Polymorphism of terthio-phene with surface confinement.

The origin of unknown polymorphic phases within thin films is still not well understood. This work reports on crystals of the molecule terthio-phene which were grown by thermal gradient crystallization using glass-plate substrates. The crystalline domains displayed a plate-like morphology with an extended lateral size of about 100 µm, but a thickness of only a few µm. Specular X-ray diffraction patterns confirmed the presence of a new polymorph of terthio-phene. Crystal structure solution from a single crystal peeled from the film revealed a structure with an extremely large unit-cell volume containing 42 independent molecules. In contrast to the previously determined crystal structure of terthio-phene, a herringbone packing motif was observed where the terminal ends of the molecules are arranged within one plane ( the molecular packing conforms to the flat substrate surface). This type of molecular packing is obtained by 180° flipped molecules combined with partially random (disordered) occupation. A densely packed interface between terthio-phene crystallites and the substrate surface is obtained, this confirms that the new packing motif has adapted to the flat substrate surface.