New chiral thiobenzoate series with antiferroelectric mesophases

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 S_A, 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 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 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.