Xing Li 李星

Xing Li received his Bachelor's degree in Chemistry and Biological Chemistry from Nanyang Technological University, Singapore, in 2014. He received his PhD degree in the NUS Graduate School for Integrative Sciences and Engineering from National University of Singapore in 2018. He is now a research fellow and working on COFs for novel applications in Prof Loh Kian Ping's group. His research interests include turning 2D COFs into organic 2D materials, applying COFs for solid-state photoluminescence, solid-state ion conduction and metal–gas batteries, and correlating structure–property relationships of functional materials.
李星于 2014 年获得新加坡南洋理工大学化学与生物化学学士学位。2018 年，他在新加坡国立大学综合科学与工程研究生院获得博士学位。他现在是 Loh Kian Ping 教授研究小组的研究员，从事 COFs 的新型应用研究。他的研究兴趣包括将二维 COFs 转化为有机二维材料，将 COFs 应用于固态光致发光、固态离子传导和金属-气体电池，以及关联功能材料的结构-性能关系。

Priya Yadav 普里亚-亚达夫

Dr Priya Yadav received her BS degree (2012) from University of Delhi and MS degree (2014) from Indian Institute of Technology Roorkee. She completed her PhD (2019) in organic material chemistry at National University of Singapore. She is now pursuing her postdoctoral research in Prof. Loh Kian Ping's group and her research interests focus on synthesis of polycyclic aromatic hydrocarbons and covalent organic frameworks.
Priya Yadav 博士于 2012 年获得德里大学学士学位，2014 年获得印度罗尔基理工学院硕士学位。她在新加坡国立大学完成了有机材料化学博士学位（2019 年）。她目前在 Loh Kian Ping 教授的研究小组从事博士后研究，研究兴趣主要集中在多环芳烃和共价有机框架的合成。

Kian Ping Loh

Kian Ping Loh is currently Provost's Chair Professor at NUS and a well-established researcher in the field of advanced 2D materials. Loh's research focuses on growth, molecular chemistry, electronic materials science and devices of 2D materials, which include 2D covalent organic frameworks, 2D hybrid perovskites and 2D inorganic materials. His awards include the President's Science Award in 2014, the University Outstanding Researcher award in 2012, University's Young Scientist award in 2008, and American Chemical Society Nano Lectureship award in 2013. He is currently an associate editor of the American Chemical Society journal Chemistry of Materials. He is also head of the 2D materials group at the Centre for Advanced 2D Materials, NUS, and also a co-director of the Shenzhen-NUS Joint Collaborative Innovation Center for Optoelectronic Science & Technology.
Kian Ping Loh 现任新加坡国立大学教务长讲座教授，是先进二维材料领域的知名研究人员。Loh 的研究重点是二维材料的生长、分子化学、电子材料科学和器件，其中包括二维共价有机框架、二维混合包光体和二维无机材料。他获得的奖项包括 2014 年的校长科学奖、2012 年的大学杰出研究员奖、2008 年的大学青年科学家奖和 2013 年的美国化学学会纳米讲座奖。他目前是美国化学学会期刊《材料化学》的副主编。他还是新加坡国立大学先进二维材料中心二维材料组组长，同时也是深圳-新加坡国立大学光电科技联合协同创新中心的联合主任。

1. Introduction
1. 引言

Although organic molecules can be synthesized with bond-to-bond, atom-to-atom precision, the cross-linking of these molecules to form a periodic framework in three-dimensional space has to compete with the fast rate of random polymerization. The synthesis of crystalline organic–inorganic polymeric materials such as zeolites,^1,2 perovskites,^3,4 and metal organic frameworks (MOFs)^5,6 has seen tremendous progress over the past few decades. Progress in the synthesis of crystalline porous organic polymers reached a milestone with the discovery of covalent organic frameworks (COFs) in 2005 by Yaghi.^7–14 The focus in COF research thus far has been heavily focused on synthesizing structures with different topologies and covalent linkages, improving crystallinity or performing various post-synthetic modifications. The ease of encoding functionalities and the abundant choice of building units and linkages give rise to a plethora of COFs with diverse properties. Despite this, a unique application that differentiates COFs from their molecular fragments or polymers has yet to be identified. This is not helped by the fact that the single-crystal structures of most COFs synthesized to date are still not confirmed. Besides the need to continually improve the crystallinity of COFs, researchers should leverage the rigid framework structures and uniform pore structures in COFs to obtain performance that is larger than the sum of their parts.
虽然有机分子的合成可以达到键对键、原子对原子的精确度，但这些分子在三维空间中形成周期性框架的交联过程必须与快速的无规聚合竞争。沸石、^1,2 过氧化物、^3、4 和金属有机框架（MOFs）^5,6 在过去几十年中取得了巨大进步。随着 2005 年 Yaghi 发现共价有机框架（COF），结晶多孔有机聚合物的合成取得了里程碑式的进展。功能编码的简易性以及构建单元和连接方式的丰富选择，催生了大量具有不同特性的 COF。尽管如此，将 COF 与其分子片段或聚合物区分开来的独特应用仍有待确定。迄今为止合成的大多数 COFs 的单晶结构仍未得到确认，这也不利于其应用。除了需要不断提高 COF 的结晶度外，研究人员还应该利用 COF 中的刚性框架结构和均匀的孔隙结构来获得大于各部分总和的性能。

Eschenmoser introduced the concept of “functional-oriented synthesis” thirty years ago; this is underscored by the concept that nature-evolved molecules are more complicated than the function encoded in them.¹⁵ Understanding structure–property correlations allows chemists to simplify the design of these molecules to perform the same function, or achieve even better performance. For instance, synthetic fentanyl has a less complicated structure and stronger biological activity than natural morphine.¹⁶ However, the concept of function-oriented synthesis is still mainly restricted to the synthesis of small molecules and has not been applied to the synthesis of framework structures.
三十年前，Eschenmoser 提出了 "功能导向合成 "的概念；自然界演化出的分子比其编码的功能更加复杂这一概念强调了这一点。¹⁶ 然而，以功能为导向的合成概念仍主要局限于小分子的合成，尚未应用于框架结构的合成。

To develop performance differentiations in COFs, chemists need to leverage the intrinsic characteristics of COFs such as their rigid framework structures, stacking order and 1-D channels, and install requisite functional groups to create synergistic interactions. In this review, we will focus on how different elements of COFs, such as their topology, chemical linkages, stacking order and porosity, can be optimized to achieve competitive performance in applications such as light emission, stimuli response, gas storage, ion conduction, and energy storage. A summary on the properties of 2D COFs and how they can be exploited for different applications is shown in Table 1.
为了开发 COF 的差异化性能，化学家们需要利用 COF 的固有特性，如其刚性框架结构、堆积顺序和一维通道，并安装必要的官能团以产生协同作用。在本综述中，我们将重点讨论如何优化 COF 的拓扑结构、化学连接、堆积顺序和孔隙率等不同元素，从而在光发射、刺激响应、气体存储、离子传导和能量存储等应用中实现具有竞争力的性能。表 1 总结了二维 COF 的特性以及如何将它们用于不同的应用。

Two-dimensional (2D) COFs are called “2D” because the covalent linkages are in-plane while the non-covalent interactions between different planes are in the out-of-plane direction. Therefore, similar to graphite, bulk 2D COFs are actually quasi-2D materials and they can become truly 2D only when reduced to the thickness of a single or a few unit cells. The relatively weak bonding along the z-axis allows 2D COFs to be delaminated into few-layer thin sheets that can have better solution-processability compared to three-dimensional (3D) COFs. Moreover, bulk 2D COFs^17–19 can act as templates for the synthesis of crystalline 2D polymers.^20,21 Inspired by breakthroughs in 2D materials, the synthesis of ultrathin 2D COF sheets has become an emerging field.^22–26 Akin to graphene²⁷ and MXenes,²⁸ the name COFene¹¹ has been coined for ultrathin 2D COF sheets, which are believed to have properties distinct to bulk COFs and conventional 2D materials such as graphene and transition metal dichalcogenides.
二维（2D）COF 之所以被称为 "2D"，是因为共价键在平面内，而不同平面之间的非共价相互作用在平面外。因此，与石墨类似，块状二维 COF 实际上是准二维材料，只有当它们的厚度减小到一个或几个单元格时，才能成为真正的二维材料。沿z轴的键合相对较弱，这使得二维 COF 可以分层成几层薄片，与三维 (3D) COF 相比，这些薄片具有更好的溶液加工性。17-19此外，块状二维 COF17-19可作为合成结晶二维聚合物的模板。20,21受二维材料突破的启发，超薄二维 COF 薄片的合成已成为一个新兴领域。^22-26 类似于石墨烯²⁷ 和二甲苯、²⁸ COFene¹¹ 这一名称是为超薄二维 COF 片而命名的、超薄二维 COF 片被认为具有不同于块状 COF 和传统二维材料（如石墨烯和过渡金属二卤化物）的特性。

This article discusses the structure–property correlations for function-oriented design and synthesis of COFs from the bulk to an ultrathin level (Fig. 1). COF structures are analyzed by deconstructing them into backbone functionalities, linkages, and topological nets, so that we can understand how these components act either singly or in concert to attain particular properties and functions.
本文讨论了面向功能设计和合成 COF 的结构-特性相关性，从块体到超薄水平（图 1）。通过将 COF 结构解构为骨干功能、连接和拓扑网，我们可以分析 COF 结构，从而了解这些成分是如何单独或协同作用以实现特定性质和功能的。

2. Function-oriented synthesis of 2D COFs
2. 面向功能的二维 COF 合成

In this section, we will highlight recent advances in the potential applications of 2D COFs, which include solid-state light emission, stimuli-responsive COFs, CO₂ capture and conversion, and ion conduction. The roles of molecular scaffolds and covalent linkages in enhancing the performance are discussed.
本节将重点介绍二维 COF 潜在应用的最新进展，包括固态光发射、刺激响应 COF、CO₂ 捕获和转换以及离子传导。本文讨论了分子支架和共价连接在提高性能方面的作用。

2.1 Solid-state emission
2.1 固态发射

2D COFs provide a framework in the form of an anisotropic, crystalline organic 2D material for understanding the photo-physics of photon and charge interactions, where exciton and charge transport in the in-plane and out-of-plane directions are expected to be different. It is expected that different stacking orders such as eclipsed, staggered or antiparallel will exert distinct influence on the optical and electronic properties. The presence of porosity suggests that the pores and channels can be used for host–guest interactions; in addition, donor–acceptor interactions can be used to tune the fluorescence.
二维 COF 以各向异性的结晶有机二维材料的形式提供了一个框架，用于理解光子和电荷相互作用的光物理学。预计不同的堆叠顺序，如倾斜、交错或反平行，将对光学和电子特性产生不同的影响。孔隙率的存在表明，孔隙和通道可用于宿主-受体相互作用；此外，供体-受体相互作用可用于调节荧光。

Many 2D COFs that are built from luminescent building blocks suffer from fluorescence quenching. Aggregation-induced quenching by π–π stacking does not account entirely for the quenching because COFs built from the same molecular building block can be fluorescent or non-fluorescent depending on whether the linkages are boronic ester type linkages or imine type.^29–32 Compared to the mechanically stiffer (but chemically unstable) boronic ester linkages, imine-type linkages are rotationally labile when photo-excited, which may result in de-excitation via non-emissive dissipative pathways. In this section, we will review selected fluorescent 2D COFs and discuss the structural design used to turn on the solid-state light emission in 2D COFs (Table 2).
许多由发光结构单元构建的二维 COF 都会出现荧光淬灭现象。π-π堆叠引起的聚集淬灭并不能完全解释这种淬灭现象，因为由相同分子构件构建的 COF 可以发出荧光或不发出荧光，这取决于链接是硼酸酯类链接还是亚胺类链接。^29-32 与机械刚性较强（但化学性质不稳定）的硼酸酯连接相比，亚胺型连接在光激发时具有旋转易变性，这可能导致通过非耗散途径去激发。在本节中，我们将回顾选定的荧光二维 COF，并讨论用于开启二维 COF 固态光发射的结构设计（表 2）。

