Bistable spin-state switching characteristic of a charge-neutral iron(II) complex

Spin-crossover (SCO) complexes that show abrupt and hysteretic spin-state switching characteristics—termed as bistable spin-state switching—are proposed suitable to realize molecule-based switching and memory elements. For realistic applications, spin-state switching needs to be demonstrated in the thin film state, requiring vacuum sublimation of SCO complexes to fabricate clean and impurity-free thin films. Charge-neutral iron(II) complexes are a class of SCO complexes that are reported to undergo sublimation, and their spin-state switching characteristics in the thin film state have been studied. However, hysteretic SCO in the thin film state is a scarcely observed phenomenon, requiring the development of iron(II) charge-neutral complexes that can undergo bistable spin-state switching in the bulk and thin film states. Herein, we report a new iron(II) charge-neutral complex—[Fe(H2Bpz2)24,4'-Br2-bpy] (H2Bpz2 = di¬hydro¬bis¬(pyrazol-1-yl)borate; 4,4'-Br2-bpy = 4,4'-dibromo-2,2'-bipyridine)—that undergoes abrupt and hysteretic spin-state switching in the bulk-state with T1/2 = 113 K and ΔT1/2 = 13 K at a scan rate of 0.25 K/min. The HS-to-LS switching of the complex is scan-rate-dependent, whereas the LS-to-HS switching is scan-rate-independent. Moreover, a reverse-SCO phenomenon was observed upon heating the sample in the 78 K-105 K temperature range at a scan rate of 3 K/minute. However, the reverse SCO was not observed when the complex was studied at scan rates of 1 and 0.5 K/min. Such observations indicate the presence of a kinetically trapped HS-fraction (frozen-in effect) during the HS-to-LS switching, when the sample was studied at the scan rate of 3 K/min. Crucially, the complex can be sublimed; efforts are on to elucidate the nature of SCO in the thin film state. Overall, a simple and easy to prepare sublimable complex—[Fe(H2Bpz2)24,4’-Br2-bpy]—shows bistable spin-state switching characteristics that can be leveraged to fabricate spin-state switchable thin film architectures.