Dual stimuli spin-crossover coordination cages in solution - theory and experiment

• Applicants: Professor Dr. Arne Lützen; Professorin Dr. Anna McConnell; Professorin Dr. Maren Podewitz
• Subject Area: Inorganic Molecular Chemistry - Synthesis and Characterisation ; Theoretical Chemistry: Electronic Structure, Dynamics, Simulation
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 560861966

The phenomenon of spin-state switching (changing from a low- to a high-spin state induced by external stimuli) has fascinated researchers for almost a hundred years. While the majority of research has focused on understanding this process in the solid state, the complex interplay of intrinsic molecular and intermolecular factors, let alone solvatomorphism and polymorphism, renders a rational bottom-up approach starting from a single molecule extremely challenging. Supramolecular approaches, allowing for both precise control of an individual transition metal centre’s ligand field and mechanical coupling of these centres by bridging ligand motifs, are promising to address this challenge and even add additional functionalities. This is particularly true for oligonuclear spin cross-over (SCO) cages; they show great potential since their spin state can be changed with a number of external stimuli, such as temperature, light, or host-guest binding. However, the influence of ligand type, guest encapsulation and interaction with the surrounding solvent on the SCO properties as well as the combination of different triggers to realize orthogonal functionalities in these cages are not well understood either experimentally or computationally. The aim of our combined experimental and theoretical proposal is to elucidate and understand the factors that control the SCO properties in supramolecular complexes and implement them in the design of dual response SCO cages as new types of multifunctional spin-state switches. In complementary approaches, the groups of McConnell and Lützen will synthesize new cage structures expanding the available ligand sets and unlocking orthogonal functionalities to trigger SCO. These efforts will be corroborated by theoretical studies of the Podewitz group to elucidate molecular properties and switching mechanisms. McConnell will expand the concept of ligand-driven light-induced spin change to multinuclear cages and will work on the assembly of lower symmetry SCO cages. These efforts will cumulate in the design of low-symmetry dual-stimuli cages, that allow selective switching of individual metal sites. Lützen will synthesize next generation host-guest-responsive SCO compounds via subcomponent self-assembly. Exploiting stereogenicity of suitable subcomponents will give rise to cages with defined stereochemistry as well as modulated host-guest binding and SCO properties. Design of cages with external guest binding sites will introduce the principle of allosteric control to spin switching by inducing conformational changes in the ligand strands, triggering switching. To provide an atomistic view of the SCO cages, Podewitz will design an efficient workflow to study these complexes in explicit solvent using multiscale modelling. Benchmarking quantum chemical methods against experimental data and devising a protocol to calculate solution entropy, will yield a fast, yet accurate strategy to study the mechanism of SCO in solution.

• DFG Programme: Priority Programmes
• Subproject of: SPP 2491: Interactive Spin-State Switching
• International Connection: Austria