105 K Wide Room Temperature Spin Transition Memory Due to a Supramolecular Latch Mechanism

Abstract

Little is known about the mechanisms behind the bistability (memory) of molecular spin transition compounds over broad temperature ranges (>100 K). To address this point, we report on a new discrete Fe II neutral complex [Fe II L 2 ] 0 ( 1 ) based on a novel asymmetric tridentate ligand 2-(5-(3-methoxy-4 H -1,2,4-triazol-3-yl)-6-(1 H -pyrazol-1-yl))pyridine (L). Due to the asymmetric cone-shaped form, in the lattice, the formed complex molecules stack into a one-dimensional (1D) supramolecular chain. In the case of the rectangular supramolecular arrangement of chains in methanolates 1-A and 1-B (both orthorhombic, Pbcn ) differing, respectively, by bent and extended spatial conformations of the 3-methoxy groups (3MeO), a moderate cooperativity is observed. In contrast, the hexagonal-like arrangement of supramolecular chains in polymorph 1-C (monoclinic, P 2 1 / c ) results in steric coupling of the transforming complex species with the peripheral flipping 3MeO group. The group acts as a supramolecular latch, locking the huge geometric distortion of complex 1 and in turn the trigonal distortion of the central Fe II ion in the high-spin state, thereby keeping it from the transition to the low-spin state over a large thermal range. Analysis of the crystal packing of 1-C reveals significantly changing patterns of close intermolecular interactions on going between the phases substantiated by the energy framework analysis. The detected supramolecular mechanism leads to a record-setting robust 105 K wide hysteresis spanning the room temperature region and an atypically large T LIESST relaxation value of 104 K of the photoexcited high-spin state. This work highlights a viable pathway toward a new generation of cleverly designed molecular memory materials.

An iron(II) complex shows bistable spin-state switching characteristic with T1/2 centred at room temperature

“…Such distortion in the crystal lattice is known to induce spin-state switching with a wide hysteresis loop, as reported for the BPP-based 9,23 and related systems. 4 Spin-state switching characteristics of 1•CH3CN…”

“…Molecular materials that show switching of a physical property-for example, magnetic [1][2][3][4][5][6][7][8][9][10][11][12] or electric [13][14][15][16][17] -accompanied with hysteresis are candidates desirable for applications. Several classes of magnetic-molecular systems such as organic radicals, 18,19 single-molecule magnets (SMMs), and spin-crossover complexes are all known to show bistable switching characteristic.…”

“…However, to the best of our knowledge, SCO complexes are the only class of molecules that frequently show bistable switching near room-temperature (RT). 4,12 For realistic applications, SCO complexes showing a sizable thermal hysteresis width (ΔT1/2)-around 70 K-with switching centred around RT are required. 4 However, preparing an SCO complex that can undergo hysteretic switching around RT is not a trivial task because spin-state switching in the solid-state is controlled by a myriad of factors, which are system-dependent.…”

“…4,12 For realistic applications, SCO complexes showing a sizable thermal hysteresis width (ΔT1/2)-around 70 K-with switching centred around RT are required. 4 However, preparing an SCO complex that can undergo hysteretic switching around RT is not a trivial task because spin-state switching in the solid-state is controlled by a myriad of factors, which are system-dependent. Therefore, it is necessary to conduct a systematic search starting from a basic ligand skeleton featuring a moderate ligand field allowing spin-state switching of a coordinated metal ion, iron(II) in the context of this study.…”

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

“…First-row transition metal complexes featuring d 4 -d 7 electronic configuration, especially iron(II) complexes, that show reversible interconversion between the low-and high-spin states-termed as spin-crossover (SCO)-are a class of switchable materials that can undergo abrupt and hysteretic -bistable-spin-state switching suitable for device applications. [1][2][3][4] For practical device applications, hysteretic switching needs to be realized in the thin film state. Preparation of thin films of SCO complexes requires solution processing or vacuum sublimation.…”