Helical Cage Rotors Switched on by Brake Molecule with Variable Fluorescence and Circularly Polarized Luminescence.

While chiral molecular rotors have unique frames and cavities to possibly generate switchable chiroptical functions, it still remains a formidable challenge to precisely restrict their rotations to activate certain functions such as fluorescence as well as circularly polarized luminescence (CPL), which are strongly related to the local molecular rotations. Herein, we design a pair of enantiopure helical cage rotors, which emit light neither at the molecular state nor in the crystal or aggregation states, although they contain luminophore groups. However, upon mounting with fluoroaromatic borane (TFPB) as a molecular brake, the phenyl rotation of the helical cage can be effectively hindered and fluorescence and CPL activities of the molecular cage are switched on. Crystal structure analysis reveals that the rotation is restricted through synergistic B-O-H-N bonding and a fluoroaromatic-aromatic (ArF-Ar) dipole interaction. Moreover, the helical cages are switched on stepwise with color-variable fluorescence and CPL by the inner brake in the molecular state and the outer brake in the supramolecular assemblies, respectively. This work not only provides the design idea of chiroptical molecular rotors but also unveils how fluorescence and CPL could be generated in cage rotor systems.

Selective Electrochemical Oxidation of Benzylic C-H to Benzylic Alcohols with the Aid of Imidazolium Radical Mediators.

Selective oxidation of benzylic C-H to benzylic alcohols is a well-known challenge in the chemical community since benzylic C-H is more prone to be overoxidized to benzylic ketones. In this work, we report the highly selective electro-oxidation of benzylic C-H to benzylic alcohols in an undivided cell in ionic liquid-based solution. As an example, the selectivity toward xanthydrol could be as high as 95.7% at complete conversion of xanthene, a typical benzylic C-H compound, on gram-scale in imidazolium bromide/H2O/DMF. Mechanism investigation reveals that the imidazolium radical generated in situ participants in a proton-coupled electron transfer process and low-barrier hydrogen bonds stabilize the reaction intermediates, together steering the redox equilibrium, favoring benzylic alcohols over benzylic ketones.

Xenon Induces Its Own Preferred Heterochiral Host from Exclusive Homochiral Assembly.

Xenon binding represents a formidable challenge, and efficient hosts remain rare. Here we report our findings that while enantiomeric bis(urea)-bis(thiourea) macrocycles form exclusive homochiral dimeric assemblies, xenon is able to overcome the narcissism and induces an otherwise-nonobservable heterochiral assembly as its preferred host. An experimental approach and fitting model were developed to obtain binding constants associated with the invisible assembly species. The determined xenon binding affinity with the heterochiral capsule reaches 1600 M-1, which is 15 times higher than that with the homochiral capsule and represents the highest record for an assembled host. The origin of the large difference in xenon affinity between the two subtle diastereotopic assemblies was revealed by single-crystal analysis. In the heterochiral capsule with S4 symmetry, the xenon atom is more tightly enclosed by van der Waals surroundings of the four thiourea groups arranged in a spherical cross-array, superior to the antiparallel array in the homochiral capsule with D2 symmetry.

From Biomass to Functional Crystalline Diamond Nanothread: Pressure-Induced Polymerization of 2,5-Furandicarboxylic Acid.

2,5-Furandicarboxylic acid (FDCA) is one of the top-12 value-added chemicals from sugar. Besides the wide application in chemical industry, here we found that solid FDCA polymerized to form an atomic-scale ordered sp3-carbon nanothread (CNTh) upon compression. With the help of perfectly aligned π-π stacked molecules and strong intermolecular hydrogen bonds, crystalline poly-FDCA CNTh with uniform syn-configuration was obtained above 11 GPa, with the crystal structure determined by Rietveld refinement of the X-ray diffraction (XRD). The in situ XRD and theoretical simulation results show that the FDCA experienced continuous [4 + 2] Diels-Alder reactions along the stacking direction at the threshold C···C distance of ∼2.8 Å. Benefiting from the abundant carbonyl groups, the poly-FDCA shows a high specific capacity of 375 mAh g-1 as an anode material of a lithium battery with excellent Coulombic efficiency and rate performance. This is the first time a three-dimensional crystalline CNTh is obtained, and we demonstrated it is the hydrogen bonds that lead to the formation of the crystalline material with a unique configuration. It also provides a new method to move biomass compounds toward advanced functional carbon materials.

