Temperature dependence of the Maier–Saupe order matrix from 2H–NMR of fluorene-d1 in the nematic, smectic A, B, and C phases

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Abstract
From 2H–NMR of perdeuterated fluorene (diphenylenemethane‐d10) dissolved in 4‐n‐butyloxybenzylidene‐4‐n‐octylaniline (4O.8) and in a mixture of 4O.8 with 4‐4′‐bis‐n‐heptyloxyazoxybenzene (HOAB, 15.1 wt. %) we have measured each of the orientational order parameters of the Maier–Saupe order matrix and their temperature dependence in the nematic, smectic A, smectic B, and smectic C phases, respectively. The fluorene‐d10 molecule, being a rigid structure of known conformation and one which contains a sufficient number of differently oriented deuterated sites, allows for the complete determination of the order matrix in terms of the measured 2H–NMR spectral splittings. The results show: (1) The principal molecular z axis (most ordered axis) is parallel to a direction which joins the centers of the six membered rings at all temperatures and in each phase studied; (2) nonlinearities are present in the temperature dependence of the ratios of the splittings and are a result of the temperature dependence of the elements of the order matrix, Szz and Sxx–Syy, demonstrating that a change in molecular conformation is not necessarily the cause of such nonlinearities as suggested in the literature; (3) splitting ratio vs ratio plots are linear throughout all phases of 4O.8 and 4O.8+HOAB mixtures; (4) the degree of ordering of the solute (fluorene) is significantly reduced in the smectic phases, in confirmation of that suggested by other solute studies reported in the literature.