Extrinsically conducting MOFs: guest-promoted enhancement of electrical conductivity, thin film fabrication and applications

Rajat Saha * and Carlos J. Gómez García *
Departamento de Química Inorgánica, Universidad de Valencia, Dr Moliner 50, 46100 Burjasot (Valencia), Spain. E-mail: Rajat.Saha@uv.es; carlos.gomez@uv.es

First published on 22nd August 2024

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Rajat Saha

Rajat Saha received his PhD from Jadavpur University, India, in 2013 on designing multifunctional MOFs and organic materials using crystal engineering techniques. He joined as an Assistant Professor at the Department of Chemistry, Kazi Nazrul University, Asansol, India, in 2016. His research interests have mainly centered on designing MOFs for versatile applications. Presently, he is working as a Maria Zambrano Postdoctoral Fellow at Universitat de Valencia, Spain. Now, he is working on conducting MOFs for their applications in spintronics and others.

Carlos J. Gómez García

Carlos J. Gómez García (Valladolid, Spain, 1964) received his PhD from the University of Valencia with the extraordinary doctoral award in 1991. After performing two post-doctoral stays at Rennes and Bordeaux (France), he joined the University of Valencia as an associate professor in 1993. In 1998 he was promoted as assistant professor and as full professor in 2007 at the Inorganic Chemistry department at the University of Valencia (after a national habilitation). Since 2014 he leads the Multifunctional Modulable Molecular Materials (M4) group. His main interests are magnetic and conducting MOFs and coordination compounds, including polyoxometalate-based materials for energy applications.

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1. Introduction

Since 1999, after the discovery of MOF-5 by Yaghi et al., metal–organic frameworks (MOFs) or porous coordination polymers (PCPs) have become the most studied functional materials in chemical science.^1–3 MOFs or PCPs, constructed using discrete metal ions or metal clusters as nodes and organic multi-topic ligands as spacers, have attracted significant attention not only due to their aesthetically beautiful architectures but also thanks to their large surface areas, chemically functionalized cavities, flexible skeletons and intriguing electronic properties.^4–6 Such structural characteristics make them potential functional materials for gas and solvent adsorption,⁷ storage,⁸ separation,⁹ catalysis,¹⁰ sensing,¹¹ drug delivery,¹² etc. Additionally, the almost infinite choice of organic bridging ligands and metal ions, with different oxidation states and coordination numbers, allows the modulation of their structures and properties in a controlled manner.^13–15

In order to obtain advanced functionalities, different crystal engineering design principles, including both pre- and post-synthetic modifications of the basic building blocks, have been employed to convert simple MOF structures into more complex ones.^16–18 Such structural complexity can be achieved by the exchange of ligands or metal ions via post-synthetic methods^19,20 or by grafting extra functional units either on the metal atom²¹ or on the ligand backbone.²² Multifunctional MOFs have also been designed by using different types of ligands and guests in simple MOFs.²³ In this regard, the most promising design principle is the utilization of the inherent void space of MOFs to encapsulate different types of guest species that may modulate and create new functionalities different from those of the host MOF and the guest.^24–27

In the last decade, electrically conductive MOFs and PCPs have gained much attention for their numerous applications in energy storage,^28,29 electrocatalysis,^30,31 chemiresistive sensing,^32,33 etc. However, the implication of MOFs in electronic and electrochemical applications is hindered by their low-charge transport properties. The poor overlap between metal d-orbitals and ligand donor p-orbitals results in weakly conducting or insulating materials.³⁴ Given the enormous opportunity offered by conducting MOFs, several strategies have been used to increase and modulate their electrical conductivity: (a) the use of ligands having soft donor atoms like S, P, Se, etc., (b) the use of extended conjugated organic ligands, (c) the use of redox-active building blocks (metal ions and organic ligands), and (d) the incorporation of guest species within the pores/voids of the frameworks, etc.^35–43 Based on the electron conduction pathway, electrically conductive MOFs are categorized into two classes: (i) intrinsically conducting MOFs (ic-MOFs), in which electron conduction proceeds only through the metal–ligand backbone, and (ii) extrinsically conducting MOFs (ec-MOFs), where electron conduction is dependent on the guest species present in the host framework.

In order to control the electrical conductivity of ic-MOFs, researchers have shown that the utilization of soft donor-based organic ligands is a very promising strategy to enhance their conductivity thanks to a better metal–ligand overlap.^44,45 A nice example is MOF [Cu₃(C₆S₆)]_n, which contains an infinite [–Cu–S–] network and shows the highest reported conductivity for any MOFs.⁴⁶ Different research groups have utilized extended conjugated organic ligands like pyrene, coronene, etc. to develop in-plane charge transport. An example of this strategy is MOF PTC-Fe (PTC = 1,2,3,4,5,6,7,8,9,10,11,12-perthiolated coronene), which has a coronene core with S donor atoms and shows a high electrical conductivity.⁴⁷ The presence of mixed-valence metal ions can also improve the electrical conductivity. Thus, Long et al. have reported a billion-fold increase in the electrical conductivity of MOFs by using mixed-valence metal ions.⁴⁸ On the other hand, the Harris group has shown that the use of radical-based ligands can increase the overall charge transport properties of MOFs and PCPs.⁴⁹ Furthermore, defect healing can improve the intrinsic charge transport properties of MOFs.⁵⁰ The detailed structural analysis has allowed a significant improvement and development of ic-MOFs and PCPs in the last decade.

Unfortunately, the incorporation of guests within the channels or cavities of these ic-MOFs and PCPs to prepare ec-MOFs induces a loss of crystallinity of the MOFs in most cases,⁵¹ rendering the study and development of ec-MOFs, in a rational way, very difficult. Nevertheless, there has been substantial growth in this field in the last few years.

Although a few review articles have focused on the guest-dependent electrical conductivity of MOFs, the major focus has been placed on intrinsically conducting frameworks.^{28,31,34–36,52–59} From the structural point of view, the design of ic-MOFs has gained much attention, and, therefore, most of the reviews are centred on the structure–property relationship of these ic-MOFs, while for the guest-dependent conducting ec-MOFs, the focus has been mostly centred on TCNQ,⁶⁰ iodine,⁶¹ polyiodides⁶² and some conducting polymers.⁶³ However, to date, there have been no comprehensive reviews highlighting all the factors associated with the guest-dependent electrical conductivity of these ec-MOFs (Fig. 1).

Herein, we present an exhaustive review of the characteristics of the guest species that have been used by different research groups to enhance the charge transport properties of MOFs. We start with the charge transport mechanisms (band and hopping transport) operative for these MOFs and with the different charge transport pathways: (a) through bond, (b) through layer, (c) through space, (d) redox hopping and (e) through guest. We then provide an overview of the formation of host–guest MOF structures through encapsulation of guest species by both in situ and post-synthetic modification methods. We present the principles that guide the choice of electroactive guest species to design ec-MOFs. Afterward, we show examples of guest-dependent electrical conductivity in MOFs revising all the guest species used to enhance charge transport in MOFs (organic⁶⁰ and inorganic⁶¹ molecules and organic conducting polymers⁶³). According to their composition, structure, host–guest interaction and charge conducting behaviour, we classify these guest species into three categories: (a) metal-based guests³⁵ (metal ions, metal nano-clusters and metal oxides), (b) molecule-based guests (TCNQ, iodine and polyiodides, ferrocene, fullerene, tetrathiafulvalene and its derivatives, metal–organic molecules, bromine, organo-metallic compounds, etc.) and (c) organic conducting polymer guests (such as PEDOT, PANI, PPy, PA and PTh) (Scheme 1). We discuss the different methods for the encapsulation of the guest species, the modification of the geometry of the framework after guest encapsulation, the charge transport pathway, the operative charge transport mechanism and the enhancement/change of electrical conductivity of the resulting guest-loaded host MOFs. Afterward, we discuss the different methods employed to develop thin films of MOFs to find out the most suitable ones for the extrinsically conducting MOFs. We have classified all the processes into two categories based on the medium used for the thin film preparation: (a) liquid phase processing (including exfoliation, electrochemical synthesis, interfacial method, epitaxial growth, etc.) and (b) gas phase processing (physical vapor deposition, atomic layer deposition and chemical vapor deposition). Then, we discussed different applications like electrocatalysis, sensing, charge storage, photoconductivity, photocatalysis, etc. using ec-MOFs. Finally, we have summarized the general aspects of this field, obstacles and perspectives. We have not considered those guest species which can’t be accommodated within the pores of MOFs.

1.1 Transport mechanisms

Charge transport in conductive MOFs usually takes place through two types of mechanisms: (i) band-like and (ii) redox-hopping (Fig. 2).⁶⁴

(i) Band transport occurs when the strong overlap between the metal and ligand orbitals of the MOFs generates continuous energy valence and conduction bands. The energy difference (E_g) between these bands defines their predictable electrical properties like metallic conductors (when both bands overlap and E_g = 0), semiconductors (when E_g < 3.6 eV), and insulators (when E_g ≥ 3.6 eV).⁶⁵ For metallic conductors, both the conduction and valence bands merge and they show a decrease in conductivity with increasing temperature due to electron–phonon coupling which restricts the free movement of electrons in the band. In semiconductors, the electrical conductivity increases with increasing temperature due to the increase in charge carriers in the conduction band. For a classical semiconductor, the conductivity follows the Arrhenius law: σ = σ₀exp(−E_a/kT) where E_a is the activation energy, which is equal to half of the band gap (E_a = E_g/2). In insulators, there should not be any change in the electrical conductivity when the temperature is varied.

