Magnetic behavior of nonet tetracarbene as a model for one-dimensional organic ferromagnets (original) (raw)
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High-spin polycarbenes as models for organic ferromagnets
Pure and Applied Chemistry, 1986
In response to the Mataga's prediction of ferromagnetic hydrocarbons (1968), we have taken up the study of two series of high-spin polycarbenes (1 and 2). The corresponding polydiazo compounds were prepared throuh a sries of unambiguous synthetic reactions and photolyzed in 2-methyltetrahydrofuran matrices and in single crystals of a benzophenone host at cryogenic temperatures. The ESR fine structures and magnetic susceptibilities were measured and analyzed to show that the highest spin states were generated as the electronic ground state of 1 and 2. Similarly, isomeric bis(diazo)-[2.2]paracyclophanes were prepared aid ph&Eolyzed to find, in good agreement with the McConnell's theory on the ferromagnetic intermolecular interaction between organic free radicals (1963), that the pseudoortho and pseudopara dicarbenes have the ground quintet state while the pseudometa isomer is in the ground singlet state. A strategy for increasing the spin ordering over the high-spin aromatic molecules by orienting the stacking mode was thus obtained. Relevance of these results to macroscopic ferromagnets is discussed.
Approaches from superhigh-spin molecules to organic ferromagnets
Pure and Applied Chemistry, 1993
There are two steps of strategy in the molecular design of organic fcrromagnets: the construction of high-spin organic molecules and the introduction of spin-aligning mechanisms into the assemblies of the open-shell molecules. The former approach requires the knowledge of how to endow organic molecules with many half-filled orthogonal orbitals. In n-conjugated diradicals, parallel alignment of the two spins can become favored, if the radical centers are placed in phase with the spin polarization of the intervening n-bonds. We have now studied a wide variety of non-Kekul6 hydrocarbons and the heteroatom analogues by EPR spectroscopy and magnetic susceptibility/magnetization measurements to find that only a part of them have high-spin ground states. Some of the superhigh-spin molecules from our laboratories will be discussed in some detail. A similar principle can be applied to effect ferromagnetic exchange coupling between high-spin molecules. Design of molecular stacking in crystals, liquid crystals and molecular layers is found to be indispensable.
A new, simple model for organic ferromagnetism and the first organic ferromagnet
Synthetic Metals, 1987
A new and simple model is presented for an organic ferromagnet containing segregated stacks of radical ions. This new model gives a conceptual framework for selecting candidate molecules, and hence appears to be a promising, general guide for designing new organic ferromagnetic materials. The discovery of the first organic ferromagnet is reported. It is based on a polymer obtained from reacting s-triaminobenzene with iodine. The reaction is complex and the resulting polymer is not very reproducible. Nevertheless, on a number of occasions a ferromagnet material has been obtained. This material remains ferromagnetic to high temperatures, until it decomposes near 400°C. NEW, SIMPLE MODEL Models for organic ferromagnets with unpaired electrons in nonbonded orbitals have been proposed by Mataga [1] and Ovchinnikov [2]. A model using ~-electrons in a mixed (o ° .DADADA° • .) stack has been proposed by McConnell [3] and extended by Breslow et al. [4]. Here we propose a new, simple model for a segregated stack of radical ions which will show which molecular parameters can cause a parallel or ferromagnetic alignment of the unpaired electronic spins of these stacked radicals. Consider a pair of adjacent radical anions, M~ and M~, which each have an unpaired electron, delocalized in a ~r-orbital over the aromatic molecule. The ground state of our two molecule dimer may be written as M~VI~ and may be either a Sing, let (S) or a Triplet (T) as indicated in the energy level diagram in Figure la. Following Mulliken, it is convenient to view the effects of the
Design of molecular ferromagnets
Journal of Chemical Sciences, 1996
A large variety of molecular ferromagnets have been synthesized since the discovery of the first organic ferromagnets, including pure organic compounds, organometallic charge-transfer complexes, metal complex-organic radical compounds, and transition metal complexes coupled to organic radicals. Besides, there are many reports on the observation of ferromagnetism in polymers and organic matrix composites. Molecular ferromagnets have great potential in different areas of technology such as low frequency magnetic shielding, magnetic imaging, magneto-optics and information storage. We provide a brief review on the current strategies for the design of molecular (organic) ferromagnets. This includes exploiting the inherent advantages of molecular systems, such as the ability to fine-tune the properties at the molecular level, and to control dimensionality, supramolecular structuring and hierarchy of spin interactions etc. for carrying out structural modifications and chemical functionalisations of stable open-shell molecules in order to generate supramolecular structures in which the natural prediction for antiparallel spin alignment (antiferromagnetism) is avoided.
