Aggregation Induced Effects on the Nonradiative Recombination Dynamics of Inverted Singlet-Triplet Heptazine-Based Materials.

Heptazine (CNH) serves as the core unit for various molecular and self-assembled compounds, finding applications in photocatalysis and optoelectronics. Some of its derivatives, such as melem and melon, are known to exhibit thermally activated delayed fluorescence (TADF). However, hindered by their insolubility and chemical inertness, a comprehensive understanding of the molecular mechanisms governing the photorelaxation dynamics of these compounds remains a matter of investigation. In this work, we present the first excited-state nonadiabatic simulations of heptazine-based molecules and aggregates, aiming to elucidate the role of nonradiative pathways in their photorelaxation processes. Our results reveal that isolated heptazine and melem (CNH) molecules return to the ground state via conical intersections within subpicosecond time scales following photoexcitation. In contrast, melem aggregation, driven by strong hydrogen bonding, markedly suppresses nonradiative photorelaxation through two mechanisms: (i) intermolecular charge transfer, which reduces the likelihood of electron-hole recombination via internal conversion, and (ii) molecular packing, which prevents ring deformation, thus reducing the occurrence of conical intersections. These findings suggest that, in addition to being a TADF material, melem's high photoluminescence quantum yield is further enhanced by aggregation-induced emission (AIE). Additionally, the findings provide valuable insights into the mechanisms underlying nonradiative recombination in organic solar cells.