(ii) Hopping transport assumes that the charges are localized and “jump” through a phonon-assisted quantum tunnelling mechanism. In this case, the conductivity follows the equation σ = σ₀exp [(−T₀/T)^α], where α = 1/(1 + d) and d is the dimensionality of the conducting lattice. Thus, α = 1/4 for three-dimensional (3D) lattices, as predicted by the Mott model.⁶⁶ In this case, the electron transport can be described as an exchange process between redox couples. In this hopping model, when the temperature increases, the electron delocalization increases, and, therefore, the conductivity also increases. The electrical conductivity can be defined by Mott's variable range hopping (VRH) theory (eqn (1)):

	(1)

with

	(2)

where the VRH exponent (γ) is calculated from the dimensionality of the transport pathway: γ = 1/(1 + d) and the values of γ are 1/4, 1/3 and 1/2, respectively, for three-, two- and one-dimensional systems. σ₀ is the conductivity at infinite temperature, k_B is the Boltzmann constant, T_Mott is the Mott characteristic temperature, N(E_F) is the density of states at the Fermi level and L_loc is the localization length. Therefore, a material with a 3D, 2D or 1D electron conduction pathway will follow a linear dependence in the plot of ln [T_Mott^1/2 σ(T)] vs. T^−γ, with γ = 1/(1 + d) and T_Mott can be calculated from the slopes of these plots. Although most MOFs are insulators, the number of semiconducting and even metallic MOFs is continuously increasing.⁶⁷ The mechanism used to explain the conductivity in these conducting MOFs is, in most cases, the band model, although the hopping model has also been used in some cases (Table 1).⁶⁸

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The electron conduction in both, ic-MOFs and ec-MOFs, proceeds through different pathways like through bond, through layer, through space and redox hopping.³⁵ However, electron transport may occur only through guest species in some ec-MOFs (as we will see later, there are many insulating MOFs that become conductive only when the guests are inserted). In through-bond transport, the metal–ligand overlap is the predominant factor in promoting the electron transport. Based on this transport idea, several ligands with soft donor atoms (S, P, Se, etc.) have been combined with different transition metal ions to construct conductive MOFs.⁴⁶ Following the electron transport in 2D materials like graphene, the design of conductive MOFs with extended π-conjugated organic ligands (like benzene, triphenylene, coronene, etc.) has gained much attention. Note that in these MOFs, the electron transport follows a through-plane pathway.⁴⁷ Interlayer π-interactions have also significant contribution in through-space charge transport for these 2D layered MOFs. The π-interactions operative between the aromatic core of organic ligands can transport electrons through the MOF as well as through space. Encapsulation of aromatic guest molecules within the void space of MOFs can promote the through space charge transfer between the aromatic cores of the organic building blocks of the host framework.³⁵ Utilization of redox-active metal ions⁴⁸ or ligands⁴⁹ or guest species⁶² may induce the charge transport within the framework thanks to the hopping of localized electrons between different redox active sites. Finally, the encapsulation of organic conductive polymers within the void space of host frameworks induces electron conduction in the MOF through the polymeric chains of the guest.⁶³ In some cases, the guest–guest interaction is also operative to conduct electrons within the host framework.

1.2 Host–guest MOFs

In MOF literature, a host–guest complex is defined as a chemical system made up of two or more subunits self-assembled together to form a super-molecular entity. In a host–guest complex, there is a control of the host–guest interactions and the guest (usually a molecule) is encapsulated within the continuous structure of the host.^76–78 Different types of intermolecular interactions like hydrophobic association,⁷⁹ hydrogen bonding,⁸⁰ electrostatic interactions,⁸¹ metal coordination,⁸² van der Waals forces,⁸³ π–π stacking interactions,⁸⁴ etc. have been found to stabilize the host–guest structure. Within the confined space of the host, the geometry, molecular orientation, supramolecular behaviour and subsequent properties of the guest molecules may be different from those in the bulk and gas phases. On the other hand, the structure-related functionalities of the host–guest complex can be modulated by changing the type and amount of the inserted guest. Different types of molecular systems having discrete inner cavities such as crown ethers,⁸⁵ cryptands,⁸⁶ cavitands,⁸⁷ calixarenes,⁸⁸ etc. have been designed for tailor-made functionalities through encapsulation of different types of guests.

Different well-established methods have been used by researchers to load guest molecules within the host MOFs. These methods can be grouped into two main categories: (a) templated in situ (TIS) and (b) post-synthetic modification (PSM).

In the TIS method, the guest species are mixed with the metal ions and the ligands during the MOF synthesis. The strong metal–ligand coordination interactions build the framework architecture around the guest molecules. The guest molecules remain in the channels and cavities anchored by weak interactions, coordinated to unsaturated metal centres or even connected to functional groups of the ligands. These guest molecules, as templates, may induce a different structure of the MOFs compared to the one obtained in the absence of these guest species. Such TIS synthesis allows the preparation of single crystals of guest-encapsulated host structures to study the host–guest interactions. This method also allows encapsulation of guests larger than the pore window of the framework that restricts the removal/insertion of the guests. Different electroactive molecules like iodine, TTF, etc. and monomers of different organic conducting polymers have been incorporated within the void space of MOFs by this method (Fig. 3).

Post-synthetic modification is a well-established method to change the basic structure of the synthesized MOFs to obtain desired functionalities. Different guest molecules or guest precursors can be inserted within the channels or pores of MOFs. Researchers have used different techniques for guest loading such as dissolving the guest in different solvents, adsorption of guests in the gas phase, ion exchange, etc. Encapsulation of guests through this PSM method depends on the size of the guest, pore window size of the host and host–guest interactions. Though this method is very easy to handle and used very often, it has several drawbacks: (i) loss of crystallinity during guest encapsulation, (ii) inhomogeneous distribution of the guests within the MOF channels and (iii) leaching. In several cases, post-treatment of encapsulated precursors to build the active guest species can damage the crystallinity of the frameworks.

Besides these two popular methods, another method namely grafting within the MOF architecture has also gained popularity. In this method, the framework is pre-functionalized either by using open metal sites or ligands having free functional organic sites, which are further utilized to load the guest species following the pre-functional sites. The accommodated guest species have been found to attach strongly to the free functional organic sites.

1.3 Principles for the choice of guests to design ec-MOFs

Electron conduction in MOFs may follow two different transport pathways: (a) through-bond and (b) through-space (Fig. 4).³⁵

The through-bond charge transport implies the presence of infinite metal–ligand coordination interactions in the MOF to conduct electrons. A good d–p (σ or π) overlap between metal orbitals and ligand functional groups with well-matched energy levels leads to small band gaps and high charge carrier mobilities. On the other hand, for the through-space charge transport, electron conduction occurs through the non-covalent π-interactions between highly conjugated planar organic moieties.

In contrast, for extrinsically conducting MOFs, electron conduction is dependent on the guest molecules and host–guest interactions. In the previous section, we have discussed the incorporation of guest species within the host MOFs where the host–guest binding may vary from covalent (including coordination) bonding to non-covalent interactions. Additionally, the incorporation of organic conductive polymers can show charge transport without any significant influence on the framework.

Based on this well-known host–guest interaction, the ec-MOFs can be designed following four different strategies: (i) MOF–guest coordination interactions: the encapsulated guest moieties can coordinate metal centres to promote the electrical charge transport pathway within the framework, as observed by Allendorf et al., who have shown a 10¹⁰ times enhancement of the electrical conductivity of HKUST-1 through encapsulation of TCNQ.⁸⁹ The encapsulation of a secondary bridging moiety provides an alternative pathway for the charge transport for the weakly conducting or insulating MOFs through well-matched metal–guest orbital overlaps. After the first report, several groups have attempted to modify the charge transport behaviour of MOFs through encapsulation of different types of charge-carrying organic bridging ligands like TCNQ and related molecules.⁹⁰ (ii) Donor–acceptor interactions: the donor–acceptor interactions between the host MOF and guest moieties have been exploited to develop ec-MOFs. In this case, the guest moiety can be grafted on the metal–organic structure or within the void space. Hupp et al. have shown the enhancement of electrical conductivity through encapsulation of fullerene moieties within the void space of a MOF.⁹¹ (iii) Extended π-interactions: the encapsulation of aromatic guest species may help to develop extended host–guest charge transport pathways.⁹² (iv) Conducting polymers: organic conductive polymers like PPy, PANI, etc. can provide an independent conducting pathway and, therefore, the incorporation of organic conducting polymers within the void space of MOFs is an excellent strategy for developing ec-MOFs.^93,94

2. Classification of guests to design extrinsically conducting MOFs

Electrical conductivity of a material is defined as the charge transport ability through the material and is characterized by both the charge carrier concentration and the charge carrier mobility. It is highly challenging to control the charge transport properties of MOFs. However, chemical modifications may modulate the electrical conductivity of the MOFs in a controlled way. In the present review, we focus only on the modulation of the electrical conductivity of MOFs by guest encapsulation within the channels or cavities i.e., extrinsically conductive MOFs (ec-MOFs).

