Synthesis, X-Ray Structure, Magnetic Properties, and a Study of Intra/Intermolecular Radical–Radical Interactions of a Triradical TEMPO Compound** Vega Lloveras, [a] Elena Badetti, [a, c] Klaus Wurst, [b] and JosØ Vidal-Gancedo* [a] 1. Introduction In the field of molecular magnetic materials, the study of the magnetic behavior of polyradical systems has attracted wide- spread interest in recent years. Triradicals are relatively rare compared with diradicals, which have received constant atten- tion over the years, and only a few higher polyradicals are known, most of which are contained within a dendritic struc- ture. [1, 2] Other recently reported polyradical structures are or- ganic radicals anchored on gold (111) forming SAMs, on gold nanoparticles or forming other supramolecular species such as metal-organic open-framework structures or others. [3] Studies on the interactions between radicals is very important to un- derstand the magnetic properties of these functionalized sys- tems. Magnetic interactions between radicals in such systems determine the most important properties of these kinds of species in applications such as conducting systems, [4] magnetic nanomaterials, [3, 5] and contrast agents for nuclear magnetic resonance imaging (MRI). [6] In particular, polynitroxides can work as organic ferromagnets, contrast agents in MRI, labels in electron magnetic resonance imaging, and radiation protectors during whole-brain radiotherapy, among others. [7] When un- paired electrons are in close proximity, the dominant interac- tion is likely to be spin-exchange coupling, mediated either by the collision of radicals or through-space and/or through- bonds. [8] The origin of such radical–radical interactions could be intra- and/or intermolecular; that is, between radicals of the same system and/or radicals anchored in different species. It is important to be able to differentiate between intra- and inter- molecular magnetic interaction pathways among the radicals operating in such polyradical species because the net magnet- ic behavior and hence their properties could be entirely differ- ent. [9] Intermolecular interactions exist only at high concentra- tions. However, intramolecular interactions exist when radical units within a structure are close enough, at both low and high concentrations, and the extent of the interaction depends on the distance between the radicals in the molecule, their dis- tribution, and the dynamics of the different branches. Electron paramagnetic resonance (EPR) spectroscopy permits us to de- termine whether radical–radical interactions are intra- and/or intermolecular by following the half-field transition band inten- sity at different concentrations. [10] Thus, it is important to know the concentration below which we can be sure that only intra- molecular interactions are observed and above which intermo- lecular interactions start to play a role. To determine this con- [a] Dr. V. Lloveras, + Dr. E. Badetti, + Dr. J. Vidal-Gancedo Institut de Cincia de Materiales de Barcelona (ICMAB-CSIC) Campus UAB s/n E-08193 Cerdanyola del Valls (Spain) and CIBER de Bioingeniería, Biomateriales y Nanomedicina (CIBER-BBN) Barcelona (Spain) E-mail : j.vidal@icmab.es [b] Dr. K. Wurst Institute of General, Inorganic and Theoretical Chemistry University of Innsbruck Center for Chemistry and Biomedicine Innrain 80-82 A-6020 Innsbruck (Austria) [c] Dr. E. Badetti + Dipartimento di Scienze Chimiche Università degli Studi di Padova Via Marzolo, 1 35131, Padova (Italy) [ + ] These authors contributed equally. [**] TEMPO = 2,2,6,6-tetramethylpiperidine-1-oxyl Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cphc.201500462. A novel triradical compound with a P=S core and three branches functionalized with 2,2,6,6-tetramethylpiperidine-1- oxyl (TEMPO) radicals is synthesized and characterized by IR, 1 H NMR, 31 P NMR, and EPR spectroscopy and MALDI-TOF mass spectrometry, and its chemical structure is confirmed by X-ray diffraction analysis. The triradical shows neither spin exchange interactions between its radical units nor detectable dipolar in- teractions. This is consistent with the separation between the radical units found in its X-ray diffraction structure, and dis- counts the existence of intramolecular interactions. This con- clusion is confirmed by an EPR concentration study. The con- centration at which intermolecular interactions start to appear is determined (5  10 3 m) and this concentration should be taken into account as a higher concentration limit when stud- ies on intramolecular radical–radical interactions in polyradicals with similar structure are required. SQUID magnetometry anal- ysis of the compound shows antiferromagnetic interactions be- tween the spin carriers of different molecules; that is, antiferro- magnetic intermolecular interactions. ChemPhysChem 2015, 16, 3302 – 3307 # 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 3302 Articles DOI: 10.1002/cphc.201500462