[en] 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

[en] In the context of luminescence from the solid state of poly(para-phenylene vinylene)-type materials, we report a study of three octyloxy-substituted five-ring oligo(para-phenylene vinylene)s, two of which bear cyano substituents on the innermost vinylene bonds. For each compound, the molecular arrangement in the single crystal has been derived from a crystallographic analysis. For the compound which has the cyano group not directly adjacent to the central ring, synthesis and structure are described here in detail. This compound, in contrast to the other two, has molecular π-stacking in the crystal lattice. Only for this compound do we observe a strongly red-shifted, long-lived emission, which is therefore ascribed to an excimer-type intermolecular excitation. The observations of excimer-type emission from cyano-substituted alkoxy-PPV's and in related copolymers fit in very well with the molecular picture derived from this study of model compounds. (Copyright (c) 1999 Elsevier Science B.V., Amsterdam. All rights reserved.)