INPLANE ANISOTROPY AND PHASE-CHANGE IN LANGMUIR-BLODGETT-FILMS OF DISK-LIKE MOLECULES

The ''in-plane anisotropy'' of triphenylene derivatives in Langmuir-Blodgett films has been investigated using infrared linear dichroism, X-ray experiments, and Brewster angle microscopy. Upon heating, one of these compounds exhibits a reversible phase transition appearing as an abrupt change from a low to a high in-plane anisotropy. It likely corresponds to the formation of the same hexagonal liquid crystal phase observed in the bulk material at almost the same temperature, with the columnar axis aligned along the dipping direction. The origin of the initial in-plane anisotropy observed in grown films is discussed with respect to the molecular structure of the low molar mass and polymeric compounds.