H-2 NMR-SPECTRA OF 2-(P-HEPTYLBENZOYLOXY)-5-(P-HEPTYLBENZENAZO)TROPONE - A NEW SIGMATROPIC LIQUID-CRYSTAL

Liquid crystals incorporating in their molecular framework a seven-membered ring are still relatively rare [1]. Recently the synthesis and thermal behaviour of a series of liquid crystal materials having a tropone moiety in their mesogenic core have been reported [2-4]. These mesogens, based on a 2-(acyloxy)tropone core structure, show intramolecular migration of the acyl substituents between the two oxygen atoms at C-1 and C-2, an effect already known for simple 2-(acyloxy)tropones in their isotropic solutions [5]. This migration involves a concerted [1,9]-sigmatropic rearrangement [2]. This rearrangement could play a major role in determining the properties of the mesophases: it has been suggested in fact that, because of this rearrangement, the mesogenic molecules acquire a mean rod-like shape which can sustain the mesophase formation [2]. In order to study the influence of the acyl migration on the liquid crystalline properties of the tropone class of liquid crystals, specifically from the point of view of the molecular orientational ordering, we have synthesized both 2-(p-heptylbenzoyloxy)-5-(p-heptylbenzenazo)tropone (1) and its partially deuteriated analogue 2: