Insights into tautomerism and pH effects on the photoluminescence of citric acid-derived molecular fluorophores

Abstract

Understanding the photoluminescent (PL) behavior of citric acid (CA)-derived carbon dots (CACDs) requires accounting for the contribution of molecular fluorophores (MFs) formed in situ during synthesis. Among these, citrazinic acid (CZA), CAGly – derived from CA and glycine, and 4-hydroxy-1H-pyrrolo[3,4-c]pyridine-1,3,6(2H,5H)-trione (HPPT) exhibit donor–acceptor groups capable of tautomerization and protonation–deprotonation equilibria, making their PL properties highly sensitive to pH and solvent effects. In this work, we combine CREST screening with DFT/TD-DFT calculations to explore the tautomeric landscape of neutral and ionized forms of these MFs in both ground (S₀) and excited (S₁) states. Our results reveal that the keto tautomer (DPR moiety) predominates in S₀ for CZA and CAGly, whereas the enol tautomer (recovering the 2-pyridone ring) becomes the main phototautomer in S₁ through an excited-state intramolecular proton transfer (ESIPT) process, accurately reproducing previously reported experimental emission wavelengths. This mechanism is strongly influenced by pH and is facilitated by water molecules that lower the ESIPT barrier, revealing the effect of solvent. For HPPT, PL arises from competing pathways involving ESIPT and excited-state deprotonation (ESDP), accounting for a large Stokes shift and pH-independent emission up to pH 10. Overall, these findings evidence that excitation-dependent fluorescence in CACDs may be better explained by tautomeric equilibria and proton-transfer dynamics rather than anti-Kasha behavior, providing a molecular-level framework for tuning optical properties in carbon-based luminescent materials.