New thermotropic mesomorphic compounds containing a [2.2]paracyclophane (PC) unit were synthesized and investigated (Scheme). Carboxylic acids were selected as the starting PC building blocks. The influence of structural features on the stability of the mesomorphic phases was studied (Figs. 1 and 2): for this purpose, the structures of the PC-carboxylate unit and the organic fragment of the aryl-ester moiety were varied systematically. Esters derived from PC-monocarboxylic acid did not exhibit liquid-crystalline (LC) properties, while diaryl PC-dicarboxylates favored mesomorphism. Dicarboxylate substituents arranged in the paraposition provided LC phases in a broad temperature range and considerably increased the mesomorphic interval in comparison with that of the structurally related pseudo-para PC derivatives.Introduction. ± Unique physical properties, providing wide application in electrooptic devices and particularly the recent discovery of new mesophases, such as antiferroelectric, ferrielectric, and twist-grain boundary phases, have put chiral liquidcrystalline (LC) materials in the centre of liquid-crystal research [1] [2]. Historically, up to now, most chiral LC compounds contain molecules incorporating a centre of chirality. More recently, compounds with planar and axial chirality are beginning to attract the growing interest of researchers dealing with LC materials. The unique geometry of this type of compounds provides a rigid interaction between the origin of the symmetry-breaking chirality and the mesogenic unit, thus resulting in a definite fixation of the source of chirality in a LC matrix.In principle, planar chirality could drastically change the physical properties of the resultant LC material: 1) In terms of molecular structure, a rigid steric coupling between the transverse dipole, chiral centre, and the mesogen rod should maximize the local ferroelectric polarization [3], allowing consideration of the above compounds as possible chiral dopants for ferroelectric compositions, which are able to confer a high value of spontaneous polarization (P s ) to the achiral host [4]. 2) Electro-optical effects exhibited by LC materials such as electroclinic effects or nonlinear optical (NLO) properties are expected to be larger for compounds with rigid interaction between the chiral and optical portions of the molecule [5].However, the bulkiness of the structural units necessary to realize an axis or a plane of chirality significantly disturbs the desired rigid rod-like shape of the molecule, thus creating a serious obstacle to achieving stable mesomorphic properties. So far, only a
