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\begin{document}

\title{Fully Conjugated Covalent Organic Frameworks: Synthesis, Structures and
Applications}



\author[1]{Yueshuai Xu}%
\author[1]{Guanshi Ren}%
\author[1]{Dianqi Zhang}%
\author[1]{Lishui Sun}%
\author[1]{Yingjie Zhao}%
\affil[1]{Qingdao University of Science and Technology}%


\vspace{-1em}



  
  \date{June 13, 2024}


\begingroup
\let\center\flushleft
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\selectlanguage{english}
\begin{abstract}
This review presents a comprehensive examination of fully conjugated
covalent organic frameworks (COFs), which constitute an emerging class
of porous materials with immense potential for diverse applications.
This article focuses on diversified fully conju-gated COFs, including
sp2 carbon-carbon linkages, pyrazine linkages, benzobisoxazole linkages,
dioxin linkages, \selectlanguage{greek}β-\selectlanguage{english}aminoalkenone linkages, etc. The synthesis techniques
and structural attributes of these COFs are expounded upon in great
detail, along with their potential applications in various fields. The
review thus provides a valuable resource for researchers keen on delving
into the synthe-sis and applications of fully conjugated COFs, thereby
highlighting their potential for developing novel functional materials
with dis-tinctive properties.%
\end{abstract}%



\sloppy


\textbf{Cite this paper:}\emph{Chin. J. Chem.} \textbf{2023} , \emph{41}
, XXX---XXX. \textbf{DOI: 10.1002/cjoc.202300XXX}

Fully Conjugated Covalent Organic Frameworks: Synthesis, Structures and
Applications

Yueshuai Xu,\emph{\textsuperscript{a}} Guanshi
Ren,\emph{\textsuperscript{a}}Dianqi Zhang,\emph{\textsuperscript{a}}
Lishui Sun,*\textsuperscript{,\emph{a}} and Yingjie
Zhao*\textsuperscript{,\emph{a}}

\textsuperscript{a} College of Polymer Science and Engineering, Qingdao
University of Science and Technology, Qingdao, 266000, China.\selectlanguage{english}
\begin{longtable}[]{@{}l@{}}
\toprule
\textbf{Keywords}\tabularnewline
\midrule
\endhead
Covalent organic frameworks \textbar{} Fully-conjugated system
\textbar{} Crystallinity\textbar{} Stability\textbar{}\tabularnewline
\textbf{Comprehensive Summary}\tabularnewline
This review presents a comprehensive examination of fully conjugated
covalent organic frameworks (COFs), which constitute an emerging class
of porous materials with immense potential for diverse applications.
This article focuses on diversified fully conjugated COFs, including
sp\textsuperscript{2} carbon-carbon linkages, pyrazine linkages,
benzobisoxazole linkages, dioxin linkages, \selectlanguage{greek}β-\selectlanguage{english}aminoalkenone linkages,
etc. The synthesis techniques and structural attributes of these COFs
are expounded upon in great detail, along with their potential
applications in various fields. The review thus provides a valuable
resource for researchers keen on delving into the synthesis and
applications of fully conjugated COFs, thereby highlighting their
potential for developing novel functional materials with distinctive
properties.\tabularnewline
\bottomrule
\end{longtable}\selectlanguage{english}
\begin{longtable}[]{@{}ll@{}}
\toprule
& \vtop{\hbox{\strut Yueshuai Xu received his BS degree from Qingdao
University of Science and Technology in 2018. He is currently studying
for his MS degree at Qingdao University of Science and Technology under
the supervision of Prof. Zhao. His research interests focus on the
design and synthesis of conjugated covalent organic
frameworks.}\hbox{\strut Lishui Sun is a Professor at Qingdao University
of Science and Technology. His current research interests focus on
organic porous materials. And he is mainly engaged in the synthesis and
application of composite porous materials.}\hbox{\strut Yingjie Zhao is
a Professor at the Qingdao University of Science and Technology. His
current research interests include crystalline organic 2D materials and
their applications in energy conversion and catalysis.}}\tabularnewline
\midrule
\endhead
\textbf{Left to Right:} Yueshuai Xu, Guanshi Ren, Dianqi Zhang, Lishui
Sun, Yingjie Zhao &\tabularnewline
\bottomrule
\end{longtable}

\section*{}

{\label{section}}

Contents

1. Introduction Page No.2

2. sp\textsuperscript{2} carbon-carbon Linkages Page No.3

2.1. Aldol reaction Page No.3

2.2. Knoevenagel reaction Page No.6

3. Pyrazine Linkages Page No.14

4. Benzobisoxazole Linkages Page No.14

5. Dioxin Linkages Page No.16

6. \selectlanguage{greek}β-\selectlanguage{english}Amino Enone Linkages Page No.17

7. Post-Synthetic Modification Page No.17

8. Multicomponent One-Pot Reaction Page No.19

9. Conclusions and Perspectives Page No.20

1. Introduction

COFs have gained significant attention due to their highly ordered
structures and precise design and
synthesis.\textsuperscript{{[}1-5{]}}These crystalline, porous two or
three-dimensional networks have accurately porous structures that make
them useful for gas storage and separation, heterogeneous catalysis, and
drug delivery.\textsuperscript{{[}6-11{]}} To synthesize these highly
ordered structures, reversible dynamic covalent bonds as linkages
between building blocks are usually
necessary.\textsuperscript{{[}12-14{]}}The mostly used linkages for
constructing COFs are limited to B-O and C=N bonds, which have a
well-known self-adjusting property that facilitates the formation of
well-ordered structures.\textsuperscript{{[}15-19{]}}However, these
bonds usually have poor stability and weak electron delocalization,
which sacrifices chemical stability and conjugation. This limitation
restricts their application in fields such as organic semiconductors,
photocatalysis, and ferromagnetic materials, which require high \selectlanguage{greek}π
\selectlanguage{english}electron delocalized systems.\textsuperscript{{[}20-23{]}} To overcome
these limitations, many efforts have been made to explore new chemistry
to construct highly stable and electron-delocalized COFs. However,
constructing fully conjugated COFs requires new linkages with poor
reversibility, which is unfavorable for the crystallinity. A perfect
equilibrium point is needed to maintain both the crystallinity and
conjugation (Figure 1). Although fully conjugated COFs are fascinating
materials, keeping the crystallinity is an extremely challenging job
because the reversibility of the conjugated bond is usually bad, which
is unfavorable for self-adjusting.\textsuperscript{{[}24-28{]}}

In this review, we focus on the recent progress of COFs with new
linkages beyond the common B-O and C=N bonds and holding fully
conjugated structures. Fully conjugated COFs with new linkages or
obtained through new chemistry synthetic strategies were included. The
potential applications of these COFs, including gas storage and
separation, heterogeneous catalysis, and drug delivery, were also
summarized.\textsuperscript{{[}29,30{]}}Finally, we present an outlook
on the future development of these COFs, which includes exploring new
chemistry strategies for constructing stable, highly ordered COFs with
excellent \selectlanguage{greek}π-\selectlanguage{english}electron delocalization and porosities suitable for various
applications.\textsuperscript{{[}31-34{]}}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image9/image9}

\end{center}
\end{figure}

\textbf{Figure 1.} Schematic diagram of the balance between
crystallinity and conjugation in COFs materials

2. sp\textsuperscript{2}carbon-carbon Linkages

The fully conjugated COFs constructed with new linkages beyond borate
and imine bonds are still less reported. The most published examples of
fully conjugated COFs focus on sp\textsuperscript{2} carbon-carbon
linked COFs (sp\textsuperscript{2}c-COFs), which possess conjugated
structures connected by sp\textsuperscript{2} carbon-carbon bonds.
Compared to the traditional imine-based COFs, the
sp\textsuperscript{2}c-COFs have excellent stability and semiconductor
properties and have shown a wide range of applications, including
catalysis, sensing, separation, energy storage, conversion, etc.
Typically, the most used synthetic reactions for the preparation of
sp\textsuperscript{2}c-COFs focus on the Aldol and the Knoevenagel
reactions.

