Stacked Reticular Frame Boosted Circularly Polarized Luminescence of Chiral Covalent Organic Frameworks

Crystalline, Porous Figure-Eight-Noded Covalent Organic Frameworks

Figure-eight macrocycles represent a fascinating class of π-conjugated units characterized by unique aesthetics and non-contact molecular crossing at the center. Despite progress in synthesis over the past century, research into inorganic, organic, and polymeric figure-eight materials remains in its infancy. Here we report the first examples of figure-eight covalent organic frameworks by condensing figure-eight knots to create extended porous figure-eight π architectures. A distinct feature is that polymerization interweaves figure-eight knots into double-decker layers, which upon supramolecular polymerization organize well-defined layer frameworks. The figure-eight frameworks exhibit a band gap of 2.3 eV and emit bright orange florescence with benchmark quantum yields. Remarkably, the donor-acceptor figure-eight skeletons convert the figure-eight knots into reduction centers and the linkers into oxidation sites upon light irradiation, enable charge transport and accumulation through π columns, while the built-in hydrophilic micropores allow rapid water and oxygen delivery via capillary effect. With these distinct features, the figure-eight frameworks function as a photocatalyst to produce hydrogen peroxide at high rate and efficiency with water/saltwater, oxygen/air, and light as sole inputs. This work paves a way to a new class of molecular frameworks, underpinning the study of well-defined figure-eight materials to explore unprecedented structures and functions so far we untouched.

Chiral Cross-Linked Covalent Organic Framework Films for Highly Sensitive Circularly Polarized Luminescence Probing

The development of covalent organic framework (COF) films featuring circular polarization luminescence (CPL) probing remains a formidable challenge. Herein, we developed a chiral cross-linked COF film to obtain uniform and dense chiral COF films (chirCOFilm) possessing highly sensitive CPL probing for enantiomers. The axial chiral cross-linkers (R-/S-BBNA) are initially introduced into the channels of a COF film (COFilm/R-BBNA or COFilm/S-BBNA) by the vapor-assisted epitaxial method. Then, olefin groups in R-/S-BBNA and COF layers undergo a chiral cross-linking reaction under UV irradiation, forming a chirCOFilm. The obtained chirCOFilms have strong chirality with mirror images, fluorescence discoloration, and intense CPL properties. A multitude of rich chiral photopatterns and chirCOFilm/PDMS flexible films are prepared taking advantage of the photochromic properties of the chirCOFilms during UV illumination, showing the potential application of advanced anticounterfeiting. More importantly, the chirCOFilms realize highly sensitive CPL probing of phenethylamine enantiomers at 5% concentration, which can hardly be achieved from their corresponding fluorescence probing. This study not only provides a new strategy for preparing chiral COF films using chiral cross-linking reaction but also opens a new avenue for achieving highly sensitive probing of enantiomers though CPL.

Generating Strong Circularly Polarized Luminescence from Self-assembled Films of Chiral Selenium Nanoparticles and Upconversion Nanoparticles

Circularly polarized luminescence (CPL) has garnered significant research attention. Achieving a high luminescence dissymmetry factor (glum) is a key challenge in this field. Herein, we reported, for the first time, the fabrication of a chiral assembled film consisting of chiral D-/L-Selenium nanoparticles (D-/L-Se NPs) and DSPE-PEG-NH2 modified upconversion nanoparticles (DPNUCNPs) with remarkable CPL properties that were generated by the interfacial self-assembly technique. The chiral Se/DPNUCNPs films, consisting of three layers (3L) of D-/L-Se films and 3L DPNUCNPs films, exhibited the highest circular dichroism (CD) response and the strongest CPL signals. Under laser excitation at 980 nm, the 3L D-/L-Se/3L DPNUCNPs assembled films displayed symmetric CPL signals between 400 and 600 nm, with a maximum |glum| value of 0.68. The interaction between DPNUCNPs and Se NPs involves energy transfer and chirality transfer, along with the formation of spin-polarized excitons, thereby resulting in CPL activity. Furthermore, the chiral Se/DPNUCNPs films were patterned for anti-counterfeit and encryption applications. Our study provides a novel guide for fabricating chiral nanomaterials with strong CPL response.

