Influence of the molecular length on the occurrence of incommensurate low ordered smectic phases for non-symmetric dimesogenic compounds

Systematic physical chemistry studies are in progress concerning the occurr ence of incommensurate low ordered smectic phases (S-ic)dagger in non-symme tric dimesogens with molecular parameters varying from those of the standar d compound KI-5(4). In the present study, the selected molecules possess th e same cholesteryl unit and the same spacer length, but the terminal chain of the aromatic mesogenic part is varied. Up to a critical molecular length , a competition between two incommensurate lengths is observed and the dime sogenic compounds respond to the frustration connected to this competition with the formation either of an incommensurate phase S-ic or of a two dimen sional modulated phase. For longer dimesogens, a commensurate lock-in of th e two characteristic lengths systematically occurs.