Mario Chiesa

EPR Characterization and Reactivity of Surface-Localized Inorganic Radicals and Radical Ions

A review. The role of surface inorg. radicals and radical ions formed at the interface between a solid and a gas phase was discussed. The formation of such species is relevant to field such as electrochem., heterogeneous catalysts, photocatalysis and corrosion. The characterization of these radicals by ESR are given. [on SciFinder(R)]

Charge trapping in TiO2 polymorphs as seen by Electron Paramagnetic Resonance spectroscopy

Electron Paramagnetic Resonance (EPR) techniques have been employed to investigate charge carrier trapping in the two main TiO2 polymorphs, anatase and rutile, with particular attention to the features of electron trapping sites (formally Ti(3+) ions). The classic CW-EPR technique in this case provides signals based on the g tensor only. Nevertheless a systematic analysis of the signals obtained in the various cases (anatase and rutile, surface and bulk centers, regular and defective sites) has been performed providing useful guidelines on a field affected by some confusion. The problem of the localization of the electron spin density has been tackled by means of Pulse-EPR hyperfine techniques on samples appositely enriched with (17)O. This approach has led to evidence of a substantial difference, in terms of wavefunction localization between anatase (electrons trapped in regular lattice sites exhibiting delocalized electron density) and rutile (interstitial sites showing localized electron density).

Room-Temperature Quantum Coherence and Rabi Oscillations in Vanadyl Phthalocyanine: Toward Multifunctional Molecular Spin Qubits

Here we report the investigation of the magnetic relaxation and the quantum coherence of vanadyl phthalocyanine, VOPc, a multifunctional and easy-processable potential molecular spin qubit. VOPc in its pure form (1) and its crystalline dispersions in the isostructural diamagnetic host TiOPc in different stoichiometric ratios, namely VOPc:TiOPc 1:10 (2) and 1:1000 (3), were investigated via a multitechnique approach based on the combination of alternate current (AC) susceptometry, continuous wave, and pulsed electron paramagnetic resonance (EPR) spectroscopy. AC susceptibility measurements revealed a linear increase of the relaxation rate with temperature up to 20 K, as expected for a direct mechanism, but τ remains slow over a very wide range of applied static field values (up to ∼5 T). Pulsed EPR spectroscopy experiments on 3 revealed quantum coherence up to room temperature with T(m) ∼1 μs at 300 K, representing the highest value obtained to date for molecular electronic spin qubits. Rabi oscillations are observed in this nuclear spin-active environment ((1)H and (14)N nuclei) at room temperature also for 2, indicating an outstanding robustness of the quantum coherence in this molecular semiconductor exploitable in spintronic devices.

Colour centres at the surface of alkali-earth oxides. A new hypothesis on the location of surface electron traps

Abstract Irradiation of highly dehydrated MgO by UV light in the presence of surface adsorbed hydrogen leads to the formation of particular types of surface colour centres indicated with F + S (H) (one-electron, paramagnetic) and F S (H) (two electrons, diamagnetic). F S centres are the surface counterparts of the well-known F colour centres formed in the bulk of ionic solids by high-energy irradiation or metal addition. In the particular case of F + S (H), the unpaired electron is in magnetic interaction with a nearby proton belonging to a hydroxyl group deriving from H 2 heterolytic dissociative chemisorption (H 2 →H + +H − ) and consequent H + stabilization on a surface oxide ion. The joint use of EPR, FT-IR and DR-UV–vis spectroscopies has allowed clarification of the mechanism of formation of these centres, which is based on the ionization of adsorbed hydryde groups by UV light and stabilization of the ionized electron into suitable positively charged surface electron traps. A fraction of these traps coincides with the site capable of stabilizing the hydride ion (in the form of bridged Mg 3 H), which is built up by an array of three Mg 2+ ions reproducing a (111) facelet of the oxide. The same site can be also seen as a O 2− 3c vacancy (3-coordinated surface anionic vacancy). Ab-initio quantum chemical calculations confirm the proposed assignment, which goes beyond the original model of 5-coordinated surface anion vacancies at the flat (100) MgO plane, which is thus left in favour of a new model that describes the surface electron traps as being localized in less coordinated regions of the surface.

Continuous wave electron paramagnetic resonance investigation of the hyperfine structure of 17O2− adsorbed on the MgO surface

The adsorption of molecular oxygen (enriched with 17O) onto high surface area MgO has been studied by electron paramagnetic resonance (EPR) spectroscopy. The oxide surface was pretreated in such a way so that surface trapped electron FS+(H) centers are produced. Subsequent dioxygen adsorption results in an electron transfer reaction from FS+(H) centers to O2, producing a surface stabilized superoxide (O2−) anion. The resulting EPR spectrum of the paramagnetic anion is complicated by the simultaneous presence of a high number of “normal” hyperfine lines along the principal axes and also by several off-axis extra features which have complicated previous interpretations of the Ayy and Azz components. By adopting a suitable adsorption procedure which suppresses the superoxide speciation, using a highly crystalline MgO material and controlling the isotopomer composition through appropriate 17O enrichments, the resolution of the EPR spectrum has been dramatically improved. Analysis of the 1H superhyperfine stru...