Rhodium-Catalyzed ON-OFF Switchable Hydrogenation Using a Molecular Shuttle Based on a [2]Rotaxane with a Phosphine Ligand.

A novel pH-responsive molecular shuttle based on a [2]rotaxane with a phosphine ligand has been designed and synthesized. In the rhodium-catalyzed hydrogenation of α,ß-dehydroamino acid esters and aryl enamides, ON/OFF-switchable catalysis was accomplished with high ON/OFF ratios by adjusting the movements of the rotaxane wheels located at the catalyst terminals with acid/base. Mechanistic studies using NMR spectroscopy and quasi in situ X-ray photoelectron spectroscopy revealed that RhIII -hydride species are possibly formed in a H2 atmosphere when the catalyst is in the OFF state. During the reaction, a heterolytic activation of dihydrogen occurs by the interlocked rotaxane dibenzylamine and RhI catalytic center acting as a frustrated Lewis pair. Subsequent homolytic splitting of dihydrogen with the newly formed RhI -hydride species generates RhIII -hydride species. These findings show that a substrate-selective hydrogenation can be achieved by using the OFF-state catalyst.

A Disulfide Switch Providing Absolute Handedness Control in Double Helices via Conversion from the Antiparallel to Parallel Helical Pattern.

A strategy to reversibly switch the parallel/antiparallel helical conformation of aromatic double helices through the formation/breakage of a disulfide bond is presented. Single-crystal X-ray structures, NMR, and circular dichroism spectroscopy demonstrate that the double helices with terminal thiol groups favor an antiparallel helical arrangement both in the solid state and in solution, while the P/M bias of helicity induced by chiral segments from another extremity of the sequence is weak in this structural motif. The antiparallel helices can be rearranged to parallel helices through the disulfide connection of the sequences. This change enhances the bias of helical handedness and results in absolute chirality control of the double helices. The handedness-mediated process can be governed by the oxidation-reduction cycle, thereby switching the structural arrangement and the enhancement of chiral bias. In addition, we find that the sequences can dimerize into an intermolecular double helix with the disulfide connection. And the helical handedness is also fully controlled due to the head-to-head structural motif.

Bioinspired Crowding Inhibits Explosive Ice Growth in Antifreeze Protein Solutions.

Antifreeze (glyco)proteins (AF(G)Ps) are naturally evolved ice inhibitors incomparable to any man-made materials, thus, they are gaining intensive interest for cryopreservation and beyond. AF(G)Ps depress the freezing temperature (Tf) noncolligatively below the melting temperature (Tm), generating a thermal hysteresis (TH) gap, within which the ice growth is arrested. However, the ice crystals have been reported to undergo a retaliatory and explosive growth beyond the TH gap, which is lethal to living organisms. Although intensive research has been carried to inhibit such an explosive ice growth, no satisfactory strategy has been discovered until now. Here, we report that crowded solutions mimicking an extracellular matrix (ECM), in which AF(G)Ps are located, can completely inhibit the explosive ice growth. The crowded solutions are the condensates of liquid-liquid phase separation consisting of polyethylene glycol (PEG) and sodium citrate (SC), which possess a nanoscale network and strong hydrogen bond (HB) forming ability, completely different to crowded solutions made of single components, that is, PEG or SC. Due to these unique features, the dynamics of the water is significantly slowed down, and the energy needed for breaking the HB between water molecules is distinctly increased; consequently, ice growth is inhibited as the rate of water molecules joining the ice is substantially reduced. The present work not only opens a new avenue for cryopreservation, but also suggests that the ECM of cold-hardy organisms, which also exhibit great water confining properties, may have a positive effect in protecting the living organisms from freezing damage.

Regulating the Structures of Self-Assembled Mechanically Interlocked Moleculecular Constructs via Dianion Precursor Substituent Effects.

