A tetrathiafulvalene salt of the nitrite (NO) anion: investigations of the spin-Peierls phase

Electrocrystallization of ortho -dimethyltetrathiafulvalene ( o -DMTTF) in the presence of a nitrite (NO 2 − ) anion affords a 2 : 1 phase, namely ( o -DMTTF) 2 NO 2 , the first example of an isolated cation radical salt with NO 2 − . The o -DMTTF molecule organizes into strongly dimerized stacks that adopt a rare chessboard organization, associated with a strongly 1D structure. As a consequence of dimerization, the salt exhibits a semiconducting behavior ( σ RT = 5 S cm −1 , E act = 0.1 eV). The bent NO 2 − anion is disordered on the inversion center but does not exhibit any anion-ordering transition. Combined SQUID magnetometry and electron paramagnetic resonance (EPR) experiments demonstrate the appearance of a spin-Peierls transition with T SP = 70 K. The temperature dependence of the spin gap and associated dimerization parameter δ across the spin-Peierls transition have been determined. The low-temperature narrow EPR line observed below T SP is attributable to intrinsic magnetic defects localized on the o -DMTTF molecules themselves. The broadening of the EPR line at higher magnetic fields is associated with the disordered NO 2 − anions surrounding these magnetic defects, contributing to the inhomogeneity of their g factor. The first nitrite salt of a tetrathiafulvalene derivative exhibits a rare chessboard-like organization of the conducting stacks, with a spin-Peierls transition at 70 K.