2.1. Aldol reaction

Zhang and co-workers successfully fabricated a series of
sp\textsuperscript{2}c-COFs utilizing pyridine units as the central
building block via the Aldol reaction. The autocatalytic acylation of
the pyridine unit by common acylation reagents, such as acetic acid,
anhydride, or acyl halide, can promote the growth of vinylidene chains.
Based on this strategy, a series of COFs constructed by vinylidene
linkages (\textbf{COF-1} , \textbf{COF-2} ) can be successfully
synthesized (Figure 2a).\textsuperscript{{[}35{]}} Notably, these COFs
exhibit intriguing spatially restricted catalytic activity towards the
esterification reactions of certain crucial drug intermediates,
demonstrating high yields, remarkable recoverability, and reusability,
as well as excellent selectivity. Subsequently, the same group reported
another two sp\textsuperscript{2}c-COFs (\textbf{COF-3}
and\textbf{COF-4} ) with an orthogonal reticulation topology through a
condensation reaction between a tetradentate monomer and a dichotomous
linear aromatic dialdehyde (Figure 2b).\textsuperscript{{[}36{]}} The
presence of the pyridine unit has the capacity to facilitate the
condensation reaction through the decomposition of the benzoic anhydride
molecule. The obtained COFs exhibit exceptional characteristics such as
high crystallinity and porosity. The remarkable semiconducting
properties and corresponding applications of both COFs can be attributed
to the \selectlanguage{greek}π-\selectlanguage{english}extended conjugated backbone made by the bipyridine unit and
the vinylidene linkages in the COFs.

Zhang and co-workers further used 1,3,5-trimethyl-2,4,6-tricyanobenzene
as a monomer to construct a series of sp\textsuperscript{2}c-COFs
(\textbf{COF-5} , \textbf{COF-6} , \textbf{COF-7} ) (Figure
3).\textsuperscript{{[}37{]}} These COFs exhibit high specific surface
area and excellent semiconductor properties, including superior \selectlanguage{greek}π-\selectlanguage{english}domain
separation, dense light-trapping ability, and tunable band structure.
They were also found to be efficient in catalyzing the oxidation of
arylboronic acids to aryl phenols in a wide range of substrates at low
loading and short reaction times. The same group reported another series
of sp\textsuperscript{2}c-COFs (\textbf{COF-8} to\textbf{11} ) by using
3,5-dicyano-2,4,6-trimethylpyridine as the key building block and
linear/trigonal aldehydes (Figure 4).\textsuperscript{{[}38,39{]}}A
systematic analysis of the photophysical and electrochemical properties
of \textbf{COF-8} , \textbf{COF-9} and \textbf{COF-10} has been
conducted. Among these sp\textsuperscript{2}c-COFs, the highly
crystalline \textbf{COF-10} has been highlighted as an exemplar and
analyzed in detail. The cyano-pyridine cores and the fully-\selectlanguage{greek}π \selectlanguage{english}conjugated
stable C=C bond endow \textbf{COF-10} with excellent light-harvesting
characteristics and stable \selectlanguage{greek}π-\selectlanguage{english}electron delocalized properties. Thus,
the\textbf{COF-10} can act as a stable photocatalyst and be capable of
hydrolysis in two half-reactions in the presence of sacrificial agents,
achieving high hydrogen generation yields. In another example, the
cyano-pyridine and the triazine cores work together with the fully-\selectlanguage{greek}π
\selectlanguage{english}conjugated stable C=C bond in \textbf{COF-11} , allowing the generation
of uninterrupted \selectlanguage{greek}π-\selectlanguage{english}electron leaving domains in the 2D direction.
Notably, a unique nanofibrous morphology was obtained for\textbf{COF-11}
. The assembly of \textbf{COF-11} with carbon nanotubes allows easy
access to flexible thin film electrodes for micro-supercapacitors.

The synthesis of ionic sp\textsuperscript{2}c-COFs (\textbf{COF-12} and
\textbf{COF-13} ) was also successfully realized by combining the ionic
monomer with multiple aromatic aldehydes by Zhang and co-workers (Figure
5).\textsuperscript{{[}40{]}} These COFs are able to be composited
uniformly with polyethylene oxide and lithium salts and exhibit good
ionic conductivity due to their regular and persistent nanochannels and
the unique charged framework. Zhang and co-workers further reported two
novel 2D sp\textsuperscript{2}c-COFs (\textbf{COF-14} and
\textbf{COF-15} ) through Aldol reaction by using
2,4,6-trimethyl-1,3,5-triazine as the central unit (Figure
6).\textsuperscript{{[}41{]}} The fully \selectlanguage{greek}π-\selectlanguage{english}conjugated 2D structures of
these two COFs bring excellent semiconductor properties such as
visible-light harvesting ability and charge separation ability. These
properties endow them with visible-light-driven photocatalytic activity
for the decomposition of water to produce hydrogen. Notably,
\textbf{COF-14} and \textbf{COF-15}can also be assembled into
microfibrous forms like the example discussed\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image10/image10}

\end{center}
\end{figure}

\textbf{Figure 2.} The synthesis and structures of \textbf{COF-1} to
\textbf{COF-4}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image11/image11}

\end{center}
\end{figure}

\textbf{Figure 3.} The synthesis and structures of \textbf{COF-5} to

\textbf{COF-7}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image12/image12}

\end{center}
\end{figure}

\textbf{Figure 4.} The synthesis and structures of \textbf{COF-8} to
\textbf{COF-11}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image13/image13}

\end{center}
\end{figure}

\par\null

\textbf{Figure 5.} The synthesis and structures of \textbf{COF-12}
and\textbf{COF-13}

above, which allows them to be fabricated in micron-thick films without
any additives. In particular, the \textbf{COF-14} film shows a
photocurrent of up to 45 \selectlanguage{greek}μ\selectlanguage{english}A cm\textsuperscript{-2} and rapid interfacial
charge transfer at 0.2 V vs RHE. It also exhibited an apparent quantum
yield of 1.06\% in the 420 nm photocatalytic hydrogen precipitation
reaction with a hydrogen yield of 14.6 \selectlanguage{greek}μ\selectlanguage{english}mol h\textsuperscript{-1}.

Perepichka and co-workers also reported the preparation of highly
crystalline sp\textsuperscript{2}c-COFs from
2,4,6-trimethyl-1,3,5-triazine and aromatic dialdehydes.\textbf{COF-16}
, \textbf{COF-17} and \textbf{COF-18} were obtained through a
bases-catalyzed procedure and exhibited high fluorescence and
crystallinities (Figure 7).\textsuperscript{{[}42{]}} Among these three
COFs, \textbf{COF-16} is highly hydrophilic, showing multiple expansions
in volume when it was in contact with protonic solvents (e.g. water).
Notably, this behavior was not observed in the amorphous samples. This
observation is significantly different from the hydrophobic behavior of
most reported COFs, including triazine-based microporous polymers.
Subsequently, they realized the transition from crystalline
ethylene-linked 2D COFs to crystalline cyclobutane-linked 3D COFs
through the illumination of \textbf{COF-16}and \textbf{COF-17} (Figure
7).\textsuperscript{{[}43{]}}Specifically, the absorption edge
irradiation of sp\textsuperscript{2}c-COFs leads to topological
{[}2+2{]} cycloaddition crosslinking of \selectlanguage{greek}π-\selectlanguage{english}stacked layers in 3D COFs. And
the crosslinking between the COFs layers can be reversed by heating the
COFs to 200 \selectlanguage{ngerman}°C, thereby opening the strained cyclobutane rings and
restoring the polyolefin network. The induced differences were probed by
optical (absorption,\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image14/image14}

\end{center}
\end{figure}

\textbf{Figure 6.} The synthesis and structures of \textbf{COF-14} and
\textbf{COF-15}

luminescence) spectroscopy and acid doping measurements. The effect of
2D-3D conversion on the gas adsorption, dephasing and ion
(Li\textsuperscript{+}, H\textsuperscript{+}) transport properties of
COFs was further elucidated.

