Liquid crystalline dendrimers

Synthesis and mesomorphic properties of "side-on" hybrid liquid crystalline silsesquioxanes

Novel hybrid silsesquioxane-based liquid crystalline derivatives with varied lengths of spacers and tails have been synthesized by hydrosilylation reactions of octakis(dimethylsiloxy)silsesquioxane and side-on mesogens via a platinum catalyst. The thermal behavior of three types of silsesquioxane-based liquid crystals (LCs), differentiated by the molecular structure of mesogens, was investigated by differential scanning calorimetry (DSC) and polarising optical microscopy (POM). Temperature-dependent small and wide-angle X-ray scattering was used to verify liquid crystalline phases, revealing that the silsesquioxane-based derivatives formed hexagonal columnar and nematic mesophases, and the effect of the molecular structure of the mesogens and the spacer length on the formation of LC phases is discussed. This investigation demonstrated that the choice of the "side-on" attachments plays a crucial role in enhancing the emergence of the nematic phase.

Dendrimer Based Binders Enable Stable Operation of Silicon Microparticle Anodes in Lithium-Ion Batteries

High-capacity anode materials (e.g., Si) are highly needed for high energy density battery systems, but they usually suffer from low initial coulombic efficiency (CE), short cycle life, and low-rate capability caused by large volume changes during the charge and discharge process. Here, a novel dendrimer-based binder for boosting the electrochemical performance of Si anodes is developed. The polyamidoamine (PMM) dendrimer not only can be used as binder, but also can be utilized as a crosslinker to construct 3D polyacrylic acid (PAA)-PMM composite binder for high-performance Si microparticles anodes. Benefiting from maximum interface interaction, strong average peeling force, and high elastic recovery rate of PAA-PMM composite, the Si electrode based on PAA-PMM achieves a high specific capacity of 3590 mAh g^-1 with an initial CE of 91.12%, long-term cycle stability with 69.80% retention over 200 cycles, and outstanding rate capability (1534.8 mAh g^-1 at 3000 mA g^-1 ). This work opens a new avenue to use dendrimer chemistry for the development of high-performance binders for high-capacity anode materials.

Gold Nanoparticles Modification with Liquid Crystalline Polybenzylic Dendrons via 1,3-Dipolar Cycloaddition

A series of six polybenzylic dendrons with an alkynyl focal point were synthesized for their incorporation to gold nanoparticles. Five of these compounds showed columnar mesomorphism in a wide range of temperatures. These dendrons were reacted with gold nanoparticles stabilized with a combination of a dodecanethiol and 11-azidoundecane-1-thiol. The azido group of the last compound allowed the functionalization of the nanoparticles with the six polybenzylic dendrons by 1,3-dipolar cycloaddition between their alkynyl groups and the terminal azido groups of the thiols. A high efficiency of the cycloaddition process (47-69%) was confirmed by several experimental techniques and no decomposition or aggregation phenomena were detected in the dendron-coated nanoparticles. The involved mechanism and the resulting percentage composition of the final materials are discussed. The results of the ulterior growth of the nanoparticles by thermal treatment are influenced by the size and the shape of the dendron and the temperature of the process. The structures of the final nanoparticles were investigated by TEM, DSC, TGA, NMR and UV-Vis spectroscopy. These nanoparticles do not show liquid crystal properties. However, a melting process between a crystalline and a fluid phase is observed. In the solid phase, the nanomaterials prepared show a short-range interaction between nanoparticles with a 2D local hexagonal order. A near-field effect was observed in the UV-vis spectra by coupling of different surface plasmon resonance bands (SPR) probably due to the short-range interactions. The main novelty of this work lies in the scarcity of previous studies of gold nanoparticles coated with dendrons forming themselves columnar mesophases. Most of the studies reported in the literature deal with gold nanoparticles coated with calamitic mesogens. Additionally, the effect of the thermal treatment, which in a previous paper was shown to increase the mean size of the nanoparticles without increasing their size polydispersity, has been studied in these materials.

