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Review

Linkage-Affected Donor–Acceptor Covalent Organic Frameworks for Photocatalytic Hydrogen Production

by
Feng-Dong Wang
1,†,
Wei Liu
1,†,
Jiao Wang
1 and
Chen-Xi Zhang
1,2,*
1
College of Chemical Engineering and Materials Science, Tianjin University of Science and Technology, Tianjin 300457, China
2
Key Laboratory of Brine Chemical Engineering and Resource Eco-Utilization, Tianjin University of Science and Technology, Tianjin 300457, China
*
Author to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Processes 2023, 11(2), 347; https://doi.org/10.3390/pr11020347
Submission received: 21 December 2022 / Revised: 13 January 2023 / Accepted: 17 January 2023 / Published: 20 January 2023
(This article belongs to the Section Materials Processes)
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Abstract

:
The depletion of traditional fossil energy and the resulting environmental pollution forces people to explore new energy sources. Direct use of solar energy is now a viable solution for solving these problems. Covalent organic frameworks (COFs) are a porous crystalline material; their well-defined two-dimensional or three-dimensional frameworks can ensure the orderly arrangement of photoelectric active units, giving them potential photoelectric conversion applications. The tunable structural features endow COFs many advantages in photocatalytic hydrogen production under visible light. This review comprehensively summarizes the research progress on photoelectronic donor–acceptor (D-A) COFs with tunable structure for photocatalytic hydrogen evolution and will provide a feasible guiding strategy for applying this type of COFs in photocatalytic hydrogen production.

Graphical Abstract

1. Introduction

Photoelectric properties are the conversion of light energy into electrical energy, which is further effectively converted into chemical energy. The photoelectric effect is widely used in modern science, especially the conversion of photoelectric signals and the of light energy into chemical energy [1,2,3,4]. The most direct and effective light source in nature is sunlight. How to effectively utilize sunlight energy has become an urgent topic in contemporary science and provides an effective method of relieving current energy and environmental issues [5,6,7]. A practical method is to use photosensitive materials to convert solar energy into usable energy. Substances with such characteristics exist in nature, which—combined with their organelles and related enzymatic substances—convert solar energy into chemical energy [8,9]. To realize the efficient conversion and utilization of solar energy, we require photoelectric conversion materials that can be prepared on a large scale and can be used at a low cost to catalyze photochemical reactions.
In the early days, inorganic semiconductors were widely studied as photoelectric conversion material. In 1972, Akira Fujishima used TiO2 to split water into oxygen and hydrogen under ultraviolet (UV) light, which led to a large-scale study of photocatalytic reactions [10]. At present, the research of most widely studied and mature semiconductor photocatalysts mainly focuses on noble metal catalysts [11,12,13] (silver-based semiconductors, tungsten-based semiconductors), transition metal oxides [14,15] (TiO2, SnO2, etc.), and metal sulfides [16,17,18,19] (CdS, ZnS, etc.). However, the above traditional semiconductor photocatalysts suffer from the problem of photogenerated electron–hole recombination; some of the low reaction kinetics of proton reduction are not active under UV or visible light [20], which leads to the inability to effectively maintain their photocatalytic performance, seriously restricting their application. As a result, these materials must be modified with other materials, such as noble metals. However, some limitations of the narrow photocatalytic region and the ability to absorb a small fraction (<5%) of incident solar irradiation and indoor light still exist [21,22]. Therefore, new photocatalysts that can overcome the above shortcomings must be developed.
As hydrogen plays an increasingly important role in various green energies, producing hydrogen from photocatalytic splitting water has become a hot topic which is also supported by the national policy of vigorously developing clean energy. However, in the conventional preparation of hydrogen, these catalysts often perform inefficiently due to the defects mentioned above. In a recent research, Chen et al., synthesized a new COF, named Tz-COF-3, which has made another breakthrough in photocatalytic hydrogen production and led us to turn our attention to this type of COFs again [23]. As a new emerging class of porous functional crystal, COFs could be used as organic semiconductors materials to broaden application prospects in energy, catalysis, and separation. Among them, COFs with sensitive photoelectric properties have attracted extensive attention as a special type of COF-based materials. The porous feature of COFs brings an enormous specific surface area, which can provide abundant active sites for photochemical reactions. At the same time, their highly ordered and adjustable skeleton facilitates the rational design of light-absorbing–light-receiving units, providing the possibility of realizing photoelectronic properties. Therefore, COFs are often used as photocatalysts in various fields, such as H2 or O2 production [24], CO2 reduction [25], etc. Zhao, Zhang, and Li et al., both reported COFs in detail from different connection modes and application fields [26,27,28]. Nonetheless, they did not sort out D-A COFs completely.
Within the large family of COF-based photocatalysts, D-A COFs are a relatively special class of members due to the electron D-A units in their skeletal structure. It is the presence of such structural units that endows COFs with many benefits, such as greatly improving photoelectric conversion efficiency, which has made the utilization of D-A COFs gradually become popular—for instance, in the fields of high-efficiency organic photoelectrode for water splitting [29], organic matter conversion, and heterogeneous photocatalysis [30], particularly photocatalytic hydrogen production (PHP). Here, a general mechanism of COF-based photocatalysts for PHP is reviewed and the recent works about donor–acceptor (D-A) COFs in this field are summarized to discuss the structure–property relationship according to different bonding methods, including amine, vinylene, and borate ester types. The purpose is to assist in the exploration of structure–performance correlation and facilitate the ability of researchers in this field to accurately find further research directions. Finally, an available outlook on how to extend and explain the structure–property relationship of D-A COFs next is put forward.

