Strategies to Improve Electrical Conductivity in Metal–Organic Frameworks: A Comparative Study

Metal–organic frameworks (MOFs), formed by the combination of both inorganic and organic components, have attracted special attention for their tunable porous structures, chemical and functional diversities, and enormous applications in gas storage, catalysis, sensing, etc. Recently, electronic applications of MOFs like electrocatalysis, supercapacitors, batteries, electrochemical sensing, etc., have become a major research topic in MOF chemistry. However, the low electrical conductivity of most MOFs represents a major handicap in the development of these emerging applications. To overcome these limitations, different strategies have been developed to enhance electrical conductivity of MOFs for their implementation in electronic devices. In this review, we outline all these strategies employed to increase the electronic conduction in both intrinsically (framework-modulated) and extrinsically (guests-modulated) conducting MOFs.


INTRODUCTION
In modern society, the implications of electronic devices are enormous.Currently the main efforts are focused on the miniaturization and more efficient device fabrication at the nanoscale. 1,2Although classic metals, such as Cu, Ag, etc., are commonly used as conducting materials, there are also many applications that require semiconductors, 3 including inorganic 4 and organic ones. 5−8 The electron conduction in these inorganic materials follows the band model. 9On the other hand, organic molecules, charge transfer donor−acceptor molecules and organic polymers have gained much attention in organic electronics. 10Electron conduction in organic molecules and polymers depends on the conjugation of π-electrons, while the electron/hole transfer occurs between the donor−acceptor pairs. 11−18 The most interesting feature of MOFs is that their structure, topology, and pore functionality can be tuned in a controlled manner through judicious selection of metal ions and bridging organic ligands. 19,20These distinct structural features of large porosity, chemically functionalized cavities, flexible skeletons, etc., make them suitable for gas and solvent adsorption, 21,22 storage 23 and separation, 24 catalysis, 25 sensing, 26 drug delivery, 27 etc.Recently, electrically conductive MOFs 28,29 have gained much attention for their numerous applications in electrocatalysis, 30 capacitors, 31 charge storage, 32 chem-resistive sensing, 33 etc., along with their porosity-based functionalities.Generally, small organic ligands, as used in conventional MOF synthesis, are weak electrical conductors, and the poor overlap between p-orbitals of ligands and d-orbitals of metal ions results in insulating or poorly conducting MOFs, which constitutes the major obstacle for practical applications of conducting MOFs. 34However, conducting MOFs have several advantages, like: (a) the structural rigidity and the doping of inorganic semiconductors can be modified through organic functionalization; (b) amorphous organic polymers can be converted into crystalline MOF structures trough bonding with metal ions; (c) the infinite choice of metal ions and organic bridging ligands provides enormous opportunity to modulate the structure and topology, i.e., the compositional versatility offers the possibility to modulate the functionality through infinite ways; (d) detailed structural analysis offers a platform to tailor the electrical properties for suitable and desired applications; and (e) the combination of metal ions and organic ligands offers the possibility to explore multifunctionality like chem-resistive sensors based on porosity and conductivity, magneto-resistive devices based on conducting and magnetic MOFs, etc.
In 2000, Coronado and Goḿez-Garci ́a et al. reported the electrical properties of a host−guest MOF formed by the encapsulation of BEDT-TTF within the interlamellar space of a 2D magnetically ordered framework. 35Four-probe electrical conductivity measurements within the plane showed a very high room temperature conductivity value of ∼250 S cm −1 and metallic conductivity within the temperature range of 2−300 K.The material also shows ferromagnetic long-range order below 5.5 K with the presence of magnetic hysteresis at 2 K. Application of a magnetic field perpendicular to the layers shows negative magnetoresistance below 10 K.They have also modified the guest species to design different conducting host− guest system. 36,37In 2009, Takaishi et al. synthesized another electrically conducting MOF, Cu[Cu(pdt) 2 ] (pdt = 2,3pyrazinedithiolate), using a thiol-based organic ligand.This mixed-valence [Cu I Cu III ] framework shows a high electrical conductivity of about 6 × 10 −4 S cm −1 at 300 K. 38 In 2010, Kobayashi and co-workers reported a similar MOF, Cu[Ni-(pdt) 2 ], with a related dithiol-based organic moiety showing an electrical conductivity of 10 −8 S cm −1 . 39Since then, many researchers have focused on this topic in order to design conducting MOFs with high conductivities and current densities, which are required for practical applications of conducting MOFs.Improvement of electrical conductivity in MOFs, i.e., design of highly conducting MOFs, is a big challenge.The proper selection through crystal engineering of metal ions and organic ligands is very important in order to obtain high charge mobilities in MOFs.
Depending on the charge transport pathway, these conducting MOFs can be classified into two different categories: (a) intrinsically conducting MOFs, when the charge transport occurs only through the metal−ligand backbone of the framework, and (b) extrinsically conducting MOFs, when the charge transport occurs through the guest species within the host framework. 34,40o date, some review articles, such as those of Zamora, 12 Dinca, 28,34 Alledorf, 41 etc., have comprehensively described the progress in this field, whereas Morris, 42 Pang, 43 and Zhu 44 have highlighted the design principles in conducting MOFs.Nevertheless, to date, there is no review article focusing on the strategies to modulate the charge transfer pathway in order to increase the electrical conductivity in MOFs and CPs.Here we show the strategies used by different groups to design both intrinsically and extrinsically conducting MOFs with high electrical conductivity.We discuss the band and hopping charge transport mechanisms operative in MOFs, and show the different charge transport pathways: (a) through-bond, (b) through-layer, (c) through-space, (d) through-guest, and (e) redox hopping in MOFs.We show that for intrinsically conducting MOFs, the charge transfer can be improved by using (i) ligands with soft donor atoms, (ii) redox-active noninnocent ligands, (iii) ligands with extended π-conjugated skeletons, (iv) mixed-valence metal centers, and (v) electronrich metals (low-valent and/or groups 8 to 12).On the other hand, the electrical conductivity of extrinsically conductive MOFs can be tuned by incorporating (a) metal ions and metal nanoclusters, (b) different types of organic, inorganic, and organometallic molecules, and (c) organic conducting polymers (Figure 1).

Conductivity Measurements.
The electrical conductivity (σ) of a conductor can be expressed as σ = neμ where n, e, and μ are the concentration, electronic charge, and mobility of the carriers, respectively.The carrier concentration decreases exponentially as the band gap increases.A way to increase the conductivity is, therefore, by oxidation or reduction, since both processes may raise the carrier concentration by increasing the electron population or decreasing the band gap.Doping of metals may lead to further supply of potential carriers, but in order to contribute to conductivity, these carriers should be mobile.Carrier mobility depends on the type of transport mechanism and pathway (interchain, single-chain, interparticle transport, etc.).The conduction mechanisms can be grouped into three models: variable range hopping model (VRH), tunnelling model, and superlocalization model (see below).
The resistance of a sample can be measured by (a) four-probe and (b) two-probe DC techniques.The four-probe method (Figure 2) can be used to measure the resistivity of any material, either bulk or thin film.The four-probe setup consists of four wires (usually tempered Cu, Pt, or Au wires of 25−100 μm diameter) connected to the sample by means of cold welding made with gold, silver, or graphite pastes (emulsions of fine powders dispersed in an organic solvent).A particular case is the four points technique, which is used when the samples are thin but firm.It uses four equidistant contacts (usually tungsten metal tips) connected by simply touching the sample (Figure 3).A high impedance current source is used to supply current through the outer two probes, and a voltmeter measures the voltage across the inner two probes to determine the sample resistivity.The typical current used ranges from 2 nA to 100 mA as in this region no Joule heating effect is observed.
The van der Pauw method (Figure 3) is used for irregular and very thin samples.In this method, the four wires are connected  to points of the periphery of the thin sample, forming an approximate square.Initially, the current is applied though two consecutive points (AB), and the voltage is measured across the other two points (CD) to obtain R CD .The resistance in the other direction, R AD , is measured in a similar way, applying the current through BC and measuring the voltage across AD.The resistivity of the sample can be calculated from the R AD and R CD values. 12he two-probe method is used for highly resistive samples, where the contact resistance is negligible compared to the sample resistance.
1.2.Conducting Mechanisms.Two different types of charge transport mechanisms are operative in solid materials: band-like charge transport and redox hopping (Figure 4). 45For most MOFs, the covalent metal−ligand bond formed by strong overlap between the metal ions and donor ligands orbitals, form continuous conduction bands.The energy gap (E g ) between the conduction and valence bands determines the electrical properties of the materials that can be classified as metallic conductors, semiconductors, and insulators.In insulators, the band gap is above ≈3.6 eV, and in semiconductors it is below ≈3.6 eV, 46 whereas in metallic conductors, the conduction and valence bands merge, with the Fermi level crossing this band.With increasing temperature, the electrical conductivity decreases for metallic conductors, while it increases for semiconductors.In metallic conductors at high temperatures, the electron−phonon coupling reduces the electron mobility and, accordingly, the electrical conductivity.For semiconductors, the increase in temperature increases the number of charge carriers in the conduction band, resulting in an increase of the conductivity.The electrical conductivity of classical semiconductors follows the Arrhenius equation: σ = σ 0 exp −(E a / kT), where E a is the activation energy, corresponding to half of the band gap (E a = E g /2).
Besides the delocalized model based on electron bands, there exists a localized model where the charges move through a socalled hopping mechanism, where charges "jump" through a phonon-assisted quantum tunnelling mechanism.In this hopping model, the conductivity is described by a general equation: σ = σ 0 exp [(−T 0 /T) α ], where the exponent α depends on the dimensionality (d) of the conducting lattice in the form: α = 1/(1 + d).For three-dimensional materials, α = 1/ 4 and the expression becomes the Mott model, 47 where the electron transport is similar to a self-exchange process between redox couples to maintain electroneutrality.In the hopping model, an increase of the temperature leads to an increase of the electron delocalization and, therefore, to an increase of the electrical conductivity.
According to Mott's variable range hopping (VRH) theory, the conductivity is given by where the VRH exponent (γ) is determined by the dimensionality of the conducting pathway: = + and for three-, two-, and one-dimensional systems, γ becomes 1/4, 1/3, and 1/2, respectively.σ 0 denotes the conductivity at infinite temperature, T Mott is the Mott characteristic temperature, k B is the Boltzmann constant, N(E F ) is the density of states at Fermi level, and L loc is the localization length.A sample with a 3D/2D/ 1D charge transport shows a linear dependence in a plot of ln [T 1/2 σ(T)] vs. T −γ (with γ = 1/4, 1/3, and 1/2 for 3D, 2D, and 1D, respectively).From the slopes of these plot, the T Mott can be determined.
Most of the MOFs are either insulators or semiconductors.Nevertheless, there is an increasing number of MOFs presenting metallic conductivity. 48In most cases, the band model is used to explain their electrical conductivity, although in a few cases, the hopping mechanism has also been claimed. 49.3.Conduction Pathways.Following the above-mentioned mechanisms, the charge transport in MOFs may occur through different pathways such as (a) through-bond, (b) through-layer, (c) through-space, (d) redox hopping, and (e) through-guest (Figure 5).
(a) Through-bond.MOFs containing (−M−X−) n lattices (where X is the donor atom of the functional group of the ligand) can show such through-bond electron transport. 50uch transport is independent of the ligand backbone.
The overlap between the metal ion (M) and donor atoms of the ligand (X) is the key factor for electron transport though this type of pathway.The use of soft donor atoms (S, P, Se, etc.) combined with transition metal ions can promote conductivity through such pathways, as observed in several MOFs showing (−M−X−) n chains. 51) Through-plane.The use of organic ligands having extended conjugated organic cores, such as benzene, triphenylene, etc., and different coordinating groups, such as hydroxido, amine, diol, dithiol, etc.) 52−54 can give rise to graphene-like 2D metal−organic sheets when bonded to transition metal ions.The charge transport of this type of 2D metal−organic layers is dependent on both the metal−ligand overlap as well as on the extended in-plane conjugation though the ligand.The extended π−d conjugation is the most operative charge transport pathway for such type of 2D metal−ligand layers. 55As expected, the interlayer π−π interaction is very important  for the charge transport perpendicular to the plane (through-space).(c) Through-space.In organic electronics, intermolecular πinteractions are used to increase the dimensionality of electrical conductors and semiconductors. 56Similarly, 1D or 2D MOFs containing aromatic rings at their ligand backbones can also participate in different interchain or interlayer weak interactions to form 3D supramolecular structures.These interchain or interlayer π-interactions have been used by different research groups as the charge transport pathway. 57,58The charge carrier efficiency is dependent on the strength of the π-interactions, which depend on the distance and geometry of the metal− organic moieties.In 1D MOFs, both the through-bond and through-space charge transport pathways can act simultaneously, and in some cases, it is difficult to predict the contribution of each of them.A similar case occurs in 2D MOFs, where both through-plane and through-space transport pathways are operative.(d) Redox Hopping.In the absence of any particular charge transport pathway, electron transport may occur through a hopping process. 59,60This mechanism is operative when the metal−ligand overlap is very poor or its nature is mostly ionic.Under these circumstances, the localized electrons jump between different redox active sites.Therefore, the presence of redox-active metals or linkers is needed to promote such charge hopping.(e) Through-guest.Taking advantage of the void space present within the MOFs, several research groups have incorporated different types of electroactive guest species: metal ions, metal nanoclusters, organic and inorganic molecules, redox-active molecules, polymers, etc., to design conducting MOFs and materials. 61,62Two different approaches have been used to develop such conducting guest@MOFs materials: (i) templated synthesis and (ii) postsynthetic incorporation.The incorporated guest molecules form charge transport pathways within the material either through host−guest or guest− guest interactions.In the case of organic conducting polymers, the charge transport pathway is provided by the continuous conjugated bond network of the polymer.In all these conducting pathways, the dimensionality of the MOFs is expected to play a key role in determining not only the electrical conductivity but also the conducting pathway.The study of the role of the dimensionality on the electrical conductivity and its mechanism would require the preparation of polymorphic conducting MOFs only differing in their dimensionalities.As far as we know, there are no published examples of conducting MOFs with the same composition (metal ions and ligands) but different dimensionality.

