Ales Omerzu

A structural and infrared study of the charge states of tetrakis (dimethylamino)ethylene (TDAE) in TDAE-C60 and (TDAE)(Cl)2

The molecular structure and vibrational spectra of the different oxidation states of tetrakis(dimethylamino)ethylene (TDAE) are calculated using structural optimization and normal-mode calculations. The predictions are compared with x-ray structural data and infrared spectra of TDAE, TDAE-C60 and (TDAE)(Cl)2. The C–N stretch vibration frequency and axial torsional angle are shown to be sensitive indicators of the charge state of the molecule. TDAE+, which occurs in TDAE-C60 is found to have a near-degenerate configurational ground state.

Hole Interactions with Molecular Vibrations on DNA

We report on a study of the interactions between holes and molecular vibrations on dry DNA using photoinduced infrared absorption spectroscopy. Laser photoexcited holes are found to have a room-temperature lifetime in excess of tau > 1 ms, clearly indicating the presence of localization. However, from a quantitative model analysis of the frequency shifts of vibrational modes caused by the holes, we find the hole-vibrational coupling constant to be relatively small, lambda approximately 0.2. This interaction leads to a change in the conformational energy of DeltaE0 approximately 0.015 eV, which is too small to cause self-trapping at room temperature. We conclude that, at least in the dry (A) form, DNA is best understood in terms of a double chain of coupled quantum dots arising from the pseudorandom chain sequence of base pairs, in which Anderson localization prevents the formation of a metallic state.

μSR studies of organic and molecular magnets

Muon-spin rotation and relaxation (μSR) experiments have been performed on a variety of novel organic and molecular magnetic systems. In these experiments, implanted muons are used to study the magnitude, distribution and dynamics of the local field at the muon site. Calculations of the spatial dependence of the dipole-field inside the unit cell are used to interpret the data and determine the muon site in certain cases. We describe and review muon experiments on nitronyl nitroxide organic ferromagnets and antiferromagnets. We discuss a muon study of the spin crossover phenomenon which has been studied in Fe(PM-PEA)2(NCS)2, and which shows Gaussian and root-exponential muon relaxation in the high-spin and low-spin phases, respectively. The effects of high temperature annealing on TDAE-C60 have also been studied with μSR. Experiments on a disc-shaped molecular complex containing Fe19 (with spin 31/2) reveal the effects of fluctuations of magnetization and allow an estimate of the fluctuation rate. These experiments demonstrate the wide range of problems which can be tackled using the μSR technique.

Ionization energy and energy gap structure of MoSI molecular wires: Kelvin probe, ultraviolet photoelectron spectroscopy, and cyclic voltammetry measurements.

The work function W of Mo(6)S(3)I(6) molecular nanowires is determined by Kelvin probe (KP) measurements, UV photoelectron spectroscopy (UPS), and cyclic voltammetry (CV). The values obtained by all three methods agree well, giving W = 4.8 ± 0.1 eV. CV measurements also give a gap between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of E(g) = 1.2 ± 0.1 eV, in agreement with recent optical measurements, but in disagreement with theoretical calculations, which predict the material to be a metal. The electronic structure of Mo(6)S(3)I(6) suggests use of the material in applications such as bulk heterostructure photovoltaics and transparent electrodes and for molecular electronics devices.

Low temperature structural analysis of a TDAE·C60 crystal

X-Ray single crystal diffraction studies of TDAE·C60 were performed at low temperature and its crystal structure could be solved and analysed on the basis of diffraction data obtained at 7–11 K.

