Versatile grafting chemistry for creation of stable molecular layers on oxides

Abstract

We demonstrate that hydroxyl containing bipyridine-based molecules can be efficiently grafted onto SiO2 and TiO2 substrates to create a stable monolayer. These surface-grafted ligands are relevant to several optoelectronic applications as well as catalysis since many metal–bipyridine complexes have been used as dye sensitizers, light emitters, and catalysts. Control experiments were carried out to infer that the grafting occurs through the formation of an ether link with available hydroxyl groups on the surface. The coverage was found to be a function of annealing temperature, annealing time, and availability of surface hydroxyl groups. Further complexation with rhenium salts resulted in a rhenium–bipyridine complex, a common electron injector in dye-sensitized solar cells. All samples were characterized by X-ray photoelectron spectroscopy (XPS) and infrared reflection absorption spectroscopy (IRRAS). This simple grafting method leads to monolayer coverage on the surface, effectively avoiding crosslinking, and shows good stability in acetonitrile and aqueous solutions for at least four weeks. We further expand the scope of this approach by grafting an electroactive ferrocene unit onto indium tin oxide. Based on these results, the formation of an ether bond via thermal annealing of small molecules offers a versatile strategy for preparing stable organic layers on a variety of oxide surfaces, and therefore, expands the tool box for functionalizing organic/inorganic interfaces.