Light-Triggered\nRolling and Unrolling of Molecular\nCrystal Microsheets

The anthracene derivative (<i>E</i>)-3-(3-(anthracen-9-yl)allylidene)-1,5-dioxaspiro[5.5]undecane-2,4-dione\n(<i><b>E</b></i><b>-ADUD</b>) undergoes an E\n→ Z photoisomerization in solution but not in crystals prepared\nby standard methods like solvent diffusion. However, the lipophilic\ncyclohexyl group facilitates precipitation of crystalline microsheets\nwith submicron thicknesses from an aqueous solution containing sodium\ndodecyl sulfate and 1-dodecanol. These microsheets have horizontal\ndimensions on the order of 200 μm and are composed of a metastable\ncrystal polymorph that permits the E → Z isomerization to proceed.\nPhotoisomerization using visible light caused these microsheets to\nrapidly roll up into multilayer microcylinders with diameters ranging\nfrom 20 to 40 μm. If the light was removed at this point, then\nthe microscrolls were stable indefinitely. Continued exposure to visible\nlight uncurled these cylindrical structures, reversing the mechanical\nprocess but not the photochemical reaction. The unrolled microsheets\nretained their crystallinity and could bend and twist under alternating\nUV and visible light but could not be rolled up again. The initial\nhigh curvature rolling up can be attributed to the creation of a surface\nlayer of the Z-isomer that interacts with the underlying E-isomer\ncrystal layer to generate a bimorph structure. Prolonged irradiation\ndistributes the photoproduct more uniformly throughout the crystal\nand relieves this interfacial stress. The photoinduced rolling and\nunrolling could prove useful for applications like antenna or stent\nopening in hard-to-reach environments.