1. Introduction

Covalent organic frameworks (COFs) are a class of porous crystalline materials which consist of organic building blocks orderly linked by strong covalent bonds ^[1], and it can also be defined as a type of two-dimensional polymer. According to the principles of reticular chemistry, the porosity and composition of COFs can be well controlled. That they are composed of lightweight elements bestows the character of low mass density on COFs, and covalent bonds linkages endow COFs with thermal stabilities. Combining the advantages of permanent porosity and high surface area makes COFs a perfect candidate material for gas storage ^[2], gas separation ^[3], catalysis ^[4], and proton conductivity ^[5]. Moreover, another significant feature of COFs is that their functionality can be tuned by carefully choosing the type of the building blocks, resulting in potential applications in various fields. Polymers with π-conjugated functional groups usually possess fluorescent and photoconductive property ^[6]. Similarly, if we use π-conjugated molecules as the monomers, COFs with photoconductive properties can be obtained as well ^[7]. Fig. 1 summarizes the π-functional molecules which can be used for synthesizing the semiconducting COFs. In this short review, we summarize the recent advance in this field.

2. Fundamentals and motivation

COFs can be developed into two-dimensional (2D) or threedimensional (3D) frameworks based on the structure of building blocks. In 2005, the first 2D COFs were designed and synthesized through the condensation reaction of bor- onate ester ^[1]. To improve the crystallization, the organic reaction for synthesizing COFs need have a good reversibility ^[8], and till now the reactions include the formation of B-O (boronate, boroxine, and borosilicate) ^[9], C=N (imine, hydrazine, and squaraine) ^[10], C-N (triazine and imide) ^[11], B-N (borazine) ^[12], and N=N (azodioxide) ^[13] bond linkages (Fig. 2).

In 2D COFs, organic building blocks are orderly linked to form 2D sheets with regular pores which then stack together by π-π interaction of the building blocks in a face-to-face way. So, there are periodically aligned columns developed by building blocks in 2D COFs. Such a well-organized structure will benefit 2D COFs applied as organic semiconductors when selecting π-functional molecules as building blocks, because the charge carriers excited from the building blocks will transport along the discrete and periodic columns ^[14]. These unidirectional pathways for charge carrier transport definitely improve the charge carrier mobility of 2D COFs as semiconductor. That is a great advantage compared with traditional organic semiconductors or polymer semiconductors which are amorphous.

3. P-type semiconducting COFs

Utilizing π-functional organic molecules especially with electron-rich feature to construct COFs which will form an extended π-conjugated system, renders the COFs p-type semiconducting behavior. In 2008, Jiang et al. reported the first example of semiconducting TP-COFs, with a pore size of 3.14 nm, by condensation reaction of 2, 3, 6, 7, 10, 11-hexahydroxytriphenylene (HHTP) and pyrene-2, 7-diboronic acid (PDBA) (Fig. 3a) ^[15]. The specific surface area and pore volume values of TP-COFs were 868 m²g^-1 and 0.7907 cm³ g^-1, respectively. The electrical conductivity of TP-COFs was measured by a two-probe method. The homogenous acetone dispersion of TP-COF was casted onto the Pt electrodes with the width of 10 mm. The I-V profile of TP-COF was linear in the air at 25 ℃, while the gap itself was silent which meat TP-COF had a semiconductor character. After TP-COF was doped with iodine, the electric current increased to 4.3 nA at 2 V bias voltage, suggesting that TP-COF was a p-type organic semiconductor (Fig. 3b). It showed a good stability, and the electric current did not deteriorate after switched on-off many times (Fig. 3c).

Jiang et al. pioneered the research of photoconductive COFs by embedding pyrene to obtain PPy-COF via the self-condensation of pyrene-2, 7-diboronic acid (PDBA) (Fig. 4a) ^[16]. PPy-COF has a pore diameter of 1.73 nm and a surface area of 923 m² g^-1, and shows an electrical conductivity similar with TP-COF (Fig. 4b and c). To investigate its photoconductivity behavior, PPy-COF thin film was sandwiched between Al and Au electrodes. When visible light (> 400 nm) irradiated from the Au side, a remarkable photocurrent is generated (Fig. 4d), and it can be switched repetitively by many times without any deterioration at an on-off ratio over 8.0 × 10⁴ (Fig. 4e).

