Study of phase transitions in 4-n-alkoxybenzilidene-4′-n-alkylanilines by one and two dimensional13C NMR

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Abstract
The phase transition of a series of homologous liquid-crystalline compounds, nO.m (4-n-alkoxybenzilidene-4′-n-alkylanilines), from the nematic phase to the smectic A phase has been studied by 13C NMR. The order parameters, determined by a two dimensional technique called separated local field spectroscopy combined with off-magic angle spinning, of different molecular segments of these compounds are related linearly to the 13C chemical shifts. Changes in the order parameters of the phenyl rings as well as those of the chains during the SA–N transition depend on the nature of the phase transition. These changes are quantitatively related to the McMillan ratio, which is defined as the ratio between the SA–N transition temperature (T SAN) and the nematic to isotropic transition temperature (T NI), i.e. M = T SAN/T NI. The SA–N transition is first order for M > M TCP, and second order for M < M TCP, where TCP is the tricritical point. The value of M TCP was found to be 0·958 ± 0·004, in excellent agreement with that obtained from spin probe studies (0·959 ± 0·005) reported by Freed and co-workers [1].