New chiral thiobenzoate series with antiferroelectric mesophases

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Abstract
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 SA, S∗Cα , 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 SC SCA SC SA 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 μm−1 and a saturation at 1 V μm−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.