Lignin and Phenylenediamine Synergy in Carbon Dots: Deciphering the Origin of Long-Wavelength Photoluminescence and the Carbon Core Growth

Abstract

Lignin, a major component of lignocellulosic biomass, is an attractive precursor for carbon dots (CDs) due to its high carbon content, intrinsic aromatic units, and abundant oxygen-containing functional groups. However, lignin-derived CDs typically exhibit photoluminescence (PL) restricted to the blue-green region, constituting a key bottleneck for their application. Although incorporating aminobenzene compounds can induce a red-shift, the molecular origin of the resulting long-wavelength emission remains elusive, and the evolution pathway in multi-component precursor systems is poorly understood. Herein, we synthesize multi-color CDs from lignin and p-phenylenediamine (PPD) with a high quantum yield. A trimeric phenazine derivative is identified as the definitive molecular fluorophore responsible for the red emission at 609 nm. This red-emitting species evolves through an oxygen-dependent conversion of a PPD-derived dimer. Concurrently, lignin undergoes depolymerization, recondensation and carbonization to form a carbon core, which confers exceptional photostability compared to CDs derived from PPD, PPD/PVA, and PPD/PEG precursors, attributed to its extended sp2-conjugated structure and radical-scavenging functional groups. We systematically unravel the synergistic interplay between lignin transformation and PPD oxidation in the formation of the carbon core and fluorophore incorporation. These findings provide molecular-level insight into the fluorescence mechanism and demonstrate a sustainable route for producing tunable, high-performance CDs.