Matthias Weil

Correlation of π-Conjugated Oligomer Structure with Film Morphology and Organic Solar Cell Performance

The novel methyl-substituted dicyanovinyl-capped quinquethiophenes 1-3 led to highly efficient organic solar cells with power conversion efficiencies of 4.8-6.9%. X-ray analysis of single crystals and evaporated neat and blend films gave insights into the packing and morphological behavior of the novel compounds that rationalized their improved photovoltaic performance.

Interrelation between Crystal Packing and Small-Molecule Organic Solar Cell Performance†

X-ray investigations on single crystals of a series of terminally dicyanovinyl-substituted quaterthiophenes and co-evaporated blend layers with C(60) give insight into molecular packing behavior and morphology, which are crucial parameters in the field of organic electronics. Structural characteristics on various levels and length scales are correlated with the photovoltaic performance of bulk heterojunction small-molecule organic solar cells.

Molecular and electronic structure of cyclo[10]thiophene in various oxidation states: polaron pair vs. bipolaron

The molecular structure of a cyclic oligothiophene, C10T, has been determined by single-crystal X-ray structure analysis. The exclusive syn-conformation of all thiophene units as confirmed in the solid state and the ring strain in this macrocycle result in its unusual and optoelectronic properties. This does not only apply to neutral C10T but also to its oxidized states, as demonstrated by absorption and ESR spectroscopy, supporting the formation of a polaron-pair structure upon oxidation of C10T to C10T2(·+) as has been discussed for linear oligothiophenes. To the best of our knowledge, C10T2(·+) represents an unambiguous example comprising a two-polaron structure (polaron-pair) of a thiophene-based conjugated macrocycle.

Synthesis and Characterization of Hydrido Carbonyl Molybdenum and Tungsten PNP Pincer Complexes

In the present study the Mo(0) and W(0) complexes [M(PNP)(CO)3] as well as seven-coordinate cationic hydridocarbonyl Mo(II) and W(II) complexes of the type [M(PNP)(CO)3H]+, featuring PNP pincer ligands based on 2,6-diaminopyridine, have been prepared and fully characterized. The synthesis of Mo(0) complexes [Mo(PNP)(CO)3] was accomplished by treatment of [Mo(CO)3(CH3CN)3] with the respective PNP ligands. The analogous W(0) complexes were prepared by reduction of the bromocarbonyl complexes [W(PNP)(CO)3Br]+ with NaHg. These intermediates were obtained from the known dinuclear complex [W(CO)4(μ-Br)Br]2, prepared in situ from W(CO)6 and stoichiometric amounts of Br2. Addition of HBF4 to [M(PNP)(CO)3] resulted in clean protonation at the molybdenum and tungsten centers to generate the Mo(II) and W(II) hydride complexes [M(PNP)(CO)3H]+. The protonation is fully reversible, and upon addition of NEt3 as base the Mo(0) and W(0) complexes [M(PNP)(CO)3] are regenerated quantitatively. All heptacoordinate complexes exhibit fluxional behavior in solution. The mechanism of the dynamic process of the hydrido carbonyl complexes was investigated by means of DFT calculations, revealing that it occurs in a single step. The structures of representative complexes were determined by X-ray single-crystal analyses.

Synthesis, Structure, and Reactivity of Co(II) and Ni(II) PCP Pincer Borohydride Complexes

The 15e square-planar complexes [Co(PCPMe-iPr)Cl] (2a) and [Co(PCP-tBu)Cl] (2b), respectively, react readily with NaBH4 to afford complexes [Co(PCPMe-iPr)(η2-BH4)] (4a) and [Co(PCP-tBu)(η2-BH4)] (4b) in high yields, as confirmed by IR spectroscopy, X-ray crystallography, and elemental analysis. The borohydride ligand is symmetrically bound to the cobalt center in η2-fashion. These compounds are paramagnetic with effective magnetic moments of 2.0(1) and 2.1(1) μB consistent with a d7 low-spin system corresponding to one unpaired electron. None of these complexes reacted with CO2 to give formate complexes. For structural and reactivity comparisons, we prepared the analogous Ni(II) borohydride complex [Ni(PCPMe-iPr)(η2-BH4)] (5) via two different synthetic routes. One utilizes [Ni(PCPMe-iPr)Cl] (3) and NaBH4, the second one makes use of the hydride complex [Ni(PCPMe-iPr)H] (6) and BH3·THF. In both cases, 5 is obtained in high yields. In contrast to 4a and 4b, the borohydride ligand is asymmetrically bound to the nickel center but still in an η2-mode. [Ni(PCPMe-iPr)(η2-BH4)] (5) loses readily BH3 at elevated temperatures in the presence of NEt3 to form 6. Complexes 5 and 6 are both diamagnetic and were characterized by a combination of 1H, 13C{1H}, and 31P{1H} NMR, IR spectroscopy, and elemental analysis. Additionally, the structure of these compounds was established by X-ray crystallography. Complexes 5 and 6 react with CO2 to give the formate complex [Ni(PCPMe-iPr)(OC(C=O)H] (7). The extrusion of BH3 from [Co(PCPMe-iPr)(η2-BH4)] (4a) and [Ni(PCPMe-iPr)(η2-BH4)] (5) with the aid of NH3 to yield the respective hydride complexes [Co(PCPMe-iPr)H] and [Ni(PCPMe-iPr)H] (6) and BH3NH3 was investigated by DFT calculations showing that formation of the Ni hydride is thermodynamically favorable, whereas the formation of the Co(II) hydride, in agreement with the experiment, is unfavorable. The electronic structures and the bonding of the borohydride ligand in [Co(PCPMe-iPr)(η2-BH4)] (4a) and [Ni(PCPMe-iPr)(η2-BH4)] (5) were established by DFT computations.

