Iodine doping covalent organic frameworks to reduce electron-hole recombination for promoting photocatalytic performance.

Covalent organic frameworks (COFs) with implemented electroactive moieties and coherent conduction channels are found to be competent for the transport of charges, in a similar manner as conventional organic (semi)conductors.Unlike densely packed organic and polymer layers that are used in organic electronics, the crystallinity of COFs is mostly supported by covalent bonds that potentially boost communication and stability.The voids inside the framework structure enable encapsulation and mass transport, allowing space for guest components such as external electron donors/acceptors as well as channels for diffusion-based functions such as sensing and switching. Covalent organic frameworks (COFs) feature covalent bond-supported crystallinity along with high encapsulating and mass transport abilities. Together with the ease of chemical addition of electroactive moieties, these properties have recently raised interest in this class of material in organic electronics. In this review, we systematically summarize the utilization of advantageous characteristics of COFs to fulfill different functions in electronic processes, resulting in various applications. Broadly, COFs have been successfully implemented as conductive or semiconductive components for electronic devices, such as organic photodetector and photovoltaics, organic transistors, organic light-emitting devices, and organic sensor and memory devices. Simultaneously, general tactics to provide electroactive functionalities are discussed, providing open considerations and inspiration for future electronic design. Covalent organic frameworks (COFs) feature covalent bond-supported crystallinity along with high encapsulating and mass transport abilities. Together with the ease of chemical addition of electroactive moieties, these properties have recently raised interest in this class of material in organic electronics. In this review, we systematically summarize the utilization of advantageous characteristics of COFs to fulfill different functions in electronic processes, resulting in various applications. Broadly, COFs have been successfully implemented as conductive or semiconductive components for electronic devices, such as organic photodetector and photovoltaics, organic transistors, organic light-emitting devices, and organic sensor and memory devices. Simultaneously, general tactics to provide electroactive functionalities are discussed, providing open considerations and inspiration for future electronic design. COFs are porous, crystalline polymers with periodically organized networks connected by covalent bonds between light atoms such as C, N, and O. Directed by reticular chemistry, 2D or 3D COFs are constructed by selecting building blocks and bonding linkages in predefined orientations [1.Lyle S.J. et al.Covalent organic frameworks: organic chemistry extended into two and three dimensions.Trends Chem. 2019; 1: 172-184Abstract Full Text Full Text PDF Scopus (124) Google Scholar]. 2D COFs have chemical bonds that extends in two dimensions, forming sheets that stack together by intermolecular interactions, whereas 3D COFs have covalent bonds that reach out in all directions in 3D space therefore forming an isotropic structure. The predesigned bottom-up synthesis (see Glossary) and rigid architecture endows this class of material with ultrahigh surface area, lightweight, precisely controlled pore and atom distribution, and high stability in a wide range of solvents and conditions [2.Liu R. et al.Covalent organic frameworks: an ideal platform for designing ordered materials and advanced applications.Chem. Soc. Rev. 2021; 50: 120-242Crossref PubMed Google Scholar,3.Geng K. et al.Covalent organic frameworks: design, synthesis, and functions.Chem. Rev. 2020; 120: 8814-8933Crossref PubMed Scopus (774) Google Scholar]. Intrigued by their unique features, a race among researchers to explore applications of COF-based materials has started. Organic electronics is a discipline that uses densely packed small organic molecules or organic polymers as functional elements in electronic devices [4.Pron A. et al.Electroactive materials for organic electronics: preparation strategies, structural aspects and characterization techniques.Chem. Soc. Rev. 2010; 39: 2577-2632Crossref PubMed Scopus (407) Google Scholar]. This is an interdisciplinary research area including materials chemistry and device engineering. Focusing on materials chemistry, the organic electroactive materials act as electrically conductive or semiconductive components that transfer charges under certain conditions. Compared with their conventional inorganic equivalents, the organic electroactive materials usually have advantages of being highly diverse, lightweight, and flexible, having low energy consumption when fabricated, and being solution processable [5.Yang Y. et al.The effects of side chains on the charge mobilities and functionalities of semiconducting conjugated polymers beyond solubilities.Adv. Mater. 