A HIGHLY ORDERED SELF-ASSEMBLED MONOLAYER FILM OF AN AZOBENZENEALKANETHIOL ON AU(111) - ELECTROCHEMICAL PROPERTIES AND STRUCTURAL CHARACTERIZATION BY SYNCHROTRON INPLANE X-RAY-DIFFRACTION, ATOMIC-FORCE MICROSCOPY, AND SURFACE-ENHANCED RAMAN-SPECTROSCOPY

The synthesis and characterization of p-HS(CH2)(11)OC6H4N=NC6H5, compo und 1d, is reported. Compound 1d self-assembles onto Au(111) substrate s 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 microscop y (AFM). We also report the characterization of SAMs of 1d on bulk sin gle crystal Au(111) by synchrotron in-plane X-ray diffraction (XRD) me asurements. 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 Angstr om nearest neighbor spacing) that is incommensurate with the underlyin g Au(111) lattice. A model is proposed to describe the SAM structure. In such a model, small bundles (similar to 80 molecules) of ordered az obenzene moieties that rest over a set of inward tilting alkyl surface tethering groups make up the individual domains. The ''bundle model'' 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 materia ls. Thermal annealing of the as-deposited SAM of 1d results in a modes t increase in domain size from similar to 45 to similar to 55 Angstrom and a change in azobenzene tilt angle from 20-30 degrees to approxima tely 0 degrees with no change in nearest neighbor spacing. The redox a ctivity of the azobenzene group is significantly affected by monolayer film structure. Only 2% of the azobenzene groups within a SAM of 1d a re electrochemically accessible through cyclic voltammetry in a THF/0. 1 M n-Bu(4)NPF(6) electrolyte. The monolayer structure impedes the inc orporation of charge compensating ions into the film, thereby regulati ng the electrochemical accessibility of the azobenzene redox centers w ithin the film. Submonolayer films of 1d and films prepared by the coa dsorption of 1d with ethanethiol on Au(111)/mica have greater electroc hemical accessibilities with regard to the azobenzene groups that do p ure SAMs of 1d. Interfacial capacitance measurements and film penetrat ion studies with Fe(CN)(6)(3-) show that SAMs of 1d are densely packed structures which form impenetrable barriers to Fe(CN)(6)(3-).