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 (i.e. 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.
Keywords: molecular crystals; single-crystalline films; specular X-ray diffraction; surface-mediated polymorphism; thermal gradient crystallization.