Crystalline Self-assembly Research Articles

Impact of the terminal fluorination of bisthienylethene-based liquid crystals on the self-assembly and mechanochromic behaviors

Four novel bisthienylethene-based rod-like liquid crystals containing different numbers of terminal fluorine atoms were designed and the effect of the terminal fluorination on liquid crystalline self-assembly, optical characteristics in solid state and mechanochromic behaviors was explored. The transition from non-liquid crystal via monotropic liquid crystal with smectic A phase to enantiotropic liquid crystal with smectic A phase can be achieved and the temperature ranges of smectic A phase gradually increase by increasing the terminal fluorination, which might be due to the increase in intermolecular interactions by increasing the number of terminal fluorine atom. All the compounds exhibited AIE characteristic and distinct solid-state fluorescence with yellow or orange color. In addition, the non-fluorosubstituted and difluorosubstituted bisthienylethene-based liquid crystals exhibited reversible mechanochromism, whereas the monofluorosubstituted and trifluorosubstituted bisthienylethene-based liquid crystals exhibited extremely weak mechanochromism due to different intermolecular interaction, molecular symmetry and molecular conformation. These investigations implied that the incorporation of fluorine atom could result in interesting self-assemblies and photophysical properties in different states, which also provided the understanding of the relationship between chemical structures and properties.

Construction of fungicide nano delivery platform through crystal engineering for sustainable fungal control

The adverse effects resulting from off-target pesticide application pose significant risks to both ecosystems and public health. Therefore, developing a pesticide delivery system that is straightforward, effective, and devoid of secondary pollution is crucial for environmental safety. Herein, self-assembled crystalline materials involved pyrimethanil (PYR) and organic acids were constructed by crystal engineering. These materials allowed for direct manipulation of the physicochemical properties and environmental fate of PYR by the building modules. The melting point of PYR was increased from 96.19 °C to 206.69 °C. The photostability and volatile stability were improved by 3.85-fold and 297-fold, respectively. Afterwards, these crystalline materials were prepared as nano cocrystal suspensions (NCSs) in water by the one-pot method. The PYR NCSs exhibited potent bioactivity against Botrytis cinerea and demonstrated slow-release behavior. Upon foliar spraying, the PYR NCSs exhibited desirable foliar wettability with a contact angle lower than 36°. By limiting leaching into the soil and reducing environmental pesticide exposure, PYR NCS ensured the safety of both earthworms and plants. The structure–property relationships of PYR NCSs were elucidated through intermolecular interaction analysis and theoretical calculations. Overall, the NCSs constructed by the supramolecular chemistry strategy are characterized by the simple preparation, high bioactivity, stability, no carrier, slow-release, and minimal environmental risk, which offers insight for improving the utilization and sustainable management of pesticides.

Mesogenic Ordering-Driven Self-Assembly of Liquid Crystalline Block Copolymers in Solution.

With the development of nanotechnology, the preparation of polymeric nanoparticles with nicely defined structures has been well-developed, and the functionalization and subsequent applications of the resultant nanostructures are becoming increasingly important. Particularly, by introducing mesogenic ordering as the driving force for the solution-state self-assembly of liquid crystalline (LC) block copolymers (BCPs), micellar nanostructures with different morphologies, especially anisotropic morphologies, can be easily prepared. This review summarizes the recent progress in the solution-state self-assembly of LC BCPs and is mostly focused on four main related aspects, including an in-depth understanding of the mesogenic ordering-driven self-assembly, precise assembly methods, utilization of these methods to fabricate hierarchical structures, and the potential applications of these well-defined nanostructures. We hope not only to make a systematic summary of previous studies but also to provide some useful thinking for the future development of this field.

Ligand Chirality Transfer from Solution State to the Crystalline Self-Assemblies in Circularly Polarized Luminescence (CPL) Active Lanthanide Systems.

