Photoswitchable Azobispyrazole Crystals Achieving Near-Quantitative Crystalline-State Bidirectional EZ Conversions

J Am Chem Soc. 2024 Oct 23;146(42):29237-29244. doi: 10.1021/jacs.4c12532. Epub 2024 Oct 14.

Abstract

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 EZ isomerization. This study presents a breakthrough in solid-state azo chemistry, where the investigated azobispyrazole molecules are capable of achieving high EZ photoconversion, ranging from 85% to nearly quantitative (96%), and quantitative ZE 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 EZ 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.