Magnetostructural relationships in [Ni(dmit)2]- radical anions

This work explores the relationship between the magnetic properties of salts based on [Ni(dmit)2]- radicals and different arrangements that these radicals can adopt in the crystals, induced by the packing constrains imposed by the counterions. Our analysis is based on difference dedicated configuration interaction calculations on models containing two neighbouring [Ni(dmit)2]- units with different interaction patterns. The amplitude and sign of these through-space interactions can be rationalized on the basis of a valence-only model that essentially analyzes the effective interactions between the atoms carrying the electronic density of singly occupied orbitals (SOMOs). Despite the simplicity of the model, it provides simple rules to predict the nature and the expected amplitude (strong/medium/weak) of the leading interactions in systems based on these [Ni(dmit)2]- radicals.

Orbital Entanglement Analysis of Exchange-Coupled Systems

A new tool for the interpretation of multiconfigurational wave functions representing the spin states of exchange-coupled transition metal complexes is introduced. Based on orbital entanglement measures, herein derived from multiconfigurational density matrix renormalization group calculations, the complexity of the wave function is reduced, thus facilitating a connection with established concepts for the interpretation of magnetically coupled systems. We show that the entanglement of localized orbitals with a small basis set is a good representation of the magnetic coupling topology and that it is sensitive to chemical changes in homologous complexes. Furthermore, we introduce a measure for the magnetic relevance of orbitals in the active subspace and a concept for the quantitative comparison of different chemical species. The approach presented here will be easily applicable to higher nuclearity clusters, providing a direct insight into all states of the Heisenberg spin ladder for systems previously accessible only by single-configurational methods.

Light-Induced Control of the Spin Distribution on Cu–Dithiolene Complexes: A Correlated Ab Initio Study

Metal dithiolene complexes—M(dmit)₂—are key building blocks for magnetic, conducting, and optical molecular materials, with singular electronic structures resulting from the mixing of the metal and dmit ligand orbitals. Their use in the design of magnetic and conducting materials is linked to the control of the unpaired electrons and their localized/delocalized nature. It has been recently found that UV–Vis light can control the spin distribution of some [Cu(dmit)₂]⁻² salts in a direct and reversible way. In this work, we study the optical response of these salts and the origin of the differences observed in the EPR spectra under UV–Vis irradiation by means of wave function-based quantum chemistry methods. The low-lying states of the complex have been characterized and the electronic transitions with a non-negligible oscillator strength have been identified. The population of the corresponding excited states promoted by the UV–Vis absorption produces significant changes in the spin distribution, and could explain the changes observed in the system upon illumination. The interaction between neighbor [Cu(dmit)₂]⁻² complexes is weakly ferromagnetic, consistent with the relative orientation of the magnetic orbitals and the crystal packing, but in disagreement with previous assignments. Our results put in evidence the complex electronic structure of the [Cu(dmit)₂]⁻² radical and the relevance of a multideterminantal approach for an adequate analysis of their properties.

The maximum occupancy condition for the localized property-optimized orbitals

It is shown analytically that the Chemist's Localized Property-optimized Orbitals (CLPOs), which are the localized orbitals obtainable from the results of ab initio calculations by the open-source program JANPA (http://janpa.sourceforge.net/) according to the recently proposed optimal property partitioning condition, form the Lewis structure with nearly maximum possible total electron occupancy. The conditions required for this additional optimality to hold are discussed. In particular, when a single-determinant wavefunction is used to describe the molecular system without a noticeable electron delocalization, CLPOs derived from this wavefunction approximately optimize the same target quantity as the Natural Bond Orbitals (NBOs), establishing in this way the link between the two sets of localized orbitals. The performance of CLPO and NBO methods is compared by using a dataset containing 7101 small molecules, and the relevant methodological features of both methods are discussed.

Theoretical study of the photoconduction and photomagnetism of the BPY[Ni(dmit)2]2 molecular crystal

UV-Vis light generates photoexcited (BPY²⁺)* radicals that interact with the close Ni(dmit)₂ units, modifying their magnetic and conducting properties.

Electronic Structure and Magnetic Interactions in the Radical Salt [BEDT-TTF]2[CuCl4]

The magnetic behavior and electric properties of the hybrid radical salt [BEDT-TTF]₂[CuCl₄] have been revisited through extended experimental analyses and DDCI and periodic DFT plane waves calculations. Single crystal X-ray diffraction data have been collected at different temperatures, discovering a phase transition occurring in the 250-300 K range. The calculations indicate the presence of intradimer, interdimer, and organic-inorganic π-d interactions in the crystal, a magnetic pattern much more complex than the Bleaney-Bowers model initially assigned to this material. Although this simple model was good enough to reproduce the magnetic susceptibility data, our calculations demonstrate that the actual magnetic structure is significantly more intricate, with alternating antiferromagnetic 1D chains of the organic BEDT-TTF⁺ radical, connected through weak antiferromagnetic interactions with the CuCl₄^2- ions. Combination of experiment and theory allowed us to unambiguously determine and quantify the leading magnetic interactions in the system. The density-of-states curves confirm the semiconductor nature of the system and the dominant organic contribution of the valence and conduction band edges. This general and combined approach appears to be fundamental in order to properly understand the magnetic structure of these complex materials, where experimental data can actually be fitted from a variety of models and parameters.

Evaluation of the Giant Ferromagnetic π-d Interaction in Iron-Phthalocyanine Molecule

The interaction between itinerant π and localized d electrons in metal-phthalocyanines, namely, J_πd interaction, is considered as responsible for the giant negative magnetoresistance observed in several phthalocyanine-based conductors, among many other important physical properties. Despite the fundamental and technological importance of this on-site intramolecular interaction, its giant ferromagnetic nature has been only recently demonstrated by the experiments conducted by Murakawa et al. in the neutral radical [Fe(Pc)(CN)₂]·2CHCl₃ ( Phys. Rev. B 2015 , 92 , 054429 ). In this article, we present the theoretical evaluation of this interaction combining wave function-based electronic calculations on isolated Fe(Pc)(CN)₂ molecules and density functional theory-based periodic calculations on the crystal. Our calculations confirm the ferromagnetic nature of the π-d interaction, with a coupling constant as large as J_πd/k_B = 570 K, in excellent agreement with the experiments, and the presence of intermolecular antiferromagnetic interactions driven by the π-π overlap of neighboring phthalocyaninato molecules. The analysis of the wave function of the ground state of the Fe(Pc)(CN)₂ molecule provides the clues of the origin of this giant ferromagnetic π-d interaction.

Twisting induces ferromagnetism in homometallic clusters

A helical chiral cluster bridging two sets of Cu₂ units is reported. The two double-stranded ligands induce a distorted tetrahedral environment for one of the two copper(ii) ions whereas the second one remains in a standard octahedral environment. Magnetic measurements and wavefunction calculations demonstrate that the copper(ii) centres are ferromagnetically coupled (J = 7.7 cm^-1). This ligand-driven ferromagnetic interaction thus appears as a proof-of-concept of an innovative strategy towards high-spin clusters.