Orientation of Solute Molecules in Nematic Liquid Crystal Solvents

Abstract

The tendency of solute molecules to align with their longest dimension parallel to the optic axis of a nematic liquid crystal solvent can be explained by a simple model, based on dispersion force interactions, relating S-values obtained from nuclear magnetic resonance measurements, to molecular dimensions.Les molCcules de solute ont tendance i s'aligner suivant leur plus grande dimension parallelement a l'axe optique du solvant constituk de cristaux liquides nkmatiques. Ceci peut Ctre expliquk par un modele simple en invoquant des interactions de forces de dispersion, qui relient les valeurs de S obtenues des mesures de resonance magnetique nucltaire, aux dimensions moleculaires.Canadian Journal o f Chemistry, 49, 2345 (1971) Nematic liquid crystals provide a means of partially orienting solute molecules in nuclear magnetic resonance experiments ( 1 4 ) . The degree of solute alignment can be described either by the order matrix {S) due to Saupe (5), or in terms of the motional constants given by Snyder (6); the two approaches are simply related (6). The number of elements of the S-matrix required to describe the degree of orientation depends upon the symmetry of the molecule and can be minimized by a proper choice of axes. If the molecule has two perpendicular planes of symmetry, but a less than three-fold symmetry axis, two elements suffce. The S-value of a given axis lies between -0.5 and + 1.0 for alignment ranging between perpendicular and parallel to the applied magnetic field and, unless a high degree of parallel alignment occurs, i.e. S > 0.5, the sign of S is not immediately determined. I t is generally observed, however, that a solute molecule is preferentially oriented with the longest molecular dimension parallel to the optic axis of the liquid crystal.Saupe has shown that, except in the case of specific interactions such as hydrogen bonding, dispersion forces are primarily responsible for the alignment of solute molecules and permanent electric dipole moments are of minor importance. Using a statistical treatment, the interaction potential energy has been correlated with localized contributions from substituent bonds in chloro-and fluoro-substituted benzene derivatives (7,8). Since dispersion forces are involved, a dependence on polarizability anisotropies and thus on molecular size is not unexpected. In the present work, a simple treatment relating S-values to molecular dimensions is developed.The average potential energy of a solute molecule is, as a function of its orientation (8), where 8 and + are the usual polar angles relating the molecule-fixed 1 , 2 , 3 axial system to the optic axis of the solvent and a i is the potential energy of interaction along the ith molecular axis. It is assumed in the subsequent discussion that the longest molecular feature lies along the 2-direction with the shortest along the 3-direction. Equation 1 is conveniently rewritten as-(a2 -a,) sin2 8 sin2 + Saupe (7) has shown that according to classical Boltzmann statistics, the two S-values required for molecules ...

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.…”

The orientational behaviour of dichlorobenzenes in nematic liquid crystal solvents

“…The derivation of eq. [3] requires that B should be independent of solvent and of temperature, but, as was observed in the case of chlorobenzene (17) and for a number of other aromatic solutes (12,15,18), there is a strong solvent dependence of the parameter B (or its equivalent). Figure 2 also shows that there is a smaller variation with temperature and concentration, where the experimental value of B increases with increasing temperature in EBBA, but decreases in 1132.…”

“…Early attempts by Saupe and Nehring (1,2) to derive such expressions used a bond additive model for various fluoro-and chloro-benzene derivatives, which gave approximate agreement with the experimental results, but, due to the lack of experimental data at that time, this approach did not examine the effects of temperature, concentration, and solvent. Subsequent studies (3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18) have shown that the behaviour is more complicated, and the mean field approach, with a dependence on dispersion forces alone, is probably an oversimplification.…”

Nematic phase nuclear magnetic resonance studies of the structure and orientation of 1,5- and 2,6-naphthyridines

“…The usual orientation of a solute molecule is with the longest molecular dimension parallel to the liquid crystal axis but pyridine, for example, aligns with the C2 axis parallel to the solvent axis whereas benzene has no preferred orientation in the xy plane. Various parameters have been suggested as suitable descriptions of the anisotropy of orientation (18)(19)(20). Anderson has used the parameter (S,, -S,,)/S,, and correlated this function with the anisotropy of the principal moments of inertia (19).…”