Lidia Piekara-Sady

A Single-Component Molecular Metal Based on a Thiazole Dithiolate Gold Complex

A single component molecular conductor has been isolated from electrocrystallization of the monoanionic gold bis(dithiolene) complex based on the N-ethyl-1,3-thiazoline-2-thione-4,5-dithiolate (Et-thiazdt) ligand. The crystal structure of the system exhibits layers built from parallel uniform one-dimensional stacks of the planar molecule. At room temperature and ambient pressure the system is semiconducting (0.33 S x cm(-1)) with a small activation energy. However, the single crystal conductivity is strongly pressure dependent reaching 1000 S x cm(-1) at 21 kbar. At 13 kbar there is a crossover between semiconducting and metallic regimes. Thus, the present system is the first well characterized single-component molecular metal without TTF dithiolate ligands. First-principles DFT calculations show that the ground state is antiferromagnetic with a very small band gap. A simulation of the effect of pressure on the electronic structure provides a rationale for the observed variations of the conductivity and gives insight on how to further stabilize the metallic state of the system.

Probing magnetic interactions in columnar phases of a paramagnetic gold dithiolene complex

A novel radical gold dithiolene complex exhibits a hexagonal columnar mesophase, as confirmed by optical microscopy, DSC analysis and X-ray diffraction. The extent of delocalization of the spin density in such a complex was analyzed by EPR. Temperature dependent magnetization measurements reveal that the global magnetic moment is remarkably affected at the liquid-crystalline phase transition with a marked hysteresis signature, rare behavior among the few described paramagnetic discotic phases. In addition, these molecules were found to strongly aggregate in solution into one-dimensional fibers with a mean diameter of 60 nm extending over micrometres, leading to the formation of gel-like structures. These fibers are stable and can be isolated on surfaces. The gelation of the system can also be detected by temperature-dependent magnetic measurements.

EPR of Graphite and Fullerenes

Abstract Analysis of the EPR results for graphite and fullerenes has led to the development of a model for the g-factor in follerene based on the analogy between graphite and fullerene. Pressure dependence of g-factor in C60 powder confirms the validity of this model. 13C hyperfine splitting ofa0.36 mT in pristine fullerene is also reported. The g-factors of C60 −1 and C60 −3 in a solution and in a different K°C60 fullerides are presented. C60 +n states should be described by the spectroscopic splitting factor larger than that of a free spin.

Molecular motions in a novel ferroelectric crystal (CH3NH3)5Bi2Br11 studied by NMR

Abstract The temperature dependence of the 1 H relaxation times T 1 , T 1 ρ and T 1 D were studied in the ferroelectric (CH 3 NH 3 ) 5 Bi 2 Br 11 . Three kinds of motions of the methylammonium cation, i.e. (1) rotations of the CH 3 and NH 3 groups about C 3 axis, (2) reorientation of the whole cation about C 3 axis forming an angle α = 13° with the C 3 axis of NH 3 and CH 3 groups, (3) 180° flip motion of the methylammonium cation, have been observed separately. The small activation energies of these motions imply that the cations experience a weak crystal field.

C60+ defect centers: effect of rubidium intercalation into C60 studied by EPR

Abstract The vanishing of C 60 + EPR centers during the intercalation of polycrystalline C 60 with rubidium is described as a two-stage process with inter- and intragrain penetration of rubidium into the C 60 sample. This process leads to quenching of C 60 + hole states through a recombination process. Two aspects of the results are discussed: 1. EPR assignment of the spectrum usually observed in pristine polycrystalline C 60 samples. 2. Similarity of the kinetics of alkali inter- and intragrain penetration into powdered C 60 to the kinetics of oxygen penetration through C 60 film, accompanied by the generation of C 60 + centers.

