W. Bietsch

Spin dynamics in oriented lithium phthalocyanine thin films investigated by pulsed electron spin resonance

Thin films of monolithium phthalocyanine (PcLi) were prepared by vacuum deposition on cleaved mica. The highly anisotropic spin diffusion in these films was studied by electron spin resonance (ESR). Using averaging techniques, we succeeded in the detection of films as thin as 100 nm using pulsed ESR and 2 nm with cw ESR. The electronic relaxation is shown to depend on the substrate temperatures Ts during film deposition which determine the morphology of the films. We find relaxation rates of films which are faster by a factor of 20 (T1e−1) to 30 (T2e−1) than those of single crystals. At ambient temperature, a low‐dimensional spin exchange mechanism is proposed for the single crystal. At low temperatures, in single crystals the electronic relaxation rates T1e−1 and T2e−1 hint at a reduced exchange with increased dimensionality d≳1. This holds for the films at all temperatures and is explained by different crystal structures of single crystals and films. A generally applicable relaxation model is developed ...

Temperature dependence of the electronic spin relaxation of DCNQI salts

For the elucidation of the charge and spin dynamics of the radical anion salts of DCNQI with metallic counterions we have performed cw- and pulsed ESR experiments (βBpp,T1e−1 andT2e−1) between 300 and 4 K at nine salts differing in counterions and sidegroups, respectively. We can explain the relaxation rates by dipolar electron-electron interaction and spin-orbit contribution. In the high temperature range we have a gradual decrease in the number of charge carriers by interband transitions without a slowing down of the mobility. With complete localization of the electron spins (no mobile electrons anymore) exchange interaction governs the spectral density, becoming strongly temperature dependent due to effective spin exchangeJeff(T), explained by an extended REHAC-model. This effective spin exchangeJeff(T) includes for the first time a contribution by the metallic counterions. For spin-orbit interaction we developed a model based on F. Adrian [1] not depending on the mixture of Bloch and spin states as given by Elliott [2]. This is achieved by the inclusion of the electronic probability on atoms with higher atomic numbers, modulated by phonons. This model explains the drastic changes in the ESR linewidth of different radical ion salts of DCNQI and allows inductively the prediction of the electronic properties of new radical ion systems of which just the molecular and crystal structure is known.

Pulsed ESR of conducting alkali-DCNQI salts: a general analysis of the electronic spin relaxation rates☆

Abstract We have determined the temperature and angular dependence of the electronic spin relaxation rates 1/ T 1e and 1/ T 2e in the radical anion salts of M(2,5-DMe-DCNQI) 2 with metallic counterions (MLi, Na, K and Rb), originating from the dipolar interaction between the spins of the conduction electrons (FS) and from contributions by spin orbit coupling (SOC). Taking into account the known crystal structures, the FS contribution could be determined quantitatively by (i) calculating the spin density distribution on the DCNQI molecule, (ii) summing over all neighboring molecules within two lattice constants, and (iii) introducing a 1D spectral density function for the electronic mobility. Using semiclassical relaxation theory for SOC interaction we introduced a new model for 1D systems based on the following considerations. Each ‘out-of-chain scattering event’ of the conduction electrons leads to an angular-dependent relaxation contribution. This depends on the relative mutual arrangement of the DCNQI molecules, i.e., the crystal structure, and possesses a spectral density resulting from a pulse-like interaction. The quantitative analysis of the experimental data gave spin diffusion constants D for the four salts of 0.05–0.4 cm 2 s −1 , anisotropies σ ‖ / σ τ of (3.8–6.5)×10 3 , and, by comparison of the conductivity data, additionally allowed the separation between the activation energies of the charge carrier concentration (about 17 meV) and that of the ‘effective mobility’.

On the anisotropy of charge and spin dynamics in the DCNQI-salts with different counterions

Abstract We present the results of the conductivity, of the ESR and of the susceptibility on the new organic metals (d 3 -, d 6 -, d 8 -DMeDCNQI) 2 Cu and the magnetic resonance experiments leading to the determination of the anisotropy (or dimensionality) of the conductivity in the quasi 3-d-copper salts and the 1-d-non copper salts, respectively.

2,3,6,7-tetrakis(methylthio)naphthalene; synthesis and cation radical salt

Abstract The three step synthesis of 2,3,6,7-Tetrakis(methylthio)naphthalene (TMTN) 1 is described. The title donor gives a 1:1 salt with hexafluoroarsenate anion. The crystal structure consists of a dimerized donor stack, with alternate stacking. The conductivity is activated, room temperature conductivity; ≈10 −3 ·10 −4 Scm −1 . The susceptibility is quite low, χ (300K) = 7·10 −7 emu/mol, and the ESR linewidth is Δ B pp (300K)≈5 G.

Pulsed ESR on the organic ion radical salt: 3,3′-diethyl-4,4′-dimethyl-2,2′-thiazolocyanine-[TCNQ]2

We present firstT1e−1 andT2e−1 measurements on the organic ion radical salt 3,3′-diethyl-4,4′-dimethyl-2,2′-thiazolocyanine-(TCNQ)2 as function of temperature and of orientation. The electronic spin diffusion constant could be determined directly by the electron spin echo field gradient technique:D (300 K)=0.03±0.02 cm2/sec. Pulsed ESR experiments have — in comparison to conventional cw-ESR — the advantage to monitor viaT1e−1 andT2e−1 the spectral density of dynamical processes at different frequencies. This is shown in a general manner on 3,3′-diethyl-4,4′-dimethyl-2,2′-thiazolocyanine-(TCNQ)2. Between 300 and 60 K,T1e−1 andT2e−1 are close in amplitude and have a similar temperature dependence. At 60 K their degeneracy is lifted, yielding a quantitative value for the effective spin exchange between localized spinsτex−1 sec−1 and via the absolute value of the relaxation an average distance of the localized centers of about 12 Å. The dynamical data as evaluated above cannot be correlated with the conductivity, clearly indicating that the conduction electrons are a minority, not being monitored by the ESR-experiments.