Homochiral Metal-organic Frameworks Research Articles

Isostructural chiral metal–organic frameworks with metal-regulated performances for electrochemical enantiomeric recognition of tyrosine and tryptophan

Two isostructural homochiral MOFs exhibit significantly different enantioselective recognition performances closely associated with the different coordination habits of the metal centers in the MOFs.

Engineering UiO-68-Typed Homochiral Metal–Organic Frameworks for the Enantiomeric Separation of Fmoc-AAs and Mechanism Study

Homochiral metal-organic frameworks (HMOFs) have been widely investigated in the application of enantiomeric separation. Nonetheless, it remains a significant challenge to explore the effect of multiple weak interactions between HMOF adsorbents and chiral adsorbates on enantiomeric separation performance still. In this work, robust chiral amine-alcohol-functionalized UiO-68-typed Zr-HMOFs 1-3 with the same hydrogen-bonding sites but slightly different π-binding sites were prepared for the enantioseparation of amino acid derivatives (Fmoc-AAs) with large π-binding groups. As a consequence of multiple host-guest interactions, these Zr-HMOFs exhibit speedy adsorption and high adsorption capacity for Fmoc-L/D-AAs and dissimilar enantioselectivity for the adsorption of their enantiomers. Materials 1 and 2 exhibit excellent enantioselective separation performance for Fmoc-valine with a single terminal π-binding group, while material 3 displays excellent enantioselective separation performance for Fmoc-phenylalanine and Fmoc-tryptophan with π-binding groups at both ends. As evidently demonstrated by our experimental and density functional theory (DFT) computational results, when the number of π-binding groups preset in the confined chiral space of adsorbents matches the number of π-binding groups of chiral adsorbates, the synergism of π-π or σ-π interactions will increase enantioselectivity; otherwise, the competition interactions from redundant identical binding sites will weaken enantioselectivity. Our case not only provides a tremendously typical system for investigating the collaborative discrimination of multiple weak interactions and exploring the impact of relatively excessive binding sites of HMOF adsorbents or chiral adsorbates on the enantioselective separation performance but also provides guidance for targeted functional modifications of high-performance chiral porous materials.

Highly Boosting Circularly Polarized Luminescence of Chiral Metal-Imidazolate Frameworks.

To develop a simple and general method for improving the circularly polarized luminescence (CPL) performances of materials is of great significance. In this work, two pairs of CPL-active homochiral metal-organic frameworks (MOFs) P/M-Et and P/M-Et(Cd) with eta topology are reported. In comparison to the reported isomorphic Zn-imidazolate MOFs P-Me and M-Me, both luminescence dissymmetry factor (glum ) and photoluminescence quantum yields (ΦPL ) of P-Et and M-Et are largely improved by simply changing the methyl group to an ethyl group of ligands in P-Et and M-Et. Furthermore, the |glum | values are significantly amplified up to 0.015 from 0.0057 by introducing the non-luminescent halogenated aromatics, while an enhanced fluorescence efficiency is observed simultaneously (from 27.2% to 47.3%). The figure of merit value is about 40 times larger than that of P-Me and M-Me. Similarly, the CPL performances of P/M-Et(Cd)are improved by about five times after encapsulatingfluorobenzene molecules. This work represents a new and simple method for developing CPL-active MOF materials.

Highly Efficient versus Null Electrochemical Enantioselective Recognition Controlled by Achiral Colinkers in Homochiral Metal-Organic Frameworks.

