Viscoelasticity, dielectric anisotropy, and birefringence in the nematic phase of three four-ring bent-core liquid crystals with an L-shaped molecular frame

Abstract

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 K 1 and bend K 3 elastic constants, splay viscosity h splay and flow viscosities h || and h t . The bend-splay anisotropy dK 31 ¼ K 3 À K 1 is negative, similar to the case of nematic LCs formed by symmetrically bent molecules of V-shape. The dielectric anisotropy D3 and birefringence are positive in the entire nematic range. The splay viscosity h splay and the flow viscosities h || and h t are smaller than the viscosities measured for the symmetric V-shaped bent-core materials at similar temperatures. The ratio G ¼ h splay /h ||,t 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 dK 31 , 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.

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

“…[15,18,19,22,23,74] and calculations have shown that it can be ascribed to the bent molecular shape. The most relevant feature is that K 3 is smaller than K 1 and remains almost constant over a wide temperature range.…”

“…[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.…”

“…[1] Initially, they attracted attention mainly because of their polar and chiral smectic phases, [2][3][4] but more recently it became clear that also their nematic phase has distinctive features, different from those of conventional rod-like systems (see ref. Most of these features remain the object of debate; they include phase biaxiality, [6][7][8][9][10][11][12][13] giant flexoelectricity, [14] unusually large viscosity, [15,16] and small twist-and bend-elastic constants, [15,[17][18][19][20][21][22][23] as well as nonstandard dielectric relaxation, [24] and electroconvection regimes. Most of these features remain the object of debate; they include phase biaxiality, [6][7][8][9][10][11][12][13] giant flexoelectricity, [14] unusually large viscosity, [15,16] and small twist-and bend-elastic constants, [15,[17][18][19][20][21][22][23] as well as nonstandard dielectric relaxation, [24] and electroconvection regimes.…”

Unusual temperature dependence of the splay elastic constant of a rodlike nematic liquid crystal doped with a highly kinked bent-core molecule

“…Although γ was increased by mixing BLC in most of the previous literature [34], our result showed a reduction of γ after doping BLC. Avci et al recently reported that BLC with an asymmetric shape could show relatively low viscosity [35]. Thus, γ is certainly deeply related to the structure of BLC.…”

Unusual polarity-dependent patterns in a bent-core nematic liquid crystal under low-frequency ac field

“…BCNs usually exhibit low-frequency dielectric relaxation and have an in verted, K 33/K ]] < 1, elastic ratio; their viscosities can be three orders of magnitude higher compared to rodlike compounds [26,[28][29][30][31]. These special characteristics of typical BCNs are attributed to their bent molecular shape, the resulting preference for a polar ordering and thus the presence of cybotactic smectic clusters [32][33][34][35][36].…”