Photoresponsive Covalent Organic Frameworks: Visible-Light Controlled Conversion of Porous Structures and Its Impacts
Yu Feng, Guangtong Wang, Ruoyang Liu, Xingyao Ye, Shanshan Tao, Matthew A. Addicoat, Zhongping Li, Qiuhong Jiang, and Donglin Jiang*
Angew. Chem., Int. Ed.2024, 63, e202400009 DOI: 10.1002/anie.202400009The Story
Covalent organic frameworks are a novel class of crystalline porous polymers that enable molecular design of extended polygonal skeletons to attain well-defined porous structures. However, construction of a framework that allows remote control of pores remains a challenge. Here we report a strategy that merges covalent, noncovalent, and photo chemistries to design photoresponsive frameworks with reversibly and remotely controllable pores. We developed a topology-guided multicomponent polycondensation system that integrates protruded tetrafluoroazobenzene units as photoresponsive sites on pore walls at predesigned densities, so that a series of crystalline porous frameworks with the same backbone can be constructed to develop a broad spectrum of pores ranging from mesopores to micropores. Distinct from conventional azobenzene-based systems, the tetrafluoroazobenzene frameworks are highly sensitive to visible lights to undergo high-rate isomerization. The photoisomerization exerts profound effects on pore size, shape, number, and environment, as well as molecular uptake and release, rendering the system able to convert and switch pores reversibly and remotely with visible lights. Our results open a way to a novel class of smart porous materials with pore structures and functions that are convertible and manageable with visible lights.
Linkage-Engineered Donor–Acceptor Covalent Organic Frameworks for Optimal Photosynthesis of Hydrogen Peroxide from Water and Air
Ruoyang Liu†, Yongzhi Chen†, Hongde Yu, Miroslav Položij, Yuanyuan Guo, Tze Chien Sum, Thomas Heine, and Donglin Jiang*
Nature Catalysis, 2024, 7, 195–206. DOI: 10.1038/s41929-023-01102-3
Charge transfer and mass transport to catalytic sites are critical factors in photocatalysis. However, achieving both simultaneously is challenging due to inherent trade-offs and interdependencies. Here we develop a microporous covalent organic framework featuring dense donor–acceptor lattices with engineered linkages. The donor–acceptor columnar π-arrays function as charge supply chains and as abundant water oxidation and oxygen reduction centres, while the one-dimensional microporous channels lined with rationally integrated oxygen atoms function as aligned conduits for instant water and oxygen delivery to the catalytic sites. This porous catalyst promotes photosynthesis with water and air to produce H2O2, combining a high production rate, efficiency and turnover frequency. This framework operates under visible light without the need of metal co-catalysts and sacrificial reagents, exhibits an apparent quantum efficiency of 17.5% at 420 nm in batch reactors and enables continuous, stable and clean H2O2 production in flow reactors.
Crystalline, Porous Helicene Covalent Organic Frameworks
Qianqian Yan, Shanshan Tao, Ruoyang Liu, Yongfeng Zhi, and Donglin Jiang*
Angew. Chem., Int. Ed. 2024, 63, e202316092. DOI:10.10o2/anie.202316092 (Hot Paper)
Helicenes are a class of fascinating chiral helical molecules with rich chemistry developed continuously over the past 100 years. Their helical, conjugated, and twisted structures make them attractive for constructing molecular systems. However, studies over the past century are mainly focused on synthesizing helicenes with increased numbers of aromatic rings and complex heterostructures, while research on inorganic, organic, and polymeric helicene materials is still embryonic. Herein, we report the first examples of helicene covalent organic frameworks, i.e., Helicene sp2c-COF-1, by condensing helicene dialdehyde with trimethyl triazine via the C=C bond formation reaction under solvothermal conditions. The resultant Helicene sp2c-COF-1 exhibits prominent X-ray diffraction peaks and assumes a highly ordered 2D lattice structure originated from the twisted configuration of Helicene unit. The C=C linked Helicene sp2c-COF-1 materials exhibited extended π conjugation and broadly tuned their absorption, emission, redox activity, photoconductivity, and light-emitting activity, demonstrating rich multifunctionalities and great potentials in developing various applications. This work opens a way to a new family of COFs as well as helicene materials, enabling the exploration of unprecedented π architectures and properties.
Covalent Organic Frameworks: Reversible 3D Coalesce via Interlocked Skeleton–Pore Actions and Impacts on π Electronic Structures
Juan Li, Lili Liu, Xuan Tang, Xi Bai, Yukun Liu, Dongsheng Wang, Shanshan Tao, Ruoyang Liu, and Donglin Jiang*
J. Am. Chem. Soc. 2023, 145, 26383–26392. DOI:10.1021/jacs.3c10280The Story
Covalent organic frameworks (COFs) create extended two-dimensional (2D) skeletons and aligned one-dimensional (1D) channels, constituting a class of novel π architectures with predesignable structural ordering. A distinct feature is that stacks of π building units in skeletons shape the pore walls, onto which a diversity of different units can be assembled to form various pore interfaces, opening a great potential to trigger a strong structural correlation between the skeleton and the pore. However, such a possibility has not yet been explored. Herein, we report reversible three-dimensional (3D) coalescence and interlocked actions between the skeleton and pore in COFs by controlling hydrogen-bonding networks in the pores. Introducing carboxylic acid units to the pore walls develops COFs that can confine water molecular networks, which are locked by the surface carboxylic acid units on the pore walls via multipoint, multichain, and multidirectional hydrogen-bonding interactions. As a result, the skeleton undergoes an interlocked action with pores to shrink over the x–y plane and to stack closer along the z direction upon water uptake. Remarkably, this interlocked action between the skeleton and pore is reversibly driven by water adsorption and desorption and triggers profound effects on π electronic structures and functions, including band gap, light absorption, and emission.
Integrated Interfacial Design of Covalent Organic Framework Photocatalysts to Promote Hydrogen Evolution from Water
Ting He, Wenlong Zhen, Yongzhi Chen, Yuanyuan Guo, Zhuoer Li, Ning Huang, Zhongping Li, Ruoyang Liu, Yuan Liu, Xu Lian, Can Xue, Tze Chien Sum, Wei Chen, and Donglin Jiang*
Nature Communications 2023, 14, 329. DOI: 10.1038/s41467-023-35999-y
Attempts to develop photocatalysts for hydrogen production from water usually result in low efficiency. Here we report the finding of photocatalysts by integrated interfacial design of stable covalent organic frameworks. We predesigned and constructed different molecular interfaces by fabricating ordered or amorphous π skeletons, installing ligating or non-ligating walls and engineering hydrophobic or hydrophilic pores. This systematic interfacial control over electron transfer, active site immobilisation and water transport enables to identify their distinct roles in the photocatalytic process. The frameworks, combined ordered π skeletons, ligating walls and hydrophilic channels, work under 300–1000 nm with non-noble metal co-catalyst and achieve a hydrogen evolution rate over 11 mmol g–1 h–1, a quantum yield of 3.6% at 600 nm and a three-order-of-magnitude-increased turnover frequency of 18.8 h–1 compared to those obtained with hydrophobic networks. This integrated interfacial design approach is a step towards designing solar-to-chemical energy conversion systems.
Covalent Organic Frameworks
Ke Tian Tan, Samrat Ghosh, Fuxiang Wen, David Rodríguez-San-Miguel, Feng Jie, Ning Huang, Wei Wang, Felix Zamora, Xinliang Feng, Arne Thomas, and Donglin Jiang*
Nature Reviews Methods Primers 2023, 3, 1. DOI:10.1038/s43586-022-00181-z
The dream to prepare well-defined materials drives the methodological evolution for molecular synthesis, structural control and materials manufacturing. Among various methods, chemical approaches to design, synthesize, control and engineer small molecules, polymers and networks offer the fundamental strategies. Merging covalent bonds and non-covalent interactions into one method to establish a complex structural composition for specific functions, mimicking biological systems such as DNA, RNA and proteins, is at the centre of chemistry and materials science. Covalent organic frameworks (COFs) are a class of crystalline porous polymers that enable the integration of organic units into highly ordered structures via polymerization. This polymerization system is unique as it deploys covalent bonds to construct the primary order structures of polymeric backbones via polycondensation and leverages on non-covalent interactions to create the high order structures of polymeric networks via supramolecular polymerization in a one-pot reaction system. This Primer covers all aspects of the field of COFs from chemistry to physics, materials and applications, and outlines the design principle, experimental methods, characterization and applications, with an aim to show a concise yet full picture of the field. The key fundamental issues to be addressed are analysed with an outlook on the future major directions from different perspectives.
Exciton Diffusion and Annihilation in An sp2 Carbon-Conjugated Covalent Organic Framework
Xinzi Zhang, Keyu Geng, Donglin Jiang*, and Gregory D. Scholes*
J. Am. Chem. Soc.2022, 144, 16423–16432. DOI:10.1021/jacs.2c04742
To optimize the optical and optoelectronic functionalities of two-dimensional (2D) covalent organic frameworks (COFs), detailed properties of emissive and nonradiative pathways after photoexcitation need to be elucidated and linked to particular structural designs. Here, we use transient absorption (TA) spectroscopy to study the colloidal suspension of the full sp2 carbon-conjugated sp2c-COF and characterize the spatial extent and diffusion dynamics of the emissive excitons generated by impulsive photoexcitation. The ∼3.5 Å stacking distance between 2D layers results in cofacial pyrene excitons that diffuse through the framework, while the state that dominates the emissive spectrum of the polycrystalline solid is assigned to an extended cofacial exciton whose 2D delocalization is promoted by C═C linkages. The subnanosecond kinetics of a photoinduced absorption (PIA) signal in the near-infrared, attributed to a charge-separated exciton, or polaron pair, reflects three-dimensional (3D) exciton diffusion as well as long-range exciton–exciton annihilation driven by resonance interactions. Within our experimental regime, doubling the excitation intensity results in a 10-fold increase in the estimated exciton diffusion length, from ∼3 to ∼30 nm, suggesting that higher lattice temperature may enhance exciton mobility in the COF colloid.
Bottom-up Interfacial Design of Covalent Organic Frameworks for Highly Efficient and Selective Electrocatalysis of CO2
Ting He, Chenhuai Yang, Yongzhi Chen, Ning Huang, Shuming Duan, Zhicheng Zhang, Wenping Hu, and Donglin Jiang*
Adv. Mater. 2022, 34, 2205186. DOI:10.1002/adma.202205186
Assembling molecular catalytic centers into crosslinked networks is widely used to fabricate heterogeneous catalysts but they often suffer loss in activity and selectivity accompanied by unclear causes. Here we report a strategy for the construction of heterogeneous catalysts to induce activity and selectivity by bottom-up introduction of segregated electron conduction and mass transport interfaces into the catalytic materials. The catalytic skeletons are designed to possess different π orderings for electron motion and the open channels are tailored to install finely engineered walls for mass transport, so that origins of activity and selectivity are correlated. The resultant covalent organic framework catalysts with ordered π skeletons and solvophobic pores increased activity by two orders of magnitude, enhanced selectivity and energy efficiency by 70-fold, and broadened voltage range, to promote CO2 transformation under ambient conditions. Our results open a way to precise interfacial design for actionable heterogeneous catalysts for producing feedstocks from CO2.
