[5]Molecular Necklaces Formed Through Zn2+-Templated Binding of Macrocycles to Dynamically Formed Imines as Guest Recognition Sites.

Herein we demonstrate an "in-ring establishing" strategy for assembling interlocked molecules through dynamic imine formation, "establishing" the host recognition sites in situ. Using Zn2+ ions to template the assembly of a pyridine-containing macrocycle with semidumbbell-shaped triazole-containing aldehyde and amine derivatives, we obtained the corresponding [2]rotaxane in high yield (85%) after subsequent imine reduction (NaBH4) and amine protonation (NH4PF6). We performed the same three steps (assembly, reduction, protonation) to prepare a stable and highly symmetrical [5]molecular necklace ([5]MN) from 12 components (two almost-90°-oriented dialdehydes, two almost-90°-oriented diamines, four macrocycles, four Zn2+ ions) in an overall yield of 69%.

Synthesis of Adhesive Polyrotaxanes through Sequential Self-Assembly via Supramolecular Interactions and Dynamic Covalent Interactions.

Self-assembly is an effective approach to construct complicated structures. Polyrotaxanes (PRs) as one of the typical polymer types with complex structure, own interlocked structures and dynamic components, in which it results in unique characteristics and functions. Currently, the synthesis of which involves covalent reactions to hinder the development of polyrotaxanes. Herein, we employed supramolecular interactions as well as dynamic covalent bonds to synthesize PRs by sequential self-assembly. First, we prepared M1 possessing two diamine structures and M2 of a bisammonium salt with two dibenzylammonium (DBA) units modified by two stoppers at its ends, then M1 and M2 self-assembled into supramolecular polymers stemming from hydrogen bonding of [N+-H···O] under high concentrations. After adding 2,6-pyridinedicarboxaldehyde (M3), the imine bond formation enabled the generation of macrocycles, transforming supramolecular polymers into PRs. Besides, the solution of polyrotaxanes was applied as the adhesive for diverse hard and soft materials. This strategy provides an important approach for synthesizing PRs, accelerating the advances of mechanically interlocked polymers.

A pseudo-Double-Network Hydrogel Built upon Layered Double Hydroxides with Self-Strengthening Properties.

While great achievements have been made in the development of mechanically robust nanocomposite hydrogels, incorporating multiple interactions on the bases of two demensional inorganic cross-linkers to construct self-strengthening hydrogels has rarely been investigated. To this end, we propose here a new method for the coupling the dynamic covalent bonds and non-covalent interactions within a pseudo double-network system. The pseudo first network, formed through the Schiff Base reation between Tris-modified layered double hydroxides (Tris-LDHs) and oxidized dextran (ODex), is linked to the second network built upon non-covalent interactions between Tris-LDHs and poly(acrylamide-co-2-acrylamido-2-methyl-propanesulfonate) (p-(AM-co-AMPS). The swelling and mechanical properties of the resulting hydrogels have been investigated as a function of the ODex and AMPS contents. The as-prepared hydrogel can swell to 420 times of its original size and retain more than 99.9 wt.% of water. Mechanical tests show that the hydrogel can bear 90 % of compression and is able to be stretched to near 30 times of its original length. Cyclic tensile tests reveal that the hydrogels are capable of self-strengthening after mechanical training. The unique energy dissipation mechanism based on the dynamic covalent and non-covalent interactions is considered to be responsible for the outstanding swelling and mechanical performances.

Dynamic Covalent Bonds in the Ebselen Class of Antioxidants Probed by X-ray Quantum Crystallography.

Dynamic bonds are essential structural ingredients of dynamic covalent chemistry that involve reversible cleavage and formation of bonds. Herein, we explore the electronic characteristics of Se-N bonds in the organo-selenium antioxidant ebselen and its derivatives for their propensity to function as dynamic covalent bonds by employing high-resolution X-ray quantum crystallography and complementary computational studies. An analysis of the experimentally reconstructed X-ray wavefunctions reveals the salient electronic features of the Se-N bonds with very low electron density localized at the bonding region and a positive Laplacian value at the bond critical point. Bond orders and percentage covalency and ionicity estimated from the X-ray wavefunctions, along with localized orbital locator (LOL) and electron localization function (ELF) analyses show that the Se-N bond is unique in its closed shell-like features, despite being a covalent bond. Time-dependent DFT calculations simulate the cleavage of Se-N bonds in ebselen in the excited state, further substantiating their nature as dynamic bonds.

