The effects of atomic electronegativity (O, S and Se atoms) on the excited-state intramolecular proton transfer (ESIPT) properties of three compounds (3-HF, 3-HTF and 3-HSeF) were systematically studied using the density functional theory (DFT) and time-dependent DFT (TD-DFT) methods. Furthermore, the antioxidant activities of the three compounds were evaluated using different parameters from the perspective of biological activities. The calculated results indicate that the generation of ESIPT is attributed to the excited-state intramolecular charge transfer owing to the change in electron density distribution on the frontier molecular orbitals and natural bond orbital (NBO) charge distribution of the O₁ and O₂ atoms at the S₀ and S₁ states. Moreover, 3-HTF shows the greatest red shift of the O₁–H₁ stretching peak and the lowest energy barrier at the S₁ state among the three compounds, implying that 3-HTF would have the strongest intramolecular hydrogen bond at the S₁ state, which is most beneficial for the occurrence of ESIPT. In addition, the decreased ionization potentials and energy gaps from 3-HF to 3-HTF and 3-HSeF means that the antioxidant activity of the compound would be enhanced along with the lowered atomic electronegativity. Interestingly, the lower ionization potentials and energy gaps of the compounds in keto forms suggest that the compound (3-HTF) easy to carry out ESIPT reaction would exhibit efficient antioxidant activity, which would establish the relationship between ESIPT reaction and antioxidant activity of the compound and provide a new notion for synthesizing more efficient antioxidants in experiments.

The photoelectrical properties of two dyes—ethyl red and carminic acid—as sensitizers of dye-sensitized solar cells were investigated in experiments herein described. In order to reveal the reason for the difference between the photoelectrical properties of the two dyes, the ground state and excited state properties of the dyes before and after adsorbed on TiO₂ were calculated via density functional theory (DFT) and time-dependent DFT (TDDFT). The key parameters including the light harvesting efficiency (LHE), the driving force of electron injection ( $\Delta G^{\mathrm{inject}}$ ) and dye regeneration ( $\Delta G^{\mathrm{regen}}$ ), the total dipole moment ( ${\mathsf{\mu }}_{\mathrm{normal}}$ ), the conduction band of edge of the semiconductor ( $\Delta E_{\mathrm{CB}}$ ), and the excited state lifetime (τ) were investigated, which are closely related to the short-circuit current density ( $J_{\mathrm{sc}}$ ) and open circuit voltage ( $V_{\mathrm{oc}}$ ). It was found that the experimental carminic acid has a larger $J_{\mathrm{sc}}$ and $V_{\mathrm{oc}}$ , which are interpreted by a larger amount of dye adsorbed on a TiO₂ photoanode and a larger $\Delta G^{\mathrm{regen}}$ , excited state lifetime (τ), ${\mathsf{\mu }}_{\mathrm{normal}}$ , and $\Delta E_{\mathrm{CB}}$ . At the same time, chemical reactivity parameters illustrate that the lower chemical hardness (h) and higher electron accepting power (ω⁺) of carminic acid have an influence on the short-circuit current density. Therefore, carminic acid shows excellent photoelectric conversion efficiency in comparison with ethyl red.

The excited state intramolecular proton transfer (ESIPT) reaction and photophysical properties of a series of 3-hydroxychromone (HC)-based derivatives (BFHC, BTHC, BSeHC, BDFHC, BDTHC and BDSeHC) were explored systematically using density functional theory (DFT) and time-dependent DFT (TD-DFT) methods. The enhanced excited-state intramolecular hydrogen bonds of the studied molecules contributed to the ESIPT reaction according to hydrogen-bond parameters and infrared-vibration analyses. The various substituent groups induced regular multi-channel signal changes (wavelength and intensity) of the fluorescence spectra of molecules in enol and keto forms. The potential energy curves showed the energy barriers in the S₁-state of the molecules followed the order BFHC < BTHC < BSeHC and BDFHC < BDTHC < BDSeHC. Combination of topology analyses of the bond critical point present in the molecules revealed that BFHC and BTHC would show an ultrafast ESIPT reaction, whereas the other molecules would have a slower ESIPT reaction. The weakened electron-withdrawing ability of an atom and longer substituent group would frustrate the ESIPT reaction of the studied molecules, thereby affecting their photophysical properties. These novel discoveries can extend the application of HC derivatives in ESIPT-based optical devices.