A light-gated regulation of the reaction site by a cucurbit[7]uril macrocycle.

Competitive inhibition can be overcome by increasing the amount of catalyst in the reaction mixture. Here we present a pseudorotaxane system that circumvents this rule. A merocyanine inhibitor linked with the substrate obstructs the binding of the macrocyclic catalyst at the electrophilic reaction site preventing catalysis. Under UV light merocyanine is converted to the spiropyran form, losing its inhibition properties, thereby allowing the catalyst to bind the reaction center and promote the reaction. Moreover, when more than one nucleophile is present in the reaction mixture, the pseudorotaxane can scavenge a selected nucleophile and change the final product ratio. This work is a step forward in the development of new types of regulation in catalytic systems with remote control.

Effective screening of Coulomb repulsions in water accelerates reactions of like-charged compounds by orders of magnitude.

The reaction kinetics between like-charged compounds in water is extremely slow due to Coulomb repulsions. Here, we demonstrate that by screening these interactions and, in consequence, increasing the local concentration of reactants, we boost the reactions by many orders of magnitude. The reaction between negatively charged Coenzyme A molecules accelerates ~5 million-fold using cationic micelles. That is ~104 faster kinetics than in 0.5 M NaCl, although the salt is ~106 more concentrated. Rate enhancements are not limited to micelles, as evidenced by significant catalytic effects (104-105-fold) of other highly charged species such as oligomers and polymers. We generalize the observed phenomenon by analogously speeding up a non-covalent complex formation-DNA hybridization. A theoretical analysis shows that the acceleration is correlated to the catalysts' surface charge density in both experimental systems and enables predicting and controlling reaction rates of like-charged compounds with counter-charged species.

Probing E/Z Isomerism Using Pillar[4]pyridinium/Gold Nanoparticle Ensembles and Their Photoresponsive Behavior.

Despite the fundamental importance and broad applicability of E/Z dicarboxylic acids, their discrimination remains challenging and greatly unexplored. Herein, we present a general approach for the recognition of E/Z diacids using supramolecular interactions coupled with plasmonic response. The method allows detecting both single isomers and their light-induced interconversion, which ultimately entails multiple reversible nanoparticle aggregations. Such a molecular recognition-coupled responsive nanoscale self-assembly resembles natural mechanisms and can be a versatile means of building artificial complexity.

Effect of Na+ and K+ on the cucurbituril-mediated hydrolysis of a phenyl acetate.

The environment around the active site affects the catalytic activity of enzymes. Studying the cucurbit[7]uril-promoted acid hydrolysis of a cationic phenyl acetate derivative, we found that the hydrophobic cavity of the macrocycle screens the reaction centre from the positively charged neighbouring group. Moreover, the chelation of alkali metal cations with the cucurbit[7]uril portal and acetyl group of the substrate reduces the hydrolysis rate of the encapsulated ester in an aqueous solution. This type of inhibition corresponds to a rare uncompetitive model in contrast to the more common competitive model that relies on substrate displacement.

Carbazole-modified thiazolo[3,2-c][1,3,5,2]oxadiazaborinines exhibiting aggregation-induced emission and mechanofluorochromism.

Two highly emissive carbazole-containing thiazole-fused oxadiazaborinines were designed and synthesized. These N,O-chelated organoboron dyes displayed large Stokes shifts and remarkable solvatofluorochromism in solutions, as well as good thermal stability and comparatively high photoluminescence quantum yields (up to 34%) in the solid state. The presence of a carbazole donor unit, linked with the oxadiazaborinine acceptor via a phenyl linker, restricted intramolecular rotation, leading to enhanced aggregation-induced emission properties of the compounds: in THF/water mixtures with a large water percentage, they demonstrated the formation of emissive nanoaggregates with an average size of 79 and 89 nm for complexes 2 and 3, respectively. The introduction of bulky tert-butyl groups attached to the carbazole moiety induced significant mechanofluorochromic properties of the compounds.

Visual discrimination of aromatic acid substitution patterns by supramolecular nanocooperativity.

Physicochemical and, in particular, visual recognition of positional isomers, due to their similar appearance and properties, is an extremely challenging task. Here we present an easy-to-prepare assay for the naked-eye differentiation of all possible isomers of phthalic acids. The desired optical response is attained through specific non-covalent interactions between the acids and a cationic macrocyclic host. These interactions are then translated to and amplified by gold nanoparticles which subsequently aggregate to various extents producing a color palette.