Yaghi and co-workers obtained an unsubstituted
sp\textsuperscript{2}c-COFs (\textbf{COF-19} ) by Br\selectlanguage{ngerman}ønsted
acid-catalyzed reaction between 2,4,6-trimethyl-triazine and
4,4'-biphenyldialdehyde (Figure 8).\textsuperscript{{[}44{]}} Due to the
chemical stability of the olefin linkages, \textbf{COF-19} was found to
maintain its crystallinity and chemical composition in the presence of
highly concentrated Brønsted acids and bases, organolithium reagents,
and Lewis acids. The chemical stability realizes the immobilization of
Lewis acid catalysts in the pores of the COFs. And they also confirmed
the activity of the Lewis acid-based-COF catalyst through the successful
catalysis of the Diels-Alder reaction.

Thomas and co-workers reported two sp\textsuperscript{2}c-COFs
(\textbf{COF-20} and \textbf{COF-21} ) by exploiting the
electron-deficient nature of 2,4,6-trimethyl-triazine (Figure
8).\textsuperscript{{[}45{]}} The acidic terminal methyl hydrogen of
2,4,6-trimethyl-triazine can be readily extracted by a base, resulting
in a stable carbon ion that further undergoes an aldol condensation

with polythematic aromatic aldehydes to form reticulated extended
crystal frameworks. This work demonstrates that crystallization of 2D
vinyl-linked COFs can be achieved by using highly electron-deficient
s-triazine nuclei under alkali-catalyzed conditions. By using UV-Vis
irradiation of \textbf{COF-20} powders, they observed that {[}2+2{]}
photocycloaddition reaction within the \selectlanguage{greek}π-π \selectlanguage{english}stacked layer of the 2D
columnar COFs. The \textbf{COF-20} remains crystalline after light
irradiation due to the formation of highly strained cyclobutane rings
within the 2D layers. This is because the pre-organized vinylidene in
the adjacent 2D layer of \textbf{COF-20} provides a platform for
{[}2+2{]} cycloaddition under UV-Vis irradiation, satisfying Schmidt's
criteria for photocyclization.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image15/image15}

\end{center}
\end{figure}

\textbf{Figure 7.} The synthesis and structures of \textbf{COF-16} to
\textbf{COF-18}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image16/image16}

\end{center}
\end{figure}

\textbf{Figure 8.} The synthesis and structures of \textbf{COF-19} to

\textbf{COF-21}

The same group then reported a series of sp\textsuperscript{2}c-COFs
(\textbf{COF-20} , \textbf{COF-21} , \textbf{COF-22} ) by using a
different synthetic strategy through acetonitrile as monomers (Figure
9). This method provides a description of the acid-catalyzed synthetic
pathway for vinylidene-linked COFs and avoids the side reactions
associated with the base-catalyzed synthesis. A nitrile trimerization
reaction with an Aldol reaction was adopted through a two-step, one-pot
reaction. The universality of this synthetic strategy was further
verified by the successful preparation of four different COFs
(\textbf{COF-19, COF-20} ,\textbf{COF-2} 1 and\textbf{COF-22}
).\textsuperscript{{[}46{]}}

Zhang and co-workers developed a solvent-free synthetic strategy that is
similar to the melt polymerization process. \textbf{COF-23} was
successfully synthesized through an Aldol reaction between the
2,5-dimethylpyrazine and 1,3,5-tricarbonylbenzene under solvent-free
conditions (Figure 10). Subsequently,
the\textbf{H\textsubscript{3}PO\textsubscript{4}@COF-23} complex was
further constructed by anchoring the proton carrier
H\textsubscript{3}PO\textsubscript{4} inside the pores of\textbf{COF-23}
.\textsuperscript{{[}47{]}}Under usual fuel cell operating conditions,
it exhibited ultra-high proton conductivity and excellent performance as
a solid electrolyte membrane. By using this solvent-free and melt
polymerization strategy, the same group further synthesized a series of
highly crystalline sp\textsuperscript{2}c-COF (\textbf{COF-24} ,
\textbf{COF-25} , and\textbf{COF-26} ) through 2,4,6-collidine and
xylene as the monomer (Figure 10).\textsuperscript{{[}48{]}}This method
enabled the fabrication of COFs with self-contained and robust foams at
the kilogram scale. The obtained COFs have an extremely low cost
(\textless{} \$50 / kg). In addition, neutral COFs can be easily
converted to ionic COFs by using one-pot or post-modification methods,
which can be used as efficient ion-exchange sorbents for the removal of
oxygen ion contaminants. This work not only expands the range of
olefin-linked COFs but also enlightens the route to industrial
production of crystalline ion-exchange adsorbents.

2.2. Knoevenagel reaction

Feng and co-workers reported the synthesis of three
sp\textsuperscript{2}c-COFs (\textbf{COF-27} ,\textbf{COF-28} , and
\textbf{COF-29} ) by utilizing
2,3,8,9,14,15-hexa(4-formylphenyl)diquinoxalino{[}2,3-a:2',3'-c{]}phenazine
(\textbf{HATN-6CHO} ) as the building block (Figure 11). In comparison
to their imine-linked COF analogs, \textbf{COF-29} demonstrates superior
chemical and electrochemical stability, rendering it a more promising
candidate for practical applications.\textsuperscript{{[}49{]}}To
further enhance the electrical conductivity of \textbf{COF-29} , an
in-situ growth on carbon nanotubes was employed to generate a core-shell
hybrid material (\textbf{COF-29@CNT} ).\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image17/image17}

\end{center}
\end{figure}

\textbf{Figure 9.} The synthesis and structures of \textbf{COF-19} to
\textbf{COF-22}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image18/image18}

\end{center}
\end{figure}

\textbf{Figure 10.} The synthesis and structures of \textbf{COF-23} to
\textbf{COF-26}

This hybrid material was then utilized as a cathode material for
lithium-ion batteries (LIBs) and demonstrated remarkable performance,
including a high capacity of 116 mA h/g and exceptional cycling
stability. Furthermore, the double thiophene-bridged
\textbf{COF-27}exhibited a narrower optical energy band gap (2.04 eV), a
higher LUMO energy level (-3.41 eV), and improved charge separation and
transport in contrast to biphenyl-bridged \textbf{COF-28} and 2D C=N
COF-B, attributed to the D-A
structure.\textsuperscript{{[}50{]}}\textbf{COF-27} also demonstrated
exceptional photocathode activity in photoelectrochemical devices with
an excellent saturation photocurrent density for water reduction.

In 2016, Feng and co-workers reported the successful synthesis of
sp\textsuperscript{2}c-COFs (\textbf{COF-30} ) by using Knoevenagel
polycondensation between 1,4-benzene diacetonitrile and
1,3,5-tris(4-formylphenyl) benzene as the monomers (Figure
12).\textsuperscript{{[}51{]}} The obtained \textbf{COF-30} exhibits a
crystalline lamellar structure with a fully-\selectlanguage{greek}π \selectlanguage{english}conjugated
sp\textsuperscript{2}-carbon bonded skeleton and pendant cyano groups.
PXRD studies and theoretical simulations verified the 2D structure. The
nitrogen physisorption measurements give a specific surface area of as
high as 880 m\textsuperscript{2}g\textsuperscript{-1}. Remarkably,
\textbf{COF-30} exhibited substantial potential for application as
supercapacitor electrodes and electrocatalysts in oxygen reduction
reactions.

