Nicolas Giuseppone

Muscle‐like Supramolecular Polymers: Integrated Motion from Thousands of Molecular Machines

Pumping iron: Double-threaded rotaxanes can be linked to coordination units and polymerized in the presence of iron or zinc ions. pH modulation triggers cooperative contractions (or extensions) of the individual rotaxanes, thus resulting in an amplified motion of the muscle-like supramolecular chains with changes of their contour lengths of several micrometers (see picture).

Macroscopic contraction of a gel induced by the integrated motion of light-driven molecular motors

Making molecular machines that can be useful in the macroscopic world is a challenging long-term goal of nanoscience. Inspired by the protein machinery found in biological systems, and based on the theoretical understanding of the physics of motion at the nanoscale, organic chemists have developed a number of molecules that can produce work by contraction or rotation when triggered by various external chemical or physical stimuli. In particular, basic molecular switches that commute between at least two thermodynamic minima and more advanced molecular motors that behave as dissipative units working far from equilibrium when fuelled with external energy have been reported. However, despite recent progress, the ultimate challenge of coordinating individual molecular motors in a continuous mechanical process that can have a measurable effect at the macroscale has remained elusive. Here, we show that by integrating light-driven unidirectional molecular rotors as reticulating units in a polymer gel, it is possible to amplify their individual motions to achieve macroscopic contraction of the material. Our system uses the incoming light to operate under far-from-equilibrium conditions, and the work produced by the motor in the photostationary state is used to twist the entangled polymer chains up to the collapse of the gel. Our design could be a starting point to integrate nanomotors in metastable materials to store energy and eventually to convert it.

Light-triggered self-construction of supramolecular organic nanowires as metallic interconnects

The construction of soft and processable organic material able to display metallic conduction properties-a large density of freely moving charges-is a major challenge for electronics. Films of doped conjugated polymers are widely used as semiconductor devices, but metallic-type transport in the bulk of such materials remains extremely rare. On the other hand, single-walled carbon nanotubes can exhibit remarkably low contact resistances with related large currents, but are intrinsically very difficult to isolate and process. Here, we describe the self-assembly of supramolecular organic nanowires between two metallic electrodes, from a solution of triarylamine derivative, under the simultaneous action of light and electric field triggers. They exhibit a combination of large conductivity values (>5 × 10(3) S m(-1)) and a low interface resistance (<2 × 10(-4) Ω m). Moreover, the resistance of nanowires in series with metal interfaces systematically decreases when the temperature is lowered to 1.5 K, revealing an intrinsic metallic behaviour.

Self-Duplicating Amplification in a Dynamic Combinatorial Library

A dynamic combinatorial library has been designed to produce a set of constituents among which one is able to self-complementarily direct its duplication. The overall molecular distribution in the library evolves along both kinetic and thermodynamic biases, leading to the amplification of the species that reproduces most efficiently and to the depletion of the other competitors.

Advances in Supramolecular Electronics – From Randomly Self-assembled Nanostructures to Addressable Self-Organized Interconnects

Supramolecular organic electronics rests on the use of bottom-up chemical self-assembly processes in order to design conducting components on the 5-100 nm scale. The challenges in this field are both the construction of 1D-nanostructures displaying optimized transport properties and their precise connections to electrodes. The present Research News highlights important advances in such materials regarding their electrical performances, from semiconductors to organic metals, but also regarding their processability. In particular, by externally controlling light-responsive supramolecular polymerization processes, and by using appropriate methods of casting with an applied electric field, it becomes possible to pre-determine the accurate positioning of organic interconnects within patterned nano-circuitry. These strategies using external stimuli to obtain addressability, thus hold promising alternatives to other conducting materials such as carbon nanotubes for further technological applications in nanosciences.

DOSY NMR Experiments as a Tool for the Analysis of Constitutional and Motional Dynamic Processes: Implementation for the Driven Evolution of Dynamic Combinatorial Libraries of Helical Strands

