A tetrathiafulvalene salt of the nitrite (NO2−) anion: investigations of the spin-Peierls phase

Abstract

Electrocrystallization of ortho-dimethyltetrathiafulvalene (o-DMTTF) in the presence of a nitrite (NO₂⁻) anion affords a 2 : 1 phase, namely (o-DMTTF)₂NO₂, the first example of an isolated cation radical salt with NO₂⁻. 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 NO₂⁻ 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 NO₂⁻ anions surrounding these magnetic defects, contributing to the inhomogeneity of their g factor.