Towards High Temperature Light-Induced Spin State Trapping in Spin Crossover materials: the Interplay of Collective and Molecular Effects

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 feted for numerous practical applications, but these remain largely unrealized – partly because of the difficulty of switching these materials at high temperatures. Here we introduce a semi-empirical microscopic model of SCO materials combining crystal field theory with elastic intermolecular interactions. We show that, 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, our model reproduces and explains the experimentally observed relationship between the critical temperature of the thermal transition, T1/2 , and the highest temperature for which the trapped state is stable, TLIESST. We propose strategies to design SCO materials with higher TLIESST: increasing the stiffness of the coordination sphere, 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 can all increase TLIESST. 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 experimentally.