Contribution of the Molecular Fluorophore IPCA to Excitation-Independent Photoluminescence of Carbon Dots

Carbon dots (CDs) are intensively studied nanomaterials with many potential applications stemming from their bright photoluminescence (PL), stability, biocompatibility, and other attractive features. However, the mechanisms of PL in CDs are not fully resolved. The intense excitation-independent PL component of CDs has been attributed to molecular fluorophores (MFs), which may be surface-bound or embedded in the CD matrix. We used a hybrid quantum mechanical/molecular mechanical method with electrostatic embedding to decipher the origins of PL in CD structures containing embedded molecules of the prototypical MF 5-oxo-1,2,3,5-tetrahydroimidazo-1,2-α-pyridine-7-carboxylic acid, IPCA. The environment of the MF and the nature of its embedding were found to significantly affect its PL intensity and excitation-emission Stokes shifts, and the predicted PL signals were generally consistent with the experiment. The good agreement between the calculated and experimental excitation-emission maps of these MF-embedded CD systems indicates that MFs are the main source of broad excitation-independent emission in CDs.