**1. Introduction**

Over last few decades, crystalline microporous materials, from zeolites, to coordination polymers and its subclass, metal organic frameworks (MOFs) have gained enormous attention in the scientific community due to their structural versatility and tailorable properties like nanoscale porosity, high surface area, and functional density [1, 2]. Metal organic frameworks have evolved in last few years as a revolutionary material that are self-assembled nanostructure [3, 4] built from metal ions and organic ligands. The first MOF, MOF-5 or IRMOF-1 (Zn4O(BDC)3) reported by Omar M. Yaghi was used in gas adsorption applications accounting to its high surface area of 2900 m2 /g [5, 6]. To date, 80,000 MOFs [7] have been reported owing to its diverse structure, compositions, tunable porosity, specific surface area, [8] ease of

functionalization, unsaturated metal sites [9] and biocompatibility [10] . As a result, MOFs were used in a wide range of applications such as gas storage and separation, drug delivery and storage, chemical separation, sensing, catalysis, and bio-imaging [3, 7, 11–13]. In terms of structural orientation, the coordination bonding between a metal ion and organic ligand results in the formation of extended networks of one, two, and three-dimensional framework with potential voids [6, 14]. The coordination bonding facilitated through a suitable molecular approach, involving reticular synthesis, provides the flexibility to alter the pore size and transform its structure, targeting specific applications. Thus, utilizing the advantage of various combinations of metal-ligands and interaction of metal-ligands, MOFs are ideal candidates in the field of material science, offering an attractive property of structural tunability, providing a pathway to introduce and tailor intrinsic characteristics, such as optical, electrical, and magnetic properties.

There has been a growing interest exploring MOF as emerging semiconducting materials to meet the current demand in the electronic devices [15]. In particular, the electronic characteristics such as electrical, optical, and magnetic properties of MOFs have become an interesting topic of research attributing to their applications in microelectronic and optical devices. The implementation of MOFs in the electronic industry was first reported by Allendorf and co-workers [16]. MOF-5 with Zn4O metal nodes and orthogonally interconnected six units of terephthalate is the most-studied MOF as a semiconductor. In 2007, Garcia and co-workers reported on the semiconducting behavior of MOF-5 synthesized at room temperature, with a bandgap of 3.4 eV [17]. Since then, intense research has been carried out to develop MOFs with semiconducting properties, opening new research domains for the scientific community in nanoscience.

The presence of narrow band gap structure either direct or indirect and charge mobility contribute to the semiconducting behavior of MOFs. To design MOFs with semiconducting behavior, significant amount of research is ongoing to identify the general structural requirements for enhancing the orbital overlapping between the building components. The main advantage of MOFs is the ability to tune the crystalline structure and functionality through phenomenal conceptual approaches such as rational designing and synthetic flexibility. In reticular chemistry, which is also known as rational designing, the coordination bonding between metal node and organic ligand provides an understanding of atomic positions precisely contributing to determine the fundamental structure–property relationships. Thus, the crystalline structure of MOFs consists of self-assembled ordered nanostructure with defined organized spatial space that is constructed via coordination chemistry between the building components.

Moreover, the sub-angstrom knowledge of atomic positions helps to eliminate any disorder in the structure that contributes to poor mobility in the structure. Considering synthetic flexibility, the electronic properties of MOFs could be tailored, resulting in potential applications such as a photovoltaic device tuned for solar cells, electroluminescent devices, field effect transistors, spintronic devices, and sensors. These developments have led many researchers to explore electrical, magnetic, and optical properties of MOFs [15, 18, 19]. However, the electrical properties of MOFs and integration of them in micro-electronic devices is still at an early stage and remain under research when compared to other types of existing conducting materials [4, 15] due to their insulating character. Although MOFs possess the properties of both organic and inorganic counterparts, they behave as electrical insulators or poor electrical conductors due to the poor overlapping between the π-orbitals of organic ligands and d-orbitals of the metal ion [20]. Yet, MOFs serving as an interface between (inorganic) hard and (organic) soft materials provide an opportunity for adapting various structure–property relationships that

**143**

**Figure 1.**

*Dimensional structures of coordination polymers.*

(**Figure 1**).

