Liquid crystalline dendrimer of the fifth generation: From lamellar to columnar structure in thermotropic mesophases

The structure of a liquid crystalline (LC) carbosilane dendrimer of the fifth generation bearing 128 terminal cyanobiphenyl mesogenic groups has been studied. This dendrimer was synthesized by a hydrosilylation reaction and then the cyanobiphenyl mesogenic groups were chemically linked to the dendritic matrix via a -OOC-(CH2)(10)-Si(CH3)(2)OSi(CH3)(2)- spacer. Structural studies carried out by polarizing optical microscopy, differential scanning calorimetry and X-ray diffraction methods revealed unusual phase behaviour. Ar room temperature the dendrimer forms a lamellar (smectic A) phase which develops in-plane ordering above 40 degrees C. This is due to a tendency to form columns of molecules which are probably perpendicular to the layers. Above 121 degrees C the material transforms into another more disordered mesophase which is probably a disordered hexagonal columnar phase. The proposed structures and molecular packing in these different types of mesophase are discussed.