Chiral materials capable of electrochemical enantiomeric recognition are highly desirable for many applications, but it is still very challenging to achieve high recognition efficiency for lack of the knowledge of structure-property relationships. Here, we report the completely distinct enantiomeric recognition related to slightly different achiral colinkers in isomorphic homochiral metal-organic frameworks with the same chiral linker. Cu-TBPBe, for which the achiral colinker has two pyridyl rings connected by ─CH═CH─, shows excellent enantioselectivity and sensitivity for electrochemical recognition of l-tryptophan (Trp) with a detection limit of 3.16 nM. The l-to-d ratio of differential pulse voltammetric (DPV) currents reaches 53, which is much higher than the values (2-14) reported for previous electrochemical sensors. By contrast, Cu-TBPBa, in which the achiral colinker has -CH2-CH2- between pyridyl rings, is incapable of discrimination between l-Trp and d-Trp. Structural and spectral analyses suggest that the achiral conjugated colinker and the chiral moieties around it cooperate to produce a chiral pocket in favor of enantioselective adsorption through multiple hydrogen-bonding and π-π stacking interactions. The work demonstrated that Cu-TBPBe can be used to fabricate reliable electrochemical sensors for ultrasensitive quantification of Trp enantiomers in racemic mixtures and in complex biological systems such as urine. The work also highlights that an achiral coligand can be of vital importance in determining enantiomeric discrimination, opening up a new avenue for the design of chiral sensing materials.

Selective SERS identification and quantification of glucose enantiomers on homochiral MOFs based enzyme-free nanoreactors

Enantiomeric identification plays a critical role in diverse fields ranging from pharmaceutics, biologics, and stereoselective synthesis to daily life. In this study, an enantioselective surface-enhanced Raman scattering (SERS) substrate was developed by integrating homochiral MIL-101(Fe) with oxidase (GOx)-mimicking Au nanoparticles (Au/Chiral-MIL-101(Fe)) for identification and quantification of chiral molecules. Using L/d-glucose (Glu) as the model enantiomers, the homochiral environment of Au/Chiral-MIL-101(Fe) exhibited selective recognition abilities for Glu enantiomers. The captured Glu was further oxidized to hydrogen peroxide (H2O2) by GOx-like activity of Au nanoparticles. The generated H2O2 molecules reduced Fe(III) nodes in MIL-101(Fe) to Fe(II) for the growth of Prussian blue (PB) on site. Benefitting from the pre-concentrated feature and confinement effect of the porous metal–organic frameworks (MOFs) structure, the Au/Chiral-MIL-101(Fe) hybrid based nanoreactors exhibited high catalytic performance for H2O2 generation and further PB growth, resulting in an excellent sensitivity for Glu quantification. The enantioselective discrimination between l-Glu and d-Glu was directly determined from the intensity of PB signals at 2150 cm−1. Using this sensing strategy, Glu enantiomers could be quantified with a limit of detection (LOD) of 0.09 µM for l-Glu and 0.08 μM for d-Glu. Owing to its enzyme-free nature and universal characteristic for the discrimination of other monosaccharide enantiomers, this design provides an attractive substrate for sensing chiral molecules by Raman technology.

Homochiral coordination polymers of Zn(II) and Pb(II) with interesting three-dimensional helicates

Based on a designed enantiomerically pure chiral ligand H3L, two interesting homochiral metal−organic frameworks (HMOFs), namely {[Zn3(L)2(H2O)2]∙2H2O}n (1) and {[Pb(HL)(H2O)]∙H2O}n (2), have been solvothermally synthesized (H3L = (S)-4'-(((1-carboxyethyl)amino)methyl)-[1,1′-biphenyl]-3,5-dicarboxylic acid). Complexes 1 and 2 are novel three-dimensional metal−organic helicates with different structural motifs. In 1, anionic (L)3− ligands bridge trimetallic Zn-clusters to form a [3,6]-connected architecture of (63)2(612·83) topology. In particular, each trimetallic Zn-cluster is the node of six strand 21 helical chains consisting of four right-handed helical chains and two left-handed helical chains. In 2, zwitterionic (HL)2− ligands link PbII ions into a 3-connected 103-topological helicate composed of 21 helical chains extending along three crystal axes, and two thirds of which are left-handed. Interesting enough, the chiral ligand H3L not only transferred its inherent chirality to the synthesized complexes 1 and 2, but also induced unexpected helical chirality. Furthermore, the morphologies, thermal stabilities and solid-state optical properties including CD spectra, and nonlinear optical and luminescent properties of compounds 1 and 2 were also carried out. Compounds 1 and 2 exhibit interesting fluorescence emissions and obvious second-order nonlinear activity in addition to the excellent thermal stability of their dehydrated frameworks, indicating that the HMOF materials have great potential in optical devices.

