Molecular ordering and motion within the smectic-Ephase:H2NMR study

Unusual deuterium NMR spectral patterns are reported for a magneto-aligned, smectic-$E$ sample consisting of a binary mixture of 4-$n$-butoxybenzylidene-4′-$n$-octylaniline-2, 3, 5, 6-$d_4$ ($4\mathrm{O}.8-d_4$) and 50 wt.% 4-$n$-octyloxycyanobiphenyl(8OCB). This spectral behavior is consistent with the para axis of the aromatic ring of 4O.8 being perfectly ordered ($S=0.99\mathrm{}\pm 0.01$) in the smectic-$E$ phase. The short axis of the ring is also ordered, however, not perfectly ordered, on the average, and undergoing $\pi$ flips about the para axis on a time scale ≲ ${10}^{-5\mathrm{}}$ sec. There are found to be two possible motional models for the orientational ordering of the short axis of the ring that are consistent with the data: (1) molecular diffusion about the herringbone lattice with angular jumps of 44° between lattice sites, or (2) on-site librational motion with an in-plane order parameter of 0.80 for the degree of order of the short axis. Upon transforming from the smectic-$B$ to the smectic-$E$ phase in this material the degree of order of the para axis of the ring is observed to change continuously.