Room‐Temperature Columnar Mesophases in Triazine–Gold Thiolate Metal–Organic Supramolecular Aggregates

Supramolecular mono‐ and dinuclear liquid‐crystalline gold(I) aggregates have been synthesized by means of hydrogen bond interactions of 2,4,6‐triarylamino‐1,3,5‐triazine with thiolate moities of gold metalloacids [Au(PR₃)(SC₆H₄COOH)] or [μ‐(binap){Au(SC₆H₄COOH)}₂], in 1:1 and 2:1 molar ratio, respectively. All of the supramolecular aggregates display a stable columnar hexagonal mesophase (Col_h) at room‐temperature. The supramolecular arrangement within the columns consists of the one‐dimensional stacking of triazine units, with the core of the attached metallic thioacid fragments acting as the fourth branch. The phosphine‐containing moieties of the metallic thioacid protrude out in the aliphatic continuum. These complexes do not show metallophilic interactions, but this strategy appears very promising for the future design of room‐temperature LC mesophases containing interacting metallic fragments.     

Liquid‐Crystalline Properties of trans‐A2B2‐Porphyrins with Extended π‐Electron Systems

Incorporation of four trialkoxyphenyl substituents combined with extending the π‐conjugated system has allowed porphyrins to display liquid‐crystalline columnar phases at room temperature. 2D and 3D columnar structures were observed as well as a biaxial smectic phase.     

Asymmetrically Substituted Benzene‐1,3,5‐tricarboxamides: Self‐Assembly and Odd–Even Effects in the Solid State and in Dilute Solution

Molecular organization : Chiral benzene tricarboxamides with methyl substituents at defined positions self‐assemble into supramolecular polymers of preferred helicity by three‐fold α‐helical‐type hydrogen bonding. The odd–even effect is operative and all derivatives are liquid crystalline showing a Col_ho phase (see figure).

     

Shape‐Persistent,Sterically Crowded Star Mesogens: From Exceptional Columnar Dimer Stacks to Supermesogens

Hexasubstituted C₃‐symmetric benzenes with three oligophenylenevinylene (OPV) arms and three pyridyl or phenyl substituents are shape‐persistent star mesogens that are sterically crowded in the center. Such molecular structures possess large void spaces between their arms, which have to be filled in condensed phases. For the neat materials, this is accomplished by an exceptional formation of dimers and short‐range helical packing in columnar mesophases. The mesophase is thermodynamically stable for the pyridyl compound. Only this derivative forms filled star‐shaped supermesogens in the presence of various carboxylic acids. The latter do not arrange as dimers, but as monomers along the columnar stacks. In this liquid crystal (LC) phase, the guests are completely enclosed by the hosts. Therefore, the host can be regarded as a new LC endoreceptor, which allows the design of columnar functional structures in the future.     

Advances in Metal‐Free Heterocycle‐Based Columnar Liquid Crystals

Liquid crystals are ordered soft materials formed by self‐organized molecules and can potentially be used as new functional materials for electron‐, ion‐ or molecular‐transport; optical; and bio‐active materials. In particular, the columnar liquid crystals are promising candidates used in various optical and electronic devices. For this purpose, design and synthesis of unconventional materials are essential. In this review, we have summarized several approaches for the synthesis of columnar liquid crystals composed of various heterocyclic systems. We also outline their liquid crystalline and other relevant properties, and their suitability for applications in diverse fields.     

Effect of Atomic‐Scale Differences on the Self‐Assembly of Thiophene‐based Polycatenars in Liquid Crystalline and Organogel States

Two series of polycatenars are reported that contain a central thiophene moiety connected to two substituted oxadiazole or thiadiazole units. The number, position, and length of the peripheral chains connected to these molecules were varied. The oxadiazole‐based polycatenars exhibited columnar phases with rectangular and hexagonal or oblique symmetry, whereas the thiadiazole‐based polycatenars exhibited columnar phases with rectangular and/or hexagonal symmetry. All of the compounds exhibited bright emission in the solution and thin‐film states. Two oxadiazole‐based molecules and one thiadiazole‐based molecule exhibited supergelation ability in hydrocarbon solvents, which is mainly supported by attractive π–π interactions. These gels showed aggregation‐induced enhanced emission, which is of high technological importance for applications in solid‐state emissive displays. X‐ray diffraction studies of the xerogel fibers of oxadiazole‐based polycatenars revealed a columnar rectangular organization, whereas a hexagonal columnar arrangement was observed for thiadiazole‐based polycatenars. Rheological measurements carried out on the samples quantitatively confirmed the formation of gels and showed that these gels are mechanically robust. The impact of an atomic‐scale difference (oxygen to sulfur, <2 % of the molecular weight) on the self‐assembly and the macroscopic properties of those self‐assembled structures are clearly visualized.     

Supermolecular Columnar Liquid‐Crystalline Phosphorus Dendrimers Decorated with Sulfonamide Derivatives

A series of supermolecular liquid crystals has been synthesized by combining phosphorus dendrimers of the zero, first, and fourth generations with sulfonamide derivatives, thus generating dendromesogens bearing 6, 12, and 96 mesogenic units on their surfaces. The relevant reactions could be monitored by ¹H, ¹⁹F, and ³¹P{¹H} NMR spectroscopies. The thermal and mesomorphic properties of the products have been studied by optical microscopy, differential scanning calorimetry, and X‐ray diffraction. All of the new macromolecules prepared in this work have been found to show mesomorphic properties over a wide temperature range; moreover, for all of the compounds, the columnar mesophases observed were maintained or vitrified at room temperature. On increasing the generation of these dendromesogens, mesophases appear at lower temperatures and remain stable over a wider temperature interval. In all cases, on the basis of X‐ray analysis, a cylindrical symmetry of the molecules can be proposed to promote the supramolecular columnar arrangement observed in the mesophases. In this type of model, the height of the dendrimer clearly increases with increasing dendrimer generation, whereas its cross‐ sectional area increases only slightly, probably due to compression of the highly hyperbranched structures as a consequence of their progressive steric constraints. The mesomorphic arrangement is governed by the peripheral sulfonamide units.     

