Polyphilic Molecules Research Articles

A self-assembled liquid crystal honeycomb of highly stretched (3-1-1)-hexagons.

A new liquid crystalline honeycomb phase is reported, containing highly stretched giant hexagonal cells with two opposing walls spanned by three consecutive end-to-end H-bonded rods, the (3-1-1) hexagons.

(Cryo)Transmission Electron Microscopy of Phospholipid Model Membranes Interacting with Amphiphilic and Polyphilic Molecules.

Lipid membranes can incorporate amphiphilic or polyphilic molecules leading to specific functionalities and to adaptable properties of the lipid bilayer host. The insertion of guest molecules into membranes frequently induces changes in the shape of the lipid matrix that can be visualized by transmission electron microscopy (TEM) techniques. Here, we review the use of stained and vitrified specimens in (cryo)TEM to characterize the morphology of amphiphilic and polyphilic molecules upon insertion into phospholipid model membranes. Special emphasis is placed on the impact of novel synthetic amphiphilic and polyphilic bolalipids and polymers on membrane integrity and shape stability.

Cluster Formation of Polyphilic Molecules Solvated in a DPPC Bilayer.

We analyse the initial stages of cluster formation of polyphilic additive molecules which are solvated in a dipalmitoylphosphatidylcholine (DPPC) lipid bilayer. Our polyphilic molecules comprise an aromatic (trans-bilayer) core domain with (out-of-bilayer) glycerol terminations, complemented with a fluorophilic and an alkyl side chain, both of which are confined within the aliphatic segment of the bilayer. Large-scale molecular dynamics simulations (1 s total duration) of a set of six of such polyphilic additives reveal the initial steps towards supramolecular aggregation induced by the specific philicity properties of the molecules. For our intermediate system size of six polyphiles, the transient but recurrent formation of a trimer is observed on a characteristic timescale of about 100 ns. The alkane/perfluoroalkane side chains show a very distinct conformational distribution inside the bilayer thanks to their different philicity, despite their identical anchoring in the trans-bilayer segment of the polyphile. The diffusive mobility of the polyphilic additives is about the same as that of the surrounding lipids, although it crosses both bilayer leaflets and tends to self-associate.

Applications of Solid-State NMR Spectroscopy for the Study of Lipid Membranes with Polyphilic Guest (Macro)Molecules.

The incorporation of polymers or smaller complex molecules into lipid membranes allows for property modifications or the introduction of new functional elements. The corresponding molecular-scale details, such as changes in dynamics or features of potential supramolecular structures, can be studied by a variety of solid-state NMR techniques. Here, we review various approaches to characterizing the structure and dynamics of the guest molecules as well as the lipid phase structure and dynamics by different high-resolution magic-angle spinning proton and 13C NMR experiments as well as static 31P NMR experiments. Special emphasis is placed upon the incorporation of novel synthetic polyphilic molecules such as shape-persistent T- and X-shaped molecules as well as di- and tri-block copolymers. Most of the systems studied feature dynamic heterogeneities, for instance those arising from the coexistence of different phases; possibilities for a quantitative assessment are of particular concern.

Skeletal cubic, lamellar, and ribbon phases of bundled thermotropic bolapolyphiles.

A series of T-shaped polyphilic molecules composed of a rigid linear biphenyl core with a polar glycerol group at each end and one swallow-tail semiperfluorinated lateral chain were synthesized and their thermotropic liquid crystalline (LC) phases were investigated by X-ray diffraction, calorimetry, and microscopy. The compounds have a long alkyl spacer between the aromatic core and the fluorinated C(n)F(2n+1) ends, where n = 4, 6, 8, and 10. Upon melting, all compounds start with lamellar LC phases, followed on heating by a rectangular columnar ribbon phase with c2mm symmetry. Unusually, a ribbon is a flat bundle of molecular cores highly aligned parallel to the ribbon axis. On further heating, for n = 8 and 10, this phase is succeeded by a bicontinuous cubic phase with Ia3d symmetry. This is a new variant of the "gyroid" phase, with axially oriented rod-like molecular cores forming the skeleton of the two infinite networks and junctions separated by exactly two molecular lengths. In this tricontinuous core-shell structure (aromatic-aliphatic-perfluoroalkyl), the polar glycerol domains of appreciable size, contained within the skeleton, can be considered as micellar.

