The quest for rationalizing the magnetism in purely organic semiquinone-bridged bisdithiazolyl molecular magnets

Semiquinone-bridged bisdithiazolyl-based radicals (XBBO) are appealing purely organic magnetic building blocks for the synthesis of new functional materials. Remarkably, for the phenyl-derivative PhBBO, the rationalization of its magnetism becomes a proof of concept that DFT can dramatically fail to evaluate JAB magnetic interactions between purely organic radical pairs. Instead, wavefunction-based methods are required. Once JAB's are fully characterized, the magnetic topology of PhBBO is disclosed to consist of ferromagnetic FM π-stacks that are very weakly coupled (by FM and AFM JAB interactions). The magnetic susceptibility χT(T) and magnetization M(H) of PhBBO are then calculated using a first-principles bottom-up approach. The study of the unit cell contraction upon cooling from room temperature to zero-Kelvin is relevant to propose a suitable model for the phase transition that occurs at 4.5 K. A simplistic picture tells us that the antiparallel-aligned 1D-FM-chains convert into domains of weakly either FM- or AFM-coupled 1D-FM-chains. Accordingly, the presence of these domains may introduce geometrical spin frustration below 4.5 K

A Definition of the Magnetic Transition Temperature Using Valence Bond Theory

Macroscopic magnetic properties are analyzed using Valence Bond theory. Commonly the critical temperature TC for magnetic systems is associated with a maximum in the energy-based heat capacity Cp(T). Here a more broadly applicable definition of the magnetic transition temperature TC is described using the spin moment expectation value (i.e., applying the spin exchange density operator) instead of energy. Namely, the magnetic capacity Cs(T) reflects variation in the spin multiplicity as a function of temperature, which is shown to be related to ∂[χT(T)]/∂T. Magnetic capacity Cs(T) depends on long-range spin interactions that are not relevant in the energy-based heat capacity Cp(T). Differences between Cs(T) and Cp(T) are shown to be due to spin order/disorder within the crystal that can be monitored via a Valence Bond analysis of the corresponding magnetic wave function. Indeed the concept of the Boltzmann spin-alignment order is used to provide information about the spin correlation between magnetic units. As a final illustration, the critical temperature is derived from the magnetic capacity for several molecular magnets presenting different magnetic topologies that have been experimentally studied. A systematic shift between the transition temperatures associated with Cs(T) and Cp(T) is observed. It is demonstrated that this shift can be attributed to the loss of long-range spin correlation. This suggests that the magnetic capacity Cs(T) can be used as a predictive tool for the magnetic topology and thus for the synthetic chemists

A first-principles analysis of the magnetism of CuII polynuclear coordination complexes: the case of [Cu4(bpy)4(aspartate)2(H2O)3](ClO4)4·2.5H2O

The magnetic structure of the [Cu4(bpy)4(aspartate)2(H2O)3](ClO4)4·2.5 H2Ocrystal - using fractional coordinates determined at room-temperature ¿ has beenanalysed in detail. This analysis has been carried out by extending our first principlesbottom-up theoretical approach, which was initially designed to study through-spacemagnetic interactions, to handle through-bond magnetic interactions. The only input datarequired by this approach are the values of the computed JAB exchange parameters for allthe unique pairs of spin-containing centres. The results allow the magnetic structure ofthe crystal, which presents two types of isolated tetranuclear CuII clusters, to be definedin quantitative terms. Each of these clusters presents ferro and antiferromagneticinteractions, the former being stronger, although outnumbered by the latter. Thecomputed magnetic susceptibility curve shows the same qualitative features as theexperimental data. However, there are small differences that are presumed to beassociated with the use of room-temperature crystal coordinates

Insights into the magnetism and phase transitions of organic radical-based materials

Organic radicals have been consistently regarded as promising building blocks for the next generation of applied materials. Multiple radical families have been developed and characterized in the last decades, fostered by the ever-growing capabilities of organic synthesis. Thiazyl-, spiro-biphenalenyl-, 1,2,4-benzotriazinyl-, and nitroxide-based radicals have furnished striking examples of metal-free switchable materials, whose phase transitions are accompanied by changes in the magnetic, optical and/or electrical response. While similar in origin, their actual mechanism, driving force(s), and spin state stabilities often depict a different landscape. Fundamental knowledge on such aspects, as well as on the underlying network of spin exchange couplings and non-covalent interactions (including pancake bonding), are key to understand their spin transition, and the tailored modification of their properties. These complex features cannot be extracted based solely on experimental input, but rely on a computational interpretation. In this Perspective article, we discuss the insight gained from computational modelling into the magnetism and phase transitions of organic radical-based materials. We focus on the key importance of dynamic effects due to the labile nature of π-stack interactions assembling those materials, the structural distortions driven by spin changes, the coupling between electronic structure and order-disorder transitions, and the dependence of spin correlation upon temperature. All these phenomena uncovered by simulations should assist in the rational design of new dynamic organic crystals

Electronic structure and properties of multifunctional systems: bisdithiazolyl-based materials

[cat] Els materials orgànics moleculars cada vegada tenen més aplicacions en la fabricació de dispositius electrònics per les seves propietats òptiques i de conducció. Quan els elements moleculars són radicals, cal tenir en compte alhora la càrrega i l'espín de l'electró desaparellat. La racionalització de l'estructura i de les propietats d'aquests materials multifuncionals requereix una descripció acurada de la seva estructura electrònica. En aquest treball s'analitza l'aplicabilitat dels models actuals en la modelització de la conducció elèctrica d'aquests materials, emprant tota una família de compostos derivats del bisditiazolil com a sistemes model.[eng] Molecular organic materials are finding increasing application in the manufacture of electronic devices thanks to their optical and conduction properties. When the molecular moieties are radicals, both charge and spin of the unpaired electron should be taken into account. The full rationalization of the structure and properties of these multifunctional materials requires a careful description of their electronic structure. In this paper, the applicability of the current models in the modeling of the electrical conduction of these materials is analyzed, using the family of bisdithiazolyl-based compounds as model systems

