Excited-State Decay Pathways of Tris(bidentate) Cyclometalated Ruthenium(II) Compounds

The synthesis, electrochemistry, and photophysical characterization are reported for 11 tris­(bidentate) cyclometalated ruthenium­(II) compounds, [Ru­(N^N)₂(C^N)]⁺. The electrochemical and photophysical properties were varied by the addition of substituents on the 2,2′-bipyridine, N^N, and 2-phenylpyridine, C^N, ligands with different electron-donating and -withdrawing groups. The systematic tuning of these properties offered a tremendous opportunity to investigate the origin of the rapid excited-state decay for these cyclometalated compounds and to probe the accessibility of the dissociative, ligand-field (LF) states from the metal-to-ligand charge-transfer (MLCT) excited state. The photoluminescence quantum yield for [Ru­(N^N)₂(C^N)]⁺ increased from 0.0001 to 0.002 as more electron-withdrawing substituents were added to C^N. An analogous substituent dependence was observed for the excited-state lifetimes, τ_obs, which ranged from 3 to 40 ns in neat acetonitrile, significantly shorter than those for their [Ru­(N^N)₃]²⁺ analogues. The excited-state decay for [Ru­(N^N)₂(C^N)]⁺ was accelerated because of an increased vibronic overlap between the ground- and excited-state wavefunctions rather than an increased electronic coupling as revealed by a comparison of the Franck–Condon factors. The radiative (<i>k</i>_r) and non-radiative (<i>k</i>_nr) rate constants of excited-state decay were determined to be on the order of 10⁴ and 10⁷–10⁸ s^–1, respectively. For sets of [Ru­(N^N)₂(C^N)]⁺ compounds functionalized with the same N^N ligand, <i>k</i>_nr scaled with excited-state energy in accordance with the energy gap law. Furthermore, an Arrhenius analysis of τ_obs for all of the compounds between 273 and 343 K was consistent with activated crossing into a single, fourth ³MLCT state under the conditions studied with preexponential factors on the order of 10⁸–10⁹ s^–1 and activation energies between 300 and 1000 cm^–1. This result provides compelling evidence that LF states are not significantly populated near room temperature unlike many ruthenium­(II) polypyridyl compounds. On the basis of the underlying photophysics presented here for [Ru­(N^N)₂(C^N)]⁺, molecules of this type represent a robust class of compounds with built-in design features that should greatly enhance the molecular photostability necessary for photochemical and photoelectrochemical applications

Rigid Medium Effects on Photophysical Properties of MLCT Excited States of Polypyridyl Os(II) Complexes in Polymerized Poly(ethylene glycol)dimethacrylate Monoliths

Higher-energy emissions from the metal-to-ligand charge-transfer (MLCT) excited states of a series of polypyridyl Os­(II) complexes were observed at the fluid-to-film transition in PEG-DMA550. The higher-energy excited states, caused by a “rigid medium effect” in the film, led to enhanced emission quantum yields and longer excited-state lifetimes. Detailed analyses of spectra and excited-state dynamics by Franck–Condon emission spectral analysis and application of the energy gap law for nonradiative excited-state decay reveal that the rigid medium effect arises from the inability of part of the local medium dielectric environment to respond to the change in charge distribution in the excited state during its lifetime. Enhanced excited-state lifetimes are consistent with qualitative and quantitative predictions of the energy gap law

Application of Degenerately Doped Metal Oxides in the Study of Photoinduced Interfacial Electron Transfer

Degenerately doped In₂O₃:Sn semiconductor nanoparticles (<i>nano</i>ITO) have been used to study the photoinduced interfacial electron-transfer reactivity of surface-bound [Ru^II(bpy)₂(4,4′-(PO₃H₂)₂-bpy)]²⁺ (RuP²⁺) molecules as a function of driving force over a range of 1.8 eV. The metallic properties of the ITO nanoparticles, present within an interconnected mesoporous film, allowed for the driving force to be tuned by controlling their Fermi level with an external bias while their optical transparency allowed for transient absorption spectroscopy to be used to monitor electron-transfer kinetics. Photoinduced electron transfer from excited-state -RuP^2+* molecules to <i>nano</i>ITO was found to be dependent on applied bias and competitive with nonradiative energy transfer to <i>nano</i>ITO. Back electron transfer from <i>nano</i>ITO to oxidized -RuP³⁺ was also dependent on the applied bias but without complication from inter- or intraparticle electron diffusion in the oxide nanoparticles. Analysis of the electron injection kinetics as a function of driving force using Marcus–Gerischer theory resulted in an experimental estimate of the reorganization energy for the excited-state -RuP^3+/2+* redox couple of λ* = 0.83 eV and an electronic coupling matrix element, arising from electronic wave function overlap between the donor orbital in the molecule and the acceptor orbital(s) in the <i>nano</i>ITO electrode, of <i>H</i>_ab = 20–45 cm^–1. Similar analysis of the back electron-transfer kinetics yielded λ = 0.56 eV for the ground-state -RuP^3+/2+ redox couple and <i>H</i>_ab = 2–4 cm^–1. The use of these wide band gap, degenerately doped materials provides a unique experimental approach for investigating single-site electron transfer at the surface of oxide nanoparticles

