Hydrogen-Bond-Mediated Through-Space Conjugation for the Fabrication of Red-Emissive Solid-State Carbon Dots and Application in LEDs

Solid-state red-emissive carbon dots (CDs) resistant to aggregation-caused quenching are highly desirable for optoelectronic materials. Here, Pd and N codoped carbon dots (CDs-N/Pd) were synthesized via a hydrothermal method using 4-aminophenylacetylene as the carbon source and PdCl₂ as the dopant. The emission of CDs-N/Pd showed a concentration- and aggregation-dependent red shift: blue at 430 nm (0.001 mg/mL), green at 490 nm (0.1 mg/mL), and red at 684 nm (shoulder peaks at 656-762 nm) in concentrated solution (50 mg/mL) and solid state. TEM analysis indicated that the red shift was associated with physical stacking of CDs. Spectroscopic studies (¹H/¹³C NMR, FT-IR, and XPS) confirmed the formation of short polyene chains, partial retention of aromatic side chains, and coordination of amino groups with Pd. DFT calculations elucidated interactions between CDs, π-electron regions, and HOMO-LUMO gaps, revealing the emission mechanism. Blue emission at low concentrations arose from short conjugation in isolated CDs-N/Pd, while at higher concentrations, hydrogen bonding between surface amino groups reduced the interparticle distance, induced through-space conjugation, extended the conjugation, and produced red-shifted emission. CDs-N/Pd also displayed aggregation-enhanced emission. Multicolor LEDs (blue, green, red) were fabricated using CDs-N/Pd. This work offers a strategy for designing solid-state red-emissive CDs.