Design of discotic liquid crystal enabling complete switching between and memory of two alignment states over a large area

The alignment control of discotic columnar liquid crystals (LCs), featuring a low motility of the constituent molecules and thus having a large viscosity, is a challenging task. Here we show that triphenylene hexacarboxylic ester, when functionalized with hybrid side chains consisting of alkyl and perfluoroalkyl groups in an appropriate ratio, gives a hexagonal columnar (Col_h) LC capable of selectively forming large-area uniform homeotropic or homogeneous alignments, upon cooling from its isotropic melt or upon application of a shear force at its LC temperature, respectively. In addition to the alignment switching ability, each alignment state remains persistent unless the LC is heated to its melting temperature. In situ X-ray diffraction analysis under the application of a shear force, together with polarized optical microscopy observations, revealed how the columnar assembly is changed during the alignment-switching process. The remarkable behavior of the discotic LC is discussed in terms of its rheological properties.

A columnar liquid crystal consisting of a triphenylene hexacarboxylic ester mesogen and semifluoroalkyl side chains shows complete switching between homeotropic and homogeneous alignments, each of which remains persistent up to its melting point.

Hierarchical lyotropic liquid crystalline behaviors of supramolecular polymers influenced by alkyl chain branching

The peripheral chain branching in monomeric structures influences the hierarchical supramolecular assembly and lyotropic liquid crystalline properties.

The Role of the 1,2,3-Triazolyl Heterocycle in the Helical Columnar Assembly and Electric Field Response

Here, we have proven the role of the 1,2,3-triazolyl group in the helical assembly and electric field (E-field) response upon comparing liquid crystal analogs 1 and 2 based on 1,2,3-triazolyl and 1,3,4-oxadiazolyl linkers, respectively. An ordered helical column was only observed in 1, driven by the hydrogen-bonding interactions between the adjacent triazolyl nitrogen and hydrogen atoms. X-ray diffraction and energy simulations indicate that the helical column is a 11₂ helix and the helical axis does not coincide with the center of the molecular long axis. The key for the formation of the helical column is the tilted conformation of 1 originating from the steric repulsion between the triazolyl C-H and C-H of the aromatic core. Analysis of the dynamics in the simple hexagonal columnar phase revealed that the in-plane rotational motion of the triazolyl linker (1) is allowed, while the oxadiazolyl linker of 2 has limited conformational flexibility. A uniform alignment under an E-field only occurs in 1, demonstrating the requirement for conformational flexibility in the polar linker. This alignment enhances the electric conductance of 1 by approximately two-fold.

Unraveling Halogen Effects in Supramolecular Polymerization

Halogens play a crucial role in numerous natural processes and synthetic materials due to their unique physicochemical properties and the diverse interactions they can engage in. In the field of supramolecular polymerization, however, halogen effects remain poorly understood, and investigations have been restricted to halogen bonding or the inclusion of polyfluorinated side groups. Recent contributions from our group have revealed that chlorine ligands greatly influence molecular packing and pathway complexity phenomena of various metal complexes. These results prompted us to explore the role of the halogen nature on supramolecular polymerization, a phenomenon that has remained unexplored to date. To address this issue, we have designed a series of archetypal bispyridyldihalogen Pt^II complexes bearing chlorine (1), bromine (2), or iodine (3) and systematically compared their supramolecular polymerization in nonpolar media using various experimental methods and theory. Our studies reveal a remarkably different supramolecular polymerization for the three compounds, which can undergo two competing pathways with either slipped (kinetic) or parallel (thermodynamic) molecular packing. The halogen exerts an inverse effect on the energetic levels of the two self-assembled states, resulting in a single thermodynamic pathway for 3, a transient kinetic species for 2, and a hidden thermodynamic state for 1. This seesaw-like bias of the energy landscape can be traced back to the involvement of the halogens in weak N-H···X hydrogen-bonding interactions in the kinetic pathway, whereas in the thermodynamic pathway the halogens are not engaged in the stabilizing interaction motif but rather amplify solvophobic effects.

