Photo-responsive supramolecular polymer bottle-brushes: The key role of the solvent on self-assembly and responsiveness.

HYPOTHESIS: Supramolecular polymer bottlebrushes (SPBs) consist in the 1D self-assembly of building blocks composed of a self-assembling core with pendant polymer arms. Kinetic hurdles often hinder their stimuli-responsiveness in solution. Changing the nature of the solvent should alleviate these hurdles by modulating the self-association strength, leading to stimuli-responsive SPBs. EXPERIMENTS: The SPBs were formed, in various solvents, by hydrogen bond-driven self-assembly of an azobenzene-bisurea decorated with poly(ethylene oxide) polymer arms. The photo-isomerization of the azobenzene unit was studied by UV/visible spectroscopy and proton NMR spectroscopy, whereas the consequences on supramolecular self-assembly were studied by small angle neutron and X-ray scattering. FINDINGS: In water, the assembly was previously shown to be driven by both hydrogen-bonds and strong hydrophobic effects, the latter rendering the system kinetically frozen and the disassembly irreversible. Here we show that in organic solvents such as toluene or chloroform, reversible light-responsive dissociation is achieved. Solvophobic effects in these solvents are expected to be much weaker than in water, which probably allows reversibility of the light-response in the former solvents. The key role of the solvent on the reversibility of the process opens up new perspectives for the design of stimuli-responsive SPBs and their applications in various fields.

Switchable supramolecular helices for asymmetric stereodivergent catalysis.

Despite recent developments on the design of dynamic catalysts, none of them have been exploited for the in-situ control of multiple stereogenic centers in a single molecular scaffold. We report herein that it is possible to obtain in majority any amongst the four possible stereoisomers of an amino alcohol by means of a switchable asymmetric catalyst built on supramolecular helices. Hydrogen-bonded assemblies between a benzene-1,3,5-tricarboxamide (BTA) achiral phosphine ligand coordinated to copper and a chiral BTA comonomer are engaged in a copper-hydride catalyzed hydrosilylation and hydroamination cascade process. The nature of the product stereoisomer is related to the handedness of the helices and can thus be directed in a predictable way by changing the nature of the major enantiomer of the BTA comonomer present in the assemblies. The strategy allows all stereoisomers to be obtained one-pot with similar selectivities by conducting the cascade reaction in a concomitant manner, i.e. without inverting the handedness of the helices, or sequentially, i.e. by switching the handedness of the supramolecular helices between the hydrosilylation and hydroamination steps. Supramolecular helical catalysts appear as a unique and versatile platform to control the configuration of molecules or polymers embedding several stereogenic centers.

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.

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.

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.

Photoactive CuI-Cross-Linked Polyurethane Materials.

Using multinuclear copper iodide complexes as cross-linking agents in a polyurethane matrix, original photoluminescent stimuli-responsive materials were synthesized. The intrinsic photoluminescence properties of the covalently incorporated copper iodide complexes are thus transferred to the materials while retaining the beneficial characteristics of the polymer host. The transparent materials exhibit room-temperature phosphorescence with emission switching properties by displaying luminescence thermochromism and solvatochromism. The luminescence thermochromism is characterized by a change in the wavelength and intensity of the emission with temperature, and the vapochromic effect presents a contrasted response of extinction or exaltation according to the nature of the solvent of exposure. By combining the luminescence characteristics of photoactive copper iodide complexes with the ease of polymer processing, the application of these luminescent materials as phosphors in LED (light-emitting diode) devices was also demonstrated. The present study shows that the use of copper iodide complexes as cross-linkers in polymeric materials is a relevant strategy to design materials with enhanced functionalities in addition to their low cost and sustainable characteristics.

Dynamics and Rheology of Supramolecular Assemblies at Elevated Pressures.

A methodology to investigate the linear viscoelastic properties of complex fluids at elevated pressures (up to 120 MPa) is presented. It is based on a dynamic light scattering (DLS) setup coupled with a stainless steel chamber, where the test sample is pressurized by means of an inert gas. The viscoelastic spectra are extracted through passive microrheology. We discuss an application to hydrogen-bonding motif 2,4-bis(2-ethylhexylureido)toluene (EHUT), which self-assembles into supramolecular structures (tubes and filaments) in apolar solvents dodecane and cyclohexane. High levels of pressure (roughly above 20 MPa) are found to slow down the terminal relaxation process; however, the increases in the entanglement plateau modulus and the associated persistence length are not significant. The concentration dependence of the plateau modulus, relaxation times (fast and slow), and correlation length is practically the same for all pressures and exhibits distinct power-law behavior in different regimes. Within the tube phase in dodecane, the relative viscosity increment is weakly enhanced with increasing pressure and reaches a plateau at about 60 MPa. In fact, depending on concentration, the application of pressure in the tube regime may lead to a transition from a viscous (unentangled) to a viscoelastic (partially entangled to well-entangled) solution. For well-entangled, long tubes, the extent of the plateau regime (ratio of high- to low-moduli crossover frequencies) increases with pressure. The collective information from these observations is summarized in a temperature-pressure state diagram. These findings provide ingredients for the formulation of a solid theoretical framework to better understand and exploit the role of pressure in the structure and dynamics of supramolecular polymers.

