Probing Weak Intermolecular Interactions in Self-Assembled Nanotubes

Extreme confinement affects the physical properties of fluids, but little quantitative data is available. We report on studies of a bisurea compound that self-assembles into nanotubes to probe solvent confinement on the angstrom scale. By applying a statistical model to calorimetric data obtained on solvent mixtures, we show that the thermodynamic stability of the nanotubes is an extremely sensitive function of the solvent composition because solvent interactions inside and outside of the nanotubes are different. We are able to measure energetic effects as small as 0.01 kT and relate them to the differences in molecular structure of the solvents

Unexpected Solvent Influence on the Rheology of Supramolecular Polymers

A very limited change in the nature of the solvent (toluene versus pentylbenzene) can have an unexpectedly strong effect on the viscosity of a hydrogen-bonded supramolecular polymer. In pentylbenzene, slightly stronger hydrogen bonds are formed than in toluene, but the supramolecular polymer solution is ca. 25 times <i>less</i> viscous. Several hypotheses can be envisaged to explain this counterintuitive result. We propose that this effect is actually related to a solvation effect involving the outer corona of the supramolecular objects. When the cohesive energy density of the solvent approaches the cohesive energy density of the outer corona of the supramolecular objects, the viscosity of solution is reduced, possibly because of faster local dynamics

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

Two-Component Self-Assemblies: Investigation of a Synergy between Bisurea Stickers

It is of interest to develop two-component systems for added flexibility in the design of supramolecular polymers, nanofibers, or organogels. Bisureas are known to self-assemble by hydrogen bonding into long supramolecular objects. We show here that mixing aromatic bisureas with slightly different structures can yield surprisingly large synergistic effects. A strong increase in viscosity is observed when a bisurea with the sterically demanding 2,4,6-trimethylbenzene spacer is combined with a bisurea 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 the supramolecular assembly triggered by the composition of the mixture. The mixture of complementary bisureas forms rodlike objects that are more stable by about 1 kJ/mol and that are thicker than the rodlike objects formed by both parent systems

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

Tunable Asymmetric Catalysis through Ligand Stacking in Chiral Rigid Rods

Chiral benzene-1,3,5-tricarboxamide (BTA) ligands, comprising one diphenylphosphino group and one or two remote chiral 1-methylheptyl side chains, were evaluated in the rhodium-catalyzed asymmetric hydrogenation of dimethyl itaconate. Despite the fact that the rhodium atom and the chiral center(s) are separated by more than 12 covalent bonds, up to 82% ee was observed. A series of control and spectroscopic experiments confirmed that the selectivity arises from the formation of chiral helical polymers by self-association of the BTA monomers through noncovalent interactions. The addition of a phosphine-free chiral BTA, acting as a comonomer for the chiral BTA ligands, increases the level of enantioselectivity (up to 88% ee). It illustrates how the selectivity of the reaction can be increased in a simple fashion by mixing two different BTA monomers. The concept was further probed by performing the same experiment with an achiral BTA ligand, i.e. a phosphine-functionalized BTA that contains two remote octyl side chains. It afforded an encouraging 31% ee, thus demonstrating the catalytically relevant transfer of chirality between the self-assembled units. It constitutes a unique example of the sergeants-and-soldiers principle applied to catalysis

Patchy Supramolecular Bottle-Brushes Formed by Solution Self-Assembly of Bis(urea)s and Tris(urea)s Decorated by Two Incompatible Polymer Arms