Fast and efficient deposition of broad range of analytes on substrates for surface enhanced Raman spectroscopy.

The majority of analytical chemistry methods requires presence of target molecules directly at a sensing surface. Diffusion of analyte from the bulk towards the sensing layer is random and might be extremely lengthy, especially in case of low concentration of molecules to be detected. Thus, even the most sensitive transducer and the most selective sensing layer are limited by the efficiency of deposition of molecules on sensing surfaces. However, rapid development of new sensing technologies is rarely accompanied by new protocols for analyte deposition. To bridge this gap, we propose a method for fast and efficient deposition of variety of molecules (e.g. proteins, dyes, drugs, biomarkers, amino acids) based on application of the alternating electric field. We show the dependence between frequency of the applied electric field, the intensity of the surface enhanced Raman spectroscopy (SERS) signal and the mobility of the studied analyte. Such correlation allows for a priori selection of parameters for any desired compound without additional optimization. Thanks to the application of the electric field, we improve SERS technique by decrease of time of deposition from 20 h to 5 min, and, at the same time, reduction of the required sample volume from 2 ml to 50 µl. Our method might be paired with number of analytical methods, as it allows for deposition of molecules on any conductive surface, or a conductive surface covered with dielectric layer.

Reversing Chemoselectivity: Simultaneous Positive and Negative Catalysis by Chemically Equivalent Rims of a Cucurbit[7]uril Host.

Enzyme catalysis has always been an inspiration and an unattainable goal for chemists due to features such as high specificity, selectivity, and efficiency. Here, we disclose a feature neither common in enzymes nor ever described for enzyme mimics, but one that could prove crucial for the catalytic performance of the latter, namely the ability to catalyze and inhibit two different reactions at the same time. Remarkably, this can be realized by two identical, spatially resolved catalytic sites. In the future, such a synchronized catalyst action could be used not only for controlling chemoselectivity, as in the present case, but also for regulating other types of chemical reactivity.

Pillar[6]pyridinium: a hexagonally shaped molecular box that selectively recognizes multicharged anionic species.

A sextuply charged cyclic cationic receptor with an extraordinary structure and unprecedented binding properties is presented. The macrocycle consists of six pyridinium ions connected by methylene linkers with an electron-deficient cavity inside. In the solid state, the cavity is padded with an organized water network that gives the macrocycle a hexagonal shape. In water, the cavity is more flexible and selectively accommodates anionic species. Of the ions of similar size the macrocycle binds most strongly those with the largest negative charge. When the net charge is the same, the most preferred are anions with delocalized charge rather that those with localized charge; remarkably, the former form inclusion complexes, while the latter are complexed externally.

Pillar[4]pyridinium: a square-shaped molecular box.

Numerous applications of Stoddart's 'blue-box', a pyridinium containing macrocycle of rectangular shape, encouraged us to seek successors of this amazing molecule. Using a one-step cyclization reaction we synthesized a square-shaped cyclic tetramer consisting of 4-methylenepyridinium units - pillar[4]pyridinium (P[4]P). Pillar[4]pyridinium is a quadruply positively charged water-soluble macrocycle with a highly symmetric, strained structure and an electron-deficient cavity. These features impel the macrocycle to assemble into channel networks in the solid state and render it an effective fluoride receptor in water.

Submicron Colloidosomes of Tunable Size and Wall Thickness.

We present a simple method for the fabrication of colloidal capsules of different sizes ranging from below 100 nm up to one micron. The capsules are produced by self-assembling 5 nm gold nanoparticles at the interface of oil-in-water emulsion droplets. The size of the capsules is regulated by tuning the wetting properties of the nanoparticles by changing either the composition of their ligand shell or the composition of the oil phase. The modified wettability affects not only the size but also the thickness of capsule walls. The wall can be thin if it is made of a single layer of the nanoparticles or thick when composed of multilayers. The durability of such capsules is quite high, although it can be improved by chemical cross-linking with UV light. Such capsules have low permeability, so they can store a molecular cargo and then release it on demand.

A halogen-free synthesis of gold nanoparticles using gold(III) oxide.