*Coordination Polymer Frameworks for Next Generation Optoelectronic Devices*

is related to wide range of parameters such as choice of metal ion, organic linker, and molecular designing approach. In general, the structure–property relationship in MOFs is a consequence of cooperative mechanism, i.e. the interaction between the metal and ligand, which could be readily identified by taking advantage of the knowledge of their detailed atomic structure, enabling fine tuning of their functionalities [7, 11]. According to the literature, Bastian Hoppe and his co-

workers reported Cu-2, 3, 6, 7, 10, 11-hexahydroxytriphenylene (Cu3hhtp2-MOF), a copper-based graphene-like framework with inherent electrical conductivity about 0.045 S cm−1 [21]. MOFs with electrical conductivity higher than 0.1 S cm−1 was achieved by Talin and co-workers [22]. Thus, the designing of MOFs with conducting or semiconducting properties is necessary to enhance the sensitivity of electrical or demonstrate a sensing concept; but rarely have MOFs been an integral part of

The purpose of this chapter is to provide comprehensive discussion on optoelectronic MOFs developed up to date and identify focus points to bring MOFs with optoelectronic properties for the realization of integrating MOFs into actual devices for electronic device applications. We first provide a MOFs chemistry and isoreticular synthesis advancements to make isoreticular MOFs (IRMOFs) with tailored optical and electronic properties. Then we summarize the current state of MOF research relevant to optoelectronics, particularly discussing the synthesis, electronic structure, and photophysical properties of three selected IRMOFs (IRMOF-1, 8, and 10). Finally, we propose a MOFs-device roadmap, focusing on MOF-based field-effect transistors, photovoltaics, thermoelectric devices, and solid-state

Coordination polymers are organic–inorganic hybrid materials where organic moieties are bonded to metal ion or metal clusters via coordination bonds. The energy of such bonding is usually between 50 and 200 KJ mol−1. Apart from strong coordination bonding, weaker interaction such as hydrogen bonds, van der Waal forces and π-π interactions also influence the formation of coordination polymers. Depending on the geometry, coordination polymers are classified into three subclasses: (1) One-dimensional (1-D) coordination polymers, (2) Two-dimensional (2-D) coordination polymers, and (3) Three-dimensional coordination polymers

The coordination polymer assembled from organic ligand and metal ion into three dimensional hierarchical crystalline structures is often regarded as metal organic framework. Since then, the term coordination polymer and metal organic

*DOI: http://dx.doi.org/10.5772/intechopen.94335*

electrolytes and lithium ion battery components.

**2.1 Dimensional classification and evolution of MOFs**

an actual device [23].

**2. Chemistry of MOFs**

*Coordination Polymer Frameworks for Next Generation Optoelectronic Devices DOI: http://dx.doi.org/10.5772/intechopen.94335*

is related to wide range of parameters such as choice of metal ion, organic linker, and molecular designing approach. In general, the structure–property relationship in MOFs is a consequence of cooperative mechanism, i.e. the interaction between the metal and ligand, which could be readily identified by taking advantage of the knowledge of their detailed atomic structure, enabling fine tuning of their functionalities [7, 11]. According to the literature, Bastian Hoppe and his coworkers reported Cu-2, 3, 6, 7, 10, 11-hexahydroxytriphenylene (Cu3hhtp2-MOF), a copper-based graphene-like framework with inherent electrical conductivity about 0.045 S cm−1 [21]. MOFs with electrical conductivity higher than 0.1 S cm−1 was achieved by Talin and co-workers [22]. Thus, the designing of MOFs with conducting or semiconducting properties is necessary to enhance the sensitivity of electrical or demonstrate a sensing concept; but rarely have MOFs been an integral part of an actual device [23].

The purpose of this chapter is to provide comprehensive discussion on optoelectronic MOFs developed up to date and identify focus points to bring MOFs with optoelectronic properties for the realization of integrating MOFs into actual devices for electronic device applications. We first provide a MOFs chemistry and isoreticular synthesis advancements to make isoreticular MOFs (IRMOFs) with tailored optical and electronic properties. Then we summarize the current state of MOF research relevant to optoelectronics, particularly discussing the synthesis, electronic structure, and photophysical properties of three selected IRMOFs (IRMOF-1, 8, and 10). Finally, we propose a MOFs-device roadmap, focusing on MOF-based field-effect transistors, photovoltaics, thermoelectric devices, and solid-state electrolytes and lithium ion battery components.

