An Inhibition Effect on an Intracrystalline Reaction by a Crystal Lattice: Analyses of the Chemiluminescence Reaction of 9,10-Diphenylanthracene Endoperoxide Initiated by Heating a Crystal Sample

Heating crystal samples of 9,10-diphenylanthracene endoperoxide (1) and its deuterated derivative (1-d10) to 200 & DEG;C led to singlet-oxygen chemiluminescence (CL) from the mixtures of the crystalline and molten states. To understand the events in the heated samples, the reactions of 1 and 1-d10 in crystals were investigated by powder X-ray diffraction measurements and thermal analyses. The used crystals of 1 and 1-d10 obtained from a mixture of ethyl acetate (EA) and n-hexane contained EA. Other crystals of 1 obtained by slow recrystallization with the same solvents had a solvent -free structure (form II) different from the reported one (form I). The results confirmed that the EA-containing crystals of 1 and 1-d10 were transformed to form II in two steps at 40-50 and 60-75 & DEG;C under heating at elevated temperature. Thus, the crystal samples had the form-II structure soon after heating to 200 & DEG;C. The behavior of the heated samples indicates that the crystal lattice inhibits the thermal reactivity of 1. Thus, the temperature for initiating the thermolytic reaction of 1 becomes higher than the melting point. An exploration of the potential energy surface (PES) obtained by density functional theory (DFT) calculations also supports that the deoxygenation of 1 is prevented by retaining the anthracene framework in the crystalline state. This study indicates that the method to use a CL-active compound is a powerful tool to analyze the reaction behavior in the crystalline state.