Highly Enantioselective Transportation Across Liquid Membranes Mediated by Porous Covalent Organic Frameworks

Chiral liquid membrane separation is crucial in pharmaceuticals and chemical synthesis for its simplicity and stability, yet designing membrane carriers that enable efficient enantioseparation remains a challenge. Here, we demonstrated for the first time that chiral porous materials can act as mobile carriers of bulk liquid membranes (BLMs) to enhance enantioselective transport and separation. We design and prepare three 2D chiral covalent organic frameworks (CCOFs) by imine condensations of a chiral dialdehyde with triamines containing ethyl, fluorine and/or isopropyl groups. These isostructural CCOFs feature ABC stacking, excellent water, acid and base tolerance, and chiral amine groups in 1D porous channels, promoting efficient enantioselective transportation of amino acid enantiomers. Among them, the CCOF with both -F and -iPr groups showing superior transport performance. Exfoliating the CCOF into chiral nanosheets creates flexible layers with accessible active sites, enabling nanosheet-mediated liquid membranes to separate chiral drug enantiomers, a feat unattainable with the pristine CCOF. This work establishes CCOFs as a promising platform for chiral BLM separations and will guide the design of high-performance BLMs using porous materials for enantioselective separation.

Recent Advance in the Synthesis and Applications of Chiral Covalent Organic Frameworks: A Mini-Review

Chiral covalent organic frameworks (CCOFs) are emerging porous materials with tunable chiral structures, abundant pores, and high surface areas, gaining significant attentions in separation, sensing, and asymmetric catalysis. This review summarizes the synthesis methods of CCOFs and their applications in chiral recognition. It discusses the advantages and limitations of three synthesis strategies, including chiral post-modification, direct synthesis, and chiral-induced synthesis. The review also highlights the potential of CCOFs in chiral separation, sensing, and asymmetric catalysis for efficient purification, detection, and synthesis of chiral molecules. Challenges and future directions for the preparation and application of CCOFs are also addressed, aiming to guide further research and practical applications.

Reticulating Crystalline Porous Materials for Asymmetric Heterogeneous Catalysis

Asymmetric catalysis is essential for addressing the increasing demand for enantiopure compounds. Recent advances in reticular chemistry have demonstrated that metal-organic frameworks (MOFs) and covalent organic frameworks (COFs) possess highly regular porous architectures, exceptional tunability, and the ability to incorporate chiral functionalities through their open channels or cavities. These characteristics make them highly effective and enantioselective catalysts for a wide range of asymmetric transformations. The chiral microenvironments within these frameworks facilitate precise control over reactant orientation and transition states, enhancing both catalytic activity and enantioselectivity, thereby offering significant advantages over traditional systems. This review overviews recent developments in chiral MOFs (CMOFs) and chiral COFs (CCOFs), focusing on their design strategies, and synthetic methods, and highlights the structure-property relationships that connect key structural features to asymmetric catalytic performance. Additionally, the current challenges and future prospects in this field are addressed, highlighting the pivotal role of reticular chemistry in the creation of chiral porous materials. It is anticipated that this review will inspire further research into the application of crystalline porous materials in asymmetric catalysis and promote the rational design of novel chiral heterogeneous catalysts for industrial use.

Conformational Chirality of Single-Crystal Covalent Organic Frameworks

The crystallization of organic polymers is often hindered by chiral units, hence resulting in chiral organic polymers typically existing as amorphous or partially crystalline phases such as natural rubber and cellulose. Similarly, as an emerging crystalline chiral polymer, chiral covalent organic frameworks (COFs) also inevitably face a delicate balance between chiral units and crystallization, limiting their production and applications in separation, catalysis, and optics. Here, we present a general strategy for producing a series of conformational chiral COFs with high crystallinity through breaking the meso conformation of achiral COFs. Conformational chirality of COF-300 was constructed by involving chiral amino-acid derivative templates during synthesis and was proven to have excellent thermodynamic (200 °C annealing in air) and dynamic stability (61% cell volume change). The stereochemistry of the conformational chiral crystals can be controllably tuned by chiral templates, resulting in wide-range circular dichroism signals from ultraviolet to infrared wavelengths and absorption dissymmetry factors (gabs) varying by up to 300%, with a maximum of gabs = 0.012. This strategy paves the way for stereochemistry modification, property enhancement, and exploration of new applications of crystalline chiral materials.

Photons, Excitons, and Electrons in Covalent Organic Frameworks

Covalent organic frameworks (COFs) are created by the condensation of molecular building blocks and nodes to form two-dimensional (2D) or three-dimensional (3D) crystalline frameworks. The diversity of molecular building blocks with different properties and functionalities and the large number of possible framework topologies open a vast space of possible well-defined porous architectures. Besides more classical applications of porous materials such as molecular absorption, separation, and catalytic conversions, interest in the optoelectronic properties of COFs has recently increased considerably. The electronic properties of both the molecular building blocks and their linkage chemistry can be controlled to tune photon absorption and emission, to create excitons and charge carriers, and to use these charge carriers in different applications such as photocatalysis, luminescence, chemical sensing, and photovoltaics. In this Perspective, we will discuss the relationship between the structural features of COFs and their optoelectronic properties, starting with the building blocks and their chemical connectivity, layer stacking in 2D COFs, control over defects and morphology including thin film synthesis, exploring the theoretical modeling of structural, electronic, and dynamic features of COFs, and discussing recent intriguing applications with a focus on photocatalysis and photoelectrochemistry. We conclude with some remarks about present challenges and future prospects of this powerful architectural paradigm.

