Bridge Mediated Electron Transfer: A Unified Description of the Thermally Activated and Superexchange Mechanisms

DNA-like Helices as Nanosized Polarizers of Electromagnetic Waves

The possibility of using a conducting double DNA-like helix as the basis of an electromagnetic wave polarizer, which converts an incident linearly polarized wave into a reflected wave with circular polarization, has been shown. A high-frequency resonance is studied, at which the wavelength of the incident radiation is approximately equal to the length of a helical turn. The simulation of a double DNA-like helix has been carried out. The electric currents arising in the helical strands under waves with circular polarization at high-frequency resonance have been analyzed. Fundamentally different behavior of the double DNA-like helix concerning waves with right-hand or left-hand circular polarization has been established, which can be called the effect of polarization selectivity. This effect is manifested in the fact that a double DNA-like helix at high-frequency resonance can create a reflected wave having only one sign of circular polarization. The electric vector of the reflected wave produces a turn in space with the opposite winding direction compared to the double helix. These studies also highlight the electromagnetic forces of interaction between helical strands. The equilibrium of the double DNA-like helix has been studied, including as an element of metamaterials and as an object with a high potential for use in nanotechnology.

TiO2-Photoanode-Assisted Direct-Solar-Energy Harvesting and Storage in a Solar-Powered Redox Cell Using Halides as Active Materials

The rapid deployment of renewable energy is resulting in significant energy security, climate change mitigation, and economic benefits. We demonstrate here the direct solar-energy harvesting and storage in a rechargeable solar-powered redox cell, which can be charged solely by solar irradiation. The cell follows a conventional redox-flow cell design with one integrated TiO₂ photoanode in the cathode side. Direct charging of the cell by solar irradiation results in the conversion of solar energy in to chemical energy. Whereas discharging the cell leads to the release of chemical energy in the form of electricity. The cell integrates energy conversion and storage processes in a single device, making the solar energy directly and efficiently dispatchable. When using redox couples of Br₂/Br^- and I₃^-/I^- in the cathode side and anode side, respectively, the cell can be directly charged upon solar irradiation, yielding a discharge potential of 0.5 V with good round-trip efficiencies. This design is expected to be a potential alternative toward the development of affordable, inexhaustible, and clean solar-energy technologies.

Site-directed electronic tunneling through a vibrating molecular network

The effect of electronic-nuclear coupling on electronic transport through a complex molecular network is studied. Electronic tunneling dynamics in a network of N donor/acceptor sites, connected by molecular bridges, is shown to be controlled by electronic-nuclear coupling at the bridges. Particularly, electronic coupling to an accepting nuclear mode at the contact site between the donor and the rest of the network is shown to affect the tunneling path selection to specific acceptors. In the "deep" tunneling regime, the network is mapped onto an N-level system using a recursive perturbation expansion, enabling analytical treatment of the electronic dynamics. The analytic formulation is applied for two model systems, demonstrating site-directed tunneling by electronic-nuclear coupling. Numerical simulations suggest that this phenomenon is not limited to the deep tunneling regime.

Understanding Electron Transfer across Negatively-Charged Aib Oligopeptides

The physicochemical effects modulating the conformational behavior and the rate of intramolecular dissociative electron transfer in phthalimide-Aibn-peroxide peptides (n = 0-3) have been studied by an integrated density functional/continuum solvent model. We found that three different orientations of the phthalimide ring are possible, labeled Phihel, PhiC7, and PhipII. In the condensed phase, they are very close in energy when the system is neutral and short. When the peptide chain length increases and the system is negatively charged, Phihel becomes instead the most stable conformer. Our calculations confirm that the 3(10)-helix is the most stable secondary structure for the peptide bridge. However, upon charge injection in the phthalimide end of the phthalimide-Aib3-peroxide, the peptide bridge can adopt an alpha-helix conformation as well. The study of the dependence of the frontier orbitals on the length and on the conformation of the peptide bridge (in agreement with experimental indications) suggests that for n = 3 the process could be influenced by a 3(10) --> alpha-helix conformational transition of the peptide chain.

Bridge mediated two-electron transfer reactions: On the influence of intersite Coulomb interactions

Donor-acceptor two-electron transfer (TET) mediated by a linear molecular bridge is described theoretically. The particular case is considered where the TET takes place in the presence of a strong electronic intersite coupling within the bridge and against the background of fast vibrational relaxation processes. For such a situation the coarse-grained description of bridge-assisted electron transfer in molecular systems can be utilized [Petrov et al., J. Phys. Chem. B 106, 3092 (2002)]. In the present case it leads to kinetic equations and rate expression for TET reactions. Our recent treatment of completely nonadiabtic TET reactions [Petrov et al., J. Chem. Phys. 120, 4441 (2004)] including a reduction to single-exponential kinetics (with overall transfer rate K(TET)) is generalized here to the case of strong intrabridge coupling and the presence of intersite Coulomb interactions. The dependence of K(TET) on the bridge length which is determined by a separate stepwise and concerted contribution is discussed in detail. It is found that the intersite Coulomb interaction favors the TET if the donor and the acceptor are uncharged in their completely reduced states (with two excess electrons present).