# Metal-Organic Frameworks Toward Electrocatalytic Applications
## 2 Charge Transport in Metal-Organic Frameworks (MOFs)
### Band Transport  
In addition to redox hopping, another approach to facilitate the charge transport in MOFs is building up the electrical conductivity. Although most MOFs are electrically insulating, MOFs with semiconducting or conducting characteristic may be synthesized via rational structural design [29; 30]. It should be noted that for most MOFs, their highest occupied molecular orbitals (HOMO), lowest unoccupied molecular orbitals (LUMO) and therefore band gaps can be experimentally measured. However, such observations do not suggest that the frameworks are semiconducting and behave like conventional inorganic semiconductors since the electrons are highly localized in most MOFs; charge carriers can not travel among unit cells through the entire MOF crystal. It is thus more reasonable to consider MOF as a periodic array of self-assembled molecular units with discrete absorption modes instead of classic semiconductors [45; 46]. Therefore, to design a \"real\" semiconducting or conductive MOF that can facilitate the long-range movement of charge carriers in the domain of the bulk crystal, the conducting pathways for electrons must be constructed periodically within the entire MOF structure. A number of strategies have been proposed to design the electrically semiconducting or conductive MOFs. For example, [PERSON] et al. synthesized the two-dimensional (2D) MOFs constructed from copper nodes and 2,3,6,7,10,11-hexahhydroxytriphenylene (HHTP) linkers, which can render the charge transport between the 2D layers and achieve an electrical conductivity of about \(2\times 10^{-1}\) S/cm [47]; recently this MOF was also found to exhibit superior supercapacitive performance [48]. Similar design of 2D MOFs with \(\pi\)-conjugation to rendering the framework conductive was also demonstrated by [PERSON] et al. [49]. The resulting 2D MOF (Ni\({}_{3}\)(HITP)\({}_{2}\), HITP = 2,3,6,7,10,11-hexaiminotriphenylene, see Figure 3a) achieved an even higher electrical conductivity (40 S/cm) and can be utilized as the active material for electrochemical charge storage [50]. A few examples of other 2D MOFs which exhibit electrical conductivity also got reported recently [51; 52; 53; 54]. The formation of such \(\pi\)-stacked pathways can also be constructed in 3D MOFs. For example, the selection of zinc-based nodes and tetrathiafulvalene(TTF)-tetrabenzoate linkers results in the 3D MOF that contains columnar stacks of TTF to facilitate charge movement [55]. In addition to the formation of \(\pi\)-stacked pathways, the electrical conductivity of MOFs can also be acquired by using sulfur-containing ligands to facilitate charge delocalization within the framework [56; 57]. Introducing both the electron-donating and electron-accepting units within the framework structure is another strategy to design the conductive MOFs [58]. For example, [PERSON] et al. synthesized the MOF constructed with the electron-donating Cu(I) ions and electron-accepting Cu(III)(2,3-pyrazinedithiolate)\({}_{2}\) linkers, and the charge transfer by virtue of donor-acceptor interactions within the obtained MOF results in an electrical conductivity of \(6\times 10^{-4}\) S/cm [59]. Similar donor-acceptor charge transfer can also be designed by incorporating either the electron donor or electron acceptor as guest molecules into the MOF consisting electron-accepting or electron-donating building blocks. In 2014, [PERSON] et al. incorporated 7,7,8,8-tetracyanoquinododimethane (TCNQ) as the electron acceptor into an electron-donating copper-based MOF (Cu\({}_{3}\)(BTC)\({}_{2}\), BTC =  
Figure 2: Charge transport via redox hopping between the pyrene units presented in one-dimensional channels of the NU-901 thin film [37]. benzene-1,3,5-tricarboxylic acid; also known as HKUST-1) [60]. The resulting thin films of TCNQ@ HKUST-1 become electrically conductive by means of the donor-acceptor charge transfer between the copper clusters in the framework and TCNQ guests (Figure 3b).