Covalent Organic Frameworks: Chemistry of Pore Interface and Wall Surface Perturbation and Impact on Functions
Lejian Deng, Zhichao Ding, Xingyao Ye, and Donglin Jiang*
Acc. Mater. Res. 2022, 3, 879–893. DOI:10.1021/accountsmr.2c00108 (Invited)
Porous structures are ubiquitous as discovered in nature ranging from biological channels in animals and plants to various pores in sediments and minerals, playing vital roles in keeping biological activities and ecosystems. Synthetic pores have been dedicated over 100 years and currently continue to be a central subject in the fields of chemistry, physics, materials science, and technology. A fundamental key issue is how to develop specific functions with pores. Pores are determined by three parameters including pore shape, size, and environment; how to design these parameters in a controlled manner is a key subject. Covalent organic frameworks (COFs) are a distinct class of crystalline porous polymer as it combines covalent and non-covalent chemistries to deliberate long-range ordered polygonal skeletons and discrete pores. Topological diagram – the principle for designing COFs, enables the predesign of not only skeletons but also pores, offering a powerful molecular platform for constructing tailor-made organic/polymeric materials. Over the past decade, progress in chemistry has greatly enhanced our capability of synthesizing COFs to achieve different structures. Especially, the pores in COFs are constructed with lightweight elements, covalent bonds, and organic components, which offer numerous combinations to design and synthesize pore shape, size, and interface. These parameters control the interplays with guest molecules and ions to determine the property and function of pores. Among various synthetic porous materials, COFs are unique in that these pore parameters are topologically designable and synthetically controllable. Two complementary strategies, i.e., topology diagram and pore surface engineering, have been developed for pore chemistry, in which the first one emphasizes the in-situ approach to design and control pores and the second one highlights the post synthetic way to tune the pore structures finely yet precisely. These two different chemistries offer individual ways to explore different pores, properties, and functional materials and systems. Looking at the features of COFs, a basic common structure of the pores is the well-defined pore interface, which is constituted by aligned surface atoms and side units at a proximate distance along the pore long-axial direction and distributed periodically over the pores. These features of pore interfaces are specific to COFs and predetermine their functions. In this Accounts, we scrutinize the chemistry of pore interface by focusing on pore shape, size, and the aligned atoms and units on pore walls to design unique properties and functions. We highlight pore wall perturbation with surface atoms and side units to demonstrate their decisive roles in structural formation and functional expression. We summarize perspectives, key issues to be addressed, and opportunities with the aim of showing a promising way to next-stage materials and functional designs.
Module-Patterned Polymerization towards Crystalline 2D sp2-Carbon Covalent Organic Framework Semiconductors
Enquan Jin,+ Keyu Geng,+ Shuai Fu,+ Matthew A. Addicoat, Wenhao Zheng, Shuailei Xie, Jun-Shan Hu, Xudong Hou, Xiao Wu, Qiuhong Jiang, Qing-Hua Xu, Hai I. Wang,* and Donglin Jiang*
Angew. Chem., Int. Ed. 2022, 61, e2021150. DOI:10.1002/anie.202115020
Despite a rapid progress over the past decade, most polycondensation systems even upon a small structural variation of building units eventually result in amorphous polymers other than desired crystalline covalent organic frameworks. This synthetic dilemma is a central and challenging issue of the field. Here we report a novel approach based on module-patterned polymerization to enable efficient and designed synthesis of crystalline porous polymeric frameworks. This strategy features a wide applicability to allow the use of various knots of different structures, enables polycondensation with diverse linkers, and develops a diversity of novel crystalline 2D polymers and frameworks, as demonstrated by using the C=C bond formation polycondensation reaction. The new sp2 carbon frameworks are highly emissive and enable up-conversion luminescence, offer low bandgap semiconductors with tunable band structures, and achieve ultrahigh charge mobilities close to theoretically predicted maxima.
Water Cluster in Hydrophobic Crystalline Porous Covalent Organic Frameworks
Ke Tian Tan, Shanshan Tao, Ning Huang, and Donglin Jiang*
Nat. Commun. 2021, 12, 6747. DOI: https://www.nature.com/articles/s41467-021-27128-4
Highlighted in phys.org; https://phys.org/news/2022-01-clusters-hydrophobic-crystalline-porous-covalent.html
Highlighted in Flipboard; https://flipboard.com/@science_x/phys.org-ti3o1bi9z/-/a-x-AC8aNJQgeJcerWvfVngQ%3Aa%3A2530880263-%2F0
Highlighted in Newsbreak; https://www.newsbreak.com/news/2440024493479/water-cluster-in-hydrophobic-crystalline-porous-covalent-organic-frameworks
Highlighted in Research News; FoS@NUS “Water clusters in hydrophobic crystalline porous covalent organic frameworks”
Highlighted in ChemistryCommunity; https://chemistrycommunity.nature.com/
Progress over the past decades in water confinement has generated a variety of polymers and porous materials. However, most studies are based on a preconception that small hydrophobic pores eventually repulse water molecules, which precludes the exploration of hydrophobic microporous materials for water confinement. Here, we demonstrate water confinement across hydrophobic microporous channels in crystalline covalent organic frameworks. The frameworks are designed to constitute dense, aligned and one-dimensional polygonal channels that are open and accessible to water molecules. The hydrophobic microporous frameworks achieve full occupation of pores by water via synergistic nucleation and capillary condensation and deliver quick water exchange at low pressures. Water confinement experiments with large-pore frameworks pinpoint thresholds of pore size where confinement becomes dominated by high uptake pressure and large exchange hysteresis. Our results reveal a platform based on microporous hydrophobic covalent organic frameworks for water confinement.
Exceptional Electron Conduction in Two-Dimensional Covalent Organic Frameworks
Enquan Jin, Keyu Geng, Shuai Fu, Sheng Yang, Narissa Kanlayakan, Matthew A. Addicoat, Nawee Kungwan, Johannes Geurs, Hong Xu, Mischa Bonn, Hai I. Wang, Jurgen Smet, Tim Kowalczyk, and Donglin Jiang*
Chem 2021, 7, 3309–3324. DOI:https://doi.org/10.1016/j.chempr.2021.08.015
Most organic/polymeric semiconductors are p-type semiconductors, whereas their n-type versions are limited in both availability and carrier mobility. How to develop high-rate n-type organic/polymeric semiconductors remains challenging. Here, we report an approach to high-rate n-type semiconductors via topology-directed polycondensation of conventional p-type knots with n-type isoindigo linkers to form non-conjugated tetragonal and hexagonal two-dimensional polymeric frameworks. The polymers are planar in conformation and show flattened frontier levels, which enable electrons to move along the non-conjugated polymeric backbones. The eclipsed face-to-face stack reduces reorganization energy and greatly strengthens electronic coupling, thus enabling band-like electron conduction perpendicular to polymer layers. A device recording electron mobility as high as 8.2 cm2 V−1 s−1 was achieved with Hall effect measurements, whereas time- and frequency-resolved terahertz spectroscopy revealed a benchmark mobility of 13.3 cm2 V−1 s−1. These new mechanistic insights with exceptional mobility open the way to high-rate n-type organic/polymeric semiconductors.
Smart Covalent Organic Frameworks: Dual Channel Sensors for Acid and Base
Jia Xin Koh, Keyu Geng, and Donglin Jiang*
Chem. Commun. 2021, 57, 9418–9421. DOI: 10.1039/D1CC03057D
The fully π-conjugated sp2 carbon covalent organic frameworks upon integration with carboxylic electrolyte sites on the pore wall develop highly luminescent sensors. The sensors feature dual channel responsiveness and enable to detect both acid and base over a wide pH range and neurotransmitter dopamine via ultrafast electron transfer under ambient conditions.
Editing Light Emission with Stable Crystalline Covalent Organic Frameworks via Wall Surface Perturbation
Zhongping Li, Keyu Geng, Ting He, Ke Tian Tan, Ning Huang, Qiuhong Jiang, Yuki Nagao, and Donglin Jiang*
Angew. Chem. Int. Ed. 2021, 60, 19419–19427. (Selected as Very Important Paper) DOI:10.1002/anie.202107179
The ordered π skeletons of covalent organic frameworks make them viable light-emitting materials but their limited tunability has precluded further implementation. Here we report the synthesis of hydrazone-linked frameworks which are stable in water, acid, and base, and demonstrate their utility as a platform for light emission. The polygonal backbone is designed to be luminescent and partially π conjugated while the pore wall is docked with single atom or unit to induce resonance, hyperconjugation, and tautomerization effects. These effects can be transmitted to the backbone, so that the framework can emit three primary colors of light. The wall can be perturbated with multiple surface sites, rendering the material able to edit diverse emission colors in a predesignable and digital way. The systems show high activity, stability, tunability, and sensibility – a set of features attractive for light-emitting and sensing applications.
Highlighted by AlphaGalileo “Tiny Tweaks to Sparkle: Editing Light-Emitting Organic Molecules Via Surface Modification” (July 06, 2021).
Highlighted by ChemistryViews “Covalent Organic Frameworks Light UP” (September 05, 2021).
Hydroxide Anion Transport in Covalent Organic Frameworks
Shanshan Tao, Hong Xu, Qing Xu, Yuh Hijikata, Qiuhong Jiang, Stephan Irle, and Donglin Jiang*
J. Am. Chem. Soc. 2021, 143, 8970–8975. DOI:10.1021/jacs.1c03268
Hydroxide anion transport is essential for alkaline fuel cells, but hydroxide anion has an inherently low conductivity owing to its small diffusion coefficient and high mass. Ordered open channels found in covalent organic frameworks are promising as pathways to enable hydroxide anion transport, but this remains to be explored. Here we report designed synthesis of anionic covalent organic frameworks that promote hydroxide anion transport across the one-dimensional channels. Engineering cationic chains with imidazolium termini onto the pore walls self-assembles a supramolecular interface of single-file hydroxide anion chains in the channels. The frameworks facilitate hydroxide anion transport to achieve an exceptional conductivity of 1.53 × 10–2 S cm–1 at 80 °C, which is 2–6 orders of magnitude higher than those of linear polymers and other porous frameworks. Impedance spectroscopy at different temperatures and studies on deuterated samples reveal that hydroxide anions transport via a proton-exchange hopping mechanism. These results open a way to design framework materials for energy conversions via engineering an anionic interface.
Ultrafast and Stable Proton Conduction in Polybenzimidazole Covalent Organic Frameworks via Confinement and Activation
Juan Li, Jing Wang, Zhenzhen Wu, Shanshan Tao, and Donglin Jiang*
Angew. Chem. Int. Ed. 2021, 60, 12918–12923.DOI: 10.1002/anie.202101400
Polybenzimidazoles are engineering plastics with superb thermal stability and this specificity has sparked a wide-ranging research to explore proton-conducting materials. Nevertheless, such materials encounter challenging issues owing to phosphoric acid proton carrier leakage and slow proton transport. We report a strategy for designing porous polybenzimidazole frameworks to address these key fundamental issues. The built-in channels are designed to be one-dimensionally extended, unidirectionally aligned, and fully occupied by neat phosphoric acid, while the benzimidazole walls trigger multipoint, multichain, and multitype interactions to spatially confine a phosphoric acid network in pores and facilitate proton conduction via deprotonation. The materials exhibit ultrafast and stable proton conduction for low proton carrier content and activation energy—a set of features highly desired for proton transport. Our results offer a design strategy for the fabrication of porous polybenzimidazoles for use in energy conversion applications.
