The synthesis, spectroscopic characterization, computational analysis, and photoelectrochemical behavior of a new iron-based chromophore, [(Cpy)2Fe(deeb)](PF6)2 (Fe(Cpy)2(deeb)), where Cpy is 1-methyl-3-(2-pyridyl)imidazole and deeb is 4,4'-(CO2CH2CH3)2-2,2'-bipyridine, is reported. Electrochemically reversible waves assigned to a metal-centered Eo(FeIII/II) = +0.48 and a ligand-centered Eo(Fe2+/+) = -1.47 V vs Fc+/0 reduction were evident in cyclic voltammetry measurements. The combination of a strong σ-donor and a π-acceptor lowered the energy of the metal-to-ligand charge-transfer (MLCT) excited state relative to the metal-centered state. Two MLCT transitions appear in the visible region at 424 and 580 nm. TDDFT calculations revealed that the lower-energy band was well formulated as Fe(II)→deeb, and the higher-energy transition was charge transfer to both the deeb and Cpy ligands. Resonance Raman spectroscopy supports these findings showing enhanced deeb vibrational modes with 532 nm excitation, both deeb and Cpy modes with 473 nm excitation, and exclusively Cpy with 405 nm excitation. Ultrafast spectroscopy reveals a short-lived (∼2 ps) MLCT excited state and a longer-lived (∼20 ps) metal-centered state. Efficient methods to deprotect the ester groups and anchor the complex to mesoporous TiO2 (anatase) thin films in high surface coverages, Fe(Cpy)2(dcb)|TiO2 σ = 3 × 10-8 mol/cm2, were established. Pulsed light excitation of Fe(Cpy)2(dcb)|TiO2 resulted in rapid excited state injection (kinj > 108 s-1) and formation of a charge-separated state, FeIII(Cpy)2(dcb)|TiO2(e), which persists on the millisecond time scale before returning cleanly to the ground state with second-order kinetics. Injection yields measured 50 ns after light excitation were found to double from Φ = 0.15 with green (532 nm) light to 0.30 with blue (457 nm) light excitation. Incident photon-to-current efficiency (% IPCE) measurements as a function of excitation wavelength in a 0.5 M LiI/I2/CH3CN electrolyte provide clear evidence for band-selective "hot carrier" injection from the remote Cpy-localized excited state. Collectively, the spectroscopic and photoelectrochemical data indicate that a semiconductor can intercept hot electrons from iron chromophores even when the excited-state dipole is oriented away from the surface-anchoring ligand.

The oxidation of metal-aquo and -hydroxo complexes to generate the high-valent metal-oxo species used in oxidative catalysis is often kinetically slow due to sluggish proton transfer between ligated -H2O/-OH in the proton-coupled electron transfer (PCET) chemistry. In this research, a ruthenium water oxidation catalyst anchored to a conductive tin-doped indium oxide (ITO) thin film, abbreviated ITO|RuII-OH2, was characterized by spectroscopic and electrochemical methods in acetate or phosphate buffers. The deprotonated intermediate, RuII-OH, was observed spectroscopically in the PCET half-reaction ITO(e-)|RuIII-OH + H+ → ITO|RuII-OH2 indicating an underlying stepwise ET-PT mechanism. In contrast, at elevated buffer concentrations, this intermediate was absent, and a 2-4 order of magnitude increase in the proton transfer rate constant was observed. Kinetic data for this PCET reaction measured as a function of the driving force provided the reorganization energy λ = 1.05 eV and was assigned to a concerted electron-proton transfer (EPT) mechanism. In addition, the standard heterogeneous rate constants for two PCET equilibria, RuIII-OH + H+ + e- ⇌ RuII-OH2 and RuIV = O + H+ + e- ⇌ RuIII-OH were enhanced by these same buffers. Collectively, the data show that the added buffers can enhance the kinetics and thermodynamics for PCET reactions relevant to oxidative catalysis.

