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

### **3.1 Road map to electrically conductive MOFs**

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 three possible charge pathways.

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

**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


#### **Table 2.**

*Significant progress in the last few years made towards developing electrically conductive MOFs and their conductive properties compared with conventional metals.*

(shown in the **Table 2**) through-space (π-π stacking) mechanism [38]. Recently, TTF-based ligand consisting of benzoate spacers is used to develop Zn based MOF reported by Dincă et al. These MOFs shows columnar stacks of TTF (3.8 Å) with the charge mobility of a magnitude that resembles some best conductive organic polymers [35, 36].

**Pathway 3:** The other alternative strategy to increase the conductivity of MOFs is via incorporating an appropriate guest molecule within the MOF. These molecules can activate long range delocalization either through bonds or through space or that can inject mobile charge carriers by oxidizing or reducing the organic ligand and metal ions [16, 30] NU-901, a MOF consisting of Zr6 (μ3-O)4 (μ3-OH)4 (H2O)4 (OH)4 nodes and tetratopic 1,3,6,8-tetrakis (p- benzoate) pyrene (TBAPy4-) linkers. These materials were chosen for the encapsulation of C60. After installation of C60, the NU-901-C60 shows electrical conductivity higher than that of NU-901 (shown in the **Table 2**). As per reports, the donor-acceptor interactions between TBAPy4-/C60 contribute to the electrical conductivity of the framework [32, 38].

#### **3.2 Synthesis and optoelectronic properties of IRMOF-1**

IRMOF-1, which is commonly known as MOF-5, invented by Yaghi and coworkers [39] in 1996, has become one of mostly studied MOF with promising application in high capacity hydrogen storage [40, 41]. MOF-5, consists of Zn4O units connected by linear 1,4-benzenedicarboxylate units to form a cubic network, having the primitive cubic unit cell. Syntheses demonstrated for MOF-5 in which the starting materials are mixed in solution at ambient temperature. Subsequent addition of triethylamine promotes the deprotonation of the organic linker to precipitate MOF-5. Depending on the addition of base either slowly by diffusion as described in the original synthesis method [39] or rapidly as an aliquot [42] the product can be either single crystal mixtures, which must be mechanically separated, or microcrystalline powders. The ambient temperature synthesis method described above following the fast addition of base, is easy to scale up. However,

**149**

**Figure 3.**

*driven self-assembly.*

*Coordination Polymer Frameworks for Next Generation Optoelectronic Devices*

metal precursor, zinc nitrate poses potential safety concerns, especially for largescale production. Furthermore, reports of such synthetic conditions have been

Later, a rapid, simple, room-temperature high yielding synthesis method was introduced by Yaghi and co-workers that can apply to make a wide range of new MOFs, including IRMOF-0, which uses acetylenedicarboxylate as the linker [45]. This synthesis method follows a room temperature synthesis, wherein separate N,Ndimethylformamide (DMF) solutions of terephthalic acid (BDC) with triethylamine and zinc acetate dihydrate are prepared, then the zinc salt solution is added to the organic solution with rapid stirring at ambient temperature. Upon immediately of the formation of a white powder followed by 2.5 hours of reaction time, pure MOF-5 is collected and confirmed by powder XRD. The same synthesis without a base (triethylamine) has also yielded pure MOF-5, confirming that addition of a base is unnecessary when zinc acetate is used as a source of Zn (II) in the MOF-5 synthesis [45]. This synthesis method has later modified by Rathnayake et.al to make IRMOFs

(IRMOF-1, 8, and 10) by cutting down the reaction time from 2.5 hours to 7–9 minutes [23]. As depicted in **Figure 3**, our group is able to make a wide range of hierarchical microstructures of highly crystalline MOFs, including IRMOF-1. Microstructures of IRMOF-1 prepared from the modified solvothermal method (**Figure 3**), are visualized using scanning electron microscope and are depicted in **Figure 4(a)**. Crystal structure of IRMOF-1, retrieved by matching its simulated XRD with experimental powder XRD is depicted in **Figure 4(b)**, and follows cubic lattice cell, which belongs to Fm3m cubic space groups. The electron density potential distribution modeled from VESTA (**Figure 4(c)**) evidences that the electron potential is localized on Zn4O clusters and there is no electron delocalization with the organic linkers, confirming no orbital overlap for energy transfer through

As a first member of isoreticular series, IRMOF-1 has explored for luminescence due to ZnO quantum dots behavior, which has been believed, contributing to luminescence. The ZnO QD absorption and emission spectra from electronic transitions have been investigated, suggesting that that the luminescent behavior of

tetrahedral Zn4O metal cluster, which has been described as a ZnO-like QD [46]. The photoluminescence emissions of IRMOF-1 with intensity peak maximum at 525 nm, was ascribed to energy harvesting and LMCT from 1,4-benzenedicarboxylate (BDC) linked to the Zn4O cluster. The nature of the luminescence transitions in IRMOF-1 nanoparticles has been investigated by Tachikawa et al. where the

*Reaction scheme for synthesis of Isoreticular MOFs using modified solvothermal method followed by solvent* 

charge-transfer transition within each

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

metal–ligand charge transfer processes.

IRMOF-1 arises from a O2 − Zn+ → O − Zn+

largely limited to MOF-5 and IRMOF-8 [42–44].

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

metal precursor, zinc nitrate poses potential safety concerns, especially for largescale production. Furthermore, reports of such synthetic conditions have been largely limited to MOF-5 and IRMOF-8 [42–44].

Later, a rapid, simple, room-temperature high yielding synthesis method was introduced by Yaghi and co-workers that can apply to make a wide range of new MOFs, including IRMOF-0, which uses acetylenedicarboxylate as the linker [45]. This synthesis method follows a room temperature synthesis, wherein separate N,Ndimethylformamide (DMF) solutions of terephthalic acid (BDC) with triethylamine and zinc acetate dihydrate are prepared, then the zinc salt solution is added to the organic solution with rapid stirring at ambient temperature. Upon immediately of the formation of a white powder followed by 2.5 hours of reaction time, pure MOF-5 is collected and confirmed by powder XRD. The same synthesis without a base (triethylamine) has also yielded pure MOF-5, confirming that addition of a base is unnecessary when zinc acetate is used as a source of Zn (II) in the MOF-5 synthesis [45].

This synthesis method has later modified by Rathnayake et.al to make IRMOFs (IRMOF-1, 8, and 10) by cutting down the reaction time from 2.5 hours to 7–9 minutes [23]. As depicted in **Figure 3**, our group is able to make a wide range of hierarchical microstructures of highly crystalline MOFs, including IRMOF-1. Microstructures of IRMOF-1 prepared from the modified solvothermal method (**Figure 3**), are visualized using scanning electron microscope and are depicted in **Figure 4(a)**. Crystal structure of IRMOF-1, retrieved by matching its simulated XRD with experimental powder XRD is depicted in **Figure 4(b)**, and follows cubic lattice cell, which belongs to Fm3m cubic space groups. The electron density potential distribution modeled from VESTA (**Figure 4(c)**) evidences that the electron potential is localized on Zn4O clusters and there is no electron delocalization with the organic linkers, confirming no orbital overlap for energy transfer through metal–ligand charge transfer processes.

