Photochromism of Neutral Spiropyran in the Crystalline State at Room Temperature

Solid-state photochromic materials are very attractive due to their promising future in advanced functional materials with reversible and tunable optical properties. However, the development of photochromic material in the solid state, especially in the crystalline state, is still a great challenge due to dense molecular packing. In this study, a solid-state optical switch based on the methanol-esterified spiropyran derivative S1 is constructed, which exhibits reversible photochromic properties in the solid-state product without further recrystallization. Moreover, the cultured single crystal (S1-C) can unexpectedly undergo photochromism at room temperature, which is a very rare example in a neutral spiropyran crystal. The crystallographic analysis reveals that weak van der Waals forces dominate in the crystal of S1-C, resulting in a relatively loose packing mode and photochromic properties. Mechanical force can destroy the restricted environment of the crystalline state and allow the S1 compound to achieve more efficient photochromism. Our study opens up an avenue for relating the molecular structure/packing mode to photochromic properties. Consequently, by utilizing the reversible and efficient photochromic properties, we successfully demonstrate the application of S1 as optical printing materials.