Abstract
Metal–organic frameworks (MOFs) are intrinsically porous extended solids formed by coordination bonding between organic ligands and metal ions or clusters. High electrical conductivity is rare in MOFs, yet it allows for diverse applications in electrocatalysis, charge storage, and chemiresistive sensing, among others. In this Review, we discuss the efforts undertaken so far to achieve efficient charge transport in MOFs. We focus on four common strategies that have been harnessed toward high conductivities. In the “through-bond” approach, continuous chains of coordination bonds between the metal centers and ligands’ functional groups create charge transport pathways. In the “extended conjugation” approach, the metals and entire ligands form large delocalized systems. The “through-space” approach harnesses the π–π stacking interactions between organic moieties. The “guest-promoted” approach utilizes the inherent porosity of MOFs and host–guest interactions. Studies utilizing less defined transport pathways are also evaluated. For each approach, we give a systematic overview of the structures and transport properties of relevant materials. We consider the benefits and limitations of strategies developed thus far and provide an overview of outstanding challenges in conductive MOFs.
Highlights
Metal−organic frameworks (MOFs) are intrinsically porous extended solids formed by coordination bonding between organic ligands and metal ions or clusters
They are relevant as active materials for many applications, including electrocatalysis,[16−20] chemiresistive sensing,[21−26] and energy storage technologies.[27−30] Their fundamental transport properties merit further study, as certain conductive MOFs have been predicted to host topologically nontrivial electronic structures.[31−36] they may contain arrangements of inorganic or organic moieties that are unprecedented in other materials, potentially giving rise to new physical phenomena.[37,38]
The discovery of free-standing graphene[130] in 2004 precipitated an avalanche of interest in the electronic properties of 2D materials.[131−136] In addition to the “top-down” methods commonly used for the preparation of these nanoscale compounds, the possibility of creating synthetic 2D materials via “bottom-up” approaches has been explored.[134−138] Conductive 2D MOFs made via solvothermal self-assembly have emerged as 2D materials with promising transport properties
Summary
Electrical conductivity is a characteristic of a material that quantifies the efficacy of the transport of electrical charge. It is well-established that the spatial separation is inversely correlated with the magnitude of the transfer integral and the charge mobility.[80] In an analogous way, linkers that have a high propensity for π−π interactions can yield MOFs with high conductivities originating from the organic components Work in this direction can be seen as extension of prior research on dense coordination polymers based on tetracyanoquinodimethane (TCNQ), which show electrical conductivities above 0.1 S/cm.[81−83]. For frameworks in which no crystallographic pathways that could facilitate band-like transport can be identified, it is reasonable to assume that conductivity proceeds via a hopping mechanism (Figure 5) In such materials, the presence of redox-active metals or linkers, as well as small spatial separation between these components, can promote charge hopping. In all cases involving this strategy, it should be noted that porosity is either significantly reduced or even completely eliminated, as may be expected by occupying the pores with bulky guests
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