Author Affiliations
1College of Chemistry and Chemical Engineering, Xi’an University of Science and Technology, Xi’an 710054, China2Center for Excellence in Nanoscience (CAS), Key Laboratory of Nanosystem and Hierarchical Fabrication (CAS), National Center for Nanoscience and Technology, Beijing 100190, Chinashow less
In short, COF-based EC materials can be synthesized by combining various redox units. The uniform porous structures guarantee fast ion diffusion, and the strong π–π stacking in COFs allows efficient charge transport. They present higher stability than conducting polymers, and they show fast switching. Large-area and cheap COF-based EC devices are expected.
The EC performance of COFs can be further adjusted by introducing electron-donating (D) and electron-accepting (A) units. Yuet al. used a N,N,N',N'-tetra(p-aminophenyl)-p-phenylenediamine (TPBD) unit (D) and a 2,1,3-benzothiadiazole-4,7-dicarboxaldehyde (BTDD) unit (A) to construct D–A type material COFTPBD-BTDD (Fig. 2(b))[15]. Differing from previous materials, COFTPBD-BTDD shows two absorption bands in the visible region. Under bias, cationic radicals and dications form, leading to dramatic color change (Fig. 3(a)). The excellent EC performance of COFTPBD-BTDD is due to the large pore volume which facilitates the ion diffusion, and the reduced bandgap results from the D–A structure (Table 1). Efficient, fast-switching and stable EC COFs can also be realized by introducing D/A units into one building unit. Bessingeret al. designed a D–A–D building unit with a central thienoisoindigo A moiety and two thienothiophene (or naphthalene) D moieties[9]. The resulted COFPy-ttTII shows strong light absorption in the visible and NIR regions and very fast switching. COFPy-ttTII shows short coloration/bleaching times of 0.38 s/0.2 s, which outperforms previous COFs by at least an order of magnitude (Table 1).
![(Color online) (a) Chemical structures for TAPA, TTDA, and COFTAPA-TTDA. (b) Schematic for the electrochromic phenomenon of oriented COFTAPA-TTDA thin film. Reprinted with permission[12], Copyright 2019, American Chemical Society. (c) The preparation of COFTAPA-TFPA nanofibers. Reprinted with permission[14], Copyright 2020, Elsevier.](/Images/icon/loading.gif)
Figure 1.(Color online) (a) Chemical structures for TAPA, TTDA, and COFTAPA-TTDA. (b) Schematic for the electrochromic phenomenon of oriented COFTAPA-TTDA thin film. Reprinted with permission[12], Copyright 2019, American Chemical Society. (c) The preparation of COFTAPA-TFPA nanofibers. Reprinted with permission[14], Copyright 2020, Elsevier.
The transparency, reflectivity and color for electrochromic (EC) materials can be changed reversibly under low bias[1]. EC materials find wide application in many fields like microelectronics, energy-saving buildings, automobiles, national defense and aerospace industry[2]. Compared with inorganic EC materials, organic EC materials have advantages like easy modification of molecular structures, rich color changes and fast-switching speed[3].
Figure 2.(Color online) (a) Schematic for the redox process of COFTAPA-TFPA nanofibers. Reprinted with permission[14], Copyright 2020, Elsevier. (b) Synthetic route for COFTPBD-BTDD. Reprinted with permission[15], Copyright 2020, Springer Nature.
Figure 3.(Color online) (a) Color switching of COFTPBD-BTDD device. Reprinted with permission[15], Copyright 2020, Springer Nature. (b) Chemical structures for TPDA, PDA, and COFTPDA-PDA. (c) The three-state electrochromic behavior of COFTPDA-PDA thin film. Reprinted with permission[16], Copyright 2021, Wiley.
Schiff-base condensation reactions provide stable aromatic imine bonds and highly conjugated systems for COFs[7]. Under the solvothermal reaction conditions, the imine exchange process can last for a long time and finally produce a thin crystalline COF film on the substrate[11]. In 2019, Haoet al. first reported a COFTAPA-TTDA with near-infrared (NIR) EC property[12]. Tris(4-aminophenyl)amine (TAPA) and thieno[3,2-b]thiophene-2,5-dicarbaldehyde (TTDA) were chosen as building units (Fig. 1(a)). These building units render COFs rich color and high stability[13]. The color of COFTAPA-TTDA can be reversibly changed from red to brown by changing potential between 0 and 1.4 V (Fig. 1(b)). The NIR absorption at 1300 nm can be attributed to the intervalence charge transfer (IVCT) between triphenylamine cation radicals. Though COFTAPA-TTDA only exhibits moderate coloration efficiency and switching speed (Table 1), it proves the feasibility of COF-based EC materials. Xionget al. reported a new EC material COFTAPA-TFPA with reversible color change from yellow to brown[14]. COFTAPA-TFPA can be viewed as a 2D triphenylamine network with the C=N bond linkage. The adhesion of COFTAPA-TFPA to substrate was improved by using an amine-functionalized ITO glass (Fig. 1(c)). The EC behavior is attributed to the reversible redox reaction of nitrogen in triphenylamine unit and C=N bond (Fig. 2(a)).
Further, the EC performance of COFs can be improved by designing novel skeletons, post-modification and developing COF-based hybrid materials. By using N,N,N’,N’-tetrakis(4-aminophenyl)-1,4-benzenediamine (TPDA) and terephthalaldehyde (PDA) units, Haoet al. developed a highly crystalline three-state NIR EC material COFTPDA-PDA (Figs. 3(b) and3(c))[16]. The three-state mix-valence derives from the adjacent triarylamine redox moieties. It causes a strong electronic coupling and improves the EC performance in the NIR region. Owing to the porous structure and IVCT interaction in the mix-valence system, dramatic absorption change and fast-switching (subsecond response) were realized (Table 1). Loading functionalized graphene oxide (FGO) and carbon nanotubes to COFs can reduce the impedance and further improve EC performance[17,18]. A FGO-COF developed by Lvet al. showed excellent cycle stability[17]. After 1800 cycles, the contrast retention even reached 109.1%.
Metal-organic frameworks (MOFs) are one type of organic EC materials[4]. Dincaet al. introduced the redox-active naphthalene diimide (NID) moiety into MOF-74 to achieve high EC performance[5]. However, the coordination bonds of MOFs are unstable to some electrolytes, thus limiting the application of MOF-based EC materials. Covalent organic frameworks (COFs) are analogues of MOFs. The building units are connectedvia covalent bonds[6]. COFs based on Schiff-base type linkages are highly stable in aqueous and most organic electrolytes[7]. COFs show many advantages for EC application. The skeletons and pore structures of COFs can be facilely adjusted[8]. The pores of COFs are permanent and continuous. Electrolyte ions can easily pass through the pores to realize fast switching and high cycle stability[9]. The strong π–π interaction and periodic columnar array structures in 2D COFs facilitate charge transport[10]. The optoelectronic properties of COFs can be facilely modulated by using different functional groups or side chains[6]. Thus, COFs are a good platform for developing high-performance EC materials.
Acknowledgements
S. Xiong thanks the National Natural Science Foundation of China (52073227). L. Ding thanks the open research fund of Songshan Lake Materials Laboratory (2021SLABFK02), the National Key Research and Development Program of China (2017YFA0206600) and the National Natural Science Foundation of China (51922032, 21961160720).
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