Solid phase-change materials exhibiting barocaloric effects (BCEs)—adiabatic temperature or isothermal entropy changes in a material induced by hydrostatic pressure—have gained considerable interest as of late, presenting barocaloric cooling as a promising alternative to conventional cooling technologies. The design of barocaloric materials focuses on capturing pressure-induced reversible transformations with large entropy changes, small hysteresis, and a high degree of pressure sensitivity. Spin-crossover (SCO)—an electronic spin-state switch due to an external stimulus—presents a compelling phase-change mechanism for realizing large and reversible BCEs. Here, I will present a molecular SCO complex that undergoes a thermally induced, reversible spin-state transformation from an <i>S</i> = 0 to <i>S</i> = 2 electronic state accompanied by a large change in volume and entropy. High-pressure DSC (0-150 bar) measurements offer insight into the pressure sensitivity, thermodynamics, and cyclability of the SCO transition and will be discussed in detail. I will further highlight (quasi)isothermal pressure cycling experiments that reveal how reversible pressure-induced SCO is accessible using modest, industrially feasible pressures, underscoring its candidacy for barocaloric cooling. Lastly, I will briefly touch on our efforts in using synthetic chemistry to systematically tune the ligand field such that the SCO temperature and thermodynamics can be predictably manipulated.

Giant barocaloric effects in a molecular spin-crossover compound

ACS Spring 2022

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