The same group furthermore developed a new strategy for the synthesis of
sp\textsuperscript{2}c-COFs, which employed Knoevenagel polycondensation
in conjunction with water-assisted Michael addition elimination. The
successful synthesis of a series of sp\textsuperscript{2}c-COFs
(\textbf{COF-28} , \textbf{COF-31} and \textbf{COF-32} ) strongly
supports the universality of this strategy (Figure 11,
12).\textsuperscript{{[}52{]}} An\emph{in-situ} high-temperature NMR
measurement was utilized on the model reaction and intermediate to
distinguish Michael-addition elimination from the hypothetical
reversible Knoevenagel reaction. Furthermore, water-assisted Michael
addition elimination was identified as effective dynamic covalent
chemistry for direct C=C bond exchange, which endowed self-correcting
properties for synthesizing extended structures. Consequently, Michael
addition elimination polymerization was demonstrated to be an efficient
approach for the synthesis of sp\textsuperscript{2}c-COFs with high
crystallinity.

Zhao and co-workers present the successful synthesis of an
sp\textsuperscript{2}c-COF (\textbf{COF-31} ) with a triazine moiety
serving as the central planar unit (Figure
12).\textsuperscript{{[}53,54{]}}In order to enhance electron
delocalization, olefin (-C=C-) bonds were utilized instead of the
commonly used imide (-C=N-) bonds as sp\textsuperscript{2}-carbon
bridges. The resulting sp\textsuperscript{2}-carbon bonds endowed the
\textbf{COF-31} with superior stability and electron delocalization.
Furthermore, the incorporation of the triazine unit as a functional
moiety positively modulated the electronic structure and semiconductor
properties of\textbf{COF-31} , making it a promising photocatalytic
candidate for photosystem I-like systems. To demonstrate the potential
of\textbf{COF-31} in catalysis, \textbf{COF-31} was employed to catalyze
coenzyme regeneration, resulting in a remarkable glutamate synthesis
yield of 97\% in 12 min. These findings highlight the potential
of\textbf{COF-31} as an efficient photocatalyst and offer promising
avenues for future research in the field of catalysis.

In 2022, Liu and co-workers reported an efficient gradient heating
strategy for the preparation of \textbf{COF-31} , resulting in the
creation of a material with high crystallinity and a large surface area
(Figure 12).\textsuperscript{{[}55{]}}This material has demonstrated
remarkable properties, including high crystallinity, large surface area,
excellent durability, and carrier mobility, rendering it particularly
suitable for solar-driven photocatalytic hydrogen production. Notably,
when triethanolamine or ascorbic acid are utilized as sacrificial
electron donors, platinum-doped \textbf{COF-31} exhibits unexpectedly
high activity in both photocatalytic systems.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image19/image19}

\end{center}
\end{figure}

\textbf{Figure 11.} The synthesis and structures of \textbf{COF-27} to

\textbf{COF-29}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image20/image20}

\end{center}
\end{figure}

\textbf{Figure 12.} The synthesis and structures of \textbf{COF-30} to
\textbf{COF-34}

Qiu and co-workers reported the synthesis of sp\textsuperscript{2}c-COF
(\textbf{COF-34} ), which exhibits excellent stability towards chemical,
thermal, and radiation exposure (Figure 12). The incorporation of
amidoxime ligands in the open one-dimensional channel imparts selective
binding of UO\textsuperscript{22+} ions. Furthermore,
\textbf{COF-34}exhibits an ultra-fast response time of 2 seconds and an
ultra-low detection limit of 6.7 nM UO\textsuperscript{22+}. These
properties make\textbf{COF-34} an ideal candidate for in situ and
real-time monitoring of UO\textsuperscript{22+} and assessment of water
quality, besides its potential application in
UO\textsuperscript{22+}extraction.\textsuperscript{{[}56{]}}

Jiang and co-workers reported a series of sp\textsuperscript{2}c-COFs
(\textbf{COF-35} , \textbf{COF-36} ,\textbf{COF-37} and \textbf{COF-38}
) utilizing pyrene as the central structural moiety through the
Knoevenagel condensation reaction (Figure 13). The systematic
investigation of the structures, applications, mechanisms, and synthesis
methods of these sp\textsuperscript{2}c-COFs has been extensively
discussed. Among these COFs, \textbf{COF-35}features a topologically
oriented mesh structure in which C=C linkages connect the pyrene nodes
at regular intervals to form a \selectlanguage{greek}π-\selectlanguage{english}conjugated 2D lattice that extends in
the x and y directions.\textsuperscript{{[}57{]}} The overlapping layers
of this lattice form a highly stable framework.\textbf{COF-35} possesses
semiconductor properties and exhibits a discrete band gap of 1.9 eV.
Through chemical oxidation, the electrical conductivity of J-COF-1 can
be significantly increased by up to 12 orders of magnitude.

The \textbf{COF-35} , \textbf{COF-36} , and \textbf{COF-37} were then
found to exhibit exceptional
stability.\textsuperscript{{[}58{]}}Furthermore, they observed that the
linkers adopt propeller conformation to constitute twisted
arylenevinylene backbones. As a result, the \selectlanguage{greek}π-\selectlanguage{english}domain transport of the
\textbf{COF-35} , \textbf{COF-36} , and\textbf{COF-37} along the x and y
directions can be regulated by the linker unit, facilitating the
adjustment of the band gap and emission color. The researchers have
demonstrated the structure-light function correlation in this study,
indicating that photochemical events are highly correlated with the
first and higher-order structures of the sp\textsuperscript{2}c-COFs by
elucidating the structural parameters that govern the photochemical
events.

In the subsequent studies, it is discovered that the
\textbf{COF-35}possesses a distinctive combination of \selectlanguage{greek}π-\selectlanguage{english}conjugation,
crystallinity, porosity, and photostability that allows it to absorb a
broad spectrum of light and maintain a steady H\textsubscript{2} release
rate of 1360 mmol[?]h\textsuperscript{-1}[?]g\textsuperscript{-1} from
water. In order to further enhance light absorption and narrow the band
gap, the researchers integrated an electron-deficient 3-ethyl-rhodanine
(ERDN) unit as an end-cap group into the periphery of the
\textbf{COF-35}lattice, resulting in the synthesis of \textbf{COF-38}
(Figure 13).\textsuperscript{{[}59{]}} The present framework embodies a
light-driven photocatalytic system, wherein a sequence of photocatalytic
phenomena comprising light absorption, exciton migration, exciton
separation, electron transfer, and charge collection are intimately
interconnected and function in tandem. This integrated mechanism
engenders the capability of sp\textsuperscript{2}c-COFs to achieve
sustained and efficient hydrogen evolution from water, utilizing a broad
spectrum of visible-to-near-infrared radiation.

To further optimize the optical and optoelectronic functionality of the
obtained 2D sp\textsuperscript{2}c-COFs, a thorough understanding of the
emitting and non-radiative pathways following photoexcitation is crucial
and should be linked to specific structural designs. Jiang and Scholes
employed transient absorption spectroscopy to examine the colloidal
suspension of \textbf{COF-35} , and to characterize the spatial extent
and diffusion kinetics of emitting excitons produced by pulsed
photoexcitation. A stacking distance of 3.5 \selectlanguage{ngerman}Å between the 2D layers
results in the diffusion of coplanar pyrene excitons within the
framework, while the dominant polycrystalline solid emission spectrum
state is attributed to an extended coplanar exciton whose
two-dimensional off-domain is facilitated by a C=C connection. The
sub-nanosecond dynamics of the photo-induced absorption signal in the
near-infrared, which is attributed to charge-separated excitons or
multiple pairs, reflects 3D exciton diffusion as well as long-range
exciton-exciton annihilation driven by resonance interactions. In their
experimental regime, doubling the excitation intensity results in a
tenfold increase in the estimated exciton diffusion length from
approximately 3 to 30 nm, suggesting that higher lattice temperatures
may enhance exciton mobility in COFs colloids.\textsuperscript{{[}60{]}}

Cooper and co-workers reported an sp\textsuperscript{2}c-COF
(\textbf{COF-39} ) containing bipyridyl and its post-synthetic
modification through the introduction of rhenium (Figure
13).\textsuperscript{{[}61{]}} In the presence of \textbf{COF-40} ,
excellent photocatalytic CO\textsubscript{2} reduction performance was
exhibited. The\textbf{COF-40} achieved a CO production rate of 1040 mmol
g\textsuperscript{-1} h\textsuperscript{-1}under 17.5 h of light CO
production rate with a selectivity of 81\% for H\textsubscript{2}. This
performance was improved by 84\% in 5 hours by dye sensitization,
resulting in a CO production rate of 1400 mmol
g\textsuperscript{-1}h\textsuperscript{-1} and a CO/H\textsubscript{2}
selectivity of 86\%. Based on a range of experimental and computational
techniques.