Covalent Organic Frameworks: A Molecular Platform for Designer Polymeric Architectures and Functional Materials (Award Accounts)
Donglin Jiang*Covalent Organic Frameworks: A Molecular Platform for Designer Polymeric Architectures and Functional Materials
The Bulletin of Chemical Society of Japan, 2021. 94, 1215–1231 (Award Accounts) (Inside front cover) . DOI:10.1246/bcsj.20200389
This award account trailed our way toward and showed our initiatives of covalent organic frameworks, the exploration of design principle and synthetic strategies, and the development of functions and properties. By elucidating various interplays, we scrutinized the unique features of materials and the origins of different functions.
Covalent Organic Frameworks: An Ideal Platform for Designing Ordered Materials and Advanced Applications
Ruoyang Liu, Ke Tian Tan, Yifan Gong, Yongzhi Chen, Zhuoer Li, Shuailei Xie, Ting He, Zhen Lu, Hao Yang, and Donglin Jiang*
Chem. Soc. Rev. 2021, 50, 120–242. DOI: 10.1039/d0cs00620c.
Covalent organic frameworks offer a molecular platform for integrating organic units into periodically ordered yet extended two- and three-dimensional polymers to create topologically well-defined polygonal lattices and built-in discrete micropores and/or mesopores. This polymer architecture is unique as it enables to predesign both primary- and high-order structures, greatly enhancing our capabilities of designing organic materials to produce predictable structures and to achieve unique properties and functions. Progress over the past 15 years in the design, synthesis and functional exploration has successively established the base of the field and these have shown a great potential of chemistry in developing a class of amazing organic materials. In this review, we focus on analysing the historic developments to uncover a full materials and application picture by providing comprehensive yet clear guidance for molecular design, synthetic control and functional exploration. We scrutinise the structural components including building blocks, reactive sites and functional groups with an aim to find the origins of structural designability and diversity, as well as multiple functionalities. We disclose strategies for designing and synthesising frameworks to construct various tailor-made interfaces, and for exploring skeletons and pores to design properties and functions. With well-defined skeletons, pores and interfaces that offer the chemical base to trigger and control interactions with photon, exciton, phonon, polaron, electron, hole, spin, ion and molecule, we illustrate the current status of our understandings of structure-property correlations, and unveil the principles for establishing a regime to design unique functions that originate from and are inherent to structures. We predict the key central issues in design and synthesis, the challenges in functional design, and the future directions from perspectives of chemistry, physics and materials science.
Covalent Organic Frameworks for Energy Conversions: Current Status, Challenges, and Perspectives
Shanshan Tao, and Donglin Jiang*
CCS Chem. 2021, 3, 2003–2024. DOI: 10.31635/ccschem.020.202000491
Energy conversion into clean fuels is critical to society’s health benefits and sustainable future; thus, exploring materials to enable and facilitate energy conversions with reduced climate-related emissions is a central subject of science and technology. Covalent organic frameworks (COFs) are a class of polymers that enables predesign of both primary- and high-order structures and precise synthesis of long-range structures through one-pot polymerization. Progress over the past 15 years in chemistry has dramatically enhanced our capability of designing and synthesizing COFs and deepening our understanding to explore energy-converting functions that originate from their ordered skeletons and channels. In this minireview, we summarize general strategies for predesigning skeletons and channels and analyze the structural requirements for each type of energy conversion. We demonstrate synthetic approaches to develop energy conversion functions, that is, photocatalytic and electrocatalytic conversions. Further, we scrutinize energy conversion features by disclosing interplays of COFs with photons, holes, electrons, and molecules, highlighting the role of structural orderings in energy conversions. Finally, we have predicted the challenging issues in molecular design and synthesis, and thought of future directions toward advancement in this field, and show perspectives from aspects of chemistry, physics, and materials science, aimed at unveiling a full picture of energy conversions based on predesignable organic architectures.
Covalent Organic Frameworks: An Amazing Chemistry Platform for Designing Polymers
Chem. 2020, 6, 2461–2483. DOI: 10.1016/j.chempr.2020.08.024
Covalent organic frameworks emerged as a class of polymers that enable the construction of ordered skeletons and pores via simple polymerization, offering a unique chemistry platform that greatly enhances our capability of designing organic structures and functions. In this perspective, we analyze the structural design and synthesis by highlighting distinct features and bottleneck problems. We scrutinize major functions by correlating structure-function relationships and disclosing the key structural parameters and interfaces that control various interplays to show the key achievements and our conceptual insights. We foresee the fundamental issues, core directions, and challenges in the viewpoints of chemistry, physics, and materials science with the aim of exploring ordered organic materials with unique structures and functions.
Covalent Organic Frameworks: Pore Design and Interface Engineering
Zhuo Li, Ting He, Yifan Gong, and Donglin Jiang*
Acc. Chem. Res. 2020, 53, 1672–1685. DOI: 10.1021/acs.accounts.0c00386
Nature evolves fascinating molecular pores to achieve unique biological functions based on a single pore or channel as observed for aquaporins and ion channels. An artificial system, on the other hand, explores porous structures to construct dense pores in materials. Progress in chemistry over the past century has greatly improved our capability to synthesize porous materials. This is evident by the advancement from inorganic to organic units, from trial-and-error tests to module fabrication and further to fully predesignable pores, and from harsh preparation protocols to ambient synthetic methods. Over the past 15 years, a molecular platform based on organic and polymer chemistry has been explored to enable the design of artificial pores to achieve different pore size, shape, wall, and interface. This becomes possible with a class of emerging polymer−covalent organic frameworks (COFs). COFs are a class of crystalline porous polymers that integrate organic units into extended molecular frameworks with periodically ordered skeletons and well-defined pores. We have focused on exploring COFs over the past 15 years to design and synthesize porous structures with the aim of developing chemistry that leads to the creation of tailor-made pore interfaces (Nagai, A. et al. Nat. Commun, 2011, 2, 536). In this Account, we summarize the general concept of our approaches to various pore interfaces by emphasizing design principle, synthetic strategy, and distinct porous features and their impacts. We illustrate pore interface design by highlighting general strategies based on direct polymerization andpore surface engineering to construct different pore walls with a diversity of functional units. One distinct feature is that these functional groups are predesigned and synthetically controlled to achieve a predetermined component, position, and density, leading to a general way to install various specific pore wall interfaces to each pore. We showcase hierarchical pore interface architectures by elucidating the nature of interplays between interfaces and molecules and ions, ranging broadly from hydrogen bond to dipole−dipole/quadrupole interactions, electrostatic interaction, acid−base interaction, coordination, and electronic interactions. We scrutinize the unique properties and functions of adsorption and separation, catalysis, energy transformation and storage, and proton and ion transport by disclosing functional design schemes and interface−function correlations. We predict the fundamental key issues to be addressed and show future directions in designing artificial pores to target at ultimate functions. This chemistry on pore interface engineering opens a way to porous materials that have remained challenging in the predesign of both structure and function.
Covalent Organic Frameworks: Polymer Chemistry and Functional Design
Keyu Geng, Vasanthakumar Arumugam, Huanjun Xu, Yanan Gao,* and Donglin Jiang*
Prog. Poly. Sci. 2020, 108,101288. DOI: 10.1016/j.progpolymsci.2020.101288
Covalent organic frameworks are a class of crystalline porous polymer that enables the covalent integration of organic units into periodically ordered skeletons and aligned polygonal pores. This structural feature sets a new polymer platform that enables to design not only structures but also functions. Especially, their primary and high-order structures are fully predesignable and synthetically controllable, offering a new diagram of chemistry in designing functional materials. Progress in chemistry and materials science over the past 15 years has greatly enhanced our capability of molecular design and functional exploration to develop a fascinating picture of this new class of polymers. In this article, we focus on exploring chemistry to show the uniqueness of covalent organic frameworks in structural design, synthetic control, and functional development. We summarize the most recent advances in exploring principle to design various polygonal topologies, outline polymerization reactions to synthesize various types of polymers, and scrutinize unique functions to disclose the structural origins of the most promising applications including catalysis, adsorption, molecular separation, and mass transport. Moreover, we propose the key fundamental issues to be addressed and show future directions from perspectives of chemistry and polymer science.
A Stable and Conductive Metallophthalocyanine Framework for Electrocatalytic Carbon Dioxide Reduction in Water
Angew. Chem., Int. Ed. 2020, 59, 16587–16593. DOI: 10.1002/anie.202005274
Transformation of carbon dioxide to high value‐added chemicals becomes a significant challenge for clean energy studies. Here a stable and conductive covalent organic framework was developed for electrocatalytic carbon dioxide reduction to carbon monoxide in aqueous solution. The cobalt(II) phthalocyanine catalysts are topologically connected via robust phenazine linkage into a two‐dimensional tetragonal framework that is stable under boiling water, acid, or base conditions. The 2D lattice enables full π conjugation along x and y directions as well as π conduction along the z axis across the π columns. With these structural features, the electrocatalytic framework exhibits a faradaic efficiency of 96 %, an exceptional turnover number up to 320 000, and a long‐term turnover frequency of 11 412 hour−1, which is a 32‐fold improvement over molecular catalyst. The combination of catalytic activity, selectivity, efficiency, and durability is desirable for clean energy production.
Topology-Templated Synthesis of Crystalline Porous Covalent Organic Frameworks
Angew. Chem. Int. Ed. 2020, 59, 12162–12169. DOI: 10.1002/anie.202004278 (Very Important Paper)
Here we report a strategy for the synthesis of crystalline porous covalent organic frameworks via topology‐templated polymerization. The template is based on imine‐linked frameworks and their (001) facets seed the C=C bond formation reaction to constitute 2D sp2 carbon‐conjugated frameworks. This strategy is applicable to templates with different topologies, enables designed synthesis of frameworks that cannot be prepared via direct polymerization, and creates a series of sp2 carbon frameworks with tetragonal, hexagonal, and kagome topologies. The sp2 carbon frameworks are highly luminescent even in the solid state and exhibit topology dependent π transmission and exciton migration; these key fundamental π functions are unique to sp2 carbon‐conjugated frameworks and cannot be accessible by imine‐linked frameworks, amorphous analogues, and 1D conjugated polymers. These results demonstrate an unprecedented strategy for structural and functional designs of covalent organic frameworks.
Confining H3PO4 Network in Covalent Organic Frameworks Enables Proton Super Flow
Nat. Commun. 2020, 11, 1981. DOI:10.1038/s41467-020-15918-1
Development of porous materials combining stability and high performance has remained a challenge. This is particularly true for proton-transporting materials essential for applications in sensing, catalysis and energy conversion and storage. Here we report the topology guided synthesis of an imine-bonded (C=N) dually stable covalent organic framework to construct dense yet aligned one-dimensional nanochannels, in which the linkers induce hyperconjugation and inductive effects to stabilize the pore structure and the nitrogen sites on pore walls confine and stabilize the H3PO4 network in the channels via hydrogen-bonding interactions. The resulting materials enable proton super flow to enhance rates by 2–8 orders of magnitude compared to other analogues. Temperature profile and molecular dynamics reveal proton hopping at low activation and reorganization energies with greatly enhanced mobility.
