Electric Fields Detected on Dye-Sensitized TiO₂ Interfaces: Influence of Electrolyte Composition and Ruthenium Polypyridyl Anchoring Group Type

Electric fields at the dye-sensitized interface of anatase TiO₂ nanocrystallites interconnected in a mesoporous thin film are reported using carboxylic acid-derivatized and phosphonic acid-derivatized ruthenium polypyridyl complexes. Systematic investigations with [Ru­(dtb)₂(dpb)]­(PF₆)₂, where dtb is 4,4′-di-<i>tert</i>-butyl-2,2′-bipyridine and dpb is 4,4′-bis-(PO₃H₂)-2,2′-bipyridine, were carried out in conjunction with its carboxylic acid structural analogue. Electric fields attributed to cation adsorption were measured from a bathochromic (red) shift of the sensitizer’s UV–visible absorption spectra upon replacement of neat acetonitrile solution with metal cation perchlorate acetonitrile electrolyte. Electric fields attributed to TiO₂ electrons were measured from the hypsochromic (blue) shift of the absorption spectra upon electrochemical reduction of the sensitized TiO₂ thin films. Electric fields, induced by either cation adsorption or electrochemically populated electrons, increase in magnitude following the same general cation-dependent trend (Na⁺ < Li⁺ < Ca²⁺ ≤ Mg²⁺ < Al³⁺), regardless of the sensitizer’s anchoring group type. For the first time, surface electric fields in the presence of trivalent cations (i.e., Al³⁺) were measured using [Ru­(dtb)₂(dpb)]­(PF₆)₂. The magnitude of electric fields detected by the carboxylic acid sensitizer was 3 times greater than that detected by the phosphonic acid structural analogue under the same experimental conditions. The influence of protons and water in the acetonitrile electrolyte was also quantified. The added water was found to decrease the electric field, whereas protons had a very similar influence as did metal cations

Electric Fields and Charge Screening in Dye Sensitized Mesoporous Nanocrystalline TiO₂ Thin Films

The photophysical and electron transfer properties of mesoporous nanocrystalline (anatase) TiO₂ thin films sensitized to visible light with [Ru­(dtb)₂(dcb)]­(PF₆)₂, where dtb is 4,4′-(<i>tert</i>-butyl)₂-2,2′-bipyridine and dcb is 4,4′-(CO₂H)₂-2,2′-bipyridine, were quantified in acetonitrile solutions that contained 100 mM concentrations of Li⁺, Na⁺, Mg²⁺, or Ca²⁺ perchlorate salts. The presence of these salts resulted in a dramatic and cation dependent bathochromic (red) shift of the metal-to-ligand charge transfer (MLCT) absorption and photoluminescence (PL) spectra of Ru­(dtb)₂(dcb)/TiO₂ relative to the value measured in neat or 100 mM TBAClO₄, where TBA is tetrabutyl ammonium cation, acetonitrile solutions. The magnitude of the shifts followed the trend: Na⁺ < Li⁺ < Ca²⁺ < Mg²⁺. The PL intensity was also found to decrease in this same order and comparative actinometry studies showed that this was due to MLCT excited state electron transfer quenching by the TiO₂ acceptor states. The Ru^III/II redox chemistry was found to be non-Nernstian; the ideality factors were cation-dependent, suggestive of an underlying electric field effect. Electrochemical reduction of the TiO₂ resulted in a black coloration and a blue shift of the fundamental (VB → CB) absorption, the normalized spectra were cation independent. Reduction of sensitized TiO₂ also resulted in a blue shift of the MLCT absorption, the magnitude of which was used to determine the surface electric fields. Under conditions where about 20 electrons were present in each anatase nanocrystallite, the electric field strength reported by the Ru compound followed the trend Na⁺ < Li⁺ < Mg²⁺ < Ca²⁺, with Na⁺ being 1.1 MV/cm and Ca²⁺ 2.3 MV/cm. In pulsed laser experiments, the first-derivative absorption signature was observed transiently after excited state injection and iodide oxidation. These absorption amplitudes were time-dependent and decayed over time periods where the number of injected electrons was constant, with behavior attributed to screening of the surface electric field by cations present in the electrolyte. The monovalent cations screened charge much more rapidly than did the dications, <i>k</i>_Li⁺,Na⁺ = 5.0 × 10⁴ s^–1 and <i>k</i>_{Mg²⁺,Ca²⁺} = 5.0 × 10² s^–1, presumably because the small number of injected electrons resulted in spatially isolated singly reduced Ti­(III) sites that were more easily screened by the monocations

