Density-functional study of the evolution of the electronic structure of oligomers of thiophene:Towards a model Hamiltonian

We present density-functional and time-dependent density-functional studies of the ground, ionic, and excited states of a series of oligomers of thiophene. We show that, for the physical properties, the most relevant highest occupied and lowest unoccupied molecular orbitals develop gradually from monomer molecular orbitals into occupied and unoccupied broad bands in the large length limit. We show that band gap and ionization potentials decrease with size, as found experimentally and from empirical calculations. This gives credence to a simple tight-binding model Hamiltonian approach to these systems. We demonstrate that the length dependence of the experimental excitation spectra for both singlet and triplet excitations can be very well explained with an extended Hubbard-like Hamiltonian, with a monomer on-site Coulomb and exchange interaction and a nearest-neighbor Coulomb interaction. We also study the ground and excited-state electronic structures as functions of the torsion angle between the units in a dimer, and find almost equal stabilities for the transoid and cisoid isomers, with a transition energy barrier for isomerization of only 4.3 kcal/mol. Fluctuations in the torsion angle turn out to be very low in energy, and therefore of great importance in describing even the room-temperature properties. At a torsion angle of 90° the hopping integral is switched off for the highest occupied molecular orbital levels because of symmetry, allowing a first-principles estimate of the on-site interaction minus the next-neighbor Coulomb interaction as it enters in a Hubbard-like model Hamiltonian

Structure and Charge Distribution of the (CH)62+ Dication

In a preceding paper Hogeveen and Kwant 1 report PMR and CMR measurements on the species (C-CH 3) 6 2+. The authors conclude that the most likely structure of this dication is a nonclassical one of approximate symmetry C 5v (fig. 1a) although classical structures of lower symmetry (fig. 1b) cannot be completely excluded. It seemed of interest therefore to carry out some exploratory ab initio mole-cular orbital calculations i n order to investigate the feasibility of symmetrical structures of the kind proposed and their stability with respect to less symmetrical alternatives. Conventional closed shell SCF-MO calculations 2 on the minimal basis-3GTO-level have been carried out for the simpler species (CH) 6 2+ in two sets. In the first set the geometry was constrained to C 5v symmetry and optimal values were determined for the C-C distance (R) in the ring and the position (h) of the CH group above the ring. All CH distances were ke pt fixed at 1.10 Å. 1671 1672 No. 19 In the second set of calculations the symmetry was relaxed to C s by moving the CH group above the ring off the C 5 axis in a plane of symmetry, in combination with various deformations of the ring. For the computations of the first set the program POLYATOM3 as well as the much faster program GAUSS 70 4 were em-ployed. Most of the calculations of the second set were done with GAUSS 70 only. In both programs a contracted basis set of 3 GTO's per atomic orbital was used. In the POLYATOM calculations contraction coefficients and exponents were chosen that minimized the ground state energies of the atoms C and H 5. In the GAUSS 70 runs the standard minimal basis set of the program was employed. For comparison purposes similar calculations were made on the ions (CH) 5 + and (CH) 5 In Table I optimal values for R and L, corresponding total energies and various Mulliken gross charges are listed which resulted from the GAUSS 70 calculations. R(Å) h(Å) Etotal (A.U.) C 1 a H

Energy-Level Alignment at Metal–Organic and Organic–Organic Interfaces

This article reports on the electronic structure at interfaces found in organic semiconductor devices. The studied organic materials are C60 and poly (paraphenylenevinylene) (PPV)-like oligomers, and the metals are polycrystalline Au and Ag. To measure the energy levels at these interfaces, ultraviolet photoelectron spectroscopy has been used. It is shown how the energy levels at interfaces deviate from the bulk. Furthermore, it is demonstrated that the vacuum levels do not align at the studied interfaces. The misalignment is caused by an electric field at the interface. Several effects are presented that influence the energy alignment at interfaces, such as screening effects, dipole layer formation, charge transfer, and chemical interaction. The combination of interfaces investigated here is similar to interfaces found in polymer light-emitting diodes and organic bulk heterojunction photovoltaic devices. The result, the misalignment of the vacuum levels, is expected to influence charge-transfer processes across these interfaces, possibly affecting the electrical characteristics of organic semiconductor devices that contain similar interfaces.

Temperature and Concentration Dependent Conductivity of Potassium Doped C60 Films in Relation to the Phase Diagram

Conductivity measurements on potassium doped thin films of C60 following various preparation procedures are reported. In the resistivity as function of x we report a sharp local maximum at x = 4 and a local minimum near x = 5 in addition to that commonly found near x = 3. These effects are only seen in films prepared with “backdoping”. For the x = 1 films we find a non-metallic quenched mestastable phase below 150°C that can be annealed at 125°C to a stable phase. The relevance of preparation methods and thermal history of the films for the existence of different phases is discussed

2kF Peierls Transition for a Half-Filled Band from a Hubbard Hamiltonian Extended with Intersite-Dependent Transfer

We report a model calculation with the Hubbard Hamiltonian, extended with an exponentially dependent intersite transfer, on a half-filled band system. The calculation shows a phase transition which changes from an electronic (at low U/t) to a spin Peierls at high U/t. In the presence of an external magnetic field a new phase (2kF + kF) was found. The specific heat, spin susceptibility, and intensity of the charge transfer absorption was calculated; the results compare reasonably well with experiment. In addition the pressure dependence of the phase transition temperature could be estimated.