Covalent organic frameworks for fluorescent sensing: Recent developments and future challenges

Sol-gel processing of a covalent organic framework for the generation of hierarchically porous monolithic adsorbents

Two-dimensional (2D) covalent organic frameworks (COFs) are composed of structurally precise, permanently porous, layered macromolecular sheets, which are traditionally synthesized as polycrystalline solids with crystalline domain lengths smaller than 100 nm. Here, we polymerize imine-linked 2D COFs as suspensions of faceted single crystals in as little as 5 min at moderate temperature and ambient pressure. Single crystals of two imine-linked 2D COFs were prepared, consisting of a rhombic 2D COF (TAPPy-PDA) and a hexagonal 2D COF (TAPB-DMPDA). The sizes of TAPPy-PDA and TAPB-DMPDA crystals were tuned from 720 nm to 4 μm and 450 nm to 20 μm in width, respectively. High-resolution transmission electron microscopy revealed that the COF crystals consist of layered, 2D polymers comprising single-crystalline domains. Continuous rotation electron diffraction resolved the unit cell and crystal structure of both COFs, which are single-crystalline in the a-b plane but disordered in the stacking c dimension. Single crystals of both COFs were incorporated into gas chromatography separation columns and exhibited unusual selective retention of cyclohexane over benzene, with single-crystalline TAPPy-PDA significantly outperforming single-crystalline TAPB-DMPDA. Polycrystalline TAPPy-PDA exhibited no separation, while polycrystalline TAPB-DMPDA exhibited poor separation and the opposite order of elution, retaining benzene more than cyclohexane, indicating the importance of improved material quality for COFs to exhibit properties that derive from their precise, crystalline structures. This work represents the first example of synthesizing imine-linked 2D COF single crystals at ambient pressure and short reaction times and demonstrates the promise of high-quality COFs for molecular separations.

Mechanistic understanding into the formation and growth of imine-linked two-dimensional (2D) covalent organic frameworks (COFs) is needed to improve their materials quality and access larger crystallite sizes, both of which limit the promise of 2D COFs and 2D polymerization techniques. Here we report a previously unknown temperature-dependent depolymerization of colloidal 2D imine-linked COFs, which offers a new means to improve their crystallinity. 2D COF colloids form at room temperature but then depolymerize when their reaction mixtures are heated to 90 °C. As the solutions are cooled back to room temperature, the 2D COFs repolymerize and crystallize with improved crystallinity and porosity, as characterized by X-ray diffraction, infrared spectroscopy and N2 porosimetry. The evolution of COF crystallinity during the solvothermal depolymerization and repolymerization processes was characterized by in situ wide angle X-ray scattering, and the concentrations of free COF monomers as a function of temperature were quantified by variable temperature 1H NMR spectroscopy. The ability of a 2D COF to depolymerize under these conditions depends on both the identity of the COF and its initial materials quality. For one network formed at room temperature (TAPB-PDA COF), a first depolymerization process is nearly complete, and the repolymerization yields materials with dramatically enhanced crystallinity and surface area. Already recrystallized materials partially depolymerize upon heating their reaction mixtures a second time. A related 2D COF (TAPB-DMTA COF) forms initially with improved crystallinity compared to TAPB-PDA COF and then partially depolymerizes upon heating. These results suggest that both high materials quality and network-dependent properties, such as interlayer interaction strength, influence the extent to which 2D COFs resist depolymerization. These findings offer a new means to recrystallize or solvent anneal 2D COFs and may ultimately inform crystallization conditions for obtaining large-area imine-linked two-dimensional polymers from solution.

ConspectusAs one of the most attractive members in the porous materials family, covalent organic frameworks (COFs) have been reported thousands of times since their first discovery in 2005, covering their design, synthesis, and applications. However, an overwhelming majority of these COFs are based on two-dimensional (2D) topologies while three-dimensional (3D) COFs are numbered fewer than 100 up to date. In fact, baring enhanced specific surface area, interconnected channels, well-exposed functional moieties, and highly adjustable structures, 3D COFs are often more competitive in various application fields like adsorption, separation, chemical sensing, and heterogeneous catalysis compared with their 2D counterparts. However, significant crystallization problems and poor chemical stabilities, which might be attributed to the highly void frameworks and the absence of π-π stacking, have raised severe limitations over the research and application of 3D COFs. To solve these problems, more elaborate synthesis regulations or more moderate functionalization conditions are required. More importantly, the strategies for enhancing chemical stabilities of 3D COFs are of vital importance for their further development and practical applications.In this Account, we review the design principles, functional approaches, and stability regulation methods toward functional 3D COFs. We begin the discussion with some essential elements in the construction of 3D COF structures, including topologies, interpenetrations, linkages, and synthetic methods. After that, we focus on several strategies for the functionalization of 3D COFs, including in situ approaches (utilizing in situ generated COF linkages as the active sites), bottom-up synthesis (embedding functional moieties from predesigned building blocks), and postsynthesis modification (covalent modification or metalation of pristine frameworks). At last, we highlight some approaches toward the durable amplification of 3D COFs, which is highly important for framework functionalization and practical application. This target could be achieved through not only the introduction of some extra strengthening force, such as hydrophobic effects, coulomb repulsion, and steric hindrance effects, but also the utilization of robust linkages, which could enhance the stability from material nature.Due to their high surface area, various interpenetrated channels, multifarious functionalities, and promising stabilities, 3D COFs demonstrated excellent performance and have great potential in a wide range of application fields including adsorption and separation, heterogeneous catalysis, energy storage, and so on. Although the development of these materials has been limited by serious crystallization problems and stability restriction, great efforts have been devoted by researchers in the past decade, and a mass of strategies have been developed in synthesis control, functionalization regulation, and stability enhancement for 3D COFs. We expect 3D COFs to be practically utilized in the future with further advances in the design, preparation, and functionalization of these materials.

