Interplay Between Hydrogen Bonding and Electron Transfer in Mixed Valence Assemblies of Triarylamine Trisamides.

Hydrogen-bonding interactions are assumed to play a critical role in the long-range transport of light or charge recently observed in supramolecular assemblies of C3 -symmetrical discotic molecules. Herein, the structure of mixed valence assemblies formed by irradiating triarylamine trisamide (TATA) molecules was determined by multifarious techniques under various conditions with the aim of probing the interplay between the hydrogen bonding network and the rate of electron transport in different states (solution, gel, film). Irradiation was performed under initial states that vary by the degree of association of TATA monomers through hydrogen bonds. Firstly, a significant shift of the N-H and C=O stretching frequencies was observed by FTIR upon irradiation thus revealing an overlooked signature of TATA⋅+ species and interacting mixed valence aggregates. Secondly, gels and films both mostly consist of hydrogen-bonded TATA polymers but their EPR spectra recorded at 293 K reveal very different behaviors: localized electrons in the gels versus fully delocalized electrons in the films. Hydrogen bonding thus appears as a necessary but not sufficient condition to get fast electron transfer rates and a packing of the TATA monomers particularly suitable for charge transport is assumed to exist in the solid state. Finally, defects in the hydrogen bonding network are detected upon increasing the number of radical species in the mixed valence assemblies present in the film state without impeding the delocalization of the unpaired electrons. A delicate balance between hydrogen bonds and packing is thus necessary to get supramolecular polarons in mixed valence TATA assemblies.

Precisely Designed Difunctionalized Cyclodextrin Produces a Solid-State Organic Porous Hierarchical Supramolecular Assembly.

Regioselective di-functionalization of a cyclodextrin allows hydrophobic domains to be directed in a geometrically controlled manner. This controlled orientation ultimately gives access to an original hierarchical assembly in the solid state. This assembly spans over three levels of hierarchy which are governed by synergistic host-guest inclusions, directed hydrophobic effect and hydrogen bonding. This combination of interactions precisely positioned in space through regioselective functionalization of a cyclodextrin creates a porous organic architecture.

Rationalizing the Extent of the "Sergeants-and-Soldiers" Effect in Supramolecular Helical Catalysts: Effect of Copper Coordination.

Aggregation of supramolecular helices, for example through interdigitation of their alkyl side chains or through more directional supramolecular interactions, leads to hierarchical architectures with original structural and chiroptical properties. However, when a chiral monomer (the "sergeant") is introduced as a minor component in these assemblies composed of a majority of achiral monomers (the "soldiers"), it is not clear how the aggregation changes the ability of the sergeant to induce a preferential helicity to the polymer main chain (the so-called "sergeants-and-soldiers" effect). This study reports a detailed investigation of the influence of [Cu(OAc)2 ⋅H2 O] coordination on the structure and chiroptical properties of helical hydrogen-bonded co-assemblies composed of a catalytically-active benzene-1,3,5-tricarboxamide (BTA) monomer, acting as the "soldier", and an enantiopure BTA monomer derived from cyclohexylalanine, playing the role of the "sergeant". The copper actually significantly influences the extent of the "sergeants-and-soldiers" effect since it acts as a crosslink that induces some chiral defects in the supramolecular helices. These crosslinks appear to be preserved during the catalytic hydrosilylation of 4-nitroacetophenone. The aggregation of helices through the formation of copper crosslinks is reversible since homochiral single helices are exclusively formed in the case of sergeant-rich assemblies. The fact that both main chain and side chain aggregation affects the chiroptical properties of supramolecular helices must be considered in the design of elaborated chiral materials.

Development of furan-2,5-dicarboxylic acid (FDCA)-based organogelators.

Organogels are used in a wide range of applications for which the development of new bio-based organogelators is highly desirable. While furan-2,5-dicarboxylic acid (FDCA) is a promising molecule for the synthesis of bio-based polyesters, it has never been used in the context of organogels. This study explores the possibility to design FDCA-based organogelators that self-assemble into fibrillar networks stabilized by hydrogen bonding. Gelation tests show the versatility of this gelator family with a wide variety of gelled liquids, especially apolar liquids. The structure of the gels was investigated by FTIR and CD spectroscopies, crystallography, powder X-ray diffraction and rheology.

Possible chemical and physical scenarios towards biological homochirality.