Surveying previous studies, we can conclude that linkage chemistry plays a vital role in inducing strong solid-state emission in 2D COFs. For instance, when an aggregation-induced emission luminogen (AIEgen) such as tetraphenylethene (TPE) was connected via boronate linkages, a highly emissive 2D COF was obtained.³⁰ In contrast, the solid-state PL was quenched when the TPE units were connected into 2D COFs via imine bonds.^29,33 Similar phenomena have been observed for non-AIE luminogen pyrene-based 2D COFs – when pyrene units were connected into 2D COFs via boronate^32,34,35 or acrylonitrile³⁶ linkages, solid-state emission was turned on, while imine linkages quenched the strong solid-state PL.³⁷ Therefore, imine bonds are apparently not the ideal linkage for building solid-state emissive 2D COFs.
综观以往的研究，我们可以得出结论：连接化学在诱导二维 COF 的强固态发射中发挥着至关重要的作用。例如，当通过硼酸盐连接^{聚合诱导发射发光剂（AIEgen）（如四苯基噻吩（TPE））时，可获得高发射率的二维 COF。30} 相反，当 TPE 单元 通过亚胺键连接成二维 COF 时，固态 PL 被淬灭。^29,33 非 AIE 发光体芘基二维 COF 也观察到类似现象--当芘单元 通过硼酸盐^{32 连接到二维 COF 时、34,35} 或丙烯腈³⁶ 连接，固态发射被打开，而亚胺连接则淬灭了强固态 PL。³⁷ 因此，亚胺键显然不是构建固态发射型二维 COF 的理想连接方式。

Here, we have summarized the privileged linkages (Fig. 2) and linkers (Fig. 3) suitable for inducing photoluminescence in solid-state 2D COFs. Guided by the 2D COF reticular chemistry, judicious choices of linkages and linkers will give rise to new solid-state photoluminescent 2D COFs.
在此，我们总结了适合在固态二维 COF 中诱导光致发光的特殊连接（图 2）和连接体（图 3）。在二维 COF 网状化学的指导下，明智地选择连接物和连接体将产生新的固态光致发光二维 COF。

To turn on the solid-state photoluminescence of 2D COFs, non-radiative pathways caused by π–π stacking or bond rotation must be suppressed.
要开启二维 COF 的固态光致发光，必须抑制由 π-π 堆叠或键旋转引起的非辐射途径。

2.1.1 Privileged covalent linkages for solid-state photoluminescence
2.1.1 用于固态光致发光的特权共价连接

Rotationally labile linkages such as imine bonds can quench the PL of 2D COFs via non-emissive release of photoexcited energy caused by free intramolecular bond rotation. Therefore, covalent linkages that restrict intramolecular bond rotation (RIR) are more likely to turn on the solid-state PL. To date, reported covalent linkages that were able to turn on the PL of 2D COFs are boronate ester, boroxine, acylhydrazone, acrylonitrile and olefin.
旋转易损连接（如亚胺键）可通过分子内自由键旋转引起的光激发能量的非消耗性释放，淬灭二维 COF 的 PL。因此，限制分子内键旋转（RIR）的共价键更有可能开启固态 PL。迄今为止，已报道的能够开启二维 COF 光激发的共价键有硼酸酯、硼氧、酰基腙、丙烯腈和烯烃。

Boronate and boroxine-type linkages have been used to turn on solid-state PL of 2D COFs (Fig. 4).^{30,32,34,35,38,39} Through condensation reactions, boron atoms covalently bond to oxygen atoms to form the six-membered ring of boroxine or the five-membered ring of benzodioxaborole, where intramolecular bond rotation can be efficiently restricted. Incorporating AIEgen tetraphenylethylene as the COF backbone functionality, highly emissive 2D COFs have been synthesized with quantum yields up to 32% in the solid state. Luminescent 2D COFs can be used in optoelectronics and ammonia sensing. The high degradability and biocompatibility of boronate or boroxine COFs suggest their applications in biology. Zhang et al. utilized boroxine COFs as smart carriers for in vivo drug delivery as well as bioimaging.⁴⁰
硼酸盐和硼氧烷型连接被用于开启二维 COF 的固态聚光（图 4）。^{30,32,34,35,38,39} 通过缩合反应，硼原子与氧原子共价键合，形成硼氧环的六元环或苯并二氧硼环的五元环，其中分子内的键旋转可被有效限制。将 AIEgen 四苯乙烯作为 COF 骨架官能团，合成出了高发射性二维 COF，固态量子产率高达 32%。发光二维 COF 可用于光电和氨传感。硼酸盐或硼氧 COF 的高降解性和生物相容性表明它们可应用于生物学。Zhang et al. 利用硼氧 COFs 作为智能载体，用于 体内给药和生物成像。

The second generation of solid-state emissive 2D COFs is based on acylhydrazone linkages (Fig. 5).^41,42 Such linkages break the conjugation, cause corrugated layer structures and therefore weaken the π–π stacking in 2D COFs.⁴¹ According to recent findings, acylhydrazone linkages together with alkyoxy side chains can induce strong dipole interactions between COF layers, thus giving rise to antiparallel stacked 2D COFs.⁴³ Both intra and interlayer hydrogen bonding are induced in the antiparallel stacked COF layers, providing in-plane rigidity for RIR to turn on PL as well as out-of-plane flexibility for excited-state interlayer proton shift (ESIPS) to induce dual emission. The acylhydrazone 2D COFs provide a platform with high tunability of a wide-ranging color from blue to yellow and even white. This can be ideal for photosensitizers with tunable bandgaps. The ease of encoding functionalities at the side chains also makes such COFs useful as sensors with high sensitivity and selectivity as well as in toxin removal.
^41,42 第二代固态发射型二维 COF 基于酰基腙连接（图 5）。⁴¹ 根据最近的研究结果，酰基腙连接和烷氧基侧链可在 COF 层之间引起强烈的偶极相互作用，从而产生反平行堆叠的二维 COF。⁴³ 在反平行堆叠的 COF 层中，层内和层间的氢键都会被诱导，从而为 RIR 提供面内刚性以开启 PL，并为激发态层间质子偏移（ESIPS）提供面外灵活性以诱导双发射。酰腙二维 COF 提供了一个平台，具有从蓝色到黄色甚至白色等多种颜色的高可调性。这对于具有可调带隙的光敏剂来说非常理想。在侧链上进行功能编码的简易性也使这种 COF 可用作具有高灵敏度和选择性的传感器，以及用于去除毒素。

Recently, a new class of CC based 2D COFs has been discovered to be light emissive in the solid state (Fig. 6).^36,44,45 Cyanostilbene species are known as good AIEgens in both small molecule^46–48 and polymer^49–51 states. Moreover, cyanostilbenes have also been reported to show high emission efficiency in both solution and the solid state.⁵² When COFs are constructed via acrylonitrile linkages, repeating cyanostilbene units are created in the framework of COFs, giving rise to solid-state fluorescence. Jiang et al. demonstrated a class of light-emitting 2D COFs based on acrylonitrile linkages using pyrene units as the backbone functionality.^36,45 This type of COFs exhibited great stability towards strong acid HCl (12 M) and strong base KOH (14 M). Besides, the COFs remained emissive in both the solid state and solution dispersions. Another type of emissive CC bonded 2D COFs were synthesized via Aldol condensation between trimethyltriazine and aldehydes.⁴⁴ The as-synthesized olefin 2D COFs displayed not only fluorescence in the solid state, but also solvatochromic emission when dispersed in various solvents. Although the fluorescence mechanism of the olefin-bonded COFs is not well understood, we propose that the fluorescence is due to the RIR as well as the disruption of π–π stacking via the antiparallel stacking of COF layers. It should be noted that the single-crystal structure of the model compound, tristyryltriazine, shows disorder between clockwise and anti-clockwise styryl arms.⁴⁴ This strongly implies that these olefin 2D COFs are likely to be antiparallel stacked, which is known to be helpful in inducing fluorescence in acylhydrazone COFs.^42,43 The chemical robustness, fluorescence, and porosity of such CC bonded COFs are highly desirable for practical applications in photocatalysis and sensing.
最近，人们发现一类新的基于 C C 的二维 COF 在固态下具有光发射性（图 6）。^36,44、45 众所周知，二苯乙烯类化合物在小分子^46-48 和聚合物^49-51 状态下都是良好的 AIEgens。52当通过丙烯腈连接构建 COF 时，会在 COF 框架中产生重复的氰苯乙烯单元，从而产生固态荧光。Jiang et al.展示了一类基于丙烯腈连接、以芘单元为骨架官能团的发光二维 COF。此外，这种 COF 在固态和溶液分散体中都保持发射性。通过三甲基三嗪和醛之间的醛缩合，合成了另一种发射性 C C 键二维 COF。⁴⁴ 合成的烯烃二维 COF 不仅在固态下显示出荧光，而且分散在各种溶剂中时也显示出溶解变色发射。虽然烯烃键合 COF 的荧光机理尚不十分清楚，但我们认为荧光是由于 RIR 以及 COF 层的反平行堆积破坏了 π-π 堆积 via 所产生的。 44这强烈暗示这些烯烃二维 COF 很可能是反平行堆叠的，众所周知，这有助于诱导酰基腙 COF 的荧光。^42,43 这种 C C 键 COF 的化学稳定性、荧光和多孔性非常适合光催化和传感领域的实际应用。

2.1.2 Solid-state emission via controlling the interlayer stacking
2.1.2 固态发射 通过控制层间堆叠

Besides restricting the intramolecular rotation of rotationally labile bonds used in constructing COFs, any strategy to improve the quantum yield of 2D COFs is not complete without considering the interlayer coupling in 2D COFs. π–π interactions between the layers provide an energy dissipation channel for the photo-excited state, analogous to aggregation-caused quenching. Enlarging the interlayer distances in eclipsed stacked COFs, or changing the interlayer stacking registry between offset, staggered or antiparallel stacking can weaken or modify the π–π interactions in 2D COFs. Although there are limited studies on how staggered or antiparallel stacking can enhance the PL in 2D COFs, chemists are now developing well-defined paths towards constructing COFs with the desired stacking orders (Fig. 7). For instance, staggered stacked COFs can be constructed by sterically bulky⁵³ or ionic⁵⁴ building units; antiparallel stacked COFs can be crystallized via dipole interactions and hydrogen bonding.⁴³
除了限制用于构建 COF 的旋转易变键的分子内旋转外，如果不考虑二维 COF 中的层间耦合，任何提高二维 COF 量子产率的策略都是不完整的。层间的π-π相互作用为光激发态提供了一个能量耗散通道，类似于聚集引起的淬火。在黯淡堆叠的 COF 中扩大层间距离，或在偏移、交错或反平行堆叠之间改变层间堆叠注册，可以削弱或改变二维 COF 中的π-π 相互作用。虽然有关交错或反平行堆叠如何增强二维 COF 的聚光效应的研究还很有限，但化学家们目前正在开发明确的途径，以构建具有所需堆叠顺序的 COF（图 7）。例如，立体笨重⁵³ 或离子⁵⁴ 构建单元可以构建交错堆叠的 COF；反平行堆叠的 COF 可通过偶极相互作用和氢键结晶。⁴³

As discussed earlier, antiparallel stacking is beneficial for inducing PL in 2D COFs. This is because it can cause misalignment of π moieties between COF layers and hence weaken the π–π interactions. Besides, such a conformation intrinsically forbids free bond rotation.
如前所述，反平行堆积有利于诱导二维 COF 中的聚光。这是因为它可以导致 COF 层之间的 π 分子错位，从而削弱 π-π 的相互作用。此外，这种构象本质上禁止自由键旋转。

In a similar vein, increasing the π–π stacking distance in eclipsed stacked solid-state 2D COFs is also helpful for inducing PL via trapping solvent molecules between COF layers. Feng et al. demonstrated a phosphorescent boronate 2D COF at cryogenic temperature (Fig. 8a).⁵⁵ Motivated by the fact that face-to-face H-aggregation in eclipsed stacked COFs could turn on phosphorescence, they integrated the benzil functionality into the COF backbone, which is a crystallization-induced phosphorescence (CIP) phosphor.⁵⁶ Interestingly, they found that guest solvent molecules (1,4-dioxane) trapped between the 2D COF layers increase the interlayer distance to 3.7 Å compared to the activated state of 3.4 Å. The 1,4-dioxane-containing BZL-COF showed discernible yellow phosphorescence with the naked eye at 77 K and a lifetime of 1.27 ms, whereas the emission was quenched at 298 K. On the other hand, the activated BZL-COF with an interlayer distance of 3.4 Å was PL-quenched for all temperatures.
与此类似，增加蚀刻堆叠固态二维 COF 的 π-π 堆叠距离也有助于通过在 COF 层之间捕获溶剂分子来诱导磷光 。Feng 等人在低温下展示了一种磷光硼酸盐二维 COF（图 8a）。⁵⁵ 由于夕照叠层 COF 中面对面的 H-aggregation 可以产生磷光，他们将苯齐尔官能团整合到 COF 骨架中，形成了结晶诱导磷光（CIP）荧光粉。⁵⁶ 有趣的是，他们发现被困在二维 COF 层之间的客体溶剂分子（1,4-二氧六环）将层间距增加到了 3.7 Å，而活化状态下为 3.4 Å。含有 1,4-二氧六环的 BZL-COF 在 77 K 时用肉眼可以看到明显的黄色磷光，寿命为 1.27 ms，而在 298 K 时发射被熄灭。

