Moisture-triggered proton conductivity switching in metal–organic frameworks: role of coordinating solvents

Abstract

Metal–organic frameworks are a good platform for investigating the correlation of structures with physical properties due to the facile coordination environment changes and their responsive structures to external stimuli such as pressure, temperature, and gas sorption. In this study, we report a proton conductivity switching behavior in Zn₅FDC, [Zn₅(μ₃-OH)₂(FDC)₄(solvent)₂] (FDC = 9H-fluorene-2,7-dicarboxylate) triggered by relative humidity (RH). Interestingly, depending on the presence and absence of coordinating molecules the MOFs show distinctively different tendencies in their proton conductivity. Two isostructural Zn₅FDC compounds, [Zn₅(μ₃-OH)₂(DEF)₂(FDC)₄] (Zn₅FDC-DEF) and [Zn₅(μ₃-OH)₂(FDC)₄] (Zn₅FDC-OMS; OMS = open metal site), are prepared, in which the three-dimensional connectivities are identical, but the local structures in the secondary building units (SBUs) are different. In the measurement of humidity-dependent conductivity, both MOFs show a dramatic proton conductivity switching phenomenon (ON/OFF ratio, approximately 10⁸), but the conductivity switching occurs at different RHs for each MOF (above RH 70% in Zn₅FDC-DEF, and above RH 90% in Zn₅FDC-OMS at 298 K). During this process water coordination in metal centers leads to their structural transformation into Zn₅FDC-H₂O, which means that the different coordination structures by the absence/presence of coordination solvents provide different water access environments to metal centers. The computational calculation supports that the structural transformation of Zn₅FDC-OMS triggered by moisture exposure occurred under higher relative humidity conditions than simple coordination solvent replacement in Zn₅FDC-DEF. This study proves that the coordination solvents play a role in conductivity variation, and it provides a new design strategy for functional solid-state proton conductors.