A Highly Ordered Self-Assembled Monolayer Film of an Azobenzenealkanethiol on Au (111): Electrochemical Properties and Structural Characterization by Synchrotron in-Plane X-ray Diffraction, Atomic Force Microscopy, and Surface-Enhanced Raman Spectroscopy

The synthesis and characterization of p-HS (CH₂)₁₁OC₆H₄N=NC₆H₅. comnound 1d. is renorted. Comnound 1d self-assembles onto Au (111) substrates into highly ordered monolayer films. Self-assembled monolayer films (SAMs) of 1d on Au (111)/mica have been characterized by ellipsometry, surface-enhanced Raman spectroscopy (SERS), and atomic force microscopy (AFM). We also report the characterization of SAMs of 1d on bulk single crystal Au (111) by synchrotron in-plane X-ray diffraction (XRD) measurements. AFM and in-plane XRD suggest that a SAM of 1d is comprised of domains of 1d which form a hexagonal lattice (4.50 ± 0.06 Å nearest neighbor spacing) that is incommensurate with the underlying Au (111) lattice. A model is proposed to describe the SAM structure. In such a model, small bundles (~80 molecules) of ordered azobenzene moieties that rest over a set of inward tilting alkyl surface tethering groups make up the individual domains. The “bundle moder for a SAM of 1d on Au (111) is a new one and provides insight into the way adsorbate molecules may arrange themselves in these novel materials. Thermal annealing of the as-deposited SAM of 1d results in a modest increase in domain size from ~45 to ~55 Å and a change in azobenzene tilt angle from 20—30° to approximately 0° with no change in nearest neighbor spacing. The redox activity of the azobenzene group is significantly affected by monolayer film structure. Only 2% of the azobenzene groups within a SAM of 1d are electrochemically accessible through cyclic voltammetry in a THF/0.1 M n-Bu₄NPF₆ electrolyte. The monolayer structure impedes the incorporation of charge compensating ions into the film, thereby regulating the electrochemical accessibility of the azobenzene redox centers within the film. Submonolayer films of 1d and films prepared by the coadsorption of 1d with ethanethiol on Au (111)/mica have greater electrochemical accessibilities with regard to the azobenzene groups than do pure SAMs of 1d. Interfacial capacitance measurements and film penetration studies with Fe (CN)₆^3- show that SAMs of 1d are densely packed structures which form impenetrable barriers to Fe (CN)₆^3-.