Nitrogen-Centered Radicals

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In this project funded by the Chemical Structure, Dynamic & Mechanism B Program of the Chemistry Division, Professor Andrzej Rajca of the Department of Chemistry at the University of Nebraska-Lincoln will investigate structure and property relationships of organic molecules and macromolecules that are relevant to the development of novel magnetic and optical technology. The goal of this research is to synthesize and study high-spin nitrogen-centered radicals (aminyls) and chiral helical radical cations. The aminyls with strong through-bond ferromagnetic coupling and with persistence at room temperature are important to the development of lightweight, soft organic magnets. In addition, such organic radicals could benefit the development of metal-free paramagnetic contrast agents for magnetic resonance imaging (MRI), agents for paramagnetic relaxation enhancement nuclear magnetic resonance (NMR) spectroscopy, as well as materials for spintronics. The helical radical cations, which possess an unpaired electron confined to a helix, could facilitate the discovery of new organic magneto-optic materials and devices. This interdisciplinary research project involves multi-step organic synthesis, diverse physical characterization of organic molecules, and computations, and is therefore well suited to the education of scientists at all levels, with exceptional opportunities to gain a broad education as well as to develop a wide range of skills. This group is also well-positioned to provide the highest level of education and training for students underrepresented in science. The proposed aminyls are high-spin radicals based upon annelated pi-systems of nitrogen-centered radicals that are made persistent by steric shielding of radical centers and isotopic substitution to prevent decay of radicals. The proposed radicals will be prepared by modern organic synthetic methodologies and characterized by electron spin resonance (EPR) spectroscopy, superconducting quantum interference device (SQUID) magnetometry, as well as computations. Aminyl diradicals with very strong pairwise ferromagnetic coupling between electron spins and persistence at room temperature, with half-life that would permit isolation of the radicals, will be investigated and compared to analogous nitroxide diradicals, to provide an insight into the degree of spin delocalization in the pi-system. Aminyl triradical will be prepared and extended to the synthesis of aminyl polyradical polymers with very large magnetic moment. The proposed chiral helical radical cations are based on nitrogen-centered radical cations of helical pi-systems, which would be configurationally persistent at room temperature. Helical radical cations are intriguing molecules with a combination of chiral pi-systems and paramagnetism. Helical, ladder-type pi-systems of n ortho-annelated aromatic rings, such as [n]helicenes, are among molecules with the strongest chiral properties. Paramagnetism of radical cations within the three-dimensional helical pi-system would render unique properties, a combination of helicene chirality and delocalized electronic spin, for development of novel paramagnetic materials with inherently strong chiral properties. Radical cations of aza-thio-[7]helicene and conjoined [5]helicene will be prepared and studied by electrochemistry, UV-vis-NIR, circular dichroism (CD), EPR spectroscopies, as well as computations.