A shape model for molecular ordering in nematics

Abstract

An explicit form for the orientating potential acting in uniaxial liquid crystal phases is derived, by analogy with the surface anchoring potential which determines the orientation of macroscopic anisometric particles. The surface of a molecule is determined by describing the molecule as an assembly of van der Waals spheres. The model is successfully applied to predict the ordering tensors for a variety of systems, namely, solutions in nematic solvents of small and rigid probes, or relatively long n-alkanes with many degrees of internal freedom, and pure nematogens formed by aromatic cores attached to flexible chains

Guest–host systems containing anthraquinone dyes with multiple visible transitions giving positive and negative dichroic order parameters: an assessment of principal molecular axes and computational methods

“…We have shown that a novel combination of molecular dynamics (MD) simulations and time-dependent density functional theory (TD-DFT) calculations, based on the application of Equation (4), can provide a rationale for observed trends in order parameters from calculations of S θ and S β that are combined to give values of S ϕ that are directly comparable with experimental values of S. These methods have resulted in the successful prediction of order parameter trends for 1,5-and 2,6-disubstituted anthraquinones dyes and have demonstrated that the choice of principal molecular axis, which affects calculations of both S θ and S β , may have a significant influence on the calculated dichroic order parameter [32]. For the structures we have studied to date, we have found that an improved match with experimental trends was obtained by using the surface tensor z-axes, which are defined by the shape of the molecular surface [33], as the principal molecular axes rather than the more conventionally used minimum moment of inertia (MOI) axes.…”

A model-free temperature-dependent conformational study of n-pentane in nematic liquid crystals

“…3 Every laboratory appears to have its own favorite model. 3,[6][7][8][9][10][11][12][13][14][15][16][17][18] In practice, the predictions of the different models show a large degree of consistency. An earlier analysis of the NMR spectrum of orientationally ordered n-pentane utilized the fact that it is a magic solute and that size-and-shape models could provide a starting point for the spectral analysis.…”

Some Correlations Between Molecular and Cholesteric Handedness

“…Recently, a promising theoretical treatment was introduced by Ferrarini et al 22 which, in selected cases, leads to the effective calculation of the helical sense and pitch of the induced cholesteric phases. 23 Attempts to relate the cholesteric handedness of lyotropic cholesterics to the helical sense of the polymers were first reported by Sato and co-workers.…”

“…72 The recent activity of Ferrarini and co-workers at Padua University aims to set up theoretical methods capable of accounting for the behavior of liquid crystals by using a realistic picture of the chemical constituents in terms of molecular geometry, charge distribution, and polarizability. This theoretical approach 22 is based on a shape model accounting for the chirality and anisotropy (affecting the orientational capability) of the interaction between solute and solvent and has been shown to interpret the twisting power of several classes of compounds. 73 The handedness and the pitch of induced chiral nematics are determined by the coupling between the molecular helicity (which is different along the different molecular directions) and the orientational behavior of the dopant (which tends to impart a preferential alignment with respect to the nematic director).…”