Stabilization of Ruthenium(II) Polypyridyl Chromophores on Nanoparticle Metal-Oxide Electrodes in Water by Hydrophobic PMMA Overlayers

We describe a poly­(methyl methacrylate) (PMMA) dip-coating procedure, which results in surface stabilization of phosphonate and carboxylate derivatives of Ru­(II)-polypyridyl complexes surface-bound to mesoporous nanoparticle TiO2 and nanoITO films in aqueous solutions. As shown by contact angle and transmission electron microscopy (TEM) measurements, PMMA oligomers conformally coat the metal-oxide nanoparticles changing the mesoporous films from hydrophilic to hydrophobic. The thickness of the PMMA overlayer on TiO2–Ru­(II) can be controlled by changing the wt % of PMMA in the dipcoating solution. There are insignificant perturbations in electrochemical or spectral properties at thicknesses of up to 2.1 nm with the Ru­(III/II) couple remaining electrochemically reversible and E 1/2 values and current densities nearly unaffected. Surface binding by PMMA overlayers results in stable surface binding even at pH 12 with up to a ∼100-fold enhancement in photo­stability. As shown by transient absorption measurements, the MLCT excited state(s) of phosphonate derivatized [Ru­(bpy)2((4,4′-(OH)2PO)2bpy)]2+ undergo efficient injection and back electron transfer with pH independent kinetics characteristic of the local pH in the initial loading solution.