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.

Two-dimensional (2D) covalent organic frameworks (COFs) represent a unique structural paradigm owing to their intriguing properties. The diverse chemical tunability and good charge carrier mobility make these materials suitable for electronics and optoelectronic applications. However, the poor conductivity and processability restrict their full potential in device fabrication. Several strategies were followed to improve the conductivity in 2D COFs, including redox-active linkers and doping. Herein, we report four 2D COFs, by modulating the substituents on one of the monomeric units. Interestingly, less-bulky groups (H and OH) yielded a Kagome lattice in ETTA-TA- and ETTA-DHTA-COFs, whereas square-grid networks were obtained for ETTA-An-ph- and ETTA-BPTA-COFs with bulky substituents (anthracene and propargylic moieties). Further, we investigated the effect of the topology and the functionalities of these COFs on electrical conductivity. The current–voltage (I–V) measurements reveal that ETTA-DHTA-COF with a Kagome lattice exhibits higher electrical conductivity of 1.3 × 10–4 S cm–1 compared to other congeners (square-grid and Kagome-type COFs) and many reported undoped COFs. The trade-off between the frameworks’ topology (Kagome vs square-grid) and the functionalities (H, anthracene vs OH, oxy-propargyl) exhibits the synergetic role in stimulating the conductivities in the resulting COFs. This strategy of switching framework topology and the effect of functional groups on electrical conductivity unveil a unique approach to fine-tuning the electronic properties in the 2D COFs.

Rationally fabricating 3D porphyrinic covalent organic frameworks with scu topology as highly efficient photocatalysts

Nitrogen-containing linkage-bonds in covalent organic frameworks: Synthesis and applications

At present, applications of covalent organic frameworks (COFs), a class of novel crystalline porous materials fabricated by organic structural blocks only containing light elements such as C, H, N, O and B through strong covalent bonds, in electrochemical sensing have entered a period of rapid development and showed great potential in various analytical fields such as environmental analysis, food analysis, pharmaceutical analysis, biological and clinical analysis owing to their unique advantages including regular porosity, large specific surface area, periodical ordered structure, extended π‐conjugated system, abundant functional groups and outstanding thermal and chemical stability. But there are still many concerns that need to be urgently solved for further electrochemical sensing application including improvement of electric conductivity and enhancement of selectivity. Limited by poor conductivity and low recognition ability of primitive COFs, they always need to be further functionalized. To further promote the development of COFs‐based electrochemical sensing technology, here, we summarize and review the recent advances on construction of two‐dimensional functional COFs materials for electrochemical sensing applications and related sensing device setups in detail, including design strategies, preparation methods, modification considerations, sensing principles and analytical applications, and tentatively propose development prospects and outlooks in future.

Covalent organic frameworks (COFs) are periodic porous polymers with relatively high crystallinity, which have become a newly emerging family of materials. The COF materials are constructed by covalent bonds (B–O, C–C, C–N etc.) of light elements, and by using various organic building blocks we can create versatile COFs with diverse topology and functionalities. The COFs often possess the characteristics of low density, high porosity and large specific surface area, and have therefore been widely applied in gas adsorption and separation, energy chemistry and the environmental fields. To develop COF materials with appreciable functionalities, extensive experimental and theoretical techniques are being employed. In particular, multiscale simulation methods have been proven to be an efficient tool to design and investigate the properties of new COFs. This article mainly introduces two rational strategies to design new COF materials. One is linker replacement or the node replacement strategy to achieve high performance gas storage COF materials with large surface areas and pore volumes. The other is functional group modification or the metal doping strategy to design functionalized COFs with strong affinity towards gases. For the two design strategies, we also give a corresponding example for further analysis. Finally, we summarize the applications of COFs in hydrogen and methane storage, as well as CO2 capture.