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.

Size-dependent compression of threaded alkyldiphosphate in head to head cyclodextrin [3]pseudorotaxanes.

The encapsulation of guests in a confined space enables unusual conformations and reactivities. In particular, the compression of akyl chains has been obtained by self-assembled molecular capsules but such an effect has not been reported in solution for pseudorotaxane architectures. By exploiting the tendency of cyclodextrin (CD) to form head to head [3]pseudorotaxanes and the hydrogen bonding abilities of phosphate groups, we have studied the effect of the CD dimer cavity on the conformation of threaded α,ω-alkyl-diphosphate axles. The formation of [2]pseudorotaxanes and [3]pseudorotaxanes was investigated by a combination of NMR, ITC and X-ray diffraction techniques. In the solid state, the [3]pseudorotaxane with a C8 axle presents a fully extended conformation with both terminal phosphate groups interacting with hydroxyl groups of the primary rim of CDs. Such hydrogen bonding interactions are also present with the C9 and C10 axles resulting in a compression of the alkyl chain with gauche conformations in the solid state. NMR studies have shown that this effect is maintained in solution resulting in a size-dependent progressive compression of the alkyl chain by the CD [3]pseudorotaxane architecture for C9, C10 and C11 axles.

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.

Colored Janus Nanocylinders Driven by Supramolecular Coassembly of Donor and Acceptor Building Blocks.

Janus nanocylinders exhibit nanometric dimensions, a high aspect ratio, and two faces with different chemistries (Janus character), making them potentially relevant for applications in optics, magnetism, catalysis, surface nanopatterning, or interface stabilization, but they are also very difficult to prepare by conventional strategies. In the present work, Janus nanocylinders were prepared by supramolecular coassembly in water of two different polymers functionalized with complementary assembling units. The originality of our approach consists in combining charge transfer complexation between electron-rich and electron-poor units with hydrogen bonding to (1) drive the supramolecular formation of one-dimensional structures (cylinders), (2) force the two polymer arms on opposite sides of the cylinders independently of their compatibility, resulting in Janus nanoparticles, and (3) detect coassembly through a color change of the solution upon mixing of the functional polymers.

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 Å3 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.

Unexpected thermo-responsiveness of bisurea-functionalized hydrophilic polymers in water.

HYPOTHESIS: Thermoresponsive polymers are important materials for various applications. However, the number of polymers that exhibit this property in the temperature range of interest remains limited. The development of novel rational design strategies through the understanding of the thermal transition's origin is therefore of utmost importance. EXPERIMENTS: Bisurea-functionalized water-soluble polymers were synthesized by RAFT polymerization. After direct dissolution in water, the supramolecular assemblies were analyzed by cryo-TEM and SANS. Their temperature-dependent water-solubility was characterized by various techniques, namely DLS, SANS, DSC, IR, to understand the origin of the temperature sensitivity. FINDINGS: The supramolecular assemblies exhibit an unexpected temperature-dependent solubility. For instance, a cloud point of only 39 °C was measured for poly(N,N-dimethylacrylamide) assemblies. This property is not restricted to one specific polymer but is rather a general feature of bisurea-functionalized polymers that form supramacromolecular bottlebrushes in water. The results highlight the existence of two distinct transitions; the first one is a visually perceptible cloud point due to the aggregation of individual micelles, presumably driven by the hydrophobic effect. The second transition is related to the dissociation of intermolecular bisurea hydrogen bonds. Finally, we show that it is possible to widely tune the cloud point temperature through the formation of co-assemblies.

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.

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.

Author Correction: Straightforward preparation of supramolecular Janus nanorods by hydrogen bonding of end-functionalized polymers.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Straightforward preparation of supramolecular Janus nanorods by hydrogen bonding of end-functionalized polymers.

Janus cylinders are one-dimensional colloids that have two faces with different compositions and functionalities, and are useful as building blocks for advanced functional materials. Such anisotropic objects are difficult to prepare with nanometric dimensions. Here we describe a robust and versatile strategy to form micrometer long Janus nanorods with diameters in the 10-nanometer range, by self-assembly in water of end-functionalized polymers. The Janus topology is not a result of the phase segregation of incompatible polymer arms, but is driven by the interactions between unsymmetrical and complementary hydrogen bonded stickers. Therefore, even compatible polymers can be used to form these Janus objects. In fact, any polymers should qualify, as long as they do not prevent co-assembly of the stickers. To illustrate their applicative potential, we show that these Janus nanorods can efficiently stabilize oil-in-water emulsions.

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.