Role of Acids in Producing Ultrabright, Dual‐Emissive Carbon Dots and their Urea/Biuret Composites with Ultralong Afterglow

Since 2015, m‐phenylenediamines (mPD) have become a popular carbon source for the synthesis of carbonized polymer dots (CPDs). However, their exact fluorescence mechanism is still obscure. To elucidate this, inorganic acids that are carbon‐free are chosen as additives for a comparative study. It is found that the green fluorescence quantum yield (nearly 80%), photostability, and reaction yield (over 90%) can be enhanced by introduction of most of inorganic acids with moderate amount. Besides, green‐blue dual emission is observed in acid‐assisted groups. UV‐vis absorption, Fourier‐transform infrared spectroscopy, and surface‐enhanced Raman scattering results indicate that the green fluorescence center is composed of quinoid rings, whereas the blue fluorophore contains benzenoid rings. Moreover, room‐temperature afterglow with lifetime up to 1.25 s is observed exclusively in acid‐assisted CPDs composites with urea/biuret. The blue chromophore is proposed to be the origin of the triplet level that induces the long afterglow. This work provides an in‐depth understanding on the macromolecular structures of CPDs derived from phenylenediamines, and contributes a new line of thought to the origin of phosphorescence in N‐doped carbon dots.