Gold nanoparticles are one of the most used nanomaterials. They are usually synthesized by the reduction of gold(III) chloride. However, the presence of halide ions in the reaction mixture is not always welcome. In some cases, these ions have detrimental influence on the morphology and structure of resulting nanoparticles. Here, we present a simple and halogen-free procedure to prepare gold nanoparticles by reduction of gold(III) oxide in neat oleylamine. The method provides the particles with an average size below 10 nm and dispersity of tens of percent. The process of nanoparticle formation was monitored using UV-Vis spectroscopy. The structure and chemical composition of the nanoparticles was determined by SEM, XPS and EDX. We also proposed the mechanism of reduction of gold(III) oxide based on MS, IR and NMR data. Importantly, the synthetic protocol is general and applicable for the preparation of other coinage metal nanoparticles from the corresponding metal oxides. For instance, we demonstrated that the absence of halogen enables efficient alloying of metals when preparing gold-silver bimetallic nanoparticles.

A Thermo- and Photo-Switchable Ruthenium Initiator For Olefin Metathesis.

A ruthenium carbene complex bearing azobenzene functionality is reported. The complex exists in the form of two isomers differing by the size of the chelate ring. Both isomers were isolated by applying kinetic or thermodynamic control during the synthesis and characterized by X-ray diffraction analysis. The isomerization of the complex was studied by UV/Vis spectroscopy. The stable isomer was tested as a catalyst in olefin metathesis. The complex was activated at about 100 °C to promote ring-closing and ring-opening polymerization metathesis reactions. The activation took place also at room temperature under middle ultraviolet radiation.

Triggering autocatalytic reaction by host-guest interactions.

The acceleration of a sequential reaction through electrostatic alteration of substrate basicity within a supramolecular host is demonstrated. In the presence of the host, the reaction, which is autocatalytic, starts much sooner and exhibits substrate size selectivity.

Nanoparticles in a capillary trap: dynamic self-assembly at fluid interfaces.

Dynamic self-assembly is an emerging scientific concept aimed to construct artificial systems of adaptative behavior. Here, we present a first nanoscopic system that is able to dynamically self-assemble in two dimensions. This system is composed of charged gold nanoparticles, dispersed at the air-water interface, which self-assemble into a dense monolayer of area of several square centimeters in response to surface tension gradient. The surface tension gradient is imposed by localized addition or removal of organic solvent from the interface. After the surface tension is equalized over the whole fluid interface, the nanoparticles return to their initial dispersed state. The arrangement of nanoparticles before and after the self-assembly was characterized using SEM microscopy and SAXS spectroscopy. The constructed self-assembling system offers a "chemical" alternative for the Langmuir-Blodgett technique. Also, it was applied for creating self-erasing nanoparticle patterns on a fluid surface.

Kinetic trapping of the host-guest association intermediate and its transformation into a thermodynamic inclusion complex.

The molecular recognition and self-assembly between host cucurbit[6]uril and guest adrenaline led to kinetic trapping and crystallization of the intermediate exclusion complex, which was characterized by X-ray diffraction. The crystalline kinetic complex undergoes slow spontaneous dissolution and subsequently recrystallizes as a thermodynamic inclusion complex.

Thiolate-protected nanoparticles via organic xanthates: mechanism and implications.

The gold nanoparticles (AuNPs) stabilized with amine ligands are converted into robust thiolate-protected AuNPs in the presence of xanthates. This enables decoration of the AuNPs with a diversity of important functional groups, in particular, to introduce the thiol-sensitive unsaturated C-C bonds. The (1)H NMR study on the reaction mechanism provides a new insight into the great mystery of nanoscience-the fate of hydrogen upon the formation of the Au-S bond.

Close-packed monolayers of charged Janus-type nanoparticles at the air-water interface.

We present a new method to obtain close-packed monolayers composed of noble metal nanoparticles (NP) possessing well-defined permanent charge of either sign. The method is based on the fact that ligands forming the protecting layer exhibit ability to rearrange at the NPs surface. We demonstrate that if the protecting layer is composed of a mixture of hydrophobic and hydrophilic charged ligands in appropriate proportion, the NP exhibits properties of a Janus-type particle with one of the hemispheres hydrophilic and the other hydrophobic. Such amphiphilic NPs self-assemble into a monolayer of well defined surface charge at the air-water interface. Due to strong stabilizing effect of the lateral electrostatic repulsions, such monolayer can be compressed to form close-packed hexagonal structure, and then easily transferred onto a solid substrate with the Langmuir-Blodgett technique.

Autonomous self-assembly of ionic nanoparticles into hexagonally close-packed lattices at a planar oil-water interface.

Let's get charged! Positively charged nanoparticles (NPs) spontaneously self-assemble into hexagonally close-packed lattices at a planar CH(2)Cl(2)-water interface. The self-assembly process is fully autonomous and occurs without any external manipulation.