Classification of the mesophase of di-isobutylsilanediol

Abstract

The thermotropic liquid crystal mesophase formed by di-isobutylsilanediol has been known for over twenty-five years, but its nature and structure have remained uncertain and the physical characteristics of the phase appeared unique and quite different from those of any other recognised type of liquid crystal. In a previous preliminary publication, it was suggested that the mesophase of di-isobutylsilanediol is in fact of the discotic type only very recently characterised in certain organic systems. Here we describe more fully our grounds for this suggestion. A model is proposed for the structure of this mesophase; this offers an explanation as to why other dialkylsilanediols do not behave similarly. A reinterpretation of earlier X-ray crystallographic data led us to suggest that the mesophase consists of hydrogen-bonded dimers. The model is supported by data from optical microscopy, thermal analysis and miscibility studies. The optical textures shown by the mesophase and those observed at the crystal mesophase boundary are discussed and compared with those of the discotic phases of the benzene hexa-n-alkanoates.