Visualization of Melem as a Solid Proton Carrier

As one of the key representatives of carbon–nitrogen-based materials, melem (2,5,8-triamino-s-heptazine) holds significant potential for applications in luminescence and catalysis. In previous studies, acid treatment has been frequently used to modify the optical and catalytic properties of melem and its family materials. However, the underlying mechanism of this process has been barely studied and remains poorly understood. In this study, we reveal that acidification results in the formation of new supramolecular structures via protonation of melem with different protonation states after reacting with strong acids. In this regard, we identified the micromechanism of the evolved optical absorption by protonation through density functional theory (DFT) calculations. Notably, when these crystals were transferred back to water, the protonated melem molecules underwent deprotonation, causing hydrogen bonds to break and the protons released to water instantly. This rapid proton release can be easily visualized by the color change of a pH indicator as the crystals are immersed in water. Interestingly, the deprotonated melem molecules recrystallized into purified melem crystals with well-defined morphologies, which varied depending on the specific acid used. The reversible structural transition between melem and protonated melem crystals can be cycled multiple times, making melem a perfect solid proton carrier and may find applications in catalysis and fuel cells.