In an attempt to design urea-based Janus nanocylinders through a supramolecular approach, nonsymmetrical bis­(urea)­s and tris­(urea)­s decorated by two incompatible polymer arms, namely, poly­(styrene) (PS) and poly­(isobutylene) (PIB), were synthesized using rather straightforward organic and polymer chemistry techniques. Light scattering experiments revealed that these molecules self-assembled in cyclohexane by cooperative hydrogen bonds. The extent of self-assembly was limited for the bis­(urea)­s. On the contrary, reasonably anisotropic 1D structures (small nanocylinders) could be obtained with the tris­(urea)­s (<i>N</i>_agg ∼ 50) which developed six cooperative hydrogen bonds per molecule. ¹H transverse relaxation measurements and NOESY NMR experiments in cyclohexane revealed that perfect Janus nanocylinders with one face consisting of only PS and the other of PIB were not obtained. Nevertheless, phase segregation between the PS and PIB chains occurred to a large extent, resulting in patchy cylinders containing well separated domains of PIB and PS chains. Reasons for this behavior were proposed, paving the way to improve the proposed strategy toward true urea-based supramolecular Janus nanocylinders

Charge Effect on the Formation of Polyoxometalate-Based Supramolecular Polygons Driven by Metal Coordination

The metal-driven self-assembly of a Keggin-based hybrid bearing two remote pyridine units was investigated. The resulting supramolecular species were identified by combination of 2D diffusion NMR spectroscopy (DOSY) and electrospray ionization mass spectrometry (ESI-MS) as a mixture of molecular triangles and squares. This behavior is different from that of the structural analogue Dawson-based hybrid displaying a higher charge, which only led to the formation of molecular triangles. This study highlights the decisive effect of the charge of the POMs in their self-assembly processes that disfavors the formation of large assemblies. An isothermal titration calorimetry (ITC) experiment confirmed the stronger binding in the case of the Keggin hybrids. A correlation between the diffusion coefficient <i>D</i> and the molecular mass <i>M</i> of the POM-based building block and its coordination oligomers was also observed. We show that the diffusion coefficient of these compounds is mainly determined by their occupied volume rather than by their shape

Fluorescent Labeling of a Bisurea-Based Supramolecular Polymer

Bisurea-based supramolecular polymer 2-ethylhexyl-3-[3-(3-(2-ethylhexyl)­ureido)-4-methyl-phenyl]­urea (EHUT) has been shown previously to self-assemble through hydrogen bonding into high-molecular-weight structures. The present publication reports the study of the thermodynamics of these tubular structures by time-resolved fluorescence spectroscopy, with the help of a tetrazine labeled monomer. Results of calorimetry and time-resolved fluorescence spectroscopy show that the as-modified monomer EHUTz does not interfere with the formation of the supramolecular assembly. When incorporated, these labeled monomers exhibit a longer fluorescence lifetime due to the electron-rich tolyl group buried in the structure. Dilution experiments allowed us to measure their partition coefficient, and to compare it with the critical aggregation concentration of EHUT. Measurements at higher dye loads, where interactions between neighboring tetrazines occur, show that EHUTz is uniformly dissolved in the supramolecular polymer. Tetrazine dye is a good reporter of events occurring in solution, such as disruption of the assembly upon heating. Our results confirm the pseudophase diagram for EHUT solution in toluene obtained previously with other techniques such as infrared spectroscopy and calorimetry

Competition Between Steric Hindrance and Hydrogen Bonding in the Formation of Supramolecular Bottle Brush Polymers

The formation of supramolecular bottle-brush polymers consisting of a noncovalent backbone assembled through directional hydrogen bonds and of poly­(isobutylene) (PIB) side-chains was investigated in cyclohexane by light scattering. Two limiting cases were observed depending on the balance between the favorable formation of hydrogen bonds and the unfavorable stretching of the PIB chains within the supramolecular bottle-brushes, in agreement with a theoretical model developed by Wang et al. On one hand, a bisurea self-assembling unit able to form four cooperative hydrogen bonds per molecule led to relatively short supramolecular bottle-brushes, the length of which could be varied by modifying steric hindrance or by using solvent mixtures. On the other hand, supramolecular bottle-brush polymers exhibiting persistent lengths of more than 300 nm could be obtained by using trisureas that are able to form six hydrogen bonds per molecule. Their easy synthesis and the fact that it is possible to control their self-assembly into long supramolecular bottle-brush polymers make polymer-decorated bisureas and trisureas an attractive alternative to cyclopeptides and shape-persistent rings for the creation of supramolecular nanostructures