Photophysics and Photochemistry of Chromophore−Quencher Assemblies on Glass and Powdered Silica

The photophysical properties of the chromophores [Ru(bpy)2(4,4‘-(CO2H)2bpy)](PF6)2 [(4,4‘-(CO2H)2bpy) is 4,4‘-dicarboxy-2,2‘-bipyridine], [Re(4,4‘-(CO2H)2bpy)(CO)3(4-Etpy)](PF6) (4-Etpy is 4-ethylpyridine), and the chromophore-quencher complex [Re(4,4‘-(CO2H)2bpy)(CO)3(py-PTZ)](PF6) have been investigated on high surface area silica particles with 1,2-dichloroethane (DCE) as the external solvent for the index of refraction matching. Surface coverages were estimated by direct absorption measurements with DCE as the external solvent. Decay of the metal-to-ligand charge transfer (MLCT) excited state of [Ru(bpy)2(4,4‘-(CO2H)2bpy)](PF6)2 on silica is exponential and independent of surface coverage from Γ= 1.5 × 10-14 to 1.3 × 10-13 mol·cm-2. For [Re(4,4‘-(CO2H)2bpy)(CO)3(4-Etpy)](PF6), MLCT emission shifts from 600 nm in DCE to 605 nm on silica and τ decreases from 487 to 255 ns. On surfaces containing both the Ru complex and the electron transfer quenchers N-methyl-N-viologenpropanoic acid bis(hexafluorophosphate) [MV-CO2H](PF6)2 or 3-(10H-phenothiazin-9-yl)propanoic acid, PTZ-CO2H, there is no evidence for cross-surface quenching from lifetime measurements. The transient absorption difference spectrum acquired following laser flash photolysis of [Re(4,4‘-(CO2H)2bpy)(CO)3(py-PTZ)](PF6) on silica resembles the spectrum in DCE, with absorptions appearing for bpy•- at ∼360 nm (with an additional shoulder at ∼390 nm) and PTZ•+ at ∼520 nm. These features are consistent with formation of the redox-separated state [Re(4,4‘-(CO2H)2bpy•-)(CO)3(py-PTZ)+]+. It returns to the ground state by bpy•- → PTZ+ electron transfer with k = 2.9 × 107 s-1.