Strengthening-Durable Trade-Off and Self-Healing, Recyclable Shape Memory Polyurethanes Enabled by Dynamic Boron-Urethane Bonds.

Addressing the demand for integrating strength and durability reinforcement in shape memory polyurethane (SMPU) for diverse applications remains a significant challenge. Here a series of SMPUs with ultra-high strength, self-healing and recyclability, and excellent shape memory properties through introducing dynamic boron-urethane bonds are synthesized. The introducing of boric acid (BA) to polyurethane leading to the formation of dynamic covalent bonds (DCB) boron-urethane, that confer a robust cross-linking structure on the SMPUs led to the formation of ordered stable hydrogen-bonding network within the SMPUs. The flexible crosslinking with DCB represents a novel strategy for balancing the trade-off between strength and durability, with their strengths reaching up to 82.2 MPa while also addressing the issue of durability in prolonged usage through the provision of self-healing and recyclability. The self-healing and recyclability of SMPU are demonstrated through rapid dynamic exchange reaction of boron-urethane bonds, systematically investigated by dynamic mechanical analysis (DMA). This study sheds light on the essential role of such PU with self-healing and recyclability, contributing to the extension of the PU's service life. The findings of this work provide a general strategy for overcoming traditional trade-offs in preparing SMPUs with both high strength and good durability.

Relief of Residual Stress in Bulk Thermosets in the Glassy State by Local Bond Exchange.

The covalently cross-linked network gives thermosets superior thermal, mechanical, and electrical properties, which, however, squarely makes the large residual stress that is inevitably induced during preparation hardly relieved in the glassy state. In this work, an incredible reduction in residual stress is successfully achieved in bulk thermosets in the glassy state through introducing highly dynamic thiocarbamate bonds by "click" reactions of thiols and isocyanates. Due to the excellent dynamic behaviors of thiocarbamate bonds, local network rearrangement is achieved through thermal stimulation, while the strong 3D cross-linked network is well maintained. Ultimately, a decrease by 44% in residual stress is detected by simply annealing samples at 30 °C below glass transition temperature (Tg), during which they could well maintain more than 98.4% of the storage modulus. After the annealing, more uniform residual stress distribution is also observed, showing a 32% decline in sample standard deviation. However, the residual stress of epoxy resin, a typical thermoset as a reference, changes little even after annealing at Tg. The results prove it a feasible strategy to reduce residual stress in bulk thermosets in the glassy state by introducing proper dynamic covalent bonds.

Open shell versus closed shell bonding interaction in cyclopropane derivatives: EDA-NOCV analyses.

Cyclopropane ring is a very common motif in organic/bio-organic compounds. The chemical bonding of this strained ring is taught to all chemistry students. This three-membered cyclic, C3 ring is quite reactive which has attracted both, synthetic and theoretical chemists to rationalize/correlate its stability and bonding with its reactivity and physical properties over a century. There are a few bonding models (mainly the Bent-Bond model and Walsh model) of this C3 ring that are debated to date. Herein, we have carried out energy decomposition analysis coupled with natural orbital for chemical valence (EDA-NOCV) to study the two most reactive bonds of cyclopropane rings of 49 different organic compounds containing different functional groups to obtain a much deeper bonding insight toward a more general bonding model of this class of compounds. The EDA-NOCV analyses of fragment orbitals and susequent bond formation revealed that the nature of the CC bond of the cyclopropane (splitting two bonds at a time out of three CC bonds) ring is preferred to form two dative covalent CC bonds (between a singlet olefin-fragment and an excited singlet carbene-fragment with a vacant sp2 orbital and a filled p-orbital) for the majority (37/49) of compounds over two covalent electron sharing bonds in some (7/49) compounds (between an excited triplet olefin and triplet carbene), while a few (5/49) compounds show flexibility to adopt either the electron sharing or dative covalent bond as both are equally possible. The effects of functional groups on the nature of chemical bond in cyclopropane rings have been studied in detail. Our bonding analyses are in line with the QTAIM analyses which produce small negative values of the Laplacian, significantly positive values of bond ellipticity, and accumulation of electron densities around the ring critical point of C3 -rings. These corresponding QTAIM parameters of C3 -rings are quite different for CC single bonds of normal hydrocarbons as expected. The chemical bonding in the majority of cyclopropane rings can be very similar to those of metal-olefin systems.