## **2. Chemistry of MOFs**

*Optoelectronics*

electrical, and magnetic properties.

scientific community in nanoscience.

between the building components.

functionalization, unsaturated metal sites [9] and biocompatibility [10] . As a result, MOFs were used in a wide range of applications such as gas storage and separation, drug delivery and storage, chemical separation, sensing, catalysis, and bio-imaging [3, 7, 11–13]. In terms of structural orientation, the coordination bonding between a metal ion and organic ligand results in the formation of extended networks of one, two, and three-dimensional framework with potential voids [6, 14]. The coordination bonding facilitated through a suitable molecular approach, involving reticular synthesis, provides the flexibility to alter the pore size and transform its structure, targeting specific applications. Thus, utilizing the advantage of various combinations of metal-ligands and interaction of metal-ligands, MOFs are ideal candidates in the field of material science, offering an attractive property of structural tunability, providing a pathway to introduce and tailor intrinsic characteristics, such as optical,

There has been a growing interest exploring MOF as emerging semiconducting materials to meet the current demand in the electronic devices [15]. In particular, the electronic characteristics such as electrical, optical, and magnetic properties of MOFs have become an interesting topic of research attributing to their applications in microelectronic and optical devices. The implementation of MOFs in the electronic industry was first reported by Allendorf and co-workers [16]. MOF-5 with Zn4O metal nodes and orthogonally interconnected six units of terephthalate is the most-studied MOF as a semiconductor. In 2007, Garcia and co-workers reported on the semiconducting behavior of MOF-5 synthesized at room temperature, with a bandgap of 3.4 eV [17]. Since then, intense research has been carried out to develop MOFs with semiconducting properties, opening new research domains for the

The presence of narrow band gap structure either direct or indirect and charge mobility contribute to the semiconducting behavior of MOFs. To design MOFs with semiconducting behavior, significant amount of research is ongoing to identify the general structural requirements for enhancing the orbital overlapping between the building components. The main advantage of MOFs is the ability to tune the crystalline structure and functionality through phenomenal conceptual approaches such as rational designing and synthetic flexibility. In reticular chemistry, which is also known as rational designing, the coordination bonding between metal node and organic ligand provides an understanding of atomic positions precisely contributing to determine the fundamental structure–property relationships. Thus, the crystalline structure of MOFs consists of self-assembled ordered nanostructure with defined organized spatial space that is constructed via coordination chemistry

Moreover, the sub-angstrom knowledge of atomic positions helps to eliminate any disorder in the structure that contributes to poor mobility in the structure. Considering synthetic flexibility, the electronic properties of MOFs could be tailored, resulting in potential applications such as a photovoltaic device tuned for solar cells, electroluminescent devices, field effect transistors, spintronic devices, and sensors. These developments have led many researchers to explore electrical, magnetic, and optical properties of MOFs [15, 18, 19]. However, the electrical properties of MOFs and integration of them in micro-electronic devices is still at an early stage and remain under research when compared to other types of existing conducting materials [4, 15] due to their insulating character. Although MOFs possess the properties of both organic and inorganic counterparts, they behave as electrical insulators or poor electrical conductors due to the poor overlapping between the π-orbitals of organic ligands and d-orbitals of the metal ion [20]. Yet, MOFs serving as an interface between (inorganic) hard and (organic) soft materials provide an opportunity for adapting various structure–property relationships that

**142**

#### **2.1 Dimensional classification and evolution of MOFs**

Coordination polymers are organic–inorganic hybrid materials where organic moieties are bonded to metal ion or metal clusters via coordination bonds. The energy of such bonding is usually between 50 and 200 KJ mol−1. Apart from strong coordination bonding, weaker interaction such as hydrogen bonds, van der Waal forces and π-π interactions also influence the formation of coordination polymers. Depending on the geometry, coordination polymers are classified into three subclasses: (1) One-dimensional (1-D) coordination polymers, (2) Two-dimensional (2-D) coordination polymers, and (3) Three-dimensional coordination polymers (**Figure 1**).

