Hierarchical Spin-Crossover Cooperativity in Hybrid 1-D Chains of FeII-1,2,4-trizole Trimers Linked by [Au(CN)2]- Bridges

Foremost, practical applications of spin-crossover (SCO) materials require control of the nature of the spin-state coupling. In existing SCO materials, there is a single, well-defined dimensionality relevant to the switching behavior. We show that a new material, consisting of 1,2,4-trizole-based trimers coordinated into 1-D chains by [Au(CN)2]- and spaced by anions and exchangeable guests, undergoes SCO defined by elastic coupling across multiple dimensional hierarchies. Detailed structural, vibrational, and theoretical studies conclusively confirm that intra-trimer coupling is an order of magnitude great than the intra-molecular coupling, which is an order of magnitude greater than inter-molecular coupling. As such, we have ascertained for the first time a clear hierarchy on the nature of elastic coupling in SCO materials, a necessary step for the technological development of molecular switching materials.