Rational Design of Covalent Bond Engineered Encapsulation Structure toward Efficient, Long-Lived Multicolored Phosphorescent Carbon Dots.

Multicolored phosphorescent materials based on carbon dots (CDs) constructed using the same or similar precursors with long lifetimes are conducive to their wide range of practical applications due to the developed compatibility. Herein, a universal method is developed to prepare long-lived multicolored phosphorescent CD-based composites for which heavy-metal doping is not required. The multicolored CDs are encapsulated in silica via silane hydrolysis, which forms many covalent SiOC and SiC bonds; hence, the vibrations and rotations of the luminescent centers on the CD surfaces are hindered. The transformation of SiOC to a more rigid SiC moiety occurs during high-temperature calcination. Furthermore, during calcination, the silica collapses, resulting in more tightly encapsulated CDs. The synergistic effect of these two calcination phenomena produces blue, green, yellow, and red phosphorescence, at wavelengths spanning 465 to 680 nm and with lifetimes of up to 2.11 s. Taking advantage of their superior phosphorescence performances, the CD-based composites are successfully applied to 3D multichannel information storage and encryption.

Development of the Colorimetric and/or Fluorescent Probes for Detecting Fluoride ions in Aqueous Solution.

Fluoride ion is a strong Lewis base and one of the essential trace elements in human body. It plays a very important role in human health and ecological balance. The deficiency or excessive intake of fluoride ions will cause serious health problems, so the development of a sensitive and accurate detection method for fluoride ions is very important. The colorimetric and/or fluorescence sensing method has been a long standing attractive technique with high sensitivity and fast response. To date, most reported probes for fluoride ion are applicable only in organic solvents or organic-containing aqueous solutions. However, the probes for fluoride ion used in aqueous solution are more practically needed in view of environment protection and human health. In this paper, the materials and designing ideas of the colorimetric and/or fluorescent probes for fluoride ion based on different detection mechanisms in recent years were reviewed. Two main categories including formation of hydrogen bonds and formation of coordination covalent bonds were discussed. The latter one is further subdivided into three types, formation of B-F bond, formation of Si-F bond and formation of Mn+-F bond.

Carbon dots as a versatile tool to monitor insulin aggregation.

The possibility to monitor peptide and protein aggregation is of paramount importance in the so-called conformational diseases, as the understanding of many physiological pathways, as well as pathological processes involved in the development of such diseases, depends very much on the actual possibility to monitor biomolecule oligomeric distribution and aggregation. In this work, we report a novel experimental method to monitor protein aggregation, based on the change of the fluorescent properties of carbon dots upon protein binding. The results obtained in the case of insulin with this newly proposed experimental approach are compared with those obtained with other common experimental techniques normally used for the same purpose (circular dichroism, DLS, PICUP and ThT fluorescence). The greatest advantage of the hereby presented methodology over all the other experimental methods considered is the possibility to monitor the initial stages of insulin aggregation under the different experimental conditions sampled and the absence of possible disturbances and/or molecular probes during the aggregation process.

On the Nature of the Partial Covalent Bond between Noble Gas Elements and Noble Metal Atoms.

