Daniel Guillon

Imidazolium-based liquid crystals: a modular platform for versatile new materials with finely tuneable properties and behaviour

Ionic liquid Crystals constitute highly versatile materials that have drawn much interest these past few years in the fields of academic research and industrial development. In this respect, the present article is intended as an update of K. Binnemans review published in 2005, but focusing exclusively on the imidazolium cation - the most widely studied. Herein, imidazolium-containing thermotropic liquid crystalline materials will be sorted by molecular structure (mono-, bis-, poly-imidazolium compounds, with symmetrical and non-symmetrical structures) and discussed. Their physico-chemical properties will be exposed in order to adduce the relevancy and potential of the imidazolium platform in various fields of research.

Nematic-like Organization of Magnetic Mesogen- Hybridized Nanoparticles

A fluid nematic-like phase is induced in monodisperse iron oxide nanoparticles with a diameter of 3.3 nm. This supramolecular arrangement is governed by the covalent functionalization of the nanoparticle surface with cyanobiphenyl-based ligands as mesogenic promoters. The design and synthesis of these hybrid materials and the study of their mesogenic properties are reported. In addition, the modifications of the magnetic properties of the hybridized nanoparticles are investigated as a function of the different grafted ligands. Owing to the rather large interparticular distances (about 7 nm), the dipolar interaction between nanoparticles is shown to play only a minor role. Conversely, the surface magnetic anisotropy of the particles is significantly affected by the surface derivatization.

Thermotropic Lamellar-to-Columnar Phase Transition Exhibited by a Biforked Compound

Abstract In this work we present a structural study of the smectic C(SC) and columnar (Colh) liquid-crystalline phases exhibited by a biforked molecule. This study was performed by means of dilatometry and X-ray diffraction measurements performed at different temperatures. Following these measurements, molecular packing within lamellar and columnar mesophases of this compound is discussed; finally, a mechanism for the lamellar-columnar transition is proposed.

Bent-core molecules with lateral halogen atoms forming tilted, synclinic and anticlinic, lamellar phases

Four homologous series of symmetric bent-core molecules containing lateral fluorine or iodine substituents in the branches were synthesised and studied. Depending on the central ring and the lateral substituent the studied compounds exhibit a B7 phase or B2 type phases with anticlinic (SmCAPA) or synclinic (SmCSPA) interlayer structures. For the SmCSPA phase the C2v symmetry structure is proposed, with regularly spaced anticlinic–ferroelectric interlayer boundaries. In the SmCAPA phase strong optical activity was detected, chiral domain segregation could be induced by applying a weak electric field.

Self-assembly and liquid-crystalline supramolecular organizations of semifluorinated block co-dendritic supermolecules

The synthesis and self-organizing behaviour into liquid-crystalline mesophases of two libraries of segmented block co-dendrimers are described. The overall structure of the dendritic supermolecules consists of two chemically incompatible molecular moieties, covalently connected by combinatory cross-coupling reactions. One block is composed of linear perfluorinated segments (2 generations, defining the two libraries), and the bulky dendritic compartment is formed by a lipophilic poly(benzyl ether)-based dendron (3 generations). The mesomorphic properties were studied as a function of the respective generations of both blocks and of the peripheral alkyl chains’ substitution pattern. The results of these investigations reveal the formation of unusual and highly segregated supramolecular liquid-crystalline mesophases, whose arrangements are subtly controlled by the lipophilic/fluorophobic balance and dendritic connectivity.

Optical textures in TGBA mesophases

In this paper, we present the results of polarized optical microscopy observations performed on different series of compounds exhibiting twist grain boundary mesophases (TGB). From all the compounds investigated, it appears there are two main typical optical textures which characterize TGBA mesophases in general. One type of texture is characteristic of a planar helical structure, while the other is a cylindrical (or marginally cone-like) domain texture, resembling the developable domains observed in columnar systems. Moreover, both textures can coexist in the same sample over the whole temperature range of the TGB phases. The detection, at the same temperature, of these two types of textures could then be considered as one of the signatures of the TGBA mesophases.

Structural study of smectic A phases in homologous series of N-alkylpyridinium alkylsulphates

The mesomorphic properties of 27 homologues of the N-alkylpyridinium alkylsulphate series were studied by X-ray diffraction and dilatometry. All of these compounds exhibit single smectic A phases whose layer spacings are between 0.64 and 0.8 times the molecular length. Polarizing optical microscopy observation shows that these peculiar layer spacings are not due to the tilting of the molecules; a new packing model is proposed to explain the particular structure of these thermotropic ionic liquid crystal phases.