Arrays of giant octagonal and square cylinders by liquid crystalline self-assembly of X-shaped polyphilic molecules

Nanopatterning by molecular self-assembly has been a topic of intense research in pursuit of 'bottom-up' methods of generating structures for use in nanotechnology. The systems most widely studied have been two- and three-dimensional morphologies of block copolymers. However, T- and X-shaped polyphilic liquid crystals have recently been shown to have great potential for generating soft honeycomb-like structures, surpassing those of polymers in both complexity and degree of order. The cell cross-section of the liquid crystals honeycombs has so far been limited by small molecular size. Here we overcome this limitation by exploiting the inability of the polyphiles to simultaneously achieve optimal packing and complete nanophase separation. This frustration results in a two-dimensional periodic honeycomb consisting of giant octagonal and square cylinders with circumferences of 12 and 8 molecules, respectively. In addition to forming large cells, the two-molecule-long bendy walls give this structure extra versatility and stability.

Self-Assembly of T-Shaped Polyphilic Molecules in Solvent Mixtures

Coarse-grained molecular simulations of a model of a T-shaped polyphilic molecule in the presence of solvents are presented. The target molecules are liquid crystal bolaamphiphiles with grafted lateral side chains. The main core of the molecule corresponds to a rigid biphenyl unit, decorated at both ends with polar associating sites. A lateral flexible chain, whose character is mostly antagonistic with respect to the rest of the molecule, imparts unique self-assembly behavior to the pure fluid. In the case herein, the pure T-shaped bolaamphiphile (TsB) forms two-dimensional honeycomb-like columnar structures, and a study is performed on the effects of solvents added to this structure. Three model solvents are considered, each corresponding to the three different parts of the original molecule. We consider the whole concentration range and a temperature at which the pure TsB fluid shows a columnar liquid crystal behavior. All three solvents show different progressions and equilibrium mesophases. The solvent compatible with the end groups swells the structure at low concentrations and at higher concentrations forces a phase split between an essentially pure solvent phase and a structured LC phase. The solvent affine with the core of the TsB does not swell the structure at low concentrations. At higher concentrations, this solvent induces a phase separation where the TsB-rich phase forms a unique liquid crystal membrane. The solvent compatible with the lateral chain will not form stable phase separations, but transforms the columnar structure into a loose, worm-like micellar network. The results reported are generic in nature and describe the phase space for the entire family of TsB's.

Axial-Bundle Phases − New Modes of 2D, 3D, and Helical Columnar Self-Assembly in Liquid Crystalline Phases of Bolaamphiphiles with Swallow Tail Lateral Chains

Two series of polyphilic molecules composed of a rigid and linear p-terphenyl core, terminated at both ends with polar glycerol groups capable of hydrogen bonding, and two branched swallow tail-type lateral chains, composed of a fluorinated and a nonfluorinated branch or two fluorinated branches, were synthesized and investigated by differential scanning calorimetry, polarizing microscopy, and X-ray diffraction (XRD) with respect to their self-assembly in thermotropic liquid crystalline (LC) phases. Hexagonal columnar phases were formed by all molecules, at least at the highest temperature. In these phases the columns are composed of a core of aromatic rods and an aliphatic shell. The aromatic rods form bundles which are rotationally averaged and lie parallel to the column long axis. This unique organization is proven by different optical and XRD methods. The aromatic and glycerol groups inside the rod bundles are segregated into alternating segments. Depending on temperature and molecular structure, long-range intercolumnar correlation of this periodicity could take place, leading to a 3D-ordered LC phase with rhombohedral R ̄3m symmetry. The bundles are embedded in the matrix of the lateral chains, which is divided into fluoroalkyl- and aliphatic-rich regions. In the 2D columnar phase the fluorinated regions take the form of either straight columns running along the edges of the hexagonal Voronoi cells or, for compounds with a higher degree of fluorination, fuse to a hexagonal honeycomb enclosing the aromatic cores. In the R ̄3m phase the fluorine-rich chains are preferentially found along right- and left-handed helices wound around the 3(1) screw axes between the main aromatic columns.

Lyotropic self-assembly mechanism of T-shaped polyphilic molecules

We present coarse-grained molecular dynamics simulations of mixtures of a model of T-shaped polyphilic bolaamphiphile liquid crystal molecules with a solvent. Based on the premise that the most important features of the liquid structure stem from the balance between the close range repulsions and the strong directional forces typical of hydrogen bonding and association, we have employed a coarse-graining approach that simplifies and minimises the attractions present in the system. The model consists of six fused rigid spheres, where the two end spheres have a significant attraction amongst themselves while the rest are repulsive in nature. A weakly self-attracting lateral chain consisting of fully flexible tangently bonded spheres is attached to one of the central spheres. Thus, the T-shaped molecule is composed of three mutually repulsive segments which allow the pure system to self-assemble into a liquid crystalline honeycomb columnar phase. The stability of the columnar phase is probed by the sequential addition of a solvent that has affinity with only one of the segments of the molecule. Our coarse-graining technique allows us to observe dynamically not only the 1st level nanoscale segregation but also the 2nd level reorganization which leads to the formation of replicated periodic structures. It is seen how this latter structuring takes place at times which are an order of magnitude longer than the former, and by itself explains the practical limitations of studying self-assembly with more detailed atomistic models. Mobility coefficients (related to diffusion constants), order parameters and direct visualization of the configurations are used to present a phase diagram for the solvated system in the whole concentration range. At low solvent density, the solvent solvates the honeycomb structure, but does not alter the order significantly. At modest volume fractions of solvent, the solvent mostly segregates into a distinct phase, while the T-shaped molecules retain a phase with columnar structure. At very large solvent concentrations, the T-shaped molecules form structureless aggregates, while at the infinite dilution limit present themselves as dimers and monomers.

