Superior Charge Transport in Ni-Diamine Conductive MOFs

Abstract

Two-dimensional conductive metal-organic frameworks (2D cMOFs) are an emerging class of crystalline van der Waals layered materials with tunable porosity and high electrical conductivity. They have been used in a variety of applications, such as energy storage and conversion, chemiresistive sensing, and quantum information. Although designing new conductive 2D cMOFs and studying their composition/structure-property relationships have attracted significant attention, there are still very few examples of 2D cMOFs that exhibit room-temperature electrical conductivity above 1 S cm^-1, the value exhibited by activated carbon, a well-known porous and conductive material that serves in myriad applications. When such high conductivities are achieved, Ni-diamine linkages are often involved, yet Ni-diamine MOFs remain difficult to access. Here, we report two new 2D cMOFs made through ortho-diamine connections: M₃(HITT)₂ (M = Ni, Cu; HITT = 2,3,7,8,12,13-hexaiminotetraazanaphthotetraphene). The electrical conductivity of Ni₃(HITT)₂ reaches 4.5 S cm^-1 at 298 K, whereas the conductivity of Cu₃(HITT)₂ spans from 0.05 (2Cu⁺Cu²⁺) to 10^-6 (3Cu²⁺) upon air oxidation, much lower than that of Ni₃(HITT)₂. Spectroscopic analysis reveals that Ni₃(HITT)₂ exhibits significantly stronger in-plane π-d conjugation and higher density of charge carriers compared to Cu₃(HITT)₂, accounting for the higher electrical conductivity of Ni₃(HITT)₂. Cu²⁺/Cu⁺ mixed valency modulates the energy level and carrier density of Cu₃(HITT)₂, allowing for a variation of electrical conductivity over 4 orders of magnitude. This work provides a deeper understanding of the influence of metal nodes on electrical conductivity and confirms ortho-diamine linkers as privileged among ligands for 2D cMOFs.