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.