Theory of zwitterionic molecular-based organic magnets William A. Shelton a , Edoardo Aprà a , Bobby G. Sumpter a,⇑ , Aldilene Saraiva-Souza b , Antonio G. Souza Filho b , Jordan Del Nero c , Vincent Meunier a,d a Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA b Departamento de Física, Universidade Federal do Ceará, 60455-900, Fortaleza, Ceará, Brazil c Departamento de Física, Universidade Federal do Pará, 66075-110, Belém, Pará, Brazil d Department of Physics, Applied Physics, and Astronomy, Rensselaer Polytechnic Institute, Troy, NY 12180-3590, USA article info Article history: Received 22 February 2011 In final form 13 June 2011 Available online 16 June 2011 abstract We describe a class of organic molecular magnets based on zwitterionic molecules (betaine derivatives) possessing donor, p bridge, and acceptor groups. Using extensive electronic structure calculations we show the electronic ground-state in these systems is magnetic. In addition, we show that the large energy differences computed for the various magnetic states indicate a high Neel temperature. The quantum mechanical nature of the magnetic properties originates from the conjugated p bridge (only p electrons) in cooperation with the molecular donor-acceptor character. The exchange interactions between electron spin are strong, local, and independent on the length of the p bridge. Ó 2011 Elsevier B.V. All rights reserved. 1. Introduction Although magnetic materials are most commonly associated with metals, metal oxides, and rare earths, a limited number of molecular-based magnets have been reported [1]. These are typi- cally composed of organic radicals [2] or mixed coordination com- pounds containing bridging organic radicals, Prussian blue compounds, and charge transfer complexes [3]. Purely organic molecules (those based on only p electrons) are rarely found to possess magnetic properties since all the electrons in carbon-based materials generally participate in covalent bonds [4]. In addition, for the relatively few organic materials that possess magnetic properties, the Curie temperature (T c ) is generally very low (<36 K) and these materials are typically crystalline. It follows that molecular-based magnetic materials with high T c usually contain a number of metallic atoms. In those compounds, the metallic atoms provide the localized magnetic moment while the organic mole- cule acts as an exchange pathway [5]. In the present Letter we use quantum mechanical modeling to show that a particular class of organic molecules, containing no radicals or metals, can display a stable magnetic ground state (difference between non-magnetic and ferromagnetic states), and in the present case a high Neel tem- perature (magnetic ordering is an anti-ferrimagnetic ground- state). Metal-free molecular magnets offer important benefits in terms of cost, weight, and efficiency. For these reasons, the pursuit to- wards their discovery and development has continued to blossom over the last two decades. The first organic magnet was discovered in 1991: it is a bulk crystal composed of a derivative of nitronyl nitroxide that contained radicals [6]. This system, while magnetic, has a very low Curie temperature (T c = 0.65 K). Fullerene-based charge transfer salts have also shown some promise as magnetic materials, but are also characterized by low Curie temperatures (e.g., T c = 16 K for C 60 -TCNQ) [7]. One of the highest Curie temper- atures reported, 36 K, is found in a molecular crystal containing sulfur free radicals [8]. Although organic free radicals are natural candidates for magnetic materials, very few are stable enough to be used as magnetic devices. This seriously limits the development of this class of promising molecules and further discoveries along this path have been very sparse. Heat-treated organic systems are examples of promising organ- ic magnets. Unfortunately, it has been very difficult to establish their intrinsic magnetic character [9]. There have been a number of studies on a polymerized form of C 60 in the rhombohedral phase with weak ferromagnetism characterized by a Curie temperature as high as 500 K [10]. Ab initio calculations initially showed that the origins of the ferromagnetism could be attributed to symmet- rical vacancies where H plays an important role in the coupling of these ferromagnetic defects [11,12]. However, it was subsequently discovered that the polymerized C 60 actually contained consider- able iron content (cementite) and that the pure rhombohedral phase is not magnetic [13]. Other allotropes of carbon such as car- bon nanofoams have also been reported to exhibit magnetic prop- erties at room temperature but these particular materials have very short-lived magnetic properties (on the time scale of hours) [14]. Boron nitride nanotubes (BN) with single atom carbon doping were predicted to be magnetic [15]. In general, non-metallic atom 0009-2614/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2011.06.028 ⇑ Corresponding author. Address: Oak Ridge National Laboratory, Computer Science and Mathematics Division, 1 Bethel Valley Rd, Oak Ridge, TN 37831 6164, USA. E-mail address: sumpterbg@ornl.gov (B.G. Sumpter). Chemical Physics Letters 511 (2011) 294–298 Contents lists available at ScienceDirect Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett