Probing Weak Intermolecular Interactions in Self-Assembled Nanotubes

Recent Advances in High-strength and High-toughness Polyurethanes Based on Supramolecular Interactions

Recent developments in supramolecular chemistry have generated increasing interest in supramolecular polymers and opened a window for the exploitation of various supramolecular polymeric materials and their multifunctional composites. High-performance polyurethanes,...

Energetics of Competing Chiral Supramolecular Polymers

Chirality can have unexpected consequences including on properties other than spectroscopic. We show herein that a racemic mixture of bis-urea stereoisomers forms thermodynamically stable supramolecular polymers that result in a more viscous solution than for the pure stereoisomer. The origin of this macroscopic property was probed by characterizing the structure and stability of the assemblies. Both racemic and non-racemic bis-urea stereoisomers form two competing helical supramolecular polymers in solution: a double and a single helical structure at low and high temperature, respectively. The transition temperature between these assemblies, as probed by spectroscopic and calorimetric analyses, is strongly influenced by the composition (by up to 70 °C). A simple model that accounts for the thermodynamics of this system, indicates that the stereochemical defects (chiral mismatches and helix reversals) affect much more the stability of single helices. Therefore, the heterochiral double helical structure predominates over the single helical structure (whilst the opposite holds for the homochiral structures), which explains the aforementioned higher viscosity of the racemic bis-urea solution. This rationale constitutes a new basis to tune the macroscopic properties of the increasing number of supramolecular polymers reported to exhibit competing chiral nanostructures.

Stabilization of Supramolecular Polymer Phase at High Pressures

We utilize dynamic light scattering (DLS) and passive microrheology to examine the phase behavior of a supramolecular polymer at very high pressures. The monomer, 2,4-bis(2-ethylhexylureido)toluene (EHUT), self-assembles into supramolecular polymeric structures in the nonpolar solvent cyclohexane by means of hydrogen bonding. By varying the concentration and temperature at atmospheric pressure, the formation of the viscoelastic network (at lower temperatures) and predominantly viscous phases, based on self-assembled tube and filament structures, respectively, has been established. The associated changes in the rheological properties have been attributed to a structural thickness transition. Here, we investigate the effects of pressure variation from atmospheric up to 1 kbar at a given concentration. We construct a temperature-pressure diagram that reveals the predominance of the viscoelastic network phase at high pressures. The transition from the viscoelastic network organization of the tubes to a weaker viscous-dominated structure of the filaments is rationalized by using the Clapeyron equation, which yields an associated volume change of about 8 ų per EHUT molecule. This change is further explained by means of Molecular Dynamics simulations of the two phases, which show a decrease in the molecular volume at the filament-tube transition, originating from increased intermolecular contacts in the tube with respect to the filament. These findings offer insights into the role of pressure in stabilizing self-assemblies.

Solute-Solvent Interactions in Modern Physical Organic Chemistry: Supramolecular Polymers as a Muse

Interactions between solvents and solutes are a cornerstone of physical organic chemistry and have been the subject of investigations over the last century. In recent years, a renewed interest in fundamental aspects of solute-solvent interactions has been sparked in the field of supramolecular chemistry in general and that of supramolecular polymers in particular. Although solvent effects in supramolecular chemistry have been recognized for a long time, the unique opportunities that supramolecular polymers offer to gain insight into solute-solvent interactions have become clear relatively recently. The multiple interactions that hold the supramolecular polymeric structure together are similar in strength to those between solute and solvent. The cooperativity found in ordered supramolecular polymers leads to the possibility of amplifying these solute-solvent effects and will shed light on extremely subtle solvation phenomena. As a result, many exciting effects of solute-solvent interactions in modern physical organic chemistry can be studied using supramolecular polymers. Our aim is to put the recent progress into a historical context and provide avenues toward a more comprehensive understanding of solvents in multicomponent supramolecular systems.

Two different pathways for assembling bis-urea in benzene and toluene

The thermodynamic stability of assemblies formed by a bis-urea-based supramolecular polymer, 2,4-bis(2-ethylhexylureido)toluene (EHUT), was investigated in solutions using either benzene or toluene as the solvent. Starting from a higher temperature in which EHUT was soluble in both solvents, molecules spontaneously self-organized into tubular assemblies upon cooling and these assemblies were stable in a wide range of temperatures. However, the systems followed different paths below a specific temperature: while the supramolecular polymer remained stable in toluene, EHUT molecules underwent precipitation in benzene. The causes for these different behaviors were explored by molecular dynamics simulations, which provided support for stronger enthalpic stabilization of the tubular assemblies in toluene as compared to benzene. This stabilization was due mainly to the better interaction energy of trapped toluene molecules instead of benzene ones. For both cases, lowering the temperature makes the solvent penetration inside the tubes less favorable, which reduces the stability of supramolecular structures upon cooling. Graphical abstract Different EHUT solubilities.