Helical nanofibers of N-(perfluorooctanoyl)cysteine ethyl ester in coordination polymers of Ag. Chem Commun (Camb) 2020;56:15133-15136. [PMID: 33165457 DOI: 10.1039/d0cc05989g]

We propose using the formation of coordination polymers of Ag+ to probe differences between the perfluorinated alkyl chain and the alkyl chain by deriving a thiol ligand, N-(perfluoroalkanoyl)cysteine. Rapid formation in EtOH of P-/M-helical nanofibrils of high thermostability was found for N-(perfluorooctanoyl)-l-/d-cysteine ethyl esters at the μM level upon mixing with Ag+, but not for the octanoyl counterpart. This difference was also observed in terms of circular dichroism-enantiomeric excess dependence.

Competing forces in the self-assembly of amide-functionalized discotic mesogens

The effect of incorporating a single amide group on the self-assembly of discotic mesogens was examined. Two series of tetraalkoxydibenzophenazines amides were prepared: tertiary diethyl amides, dEt(n) incapable of hydrogen bonding, and secondary amides HBu(n) that can act as both H-bond donors and acceptors. These compounds exhibit markedly different behavior in solution; NMR studies of dEt(n) show no evidence of self-association, whereas HBu(n) strongly associate via H-bonding and π-stacking. Compounds HBu(n) also act as small molecule gelators in a range of solvents, a property not observed for the corresponding tertiary amides. All compounds were found to form Colh liquid crystal phases; variable temperature XRD experiments indicate that each column has a diameter approximately equal to that of a single molecule. A comparison of the phase behavior of HBu(n) and dEt(n) suggests that the columnar phases of the former are stabilized by hydrogen bonding, likely at the expense of local parallel alignment of these molecules. Single crystal X-ray diffraction studies revealed that dEt(6) adopts an antiparallel arrangement in the solid state, in keeping with previous theories of packing within columnar LC phases. These studies highlight the interplay between competing factors, such as hydrogen bonding, π-stacking and dipole-dipole interactions that affect the stability of the LC phases.

Droplet Core Intermolecular Interactions and Block Copolymer Composition Heavily Influence Oil-In-Water Nanoemulsion Stability

Colloidal oil-in-water nanoemulsions are gaining increasing interest as a nanoparticle delivery system because of their large oil droplet core that can carry a large payload. In order to formulate these particles with long-term stability, an appropriate oil media and block copolymer pair must be selected. The interaction between the nanoemulsion core and the polymer shell is critical to forming stable nanoparticles. Herein, we probed how interactions between various polymers with hydrocarbon and perfluorocarbon oil media influenced nanoemulsion formation, stability, and size. Through a series of nanoemulsions with unique polymer/oil media combinations, we examined the effects of oil core hydrophobicity, fluorophilicity, surface charge, and volume as well as the effects of polymer tail composition. Surprisingly, we found that nanoemulsions formulated with pure perfluorocarbon oil cores versus perfluoro poly(ether) oil cores exhibited very different characteristics. We also found that both hydrocarbon and fluorocarbon polymer tails interacted favorably with perfluoro poly(ethers) as well as hydrocarbon oil cores forming stable nanoemulsions. We believe these results are focused on the unique properties of perfluorocarbons especially their rigidity, low polarizability, and near-zero surface charge. Interestingly, we saw that perfluoro poly(ethers) deviated from these expected properties resulting in an increased versatility when formulating nanoemulsions with perfluoro poly(ether) oil cores compared to pure perfluorocarbon oil cores. Nanoemulsion size, stability, growth rate, and life time were explored to probe these factors. Experimental and computational data are presented as a rationale.

Synthesis and characterization of new H-shaped triphenylene discotic room-temperature liquid crystal tetramers by a copper-free click reaction

A series of new H-shaped triphenylene discotic liquid crystal tetramers has been designed and synthesized using a copper-free [3+2] cycloaddition reaction.