Zhao and co-workers reported three sp\textsuperscript{2}c-COF
(\textbf{COF-41} , \textbf{COF-42} and \textbf{COF-43} ), which
incorporate benzobisthiazole as an electron-withdrawing group, resulting
in an electron-deficient \selectlanguage{greek}π-\selectlanguage{english}conjugated C2-symmetric building block in
combination with a cyano ethylene bond (Figure
14).\textsuperscript{{[}62{]}} Moreover, C3 symmetric units such as
triazine and benzotrithiophene were introduced into the COFs skeleton as
photoactive functional groups. The electronic properties of the C3
symmetric unit can be modified through the structure adjustment,
allowing modulation of the semiconductor properties of the \selectlanguage{greek}π-\selectlanguage{english}conjugated
COF. \textbf{COF-43} , which contains electron-rich benzotrithiophene as
a donor, exhibits a strong donor-acceptor (D-A) effect, leading to
excellent photocatalytic hydrogen evolution reaction activity of 15.1
mmol/h\textsuperscript{-1}g\textsuperscript{-1} under visible light
irradiation. This performance is superior to that of
\textbf{COF-41}containing triazine (10.5 mmol/h
h\textsuperscript{-1}g\textsuperscript{-1}) and
benzene-containing\textbf{COF-42} (1.2
mmol/h\textsuperscript{-1}g\textsuperscript{-1}), which demonstrates
that the sp\textsuperscript{2}c-COF material with a D-A system has
remarkable photocatalytic hydrogen evolution reaction performance.
Furthermore, this work presents a facile method for the preparation of
2D sp\textsuperscript{2}c-COFs through Knoevenagel condensation, where
neutral AcONH\textsubscript{4} is used to catalyze the condensation,\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image21/image21}

\end{center}
\end{figure}

\textbf{Figure 13.} The synthesis and structures of \textbf{COF-35} to
\textbf{COF-40}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image22/image22}

\end{center}
\end{figure}

\textbf{Figure 14.} The synthesis and structures of \textbf{COF-41} to
\textbf{COF-43}

thereby improving the reversibility of the initial step. This approach
simplifies the traditionally challenging preparation and synthesis of
sp\textsuperscript{2}c-COFs.

The approach of polymerization via a template, as proposed by Jiang and
co-workers, offers a convenient pathway toward the synthesis of
sp\textsuperscript{2}c-COFs. The study revealed that the imine-linked
COFs are capable of seeding the C=C bond formation reaction and
confining the growth of all sp\textsuperscript{2}carbon lattice onto the
x-y plane, so that the polymer chains can nucleate into the same
topological tiling to form 2D all sp\textsuperscript{2} carbon
conjugated COFs. Through this method, a series of
sp\textsuperscript{2}c-COFs (\textbf{COF-30} , \textbf{COF-35} , and
\textbf{COF-44} ) were successfully synthesized, each exhibiting
tetragonal, hexagonal, and Kagome topologies (Figures 12, 13, and
15).\textsuperscript{{[}63{]}} The use of template aggregation provides
a structural foundation for unraveling the unprecedented functionalities
of \selectlanguage{greek}π \selectlanguage{english}transport and exciton leap in the two-dimensional
sp\textsuperscript{2} carbon backbone, which is controlled by the
lattice topology.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image23/image23}

\end{center}
\end{figure}

\textbf{Figure 15.} The synthesis and structures of \textbf{COF-44}

In 2022, Jiang and co-workers developed a module-patterned
polycondensation strategy for the efficient and designed synthesis of
crystalline porous covalent organic
frameworks.\textsuperscript{{[}64{]}}This strategy uses four branching
segments as knot modules, and these knots with different \selectlanguage{greek}π \selectlanguage{english}backbones can
polymerize with different linkers, thus easily producing crystalline
porous COFs in high yields. And it is widely applicable to a broad
diversity of both knot and liker units with different structures,
enabling the construction of a diversity of unprecedented crystalline 2D
polymers and open frameworks. A series of sp\textsuperscript{2}c-COFs
(\textbf{COF-45} to \textbf{COF-53} ) were successfully synthesized
(Figure 16). The adaptable nature of C2-symmetric junction modules,
which can be extended to comprise distinct \selectlanguage{greek}π-\selectlanguage{english}skeletons and paired with
varying linker units, engendered the creation of an unparalleled library
of 2D sp\textsuperscript{2}-carbon polymers and open frameworks.

Gu and co-workers reported an approach to synthesize cyano-substituted
benzofuran bonded sp\textsuperscript{2}c-COFs via hydroxybenzaldehyde
and acetonitrile building blocks.\textsuperscript{{[}65{]}} Two
cyano-substituted benzofuran-linked skeletons (\textbf{COF-54}
to\textbf{COF-57} ), which possess both crystallinity and porosity, were
successfully prepared (Figure 17). In the initial process, Knoevenagel
condensation generates the backbone reversibly, while successive cyanide
migration, ring closure, and oxidation reactions produce irreversible
heterocyclic cyano-substituted benzofuran bonds. This advancement
extends the synthesis of sp\textsuperscript{2}c-COFs by integrating
crystallinity, porosity, and post-synthetic modification feasibility
into a chemically robust framework using irreversible reactions. The
high stability of the benzofuran bond allows for the introduction of new
functionality through post-synthetic modifications under harsh
conditions. In a demonstration of its proof-of-concept, the
acidified\textbf{COF-57} displayed a remarkable super protonic
conductivity of 4.35 \selectlanguage{ngerman}× 10\selectlanguage{english}\textsuperscript{-2} S cm\selectlanguage{english}\textsuperscript{-1},
thereby substantiating the viability and practical utility of COF
materials of this nature.