The coordination polymer assembled from organic ligand and metal ion into three dimensional hierarchical crystalline structures is often regarded as metal organic framework. Since then, the term coordination polymer and metal organic

**Figure 1.** *Dimensional structures of coordination polymers.*

framework have been used interchangeably. The term MOFs was first introduced by Omar Yaghi in 1995 [4, 9]. The framework of MOFs is either porous or non-porous. However, the porosity of MOFs was reported to be reversible due to various environmental factors (temperature, pressure, light intensity) contributing to the weak intermolecular interactions between building components. Thus, efforts have been made to modulate the strong structural rigidity that could incorporate permanent porosity. Based on this, in 1998 Kitagawa classified MOFs into three categories; 1st, 2nd, and 3rd generation coordinated network. Among three generations of coordinated networks, 3rd generation coordinated networks were defined to have permanent porosity with structural flexibility [10]. This led to numerous applications and implementation of coordinated networks in the gas storage community. The intermolecular interaction between organic ligand and metal ions, choice of building units, crystallization, environment, and guest molecules determine the crystal structural rigidity and dimensionality of MOF's coordination network. This major advance in the field of coordination polymer depicted that coordinated networks of MOFs could be modified and developed in a highly periodic manner, with a defined understanding of the crystalline structure, porosity, and chemical functionality. Thus, the ability to design and control the arrangement of metal ions with extended organic spaces in three-dimensional fashion led to the origin of the term reticular chemistry which was first introduced by Yaghi and coworkers [4].

With the variability of organic and inorganic components and their interaction, the freedom of modulating the structure of MOFs into highly ordered hierarchical structures with tunable pore volume and adjustable surface area has become feasible that made MOFs stand out compared to the other porous materials. Taking advantage of one of these hallmarks of MOFs i.e. designing of topologically diverse structures with desirable properties has been explored extensively attracting wide range of applications in gas storage, separation, catalysis, sensing and drug delivery [5]. Since 1990s, this area of chemistry has experienced tremendous growth in the field of material science and modern chemistry [4]. The flexibility with geometry, size, and functionality led to the "design" of a large number of MOFs. The organic units are generally ditopic or polytopic organic carboxylates, linked to metal-containing units, such as transition metals (e.g., Cu, Zn, Fe, Co, and Ni), alkaline earth elements (e.g. Sr., Ba), p-block elements (e.g. In, Ga), and actinides (e.g. U, Th) [6]. A major advance in the chemistry of MOFs came in 1999 with the invention of two structures i.e. MOF-5 (IRMOF-1) and HKUST-1 [11] reported by Omar et al. and Chui et al., respectively. Subsequently, in the coming years around 2002, the flexible and nonflexible structures of MIL-88/53 [12] was reported by Ferey et al.

#### **2.2 Reticular chemistry and isoreticular MOFs**

The demand for the synthesis of new materials to perform highly specific and cooperative functions has been increasing rapidly in parallel with advanced technology [13, 14]. Recently, the field of metal organic framework has evolved significantly due to its practical and conceptual approach to design and develop the target material. Intrinsically, the reticular chemistry is described as the process of assembly of molecular building blocks held together by strong bonding that pattern into periodic arrays of the ordered net like structures [13–16]. Some of the advantages of this approach are: (1) Molecular approach, which provides the ability to design and control the structure of frameworks [17]; (2) Bonding in which the strong bonding between the building blocks could impart superior functionalities like thermal and chemical stability into the framework; and (3) Engineered crystallinity, which is based on the type of the interactions (intermolecular or intramolecular) design and synthesis with controlled and desired properties.

**145**

**Figure 2.**

*Coordination Polymer Frameworks for Next Generation Optoelectronic Devices*

After the introduction of the parent MOF, MOF-5, taking advantage of reticular chemistry that includes reticulating metal ions and organic carboxylate, the group of Omar M. Yaghi synthesized a new class of materials called IRMOFs. Thus, the theory of isoreticular chemistry was established in the year 2002 with the development of IRMOFs. These class of materials were developed to improve the surface area and pore volume by incorporation of different topological linkers. In IRMOF, IR stands for isoreticular, which means it is a series of MOFs with the same topology, but different pore size [14, 20, 22, 23]. A series of different IRMOFs share similar *pcu* topology of IRMOF-n (n = 1–16). As shown in the **Figure 2**, the pore volume and porosity vary with the variation in the organic linker. Applying the concept of isoreticular chemistry, various kinds of MOFs were developed.

