Search

Star‐shaped mesogens with a phloroglucinol or a trimesic acid core and oligobenzoate arms with up to five repeating units have been synthesised. These non‐conventional mesogens form various columnar mesophases over a broad temperature range. The liquid‐crystal phases were characterised by optical microscopy, differential scanning calorimetry, X‐ray diffraction, dilatometry and solid‐state NMR spectroscopy. In addition to the high‐temperature hexagonal columnar phases, the columnar self‐assemblies undulate upon cooling and consequently form higher‐ordered body‐centred orthorhombic columnar 3D structures. A model of E‐shaped folded conformers helically displaced along the columns is proposed. Helical preorganisation in the hexagonal phase precedes the transition to the low‐temperature phases. Space filling and nano‐segregation compete in the self‐organisation process, thus aliphatic chains and the polar oligobenzoate scaffold are not perfectly separated in these star‐shaped mesogens.

First‐ and second‐generation dendrimers with an isocyanide group as the focal functional point (CN‐Gn; n: 1,2) and their corresponding organometallic complexes [MCl(CN‐Gn)] (M: Au, Cu), [{CuCl(CN‐Gn)2}2], and trans‐[MI2(CN‐Gn)2] (M: Pd, Pt) have been synthesized. The free ligands and the first‐generation complexes do not show mesogenic behavior, but all of the second‐generation complexes display a thermotropic micellar cubic mesophase, over a large temperature range, and some of them directly at room temperature. The structure of the mesophase consists of the packing of two, discrete polyhedral micellar aggregates in a three‐dimensional cubic Im$\bar 3$mlattice.

The connection of twelve peripheral and divergent dodecyloxy chains to a central tridentate aromatic binding unit provides the dodecacatenar ligand L11, for which room‐temperature mesomorphism is detected. An enthalpically unbalanced large melting entropy (Δ${S{{{\rm L11}\hfill \atop {\rm m}\hfill}}}$=226 J mol−1K−1) results from the programmed microsegregation induced in the crystalline phase, a phenomenon which is maintained in the associated lanthanide complexes [Ln(L11)(NO3)3] and [Ln(L11)(CF3CO2)3]2. Low‐temperature melting processes (−43≤Tm≤−25 °C) producing room‐temperature hexagonal columnar liquid‐crystalline phases thus result for these novel lanthanidomesogens. A combined photophysical (high‐resolution emission spectroscopy), thermodynamic (differential scanning calorimetry, DSC) and structural (small‐angle X‐ray diffraction, SA‐XRD) investigation of the melting process shows minor structural changes occurring between the crystal (Cr) and the hexagonal columnar mesophase (Colh) in the complexes, which allows the assignment of the existence of these unusual first‐order phase transitions to the negligible mixing entropy produced by the two viscous phases at low temperature. Extension of the concept of chemical tuning of melting entropies for the global design of room‐temperature metallomesogens and liquid crystals is discussed.

A number of substituted zinc(II) porphycenes and porphyrins have been synthesized as potentially mesogenic materials. One of the resulting porphycenes, bearing eight decyloxy chains, exhibits two mesophases, a transient lamellar phase (Lam) and a highly ordered lamello‐columnar phase (LCol), with remarkably different structural characteristics. The same zinc(II) porphycene also forms an electron donor–acceptor (EDA) complex with tetracyanoquinodimethane (TCNQ), generating a hexagonal columnar mesophase (Colh) that is thermally stable up to ca. 200 °C. The EDA interaction between porphycene and TCNQ has been probed using electronic and vibrational spectroscopy. A mixture of zinc(II) porphyrins, isomeric with the above porphycene complex, forms a rectangular columnar mesophase (Colr).

The structure of hexacatenar bent‐shape molecules has been systematically modified in order to determine the main molecular factors responsible for the appearance of the axially polar columnar mesophase. It was found that the stability of the polar phase is very sensitive to the subtle modifications of the molecular shape: the phase is solely preserved if the modification is made at the terminal parts of the mesogenic core, whilst any other modifications destabilize the phase. It can be concluded that the main factor driving the transition between the phase made of flat supramolecular discs and the axially polar phase made of the cone‐like units is the ability to fulfill close packing conditions in order to eliminate voids between neighboring molecular rigid cores.

