SAMs of azobenzene-functionalized alkanethiols

Self-assembled monolayers (SAMs) of 4-trifluoromethyl-azobenzene-4’methyleneoxy- alkanethiols on (111)-oriented poly-crystalline gold films on mica are 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 C1s XPS 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 ultrafast charge transfer among the densely packed aromatic moieties. We propose that quenching of the optical excitation within the molecular layer is thus one major reason for the low trans to cis photoisomerization rate of azobenzene in aromatic-aliphatic SAMs.