Covalent Organic Frameworks: Design, Synthesis, and Functions
Chem. Rev. 2020, 120, 8814–8933. DOI: 10.1021/acs.chemrev.9b00550
Covalent organic frameworks (COFs) are a class of crystalline porous organic polymers with permanent porosity and highly ordered structures. Unlike other polymers, a significant feature of COFs is that they are structurally predesignable, synthetically controllable, and functionally manageable. In principle, the topological design diagram offers geometric guidance for the structural tiling of extended porous polygons, and the polycondensation reactions provide synthetic ways to construct the predesigned primary and high-order structures. Progress over the past decade in the chemistry of these two aspects undoubtedly established the base of the COF field. By virtue of the availability of organic units and the diversity of topologies and linkages, COFs have emerged as a new field of organic materials that offer a powerful molecular platform for complex structural design and tailor-made functional development. Here we target a comprehensive review of the COF field, provide a historic overview of the chemistry of the COF field, survey the advances in the topology design and synthetic reactions, illustrate the structural features and diversities, scrutinize the development and potential of various functions through elucidating structure–function correlations based on interactions with photons, electrons, holes, spins, ions, and molecules, discuss the key fundamental and challenging issues that need to be addressed, and predict the future directions from chemistry, physics, and materials perspectives.
Covalent Organic Frameworks: Chemical Approaches to Designer Structures and Built-in Functions
Angew. Chem. Int. Ed. 2020, 59, 5050–5091. DOI: 10.1002/anie.201904291
A new approach has been developed to design organic polymers using topology diagrams. This strategy enables covalent integration of organic units into ordered topologies and creates a new polymer form, that is, covalent organic frameworks. This is a breakthrough in chemistry because it sets a molecular platform for synthesizing polymers with predesignable primary and high‐order structures, which has been a central aim for over a century but unattainable with traditional design principles. This new field has its own features that are distinct from conventional polymers. This Review summarizes the fundamentals as well as major progress by focusing on the chemistry used to design structures, including the principles, synthetic strategies, and control methods. We scrutinize built‐in functions that are specific to the structures by revealing various interplays and mechanisms involved in the expression of function. We propose major fundamental issues to be addressed in chemistry as well as future directions from physics, materials, and application perspectives.
Designing Covalent Organic Frameworks with Tailored Ionic Interface for Ion Transportation across One-Dimensional Channels
Angew. Chem. Int. Ed. 2020, 59, 4557–4563. DOI: 10.1002/anie.201915234
A strategy based on covalent organic frameworks for ultrafast ion transport involves designing an ionic interface to mediate ion motion. Electrolyte chains were integrated onto the walls of one‐dimensional channels to construct ionic frameworks via pore surface engineering, so that the ionic interface can be systematically tuned at the desired composition and density. This strategy enables a quantitative correlation between interface and ion transport and unveils a full picture of managing ionic interface to achieve high‐rate ion transport. Moreover, the effect of interfaces was scaled on ion transport; ion mobility is increased in an exponential mode with the ionic interface. This strategy not only sets a benchmark system but also offers a general guidance for designing ionic interface that is key to systems for energy conversion and storage.
sp2 Carbon-Conjugated Covalent Organic Frameworks for Photocatalytic Hydrogen Production from Water
Chem 2019, 5, 1632–1647. DOI: 10.1016/j.chempr.2019.04.015
Highlighted by Phys. Org. October 9, 2019.
Water is a sustainable energy resource on this planet. Using another sustainable energy resource, i.e., sunlight to decompose water into hydrogen, a fuel that is green to our environment and society, is attracting great scientific interest and public concern. However, this conversion never happens spontaneously, and it requires a complex system that allows a flow of electrons from light into water. To realize this function, this work creates a purely organic yet robust material in which carbon-based building blocks are connected with a specific bond in a topologically pre-designable ordered manner. This unique structure not only collects sunlight efficiently but also injects electrons to water through a built-in interface, enabling an immediate yet continuous stable hydrogen production from water upon irradiation. We anticipate that this work will offer the structural and mechanistic base for scalable and sustainable fuel production from water and sunlight.
High Precision Size Recognition and Separation in Synthetic 1D Nanochannels
Angew. Chem. Int. Ed. 2019, 58, 15922–15928. Doi/10.1002/anie.201909851
Covalent organic frameworks (COFs) allow elaborate manufacture of ordered one-dimensional channels in the crystal. While this makes designing by topology diagram particularly attractive as a tool for assembling straight pores to facilitate mass transport, molecular recognition has been so far inaccessible. We define a superlattice of COFs, by engineering channels with persistent triangular shape and discrete pore size. We observe a size recognition regime that is different from the characteristic adsorption of COFs, where pore window and walls are cooperative so that triangular apertures sort molecules of one-atom difference and notch nanogrooves confine them into single-file molecular chains. The recognition and confinement are accurately described by sensitive spectroscopy and femto-second dynamic simulations. The resulting COFs enable instantaneous separation of mixture to achieve infinite selectivity and 100% purity at ambient temperature and pressure. Our findings offer an approach to merge precise recognition, selective transport, and instantaneous separation in synthetic 1D channels.
Designed Synthesis of Stable Light-Emitting Two-Dimensional sp2 Carbon-Conjugated Covalent Organic Frameworks
Nature Commun. 2018, 9, 6719. DOI: 10.1038/s41467-018-06719-8 (Also the story behind the paper at Nature research chemistry community https://chemistrycommunity.nature.com/users/177332-jiang-donglin/posts/39712-designed-synthesis-of-stable-light-emitting-two-dimensional-sp2-carbon-conjugated-covalent-organic-frameworks)
Covalent organic frameworks enable the topological connection of organic chromophores into π lattices, making them attractive for creating light-emitting polymers that are predesignable for both the primary- and high-order structures. However, owing to linkages, covalent organic frameworks are either unstable or poor luminescent, leaving the practical synthesis of stable light-emitting frameworks challenging. Here, we report the designed synthesis of sp2 carbon-conjugated frameworks that combine stability with light-emitting activity. The C=C linkages topologically connect pyrene knots and arylyenevinylene linkers into two-dimensional all sp2 carbon lattices that are designed to be π conjugated along both the x and y directions and develop layer structures, creating exceptionally stable frameworks. The resulting frameworks are capable of tuning band gap and emission by the linkers, are highly luminescent under various conditions and can be exfoliated to produce brilliant nanosheets. These results suggest a platform based on sp2carbon frameworks for designing robust photofunctional materials.
Light-Emitting Covalent Organic Frameworks: Fluorescence Improving via Pinpoint Surgery and Selective Switch-On Sensing of Anions
J. Am. Chem. Soc. 2018, 140, 12374–12377. DOI: 10.1021/jacs.8b08380
Covalent organic frameworks (COFs) offer ordered π structures that are useful for developing light-emitting materials. However, most COFs are weak in luminescence. Here we report the conversion of less emissive COFs into light-emitting materials via a pinpoint surgery on the pore walls. Deprotonation of the N–H bond to form an anionic nitrogen species in the hydrazone linkage can eliminate the nitrogen-related fluorescence quenching pathway. The resulting COF enhances the fluorescence in a linear proportion to the progress of deprotonation, achieving a 3.8-fold improved emission. This pinpoint N–H cleavage on the pore walls can be driven only by the fluoride anion while other halogen anions, including chloride, bromide, and iodide, remain inactive, enabling the selective fluorescence switch-on sensing of the fluoride anion at a ppb level.
Ion Conduction in Polyelectrolyte Covalent Organic Frameworks
J. Am. Chem. Soc. 2018,140, 7429–7432. DOI: 10.1021/jacs.8b03814
Covalent organic frameworks (COFs) with ordered one-dimensional channels could offer a predesigned pathway for ion motion. However, implanting salts into bare channels of COFs gives rise to a limited ion conductivity. Here, we report the first example of polyelectrolyte COFs by integrating flexible oligo(ethylene oxide) chains onto the pore walls. Upon complexation with lithium ions, the oligo(ethylene oxide) chains form a polyelectrolyte interface in the nanochannels and offer a pathway for lithium ion transport. As a result, the ion conductivity was enhanced by more than three orders of magnitude compared to that of ions across the bare nanochannels. The polyelectrolyte COFs promoted ion motion via a vehicle mechanism and exhibited enhanced cycle and thermal stabilities. These results suggest that the strategy for engineering a polyelectrolyte interface in the 1D nanochannels of COFs could open a new way to solid-state ion conductors.
Exceptional iodine capture in covalent organic frameworks
Advanced Materials, 2018, 30, 1801911, DOI: 10.1002/adma.201801990
Progress in chemistry over the past four decades has generated a variety of porous materials for removing iodine – a radioactive emission accompanying nuclear fission. However, most studies are still based on a notion that entangled pores together with specific binding sites are essential for iodine capture. Here, we disclose an unraveled physical picture of iodine capture that overturns the preconception by exploring one-dimensionally channeled porous materials. We constructed two-dimensional covalent organic frameworks in a way so that they are free of interpenetration and binding sites but consist of one-dimensional (1D) open channels. As verified with different channels shaping from hexagonal to tetragonal and trigonal and ranging from micropores to mesopores, all the 1D channels enable a full access to iodine, generalizing a new paradigm that the pore volume determines the uptake capacity. These results are of fundamental importance to understanding iodine uptake and designing materials to treat coagulative toxic vapors.
Template Conversion of Covalent Organic Frameworks into 2D Conducting Nanocarbons for Catalyzing Oxygen Reduction Reaction
Adv. Mater. 2018, 30, 1706330. DOI: 10.1002/adma.201706330
Progress over the past decades in porous materials has exerted great effect on the design of metal‐free carbon electrochemical catalysts in fuel cells. The carbon material must combine three functions, i.e., electrical conductivity for electron transport, optimal pores for ion motion, and abundant heteroatom sites for catalysis. Here, an ideal carbon catalyst is achieved by combining two strategies—the use of a 2D covalent organic framework (COF) and the development of a suitable template to guide the pyrolysis. The COF produces nanosized carbon sheets that combine high conductivity, hierarchical porosity, and abundant heteroatom catalytic edges. The catalysts achieve superior performance to authentic Pt/C with exceptional onset potential (0 V vs −0.03 V), half‐wave potentials (−0.11 V vs −0.16 V), high limit current density (7.2 mA cm−2 vs 6.0 mA cm−2), low Tafel slope (110 mV decade−1 vs 121 mV decade−1), long‐time stability, and methanol tolerance. These results reveal a novel material platform based on 2D COFs for designing novel 2D carbon materials.
Two-Dimensional sp2 Carbon-Conjugated Covalent Organic Frameworks
Science 2017, 357, 673–676. DOI: 10.1126/science.aan0202
Although graphene and related materials are two-dimensional (2D) fully conjugated networks, similar covalent organic frameworks (COFs) could offer tailored electronic and magnetic properties. Jin et al. synthesized a fully π-conjugated COF through condensation reactions of tetrakis(4-formylphenyl)pyrene and 1,4-phenylenediacetonitrile. The reactions were reversible, which provides the self-healing needed to form a crystalline material of stacked, π-bonded 2D sheets. Chemical oxidation of this semiconductor with iodine greatly enhanced its conductivity, and the radicals formed on the pyrene centers imparted a high spin density and magnetism.