DESIGN STRATEGIES TO IMPROVE CHARGE TRANSPORT IN MOFS
In the past decade, the number of conducting MOFs has experienced a remarkable increase.Based on the charge transport pathway, conducting MOFs are classified into intrinsically and extrinsically conducting MOFs.Different presynthetic and post-synthetic strategies have been developed to obtain high conductivity in MOFs.

Intrinsically Conducting MOFs.
The electrical conductivity of the intrinsically conducting MOFs is an inherent property and thus can be modulated through the modification of both metal ions and organic ligands in the MOF.Several strategies have been developed in order to enhance the conductivity of intrinsically conducting MOFs.
2.1.1.Incorporation of Redox-Active Ligands.Organic bridging ligands having a conjugated organic core with variable oxidation states have gained significant attention in the design of conducting molecules and MOFs. 63Significant attention has been paid to those ligands having a catechol moiety (dihydroxybenzene, chloranilic acid, etc.) 64,65 since they present several stable and easily interconvertible oxidation states through protonation and deprotonation.During the synthesis, the ligands can be easily reduced or oxidized to form redoxactive frameworks with different charges or protonation degree.When these redox-active ligands form extended networks with extended conjugation, these frameworks may be highly conducting thanks to the electron hopping between the metal ion and the redox-active ligand (usually due to valence tautomerism).On the other hand, the electron conduction in MOFs is also dependent on the metal−ligand frontier orbitals overlap.The presence of an open shell in the outer orbital of both, the metal ion and ligand, may lead to optimal overlap that facilitates the electron conduction throughout the framework.One of the most studied redox-active ligands is 2,5dihydroxybenzoquinone (H 2 dhbq, Figure 6, X = H) that has frontier orbitals with similar energies to those of many transition metals.This ligand may show up to three different valence states in metal−organic motifs (Figure 6).This valence tautomerism may result in an increase of the electrical properties and in a modulation of other properties.In this section, we discuss the conducting MOFs formed by using mixed-valence tetraoxolenebased ligands such as H 2 dhbq and the derivatives with X = Cl and Br, known as chloranilate (C 6 O 4 Cl 2 ) 2− and bromanilate (C 6 O 4 Br 2 ) 2− , respectively (Figure 6).tion with Na of the oxidized compound (Table 1). 667 Both oxidized products have a −2 oxidation state in the ligands, while the reduced form contains mixed-valence C 6 O 4 Cl 2 3− ligands.The room-temperature electrical conductivity values are 1.14 × 10 −13 and 1.45 × 10 −13 S cm −1 for 3-ox and 3-reox, respectively, while 3-red has a  Two-probe on pressed pellets.

