Orientational order of uniaxial nematic liquid crystals using electron spin resonance of solute molecules

We present here measurements of hyperfine splittings andg factors of two nitroxide spin probes dissolved inp-azoxyanisole, a wellknown uniaxial nematic liquid crystal, about one part to a million in their molecular ratio. The spin probe molecules used are 1) 17β-hydroxy-4′, 4′-dimethylspiro-/5α-androstane-3, 2′-oxazolidin/-3′-yloxyl, and 2) 2-(3-Carboxypropyl)-4, 4-dimethyl-2-tridecyl-3-oxazolidinyloxyl methyl ester; the former is known to have rigid and the latter flexible structures. These results are used to demonstrate the validity of our model that relates the orientational-order tensor\(\mathop S\limits^ \leftrightarrow\) of liquid crystals to the above quantities obtained by the electron spin resonance technique. The model is free of restrictions on the nature of the solute molecules, rigid or flexible. The relation between these directly measured quantities and the components of\(\mathop S\limits^ \leftrightarrow\) is established in two steps: first by relating them to\({\mathop S\limits^ \leftrightarrow}^{(p)}\), an order parameter tensor defined by a unit vector along the so-called long axis of solute molecules, and then by the use of\({\mathop S\limits^ \leftrightarrow}^{(p)} = k\mathop S\limits^ \leftrightarrow\), wherek is a scalar constant. This latter relation is obtained on the plausible arguments of solute-solvent and solvent-solvent interactions giving rise to\({\mathop S\limits^ \leftrightarrow}^{(p)}\) and\(\mathop S\limits^ \leftrightarrow\), respectively, andk representing their relative strengths. In spite of appreciable structural differences existing between these two solute molecules, very satisfactory agreements between the theory and experiment are observed for both. Such agreements are essentially independent of uncertainties in our knowledge of the parameters involved. Furthermore, we show that important molecular parameters can also be obtained from such measurements.