The synthesis of a family of chiral and enantiomerically pure pyridyl-diamide (pda) ligands that upon complexation with europium [Eu(CF3SO3)3] result in chiral complexes with metal centered luminescence is reported; the sets of enantiomers giving rise to both circular dichroism (CD) and circularly polarized luminescence (CPL) signatures. The solid-state structures of these chiral metallosupramolecular systems are determined using X-ray diffraction showing that the ligand chirality is transferred from solution to the solid state. This optically favorable helical packing arrangement is confirmed by recording the CPL spectra from the crystalline assembly by using steady state and enantioselective differential chiral contrast (EDCC) CPL Laser Scanning Confocal Microscopy (CPL-LSCM) where the two enantiomers can be clearly distinguished.

Impact of the position of nitrogen atom on the liquid crystal, optical and multi-stimuli response behaviors of rod-like mesogens containing multiple functional building blocks

Two novel rod-like mesogens containing one cyanostyrene, one pyridine and one pyrene as π-conjugate rigid core were designed and synthesized by Suzuki coupling and Knoevenagel reactions. The effect of the position of nitrogen-atom on the thermodynamic behavior, liquid crystalline self-assembly, optical behavior, mechanochromism behavior and acidochromism behavior was investigated. The rod-like mesogen in which the nitrogen atom is adjacent to the pyrene displays a phase transition from smectic A to smectic C by increasing temperature, whereas the other rod-like mesogen can only self-assemble into smectic C phase. This difference in mesophase transition behavior might be attributed to the position of nitrogen-atom which induces the change in molecular configuration. Both mesogens exhibit positive solvatochromism and high fluorescence in both solution state and aggregated state which might be attributed to the intramolecular charge transfer and twisted molecular configurations, respectively. Reversible mechanochromism behavior due to changes in molecular stacking and acidochromism behavior due to protonation and deprotonation were also achieved. These multi-stimuli responsive behaviors of the designed rod-like mesogens demonstrate potential applications in the field of multifunctional materials.

Synthesis, liquid crystalline self-assembly, organogel behavior, and DFT investigation of first phenolphthalein-based hexacatenar

ABSTRACT Different building blocks in organic molecules could induce complex intermolecular interactions to lead to different supramolecular micro/nanostructures in bulk and solution states. Herein, the first new phenolphthalein-based hexacatenar containing a central phenolphthalein fluorochrome functionalized with two dendritic wings is synthesized by click reaction. The polarized optical microscopy, differential scanning calorimetry, and small-angle X-ray diffraction data demonstrated that the reported phenolphthalein-based hexacatenar could self-assemble into hexagonal columnar mesophase in the bulk state and could form organogel with distinct morphologies in some common organic solvents due to intermolecular interactions. Density functional theory calculation results showed highly twisted molecular conformation, distinct charge distribution, and asymmetrical electronic property, which might be essential for the stabilization of columnar mesophase and organogel. Therefore, this work provides a method to develop soft materials containing different functional building blocks with complex self-assemblies.

Topological solitons in chiral liquid crystals

ABSTRACT Topological solitons are quasi-particles of field deformations that are topologically nontrivial. Conceptualised by celebrated mathematicians and scientists including Carl Friedrich Gauss, Lord Kelvin, and Tony Skyrme, the ideas of knots, topology, and solitons have found a wealth of experimental embodiments in chiral liquid crystals in recent years. This is largely due to the exceptional capacity of the material for stabilising, characterising, and controlling topological solitons. In this review, I report the recent progress in realising and characterising topological solitons in chiral liquid crystals. Following the homotopy classification, I begin with the 1D twist walls and subsequently introduce 2D skyrmions and skyrmion bags, followed by 3D Hopf solitons including both hopfions and heliknotons. Physical properties of topological solitons are discussed, which include stability, soliton–soliton interaction, field-controlled transformation, field-activated dynamics, and crystalline self-assembly. Finally, the review ends by illustrating how liquid-crystal solitons can provide insights into topological solitons in other physical systems, and perspectives of future studies on the subject. The purpose of this review is to provide a fundamental overview of topological solitons in chiral liquid crystals.

