Matrix-free synthesis of spin crossover micro-rods showing a large hysteresis loop centered at room temperature.

An original simple homogeneous acid medium was used to synthesize polymer/surfactant-free acicular micro-rod particles (10-40 μm long by 0.1-0.3 μm diameter) of the novel [Fe(Htrz)3](CF3SO3)2 complex. The study of the spin crossover properties reveals a rare 50 K hysteretic behavior perfectly centered at room temperature accompanied by a pronounced thermochromism effect, purple in the low spin state and white in the high spin state.