In 2014, as the pioneers in this field, Lotsch et al., successfully used COF (TFPT-COF, hydrazone-linked) to produce H₂ for the first time and achieved an excellent hydrogen evolution rate (HER) of 230 μmol·h⁻¹·g⁻¹ under standardized conditions (assisted by proton reduction catalyst Pt and using sodium ascorbate as SED), which proved that hydrogen evolution is an intrinsic photoinduced process instead of stoichiometric decomposition ^[1][37]. In the following experiment, they changed sacrificial electron donor to a 10 vol% aqueous TEOA solution and achieved a higher HER of 1970 μmol·h⁻¹·g⁻¹. Since then, COFs have begun to be used as a new tool to explore the production of hydrogen and are constantly updated. In these works, the essential and vital fact that COFs still possess tremendous potential in the field of PHP has been confirmed, while finding a method of simulating that potential and make perfect use of is is a problem that all researchers in this field want to solve.

At present, most reported D-A COFs’ building blocks are limited to be linked by covalent bonds, such as C=N (imine or amine, imide), C=C (vinylene), B-O (borate ester), etc. The linkage tends to be associated not only with chemical stability but also with the transportation of photogenerated charge carriers ^[2][33], thus improving PHP efficiency. Additionally, different linkages endow COFs with different electron structures and interfacial properties because the linkages also tend to determine electronic communication of COFs, which is of significance for the photochemical process ^[3][38].

The formation of amine bonds in D-A COFs is usually attributed to the Schiff base reversible condensation reaction. Currently, most of the reported D-A COFs are synthesized using this classic and useful reaction because COFs connected by amine bonds usually have good stability and ability to extend a wider visible light absorption region. Numerous amine-linked D-A COFs with excellent photoelectronic properties have been developed for photocatalytic applications.

Based on previous studies ^[4][5][39,40], due to the necessary use of AA in most PHP tests, the imine bonds in imine-linked COFs could be protonated. This character will make the structure of such COFs more planar and the degree of conjugation higher, thus further enhancing the range of visible light absorption, which also means that the water-splitting efficiency for imine-linked COFs may be improved. This was confirmed again by synthesizing three highly crystalline imine-linked D-A COFs (TtaTfa, TpaTfa and TtaTpa) in 2021 by Yang and coworkers ^[6][41]. Among them, combined by the strongest donor Tfa and acceptor Tta, the obtained TtaTfa-COF exhibited the highest HER values of 20.7 mmol·h⁻¹·g⁻¹ because of the enhanced D-A effect. Interestingly, during the immersion of these three imine COFs with AA, their colors gradually changed to deep red (TpaTfa and TtaTfa) or orange (TtaTpa), which could be considered evidence of the enhancement of light absorption. Therefore, when AA was served as SED, imine COFs were supposed to be protonated, and it was the protonated form of COFs that improved the PHP activity instead of the pristine COFs, which was confirmed via UV–Vis DRS. Moreover, with the modification of AA, the band gaps of these three COFs obviously decrease from 2.60, 2.52, and 2.73 eV to 1.89, 1.90, and 2.22 eV for TpaTfa, TtaTfa, and TtaTpa, respectively. The decrease reflects the enhanced visible light absorption of the three COFs. Signal intensity of TtaTfa with AA in electron paramagnetic resonance (EPR) spectroscopy is much higher than that of pristine TtaTfa-COF, which suggests a significantly improved charge separation efficiency of the protonated COF. Thus, the enhanced photogenerated charge separation capability brings high PHP efficiency.

Similarly, as PETZ-COF containing imine linkages could be protonated, the hydrophilia of PETZ-COF was significantly improved and facilitated the HER synergistically. Yu et al., used TzDA as an acceptor unit and PE as a donor unit to construct PETZ-COF, whose crystallinity was better than that of PEBP-COF (the control example, non-D-A COF), which resulted in faster charge transfer efficiency in PETZ-COF ^[7][42]. Compared to PEBP-COF, the emission peak intensity of PETZ-COF in steady-state photoluminescence (PL) spectra is much lower, while PETZ-COF has a stronger photovoltage value, as shown in the transient surface photovoltage (TS-SPV) spectra. Such a difference indicated that the introduction of the D-A structure resulted in a much higher charge separation of PETZ-COF. The ordered D-A structure also brings PETZ-COF a plane with a higher degree of conjugation, which further accelerates charge transfer. Moreover, the longer fluorescence lifetime of PETZ-COF (3.6 ns) than PEBP-COF (1.6 ns) proves that the strong electron-withdrawing effect of the TZ acceptor would make electrons delocalize and remain in an excited state for a longer period of time, which facilitates the separation of excitons. PETZ-COF gets a good HER value of 7204.3 μmol·h⁻¹·g⁻¹ under the irradiation of visible light (λ > 420 nm) with 3 wt% Pt served as the cocatalyst, and AA served as the SED.

