Selected short abstracts
Electrochemical and Photochemical Cyclization and Cycloreversion of Diarylethenes and Diarylethene-Capped Sexithiophene Wires
A combined theoretical and experimental study was performed on diarylethenes and diarylethene-capped sexithiophenes aiming at an improved understanding of the electrochemical and photochemical ring-opening and ring-closing mechanisms. Theoretical calculations, based on DFT and TDDFT, suggested that the spatial distribution and the occupancy of the frontier orbitals determine and control the diarylethenes' ring-opening and ring-closing upon photoirradiation and electrochemical oxidation. Optimized geometries, potential energy surfaces, and activation energies between the open-ring and closed-ring forms were calculated for diarylethenes in the neutral ground state, excited states, and mono- and dicationic states. Analysis of the frontier orbitals was employed to understand the cyclization and cycloreversion of diarylethenes and to predict and explain the switching properties of diarylethene-capped sexithiophene molecular wires. The TDDFT data were verified with experimentally measured UV/vis spectra. The DFT calculations estimated open-shell ground states of diarylethene-capped sexithiophene dications, which were verified with EPR spectroscopy, and the broadening of the peaks in the EPR spectra were explained with the calculated singlet-triplet splitting. The good agreement of experiment and theory allows for the understanding of switching behavior of diarylethenes in solutions, in metal break junctions, in monolayers on metal surfaces, and as a part of complex organic molecular wires.
DNA conductance
The conductance through short DNA molecules connected to gold electrodes is studied with quantum mechanical methods. The anchoring of the molecules to the electrodes is investigated and in addition to the covalent S-Au bond, weak molecule-surface interactions between the aromatic heterocyclic bases and the electrodes are found. These weak interactions are important for the electron transport through DNA molecules. Tunneling mechanism is suggested and the conductive properties of the nucleotides in a metal-molecule-metal junction are compared. Different four-nucleotide DNA sequences are investigated. Significant value for the current is calculated for 1.5 V applied bias for a DNA sequence consisting of guanine and cytosine nucleotides. It is shown that adenine-thymine nucleotide pairs introduce potential barriers for the electron transport and therefore significantly decline the conductance. The obtained results are compared with recent experimental observations (Nanotech. 2009, 20, 115502) and previous theoretical studies (ChemPhysChem 2003, 4, 1256).
Donor-π-Bridge-Acceptor Unimolecular Electric Rectifier
The electrical rectifying properties of a single-molecule nanowire from the type donor-π-bridge-acceptor are investigated by means of the nonequilibrium Green’s function method, combined with density functional theory (NEGF-DFT). The investigated nanowire is an oligo-1,4-phenylene ethylene with π-donor and π-acceptor groups attached on opposite sides of the molecule. The donor and acceptor wires are separated by a π-bridge, in contrast to the Aviram-Ratner rectifier, which is a donor-σ-bridge-acceptor diode. A model more similar to the real molecular electronic device is considered with relaxation of the molecular geometry, under the interaction with external electric field, taking into account its influence on the electronic properties of the nanowire. An asymmetric current-bias (I-V) diagram is observed, with a conductance ratio of 7. The analysis of the spatial distribution of frontier orbitals, the highest occupied molecular orbital-lowest unoccupied molecular orbital ( HOMO-LUMO) gaps, and the transmission spectra give an inside view of the observed results.
