Key words β-ketoenamine - covalent organic frameworks - photocatalytic - hydrogen
Introduction
As a green and sustainable energy, hydrogen is one of the most promising alternative
candidates for traditional fossil energy, and expected to solve the current energy
crisis and severe environmental deterioration.[1 ] Among many approaches to hydrogen, the photocatalysis technology driven by inexhaustible
solar energy undoubtedly offers a green and sustainable path.[2 ] It is worth mentioning that there are three key processes in the photoinduced water
splitting[3 ]: (a) the light adsorption of the photocatalyst, (b) the separation and transfer
of photoinduced charge-carriers in the catalyst, and (c) the acquisition of electrons
by H+ on the surface of the catalyst to form hydrogen. Therefore, the development of catalysts
with high performance is one of the key scientific issues in this field. Over the
past few decades, numerous types of photocatalysts have been reported, and the regulation
and modification based on the structure, morphological structure and composite catalysts
have been fully explored.[4 ] Although some achievements have been made in the development of photocatalysts,
there are still some defects,[5 ] such as the narrow light absorption, the recombination of photogenerated electron-charge
pairs, low specific surface area and poor stability. Therefore, it is urgent to develop
photocatalysts with strong visible-light absorption and high charge-carrier utilization,
so as to realize efficient capture and utilization of solar energy and industrial
application of photocatalytic water splitting. Covalent organic frameworks (COFs)
are particularly prominent in the new generation of semiconductor photocatalysts due
to their excellent visible-light absorption, structural designability, high permanent
porosity and ultra-high physical and chemical stability.[5b ],[6 ] Since Lotsch et al.[7 ] first reported a hydrazone-based COF as a photocatalyst to achieve photocatalytic
hydrogen evolution in 2014, a large number of COF-based photocatalysts have been successfully
constructed for photocatalytic water splitting with amazing catalytic performances.[8 ] Among these COFs, β-ketoamide COFs (Tp-COFs, [Figure 1 ]) obtained by Schiff base reaction and irreversible enol-to-keto tautomerism with
1,3,5-triformylphloroglucinol (Tp) and amines as building blocks[9 ] exhibited pre-eminent hydrogen evolution activity.[4b ],[8e ],[10 ] The splendid photocatalytic activity of Tp-COFs may be related to the following
factors: 1) the abundant conjugated structure broadens the light absorption range
of the photocatalyst[11 ]; 2) the irreversible β-ketoamide bonds endow Tp-COFs with ultra-high thermal and
chemical stability, which was conducive to maintaining the stability of the structure
during catalysis[9 ]; 3) the carbonyl oxygen and enamine nitrogen atoms in the skeleton supply binding
sites for metal ions[10f ],[12 ]; 4) the strong π–π stacking between adjacent layers provides another path for charge-carrier
transfer[13 ]; 5) the high porosity and specific surface area offer more active sites for catalytic
reaction[14 ]; 6) the high crystallization reduces the recombination of photogenerated electron–hole
pairs in bulk defects[4a ] and offers more insight into the catalyst structure. At present, the optimization
strategy of Tp-COFs catalysts mainly focuses on two points: molecular and composite
engineering. In spite of the fact that the composite strategy is simple, general and
easy to operate, some intrinsic properties such as the difficulty of forming a controllable
composite interface and poor interface stability still restrict its widespread application
in the efficient photoinduced water splitting. By contrast, the molecular engineering
strategy shows more advantages in terms of regulable band structure and catalytic
site at the molecular level. Thus, it is necessary to summarize the research studies
of Tp-COFs in photocatalytic hydrogen evolution based on the molecular engineering
strategy. Previous reviews of Tp-COFs focused on synthesis[15 ] and photocatalytic hydrogen evolution with a preference for composite engineering
strategies.[16 ] Different from these reviews, this paper is dedicated to concentrating on Tp-COF
photocatalysts from the perspective of molecular engineering, including the construction
of planarization, optimization of ordered structure, precise organic functionalization
and effect of morphology. Finally, we also provide an outlook of the challenges and
some enlightenments for the subsequent construction of high-efficiency Tp-COF photocatalysts.
Figure 1 Schematic diagram of the synthesis of Tp-COFs (Tp-Pa-1 and 2).
Brief Introduction of Synthesis of Tp-COFs
Brief Introduction of Synthesis of Tp-COFs
To achieve a wide-range application of Tp-COF photocatalysts, it is particularly important
to develop convenient, economical and efficient synthesis methods. Additionally, since
synthesis methods have a strong impact on the photocatalytic hydrogen evolution performance
of Tp-COFs due to the different crystallinity, morphology and specific surface areas,
herein, the syntheses of Tp-COFs are also discussed (summarized in [Table 1 ]).
Table 1 The synthesis method of Tp-COFs
Method
Condition
Product form
Advantage
Optimization method
Ref.
