Nonadiabatic Electron Transfer Research Articles

Latent Electronic (Anti-)Ferroelectricity in BiNiO_{3}.

BiNiO_{3} exhibits an unusual metal-insulator transition from Pnma to P1[over ¯] that is related to charge ordering at the Bi sites, which is intriguingly distinct from the charge ordering at Ni sites usually observed in related rare-earth nickelates. Here, using first principles calculations, we first rationalize the phase transition from Pnma to P1[over ¯], revealing an overlooked intermediate P2_{1}/m bridging phase and a complex interplay between distinct degrees of freedom. Going further, we point out that the charge ordering at Bi sites in the P1[over ¯] phase is not unique. We highlight an alternative polar ordering giving rise to a ferroelectric Pmn2_{1} phase nearly degenerated in energy with P1[over ¯] and showing an in-plane electric polarization of 53 μC/cm^{2} directly resulting from the charge ordering. The close energy of Pmn2_{1} and P1[over ¯] phases, together with low energy barrier between them, make BiNiO_{3} a potential electronic antiferroelectric in which the field-induced transition from nonpolar to polar would relate to nonadiabatic intersite electron transfer. We also demonstrate the possibility to stabilize an electronic ferroelectric ground state from strain engineering in thin films, using an appropriate substrate.

Nonadiabatic Proton-Coupled Electron Transfer at a Graphitic Surface Immobilized Cobalt Porphyrin

Nonadiabatic Proton-Coupled Electron Transfer at a Graphitic Surface Immobilized Cobalt Porphyrin

Unraveling abnormal collective effects via the non-monotonic number dependence of electron transfer in confined electromagnetic fields.

Strong light-matter coupling within an optical cavity leverages the collective interactions of molecules and confined electromagnetic fields, giving rise to the possibilities of modifying chemical reactivity and molecular properties. While collective optical responses, such as enhanced Rabi splitting, are often observed, the overall effect of the cavity on molecular systems remains ambiguous for a large number of molecules. In this paper, we investigate the non-adiabatic electron transfer process in electron donor-acceptor pairs influenced by collective excitation and local molecular dynamics. Using the timescale difference between reorganization and thermal fluctuations, we derive analytical formulas for the electron transfer rate constant and the polariton relaxation rate. These formulas apply to any number of molecules (N) and account for the collective effect as induced by cavity photon coupling. Our findings reveal a non-monotonic dependence of the rate constant on N, which can be understood by the interplay between electron transfer and polariton relaxation. As a result, the cavity-induced quantum yield increases linearly with N for small N (as predicted by a simple Dicke model) but shows a turnover and suppression for large N. We also interrelate the thermal bath frequency and the number of molecules, suggesting the optimal number for maximizing enhancement. The analysis provides an analytical insight for understanding the collective excitation of light and electron transfer, helping to predict the optimal condition for effective cavity-controlled chemical reactivity.

Photoinduced Electron Transfer in Inclusion Complexes of Carbon Nanohoops.

ConspectusPhotoinduced electron transfer (PET) in carbon materials is a process of great importance in light energy conversion. Carbon materials, such as fullerenes, graphene flakes, carbon nanotubes, and cycloparaphenylenes (CPPs), have unusual electronic properties that make them interesting objects for PET research. These materials can be used as electron-hole transport layers, electrode materials, or passivation additives in photovoltaic devices. Moreover, their appropriate combination opens up new possibilities for constructing photoactive supramolecular systems with efficient charge transfer between the donor and acceptor parts. CPPs build a class of molecules consisting of para-linked phenylene rings. CPPs and their numerous derivatives are appealing building blocks in supramolecular chemistry, acting as suitable concave receptors with strong host-guest interactions for the convex surfaces of fullerenes. Efficient PET in donor-acceptor systems can be observed when charge separation occurs faster than charge recombination. This Account focuses on selected inclusion complexes of carbon nanohoops studied by our group. We modeled charge separation and charge recombination in both previously synthesized and computationally designed complexes to identify how various modifications of host and guest molecules affect the PET efficiency in these systems. A consistent computational protocol we used includes a time-dependent density-functional theory (TD-DFT) formalism with the Tamm-Dancoff approximation (TDA) and CAM-B3LYP functional to carry out excited state calculations and the nonadiabatic electron transfer theory to estimate electron-transfer rates. We show how the photophysical properties of carbon nanohoops can be modified by incorporating additional π-conjugated fragments and antiaromatic units, multiple fluorine substitutions, and extending the overall π-electron system. Incorporating π-conjugated groups or linkers is accompanied by the appearance of new charge transfer states. Perfluorination of the nanohoops radically changes their role in charge separation from an electron donor to an electron acceptor. Vacancy defects in π-extended nanohoops are shown to hinder PET between host and guest molecules, while large fully conjugated π-systems improve the electron-donor properties of nanohoops. We also highlight the role of antiaromatic structural units in tuning the electronic properties of nanohoops. Depending on the aromaticity degree of monomeric units in nanohoops, the direction of electron transfer in their complexes with C60 fullerene can be altered. Nanohoops with aromatic units usually act as electron donors, while those with antiaromatic monomers serve as electron acceptors. Finally, we discuss why charged fullerenes are better electron acceptors than neutral C60 and how the charge location allows for the design of more efficient donor-acceptor systems with an unusual hypsochromic shift of the charge transfer band in polar solvents.