In-situ grown metal organic framework synergistic system for the enantioseparation of three drugs in open tubular capillary electrochromatography.

Metal organic frameworks have received great attention as the chiral stationary phase for racemic drug separation because of their fascinating structures and properties. However, the most homochiral metal organic frameworks were constructed by rare and precious chiral organic ligands. In this work, an achiral metal organic framework, together with a natural chiral selector carboxymethyl β-cyclodextrin built a synergistic separation system in the open tubular capillary electrochromatography. The novel coated columns were developed by inducing metal organic framework nanoparticles to grow on the imidazolyl functional capillary inner wall. The baseline separations of hydroxychloroquine, ofloxacin, and atenolol were achieved in the synergistic separation system. The effects of the concentration of chiral selector, pH, voltage, and the concentration of organic additives were studied. Compared with chiral selector auxiliary bare capillary, the resolutions of three drugs were remarkably improved. The relative standard deviations for the retention time of intraday (n=6), interday (n=6), and column-to-column were less than 2.1, 2.6, and 5.2%, respectively. These results demonstrate that affordable synergistic separation systems are prospective for racemic drug enantioseparation in capillary electrochromatography.

Formation of homochiral 3D frameworks using zinc (II) and rigid achiral ligands

Spontaneous resolution based on achiral ligands has become an important method to obtain homochiral metal–organic frameworks (HMOFs) for its low cost. In this work, we use rigid ligand 5-(hydroxymethyl)isophthalic acid (H2HIPA) to react with Zn(Ⅱ) ions to build a 3D cage-shaped complex [Zn4O(HIPA)4]n (1) under hydrothermal condition. Complex 1 crystallizes in space group I2 with near-zero Flack parameter. Solid CD spectra further firms that complex 1 is comprised of a pair of enantiomers and spontaneous resolution takes place in reaction system. According to the similar process, 4, 4′-bis(2-methyl-1H-imidazol-1-yl)-1,1′-biphenyl (MIBI) as second ligand participates in constructing another 3D helical complex [Zn(HIPA)(MIBI)]n (2). Complex 2 also crystallize in chiral space group with near-zero Flack parameter. With the help of enantiomeric helical structures, enantiomers 2P and 2 M from complex 2 can be distinguished by crystal X-ray diffraction.

MIL-53-based homochiral metal-organic framework as a stationary phase for open-tubular capillary electrochromatography

Homochiral metal-organic frameworks (MOFs) have attracted considerable attention in many fields of research, such as chiral catalysis and chiral chromatography. However, only few homochiral MOFs can be effectively used in capillary electrochromatography (CEC) and their performances are far from adequate. In this study, we successfully synthesized achiral nanocrystalline MIL-53. A facile post-synthetic modification strategy was then implemented to functionalize the product, yielding a homochiral MOF: l-His-NH-MIL-53. This MOF was then employed as a chiral coating in open-tubular CEC mode (OT-CEC), and, as such, it exhibited high enantioselectivities for several racemic drugs. The homochiral MOF and the fabricated capillary coating were systematically characterized using transmission electron microscopy, scanning electron microscopy (with energy-dispersive X-ray spectrometry), Fourier-transform infrared spectroscopy, X-ray diffractometry, thermogravimetric analysis, circular dichroism spectroscopy, Brunauer-Emmett-Teller surface area measurements, and X-ray photoelectron spectroscopy. This study is expected to provide a new strategy for the design and establishment of MOF-based chiral OT-CEC systems.