Controlling pairing of pi-conjugated electrons in 2D covalent organic radical frameworks via in-plane strain

Controlling the electronic states of molecules is a fundamental challenge for future sub-nanoscale device technologies. π-conjugated bi-radicals are very attractive systems in this respect as they possess two energetically close, but optically and magnetically distinct, electronic states: the open-shell antiferromagnetic/paramagnetic and the closed-shell quinoidal diamagnetic states. While it has been shown that it is possible to statically induce one electronic ground state or the other by chemical design, the external dynamical control of these states in a rapid and reproducible manner still awaits experimental realization. Here, via quantum chemical calculations, we demonstrate that in-plane uniaxial strain of 2D covalently linked arrays of radical units leads to smooth and reversible conformational changes at the molecular scale that, in turn, induce robust transitions between the two kinds of electronic distributions. Our results pave a general route towards the external control, and thus technological exploitation, of molecular-scale electronic states in organic 2D materials

Pitfalls on evaluating pair exchange interactions for modelling molecule-based magnetism

Molecule-based magnetism is a solid-state property that results from the microscopic interaction between magnetic centres or radicals. The observed magnetic response is due to unpaired electrons whose coupling leads to a particular magnetic topology. Therefore, to understand the magnetic response of a given molecule-based magnet and reproduce the available experimental magnetic properties by means of statistical mechanics, one has to be able to determine the value of the JAB magnetic exchange coupling between radicals. The calculation of JAB is thus a key point for modelling molecule-based magnetism. In this Perspectives article, we will build upon our experience in modelling molecular magnetism to point out some pitfalls on evaluating JAB couplings. Special attention must be paid to the cluster models used to evaluate JAB, which should account for cooperative effects among JAB interactions and also consider the environment (counterions, hydrogen bonding) of the two radicals whose interaction has to be evaluated. It will be also necessary to assess whether a DFT-based or a wavefunction-based method is best to study a given radical. Finally, in addition to model and method, the JAB couplings have to be able to adapt to changes in the magnetic topology due to thermal fluctuations. Therefore, it is most important to appraise in which systems molecular dynamics simulations would be required. Given the large number of issues one must tackle when choosing the correct model and method to evaluate JAB interactions for modelling magnetic properties in molecule-based materials, the "human factor" is a must to cross-examine and challenge computations before trusting any result

Magnetic fingerprint of dithiazolyl-based molecule magnets

Magnetic bistability in organic-radical based materials has attracted significant interest due to its potential application in electronic devices. The First-Principles Bottom-Up study herein presented aims at elucidating the key factors behind the different magnetic response of the low and high temperature phases of four different switchable dithiazolyl (DTA)-based compounds. The drastic change in the magnetic response upon spin transition is always due to changes in the JAB magnetic interactions between adjacent radicals along the π-stacks of the crystal, which in turn are driven mostly by changes in interplanar distance and degree of lateral slippage, according to the interpretation of a series of magneto-structural correlation maps. Furthermore, specific geometrical dispositions have been recognized as a ferromagnetic fingerprint in such correlations. Our results thus show that a proper substitution of the chemical skeleton attached to the DTA ring could give rise to new organic materials with dominant ferromagnetic interactions

2D Hexagonal covalent organic radical frameworks as tunable correlated electron systems

Quantum materials hold huge technological promise but challenge the fundamental understanding of complex electronic interactions in solids. The Mott metal-insulator transition on half‐filled lattices is an archetypal demonstration of how quantum states can be driven by electronic correlation. Twisted bilayers of 2D materials provide an experimentally accessible means to probe such transitions, but these seemingly simple systems belie high complexity due to the myriad of possible interactions. Herein, it is shown that electron correlation can be simply tuned in experimentally viable 2D hexagonally ordered covalent organic radical frameworks (2D hex‐CORFs) based on single layers of half‐filled stable radical nodes. The presented carefully procured theoretical analysis predicts that 2D hex‐CORFs can be varied between a correlated antiferromagnetic Mott insulator state and a semimetallic state by modest out‐of‐plane compressive pressure. This work establishes 2D hex‐CORFs as a class of versatile single‐layer quantum materials to advance the understanding of low dimensional correlated electronic systems

Two different mechanisms of stabilization of regular pi-stacks of radicals in switchable dithiazolyl-based materials

Materials based on regular π-stacks of planar organic radicals are intensively pursued by virtue of their technologically relevant properties. Yet, these π-stacks are commonly unstable against π-dimerization. In this computational study, we reveal that regular π-stacks of planar dithiazolyl radicals can be rendered stable, in some range of temperatures, via two different mechanisms. When the radicals of a π-stack are both longitudinally and latitudinally slipped with respect to each other, the corresponding regular π-stacked configuration is associated with a locally stable minimum in the potential energy surface of the system. Conversely, those regular π-stacks in which radicals are latitudinally slipped with respect to each other are stable as a result of a dynamic interconversion between two degenerate dimerized configurations. The existence of two stabilization mechanisms, which can be traced back to the bonding properties of isolated π-dimers, translates into two different ways of exploiting spin-Peierls-like transitions in switchable dithiazolyl-based materials