CPB: cetylpyridinium bromide; Pa: p -phenylenediamine; PAN: polyacrylonitrile; rt: room temperature.
Solvothermal method
Mesitylene, dioxane, acetic acid, 120°C, 3 d
Powder
High crystallinity
Replacement solvent, pre-protection, construction of dynamic imine bonds, microwave
assisted
[9 ],[13a ],[17 ]–[20 ]
Mechanochemical method
1 – 2 Drops of mesitylene: dioxane (1 : 1), rt, 45 min
Powder
Rapid synthesis
Liquid-assisted mechanochemical method
[21 ]–[24 ]
Emulsion polymerization
CPB, dichloromethane, rt, 10 min
Powder
Rapid synthesis, high crystallinity and controlled morphology
–
[25 ]
Interfacial strategy
Dichloromethane, water, rt, 72 h, undisturbed condition
Film
Film-type catalyst
–
[27 ]
Electrospinning technology with solvothermal method
1. Pa/PAN film (electrospinning technology)
2. Tp-COF/PAN film (solvothermal method)
Film
Film-type catalyst, self-standing film, high flexibility
–
[28 ]
Generally, Tp-COFs are synthesized via the solvothermal method. Specifically, all
reactants are fully mixed in solvents and then sealed in Pyrex tubes for 3 days or
more at a certain temperature. In 2012, Banerjee et al.[9 ] first reported the successful synthesis of Tp-Pa-1/2 in mesitylene and dioxane solvents
with acetic acid as a catalyst in sealed Pyrex tubes at 120°C for 3 days. In the subsequent
study, other common solvents, such as N,N -dimethylformamide (DMF), dimethyl sulfoxide (DMSO) and N,N -dimethylacetamide (DMAc), were also used to synthesize Tp-COFs.[13a ],[17 ] Moreover, in 2017, Dichtel et al.[18 ] pre-protected diamines based on a formal transamination strategy to obtain Tp-COF
with an ultra-high specific surface area (> 2600 m2 /g). In 2019 and 2021, the crystallinity of Tp-COFs was improved by the construction
of dynamic imine bonds.[19 ] Furthermore, in 2015, Wang et al.[20 ] took the microwave-assisted solvothermal method to quickly construct Tp-COFs within
60 min; however, sealing conditions and high boiling solvent were still required.
Although it was inclined to form good crystalline Tp-COFs via solvothermal method,
the harsh reaction conditions and milligram-scale synthesis seriously limit its practical
application. Banerjeeʼs research group has done a lot of outstanding works in the
practical synthesis of Tp-COFs. In 2013, Banerjee et al.[21 ] efficiently prepared a series of Tp-COFs (TpPa-1, TpPa-1 and TpBD) with a graphene-like
layered morphology through solvent-free mechanochemical synthesis (named the grinding
method). Similarly, Tp-COF nanosheets were also prepared by this method,[22 ] such as TpPa-F4 , TpPa-(Me)2 , TpPa-(OMe)2 , TpPa-NO2 and TpPa-(NO2 )2 . However, these COFs exhibited poor crystallinity and porosity. To improve crystallinity,
this group also reported a liquid-assisted mechanochemical technique in the presence
of trace amounts of solvent (so-called microsolution), such as DMF, o -dichlorobenzene and acetic acid.[23 ] The microsolution was beneficial for dispersing reactants and thereby improving
the crystallinity of Tp-COFs to some extent. They also found that acid catalysts,
such as p -toluenesulfonic, and Tp-COF precursors would help maintain the reversibility of the
COF formation reaction to prepare highly crystalline Tp-COFs.[24 ] Very recently, emulsion polymerization has also been used to construct Tp-COFs.
This method can not only gently and rapidly produce COFs, but also avoid the use of
acidic catalysts and a large number of organic solvents.[25 ]
Based on the actual demand of photocatalysts, such as easy recovery after the reaction
and integration, the development of film-type COF catalysts is the trend. In recent
years, due to the challenges of constructing stable, continuous, highly crystalline
and porous COF films, there have been relatively few reports on the preparation of
Tp-COF films. In 2015, Wang et al.[26 ] demonstrated a one-way diffusion strategy to grow Tp-COF films on PEI-modified polyethersulfone
(PES) substrates. In 2017, Banerjee et al.[27 ] successfully fabricated four Tp-COF films with high crystallinity and specific surface
area at the dichloromethane/water interface via a bottom-up interfacial strategy,
also known as interface synthesis techniques. Recently, Agarwal et al.[28 ] reported the preparation of flexible self-standing Tp-COF membranes with high specific
surfaces and strong mechanical stability by the combination of the electrospinning
technology with the solvothermal method.