This article provides a discussion on the nature of bonding between noble gases (Ng) and noble metals (M) from a quantum chemical perspective by investigating compounds such as NgMY (Y=CN, O, NO3, SO4, CO3), [NgM-(bipy)]+, NgMCCH, and MCCNgH complexes, where M=Cu, Ag, Au and Ng=Kr-Rn, with some complexes containing the lighter noble gas atoms as well. Despite having very low chemical reactivity, noble gases have been observed to form weak bonds with noble metals such as copper, gold, and silver. In this study, we explore the factors that contribute to this unusual bonding behavior, including the electronic structure of the atoms involved and the geometric configuration of the concerned fragments. We also investigate the metastable nature of the resulting complexes by studying the energetics of their possible dissociation and internal isomerization channels. The noble gas-binding ability of the bare metal cyanides are higher than most of their bromide counterparts, with CuCN and AgCN showing higher affinity than their chloride analogues as well. In contrast, the oxides seem to have lower binding power than their corresponding halides. In the oxide and the bipyridyl complexes, the Ng-binding ability follows the order Au > Cu > Ag. The dissociation energies calculated, considering the zero-point energy correction for possible dissociation channels, increase as we move down the noble gas group. The bond between the noble gases and the noble metals in the complexes are found to have comparable weightage of orbital and electrostatic interactions, suggestive of a partial covalent nature. The same is validated from the topological analysis of electron density.

A Multifunctional Coating on Sulfur-Containing Carbon-Based Anode for High-Performance Sodium-Ion Batteries.

A sulfur doping strategy has been frequently used to improve the sodium storage specific capacity and rate capacity of hard carbon. However, some hard carbon materials have difficulty in preventing the shuttling effect of electrochemical products of sulfur molecules stored in the porous structure of hard carbon, resulting in the poor cycling stability of electrode materials. Here, a multifunctional coating is introduced to comprehensively improve the sodium storage performance of a sulfur-containing carbon-based anode. The physical barrier effect and chemical anchoring effect contributed by the abundant C-S/C-N polarized covalent bond of the N, S-codoped coating (NSC) combine to protect SGCS@NSC from the shuttling effect of soluble polysulfide intermediates. Additionally, the NSC layer can encapsulate the highly dispersed carbon spheres inside a cross-linked three-dimensional conductive network, improving the electrochemical kinetic of the SGCS@NSC electrode. Benefiting from the multifunctional coating, SGCS@NSC exhibits a high capacity of 609 mAh g-1 at 0.1 A g-1 and 249 mAh g-1 at 6.4 A g-1. Furthermore, the capacity retention of SGCS@NSC is 17.6% higher than that of the uncoated one after 200 cycles at 0.5 A g-1.

B-N/B-O Contained Heterocycles as Fusion Locker in Multi-Resonance Frameworks towards Highly-efficient and Stable Ultra-Narrowband Emission.

Fusing condensed aromatics into multi-resonance (MR) frameworks has been an exquisite strategy to modulate the optoelectronic properties, which, however, always sacrifices the small full width at half maxima (FWHM). Herein, we strategically embed B-N/B-O contained heterocycles as fusion locker into classical MR prototypes, which could enlarge the π-extension and alleviate the steric repulsion for an enhanced planar skeleton to suppress the high-frequency stretching/ scissoring vibrations for ultra-narrowband emissions. Sky-blue emitters with extremely small FWHMs of 17-18 nm are thereafter obtained for the targeted emitters, decreased by (1.4-1.9)-fold compared with the prototypes. Benefiting from their high photoluminescence quantum yields of >90 % and fast radiative decay rates of >108  s-1 , one of those emitters shows a high maximum external quantum efficiency of 31.9 % in sensitized devices, which remains 25.8 % at a practical luminance of 1,000 cd m-2 with a small FWHM of merely 19 nm. Notably a long operation half-lifetime of 1,278 h is also recorded for the same device, representing one of the longest lifetimes among sky-blue devices based on MR emitters.

Proton and Lithium Cations Linked to π-Electron and σ-Electron Systems: Are Such Interactions beyond or within the Definition of Hydrogen/Lithium Bond?

