Molecular ordering in bis(phenylenyl)bithiophenes

The crystal structure of phenylene-bithiophene oligomers (PTTP) has been investigated by powder X-ray diffraction measurements and ab initio Monte Carlo modelling. Two new hydroxyl-terminated PTTP powders, 5,5'-bis-4-(6-hydroxyhexyloxy)-phenyl-2,2'-bithiophene and the shorter alkyl chain substituted oligomer 5,5'-bis(4-hydroxyphenyl)-2,2'-bithiophene, are compared to single crystals of 5,5'-bis(4-ethylphenyl)-2,2'-bithiophene. The new molecules are characterized by a close packed, distorted hexagonal array with tilted short axes in a herringbone-type structure. Neighbouring molecules within layers are displaced along the long axis. Modelling the electrostatic potential surface suggests that electrostatic interactions are responsible for the observed herringbone-like ordering. Alkyl chains facilitate a partial mixing through interdigitation of aromatic and aliphatic moieties.