Therefore, in the subsequent design of high-performance Tp-COFs based on the molecular
engineering strategy, the selection of synthesis path is also particularly important.
In addition, it is worth pointing out that the research of COF membranes is still
in initial stage, and the development of Tp-COF-based plate reactors should be valued
highly.
Optimization of Photocatalytic Hydrogen Evolution by Molecular Engineering Strategies
Optimization of Photocatalytic Hydrogen Evolution by Molecular Engineering Strategies
Due to the replaceability of the building blocks, COFs have more opportunities for
regulation and modification of photocatalytic activity at the molecular level. Based
on regulations of group electronic properties, steric effect, conjugation degree and
so on, it is easy to design Tp-COF catalysts with different properties. Moreover,
the molecular engineering strategy avoids the regulation of complicated interfaces.
At present, Tp-COF photocatalysts constructed via the molecular engineering strategy
exhibit excellent hydrogen evolution, which is summarized in [Table 2 ]. The structure optimization of the molecular engineering strategy mainly focuses
on the following aspects.
Table 2 Summary of the photocatalytic hydrogen evolution performances of Tp-COFs constructed
via the molecular engineering strategy
COFs
Sacrificial agent
H2 evolution rate (mmol · h−1 · g−1 )
Light
Ref.
a Acetic acid as a catalyst. b Pyrrolidine as a catalyst. Overall water splitting. TEOA: triethanolamine; SA: sodium
ascorbate; AA: ascorbic acid. Pt co-catalyst was used unless otherwise illustrated.
TP-EDDA
TEOA
0.324
λ > 395 nm
[29 ]
TP-BDDA
TEOA
0.03
λ > 395 nm
[29 ]
Tp-DTP
SA
4.76 µmol · m−2 · h−1
λ > 400 nm
[30 ]
TpBD
SA
7.19 µmol · m−2 · h−1
λ > 400 nm
[30 ]
TpPa-H
SA
11.13 µmol · m−2 · h−1
λ > 400 nm
[30 ]
TpPa-Cl2
SA
11.73 µmol · m−2 · h−1
λ > 400 nm
[30 ]
TpPa-SO3 H
SA
4.44 µmol · m−2 · h−1
λ > 400 nm
[30 ]
TpPa-(CH3 )2
SA
3.62 µmol · m−2 · h−1
λ > 400 nm
[30 ]
AntCOF-150
TEOA
0.055
λ > 395 nm
[31 ]
BtCOF-150
TEOA
0.75
λ > 395 nm
[31 ]
TpCOF-150
TEOA
0.05
λ > 395 nm
[31 ]
TzCOF-150
TEOA
0
λ > 395 nm
[31 ]
COF-BBT
SA
48.7
λ > 420 nm
[32 ]
S-COF
AA
4.44
λ > 420 nm
[33 ]
FS-COF
AA
10.1
λ > 420 nm
[33 ]
BT-COFa
AA
2.02
λ > 420 nm
[34 ]
BT-COFb
AA
7.7
λ > 420 nm
[34 ]
30%PEG@BT-COF
AA
11.14
λ > 420 nm
[34 ]
RC-COF-1
AA
27.98
λ > 420 nm
[35 ]
COF-935
AA
67.55
λ > 420 nm
[36 ]
e -Tp-Pa-COF
AA
133.9
AM 1.5
[25 ]
TpBpy-Ni2%
AA
51.3
λ > 420 nm
[37 ]
Tp-DB-(OCH3 )2
SA
1.23
λ > 400 nm
[10g ]
Tp-DB-(CH3 )2
SA
0.81
λ > 400 nm
[10g ]
Tp-DB
SA
0.60
λ > 400 nm
[10g ]
Tp-DB-(NO2 )2
SA
0.015
λ > 400 nm
[10g ]
TpPa-COF
SA
1.56
λ > 420 nm
[38 ]
TpPa-COF-NO2
SA
0.22
λ > 420 nm
[38 ]
TpPa-COF-(CH3 )2
SA
8.33
λ > 420 nm
[38 ]
Tp-DBN
SA
1.8
λ > 420 nm
[10h ]
Pt@TpBpy-NSc
–
0.132
λ > 420 nm
[39 ]
Pt@TpBpy-2-NSc
–
41.3
λ > 420 nm
[39 ]
Tp-2C/BPy2+ -COF
AA
34.6
λ > 420 nm
[8a ]
BT-COF
AA
3.40
λ > 420 nm
[40 ]
HBT-COF
AA
19.00
λ > 420 nm
[40 ]
COF-H
AA
5.03
AM 1.5
[41 ]
COF-Cl
AA
5.84
AM 1.5
[41 ]
COF-F
AA
10.58
AM 1.5
[41 ]
CYANO-COF
AA
60.85
λ > 420 nm
[42 ]
CYANO-CN
AA
134.2
λ > 420 nm
[42 ]
BD-COF
AA
19.75
λ > 420 nm
[42 ]
BD-CN
AA
79.5
λ > 420 nm
[42 ]
TpBT-COF
AA
1.447
λ > 420 nm
[43 ]
Tp(BT0.5 TP0.5 )-COF
AA
9.839
λ > 420 nm
[43 ]
Tp(BT0.25 TP0.75 )-COF
