Fe-Doped 4-Aminophenylacetylene-Derived Red Emissive Polymer Carbon Dots: Synthesis and Anti-Counterfeiting Applications

Phenylacetylene derivatives serve as typical monomers for polyaddition reactions. In this study, we present a straightforward one-step protocol for synthesizing polyacetylene P0 (undoped), P0.09 (doped with 0.09 mg/mL Fe³⁺), and carbonized polymer dots CPDs-Fe (doped with 0.13 mg/mL Fe³⁺) using 4-aminophenylacetylene and varying concentrations of Fe³⁺ via solvothermal addition and carbonization (EtOH, 200 °C, 6 h, high-pressure reactor). The resulting P0, P0.09, and CPDs-Fe exhibit green, light red, and red luminescence with maxima at 514 nm (quantum yield, QY = 20.08%), 623 nm (QY = 8.17%), and 645 nm (QY = 9.03%), respectively. Structural and morphological analyses indicate that altering the doping concentration of Fe³⁺ and the reaction temperature results in a transformation from the amorphous, short-conjugated structure of P0 to the low-crystallinity, fibrous, and longer-conjugated structure of P0.09, and finally to the highly crystalline, elliptical, and largest-conjugated structure of CPDs-Fe, respectively. These structural and morphological changes lead to a shift in emission from green in P0 to red in CPDs-Fe. Density functional theory (DFT) calculations on the CPDs-Fe substructures reveal that coordination with Fe³⁺/Fe²⁺ and the elongation of the alkene chain enhances conjugation interactions, reduces the band gap, and consequently induces a red shift in emission. CPDs-Fe can be incorporated into poly(vinyl alcohol) (PVA) to form a CPDs-Fe@PVA composite film, which exhibits dual-mode fluorescence and room-temperature phosphorescence with tunable lifetimes, making it suitable for anti-counterfeiting applications based on phosphorescence. This study provides a strategy for converting 4-aminophenylacetylene into tunable CPDs with tailored structural and photonic properties, offering potential applications in fluorescence and phosphorescence-based technologies.