Ether Naphthotube Host-Guest Complexes and [2]Rotaxanes with Dications.

The interactions between ether naphthotube and a series of dication guests in organic solution were investigated. It was found that ether naphthotube formed stable host-guest complexes selectively with these guests in a 1 : 1 stoichiometric ratio with association constants ranging from 102 to 106 M-1, which were confirmed by 1H-NMR spectra and ITC experiments. The host-guest interactions are driven by enthalpy change as the entropic factors are unfavorable. Positive correlations between ΔH and ΔS have been observed in the host-guest complexes. Furthermore, the para-substitution of the guests can significantly affect the binding affinities through a combination of field/inductive and resonance effects by following a linear free energy relationship. Based on the host-guest complexes composed of ether naphthotube and organic cations, two interlocked [2]rotaxanes were prepared by cationization reaction and Huisgen cycloaddition between the cations and the stopper components. The ether naphthotube-based host-guest complexes are useful for creating sophisticated interlocked molecules.

N-aminomorpholine-functionalized bromine-doped carbon dots for hypochlorous acid detection in foods and imaging in live cells.

Hypochlorous acid (HClO) is used in food preservation. However, excessive HClO can deteriorate nutritional composition of food, compromise its quality, and potentially induce various diseases. Consequently, the development of multifunctional fluorescent probes for the sensitive and selective detection of HClO is highly anticipated for food safety. In this work, we designed a nanoprobe using N-aminomorpholine (AM)-functionalized bromine-doped carbon dots (Br-CDs-AM) for sensing HClO. This nanoprobe exhibits pH stability, strong resistance to photobleaching, superior long-term photostability (12 weeks), high sensitivity (19.3 nM), and an ultrarapid response (8 s) for detecting HClO residues in food matrices with percentage recovery (96.5 %-108 %) and RSDs less than 5.34 %. In addition, extremely low cytotoxicity and outstanding biocompatibility enable the nanoprobe to be used primarily for lysosome tracking and rapidly visualizing HClO in live cells. Thus, this study provides a new pathway to design unconventional nanoprobes for food safety assessment and subcellular organelle-specific imaging HClO.

Precise Recognition in Water by an Endo-Functionalized Cavity: Tuning the Complementarity of Binding Sites.

Precise binding towards structurally similar substrates is a common feature of biomolecular recognition. However, achieving such selectivity-especially in distinguishing subtle differences in substrates-with synthetic hosts can be quite challenging. Herein, we report a novel design strategy involving the combination of different rigid skeletons to adjust the distance between recognition sites within the cavity, which allows for the highly selective recognition of hydrogen-bonding complementary substrates, such as 4-chromanone. X-ray single-crystal structures and density functional theory calculations confirmed that the distance of endo-functionalized groups within the rigid cavity is crucial for achieving high binding selectivity through hydrogen bonding. The thermodynamic data and molecular dynamics simulations revealed a significant influence of the hydrophobic cavity on the binding affinity. The new receptor possesses both high selectivity and high affinity, which provide valuable insights for the design of customized receptors.

Chiral recognition of neutral guests by chiral naphthotubes with a bis-thiourea endo-functionalized cavity.

Developing chiral receptors with an endo-functionalized cavity for chiral recognition is of great significance in the field of molecular recognition. This study presents two pairs of chiral naphthotubes containing a bis-thiourea endo-functionalized cavity. Each chiral naphthotube has two homochiral centers which were fixed adjacent to the thiourea groups, causing the skeleton and thiourea groups to twist enantiomerically through chiral transfer. These chiral naphthotubes are highly effective at enantiomerically recognizing various neutral chiral molecules with an enantioselectivity up to 17.0. Furthermore, the mechanism of the chiral recognition has been revealed to be originated from differences in multiple non-covalent interactions. Various factors, such as the shape of cavities, substituents of guests, flexibility of host and binding modes are demonstrated to contribute to creating differences in the non-covalent interactions. Additionally, the driving force behind enantioselectivity is mainly attributed to enthalpic differences, and enthalpy -entropy compensation has also been observed to influence enantioselectivity.

Synthetic strategies, properties and sensing application of multicolor carbon dots: recent advances and future challenges.

