Browsing by Author "Schweitzer, Dieter"

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Open Access
1,3,5-Tris(dimethylamino)benzol mit Iod und 1,3,5-Tricyanobenzol : eine ungewöhnliche Dimerisierung und ein "normaler" Donor/Akzeptor-Komplex
(1988) Keller, Heimo J.; Niebl, Roland; Renner, Gerd; Ruhr, Dorothea von der; Schweitzer, Dieter
Wir sind auf der Suche nach "organischen" Ferromagneten. Als Zielsubstanz haben wir kristallisierte Radikal-Ionen mit bahnentarteten Grundzuständen ausgewählt. Molekulare Festkörper aus planaren organischen Radikalionen zeigen ein breites Spektrum teilweise recht interessanter physikalischer Eigenschaften, die durch kollektives Elektronenverhalten zu erklären sind. In den letzten Jahren wurde vor allem das elektrische Verhalten dieser Substanzen untersucht. Dies führte zur Entdeckung einer Vielzahl "organischer Metalle", von denen einige bei tiefer Temperatur auch supraleitend werden.
Open Access
1,8-dipyrenylnaphthalenes : syntheses, molecular structure, and spectroscopic properties
(1984) Wahl, Peter; Krieger, Claus; Schweitzer, Dieter; Staab, Heinz A.
Syntheses of the 1,8-dipyrenylnaphthalenes 1-3 are reported. The stereoisomers 1 and 2 were separated; their structural assignment is based on 1H NMR, on the optical activity of 2, and on X-ray structure analyses of 1 and 2. Kinetic parameters for the isomerisation 2 = 1 were determined by optical rotation measurements. - Emission spectra of 1 - 3 are discussed in comparison to monopyrenyl compounds 4 and 8. For 1 and 3 typical 'excimer-like' fluorescence is observed. The difference between 1 and 2 clearly demonstrates the dependence of excimer interactions between the pyrene units on the mutual orientation of the π-systems involved. - On the basis of X-ray analyses the molecular structures of 1- 3 are discussed with emphasis on π···π-interactions between the pyrene units.
Open Access
13C nuclear magnetic relaxation studies at 62 MHz
(1971) Jaeckle, H.; Haeberlen, Ulrich; Schweitzer, Dieter
An apparatus suitable to measure 13C relaxation times T1 and T2 in liquids at 62 MHz is described. The required field of 58 kG is generated by a superconducting magnet. Results of T1 measurements on a variety of liquids, including benzene, derivatives of benzene, saturated rings, CS2, and others are reported. They are discussed in terms of dipole-dipole interactions, spin-rotation interactions and anisotropic chemical shifts, the latter of which turn out to play only a minor role. Intramolecular dipole-dipole interactions are found to provide by far the most important spin-lattice relaxation mechanism, whenever protons are bound directly to the carbons under investigation, even when the samples contain dissolved oxygen.
Open Access
15N and 13C spin lattice relaxation in neat liquids at high magnetic fields
(1975) Schweitzer, Dieter; Spiess, Hans Wolfgang
General features of 15N spin lattice relaxation in nitrobenzene, pyridine and triethanolamine are discussed and compared with l3C relaxation mechanisms.
Open Access
[2.2]- and [3.3]tetrathiafulvalenophanes
(1980) Ippen, Joachim; Chu, Tao-pen; Starker, Barbara; Schweitzer, Dieter; Staab, Heinz A.
In the "metallic" conducting 1:1 complex of tetrathiafulvalene (TTF) and tetracyanoquinodimethane (TCNQ), donor and acceptor molecules form separate stacks in the crystal. In order to enforce other TTF/TCNQ arrangements by altering the molecular architecture the phane-like bridged TTF-paracyclophanes (1), n = 3, and (2) were first synthesized. To do this a novel reaction sequence had to be developed which also facilitated synthesis of the tetrathiafulvalenophanes (3)/(4) (and the corresponding [2.2]-derivatives). (3)/(4) and TCNQ form a 1:4 complex having a conductivity of σ = 5 × 10−3 to 10−2 [Ω cm]−1 (single crystal, long axis, 300 K).