(Keynote) Electrochemical Proton-Coupled Electron Transfer Theory

Proton-coupled electron transfer (PCET) plays a vital role in a wide range of electrochemical processes. This talk will describe theoretical and computational methods that have been developed to study electrochemical PCET and a variety of applications to molecular and heterogeneous electrocatalysis. My group has formulated a general PCET theory that includes the quantum mechanical effects of the electrons and transferring protons, as well as the motions of the donor-acceptor modes and solvent or protein environment. This PCET theory enables the calculation of rate constants and kinetic isotope effects for comparison to experiment. Our extension of this theory to electrochemical PCET incorporates the electronic structure of the electrode and the interfacial electric fields arising from the electrical double layer. Theoretical formulations for both homogeneous and heterogeneous electrochemical PCET provide analytical expressions for the rate constants and current densities as functions of applied potential. This theory has been applied to proton discharge on metal electrodes, as well as PCET at metal oxides and graphite-conjugated catalysts. These applications highlight the importance of using a theory that quantizes the transferring proton and includes the effects of hydrogen tunneling and excited electron-proton vibronic states. The insights from these theoretical studies are useful for the design of electrocatalytic systems to control the movement and coupling of electrons and protons for energy conversion processes.References Venkataraman, A. V. Soudackov, and S. Hammes-Schiffer, Theoretical formulation of nonadiabatic electrochemical proton-coupled electron transfer at metal-solution interfaces, J. Phys. Chem. C 112, 12386-12397 (2008).K. Goldsmith, Y. C. Lam, A V. Soudackov, and S. Hammes-Schiffer, Proton discharge on a gold electrode from triethylammonium in acetonitrile: Theoretical modeling of potential-dependent kinetic isotope effects, J. Am. Chem. Soc. 141, 1084-1090 (2019).C. Lam, A. V. Soudackov, and S. Hammes-Schiffer, Kinetics of proton discharge on metal electrodes: Effects of vibrational nonadiabaticity and solvent dynamics, J. Phys. Chem. Lett. 10, 5312-5217 (2019).E. Warburton, P. Hutchison, M. N. Jackson, M. L. Pegis, Y. Surendranath, and S. Hammes-Schiffer, “Interfacial field-driven proton-coupled electron transfer at graphite-conjugated organic acids,” J. Am. Chem. Soc. 142, 20855-20864 (2020).E. Warburton, A. V. Soudackov, and S. Hammes-Schiffer, Theoretical modeling of electrochemical proton-coupled electron transfer, Chem. Rev. 122, 10599-10650 (2022).

Impact of Molecular Orientation on Lateral and Interfacial Electron Transfer at Oxide Interfaces.

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.

