Single Crystals of Electrically Conductive 2D MOFs: Structural and Electrical Transport Properties

Abstract

Crystalline, electrically conductive, and intrinsically porous materials are rare. Layered 2D metal-organic frameworks (MOFs) break this trend. They are porous crystals that exhibit high electrical conductivity and are novel platforms for studying fundamentals of electricity and magnetism in two dimensions.1-8 Despite demonstrated applications,9-13 electrical transport in these remains poorly understood because of a lack of single crystal studies. Here, studies of single crystals of two 2D MOFs, Ni3(HITP)2 and Cu3(HHTP)2, uncover critical insights into their structure and transport. Conductivity measurements down to 0.3 K suggest metallicity for mesoscopic single crystals of Ni3(HITP)2, which contrasts with apparent activated conductivity for polycrystalline films. Microscopy studies further reveal that these MOFs are not isostructural as previously reported.14 Notably, single rods exhibit conductivities up to 150 S/cm, which persist even after prolonged exposure to the ambient. These single crystal studies confirm that 2D MOFs hold promise as molecularly tunable platforms for fundamental science and applications where porosity and conductivity are critical.<br>

Electrically Conductive Metal–Organic Frameworks

“…Moreover, only one study on single crystals contacted along the conjugation direction has been reported. 79 Hence, interlayer effects are expected to influence the reported bulk properties. Molecular metal–dithiolene and diiminobenzosemiquinonate complexes exhibit semiconducting or metallic behavior with the highest conductivities along the stacking direction, 142 , 144 suggesting that transport facilitated by noncovalent c direction interactions may also contribute to the conductivities measured in powder samples of 2D MOFs.…”

“…This π–d conjugation allows for efficient delocalization of charge carriers within the plane, with such frameworks exhibiting the highest conductivities for MOFs. 21 , 30 , 75 − 79 …”

“…However, recent studies on triphenylene-based MOFs suggest charge transport through π–π stacking indeed contributes to the high bulk conductivities observed. 79 , 92 …”

Conductive metal‐organic frameworks: Recent advances in electrochemical energy‐related applications and perspectives

“…The product synthesized by nickel chloride/copper sulfate and TAB·4HCl (TAB = 1,2,4,5‐tetraaminobenzene tetrahydrochloride) in the aqueous solution at 65°C to 70°C is defined as MTIB (M = Ni, Cu) 39 . M 3 (HHTP) 2 (M = Cu, Ni) is the product of metal salt and HHTP, which reacts at 85°C in an aqueous solution 47 . Anyway, some MOFs are still obtained at higher synthesis temperature.…”

Ligand Engineering in Cu(II) Paddle Wheel Metal-Organic Frameworks for Enhanced Electrical Conductivity

“…The number of known intrinsically conductive MOFs is small, although it has increased substantially during the last decade [9][10][11][12][13] . Charge carrier movement within the frameworks is often supressed due to poor conjugation pathways across node and ligand, arising from a mismatch in energy and/or symmetry of the frontier orbitals of the building blocks.…”