UV Protection and Singlet Oxygen Quenching Activity of Aloesaponarin I

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.

The theoretical study of excited-state intramolecular proton transfer of 2,5-bis(benzoxazol-2-yl)thiophene-3,4-diol

The symmetrical structures 2,5-bis(benzoxazol-2-yl)thiophene-3,4-diol (BBTD) can take shape two intramolecular hydrogen bonds in chloroform. In order to research the molecular dynamic behavior of BBTD upon photo-induced process, we utilize density functional theory (DFT) and time-dependent density functional theory (TDDFT) to complete theoretical calculation. Through the comparison of bond length, bond angle, IR spectra, and frontier molecular orbitals between ground state (S₀) and first excited state (S₁), it clearly indicates that photoexcitation have slightly influence for intensity of hydrogen bond. For the sake of understanding the mechanism of excited state intramolecular proton transfer (ESIPT) of BBTD in chloroform, potential energy surfaces have been scanned along with the orientation of O₁-H₂ and O₄-H₅ in S₀ and S₁ state, respectively. A intrigued hydrogen bond dynamic phenomenon has been found that ESIPT of BBTD is not a synergetic double proton transfer process, but a stepwise single proton transfer process BBTD→BBTD-S→BBTD-D. Moreover, the proton transfer process of BBTD-S→BBTD-D is easier to occur than that of BBTD→BBTD-S in S₁ state.

Photoinduced excited state intramolecular proton transfer and spectral behaviors of Aloesaponarin 1

The novel spectral behaviors of Aloesaponarin 1 (AS1) are investigated by studying the dynamics process of excited state intramolecular proton transfer (ESIPT). Two intramolecular hydrogen bonds (HB1 and HB2) are formed between hydroxyl and carbonyl groups of AS1. The calculated potential energy curves of AS1 demonstrate that the ESIPT process along HB1 is energy favorable while not along HB2. The analysis of potential energy curves describes clearly the dynamic behaviors of the proton transfer process from hydroxyl group to carbonyl group along HB1. The infrared spectra of AS1 confirm that the stretching absorption peak of hydroxyl group in HB1 disappears and that a new peak corresponding to hydroxyl group appears in the first excited state, which depicts the ESIPT process indirectly. The fluorescence peaks of AS1 (636nm), AS2 (Aloesaponarin 1 3-O-methyl ether, 629 nm) and AS3 (Aloesaponarin 1 8-O-methyl ether, 522 nm) demonstrate that the fluorescence behavior of AS1 is primarily effected by HB1 rather than HB2. The large Stokes shifts of AS1 (206 nm) indicate that the absorbed energy is partly transferred to non-harmful long fluorescence through ESIPT process, which plays important role in the explanation for the UV protection property of AS1. The inducement and influence factors of ESIPT process of AS1 are illustrated by analyzing electrostatic potential, molecular orbital and natural bond orbital.

Singlet oxygen quenching by trolox C in aqueous micelle solutions

Singlet oxygen ((1)O(2)) quenching by trolox C (TC, 6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid), which is a water-soluble vitamin E analogue, in ethanol-water solutions and in aqueous SDS and CTAC micelles was studied by measuring the time-profiles of (1)O(2) phosphorescence at 1274 nm. The second-order rate-constant for (1)O(2) quenching by TC was determined in the ethanol-water solution to be 1.03 x 10(8), 6.22 x 10(7) and 6.23 x 10(7) M(-1) s(-1) at pH 2.0, 7.0, and 8.4, respectively. These values mean that the non-dissociated form of TC under acidic conditions has superior activity to the mono-anion form. In aqueous micelle systems, the decay rate of (1)O(2) at first decreased and then increased with increase of the concentration of TC. This behavior is explained in terms of the (1)O(2) quenching by TC in the bulk phase and in terms of shifting the environment surrounding (1)O(2) to lipophilic by dissolving TC in the hydrophobic region inside the micelle. The present investigation on (1)O(2) emission dynamics in inhomogeneous solutions made it possible to detect a little change in the solutions, which affects the environment around (1)O(2), such as the micelle formation and dissolving solute in the micelle.