Q-band EPR investigations of neuromelanin in control and Parkinson's disease patients

New insights into the understanding of the changes induced in the iron domain of neuromelanin (NM) upon development of Parkinsons disease (PD) have been gained by electron paramagnetic spectroscopy (EPR). The results of this study are compared with a previously reported variable temperature analysis of X-band EPR spectra of a NM specimen obtained from control brain tissues. The availability of high sensitivity instruments operating in the Q-band (34.4 GHz) allows us to deal with the low amounts of NM available from PD brains. The organization of iron in NM is in the form of polynuclear superparamagnetic/antiferromagnetic aggregates, but the lack of one or more signals in the EPR spectra of NM from PD suggests that the development of the pathology causes NM to decrease its ability to bind iron. Furthermore, the detection of the Mn(II) signal in the Q-band spectra is exploited as an additional internal probe to assess minor structural differences in iron domains of PD and control NM specimens.

Quantum Coherence Times Enhancement in Vanadium(IV)-based Potential Molecular Qubits: the Key Role of the Vanadyl Moiety

In the search for long-lived quantum coherence in spin systems, vanadium(IV) complexes have shown record phase memory times among molecular systems. When nuclear spin-free ligands are employed, vanadium(IV) complexes can show at low temperature sufficiently long quantum coherence times, Tm, to perform quantum operations, but their use in real devices operating at room temperature is still hampered by the rapid decrease of T1 caused by the efficient spin-phonon coupling. In this work we have investigated the effect of different coordination environments on the magnetization dynamics and the quantum coherence of two vanadium(IV)-based potential molecular spin qubits in the solid state by introducing a unique structural difference, i.e., an oxovanadium(IV) in a square pyramidal versus a vanadium(IV) in an octahedral environment featuring the same coordinating ligand, namely, the 1,3-dithiole-2-thione-4,5-dithiolate. This investigation, performed by a combined approach of alternate current (ac) susceptibility measurements and continuous wave (CW) and pulsed electron paramagnetic resonance (EPR) spectroscopies revealed that the effectiveness of the vanadyl moiety in enhancing quantum coherence up to room temperature is related to a less effective mechanism of spin-lattice relaxation that can be quantitatively evaluated by the exponent n (ca. 3) of the temperature dependence of the relaxation rate. A more rapid collapse is observed for the non-oxo counterpart (n = 4) hampering the observation of quantum coherence at room temperature. Record coherence time at room temperature (1.04 μs) and Rabi oscillations are also observed for the vanadyl derivative in a very high concentrated material (5 ± 1%) as a result of the additional benefit provided by the use of a nuclear spin-free ligand.

Probing the Coordinative Unsaturation and Local Environment of Ti3+ Sites in an Activated High‐Yield Ziegler–Natta Catalyst

The typical activation of a fourth generation Ziegler-Natta catalyst TiCl4/MgCl2/phthalate with triethyl aluminum generates Ti(3+) centers that are investigated by multi-frequency continuous wave and pulse EPR methods. Two families of isolated, molecule-like Ti(3+) species have been identified. A comparison of the experimentally derived g tensors and (35,37)Cl hyperfine and nuclear-quadrupole tensors with DFT-computed values suggests that the dominant EPR-active Ti(3+)  species is located on MgCl2(110) surfaces (or equivalent MgCl2 terminations with tetra-coordinated Mg). O2 reactivity tests show that a fraction of these Ti sites is chemically accessible, an important result in view of the search for the true catalyst active site in olefin polymerization.

Probing the Local Environment of Ti3+ Ions in TiO2 (Rutile) by 17O HYSCORE

Reduced states in TiO(2) : (17)O hyperfine sublevel correlation spectroscopy was used to monitor the local environment of stable Ti(3+) ions generated in a (17)O-enriched polycrystalline TiO(2) (rutile) sample. A hyperfine interaction of about 8 MHz is found, which is analogous to that observed for molecular Ti(3+) aqua complex cations and suggests a localized nature of the unpaired electron wave function for these centers at 4 K.

Elucidating the Nature and Reactivity of Ti Ions Incorporated in the Framework of AlPO-5 Molecular Sieves. New Evidence from 31P HYSCORE Spectroscopy

The incorporation of Ti ions within the framework of aluminophosphate zeotype AlPO-5 and their chemical reactivity is studied by means of CW-EPR, HYSCORE, and UV-vis spectroscopies. Upon reduction, Ti(3+) ions are formed, which exhibit large (31)P hyperfine couplings, providing direct evidence for framework substitution of reducible Ti ions at Al sites.