One-step synthesis of spin crossover nanoparticles using flow chemistry and supercritical CO2

Molecular switches are increasingly implemented in functional materials for sensing applications or in molecular electronics. Among the switchable systems under study, spin crossover (SCO) materials are widely investigated with regard to their ability to change their electronic configuration in response to external stimuli (temperature, pressure, magnetic field, light, adsorption of molecules). The vast majority of SCO compounds reported so far concerns Feˡˡ and Feˡˡˡ complexes and especially the 1D coordination polymers based on 4-R-1,2,4-triazole ligands and Feˡˡ salts (SCO-triazole family). They exhibit a room-temperature (or close to) thermochromism, related to the switching between high-spin (HS, paramagnetic) and low-spin (LS, diamagnetic) states, which can result in memory effects of interest in displays and sensors.

This increasing involvement of such switchable material into various kind of devices requires the development of reliable and green synthetic processes.

The current study explored a synthesis process able to produce in one-step dried powders of switchable nanoparticles. To do so, flow chemistry was used to promote a continuous reaction between the reactants and supercritical CO2 was introduced to dry the resulting product. By doing so, the production of several grams of dried powder can be obtained in few minutes while several hours are required in a classical approach. Moreover, this process allows to drastically reduce the size of the synthesized particles (by a factor 12) without the use of any chemically polluting template.
This one-step process constitutes and important step towards the reliable production of well-defined nanoparticles, limiting the number of manipulation of nanoparticles and the use of organic surfactants. This is of importance for the green production of such materials.