MP2/aug-cc-pVTZ calculations were performed on systems containing a proton or a lithium cation located between two π-electron systems or between π-electron and σ-electron units. The proton or the lithium cation attached to the acetylene or its derivative may be treated as the Lewis acid unit while the remaining part of the complex, the π-electron species or the dihydrogen, act as the Lewis base through their π-electrons or σ-electrons, respectively. The complexes analysed here are linked by the π⋅⋅⋅H+ /Li+ ⋅⋅⋅π and π⋅⋅⋅H+ /Li+ ⋅⋅⋅σ interactions. It is discussed whether these interactions are classified as hydrogen and lithium bonds. Therefore, different definitions of the latter interactions are presented. The Electron Localization Function (ELF) and the Natural Bond Orbital (NBO) approaches were applied to analyse the above-mentioned complexes. The unique properties of interactions with the proton and with the lithium cation that occur in complexes analysed here are described.

Polymeric Emissive Materials Based on Dynamic Covalent Bonds.

Dynamic covalent polymers, composed of dynamic covalent bonds (DCBs), have received increasing attention in the last decade due to their adaptive and reversible nature compared with common covalent linked polymers. Incorporating the DCBs into the polymeric material endows it with advanced performance including self-healing, shape memory property, and so forth. However, the emissive ability of such dynamic covalent polymeric materials has been rarely reviewed. Herein, this review has summarized DCBs-based emissive polymeric materials which are classified according to the different types of DCBs, including imine bond, acylhydrazone bond, boronic ester bond, dynamic C-C bond, as well as the reversible bonds based on Diels-Alder reaction and transesterification. The mechanism of chemical reactions and various stimuli-responsive behaviors of DCBs are introduced, followed by typical emissive polymers resulting from these DCBs. By taking advantage of the reversible nature of DCBs under chemical/physical stimuli, the constructed emissive polymeric materials show controllable and switchable emission. Finally, challenges and future trends in this field are briefly discussed in this review.

A Novel Mechanism for SIRT1 Activators That Does Not Rely on the Chemical Moiety Immediately C-Terminal to the Acetyl-Lysine of the Substrate.

SIRT1, an NAD+-dependent deacetylase, catalyzes the deacetylation of proteins coupled with the breakdown of NAD+ into nicotinamide and 2'-O-acetyl-ADP-ribose (OAADPr). Selective SIRT1 activators have potential clinical applications in atherosclerosis, acute renal injury, and Alzheimer's disease. Here, we found that the activity of the potent SIRT1 activator CWR is independent of the acetylated substrate. It adopts a novel mechanism to promote SIRT1 activity by covalently bonding to the anomeric C1' carbon of the ribose ring in OAADPr. In addition, CWR is highly selective for SIRT1, with no effect on SIRT2, SIRT3, SIRT5, or SIRT6. The longer distance between the anomeric C1' carbon of the ribose ring in OAADPr and Arg274 of SIRT1 (a conserved residue among sirtuins) than that between the anomeric C1' carbon in OAADPr and the Arg of SIRT2, SIRT3, SIRT5, and SIRT6, should be responsible for the high selectivity of CWR for SIRT1. This was confirmed by site-directed mutagenesis of SIRT3. Consistent with the in vitro assays, the activator also reduced the acetylation levels of p53 in a concentration-dependent manner via SIRT1 in cells. Our study provides a new perspective for designing SIRT1 activators that does not rely on the chemical moiety immediately C-terminal to the acetyl-lysine of the substrate.

Mechanochromic, Shape-Programmable and Self-Healable Cholesteric Liquid Crystal Elastomers Enabled by Dynamic Covalent Boronic Ester Bonds.

Endowing a cholesteric liquid crystal elastomer (CLCE) exhibiting a helicoidal nanostructure with dynamically tailorable functionalities is of paramount significance for its emerging applications in diverse fields such as adaptive optics and soft robotics. Here, a mechanochromic, shape-programmable and self-healable CLCE is judiciously designed and synthesized through integrating dynamic covalent boronic ester bonds into the main-chain CLCE polymer network. The circularly polarized reflection of CLCEs can be reversibly and dynamically tuned across the entire visible spectrum by mechanical stretching. Thanks to the introduction of dynamic boronic ester bonds, the CLCEs were found to show robust reprogrammable and self-healing capabilities. The research disclosed herein can provide new insights into the development of 4D (color and 3D shape) programmable photonic actuators towards bioinspired camouflage, adaptive optical systems, and next-generation intelligent machines.