2019; 311903104Crossref PubMed Scopus (93) Google Scholar]. Organic conductive materials with high electrical conductance can be used for ductile and transparent electrodes for flexible circuits, whereas organic semiconductive materials are applied in organic field-effect transistors (OFETs), organic light-emitting diodes (OLEDs), organic photovoltaics (OPVs), and organic lasers, among others. As an emerging organic material, COFs are now being designed for electroactivity, releasing a new choice of functional material for organic electronics (Box 1) [6.Yusran Y. et al.Electroactive covalent organic frameworks: design, synthesis, and applications.Adv. Mater. 2020; 322002038Crossref Scopus (66) Google Scholar,7.Allendorf M.D. et al.Electronic devices using open framework materials.Chem. Rev. 2020; 120: 8581-8640Crossref PubMed Scopus (82) Google Scholar]. Compared to conventional organic electronic materials, electroactive COFs show some unique advantages that remedy traditional limitations and might even lead to device innovations. First, the covalent bond-supported crystallinity of COFs vastly surpasses the intermolecular-force-supported crystallinity of semiconducting molecules/polymers, considering enhanced long-range communication and higher stability. The charge transfer behavior is profoundly affected by crystallinity and COFs allow a stable, long range, and a priori predictable crystallinity. Second, porous COFs enable mass transport within the tunnels of electroactive layers, which is uncommon for traditional conductive/semiconductive materials that are too densely packed to allow high mass diffusion of molecules. Furthermore, the precisely controlled pores of COFs provide a space for guest materials (e.g., dopants), which could interact with the COF host for property modification or to integrate multiple functionalities. Considering these benefits, electroactive COFs are an attractive alternative to traditional materials in a vast number of electronic applications.Box 1The branches of COF-based electronic applications and processesDepending on the different electronic processes (shown in rectangles in Figure I), electroactive COFs can be categorized as and applied to different device applications (shown in colored boxes in Figure I). If the COF can transfer charges between two electrodes under bias voltage, it is electrically conductive and can be defined as a conductive COF. Conductive behavior is the most basic and widely researched aspect of electroactive COF materials. If the COF can be thermally activated at room temperature, the COF is intrinsically conductive. Light and chemicals (dopants) can also be the source of activation, corresponding to light activation and doping activation, respectively. Furthermore, if conduction is triggered by a gate voltage from a third electrode, it belongs to the scope of three-terminal devices, OFETs. For a photoactivated conductive material, if the current change on illumination is large, the material shows photoresponsive characteristics and can be utilized in photodetectors. If the formed excitons can dissociate and accumulate in different phases followed by extraction at the electrodes, the materials can be exploited in organic solar cells. For electroactive COFs showing conduction, if the COFs: (i) reversibly trap ions/charges under voltage switching, the device behaves as a memory device for signal storage; (ii) interact with chemicals causing a current change, the device acts as a sensor for chemical detection; or (iii) change light absorption due to an electrochemical redox reaction, it can be used in an EC device. Furthermore, an electroactive and emissive COF can be integrated as the active layer in an OLED. Depending on the different electronic processes (shown in rectangles in Figure I), electroactive COFs can be categorized as and applied to different device applications (shown in colored boxes in Figure I). If the COF can transfer charges between two electrodes under bias voltage, it is electrically conductive and can be defined as a conductive COF. Conductive behavior is the most basic and widely researched aspect of electroactive COF materials. If the COF can be thermally activated at room temperature, the COF is intrinsically conductive. Light and chemicals (dopants) can also be the source of activation, corresponding to light activation and doping activation, respectively. Furthermore, if conduction is triggered by a gate voltage from a third electrode, it belongs to the scope of three-terminal devices, OFETs. For a photoactivated conductive material, if the current change on illumination is large, the material shows photoresponsive characteristics and can be utilized in photodetectors. If the formed excitons can dissociate and accumulate in different phases followed by extraction at the electrodes, the materials can be exploited in organic solar cells. For electroactive COFs showing conduction, if the COFs: (i) reversibly trap ions/charges under voltage switching, the device behaves as a memory device for signal storage; (ii) interact with chemicals causing a current change, the device acts as a sensor for chemical detection; or (iii) change light absorption due to an electrochemical redox reaction, it can be used in an EC device. Furthermore, an electroactive and emissive COF can be integrated as the active layer in an OLED. Being electrically conductive, meaning that the material can transfer charges under a bias voltage, is the most