Uncovering the “Motion-Induced Enhancement” Mechanism in Luminescent Aggregates Via Experimental Photo-Difference Electron Density

Abstract

As pivotal photosensitive materials, luminescent organics are indispensable in modern life. Nonetheless, their luminescence behavior in aggregated states remains poorly understood. Here, in situ experimental studies on the electronic structure of two representative model compounds, 1,8-naphthalimide and fluoranthene is conducted. Topological quantification of photo-difference electron densities identifies the naphthyl in both compounds as the photosensitive functional motif. Notably, a photo-induced molecular motion is observed within both crystals, disrupting the strong π‒π stacking between adjacent parallel molecules. This structural evolution facilitates larger excitation energy and higher radiative transition rates, as demonstrated by theoretical calculations, ultimately leading to enhanced emission. The uncovered motion-induced enhancement mechanism clarifies the long-standing debate on the photophysical process in luminescent aggregates, paving the way for the development of novel luminescent materials with improved performance and versatility.