On the Ordering Matrix for the Spin Probe (3-spiro [2′-N-oxyl-3′,3′-dimethyloxazolidine])-5α-cholestane, in the Nematic Mesophase of 4,4′-dimethoxyazoxybenzene

Abstract

Spin probes, such as (3-spiro[2′-N-oxyl-3′,3′-dimethyloxazolidine])-5α-cholestane (in Figure 1) have proved to be extremely valuable for studies of the long range orientational order and rates of molecular reorientation in a wide variety of liquid crystals. Their value stems, in part from their elongated form which results in a high alignment of the long molecular axis in an anisotropic environment. The extent of alignment is invariably described by a single order parameter associated with the long axis. This implies that the ordering matrix for the spin probe is cylindrically symmetric and such a description has considerable advantages for the theoretical analysis of both the spectral linewidths and the order parameter. However the spin probe does not possess a three-fold or higher symmetry axis and so the cylindrical symmetry of the ordering matrix must be established by direct measurement. There have been several investigations of 1 in lyotropic mesophases which conclude that the ordering matrix exhibits significant deviations from cylindrical symmetry.¹ This conclusion contrasts with the cylindrically symmetric ordering matrix found for I in a thermotropic liquid crystal.² We believe that this disagreement is caused by the use of different principal coordinate systems for the ordering matrix in these investigations, rather than the nature of the mesophase. In this Paper we present the results of a detailed investigation of I dissolved in the nematic mesophase of 4,4′-dimethoxyazoxybenzene and justify, with the aid of theory, our choice of principal coordinate system in which the ordering matrix is found to approximate closely to cylindrical symmetry.