Metalcone thin films, composed of inorganic-organic hybrids, are synthesized using molecular layer deposition (MLD) through reactions between organometallic precursors (e.g., Sn, Al, and Ti) and organic reactants (e.g., ethylene glycol and glycerol). Despite their unique properties, metalcones exhibit significant vulnerability to water due to their organic components, limiting their potential in electrochemical applications. This study focuses on enhancing the photoelectrochemical stability of tincone thin films in aqueous electrolyte while preserving their hybrid characteristics through mild annealing in air at 250 degrees C. As-deposited and vacuum-annealed tincone thin films exhibited significant degradation under these conditions, while high-temperature-annealed (500 degrees C) tincone thin films offered improved stability with a significant decline in charge transfer efficiency. In contrast, mild annealing in air maintained the C-O bond at half level and improved the stability and charge transport without compromising the unique characteristics of tincone. This was confirmed by ellipsometry, X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectroscopy (FTIR). Mild-annealed tincone deposited on a lightly doped p-type silicon (p-Si) photocathode produced a 20-fold increase in CO volume compared to high-temperature annealed tincone in a CO2-saturated potassium bicarbonate (KHCO3) electrolyte with dispersed graphene oxide-cobalt phthalocyanine (GO-CoPc) under 1 sun illumination at 0.9 V vs reversible hydrogen electrode (RHE), while maintaining the faradaic efficiency for CO and H-2. These results suggest that mild-annealed tincone thin films hold significant potential as protective charge transport layers on silicon photocathodes for the aqueous CO2 reduction reaction (CO2RR).

Semiconductor photoelectrodes are regularly coupled to solid-state heterogeneous catalysts to perform solar-driven reduction of CO2. Less frequently, molecular catalysts are employed to better control the reactivity toward desired products, yet the development of robust semiconductor/molecule interfaces has proven challenging. Here, we demonstrate that a 2-3 nm thermal oxide layer on Si exhibits stability in aqueous solution, high photovoltage, and a photocurrent density of ∼10 mA/cm2 for the solar-driven photoelectrochemical reduction of a homogeneous molecular catalyst, producing syngas with an ∼2:1 H2 to CO ratio. Because of a low defect density, the oxide interface forms an electron inversion layer with metal-like electron density at cathodic potentials. This inversion layer facilitates electron transfer to redox-active molecules via tunneling even if the molecule's reduction potential is beyond the semiconductor's conduction band edge. Using an electrolyte solution composed of a homogeneous cobalt bis(terpyridine) catalyst in a water/organic solvent mixture, stable photoelectrochemistry was observed under 1-sun illumination, exhibiting an ∼30% Faradaic efficiency for CO that was similar to a glassy carbon electrode under comparable conditions. The results demonstrate that an ultrathin thermal oxide interface is a robust platform for development of aqueous-stable, molecule-driven photoelectrocatalysis.

Photoelectrochemical generation of a potent organic hydride donor at silicon is demonstrated. Two different oxide-coated p-type silicon photoelectrodes reduced 1,2,3,5,6-pentamethyl-1H-benzo[d]imidazol-3-ium hexafluorophosphate, [PMBI][PF6], to its corresponding imidazole hydride, PMBIH, in the presence of a proton source. Under 1 sun illumination, in acetonitrile with 1,5-diazabicyclo[4.3.0]non-5-ene (DBN) buffer, the p-Si photoelectrodes convert PMBI+ to PMBIH with good Faradaic efficiencies (FEs): 78% FE at -2.3 V vs Fc+/0 for Si|TiO2 and 83% FE at -2.6 V vs Fc+/0 for Si|SiO2 (where Si|SiO2 represents silicon coated with an oxide layer). Generally, the Si|TiO2 catalyzed the reaction at milder potentials than Si|SiO2, but the Si|SiO2 had better selectivity for PMBIH generation over H2 evolution than Si|TiO2. In light of prior studies of these photoelectrodes, the differences in selectivity and onset potential suggest a difference in mechanism, likely an outer-sphere electron transfer (ET) mechanism at the SiO2 surface and potentially a proton-coupled ET process at the TiO2 surface. To help understand reaction efficiency and identify areas of improvement, a thermochemical model for understanding net hydride transfer from the semiconductor to an acceptor in solution was developed. We find that the reactions in the present system are quite downhill. The high overpotentials (even while maintaining selectivity over H2 evolution) emphasize the need for improved catalysts. This approach to evaluate the thermodynamics of net hydride transfer should be broadly valuable for electrochemical and photoelectrochemical processes.