Primary Amine-Functionalized Chiral Covalent Organic Framework Enables High-Efficiency Asymmetric Catalysis in Water

Aqueous asymmetric catalysis using chiral covalent organic frameworks (COFs) represents a significant advancement but remains to be explored. Herein, we present the first example of aqueous asymmetric catalysis catalyzed by a primary amine-tagged chiral D-ADP-TAPB COF. The D-ADP-TAPB COF was synthesized by the postsynthetic deprotection of D-ADP-TAPB-Boc bearing a protective tert-butoxycarbonyl (Boc) group, which was constructed by a Schiff-base reaction between an alanine-derived chiral building block (D-ADP-Boc) and 1,3,5-tris(4-aminophenyl)benzene (TAPB). The crystalline D-ADP-TAPB COF exhibits a uniform, spherical morphology with abundant, well-distributed chiral primary amines, rendering it highly active in the asymmetric aldol reaction between cyclohexanone and 4-nitrobenzaldehyde. Notably, this reaction is conducted entirely in water, achieving impressive yields and enantiomeric excess (ee) values of up to 90 and 85%, respectively. To the best of our knowledge, D-ADP-TAPB COF represents the first chiral COF catalyst with high reactivity and enantioselectivity for an asymmetric aldol reaction solely in water, eliminating the need for conventional organic solvents. Moreover, a plausible mechanism for D-ADP-TAPB COF-mediated aqueous asymmetric aldol reactions is elucidated. This work not only expands the toolbox for designing rare primary amine-functionalized chiral COFs for asymmetric catalysis but also opens exciting avenues for developing green and water-based enantioselective catalysis.

Recent Advances in the Utilization of Chiral Covalent Organic Frameworks for Asymmetric Photocatalysis

The use of light energy to drive asymmetric organic transformations to produce high-value-added organic compounds is attracting increasing interest as a sustainable strategy for solving environmental problems and addressing the energy crisis. Chiral covalent organic frameworks (COFs), as porous crystalline chiral materials, have become an important platform on which to explore new chiral photocatalytic materials due to their precise tunability, chiral structure, and function. This review highlights recent research progress on chiral COFs and their crystalline composites, evaluating their application as catalysts in asymmetric photocatalytic organic transformations in terms of their structure. Finally, the limitations and challenges of chiral COFs in asymmetric photocatalysis are discussed, with future opportunities for research being identified.

Self-Standing Chiral Covalent Organic Framework Thin Films with Full-Color Tunable Guest-Induced Circularly Polarized Luminescence

Exploring self-standing chiral covalent organic framework (COF) thin films with controllable circularly polarized luminescence (CPL) is of paramount significance but remains a challenging task. Herein, we demonstrate the first example of self-standing chiral COF films employing a polymerization-dispersion-filtration strategy. Pristine, low-quality chiral COF films were produced by interfacial polymerization and then re-dispersed into COF colloidal solutions. Via vacuum assisted assembly, these COF colloids were densely stacked and assembled into self-standing, pure chiral COF films (L-/D-CCOF-F) that were transparent, smooth, crack-free and highly crystalline. These films were tunable in thicknesses, areas, and roughness, along with strong diffuse reflectance circular dichroism (DRCD) and cyan CPL signals, showing an intrinsic luminescence asymmetric factor (glum) of ~4.3×10-3. Furthermore, these COF films served as host adsorbents to load various achiral organic dye guests through adsorption. The effective chiral transfer and energy transfer between CCOF-F and achiral fluorescent dyes endowed the dyes with strong chirality and tunable DRCD, resulting in intense, full-color-tunable solid-state CPL. Notably, the ordered arrangement of dye guest molecules within the preferentially oriented chiral pores of CCOF-F contributed to an amplified |glum| factor of up to 7.2×10-2, which is state-of-the-art for COF-based CPL materials. This work provides new insights into the design and fabrication of self-standing chiral COF films, demonstrating their great potential for chiroptical applications.