Self-assembly of carboxylate-containing organic ligands with metal ions results in poor metal–ligand overlap with large energy gaps and poor conductivity.⁹⁵ Additionally, the presence of inherent porosity cuts off the extended π-conjugation among the aromatic ligands of the framework. In this respect, encapsulation of electroactive guest species within the channels or cavities of these MOFs can build up an alternative charge transport pathway throughout the framework.⁹⁶ The charge transport of such guest-encapsulated MOFs follows two different mechanisms: (a) host–guest charge transfer interaction and (b) charge transport through the guest. The presence of discrete guests within the pores of MOFs may enhance the first mechanism resulting in an increase of the electrical conductivity through host–guest charge transfer interactions. In contrast, conducting polymers enhance the second mechanism since they provide extended conjugation assemblies throughout the MOF as a charge transport pathway.^97,98 In some cases, a guest–guest charge transfer interaction has also been found.⁹⁹ To date, many studies have been done on this topic and, herein, we summarize all these efforts to construct extrinsically conducting MOFs by guest insertion. Thus, in this review, we present the design of conductive MOFs based on the encapsulation of different types of guests within the voids or channels.

To date, several types of guest species like metal ions, metal nanoparticles, metal oxides, inorganic and metal–organic complexes, different organic molecules, halogens, anions, organic polymers and so on, have been accommodated within the void space of MOFs, following the dimension of the pores and understanding the host–guest interactions. In the present review, to discuss the modulation of electrical conductivity of porous MOFs through encapsulation of guest species, we have classified the guests into three categories: (i) metal-based guests, (ii) molecule-based guests and (iii) conducting polymers. This classification is based on their chemical composition, geometry, host–guest interaction and charge transport pathway.

In the first category, metal-based guests, we include metal ions, metal nanoparticles (MNPs) and metal-oxides, as the main component of these guests is the metal ion itself. Metal ions may be present in the pores either as a single metal ion or as a cluster attached to the framework by electrostatic interactions or coordination bonds. An example of this type is the incorporation of Cd²⁺ ions within a framework through Cd²⁺⋯S interaction of the ligand.⁷² MNPs may agglomerate within the pores of the MOFs through electrostatic interaction. Thus, Ag⁺ ions can be loaded within the Rb-CD-MOFs through electrostatic interactions between the metal ions and hydroxide groups that also serve to reduce Ag⁺ to form Ag-NPs.⁷³ In a similar way, metal-oxides can be incorporated into MOFs through electrostatic interactions with the hydroxide groups of the framework.⁷⁵ In the second group of guests, molecules and molecular entities, we will discuss different types of inorganic molecules like ferrocene, nickel(II)carbolide complexes, iodine/polyiodides, etc. as well as organic molecules like TCNQ, fullerene, TTFs, etc. These molecules or molecular entities have their own molecular structure and are loaded within the framework through different host–guest interactions, depending on the framework and the guest. Their charge transfer pathways may also vary with the guest. In the third group of guests, organic conductive polymers, we include well-known conductive organic polymers like PEDOT, PPy, PANI, etc. that have 1D structures formed by a purely organic backbone and interact with the host framework mostly through π-interactions.

2.1 Metal-based guests

Encapsulation of different metal-based guest species within porous MOFs has gained much attention in promoting different physical and chemical properties of the host framework. Thus, Bu et al. have demonstrated how the encapsulation in MOFs of different metal-based guest species can modulate their catalytic, sensing and luminescent properties.²⁴ Of course, metal-based guests have also been often used for the enhancement of electrical conductivity of MOFs. Based on their composition, we have classified these metal-based guests into three main sub-categories: (a) metal ions, (b) metal nano-particles (MNPs) and (c) metal oxides. Such metal-based guest species help to promote the charge transfer within the guest@host MOF through electrostatic interactions and a redox hopping mechanism.

2.2 Molecule-based guests

Different types of electroactive molecules and molecular entities including TCNQ (7,7,8,8-tetracyanoquinodimethane), iodine, polyiodides, ferrocene, fullerene, tetrathiafulvalene and its derivatives, methyl viologen, etc. have been encapsulated within the channels of MOFs to induce and enhance conducting properties of the parent MOFs (Table 2). In most cases, the guest molecules are incorporated by post-synthetic methods within the voids of pre-synthesized MOFs, although, in some cases, guest molecules are used as templates and incorporated by in situ methods. The encapsulated guest species remain either attached to the metal/ligand framework by coordination/covalent linkages or within the voids through non-covalent interactions.

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Such incorporation of electroactive molecules can induce or enhance the electrical conductivity by several mechanisms: (i) by creating a new electron conducting pathway throughout the framework thanks to the formation of covalent/coordination linkages between the metal centres, (ii) by developing a donor–acceptor (D–A) host–guest charge transfer pathway within the framework, and (iii) through guest–guest interactions.

In the first mechanism, the encapsulated guest acts as a bridge between metal centres and provides an alternative electron conduction pathway. It is obvious that the guest should be a good electronic conductor as TCNQ, which is a π-conjugated organic building block that may induce high conductivity through the formation of linkages between different metal centres of the MOFs. Similarly, the formation of polyiodide chains within the channels of MOFs can substantially enhance their electrical conductivity by formation of coordination bonds between metal centres and polyiodides. While the donor–acceptor (D–A) charge transfer interaction is a well-established phenomenon in co-crystals and has been used for applications in optoelectronic devices, it is less studied in MOFs. The formation of MOFs using metal–ligand coordination also reduces the electron density on the ligand backbone through the partial transfer of electron density from ligands to the cationic metal centres that act as electron acceptors. The incorporation of electron-rich guest species (iodides) within the voids/channels of MOFs produces donor–acceptor electron transfer pathways throughout the framework, inducing higher electrical conductivity. Again, the utilization of conjugated organic ligands for the synthesis of MOFs results in electron-rich skeletons, and therefore, the incorporation of electron acceptor molecules can also create D–A charge transfer pathways throughout the framework.

2.3 Organic polymer-based guests

Following their discovery in 1920,¹⁷³ the synthesis of organic polymers, bearing repetitive building blocks, has experienced extraordinary development thanks to their countless applications in many different fields. Since the mid-1990s, controlled polymer synthesis in terms of size, dimension, molecular weight, etc. has gained much attention. In this regard, the combination of polymer chemistry with coordination chemistry may create a new generation of novel functional materials. Thus, the integration of polymers within the channels of MOFs has become a major focus in order to combine the novel functionalities of MOFs with the useful properties of polymers.

The encapsulation of polymers within the nano-porous channels of MOFs provides an emerging category of materials with a wide variety of applications. The confined polymer chains are affected by their orientation, conformational arrangement, molecular weight, regularity, polymer composition, etc. This host–guest effect is double since, on one side, within the confinement space, polymers show properties different from those of their bulk phases. On the other side, the encapsulation of polymers within the nano-channels of frameworks may have a significant effect on the physical and chemical properties of the host MOF.

Encapsulation of organic conducting polymers has been done using several methods. In most cases, monomers are loaded within the channels of the MOF using an in situ synthetic method like a ‘ship-in-a-bottle’ approach. The small monomers can be easily loaded within the channels of the framework and then different types of polymerization methods like radical, electrochemical, ionic, ring opening methods, etc. are applied to form the extended polymeric chains within the channels of the MOF. The so-formed polymeric chains are trapped within the channels and thus cannot be easily released. In some cases, the monomers are loaded by soaking the MOF in the monomers or in a solution of the monomers in different solvents. Subsequently, the loaded guest monomers are converted to polymers. Recently, grafting has emerged as a new method to load the monomers in the MOF. In this case, pre-functionalized MOFs, in both, ligand and metal nodes, interact with a monomer or oligomer having a counter-functional site, facilitating the incorporation of the guest species through strong host–guest interaction within the channels of the framework. The loaded guests can attach to the ligand site by covalent bonding or to the metal site by coordination bonds. These grafted monomers/oligomers can be afterward polymerized within the channels. In a few cases, the polymer chains are directly loaded within the channels of the MOFs. MOFs are soaked in a solution of the polymer or in a melted polymer leading to the direct insertion of the polymer chains within the channels. Such direct insertion becomes feasible due to strong interactions between the polymer chains and the pore walls.

As expected, most of the attempts to increase the electrical conductivity of polymer-doped MOFs have been performed with π-conjugated organic polymers since they are well-known for their electrical conducting properties¹⁷⁴ and their applications in many different electronic devices. Therefore, the incorporation of conducting polymers like PEDOT (polymer of 3,4-ethylenedioxythiophene),¹⁷⁵ polyaniline (PANI),¹⁷⁶ polypyrrole (PPy),¹⁷⁷ polythiophene (PTh),¹⁷⁸ polyacetylene (PA),¹⁷⁹ etc. into different MOFs, is expected to increase their carrier density leading to higher conductivities for the guest@MOF materials.

