Occurrence of Reentrant Nematic and Reentrant Smectic A Phases in Some Liquid Crystal Series

Abstract

The first pure compound exhibiting at atmospheric pressure a stable reentrant nematic but also a reentrant smectic A phase was discovered a few months ago [1]. The nature of these phases was confirmed in particular by miscibility diagrams [2], magnetic measurements [2] and X-ray experiments [3]. From the squeletton of this previous molecule, the octyloxybenzoyloxy-cyanostilbene, we have synthetized numerous other compounds with three phenyl rings. A comparative study of these liquid crystals series (including also some materials synthesized in other laboratories) in which the molecular chemical structure is not randomly modified but keep some common features with the original compound has been undertaken. The properties of these rather similar substances can be very different since in some cases the reentrant phenomenon is ruled out. In this manner, we can define some essential characteristics for families apt to give reentrant phases at 1 atm:

• the cyano end group is essential-This seems to be the consequence of a strong dipole connected with relatively small dimensions.

• the electronic distribution along the molecule plays also an important role as schematized: (→sense of dipoles), the absence of donor group, a linkage which stops the electronic transfert throughout the core, a strong transverse dipole withdraws the reentrant phases.

• an alkoxy chain is more favourable to the reentrance than an alkyl chain for which no reentrant smectic A phase is observed.

• the comparison of the various series allows to classify the different Y linkages (with a constant X=C00 linkage) with regard to their ability to induce reentrant phases: CH=CH >; N=N, C≡C > CH=N, nothing > C00. The role of X=C00 linkage remains to be specified.