Air-Stability and Carrier Type in Conductive M₃(Hexaaminobenzene)_2, (M = Co, Ni, Cu)

Herein, we investigate the effects of changing the metal ions in the M-HAB system, with HAB = hexaaminobenzene ligands and M = Co, Ni, Cu. The phyiscal characteristics of this MOF family are insensitive to changes in the metal cation, which enables systematic evaluation of the effect of metal cation identity on electrical transport properties. We observe that the metal ion profoundly influences the electrical conductivity and dominant carrier type in the resulting MOF and the air-stability thereof. Cu-HAB and Co-HAB are determined to exhibit n-type conduction under both ambient and nitrogen conditions; Ni-HAB is found to be ambipolar, with its dominant carrier type dramatically affected by the environment. We examine these results through calculation of the band structure, the partial density of states, and charge transfer analysis. Unlike traditional conductive organic materials, we find that the air-stability is not well predicted by the LUMO level of these n-type MOFs but instead is additionally dependent on the occupancy and orientation of the metal ion's d-orbitals and the resulting interaction between the metal ion and ligand. This study provides fundamental insights for rational design of air-stable, electronically conductive MOFs.