Direct observation of reversible single-crystal-to-single-crystal E⇄Z photoisomerization of an azo molecule

Single-crystal-to-single-crystal (SCSC) photoisomerization is direct structural transformation of crystalline photoswitches in the solid state, preserving the long-range order in the crystal lattice. Azo molecules, as one of the most widely used photoswitches, has not yet achieved SCSC isomerization. The relatively large free volume required for isomerization poses significant challenge for its SCSC transformation. Here we demonstrated for the first time that an azobispyrazole underwent high-quality reversible E⇄Z SCSC isomerization by light irradiations. We found that the similarity in crystal lattice parameters between E and Z crystals and the efficient photoisomerization ability of azo molecules are key factors facilitating the single crystal to single crystal photoisomerization of azo molecules. Our work opens an avenue for the single crystal photochemistry of azo molecules and offers new opportunities for developing light-controlled single-crystal materials and devices.