Growth of branched single-crystalline GaAs whiskers on Si nanowire trunks

In this paper we present the hetero-epitaxial growth of single-crystalline GaAs whiskers on Si(111)-nanowire trunks forming hierarchical star-like structures with a six-fold symmetry. These hierarchical nanostructures have been successfully formed utilizing both vapor?liquid?solid (VLS) growth by low-pressure chemical vapor deposition (LPCVD) and molecular-beam epitaxy (MBE) techniques. High-resolution transmission electron microscopy (HRTEM) studies revealed the [111] growth direction of the core Si nanowires (Si-NWs) with six {112} facet planes. The sequentially grown branches are single-crystalline hexagonal GaAs nanowhiskers which grow preferably in the [0001] direction and are perpendicular to the {112} facets of the Si-NW backbone. Photoluminescence (PL) measurements confirm the good crystalline quality of the GaAs nanowhiskers and a blueshift of about 30?meV compared to bulk zinc blende-type GaAs. The ability to prepare rotationally branched NW structures should open new opportunities for both fundamental research and applications including monolithic three-dimensional nanoelectronics and nanophotonics.

Complex Magnetism and Magnetic-Field-Driven Electrical Polarization in Co3TeO6

The magnetic and electrical properties of Co(3)TeO(6) single crystals with corundum-related structure reveal a magnetic-field-induced polarization below 21 K. A sharp peak in the specific heat at approximate to 18 K indicates a reconstructive-type first-order phase transition. From second-harmonic generation measurements, breaking of inversion symmetry is evident and the point-group symmetry was determined as m. The temperature and magnetic-field dependences of the magnetic and electrical polarizations are discussed in the light of the SHG results.

New type of incommensurate magnetic ordering in Mn3TeO6

The complex metal oxide Mn(3)TeO(6) exhibits a corundum related structure and has been prepared both in forms of single crystals by chemical transport reactions and of polycrystalline powders by a solid state reaction route. The crystal structure and magnetic properties have been investigated using a combination of X-ray and neutron powder diffraction, electron microscopy, calorimetric and magnetic measurements. At room temperature this compound adopts a trigonal structure, space group R (3) over bar with a = 8.8679(1) angstrom. c = 10.6727(2) angstrom. A long-range magnetically ordered state is identified below 23 K. An unexpected feature of this magnetic structure is several types of Mn-chains. Under the action of the incommensurate magnetic propagation vector k = [0, 0, 0.4302(1)] the unique Mn site is split into two magnetically different orbits. One orbit forms a perfect helix with the spiral axis along the c-axis while the other orbit has a sine wave character along the c-axis.

Spin and Dipole Ordering in Ni2InSbO6 and Ni2ScSbO6 with Corundum-Related Structure

The complex metal oxides Ni2InSbO6 (NISO) and Ni2ScSbO6 (NSSO) have been prepared in form of polycrystalline powders by a solid state reaction route. The crystal structure and magnetic properties of the compounds were investigated using a combination of X-ray and neutron powder diffraction, electron microscopy, calorimetric and magnetic measurements. The compounds adopt a trigonal structure, space group R3, of the corundum related Ni3TeO6 (NTO) type. Only one of the octahedral Ni positions (Ni(2)) of the NTO structure was found to be occupied by In (Sc). NTO has non-centrosymmetric structure and is ferroelectric below 1000 K, dielectric and second harmonic measurements suggest that also NISO and NSSO are correspondingly ferroelectric. Magnetization measurements signified antiferromagnetic ordering below TN=60 K (NSSO) and 76 K (NISO). The magnetic structure is formed by two antiferromagnetically coupled incommensurate helices with the spiral axis along the b-axis and propagation vector k = [0, ky,0] with ky= 0.036(1) (NSSO) and ky= 0.029(1) (NISO). The observed structural and magnetic properties of NISO and NSSO are discussed and compared with those of NTO.

Sr3TeO6 and Ba3TeO6: double perovskites with pronounced superstructures

Abstract The orthotellurates (VI) of strontium and barium were synthesized at 1373 K in air by oxidation of TeO2 in a melt of SrBr2 and BaBr2, respectively. Their unprecedented structures were determined from room temperature single crystal X-ray diffraction data. Sr3TeO6, [F-1, Z = 32, a = 16.5338(2) Å, b = 16.6308(2) Å, c = 16.7112(3) Å, α = 90.031(2)°, β = 90.035(2)°, γ = 90.1370(10)°, R[F2 > 2σ(F2)] = 0.035, 19195 F2 data, 375 variable parameters] and Ba3TeO6 [I41/a, Z = 80, a = 19.3878(10) Å, c = 34.909(2) Å, R[F2 > 2σ(F2)] = 0.034, 19576 F2 data, 453 variable parameters] are eight- and 40-fold superstructures which can be described as a pseudo-cubic close-packing of [TeO6]6– octahedra with M2+ (M = Ba, Sr) atoms located in the tetrahedral and octahedral voids. Sr3TeO6 and Ba3TeO6 are hettotypes of the cubic aristotype with the double perovskite structure crystallizing in space group Fm-3m. A comparative discussion of the crystal structures and their group-subgroup relations are given, including different approaches for the structure descriptions.