Introduction
Metal–organic frameworks (MOFs) are crystalline, porous coordination solids formed by the self-assembly of metal ions and organic ligands. Reference Hoskins and Robson1,Reference Yaghi, Li and Li2 During the past two decades, MOFs have emerged as a highly versatile platform for wide-ranging potential applications from gas storage to separations, heterogeneous catalysis, and biomedicine, among others. Reference Batten, Neville and Turner3,Reference Farrusseng4 One property that has generated recent attention is electrical conductivity, which, if realized, could provide vast potential technological impacts across the areas of chemiresistive sensing, thermoelectric, and clean-energy technologies (e.g., fuel cells, electrocatalysts, energy storage). While the inherent porosities of MOFs are conducive to applications in gas storage, separations, and catalysis, as well as ionic conductors, this inherent porosity often impedes long-range charge transport. Moreover, many robust MOFs are constructed from redox-inactive metal centers (e.g., ZnII), and carboxylate-based ligands, which do not mediate overlap of the frontier orbitals required to bring about charge delocalization. As a result, MOFs are typically electrical insulators.
Despite the inherent research challenges, the past five years have yielded the first examples of MOFs that exhibit semiconducting and conducting capabilities. Reference D’Alessandro, Kanga and Caddy5–Reference D’Alessandro10 In this article, we discuss the exciting research frontier of conducting MOFs, with an emphasis on experimental reports of systems that display intrinsic conductivity, wherein conductivity is an inherent property, rather than being induced by augmentations such as the exchange of guest molecules Reference Allendorf, Foster, Léonard, Stavila, Feng, Doty, Leong, Ma, Johnston and Talin9 or ionic conduction through the framework pores. We also provide a brief historical perspective of the field, given that conductivity in the broader context of coordination polymers has been of interest for more than two decades. Reference Givaja, Amo-Ochoa, Gomez-Garcia and Zamora6
General design considerations
A prerequisite for conductivity in MOFs is delocalization of charge, either through a highly delocalized connector or by virtue of the close proximity of the components in space. While an understanding of the mechanisms of charge transport in MOFs is in its infancy, it is generally believed that charge hopping between components predominates for semiconducting MOFs, including mixed-valence frameworks such as Prussian blue, Reference England, Kathirgamanathan and Rosseinsky11,Reference Rosseinsky, Tonge, Berthelot and Cassidy12 and can explain electron transfer in MOF films during electrochemical experiments. Reference Ahrenholtz, Epley and Morris13 The variable-range hopping model, postulated by Mott Reference Mott14 and originally developed for highly disordered systems with localized charge carriers, describes low-temperature conductivity (σ) in semiconductor solids with localized charge-carrier states, where conduction in three-dimensional systems follows a temperature (T) dependence of log σ ∼ T –1/4. This model has had mixed success in the interpretation of MOF conductivity data. Reference Amo-Ochoa, Welte, Gonzalez-Prieto, Sanz Miguel, Gomez-Garcia, Mateo-Marti, Delgado, Gomez-Herrero and Zamora15 As electronic delocalization increases, the distinction between redox components diminishes because the thermal barrier to electron transfer between them ceases to exist. In the high delocalization limit, the material approaches a metallic state with essentially “free” charge carriers and the conductivity is perhaps best described by employing band theory.
In light of these general mechanisms, a number of synthesis approaches for the development of MOFs have been identified that promote long-range charge-transport pathways via either charge hopping or band transport. Reference Sun, Campbell and Dincă7 Many of these exploit donor–acceptor or π-interactions (which can be through-bond or through-space interactions). Through-bond mechanisms take advantage of covalent bonding between appropriately matched metals and ligands; through-space mechanisms rely on noncovalent interactions. While not covered in this article, it is also important to note that post-synthesis modulation (such as via redox-state changes Reference D’Alessandro10 or intercalation of guest molecules Reference Allendorf, Foster, Léonard, Stavila, Feng, Doty, Leong, Ma, Johnston and Talin9 ) has also been shown to modulate the conductivities of some MOFs.
