【Origin】

At present, the application research of conductive metal-organic framework (MOF) in charge storage materials, electrocatalysts and chemical resistance sensors has made important progress. However, much of the work has focused on extending existing conductive porous MOFs, but most of these materials exhibit charge transport that occurs preferentially along low-dimensional paths , such as one-dimensional metal-ligand chains, π-π stacking directions, or π- Conjugate net. The rapid development of low-dimensional conductive materials (such as graphene, silicon nanowires) proves that anisotropic electron transport in structures is sometimes necessary , but the development of conductive MOFs with isotropic charge transport properties also has important fundamental significance. However, there is very little research in this field. Only two MOFs with intrinsic (non-guest-based) conductivity have been reported.

So far, it has been reported that MOFs constructed from ligands derived from catechol and semiquinone-derived functional groups (such as hexaaminotriphenylene and hexahydroxytriphenylene (HHTP), etc.) exhibit the highest electronic conductivity. These ligands are particularly suitable for making conductive skeletons due to some characteristics, for example: strong π-d conjugation is formed between metals and ligands, ligands tend to π-π stacking, and the most important thing is their redox activity . The catechol groups can be oxidized to form semiquinone free radicals, which can act as charge carriers in the material. The cubic skeleton [Fe 2 (DHBQ) 3 ] [Bu 4 N] 2 formed by dihydroxyp-benzoquinone (DHBQ) shows a conductivity as high as 0.16 S / cm. Its electronic conductivity can be controlled by a post-synthesis redox method, in which the semiquinone radical can be reduced to the form of catechol to reduce the free electron concentration. But because the charge-compensating tetrabutylammonium cation blocks the pores, the frame is non-porous. Inspired by this, a larger HHTP ligand with similar chemical properties was selected in this paper to synthesize a potentially porous, isotropic MOF to achieve high electronic conductivity.   

[ MOF synthesis]

Fig. 1 M 6 HOTP 2 (M = Y, Eu), crystal structure along (111) crystal plane.

MOF synthesis adopts solvothermal method to react at 150 ° C, and finally obtain yellow octahedral crystal [M 6 (μ 6 -NO 3 ) (HOTP) 2 ] ⁵⁺ (M 6 HOTP 2 , M = Y, Eu; HOTP = 2,3,6,7,10,11-hexaoxytriphenylene). It is balanced by five hydroxides or nitrates, which indicates that the ligand is completely reduced by HOTP ^6- . Single crystal X-ray diffraction analysis showed that all three compounds crystallized in the cubic space group Fd3-m. The second-level building unit (SBU) consists of six-core μ 6 -catechol nitrate clusters (Figure 2b), where metal ions are alternately arranged above and below the central nitrate ion plane. Each SBU is connected to six HHTP ligands, and adjacent SBUs are connected by HHTP to form a tetrahedral cage (Figure 2c). The cage itself is connected at the apex to provide a network structure with 6 and 3 connection nodes and a SPN topology (Figure 2a). The same topological structure has been observed in multiple MOFs, such as MOF-808, which is homologous to the faujasite structure type.

Figure 2 Crystal structure of M 6 HOTP 2 .

【Structure Analysis】

In view of the coordination flexibility of rare earth ions, SBU exhibits obvious crystallographic disorder. The metal atoms in Eu 6 HOTP 2 and Y 6 HOTP 2 have a similar eight coordination environment, each metal is connected to one oxygen atom of the central bridged nitrate and four oxygen atoms from three different HOTP ligands and The disordered combination of water and nitrate coordinates towards the hole or lumen of the tetrahedral cage. This type of SBU first appeared in the field of MOF . The discovery of the new SBU can promote the development of MOF mesh chemistry, but there are fewer and fewer reports that the structural characteristics of the new SBU in different metals have been retained. The synthesis of the new SBU in this work is of great significance, because the catechol ligands have so far only been able to obtain single-core structures. Similar to the carboxylate SBU, the six-core SBU reported here is also similar to the reported six-core cluster of lanthanide molecules. This similarity makes it possible to use the high-symmetry cluster synthesis strategy to rationally design the structure, which is a common strategy in network chemistry.

