Tuning the charge flow between Marcus regimes in an organic thin-film device.

Marcus's theory of electron transfer, initially formulated six decades ago for redox reactions in solution, is now of great importance for very diverse scientific communities. The molecular scale tunability of electronic properties renders organic semiconductor materials in principle an ideal platform to test this theory. However, the demonstration of charge transfer in different Marcus regions requires a precise control over the driving force acting on the charge carriers. Here, we make use of a three-terminal hot-electron molecular transistor, which lets us access unconventional transport regimes. Thanks to the control of the injection energy of hot carriers in the molecular thin film we induce an effective negative differential resistance state that is a direct consequence of the Marcus Inverted Region.

Influence of structural defects and oxidation onto hole conductivity in P3HT.

The effect of structural imperfections as well as oxygen impurities on the quantum conductance of poly(3-hexylthiophene) is calculated from first-principles by solving the scattering problem for molecular structures obtained within density functional theory. It is shown that the conductivity of molecular crystals perpendicular to the polymer chains depends strongly on the stacking geometry and is roughly described within the Wentzel-Kramers-Brillouin approximation. Furthermore, it is found that local relaxation for twisted or bent polymer chains efficiently restores the conductance that drops substantially for sharp kinks with curvature radii smaller than 17 Å and rotations in excess of ∼60°. In contrast, isomer defects in the coupling along the chain direction are of minor importance for the intrachain transmission. Also, oxidation of the side chains as well as molecular sulfur barely changes the coherent transport properties, whereas oxidation of thiophene group carbon atoms drastically reduces the conductance.

Guanine crystals: a first principles study.

We report first principles density functional theory studies on the basic ground state characteristics, dynamic properties, and the electronic structure of guanine crystals. The effect of water molecules within the crystal is studied in detail, and we discuss their influence on the structural, vibrational, and electronic properties. The geometries calculated for various crystal structures are compared with gas-phase calculations and available experimental data. Phonon frequencies and eigenvectors are predicted for intermolecular and intramolecular lattice vibrations. Vibrational and electronic density-of-states are presented and analyzed. The electronic band structure near the fundamental gap is calculated from the Kohn-Sham approach. We find that the former molecular HOMO states form a dispersive band in the pi-pi stacking direction upon condensation resulting in a large bandwidth of 0.83 eV. Consequences for the charge transport in layered van der Waals bonded organic molecular crystals are discussed.

Electronic excitations of glycine, alanine, and cysteine conformers from first-principles calculations.

The electronic and optical properties are studied for three conformers of amino acid molecules using gradient-corrected (spin-) density functional theory within a projector-augmented wave scheme and the supercell method. We investigate single-particle excitations such as ionization energies and electron affinities as well as pair excitations. By comparing eigenvalues resulting from several local and nonlocal energy functionals, the influence of treatment of exchange and correlation is demonstrated. The excitations are described within the Delta-self-consistent field method with an occupation number constraint to obtain excitation energies and Stokes shifts. The results are used to also discuss the optical absorption properties. In contrast to the lowest single- and two-particle excitation energies, remarkable changes are found in absorption spectra in dependence on the conformation of the molecule geometry.

DFT studies using supercells and projector-augmented waves for structure, energetics, and dynamics of glycine, alanine, and cysteine.

A large variety of gas phase conformations of the amino acids glycine, alanine, and cysteine is studied by numerically efficient semi-local gradient-corrected density functional theory calculations using a projector-augmented wave scheme and periodic boundary conditions. Equilibrium geometries, conformational energies, dipole moments, vibrational modes, and IR optical spectra are calculated from first principles. A comparison of our results with values obtained from quantum-chemistry methods with localized basis sets and nonlocal exchange-correlation functionals as well as with experimental data is made. For conformations containing strong intramolecular hydrogen bonds deviations in their energetic ordering occur, which are traced back to different treatments of spatial nonlocality in the exchange-correlation functional. However, even for these structures, the comparison of calculated and measured vibrational frequencies shows satisfying agreement.

Attracted by long-range electron correlation: adenine on graphite.

The adsorption of adenine on graphite is analyzed from first-principles calculations as a model case for the interaction between organic molecules and chemically inert surfaces. Within density-functional theory we find no chemical bonding due to ionic or covalent interactions, only a very weak attraction at distances beyond the equilibrium position due to the lowering of the kinetic energy of the valence electrons. Electron exchange and correlation effects are much more important for the stabilization of the adsystem. They are modeled by the local density or generalized gradient approximation supplemented by the London dispersion formula for the van der Waals interaction.

[Dietrich Geotg Kieser (1779-1862)--merely a romantic physician?]. / Dietrich Georg Kieser (1779-1862)--nur ein romantischer Mediziner?

As an adherent of Schelling Dietrich Georg Kieser (1779-1862) was an outstanding representative of the German medical romanticism. From 1812 to his death he was professor of medicine in Jena. Despite natural philosophical-speculative theories his general pathology contains dynamic views of the disease including psychosomatic opinions. The basis for this is the holism of man under the aspect of developmental history. Kieser did an eminent good work in the field of medical practice and organisation. He was the head of several clinics and from 1847 he was the first professor in Jena who only worked in the field of psychiatry. 1858 he was elected president of the Imperial Leopoldino-Carolinic German Academy of Scientists.