As a first member of isoreticular series, IRMOF-1 has explored for luminescence due to ZnO quantum dots behavior, which has been believed, contributing to luminescence. The ZnO QD absorption and emission spectra from electronic transitions have been investigated, suggesting that that the luminescent behavior of IRMOF-1 arises from a O2 − Zn+ → O − Zn+ charge-transfer transition within each tetrahedral Zn4O metal cluster, which has been described as a ZnO-like QD [46]. The photoluminescence emissions of IRMOF-1 with intensity peak maximum at 525 nm, was ascribed to energy harvesting and LMCT from 1,4-benzenedicarboxylate (BDC) linked to the Zn4O cluster. The nature of the luminescence transitions in IRMOF-1 nanoparticles has been investigated by Tachikawa et al. where the

**Figure 3.**

*Reaction scheme for synthesis of Isoreticular MOFs using modified solvothermal method followed by solvent driven self-assembly.*

*Optoelectronics*

Metals Cu Au Fe

Organic polymers Polyacetylene Polythiophenes Rubrene

Cu3(BTC)2- TCNQ NU-901-C60

Fe2(DSBDC) {[Cu2(6-Hmna) (6-mn)] ·NH4]n Cu [Ni(pdt)2]

**Table 2.**

TTF-TCNQ Ni3 (HITP)2 Zn2 (TTFTB)

**Materials Mechanism Conductivity** 

Charge transfer (electron hopping)

*conductive properties compared with conventional metals.*

polymers [35, 36].

(shown in the **Table 2**) through-space (π-π stacking) mechanism [38]. Recently, TTF-based ligand consisting of benzoate spacers is used to develop Zn based MOF reported by Dincă et al. These MOFs shows columnar stacks of TTF (3.8 Å) with the charge mobility of a magnitude that resembles some best conductive organic

*Significant progress in the last few years made towards developing electrically conductive MOFs and their* 

**(Scm−1)**

4.1x105 1.0 x 105

> 10–9 1975

40 4.0x10–6

1x10–3

10.96 1x10–4

Tunneling 6.5 x105

Through-space 700

Guest molecule 0.07

Through-bond 1x10–6

**Charge carrier**

**Mobility (cm2**

e 46 [31]

4

0.2

h [33, 34]

h 1–10

h or e 48.6

**V−1 S−1)**

**Ref.**

[31]

[16, 31, 32]

[18, 35–37]

**3.2 Synthesis and optoelectronic properties of IRMOF-1**

**Pathway 3:** The other alternative strategy to increase the conductivity of MOFs is via incorporating an appropriate guest molecule within the MOF. These molecules can activate long range delocalization either through bonds or through space or that can inject mobile charge carriers by oxidizing or reducing the organic ligand and metal ions [16, 30] NU-901, a MOF consisting of Zr6 (μ3-O)4 (μ3-OH)4 (H2O)4 (OH)4 nodes and tetratopic 1,3,6,8-tetrakis (p- benzoate) pyrene (TBAPy4-) linkers. These materials were chosen for the encapsulation of C60. After installation of C60, the NU-901-C60 shows electrical conductivity higher than that of NU-901 (shown in the **Table 2**). As per reports, the donor-acceptor interactions between TBAPy4-/C60 contribute to the electrical conductivity of the framework [32, 38].

IRMOF-1, which is commonly known as MOF-5, invented by Yaghi and coworkers [39] in 1996, has become one of mostly studied MOF with promising application in high capacity hydrogen storage [40, 41]. MOF-5, consists of Zn4O units connected by linear 1,4-benzenedicarboxylate units to form a cubic network, having the primitive cubic unit cell. Syntheses demonstrated for MOF-5 in which the starting materials are mixed in solution at ambient temperature. Subsequent addition of triethylamine promotes the deprotonation of the organic linker to precipitate MOF-5. Depending on the addition of base either slowly by diffusion as described in the original synthesis method [39] or rapidly as an aliquot [42] the product can be either single crystal mixtures, which must be mechanically separated, or microcrystalline powders. The ambient temperature synthesis method described above following the fast addition of base, is easy to scale up. However,

**148**

**Figure 4.**

*(a) A SEM image of IRMOF-1 microstructures, (b) crystal structure of IRMOF-1 retrieved from crystallographic open database, and (c) electron density potential distribution of IRMOF-1 modeled from VESTA.*

transition responsible for the green emission of IRMOF-1 is similar to that of ZnO [47]. Therefore, the emission observed in IRMOF-1 has been speculated to originate from the ZnO QD not from the ligand. However, Further investigations demonstrated that ZnO impurities in the material gave rise to the emission assigned to the quantum dot like luminescence and that pure MOF-5 displays a luminescence behavior that is more closely relevant to that of the ligand. [9] However, the exact nature of the luminescence of MOF-5 is still under dispute with ligand− ligand charge transfer, [10] ligand-centered, [9] and ligand–metal charge transfer [11] mechanisms as primary suggestions.

In an on-going study, our group has been investigating optoelectronic behavior of IRMOF-1. As depicted in **Figure 5**, UV–vis absorption spectrum shows absorption vibrionic features similar to the linker with two absorptions peaks at 208 nm and 240 nm along with a shoulder peak at 285 nm. The emission spectrum collected by exciting at 240 nm exhibits linker-based emission with three well-resolved vibrionic transitions at 328 nm, 364 nm, and 377 nm. We observed a small high energy shoulder peak at 464 nm, which corresponds to an excitonic transition of Zn4O nodes. However, we have no observed a longer wavelength emission peak at 525 nm, which has claimed in prior studies to energy harvesting and LMCT from 1,4-benzenedicarboxylate (BDC) linked to the Zn4O cluster. Therefore, our findings support that IRMOF-1's luminescence comes from linker emission rather than the charge transfer processes. This further excludes the emission originating from the ZnO quantum dots like clusters of Zn4O. The optical band gap calculated from the UV–visible spectrum on-set is found to be 3.97 eV. There are no experimental band gaps reported for IRMOF-1 up to date.

#### **Figure 5.**

*Photophysical properties of IRMOF-1 – (a) UV–visible spectrum and (b) photoluminescence spectrum in solution (ethanol).*

**151**

**Figure 6.**

*(d) its space filling view.*

*Coordination Polymer Frameworks for Next Generation Optoelectronic Devices*

Database (COD) and generated using VESTA are depicted in **Figure 6**.

pattern (**Figure 7(c)** and **(d)**), benefiting for optoelectronic behavior.

*Crystal structures of: (a) non-interpenetrated IRMOF-8 and (b) its space filling view, (c) INT-IRMOF-8 and* 

Recently, our group has introduced a modified solvothermal synthesis method, which involves a solvent polarity driven self-assembly process to make hierarchical microstructures of INT- IRMOF-8, exhibiting promising optoelectronic properties for the first time [23]. Instead using zinc nitrate as the metal precursor, the synthesis we developed utilizes zinc(II)acetate as the metal precursor. Hierarchical microstructures of INT-IRMOF-8 nanocrystals can be prepared in high yield in the presence of minimum volume of dimethyl formamide by mixing zinc(II) precursor with naphthalene-2,6-dicarboxylic acid at room temperature followed by subjecting to solvothermal annealing at 260°C for 7 minutes [23]. Microstructures visualized under TEM (**Figure 7(b)**) reveal that they are hierarchical layers of self-assembled nanocrystals with randomly arranged voids among the nanocrystals. The wide-angel X-ray scattering (WAXS) pattern along with the selective area electron diffraction (SAED) pattern have shown that the microstructures are made from self-assembled nanocrystals of INT-IRMOF-8, which exhibits lamella packing