Crystal Growth & Design
higher conductivity value of 2.27 × 10 −8 S cm −1 .The activation energy of the reduced MOF 3-red (489 meV) is lower than that of the parent MOF 3-ox (740 meV).We can conclude that, in this case, chemical reduction injects free electrons to the system, increasing the conductivity by 5 orders of magnitude.
Wentz et al. have synthesized a porous 3D MOF using the redox-active naphthalenediimide ligand.The reduced framework shows 10 6 times higher conductivity than the original framework. 68ollowing the inner sphere redox-active metal−ligand pair, Cleŕac et al. have developed an interesting 2D layered metal− organic framework [Cr III Cl 2 (pyz) •− ] n (4) that shows both interesting ferrimagnetism and high electrical conductivity. 69It is formed by connecting metal centers with four pyrazine ligands while the chlorides occupy the trans-axial positions.The complex undergoes inner-sphere electron transfer to produce [Cr III Cl 2 (pyz) •− ] n (4) through electron transfer from metal to ligand to form Cr 3+ and half-reduced (pyz 2 ) − .This electronic configuration creates strong magnetic interaction between S = 3/2 Cr III and delocalized S = 1/2 pyrazine spin, and [Cr III Cl 2 (pyz) •− ] n shows strong ferrimagnetic interaction below 55 K.The charge delocalization also provides a pathway for electron transport, and this compound shows a high electrical conductivity of 0.032 S cm −1 at room temperature (two-probe method using pressed pellet).Using the same strategy, they have prepared two other 2D layered materials: [V II Cl 2 (pyz) 2 ] n (5)  and [Ti II Cl 2 (pyz) 2 ] n (6). 70Detailed analysis reveals that the former complex remains in its nonreduced state, while the latter undergoes inner-sphere electron transfer to produce [Ti III Cl 2 (pyz) •− ] n (6).Electrical conductivity measurement shows that the V-complex is insulating (σ = 10 −10 S/cm) in nature while the Ti-complex shows metallic conductivity (σ = 5.3 S/cm) due to similar metal−ligand inner-sphere electron transfer.[Ti III Cl 2 (pyz) •− ] n also shows a magnetoresistance of 25% at 1.8 K and ±9 T. Based on these results, they have established that electrical conductivity depends on the energy of the d-orbitals.
2.1.2.Extended Conjugated Organic Ligands.Another strategy to increase the electrical conductivity is the use of organic ligands having highly conjugated cores like triphenylene, phthalocyanine, napthalocyanine, etc.Thus, Park et al. synthesized MOF Cu 3 (HHB) 2 (7) using the ligand hexahydroxybenzene (H 6 HHB, Scheme 1). 71This MOF shows a honeycomb layered structure with Cu II ions in a four coordinated square planar geometry (Figure 8).Two-probe electrical conductivity measurements show that this MOF is a modest conductor with a room-temperature conductivity of 7.3 × 10 −8 S cm −1 , and the corresponding activation energy is 450 meV (Table 2).This conductivity value is a combination of both in-plane and out-of-plane conductivity of the MOF.
A similar MOF, Cu 3 (HHTP) 2 (8), with the extended conjugated ligand 2,3,6,7,10,11-hexahydroxytriphenylene (H 6 HHTP, Scheme 1) has been synthesized by Hmadeh and co-workers. 72The electrical conductivity measurements performed on single crystals of the MOF shows a very high conductivity of 0.1 S cm −1 at room temperature, similar to the values measured on powder and on a thin film of the MOF (in the range of 0.02 to 0.2 S cm −1 ). 73,74Song et al. have measured the electrical conductivity of thin films of Cu 3 (HHTP) 2 (8-film) along the ab-plane with the four-probe method. 75These measurements show a high in-plane conductivity value of 0.29 S cm −1 with an activation energy of 130 meV (Table 2).Day and co-workers prepared hexagonal flakes of the 2D layers (8-hex) by a sonication-assisted liquid-phase exfoliation method. 76our-probe conductivity measurement shows an out-of-plane conductivity of 1.5 S cm −1 while the in-plane conductivity is 0.5 S cm −1 .Yao et al. synthesized a hybrid of Cu 3 (HHTP) 2 and Cu 3 (HHB) 2 with the formula Cu 3 (HHTP)(HHB) (9). 77Twoprobe electrical conductivity measurements on pressed pellets show a moderate conductivity of 2.53 × 10 −5 S cm −1 , which is lower than the value of Cu 3 (HHTP) 2 but much higher than that of Cu 3 (HHB) 2 .These results indicate that the presence of extended conjugated organic ligands facilitates the charge transport and results in highly conducting frameworks (Table 2).
Hmadeh et al. also reported two other MOFs with formula M 9 (HHTP) 4 , with M = Co (10) and Ni (11).In contrast to Cu 3 (HHTP) 2 , where the metal ions are four-coordinated square planar, in Co 9 (HHTP)    12) shows a 10 times higher conductivity (2.5 × 10 −12 S cm −1 ) than [Mn 2 (DOBDC)] (13) (3.9 × 10 −13 S cm −1 ).In the Fe II derivative, the conductivity of [Fe 2 (DSBDC)] ( 14) (3.9 × 10 −6 S cm −1 ) is also 10 times higher than that of the phenolate analogue [Fe 2 (DOBDC)] (15) (3.2 × 10 −7 S cm −1 ) (Table 3).This result indicates that an efficient orbital overlap between the metal d-orbitals and ligand p-orbitals by matching their energy levels is an appropriate strategy to promote electrical conductivity in MOFs.Thus, the (−M−S−) n chains facilitate the electron transport better than the (−M− O−) n chains.The higher conductivity shown by the Fe-based    MOFs compared to the Mn ones is attributed to the presence of loosely bound β-spin d-electrons in Fe II , not present in Mn II .N-Donor Ligands.Amine-functionalized conjugated organic ligands such as hexaiminobenzene (HIB, Scheme 2), 82 hexaiminotriphenylene (H 6 HITP, Scheme 2), and octaiminophthalocyanine (H 8 OIPc, Scheme 2) have also been used to prepare conducting MOFs.Thus, Dou et al. have reported metallic conductivity in Ni 3 (HIB) 2 ( 16) and Cu 3 (HIB) 2 (17).In both cases, hexagonal honeycomb layers are stacked in a slipped-parallel fashion (Figure 10). 83Electrical conductivity measurements on pressed pellets by the van der Pauw method show conductivity values in the range of 0.7 to 10 S cm −1 and 0.11 to 0.7 S cm −1 for Ni 3 (HIB) 2 (16) and Cu 3 (HIB) 2 (17), respectively.Park et al. reported conductivity values in the range of 0.1 to 1.57 S cm −1 for Co 3 (HIB) 2 (18), measured by the two-probe method on pressed pellets. 84Lee and co-workers reported very high conductivity values for the Fe 3 (HIB) 2 (19) and Mn 3 (HIB) 2 (20) derivatives of this series (Table 4). 85he ligand hexaiminotriphenylene (HITP, Scheme 2) has also been used to prepare several conducting MOFs.Sheberla et al. reported metallic-like conductivity in the MOF Ni 3 (HITP) 2 (21), 86 where the honeycomb hexagonal layers show a neareclipsed packing.Four-probe electrical conductivity measurements on thin films (21-film) and pressed pellets (21) of this MOF show room-temperature conductivities of 40 and 10 S cm −1 , respectively, and a semiconducting behavior, although the charge transport phenomena is not clear.Day et al. have measured electrical conductivity on single crystals of Ni 3 (HITP) 2 and found a value of ∼150 S cm −1 at 295 K and nonzero conductivity at 0.3 K. 74 Charge carrier mobility measurements of thin films of the Ni 3 (HITP) 2 MOF show a value of 40 cm 2 V −1 s −1 . 87The high conductivity arises from the contribution of both in-plane and out-of-plane charge transport.Campbell et al. have found a conductivity value of 0.2 S cm −1 on pressed pellets of Ni 3 (HITP) 2 measured by the two-probe method. 88Lian and co-workers have synthesized the Co 3 (HITP) 2 (22) and Co x Ni 3−x (HITP) 2 (23) derivatives.The electrical conductivity of Co 3 (HITP) 2 ( 22) is very small (8 × 10 −4 S cm −1 ), whereas the mixed-metal MOF shows a much higher conductivity (0.032 S cm −1 ) that increases with increasing the Ni content. 89inally, Jia and co-workers have synthesized the highly conjugated 2D MOF [Ni 2 (Ni-OIPc)] ( 24) using the metallophthalocyanine-based organic linker Ni-OIPC (Scheme 2). 90our-probe electrical conductivity measurement on a pressed pellet of the sample shows a high room-temperature conductivity value of ∼0.2 S cm −1 (Table 4).
S-Based Ligands.Different thiol-based ligands have gained significant attention in designing conductive MOFs (Scheme 3).Takaishi et al. used 2,3-pyrazinedithiolate (pdt) to construct a porous 3D MOF formulated as Cu[Cu(pdt) 2 ] ( 25) that shows a conductivity value of 6 × 10 −4 S cm −1 at 300 K. 38 The temperature dependence of the electrical conductivity indicates that this MOF is a semiconductor with an activation energy of 193 meV.
Chen and co-workers synthesized two different 2D MOFs formulated as Ag 3 BHT 2 (29) and Au 3 BHT 2 (30) using benzenehexathiol (H 6 BHT, Scheme 3) as the ligand with a liquid−liquid interfacial method. 92The honeycomb hexagonal layers are almost eclipsed for Ag 3 BHT 2 (29) but slightly displaced for Au 3 BHT 2 (30).The electrical conductivity of the Ag-based MOF is higher than the Au one, and the conductivity is dependent on the thickness of the films.Thus, for thin (≈ 0.276 μm) and thick (≈ 64 μm) films, the electrical conductivity values change from 363 to 19.8 S cm −1 , respectively, in the Ag 3 BHT 2 MOF.For the Au 3 BHT 2 MOF, these values are 1.12 × 10 −4 (for thin films of ≈0.325 μm) and 8.07 × 10 −5 S cm −1 (for thick films of ≈89 μm).The large differences in the conductivity values can be easily explained by the better packing of the layers in the Ag-MOF (almost eclipsed), compared to the Au-MOF (slightly alternated), which leads to stronger interlayer π−π interactions in the Ag-MOF, responsible for the much higher electrical conductivity of the Ag-MOF.(31). 93Four-probe electrical conductivity measurements of thin films of this MOF show very high conductivity values of ≈250 S cm −1 at room temperature and an increase of the conductivity with temperature, typical of semiconductors.The activation energy of the MOF also increases with temperature.This high conductivity is based on both Ag−S bonding in combination with the hopping process between neighboring Ag-BHT nanocrystals within the thin film.The same group also prepared a Cu-MOF with the same ligand, formulated as [Cu 3 (BHT)] n (32). 58This MOF presents the highest conductivity value reported to date in a MOF: 750−1580 S cm −1 , measured on highly crystalline thin films prepared by a liquid−liquid interface method.Four-probe conductivity measurements on thin films of this MOF with different thickness show that the conductivity values do not vary with the thickness of the films.The conductivity of these films decreases when the temperature is lowered and reaches a minimum value of 1360 S cm −1 at 2 K.The activation energy rises continuously with temperature, from 0.12 meV at 40 K to 2.06 meV at 300 K. 58 Furthermore, this 2D MOF shows the appearance of superconductivity at 0.25 K at ambient pressure. 59ambe et al. have established that the electrical conductivity of the 2D metal-dithiolene MOF Ni 3 (BHT) 2 (33) varies with the oxidation state. 39,53For the pristine MOF Ni 3 (BHT) 2 (33), both the IR and XPS analysis reveal that the average charge on the metal center is −3/4.These authors, therefore, attempted to oxidize and reduce the framework to get pure 0 and −1 oxidation states (Figure 12).IR and XPS studies show that the reduced framework obtained using NaTCNQ shows a charge of −1 (33red). 39Two-probe electrical conductivity measurements on pressed pellets of the parent and reduced MOFs show conductivity values of 0.15 and 6.7 × 10 −3 S cm −1 at 298 K.The oxidation of the parent MOF using tris(4-bromophenyl)ammonium hexachloroantimonate leads to a sample with a much higher conductivity of 160 S cm −1 , measured by the fourprobe van der Pauw method to avoid grain boundary effect and contact resistance. 53In fact, with the four-probe method, the conductivity of the parent MOF increases from 0.15 to 2.8 S cm −1 .
Pal et al. have prepared another similar MOF: [Pd 3 (BHT) 2 ] (34) by a liquid−liquid interface method, although it was contaminated with PdNPs. 94Accordingly, they used K 3 [Fe-(CN) 6 ] as an oxidizing agent to prevent the formation of PdNPs.Four-probe electrical conductivity measurements on pressed pellets of the sample shows a conductivity at room temperature of 2.80 × 10 −2 S cm −1 .
Another highly extended conjugated thiol-based organic ligand that has been used to prepare conducting MOFs is triphenylenehexathiol (H 6 TPHT, Scheme 3).Cui et al. reported that the reaction between H 6 TPHT with Pt(CH 3 CN) 2 Cl 2 produces a red-colored compound under N 2 atmosphere that turned black upon exposure to open air. 95ICP analysis of the compound reveals that the formula of the compound is (Pt) 1.5 (Na) 0.9 TPHT (35) and the 2D framework is anionic in nature.The anionic framework is easily oxidized by I 2 to form Pt 3 (TPHT) 2 (36).The hexagonal honeycomb layers are stacked in a staggered conformation.Two-probe electrical conductivity   measurement shows conductivity values of 2.47 × 10 −6 and 1.09 × 10 −6 S cm −1 for TPHT(Pt) 1.5 (Na) 0.9 (35) and Pt 3 (TPHT) 2 (36), respectively.
Clough et al. synthesized another similar 2D MOF formulated as Co 3 (TPHT) 2 (37) where the hexagonal honeycomb layers are eclipsed (Figure 13). 96Four-probe van der Pauw measurements show that the electrical conductivity of a pressed pellet of the MOF with thickness 0.24(2) mm is 1.4 × 10 −3 S cm −1 with an activation energy of 173 meV.Interestingly, this MOF shows a transition from a semiconductor to metallic state on lowering the temperature from 130 to 50 K (Figure 13), although this transition is dependent on the thickness of the pressed pellets and might be an artifact.Such high electrical conductivity is due to both strong metal−ligand orbital overlap and highly conjugated organic ligand present within the 2D layered material.
Dong and co-workers have synthesized another 2D MOF formulated as Fe 3 (TPHT) 2 (NH 4 ) 3 (38) by liquid−liquid interfacial method with time-dependent variable thicknesses. 97n this compound, the honeycomb hexagonal layers show a tilt between them.Exposure to open air leads to oxidation of both metal and TPHT ligand, which consequently converts this material from semiconducting to metallic conductor.Four-point van der Pauw conductivity measurements show a conductivity value of 3.4 × 10 −2 S cm −1 at 300 K (Table 5).
Yang et al. used a highly conjugated thiol-based organic ligand containing a coronene core to prepare a conducting MOF formulated as K 3 Fe 2 [PcFe-O 8 ] (39), with PcFe-O 8 = (2,3,9,10,16,17,23,24-octahydroxyphthalocyaninato)iron. 98his MOF shows alternating planar metal−organic layers.The conductivity of the sample, measured with the van der Pauw method on pressed pellets, shows a conductivity value of 0.2 S cm −1 at room temperature (Table 5).(42) that presents honeycomb hexagonal layers with an eclipsed stacking (Figure 14). 101Four-probe electrical conductivity measurements on        45) (where DCTP = 4′-(3,5-dicarboxyphenyl)-4,2′:6′,4″-terpyridine) with a narrow bandgap and photocatalytic hydrogen evolution and degradation of organic dyes, based on photogenerated electrons and holes. 104un and co-workers have attempted to partially oxidize iron (from Fe 2+ to Fe 3+ ) on single crystals of MOF Fe 2 (BDT) 3 (46), where H 2 BDT = 5,5′-(1,4-phenylene)bis(1H-tetrazole), upon exposure to ambient atmosphere. 105The electrical conductivity of the as-synthesized red crystals is 6 × 10 −5 S cm −1 at 296 K, but the conductivity value increases with time when exposed to air.After 7 days, the conductivity values increase to 0.3 S cm −1 and after 30 days to 1.2 S cm −1 (Table 6).The temperature dependence of the conductivity shows a classical Arrhenius-type semiconducting behavior with an activation energy of 160 meV.This MOF shows how the presence of a mixed-valence state reduces the activation barrier for charge transfer and increases the electrical conductivity up to 5 orders of magnitude, 106 confirming that the use of redox-active metals within a MOF has a similar effect to the use of redox-active ligands.
Park et al. also reported the gradual increase in the electrical conductivity of the Fe II -containing MOF [Fe(tri) 2 ] (tri = 1,2,3triazolate) by chemically oxidizing it to the mixed-valence MOF [Fe(tri) 2 (BF 4 ) x ], with x = 0.09, 0.22, and 0.33 (47). 107The experimental results show that the derivative [Fe(tri) 2 (BF 4 ) 0.33 ] (47) has a high conductivity (0.3 S cm −1 ) and the electron transfer between Fe II / III ions is the main reason behind such phenomenon (Figure 15).Goḿez-Garci ́a et al. synthesized two different mixed-valence MOFs formulated as (48) and Br (49), having two different halide-substituted anilato ligands.These layered MOFs contain mixed-valence Fe II /Fe III ions as shown by EPR and structural data analysis and, besides a magnetic ordering at low temperatures, show high in-plane electrical conductivities of 0.03 and 0.003 S cm −1 , respectively. 55.1.5.Electronic Structure and Size of Metal Ions.There are some MOFs where the electrical conductivity (and even the conduction mechanism) can be modulated by changing the metal cations since their electronic structure and size may modulate the intermolecular contacts and, accordingly, the electronic conductivity.Long et al. have recently shown a very interesting case of three isostructural MOFs where the electrical conductivity mechanism depends on the electronic structure of the metal ion (Table 7).Thus, they have prepared three  (56).These authors observed that the electrical conductivity depends on the cation size (Table 7). 109wo-probe conductivity measurement on single crystals of these MOFs shows values of 3.95 × 10 −6 , 1.49 × 10 −5 , 8.64 × 10 −5 , and 2.86 × 10 −4 S cm −1 for M = Zn, Co, Mn, and Cd, respectively, and semiconducting behavior.The cation radius determines the S•••S distance between neighboring TTF cores and, therefore, the intermolecular interactions and, thus, the conductivity of the MOFs.The shorter the S•••S distance, the higher the electrical conductivity.In this family, the S•••S distance is inversely proportional to the ionic radius of the metal ions.Larger cations increase the chemical pressure and produce a contraction of the S•••S distance that results in better orbital overlap between the p z orbitals of neighboring atoms and in an increase of the electrical conductivity.Accordingly, the conductivity values follow the same order as the cation radii and the electrical conductivities can thus be tuned by tuning the cation size of this class of MOFs with π-stacked motifs. 109.1.6.π•••π Stacking Interactions.Planar π-conjugated organic moieties can form π•••π interactions within the frameworks, giving rise to electrical conducting networks (Table 8).Based on this idea, different organic ligands like anthracene, naphthalene, phenanthroline, etc.  110 The distance between the π-assembled 4-ClBA ligands is 3.92 Å. Two-probe conductivity measurement shows that the electrical conductivity of this MOF is 2.8 × 10 −6 S cm −1 , which increases to 3.9 × 10 −6 S cm −1 upon desolvation (57-des).The authors suggest that the desolvation may increase the extended conjugation within the coordination chains.The same group has also reported the semiconducting 2D MOF [Cu(ndc)(1,10-phen)] (58) (ndc = 2,6-napthalenedicarboxylate and 1,10-phen = 1,10-phenanthroline, Scheme 4).This MOF presents interlayer π•••π interactions and forms an interpenetrated 3D supramolecular network. 60he average distance between the aromatic rings is ∼3.50 Å.This MOF is also semiconducting with a room temperature conductivity of 3 × 10 −3 S cm −1 that increases 16 times on decomposition of the framework to CuO-NPs.