The conceptual approach of designing and assembling a metal–organic framework follows reticular synthesis and is based upon identification of how building blocks come together to form a net, or reticulate. The synthesis of MOFs is often regarded as "design" which implies to construct, built, execute, or create according to the target material. The synthesis approach for a new MOF should follow several factors asides from the geometric principles that are considered during its design. Among such factors, by far the most important is the maintenance of the integrity of the building blocks. A great deal of research effort has been demonstrated on the synthesis of a novel organic link and synthesis conditions that are mild enough to maintain the functionality and conformation of organic ligand, yet reactive enough to establish the metal–organic bonds. In situ generation of a desired secondary subunit (SBU) is required carful design of synthetic conditions and must be compatible with the mobilization and preservation of the linking units [24]. Typically, this is achieved by precipitation of the product from a solution of the precursors where solubility is a necessary attribute of the building blocks

but is quite often circumvented by using solvothermal techniques [24].

*Crystal structures of IRMOFs-n series [n = 1–16]. The non-interpenetrated structures from* 

*C in gray, Br atoms in Orange, and amino groups in blue [17].*

*(n = 1,2,3,4,5,6,7,8,10,12,14,16). The yellow sphere represents the pore volume. Zn atoms are in green, O in red,* 

*DOI: http://dx.doi.org/10.5772/intechopen.94335*

**2.3 Synthetic advancements of MOFs**

*Coordination Polymer Frameworks for Next Generation Optoelectronic Devices DOI: http://dx.doi.org/10.5772/intechopen.94335*

After the introduction of the parent MOF, MOF-5, taking advantage of reticular chemistry that includes reticulating metal ions and organic carboxylate, the group of Omar M. Yaghi synthesized a new class of materials called IRMOFs. Thus, the theory of isoreticular chemistry was established in the year 2002 with the development of IRMOFs. These class of materials were developed to improve the surface area and pore volume by incorporation of different topological linkers. In IRMOF, IR stands for isoreticular, which means it is a series of MOFs with the same topology, but different pore size [14, 20, 22, 23]. A series of different IRMOFs share similar *pcu* topology of IRMOF-n (n = 1–16). As shown in the **Figure 2**, the pore volume and porosity vary with the variation in the organic linker. Applying the concept of isoreticular chemistry, various kinds of MOFs were developed.

### **2.3 Synthetic advancements of MOFs**

*Optoelectronics*

framework have been used interchangeably. The term MOFs was first introduced by Omar Yaghi in 1995 [4, 9]. The framework of MOFs is either porous or non-porous. However, the porosity of MOFs was reported to be reversible due to various environmental factors (temperature, pressure, light intensity) contributing to the weak intermolecular interactions between building components. Thus, efforts have been made to modulate the strong structural rigidity that could incorporate permanent porosity. Based on this, in 1998 Kitagawa classified MOFs into three categories; 1st, 2nd, and 3rd generation coordinated network. Among three generations of coordinated networks, 3rd generation coordinated networks were defined to have permanent porosity with structural flexibility [10]. This led to numerous applications and implementation of coordinated networks in the gas storage community. The intermolecular interaction between organic ligand and metal ions, choice of building units, crystallization, environment, and guest molecules determine the crystal structural rigidity and dimensionality of MOF's coordination network. This major advance in the field of coordination polymer depicted that coordinated networks of MOFs could be modified and developed in a highly periodic manner, with a defined understanding of the crystalline structure, porosity, and chemical functionality. Thus, the ability to design and control the arrangement of metal ions with extended organic spaces in three-dimensional fashion led to the origin of the term reticular

chemistry which was first introduced by Yaghi and coworkers [4].

flexible structures of MIL-88/53 [12] was reported by Ferey et al.

**2.2 Reticular chemistry and isoreticular MOFs**

synthesis with controlled and desired properties.

