Self-assembly of highly rigid ferrocenyl nanostructures monitored by optical second harmonic generation

摘要

Most compounds used for thin film formation with nanometer dimensions in the past are intrinsically "soft" due to flexible spacer units, which limit the overall film ordering. In order to resolve this problem we have synthesised compounds bearing very rigid π-spacers to connect the 2,2':6',2"-terpyridin-4'-yl head groups and redox-active ferrocenyl functional groups. Subsequently, we have prepared highly ordered nanostructured surfaces by self-assembly of these rigid ferrocenyl functionalised terpyridines. Adsorption and assembly from solution on different substrates were studied in situ by optical second harmonic generation (SHG). With this technique a signal can be generated exclusively at the interface which makes readily possible investigations of adsorption and film assembly in the submonolayer regime. The experimental data clearly show formation of monomolecular films on the substrate on a timescale of typically several minutes. This process can be described by diffusion-limited Langmuir kinetics. Most importantly, we have also found that film growth is followed by self-assembly of highly ordered ferrocenyl nanostructures on a much longer timescale. This opens the door to exploit these rigid, oriented molecules as templates for the preparation of novel structures consisting, for example, of metal nanoparticle arrays or multilayer architectures.