Ingeniously regulating the antioxidant activities of hydroxyanthraquinone-based compounds via ESIPT reaction: Combining experiment and theory methods

Effects of functional groups on ESIPT and antioxidant activity of apigenin: A non-existent enol* state fluorescein

The development and enhancement of antioxidant drugs, which are aimed at mitigating DNA damage, mutations, and cancer, are of paramount significance in the biomedical sphere. In recent years, antioxidant drug molecules with photoluminescence have sprung up like mushrooms. Apigenin (AP), characterized by its distinctive property of excited state intramolecular proton transfer (ESIPT), plays a pivotal role in mediating antioxidant and anticancer activities. Despite being a representative molecule of the non-existent enol form (E*) state with ESIPT nature, there is a notable lack of theoretical investigations into its antioxidant properties. Herein, density functional theory (DFT) and time-dependent DFT methodologies were utilized to explore the effects of various functional groups on AP molecules in a methanol solvent. Studies have demonstrated that for the non-existent E* state fluorescence molecule AP, the ESIPT process can significantly enhance the antioxidant potency of AP and its derivatives. However, the introduction of electron-withdrawing groups significantly accelerated the ESIPT process while simultaneously suppressing the antioxidant activity of AP-CN. Conversely, the incorporation of electron-donating groups effectively inhibited the ESIPT process, yet markedly enhanced the antioxidant activity of AP-NH2. This investigation furnishes vital perspectives and sources of reference for the conception and advancement of groundbreaking antioxidant medications that aim to tackle non-existent E* state molecules.

Distinguishing the responsive mechanisms of fluorescent probes to hydrogen peroxide, proteins, and DNA/RNA

The responsive mechanisms of cationic quinolinium-vinyl-N,N-dimethylaniline boronate (QVD-B) derivative probes to hydrogen peroxide (H2O2), proteins and DNA/RNA are theoretically investigated in this study. The potential energy curves of QVD-B scanned on a dihedral angle (N+-C-CC) in the first singlet (S1) state exhibit large torsional energy barriers. Additionally, the energy of the lowest unoccupied molecular orbital (LUMO) of an acceptor moiety (-3.14 eV) is lower than that of a donor moiety (-1.13 eV) in QVD-B. This demonstrates that photoinduced electron transfer (PET) quenches the fluorescence of QVD-B, as opposed to the previous report of intramolecular single-bond rotation. After reacting with H2O2, the reaction product of quinoline-vinyl-N,N-dimethylaniline (QVD) turns off the PET pathway and turns on the fluorescence at 550 nm, which is consistent with the experimental results (580 nm). Among the possible configurations of QVD-B that forms with proteins and DNA, the calculated fluorescence values of corresponding twisted QVD-B-P (638 nm) and QVD-B-D (686 nm) are consistent with the experimental results (632 and 688 nm). The frontier molecular orbital and electron-hole analysis show that the charge transfer distance follows the order of QVD (1.88 Å) < QVD-B-P (4.49 Å) < QVD-B-D (6.39 Å), which induces the fluorescence red-shifts of QVD-B-P and QVD-B-D compared to that of QVD. The multi-detection mechanism of the fluorescent probe QVD-B is attributed to PET progress and different degrees of local charge transfer after photoexcitation.

Simultaneously regulating absorption capacities and antioxidant activities of four stilbene derivatives utilizing substitution effect: A theoretical and experimental study against UVB radiation

With the continued depletion of the ozone layer, the sun protection consciousness of humans has gradually enhanced. Long-term ultraviolet (UV) rays exposure will lead to skin tanning, even skin cancer in severe cases, and generate free radicals to cause skin aging. To better protect human skin against UV rays, this work explores the absorption capacities and antioxidant activities of four stilbene derivatives (EHDB, EDMB, EAPD, and HPTP) through the computational chemistry method and DPPH radical scavenging experiment. The research results indicate that their absorption spectra cover the entire UV region, and can effectively protect against UVB radiation. Moreover, three prevailing antioxidant mechanisms: hydrogen atom transfer, sequential proton loss electron transfer, and single electron transfer followed by proton transfer mechanisms, were used to evaluate their antioxidant activities in the ground state. It can be concluded that the O1H1 sites of EHDB and HPTP are the most active, and the SPLET mechanism is the most preferred for the four compounds in ethanol solvent. Furthermore, the DPPH radical scavenging experiment compensates for the theoretical calculation deficiency in the excited state, revealing that the EHDB and HPTP are the most suitable for sunscreen due to their excellent performance on antioxidant capacities, whether before or after sunlight. This work will facilitate EHDB and HPTP to be applied in sunscreen and provide a novel idea in sunscreen research.

