Comparison of the excited-state proton transfer and single electron transfer mechanisms of the natural antioxidant Juglone and its dimer 3,3′-bijuglone

• The ESIDPT reaction for BJu is stepwise rather than concerted. • ICT process causes the fluorescence quenching phenomenon of Ju. • Dimerization enhances the antioxidant activity of Ju in the excited state. • ESDPT process has improved the antioxidant activity of BJu effectively. In this work, the excited-state intramolecular proton transfer (ESIPT) mechanism and antioxidant activity of Juglone (Ju) and its dimer 3,3′-bijuglone (BJu) were systematically compared and analyzed by using the density functional theory (DFT) and time-dependent DFT (TD-DFT) methods. Via comparing the primary hydrogen bond parameters, reduced density gradient (RDG) versus sign(λ 2 )ρ(r) scatter plots, interaction region indicator (IRI) isosurface, topology descriptors, and infrared (IR) spectra, it can be confirmed that all the intramolecular hydrogen bonds (IHBs) of Ju and its dimer 3,3′-bijuglone (BJu) have been enhanced in the lowest excited state (S L ). The intramolecular charge transfer (ICT) triggered the fluorescence quenching mechanism for Ju was confirmed based on the hole-electron analysis. Moreover, the calculated S 0 - and S L -stated potential energy surfaces (PESs) have proven that the excited-state intramolecular double proton transfer (ESIDPT) of BJu is stepwise rather than concerted and based on which the detailed proton-transfer (PT) process has been clarified. Compared with the first step PT behavior of BJu, the ESIPT process of Ju has barely been influenced by dimerization. Besides, the computed ionization potential (IP) has substantiated that BJu shown the better antioxidant capacity via the single electron transfer (SET) mechanism than the corresponding monomer Ju in the S L state. The relationship between the PT process and antioxidant activity of Ju and BJu has also been successfully established, which is conducive to design and synthesize highly effective PT-based antioxidants.