Ru(II) polypyridine complexes: photophysics, photochemistry, eletrochemistry, and chemiluminescence

About Supramolecular Assemblies of π-Conjugated Systems

Most associate liquid crystals with their everyday use in laptop computers, mobile phones, digital cameras, and other electronic devices. However, in contrast to their rodlike (calamitic) counterparts, first described in 1907 by Vorlander, disklike (discotic, columnar) liquid crystals, which were discovered in 1977 by Chandrasekhar et al., offer further applications as a result of their orientation in the columnar mesophase, making them ideal candidates for molecular wires in various optical and electronic devices such as photocopiers, laser printers, photovoltaic cells, light-emitting diodes, field-effect transistors, and holographic data storage. Beginning with an overview of the various mesophases and characterization methods, this Review will focus on the major classes of columnar mesogens rather than presenting a library of columnar liquid crystals. Emphasis will be given to efficient synthetic procedures, and relevant mesomorphic and physical properties. Finally, some applications and perspectives in materials science and molecular electronics will be discussed.

Discotic Liquid Crystals: From Tailor-Made Synthesis to Plastic Electronics

This critical review begins with a brief, but essential, introduction to the special nature of liquid crystal materials, their peculiar properties, and their commercial applications, followed by an introductory insight into the remarkable nature of the fluoro substituent, and its fascinating influence on the properties of organic compounds. However, the main focus of the review is to discuss the enormous amount of exciting research on fluorinated liquid crystals that has been reported. The small size of the fluoro substituent enables its incorporation into all types of liquid crystal, including calamitic, discotic, banana, lyotropic, and polymers, without ruining the liquid crystalline nature of the material. However the fluoro substituent is larger than hydrogen, and hence causes a significant steric effect, which combined with the high polarity, confers many fascinating, and often remarkable, modifications to melting point, mesophase morphology and transition temperatures, and the many other very important physical properties, such as dielectric anisotropy, optical anisotropy, and visco-elastic properties. There are many different positions within a liquid crystal structure where a fluoro substituent can be located, including (i) a terminal position, (ii) within a terminal chain, as a semi-fluorinated or as a perfluorinated chain, or as one fluoro substituent at a chiral centre, (iii) as part of a linking group, and (iv) a lateral position in the core section. Such variety enables the interesting and advantageous tailoring of properties, both for the fundamental purposes of establishing structure–property relationships, and for materials targeted towards commercially-successful liquid crystal display applications.

Fluorinated liquid crystals – properties and applications

Discotic Liquid Crystals.

The results of a recent investigation of the nematic biaxiality in a bent-core mesogen (A131) are in apparent disagreement with earlier claims. Samples of mesogen A131 used in the two studies were investigated with polarized optical microscopy, conoscopy, carbon-13 NMR, and crossover frequency measurements. The results demonstrate that textural changes associated with the growth of biaxial nematic order appear at similar to 149 degrees C. The Maltese cross observed in the conoscopic figure gradually splits into two isogyres at lower temperatures indicating phase biaxiality. Presence of the uniaxial to biaxial nematic phase transition is further confirmed by temperature trends of local order parameters based on (13)C chemical shifts in NMR experiments. Frequency switching measurements also clearly reveal a transition at 149 degrees C. Differences between the two reports appear to be related to the presence of solvent, impurities, and/or adsorbed gases in samples of A131 used in the study of Van Le et al. [Phys. Rev. E 79, 030701 (2009)]

Nematic biaxiality in a bent-core material

The paper deals with a study of the thermotropic mesophases observed in systems of disc-like molecules, viz., pure benzene-hexa-n-alkanoates and their mixtures with benzene. The mesophases exhibit optical textures that are typical of a lamellar type of liquid crystal. The X-ray data for the pure compounds are consistent with a structure in which there is lamellar order with hexagonal symmetry in two dimensions and liquid-like disorder in the third. Preliminary NMR results are described, which again indicate that the degree of molecular order in the mesophase is intermediate between those in the solid and isotropic phases. Pressure-temperature phase diagrams are presented for two of the homologues, the hexanoate and octanoate compounds.

Disc-like mesogens

The Langmuir monolayer of cholesterol at the air-water interface exhibits a condensed phase in which the cholesterol molecules are aligned normal to the water surface. We have transferred the monolayer from water surface to different substrates by Langmuir-Blodgett (LB) technique and have studied their assembly by atomic force microscope (AFM). Our studies reveal that the aggregation of cholesterol molecules on hydrophobic surfaces leads to interesting structures. The cholesterol molecules assemble into a uniform film, elongated domains and uniformly distributed torus-shaped domains (doughnuts) for one, two and four cycles of deposition, respectively. Beyond four cycles, the molecules adsorb and desorb by an equal amount resulting in no further deposition. The formation of uniformly distributed doughnuts can be attributed to the hydrophobic interaction and reorganization of the molecules due to successive adsorption and desorption during deposition cycles. Our studies on hydrophilic surfaces show that cholesterol cannot form more than one layer of deposition.

AFM studies on Langmuir-Blodgett films of cholesterol.

Discotic molecules are known to form highly anisotropic structures at the air-water (A-W) interface. We have studied two novel ionic discotic mesogenic molecules, viz., pyridinium tethered with hexaalkoxytriphenylene with bromide counterion (Py-Tp) and imidazolium tethered with hexaalkoxytriphenylene with bromide counterion (Im-Tp) at A-W and air-solid interfaces. The monolayer phases were investigated at the A-W interface employing surface manometry and Brewster angle microscopy techniques. They indicate a uniform monolayer phase which shows negligible hysteresis on expanding and compressing. Also, in both the systems the collapsed state completely reverts to the monolayer state. These monolayer films transferred at different surface pressures by Langmuir-Blodgett technique were studied by employing atomic force microscopy. The topographies of these films transferred at the low and high surface pressure region of the isotherm indicate a transformation of the monolayer from face-on to edge-on structure.

Films of novel mesogenic molecules at air-water and air-solid interfaces.

Nous avons prepare des monocouches de Langmuir avec des molecules organiques qui forment des phases cristal liquide en volume. Tous les polymeres et monomeres utilises presentent le meme type de structure avec un cœur central aromatique rigide contenant plusieurs groupes esters hydrophiles et deux ou plusieurs chaines aliphatiques hydrophobes. Un etalement bi-dimensionnel satisfaisant est obtenu dans la plupart des cas, avec une augmentation monotone de la pression superficielle en fonction de la concentration dans la monocouche. Aux pressions les plus fortes, l'aire minimum correspond a une orientation verticale des molecules avec decollement de certains groupes hydrophiles initialement en contact avec le substrat aqueux. Dans la region de basses pressions, les donnees experimentales tendent a prouver que les molecules sont orientees parallelement a l'interface

https://hal.archives-ouvertes.fr/jpa-00209984/document

K. A. Suresh

Papers

Nematic biaxiality in a bent-core material

Disc-like mesogens

AFM studies on Langmuir-Blodgett films of cholesterol.

Films of novel mesogenic molecules at air-water and air-solid interfaces.

Langmuir monolayers of mesomorphic monomers and polymers