Uncovering the dependence of ESIPT behaviors and fluorescence properties of two new benzothiazole-based fluorophores on solvent polarity: A TD-DFT study

We have studied the spectral features and excited state intramolecular proton transfer (ESIPT) processes of 2-(2',4'-dihydroxyphenyl)benzothiazole (OHBT) and 2-(2'-hydroxy-5'-chlorophenyl)benzothiazole (CHBT) using density functional theory (DFT) and time-dependent density functional theory (TD-DFT). To consider the impact of solvent polarity and intermolecular hydrogen bond (H-bond) on the ESIPT behavior and photophysical properties, four solvents including toluene (TL), tetrahydrofuran (THF), methanol (MeOH) and dimethylsulfoxide (DMSO) were used. The simulated absorption and fluorescence wavelengths of OHBT and CHBT are well consistent with the experimental values. According to the results of structures, electron density and infrared (IR) vibrational frequencies, we found that the intramolecular H-bonds in OHBT/CHBT and OHBT-MeOH/CHBT-MeOH are strengthened in the first singlet excited state (S₁), which will be benefical to the ESIPT process. The potential energy curves (PECs) verified that the ESIPT processes in OHBT/CHBT and OHBT-MeOH/CHBT-MeOH can take place much easier because of their lower energy barrier. The influences of solvent polarity on ESIPT behaviors and photophysical properties of OHBT and CHBT are summarized below. As the solvent polarity becomes stronger from TL to DMSO, the energy gaps enlarges a little, the maximum absorption and fluorescence peaks at normal form red-shift slightly, and the strengths of H-bond in S₁ state become weaker, which makes the ESIPT process occur much harder. The formation of intermolecular H-bond between OHBT/CHBT and MeOH is conducive to promote the ESIPT process of OHBT/CHBT.

Hydrogen-bond activated ESIPT in naphthalimide-based fluorescent probe for sensing volatile amines

Amines levels present important indicative value in food safety and human health. Although they are involved in some normal physiological responses of the organism, their overproduction or intake may cause pathological responses. Herein, we report a recyclable visual packaging bag for volatile amines detections based on the naphthylamide derivative N-S and its positive PL characteristics. Specifically, handmade test strips based on compound N-S have been applied to fish freshness labeling, and the cyclic fumigation experiment shows its restorable PL effect and efficiency. The possible PL transfer mechanism of naphthylamide derivative N-S is uncovered by the density functional theory (DFT) calculation and titration mass spectrometer and ¹H NMR. This work expands a conjugation in a molecule by hydrogen-bond activated ESIPT (H-ESIPT) and provides a portable detection method for volatile amines detection.

Enhancing fluorescence of benzimidazole derivative via solvent-regulated ESIPT and TICT process: A TDDFT study

In this work, we use density functional theory and time dependent density functional theory to explore the ESIPT and TICT process of 6-(1H-Benzoimidazol-2-yl)-2,3-dimethoxy-phenol (BIDOP) in cyclohexane (CHX) and tetrahydrofuran (THF) solvent, respectively. It reveals that ESIPT process of BIDOP can occur in both CHX and THF solvent at the first excited state with similar reaction barrier. Remarkably, compared to barrierless from keto (K*) to TICT state of BIDOP in THF solvent, the reaction barrier between K* and TICT state is up to 20.28 kcal/mol for in CHX that TICT process is inhibited in CHX solvent. The absence of nonradiative decay TICT state of BIDOP in CHX solvent induces higher fluorescence in CHX compared to in THF solvent. These findings indicate that CHX solvent can effectively enhance fluorescence of BIDOP. Our study highlights a convenient approach for enhancing fluorescence and is significant for photophysics and photobiology field.