Optical Properties and Applications of Crystalline Materials

Recent years have witnessed the prosperous developments of crystalline materials based on diverse organic/inorganic small molecules, metal organic frameworks (MOFs), covalent organic frameworks (COFs), etc. Crystalline materials are highly attractive for their highly ordered molecular and supramolecular architectures, allowing for the rational design of materials with brilliant optical properties and advanced photonic applications. In particular, crystalline materials often feature well-defined morphologies in wavelength and subwavelength scales, and are therefore ideal platforms for the realization of many advanced optical functionalities such as waveguiding, lasing, and optical logics. In recognition of the importance of this field, we have gladly taken the opportunity to serve as the Guest Editors to organize this themed special issue, with the aim to highlight the state of the art in the area of crystalline materials for optical applications. This special issue, consisting of 14 reviews and 6 research papers, covers a broad spectrum of the recent advances in crystalline optical materials based on MOFs, COFs, perovskites, microporous zeolites, and small molecules. It encompasses some of the highly important research achievements, efforts and practices of crystalline materials in many well-established optical applications including light emitting diodes (LEDs), waveguides, optical sensing, nonlinear optics (NLO), as well as the emerging photonic technologies such as terahertz (THz) generation and detection. MOFs are crystalline materials assembled by the coordination interactions between metal-containing nodes and organic linkers. Luminescent MOFs, with diversified architectures, devisable pore structures and affluent luminescence, provide great potential for the development of novel phosphors. In their review article, Prof. Guodong Qian and co-workers focus on the phosphor-converted white LEDs based on MOF matrixes doped with luminescent lanthanide ions, organic ligands, and/or encapsulated with diverse luminescent guest species including organic dyes, metal complexes, quantum dots (adom.202001817). In terms of luminescent species encapsulated host–guest MOFs, Prof. Xian-He Bu and co-workers provide a comprehensive review on the construction strategies and optical applications of luminescent particles hybridized MOFs materials (adom.202100283). Similar encapsulation strategy can be applied in other porous materials. Dr. Eduardo Coutino-Gonzalez and colleagues present the tunable luminescence behaviours from stable silver nanoclusters confined in microporous zeolites (adom.202100526). Many fascinating optical effects and proccesses have been explored in MOF materials. In their review article, Prof. Shuang-Quan Zang et al. discuss on MOF materials with thermally activated delayed fluorescence (TADF) properties (adom.202100081). The TADF characteristics of metal–organic complexes, organic ligand-protected metal clusters, and coordination polymers have been summarized. Prof. Mei Pan and colleagues present an overview on another highly important optical process, namely the excited-state intramolecular proton transfer (ESIPT), in the solid-state materials including MOFs, COFs, and small organic molecules (adom.202001952). In their research article, Prof. Dan Li, Prof. Xiao-Ping Zhou, et al. present a strategy to turn-on the circularly polarized luminescence in a family of homochiral MOF materials constructed by conformation tuning of ligands and self-assembly control (adom.202002096). Lead halide perovskites have recently emerged as highly promising optoelectronic materials owing to their low-cost solution fabrication, chemical versatility, tolerance to defects, large absorption coefficient, and high charge carrier mobility. Prof. Zhang Chuang and colleagues present a review to summarize the recent progress on (linearly and circularly) polarized photoluminescence from lead halide perovskites (adom.202002236). In their research article, Prof. Wei Li, Prof. Xiang Wu, Prof. Xian-He Bu, et al. report on thermal and pressure dual-stimuli-responsive luminescence of a pair of new enantiomeric 2D Dion–Jacobson hybrid organic–inorganic perovskites (adom.202100003). Meanwhile, Dr. Elke Debroye, Prof. Johan Hofkens, et al. provide experimental evidence of chloride induced trap passivation in lead halide perovskites through single particle blinking studies (adom202002240). On the other hand, Dr. Valentin