Azobenzene-functionalized alkanethiols in self-assembled monolayers on gold

Abstract

Abstract Self-assembled monolayers (SAMs) of 4-trifluoromethyl-azobenzene-4′-methyleneoxy-alkanethiols (CF 3 – C 6 H 4 –N=N–C 6 H 4 –O–(CH 2 ) 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 2 p spectra of the investigated SAMs, consistent with a thiolate bond of the molecule to the gold surface. The C 1 s 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 1 s core holes. While the carbon 1 s 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 1 s 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.