Earth-abundant, visible light absorbing zirconium photosensitizers are of great interest for their long-lived excited states. These photosensitizers consist of two electron rich tridentate ligands ligated to an electron deficient d0 Zr(IV) metal center, coordination chemistry that gives rise to ligand-to-metal charge transfer (LMCT) excited states. The vectoral nature of an LMCT transition was expected to give rise to an excited state dipole moment that electroabsorption data reported herein show is in fact absent for these photosensitizers. Electroabsorption spectra of three Zr(IV) photosensitizers bearing two tridentate pyridinedipyrrolide ligands, abbreviated Zr(PDP)2, were measured in a 2-MeTHF glass. Analysis of both traditional and higher-order electroabsorption spectra revealed significant changes in the polarizability between the ground and excited state without a dipole moment change. The spectral data indicates that upon the absorption of a single photon a quadrupolar excited state is formed via a simultaneous and equal charge transfer from both tridentate ligands to the Zr metal center. The data presented herein provides the first direct experimental evidence for such behavior in transition metal complexes with charge transfer excited states. The implications for the design of LMCT photosensitizers and our fundamental understanding of symmetry breaking in transition metal complexes are discussed.

Efficient and stable photoelectrochemical reduction of CO2 into highly reduced liquid fuels remains a formidable challenge, which requires an innovative semiconductor/catalyst interface to tackle. In this study, we introduce a strategy involving the fabrication of a silicon micropillar array structure coated with a superhydrophobic fluorinated carbon layer for the photoelectrochemical conversion of CO2 into methanol. The pillars increase the electrode surface area, improve catalyst loading and adhesion without compromising light absorption, and help confine gaseous intermediates near the catalyst surface. The superhydrophobic coating passivates parasitic side reactions and further enhances local accumulation of reaction intermediates. Upon one-electron reduction of the molecular catalyst, the semiconductor-catalyst interface changes from adaptive to buried junctions, providing a sufficient thermodynamic driving force for CO2 reduction. These structures together create a unique microenvironment for effective reduction of CO2 to methanol, leading to a remarkable Faradaic efficiency reaching 20% together with a partial current density of 3.4 mA cm(-2), surpassing the previous record based on planar silicon photoelectrodes by a notable factor of 17. This work demonstrates a new pathway for enhancing photoelectrocatalytic CO2 reduction through meticulous interface and microenvironment tailoring and sets a benchmark for both Faradaic efficiency and current density in solar liquid fuel production.

The utilization of visible light to mediate chemical reactions in fluid solutions has applications that range from solar fuel production to medicine and organic synthesis. These reactions are typically initiated by electron transfer between a photoexcited dye molecule (a photosensitizer) and a redox-active quencher to yield radical pairs that are intimately associated within a solvent cage. Many of these radicals undergo rapid thermodynamically favored "geminate" recombination and do not diffuse out of the solvent cage that surrounds them. Those that do escape the cage are useful reagents that may undergo subsequent reactions important to the above-mentioned applications. The cage escape process and the factors that determine the yields remain poorly understood despite decades of research motivated by their practical and fundamental importance. Herein, state-of-the-art research on light-induced electron transfer and cage escape that has appeared since the seminal 1972 review by J. P. Lorand entitled "The Cage Effect" is reviewed. This review also provides some background for those new to the field and discusses the cage escape process of both homolytic bond photodissociation and bimolecular light induced electron transfer reactions. The review concludes with some key goals and directions for future research that promise to elevate this very vibrant field to even greater heights.