fundamental property of electroactive materials. Considerable effort has been devoted to construct conductive COFs, and achievements are marked by continuously increasing conductivities (Table 1). From recent research, some design rules for highly conductive COFs can be extracted (Box 2). In the following section, we expand on these strategies.Table 1Comparison of electrical conductivities for COFs reported to date and electrical conductivities of some representative MOFs and polymersMaterial typeMaterialConductivity (S m−1)MethodSample typeRefs2D COFI2@TTF-Ph-COFI2@TTF-Py-COF10−310−4Two probePellet[10.Jin S. et al.Two-dimensional tetrathiafulvalene covalent organic frameworks: towards latticed conductive organic salts.Chem. Eur. J. 2014; 20: 14608-14613Crossref PubMed Scopus (115) Google Scholar]I2@TTF-COF1.8 × 10−4Two probePellet[8.Ding H. et al.A tetrathiafulvalene-based electroactive covalent organic framework.Chem. Eur. J. 2014; 20: 14614-14618Crossref PubMed Scopus (123) Google Scholar]TTF-COFI2@TTF-COF1.2 × 10−40.28Two probeFilm[9.Cai S.-L. et al.Tunable electrical conductivity in oriented thin films of tetrathiafulvalene-based covalent organic framework.Chem. Sci. 2014; 5: 4693-4700Crossref Google Scholar]TTF-DMTA1.8 × 10−4Two probeFilm[25.Cai S. et al.Reversible interlayer sliding and conductivity changes in adaptive tetrathiafulvalene-based covalent organic frameworks.ACS Appl. Mater. Interfaces. 2020; 12: 19054-19061Crossref PubMed Scopus (24) Google Scholar]COF-DC-8I2@COF-DC-82.51 × 10−3~1Four probePellet[12.Meng Z. et al.Two-dimensional chemiresistive covalent organic framework with high intrinsic conductivity.J. Am. Chem. Soc. 2019; 141: 11929-11937Crossref PubMed Scopus (164) Google Scholar]POR-COFI2@POR-COF4.6 × 10−91.52 × 10−5Two probePellet[15.Nath B. et al.A new azodioxy-linked porphyrin-based semiconductive covalent organic framework with I2 doping-enhanced photoconductivity.CrystEngComm. 2016; 18: 4259-4263Crossref Google Scholar]TAPP−TFPP−COFI2@TAPP−TFPP−COF1.12 × 10−81.46 × 10−5Two probePellet[26.Xu X. et al.Semiconductive porphyrin-based covalent organic frameworks for sensitive near-infrared detection.ACS Appl. Mater. Interfaces. 2020; 12: 37427-37434Crossref PubMed Scopus (35) Google Scholar]I2@TANG-COF1Four probePellet[18.Lakshmi V. et al.A two-dimensional poly(azatriangulene) covalent organic framework with semiconducting and paramagnetic states.J. Am. Chem. Soc. 2020; 142: 2155-2160Crossref PubMed Scopus (40) Google Scholar]WBDT[email protected]2.70 × 10−43.67vdPaAbbreviation: vdP, van der Pauw.Pellet[19.Rotter J.M. et al.Highly conducting Wurster-type twisted covalent organic frameworks.Chem. Sci. 2020; 11: 12843-12853Crossref PubMed Google Scholar]1-S1-Se1-Te3.7(± 0.4) × 10−88.4 (± 3.8) × 10−71.3 (± 0.1) × 10−5Two probePellet[20.Duhović S. Dincă M. Synthesis and electrical properties of covalent organic frameworks with heavy chalcogens.Chem. Mater. 2015; 27: 5487-5490Crossref Scopus (77) Google Scholar]BDT-COF1~5 × 10−5Two probeFilm[21.Medina D.D. et al.Directional charge-carrier transport in oriented benzodithiophene covalent organic framework thin films.ACS Nano. 2017; 11: 2706-2713Crossref PubMed Scopus (87) Google Scholar]I2@sp2c-COF7.1 × 10−2Two probePellet[23.Jin E. et al.Two-dimensional sp2 carbon-conjugated covalent organic frameworks.Science. 2017; 357: 673-676Crossref PubMed Scopus (533) Google Scholar]PyVg-COF0.4Two probeFilm[27.Wang L. et al.A highly soluble, crystalline covalent organic framework compatible with device implementation.Chem. Sci. 2019; 10: 1023-1028Crossref PubMed Google Scholar][email protected]110Two probePellet[5.Yang Y. et al.The effects of side chains on the charge mobilities and functionalities of semiconducting conjugated polymers beyond solubilities.Adv. Mater. 2019; 311903104Crossref PubMed Scopus (93) Google Scholar]3D COFI2@JUC-5182.7 × 10−2 (25oC)1.4 (120oC)Two probePellet[11.Li H. et al.Three-dimensional tetrathiafulvalene-based covalent organic frameworks for tunable electrical conductivity.J. Am. Chem. Soc. 2019; 141: 13324-13329Crossref PubMed Scopus (93) Google Scholar][email protected]3.4Two probeFilm[29.Yang Y. et al.A highly conductive all-carbon linked 3D covalent organic framework film.Small. 2021; 17e2103152Crossref PubMed Scopus (2) Google Scholar]2D MOF{[Cu2(6-Hmna)(6-mn)]NH4}n1096Four probeCrystal[33.Pathak A. et al.Integration of a (–Cu–S–)n plane in a metal–organic framework affords high electrical conductivity.Nat. Commun. 2019; 10: 1721Crossref PubMed Scopus (83) Google Scholar]Ni3(HITP)25540Four probePellet[34.Chen T. et al.Continuous electrical conductivity variation in M3(hexaiminotriphenylene)2 (M = Co, Ni, Cu) MOF alloys.J. Am. Chem. Soc. 2020; 142: 12367-12373Crossref PubMed Scopus (76) Google Scholar][Ag5(C6S6)]n25 000Four probePellet[35.Huang X. et al.Highly conducting neutral coordination polymer with infinite two-dimensional silver–sulfur networks.J. Am. Chem. Soc. 2018; 140: 15153-15156Crossref PubMed Scopus (68) Google Scholar]PolymerPEDOT:PSS(H2SO4 treated)438 000Four probeFilm[37.Kim N. et al.Highly conductive PEDOT:PSS nanofibrils induced by solution-processed crystallization.Adv. Mater. 