Superior Charge Transport in Ni-Diamine Conductive MOFs.

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, 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: M(HITT) (M = Ni, Cu; HITT = 2,3,7,8,12,13-hexaiminotetraazanaphthotetraphene). The electrical conductivity of Ni(HITT) reaches 4.5 S cm at 298 K, whereas the conductivity of Cu(HITT) spans from 0.05 (2CuCu) to 10 (3Cu) upon air oxidation, much lower than that of Ni(HITT). Spectroscopic analysis reveals that Ni(HITT) exhibits significantly stronger in-plane π-d conjugation and higher density of charge carriers compared to Cu(HITT), accounting for the higher electrical conductivity of Ni(HITT). Cu/Cu mixed valency modulates the energy level and carrier density of Cu(HITT), 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.