Ammathnadu S. Achalkumar

Luminescent, liquid crystalline tris(N-salicylideneaniline)s: synthesis and characterization.

A new class of discotics derived from tris(N-salicylideneaniline)s have been synthesized and their thermal and photophysical properties are investigated. These systems with outer 1,3,4-oxadiazole wings exist in an inseparable mixture of two keto-enamine tautomeric forms with C(3h) and C(s) rotational symmetries, and self-assemble into fluid columnar phase over a wide thermal range as evidenced by several complementary studies. They possess emissive characteristics in both solution and columnar states; the blue light (lambda = 474 nm) emission has been evidenced for the former state.

Columnar self-assembly of star-shaped luminescent oxadiazole and thiadiazole derivatives

A new class of blue light emitting liquid crystalline star-shaped molecules based on 1,3,4-thiadiazoles have been designed and synthesized. These compounds were investigated using polarizing optical microscopy, differential scanning calorimetry, X-ray diffraction, cyclic voltammetry and photophysical studies. In comparison to their 1,3,4-oxadiazole counterparts, these thiadiazole-based molecules are promising as they stabilize the hexagonal columnar phases over a broad thermal range. The thermal behavior and photophysical properties of these new star-shaped molecules are extremely dependent on the number and types of peripheral tails in the molecular structure. 1,3,4-Thiadiazole derivatives exhibit sky-blue emission in solution, unlike the deep blue emission of 1,3,4-oxadiazole derivatives. They also exhibit a lower band gap as compared to their oxadiazole counterparts and offer great potential in organic light emitting diode applications.

Dielectric spectroscopy of unsymmetrical liquid crystal dimers showing wide temperature range TGBA and TGBC* phases

The electrical properties of frustrated twist grain boundary (TGB) phase are a matter of curiosity. Some studies have indicated the existence of soft and Goldstone modes in TGBA and TGBC* phases respectively. However, the experimental results are still not very conclusive. In the present work, we report dielectric studies of wide temperature range TGBA and TGBC* phases of an optically active dimeric compound 4‐n‐decyloxy‐4′‐(cholesteryloxycarbonyl‐1‐butyloxy) chalcone in the frequency range of 1 Hz to 35 MHz for the planar and homeotropic anchoring of the molecules. Two different relaxation processes have been detected for the planar anchoring of molecules in the TGBA and TGBC* phases. The soft mode like behaviour is obtained due to tilt fluctuation of molecules in the megahertz region for both TGBA and TGBC* phases. Goldstone mode like behaviour due to phase fluctuation of molecules has been detected for the TGBC* phase in the low frequency region (∼200–300 Hz). Activation energies for DC conductivity have also been determined for various phases of the material.

The first examples of optically active tris(N-salicylideneaniline)s: manifestation of chirality from molecules to fluid columnar phases

Tris(N-salicylideneaniline)s comprising chiral tails in varying numbers self-organize into a room temperature helical columnar mesophase resulting from the chiral stacking of the constituent molecules differing in their rotational symmetry and ratio as evidenced by optical, chiroptical, calorimetric and X-ray studies.

Cholesterol-based anchors and tethers for phospholipid bilayers and for model biological membranes

This review covers the range of cholesterol-based anchors and tethers and the ways in which they are being used. These cholesterol conjugates provide us with a very flexible ‘tool-box’ that can be used to tether phospholipid bilayers to surfaces, to join bilayers together (bilayer-to-bilayer, bilayer-to-vesicle, vesicle-to-vesicle, etc.) or to anchor molecules, biomolecules, macromolecules or particulate species to the surface of the bilayer. Model biomembranes tethered to a solid support provide a stable platform for addressing membrane components in vitro using force field and spectroscopic methods and since these membranes generally remain fluid and retain much of their biological activity, solid-supported membranes can also be use to study aspects of membrane biology and biochemistry. Potential medical applications are developing out of our ability to anchor ‘markers’ and antibodies to phospholipid vesicles. Compared to anchors in which the anchoring group is a phospholipid (or phospholipid-like) moiety, cholesterol-based anchors are generally easier to make and purify but the anchoring is less strong and this can be an issue when the ‘payload’ is large and water-soluble. In the case of nucleic acid functionalised systems this problem has been addressed by anchoring each oligonucleotide through two cholesteryl end-groups.

The first examples of discotic radicals: columnar mesomorphism in spin-carrying triphenylenes

The synthesis, columnar behaviour and magnetic properties of the first examples of radical discotics are reported. These organic molecular systems with spin sources have been accomplished by covalently linking triphenylene with molecular radicals. The columnar mesomorphism of these new radicals has been established unambiguously using several complementary techniques. Magnetic susceptibility measurements in the fluid columnar, frozen columnar and solid states reveal the antiferromagnetic intermolecular interactions of Curie–Weiss behaviour.

