Monodisperse π–π Stacking Anthracene Dimer under Pressure: Unique Fluorescence Behaviors and Experimental Determination of Interplanar Distance at Excimer Equilibrium Geometry

Abstract Organic light‐emitting materials are centered on their excited states. However, excited‐state geometry is hardly solved. Herein, the planar distance of an anthracene excimer is determined by integrating a monodisperse dimeric anthracene stacking in crystal and high‐pressure technique. The crystal exhibits the pure excimer fluorescence at ambient conditions, and a unique photophysical phenomenon is observed under pressure: fluorescence peak keeps constant first and then starts a gradual redshift. The theoretical potential energy curve indicates that this starting point of redshift corresponds to the excimer equilibrium geometry, which will be innovative to determine the excited‐state π–π interplanar distance (R_e) in experiment. Moreover, high‐pressure synchrotron powder X‐ray diffraction experiment tactfully determines R_e = 3.330 Å at the pressure point of initial redshift. This work not only suggests the R_e determination by an original pathway but also reports a unique excimer photophysics, which may provide a reference benchmark for the further theoretical and experimental studies on aromatic excimer.