Nadine E. Gruhn

Impact of Perfluorination on the Charge-Transport Parameters of Oligoacene Crystals

The charge-transport parameters of the perfluoropentacene and perfluorotetracene crystals are studied with a joint experimental and theoretical approach that combines gas-phase ultraviolet photoelectron spectroscopy and density functional theory. To gain a better understanding of the role of perfluorination, the results for perfluoropentacene and perfluorotetracene are compared to those for their parent oligoacenes, that is, pentacene and tetracene. Perfluorination is calculated to increase the ionization potentials and electron affinities by approximately 1 eV, which is expected to reduce significantly the injection barrier for electrons in organic electronics devices. Perfluorination also leads to significant changes in the crystalline packing, which greatly affects the electronic properties of the crystals and their charge-transport characteristics. The calculations predict large conduction and valence bandwidths and low hole and electron effective masses in the perfluoroacene crystals, with the largest mobilities expected along the pi-stacks. Perfluorination impacts as well both local and nonlocal vibrational couplings, whose strengths increase by a factor of about 2 with respect to the parent compounds.

Characterization of the molecular parameters determining charge transport in anthradithiophene

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.

Investigation of metal-dithiolate fold angle effects: implications for molybdenum and tungsten enzymes.

Gas-phase photoelectron spectroscopy and density functional theory have been used to investigate the interactions between the sulfur π-orbitals of arene dithiolates and high-valent transition metals as minimum molecular models of the active site features of pyranopterin Mo/W enzymes. The compounds (Tp*)MoO(bdt) (compound 1), Cp2Mo(bdt) (compound 2), and Cp2Ti(bdt) (compound 3) [where Tp* is hydrotris(3,5-dimethyl-1-pyrazolyl)borate, bdt is 1,2-benzenedithiolate, and Cp is η5- cyclopentadienyl] provide access to three different electronic configurations of the metal, formally d1, d2, and d0, respectively. The gas-phase photoelectron spectra show that ionizations from occupied metal and sulfur based valence orbitals are more clearly observed in compounds 2 and 3 than in compound 1. The observed ionization energies and characters compare very well with those calculated by density functional theory. A “dithiolate-folding-effect” involving an interaction of the metal in-plane and sulfur-π orbitals is proposed to be a factor in the electron transfer reactions that regenerate the active sites of molybdenum and tungsten enzymes.

Hole-vibronic coupling in oligothiophenes: impact of backbone torsional flexibility on relaxation energies

Density functional theory calculations together with highly resolved gas-phase ultraviolet photoelectron spectroscopy have been applied to oligothiophene chains with up to eight thiophene rings. One of the important parameters governing the charge transport properties in the condensed phase is the amount of energy relaxation upon ionization. Here, we investigate the impact on this parameter of the backbone flexibility present in oligothiophenes as a result of inter-ring torsional motions. With respect to oligoacenes that are characterized by a coplanar and rigid backbone, the torsional flexibility in oligothiophenes adds to the relaxation energy and leads to the broadening of the first ionization peak, making its analysis more complex.

Valence electronic structure of benzo-2,1,3-chalcogenadiazoles studied by photoelectron spectroscopy and density functional theory

The He I photoelectron spectra of benzo-2,1,3-thia-, selena-, and telluradiazole were measured, and the observed ionization bands were assigned by comparison with the results of DFT calculations. Whereas the B3LYP exchange-correlation functional provided orbital energies that permitted a preliminary assignment by application of Koopmans theorem, a more-accurate interpretation was established by calculation of the vertical ionization energies with the PW91 functional and analysis of the correlation of energy levels along the homologous series. This strategy clarified earlier disagreements in the assignment of the spectrum of benzo-2,1,3-thiadiazole.

Semiempirical MO and voltammetric estimation of ionization potentials of organic pigments. Comparison to gas phase ultraviolet photoelectron spectroscopy

Abstract A number of organic pigments were identified by semiempirical molecular orbital calculations, using the PM3 method, as having ionization potential (IP) values of 7.0–9.5 eV. Based on photostability, solubility and commercial availability twelve (quinacridone, isoviolanthrone, indanthrone, indigo, 3,4,9,10 perylenetetracarboxylic dianhydride, bis( p -chlorophenyl)1,4-diketopyrrolo (3,4-C) pyrrole, pyranthrone, indanthrene yellow GCN, 16,17-dimethoxyviolanthrone, indanthrene gold orange, 4,4′-diamino-9,9′,10,10′-tetrone [1,1′ bianthracene], and N,N ′ ditridecyl-3,4,9,10-perlenetetracarboxylic diimide) were chosen for further study. The accuracy of the MO calculations was confirmed by experimental measurement of the ionization potentials for eight of the pigments, using gas phase ultraviolet photoelectron spectroscopy. For compounds having at least three fused rings and containing oxygen, nitrogen, or both, the theoretical and experimental IPs have a linear relationship defined by the equation IP exp =0.694IP Calc +1.9049. Lewis acid pigment solubilization (LAPS) was shown to be a viable approach to preparing electrodes for cyclic voltammetry of pigment solid films. The results of the cyclic voltammetry experiments were utilized to formulate the equation E ox (V vs. NHE)=0.5488 IP Calc −3.042, which relates the experimental oxidation potential to the theoretical IP.

Electrochemical and chemical oxidation of dithia-, diselena-, ditellura-, selenathia-, and tellurathiamesocycles and stability of the oxidized species.

