Assignment of Core and Surface States in Multicolor‐Emissive Carbon Dots

It is important to reveal the luminescence mechanisms of carbon dots (CDs). Herein, CDs with two types of optical centers are synthesized from citric acid in formamide by a solvothermal method, and show high photoluminescence quantum yield reaching 42%. Their green/yellow emission exhibits pronounced vibrational structure and high resistance toward photobleaching, while broad red photoluminescence is sensitive to solvents, temperature, and UV–IR. Under UV–IR, the red emission is gradually bleached due to the photoinduced dehydration of the deprotonated surface of CDs in dimethyl sulfoxide, while this process is hindered in water. From the analysis of steady‐state and time‐resolved photoluminescence and transient absorption data together with density functional theory calculations, the green/ yellow emission is assigned to conjugated sp2‐domains (core state) similar to organic dye derivatives stacked within disk‐shaped CDs; and the broad red emission—to oxygen‐containing groups bound to sp2‐domains (surface state), whereas energy transfer from the core to the surface state can happen. Based on comprehensive analysis of structural and optical properties of carbon dots, their multicolor emission centers are assigned. The green/yellow emission comes from conjugated sp2‐domains (core state) similar to organic dye derivatives stacked within disk‐shaped carbon dots; and the broad red emission—from oxygen‐containing groups bound to sp2‐domains (surface state), whereas energy transfer from the core to surface states can happen.