Effects of Order on Spectral Properties of Discotic Liquid Crystal Heptyloxydibenzopyrene

The spectral properties of discotic heptyloxydibenzopyrene were studied by UV-VIS absorption, fluorescence, and infrared spectroscopy as a function of temperature in two different types of spectroscopic cells. The molecule was found to be a sensitive spectroscopic probe of molecular orientation on a surface. Capillary-filled cells caused the liquid crystal to assume a parallel orientation with respect to the surface, whereas spin-coating caused the orientation of columns to be statistically more nonuniform. It was shown from the spectroscopic studies that the molecular orientation changed on going from crystal to liquid crystalline state and a conclusion was drawn that bulk molecular alignment of columnar phase was defined by an interface imposed by an external surface.