Molecular approaches to solar-energy conversion require a kinetic optimization of light-induced electron-transfer reactions. At molecular-semiconductor interfaces, this optimization has previously been accomplished through control of the distance between the semiconductor donor and the molecular acceptor and/or the free energy that accompanies electron transfer. Here we show that a kinetic pathway for electron transfer from a semiconductor to a molecular acceptor also exists and provides an alternative method for the control of interfacial kinetics. The pathway was identified by the rational design of molecules in which the distance and the driving force were held near parity and only the geometric torsion about a xylyl- or phenylthiophene bridge was varied. Electronic coupling through the phenyl bridge was a factor of ten greater than that through the xylyl bridge. Comparative studies revealed a significant bridge dependence for electron transfer that could not be rationalized by a change in distance or driving force. Instead, the data indicate an interfacial electron-transfer pathway that utilizes the aromatic bridge orbitals.

The total reorganization energy, λ, for interfacial electron transfer, ET, from a conductive electrode to redox-active molecules at fixed positions within the electric double layer, EDL, has been determined experimentally. Conductive indium–tin-oxide (ITO, In2O3:Sn) mesoporous films were functionalized with 4-[N,N-di­(p-tolyl)-amino]­benzylphosphonic acid (TPA) and/or [RuII(bpy)2(4,4′-(PO3H2)2-bpy)]2+ (RuP), where bpy is 2,2′-bipyridine. The small inner-sphere reorganizations, λi, for RuIII/IIP and TPA+/0 make them excellent probes of outer-sphere reorganization energy, λo, as λi ≪ λo such that λ = λi + λo ≈ λo. Consecutive layer-by-layer addition of ZrIV-bridged methylenediphosphonic acid enabled positioning at distances from 4 to 27 Å from the ITO. Excited-state injection into the ITO by RuP* generated ITO­(e–)|RuIIIP. For ITO cofunctionalized with TPA and RuP, subnanosecond lateral ET yielded ITO­(e–)|TPA+. The kinetics for ET from ITO to RuIIIP or TPA+ were quantified spectroscopically as a function of applied potential (E app) and hence driving force, −ΔG°. Marcus–Gerischer analysis of this data provided λ. Significantly, λo was near zero at close electrode proximity, λ = 0.11 eV at a distance of ∼4 Å, as manifest by kinetics largely insensitive to E app. In agreement with dielectric continuum theory, λ increased to values expected in CH3CN solution when the molecule was positioned at a distance of ∼27 Å (λ = 0.94 eV). The data reveal small intrinsic barriers for electron transfer proximate to conductive interfaces, which is an exploitable behavior in solar energy conversion and other applications that utilize transparent conductive oxides to accept or deliver electrons.

Three iridium photosensitizers, [Ir(dCF 3 ppy) 2 (N−N)] + , where N−N is 1,4,5,8-tetraazaphenanthrene (TAP), pyrazino[2,3-a]phenazine (pzph), or benzo[a]pyrazino[2,3-h]phenazine (bpph) and dCF 3 ppy is 2-(3,5-bis(trifluoromethylphenyl)pyridine), were found to be remarkably strong photo-oxidants with enhanced light absorption in the visible region. In particular, judicious ligand design provided access to Ir-bpph, with a molar absorption coefficient, ε = 9800 M −1 cm −1 , at 450 nm and an excited-state reduction potential, E(Ir + * /0 ) = 1.76 V vs NHE. These complexes were successful in performing light-driven charge separation and energy storage, where all complexes photo-oxidized seven different electron donors with rate constants (0.089−3.06) × 10 10 M −1 s −1 . A Marcus analysis provided a total reorganization energy of 0.7 ± 0.1 eV for excited-state electron transfer.

Electron-transfer theories predict that an increase in the quantum-mechanical mixing (H) of electron donor and acceptor wavefunctions at the instant of electron transfer drives equilibrium constants toward unity. Kinetic and equilibrium studies of four acceptor-bridge-donor (A-B-D) compounds reported herein provide experimental validation of this prediction. The compounds have two redox-active groups that differ only by the orientation of the aromatic bridge: a phenyl-thiophene bridge (p) that supports strong electronic coupling of H > 1,000 cm; and a xylyl-thiophene bridge (x) that prevents planarization and decreases H < 100 cm without a significant change in distance. Pulsed-light excitation allowed kinetic determination of the equilibrium constant, K In agreement with theory, K(p) were closer to unity compared to K(x). A van't Hoff analysis provided clear evidence of an adiabatic electron-transfer pathway for p-series and a nonadiabatic pathway for x-series. Collectively, the data show that the absolute magnitude of the thermodynamic driving force for electron transfers are decreased when adiabatic pathways are operative, a finding that should be taken into account in the design of hybrid materials for solar energy conversion.

A central theme in introductory and advanced chemical education courses pertains directly to the transfer of electrons between atoms, ions, or molecules. This article presents theoretical treatments of electron transfer with specific attention toward applying these principles to experiment. The goal is to revitalize teaching electron transfer at the undergraduate and graduate levels. Central theoretical aspects are presented through the construction of Gibbs free (potential) energy surfaces with definitions and semiquantitative descriptions of the three key parameters necessary to compute electron transfer rate constants with Marcus theory: (1) the Gibbs free energy change, ΔG°; (2) the reorganization energy, λ; and (3) the electronic coupling between D and A wave functions, H DA. A simplified description of this theory is presented with classical free energy surfaces for the electron donor and acceptor wherein the force constant in Hooke’s Law is replaced by λ. Variation of ΔG° results in a Gaussian distribution of activation energies that give rise to Marcus normal, activationless, and inverted kinetic behaviors. Classical and contemporary experimental examples that have tested and utilized Marcus theory are described, including the first validation of the inverted kinetic region. It is shown that, as the donor–acceptor coupling increases, adiabatic electron transfer may result where it becomes more difficult to decouple the Marcus parameters through experiment. The trials and tribulations of doing so are described that provide context and enable readers to understand the prior electron transfer literature and use the pedagogical foundations presented herein for their own learning and pleasure.