A model analysis of centimeter-long electron transport in cable bacteria.

The recent discovery of cable bacteria has greatly expanded the known length scale of biological electron transport, as these multi-cellular bacteria are capable of mediating electrical currents across centimeter-scale distances. To enable such long-range conduction, cable bacteria embed a network of regularly spaced, parallel protein fibers in their cell envelope. These fibers exhibit extraordinary electrical properties for a biological material, including an electrical conductivity that can exceed 100 S cm-1. Traditionally, long-range electron transport through proteins is described as a multi-step hopping process, in which the individual hopping steps are described by Marcus electron transport theory. Here, we investigate to what extent such a classical hopping model can explain the conductance data recorded for individual cable bacterium filaments. To this end, the conductive fiber network in cable bacteria is modelled as a set of parallel one-dimensional hopping chains. Comparison of model simulated and experimental current(I)/voltage(V) curves, reveals that the charge transport is field-driven rather than concentration-driven, and there is no significant injection barrier between electrodes and filaments. However, the observed high conductivity levels (>100 S cm-1) can only be reproduced, if we include much longer hopping distances (a > 10 nm) and lower reorganisation energies (λ < 0.2 eV) than conventionally used in electron relay models of protein structures. Overall, our model analysis suggests that the conduction mechanism in cable bacteria is markedly distinct from other known forms of long-range biological electron transport, such as in multi-heme cytochromes.

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. We describe the dependence of these regimes on D-B-A many-electron state energetics and on D-B (A-B) one-electron and two-electron matrix elements. This semianalytical approach can be used to guide computational and experimental searches for D-B-A systems with tuned singlet fission rates. We use this approach to interpret the bridge-resonance effect of singlet fission that has been observed in recent experiments.

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. Apart from Landauer tunneling, incoherent hopping can be an important mechanism in molecular junctions. We propose a donor-bridge-acceptor molecular junction, functioning in the incoherent hopping regime, that is suited for establishing direct correlations between the electrode-to-electrode current and the intramolecular donor-to-acceptor electron transfer rate. We suggest that this type of junction may be used to observe the Marcus-inverted-parabola dependence of the intramolecular rate on energy gap by varying the bias voltage. The realization of such an experiment would enable meaningful comparisons between solution-phase electron transfer rates and molecular-junction currents for the same molecule.