Significant research efforts have demonstrated successful synthesis of a variety of isoreticular MOFs (IRMOFs) with the formula of Zn4O(L)3 (where L is a rigid linear dicarboxylates) using traditional solvothermal method, which uses zinc nitrate as metal precursor and the respective organic ligands in an amide-based solvent system. These IRMOFs follow the same cubic topology as the prototypical MOF-5, a framework with octahedral Zn4O(CO2)6 clusters, which are linked along orthogonal axes by phenylene rings [3, 26, 48, 49]. This family of IRMOFs-n (n = 1–16) gained significant attention in gas storage community due to its high pore volume and surface area. Among the IRMOFs series, IRMOF-1 and 8 have been extensively studied for gas adsorption and photoluminescence properties [39, 50, 51] but have not explored their optoelectronic properties until recently. IRMOF-8 is constructed from the linkage of basic zinc acetate clusters and naphthalene-2,6-dicarboxylic acid units (NDC). Originally reported IRMOF-8 with non-interpenetrated cubic crystal lattice has only been extensively studied for gas sorption and storage applications [50, 51]. Later, a number of interpenetrated phases of Zn4O(ndc)3-based systems have been discovered [52–54]. Although the synthesis of interpenetrated IRMOF-8 (INT-IRMOF-8) are similar to that of IRMOF-8, the possibility that typical solvothermally synthesized IRMOF-8 contains at least a significant amount of an interpenetrated phase. There are modified synthesis methods have been introduced to make fully non-interpenetrated IRMOF-8 [55] and INT-IRMOF-8 [23, 55]. The crystal structures of non-interpenetrated IRMOF-8 and INT-IRMOF-8 along with their space filling structures, acquired from the Crystallographic Open

**3.3 Synthesis and optoelectronic properties of IRMOF-8**

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

### **3.3 Synthesis and optoelectronic properties of IRMOF-8**

*Optoelectronics*

**Figure 4.**

*VESTA.*

mechanisms as primary suggestions.

band gaps reported for IRMOF-1 up to date.

transition responsible for the green emission of IRMOF-1 is similar to that of ZnO [47]. Therefore, the emission observed in IRMOF-1 has been speculated to originate from the ZnO QD not from the ligand. However, Further investigations demonstrated that ZnO impurities in the material gave rise to the emission assigned to the quantum dot like luminescence and that pure MOF-5 displays a luminescence behavior that is more closely relevant to that of the ligand. [9] However, the exact nature of the luminescence of MOF-5 is still under dispute with ligand− ligand charge transfer, [10] ligand-centered, [9] and ligand–metal charge transfer [11]

*(a) A SEM image of IRMOF-1 microstructures, (b) crystal structure of IRMOF-1 retrieved from crystallographic open database, and (c) electron density potential distribution of IRMOF-1 modeled from* 

In an on-going study, our group has been investigating optoelectronic behavior of IRMOF-1. As depicted in **Figure 5**, UV–vis absorption spectrum shows absorption vibrionic features similar to the linker with two absorptions peaks at 208 nm and 240 nm along with a shoulder peak at 285 nm. The emission spectrum collected by exciting at 240 nm exhibits linker-based emission with three well-resolved vibrionic transitions at 328 nm, 364 nm, and 377 nm. We observed a small high energy shoulder peak at 464 nm, which corresponds to an excitonic transition of Zn4O nodes. However, we have no observed a longer wavelength emission peak at 525 nm, which has claimed in prior studies to energy harvesting and LMCT from 1,4-benzenedicarboxylate (BDC) linked to the Zn4O cluster. Therefore, our findings support that IRMOF-1's luminescence comes from linker emission rather than the charge transfer processes. This further excludes the emission originating from the ZnO quantum dots like clusters of Zn4O. The optical band gap calculated from the UV–visible spectrum on-set is found to be 3.97 eV. There are no experimental

*Photophysical properties of IRMOF-1 – (a) UV–visible spectrum and (b) photoluminescence spectrum in* 

**150**

**Figure 5.**

*solution (ethanol).*

Significant research efforts have demonstrated successful synthesis of a variety of isoreticular MOFs (IRMOFs) with the formula of Zn4O(L)3 (where L is a rigid linear dicarboxylates) using traditional solvothermal method, which uses zinc nitrate as metal precursor and the respective organic ligands in an amide-based solvent system. These IRMOFs follow the same cubic topology as the prototypical MOF-5, a framework with octahedral Zn4O(CO2)6 clusters, which are linked along orthogonal axes by phenylene rings [3, 26, 48, 49]. This family of IRMOFs-n (n = 1–16) gained significant attention in gas storage community due to its high pore volume and surface area. Among the IRMOFs series, IRMOF-1 and 8 have been extensively studied for gas adsorption and photoluminescence properties [39, 50, 51] but have not explored their optoelectronic properties until recently.

IRMOF-8 is constructed from the linkage of basic zinc acetate clusters and naphthalene-2,6-dicarboxylic acid units (NDC). Originally reported IRMOF-8 with non-interpenetrated cubic crystal lattice has only been extensively studied for gas sorption and storage applications [50, 51]. Later, a number of interpenetrated phases of Zn4O(ndc)3-based systems have been discovered [52–54]. Although the synthesis of interpenetrated IRMOF-8 (INT-IRMOF-8) are similar to that of IRMOF-8, the possibility that typical solvothermally synthesized IRMOF-8 contains at least a significant amount of an interpenetrated phase. There are modified synthesis methods have been introduced to make fully non-interpenetrated IRMOF-8 [55] and INT-IRMOF-8 [23, 55]. The crystal structures of non-interpenetrated IRMOF-8 and INT-IRMOF-8 along with their space filling structures, acquired from the Crystallographic Open Database (COD) and generated using VESTA are depicted in **Figure 6**.

Recently, our group has introduced a modified solvothermal synthesis method, which involves a solvent polarity driven self-assembly process to make hierarchical microstructures of INT- IRMOF-8, exhibiting promising optoelectronic properties for the first time [23]. Instead using zinc nitrate as the metal precursor, the synthesis we developed utilizes zinc(II)acetate as the metal precursor. Hierarchical microstructures of INT-IRMOF-8 nanocrystals can be prepared in high yield in the presence of minimum volume of dimethyl formamide by mixing zinc(II) precursor with naphthalene-2,6-dicarboxylic acid at room temperature followed by subjecting to solvothermal annealing at 260°C for 7 minutes [23]. Microstructures visualized under TEM (**Figure 7(b)**) reveal that they are hierarchical layers of self-assembled nanocrystals with randomly arranged voids among the nanocrystals. The wide-angel X-ray scattering (WAXS) pattern along with the selective area electron diffraction (SAED) pattern have shown that the microstructures are made from self-assembled nanocrystals of INT-IRMOF-8, which exhibits lamella packing pattern (**Figure 7(c)** and **(d)**), benefiting for optoelectronic behavior.

#### **Figure 6.**

*Crystal structures of: (a) non-interpenetrated IRMOF-8 and (b) its space filling view, (c) INT-IRMOF-8 and (d) its space filling view.*

**Figure 7.**

*(a) Interpenetrated view of INT-IRMOF-8's crystal structure; (b) a TEM image of a microstructure; (c) the SAED pattern of a microstructure taken from the TEM under dark field diffraction mode along with (d) a TEM image of the respective microstructure. [Figure 7 is re-created from the original data].*

The photophysical properties of INT-IRMOF-8 exhibit mainly linker based optical properties. The presence of high intensity absorption peak set from 220 nm to 360 nm, which corresponds to vibronic π-π\* absorption transitions of naphthalene core, evidencing the linker-based absorption, resulting from the lack of favorable spatial and energetic overlap of the metal and the ligand orbitals [21, 49]. Typically, MOFs' photoluminescence behavior arises as a result of different types of charge transfer processes, which include metal-to-ligand charge transfer (MLCT), ligandto-metal charge transfer (LMCT), ligand–ligand charge transfer (LLCT), ligandcentered luminescence, and metal-to-metal charge transfer (MMCT) processes [56]. However, this metal-centered luminescence depends on the metal type, ligand type, and their spatial orientations. The emission spectrum of INT-IRMOF-8 microstructures exhibits three emission bands, in which vibrionic transitions corresponds to only linker-based emission with no indication of additional emissions due to any charge transfer processes. The optical band gap reported for INT-IRMOF-8 is 2.82 eV [23] and the theoretical band gap reported in the past for non-interpenetrated IRMOF-8 was ranged from 2.83 eV to 3.27 eV [57]. There are no experimental band gaps reported for IRMOF-8 up to date.

