Band gap, sorption properties and fluorescence sensing behaviour of a novel 1D→2D cathenane-like cobalt(II)-organic framework.

A novel hydrolytic stable Co-organic framework, namely poly[[bis(2-amino-4-sulfonatobenzoato-κO)tetraaquatris{μ-1,4-bis[(imidazol-1-yl)methyl]benzene-κN:N}dicobalt(II)] tetrahydrate], {[Co(CHNOS)(CHN)(HO)]·2HO}, (1), based on multifunctional 2-amino-5-sulfobenzoic acid (Hasba) and the auxiliary flexible ligand 1,4-bis[(imidazol-1-yl)methyl]benzene (bix), was prepared using the solution evaporation method. The purity of (1) was confirmed by elemental analysis and powder X-ray diffraction (PXRD) analysis. Complex (1) shows a novel 1D→2D interpenetrating network, which is further extended into a 3D supramolecular framework with channels occupied by the lattice water molecules. The 2-amino-4-sulfonatobenzoate (asba) ligand adopts a monodentate coordination mode. The bix ligands exhibit gauche-gauche (GG) and trans-trans (TT) conformations. A detailed analysis of the solid-state diffuse-reflectance UV-Vis spectrum reveals that an indirect band gap exists in the complex. The band structure, the total density of states (TDOS) and the partial density of states (PDOS) were calculated using the CASTEP program. The calculated band gap (E) matches well with the experimental one. The complex exhibits a reversible dehydration-rehydration behaviour. Interestingly, gas sorption experiments demonstrate that the new fully anhydrous compound obtained by activating complex (1) at 400 K shows selective adsorption of CO over N. Complex (1) retains excellent framework stability in a variety of solvents and manifests distinct solvent-dependent fluorescence properties. Moreover, the complex shows multiresponsive fluorescence sensing for some nitroaromatics in aqueous medium.