Magnetism organic radicals

The concept of spin-polarization has been found to be extremely useful for understanding the magnetic HFCs of organic radicals which are dominated by the Fermi contact contribution. The situation for transition metal complexes is rather different in several respects. The idea of spin-polarization is relatively simple and is best... 

When several magnetically equivalent nuclei are present in a radical, some of the multiplet lines appear at exactly the same field position, i.e., are degenerate , resulting in variations in component intensity. Equivalent spin-1/2 nuclei such as 1H, 19F, or 31P result in multiplets with intensities given by binomial coefficients (1 1 for one nucleus, 1 2 1 for two, 1 3 3 1 for three, 1 4 6 4 1 for four, etc.). One of the first aromatic organic radical anions studied by ESR spectroscopy was the naphthalene anion radical,1 the spectrum of which is shown in Figure 2.2. The spectrum consists of 25 lines, a quintet of quintets as expected for hyperfine coupling to two sets of four equivalent protons. 

Figure 8.9 TbPc2-based single ion magnets (SIMs) [90] (a) Schematic representation molecular localization of the three spin-systems characteristics of the [TbPc2]° complex J = 6, the uniaxial magnetic moment of the 4f configuration / = 3/2 nuclear spin of the only stable and naturally occurring 159Tb isotope S = 1/2 organic radical delocalized over the two Pc ligands. The radical...

Diverse, chemically stable pedant polarons can be introduced into a polymer chain. The resultant poly(cation-radical)s are really stable and easily handled. They are described as monthly stable at room temperature in air (Murata et al. 2004, 2005). These properties open a way to advanced applications of magnetic organic molecules in the future. 

Usually, an organic radical will have more than one magnetic nucleus. In such instances, the EPR spectrum can display rich hyperfine structure, which is useful both for identifying the species and for extracting information about the electronic structure of the radical. The origin of such hyperfine patterns is straightforward, but is most easily understood in terms of concrete examples. 

Commonly, the frequency v is about 9000 Mc/s which means that fields in the region 3000 G are required for resonance. The -factor is generally very close to the free-spin value for organic radicals deviations occur if there is an appreciable electronic orbital magnetic moment, and this will mean that resonance will occur either at higher or lower fields than that required for the free electron. This variation in the -value is discussed in Section V, D. 

Very few aromatic 77-radicals have been studied in the solid state. It has been stressed that magnetically dilute crystals are required, and these are not readily prepared. One very important example is that of a.a-diphenyl-jS-picrylhydrazyl. This was incorporated in small quantities in single crystals of the corresponding hydrazine and the 14N hyperfine and gf-tensors derived in the usual manner (Zeldes e.t al., 1960). This method of studying radicals, whilst normal for transition metal ions, is obviously inapplicable to most organic radicals whether stable or unstable. Fortunately, the method of radiation damage beautifully accomplishes this difficult task. This is discussed in Sections V and IX. 

Occasionally such intermolecular effects are specific and have sufficient lifetime to contribute individually to the spectrum. This is the case, for example, when organic radical-anions are studied in solvents of low ionizing power such as tetrahydrofuran. Ion-pairing then becomes important and, when alkali-metal cations are involved, the effect on the spectrum is to split each hyperfine component into four lines, each having one-quarter of the original intensity. This may convert a complicated spectrum into one that is quite uninterpretable, and can be avoided by using a better solvent or non-magnetic cation. However, it also provides evidence that contact ion-pairs are important in such solutions, and yields structural details unobtainable by other techniques. 

Organic radical magnetic materials lacking directional crystal assembly functionality 130... 

We now turn to considering exchange linkage between spin units in a more general fashion, as a transition from organic radical spin units to organic magnetic materials. 

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