Achieving High Permittivity Paraelectric Behavior in Mesogen-Free Sulfonylated Chiral Polyethers with Smectic C Liquid Crystalline Self-Assembly

Ferroelectric liquid crystalline polymers (LCPs) with a high spontaneous polarization (Ps) are of great interest for the fabrication of advanced electronic devices. However, conventional ferroelectric LCPs typically exhibit low spontaneous polarization, only around 1 mC/m2. To increase the orientational polarization for ferroelectric LCPs, in this work, we designed mesogen-free comb-shaped LCPs based on isotactic polyepichlorohydrin, which contained highly dipolar sulfonyl groups (dipole moment of 4.5 Debye) in the side chains. The goal was to increase dipole density and induce ferroelectric switching. In this new series of mesogen-free LCPs, the sulfonyl groups were first moved away from the main chain to mitigate strong dipole–dipole interactions. Second, methyl-branched alkyl side chains were implemented to decrease the crystallizability and induce the Smectic C (SmC) self-assembly. Intriguingly, these SmC samples, for the first time, exhibited typical paraelectric behavior (i.e., slim S-shaped single hysteresis loops) with a high dielectric constant in the range of 20–30. Learning from the knowledge obtained from this study, we will be able to design new mesogen-free LCPs with tailor-made dipole–dipole interactions and realize ferroelectricity.

New opportunities for cyclodextrins in supramolecular assembly: metal organic frameworks, crystalline self-assembly, and catalyzed assembly.

Cyclodextrins (CDs) are widely used macrocycles in supramolecular assembly due to their easy availability, versatile functionality and excellent biocompatibility. Although they are well-known for forming host-guest complexes with a wide range of guests and this host-guest chemistry has long been utilized in industry and academia, new opportunities have arisen in recent years, particularly in supramolecular assembly. In the present review, we will first provide a basic introduction to CDs and then summarize their emerging roles in the fields of supramolecular chemistry and materials. This includes their involvement in hybrid frameworks with inorganic components such as metal ions and polyoxometalates, crystalline self-assembly with amphiphilic molecules, and their new possibility of "catassembly" and induced chiral supramolecular structures that have previously been overlooked. Finally, we will comment on the future perspectives of CDs to inspire more ideas and efforts, with the aim of promoting diverse applications of CDs in supramolecular materials.

Tailoring liquid crystalline self-assembly and de Vries behavior of azulenes via lateral and core substitution.

The azulene moiety is a highly attractive building block in optoelectronic applications due to its unique properties. For high-performing devices, the molecular orientation is crucial and can be controlled through liquid-crystalline self-assembly. Recent work showed that liquid crystalline derivatives bearing the 2-phenyl-azulene-1-nitrile core formed broad de Vries-type SmA and SmC phases. For exact understanding of the structure-property relationship, a series of 2-(hetero)aryl-azulenes has been synthesized varying the chain linkage, the lateral substituent, and the aromatic ring. Small changes of the molecular structure determined whether the orthogonal SmA phase or the tilted SmC phase is predominant. Implementation of alkyne chains instead of alkoxy chains resulted in the reduction of phase transition temperatures and formation of mesophases at room temperature. Furthermore, de Vries-like behavior was investigated and reduction values between R = 0.35 and 0.74 were measured which supported the hypothesis that in this system de Vries-like behavior is caused by steric repulsion of the lateral substituent. The control of the phase geometry by the molecular structure might be used for improved molecular orientation in optoelectronic materials.

