Classification of crystal structures of thiophene-containing organic semiconductors

Oligothiophenes and thienoacenes are essential components of organic semiconductors and usually form herringbone structures with dihedral angles of theta = 50-60 degrees. However, when more than three thiophene rings are fused, a stacking structure with theta = 125-130 degrees appears. Since the molecules are located on a lattice point, the lattice constants as well as the intermolecular geometry are obtained by a simple relation of theta and the molecular size. Stacking structures are preferred when the peripheral hydrogen atoms are lost or when polar oxygen atoms are included. Coronene and ovalene with more than a three-ring width form a stacking structure called the gamma-structure with theta = 90 degrees, and some molecules form pitched pi-stacking with a nonparallel terminal contact, where the intermolecular geometry is obtained by the same relation as the herringbone structures. For the molecular rotation of the gamma-structure within the molecular plane, the nonparallel contact is usually formed using the molecular zigzag edge. Crystal structures of oligothiophenes and thienoacenes are classified based on the geometrical relations between the nonparallel molecules.