Photoexcited Species Localize on Solvent-Accessible Fluorophore-Rich Domains inside Carbon Dots

Understanding the optical properties of luminescent carbon dots (CDs) at the electronic level is essential for engineering their light-responsive behavior. The localization of photoexcited species and the pathways of their de-excitation govern CD performance in sensing, bioimaging, and emerging photocatalytic applications. Yet, the underlying mechanisms remain unresolved. Here, we combine multiscale simulations with experiments on CDs synthesized from citric acid (CA) and ethylenediamine (EDA), precursors capable of forming the molecular fluorophore 5-oxo-1,2,3,5-tetrahydroimidazo[1,2-α]pyridine-7-carboxylic acid (IPCA). All-atom molecular dynamics simulations in water reveal that CA–EDA oligomeric condensation products containing IPCA units spontaneously assemble into dynamic ~2 nm nanoparticles with amorphous internal structures and stacked domains reminiscent of those observed in transmission electron microscopy images of CDs. Time-dependent density functional theory (TD-DFT) calculations show that photoexcited carriers are generated in these domains and remain spatially distributed, not confined to the CD core. Quenching experiments with Hg²⁺ confirm their accessibility to the environment. We therefore propose a structural model of fluorophore-rich domains embedded in an amorphous carbonaceous matrix, explaining the quasi-spherical morphology and characteristic blue photoluminescence. This model provides a mechanistic basis for fluorescence sensing and photocatalysis and establishes a framework for rational design of CDs with tailored photophysical and catalytic properties.从电子层面理解发光碳点(CDs)的光学性质，对于调控其光响应行为至关重要。光激发物种的局域化及其退激发路径，决定了碳点在传感、生物成像及新兴光催化应用中的性能表现。然而，其背后的核心机制仍未阐明。本研究针对以柠檬酸(CA)与乙二胺(EDA)为前驱体合成的碳点展开研究，二者可形成分子荧光团5-氧代-1,2,3,5-四氢咪唑并[1,2-α]吡啶-7-羧酸(IPCA)。水中的全原子分子动力学模拟结果显示，含有IPCA单元的CA-EDA低聚缩合产物会自发组装为直径约2 nm的动态纳米颗粒，其内部呈现无定形结构，且存在与碳点透射电子显微镜图像中观测到的类似堆叠结构域。含时密度泛函理论(TD-DFT)计算表明，光激发载流子在这些结构域中产生并呈空间分散状态，并未局限于碳点核心。Hg²+淬灭实验证实了这类结构域对环境的可及性。据此，我们提出了一种嵌入无定形碳基质的富荧光团结构域结构模型，该模型可解释碳点的准球形形貌与特征性蓝色光致发光行为。此模型为荧光传感与光催化应用提供了机制基础，并为合理设计具备定制化光物理与催化性能的碳点建立了理论框架。