NEW CHIRAL THIOBENZOATE SERIES WITH ANTIFERROELECTRIC MESOPHASES

A new series of chiral homologous thiobenzoates with non-chiral end ch ains ranging from heptyloxy to octadecyloxy has been synthesized and c haracterized. The mesomorphic properties have been analysed by optical microscopy on pure compounds and mixtures, DSC, and electro-optical a nd pitch measurements. They display a very rich polymesomorphism inclu ding S-A, S-Ca, S-C*, S-CFI*, S-CA* phases with reference to the comp ound MHPOBC through miscibility tests. Peculiarly noticeable are: (i) two S-CFI phases sandwiched between S-C* and S-CA* phases in some com pounds; (ii) full miscibility between the S-CA phase and the S-O* pha se 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 antif erroelectric 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 ass essed by a phase diagram. The electro-optical measurements were carrie d out with the classical SSFLC geometry. The field threshold is determ ined for each titled mesophase. One can note: (i) a low threshold what ever the phase; (ii) a first plateau at 0.4 V mu m(-1) and a saturatio n at 1 V mu m(-1) in the S-CFI phase; (iii) strong pretransitional va riations of the pitch at the S-CA-S-CFI* and S-CFI*-S-C* transitions.