EPR of fullerene ions and superconductivity in K-fullerides at low doping levels

Abstract Early stages of the potassium-doping process of C 60 were investigated with the EPR and MMMA techniques. Weak superconductivity of the (K + C 60 ) mixture was found without heating or annealing the mixture. Upon heating two well-separated C 60 1− and C 60 3− EPR signals were found. Two superconducting phases with T c (1) = (21 ± 0.5) K and T c (2) = (18.5 ± 0.5) K were also distinctly separated by increase of the external magnetic field. To explain the appearance of the superconducting phase with T c (1) which is 2.5 K larger compared to the standard T c (2) of K 3 C 60 , a model has been developed.

EPR of a Fullerene-Molecule-Derived Paramagnetic Center as a Mesoscopic Conducting Object

Discussion of theg-factor value of fullerene is based on the model of itinerant electrons restricted to the surface of the fullerene molecule C60. The Ag shift, i.e., the difference between the experimentalg-factor and theg-factor of free electron Δg = g − 2.0023 for C60−1 is negative as for a very small metallic conducting particle.g-factor value is proportional to the interaction between itinerant electrons in the conduction band, thus the Δg is negative for C60−1 and C60−3 having less than half filled conduction band, while Δg is positive for C60+ where the conduction band is almost filled.

Do structural defects affect semiconducting properties of fullerene thin films

Abstract We report on the time development of EPR signals (g=2.0026±0.0002) from C60 films with various crystalline structure under air/light exposure. The time development consists of two clearly distinguished regions of fast and slow growth. Improvement of the film structure, and in particular the increase in grain size, leads to a deceleration of the “fast” growth. The results are explained assuming that EPR signal growth is controlled by oxygen diffusion, along grain boundaries and into grains, during the “fast ” and “slow” periods, respectively. Fast decrease of the EPR signal as a result of in situ pumping strongly supports this model and indicates a correlation between crystalline structure and oxygen diffusion in C60 films. Such correlation is considered as one of the possible mechanisms which govern the semiconducting properties of the material.

Dithiolene complexes as metallo-ligands: a crown-ether approach

A nickel dithiolene complex substituted with crown ether cyclic moieties incorporating four O atoms, abbreviated as [Ni(S2O4)2]1−,0, is isolated in its radical anionic (as Na+ salt) and neutral forms. The Na+ cation is six-coordinated with short Na⋯O distances (2.46–2.52 A), involving two crown ether moieties of two different complexes. The oxidized neutral complex [Ni(S2O4)2]0 was also isolated and structurally characterized. In the absence of alkaline cations during the synthesis, a mixed salt associating [Ni(S2O4)2]1− with Ni2+ was isolated, and formulated as (Ni,H2O)[Ni(S2O4)2]2, with the Ni2+ cation weakly bonded to two crown ether moieties. The salt exhibits an unprecedented solid state association with extremely short S⋯S intermolecular contacts [3.332(2) A], leading to a pairing of the radical [Ni(S2O4)2]− into antiferromagnetic uniform spin chains.

Evidence of hydrogen bonds in [Ni(NH3)6](NO3)2

Abstract The thermal expansion of polycrystalline [Ni(NH3)6](NO3)2 has been measured in the temperature range from 77 to 300 K. Beside volume discontinuities with a phase transition I⇔II at 243 K, a considerable increase in the length of the sample was observed during the transition from phase II to III (1.2%). Vanishing of the phase III on heating takes place gradually and with considerable temperature hysteresis of about 100 K. The anomaly of the phase transition II⇔III is explained based on the assumption of a hydrogen bond of the type N-H…O existing only in the low-temperature phase III. Hydrogen bond energy of about 17 kJ/mol is evaluated. The pressure coefficient dTc2/dp = −1.4 K/MPa results from the Clapeyron-Clausius equation. The phase diagram of [Ni(NH3)6](NO)6 is completed and is compared with those previously obtained for nickel hexammines with other complex anions (ClO-4, BF-4). Effect of [Ni(NH3)6](NO3)2 ageing on the phase transitions is also reported.