Toward Better Understanding of Molecular Fluorophore Covalent Binding to Carbon Dots

The mechanism of the molecular photoluminescence of carbon dots (CDs) has gained much attention recently, due to several reports of formation of small fluorophore species at reaction conditions applied in CD synthesis. In this work, two novel green‐emitting species are discovered in postsynthetic mixtures of citric acid (CA)–urea CDs and identified as 3‐(4‐hydroxy‐1,3,6‐trioxo‐5 H ‐pyrrolo[3,4‐ c ]pyridine‐7‐yl)‐2,6‐dioxo‐piperidine‐4‐carboxyamide (HTPPC) and 7‐[(2,5‐dioxopyrrolidin‐3‐yl)methyl]‐4‐hydroxy‐5 H ‐pyrrolo[3,4‐ c ]pyridine‐1,3,6‐trione (DMHPPT). The less‐considered covalent character of interactions between the molecular fluorophores and the CD structure is revealed during the first steps of formation of the frequently studied CA–urea CDs. Thus, specific reaction sites in the HPPT fluorophore (4‐hydroxy‐1 H ‐pyrrolo[3,4‐ c ]pyridine‐1,3,6(2 H ,5 H )‐trione), involved in the formation of covalent bonds with CDs, are identified; the origin of the reactive species in the synthesis mixture is revealed; and the spectroscopic consequences of this binding are demonstrated using steady‐state and time‐resolved spectroscopy methods accompanied by quantum chemical modeling. The hypothesis suggesting the formation of CDs at the expense of molecular fluorophores is validated experimentally. The covalent nature of bonding of molecular fluorophores to the CDs involving a reaction with double bond functionalities of CDs is successfully expanded to citrazinic acid fluorophores, suggesting that a similar mechanism can be present in other carbon dot systems.