Intra-strand phosphate-mediated pathways in microsolvated double-stranded DNA.

We argue that dry DNA charge transport in molecular junctions, over distances of tens of nanometers, can take place via independent intra-strand pathways involving the phosphate groups. Such pathways explain recent single-molecule experiments that compare currents in intact and nicked 100 base-pair double-stranded DNA. We explore the conditions that favor independent intra-strand transport channels with the participation of the phosphate groups, as opposed to purely base-mediated transport involving the pi-stacked bases and inter-strand transitions.

Molecular Wires for Efficient Long-Distance Triplet Energy Transfer.

We propose design rules for building organic molecular bridges that enable coherent long-distance triplet-exciton transfer. Using these rules, we describe example polychromophoric structures with low inner-sphere exciton reorganization energies, low static and dynamic disorder, and enhanced π-stacking interactions between nearest-neighbor chromophores. These features lead to triplet-exciton eigenstates that are delocalized over several units at room temperature.

The Role of Bridge-State Intermediates in Singlet Fission for Donor-Bridge-Acceptor Systems: A Semianalytical Approach to Bridge-Tuning of the Donor-Acceptor Fission Coupling.

We describe a semianalytical/computational framework to explore structure-function relationships for singlet fission in Donor (D)-Bridge (B)-Acceptor (A) molecular architectures. The aim of introducing a bridging linker between the D and A molecules is to tune, by modifying the bridge structure, the electronic pathways that lead to fission and to D-A-separated correlated triplets. We identify different bridge-mediation regimes for the effective singlet-fission coupling in the coherent tunneling limit and show how to derive the dominant fission pathways in each regime.

Backbone charge transport in double-stranded DNA.

Understanding charge transport in DNA molecules is a long-standing problem of fundamental importance across disciplines. It is also of great technological interest due to DNA's ability to form versatile and complex programmable structures. Charge transport in DNA-based junctions has been reported using a wide variety of set-ups, but experiments so far have yielded seemingly contradictory results that range from insulating or semiconducting to metallic-like behaviour.

Initial-state preparation effects in time-resolved electron paramagnetic resonance experiments.

We explain a recent experimental observation that the time-resolved electron paramagnetic resonance spectra of an organic molecule for optical excitation within a highly absorbing region of the molecule has similar intensities to the spectra for optical excitation in a nonabsorbing region [D. L. Meyer et al.

Observing Donor-to-Acceptor Electron-Transfer Rates and the Marcus Inverted Parabola in Molecular Junctions.

A recurring theme in molecular electronics is the relationship between the intramolecular electron transfer rate in a donor-bridge-acceptor system and the conductance at low bias in the corresponding electrode-bridge-electrode junction. The similarities between through-bridge donor-to-acceptor electron tunneling and through-bridge electrode-to-electrode Landauer transport led to the suggestion of an approximate linear relationship between the rate and the conductance for any given bridge. A large body of work indicates that the two quantities are not necessarily linearly related, showing different behaviors as a function of temperature, voltage and bridge length.

Quantum interferences among Dexter energy transfer pathways.

Dexter energy transfer in chemical systems moves an exciton (i.e., an electron-hole pair) from a donor chromophore to an acceptor chromophore through a bridge by a combination of bonded and non-bonded interactions.

How can infra-red excitation both accelerate and slow charge transfer in the same molecule?

A UV-IR-Vis 3-pulse study of infra-red induced changes to electron transfer (ET) rates in a donor-bridge-acceptor species finds that charge-separation rates are slowed, while charge-recombination rates are accelerated as a result of IR excitation during the reaction. We explore the underpinning mechanisms for this behavior, studying IR-induced changes to the donor-acceptor coupling, to the validity of the Condon approximation, and to the reaction coordinate distribution. We find that the dominant IR-induced rate effects in the species studied arise from changes to the density of states in the Marcus curve crossing region.

A Nonequilibrium Molecular Dynamics Study of Infrared Perturbed Electron Transfer.

Infrared (IR) excitation is known to change electron-transfer kinetics in molecules. We use nonequilibrium molecular dynamics (NEqMD) simulations to explore the molecular underpinnings of how vibrational excitation may influence nonadiabatic electron-transfer. NEqMD combines classical molecular dynamics simulations with nonequilibrium semiclassical initial conditions to simulate the dynamics of vibrationally excited molecules.

Dexter energy transfer pathways.

Energy transfer with an associated spin change of the donor and acceptor, Dexter energy transfer, is critically important in solar energy harvesting assemblies, damage protection schemes of photobiology, and organometallic opto-electronic materials. Dexter transfer between chemically linked donors and acceptors is bridge mediated, presenting an enticing analogy with bridge-mediated electron and hole transfer. However, Dexter coupling pathways must convey both an electron and a hole from donor to acceptor, and this adds considerable richness to the mediation process.

Vibrational control of electron-transfer reactions: a feasibility study for the fast coherent transfer regime.