Extrinsically Conducting MOFs.
In extrinsically conducting MOFs, the electrical conductivity is strongly dependent on guest molecules, incorporated either through in situ methods or by post-synthetic modifications within their pores.These guest molecules can be classified as (a) metal-based guests, (b) organic, inorganic and organometallic guest molecules and (c) organic conducting polymers.In the following section, we will show the most important examples of extrinsically conducting MOFs prepared with different types of guests.

Metal-Based
Guest.The insertion of metal atoms and metallic nanoparticles has been used by different research groups as a strategy to increase the electrical conductivity of porous MOFs.Morsali and co-workers have shown the possibility to modulate the electrical conductivity of the amine-functionalized MOF [Zn(OBA)(DPTHD)]•DMF (63), where H 2 OBA = 4,4′-oxybisbenzoic acid and DPTHD = 5,6-di(pyridin-4-yl)-1,2,3,4-tetrahydropyrazine, by incorporation of Cd 2+ ions.(Figure 17).The encapsulation of Cd 2+ ions within this framework increases the electrical conductivity from 5.8 × 10 −6 S cm −1 to 1.8 × 10 −2 S cm −1 . 115everal groups have used metal nanoparticles (NPs) as the guest species in order to increase the electrical conductivity of different MOFs (Table 9).Thus, Han et al. synthesized a semiconducting MOF through encapsulation of AgNPs within the insulating framework. 116A cyclodextrin-based Rb-CD-MOF (64) adsorbs Ag + ions upon soaking the MOF crystals in AgNO 3 solutions.The adsorbed Ag + ions are reduced by the hydroxide groups of the cyclodextrin moieties to form AgNPs.Such encapsulation of AgNPs within the MOF increases the electrical conductivity to 3.1 × 10 −9 S cm −1 .Kung and co-workers successfully enhanced electrical conductivity of NU-1000 MOF through incorporation of SnO 2 within the pores. 117SnO 2 was deposited within pores of the MOF by consequent solution and vapor phase treatment of Sn(amd) 2 , where amd = bis(N,N′-di-ipropylacetamidinato) (Figure 18).After three consecutive cycles of incorporation, the obtained SnO 2 @NU-1000 MOF (65) shows a conductivity value of ∼1.8 × 10 −7 S cm −1 , while the parent MOF is an insulator.The formation of SnO 2 pillars within the void space of MOF acts as the charge transport pathway within the framework.
2.2.2.Molecular Guests.−120 In 2000, Coronado, Goḿez-Garci ́a et al. encapsulated BEDT-TTF molecules within the interlamellar space of a 2D magnetically ordered framework by an in situ electrocrystallization synthetic method.Within the host−guest (BEDT-  2− = oxalate dianion), the BEDT-TTF cations form pseudohexagonal β-type layers intercalated within honeycomb 2D coordination layers of [MnCr(C 2 O 4 ) 3 ] − .Four-probe electrical conductivity measurements within the plane showed a very high room temperature conductivity value of 250 S cm −1 and a metallic conductivity within the temperature range 2−300 K. 35 Furthermore, the anionic honeycomb layers display a long-range ferromagnetic order at low temperatures.In another report, the same group synthesized a similar type of framework with the seleniumsubstituted organic donor bis(ethylenedithio)-tetraselanafulvalene (BEDT-TSF): (BEDT-TSF) 3 [MnCr(C 2 O 4 ) 3 ] (67) (Figure 19). 36As in the BEDT-TTF compound, the BEDT-TSF moieties are located within the interlamellar space of 2D honeycomb layers of [MnCr(C 2 O 4 ) 3 ] − , and the average charge on each BEDT-TSF moiety is 1/3.The framework also shows a ferromagnetic long-range order at low temperatures.The room temperature electrical conductivity of the Se-derivative is 1 S cm −1 , and it shows metallic conductivity within the range of 150−300 K.
The same group also prepared the host−guest charge transfer complex formulated as (BEDT-TTF) x [MnRh(C 2 O 4 ) 3 ]•CH 2 Cl 2 (68) (where x = 2.526(1)) by replacing Cr with Rh.The room-temperature electrical conductivity of this compound is 13 S cm −1 and shows a metallic behavior, reaching a value of 28 S cm −1 at 103 K. 37 The electrical conductivity of MOFs having nanopores can be tuned by filling them up with redox-active conjugated guest molecules such as 7,7,8,8-tetracyanoquinodimethane (TCNQ). 121Thus, Allendorf et al. encapsulated TCNQ within a thin film of MOF Cu 3 (BTC) 2 (HKUST-1, 69), where H 3 BTC = benzene-1,3,5-tricarboxylic acid, by soaking the MOF in a saturated solution of TCNQ. 122Spectroscopic and structural analysis shows a coordination between the Cu 2+ centers and TCNQ (Figure 20).Such encapsulation increases the electrical conductivity of the framework from 10 −8 S cm −1 to 7 × 10 −2 S cm −1 at room temperature.Later, Allendorf, Fischer, and coworkers synthesized TCNQ-incorporated HKUST-1 with the precise molecular formula TCNQ 1.0 @Cu 3 BTC 2 that shows an electrical conductivity value of 1.5 × 10 −4 S cm −1 . 123Thurmer et al. have also prepared thin films of HKUST-1 on ITO, and then TCNQ was inserted within the pores of the MOF.The resultant TCNQ@HKUST-1 has been found to be 10 10 times more conductive (10 −1 S cm −1 ) than the parent MOF (10 −11 S cm −1 ). 124In another report, Deep et al. prepared thin films of TCNQ@HKUST-1 on a gold-screen printed electrode through Impedance on pressed pellet.b Two-probe on single crystal.c Two-probe on thin film.The resultant TCNQ@2D-MOF shows a conductivity of 10 −6 S cm −1 , which is 10 3 times higher than that of the parent MOF (10 −9 S cm −1 ). 126ncorporation of iodine and polyiodides within the channels of MOFs is one of the easiest and most attractive strategies to enhance the electrical conductivity of MOFs.Due to its large size, electron density, and low electronegativity, I 2 becomes an ideal dopant in the form of either vapor or solution or even as a template. 127Zeng et al. loaded I 2 within the 1D channels (∼11 × 10 Å 2 ) of MOF [Zn 3 (D,L-lac) 2 (pybz) 2 ]•2.5DMF (71) (where D,L-lac = lactate and pybz = 4-pyridine benzoate anions) by soaking the MOF in a cyclohexane solution of I 2 (Figure 21).The electrical conductivity of I 2 @MOF is 3.42 × 10 −3 S cm −1 along the channels and 1.65 × 10 −4 S cm −1 in the direction perpendicular to the channels.Both values are much higher than those of the pristine MOF (7 × 10 −6 S cm −1 ). 128n  75) by soaking the crystals in methanol.The former compound with polyiodides shows a 100 times higher conductivity (8.11 × 10 −7 S cm −1 ) than the later one with I − (8.04 × 10 −9 S cm −1 ). 131ung and co-workers modified the electrical conductivity of the porous NU-1000 MOF [Zr 6 (μ 3 -OH) 8 (OH) 8 (TBAPy) 2 ] (76) (H 4 TBAPy = 1,3,6,8-tetrakis(p-benzoic acid)pyrene) by introducing electron-deficient [Ni(dicarbollide) 2 ] (NiCB) molecules in the pores of the framework containing an electron-rich pyrene core as the ligand backbone (Figure 22).
Single crystal X-ray structural analysis reveals that NiCB molecules are loaded within the narrow triangular channels but not in the large hexagonal channels.Both NiCB and parent MOF are insulating, whereas the conductivity of NiCB@MOF (76) is 2.7 × 10 −7 S cm −1 (Figure 23). 132Such enhancement of electrical conductivity is due to donor−acceptor charge transfer between NiCB and the pyrene-based organic ligand of the framework.

