Manipulating Metal-to-Metal Charge Transfer for Materials with Switchable Functionality

Metal-to-metal charge transfer (MMCT) describes electron transfer between metal ions, to generate valence isomers with markedly different electronic configurations. In particular, MMCT changes the spin states of single-metal sites and the coupling interactions between them, while also changing the symmetry in charge distribution. The result is a drastic change in both magnetic and electric properties of the affected material. Moreover, MMCT causes significant variation in bond length and absorption spectra, and induces unusual thermal expansion and photochromic behavior. Thus, materials demonstrating MMCT in response to external stimuli are excellent candidates for switchable multifunctional devices with synergistic responses. In this Minireview, recent progress in utilizing MMCT units as actuators to tune magnetic, electric, thermal expansion, and photochromic properties in cyanide-bridged systems is highlighted, and emphasis is given to the remaining challenges and future perspectives in the field.