With the variability of organic and inorganic components and their interaction, the freedom of modulating the structure of MOFs into highly ordered hierarchical structures with tunable pore volume and adjustable surface area has become feasible that made MOFs stand out compared to the other porous materials. Taking advantage of one of these hallmarks of MOFs i.e. designing of topologically diverse structures with desirable properties has been explored extensively attracting wide range of applications in gas storage, separation, catalysis, sensing and drug delivery [5]. Since 1990s, this area of chemistry has experienced tremendous growth in the field of material science and modern chemistry [4]. The flexibility with geometry, size, and functionality led to the "design" of a large number of MOFs. The organic units are generally ditopic or polytopic organic carboxylates, linked to metal-containing units, such as transition metals (e.g., Cu, Zn, Fe, Co, and Ni), alkaline earth elements (e.g. Sr., Ba), p-block elements (e.g. In, Ga), and actinides (e.g. U, Th) [6]. A major advance in the chemistry of MOFs came in 1999 with the invention of two structures i.e. MOF-5 (IRMOF-1) and HKUST-1 [11] reported by Omar et al. and Chui et al., respectively. Subsequently, in the coming years around 2002, the flexible and non-

The demand for the synthesis of new materials to perform highly specific and cooperative functions has been increasing rapidly in parallel with advanced technology [13, 14]. Recently, the field of metal organic framework has evolved significantly due to its practical and conceptual approach to design and develop the target material. Intrinsically, the reticular chemistry is described as the process of assembly of molecular building blocks held together by strong bonding that pattern into periodic arrays of the ordered net like structures [13–16]. Some of the advantages of this approach are: (1) Molecular approach, which provides the ability to design and control the structure of frameworks [17]; (2) Bonding in which the strong bonding between the building blocks could impart superior functionalities like thermal and chemical stability into the framework; and (3) Engineered crystallinity, which is based on the type of the interactions (intermolecular or intramolecular) design and

**144**

The conceptual approach of designing and assembling a metal–organic framework follows reticular synthesis and is based upon identification of how building blocks come together to form a net, or reticulate. The synthesis of MOFs is often regarded as "design" which implies to construct, built, execute, or create according to the target material. The synthesis approach for a new MOF should follow several factors asides from the geometric principles that are considered during its design. Among such factors, by far the most important is the maintenance of the integrity of the building blocks. A great deal of research effort has been demonstrated on the synthesis of a novel organic link and synthesis conditions that are mild enough to maintain the functionality and conformation of organic ligand, yet reactive enough to establish the metal–organic bonds. In situ generation of a desired secondary subunit (SBU) is required carful design of synthetic conditions and must be compatible with the mobilization and preservation of the linking units [24]. Typically, this is achieved by precipitation of the product from a solution of the precursors where solubility is a necessary attribute of the building blocks but is quite often circumvented by using solvothermal techniques [24].

#### **Figure 2.**

*Crystal structures of IRMOFs-n series [n = 1–16]. The non-interpenetrated structures from (n = 1,2,3,4,5,6,7,8,10,12,14,16). The yellow sphere represents the pore volume. Zn atoms are in green, O in red, C in gray, Br atoms in Orange, and amino groups in blue [17].*

Traditional goal of MOF synthesis is to obtain high quality single crystal for deducing the structure and understand the crystal packing, geometry, and pore volume with respect to the organic ligand's length. Thus, prior to begin elucidating the concept of reticular synthesis, most early studies were exploratory and early stage synthesis has mainly involved simple, highly soluble precursors, and labile metal ions of the late transition series. The polymerization that leads to make 3D-network of MOFs needs an assembly process where an insoluble entity is quickly formed that precludes recrystallization. Fortunately, it differs in the degree of reversibility of the bond formation event, allowing detachment of incoherently matched monomers followed by reattachment with continued defect free crystal growth. The framework assembly occurs as a single synthetic step, where all of the desired attributes of the target material constructs by the building blocks. This often requires a combinatorial approach, which involves subtle changes in concentration, solvent polarity, pH, or temperature. Any subtle changes in these parameters leads to poorer quality crystals, reduced yields, or the formation of entirely new phases [24].

Augmenting simple crystal growth processes used to grow simple inorganic salts, early efforts of producing highly crystalline MOFs involved the slow introduction of the building blocks to reduce the rate of crystallite nucleation. Methods included slow evaporation of a solution of the precursors, layering of solutions, or slow diffusion of one component solution into another through a membrane or an immobilizing gel [24]. During the nucleation stage, the ligand deprotonation prior to the coordination onto metal ion is catalyzed introducing a volatile amine gradually via vapor diffusion. Just as for many of the polar solvents used, suitable choice of base is necessary to avoid competitive coordination with the organic links for the available metal sites. While in some cases, blocking of metal coordination sites is necessary for the formation of a particular SBU. However, this approach has generally been regarded as leading to low-dimensional structures that are less likely to define an open framework.