Recently, carbon dots (CDs) as newly developed carbon-based nanomaterials due to advantages such as excellent photostability and easy surface functionalization have generated wide application prospects in fields such as biological imaging and chemical sensing. The multicolor emission carbon dots (M-CDs) were acquired through the selection of different carbon source precursors, change of synthesis conditions and synthesis environment. Therefore, the aim of this review is to summarize the latest research progress in polychromatic CDs from the perspectives of synthesis strategies, luminescent mechanisms, luminescent properties and applications. This review focuses on how to prepare MCDs by changing raw materials and synthesis conditions such as reaction temperature, synthesis time, synthesis pH, and synthesis solvent. This review also presents the optical properties of MCDs, concentration effects, solvent effects, pH effects, elemental doping, and surface passivation on them, as well as their creative applications in the field of sensing applications. It is anticipated that this review will serve as a guide for the development of multifunctional M-CDs and inspire future research on controllable design and preparation of M-CDs.

Selective adsorption of trace morpholine impurities over N-ethyl morpholine by tetralactam solids.

The separation and especially the removal of morpholine (MOR) impurities from N-ethyl morpholine (NEM) is of great significance in the fine chemical industry. Herein, we offer a novel strategy for the selective adsorption of MOR over NEM by tetralactam solids. The adsorbent realized the purification of NEM by adsorbing traces of MOR impurities, such that the purity can be improved from around 98% to over 99.5%. Single crystal structures indicate that N-H⋯O and N-H⋯N hydrogen bonding interactions are essential in the selective separation.

Tetralactam macrocycle based indicator displacement assay for colorimetric and fluorometric dual-mode detection of urinary uric acid.

An indicator displacement assay for colorimetric and fluorometric dual-mode detection of urinary uric acid (UA) was constructed using a water-soluble naphthalene-based tetralactam macrocycle and the phenoxazine dye, resorufin (RF). The visual detection of UA levels of volunteers was successfully realized using modified paper assays, which could be used for the home monitoring of urinary UA.

Mechanical Training Enabled Reinforcement of Polyrotaxane-Containing Hydrogel.

Many strategies have been developed for constructing anisotropic hydrogels, however, it remains a challenge to fabricate hydrogels with anisotropic nanocrystalline domains from intrinsically soft networks. Here, we report a naphthotube-based polyrotaxane-containing hydrogel that can be reinforced via mechanical training. During the training process, the hydrogel can adopt reorientation of polymer chains to form anisotropic structures driven by external uniaxial force. Due to the multiple hydrogen bonding sites and movable feature of naphthotube, the sliding of naphthotube on PEG chains simultaneously inducing the zipping of adjacent polymer chains to form densely anisotropic nanocrystalline domains through hydrogen bonded networks. Thus, the trained hydrogel exhibits an enhanced tension stress of ≈110 kPa, which realize a remarkable enhancement of ≈10 times compare to initial state. This study provides a new tactic for improving the mechanical performance of soft materials.

Conformational Effects on the Threading Kinetics of Dumbbell-Shaped Guests into the Cavity of Oxatub[4]arene.

Unprecedented threading kinetics were revealed between viologen-based guests and conformationally adaptive oxatub[4]arene. Three representative conformations of oxatub[4]arene are involved in the kinetic and thermodynamic products which follow the opposite orders in their rankings. Consequently, error correction was involved and a complex kinetic process was observed in a simple two-component system. Moreover, it was found that some viologen-based guests have much faster threading kinetics than those of DABCO-based with the same stoppers. This was enabled by an unprecedented threading mechanism in which a tilted conformation of the guests is adopted by involving one linear alkyl group on the 3,5-dialkoxybenzyl stoppers, the viologen core, and the methylene spacers in the transition states. This new mechanism even allows the viologen-based guests with the 3,5-dicetyloxybenzyl stoppers to form a pseudorotaxane with oxatub[4]arene.

Synthesis, Configurational Analysis, Molecular Recognition and Chirality Sensing of Methylene-Bridged Naphthotubes.