Experimental milestones in intrinsically conducting MOFs
In this section, we review key developments in the field within the context of different strategies that have been employed to achieve long-range conductivity in MOFs.
Promoting conduction pathways via redox matching: Exploiting mixed-valency and donor–acceptor interactions
The integration of donor and acceptor components into MOFs to promote charge-transfer (CT) interactions has shown promise for the design and construction of conductive systems. Reference D’Alessandro, Kanga and Caddy5,Reference D’Alessandro10,Reference Faust and D’Alessandro16 Here, appropriate matching of the electronic energy levels of the components can be exploited to bring about CT, which occurs when electron donor (D) and acceptor (A) moieties are arranged in close proximity, interacting either through-bond or through-space (i.e., D to A [D0A0 ↔ Dδ+Aδ−, where 0 < δ < 1]). Reference Miyasaka17 The degree of CT (ρ) is largely dictated by the overlap in frontier orbitals and by the ionization potential and electron affinity of D and A, respectively.
The donor–acceptor strategy has also drawn inspiration from the historically rich field of chemistry known as “organic metals.” This area flourished following the 1973 discovery of the archetypal CT material tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ), which was found to exhibit metallic conductivity comparable to that of copper (1.47 × 104 S·cm–1 versus 6 × 105 S·cm–1 in copper) ( Figure 1a–b). Reference Ferraris, Cowan, Walatka and Perlstein18,Reference Alves, Molinari, Xie and Morpurgo19 Of particular interest was the anisotropy of conductivity with respect to the direction of D and A stacking in a TTF-TCNQ crystal. Reference Cohen, Coleman, Garito and Heeger20 Systematic investigations of purely organic CT complexes experimentally demonstrated the “Torrance V-shaped diagram” (see Figure 1c), which shows the relationship between the redox match of the D and A components (difference in their redox potentials) and the energy of the CT transition. Reference Torrance, Vazquez, Mayerle and Lee21 The extent of the redox match, and therefore the degree of ρ, gives rise to three regimes: an ionic system where ρ = 1, a neutral system where ρ = 0, and an intermediate region where 0 < ρ < 1. It is in this intermediary region where transitions can occur between neutral and ionic states upon application of external stimuli such as temperature, pressure, and light. Reference Saito and Yoshida22 Importantly, CT complexes exhibit metallic behavior in the “sweet spot,” where 0.5 < ρ < 0.75 (the minimum in Figure 1c), Reference Goetz, Vermeulen, Payne, Kloc, McNeil and Jurchescu23 which is known to be highly dependent on the stacking arrangement of donor and acceptor units (i.e., mixed-stack materials) (⋅⋅⋅DADADA⋅⋅⋅) typically exhibit insulator or semiconducting behavior, whereas segregated stack materials (⋅⋅⋅DDDDD⋅⋅⋅ and ⋅⋅⋅AAAAA⋅⋅⋅ such as TTF-TCNQ) tend toward metallic conductivity.
Figure 1. (a–b) Structures of tetrathiafulvalene (TTF) and tetracyanoquinodimethane (TCNQ) with their charge-transfer (CT) salt, a TTF–TCNQ compound in which there is a partial degree of charge transfer between the donor TTF and acceptor TCNQ, showing the segregated stacking arrangement of TTF and TCNQ. Individual columns of each molecule are stacked along the crystallographic b-axis, into the page. (a) Reprinted with permission from Reference Reference Alves, Molinari, Xie and Morpurgo19. © 2008 Nature Publishing Group. (b) Adapted with permission from Reference Reference Cohen, Coleman, Garito and Heeger20. © 1974 IOP Publishing. (c) Schematic of Torrance’s “V-shaped” diagram showing the relationship between the energy of CT, ΔE CT, and the difference between donor and acceptor ionization potential and electron affinity, respectively. The red region highlights the range in which partial CT and metallic conductivity are observed. Reprinted with permission from References Reference Torrance, Vazquez, Mayerle and Lee21 and Reference Goetz, Vermeulen, Payne, Kloc, McNeil and Jurchescu23. © 1981 American Physical Society and 2014 Royal Society of Chemistry, respectively. Note: h, Planck’s constant; ν, frequency; HOMO, highest occupied molecular orbital; LUMO, lowest unoccupied molecular orbital; HOMOdonor, highest occupied molecular orbital for the donor; LUMOacceptor, lowest unoccupied molecular orbital for the acceptor; E donor, donor energy; E acceptor, acceptor energy.
