Spin Crossover Phenomena in Coordination Complexes

Spin crossover in coordination complexes arises when certain transition metal centres, notably iron(ii), adopt two distinct electronic configurations in response to external stimuli such as temperature, pressure or light. In the high-spin state unpaired electrons occupy higher-energy orbitals, yielding larger metal–ligand distances, stronger paramagnetism and characteristic optical signatures; in the low-spin state electrons pair in lower-energy orbitals, resulting in shorter bond lengths, diamagnetism or weak paramagnetism and altered absorption spectra. The balance between these states is dictated by the ligand field strength and by intramolecular and intermolecular interactions that promote cooperativity, often leading to abrupt transitions and thermal hysteresis. Embedding spin-crossover units into extended frameworks or nanostructures harnesses their bistability for sensing, data storage, molecular electronics and spintronics. Modulation of the spin transition temperature and cooperativity has been achieved through rational ligand design, control of supramolecular architecture and guest-molecule interactions, yielding tailored switchable materials operable at or near room temperature. Ongoing research explores multi-functional materials combining spin crossover with photoluminescence, dielectric switching and other emergent properties, underlining the global significance of this versatile class of molecular switches.

Research from Nature Portfolio

Recent studies have introduced mechanically interlocked spin-crossover frameworks that combine fluorescence emission with dielectric switching. In one case, catenated metal–organic frameworks assembled from a naphthalene diimide-based ligand, coinage metal cyanides and iron(ii) centres exhibit both thermal and light-induced spin crossover. Upon switching, synchronous increases in emission intensity and dielectric constant were detected, demonstrating a dual-functional bistable material. Another investigation of two-dimensional spin-crossover layers on metallic substrates employed scanning tunnelling microscopy and vibrational spectroscopy to distinguish high-spin and low-spin states at the molecular scale. Light-driven collective transformations between mixed spin-state phases revealed distinct dynamical regimes and opened pathways for true molecular-scale control of spin crossover in two dimensions.

Spin Crossover Phenomena in Coordination Complexes publication trend

The graph below shows the total number of articles in spin crossover phenomena in coordination complexes across all publications each year (not limited to Nature Index journals).

Technical terms

Spin crossover (SCO): A reversible transition between high-spin and low-spin electronic configurations in a coordination complex driven by external stimuli.

High-spin state: An electronic arrangement with maximum number of unpaired electrons, resulting in elongated metal–ligand bonds and strong paramagnetism.

Low-spin state: An electronic arrangement with paired electrons in lower-energy orbitals, yielding shorter bonds and weakened or diamagnetic behaviour.

Cooperativity: Interactions between spin-crossover centres that lead to concerted, often abrupt transitions and thermal hysteresis.

Thermal hysteresis: A temperature range over which the transition between spin states shows a lag on heating and cooling, indicative of bistability.

Metal–organic framework (MOF): A crystalline network of metal ions or clusters coordinated by organic ligands, offering high porosity and tunable properties.

References

1. Integrating spin-dependent emission and dielectric switching in FeII catenated metal-organic frameworks. Nature Communications (2024).
2. Molecular-scale dynamics of light-induced spin cross-over in a two-dimensional layer. Nature Communications (2016).
3. Hybrid nanostructures based on gold nanoparticles and functional coordination polymers: Chemistry, physics and applications in biomedicine, catalysis and magnetism. Coordination Chemistry Reviews (2023).
4. Sublimable Spin‐Crossover Complexes: From Spin‐State Switching to Molecular Devices. Angewandte Chemie International Edition (2020).
5. Increasing spin crossover cooperativity in 2D Hofmann-type materials with guest molecule removal. Chemical Science (2018).