Photo- and thermal-processing of azobenzene-containing star-shaped liquid crystals

A new class of star-shaped, liquid crystalline, low-molecular weight compounds functionalized with photochromic azobenzene and mesogenic groups was investigated in terms of light-induced anisotropy. The behaviour of the materials under the action of light with simultaneous or subsequent thermal treatment was examined with respect to the induction of anisotropy. The unconventional UV light treatment prior to the irradiation with linearly polarized light allowed induction of very high values of anisotropy (D = 0.77) at room temperature. Moreover, the simultaneous action of light and temperature led to the induction of higher values of dichroism in comparison with anisotropy generated by the standard procedure. Subsequent thermal treatment led to dewetting and the formation of 3D macroscopic stripe- and dome-like structures for one of the investigated compounds. Despite photoinduction of anisotropy by a single beam, the formation of polarization and surface relief gratings by two-beam interference pattern was also investigated.

Heterolithic azobenzene-containing supermolecular tripedal liquid crystals self-organizing into highly segregated bilayered smectic phases

Synthesis, self-organization, and optical properties of supermolecular tripedal liquid crystals incorporating various prototypical mesogenic units such as alkoxy-azobenzene (AZB), alkoxy-biphenylene (BPH) or alkoxy-cyanobiphenyl (OCB) derivatives are reported. Different molecular systems were designed in order to sequentially incorporate the smectogenic-like alkoxy-azobenzene-based chromophore within the molecular structure, whose relative proportion is selectively varied by exchanging with the other mesogens. A divergent synthetic mode was elaborated for their synthesis, starting from the regioselective functionalization of the phloroglucinol-based (PG) inner core. This methodology allowed the preparation of several sets of unconventional tripedal oligomers with conjugated heterolithic structures (made of different blocks, e.g. PG6AZBxBPH3−x and PG6AZBxOCB3−x, x = 1 or 2) along the homolithic parents (all identical blocks, e.g. PGzAZB3, z = 6 or 11, z is the number of methylene in the spacer between PG and the protomesogen, PG6BPH3, and PG6OCB3), respectively. Essentially all the synthesized systems behave as thermotropic liquid crystals and show various types of highly segregated multilayered smectic phases, or, in one case, a nematic phase, depending on the nature of the constitutive anisotropic blocks and on the molecular topology (homolithic versus heterolithic, mesogenic ratio x:3 − x). The effects of these structural modifications on the mesomorphism (mesophase structures, temperature ranges, and thermodynamic stability) have been investigated by differential scanning calorimetry and small-angle X-ray diffraction experiments combined with dilatometric measurements. Models describing the various supramolecular organizations of these tripedes into such multilayered structures are proposed and discussed. Preliminary results of the investigations of their optical properties will also be presented.

Influence of linear and branched perfluoroalkylated side chains on the π–π stacking behaviour of hexa-peri-hexabenzocoronene and thermotropic properties

The thermotropic properties and self-assembly of two different series of hexa-peri-hexabenzocoronenes (HBC) bearing either linear or branched perfluoroalkylated side chains, each with a wide range of alkyl spacer and perfluorinated tail lengths, have been studied. Correlations between thermogravimetric analysis, differential scanning calorimetry, polarised optical microscopy and small-angle X-ray scattering experiments revealed, as expected, that the transition temperatures and phase stability are influenced by the determining roles of the aliphatic spacer length and odd/even nature, as well as the size and structure (linear vs. bifurcation) of the perfluorinated sections. Most of the investigated HBC derivatives showed a single-column hexagonal columnar phase, where the columns are structured by a double segregation process between HBC aromatic cores and aliphatic spacers, on the one hand, and between aliphatic spacers and fluorinated tails, on the other hand. For the derivatives with long linear spacers, cores of untilted or quasi-untilted HBC stacks are surrounded by a cylindrical aliphatic envelope segregated from the fluorinated periphery, whereas for those with shorter linear spacers, the same structure is kept, except that the aliphatic envelope deviates from cylindricality and causes a symmetry break to rectangular envelope for the first term of the series. Of the four HBC with branched spacers, two are amorphous, whereas a columnar phase is maintained for the other two derivatives, but with tilted HBC stacks. Consequently, the evolution of the polymorphism in the series could be correlated with the variation of both interface areas.