Macrocyclic hosts with a well-defined cavity are particularly appealing for supramolecular chemistry, but they are still rare. In this research, we reported a new class of macrocyclic hosts, namely methylene-bridged naphthotubes, with well-defined cavities. They were synthesized through TFA-catalyzed Friedel-Crafts reactions between alkoxy-substituted bisnaphthalenes and paraformaldehyde. A configurational selection was observed. Three-membered macrocycles possess a single cavity, while four-membered macrocycles adopt a double-cavity conformation or a self-filling conformation depending on the alkoxy length. The small homologue shows quite strong binding affinities (up to 108  M-1 ) to organic cations, which is better than structurally similar but flexible macrocycles. This demonstrates the advantage of a well-defined cavity in molecular recognition. Moreover, these naphthotubes can be used as chirality sensors for chiral organic cations through different chirality transfer mechanisms.

Molecular recognition and spectral tuning of organic dyes in water by amide naphthotubes.

Molecular recognition and spectral tuning of 13 organic dyes were achieved in water by amide naphthotubes. The association affinity to a styryl derivative is up to 4.5 × 107 M-1, which is the highest among all the known hosts. In addition, great fluorescence enhancement was observed for styryl derivatives. This would lay a basis for the potential analysis application.

Molecular recognition and photoprotection of riboflavin in water by a biomimetic host.

A water-soluble tetralactam macrocycle with 2,6-diethoxynaphthalene groups as side walls is able to strongly bind riboflavin (Ka >107 M-1) in water through hydrogen bonding and the hydrophobic effect. The encapsulated riboflavin can be stabilized by the host against photo-degradation under UV-vis irradiation, which may be harnessed to extend the shelf life of riboflavin.

Biomimetic Recognition of Quinones in Water by an Endo-Functionalized Cavity with Anthracene Sidewalls.

Selective molecular recognition in water is the foundation of numerous biological functions but is a challenge for most synthetic hosts. We employ the concept of endo-functionalized cavity and the strategy of simultaneous construction to address this issue. The concept and the strategy were demonstrated in the construction of a biomimetic host for selectively recognizing quinones in water. The host was synthesized by joining two pieces of bent anthracene dimer through amide bond formation, affording a deep hydrophobic cavity and inward-directing hydrogen bonding sites. The host can recognize quinones over their close analogues in water, and its association affinity to p-benzoquinone is the highest among all the known hosts and is even comparable to that of the bioreceptor. The binding with an anthraquinone reaches nanomolar affinity. Shielded hydrogen bonding, C-H⋅⋅⋅π, and charge transfer interactions, and the hydrophobic effect are responsible for the high binding affinity and selectivity.

Fluorescence detected circular dichroism (FDCD) for supramolecular host-guest complexes.

Fluorescence-detected circular dichroism (FDCD) spectroscopy is applied for the first time to supramolecular host-guest and host-protein systems and compared to the more known electronic circular dichroism (ECD). We find that FDCD can be an excellent choice for common supramolecular applications, e.g. for the detection and chirality sensing of chiral organic analytes, as well as for reaction monitoring. Our comprehensive investigations demonstrate that FDCD can be conducted in favorable circumstances at much lower concentrations than ECD measurements, even in chromophoric and auto-emissive biofluids such as blood serum, overcoming the sensitivity limitation of absorbance-based chiroptical spectroscopy. Besides, the combined use of FDCD and ECD can provide additional valuable information about the system, e.g. the chemical identity of an analyte or hidden aggregation phenomena. We believe that simultaneous FDCD- and ECD-based chiroptical characterization of emissive supramolecular systems will be of general benefit for characterizing fluorescent, chiral supramolecular systems due to the higher information content obtained by their combined use.

Effective and Rapid Removal of Polar Organic Micropollutants from Water by Amide Naphthotube-Crosslinked Polymers.

