Intermediate-paramagnetic phases with a half and a quarter spin entities in fluorinated biphenyl-3,5-diyl bis(tert-butyl nitroxides)

Abstract

A new ground-triplet biradical, 2′,5′-difluorobiphenyl-3,5-diyl bis(tert-butyl nitroxide), showed spin-transition-like behaviour at T_C↑ = 182 K and T_C↓ = 181 K. This compound had a quarter of the nominal amount of paramagnetic spins in the low-temperature phase and a half in the high-temperature phase. Another new 3′-monofluoro derivative maintained a half amount of the spins as a single solid phase, which is isomorphous to the high-temperature phase of the difluoro analogue. During the spin transition, a reversible bond formation–cleavage between nitrogen and oxygen atoms takes place at neighbouring nitroxide groups in an intermolecular fashion. These intermediate states emerge during multi-step spin transition from the unsymmetrical biradicals as designed.