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

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 β, but different c for different numbers of thiophene rings: a=0.598nm, b=0.89nm, and β=98°; 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. Polymorphism is found, with coexistence of monoclinic and orthorhombic crystal structures.