The atypical hysteresis of [Fe(C6F5Tp)2]: Interplay between spin-crossovers and symmetry-breaking phase transition.

Abstract

The [Fe II (C 6 F 5 Tp) 2 ] spin-crossover complex is an atypical molecular switch, which can be converted upon annealing between two archetypal spin-crossover behaviors: from an extremely gradual spin-crossover to a broad hysteretic spin-transition (of ca . 65 K). The hysteresis shows an uncommon rounded shape that is reproducible upon cycling temperature. In depth structural studies reveal a first crystal phase transition, which occurs upon melting and recrystallizing at high temperature. This first irreversible transition is associated to a radical change in the crystal packing. More importantly, the rounded and broad hysteretic transition is shown to occurs in a non-cooperative SCO system and is associated to the occurrence of a symmetry-breaking phase transition that appears when roughly ca . 50% of SCO are switched.