The opposite orientation of the ester spacers in the rodlike ligands L 4C12(benzimidazole‐OOC‐phenyl) and L 5C12(benzimidazole‐COO‐phenyl) drastically changes the electronic structure of the aromatic systems, without affecting their meridional tricoordination to trivalent lanthanides, LnIII, and their thermotropic liquid crystalline (i.e., mesomorphic) behaviors. However, the rich mesomorphism exhibited by the complexes [Ln(L 4C12)(NO3)3] (Ln=La–Lu) vanishes in [Ln(L 5C12)(NO3)3], despite superimposable molecular structures and comparable photophysical properties. Density functional theory (DFT) and time‐dependant DFT calculations performed in the gas phase show that the inversion of the ester spacers has considerable effects on the electronic structure and polarization of the aromatic groups along the strands, which control residual intermolecular interactions responsible for the formation of thermotropic liquid‐crystalline phases. As a rule of thumb, an alternation of electron‐poor and electron‐rich aromatic rings favors intermolecular interactions between the rigid cores and consequently mesomorphism, a situation encountered for L 4C12, L 5C12, [Ln(L 4C12)(NO3)3], but not for [Ln(L 5C12)(NO3)3]. The intercalation of an additional electron‐rich diphenol ring on going from [Ln(L 5C12)(NO3)3] to [Ln(L 6C12)(NO3)3] restores mesomorphism despite an unfavorable orientation of the ester spacers, in agreement with our simple predictive model.

We report on the design and synthesis of three series of segmented amphiphilic block codendrimers, and on their self‐organizing behavior in liquid‐crystalline mesophases. Connecting two prefunctionalized monodendrons, each differing in their chemical constitution and generation number, yielded these diblock supermolecules. One wedge of the codendrimer was made hydrophobic, and is based on a branched poly(benzyl ether) monodendron functionalized at the periphery by lipophilic aliphatic fragments (also known as Percec dendrons). The other segment was made hydrophilic by the grafting of hydroxyl‐containing moieties onto the focal functions of the former dendrons. Both types of dendrons were prepared independently by convergent methods and then joined in the ultimate stage of the synthetic procedure by cross‐coupling reactions. In this way, the proportion of the dendritic blocks was varied independently to allow control of the hydrophilic/hydrophobic balance (HHB), the hydrogen‐bonding ability, and consequently the capacity to tune the mesomorphic properties of the resulting “superamphiphiles” was anticipated. Essentially all the dendritic compounds display a thermotropic mesomorphism directly at or near room temperature as determined by using X‐ray diffraction, polarized optical microscopy, and differential scanning calorimetry. The nature and the supramolecular organization of the mesophases, namely columnar and cubic phases, are correlated to the size of the respective block monodendrons and the chemical structures of the dendromesogens. The molecular organization within the cubic phases can be geometrically described and well understood by the space‐filling polyhedron model.

A series of bischelate ionic silver complexes [Ag(L*)2][X] was prepared by complexation of a newly synthesized 2,2′‐bipyridine containing chiral alkoxy chains in the 4,4′ positions. The appropriate choice of the construction motifs allows the preparation of new materials in which several functionalities can be introduced. Indeed, when the anion X−is a triflate or a dodecylsulfate group, the right combination of intermolecular interactions promotes the production of liquid crystalline mesophases. Therefore, the presence of coordinating anions, which drives the supramolecular assembly, is essential to generate, at the same time, room‐temperature columnar hexagonal mesomorphism, the columnar helical supramolecular structure, and excimeric emission.

A rational synthetic strategy is developed to provide compact and simple terpyridine (terpy) mesogens that show liquid‐crystallinity both as pure compounds and in organic solution (amphotropic compound). The use of a central 4‐methyl‐3,5‐diacylaminophenyl platform equipped with two lateral aromatic rings, each bearing three appended aliphatic chains, allows connection of a 2,2′:6′,2′′‐terpyridine fragment through a polar group such as an ester, amide, or flat conjugated alkyne linker. For the T12ester and T12amide scaffolds, the mesophase is best described as a lamellar phase, in which the molecules self‐assemble into columnar stacks held together in layers. In the T12amide case, the additional amide link results in significant stabilization of the lamellar phase. The driving forces for the appearance of columnar ordering are the hydrogen‐bonding interactions of the amide groups, which induce head‐to‐tail π‐stacking of the terpy subunits. Replacing the polar linker by a nonpolarized but linear alkyne spacer, as in the T12ethynyl compound, provides a columnar mesophase organized in a rectangular lattice of p2ggsymmetry. In this arrangement, two nondiscotic molecules arranged into dimers by hydrogen bonding and π–π stacking pile up in a head‐to‐tail manner to form columns. In addition, the T12amide compound proves to be an excellent gelator of cyclohexane, linear alkanes, and DMSO. The resulting robust and transparent gels are birefringent and formed by large aggregates that are readily aligned by shear‐flow. TEM and freeze‐fracture microscopy reveal that the gels have an original layered morphology made of fibers.