Unexpected Roles of Triethanolamine in the Photochemical Reduction of CO₂ to Formate by Ruthenium Complexes

A series of 4,4′-dimethyl-2,2′-bipyridyl ruthenium complexes with carbonyl ligands were prepared and studied using a combination of electrochemical and spectroscopic methods with infrared detection to provide structural information on reaction intermediates in the photochemical reduction of CO2 to formate in acetonitrile (CH3CN). An unsaturated 5-coordinate intermediate was characterized, and the hydride-transfer step to CO2 from a singly reduced metal-hydride complex was observed with kinetic resolution. While triethanolamine (TEOA) was expected to act as a proton acceptor to ensure the sacrificial behavior of 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo­[d]­imidazole as an electron donor, time-resolved infrared measurements revealed that about 90% of the photogenerated one-electron reduced complexes undergo unproductive back electron transfer. Furthermore, TEOA showed the ability to capture CO2 from CH3CN solutions to form a zwitterionic alkylcarbonate adduct and was actively engaged in key catalytic steps such as metal-hydride formation, hydride transfer to CO2 to form the bound formate intermediate, and dissociation of formate ion product. Collectively, the data provide an overview of the transient intermediates of Ru­(II) carbonyl complexes and emphasize the importance of considering the participation of TEOA when investigating and proposing catalytic pathways

Unexpected Roles of Triethanolamine in the Photochemical Reduction of CO₂ to Formate by Ruthenium Complexes

A series of 4,4′-dimethyl-2,2′-bipyridyl ruthenium complexes with carbonyl ligands were prepared and studied using a combination of electrochemical and spectroscopic methods with infrared detection to provide structural information on reaction intermediates in the photochemical reduction of CO2 to formate in acetonitrile (CH3CN). An unsaturated 5-coordinate intermediate was characterized, and the hydride-transfer step to CO2 from a singly reduced metal-hydride complex was observed with kinetic resolution. While triethanolamine (TEOA) was expected to act as a proton acceptor to ensure the sacrificial behavior of 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo­[d]­imidazole as an electron donor, time-resolved infrared measurements revealed that about 90% of the photogenerated one-electron reduced complexes undergo unproductive back electron transfer. Furthermore, TEOA showed the ability to capture CO2 from CH3CN solutions to form a zwitterionic alkylcarbonate adduct and was actively engaged in key catalytic steps such as metal-hydride formation, hydride transfer to CO2 to form the bound formate intermediate, and dissociation of formate ion product. Collectively, the data provide an overview of the transient intermediates of Ru­(II) carbonyl complexes and emphasize the importance of considering the participation of TEOA when investigating and proposing catalytic pathways

Electronic and Electrochemical Control of Isostructural Ruthenium Hydricities and the Implications for Catalytic Overpotentials

