Regulating Reversible Untwisting and Twisting Motions in Helical Dynamic Molecular Crystals

Dynamic molecular crystals with a desirable morphology and controllable responses to external stimuli are highly desired but remain challenging. Here, we present a new photoactive molecular crystal (MCA) featuring a naphthalene backbone linked with a vinyl structure and an asymmetric tail. It can undergo reversible head-to-tail [2 + 2] photodimerization under visible light illumination and heating. Helicoidally twisted crystalline fibers can be easily prepared by drop-casting an MCA ethanol solution onto a glass surface. The thread length and fiber width can be regulated by changing the solvent evaporation temperature. An MCA fiber untwists upon light irradiation and twists back under mild heating due to the dissociation of the metastable photodimer (d-MCA), enabling a highly reverse transformation. Crystallinity and overall helical morphology can be reserved well during the untwisting and twisting cycles, allowing multiple repetitions of the dynamic motions. Under interval light excitation and continuous heating, an MCA fiber can perform cyclical untwisting-twisting motions over 200 times for 5 h. The amount of photodimer significantly affects the photoresponse, resulting in fully reversible cycles, partial reverse twisting, or complete untwisting at different dimer fractions. We found that about 10% of d-MCA is enough to drive the circular movements. Higher photoproduct conversion makes the fibers prone to fatigue and lose the cyclic responses. Our results provide an excellent example for fabricating new dynamic organic crystals for soft robotics and photoactuators based on spiral twisting movements.