Constitutional dynamic chemistry (CDC) is the chemistry of molecular or supramolecular species and libraries of species, generated from components connected either by reversible covalent bonds or by noncovalent interactions, respectively. CDC takes advantage of these dynamic linkages for the expression of (supra)molecular diversity through crossover recombination of a set of building blocks. At thermodynamic equilibrium, specific changes in the environmental parameters can lead to the amplification/selection of preferred constituents generated in the libraries. This selection is of great interest for drug discovery purposes; for example, the presence of molecular targets such as enzymes, can discriminate against the best inhibitor through an in situ dynamic screening of the equilibrating mixture. CDC has also recently been shown to respond to various external chemical or physical stimuli such as protons, phase transitions, temperature, or electric field modulation. These libraries can afford the tuning of various physical properties by controlling the molecular, supramolecular, or macromolecular constitution of their dynamic functional entities, thus extending CDC to the domain of materials science. Whatever the application domain of CDC, one of the crucial prerequisites concerns the analysis of the libraries. For example, HPLC methods often require one to freeze the component exchange before analysis because the chromatographic interactions themselves can disturb thermodynamic equilibrium and affect the ratio between the library constituents. On the other hand, NMR spectroscopy methods do not interfere with the constitutional expression of the libraries, but only a few compounds can be characterized in a mixture. Thus, the need for deconvolution methods not involving chemical modifications appears to be of special interest for the analysis of more complex dynamic combinatorial libraries (DCLs) containing constituents generated by recombination of the components. A diffusion ordered spectroscopy (DOSY) NMR experiment represents an attractive noninvasive method for the analysis of DCLs because it allows the measurement of the diffusion coefficient of a certain molecular species that is directly related to its hydrodynamic radius according to the Stokes–Einstein equation. DOSY NMR techniques provide two-dimensional maps in which one axis corresponds to the chemical shift and the other one corresponds to the diffusion coefficient. Supramolecular entities, as well as mixtures of molecules, have been studied by this method, but no implementation for CDC purposes has been reported. Herein we discuss the analysis of a previously described DCL composed of helical molecular strands and its subsequent molecular evolution toward [2 < 2] gridlike arrays in the presence of Zn ions by DOSYNMRmethods. We highlight this method as a powerful complementary tool for the analysis of equilibrated mixtures in which minimal structural changes take place. We first characterized the spatial dimensions of individual compounds 1–6 in solution by DOSY NMR techniques to evaluate the potential of the diffusion methodology for the deconvolution of closely related structures in combinatorial mixtures (Figure 1). Molecular strands derived from the linking of pyrimidine and pyridine units by hydrazone bonds are known to display a persistent helical shape in solution. They were chosen as references because their structures have been well characterized by X-ray radiocrystallography, allowing a direct comparison of the solid-state structure with the calculated dimensions of the objects in solution (Figure 2 and Table 1). Compound 1 is a bow-shaped flat structure in the crystal form, and displays a diffusion coefficient (D) of 750 mms 1 at room temperature, corresponding to a hydrodynamic radius of 5.3 C. The calculations used to fit this value with an oblate ellipsoidal object lead to dimensions of 15 < 8.4 C, which are close to those values found in the crystal form (15< 8.8 C). The hydrodynamic radii for the series of helical strands 1, 2, and 4 increase progressively, yielding dimensions for an oblate ellipsoid which are in agreement with those calculated on the basis of solid-state structures (Table 1). The introduction of the larger trimethoxyphenyl residues in 3, 5, and 6 leads to a significant increase in DOSY-derived [*] Prof. Dr. N. Giuseppone, Dr. J.-L. Schmitt, Prof. Dr. J.-M. Lehn Institut de Science et d’Ing+nierie Supramol+culaires Universit+ Louis Pasteur 8 All+e Gaspard-Monge, BP 70028, 67083 Strasbourg (France) Fax: (+ 33)3-9024-5140 E-mail: lehn@isis.u-strasbg.fr

MUKAIYAMA ALDOL AND MICHAEL REACTIONS CATALYZED BY LANTHANIDE IODIDES

Abstract Samarium diiodide is an efficient catalyst precursor which allows the formation of condensation products between various carbonyl compounds and ketene silyl acetals or enoxysilanes. With α,β-unsaturated carbonyl compounds, 1,2- or 1,4-additions are observed according to the structure of the substrate. α,β-Unsaturated ketones yield to enoxysilanes by selective Michael additions. Aldol poducts are isolated as silyl ethers. The mechanisms of the reactions are discussed.

Electric-field triggered controlled release of bioactive volatiles from imine-based liquid crystalline phases.

Application of an electric field to liquid crystalline film forming imines with negative dielectric anisotropy, such as N-(4-methoxybenzylidene)-4-butylaniline (MBBA, 1), results in the expulsion of compounds that do not participate in the formation of the liquid crystalline phase. Furthermore, amines and aromatic aldehydes undergo component exchange with the imine by generating constitutional dynamic libraries. The strength of the electric field and the duration of its application to the liquid crystalline film influence the release rate of the expelled compounds and, at the same time, modulate the equilibration of the dynamic libraries. The controlled release of volatile organic molecules with different chemical functionalities from the film was quantified by dynamic headspace analysis. In all cases, higher headspace concentrations were detected in the presence of an electric field. These results point to the possibility of using imine-based liquid crystalline films to build devices for the controlled release of a broad variety of bioactive volatiles as a direct response to an external electric signal.

Hierarchical Self-Assembly of Supramolecular Muscle-Like Fibers

An acid-base switchable [c2]daisy chain rotaxane terminated with two 2,6-diacetylamino pyridine units has been self-assembled with a bis(uracil) linker. The complementary hydrogen-bond recognition patterns, together with lateral van der Waals aggregations, result in the hierarchical formation of unidimensional supramolecular polymers associated in bundles of muscle-like fibers. Microscopic and scattering techniques reveal that the mesoscopic structure of these bundles depends on the extended or contracted states that the rotaxanes show within individual polymer chains. The observed local dynamics span over several length scales because of a combination of supramolecular and mechanical bonds. This work illustrates the possibility to modify the hierarchical mesoscopic structuring of large polymeric systems by the integrated actuation of individual molecular machines.

Reversible Native Chemical Ligation: A Facile Access to Dynamic Covalent Peptides

The broad interest of using reversible covalent bonds in chemistry, in particular at its interfaces with biology and materials science, has been recently established through numerous examples in the literature. However, the challenging exchange of peptide fragments using a dynamic covalent peptide bond has not yet been achieved without enzymatic catalysis because of its high thermodynamic stability. Here we show that peptide fragments can be exchanged by a chemoselective and reversible native chemical ligation (NCL) which can take place at N-(methyl)-cysteine residues. This very mild reaction is efficient in aqueous solution, is buffered at physiological pH in the presence of dithiothreitol (DTT), and shows typical half-times of equilibration in the 10 h range.