PyTz-COF, reported in 2020 by Li et al., owing to its excellent optoelectronic properties and photocatalytic ability, could be used in sunlight-driven photocatalytic hydrogen evolution ^[8][43]. To test the conjecture, PyTz-COF was synthesized by combining TzDA as an electron-deficient unit with PyTA as an electron-rich donor monomer. It turned out that this D-A structure of COF indeed relatively enhanced the light absorption capacity. The overlapping π-orbitals of PyTA and TzDA consequently enhanced the electron push–pull effect, which resulted in more rapid charge separation in COF. Additionally, the narrow band gap of PyTz-COF (2.20 eV) implied that intramolecular charge from the donor to the acceptor could immigrate faster, which ensured a broaden light absorption region. The photoluminescence intensity of PyTz-COF is much weaker than that of PyBp-COF (the control sample) at 425 nm, which indicates that the photogenerated excitons recombination in PyTz-COF is greatly suppressed. This fact is further proved using time-resolved photoluminescence (TRPL) spectroscopy, which exhibits that the average fluorescence lifetime of PyTz-COF (4.4 ns) is much longer than that of PyBp-COF (2.3 ns) at 320 nm. The fitting results in the nanosecond transient time profile of hole-polarons revealed that the charge-separated state of PyTz-COF has a long lifetime value of 6.93 ns. Moreover, PyTz-COF also has a stronger transient absorption signal than PyBp-COF. The above evidence proves that PyTz-COF possesses exceptional charge separation ability and greatly facilitates PHP activity.

Similarly, in 2021, inspired by Photosystem I in nature, Chen et al., designed and synthesized a series of D-A COFs (NKCOFs). Their donors and acceptors are derivatives of pyrene and benzothiadiazole, respectively, and play a vital role in enhancing light-harvesting and light-absorbing abilities ^[9][44]. Among the four NKCOFs, NKCOF-108 (monofluorinated benzothiazole served as the acceptor) reflects the biggest degrees of redshift in the optical absorption onset from 500 to 700 nm in UV-vis DRS and reveals the widest light absorption region, which also matches the narrowest optical band gap of NKCOF-108 (1.82 eV). In the PL spectra, compared to NKCOFs-111 and -110, NKCOF-108 exhibits a much lower PL intensity, which implies that the recombination of photogenerated electron-hole pairs is greatly suppressed. Moreover, according to the result calculated by DFT equation, benzothiadiazole derivatives are beneficial to π-delocalization for the immigration of charge. In PHP experiment, without Pt cocatalyst, all the NKCOFs could also exhibit PHP activity to a certain degree. While under the optimized conditions of 5 wt%, Pt served as cocatalyst, NKCOF-108 provided the best HER value of 12.0 mmol·h⁻¹·g⁻¹, and the apparent quantum yield (AQY) was 2.96 % at 520 nm.

Recently, Hao et al., investigated the structure–property relationship of two D-A COFs (TeTz-COF1 and TeTz-COF2) with highly similar structures synthesized via Suzuki cross coupling reaction, which were imine-linked and alkyne-linked thiadiazole-based COFs, respectively ^[10][45]. TeTz-COF1 exhibited an HER value 19 times higher (2.10 mmol·h⁻¹·g⁻¹) than that of TeTz-COF2 under general conditions (AA was served as SED; 3% Pt was served as cocatalyst). The apparent quantum efficiency (AQE) of TeTz-COF1 reached 3.5% at 475 nm, which could be due to the existence of imine bonds. The longer average fluorescence lifetime of TeTz-COF1 (2.84 ns) reflects that the excited electrons and holes are more likely to remain stable, which could be because the recombination of electrons and holes is more heavily inhibited. One of the reasons that the HER value of the two COFs varied greatly can also be found in the energy diagram, in which the conduction band potential of TeTz-COF1 is more negative than that of TeTz-COF2, which reveals that the electrons in TeTz-COF1 have stronger proton reduction ability. Furthermore, the work function calculation theoretically explains that the electrons and holes of TeTz-COF1 could more easily reach the excited state, which furtherly enhances its electrical conductivity.