Open Access
[2.2]- und [3.3]Tetrathiafulvalenophane
(1980) Ippen, Joachim; Chu, Tao-pen; Starker, Barbara; Schweitzer, Dieter; Staab, Heinz A.
Um bei Charge-Transfer-Systemen, die als elektrische Halbleiter oder Leiter interessant sein könnten, durch Änderung der Molekularchitektur das Kristallgitter systematisch zu variieren, synthetisierten wir [2.2]- und [3.3]Tetrathiafulvalenophane ("TTF-Phane").
Open Access
[3.3]- and [4.4](2,7)pyrenophanes as excimer models : synthesis, molecular structure, and spectroscopic properties
(1984) Staab, Heinz A.; Riegler, Norbert; Diederich, Francois; Krieger, Claus; Schweitzer, Dieter
As excimer models the [3.3]- and [4.4]pyrenophanes 1 and 2 were synthesized. As the key precursor for the synthesis of 1 the octahydro-dithia[4.4]pyrenophane 9 was obtained by cyclisation of 4 and 8, the syntheses of which are described. Disulfone 10, derived from 9, on vapour-phase pyrolysis yielded 11 which by dehydrogenation was converted into 1. In an analogous route 2 was obtained via 16, 17, and 18 starting from 4 and 15 (prepared in the reaction sequence 12 - 13 - 14 - 15). - The molecular structures of 1 and 2, determined by X-ray structure analyses, are discussed with regard to transanular distances and deviations from planarity of the pyrene units. - Absorption spectra of 1 and 2 are discussed with reference to transanular interactions. The fluorescence emissions of 1 and 2 are found to be 'excimer-like'. Results obtained by ODMR measurements of 1 are reported.
Open Access
α- and β-(BEDT-TTF)2+I3-: two dimensional organic metals
(1985) Hennig, Ingolf; Bender, Klaus; Schweitzer, Dieter; Dietz, Klaus; Endres, Helmut; Keller, Heimo J.; Gleitz, Arno; Helberg, Hans W.
Electronic properties of α- and β-(BEDT-TTF)2I3 crystals are reported.
Open Access
α- and β-(BEDT-TTF)2I3 - two modifications with contrasting groundstate properties: insulator and volume superconductor
(1985) Schwenk, Helmut; Gross, Frieder; Heidmann, Claus Peter; Andres, Klaus; Schweitzer, Dieter; Keller, Heimo J.
The α-modification of (BEDT-TTF)2I3 undergoes a metal-to-insulator transition at T = 135 K. Application of hydrostatic pressure shifts the transition temperature to lower values. Above 12 kbar semimetallic behavior is observed, but no superconductivity is detected up to a pressure of 17 kbar and temperatures down to l00 mK. The β-modifcation, on the other hand, exhibits ambient pressure volume superconductivity below Tc = 1.05 K. Measurements of the anisotropy of the diamagnetic shielding- as well as the Meissner-magnetization are presented.
Open Access
Anisotropic chemical shifts and spin rotation constants of 15N from liquid and solid state NMR: Nitrobenzene
(1974) Schweitzer, Dieter; Spiess, Hans Wolfgang
The study of the 15N spin-lattice relaxation time T1 in nitrobenzene at 14 and 32 MHz from −10 to 60°C showed that at 32 MHz relaxation due to anisotropic chemical shift is predominant. At low frequencies, the relaxation is caused mainly by spin rotation and at low temperature also by intermolecular dipole-dipole interaction. From the powder spectrum in solid nitrobenzene, the principal elements of the shielding tensor σ were obtained: σxx = −273 ± 10 ppm, σyy = +94 ± 10 ppm, and σzz = +156 ± 10 ppm relative to liquid nitrobenzene, Δσ = σ xx - ½(σ yy + σ zz) = -398 ± 20ppm. From the almost axially symmetric σ -tensor, the spin rotation constants were calculated: C‖ = 11.4 ± 1.5 kHz and C⊥ = 1.35 ± 0.5 kHz, where C‖ is the component parallel to the twofold axis of the molecule. These values for Δσ and the spin rotation constants are in excellent agreement with those obtained by analysis of the relaxation data. A comparison of anisotropic chemical shifts and spin rotation constants for 15N and 13C in isoelectronic compounds is given.
