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Phase transitions in the radical ion salts of Cu(2,5-dimethyl-dicyanoquinonediimine)2 (Cu(DCNQI)2) can be achieved either by ‘‘external’’ pressure or by ‘‘internal’’ pressure. The latter is introduced by chemical modifications at the DCNQI-molecules (e.g., deuteration) or by alloying deuterated Cu(2,5-(CD3)2-DCNQI)2 (d6) with undeuterated Cu(2,5-(CH3)2-DCNQI)2 (h8), giving the mixture (h8/d6) in different ratios. In this work we present simultaneous conductivity (σ) and electron spin resonance (ESR) experiments on differently deuterated Cu(DCNQI)2-systems and on alloys (h8/d6) under external pressure. The anticoincidence of σ and ESR allows the determination of phase transition temperatures even in the absence of electrical contacts. For each system an individual phase diagram is established. Introducing an effective pressure peff=p0+p with p0 being the ‘‘internal’’ (chemical) pressure, a general phase diagram could be constructed by determining the individual p0 value for all systems. For the alloys (h8/d6) the relation ∂p0/∂q=5.0 bar/% (q is the percentage of d6) could be evaluated. The occurrence of a phase transition is associated with a change in the lattice parameters. For that, a structural model of a v-shaped temperature dependence of the unit cell volume V is suggested. For thermodynamic considerations, V is used as order parameter. If this order parameter V crosses critical values Vcbig and Vcsmall from above (cooling) or below (heating), phase transitions from conducting to insulating phases or vice versa are induced. This model explains the phase transition temperatures, the re-entry and the hysteresis effects of all systems qualitatively. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The deuterated dicyanoquinonediimine salt Cu(2-CD3,5-CD3-DCNQI)2, (d6), a highly conductive organic metal, exhibits a phase transition from the metallic to the insulating state at Tc≊60 K. In a wide temperature range we observe an anticoincidence of high conductivity and electron spin resonance (ESR). These experiments were performed simultaneously in our ESR apparatus equipped for application of He pressure up to 200 bar. Near Tc there is a coexistence of conducting (ESR-silent) and insulating (ESR-detectable) domains. This correlation of ESR and conductivity σ points to a percolation limited conductivity. Upper and lower limits of the size of the domains are given. Light shifts Tc to lower temperatures, pressure to higher ones. This could be due to shrinking of the lattice which induces the phase transition when critical structural parameters are approached. This is most probably explained by a simultaneous development of a charge density wave (CDW) and a spin pairing within the DCNQI stack together with the trimerization of the lattice. Systematic measurements by variation of temperature, pressure, and light led to phase diagrams of (d6) and to the understanding of the mechanism of the phase transitions. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

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 which describes the transition from one-dimensional to three-dimensional spin propagation. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Zero field absorption detected magnetic resonance hole burning measurements were performed on photosynthetic reaction centers of the bacteria Rhodobacter sphaeroides R26 and Rhodopseudomonas viridis. Extrapolation to zero microwave power yielded pseudohomogeneous linewidths of 2.0 MHz for Rhodopseudomonas viridis, 1.0 and 0.9 MHz for the protonated forms of Rhodobacter sphaeroides R26 with and without monomer bacteriochlorophyll exchanged, and 0.25 MHz as an upper limit for fully deuterated reaction centers of Rhodobacter sphaeroides R26. The measured linewidths were interpreted as being due to unresolved hyperfine interaction between the nuclear spins and the triplet electron spin, the line shape being determined by spectral diffusion among the nuclei. The difference in linewidths between Rhodobacter sphaeroides R26 and Rhodopseudomonas viridis is then explained by triplet delocalization on the special pair in the former, and localization on one dimer half on the latter. In the fully deuterated sample, four quadrupole satellites were observed in the hole spectra arising from the eight 14N nitrogens in the special pair. The quadrupole parameters seem to be very similar for all nitrogens and were determined to κ=1.25±0.1 MHz and η=0.9±0.1 MHz. [ABSTRACT FROM AUTHOR]

We have determined the electron spin diffusion constant in (dicyclopentaheptalene)2SbF6 by means of a magnetic field gradient electron spin resonance spin echo technique. Parallel to the chain axis we obtain a value of D||≊0.064 cm2 s-1. From the comparison of temperature dependent conductivity and susceptibility measurements it is concluded, that the charge carriers are localized in this compound. These localized charge carriers can be described as a system of 1d-Heisenberg spin chains. The measured D|| value therefore corresponds to an effective exchange coupling constant of J≊4.5 meV (J/kB≊51 K). [ABSTRACT FROM AUTHOR]

Triplet states on conjugated polydiacetylene chains which are created by UV excitation are examined experimentally using ODMR spectroscopy. The observed fine structure shows that the triplet state can be ascribed to the conjugated chain rather than to the side groups and that it is localized. This leads to the suggestion of an exciton polaron. In the theoretical part the wave function of the exciton polaron is calculated using the configuration model in analogy to the description of pz radical electrons on carbene chain ends of reactive short-chain intermediates. The total fine structure tensor is obtained by summing up the fine structure tensors of the individual configurations weighted by their probability densities. The transfer integral t of the configuration model is fitted with respect to good agreement between experimentally observed and calculated fine structure parameters. Finally, comparison between the experimentally observed ESR linewidth and the calculated hyperfine structure splitting shows that the linewidth is motionally narrowed leading to the conclusion that the exciton polaron is mobile. [ABSTRACT FROM AUTHOR]