The single chirality of biological molecules in terrestrial biology raises more questions than certitudes about its origin. The emergence of biological homochirality (BH) and its connection with the appearance of life have elicited a large number of theories related to the generation, amplification and preservation of a chiral bias in molecules of life under prebiotically relevant conditions. However, a global scenario is still lacking. Here, the possibility of inducing a significant chiral bias "from scratch", i.e. in the absence of pre-existing enantiomerically-enriched chemical species, will be considered first. It includes phenomena that are inherent to the nature of matter itself, such as the infinitesimal energy difference between enantiomers as a result of violation of parity in certain fundamental interactions, and physicochemical processes related to interactions between chiral organic molecules and physical fields, polarized particles, polarized spins and chiral surfaces. The spontaneous emergence of chirality in the absence of detectable chiral physical and chemical sources has recently undergone significant advances thanks to the deracemization of conglomerates through Viedma ripening and asymmetric auto-catalysis with the Soai reaction. All these phenomena are commonly discussed as plausible sources of asymmetry under prebiotic conditions and are potentially accountable for the primeval chiral bias in molecules of life. Then, several scenarios will be discussed that are aimed to reflect the different debates about the emergence of BH: extra-terrestrial or terrestrial origin (where?), nature of the mechanisms leading to the propagation and enhancement of the primeval chiral bias (how?) and temporal sequence between chemical homochirality, BH and life emergence (when?). Intense and ongoing theories regarding the emergence of optically pure molecules at different moments of the evolution process towards life, i.e. at the levels of building blocks of Life, of the instructed or functional polymers, or even later at the stage of more elaborated chemical systems, will be critically discussed. The underlying principles and the experimental evidence will be commented for each scenario with particular attention on those leading to the induction and enhancement of enantiomeric excesses in proteinogenic amino acids, natural sugars, and their intermediates or derivatives. The aim of this review is to propose an updated and timely synopsis in order to stimulate new efforts in this interdisciplinary field.

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.

Tuning the Organogelating and Spectroscopic Properties of a C3 -Symmetric Pyrene-Based Gelator through Charge Transfer.

Two-component organogels and xerogels based on a C3 -symmetric pyrene-containing gelator have been deeply characterized through a wide range of techniques. Based on the formation of charge transfer complexes, the gelation phenomenon proved to be highly dependent on the nature of the electron poor dopant. This parameter significantly influenced the corresponding gelation domains, the critical gelation concentrations of acceptor dopants, the gel-to-sol transition temperatures, the microstructures formed in the xerogel state and their spectroscopic properties. In particular, titrations and variable-temperature UV-visible absorption spectroscopy demonstrated the key role of donor-acceptor interactions with a remarkable correlation between the phase transition temperatures and the disappearance of the characteristic charge transfer bands. The assignment of these electronic transitions was confirmed through time-dependent density functional theory (TD-DFT) calculations. Eventually, it was shown that the luminescent properties of these materials can be tuned with the temperature, either in intensity or emission wavelength.

Dissecting the Role of the Sergeants in Supramolecular Helical Catalysts: From Chain Capping to Intercalation.

Controlling the properties of supramolecular assemblies requires unveiling the specific interactions between their components. In the present work, the catalytic properties and structure of co-assemblies composed of a benzene-1,3,5-tricarboxamide (BTA) ligand coordinated to copper (the soldier) and seven enantiopure BTAs (the sergeants) have been determined. Whatever the sergeant, the enantioselectivity of the reaction is directly proportional to the optical purity of the supramolecular helices. More strikingly, the role played by the sergeant in the co-assembly process differs significantly: from almost pure intercalator (when it is incorporated in the stacks of the soldier and generates long homochiral helices) to pure chain capper (when it leads to the formation of partly helically biased and short assemblies). The former situation leads to optimal enantioselectivity for the catalytic system under study (58 % ee) while the latter situation leads to very low selectivity (8 % ee). The successful rationalization of this high and unexpected difference is crucial for the development of more efficient catalysts and more elaborate supramolecular systems.

Emergence of Homochiral Benzene-1,3,5-tricarboxamide Helical Assemblies and Catalysts upon Addition of an Achiral Monomer.

