Rational Dopant Design Uncovers Precursor–Structure Rules for Room-Temperature Phosphorescence in Carbonized Polymer Dots

This study unveils how heteroatom incorporation and the structural geometry of dopant precursors tune the fluorescence and room-temperature phosphorescence (RTP) of carbonized polymer dots (CPDs). Citric acid was selected as the carbon source, while cysteine, homocysteine, cysteamine, N-acetylcysteine, and thiomalic acid served as structurally related dopant precursors. Although CA–HCys–CPDs, synthesized from hydrothermal treatment of citric acid and homocysteine, did not exhibit the highest total emission quantum yield, they possessed a comparable phosphorescence quantum yield and longer phosphorescence lifetimes and showed persistent afterglow. The enhanced performance arises from the additional methylene in homocysteine, which increases chain spacing and promotes a confined-domain cross-link-enhanced emission effect, as well as from the formation of itaconic anhydride–related compounds, which generate fluorescence and phosphorescence through cluster-triggered emission. Additionally, X-ray photoelectron spectroscopy confirms the indispensable role of nitrogen in enabling the RTP of the CPDs. Time-dependent dialysis experiments demonstrate that high-molecular-weight polymeric luminophores and polymer-integrated carbon dots dominate the phosphorescence of the CPDs. Importantly, the combination of reversed-phase high-performance liquid chromatography coupled with electrospray ionization mass spectrometry and time-dependent density functional theory simulations identified two itaconic anhydride–derived molecules as core luminophores governing both fluorescence and phosphorescence. These findings offer new guidelines for the rational design of CPDs with enhanced and tunable phosphorescent properties.