Molecular vibrations and electron-vibrational interactions are central to the control of biomolecular electron and energy-transfer rates. The vibrational control of molecular electron-transfer reactions by infrared pulses may enable the precise probing of electronic-vibrational interactions and of their roles in determining electron-transfer mechanisms. This type of electron-transfer rate control is advantageous because it does not alter the electronic state of the molecular electron-transfer system or irreversibly change its molecular structure.

Two-Electron Transfer Pathways.

The frontiers of electron-transfer chemistry demand that we develop theoretical frameworks to describe the delivery of multiple electrons, atoms, and ions in molecular systems. When electrons move over long distances through high barriers, where the probability for thermal population of oxidized or reduced bridge-localized states is very small, the electrons will tunnel from the donor (D) to acceptor (A), facilitated by bridge-mediated superexchange interactions. If the stable donor and acceptor redox states on D and A differ by two electrons, it is possible that the electrons will propagate coherently from D to A.

Long-range charge transport in single G-quadruplex DNA molecules.

DNA and DNA-based polymers are of interest in molecular electronics because of their versatile and programmable structures. However, transport measurements have produced a range of seemingly contradictory results due to differences in the measured molecules and experimental set-ups, and transporting significant current through individual DNA-based molecules remains a considerable challenge. Here, we report reproducible charge transport in guanine-quadruplex (G4) DNA molecules adsorbed on a mica substrate.

Charge transfer in dynamical biosystems, or the treachery of (static) images.

CONSPECTUS: The image is not the thing. Just as a pipe rendered in an oil painting cannot be smoked, quantum mechanical coupling pathways rendered on LCDs do not convey electrons. The aim of this Account is to examine some of our recent discoveries regarding biological electron transfer (ET) and transport mechanisms that emerge when one moves beyond treacherous static views to dynamical frameworks.

Biological charge transfer via flickering resonance.

Biological electron-transfer (ET) reactions are typically described in the framework of coherent two-state electron tunneling or multistep hopping. However, these ET reactions may involve multiple redox cofactors in van der Waals contact with each other and with vibronic broadenings on the same scale as the energy gaps among the species. In this regime, fluctuations of the molecular structures and of the medium can produce transient energy level matching among multiple electronic states.

Fragment orbital based description of charge transfer in peptides including backbone orbitals.

Charge transfer in peptides and proteins can occur on different pathways, depending on the energetic landscape as well as the coupling between the involved orbitals. Since details of the mechanism and pathways are difficult to access experimentally, different modeling strategies have been successfully applied to study these processes in the past. These can be based on a simple empirical pathway model, efficient tight binding type atomic orbital Hamiltonians or ab initio and density functional calculations.

Review: Probing protein electron transfer mechanisms from the molecular to the cellular length scales.

The mechanisms of bridge-mediated electron transfer (ET) reactions vary from coherent deep tunneling to thermally activated hopping. This short review focuses on some developments in the study of protein ET mechanisms at the molecular and at the cellular levels. It explains experimental and theoretical work on the influence of electronic-energy and electronic-coupling fluctuations on ET rates and on the switch from the tunneling to the hopping regimes.

Physical constraints on charge transport through bacterial nanowires.

Extracellular appendages of the dissimilatory metal-reducing bacterium Shewanella oneidensis MR-1 were recently shown to sustain currents of 10(10) electrons per second over distances of 0.5 microns [El-Naggar et al., Proc.

Floquet analysis for vibronically modulated electron tunneling.

Electron tunneling provides the primary reaction channel for electron transfer (ET) in many molecular systems. The analysis of such systems therefore requires the consideration of electronic coherence and interference effects. A model system for which tunneling may be either symmetry forbidden or allowed is considered here in the presence of a driving infrared (IR) field.

Coherence in electron transfer pathways.

Central to the view of electron-transfer reactions is the idea that nuclear motion generates a transition state geometry at which the electron/hole amplitude propagates coherently from the electron donor to the electron acceptor. In the weakly coupled or nonadiabatic regime, the electron amplitude tunnels through an electronic barrier between the donor and acceptor. The structure of the barrier is determined by the covalent and noncovalent interactions of the bridge.

Steady-state current transfer and scattering theory.

The correspondence between the steady-state theory of current transfer and scattering theory in a system of coupled tight-binding models of one-dimensional wires is explored. For weak interwire coupling both calculations give nearly identical results, except at singular points associated with band edges. The effect of decoherence in each of these models is studied using a generalization of the Liouville-von Neuman equation suitable for steady-state situations.

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.

Modulating unimolecular charge transfer by exciting bridge vibrations.

Ultrafast UV-vibrational spectroscopy was used to investigate how vibrational excitation of the bridge changes photoinduced electron transfer between donor (dimethylaniline) and acceptor (anthracene) moieties bridged by a guanosine-cytidine base pair (GC). The charge-separated (CS) state yield is found to be lowered by high-frequency bridge mode excitation. The effect is linked to a dynamic modulation of the donor-acceptor coupling interaction by weakening of H-bonding and/or by disruption of the bridging base-pair planarity.