Tetrahedratic mesophases, ambidextrous chiral domains and helical superstructures produced by achiral 1,1′-disubstituted ferrocene derivatives

A series of symmetrically and asymmetrically 1,1′-disubstituted mesogenic ferrocene derivatives exhibiting nonconventional lamellar and nematic phases has been synthesized. Liquid-crystal properties of these compounds were investigated by polarizing optical microscopy (POM), thermal analysis (DSC) and X-ray scattering methods (XRD). XRD patterns and optical textures of the mesophases have a complex appearance, standing apart from the signatures of previously classified liquid crystals. The most important features of the new mesophases are their spontaneous separation into large macroscopic chiral domains of opposite handednesses, intricate helical and myelinic supramolecular structures and a few signs of possible optical biaxiality. A reasonable explanation of these findings can be drawn from the tetrahedral liquid crystal order. Free rotation of the cyclopentadiene rings in the ferrocene moiety around their normal axis makes viable bent conformations of the rod-like molecules and further stabilization of the conformational enantiomers through assembly into tetrahedral molecular associates. Thus, fundamentally new types of mesophases arise from such a molecular arrangement. In addition, a nontrivial case of mixed packing of bent and nonbent molecules into columnar mesophases was detected in one of the symmetrically substituted ferrocene derivatives. Basic relationships between mesomorphic properties, supramolecular organization and chemical structures of the ferrocenomesogens have been outlined.