Four new series of non-disklike complexes of general formula [MCl2(Lⁿ)] based upon substituted 2,6-bis(3',4',5'-trialkoxyphenyliminomethyl)pyridine ligands (Lⁿ) and with M=Zn^II, Co^II, Mn^II, and Ni^II have been prepared and examined for liquid crystallinity. A complete analysis of the thermal behavior by polarized-light optical microscopy, differential scanning calorimetry, and small-angle Xray scattering revealed a rich and varied mesomorphism. Moreover, the high thermal stability of the compounds leads to rather extended mesomorphic ranges. The nature and thermal stability of each mesophase depend on both the length of the six terminal alkoxy chains, n (n=8, 10, 12, 14, 16), and on the metal ions. As demonstrated by small-angle Xray diffraction experiments, the mesomorphism of these complexes is solely of the columnar type. One compound shows an oblique columnar phase, while most of them show a hexagonal columnar phase, Colh, and several types of rectangular columnar phase, Colr. Xray single-crystal structures obtained for three methoxy derivatives confirm the 1:1 metal–ligand stoichiometry of the complexes, in which the metal is pentacoordinate with a distorted, trigonal bipyramidal geometry. The crystalline structures also reveal the existence of some columnar organization in the solid state, the columns resulting from an alternated stacking of the complexes in one direction. By combining these results with those obtained from dilatometry experiments, a model for the molecular organization within the mesophases is proposed in which an antiparallel arrangement of the metallomesogens is retained in the mesophase.

Four new series of non‐disklike complexes of general formula [MCl2(Ln)] based upon substituted 2,6‐bis(3′,4′,5′‐trialkoxyphenyliminomethyl)pyridine ligands (Ln) and with M=ZnII, CoII, MnII, and NiIIhave been prepared and examined for liquid crystallinity. A complete analysis of the thermal behavior by polarized‐light optical microscopy, differential scanning calorimetry, and small‐angle Xray scattering revealed a rich and varied mesomorphism. Moreover, the high thermal stability of the compounds leads to rather extended mesomorphic ranges. The nature and thermal stability of each mesophase depend on both the length of the six terminal alkoxy chains, n(n=8, 10, 12, 14, 16), and on the metal ions. As demonstrated by small‐angle Xray diffraction experiments, the mesomorphism of these complexes is solely of the columnar type. One compound shows an oblique columnar phase, while most of them show a hexagonal columnar phase, Colh, and several types of rectangular columnar phase, Colr. Xray single‐crystal structures obtained for three methoxy derivatives confirm the 1:1 metal–ligand stoichiometry of the complexes, in which the metal is pentacoordinate with a distorted, trigonal bipyramidal geometry. The crystalline structures also reveal the existence of some columnar organization in the solid state, the columns resulting from an alternated stacking of the complexes in one direction. By combining these results with those obtained from dilatometry experiments, a model for the molecular organization within the mesophases is proposed in which an antiparallel arrangement of the metallomesogens is retained in the mesophase.

This paper describes the first examples of heteropolynuclear metallomesogens that contain both a transition metal ion and a trivalent lanthanide ion. Adducts were formed between a mesomorphic [Cu(salen)] complex (salen=2,2′‐N,N′‐bis(salicylidene)ethylenediamine) with six terminal tetradecyloxy chains and a lanthanide nitrate (Ln=La, Gd). Different stoichiometries were found, depending on the lanthanide ion: a trinuclear copper–lanthanum–copper complex [La(NO3)3{Cu(salen)}2] and a binuclear copper‐gadolinium complex [Gd(NO3)3Cu(salen)]. The compounds exhibit a hexagonal columnar mesophase (ColH) over a wide temperature‐range with rather low melting temperatures. Although the clearing point could be observed for the parent [Cu(salen)] complex, the mixed f–d complexes decomposed in the high‐temperature part of the mesomorphic domain before clearing. On the basis of X‐ray diffraction measurements and molecular modelling, a structural model for the mesophase of the metal complexes is proposed.

A new series of liquid crystalline poly(amidoamine) (PAMAM) dendrimers is described. These dendrimers are made by attaching to the 0‐, 1‐, 2‐, 3‐, and 4‐generation of PAMAM‐terminal promesogenic units that carry two decyloxy chains in the 3‐ and 4‐positions of their peripheral aromatic ring. X‐ray diffraction studies show that all the compounds display a hexagonal columnar mesophase. A high density of aliphatic chains imposes a curved interface with the promesogenic units that forces the molecules to adopt a radial conformation, and therefore, the columnar structure. A model for the supramolecular organization of the different generations within the columnar mesophase is proposed based on the variation of some of the structural parameters.

The bottom‐up designof luminescent lanthanide‐containing thermotropic liquid crystals is depicted on the cover. In their Full Paper on page 1674 ff., C. Piguet, P.‐Y. Morgantini et al. describe how the specific orientation of the carboxylic spacers in the molecular building blocks induces significant changes in the aromatic polarizations as measured by the electrostatic potential calculated on the Connolly surface. Only a sufficient set of polarization alternances produces enough intermolecular enthalpic cohesion compatible with the formation of liquid‐crystalline phases.