The bonding of 1,4,5,8-naphthalene-tetracarboxylicacid-dianhydride (NTCDA) on Ag(111) and its molecular orientation were investigated by X-ray photoelectron (XPS) and near edge X-ray absorption fine-structure spectroscopy (NEXAFS). Vapour-deposited NTCDA films show high orientational order in the mono-and multi-layer regime. In the monolayer regime the molecules are strongly bound to the substrate via the conjugated π-system of the naphthalene core which leads to a parallel orientation of the molecular plane to the Ag(111) substrate. For thicker films (e.g., 150 Å) the molecular orientation strongly depends on the substrate temperature during preparation. Film preparation at 285 K results in upright standing molecules, whereas low substrate temperatures yield a uniform molecular orientation with the molecular plane parallel to the substrate. This behaviour is explained by the weak intermolecular interaction and the adsorption kinetics.

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We report on C 1s and N 1s near-edge X-ray absorption (NEXAFS) investigations of highly oriented ultrathin films of 2,5-dimethyl-N,N‘-dicyanoquinonediimine (DMe-DCNQI), tetracyano-quinonedimethane (TCNQ), and 2,5-dimethyl-1,4-benzoquinone (DMe-BQ) adsorbed on Ag(111). Several sharp resonances are resolved in the spectra indicating the localized electronic structure of these quinoide molecules. The comparison of the angle-dependent DMe-DCNQI results with those from other molecules and literature data on cyano-containing molecules leads to the conclusion that the common building-block approach is not sufficient, but that the interpretation of most valence orbitals being delocalized within the whole molecule appears to be more appropriate as confirmed by semiempirical INDO/S SDCI calculations. The NEXAFS spectra of the related DMe-DCNQI charge transfer salts also reflect strongly delocalized molecular orbitals very different from those of unreacted DCNQI.