Understanding the Key Roles of pH Buffer in Accelerating Lignin Degradation by Lignin Peroxidase.

pH buffer plays versatile roles in both biology and chemistry. In this study, we unravel the critical role of pH buffer in accelerating degradation of the lignin substrate in lignin peroxidase (LiP) using QM/MM MD simulations and the nonadiabatic electron transfer (ET) and proton-coupled electron transfer (PCET) theories. As a key enzyme involved in lignin degradation, LiP accomplishes the oxidation of lignin via two consecutive ET reactions and the subsequent C-C cleavage of the lignin cation radical. The first one involves ET from Trp171 to the active species of Compound I, while the second one involves ET from the lignin substrate to the Trp171 radical. Differing from the common view that pH = 3 may enhance the oxidizing power of Cpd I via protonation of the protein environment, our study shows that the intrinsic electric fields have minor effects on the first ET step. Instead, our study shows that the pH buffer of tartaric acid plays key roles during the second ET step. Our study shows that the pH buffer of tartaric acid can form a strong H-bond with Glu250, which can prevent the proton transfer from the Trp171-H•+ cation radical to Glu250, thereby stabilizing the Trp171-H•+ cation radical for the lignin oxidation. In addition, the pH buffer of tartaric acid can enhance the oxidizing power of the Trp171-H•+ cation radical via both the protonation of the proximal Asp264 and the second-sphere H-bond with Glu250. Such synergistic effects of pH buffer facilitate the thermodynamics of the second ET step and reduce the overall barrier of lignin degradation by ∼4.3 kcal/mol, which corresponds to a rate acceleration of 103-fold that agrees with experiments. These findings not only expand our understanding on pH-dependent redox reactions in both biology and chemistry but also provide valuable insights into tryptophan-mediated biological ET reactions.

Effect of the number of graphene layers on the electron transfer kinetics at metal/graphene heterostructures

We theoretically investigated the kinetics of outer-sphere electron transfer (ET) at graphene with different number of layers in the presence and absence of metal substrates using density functional theory (DFT) calculations. It is shown that the experimental data on the electrocatalytic activity of the metal substrate for few-layer graphene can be explained under the assumption of the nonadiabatic electron transfer. However, the electron transfer at the metal-supported single-layer graphene is likely adiabatic. The enhanced electron transfer in the presence of the metal substrate is explained by the hybridization of the metal and graphene electronic states, which provides more efficient interaction of the metal substrate with the reactant near the graphene surface.

Reorganization Energies for Interfacial Electron Transfer across Phenylene Ethynylene Rigid-Rod Bridges.

A family of three ruthenium bipyridyl rigid-rod compounds of the general form [Ru(bpy)2(LL)](PF6)2 were anchored to mesoporous thin films of tin-doped indium oxide (ITO) nanocrystals. Here, LL is a 4-substituted 2,2-bipyridine (bpy) ligand with varying numbers of conjugated phenylenethynylene bridge units between the bipyridine ring and anchoring group consisting of a bis-carboxylated isophthalic group. The visible absorption spectra and the formal potentials, Eo(RuIII/II), of the surface anchored rigid-rods were insensitive to the presence of the phenylene ethynylene bridge units in 0.1 M tetrabutyl ammonium perchlorate acetonitrile solutions (TBAClO4/CH3CN). The conductive nature of the ITO enabled potentiostatic control of the Fermi level and hence a means to tune the Gibbs free energy change, -ΔG°, for electron transfer from the ITO to the rigid-rods. Pseudo-rate constants for this electron transfer reaction increased as the number of bridge units decreased at a fixed -ΔG°. With the assumption that the reorganization energy, λ, and the electronic coupling matrix element, Hab, were independent of the applied potential, rate constants measured as a function of -ΔG° and analyzed through Marcus-Gerischer theory provided estimates of Hab and λ. In rough accordance with the dielectric continuum theory, λ was found to increase from 0.61 to 0.80 eV as the number of bridge units was increased. In contrast, Hab decreased markedly with distance from 0.54 to 0.11 cm-1, consistent with non-adiabatic electron transfer. Comparative analysis with previously published studies of bridges with an sp3-hybridized carbon indicated that the phenylene ethynylene bridge does not enhance electronic coupling between the oxide and the rigid-rod acceptor. The implications of these findings for practical applications in solar energy conversion are specifically discussed.

