Rapid solvent-free synthesis of carbon dots with yellow to red emission for LED spectral conversion

A rapid and solvent-free, microwave-assisted strategy was developed to synthesize carbon dots (CDs) exhibiting intrinsic yellow-to-red emission, enabling their direct use in LED spectral conversion. Remarkably, this green approach enables the formation of yellow to red emissive CDs within approximately 1 min, without the use of any solvents, highlighting a highly sustainable and time-efficient route compared to conventional wet-chemical methods. The thermal treatment yielded CDs with diverse nitrogen and oxygen surface groups, producing separable emissive fractions. Yellow and red-emitting (y-CDs and r-CDs) were isolated from a single synthetic batch via centrifugation, driven by surface polarity. Graphitic-N emerged as the key configuration modulating the emission redshift, supported by both structural and computational analyses. The r-CDs exhibited broader lattice fringes (∼0.31 nm) and approximately twice the graphitic-N and pyridinic-N content compared to y-CDs (∼0.21 nm), which contained higher oxygen levels and pyrrolic-N dominance. Time-dependent density functional theory (TD-DFT) calculations revealed that only graphitic-N significantly narrows the HOMO–LUMO gap, inducing a shift in emission spectra. Dipole moment and electrophilicity index calculations provided mechanistic insight into polarity-driven separation of the emissive fractions. When embedded into transparent polymer matrices, the CDs demonstrated moderate photostability and enabled efficient yellow, red, and white light-emitting diodes (LEDs). The white LED exhibited CIE coordinates of (0.312, 0.353), CRI of 88, and a CCT of 6376 K, producing bright, cool-white light. This work establishes a rapid, green synthesis platform for broad-spectrum emissive CDs, elucidates a mechanistic framework for nitrogen-driven emission tuning and polarity-based separation, and expands the analytical potential of carbon-based fluorophores for spectral conversion and fluorescence-based sensing applications.