Structure property relationships in organic biradicals
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In this work, structure-property relationships of several organic biradical ligands are explored. A basic understanding of these relationships is important in designing new molecular magnetic materials. The utility of studying biradicals is, that they represent the simplest high-spin system with which to build upon. All of the spin groups studied are air stable and are synthetically adaptable organic radicals. A series of bis(verdazyl) biradicals connected (at the nodal position of the verdazyl) to several different aromatic spacer groups (couplers), has been prepared to compare their properties to those of the non-spaced 1,1’,5,5’-tetramethyl-6,6’-dioxo-3,3’- bisverdazyl (1) and 1,1’,5,5’-tetramethyl-6,6’-dithio-3,3’-bisverdazyl (7). Cyclic voltammograms of 1,4-bis(1,5-dimethyl-6-oxo-3-verdazyl)benzene (5), 2,5-bis(1,5- dimethyl-6-oxo-3-verdazyl)thiophene (6), and 1,3-bis(1,5-dimethyl-6-oxo-3- verdazyl)benzene (4) show a single two-electron oxidation wave near 700 mV vs. SCE. In contrast 1 and 7 (non-spaced verdazyls) display two one-electron oxidation waves, with the first oxidative wave appearing also near 700 mV vs. SCE. The absorption spectrum of each of these biradicals is red-shifted from the maximum observed for 1. Biradicals 4, 5, and 6 exhibited linear Curie plots, although a curved Curie plot was observed for 7 with J = -280 cm-1 . A magneto-structural correlation of conformational exchange modulation within an isostructural series of TMM-type bis(semiquinone) biradical complexes is presented. Zero-field splitting parameters, X-ray crystal structures, and variable-temperature magnetic susceptibility measurements were used to evaluate electron spin exchange in this series of molecules. Our combined results indicate that the ferromagnetic portion of the exchange coupling occurs via the cross-conjugated p-system, while the antiferromagnetic portion occurs through space and is equivalent to incipient bond formation. Molecular conformation controls the relative amounts of ferro- and antiferromagnetic contributions to exchange coupling. In fact, the exchange parameter correlates with average semiquinone ring torsion angles via a Karplus-Conroy-type relation: J = 213 cos2 f- 44. This is the first clear assignment of ferro- and antiferromagnetic components of exchange coupling to specific molecular structure features in a series of biradicals. The molecular structures and magnetic properties of six TMM-type dinitroxide biradicals are also described. Five of the dinitroxides are trimethylenemethane-type (TMM-type) biradicals, i.e., the intramolecular exchange parameter, J, is modulated by a carbon-carbon double bond. However, the efficacy of the carbon-carbon double bond as an exchange coupler is determined by the molecular conformation. Our results show that the exchange parameters correlate with phenyl-ring torsion angles (f) via a simple Karplus-Conroy-type relation: J = 44 cos2 f- 17. Comparison of these results to those obtained for our isostructural series of bis(semiquinone) biradicals shows that both the magnitude of J and the resistance of ferromagnetic J to bond torsions is proportional to the spin density adjacent to the exchange coupler.