Growth and characterization of melem hydrate crystals with a hydrogen-bonded heptazine framework

In carbon nitride (CN) compounds, hydrogen bonds play a major role in cohesion, in addition to dispersion forces. The crystal structures of CN compounds produced via thermal polymerization are complex, but they possess unique and attractive features. Melem is a well-known building unit of CN compounds such as melon and g-C₃N₄, which have recently attracted attention as photocatalysts. Melem hydrate (Mh) forms hexagonal prismatic crystals that are sufficiently porous to accommodate small molecules. In this study, we grew and characterized single crystals of Mh and investigated their optical properties and hygroscopicity. By precisely adjusting the hydration conditions, we succeeded in growing a well-formed hexagonal prismatic single crystal of Mh (Mhr) with a length measuring several tens of micrometers. Furthermore, we discovered a parallelogram-shaped Mh single crystal (Mhp), which possessed a different crystal structure and optical properties from those of Mh and melem crystals. Although the crystal structure of Mh was greatly disrupted by dehydration, it exhibited hygroscopicity and could absorb moisture even in air, restoring the crystal structure of Mh. In addition, Mh demonstrated a high photoluminescence quantum yield and long lifetime delayed fluorescence, similar to melem crystal. The high quantum yield of Mh can be attributed to the effect of the strong anchoring of the melem molecule by several hydrogen bonds in the Mh crystal, since the strongly anchored molecule is less likely to undergo radiation-free deactivation due to the small displacement of atomic positions in the excited state after light absorption. The findings obtained in this study shed light not only on the application of CN compounds as photocatalysts, but also on a wider range of applications based on their optoelectronic functions.