Theory of the Relaxation of Trapped Spin-States in Spin Crossover Materials: Drosophila for Complex Dynamics

Understanding the underlying factors that give rise to complex kinetic processes is of fundamental interest to many research areas, such as protein folding, photochemistry, and quantum materials. Spin crossover (SCO) materials are relatively simple, highly tunable systems that offer a unique playground to study the universal aspects of complex dynamics. Experimentally, a diverse range of relaxation dynamics of trapped spin-states are observed in SCO materials, including exponential, sigmoidal, stretched exponential, multi-step, and mixed kinetics. Here we reproduce and explain this full range of relaxation behaviours using a semi-classical model that combines crystal field theory with elastic inter-molecular interactions. We show that frustrated intermolecular interactions lead to multiple energetically competitive ordered phases even in systems that contain only one crystallographically distinct SCO site. This rugged free energy landscape leads to dynamic disorder and thence complex dynamics. We show that the same frustrated interactions are responsible for multistep thermal transitions.