Chirality amplification refers to the ability of a small chiral bias to fully control the main chain helicity of polymers and assemblies. Further implementation of functional chirally amplified helices as switchable asymmetric catalysts, chiral sensors, and circularly polarized light emitters will require a greater control of the energetics governing these chirality amplification effects. In this work, we report on the counterintuitive ability of an achiral molecule to suppress conformational defects in supramolecular helices, thus leading to the emergence of homochirality in a system containing a very small chiral bias. We focus our investigation on supramolecular helices composed of an achiral benzene-1,3,5-tricarboxamide (BTA) ligand, coordinated to copper, and an enantiopure BTA comonomer. Amplification of chirality as probed by varying the amount (sergeants and soldiers effect) or the optical purity (diluted majority-rules effect) of the enantiopure comonomer are modest in this initial system. However, both effects are hugely enhanced upon addition of a second achiral BTA monomer, leading to a perfect control of the helicity either by means of a remarkably low amount of sergeants (0.5%) or a small bias from a racemic mixture of enantiopure comonomers (10% ee). Such an enhancement in the amplification of chirality is only achieved by mixing the three components, i.e. the two achiral and the enantiopure comonomers, highlighting a synergistic effect upon coassembly of the three monomers. Investigation of the role of the achiral additive by multifarious analytical techniques supports its ability to stabilize the helical coassemblies and suppress helix reversals: i.e., conformational defects. Implementation of these helical copper precatalysts in the hydrosilylation of 1-(4-nitrophenyl)ethanone confirms that the effect of the achiral BTA additive is also operative under the conditions of the catalytic experiment. A highly enantioenriched product (90% ee) is produced by a supramolecular catalyst operating with ppm levels of chiral species.

Nonmonotonic Stress Relaxation after Cessation of Steady Shear Flow in Supramolecular Assemblies.

Stress relaxation upon cessation of shear flow is known to be described by single-mode or multimode monotonic exponential decays. This is considered to be ubiquitous in nature. However, we found that, in some cases, the relaxation becomes anomalous in that an increase in the relaxing stress is observed. Those observations were made for physicochemically very different systems, having in common, however, the presence of self-associating units generating structures at large length scales. The nonmonotonic stress relaxation can be described phenomenologically by a generic model based on a redistribution of energy after the flow has stopped. When broken bonds are reestablished after flow cessation, the released energy is partly used to locally increase the elastic energy by the formation of deformed domains. If shear has induced order such that these elastic domains are partly aligned, the reestablishing of bonds gives rise to an increase of the overall stress.

N-Substituted Benzene-1-Urea-3,5-Biscarboxamide (BUBA): Easily Accessible C2 -Symmetric Monomers for the Construction of Reversible and Chirally Amplified Helical Assemblies.

Non-C3 -symmetric supramolecular helices are gaining interest for the design of hierarchical assemblies, for the compartmentalisation or the self-assembly of polymer chains and for application in asymmetric catalysis. Herein, N-substituted benzene-1-urea-3,5-biscarboxamide (BUBA) monomers, which consist of one urea and two carbon-connected amide functions linked to an aromatic ring, are introduced as an easily accessible class of C2 -symmetric supramolecular synthons. In apolar solvents, BUBA monomers assemble into long helical assemblies by means of hydrogen-bonding and aromatic interactions, as assessed by several analytical techniques. To probe the influence of the urea function, BUBA and related benzene-1,3,5-tricarboxamide (BTA) helical polymers have been compared, in terms of their thermodynamics of formation, stability, reversibility and chiral amplification properties. Similar to BTA, BUBA monomers form long helices reversibly through a highly cooperative mechanism and the helicity of their assemblies is governed by chiral amplification effects. However, precise quantification of their properties reveals that BUBA monomers assemble in a more cooperative manner. Also, chiral amplification operates to a higher extent in BUBA helices, as probed by both sergeants-and-soldiers and majority-rules experiments. Compatibility between urea and amide functions also allows the formation of co-assemblies that incorporate both BUBA and BTA monomers. Importantly, a small amount of chiral BUBA monomers in these co-assemblies is sufficient to obtain single-handed helices; thus paving the way towards the development of functional supramolecular helices.

Organogel Formation Rationalized by Hansen Solubility Parameters: Shift of the Gelation Sphere with the Gelator Structure.

To rationalize how the gelation ability of a low molecular weight gelator is influenced by its molecular structure, we performed extensive solubility tests of a group of thiazole-based gelators and made use of Hansen solubility parameter formalism. We observe that the increase of a linear alkyl chain in these gelators promotes an increase of the radius of the gelation sphere as well as a gradual shift of its center to lower values of the polar (δP) and hydrogen bonding (δH) components. The molecular packing within the fibers and the crystal habit were determined by a combination of X-ray diffraction and molecular modeling. We attribute the gradual and linear shift of the gelation sphere to the fact that all of the studied gelators share the same molecular packing, so that an increasing length of the alkyl chain reduces the proportion of polar groups at the surface, resulting in a gradual increase in the contact between apolar parts of the fiber and the solvent.