The Solution of the Puzzle of Smectic‐Q: The Phase Structure and the Origin of Spontaneous Chirality

The so‐called smectic‐Q (SmQ) liquid crystal phase was discovered in 1983 in rod‐like molecules, but its structure remain unclear in spite of numerous attempts to solve it. Herein, we report what we believe to be the solution: A unique bicontinuous phase that is non‐cubic and is made up of orthogonal twisted columns with planar 4‐way junctions. While SmQ had only been observed in chiral compounds, we show that this chiral phase forms also in achiral materials through spontaneous symmetry breaking. The results strongly support the idea of a helical substructure of bicontinuous phases and long‐range homochirality being sustained by helicity‐matching at network junctions. The model also explains the triangular shape of double‐gyroid domains growing within a SmQ environment. SmQ‐forming materials hold potential for applications such as circularly polarized light emitters that require no alignment or asymmetric synthesis.     

Solution‐Phase Racemization in the Presence of an Enantiopure Solid Phase

Solution‐phase racemization drives the evolution of single chirality in the solid phase by the “chiral amnesia” process first described by Viedma. The current investigations lay the basis for a better understanding of the mechanism of the solid‐phase deracemization by uncoupling the chemical rate processes associated with the interconversion of enantiomers in the solution phase from the physical processes associated with solution–solid phase transfer via dissolution and reaccretion of molecules onto crystals. In addition, the enantiomer concentration profiles presented in this work, together with an analytical treatment of the racemization process in the presence of excess enantiopure solid, unequivocally reconfirm the validity of the Meyerhoffer double solubility rule for systems under solution racemization conditions.     

Cubic and Columnar Supramolecular Architectures of Rod–Coil Molecules in the Melt State

The liquid crystalline behavior of compounds 1 (n = 7, 12, 15) differs significantly from that exhibited for conventional rodlike molecules. They organize into layered smectic, bicontinuous cubic or hexagonal columnar mesophases depending on the temperature or the volume fraction of coil segments.     

Cocrystallization‐Induced Spontaneous Deracemization: A General Thermodynamic Approach to Deracemization

Processes leading to enantiomerically pure compounds are of utmost importance, in particular for the pharmaceutical industry. Starting from a racemic mixture, crystallization‐induced diastereomeric transformation allows in theory for 100 % transformation of the desired enantiomer. However, this method has the inherent limiting requirement for the organic compound to form a salt. Herein, this limitation is lifted by introducing cocrystallization in the context of thermodynamic deracemization, with the process applied to a model chiral fungicide. We report a new general single thermodynamic deracemization process based on cocrystallization for the deracemization of (R,S)‐4,4‐dimethyl‐1‐(4‐fluorophenyl)‐2‐(1H‐1,2,4‐triazol‐1‐yl)pentan‐3‐one. This study demonstrates the feasibility of this novel approach and paves the way to further development of such processes.     

Self‐Assembly in Helical Columnar Mesophases and Luminescence of Chiral 1H‐Pyrazoles

Chiral polycatenar 1H‐pyrazoles self‐assemble to form columnar mesophases that are stable at room temperature. X‐ray diffraction and CD studies in the mesophase indicate a supramolecular helical organization consisting of stacked H‐bonded dimers. The liquid‐crystalline compounds reported are 3,5‐bis(dialkoxyphenyl)‐1H‐pyrazoles that incorporate two or four dihydrocitronellyl chiral tails. It can be observed that the grafting of these branched chiral substituents onto the 3,5‐diphenyl‐1H‐pyrazole core has a beneficial role in inducing mesomorphism, because isomeric linear‐chain compounds are not liquid crystalline; this is not the usual scheme of behavior. Furthermore, the molecular chirality is transferred to the columnar mesophase, because preferential helical arrangements are observed. Films of the compounds are luminescent at room temperature and constitute an example of the self‐organization of nondiscoid units into columnar liquid‐crystalline assemblies in which the functional molecular unit transfers its properties to a hierarchically built superstructure.     

Conformation‐Specific Circular Dichroism Spectroscopy of Cold,Isolated Chiral Molecules

The CD spectroscopy of a chiral compound in solution yields an average CD value derived from all of the conformations of a chiral molecule. By contrast, CD spectroscopy of cold chiral molecules in the gas phase distinguishes specific conformers of a chiral molecule, but the weak CD effect has limited the practical application of this technique. Reported herein is the first resonant two‐photon ionization CD spectra of ephedrines in a supersonic jet using circularly polarized laser pulses, which were generated by synchronizing the oscillation of the photoelastic modulator with the laser firing. The spectra exhibited well‐resolved CD bands which were specific for the conformations and vibrational modes of each enantiomer. The CD signs and magnitudes of the jet‐cooled chiral molecules were very sensitive to their conformations and thus offered crucial information for determining the three‐dimensional structures of chiral species, as conducted in combination with quantum chemical calculations.