Three-dimensional (3D) covalent organic frameworks (COFs) possess higher surface areas, more abundant pore channels, and lower density compared to their two-dimensional counterparts which makes the development of 3D COFs interesting from a fundamental and practical point of view. However, the construction of highly crystalline 3D COF remains challenging. At the same time, the choice of topologies in 3D COFs is limited by the crystallization problem, the lack of availability of suitable building blocks with appropriate reactivity and symmetries, and the difficulties in crystalline structure determination. Herein, we report two highly crystalline 3D COFs with pto and mhq-z topologies designed by rationally selecting rectangular-planar and trigonal-planar building blocks with appropriate conformational strains. The pto 3D COFs show a large pore size of 46 Å with an extremely low calculated density. The mhq-z net topology is solely constructed from totally face-enclosed organic polyhedra displaying a precise uniform micropore size of 1.0 nm. The 3D COFs show a high CO2 adsorption capacity at room temperature and can potentially serve as promising carbon capture adsorbents. This work expands the choice of accessible 3D COF topologies, enriching the structural versatility of COFs.

Recent advances in the construction of functionalized covalent organic frameworks and their applications to sensing

Covalent Organic Frameworks (COFs), which are frameworks composed of light atoms held together by strong covalent bonds, are generating interest as potential materials for applications such as renewable energy and gas capture. We employed Density Functional Theory (DFT) calculations, as implemented in the VASP code, to look at both 2D and 3D COFs. We systematically analyzed various properties, including structural stability, phonon dispersion, electronic structures, density of states, adsorption behavior, and mechanical properties. To get better accuracy, we took into account van der Waals interactions and even used hybrid functionals. What we found was that 3D COFs generally exhibit greater mechanical strength and, in most cases, better gas adsorption, which seems to come from their interconnected pore structures. On the other hand, 2D COFs exhibit enhanced π-electron delocalization and direct band gaps of approximately 2.5eV, which may be helpful in sensors and optoelectronics. Phonon analyses verified the dynamical stability of both structures. Ultimately, these results underscore the importance of dimensionality in tailoring COF properties for energy and electronic applications. First-principles simulations were performed using Density Functional Theory (DFT) within the Vienna Ab initio Simulation Package (VASP). To account for exchange-correlation effects, we employed the Generalised Gradient Approximation (GGA) in the Perdew-Burke-Ernzerhof (PBE) formulation, and we also utilised projector-augmented wave (PAW) pseudopotentials. Hybrid functional (HSE06) and DFT-D3 van der Waals corrections were introduced to improve the accuracy of our band gap prediction. The plane-wave cutoffs were set at 500eV for the calculations, and Monkhorst-Pack k-point meshes were used with a 3 × 3 × 1 (2D) and 2 × 2 × 2 (3D) grid. Evaluated were structural optimisations, band structures, total and projected DOS, adsorption energies, and charge transfer (using Bader analysis). Assessment of bonding features utilised the Electron Localisation Function (ELF) and charge density difference (Δρ) visualisations. The Phonopy package was used to calculate Phonon dispersions and thus confirm the dynamic stability of the COFs.

Due to their high structural tunability, remarkable internal surface areas, readily accessible pore space, and host of possible applications, covalent organic frameworks (COFs) remain at the forefront of materials science research. Unfortunately, many COFs suffer from structural distortions or pore collapse during activation, which can lead to a substantial loss of crystallinity and functionality. Thus, herein, we demonstrate a facile method to address this issue by introducing polymer guests. The polymer adheres to the COF internal pore wall, acting as a supporting pillar during activation and effectively preserving the COF porosity and crystallinity. In fact, the surface area of one COF/polymer composite, known as TAPB-TA/PDA, was boosted by a factor of 16 when compared to the parent COF, TAPB-TA. More importantly, the now robust COF structure was able to resist layer shifting and order loss during both solvent immersion and removal. The introduction of functional polymer guests not only solidifies the COF structure and preserves its high porosity but is also shown to enhance the transport and separation of photogenerated charge carriers, thereby facilitating hydrogen evolution during photocatalytic water splitting. Molecular dynamics simulations further support experimental observations that the incorporation of PDA within the COF pores reinforces the walls, preventing its collapse. The proposed mechanism is based on the adsorption of PDA oligomers along the c direction of the unit cell, fastening the COF layers in place via van der Waals interactions. This kind of interaction locks -N═CH-Ph-CH═N- units in a trans-configuration in the COF pores.

Although a lot of covalent organic frameworks (COFs) have been constructed over the past 10 years, the topologies of COFs are still quite limited. In this work, we report one-step construction of a COF with an unprecedented topology from the combination of C3-symmetrical and C2-symmetrical building blocks. It contains two types of triangular micropores of different sizes and chemical environments: one is 11.3 A and the other is 15.2 A. The structure of the dual-pore COF was confirmed by powder X-ray diffraction (PXRD) investigation, nitrogen adsorption–desorption study, and theoretical calculations.