Decoding Heptazine Architectures: From Molecular Association to Structural Insight

Heptazine (Hpz) derivatives display remarkable supramolecular organization governed by noncovalent interactions. Here, we explore how molecular chirality and structural variation influence the self-association and organization of three Hpz derivatives (two chiral and one nonchiral) in solution and in the condensed phase. Variable-temperature and concentration-dependent NMR studies allowed the determination of dimerization and supramolecular aggregation constants, as well as the calculation of the thermodynamic parameters of these processes. Computational studies -including conformational analysis and molecular dynamics-corroborated the experimental findings, confirming the energetic stability and geometry of the proposed dimers. Moreover, photophysical studies showed aggregation-induced emission (AIE) behavior, with blue light emission upon molecular association. Additionally, the introduction of chiral chains resulted in mirror-image circular dichroism (CD) responses for the two Hpz enantiomers, demonstrating retention of chirality in bulk. In contrast, the nonchiral analogue displayed liquid crystalline (LC) behavior at high temperatures, characterized by a smectic mesophase. These results reveal how subtle structural variations modulate the supramolecular, optical, and mesomorphic properties of Hpz derivatives, highlighting the versatility of the heptazine core for multifunctional supramolecular and optoelectronic materials.