The charge transfer ability of IRMOF-8 for the first time is evaluated by our group. The average electrical conductivity of INT-IRMOF-8 microstructures was ranged from 3.98 x 10−2 to 2.16 x 10−2 S cm−1, which is higher than the electrical conductivities reported for most MOFs (<10−10 S/cm). The interpenetrated structure, high crystallinity, and narrow band gap contribute to the to the electrical conductivity of hierarchical structures of INT-IRMOF-8 nanocrystals.

#### **3.4 Synthesis and optoelectronic properties of IRMOF-10**

Among the series of IRMOFs (IRMOF 1–16) introduced by Yaghi and coworkers [58–60], several IRMOFs have shown effective selective preconcentration properties, including IRMOF-10 [61–64].

Compared to IRMOF-1, physicochemical, optical, and electronic properties of IRMOF-10 with its 4,4′-biphenyldicarboxylate linkers has received much less attention. IRMOF-10 was first synthesized by Yaghi and coworkers [50, 58–60], Owing to its higher surface area and larger pore sizes, use of IRMOF-10 for gas absorption and separation and hydrogen storage have been widely investigated, but scarce attention has been paid to other properties of IRMOF-10, such as structural stability, optical and electrical properties, electronic structure, and chemical bonding. The first publication about biphenyl MOFs already anticipated the major challenges related to Zn-biphenyl MOFs: the growth of single-crystals and interpenetration. A structure from single-crystal XRD of non-functionalized IRMOF-10 is not yet available. A single-crystal X-ray structure analysis of a non-interpenetrated

**153**

**Figure 8.**

*VESTA.*

*Coordination Polymer Frameworks for Next Generation Optoelectronic Devices*

IRMOF-10 derivative was not reported until the breakthrough of the group of Telfer, which showed how interpenetration can be effectively suppressed by using thermolabile protecting groups in the synthesis of amino-MOFs [65]. Following the modified solvothermal synthesis method introduced by Rathnayake et al., microstructures of non-interpenetrated IRMOF-10 was successfully synthesized, and crystal structure was confirmed by matching the powder XRD traces with the simulated XRD pattern. The microstructures morphology is depicted in **Figure 8(a)** and crystal structure retrieved from the Crystallographic Open Database is depicted in **Figure 8(b)**. IRMOF-10's single crystal structure reveals three-dimensional coordination framework, formed by periodic arrangement of Zn(II) atoms, which is tetrahedrally coordinated by four oxygen atoms from four biphenyl linker units,

following the unit formula of Zn4O(L)3 with cubic topology as IRMOF-1.

The electron potential density localization surrounding the metal oxide nodes and organic linker units in IRMOF-10's unit cell reveals the electron density distribution with respect to the biphenyl conjugation length. As shown in **Figure 8(c)**, the electron potential is delocalized within metal oxide nodes and bi-phenyl units, and partial distribution of charges has increased around bi-phenyl units compared to naphthalene units of IRMOF-8. Thus, the findings suggest that linker length has more pronounced effect on the individual material's electronic band structure and density of state, providing clear visualization on the localization of electronic potential within the crystal lattice. The delocalization of electron density potential through biphenyl linkers evidences its potential to be used as optoelectronic materials. Thus, exploring its electronic structure, band gap, optical, and electrical properties is a major interest to the materials science community. However, despite computational investigations on theoretical prediction of optoelectronic properties, [66] there are no experimental investigations on IRMOF-10's optoelectronic

The equilibrium solid-state structure, electronic structure, formation enthalpy, chemical bonding, and optical properties of IRMOF-10 have investigated with density functional calculations. Electronic density of states and band structures study have shown that the band gap for the IRMOF-10 is ranged from 2.9 eV to 3.0 eV, resulting in a nonmetallic character [66]. Until now, there are no experimental studies available to verify theoretical predictions on IRMOF-10's electronic structure. The calculated optical properties of IRMOF-10 provide useful information for future experimental exploration. The optical properties (dielectric function, refractive index, absorption coefficient, optical conductivity s(v), reflectivity, and electron energy-loss spectrum of the IRMOF-10 have computed in the past, [66]

but there is no experimental investigation conducted up to date.

*(a) A SEM image of IRMOF-10 microstructures, (b) crystal structure of IRMOF-10 retrieved from crystallographic open database, and (c) electron density potential distribution of IRMOF-10 modeled from* 

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

behavior has been conducted up to date.

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

IRMOF-10 derivative was not reported until the breakthrough of the group of Telfer, which showed how interpenetration can be effectively suppressed by using thermolabile protecting groups in the synthesis of amino-MOFs [65]. Following the modified solvothermal synthesis method introduced by Rathnayake et al., microstructures of non-interpenetrated IRMOF-10 was successfully synthesized, and crystal structure was confirmed by matching the powder XRD traces with the simulated XRD pattern. The microstructures morphology is depicted in **Figure 8(a)** and crystal structure retrieved from the Crystallographic Open Database is depicted in **Figure 8(b)**. IRMOF-10's single crystal structure reveals three-dimensional coordination framework, formed by periodic arrangement of Zn(II) atoms, which is tetrahedrally coordinated by four oxygen atoms from four biphenyl linker units, following the unit formula of Zn4O(L)3 with cubic topology as IRMOF-1.

The electron potential density localization surrounding the metal oxide nodes and organic linker units in IRMOF-10's unit cell reveals the electron density distribution with respect to the biphenyl conjugation length. As shown in **Figure 8(c)**, the electron potential is delocalized within metal oxide nodes and bi-phenyl units, and partial distribution of charges has increased around bi-phenyl units compared to naphthalene units of IRMOF-8. Thus, the findings suggest that linker length has more pronounced effect on the individual material's electronic band structure and density of state, providing clear visualization on the localization of electronic potential within the crystal lattice. The delocalization of electron density potential through biphenyl linkers evidences its potential to be used as optoelectronic materials. Thus, exploring its electronic structure, band gap, optical, and electrical properties is a major interest to the materials science community. However, despite computational investigations on theoretical prediction of optoelectronic properties, [66] there are no experimental investigations on IRMOF-10's optoelectronic behavior has been conducted up to date.

The equilibrium solid-state structure, electronic structure, formation enthalpy, chemical bonding, and optical properties of IRMOF-10 have investigated with density functional calculations. Electronic density of states and band structures study have shown that the band gap for the IRMOF-10 is ranged from 2.9 eV to 3.0 eV, resulting in a nonmetallic character [66]. Until now, there are no experimental studies available to verify theoretical predictions on IRMOF-10's electronic structure. The calculated optical properties of IRMOF-10 provide useful information for future experimental exploration. The optical properties (dielectric function, refractive index, absorption coefficient, optical conductivity s(v), reflectivity, and electron energy-loss spectrum of the IRMOF-10 have computed in the past, [66] but there is no experimental investigation conducted up to date.