Stable Noble Gas Compounds Based on Superelectrophilic Anions [B12 (BO)11 ]- and [B12 (OBO)11 ].

Superelectrophilic monoanions [B12 (BO)11 ]- and [B12 (OBO)11 ]- , generated from stable dianions [B12 (BO)12 ]2- and [B12 (OBO)12 ]2- , show great potential for binding with noble gases (Ngs). The binding energies, quantum theory of atoms in molecules (QTAIM), natural population analysis (NPA), energy decomposition analysis (EDA), and electron localization function (ELF) were carried out to understand the B-Ng bond in [B12 (BO)11 Ng]- and [B12 (OBO)11 Ng]- . The calculated results reveal that heavier noble gases (Ar, Kr, and Xe) bind covalently with both [B12 (BO)11 ]- and [B12 (OBO)11 ]- with large binding energies, making them potentially feasible to be synthesized. Only [B12 (OBO)11 ]- could form a covalent bond with helium or neon but the small binding energy of [B12 (OBO)11 He]- may pose a challenge for its experimental detection.

How anisotropic and isotropic atomic displacement parameters monitor protein covalent bonds rigidity: isotropic B-factors underestimate bond rigidity.

Under the assumption that covalent bonds are rigid, it is possible to compare the estimations of rigidity based on anisotropic and isotropic B-factors. This is done by computing the difference of the mean-square displacements (Delta-u) of atoms A and Z along the covalent bond A-Z, which must be close to zero for a rigid bond. The analysis of a high-quality set of protein structures, refined at a resolution better than (or equal to) 0.8 Angstroms, showed that Delta-u is significantly close to zero when anisotropic B-factors are used, with an average 60% Delta-u reduction. This reduction is larger for larger B-factors and this suggests that care should be taken in data-mining procedures that involve isotropic B-factors, especially at lower resolution, when anisotropic B-factors cannot be determined and when the average B-factor increases.

Injectable stress relaxation gelatin-based hydrogels with positive surface charge for adsorption of aggrecan and facile cartilage tissue regeneration.

BACKGROUND: Cartilage injury and pathological degeneration are reported in millions of patients globally. Cartilages such as articular hyaline cartilage are characterized by poor self-regeneration ability due to lack of vascular tissue. Current treatment methods adopt foreign cartilage analogue implants or microfracture surgery to accelerate tissue repair and regeneration. These methods are invasive and are associated with the formation of fibrocartilage, which warrants further exploration of new cartilage repair materials. The present study aims to develop an injectable modified gelatin hydrogel. METHOD: The hydrogel effectively adsorbed proteoglycans secreted by chondrocytes adjacent to the cartilage tissue in situ, and rapidly formed suitable chondrocyte survival microenvironment modified by ε-poly-L-lysine (EPL). Besides, dynamic covalent bonds were introduced between glucose and phenylboronic acids (PBA). These bonds formed reversible covalent interactions between the cis-diol groups on polyols and the ionic boronate state of PBA. PBA-modified hydrogel induced significant stress relaxation, which improved chondrocyte viability and cartilage differentiation of stem cells. Further, we explored the ability of these hydrogels to promote chondrocyte viability and cartilage differentiation of stem cells through chemical and mechanical modifications. RESULTS: In vivo and in vitro results demonstrated that the hydrogels exhibited efficient biocompatibility. EPL and PBA modified GelMA hydrogel (Gel-EPL/B) showed stronger activity on chondrocytes compared to the GelMA control group. The Gel-EPL/B group induced the secretion of more extracellular matrix and improved the chondrogenic differentiation potential of stem cells. Finally, thus hydrogel promoted the tissue repair of cartilage defects. CONCLUSION: Modified hydrogel is effective in cartilage tissue repair.