For C=C linked COFs or sp2-carbon COFs, the below report is the first example.
We have reported the C=C linked COFs on March 24, 2016, in the 96th Spring Annual Meeting of Chemical Society of Japan, Doshisha University, Kyoto.
Enquan JIN, Sasanka DALAPATI, Cheng GU, Ning HUANG, Shanshan TAO, Ping WANG, Lipeng ZHAI presented oral lectures in the 96th CSJ annual meeting, Doshisha U., Kyoto.
On 24 March 2016, Enquan JIN reported the first example of sp2-Carbon Covalent Organic Frameworks in the 96th Annual Meeting of Chemical Society of Japan. This is the first report and publication on C=C linkage for the design and synthesis of covalent organic frameworks.
Enquan Jin, Donglin Jiang, An All sp2-Carbon-Based Covalent Organic Framework, Proceeding of the 96th Annual Meeting of Chemical Society of Japan, March 10, 2016, 1D1-09.
A pdf file of the 96th Spring Annual Meeting of Chemical Society of Japan can be seen here program on Page 57 and abstract.
Ionic Covalent Organic Frameworks: Design of a Charged Interface Aligned on 1D Channel Walls and Its Unusual Electrostatic Functions
Angew. Chem., Int. Ed. 2017, 56, 4982–4986. DOI: 10.1002/anie.201611542
Covalent organic frameworks (COFs) have emerged as a tailor‐made platform for designing layered two‐dimensional polymers. However, most of them are obtained as neutral porous materials. Here, we report the construction of ionic crystalline porous COFs with positively charged walls that enable the creation of well aligned yet spatially confined ionic interface. The unconventional reversed AA‐stacking mode alternately orientates the cationic centers to both sides of the walls; the ionic interface endows COFs with unusual electrostatic functions. Because all of the walls are decorated with electric dipoles, the uptake of CO2 is enhanced by three fold compared to the neutral analog. By virtue of sufficient open space between cations, the ionic interface exhibits exceptional accessibility, efficiency, and selectivity in ion exchange to trap anionic pollutants. These findings suggest that construction of the ionic interface of COFs offers a new way to structural and functional designs.
Stable Covalent Organic Frameworks for Exceptional Mercury Removal from Aqueous Solutions
J. Am. Chem. Soc. 2017, 139, 2428–2434. DOI: 10.1021/jacs.6b12328
The pre-designable porous structures found in covalent organic frameworks (COFs) render them attractive as a molecular platform for addressing environmental issues such as removal of toxic heavy metal ions from water. However, a rational structural design of COFs in this aspect has not been explored. Here we report the rational design of stable COFs for Hg(II) removal through elaborate structural design and control over skeleton, pore size, and pore walls. The resulting framework is stable under strong acid and base conditions, possesses high surface area, has large mesopores, and contains dense sulfide functional termini on the pore walls. These structural features work together in removing Hg(II) from water and achieve a benchmark system that combines capacity, efficiency, effectivity, applicability, selectivity, and reusability. These results suggest that COFs offer a powerful platform for tailor-made structural design to cope with various types of pollution.
Covalent Organic Frameworks: A Materials Platform for Structural and Functional Designs
Nature Reviews Materials 2016, 1, 16068. DOI: 10.1038/natrevmats.2016.68
Covalent organic frameworks (COFs) are a class of crystalline porous polymer that allows the atomically precise integration of organic units into extended structures with periodic skeletons and ordered nanopores. One important feature of COFs is that they are designable; that is, the geometry and dimensions of the building blocks can be controlled to direct the topological evolution of structural periodicity. The diversity of building blocks and covalent linkage topology schemes make COFs an emerging materials platform for structural control and functional design. Indeed, COF architectures offer confined molecular spaces for the interplay of photons, excitons, electrons, holes, ions and guest molecules, thereby exhibiting unique properties and functions. In this Review, we summarize the major progress in the field of COFs and recent achievements in developing new design principles and synthetic strategies. We highlight cutting-edge functional designs and identify fundamental issues that need to be addressed in conjunction with future research directions from chemistry, physics and materials perspectives.
Multiple-Component Covalent Organic Frameworks
Nature Communications 2016, 7, 12325. DOI: 10.1038/ncomms12325
Covalent organic frameworks are a class of crystalline porous polymers that integrate molecular building blocks into periodic structures and are usually synthesized using two-component [1+1] condensation systems comprised of one knot and one linker. Here we report a general strategy based on multiple-component [1+2] and [1+3] condensation systems that enable the use of one knot and two or three linker units for the synthesis of hexagonal and tetragonal multiple-component covalent organic frameworks. Unlike two-component systems, multiple-component covalent organic frameworks feature asymmetric tiling of organic units into anisotropic skeletons and unusually shaped pores. This strategy not only expands the structural complexity of skeletons and pores but also greatly enhances their structural diversity. This synthetic platform is also widely applicable to multiple-component electron donor–acceptor systems, which lead to electronic properties that are not simply linear summations of those of the conventional [1+1] counterparts
Highly Emissive Covalent Organic Frameworks
J. Am. Chem. Soc. 2016, 138, 5797–5800. DOI: 10.1021/jacs.6b02700
Highly luminescent covalent organic frameworks (COFs) are rarely achieved because of the aggregation-caused quenching (ACQ) of π–π stacked layers. Here, we report a general strategy to design highly emissive COFs by introducing an aggregation-induced emission (AIE) mechanism. The integration of AIE-active units into the polygon vertices yields crystalline porous COFs with periodic π-stacked columnar AIE arrays. These columnar AIE π-arrays dominate the luminescence of the COFs, achieve exceptional quantum yield via a synergistic structural locking effect of intralayer covalent bonding and interlayer noncovalent π–π interactions and serve as a highly sensitive sensor to report ammonia down to sub ppm level. Our strategy breaks through the ACQ-based mechanistic limitations of COFs and opens a way to explore highly emissive COF materials.
Two-Dimensional Artificial Light-Harvesting Antennae with Predesigned High-Order Structure and Robust Photosensitising Activity
Scientific Reports 2016, 6, 32944. DOI: 10.1038/srep32944
Highly ordered discrete assemblies of chlorophylls that are found in natural light-harvesting antennae are key to photosynthesis, which converts light energy to chemical energy and is the principal producer of organic matter on Earth. Porphyrins and phthalocyanines, which are analogues of chlorophylls, exhibit a strong absorbance of visible and near-infrared light, respectively. A highly ordered porphyrin-co-phthalocyanine antennae would harvest photons over the entire solar spectrum for chemical transformation. However, such a robust antennae has not yet been synthesised. Herein, we report a strategy that merges covalent bonds and noncovalent forces to produce highly ordered two-dimensional porphyrin-co-phthalocyanine antennae. This methodology enables control over the stoichiometry and order of the porphyrin and phthalocyanine units; more importantly, this approach is compatible with various metalloporphyrin and metallophthalocyanine derivatives and thus may lead to the generation of a broad structural diversity of two-dimensional artificial antennae. These ordered porphyrin-co-phthalocyanine two-dimensional antennae exhibit unique optical properties and catalytic functions that are not available with single-component or non-structured materials. These 2D artificial antennae exhibit exceptional light-harvesting capacity over the entire solar spectrum as a result of a synergistic light-absorption effect. In addition, they exhibit outstanding photosensitising activities in using both visible and near-infrared photons for producing singlet oxygen.
Proton Conductions in Crystalline and Porous Covalent Organic Frameworks
Nature Materials 2016, 15, 722–727. DOI: 10.1038/NMAT4461
Progress over the past decades in proton-conducting materials has generated a variety of polyelectrolytes and microporous polymers. However, most studies are still based on a preconception that large pores eventually cause simply flow of proton carriers rather than efficient conduction of proton ions, which precludes the exploration of large-pore polymers for proton transport. Here, we demonstrate proton conduction across mesoporous channels in a crystalline covalent organic framework. The frameworks are designed to constitute hexagonally aligned, dense, mesoporous channels that allow for loading of N-heterocyclic proton carriers. The frameworks achieve proton conductivities that are 2–4 orders of magnitude higher than those of microporous and non-porous polymers. Temperature-dependent and isotopic experiments revealed that the proton transport in these channels is controlled by a low-energy-barrier hopping mechanism. Our results reveal a platform based on porous covalent organic frameworks for proton conduction.
Porous Organic Polymers with Tunable Work Functions and Selective Hole and Electron Conductions for Energy Conversions
Angew. Chem., Int. Ed. 2016, 55, 3049–3053. DOI: 10.1002/anie.201510723
Organic optoelectronics are promising technologies for energy conversion. However, the electrode interlayer, a key material between active layers and conducting electrodes that controls the transport of charge carriers in and out of devices, is still a chemical challenge. Herein, we report a class of porous organic polymers with tunable work function as hole‐ and electron‐selective electrode interlayers. The network with organoborane and carbazole units exhibits extremely low work‐function‐selective electron flow; while upon ionic ligation and electro‐oxidation, the network significantly increases the work function and turns into hole conduction. We demonstrate their outstanding functions as anode and cathode interlayers in energy‐converting solar cells and light‐emitting diodes.
π-Conjugated Microporous Polymer Films: Designed Synthesis, Conducting Properties and Photoenergy Conversions
Angew. Chem., Int. Ed. 2015, 54, 13594–13598. (Hot Paper) DOI: 10.1002/anie.201506570
Conjugated microporous polymers are a unique class of polymers that combine extended π‐conjugation with inherent porosity. However, these polymers are synthesized through solution‐phase reactions to yield insoluble and unprocessable solids, which preclude not only the evaluation of their conducting properties but also the fabrication of thin films for device implementation. Here, we report a strategy for the synthesis of thin films of π‐conjugated microporous polymers by designing thiophene‐based electropolymerization at the solution–electrode interface. High‐quality films are prepared on a large area of various electrodes, the film thickness is controllable, and the films are used for device fabrication. These films are outstanding hole conductors and, upon incorporation of fullerenes into the pores, function as highly efficient photoactive layers for energy conversions. Our film strategy may boost the applications in photocatalysis, energy storage, and optoelectronics.
Designed Synthesis of Double-Stage Two-Dimensional Covalent Organic Frameworks
Scientific Reports 2015, 5, 14650. DOI: 10.1038/srep.14650
Covalent organic frameworks (COFs) are an emerging class of crystalline porous polymers in which organic building blocks are covalently and topologically linked to form extended crystalline polygon structures, constituting a new platform for designing π-electronic porous materials. However, COFs are currently synthesised by a few chemical reactions, limiting the access to and exploration of new structures and properties. The development of new reaction systems that avoid such limitations to expand structural diversity is highly desired. Here we report that COFs can be synthesised via a double-stage connection that polymerises various different building blocks into crystalline polygon architectures, leading to the development of a new type of COFs with enhanced structural complexity and diversity. We show that the double-stage approach not only controls the sequence of building blocks but also allows fine engineering of pore size and shape. This strategy is widely applicable to different polymerisation systems to yield hexagonal, tetragonal and rhombus COFs with predesigned pores and π-arrays.