With the need for more robust frameworks, having larger pore volumes and higher surface area, introducing bulker, longer length organic linkers are necessary, but greater difficulties in crystal growth were encountered. Thus, later, solvothermal techniques were found to be a convenient solution to overcome this challenge and have largely benefit over often time-consuming methods involving slow coupling of the coordinating species. The typical solvothermal method combines the precursors as dilute solutions in polar solvents such as water, alcohols, acetone or acetonitrile and heated in sealed vessels such as Teflon-lined stainless-steel bombs or glass tubes, generating autogenous pressure. The crystal growth process is enhanced by using mixed solvent systems where the solution polarity and the kinetics of solvent-ligand exchange can tune to achieve rapid crystal growth. It has found that, exposing the growing framework to a variety of space-filling solvent molecules may also be an effective way to stabilize its defect-free construction as they efficiently pack within the defined channels [24]. For deprotonation of the linking molecule alkyl formamides and pyrrolidinones have been particularly useful, as they are also excellent solubilizing agents.

In recent years, modifying the solvothermal method, there are several rapid synthesis methods were proposed by researchers to develop MOF crystals within a short duration of time. Some of the external parameters implemented to develop MOFs include the use of Microwave energy (Microwave synthesis), [25] Ultrasonic waves (Sonochemical synthesis), Mechanical energy (Mechano-chemical synthesis) and electrical energy (Electrochemical synthesis). The synthetic strategies developed up to date to make different type of MOFs are summarized in the **Table 1** along with reaction conditions [26]. Additionally, a surfactant driven-templating method, [22] a CO2-expanded liquid route, [27] a post-synthetic method, [28] and an ionic liquid-based method [29] are developed to create hierarchical mesoporous microstructures and thin films of MOFs [25, 27–29].

**147**

*Coordination Polymer Frameworks for Next Generation Optoelectronic Devices*

**Synthesis method Reaction time Temperature (K)** Slow evaporation 7 days to 7 months 298 Sonochemical method 30–180 mins 272–313 Solvothermal method 48–96 hours 353–453 Mechano-chemical method 30 min to 2 hours 298 Electrochemical method 10–30 mins 273–303 Microwave Synthesis 4 mins to 4 hours 303–373

**3. Zn4O(L)3-based isoreticular MOFs with cubic topology for** 

In the area of MOFs, the main desire is to design MOFs with optoelectronic properties and to optimize the charge transport mechanism suitable for developing electronic devices. Although numerous applications of MOFs with different types of synthesis methods are being investigated, a versatile and scalable synthesis approach for the preparation of MOFs with semiconducting properties for optoelectronic devices are still in the early stage and a little research work so far done towards tailoring MOFs structure–property relationship to use as active materials in optoelectronic devices, such as solar cells, field-effect transistors, and photoluminescence devices. To introduce MOFs as semiconducting materials, tuning of band gap such as lowering the bandgap or increasing the charge mobility is required. This tunability is again dependent upon the type of interaction i.e. Intermolecular interaction: metal ion and the organic ligand or Intramolecular interaction - π stacking [18]. The two key factors responsible for poor electrical conductivity in MOFs are: (1) the insulating character of organic ligand and (2) due to poor overlapping between the π-orbitals of organic ligand and d-orbitals of metal ions [16]. The common strategies for constructing MOFs with electrical conductivity involves

**Pathway 1:** A long range of charge transport in this pathway is facilitated through bonds. This mechanism is promoted by interaction between ligand π and metal d orbital [16]. This mechanism is based on the tunneling of electron between the donor and acceptor portions of the framework. Typically, the electrical conductivity in the range 10−7 to 10−10 S cm−1 is considered as insulator. This is caused due to poor overlapping between the metal ion and organic linker as the electronegative nature of oxygen atom in the carboxylate group of the linker is so high that it requires high voltage for tunneling of the electrons [30]. Various MOFs that exhibit conductivity through this mechanism have been reported, of which [[Cu2(6-Hmna) (6-mn)]·NH4]n, a copper-sulfur based MOF constructed from 1,6-Hmna = 6-mercaptonicotinic acid, 6-mn = 6-mercaptonicotinate shows highest

**Pathway 2:** In this pathway, the charge transport is facilitated through space via π stacked aromatic ligands which was proposed as an alternative to through bond strategy. This mechanism typically promotes electron hopping mechanism by employing electroactive molecules [16, 30]. TTF-TCNQ i.e. tetrathiafulvalenetetracyano quinomethane is one of the MOFs that demonstrate metallic conductivity

**3.1 Road map to electrically conductive MOFs**

*Synthesis methods developed up to date to make MOFs.*

*DOI: http://dx.doi.org/10.5772/intechopen.94335*

**optoelectronics**

**Table 1.**

three possible charge pathways.

electrical conductivity of 10.96 S/cm (**Table 2**).

