Solvent mixing to induce molecular motor aggregation into bowl-shaped particles: underlying mechanism, particle nature and application to control motor behavior

The control over dynamic functions in larger assemblies is key to many molecular systems ranging from responsive materials to molecular machines. Here we report a molecular motor that forms bowl-shaped particles in water and how confinement of the molecular motor effects rotary motion. Studying the aggregation process in a broader context we provide evidence that in the case of bowl-shaped particles the structures are not the product of self-assembly, but a direct result of the mixing a good solvent and a (partial) non-solvent and highly independent of the molecular design. Under influence of the non-solvent, droplets are formed, of which the exterior is hardened due to the increased glass-transition temperature by the external medium, while the interior of the droplets remains plasticized by the solvent resulting in the formation of stable bowl-shaped particles with a fluid interior, a glass-like exterior and a very specific shape; dense spheres with a hole in their side. Applying this to a bulky first generation molecular motor allowed us to change its isomerization behavior. Furthermore, the motor shows in situ photo-switchable aggregation-induced emission (AIE). Strong confinement prohibits the thermal helix inversion step while altering the energy barriers that determine the rotary motion, such that it introduces a reverse trans-cis isomerization by heating. These studies show a remarkable control of forward and backward rotary motion by simple changing solvent ratios and extend of confinement.