AA
7.398
λ > 420 nm
[43 ]
Tp(BT0.1 TP0.9 )-COF
AA
5.822
λ > 420 nm
[43 ]
Tp(BT0.05 TP0.95 )-COF
AA
5.695
λ > 420 nm
[43 ]
TpTP-COF
AA
6.04
λ > 420 nm
[43 ]
2Me-OMe-COF
AA
33.1
λ > 420 nm
[44 ]
Me-2OMe-COF
AA
19.5
λ > 420 nm
[44 ]
TP-TTA/SiO2 -1
AA
153.2
λ > 420 nm
[45 ]
Construction of Planarization
Construction of Planarization
It has been proved that the improvement of structural conjugation would reduce the
Coulomb binding force of electrons and holes, thereby increasing the exciton dissociation
rate.[46 ] Therefore, the introduction of highly conjugated units is one of the effective ways
to improve the performance of COF photocatalysts. In 2017, Thomasʼ group successfully
synthesized the acetylene-bridged COFs (Tp-EDDA and Tp-BDDA, [Figure 2 ]) of which the highly conjugated structure endowed charge carriers with super mobility.[29 ] It was worth noting that the hydrogen evolution of Tp-BDDA (diacetylene-containing
COF) was much better than that of Tp-EDDA (acetylene-containing COF). This indicates
that the diacetylene fraction has a profound effect on the photocatalytic activity
of COFs, and the conjugated diacetylene moiety could accelerate the electron transformation,
that is, the photogenerated excitons are more likely to migrate to the surface of
the photocatalyst.
Figure 2 Structures of Tp-EDDA and TP-BDDA. Reprinted with permission from Ref. [29 ]. Copyright 2018 American Chemical Society.
Subsequently, in 2021, Li et al. reported[30 ] that the photocatalytic performance of β-keto-enamine-based COFs decayed along with
the length of the diamine linker (TpPa-H: 11.13 µmol · m−2 · h−1 ; TpBD: 7.19 µmol · m−2 · h−1 ; Tp-DTP: 4.76 µmol · m−2 · h−1 ), mainly due to the fact that the increase of Tp-COFʼs torsion angles reduced the
conjugation and planarity of the backbone, thus expanding the band gap and hindering
carrier transfer and separation. Besides, Seki and partners prepared four Tp-COFs
with different torsion angles between the central aromatic ring and the peripheral
benzene ring (Ant, 66°; Bt, 39°; Tp, 27°; Tz, 0°, [Figure 3 ]) by condensation of Tp with 4,4′-diamino-substituted p -terphenyl or its analogous derivatives.[31 ] It was found that the crystallinity (AntCOF150, amorphous; BtCOF150, semicrystalline;
TpCOF150, semicrystalline and TzCOF150, crystalline) and specific surface area of
these Tp-COFs were improved with a only BtCOF150 showed the highest hydrogen evolution
in the presence of 1 wt% Pt as a cocatalyst in all constructed Tp-COFs. Apart from
the torsional angle, the donor–acceptor (D-A) structure is also a key factor for photocatalytic
hydrogen production. The LUMO suffers from fall with the increase of the acceptor
strength, resulting in insufficient driving force of TzCOF for hydrogen evolution.
Recently, Li et al. reported[32 ] a Tp-COF contained benzobisthiazole (BBT) unit with high crystallinity and wettability,
which showed excellent photocatalytic performance ascribing to the enhanced interlayer
electron delocalization and π–π stacking by the rigid and planar BBT unit.
Figure 3 The synthesis and structures of Tp-COFs (AntCOF150, BtCOF150, TpCOF150 and TzCOF150;
150 represent the reaction temperature). Adapted with permission from Ref. [31 ]. Copyright 2020 American Chemical Society.
