NEW CHIRAL THIOBENZOATE SERIES WITH ANTIFERROELECTRIC MESOPHASES

A new series of chiral homologous thiobenzoates with non-chiral end chains ranging from heptyloxy to octadecyloxy has been synthesized and characterized. The mesomorphic properties have been analysed by optical microscopy on pure compounds and mixtures, DSC, and electro-optical and pitch measurements. They display a very rich polymesomorphism including S-A, S-Ca*, S-C*, S-CFI*, S-CA* phases with reference to the compound MHPOBC through miscibility tests. Peculiarly noticeable are: (i) two S-CFI* phases sandwiched between S-C* and S-CA* phases in some compounds; (ii) full miscibility between the S-CA* phase and the S-O* phase of the racemic reference compound MHTAC which leads us to question whether the S-CA* or S-O* symbol should be used to designate the antiferroelectric smectic C* phase; (iii) odd-even cancellation of the S-CA* phase in relation to both the chiral chain and the alkyloxy chain; (iv) a reentrant sequence, C S-C S-CA S-C S-A I, in a pure compound assessed by a phase diagram. The electro-optical measurements were carried out with the classical SSFLC geometry. The field threshold is determined for each titled mesophase. One can note: (i) a low threshold whatever the phase; (ii) a first plateau at 0.4 V mu m(-1) and a saturation at 1 V mu m(-1) in the S-CFI* phase; (iii) strong pretransitional variations of the pitch at the S-CA*-S-CFI* and S-CFI*-S-C* transitions.