#### **Figure 8.**

*Optoelectronics*

**Figure 7.**

The photophysical properties of INT-IRMOF-8 exhibit mainly linker based optical properties. The presence of high intensity absorption peak set from 220 nm to 360 nm, which corresponds to vibronic π-π\* absorption transitions of naphthalene core, evidencing the linker-based absorption, resulting from the lack of favorable spatial and energetic overlap of the metal and the ligand orbitals [21, 49]. Typically, MOFs' photoluminescence behavior arises as a result of different types of charge transfer processes, which include metal-to-ligand charge transfer (MLCT), ligandto-metal charge transfer (LMCT), ligand–ligand charge transfer (LLCT), ligandcentered luminescence, and metal-to-metal charge transfer (MMCT) processes [56]. However, this metal-centered luminescence depends on the metal type, ligand type, and their spatial orientations. The emission spectrum of INT-IRMOF-8 microstructures exhibits three emission bands, in which vibrionic transitions corresponds to only linker-based emission with no indication of additional emissions due to any charge transfer processes. The optical band gap reported for INT-IRMOF-8 is 2.82 eV [23] and the theoretical band gap reported in the past for non-interpenetrated IRMOF-8 was ranged from 2.83 eV to 3.27 eV [57]. There are no experimental

*(a) Interpenetrated view of INT-IRMOF-8's crystal structure; (b) a TEM image of a microstructure; (c) the SAED pattern of a microstructure taken from the TEM under dark field diffraction mode along with (d) a* 

*TEM image of the respective microstructure. [Figure 7 is re-created from the original data].*

The charge transfer ability of IRMOF-8 for the first time is evaluated by our group. The average electrical conductivity of INT-IRMOF-8 microstructures was ranged from 3.98 x 10−2 to 2.16 x 10−2 S cm−1, which is higher than the electrical conductivities reported for most MOFs (<10−10 S/cm). The interpenetrated structure, high crystallinity, and narrow band gap contribute to the to the electrical

Among the series of IRMOFs (IRMOF 1–16) introduced by Yaghi and coworkers [58–60], several IRMOFs have shown effective selective preconcentration proper-

Compared to IRMOF-1, physicochemical, optical, and electronic properties of IRMOF-10 with its 4,4′-biphenyldicarboxylate linkers has received much less attention. IRMOF-10 was first synthesized by Yaghi and coworkers [50, 58–60], Owing to its higher surface area and larger pore sizes, use of IRMOF-10 for gas absorption and separation and hydrogen storage have been widely investigated, but scarce attention has been paid to other properties of IRMOF-10, such as structural stability, optical and electrical properties, electronic structure, and chemical bonding. The first publication about biphenyl MOFs already anticipated the major challenges related to Zn-biphenyl MOFs: the growth of single-crystals and interpenetration. A structure from single-crystal XRD of non-functionalized IRMOF-10 is not yet available. A single-crystal X-ray structure analysis of a non-interpenetrated

conductivity of hierarchical structures of INT-IRMOF-8 nanocrystals.

**3.4 Synthesis and optoelectronic properties of IRMOF-10**

band gaps reported for IRMOF-8 up to date.

ties, including IRMOF-10 [61–64].

**152**

*(a) A SEM image of IRMOF-10 microstructures, (b) crystal structure of IRMOF-10 retrieved from crystallographic open database, and (c) electron density potential distribution of IRMOF-10 modeled from VESTA.*

**Figure 9.**

*Photophysical properties of IRMOF-10 – (a) UV–visible spectrum and (b) photoluminescence spectrum in solution (ethanol).*

Recently, our group has studied optoelectronic behavior of non-interpenetrated IRMOF-10. As shown in **Figure 9**, we explored the photophysical properties of noninterpenetrated IRMOF-10 and calculated its optical band gap. IRMOF-10 exhibits linker-based absorption with absorption maximum at 282 nm along with a shoulder peak at 222 nm. IRMOF-10 shown blue luminescence with broader emission ranged from 310 nm to 450 nm along with the emission maximum at 353 nm. The optical band gap calculated from the UV–visible spectrum on-set is 3.80 eV, which is narrower than the optical band gap of IRMOF-1 and larger than the theoretical band gap predicted from computational analysis.

In summary, the conjugation length of the organic linker in IRMOFs contributes to the semiconducting properties rather than the periodic pattern or the distances between the Zn4O clusters. The conjugation length of organic linkers of IRMOF-1, 8, and 10 described here differs from one aromatic unit (benzene) to one and half aromatic unit (naphthalene) to two aromatic units (biphenyl). The resonance effect arises due to the conjugation speaks directly to the photophysical behavior and optical band structure characteristics, reflecting a clear trend in narrowing the band gap with gradual increase in the conjugation length of the ligand. The dramatic change in the optical band gap upon changing the organic linker in the MOF structure has also been reported in the past [66]. Thus, these studies evidences that the semiconducting properties of MOFs strongly depends on the resonance effects from the organic linker [67].

#### **4. Future prospective**

With a growing demand for continuous miniaturization and functional scaling, the complementary metal-oxide semiconductor (CMOS) platform continues to drive advances in integrated circuits (IC), nanoelectronics and information processing technologies. While it is now possible to produce an amazing array of nanoscale materials and morphologies, the assembly and integration of these nanostructures into ordered arrays, with other materials, remain key challenges. Moore's Second Law projects a need for new, high throughput fabrication approaches that can produce useful and defect free nanostructures for future silicon-based CMOS related technologies. Recent advances in nanomaterial synthesis enable new families of emerging research materials (ERMs) that show potential for extending and augmenting existing CMOS technology, with respect to wafer level manufacturability, uniformity, reliability, performance and cost, and they warrant additional

**155**

the properties of an ideal template.

*Coordination Polymer Frameworks for Next Generation Optoelectronic Devices*

research focus and verification. The integration of More-than-Moore, application specific, materials and structures on a CMOS platform leverages the best of both technologies, though this added complexity also challenges the extensibility of conventional fabrication and patterning methods. Consequently, there remains a need for simple fabrication methods that can create two- and three-dimensional ordered functional nanostructures, which can adapt to a wide variety of materials,

Highly crystalline microstructures of MOFs have been paving the path, addressing the current challenges in fabrication needs that create two- and three-dimensional ordered structures and which are adaptable to a wide variety of materials specific applications. These nanoscale building blocks, and their assemblies combine the flexibility, conductivity, transparency, and ease of processability of soft matter (organic) with electrical, thermal, and mechanical properties of hard matter (inorganic). They offer a new window for fine-tuning structural nodes with known geometries and coordination environments. With respect to the fabrication of ordered nanoscale structures, MOFs have several advantages. First, since they are themselves a highly ordered self-assembled nanostructure, as a result of their crystallinity, their pore dimensions are

completely defined, making knowledge of atomic positions possible. Second, the nanoporosity of their structure results from geometric factors associated with the bonding between their inorganic and organic components, enabling rational template design [68]. Third, unlike the conventional template materials, MOFs possess a high degree of synthetic flexibility with potentially widely tunable electrical, optical, and mechanical properties. Surely, the development of simple, versatile low-cost methodologies for the design, production, and nanoscale manipulation of innovative functional organic–inorganic hybrid building blocks will provide a powerful new capability for designing, integrating, and patterning new nanoscale materials with tunable properties onto a CMOS platform. Recent milestones of MOFs in photovoltaic, optical and chemical sensing, and field effect transistors highlight the potential of these materials for future electronic devices, heterogeneous platforms, non-traditional patterning opportunities [16, 69–71]. Interest in using these materials in fields such as gas storage [72], separations [73], [sensing [21], and catalysis [74] is rapidly accelerating. The advantages of MOFs for above applications are promising due to the wide range of possibilities of the rational design inherited in these materials. Thus, superior properties and new understanding with respect to the interaction of small molecules with nanoporous materials are being achieved. Although most MOFs are found to be dielectrics, a few semiconducting frameworks are known [23, 37, 75, 76]. The theoretical predictions conduced up to date on variety of MOFs suggest that there are possible MOFs with semiconducting properties [77–79] . MOFs that are magnetic [80], ferroelectric [81, 82], proton-conducting [83–86], and luminescent [87, 88] are also known. Additionally, their porosity creates the potential to introduce non-native functionality to a given structure by infusing the accessible volume with a second molecule or material. Moreover, because the chemical environment within the pore can be modified, it is possible to tailor the interface between the MOF and a templated material to stabilize specific materials or nanostructures. Consequently, MOFs and the coordination polymers of crystalline nanoporous frameworks possess many of

Despite the endless possibilities for how MOFs could be used for device applications, when using MOFs for semiconductor microelectronic devices such as sensors, field-effect transistors, light harvesting and absorbing, light-emitting diodes, thermoelectric devices, energy storages and lithium ion batteries, and scintillators, it is necessary to understand how these materials function within the device and how

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

patterning, and application needs.

