Molecular Motion in the Solid State

All matter is in motion, and such behavior has been well studied in the gas and liquid states. However, the study of solid-state molecular motion is still in its infancy, mainly, because of the absence of valid characterization methods. Aggregation-induced emission (AIE) is a cutting-edge fluorescence technology related to aggregate-state luminescence. Thus, it holds great potential to address this issue. AIE luminogens (AIEgens) are often propeller-like in shape and are, therefore, born with molecular mobility. The emission of AIEgens is determined by molecular motion, and thus, the motion in the solid state can be visualized by fluorescence variation. The active molecular motion of AIEgens promotes nonradiative decay of excitons to release heat in the nanoparticles. This makes AIEgens find application in photothermal therapy and photoacoustic imaging. In this Viewpoint, we briefly introduce a newly emerging frontier derived from AIE: molecular motion in the solid state with the hope to stimulate new ideas and inspire new endeavors in this area.