The preassociation of reactants in a photoinitiated redox reaction through the use of noncovalent interactions can have a significant impact on excited state reactivity. As these noncovalent interactions render some stabilization to the associated species, they impact the kinetics and thermodynamics of photoinitiated electron transfer. Reported herein is a novel iridium(III) photocatalyst, equipped with an anion-sensitive, amide-substituted bipyridine ligand, and its reactivity with the halides (X = I-, Br-, Cl-) in acetonitrile and dichloromethane. A noteworthy periodic trend was observed, where the size and electron affinity dramatically altered the observed photoredox behavior. The binding affinity for the halides increased with decreasing ionic radius (K-eq similar to 10(3) to >10(6)) in a polar medium but association was stoichiometric for each halide in a nonpolar medium. Evidence for the static quenching of iodide and bromide is presented while dynamic quenching was observed with all halides. These results highlight how the photophysics of halide adducts and the thermodynamics of intra-ionic photo-oxidation are impacted as a consequence of preassociation of a quencher through hydrogen bonding.

Surface modification with CoFe-based overlayers has been widely studied to improve the performance of WO3/BiVO4 photoanodes for photoelectrochemical water oxidation because such overlayers can increase the photocurrent and shift the onset potential to more favorable values. Herein, we present a transient absorption spectroscopic analysis of WO3/BiVO4 photoanodes coated with cobalt iron oxide or cobalt iron Prussian blue overlayers, designed to establish the underlying mechanisms for these enhancements on the picosecond-to-second time scale. The data reveal that the overlayer suppresses recombination of trapped holes in BiVO4, with free and trapped electrons, and accepts photogenerated holes. These results show that the observed boost in efficiency for water oxidation can be explained by the dual role of the overlayer in inhibiting charge recombination and enhancing charge extraction.

Fundamental insight into multielectron, multiproton redox reactions with organometallic catalysts is greatly facilitated by knowledge of the formal oxidation state of the metal center in each of the elementary reaction steps that comprise the catalytic cycle. X-ray absorption near edge structure (XANES) is utilized herein to quantify the oxidation states and coordination environment of the organometallic resting state and intermediates in a newly proposed catalytic reduction of carbon monoxide to methanol.

A new homoleptic Ru polypyridyl complex bearing two aldehyde groups on each bipyridine ligand, [Ru(dab)(3)](PF6)(2), where dab is 4,4 '-dicarbaldehyde-2,2 '-bipyridine, was synthesized, characterized, and utilized for iodide photo-oxidation studies. In acetonitrile (CH3CN) solution, the complex displayed an intense metal-to-ligand charge transfer (MLCT) absorbance maximum at 475 nm (epsilon = 22,000 M-1 cm(-1)) and an infrared (IR) band at 1712 cm(-1) assigned to the pendent aldehyde groups. Visible light excitation in air-saturated solution resulted in room temperature photoluminescence (PL) with a maximum at 675 nm, a quantum yield, phi(PL) = 0.048, and an excited state lifetime, tau(omicron) = 440 ns, from which radiative and nonradiative relaxation rate constants were extracted, k(r) = 9.1 x 10(4) s(-1) and k(nr) = 1.8 x 10(6) s(-1). Pulsed visible light excitation yielded transient UV-vis and IR absorption spectra consistent with an MLCT excited state; relaxation occurred with the maintenance of two isosbestic points in the visible region, and a lifetime that agreed with that measured by time-resolved PL. Cyclic voltammetry studies in a CH3CN solution with 0.1 M TBAPF(6) electrolyte revealed a quasi-reversible oxidation, E degrees(Ru-III/II) = +1.25 V vs. Fc(+/0), and three sequential one-electron reductions at -1.10, -1.25, and -1.54 V vs. Fc(+/0). An excited state reduction potential of E degrees(Ru*(2+/+)) = +0.89 V vs. Fc(+/0) was estimated with the Rehm-Weller expression. Titration of tetrabutylammonium iodide, TBAI, into a CD3CN solution of [Ru(dab)(3)](PF6)(2) resulted in significant shifts in the aldehyde H atom and 3,3 '-biypridyl resonances that were analyzed with a 1:1 equilibrium model, from which K-eq = 460 M-1 was extracted, increasing to 5800 M-1 when the solvent was changed to acetone-d(6). Iodide titrations resulted in a significant quenching of the [Ru(dab)(3)]*(2+) lifetime and quantum yield in both CH3CN and acetone solvents. In CH3CN, the quenching was mainly dynamic and well described by the Stern-Volmer model, from which a quenching rate constant, k(q), of 4.5 x 10(10) M-1 s(-1) and an equilibrium constant, K-eq, of 8.3 x 10(3) M-1 were obtained. In acetone, the static quenching pathway by iodide was greatly enhanced, with a K-eq of 1.2 x 10(4) M-1 and a higher k(q) of 9.2 x 10(10) M-1 s(-1).