Unexpectedly, the reported β-ketoenamine-linked D-A COFs have one thing in common, which is that their donor or acceptor units are all hydroxy derivatives of benzene-1,3,5-tricarbaldehyde (BTA), such as 2-hydroxybenzene-1,3,5-tricarbaldehyde or 2,4,6-trihydroxybenzene-1,3,5-tricarbaldehyde. As for the latter, it has higher symmetry (C₃h) and planar structure. The three active hydroxyl groups on the phenyl ring undergo irreversible tautomerization from enol to keto during COFs synthesis, forming carbonyls strong electron-withdrawing capacity, which enhances the local polarity, increases a higher degree of conjugation and greatly improves the charge transfer rate, thus contributing to the PHP promotion.

For instance, Liu and coworkers designed and synthesized a series of benzobisthiazole-bridged covalent organic frameworks (Tz-COF-1, 2, 3, Tz as the donor; hydroxy derivatives of BTA as the acceptor), and a built-in control of the D-A interaction strategy has been implemented on them to accelerate exciton dissociation, thus generating more long-lived photogenerated charge carriers for photoredox reactions ^[11][23]. Surprisingly, the strategy worked best with Tz-COF-3 optimizing the D-A effect in it. Then, the photocatalytic hydrogen evolution experiments were performed by suspending Tz-COF-3 in 0.8 M AA solution with 3 wt% Pt cocatalyst under the irradiation of a 300 W Xe lamp, which exhibited a high HER of 43.2 mmol·h⁻¹·g⁻¹. The E_g values of the three COFs are 2.35, 2.15 and 1.96 eV, respectively. This is due to the increase in the number of carbonyl groups in the COFs, and the root of the rule is the enhanced D-A interactions caused by the electron-withdrawing acceptor. Moreover, the enhancement of the D-A effect is also reflected by their light absorption range, which increases gradually from Tz-COF-1 to Tz-COF-3. The authors found that with the increasing number of carbonyl groups, the exciton binding energy (E_b) decreased considerably, which indicated that the improved D-A interactions in Tz-COFs greatly reduced the E_b. Due to the E_b of Tz-COF-3 (29.8 meV) being very close to the thermal disturbance energy at room temperature (∼26 meV), the excitons could easily overcome thermodynamic constraints and dissociate into free electrons and holes at room temperature. This evidence further confirmed the feasibility and correctness of the strategy above.

The D-A effect can be further extended by using electron-deficient monomers with stronger electron-withdrawing capacity as acceptor units to achieve enhanced visible light absorption range. Similarly, in 2021, Li and coworkers synthesized a CN-COF linked by β-ketoenamine with Tp (served as donor) and BDCN (severed as acceptor) via Schiff base condensation reaction ^[12][46]. In order to compare the effect of forming the D-A effect after the introduction of cyano (structure-function correlation), BD-COF with similar linkage and topology structure to CN-COF but without cyano group as an acceptor was synthesized by TP and benzidine (BD) unit. The PHP experiments were performed in 0.1 M AA aqueous solution with 1 wt% Pt cocatalyst under visible light (λ > 420 nm). The HER values of CN-COF and BD-COF under the optimized conditions were 60.85 and 1.98 mmol·h⁻¹·g⁻¹. The huge difference in HER values between the two catalysts implied the enhancement electron push–pull effect of introducing cyano group into CN-COF. This is attributed to the light absorption of CN-COF exhibiting an obvious redshift compared to that of BD-COF. The optical band gaps of CN-COF and BD-COF are 2.17 and 2.24 eV, which reflects that introducing cyano groups could narrow the band gaps and thus enhanced visible light capture ability of CN-COF. Afterwards, the COFs nanosheet (CN-CON) even exhibited two times higher HER value of 134.2 mmol·h⁻¹·g⁻¹ than CN-COF under the optimized conditions. The E_b of CN-CON (31.2 meV) was much lower than that of BD-CON (44.2 meV), which indicates that the excitons in CN-CON dissociated easier than those in BD-COF, limiting the recombination of excitons and increasing the ratio of free charge carriers in CN-CON, which in turn increased the photocatalytic activity. With the help of fs-TAS, the charge carrier lifetime of CN-CON (14.2 ± 2.3) was three times longer than that of BD-CON (4.3 ± 0.6), which implied that the electron–hole recombination probability in CN-CON was much lower. In general, the presence of cyano-withdrawing groups in CN-COF (or CN-CON) enhances the D-A effect and extends the lifetime of charge carriers, which finally improves the PHP activity.

