Organogel formation rationalized by Hansen solubility parameters: improved methodology

International audienceAn organogel is obtained when a low molar mass compound forms a network of anisotropic fibres in a liquid that is therefore transformed into a macroscopic solid. Various approaches have been proposed to correlate organogel formation and Hansen solubility parameters. These approaches are well adapted to specific experimental datasets but lack universality. A general method to determine the gelation domain from the solubility data of low molecular weight gelators is here reported

Ring distortions and nano-architecture formed by purpose designed phthalocyanines

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Direct probing of the free-energy penalty for helix reversals and chiral mismatches in chiral supramolecular polymers

The amplification of chirality, where a small imbalance in a chiral constituent is propagated into a strong optical purity, can occur in the spontaneous formation of helical 1-D stacks of molecules stabilized by hydrogen bonding, also known as supramolecular polymers. We have extended a statistical model by van Gestel et al. describing the highly nonlinear relationship between supramolecular helicity and enantiomeric excess for mixtures of enantiomers (the majority-rules effect) and quantitatively account for how this affects the thermodynamic stability of the assemblies. Our method allows for a direct comparison with experimental data, providing an unambiguous determination of the key parameters of the model (i.e., the mismatch and the helix reversal penalties). We demonstrate the successful application of this model to calorimetry data for bis-urea-based helical nanotubes, showing that reversals in the handedness of these nanotubes are not all that rare even though the helix reversal penalty is fairly large. By contrast, the mismatch penalty we obtain is small, implying that a large proportion of enantiomers are present in tube fractions not of their preferred handedness

Conformational Control of Hydrogen‐Bonded Aromatic Bis‐Ureas

International audiencehe phenylurea moiety is a ubiquitous synthon in supramolecular chemistry because it contains strong complementary hydrogen bonding groups and is synthetically very accessible. Here we investigate the possibility to strengthen self-association by conformational preorganization of the phenylurea moiety. In fact, we show that it is possible to strongly enhance intermolecular interactions between hydrogen bonded aromatic bis-ureas by substitution at the ortho positions of the phenylurea groups. Ortho substituents enforce a non-coplanar conformation of the urea and phenyl moieties better suited for hydrogen bonding. Substitution by methyl groups is more efficient than with larger groups, probably because of reduced steric hindrance. These effects have been demonstrated in the case of two different supramolecular architectures, which points to the probable generality of the phenomenon. In addition, this study has lead to the discovery of a new bis-urea able to form very stable self-assembled nanotubes in toluene up to high temperatures (boiling point) or low concentrations (10-7M) and in chloroform down to 3x10-4M

Orthohalogen substituents dramatically enhance hydrogen bonding of aromatic ureas in solution

International audienceThe phenylurea moiety is a ubiquitous synthon in supramolecular chemistry. Here we report that the introduction of chlorine or bromine atoms in the ortho positions to the urea unit is a simple and very efficient way to improve its intermolecular hydrogen bond (HB) donor character. This effect was demonstrated in solution both in the context of bis-ureas self-association and of mono-ureas hydrogen bonding to strong HB acceptors

Two-component self-assemblies: investigation of a synergy between bis-urea stickers

International audienceIt is of interest to develop two-component systems for added flexibility in the design of supramolecular polymers, nanofibers or organogels. Bis-ureas are known to self-assemble by hydrogen bonding into long supramolecular objects. We show here that mixing aromatic bis-ureas with slightly different structures can yield surprisingly large synergistic effects. A strong increase in viscosity is observed when a bis-urea with the sterically demanding 2,4,6-trimethylbenzene spacer is combined with a bis-urea bearing no methyl group in position 2 of the aromatic spacer (i.e. 4-methylbenzene or 4,6-dimethylbenzene). This effect is the consequence of a change in supramolecular assembly triggered by the composition of the mixture. The mixture of complementary bis-ureas forms rod-like objects that are more stable by about 1 kJ/mol, and that are thicker than the rod-like objects formed by both parent systems

Rational Design of Urea-Based Two-Component Organogelators

International audienceLow molecular weight gelators are versatile and responsive gel-forming systems. However, it is still a challenge to develop a new organogelator for a precise application, i.e., to gel a predetermined liquid. We propose a simple concept of a two-component gelling system that can be rationally adaptedto gel liquids ranging in polarity from silicone oil to acetonitrile

A Competing Hydrogen Bonding Pattern to Yield Thermo-Thickening Supramolecular Polymer

International audienceIntroduction of competing interactions in the design of a supramolecular polymer (SP) creates pathway complexity. Ester–bis‐ureas contain both a strong bis‐urea sticker that is responsible for the build‐up of long rod‐like objects by hydrogen bonding and ester groups that can interfere with this main pattern in a subtle way. Spectroscopic (FTIR and CD), calorimetric (DSC), and scattering (SANS) techniques show that such ester–bis‐ureas self‐assemble into three competing rod‐like SPs. The previously unreported low‐temperature SP is stabilized by hydrogen bonds between the interfering ester groups and the urea moieties. It also features a weak macroscopic alignment of the rods. The other structures form isotropic dispersions of rods stabilized by the more classical urea‐urea hydrogen bonding pattern. The transition from the low‐temperature structure to the next occurs reversibly by heating and is accompanied by an increase in viscosity, a rare feature for solutions in hydrocarbons