2014; 26: 2268-2272Crossref PubMed Scopus (677) Google Scholar]PBTTT(FeCl3 doped)100 000vdPaAbbreviation: vdP, van der Pauw.Film[38.Jacobs, I.E. et al. High-efficiency ion-exchange doping of conducting polymers. Adv. Mater. Published online August 21, 2021. https://onlinelibrary.wiley.com/doi/full/10.1002/adma.202102988Google Scholar]a Abbreviation: vdP, van der Pauw. Open table in a new tab Box 2Key elements for conductive COFsAs a category of organic materials, COFs usually have a common wide bandgap that is too large to allow electrons to be thermally promoted from the valence band to the conduction band under ambient conditions, and thus show electrical insulating properties. To endow the COF with metallic electrical conductivity, three elements are important: electroactive moieties (Figure IA), activating factors (Figure IB), and conduction channels (Figure IC). Specifically, the electroactive moieties introduced into frameworks act as functional sites ready to produce mobile charge carriers. These functional moieties are further activated by activating factors (e.g., heat, light, dopants) to form active sites that provide mobile charges. Finally, the mobile charges need coherent conduction channels for efficient transportation. The optimization of all three elements cooperatively contributes to the conductivity. As a category of organic materials, COFs usually have a common wide bandgap that is too large to allow electrons to be thermally promoted from the valence band to the conduction band under ambient conditions, and thus show electrical insulating properties. To endow the COF with metallic electrical conductivity, three elements are important: electroactive moieties (Figure IA), activating factors (Figure IB), and conduction channels (Figure IC). Specifically, the electroactive moieties introduced into frameworks act as functional sites ready to produce mobile charge carriers. These functional moieties are further activated by activating factors (e.g., heat, light, dopants) to form active sites that provide mobile charges. Finally, the mobile charges need coherent conduction channels for efficient transportation. The optimization of all three elements cooperatively contributes to the conductivity. The electroactive moieties in COFs endow electrical conductivity to the bulk material. Efficient electroactive moieties are usually conjugated functional building blocks that can be easily oxidized or reduced to form a stable open shell species (i.e., a radical cation or anion), which serves as a source for mobile charge carriers. Due to the predesigned synthesis for the construction of COFs, these functional building blocks can be easily introduced into the framework structure with precisely controlled density and order (see Figure 1 for examples). To be conductive, the synthesized electroactive COFs need to be activated. This involves the removal of electrons in the valence band and/or the addition of electrons to the conduction band. Methods of activation include thermal excitation, photoexcitation, and doping. Among activating factors, doping is the most efficient way to create a large amount of charge carriers. Tetrathiafulvalene (TTF) is a widely utilized electroactive building block to make conductive COFs due to its strong electron-donating ability [8.Ding H. et al.A tetrathiafulvalene-based electroactive covalent organic framework.Chem. Eur. J. 2014; 20: 14614-14618Crossref PubMed Scopus (123) Google Scholar]. When TTF is introduced into COFs, it can be oxidized into highly ordered radical cations by the addition of a dopant [9.Cai S.-L. et al.Tunable electrical conductivity in oriented thin films of tetrathiafulvalene-based covalent organic framework.Chem. Sci. 2014; 5: 4693-4700Crossref Google Scholar,10.Jin S. et al.Two-dimensional tetrathiafulvalene covalent organic frameworks: towards latticed conductive organic salts.Chem. Eur. J. 2014; 20: 14608-14613Crossref PubMed Scopus (115) Google Scholar], resulting in the partially filled band structure that is necessary for a conductive behavior. TTF-based 2D [9.Cai S.-L. et al.Tunable electrical conductivity in oriented thin films of tetrathiafulvalene-based covalent organic framework.Chem. Sci. 2014; 5: 4693-4700Crossref Google Scholar,10.Jin S. et al.Two-dimensional tetrathiafulvalene covalent organic frameworks: towards latticed conductive organic salts.Chem. Eur. J. 2014; 20: 14608-14613Crossref PubMed Scopus (115) Google Scholar] (Figure 1A) and 3D [11.Li H. et al.Three-dimensional tetrathiafulvalene-based covalent organic frameworks for tunable electrical conductivity.J. Am. Chem. Soc. 2019; 141: 13324-13329Crossref PubMed Scopus (93) Google Scholar] COFs (Figure 1B) both show electrical conductivity after doping, indicated by linear I–V characteristics, with conductivity as high as 1.4 S m−1 at 120°C [11.Li H. et al.Three-dimensional tetrathiafulvalene-based covalent organic frameworks for tunable electrical conductivity.J. Am. Chem. Soc. 2019; 141: 13324-13329Crossref PubMed Scopus (93) Google Scholar]. Phthalocyanines and porphyrins both feature cyclic conjugation involving 18e− and belong to an active category of building blocks used in electroactive materials. Phthalocyanine/porphyrin-based 2D COFs can be thermally excited [12.Meng Z. et al.Two-dimensional chemiresistive covalent organic framework with high intrinsic conductivity.J. Am. Chem. Soc. 2019; 141: 11929-11937Crossref PubMed Scopus (164) Google Scholar,13.Wan S. et al.Covalent organic frameworks with high charge carrier mobility.Chem. Mater. 