Experimental\nand Theoretical Investigation of Anisotropic\nProton Conduction in Two-Dimensional Metal–Organic Frameworks

Abstract

Two-dimensional (2D) materials are known for their potential\nto\nexhibit anisotropic transport properties due to their layered structures.\nHowever, the anisotropic ion conduction of 2D metal–organic\nframeworks (MOFs) has been rarely explored. In this study, we investigated\nthe anisotropic proton conduction along the in-plane and stacking\ndirections of two analogs of undulating 2D MOFs: [Mn(salen)]₂[Pt(CN)₄]·H₂O (<b>MnPt</b>) and\n[Mn(salen)]₂[PtI₂(CN)₄]·H₂O (<b>MnPtI</b>). This investigation was conducted using\nboth experimental methods, involving single crystals, and theoretical\ncalculations. Compared to the relatively isotropic proton conduction\nof <b>MnPt</b> at 85 °C and 95% relative humidity (RH),\nwith a stacking direction conductivity (σ_stacking) of 1.8 × 10^–5 S/cm, which is approximately\n2.9 times the in-plane conductivity (σ_in‑plane), <b>MnPtI</b> exhibited highly anisotropic proton conduction.\nThe σ_stacking of <b>MnPtI</b> under the same\nconditions (85 °C, 95% RH) was 1.5 × 10^–4 S/cm, which is 83 times higher than its σ_in‑plane. Additionally, the activation energy for proton conduction in <b>MnPtI</b> ranged from 0.65 to 0.73 eV, which is higher than the\n0.48 eV observed for <b>MnPt</b>. Theoretical calculations confirmed\nthat slight differences in local structures, including node distortions\nbetween <b>MnPt</b> and <b>MnPtI</b>, significantly influenced\nthe activation energies for water migration. This was attributed to\nthe formation of hydrogen bonds between layers and water molecules.