H2-NMR investigation of the two-dimensional smectic F and I phases

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Abstract
The smectic F and I phases have been reported to be quasi-two dimensional systems. The SF phase was shown to be the first tilted version of the hexatic phase whereas the SI phase was shown to exhibit a 2D solid-like order. We have studied the terephtal-bis (4-n decylaniline) compound (TBDA) in which the SF phase surprisingly occurs at lower temperatures than the SI phase with a higher degree of positional order. To further investigate this puzzling problem we have studied the molecular behaviour by means of H2-NMR experiments. Two deuterated samples of TBDA have been prepared so as to allow separate investigations of the aromatic core and end-chain orderings. Concerning the aromatic part the data have been interpreted in a simple model based on the reorientations of the phenyl rings about their para-axis. This analysis suggests a possible scenario for the intramolecular disordering and its influence on the intermolecular ordering through the smectic F and I phases