Short-range correlations seen in the nematic phase of bent-core liquid crystals by dielectric and electro-optic studies

Abstract

Three bent-core nematic liquid crystals having the same core but with different terminal groups, short (C4) and long (C7,C9) tails, are investigated by dielectric and electro-optic contrast spectroscopic techniques. C4 shows sign reversal in the dielectric anisotropy as a function of both temperature and frequency, whereas C9 shows only negative in the entire mesophasic region. The behavior of C7 is intermediate of the two. Results of a dielectric study show that both C7 and C9 exhibit strong short-range polar correlations normal to the director. The correlation lengths of these interactions are found to be similar to those from the x-ray scattering. An increased hindered rotation for C9 compared to C4 moves the dielectric dispersion for to much lower frequencies, such that C9 shows only negative over the entire temperature range. The molecular structure of a liquid-crystalline compound has been found to have a significant influence on its properties and the phase behavior. The typical structure of a calamitic mesogen consists of a rigid core having a couple of phenyl rings and one or two flexible terminal chains. The anisotropic properties of the core moiety plays a key role in the formation of the orientational order of liquid-crystalline (LC) phases, while the terminal chains stabilize positional order, reduce the melting points and extend the phase ranges. The bent-core or banana-shaped mesogens give rise to special kind of nematic phases, in addition to the well-known B1 to B8 phases [1,2], and during the past few years the isotropic-to-nematic (I-N) transition temperature has been consistently lowered and the range of the nematic phase extended [3]. These materials in their nematic phase display unusual physical properties. Some of these are giant flexoelectricity [4], unprecedented electroconvection [5], and splitting in small-angle scattering patterns [6]. The splitting in small x-ray scattering has been interpreted either in terms of a presence of smectic-C (SmC)-like nanostructures [7][8][9] in the nematic phase or in terms of the form factor of the banana system [10,11]. A strong debate about the appropriateness of such an interpretation is continuing [12]. In this Rapid Communication, dielectric and electro-optic spectroscopic studies of the two representatives of cyanoresorcinols, 4-cyanoresorcinols (C4 and C9), specifically are compared. The results of C7, whose behavior is in between the two, are also given [13]. Interestingly we find that C9 exhibits only negative dielectric anisotropy whereas C4 shows a reversal in from positive to negative both with temperature and frequency. We later examine in detail the various causes for the observed sign reversal in given in the recent literature. In contrast, however, we find that the Maier-Meier model, which includes the anisotropic Kirkwood correlation factors [14], explains the observed behavior extremely well. The unusual dielectric behavior of C9 is interpreted in terms of the short-range * jvij@tcd.ie molecular associations along and normal to the d...

Effect of cybotactic clusters on the elastic and flexoelectric properties of bent-core liquid crystals belonging to the same homologous series

“…This means that both e and K depend on temperature similarly and both follow S 2 dependence, where S is the orientational order parameter. The conformational freedom of molecules is thus restricted and this agrees with the conclusions derived from dielectric studies reported by Jang et al [23]. The magnitude of the effective flexoelectric coefficient |(e 1 − e 3 )| is obtained using Eq.…”

“…Further studies by Jang et al [23] of this series of cyanoresorcinols revealed that the sign reversal of ε as a function of frequency was related to the relaxation of the longitudinal component of the dipole moment occurring at much lower frequencies compared to the transverse component. The temperature dependence of permittivity was found to be related to a change in the strong anisotropic correlations among the molecules taking place with temperature.…”

“…The values in the round brackets refer to the monotropic transitions observed on cooling. In general, number of carbon atoms on the left (m) of the core structure can be different than on the right (n), but here m = n. different relaxation dynamics for both ε and ε ⊥ are discussed in detail by Jang et al [23].…”

“…The temperature dependence of permittivity was found to be related to a change in the strong anisotropic correlations among the molecules taking place with temperature. They ruled out the possibility of conformational change (used to describe the sign reversal phenomena in calamitic molecules) affecting the value of ε by a change in the bent angle and the dipole moments [23]. ε values for bent-core systems in the nematic phase are unusually low for the reason that dipole moments along the bow and the long axes are comparable to each other.…”

“…Some of these include intrinsic biaxiality [19] and the optical biaxiality measured directly [20] and induced biaxiality [21], a reversal in the sign of the elastic and dielectric anisotropies [22][23][24][25], and the observed chiral domains of the opposite polarity in C5 [26] and C9 [26]. For a widely studied conventional calamitic such as 4-cyano-4 -pentylbiphenyl (5CB), K 33 is greater than K 11 and the ratio of K 33 /K 11 varies from 2 to 3 [27].…”