First isoflavone-based hexacatenar derivatives with columnar liquid crystalline self-assembly and binding selectivity behavior

ABSTRACT The incorporation of different functional groups into organic molecules is a common method to construct soft liquid crystalline materials with multiple properties and adjust mesophases from one-dimensional to three-dimensional structures. This article reports the synthesis, characterization, self-assembly, and binding selectivity behaviors of the first isoflavone-based hexacatenar liquid crystalline materials with an isoflavone as central rigid core and two 1,2,3-triazole dendritic wings as terminal functional units. Polarized optical microscopy, differential scanning calorimetry, and small-angle X-ray scattering (SAXS) investigations indicated that all the compounds could self-assemble into rectangular columnar mesophase. The differences in temperature range of mesophase and sequence of mesophase compared with previously reported coumarin-based polycatenars were attributed to the change of position of carbonyl. The optical spectra indicated that these compounds display selectivity for Fe2+ among the different and cations. FT-IR and density functional theory calculations before and after interaction with Fe2+ were employed to demonstrate the recognition sites. The design strategy here provides a simple method to construct supramolecular self-assembly materials with wide properties and potentials.

Tailoring boron liquid crystals: Mesomorphic properties of iminodiacetic acid boronates

The highly polar N-methyl iminodiacetic acid (MIDA) unit induces liquid crystalline self-assembly when the MIDA boronate is attached to conventional mono-, bis- or trisalkoxyphenyl mesogens. In order to understand the influence of the rigid mesogenic unit and the N-substituent on the mesomorphic properties six series of IDA boronates carrying N-ethyl in combination with classic mesogenic aryl units (Ph(n)kEIDA, n = 10 – 16, k = 1 – 3), or N-methyl in combination with bend meta-alkoxyphenyl (Ph(n)metaMIDA), linear alkoxy-pyridyl (Pyr(n)1MIDA) and -pyrimidinyl (Pym(n)1MIDA) respectively, were prepared. Analysis by differential scanning calorimetry (DSC), polarizing optical microscopy (POM) and X-ray diffraction (WAXS, SAXS) revealed that N-ethyl and bend-shaped aryl units mostly lowered the clearing transition temperatures, decreasing the tendency for thermal decomposition and affected the preference of SmA phases, while heterocyclic units were beneficial for stable SmA phases.

Liquid crystalline self-assembly of azulene-thiophene hybrids and their applications as OFET materials.

Orientational control within thin films is crucial for the preparation of organic field effect transistors (OFETs). The highly ordered liquid crystalline smectic E phase (SmE) is known as a powerful template for solution processed thin films. Here, we describe the synthesis and characterization of three novel azulene-thiophene hybrid materials. Liquid crystalline characterization showed the presence of wide SmE phases. Thin films were prepared by spin-coating at mesophase temperature. Due to the self-aligning properties of the SmE phase uniformly flat films with good molecular alignment were manufactured. Top contact bottom gate OFETs showed mobilities up to (3.3 ± 0.5) × 10-3 cm2 V-1 s-1.

Merging liquid crystalline self-assembly and linear optical properties of merocyanines via tailored donor units.

Aiming at merocyanine dyes with good linear optical and self-assembly properties, a series of rigid mono-, bi- and tricyclic merocyanines with O- and N-donor units as well as keto or malodinitrile acceptor units was prepared by a convergent approach. With particular focus on tailoring the donor unit, a selection of appropriate derivatives was investigated with respect to their dye properties in solution and in the bulk (UV/Vis, fluorescence, temperature-dependent fluorescence, lifetime). Determination of fluorescence quantum yields revealed the importance of the donor unit and the chromophore size. Larger chromophores and N-donors were beneficial for strong emission in solution, whereas small chromophores and O-donors favored emission in the solid state. To rationalize the different optical properties depending on their donor unit, density functional theory (DFT) calculations were performed. Liquid crystalline derivatives were additionally studied by optical polarization microscopy, differential scanning calorimetry, and X-ray diffraction experiments. For merocyanines with O-donor, fluorinated side chains were mandatory to get stable enantiotropic SmA phases regardless of chromophore size, side chain lengths or acceptor unit. Increased mesophase widths (up to 134 K) were observed upon increasing the chromophore lengths, chain lengths (up to C12) and F/C ratio in the side chain. On the other hand, merocyanines with N-donor and keto acceptor showed enantiotropic SmA phases in the presence of simple alkoxy side chains. The tricyclic merocyanine with N-donor shows an additional SmE phase at lower temperatures. The results revealed the importance of the donor unit to balance optical and mesomorphic properties in merocynanines.