Safety Challenges and Application Strategies for the Use of Dendrimers in Medicine

Dendrimers are used for a variety of applications in medicine but, due to their host–guest and entrapment characteristics, are particularly used for the delivery of genes and drugs. However, dendrimers are intrinsically toxic, thus creating a major limitation for their use in biological systems. To reduce such toxicity, biocompatible dendrimers have been designed and synthesized, and surface engineering has been used to create advantageous changes at the periphery of dendrimers. Although dendrimers have been reviewed previously in the literature, there has yet to be a systematic and comprehensive review of the harmful effects of dendrimers. In this review, we describe the routes of dendrimer exposure and their distribution in vivo. Then, we discuss the toxicity of dendrimers at the organ, cellular, and sub-cellular levels. In this review, we also describe how technology can be used to reduce dendrimer toxicity, by changing their size and surface functionalization, how dendrimers can be combined with other materials to generate a composite formulation, and how dendrimers can be used for the diagnosis of disease. Finally, we discuss future challenges, developments, and research directions in developing biocompatible and safe dendrimers for medical purposes.

Functional Chirality: From Small Molecules to Supramolecular Assemblies

Many structures in nature look symmetric, but this is not completely accurate, because absolute symmetry is close to death. Chirality (handedness) is one form of living asymmetry. Chirality has been extensively investigated at different levels. Many rules were coined in attempts made for many decades to have control over the selection of handedness that seems to easily occur in nature. It is certain that if good control is realized on chirality, the roads will be ultimately open towards numerous developments in pharmaceutical, technological, and industrial applications. This tutorial review presents a report on chirality from single molecules to supramolecular assemblies. The realized functions are still in their infancy and have been scarcely converted into actual applications. This review provides an overview for starters in the chirality field of research on concepts, common methodologies, and outstanding accomplishments. It starts with an introductory section on the definitions and classifications of chirality at the different levels of molecular complexity, followed by highlighting the importance of chirality in biological systems and the different means of realizing chirality and its inversion in solid and solution-based systems at molecular and supramolecular levels. Chirality-relevant important findings and (bio-)technological applications are also reported accordingly.

Converting non-Mesogenic to Mesogenic Stacking of Amino-s-Triazine-Based Dendrons with p-CN Phenyl Unit by Eliminating Peripheral Dipole

Three new amino-s-triazine-based dendrons, 1a, 1b, and 1c, containing an aryl-CN moiety in the dendritic skeleton were prepared in 72–81% yields (1a: R¹ = − N(n-C₈H₁₇)₂, R² = n-OC₈H₁₇, 1b: R¹ = R² = − N(n-C₈H₁₇)₂, 1c: R¹ = − N(n-C₈H₁₇)₂, R² = − N(n-C₄H₉)₂). Dendrons 1a with N(n-C₈H₁₇)₂ and n-OC₈H₁₇ peripheral substituents, surprisingly, did not show any mesogenic phase during the thermal process. However, non-mesogenic 1a can be converted to mesogenic 1b or 1c by eliminating the peripheral dipole arising from the alkoxy substituent; dendron 1b only comprising the same N(n-C₈H₁₇)₂ peripheral groups showed a ~25 °C mesogenic range on heating and ~108 °C mesogenic range on cooling. In contrast, dendron 1c possessing different N(n-C_mH_2m+1)₂ (m = 8 versus m = 4) peripheral units, having similar stacking as 1b, exhibited a columnar phase on thermal treatment, but its mesogenic range (~9 and ~66 °C on heating and cooling, respectively) was much narrower than that of 1b, attributed to 1c’s less flexible alkyl chains in the peripheral part of dendron. Dendron 1a with the alkoxy substituent in the peripheral skeleton, creating additional dipole correspondingly, thus, leads to the dendritic molecules having a non-mesogenic stacking. Without the peripheral dipole for intermolecular side-by-side interaction, dendrons 1b and 1c exhibit a columnar phase on thermal treatment because of the vibration from the peripheral alkyl chain.