Substituent effects play critical roles in both modulating reaction chemistry and supramolecular self-assembly processes. Using substituted terephthalate dianions (p-phthalic acid dianions; PTADAs), the effect of varying the type, number, and position of the substituents was explored in terms of their ability to regulate the inherent anion complexation features of a tetracationic macrocycle, cyclo[2](2,6-di(1H-imidazol-1-yl)pyridine)[2](1,4-dimethylenebenzene) (referred to as the Texas-sized molecular box; 14+), in the form of its tetrakis-PF6- salt in DMSO. Several of the tested substituents, including 2-OH, 2,5-di(OH), 2,5-di(NH2), 2,5-di(Me), 2,5-di(Cl), 2,5-di(Br), and 2,5-di(I), were found to promote pseudorotaxane formation in contrast to what was seen for the parent PTADA system. Other derivatives of PTADA, including those with 2,3-di(OH), 2,6-di(OH), 2,5-di(OMe), 2,3,5,6-tetra(Cl), and 2,3,5,6-tetra(F) substituents, led only to so-called outside binding, where the anion interacts with 14+ on the outside of the macrocyclic cavity. The differing binding modes produced by the choice of PTADA derivative were found to regulate further supramolecular self-assembly when the reaction components included additional metal cations (M). Depending on the specific choice of PTADA derivatives and metal cations (M = Co2+, Ni2+, Zn2+, Cd2+, Gd3+, Nd3+, Eu3+, Sm3+, Tb3+), constructs involving one-dimensional polyrotaxanes, outside-type rotaxanated supramolecular organic frameworks (RSOFs), or two-dimensional metal-organic rotaxane frameworks (MORFs) could be stabilized. The presence and nature of the substituent were found to dictate which specific higher order self-assembled structure was obtained using a given cation. In the specific case of the 2,5-di(OH), 2,5-di(Cl), and 2,5-di(Br) PTADA derivatives and Eu3+, so-called MORFs with distinct fluorescence emission properties could be produced. The present work serves to illustrate how small changes in guest substitution patterns may be used to control structure well beyond the first interaction sphere.

Highly Selective Binding and Inhibition of Pyr-His-Pro-NH2 (TRH) Function using a Polypyridinyl Macrocyclic Receptor with an Amphiphilic Cavity.

Macrocycle, cyclo[4] [(1,3-(4,6)-dimethylbezene)[4](2,6-(3,5)-dimethylpyridine (B4P4), shows highly selective binding affinity with protirelin (Pyr-His-Pro-NH2 ; TRH) among the tested 26 drug or drug adductive substrates. The stable complexation in a 1:1 manner was fully characterized in solution, gas phase, and solid state study. Furthermore, B4P4 acts as an efficient TRH inhibitor even at [macrocycle]:[drug] <1:300, both in membrane transport and cellar incubation. The current work provides an unprecedented strategy for macrocycles to be efficiently used in drug target therapy.

Hydrogen-Bonding Catalyzed Ring-Closing C-O/C-O Metathesis of Aliphatic Ethers over Ionic Liquid under Metal-Free Conditions.

O-heterocycles have wide applications, and their efficient and green synthesis is very interesting. Herein, we report hydrogen-bonding catalyzed ring-closing metathesis of aliphatic ethers to O-heterocycles over ionic liquid (IL) catalyst under metal- and solvent-free conditions. The IL 1-butylsulfonate-3-methylimidazolium trifluoromethanesulfonate ([SO3 H-BMIm][OTf]) is discovered to show outstanding performance, better than the reported catalysts. An interface effect plays an important role in mediating the reaction rate due to the immiscibility between the products and the IL catalyst, and the products can be spontaneously separated. NMR analysis and DFT calculation suggest that a pair of cation and anion of [SO3 H-BMIm][OTf] could form three strong H-bonds with an ether molecule, which catalyze the ether transformation via a cyclic oxonium intermediate. A series of O-heterocycles including tetrahydrofurans, tetrahydropyrans, morpholines and dioxane can be obtained from their corresponding ethers in excellent yields (e.g., >99 %). This work opens an efficient and metal-free way to produce O-heterocycles from aliphatic ethers.

Excimer Disaggregation Enhanced Emission: A Fluorescence "Turn-On" Approach to Oxoanion Recognition.

