Antagonistic Effects of Distance and Overlap toward Anomalous Pressure-Induced Blueshift of π-π Excimer Fluorescence in 9-(2,2-Diphenylvinyl)anthracene Crystals.

Piezochromic materials usually exhibit a gradual redshift of emission as pressure increases due to the formation of a low-energy "dark" state, e.g., excimer. However, our study presents an anomalous excimer-based pressure-induced emission blueshift. A crystal was investigated here with a discrete π-π anthracene dimer stacking and excimer emission, and the dimer is characterized by an overshifted off-center stacking pattern. Intriguingly, under isotropic hydrostatic pressure, this crystal exhibits negative linear compressibility almost along the -axis of the unit cell. Furthermore, an antagonistic effect between overlap ratio (S) and interplanar distance (D) within the dimer on emission was identified: reduced D typically dominates the emission redshift, while decreasing S can cause emission blueshift. When the pressure reaches around 5.00 GPa, the π-π excimer fluorescence exhibits an unexpected blueshift, indicating the reign of decreasing S. This study not only sheds light on the modulation of fluorescence properties by noncovalent interactions but also introduces an innovative approach to anomalous piezochromism.