Mechanism of ferromagnetism in TDAE-C60 : ESR study under pressure

The low-frequency electron spin resonance (ESR) under hydrostatic pressure has been studied in the soft-ferromagnet TDAE-C 60 , where TDAE is tetrakis-dimethylamino-ethylene. A demagnetising field causes ESR line shift proportional to the spontaneous magnetisation. The pressure dependence of T C deduced from the ESR line shift is found to follow a relationship of T C (P) T C (0)[1 - (P/P C ) 2 ], where T C 16 K and P C 9 kbar. The antiferro-orbital-ordering model of Jahn-Teller distorted C 60 - is investigated as a possible mechanism for the ferromagnetism. With the realistic magnitudes of parameters a prediction of this model is in good quantitative agreement with the data. The pressure above 10 kbar promotes an irreversible phase change; a reduced lattice constant along c-axis, a reduced ESR line width, an increased number of spins and an increased g-shift. From the c-lattice constant of 9.17±0.07 A, we concluded that a new β-phase induced by pressure is a [2+2] cycloadditive 1D polymer phase, where the spin revives on TDAE + , missing in the ferromagnetic α-phase.

Ferromagnetic and merohedral ordering in powder and crystalline tetrakis (dimethylamino) ethylene (TDAE)-C60

Abstract A.c. susceptometry and electron spin resonance (ESR) measurements on crystals and powders show direct evidence of coupling between spin and molecular merohedral degrees of freedom in tetrakis(dimethylamino)ethylene (TDAE)-C 60 (16 T c 60 molecules at high temperature influences the low-temperature magnetic transition temperature. The merohedral disorder gives rise to a distribution of π-electron exchange interactions between spins on neighbouring C 60 molecules, suggesting a microscopic origin for the observed spin-glass-like behaviour in the magnetic state. ESR measurements on single crystals of TDAE-C 60 show a frequency shift which is maximum when the absolute value of applied field ¦ H ext ¦ is along the crystallographic a axis. On the basis of the present data we suggest the material to be a ferromagnet, with the magnetic axis along a and with the dipole-dipole interactions being responsible for the large ESR line shifts and line broadening in the magnetic phase. New IR reflectivity data suggest a low-frequency excitation unobserved to date, whose origin could be a Drude peak, indicating a metallic ground state or a possible low-frequency charge density wave (CDW) excitation.

A μ+ SR study of ferromagnetic exchange in the fullerence charge transfer salt TDAE-C60

Zero field muon spin relaxation measurements have been made on the charge transfer salt (tetrakisdimethylaminoethylene)C 60 [TDAE-C 60 ]. The effects of high temperature annealing on the dynamics and local magnetic properties of the compound are investigated. With the sample in the ferromagnetic α-phase, a signal characteristic of magnetic order is observed below 16 K. The initial asymmetry shows a discontinuity around 200 K, while the relaxation rate shows temperature activated behaviour above around 120 K. Annealing at 383 K is found to destroy the ferromagnetic signal and results in a relaxation rate peaked at 10 K, which may be attributable to the freezing out of a dynamic Jahn-Teller distortion.

C60 molecular configurations leading to ferromagnetic exchange interactions in TDAE-C60

The charge-transfer salt tetrakis(demethylamino)ethylene-fullerene [C60] or TDAE-C60 is a rare exception among pure organic crystalline systems because it shows a transition to a ferromagnetic (FM) state with fully saturated s=1/2 molecular spins at a respectable Tc=16 K. In spite of extensive experimental and theoretical work in the last ten years, the origin of the ferromagnetism in TDAE-C60 has remained mysterious. To resolve this problem we have performed a comparative structural study of two different magnetic forms of TDAE-C60 crystals, one being magnetic and the other nonmagnetic, at low temperatures, fully correlating the structural properties - and particularly the inter-molecular orientations-with the magnetic properties. We have identified the relative orientations of C60 molecules along the c-axis as the primary variable controlling the ferromagnetic order parameter and have shown that both FM and low-temperature spin-glass-like ordering are possible in this material, depending on the orientat...

Direct observation of spontaneous magnetization in TDAE-C60

Abstract We have succeeded in preparation of high-quality single crystals of TDAE-C 60 (TDAE; tetrakis(dimethylamino) ethyiene) with saturation magnetization as high as 1.0μ B /C 60 and obtained the first unambiguous experimental evidence for the presence of ferromagnetic order, through a direct observation of spontaneous magnetization.