J.U. von Schütz

A new crystalline phase of the electroluminescent material tris(8-hydroxyquinoline) aluminum exhibiting blueshifted fluorescence

We report on two different crystalline phases of tris(8-hydroxyquinoline) aluminum (Alq3) which were obtained by thermal sublimation in a horizontal glass tube. These phases are investigated by x-ray powder diffraction, Raman and infrared spectroscopy, and low temperature photoluminescence measurements. Apart from the already known α phase we could identify a new crystalline phase of Alq3 (δ-Alq3) showing blueshifted fluorescence. As compared to the α phase this new phase is characterized by a larger unit cell volume, a reduced number of Raman lines in the energy range between 70 and 700 cm−1, a blueshift of the photoluminescence maximum by about 0.2 eV, and a decreased intersystem crossing to the triplet state. These differences are interpreted in terms of the isomery of the Alq3 molecule. It is supposed that the new phase contains the facial isomer, whereas in the other phases only the meridianal isomer was reported. Low temperature photoluminescence spectra show a well-resolved vibronic progression wit...

Radical anion salts of N,N′-dicyanoquinonediimine (DCNQI): Conductivity and magnetic properties

Abstract DC-conductivity (σ), static susceptibility (χ), ESR-intensity, resonance position and linewidth of (R 1 ,R 2 -N,N′ -DCNQI) 2 Me ( R 1 = CH 3 ; R 2 = CH 3 , Cl , Br ; Me = Ag , Cu ) between 300 K and 1.2 K are reported. All copper salts have a high χ of about 5 – 6.10 −4 emu/mole in the conducting temperature range. This can be explained by strong electron-electron repulsion. In two salts, a CDW driven phase transition is seen and antiferromagnetic resonance (AFMR) is found below 10 K. The lack of any ESR signal in the conducting state is due to strong spin-orbit coupling (SOC) given by the nitrogen(p)-Cu(d) admixture which implies some electronic transport across the stacks. The silver salts do not exhibit these features. They behave as quasi-one dimensional conductors with reduced SOC effects. The conductivity is limited solely to the anion stack. Consequently, 1D-localization effects dominate the temperature dependence of the conductivity.

Proton spin-lattice relaxation and exchange splitting of the torsional ground state in NH4ClO4

Abstract The proton spin-lattice relaxation time T 1 has been measured in a crystalline powder of NH 4 ClO 4 from room temperature to 4°K at Larmor frequencies of 22 and 56 MHz. At temperatures below 20°K the recovery of the magnetization is given by a sum of two exponentials. Three T 1 -minima are observed, and one of them can be explained by the large exchange splitting of the weakly hindered NH + 4 ion.

Magnetic resonance experiments on the radical cation salt (fluoranthenyl)2+ PF6−. Correlation of proton-NMR, fluorine-NMR and ESR

Abstract The longitudinal relaxation rates of the protons and the fluorine were determined for the radical cation salt (fluoranthenyl) 2 + PF 6 − as a function of the temperature. These data are compared with the temperature dependence of the pulsed ESR signal amplitude at the same Larmor frequency (44 MHz). Whereas the fluorines are relaxed mainly by the reorientational motion of the anions and by the interaction with fixed paramagnetic impurities, the protons are relaxed additionally above 150 K predominantly by highly mobile paramagnetic species, whose concentration could be determined directly via the signal amplitudes of the NMR and ESR signals in the same set up. The Pauli susceptibility χ c s = 3.1 ×10 −5 cm 3 /mole which is derived from this experiment and the Korringa relation ( T 1H 4 − ) −1 = constant of the proton relaxation leads to the assumption of a metal-like behavior of the salt above 183 K. The observation that the protons of the cation stacks, rather than the fluorines on the anions are relaxed by the mobile paramagnetic species favors the assumption of one-dimensional spin transport within the fluoranthenyl stacks, which is further supported by an ω −1 2 dependence of T 1H −1 .

