Alessandro Lunghi

The role of anharmonic phonons in under-barrier spin relaxation of single molecule magnets

The use of single molecule magnets in mainstream electronics requires their magnetic moment to be stable over long times. One can achieve such a goal by designing compounds with spin-reversal barriers exceeding room temperature, namely with large uniaxial anisotropies. Such strategy, however, has been defeated by several recent experiments demonstrating under-barrier relaxation at high temperature, a behaviour today unexplained. Here we propose spin–phonon coupling to be responsible for such anomaly. With a combination of electronic structure theory and master equations we show that, in the presence of phonon dissipation, the relevant energy scale for the spin relaxation is given by the lower-lying phonon modes interacting with the local spins. These open a channel for spin reversal at energies lower than that set by the magnetic anisotropy, producing fast under-barrier spin relaxation. Our findings rationalize a significant body of experimental work and suggest a possible strategy for engineering room temperature single molecule magnets.

Giant spin–phonon bottleneck effects in evaporable vanadyl-based molecules with long spin coherence

Vanadium(iv) complexes have recently shown record quantum spin coherence times that in several circumstances are limited by spin-lattice relaxation. The role of the environment and vibronic properties in the low temperature dynamics is here investigated by a comparative study of the magnetization dynamics as a function of crystallite size and the steric hindrance of the β-diketonate ligands in VO(acac)2 (1), VO(dpm)2 (2) and VO(dbm)2 (3) evaporable complexes (acac- = acetylacetonate, dpm- = dipivaloylmethanate, and dbm- = dibenzoylmethanate). A pronounced crystallite size dependence of the relaxation time is observed at unusually high temperatures (up to 40 K), which is associated with a giant spin-phonon bottleneck effect. We model this behaviour by an ad hoc force field approach derived from density functional theory calculations, which evidences a correlation of the intensity of the phenomenon with ligand dimensions and the unit cell size.

Spin Dynamics and Low Energy Vibrations: Insights from Vanadyl-Based Potential Molecular Qubits

Here we report the investigation of the magnetization dynamics of a vanadyl complex with diethyldithiocarbamate (Et2dtc-) ligands, namely [VO(Et2dtc)2] (1), in both solid-state and frozen solution. This showed an anomalous and unprecedentedly observed field dependence of the relaxation time, which was modeled with three contributions to the relaxation mechanism. The temperature dependence of the weight of the two processes dominating at low fields was found to well correlate with the low energy vibrations as determined by THz spectroscopy. This detailed experimental comparative study represents a fundamental step to understand the spin dynamics of potential molecular quantum bits, and enriches the guidelines to design molecule-based systems with enhanced quantum coherence.

cis-Pt I2(NH3)2: a reappraisal

The investigation of cis-PtI2(NH3)2, the diiodido analogue of cisplatin (cisPtI2 hereafter), has been unjustly overlooked so far mainly because of old claims of pharmacological inactivity. Some recent - but still fragmentary - findings prompted us to reconsider more systematically the chemical and biological profile of cisPtI2 in comparison with cisplatin. Its solution behaviour, interactions with DNA and cytotoxic properties versus selected cancer cell lines were thus extensively analysed through a variety of biophysical and computational methods. Notably, we found that cisPtI2 is highly cytotoxic in vitro toward a few solid tumour cell lines and that its DNA platination pattern closely reproduces that of cisplatin; cisPtI2 is also shown to completely overcome resistance to cisplatin in a platinum resistant cancer cell line. The differences in the biological actions of these two Pt complexes are most likely related to slight but meaningful differences in their solution behaviour and reactivity. Overall, a very encouraging and unexpected pharmacological profile emerges for cisPtI2 with relevant implications both in terms of mechanistic knowledge and of prospective clinical application. An ab initio DFT study is also included to support the interpretation of the solution behaviour of cisPtI2 under physiological and slightly acidic pH conditions.

DFT magnetic characterization of a Fe4 SMMs series: from isotropic exchange interactions to multi-spin zero field splitting

Single molecule magnets represent one of the recent hottest research topics both at the experimental and theoretical levels. However, the tailoring of their magnetic properties is still far from being fully understood and a lot of work has to be done in order to rationalize it. In this framework, we want to present here a comprehensive magnetic characterization performed at the DFT level for a selected series of star shaped Fe4 SMMs. In this work we have exploited the already established procedure to compute single ion anisotropy tensors to extend it in order to include the determination of exchange anisotropy tensors, both spin–spin and spin–orbit contributions, valid also for ferrimagnetic systems. In this framework, it has been possible to perform a detailed analysis of the magneto-structural correlations that govern the Fe4 familys magnetic properties. Indeed, the analysis in terms of a multi-spin Hamiltonian of the anisotropy parameters led to the conclusion that the differences in the anisotropy parameters in the Fe4 SMMs series are to be mainly ascribed to the non-collinearity and the magnitude of single ion anisotropy tensors of the peripheral ions. The results obtained at PBE and PBE0 level have also been discussed.

The disclosure of mesoscale behaviour of a 3d-SMM monolayer on Au(111) through a multilevel approach

Here we present a computational study of a full- and a half-monolayer of a Fe4 single molecule magnet ([Fe4(L)2(dpm)6], where H3L = 2-hydroxymethyl-2-phenylpropane-1,3-diol and Hdpm = dipivaloylmethane, Fe4Ph) on an unreconstructed surface of Au(111). This has been possible through the application of an integrated approach, which allows the explicit inclusion of the packing effects in the classical dynamics to be used in a second step in periodic and non-periodic high level DFT calculations. In this way we can obtain access to mesoscale geometrical data and verify how they can influence the magnetic properties of interest of the single Fe4 molecule. The proposed approach allows to overcome the ab initio state-of-the-art approaches used to study Single Molecule Magnets (SMMs), which are based on the study of one single adsorbed molecule and cannot represent effects on the scale of a monolayer. Indeed, we show here that it is possible to go beyond the computational limitations inherent to the use, for such complex systems, of accurate calculation techniques (e.g. ab initio molecular dynamics) without losing the level of accuracy necessary to gain new detailed insights, hardly reachable at the experimental level. Indeed, long-range and edge effects on the Fe4 structures and their easy axis of magnetization orientations have been evidenced as their different contributions to the overall macroscopic behavior.

The Role of Anisotropic Exchange in Single Molecule Magnets: A CASSCF/NEVPT2 Study of the Fe 4 SMM Building Block [Fe 2 (OCH 3 ) 2 (dbm) 4 ] Dimer

The rationalisation of single molecule magnets’ (SMMs) magnetic properties by quantum mechanical approaches represents a major task in the field of the Molecular Magnetism. The fundamental interpretative key of molecular magnetism is the phenomenological Spin Hamiltonian and the understanding of the role of its different terms by electronic structure calculations is expected to steer the rational design of new and more performing SMMs. This paper deals with the ab initio calculation of isotropic and anisotropic exchange contributions in the Fe(III) dimer [Fe2(OCH3)2(dbm)4]. This system represents the building block of one of the most studied Single Molecule Magnets ([Fe4RC(CH2O)3)2(dpm)6] where R can be an aliphatic chain or a phenyl group just to name the most common functionalization groups) and its relatively reduced size allows the use of a high computational level of theory. Calculations were performed using CASSCF and NEVPT2 approaches on the X-ray geometry as assessment of the computational protocol, which has then be used to evinced the importance of the outer coordination shell nature through organic ligand modelization. Magneto-structural correlations as function of internal degrees of freedom for isotropic and anisotropic exchange contributions are also presented, outlining, for the first time, the extremely rapidly changing nature of the anisotropic exchange coupling.