In 2021, Lin et al., successfully synthesized two D-A COFs, HBT-COF and BT-COF (the structural difference between the two COFs was shown by the extra hydroxyl group on the donor of HBT-COF) ^[13][47]. However, this minor modification led to great difference in optoelectronic properties and photocatalytic activity. Under the irradiation of visible light, HBT-COF (19.00 μmol·h⁻¹) exhibited an HER value five times higher than that of BT-COF (3.40 μmol·h⁻¹). Because of the existence of D-A structure, the absorption onset of both HBT-COF and BT-COF showed redshift compared to their pristine polymers. Meanwhile, compared to BT-COF, the absorption onset of HBT-COF was even redshifted by 81 nm, which implied that the D-A effect of HBT-COF was relatively stronger than that of BT-COF. This result also demonstrates that it is feasible to change the performance of COFs through fine structural adjustments. The conduction band (CB) levels of HBT-COF (−1.39 V vs. NHE) and HBT monomer (−1.41 V vs. NHE) were more negative than those of BT-COF and its monomer, respectively, which implied that HBT-COF had stronger proton reduction ability than of BT-COF. Additionally, the narrowest band gap of 1.94 eV is the main reason that HBT-COF has the widest visible light absorption range.

A suitable narrow band gap can balance the relationship between recombination and migration of electron–hole pairs, which can not only inhibit their recombination but also maximize the transfer of electrons and holes to the surface of the catalyst for redox reactions with substrates. In addition, chemical stability of the imine-based COFs can be further enhanced by introducing irreversible tautomerization of the enol-imine to produce keto-enamine, which broadens their photoelectronic applications ^[14][15][48,49]. Recently, Liu et al., successfully developed and first reported two phenanthroimidazole-based COFs (PIm-COF1 and PIm-COF2) with good photocatalytic properties in which PIm-COF2 showed inherently photocatalytic activity ^[16][50]. With AA serving as the SED and Pt nanoparticles as the cocatalyst, the HER values of the two COFs were evaluated in AA aqueous solution with the irradiation of visible light (λ ≥ 420 nm). Under optimal reaction conditions, PIm-COF2 exhibits an excellent HER value of 7417.5 μmol·h⁻¹·g⁻¹, which is 20 times higher than that of PIm-COF1 (358.5 μmol·h⁻¹·g⁻¹). It is obvious that PIm-COF2 possesses a broader light absorption range than PIm-COF1 and PIDA (the monomer for the synthesis of PIm-COF1, 2, served as acceptor). This difference could be attribute to the enhanced D-A effect because of the existence of great conjugated structure between the donor and acceptor units connected by the β-ketoenamine linkage. Moreover, PIm-COF2 has a narrower optical band gap than PIm-COF1 and they are 2.13 eV and 2.34 eV, respectively, which results in the higher PHP activity of PIm-COF2. As expected, the results of electrochemical measurements imply that the charge transfer resistance of PIm-COF2 is lower than PIm-COF1, which also indicates that the photoinduced electron of PIm-COF2 moved faster. The higher photocatalytic activity of PIm-COF2 is reflected in the emission peak of PIm-COF2 at 620 nm in the PL spectrum, which is much lower than that of PIm-COF1 at 588 nm, indicating that the recombination rate of electrons and holes in PIm-COF2 was lower than PIm-COF1.

Organic conjugated polymers with electron-donor and -acceptor moieties usually owe advantages of high carrier mobility or electrochemical activity ^[17][51]. The synthesis of organic heterostructures with g-C₃N₄ is one of the feasible methods to promote charge separation. In 2018, Lin et al., used TP as the electron donor and BTDA as the electron acceptor to synthesize the D-A COF (TBTA) and then combined TBTA and g-C₃N₄ (mass ratio 2.5:100 for TBTA and g-C₃N₄) to form 2.5-TBTA/g-C₃N₄ hybrid materials ^[18][52]. The PHP experiments of TBTA, g-C₃N₄ and 2.5-TBTA/g-C₃N₄ were conducted at λ ≥ 420 nm visible light with AA as SED under optimized conditions. The results shows that their HER values were 2.47, ~0, and 11.73 mmol·h⁻¹·g⁻¹, respectively. The higher PHP activity of 2.5-TBTA/g-C₃N₄ is attributed to the combination of TBTA and g-C₃N₄ enhancing visible light absorption and charge separation. TBTA’s PL intensity in 2.5-TBTA/g-C₃N₄ at 615 nm is obviously decreased compared to that of TBTA alone, which indicates an effective charge transfer effect existed between TBTA and g-C₃N₄.