Chiral Induction in 2D Borophene Nanoplatelets through Stereoselective Boron-Sulfur Conjugation

Chirality is a structural metric that connects biological and abiological forms of matter. Although much progress has been made in understanding the chemistry and physics of chiral inorganic nanoparticles over the past decade, almost nothing is known about chiral two-dimensional (2D) borophene nanoplatelets and their influence on complex biological networks. Borophene's polymorphic nature, derived from the bonding configurations among boron atoms, distinguishes it from other 2D materials and allows for further customization of its material properties. In this study, we describe a synthetic methodology for producing chiral 2D borophene nanoplatelets applicable to a variety of structural polymorphs. Using this methodology, we demonstrate feasibility of top-down synthesis of chiral χ3 and β12 phases of borophene nanoplatelets via interaction with chiral amino acids. The chiral nanoplatelets were physicochemically characterized extensively by various techniques. Results indicated that the thiol presenting amino acids, i.e., cysteine, coordinates with borophene in a site-selective manner, depending on its handedness through boron-sulfur conjugation. The observation has been validated by circular dichroism, X-ray photoelectron spectroscopy, and 11B NMR studies. To understand how chiral nanoplatelets interact with biological systems, mammalian cell lines were exposed to them. Results showed that the achiral as well as the left- and right-handed biomimetic χ3 and β12 borophene nanoplatelets have distinct interaction with the cellular membrane, and their internalization pathway differs with their chirality. By engineering optical, physical, and chemical properties, these chiral 2D nanomaterials could be applied successfully to tuning complex biological events and find applications in photonics, sensing, catalysis, and biomedicine.

Polymers with Circularly Polarized Luminescent Properties: Design, Synthesis, and Prospects

Chiral materials with circularly polarized luminescence (CPL) have garnered significant attention owing to their distinctive luminescent properties and wide array of applications. CPL enables the selective emission of left and right circularly polarized light. The fluorescence quantum yield and dissymmetry factor play pivotal roles in the generation of CPL. Helical polymers exhibit immense promise as CPL materials due to their inherent chirality, structural versatility, modifiability, and capacity to incorporate diverse chromophores. This Review provides a brief review of the synthesis of CPL materials based on helical polymers. The CPL can be realized by aggregation-induced CPL of non-emissive helical polymers, and helices bearing chromophores on the pendants and on the chain end. Furthermore, future challenges and potential applications of CPL materials are summarized and discussed.

Observation of Chiral Channels in Helical Covalent Organic Frameworks

Unraveling the mechanism of chirality transfer across length scales is crucial to the rational development of functional materials with hierarchical chirality. The key obstacle is the lack of structural information, especially at the mesoscopic level. We report herein the structural identification of helical covalent organic frameworks (heliCOFs) with hierarchical chirality, which integrate molecular chirality, channel chirality, and morphology chirality into one crystalline entity. Specifically, benefiting from the highly ordered structure of heliCOFs, the existence of chiral channels at the mesoscopic level has been confirmed by electron crystallography, and the handedness of these chiral channels has been directly determined through the stereopair imaging technique. Accordingly, the chirality transfer in heliCOFs from microscopic to macroscopic levels could be rationalized with a layer-rotating model that has been supported by both crystal structure analysis and theoretical calculations. Observation of chiral channels in heliCOFs not only provides unprecedented data for the understanding of the chirality transfer process but also sheds new light on the rational construction of highly ordered polymeric materials with hierarchical chirality.

High Internal Phase Emulsion for Constructing Chiral Helical Polymer-Based Circularly Polarized Luminescent Porous Materials

Polymerized high internal phase emulsions (polyHIPEs) with circularly polarized luminescence (CPL), as an interesting class of porous materials, are of great significance for the development of CPL porous materials but have not been reported so far. Herein, we report the construction of polyHIPE-based CPL porous materials, taking advantage of an adsorption strategy. The pristine polyHIPEs constructed by chiral helical polymers, which acted as a chiral microenvironment, were fabricated by coordination polymerization of chiral acetylene monomers (R/S-SA) using HIPEs as templates. Achiral fluorescent small molecules were dispersed in the pores of the 3D porous organic chiral polymer matrix provided by polyHIPEs through the adsorption strategy, and CPL-active porous materials with blue, cyan, and green emissions were constructed using a fluorescence-selective absorption mechanism that does not rely on chirality transfer at the molecular level. The maximum luminescence dissymmetry factor (glum) value was -2.6 × 10-2. This work establishes a new and simple way for developing CPL porous materials.

Constructing Chiral Covalent-Organic Frameworks for Circularly Polarized Light Detection

The use of chiral covalent organic frameworks (COFs) as active elements in photodetectors to directly identify circularly polarized light (CPL) can meet the requirement of integration and miniaturization of the as-fabricated devices. Herein, the design and synthesis of two isoreticular chiral two-dimensional (2D) COFs (CityU-7 and CityU-8) by introducing photosensitive porphyrin-based amines (5,10,15,20-tetrakis(4-aminophenyl)porphyrin) to enhance the optical absorption and chiral aldehyde linkage (2,5-bis((S/R))-2-methylbutoxy)terephthalaldehyde) to engender chirality for direct CPL detection  are  reported. Their crystalline structures  were  confirmed by powder X-ray diffraction, Fourier-transform infrared spectroscopy, and low-dose transition electron microscopy. Employing both chiral COFs as the active layers in photodetectors, left-handed circularly (LHC) and right-handed circularly (RHC) polarized light at 405 nm can be well distinguishable with short response time, high responsivity, and satisfying detectivity. The study provides the first example on the design and synthesis of chiral COFs for direct detection of CPL.