Structural gating provides a molecular means to transfer electrons preferentially in one desired vectorial direction, a behavior needed for applications in artificial photosynthesis. At the interfaces utilized herein, visible-light absorption by a transition metal complex opens a "structural gate" by planarization of otherwise rotating phenyl rings in p-phenylene ethynylene (PE) bridge units. Planarization provides a conjugated pathway for electron flow toward a conductive oxide surface. Interfacial electron transfer to the oxide restores rotation and closes the gate to the unwanted recombination reaction. This structural gating results in nearly quantitative long-distance (>20 Å) interfacial electron transfer that occurs ∼1000 times faster than transfer in the opposite direction. A comparative kinetic study of these complexes with those that contain ionic bridge units, without gating function, as a function of the applied potential and hence -ΔG° provided a physical basis for the structural gating. A small distance-dependent reorganization energy with weak electronic coupling underlies the success of this gate that enables efficient long-distance electron transfer and slow recombination.

Proton-coupled electron transfer (PCET) reactions play a crucial role in the interconversion of metal-aqua and metal-hydroxo species present in transition metal complexes and oxide surfaces (M(III)-OH + e− + H+ M(II)-OH2). For ruthenium-based water oxidation catalysts, PCET reactions involved in the mechanism of oxygen evolution have demonstrated a strong dependence on the identity and concentration of the proton donor and acceptor with significant rate enhancements observed for electrocatalysis performed in acetate, phosphate, and borate buffered electrolytes. However, the systematic study of this phenomenon has been hampered by the inability to independently measure discrete rates for electron transfer (ET) and proton transfer (PT) under electrochemical applied potentials. Herein, the PCET kinetics and mechanism of metal aqua bond formation in a ruthenium water oxidation catalyst [Ru(tpy)(bpy′)H2O]2+, Ru(II)−OH2 where tpy is 2,2′:6′,2″-terpyridine and bpy′ is 4,4′-diaminopropylsilatrane-2,2′-bypyridine were investigated at a conductive metal oxide interface as a function of buffer identity and concentration. The reaction of interest was triggered by visible light excitation of the catalyst and the kinetics of the independent ET and PT steps of the PCET mechanism were determined through nanosecond transient absorption spectroscopy. Kinetic measurements performed in aqueous acetate, phosphate, or borate buffer solutions revealed two distinct regimes of PT kinetics solely dependent on the buffer concentration. At the greatest buffer concentrations investigated (2 M acetate) spectral signals corresponding to the discreet ET and PT steps were absent indicative in a change in underlying PCET mechanism. Likewise, kinetic modeling indicated that PT from protonated acetate or phosphate occurred with rate constants that were 2-4 orders of magnitude greater than those for bulk water. In all, these results suggest that the presence of buffer-bases can significantly enhance PCET rates and, in this reaction, may alter the underlying mechanism.

