Switching between Fluorescence and Room Temperature Phosphorescence in Carbon Dots: Key Role of Heteroatom Functionalities

Herein, we have synthesized three different types of carbon dots, i.e., (a) CDs from citric acid, (b) nitrogen atom functionalized CDs from citric acid and ammonia, and (c) phosphorus atom functionalized CDs from citric acid and sodium dihydrogen orthophosphate through a simple bottom-up carbonization technique. Detailed morphological and elemental features are investigated by high-resolution transmission electron microscopy and X-ray photoelectron spectroscopy study, and it is further correlated to the ongoing photophysical properties. To investigate the specific role of the heteroatom functionalities on the room temperature phosphorescence, we have incorporated all these three types of CDs in a boric acid matrix to diminish the flexibility of surface functional groups and decrease the nonradiative relaxation processes. Various heteroatom functionalities play a very specific role to tune the afterglow properties by altering the energy gap (ΔEST) between the lowest excited singlet (S1) and triplet state (T1) and the spin–orbit coupling constant which eventually control the radiative recombination from the triplet state. Finally, a switchable fluorescence and room temperature phosphorescence have been observed depending on the specific heteroatom functionalities. A detailed temperature-dependent study has been performed to investigate the tunability between prompt fluorescence and phosphorescence properties. This is further correlated to the conversion of phosphorescence with thermally activated delayed fluorescence (TADF). Computational studies based on time-dependent density functional theory (TD-DFT) have been performed by using optimized model systems in connection to the elemental study, which nicely support our experimental findings.