Solid state physicochemical properties and applications of organic and metallo-organic fullerene derivatives

We review the fundamental properties and main applications of organic derivatives and complexes of fullerenes in the solid-state form. We address in particular the structural properties, in terms of crystal structure, polymorphism, orientational transitions and morphology, and the electronic structure and derived properties, such as chemical activity, electrical conduction mechanisms, optical properties, heat conduction and magnetism. The last two sections of the review focus on the solid-state optoelectronic and electrochemical applications of fullerene derivatives, which range from photovoltaic cells to field-effect transistors and photodetectors on one hand, to electron-beam resists, electrolytes and energy storage on the other.Peer ReviewedPreprin

Characterization of high-quality MgB2(0001) epitaxial films on Mg(0001)

High-grade MgB2(0001) films were grown on Mg(0001) by means of ultra-high-vacuum molecular beam epitaxy. Low energy electron diffraction and x-ray diffraction data indicate that thick films are formed by epitaxially oriented grains with MgB2 bulk structure. The quality of the films allowed angle-resolved photoemission and polarization dependent x-ray absorption measurements. For the first time, we report the band mapping along the Gamma-A direction and the estimation of the electron-phonon coupling constant l ~ 0.55 for the surface state electrons.Comment: 15 text pages, 6 figures Submitted for publicatio

Reversible phase transformation and doubly-charged anions at the surface of simple cubic RbC60

The simple cubic phase of a RbC60 thin film has been studied using photoelectron spectroscopy. The simple cubic-to-dimer transition is found to be reversible at the film surface. A sharp Fermi edge is observed and a lower limit of 0.5 eV is found for the surface Hubbard U, pointing to a strongly-correlated metallic character of thin-film simple cubic RbC60. A molecular charge state is identified in the valence band and core level photoemission spectra which arises from C602- anions and contributes to the spectral intensity at the Fermi level.Comment: 13 pages, 3 figure

Glass transition, crystallization kinetics, and inter-conformer relaxation dynamics of amorphous mitotane and related compounds

We employ differential scanning calorimetry, broadband dielectric spectroscopy and optical microscopy to investigate the glass transition, molecular relaxation dynamics, and isothermal recrystallization kinetics of amorphous mitotane, the only drug approved for the pharmacological treatment of adrenocortical carcinoma. Amorphous mitotane displays a glass transition at Tg = 243 ± 1 K, characterized by relatively low fragility index of 68 ± 2. Besides the structural and Johari-Goldstein relaxations, amorphous mitotane displays an intramolecular relaxation with activation energy of 25 ± 1 kJ mol-1. The same relaxation process, with virtually the same activation energy and relaxation times, is observed in the closely-related o,p’-dichlorobenzophenone compound, which allows identifying it as the rotation of the chlorobenzene ring with the chlorine closest to the central carbon. Such conformational relaxation is active at human body temperature, and may thus be potentially relevant for the mechanism of action of the drug. Our study shows that the comparative study of the relaxation map of related molecular species is a powerful tool to identify and classify secondary relaxation processes. The amorphous drug is found to be unstable against recrystallization at as well as slightly below room temperature, and to display-two-dimensional growth with only sporadic nucleation, characterized by an Avrami kinetic exponent of 2.05 ± 0.05. The kinetic stability of the amorphous form of mitotane, observed at room temperature in micellar formulations, is therefore limited to the nanoconfined sample and is not observed in the bulk compound.Peer ReviewedPostprint (published version

Orientational relaxations in solid (1,1,2,2)tetrachloroethane

We employ dielectricspectroscopy and molecular dynamic simulations to investigate the dipolar dynamics in the orientationally disordered solid phase of (1,1,2,2)tetrachloroethane. Three distinct orientational dynamics are observed as separate dielectric loss features, all characterized by a simply activated temperature dependence. The slower process, associated to a glassytransition at 156 ± 1 K, corresponds to a cooperative motion by which each molecule rotates by 180° around the molecular symmetry axis through an intermediate state in which the symmetry axis is oriented roughly orthogonally to the initial and final states. Of the other two dipolar relaxations, the intermediate one is the Johari-Goldstein precursor relaxation of the cooperative dynamics, while the fastest process corresponds to an orientational fluctuation of single molecules into a higher-energy orientation. The Kirkwood correlation factor of the cooperative relaxation is of the order of one tenth, indicating that the molecular dipoles maintain on average a strong antiparallel alignment during their collective motion. These findings show that the combination of dielectricspectroscopy and molecular simulations allows studying in great detail the orientational dynamics in molecular solids.Peer ReviewedPostprint (author's final draft