Optimization of Ordered Structure
Optimization of Ordered Structure
Generally, the crystallinity of materials is one of the most important factors in
photogenerated carrier migration.[47 ] For instance, amorphous organic conjugated polymers exhibit local charge transport
properties, partially due to local structural deformation caused by the disordered
nature of the polymer blends.[48 ] Because of the long-range ordered structure, COFs usually show delocalized electronic
states, which facilitate electron transport and intensify the reaction kinetics by
the aggregation of photogenerated electrons. Therefore, in recent years, the research
on organic photocatalysts has gradually shifted from amorphous and semi-crystalline
polymers to crystalline COFs. For example, in 2016, Cooperʼs group fused phenylenes
by the introduction of bridging functionality (dibenzo[b,d ]thiophene sulfone, DBTS) to prepare a linear conjugated copolymer (P7 , [Figure 4 ])[46b ] with high photocatalytic hydrogen evolution activity (1.49 mmol · g−1 · h−1 ) and an apparent quantum yield as high as 2.3% at 420 nm. The high photocatalytic
activity was attributed to the fact that the rigid DBTS units in the P7 copolymer
accelerated the generation and transport of charge carriers, as mentioned above. Then,
in 2018, this group set out to incorporate the DBTS unit into an ordered Tp-COF on
the basis of semi-crystalline P7 , and successfully synthesized sulfone-containing COFs (S-COF and FS-COF, [Figure 4 ]).[8e ] They found that the crystallinity of FS-COF was much better than that of S-COF due
to the fused and extended planar linker in FS-COF. As a result, the hydrogen evolution
of highly crystalline FS-COF (10.1 mmol · g−1 · h−1 ) strongly outperformed that of relatively low crystalline S-COF (4.44 mmol · g−1 · h−1 ) and semi-crystalline copolymer P7 (1.49 mmol · g−1 · h−1 ). On the contrary, the amorphous FS-COF analogue (FS-P) was also synthesized. As
expected, the amorphous FS-P displayed a much lower photocatalytic activity, with
hydrogen evolution rate of only 1.49 mmol · g−1 · h−1 . Furthermore, the photocatalytic activity of FS-COF was still preserved when it was
cast on the substrate in the form of thin film.
Figure 4 Structures and construction ideas of P7, S-COF and FS-COF. Adapted with permission
from Ref. [8e ]. Copyright 2018 Springer nature.
Besides, two strategies including monomer exchange (based on dynamic imine bonds)
and molecular reconstruction were also used to improve the crystallinity of Tp-COFs,
and great successes have been achieved. In 2021, Guo et al.[19a ] adopted pyrrolidine instead of acetic acid as a catalyst to enhance the controllability
of crystal growth kinetic by monomer exchange strategies[19b ] and, in consequence, Tp-COFs with better crystallinity were acquired. The follow-up
research exhibited that the photocatalytic activity of low-crystalline BT-COF (acetic
acid as a catalyst, 2.02 mmol · g−1 · h−1 ) was far less than that of high-crystalline BT-COF (pyrrolidine as catalyst, 7.7 mmol · g−1 · h−1 ).[34 ] Cooper et al.[35 ] found that framework reconstruction featuring synchronous hydrolysis of COFs and
in situ polymerization was beneficial to improve the crystallinity of COFs. Based
on this, ultra-high crystalline RC-COF-1 was synthesized by the reaction of Tp and
urea ([Figure 5a ]), which showed a photocatalytic hydrogen evolution as high as 27 mmol · g−1 · h−1 . The authors speculated that high crystallinity brought up fast carrier transfer.
Figure 5 (a) Schematic diagram of the synthesis of RC-COFs using the molecular reconstruction
strategy. Adapted from Ref. [35 ] published under a creative commons license (CC BY). (b) Growth mechanism diagram
of COF-935 using dynamic imine bonds. Adapted with permission from Ref. [36 ]. Copyright 2023 Wiley.
Moreover, the irreversible enol-to-keto tautomerization and dynamic imine bonds played
an important role in improving COF ordered structures. For instance, high-crystalline
COF-935 was rapidly synthesized based on the formation of hexagonal intermediates
([Figure 5b ]).[36 ] The dynamic imine bond helped to maintain the reversibility of COF formation, which
provided an opportunity for the modification and reconstruction of the framework.
As a result, when it was exposed to visible light, COF-935 exhibited extremely high
hydrogen evolution up to 67.55 mmol · g−1 · h−1 with 3 wt% Pt as a cocatalyst. More interestingly, the hydrogen evolution of COF-935
was still as high as 19.80 mmol · g−1 · h−1 with 0.1 wt% Pt.