2011; 23: 4094-4097Crossref Scopus (510) Google Scholar], photoexcited [14.Ding X. et al.Synthesis of metallophthalocyanine covalent organic frameworks that exhibit high carrier mobility and photoconductivity.Angew. Chem. Int. Ed. 2011; 50: 1289-1293Crossref PubMed Scopus (387) Google Scholar], or doping activated [15.Nath B. et al.A new azodioxy-linked porphyrin-based semiconductive covalent organic framework with I2 doping-enhanced photoconductivity.CrystEngComm. 2016; 18: 4259-4263Crossref Google Scholar] to gain conductivity. Interestingly, the charge-carrier type is tunable by changing the coordinated central metal of the phthalocyanine/porphyrin (Figure 1C) [16.Feng X. et al.High-rate charge-carrier transport in porphyrin covalent organic frameworks: switching from hole to electron to ambipolar conduction.Angew. Chem. Int. Ed. 2012; 51: 2618-2622Crossref PubMed Scopus (289) Google Scholar,17.Ding X. et al.Conducting metallophthalocyanine 2D covalent organic frameworks: the role of central metals in controlling π-electronic functions.Chem. Commun. 2012; 48: 8952-8954Crossref PubMed Scopus (107) Google Scholar]. There are also other excellent recently introduced building blocks [18.Lakshmi V. et al.A two-dimensional poly(azatriangulene) covalent organic framework with semiconducting and paramagnetic states.J. Am. Chem. Soc. 2020; 142: 2155-2160Crossref PubMed Scopus (40) Google Scholar, 19.Rotter J.M. et al.Highly conducting Wurster-type twisted covalent organic frameworks.Chem. Sci. 2020; 11: 12843-12853Crossref PubMed Google Scholar, 20.Duhović S. Dincă M. Synthesis and electrical properties of covalent organic frameworks with heavy chalcogens.Chem. Mater. 2015; 27: 5487-5490Crossref Scopus (77) Google Scholar, 21.Medina D.D. et al.Directional charge-carrier transport in oriented benzodithiophene covalent organic framework thin films.ACS Nano. 2017; 11: 2706-2713Crossref PubMed Scopus (87) Google Scholar] with strong electron-donating properties that give the TANG (Figure 1D) [18.Lakshmi V. et al.A two-dimensional poly(azatriangulene) covalent organic framework with semiconducting and paramagnetic states.J. Am. Chem. Soc. 2020; 142: 2155-2160Crossref PubMed Scopus (40) Google Scholar] and WBDT (Figure 1E) COFs [19.Rotter J.M. et al.Highly conducting Wurster-type twisted covalent organic frameworks.Chem. Sci. 2020; 11: 12843-12853Crossref PubMed Google Scholar] high electrical conductivities of 1 S m−1 and 3.67 S m−1, respectively, after doping. To improve the doping efficiency for electroactive COFs, the selection of the matching dopant is important. It is generally difficult to predict which dopant will give the best result. Dopant screening is therefore necessary for COF materials. For instance, among dopants such as SbCl5, I2, and F4TCNQ, the latter has the best doping efficiency for WBTD (Figure 1E) [19.Rotter J.M. et al.Highly conducting Wurster-type twisted covalent organic frameworks.Chem. Sci. 2020; 11: 12843-12853Crossref PubMed Google Scholar]. Having a large uptake of dopants is also important to ensure that all active sites are doped. Fortunately, COF materials have the natural advantage of high porosity, exemplified by the TANG-COF showing close-to-complete doping of active sites using I2 [18.Lakshmi V. et al.A two-dimensional poly(azatriangulene) covalent organic framework with semiconducting and paramagnetic states.J. Am. Chem. Soc. 2020; 142: 2155-2160Crossref PubMed Scopus (40) Google Scholar]. In this respect, 3D COFs have the potential to achieve higher dopant capture as they exhibit greater porosity than 2D COFs (Figure 1F) [22.Wang C. et al.A 3D covalent organic framework with exceptionally high iodine capture capability.Chem. Eur. J. 2018; 24: 585-589Crossref PubMed Scopus (151) Google Scholar]. It is worth mentioning that different types of activation sometimes can be overlapped to generate better performance [12.Meng Z. et al.Two-dimensional chemiresistive covalent organic framework with high intrinsic conductivity.J. Am. Chem. Soc. 2019; 141: 11929-11937Crossref PubMed Scopus (164) Google Scholar,15.Nath B. et al.A new azodioxy-linked porphyrin-based semiconductive covalent organic framework with I2 doping-enhanced photoconductivity.CrystEngComm. 2016; 18: 4259-4263Crossref Google Scholar]. Once mobile charges are formed, extended conduction channels are needed for efficient charge transfer. Within COF materials, charge transfer occurs through bonds and through space (Box 2 and Figure 1C). For 2D COFs, the bond channel is constructed by in-plane conjugation within individual COF layers. Thus, enhancing the overall in-plane delocalization facilitates through-bond transfer [23.Jin E. et al.Two-dimensional sp2 carbon-conjugated covalent organic frameworks.Science. 2017; 357: 673-676Crossref PubMed Scopus (533) Google Scholar,24.Kim S. Choi H.C. Light-promoted synthesis of highly-conjugated crystalline covalent organic framework.Commun. Chem. 2019; 2: 60Crossref Scopus (49) Google Scholar]. An example of this is sp2c-COF (Figure 1G), which has a structure sp2 showing a conductivity of × 10−2 S m−1 after I2 doping [23.Jin E. et al.Two-dimensional sp2 carbon-conjugated covalent organic frameworks.Science. 