Exohedral Functionalization of Fullerene by Substituents Controlling of Molecular Organization for Spontaneous C60 Dimerization in Liquid Crystal Solutions and in a Bulk Controlled by a Potential

A study was carried out on the possibility of orderly and spontaneous dimerization at room temperature of C₆₀ cages in fullerene liquid crystal fullerene dyads (R-C₆₀). For this purpose, dyads with a structural elements feature supporting π-stacking and Van der Waals interactions were tested, due to the presence of terthiophene donors linked through an α-position or dodecyloxy chains. In addition, this possibility was also tested and compared to dyads with shorter substituents and the pristine C₆₀. Research has shown that only in dyads with the features of liquid crystals, π-dimerization of C₆₀ units occurs, which was verified by electrochemical and spectroelectrochemical (ESR) measurements. Cyclic voltammetry and differential voltammetry studies reveal π-dimerization in liquid crystal dyad solution even without the possibility of previous polymerization (cathodic or anodic) under conditions in the absence of irradiation and without the availability of reaction initiators, and even with the use of preliminary homogenization. These dyads undergo six sequential, one-electron reductions of π-dimer (R-C₆₀···C₆₀-R), where two electrons are added successively to each of the two fullerene cages and first form two radical anion system (R-C₆₀)^•−(R-C₆₀)^•− without pairing with the characteristics of two doublets. Similarly, the second reductions of π-dimer occur at potentials that are close to the reduction potential for the conversion to a system of two triplet dianions (R-C₆₀)²⁻(R-C₆₀)²⁻. Electron paramagnetic resonance spectra indicate a significant interaction between C₆₀ cages. Interestingly, the strength of intermolecular bonds is so significant that it can overcome Coulombic repulsion, even with such highly charged particles as dianions and trianions. Such behavior has been revealed and studied so far only in covalently bonded C₆₀ dimers.

Unconventional Approaches to Prepare Triazine-Based Liquid Crystal Dendrimers

Most triazine-based liquid crystalline (LC) dendrimers reported thus far are the main-chain LC macromolecules with long flexible chains at their periphery and attached to internal rigid or semi-rigid frameworks. Their formation of mesogenic phases often depends on the intermolecular face-to-face π-π interactions between dendritic molecules, which are unusual. Their mesogenic phases can also be formed by incorporation of mesogenic units to the dendritic skeletons through long flexible chains, as most side-chain LC dendrimers, in which the peripheral mesogenic units generally play the important roles. For main-chain triazine-based dendrimers, their morphology is maintained by restricted freedom of rigid or semi-rigid connecting units, and their formations of LC phases are therefore not straightforward to be controlled. In this review, we thus describe modulating of the intermolecular face-to-face π-π interactions between the triazine-based dendritic molecules, with the aim of forming LC phases through molecular design.

Dendrimers: A New Race of Pharmaceutical Nanocarriers

Dendrimers are nanosized, symmetrical molecules in which a small atom or group of atoms is surrounded by the symmetric branches known as dendrons. The structure of dendrimers possesses the greatest impact on their physical and chemical properties. They grow outwards from the core-shell which further reacts with monomers having one reactive or two dormant molecules. Dendrimers' unique characteristics such as hyperbranching, well-defined spherical structure, and high compatibility with the biological systems are responsible for their wide range of applications including medical and biomedical areas. Particularly, the dendrimers' three-dimensional structure can incorporate a wide variety of drugs to form biologically active drug conjugates. In this review, we focus on the synthesis, mechanism of drug encapsulations in dendrimers, and their wide applications in drug delivery.

Dendrimer-mediated columnar mesophase of surfactants

The columnar mesophase, in which the molecular or supramolecular building blocks with rod-like geometry pack into two-dimensional (2D) lattices, is an important class of mesomorphic structure having been found in various liquid crystalline materials for practical applications. The cylindrical micelles assembled by amphiphilic surfactants may also form columnar mesophases with the micelle packing symmetry being tunable by the molecular characteristics of the surfactants. In this study, we demonstrate that a positively charged tree-like polymer, poly(amidoamine) (PAMAM) G4 dendrimer, acted as an effective structure-directing agent for the columnar mesophase of a common anionic surfactant, sodium dodecyl sulfate (SDS), via their electrostatic interaction. By adjusting the dendrimer charge density and the nominal binding ratio (Xn) of SDS to dendrimer, the electrostatic complexes self-assembled to form a body-centered cubic (BCC) sphere phase, wherein the dendrimers were staggered between the interspaces of the SDS spherical micelles packed in the BCC lattice. Four types of 2D columnar mesophase composed of SDS cylindrical micelles and dendrimers were accommodated within the interstitial tunnels, including the hexagonal columnar phase (Colhex), simple rectangular columnar phase (Colsr), oblique columnar phase (Colob) and centered rectangular columnar phase (Colcr). A detailed analysis of the geometry of the dendrimer in the columnar mesophases revealed that the structural transition was governed by the interplay among the lateral and axial deformations of the dendrimer, and the deformation of the SDS micelle cross section for achieving effective charge matching and accommodation of the dendrimer. The present study demonstrated the power of the dendrimer in directing the long-range ordered packing of oppositely charged cylinders to yield a rich structural polymorphism of the columnar mesophase that may be exploited for the development of functional materials.