Organic Conducting Polymers
Guests.The insertion of organic conducting polymers like PEDOT (polymer of 3,4ethylenedioxythiophene), 134 polyaniline (PANI), 135 and poly-  pyrrole (PPy) 136 within the channels of MOFs has also been used in order to combine the novel functionalities of MOFs with the useful properties of conducting polymers.
Although not within a MOF, as far as we know, the first attempts to insert a conducting polymer within a porous material were performed by inserting PEDOT inside porous NaX and NaY faujasites. 137Conductivity measurements on thin films of this material show a huge increase in the conductivity of PEDOT@NaY with a value similar to that found in bulk PEDOT and much higher than that measured in the pristine NaY faujasite.Following the same strategy, Kitagawa and coworkers synthesized PEDOT encapsulated within the void space of MIL-101(Cr) (78) by using the monomer 3,4-ethylenedioxythiophene (EDOT) following a two-step method: adsorption of monomer and then I 2 -mediated oxidative polymerization.These authors observed that PEDOT@MIL-101(Cr) has 10 8 times higher conductivity (1.1 × 10 −3 S cm −1 ) in the presence of I 2 as a dopant than the parent MOF (∼10 −11 S cm −1 ).They also found that the conductivity of PEDOT@MIL-101(Cr) varies with the amount of polymer present within the channels. 138Ballav and co-workers have modulated the insulating behavior of the biporous UiO-66 (79) MOF by incorporating two different types of conducting polymers (PEDOT and PPy) within the pores in a similar two-step method (Figure 24).Four-probe electrical conductivity measurements show room-temperature electrical conductivity values of 2 × 10 −2 S cm −1 and 10 −3 S cm −1 for UiO-66-PPy and UiO-66-PEDOT, respectively, while UiO-66 ( 79) is insulating, and the electrical conductivities of the physical mixtures of UiO-66 (79) with the bulk polymers vary between 10 −6 and 10 −7 S cm −1 . 139ang and co-workers observed the enhancement of the electrical conductivity of a porous MOF upon incorporation of conducting PPy chains within the nanochannels.They prepared polymeric chains of pyrrole (PPy) within the 1D channels of MOF [Zn 3 (D,L-lactate) 2 (4-pyridylbenzoate) 2 ] (80) (Figure 25) via oxidative polymerization in the presence of 0.05 M I 2 .Conductivity measurements show that PPy@MOF has 10 5 times higher conductivity (10 −2 S cm −1 ) than bulk PPy (10 −7 S cm −1 ) and 10 times higher conductivity than I 2 @MOF. 140The encapsulated PPy acts as the main charge transport pathway.
Aniline molecules were adsorbed within the pores of MIL-101(Cr) (78) under reduced pressure and then polymerized.At low concentrations, the polymers are formed within the pores, while at high concentrations, the size of the polymer chains becomes larger and the polymers cover the surface.Electrical conductivity measurements show that increasing the concentration of PANI within the MOF pores leads to higher conductivity values with a maximum conductivity of 0.55 S cm −1 for a 20% loading and a decrease for higher PANI loadings (Table 11). 142Shao and co-workers also loaded PANI within the pores of UiO-66 MOF (79) under similar conditions.The incorporation of PANI within UiO-66 MOF (79) was verified by XRPD, IR, UV−vis, SEM, and gas adsorption analyses.The electrical conductivity of PANI@UiO-66 (≈0.2 S cm −1 ) is higher than that of PANI itself, while the MOF is an insulator. 143

MOF COMPOSITES
The combination of MOFs with other conducting carbon-based materials like carbon nanotubes, graphene, graphene oxide, reduced graphene oxide, etc., is another important strategy to improve the bulk electrical conductivity of MOFs as well as their applications (Table 12). 43,44,144There are two different ways for  such composite formation: physical mixture of MOF with conducting substrates and growth of MOF nanostructures (nano rods, nano wires, nano sheets, etc.) on the conductive substrates.
Lu and co-workers have constructed 3D MOF nanostructures of three different MOFs (HKUST-1, Zn 2 (BDC) 2 DABCO, and MIL-68) on the layers of conducting polyaniline substrates deposited on Pt electrodes. 145The obtained composite HKUST-1/polyaniline/Pt (82) shows high porosity and large enhancement of electrical conductivity to 0.125 S cm −1 (fourprobe on thin films) upon I 2 adsorption, while the HKUST-1 is itself insulating in nature.
Kim et al. have developed a nanocomposite of ZIF-8/ graphene (83), by templated growth of the MOF nanoparticles on graphene followed by annealing at 650 °C to reduce the graphene oxide. 146Such subsequent composite formation and reduction of graphene oxide increase the electrical conductivity as well as the porosity.They have also observed that the conductivity is dependent on the amount of graphene oxide present in the composite.The conductivity value (0.02 S cm −1 ) increased to 0.64 S cm −1 with increasing the amount of graphene from 2.5 to 20% wt (measured with the four-probe method on pressed pellets of the composite), while both ZIF-8 and graphene oxide are insulating in nature.
Ellis et al. have reported the controlled growth of ZIF-8 nanoparticles on single-walled carbon nanotubes (SWCNTs).Two-probe electrical conductivity measurement on thin films of the ZIF-8/SWCNTs composite (84) shows a large enhancement of the electrical conductivity to 0.056 S cm −1 . 147hrenholtz et al. have reported the solvothermal growth of thin films of [Co II (Co III TCPP)] (85) on conducting fluorinedoped tin oxide substrates (FTO) (CoTCPP = [5,10,15,20-(4carboxyphenyl)porphyrin]cobalt(III)). 148The obtained composite shows charge transport through a redox hopping mechanism with a moderate electrical conductivity of 3.62 ×    Alfèand co-workers have developed a series of composite of HKUST-1 with different amounts of graphene (86). 149Fourprobe conductivity measurements on pressed pellets show that such composite has an electrical conductivity of 10 −4 S cm −1 (with 40% wt graphene).

THIN FILM FABRICATION OF MOFS
Thin film fabrication of MOFs is a key step in the development of electronic and optoelectronic technologies. 150,151−157 The methods employed to fabricate thin films of conductive MOFs can be classified into two categories: (a) liquid-phase fabrication and (b) gas-phase fabrication.
In liquid-phase, the main fabrication methods are (i) exfoliation, (ii) drop-cast, (iii) in situ growth from mother solution, (iv) electrochemical deposition, (v) interfacial method, (vi) liquid-phase epitaxy and (vii) layer by layer growth.In gas-phase, the main methods are (i) chemical vapor deposition, (ii) atomic layer deposition and (iii) physical vapor deposition.Herein, we will discuss only those methods which have been more frequently used to fabricate thin films of MOFs.
Ultrasonication-assisted exfoliation of thin films of 2D layered MOFs is an attractive approach.Benmansour et al. have designed highly conductive 2D layered MOFs of mixed-valent Fe-anilates with different halogens in the organic building blocks.Afterward, they exfoliated these 2D MOFs into nanosheets (∼7 nm) by ultrasonication in methanol. 55Another approach is drop-casting of the    colloidal solution containing microparticles of presynthesized MOFs in the presence/absence of binder.Horiuchi and co-workers have fabricated thin films of MIL-101(Fe) on fluorine-doped tin oxide substrates (FTO) glass substrates with Nafion. 152(b) Layer by layer growth.In this method, the surface of the conducting substrate is modified by organic building blocks to form self-assembled monolayers (SAM) with free functional groups like −SH, −OH, −NH 2 , −COOH, etc.These free functional groups are reacted with metal ions and then with the organic building blocks to construct MOFs on the SAM layers by alternatively soaking in the solution of metal ions and organic building blocks.The thickness and number of layers in the deposited MOF thin films can be controlled through the repetition of soaking.Using this method, Shekhah et al.
have prepared thin films of HKUST-1 on Au substrates modified with 16-mercaptohexadecanoic acid (MHDA) followed by soaking the SAM coated substrate into solutions of copper(II) acetate and H 3 BTC alternatively. 153(c) Interfacial growth method.In this method, thin films of MOFs are fabricated at the interface of two immiscible solvents.Ameloot et al. have prepared thin films of HKUST-1 by injecting a solution of copper(II)-acetate in water with 4 wt % poly(vinyl alcohol) into a solution of H 3 BTC in octaethanol at the interface of the two solutions. 154Bai and co-workers have introduced spray methods to prepare thin films at the liquid−liquid interface.The thin films are prepared by spraying an acetonitrile solution of the metal ion (Cu 2+ ) over a DMF solution of BDC 2− ligand to form nanosheets of [Cu(BDC)] MOF with a negligible growth in the direction perpendicular to the nanosheets. 155 MOF thin films are formed on the surface of electrodes using electrochemical reactions.Depending on the type of redox reaction, oxidation or reduction, we can distinguish two types of deposition methods: anodic and cathodic.In the anodic deposition, the metal itself is used as the anode and is oxidized, with an applied voltage, to generate metal ions in the solution that react with the ligand.MOF particles start to grow on the surface of the anode.Ameloot and co-workers have prepared thin films of [Cu 2 (BTC) 3 ] (HKUST-1) by immersing a copper electrode in a solution of H 3 BTC with methyltributylammonium methyl sulfate (MTBS) as the electrolyte using different applied voltages and times. 157In the cathodic deposition, both metal ions and ligands are present in the electrolyte solution.Ligand deprotonation occurs through electrochemical reactions, and the MOF particles are formed at the cathode surface.Dinca et al. have obtained thin films of Zn 4 O(BDC) 3 (MOF-5) by this method. 158Hupp et al. have fabricated thin films of four different MOFs: NU-1000, UiO-66, HKUST-1, and MIL-53 by using an electrophoretic deposition method. 159In this case, the MOF thin films are prepared by using presynthesized MOF particles dispersed in a colloidal solution of the electrolyte.The MOF thin films grow on the surface of the electrode through electrostatic interactions.