Construction of 2D COFs with staggered stacking is an effective way to turn on the solid-state emission. In 2018, Zamora et al. reported a solid-state fluorescent imine 2D COF with staggered stacking (Fig. 8d).⁵⁷ They utilized a pyrene-derived amine to construct an eclipsed stacked and a staggered stacked 2D COF. Normally, the π–π stacking of the pyrene units together with the rotationally labile imine linkages would lead to PL quenching in solid-state 2D COFs. However, the staggered-stacked imine COF exhibits conspicuous green fluorescence upon excitation at 365 nm with an absolute PL quantum yield of 3.5%. In contrast, the amorphous polymer based on the same building blocks and the eclipsed-stacked COF were non-emissive.
构建交错堆叠的二维 COF 是开启固态发射的有效方法。2018 年，Zamora et al.报告了一种具有交错堆积的固态荧光亚胺二维 COF（图 8d）。⁵⁷ 他们利用源自芘的胺构建了一个黯淡堆叠和一个交错堆叠的二维 COF。通常情况下，芘单元的π-π堆积以及旋转易变的亚胺连接会导致固态二维 COF 中的聚光淬灭。然而，交错堆积的亚胺 COF 在 365 纳米波长的激发下会发出明显的绿色荧光，其绝对 PL 量子产率为 3.5%。相比之下，基于相同结构单元的无定形聚合物和交错堆叠的 COF 则没有荧光。

2.1.3 An outlook for emissive 2D COFs and future applications
2.1.3 发射型二维 COF 的展望和未来应用

Tunable emission in solid-state 2D COFs is an intriguing research topic since all the intrinsic properties of 2D COFs, such as crystallinity, porosity, conjugation and stacking order, exert varying degrees of influence on the PL. Currently, there is still a lot of room for improving the emission color, tunability and PL quantum efficiency of COFs as compared to polymers. Furthermore, it is of great interest to realize room-temperature phosphorescence in COFs.
固态二维 COF 的可调谐发射是一个引人入胜的研究课题，因为二维 COF 的所有固有特性，如结晶度、孔隙率、共轭和堆叠顺序等，都会对 PL 产生不同程度的影响。目前，与聚合物相比，COFs 的发射颜色、可调性和 PL 量子效率仍有很大的改进空间。此外，在 COFs 中实现室温磷光也是一个非常有意义的课题。

Emissive 2D COFs have been successfully applied in sensing,^{30,31,41,58–60} photocatalysis,^61,62 displays,⁶³ photodynamic therapy,⁶⁴ thermometers⁶⁵ and diagnostics.⁶⁴ The enriched pores in 2D COFs can serve as smart carriers for drug delivery.^66–68 Emissive COFs can also enable the bioimaging of the drug delivery process.⁴⁰ For the latter purpose, it may be necessary to convert the COFs to nanoparticles or flakes to allow better dispersion in solution.
发射型二维 COF 已成功应用于传感、^{30,31,41,58-60} 光催化、^61,62 显示、⁶³ 光动力疗法、⁶⁴ 温度计⁶⁵ 和诊断。⁶⁴ 二维 COF 中富集的孔隙可作为药物输送的智能载体。^66-68 发射型 COF 还能对药物输送过程进行生物成像。⁴⁰ 出于后一种目的，可能有必要将 COF 转化为纳米颗粒或薄片，以便在溶液中更好地分散。

2.2 Stimuli responsive 2D COFs
2.2 刺激响应性二维 COF

Stimuli-responsive 2D COFs are engineered in such a way that changes in intermolecular or intramolecular interactions, or molecular reorganization occur in response to external stimulation by light, moisture, pH or heat, leading to changes in the emission wavelength or intensity. In the following section, we will review representative examples of stimuli responsive 2D COFs based on solvatochromism and photoisomerization, and further discuss how these properties are induced based on structural design.
刺激响应型二维 COF 的设计方式可使分子间或分子内的相互作用或分子重组发生变化，以响应光、湿度、pH 值或热量等外部刺激，从而导致发射波长或强度发生变化。在下一节中，我们将回顾基于溶解变色和光异构化的刺激响应型二维 COF 的代表性实例，并进一步讨论如何根据结构设计诱导这些特性。

2.2.1 Solvatochromism
2.2.1 Solvatochromism

A solvatochromic COF exhibits different light absorption or emission properties in different solvents. At the fundamental level, this can be a consequence of the screening of the excitonic states in the COF by the surrounding dielectric medium, which depends on the dielectric properties of the solvent. A strong solvatochromic effect can be achieved by introducing enol–keto tautomerism in the molecular building block because of the presence of distinct intramolecular hydrogen bonding states in the different conformers that are sensitive to the proticity of the solvent. In this section, we will review three types of solvatochromic 2D COFs and discuss the structural origins of their solvatochromism, where the key functionalities are summarized in Fig. 9.
溶解变色 COF 在不同溶剂中表现出不同的光吸收或发射特性。从根本上说，这可能是 COF 中的激发态被周围介电介质屏蔽的结果，而这取决于溶剂的介电性质。在分子构件中引入烯醇-酮同分异构体可以产生强烈的溶解变色效应，这是因为不同构象存在不同的分子内氢键态，而这些态对溶剂的质性非常敏感。在本节中，我们将回顾三种溶解变色二维 COF，并讨论其溶解变色的结构起源，其中的关键功能在 图 9 中进行了总结。

A major category of solvatochromic 2D COFs is based on salicylidene-analogue functionalities, which can undergo enol–keto tautomerization triggered by a protic solvent when integrated into 2D COFs (Fig. 10).^63,69–72 Traditionally, chromism in salicylideneaniline-based small molecules has to be triggered by high energy input such as light irradiation or heat, leading to a reversible tautomeric process between enol, cis-keto and trans-keto forms.^73–75 Interestingly, no heat or light is needed for triggering chromism in this class of COFs because the presence or absence of moisture alone can induce the reversible tautomerization. The well-ordered π-stacked arrays and 1-D channels in 2D COFs allow protons to activate collective molecular reorganization. The solvatochromism is caused by enhanced charge transfer between the electron donor and acceptor dyads. Using a salicylideneaniline COF as an example, the salicylideneaniline units become stronger electron acceptors via enol–keto tautomerization, therefore facilitating the charge transfer process and the narrowing of the band gap.⁷¹
溶解变色二维 COF 的一个主要类别是基于水杨醛类似官能团，当这些官能团整合到二维 COF 中时，可在质子溶剂的触发下发生烯醇酮同分异构（图 10）。^63,69-72 传统上，水杨酰苯胺基小分子的发色作用必须通过高能量输入（如光照射或热）来触发、导致烯醇、顺式酮和反式酮之间的可逆同分异构过程。^73-75 有趣的是，这类 COFs 的发色无需热量或光照，因为仅存在或不存在水分就能诱导可逆的同分异构。二维 COF 中井然有序的 π 叠层阵列和一维通道允许质子激活集体分子重组。电子供体和受体二元体之间的电荷转移增强，从而产生溶解变色现象。以水杨酰苯胺 COF 为例，水杨酰苯胺单元通过烯醇酮缩聚反应成为更强的电子受体，从而促进了电荷转移过程并缩小了带隙。

Beside salicylidene-type functionalities, Bein and Auras synthesized a solvatochromic imine 2D COF based on electron donor–acceptor dyads between pyrene and thienothiophene building units (Fig. 12).⁷⁶ Their time-dependent density functional theory (TD-DFT) calculation suggested that one-electron charge transfer from pyrene to thienothiophene units occurs in the excited state, while surrounding protic solvents can redshift the light absorption. Furthermore, the authors prepared oriented films of the Py-TT COF with a thickness of 360 nm, which is highly responsive to humidity. The dried film exhibited a fast response to H₂O-saturated gas streams (0.21 s), while the wetted film showed an even faster response to dry gas streams (0.15 s).
除了水杨醛型功能之外，Bein 和 Auras 还合成了一种基于芘和噻吩之间电子供体-受体二元结构单元的溶变色亚胺二维 COF（图 12）。⁷⁶ 他们的时变密度泛函理论（TD-DFT）计算表明，芘与噻吩噻吩单元之间的单电子电荷转移发生在激发态，而周围的原生溶剂会使光吸收发生重移。此外，作者还制备了厚度为 360 纳米的 Py-TT COF 拉伸薄膜，这种薄膜对湿度的反应非常灵敏。干燥薄膜对 H₂O 饱和气流的响应速度很快（0.21 秒），而湿润薄膜对干燥气流的响应速度更快（0.15 秒）。

Recently, Perepichka and coworkers reported a type of arylene vinylene 2D COFs with triazine functionalities showing solid-state fluorescence.⁴⁴ Interestingly, the COF exhibited solvatochromic fluorescence when dispersed in various solvents, although tautomerization was not involved; instead the mechanism may involve charge transfer between the COF and the solvent.
⁴⁴ 最近，Perepichka 和同事们报道了一种具有三嗪官能团的芳基乙烯二维 COF，这种 COF 显示出固态荧光。

Solvatochromism in 2D COFs is highly sensitive and even moisture can trigger a conspicuous color change. Thus, humidity sensors have been built using solvatochromic COFs.^69,72,76 In addition, the presence of basic enamine groups in the 1D channel of salicylidene-type COFs provides the basis for chemoselective separation.⁷⁰ Enol–keto tautomerization in 2D COFs as a result of external stimuli has been exploited to reversibly engineer the bandgaps in COFs since the electronic structure is affected by molecular reorganisation. In one case, the bandgap narrowing allows better optical limiting performance.⁷¹ Besides sensing, the solvatochromic fluorescence in 2D COFs also provides a means to tune light emission, producing even a white color (Fig. 11).⁶³
二维 COF 中的溶致变色具有很高的灵敏度，即使是湿气也会引发明显的颜色变化。69,72,76此外，水杨醛型 COF 的一维通道中存在碱性烯胺基团，这为化学选择性分离提供了基础。⁷⁰ 由于电子结构会受到分子重组的影响，因此人们利用二维 COF 中的烯醇酮同分异构现象来可逆地设计 COF 的带隙。在其中一种情况下，带隙变窄可实现更好的光学限制性能。⁷¹ 除了传感之外，二维 COF 中的溶变色荧光还提供了一种调节光发射的方法，甚至可以产生白色（Fig.11 ）。

One interesting observation is that all solvatochromic COFs reported to date are 2D COFs. It can be inferred that the periodic π-stacked arrays in 2D COFs are vital to induce solvatochromism. This also suggests that solvatochromism can be used as a property to differentiate 2D COFs from 3D ones.
一个有趣的现象是，迄今为止报道的所有溶解变色 COF 都是二维 COF。由此可以推断，二维 COF 中的周期性 π 叠层阵列对于诱导溶解变色至关重要。这也表明溶解变色可以作为区分二维 COF 和三维 COF 的一种特性。

2.2.2 Photoisomerization
2.2.2 光异构化

Other than solvatochromism, photostimuli-triggered chemical transformation in 2D COFs has been receiving increasing research attention in recent years. Such behavior in COFs can generate fascinating properties for electronic bandgap tuning, switchable photodetection, controllable cargo loading/releasing, etc. In this section, we will briefly introduce the available functionalities for photoswitchable isomerization in 2D COFs (Fig. 13).
除溶解变色作用外，二维 COF 中的光刺激触发化学变化近年来也受到越来越多的研究关注。COF 中的这种行为可以产生令人着迷的特性，如电子带隙调整、可切换光探测、可控货物装载/释放等。在本节中，我们将简要介绍二维 COF 中光开关异构化的可用功能（图 13）。