Free Energy Dependencies for Interfacial Electron Transfer from Tin-Doped Indium Oxide (ITO) to Molecular Photoredox Catalysts

John B. Goodenough proposed that interfacial electron transfer kinetics from main group metal oxides should be fundamentally different from that of transition metal oxides, an expectation that has not been widely tested. Herein, the kinetics for interfacial electron transfer from mesoporous transparent conductive oxide Tin-doped Indium Oxide (ITO) to four photoredox catalysts (PCs) were characterized in acetonitrile electrolytes. The photocatalysts had the form: [Ru(4,4ʹ-R2-2,2′-bipyridine)2(4,4ʹ-(PO3H2)2-2,2′-bipyridine)]2+, where R was H, methoxy, tert-butyl, and Br. The impact of the surface binding group was characterized with [Ru(2,2′-bipyridine)2(4,4ʹ-(CO2H)2-bpy)]2+. The interfacial electron transfer reaction ITO(e−)∣PC+ → ITO∣PC was quantified by nanosecond absorption spectroscopy as a function of the applied potential (and hence ‒ΔG°). Specific conditions of applied potential were identified where the kinetics were sensitive to the incident irradiance. A layer-by-layer method was used to insert ionic methylene bridge(s) between the PC and the oxide surface. Marcus-Gerischer analysis of the kinetic data indicates non-adiabatic interfacial electron transfer with total reorganization energies that increase when bridges were placed between the photocatalyst and the ITO surface.

Modulation of the kinetics of outer-sphere electron transfer at graphene by a metal substrate.

Solid-supported graphene is a typical configuration of electrochemical devices based on single-layer graphene. Therefore, it is necessary to understand the electrochemical features of such heterostructures. In this work, we theoretically investigated the effect of the metal type on the nonadiabatic electron transfer (ET) at the metal-supported graphene using DFT calculations. We considered five metals Au, Ag, Pt, Cu, and Al on which graphene is physically adsorbed. It is shown that all metals catalyze the ET. The electrocatalytic effect increases in the following series Al < Au ≲ Ag ≈ Cu < Pt. The enhanced ET in the presence of the metal substrate is explained by the hybridization of metal and graphene states, due to which the coupling between the reactant in an electrolyte and metal is increased. Metal-dependent electrocatalytic effect is explained both by different densities of states at the Fermi level of the systems and by differences in the behaviour of the tails of hybridized wave functions in the electrolyte region. The shift of the Fermi level with respect to the Dirac point in graphene when charging at the metal/graphene/electrolyte interface does not affect the kinetics due to the small contribution of graphene states to the electron transfer.

Adiabatic versus non-adiabatic electron transfer at 2D electrode materials

2D electrode materials are often deployed on conductive supports for electrochemistry and there is a great need to understand fundamental electrochemical processes in this electrode configuration. Here, an integrated experimental-theoretical approach is used to resolve the key electronic interactions in outer-sphere electron transfer (OS-ET), a cornerstone elementary electrochemical reaction, at graphene as-grown on a copper electrode. Using scanning electrochemical cell microscopy, and co-located structural microscopy, the classical hexaamineruthenium (III/II) couple shows the ET kinetics trend: monolayer > bilayer > multilayer graphene. This trend is rationalized quantitatively through the development of rate theory, using the Schmickler-Newns-Anderson model Hamiltonian for ET, with the explicit incorporation of electrostatic interactions in the double layer, and parameterized using constant potential density functional theory calculations. The ET mechanism is predominantly adiabatic; the addition of subsequent graphene layers increases the contact potential, producing an increase in the effective barrier to ET at the electrode/electrolyte interface.

Ab Initio Nonadiabatic Surface-Hopping Trajectory Simulations of Photocatalytic Water Oxidation and Hydrogen Evolution with the Heptazine Chromophore.

In the past decade, polymeric carbon nitrides consisting of heptazine (Hz) building blocks emerged as highly promising materials for photocatalytic hydrogen evolution from water or sacrificial electron donors with near-ultraviolet light. However, the complexity of these materials and their poor characterization at the atomic level are detrimental to the unraveling of the detailed mechanisms of the reactions leading to hydrogen evolution. Recently, it has been shown that a derivative of the Hz molecule, trianisole-heptazine (TAHz), is a potent photobase, which readily oxidizes various derivatives of phenol and even water by an excited-state proton-coupled electron-transfer (PCET) reaction. Energy profiles along minimum-energy reaction paths and relaxed PCET potential-energy surfaces, which previously were computed with ab initio electronic-structure methods for complexes of Hz and TAHz with protic substrates, led to qualitative insights. To obtain more quantitative insight, reaction dynamics simulations are required. In the present work, the time scales of the electron and proton transfer processes and the branching ratios of competing channels were explored with ab initio on-the-fly quasiclassical surface-hopping trajectory simulations for the hydrogen-bonded Hz-H2O complex. By the analysis of representative trajectories, detailed insight into the interplay of various nonadiabatic electronic transitions, electron transfer, proton transfer, and vibrational energy relaxation is obtained. The HzH radicals which are formed by the photoreduction of Hz can disproportionate to Hz and HzH2 in an exothermic reaction with a low reaction barrier. The time scales and branching ratios of competing channels in H-atom photodetachment from the HzH2 molecule also were explored with ab initio surface-hopping simulations. These results delineate for the first time a quantitatively supported scenario of water oxidation and hydrogen evolution with a molecular carbon nitride photocatalyst.