The diverse electrochemical and chemical oxidations of dichalcogena-mesocycles are analyzed, broadening our understanding of the chemistry of the corresponding radical cations and dications. 1,5-Diselenocane and 1,5-ditellurocane undergo reversible two-electron oxidation with inverted potentials analogous to 1,5-dithiocane. On the other hand, 1,5-selenathiocane and 1,5-tellurathiocane undergo one-electron oxidative dimerization. The X-ray crystal structures of the Se-Se dimer of the 1,5-selenathiocane one-electron oxidized product and the monomeric two-electron oxidized product (dication) of 1,5-tellurathiocane are reported. 1,5-Dithiocanes and 1,5-diselenocanes with group 14 atoms as ring members undergo irreversible oxidation, unlike the reversible two-electron oxidation of the corresponding silicon-containing 1,5-ditellurocanes. These results demonstrate the chemical consequences of the dication stabilities Te(+)-Te(+) > Se(+)-Se(+) > S(+)-S(+), as well as Se(+)-Se(+) > Se(+)-S(+) and Te(+)-Te(+) > Te(+)-S(+).

Interactions of arenes and thioethers resulting in facilitated oxidation

Synthesis of 6-endo and 6-exomethylthio-2-endoarylbicyclo[2.2.1]heptanes was accomplished stereoselectively. The ionization energies, determined by photoelectron spectroscopy, and electrochemical oxidation potentials, determined by cyclic voltammetry, were lower for the 6-endomethylthio compounds than for their 6-exomethylthio analogues. Calculations supported the notion that facilitation of electron transfer in the 6-endomethylthio compounds results from through-space S...pi interaction.

Electronic structure of exohedral interactions between C60 and transition metals

Abstract The electron distribution and orbital interactions of C 60 with metals coordinated at different sites on the outside of the fullerene are evaluated. These sites include the position of a metal atom directly above a carbon atom (η 1 site), the metal atom centered above two carbons of a pentagon or above two carbons between two pentagons (both η 2 sites), the metal atom centered above a pentagon (η 5 site), and the metal atom centered above a hexagon (η 6 site). The frontier orbitals of C 60 are illustrated first with three-dimensional orbital contour plots. A palladium atom is then used to probe the bonding at the different sites on the C 60 surface. The results with Pd 0 are compared to our earlier study with the harder Ag + ion in order to examine the effects of metal electron richness and size. In addition, these results are compared with the bonding to more traditional ligands that represent the hapticity of these sites, such as methyl (η 1 ), ethylene (η 2 ), cyclopentadienyl (η 5 ), and benzene (η 6 ). The strength of the metal-C 60 interaction and the amount of charge delocalized from the metal to C 60 is sensitive to the site of coordination, the electron richness of the metal, and distortions in the geometry of C 60 . As discussed in our previous work, the frontier orbitals of C 60 are well-suited for synergistic bonding of a metal atom to a carbon-carbon pair in an alkene-like fashion, in which the HOMO of C 60 donates carbon-carbon π bonding electron density to the metal, and the LUMO of C 60 accepts electron density from the metal into a carbon-carbon π* antibonding orbital. Although the HOMO and LUMO of C 60 describe the basic interaction, many frontier orbitals are involved. The site above the CC bond between two pentagons is favored over the site above the CC bond within a pentagon, and the interaction above the other sites is indicated to be net repulsive by these calculations. The differentiation between these sites increases with the electron richness of the metal center. The bonding of the metal to C 60 is generally weaker than to the small ligands, except for very electron rich metal centers where the bonding to the η 2 site between pentagons apparently becomes stronger than the bonding to ethylene.

Ligand-mediated metal-metal interactions and localized versus delocalized mixed-valence cation states of biferrocene and bis(μ-fulvalenediyl)diiron characterized in the gas phase by valence photoelectron spectroscopy

Abstract Gas-phase photoelectron spectroscopy is used to investigate metal–metal interactions and the mixed-valence positive ion states of biferrocene and bis(μ-fulvalenediyl)diiron. The spectra of phenylferrocene and 1,1′-diphenylferrocene are used to show that, in comparison to ferrocene, the extension of the ligand π system and the reduced ligand symmetry do not have an appreciable effect on the band profile of the metal-based ionizations. In contrast, the initial ionization bands of both bimetallic molecules, which derive from the metal-based 2 E 2g ionizations of ferrocene, are spread over a wide energy range, indicating delocalization across the two metal halves of the molecule and formal oxidation states of +2 1 / 2 for each metal atom in these cation states. The broadening and splitting of this ionization band for bis(μ-fulvalenediyl)diiron is twice that observed for biferrocene, consistent with a through-bond ligand-mediated mechanism of interaction. Ionizations of the bimetallic molecules that derive from the metal-based 2 A 1g ionizations of ferrocene occur in a single narrow band, indicating that both through-space and through-ligand interactions are not appreciable for the d z 2 -based orbitals. The difference between the metal–metal interactions in these positive ion states follows from the different overlap and energy match of the metal orbitals with fulvalendiyl orbitals of the appropriate symmetry. Most important to the metal–metal interaction in the ground ion state are empty fulvalendiyl orbitals with two nodes perpendicular to the C 5 planes and gerade and ungerade symmetries with respect to the inversion centers of the molecules. In the gas phase, both species are found to be strongly interacting, delocalized mixed-valence compounds in their ground ion states.