Electrically Conductive Metal-Organic Frameworks

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.

Figure 1

Histogram showing cumulative number of papers focusing on aspects of electrical conductivity in MOFs from 2008 to 2019, grouped according to the design strategy employed.

Figure 2

Schematic representations of (a) ballistic band-like charge transport and (b) hopping charge transport. In both modes of transport, electrons move from high to low electric potential energy (denoted as E). In band-like transport, movement of electrons proceeds along a smooth energy landscape, while in hopping transport, movement of electrons is gated by activation barriers (denoted as E_a).

Figure 3

Electronic structures of a generic insulator, semiconductor, and metal, where E_g is the band gap and E_F is the Fermi level.

Figure 4

Orbital representations of potential charge transport pathways operative in MOFs. (a) The through-bond pathway involves orbitals from the metal and ligand functional groups. (b) The extended conjugation pathway also involves π–d conjugation including the ligand core (both are shown here for M = a transition metal). (c) The through-space pathway involves π–π stacking of organic moieties (E = S for tetrathiafulvalene, a common component in MOFs).

Figure 5

Schematic representations of electron transport via (a) a redox hopping mechanism between organic linkers and (b) a guest-promoted pathway involving host–guest interactions between the inorganic framework nodes and guest molecules.

Figure 6

Calculated electronic band structures and densities of states for an insulating MOF, Zn₄O(BDC)₃ (MOF-5); a semiconducting MOF, Zn₂(TTFTB); and a metallic MOF, Ni₃(HIB)₂.

Figure 7

Measurement techniques commonly used for measuring electrical conductivity of MOFs.I and V denote current and voltage, respectively, and L, w, and t denote the length, width, and thickness of the samples, respectively. s denotes the spacing between contacts. R_AB and R_AC are the resistances measured between contacts A and B and A and C, respectively. F₁ and F₂ are correction factors depending on the geometry of the sample and the values of R_AB and R_AC, respectively.

Figure 8

Structure of Fe₂(DSBDC), showing (a) the proposed through-bond conduction pathway comprising (−Fe–S−)_∞ chains, as well as bridging ligand carboxylates and oxygens from coordinated DMF molecules, and (b) hexagonal 1D pores.

Figure 9

Structures of Fe–azolate MOFs. (a–c) SBUs of Fe(1,2,3-triazolate)₂, Fe₂(BDT)₃, and Fe₂(BDP)₃, respectively, showing continuous (−Fe–N–N−)_∞ chains. (d) Connectivity of Fe(1,2,3-triazolate)₂, showing continuous 3D diamondoid network of Fe–N bonds. (e and f) Connectivities of Fe₂(BDT)₃ and Fe₂(BDP)₃, respectively, showing 1D channels.

Figure 10

Redox series of 2–, 1–, and 0 charge states for a deprotonated catecholoid fragment, a common motif in conductive MOFs with extended conjugation.

Figure 11

Structure of [Fe₂(Cl₂dhbq)₃]^2–, showing a honeycomb arrangement of Fe^II/III and Cl₂dbhq^2– within a single 2D layer (charge-balancing [Me₂NH]⁺ cations occupying pore volume not shown).

Figure 12

Redox series of 4–, 3–, and 2– charge states for linkers based on dihydroxybenzoquinone (dhbq).

Figure 13

Structure of [Fe₂(dhbq)₃]^2–, showing (a) a single sublattice illustrating local coordination environments of Fe^III and dhbq^2–/3–•, and (b) two interpenetrated nets (charge-balancing Bu₄N⁺ cations not shown).

Figure 14

Representations of the structures of 2D honeycomb sheets in porous MOFs exhibiting extended conjugation with (a) triphenylene-based linkers and (b) benzene-based linkers, both with various functional groups (X) and framework metals (M).

Figure 15

Structure of Co₉(HOTP)₄, showing (a) view of 2D honeycomb layers with octahedrally coordinated Co centers bridging triphenylene-based linkers, alternating with layers of trinuclear molecular clusters, and (b) stacking arrangement of the extended and molecular layers in the material.

Figure 16

Structure of Cu₃(HOTP)₂, showing (a) 2D honeycomb layers with square planar Cu centers bridging triphenylene-based linkers and (b) continuous slipped stacking arrangement of layers.

Figure 17

Structure of Nd_1+x(HOTP), showing (a) Nd coordination environment and close π–π stacking distances among triphenylene linkers and (b) 1D channels along the ligand stacking direction.

Figure 18

Representation of the structure of square 2D sheets in MOFs based on phthalocyanine (Pc) linkers with various functional groups (X) and framework and central Pc metals (M and M′, respectively).

Figure 19

Representation of the structure of a 2D layer of the coordination polymer Cu₃(BHT), showing dense coordination of square planar Cu centers by benzenehexathiolate linkers.

Figure 20

Structure of Ni₃(HITP)₂, showing (a) 2D honeycomb layers with square planar Ni centers bridging triphenylene-based linkers, and (b) ABAB stacking arrangement of layers.^,

Figure 21

Structure of Cd₂(TTFTB) showing (a) infinite 1D helical stacking of tetrathiafulvalene cores with a continuous close S···S contact of 3.654(2) Å and (b) 1D channels parallel to the stacking direction.

Figure 22

Structures of (a) La₄(TTFTB)₄, (b) Tb₃(TTFTB)₂(OAc)(OH), (c) Tb₄(TTFTB)₃, and (d) La₄(TTFTB)₃ with the longest S···S contact distances indicated.⁻

Figure 23

Pressed pellet conductivities of MOFs with the TTFTB ligand and different metals plotted versus the longest crystallographic S···S contact distance between two neighboring ligands.

Figure 24

Structure of Cu[Cu(pdt)₂] showing square channels formed by pyrazine-linked Cu centers and redox-active [Cu(pdt)₂]^2– units.

Figure 25

(a) Scheme showing photoswitchable isomerization of “open” and “closed” forms of diarylethene derivatives and (b) structure of Zn₂(SDC)₂(BPMTC) with the “open” form of the diarylethene linker.

Figure 26

Structure of Tb(Cu₄I₄)(PCA)₃ containing crystallographically resolved I₂ guests with close I^–···I₂···I^– contacts highlighted.

Figure 27

Predicted structure of TCNQ@Cu₃(BTC)₂ with TCNQ bridging the Cu^II paddlewheel clusters of the MOF to form a continuous charge transport pathway.^,

Figure 28

Illustration of a chain of polypyrrole occupying the pore volume of Zn₃(lac)₂(pybz)₂.^,