Electronic tuning of metal hydrides enables precise control over potentials, mechanisms, selectivity, and rates of electrocatalytic reactions by regulating bond dissociation free energies such as the hydricity (ΔGH–°) and pKa of the catalyst. Here, we investigate a series of electronically tuned ruthenium hydrido complexes that are isostructural at the metal center: [Ru­(4,4′-R2-bpy)2(CO)­H]+ (R = CF3, Cl, H, CH3, and CH3O; bpy = 2,2′-bipyridine) (denoted as (R)­Ru–H+). A substantial 22 kcal mol–1 hydricity range is available across five complexes in three stable oxidation states: (R)­Ru–H+, (R)­Ru–H0, and (R)­Ru–H–. Thermodynamic and mechanistic predictions of electrocatalytic proton reduction were tested experimentally by reducing protons from weak acids to H2. Two mechanisms are observed, depending on the acid strength and the catalyst hydricity. The rate constants for hydride transfer and protonation of the catalyst were, in some cases, extracted from the analysis of cyclic voltammetry data. A key finding is a 400 mV decrease in the catalytic overpotential for H2 production by using a doubly reduced electron-poor metal hydride instead of a singly reduced electron-rich metal hydride. The former also exhibits a higher rate constant for hydride transfer, representing a strategy to disconnect rate and free energy relationships

Unexpected Roles of Triethanolamine in the Photochemical Reduction of CO₂ to Formate by Ruthenium Complexes

A series of 4,4′-dimethyl-2,2′-bipyridyl ruthenium complexes with carbonyl ligands were prepared and studied using a combination of electrochemical and spectroscopic methods with infrared detection to provide structural information on reaction intermediates in the photochemical reduction of CO2 to formate in acetonitrile (CH3CN). An unsaturated 5-coordinate intermediate was characterized, and the hydride-transfer step to CO2 from a singly reduced metal-hydride complex was observed with kinetic resolution. While triethanolamine (TEOA) was expected to act as a proton acceptor to ensure the sacrificial behavior of 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo­[d]­imidazole as an electron donor, time-resolved infrared measurements revealed that about 90% of the photogenerated one-electron reduced complexes undergo unproductive back electron transfer. Furthermore, TEOA showed the ability to capture CO2 from CH3CN solutions to form a zwitterionic alkylcarbonate adduct and was actively engaged in key catalytic steps such as metal-hydride formation, hydride transfer to CO2 to form the bound formate intermediate, and dissociation of formate ion product. Collectively, the data provide an overview of the transient intermediates of Ru­(II) carbonyl complexes and emphasize the importance of considering the participation of TEOA when investigating and proposing catalytic pathways

Electronic and Electrochemical Control of Isostructural Ruthenium Hydricities and the Implications for Catalytic Overpotentials

Electronic tuning of metal hydrides enables precise control over potentials, mechanisms, selectivity, and rates of electrocatalytic reactions by regulating bond dissociation free energies such as the hydricity (ΔGH–°) and pKa of the catalyst. Here, we investigate a series of electronically tuned ruthenium hydrido complexes that are isostructural at the metal center: [Ru­(4,4′-R2-bpy)2(CO)­H]+ (R = CF3, Cl, H, CH3, and CH3O; bpy = 2,2′-bipyridine) (denoted as (R)­Ru–H+). A substantial 22 kcal mol–1 hydricity range is available across five complexes in three stable oxidation states: (R)­Ru–H+, (R)­Ru–H0, and (R)­Ru–H–. Thermodynamic and mechanistic predictions of electrocatalytic proton reduction were tested experimentally by reducing protons from weak acids to H2. Two mechanisms are observed, depending on the acid strength and the catalyst hydricity. The rate constants for hydride transfer and protonation of the catalyst were, in some cases, extracted from the analysis of cyclic voltammetry data. A key finding is a 400 mV decrease in the catalytic overpotential for H2 production by using a doubly reduced electron-poor metal hydride instead of a singly reduced electron-rich metal hydride. The former also exhibits a higher rate constant for hydride transfer, representing a strategy to disconnect rate and free energy relationships