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

research focus and verification. The integration of More-than-Moore, application specific, materials and structures on a CMOS platform leverages the best of both technologies, though this added complexity also challenges the extensibility of conventional fabrication and patterning methods. Consequently, there remains a need for simple fabrication methods that can create two- and three-dimensional ordered functional nanostructures, which can adapt to a wide variety of materials, patterning, and application needs.

Highly crystalline microstructures of MOFs have been paving the path, addressing the current challenges in fabrication needs that create two- and three-dimensional ordered structures and which are adaptable to a wide variety of materials specific applications. These nanoscale building blocks, and their assemblies combine the flexibility, conductivity, transparency, and ease of processability of soft matter (organic) with electrical, thermal, and mechanical properties of hard matter (inorganic). They offer a new window for fine-tuning structural nodes with known geometries and coordination environments. With respect to the fabrication of ordered nanoscale structures, MOFs have several advantages. First, since they are themselves a highly ordered self-assembled nanostructure, as a result of their crystallinity, their pore dimensions are completely defined, making knowledge of atomic positions possible. Second, the nanoporosity of their structure results from geometric factors associated with the bonding between their inorganic and organic components, enabling rational template design [68]. Third, unlike the conventional template materials, MOFs possess a high degree of synthetic flexibility with potentially widely tunable electrical, optical, and mechanical properties. Surely, the development of simple, versatile low-cost methodologies for the design, production, and nanoscale manipulation of innovative functional organic–inorganic hybrid building blocks will provide a powerful new capability for designing, integrating, and patterning new nanoscale materials with tunable properties onto a CMOS platform. Recent milestones of MOFs in photovoltaic, optical and chemical sensing, and field effect transistors highlight the potential of these materials for future electronic devices, heterogeneous platforms, non-traditional patterning opportunities [16, 69–71].

Interest in using these materials in fields such as gas storage [72], separations [73], [sensing [21], and catalysis [74] is rapidly accelerating. The advantages of MOFs for above applications are promising due to the wide range of possibilities of the rational design inherited in these materials. Thus, superior properties and new understanding with respect to the interaction of small molecules with nanoporous materials are being achieved. Although most MOFs are found to be dielectrics, a few semiconducting frameworks are known [23, 37, 75, 76]. The theoretical predictions conduced up to date on variety of MOFs suggest that there are possible MOFs with semiconducting properties [77–79] . MOFs that are magnetic [80], ferroelectric [81, 82], proton-conducting [83–86], and luminescent [87, 88] are also known. Additionally, their porosity creates the potential to introduce non-native functionality to a given structure by infusing the accessible volume with a second molecule or material. Moreover, because the chemical environment within the pore can be modified, it is possible to tailor the interface between the MOF and a templated material to stabilize specific materials or nanostructures. Consequently, MOFs and the coordination polymers of crystalline nanoporous frameworks possess many of the properties of an ideal template.

Despite the endless possibilities for how MOFs could be used for device applications, when using MOFs for semiconductor microelectronic devices such as sensors, field-effect transistors, light harvesting and absorbing, light-emitting diodes, thermoelectric devices, energy storages and lithium ion batteries, and scintillators, it is necessary to understand how these materials function within the device and how

*Optoelectronics*

**Figure 9.**

*solution (ethanol).*

Recently, our group has studied optoelectronic behavior of non-interpenetrated IRMOF-10. As shown in **Figure 9**, we explored the photophysical properties of noninterpenetrated IRMOF-10 and calculated its optical band gap. IRMOF-10 exhibits linker-based absorption with absorption maximum at 282 nm along with a shoulder peak at 222 nm. IRMOF-10 shown blue luminescence with broader emission ranged from 310 nm to 450 nm along with the emission maximum at 353 nm. The optical band gap calculated from the UV–visible spectrum on-set is 3.80 eV, which is narrower than the optical band gap of IRMOF-1 and larger than the theoretical band

*Photophysical properties of IRMOF-10 – (a) UV–visible spectrum and (b) photoluminescence spectrum in* 

In summary, the conjugation length of the organic linker in IRMOFs contributes to the semiconducting properties rather than the periodic pattern or the distances between the Zn4O clusters. The conjugation length of organic linkers of IRMOF-1, 8, and 10 described here differs from one aromatic unit (benzene) to one and half aromatic unit (naphthalene) to two aromatic units (biphenyl). The resonance effect arises due to the conjugation speaks directly to the photophysical behavior and optical band structure characteristics, reflecting a clear trend in narrowing the band gap with gradual increase in the conjugation length of the ligand. The dramatic change in the optical band gap upon changing the organic linker in the MOF structure has also been reported in the past [66]. Thus, these studies evidences that the semiconducting properties of MOFs strongly depends on the resonance effects

With a growing demand for continuous miniaturization and functional scaling, the complementary metal-oxide semiconductor (CMOS) platform continues to drive advances in integrated circuits (IC), nanoelectronics and information processing technologies. While it is now possible to produce an amazing array of nanoscale materials and morphologies, the assembly and integration of these nanostructures into ordered arrays, with other materials, remain key challenges. Moore's Second Law projects a need for new, high throughput fabrication approaches that can produce useful and defect free nanostructures for future silicon-based CMOS related technologies. Recent advances in nanomaterial synthesis enable new families of emerging research materials (ERMs) that show potential for extending and augmenting existing CMOS technology, with respect to wafer level manufacturability, uniformity, reliability, performance and cost, and they warrant additional

gap predicted from computational analysis.

from the organic linker [67].

**4. Future prospective**

**154**

they will interface with other functional and structural elements. Therefore, this section focuses on providing a future prospective for advances that must be made for their realization in electronic devices. A possible MOF-device roadmap, which identifies MOFs applications in electronic devices along with machine learning for new MOFs developments and MOFs database screening for novel properties is depicted in **Figure 10**. Our intention of providing this roadmap is to stimulate future endeavors of MOFs roadmap for electronic industry by translating current MOFs basic research agenda into applied research in the future. The roadmap that we identified here is created by combining the prospective previously provided by Allendorf et al., focusing five major fields pertinent to device fabrication [89]. These previously proposed areas include: (1) Fundamental Properties, (2) Thin film growth and processing, (3) MOFs hybrid and multilevel structures, (4) Device integration, and (5) Manufacturing issues. Our prospective for the proposed potential MOF-device roadmap particularly concentrated on member-specific applications in electronic industry, where functional density of MOFs can be utilized in subcategories of a wide variety of electronic devices. As the MOF-based optoelectronic field is fairly new and fall within the basic research stage, our roadmap is structured based on MOFs relative progress made so far and build upon the future road map comparing with the progress made in the field of organic electronics.