Supramolecular Assembly of Phosphole Oxide Based Alkynylplatinum(II) 2,6-Bis(N-alkylbenzimidazol-2'-yl)pyridine Complexes-An Interplay of Hydrophobicity and Aromatic π-Surfaces

A new class of phosphole oxide based alkynylplatinum(II) 2,6-bis(N-alkylbenzimidazol-2'-yl)pyridine (bzimpy) complexes were synthesized and characterized. Their self-assembly was driven by hydrophobic-hydrophobic and π-π stacking interactions. The self-assembly properties were also investigated by UV/Vis absorption spectroscopy, which revealed that the alkyl-chain length of the bzimpy moiety and the π-surface area of the alkynyl ligand have significant influence on the overall self-assembly process. The alkyl-chain length also affected the morphological structures of the aggregates, which were studied by transmission electron microscopy and scanning electron microscopy.

Probing halogen-halogen interactions in solution

Halogen-halogen interactions are a particularly interesting class of halogen bonds that are known to be essential design elements in crystal engineering. In solution, it is likely that halogen-halogen interactions also play a role, but the weakness of this interaction makes it difficult to characterize or even simply detect. We have designed a supramolecular balance that allows detecting BrBr interactions between CBr₃ groups in solution and close to room temperature. The sensitivity and versatility of the chosen platform have allowed accumulating consistent data. In halogenoalkane solvents, we propose estimates for the free energy of these weak halogen bond interactions. In toluene solutions, we show that the interactions between Br atoms and the solvent aromatic groups dominate over the BrBr interactions.

A theoretical study of weak interactions in phenylenediamine homodimer clusters

Utilizing dispersion-corrected density functional theory (DFT) calculations, we demonstrate the weak intermolecular interactions of phenylenediamine dimer (pdd) clusters, emphasizing the local lowest energy structures and decomposition of interaction energies by natural bond orbital (NBO) and atoms in molecule (AIM) analyses.

Dynamic Landau theory for supramolecular self-assembly

Although pathway-specific kinetic theories are fundamentally important to describe and understand reversible polymerisation kinetics, they come in principle at a cost of having a large number of system-specific parameters. Here, we construct a dynamical Landau theory to describe the kinetics of activated linear supramolecular self-assembly, which drastically reduces the number of parameters and still describes most of the interesting and generic behavior of the system in hand. This phenomenological approach hinges on the fact that if nucleated, the polymerisation transition resembles a phase transition. We are able to describe hysteresis, overshooting, undershooting and the existence of a lag time before polymerisation takes off, and pinpoint the conditions required for observing these types of phenomenon in the assembly and disassembly kinetics. We argue that the phenomenological kinetic parameter in our theory is a pathway controller, i.e., it controls the relative weights of the molecular pathways through which self-assembly takes place.

Cooperative polymerization of one-patch colloids

We numerically investigate cooperative polymerization in an off-lattice model based on a pairwise additive potential using particles with a single attractive patch that covers 30% of the colloid surface. Upon cooling, these particles self-assemble into small clusters which, below a density-dependent temperature, spontaneously reorganize into long straight tubes. We evaluate the partition functions of clusters of all sizes to provide an accurate description of the chemical reaction constants governing this process. Our calculations show that, for intermediate sizes, the partition functions retain contributions from two different structures, differing in both energy and entropy. We illustrate the microscopic mechanism behind the complex polymerization process in this system and provide a detailed evaluation of its thermodynamics.

Conformational analysis of chiral supramolecular aggregates: modeling the subtle difference between hydrogen and deuterium

A detailed analysis of the conformational states of self-assembled, stereoselectively deuterated benzene-1,3,5-tricarboxamides ((S,S,S)-D-BTAs) reveals four different conformers for the supramolecular polymers. The relative amount of the conformers depends on the solvent structure and the temperature. With the help of a model, the thermodynamic parameters that characterize the different conformational states were quantified as well as the amount of the species that occur at different stages of the polymerization process. The results show that small changes in the stability between different types of conformers formed by (S,S,S)-D-BTAs—in the order of a few J mol(–1)—arise from the combination of interactions between the solvent/supramolecular aggregate, temperature, and solvent structure. While the introduction of a deuterium label allows to sensitively probe the solvophobic effects in the supramolecular aggregation, a rationalization of the observed effects on a molecular level is not yet straightforward but is proposed to result from subtle effects in the vibrational enthalpy and entropy terms of the isotope effect.

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.

Tuning the viscoelastic properties of bis(urea)-based supramolecular polymer solutions by adding cosolutes

Polymers formed by the self-assembly of a bis(urea)-based polymer, 2,4-bis(2-ethylhexylureido)toluene (EHUT), in organic solvents such as octane are promising systems with remarkable rheological properties. This is the first self-assembled polymer recently reported as a hydrodynamic drag reducer for hydrocarbons. The rheology of diluted and semidiluted EHUT solutions can be tuned by specific interactions between the chains, modulated by the nature of the solvent and the presence of additives. In this article, rheological, thermal and SANS measurements were performed in order to investigate the competition between EHUT self-assembly and its interaction with specific molecules (benzene, benzyl alcohol, and ethanol) that can interact with EHUT unimers via hydrogen bonds and π-π interactions. No substantial rheological, thermal, or structural effect is observed when benzene is added to the systems. However, ethanol and benzyl alcohol interact with EHUT unimers through hydrogen bonds, drastically decreasing the viscoelasticity of the solutions. In addition, benzyl alcohol can interact with EHUT polymers by π-stacking interactions, playing an important role in tuning the rheological properties of the systems.

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.