Inter-enantiomer conversion dynamics and Johari–Goldstein relaxation of benzophenones

We employ temperature- and pressure-dependent dielectric spectroscopy, as well as differential scanning calorimetry, to characterize benzophenone and the singly-substituted ortho-bromobenzophenone derivative in the liquid and glass states, and analyze the results in terms of the molecular conformations reported for these molecules. Despite the significantly higher mass of the brominated derivative, its dynamic and calorimetric glass transition temperatures are only ten degrees higher than those of benzophenone. The kinetic fragility index of the halogenated molecule is lower than that of the parent compound, and is found to decrease with increasing pressure. By a detailed analysis of the dielectric loss spectra, we provide evidence for the existence of a Johari–Goldstein (JG) relaxation in both compounds, thus settling the controversy concerning the possible lack of a JG process in benzophenone and confirming the universality of this dielectric loss feature in molecular glass-formers. Both compounds also display an intramolecular relaxation, whose characteristic timescale appears to be correlated with that of the cooperative structural relaxation associated with the glass transition. The limited molecular flexibility of ortho-bromobenzophenone allows identifying the intramolecular relaxation as the inter-enantiomeric conversion between two isoenergetic conformers of opposite chirality, which only differ in the sign of the angle between the brominated aryl ring and the coplanar phenyl-ketone subunit. The observation by dielectric spectroscopy of a similar relaxation also in liquid benzophenone indicates that the inter-enantiomer conversion between the two isoenergetic helicoidal ground-state conformers of opposite chirality occurs via a transition state characterized by a coplanar phenyl-ketone moiety.Peer ReviewedPostprint (published version

Tuning molecular dynamics by hydration and confinement: Antiplasticizing effect of water in hydrated prilocaine nanoclusters

In glass-forming substances, the addition of water tends to produce the effect of lowering the glass transition temperature, Tg. In a previous work by some of us (Ruiz et al., Sci. Rep., 2017, 7, 7470) we reported on a rare anti-plasticizing effect of water on the molecular dynamics of a simple molecular system, the pharmaceutically active prilocaine molecule, for which the addition of water leads to an increase of Tg. In the present work, we study pure and hydrated prilocaine confined in 0.5 nm and 1 nm pore size molecular sieves, and carry out a comparison with the bulk compounds in order to gain a better understanding of the microscopic mechanisms that result in this rare effect. We find that the Tg of the drug under nanometric confinement can be lower than the bulk value by as much as 17 K. Through the concurrent use of differential scanning calorimetry and broadband dielectric spectroscopy we are able to observe the antiplasticizing effect of water in prilocaine also under nanometric confinement, finding an increase of Tg of up to almost 6 K upon hydration. The extension of our analysis to nanoconfined systems provides a plausible explanation for the very uncommon antiplasticizing effect, based on the formation of water-prilocaine molecular complexes. Moreover, this study deepens the understanding of the behavior of drugs under confinement, which is of relevance not only from a fundamental point of view, but also for practical applications such as drug delivery.Peer ReviewedPostprint (author's final draft

Water-triggered conduction and polarization effects in a hygroscopic fullerene salt

Impedance spectroscopy is employed to probe the frequency-dependent conductivity and dielectric response of the crystalline C60O24Na24 fulleride, both in its pure form obtained by heating to 473 K and in its bulk-hydrate form stable only below 390 K, of chemical formula C60O24Na24 ·16 H2O. A dielectric loss feature is visible in both the pure material and the hydrate, displaying different strength and activated behavior in different temperature ranges.Peer ReviewedPostprint (published version

Measurement of the dispersion of air and of refractive index anomalies by wavelength- dependent nonlinear interferometry

We carry out wavelength-dependent second harmonic interference experiments using thin films of an organic dye as nonlinear optical sources. While the measured difference of refractive index between the fundamental and second harmonic wavelengths follows the theoretical expectation for air in a wide spectral region, anomalous dispersion is observed when the second harmonic light lies in the absorption band of the dye. The sensitivity of the technique to small refractive index variations may prove useful for sensing applications as well as for testing models of light dispersion in weakly dispersing and absorbing media.Preprin