Herein, it should be briefly stated that the crystallinity of COF in the process of
photocatalytic reaction was very likely to be destroyed owing to the disruption of
stack order by breaking partial π–π stacking between adjacent layers. Therefore, it
is often observed that both X-ray diffraction peaks and photocatalytic activities
of COFs decreased after photocatalysis.[7 ],[49 ] In other words, the dislocation between adjacent layers breaks the π-stacking array,
resulting in imitation of charge-carrier transport. In 2021, Guoʼs group[34 ] proposed to use the linear polymer polyethylene glycol (PEG) to fill one-dimensional
pores of BT-COF to stabilize and enhance the π-stacking of COFs, as a consequence
that PEG@BT-COF deposited by Pt displayed excellent hydrogen performance of 11.14 mmol · g−1 · h−1 , which was almost 1.5 times that of pristine BT-COF. This result was attributed to
the fact that the filled PEG was anchored to the framework of BT-COF via H-bonds and
thereby inhibited the sliding of COF adjacent layers during Pt-cocatalyst deposition
([Figure 6 ]). As a result, the PEG@BT-COF photocatalytic material facilitated charge-carrier
transfer and extended exciton lifetime. Recently, this group[37 ] weakened the interlayer interaction of Tp-Bpy-COF by a solvothermal method to transform
the twisted bipyridine part into a planar conformation, thereby favoring of the coordination
with Ni2+ . The obtained TpBpy-Ni2%-COF displayed outstanding hydrogen evolution up to 51.3 mmol · g−1 · h−1 , and still had a hydrogen production capacity under 700 nm light irradiation. The
panchromatic photocatalytic hydrogen evolution is derived from the coaxially ordered
stacking that facilitated the coordination between metal ions and COF frameworks,
thus promoting the metal-to-ligand transfer.
Figure 6 Schematic diagram of structural transformation of BT-COF and PEG@BT-COF during cocatalyst
(Pt) deposition. Adapted from Ref. [34 ] published under a creative commons license (CC BY).
Precise Organic Functionalization Strategy
It is clear that functional substituents on the COF framework with different electron
push-pull properties will affect the band gap of COFs as well as light absorption
and exciton dissociation for photogenerated electron transfer. Two studies have reported
that the introduction of electron-donating functional groups into Tp-COFs enhanced
π-electron delocalization of Tp-COF skeletons. The enhanced π-electron delocalization
optimized charge-carrier transport between and/or within covalent layers,[10g ],[38 ] resulting in better photocatalytic hydrogen evolution.
Meanwhile, the strongly electronegative substituents can also reinforce π-electron
delocalization over the COF framework and accelerate photo-induced exciton dissociation
by strengthening the polarization of the local charges.[50 ] In 2021, Li and co-workers introduced electron-withdrawing groups (-Cl and -SO3 H) into TpPa-H to obtain TpPa-Cl2 and TpPa-SO3 H,[30 ] where TpPa-Cl2 had superior hydrogen evolution (11.73 µmol · m−2 · h−2 ) and apparent quantum efficiency (17%, 400 nm). This result was attributed to the
strong electron-withdrawing ability of halogens, reasonable band structure, high carrier
separation and so on. Besides halogens, the cyano group is also a typical electron-withdrawing
group. In 2021, Chen et al.[10h ] reported a cyano-conjugation, Tp-DBN-COF ([Figure 7a ]), via aldehyde-imine Schiff-base condensation between Tp and 2,5-diaminobenzonitrile
(DBN). The functionalized Tp-DBN-COF showed better photocatalytic hydrogen evolution
(Tp-DBN-COF: 1.8 mmol · g−1 · h−1 ; Tp-PDA-COF: 0.6 mmol · g−1 · h−1 ) in comparison with the pristine Tp-PDA (Tp-Pa-1). The density functional theory
calculation results revealed that the introduction of cyano to the backbones redistributed
electrons in the π-conjugated framework and reduced the energy barrier generated by
the H-intermediate. It is worth noting that most of the current COFs do not show activity
for photocatalytic overall water splitting because the oxygen evolution reaction (OER)
involves the sluggish four-electron process.[51 ] Some pioneering work has demonstrated that photocatalysts with N-containing aromatic
heterocyclic structure could realize overall water splitting, such as g-C3 N4
[52 ] and covalent triazine frameworks.[53 ] Inspired by this, in 2023, Lanʼs group introduced two bipyridine-containing fragments
into Tp-COFs ([Figure 7b ]), and found that Pt@TpBpy-NS and Pt@TpBpy-2-NS displayed activity of overall water
splitting. However, Pt@TpBD-NS-containing biphenyl fragments exhibited only hydrogen
half-reactionʼs activity.[39 ] Further study showed that the N-siteʼs position in the dipyridyl section had an
important effect on the electron transfer from dipyridine to Tp, which made the sp2 -hybridized C2 active sites more favorable to the OER path. The functional COFs mentioned
above all originate from pre-designed building blocks. However, it is difficult to
synthesize structurally diverse functional building blocks. Hence, a post-synthetic
functionalization strategy offers a general approach to introduce a broad range of
functional fragments into COFs without changing the ordered structure of COFs. Using
a post-synthesis strategy, Guo et al.[8a ] incorporated the electron transfer module (viologen derivatives) into Tp-COF and
constructed a dual-function Tp-nC/BPy2+ -COF with photosensitizing and electron transfer units. The synergistic effect of
dual modules accelerates the carrier mobility and the overall reaction kinetics ([Figure 7c ]), resulting in the excellent activity (34.6 mmol · g−1 · h−1 ) of Tp-2C/BPy2+ -COF under visible-light irradiation.