2. Photocatalytic Mechanisms

2.1. General Photocatalytic Mechanism of COF-Based Photocatalysts

Deeply understanding the working principle of photocatalysts or photocatalyst systems in water is helpful in guiding catalyst design and optimizing photocatalytic efficiency. As shown in Figure 1, the latest principle of photocatalytic water splitting for producing hydrogen over COF semiconductor catalysts mainly consists of three steps [23,31,32]: (i) owing to the inherently low dielectric constants, the catalyst absorbs photons and is excited to produce excitons (bound photogenerated electron–hole pairs); (ii) the excitons dissociate, the charge carriers (photogenerated electrons and holes) generate, separate, and transfer (to the active centers or catalyst interface); and (iii) the electrons on the surface interact with substrate in the hydrogen evolution reaction and oxygen evolution reaction. Due to the water splitting reaction being a process whose Gibbs free energy increases, the theoretical band gap of the semiconductor photocatalyst should be over 1.23 eV, and it also needs to be narrower than 3.10 eV to realize efficient utilization of the visible light (λ > 400 nm) [33]. Indeed, since this paper only focuses on the application of COFs in photocatalytic hydrogen evolution, we only need to consider the conditions for accelerating the semireaction of hydrogen reduction. For simultaneous oxidation reaction, it is often necessary to offer sacrificial electron donors (SED) for hole consumption—for example, ascorbic acid (AA) and triethanolamine (TEOA).

