Alternately π‐Stacked Systems Assisted by o‐Carborane: Dual Excimer Emission and Color Modulation by Bcage‐Methylation.

Herein, we report the unique solid‐state excimer emission of three types of acridine‐tethered o‐carboranes with variable degrees of methylation at the o‐carborane unit. They all showed columnar packing structures based on dimer formation, and two types of π‐overlapping motifs were alternately stacked. From the photoluminescence (PL) measurements on the crystalline samples, it was found that three types of luminescence bands can simultaneously appear: monomer emission, excimer emission from the moderately π‐stacked intra‐dimer unit, and excimer emission from the widely π‐stacked inter‐dimer unit. Consequently, the PL colors were drastically changed by the steric effect of the methyl groups, with a strong correlation found between the π‐overlapping and excimer character. In addition, variable‐temperature PL measurements revealed that these PL species should be in thermal equilibrium at room temperature, with the intensity ratios sensitive toward temperature changes. [ABSTRACT FROM AUTHOR]