In 2015, Jiang et al. demonstrated a photo-responsive 2D boronate COF that was constructed from anthracene backbone functionalities.⁷⁷ Due to the eclipsed stacking in the 2D COF, the anthracene moieties are arranged face-to-face and undergo photo-induced dimerization via interlayer [4π+4π] cycloaddition. Heating to 100 °C reverses the dimerization reaction. By repeating UV irradiation and heating, the light absorption of the COF can be reversibly tuned.
2015 年，Jiang et al.展示了一种由蒽骨架官能团构建的光响应型二维硼酸盐 COF。⁷⁷ 由于二维 COF 中的蚀刻堆积，蒽分子面对面排列，并通过 层间 [4π+4π] 环加成发生光诱导二聚化。加热至 100 ℃ 可以逆转二聚反应。通过重复紫外线照射和加热，可以可逆地调节 COF 的光吸收。

In 2019, Zhang et al. integrated photo-responsive 1,2-bis(2-methylthien-3-yl)cyclopentene (DAE) into a 2D imine COF.⁷⁸ The DAE units can undergo reversible Cope rearrangement between an open state and a closed state upon light irradiation at 365 nm and >550 nm, respectively. When the COF was prepared in the form of a film (0.6 μm), the conductivity increased from (1 ± 0.25) × 10⁻⁷ S cm⁻¹ in the open state to (2 ± 0.23) × 10⁻⁵ S cm⁻¹ in the closed state, which is a ∼200-fold increase, after irradiation for 6 min at room temperature.
2019 年，张等人将光响应性 1,2-双（2-甲基噻吩-3-基）环戊烯（DAE）集成到二维亚胺 COF 中。⁷⁸ 在 365 纳米和 >550 纳米的光照射下，DAE 单元可分别在开放态和封闭态之间发生可逆的科普重排。当 COF 以薄膜（0.6 μm）的形式制备时，电导率从开放态的（1 ± 0.25）×10^-7 S cm^-1 增加到（2 ± 0.23) × 10^-5 S cm^-1 在室温下照射 6 分钟后，闭合状态下的 S cm^-1 增加了 ∼ 200 倍。

Azobenzene is famous for its trans–cis photoisomerization.⁷⁹ However, when the azobenzene units are integrated into the backbone of COFs, the close-packed rigid framework structures inhibit this trans–cis isomerization due to the high energy barriers. Recently, Trabolsi and coworkers reported a 2D imine COF as a light-operated reservoir.⁸⁰ Interestingly, they introduced the azobenzene units as dangling groups in the COF pore structure. When the azobenzene units are in the trans state, the dangling groups occupy the COF channels as the pore walls, mimicking a honeycomb network but with periodic disconnection/defects (Fig. 14). The free volume of the COF pores and the disconnection at one side of the azobenzene moieties allow the photoisomerization process. They found that the light absorption and emission behavior of the COF dispersion in ethanol, as well as its hydrophilicity, can be reversibly tuned via photo-induced trans–cis isomerization. Furthermore, the trans–cis isomerization enables a reversible change of pore sizes of 1.2 nm for the trans state and 2.7 nm for the cis state. Based on the photoswitching of hydrophilicity and pore size, their COF realized the light-induce capture and release of rhodamine B, which is promising for drug delivery applications.
偶氮苯以其反式-顺式光异构化而闻名。⁷⁹ 然而，当偶氮苯单元被整合到 COF 的骨架中时，由于能垒较高，紧密堆积的刚性框架结构会抑制这种 反式- 顺式异构化。⁸⁰ 有趣的是，他们在 COF 孔结构中引入了偶氮苯单元作为悬挂基团。当偶氮苯单元处于反式状态时，悬挂基团占据 COF 通道作为孔壁，模仿蜂巢网络，但具有周期性断开/缺陷（图 14）。COF 孔隙的自由体积和偶氮苯分子一侧的断开使光异构化过程成为可能。他们发现，乙醇中 COF 分散体的光吸收和发射行为及其亲水性可通过光诱导反式 顺式异构化进行可逆调节。此外，trans-cis 异构化使孔隙尺寸发生可逆变化，trans 态为 1.2 nm，cis 态为 2.7 nm。基于亲水性和孔径的光开关作用，它们的 COF 实现了罗丹明 B 的光诱导捕获和释放，具有良好的给药应用前景。

2.3 CO₂ storage and conversion
2.3 CO₂ 的储存和转化。

The rising level of carbon dioxide (CO₂) in the environment is a major concern. A key step in reducing carbon footprints is the storage and fixation of CO₂ into fuels. Materials scientists have created a wide range of porous materials, some of which show potential as CO₂ capture and storage materials.^81,82 COFs are potential CO₂ sorbents in view of their porosity, thermal stability and tunable functionality.^8,83–87 Various catalysts can also be immobilized in the channels in COFs to convert CO₂ to useful chemicals. The different sorption capacities in COFs are attributed to the interplay of various complex factors such as surface area, pore geometry and chemical functionality. In this section, we will discuss the strategies to improve the gas sorption capacity of 2D COFs for CO₂ capture and conversion.
环境中二氧化碳（CO₂）含量的不断上升是一个重大问题。减少碳足迹的一个关键步骤是将 CO₂ 储存和固定为燃料。^81,82 COFs 具有多孔性、热稳定性和可调功能，是潜在的 CO₂ 吸附剂。^8,83-87 还可在 COF 的通道中固定各种催化剂，将 CO₂ 转化为有用的化学物质。COF 中不同的吸附能力归因于各种复杂因素的相互作用，如表面积、孔隙几何形状和化学功能。在本节中，我们将讨论提高二维 COF 的气体吸附能力以实现 CO₂ 捕获和转化的策略。

The primary factor for the CO₂ intake capacity is the surface area of COFs. Despite the large surface area of 3D COFs, most structures are interpenetrated, blocking the pores needed for CO₂ capture. In contrast, eclipsed-stacked 2D COFs have unblocked channels for gas storage. Although the ideal surface areas of COFs for various sorbents can be estimated via simulation, the experimental values are often lower than the theoretical ones due to defects, polycrystallinity, and insufficient activation. Routine activation procedures have been established, including supercritical CO₂ extraction, Soxhlet extraction, vacuum at temperature, etc. Here, we will review the strategies to improve the experimental surface areas of COFs.
CO₂ 摄入量的主要因素是 COF 的表面积。尽管三维 COF 的表面积很大，但大多数结构都是相互渗透的，从而堵塞了捕获 CO₂ 所需的孔隙。相比之下，层叠的二维 COF 具有畅通无阻的气体存储通道。虽然可以通过模拟估算出各种吸附剂的 COF 理想表面积，但由于缺陷、多晶度和活化不足等原因，实验值往往低于理论值。常规活化程序已经建立，包括超临界 CO₂ 萃取、索氏萃取、温度真空等在此，我们将回顾改善 COF 实验表面积的策略。

In 2017, Banerjee et al. demonstrated an organic terracotta method to construct ultraporous 2D COFs (Fig. 15).⁸⁸ Using a solid-state reaction at 170 °C and p-toluenesulfonic acid (PTSA) as the catalyst, they obtained 2D COFs with Brunauer–Emmett–Teller (BET) surface areas as high as 3109 m² g⁻¹, which are 2 to 3-fold higher than those synthesized via solvothermal conditions. The large surface area was attributed to the larger particle dimensions, ordered pore channels, and long-range periodicity of the COF. The suitable choice of acid catalyst creates PTSA-amine salts with H-bonded lamellar structures, which act as templates for 2D COF growth. Their study also revealed that high temperature was beneficial for COFs to grow into larger particles, and a solvent-free (or less) condition could prevent the COF pores from being blocked by the solvent molecules.
2017 年，Banerjee et al.展示了一种构建超多孔二维 COF 的有机陶土法（图 15）。⁸⁸ 使用 170 °C 固态反应和 p 甲苯磺酸 (PTSA) 作为催化剂、他们获得了二维 COF，其布鲁诺-艾美特-泰勒（BET）表面积高达 3109 m² g^-1 ，比 溶热条件合成的 COF 高出 2 到 3 倍。大表面积归因于 COF 较大的颗粒尺寸、有序的孔道和长程周期性。选择合适的酸催化剂可生成具有 H 键层状结构的 PTSA-胺盐，这种结构可作为二维 COF 生长的模板。他们的研究还发现，高温有利于 COF 长成更大的颗粒，无溶剂（或更少）条件可防止 COF 孔隙被溶剂分子堵塞。

In 2018, we reported a strategy to enhance the surface area via construction of a lacunary 2D COF with frustrated bonding (Fig. 16).⁸⁹ Using π-rigid and rotationally flexible tetraphenylethylene as the backbone, two distinct 2D COFs can be divergently synthesized under different combinations of solvents. One of the COFs (TPE-COF-I) was fully bonded via a [4+4] pathway, while the other (TPE-COF-II) had frustrated bonding with dangling aldehyde groups via a [4+2] pathway. The lacunary structure of the frustrated bonding TPE-COF-II gives rise to an enhanced pore volume of 2.14 cm³ g⁻¹ (P/P₀ = 0.984) compared to 1.65 cm³ g⁻¹ (P/P₀ = 0.984) for the fully bonded TPE-COF-I. Due to the large BET surface area (2168 m² g⁻¹) of the frustrated COF, it exhibits a great CO₂ uptake capacity of 118.8 cm³ g⁻¹ (23.2 wt%, 1 atm, 273 K). The CO₂ intake per unit surface area at 273 K was calculated to be 54.8 mm³ m⁻² for TPE-COF-II and 39.4 mm³ m⁻² for TPE-COF-I, suggesting that the presence of dangling formyl groups at pore walls is helpful in binding CO₂ molecules.
2018 年，我们报道了一种通过构建具有挫折键的裂隙型二维 COF 来增强表面积的策略（图 16）。⁸⁹ 以具有π刚性和旋转柔性的四苯基乙烯为骨架，可以在不同的溶剂组合下合成两种不同的二维 COF。其中一种 COF（TPE-COF-I）是通过[4+4]途径完全键合的，而另一种 COF（TPE-COF-II）则是通过[4+2]途径与悬垂醛基挫折键合的。受挫键 TPE-COF-II 的裂隙结构使孔隙体积增大到 2.14 cm³ g^-1 (P/P₀ = 0.984），而完全粘合的 TPE-COF-I 为 1.65 cm³ g^-1 (P/P₀ = 0.984)。由于受挫 COF 具有较大的 BET 表面积（2168 m² g^-1 ），它对 CO₂ 的吸收能力高达 118.8 cm³ g^-1 (23.2 wt%, 1 atm, 273 K)。根据计算，在 273 K 下，单位表面积的 CO₂ 摄入量对于 TPE-COF-II 为 54.8 mm³ m^-2 ，对于 TPE-COF-II 为 39.4 mm³ m^-2 。4 mm³ m^-2 ，表明孔壁存在悬垂甲酰基有助于结合 CO₂ 分子。

Other than the surface area, the functional groups in COFs also play an important role in sorption. Useful CO₂-philic functionalities have been summarized in Fig. 17. The interaction mechanism involves either acid–base or dipole interactions between CO₂ molecules and the built-in functionality. However, COFs with high affinity to CO₂ do not necessarily show high CO₂ intake, limited by their surface areas. Therefore, balanced methods need to be developed to introduce CO₂-philic groups without sacrificing the surface area in COFs.
除了表面积，COF 中的官能团也在吸附过程中发挥着重要作用。图 17 总结了有用的 CO₂ 亲和官能团。相互作用机理涉及 CO₂ 分子与内置官能团之间的酸碱或偶极相互作用。然而，对 CO₂ 具有高亲和力的 COF 并不一定具有高 CO₂ 摄取量，这受到其表面积的限制。因此，需要开发平衡的方法，在不牺牲 COF 表面积的情况下引入 CO₂ 亲水基团。

A more efficient way of utilizing these materials is to incorporate catalytic capability so that the sequestered CO₂ can be converted into value-added carbon products. Although simultaneous CO₂ fixation and conversion is still a big challenge using COFs, they have been explored in photocatalytic or electrocatalytic systems for CO₂ conversion. Herein, we will review a few representative examples of CO₂ conversion using 2D COFs.
利用这些材料的一个更有效的方法是加入催化功能，以便将被封存的 CO₂ 转化为高附加值的碳产品。尽管使用 COFs 同时进行 CO₂ 固定和转化仍然是一个巨大的挑战，但人们已经在光催化或电催化系统中探索 CO₂ 转化。在此，我们将回顾几个利用二维 COF 进行 CO₂ 转化的代表性实例。