2.2. Mechanism of Enhancing Photocatalysis

However, even if some methods to improve PHP efficiency have been reported in the early years, the current methods of using photocatalysts are still in a relatively inefficient stage. According to the most reported works, it can be found that the COFs used to produce hydrogen have the following advantages: (1) stable structure ensures that COFs can continue to function in most solvents (e.g., base, acid, or others) and in harsh conditions (e.g., high temperature or at least the corresponding experimental conditions); (2) highly ordered and conjugated structure broadens light absorption range and provides sufficient energy for the accelerated immigration of photoelectrons and holes; and (3) a feasible band gap and suitable potentials of the conduction band (CB) and the valence band (VB). Good crystallinity is already one of the basic requirements of D-A COFs. Consequently, in line with the mechanism, there are three critical strategies in total for boosting photocatalytic performance of COFs: (1) broadening the light absorption region and its efficiency; (2) promoting charge charrier separation–transfer rate; and (3) enhancing surface catalytic reaction [35], such as by constructing a D-A structure, loading cocatalyst, and so on. Construction of D-A system is a novel, effective and simple method of modulating the optical and electronic features to meet the requests mentioned above. That is to say, the COFs containing D-A system almost all possess appropriate pretuned CB and VB positions in which CB is more negative than the reduction potential (H+/H2 = 0 V vs. normal hydrogen electrode (NHE), pH = 0) to meet the thermodynamic driving force.
The D-A COFs refers to COFs that have been introduced into D-A units. Typically, electron-deficient building blocks act as acceptors and electron-rich building blocks act as donors. The donor may offer electrons and the acceptor may accept electrons, leading to the generation of a polarized electric field, which facilitates photocatalytic activity. When they were linked by covalent bonds, the push–pull effect (or D-A effect, the ability to adjust the electronic properties) between the building blocks was formed and then spread in the COF systems. D-A units in the COFs play an important role. For example, the alternating electron-donating and electron-withdrawing motifs further extend the π-conjugated system to expand the light absorption region and promote light-harvesting, thus stimulating charge separation and transport of electrons [36]. Moreover, integrating the D-A system into the π-conjugated skeleton structure may endow COFs with a stronger effect of electron transportation, preventing the problem that seriously limits photocatalytic activity, the recombination of electron (e−)–hole (h+) pairs. Importantly, modifying the D-A structure means modulating the molecular orbital energy levels, which could be adjusted by utilizing different electron donor and electron acceptor building blocks. All these merits offer some measures for improving the PHP capacity of D-A COF photocatalysts.

3. D-A COFs for PHP

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 [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. The different unit of D-A COF reported recently are summarized in Table 1.
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 [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 [38]. Table 1 is aimed at providing a brief review on the structure of COFs containing the D-A system, creating a simple statistic of the donors, acceptors, and their roles in the skeletons for HER processes. In this section, we summarized D-A COFs with three aforementioned representative linkages for PHP application and had a discussion to compare their difference in this field on the basis of different linkers. Moreover, due to the linkers possibly having influence on the optical properties and because they will further affect the semiconductor properties of the COFs, it is feasible to use the linkers as a classification basis to study the differences of D-A COFs in PHP.

3.1. Amine-Linked D-A COFs

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.

3.1.1. Imine-Linked D-A COFs

Based on previous studies [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 [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 (Figure 2a,b), 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 (Figure 3a). 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 [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 (Figure 3b). 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 [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 (Figure 4c), 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, Figure 5b,c). Their donors and acceptors are derivatives of pyrene and benzothiadiazole, respectively, and play a vital role in enhancing light-harvesting and light-absorbing abilities [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 (Figure 5c) 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 (Figure 5d), 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 [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) (Figure 6b). 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 (Figure 6). 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.

3.1.2. β-Ketoenamine-Linked D-A COFs

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 [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 Eg 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 (Figure 7a). The authors found that with the increasing number of carbonyl groups, the exciton binding energy (Eb) decreased considerably, which indicated that the improved D-A interactions in Tz-COFs greatly reduced the Eb. Due to the Eb 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 real-time dynamic behavior of free charge carriers in Tz-COF-3 is shown in Figure 7b,c, in which there is a clear negative signal on the behalf of the presence of long-lived free charge carriers. The implemented built-in control strategy enhances the D-A interaction in Tz-COFs, which is beneficial to the reduction of Eb. As Eb decreased, the excitons were more likely to dissociate to generate long-lived free charge carries, which boosts the photocatalytic process.
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 [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 Eb 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) [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. As shown in Figure 8b, at λem = 665 nm, the very low PL intensity of HBT-COF is clearly displayed, which indicates that PL emission of HBT-COF is quenched more heavily. In summary, the recombination of electron–hole pairs in HBT-COF are suppressed more heavily, which results in the enhanced separation and immigration of charge in HBT-COF. The evidence in Figure 8c and the electrochemical investigation could also prove that charges in HBT-COF are able to separate and transfer rapidly. Their work had implications for exploring the structure–function relationship.
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 [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 (Figure 9a) [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) (Figure 9b). 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 (Figure 9c), 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 [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 [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. A shortest average lifetime of 2.5-TBTA/g-C3N4 also proves that the fastest charge transfer is between TBTA and g-C3N4, which was also shown in Figure 10b,c. The evidence above proves that the hybrid material 2.5-TBTA/g-C3N4 has strong absorption efficiency for visible light and could accelerate electron immigration, which further influences the PHP activity of the hybrid by promoting photoinduced charge dissociation in it.