Open Access
The anisotropy of conductivity of n-type germanium in strong d.c. fields
(1965) Schweitzer, Dieter; Seeger, Karlheinz
The electric conductivity of n-type germanium at large d.c. field intensities has been measured in three directions of symmetry of the cubic lattice. From these data the repopulation of valleys of the conduction band has been determined. Measurements were done with three materials of different purity. The variation of repopulation between these materials can be explained qualitatively by the influence of Coulomb scattering at ionized impurities. The data are compared with those of Nathan and theoretical data of Franz and Reik et al. The relations of these data to those of the current component perpendicular to the field direction (Sasaki et al.) are also given.
Open Access
Assignment of fundamental vibrations and estimation of electron-molecular vibration coupling constants for bis(ethylenedioxy)tetrathiafulvalene (BEDO)
(1993) Pokhodnia, Konstantin I.; Kozlov, Mikhail E.; Onischenko, V. G.; Schweitzer, Dieter; Moldenhauer, Jörg; Zamboni, Roberto
The 1.r. absorption and Raman spectra of BEDO are investigated. All fundamental vibrations are assigned using correlations between the obtained data and those for a well-known compound BEDT-TTF (ET). Comparative normal coordinate analysts has been done. The results are used for calculation of electron-intramolecular vibration coupling constants of BEDO based superconducting charge-transfer complexes. Comparison of these values with the constants obtained earlier for ET shows that BEDO is a perspective donor for searching new superconductors on its basis.
Open Access
Band structure calculation and tunneling measurements in (BEDT-TTF)2X (X=I3, IAuI)
(1990) Weger, Meir; Nowack, Andreas; Schweitzer, Dieter; Kübler, Jürgen; Bentum, Jan M. van; Sommers, Charles S.
At about the same time when little proposed room-temperature superconductivity in organic polymers, it was suggested that the high-Tc of the more conventional A-15's is associated with their one-dimensional electronic band structure. When TTF-TCNQ was discovered in 1973, it was suggested that the electron-phonon coupling in this 1-D organic molecular crystal is responsible for the metal-to-insulator (Peierls) transition at 52 K2, and reducing λ will cause a crossover to a superconducting state. Since then, the electronic structure, the conduction mechanism, and the superconducting mechanism were subject to controversy. Therefore, it is of some importance to establish whether the electronic band structure, and conduction mechanism, are similar to those in more conventional metals, and whether the superconductivity mechanism is the normal BCS phonon-mediated interaction.
Open Access
(BEDO-TTF)2ReO4·(H2O): a new organic superconductor
(1991) Kahlich, Siegfried; Schweitzer, Dieter; Heinen, Ilsabe; Song, En Lan; Nuber, Bernhard; Keller, Heimo J.; Winzer, Klaus; Helberg, Hans W.
The structure and the temperature dependence of the resistivity, thermopower and ac-susceptibility of the new organic metal (BEDO-TTF)2ReO4(H2O) was investigated. The resistivity and thermopower data indicate phase transitions at 213K, around 90K and 35K. Below 2.5K an onset to superconductivity is observed in the resistivity data. Superconductivity was suppressed in the resistivity at 1.3K by applying a magnetic field of about 0.2T. Ac-susceptibility data indicate that superconductivity is a bulk effect in (BEDO-TTF)2ReO4(H2O) but the onset for superconductivity observed in the ac-susceptibility is only at 0.9K and the transition seems to be complete only at temperatures below 50 mK. This broad transition might be due to some disorder in the structure created by the low temperature phase transitions.
Open Access
BEDT-TTF radical salts: organic metals and superconductors
(1987) Schweitzer, Dieter
A review of the structural, electronic and superconducting properties of some BEDT-TTF-radical salts is given.
Open Access
BEDT-TTF salts with square platinates(II) as counterions : [BEDT-TTF]4[Pt(C2O4)2], a new organic metal
(1989) Gärtner, Stephan; Heinen, Ilsabe; Schweitzer, Dieter; Nuber, Bernhard; Keller, Heimo J.