Design, Synthesis and Self-Assembly of Functional Amphiphilic Metallodendrimers

A new family of alkynylated, amphiphilic dendrimers consisting of amidoamine linkers connected to 5,5'-functionalized 2,2'-bipyridine cores has been developed and evaluated in the formation of metallodendrimers of different generations and in self-assembly protocols. A convergent synthetic strategy was applied to provide dumbbell-shaped amphiphilic dendrimers, where the 2,2'-bipyridine cores could be coordinated to FeII centers to afford corresponding metallodendrimers. The ability of the metallic- and non-metallic dendritic structures to self-assemble into functional supramolecular aggregates were furthermore evaluated in aqueous solution. Spherical aggregates with sizes of a few hundred nanometers were generally produced, where controlled disassembly of the metallodendrimers through decomplexation could be achieved.

Molecular ejection transition in liquid crystal columns self-assembled from wedge-shaped minidendrons

Fan-shaped molecules with aromatic head-groups and two or more flexible pendant chains often self-assemble into columns that form columnar liquid crystals by packing on a 2d lattice. Such dendrons or minidendrons are essential building blocks in a large number of synthetic self-assembled systems and organic device materials. Here we report a new type of phase transition that occurs between two hexagonal columnar phases, Colh1 and Colh2, of Na-salt of 3,4,5-tris-dodecyloxy benzoic acid. Interestingly, the transition does not change the symmetry, which is p6mm in both phases, but on heating it involves a quantised drop in the number of molecules n in the cross-section of a column. The drop is from 4 to 3.5, with a further continuous decrease toward n = 3 as temperature increases further above Tc. The finding is based on evidence from X-ray diffraction. Using a transfer matrix formulation for the interactions within a column, with small additional mean field terms, we describe quantitatively the observed changes in terms of intermolecular forces responsible for the formation of supramolecular columns. The driving force behind temperature-induced molecular ejection from the columns is the increase in conformational disorder and the consequent lateral expansion of the alkyl chains. The asymmetry of the transition is due to the local order between 4-molecule discs giving extra stability to purely n = 4 columns.

Liquid-Crystalline Tris[60]fullerodendrimers

Liquid-crystalline tris[60]fullerodendrimers based on first- and second-generation poly(arylester)dendrons carrying cyanobiphenyl mesogens were synthesized for the first time by the olefin cross-metathesis reaction between type I (terminal) and type II (α,β-unsaturated carbonyl) olefinic precursors, using a second-generation Grubbs or Hoveyda-Grubbs catalyst. The modular synthetic approach developed here also allowed the selective preparation of the [60]fullerene-free, mono[60]fullerodendrimer, and bis[60]fullerodendrimer derivatives from the appropriate precursors. As revealed by polarized optical microscopy, differential scanning calorimetry, and small-angle X-ray scattering, all of the materials displayed liquid-crystalline properties. In agreement with the nature of the dendritic building blocks, the emergence of lamellar mesophases (smectic C and/or smectic A phases), with the segregation of the various constitutive parts, was systematically observed. The small variation of the mesomorphic temperature range and of the mesophase stability suggested that the mesomorphism is essentially dominated by the dendrimer itself and is regulated by a subtle adaptive mechanism, in which the proportion of monolayering and bilayering arrangements of the multisegregated lamellar mesophases is modified in order to compensate the space requirements of each of the elementary building blocks, namely, the [60]fullerene units, the cyanobiphenyl mesogens, and the dendritic matrix.

Guanine-oligothiophene conjugates: liquid-crystalline properties, photoconductivities and ion-responsive emission of their nanoscale assemblies

Liquid-crystalline conjugated guanine–oligothiophene derivatives show electron or ambipolar carrier transport properties and ion-induced phase transition accompanied by a luminescence colour change.

