Electron transport through a spin crossover junction. Perspectives from a wavefunction-based approach.

The present paper reports the application of a computational framework, based on the quantum master equation, the Fermi's golden Rule, and conventional wavefunction-based methods, to describe electron transport through a spin crossover molecular junction (Fe(bapbpy) (NCS)2, 1, bapbpy = N-(6-(6-(Pyridin-2-ylamino)pyridin-2-yl)pyridin-2-yl)-pyridin-2-amine). This scheme is an alternative to the standard approaches based on the relative position and nature of the frontier orbitals, as it evaluates the junction's Green's function by means of accurate state energies and wavefunctions. In the present work, those elements are calculated for the relevant states of the high- and low-spin species of 1, and they are used to evaluate the output conductance within a given range of bias- and gate-voltages. The contribution of the ground and low-lying excited states to the current is analyzed, and inspected in terms of their 2S + 1 Ms-states. In doing so, it is shown the relevance of treating not only the ground state in its maximum-Ms projection, as usually done in most computational-chemistry packages, but the whole spectrum of low-energy states of the molecule. Such improved representation of the junction has a notable impact on the total conductivity and, more importantly, it restores the equivalence between alpha and beta transport, which means that no spin polarization is observed in the absence of Zeeman splitting. Finally, this work inspects the strong- and weak-points of the suggested theoretical framework to understand electron transport through molecular switchable materials, identifies a pathway for future improvement, and offers a new insight into concepts that play a key role in spintronics.

Assembly of Ferrocene Molecules on Metal Surfaces Revisited.

Metallocene (MCp2) wires have recently attracted considerable interest in relation to molecular spintronics due to predictions concerning their half-metallic nature. This exciting prospect is however hampered by the little and often-contradictory knowledge we have concerning the metallocene self-assembly and interaction with a metal. Here, we elucidate these aspects by focusing on the adsorption of ferrocene on Cu(111) and Cu(100). Combining low-temperature scanning tunneling microscopy and density functional theory calculations, we demonstrate that the two-dimensional molecular arrangement consists of vertical- and horizontal-lying molecules. The noncovalent T-shaped interactions between Cp rings of vertical and horizontal molecules are essential for the stability of the physisorbed molecular layer. These results provide a fresh insight into ferrocene adsorption on surfaces and may serve as an archetypal reference for future work with this important variety of organometallic molecules.

Impact of short and long-range effects on the magnetic interactions in neutral organic radical-based materials.

The mutual influence of electronic structure and environment of the constituent units of neutral organic radical-based materials (radical dimers) is analysed by means of wave function calculations (Difference Dedicated Configuration Interaction, DDCI). Focus is put on the magnetic property modulations of two classes of neutral organic materials by inspecting both short- and long-range effects. The exchange coupling constant J for the high-temperature phase of the 1,3,5-trithia-2,4,6-triazapentalenyl (TTTA) material is calculated to be J= -95 cm(-1) at the DDCI level. The environmental electronic polarization is taken into account self-consistently using the individual polarizabilities of the atoms in a finite block of the crystal lattice (Discrete Reaction Field, DRF) and accounts for less than 5% of the calculated J value in TTTA. Furthermore, taking advantage of the chemical flexibility of the verdazyl radical family, the contribution of strong electron-withdrawing groups is analysed by extracting the J, U, t and K parameters from pairs of substituted verdazyl-based radicals. Our ab initio calculations of verdazyl radical pairs suggest that the addition of NO2 groups cause (i) the variations of the ferromagnetic and antiferromagnetic contributions to cancel out, leaving an almost constant exchange coupling constant, ca. J≈ 20 cm(-1), and that (ii) enhanced conduction properties can be anticipated. In contrast to inorganic analogues, one may conclude that the magnetic behaviour of neutral organic radical-based materials is mostly governed by the supramolecular arrangement, whereas environmental effects have a lesser impact.

Spin filtering in molecular junction: magnetoresistance evaluation from wave-function calculations.

The conductance of magnetic molecules opens new ways to probe the electronic structure of correlated systems. Based on a 2-electron/2-molecular orbital prototype system, the current-potential characteristics is inspected as a function of the differential magnetization of the electrodes sandwiching the molecule within a multideterminantal framework. The bias-dependent magnetoresistance effect along the junction reflects the nature and energetics of the different multiplets, obtained within the multiconfigurational wave-function approach. From the wave-function description, a modulation of the magnetoresistance ratio is anticipated and both direct and inverse regimes are observed depending on the electronic structure of the junction.

Seeking hidden magnetic phenomena by theoretical means in a thiooxoverdazyl adduct.

The oxidation of 1,5-dimethyl-3-(2'-pyridyl)-6-thiooxotetrazane (SvdH(3) py) by benzoquinone leads to a 1:1 adduct of 1,5-dimethyl-3-(2'-pyridyl)-6-thiooxoverdazyl radical (Svdpy) with hydroquinone (hq). The single-crystal X-ray diffraction of this adduct at room temperature (RT) shows that the radicals exhibit a slight curvature that leads to the formation of alternating head-to-tail (antiparallel) stacked 1D chains. Moreover, temperature-dependent X-ray measurements at 100, 200, and 303 K reveal that the lateral slippages between the radicals of the stacks |δ(1) | and |δ(2) | vary from 0.64 to 0.78 Šand 0.54 to 0.40 Šbetween 100 and 303 K. Despite the alternation of the inter-radical distances and lateral slippages, the magnetic susceptibility data can be fitted with excellent agreement using a regular one-dimensional antiferromagnetic chain model with J=-5.9 cm(-1). Wavefunction-based calculations indicate an alternation of the magnetic interaction parameters correlated with the structural analysis at RT. Moreover, they demonstrate that the thermal slippage of the radicals induces a switching of the physical behavior, since the exchange interaction changes from antiferromagnetic (-0.9 cm(-1)) at 100 K to ferromagnetic (1.4 cm(-1)) at 303 K. The theoretical approach thus reveals a much richer magnetic behavior than the analysis of the magnetic susceptibility data and ultimately questions the relevance of a spin-coupled picture based on temperature-independent parameters.

Magnetic and conduction properties in 1D organic radical materials: an ab initio inspection for a challenging quest.

The chemical control of magnetic and conduction properties for organic radicals is mainly based on t, the resonance integral, and U, the on-site repulsion, used in the Hubbard model. A qualitative analysis based on the competition between the kinetic and the Coulomb contribution, and the expression of the magnetic exchange coupling suggests that U should be roughly 800 cm(-1) while the resonance integral |t| should be 200 cm(-1) to reach bifunctionality. Ab initio wavefunction-based calculations allowed us to quantitatively measure those quantities for several organic materials considered as 1D systems starting from their reported crystal structures. The extraction of t and U parameters from the exchange coupling constants between neighbouring radicals allowed us to anticipate a possible metallic behaviour. Finally, the impact of chemical changes in the constitutive units is measured to rationalize the macroscopic behaviour modifications. It is shown that the intriguing regime characterized by simultaneous itinerant and localized electrons might be achieved by molecular engineering.