While the donor and acceptor can have different identities, they may also be alike in all respects other than their formal oxidation states. In cases where the D and A units are the same, mixed valency may be found, as exemplified by Prussian blue, with formula Fe4 III[FeII(CN)6]3·xH2O, which has been used as a pigment since the early 17th century. Reference Buser, Schwarzenbach, Petter and Ludi24 While not strictly a MOF, this framework comprises the donor and acceptor units FeII and FeIII, respectively, and exhibits intervalence CT (IVCT) between D and A units (i.e., rapid oscillation of electrons between these units). In this case, IVCT is mediated by CN– bridges (through-bond) and accounts for the semiconductor-type conductivity (5.5 × 10–5 S·cm–1) in this material. Reference Robin25,Reference Behera, D’Alessandro, Soheilnia and Long26 The conductivity can be improved through substitution of FeII/III with second-row transition metals such as RuII/III. In the latter case, K1.2RuIII 3.6[RuII(CN)6]3·16H2O exhibited a markedly improved conductivity of 5.69 × 10–3 S·cm–1, which was attributed to the more diffuse frontier orbitals of ruthenium. Reference Behera, D’Alessandro, Soheilnia and Long26
More recently, Long and co-workers reported the highest intrinsic conductivity in a three-dimensional MOF to date (0.16(1) S·cm–1) ( Figure 2 ). Reference Darago, Aubrey, Yu, Gonzalez and Long27 [(NBu4)2FeIII 2(dhbq)3] (where NBu4 = tetra-n-butylammonium and dhbq = dihydroxybenzoquinone [Figure 2]) exhibited through-bond mixed valency due to dhbq2−/3− ligand-based IVCT (Figure 2d). Chemical reduction of the MOF to Na0.9([NBu4]1.8Fe2[dhbq]3) produced more of the radical dhbq3− species, resulting in a lowering in the intensity of the IVCT band (Figure 2c) as well as a decrease in the material’s conductivity (0.0062(1) S·cm–1 [Figure 2b]). It is interesting to note the link between the spectroscopic IVCT transitions and the conductivity properties, which are discussed further. Reference Darago, Aubrey, Yu, Gonzalez and Long27,Reference Talin, Centrone, Ford, Foster, Stavila, Haney, Kinney, Szalai, El Gabaly, Yoon, Leonard and Allendorf28
Figure 2. (a) Crystal structure of (NBu4)2[FeIII 2(dhbq)3] showing the redox-active unit (inset). (Na)0.9(NBu4)1.8[FeIII 2(dhbq)3] is formed on chemical reduction, leading to a decrease in both (b) the conductivity (σ) and (c) the intensity of the intervalence charge-transfer band. For (a), orange, red, and gray spheres represent Fe, O, and C atoms, respectively. In (b), black lines are fits to the conductivity profiles. (d) The observed reduction in conductivity is consistent with the reduction of dhbq2– to dhbq3–, R = H. Reprinted with permission from Reference Reference Darago, Aubrey, Yu, Gonzalez and Long27. © 2015 American Chemical Society. Note: NBu4, tetra-n-butylammonium; dhbq, dihydroxybenzoquinone; T, temperature; F (R), Kubelka-Munk function; R, reflectance.