Diez-Cabanes, Prof. David Beljonne, et al. studied from the theoretical point of view the electronic structures and optical properties of mixed iodine/bromine lead perovskites (adom.202001832). Lead halide perovskites have been also widely applied in NLO. In this aspect, Prof. Wei Ji, Prof. Shunbin Lu and colleagues present a comprehensive review on multiphoton absorption properties and applications of perovskite materials (adom.202100292). In another review article, Prof. Marek Samoć and co-workers introduce the concept of “NLO pigments” based on different nano- and microcrystalline materials including MOFs, perovskites, 0D/1D materials, as well as 2D structures such as black phosphorus, transition metal dichalcogenides, MXenes, and topological insulators (adom.202100216). Other NLO materials such as molecular materials based on small organic compounds have been also explored. Prof. Theo Rasing and colleagues show their particular interest in a family of fluorenone based molecular materials (adom.202100327), while Dr. Mojca Jazbinsek, Prof. Fabian Rotermund, Prof. O-Pil Kwon, et al. shed light on the terahertz wave generation, detection, and applications of some highly efficient NLO organic crystals (adom.202101019). Many more optical and photonic functionalities have been realized in small molecule based crystalline materials other than NLO. Prof. Dongpeng Yan and colleagues introduce the recent advances on molecular crystalline luminescent materials for optical waveguides (adom.202001768), while Prof. Yulan Chen and colleagues pay special attention to the waveguide behaviours of polycyclic arenes based organic crystals (adom.202002264). Prof. Akimitsu Narita et al. provide an overview of the latest developments in the chemical synthesis and spectroscopic investigations of atomically precise graphene quantum dots based on polycyclic aromatic hydrocarbons and their applications in lasing and optical imaging (adom.202100508). Prof. Yang Zhang et al. present the recent advances in the field of optical control over ferroelectric polarization in various ferroelectrics including oxide perovskites, van der Waals ferroelectrics, and ferroelectric heterostructures (adom.202002146). In their research article, Prof. Giancarlo Morelli, Prof. Gil Rosenman, et al. observe a type of fold-sensitive visible fluorescence in both biological amyloid and bioinspired amyloidogenic assemblies, arising at the earliest stages of seeding and nucleation of β-sheet nanofibers (adom.202002247). Prof. Jing Liu, Prof. Joseph M. K. Irudayaraj, Prof. Gary J. Cheng, et al. realized enhanced energy transfer from nitrogen-vacancy centers to three-dimensional graphene heterostructures by laser nanoshaping (adom.202001830). This themed special issue might not be exhaustive, but it highlights some of the recent advances in the optical properties and applications of crystalline materials. We hope that the special issue will provide a timely, meaningful and valuable reference and perspective for the research community in this important field. We would like to thank Dr. Anja Wecker (Editor-in-Chief) and Dr. Jipei Yuan (Deputy Editor) for giving us the opportunity to organize this special issue. Our appreciation extends to the editorial team of Advanced Optical Materials, for their professionalism throughout the editing process. We are also very grateful to all the contributing authors for their efforts and enthusiastic support. Jialiang Xu is a Professor of Materials Chemistry at Nankai University. He obtained his PhD from ICCAS in 2010 under the supervision of Prof. Yuliang Li, and then worked as Marie-Curie Fellow at Radboud University, Nijmegen. In 2013, he was awarded the NWO-VENI grant, with which he developed his research line at the interface between chemistry and physics to study the coupling between light and (supra)molecular materials. He joined Tianjin University in 2015, and relocated to Nankai University in 2018. Xian-He Bu is a Cheung Kong Scholar Professor of Chemistry at Nankai University (from 2004). He obtained his BS and PhD degrees from Nankai University in 1986 and 1992 under the supervision of Prof. Rong-Ti Chen. Since 1995, he has been a full professor at Nankai University. He now serves as the dean of School of Material Science and Engineering, and a director of Tianjin Key Lab of Metal and Molecule-Based Material Chemistry. His current research focuses on the syntheses and applications of multifunctional coordination compounds, crystal engineering, magnetic materials, etc.