Cui and co-workers reported two chiral sp\textsuperscript{2}c-COFs
(\textbf{COF-58} and \textbf{COF-59} ) with a 2D lamellar tetragonal
structure, employing olefin linkages as building blocks (Figure
18).\textsuperscript{{[}66{]}} Reduction of the olefin linkages of the
prepared COFs generated C-C single bonds linked frameworks, which
maintained high crystallinity, porosity, and superior chemical stability
even in strongly acidic or basic conditions. The newly formed C-C bond
linkage led to a blue-shifted emission, enhanced quantum yield, and
fluorescence lifetime compared to the original structure. Furthermore,
the enantioselectivity of the sp\textsuperscript{2}c-COFs was higher
than its reduced structure when utilized as a fluorescent sensor for
chiral amino alcohols. This study highlights the potential of utilizing
COFs with tunable chemical and structural properties for the development
of advanced materials with tailored properties.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image24/image24}

\end{center}
\end{figure}

\textbf{Figure 16.} The synthesis and structures of \textbf{COF-45} to
\textbf{COF-53}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image25/image25}

\end{center}
\end{figure}

\textbf{Figure 17.} The synthesis and structures of \textbf{COF-54} to
\textbf{COF-57}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image26/image26}

\end{center}
\end{figure}

\textbf{Figure 18.}The synthesis and structures of\textbf{COF-58} to
\textbf{COF-61}

The 2D sp\textsuperscript{2}c-COFs often encounter a bottleneck in
charge transport between the layers. In this case, 3D
sp\textsuperscript{2}c-COFs offer extended \selectlanguage{greek}π-\selectlanguage{english}conjugation in the x, y,
and z directions simultaneously, making them more favorable for charge
transport in theory. However, the examples of reported 3D COFs have
fallen far behind that of 2D COFs since Yaghi reported the first example
of 3D COFs in 2007. Cao and co-workers reported an example of 3D
sp\textsuperscript{2}c-COFs.\textsuperscript{{[}67{]}}Cyclooctatetrathiophene
was utilized as a building block to construct this 3D
sp\textsuperscript{2}c-COFs (\textbf{COF-62} ) (Figure 19). The highly
ordered nature of I\textsubscript{2}-doped \textbf{COF-62} led to an
enhanced Hall electron mobility of up to 2.62
cm\textsuperscript{2}V\textsuperscript{-1} s\textsuperscript{-1} in
ambient air, which typically exhibits ferromagnetism without metals.

Fang and co-workers synthesized two 3D sp\textsuperscript{2}c-COFs
(\textbf{COF-63} , \textbf{COF-64} ) by using tetra4-formyl
phenylene)silane as the central building block (Figure
19).\textsuperscript{{[}68{]}} The potential application of
\textbf{COF-64} with a larger pore as a cancer drug (cisplatin, CIS)
carrier is also explored. Furthermore, materials relating to the
carrying of drugs (\textbf{CIS@COF-64} ) not only functioned as a good
drug-carrying and slow-release material but also as a potential cellular
fluorescent dye that provided a long-lasting fluorescent tracer for drug
transport.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image27/image27}

\end{center}
\end{figure}

\textbf{Figure 19.} The synthesis and structures of \textbf{COF-62} to
\textbf{COF-68}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image28/image28}

\end{center}
\end{figure}

\textbf{Figure 20.} The synthesis and structures of \textbf{COF-69} to

\textbf{COF-74}

Qiu and co-workers constructed a 3D COFs electroextraction platform for
uranium adsorption from seawater. A 3D sp\textsuperscript{2}c-COF
(\textbf{COF-65} ) was synthesized through post-synthetic modification
with ketamine oxime (Figure 19).\textsuperscript{{[}69{]}} After
functionalization with an alkylamine oxime group, \textbf{COF-66} was
obtained, which exhibited a high affinity for uranyl ions. Under the
action of an electric field, a strong sorption capacity toward uranyl
ions was obtained.

Zhao and co-workers synthesized\textbf{COF-67} through {[}3+4{]} imine
condensation reaction.\textsuperscript{{[}70{]}}\textbf{COF-67} was then
transformed into quinoline-linked COFs (\textbf{COF-68} ) (Figure 19).
The nitrogen-riched triazine and pyrazine fractions in \textbf{COF-68}
can bind Li ions and provide multiple ion-adsorption active sites and
ion storage space. The LIBs assembled with \textbf{COF-68} electrodes
demonstrated high Li-ion storage capacity and good cycling stability due
to high chemical stability, \selectlanguage{greek}π-\selectlanguage{english}conjugation, and good electronic and ionic
conductivity.

3. Pyrazine Linkages

Pyrazine-linked COFs are usually constructed through the reaction of
Pictet-Spengler between 1,2-cyclohexanedione and 1,2-phenylenediamine.
The pyrazine linkages bring uninterrupted \selectlanguage{greek}π-\selectlanguage{english}electron conjugation and
make them a promising material for semiconductor applications. Jiang and
co-workers reported a chemically stable
\selectlanguage{greek}π-\selectlanguage{english}electron-conjugated\textbf{COF-69} (Figure
20).\textsuperscript{{[}71{]}}\textbf{COF-69} with uninterrupted
\selectlanguage{greek}π-\selectlanguage{english}electron conjugation, which extends throughout the entire sheet of
COFs, exhibits exceptional physical properties and three-dimensional
periodic ordering that are typically absent in conjugated polymers.

The same group integrated cobalt (II) phthalocyanine into 2D COFs
(\textbf{COF-70} ) via a robust phenazine bond formation (Figure
20).\textsuperscript{{[}23{]}} Excellent stability, even under boiling
water, acid, or base conditions, was obtained. \textbf{COF-70} can be
used as an electrocatalyst which is unique in its ability. By
integrating all-\selectlanguage{greek}π \selectlanguage{english}conjugated, stable backbone and catalytic sites into a
single lattice, \textbf{COF-70} exhibits excellent stability,
conductivity, and catalytic activity in catalyzing the reduction of
CO\textsubscript{2} to CO in water.

Hoberg and co-workers reported a series of COFs (\textbf{COF-71} to
\textbf{73} ) that possess highly stable semiconducting aromatic
backbones and intrinsically ordered nanoscale pores (Figure
20).\textsuperscript{{[}72{]}} This family of COFs can be functionalized
either through the monomer design and synthesis or the post-modification
of the obtained COFs to incorporate various desired functional groups.
This new bottom-up approach has achieved the objective of producing
highly ordered and modifiable materials with stable aromatic backbones
and functionalized pores. The condensation of readily available small
precursor molecules in combination with well-established high-yield
coupling reactions enables the replacement of the bromine placed on the
aromatic ring of aligned pores, resulting in different functionalities.

Feng and co-workers reported a family of pyrazine-linked
metal-phthalocyanine-based conjugated 2D COFs (\textbf{COF-74-M} , M =
Zn, Cu, Ni) that assemble in van der Waals (vdWs) layer stacking
structures (Figure 20).\textsuperscript{{[}73,74{]}}This work emphasizes
the potential of fully conjugated 2D COFs with high mobility as
optoelectronic semiconductors and provides a rational methodology for
determining structure-property relationships. This work provides a
dependable route to designing and developing semiconductor COF materials
with enhanced charge transport properties.

The synthesis of fully conjugated COFs through the condensation
reactions of various building blocks has resulted in the production of
diverse materials with tunable properties. Mirica and co-workers
investigated enhanced proton transport properties of COFs through the
systematic study of two aza-COF analogues (\textbf{COF-75}
and\textbf{COF-76} ) (Figure 21).\textsuperscript{{[}75{]}} The chemical
composition, porosity, and crystallinity of these materials were found
to play important roles in their proton transport properties, providing
insight into the design and optimization of COFs for proton conduction.

Chen and co-workers reported the synthesis of a honeycomb-shaped
nitrogen-rich COF (\textbf{COF-77} ) with multiple carbonyl groups
through a triple condensation reaction (Figure
21).\textsuperscript{{[}76{]}} This study also led to the discovery of
materials with excellent electrochemical properties. \textbf{COF-77}
electrode demonstrated a high specific capacity, excellent cycling
stability, and good rate capability, making it a promising candidate for
use in high-energy and high-power sodium batteries. This work highlights
the potential of COFs with multiple redox sites as high-performance
electrode materials and pro- vides a basis for the design and
application of such materials in energy storage devices.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image29/image29}

