Optical, dielectric and visco-elastic properties of a few hockey stick-shaped liquid crystals with a lateral methyl group

Abstract

Measurements of birefringence (Dn), dielectric permittivity (3 k , 3 t ), elastic moduli (K 11 , K 33 ) and rotational viscosity (g 1 ) have been carried out on the nematic (N) phase of a few hockey stick-shaped compounds 4-(3n-alkyloxy-2-methyl-phenyliminomethyl)phenyl 4-n-alkyloxycinnamates with a lateral methyl group inserted between the m-alkyloxy chain and the azomethine connecting group. Interestingly, a dual characteristic (i.e. partially calamitic like and partially bent-core like) is revealed in the N phase of these compounds. The SmC a -N transition is found to be of first order in nature while the SmC s -N transition is either second order or weakly first order. All the mesogens exhibit a temperature dependent inversion in the static dielectric anisotropy (D3 ¼ 3 k À 3 t ) from positive to negative values on entering the SmC a phase. Remarkably in the entire nematic range, the bend elastic modulus (K 33 ) is substantially lower than the corresponding splay modulus (K 11 ). The rotational viscosity coefficient (g 1 ) as obtained by extracting the relaxation time (s 0 ) values from two precise, independent probing methods, viz. the capacitive decay technique and the optical phase-decay-time measurement method, are slightly higher than those of many known calamitic systems. Moreover, the activation energy (E a ) calculated from the viscosity data is found to be considerably higher in the nematic phase than those obtained for conventional calamitics. The observed behaviours are accounted for by considering the intriguing shape-determined inter-molecular interactions and molecular associations appearing in the mesophase.

From the Molecular Structure to Spectroscopic and Material Properties: Computational Investigation of a Bent‐Core Nematic Liquid Crystal

“…Bent-core mesogens were predicted to promote spontaneous bend of the nematic director, [68] and lowering of the bend elastic constant has been evidenced upon doping conventional nematics with bent-core mesogens. [69][70][71] Very recently, the temperature dependence of the elastic constants was measured for the nematic phase of a number of bent-shaped mesogens, [18][19][20][21][22][23][72][73][74] with results similar to those for A131. In ref.…”

“…[17]. [69][70][71] Very recently, the temperature dependence of the elastic constants was measured for the nematic phase of a number of bent-shaped mesogens, [18][19][20][21][22][23][72][73][74] with results similar to those for A131. For the bend-elastic constant, theoretical predictions are in good agreement with the experimental trend in the temperature region far from the smectic C transition: K 3 < K 1 is found, with K 3 scarcely dependent on temperature.…”

Negative Birefringence in the Higher Homologs of the 5O.m Series of Liquid Crystals

“…The schematic diagram of the thin-prism setup is shown in Figure More details of this method are reported in the literature. ,,− ,− …”

“…Refractive indices of LC samples are used to determine the birefringence of the LC samples. Various theoretical models and experimental techniques have established the temperature dependence of the refractive indices of LCs. − …”

Chiral random grain boundary phase of achiral hockey-stick liquid crystals

“…[21][22][23][24][25][26][27][28][29][30] These molecules typically comprise a central core to which arms of substantially different length are attached. Hockey-stick mesogens exhibit several exotic phases and effects associated with their asymmetric shape, including the dual-characteristic nematic phase (which displays both calamitic and bent-core properties), 31 the occurrence of two polymorphic, tilted smectic phases (the SmC S and SmC A phases) induced by a molecular conformation change, [32][33][34] the coexistence of two different molecular tilts at the nematic-SmC transition, 35 the macroscopically chiral nematic and dark conglomerate phases, 36 the frustrated blue and twist grain boundary phase induced by chiral, rod-like mesogens, 37 and a layer thinning transition. 38 In this paper, we report a new hockey-stick mesogen (COBOXD), an achiral molecule with a exible alkyl chain attached to one end of an asymmetric, rigid, bent core.…”