Effect of regioisomerism on the self-assembly and photophysical behavior of 1,3,4-thiadiazole-based polycatenars

A new class of polycatenars, where the central benzene ring is connected to two arms derived from substituted 1,3,4-thiadiazoles at the 1,3- and 1,4-positions, was synthesized and characterized. These thiadiazole-based molecules are promising as they stabilize columnar phases over a wide temperature range, in comparison to their oxadiazole analogues. The para-substituted polycatenars exhibited a columnar hexagonal and/or a columnar oblique phase, while the meta-substituted polycatenars exhibited solely a columnar oblique phase. The para-substituted polycatenars exhibited green emission, while the meta-substituted polycatenars exhibited blue emission in solution and film states. Stabilization of a broad range columnar phase and luminescence in the solid state make these new compounds promising from the viewpoint of applications in emissive displays. The self-assembly and luminescence of these regioisomers was greatly influenced by the molecular structure.

Controlling Liquid Crystal Alignment Using Photocleavable Cyanobiphenyl Self-Assembled Monolayers

We report on the development of novel cyano-biphenyl-based thiolate self-assembled monolayers designed to promote homeotropic alignment of calamitic liquid crystals. The molecules developed contain an ortho-nitrobenzyl protected carboxylic acid group that on irradiation by soft UV (365 nm) is cleaved to yield carboxylic acid groups exposed at the surface that promote planar alignment. Using a combination of wetting, X-ray photoelectron spectroscopy, Fourier transform-infrared reflection absorption spectroscopy, and ellipsometry we show that high photolysis yields (>90%) can be achieved and that the patterned SAMs are suitable for the controlled alignment of calamitic liquid crystals. This study further shows that such photo-patterned SAMs can be used to control the formation of focal conic domains (FCDs) in the smectic-A phase in terms of positioning and size confinement on surfaces.

Non-symmetric dimers comprising chalcone and cholesterol entities: an investigation on structure–property correlations

Four series of new dimers formed by interlinking chalcone and cholesterol mesogenic entities through spacers of varying length and parity have been synthesized and characterized by polarizing optical microscopy, differential scanning calorimetry, X-ray diffraction and electrical switching studies. The structure of the chalcone core has been systematically modified to investigate the effect of the molecular structure on the thermal behaviour of the dimers. The study demonstrates the dramatic dependence of thermal behaviour of these dimers on the nature of the chalcone as well as the length and parity of the spacer.

Tunable chiral reaction media based on two-component liquid crystals: regio-, diastereo-, and enantiocontrolled photodimerization of anthracenecarboxylic acids.

Three kinds of enantiopure amphiphilic amino alcohols (1a-c) were newly synthesized, of which the stereochemistry of the stereogenic carbons adjacent to the amino (C2) and hydroxy (C1) groups was systematically varied. By using these amino alcohols and four photoreactive carboxylic acids, 12 kinds of salts were prepared. The structure and thermal behavior of the salts were thoroughly investigated by various techniques, which revealed that the stereochemistry of the amino alcohol unit has significant effects on the properties of the salts; the salts of 1a with (1R,2S)-configuration did not exhibit any liquid crystal (LC) phase but showed high crystallinity, whereas 1b and 1c with (1S,2S)- and (1S)-configurations, respectively, generally afforded stable LC salts with smectic structure(s). Within the matrix of these amphiphilic salts, the in situ photodimerizations of 2-anthracenecarboxylic acid (2c) and 1-anthracenecarboxylic acid (2d) were conducted by the irradiation with UV/vis light (500 W, a high-pressure mercury arc lamp, >380 nm). Concerning reactivity and regio-/diastereo-/enantioselectivities, the LC phases were found to be superior to isotropic and crystalline phases. For the two substrates 2c and 2d, every LC phase promoted the photodimerization with unprecedentedly high head-to-head selectivity. Particularly in the case of 2c, diastereoselecitivity (syn(HH) vs anti(HH)) could be rationally controlled by the choice of the amino alcohol unit and mesophase (syn(HH):anti(HH) = 61:37 to 26:72). Moreover, one of the LC phases exhibited by 1b·2c afforded the anti(HH)-dimer of 2c with excellent enantioselectivity (up to 86% ee). On the basis of the hypothesis that the present photochemical outcome arises from the preorientation of the substrates, a preliminary structural model of these LCs was proposed.