Enhancing Intramolecular Ferromagnetic Coupling in Tetrathiafulvalene–Nitronyl Nitroxide‐based Compounds through Spin Polarization Mechanism

Spin-polarized-donor radicals based on tetrathiafulvalene (TTF) and nitronyl-nitroxide (NN) in which one-electron oxidation involves the HOMO instead of the SOMO are well-known for exhibiting magnetoresistance. In particular, BTBN consists of one dibromo-TTF and one NN radical, which are linked by a phenyl coupler group. One of the key factors driving magnetoresistance is the presence of intramolecular ferromagnetic (FM) coupling between the TTF+•-oxidized π-donor (D-unit) and NN (R-unit). Here, a theoretical study is conducted to assess suitable candidates with enhanced FM-coupling with respect BTBN. The study is pursued via in-silico chemical modification of the substituents of the BTBN basic functional units (D/R-radicals, C-coupler) to benefit from the spin polarization mechanism to boost the intramolecular FM coupling, aiming to minimize distortion of the BTBN π-stacking. NICSiso(1) and Wiberg's Bond-Order are analyzed to assist in identifying promising candidates, since decrease in aromaticity is expected to enhance diradical character and lead to larger magnetic coupling values. The most favorable diradical to replace BTBN results from using a hydroxyl-ethylene (–(H)C=C(OH)–) as coupler preserving BTBN original NN and TTF+•-units. The feasibility of improving the intramolecular FM-coupling between radicals is here fully illustrated, as a step towards the synthesis of new materials with (possibly) enhanced magnetoresistance properties.