Restricted electron motion in 1D organic conductors: PGSE-ESR in (PE)2PF6 and (FA)2PF6

The pulsed-gradient spin echo electron spin resonance technique has been used to study the restricted motion of the conduction electrons in the one-dimensional organic conductors (PE)₂PF₆ and (FA)₂PF₆. Data were collected as a function of temperature, between room temperature and the Peierls transition temperature. Improved signal-to-noise ratio and reduced eddy currents were achieved. Echo attenuation for (PE)₂PF₆ was found to fit a model assuming diffusional motion between scattered, totally reflecting, barriers. The free diffusion constant and average barrier distance were successfully best fitted simultaneously for all temperatures, and the temperature dependence of the diffusion constant was found to scale with the previously measured AC conductivity. In contrast, PGSE results for the (FA)₂PF₆ crystals do not fit the above model, possibly due to the existence of basically different restricting factors.     

Structural origin of inhomogeneous properties in (fluoranthene)2PF6 crystals

Twinned and untwinned (FA)₂PF₆ crystals are analysed by X-ray scattering at varied temperature. A rotation by 180° around the a axis (stacking axis) is identified as the typical twinning rule. The influence of twinning on the crystal quality in general is derived, as well as its irrelevance for the Peierls transition at T_P = 180 K and for the high-temperature structural transition at T_S = 200 K, accompanied by the loss of the FA-stack's mirror plane (space group A2/m to P2₁/c). These observations are discussed with respect to recent electron spin resonance imaging results. Differing concentration of localized defects in the different growth sectors of macroscopic single crystals is identified as an obvious explanation of inhomogeneous ESR properties.     

Electron spin resonance of thin films of organic light-emitting material tris(8-hydroxyquinoline) aluminum doped by magnesium

We have successfully observed electron spin resonance (ESR) signals of radical anions in thin films of tris(8-hydroxyquinoline) aluminum (Alq₃), a compound widely used as electron transporting and luminescent layers in organic light-emitting diodes. To obtain definitely defined radical-anion states in Alq₃, we doped Alq₃ with Mg by co-evaporating these materials. The obtained g value and peak-to-peak ESR linewidth ΔH_pp of Alq₃ radical anions are 2.0030 and 2.19 mT, respectively. Theoretical g value and hyperfine interactions were calculated by density functional theory method, which are in good agreement with the experimental results. A quantitative evaluation of doping concentration was performed. We confirmed that doped charges are localized at deep trapping sites by the lineshape analysis and temperature dependence of the ESR signals. Morphological investigation using transmission electron microscopy clarified that the co-evaporated Mg atoms form clusters.     

Photo-induced charge separation process in (PCBM-C120O)/(M3EH-PPV) blend solid film studied by means of X and K-bands ESR at 77 and 120 K

The new fullerene dimer adduct PCBM-C₁₂₀O was studied as an electron acceptor by light-induced ESR (LESR) when blended with the electron donor-conjugated polymer M3EH-PPV in a thin solid film. ESR parameters for the PCBM-C₁₂₀O anion radical and the M3EH-PPV polaron (triaxial g_x,y,z factors) were determined by X- and K-band ESR techniques at temperatures of 77 and 120 K. It was found that isotropic g₀ of PCBM-C₁₂₀O anion radical ESR spectra obtained in this blend under illumination is close to g-factors of electrochemically reduced (C₁₂₀-O)^? and does not coincide with the analogous parameter of (C₁₂₀-O)^2? (reported in literature). Spin relaxation parameters (T₁ and T₂) of the PCBM-C₁₂₀O anion radical and M3EH-PPV positive polaron are similar to T₁, T₂ for light-induced ion radicals in the P3HT/PCBM blend.     

‘Real’ single crystals of the organic conductor (fluoranthene)2PF6: An N.M.R. study

We report on extensive n.m.r. investigations of the quasi-one-dimensional organic conductor (fluoranthene)₂PF₆. N.m.r. signatures of sample aging and decomposition were established. We analysed the frequency dependence of proton spin-lattice relaxation of individual, well-characterized single crystals. These data could be described quantitatively based on parameters that were independently derived earlier. It is concluded that the intrinsic anisotropy of the conduction electron scattering timesτ_⊥/τ_| for (fluoranthene)₂PF₆ must be larger than 3 × 10⁴, i.e., more than one order of magnitude larger than for TTF-TCNQ and its derivatives.     

