This study uses density functional and time-dependent density functional theory to investigate excited-state intramolecular proton transfer (ESIPT) reactions of 1-(acylamino)-anthraquinons (AYAAQs). We report that hydrogen-bond intensity in excited states is affected by the different AYAAQs substituted groups in ethanol for the first time. Absorption and emission spectra were also calculated and show that the changed AYAAQs spectra can be explained perfectly by combining the AYAAQs potential energy curves. The theoretical spectral values show good agreement with experimental results. The theoretical calculations indicate that proton transfer reactions can be implemented in the first excited (S1) state. The hydrogen-bond strengthening mechanism was confirmed, where hydrogen-bond interaction acts as the driving force for the ESIPT reactions. Therefore, ESIPT reactions are more likely to occur from AAAQ → CAAQ → DCAQ → TFAQ molecules, which are different substituted derivatives.

The proton transfer reaction in methyl 5-R-salicylate is found to be highly sensitive to the presence of specific substituents in resonance with the hydroxyl group, leading to different fluorescence behaviors of methyl 5-R-salicylate with different substituents (J. Catalán, J. Phys. Chem. B, 2015, 119, 2132). But a detailed survey of its reaction mechanism is lacking. In our research, the hydrogen bond strengthening behavior in excited states is affected by the different substituents that have been reported for the first time. Absorption and emission spectra calculated for the work presented here agree well with experimental results. At the same time, in order to provide a reliable description of the reaction energy profiles, we compare the barrier differences obtained using CAM-B3LYP and B3LYP methods, and we visually observe the effect of different substituents on the ESIPT reactions in methyl 5-R-salicylates by combining the potential energy curves. So the excited state intramolecular proton transfer (ESIPT) reactions in methyl 5-R-salicylate molecules are investigated in detail using density functional theory (DFT) and time dependent density functional theory (TDDFT) methods. It can be confirmed that the mobility of the intramolecular π electrons is affected by an increase in the resonant strength of the different substituents and hydroxyl groups. As a consequence, a hydrogen bonding interaction gradual weakening mechanism has been perfectly verified, that is, the ESIPT reaction is more difficult to occur from MS → 5MeMS → 5FMS → 5ClMS → 5BrMS → 5MeOMS → 5AmMS molecules.

By utilizing the density functional theory (DFT) and the time-dependent density functional theory (TDDFT), the excited state intramolecular proton transfer (ESIPT) mechanism of o-hydroxynaphthyl phenanthroimidazole (HNPI) is studied in detail. Upon photo is excited, the intramolecular hydrogen bond is obviously enhanced in the S 1 state, which thus promotes the ESIPT process. Hydrogen bond is shown to be strengthened via comparing the molecular structures and the infrared vibration spectra of the S 0 and S 1 states. Through analyzing the frontier molecular orbitals, we can conclude that the excitation is a type of the intramolecular charge transfer excitation, which also indicates the trend of proton transfer in S 1 state. The vertical excitation based on TDDFT calculation can effectively repeat the absorption and fluorescence spectra of the experiment. However, the fluorescence spectrum of normal structure, which is similar to the spectrum of isomer structure is not detected in the experiment. It can be concluded that the fluorescence measured in the experiment is attributed to both structures. In addition, by analyzing the potential energy curves (PECs) calculated by the B3LYP functional method, it can be derived that since the molecule to cross the potential barrier in the S 1 state is smaller than in the S 0 state and the reverse proton transfer process in the S 1 state is more difficult than in the S 0 state, the ESIPT occurs in the S 1 state.

The novel photodeamination process of cresol derivatives 1 and 3 has been reported experimentally ( J. Org. Chem . 2015 , 80 , 10817 ). However, a full theoretical interpretation of the mechanism is still lacking. In the present study, we aim to provide insight into the factors that promote the deamination reaction through density functional theory (DFT) and time-dependent DFT methods. Calculated absorption and emission spectra are in good agreement with the experimental results. Hydrogen-bond strengthening in the excited state has been verified by analyzing relevant bond parameters and vibrational frequencies as well as frontier molecular orbitals (FMOs), implying that hydrogen-bond interaction acts as the important parameter for the excited-state intramolecular proton-transfer (ESIPT) reaction. The proton-transfer and deamination reactions have been qualitatively analyzed through Gibbs free-energy reaction profiles in different electronic states. It can be concluded that the ESIPT and photodeamination reactions occur in the excited state. To further illustrate the photodeamination mechanism, the constructed 2D potential-energy surface indicates that the photodeamination reaction is infeasible without the ESIPT reaction. This work provides the first theoretical rationale for ESIPT-induced photodeamination occurring spontaneously because of protonation of a basic nitrogen atom.

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