Dual-Mode Emission and Solvent-Desorption Dependent Kinetic Properties of Crystalline-State Chemiluminescence Reaction of 9-Phenyl-10-(2-phenylethynyl)anthracene Endoperoxide

The chemiluminescence (CL) feature and reactivity of the aromatic endoperoxide 9-phenyl-10-(2-phenylethynyl)anthracene endoperoxide (PPEA-O₂) were investigated in the crystalline state. For this, PPEA-O₂ crystals were prepared using dichloromethane and <i>n</i>-hexane. These crystals exhibited an α-phase structure containing <i>n</i>-hexane as a crystal solvent. The crystal structure of nonperoxidic anthracene (i.e., PPEA) was also confirmed. After optimizing heating conditions to 120 °C for the thermolytic reaction of PPEA-O₂ in crystals while maintaining the solid state, its CL characteristic and reactivity were investigated. Two key findings were derived: (1) dual-mode emission with maxima at 510 and 1275 nm and (2) distinct observation of CL emission at the first 2-3 min after the start of heating owing to the rapid thermolytic reaction coupled with <i>n</i>-hexane desorption. The 510 and 1275 nm emissions were attributed to the PPEA excimer and ¹O₂ (¹Δ_g), respectively. We proposed a mechanism involving the triplet-triplet annihilation of the excited triplet states of PPEA to explain excimer production with postulated pathways for generating these triplet states from PPEA-O₂. The rapid thermolytic reaction of PPEA-O₂ in α-phase crystals with simultaneous <i>n</i>-hexane desorption was attributed to the formation of transient vacant spaces, which increased the molecular freedom necessary for the reaction ("transient vacant space effect"). Thus, the CL of PPEA-O₂ proved useful for identifying characteristic reactivity and analyzing the luminescence mechanism of aromatic endoperoxides in the crystalline state.