Mesomorphism of hybrid siloxane-triphenylene star-shaped oligomers

A. ZELCER; B. DONNIO; F. D. CUKIERNIK; D. GUILLON

Keystone, Colorado, USA

Congreso; 21ST INTERNATIONAL LIQUID CRYSTALS CONFERENCE; 2006

International Liquid Crystals Sciety

Triphenylene-based discotic liquid crystals are seen as promising organic semiconductors for applications in the domain of molecular electronics, optoelectronics, photoconductivity, photovoltaic, and electro-luminescent devices. The freeze-in of the columnar order into stable, room temperature glasses appear to be an attractive strategy for increasing the extent of ordering and improving the performances of the charge mobility in this kind of materials. Hybrid siloxane-containing liquid crystals form an interesting class of mesogenic materials since they combine properties of mesomorphic materials with those of their poly(dimethyl)siloxane polymer analogues, thus maintaining the liquid crystalline property but considerably reducing the transition temperatures. In this context we were interested in designing and synthesising new hybrid discotic oligomeric materials and in the investigation of their thermal behaviour. We report the synthesis and the liquid-crystalline properties of hexa(alkoxy)-substituted triphenylenes containing either terminal double bonds or terminal disiloxane groups. In order to obtain stabilized columnar mesophases where lateral slippage of molecules from one column to the adjacent one is strongly prevented, larger molecules including disiloxane-bridged dimer, isomeric star-like tetramers and a star-like heptamer have been prepared and studied by polarized optical microscopy, differential scanning calorimetry, and variable temperature X-ray diffraction. All of the studied compounds showed hexagonal columnar phases, most of them being ordered. The mesophase structure of the star-like heptamer shows a superlattice, induced by steric hindrance between siloxane groups. The use of a tri-block architecture based on triphenylene aromatic cores, and alkane and siloxane chains, together with the typical characteristics of liquid crystalline oligomers gave rise to improved thermal behavior: lower melting points and glass transition temperatures, higher isotropization points and little tendency to crystallize. The thermal behavior of these compounds is strongly dependent on the geometry in the case of the crystalline to columnar transition, but little difference has been found for the columnar to isotropic transition. One compound could be oriented by simply rolling a sample between two glasses. Punctual beam XRD patterns show two perpendicular sets of signals corresponding to the 10 reflection of the hexagonal lattice and to the π-π stacking