Dissipative particle dynamics simulation of T- and X-shaped polyphilic molecules exhibiting honeycomb columnar phases

Dissipative particle dynamics simulations are used to investigate the structure of the columnar phases in T- and X-shaped bolaamphiphiles. These amphiphilic molecules consist of a rod-like aromatic core, with a polar group at each end. A single lateral chain is present in the T-shaped molecules and two lateral chains are present for X-shaped molecules. The simulations indicate that both types of molecules exhibit square and hexagonal columnar phases, in which the walls of the columns are formed by the aromatic core, the polar groups are located at the edges and the lateral chains fill the interiors of the columns. For the T-shaped molecules, the columns are always double walled. In contrast, for the X-shaped molecules, the columnar phases are single walled, in agreement with X-ray diffraction studies.

Molecular dynamics simulation of the mesophase behaviour of a model bolaamphiphilic liquid crystal with a lateral flexible chain

We present simulations of a coarse-grained model of a bolaamphiphile liquid crystal molecule with a grafted flexible side chain. The coarse-graining approach employed is based on minimising the attractions present in the system, on the premise that the most important features of the liquid structure stem from the balance between the close range repulsions and the strong directional forces typical of hydrogen bonding and association. The model consists of six fused rigid spheres, where the two end spheres have a significant attraction amongst themselves while the rest are repulsive in nature. A weakly self-attracting lateral chain consisting of fully flexible tangently bonded spheres is attached to one of the central spheres. A parametric study is made of the configurations of collectives of these molecules at temperatures that span from the isotropic fluid range down to the onset of crystallisation. The underlying rigid core molecules (with no side chain) are set up to exhibit a smectic liquid crystal behaviour. Upon increasing the number of spheres in the lateral chains from 1 to 12, the liquid regions exhibit a rich variety of self-assembled structures; for small number of lateral chain spheres columnar arrays of different cross sections (triangular, square, rectangular, hexagonal) are obtained and for the longer chains lamellar structures of different interlayer spacing are observed. We showcase and give a rational physical explanation for the global phase behaviour of the model, based on pertinent order parameters and apparent diffusivities of the several regimes encountered. Although no attempt has been made to fit the parameters of the model to real molecules, the model is inspired by the reported synthesis of a family of T-shaped polyphilic molecules (C. Tschierske, Chem. Soc. Rev., 2007, 36, 1930–1970) where some of the above mentioned phases have been inferred from experimental measurements.

Structure and molecular dynamics of the mesophases exhibited by an organosiloxane tetrapode with strong polar terminal groups

The polymorphism of a new organosiloxane tetrapode compound with cyano terminal polar groups was characterized by means of polarizing optical microscopy and x-ray diffraction. The compound exhibits smectic- A and smectic- C phases with a partial bilayer arrangement due to a certain degree of head-to-head association of the mesogenic units through their cyano end groups. On the basis of x-ray diffraction results, evidencing the microsegregation of polyphilic molecules, packing models for the smectic- A and smectic- C phases are proposed. A high degree of smectic positional order and a relatively low value of the tilt angle in the smectic- C phase are indicated. Molecular dynamics of the studied compound was investigated by means of proton NMR relaxometry. The frequency dispersions of the spin-lattice relaxation time (T1) show that the relaxation is induced by three rotational modes of individual dendrimer arms with frequencies between 10;{6} and 10;{9}Hz . In the smectic phases, the effect of individual rotations is overwhelmed by a well expressed contribution of layer undulations at Larmor frequencies below approximately 10MHz . The appearance of this relaxation mechanism over the frequency range of three decades is so far unique in the case of thermotropic liquid crystals. The analysis of the layer undulations contribution supports the microsegregation model of the smectic phases by revealing a slowing-down of translational diffusion and the lack of interactions among the sublayers formed by the mesogenic groups.