Spin dynamics in undoped trans-(CH)x

Dynamics of neutral soliton (domain wall) of undoped trans-polyacetylene t-(CH)_x has been studied by analyzing ESR line width as a function of frequency from 3 to 500 MHz at temperatures from 10 to 300 K. Comparing the result analyzed at f>6 MHz with that of t-(CD)_x, it was concluded that the spin dynamics both in t-(CH)_x and t-(CD)_x are almost the same as each other; the diffusion rate along the chain D_// is 5 × 10¹³ rad/sec, which is consistent with that of NMR T₁, and varies approximately as T² between 10 and 300 K. An anomalous line broadening of ESR at frequencies lower than 6 MHz was found at all the temperatures studied. This can be attributed to a non-secular broadening of the width by soliton trapping. This gives the soliton spin density ϱ ≈ 0.1 at the center of neutral soliton.     

The new salt κ-ET2[Hg(SCN)2I]: crystal structure and physical properties

The structure, resistivity and ESR of a new organic conductor κ-ET₂[Hg(SCN)₂I](ET=bis(ethylenedithio) tetrathiafulvalene) are studied. Its main crystallographic parameters are found to be: M-1213.2, a-38.03(1), b= 11.80(1), c = 8.329(9) Å; γ=98.1(2)°; V-3700(2) ų; space group P2₁Ib; Z = 4. It is shown that the radical cations are packed according to the κ-type in the organic sheets, and the anions form polymerized chains. Two different radical cation layers with different amounts of shortened S···S contacts and distinct interaction between anionic and cationic sheets are found. ESR linewidth is found to be 9-11G (300 K), which is substantially narrower than a typical linewidth for ET-based radical cation salts of the κ-type. Based on temperature dependences of the resistance anisotropy and ESR parameters, we suggest that a structure rearrangement with electron localization may take place around 50 K. The comparison of crystal structure and properties of the title compound with other salts of the family ET₂[Hg(SCN)_3−nX_n], where X=Cl, Br, and n = 1.2, is camed out.     

Anisotropic diffusion and electron spin relaxation in the single crystal (fluoranthenyl)2AsF6 above and below the metal-semiconductor phase transition

We have determined the anisotropic conduction electron diffusion constants parallel (D_∥) and perpendicular (D_⊥) to the molecular stack axis, and the electronic spin relaxation rates T₁⁻¹ and T₂⁻¹ in the temperature range 140 K to 300 K by using the spin echo field gradient technique. Values of D_∥ = (0.5 – 1.6) cm² s⁻¹ and D_⊥ = (0.7 – 1.5) 10⁻³ cm² s⁻¹ are obtained.The temperature dependence is very weak in the metallic regime above the phase transition temperature T_c = 180 K and moderate in the semiconducting regime.The temperature dependence of the electron spin relaxation rates T₁⁻¹ and T₂⁻¹, however, shows a drastic increase in the anisotropy below T_c. The high mobility of the spins and the increase in relaxation anisotropy are attributed to the development of a high dynamical order below T_c.     

Charge ordering state of mixed-valence (TP-EDTT)3(PF6)2

4,5-Ethylenedithio-2-(thiopyran-4-ylidene)-1,3-dithiole (TP-EDTT) and its radical cation salts were prepared. Conventional galvanostatic electrocrystallization gave (TP-EDTT)₂SbF₆, (TP-EDTT)GaCl₄, (TP-EDTT)ReO₄, and (TP-EDTT)₃(PF₆)₂. The variation of bond lengths upon the charge of each molecule was examined based on 0, +0.5, and +1.0-charged TP-EDTT molecules. Applying the relationship between the charge and the bond lengths to the donor molecules, (TP-EDTT)₃(PF₆)₂ was proved to be a charge ordering system.     

A new phase in stoichiometric Cu6Sn5

The intermetallic compound Cu₆Sn₅ is a significant microstructural feature of many electronic devices where it is present at the solder–substrate interfaces. The time- and temperature-dependent thermomechanical properties of Cu₆Sn₅ are dependent on the nature and stability of its crystal structure, which has been shown to exist in at least four variants (η, η′, η⁶ and η⁸). This research details an additional newly identified monoclinic-based structure in directly alloyed stoichiometric Cu₆Sn₅ using variable-temperature synchrotron X-ray diffraction (XRD) and transmission electron microscopy. The phase is associated with a departure from the equilibrium temperature of the polymorphic monoclinic–hexagonal transformation temperature. The new monoclinic phase can be treated as a modulation of four η⁸-Cu₅Sn₄ unit cells plus one η′-Cu₆Sn₅ unit cell. It has been labeled as η⁴⁺¹ and has cell parameters of a = 92.241 Å, b = 7.311 Å, c = 9.880 Å and β = 118.95° determined from electron diffraction patterns. The XRD results could be fitted well to a Rietveld refinement using the new crystal parameters.     