2D COFs provide a well-defined platform with tunable bandgaps and open sites for coordinating to metals. In addition, their structural tunability and porosity with high surface area make COFs promising catalyst candidates for both photocatalytic and electrocatalytic systems. A pool of useful building units for CO₂ conversion is given in Fig. 18. Yaghi et al. demonstrated a 2D COF impregnated with a cobalt porphyrin catalyst for the electrochemical conversion of CO₂ into CO in water.⁹⁰ Using an overpotenial of −0.55 V, a high faradaic efficiency of 90% with a turnover number up to 290 000 under neutral conditions was achieved. To address the issue of low charge mobility in COFs, Lan et al. introduced tetrathiafulvalene units into the porphyrin COF system and exfoliated the COF into nanosheets, achieving a maximum faradaic efficiency of 99.7% with an overpotential of −0.8 V.⁹¹ The electrical conductivity of COFs is a bottleneck in many applications requiring shuttling of electrons, and hopefully that can be improved by developing new conjugated covalent linkages and building units.⁹² Alternatively, COFs with channels with immobilized nanocatalysts can be utilized as diffusion layers for both CO₂ and ions, achieving both gas fixation and conversion into energy storage intermediates.⁹³
二维 COF 提供了一个定义明确的平台，具有可调整的带隙和开放的金属配位位点。此外，COFs 结构的可调性和高比表面积的多孔性使其成为光催化和电催化系统的理想候选催化剂。图 18 列出了用于 CO₂ 转化的有用构建单元库。Yaghi et al. 演示了一种浸渍有卟啉钴催化剂的二维 COF，用于在水中将 CO₂ 电化学转化为 CO。⁹⁰ 利用-0.55 V的过电位，在中性条件下实现了 90% 的高远电效率，周转次数高达 290 000。为了解决 COF 中电荷迁移率低的问题，Lan et al. 在卟啉 COF 系统中引入了四硫富戊烯单元，并将 COF 剥离成纳米片，在过电位为 -0.8 V.91在许多需要电子穿梭的应用中，COF的导电性是一个瓶颈。⁹² 或者，带有固定纳米催化剂通道的 COF 可用作 CO₂ 和离子的扩散层，实现气体固定和转化为储能中间体。⁹³.

Photocatalytic systems using 2D COFs have also been developed for CO₂ conversion.^94–97 A good design has the backbone functionalities serving as the coordination sites for the metal catalyst, while the organic COF skeleton plays the role of photosensitizers. The current benchmark performance of COF photocatalysts for CO₂ conversion to CO is 1400 μmol g⁻¹ h⁻¹ with the addition of a dye as a photosensitizer.⁹⁶ In fact, a myriad of different combinations between COFs and metal catalysts can be screened to identify the optimal system.
^94-97 一种好的设计是将骨架官能团作为金属催化剂的配位位点，而有机 COF 骨架则起到光敏剂的作用。目前 COF 光催化剂将 CO₂ 转化为 CO^-1 h^-1 的基准性能为 1400 μmol g^-1 h^-1 ，并添加了染料作为光敏剂。⁹⁶ 事实上，可以对 COF 和金属催化剂之间的无数不同组合进行筛选，以确定最佳系统。

2.4 Ion conduction
2.4 离子传导

COFs possess aligned 1D channels that can be modified chemically to enable ion conduction in a pre-defined manner. The porous structures of COFs endow them with abundant free space for ion diffusion, yet their framework rigidity ensures that these pores do not collapse over a wide temperature range;^98,99 thus they have good potential as solid-state ion conductors. Their rigidity and phase stability contrast with the temperature-driven phase transition in conventional polymer electrolytes. The glassy state is necessary in some polymer electrolytes to create free volume for ion transport, whereas well-designed COFs can be considered as true solid-state electrolytes because they have no glass transition state.¹⁰⁰
COF 具有排列整齐的一维通道，可以通过化学方法对其进行修饰，从而以预先确定的方式实现离子传导。COF 的多孔结构为离子扩散提供了充足的自由空间，而其框架的刚性又确保了这些孔隙在很宽的温度范围内不会塌陷；^98,99 因此，它们很有可能成为固态离子导体。它们的刚性和相稳定性与传统聚合物电解质中由温度驱动的相变形成鲜明对比。在某些聚合物电解质中，玻璃态是形成离子传输自由体积的必要条件，而设计良好的 COF 则可以被视为真正的固态电解质，因为它们没有玻璃转变态¹⁰⁰ 。

In recent years, proton conducting materials have spurred tremendous interest among researchers due to their application in fuel cells, sensors, and electronic devices.^101–103 Reported proton conductive 2D COFs are summarized in Table 3. The covalent coupling of proton donating groups in the COF skeleton can promote proton conductivity and single-ion conduction. However, the intrinsic conductivity of COFs is usually low because the number of acidic groups that can be anchored in a unit cell is limited. One way to boost the proton conductivity is to dope external proton donors inside the channels of COFs. Overall, there are two strategies for the synthesis of proton conductive COFs. One is doping external proton carriers inside the pores of COFs, utilizing solely the channels for proton conduction (Fig. 19); the other is encoding functionalities such as ionic, acidic, and basic groups onto the COF pore walls to facilitate proton conduction from external donors (Fig. 21).
101-103表 3总结了近年来报道的质子传导二维 COF。COF 骨架中质子捐赠基团的共价偶联可促进质子传导和单离子传导。然而，COF 的固有电导率通常较低，这是因为单位晶胞中可锚定的酸性基团数量有限。提高质子传导性的一种方法是在 COF 沟道内掺入外部质子供体。总的来说，合成质子传导 COF 有两种策略。一种是在 COF 孔内掺入外部质子载体，仅利用通道进行质子传导（图 19）；另一种是在 COF 孔壁上编码离子、酸性和碱性基团等功能，以促进外部供体的质子传导（图 21）。

To use COFs as proton transport membranes, processability and stability are important considerations besides conductivity. Jiang et al. reported a highly crystalline and robust COF with large pore volume for proton conduction.¹⁰⁴ Due to the high BET surface area, they doped triazole or imidazole as proton carries into the COF channels with a high loading of 180 wt% and 155 wt%, achieving proton conductivities of 1.1 and 3.78 mS cm⁻¹ at 403 K, respectively. Using a one-pot synthesis, Zamora et al. trapped acetic acid into the COF with 3.5 molecules per formula unit, achieving a proton conductivity of 0.525 mS cm⁻¹ at 313 K (100% relative humidity). They further processed the COF into thin films for use in fuel cells, achieving a power density of 12.95 mW cm⁻² and a maximum current density of 53.1 mA cm⁻².¹⁰⁵
要将 COF 用作质子传输膜，除了导电性之外，加工性和稳定性也是重要的考虑因素。Jiang et al.¹⁰⁴ 由于具有较高的 BET 表面积，他们在 COF 通道中掺入了三唑或咪唑作为质子载体，掺入量分别高达 180 wt% 和 155 wt%，在 403 K 时质子传导率分别达到 1.1 和 3.78 mS cm^-1 。Zamora et al.采用单锅合成法，在 COF 中捕获了醋酸，每式单位为 3.5 个分子，在 313 K（100% 相对湿度）条件下实现了 0.525 mS cm^-1的质子传导率。他们进一步将 COF 加工成薄膜，用于燃料电池，功率密度达到 12.95 mW cm^-2 ，最大电流密度达到 53.1 mA cm^-2105 。

Immobilizing basic groups in the 1-D channels of a COF can facilitate proton dissociation from external carriers, thereby enhancing the proton conductivity. Besides, acidic groups can improve the intrinsic proton conductivity of COFs. The beneficial functional groups for enhancing proton conduction are summarized in Fig. 20. Banerjee and coworkers reported the first proton conductive COF, which bears basic azobenzene functional groups for loading phosphoric acid.¹⁰⁶ Compared to an isoreticular COF without azo groups, the azo-COF achieved a proton conductivity of 0.99 mS cm⁻¹ at 332 K under 98% relative humidity. In following work, the authors further demonstrated a multi-component hybrid COF with both acidic sulfonyl and basic pyridinic functionalities in the hybrid COF skeleton.¹⁰⁷ Using phytic acid as the proton donor, they achieved anhydrous proton conductivity up to 0.5 mS cm⁻¹ at 393 °C. In 2018, they reported a method to prepare self-standing flexible COF membranes with superprotonic conductivities.¹⁰⁸ Loaded with p-toluene sulfonic acid as proton carriers, the COF membrane with azo functional groups achieved a proton conductivity of 63 mS cm⁻¹ at 303 K under 95% relative humidity. The proton exchange membrane fuel cell based on the COF membrane achieved a power density of 24 mW cm⁻². In line with the strategy of constructing COFs with both acid and base groups, Zhang et al. designed a new COF building unit with both acidic phenol and basic azo functionalities.¹⁰⁹ The neat COFs achieved an intrinsic proton conductivity up to 7.08 mS cm⁻¹ at 353 K under 98% relative humidity. When loaded with phosphoric acid (≤8.1 wt%) as the external proton donor, the proton conductivity increased to 113 mS cm⁻¹ at 353 K under 98% relative humidity.
在 COF 的一维通道中固定碱性基团可促进质子与外部载流子的解离，从而提高质子传导性。此外，酸性基团也能提高 COF 的内在质子传导性。图 20 总结了对增强质子传导有益的官能团。106与不含偶氮基团的等价 COF 相比，偶氮 COF 在相对湿度为 98% 的 332 K 条件下的质子传导率达到了 0.99 mS cm^-1。¹⁰⁷ 在接下来的工作中，作者进一步展示了一种多组分混合 COF，在混合 COF 骨架中同时具有酸性磺酰基和碱性吡啶基官能团。¹⁰⁷ 他们使用植酸作为质子供体，在 393 °C 时实现了高达 0.5 mS cm^-1 的无水质子电导率。2018 年，他们报告了一种制备具有超质子电导率的自立柔性 COF 膜的方法。¹⁰⁸ 以 p- 甲苯磺酸为质子载体，带有偶氮官能团的 COF 膜在 303 K、相对湿度为 95% 的条件下实现了 63 mS cm^-1 的质子电导率。基于 COF 膜的质子交换膜燃料电池的功率密度达到了 24 mW cm^-2。张等人根据构建具有酸基和碱基的 COF 的策略，设计了一种基于 COF 膜的质子交换膜燃料电池。¹⁰⁹ 在相对湿度为 98% 的 353 K 条件下，纯 COF 的本征质子电导率高达 7.08 mS cm^-1 。当添加磷酸（≤8.1 wt%）作为外部质子供体时，在相对湿度为 98% 的 353 K 条件下，质子电导率增加到 113 mS cm^-1 。

Ma et al. demonstrated a proton conductive 2D COF with cationic backbone functionality.¹¹⁰ The anions in the COF channels can be exchanged with PW₁₂O₄₀³⁻, achieving a proton conductivity of 3.32 mS cm⁻¹ at room temperature under 97% relative humidity. The authors postulated that the hydrophilic polyoxometalate anions in the COF channels formed water clusters, providing interconnected hydrogen bonding networks to boost the proton conductivity. In their experiments, they used H₃PW₁₂O₄₀ for anion exchange with the COF, which may result in partial ion exchange and trapping of HPW₁₂O₄₀²⁻ and H₂PW₁₂O₄₀⁻. Proton donors enhance the proton conductivity and a superacid with a high pK_a value is beneficial (Fig. 21).
Ma et al.展示了一种具有阳离子骨架功能的质子传导二维 COF。¹¹⁰ COF 通道中的阴离子可与 PW₁₂O₄₀^3- 交换，从而实现了 3.32 mS cm^-1 ，室温下相对湿度为 97%。作者推测，COF 通道中的亲水性聚氧化金属阴离子形成了水簇，提供了相互连接的氢键网络，从而提高了质子传导性。在实验中，他们使用 H₃PW₁₂O₄₀ 与 COF 进行阴离子交换、这可能会导致部分离子交换和 HPW₁₂O₄₀^2- 和 H₂PW₁₂O₄₀^- 。质子供体会增强质子传导性，pK_a 值高的超级酸会带来好处（图 21）。