The fact that COFs have a narrow band gap and a wide range of visible light absorption does not necessarily prove that they perform well in hydrogen evolution. A novel imide-bond-linked D-A COF (PMDA-COF) was reported by Lu et al., in 2021 ^[19][53]. The interactions between each layer in PMDA-COF are stronger than those of its control examples (DHTA-COF and TPAL-COF, two imine-linked COFs), which indicates the relatively stable chemical properties of PMDA-COF. The D-A molecular heterojunction consisted of triazine donor and dianhydride acceptor. Due to the presence of the D-A heterojunction, the intramolecular charge transfer (ICT) was accelerated and hence promoted Frenkel exciton immigration. Meanwhile, E_g of PMDA-COF (1.99 eV) becomes narrower, and the visible light absorption is widest among the three COFs because they possess the greatest conjugated degrees. In the main part of the PHP activity experiment, PMDA-COF exhibited a higher HER value of 435 μmol·h⁻¹·g⁻¹ than that of DHTA-COF and TPAL-COF under visible light irradiation by employing TEOA as SED with Pt as cocatalyst. Nevertheless, the HER value is not satisfied compared to COFs reported above. The reason could be summarized that the CB potential of PMDA-COF is not negative enough, and thus, its ability to reduce the protons is heavily limited.

The polarization of the carbon–nitrogen bond in the above-mentioned C=N linkage cannot promote efficient conjugation through 2D backbone ^[20][54], while vinylene-linked COFs (could also be termed sp²-Carbon (sp²-C) linked COFs) with the existence of vinylene (C=C) linkages between building blocks usually cause COFs to form a fully conjugated structure. The higher conjugation degree, on the one hand, endows vinylene linked COFs with an ability to enhance electron delocalization that is more powerful than that of amin-linked COFs. On the other hand, the charge transfer and separation efficiency are significantly improved. When integrating sp² hybridized C=C linkers into the skeleton of COF, the stability will be significantly enhanced ^[21][55]. Adding D-A structure into vinylene linked COFs could also facilitate photocatalytic activity. In the synthetic dimension, Knoevenagle condensation reaction is the main method. However, synthesizing vinylene-linked COFs is still challenging compared to amine-linked COFs above mentioned. There are very limited reports on the application of vinylene-linked D-A COFs in PHP, but they have an important guiding significance for the development of this field.

In 2019, Jin et al., constructed a fully π-conjugated 2D sp²c-COF via Knoevenagle condensation reaction and the 3-ethylrho-danine (ERDN) unit as an end-capping group was introduced into its lattice at the same time (sp²c-COF_ERDN) ^[22][56]. The presence of ERDN (served as electron-deficient groups) adjusted polarity to enhance the push-and-pull effect in the COF. The obvious result is that the extended light harvest region of sp²c-COF_ERDN, which is shifted from 620 nm to nearly 800 nm. In addition, the band gap of sp²c-COF is also narrowed after the introduction of the end group. Although the ERDN units only modify the outermost edge position of the sp²c-COF, they have a great influence on PHP activity. sp²c-COF_ERDN exhibited an HER value almost 1.6-fold that of the pristine sp²c-COF under otherwise identical conditions. With the increase in the conjugation degree of the sp²c-COF skeleton, electron migration efficiency is greatly improved and the charge carrier migration distance is shortened, which is more conducive to transferring more electrons to the catalyst surface to react with the substrates.

