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Keywords:

  • organic thin films;
  • scanning tunneling microscopy;
  • NEXAFS;
  • functional group;
  • template-induced growth

Abstract

The ordering and conformational properties of dicarbonitrile-para-oligophenyls are studied with complementary methods, namely X-ray structure analysis, low-temperature scanning tunneling microscopy, and near-edge X-ray absorption fine-structure spectroscopy. The packing of the functionalized variants differs from their technologically interesting para-oligophenyl counterparts, both in the bulk crystal phase and in thin films grown by organic molecular beam epitaxy (OMBE) under ultra-high vacuum conditions on the Ag(111) surface. In the crystal phase, the conformation depends on the number n of phenyl rings, exhibiting an intriguing screw-like structure in the case of n = 4 at room temperature as well as at 180 K. For OMBE-grown thin films, the whole series acquires the same type of conformation, characterized by alternately twisted phenyl rings, similar to the pure oligophenyl species. However, for all tested molecules, the orientation of the molecular reference plane is uniform within the entire film and coincides with the surface plane. This contrasts with the herringbone ordering adopted by the phenyl backbones without the carbonitrile groups. Our results demonstrate how the functionalization of moieties with extended conjugated electron systems can help to improve the structural homogeneity in technologically relevant organic thin films.