Revealing the Relationship between Semiconductor Electronic Structure and Electron Transfer Dynamics at Metal Oxide–Chromophore Interfaces

Interfacial charge recombination dynamics in nanocrystalline SnO₂ and TiO₂ thin films sensitized with phosphonate-derivatized ruthenium chromophores (Ru­(bpy)₂(4,4′-(PO₃H₂)₂bpy)]²⁺, RuP) have been investigated in aqueous media by nanosecond transient absorption spectroscopy_. Back electron transfer (BET) rates for RuP–SnO₂ were observed to be 2–3 times greater than for RuP–TiO₂. Additionally, rates of charge recombination for RuP–TiO₂ show a significant pH dependence, while only a subtle influence of pH is observed for BET in RuP–SnO₂. Cyclic voltammetry measurements indicate the exponential distribution of intra-band-gap trap states varies with pH for both SnO₂ and TiO₂ nanocrystalline thin films. BET rates for RuP–SnO₂ and RuP–TiO₂ are correlated with the distribution, identity, and occupation of localized trap states within the nanocrystalline metal oxide films, which are pH specific. Recombination between injected electrons and oxidized chromophores is influenced by the identity of metal oxide localized trap states populated and the specific pathways by which BET can proceed

Controlling Ground and Excited State Properties through Ligand Changes in Ruthenium Polypyridyl Complexes

The capture and storage of solar energy requires chromophores that absorb light throughout the solar spectrum. We report here the synthesis, characterization, electrochemical, and photophysical properties of a series of Ru­(II) polypyridyl complexes of the type [Ru­(bpy)₂­(N–N)]²⁺ (bpy = 2,2′-bipyridine; N–N is a bidentate polypyridyl ligand). In this series, the nature of the N–N ligand was altered, either through increased conjugation or incorporation of noncoordinating heteroatoms, as a way to use ligand electronic properties to tune redox potentials, absorption spectra, emission spectra, and excited state energies and lifetimes. Electrochemical measurements show that lowering the π* orbitals on the N–N ligand results in more positive Ru^3+/2+ redox potentials and more positive first ligand-based reduction potentials. The metal-to-ligand charge transfer absorptions of all of the new complexes are mostly red-shifted compared to Ru­(bpy)₃²⁺ (λ_max = 449 nm) with the lowest energy MLCT absorption appearing at λ_max = 564 nm. Emission energies decrease from λ_max = 650 nm to 885 nm across the series. One-mode Franck–Condon analysis of room-temperature emission spectra are used to calculate key excited state properties, including excited state redox potentials. The impacts of ligand changes on visible light absorption, excited state reduction potentials, and Ru^3+/2+ potentials are assessed in the context of preparing low energy light absorbers for application in dye-sensitized photoelectrosynthesis cells

An Amide-Linked Chromophore–Catalyst Assembly for Water Oxidation

The synthesis and analysis of a new amide-linked, dinuclear [Ru­(bpy)₂(bpy-ph-NH-CO-trpy)­Ru­(bpy)­(OH₂)]⁴⁺ (bpy = 2,2′-bipyridine; bpy-ph-NH-CO-trpy = 4-(2,2′:6′,2″-terpyridin-4′-yl)-<i>N</i>-[(4′-methyl-2,2′-bipyridin-4-yl)­methyl]­benzamide) assembly that incorporates both a light-harvesting chromophore and a water oxidation catalyst are described. With the saturated methylene linker present, the individual properties of both the chromophore and catalyst are retained including water oxidation catalysis and relatively slow energy transfer from the chromophore excited state to the catalyst

Structure–Property Relationships in Phosphonate-Derivatized, Ru^II Polypyridyl Dyes on Metal Oxide Surfaces in an Aqueous Environment

The performance of dye-sensitized solar and photoelectrochemical cells is strongly dependent on the light absorption and electron transfer events at the semiconductor–small molecule interface. These processes as well as photo/electrochemical stability are dictated not only by the properties of the chromophore and metal oxide but also by the structure of the dye molecule, the number of surface binding groups, and their mode of binding to the surface. In this article, we report the photophysical and electrochemical properties of a series of six phosphonate-derivatized [Ru­(bpy)₃]²⁺ complexes in aqueous solution and bound to ZrO₂ and TiO₂ surfaces. A decrease in injection yield and cross surface electron-transfer rate with increased number of diphosphonated ligands was observed. Additional phosphonate groups for surface binding did impart increased electrochemical and photostability. All complexes exhibit similar back-electron-transfer kinetics, suggesting an electron-transfer process rate-limited by electron transport through the interior of TiO₂ to the interface. With all results considered, the ruthenium polypyridyl derivatives with one or two 4,4′-(PO₃H₂)₂bpy ligands provide the best balance of electron injection efficiency and stability for application in solar energy conversion devices

Pathways Following Electron Injection: Medium Effects and Cross-Surface Electron Transfer in a Ruthenium-Based, Chromophore–Catalyst Assembly on TiO₂

Interfacial dynamics following photoexcitation of the water oxidation assembly [((PO₃H₂)₂bpy)₂Ru^II(bpy-bimpy)­Ru^II(tpy)­(OH₂)]⁴⁺, −[Ru_a^II–Ru_b^II–OH₂]⁴⁺, on nanocrystalline TiO₂ electrodes, starting from either −[Ru_a^II–Ru_b^II–OH₂]⁴⁺ or −[Ru_a^II–Ru_b^III–OH₂]⁵⁺, have been investigated. Transient absorption measurements for TiO₂–[Ru_a^II–Ru_b^II–OH₂]⁴⁺ in 0.1 M HPF₆ or neat trifluoroethanol reveal that electron injection occurs with high efficiency but that hole transfer to the catalyst, which occurs on the electrochemical time scale, is inhibited by local environmental effects. Back electron transfer occurs to the oxidized chromophore on the microsecond time scale. Photoexcitation of the once-oxidized assembly, TiO₂–[Ru_a^II–Ru_b^III–OH₂]⁵⁺, in a variety of media, generates −[Ru_a^III–Ru_b^III–OH₂]⁶⁺. The injected electron randomly migrates through the surface oxide structure reducing an unreacted −[Ru_a^II–Ru_b^III–OH₂]⁵⁺ assembly to −[Ru_a^II–Ru_b^II–OH₂]⁴⁺. In a parallel reaction, −[Ru_a^III–Ru_b^III–OH₂]⁶⁺ formed by electron injection undergoes proton loss giving −[Ru_a^II–Ru_b^IVO]⁴⁺ with possible conversion to −[Ru_a^II–Ru_b^II–OH₂]⁴⁺ by an electrolyte-mediated reaction. In the following slow step, re-equilibration on the surface occurs either by reaction with added Fe^III/II or by cross-surface electron transfer between spatially separated −[Ru_a^II–Ru_b^IVO]⁴⁺ and −[Ru_a^II–Ru_b^II–OH₂]⁴⁺ assemblies to give −[Ru_a^II–Ru_b^III–OH₂]⁵⁺ with a half-time of <i>t</i>_1/2 ∼ 68 μs. These results and analyses show that the transient surface behavior of the assembly and cross-surface reactions play important roles in producing and storing redox equivalents on the surface that are used for water oxidation