Nitrogen-Centered Radicals

内布拉斯加大学林肯分校化学系的Andrzej Rajca教授将研究与新型磁性和光学技术发展相关的有机分子和大分子的结构和性质关系，该项目由化学学部化学结构、动态和机制B项目资助。本研究的目的是合成和研究高自旋氮中心自由基（氨基基）和手性螺旋自由基阳离子。具有强全键铁磁偶联和在室温下具有持久性的胺基对开发轻质软有机磁体具有重要意义。此外，这种有机自由基可以促进无金属磁共振成像（MRI）顺磁造影剂、顺磁弛豫增强核磁共振（NMR）波谱剂以及自旋电子学材料的发展。螺旋自由基阳离子具有一个被限制在螺旋上的未配对电子，可以促进新的有机磁光材料和器件的发现。这个跨学科的研究项目涉及多步骤有机合成，有机分子的各种物理特性和计算，因此非常适合各级科学家的教育，有特殊的机会获得广泛的教育，并发展广泛的技能。这个群体也处于有利地位，可以为科学领域代表性不足的学生提供最高水平的教育和培训。所提出的氨基基是高自旋自由基，基于氮中心自由基的合成pi系统，通过自由基中心的空间屏蔽和同位素取代来防止自由基的衰变。所提出的自由基将采用现代有机合成方法制备，并通过电子自旋共振（EPR）光谱、超导量子干涉装置（SQUID）磁强计和计算进行表征。在电子自旋和室温下的持久性之间具有很强的对铁磁耦合的氨基基双自由基，其半衰期允许自由基的分离，将被研究并与类似的氮氧化物双自由基进行比较，以提供对pi系统中自旋离域程度的深入了解。胺基三自由基将被制备并扩展到具有非常大磁矩的胺基多自由基聚合物的合成。所提出的手性螺旋自由基阳离子是基于螺旋pi体系的氮中心自由基阳离子，其构型在室温下是持久的。螺旋自由基阳离子是一种具有手性系统和顺磁性的有趣分子。n邻环芳香环的螺旋阶梯型pi-体系，如[n]螺旋烯，是具有最强手性的分子之一。三维螺旋pi-体系中自由基阳离子的顺磁性将呈现出独特的性质，螺旋手性和离域电子自旋的结合，为开发具有固有强手性的新型顺磁性材料提供了条件。利用电化学、紫外-可见-近红外光谱、圆二色性（CD）、EPR光谱和计算方法，制备了氮杂硫-[7]螺旋烯和连接[5]螺旋烯的自由基阳离子。

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.