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Design and fine-tuning of magnetic properties in organic salts of semiquinone radical

Semiquinones are a class of stable organic radical anions ; especially stable are ones with four electronegative substituents (Fig. 1a), which enhance delocalisation of the unpaired electron. Therefore, they are potential candidates for design of functional materials with fine-tuned magnetic properties. Semiquinoid anions in crystals are usually stacked by π-interactions, and their magnetic properties depend on their interplanar distance. Most often, the radicals form closely bound dimers with coupled spins (type I, Fig 1c) [1, 2], resulting in diamagnetic properties. However, crystal engineering can be used to enlarge the ring separation distance with equidistant-ring stacks generating antiferromagnetic [3] or paramagnetic crystals (type II, Fig 1c). Our research has been aimed to obtain stable semiquinone systems with fine-tuned magnetic properties by crystal engineering: we varied size of substituents on the quinoid ring (Fig. 1a) and aromatic organic cations (Fig. 1b). Therefore, we varied substituents on the semiquinone ring: tetrachloro- (CA), tetrabromo- (BA) and 2, 3-dicyano-5, 6-dichlorosemiquinone (DDQ) were used as anions. Aromatic, approximately planar organic cations [pyridinium (py), N-methylpyridinium (N-MePY) and diquat] were used to check for possible π-interactions between cations and anions. Crystallographic, magnetic (EPR, SQUID), electrical (impedance spectroscopy) and thermal (TG/DTA, DSC) methods were used to obtain data on structure, stability, magnetic and electrical properties of the crystalline compounds. A pair of analogous salts, N-MePy∙CA and N-MePy∙BA is especially interesting: in CA salt closely bound radical dimers occur, similar to previously known K∙CA∙Me2CO [1] and similar alkali salts of DDQ [2], and the crystals are diamagnetic due to spin pairing. In BA salt radicals are equdistant, similar to high-temperature polymorph of K∙CA∙MeCOEt [3] ; due to larger interplanar distances, the crystals are paramagnetic. To gain insight into spin coupling/decoupling and transport properties, impedance spectroscopy was done on single crystals with electrical contacts applied in the direction of stacking. [1]Molčanov, Kojić-Prodić, Babić, Žilić, Rakvin, CrystEngComm, 13 (2011), 5170. [2] Molčanov, Babić, Kojić-Prodić, Stare, Maltar-Strmečki, Androš, Acta Crystallogr. B, 70 (2014), 181. [3] Molčanov, Kojić-Prodić, Babić, Pajić, Novosel, Zadro, CrystEngComm, 14 (2012), 7958.

semiquinone radical ; magnetic properties ; crystal structure ; conductivity

Djelomično financirala Zaklada HAZU (za 2014. godinu) i hrvatsko-francuska bilaterala - Cogito.