Figure 7 (a) Schematic illustration of the synthesis of Tp-COFs (Tp-PDA and Tp-DBN). Adapted
with permission from Ref. [10h ]. Copyright 2018 Wiley. (b) Structures of Tp-COFs (Tp-Bpy-NS, Tp-Bpy-2-NS, Tp-BD-NS).
Adapted from Ref. [39 ] published under a creative commons license (CC BY). (c) Schematic diagram for Tp-nC/BPy2+ -COF accelerating electron transfer. Reprinted with permission from Ref. [8a ]. Copyright 2021 Wiley.
The design of D-A conformation based on electron regulation is also an important idea
for a precise organic functionalization strategy. In the D-A structure, a strong dipole
moment is generated and a build-in electric field is formed to drive electrons from
D to A due to the obviously different electron affinities between D and A, so as to
improve the exciton dissociation and mobility of carriers.[54 ] So far, the design of D-A configuration has made great achievements in photocatalysis,[55 ] including Tp-COFs. In 2021, Zhuangʼs group constructed D-A type HBT-COF and BT-COF
by Schiff-base condensation of benzene-1,3,5-tricarbaldehyde (BT) and 2-hydroxybenzene-1,3,5-tricarbaldehyde
(HBT) or 4,4′-(benzo[c ][1, 2, 5]thiadiazole-4,7-diyl)dianiline containing a strong electron unit (benzothiadiazole
moiety), respectively[40 ] ([Figure 8a ]). Notably, HBT-COF presented superior hydrogen evolution activity, which was ascribed
to the introduction of hydroxyl groups to enhance the D-A effect and wettability.
Meanwhile, the introduction of hydroxyl group resulted in a partial transformation
of imine bonds into a β-ketoamide structure, as a consequence of that the crystallinity
of HBT-COF was optimized to facilitate electron transfer. Li and co-workers constructed
Tp-COFs (COF-Cl and COF-F, [Figure 8b ]) with strong D-A effect by introducing electronegative Cl or/and F atoms into the
benzothiadiazole moiety.[41 ] The incorporation of halogen atoms enhanced the intrinsic driving force for charge
separation. In addition, due to the intramolecular hydrogen bond between F and hydrogen
atoms on the adjacent aromatic rings, COF-F exhibited a planar structure, further
facilitating charge transport, as described above. In a similar way, Li et al.[42 ] replaced benzidine (BD) with 4,4′-diamino-[1,1′-biphenyl]-3,3′-dicarbonitrile as
the building block to obtain CYANO-COF with a ketene-cyano (D-A) pair, and acquired
COF nanosheets (CYANO-CON) by ball milling. In comparison with bulk CYANO-COF and
H2 BD-CON (nanosheets), the CYANO-CON nanosheets showed a higher photocatalytic activity
with apparent quantum yield of CYANO-CON at 450 nm reaching up to 82.6%, which was
one of the highest efficiencies achieved so far. It could be ascribed to the fact
that the introduction of cyanide group shortens the band group, enhances light capture,
and constructs D-A pair to facilitate charge separation. In addition, the nanosheets
also play a key role in shortening the distance of carrier transport and exposing
more active sites.
Figure 8 (a) Structures of BT-COF and HBT-COF. Reprinted with permission from Ref. [40 ]. Copyright 2021 American Chemical Society. (b) TP-BT-X-COFs (X = H, F and Cl). Adapted
with permission from Ref. [41 ]. Copyright 2023 American Chemical Society.
The synergistic effect of multi-component building blocks in COFs can further modulate
the D-A structure to increase exciton dissociation. For example, Guo et al.[43 ] reported β-ketoenamine-linked Tp(BT
x
TP
1−x
)-COFs obtained by condensation of 4,4′-diamino-p -terphenyl (TP) and 4,4′-(benzo-2,1,3-thiadiazole-4,7-diyl)dianiline (BT) units with
Tp as a fixed node. The D-A pair produced by the introduction of electronegative BT
accelerated the exciton dissociation. However, with the further increase of BT content,
the formation of the induced excimer state acted as an exciton trap site and inhibited
the long-distance diffusion of excitons to the catalytic sites. Although this multi-component
COF exhibits good crystallinity, the local structure of the COF is not clear from
a microscopic point of view, which hinders the study of actual structure–activity
relationship and runs counter to the original intention of COF structure precision.
For this reason, the synthesis of multi-component ordered COFs is extremely in demand
and has full of challenges. In 2023, Jiangʼs group creatively adopted a hierarchical
synthesis strategy to pre-polymerize the two building blocks according to stoichiometric
ratios, and then assembled them with the third building block to form COFs ([Figure 9 ]).[44 ] This approach reduced the complexity of ternary polymerization and achieved precise
control at the binary level. As a result, both 2Me-OMe-COF and Me-2OMe-COF exhibited
remarkable hydrogen evolution activity, which could be attributed to multi-factor
coupling such as absorbance, crystallinity and charge transport capacity; besides,
the presence of the D1 -A – D2 structure in COFs might also affect the electron transfer path.