3.1.3. Imide-Linked D-A COFs

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 [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 (Figure 11a). 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, Eg 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 (Figure 11b). 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.

3.2. Vinylene-Linked D-A COFs

The polarization of the carbon–nitrogen bond in the above-mentioned C=N linkage cannot promote efficient conjugation through 2D backbone [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 [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-COFERDN) (Figure 12a) [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-COFERDN, 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-COFERDN 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) [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 (Figure 13), 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 (Figure 14a) [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 (Figure 14b). 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 [59]. TFBTP was replaced with DCTMP to synthesize the control example (termed g-C52N6-COF). The high crystallinity of these two COFs is shown in Figure 15a,b. 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 (Figure 15c). 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.

3.3. Borate-Ester-Linked D-A COFs

In 2013, Jin et al., reported a type of borate-ester-linked D-A COF, DZnPc-ANDI-COF, which has good photocatalytic potential because of its excellent charge dynamic properties [60]. The highly ordered and conjugated structure endows DZnPc-ANDI-COF with a wide visible and near-infrared light absorption region even up to 1100 nm (Figure 16a). Femtosecond transient absorption spectra of DZnPc-ANDI-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 (Figure 16b). 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 (Figure 16c), 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 DMPc-ADI-COFs using other metallophthalocyanine units to correlate their structure–function relationships [61], and they found that these long-lived charge separation states in DMPc-ADI-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 [62].

4. Summary and Outlook

The introduction of D-A structure into COFs materials is a feasible and important way to improve the utilization efficiency and performance of these materials in various applications, especially in the production of hydrogen by photocatalytic water splitting. D-A COFs will still be a kind of photocatalyst with great potential for improvement. In terms of synthetic means, for instance, some new methods (Wittig reaction and its improved method, Horner–Wadsworth–Emmons (HWE) reaction) have been applied to the synthesis of vinylene linked COFs, which will also make the further progress of D-A COFs possible. At present, it has been the mainstream operation to modify the band gap to broaden the absorption range of visible light, restraining the recombination of electron–hole pairs to increase the electron transfer rate, and even enhancing the interfacial features of COFs (such as hydrophilic ability) to promote the synergistic interactions among the multiple components in substrates by using various donors and acceptors. There are many linkers for COFs. However, the linkers that could be used for D-A COFs and that exhibited good performance in the meantime are still limited. Hence, developing more and new linkers to broaden the family of D-A COFs would be a trend in the future. Moreover, the structure–function correlation still requires further exploration, which is also one of the reasons for limiting the large-scale production of COFs. Currently, amine-linked D-A COFs are more practical than vinylene-linked D-A COFs, as shown by their generally higher HER values. However, traditional characterization tools cannot satisfy the exploration of the fundamental kinetic reasons for carries migration, so more advanced instruments, such as fs-TAS, atomic force microscopy (AFM), etc., should be emphasized. Interestingly, compared to traditional trial-and-error research, a new class of fundamentals for the development of chemical materials, such as theoretical calculation and machine learning, could identify or predict the best materials or synthesis methods more accurately. In terms of material structure, making D-A COFs into films may also have a positive enhancement of photocatalytic activity, but there are few relevant works at present, and the topic has great research prospects in the future.

Author Contributions

All authors conceived the research idea and the framework of this study. F.-D.W. and W.L. wrote and edited the manuscript. J.W. prepared the figures and analyzed the structure of the manuscript. C.-X.Z. reviewed the manuscript and funded the article. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the National Natural Science Foundation of China, grant numbers 21771111, 21371104, 20771081, 21101096, and 21471084.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Conflicts of Interest

The authors declare no conflict of interest.

