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}<i>_n</i> (<b>1</b>) and {[Cd(2,6-bip)(suc)](xSol)}<i>_n</i> (<b>2</b>) were constructed by flexible di(1<i>H</i>-imidazol-1-yl)methane (dim) and rigid 2,6-bis(1<i>H</i>-imidazol-2-yl)pyridine (2,6-bip) linkers, respectively. Compound <b>1</b> 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)}<i>_n</i> (<b>1a</b>), with no accessible nucleophilic center; therefore, it does not show any changes in fluorescence spectra in the presence of metal ions. Compound <b>2</b>, 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 <b>2</b>, an SCSC structural transformation occurs, to produce <b>2a</b>, {[Cd(2,6-bip)(Suc)(imi)](H₂O)}<i>_n</i>, with pendent imidazole ligand. The free nitrogen-based nucleophilic centersof imidazole is responsible for the fluorescence enhancement of <b>2a</b> 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 <b>1</b> and <b>2a</b> 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.