Structured layer of rhenium dye on SiO₂ and TiO₂ surfaces by Langmuir-Blodgett technique

We demonstrate the Langmuir-Blodgett assembly of two rhenium-bipyridine complexes containing a flexible or an aromatic bridge, and transfer of the monolayer to SiO2 and single crystal TiO2 substrates. Both of the complexes (ReEC and Re2TC) have a hydrophilic carboxylic acid group, which preferentially anchors into the water subphase, and forms stable monolayers at surface pressures up to 40 mN/m. The optimum conditions for the formation of complete monolayers of both ReEC and Re2TC were identified through characterization of the morphology by atomic force microscopy (AFM), the thickness by ellipsometry, and the surface coverage by X-ray photoelectron spectroscopy (XPS). X-ray reflectivity measurements (XRR) are consistent with the orientation of the molecules normal to the substrate, and their extension to close to their calculated maximum length. Parameters derived from XRR analysis show that there is a higher packing density for Re2TC monolayers than for ReEC monolayers, attributable to the more rigid bridge in the Re2TC molecule.