Switchable Molecular Materials – From Structure to Function

Many crystalline materials undergo displacive phase transitions that result in changes to their physical properties. The Spin Crossover (SCO) phenomenon leads to a redistribution of electrons within the d-orbitals of some transition metal complexes as a result of an external perturbation. The transition between high spin (HS) and low spin (LS) states involves a significant change in volume and is often cooperative in crystalline materials, resulting in dramatic changes in the optical, mechanical and magnetic properties. Single crystal X-ray diffraction techniques at elevated pressure and variable temperatures, complimented by spectroscopic and magnetic studies, have been used to probe the structure-properties relationship of a series of molecular FeII derivatives. A range of fascinating pressure-induced behaviours has been observed providing unprecedented insight into the driving forces behind the dynamic solid state processes. Examples include negative linear compression that is structurally antagonistic to the requirements of SCO [1] and pressure-induced stepped SCO accompanied by symmetry breaking [2].
While we have been observing the structural changes in these systems for decades, no one has previously exploited the huge spontaneous strain that accompanies the spin transition for application. This presentation also describes a recently developed proof-of-concept actuator device made from anisotropic single crystals of a SCO framework material [3]. Macroscopic motion of the device is shown to be a direct consequence of the huge volume change associated with the spin state switching. Dynamic actuation of the device may be induced by light irradiation or changes in temperature. By extending the concept to polycrystalline SCO materials embedded in a polymer matrix, it is possible to employ virtually any SCO complex as the active component of the actuator, making such a platform extremely versatile [4].