Driving Force Dependent, Photoinduced Electron Transfer at Degenerately Doped, Optically Transparent Semiconductor Nanoparticle Interfaces

Photoinduced, interfacial electron injection and back electron transfer between surface-bound [Ru^II(bpy)₂(4,4′-(PO₃H₂)₂-bpy)]²⁺ and degenerately doped In₂O₃:Sn nanoparticles, present in mesoporous thin films (nanoITO), have been studied as a function of applied external bias. Due to the metallic behavior of the nanoITO films, application of an external bias was used to vary the Fermi level in the oxide and, with it, the driving force for electron transfer (Δ<i>G</i>^o′). By controlling the external bias, Δ<i>G</i>^o′ was varied from 0 to −1.8 eV for electron injection and from −0.3 to −1.3 eV for back electron transfer. Analysis of the back electron-transfer data, obtained from transient absorption measurements, using Marcus–Gerischer theory gave an experimental estimate of λ = 0.56 eV for the reorganization energy of the surface-bound Ru^III/II couple in acetonitrile with 0.1 M LiClO₄ electrolyte

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

Efficient Photochemical Dihydrogen Generation Initiated by a Bimetallic Self-Quenching Mechanism

Artificial photosynthesis relies on coupling light absorption with chemical fuel generation. A mechanistic study of visible light-driven H₂ production from [Cp*Ir­(bpy)­H]⁺ (<b>1</b>) has revealed a new, highly efficient pathway for integrating light absorption with bond formation. The net reaction of <b>1</b> with a proton source produces H₂, but the rate of excited state quenching is surprisingly acid-independent and displays no observable deuterium kinetic isotopic effect. Time-resolved photoluminescence and labeling studies are consistent with diffusion-limited bimetallic self-quenching by electron transfer. Accordingly, the quantum yield of H₂ release nearly reaches unity as the concentration of <b>1</b> increases. This unique pathway for photochemical H₂ generation provides insight into transformations catalyzed by <b>1</b>

Ultrafast Recombination Dynamics in Dye-Sensitized SnO₂/TiO₂ Core/Shell Films

Interfacial dynamics are investigated in SnO₂/TiO₂ core/shell films derivatized with a Ru­(II)-polypyridyl chromophore ([Ru^II(bpy)₂(4,4′-(PO₃H₂)₂bpy)]²⁺, <b>RuP</b>) using transient absorption methods. Electron injection from the chromophore into the TiO₂ shell occurs within a few picoseconds after photoexcitation. Loss of the oxidized dye through recombination occurs across time scales spanning 10 orders of magnitude. The majority (60%) of charge recombination events occur shortly after injection (τ = 220 ps), while a small fraction (≤20%) of the oxidized chromophores persists for milliseconds. The lifetime of long-lived charge-separated states (CSS) depends exponentially on shell thickness, suggesting that the injected electrons reside in the SnO₂ core and must tunnel through the TiO₂ shell to recombine with oxidized dyes. While the core/shell architecture extends the lifetime in a small fraction of the CSS, making water oxidation possible, the subnanosecond recombination process has profound implications for the overall efficiencies of dye-sensitized photoelectrosynthesis cells (DSPECs)

Synthesis and Photophysical Properties of a Covalently Linked Porphyrin Chromophore–Ru(II) Water Oxidation Catalyst Assembly on SnO₂ Electrodes

