Efficient and tunable one-dimensional charge transport in layered lanthanide metal–organic frameworks

The emergence of electrically conductive metal–organic frameworks (MOFs) has led to applications in chemical sensing and electrical energy storage, among others. The most conductive MOFs are made from organic ligands and square-planar transition metal ions connected into two-dimensional (2D) sheets stacked on top of each other. Their electrical properties are thought to depend critically on the covalency of the metal–ligand bond, and less importance is given to out-of-plane charge transport. Here, we report a series of lanthanide-based MOFs that allow fine tuning of the sheet stacking. In these materials, the Ln3+ ions lie between the planes of the ligands, thus connecting organic layers into a 3D framework through lanthanide–oxygen chains. Here, efficient charge transport is found to occur primarily perpendicular to the 2D sheets. These results demonstrate that high conductivity in layered MOFs does not necessarily require a metal–ligand bond with highly covalent character, and that interactions between organic ligands alone can produce efficient charge transport pathways. High electrical conductivities in metal–organic frameworks—attractive for applications in sensing and energy storage—typically arise in layered MOFs from metal–ligand bonds with strong covalent character. Now, lanthanide-based MOFs have shown high out-of-plane conductivities originating instead from the π-stacking of organic ligands.