Mixed valency has also been implicated as the origin of conductivity in Cu[Cu(pdt)2] (pdt = 2,3-pyrazinedithiolate) developed by Kitagawa and co-workers, which contains the donor CuI and acceptor [CuIII(pdt)2]–. Reference Takaishi, Hosoda, Kajiwara, Miyasaka, Yamashita, Nakanishi, Kitagawa, Yamaguchi, Kobayashi and Kitagawa29 The reasonably high conductivity of 6 × 10−4 S·cm–1 at 300 K was attributed to bistability of the mixed-valence CuI[CuIII(pdt)2] and CuII[CuII(pdt)2] states. The possibility of an IVCT band in the infrared spectrum of Cu[Cu(pdt)2] also suggested the significance of a relationship between low-energy CT interactions and conductivity. A related material, Cu[Ni(pdt)2], was found to have significantly lower conductivity (1 × 10−8 S·cm–1), consistent with its larger optical bandgap. Reference Kobayashi, Jacobs, Allendorf and Long30
Seminal work by Miyasaka and Dunbar involved two-dimensional (2D) layered materials with the chemical formula [(M2(O2CCF3)4)2(TCNQ)]·C7H8 (where M = Ru, Rh, O2CCF3 = trifluoroacetate, and C7H8 = toluene) which integrated the electron-rich dinuclear complex [M2(O2CCF3)4] and electron–acceptor TCNQ ( Figure 3 ). Reference Miyasaka, Campos-Fernández, Clérac and Dunbar31,Reference Miyasaka, Motokawa, Matsunaga, Yamashita, Sugimoto, Mori, Toyota and Dunbar32 Partial CT was observed from the metal node to TCNQ, which was attributed to significant metal–ligand π-backbonding. As a result, the materials are mixed valence. By substituting TCNQ for its more electron-deficient derivative TCNQF4, one-electron transfer from [Ru2(O2CCF3)4] to TCNQF4 was observed. Reference Miyasaka, Izawa, Takahashi, Yamashita and Dunbar33 This manifested in strong intralayer magnetic coupling as well as 100× greater conductivity (4.6 × 10−4 S·cm–1).
Figure 3. (a) General scheme for the construction of [(Ru2(O2CCF3)4)2(μ4-TCNQ)]·(C7H8) frameworks showing the donor [Ru2(O2CCF3)4] and acceptor TCNQR x components, (b) the charge-transfer (CT) interactions leading to intervalence CT (IVCT), and (c) a packing diagram of the 2D network projected along the c-axis. Each of the four TCNQ cyanide arms is bound to a ruthenium center (indicated by μ4). Pink, red, green, and gray lines represent Ru, O, H, and C atoms, respectively. In (c), the gray box represents the unit cell for the packing diagram projected along the c-axis. Reprinted with permission from Reference Reference Miyasaka, Motokawa, Matsunaga, Yamashita, Sugimoto, Mori, Toyota and Dunbar32. © 2010 American Chemical Society. Note: TCNQ, tetracyanoquinodimethane; S, spin component; R, substituents on the central ring, H4 for TCNQ.
Clearly, the strategy of redox matching between donor and acceptor components can be exploited to modulate CT, leading to conducting behavior. Dincă and co-workers developed analogues of the MOF-74 series of frameworks in which the oxygen donor atoms were replaced with sulfur donors, [M2(DSBDC)] (M = MnII and FeII, DSBDC = 2,5-disulfhydrilbenzene-1,4-dicarboxylate). Reference Sun, Miyakai, Seki and Dincă34 In the MnII analogue, a relatively high AC charge mobility of 0.01 cm2 V–1 s–1 was derived from time-resolved microwave conductivity measurements, which is comparable to organic semiconductors. Infinite Mn–S chains found within the framework were proposed to serve as conduction pathways. Substitution of the O for S as the donor atom resulted in an order of magnitude increase in conductivity, from 3.0 × 10–13 to 1.2 × 10–12 S·cm–1 for the MnII analogue and 4.6 × 10–8 to 5.8 × 10–7 S·cm–1 for the FeII analogue. Reference Sun, Hendon, Minier, Walsh and Dincă35 Redox matching has also been implicated in an iron 1,2,3-triazolate framework, [Fe(C2N3H2)2] (C2N3H2 = 1H-1,2,3-triazole), which showed a room-temperature conductivity of 0.77 × 10–4 S·cm–1; however, the origins of this behavior were not elucidated. Reference Gandara, Uribe-Romo, Britt, Furukawa, Lei, Cheng, Duan, O’Keeffe and Yaghi36
Exploiting π-interactions (π-π stacking and π-conjugation)