Silicon-Containing Polyphilic Bent-Core Molecules: The Importance of Nanosegregation for the Development of Chirality and Polar Order in Liquid Crystalline Phases Formed by Achiral Molecules

Polyphilic molecules composed of a bent aromatic core, oligo(siloxane) units, and alkyl segments were synthesized, and the self-organization of these molecules was investigated. Most materials organize into polar smectic liquid crystalline phases. The switching process of these mesophases changes from antiferroelectric for the nonsilylated compounds via superparaelectric to surface-stabilized ferroelectric with increasing segregation of the silylated segments. It is proposed that the siloxane sublayers stabilize a polar synclinic ferroelectric (SmC(s)P(F)) structure, and the escape from a macroscopic polar order as well as steric effects leads to a deformation of the layers with formation of disordered microdomains, giving rise to optical isotropy. Another striking feature is the spontaneous formation of chiral domains with opposite handedness. For two compounds, a temperature-dependent inversion of the optical rotation of these domains was found, and this is associated with an increase of the tilt angle of the molecules from < 45 degrees to > 50 degrees. This observation confirms the recently proposed concept of layer optical chirality (Hough, L. E.; Clark, N. A. Phys. Rev. Lett. 2005, 95, 107802), which is a new source of optical activity in supramolecular systems. With increasing length of the alkyl chains, segregation is lost and a transition from smectic to a columnar phase is found. In the columnar phase, the switching process is antiferroelectric and takes place by rotation of the molecules around the long axes, which reverses the layer chirality; that is, the racemic ground-state structure is switched into a homogeneous chiral structure upon application of an electric field.

Smectic layering in polyphilic liquid crystals : X-ray diffraction and infra-red dichroism study

We present X-ray diffraction measurements of lamellar ordering for a new class of mesogens-polyphilic compounds-which were reported to form achiral ferroelectrics. Two phases called smectic X and X' manifest polar properties. Analysis of the scattering profiles parallel and perpendicular to the smectic layers provides detailed data on the structure of smectic A, smectic X and X' phases of these compounds. Smectic X' phase corresponds to a strongly defective layered structure of the smectic C type with longitudinal correlation length ξ ∥≤200 A. The molecules are in a zigzag conformation and are tilted with respect to the layer normals. Infra-red dichroism data show that the biphenyl moiety and the polyfluorinated chain of molecules form angles of 56 o and 26 o with respect to the layer normals. In the smectic X phase, three types of layering coexist: (i) polar layers with tilted molecules, (ii) a modulated structure of the same type consisting of layers periodically shifted with respect to each other and (iii) the orthogonal smectic A layers with a spacing of 41 A incommensurate with period 35 A of the polar smectic layers. The coexistence of such incommensurate structures is only possible in a strongly disordered medium. Infra-red dichroism data are consistent with this picture. The in-plane structure factor in smectic X, X' phases reveals two features of polyphilic compounds: a dramatic increase in the short-range positional correlations (ξ ⊥=35±40 A or approximately 7 or 8 molecules in the local in plane order) and splitting of the intensity profiles into three peaks. The latter corresponds to the appearance of nearest-neighbour molecular stacking at different distances. The formation of polar, either uniform or modulated layered structures in the system of polyphilic molecules is discussed

Physical properties of achiral mesomorphic ferroelectrics

Physical properties of new mesomorphic organic ferroelectrics composed of achiral polyphilic molecules are described. Ferroelectricity is demonstrated by the measurements of the acoustically induced piezoelectric response, pyroelectric effect and repolarization currents. For the first time, a ferroelectric repolarizable mesophase is obtained with non-chiral molecular units. The mechanism of ferroelectricity is therefore different from that found in conventional chiral smectics C ∗.

Synthesis of polyphilic compounds Evidence for ferroelectricity in a non-chiral mesophase

Abstract The synthesis and properties of new mesomorphic ferroelectrics composed of nonchiral polyphilic molecules are described. Polyphilic molecular units are made up of a sequence of chemical fragments differing by their chemical nature. The segregation between unlike species is used to favour a polar packing. Ferroelectricity is demonstrated by the measurement of an acoustically induced piezoelectric response and the determination of repolarization currents. It is the first time that a ferroelectric repolarizable mesophase is obtained with non-chiral molecular units. The mechanism of ferroelectricity is therefore different from that found in conventional chiral smectic C.

Polyphilic molecules: Synthesis and mesomorphic properties of a four-block molecule

A four-block mesogen composed of three chemically different moieties is synthesized. Its mesomorphic properties are characterized by optical microscopy, differential thermal analysis and X-ray diffraction at small angles, demonstrating a smectic C structure between 92°C and 111°C. The notion of a polyphile is defined and the expected peculiarities of such compounds are emphasized.