Tailoring Multifunctional Carbon Dots via Precursor Stoichiometry: Switching Between Solid‐State Fluorescence and Broadband Absorption Through Aggregation Control

ABSTRACT Achieving precise control over the solid‐state optical properties of carbon dots (CDs) remains a significant challenge, primarily due to aggregation‐caused quenching (ACQ) and their intrinsically narrow absorption. Herein, we establish a framework that links surface chemistry to aggregation behavior by demonstrating that precursor stoichiometry programs the surface functionality of CDs, which in turn dictates their packing geometry. By varying the molar ratio of 2,3‐diaminonaphthalene to o ‐phthalaldehyde, CDs with distinct surface chemistries are engineered. Under aldehyde‐rich conditions, CDs with aldehyde‐enriched surfaces self‐assemble into ordered, layered structures through directional hydrogen bonding. This ordered arrangement effectively suppresses π – π stacking and enables efficient red solid‐state fluorescence (SSF). Conversely, amine‐rich surfaces promote face‐to‐face π – π stacking, leading to compact, spherical aggregates. Such dense packing establishes a continuous donor–acceptor interface, facilitating broadband visible absorption through efficient intermolecular charge transfer (ICT). This work establishes a general design principle according to which aldehyde‐rich surfaces enable SSF, while amine‐rich surfaces confer broadband absorption and photothermal capability. Our findings demonstrate that precursor stoichiometry serves as a powerful tool for directing CDs aggregation and provides a unified platform for developing multifunctional CDs‐based materials.