2017; 357: 673-676Crossref PubMed Scopus (533) Google Scholar]. The space channel is constructed by between different layers (Figure Thus, enhancing interlayer by the layer and/or a sliding of layers facilitates transfer S. et al.Two-dimensional tetrathiafulvalene covalent organic frameworks: towards latticed conductive organic salts.Chem. Eur. J. 2014; 20: 14608-14613Crossref PubMed Scopus (115) Google Scholar]. For instance, COFs with an shows higher conductivity than S. et al.Reversible interlayer sliding and conductivity changes in adaptive tetrathiafulvalene-based covalent organic frameworks.ACS Appl. Mater. Interfaces. 2020; 12: 19054-19061Crossref PubMed Scopus (24) Google Scholar]. Fortunately, COF materials usually have a ability to in the most X. et al.Semiconductive porphyrin-based covalent organic frameworks for sensitive near-infrared detection.ACS Appl. Mater. Interfaces. 2020; 12: 37427-37434Crossref PubMed Scopus (35) Google Scholar]. It is worth mentioning that the bond channel and space channel usually to the conductivity, resulting in conductivities for 2D COFs [18.Lakshmi V. et al.A two-dimensional poly(azatriangulene) covalent organic framework with semiconducting and paramagnetic states.J. Am. Chem. Soc. 2020; 142: 2155-2160Crossref PubMed Scopus (40) Google L. et al.A highly soluble, crystalline covalent organic framework compatible with device implementation.Chem. Sci. 2019; 10: 1023-1028Crossref PubMed Google S. et and synthesis of two-dimensional covalent organic frameworks with of ambipolar 2019; Google Scholar]. This is the in-plane electron delocalization and create different types of conductive corresponding to through-bond and transfer respectively. There are two to efficient conduction is the of COF into in a material with and even which the mobile charges and even the conduction This can be by to make COF films D.D. et al.Directional charge-carrier transport in oriented benzodithiophene covalent organic framework thin films.ACS Nano. 2017; 11: 2706-2713Crossref PubMed Scopus (87) Google L. et al.A highly soluble, crystalline covalent organic framework compatible with device implementation.Chem. Sci. 2019; 10: 1023-1028Crossref PubMed Google Scholar]. For instance, films in a show high with the conductivity S m−1 after doping by Y. et al.A highly conductive all-carbon linked 3D covalent organic framework film.Small. 2021; 17e2103152Crossref PubMed Scopus (2) Google Scholar]. are the or in the framework R. the of and chemical doping on

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Various molecular design strategies of covalent organic frameworks (COFs) are employed to enable highly efficient and selective photocatalytic reduction of CO2 for carbon neutralization and the production of value‒added chemical products. Instead of frequently‒studied variation in main frameworks of COFs, side‒chain engineering is adopted in this study to tailor their photocatalytic CO2 reduction performance. Alkyl side chains with different lengths are attached to benzo[d][1,2,3]triazole‒based β‒ketoenamine COFs. It is found that alkyl side chains can alter the properties of the as‒synthesized COFs, including interlayer stacking, crystallinity, specific surface area, light harvesting and charge transfer behavior. After loading Co2+, COFs featuring a moderate ethyl side chain length exhibit superior photocatalytic performance compared to those with shorter methyl or longer butyl side chains. The CO production rate of 21.74mmol g-1 h-1 and apparent quantum yield of 13.3% rank at the top among COFs-based photocatalytic systems. This study may not only help to get in‒depth understanding of photocatalytic mechanism of COFs, but also offer an alternative approach for achieving efficient and selective photocatalytic CO2 reduction.

Covalent organic frameworks (COFs) are emerging as a transformative platform for photocatalytic hydrogen peroxide (H2O2) production due to their highly ordered structures, intrinsic porosity, and molecular tunability. Despite their potential, the inefficient utilization of photogenerated charge carriers in COFs significantly restrains their photocatalytic efficiency. This study presents two regioisomeric COFs, α-TT-TDAN COF and β-TT-TDAN COF, both incorporating thieno[3,2-b]thiophene moieties, to investigate the influence of regioisomerism on the excited electron distribution and its impact on photocatalytic performance. The β-TT-TDAN COF demonstrates a remarkable solar-to-chemical conversion efficiency of 1.35%, outperforming its α-isomeric counterpart, which is merely 0.44%. Comprehensive spectroscopic and computational investigations reveal the critical role of subtle substitution change in COFs on their electronic properties. This structural adjustment intricately connects molecular structure to charge dynamics, enabling precise regulation of electron distribution, efficient charge separation and transport, and localization of excited electrons at active sites. Moreover, this finely tuned interplay significantly enhances the efficiency of the oxygen reduction reaction. These findings establish a new paradigm in COF design, offering a molecular-level strategy to advance COFs and reticular materials toward highly efficient solar-to-chemical energy conversion.