Covalent Organic Frameworks with Tunable Chirality for Chiral-Induced Spin Selectivity

Chiral covalent organic frameworks (CCOFs) have attracted extensive interest for their potential applications in various enantioselective processes. However, the exploitation of chirality-induced spin selectivity (CISS) that enables a new technology for the injection of spin polarized current without the need for a permanent magnetic layer within CCOFs remains a largely untapped area of research. Here, we demonstrate that, for the first time, COFs can be an attractive platform to develop spin filter materials with efficient CISS. This facilitates the design and synthesis of a new family of Zn(salen)-based 2D CCOFs, namely, CCOFs-9-12, by imine condensation of chiral 1,2-diaminocyclohexane and tri- or tetra(salicylaldehyde) derivatives. CCOF-9, distinguished by its unique C2 symmetric "armchair" tetrasubstituted pyrene conformation, exhibits the most pronounced chirality among these materials and serves as a solid-state host, enabling the enantioselective adsorption of racemic drugs with an enantiomeric excess (ee) of up to 97%. After substituting diamagnetic zinc(II) ions for paramagnetic cobalt(II), the resulting CCOF-9-Co not only retains its high crystallinity, porosity, and exceptional chirality but also exhibits enhanced conductivity, a crucial factor for the effective observation of CISS. Magnetic conductive atomic force microscopy showed that CCOF-9-Co exhibited a remarkable CISS effect with up to an 88-94% spin polarization ratio. This phenomenon is further confirmed by the increased intensity in the magnetic circular dichroism (MCD) when CCOF-9-Co is under an external magnetic field. This work therefore shows the tremendous potential of CCOFs for controlling spin selectivity and will stimulate the creation of new types of crystalline polymers with strong CISS effects for spin filters.

Raising the Asymmetric Catalytic Efficiency of Chiral Covalent Organic Frameworks by Tuning the Pore Environment

Chiral covalent organic frameworks (COFs) hold considerable promise in the realm of heterogeneous asymmetric catalysis. However, fine-tuning the pore environment to enhance both the activity and stereoselectivity of chiral COFs in such applications remains a formidable challenge. In this study, we have successfully designed and synthesized a series of clover-shaped, hydrazone-linked chiral COFs, each with a varying number of accessible chiral pyrrolidine catalytic sites. Remarkably, the catalytic efficiencies of these COFs in the asymmetric aldol reaction between cyclohexanone and 4-nitrobenzaldehyde correlate well with the number of accessible pyrrolidine sites within the frameworks. The COF featuring nearly one pyrrolidine moiety at each nodal point demonstrated excellent reaction yields and enantiomeric excess (ee) values, reaching up to 97 and 83%, respectively. The findings not only underscore the profound impact of a deliberately controlled chiral pore environment on the catalytic efficiencies of COFs but also offer a new perspective for the design and synthesis of advanced chiral COFs for efficient asymmetric catalysis.

Stepwise Solution-Interfacial Nanoarchitectonics for Assembled Film with Full-Color and White-Light Circularly Polarized Luminescence

The fabrication of chiral thin films with tunable circularly polarized luminescence (CPL) colors is important in developing chiroptical materials but remains challenging due to the lack of assembly-initiated chiral film formation methodology. Here, by adopting a combined solution aggregation and interfacial assembly strategy, we report the fabrication of chiral film materials with full-color and white-light CPL. A biquinoline glutamic acid ester (abbreviated as BQGE) shows a typical aggregation-induced emission property with blue CPL after solution aggregation. Subsequent interfacial assembly of these solution aggregates on a solid substrate leads to the formation of a CPL active film consisting of nanobelt structures. Since the BQGE molecule has a coordination site, the CPL emission of an individual BQGE film can be extended from blue to green emission upon coordination with a zinc ion, accompanied by morphology transition from nanobelts to nanofibers. Further extension to red-color CPL is successfully achieved by coassembly with an achiral acceptor dye. Interestingly, the proper combination of coordination ratio and acceptor loading ratio provides bright white-light CPL emission from the BQGE/Zn²⁺/PDA triad composite film. This work provides a new approach to fabricating chiroptical film materials with controlled microscopic morphology and tunable CPL properties.

