THE EFFECT OF LOW-MOLECULAR-WEIGHT ORGANOSILOXANE SUBSTITUENTS ON MESOPHASE FORMATION AND STRUCTURE IN NONSYMMETRICAL NICKEL(II) COMPLEXES

Organosiloxane groups have been used as components of metal-containing liquid crystals (metallomesogens). It has been found that the disilox ane unit aids mesophase formation, stabilising an enantiotropic smecti c A phase over a wide temperature range. The overall effect of an orga nodisiloxane unit is to lower the melting point relative to an alkyl s ubstituted complex while increasing the clearing temperature, or leavi ng it unchanged, and so increasing mesophase temperature range. A mode l for the mesophase is proposed based on thermo-optical polarized ligh t microscopy and small angle X-ray scattering studies. This proposes t hat weak nickel-nickel interactions, which are observed in the solid s tate, are maintained in the mesophase. The unusual steric shape of the organosiloxane group stabilizes the smectic A structure, allowing mor e efficient packing of the flexible chains. (C) 1998 Elsevier Science S.A.