Abbreviation

1.
Tz
= benzo [1,2-b:4,5-d] bisthiazole-2,6-diamine
2.
Tfa
= triphenylamine
3.
Tta
= triazine
4.
Tpa
= tris(4-formylphenyl)benzene (Tpa-CHO) and 1,3,5-Tris(4-aminophenyl)benzene (Tpa-NH2)
5.
PE
= 4,4′,4′’,4′’’-(ethene-1,1,2,2-tetrayl)tetraaniline
6.
TzDA
= 4,4’-(thiazolo [5,4-d]thiazole-2,5-diyl)dibenzaldehyde
7.
PyTA
= 4,4’,4’’,4’’’-(pyrene-1,3,6,8-tetrayl)tetraaniline
8.
Tp
= 1,3,5-triformylphloroglucinol
9.
BDCN
= 4,4’-diamino-[1,1’-biphenyl]-3,3’-dicarbonitrile
10.
TP
= 2,4,6-triformylphloroglucinol
11.
BTDA
= 4,4′-(benzo-1,2,5-thiadiazole-4,7-diyl)dianiline

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Figure 1. The complete process of photocatalytic hydrogen evolution reaction. Reproduced with permission from Ref. [34]. Copyright: The Chemical Society of Japan, 2021.
Figure 1. The complete process of photocatalytic hydrogen evolution reaction. Reproduced with permission from Ref. [34]. Copyright: The Chemical Society of Japan, 2021.
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Figure 2. (a) Synthesis of the COFs studied in this work. AC = ascorbic acid modification. EPR conduction band electron spectra of (b) the protonation of TtaTfa with L-ascorbic acid. (c) Color of TtaTfa powder and dispersion at different reaction conditions. Reproduced with permission from Ref. [41]. Copyright: Angewandte Chemie International Edition, 2021.
Figure 2. (a) Synthesis of the COFs studied in this work. AC = ascorbic acid modification. EPR conduction band electron spectra of (b) the protonation of TtaTfa with L-ascorbic acid. (c) Color of TtaTfa powder and dispersion at different reaction conditions. Reproduced with permission from Ref. [41]. Copyright: Angewandte Chemie International Edition, 2021.
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Figure 3. (a) Synthesis of the D-A COFs in this work. (b) Steady-state PL spectra of PETZ–COF and PEBP-COF excited by using a 540 nm laser. Reproduced with permission from Ref. [42]. Copyright: Royal Society of Chemistry, 2022.
Figure 3. (a) Synthesis of the D-A COFs in this work. (b) Steady-state PL spectra of PETZ–COF and PEBP-COF excited by using a 540 nm laser. Reproduced with permission from Ref. [42]. Copyright: Royal Society of Chemistry, 2022.
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Figure 4. (a) Synthetic route and top view of the PyTz-COF. (b) PL spectra of PyTz-COF and PyBp-COF (excitation at 320 nm). (c) TRPL spectra of PyTz-COF and PyBp-COF (excitation at 320 nm). Reproduced with permission from Ref. [43]. Copyright: Angewandte Chemie International Edition, 2020.
Figure 4. (a) Synthetic route and top view of the PyTz-COF. (b) PL spectra of PyTz-COF and PyBp-COF (excitation at 320 nm). (c) TRPL spectra of PyTz-COF and PyBp-COF (excitation at 320 nm). Reproduced with permission from Ref. [43]. Copyright: Angewandte Chemie International Edition, 2020.
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Figure 5. (a) The relationship of donor−acceptor moieties in PS I. (b) Mechanism of hydrogen production from photocatalytic water splitting for NKCOFs-108 (via analogy with PS I). (c) Structural models for NKCOFs with perfectly eclipsed AA stacking shown parallel to the pore channel along the crystallographic c axis. Reproduced with permission from Ref. [44]. Copyright: American Chemical Society, 2021.
Figure 5. (a) The relationship of donor−acceptor moieties in PS I. (b) Mechanism of hydrogen production from photocatalytic water splitting for NKCOFs-108 (via analogy with PS I). (c) Structural models for NKCOFs with perfectly eclipsed AA stacking shown parallel to the pore channel along the crystallographic c axis. Reproduced with permission from Ref. [44]. Copyright: American Chemical Society, 2021.
