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Aminoxyl radicals: pure organic materials with tunable magnetic and sensing properties

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Borozdina, Y. (2012). Aminoxyl radicals: pure organic materials with tunable magnetic and sensing properties. PhD Thesis, Johannes Gutenberg Universität, Mainz, Germany.

Cite as: http://hdl.handle.net/21.11116/0000-0001-A66D-A
Stable radicals have demonstrated their usefulness in solving many problems in chemistry, physics, biology and biomedicine. In the past decades there has been renewed interest in these compounds based, to a large extend on the advantages of the nitroxides application as redox-probes and magnetic materials. Chapter 1 gives a brief outline of the classes of stable radicals. This chapter deals with the general theoretical background, which is commonly considered for magnetic materials design. Some examples of stable radicals complexes with paramagnetic metals, biradical and polyradical systems are also described. rnThe synthesis of hybrid fluorophore-nitronyl nitroxide and -imino nitroxide radicals, their characterization by IR, CV, EPR techniques, and X-ray structural analysis are reported in Chapter 2. UV-Vis studies form the basis for the evaluation of nitroxides potential as redox-probes. The results of subsequent PL investigation of the sensing properties of the obtained radicals confirmed preliminary assumptions. Moreover, the possibility to utilize pyrene-pyrazole nitronyl nitroxide as NO-probe is elucidated. rnAdvantages in the area of design and investigation of magnetic materials on the basis of nitroxides are the main subjects of Chapter 3. Following classical Ullman procedure nitronyl nitroxide and imino nitroxide biradicals with various pi-bridges between the radical centers were synthesized. Magnetic measurements revealed the existence of moderately strong antiferromagnetic intramolecular exchange for the majority of biradicals under study. However, 3,3’- diazatolane derivative featured an exceptionally high exchange coupling, which can only be explained by the presence of strong intermolecular interactions. From DFT calculations performed on the basis of the X-ray crystal structure of tolane-bridged biradical 87c a theoretical value of the intra-dimer coupling constant Jintra = -8.6 K is obtained. Direct proof for also intermolecular arrangement with Jinter ~ -2 K was provided by low temperature AC studies of 87c. According to the magnetic characterization, the nitronyl biradical 87c is a promising candidate for a purely organic low-dimensional quantum magnet.rnThe idea behind this investigation was to show how, by using a set of structural units, the sensitivity of the nitroxide containing probes and the intramolecular exchange interactions in зш-conjugated spin systems could be adjusted to obtain molecules with the desired sensing or magnetic properties.