Synchronous or stepwise Mechanism? a theoretical study on the Excited-State double proton transfer properties of shikonin and acetylshikosin

Abstract

The excited state double proton transfer (ESDPT) mechanism of shikonin (Shk) and its derivative acetylshikosin (AcShk) were studied by density functional theory (DFT) and time-dependent density functional theory (TD-DFT) methods. The potential energy curves scanned along the coordinates of proton transfer indicate a preference for the ESDPT reaction to occur step by step. The AcShk molecule possesses an additional reaction pathway in comparison to the Shk molecule. Furthermore, efforts have been made to compute the absorption and fluorescence peak, which exhibits favorable conformity with the experimental findings of the system investigated. The fluorescence spectra in cyclohexane and acetonitrile solvents indicate that the solvent polarity affects the location of the ESDPT fluorescence peak in both Shk and AcShk systems. The fluorescence spectra concentrated in the green light region (504 nm ∼ 550 nm) are obtained, which has the potential to promote human health through disinfection and boosting the immune system.