Effect of spin ½ radicals on the ultrafast photoexcitation dynamics in RR-P3HT/PCBM blends for photovoltaic applications

We report the effect of spin ½ radical impurity, Galvinoxyl on the ultrafast photoexcitation dynamics in annealed films of regio-regular poly(3-hexylthiophene) (RR-P3HT)/[6,6]-phenyl C₆₁ butyric acid methyl ester (PCBM) blend. The addition of Galvinoxyl radical impurities to the blend reduces the geminate recombination rate of photogenerated polaron pairs. Consequently organic photovoltaic solar cells made from the radical/blend mixture as the active layer show increased short-circuit current (J_sc), fill-factor, and power conversion efficiency. We speculate that the spin–spin interaction of the radical impurity with the electron and hole forming the polaron pairs is the main mechanism responsible for the obtained reduced recombination rate.     

A proton magnetic relaxation study on pyrolysates of polyacrylonitrile (PAN) fibres: Motion of electrons

Polyacrylonitrile (PAN) fibres heat treated at temperatures between 400 and 800 °C (PAN400-PAN800) have been studied by means of proton NMR. The spin-lattice relaxation rate (T₁⁻¹) was measured as a function of NMR frequency over a wide range (4–300MHz). T₁⁻¹ was found to show the singular dependence upon frequency characteristic of the low-dimensional (one- and two-dimensional) diffusive motion of electrons. A distinct change of the dimensionality of the electronic motion from one dimensional to two dimensional was observed in PAN500. In addition, the dependence of T₁⁻¹ on the heat-treatment temperature (HTT) is in accordance with that of the unpaired electron spin number measured by ESR. It was interpreted in terms of polaron and bipolaron formation.     

Synthesis and physical properties of new metallic compound LixC60(THF)y

Electrocrystallization of C₆₀ in chlorobenzene and lithium tetraphenylborate in THF efficiently produced the new metallic species Li_xC₆₀(THF)_y; the ESR spectra show clearly the existence of a typical broad signal due to the C₆₀ anion radical. The change of the intensity of the ESR signal corresponds to the metal-insulator transition at 150 K, and the ¹³C and ⁷Li solid-state NMR spectra are also reported.     

Synthesis,structure of a new acceptor TBA2S6 and preparation,physical properties of ET3S6

A general procedure for the synthesis of the tetrabutylammoium hexasulphide, TBA₂S₆, is first described. The structure of TBA₂S₆ has been determined by X-ray crystallography. Lattice parameters and space group information are as follows: a=15.039(5) Å, b=16.086(5) Å, c=17.078(6) Å, α=β=γ=90.00°, V=4131.5(24) ų, orthorhombic, Pbnb (Z=4). Diffraction data (MoK_α radiation, 2θ_max=50) is collected by Rigaku-AFC6 diffract meter. The structure was solved and refined by direct method and full-matrix least-squares procedures to R-value of 0.0645. The complex ET₃S₆ has been prepared through electrocrystallization ways. The conductivity of this salt at room temperature is 2.3 S cm⁻¹. It shows weak metallic behavior above 240 K. Below this temperature, it becomes a semiconductor. The XPS spectra indicated the presence of three different kinds of S atoms in the salt. The ESR line width is found to be 44.478 G at room temperature.     

Electron spin resonance and resistivity studies of charge-ordered Bi(1−x)SrxMnO3

In this paper we report the temperature dependent electron spin resonance (ESR) and electrical resistivity studies of Bi_(1−x)Sr_xMnO₃ (x = 0.3, 0.4, 0.45, and 0.5). The double integrated (DI) intensity of the ESR signal vs temperature (T) and ln DI vs 1000/T plots have been used to get information about magnetic interactions in the sample. A sharp change in the slope of this plot indicates the onset of long range antiferromagnetic (AFM) order. The Neel temperatures (T_N) of the sample increases with increase in Sr content. In the temperature range T > T_N, domains of ferromagnetic (FM) and AFM correlations co-exist. The contributions of AFM correlations increase with the increase in Sr content. The observation of weak ESR signal in the temperature range below T_N indicates the freezing of FM microdomains/inhomogeneities in the AFM long range ordered state. The resistivity (ρ) data are analyzed in view of polaron model and variable range hopping (VRH) models given by Mott and Efros-Shklovskii (ES). As per Motts VRH model, ln ρ varies linearly with T^−1/4 with a change in slope at T_N. The estimated values of hopping distance and localization length are reasonable. It is also found that ln ρ varies linearly with T^−1/2 in accordance with the ES VRH model. However, the estimated values of the model parameters are unrealistic. The ESR and resistivity data are also analyzed in view of phase separation (PS) model.     

