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.

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