Synthesis and photophysical characterization of porphyrin and porphyrin-Ru(II) polypyridyl chromophore-catalyst assemblies on mesoporous metal oxides

A layer-by-layer procedure has been used to prepare chromophore-catalyst assemblies consisting of phosphonate-derivatized porphyrin chromophores and a phosphonate-derivatized Ru(II) water oxidation catalyst on the surfaces of SnO2 and TiO2 mesoporous, nanoparticle films. In the procedure, initial surface binding of the phosphonate-derivatized porphyrin is followed in sequence by reaction with ZrOCl2 and then with the phosphonate-derivatized water oxidation catalyst [Ru-II(2,6-bis-(1-methylbenzimidazole-2-yl)pyridine)(2,2'-bipyridine-4,4'-hydroxymethylphosphonate)(H2O)](2+), [Ru-II(Mebimpy)(4,4'-(PO(OH)(2)-CH2) (2)-bpy)(OH2)](2+). Fluorescence from both the free base and Zn(II) porphyrin derivatives on SnO2 is quenched; substantial emission quenching of the Zn(II) porphyrin occurs on TiO2. Transient absorption difference spectra provide direct evidence for appearance of the porphyrin radical cation on SnO2 via excited-state electron injection. For the chromophore-catalyst assembly on SnO2, transient absorption difference spectra demonstrate rapid intra-assembly electron transfer oxidation of the catalyst following excitation and injection by the porphyrin chromophore.