de Thieulloy Laure, Oliboni Robson S, de Silva Piotr, Rego Luis G C
Department of Energy Conversion and Storage, Technical University of Denmark, 2800 Kongens Lyngby, Denmark.
Department of Chemistry, Universidade Federal de Pelotas, Pelotas, Rio Grande do Sul 96010-900, Brazil.
J Phys Chem A. 2025 Jun 19;129(24):5220-5233. doi: 10.1021/acs.jpca.5c00709. Epub 2025 Jun 4.
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.
七嗪（CNH）是各种分子和自组装化合物的核心单元，在光催化和光电子学领域有应用。已知其一些衍生物，如蜜勒胺和蜜勒腈，表现出热激活延迟荧光（TADF）。然而，由于它们的不溶性和化学惰性，全面了解控制这些化合物光弛豫动力学的分子机制仍是一个研究课题。在这项工作中，我们首次对基于七嗪的分子和聚集体进行了激发态非绝热模拟，旨在阐明非辐射途径在其光弛豫过程中的作用。我们的结果表明，孤立的七嗪和蜜勒胺（CNH）分子在光激发后，会在亚皮秒时间尺度内通过锥形交叉回到基态。相比之下，由强氢键驱动的蜜勒胺聚集通过两种机制显著抑制非辐射光弛豫：（i）分子间电荷转移，通过内转换降低电子 - 空穴复合的可能性；（ii）分子堆积，防止环变形，从而减少锥形交叉的发生。这些发现表明，除了作为一种TADF材料外，蜜勒胺的高光致发光量子产率还通过聚集诱导发光（AIE）进一步提高。此外，这些发现为有机太阳能电池中非辐射复合的潜在机制提供了有价值的见解。