Both through-bond and through-space π-interactions have proven to be particularly effective in promoting conductivity in MOFs. Reference Panda and Banerjee37 Dincă and co-workers employed a ligand, based on the well-known electron donor TTF, which was decorated with benzoate groups for incorporation into the family of MOFs [M2(TTFTB)] (M = MnII, CoII, ZnII, CdII; TTFTB4– = tetrathiafulvalene tetrabenzoate). Reference Narayan, Miyakai, Seki and Dincă38,Reference Park, Hontz, Sun, Hendon, Walsh, Van Voorhis and Dincă39 These MOFs exhibited a similar stacking arrangement between the TTF moieties as that observed in the aforementioned “organic-metal” TTF-TCNQ (S···S stacking distance ≈3.8 Å in both) (Figure 1). Reference Alves, Molinari, Xie and Morpurgo19 As shown in Figure 4 , the MOFs are composed of alternating channels and infinite π-π stacked helical TTF moieties along the c-axis. Reference Narayan, Miyakai, Seki and Dincă38,Reference Park, Hontz, Sun, Hendon, Walsh, Van Voorhis and Dincă39 The conductivity was found to be strongly dependent on the separation between the TTF layers, which was in turn dictated by the size of metal ions, with single-crystal conductivity values ranging from approximately 10–6 S·cm–1 (M = ZnII and CoII) to 10–5 and 10–4 S·cm–1 for M = MnII and CdII, respectively, in accord with the increase in the cation radii (and the corresponding decrease in the S···S contact distance). The origin of the intrinsic conductivity was attributed to partial oxidation of the TTF moieties to radical cations in the as-synthesized state, which improves the charge-carrier density. The incorporation of stable organic radicals thus represents an important tactic for improving long-range charge mobility. Reference Faust and D’Alessandro16
Figure 4. A helical tetrathiafulvalene (TTF) stack in the crystal structure of [Zn2(TTFTB)] showing the shortest intermolecular S···S contact (dashed purple line). The tetrathiafulvalene tetrabenzoate (TTFTB) ligands (shown in the inset) run parallel to infinite chains of metal carboxylates. Orange, yellow, red, and gray spheres represent Zn, S, O, and C atoms, respectively. Reprinted with permission from Reference Reference Narayan, Miyakai, Seki and Dincă38. © 2012 American Chemical Society.
Planar MOFs with extended 2D π-conjugation and graphene-like structures are currently among the most conductive frameworks known. These materials exhibit stacked honeycomb lattices that often incorporate benzene or triphenylene-based ligands with square planar metal centers ( Figure 5 ). An interesting property is the relative disposition of the sheets, which can be eclipsed, stepped-parallel, or staggered, giving rise to one-dimensional pores of differing dimensions. One reason for the high conductivities of these materials is believed to be the presence of increased charge density: to achieve charge balance and a neutral framework with the MII centers, each of the ligands is expected to be oxidized. Reference Sun, Campbell and Dincă7
Figure 5. (a–b) Schematic structure of [Ni3(BHT)2] where the electron is delocalized throughout the entire 2D nanosheet. In (a), red spheres represent the nickel centers, and each blue triangle represents a benzenehexathiol (BHT) ligand. Yellow circle indicates a fragment of the structure that is represented in (b). In (b), the green, yellow, and gray spheres (lightly shaded below the chemical structure) represent the Ni, S, and C atoms, respectively. (c) Scanning tunneling microscope image of a single layer of the metal–organic framework on highly oriented pyrolytic graphite (HOPG) showing a cross-sectional analysis (inset) with a zoomed-in view of the hexagonal structure (which is a moiré interference pattern) in the white box, also shown in (d). The cross-sectional analysis in the inset plot shows the thickness of the 2D nanosheet (called nano-1) as a function of the distance. These data show that the thickness was estimated to be 0.6 nm. The red line shows the distance over which the scan was conducted. (d) The insets show the fast Fourier transform (FFT) of the image (upper right) and the FFT-filtered image (lower left). (a) Reprinted with permission from Reference Reference Kambe, Sakamoto, Hoshiko, Takada, Miyachi, Ryu, Sasaki, Kim, Nakazato, Takata and Nishihara41. © 2013 American Chemical Society. (a,d) Reprinted with permission from Reference Reference Kambe, Sakamoto, Kusamoto, Pal, Fukui, Hoshiko, Shimojima, Wang, Hirahara, Ishizaka, Hasegawa, Liu and Nishihara43. © 2014 American Chemical Society.