Stable, Crystalline, Porous, Covalent Organic Frameworks as A Platform for Chiral Organocatalysts
Nature Chemistry, 2015, 7, 905–912. DOI: 10.1038/NCHEM2352
Highlighted by EurekAlert&AAAS “Exploration of stable, crystalline, porous covalent organic frameworks”; Chemistry World “Firming COFs up takes Michael reaction catalysis forward”, by ANDY EXTANCE; Synfacts, 2015, 11, 1269 “Persistent COF: A Stable Platform for Asymmetric Organocatalysis” by Timothy M. Swage, Lily Chen
The periodic layers and ordered nanochannels of covalent organic frameworks (COFs) make these materials viable open catalytic nanoreactors, but their low stability has precluded their practical implementation. Here we report the synthesis of a crystalline porous COF that is stable against water, strong acids and strong bases, and we demonstrate its utility as a material platform for structural design and functional development. We endowed a crystalline and porous imine-based COF with stability by incorporating methoxy groups into its pore walls to reinforce interlayer interactions. We subsequently converted the resulting achiral material into two distinct chiral organocatalysts, with the high crystallinity and porosity retained, by appending chiral centres and catalytically active sites on its channel walls. The COFs thus prepared combine catalytic activity, enantioselectivity and recyclability, which are attractive in heterogeneous organocatalysis, and were shown to promote asymmetric C–C bond formation in water under ambient conditions.
Rational Design of Crystalline Supermicroporous Covalent Organic Frameworks with Triangular Topologies
Nature Communications, 2015, 6, 7786. DOI: 10.1038/ncomms8786.
Covalent organic frameworks (COFs) are an emerging class of highly ordered porous polymers with many potential applications. They are currently designed and synthesized through hexagonal and tetragonal topologies, limiting the access to and exploration of new structures and properties. Here, we report that a triangular topology can be developed for the rational design and synthesis of a new class of COFs. The triangular topology features small pore sizes down to 12 Å, which is among the smallest pores for COFs reported to date, and high π-column densities of up to 0.25 nm−2, which exceeds those of supramolecular columnar π-arrays and other COF materials. These crystalline COFs facilitate π-cloud delocalization and are highly conductive, with a hole mobility that is among the highest reported for COFs and polygraphitic ensembles.
Creation of Superheterojunction Polymers via Direct Polycondensation: Segregated and Bicontinuous Donor-Acceptor π-Columnar Arrays in Covalent Organic Frameworks for Long-Lived Charge Separation
J. Am. Chem. Soc. 2015, 137, 7817–7828. DOI: 10.1021/ja5b03553
By developing metallophthalocyanines and diimides as electron-donating and -accepting building blocks, herein, we report the construction of new electron donor–acceptor covalent organic frameworks (COFs) with periodically ordered electron donor and acceptor π-columnar arrays via direct polycondensation reactions. X-ray diffraction measurements in conjunction with structural simulations resolved that the resulting frameworks consist of metallophthalocyanine and diimide columns, which are ordered in a segregated yet bicontinuous manner to form built-in periodic π-arrays. In the frameworks, each metallophthalocyanine donor and diimide acceptor units are exactly linked and interfaced, leading to the generation of superheterojunctions—a new type of heterojunction machinery, for photoinduced electron transfer and charge separation. We show that this polycondensation method is widely applicable to various metallophthalocyanines and diimides as demonstrated by the combination of copper, nickel, and zinc phthalocyanine donors with pyrommellitic diimide, naphthalene diimide, and perylene diimide acceptors. By using time-resolved transient absorption spectroscopy and electron spin resonance, we demonstrated that the COFs enable long-lived charge separation, whereas the metal species, the class of acceptors, and the local geometry between donor and acceptor units play roles in determining the photochemical dynamics. The results provide insights into photoelectric COFs and demonstrate their enormous potential for charge separation and photoenergy conversions.
Tailor-Made Pore Surface Engineering in Covalent Organic Frameworks: Systematic Functionalization for Performance Screening
J. Am. Chem. Soc. 2015, 137, 7079–7082. DOI: 10.1021/ja5b04300
Imine-linked covalent organic frameworks (COFs) were synthesized to bear content-tunable, accessible, and reactive ethynyl groups on the walls of one-dimensional pores. These COFs offer an ideal platform for pore-wall surface engineering aimed at anchoring diverse functional groups ranging from hydrophobic to hydrophilic units and from basic to acidic moieties with controllable loading contents. This approach enables the development of various tailor-made COFs with systematically tuned porosities and functionalities while retaining the crystallinity. We demonstrate that this strategy can be used to efficiently screen for suitable pore structures for use as CO2 adsorbents. The pore-surface-engineered walls exhibit an enhanced affinity for CO2, resulting in COFs that can capture and separate CO2 with high performance.
A Photoresponsive Smart Covalent Organic Framework
Angew. Chem., Int. Ed. 2015, 54, 8704–8707. (VIP) DOI: 10.1002/anie.201503902
Ordered π‐columnar structures found in covalent organic frameworks (COFs) render them attractive as smart materials. However, external‐stimuli‐responsive COFs have not been explored. Here we report the design and synthesis of a photoresponsive COF with anthracene units as the photoresponsive π‐building blocks. The COF is switchable upon photoirradiation to yield a concavo‐convex polygon skeleton through the interlayer [4π+4π] cycloaddition of anthracene units stacked in the π‐columns. This cycloaddition reaction is thermally reversible; heating resets the anthracene layers and regenerates the COF. These external‐stimuli‐induced structural transformations are accompanied by profound changes in properties, including gas adsorption, π‐electronic function, and luminescence. The results suggest that COFs are useful for designing smart porous materials with properties that are controllable by external stimuli.
Radical Covalent Organic Frameworks: A General Strategy to Immobilize Open-Accessible Polyradicals and High-Performance Capacitive Energy Storage
Angew. Chem., Int. Ed. 2015, 54, 6814–1818. DOI: 10.1002/anie.201501706
Ordered π‐columns and open nanochannels found in covalent organic frameworks (COFs) could render them able to store electric energy. However, the synthetic difficulty in achieving redox‐active skeletons has thus far restricted their potential for energy storage. A general strategy is presented for converting a conventional COF into an outstanding platform for energy storage through post‐synthetic functionalization with organic radicals. The radical frameworks with openly accessible polyradicals immobilized on the pore walls undergo rapid and reversible redox reactions, leading to capacitive energy storage with high capacitance, high‐rate kinetics, and robust cycle stability. The results suggest that channel‐wall functional engineering with redox‐active species will be a facile and versatile strategy to explore COFs for energy storage.
Locking Covalent Organic Frameworks with Hydrogen Bonds: General and Remarkable Effects on Crystalline Structure, Physical Properties, and Photochemical Activities
J. Am. Chem. Soc. 2015, 137, 3241–3247. DOI: 10.1021/ja509602c
A series of two-dimensional covalent organic frameworks (2D COFs) locked with intralayer hydrogen-bonding (H-bonding) interactions were synthesized. The H-bonding interaction sites were located on the edge units of the imine-linked tetragonal porphyrin COFs, and the contents of the H-bonding sites in the COFs were synthetically tuned using a three-component condensation system. The intralayer H-bonding interactions suppress the torsion of the edge units and lock the tetragonal sheets in a planar conformation. This planarization enhances the interlayer interactions and triggers extended π-cloud delocalization over the 2D sheets. Upon AA stacking, the resulting COFs with layered 2D sheets amplify these effects and strongly affect the physical properties of the material, including improving their crystallinity, enhancing their porosity, increasing their light-harvesting capability, reducing their band gap, and enhancing their photocatalytic activity toward the generation of singlet oxygen. These remarkable effects on the structure and properties of the material were observed for both freebase and metalloporphyin COFs. These results imply that exploration of supramolecular ensembles would open a new approach to the structural and functional design of COFs.
Cascade Exciton-Pumping Engines with Manipulated Speed and Efficiency in Light-Harvesting Porous π-Network Films
Scientific Reports 2015, 5, 8867 (2015). DOI:10.1038/srep08867
Light-harvesting antennae are the machinery for exciton pumping in natural photosynthesis, whereas cascade energy transfer through chlorophyll is key to long-distance, efficient energy transduction. Numerous artificial antennae have been developed. However, they are limited in their cascade energy-transfer abilities because of a lack of control over complex chromophore aggregation processes, which has impeded their advancement. Here we report a viable approach for addressing this issue by using a light-harvesting porous polymer film in which a three-dimensional π-network serves as the antenna and micropores segregate multiple dyes to prevent aggregation. Cascade energy-transfer engines are integrated into the films; the rate and efficiency of the energy-funneling engines are precisely manipulated by tailoring the dye components and contents. The nanofilms allow accurate and versatile luminescence engineering, resulting in the production of thirty emission hues, including blue, green, red and white. This advance may open new pathways for realising photosynthesis and photoenergy conversion.
Electrochemically Active, Crystalline, Mesoporous Covalent Organic Frameworks on Carbon Nanotubes for Synergistic Lithium Battery Energy Storage
Scientific Reports 2015, 5, 8225. DOI:10.1038/srep08225
Organic batteries free of toxic metal species could lead to a new generation of consumer energy storage devices that are safe and environmentally benign. However, the conventional organic electrodes remain problematic because of their structural instability, slow ion-diffusion dynamics and poor electrical conductivity. Here, we report on the development of a redox-active, crystalline, mesoporous covalent organic framework (COF) on carbon nanotubes for use as electrodes; the electrode stability is enhanced by the covalent network, the ion transport is facilitated by the open meso-channels and the electron conductivity is boosted by the carbon nanotube wires. These effects work synergistically for the storage of energy and provide lithium-ion batteries with high efficiency, robust cycle stability and high rate capability. Our results suggest that redox-active COFs on conducting carbons could serve as a unique platform for energy storage and may facilitate the design of new organic electrodes for high-performance and environmentally benign battery devices.
Two-Dimensional Covalent Organic Frameworks for Carbon Dioxide Capture via Channel-Wall Functionalization
Angew. Chem., Int. Ed. 2015, 54, 29860–2990. DOI: 10.1002/anie.201411262
Ordered open channels found in two‐dimensional covalent organic frameworks (2D COFs) could enable them to adsorb carbon dioxide. However, the frameworks’ dense layer architecture results in low porosity that has thus far restricted their potential for carbon dioxide adsorption. Here we report a strategy for converting a conventional 2D COF into an outstanding platform for carbon dioxide capture through channel‐wall functionalization. The dense layer structure enables the dense integration of functional groups on the channel walls, creating a new version of COFs with high capacity, reusability, selectivity, and separation productivity for flue gas. These results suggest that channel‐wall functional engineering could be a facile and powerful strategy to develop 2D COFs for high‐performance gas storage and separation.
High-Performance Heterogeneous Catalysis with Surface-Exposed Stable Metal Nanoparticles
Scientific Reports 2014, 4, 7228. DOI:10.1038/srep07228
Protection of metal nanoparticles from agglomeration is critical for their functions and applications. The conventional method for enhancing their stability is to cover them with passivation layers to prevent direct contact. However, the presence of a protective shell blocks exposure of the metal species to reactants, thereby significantly impeding the nanoparticles’ utility as catalysts. Here, we report that metal nanoparticles can be prepared and used in a surface-exposed state that renders them inherently catalytically active. This strategy is realised by spatial confinement and electronic stabilisation with a dual-module mesoporous and microporous three-dimensional π-network in which surface-exposed nanoparticles are crystallised upon in situ reduction. The uncovered palladium nanoparticles serve as heterogeneous catalysts that are exceptionally active in water, catalyse unreactive aryl chlorides for straightforward carbon–carbon bond formation and are stable for repeated use in various types of cross couplings. Therefore, our results open new perspectives in developing practical heterogeneous catalysts.
