STUDY OF PHASE-TRANSITIONS IN 4-N-ALKOXYBENZILIDENE-4'-N-ALKYLANILINES BY ONE-DIMENSIONAL AND 2-DIMENSIONAL C-13 NMR

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 C-13 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 C-13 chemical shifts. Changes in the order parameters of the phenyl rings as well as those of the chains during the S(A)-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 S(A)-N transition temperature (T(S)A(N)) and the nematic to isotropic transition temperature (T(NI)), i.e. M = T(S)A(N)/T(NI). The S(A)-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].