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The interaction between 2,20-bithiophene-5-carboxylic acid (PT2) sublimed under ultra-high vacuum conditions and anatase (101) and rutile (110) TiO2 single crystal surfaces is investigated by studying the electronic spectral density near the Fermi level with synchrotron-based spectroscopy. The experimental results are compared to density functional theory calculations of the isolated PT2 molecule and of the molecule adsorbed on an anatase TiO2 (101) cluster. The relative concentrations of Ti, C, and S atoms indicate that the adsorbed molecule remains intact upon deposition, which is typical of a Stranski-Krastanov growth mode. The analysis of the O1s spectrum suggests a predominant bidentate geometry of the adsorption with both rutile and anatase surfaces, as supported by previous theoretical simulations. It is also theoretically and experimentally demonstrated that the PT2 adsorption causes the appearance of new electronic states in the gap near the TiO2 valence band. A pinning effect of the LUMO level of the dye is also theoretically predicted. [ABSTRACT FROM AUTHOR]

The quantum-chemical study of the electronic and optical properties of unsubstituted oligo(phenylene vinylene) radical cations was reported. The goal is to distinguish the impact of the choice of molecular geometry from the impact of the choice of quantum-chemical method, on the calculated optical transition energies and the optical excitation is localized with HF-CI and delocalized with TD-DFT, almost irrespective of the molecular geometry.

Research has been conducted on the photoinduced charge transfer. The quantum-chemical techniques used in investigating the photoinduced charge transfer mechanism and rates are described.

Research was conducted to examine quantum-chemical characterization performed on interacting conjugated chains. The objective is to provide a better understanding of the effects of interchain coupling on the optical and electronic properties with respect to those calculated for isolated chains, thereby, understanding the effect of interchain interactions on the emission and absorption. Results indicate that a transfer of oscillator strength occurs between the highest energy optical transition and the lowest excited state in configurations.

Configuration interaction calculations on the lowest singlet and triplet excitations in oligothiophenes and their substituted derivatives reveal a decrease in singlet-singlet excitation energies with chain length, and an intersystem crossing between the singlet and triplet forms. This crossing populates the lowest triplet state and creates an absorption red-shift. An extended pi-conjugated path decreases the singlet-singlet excitation energies whereas exchange between two like-spins creates only a marginal effect in the lowest triplet excited state. Geometry relaxation phenomena are also noted.

Novel optical pump-push – photocurrent probe ultrafast spectroscopy experiments on organic photoconversion systems show that excessive excitation energy in such systems is not lost but used to reach delocalised states that act as the gateway for long-range charge separation. We also show that the developed experimental approach can be generalised to inorganic and hybrid photoconversion systems.

The molecular parameters that govern charge transport in anthradithiophene (ADT) are studied by a joint experimental/theoretical approach involving high-resolution gas-phase photoelectron spectroscopy and quantum-mechanical methods. The hole reorganization energy of ADT has been determined by an analysis of the vibrational structure of the lowest ionization band in the gas-phase photoelectron spectrum as well as by density-functional theory calculations. In addition, various dimers and clusters of ADT molecules have been considered in order to understand the effect of molecular packing on the hole and electron intermolecular transfer integrals. The results indicate that the intrinsic electronic structure, the relevant intramolecular vibrational modes, and the intermolecular interactions in ADT are very similar to those in pentacene. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Internal reorganization energies and interchain transfer integrals are two key parameters governing the charge-transport properties of organic semiconducting materials. Here, in order to model some aspects of device operation in field-effect transistors based on conjugated oligomers, we investigate via semiempirical quantum-chemical calculations the way these two parameters are modified when a static electric field in the range 10[sup 6]–10[sup 8] V/cm is applied along the long axis of pentacene and sexithienyl molecules. For the highest fields, a pronounced redistribution of the charges along the oligomer chains occurs, which is accompanied by significant geometric distortions. However, these charge redistribution effects are found not to impact significantly the transport parameters. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We report a detailed quantum-chemical characterization of the electronic and optical properties of polyfluorene chains and compare them to those in copolymers containing alternating fluorene and benzothiadiazole or ethylenedioxythiophene units. The introduction of the comonomer can strongly modify the excitonic properties as well as the efficiency of charge- and energy-transfer processes. The choice of the comonomer is thus critical in targeting specific optical properties while maintaining good transport properties. [ABSTRACT FROM AUTHOR]

Quantum-chemical calculations coupled with a tight binding band model are used to study the charge carrier mobilities in oligoacene crystals. The transfer integrals for all nonzero interactions in four crystalline oligoacenes (naphthalene, anthracene, tetracene, and pentacene) were calculated, and then used to construct the excess electron and hole band structures of all four oligoacene crystals in the tight binding approximation. From these band structures, thermal-averaged velocityvelocity tensors in the constant-free-time and the constant-free-path approximations for all four materials were calculated at temperatures ranging from 2 to 500 K. The bandwidths for these oligoacenes were found to be of the order of 0.1-0.5 eV. Furthermore, comparison of the thermal-averaged velocity-velocity tensors with the experimental mobility data indicates that the simple band model is applicable for temperatures only up to about 150 K. A small-polaron band model is also considered, but the exponential band narrowing effect is found to be incompatible to experimental power law results. [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]