*Coordination Polymer Frameworks for Next Generation Optoelectronic Devices DOI: http://dx.doi.org/10.5772/intechopen.94335*


**Table 1.**

*Optoelectronics*

Traditional goal of MOF synthesis is to obtain high quality single crystal for deducing the structure and understand the crystal packing, geometry, and pore volume with respect to the organic ligand's length. Thus, prior to begin elucidating the concept of reticular synthesis, most early studies were exploratory and early stage synthesis has mainly involved simple, highly soluble precursors, and labile metal ions of the late transition series. The polymerization that leads to make 3D-network of MOFs needs an assembly process where an insoluble entity is quickly formed that precludes recrystallization. Fortunately, it differs in the degree of reversibility of the bond formation event, allowing detachment of incoherently matched monomers followed by reattachment with continued defect free crystal growth. The framework assembly occurs as a single synthetic step, where all of the desired attributes of the target material constructs by the building blocks. This often requires a combinatorial approach, which involves subtle changes in concentration, solvent polarity, pH, or temperature. Any subtle changes in these parameters leads to poorer quality

crystals, reduced yields, or the formation of entirely new phases [24].

Augmenting simple crystal growth processes used to grow simple inorganic salts, early efforts of producing highly crystalline MOFs involved the slow introduction of the building blocks to reduce the rate of crystallite nucleation. Methods included slow evaporation of a solution of the precursors, layering of solutions, or slow diffusion of one component solution into another through a membrane or an immobilizing gel [24]. During the nucleation stage, the ligand deprotonation prior to the coordination onto metal ion is catalyzed introducing a volatile amine gradually via vapor diffusion. Just as for many of the polar solvents used, suitable choice of base is necessary to avoid competitive coordination with the organic links for the available metal sites. While in some cases, blocking of metal coordination sites is necessary for the formation of a particular SBU. However, this approach has generally been regarded as leading to low-dimensional structures that are less likely to define an open framework. With the need for more robust frameworks, having larger pore volumes and higher surface area, introducing bulker, longer length organic linkers are necessary, but greater difficulties in crystal growth were encountered. Thus, later, solvothermal techniques were found to be a convenient solution to overcome this challenge and have largely benefit over often time-consuming methods involving slow coupling of the coordinating species. The typical solvothermal method combines the precursors as dilute solutions in polar solvents such as water, alcohols, acetone or acetonitrile and heated in sealed vessels such as Teflon-lined stainless-steel bombs or glass tubes, generating autogenous pressure. The crystal growth process is enhanced by using mixed solvent systems where the solution polarity and the kinetics of solvent-ligand exchange can tune to achieve rapid crystal growth. It has found that, exposing the growing framework to a variety of space-filling solvent molecules may also be an effective way to stabilize its defect-free construction as they efficiently pack within the defined channels [24]. For deprotonation of the linking molecule alkyl formamides and pyrrolidinones have been particularly useful, as they are also excellent solubilizing agents. In recent years, modifying the solvothermal method, there are several rapid synthesis methods were proposed by researchers to develop MOF crystals within a short duration of time. Some of the external parameters implemented to develop MOFs include the use of Microwave energy (Microwave synthesis), [25] Ultrasonic waves (Sonochemical synthesis), Mechanical energy (Mechano-chemical synthesis) and electrical energy (Electrochemical synthesis). The synthetic strategies developed up to date to make different type of MOFs are summarized in the **Table 1** along with reaction conditions [26]. Additionally, a surfactant driven-templating method, [22] a CO2-expanded liquid route, [27] a post-synthetic method, [28] and an ionic liquid-based method [29] are developed to create hierarchical mesoporous

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microstructures and thin films of MOFs [25, 27–29].

*Synthesis methods developed up to date to make MOFs.*
