High-spin polycarbenes as models for organic ferromagnets (original) (raw)
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Increasing the Spin Ordering Over the High�spin Aromatic Molecules
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.
Magnetic behavior of nonet tetracarbene as a model for one-dimensional organic ferromagnets
Journal of the American Chemical Society, 1986
Tetracarbene (1) was generated by photolysis of the corresponding tetradiazo compound (2) in a 2-methyltetrahydrofuran glass or a single crystal of benzophenone at cryogenic tempreatures. The temperature dependence of paramagnetic susceptibility revealed its nonet spin multiplicity in the ground state. A behavior suggesting the presence of antiferromagnetic intermolecular interaction was also found when generated in the glass. The field dependence of magnetization of 1 was analyzed in terms of the Brillouin function and the experimental data fit closely to that of the theoretical value for J = */*. This is independent evidence for the nonet spin multiplicity of 1. The characteristic saturation behavior of magnetization in 1 was found to be due to its high spin multiplicity. Thus 1 may be regarded as a molecular superparamagnet. In the polycarbenes, the localized n spins at carbenic centers are aligned all in parallel through the electron correlation between the mobile K spins, resulting in the intramolecular ferromagnetic spin ordering. Although the mechanism of spin ordering of a spins in polycarbenes is entirely different from that in metallic ferromagnets, the interaction between n and K spins resembles s-d interaction in dilute alloys. The potentiality of polycarbenes as a microdomain in macroscopic ferromagnets will be 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.
Journal of the American Chemical Society, 1994
Preparation and characterization of several new polymers (1-7) that are considered one-dimensional prototypes for the polaronic ferromagnet are reported. Synthesis involved either Wittig or Suzuki coupling to produce polymers with extended ?r systems. Oxidative doping (I2 or AsF5) produces radical cations (polarons) that are stable at room temperature. Magnetic characterization of the doped polymers, using SQUID-based magnetometry, indicates that in several instances ferromagnetic coupling of polarons occurs along the polymer chain. Comparison of the various polymers reveals useful design principles and suggests new directions for the development of magnetic organic materials.
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
Studies of the electronic structure of polyradicals by means of their magnetic properties
Pure and Applied Chemistry, 1998
Having identified the limitations of the commonly used ESR spectroscopic method, we have employed a versatile method of measuring magnetic susceptibility and magnetization for the characterization of the ground state spins of di-, oligo-and polyradicals. Under these subjects are specifically included such items as the determination of a small singlet-triplet energy gap of < 1 K in conformationally fixed tetramethyleneethane (l), the demonstration of spin frustration or competing interaction in the ground state of antiferromagnetically coupled spins in triradical 3 and the dinuclear Mn(I1) complex 5, and the analyses of super high-spin (S = 7-9) poly-carbenes 6 and 7. EPR SPECTROSCOPY VERSUS PARAMAGNETIC SUSCEPTIBILITY/ MAGNETIZATION STUDIES EPR spectroscopy serves as a standard method for detecting triplet diradicals and higher spin polyradicals and for studying their electronic states (1). The fine structures due to the dipolar interaction of the spins within the polyradical molecules are analyzed and AmS = 2 or higher transitions are often detected in the g = 4 or higher regions. The D and E parameters of the zero-field tensor of the radical molecules thus 1 Cexp(-AEsT 1 kBT) Znt =-T [1+ 3exp(-AE~~ 1 ~B T) ]
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.
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.
Theoretical design of a high spin organic molecule. II
Chemical Physics Letters, 2002
Novel stable high spin molecules possessing three different arranged fashions are designed with-•N-N＜ as a spin-containing (SC) fragment, various aromatic, such as benzene (1), pyridine (2), pyridazine (3), pyrimidine (4), pyrazine (5), triazine (6) as end groups (EG) and phenyl as a ferromagnetic coupling (FC) unit. The effects of a different end groups on the spin multiplicities of the ground states and their stabilities were investigated by means of AM1-CI approach. It has been found that the spin densities on the two atoms of the SC fragment are different from delocalization resulting in the specific stability of-•N-N＜. In these molecules, the stabilities of the triplet states decrease when the distance between the atoms of central SC (-N-) increases. The orders of the stability of triplet states for 1an, 1bn, 1cn [They are isomers in which SC is connected with FC in different way (1an, N 1 NNN 1 ; 1bn, N 1 N N 1 N; 1cn, NN 1 N 1 N) and six heterocycles are EG] show that the stability of triplet states with heterocycles as end groups is higher than that with phenyl as end groups, and in the order: triazine (EG)＞pyrimidine, pyrazine＞ pyridine, pyridazine.