Polymorphic smectic A phases in perfluoroalkylated mesogenic dimers

New homologous symmetric dimers are presented in which two-ring monomers ar e connected by a flexible spacer and perfluorinated chains are introduced i n terminal positions. The mesophase behaviour depends on the length and str ucture of the spacer and on the length of the terminal chains, The most int eresting finding is the occurrence of dimorphic SmA phases in the longer ch ain homologues with odd-numbered spacer. The transition between the dimorph ic SmA phases is accompanied by a relatively high enthalpy. Considering the results of XRD measurements and Computer simulations. structural models ar e proposed. based on different conformations of the molecules.