Circularly polarized luminescence based on cholesterol-tetraphenylethylene-perylene liquid crystal

Perylene derivative with circularly polarized luminescence (CPL) at aggregated state was seldom reported due to the strong ACQ (aggregation-caused quench) effect at aggregation. In this work, a novel cholesterol-tetraphenylethylene-perylene derivative (TPE-P) was designed and synthesized in moderate yield. It exhibited liquid crystalline behavior with orderly hexagonal columnar mesophase and good fluorescence emission at long wavelength (600-700 nm) not only in solution but also at aggregated states based on the AIE (aggregation-induced emission)-FRET (fluorescence resonance energy transfer) effect between tetraphenylethylene unit and perylene moiety. Moreover, the circular dichroism (CD) and CPL studies suggested the effective chiral transfer from cholesterol unit to tetraphenylethylene unit and perylene skeleton due to the spiral liquid crystalline self-assembly. The CD and CPL signals showed the order of THF < THF-hexane < solid film < meosphase, indicating that the higher spiral orderly degree resulted in the stronger chiral transfer. The largest |glum| value for mesophase excited at 320 nm was as high as 1.5×10−2 based on the combining effect of AIE-FRET and chiral transfer. This research not only reported a novel CPL perylene derivative at aggregated state, but also confirmed that the combination of AIE-FRET effect and chiral transfer of liquid crytalline phase was an effective method to construct normal dye with excellent CPL property in aggregated state.

Ordered hexagonal columnar liquid crystalline self-assembly of mesogen-free sulfonylated side-chain chiral polyethers and their high dielectric property

To achieve large spontaneous polarization for ferroelectric liquid crystalline polymers, we have designed a series of mesogen-free isotactic polyethers with sulfonylated side chains. Different from prior studies using linear alkyl side chains, in this work a methyl branch was implemented in the alkyl side chains of the chiral polyethers. As a result of the continuous twist from the methyl branches and strong dipole-dipole interactions among neighboring sulfonyl groups, a helical chain conformation with tilted side chains was achieved. When the branched side chains were short (e.g., C5) and double chirality existed in both the main chain and side chains, a crystalline structure was obtained. When the branched side chains were long (e.g., ≥ C8) and single chirality existed in the main chain, an ordered hexagonal columnar liquid crystalline phase was induced. Their high-field dielectric property was studied by electric displacement-electric field (D-E) loop tests; however, no ferroelectric behavior was observed up to 200 MV/m. It was considered that the dipole-dipole interaction among neighboring sulfonyl groups in the helical chains was too strong to allow easy ferroelectric flipping.

The Observation of Interchain Motion in Self-Assembled Crystalline Platinum(II) Complexes: An Exquisite Case but By No Means the Only One in Molecular Solids.

In organic and organometallic solids, upon electronic excitation, most intermolecular structural relaxations follow a pathway along the π-π stacking direction or metal-metal bond with significant coupling strength. Differently, we discovered that the self-assembled platinum(II) complexes, Pt(fppz)2, exhibit an unusual interchain contraction. The ground-state and excited-state multiple local minima were distinguished by temperature-dependent excitation/emission spectra, indicating the existence of multiple local minima. The time-resolved emission decay revealed the excited-state structural relaxation lifetime with τobs = 41 ns at 298 K. Temperature-dependent X-ray diffraction analysis showed that the packing geometries contract 0.6 Å along the interchain direction (a-axis) at 50 K compared to the geometries at 298 K. Such structural displacements render the slow internal conversion rate in the excited states. We thus demonstrate the correlation between the packing geometries and the excited-state dynamics of the self-assembled Pt(II) complexes, shedding light on the unique direction of interchain structural deformation of the molecular aggregates.

