Electron Transfer Reactions of C-shaped Molecules in Alkylated Aromatic Solvents: Evidence that the Effective Electronic Coupling Magnitude Is Temperature-Dependent

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.

Choosing sides: unusual ultrafast charge transfer pathways in an asymmetric electron-accepting cyclophane that binds an electron donor

Photo-driven electron transfer is faster from an electron donor guest to the harder to reduce acceptor in an asymmetric cyclophane host.

Constructing functional molecular systems for solar energy conversion and quantum information science requires a fundamental understanding of electron transfer in donor–bridge–acceptor (D–B–A) systems as well as competitive reaction pathways in acceptor–donor–acceptor (A–D–A) and acceptor–donor–acceptor′ (A–D–A′) systems. Herein we present a supramolecular complex comprising a tetracationic cyclophane having both phenyl-extended viologen (ExV²⁺) and dipyridylthiazolothiazole (TTz²⁺) electron acceptors doubly-linked by means of two p-xylylene linkers (TTzExVBox⁴⁺), which readily incorporates a perylene (Per) guest in its cavity (Per ⊂ TTzExVBox⁴⁺) to establish an A–D–A′ system, in which the ExV²⁺ and TTz²⁺ units serve as competing electron acceptors with different reduction potentials. Photoexcitation of the Per guest yields both TTz⁺˙–Per⁺˙–ExV²⁺ and TTz²⁺–Per⁺˙–ExV⁺˙ in <1 ps, while back electron transfer in TTz²⁺–Per⁺˙–ExV⁺˙ proceeds via the unusual sequence TTz²⁺–Per⁺˙–ExV⁺˙ → TTz⁺˙–Per⁺˙–ExV²⁺ → TTz²⁺–Per–ExV²⁺. In addition, selective chemical reduction of TTz²⁺ gives Per ⊂ TTzExVBox³⁺˙, turning the complex into a D–B–A system in which photoexcitation of TTz⁺˙ results in the reaction sequence ²*TTz⁺˙–Per–ExV²⁺ → TTz²⁺–Per–ExV⁺˙ → TTz⁺˙–Per–ExV²⁺. Both reactions TTz²⁺–Per⁺˙–ExV⁺˙ → TTz⁺˙–Per⁺˙–ExV²⁺ and TTz²⁺–Per–ExV⁺˙ → TTz⁺˙–Per–ExV²⁺ occur with a (16 ± 1 ps)^–1 rate constant irrespective of whether the bridge molecule is Per⁺˙ or Per. These results are explained using the superexchange mechanism in which the ionic states of the perylene guest serve as virtual states in each case and demonstrate a novel supramolecular platform for studying the effects of bridge energetics within D–B–A systems.

Ultrafast dynamics of nonequilibrium electron transfer in photoinduced redox cycle: solvent mediation and conformation flexibility

We report here our systematic characterization of a photoinduced electron-transfer (ET) redox cycle in a covalently linked donor-spacer-acceptor flexible system, consisting of N-acetyl-tryptophan methylester as an electron donor and thymine as an electron acceptor in three distinct solvents of water, acetonitrile, and dioxane. With femtosecond resolution, we determined all the ET time scales, forward and backward, by following the complete reaction evolution from reactants to intermediates and finally to products. Surprisingly, we observed two distinct ET dynamics in water, corresponding to a stacked configuration with ultrafast ET in 0.7 ps and back ET in 4.5 ps and a partially folded C-clamp conformation with ET in 322 ps but back ET in 17 ps. In acetonitrile and dioxane, only the C-clamp conformations were observed with ET in 470 and 1068 ps and back ET in 110 and 94 ps, respectively. These relatively slow ET dynamics in hundreds of picoseconds all showed significant conformation heterogeneity and followed a stretched decay behavior. With both forward and back ET rates determined, we derived solvent reorganization energies and coupling constants. Significantly, we found that solvent molecules intercalated in the cleft of the C-clamp structure mediate electron transfer with a tunneling parameter (β) of 1.0-1.4 Å(-1) and the high-frequency vibration modes in the product(s) couple with the back ET process, leading to the ultrafast back ET dynamics in tens of picoseconds. These findings provide mechanistic insights of nonequilibrium ET dynamics modulated by conformation flexibility, mediated by unique solvent configuration, and accelerated by vibrational coupling.

Electron transfer triggered by optical excitation of phenothiazine-tris(meta-phenylene-ethynylene)-(tricarbonyl)(bpy)(py)rhenium(I)

We have investigated excited-state electron transfer in a donor-bridge-acceptor complex containing phenothiazine (PTZ) linked via tris(meta-phenylene-ethynylene) to a tricarbonyl(bipyridine)(pyridine)Re(I) unit. Time-resolved luminescence experiments reveal two excited-state (*Re) decay regimes, a multiexponential component with a mean lifetime of 2.7 ns and a longer monoexponential component of 530 ns in dichloromethane solution. The faster decay is attributed to PTZ → *Re electron transfer in a C-shaped PTZ-bridge-Re conformer (PTZ-Re ≈ 7.5 Å). We assign the longer lifetime, which is virtually identical to that of free *Re, to an extended conformer (PTZ-Re > 20 Å). The observed biexponential *Re decay requires that interconversion of PTZ-bridge-Re conformers be slower than 10(6) s(-1).

