Spin crossover (SCO) in transition metal complexes leads to large changes in the magnetic, structural, and optical properties of the material, due to drastic difference in the characteristics of the low-spin and high-spin electronic configurations. These large changes render SCO materials appealing targets for applications in sensors, nanothermometers, memory devices, etc. Minituarization of electronic devices has caused an increased interest in the preparation of thin or ultrathin structures based on SCO complexes. The central challenges in these studies are the ability to deposit the SCO complex by vapor sublimation, spin coating, or exfoliation, and the preservation of the SCO behavior in the ultrathin structure. To address these challenges, we use an asymmetric ligand that retains the possibility for strong intermolecular interactions on one side of the complex, to preserve the cooperative SCO behavior, and uses bulky tertbutyl groups on the other side, to increase the volatility. We show that an Fe(II) complex, Fe[(tBu)2qsal]2, exhibits crystal packing in which layers of neutral SCO molecules are separated by van der Waals interactions between the t-Bu groups. As a result, the complex easily sublimes at 300 °C and 10–4 Torr. Furthermore, the weak interlayer forces make it possible to mechanically exfoliate this material down to a single molecular layer thickness. We demonstrate that the abrupt hysteretic SCO observed in the bulk crystal is also preserved in the thin films and ultrathin flakes. The differences in the behavior of the bulk sample and thin-film structures will be also discussed.

Asymmetric ligand to design volatable and exfoliable of spin-crossover complexes

ACS Spring 2022

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