Choosing large and planar conjugated monomers as the building block will definitely increase the electronic conductivity of COFs. For instance, phthalocyanines and porphyrins are extensively adopted to synthesize semiconducting COFs ^[17]. In 2011, Jiang et al. and Yaghi et al. reported the phthalocyanine- based and porphyrin-based COFs by solvothermal method, respectively ^{[14, 18]}. (2, 3, 9, 10, 16, 17, 23, 24-octahydroxyphthalo- cyaninato)nickel(II), [(OH)₈PcNi], and 1, 4-benzenediboronic acid (BDBA) reacted in dimethylacetamide (DMAc)/o-dichlorobenzene via boronate esterification reaction, forming crystalline NiPc COF in high yield (Fig. 5a) ^[18]. NiPc COF exhibits a linear I-V curve when it is sandwiched between Au and Al electrodes (Fig. 5b), suggesting its perfect conductive property. Its intrinsic charge-carrier mobility was measured by laser flash-photolysis time-resolved microwave conductivity measurements (FP-TRMC) (Fig. 5c and d). NiPc COF's carrier species was demonstrated to originate from holes with a mobility as high as 1.3 cm² V^-1 s^-1. They also found that, due to the central metal's influence on the π-electron density of metallophthalocyanine, the metal species could significantly influence the carrier mobilities of the phthalocyanine-based COFs (CuPc-COF < ZnPc-COF < CoPc-COF) ^[19]. Moreover, Yaghi et al. synthesized porphyrin-based COFs (COF-366 and COF-66) by Schiff Base reaction and boronate esterification reaction (Fig. 6a and b) ^[14]. Compared with COF-66 (hole mobility 3.0 cm² V^-1 s^-1), COF-366 has a much higher hole mobility (8.1 cm²V^-1 s^-1), which is also the highest in semiconducting COFs till now (Fig. 6c and d). It should be attributed to the linkage of imine in COF-66, which improves the conjugacy of the frameworks.

Tetrathiafulvalene (TTF) is a typical organic semiconductor molecule with a strong electron donor behavior. COFs with TTF building block were synthesized by reacting 2, 3, 6, 7-tetra(4- formylphenyl) tetrathiafulvalene with 1, 4-phenyldiamine or 1, 3, 6, 8-tetra(4-aminophenyl)pyrene (Fig. 7a) in 2014 ^[20]. TTF- Ph-COF and TTF-Py-COF are not only thermally stable (no obvious weight loss until 450 ℃), but also have a good chemical stability in organic solvents. TTF-Ph-COF is a mesoporous material with pore size of 2.2 nm and surface area as high as 1014 m² g^-1, while TTF- Py-COF is a microporous material with pore size of 1.3 nm and surface area of 817 m² g^-1. TTF itself is a hole-transporting material, so TTF-based COFs are p-type semiconductors. The carrier mobilities of TTF-Ph-COF and TTF-Py-COF are 0.2 and 0.08 cm² V^-1 s^-1, respectively. Moreover, when doping TTF-based COFs with electron acceptors such as I₂ or tetracyanoquinodi- methane (TCNQ), their conductivity can be improved significantly (Fig. 7b-e) ^[21].

Due to blocking by the linkages, typical COFs lack intra-sheet π- conjugation. The fully intra-sheet π-conjugated COFs will definitely increase both the chemical stability and the electrical conductivity. In 2013, Jiang et al. used triphenylene hexamine (TPHA) and tert-butylpyrene tetraone (PT) as the monomers to synthesize the crystalline phenazine-linked CS-COF through topological ring fusion reaction (Fig. 8a) ^[22]. Owing to the π-electron delocalization, CS-COF performs high-rate hole-conducting with mobility up to 4.2 cm² V^-1 s^-1. Furthermore, they loaded fullerene into the pores of CS-COF forming a donor-acceptor system (CS-COF⊃C₆₀) (Fig. 8b). The sandwiched devices with CS- COF⊃C₆₀ as the photoconductive layer display a rapid response to light irradiation with an on-off ratio up to 5.9 × 10⁷ (Fig. 8c and d). When using CS-COF⊃C₆₀ in solar cells, the power conversion efficiency is about 0.9% (Fig. 8e). Bein et al. reported a thienothiophene-based covalent organic (TT-COF) ^[23]. After loading TT-COF with electron acceptor [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM), it can be used in solar cells with a power conversion efficiency only about 0.053%, much lower than the fully intra-sheet π-conjugated COFs.

