Spontaneous and Fast Molecular Motion at Room Temperature in the Solid State

The development of molecular machines requires new building blocks which are easy to characterize and visualize to realize a complexity comparable to their natural counterparts such as biological enzymes. Furthermore, with the desire to build functional nanobots capable of navigating living organisms, it is necessary that the building blocks show mobility even in the solid state. Herein we report a system which is emissive in the amorphous state but is non‐fluorescent in the crystalline state due to the formation of extensive π‐π interactions. This dual nature could be exploited for easy visualization of its solid‐state molecular rearrangement. The emission of the amorphous film was quenched as the molecules spontaneously formed π‐π interactions even in the solid state. Scratching the non‐emissive film destroyed the interactions and restored the emission of the film. The emission quickly disappeared with an average lifetime of 20 s as the compound reformed the π‐network even at room temperature. Beacons lit by molecular motion: Intermolecular forces drive the formation of π‐π interactions even in the solid state. Shear forces/scratching disrupt the interactions, turning on emission. The quenched π‐π‐interactions quickly reform even in a film, allowing the visualization of the molecular motions. Theoretical calculations show that the formation of π‐interactions is highly favorable and driving this motion.