Photoelectric Covalent Organic Frameworks: Converting Open Lattices into Ordered Donor-Acceptor Heterojunctions
J. Am. Chem. Soc. 2014, 136, 9806–9808. DOI: 10.1021/ja502692w
Ordered one-dimensional open channels represent the typical porous structure of two-dimensional covalent organic frameworks (COFs). Here we report a general synthetic strategy for converting these open lattice structures into ordered donor–acceptor heterojunctions. A three-component topological design scheme was explored to prepare electron-donating intermediate COFs, which upon click reaction were transformed to photoelectric COFs with segregated donor–acceptor alignments, whereas electron-accepting buckyballs were spatially confined within the nanochannels via covalent anchoring on the channel walls. The donor–acceptor heterojunctions trigger photoinduced electron transfer and allow charge separation with radical species delocalized in the π-arrays, whereas the charge separation efficiency was dependent on the buckyball content. This new donor–acceptor strategy explores both skeletons and pores of COFs for charge separation and photoenergy conversion.
Controlled Synthesis of Conjugated Microporous Polymer Films: Versatile Platforms for Highly Sensitive and Label-Free Chemo- and Bio-Sensings
Angew. Chem., Int. Ed. 2014, 53, 4850–4855. DOI: 10.1002/anie.201402141
Conjugated microporous polymers (CMPs), in which rigid building blocks form robust networks, are usually synthesized as insoluble and unprocessable powders. We developed a methodology using electropolymerization for the synthesis of thin CMP films. The thickness of these films is synthetically controllable, ranging from nanometers to micrometers, and they are obtained on substrates or as freestanding films. The CMP films combine a number of striking physical properties, including high porosity, extended π conjugation, facilitated exciton delocalization, and high‐rate electron transfer. We explored the CMP films as versatile platforms for highly sensitive and label‐free chemo‐ and biosensing of electron‐rich and electron‐poor arenes, metal ions, dopamine, and hypochloroic acid, featuring rapid response, excellent selectivity, and robust reusability.
An Azine-Linked Covalent Organic Framework
J. Am. Chem. Soc. 2013, 135, 17310–17313. DOI: 10.1021/ja4103293
Condensation of hydrazine with 1,3,6,8-tetrakis(4-formylphenyl)pyrene under solvothermal conditions yields highly crystalline two-dimensional covalent organic frameworks. The pyrene units occupy the vertices and the diazabutadiene (−C═N–N═C−) linkers locate the edges of rohmbic-shaped polygon sheets, which further stack in an AA-stacking mode to constitute periodically ordered pyrene columns and one-dimensional microporous channels. The azine-linked frameworks feature permanent porosity with high surface area and exhibit outstanding chemical stability. By virtue of the pyrene columnar ordering, the azine-linked frameworks are highly luminescent, whereas the azine units serve as open docking sites for hydrogen-bonding interactions. These synergestic functions of the vertices and edge units endow the azine-linked pyrene frameworks with extremely high sensitivity and selectivity in chemosensing, for example, the selective detection of 2,4,6-trinitrophenol explosive. We anticipate that the extension of the present azine-linked strategy would not only increase the structural diversity but also expand the scope of functions based on this highly stable class of covalent organic frameworks.
Conjugated Organic Framework with Three-Dimensionally Ordered Stable Polymer with Delocalized π Clouds
Nature Communications 2013, 4, 2736. DOI: 10.1038/ncomms3736
Covalent organic frameworks are a class of crystalline organic porous materials that can utilize π–π-stacking interactions as a driving force for the crystallization of polygonal sheets to form layered frameworks and ordered pores. However, typical examples are chemically unstable and lack intrasheet π-conjugation, thereby significantly limiting their applications. Here we report a chemically stable, electronically conjugated organic framework with topologically designed wire frameworks and open nanochannels, in which the π conjugation-spans the two-dimensional sheets. Our framework permits inborn periodic ordering of conjugated chains in all three dimensions and exhibits a striking combination of properties: chemical stability, extended π-delocalization, ability to host guest molecules and hole mobility. We show that the π-conjugated organic framework is useful for high on-off ratio photoswitches and photovoltaic cells. Therefore, this strategy may constitute a step towards realizing ordered semiconducting porous materials for innovations based on two-dimensionally extended π systems.
Control Crystallinity and Porosity of Covalent Organic Frameworks through Managing Interlayer Interactions Based on Self-Complementary π-Electronic Force
J. Am. Chem. Soc. 2013, 135, 546–549. DOI: 10.1021/ja3100319
Crystallinity and porosity are crucial for crystalline porous covalent organic frameworks (COFs). Here we report synthetic control over the crystallinity and porosity of COFs by managing interlayer interactions based on self-complementary π-electronic forces. Fluoro-substituted and nonsubstituted aromatic units at different molar ratios were integrated into the edge units that stack to trigger self-complementary π-electronic interactions in the COFs. The interactions improve the crystallinity and enhance the porosity by maximizing the total crystal stacking energy and minimizing the unit cell size. Consequently, the COF consisting of equimolar amounts of fluoro-substituted and nonsubstituted units showed the largest effect. These results suggest a new approach to the design of COFs by managing the interlayer interactions.
A Squaraine-Linked Covalent Organic Framework
Angew. Chem., Int. Ed. 2013, 52, 3770–3774. (Hot Paper). DOI: 10.1002/anie.201300256
π in the sky: A squaraine‐linked, conjugated two‐dimensional porphyrin covalent organic framework (COF; see scheme; C: gray, H: white, N: blue, Cu: red) was synthesized. Owing to the π‐conjugated linkage together with the eclipsed stacking of the units, this COF exhibits enhanced chemical and thermal stabilities. It absorbs a broad range of light, from the ultraviolet to the visible, and near‐infrared regions, and shows potential as a photocatalyst.
Charge Dynamics in a Donor–Acceptor Covalent Organic Framework with Periodically Ordered Bicontinuous Heterojunctions
Angew. Chem., Int. Ed. 2013, 52, 2017–2021. (Inside Cover). DOI: 10.1002/anie.201209513
Light works: Mechanistic insights into the photochemical events and charge dynamics of a donor–acceptor covalent organic framework were given by time‐resolved transient absorption spectroscopy and time‐resolved electron spin resonance spectroscopy (see picture). The organic framework triggers ultrafast electron transfer and enables long‐distance charge delocalization and exceptional long‐term charge separation.
Conjugated Microporous Polymers: Design, Synthesis and Application
Chem. Soc. Rev. 2013, 42, 8012–8031. (Front Cover Page) DOI: 10.1039/C3CS60160A
Conjugated microporous polymers (CMPs) are a class of organic porous polymers that combine π-conjugated skeletons with permanent nanopores, in sharp contrast to other porous materials that are not π-conjugated and with conventional conjugated polymers that are nonporous. As an emerging material platform, CMPs offer a high flexibility for the molecular design of conjugated skeletons and nanopores. Various chemical reactions, building blocks and synthetic methods have been developed and a broad variety of CMPs with different structures and specific properties have been synthesized, driving the rapid growth of the field. CMPs are unique in that they allow the complementary utilization of π-conjugated skeletons and nanopores for functional exploration; they have shown great potential for challenging energy and environmental issues, as exemplified by their excellent performance in gas adsorption, heterogeneous catalysis, light emitting, light harvesting and electrical energy storage. This review describes the molecular design principles of CMPs, advancements in synthetic and structural studies and the frontiers of functional exploration and potential applications.
High-Rate Charge Carrier Transport in Porphyrin Covalent Organic Frameworks: Switching from Hole to Electron, and to Ambipolar
Angew. Chem., Int. Ed. 2012, 51, 2618–2622. DOI: 10.1002/anie.201106203
Well conducted: A two‐dimensional porphyrin covalent organic framework is described. Owing to the eclipsed stacking alignment, the framework is conductive and allows high‐rate carrier transport through the porphyrin columns (see picture). The central metal in the porphyrin rings changes the conducting nature of the material from hole to electron, and to ambipolar conduction. It also drives the high on–off ratio photoconductivity of the framework.
An Ambipolar Covalent Organic Framework with Self-Sorted and Periodic Electron Donor-Acceptor Ordering
Adv. Mater. 2012, 24, 3026–3031. DOI: 10.1002/adma.201201185
A donor‐acceptor built‐in covalent organic framework with segregated, periodic, and bicontinuous electron donor‐acceptor ordering is reported. The framework consists of pre‐organized periodic independent pathways for ambipolar electron and hole conduction and constitutes vertically ordered p‐n heterojunctions with a broad D‐A interface for enhanced photoconductivity.
Covalent Organic Frameworks
Chem. Soc. Rev. 2012, 41, 6010–6022. DOI: 10.1039/C2CS35157A
Covalent organic frameworks (COFs) are a class of crystalline porous polymers that allow the atomically precise integration of organic units to create predesigned skeletons and nanopores. They have recently emerged as a new molecular platform for designing promising organic materials for gas storage, catalysis, and optoelectronic applications. The reversibility of dynamic covalent reactions, diversity of building blocks, and geometry retention are three key factors involved in the reticular design and synthesis of COFs. This tutorial review describes the basic design concepts, the recent synthetic advancements and structural studies, and the frontiers of functional exploration.
Conjugated Microporous Polymers as Molecular Sensing Devices: Microporous Architecture Enables Rapid Response and Enhances Sensitivity in Fluorescence-On and Fluorescence-Off Sensing
J. Am. Chem. Soc. 2012, 134, 8738–8742. DOI: 10.1021/ja303448r
Conjugated polymers are attractive materials for the detection of chemicals because of their remarkable π-conjugation and photoluminescence properties. In this article, we report a new strategy for the construction of molecular detection systems with conjugated microporous polymers (CMPs). The condensation of a carbazole derivative, TCB, leads to the synthesis of a conjugated microporous polymer (TCB-CMP) that exhibits blue luminescence and possesses a large surface area. Compared with a linear polymer analogue, TCB-CMP showed enhanced detection sensitivity and allowed for the rapid detection of arenes upon exposure to their vapors. TCB-CMP displayed prominent fluorescence enhancement in the presence of electron-rich arene vapors and drastic fluorescence quenching in the presence of electron-deficient arene vapors, and it could be reused without a loss of sensitivity and responsiveness. These characteristics are attributed to the microporous conjugated network of the material. Specifically, the micropores absorb arene molecules into the confined space of the polymer, the skeleton possesses a large surface area and provides a broad interface for arenes, and the network architecture facilitates exciton migration over the framework. These structural features function cooperatively, enhancing the signaling activity of TCB-CMP in fluorescence-on and fluorescence-off detection.