Since HHTP has a tendency of π stacking, a 3D structure is rarely formed. Only one case of isotropic 3D MOF formed by HHTP has been reported so far. In contrast, such ligands are generally easy to form stacked structures. The electrical conductivity is enhanced by the intimate contact of the extended aryl groups. It is worth noting that although the 2D layer and intimate contact in most HHTP-based materials result in the highest electrical conductivity in porous MOFs, the charge transport in these materials is anisotropic. In fact, almost all the reported symmetry of conductive MOF is lower than cubic symmetry, so it shows anisotropic transmission.

【Structure characteristics】

      Pass through the N 2 adsorption measurements show that the activation of the Y . 6 the HOTP 2 shows permanent long pores, a BET surface area of 780m ² / G. This value is similar to the surface area of the topologically equivalent MOF-808 (1140 m ² / g) (MOF-808 and Y 6 HOTP 2 have almost the same unit cell parameters). At the same time, although the crystals of M 6 HOTP 2 are yellow, even under strict air-free conditions, they will slowly darken over time, which may indicate that the catechol group in the HOTP ^6- ligand is partially oxidized to half Quinone . This transformation will continue for several weeks at room temperature, but heating or exposure to air will cause the transformation to accelerate significantly. Prolonged exposure to air will cause it to eventually lose its crystallinity until it becomes completely amorphous, but heating under inert conditions can retain the structure and accelerate the color change.

Figure 3 The diffuse reflectance spectrum of Y 6 HOTP 2 (black, initial spectrum; yellow, final spectrum). The broadband of 1000-1200 nm is generally regarded as the characteristic peak of semiquinone radical.

The author tracked this color change through in-situ diffuse reflectance spectroscopy (slowly heated to 90 ° C in a nitrogen atmosphere), as shown in Figure 3. Its spectrum is similar to molecular clusters with similar chemical composition (such as trinuclear Co (III) HOTP complex and trinuclear Fe (III) complex with hexaaminotriphenylene ligand). The spectral changes observed after heating of Y 6 HOTP 2 are also similar to the behavior of Fe (III) complex during oxidation. The spectrum of MOF shows a decrease in the relative intensity between 300-450nm (due to the π-π * transition in the aromatic core) and 800 nm (due to the charge transfer from the ligand to the metal), and a The new band at 1200 nm is usually attributed to the π-π * transition of the semiquinone radical.

【Performance characteristics】

The spectral changes discussed above confirm that partial oxidation of the material may improve the formation of free charge carriers of electrical conductivity . In fact, Y 6 HOTP 2 and Eu 6 HOTP 2 after activation showed  electrical conductivity of 10 ^-6 to 10 ^-5 S / cm in different synthesis batches (Figure 4). Although the overall conductivity of HHTP-derived MOF (10 ^-4 –10 ^-2 S / cm), which is not as densely packed as HOTP , is high, it is amazing for 3D isotropic porous MOF, and the charge is expected to be along three dimensions The direction is transmitted with the same efficiency .

Fig. 4 Y 6 HOTP 2 current-voltage curve. The linear fit of the data (dashed line) indicates that the conductivity of this representative device is 2.0 × 10 ^-5  S / cm.

【to sum up】

It is rare for a material to have both inherent porosity and electrical conductivity, and it is even more rare that the structure is isotropic . This series of MOF combines significant porosity and electrical conductivity in a cubic frame. These results indicate that higher conductivity is not necessary for directional charge transport through low-dimensional paths. The authors believe that this result can stimulate more detailed study of the charge transport path in MOF materials, and ultimately give people a better understanding of how the porosity and conductivity interact.

Original link: https://pubs.acs.org/doi/10.1021/jacs.0c01713

Source of information: COF people