S-C*A and S((C)over-tilde*) phases in chiral non-symmetric dimesogens

We have synthesized two separate homologous series of chiral non-symmetric dimesogens: these have a two phenyl ring moiety bearing a terminal alkoxy chain linked via a methylenic spacer to a chiral three phenyl ring mesogen which itself displays antiferroelectric properties. Depending on the parity of the number of carbons constituting the spacer, two kinds of mesomorphic behaviour have been found. When the spacer is even, various S-C* phases (S-C*(A), S-C*(FI), S-C*, S-C*(alpha)) have been identified without doubt, whereas when the spacer is odd, a tilted modulated phase (S (C) over bar*) has been observed and characterized. In each case, we study the effect of the length of the alkoxy chain at the end of the shorter moiety. The alternation of the mesomorphic behaviour with the spacer parity has been interpreted in terms of molecular shape, but we have also discussed the influence of some chemical parameters on the stability of the S-C*(A) phase.