Fluctuations in biological and bioinspired electron-transfer reactions

Central to theories of electron transfer (ET) is the idea that nuclear motion generates a transition state that enables electron flow to proceed, but nuclear motion also induces fluctuations in the donor-acceptor (DA) electronic coupling that is the rate-limiting parameter for nonadiabatic ET. The interplay between the DA energy gap and DA coupling fluctuations is particularly noteworthy in biological ET, where flexible protein and mobile water bridges take center stage. Here, we discuss the critical timescales at play for ET reactions in fluctuating media, highlighting issues of the Condon approximation, average medium versus fluctuation-controlled electron tunneling, gated and solvent relaxation controlled electron transfer, and the influence of inelastic tunneling on electronic coupling pathway interferences. Taken together, one may use this framework to establish principles to describe how macromolecular structure and structural fluctuations influence ET reactions. This framework deepens our understanding of ET chemistry in fluctuating media. Moreover, it provides a unifying perspective for biophysical charge-transfer processes and helps to frame new questions associated with energy harvesting and transduction in fluctuating media.

Temperature-Dependent Mechanistic Transition for Photoinduced Electron Transfer Modulated by Excited-State Vibrational Relaxation Dynamics

The electron transfer (ET) dynamics of an unusually rigid pi-stacked (porphinato)zinc(II)-spacer-quinone (PZn-Q) system, [5-[8'-(4' '-[8' ''-(2' '' ',5' '' '-benzoquinonyl)-1' ''-naphthyl]-1' '-phenyl)-1'-naphthyl]-10,20-diphenylporphinato]zinc(II) (2a-Zn), in which sub-van der Waals interplanar distances separate juxtaposed porphyryl, aromatic bridge, and quinonyl components of this assembly, have been measured by ultrafast pump-probe transient absorption spectroscopy over a 80-320 K temperature range in 2-methyl tetrahydrofuran (2-MTHF) solvent. Analyses of the photoinduced charge-separation (CS) rate data are presented within the context of several different theoretical frameworks. Experiments show that at higher temperatures the initially prepared 2a-Zn vibronically excited S1 state relaxes on an ultrafast time scale, and ET is observed exclusively from the equilibrated lowest-energy S1 state (CS1). As the temperature decreases, production of the photoinduced charge-separated state directly from the vibrationally unrelaxed S1 state (CS2) becomes competitive with the vibrational relaxation time scale. At the lowest experimentally interrogated temperature ( approximately 80 K), CS2 defines the dominant ET pathway. ET from the vibrationally unrelaxed S1 state is temperature-independent and manifests a subpicosecond time constant; in contrast, the CS1 rate constant is temperature-dependent, exhibiting time constants ranging from 4x10(10) s(-1) to 4x10(11) s(-1) and is correlated strongly with the temperature-dependent solvent dielectric relaxation time scale over a significant temperature domain. Respective electronic coupling matrix elements for each of these photoinduced CS1 and CS2 pathways were determined to be approximately 50 and approximately 100 cm-1. This work not only documents a rare, if not unique, example of a system where temperature-dependent photoinduced charge-separation (CS) dynamics from vibrationally relaxed and unrelaxed S1 states can be differentiated, but also demonstrates a temperature-dependent mechanistic transition of photoinduced CS from the nonadiabatic to the solvent-controlled adiabatic regime, followed by a second temperature-dependent mechanistic evolution where CS becomes decoupled from solvent dynamics and is determined by the extent to which the vibrationally unrelaxed S1 state is populated.

Competition between Superexchange-Mediated and Sequential Electron Transfer in a Bridged Donor-Acceptor System

The temperature- and solvent-dependence of photoinduced electron-transfer reactions in a porphyrin-based donor-bridge-acceptor (DBA) system is studied by fluorescence and transient absorption spectroscopy. Two competing processes occur: sequential and direct superexchange-mediated electron transfer. In a weakly polar solvent (2-methyltetrahydrofuran), only direct electron transfer from the excited donor to the appended acceptor is observed, and this process has weak temperature dependence. In polar solvents (butyronitrile and dimethylformamide), both processes are observed and the sequential electron transfer shows strong temperature dependence. In systems where both electron transfer processes are observed, the long-range superexchange-mediated process is more than two times faster than the sequential process, even though the donor-acceptor distance is significantly larger in the former case.

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.

Hole Transfer in a C-Shaped Molecule: Conformational Freedom versus Solvent-Mediated Coupling

The electronic coupling matrix elements attending the charge separation reactions of a C-shaped molecule containing an excited pyrene as the electron acceptor and a dimethylaniline as the donor are determined in aromatic, ether, and ester solvents. Band shape analyses of the charge-transfer emission spectra (CT --> S(0)) provide values of the reaction free energy, the solvent reorganization energy, and the vibrational reorganization energy in each solvent. The free energy for charge separation in benzene and toluene solvents is independently determined from the excited state equilibrium established between the locally excited pyrene S(1) state and the charge-transfer state. Analyses of the charge separation kinetics using the spectroscopically determined reorganization energies and reaction free energies indicate that the electronic coupling is solvent independent, despite the presence of a cleft between the donor and acceptor. Hence, solvent molecules are not involved in the coupling pathway. The orientations of the donor and acceptor units, relative to the spacer, are not rigidly constrained, and their torsional motions decrease solvent access to the cleft. Generalized Mulliken-Hush calculations show that rotation of the pyrene group about the bond connecting it to the spacer greatly modulates the magnitude of through-space coupling between the S(1) and CT states. The relationship between the torsional dynamics and the electron-transfer dynamics is discussed.