The proton-coupled electron transfer (PCET) mechanism for the reaction Mox-OH + e- + H+ → Mred-OH2 was determined through the kinetic resolution of the independent electron transfer (ET) and proton transfer (PT) steps. The reaction of interest was triggered by visible light excitation of [RuII(tpy)(bpy')H2O]2+, RuII-OH2, where tpy is 2,2':6',2″-terpyridine and bpy' is 4,4'-diaminopropylsilatrane-2,2'-bipyridine, anchored to In2O3:Sn (ITO) thin films in aqueous solutions. Interfacial kinetics for the PCET reduction reaction were quantified by nanosecond transient absorption spectroscopy as a function of solution pH and applied potential. Data acquired at pH = 5-10 revealed a stepwise electron transfer-proton transfer (ET-PT) mechanism, while kinetic measurements made below pKa(RuIII-OH/OH2) = 1.3 were used to study the analogous interfacial reaction, where electron transfer was the only mechanistic step. Analysis of this data with a recently reported multichannel kinetic model was used to construct a PCET zone diagram and supported the assignment of an ET-PT mechanism at pH = 5-10. Ultimately, this study represents a unique example among Mox-OH/Mred-OH2 reactivity where the protonation and oxidation states of the intermediate were kinetically and spectrally resolved to firmly establish the PCET mechanism.

The electroabsorption and absorption spectra of eight homoleptic complexes of the general form [M(LL)(3)](2+) where M = Ru, Fe, and LL = 1,10-phenanthroline (phen), 2,2 '-bipyridine (bpy), and 4,4 ',-(R)(2)-bpy where R = -OCH3, -CF3, were quantified at 77 K in a butyronitrile glass. Intense metal-to-ligand charge transfer (MLCT) absorption bands were evident in the visible region. Electroabsorption spectra measured with applied electric fields >0.2 MV/cm were analyzed by the two-state Liptay model. Significant light-induced dipole moment changes of Delta mu() = 4-13 D were found consistent with a metal-to-ligand charge transfer (MLCT) excited state comprised an electron localized on a single diimine ligand, [M-III(LL-)(LL)(2)]*(2+), in the initially formed Franck-Condon excited state. A low energy feature evident in the electroabsorption spectra was assigned to a direct singlet-to-triplet MLCT excited state. The identity of the diimine ligand had an unexpected and large impact on these transitions. Analysis relative to the higher energy absorption provides a comparison of spin-allowed and disallowed transitions for first- and second-row transition metal complexes. With the notable exception of [Fe(CF(3)bpy)(3)](2+), the change in dipole moment for the (MLCT)-M-3 excited states was less than or equal to that of the (MLCT)-M-1 excited states. The charge transfer distances for the iron complexes were generally larger than those for the Ru complexes, a behavior attributed to a smaller degree of iron-diimine coupling in the ground state. A striking result was the sensitivity of the extinction coefficient and spectral profile of the low energy electroabsorption assigned to the identity of the diimine ligand; data that suggests electronic coupling with ligand localized triplet states and high spin metal centered states must be considered when modeling the Franck-Condon excited state.