Engineering Homochiral MOFs in TiO2 Nanotubes as Enantioselective Photoelectrochemical Electrode for Chiral Recognition.

Enantioselective sensing of chiral molecules is an important issue for both biomedical research and the pharmaceutical industry. Here, an enantioselective photoelectrochemical (PEC) sensor was constructed by integrating TiO2 nanotubes (NTs) with metal-organic frameworks (MOFs) for the identification of enantiomers. TiO2 NTs prepared by electrochemical anodization can not only be used as the PEC platform but also as the metal-ion precursor to react with terephthalic acid (BDC) to form MIL-125(Ti) in situ. A postsynthetic exchange (PSE) method was used for exchanging the ligand of MIL-125 by 2-aminoterephthalic acid (BDC-NH2) for further functionalization. Homochirality was then successfully introduced into achiral MIL-125-NH2 by postsynthetic modification (PSM) with l-histidine (l-His). The resulting homochiral metal-organic frameworks (MOF)-in-NT architecture exhibits excellent discrimination ability for the chiral recognition of 3,4-dihydroxyphenylalanine (l/d-DOPA) enantiomers. Moreover, by adjusting the charge-carrier separation-induced photocurrent variation mechanism, the as-proposed homochiral PEC electrode exhibits a broad application potential for the discrimination of enantiomers. Because of the construction of binder-free monochiral MOF-in-NT structure directly on a Ti-metal substrate, the valuable feature is that the PEC sensing platform can be used directly, thereby providing a stable, simplified, and low-cost sensing device for the recognition of chiral enantiomers.

Fabrication of Homochiral Metal-Organic Frameworks in TiO2 Nanochannels for In Situ Identification of 3,4-Dihydroxyphenylalanine Enantiomers.

Enantioselective identification of chiral molecules is important for biomedical and pharmaceutical research. However, owing to identical molecular formulas and chemical properties of enantiomers, signal transduction and amplification are still the two major challenges in chiral sensing. In this study, we developed an enantioselective membrane by integrating homochiral metal-organic frameworks (MOFs) with nanochannels for the sensitive identification and quantification of chiral compounds. The membrane was designed using a TiO2 nanochannel membrane (TiNM) as the metal ion precursor of MOFs (using MIL-125(Ti)) and incorporating l-glutamine (l-Glu) into the framework of MIL-125(Ti). Using 3,4-dihydroxyphenylalanine (DOPA) as the model analyte, the as-prepared homochiral l-Glu/MIL-125(Ti)/TiNM exhibits a remarkable chiral recognition to d-DOPA than l-DOPA. More importantly, benefiting from the highly enlarged surface area and confinement effect provided by the MOFs-in-nanochannel architecture, the discrimination for chiral recognition is largely amplified through the chelation interaction of Fenton-like activity of Fe3+ onto DOPA. Using 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonate) (ABTS) as the substrate, the positively charged ABTS•+ product via Fenton-like reaction induces significant ionic transport changes in nanochannels, which in turn provides information about chiral recognition. This innovative signal amplification strategy on homochiral nanochannels might pave a new way for sensitive monitoring and chiral recognition.

Solvent-induced enantioselectivity reversal in a chiral metal organic framework.

Solvent-induced enantioselectivity reversal is a rarely reported phenomenon in porous homochiral materials. Similar behavior has been studied in chiral high performance liquid chromatography, where minor modifications to the mobile phase can induce elution order reversal of two enantiomers on a chiral stationary phase column. We report the first instance of solvent-induced enantioselectivity reversal in a homochiral metal organic framework. Further, we highlight the complex enantioselectivity behavior of homochiral metal organic frameworks toward racemic mixtures in the presence of solvents through racemate-solvent enantioselectivity and loading experiments as well as enantiopure-solvent loading experiments. We hypothesize that this interesting selectivity reversal behavior is likely to be observed in other competitive adsorption, nonchiral selective processes involving a solvent.

Molecular fingerprint and machine learning to accelerate design of high‐performance homochiral metal–organic frameworks

AbstractComputational screening was employed to calculate the enantioseparation capabilities of 45 functionalized homochiral metal–organic frameworks (FHMOFs), and machine learning (ML) and molecular fingerprint (MF) techniques were used to find new FHMOFs with high performance. With increasing temperature, the enantioselectivities for (R,S)‐1,3‐dimethyl‐1,2‐propadiene are improved. The “glove effect” in the chiral pockets was proposed to explain the correlations between the steric effect of functional groups and performance of FHMOFs. Moreover, the neighborhood component analysis and RDKit/MACCS MFs show the highest predictive effect on enantioselectivities among the four ML classification algorithms with nine MFs that were tested. Based on the importance of MF, 85 new FHMOFs were designed, and a newly designed FHMOF, NO2‐NHOH‐FHMOF, with high similarity to the optimal MFs achieved improved chiral separation performance, with enantioselectivities of 85%. The design principles and new chiral pockets obtained by ML and MFs could facilitate the development of new materials for chiral separation.