A new approach to anion sensing that involves excimer disaggregation induced emission (EDIE) is reported. It involves the anion-mediated disaggregation of the excimer formed from a cationic macrocycle. This leads to an increase in the observed fluorescence intensity. The macrocycle in question, cyclo[1] N2, N6-dimethyl- N2, N6-bis(6-(1 H-imidazolium-1-yl)pyridin-2-yl)pyridine-2,6-diamine[1]1,4-dimethylbenzene (12+; prepared as its PF6- salt), is obtained in ca. 70% yield via a simple cyclization. X-ray diffraction analyses of single crystals revealed that, as prepared, this macrocycle exists in a supramolecular polymeric form in the solid state. Macrocycle 12+ is weakly fluorescent in acetonitrile. The emission intensity is concentration dependent, with the maximum intensity being observed at [12+] ≈ 0.020 mM. This finding is ascribed to formation of an excimer, followed possibly by higher order aggregates as the concentration of 12+ is increased. Addition of tetrabutylammonium pyrophosphate (HP2O73-) to 12+ (0.020 mM in acetonitrile) produces a ca. 200-fold enhancement in the emission intensity (λex = 334 nm; λem = 390-650 nm). These findings are rationalized in terms of the HP2O73- serving to break up essentially non-fluorescent excited-state dimers of 12+ through formation of a highly fluorescent anion-bound monomeric complex, 12+·HP2O73-. A turn-on in the fluorescence intensity is also seen for H2PO4- and, to a lesser extent, HCO3-. Little (HSO4-, NO3-) or essentially no (N3-, SCN-, F-, Cl-, Br- and I-) response is seen for other anions. Solid-state structural analysis of single crystals obtained after treating 12+ with HP2O73- in the presence of water revealed a salt form wherein a H2P2O72- anion sits above the cone-like macrocycle.

From CO2 to 4 H-Quinolizin-4-ones: A One-Pot Multicomponent Approach via Ag2O/Cs2CO3 Orthogonal Tandem Catalysis.

Using carbon dioxide as a C1 precursor, here we report relatively simple and cost-effective orthogonal tandem catalysis, namely Ag2O in conjunction with Cs2CO3 serves to promote a multicomponent tandem reaction forming two new C-C and one new C-N bonds. 4 H-Quinolizin-4-ones, key skeletal components in a variety of biologically active molecules, were obtained with yields up to 99%. The present approach features a broad substrate scope and mild reaction conditions and benefits from using cost-effective reaction and catalysts.

Effect of Polyvinyl Alcohol on Ice Formation in the Presence of a Liquid/Solid Interface.

Tuning ice formation is of great importance in biological systems and some technological applications. Many synthetic polymers have been shown to affect ice formation, in particular, polyvinyl alcohol (PVA). However, the experimental observations of the effect of PVA on ice formation are still conflicting. Here, we introduced colloidal silica (CS) as the model liquid/solid interface and studied the effect of PVA on ice formation in detail. The results showed that either PVA or CS promoted ice formation, whereas the mixture of these two (CS-PVA) prevented ice formation (antifreezing). Using quantitative analysis based on classical nucleation theory, we revealed that the main contribution came from the kinetic factor J0 rather than the energy barrier factor Γ. Combined with the PVA adsorption behavior on CS particles, it is strongly suggested that the adsorption of PVA at the interface has significantly reduced ice nucleation, which thus may provide new ideas for developing antifreezing agents.

Novel fluorescent cationic benzothiazole dye that responds to G-quadruplex aptamer as a novel K+ sensor.

A fluorescent cationic benzothiazole dye that selectively targets a G-quadruplex aptamer was designed and synthesized as a K+ sensor. The K+-driven aptamer sensor is based on the strategy of conformational transition from single-stranded DNA to G-quadruplex structure, leading to an amplified fluorescence signal in the reporter. This fluorescent sensor displayed high selectivity for K+, suggesting great potential for practical applications.

Directly lighting up RNA G-quadruplexes from test tubes to living human cells.

RNA G-quadruplexes (G4s) are one of the key components of the transcriptome that act as efficient post-transcriptional regulatory elements in living cells. To conduct further studies of the unique biological functions of RNA G4s, techniques need to be developed that can efficiently recognize RNA G4 structures under various conditions, in fixed cells and living cells, as well as in vitro. This paper presents the development of such a method, a new technique using a cyanine dye called CyT, which can detect both canonical and non-canonical RNA G4 structures from test tubes to living human cells. The ability of CyT to distinguish between G4 and nonG4 RNA offers a promising tool for future RNA G4-based biomarker discovery and potential diagnostic applications.

Selective Utilization of the Methoxy Group in Lignin to Produce Acetic Acid.

Selective transformation of lignin into a valuable chemical is of great importance and challenge owing to its complex structure. Herein, we propose a strategy for the transformation of methoxy group (-OCH3 ) which is abundant in lignin into pure highly valuable chemicals. As an example to apply this strategy, a route to produce acetic acid with high selectivity by conversion of methoxy group of lignin was developed. It was demonstrated that the methoxy group in lignin could react with CO and water to generate acetic acid over RhCl3 in the presence of a promoter. The conversions of methoxy group in the kraft lignin and organosolv lignin reached 87.5 % and 80.4 %, respectively, and no by-product was generated. This work opens the way to produce pure chemicals using lignin as the feedstock.

