Structural studies on thin films of an unsubstituted oligo(para-phenylenevinylene)

Typically, thin films of the conjugated oligomer p-bis[(p-styryl)styryl]benzene (P5V4) prepared by vacuum deposition on substrates like silicon wafers and quartz plates are polycrystalline. Atomic force and electron microscopy reveal elongated clusters with characteristic dimensions of 20x80 nm2. X-ray and electron diffraction (ED) measurements indicate a close correspondence between the crystal structure of these micro-crystallites and the reported structure of single crystals grown from solution. However, we argue on the basis of ED measurements that P5V4 micro-crystallites found in thin films vapor-deposited on amorphous carbon substrates adopt a morphology that differs from the single-crystal structure. In the proposed polymorph, the molecular axis is nearly perpendicular to the substrate plane, though the molecular arrangement is otherwise very similar to that in the single crystal. We suggest that several polymorphs may coexist in the thin films