Benjamin Isare

Chirality in dynamic supramolecular nanotubes induced by a chiral solvent.

Amplification of chirality has been reported in polymeric systems. It has also been shown that related effects can occur in polymer-like dynamic supramolecular aggregates, if a subtle balance between noncovalent interactions allows the coupling between a chiral information and a cooperative aggregation process. In this context, we report a strong majority-rules effect in the formation of chiral dynamic nanotubes from chiral bisurea monomers. Furthermore, similar helical nanotubes (with the same circular dichroism signature) can be obtained from racemic monomers in a chiral solvent. Competition experiments reveal the relative strength of the helical bias induced by the chiral monomer or by the chiral solvent. The nanotube handedness is imposed by the monomer chirality, whatever the solvent chirality. However, the chirality of the solvent has a significant effect on the degree of chiral induction.

Structure and dynamics of a bisurea-based supramolecular polymer in n-dodecane.

The structure and dynamics of a supramolecular polymer formed by a bisurea-type compound, 2,4-bis(2-ethylhexylureido)toluene (EHUT), in an apolar solvent, n-dodecane (C12), were examined in detail. The EHUT/C12 organo-gel system forms long, dynamic chain-like supramolecular polymers, which lead to an entangled network showing remarkable viscoelastic behavior with two major relaxation modes. A slow relaxation mode with an approximately constant relaxation time, tauS, was observed in a flow region and the other, fast, relaxation mode with a time tauF1 (<tauS) was observed in a high-frequency range. Because no dielectric relaxation behavior was observed over a frequency region including the mechanical tauS and tauF1 relaxation modes, the formed supramolecular polymer does not possess any total dipole moment due to antiparallel intermolecular hydrogen bonding of the two ureido groups of each EHUT unit. A structural model for the supramolecular polymer formed in EHUT/C12 is proposed based on force-field simulations. This proposed model is consistent with all the experimental data concerning this system: flow birefringence measurements, dielectric spectroscopy, SANS, and FTIR.

Supramolecular balance: using cooperativity to amplify weak interactions

Gathering precise knowledge on weak supramolecular interactions is difficult yet is of utmost importance for numerous scientific fields, including catalysis, crystal engineering, ligand binding, and protein folding. We report on a combined theoretical and experimental approach showing that it is possible to vastly improve the sensitivity of current methods to probe weak supramolecular interactions in solution. The concept consists of using a supramolecular platform involving a highly cooperative configurational transition, the perturbation of which (by the modification of the molecular building blocks) can be monitored in a temperature scanning experiment. We tested this concept with a particular bisurea platform, and our first results show that it is possible to detect the presence of interaction differences as low as 60 J/mol, which may be due to steric repulsion between vinyl and alkyl groups or may be the result of solvation effects.

Hydrogen bonded supramolecular polymers in moderately polar solvents.

Hydrogen bonded assemblies are usually decomposed by polar organic solvents. However, we have succeeded in preparing a strongly associated supramolecular polymer which forms viscous solutions in competitive solvents such as tetrahydrofuran.

The weak help the strong: low-molar-mass organogelators harden bitumen.

Low-molar-mass organogelators (LMOG) can turn liquids into thermoreversible gels because they self-assemble into a fibrous network. In contrast, using the same kind of low-molar-mass additives to harden materials, which are already solidlike on their own, has been hardly exploited. We show here that simple dicarboxylic acids are very efficient low-molar-mass organogelators (LMOG) for bitumen. Indeed, they increase the range of temperature where bitumen is a solid. Moreover, the hardness and elastic modulus of bitumen at room temperature are also improved. This concept of improving the mechanical properties of a solid with an LMOG can probably be applied to other materials.

Conformational plasticity of hydrogen bonded bis-urea supramolecular polymers.

We report a detailed structural investigation of supramolecular polymers formed by hydrogen bonded self-assembly of bis-urea monomers. The careful exploration of the energy landscape by molecular mechanics/molecular dynamics (MM/MD) simulations has allowed us to identify three distinct self-assembled structures of similar stabilities. These structures have been compared to X-ray crystal data. We observe that a slight change in the molecular structure can favor a particular structure over the others. Detailed analysis shows that hydrogen bonds stabilize all three structures to a similar extent. Therefore, it is the interactions among the lateral substituents, and with the filament environment, that are the decisive factors in the competition between the possible self-assembled structures. This study constitutes a clear reminder that the conformation of a supramolecular polymer is a sensitive function of the molecular structure and may significantly differ from the solid-state conformation of a model compound.

Engineering the cavity of self-assembled dynamic nanotubes

By analogy with hydrogen-bonded molecular capsules, self-assembled nanotubes are of interest because they can temporarily isolate guest molecules from the solution. We show here that the stability of a particular bis-urea based dynamic self-assembled nanotube is related to the possibility for solvent molecules to fit inside the tubular cavity. The diameter of the cavity can be finely tuned by introducing a modified monomer in controlled amount.

Conformational control of hydrogen-bonded aromatic bis-ureas.

The 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 noncoplanar 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 led 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(-7) M) and in chloroform down to 3 × 10(-4) M.

Tuning reversible supramolecular polymer properties through co-monomer addition

Unlike most low molar mass organogelators, which form strong gels due to the entanglement of strong molecular aggregates such as crystalline fibres, a few reversible supramolecular polymers form viscoelastic solutions due to the formation of both rigid and dynamic filaments. By using three different bisureas which self-assemble according to the same hydrogen-bonding pattern and form such rigid reversible supramolecular polymers, we have shown that it is possible to finely tune the properties of the resulting solutions. The critical temperature below which viscoelastic solutions are obtained and the viscosity of the dilute solutions can be adjusted by changing the co-monomer content. This clearly facilitates gel formulation, which is an important step as far as applications are concerned. By using three different bis-ureas which self-assemble according to the same hydrogen bonding pattern and form rigid reversible supramolecular polymers, we show that it is possible to finely tune the properties of the resulting viscoelastic solutions.

Structural Control of Bisurea-Based Supramolecular Polymers: Influence of an Ester Moiety

A few examples of monomers are known that self-assemble into various high molar mass structures in solution. Controlling the morphology of the resulting supramolecular polymers is a highly desirable goal for many applications. Herein, we compare the self-assembling properties of newly prepared ester bisurea monomers with those of previously investigated alkyl bisurea monomers. The ester functionality decreases the hydrogen bonding strength of the bisurea monomers but does not prevent the formation of long assemblies in nonpolar solvents: gels are formed at millimolar concentration. Surprisingly, ester bisureas self-assemble at room temperature into rod-like urea-bonded supramolecular polymers that are different from the ones formed by alkyl bisureas. The rods formed by ester bisurea supramolecular polymers are compact (instead of tubular in the case of alkyl bisureas) and display two monomers in the cross-section (instead of three in the case of alkyl bisureas). The stability of the structures formed by ester bisureas can be easily tuned by changing the nature of the substituent in the α-position of the urea functions and/or the nature of the alkyl side chains.