Design of Metal–Organic Frameworks with Strong and Weak Field Ligands for Fluorescence–Based Segregation of Metal Ion Redox States

Two Cd(II)-succinate-based metal-organic frameworks (MOFs) were synthesized to evaluate the effect of hard/soft nucleophilic centers to identify oxidation states of metals. Here, the MOFs {[Cd(dim)₂(H₂O)₂](suc)(H₂O)₃}_n (1) and {[Cd(2,6-bip)(suc)](xSol)}_n (2) were constructed by flexible di(1H-imidazol-1-yl)methane (dim) and rigid 2,6-bis(1H-imidazol-2-yl)pyridine (2,6-bip) linkers, respectively. Compound 1 possesses a noncoordinating weak field succinate linker and exhibits fluorescence quenching to different extent for Cu(II) and Cu(I) ions. A single-crystal-to-single-crystal (SCSC) structural transformation was carried out using either heating or K₂CO₃ treatment, yielding a 2D MOF, {[Cd(dim)(suc)](H₂O)}_n (1a), with no accessible nucleophilic center; therefore, it does not show any changes in fluorescence spectra in the presence of metal ions. Compound 2, having a 2D framework, has no accessible nucleophilic centers and is also unaffected by emission behavior in the presence of any metal ions. Upon air exposure of 2, an SCSC structural transformation occurs, to produce 2a, {[Cd(2,6-bip)(Suc)(imi)](H₂O)}_n, with pendent imidazole ligand. The free nitrogen-based nucleophilic centersof imidazole is responsible for the fluorescence enhancement of 2a in presence of Cu(II) ion. This phenomena provides an excellent opportunity to use this MOF for Cu(II) sensing. The limits of detection of Cu(II) for 1 and 2a are 6 × 10^-6 and 2 × 10^-6 (M), respectively. Photoluminescence-based sensing was also performed with some other metal ions of different oxidation states, for comparison. Time-resolved fluorescence spectroscopy nicely substantiates the steady-state fluorescence findings.