Exploring electronic properties, such as electronic structure, band gap, conductivity, electron, and hole mobilities, and dielectric constants of MOFs need to be one of the priority areas in the next decades and must be understood. Additionally, understanding lattice defects and their relationship to electronic properties must be explored combining theoretical and experimental approaches as they likely will limit the ultimate performance of a device. The field-effect transistor (FETs), which is the basic device building block for modern electronics, dictates the materials properties relevant to electronic applications. The FET performance is determined by the carrier mobility, source and drain contact resistance, and the capacitance of the gate electrode. Si is the preeminent materials for FET fabrication because of its bandgap of 1.1 eV, high carrier mobility, availability of multiple n- and p-type dopants, environmental stability, stable oxide, and high terrestrial abundance. However, Si based device fabrication requires enormous capital investment and Si is not compatible with a variety of low cost, flexible, transparent, and low melting

**157**

*Coordination Polymer Frameworks for Next Generation Optoelectronic Devices*

temperature substrates. For these reasons, alternative materials including polymers, organic molecules, and more recently nanotubes and nanowires have been gaining a lot of attention for various emerging applications. The long-range crystalline order of MOFs implies that charge transport through delocalized conduction and valence bands typical of crystalline inorganic semiconductors is possible. Emergence of delocalized bands in MOFs will require that the π orbitals in the linker groups overlap effectively with the metal d orbitals. Such overlap is absent in the majority of synthetically known MOFs where carboxylate oxygen atoms are coordinated to the metal center through σ bonds. Therefore, most MOFs are electrical insulators. This barrier needs to be overcome in next decades, perhaps by synthesizing novel MOFs using higher order conjugated linkers and increasing the functional density of the MOFs. Modifying the linker structure could lead to better charge transfer between linker and the metal cations of the framework. One possible route has suggested replacing the carboxylate terminating linkers with isocyanide groups [89]. It has been shown that, Prussian Blue, a mixed valence crystalline compound with Fe(II) and Fe(III) ions coordinated with isocyanide ligands, is electrically conducting [90]. Another approach, suggested by Allendorf et al., is to introduce conducting phases into the MOF channels [89]. Some other approaches have been taken place to enhance electronic transport properties of MOFs by introducing other conductive nanomaterials, inorganic oxides, polymers, and carbon nanotubes into MOFs

Atomic level fundamental understanding that cannot be obtained readily from experimental methods, is necessary to address MOFs electronic band structure, density of state, band gap, and electron and hole mobilities. There has been increasing accuracy of predictive results using molecular dynamics (MD) force fields (FF) and DFT approximations for various MOFs' property studies [94–100]. Density Functional Theory (DFT) methods using periodic boundary conditions have been popular for predicting the electronic structure of MOFs [57, 67, 101, 102]. However, DFT-based computational calculations are underestimate excited state energies by about a factor of two. Adapting high-accuracy methods, such as Quantum Monte Carlo (QMC), DFT + U, and GW are not feasible for systems with large numbers of electrons. For example, practical QMC calculations currently could not apply to the systems that exceed 1000 electrons. One formula unit of IRMOF-1 has 760 electrons and 106 atoms. Owing to these limitations in current computational approaches, MOFs are much more challenging than traditional electronic materials with much smaller unit cells. The computational methods for predicting properties of MOFs are at an early stage of development, in particularly for predicting electronic properties of MOFs [57, 101]. Developing simple and rapid analytical approaches are not only a necessary tool to experimental investigations but also can be used as an accelerate investigation and predictive tool by themselves to screen semiconducting MOFs from the database of synthetically known MOFs. Such analytical approaches combined with computational studies will eventually enable the design of machine learning approaches for large-scale screening of existing and hypothetical MOF

MOFs are also showing promise in their use as electrolytes due to their low electronic conductivity, tunable polarity, and high porosity [106]. There are many ways that MOFs have been employed to elevate the downfalls of current electrolytes. For example, they have been using as hosts for liquid electrolyte solutions or ionic liquids [107, 108]. However, the drying of the electrolyte solution within the MOFs presents an issue since the ion transport is mostly achieved by the solvent molecules within the electrolyte rather than by the MOF itself. Furthermore, MOFs is used as a filler to reduce the crystallinity of SPEs [107, 108]. However, up to date MOFs have not been explored to be used as a solid electrolyte excepts in a composite form [109].

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

framework [91–93].

structures for specific applications [103–105].

**Figure 10.**

*A possible MOF-device roadmap for electronic industry proposed by Rathnayake.*

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

temperature substrates. For these reasons, alternative materials including polymers, organic molecules, and more recently nanotubes and nanowires have been gaining a lot of attention for various emerging applications. The long-range crystalline order of MOFs implies that charge transport through delocalized conduction and valence bands typical of crystalline inorganic semiconductors is possible. Emergence of delocalized bands in MOFs will require that the π orbitals in the linker groups overlap effectively with the metal d orbitals. Such overlap is absent in the majority of synthetically known MOFs where carboxylate oxygen atoms are coordinated to the metal center through σ bonds. Therefore, most MOFs are electrical insulators. This barrier needs to be overcome in next decades, perhaps by synthesizing novel MOFs using higher order conjugated linkers and increasing the functional density of the MOFs. Modifying the linker structure could lead to better charge transfer between linker and the metal cations of the framework. One possible route has suggested replacing the carboxylate terminating linkers with isocyanide groups [89]. It has been shown that, Prussian Blue, a mixed valence crystalline compound with Fe(II) and Fe(III) ions coordinated with isocyanide ligands, is electrically conducting [90]. Another approach, suggested by Allendorf et al., is to introduce conducting phases into the MOF channels [89]. Some other approaches have been taken place to enhance electronic transport properties of MOFs by introducing other conductive nanomaterials, inorganic oxides, polymers, and carbon nanotubes into MOFs framework [91–93].

Atomic level fundamental understanding that cannot be obtained readily from experimental methods, is necessary to address MOFs electronic band structure, density of state, band gap, and electron and hole mobilities. There has been increasing accuracy of predictive results using molecular dynamics (MD) force fields (FF) and DFT approximations for various MOFs' property studies [94–100]. Density Functional Theory (DFT) methods using periodic boundary conditions have been popular for predicting the electronic structure of MOFs [57, 67, 101, 102]. However, DFT-based computational calculations are underestimate excited state energies by about a factor of two. Adapting high-accuracy methods, such as Quantum Monte Carlo (QMC), DFT + U, and GW are not feasible for systems with large numbers of electrons. For example, practical QMC calculations currently could not apply to the systems that exceed 1000 electrons. One formula unit of IRMOF-1 has 760 electrons and 106 atoms. Owing to these limitations in current computational approaches, MOFs are much more challenging than traditional electronic materials with much smaller unit cells. The computational methods for predicting properties of MOFs are at an early stage of development, in particularly for predicting electronic properties of MOFs [57, 101]. Developing simple and rapid analytical approaches are not only a necessary tool to experimental investigations but also can be used as an accelerate investigation and predictive tool by themselves to screen semiconducting MOFs from the database of synthetically known MOFs. Such analytical approaches combined with computational studies will eventually enable the design of machine learning approaches for large-scale screening of existing and hypothetical MOF structures for specific applications [103–105].

MOFs are also showing promise in their use as electrolytes due to their low electronic conductivity, tunable polarity, and high porosity [106]. There are many ways that MOFs have been employed to elevate the downfalls of current electrolytes. For example, they have been using as hosts for liquid electrolyte solutions or ionic liquids [107, 108]. However, the drying of the electrolyte solution within the MOFs presents an issue since the ion transport is mostly achieved by the solvent molecules within the electrolyte rather than by the MOF itself. Furthermore, MOFs is used as a filler to reduce the crystallinity of SPEs [107, 108]. However, up to date MOFs have not been explored to be used as a solid electrolyte excepts in a composite form [109].

