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The affects of alkali-metal atoms on the electronic structure of disordered and highly ordered thin films of the medium-size aromatic hydrocarbon hexa-peri-hexabenzocoronene (HBC) have been investigated by valence and core level photoelectron spectroscopies—ultraviolet photoelectron spectroscopy (UPS) and x-ray photoelectron spectroscopy (XPS)—and accompanying quantum-chemical calculations. Deposition of Li or Na atoms in situ leads to new spectral features in the UPS spectra, which are related to formerly unoccupied molecular states. The binding energies and intensities of these features depend on the nature of the counterion. The smaller Li ion exhibits a stronger influence on the electronic structure than its sodium counterpart. In the intercalation of sodium into ordered films, a high degree of molecular order is preserved, and, at high deposition levels, a surface dipole is formed that is associated with the layered structure of the compound. Remarkably, high levels of alkali-metal storage of at least one alkali-metal atom for each four carbon atoms have been observed, indicating clearly the potential use of these graphene materials in lithium-ion batteries with a high charge-storage capacity. [ABSTRACT FROM AUTHOR]

The dependence of the Mössbauer line shape on the double-exchange interaction is studied for dimeric mixed-valence systems in the framework of a model that treats the entire nuclear-electron system quantum-mechanically. The coherent tunneling motion of the "excess" electron between the metal sites produces a resonance splitting of the localized states. In the case of weak double-exchange interaction, this motion shows up as a splitting of Mössbauer lines. This effect however, is readily suppressed by an asymmetry distortion, which destroys the resonance conditions. The origin of different patterns in the temperature dependence of the Mössbauer line shape is discussed. [ABSTRACT FROM AUTHOR]

The electronic structure of the propeller-shaped spiro-type molecules 2,2', 7,7-'tetraphenyl-9,9-' spirobifluorene (spiro-quarterphenyl) and 2,2,', 7,7'-tetrakis(biphenyl-4-yl)-9,9-spirobifluorene (spiro-sexiphenyl) and its evolution upon doping with Li atoms or Na atoms have been studied by means of photoelectron spectroscopy. With lithium atoms as the dopant, two charges are stored on the same spiro branch in the form of bipolarons, for both spiro-quarterphenyl and spiro-sexiphenyl. In contrast, for doping with sodium atoms, the size of the counter ion makes it less energetically desirable to store two charges onto a single branch, and the charged species are polarons independent of the level of doping; this result is confirmed by optical absorption data. [ABSTRACT FROM AUTHOR]

We apply correlated quantum-chemical techniques to study the origin of the large two-photon absorption (TPA) cross sections, δ, in stilbene derivatives in which electron-donating and electron-withdrawing substituents provide a quadrupolar charge-transfer arrangement. An additional field created by a set of point charges is used to systematically modify the ground-state polarization to determine its consequences for the TPA response. The effect on the molecular structure can be quantified by the evolution of the π-bond-order alternation (π-BOA) of the conjugated backbone. For moderate ground-state polarizations, a marked increase of the TPA response occurs; for large polarizations, δ peaks and then drops dramatically. Insight into the origin of this evolution is gained by comparing the values of δ obtained via the converged sum-over-states approach to the results of a simple three-state model. [ABSTRACT FROM AUTHOR]

The electronic structure of the conjugated polymer poly(9,9-dioctylfluorene) and the charge storage mechanism upon doping with lithium atoms have been studied using a combined experimental–theoretical approach. The density of states in the valence band region was measured using ultraviolet photoelectron spectroscopy, and the spectra interpreted with the help of the results of quantum-chemical calculations carried out at the Austin Model 1/Valence Effective Hamiltonian level. The valence band spectra as well as the calculated band structure and derived density of valence states of poly(9,9-dioctylfluorene) are compared to those of poly(p-phenylene) and a ladder-type derivative of poly(p-phenylene). The deposition of lithium atoms, in situ, onto the surface of the polymer films resulted in uniform (bulk) diffusion of the lithium atoms, charge transfer to the polymer (resulting in lithium ions), and the appearance of new electronic states in the gap. At low doping levels (up to about 0.1 Li-atom per repeat unit of the polymer), these states are polarons, and, as the doping level increases, they evolve into bipolarons. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A theoretical model is developed to describe the intervalence transitions in the mixed-valence N,N,N,[sup ′]N[sup ′]-tetrakis(4-methoxyphenyl)-1, 4-benzenediamine system. The excited-state energies and transition dipole moments are evaluated by means of time-dependent density functional theory while the shape of the lowest absorption band is analyzed in the framework of a dynamic vibronic model. Our results are discussed in relation with recent experimental data. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A simple approach is proposed to predict the magnitude of charge transfer between a polarized transition metal electrode and a chemisorbed molecule in an electrolytic solution. The approach is based on the equalization of the chemical potentials, as defined in density-functional theory (DFT). The system is modeled by a metal cluster, a molecule, and a set of point charges (counter ions) and the properties evaluated using methods derived from DFT; specifically, we study here the chemisorption of acrylonitrile on nickel. This approach allows us to point out the various factors determining the charge transfer. The results show a linear increase in the amount of charge transferred to the chemisorbed molecule with the polarization of the metal electrode. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We have used polyvinylcarbazole, PVK, in combination with poly(p-phenylene vinylene), PPV and a green-emitting, soluble derivative, poly(4,4′-diphenylene diphenylvinylene) or briefly PDPV, to fabricate triple-layer light-emitting diodes PPV/PVK/PDPV/metal, with indium tin oxide as the anode. We report luminances in excess of 1300 cd/m2 with Al cathodes, turn-on voltages of ∼8 V in structures 300 nm thick, and internal efficiencies in the range 0.4%–0.8% for Al and Ca cathodes; at least ten times higher than for single-layer PDPV devices. Quantum efficiencies for Ca or Al electrodes are only a factor 1.5 different. The improvements over single-layer PDPV devices are considered to be due to better hole injection by PPV and to the formation of a hole barrier at the PPV/PVK interface, which forces a better balance of the electron and hole currents. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]