Ter-Ionic Complex that Forms a Bond Upon Visible Light Absorption

A “ter-ionic complex” composed of a tetracationic Ru­(II) complex and two iodide ions was found to yield a covalent I–I bond upon visible light excitation in acetone solution. ¹H NMR, visible absorption and DFT studies revealed that one iodide was associated with a ligand while the other was closer to the Ru metal center. Standard Stern–Volmer quenching of the excited state by iodide revealed upward curvature with a novel saturation at high concentrations. The data were fully consistent with a mechanism in which the Ru metal center in the excited state accepts an electron from iodide to form an iodine atom and, within 70 ns, that atom reacts with the iodide associated with the ligand to yield I₂^•–. This rapid formation of an I–I bond was facilitated by the supramolecular assembly of the three reactant ions necessary for this ter-ionic reaction that is relevant to solar fuel production

Evidence that Δ<i>S</i>^‡ Controls Interfacial Electron Transfer Dynamics from Anatase TiO₂ to Molecular Acceptors

Recombination of electrons injected into TiO₂ with molecular acceptors present at the interface represents an important loss mechanism in dye-sensitized water oxidation and electrical power generation. Herein, the kinetics for this interfacial electron transfer reaction to oxidized triphenylamine (TPA) acceptors was quantified over a 70° temperature range for <i>para</i>-methyl-TPA (<b>Me</b>-TPA) dissolved in acetonitrile solution, 4-[<i>N</i>,<i>N</i>-di­(<i>p</i>-tolyl)­amino]­benzylphosphonic acid (<b><i>a</i>-TPA</b>) anchored to the TiO₂, and a TPA covalently bound to a ruthenium sensitizer, [Ru­(tpy-C₆H₄-PO₃H₂)­(tpy-TPA)]²⁺ “<b>RuTPA</b>”, where tpy is 2,2′:6′,2′′-terpyridine. Activation energies extracted from an Arrhenius analysis were found to be 11 ± 1 kJ mol^–1 for <b>Me</b>-TPA and 22 ± 1 kJ mol^–1 for <b><i>a</i>-TPA</b>, values that were insensitive to the identity of different sensitizers. Recombination to <b>RuTPA</b>^<b>+</b> proceeded with <i>E</i>_a = 27 ± 1 kJ mol^–1 that decreased to 19 ± 1 kJ mol^–1 when recombination occurred to an oxidized <i>para</i>-methoxy TPA (<b>MeO</b>-TPA) dissolved in CH₃CN. Eyring analysis revealed a smaller entropy of activation |Δ<i>S</i>^‡| when the <b><i>a</i>-TPA</b> was anchored to the surface or covalently linked to the sensitizer, compared to that when <b>Me</b>-TPA was dissolved in CH₃CN. In all cases, Eyring analysis provided large and negative Δ<i>S</i>^‡ values that point toward unfavorable entropic factors as the key contributor to the barrier that underlies the slow recombination kinetics that are generally observed at dye-sensitized TiO₂ interfaces

Ligand Control of Supramolecular Chloride Photorelease

Supramolecular assembly is shown to provide control over excited-state chloride release. Two dicationic chromophores were designed with a ligand that recognizes halide ions in CH₂Cl₂ and a luminescent excited state whose dipole was directed toward, <b>1</b>^<b>2+</b>, or away, <b>2</b>^<b>2+</b>, from an associated chloride ion. The dipole orientation had little influence on the ground-state equilibrium constant, <i>K</i>_eq ∼ 4 × 10⁶ M^–1, but induced a profound change in the excited-state equilibrium. Light excitation of <b>[1</b>^<b>2+</b>,<b>Cl</b>^<b>–</b><b>]</b>^<b>+</b> resulted in time-dependent shifts in the photoluminescence spectra with the appearance of biexponential kinetics consistent with the photorelease of Cl^–. Remarkably, the excited-state equilibrium constant was lowered by a factor of 20 and resulted in nearly 45% dissociation of chloride. In contrast, light excitation of <b>[2</b>^<b>2+</b>,<b>Cl</b>^<b>–</b><b>]</b>^<b>+</b> revealed a 45-fold increase in the excited-state equilibrium constant. The data show that rational design and supramolecular assembly enables the detection and photorelease of chloride ions with the potential for future applications in biology and chemistry