Multi-state photoluminescent properties of an overcrowded alkene-based molecular motor in aggregates

Photoisomerization and photoluminescence are two distinct energy dissipation pathways in light-driven molecular motors. The photoisomerization properties of discrete molecular motors have been well established in solution, but their photoluminescent properties have been rarely reported—especially in aggregates. Here, it is shown that an overcrowded alkene-based molecular motor exhibits distinct dynamic properties in solution and aggregate states, for example, gel and solid states. Despite the poor emissive properties of molecular motors in solution, a bright emission is observed in the aggregate states, including in gel and the crystalline solid. The emission wavelength is highly dependent on the nature of the supramolecular packing and order in the aggregates. As a result, the fluorescent color can be readily tuned reversibly via mechanical grinding and vapor fuming, which provides a new platform for developing multi-stimuli functional materials.