3. Fabrication of MOF thin films

For the applications of MOFs in electronic and optoelectronic devices, the fabrication of MOF thin films on conducting substrates is highly desirable.^203,204 The fruitful integration can be achieved by establishing strong contacts between the MOFs and the substrates. However, most of the MOFs are synthesized as crystalline powders, hindering their use in device fabrication. Most of the inorganic–organic hybrid MOFs are not as thermally stable as inorganic materials and are not soluble in organic solvents, in contrast to organic compounds. These differences preclude the use of well-established techniques for thin film fabrication.^205,206 It is highly important to understand the interfacial interactions and the charge transport mechanism between the substrate and the thin film.²⁰⁶ In order to do so, the first step involves the deposition of thin films on the surface of the substrate. The most important parameters of the deposited thin films are thickness, composition, morphology, crystallinity, homogeneity, robustness, orientation, etc. for their real-world applications.²⁰⁷ Since the properties of thin films may be different from those of the bulk MOFs, the complete characterization of the thin film is also required.^206,207 However, the quality of the thin films may vary with the method of deposition.^208,209 Thus, it is necessary to gain proper knowledge of the thin film fabrication method. Another important criterion for developing thin films is the proper choice of the substrate. Different materials like silicon, silica, silicon nitride, gold, aluminium, graphite, copper, indium tin oxide (ITO), fluorine-doped indium tin oxide (FTO), etc. have been chosen for this purpose. In some cases, functionalization of substrates like self-assembled monolayer (SAM) formation or polymer coating has been performed to form uniform thin films. There are also a few reports of thin film fabrication using extrinsically conducting MOFs, and, therefore, we describe examples of thin film fabrication of MOFs using both intrinsically and extrinsically conducting MOFs.

To date, different methods have been developed for depositing MOF thin films on solid substrates, but it is difficult to use only a particular method for all types of MOFs. Several review articles have highlighted this topic.^203–209 Here, we attempt to provide a better understanding of thin film methods to find out the most suitable ones applicable for extrinsically conducting MOFs. We have classified all the methods to fabricate thin films of MOFs into two categories: (a) liquid phase and (b) gas phase.

3.1 Liquid phase processing

Here, the solutions of either pre-synthesized MOF or MOF precursors are used for the thin film fabrication.

3.2 Gas phase processing

In order to avoid solvent interference in the morphology and/or quality of thin films; recently, gas phase thin film fabrication has become a major research focus in conductive MOFs. Gas phase fabrication proceeds through three different steps: (a) vaporization of the metal ions and ligands, (b) transport of the vapours and (c) deposition.²⁵⁶ To date, three different types of gas phase processing have been developed: (i) physical vapour deposition (PVD), (ii) chemical vapour deposition (CVD), and (iii) atomic layer deposition (ALD). These processes are very new in the field of thin film fabrication of MOFs and will require further studies for the development of optimal conditions.

3.3 Other methods

There are several other methods reported for the synthesis of thin films of MOFs which include seed-assisted growth, sono-chemical, ultrasonic, microwave, electrospray, hot pressing, etc.

4. Applications of extrinsically conducting MOFs

Conductive MOFs, at the conjunction of organic semiconductors and inorganic metallic conductors, have aroused much attention as multifunctional materials due to their large surface area, tuneable structure and moderate-to-high charge transport properties. To date, conductive MOFs are used for different applications such as sensing, electrocatalysis, photoconductivity, charge storage, etc. Here, we briefly summarize the applications of ec-MOFs in these relevant fields.

4.1 Electrocatalysis

Large surface area and high porosity with an easily accessible large number of transition metal sites with labile exchangeable coordination sites make these conductive MOFs high potential electrocatalysts. Several review articles have covered this topic.^{30,31,68,274,275} Herein, we only focus on the electrocatalytic applications of guest@MOFs.

4.2 Electronic sensing

Electronic sensors are analytical devices that show the presence of a particular analyte or their existence above a certain concentration by transducing different electronic parameters like resistivity, capacitance, etc.³⁰⁷ Several important parameters like technical simplicity, easy fabrication, cost-effectiveness, sensitivity and selectivity make these electronic sensors highly attractive in analytical research.³⁰⁸ The chemical tuneability of MOFs provides an enormous opportunity to tune the electrochemical performance and recognition behaviour of the fabricated devices. The inherent void space can also accommodate different analytes, and the large surface area of MOFs facilitates their interaction with the metal centres or with the organic building blocks. Again, the easy fabrication of MOF thin films in electronic devices also boosts their application in sensing devices.³⁰⁹ To date, different methods like chemiresistive, chemicapacitive, impedance, Kelvin probe, FET, etc. have been adopted for this purpose.^310–312 A number of MOFs and MOF composites with other conducting substances have been developed for sensing various types of analytes: gases, ions, toxic molecules and biomolecules. Several reviews have already been published on this topic which include either pristine MOF or MOF composite as the sensing electronic platform but there is no review that highlights the sensing properties of extrinsically conducting MOFs. Here, we discuss some sensing behaviour of extrinsically conducting MOFs by classifying analytes into different categories.

4.3 Charge storage

Electrochemical energy storage has gained much interest in the field of conducting porous MOFs for their high charge storage densities, fast charging processes, large number of cycles and large surface areas.^321,322

4.4 Photo-conductivity

Photoconductivity is a useful property for applications such as solar cells, photo-electrocatalysis etc. In a photoconductive material, upon light excitation, electrons will jump from the valence band to the conduction band by creating holes in the valence band. In order to develop a photoconductive MOF, a donor–acceptor pair is required.²⁰³ So, to design an extrinsically photo-conducting MOF, a donor–acceptor pair should be formed within a host MOF. There are two different possibilities: either the MOF acts as a donor and the guest as an acceptor or vice versa. Liu and co-workers have studied the photoconductivity of thin films of three different MOFs upon C₆₀ encapsulation.¹⁵² They have prepared thin films of C₆₀@Cu-BPDC (BPDC = biphenyl-dicarboxylate), C₆₀@ZnTPP (TPP = 5,15-bis-(3,4,5-trimethoxyphenyl)-10,20-bis(4-carboxyphenyl)porphyrine) and C₆₀@ZnDAP (DAP = 10,20-bis(4-carboxyphenyl)-5,15-diazaporphyrine) with the LBL technique. Two-probe conductivity measurements show that Cu-BPDC (2 × 10⁻¹³ Sm⁻¹) shows an enhancement of 4 orders of magnitude due to encapsulation of C₆₀. However, the enhancement of conductivity is less than 10% for C60@Cu-BPDC upon illumination. C₆₀@ZnTPP shows a conductivity value of ∼1.5 × 10⁻¹¹ S cm⁻¹ while the parent MOF has a conductivity value of 1.5 × 10⁻¹³ S cm⁻¹ and the conductivity value rises by 100 times upon illumination with light of 455 nm (Fig. 64).

This difference is obviously due to the cooperative charge transfer between the donor porphyrin and the acceptor C₆₀ moieties. The related MOF C₆₀@ZnDAP also shows a 100 times enhancement of the electrical conductivity upon excitation with 455 nm wavelength. This different photoelectric behaviour of these porphyrin-based MOFs is due to their different electronic structure. In order to explain the photoconductive behaviour of these MOFs, these authors have performed quantum mechanical calculations which reveal that the LUMO of TPP is lower in energy than in the C₆₀@ZnTPP MOF and this facilitates both the direct and photo-induced electron transfers between porphyrin and C₆₀. Upon excitation, the Soret band of the porphyrin was activated to generate electron–hole pairs and promote their transfer by hampering their recombination and electron back transfer. Han and co-workers have shown the photoconductive behaviour of AgNC@Rb-CD-MOF. AgNC@MOF shows 10⁴ times enhancement of the electrical conductivity upon illumination to reach 2.15 × 10⁻⁷ S cm⁻¹.⁷³ They have established that direct optical excitation and an indirect thermal excitation contribute to the overall enhancement of the electrical conductivity upon illumination and also proposed a tunnelling mechanism for charge transport. Hu et al. have shown the photoconductive behaviour of the MOF [Tb(Cu_4I4)(INA)₃(DMF)] after iodine adsorption.¹²⁴ Upon illumination with a Xe lamp, the photocurrent increases from 3.8 to 5.8 μA.

Some of these guest-encapsulated photoconductive MOFs may also be used for photovoltaic cells to convert solar light into electricity. Lee et al. have shown the photovoltaic activity of thin films of two different MOFs, Co-BDC (H₂BDC = 1,4-benzene dicarboxylic acid) and Co-NDC (NDC = 2,6-naphthalenedicarboxylate).¹²² Thin films of these MOFs are prepared via layer-by-layer and iodine was loaded by soaking the thin films within an iodine saturated acetonitrile solution. Absorption spectroscopy confirms the encapsulation of iodine as well as oxidation of Co²⁺ to Co³⁺. Such iodine encapsulation converts these two insulating MOFs to semiconductors. Co-NDC shows better conductivity than Co-BDC due to the encapsulation of a larger number of iodine molecules. Both iodine-doped MOFs are used as light-harvesting sensitizers in a TiO₂-based photovoltaic cell. The Co-NDC MOF has a power conversion efficiency of 1.12%.¹²² In another article, Zhang and co-workers reported the solar cell activity of a polyoxometalate-encapsulated MOF.³²⁵ The MOF [Fe(BDC)(4,4′-bpy)] was used to encapsulate H₃PW₁₂O₄₀ Keggin POMs inside the MOF. This MOF shows an optical response with λ_max ∼ 750 nm. The photovoltaic performance of the POM@MOF modified ZnO photoanode shows better performance than bare ZnO. The power conversion efficiency is ∼0.073%.