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Figure 6. (a) Highly similar structures of TeTz-COF1 and TeTz-COF2. (b) Mechanism of photocatalytic HER of TeTZ–COFs. Reproduced with permission from Ref. [45]. Copyright: Royal Society of Chemistry, 2022.
Figure 6. (a) Highly similar structures of TeTz-COF1 and TeTz-COF2. (b) Mechanism of photocatalytic HER of TeTZ–COFs. Reproduced with permission from Ref. [45]. Copyright: Royal Society of Chemistry, 2022.
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Figure 7. (a) Schematic illustration of the synthesis of Tz-COFs with adjustable D-A interactions toward photocatalysis. (b) Integrated PL intensity as a function of temperature of Tz-COF-3 (inset: temperature-dependent PL spectra from 15 to 273 K). (c) Femtosecond time-resolved transient absorption spectra (fs-TAS) (time unit: ps) of Tz-COF-3. Reproduced with permission from Ref. [23]. Copyright: American Chemical Society, 2022.
Figure 7. (a) Schematic illustration of the synthesis of Tz-COFs with adjustable D-A interactions toward photocatalysis. (b) Integrated PL intensity as a function of temperature of Tz-COF-3 (inset: temperature-dependent PL spectra from 15 to 273 K). (c) Femtosecond time-resolved transient absorption spectra (fs-TAS) (time unit: ps) of Tz-COF-3. Reproduced with permission from Ref. [23]. Copyright: American Chemical Society, 2022.
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Figure 8. (a) Top views of the ideal eclipsed structures of HBT-COF (top) and BT-COF (bottom). (b) PL spectra (λex = 450 nm). (c) TRPS (λex = 475 nm, λem = 665 nm). Reproduced with permission from Ref. [47]. Copyright: American Chemical Society, 2021.
Figure 8. (a) Top views of the ideal eclipsed structures of HBT-COF (top) and BT-COF (bottom). (b) PL spectra (λex = 450 nm). (c) TRPS (λex = 475 nm, λem = 665 nm). Reproduced with permission from Ref. [47]. Copyright: American Chemical Society, 2021.
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Figure 9. (a) Side views of the ideal eclipsed structures of PIm-COF1 (left) and PIm-COF2 (right). (b) UV−Vis DSR spectra of PIm-COF1 (blue curve), PIm-COF2 (orange curve), and ligand PIDA (black curve). (c) PL intensity of PIm-COFs. Reproduced with permission from Ref. [50]. Copyright: ChemRxiv, 2022.
Figure 9. (a) Side views of the ideal eclipsed structures of PIm-COF1 (left) and PIm-COF2 (right). (b) UV−Vis DSR spectra of PIm-COF1 (blue curve), PIm-COF2 (orange curve), and ligand PIDA (black curve). (c) PL intensity of PIm-COFs. Reproduced with permission from Ref. [50]. Copyright: ChemRxiv, 2022.
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Figure 10. (a) The synthesis of the 2.5-TBTA/g-C3N4 hybrid. (b,c) Charge density differences in the constructed cluster model of TBTA/g-C3N4 (blue: accumulation region; yellow: depletion region). (d) The possible photocatalytic mechanism. Reproduced with permission frrom Ref. [51]. Copyright: Royal Society of Chemistry, 2021.
Figure 10. (a) The synthesis of the 2.5-TBTA/g-C3N4 hybrid. (b,c) Charge density differences in the constructed cluster model of TBTA/g-C3N4 (blue: accumulation region; yellow: depletion region). (d) The possible photocatalytic mechanism. Reproduced with permission frrom Ref. [51]. Copyright: Royal Society of Chemistry, 2021.
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Figure 11. (a) Schematic illustration of the chemical structure of PMDA-COF. (b) UV–Vis diffuse reflectance spectra and (c) H2 evolution rates of the three COFs. Reproduced with permission from Ref. [53]. Copyright: Elsevier, 2021.
Figure 11. (a) Schematic illustration of the chemical structure of PMDA-COF. (b) UV–Vis diffuse reflectance spectra and (c) H2 evolution rates of the three COFs. Reproduced with permission from Ref. [53]. Copyright: Elsevier, 2021.