Directional Molecular Transportation Based on a Catalytic Stopper-Leaving Rotaxane System.

Ratchet mechanism has proved to be a key principle in designing molecular motors and machines that exploit random thermal fluctuations for directional motion with energy input. To integrate ratchet mechanism into artificial systems, precise molecular design is a prerequisite to control the pathway of relative motion between their subcomponents, which is still a formidable challenge. Herein, we report a straightforward method to control the transportation barrier of a macrocycle by selectively detaching one of the two stoppers using a novel DBU-catalyzed stopper-leaving reaction in a rotaxane system. The macrocycle was first allowed to thread onto a semidumbbell axle from the open end and subsequently thermodynamically captured into a nonsymmetrical rotaxane. Then, it was driven energetically uphill until it reached a kinetically trapped state by destroying its interaction with ammonium site, and was finally quantitatively released from the other end when the corresponding stopper barrier was removed. Although the directional transportation at the present system was achieved by discrete chemical reactions for the sake of higher transportation efficiency, it represents a new molecular transportation model by the strategy of using stopper-leavable rotaxane.

Flexible, Linear Chains Act as Baffles To Inhibit the Intramolecular Rotation of Molecular Turnstiles.

In artificial molecular devices, flexible, linear chains typically exhibit very weak capability in inhibiting molecular motion. Herein, we describe the dynamic properties of a series of molecular turnstiles consisting of a rigid frame and a phenyl rotator flanked with linear alkoxymethyl substituents. The long, flexible substituents act as elastic baffles to inhibit the rotations of the rotator at medium to fast speeds on the NMR time scale. When the rotator moves slowly, the substituents become more relaxed, thus obtaining an opportunity to completely thread through the cavity of the turnstiles. These findings reveal a basic but missing correlation between steric hindrance and speed of motion for flexible, linear chains in dynamic molecular devices, thus opening up a new direction toward molecular machines with more elaborate dynamic functions.

Self-Assembly of a [2]Pseudorotaxane by an Inchworm-Motion Mechanism.

The threading of biomolecules through pores or channels in membranes is important to validate the physiological activities of cells. To aid understanding of the controlling factors required for the translocation in space with confined size and distorted conformation, it is desirable to identify experimental systems with minimized complexity. We demonstrate the mechanism of a linear guest L1 threading into a tris(crown ether) host TC with a combinational distorted cavity to form a triply interlocked [2]pseudorotaxane 3in-[L1⊂TC]. An inchworm-motion mechanism is proposed for the process. For the forward-threading steps that lead to the formation of higher-order interlocked species, guest L1 must adopt a bent conformation to find the next crown ether cavity. Two simplified models are applied to investigate the self-assembly dynamic of 3in-[L1⊂TC]. Kinetic NMR spectroscopic and molecular dynamics (MD) studies show that formation of the singly penetrated species is fast, whereas formation of the doubly and triply threaded species is several orders of magnitude slower. During threading the freedom of both the guest L1 and host TC gradually decrease due to their interactions. This results in a significant entropy effect for the threading dynamic, which is also observed for the threading of a biomolecular chain through a channel.

Characterizing the Adsorption of Poly(vinyl alcohol) on Colloidal Silica with Aggregation-Induced Emission Fluorophore.

The adsorption of polymer on colloidal particle has significant influence on colloid structure and dynamics. Here we introduce a new method to monitor the adsorption in situ, based on the different emission behavior of aggregation-induced emission (AIE) luminogen in different micro environments. Poly(vinyl alcohol) (PVA) and colloidal silica (CS) were used as a model system. It was found that AIE molecules exhibited extremely low fluorescence intensity in water and PVA solution, while their emission efficiency was enhanced when adsorbed on CS, and became significantly boosted when PVA was adsorbed on CS at the same time. The fluorescence intensity increases with the amount of added PVA and reaches a saturation point, which is earlier than that obtained by the well-established solvent relaxation NMR method, due to their different sensitivities for adsorption segments in specific conformation. This new method is advantageous in quick response, where the measurement can be finished within 2 min, while others usually take hours. Therefore, it is expected that this new method may be used to monitor the dynamical adsorption process of polymer on colloidal particles.