The Role Played by Orbital Energetics in Solvent Mediated Electronic Coupling†

Molecular Dynamics Simulations of Bimolecular Electron Transfer: the Distance-Dependent Electronic Coupling

Understanding the distance dependence of the parameters underpinning Marcus theory is imperative when interpreting the results of experiments on electron transfer (ET). Unfortunately, most of these parameters are difficult or impossible to access directly with experiments, necessitating the use of computer simulations to model them. In this work, we use molecular dynamics simulations in conjunction with constrained density functional theory calculations to study the distance dependence of the electronic coupling matrix element, |H_RP|, for bimolecular ET. Contrary to what is typically assumed for such intermolecular reactions, we find that the magnitude of |H_RP| does not decay exponentially with the center-of-mass separation of the reactants, r_COM. The addition of other simple measures of donor/acceptor (D/A) orientation did not improve the correlation of |H_RP| with r_COM. Using the minimum distance separation, r_min, of the reactants as the structural descriptor allowed the system to be partitioned into high-coupling/close-contact and low-coupling/non-contact regimes, but large fluctuations of |H_RP| were still found for the close-contact reactant pairs. Despite the persistent large fluctuations of |H_RP|, its mean value was found to decay piecewise exponentially with increasing r_min, which was attributed to significant changes in the average D/A pair structure. The results herein advise one to use caution when interpreting the experimental results derived from spherical reactant models of bimolecular ET.

Defining Direct Orbital Pathways for Intermolecular Electron Transfer Using Sensitized Semiconducting Surfaces

High-performance electronic materials and redox catalysts often rely on fast rates of intermolecular electron transfer (IET). Maximizing IET rates requires strong electronic coupling (H_DA) between the electron donor and acceptor, yet universal structure-property relationships governing H_DA in outer-sphere IET reactions have yet to be developed. For ground-state IET reactions, H_DA is reasonably approximated by the extent of overlap between the frontier donor and acceptor orbitals involved in the electron-transfer reaction. Intermolecular interactions that encourage overlap between these orbitals, thereby creating a direct orbital pathway for IET, have a strong impact on H_DA and, by extension, the IET rates. In this Forum Article, we present a set of intuitive molecular design strategies employing this direct orbital pathway principle to maximize H_DA for IET reactions. We highlight how the careful design of redox-active molecules anchored to solid semiconducting substrates provides a powerful experimental platform for elucidating how electronic structure and specific intermolecular interactions affect IET reactions.

Resolving orbital pathways for intermolecular electron transfer

Over 60 years have passed since Taube deduced an orbital-mediated electron transfer mechanism between distinct metal complexes. This concept of an orbital pathway has been thoroughly explored for donor-acceptor pairs bridged by covalently bonded chemical residues, but an analogous pathway has not yet been conclusively demonstrated for formally outer-sphere systems that lack an intervening bridge. In our present study, we experimentally resolve at an atomic level the orbital interactions necessary for electron transfer through an explicit intermolecular bond. This finding was achieved using a homologous series of surface-immobilized ruthenium catalysts that bear different terminal substituents poised for reaction with redox active species in solution. This arrangement enabled the discovery that intermolecular chalcogen⋯iodide interactions can mediate electron transfer only when these interactions bring the donor and acceptor orbitals into direct contact. This result offers the most direct observation to date of an intermolecular orbital pathway for electron transfer.

C- and Z-Shaped Coordination Compounds. Synthesis, Structure, and Spectroelectrochemistry of cis- and trans-[Re(CO)3(L)]2-2,2‘-bisbenzimidizolate with L = 4-Phenylpyridine, 2,4‘-Bipyridine, or Pyridine

A series of C- and Z-shaped complexes of the form cis- and trans-[Re(CO)3(L)]2BiBzIm, where L = 4-phenylpyridine, 2,4'-bipyridine, or pyridine and BiBzIm = 2,2'-bisbenzimidizolate, have been synthesized by the reaction of [Re(CO)4]2BiBzIm with a slight excess of L in refluxing tetrahydrofuran. Five of the six compounds have been isolated and crystallographically and electrochemically characterized. Formation of the sixth, the cis form of the [Re(CO)3(4-phenylpyridine)]2BiBzIm, is evidently inhibited by the torsional steric demands of proximal 4-phenylpyridines. The compounds are acyclic analogues of recently studied tetrarhenium molecular rectangles and are of interest, in part, because of their potential to form ligand-centered mixed-valence (LCMV) compounds upon reduction by one electron. Spectroelectrochemical measurements corroborated the formation of a LCMV version of cis-[Re(CO)3(L)]2BiBzIm but failed to uncover a ligand-based intervalence transition. Electrochemical measurements revealed isomer-dependent L/L electrostatic effects, resulting in greater mixed-valence ion comproportionation for C-shaped assemblies versus Z-shaped assemblies.

Intramolecular Electron and Energy Transfer in an Axial ZnP−Pyridylfullerene Complex As Studied by X- and W-Band Time-Resolved EPR Spectroscopy

Light-driven electron transfer (ET) and energy transfer (EnT) in a self-assembled via axial coordination Zn-porphyrin-pyridylfullerene (ZnP-PyrF) complex were studied by time-resolved electron paramagnetic resonance (TREPR) spectroscopy at 9.5 GHz (X-band) and 95 GHz (W-band). The studies over a wide temperature range were carried out in media of different polarity, including isotropic toluene and tetrahydrofuran (THF), and anisotropic nematic liquid crystals (LCs), E-7 and ZLI-4389. At low temperatures (frozen matrices), photoexcitation of the ZnP donor results mainly in singlet-singlet EnT to the pyridine-appended fullerene acceptor. In fluid phases ET is the dominant process. Specifically, in isotropic solvents the generated radical pairs (RPs) are long-lived, with lifetimes exceeding that observed for covalently linked donor-acceptor systems. It is concluded that in liquid phases of both polar and nonpolar solvents the separation of the tightly bound complex into the more loosely bound structure slows down the back ET (BET) process. Photoexcitation of the donor in fluid phases of LCs does not result in the creation of the long-lived RPs, since the ordered LC matrix hinders the separation of the complex constituents. As a result, fast intramolecular BET takes place in the tightly bound complex. Contrarily to the behavior of covalently linked donor-acceptor systems in different LCs, the polarity of the LC matrix affects the ET process. Moreover, in contrast to covalently linked D-s-A systems, utilization of LCs for the coordinatively linked D-s-A complexes does not reduce the ET rates significantly.

Dynamic nature of the intramolecular electronic coupling mediated by a solvent molecule: a computational study

We present a combined Molecular Dynamics/Quantum Chemical study of the solvent-mediated electronic coupling between an electron donor and acceptor in a C-clamp molecule. We characterize the coupling fluctuations due to the solvent motion for different solvents (acetonitrile, benzene, 1,3-diisopropyl-benzene) for the charge separation and the charge recombination processes. The time scale for solvent-induced coupling fluctuation is approximately 0.1 ps. The effect of these fluctuations on the observed rate is discussed using a recently developed theoretical model. We show that, while the microscopic charge transfer process is very complicated and its computational modeling very subtle, the macroscopic phenomenology can be captured by the standard models. Analyzing the contribution to the coupling given by different solvent orbitals, we find that many solvent orbitals mediate the electron transfer and that paths through different solvent orbitals can interfere constructively or destructively. A relatively small subset of substrate-solvent configurations dominate contributions to solvent-mediated coupling. This subset of configurations is related to the electronic structure of the C-clamp molecule.