Energy natural orbitals.

We propose and numerically demonstrate that highly correlated electronic wavefunctions such as those of configuration interaction, the cluster expansion, and so on, and electron wavepackets superposed thereof can be analyzed in terms of one-electron functions, which we call energy natural orbitals (ENOs). As the name suggests, ENOs are members of the broad family of natural orbitals defined by Löwdin, in that they are eigenfunctions of the energy density operator. One of the major characteristics is that the (orbital) energies of all the ENOs are summed up exactly equal to the total electronic energy of a wavefunction under study. Another outstanding feature is that the population of each ENO varies as the chemical reaction proceeds, keeping the total population constant though. The study of ENOs has been driven by the need for new methods to analyze extremely complicated nonadiabatic electron wavepackets such as those embedded in highly quasi-degenerate excited-state manifolds. Yet, ENOs can be applied to scrutinize many other chemical reactions, ranging from the ordinary concerted reactions, nonadiabatic reactions, and Woodward-Hoffman forbidden reactions, to excited-state reactions. We here present the properties of ENOs and a couple of case studies of numerical realization, one of which is about the mechanism of nonadiabatic electron transfer.

Crossover between the adiabatic and nonadiabatic electron transfer limits in the Landau-Zener model

The semiclassical models of nonadiabatic transition were proposed first by Landau and Zener in 1932, and have been widely used in the study of electron transfer (ET); however, experimental demonstration of the Landau-Zener formula remains challenging to observe. Herein, employing the Hush-Marcus theory, thermal ET in mixed-valence complexes {[Mo2]-(ph)n-[Mo2]}+ (n = 1–3) has been investigated, spanning the nonadiabatic throughout the adiabatic limit, by analysis of the intervalence transition absorbances. Evidently, the Landau-Zener formula is valid in the adiabatic regime in a broader range of conditions than the theoretical limitation known as the narrow avoided-crossing. The intermediate system is identified with an overall transition probability (κel) of ∼0.5, which is contributed by the single and the first multiple passage. This study shows that in the intermediate regime, the ET kinetic results derived from the adiabatic and nonadiabatic formalisms are nearly identical, in accordance with the Landau-Zener model. The obtained insights help to understand and control the ET processes in biological and chemical systems.

Marcus’ electron transfer rate revisited via a Rice-Ramsperger-Kassel-Marcus analogue: A unified formalism for linear and nonlinear solvation scenarios

In the pioneering work by R. A. Marcus, the solvation effect on electron transfer (ET) processes was investigated, giving rise to the celebrated nonadiabatic ET rate formula. In this work, on the basis of the thermodynamic solvation potentials analysis, we reexamine Marcus’ formula with respect to the Rice-Ramsperger-Kassel-Marcus (RRKM) theory. Interestingly, the obtained RRKM analogue, which recovers the original Marcus’ rate that is in a linear solvation scenario, is also applicable to the nonlinear solvation scenarios, where the multiple curve-crossing of solvation potentials exists. Parallelly, we revisit the corresponding Fermi’s golden rule results, with some critical comments against the RRKM analogue proposed in this work. For illustration, we consider the quadratic solvation scenarios, on the basis of physically well-supported descriptors.

Effect of a Au underlayer on outer-sphere electron transfer across a Au/graphene/electrolyte interface.

The effect of a gold underlayer on the outer-sphere non-adiabatic electron transfer on a graphene surface is investigated theoretically using both periodic and cluster DFT calculations. We propose a model that describes the alignment of energy levels and charge redistribution at the metal/graphene/redox electrolyte interface. Model calculations were performed for the [Fe(CN)6]3-/4- and [Ru(NH3)6]3+/2+ redox couples. It is shown that the gold support increases the rate constant of electron transfer. Gold electronic states hybridize with graphene wave functions, which provides an effective overlap with reactant orbitals outside the graphene layer and favors an increasing reaction rate. Although the Fermi level shift relative to the Dirac point in graphene depends significantly on the redox couple, this weakly affects the electron transfer kinetics at the Au(111)/graphene/electrolyte interface due to a small contribution of graphene states to the rate constant as compared to gold ones.