C3 -Symmetric Tricyclo[2.2.1.02,6 ]heptane-3,5,7-triol

A straightforward access to a hitherto unknown C₃ -symmetric tricyclic triol both in racemic and enantiopure forms has been developed. Treatment of 7-tert-butoxynorbornadiene with peroxycarboxylic acids provided mixtures of C₁ - and C₃ -symmetric 3,5,7-triacyloxynortricyclenes via transannular π-cyclization and replacement of the tert-butoxy group. By refluxing in formic acid, the C₁ -symmetric esters were converted to the C₃ -symmetric formate. Hydrolysis gave diastereoisomeric triols, which were separated by recrystallization. Enantiomer resolution via diastereoisomeric tri(O-methylmandelates) delivered the target triols on a gram scale. The pure enantiomers are useful as core units of dopants for liquid crystals.

Multicompartment Theranostic Nanoemulsions Stabilized by a Triphilic Semifluorinated Block Copolymer

The presence of a perfluorocarbon block in a multiblock polymer has been shown to be an additional driving force toward nanoparticle assembly. In the preparation of nanoemulsions, this perfluorocarbon block also provides enhanced particle stability. Herein, the synthesis of a new triphilic, semifluorinated copolymer, M2F8H18, is introduced. This ABC type block copolymer can be used to formulate extremely stable nanoemulsions, assembled around a lipophilic droplet, with lifetimes of one year or more. The central oil droplet can stably solubilize high concentrations of hydrophobic drugs, making this system an ideal drug delivery vehicle. The incorporation of the perfluorocarbon block modulates drug release from the lipophilic core via the surrounding fluorous shell. Fluorous imaging agents incorporated into the fluorous shell prolong drug release even further as well as provide potent ¹⁹F-MRI contrast ability. In vitro studies show that these nanoemulsions efficiently inhibit cancer cell growth, thus providing a theranostic drug delivery system.

Temperature-Induced, Selective Assembly of Supramolecular Colloids in Water

In this article, we report the synthesis and physical characterization of colloidal polystyrene particles that carry water-soluble supramolecular N,N',N″,-trialkyl-benzene-1,3,5-tricarboxamides (BTAs) on their surface. These molecules are known to assemble into one-dimensional supramolecular polymers via noncovalent interactions. By tethering the BTAs to charge-stabilized particles, the clustering behavior of the resulting colloids was dictated by a balance between interparticle electrostatic repulsion and the BTA-mediated attractions. Through careful tuning of the dispersing medium's ionic strength, a regime was found in which particle aggregation could be reversibly induced upon heating the dispersion. These findings clearly indicate that hydrophobic interactions, which become stronger upon heating, play an important role during the clustering process. Besides the thermoreversible nature of the generated hydrophobic interparticle attractions, we found the clustering to be selective, that is, the BTA-functionalized colloids do not interact with nonfunctionalized hydrophobic polystyrene particles. This selectivity in the association process can be rationalized by the preferred stacking of the surface-tethered BTAs. These selective intermolecular/particle bonds are likely stabilized by the formation of hydrogen bonds, as previously observed for analogous molecular BTA assemblies. The resulting driving force responsible for particle clustering is therefore dual in nature and depends on both hydrophobic attractions and hydrogen bonding.

The effects of hexafluoroisopropanol on guest binding by water-soluble capsule and cavitand hosts

Encapsulation of amphiphilic guests in a water-soluble cavitand is enhanced by the addition of hexafluoroisopropanol (HFIP). While binding of n-alkanes in cavitands in HFIP/D2O mixtures was similar to that observed in 100% D2O, the binding of guests with terminal polar groups was quite different. Several α,ω-bolaamphiphiles: alkyldiols (C10-C12), a dinitrile (C14) and a diacid (C16) became encapsulated in HFIP/D2O solutions. As little as 15% HFIP v/v in D2O moves the guest from cavitand to the dimeric capsule. The unusual binding of polar functional groups inside the confined space is deduced from NMR COSY spectra and supported by DFT calculations. Alkane guests are also encapsulated in 100% HFIP.

