Density functional theory calculations are performed to explain the electrical behavior of a π-conjugated oligo(phenylene ethynylene) resembling a resonant tunneling diode. Results of this theoretical study are compatible with the assumptions that electron transport occurs through the lowest unoccupied molecular orbital, that the conduction barrier is determined by the molecule chemical potential, and that the molecule becomes charged as the external potential increases. We are able to explain the nonlinear character of the current-voltage characteristic of the molecule and its temperature dependence. The intrinsic controlledamplification feature of these molecules is indicated.

A density-functional-theory treatment has been carried out on chalcogenide- and isonitrile-containing molecular systems (alligator clips) involved at the interface of molecule/Au-electrode contacts. The B3PW91 functional was used with effective core potentials provided within the LANL2DZ potentials and basis set. An extended basis set, LANL-E, was implemented by combining the valence, diffuse, and polarization basis from the 6-311++G** for H, C, N, S, and Se and by adding polarization and diffuse functions for Te and Au atoms. Results, including bond lengths and angles, ionization potentials, electron affinities, and binding energies for small systems containing Au atoms, were obtained with acceptable precision for those systems with available experimental information. Predicted quantities are reported for other systems that, as yet, have no experimental information available. This study indicates that, of the alligator clips studied, S would provide the best embodiment followed closely by Se- and Te-terminated molecules. This study also indicates that the precision obtained with calculations of first- and second-row atom-containing molecules can also be achieved with systems possessing heavier elements such as Au.

Quantum density functional theory and classical molecular dynamics studies of tolane molecules are carried out to interpret results of conductivity measurements on a monolayer of thiotolane molecules self-assembled on a gold surface and sandwiched by a titanium layer. Density functional theory techniques have been used to determine the ground state conformations and electronic structure, while classical molecular dynamics accounts for the effects of pressure and temperature for a cluster of five thiotolane molecules arranged between titanium and gold surfaces used to simulate the experimental system. On the basis of the theoretical results, it can be concluded, in agreement with the experimental findings, that the relative angle between two benzene rings in each tolane molecule determines its conductivity, with a maximum at 0° and a minimum at 90°. Therefore, this system would work as an unbiased controller, where the current through the molecule is controlled by the angle of one phenyl ring with respect to the other.

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The electrical behavior of π-conjugated oligo(phenyleneethynylene) systems functioning as memory devices is studied using quantum chemistry methods, including density functional and Green function formalisms combined in a fully self-consistent manner. Electron charge alters a molecule impedance characteristic providing in some cases distinguishable "impedance states" that can serve to determine experimentally the state of charge of the molecule. Conducting and nonconducting states can be strategically engineered by arranging substituents in a molecule. The NH 2 group localizes the highest energy occupied electronic states whereas the NO 2 group localizes the lowest energy unoccupied orbitals of the oligomer systems. These effects yield two complementary molecular memories, each occupying a volume smaller than 1 nm 3 .

A proposed dinitro device, Au-(2′-nitro-4-ethynylphenyl-4′-ethynylphenyl-5′-nitro-1-benzene thiolate)-Au is analyzed using a combination of density functional and Green function theories complemented with information from theoretical and experimental studies of a similar nitroamino device, Au-(2′-amino-4-ethynylphenyl-4′-ethynylphenyl-5′-nitro-1-benzenethiolate)-Au. The dinitro compound might also perform as a molecular memory but with different characteristics than those of the nitroamino, showing well-defined charge states; however, the neutral charge state of the nitroamino presents well-defined resonant tunneling characteristics and a larger intrinsic dipole moment. Density of states, transmission functions, and current–voltage characteristics for the neutral, anion, and dianion of the two molecules are compared. The effect of the bias potential is explicitly considered in the calculations as well as the effect of the contacts and the spin states of the open shell systems. The theoretical results for the training molecule are in good agreement with experiment. It is concluded that observed negative differential resistance is due mainly to charge effects combined in less degree with resonant tunneling intrinsic to single molecules.

A density functional theory calculation for determining the I−V characteristics (admittances of molecules) in molecular-based junctions is presented here. The efficacy of this method is shown by calculations of I(V) characteristics on an S−(p-C6H4)−S between proximal Au atoms and comparing the data to the results obtained experimentally. Over the range studied experimentally, the calculations here corroborate well with the I(V) characteristics found in molecular junction experiments.

Chlorophyll a (Chl a) is the most abundant pigment on earth. In all plants, algae, and cyanobacteria, it plays a pivotal role as an antenna and reaction center pigment in the primary steps of photosynthesis. In the past, a true three-dimensional (3D) experimental determination of the Qy electronic transition dipole moment orientation could not be obtained. With combined femtosecond polarization resolved VIS pump-IR probe experiments and theoretical calculations of the infrared transition dipole moments (tdm's) in the electronic ground state, we determined the 3D orientation of the Qy electronic tdm of Chl a within the molecular structure. Polarization resolved experiments provided angles of the Qy electronic tdm with three different infrared tdm's, whose orientations within the molecular structure were taken from our theoretical calculations. The orientation of the Qy tdm results from the intersection of all three angles and was found to have an angle of (78 +/- 3)degrees with the x-axis, (12 3)degrees with the y-axis, and (86 +/- 2)degrees with the z-axis.