Stabilized Chiral Organic Material Containing BINAP Oxide Units as a Heterogeneous Asymmetric Organocatalyst for Allylation of Aldehydes

Condensation of BINAPO-(PhCHO)₂ and 1,3,5-tris(4-aminophenyl)benzene (TAPB) results in a new imine-based chiral organic material (COM) that can be further post-functionalized through reductive transformation of imine linkers to amines. While the imine-based material does not show the necessary stability to be used as a heterogeneous catalyst, the reduced amine-linked framework can be efficiently employed in asymmetric allylation of different aromatic aldehydes. Yields and enantiomeric excesses found are comparable to those observed for the molecular BINAP oxide catalyst, but importantly, the amine-based material also permits its recyclability.

Postmodification of an Amine-Functionalized Covalent Organic Framework for Enantioselective Adsorption of Tyrosine

The development of chiral covalent organic frameworks (COFs) by postsynthetic modification is challenging due to the common occurrences of racemization and crystallinity decrement under harsh modification conditions. Herein, we employ an effective site-selective synthetic strategy for the fabrication of an amine-functionalized hydrazone-linked COF, NH₂-Th-Tz COF, by the Schiff-base condensation between aminoterephthalohydrazide (NH₂-Th) and 4,4',4″-(1,3,5-triazine-2,4,6-triyl)tribenzaldehyde (Tz). The resulting NH₂-Th-Tz COF with free amine groups on the pore walls provides an appealing platform to install desired chiral moieties through postsynthetic modification. Three chiral moieties including tartaric acid, camphor-10-sulfonyl chloride, and diacetyl-tartaric anhydride were postsynthetically integrated into NH₂-Th-Tz COF by reacting amine groups with acid, acyl chloride, and anhydride, giving rise to a series of chiral COFs with distinctive chiral pore surfaces. Moreover, the crystallinity, porosity, and chirality of chiral COFs were retained after modification. Remarkably, the chiral COFs exhibited an exceptional enantioselective adsorption capability toward tyrosine with a maximum enantiomeric excess (ee) value of up to 25.20%. Molecular docking simulations along with experimental results underscored the pivotal role of hydrogen bonds between chiral COFs and tyrosine in enantioselective adsorption. This work highlights the potential of site-selective synthesis as an effective tool for the preparation of highly crystalline and robust amine-decorated COFs, which offer an auspicious platform for the facile synthesis of tailor-made chiral COFs for enantioselective adsorption and beyond.

Stepwise Amplification of Circularly Polarized Luminescence in Chiral Metal Cluster Ensembles

Chiral metal-organic frameworks (MOFs) are usually endowed by chiral linkers and/or guests. The strategy using chiral secondary building units in MOFs for solving the trade-off of circularly polarized luminescence (CPL)-active materials, high photoluminescence quantum yields (PLQYs) and high dissymmetry factors (|g_lum |) has not been demonstrated. This work directionally assembles predesigned chiral silver clusters with ACQ linkers through reticular chemistry. The nanoscale chirality of the cluster transmits through MOF's framework, where the linkers are arranged in a quasi-parallel manner and are efficiently isolated and rigidified. Consequently, this backbone of chiral cluster-based MOFs demonstrates superb CPL, high PLQYs of 50.3%, and |g_lum | of 1.2 × 10^-2 . Crystallographic analyses and DFT calculations show the quasi-parallel arrangement manners of emitting linkers leading to a large angle between the electric and magnetic transition dipole moments, boosting CPL response. As compared, an ion-pair-direct assembly without interactions between linkers induces one-ninth |g_lum | and one-sixth PLQY values, further highlighting the merits of directional arrangement in reticular nets. In addition, a prototype CPL switching fabricated by a chiral framework is controlled through alternating ultraviolet and visible light. This work is expected to inspire the development of reticular chemistry for high-performance chiroptical materials.

Construction of Monophosphine-Metal Complexes in Privileged Diphosphine-Based Covalent Organic Frameworks for Catalytic Asymmetric Hydrogenation

Privileged diphosphine ligands that chelate many transition metals to form stable chelation complexes are essential in a variety of catalytic processes. However, the exact identity of the catalytically active moieties remains ambiguous because the chelated metal catalysts may undergo rearrangement during catalysis to produce monophosphine-metal complexes, which are hard to isolate and evaluate the activities. By taking advantage of the isolation of two phosphorus atoms, we demonstrate here the successful construction of chiral monophosphine-Ir/Ru complexes of diphosphine ligands in covalent organic frameworks (COFs) for enantioselective hydrogenation. By condensation of the tetraaldehyde of enantiopure MeO-BIPHEP and linear aromatic diamines, we prepare two homochiral two-dimensional COFs with ABC stacking, in which the two P atoms of each diphosphine are separated and fixed far apart. Post-synthetic metalations of the COFs thus afford the single-site Ir/Ru-monophosphine catalysts, in contrast to the homogeneous chelated analogues, that demonstrated excellent catalytic and recyclable performance in the asymmetric hydrogenation of quinolines and β-ketoesters, affording up to 99.9% enantiomeric excess. Owing to the fact that the porous catalyst is capable of adsorbing and concentrating hydrogen, the catalytic reactions are promoted under ambient/medium pressure, which are typically performed under high pressure for homogeneous catalysis. This work not only shows that monophosphine-metal complexes of diphosphines can be catalytically active centers for asymmetric hydrogenation reactions but also provides a new strategy to prepare new types of privileged phosphine-based heterogeneous catalysts.