Vinyl-functionalized covalent organic framework via tuning π-conjugation effectively promotes photocatalytic hydrogen evolution

Heteroaromatic molecular building blocks can be assembled in the highly ordered spatial environment of covalent organic frameworks (COFs), forming two-dimensional (2D) and 3D crystalline architectures. For example, photoactive molecular moieties such as porphyrins and other chromophores can be spatially integrated into their crystalline lattice, allowing us to create models for organic bulk heterojunctions, chemical sensors and porous electrodes for photoelectrochemical systems.Here, we will discuss different strategies aimed at creating electroactive networks capable of light-induced and electrochemical charge transfer. In earlier work, we have developed COF-based heterojunctions containing stacked thienothiophene-based building blocks acting as electron donors with a 3 nm open pore system, which show light-induced charge transfer to an intercalated fullerene acceptor phase.[1] We now create interpenetrated donor-acceptor COF phases with novel dibenzochrysene-based building blocks that can be viewed as constrained propeller-shaped tetraphenylethylene with reduced curvature engaging in very tight π-stacking.[2] Thienothiophene- (TT) and benzodithiophene-2,6-dicarboxaldehyde based 2D kagome COFs were synthesized in situ with a fullerene derivative to create interpenetrated electron-donor/acceptor host-guest systems showing efficient charge transfer within ps from the COF to the fullerene guest.Contrasting the above approach, one can design COF integrated heterojunctions consisting of alternating columns of stacked donor and acceptor molecules, promoting the photo-induced generation of mobile charge carriers inside the COF network.[3] Synthetic efforts have led to several COFs integrating extended chromophores capable of efficient harvesting of visible and near infrared light, for example.[4] Notably, heterocyclic regioisomers that can be embedded in the same COF crystal structure allow for fine-tuning of optical absorption and luminescence.[5]Extending thin film growth methodology to create a solvent-stable oriented 2D COF photoabsorber structure, COF films can serve in photoelectrochemical water splitting systems.[6] The detailed mechanism of excited state dynamics in light-harvesting conjugated COFs has been revealed by means of transient absorption spectroscopy,[7] while ongoing work establishes efficient excited state diffusion – even across grain boundaries – in 2D COF thin films. Many optoelectronic applications of COFs depend on significant electrical conductivity. Here, Wurster-type structural motifs are attractive building blocks for imparting high conductivity in the corresponding COFs,[8] which feature tunable optical properties upon integrating donor-acceptor moieties. COF films can also act as ultrafast solvatochromic chemical sensors,[9] as photodetectors,[10] and show very efficient electrochromic response.[11]While guest-responsive (breathing) MOFs have been studied for some time, to date few guest-responsive COF structures have been described. We have now developed dynamic two-dimensional COFs that can open and close their pores upon uptake or removal of guests while fully retaining their crystalline long-range order.[12] Here, a wine rack design based on rigid, π-stacked columns of perylene diimides (PDIs) interconnected by non-stacked, flexible bridges shows stepwise phase transformations between contracted pore and open pore forms with up to 40% increase in unit cell volume. By reversibly tuning π-interactions via guest sorption, we modulate excitonic coupling within the COFs, where the PDI moieties can be switched from excimer-forming H-aggregates to null-aggregates with monomer-like absorption and emission characteristics. These findings open new pathways towards designing stimuli-responsive optical, electronic, or spintronic materials.Ongoing work focuses on the design of ultra-large pore donor-acceptor COFs with extended light-harvesting abilities and optimized charge separation, illustrating their intriguing structural diversity leading to enhanced optoelectronic functionality.[13]References Dogru et al., Chem. Int. Ed. 2013, 52, 2920.Xue et al., to be submitted Calik et al., J. Am. Chem. Soc. 2014, 136, 17802.Keller et al., J. Am. Chem. Soc. 2017, 139, 8194.Guntermann et al., 2024, in revision. Sick et al., J. Am. Chem. Soc. 2018, 140, 2085.Jakowetz et al., J. Am. Chem. Soc. 2019, 141, 11565.Rotter et al., Chem. Sci. 2020, 11, 12843.Ascherl et al., Nature Commun. 2018, 9, 3802.Bag et al., J. Am. Chem. Soc. 2023, 145, 1649.Muggli et al., ACS Nanosci. Au 2023, 3, 153.Auras et al., Nature Chem. 2024, 16, 1373. Blätte, Ortmann, Bein, J. Am. Chem. Soc., 2024, 146, 32161.

When new covalent organic frameworks (COFs) are designed, the main efforts are typically focused on selecting specific building blocks with certain geometries and properties to control the structure and function of the final COFs. The nature of the linkage (imine, boroxine, vinyl, etc.) between these building blocks naturally also defines their properties. However, besides the linkage type, the orientation, i.e., the constitutional isomerism of these linkages, has rarely been considered so far as an essential aspect. In this work, three pairs of constitutionally isomeric imine-linked donor-acceptor (D-A) COFs are synthesized, which are different in the orientation of the imine bonds (D-C=N-A (DCNA) and D-N=C-A (DNCA)). The constitutional isomers show substantial differences in their photophysical properties and consequently in their photocatalytic performance. Indeed, all DCNA COFs show enhanced photocatalytic H2 evolution performance than the corresponding DNCA COFs. Besides the imine COFs shown here, it can be concluded that the proposed concept of constitutional isomerism of linkages in COFs is quite universal and should be considered when designing and tuning the properties of COFs.

