A theoretical investigation on excited-state single or double proton transfer process for aloesaponarin i

The excited-state proton transfer (ESPT) dynamical behavior of aloesaponarin I (ASI) was studied using density functional theory (DFT) and time-dependent DFT (TDDFT) methods. Our calculated vertical excitation energies based on TDDFT reproduced the experimental absorption and fluorescence spectra well [Nagaoka et al. J. Phys. Chem. B, 117, 4347 (2013)]. Two intramolecular hydrogen bonds were confirmed to be strengthened in the S₁ state, which makes ESPT possible. Herein, the ESPT process is more likely to happen, along with one hydrogen bond (O₁-H₂⋯O₃). Qualitative analyses about charge distribution further demonstrate that the ESPT process could occur because of the intramolecular charge transfer. Our constructed potential energy surfaces of both S₀ and S₁ states show that a single proton transfer reactive is more reasonable along with the intramolecular hydrogen bond (O₁-H₂⋯O₃) rather than O₄-H₅⋯O₆ in the S₁ stated potential energy surface. Then, ASI-SPT∗ decays to the ground state with a 640 nm fluorescence; subsequently, the ASI-SPT form shows that reverse ground state single-proton transfer back to the ASI structure occurs. Particularly, dependent on relatively accurate potential energy barriers among these excited-state stable structures, we confirmed the excited-state single proton transfer process rather than using the controversial nodal plane model.