It is challenging to remove polar organic micropollutants from water through adsorption-mediated processes. Macrocycle-crosslinked polymers were recently shown to be effective adsorbents for nonpolar or charged organic micropollutants through specific host-guest binding, but are rarely used for the treatment of neutral and polar organic micropollutants. This is due to the challenge of recognizing polar molecules in water by macrocyclic hosts. In this research, we report two amide naphthotube-crosslinked polymers which can effectively and rapidly adsorb a wide scope of polar organic micropollutants from water through biomimetic molecular recognition. Amide naphthotubes possess hydrogen bonding sites in their deep hydrophobic cavities and can effectively bind polar organic micropollutants in water through the hydrophobic effects and shielded hydrogen bonds. The cross-linked polymers containing amide naphthotubes are even able to remove a complex mixture of organic micropollutants from water and the used materials can be easily regenerated through washing with MeOH or EtOH. This research provides a solution for the treatment of polar organic micropollutants by using biomimetic molecular recognition in water.

Multiple Stimuli-Responsive Conformational Exchanges of Biphen[3]arene Macrocycle.

Conformational exchanges of synthetic macrocyclic acceptors are rather fast, which is rarely studied in the absence of guests. Here, we report multiple stimuli-responsive conformational exchanges between two preexisting conformations of 2,2',4,4'-tetramethoxyl biphen[3]arene (MeBP3) macrocycle. Structures of these two conformations are both observed in solid state, and characterized by 1H NMR, 13C NMR and 2D NMR in solution. In particular, conformational exchanges can respond to solvents, temperatures, guest binding and acid/base addition. The current system may have a role to play in the construction of molecular switches and other stimuli-responsive systems.

Stabilization of the Closed-Ring Isomer of Spiropyran by Amide Naphthotube in Water and Its Application in Naked-Eye Detection of Toxic Paraoxon.

The thermodynamically unstable, colourless closed-ring isomer of spiropyran can be stabilized in water by the anti-configurational isomer of amide naphthotube. The influence of the binding on the thermodynamics and kinetics of spiropyran have been studied. The complex was further used to prepare a test paper that allows naked-eye detection of toxic paraoxon.

Adsorptive Separation of Benzene, Cyclohexene, and Cyclohexane by Amorphous Nonporous Amide Naphthotube Solids.

Benzene hydrogenation is an important industrial process. The reaction is incomplete, resulting in a mixture of benzene, cyclohexane, and/or cyclohexene that have to be separated before any further reactions. The currently used extractive and azeotropic distillations are operationally complex and energy intensive. Adsorptive separation provides an alternative energy-efficient method. However, the separation of the ternary mixture by adsorptive separation has not yet been reported. In the present research, we report two macrocyclic hosts with hydrogen-bonding sites in their cavities that are able to separate the ternary mixture of benzene, cyclohexene, and cyclohexane. N-H⋅⋅⋅π interactions were found to play a key role in the selective separation. In addition, fast adsorption, high loading ratios, and easy recycling are achieved with the present system, which is promising for practical applications.

A supramolecular system that strictly follows the binding mechanism of conformational selection.

Induced fit and conformational selection are two dominant binding mechanisms in biology. Although induced fit has been widely accepted by supramolecular chemists, conformational selection is rarely studied with synthetic systems. In the present research, we report a macrocyclic host whose binding mechanism is unambiguously assigned to conformational selection. The kinetic and thermodynamic aspects of this system are studied in great detail. It reveals that the kinetic equation commonly used for conformational selection is strictly followed here. In addition, two mathematical models are developed to determine the association constants of the same guest to the two host conformations. A "conformational selectivity factor" is defined to quantify the fidelity of conformational selection. Many details about the kinetic and thermodynamic aspects of conformational selection are revealed by this synthetic system. The conclusion and the mathematical models reported here should be helpful in understanding complex molecular recognition in both biological and synthetic systems.

Conformationally adaptive macrocycles with flipping aromatic sidewalls.

Conformationally adaptive macrocycles possess multiple well-defined conformations through quickly flipping their aromatic sidewalls. The macrocycles combine the binding power of all the conformations. Upon binding a guest, one or a combination of conformations are selected to achieve the maximized binding affinity. In addition, the complex conformational network is responsive to changes in temperature or solvent. It has been demonstrated that these macrocycles have unique properties in chirality sensing, stimuli-responsive self-assembly, and molecular switches. In this tutorial review, we summarize recent advances on conformationally adaptive macrocycles with an emphasis on our own research. We believe that this class of macrocycles will have a bright future in supramolecular chemistry and beyond.