Photophysical Characterization of a Chromophore/Water Oxidation Catalyst Containing a Layer-by-Layer Assembly on Nanocrystalline TiO₂ Using Ultrafast Spectroscopy

Abstract

Femtosecond transient absorption spectroscopy is used to characterize the first photoactivation step in a chromophore/water oxidation catalyst assembly formed through a “layer-by-layer” approach. Assemblies incorporating both chromophores and catalysts are central to the function of dye-sensitized photoelectrosynthesis cells (DSPECs) for generating solar fuels. The chromophore, [Ru_a^II]²⁺ = [Ru­(pbpy)₂(bpy)]²⁺, and water oxidation catalyst, [Ru_b^II-OH₂]²⁺ = [Ru­(4,4′-(CH₂PO₃H₂)₂bpy)­(Mebimpy)­(H₂O)]²⁺, where bpy = 2,2′-bipyridine, pbpy = 4,4′-(PO₃H₂)₂bpy, and Mebimpy = 2,6-bis­(1-methylbenzimidazol-2-yl)­pyridine), are arranged on nanocrystalline TiO₂ via phosphonate-Zr­(IV) coordination linkages. Analysis of the transient spectra of the assembly (denoted TiO₂-[Ru_a^II-Zr-Ru_b^II-OH₂]⁴⁺) reveal that photoexcitation initiates electron injection, which is then followed by the transfer of the oxidative equivalent from the chromophore to the catalyst with a rate of <i>k</i>_ET = 5.9 × 10⁹ s^–1 (τ = 170 ps). While the assembly, TiO₂-[Ru_a^II-Zr-Ru_b^II-OH₂]⁴⁺, has a near-unit efficiency for transfer of the oxidative equivalent to the catalyst, the overall efficiency of the system is only 43% due to nonproductive photoexcitation of the catalyst and nonunit efficiency for electron injection. The modular nature of the layer-by-layer system allows for variation of the light-harvesting chromophore and water oxidation catalyst for future studies to increase the overall efficiency