Self-Assembled Monolayers of Azobenzene-Incorporated Alkanethiol on Silver

In the past decade, self-assembled monolayers (SAMs) have received a great deal of attention for their fundamental importance in understanding interfacial properties as well as for their potential applications in molecular technologies. SAMs have been used in the design of various interfaces for chemical sensors, nonlinear optical materials, optical switches, and high density memory devices. The principal ingredient for obtaining SAMs is a relatively strong interfacial binding asymmetry of the molecular constituents. For the most frequently studied alkanethiol SAMs on Au or Ag, this is obviously provided by the sulfur affinity for gold and silver and a comparatively strong lateral interaction arising from the van der Waals forces between the chains. In this sense, a better understanding of the molecular details for various SAM systems is needed in order to engineer nanoscale surfaces. Recently, azobenzene-based alkanethiol SAMs have attracted much attention not only from the structural point of view but also from that of the photo and electrochemical characteristics of the azobenzene group; azobenzene-based molecules have been proved to be a potential medium for high-density recording elements and molecular switches. Simulations and infrared experiments suggested that such a bulky aromatic unit within an alkanethiol SAM could change the packing density as well as the lattice parameter. For instance, Han et al. reported that azobenzeneincorporated alkanethiol SAMs on Au(111) should possess near hexagonal lattices on a Au(111) surface, with nearest spacing of ca. 5.5 Å. The experimental data could be understood by bundle model. The azobenzene moieties condense to form small bundles, thereby causing the alkyl chains to be projected outward from the azobenzene moieties. This clearly indicates that the azobenzene group, although not completely independent from Au(111) surface influence, plays a significant role in dictating the overall monolayer structure regardless of its actual position in alkanethiols. Herein we aim to report a study performed to further increase our understanding of the structure of azobenzenebased alkanethiol SAMs on Ag. Since most of the previous studies have dealt with SAMs on Au, we have investigated the effect of the azobenzene moiety on the structure and stability of SAMs on Ag surface with an azobenzene-based alkanethiol molecule, i.e., 4-dodecoxy-4'-(6-mercaptohexoxy)azobenzene (C12AzoC6SH). Experimental Section