Toward High-Temperature Light-Induced Spin-State Trapping in Spin-Crossover Materials: The Interplay of Collective and Molecular Effects

Abstract

Spin-crossover (SCO) materials display many fascinating behaviors including collective phase transitions and spin-state switching controlled by external stimuli, e.g., light and electrical currents. As single-molecule switches, they have been fêted for numerous practical applications, but these remain largely unrealized–partly because of the difficulty of switching these materials at high temperatures. We introduce a semiempirical microscopic model of SCO materials combining crystal field theory with elastic intermolecular interactions. For realistic parameters, this model reproduces the key experimental results including thermally induced phase transitions, light-induced spin-state trapping (LIESST), and reverse-LIESST. Notably, we reproduce and explain the experimentally observed relationship between the critical temperature of the thermal transition, T 1/2, and the highest temperature for which the trapped state is stable, T LIESST, and explain why increasing the stiffness of the coordination sphere increases T LIESST. We propose strategies to design SCO materials with higher T LIESST: optimizing the spin–orbit coupling via heavier atoms (particularly in the inner coordination sphere) and minimizing the enthalpy difference between the high-spin (HS) and low-spin (LS) states. However, the most dramatic increases arise from increasing the cooperativity of the spin-state transition by increasing the rigidity of the crystal. Increased crystal rigidity can also stabilize the HS state to low temperatures on thermal cycling yet leave the LS state stable at high temperatures following, for example, reverse-LIESST. We show that such highly cooperative systems offer a realistic route to robust room-temperature switching, demonstrate this in silico, and discuss material design rationale to realize this.

Understanding kinetically controlled spin transitions in bistable spin crossover materials

“…To explain the molecular contribution to the T 0 lines, the rigidity of the coordination sphere has been proposed, and it has been related to the frequency of the breathing VNM. 67 This is in agreement with our results that the Fe–N breathing mode is the most important VNM to find the MECP. What remains to be seen is whether the rigidity depends exclusively on the ligand denticity or if any other ligand properties play a role.…”

“…25). Overall, the combination of advanced physical models, 67 and atomistic approaches on real systems such as this one, will enable a better understanding of the T 1/2 vs. T LIESST empirical relationships governing SCO complexes. Particularly, to understand how the different T 0 lines are related to the MECP, and how this MECP relates to the VNM of complexes with different structures, chemical compositions, and denticities.…”

Electrically Detectable Photoinduced Polarization Switching in a Molecular Prussian Blue Analogue

“…Along this consideration, by proper molecular design such as introduction of rigidity to the inner coordination sphere, it is feasible to stabilize the photoinduced MS states at elevated temperatures, potentially reaching room temperature. 63 Remarkably, in the molecular Prussian Blue [Fe 4 Co 4 ] cage, such states have been maintained up to 180 K. 50 On the other hand, the polar to nonpolar transition in FeRAM devices is closely mimicked in a light-controllable and noncontact manner in our system. Our findings represent a proof of concept for using polar ETCST crystals as the photoresponse motif in polarization switching devices.…”

“…While this report primarily focuses on the current signal associated with light-induced polarization switching at 10 K, the operational temperature can be extended to the conditions under which the MS states are sustained. Along this consideration, by proper molecular design such as introduction of rigidity to the inner coordination sphere, it is feasible to stabilize the photoinduced MS states at elevated temperatures, potentially reaching room temperature . Remarkably, in the molecular Prussian Blue [Fe 4 Co 4 ] cage, such states have been maintained up to 180 K .…”

Transition Temperature for Spin-Crossover Materials with the Mean Value Ensemble Hubbard-U Correction

“…A contemporary experimental goal is to find appropriate combinations of transition metals and ligands to give a T 1/2 value at or close to room temperature. − Pursuit of operation at or near ambient temperature, rather than low temperatures, is motivated by the prospect of myriad applications of these materials as actuators, displays, sensors, memory storage, magnetic resonance imaging, spintronics, or molecular electronics. − Understanding the effects of the physical and chemical characteristics of the ligand choice upon the spin transition is key to guiding the design of materials with such tailored properties …”