Photoswitchable Azobispyrazole Crystals Achieving Near-Quantitative Crystalline-State Bidirectional <i>E</i> ⇆ <i>Z</i> Conversions

Azo molecules, being extensively studied as photoswitches, have demonstrated versatile photoswitching performance and applications in solution-phase systems. However, the dense molecular packing and insufficient conformational freedom in the solid/crystalline state typically pose a challenge to their <i>E</i> ⇆ <i>Z</i> isomerization. This study presents a breakthrough in solid-state azo chemistry, where the investigated azobispyrazole molecules are capable of achieving high <i>E</i> → <i>Z</i> photoconversion, ranging from 85% to nearly quantitative (96%), and quantitative <i>Z</i> → <i>E</i> photoswitching in their crystalline states. To the best of our knowledge, azobispyrazoles are the first photoswitchable azo crystals that achieve high-yield bidirectional conversions, particularly the challenging thermodynamically stable-to-metastable <i>E</i> → <i>Z</i> transformation. Crystallographic and computational analyses provide in-depth insights into the photoswitching mechanism and propose that locally distributed free spaces and weak intermolecular interactions within the crystal structures are key factors contributing to the crystalline-state conversion. This work opens up new avenues for the development of promising photoswitchable azo crystals and also underscores the potential application of azobispyrazole crystals as light-responsive materials.