Disulfide-Containing Nitrosoarenes: Synthesis and Insights into Their Self-Polymerization on a Gold Surface

Disulfide-containing nitrosoarenes with [bis(4-nitrosobenzyl) disulfide, NOBnDS (1)] or without [4-nitrosophenyl disulfide, NOPDS (2), and 1,2-bis(4'-nitroso-[1,1'-biphenyl]-4-yl)disulfane, NOBPDS (3)] an alkyl spacer between the sulfur headgroup and the aromatic moiety (phenyl in NOPDS (2) or biphenyl in NOBPDS (3)) were synthesized and used as precursors to form azodioxy thiolate films on Au(111) substrates. Due to the incorporated disulfide functionalities, these specifically designed nitrosoarenes are enabled to self-polymerize through azodioxy bonds on a gold surface. Thin films of NOBnDS (1), NOPDS (2), and NOBPDS (3) were prepared at different adsorption times via the solution-phase self-assembly of molecules onto the Au(111) surface and characterized by Raman spectroscopy, ellipsometry, water contact angle measurements, atomic force microscopy (AFM), and scanning tunneling microscopy (STM). For comparison, films of structurally analogous disulfide-containing nitro derivatives on Au(111), which can form only monolayer films, thus serving as a reference, were also fabricated and characterized. Raman spectra of NOBnDS (1), NOPDS (2), and NOBPDS (3) films on the Au(111) substrates revealed the presence of a N═N stretching band of the E-azodioxy group. Ellipsometry showed that nitrosoarenes form thicker films compared with their nitro counterparts, indicating the formation of azodioxy oligomer films. AFM revealed an island-like morphology of the nitrosoarene films, in contrast to the rather homogeneous one observed for the monolayer films of nitro derivatives. STM images of nitrosoarene films indicated the formation of poorly organized surface structures in which oligomer chains are possibly interdigitated and substantially tilted relative to the surface normal, which agrees with the relatively low values of the ellipsometry-derived thicknesses. The obtained results emphasized the influence of the substituent electronic effects on the on-surface formation of azodioxides and could be used for the future design of azodioxy thiolate films on gold surfaces.