\end{center}
\end{figure}

\textbf{Figure 21.} The synthesis and structures of \textbf{COF-75} to

\textbf{COF-77}

4. Benzobisoxazole Linkages

McGrier and co-workers reported benzobisoxazole-linked COFs by using
cyanide as a catalyst. \textbf{COF-78} and \textbf{COF-79} were
successfully synthesized utilizing C3-symmetric formyl and C2-symmetric
o-aminophenol-substituted molecular building blocks (Figure
22).\textsuperscript{{[}77{]}} These COFs exhibit exceptional water
stability, high surface area, and remarkable CO\textsubscript{2}
absorption capacity. Wang and co-workers synthesized highly crystalline
benzobisoxazole-linked COFs (\textbf{COF-79 to 81} ) by employing
N-methyl-2-pyrrolidone/resorcinol as a solvent mixture with benzoxazole
as an additive (Figure 22).\textsuperscript{{[}78{]}} These COFs display
excellent stability in the presence of acids, bases, and sunlight. In
addition, the incorporation of the benzothiazole moiety in the
\selectlanguage{greek}π-\selectlanguage{english}conjugated COF framework diminishes the optical band gap and enhances
the visible light absorption capacity. Benefiting from the reinforced
covalent linkage in its structure, \textbf{COF-79} exhibits excellent
recoverability, and its activity and crystallinity remain unaffected
even after 20 runs.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image30/image30}

\end{center}
\end{figure}

\textbf{Figure 22.} The synthesis and structures of \textbf{COF-78} to

\textbf{COF-81}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image31/image31}

\end{center}
\end{figure}

\textbf{Figure 23.} The synthesis and structures of \textbf{COF-82 to
COF-84}

\par\null

Dong and co-workers have employed a natural-sunlight-mediated
photocatalytic polymerization technique for the synthesis of
COFs.\textsuperscript{{[}79{]}} By employing TBA-eosin Y as an
assistant, the reaction of 2,5-diamino-1,4-benzenediol dihydrochloride
with 1,3,5-tris(4-formylphenyl)triazine resulted in the facile
production of benzoxazole-linked COF \textbf{COF-81} in satisfactory
yield via solar photocatalytic synthesis (Figure 22). This eco-friendly
approach, which is triggered by clean energy, enables the COF synthesis
to be more sustainable than the conventional solvothermal synthesis
of\textbf{COF-81} , which involves high temperature, high pressure, and
stoichiometric amounts of additives. The resulting \textbf{COF-81} can
serve as a reusable photocatalyst that efficiently promotes the
visible-light-driven oxidation of sulfides to sulfoxides under mild
conditions.

Baek and co-workers reported the transformation of
reversible\textbf{COF-82} to irreversible \textbf{COF-83} via
post-oxidative cyclization (Figure
23).\textsuperscript{{[}80{]}}2,3-dichloro-5,6-dicyano-1,4-benzoquinone
(DDQ) was chosen as the oxidizing agent, which not only promotes the
reaction directly but can also be easily removed after the completion of
the reaction. This post-oxidative cyclization strategy has yielded
\textbf{COF-83} with high crystallinity and porosity. The resulting
\textbf{COF-83} exhibits superior thermal and chemical stability
compared to \textbf{COF-82} due to the stable and rigid benzoxazole
linkage.

The same group further produced \textbf{COF-84} , which possessed a
densely aromatic structure (Figure 23).\textsuperscript{{[}81{]}} The
resultant \textbf{COF-84} displayed both crystallinity and porosity.
Partial carbonization occurred in the grain boundaries of
the\textbf{COF-84} subsequent to heat treatment (\selectlanguage{ngerman}\textbf{COF-84-600°C}
). This treatment led to improved electrical conductivity. Notably, the
immobilization of platinum single atoms onto the nitrogen sites within
the pores of \textbf{COF-84-600°C} resulted in superior electrocatalytic
properties for the acidic hydrogen evolution reaction.

5. Dioxin Linkages

In 2019, Yaghi and co-workers reported a novel class of COFs referred to
as dioxin-linked COFs, namely \textbf{COF-86} and \textbf{COF-87}
(Figure 24).\textsuperscript{{[}82{]}} In contrast to the typical
reversible condensation reactions employed in most COF synthesis,
irreversible nucleophilic aromatic substitution reactions were utilized.
This unique strategy resulted in these COFs with exceptional chemical
stability in both acidic and basic environments. \textbf{COF-86} can be
post-modified by concentrated sodium hydroxide solution and
hydroxylamine, resulting in the production of \textbf{COF-88} and
\textbf{COF-89} . Notably, the crystallinity of both COFs remained
intact following post-modification. Because these reactions are
irreversible, the resultant frameworks have high chemical stability in
both acid and base.

Qiu and co-workers reported the synthesis of polyarylether-based COFs
(\textbf{COF-85} , \textbf{COF-86} , \textbf{COF-90} and \textbf{COF-91}
) via nucleophilic aromatic substitution reactions between\emph{o}
-difluorobenzene and catechol (Figure 24).\textsuperscript{{[}83{]}} The
resulting materials exhibited remarkable chemical stability in boiling
water, strong acids, and bases, as well as oxidizing and reducing
conditions. Moreover, these polyarylether-based COFs can be
post-synthetically functionalized with carboxyl or amino groups, keeping
porosity, high stability, and recyclability. Such features make these
materials promising candidates for various applications, including the
removal of antibiotics from water across a broad range of pH values.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image32/image32}

\end{center}
\end{figure}

\textbf{Figure 24.} The synthesis and structures of \textbf{COF-85} to
\textbf{COF-91}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image33/image33}

\end{center}
\end{figure}

\textbf{Figure 25.} The synthesis and structures of \textbf{COF-92} to
\textbf{COF-96}

6. \selectlanguage{greek}β-\selectlanguage{english}Amino Enone Linkages

Perepichka and co-workers reported \selectlanguage{greek}β-\selectlanguage{english}amino enone-linked COFs
(\textbf{COF-92 to 95} ) through dynamic condensation of Michael
addition-elimination reactions between \selectlanguage{greek}β-\selectlanguage{english}ketones and aromatic amines
(Figure 25).\textsuperscript{{[}84{]}} The obtained COFs exhibit better
hydrolytic stability and more efficient \selectlanguage{greek}π-\selectlanguage{english}electron delocalization. The
emission of fluorescent \textbf{COF-94}and \textbf{COF-95} can be
effectively quenched in the presence of nitroaromatic (picric acid) and
peroxide explosives. This synthetic strategy paves the way for the
development of electronically functional COFs and their potential
applications in semiconductors and sensing devices.

Bojdys and co-workers reported a 2D COF (\textbf{COF-96} ) condensed
from the C2-symmetric keto-enol and the C3-symmetric triamine (Figure
25).\textsuperscript{{[}85{]}}\textbf{COF-96} was found to be capable of
real-time, reversible optical and electronic sensing of volatile acids
and bases. The optical and electronic activity of the \textbf{COF-96}
was achieved through preferential protonation of triazine nitrogen,
leading to two-orders of magnitude increase in optical response and bulk
conductivity that was visible to the naked eye. The single-point
protonation triggered the entry of \selectlanguage{greek}π \selectlanguage{english}electrons into the triazine ring,
accompanied by a fluorescence burst. Both optical and electronic effects
involved the entire \selectlanguage{greek}π-\selectlanguage{english}aromatic skeleton. \textbf{COF-96} was not only
chemically stable to the corrosive trigger molecule but also permanently
porous to the chemisorbed guest.

7. Post-Synthetic Modification

The synthesis of fully conjugated COFs with enhanced chemical stability
and optoelectronic properties can be achieved through post-synthetic
modification. In this approach, Imine-linked COFs are converted into
ultra-stable porous aromatic frameworks. For instance, Lotsch and
co-workers have reported the topochemical conversion of triphenyl
triazine-based Imine-linked COFs (\textbf{COF-99} ) into thiazole-linked
COFs (\textbf{COF-100} ) through a post-synthesis locking strategy
involving elemental sulfur (Figure 26).\textsuperscript{{[}86{]}} This
method leads to a change in the symmetry of the COF crystals while
maintaining their original topology, thereby improving their chemical
stability. Similarly, Liu and co-workers have converted dynamic imine
linkages into a more stable quinoline aromatic ring system via the
za-Diels-Alder cycloaddition reaction, obtaining quinoline-linked COFs
(\textbf{COF-98} ) with significantly enhanced chemical stability
(Figure 26).\textsuperscript{{[}87{]}}

Yaghi and co-workers exhibited that the imine-linked COFs
(\textbf{COF-101} ) can be transformed into two distinct heterogeneous
COFs, namely \textbf{COF-102} and \textbf{COF-103} , featuring thiazole
and oxazole linkages, respectively (Figure
26).\textsuperscript{{[}88{]}} This conversion was achieved through
successive substitution and oxidative cyclization procedures. These
materials exhibit higher chemical stability compared to their
amine-substituted starting materials.