*Optoelectronics*

they will interface with other functional and structural elements. Therefore, this section focuses on providing a future prospective for advances that must be made for their realization in electronic devices. A possible MOF-device roadmap, which identifies MOFs applications in electronic devices along with machine learning for new MOFs developments and MOFs database screening for novel properties is depicted in **Figure 10**. Our intention of providing this roadmap is to stimulate future endeavors of MOFs roadmap for electronic industry by translating current MOFs basic research agenda into applied research in the future. The roadmap that we identified here is created by combining the prospective previously provided by Allendorf et al., focusing five major fields pertinent to device fabrication [89]. These previously proposed areas include: (1) Fundamental Properties, (2) Thin film growth and processing, (3) MOFs hybrid and multilevel structures, (4) Device integration, and (5) Manufacturing issues. Our prospective for the proposed potential MOF-device roadmap particularly concentrated on member-specific applications in electronic industry, where functional density of MOFs can be utilized in subcategories of a wide variety of electronic devices. As the MOF-based optoelectronic field is fairly new and fall within the basic research stage, our roadmap is structured based on MOFs relative progress made so far and build upon the future road map comparing

Exploring electronic properties, such as electronic structure, band gap, conductivity, electron, and hole mobilities, and dielectric constants of MOFs need to be one of the priority areas in the next decades and must be understood. Additionally, understanding lattice defects and their relationship to electronic properties must be explored combining theoretical and experimental approaches as they likely will limit the ultimate performance of a device. The field-effect transistor (FETs), which is the basic device building block for modern electronics, dictates the materials properties relevant to electronic applications. The FET performance is determined by the carrier mobility, source and drain contact resistance, and the capacitance of the gate electrode. Si is the preeminent materials for FET fabrication because of its bandgap of 1.1 eV, high carrier mobility, availability of multiple n- and p-type dopants, environmental stability, stable oxide, and high terrestrial abundance. However, Si based device fabrication requires enormous capital investment and Si is not compatible with a variety of low cost, flexible, transparent, and low melting

with the progress made in the field of organic electronics.

*A possible MOF-device roadmap for electronic industry proposed by Rathnayake.*

**156**

**Figure 10.**

One way to achieve this is designing lithium-based metal organic frameworks (Li MOF) where excess lithium is transferred through the defects in the MOF structure. However, research regarding Li MOFs as solid electrolytes is currently lacking. The majority of MOF/electrolyte studies are only focused on employing MOFs as a host of ionically conductive materials rather than utilizing MOFs as solid-state electrolytes. Therefore, we identified this area of research in the proposed roadmap to stimulate investigating the potential of Li-based MOFs as solid electrolytes. There are different types of Li-MOFs already developed [110–112], but many of them are designed for applications other than battery electrolytes. We believe that Li- MOF structures can be tuned for lithium transport. Overall, Li MOFs show potential for the use as solid ionic conductors and much research should be performed to explore their possibility for solid state electrolytes and battery components.

Exploring thermoelectric properties of MOFs emerges five years ago along with exploring the electronic properties of MOFs by systematic structural modifications and introducing guest molecules onto MOFs. The first thermoelectric property measurements on MOFs has introduced by Erikson in 2015 [113]. then, up to date, there have been less than ten publications in thermoelectric MOFs, thus this field of research is relatively new. Highly nanoporous MOFs are promising since porosity can reduce the lattice thermal conductivity. The effect the conjugation length of the organic linker that tailors the pore dimension for lattice thermal conductivity must be investigated. The thermoelectric figure of merit that measures the efficiency of a thermoelectric device can be improved by decreasing the lattice thermal conductivity. It is believed that changing the conjugation length or the complexity of the organic linker changes phonon scattering, thereby changing the lattice thermal conductivity [77, 114]. The ligand modifications can be successfully achieved by isoreticular synthesis approaches. Also, increasing the porosity of MOFs increases phonon scattering that also reduces thermal conductivity [114]. Therefore, in order to utilize MOFs as active materials in thermoelectric devices, understanding the contribution of phonon vibrations to lattice thermal conductivity is essential and must be investigated. Directing future research on thermoelectric MOFs towards experimentally investigating thermoelectric properties of MOF based thin films to find ways of decreasing thermal conductivity by structural modifications to the organic ligand is beneficial.

In order to use MOFs as photoactive layer for energy harvesting and conversion, MOFs should possess decent light harvesting capability in the region from visible light to near-infrared (NIR). As the material's light-harvesting window is primarily determined by its band gap, synthesizing a MOF with a semiconducting band gap that can absorb light in the solar spectrum should be a requirement for it to serve as the photoactive material. Given that the electronic configuration of MOFs is contributed by both the constituent metal ion and the organic linker, the resultant bandgap and semiconducting properties of MOFs can thus be tailored by their structural design and engineering. Since most MOFs possess large band gap due to lack of overlap between metal ion and the organic linker and low degree of conjugation, they cannot effectively absorb light in the solar spectrum. The ligand center of MOFs plays a dominant role in its resulting light harvesting behavior [77, 114]. Tailoring the structure and its composition, MOFs charge transfer processes can be improved to enable the photocurrent of MOFs and fulfilling the photoactive functions.

To effectively reduce the band gap of MOFs and enrich their semiconducting properties for photovoltaic applications, three strategies can be implemented and have been identified [115]. These strategies are: (1) selecting electron rich metal nodes and conjugated-based organic molecules, (2) enhancing the conjugation of the organic linker, and (3) functionalizing the organic linker with

**159**

devices.

**Acknowledgements**

**Conflict of interest**

the NSF (Grant ECCS-1542174).

There is no conflict of interest to declare.

*Coordination Polymer Frameworks for Next Generation Optoelectronic Devices*

electron-donating groups, such as hydroxyl, nitro, and amino groups. Additionally, facilitating electron delocalization through guest-mediated p-donor/acceptor stacks can also effectively diminish the band gaps of the materials [115]. Besides narrowing the band gap, electronic structure that contributes the semiconducting properties of MOFs also play a vital role as sufficient dissociation of the photoexcitons generated in the MOFs is required to produce a reasonable photocurrent. In this regard, MOFs exhibit critical barriers to use as the photoactive materials directly and impedes its progress in photovoltaic applications to date. However, up to date, besides acting as the photoactive materials, the MOFs has been contributing to the photovoltaic community by serving as functional additives or interlayers to improve the performance and stability of the derived solar cell devices. In order to utilize MOFs for photoactive layer in photovoltaics, it is necessary to design electrically conductive MOFs. The research efforts developing more functional conducting

Owing to synthetic processability using reticular chemistry, MOFs offer unusual properties paving the path for many opportunities and their use in optoelectronic devices. Their use in devices so far is limited to sensors and gas storage. However, MOFs field is moving towards exploring their optical and electrical properties to use in electronic devices. There are many MOFs with tunable bandgap, both ultralow-k and high-k dielectric constants, varied magnetic properties, luminescence, and a few with semiconducting behavior, suggesting MOFs as emerging material with unique properties exceeding any other class of materials. Combining the solvothermal synthesis method with self-assembly processes, we can achieve highly ordered nanoporous structures with precise dimensionality that creates the potential for electronics and self-assembly with atomic-scale resolution and precision. In order to become MOFs for electronic devices, many challenges must be solved, and electronic structures of MOFs should be revealed. The MOFs-device

roadmap should be one meaningful way to reach MOFs milestones for optoelectronic devices and will enable MOFs to be performed in their best, as well as allowing the necessary integration with other materials to fabricate fully functional

Authors acknowledge the Joint School of Nanoscience and Nanoengineering, a member of the South-eastern Nanotechnology Infrastructure Corridor (SENIC) and National Nanotechnology Coordinated Infrastructure (NNCI), supported by

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

MOFs are required in the coming decade.

**5. Conclusions**

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

electron-donating groups, such as hydroxyl, nitro, and amino groups. Additionally, facilitating electron delocalization through guest-mediated p-donor/acceptor stacks can also effectively diminish the band gaps of the materials [115]. Besides narrowing the band gap, electronic structure that contributes the semiconducting properties of MOFs also play a vital role as sufficient dissociation of the photoexcitons generated in the MOFs is required to produce a reasonable photocurrent. In this regard, MOFs exhibit critical barriers to use as the photoactive materials directly and impedes its progress in photovoltaic applications to date. However, up to date, besides acting as the photoactive materials, the MOFs has been contributing to the photovoltaic community by serving as functional additives or interlayers to improve the performance and stability of the derived solar cell devices. In order to utilize MOFs for photoactive layer in photovoltaics, it is necessary to design electrically conductive MOFs. The research efforts developing more functional conducting MOFs are required in the coming decade.
