Nandiraju V. S. Rao

Aggregation-induced emitters or AIEgens are generally signified by their stronger photoluminescence in aggregation than in the solution state. Due to high emission efficiency in aggregate and solid states and good processability, organic AIEgens drew attention to the development of advanced luminescent materials. However, as mesogenic materials self-assemble to a different molecular arrangement in different phases, achieving liquid crystallinity and AIE properties in the same molecule would provide a valuable tool to develop solvent-independent AIEgenic materials. With this goal, the present work reports the synthesis of new organic thermotropic liquid crystalline compounds exhibiting aggregation-induced emission (AIE). The synthesized compounds exhibit strong green luminescence in a solid state which sharply quenches upon entering smectic mesophase by heating. This is in addition to the exhibition of dispersion medium (solvent)-dependent emission, thus providing a dual mode of AIE. The mesogenic property of the synthesized compounds was studied by XRD, POM, and DSC. The AIE was studied by fluorescence spectroscopy and variable temperature fluorescence microscopy. A DFT study was carried out to gain an insight into the AIEgenic behavior of the material.

Single-walled carbon nanotubes (SWCNTs) are dispersed in (4’-fluoro phenyl azo) phenyl-4-yl 3-[N-(4’-n-hecyloxy 2-hydroxybenzylidene)amino]-2-methylbenzoate (6–2M-F) a bent-core nematic (BCN) liquid crystalline medium composed of bent-shaped molecules with short core, reduced bend angle possessing polar fluoro substituent in longitudinal direction and methyl group in bent direction. Such molecules are at the borderline of typical bent-core and rod-like molecules resembling hockey stick shape with intermediate properties. The elastic anisotropy is negative for 6–2M-F (bend elastic constant K₃₃ < splay elastic constant K₁₁); similar to other BCNs reported earlier with smectic-like clusters; but turns to high positive (K₃₃ > K₁₁) value by insertion of SWCNT (concentration ≥0.05 wt.%) in 6–2 M-F. The ratio of K₃₃/K₁₁ becomes comparable to the calamitic liquid crystals (LCs) in doped system. Dielectric anisotropy increases in the nanocomposite implying enhanced nematic ordering due to π–π electron interaction between CNTs and the LC molecules. Threshold voltage at first increases and then decreases with increasing CNT concentration owing to the respective variations in splay viscosity of the system. The present study demonstrated the interaction of SWCNTs with BCN molecules and reveals significant modifications in viscoelastic, dielectric and ionic properties of the host. Graphical Abstract

Design, synthesis and characterization of a four-ring achiral bent-core liquid crystal with a broad range nematic phase which can be cooled down without crystallization.

In a newly designed four-ring asymmetrical bent-core compound, we observed smectic-C-type diffuse layer reflection over the entire nematic temperature range. At the nematic–smectic C phase transition, a sharp layer reflection emerges in addition to the diffuse reflection with different layer tilt angles.

Molecular shape is an important factor in determining the material properties of thermotropic liquid crystals (LCs). We synthesized and investigated several LC compounds formed by asymmetrically bent molecules with a rigid four-ring core in the shape of the letter ‘L’. We measured the temperature dependencies of dielectric permittivities, birefringence, splay K₁ and bend K₃ elastic constants, splay viscosity η_splay and flow viscosities η_|| and η_⊥. The bend–splay anisotropy δK₃₁ = K₃ − K₁ is negative, similar to the case of nematic LCs formed by symmetrically bent molecules of V-shape. The dielectric anisotropy Δε and birefringence are positive in the entire nematic range. The splay viscosity η_splay and the flow viscosities η_|| and η_⊥ are smaller than the viscosities measured for the symmetric V-shaped bent-core materials at similar temperatures. The ratio Γ = η_splay/η_||,⊥ is in the range 5–4 that is typical for rod-like LCs. The reported L-shaped bent-core nematic LCs combine the useful features of bent-core LCs (such as a negative δK₃₁, suitable for formulation of broad-range blue phases) with the relatively low viscosities, a property typical for rod-like LCs and beneficial for electro-optic switching applications.

The synthesis, liquid crystalline and optical properties of polar Schiff's bases, viz. N-(4-n-hexadecyloxysali cylidene)-4′-substituted anilines with methyl, nitro and cyano substituents in the 4-position of N-aryl moiety and their d- and f-block metal complexes, are presented. The polar ligands exhibited a smectic A phase, while methyl homologue exhibited a nematic phase. The majority of the complexes exhibited a smectic A phase with some of them exhibiting smectic E phases. The ultraviolet-visible and photo-physical properties of the ligands and complexes are also discussed.

New lanthanide complexes, Tb(III), Dy(III) and Gd(III) of N-aryl based Schiff bases exhibiting mesomorphism have been synthesised and characterised. The crossover phenomena of calamitic lamellar to discotic columnar phase behaviour in these lanthanidomesogens is found to be controlled by the variation in chain length of the single alkyl substituent on the terminal N-aryl ring, hitherto the first case of a metallomesogen with only one alkyl chain length at each end of the ligand as evidenced by thermal microscopy studies and complemented by X-ray studies. It is also observed as a function of temperature in a single component. The electronic properties of these systems are dominated by the donor–acceptor substituted organic chromophore ligand, and the emission maxima revealed a large Stokes shift of 160 nm. The ligand–ligand charge transfer is reflected in emission spectra and is due to the presence of phenyl rings in the ligands.

Achiral molecules based on a novel four-ring core with an ester linkage at the molecular bend are shown to exhibit smectic liquid crystal phases with spontaneously chiral and polar layers (SmCP phases), including their polarization splay modulated and layer undulated (PMLU) variants, B7 and B1. Additionally, these compounds exhibit strong photoluminescence, the first family of SmCP phases to do so.

The results of experiments with stable five-ring banana-shaped molecules consisting of laterally 4-fluoro substituted 1,3-phenylene ring as the central unit in Bis-[4-(4′-n-alkyloxybenzoyloxy)salicylidene]-phenylene-4-fluoro-1,3-diamines are presented. These compounds are thermally and hydrolytically stable due to intermolecular or intramolecular hydrogen bonding and exhibit SmCP_A and low-temperature crystalline B_X phases. The phases had been characterised by thermal microscopy and differential scanning calorimetry. A representative example has also been characterised by X-ray diffraction studies and electric field effects. The influence of a lateral fluoro substituent in the central core is clearly demonstrated with the induction of the SmCP_A phase which is absent in the parent compound without the substituent. Further these compounds exhibit a large tilt angle. We also describe a comparison of phase behaviour with similar analogous compounds, taking the position of fluoro substitution, changes in the linking groups and the direction of the linking groups into consideration.