APPLICATIONS OF CONDUCTING MOFS
Conducting MOFs are well-known for several electrochemical and electronic applications in combination with the other established functionalities of porous MOFs.Here, we will discuss some examples of applications of conducting MOFs like charge storage 160 (supercapacitors, Li-ion batteries, Li−S batteries, etc.), electrocatalysis 161−163 (hydrogen evolution reaction, HER, oxygen evolution reaction, OER, oxygen reduction reaction, ORR, CO 2 reduction reaction, CO 2 RR, etc.), sensing 164−166 (gas sensing, organic molecule sensing, and biomolecule sensing), electronic devices, etc. 5.1.Charge Storage.Electrochemical charge storage has become a hot topic in order to obtain batteries with high charge storage densities, fast charging processes, large number of cycles, etc. 167 Porous MOFs, with easily accessible metal sites along with large surface area, have become an interesting platform to study their charge storage properties. 168Dinca and co-workers have shown capacitance behavior of [Ni 3 (HITP) 2 ] MOF by developing electrochemical double layers.The pristine material shows a capacitance of 18 μF cm −2 with excellent retention over 90% even after 10,000 cycles in TEABF 4 /ACN electrolyte.The authors have demonstrated that the presence of large pores within the MOF allows the diffusion of electrolytes, and this promotes the large recyclability over charging−discharging. 169 Wang and co-workers have shown an excellent supercapacitor activity of [Cu 3 (HHTP) 2 ] deposited on reduced graphene oxide.The authors prepared MOF nanowires on the substrate with a capacitance of 44.6 mF cm −2 at 5 mV s −1 with good recyclability and mechanical flexibility based on its large porosity and high electrical conductivity. 170Wechsler and co-workers have fabricated nanosheets of [Ni 3 (HIB) 2 ] on Ni-foam.The obtained material shows a capacitance of 13.63 mF cm −2 at 0.1 mA cm −2 with 81% retention after 50,000 cycles. 171Bao and coworkers have reported high capacitance of Ni-HIB and Cu-HIB MOFs.Both redox-active frameworks show pseudocapacitance behavior with gyrometric capacitance values of 420 F g −1 and 215 F g −1 for the Ni-HIB and Cu-HIB MOFs, respectively, with a retention of around 90% after 12,000 cycles. 172Andikaey et al. have deposited thin films of NiCo-MOF on a glucose-modified liquid-phase exfoliated reduced graphene oxide substrate.The glucose moieties on the surface of the exfoliated graphene layers provide a large amount of donor hydroxide groups to coordinate the metal centers for the construction of MOFs on the substrate.The fabricated NiCo-MOF on the modified graphene layers shows excellent supercapacitor activity of 244 F g −1 with a specific capacitance of 4077.3F g −1 at 2.5 A g −1 and a high charge density of 76.3 W h g −1 . 173Jia and co-workers have deposited thin films of NiPc-MOF on Ni-foam to prepare a material with a specific capacitance of 22.1 mF cm −2 in an organic electrolyte and 11.5 mF cm −2 in water at 0.1 mA cm −2 with good recyclability and mechanical stability. 174Mohan et al. have developed a composite of a Ni/Co-MOF with reduced graphene oxide.The specific capacitance value increases from 978 F g −1 of the bare Ni/Co-MOF to 1162 F g −1 due to composite formation.The composite also shows high retention of capacitance after 5000 cycles. 175Jayaramulu and co-workers have made thin films of UiO-66 NH 2 MOF on carboxylatefunctionalized graphene through covalent attachment and have fabricated a device with a high capacitance of 651 F g −1 with excellent retention of 88% after 10,000 cycles of charging− discharging. 176ithium-ion batteries (LIB) can store and release charge through the back and forth movement of Li + ions between the anode and cathode. 177Li + ions can be incorporated within the MOFs either on the metal nodes or loading it within the void space through ion exchange.Nishihara and co-workers have shown LIB application of the [Ni 3 (HITP) 2 ] MOF based on the electron-carrying capacity of Li + and PF 6 − ions.This highly conductive MOF shows a high storage capacity of 115 mA h g −1 with a large specific energy density of 434 W h kg 1− at 10 mA g −1 with a retention of capacity after 300 cycles. 178Guo and coworkers have synthesized nanowires of Cu-CAT [Cu 3 (2,3,6,7,10,11-hexahydroxytriphenylene) 2 ].This conducting porous MOF shows LIB application based on Li + ion diffusion.It shows a specific capacity of 631 mA h g −1 at 0.2 A g −1 with retention of ∼81% capacity after 500 cycles. 179Long et al. have shown LIB application of (H 2 NMe 2 ) 2 Fe 2 (Cl 2 dhbp) 3 and (H 2 NMe 2 ) 2 Fe 3 (Cl 2 dhbp) 3 (SO 4 ) 2 .The MOFs show specific capacities of 142 and 165 mA h g −1 , respectively, in the presence of 0.1 M LiBF 4 propylene electrolyte at 10 mA g −1 with recyclability. 180Yan et al. have reported LIB application of another MOF: Cu-HHTQ (HHTQ = 2,3,7,8,12,13-hexahydroxytricycloquinazoline) with a specific capacity of 657 mA h g −1 at 600 mA g −1 with a retention of capacity of ∼82% after 200 cycles. 181Finally, Wei et al. have deposited a fluorine-doped Co-MOF on reduced graphene oxide.This material shows a high lithium storage capacity of 1202 mA h g −1 at 0.1 A g −1 with good recyclability. 182ithium−sulfur batteries rely on the charge storage between a sulfur cathode and a metallic Li anode. 183Porous conductive MOFs have gained much attention in this field based on their high charge transport properties, loading of sulfur/polysulfides within the void space, volume change during cycling, easy diffusion of electrolytes, reduced dissolution of polysulfides, etc. Zhao et al. have shown a large encapsulation of sulfur within Cu-BHT MOF through mutual interaction of guest sulfides with the metal centers of the framework and Li + ions with S-donor sites of the ligand backbone.These authors propose (Li−S) n @Cu-BHT as a potential electrode for Li−S batteries. 184Cai and co-workers have shown Li−S battery application of porous conductive nanosheets of the 2D MOF [Ni 3 (HITP) 2 ]. 185 The S-loaded framework: S@Ni-HITP was mixed with carbon nanotubes (CNTs) to prepare the cathode.Under the experimental condition, the cathode shows a high storage capacity of 1302.9 mA h g −1 with excellent capacity retention after 300 cycles, which is far better than S@CNTs or S@ZIF-67.The authors have suggested several reasons for such extraordinary performance, like high conductivity, mutual cooperation between MOF and CNTs, and porosity of the MOF to accommodate polysulfides.Wang and co-workers have reported the sulfur loading within the void space of Ni-HHTP MOF on selfsupported carbon paper. 186The obtained material shows a high capacitance of 892 mA h g −1 .Zhao et al. have reported Li−S battery applications of a composite of MIL-101(Cr) with graphene and S. 187 The composite shows a high storage capacity of 809 mA h g −1 with 95% capacity retention after 134 cycles.Finally, Geng et al. have shown that the PPy-ZIF-67-S composite can act as a cathode in Li−S batteries.The composite shows a high specific capacitance of 1092.5 mA h g −1 . 188.2.Electrocatalysis.Electrocatalysis plays, and will continue to play, an important role in solving the major problems of fossil fuel consumption and environment related issues through electrochemical energy storage and conversion. 189−163 However, their poor electrical conductivity and low chemical stability under the electrocatalytic conditions are the major problems for their electrocatalytic applications.The development of electrical conducting MOFs provides good opportunities for their electrocatalytic applications, though long-term stability is still a major concern.In this section, we will discuss some interesting examples of electrocatalytic applications of MOFs in hydrogen evolution reaction (HER), oxygen evolution reaction (OER), oxygen reduction reaction (ORR), and CO 2 reduction reaction (CO 2 RR).
The hydrogen evolution reaction (HER) is a two-electron half-cell reaction in water splitting which occurs at the cathode over a wide pH range. 190Proton reduction takes place through a two-step process: (a) Volmer step, to produce a hydrogen intermediate from water or a proton by one-electron reduction and (b) Tafel or Heyrovsky step, to generate molecular hydrogen. 191Significant development has already been achieved by using transition-metal-based MOFs.Feng et al. have reported electrocatalytic HER activity of 2D nanosheets of Ni-TPHT (H 6 TPHT = 1,2,5,6,9,10-triphenylenehexathiol).The Nidithiolene moieties within the framework allow hydrogen evolution with an overpotential of 110 mV (10 mA cm −2 at 333 mV), which is far better than other doped-graphene materials. 192Marinescu et al. have reported electrocatalytic HER activity of two different MOFs with similar cobaltdithiolene moieties: Co-BHT (H 6 BHT = 1,2,3,4,5,6-benzenehexathiol) and Co-TPHT (H 6 TPHT = 1,2,5,6,9,10-triphenylenehexathiol) which show substantial stability in acidic pH.At pH = 1.3, the overpotential values are 340 and 530 mV with a current density of 10 mA cm −2 , for Co-BHT and Co-TPHT, respectively. 193he oxygen evolution reaction (OER), the anodic reaction of water splitting, is a four-electron oxidation process that is kinetically slower and more complex in nature than HER.Dubey and co-workers have reported OER activity of two different Copdc MOFs (pdc = 2,5-pyridinedicarboxylate) of different dimensionalities within the pH range 7−14.At neutral pH, the OER overpotentials are 0.77 and 0.46 mV for the 2D and 1D MOFs, respectively, for a current density of 1 mA cm −2 .These values reduce to 0.36 and 0.25 mV at pH = 14. 194Yang et al. have reported OER activity of MIL-53(Fe)-2OH at an overpotential of 215 mV with a current density of 10 mA cm −2 in 1 M KOH. 195ang et al. have shown electrocatalytic OER activity of ultrathin nanosheets of 2D NiCo-UMOFN at an overpotential of 180 mV with a current density of 10 mA cm −2 in 1 M KOH. 196Luo and co-workers have reported OER activity of CoCl 2 @Th-BPYDC framework (BPYDC = [2,2′-bipyridine]-5,5′-dicarboxylate) in acidic pH (0.1 M HClO 4 ). 197−200 It may be a two-or four-electron process and can be conducted in both alkaline and acidic medium.Highly electrically conducting MOFs with M−N x or M−O x units are considered as potential ORR catalysts.Dinca and co-workers have reported the electrocatalytic ORR activity of the conducting [Ni 3 (HITP) 2 ] MOF (HITP = 2,3,6,7,10,11hexaiminotriphenylene) having Ni−N 4 units on a glassy carbon electrode.The electrocatalytic onset potential is 0.82 V at a current density of 0.05 mA cm −2 in 0.1 M KOH solution. 201oon et al. have shown the four-electron electrocatalytic reduction of O 2 by Co 0.27 Ni 0.73 -CAT (H 6 CAT = 2,3,6,7,10,11hexahydroxytriphenylene) in a 0.1 M NaClO 4 and 0.02 M PBS electrolyte solution.The catalytic cathodic peak appears at 0.41 V vs. reversible hydrogen electrode (RHE), which is much higher than the values of the corresponding monometallic Co-CAT or Ni-CAT MOFs. 202Mao et al. have reported the electrocatalytic ORR activity of two different MOFs: HKUST-1 and Cu-bipy-BTC (bipy-BTC = 2,2′-bipyridine-benzene-1,3,5tricarboxylate). 203he CO 2 reduction reaction (CO 2 RR) to value-added chemicals like CO, HCOOH, CH 4 , CH 3 OH, etc., is highly desirable to develop another alternative and sustainable source of energy and reduce CO 2 emissions. 204The high chemical stability of the CO 2 molecule is the main challenge in this technology.It is considered that CO 2 RR proceeds through three different steps: (a) binding of CO 2 to the active sites, (b) reduction of CO 2 via either electron transfer or H + attachment to the CO 2 molecule, and (c) rearrangement and detachment of the product from the catalyst. 205Different conducting MOFs such as copper-based MOFs and MOFs containing porphyrinbased organic moieties have been studied for electrochemical CO 2 reduction.Gu et al. have reported the electrocatalytic CO 2 reduction by a porphyrin-based MOF [Cu 2 (CuTCPP)] (CuTCPP = 5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin-Cu(II)).The MOF shows CO 2 RR at an overpotential of −1.55 V vs. Ag/Ag + to convert CO 2 to formate with a faradaic efficiency (FE) of 68.4%. 206Kumar et al. have reported the electrocatalytic CO 2 reduction by HKUST-1 deposited on a glassy carbon electrode with formation of oxalic acid as the major product.The MOF-coated glassy carbon electrode shows electrocatalytic CO 2 RR activity at an overpotential of −1.12 V vs. Ag/Ag + with 51% FE. 207 Hod et al. have shown the electrocatalytic CO 2 reduction of a MOF-525 containing Zr 6 metal nodes and Fe-TCPP as the building blocks.The electrocatalysis was carried out by depositing thin films of MOF-525 onto FTO substrates.These thin films show electrocatalytic CO 2 reduction to CO at an overpotential of −1.3 V vs NHE with 100% FE in the presence of trifluoroethanol. 208Bao et al. have studied the electrocatalytic CO 2 reduction by a series of MOFs with similar sodalite topology: ZIF-7, ZIF-8, and ZIF-108.ZIF-8 shows CO 2 reduction at an overpotential of −1.1 V vs. RHE to CO with 81% FE, while ZIF-108 shows a similar behavior at an overpotential of −1.3 V vs. RHE. 209.3.Electrochemical Sensors.Highly selective and sensitive electrochemical sensing has several important applications, like environmental monitoring, disease diagnosis, rare element detection, and so on.In an electrochemical sensing device, the analyte interacts with the surface of the sensor that produces an electrical signal in response to the analyte−sensor interaction. 210Conductive MOFs are used as potential electrochemical sensors based on their excellent properties such as high porosity, tunable functionalization, reproducibility, efficient charge transfer properties, etc. 164,165,211 In some cases, MOFs are also assembled with carbon paste electrodes (CPE) and glassy carbon electrodes (GCE) to implement them in electrochemical sensing devices.To date, different types of electrochemical sensing methods, such as chemresistive, chemcapacitive, impedance, Kelvin probe, and field effect transistors, have been developed to detect different types of analytes. 212In the next sections, we will briefly discuss the electrochemical sensing properties of different MOFs and MOF-composites by classifying analytes into four categories: gases, ions, molecules, and biomolecules.
Gas Sensing.Dinca group have reported the first example of chemresistive sensing of ammonia gas using 2D conductive [Cu 3 (HITP) 2 ] within the detection limit 0.5−10 ppm.The interaction between ammonia and copper atoms within the MOF is the underlying reason for such selective sensing of ammonia. 213These authors have also developed a cross-reactive chemresistive sensor array using Cu 3 (HHTP) 2 , Cu 3 (HITP) 2 , and Ni 3 (HITP) 2 to detect and classify 16 organic compounds based on their functional groups: alcohol, ether, ketone, aromatic, amine, and aliphatic. 88Xu et al. have fabricated thin films of [Cu 3 (HHTP) 2 ] by the spray layer by layer method that show highly selective detection of ammonia in the range 1−100 ppm. 214Mirica et al. have shown highly selective detection of NO and H 2 S (with detection limits of 0.16 and 0.23 ppm, respectively) by self-organizing [Ni 3 (HHTP) 2 ] on textiles. 2152 The same group also reported the chemresistive sensing of trimethylamine by [Co(im) 2 ] n MOF with a detection limit of 2 ppm. 223Zhan et al. have reported the selective detection of H 2 O 2 by ZnO@ZIF-8 nanostructures deposited on a FTO substrate. 224Hosseini et al. have fabricated an electrochemical sensing device by immobilizing Au-SH-SiO 2 nanoparticles on HKUST-1 that can detect and electrocatalytically oxidize hydrazine with a detection limit of 10 −8 M. 225 Biomolecule Sensing.Li et al. have developed an electrode material by depositing Ag nanoparticles on MIL-101(Fe) on modified glassy carbon electrode which shows selective detection of tryptophan.The modified electrode presents a higher oxidation current in the presence of tryptophan, which increases with the amount of tryptophan, with a detection limit of 140 μM. 226Deep et al. have developed a thin film of the MOF Cd-ATC (ATC = 2-aminoterephthalate) on ITO for pesticide detection.The thin film was modified with antiparathion antibody and then used for the detection of parathion with a detection range of 0.1−20 ng mL −1 ). 227.4.Electronic Devices.Prasoon et al. have prepared a TCNQ-doped MOF thin film with diode-like behavior.The thin films were prepared by depositing Cu-BTEC (BTEC = 1,2,4,5benzenetetracarboxylate) on self-assembled multilayers (SAM) with the layer by layer (LBL) technique.The films were doped with TCNQ molecules through coordination of the cyanogroups of TCNQ with the metal sites of the framework.I−V measurement shows that the TCNQ@Cu-BTEC thin films act as a Schottky diode with 10 5 rectification ratio. 228Wu et al. have developed a porous field-effect transistor (FET) based on thin films of the highly conducting [Ni 3 (HITP) 2 ] MOF prepared using the gas−liquid interfacial method.The obtained thin film was deposited on a SiO 2 substrate by the stamp method, and then Au electrodes were patterned on the surface of the MOF film to develop the FET device.This FET device shows p-type behavior with an on/off ratio of 2 × 10 3 and field effect mobility of 48.6 cm 2 V −1 s −1 . 229Wang et al. have developed a FET device using the same [Ni 3 (HITP) 2 ] MOF on a Si/SiO 2 substrate using in situ solid−liquid interfacial method.The FET device shows an on/off ratio of 2.29 × 10 3 with a mobility of 45.4 cm 2 V −1 s −1 . 230