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Figure 12. (a) Schematic representation of the synthesis of sp2c-COFERDN and (b) PHP from water of sp2c-COFERDN. Reproduced with permission from Ref. [56]. Copyright: Elsevier, 2019.
Figure 12. (a) Schematic representation of the synthesis of sp2c-COFERDN and (b) PHP from water of sp2c-COFERDN. Reproduced with permission from Ref. [56]. Copyright: Elsevier, 2019.
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Figure 13. (a) Two types of band gap in COFs and corresponding energy level alignment of frontier orbitals in building blocks. (b) Schematic illustration of the stacking structure of D-A COFs and the separation of one-dimensional transport channels for electron and hole. (c) Stacking structure of PPy-BT. Reproduced with permission from Ref. [57]. Copyright: American Chemical Society, 2021.
Figure 13. (a) Two types of band gap in COFs and corresponding energy level alignment of frontier orbitals in building blocks. (b) Schematic illustration of the stacking structure of D-A COFs and the separation of one-dimensional transport channels for electron and hole. (c) Stacking structure of PPy-BT. Reproduced with permission from Ref. [57]. Copyright: American Chemical Society, 2021.
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Figure 14. (a) Synthetic routes and chemical structures of COFs BTH-3. (b) AQY of BTH-1, 2, 3 under different monochromatic light irradiation. Reproduced with permission from Ref. [58]. Copyright: Nature, 2022.
Figure 14. (a) Synthetic routes and chemical structures of COFs BTH-3. (b) AQY of BTH-1, 2, 3 under different monochromatic light irradiation. Reproduced with permission from Ref. [58]. Copyright: Nature, 2022.
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Figure 15. PXRD patterns and comparisons between (a) g-C54N6-COF and (b) g-C52N6-COF. (c) The octupolar structure with acceptors and donors alternating around a benzene ring or a triazine ring, respectively (top), and the aligned octupolar structure of g-C54N6-COF (bottom). Reproduced with permission from Ref. [59]. Copyright: Angewandte Chemie International Edition, 2020.
Figure 15. PXRD patterns and comparisons between (a) g-C54N6-COF and (b) g-C52N6-COF. (c) The octupolar structure with acceptors and donors alternating around a benzene ring or a triazine ring, respectively (top), and the aligned octupolar structure of g-C54N6-COF (bottom). Reproduced with permission from Ref. [59]. Copyright: Angewandte Chemie International Edition, 2020.
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Figure 16. Illustration of a 2 × 2 cell of the 0.8 Å slipped AA-stacked COFs and photochemical events. (b) Femtosecond transient absorption spectra at an initial time of 1.4 ps. (c) The time profile of the nanosecond transient absorption band at 480 nm of a benzonitrile-dispersed COF suspension at 298 K (red dot). The lifetime of the charge-separated state was estimated to be 10 ms using curve-fitting (dotted black curve). Reproduced with permission from Ref. [60]. Copyright: Angewandte Chemie International Edition, 2013.
Figure 16. Illustration of a 2 × 2 cell of the 0.8 Å slipped AA-stacked COFs and photochemical events. (b) Femtosecond transient absorption spectra at an initial time of 1.4 ps. (c) The time profile of the nanosecond transient absorption band at 480 nm of a benzonitrile-dispersed COF suspension at 298 K (red dot). The lifetime of the charge-separated state was estimated to be 10 ms using curve-fitting (dotted black curve). Reproduced with permission from Ref. [60]. Copyright: Angewandte Chemie International Edition, 2013.
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Table
Table 1. Summary of different structure units of D-A COFs containing for H2 evolution.
Table 1. Summary of different structure units of D-A COFs containing for H2 evolution.
PhotocatalystDonorAcceptorConnection ModeRole of SystemHER
(mmol h−1 g−1)