We describe here the preparation and surface photophysical properties of a covalently linked, chromophore-catalyst assembly between a phenyl phosphonate-derivatized pentafluorophenyl-substituted porphyrin and the water oxidation catalyst, [Ru^II(terpyridine)­(2-benzimidazolylpyridine)­(OH₂)]²⁺, in a derivatized assembly of porph-Ru^II–OH₂²⁺. The results of nanosecond transient absorption measurements on nanoparticle SnO₂ electrodes in aqueous acetate buffer at pH 4.7 are consistent with rapid electron injection into SnO₂ with transfer of oxidative equivalents to the assembly. Electron transfer from the singlet excited state of the porphyrin to the conduction band of the electrode, SnO₂(e^–)|-porph⁺-Ru^II–OH₂²⁺, is favored as the porphyrin singlet excited state lies 0.44 eV above the SnO₂ conduction band edge. Electron injection is rapid (⟨τ_inj⟩ < 10^–8 s), and occurs with high efficiency. Based on measured redox potentials, following excitation and injection, intra-assembly oxidation of the catalyst, -porph⁺-Ru^II–OH₂²⁺ → -porph-Ru^III–OH²⁺ + H⁺, is favored in the transient equilibrium state by 0.62 eV at pH 4.7. However, immediately after the flash, a distribution exists at the surface between isomers with SnO₂(e^–)|-porph⁺-Ru^II–OH₂²⁺ undergoing back electron transfer to the surface with an average lifetime of ⟨τ₁⟩ ∼ 10^–7 s and a slower component for back electron transfer from SnO₂(e^–)|-porph-Ru^III–OH²⁺ with ⟨τ₂⟩ ∼ 4 × 10^–5 s

Stabilization of Ruthenium(II) Polypyridyl Chromophores on Nanoparticle Metal-Oxide Electrodes in Water by Hydrophobic PMMA Overlayers

We describe a poly­(methyl methacrylate) (PMMA) dip-coating procedure, which results in surface stabilization of phosphonate and carboxylate derivatives of Ru­(II)-polypyridyl complexes surface-bound to mesoporous nanoparticle TiO₂ and nanoITO films in aqueous solutions. As shown by contact angle and transmission electron microscopy (TEM) measurements, PMMA oligomers conformally coat the metal-oxide nanoparticles changing the mesoporous films from hydrophilic to hydrophobic. The thickness of the PMMA overlayer on TiO₂–Ru­(II) can be controlled by changing the wt % of PMMA in the dipcoating solution. There are insignificant perturbations in electrochemical or spectral properties at thicknesses of up to 2.1 nm with the Ru­(III/II) couple remaining electrochemically reversible and <i>E</i>_1/2 values and current densities nearly unaffected. Surface binding by PMMA overlayers results in stable surface binding even at pH 12 with up to a ∼100-fold enhancement in photo­stability. As shown by transient absorption measurements, the MLCT excited state(s) of phosphonate derivatized [Ru­(bpy)₂((4,4′-(OH)₂PO)₂bpy)]²⁺ undergo efficient injection and back electron transfer with pH independent kinetics characteristic of the local pH in the initial loading solution

Competing Pathways in the <i>photo-</i>Proton-Coupled Electron Transfer Reduction of <i>fac</i>-[Re(bpy)(CO)₃(4,4′-bpy]^+* by Hydroquinone

The emitting metal-to-ligand charge transfer (MLCT) excited state of <i>fac</i>-[Re^I(bpy)(CO)₃(4,4′-bpy)]⁺ (<b>1</b>) (bpy is 2,2′-bipyridine, 4,4′-bpy is 4,4′-bipyridine), [Re^II(bpy^–•)(CO)₃(4,4′-bpy)]⁺*, is reductively quenched by 1,4-hydroquinone (H₂Q) in CH₃CN at 23 ± 2 °C by competing pathways to give a common electron–proton-transfer intermediate. In one pathway, electron transfer (ET) quenching occurs to give Re^I(bpy^–•)(CO)₃(4,4′-bpy)]⁰ with <i>k</i> = (1.8 ± 0.2) × 10⁹ M^–1 s^–1, followed by proton transfer from H₂Q to give [Re^I(bpy)(CO)₃(4,4′-bpyH^•)]⁺. Protonation triggers intramolecular bpy^•– → 4,4′-bpyH⁺ electron transfer. In the second pathway, preassociation occurs between the ground state and H₂Q at high concentrations. Subsequent Re → bpy MLCT excitation of the adduct is followed by electron–proton transfer from H₂Q in concert with intramolecular bpy^•– → 4,4′-bpyH⁺ electron transfer to give [Re^I(bpy)(CO)₃(4,4′-bpyH^•)]⁺ with <i>k</i> = (1.0 ± 0.4) × 10⁹ s^–1 in 3:1 CH₃CN/H₂O