CHALLENGES AND PERSPECTIVES
After decades of intense research, the field of conducting MOFs has accomplished important results and has shown that electrical conducting MOFs may act as functional materials in fields such as charge storage, electrocatalysis, electrochemical sensing, etc.However, one of the major challenges for their industrial application is their nonscalable synthesis and high cost, in most cases.Moreover, their limited stability under experimental conditions also hinders their use.Therefore, there are still several challenges to be faced for their industrial development: (a) High electrical conductivity.There are still very few MOFs reported with high electrical conductivities (>1 S cm −1 ), but most electronic applications of MOFs require high conductivity.Therefore, there are still much effort to be done to develop highly conducting MOFs.Most of the highly conducting MOFs are obtained using planar organic S-or N-donor ligands since they present a high metal−ligand orbital overlap.Therefore, research efforts should be carried out to find out adequate metal−donor atom overlaps for better conductivity using other donor atoms such as O, Se or P and other transition metal atoms.Radical-based ligands also provide opportunities to develop highly conducting MOFs.Besides anilatederivative ligands, other redox-active ligands should be used to design new conducting MOFs, especially those forming stable organic radicals.(b) MOF-composites interactions.The synthesis of MOFcomposites with synergetic interactions between MOFs and conducting materials is a second important challenge to be faced.Unfortunately, in most cases, the composites are formed by simply mixing the MOFs with the conducting matrices.This fact leads to nonuniformity of the material with weak interaction between the MOFs and the conducting material, limiting their stability.Furthermore, the weak interactions between the MOF and the conducting matrix also limits the charge transport.A possible way to overcome these low interactions may be the direct synthesis of the MOFs as nanoparticles inside the functionalized conducting matrices.(c) Stability.One of the major challenges in MOF chemistry (not only in conducting MOFs), is their limited stability under experimental conditions.Most highly conducting 2D MOFs contain metal centers with square planar coordination environments that make them highly suitable for electrocatalytic applications but reduce their stability due to potential solvent coordination to the axial vacant sites.The design of conducting MOFs with strong metal−ligand coordination bonds is a good strategy to increase their stability.(d) Transport Mechanisms analysis.In only a few cases, the charge transport mechanism has been established including both intrinsically and extrinsically conducting MOFs.The charge carrier types (holes or electron), charge carrier density, and their mobility are also important parameters to characterize conducting MOFs.Therefore, mechanistic studies need to be performed in order to design suitable MOFs for different applications.
A combination of experimental and theoretical investigations may help to establish the detailed charge transport mechanisms in conducting MOFs.(e) Applications.For charge-storage MOFs, it is highly desirable to correlate the charge storage with the active sites of the MOFs.The uptake/release of metal ions through the channels of the MOFs is a key factor in metalion batteries.These phenomena should be studied in detail in order to develop MOFs with a high storage capacity and charge−discharge recyclability.Electrocatalysis (HER, OER, ORR, and CO 2 RR) is one of the most important applications of conducting MOFs.However, their performances and stability are still far behind noble metal catalysts.Further investigations are required to improve the electrocatalytic performance of conducting MOFs.The use of conducting MOFs with two (or more) different metal ions may be an appropriate strategy to increase the catalytic performance, as already observed in zeolites and other heterogeneous catalysts.Finally, electrochemical sensors based on conducting MOFs require the presence of strong interactions between the sensor and the analyte.A possible way to increase these interactions may be the use of ligands with many functional groups.The use of conducting MOFs with different metal ions and ligands simultaneously is expected to increase the selectivity of these sensors.

CONCLUSIONS
The synthesis of conducting MOFs has become a very active area in recent years since the addition of electrical conductivity to these multifunctional materials may lead to applications in fields such as electrochemical sensors, photovoltaic devices, electrocatalysis, batteries, etc.The intrinsic hybrid tunable structures of most MOFs and their capacity to insert many different types of guests have led to the preparation of an increasing number of conducting MOFs and, more importantly, to the rational improvement of the conductivity in some of these examples.However, most of these conducting MOFs show low roomtemperature conductivities and semiconducting behaviors, which largely limits their use in the aforementioned fields.Luckily, as we have shown here, the work of many qualified and active research groups is changing this trend by developing different strategies to increase the electrical conductivity in MOFs.The first results show that this is, indeed, possible, as shown by some synthesized MOFs with high conductivities and even with metallic behavior.
For the intrinsically conducting MOFs (those where the electrical conductivity takes place through the framework of the MOF), these strategies can be summarized in (i) the use of mixed-valence metal ions and/or ligands, which favor the electron transport, (ii) the use of soft donor-atom-based ligands that lead to better metal−ligand orbital overlap, favoring the through-bond electron transport, and (iii) the use of planar and highly conjugated ligands to favor the interchain/interlayer π−π interactions, favoring the through-space charge transport.
In the case of the extrinsically conducting MOFs (those where the electrical conductivity is achieved by inserting electroactive guests species that facilitate the electron transfer), the strategies to improve conductivity can be summarized in (i) the insertion of metal ions and/or metal nanoparticles that may create a pathway for the electron transfer, (ii) the insertion of electroactive molecules (either donors or acceptors) that may change the electron density in the framework and facilitate the electron transfer (either through the framework or through the guest), and (iii) the incorporation of well-known organic conducting polymers in channels or in the interlayer space of the host MOFs, to create a conducting pathway inside the MOF.In most cases, the insertion of the guests can be done either postsynthetically or by in situ template-assisted synthesis.In the case of the organic conducting polymers, the insertion of monomers and their posterior polymerization constitutes a very useful strategy.

Figure 1 .
Figure 1.Schematic diagram for designing and improving electrical conductivity in both intrinsically (left side) and extrinsically (right side) conducting MOFs.Color code: C = gray, S = yellow, Se = pink, N = blue, O = red, Cl = green.