We here report the supramolecular self-assembly of hydrogen-bonded motifs for the development of nanostructured materials that exhibit dynamic functions such as stimuli-responsive properties and molecular recognition behaviour. We have designed and synthesised new thermotropic bicontinuous and columnar liquid-crystalline (LC) guanine–oligothiophene conjugates tethered with lipophilic chains, which exhibit ionic, electronic and photoluminescence properties. Their potassium salt complexes self-assemble into thermotropic columnar LC phases. Time-of-flight photoconductivity measurements have revealed that the guanine–oligothiophene conjugates in the LC states possess charge transport abilities with either electron or ambipolar mobility values of 10^–4 to 10^–3 cm² V^–1 s^–1. Furthermore, we have found that the complexation of potassium ions with the guanine motif could lead not only to structural change and thermal stabilization of the LC phases but also to a photoluminescence colour change in the solid states. The strategy presented in this work could lead to the design of new functional LC materials that could potentially be applicable as sensors and electronic devices.

Body-centred cubic packing of spheres - the ultimate thermotropic assembly mode for highly divergent dendrons

We have synthesized sodium tris(alkoxy)benzoates in which one of the three alkyl chains branches further into three C₁₈H₃₇ chains. These AB5 hyperbranched minidendrons melt directly into a body-centred cubic (BCC) mesophase formed by spherical "micelles". In contrast, their non-branched counterparts display various mesophases before they turn into BCC upon heating. This agrees with the predictions from a numerical geometric model that relates the shape of the molecular wedge to the type of mesophase they adopt. The spheres were found to shrink in volume upon heating and expand upon cooling, as molecules, in some cases nearly half of them, are ejected and reintegrated in the spheres. The ejection of dendrons is caused by their lateral thermal expansion. The BCC appears to be the ultimate mesophase for the extremely divergent wedges such as the hyperbranched minidendrons. In dendrons with chains of unequal length, the sphere size is fixed by the shorter chains, the longer ones back-folding or interdigitating to effectively widen the wedge. This new understanding of their assembly will help in designing new dendrons, e.g. for better encapsulation of guest molecules.

Ionic Liquid Crystals: Versatile Materials

This Review covers the recent developments (2005-2015) in the design, synthesis, characterization, and application of thermotropic ionic liquid crystals. It was designed to give a comprehensive overview of the "state-of-the-art" in the field. The discussion is focused on low molar mass and dendrimeric thermotropic ionic mesogens, as well as selected metal-containing compounds (metallomesogens), but some references to polymeric and/or lyotropic ionic liquid crystals and particularly to ionic liquids will also be provided. Although zwitterionic and mesoionic mesogens are also treated to some extent, emphasis will be directed toward liquid-crystalline materials consisting of organic cations and organic/inorganic anions that are not covalently bound but interact via electrostatic and other noncovalent interactions.

Homogeneous alignment of liquid crystalline dendrimers confined in a slit-pore. A simulation study

In this work we present results from isobaric-isothermal (NPT) Monte Carlo simulation studies of model liquid crystalline dendrimer (LCDr) systems confined in a slit-pore made of two parallel flat walls. The dendrimers are modelled as a collection of spherical and ellipsoidal particles corresponding to the junction points of the dendritic core and to the mesogenic units respectively. Assuming planar uniform (unidirectional) soft anchoring of the mesogenic units on the substrates we investigate the conformational and alignment properties of the LCDr system at different thermodynamic state points. Tractable coarse grained force fields have been used from our previous work. At low pressures the interior of the pore is almost empty, since almost all LCDrs are anchored to the substrates forming two-dimensional smectic-like structures with the mesogens aligned along the aligning direction of the substrates. As the pressure grows the LCDrs occupy the whole pore. However, even at low temperatures, the smectic organization does not transmit in the interior of the pore and is preserved for distances of 2-3 mesogenic diameters from the walls. For this reason, the global orientational order decreases with increasing pressure (density). In the vicinity (2-3 mesogenic diameters) of the pore walls, mesogenic units preserve the smectic structure whose layers are separated by layers of spherical beads. In this region individual LCDrs possess a rod like shape.

