) to two-dimensional (2D) frameworks. The structural evolution is driven by the gradational size variation of the pyridine-based ligands, L1 (9,10-di(4-pyridyl)anthracene), L2 (1,4-bis(pyridyl)benzene), and L3 (4,4′-bipyridine), resulting in distinct spatial packing and coordination modes. The polymers, designated as {[Zn₂(L)₂(L1)]}_n (1), {[Zn₂(L)₂(L2)]}_n (2), and {[Zn₂(L)₂(L3)]}_n (3), feature a structural evolution from 3D interpenetrated to 2D alternating structures, significantly influencing their fluorescent properties for ion detection. Notably, compound 3 exhibits marked selectivity and sensitivity toward trivalent metal ions (Cr³⁺, Al³⁺, and Fe³⁺) with detection limits of 0.19 μM for Cr³⁺, 0.30 μM for Al³⁺, and 0.37 μM for Fe³⁺. This research provides valuable insights into the designability of metal–organic frameworks and advances the development of MOF-based fluorescent probes for selective ion detection, highlighting the impact of systematic structural adjustments on ion selectivity and sensing performance.