Pore Surface Engineering in Covalent Organic Frameworks
Nature Communications 2011, 2, 536. doi: 10.1038/ncomms1542 (2011). DOI: 10.1038/ncomms1542
Covalent organic frameworks (COFs) are a class of important porous materials that allow atomically precise integration of building blocks to achieve pre-designable pore size and geometry; however, pore surface engineering in COFs remains challenging. Here we introduce pore surface engineering to COF chemistry, which allows the controlled functionalization of COF pore walls with organic groups. This functionalization is made possible by the use of azide-appended building blocks for the synthesis of COFs with walls to which a designable content of azide units is anchored. The azide units can then undergo a quantitative click reaction with alkynes to produce pore surfaces with desired groups and preferred densities. The diversity of click reactions performed shows that the protocol is compatible with the development of various specific surfaces in COFs. Therefore, this methodology constitutes a step in the pore surface engineering of COFs to realize pre-designed compositions, components and functions.
Light-Emitting Conjugated Polymers with Microporous Network Architecture: Interweaving Scaffold Promotes Electronic Conjugation, Facilitates Exciton Migration, and Improves Luminescence
J. Am. Chem. Soc. 2011, 133, 17622–17625. DOI: 0.1021/ja208284t
Highlighted by ACS Noteworthy Chemistry, December 19, 2011 “Switch on Luminescence by Locking Molecular Rotors”
Herein we report a strategy for the design of highly luminescent conjugated polymers by restricting rotation of the polymer building blocks through a microporous network architecture. We demonstrate this concept using tetraphenylethene (TPE) as a building block to construct a light-emitting conjugated microporous polymer. The interlocked network successfully restricted the rotation of the phenyl units, which are the major cause of fluorescence deactivation in TPE, thus providing intrinsic luminescence activity for the polymers. We show positive “CMP effects” that the network promotes π-conjugation, facilitates exciton migration, and improves luminescence activity. Although the monomer and linear polymer analogue in solvents are nonemissive, the network polymers are highly luminescent in various solvents and the solid state. Because emission losses due to rotation are ubiquitous among small chromophores, this strategy can be generalized for the de novo design of light-emitting materials by integrating the chromophores into an interlocked network architecture.
An n-Channel Two-Dimensional Covalent Organic Framework
J. Am. Chem. Soc. 2011, 133, 14510–14513. DOI: 10.1021/ja2052396
Highlighted byChemical & Engineering News, September 12, 2011,Volume 89 Issue 37 | p. 21 | Concentrates “Framework Compound Conducts Electrons – Availability of p-type and n-type conductors may drive these crystalline materials toward electronic applications”
Co-condensation of metallophthalocyanine with an electron-deficient benzothiadiazole (BTDA) block leads to the formation of a two-dimensional covalent organic framework (2D-NiPc-BTDA COF) that assumes a belt shape and consists of AA stacking of 2D polymer sheets. Integration of BTDA blocks at the edges of a tetragonal metallophthalocyanine COF causes drastic changes in the carrier-transport mode and a switch from a hole-transporting skeleton to an electron-transporting framework. 2D-NiPc-BTDA COF exhibits broad and enhanced absorbance up to 1000 nm, shows panchromatic photoconductivity, is highly sensitive to near-infrared photons, and has excellent electron mobility as high as 0.6 cm2 V–1 s–1.
Supercapacitive Energy Storage and Electric Power Supply Using an Aza-Fused Conjugated Microporous Framework
Angew. Chem., Int. Ed. 2011, 50, 8753–8757. (VIP) DOI: 10.1002/anie.201103493 (Very Important Paper)
Highlighted by Nature Nanotechnology “porous polymer charged up”, September 6, 2011
Supercapacitor: A π conjugated microporous polymer with an aza‐fused framework is reported. The porous framework is conductive and allows electrolyte ions to move into the pores because of structural features (see picture). The material becomes highly co‐operative in the formation of electrostatic charge‐separation layers, shows exceptional capacitance in supercapacitive energy storage, provides high energy densities, and offers an excellent cycle life.
Highly Efficient Activation of Molecular Oxygen with Nanoporous Metalloporphyrin Frameworks in Heterogeneous Systems
Adv. Mater. 2011, 23, 3149–3154. DOI: 10.1002/adma.201100974
A nanoporous metalloporphyrin framework exhibits high activity in the activation of molecular oxygen and excellent catalytic activity in the aerobic epoxidation of olefins, with high conversion, outstanding selectivity, and broad substrate applicability. The porous framework is reusable and allows large‐scale transformation. These catalytic features are correlated with the structural characteristics of the porous framework and mark a breakthrough for this classic reaction.
Synthesis of Metallophthalocyanine Covalent Organic Frameworks That Exhibit High Carrier Mobility and Photoconductivity
Angew. Chem., Int. Ed. 2011, 50, 1289–1293. DOI: 10.1002/anie.201005919
A light COF: Two‐dimensional covalent organic frameworks of a nickel phthalocyanine have been synthesized. Owing to well‐ordered stacking of the phthalocyanine units, the resulting 2D framework provides enhanced and broad light absorbance and facilitates charge transport. The material becomes highly photoconductive and is exceptionally sensitive to deep‐red visible and near‐infrared light.
CMPs as Scaffolds for Constructing Porous Catalytic Frameworks: A Built-In Heterogeneous Catalyst with High Activity and Selectivity Based on Nanoporous Metalloporphyrin Polymers
J. Am. Chem. Soc. 2010, 132, 9138–9143. DOI: 10.1021/ja1028556
This article describes the synthesis and functions of a porous catalytic framework based on conjugated micro- and mesoporous polymers with metalloporphyrin building blocks (FeP-CMP). FeP-CMP was newly synthesized via a Suzuki polycondensation reaction and was developed as a heterogeneous catalyst for the activation of molecular oxygen to convert sulfide to sulfoxide under ambient temperature and pressure. FeP-CMP is intriguing because the polymer skeleton itself is built from catalytic moieties and serves as built-in catalysts, bears inherent open nanometer-scale pores that are accessible for substrates, and possesses large surface areas (1270 m2 g−1) that facilitate the transformation reaction. It is highly efficient with high conversion (up to 99%) and a large turnover number (TON = 97,320), is widely applicable to various sulfides covering from aromatic to alkyl and cyclic substrates, displays high selectivity (up to 99%) to form corresponding sulfoxides, and is highly chemoselective for the oxidation of a sulfide group even in the coexistence of other oxidative functionalities. Owing to the covalent linkages between catalytic sites in the frameworks, FeP-CMP can be recycled with good retention of its porous structure and allows for large-scale transformation. These unique characteristics clearly originate from the covalent porous catalytic framework structure and demonstrate the usefulness of CMPs in the exploration of built-in heterogeneous catalysts, a new potential of these materials that have thus far been reported to exhibit noteworthy gas adsorption functions.
Light-Harvesting Conjugated Microporous Polymers: Rapid and Highly Efficient Flow of Light Energy with a Porous Polyphenylene Framework as Antennae
J. Am. Chem. Soc. 2010, 132, 6624–6628. (Cover Page) DOI: 10.1021/ja100327h
The molecular design of light-harvesting antennae requires not only the segregation of a large number of chromophore units in a confined nanospace but also the cooperation of these units in achieving highly efficient energy transduction. This article describes the synthesis and functions of a polyphenylene-based conjugated microporous polymer (PP-CMP). PP-CMP was recently designed and synthesized by Suzuki polycondensation reaction and used as an antenna for the noncovalent construction of a light-harvesting system. In contrast to linear polyphenylene, PP-CMP consists of conjugated three-dimensional polyphenylene scaffolds and holds inherent porous structure with uniform pore size (1.56 nm) and large surface area (1083 m2g−1). It emits blue photoluminescence, is capable of excitation energy migration over the framework, and enables rapid transportation of charge carrier with intrinsic mobility as high as 0.04 cm2 V−1 s−1. The microporous structure of PP-CMP allows for the spatial confinement of energy-accepting coumarin 6 molecules in the pores and makes the high-throughput synthesis of light-harvesting systems with designable donor−acceptor compositions possible. Excitation of the PP-CMP skeleton leads to brilliant green emission from coumarin 6, with an intensity 21-fold as high as that upon direct excitation of coumarin 6 itself, while the fluorescence from PP-CMP itself is wholly quenched as a result of energy transfer from the light-harvesting PP-CMP framework to coumarin 6. The PP-CMP skeleton is highly cooperative, with an average of 176 phenylene units working together to channel the excitation energy to one coumarin 6 molecule, and features the energy-transfer process with quick, efficient, and vectorial character. These unique characteristics clearly originate from the conjugated porous structure and demonstrate the usefulness of CMPs in the exploration of π-electronic functions, in addition to their gas adsorption properties thus far reported.
Noncovalently Netted, Photoconductive Sheets with Extremely High Carrier Mobility and Conduction Anisotropy from Triphenylene-Fused Metal Trigon Conjugates
J. Am. Chem. Soc. 2009, 131, 7287–7292. DOI: 10.1021/ja901357h
Supramolecular assembly of small molecules via noncovalent interaction is useful for bottom-up construction of well-defined macroscopic structures. This approach is attracting increasing interest due to its high potential in manufacturing novel molecular electronic and optoelectronic devices. This Article describes the synthesis and functions of a sheet-shaped assembly from novel triphenylene-fused metal trigon conjugates. These conjugates were recently designed and synthesized by a divergent method and used for the supramolecular self-assembly of sheet-like objects. In contrast to triphenylene, which absorbs photons in ultraviolet region, the triphenylene-fused metal trigon conjugate shows a strong absorption band in the visible region. The metal trigon conjugate emits green photoluminescence with significantly enhanced quantum yield and allows intramolecular energy migration, as a result of extended π-conjugation over metal sites. It assembles via physical gelation to form noncovalent sheets that collect a wide wavelength range of photons from ultraviolet to visible regions. The noncovalent sheets allow exciton migration and are semiconducting with an extremely large intrinsic carrier mobility of 3.3 cm2 V−1 s−1. They are highly photoconductive, produce photocurrent with a quick response to light irradiation, and are capable of repetitive on−off switching. Moreover, these sheets facilitate a conduction path perpendicular to the sheet plane, thus exhibiting a spatially distinctive anisotropy in conduction. The noncovalent sheet assemblies with these unique characteristics are important for molecular optoelectronic devices based on solution-processed soft materials.
A Photoconductive Covalent Organic Framework: Self-Condensed Arene Cubes with Eclipsed 2D Polypyrene Sheets for Photocurrent Generation
Angew. Chem. Int. Ed. 2009, 48, 5439–5442. DOI: 10.1002/anie.200900881
On again, off again: A pyrene‐based covalent organic framework (see structure: blue pyrene, white B, red O) facilitates exciton migration and carrier transportation, harvests visible‐light photons, and responds quickly to irradiation with light to enable the generation of a significant photocurrent. The framework is capable of repetitive photocurrent switching with a large on–off ratio.
A Belt-Shaped, Blue Luminescent and Semiconducting Covalent Organic Framework
Angew. Chem., Int. Ed. 2008, 47, 8826–8830. (VIP) DOI: 10.1002/anie.200803826
Selected as a frontispiece of ACIE. Highlighted by Chemical & Engineering News. Jyllian N. Kemsley, “Covalent Conducting Belts”, C & EN, October 13, 2008 Volume 86, Number 41 P. 29.
Blue belt: Condensation polymerization of pyrene (blue) and triphenylene (green) monomers leads to the formation of a hexagonal mesoporous covalent organic framework (see picture). This material exists in a belt shape, absorbs photons over a wide wavelength range to emit them as blue luminescence, and is semiconducting, as well as being capable of repetitive on–off switching.