Theory of coupled ion-electron transfer kinetics

The microscopic theory of chemical reactions is based on transition state theory, where atoms or ions transfer classically over an energy barrier, as electrons maintain their ground state. Electron transfer is fundamentally different and occurs by tunneling in response to solvent fluctuations. Here, we develop the theory of coupled ion-electron transfer, in which ions and solvent molecules fluctuate cooperatively to facilitate non-adiabatic electron transfer. We derive a general formula of the reaction rate that depends on the overpotential, solvent properties, the electronic structure of the electron donor/acceptor, and the excess chemical potential of ions in the transition state. For Faradaic reactions, the theory predicts curved Tafel plots with a concentration-dependent reaction-limited current. For moderate overpotentials, our formula reduces to the Butler–Volmer equation and explains its relevance, not only in the well-known limit of large electron-transfer (solvent reorganization) energy, but also in the opposite limit of large ion-transfer energy. The rate formula is applied to Li-ion batteries, where reduction of the electrode host material couples with ion insertion. In the case of lithium iron phosphate, the theory accurately predicts the concentration dependence of the exchange current measured by in operando X-Ray microscopy without any adjustable parameters. These results pave the way for interfacial engineering to enhance ion intercalation rates, not only for batteries, but also for ionic separations and neuromorphic computing.

Solvent influence on non-adiabatic interfacial electron transfer at conductive oxide electrolyte interfaces.

The kinetics for interfacial electron transfer (ET) from a transparent conductive oxide (tin-doped indium oxide, ITO, Sn:In2O3) to molecular acceptors 4-[N,N-di(p-tolyl)amino]benzylphosphonic acid, TPA, and [RuII(bpy)2(4,4'-(PO3H2)2-bpy)]2+, RuP, positioned at variable distances within and beyond the electric double layer (EDL), were quantified in benzonitrile and methanol by nanosecond absorption spectroscopy as a function of the thermodynamic driving force, -ΔG°. Relevant ET parameters such as the rate constant, ket, reorganization energy, λ, and electronic coupling, Hab, were extracted from the kinetic data. Overall, ket increased as the distance between the molecular acceptor and the conductor decreased. For redox active molecules within the Helmholtz planes of the EDL, ket was nearly independent of -ΔG°, consistent with a negligibly small λ value. Rips-Jortner analysis revealed a non-adiabatic electron transfer mechanism consistent with Hab < 1 cm-1. The data indicate that the barrier for electron transfer is greatly diminished at the conductor-electrolyte interface.

Perspectives on Dye Sensitization of Nanocrystalline Mesoporous Thin Films.

Recent advances in our mechanistic understanding of dye-sensitized electron transfer reactions occurring at metal oxide interfaces are described. These advances were enabled by the advent of mesoporous thin films, comprised of anatase TiO2 nanocrystallites, that are amenable to spectroscopic and electrochemical characterization in unprecedented molecular-level detail. The metal-to-ligand charge transfer (MLCT) excited states of Ru polypyridyl compounds serve as the dye sensitizers. Excited-state injection often occurs on ultrafast time scales with yields that can be tuned from unity to near zero through modification of the sensitizer or the electrolyte composition. Transport of the injected electron and the oxidized sensitizer (hole hopping) are both operative in the composite mechanism for charge recombination between the injected electron and the oxidized sensitizer. Sensitizers that contain a pendant electron donor, as well as core/shell SnO2/TiO2 nanostructures, often prolong the lifetime of the injected electron and provide fundamental insights into adiabatic and nonadiabatic electron transfer mechanisms. Regeneration of the oxidized sensitizer by iodide is enhanced through halogen bonding, orbital pathways, and ion pairing. A substantial ∼10 MV cm-1 electric field is created by electron injection into TiO2 nanocrystallites that induces ion migration, reports on the sensitizer dipole orientation, and (in some cases) reorients or flips the sensitizer. Dye-sensitized conductive oxides also promote long-lived charge separation with bias dependent kinetics that provide insights into the reorganization energies associated with electron and proton-coupled electron transfer in the electric double layer.