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

Spin-crossover (SCO) materials display many fascinatingbehaviors including collective phase transitions and spin-state switchingcontrolled by external stimuli, e.g., light and electrical currents. As single-molecule switches, they have been fe??ted for numerous practicalapplications, but these remain largely unrealized-partly because of thedifficulty of switching these materials at high temperatures. We introducea semiempirical microscopic model of SCO materials combining crystalfield theory with elastic intermolecular interactions. For realisticparameters, this model reproduces the key experimental results includingthermally induced phase transitions, light-induced spin-state trapping(LIESST), and reverse-LIESST. Notably, we reproduce and explain theexperimentally observed relationship between the critical temperature ofthe thermal transition,T1/2, and the highest temperature for which the trapped state is stable,TLIESST, and explain why increasing thestiffness of the coordination sphere increasesTLIESST. We propose strategies to design SCO materials with higherTLIESST: optimizingthe spin-orbit coupling via heavier atoms (particularly in the inner coordination sphere) and minimizing the enthalpy differencebetween the high-spin (HS) and low-spin (LS) states. However, the most dramatic increases arise from increasing the cooperativityof the spin-state transition by increasing the rigidity of the crystal. Increased crystal rigidity can also stabilize the HS state to lowtemperatures on thermal cycling yet leave the LS state stable at high temperatures following, for example, reverse-LIESST. We showthat such highly cooperative systems offer a realistic route to robust room-temperature switching, demonstrate thisin silico, anddiscuss material design rationale to realize this.