Figure 2 .
Figure 2. Schematic diagram of four-probe (left) and two-probe (right) methods for conductivity measurements.Dimensions l, a, and d are the voltage probes distance, the sample width, and sample thickness, respectively.Reproduced with permission from ref 12.Copyright 2012 Royal Society of Chemistry.

Figure 3 .
Figure 3.The four points (left) and van der Pauw (right) methods used for measuring electrical conductivity in very thin samples.Reproduced with permission from ref 12.Copyright 2012 Royal Society of Chemistry.

Figure 4 .
Figure 4. Two most probable charge-transport mechanisms applicable for MOFs.Reproduced with permission from ref 45.Copyright 2019 Royal Society of Chemistry.

Figure 5 .
Figure 5. (a) Two different charge-transport pathways, through-bond and through-space, applicable for MOFs.Reproduced with permission from ref 45.Copyright 2019 Royal Society of Chemistry.(b) Through-plane charge transport based on metal−ligand d−π orbital overlap.Reproduced from ref 92.Copyright 2018 American Chemical Society.(c) Conductive polymeric guest-mediated charge transport within porous MOF and (d) two possible redox hopping mechanisms (lattice-to-guest or guest-to-lattice) for charge transport.Color code: C = gray, S = yellow, N = blue, O = red, and Fe = green.
Darago et al. have synthesized both the oxidized and reduced c o m p l e x e s o f d h b q n − a n d F e I I I w i t h f o r m u l a s ( N B u 4 ) 2 [ F e I I I 2 ( d h b q ) 3 ] ( 1 -o x ) a n d Na 0.9 (NBu 4 ) 1.8 [Fe III 2 (dhbq) 3 ] (1-red) through selective reduc-

Figure 6 .
Figure 6.Different redox states (−2, −3, and −4) of the C 6 O 4 X 2 n− 4 and Ni 9 (HHTP) 4 , the metal ions present a hexacoordinated octahedral geometry with two extra coordination positions occupied by two water molecules.A second difference between the Cu and the Ni and Co derivatives is the ligand charge.Thus, in Co 9 (HHTP) 4 and Ni 9 (HHTP) 4 , the average charge on the ligand is −4.5, while it is −3 in Cu 3 (HHTP) 2 .Electrical conductivity measurement shows that the conductivity values vary in the range of 0.1 to 0.003 S cm −1 and 0.01 to 3 × 10 −6 S cm −1 for Co 9 (HHTP) 4 and Ni 9 (HHTP) 4 , respectively. 74,78,79

Figure 7 .
Figure 7. X-ray crystal structure of the [Fe 2 (C 6 O 4 Cl 2 ) 3 ] 3− layer as observed in 2-red, viewed perpendicular to the anionic layer.Cations and DMF molecules are omitted for clarity.Color code: Fe = orange, Cl = green, O = red, and C = gray.Reproduced from ref 64.Copyright 2017 American Chemical Society.

Figure 9 .
Figure 9. (a) Structure of M 2 (DEBDC) (E = S, O) MOFs.(b) Electrical conductivity as a function of 1/T for four different derivatives.Color code: C = gray, S = yellow, and M = blue.Reproduced from ref 80.Copyright 2018 American Chemical Society.

Figure 10 .
Figure 10.(a) Calculated structures of M 3 (HIB) 2 (M = Ni and Cu) (b) Variable-temperature electrical conductivity of pressed pellets of M 3 (HIB) 2 measured by the van der Pauw method.Color code: C = gray; M = pink, and N = blue.Reproduced from ref 83.Copyright 2017 American Chemical Society.

Figure 11 .
Figure 11.(a) View of the structure of [Ag 5 (BHT)] n (31).Color code: Ag = blue, S = yellow, and C = gray.The Ag-centered tetrahedra are shown in pale blue.(b) Electrical conductivity of a thin film of [Ag 5 (BHT)] n (31) as a function of temperature.Reproduced from ref 93.Copyright 2017 American Chemical Society.

Figure 12 .
Figure 12.(a) Illustration of the chemical structure of the Ni 3 (BHT) 2 (33) nanosheets.(b) Schematic illustration of the redox control in 33.Reproduced from ref 53.Copyright 2014 American Chemical Society.

a
Two-probe on single crystal.b Two-probe on pressed pellet.c Four-probe on thin film.d Two-probe on thin film.e Van der Pauw on thin film.f Four-probe on pressed pellet.g Van der Pauw on pressed pellet.h E a at 40 K.
pressed pellets of the sample show a very high conductivity of ∼110 S cm −1 at 300 K that increases with temperature from 10 to 400 K, showing a semiconducting behavior.Enriched metal− ligand overlap between copper atoms and the Se atoms of the ligand provides the efficient charge transport pathway through the 2D metal−organic layer.Another example of Se-based conducting MOF was prepared by Huang et al., which synthesized a special type of MOF formulated as PhSeAg(43), containing inorganic layers of AgSe with phenyl groups on both sides of the inorganic layer.Twoprobe conductivity measurement reveals a conductivity at room temperature of 2.93 × 10 −11 that increases to 3.95 × 10 −8 S cm −1 at 418 K, indicative of a semiconducting behavior.1022.1.4.Mixed-Valence Metal Ions.As already described for the mixed-valence ligands, metal ions with tunable valences are also utilized to design conducting MOFs (Table6) since the redox behavior of metal cations inside the MOFs may provide a pathway for electron transfer.One example is the MOF Cu[Cu(pdt) 2 ] (25) (pdt = 2,3-pyrazinedithiolate) reported byTakaishi et al. in 2009.This mixed-valence MOF contains Cu I cations and [Cu III (pdt) 2 ] − anions and shows a conductivity of 6 × 10 −4 S cm −1 at 300 K.38 Okubo et al. reported a semiconductor based on a mixed-valence Cu I −Cu II coordination polymer formulated as [Cu 3 I Cu II Br 3 (3,5-Dmpip-dtc) 2 ] n (44) (3,5-Dmpip-dtc = 3,5-dimethylpiperidine dithiocarbamate).This MOF has a room-temperature conductivity of 6.5 × 10 −8 with an activation energy of 390 meV. 103In 2016, Wu et al. reported the mixed-valence semiconducting 3D MOF [Cu Ι Cu 2 ΙΙ (DCTP) 2 ](NO 3 )•1.5DMF(

=
i s o s t r u c t u r a l 2 D a n i l a t o -b a s e d f r a m e w o r k s (H 2 NMe 2 ) 2 [M 2 (C 6 O 4 Cl 2 ) 3 ] with M = Ti (50) and V (51) and (H 2 NMe 2 ) 1.5 [Cr 2 (C 6 O 4 Cl 2 ) 3 ] (52).108The room-temperature electrical conductivities of these Cr, Ti, and V-containing MOFs, measured under Ar atmosphere with the two-probe technique, are 1.2 × 10 −4 , 2.7 × 10 −3 , and 0.45 S cm −1 respectively.The thermal dependence of the conductivity with temperature show that the conductivity of the Ti and Cr-MOFs is due to redox-hopping between neighboring ligands in different valence states, rather than through excitation into a delocalized band.The activation energies of the Ti and Cr-based MOFs were calculated, using the Arrhenius model, as 270 and 440 meV, respectively.In contrast, the V-containing MOF follows a variable-range hopping (VRH) mechanism for charge transport with activation energies of 64 meV at 300 K and 11 meV at 20 K.The variable-temperature conductivity of the V-MOF was satisfactorily fit with the Efros−Shklovskii variablerange hopping model:Another interesting example of cation-dependent conductivity in MOFs was reported by Park et al. in a series of four isostructural MOFs formulated as M 2 (TTFTB) (TTFTB = tetrathiafulvalene-tetrabenzoate) with M 2+ = Mn 2+ (53), Co 2+ (54), Zn 2+ (55), and Cd 2+

Figure 15 .
Figure 15.Modulation of the electrical conductivity of an Fecontaining MOF due to change in the oxidation states.Color code: C = gray, Fe = orange, N = blue, and Cl = green.Reproduced from ref 107.Copyright 2018 American Chemical Society.

Figure 20 .
Figure 20.(a) TCNQ molecule shown above a Cu 3 (BTC) 2 MOF; arrow points into the pore.Color code: H = white, N = blue, C = cyan, O = red, and Cu = light brown.(b) SEM image of a MOF-coated device; insets are optical images of devices before and after TCNQ infiltration.(c) XRD data for powders and grazing incidence XRD for a thin film.(e) Minimum-energy configuration for TCNQ@Cu 3 (BTC) 2 obtained from ab initio calculations.(f) Possible configuration that would provide a conductive channel through the MOF unit cell.Color code: C = cyan, Cu = yellow, and O = red.Reproduced with permission from ref 122.Copyright 2013 The American Association for the Advancement of Science.

Figure 22 .
Figure 22.Crystal structure of NiCB@NU-1000.The triangular channels of NiCB@NU-1000 are shown in a space-filling model to present the close interaction between NiCB and three surrounding pyrenes.H-atoms have been omitted for clarity.The structure of NiCB is also presented.Reproduced from ref 132.Copyright 2018 American Chemical Society.

133 a
Two-probe on single crystal.b Four-probe on single crystal.c Two-probe on thin film.d Two-probe on pressed pellet.e Four-probe on thin film.f Four-probe on pressed pellet.g Thin film.

Figure 24 .
Figure 24.Reaction scheme for the synthesis of PEDOT@UiO-66 and Ppy@UiO-66.Left panel: Visualizing two different pores in UiO-66 (color code: Zr = green, O = red, and C = gray; H atoms are omitted for clarity.Right panel: Optical images of products obtained after each step.Reproduced with permission from ref 139.Copyright 2019 John Wiley and Sons.

Figure 26 .
Figure 26.Schematic illustration of the fabrication process and the interaction between PANI and MIL-101(Cr).Reproduced with permission from ref 142.Copyright 2018 Elsevier.

141 a
Impedance on pressed pellet.b Four-probe on pressed pellet.c Two-probe on single crystal.d Two-probe on pressed pellet.

Table 2 .
Conducting MOFs Prepared with Extended Conjugated Ligands

Table 3 .
Conducting MOFs with O/S-Donor Atom-Based Organic Ligands a Two-probe on pressed pellets Scheme 2. N-Donor-Based Organic Ligands for Designing Conducting MOFs

Table 4 .
Conducting MOFs with N-Donor Atom-Based Organic Ligands Van der Pauw on pressed pellet.b Four-probe on pressed pellet.c Impedance on pressed pellet.d Two-and four-probe on single crystal.e Fourprobe on pressed pellet.f Four-probe on thin film. a

Table 5 .
Conducting MOFs with S, Se, and Mixed Donor-Atom-Based Organic Ligands

Table 7 .
Conducting MOFs with Cation Size and Electronic Structure Dependent Conductivity a Two-probe on pressed pellet.b Two-probe on single crystal.

Table 8
a Distance between neighboring π-ligands.b Four-probe on pressed pellet.c Two-probe on single crystal.

Table 9 .
Conducting MOFs with Metal-Based Guests a

Table 10 .
Conducting MOFs with Molecular Guests

Table 12 .
Conducting MOF-Composites a Four-probe on thin film.b Four-probe on pressed pellet.c Impedance on thin film.