Overcrowded alkene-based molecular motors are molecular machines that can execute precise and controlled unidirectional 360° rotations under light irradiation. This rotation has been well-established in solution and supramolecular assemblies. It can serve as a useful tool in the design of, e.g., nanomachines and responsive materials. However, understanding the photoresponsive behavior of these rotary motors in aggregate states—especially in solvent-free solid states—has remained challenging so far.

Da-hui Qu, East China University of Science and Technology, Shanghai, China, Ben L. Feringa University of Groningen, The Netherlands, and East China University of Science and Technology, and colleagues have designed a new overcrowded alkene-based molecular motor (pictured) that shows distinct dynamic properties in various states, e.g., in solution, gels and solids. The team’s molecular motor features four amide groups, two at each half of its structure, and long alkyl chains, which can form hydrogen bonds and strong van-der-Waals interactions, respectively. The molecular motor was synthesized using an amidation and a Miyaura borylation reaction to generate two building blocks, and a Suzuki coupling reaction to connect them.

The fluorescence of the molecular motor is poor in solution, but a strong emission is observed in aggregate states of gels and crystalline solids. In addition, the emission wavelength of the crystalline solid strongly depends on the degree of supramolecular stacking and order in the aggregates, which can be tuned reversibly via mechanical grinding and vapor fuming (pictured schematically). Overall, the work could be a foundation for the integration of molecular motors into multifunctional solid-state fluorescent nanomaterials.

• Multi‐state photoluminescent properties of an overcrowded alkene‐based molecular motor in aggregates,
  Yahan Shan, Jinyu Sheng, Qi Zhang, Marc C. A. Stuart, Da‐Hui Qu, Ben L. Feringa,
  Aggregate 2024.
  https://doi.org/10.1002/agt2.584