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 EZ photoisomerization in solution but is inactive when crystals are grown using 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 m and are composed of a metastable crystal polymorph that permits the EZ isomerization to proceed. EZ photoisomerization using visible light caused these microsheets to rapidly roll up into multilayer micro-cylinders with diameters ranging from 20-40 m. If the light was removed at this point, 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, behaving as a photoreversible crystal but without additional rolling up. The initial high curvature rolling up can be attributed to the creation of a surface layer of Z-isomer, while prolonged irradiation distributes this photoproduct more uniformly throughout the crystal and relieves the interfacial stress. This photoinduced rolling and unrolling could prove useful for applications like antenna or stent opening in hard-to-reach environments.