MOLECULAR-LEVEL ELECTRON-TRANSFER AND EXCITED-STATE ASSEMBLIES ON SURFACES OF METAL-OXIDES AND GLASS

A general procedure is described for the attachment to antimony-doped tin dioxide (SnO2:Sb), tin-doped indium oxide (In2O3:Sn), or glass sur faces of molecules with known electron transfer or excited state prope rties, e.g. [Ru-(bpy)2(4,4'-(CO2H)2bpy)](PF6)2 (bpy = 2,2'-bipyridine, 4,4'-(CO2H)2bpy = 4,4'-dicarboxy-2,2'-bipyridine), based on the inter action between surface hydroxyls and carboxylic acid groups. Integrati ons of cyclic voltammetric waveforms on the metal oxide electrode give maximum surface coverages of GAMMA approximately 1 x 10(-10) mol/cm2 for the ruthenium complex, which corresponds to a monolayer coverage. Atomic force microscope (AFM) measurements reveal that the metal oxide surfaces are highly roughened with root mean square roughnesses in th e range 4-6.5 nm for tin oxide. The smaller organics, N-methyl-N-violo genpropanoic acid bis(hexafluorophosphate), [MV-CO2H](PF6)2, and 10H-p henothiazine-10-propanoic acid, PTZ-CO2H, display similar surface cove rages. Resonance Raman measurements on surfaces containing the rutheni um complex imply that attachment to SnO2, In2O3, and TiO2 is via an es ter bond. For SiO2, two modes of binding are suggested, a majority by a chelating carboxylato link and a minority by ester formation. Bindin g constants for surface attachment were measured in CH2Cl2 at 298 K by equilibration, which gave K = 8 x 10(4) M-1 on both SnO2:Sb and In2O3 :Sn. Surface molecular assemblies have been prepared containing [Ru(bp y)2(4,4'-(CO2H)2bpy)](PF6)2 and [Os(bpy)2(4,4'-(CO2H)2 bpy)](PF6)2, [M V-CO2H}(PF6)2, and PTZ-CO2H. In these assemblies, separate waves are o bserved for the different redox couples at potentials near those found for surfaces containing only a single component. Emission decay of th e metal-to-ligand charge transfer (MLCT) excited state of [Ru(bpy)2(4, 4'-(CO2H)2bpy)](PF6)2 attached to the glass backings of metal oxide el ectrodes or to glass slides was found to be nonexponential with averag e lifetimes ([tau]) from < 5 to 600 ns with CH2Cl2 in the external sol ution. [tau] increases as surface coverage decreases. There is evidenc e for excited state-ground state interactions by a red-shift in the em ission maximum as surface coverage increases. Emission decay remains n onexponential even on surfaces that are lightly covered. Emission is n early completely quenched on the semiconductor surfaces, with [tau] < 5 ns. The bound Ru(II) emitters on glass were quenched by electron or energy transfer to the coattached quenchers [MV-CO2H](PF6)2, PTZ-CO2H, or [Os(bpy)2(4,4'-(CO2H)2-bpy)](PF6)2, suggesting that lateral electr on and energy transfer can occur across the surface. Surface lifetime quenching also occurred in the presence of added 10-methyl-10-phenothi azene in the external CH2Cl2 solution. The kinetics of lifetime quench ing did not follow Stern-Volmer kinetics but could be fit to a model i n which there are both quenchable and unquenchable sites on the same s urface.