4.5 Photocatalysis

Some of the conducting MOFs are also used for photocatalytic applications like CO₂ reduction, water splitting etc. Yuan et al. have studied the photocatalytic CO₂ reduction behaviour of PPy-loaded ZIF-67.³³⁴ Due to high electrical conductivity, high aqueous stability and large surface area, PPy@ZIF-67 is used as a co-catalyst for CO₂ reduction under visible light in the presence of [Ru(bpy)₃]²⁺ as a photosensitizer. Under these catalytic reaction conditions, PPy@ZIF-67 produces CO and H₂ with a CO selectivity of 63%. The encapsulated PPy acts as an electron harvester and transporter for the photogenerated electron from the photosensitizer to the metal nodes. Upon excitation at 500 nm, [Ru(bpy)₃]²⁺ shows the characteristic emission peak at 606 nm, which is quenched by the presence of the co-catalyst PPy@ZIF-67. The weak interactions between PPy and [Ru(bpy)₃]²⁺ enable a more efficient electron transfer within the MOF.³³⁴

In another article, Fang et al. have shown a similar photocatalytic CO₂ reduction of PPy@Al-PMOF-Cu. Porphyrin moieties with Al-PMOF were post-metalated with copper and then pyrrole monomers were encapsulated within the void space. Oxidative polymerization to form PPy was done within the void space and confirmed by TEM, XRPD and IR studies. Such encapsulation of PPy reduces porosity but enhances electrical conductivity. Photocatalytic CO₂ reduction to CO (90 μmol g⁻¹ h⁻¹) was obtained by using triethanolamine as the sacrificial agent under visible light irradiation. The porphyrin moiety acts as a photosensitizer, PPy as an electron transporter and the Cu sites as the catalytic active centres.¹⁹³

4.6 Other applications

Xu et al. have shown thermoelectric properties of PANI-loaded MOFs by measuring the temperature dependence of their Seebeck coefficients.¹⁹⁷ The Seebeck coefficients for all the PANI-loaded MOFs (PANI@Co-MOF, PANI@Co-MOF-Br and PANI@Co-Ag-MOF) increase with temperature and all show a p-type thermoelectric behaviour. PANI@Co-MOF-Br has the highest Seebeck coefficient (66.5 μV K⁻¹ at 400 K), much higher than PANI@Co-MOF (46.3 μV K⁻¹) and PANI@Co-Ag-MOF (59.5 μV K⁻¹). They have observed that this trend is different from the electrical conductivity of MOFs due to the coupling effect and the mobility and carrier concentration.¹⁹⁷ Jiao and co-workers have reported the electromagnetic absorption (EMA) of PPy@ZIF by evaluating the reflection loss (RL) value. They have observed that the electromagnetic absorption is also dependent on the electrical conductivity of the samples. The higher the electrical conductivity, the higher the absorbance. Pure ZIFs have low EMA performance but it improves with the introduction of PPy within the channels. PZ-3 containing 5% of PPy shows the maximum EMA performance and this can also be tuned by the sample thickness and the frequency of the electromagnetic radiation. The maximum EMA performance for PZ-3 is −49 dB with a sample thickness of 2.9 nm.¹⁹¹ Prasoon and co-workers have shown a diode-like behaviour of thin films of TCNQ-doped Cu-BTEC MOF with a rectification ratio of ∼10⁵.³³⁵ Liu and co-workers have shown the photothermal energy storage behaviour of PPy composite with ODA@Cr-MIL-101-NH₂ (ODA = 1-octadecanol). The photothermal conversion and storage efficiency of ODA@MOF/PPy-6% reaches values as high as 88.3%.³³⁶

5. Summary and outlook

During the last two decades, MOFs have become a major research topic thanks to their interesting properties such as ultrahigh surface area, tuneable structure, gas and solvent sorption, size-selective heterogeneous catalysis, sensing, etc. The development of intrinsically conductive MOFs for multiple applications combining electronic properties and structure-based functionalities is hindered by the limited choice of electroactive organic ligands and metal ions to construct these MOFs. In this scenario, the encapsulation of electroactive guest species within the void space of MOFs has gained much attention due to the endless possibilities of guest species and host–guest interactions. A large number of electroactive species can be used as guests to design ec-MOFs by generating mobile charge carriers, host–guest charge transfer interactions or individual conductive pathways. Although significant progress has been made in this field, there are still several limitations such as device fabrication, stability, mechanical properties, etc. In conducting MOFs, the formation of efficient charge transport pathways is the main requisite for the development of highly conductive materials. Based on this conducting pathway, MOFs and PCPs can be classified into two categories: intrinsically (ic-MOFs) and extrinsically conducting MOFs (ic-MOFs). The main differences between both types of conducting MOFs can be grouped in different aspects as (i) design, (ii) synthesis, (iii) structural studies and (iv) conductivity studies.

(i) Design: in ic-MOFs, the charge transport depends on the metal–ligand backbone of the framework. Different strategies have been adopted to design ic-MOFs including metal–ligand overlap (use of soft donor-based ligands), redox hopping (use of mixed-valence metal ions), extended organic conjugation (use of highly conjugated ligands) and redox-active radical-based ligands. Thus, the design of ic-MOFs is critically dependent on the metal ions and ligands. However, the limited choice of electroactive ligands, compared to general MOFs, strongly reduces the design possibilities.

In contrast, in ec-MOFs, the charge transport depends on the guest and host–guest interactions. The guests can conduct electrons on their own (as organic conducting polymers) within the void space of the host MOF. In other cases, host–guest interactions create new charge transport pathways within the guest@MOF structure through π-interaction, redox hopping, metal–guest interactions, etc. Here, the large number of possible guests and host–guest interactions provide almost infinite opportunities to design ec-MOFs.

(ii) Synthesis: ic-MOFs are generally synthesized by in situ methods through direct mixing of the chemical components under solvo- or hydrothermal conditions by self-assembly of organic ligands and metal ions. Additionally, some post-synthetic modification can be done by metal ions or organic ligands exchange while keeping the basic MOF architecture.

In contrast, ec-MOFs are very rarely obtained by in situ methods. In most cases, the guest encapsulation is done via the post-synthetic methods. The encapsulated guest species remain either in the void space or grafted to the metal ions or to the ligands. Such post-synthetic encapsulation leads to the non-homogeneous distribution of guests within the host structure. So, the synthesis of ec-MOF is somewhat a bigger challenge compared to ic-MOFs.

(iii) Structural analysis. For the intrinsically conducting frameworks, the detailed structural information enables the rational development of such materials by simply using mixed-valence metal ions, conjugated ligands with an adequate metal–ligand overlap, etc. In contrast, for extrinsically conducting materials, there is little crystallographic evidence of the host–guest interactions and their contributions to the charge transport phenomena of the resultant host–guest materials. However this host–guest interaction has already shown a high potential to enhance the electrical conductivity by several orders of magnitude. Unfortunately, in most cases, the inclusion of guests within the regular confinement spaces of the frameworks reduces the crystallinity of the host and precludes the single crystal X-ray structure determination.

(iv) Conductivity studies. The electrical conductivity of MOFs is measured either on single crystals or on microcrystalline powders. Single crystals of ec-MOFs are very rare as the post-synthetic encapsulation of guest species within the pores of MOFs reduces the single crystalline nature. Therefore, the electrical conductivity of ec-MOFs is measured mostly on pressed pellets or thin films. In the case of ic-MOFs, there are more reports on single-crystal electrical conductivity measurement. The electrical conductivity measurement using pressed pellets or thin films of MOFs is less reliable than single crystal measurements due to the influence of grain-boundary effects. Although for real-world applications, devices can be prepared by using MOF thin films, in order to get a deep understanding of the structure-properties relationships, we need accurate structural and electrical studies.