Zhao and co-workers reported the synthesis of two ultra-stable fully
conjugated two-dimensional COFs (\textbf{COF-105} and \textbf{COF-107} )
linked by thieno{[}3,2-c{]}pyridine units using post-oxidative
cyclization (Figure 27).\textsuperscript{{[}89{]}} The dynamic imine
bond and the adjacent thiophene can be converted to
thieno{[}3,2-c{]}pyridine by oxidative cyclization.
Thieno{[}3,2-c{]}pyridines have not been previously used as linkages for
COFs construction. The resulting COFs exhibit superstability and good
electron delocalization. Notably, the small structural differences can
lead to completely different photocatalytic activities. The fully
\selectlanguage{greek}π-\selectlanguage{english}conjugated \textbf{COF-107} with a triazine core showed an excellent
photocatalytic NADH regeneration rate of 74\% within 10 min. Overall,
these post-synthetic modification strategies offer a powerful approach
to enhance the chemical stabilities and optoelectronic activities of
COFs, opening new avenues for their applications in various fields.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image34/image34}

\end{center}
\end{figure}

\textbf{Figure 26.} The synthesis and structures of \textbf{COF-97} to
\textbf{COF-103}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image35/image35}

\end{center}
\end{figure}

\textbf{Figure 27.} The synthesis and structures of \textbf{COF-104} to
\textbf{COF-107}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image36/image36}

\end{center}
\end{figure}

\par\null

\textbf{Figure 28.} The synthesis and structures of \textbf{COF-108} to
\textbf{COF-111}

8. Multicomponent One-Pot Reaction

Wang and co-workers reported a novel methodology for the synthesis of
robust COFs (\textbf{COF-108} to \textbf{111} ) through multicomponent
reactions (Figure 28).\textsuperscript{{[}90{]}} By inducing the
formation of five covalent bonds within each cyclic imidazole ring in
situ, a series of imidazole-linked COFs was fabricated in a robust
manner from three readily available components via the
Debus-Radziszewski multicomponent reaction. This study opens up a new
avenue for the precise covalent assembly of crystalline COFs.

Dong and co-workers developed a strategy for the synthesis of COFs
through multicomponent one-pot in situ
reactions.\textsuperscript{{[}91{]}}Specifically, they obtained a series
of \selectlanguage{greek}α-\selectlanguage{english}aminonitrile and quinoline-linked COFs (\textbf{COF-112, 113, 115}
to \textbf{COF-124} ) by employing a three-component one-pot in situ
Strecker and Povarov reaction under solvent-heated conditions (Figure
29). The obtained COFs exhibited high crystallinity and permanent
porosity. Compared to the stepwise post-synthetic modification methods,
the multicomponent one-pot synthetic strategy can avoid the collapse of
the frameworks and maintain crystallinity. This work presents a novel
synthetic method that guarantees high yields while maintaining high
crystallinity and porosity.

Cooper and co-workers reported a simple and efficient three-component
assembly reaction between readily available aldehydes, amines and
elemental sulfur via C-H functionalization and oxidative cyclization
under transition-metal-free conditions.\textsuperscript{{[}92{]}}This
one-pot method successfully prepared five highly stable thiazole-linked
COFs (\textbf{COF-125} to \textbf{COF-129} ) through a range of
functionalized aldehydes and amines (Figure 30). Among these COFs,
\textbf{COF-129} exhibits high photocatalytic activity toward hydrogen
evolution from water. A maximum rate of 4296 \selectlanguage{greek}μ\selectlanguage{english}mol h\textsuperscript{-1}
g\textsuperscript{-1} under visible light irradiation was achieved, as
well as high recoverability for 50 hours of continuous hydrogen
evolution.

9. Conclusions and Perspectives

Fully conjugated COFs have emerged as a significant development in the
field of COFs since their successful preparation. This review provides a
comprehensive overview of recent developments in the structure,
synthesis, and applications of fully conjugated COFs, which possess
outstanding chemical stability, continuous \selectlanguage{greek}π-\selectlanguage{english}conjugation, and high
crystallinity. These properties have led to the application of fully
conjugated COFs in a diverse range of fields, including LIBs,
photocatalytic hydrogen production, supercapacitors, and multiphase
catalysis. However, despite the impressive fully conjugated COFs face a
number of challenges that must be overcome.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image37/image37}

\end{center}
\end{figure}

\textbf{Figure 29.} The synthesis and structures of \textbf{COF-112} to
\textbf{COF-124}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image38/image38}

\end{center}
\end{figure}

\textbf{Figure 30.} The synthesis and structures of \textbf{COF-125} to
\textbf{COF-127}

One of the most important issues is the lack of a deep understanding of
the growth and nucleation processes of fully conjugated COFs, which
limits researchers to the predictive synthesis and rational upgrading of
fully conjugated COFs. Single-crystal preparation of fully conjugated
COFs is currently needed because a high degree of structural integrity
is essential to elucidate the nature and origin of fully conjugated
COFs. In addition, how to translate the structural designability of
fully conjugated COFs into the designability of properties is worthy of
deeper investigation. Clarifying the correlation between structure and
performance will greatly expand the application area of fully conjugated
COFs. A deeper investigation of the correlation between the structure
and performance of fully conjugated COFs is necessary to expand their
application areas. Moreover, the limited range of applications of fully
conjugated COFs, primarily in photocatalysis, calls for urgent
expansion. In contrast, other types of COFs, such as imine-linked COFs,
are widely used in gas separation, sensing, seawater desalination,
chiral chemistry and sensing, among others. Advancements in the
application of fully conjugated COFs will promote their further
development.

Additionally, the huge trial and error work in the synthesis of fully
conjugated COFs hinders its development and applications. Therefore, the
development of more widely applicable synthesis methods through
innovation in monomers, catalysts and other factors is crucial to
accelerate the synthesis of fully conjugated COFs. Finally, fully
conjugated COFs are still in their infancy and require more scientific
effort. Only a few 3D COFs have been reported successfully thus far,
necessitating the development of more diverse synthesis methods and
structures to expand the library of 3D fully conjugated COFs and support
their further development in the future. Despite these challenges, the
unique structure and excellent properties of fully conjugated COFs
position them at the forefront of COFs. We envision that the combination
of post-synthetic modification and multicomponent one-pot reaction will
further diversify the portfolio of fully conjugated COFs in an easier
way.

Innovations in energy sources, monomer design, catalysts, nucleation
control, and workup activation, as well as computational high-throughput
screening, can accelerate the discovery process of fully conjugated
COFs. Furthermore, the integration of fully conjugated COFs with
functional materials such as metal nanoparticles, enzymes, MOFs, and
polymers can lead to the development of fully conjugated COFs-based
composites, which is still in its nascent stage and merits more
scientific efforts. Lastly, a mechanistic understanding of the formation
and performance of fully conjugated COFs in specialty applications is of
paramount importance for their mechanism-based synthesis and rational
improvement. Although fully conjugated COFs are still in a nascent
stage, continuous and interdisciplinary scientific endeavors along these
promising directions will lay a solid groundwork for their bright
future.

Acknowledgement

This work was supported by the National Natural Science Foundation of
China (51972185, 22175101) and the Natural Science Foundation of
Shandong Province (ZZR2020ZD38).

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