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

Electrocrystallization of ortho-dimethyltetrathiafulvalene (o-DMTTF) in the presence of a nitrite (NO2-) anion affords a 2 : 1 phase, namely (o-DMTTF)(2)NO2, 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 (sigma(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 delta 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.