The investigation of excited-state intramolecular proton transfer (ESIPT) has been carried out via the density functional theory (DFT) and the time-dependent density functional theory (TDDFT) method for natural product quercetin in dichloromethane (DCM) solvent. For distinguishing different types of intramolecular interaction, the reduced density gradient (RDG) function also has been used. In this study, we have clearly clarified the viewpoint that two kinds of tautomeric forms (K₁, K₂)originated from ESIPT processconsist inthe first electronic excited state (S₁). The phenomenon of hydrogen bonding interaction strengtheninghas been proved by comparing the changes of infrared (IR) vibrational spectra and bond parameters of the hydrogen bonding groups in the ground state with that in the first excited state. The frontier molecular orbitals (MOs)provided visual electron density redistribution have further verified the hydrogen bond strengthening mechanism. It should be noted that the ESIPT process of the K₂form is easier to occur than that of the K₁form via observing the potential energy profiles. Furthermore, the RDG isosurfaces has indicated that hydrogen bonding interaction of the K₂form is stronger than that of the K₁formin the S₁state, which is also the reason why the ESIPT process of the K₂form is easier to occur.

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.

The detection of toxic phosgene is extraordinarily crucial. A fluorescence probe 2-(2-aminophenyl)benzothiazole (abbreviated as probe 1), designed to detect phosgene, had been developed based on an excited state intramolecular proton transfer (ESIPT) process in experiment (L. Y. Chen, D. Wu, J. M. Kim and J. Yoon, Anal. Chem., 2017, 89, 12596–12601). However, the ESIPT mechanism failed to be qualitatively investigated and it is questionable that ESIPT emission of probe 1 was not observed in experiment. In this work, we give in-depth insight into the ESIPT reaction process and non-radiative deactivation mechanism of probe 1 for the first time. Theoretical absorption and fluorescence-emission spectra matched the experimental results. The excited state hydrogen bond strengthening mechanism was verified. The torsional isomer structure of probe 1 was found by building and analyzing the potential energy curves that demonstrate proton transfer just occurs in the excited state. The obtained cross point of the potential energy curves demonstrated that intersystem crossing and phosphorescence are inactivation channels, and the calculated phosphorescence value is 728 nm, which explains the reason behind the unobserved ESIPT-emission spectrum. The frontier molecular orbitals were shown and displayed charge redistribution, which further explains that the quenched fluorescence originates from the twisting intramolecular charge transfer (TICT) character of photoisomerization.