Figure 9 Schematic diagram of the synthesis of multi-component Tp-COFs. Adapted with permission
from Ref. [44 ]. Copyright 2023 Springer.
Effect of Morphology
It has been mentioned above that the reduction of charge-carrier transmission distance
could effectively improve its utilization. Therefore, the preparation of single-layered
or few-layered COFs is also an important candidate to reduce the recombination of
photoinduced carriers. In 2022, Yang et al.[45 ] reported that Tp-COF colloids were deposited on a strong-affinity carrier (SiO2 ) in a self-exfoliating way to get a near-single-layer COF (SLCOF, TP-TTA/SiO2 -1). The near-SLCOF exhibited a remarkable hydrogen evolution rate, up to 153.2 mmol · g−1 · h−1 , which was one of the highest hydrogen evolution performances reported to date. Deposition
of a near-monolayer COF on the carrier not only reduced the number of expensive COFs,
but also shortened the migration distance of photogenerated charge carriers, thus
improving the efficiency of photocatalysis.
Recently, Jin and co-workers successfully prepared high-crystalline TpPa-COF with
different morphologies (spheres, bowls and fibers) by emulsion polymerization ([Figure 10 ]).[25 ] The spherical morphology with a higher specific surface area exposed more active
sites and the smaller size COF shortened the carrier transport distance. As a result,
the e -TpPa-COF (sphere morphology) showed the best hydrogen evolution of 133.9 mmol · g−1 · h−1 (with fresh emulsion and 0.9 wt% Pt), which was comparable to the values of most
advanced COFs reported to date.
Figure 10 Schematic diagram of the synthesis of Tp-COFs by emulsion polymerization. Reprinted
with permission from Ref. [25 ]. Copyright 2020 American Chemical Society.
Conclusions and Outlook
The unique properties of Tp-COFs endow them with the potential to be prominent photocatalysts.
Moreover, with the continuous exploration of the synthesis of Tp-COFs, large-scale
and simple preparation has been realized, so the improvement of Tp-COFsʼ photocatalytic
efficiency is a key factor to achieve its industrial applications. To date, Tp-COFs
and their composites displayed excellent photocatalytic hydrogen evolution rates,
surging new highs again and again. The construction and modification based on the
molecular engineering strategy are more likely to obtain high-performance Tp-COF photocatalysts
because it maintains maximum crystallinity and avoids interface problems. Therefore,
this paper reviews the research process of Tp-COFs based on the molecular engineering
strategy, so as to bring theoretical guidance for follow-up optimization.
Even if the construction of Tp-COF photocatalysts based on the molecular engineering
strategy possesses unparalleled advantages, the following challenges still seriously
hinder its development. 1) Since the structure–activity relationship of Tp-COFs has
not been fully defined, there is insufficient guidance for the design of high-activity
Tp-COF photocatalysts. 2) Due to great challenge in the preparation of multi-component
Tp-COFs with specific microstructure, the expected hydrogen evolution effect cannot
be achieved through multi-component regulation. 3) So far, most of the research studies
on photocatalytic hydrogen evolution have focused on the half-reaction for the reason
that it is very difficult to achieve simultaneous hydrogen and oxygen evolution by
synergistically regulating the band structure and catalytic sites. This runs counter
to the original intention of developing photocatalysts. Therefore, the realization
of Tp-COFsʼ photocatalytic overall water splitting is also one of the key scientific
problems to be solved urgently. 4) The construction of film-typed Tp-COF photocatalysts
is on the primary stage, and the development of commercial plate reactors still seems
a long way off. Therefore, more attention should be paid to the structure–activity
relationship, the synthesis of multi-component Tp-COFs, Tp-COFsʼ photocatalytic overall
water splitting and the development of plate reactors in follow-up research, so as
to accelerate the industrial application of Tp-COF photocatalysts.
Funding Information
This work was supported by the National Natural Science Foundation of China (52 173 163),
the National 1000-Talents Program, the Innovation Fund of WNLO, Huazhong University
of Science and Technology (HUST, 2023BR021), Hubei Provincial Natural Science Foundation
of China (Project No. 2023AFB479) and the Hubei University of Science and Technology
Doctoral Research Initiation Project (Project No. BK202325). The authors acknowledge
projects funded by China Postdoctoral Science Foundation (2022M710 849 and 2023 T160 137),
Department of Education of Hubei Province (Q20221004) and Overseas Expertise Introduction
Center for Discipline Innovation (D18025).