Construction of Covalent Organic Frameworks via Multicomponent Reactions

Multicomponent reactions (MCRs) combine at least three reactants to afford the desired product in a highly atom-economic way and are therefore viewed as efficient one-pot combinatorial synthesis tools allowing one to significantly boost molecular complexity and diversity. Nowadays, MCRs are no longer confined to organic synthesis and have found applications in materials chemistry. In particular, MCRs can be used to prepare covalent organic frameworks (COFs), which are crystalline porous materials assembled from organic monomers and exhibit a broad range of properties and applications. This synthetic approach retains the advantages of small-molecule MCRs, not only strengthening the skeletal robustness of COFs, but also providing additional driving forces for their crystallization, and has been used to prepare a series of robust COFs with diverse applications. The present perspective article provides the general background for MCRs, discusses the types of MCRs employed for COF synthesis to date, and addresses the related critical challenges and future perspectives to inspire the MCR-based design of new robust COFs and promote further progress in this emerging field.

Self-Assembly of Helical Nanofibrous Chiral Covalent Organic Frameworks

Despite significant progress on the design and synthesis of covalent organic frameworks (COFs), precise control over microstructures of such materials remains challenging. Herein, two chiral COFs with well-defined one-handed double-helical nanofibrous morphologies were constructed via an unprecedented template-free method, capitalizing on the diastereoselective formation of aminal linkages. Detailed time-dependent experiments reveal the spontaneous transformation of initial rod-like aggregates into the double-helical microstructures. We have further demonstrated that the helical chirality and circular dichroism signal can be facilely inversed by simply adjusting the amount of acetic acid during synthesis. Moreover, by transferring chirality to achiral fluorescent molecular adsorbents, the helical COF nanostructures can effectively induce circularly polarized luminescence with the highest luminescent asymmetric factor (g_lum ) up to ≈0.01.

Recent Trends in the Design, Synthesis and Biomedical Applications of Covalent Organic Frameworks

The most recent and advanced class of crystalline and permeable compounds are covalent organic frameworks (COFs). Due to their exceptional qualities, such as their porous structure, high surface area, strong chemical and thermal stabilities, low density, good water stability, luminescent nature, and so on, COFs have seen remarkable growth over the past ten years. COFs have been successfully researched for a number of applications based on these characteristics. The current state of COFs has been reported in this study, with particular attention paid to their design, topology, synthesis, and a variety of biological applications, including drug delivery systems, photodynamic and photothermal therapy, biosensing, bioimaging, etc. Moreover, several miscellaneous applications, such as catalysis, gas storage and separation, photocatalysis, sensors, solar cells, supercapacitors, and 3D printers, have also been explored. It is significant that we have examined current research on COFs with a focus on the biological applications, which are infrequently covered in the literature. Descriptions of the difficulties and prospective outcomes have also been given.

2D Hexagonal Assemblies of Amphiphilic Double-Helical Poly(phenylacetylene) Homopolymers with Enhanced Circularly Polarized Luminescence and Chiral Self-Sorting

Two-dimensional (2D) chiral materials have been attracting immense attentions owing to their unique properties. Herein, we successfully developed a unique assembly strategy of amphiphilic homopolymers to construct stable free-standing 2D chiral nanosheets in solution. The amphiphilic poly(phenylacetylene) (PPA) homopolymers bearing the hydrophobic and hydrophilic dendritic side chains adopt a DNA-like double-helical conformation. The regular hexagonal nanosheets were formed in THF/EtOH through nucleation and epitaxial growth. The sizes of the nanosheets can be modulated from nanometers to submillimeters upon varying the ratio of binary solvents, while the thickness is linearly correlated with the molecular weights. The 2D architecture can significantly enhance the CPL of polymers with a high dissymmetry factor ≈0.1. Driven by a discrimination of helical conformation, the PPAs can self-sort into homochiral 2D nanosheets, as directly visualized by using fluorescent microscopy.