In the plotted the electron state density distribution of valence band maximum (VBM) and conduction band minimum (CBM) from density functional theory (DFT) calculations, the VBM electron state density is mostly distributed in the donor units, while the CBM is mostly located at the acceptor units. In other words, the charges could migrate readily from the donor units to the acceptor units under light irradiation. In 2021, Yu et al., designed and synthesized eight vinylene-linked, fully conjugated D-A sp²C-COFs with a pyrene (PPy) donor node, five of which possessed hydrogen evolution potentials under visible light irradiation in neutral solution without the help of a cocatalyst (PPy-BT, PPy-BT(F), PPy-PT, PPy-TzBI, and PPy-Q) ^[23][57]. They took D-D COFs (PPy-Ph) as a reference and carried out modifications for the presynthesized COFs, including narrowing band gaps, modulating band edge positions, decreasing exciton binding energy, and suppressing overpotentials. Therefore, because the highest occupied molecular orbital (HOMO) of the donor node and the lowest unoccupied molecular orbital (LUMO) of the acceptor edge are staggered in these D-A COFs, the energy offset between them is smaller than that of PPy-Ph D-D COF, which implies that D-A COFs could possess narrower band gaps. The narrowed band gaps have influence on the visible light absorption of D-A COFs; for instance, the absorption ranges of PPy-Q, -BT and -PT achieve different degrees of extension, among which the absorption of PPy-PT redshifted nearly 100 nm.

Recently, Wang et al., rationally designed and successfully synthesized three vinylene-linked, fully sp²-C-conjugated 2D COFs—BTH-1, 2, 3—among which BTH-3 possessed a good PHP performance of 15.1 mmol·h⁻¹·g⁻¹ due to the enhanced D-A effect in itself ^[24][58]. Thanks to the strong D-A effect, BTH-3 showed a perfect absorption capacity and efficiency for visible light, even at ~800 nm. Meanwhile, the band gaps were 1.91, 2.02, and 1.42 eV for BTH-1, 2, 3, respectively. It is the narrowest band gap that made the charge in BTH-3 transfer more rapidly, thus efficiently suppressing the recombination of electron–hole pairs. The point could be also confirmed by the highest photocurrent response and the smallest impedance in the electrochemical investigation. Excitingly, BTH-3 showed 0.883% AQY at 600 nm, which was not found in other COFs.

Moreover, Xu et al., also synthesized a vinylene-linked COF (termed g-C₅₄N₆-COF) from two D₃h symmetric monomers (tricyanomesitylene and TFBTP) via Knoevenagel condensation ^[25][59]. TFBTP was replaced with DCTMP to synthesize the control example (termed g-C₅₂N₆-COF). Interestingly, comparison to g-C₅₂N₆-COF, g-C₅₄N₆-COF with octupolar conjugated features showed similar solvatochromic effect to TtaTfa, TpaTfa, and TtaTpa. This was later proved to be due to the promoted light-harvesting ability of g-C₅₄N₆-COF. Notably, there were two different but ordered D-A units in g-C₅₄N₆-COF, in which tricyanomesitylene and 1, 3, 5-triazine served as acceptors, respectively, and the benzene ring served as the common donor. Accordingly, the two elemental octupolar substructures significantly facilitated charge delocalization in g-C₅₄N₆-COF. The low charge transfer resistance in electrochemical studies and the long exciton lifetime in PL spectra consistently prove that g-C₅₄N₆-COF should have better HER ability of 2518.9 μmol·h⁻¹·g⁻¹, which is two times higher than that of g-C₅₂N₆-COF.

In 2013, Jin et al., reported a type of borate-ester-linked D-A COF, D_ZnPc-A_NDI-COF, which has good photocatalytic potential because of its excellent charge dynamic properties ^[26][60]. The highly ordered and conjugated structure endows D_ZnPc-A_NDI-COF with a wide visible and near-infrared light absorption region even up to 1100 nm. Femtosecond transient absorption spectra of D_ZnPc-A_NDI-COF showed that the charge separation and delocalization in the D-A structure were so significantly efficient that they could be accomplished in 1.4 ps. The long lifetime (10 μs, 11 μs) of the charge-separated state is clearly shown in the nanosecond transient time profile at 480 nm in both polar and nonpolar solvents, which is attributed to the suppressed charge recombination caused by the charge delocalization effect in the bicontinuous donor–acceptor units. Afterwards, they developed eight more D_MPc-A_DI-COFs using other metallophthalocyanine units to correlate their structure–function relationships ^[27][61], and they found that these long-lived charge separation states in D_MPc-A_DI-COFs offered the great chance to extract holes and electrons for electric current production. Unfortunately, the borate ester COFs were prone to be hydrolyzed in acid and basic solvents, which inhibits their applications in the photocatalytic field ^[28][62].