Increasingly, COF research will focus on multifunctional applications and these necessitate the integration of various components to work synergistically. The tailorable 1-D channels in 2D COFs are useful for gas storage and ion conduction. Combination of these properties engenders new applications of COFs such as metal-ion–gas batteries, which require a stable cathode with high surface area to capture gas molecules as well as the ability to facilitate the diffusion of redox species. As a proof of concept, we reported COF-based Li–CO₂ batteries with enhanced energy capacity and rate performance (Fig. 22).⁹³ An acylhydrazone COF was used as the cathode in the Li–CO₂ battery to provide diffusion channels for both CO₂ gas and Li ions. In addition, coordinated Ru nanoparticles are well-positioned to catalyse CO₂ conversion via rimming the pores of the acylhydrazone COF. Such synergetic design greatly facilitates the kinetics of charge/discharge cycles in the batteries, leading to enhanced battery rate performance and cycle life. The COF-based Li–CO₂ battery achieved a high energy capacity of 27 348 mA h g⁻¹ (7.38 mA h cm⁻²) at a current density of 200 mA g⁻¹ and can endure a maximum current density of 4 A g⁻¹ without decay of the discharge/charge voltage. The battery ran for 200 cycles at a current density of 1 A g⁻¹ within a limiting capacity of 1000 mA h g⁻¹.
COF 研究的重点将越来越多地放在多功能应用上，这就需要整合各种元件，使其协同工作。二维 COF 中可定制的一维通道可用于气体存储和离子传导。这些特性的结合为 COF 带来了新的应用，如金属离子-气体电池，它需要一个具有高表面积的稳定阴极来捕获气体分子，并能促进氧化还原物种的扩散。作为概念验证，我们报告了基于 COF 的锂-CO₂ 电池，其能量容量和速率性能均有所提高（图 22）。⁹³ 在锂-CO₂ 电池中使用了酰基腙 COF 作为阴极，为 CO₂ 气体和锂离子提供扩散通道。此外，配位的 Ru 纳米粒子还能很好地催化 CO₂ 转换 通过环绕酰化腙 COF 的孔隙。这种协同设计极大地促进了电池的充放电循环动力学，从而提高了电池的速率性能和循环寿命。基于 COF 的锂-CO₂ 电池实现了 27 348 mA h g^-1 （7.在电流密度为 200 mA g^-1 时，电池的能量容量高达 27 348 mA h g^-1 (7. 38 mA h cm^-2) ，并能承受最大电流密度为 4 A g^-1 时的放电/充电电压而不衰减。电池在 1 A g^-1 的电流密度下运行了 200 次，极限容量为 1000 mA h g^-1 。

3. COFenes

Reducing the thickness of 3D solids into 2D sheets generates novel properties due to quantum confinement effects and dimensionality-dependent electronic and optical properties, and this has spurred a huge research field on 2D materials.^115,116 Considering their layered structure, 2D COFs provide opportunities to access a new class of organic 2D materials that are highly tunable in structure and properties. Similar to graphene, single or few layer COF flakes have to be exfoliated from bulk 2D COFs. COFenes are defined as monolayer crystals generated by decoupling the non-covalent interactions in bulk 2D COF layers. To distinguish COFenes from amorphous 2D polymers such as covalent organic nanosheets (CONs),¹²⁴ COFenes should possess the following features: (1) the covalent linkages propagate in a 2D plane; (2) the material exhibits a crystalline network structure inherited from the parent COF; and (3) a thickness less than a few unit cells. In this review, we will categorize qualified examples as COFenes, some of which were addressed as CONs or COF nanosheets in the past but fulfil the above requirements.
^115,116 考虑到二维 COF 的层状结构，二维 COF 为获得结构和性质高度可调的新型有机二维材料提供了机会。与石墨烯类似，单层或少层 COF 薄片必须从块状二维 COF 中剥离出来。COFenes 被定义为通过解耦块状二维 COF 层中的非共价相互作用而生成的单层晶体。为了将 COFenes 与共价有机纳米片（CONs）等无定形二维聚合物区分开来，¹²⁴ COFenes 应具有以下特征：(1) 共价键在二维平面上传播；(2) 材料表现出从母体 COF 继承而来的结晶网络结构；(3) 厚度小于几个单元格。在本综述中，我们将把符合条件的例子归类为 COFenes，其中一些在过去被作为 CON 或 COF 纳米片处理，但符合上述要求。

3.1 Advantages of functional COFenes
3.1 功能性 COFenes 的优点

Do COFenes offer new properties different from those of their bulk counterparts? Research on bulk 2D COFs has shown that excitons can move both across the covalent linkages in the xy plane and the π columns along the z axis.^45,117,118 By reducing bulk 2D COFs into COFenes, the movement of excitons is confined in a 2D plane, giving rise to organic quantum wells with new electronic properties and band structures.¹¹⁹ The absence of interlayer interactions in COFenes may allow fluorescence to be turned on, whereas this would be quenched by π–π interactions in the bulk.^31,120 In addition, open-shell radical structures incorporated in the scaffold of COFenes are free from interlayer interactions, thereby stabilizing the ferromagnetic ground state in these systems. Wu et al. reported a 2D covalent organic radical framework via interfacial synthesis (Fig. 23).¹²¹ Polychlorotriphenylmethyl radical moieties were connected via diacetylene linkages in the ordered framework, giving rise to a moderate anti-ferromagnetic exchange interaction of J = −375 cm⁻¹. The authors reported these results on bulk-like films as thick as 95 nm. There is a possibility that if a COFene is generated from such materials, the ferromagnetic properties can be enhanced.
二维 COFenes 是否具有不同于块体 COFenes 的新特性？对块状二维 COF 的研究表明，激子既可以在 xy 平面的共价键上移动，也可以沿 z 轴在π 柱上移动。^45,117,118 通过将块状二维 COF 还原成 COFenes，激子的运动被限制在二维平面内，从而产生了具有新的电子特性和带状结构的有机量子阱。¹¹⁹ COFenes 中不存在层间相互作用，因此可以开启荧光，而在大体积中，π-π 相互作用会熄灭荧光。^31,120 此外，COFenes 支架中的开壳自由基结构不受层间相互作用的影响，从而稳定了这些系统中的铁磁基态。Wu et al.报告了一种二维共价有机基框架通过界面合成（图 23）。¹²¹ 多氯三苯甲基自由基分子通过 二乙炔连接在有序框架中、J = -375 cm^-1 的中度反铁磁交换相互作用。作者在厚度为 95 nm 的块状薄膜上报告了这些结果。如果从这些材料中生成 COFene，铁磁特性有可能得到增强。

Another benefit of having a COFene is that the exposed surface functional groups on either of its faces should provide readily available catalytic sites for heterogenous catalysis. In 2019, Jiang and coworkers demonstrated the use of COFenes for photocatalytic CO₂ reduction (Fig. 23).¹²² Using co-coordinated porphyrin as the backbone functionality, the COF-367-Co COFene exhibited a CO₂ production rate of 10 162 μmol g⁻¹ and a selectivity of ca. 78%, which was much higher than the bulk counterpart's CO production rate of 124 μmol g⁻¹ and selectivity of 13%. They found that Co atoms are vital for adsorbing CO₂ molecules. Although the bulk COF possesses a larger BET surface area, the Co atoms are buried deep within the framework, resulting in a lower adsorption capacity of CO₂. This suggests that the catalytic activity of the COFene is largely enhanced on account of the exposed backbone functionalities. Very recently, Segura et al. reported a naphthalene diimide-based COFene for the electrocatalytic oxygen reduction reaction (ORR) (Fig. 23).¹²³ In electrocatalysis, a well-dispersed thin catalyst layer can enhance the contact between the electrode and the catalytic sites. A 6–7 nm thick NDI-COFene was coated onto the glassy carbon (GC) electrode. Linear sweep voltammetry revealed good ORR reactivity of the COFene catalyst with an onset potential of (−0.25 V vs. SCE) and a limiting current of ∼3.8 mA cm⁻². It is noteoworthy that the amorphous NDI polymer exhibited almost no ORR reactivity because of the lack of well-defined pores for O₂ diffusion; this clearly evidenced that the crystallinity of COFenes distinguishes themselves from amorphous materials. Furthermore, we believe that COFenes may serve as a new class of carbocatalysts like graphene oxide,¹²⁴ with the possibility of attaining higher selectivity.
COFene 的另一个好处是，其任一表面上暴露的表面官能团都能为异质催化提供现成的催化位点。2019 年，Jiang 及其同事证明了 COFene 可用于光催化 CO₂ 还原（图 23）。¹²² 使用共配位卟啉作为骨架官能团、COF-367-Co COFene 的 CO₂ 生成率为 10 162 μmol g^-1 ，选择性为 ca.他们发现，Co 原子对吸附 CO₂ 分子至关重要。虽然块状 COF 具有较大的 BET 表面积，但 Co 原子深埋在框架中，导致 CO₂ 的吸附能力较低。这表明 COFene 的催化活性在很大程度上由于骨架官能团的暴露而得到了增强。最近，Segura et al. 报道了一种基于萘二亚胺的 COFene，用于电催化氧还原反应（ORR）（图 23）。¹²³ 在电催化过程中，分散良好的催化剂薄层可以增强电极与催化位点之间的接触。将 6-7 nm 厚的 NDI-COFene 涂覆在玻璃碳 (GC) 电极上。线性扫描伏安法显示 COFene 催化剂具有良好的 ORR 反应活性，起始电位为（-0.25 V vs. SCE）和 3.8 mA cm^-2 的极限电流。值得注意的是，无定形的 NDI 聚合物几乎不表现出 ORR 反应性，因为缺乏明确的孔隙供 O₂ 扩散；这清楚地证明了 COFenes 的结晶性有别于无定形材料。此外，我们认为 COFenes 可以像氧化石墨烯一样，¹²⁴ 成为一类新的碳催化剂，并有可能获得更高的选择性。

The presence of ionic groups or polarized species on COFenes means they have better solution-processability than 2D COFs and are amenable to the membrane fabrication needed for filtration applications. The pore sizes of 2D COFs are usually in the range of nanometers, which are too large for selective filtration of metal ions or gas molecules.¹²⁵ Through a delamination and restacking process, the channels in COFenes become partially overlapped, achieving a pore narrowing effect. Compared to membranes based on conventional 2D materials such as graphene,¹²⁶ COFene membranes with well-defined pores can provide much higher permeance as well as better selectivity.
与二维 COF 相比，COFenes 上离子基团或极化物种的存在意味着它们具有更好的溶液可加工性，适合过滤应用所需的膜制造。二维 COF 的孔径通常在纳米范围内，对于金属离子或气体分子的选择性过滤来说过大。¹²⁵ 通过分层和重新堆积过程，COFenes 中的通道部分重叠，实现了孔径变窄的效果。与基于传统二维材料（如石墨烯）的膜相比，¹²⁶ COFene 膜具有明确的孔隙，可提供更高的渗透率和更好的选择性。

3.2 Functionality-directed synthesis of COFenes
3.2 功能定向合成 COFenes

We will review the two major methods to obtain COFenes: top-down exfoliation from bulk 2D COFs and bottom-up direct synthesis of COFenes via interface confinement (Fig. 24). Each method possesses its own advantages and drawbacks. For the top-down method, scalable CONs can be obtained via mechanical or chemical exfoliation from a big pool of 2D COFs; however, exfoliation via brutal forces or harsh chemical conditions is destructive to the crystallinity and less controlled, resulting in uncontrollable thickness and ill-defined morphologies such as nanoparticles instead of thin sheets. In the bottom-up method, COFenes can be obtained with better crystallinity and better-controlled thickness; however, this method is difficult to scale up and the choice of building units is rather limited.
我们将回顾获得 COFenes 的两种主要方法：从块状二维 COFs 自上而下剥离和通过界面约束自下而上直接合成 COFenes（图 24）。每种方法都有自己的优点和缺点。对于自上而下法，可通过机械或化学剥离从一大池二维 COF 中获得可扩展的 CON；但是，通过野蛮力或苛刻的化学条件进行剥离会破坏结晶度，且控制性较差，导致厚度不可控和形态不清晰，如纳米颗粒而非薄片。采用自下而上的方法，可以获得结晶度更好、厚度更可控的 COFenes；但这种方法难以推广，而且可供选择的构建单元相当有限。

In this section, we will summarize the privileged functionalities used in the preparation of COFenes, as well as elaborate on the new properties afforded by these materials.
在本节中，我们将总结制备 COFenes 时所使用的特殊官能团，并详细阐述这些材料所具有的新特性。

