Manoj Kumar Paul

Four-ring achiral unsymmetrical bent core molecules forming strongly fluorescent smectic liquid crystals with spontaneous polar and chiral ordered B7 and B1 phases

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.

Hockey-stick-shaped mesogens based on 1,3,4-thiadiazole: synthesis, mesomorphism, photophysical and DFT studies

ABSTRACT New hockey-stick-shaped 2,5-diphenyl-1,3,4-thiadiazole-containing mesogens have been designed and synthesised. These molecules possess 4-n-alkoxy chain at the elongated arm whereas and 4-n-butyloxy chain at the shorter arm of the molecule. The compounds are fluorescent and emission located in the violet to blue region with a large stoke shift observed in polar solvents. No substantial solvatochromic effect was found in the absorption maxima, but in the emission, a red shift was observed with increasing the solvent polarity. In the solid thin film, compounds emitted green light. The compounds having longer chain (n > 8) exhibit SmA and SmC phase, while the compounds with shorter chain at one end of the molecule display nematic and SmC phase. The density functional theory study has been carried out to investigate the stable configuration and the influence of solvent on bond length, bond angle, bending angle, dipole moment, polarisability of the molecule. It also enables us to predict the possible reactive sites and effect of solvent on the chemical reactivity descriptors. Our study concludes that the molecule becomes more reactive in solvent compared to gas. The large voltage holding ratio confirms that this molecule may be used for active matrix display applications. Graphical Abstract

Effect of methoxy group instead of polar group in the nematic phase of four-ring bent-core liquid crystals

Two achiral four-ring bent-core liquid crystal possessing an alkoxy chain at one end of the molecules and methoxy group at the other end are investigated by using the optical polarizing microscope, electro-optical studies, and dielectric spectroscopy. Both the compounds exhibit enantiotropic nematic phase over a wide range of temperatures. In the dielectric spectroscopic studies of the compounds, only one sharp peak in the loss curve is observed in the entire nematic range. In addition, the dielectric permittivity of the sample is very large. The molecular structures of these two compounds are similar to the compounds studied by Ghosh et al., which are reported to exhibit the ferroelectric-like switching and electro-convection pattern. However, the differences are (a) replacement of the polar halogen substituent at one end of the molecule by a methoxy group and (b) an increased length of the alkyl chain at the other end. These studies confirm the requirement of a polar moiety for the ferro-nematic phase to exist in these four-ring bent-core compounds.

Synthesis, mesomorphic, photophysical and computational studies of new achiral four-ring unsymmetrical bent-core mesogens and their Copper(II) complexes

New achiral four-ring unsymmetrical bent-core mesogens derived from 2,5-dihydroxybenzaldehyde and their copper(II) complexes have been synthesised as a new design with an imine and ester linkage. These new bent-core molecules resemble hockey-stick shape, which possesses 4-n-alkyloxy chain (4-n-hexyloxy and 4-n-decyloxy) at one end and methyl or methoxy group at the other end of the molecule. The synthesis, spectroscopic characterisation, phase transition temperature and characterisation of phase behaviour are reported. The bent-core molecules exhibited monotropic nematic and smectic A phase depending on the terminal chain length. Interestingly, copper(II) complexes of bent-core molecules displayed monotropic nematic phase. This is the first report on copper(II) complexes of bent-core molecules that exhibited nematic phase. The four-ring bent-core molecule exhibited fluorescence with large stoke shift. The density functional theory calculations of bent-core molecules and their copper(II) complexes are carried out using Gaussian 09 program at B3LYP level to obtain the stable molecular conformation, dipole moment, highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO) energies and bending angle of the compounds. The natural atomic charges and electronic configurations of the atoms of free ligands as well as the complexes have been evaluated.

Low-temperature nematic phase in asymmetrical 1,3,4-oxadiazole bent-core liquid crystals possessing lateral methoxy group

ABSTRACT Asymmetrical bent-core molecules based on 1,3,4-oxadiazole bent-core unit have been synthesised as a new design with a lateral methoxy group at outer phenyl ring of the molecule. These new asymmetrical bent-core molecules resemble hockey-stick shaped due to the presence of two different arms of different lengths. One arm of these molecules is elongated having two phenyl rings and possesses a 4-n-alkyloxy chain of a different number of carbon atoms (n = 4, 8, 12 and 18) and other arm is short and has one phenyl ring with fixed 4-n-octyloxy chain. The bent-core molecules possess a lateral polar methoxy group at the elongated arm of the molecule. These bent-core compounds exhibited fluorescence emission in the UV wavelength region (~377–386 nm) whereas in acetonitrile and dimethylformamide, solvent displays blue emission peak with a large stoke shift.The bent-core molecules with the number of carbon atoms (n = 4, 8 and 12) at the elongated arm exhibited monotropic nematic phase at low temperature, while the 4-n-octadecyloxy chain at the elongated arm displayed smectic A phase. Dielectric studies were performed in the nematic phase of the bent-core mesogens confirm the formation of the cybotactic cluster in the nematic mesophases. GRAPHICAL ABSTRACT

Design, synthesis and mesomorphic behaviour of a four-ring achiral bent-core liquid crystal in the nematic phase

Bent-core nematics have attracted growing interest because of the unconventional properties and extraordinary effects exhibited by these liquid crystalline phases. In this report we design, synthesize and characterize four different four-ring achiral bent-core liquid crystals containing a methyl group in the central phenyl ring. The mesomorphic behaviour of compounds has been investigated by optical polarizing microscopy, differential scanning calorimetry, electro-optical investigation, and dielectric spectroscopy. Calculations based on density functional theory of the molecules were also carried out. The addition of a methyl group in the central phenyl ring not only induces the lowering of the clearing point but also enhances the nematic phase range. Dielectric spectroscopy reveals a low frequency relaxation peak and a very large dielectric permittivity; which is the characteristic of cybotactic clusters. In electro-optic studies, spontaneous polarization peaks are not observed. However, a periodic pattern is observed under the application of an ac electric field. Finally, the compounds show a weak tendency to crystallize; which makes it possible to supercool the cybotactic nematic phase down to room temperature.

Mesomorphic and photophysical behaviour of 1,3,4-oxadiazole based hockey stick reactive mesogens

ABSTRACT New hockey stick mesogens derived from 1,3,4-oxadiazole as a bent-core unit have been synthesised. The molecules resemble hockey stick shape due to the presence of two arms containing a different number of phenyl rings attached with the 1,3,4-oxadiazole bending unit. The shorter arm of the molecule consists of one phenyl ring and 4-n-alkyloxy terminal chains whereas the long arm of the molecule possesses containing two phenyl rings which are linked via imine linkage and reactive 4-n-undecenyloxy as a terminal chain. The thermal stabilities of the newly synthesised compounds were carried out by thermogravimetric analysis (TGA). The mesomorphic behaviour was investigated by polarising optical microscopy (POM) and differential scanning calorimetry (DSC). All the compounds exhibit enantiotropic nematic phase along with smectic phases (SmA and SmC phases). Interestingly, the compounds with lower 4-n-alkyloxy terminal chains (n = 4 and 6) exhibit a wide range of optically isotropic DC phase. On increasing, the terminal 4-n-alkoxy chain length the DC phase disappears. The photophysical properties of the compounds were investigated in different solvents and in the solid state. It was observed that the compound exhibit absorption in UV region and emission in the green region. GRAPHICAL ABSTRACT