Most metal-organic frameworks feature highly ionic bonds, resulting in low charge mobility throughout the framework.^1,2,3 The highest performing conductive MOFs, on the other hand, are 2D-connected and feature extended conjugation and non-covalent π-overlap. In those systems, charge delocalization is promoted by delocalization of charge carriers in the out-of-plane direction.¹ However, the intersheet stacking configurations likely play a significant role in determining the conductivity, as the extent of π-overlap decreases as the sheet move away from eclipsed. Thus, there is an outstanding goal of creating a 3D connected conductor, that maintains high, cyclable porosity.
In this paper, we discuss a retrofitting approach to “rewire” 2D conductive MOFs into 3D conductive MOFs. The highest performing 2D MOF, Ni₃(hexaiminotriphenylene)₂, is semiconducting in-plane.⁵ Motivated by this, we elected to study the 2D MOF, Ni₃(hexaiminobenze)₂,⁶ an in-plane metal, with the hypothesis that it may be easier to form a 3D connected conductor from a MOF sheet that itself is metallic, rather than semiconducting. We present a series of plausible post-synthetically tractable modifications to the known conductor, and assess the dynamic stability of these materials using quantum chemical simulation. The electronic properties of the modified, retrofitted MOFs revealed the role of the d_z2 orbitals in retaining the in-plane charge mobility, and highlight how redox activity affects the charge mobility of carriers in the frontier bands. Together, this paper elucidates the key observation that the development of new conductive MOFs may feature ligand pillaring and metal exchange that perhaps gave rise to a novel class of 3D-connected electrical conductors.⁷

Christopher Hendon

University of Oregon

Appearances