3.2.1 Top-down methods
3.2.1 自顶向下方法

The first example of a top-down produced COFene was demonstrated by Zamora et al. (Fig. 25).¹⁷ A boronate 2D COF was delaminated by a facile approach via tip ultrasonication in dichloromethane. The resulting suspension was centrifuged, diluted and drop-casted on to mica and silica substrates, giving rise to COFene-8 flakes that are 4–10 nm thick according to atomic-force microscopy (AFM) analysis, which corresponds to 10 to 25 layers. The in-plane and out-of-plane crystallinity of the COFene was maintained, as evidenced by transmission electron microscopy (TEM) images. The retention of the in-plane crystallinity of COFene-8 suggests that the boronate linkage is robust to mechanical turbulence such as ultrasonication. Besides this work, there were also reported boronate ester 2D COFs showing high in-plane crystallinity under TEM.^127–129 We infer that unlike the imine-type or CC linkages, boronate linkages form a five-membered ring with two B–O bonds; the more rigid nature of these bonds restricts the bond rotation and planarizes the 2D porous organic sheets during the delamination process. Therefore, linkages with multiple connection bonds can enhance the in-plane robustness of 2D COFs, facilitating the production of COFenes via the top-down exfoliation method. For instance, TEM studies revealed that thin sheets of phenazine-linked 2D COFs showed good in-plane crystallinity.^130,131 In a similar vein, the delamination of arylether-based 2D COFs^132,133 should be explored (Fig. 26).
Zamora et al.（图 25）展示了第一个自上而下生产 COFene 的实例。所得悬浮液经离心、稀释后滴铸到云母和二氧化硅基底上，根据原子力显微镜（AFM）分析，COFene-8 薄片的厚度为 4-10 nm，相当于 10 到 25 层。透射电子显微镜（TEM）图像显示，COFene 的面内和面外结晶度均保持不变。COFene-8 面内结晶度的保持表明，硼酸酯连接对机械湍流（如超声）具有很强的耐受性。除这项工作外，也有报道称硼酸酯二维 COF 在 TEM 下显示出很高的面内结晶度。^127-129 我们推断，与亚胺型或 C C 连接不同，硼酸酯连接形成了具有两个 B-O 键的五元环；这些键的刚性更强，在分层过程中限制了键的旋转并使二维多孔有机片平面化。因此，具有多个连接键的连接体可以增强二维 COF 的平面稳固性，从而促进通过自上而下剥离法生产 COFenes 。例如，TEM 研究表明，酚嗪连接的二维 COF 薄片显示出良好的面内结晶性。^130,131 同样，还应探索芳基醚基二维 COF 的分层^132,133 （图 26）。

Other than specific COF linkages which can reinforce in-plane robustness, the structure of the molecular scaffolds also facilitates the exfoliation of 2D COFs. In 2013, Dichtel et al. reported a 2D acylhydrazone COF which can be exfoliated into a COFene via ultrasonication in solvents such as 1,4-dioxane and water (Fig. 27).¹³⁴ COF-43 was sonicated in tetrahydrofuran (THF) to afford nanoparticles that were hundreds of nanometers thick. On the other hand, sonication of COF-43 in 1,4-dioxane produced COFene-43 with a thickness of 1.32 ± 0.37 nm, corresponding to 3–5 layers. Interestingly, selected area electron diffraction (SAED) of COFene-43 displayed a hexagonal diffraction pattern, indicating in-plane long-range order. We infer that the strong in-plane robustness of COF-43 comes from the intralayer hydrogen bonding as mentioned in the section of light emissive 2D COFs. The NH⋯O hydrogen bonding within the six-membered ring can restrict intramolecular bond rotation, giving rise to a highly stable in-plane structure during the exfoliation process.
除了特定的 COF 连接可以增强平面内的稳固性之外，分子支架的结构也有助于二维 COF 的剥离。2013 年，Dichtel et al.报道了一种二维酰腙 COF，这种 COF 可以在 1,4- 二恶烷和水等溶剂中 通过超声作用剥离成 COFene（图 27）。¹³⁴ COF-43 在四氢呋喃（THF）中进行超声处理，可得到数百纳米厚的纳米颗粒。另一方面，在 1,4-二氧六环中超声 COF-43 产生的 COFene-43 厚度为 1.32 ± 0.37 纳米，相当于 3-5 层。有趣的是，COFene-43 的选区电子衍射（SAED）显示出六角形衍射图样，表明了面内长程有序。我们推断，COF-43 强大的面内稳固性来自层内氢键，这一点在 "发光的二维 COF "一节中已经提到。六元环内的 NH⋯O 氢键可限制分子内键的旋转，从而在剥离过程中形成高度稳定的面内结构。

Bein's group demonstrated that tetraphenylethylene³³ and pyrene³⁷ units are beneficial for growing highly crystalline 2D COFs. Judging from their TEM images, such COFs are expected to survive strong mechanical disturbance and produce crystalline exfoliated COFenes. In Bein's work on highly crystalline TPE COFs, they also showed that propeller-shaped triphenylamine (TPA) units are beneficial for enhancing the crystallinity of 2D COFs.³³ In 2017, Han, Zhang and coworkers reported the exfoliation of a TPA-based imine 2D COF into COFene (Fig. 28).¹⁹ The authors claimed that the judicious choice of two flexible TPA building units (amine and aldehyde) weakened the interlayer stacking, leading to easy exfoliation into 2D nanosheets. A colloidal solution exhibiting the Tyndall effect could be obtained by sonication of the TPA-COF in ethanol. Low-dose TEM allowed the visualization of the highly ordered porous structure of the TPA-COFene. AFM measurements revealed that the exfoliated nanosheets had a thickness of 3.5 ± 0.3 nm, corresponding to 9 ± 1 layers. The exfoliated TPA-COFene was adopted as a platform for DNA detection. The TPA-COFene acted as a fluorescence quencher for a dye-labelled DNA probe. Upon the hybridization of the probe with the target DNA, the fluorescence tag was removed from the COFene, leading to the recovery of fluorescence; the quantitative detection of target DNA using this method had a detection limit as low as 20 pM.
Bein 的研究小组证明，四苯乙烯³³ 和芘³⁷ 单元有利于生长高度结晶的二维 COF。从它们的 TEM 图像来看，这种 COF 可以经受住强烈的机械干扰，并产生结晶剥离的 COFenes。在 Bein 对高结晶 TPE COF 的研究中，他们还发现螺旋桨状的三苯胺（TPA）单元有利于提高二维 COF 的结晶度。³³ 2017 年，Han、Zhang 和同事们报道了将基于 TPA 的亚胺二维 COF 剥离成 COF 烯（Fig.19作者声称，明智地选择两种柔性 TPA 构建单元（胺和醛）削弱了层间堆叠，从而使其易于剥离成二维纳米片。通过在乙醇中对 TPA-COF 进行超声处理，可获得具有廷德尔效应的胶体溶液。低剂量 TEM 可以观察到 TPA-COFene 高度有序的多孔结构。原子力显微镜测量显示，剥离的纳米片厚度为 3.5 ± 0.3 nm，相当于 9 ± 1 层。剥离的 TPA-COFene 被用作 DNA 检测平台。TPA-COFene 可作为染料标记 DNA 探针的荧光淬灭剂。当探针与目标 DNA 杂交时，COFene 上的荧光标签被去除，从而恢复荧光；使用这种方法定量检测目标 DNA 的检测限低至 20 pM。

In 2016, Zang and coworkers reported a cationic imine 2D COF based on ethidium bromide building units.¹¹⁰ The charge repulsion between the COF backbone functionalities is expected to facilitate the exfoliation process. In 2018, Ajayaghosh and coworkers demonstrated the exfoliation of this 2D COF (Fig. 29).¹³⁵ According to their report, EB-TFP could be self-exfoliated at room temperature when suspended in water for 2 days without any physical or chemical disturbance. The exfoliated COFene showed an AFM thickness of 1.5 ± 0.3 nm, and good crystallinity under HRTEM. The cationic COFene exhibited fluorescence at 510 nm. A new emission band at 600 nm was observed when negatively charged calf thymus DNA (ctDNA) binds to the COFene to form a COFene–DNA reassembly. They further found that the perfectly matched double-strand DNA showed a higher PL enhancement compared to single-strand DNA of double-strand DNA with mismatched sequences, inidcating that the COFene could function as a sensor for double-strand DNA.
2016 年，Zang 和同事报道了一种基于溴化乙锭构建单元的阳离子亚胺二维 COF。¹¹⁰ COF 主干官能团之间的电荷斥力有望促进剥离过程。2018 年，Ajayaghosh 及其同事展示了这种二维 COF 的剥离（Fig. 29）。¹³⁵ 根据他们的报告，EB-TFP 在室温下悬浮于水中 2 天，在没有任何物理或化学干扰的情况下可以自剥离。剥离的 COFene 的 AFM 厚度为 1.5 ± 0.3 nm，在 HRTEM 下具有良好的结晶性。阳离子 COFene 在 510 纳米波长处发出荧光。当带负电荷的小牛胸腺 DNA（ctDNA）与 COFene 结合形成 COFene-DNA 重新组合时，在 600 纳米处观察到新的发射带。他们进一步发现，与单链 DNA 和序列不匹配的双链 DNA 相比，完全匹配的双链 DNA 显示出更高的聚光增强，这表明 COFene 可用作双链 DNA 的传感器。

In 2019, Gu, Ma et al. reported a highly soluble 2D imine COF based on viologen building units (Fig. 30).⁵⁴ Upon imine condensation between a pyrene-amine and an aryl viologen-aldehyde, they obtained a staggered-stacked 2D COF. The PyVg-COF readily dissolves in various organic solvents such as N-methyl pyrrolidone (NMP), dimethyl sulfoxide (DMSO), N,N-dimethylformamide (DMF), N,N-diethylformamide (DEF), etc. The small-angle neutron scattering (SANS) measurements of a concentrated solution of the PyVg-COF in DMSO-d₆ indicated that the COFene sheets were larger than 1 μm with a thickness of 10–15 nm. The SAED pattern confirmed the good crystallinity of the COFene. Remarkably, the as-prepared COFene film showed a vertical and horizontal electrical conductivity of 0.4 S m⁻¹ and 1.8 × 10⁻¹⁰ S m⁻¹. The anisotropic conductivity suggested that holes or electrons could hop through the individual interlayer donor or acceptor π-columnar arrays, while intralayer conduction was inhibited due to charge recombination in the 2D network. The carrier mobility at zero electric field was measured to be 9 × 10⁻⁴ and 1.4 × 10⁻⁵ cm² V⁻¹ s⁻¹ for holes and electrons, respectively.
2019 年，Gu、Ma et al.报道了一种基于紫罗兰烯构建单元的高可溶性二维亚胺 COF（图 30）。⁵⁴ 在芘胺和芳基紫杉醛之间进行亚胺缩合后，他们得到了交错堆积的二维 COF。PyVg-COF 很容易溶解在各种有机溶剂中，如N-甲基吡咯烷酮（NMP）、二甲基亚砜（DMSO）、N、N-二甲基甲酰胺（DMF）、N、N-二乙基甲酰胺（DEF）、等。 对 PyVg-COF 在 DMSO-d₆ 中的浓缩溶液进行的小角中子散射（SANS）测量表明，COF 烯片大于 1 μm，厚度为 10-15 nm。SAED 图样证实了 COFene 的良好结晶性。值得注意的是，制备的 COFene 薄膜显示出 0.4 S m^-1 和 1.8 × 10^-10 S m^-1 的垂直和水平电导率。各向异性的传导性表明，空穴或电子可以跳过单个层间供体或受体π柱阵列，而由于二维网络中的电荷重组，层内传导受到抑制。据测量，在零电场下，空穴和电子的载流子迁移率分别为 9 × 10^-4 和 1.4 × 10^-5 cm² V^-1 s^-1 。

In short, we have summarized the useful building units for construction of COFenes via the top-down approach (Fig. 31). These molecular scaffolds can reinforce the in-plane robustness of the 2D network or/and facilitate the delamination process by weakening the charge repulsion.
总之，我们总结了通过自上而下的方法（图 31）构建 COFenes 的有用构建单元。这些分子支架可以加强二维网络的面内稳固性，或/和通过削弱电荷排斥力来促进分层过程。

3.2.2 Bottom-up direct growth of COFenes
3.2.2 COFenes 自下而上的直接生长

The bottom-up approach is advantageous in synthesizing thinner COFenes of better crystallinity and larger continuous flake area than the top-down method. Besides, it does away with any tedious exfoliation process. However, this method is difficult to scale up and the choices of building units are rather limited for direct synthesis of COFenes.
与自上而下的方法相比，自下而上的方法在合成结晶度更好、连续薄片面积更大的更薄的 COFenes 方面更具优势。此外，它还省去了繁琐的剥离过程。不过，这种方法难以推广，而且直接合成 COFenes 的构建单元选择相当有限。