The title compound, [BEDT-TTF]4[Pt(C2O4)2] is obtained by electrocrystallization, which yields black platelets of stoichiometry C44H32O8PtS32, Mr = 1909.87. Crystal structure determination at room temperature gives a triclinic cell Pi, with a = 8.678(2) A, b =11.878(5) A, c = 15.757(7) A,α = 105.49(3)°, β= 91.05(3)", 'γ= 91.96(3)", V = 1563.64 A3,d c = 2.03 g/cm 3, Z = 1. The solid contains sheets of BEDT-TTF cations separated by sheets of [Pt(C2O4)2]2-- counterions. The BEDT-TTF sheets are made up of columns that interact strongly through S-S contacts. The columns are built up by pairs of BEDT-TTF molecules. There are strongly varying bond distances in the two different BEDT-TTF moieties that make up a pair. This fact is especially clearly expressed in the surprisingly short C-C distances of the central C=C double bond of the two BEDT-TTF ions:1.273 A and 1.327 A respectively. These are by far the shortest distances for the central C=C bonds observed so far in conductive BEDT-TTF salts. The compound behaves like a metal down to about 60 K. Temperature-dependent d.c. conductivity, e.s.r. and thermopower measurements show metallic behaviour at room temperature (σ300 = 20 S/cm) with a metal-to-metal phase transition around 200 K. At about 60 K a second broad phase transition occurs and the crystals become semiconducting.
Open Access
(BEDT-TTF)2+J3- : a two-dimensional organic metal
(1984) Bender, Klaus; Dietz, Klaus; Endres, Helmut; Helberg, Hans W.; Hennig, Ingolf; Keller, Heimo J.; Schäfer, Herbert; Schweitzer, Dieter
Temperature-dependent thermopower-, dc- and microwave-conductivity measurements on the two-dimensional organic metal (BEDT-TTF)2+J3- are reported.
Open Access
(BEDT-TTF)2X(TCE)0,5[X:BF4, FSO3, IO4] new organic metals
(1991) Armbruster, Kurt; Bele, Petra; Brunner, Hermann; Chen, Benming; Dahm, Stefan; Geiger, Rolf; Heinen, Ilsabe; Keller, Heimo J.; Schweitzer, Dieter
The structure, the temperature dependence of the resistivity, thermopower, ESR-linewidth and the spin susceptibility of a new series of organic metals (BEDT-TTF)2X·(TCE)0,5 [X = BF4, FS03 and IO4] is presented and discussed.
Open Access
BEDT-TTF-Salze mit quadratischen Platinaten(II) als Gegenionen: [BEDT-TTF]4[Pt(CN)4]
(1990) Gärtner, Stephan; Heinen, Ilsabe; Keller, Heimo J.; Niebl, Roland; Nuber, Bernhard; Schweitzer, Dieter
Im folgenden beschreiben wir Synthese, Kristallstruktur und das physikalische Verhalten verschiedener Modifikationen einer weiteren Verbindung aus dieser Reihe, dem [BEDT-TTF]4[Pt(CN)4]. Über eine bei Raumtemperatur metallische Modifikation dieses Festkörpers wurde kürzlich berichtet. Diese Modifikation wird im folgenden als [BEDT-TTF]4[Pt(CN)4'] bezeichnet und hat die folgenden Zelldaten: triklin,α=11,002(9) A,b=17.906(9)A,c=16,625(7)A, a=77.28(4)°,β=84,17(5)°,γ=81.26(6)°. Raumgruppe CI,Z=2[5]. Uns gelang nun die Isolierung und die physikalische Charakterisierung von drei weiteren Festkörpern mit der genannten Zusammensetzung. Wir nennen diese β-,γ- und δ-Phase.
Open Access
Bulk superconductivity at ambient pressure in polycrystalline pressed samples of organic metals
(1990) Schweitzer, Dieter; Kahlich, Siegfried; Gärtner, Stephan; Gogu, Emil; Grimm, Hans; Zamboni, Roberto; Keller, Heimo J.
Bulk superconductivity in polycrystalline pressed samples of αt-(BEDT-TTF)2I3 and βp-(BEDT-TTF)2I3 is reported. This finding shows that organic superconductors can be used in principle for the preparation of electronic devices and superconducting cables.