Phase behaviour of alkynyl-terminated bicyclo[3.3.0]octa-1,4-diene ligands: a serendipitous discovery of novel calamitic liquid crystals

ABSTRACT Chiral bicyclo[3.3.0]octadienes carrying two alkoxy- or alkynyloxy side chains are highly valuable ligands for asymmetric catalysis but have never been explored regarding their liquid crystalline self-assembly. Therefore, a series of bicyclo[3.3.0]octadienes was synthesised and studied by differential scanning calorimetry (DSC), polarising optical microscopy (POM) and X-ray diffraction (XRD) to understand the role of the terminal alkyne unit in the side chain. Dienes with hexinyloxy or octinyloxy side chains behaved similar to the diene with hexyloxy chains, showing broad SmA phases and a pronounced tendency for supercooling. In contrast, dienes with mono- or diethylenglycolpropargylic ether chains displayed a decreased phase range and monotropic behaviour in the latter case, while the corresponding diene with propargylether side chains was non-mesomorphic.

Porphyrin with circularly polarized luminescence in aggregated states

Porphyrin with circularly polarized luminescence (CPL) in aggregated state was seldom presented due to the strong aggregation-caused quenching (ACQ) effect at aggregation. This work designed and synthesized a novel porphyrin derivative (CPL-P) with good CPL behaviour in aggregated states based on the strong AIE-FRET effect and good chirality transfer of liquid crystalline porphyrin, which was constructed by using porphyrin unit as core with four AIE cyanostilbene groups bearing peripheral chiral alkyl chains. CPL-P exhibited orderly hexagonal columnar mesophase between 62.5 °C and 113.2 °C. It displayed excellent fluorescence emission at 640–760 nm in aggregated states due to the AIE-FRET effect between cyanostilbene unit and porphyrin moiety. Moreover, CPL-P showed the strong circular dichroism and CPL properties in aggregated states based on the effective chiral transfer of the spiral liquid crystalline self-assembly. The |glum| value for mesophase attained 6.6 × 10−3 based on the combining effect of AIE-FRET and chiral transfer. This research supplied a new strategy for transferring typical achiral porphyrin with ACQ effect to luminescent molecule with good CPL behaviour in aggregated state.

Visual recognition of ortho-xylene based on its host-guest crystalline self-assembly with α-cyclodextrin

HypothesisDistinguishing substituted aromatic isomers is a challenging task because of the great similarity of their physicochemical properties. Considering xylene isomers have drastically different geometrical shapes, we predict this would show great impact on the self-assembling behavior of various xylene isomer@cyclodextrin inclusion complex. ExperimentsThrough host–guest crystalline self-assembly, among three isomers, only ortho-xylene is capable to form hydrogels with α-cyclodextrin. ROESY NMR, molecular simulations and circular dichroism spectra suggest that the ortho selectivity comes from the difference in the conformation of host–guest building block. The larger volume, and steric hinderance of the ortho isomer make it most possibly decrease their tendency to adopt more mobile orientations in cyclodextrin-based complex as meta and para isomers do, resulting in gel formation. FindingsHerein, we report a novel, facile and environmentally-friendly protocol on the recognition of ortho benzene isomers using α-cyclodextrin through host–guest crystalline self-assembly. Visual recognition of ortho-xylene is achieved through amplifying the structural difference of xylene isomers at molecular scale into macroscopic scale. We believe this work unveils subtle rules to control macroscopic assemblies at the molecular level and highlights the potential of using macrocyclic compounds to improve the quality and reduce the energy bill for separation in petrochemical industry.