4. Ambipolar semiconducting COFs

Ambipolar semiconducting COFs usually have both electron donor (D) and electron acceptor (A) building blocks, which can be obtained by post-synthesis functionalizing or directly using D and A monomers in the synthesis ^[24]. Utilizing two different kinds of building blocks, D and A molecules, to construct COFs will form a segregated and bi-continuous D-A structure, which produces periodic and unidirectional D-on-D and A-on-A columnar arrays. Compared with the amorphous D-A polymers in which D and A units stack disorderly, D-A COFs have a better control on the charge dynamics. The first example of D-A COFs was reported in 2012, in which 2, 3, 6, 7, 10, 11-hexahydroxytriphenylene (HHTP) was used as the electron donor and 2, 1, 3-benzothiadiazole-4, 7- diboronic acid (BTDADA) was used as the electron donor (Fig. 9a and b) ^[25]. The D column is the pathway for hole transport, while the A column is for electron transport. It hole and electron mobilities were measured by FP-TRMC method, which were 0.01 and 0.04 cm² V^-1 s^-1, respectively (Fig. 9c and d). A photocurrent with linear I-V curve was observed when the D-A COF was irradiated by visible light (Fig. 9e), whereas the mixture of D and A molecule with a molar ratio of 3:2 did not response to light irradiation. Furthermore, this D-A COF has a quick response to light irradiation, and the photocurrent can be switched on and off many times without deterioration as shown in Fig. 9f.

Bein et al. developed a triphenylene-porphyrin (TP-Por) COF with highly defined interdigitated donor-acceptor heterojunctions, and grew an oriented TP-Por COF thin film on ITO-covered glass (Fig. 10a and b) ^[26]. Using the thin film, they fabricated a photovoltaic device in which the COF itself provided the photoactive junction and investigated the charge carrier generation and extraction (Fig. 10c). The D and A units in the TP-Por COF film formed one-dimensional stacked columns that extended along the substrate. The short-circuit current was about 30 times higher than that of a reference device based on a randomly intermixed blend of the two building blocks (Fig. 10d). Moreover, the external quantum efficiency of this COF-based device could be boosted to more than 30% at 350 nm under reverse bias, which is highly enough to replace small molecule/fullerene-containing devices (Fig. 10e).

In 2013, two kinds of D-A COFs with the largest pore size among known COFs, as large as 5.3 nm, were reported ^[27]. Triphenylene and diimide were selected as vertices and edge units respectively to construct hexagonal triphenylene-diimide COFs (Fig. 11a, D_TP- A_NDI-COF and D_TP-A_PyrDI-COF). The charge transfer was detected by steady-state electronic absorption spectra. D_TP-A_PyrDI-COF exhibited a clear charge-transfer (Fig. 11c), while no clear charge- transfer band was observed for D_tp-A_ndi-COF (Fig. 11b). It is because that the D and A columns in D_TP-A_NDI-COF are indepe at the ground state leading to a neutral system ready for separation. The lifetime of charge-separated was also measu time resolved electron spin resonance spectroscopy (TR-E track the formation of radical spins and their dynamics. irradiation by laser (355 nm, 2 mJ), the TR-ESR signal of D_TP COF increases rapidly to 2.5 μs, as the result of charge sepa (Fig. 11d-f). However, D_TP-A_PyrDI-COF did not have any sign the TR-ESR measurements of which reason may be the ac structure. That means pairing donor and acceptor is a key fa design COFs for solar energy conversion.

To further investigate the effect of different electron don acceptor pairs on COFs' charge transfer and separation, et al. employed different electron-donating metallophtha nines and electron-accepting diimide derivatives to constru COFs (Fig. 12), and showed that the metal species in phtha nines and the class of acceptor could influence the photoch dynamics of the D-A COFs ^[28].