A number of relatively highly conductive 2D MOFs have been developed. These include the family of metal catecholate frameworks of NiII and CoII constructed from hexahydroxytriphenylene (H6HHTP), which consist of 2D sheets layered between discrete HHTP complexes. Reference Hmadeh, Lu, Liu, Gandara, Furukawa, Wan, Augustyn, Chang, Liao, Zhou, Perre, Ozolins, Suenaga, Duan, Dunn, Yamamto, Terasaki and Yaghi40 More recently, the groups of Nishihara and Dincă have reported the 2D MOFs [Ni3(BHT)2] (BHT = benzenehexathiol) Reference Kambe, Sakamoto, Hoshiko, Takada, Miyachi, Ryu, Sasaki, Kim, Nakazato, Takata and Nishihara41 and [Ni3(HITP)2] (HITP = hexaiminotriphenylene), respectively. Reference Sheberla, Sun, Blood-Forsythe, Er, Wade, Brozek, Aspuru-Guzik and Dincă42 These materials exhibited strong π-conjugation within their structures, with conductivities of 0.15 and 2 S·cm–1, respectively.
The materials [Ni3(BHT)2] and [Ni3(HITP)2] also serve to illustrate the importance of fabrication methods and growth conditions for conductive MOFs. For example, a bulk powder of [Ni3(BHT)2] consisted of staggered honeycomb layers, but single nanosheets produced at the liquid–liquid interface revealed a conductivity of 160 S·cm–1. Reference Kambe, Sakamoto, Kusamoto, Pal, Fukui, Hoshiko, Shimojima, Wang, Hirahara, Ishizaka, Hasegawa, Liu and Nishihara43 An analogous CuII structure produced as a thin film revealed a conductivity of 1580 S·cm–1—a record value at room temperature. Reference Huang, Sheng, Tu, Zhang, Wang, Geng, Zou, Di, Yi, Sun, Xu and Zhu44
A number of related 2D MOFs have subsequently been reported, Reference Pal, Kambe, Kusamoto, Foo, Matsuoka, Sakamoto and Nishihara45 and their range of applications have been expanded to include electrocatalysis Reference Cui and Xu46–Reference Dong, Pfeffermann, Liang, Zheng, Zhu, Zhang and Feng49 and chemiresistive sensing. Reference Campbell, Liu, Swager and Dincă50,Reference Campbell, Sheberla, Liu, Swager and Dincă51 Extensive computational modeling is also underway to understand the influence of structural features such as the stacking geometries. Reference Shojaei, Hahn and Kang52
Conclusions and outlook
The remarkable structural characteristics of MOFs, coupled with their permanent porosities and the exquisite levels of control that can now be achieved in their synthesis, provides an extraordinary foundation for the discovery of conducting microporous solids. During the past five years, important advances have been made in this area. Building upon the foundations laid in coordination polymers, Reference Givaja, Amo-Ochoa, Gomez-Garcia and Zamora6 a number of clear design parameters have now been outlined that provide a future roadmap for the field.
