Teresa Sierra

Helical supramolecular organizations from metal-organic liquid crystals

Abstract The present paper is aimed at providing a survey of investigations undertaken regarding the preparation and characterization of helical supramolecular architectures using a strategy based on two concepts: coordination chemistry and mesomorphic behavior. Metal-coordination procedures have been used to design and synthesize molecular units capable of organizing into helical superstructures. In order to ensure this capability, the molecular building blocks have been complemented with a pro-mesogenic character, most commonly columnar, so that this type of mesophase is used as a template to build up helical supramolecular architectures with long-range orientational order and long-range positional order. For example, metal-containing liquid crystals have been described that organize into helical columnar mesophases whose structure has been fully investigated. To date, molecular units that display this organizational behavior have been found to be very different in shape, and hence the formation of helical structures has a different origin in each case. In this paper, three molecular structures are discussed (i.e. ‘disk-like’ molecules, ‘propeller-like’ molecules and polycatenar elongated molecules) that have been shown to give rise to helical organizations. Models have been proposed for the three systems described and in the present overview these have been classified as azimuthal rotation of planar disk-like complexes, interdigitated stacking of octahedral complexes and staggered stacking of polycatenar elongated planar complexes, respectively. Throughout this paper, emphasis is placed on studies carried out in the mesophase that have allowed authors to propose the corresponding models.

Structure–activity studies of ferroelectric and antiferroelectric imine ligands and their square-planar complexes

The synthesis and characterization of the mesomorphic and dielectric properties of a series of new imine-based liquid crystalline compounds that exhibit tilted SmC* phases is reported. The presence of an imino linkage within the mesogenic nucleus of these compounds is significant due to the ability of salicylaldimines to coordinate to metals. Structure–activity studies have also been carried out by varying the structural elements in the ligands. The structural variations include changing the length of the chiral chain and incorporating fluorocarbon segments in the achiral terminal chain.

The first discotic liquid crystal with a tetrathiafulvalene central core

Abstract The synthesis and characterization of tetrasubstituted tetrathiafulvalenes 7 a-b and 18 a-b, which bear promesogenic units such as 4-decyloxybenzoyl and 3,4,5-tris(decyloxy)benzoyl groups, are described. Such units are linked to the TTF core through spacers of different lengths. Compound 18b exhibits a metastable discotic mesophase and constitutes the first example of a discotic liquid crystal with a tetrathiafulvalene central core.

Structure-activity studies of ferroelectric and antiferroelectric imine ligands and their palladium(II) complexes. An antiferroelectric metallomesogen

The synthesis and characterization of the mesomorphic and dielectric properties of new imine-based liquid crystalline compounds that exhibit tilted SmC* phases is reported. The presence of an imino linkage within the mesogenic nucleus of these compounds is significant due to the ability of benzaldimines to coordinate to metals. Complexation to a divalent metal such as Pd(II) has been studied in order to assess the effects of antiparallel dimerization of organic ligands, through metal-coordination, on the appearance of antiferroelectric order in the mesophase. An organometallic palladium(II) complex has been prepared and this shows antiferroelectric behaviour over the whole range of the tilted smectic mesophase. Structure–activity studies have also been carried out by varying structural elements in the composition of the ligands. The changes include the disposition of the linking groups within the mesogenic core, and presence of a lateral hydroxy group.

Dielectric behaviour of the relaxational modes in a ferroelectric liquid crystal

Abstract The dielectric response in 2-hydroxy-4-decyloxybenzilidene-4′-(2S-2-chloro-3-methylbutyloxy) aniline, a ferroelectric liquid crystal which shows a C-Sc*-SA-I phase transition sequence has been determined. The influence of a bias electric field, applied parallel to the smectic layers, on the parameters that characterize the contributions of the different modes to this response has been analysed. In addition to the soft mode and Goldstone mode contributions we have observed a low frequency relaxation mechanism which could be related to the defect structure present in samples with planar alignment. Moreover, the deconvolution process has allowed us to characterize the soft mode in the ferroelectric phase without unwinding the helical structure.

Polar Switching in a Lyotropic Columnar Nematic Liquid Crystal Made of Bowl‐Shaped Molecules

A polar response in a lyotropic columnar nematic material is reported. The material accommodates bowl-shaped molecules with strong axial dipole moments in column segments without head-to-tail invariance. Optical second-harmonic-generation methods confirm that the nematic columns align unidirectionally under an applied electric field and the material develops remnant macroscopic polarization observable for hours. The switching takes place by a flip of the columns.

Tetrathiafulvalene-containing liquid crystals

Abstract A range of tetrathiafulvalenes incorporating 4′-cyano-4-alkoxybiphenyl groups have been prepared. One of these compounds shows mesomorphic properties both before and after doping with iodine.

Chiral hexacatenar metallomesogens: supramolecular organization versus steric demand of chiral cores

Hexacatenar chiral oxazoline complexes were prepared in a stereospecific synthesis. Metals (Pd, Ni, Cu, Zn), lateral alkoxy chains (OC6H13, OC12H25), sterically demanding alkyl groups (CH3, C4H9) on the chiral rigid centre and the number of nuclei incorporated in the complex core (mononuclear, orthopalladated dinuclear) were varied in order to achieve supramolecular organization of the phasmid-like molecules in liquid crystal phases. Mesomorphic properties were not observed in neat materials, but phase diagrams of binary mixtures with TNF demonstrate that steric repulsion can be overcome by intercalation of the electron-acceptor TNF. In spite of the presence of six lateral chains, SmA phases were found exclusively. The structures of the SmA phases were found to have additional order with respect to SmA phases formed by calamitic molecules. Further examination nof the complexes in host nematic materials demonstrated their use as chiral dopants to induce cholesteric LC phases.

Wide temperature range mesomorphic behaviour of highly fluorinated 15‐membered macrocycles and their open trisulphonamide precursor

The mesomorphic properties of 15‐membered azamacrocycles containing long hydrocarbon or highly fluorinated long chains have been explored along with those of their open precursors. The liquid crystalline behaviour is determined by the presence of different heteroatoms or long fluorinated chains and the substitution pattern of the aryl units of the sulphonamides. The presence of highly fluorinated long chains in the para position with regard to the sulphonamide group was found to favour the formation of smectic mesophases with wide temperature ranges. All of the mesophases were studied by polarizing optical microscopy and differential scanning calorimetry. **Deceased on 20th February 2006.

Low and High Frequency Relaxations of a Ferroelectric Liquid Crystal

Abstract Broadband dielectric measurements on a ferroelectric liquid crystal have been carried out. The alignment was planar and the cell and sample holder were the same in the whole frequency range. Five different relaxations have been obtained and characterized: soft mode, Goldstone mode, molecular reorientation around the transversal axis, molecular reorientation around the longitudinal axis and a ferroelectric domain mode. Strengths and frequencies of these modes have been obtained in the different phases for different bias fields.