Light-Triggered Rolling and Unrolling of Molecular Crystal Microsheets

The anthracene derivative (E)-3-(3-(anthracen-9-yl)allylidene)-1,5-dioxaspiro[5.5]undecane-2,4-dione (E-ADUD) undergoes an E → Z photoisomerization in solution but not in crystals prepared by standard methods like solvent diffusion. However, the lipophilic cyclohexyl group facilitates precipitation of crystalline microsheets with submicron thicknesses from an aqueous solution containing sodium dodecyl sulfate and 1-dodecanol. These microsheets have horizontal dimensions on the order of 200 μm and are composed of a metastable crystal polymorph that permits the E → Z isomerization to proceed. Photoisomerization using visible light caused these microsheets to rapidly roll up into multilayer microcylinders with diameters ranging from 20 to 40 μm. If the light was removed at this point, then the microscrolls were stable indefinitely. Continued exposure to visible light uncurled these cylindrical structures, reversing the mechanical process but not the photochemical reaction. The unrolled microsheets retained their crystallinity and could bend and twist under alternating UV and visible light but could not be rolled up again. The initial high curvature rolling up can be attributed to the creation of a surface layer of the Z-isomer that interacts with the underlying E-isomer crystal layer to generate a bimorph structure. Prolonged irradiation distributes the photoproduct more uniformly throughout the crystal and relieves this interfacial stress. The photoinduced rolling and unrolling could prove useful for applications like antenna or stent opening in hard-to-reach environments.