Light-Induced Molecular Motion Breaks Fluorescence Quenching in Aggregated π-Conjugated Luminophores

Luminescent organic materials play an indispensable role in various aspects of human life and modern society; however, their photophysical processes in the aggregated state remain inadequately understood. Here, we introduce an experimental scheme to monitor in situ the evolution of charge density and noncovalent interactions in aggregated molecules under dark conditions and 360 nm laser irradiation. Application to 9,10-diphenylanthracene (DPA) crystals reveals an unexpected light-induced molecular shift and rotation. Topological analysis of the in situ charge density indicates that the overall molecular shift within DPA crystals strengthens intermolecular C–H···π interactions while weakening π···π interactions. Moreover, significant rotation of the phenyls occurs upon light exposure, resulting in a change in the dihedral angle between their plane and the anthracene core from 66.621 to 66.344°, which enhances the intramolecular conjugation across the entire molecule. This structural evolution leads to a reduction in excitation energy and an increase in the radiative transition rate, as demonstrated by theoretical calculations, ultimately resulting in the observation of strong emission. The motion-induced enhancement mechanism uncovered in this study addresses the long-standing debate surrounding photophysical processes in traditional luminescent aggregates, providing a foundation for the development of novel luminescent materials with improved performance and versatility.