Spin‐Crossover Materials

Abstract Smart materials respond to their environment as illustrated by photochromic eyeglasses, that darken upon exposure to ultraviolet light to attenuate additional ultraviolet light. Hence, materials that have fast reversible responses to environmental stimuli are sought as components of smart systems. Similar to photochromic materials, thermochromic materials reversibly respond to heat and exhibit substantial color changes upon small changes in temperature. Spin‐crossover materials are a class of thermochromic materials that possess fast, reversible color changes amenable to display and memory devices. These color changes can also be induced by light (photochromic) or pressure (piezochromic) as well as heat. Due to the nature of the mechanism of their thermo‐, photo‐, or piezochromic responses (i.e., redistribution of the electron density at a metal ion site within the molecule), they are extremely fast and reversible. As a consequence of the fast color change, strong contrast between colors, and the intermolecular interactions within the solid, the differing colors can be maintained for a long period of time, and due to the lack of moving parts, these materials are completely recyclable and amenable to fast, low power‐consuming, high‐data‐density display and storage devices and “smart” materials and systems of the future.