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 c-axis of the unit cell. Furthermore, an antagonistic effect between overlap ratio (S_π-π) and interplanar distance (D_π-π) within the dimer on emission was identified: reduced D_π-π typically dominates the emission redshift, while decreasing S_π-π 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_π-π. This study not only sheds light on the modulation of fluorescence properties by noncovalent interactions but also introduces an innovative approach to anomalous piezochromism.