Enhanced Magnetic Anisotropy via Quasi-Molecular Magnet at Organic-Ferromagnetic Contact
The Journal of Physical Chemistry Letters, 2013
To realize the origin of efficient spin injection at organic-ferromagnetic contact in organic spintronics, we have implemented the formation of quasi-molecular magnet via surface restructuring of a strong organic acceptor, tetrafluoro-tetracyanoquinodimethane (F4-TCNQ), in contact with ferromagnetic cobalt. Our results demonstrate a spin-polarized F4-TCNQ layer and a remarkably enhanced magnetic anisotropy of the Co film. The novel magnetic properties are contributed from strong magnetic coupling caused by the molecular restructuring that displays an angular anchoring conformation of CN and upwardly protruding fluorine atoms. We conclude that the π bonds of CN, instead of the lone-pair electrons of N atoms, contribute to the hybridization-induced magnetic coupling between CN and Co and generate a superior magnetic order on the surface. SECTION: Surfaces, Interfaces, Porous Materials, and Catalysis O rganic spintronics with integrated organic electronics
The Journal of Physical Chemistry B, 2000
A model compound for purely organic ferrimagnets has been studied by single-crystal cw-ESR spectroscopy. A heterospin system under study is composed of two kinds of nitronylnitroxide molecules with the ground states of S ) 1 / 2 and S ) 1. These molecules are stacked in an alternating chain in the crystal. The cw-ESR signal of the compound was found to split into two signals below 10 K, which were reproduced by the superposition of two Lorentzian signals. The X-ray measurements at 9.6 K disclosed that the crystal structure remained unchanged at low temperatures, indicating that the origin of the ESR line splitting is not attributable to symmetry reduction associated with structural change but to some change in the spin state: Two distinguishable paramagnetic species are found from the ESR spectra. The appearance of two kinds of paramagnetic species in the alternating chain at finite temperature is expected as a primitive model of thermal excitation in the ferrimagnetic chain which is based on a theoretical calculation. The present experimental results demonstrate that the classical picture of ferrimagnetic spin state (antiparallel alignment of adjacent spins with different spin quantum numbers) is invalidated to describe organic molecule-based exchangecoupled spin systems and overlooks an essential part of the nature of ferrimagnetics.
Chemistry of Materials, 1994
A crystalline phase of the 2,4-hexadiynylenedioxybis[2-(p-phenylene)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-l-oxyl] diradical, 1, has been prepared and characterized by X-ray diffraction, IR, W-vis, and EPR spectroscopies, and magnetic susceptibilit measurements. This phase belongs to the C21c space group [a = 16.57(2) A, b = 16.116(2) i, c = 13.10(1) A, / 3 = 123.05(4)", V = 2931(4) A3, 2 = 4, dcalc = 1.30 g ~m-~, T = 21 "C, R, = 0.092, R, = 0.1161. The molecular structure of the diradical is characterized by an asymmetrical Z-shaped conformation. The most relevant features observed in the molecular packing are the large interdiacetylene separations-the shortest one is 8.285 A-and the alternation in the characteristics of the intermolecular contacts between the radical side groups of the DA; which are joined by hydrogen bonds between the oxygen atoms of NO groups and aromatic hydrogen atoms. On the basis of accepted structural criteria, this solid-state structure should not support a single-crystal topochemical polymerization and, accordingly, the W-induced polymerization was not achieved. Thermal treatment, however, turns the crystals from blue to dark brown. Thermal analyses under nitrogen, performed with DSC and TGA techniques, reveal an explosive and complex decomposition, at temperatures higher than 90 "C, with an evolution of gaseous NO (GC-MS) and a destruction of most of the radical centers of diradical molecules, as demonstrated by EPR and magnetic measurements. The study of the temperature dependence of the EPR signals of very diluted solutions of diradical 1 shows that it has a thermally modulated intramolecular exchange interaction due to the flexibility of the spacers joining the two radical centers and, furthermore, that when this diradical adopts a rigid conformation the two radical moieties are magnetically isolated (Jintra/k-0 K). EPR studies on polycrystalline samples of diradical 1 provide evidences for significant intermolecular exchange couplings between radical side groups of neighbor diradicals. The magnetic susceptibility of 1 suggests the simultaneous presence of dominant antiferromagnetic interactions together with very weak ferromagnetic ones; in agreement with the observed alternation of structural characteristics and the solid state EPR spectrum. This magnetic behavior is quantitatively described by a linear Heisenberg chain of S = l/2 spins with alternating F-AF intermolecular interactions of Jminter/k =-3.9 K and J F~~~~~/~-+1.2 K. The presence of ferromagnetic interactions in 1 is attributed t o the substituted diacetylenic unit, which, by structural reasons, obstructs the natural tendency of radical centers to interact only in an antiferromagnetic fashion. Finally, a reinterpretation of the magnetic data of other related organic compounds showing alternating F-AF interactions is presented, and the resulting exchange parameters correlated with the structural features of the solids.
2012
An overview is given on recent results in organic spintronic research. In particular, so-called spinterfaces, spininjecting interfaces involving organic semiconductor (OSC) molecules and ferromagnetic metals, are discussed. The interfaces are classified in different categories depending on the type and strength of interface interaction and the relevant physics concerning energy level alignment and spin polarization of interface states are explained. Examples are given on characterization of both interface energetics and spin-related properties obtained from a wide variety of experimental techniques, highlighting the different ways contacting can modify the electronic and magnetic properties of the OSC molecules and the ferromagnetic metals at the resulting spinterfaces. Finally, models for spin injection at spinterfaces are presented and discussed, followed by some speculations on consequences for device design and performance.
Spin-dependent transport in organic-ferromagnets
The European Physical Journal B, 2009
Based on the modified Su-Schrieffer-Heeger model and the non-equilibrium Green's function current formula, the spin polarization of the ferromagnet-electrode connected organic ferromagnet is theoretically studied. The spin polarization can be suppressed by atomic dimerization and be driven by an applied electric field. We investigate the spin polarization from the viewpoint of energy competitions in different interactions under the electric field. In addition, the ferromagnetic electrodes significantly enhance the spin polarization.