A polymorph crystal structure of hexaphenyl observed in thin films

Highly oriented thin films of hexaphenyl - which are used in organic opto-e lectronic applications - are characterised in terms of their crystal struct ures. Two different crystal structures of hexaphenyl (C36H26) are observed when the films are prepared by physical vapour deposition at various substr ate temperatures. If the substrate is kept at room temperature, hexaphenyl crystallises within a structure which is already known from single crystal investigations. However, when the thin films are grown at a substrate tempe rature of 160 degreesC a new crystalline phase appears. This structure was characterised by Xray and transmission electron diffraction. Due to the str ong preferred orientation of the crystallites within the thin films, the la ttice constants as well as main features of the new crystal structure could be determined. The lattice is indexed as monoclinic with: a = 7.98 Angstro m, b = 5.54 Angstrom, c = 27.64 Angstrom, and beta = 99.8 degrees. The new crystal structure has high similarity to the already known crystal structur e: Both structures are built by layers of hexaphenyl molecules, within one layer the aromatic planes of the hexaphenyl molecules are packed in a herri ngbone pattern. The characteristic feature of the new structure is that the long axes of the hexaphenyl molecules are arranged absolutely perpendicula r to the layers, whereas, within the already known structure the long axes show an tilt angle of 17 degrees to the layer normal direction.