Negative and Positive Anisotropic Thermal Expansions in a Hexagonally Packed Columnar Discotic Liquid Crystal Thin Film

The negative thermal expansion showing contraction of unit cell dimensions on heating is an unusual phenomenon which is rarely observed in organic materials. Here we report that a highly aligned hexagonal columnar liquid crystalline phase of discotic liquid crystal, cobalt octakis(n-decylthio)porphyrazine (CoS10), in thin films shows negative thermal expansion in the transverse plane and positive thermal expansion along the column axis. This exceptional thermal expansion behavior of CoS10 at the hexagonal columnar liquid crystalline phase was understood as the synergetic effect of the increased vibrational motion of n-alkyl side chains perpendicular to the chain axes and the increased tilting of the cores with temperature.