The maximum electrical conductivity reported to date for ic-MOFs is 1580 S cm⁻¹ for the MOF Cu-BHT (BHT = benzenehexathiolate), measured on thin films by D. Zhu et al.²⁴² whereas the maximum electrical conductivity reported to date for an ec-MOF is 250 S cm⁻¹, measured on single crystals of a BEDT-TTF donor–acceptor charge-transfer based MOF reported by Coronado, Gómez-García et al.¹³⁷

In this review, we have focused on these guest-containing extrinsically conducting materials. We have classified the guests into three categories: (a) metal-based guest, (b) molecule-based guest and (c) organic conducting polymers. For the reported examples, we have discussed: (i) the guest loading process within the pores/channels of the MOFs and PCPs, (ii) the structural modifications after guest loading (when data is available), (iii) the (most times huge) change in the electrical conductivity and (iv) the charge transport pathway and mechanisms proposed. The review of these extrinsically conducting MOFs shows the following conclusions:

(a) Metal-based dopants: the dopants (metal ions, metal nanoclusters and metal oxides) are incorporated into the MOFs via post-synthetic techniques. The incorporation of metal ions can be done by simple intercalation. This method usually maintains the crystallinity and allows the study of the crystal structure of the intercalated material. Structural studies show that the metal ions are mainly located around the ligand donor sites of the framework and, accordingly, the electrical conductivity is obtained thanks to a charge transport hopping mechanism. The metal nanoclusters (MNCs) are usually loaded by intercalation of metal ions followed by a reduction to form metal nanoparticles. The reduction can be done by either incorporating a secondary reducing agent or by the MOF itself, although this chemical reaction decreases the crystalline nature of the frameworks. The enhancement of the electrical conductivity is due to the easy charge transport through the MNCs within the MNC@MOFs by the hopping mechanism. Metal-oxide encapsulated within MOFs is very rare and is performed by post-synthetic methods. The highest conductivity value obtained with this strategy is 1.8 × 10⁻² S cm⁻¹, reported for a MOF with Cd²⁺ ions encapsulated.⁷³

(b) Molecules and molecular entities: different molecules and molecular entities have been loaded within the void space of many different MOFs and PCPs by both, templated and post-synthetic methods. The incorporated guest molecules enhance the electrical conductivity either by developing an alternative pathway of charge transport or by host–guest charge transfer interactions. In some cases, the incorporation of guest molecules may increase the conductivity more than one million times, as in TCNQ@HKUST-1, where metal–guest coordination interactions provide the pathway for electron transport and show almost 10⁷ times enhancement of the electrical conductivity. For iodine, TTF-type donors, fullerene, ferrocene, etc., the appearance of donor–acceptor charge transfer interactions with the host framework may also lead to an increase of several orders of magnitude of the electrical conductivity. The host–guest (BEDT-TTF)₃[MnCr(C₂O₄)₃] framework shows the highest conductivity in this category (250 S cm⁻¹).¹³⁷ For this category of host–guest structures, the electrical conductivity value varies depending on the guest, the host as well as the host–guest interactions.

(c) Organic conducting polymers: polymerization of different conducting organic polymers within the channels of the frameworks is usually done in two steps: loading of monomers within the pores and then polymerization. Loading of monomers or molecular fragments can be done by both template and post-synthetic methods. The loaded monomers interact with the MOFs either by weak interactions or forming covalent or coordination bonds with the ligands or metals of the framework. The polymerization can be performed by different mechanisms such as oxidative polymerization, radical polymerization, electro-polymerization, etc., depending on the monomer and MOF. The charge transport for most of the polymer-encapsulated frameworks is due to the extended π-conjugated polymeric chains, interchain π-interactions as well as polymer–host interactions. In most cases, these frameworks with encapsulated organic conducting polymers show moderate conductivity in the range of 10⁻⁴ to 10⁻² S cm⁻¹ with a maximum reported conductivity of 1.5 S cm⁻¹ for PPy encapsulated in ZIF-67.¹⁹¹ In all cases, the electrical charge transport occurs through the encapsulated organic polymer, although the role of host–guest interaction is non-negligible.

The design of conducting host–guest structures, through encapsulation of different electroactive guest species within the cavities and channels of porous materials, is an area of growing interest. Despite the intense efforts performed in the last decades, there are still several challenges that need to be addressed for further development:

1. Structural aspects and rational design: the advance of single crystal X-ray crystallography has enabled the establishment of the structure–property relationships for different assembled materials like MOFs, PCPs and COFs and their rational design. Thanks to the knowledge of crystal engineering, material chemists have designed and modulated the structure of these materials to obtain the desired properties. Nevertheless, the lack of precise crystallographic information in many guest@MOFs precludes the rational design to build highly conducting guest encapsulated frameworks. Some guest@MOFs formed by template-assisted methods may allow the precise determination of the crystal structure but, in most cases, guest encapsulation within pre-synthesized frameworks using post-synthetic methods, reduces the crystallinity. The formation of conducting polymeric chains within the channels of MOFs is the best example of such loss of crystallinity of the frameworks. Therefore, the synthesis of high-quality single crystals of guest@MOFs, remains one of the most important challenges in conducting MOF chemistry, especially in polymer-encapsulated frameworks. The use of Rietveld analysis of XRPD data and neutron diffraction data, pair distribution function analysis etc., may also be helpful to resolve the crystal structure of homogeneous, single-phase microcrystalline guest@MOF samples.

2. Host–guest interactions: rational design of conducting MOFs through appropriate encapsulation of guests within the cavities of host frameworks is strongly dependent on the host–guest interaction. The lack of precise crystallographic information limits the analysis of these host–guest interactions. Therefore, the synthesis of single crystals of guest@MOFs and their structural determination constitutes a key step to studying and understanding the host–guest interactions that govern the conductivity, in order to improve the electrical properties of these porous materials.

3. Homogeneous distribution of the guests: one of the major challenges in this field is the lack of homogeneous loading of the guest molecules within the channels of the MOFs. Homogeneous loading of guest molecules within the channels of MOFs is only achieved by template-assisted synthetic methods, since guest-loading through post-synthetic methods usually leads to an inhomogeneous distribution of the guests within the channels. The presence of pores that are much larger than the guests may lead to the disordered accumulation of guests within the channels as well as the chance to increase the leaching. Therefore, the choice of complementary hosts and guests for the homogeneous encapsulation of the guests within the channels of the hosts constitutes a very challenging task in this field. Thus, the proper structural analysis is the primary criterion for the rational design and further development of this field. The introduction of similar guests with identical geometrical parameters within a single MOF may also highlight the host–guest interactions. Theoretical calculations may also predict the ideal candidates of host and guest to design ec-MOFs. Advanced simulations and molecular modelling could assist in the synthesis by optimizing proper building blocks towards predesigned structures with predefined functionalities and host–guest interactions.

4. Mechanistic studies: although in some cases high electrical conductivities at room temperature have been obtained (up to 250 S cm⁻¹), most of the conducting host–guest materials behave as semiconductors. In fact, only very few are metallic and no superconducting host–guest MOF has been reported to date. The development of metallic conductors and superconductors constitutes, therefore, a very appealing challenge. A possible way to achieve these goals maybe the incorporation of charge transfer active guest species within the cavities of highly conducting MOFs containing highly conjugated ligands and mixed-valence metal ions. These features are expected to facilitate the electron delocalization and lead to metallic or even superconducting MOFs with electron delocalization through the guest, through the MOF or even through both. Nevertheless, such development requires a deep understanding of the charge transfer mechanisms as well as the charge carrier mobility and a control of both. Such mechanistic studies with ec-MOFs have been done in very rare cases.

5. Identify charge carriers and their properties: the electrical conductivity of a material is determined by the charge carrier density and charge carrier mobility. Only in very few examples, these studies have been performed. Therefore, more studies to identify the charge carriers (electrons or holes) and determine their properties (density and mobility) as well as the conduction mechanisms are needed. These studies are expected to contribute to a better understanding of the key parameters that control the electron delocalization mechanisms and, therefore, the electrical properties of these guest@MOFs. Further, in-depth analysis is highly necessary to identify the charge transfer mechanism.

6. Control over the charge transport: one of the major advantages of the design of guest-mediated conducting MOFs is that their conductivity can be tuned by changing the amount of guest loading. Only a few studies have been done to modulate the electrical conductivity of MOFs by controlling the loading of guests, and, therefore, there is still a lot of work to be done in this direction. This work is expected to lead to the easy control and modulation of the electrical properties of these guest@MOFs materials by simply changing the nature and amount of the guest. The final aim is the synthesis of guest-dependent conducting materials at will. Furthermore, these materials may tune their electrical conductivity by gas or ionic absorption in solution, rendering them as ideal candidates for the preparation of gas and ionic sensors.

7. Thin film fabrication: for advanced applications, it is highly necessary to deposit defect-free and homogeneous thin films of conductive MOFs on different substrates. Thickness, coverage, roughness, orientation, homogeneous distribution over the substrate and crystallinity after fabrication, are the important parameters of a MOF thin film. Following their many potential applications, several research groups have developed different thin film fabrication methods and explored their applications. Still, there are several challenges to be addressed for further development: (a) Improvement of the thin films quality and thickness, their homogeneity at the surface, orientation, low defect density, etc. There is no ideal method to reach these goals and, therefore, extensive research should be done to address this issue. (b) Stability and adhesion of the MOF particles to the substrate is also important for device performance. The growth of MOF thin films on functionalized substrates may be a good choice to increase the stability and adhesion. (c) Control over defects, as defects may have an important influence on the properties and functionality of thin films of conductive MOFs. Defects can disrupt the charge transport behavior in the thin film although they can promote the electrocatalytic and electrochemical sensing properties. So, it is important to understand the relationship between defects and the charge transport properties. (d) Mechanistic studies of the growth process of thin film should be performed to improve their synthetic methods. Theoretical analysis is also an important aspect. Besides, the thermal, chemical and mechanical stabilities of the thin films are also crucial for practical applications.