Molecular dyes, called sensitizers, with a cis-[Ru(LL)(dcb)(NCS)2] structure, where dcb is 4,4'-(CO2H)2-2,2'-bipyridine and LL is dcb or a different diimine ligand, are among the most optimal for application in dye-sensitized solar cells (DSSCs). Herein, a series of five sensitizers, three bearing two dcb ligands and two bearing one dcb ligand, were anchored to mesoporous thin films of conducting tin-doped indium oxide (ITO) or semiconducting TiO2 nanocrystallites. The number of dcb ligands impacts the surface orientation of the sensitizer; density functional theory (DFT) calculations revealed an ∼1.6 Å smaller distance between the oxide surface and the Ru metal center for sensitizers with two dcb ligands. Interfacial electron transfer kinetics from the oxide material to the oxidized sensitizer were measured as a function of the thermodynamic driving force. Analysis of the kinetic data with Marcus-Gerischer theory indicated that the electron coupling matrix element, Hab, was sensitive to distance and ranged from Hab = 0.23 to 0.70 cm-1, indicative of nonadiabatic electron transfer. The reorganization energies, λ, were also sensitive to the sensitizer location within the electric double layer and were smaller, with one exception, for sensitizers bearing two dcb ligands λ = 0.40-0.55 eV relative to those with one λ = 0.63-0.66 eV, in agreement with dielectric continuum theory. Electron transfer from the oxide to the photoexcited sensitizer was observed when the diimine ligand was more easily reduced than the dcb ligand. Lateral self-exchange "hole hopping" electron transfer between surface-anchored sensitizers was found to be absent for sensitizers with two dcb ligands, while those with only one were found to hop with rates similar to those previously reported in the literature, khh = 47-89 μs-1. Collectively, the kinetic data and analysis reveal that interfacial kinetics are highly sensitive to the surface orientation and sensitizers bearing two dcb ligands are most optimal for practical applications of DSSCs.

The pH dependence of proton-coupled electron transfer (PCET) reactions, which are critical to many chemical and biological processes, is a powerful probe for elucidating their fundamental mechanisms. Herein, a general, multichannel kinetic model is introduced to describe the pH dependence of both homogeneous and electrochemical PCET reactions. According to this model, a weak pH dependence can arise from the competition among multiple sequential and concerted PCET channels involving different forms of the redox species, such as protonated and deprotonated forms, as well as different proton donors and acceptors. The contribution of each channel is influenced by the relative populations of the reactant species, which often depend strongly on pH, leading to complex pH dependence of PCET apparent rate constants. This model is used to explain the origins of the experimentally observed weak pH dependence of the electrochemical PCET apparent rate constant for a ruthenium-based water oxidation catalyst attached to a tin-doped In2O3 (ITO) surface. The weak pH dependence is found to arise from the intrinsic differences in the rate constants of participating channels and the dependence of their relative contributions on pH. This model predicts that the apparent maximum rate constant will become pH-independent at higher pH, which is confirmed by experimental measurements. Our analysis also suggests that the dominant channels are electron transfer at lower pH and sequential PCET via electron transfer followed by fast proton transfer at higher pH. This work highlights the importance of considering multiple competing channels simultaneously for PCET processes.

Photovoltages for hydrogen-terminated p-Si(111) in an acetonitrile electrolyte were quantified with methyl viologen [1,1'-(CH3)2-4,4'-bipyridinium](PF6)2, abbreviated MV2+, and [Ru(bpy)3](PF6)2, where bpy is 2,2'-bipyridine, that respectively undergo two and three one-electron transfer reductions. The reduction potentials, E°, of the two MV2+ reductions occurred at energies within the forbidden bandgap, while the three [Ru(bpy)3]2+ reductions occurred within the continuum of conduction band states. Bandgap illumination resulted in reduction that was more positive than that measured with a degenerately doped n+-Si demonstrative of a photovoltage, Vph, that increased in the order MV2+/+ (260 mV) < MV+/0 (400 mV) < Ru2+/+ (530 mV) ∼ Ru+/0 (540 mV) ∼ Ru0/- (550 mV). Pulsed 532 nm excitation generated electron-hole pairs whose dynamics were nearly constant under depletion conditions and increased markedly as the potential was raised or lowered. A long wavelength absorption feature assigned to conduction band electrons provided additional evidence for the presence of an inversion layer. Collectively, the data reveal that the most optimal photovoltage, as well as the longest electron-hole pair lifetime and the highest surface electron concentration, occurs when E° lies energetically within the unfilled conduction band states where an inversion layer is present. The bell-shaped dependence for electron-hole pair recombination with the surface potential was predicted by the time-honored SRH model, providing a clear indication that this interface provides access to all four bias conditions, i.e., accumulation, flat band, depletion, and inversion. The implications of these findings for photocatalysis applications and solar energy conversion are discussed.