Tunable Circularly Polarized Luminescence via Chirality Induction and Energy Transfer from Organic Films to Semiconductor Nanocrystals

Circularly polarized luminescent (CPL) films with high dissymmetry factors hold great potential for optoelectronic applications. Herein, we propose a strategy for achieving strongly dissymetric CPL in nanocomposite films based on chirality induction and energy transfer to semiconductor nanocrystals. First, focusing on a purely organic system, aggregation-induced emission (AIE) and CPL activity of organic liquid crystals (LCs) forming helical nanofilaments was detected, featuring green emission with high dissymmetry factors g_lum ∼ 10^-2. The handedness of helical filaments, and thus the sign of CPL, was controlled via minute amounts of a small chiral organic dopant. Second, nanocomposite films were fabricated by incorporating InP/ZnS semiconductor quantum dots (QDs) into the LC matrix, which induced the chiral assembly of QDs and endowed them with chiroptical properties. Due to the spectral matching of the components, energy transfer (ET) from LC to QDs was possible enabling a convenient way of tuning CPL wavelengths by varying the LC/QD ratio. As obtained, composite films exhibited absolute g_lum values up to ∼10^-2 and thermally on/off switchable luminescence. Overall, we demonstrate the induction of chiroptical properties by the assembly of nonchiral building QDs on the chiral organic template and energy transfer from organic films to QDs, representing a simple and versatile approach to tune the CPL activity of organic materials.

Challenges and opportunities for chiral covalent organic frameworks

As highly versatile crystalline porous materials, covalent organic frameworks (COFs) have emerged as an ideal platform for developing novel functional materials, attributed to their precise tunability of structure and functionality. Introducing chiral functional units into frameworks produces chiral COFs (CCOFs) with chiral superiorities through chirality conservation and conversion processes. This review summarises recent research progress in CCOFs, including synthetic methods, chiroptical characterisations, and their applications in asymmetric catalysis, chiral separation, and enantioselective recognition and sensing. Challenges and limitations are discussed to uncover future opportunities in CCOF research.

Enantiomorphic Single Crystals of Linear Lead(II) Bromide Perovskitoids with White Circularly Polarized Emission

Chiroptical hybrid organic-inorganic perovskites are emerging as a new class of promising materials with mirror optical signal responses for optoelectronic applications. However, chiroptical white-emission materials have been scarcely unearthed. Herein, four pairs of hybrid lead(II) bromide perovskitoids were obtained, namely, (R)- and (S)-(H₂ MPz)PbBr₄ (R/S-MPz=(R)-(-)/(S)-(+)-2-methylpiperazine) (1 and 2), (R)- and (S)-(H₂ MPz)₃ Pb₂ Br₁₀ ⋅2 DMAc (3 and 4), (R)- and (S)-(H₂ MPz)PbBr₄ ⋅0.5 MeCN (5 and 6) and (R)- and (S)-(H₂ MPz)₂ Pb₂ Br₈ ⋅DCM (7 and 8). Notably, they all exhibit ultrabroadband emission and chiroptical signals. Perovskitoids 3-6 even achieve white circularly polarized emission with a high dissymmetric factor (g_lum ) (±3×10^-3 for 3 and 4; ±8×10^-3 for 5 and 6). This new type of hybrid perovskitoids will attract attention and find applications in chiroptical fields because of the extensively and easily tunable photophysical properties.

Chiral-Induced Ultrathin Covalent Organic Frameworks Nanosheets with Tunable Circularly Polarized Luminescence

Development of covalent organic frameworks (COFs) with circularly polarized luminescence (CPL) is still challenging. Here we first reported ultrathin COFs nanosheets (NS) based CPL materials using a chiral induced-synthesis strategy. Chiral amines served as chiral inducers to give COF TpBpy with chirality and participated in the modification of TpBpy, inhibiting the fluorescence quenching caused by π-π stacking to form ultrathin luminescent chiral COFs (chirCOFs) NS. The obtained chirCOFs R-/S-TpBpy NS had strong chirality and intense red CPL property with a |g_lum| of ∼0.02. Afterward, the carboxyl containing green and blue fluorescent dye molecules were postmodified onto the chirCOFs NS (chirCOFs/Dyes) to achieve color-adjustable CPL. Due to the chirality and energy transfer between chirCOFs and dye groups, the obtained chirCOFs/Dyes showed strong chirality and increased and tunable photoluminescence, exhibiting excellent, tunable, and amplified CPL performance with a maximum |g_lum| of ∼0.1, which was ∼5 times stronger than that of as-prepared chirCOFs NS. Moreover, the corresponding chirCOFs NS were dispersed into a polydimethylsiloxane (PDMS) matrix to form wafer size, highly transparent, and flexible COFs/PDMS films for practical CPL application. This study opens a new strategy to prepare ultrathin chirCOFs NS with strong and tunable CPL by chiral induction and provides a new approach for the preparation of transparent, large size, and flexible COFs composite films in chiral optical applications.