Turn-On Fluorescence Enantioselective Sensing of Hydroxyl Carboxylic Enantiomers by Metal-Organic Framework Nanosheets with a Homochiral Tetracarboxylate of Cyclohexane Diamide.

Two-dimensional (2D) metal-organic frameworks (MOFs) have attracted growing interest due to excellent performance in gas separation, energy conversion and storage, catalysis, and sensing, but their homochirality and exfoliation as well as related enantioselective catalysis and sensing remain a stage of pending exploration owing to the scarcity of homochiral MOFs and intrinsic aggregation of nanosheets. Herein, a homochiral 2D MOF (HMOF-3) with polymeric chirality, good thermostability, and solvent stability is designed and constructed by a homochiral organic ligand 5,5'-((1R,2R)-cyclohexane dicarbonyl bis(azanediyl)) diisophthalic acid (R,R-CHCAIP), a ditopic coligand 4,4'-bipyridine, and Zn salts. Remarkably, HMOF-3 can be exfoliated via solvent-assisted sonication to achieve 2D HMOF-3 nanosheets (HMOF-3-NS), which exhibit a sensitive turn-on effect with the fluorescence enhancement up to 63.5 times in the presence of R/S-mandelic acid, d/l-tartaric acid, d/l-lactic acid, d/l-alanine, and d/l-tryptophan. More importantly, the high surface area, polymeric chirality environment, and highly accessible functional sites on the surface of HMOF-3 nanosheets enable close contact with probed enantiomers, leading to highly enantioselective and sensitive sensing. The turn-on mechanism of host-guest-assisted electronic transfer is confirmed by DFT calculation and the relative experiment. This work highlights the promise of homochiral 2D MOF nanosheets for enantioselective sensing applications.

Asymmetric catalysis using metal-organic frameworks

• Principles of designing MOFs for asymmetric catalytic applications. • Comprehensive overview of MOF-catalyzed asymmetric organic reactions. • Discussion of size- and confined-space effects, sequential/cascade reactions, etc. To date, porous coordination polymers (metal–organic frameworks, MOFs) have been extensively studied in multiple applications, including the use of homohiral or chirally modified MOFs in stereoselective catalysis. Homochiral MOFs, assembled from enantiopure chiral building blocks, have sufficient chemical robustness, demonstrate chiral recognition capabilities, hold rich possibilities of synthetic modifications in order to introduce catalytically active sites, and, in addition to catalyst-substrate interactions, may impose size and confined-space effects, thus providing additional means of finely tuning the catalytic activity and selectivity. Herein, the progress in the area of asymmetric catalysis using metal–organic frameworks, achieved during the twenty years of investigation, is summarized.

Enantioselective SERS sensing of pseudoephedrine in blood plasma biomatrix by hierarchical mesoporous Au films coated with a homochiral MOF

Here, we present a new family of hierarchical porous hybrid materials as an innovative tool for ultrasensitive and selective sensing of enantiomeric drugs in complex biosamples via chiral surface-enhanced Raman spectroscopy (SERS). Hierarchical porous hybrid films were prepared by the combination of mesoporous plasmonic Au films and microporous homochiral metal-organic frameworks (HMOFs). The proposed hierarchical porous substrates enable extremely low limit of detection values (10−12 M) for pseudoephedrine in undiluted blood plasma due to dual enhancement mechanisms (physical enhancement by the mesoporous Au nanostructures and chemical enhancement by HMOF), chemical recognition by HMOF, and a discriminant function for bio-samples containing large biomolecules, such as blood components. We demonstrate the effect of each component (mesoporous Au and microporous AlaZnCl (HMOF)) on the analytical performance for sensing. The growth of AlaZnCl leads to an increase in the SERS signal (by around 17 times), while the use of mesoporous Au leads to an increase in the signal (by up to 40%). In the presence of a complex biomatrix (blood serum or plasma), the hybrid hierarchical porous substrate provides control over the transport of the molecules inside the pores and prevents blood protein infiltration, provoking competition with existing plasmonic materials at the limit of detection and enantioselectivity in the presence of a multicomponent biomatrix.

