Proton conduction and luminescent sensing property of two newly constructed positional isomer-dependent redox-active Mn(II)-organic frameworks

Abstract

Abstract Recently, investigation of proton conduction and luminescent sensing property of crystalline MOFs-based materials have attracted intensive attention. Herein, two luminescent and redox-active Mn(II)-organic frameworks (MOFs) based on the multi-functional 5-iodoisophthalic acid (H2iip) and a pair of positional isomers N,N’-bis(3-pyridylmethyl)oxalamide (3bpmo) and N,N’-bis(4-pyridylmethyl)oxalamide (4bpmo), {[Mn(iip)(3bpmo)0.5·(H2O)]·0.5(3bpmo)}n (1) and {[Mn(iip)(4bpmo)·(H2O)]·(H2O)}n (2), were obtained at ambient conditions. 1 is a 2D (4, 4) topological framework which further extends into a 3D supramolecular architecture pillared by the interstitial 3bpmo molecule via 1D hydrogen-bonded tape between the coordinated 3bpmo ligand and lattice 3bpmo molecule. 2 shows a 2D centro-symmetric double-layer structure. The R22(8) hydrogen-bonded ring further holds the adjacent 2D layers into a 3D supramolecular network. The wide indirect band gap exists in both 1 and 2, which is further illustrated by the calculation based on the Density Functional Theory using Materials studio software. The electrochemical studies suggest that the positional isomers 3bpmo and 4bpmo have great influence on the redox characteristics of 1 and 2. The proton-conductive properties of 1 and 2 in both the solid state and the composite Nafion-MOFs membranes were studied by AC impedance spectroscopy. The proton conductivities of 2 are higher than those of 1 in both cases. 1 and 2 show greatly solvent-dependent luminescent emissions and the strongest emission occurs in water. 1 and 2 exhibit efficient and selective turn off sensing towards Fe3+. Moreover, 1 and 2 show quite different sensing behavior to the Cr(VI) anions. 1 has nearly no fluorescence response to Cr2O72- and CrO42-, while 2 demonstrates sensitive sensing toward both Cr2O72- and CrO42-. The sensing for the Fe3+ cation and the Cr(VI) anions is quite recyclable. The limits of detection (LOD) for these analytes are in ppm order.