Molecular assembly of highly symmetric molecules under a hydrogen bond framework controlled by alkyl building blocks: a simple approach to fine-tune nanoscale structures

To date, molecular assemblies under the contribution of hydrogen bond in combination with weak interactions and their consequent morphologies have been variously reported; however, how the systematic variation of the structure can fine-tune the morphologies has not yet been answered. The present work finds an answer through highly symmetric molecules, i.e. diamine-based benzoxazine dimers. This type of molecule develops unique molecular assemblies with their networks formed by hydrogen bonds at the terminal, while, at the same time, their hydrogen bonded frameworks are further controlled by the hydrophobic segment at the center of the molecule. When this happens, slight differences in hydrophobic alkyl chain lengths (, , and ) bring a significant change to the molecular assemblies, thus resulting in tunable morphologies, i.e. spheres, needles and dendrites. The superimposition between the crystal lattice obtained from X-ray single crystal analysis and the electron diffraction pattern obtained from transmission electron microscopy allows us to identify the molecular alignment from single molecules to self-assembly until the morphologies developed. The present work, for the first time, shows the case of symmetric molecules, where the hydrophobic building block controls the hydrogen bond patterns, leading to the variation of molecular assemblies with tunable morphologies.

Nanoobjects formed by ionic PAMAM dendrimers: hydrophilic/lipophilic modulation and encapsulation properties

In this paper, we present the preparation and properties of some ionic PAMAM derivatives, which combine hydrophilic and lipophilic carboxylic acid chains as counter-ions for all protonable inner and outer amino groups. The amphiphilic nature of the final ionic codendrimers and, hence, their self-assembling features can be modulated by using different ratios between hydrophilic and lipophilic chains. In the bulk, these new materials self-organize into smectic A liquid crystal phases. In water, they self-assemble into different types of nano-objects depending on the molecular composition. The study of the morphology of these nano-structures, their cytotoxicity and their capability to encapsulate a lipophilic anticancer drug are reported herein. Some of these nanoobjects are non-cytotoxic and present good drug trapping ability, which make them interesting nanocarriers for applications in nanotechnology and biomedicine.

Efficient Syntheses of Star-Branched, Multifunctional Mesogens

Star-branched molecular architectures lend themselves to convergent synthesis strategies for creating materials that combine three or more functional modalities, but these approaches require a core moiety with several reactive groups that are orthogonal to one another. The direct synthesis of three-arm, star-branched mesogens has been investigated via the Passerini three-component reaction to demonstrate how multicomponent reactions circumvent the need to identify and synthesize specialized branched core molecules.

1D, 2D and 3D liquid crystalline phases formed by bent-core mesogens

The type of the mesophase is altered by a small change in the molecular architecture, i.e. increasing the number of alkyl chains attached to the mesogenic core at terminal positions. Lamellar (1D) and columnar (2D) phases are formed for molecules with one and three terminal chains, respectively. For those with two chains 3D phases are observed with either cubic or triclinic symmetry structure.

Hierarchical superstructures from a star-shaped molecule consisting of a cyclic oligosiloxane with cyanobiphenyl moieties

Unconventional star-shaped liquid crystals (abbreviated as SiLCs) were successfully synthesized by chemically connecting four cyanobiphenyl anisotropic mesogens to the periphery of a super-hydrophobic and ultra-flexible cyclic tetramethyltetrasiloxane ring with flexible hexyl chains. Based on the combined experimental techniques of differential scanning calorimetry (DSC), cross-polarized optical microscopy (POM), solid-state carbon-13 ((13)C) nuclear magnetic resonance (NMR) spectroscopy and one-dimensional (1D) wide-angle X-ray diffraction (WAXD), it was found that the SiLC molecule exhibited the monotropic phase transition from a LC phase to a crystalline phase. The crystalline phase was only detected during slow heating processes above its glass transition temperature, while a LC phase was formed both during cooling and during heating processes. The hierarchical superstructures were identified from the structure-sensitive 2D WAXD of the macroscopically oriented SiLC film and confirmed by selected area electron diffraction (SAED) of the SiLC single crystals. The molecular packing symmetry in the monoclinic unit cell was further investigated by computer simulations on the real and reciprocal spaces. Macroscopically oriented SiLC hierarchical superstructures on the different length scales may provide the targeted physical properties, which can allow us to apply SiLC molecules in the fields of electro-optical devices and nonlinear optics.

2D assemblies of ionic liquid crystals based on imidazolium moieties: formation of ion-conductive layers

Organization of imidazolium ionic liquids into 2D ion-conductive pathways enhances the ionic conductivity of the imidazolium-based ionic liquid crystals.