Turn‐On Circularly Polarized Luminescence in Metal–Organic Frameworks

AbstractThe fabrication of circularly polarized luminescence (CPL) active materials by self‐assembly is still in its challenge. In this work, a family of homochiral metal–organic frameworks (MOFs) and metal–organic cages (MOCs) are constructed by solvothermal subcomponent self‐assembly. These MOFs feature an eta topology with trifold helical chains, while the MOCs adopt a cubic cage structure. The chiral ligands show two distinct types of conformations: “opened” and “closed” in MOFs and MOCs, respectively. Although homochiral MOFs and MOCs show similar spectra of circular dichroism and photoluminescence with similar quantum yields and lifetimes, the MOFs yield clear CPL signals and the CPL of MOCs are silent. The turn‐on CPL in MOFs achieved by tuning the conformation of ligands and controlling self‐assembly provides a new approach for development of CPL‐active MOF materials.

Fabrication of a "turn-on"-type enantioselective fluorescence sensor via a modified achiral MOF: applications for synchronous detection of phenylalaninol enantiomers.

Homochiral metal-organic frameworks (HMOFs) have garnered considerable attention due to their extrachiral properties and broad application for chiral recognition. However, assembling a pair of high-quality chiral MOFs for sensing enantiomers precisely is a formidable challenge because of the complicated chiral environment and uncontrollable coordinated conditions. Herein, one pair of homochiral UiO-66 analogues, S-1 (l-AP@UiO-66-(COOH)2) and R-1 (d-AP@UiO-66-(COOH)2), are reported for chiral recognition. They were fabricated via a condensation reaction between the carboxyl groups of UiO-66-(COOH)2 and amino groups of l/d-amino propanol (l/d-AP). These novel fluorescent probes exhibited highly enantioselective fluorescence enhancement towards l/d-phenylalaninol (l/d-PA). For example, when S-1 and R-1 were treated with l-PA or d-PA, they displayed different fluorescence responses: the enantiomeric fluorescence enhancement ratio (ef) was 2.51 and 0.41 for S-1 and R-1, respectively. Hence, a visible difference in fluorescence enhancement for l-PA and d-PA and excellent enantioselective behavior between S-1 and l-PA (or R-1 and d-PA) was displayed. Measurements of fluorescence lifetime, powder X-ray diffraction, molecular-dynamic simulations and Benesi-Hildebrand plots were employed to determine the observed high enantioselectivity for l/d-PA. In brief, we found that two post-modified HMOFs, S-1 and R-1, were outstanding enantioselective sensors for detecting l-PA and d-PA. They had a prominent difference in ef and remarkable enantioselectivity factor α and ΔΔG based on steric hindrance and stereochemical difference.

Homochiral metal-organic frameworks for enantioseparation.

Obtaining homochiral compounds is of high importance to human health and environmental sustainability. Currently, enantioseparation is one of the most effective approaches to obtain homochiral compounds. Thanks to their controlled synthesis and high efficiency, homochiral metal-organic frameworks (HMOFs) are one of the most widely studied porous materials to enable enantioseparation. In this review, we discuss the chiral pocket model in depth as the key to unlock enantioselective separation mechanisms in HMOFs. In particular, we classify our discussion of these chiral pockets (also regarded as "molecular traps") into: (a) achiral/chiral linker based helical channels as a result of packing modality; and (b) chiral pores inherited from chiral ligands. Driven by a number of mechanisms of enantioseparation, conceptual advances have been recently made in the design of HMOFs for achieving high enantioseparation performances. Herein, these are systematically categorised and discussed. Further we elucidate various applications of HMOFs as regards enantioseparation, systematically classifying them into their use for purification and related analytical utility according to the reported examples. Last but not the least, we discuss the challenges and perspectives concerning the rational design of HMOFs and their corresponding enantioseparations.