Dark conglomerate phases of azobenzene derived bent-core mesogens - relationships between the molecular structure and mirror symmetry breaking in soft matter

New 4-bromoresorcinol based bent-core molecules with peripheral fluoro substituted azobenzene wings have been synthesized and the liquid crystalline self-assembly was investigated by differential scanning calorimetry (DSC), optical polarizing microscopy (POM), electro-optic studies and X-ray diffraction (XRD). A new type of optically isotropic mesophase composed of chiral domains with opposite handedness (dark conglomerate phases, DC phases) is observed, which for some homologues with medium alkyl chain length is stable down to ambient temperature. It is proposed that these DC phases are formed by helical twisted nano-domains of limited size and composed of the crystallized aromatic cores which are separated by the disordered alkyl chains. This structure is distinct from the previously known soft helical nano-filament phases (HNF phases, B4 phases) formed by extended crystalline nano-filaments and also distinct from the fluid sponge phases composed of deformed fluid layers. Comparison with related bent-core molecules having H, F, Cl, I, CH3 and CN groups in the 4-position at the resorcinol core, either with or without additional peripheral fluorines, provided information about the effects of these substituents on the tendency to form DC phases. Based on these relationships and by comparison with the minimum energy conformations obtained by DFT calculations a hypothesis is provided for the formation of DC phases depending on the molecular structure.

Triple helix formation in amphiphilic discotics: demystifying solvent effects in supramolecular self-assembly

A set of chiral, amphiphilic, self-assembling discotic molecules based on the 3,3'-bis(acylamino)-2,2'-bipyridine-substituted benzene-1,3,5-tricarboxamide motif (BiPy-BTA) was prepared. Amphiphilicity was induced into the discotic molecules by an asymmetrical distribution of alkyl and oligo(ethylene oxide) groups in the periphery of the molecules. Small-angle X-ray scattering, cryogenic transmission electron microscopy, and circular dichroism spectroscopy measurements were performed on the discotic amphiphiles in mixtures of water and alcohol at temperatures between 0 °C an 90 °C. The combined results show that these amphiphilic discotic molecules self-assemble into supramolecular fibers consisting of either one or three discotic molecules in the fiber cross-section and that the presence of water induces the bundling of the supramolecular fibers. The rich phase behavior observed for these molecules proves to be intimately connected to the mixing thermodynamics of the water-alcohol mixtures.

Coordination polymer gels with important environmental and biological applications

Coordination Polymer Gels (CPGs) constitute a subset of solid-like metal ion and bridging organic ligand structures (similar to metal-organic frameworks) that form multi-dimensional networks through a trapped solvent as a result of non-covalent interactions. While physical properties of these gels are similar to conventional high molecular weight organic polymer gels, coordination polymer gel systems are often fully reversible and can be assembled and disassembled in the presence of additional energy (heat, sonication, shaking) to give a solution of solvated gelators. Compared to gels resulting from purely organic self-assembled low molecular weight gelators, metal ions incorporated into the fibrilar networks spanning the bulk solvent can impart CPGs with added functionalities. The solid/liquid nature of the gels allows for species to migrate through the gel system and interact with metals, ligands, and the solvent. Chemosensing, catalysis, fluorescence, and drug-delivery applications are some of the many potential uses for these dynamic systems, taking advantage of the metal ion's coordination, the organic polydentate ligand's orientation and functionality, or a combination of these properties. By fine tuning these systems through metal ion and ligand selection and by directing self-assembly with external stimuli the rational synthesis of practical systems can be envisaged.

Biomolecules at interfaces: chiral, naturally

Interfaces are a most important environment in natural and synthetic chemistries for a wide variety of processes, such as catalysis, recognition, separation, and so on. Naturally occurring systems have evolved to one handedness and the study of interfaces where biomolecules are located is a potentially revealing pursuit with regard to understanding the reasons and importance of stereochemistry in these environments. Equally, the spontaneous resolution of achiral and chiral compounds at interfaces could lead to explanations regarding the emergence of single handedness in proteins and sugars. Also, the attachment of biomolecules to surfaces leads to systems capable of stereoselective processes which may be useful for the applications mentioned above. The review covers systems ranging from small biomolecules studied under ultrapure conditions in vacuum to protein adsorption to surfaces in solution, and the techniques that can be used to study them.