5. n-type semiconducting COFs

p-type and ambipolar semiconducting COFs have been si intensively; however, only a few n-type semiconducting were reported. In 2011, Jiang et al. reported an n-ch conducting COF with high electron mobility and prom photoconductivity ^[29]. Their strategy was to employ ele withdrawing building block, benzothiadiazole (BTDA), at the of a two-component tetragonal nickel(II) phthalocyanine CO NiPc-BTDA COF) (Fig. 13a), which gave rise to a drastic cha the carrier-transport mode from hole-transporting to ele conducting. The 2D-NiPc-BTDA COF was synthesized by co sation of (2, 3, 9, 10, 16, 17, 23, 24-octahydroxyphthalocyani nickel(II) ([OH]₈PcNi) and BTDADA in a mixture dichlorobenzene and dimethylacetamide under solvoth condition in 85% isolated yield. 2D-NiPc-BTDA COF has tetr pores with a pore size of 2.2 nm and its surface area was evaluated to be 877 m²g^-1. Its intrinsic carrier mobility was measured by FP-TRMC method under different atmospheres, which demonstrated an n-type semiconducting behavior with electron mobility as high as 0.6 cm² V^-1 s^-1 (Fig. 13b and c). When irradiating the 2D-NiPc-BTDA COF by light with different wavelengths, obvious photoconductive behavior was detected, and it could be switched on-off by many times without any deterioration (Fig. 13d and e).

Another example of n-type semiconducting COFs is Cu porphyrin-based COF (CuP-COF), which has minimum electron mobilities up to 0.19 cm² V^-1 s^{-1 [30]}. Interestingly, removing or changing the central metal of the building block of porphyrin would switch conducting nature of the porphyrin-based COF (MP- COF, M = H₂, Zn, and Cu) (Fig. 14a). The stacking macrocycles of H₂P-COF without central metals formed a path for the hole transport. When inserting a metal species into the porphyrin macrocycles, it formed another path, metal-on-metal channel, for the electron transport (Fig. 14c). The insertion of a central metal to the porphyrin macrocycle lowered the electron density of the macrocycles. According to the central metal's ability of exhibiting ligand-to-metal charge transfer, copper metal reduced much more electron density of the porphyrin macrocycles. Thus, CuP-COF favored electron transport through the metal-on-metal channel, whereas ZnP-COF allowed balanced transport through both channels (Fig. 14b). However, ZnP-COF exhibited better photoconductivity than CuP-COF with an on-off ratio of 5 × 10⁴(Fig. 14d and e).

6. Conclusion

In summary, this mini review summarized different types of semiconducting COFs and the strategies to choose π-functional building blocks. COF materials themselves possess the advantages of low density, large surface areas and permanent porosity. The most important one is that their properties and functionalities can be tuned by the building blocks and covalent linkages. Using building blocks with rich conjugated π-electron tend to obtain p- type semiconducting COFs, whereas integrating electron-withdrawing building blocks can switch them to ambipolar or n-type semiconductors. Because of their ordered structure, the charge mobilities are normally higher than amorphous conducting polymers and organic molecules, however some challenges still remain, including: (1) the study on semiconducting COFs is still confined to their semiconducting properties. Therefore, applying them on photoelectric devices is the primary challenge to overcome in future; (2) n-type semiconductor is an essential part in photovoltaic devices ^[31], but only two examples of n-type semiconducting COFs have been reported. Thus, more effort toward new n-type semiconducting COFs is required; (3) COFs materials are insoluble in various organic solvents, giving rise to a processing problem when fabricating the optoelectronic devices. To develop a reliable method to prepare COF thin films is of great importance for its practical application ^[32]; (4) in terms of the synthetic methods, crystalline COFs are usually synthesized by solvothermal method, which is hard to be scaled up and needs a long reaction time. To develop methods under mild reaction conditions for large scale production of crystalline COFs is still a big challenge ^[33]. We believe, as a novel type of 2D conducting polymers, the semiconducting COFs have great potential to be a new generation of organic semiconductors, and would have a promising future in the application of high performance optoelectronic devices.

Acknowledgment We are grateful to the support from National Program for Thousand Young Talents of China, the National Natural Science Foundation of China (No. 21544001), and Fudan University.