TtaTfa		Processes 11 00347 i001Processes 11 00347 i0023 + 3
Imine-linked


enhanced the light absorption ability and promoted the charge separation efficiency
20.7

TpaTfa		Processes 11 00347 i003Processes 11 00347 i0043 + 3
Imine-linked		14.9

TtaTpa		Processes 11 00347 i005Processes 11 00347 i0063 + 3
Imine-linked		10.8

PyTz-COF		Processes 11 00347 i007
Processes 11 00347 i008		4 + 2
Imine-linked		enabled effective photogenerated charge separation and efficient charge migration
2.01

NKCOF-108		Processes 11 00347 i009
Processes 11 00347 i010		4 + 2
Imine-linked		promoted the separation of photogenerated carrier and broadened visible light response range
12.0

PETZ-COF		Processes 11 00347 i011
Processes 11 00347 i012		4 + 2
Imine-linked		increased visible light response range, improved the separation efficiency of excitons and accelerated the electron transport
7.32

TeTz-COF1		Processes 11 00347 i013Processes 11 00347 i0143 + 2
Imine-linked		tuning the charge separation and transport for efficient photocatalysis
2.1

TeTz-COF2		Processes 11 00347 i015Processes 11 00347 i0163 + 2
Alkyne-linked		theoretically enhance the photocatalytic hydrogen evolution of COFs, while it has not been demonstrated in this work
0.11


Tz-COF-3
Processes 11 00347 i017
Processes 11 00347 i018
3 + 2
β-ketoenamine-linked
accelerated exciton dissociation and generated more long-lived photo-generated charge carriers
43.2

2.5-TBTA/g-C3N4
Processes 11 00347 i019
Processes 11 00347 i020
3 + 2
β-ketoenamine-linked		enabled the enhancement of visible light absorption and photoinduced charge separation
11.73

PIm-COF2		Processes 11 00347 i021Processes 11 00347 i0223 + 2
β-ketoenamine-linked		enhanced conjugation effect, broadened light absorption region and narrowed optical band gap
7.42

CN-COF		Processes 11 00347 i023Processes 11 00347 i0243 + 2
β-ketoenamine-linked		enhanced charge separation and immigration efficiency
60.85
HBT-COFProcesses 11 00347 i025Processes 11 00347 i0263 + 2
β-ketoenamine-linked		broadened visible light absorption range and led to efficient photoinduced charge separation and transfer
3.8
PMDA-COFProcesses 11 00347 i027Processes 11 00347 i0283 + 2
Imide-linked		modified the electronic band structure and separate the reduction site
0.44
sp2c-COFProcesses 11 00347 i029Processes 11 00347 i030
Processes 11 00347 i031		2 + 2 + 1
Vinylene-linked		harvested a broad range of visible and near-infrared light
2.12
PPy-BT




Processes 11 00347 i032		Processes 11 00347 i033




4 + 2
Vinylene-linked




reduced exciton binding energies to promote electron-hole separation and modulated band gaps




Potential for PHP
PPy-BT(F)Processes 11 00347 i034
PPy-PTProcesses 11 00347 i035
PPy-TzBIProcesses 11 00347 i036
PPy-Q(F)Processes 11 00347 i037

BTH-3		Processes 11 00347 i038Processes 11 00347 i0393 + 2
Vinylene-linked		harvested a broad range of visible and near-infrared light
15.1

g-C54N6-COF		Processes 11 00347 i040
Processes 11 00347 i041		3 + 3
Vinylene-linked		formed two types of octupolar conjugated structure to enhance light-harvesting and photo-induced charge generation & separation
2.52
DZnPc-ANDI-COFProcesses 11 00347 i042Processes 11 00347 i0434 + 2
Borate-Ester-linked		suppressed charge recombination and retained the charges for a prolonged period of timePotential for PHP
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Wang, F.-D.; Liu, W.; Wang, J.; Zhang, C.-X. Linkage-Affected Donor–Acceptor Covalent Organic Frameworks for Photocatalytic Hydrogen Production. Processes 2023, 11, 347. https://doi.org/10.3390/pr11020347

AMA Style

Wang F-D, Liu W, Wang J, Zhang C-X. Linkage-Affected Donor–Acceptor Covalent Organic Frameworks for Photocatalytic Hydrogen Production. Processes. 2023; 11(2):347. https://doi.org/10.3390/pr11020347

Chicago/Turabian Style

Wang, Feng-Dong, Wei Liu, Jiao Wang, and Chen-Xi Zhang. 2023. "Linkage-Affected Donor–Acceptor Covalent Organic Frameworks for Photocatalytic Hydrogen Production" Processes 11, no. 2: 347. https://doi.org/10.3390/pr11020347

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