Interactions of Organosilanes with Fibrinogen and Their Influence on Muscle Cell Proliferation in 3D Fibrin Hydrogels.

Silanization of biomacromolecules has emerged as a fruitful approach to prepare hybrid biohydrogels. However, very little is known about interactions between organosilanes and biopolymers in solution. Here we focused on fibrin, a protein of interest in the biomedical field, whose self-assembly process and resulting gel structure are highly sensitive to experimental conditions. Three main silanes were selected to decipher the relative influence of the silanol groups and organic functions. Whereas no protein denaturation was observed, silanes bearing hydrophobic groups had a surfactant-like behavior and could improve the dispersion of fibrinogen molecules, impacting gel formation kinetics and rheological properties. 3D cultures of myoblasts evidenced that organosilanes could promote or impede cell proliferation, suggesting interactions of silanols with fibrin. These results demonstrate that the two sides of the coin of organosilane reactivity are relevant at different stages of fibrin gel formation and must be considered for future development of hybrid biomaterials.

Beyond Simple AB Diblock Copolymers: Application of Bifunctional and Trifunctional RAFT Agents to PISA in Water.

The influence of the macromolecular reversible addition-fragmentation chain transfer (macro-RAFT) agent architecture on the morphology of the self-assemblies obtained by aqueous RAFT dispersion polymerization in polymerization-induced self-assembly (PISA) is studied by comparing amphiphilic AB diblock, (AB)2 triblock, and triarm star-shaped (AB)3 copolymers, constituted of N,N-dimethylacrylamide (DMAc = A) and diacetone acrylamide (DAAm = B). Symmetrical triarm (AB)3 copolymers could be synthesized for the first time in a PISA process. Spheres and higher order morphologies, such as worms or vesicles, could be obtained for all types of architectures and the parameters that determine their formation have been studied. In particular, we found that the total DPn of the PDMAc and the PDAAm segments, i.e., the same overall molar mass, at the same Mn (PDMAc)/Mn (PDAAm) ratio, rather than the individual length of the arms determined the morphologies for the linear (AB)2 and star shaped (AB)3 copolymers obtained by using the bi- and trifunctional macro-RAFT agents.

Oligo-Urea with No Alkylene Unit Self-Assembles into Rod-Like Objects in Water.

Long and rigid objects formed by self-assembly in water are useful as templates or for their rheological or biological properties. They are usually obtained by combining hydrogen bonding and strong hydrophobic interactions brought by an alkyl or alkylene chain. A simple access to well-defined rod-like assemblies in water is reported based on a penta-urea sticker directly connected to poly(ethylene oxide) side chains. These assemblies are characterized by an average length of several hundreds of nanometers and a monodisperse radius (4.5 nm) resulting from a reduced lateral aggregation of the stickers.

Templated PISA: Driving Polymerization-Induced Self-Assembly towards Fibre Morphology.

Dispersions of block copolymer fibres in water have many potential applications and can be obtained by polymerization-induced self-assembly (PISA), but only under very restricted experimental conditions. In order to enlarge this experimental window, we introduced a supramolecular moiety, a hydrogen-bonded bis-urea sticker, in the macromolecular reversible addition fragmentation chain transfer (RAFT) agent to drive the morphology of the nano-objects produced by RAFT-mediated PISA towards the fibre morphology. This novel concept is tested in the synthesis of a series of poly(N,N-dimethylacrylamide)-b-poly(2-methoxyethyl acrylate) (PDMAc-b-PMEA) diblock copolymers prepared by dispersion polymerization in water. The results prove that the introduction of the templating bis-urea stickers into PISA greatly promotes the formation of fibres in a large experimental window.

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

Introduction 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.

Organogel formation rationalized by Hansen solubility parameters: improved methodology.

An 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.

Bridging ß-Cyclodextrin Prevents Self-Inclusion, Promotes Supramolecular Polymerization, and Promotes Cooperative Interaction with Nucleic Acids.

A bridge to assemble: Cyclodextrins bridged with an ammonium linker bearing a hydrophobic substituent can efficiently form supramolecular polymers and avoid the competing self-inclusion and head-to-head processes. Furthermore, the self-assembling cyclodextrin derivative interacts in a highly cooperative manner with DNA, as demonstrated by compaction experiments. It also interacts cooperatively with siRNA and allows its transfection.

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 D and the molecular mass M 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.