Covalent organic frameworks (COFs) are crystalline porous materials constructed from molecular building blocks using diverse linkage chemistries. Their modular construction system allows not only for ...tailor-made design but also for an immense variety of building blocks, opening the door to numerous different functionalities and potential applications. As a consequence, a large number of building blocks that can act as light-harvesters, semiconductors, ligands, binding sites or redox centers have recently been integrated into the scaffolds of COFs. This unique combination of reticular chemistry with the molecular control of intrinsic properties paves the way towards the design of new semiconducting materials for (opto-)electronic applications such as sensors, photocatalysts or -electrodes, supercapacitor and battery materials, solar-harvesting devices or light emitting diodes. With new developments regarding the linkage motif, highly stable but still tunable COFs have been developed for applications even under harsh conditions. Further, the molecular stacking modes and distances in the COFs have been investigated as a powerful means to control optical and electrical characteristics of these self-assembled frameworks. Advanced understanding of optoelectronic processes in COFs has enabled their implementation in optoelectronic devices with promising potential for real-world applications. This review highlights the key developments of design concepts for the synthesis of electro- and photoactive COFs as well as our understanding of optoelectronic processes in these frameworks, hence establishing a new paradigm for the rational construction of well-defined novel optoelectronic materials and devices.
Covalent organic frameworks (COFs) are crystalline porous materials constructed from molecular building blocks using diverse linkage chemistries. The image illustrates electron transfer in a COF-based donor-acceptor system. Image by Nanosystems Initiative Munich.
Covalent organic frameworks (COFs) are crystalline porous polymers formed by a bottom‐up approach from molecular building units having a predesigned geometry that are connected through covalent ...bonds. They offer positional control over their building blocks in two and three dimensions. This control enables the synthesis of rigid porous structures with a high regularity and the ability to fine‐tune the chemical and physical properties of the network. This Feature Article provides a comprehensive overview over the structures realized to date in the fast growing field of covalent organic framework development. Different synthesis strategies to meet diverse demands, such as high crystallinity, straightforward processability, or the formation of thin films are discussed. Furthermore, insights into the growing fields of COF applications, including gas storage and separations, sensing, electrochemical energy storage, and optoelectronics are provided.
Covalent organic frameworks are porous crystalline polymers that are built up from organic linkers via covalent bond formation, thereby offering positional control over their building blocks in two and three dimensions. A comprehensive overview over the structures realized to date is provided and advanced synthesis strategies and applications including gas storage, sensing, electrochemical energy storage, and optoelectronics are discussed.
Covalent organic frameworks (COFs) are a novel class of porous crystalline organic materials assembled from molecular building blocks. The construction principles of these materials allow for the ...design of precisely controllable structures since their chemical and physical properties can be easily tuned through the selection of the building blocks and the linkage motif. Their extraordinary and versatile properties impart functionality that is of great interest in areas such as gas storage, separation, catalysis and optoelectronics. This feature article discusses key aspects of the design of covalent organic frameworks with a focus on electroactive COFs for potential optoelectronic and photovoltaic applications.
This Feature Article discusses key aspects of the design of covalent organic frameworks with a focus on electroactive COFs for potential optoelectronic and photovoltaic applications.
Fattakhova-Rohlfing et al examine three-dimensional titanium dioxide (TiO2) nanomaterials. They focus on porous films, porous spheres, hierarchical hollow spheres, porous fibers, and ...three-dimensional titania materials with hierarchical morphology.
Electrochromic coatings are promising for applications in smart windows or energy-efficient optical displays. However, classical inorganic electrochromic materials such as WO3 suffer from low ...coloration efficiency and slow switching speed. We have developed highly efficient and fast-switching electrochromic thin films based on fully organic, porous covalent organic frameworks (COFs). The low band gap COFs have strong vis–NIR absorption bands in the neutral state, which shift significantly upon electrochemical oxidation. Fully reversible absorption changes by close to 3 OD can be triggered at low operating voltages and low charge per unit area. Our champion material reaches an electrochromic coloration efficiency of 858 cm2 C–1 at 880 nm and retains >95% of its electrochromic response over 100 oxidation/reduction cycles. Furthermore, the electrochromic switching is extremely fast with response times below 0.4 s for the oxidation and around 0.2 s for the reduction, outperforming previous COFs by at least an order of magnitude and rendering these materials some of the fastest-switching frameworks to date. This combination of high coloration efficiency and very fast switching reveals intriguing opportunities for applications of porous organic electrochromic materials.
Hierarchical functional materials with properties encoded for a defined functionality such as light‐induced charge separation are highly desirable targets of numerous synthetic efforts. In the field ...of reticular chemistry, 2D covalent organic frameworks are an emerging class of porous and crystalline materials obtained by bottom‐up condensation of molecular building blocks assisted by non‐bonding interactions. In this research news article, an overview is provided on newly emerging photoactive and conducting 2D covalent organic frameworks, sketching the creative trajectory from subunit design strategies to framework construction and resulting functionalities.
Hierarchical functional materials with properties encoded for a defined functionality such as light‐induced charge separation are highly desirable targets of numerous synthetic efforts. In this research news article, an overview is provided on newly emerging photoactive and conducting 2D covalent organic frameworks, sketching the creative trajectory from subunit design strategies to framework construction and resulting functionalities.
Formamidinium lead iodide (FAPbI3) has the potential to achieve higher performance than established perovskite solar cells like methylammonium lead iodide (MAPbI3), while maintaining a higher ...stability. The major drawback for the latter material is that it can crystallize at room temperature in a wide bandgap hexagonal symmetry (P63 mc) instead of the desired trigonal (P3m1) black phase formed at a higher temperature (130 °C). Our results show that employing a mixture of MAI and FAI in films deposited via a two-step approach, where the MAI content is <20%, results in the exchange of FA molecules with MA without any significant lattice shrinkage. Additionally, we show with temperature-dependent X-ray diffraction that the trigonal phase exhibits no phase changes in the temperature range studied (25 to 250 °C). We attribute the stabilization of the structure to stronger interactions between the MA cation and the inorganic cage. Finally, we show that the inclusion of this small amount of MA also has a positive effect on the lifetime of the photoexcited species and results in more efficient devices.
Filling a honeycomb: The thienothiophene‐based covalent organic framework (COF) can be loaded with a complementary semiconductor, such as a fullerene derivative, and electronic interactions are ...observed (see figure). The novel periodic interpenetrated donor–acceptor system shows the spectroscopic signatures of efficient charge transfer on the nanoscale. Photovoltaic activity was demonstrated upon integrating the COF:fullerene film into a device.
Traditionally, the properties and functions of covalent organic frameworks (COFs) are defined by their constituting building blocks, while the chemical bonds that connect the individual subunits have ...not attracted much attention as functional components of the final material. We have developed a new series of dual-pore perylene-based COFs and demonstrated that their imine bonds can be protonated reversibly, causing significant protonation-induced color shifts toward the near-infrared, while the structure and crystallinity of the frameworks are fully retained. Thin films of these COFs are highly sensitive colorimetric acid vapor sensors with a detection limit as low as 35 μg L–1 and a response range of at least 4 orders of magnitude. Since the acidochromism in our COFs is a cooperative phenomenon based on electronically coupled imines, the COFs can be used to determine simultaneously the concentration and protonation strength of nonaqueous acid solutions, in which pH electrodes are not applicable, and to distinguish between different acids. Including the imine bonds as function-determining constituents of the framework provides an additional handle for constructing multifunctional COFs and extending the range of their possible applications.
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