Conformational Behavior of Chemisorbed Azobenzene Derivatives in External Electric Fields: A Theoretical Study

Azobenzene derivatives have been shown to act as molecular switches when exposed to an applied electric field or tunneling electrons. Many applications require the switching molecule to be adsorbed on a surface. However, stable conformations and the isomerization energetics of adsorbed azobenzenes can be very different from the analogous more thoroughly studied behaviors in the gas phase or liquid solution. In this study, we investigate the zero-density limit behavior of cis and trans N-(2-mercaptoethyl)-4-phenylazobenzamide chemisorbed on a Au(111) surface. For all calculations, we employ the Perdew-Burke-Erzenhof functional as implemented in the SIESTA package, with a double-zeta plus polarization basis set. A large number of starting geometries were equilibrated, and several stable configurations were identified for both the trans- and cis-adsorbed isomers. The most stable are those in which the azobenzene moiety is parallel to the surface. Applied external electric fields in the usual STM range of +/-1-3 V/nm produce minimal changes in these geometries. We find that the strength of the dispersive interactions between the extended conjugated system and the metallic surface is such that switching between parallel and upright geometries of single-molecules is unlikely to occur because of coupling to such a field. Although the presence of the surface slightly modifies the ground electronic state pathways for isomerization, this process is also not accessible through simple field-molecule coupling effects.