Facile synthesis of multicolor photoluminescent polymer carbon dots with surface-state energy gap-controlled emission

The facile and effective synthesis of truly multicolor photoluminescent (PL) polymer carbon dots (PCDs) is still a considerable challenge. Here, a mild and facile synthetic method to prepare multicolor emissive PCDs by self-oxidation and autopolymerization using hydroquinone and ethylenediamine as precursors is reported. The resulting PCDs were separated via silica-gel column chromatography, which gave three types of PCDs emitting bright and stable yellow, green, and blue fluorescence. The three types of PCDs exhibited attractive features such as excellent solubility, excitation wavelength-independent photoluminescence, high stability, monoexponential fluorescence lifetimes, and homogeneous optical properties. The uniform optical properties of the as-prepared PCDs helped us to explore their multicolor emissive PL mechanism. Detailed characterization and investigation of the PCDs revealed that their surface state was primarily responsible for the observed multicolor emissive photoluminescence. A PL mechanism that involved control of the energy gap by the surface state was proposed to explain the effect of the surface functional groups on the PL properties of the PCDs. The emission of the PCDs was strongly associated with the CQN functional groups on their surface. The band gap narrowed with increasing content of CQN on the PCD surface, resulting in the red shift of the PL emission peak.