Several thin film fabrication methods are well-known for conducting MOFs with each method having its own advantages and limitations. Here, we have summarized the thin film fabrication methods of ec-MOFs and pointed out their challenges and possible solutions. For the fabrication of ec-MOFs, synthesized by in situ methods, it is highly challenging to control the leaching of weakly bonded guest species from the host MOF. So, it is more convenient to fabricate the thin film of the host MOF on the substrate and then encapsulate the guest species.⁶⁰ Such guest encapsulation has also several challenges. A thin film of the host MOF should interact with the guest present in any solvent – so there will be a chance of solvent interference. Solvent molecules may penetrate within the void space of the MOFs. The use of non-coordinating and volatile solvents may be a good choice to avoid solvent interference. Vapor-phase guest encapsulation can also be done by chemical vapor adsorption. The next challenge may arise from the accumulation of guest species at the pore window of the host MOF which may block further encapsulation of guests. Similarly, non-homogeneous distribution is also a big challenge for ec-MOF fabrication. The next issue may arise from the stability of the host framework during encapsulation. It is important to note that the host MOF structure can change or collapse during thin film fabrication. Under such circumstances, the guest loading will be different from the theoretical prediction or even zero loading may be found. So, it is highly important to characterize the host MOF after fabrication as well as after guest loading. After guest encapsulation, it is necessary to measure the electrical conductivity of the guest@MOF thin film to point out the host–guest interactions and the differences from the bulk phase. More studies of the mechanism of guest encapsulation, host–guest interaction and their effects on the electrical conductivity of the guest@MOF thin films are needed. The complete characterization of the thin films of the MOFs before and after encapsulation of the guests is also necessary.

8. Patterning: the patterning of conductive MOF thin films is another necessary step for their integration in electronic devices. The control over the position of MOF thin films on the surface of the substrate has paramount importance in exploring the applications of MOF thin films in sensors and microelectronics. Here, the problem is that MOFs are usually grown as microparticles or as single crystals and patterning of MOF thin films is challenging. Several methods have been adopted for the successful patterning of MOF thin films including: (a) masking the substrate and functionalizing the desired area for the preferential growth of the MOF thin film, (b) patterned deposition of chemical components of MOFs and then growth, and (c) lithographic technique. Photolithography is also an important area of research in MOF patterning. There are several challenges to be addressed including the development of an ideal technique that should be fast, easy, cheap, while allowing the control of the thickness, position, size and crystal orientation. Another important parameter is the adhesion of the MOF particles on the substrate, mechanical properties and durability. For the ec-MOFs, the guest moieties can be encapsulated within the void space after patterning the thin films of the host MOF.

9. Applications: the main aim of electrically conductive MOF research is the development of novel functional materials for applications in electrocatalysis, chemiresistive sensing, charge storage, photovoltaics, photocatalysis, etc.

Electrocatalysis: the design of efficient and cheap electrocatalysts for the energy-related applications like electrocatalytic HER, OER, ORR, CO₂RR, NRR, etc. is important to find an alternative and sustainable source of energy to control fossil fuel consumption. The suitable electrocatalysts must have the following properties: high electrical conductivity, easily accessible and homogeneously distributed active sites, optimized pore dimension for efficient mass transfer, stability under the electrocatalytic reaction conditions and durability. To date, several ec-MOFs have been reported with electrocatalytic applications containing diverse types of guest species like metal ions, metal oxides, metal chalcogenides, inorganic molecular catalysts and others. However, their catalytic activity and stability are still questionable. For electrocatalytic applications, thin film fabrication of MOFs on the electrode surface is the primary criterion – such thin film fabrication improves the electrical conductivity but decreases the number of active sites due to the necessity of Nafion-like binders. Most of the MOFs are not stable in the solution phase under acidic/basic conditions while both media are required for many electrocatalytic studies. Therefore, it is important to characterize the MOFs after the electrocatalytic experiments. In several cases, MOFs are attached to the electrode surface through non-covalent interactions, limiting the durability of the catalysts. Detailed studies of the mechanism of these electrocatalytic processes using ec-MOFs are also required. Theoretical study may provide some insights on this topic. Following the high efficiency of single-atom catalysts, it will be desirable to incorporate catalytically active single atoms as the guest species within the host MOFs to study their activity. The confined space of MOFs may provide extra stability to the single atoms within the MOF structure. Highly efficient molecular catalysts can also be incorporated within the MOFs to design ec-MOF-based electrocatalysts. Further, the design of bimetallic host–guest MOFs through encapsulation of different metallic species within an MOF will also be a fruitful strategy. Substrate conversion is also another hot topic in electrocatalysis that has gained attention very recently. In summary, there are many aspects to improve on ec-MOF-based electrocatalytic applications.

Charge storage: several ec-MOFs have been studied for different types of charge storage applications such as supercapacitors, Li-ion batteries and Li–S batteries. However, the design of highly conducting MOFs to prepare supercapacitors is still at its initial stage. The ideal supercapacitor should possess the following characteristics: metallic conductivity, easily accessible and enriched active sites, high specific capacitance, large surface area, optimized porosity, high chemical and physical stability, durability during operation and cost-effectiveness. To date, not only the electrical conductivity of MOFs is far lower than metallic behavior but also the specific capacity is much smaller than the desired value (around 1000 F g⁻¹). Moreover, structural stability and high cost further hinder their use. Again, further studies with ec-MOFs are needed for potential supercapacitor applications. MOFs with encapsulated alkali metal ions and poly-sulfides are important for the development of metal-ion and sulfur-based batteries. The unique properties of such devices are generally high porosity, controlled uptake and release of guest metal ions, reversible redox behavior, prolonged stability, high storage capacity, large charge–discharge cycling behavior, durability, etc. The number of studies conducted with ec-MOFs for battery applications is still very limited. Some of the challenges that should be addressed are the device fabrication with powdered samples of the ec-MOFs and the diffusion of guest moieties through the channels of the framework.

Electrochemical sensors: conducting MOFs are highly promising materials for sensing different types of analytes (gases, ions, molecules, liquids, biomolecules,…) due to their tunable porosity, large surface area, chemically functionalized pores, functionalized organic ligands and strong sensor–analyte interactions. Research on MOF-based sensors has increased rapidly with the inclusion of electronic properties in the frameworks. The development of highly conductive MOFs and the advantages of chemiresistive sensing devices based on them, such as ease of fabrication and integration, low power consumption, etc., has stimulated the development of these devices. The most important characteristics of a sensing material are sensitivity, selectivity, stability, reliability and reproducibility. Although significant progress has been made in this field (several ec-MOFs are used for sensing different analytes), still there are several drawbacks. For the integration of devices, the most important part is the formation of thin films, an area which is already in progress. High selectivity and sensitivity can be achieved through rational design. Various studies have revealed that mixed metal MOFs are very promising as electrochemical sensors and, therefore, the encapsulation of different metallic species within host MOF structures may be a good strategy to achieve these properties. Furthermore, no mechanistic analysis of electrochemical sensing devices has been reported. Computational studies may help to identify the potential ec-MOF candidates for the selective identification of particular analytes as well as highlight their mechanism. Finally, it is worth mentioning that there are many other different applications of ec-MOFs such as photocatalysis, photoconductivity, magnetoresistance, etc. Nevertheless, these studies are at the very initial stages, and they should be explored in the near future. Despite the multiple challenges above mentioned, we believe that further study will provide more opportunities to explore the multiple applications of ec-MOFs.

10. Commercialization: although commercialization is not the only goal of scientific research, patenting and real-world applications of a material boost the scientific community. The finding of possible commercialization of a material attracts interest from different and multiple technological directions. It is also obvious that real-world applications require multiple, coherent and interdisciplinary actions. The commercial success, gross profit and benefit for society will be the main target for such commercialization.

In MOF chemistry, the initial targets were to develop suitable MOFs for adsorption and separation of gases, (several patents have been granted) and scale up of some MOF-based materials is also in process for industrial applications. Very recently, the exploration of conductive MOFs has become a prime focus for their unique electronic applications and thus we hope to see real-world applications of conductive MOFs very soon. The self-assembly of metal ions and organic ligands provides an opportunity to overlap their inherent properties along with newly evolved structure-related functionalities. Additionally, the encapsulation of electroactive guest species within the void space of MOFs further allows modulation of the electronic and other properties of the host MOF in a controlled manner. The major obstacle is the lack of complete structural information in most cases. Theoretical calculations may provide insight and help to find the most suitable candidates for this purpose. One of the major concerns for real-world application of MOFs is the formation of thin films, which can be used in optoelectronic devices, bio-sensing, electrocatalysis, etc. There are several methods for thin film fabrication of ec-MOFs, although this field is still in its infancy. Here, we present the first attempt to summarize and highlight thin film fabrication of ec-MOFs. Furthermore, chemical stability, mechanical flexibility, durability and other properties should be analyzed thoroughly for practical applications. Thus, we believe that the coherent strategy from various fields like synthetic chemistry (for the synthesis), engineering (device fabrication), characterization of thin films (solid state physics), material integration, etc. can provide a better understanding of real-world applications of ec-MOFs.

