Enhancement of Solid‐State Reversible Photochromism by Incorporation of Rigid Steric Hindrance Groups

Abstract Solid‐state materials with reversible properties are highly attractive, owing to versatile applications in security, sensors, bioimaging, and information storage. Unfortunately, obtaining these materials in the solid state remains a huge challenge. Efficient isomerization usually requires sufficient conformational freedom and cannot be achieved without solvents or matrices. In this study, four structurally simple spiropyran derivatives are prepared that show excellent reversible photochromism under solvent‐free conditions. The large free volumes induced by different rigid aromatic ring groups enable an efficient photoswitching between closed‐ring compounds and their photoisomers. Furthermore, these compounds exhibit mechanochromic properties upon application of a mechanical force. Their color can be maintained for at least 2 days without fading under visible light irradiation at 30 °C, and can be easily erased by heating at 60 °C for 5 min. At least 30 consecutive print–erase cycles can be performed without apparent fatigue. The excellent solid‐state photochromic properties of the present compounds can find application in both UV printing and rewritable paper.