The quest for spin crossover molecules for low-power nonvolatile memory applications

A spin crossover molecular complex consists of a transition metal atom core, usually with a 3d4 to 3d7 electronic configuration, that can be switched from its low-spin to high-spin state with an external perturbation. Of relevance to this talk is the voltage control of spin-state switching leading to nonvolatile conductance change, which shows promise for applications in high-density, low-power nonvolatile memory. Yet, there is little fundamental understanding of how to reliably design a spin crossover complex with low enough resistance while at the same time retaining a very high (~10⁴) ON/OFF ratio. In this talk, I will summarize recent understanding of the characteristics of spin crossover complexes, as single molecules, in organized layers, and as supported on functional low dimensional materials and organic ferroelectrics. The interactions of spin crossover complexes with substrates will be explored with the goal of unraveling the effect of ferroelectric polarization on the molecular spin states. Through examples of a handful of “good SCO complexes” such as the Fe(II) complex Fe[H₂B(Pz)₂]₂(bipy), some possible descriptors of “goodness” will be presented that may accelerate the discovery of novel spin crossover complexes. Could we design redox-active ligands for substantial charge delocalization and enhanced intermolecular charge transfer in the solid state that may promote high conductivity in the ON state of the device? Could AI help us discover new “good SCO complexes” that would make molecular electronics with spin crossover complexes feasible?  Answers to these and related questions will be discussed.