The dynamics of triplet exciton trapping in 1,2,4,5- tetrachlorobenzene

Abstract The phosphorescence intensifies and decay times of the triplet exciton and X 2 -trap emission in neat and doped 1,2,4,5-tetrachlorobenzene crystals were measured as a function of temperature. The comparison of the results found on neat and doped crystals allows the statement, that exciton trapping is not limited by the motion of the excitons towards the traps but by the trapping step on the trap molecule solely. For the individual trapping step a rate of about 3 × 10 6 S −1 was determined.

Synthesis and structure of new anion radical salts from DCNQIs

Abstract Anion radical salts [2-R 1 ,5-R 3 -DCNQI] m M n (M=metal or organic cation) are described, which are derived from the newly introduced class of acceptors, N,N′-dicyanoquinonediimines. Single crystals were easily prepared from DCNQIs and metal wires. The salts are discussed in terms of structural properties and (temperature dependent) conductivities together with the behaviour of alloys.

Pressure-Induced Metal-to-Insulator Phase Transitions in the Organic Conductor (2,5 DM-DCNQI)2Cu

The electrical resistivity along the high-conductivity axis has been measured at pressures up to 9 kbar in the novel organic conductor (2,5 DM-DCNQI)2Cu. Deviations from the usual linear behaviour of the conductivity under pressure at room temperature have been observed. Moreover, at lower temperatures metal-to-insulator phase transitions have been induced with values of transition temperatures depending on the applied pressure. The obtained results are discussed and compared with corresponding data for other organic compounds. A tentative interpretation of this novel and unusual phase diagram is proposed in terms of a mixed valence state for copper ions.

The conducting salts of N,N′-dicyanoquinonediimine (DCNQI)

Abstract New results on a series of X,Y-N,N′-DCNQI salts with metallic counterions are presented (X,Y = H, CH3 ≡ Me, Cl, Br, I). The comparison of conductivity, ESR (linewidth and susceptibility) and NMR (Proton spin relaxation) of these radical anions salts indicates that all salts can be separated into two groups: salts containing copper as counterion, which can be characterised as organic metals (at least at high temperatures); and salts with other counterions, whose physical properties are strongly influenced by low dimensionality. The metallic nature of (DMe-DCNQI)2Cu has been demonstrated directly by UPS-photoelectron spectroscopy, which reveals a high density of states near the Fermi energy.

Time-resolved ESR investigation of electron-spin diffusion in the radical cation salt (fluoranthenyl)2+ AsF6−

Abstract The diffusion constant D of electronic spins has been determined for the radical cation salt (fluoranthenyl) 2 + AsF 6 − by means of electron-spin echo decays in a magnetic field gradient. An average value of D = 1.3 cm 2 s −1 is obtained in the “metallic region” (180 ⩽ T ⩽ 300 K). Comparison with electronic spin relaxation times ( T 1 , T 1ρ , T 2 ) at 180 MHz and 9.8 GHz reveals the temperature dependence of the transport process.

The electronic structure of (2-X-5-M-DCNQI)2Cu

Abstract Using XPS and UPS photoelectron spectroscopy, we have studied the electronic structure of single-crystalline needles of 2-Cl-5-M-, 2-Br-5-M- and 2,5-DM-substituted (DCNQI) 2 Cu charge-transfer salts, prepared in situ. The results indicate that the oxidation state of the counterion is Cu + . The electronic structure of the valence band contains contributions from C 2p and N 2p states, but also from Cu 3d atomic levels. There is a metallic density of states near the Fermi energy for (DM-DCNQI) 2 Cu. This is the first experimental evidence for the existence of an organic metal with an admixture of Cu 3d states. The admixture from Cu states near the Fermi energy is reduced by substitution of Cl and disappears in amorphous films, demonstrating the strong influence of local structure. The implication of these spectroscopic results on the conductivity mechanism is discussed.