Covalent organic frameworks (COFs) represent an emerging class of organic photocatalysts. However, their complicated structures lead to indeterminacy about photocatalytic active sites and reaction mechanisms. Herein, we use reticular chemistry to construct a family of isoreticular crystalline hydrazide-based COF photocatalysts, with the optoelectronic properties and local pore characteristics of the COFs modulated using different linkers. The excited state electronic distribution and transport pathways in the COFs are probed using a host of experimental methods and theoretical calculations at a molecular level. One of our developed COFs (denoted as COF-4) exhibits a remarkable excited state electron utilization efficiency and charge transfer properties, achieving a record-high photocatalytic uranium extraction performance of ~6.84 mg/g/day in natural seawater among all techniques reported so far. This study brings a new understanding about the operation of COF-based photocatalysts, guiding the design of improved COF photocatalysts for many applications.

Optimizing the electronic structure of covalent organic framework (COF) photocatalysts is essential for maximizing photocatalytic activity. Herein, we report an isoreticular family of multivariate COFs containing chromenoquinoline rings in the COF structure and electron‐donating or withdrawing groups in the pores. Intramolecular donor‐acceptor (D‐A) interactions in the COFs allowed tuning of local charge distributions and charge carrier separation under visible light irradiation, resulting in enhanced photocatalytic performance. By optimizing the optoelectronic properties of the COFs, a photocatalytic uranium extraction efficiency of 8.02 mg/g/day was achieved using a nitro‐functionalized multicomponent COF in natural seawater, exceeding the performance of all COFs reported to date. Results demonstrate an effective design strategy towards high‐activity COF photocatalysts with intramolecular D‐A structures not easily accessible using traditional synthetic approaches.

Optimizing the electronic structure of covalent organic framework (COF) photocatalysts is essential for maximizing photocatalytic activity. Herein, we report an isoreticular family of multivariate COFs containing chromenoquinoline rings in the COF structure and electron-donating or withdrawing groups in the pores. Intramolecular donor-acceptor (D-A) interactions in the COFs allowed tuning of local charge distributions and charge carrier separation under visible light irradiation, resulting in enhanced photocatalytic performance. By optimizing the optoelectronic properties of the COFs, a photocatalytic uranium extraction efficiency of 8.02 mg/g/day was achieved using a nitro-functionalized multicomponent COF in natural seawater, exceeding the performance of all COFs reported to date. Results demonstrate an effective design strategy towards high-activity COF photocatalysts with intramolecular D-A structures not easily accessible using traditional synthetic approaches.

Since the linkages structured in covalent organic frameworks (COFs) usually impact the charge transfer behavior during photocatalytic hydrogen evolution reaction (pc-HER), linkage dependence on charge transfer kinetics should be further claimed. Herein, COFs with N-based linkages and pyrene-based building nodes are constructed to enable us to obtain new clues about the charge transfer behavior and evolution tendency relevant to linkages at a molecular level for pc-HER. It is demonstrated that photo-excited electrons preferably move to the N sites in C=N linkage for pc-HER and are trapped around NN linkage as well. A high electron transfer rate does not point to high photocatalytic activity directly, while a small difference between the electron transfer rate and electron recombination rate ΔkCT - CR predicts the inefficiency of charge transfer in Azod-COFs. Contrarily, large value of ΔkCT - CR in the case of Benzd-COFs, demonstrats an unimpeded charge transfer process to result in boosted pc-HER rate (2027.3 µmol h-1 g-1 ). This work offers a prominent strategy for the reasonable design of efficient photocatalysts at the molecular level for structural regulation and achieves an efficient charge transfer process for the pc-HER process.

Recent progress in designing heterogeneous COFs with the photocatalytic performance

Many aspects of the correlation between the physical structure, light harvesting, and excitonic properties of covalent organic frameworks (COFs) remain unclear despite being key properties determining their photocatalytic function. One area of COF research that could bring clarity is using both electronic structure theory and time-resolved spectroscopic analysis over a series of systematically varied COFs. Here, we show structure–property relationships between four imine COFs built from a combination of ditopic and tritopic monomers using transient absorption spectroscopy together with time-dependent density functional theory. We find that monomer selection only moderately affects the charge transfer (CT) behavior of the COFs. Instead, we infer that imine chemistry profoundly impacts CT by acting as a CT mediator. Moreover, we discover two distinct valence bands arising from varying degrees of locally excited/CT mixing, which is responsible for energy-dependent exciton dynamics. Finally, we use theory to hypothesize that interlayer interactions can modify excitonic properties that we correlate with tail states commonly observed but rarely investigated in COFs. These results reveal that imine chemistry should be recognized as a very important factor to consider in the development of COF photocatalysts and the correlation of their structural environment with light-harvesting and CT properties that should ultimately determine their photocatalytic function.