Crystal structure and polymorphism of dimethyl-oligothiophenes crystallized epitaxially on highly oriented PTFE thin films

Summary form only given. The crystal structure of dimethyl- oligothiophenes grown by epitaxial crystallization on highly oriented PTFE thin film has been studied by transmission electron microscopy. The dimethyl-oligothiophenes crystallize principally in a monoclinic phase with common lattice parameters a,b, and /spl beta/, but different c for different numbers of thiophene rings: a=0.598nm, b=0.789nm, and /spl beta/=98/spl deg/; and c=1.866nm for tetramer, c=2.234nm for the pentamer and c=2.596nm for the hexamer. The space group is C2/m. The calculated crystallographic densities are 1.36g/cm/sup 3/, 1.39g/cm/sup 3/ and 1.40g/cm/sup 3/ for the tetramer, pentamer and hexamer respectively. Polymorphism is found, with coexistence of monoclinic and orthorhombic crystal structures. The dominant phase, however, is monoclinic. The relationship between the different phases is discussed.