Cyclo-Octasulfur Crystals as Light-Controlled Molecular Muscles

Here, we describe the synthesis and photo-responsive properties of 2D cyclo-octasulfur microcrystals (α-S8 MCs), produced using a quick, simple, cost-effective, and environmentally friendly hydrothermal method. Controlled 385-nm irradiation of these crystals induces an immediate, reversible, and sustained bending effect. The time required for the crystals to return to their initial shape is significantly reduced when exposed to 475-nm light, completing the entire excitation-relaxation cycle in less than 2 s. Moreover, this process can be repeated up to 50 times as long as the crystals remain in water. The dependence of recovery time on light wavelength is rationalized qualitatively via highly correlated quantum-chemical calculations of the absorption spectra of S8 chains. The underlying mechanism involves a combination of ring-to-chain and chain-to-ring transformations: the breaking of α-S8 rings by 385-nm radiation induces bending in the α-S8 MCs, while the excitation of the chains with 475-nm light facilitates an accelerated recovery process, allowing the S8 molecules to swiftly regain their ring shape. Thus, our study demonstrates that α-S8 MCs represent intelligent actuators as light-controlled molecular muscles with the simplest inorganic composition reported to date.