Assessing the impact of cooperativity in spin crossover complexes by integration into polymers

• Applicants: Dr. Franziska Fennel; Professorin Dr. Stefanie Gräfe; Professor Dr. Ulrich S. Schubert; Professorin Dr. Birgit Weber
• Subject Area: Polymer Materials ; Inorganic Molecular Chemistry - Synthesis and Characterisation ; Physical Chemistry of Molecules, Liquids and Interfaces, Biophysical Chemistry ; Theoretical Chemistry: Molecules, Materials, Surfaces
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 560965699

Spin-crossover (SCO) complexes are bistable molecular complexes with two different spin configurations, i.e. high-spin and low-spin, which can be switched by external stimuli such as light, magnetic fields, pressure, temperature. In particular, Fe(II) complexes are known for such a behavior. SCO involves the re-organization of electrons; the stimuli-induced occupation of orbitals with different character (bonding or antibonding) results in a change in the metal-ligand bond length and a variation in the corresponding molecular volume of an individual SCO-complex. These structural changes of an assembly of these SCO complexes, are discussed to be the source of cooperative interactions during the SCO in the bulk, being responsible for hysteresis and bistability. If the interaction between the SCO centers is negligible, e.g., in solution, only gradual spin crossover is observed. Cooperativity can, thus, be influenced by the surrounding environment of the complexes, e.g., a polymer matrix. Cooperativity can, thus, be influenced and steered by the surrounding environment of the complexes, e.g., a polymer matrix and by the size of a SCO agglomerates. However, the question of how many of those interacting complexes are needed to achieve bistability, and how the environment influences SCO and cooperativity, has not been satisfactorily answered. In this joint experimental-theoretical project, we propose a new bottom-up approach to design and investigate assemblies of spin-crossover complexes in a (matrix) environment. By this approach both, the environment and the size of the agglomeration can be controlled, allowing to customize cooperative effects. For this purpose, various new metallopolymers containing Fe(II) SCO complexes will be synthesized. By a tailor-made design, the assembly of the SCO-complexes in the polymeric environment will be adjusted to enable or to disable agglomeration of the complexes, thereby tuning the number of SCO complexes, and, correspondingly, the degree of cooperativity. Combined with a detailed characterization of the spin-crossover properties and the underlying mechanisms by spectroscopic and theoretical approaches, it will be possible to reveal the influence of the polymer matrix on the spin-crossover properties as well as to answer the question of how many complexes are required to achieve bistability through cooperative interactions.

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