Single Molecule Swimmer Propelled by Photoredox Brownian Ratchet

The ability to control and create motion at various scales has been a key driver of scientific advancements and economic growth throughout human history. Over the past decades, efforts have focused on developing nano- and molecular machines, ranging from Nobel Prize-winning molecular machines to contemporary pursuits of active colloid/microswimmers, in which chemical energy or light is converted into mechanical work. As the ultimate tools for controlling chemical equilibrium, biological processes, and material phase, molecular swimmers may lead to revolutionary developments in green chemistry and precision medicine due to their ability to execute intricate molecular tasks with nanometer precision. While the concept of reaction-induced propulsion presents intriguing possibilities, the feasibility of molecular swimmers remains a subject of ongoing debate. This study provides direct evidence that, during photoredox reaction, the dynamics of dye molecules embedded in viscous lipid layers deviate from Brownian fluctuation and exhibit Lévy flight behavior. After ruling out possible artifacts from thermal and fluid convection, this result suggests that mechanical propulsion can be extended to the molecular scale. As inspired by micrometer-scaled counterparts, we propose that during photoredox reaction, the molecules form a pseudo-stable complex that undergoes asymmetric surface potential switching, resulting in the directional propulsion of polar solvent molecules (e.g., water or lipid) via a Brownian ratchet mechanism, thereby realizing molecular-scale self-propelling swimmer. Notably, with excellent biocompatibility, we demonstrated that these photoredox molecular swimmers can be applied to live cells to regulate cell membrane permeability and modulate lipid droplet transport. Leveraging the simplicity and prevailing presence of photoredox system, our findings present a promising molecular engine for the coming round development of molecular robots with free motion ability.