Azobenzene-functionalized alkanethiols in self-assembled monolayers on gold |
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Authors: | Roland Schmidt Erik McNellis Wolfgang Freyer Daniel Brete Tanja Gießel Cornelius Gahl Karsten Reuter Martin Weinelt |
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Institution: | 1. Max-Born-Institut, Max-Born-Str. 2A, 12489, Berlin, Germany 2. Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195, Berlin, Germany 4. BESTEC GmbH, Carl-Scheele Str. 14, 12489, Berlin, Germany 3. Fachbereich Physik, Freie Universit?t Berlin, Arnimallee 14, 14195, Berlin, Germany
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Abstract: | Self-assembled monolayers (SAMs) of 4-trifluoromethyl-azobenzene-4′-methyleneoxy-alkanethiols (CF3– C6H4–N=N–C6H4–O–(CH2)
n
–SH on (111)-oriented poly-crystalline gold films on mica were examined by X-ray photoelectron spectroscopy (XPS) and X-ray
absorption spectroscopy (XAS). The spectra are analyzed with the help of density-functional-theory calculations of the isolated
molecule. Only one doublet is detected in the sulphur 2p spectra of the investigated SAMs, consistent with a thiolate bond of the molecule to the gold surface. The C 1s XP spectra and the corresponding XAS π
* resonance exhibit a rich structure which is assigned to the carbon atoms in the different chemical surroundings. Comparing
XPS binding energies of the azobenzene moiety and calculated initial-state shifts reveals comparable screening of all C 1s core holes. While the carbon 1s XPS binding energy lies below the π
*-resonance excitation-energy, the reversed order is found comparing core ionization and neutral core excitation of the nitrogen 1s core-hole of the azo group. This surprising difference in core-hole binding energies is interpreted as site-dependent polarization
screening and charge transfer among the densely packed aromatic moieties. We propose that a quenching of the optical excitation
within the molecular layer is thus one major reason for the low trans to cis photo-isomerization rate of azobenzene in aromatic-aliphatic
SAMs. |
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Keywords: | PACS" target="_blank">PACS 82 80 Pv 64 75 Yz 68 43 -h |
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