11. Future generation materials: encapsulation of different guest species within the void space of MOFs to tune their electrical conductivity, most probably, will become one of the hottest research topics in materials science within the next few years. We also look forward to see some interesting examples of conducting MOFs, particularly ec-MOFs in highly relevant applications for future technologies like:

Dual conducting MOFs: very recently, dual conducting materials, i.e., simultaneous electronic and protonic conductors, have gained much attention for various applications (like CO₂RR, OER, etc.) where proton-coupled electron transfer is the main step. In this regard, the encapsulation of electroactive guest species within the void space of proton-conducting MOFs or vice versa will be a good strategy for designing dual-conducting MOFs.

Tuning conductivity by external stimuli. Further control over the electronic conductivity of guest@MOFs through external physical stimuli such as light, heat, etc., or even chemical ones (such as gasses, solvents…) may also lead to the construction of advanced electronic materials, sensors and opto-electronic switches.

Quantum computing. The design of electrically conducting MOF in combination with ferromagnetic behavior may provide a pathway to develop spintronic devices for their applications in quantum computation.

Biomedical applications. The encapsulation of electroactive drug molecules within the host framework may also allow the controlled release of the guest drug molecules at the targeted site under a particular bias voltage – this may be fascinating in biomedical research.

Motor. We wish to see the development of MOF motors, rotors and machines using ec-MOFs. Under external stimuli like voltage, light, pressure, etc., the encapsulated guest moieties may show rudimentary motions like shuttle, rotor, motor, etc. following the perfect host–guest interaction and structure.

In conclusion, MOF chemistry has grown rapidly as advanced functional materials over the last two decades. To date, more than 90000 MOFs have been reported with tunable structural topology, chemical versatility, inherent porosity, large surface areas, etc. and diverse applications. The poor charge transport properties hinder the use of these MOFs in electronic applications like electrocatalysis, chemiresistive sensing, photovoltaics, etc. The design and synthesis of electronically conductive MOFs with enhanced electron transport properties have gained significant attention in the last decade. These electrically conductive MOFs can be classified as ic-MOF and ec-MOF, depending on the conducting entity (the host or the guest, respectively). Following the structural details, ic-MOFs have gained much attention for rational design although the limited choice of electroactive ligands has become a major problem for further development. In contrast, ec-MOFs show many more possibilities given the huge number of electroactive guest species and the many different possible host–guest interactions. However, ec-MOFs lack, in most cases, detailed structural information and the synthetic methods present some drawbacks such as low guest insertion, non-homogeneous guest distribution, guest-leaching, etc. Additionally, there are not adequate scientific guidelines to measure and report the electrical conductivity, device fabrication, measurement conditions, charge transport mechanistic analysis, transport pathway, etc. Although thin film fabrication of ec-MOFs is the main requirement for their integration into electronic devices, still there is no ideal technique for the preparation of high quality, defect-free thin films of ec-MOFs. The detailed characterization of these thin films is also required. In the last decade, the research on ec-MOFs has seen significant growth due to their significant electronic applications in electrocatalysis, chemiresistive sensing, charge storage, and so on. Still, we look forward to further advances in this field including: (i) the rational synthesis of improved ec-MOFs based on synergistic and cooperative host–guest interactions, (ii) the complete characterization of these ec-MOFs, (iii) the improvement of the methods for thin films and devices preparation and (iv) more in-depth theoretical studies to facilitate and guide the synthesis of better conductive MOFs and help to understand, not only the electron transport mechanism but also the other properties of these MOFs. These advances will lead to a new generation of (multi)functional materials and conductive MOFs in the near future.

Abbreviations

MOF	Metal–organic framework
PCP	Porous coordination polymer
TCNQ	7,7,8,8-Tetracyanoquinodimethane
TTF	Tetrathiafulvalene
PEDOT	Poly(3,4-ethylenedioxythiophene)
PANI	Polyaniline
PPy	Polypyrrole
PTh	Polythiofene
PA	Polyacetylene
PTC	1,2,3,4,5,6,7,8,9,10,11,12-Perthiolated coronene
H4dobdc	2,5-Dihydroxy-benzene-1,4-dicarboxylic acid
EC	Ethylene carbonate
DEC	Diethyl carbonate
iPr	Isopropyl
UiO	University of Oslo
H2OBA	4,4′-Oxybisbenzoic acid
PSM	Post-synthetic modification
MNCs	Metal nano-clusters
CD	Cyclodextrin
NU	Northwestern University
PDA	Polydopamine
Hmim	2-Methylimiazole
TEM	Tunnelling electron microscope
XRPD	X-ray powder diffraction
NP	Nanoparticle
ZIF	Zeolitic imidazolate framework
amd	Bis(N,N′-di-i-propylacetamidinato)
DED	Difference electron density
BET	Brunauer–Emmett–Teller
H3BTC	Benzene-1,3,5-tricarboxylic acid
HKUST	Hong-Kong University of Science and Technology
H3TATAB	4,4′,4′′-((1,3,5-Triazine-2,4,6-triyl)tris(azane diyl))tribenzoic acid
DL-lac	Lactate
pybz	4-Pyridine benzoate
pdt2−	Pyrazine-2,3-dithiolate
NDC	2,6-Napthalenedicarboxylic acid
H2BDC	1,4-Benzenedicarboxylic acid
H2bpz	3,3′,5,5′-Tetra methyl-4,4′-bipyrazole
INA	Isonicotinate
H4L	Biphenyl-3,3′,5,5′-tetracarboxylic acid
MUDA	Mercaptoundecanoic acid
EPR	Electron paramagnetic resonance
H3BPT	Biphenyl-3,4′,5-tricarboxylate
DMF	N,N′-Dimethylformamide
TTFTB	Tetrathiafulvalene tetrabenzoate
dca	Dicyanamide
XPS	X-Ray photoelectron spectroscopy
BEDT-TTF	Bis(ethylenedithio)-tetrathiofulvalene
BEDT-TSF	Bis(ethylenedithio)-tetraselanafulvalene
TBAPy	1,3,6,8-Tetrakis(p-benzoate)pyrene
DFT	Density functional theory
H4TTFTB	Tetrathiafulvalene tetrabenzoic acid
TPP	5,15-Bis-(3,4,5-trimethoxyphenyl)-10,20-bis-(4-carboxyphenyl) porphyrinato
BPDC	4,4′-Biphenyl-dicarboxylate
Fc	Ferrocene
FeCp*2+	Decamethylferrocenium cations
ndc	1,4-Napthalenedicarboxylate
dabco	1,4-Diazabicyclo-[2.2.2]-octane
HDT	Hexadecanethiol
CMMT	9-Carboxy-10-(mercaptomethyl) triptycene
H4TBAPy	1,3,6,8-Tetrakis(p-benzoic acid)pyrene
NiCB	[Ni(dicarbollide)2]
IDE	Interdigitated electrodes
BPDPNDI	N,N′-Bis(4-pyridyl)-2,6-dipyrrolidyl naphthalenediimide
TCPB	1,2,4,5-Tetrakis-(4-carboxy phenyl)benzene
MV2+	Methyl viologen
DFDNB	1,5-Difluoro-2,4-dinitrobenzene
DNT	Dinitrotoluene
TTFTB4−	Tetrathiafulvalene tetrabenzoate
TCNE	Tetracyanoethylene
MIL	Matérial Institut Lavoisier
dpzNDI	Dipyrazolate-naphthalenediimide
H2bzpdc	Benzophenone-4,4≤-dicarboxylic acid
dmen	1,1-Dimethylethylenediamine
PhBSO3	p-Phenylbenzenesulfonate
H2NDC	2,6-Napthalenedicarboxylicacid
HPCA	4-Pyridinecarboxylic
H2-TCPP	Tetrakis((4-carboxyphenyl)porphyrin)
BTB	1,3,5-Benzentrisbenzoate
SURMOF	Surface-mounted metal–organic framework
MHDA	Mercaptohexadecanoic acid
SAM	Self-assembled monolayer
TBAHFP	Tetrabutylammonium hexafluorophosphate
FIB	Focused ion beam
SEM	Scanning electron microscope
EDX	Energy dispersive X-ray spectroscopy
MALDI	Matrix-assisted laser desorption/ionization
EP	Electro-polymerization

Author contributions

The manuscript was written through the contributions of both authors.

Data availability

No primary research results, software or code have been included and no new data were generated or analysed as part of this review.

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

R. S. is the beneficiary of the grant (Z21-070) for the requalification of the Spanish University system from the Ministry of Universities of the Government of Spain, modality “Maria Zambrano” financed by the European Union. This study forms part of the Advanced Materials program and was supported by the Spanish MCIN with funding from European Union NextGeneration EU (PRTR-C17.I1) and the Generalitat Valenciana (project MFA-2022-057). We also thank the Grant PID2021-125907NB-I00 funded by MCIN/AEI/10.13039/501100011033 and by “ERDF A way of making Europe” and project CIPROM-2022-060 from the Generalitat Valenciana, for financial support.

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