Early research in the bottom-up synthesis of COFenes mainly focused on the surface-assisted method, which was carried out in ultrahigh vacuum conditions for scanning tunneling microscopy imaging.¹³⁶ However, only sub-micron sized small islands could be grown and the choice of linkers is limited. Herein, the useful building units for bottom-up synthesis of COFenes are summarized in Fig. 32.
早期自下而上合成 COFenes 的研究主要集中在表面辅助法，这种方法是在超高真空条件下进行的，用于扫描隧道显微镜成像¹³⁶ 。图 32 总结了自下而上合成 COFenes 的有用构建单元。

In 2016, Feng et al. reported a wafer-scale porphyrin-based imine COFene prepared via the interface growth method (Fig. 33).¹³⁷ By carrying out confined growth at the air–water or liquid–liquid interface, monolayer or multilayer COFenes were successfully prepared. The monolayer COFene displayed an AFM thickness of ∼0.7 nm, and its in-plane crystallinity was proved by SAED. The multilayer COFene displayed an AFM thickness of 20 nm, and its out-of-plane crystallinity was confirmed by TEM. Interestingly, the monolayer COFene exhibited a Young's modulus of 267 ± 30 GPa, which is comparable to the lower end boundary of graphene (200–1000 GPa). The thin film transistor fabricated using the monolayer COFene exhibited a mobility of 1.3 × 10⁻⁶ cm² V⁻¹ s⁻¹ and an on/off ratio of 10². When doped with iodine, the mobility increased to 1.6 × 10⁻⁴ cm² V⁻¹ s⁻¹.
2016 年，Feng et al. 报道了通过界面生长法制备的晶圆级卟啉基亚胺 COFene（图 33）。¹³⁷ 通过在空气-水或液体-液体界面进行封闭生长，成功制备出单层或多层 COFene。单层 COFene 的 AFM 厚度为 ∼0.7 nm，其平面内结晶度通过 SAED 得到证实。多层 COFene 的原子力显微镜厚度为 20 nm，其平面外结晶性由 TEM 证实。有趣的是，单层 COFene 的杨氏模量为 267 ± 30 GPa，与石墨烯的下限（200-1000 GPa）相当。使用单层 COFene 制造的薄膜晶体管的迁移率为 1.3 × 10^-6 cm² V^-1 s^-1 和 10² 的导通/截止比。当掺入碘时，迁移率增加到 1.6 × 10^-4 cm² V^-1 s^-1 。

In 2017, our group demonstrated a highly conjugated COFene with C–C bond linkages (Fig. 34).¹³⁸ Using the surface-assisted growth method and an Ulman coupling reaction, a C–C bonded COFene with brickwall topology was synthesized and imaged by STM. The material can also be prepared in the bulk form by crystallizing the “H” shaped monomers into a pre-packed molecular crystal, followed by solid-station polymerization. The bulk material exhibits eclipsed stacking suggested by the PXRD pattern, and can be mechanically exfoliated into micrometre-sized sheets with an AFM thickness of 1 nm. The highly conjugated structure and 1D open channels in the bulk material afford good electrical and ion conductivity. The material is useful as an anode material for sodium ion batteries, in which long-term cycling over 7700 cycles with retention of 70% of the initial capacity at a high current density of 5 A g⁻¹ could be obtained.
2017 年，我们小组展示了一种具有 C-C 键连接的高度共轭 COFene（Fig. 34）。¹³⁸ 利用表面辅助生长方法和乌尔曼偶联反应，合成了一种具有砖墙拓扑结构的 C-C 键 COFene，并通过 STM 进行了成像。通过将 "H "形单体结晶成预包装分子晶体，然后进行固态聚合，还可以制备出块状材料。从 PXRD 图谱上看，这种块状材料呈现出蚀刻堆叠，可以机械剥离成微米大小的薄片，AFM 厚度为 1 纳米。块状材料的高度共轭结构和一维开放通道具有良好的导电性和离子导电性。这种材料可用作钠离子电池的阳极材料，在 5 A g^-1 的高电流密度下，可长期循环 7700 次以上，并保持初始容量的 70%。

In 2018, Liu, Wang et al. reported an interface confinement method to prepare COFenes within a superspreading water layer.²⁵ As shown in Fig. 35, the authors adopted a liquid/liquid/gel triphase system for the confined synthesis of COFenes. The amine linkers and aldehyde linkers were introduced into the hydrogel and oil phase, respectively, and diffused into the thin superspreading water layers between the oil and hydrogel phases to form the COFene thin film. Large area COF films with AFM thicknesses ranging from 4 to 150 nm were successfully prepared. Oriented crystallinity was verified using synchrotron radiation grazing incidence wide-angle X-ray scattering (GIWAXS). The Young's modulus of the COFene film measured by the AFM indentation method was 25.9 ± 0.6 GPa. The COFene film was able to generate ∼1.2 μA cm⁻² photocurrent in a 0.1 M Na₂SO₄ solution with 0.5 mM ascorbic acid as an electron donor under white light irradiation. Using its photoelectrochemical (PEC) activity, the COFene film could detect Ru³⁺ selectively.
2018 年，Liu、Wang et al.报道了一种在超展宽水层内制备 COFenes 的界面约束方法。²⁵ 如图 35所示，作者采用液/液/凝胶三相体系进行 COFenes 的封闭合成。胺连接体和醛连接体分别被引入水凝胶相和油相，并扩散到油相和水凝胶相之间的超扩散水薄层中，形成 COFene 薄膜。成功制备出大面积 COF 薄膜，AFM 厚度从 4 纳米到 150 纳米不等。使用同步辐射掠入射广角 X 射线散射 (GIWAXS) 验证了定向结晶性。用原子力显微镜压痕法测得的 COFene 薄膜的杨氏模量为 25.9 ± 0.6 GPa。在白光照射下，COFene 膜能够在以 0.5 mM 抗坏血酸为电子供体的 0.1 M Na₂SO₄ 溶液中产生 1.2 μA cm^-2 的光电流。利用其光电化学（PEC）活性，COFene 薄膜可以选择性地检测 Ru³⁺ 。

Very recently, Kaiser, Zheng, Feng et al. reported a surfactant-assisted method to prepare porphyrin COFenes via imide or amide linkages (Fig. 36).²⁶ Usually, preparation of polyimide COFs requires a base catalyst and high temperature due to the low reactivity and reversibility of the imide condensation reaction. In this work, aided by surfactant (sodium oleyl sulfate) monolayers, few-layer polyimide COFenes (2DPI) with a thickness of ∼2 nm and average crystal domain size of ∼3.5 μm² were successfully prepared on the water surface by the reaction between amine and anhydride monomers at room temperature and under weak acidic conditions; GIWAXS and aberration-corrected HRTEM provided evidence for the good crystallinity of the COFene. The authors also reported the preparation of a polyamide COFene (2DPA) under similar conditions save for using trifluoromethanesulfonic acid as a catalyst. Interestingly, by changing the surfactant from sodium oleyl sulfate to stearic acid, they could switch the face-on configuration of the 2DPA COFene to edge-on-oriented multilayer COFs.
最近，Kaiser、Zheng、Feng et al.报告了一种表面活性剂辅助方法，通过亚胺或酰胺连接制备卟啉 COFenes （图 36）。²⁶ 通常，由于酰亚胺缩合反应的反应性和可逆性较低，制备聚酰亚胺 COFs 需要碱催化剂和高温。在这项工作中，借助表面活性剂（油醇硫酸钠）单层，制备出了厚度为 2 nm、平均晶体畴尺寸为 3.5 μm² 在室温和弱酸性条件下，通过胺和酸酐单体之间的反应，成功地在水面上制备出了；GIWAXS 和畸变校正 HRTEM 为 COFene 的良好结晶性提供了证据。除使用三氟甲磺酸作为催化剂外，作者还报告了在类似条件下制备聚酰胺 COFene（2DPA）的情况。有趣的是，通过将表面活性剂从油醇硫酸钠改为硬脂酸，他们可以将 2DPA COFene 的面朝上构型转换为面向边缘的多层 COF。

In 2019, Jiang et al. reported a general synthetic method for the preparation of imine COFenes (Fig. 37).¹²² By using an excess amount of 2,4,6-trimethylbenzaldehyde (TBA) as a growth inhibitor, the axial π–π stacking of the COFenes was hindered, thereby allowing the anisotropic growth of the COF NSs along the planar directions to form few-layer COFenes. Based on this strategy, they produced five different COFenes using the solvothermal method, with various AFM thicknesses from 1.1 to 2.1 nm. Impressively, the SAED patterns of all the five COFenes exhibit sharp diffraction spots that reflect the good crystallinity of the COFenes. STM study revealed long-range order of porphyrin-based COFene-367. With [Ru(bpy)₃]Cl₂ as the photosensitizer and ascorbic acid as the electron donor, the porphyrin-based COFene shows excellent photocatalytic properties for CO₂ reduction, with a CO production rate of 10 162 μmol g⁻¹ h⁻¹ and a selectivity of ca. 78% in aqueous media under visible light irradiation.
2019 年，蒋等人报道了一种制备亚胺 COFenes 的通用合成方法（图 37）。¹²² 通过使用过量的 2,4,6-三甲基苯甲醛 (TBA) 作为生长抑制剂，阻碍了 COFenes 的轴向 π-π 堆积，从而使 COF NS 沿着平面方向各向异性地生长，形成少层 COFenes。基于这一策略，他们利用溶热法制备了五种不同的 COFenes，其 AFM 厚度从 1.1 纳米到 2.1 纳米不等。令人印象深刻的是，所有五种 COFenes 的 SAED 图样都显示出尖锐的衍射点，反映出 COFenes 具有良好的结晶性。STM 研究揭示了卟啉基 COFene-367 的长程有序性。以[Ru(py)₃]Cl₂ 为光敏剂，以抗坏血酸为电子供体，卟啉基 COFene 在还原 CO₂ 时表现出优异的光催化性能、CO 生成率为 10 162 μmol g^-1 h^-1 ，选择性为 ca.78%。

4. Conclusion
4. 结论

Chemists are drawn to the intellectual challenge of designing COFs from the bottom up, and to developing synthetic methodologies to make COFs with new topological structures and covalent linkages. Materials scientists examine the synthesized COF and evaluate its potential for application. Function-oriented design and synthesis of COFs require a truly multidisciplinary approach. In this review, we have analyzed the structures of 2D COFs in terms of their building blocks and linkages and correlated these to emerging applications in solid-state light emission, solvatochromism, gas storage, ion conduction and energy storage. Analysing the structure–property correlations for a large class of COFs provides useful guidance for the function-oriented synthesis of 2D COFs. Furthermore, we discuss the synthesis and properties of a new class of molecular 2D materials called COFenes, derived from the exfoliation of 2D COF thin sheets. The ability to fabricate COFene sheets with high in-plane crystallinity requires the structure and linkages of the parent COF to be robust. COFenes are solution-processible and can be restacked into lamellar films like graphene oxide, and thus they open up possibilities in many applications where processing issues prevented bulk COFs from being deployed.
化学家们被自下而上设计 COF 的智力挑战所吸引，并致力于开发合成方法，以制造具有新拓扑结构和共价连接的 COF。材料科学家会对合成的 COF 进行检查，并评估其应用潜力。以功能为导向的 COF 设计与合成需要真正的多学科方法。在本综述中，我们分析了二维 COF 结构的构件和链接，并将其与固态光发射、溶解变色、气体存储、离子传导和能量存储等新兴应用联系起来。分析一大类 COF 的结构-性能相关性为以功能为导向合成二维 COF 提供了有用的指导。此外，我们还讨论了一类名为 COFenes 的新型二维分子材料的合成和性质，该材料源自二维 COF 薄片的剥离。要制造出具有高面内结晶度的 COFene 薄片，需要母体 COF 的结构和链接具有稳健性。COFenes 可在溶液中加工，并可像氧化石墨烯一样重新堆积成片状薄膜，因此它们为许多应用提供了可能性，而在这些应用中，由于加工问题，无法使用块状 COF。

Conflicts of interest  利益冲突

There are no conflicts to declare.
没有需要声明的冲突。

Acknowledgements 致谢

K. P. L. acknowledges NRF-CRP grant “Two-dimensional Covalent Organic Framework: Synthesis and Applications”. Grant number NRF-CRP16-2015-02, funded by National Research Foundation, Prime Minister's Office, Singapore.
K.K. P. L. 感谢 NRF-CRP 资助 "二维共价有机框架：合成与应用"。拨款编号 NRF-CRP16-2015-02，由新加坡总理公署国家研究基金会资助。

References 参考资料