The influence of structure on the elastic, optical and dielectric properties of nematic phases formed from bent-core molecules

Abstract

The physical properties of the nematic phases formed by four bent-core oxadiazole based materials are reported. In particular, the splay (K11), twist (K22) and bend (K33) elastic constants, the birefringence and the dielectric anisotropy of the materials are described and the effect of chain length and the presence of fluoro-substituents at the outer phenylene group of the aromatic core structure on these parameters is determined. The birefringence and order parameter are found to be independent of the modification of molecular structure. The dielectric anisotropy is quite strongly dependent on molecular structure; the fluoro-substituted material has the largest magnitude of dielectric anisotropy while the alkyl-substituted compound has the smallest. Changes in the molecular length and fluoro-substitution in the bent-core materials are found to have little influence on the splay, twist and bend elastic constants at equivalent reduced temperatures. However, the material substituted with an alkyl terminal chain exhibits both smaller elastic constants and a less marked dependence on temperature than the alkoxy-substituted compounds. A possible insight into the behaviour of the elastic constants relevant to the formation of the dark conglomerate phase, which underlies the nematic phase in one of the compounds studied, is suggested by following the analysis proposed by Berreman and Meiboom. Importantly, using molecular field theory and atomistic modelling, we calculate elastic constants that are in excellent agreement with the experimental values. Our conclusion that the elasticity in the nematic phase formed from bent-core molecules is not strongly influenced by changes to the terminal chains or the presence of fluoro- substituents at the outer phenylene group of the aromatic core structure is in agreement with our previous work showing that the dominant parameter is the bend angle

Unusual electric-field-induced transformations in the dark conglomerate phase of a bent-core liquid crystal

“…It is likely that the formation of long range order and removal of the sponge-like structure in some materials and the absence of such a feature in others is related to the elastic constants of that mesogen. A study of the nature of the elastic constants in the nematic phase of our material has shown that this mesogen possesses relatively low splay, twist and bend elastic constants (K 11 = 8 pN, K 22 = 1 pN, K 33 = 3 pN close to N-DC phase transition) [33,34]. The standard elastic approach predicts that the saddle-splay elastic constant K 24 ≤ 2K 11 + K 22 or K 24 < K 22 whichever is smaller [39][40][41].…”

“…Also, the nematic phase shows low splay (K 11 ), twist (K 22 ) and bend (K 33 ) elastic constants compared to other members in the homologues series [33,34]. It has low twist and bend elastic constants compared to calamitics [34].…”

“…In the whole nematic and DC phase temperature range, the material showed six Raman peaks positioned at 1167 cm -1 (phenyl C-H in-plane deformation), 1422 cm -1 (C-C stretching in aromatic rings), 1496 cm -1 (symmetric stretching of alkyl units), 1558 cm -1 (C-C stretching in aromatic rings) 1608 cm -1 (C-C stretching in aromatic rings) and 1736 cm -1 (C=O stretching) ( Figure S2a) (More details on the Raman spectroscopy of this material in the nematic phase are given in Ref. 34) . No shift in the peak position was observed over the entire mesophase temperature range.…”

“…Several studies indicate that there are two distinctive (possibly uniaxial and biaxial) regimes in the nematic phase [30][31][32]. Also, the nematic phase shows low splay (K 11 ), twist (K 22 ) and bend (K 33 ) elastic constants compared to other members in the homologues series [33,34]. It has low twist and bend elastic constants compared to calamitics [34].…”

Optical Simulation of Viewing Angle Property of Biaxial Nematic Bent-Core Liquid Crystal

“…(4). We referred to the general refractive indices nx and ny in the previous literatures for the Nb-LCD simulation [20,21]. The refractive indices of the VA-and the IPS-LCD were set to be ne = 1.70 and nz = 1.50.…”