Electrical properties and spectroscopic studies of HClO4-doped poly(p-diethynylbenzene)

Electrical properties of poly(p-diethynylbenzene) (PDEB), chemically doped with perchloric acid (HClO₄), were investigated as a function of dopant concentration and relative humidity. The doped polymer was characterized by X-ray photoelectron spectroscopy (XPS), UV—Vis, IR, electron spin resonance (ESR) and scanning electron microscopy (SEM). A possible structure of the HClO₄-doped PDEB was proposed on the basis of spectroscopic studies. In the presence of humidity, HClO₄-doped PDEB follows the proton conduction mechanism.     

Characterization of CsC24 prepared from carbon materials with different graphitization degree

Samples of CsC₂₄ were prepared from carbon materials (derived from petroleum cokes) with different graphitization degree and were characterized by X-ray diffraction (XRD), electron spin resonance (ESR) and Raman spectroscopy. The XRD measurement showed that all the host carbon samples heat treated from 1300 to 2800°C being contacted with CsC₈ planar specimen allowed the intercalation of Cs. Correspondingly, the resulting CsC₂₄ samples were able to sorb hydrogen molecules, which is a clear evidence of the formation of the nanospace. The broad ESR signal due to conduction electron observed for carbon samples (heat-treatment temperature, HTT above 2200°C) disappeared after intercalation of Cs because of the spin–orbit interaction caused by the intercalated Cs. The Raman G-band of the CsC₂₄ samples (HTT above 1750°C) shifted from 1584 cm⁻¹ of the host carbon to higher wave number (∼1602 cm⁻¹) in agreement with the reported data for CsC₂₄. In addition, it was confirmed that the D-band signal disappeared for the CsC₂₄ samples (HTT above 2000°C).     

A high-frequency light-induced electron spin resonance study of conjugated polymer/fullerene composites

Light-induced electron spin resonance (LESR) measurements are reported in composites of MDMO-PPV and PCBM, a soluble derivative of C₆₀. After illumination of the sample, two radicals are formed. One is the remaining positive polaron P⁺ on the polymer backbone and the other the C₆₁⁻ radical anion. Using high-frequency (95 GHz) LESR, it was possible to separate these two contributions to the spectrum on the basis of their g-parameters, and to resolve the g-anisotropy of the radicals. The P⁺-polaron possesses axial symmetry whereas for C₆₁⁻ a lower, rhombic symmetry was observed.     

The spectroelectrochemical, magnetic, and structural characterization of reduced hexaazatriphenylenehexacarbonitrile, HAT(CN)6

The cyanoazacarbon, hexaazatriphenylenehexacarbonitrile, or HAT(CN)₆, is readily reduced and the spectroelectrochemical properties associated with the multiple reductions are described. The singly reduced radical monoanion forms stable salts and we report the crystal structure and magnetic properties for the tetrabutylammonium salt. [Tetra-n-butylammonium] [HAT(CN)₆] behaves ferromagnetically below 3 K and follows Curie Law behavior at higher temperatures after correction for Pauli-type susceptibility. The room temperature conductivity of the powdered salt is 3.8 × 10⁻⁸ S/cm. The crystal structure shows closely bound pairs of radical anions with slip-stacking of these pairs to form a staircase, features that elucidate the observed properties. Thin films of HAT(CN)₆ were found to support negative charge transport by electron time-of-flight measurements, yielding electron mobilities of 10⁻⁴ cm²/Vs at room temperature.     

Electrically conducting complexes of poly(3,6-N-methylcarbazolyl methylene

Poly(3,6-N-methylcarbazoly methylene) prepared by acid-catalysed condensation polymerization of N-methylcarbazole with formaldehyde exhibits p-type semiconducting properties when doped with electron acceptors such as iodine, bromine, nitrosyl tetrafluoroborate and nitrosyl hexafluoroantimonate. The polymer samples have T_g in the range 100 – 148 °C, chain length in the range 13 – 25, and a molecular weight distribution of 1.17 –1.51. The polymer complexes with dopant anion (I₃^?, Br₃^?, BF₄^?) to polymer repeating unit ratio of 0.67 – 0.95 have a d.c. conductivity of 10^?3 to 10^?1 ohm^?1 cm^?1 and a positive thermoelectric voltage at 23 °C.An important new feature of the methylene-bridged polycarbazole conducting polymers is doping-induced polymer backbone conjugation of the form CH₂→=CH. Evidence for this oxidation mechanism, converting methylene linkage to methine linkages, includes elemental analysis, infrared spectra, proton NMR and electron spin resonance results.