Elucidating these structure–function relationships in conductive MOFs, however, is still in its infancy. In tandem with experimental work, theoretical calculations are now emerging that tackle the significant challenge of understanding charge transport in large, periodic, multidimensional systems. The long-range connectivity and, unavoidably, the nonuniform or defective nature of MOFs pose challenges for reconciling computational calculations with experimental observations. Reference Cheetham, Bennett, Coudert and Goodwin53
A critical issue mentioned in this article and discussed in detail elsewhere, Reference Givaja, Amo-Ochoa, Gomez-Garcia and Zamora6,Reference Sun, Campbell and Dincă7 is the enormous variability in measurement methods used to assess conductive MOFs. The most commonly used method is the four-point probe technique on pressed pellets; Reference Givaja, Amo-Ochoa, Gomez-Garcia and Zamora6 however, there are issues with grain-boundary effects, to which single crystals represent a superior option, enabling measurements of the conduction anisotropy. To circumvent the existence of a contact–sample interface, which can introduce significant variation into the measurements, the development of nondestructive contactless techniques has received considerable attention. One such technique, microwave conductivity, has been widely used to determine the intrinsic conductivity of semiconducting and superconducting materials and has recently been applied to MOFs. Reference Sun, Miyakai, Seki and Dincă34,Reference Narayan, Miyakai, Seki and Dincă38
Despite the range of different conductivity techniques available, no direct comparison between them has been made to date, which is essential for the standardization of results. It is also interesting to note the connection drawn between spectroscopic measurements of the CT bands and conducting properties. This spectroscopic approach has received limited attention; Reference England, Kathirgamanathan and Rosseinsky11,Reference Darago, Aubrey, Yu, Gonzalez and Long27,Reference Talin, Centrone, Ford, Foster, Stavila, Haney, Kinney, Szalai, El Gabaly, Yoon, Leonard and Allendorf28 however, it may become an important complement to conventional conductivity measurements. While the as-synthesized state of a MOF may not be conducting, others may be, and solid-state spectroelectrochemical methods have emerged as useful tools for exploring redox-accessible states. Reference D’Alessandro10,Reference Usov, Fabian and D’Alessandro54,Reference Leong, Chan, Faust and D’Alessandro55
As a relatively new field of research, MOFs have demonstrated myriad unique and desirable properties that are not found in discrete or polymeric systems. Though faced with a number of key challenges, the development of conductive MOFs is envisioned to produce an emergent class of tunable solid-state conductors with substantial untapped potential for areas of technological and industrial interest.
Acknowledgment
The authors gratefully acknowledge the support of the Australian Research Council.
Chanel F. Leong is currently pursuing her PhD degree in The University of Sydney’s School of Chemistry. She completed her bachelor of science degree at the University of Technology, Sydney, in 2011, and her bachelor of science degree at The University of Sydney in 2012. Her research focuses on the development of electroactive metal–organic frameworks (MOFs) featuring charge-transfer interactions as a strategy for the development of conductive MOFs. Leong can be reached by phone at +61 2 93517482 or by email at chanel.leong@sydney.edu.au.
Pavel M. Usov joined Virginia Polytechnic Institute and State University in January 2016 as a postdoctoral associate. He received his bachelor of science degree at The University of Adelaide, South Australia, in 2010, and his bachelor of science degree at The University of Sydney in 2011. He completed his PhD degree in 2015, where he studied the development of solid-state spectroelectrochemical techniques for interrogating electroactive metal–organic frameworks. Usov can be reached by phone at 540-231-5391 or by email at pusov@vt.edu.
Deanna M. D’Alessandro created her independent research group at The University of Sydney exploring the development of electroactive metal–organic frameworks (MOFs), including new spectroscopic methods for their study in 2011. She completed her PhD degree at James Cook University, Australia, in 2006. She held a Dow Chemical Company Postdoctoral Fellowship of the American Australian Association, and a Royal Commission for 1851 Fellowship at the University of California, Berkeley, from 2007 to 2009, where she worked on CO2 capture in MOFs. D’Alessandro can be reached by phone at +61 2 9351 3777 or by email at deanna.dalessandro@sydney.edu.au.
