Chiral spin crossover nanoparticles and gels with switchable circular dichroism

Abstract

Spin crossover complexes represent spectacular examples of molecular switchable materials. We describe a new approach towards homochiral coordination nanoparticles of [Fe(NH₂trz)₃](L-CSA)₂ (NH₂trz = 4-amino-1,2,4-triazole, L-CSA = L-camphorsulfonate) that display an abrupt switch of chiral properties associated with a cooperative spin transition. This is an original method that generates stable and additive-free colloidal solutions of nanoparticles with a spin transition around room temperature. The introduction of a chiral anion to the coordination framework makes these nanoparticles display specific chiro-optical (circular dichroism) properties that are different in high and low spin states. We also illustrate here an effect of the “chiral memory” which is caused by the hysteretic spin transition. In addition, a bistable chiral composite supramolecular system – a [Fe(NH₂trz)₃](L-CSA)₂–CHCl₃ gel – is described here.