In view of the enormous potential of fluorescence chemosensors in recent years, more and more people focus on their developments. In the present work, we theoretically investigate a novel fluorescence sensor 2-{[2-(2-Hydroxy-phenyl)-1H-benzoimidazo-5-yl]-phenyl-methylene}-malononitrile (HBPMM) [J. Lumin. 2016, 173, 165] about its excited state intramolecular proton transfer (ESIPT) and probe response mechanism. Based on density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods, we focus on the $S_0$-state and $S_1$-state hydrogen bonds dynamical behaviors and confirm that the strengthening intramolecular hydrogen bond in the $S_1$ state may promote the ESIPT reaction. In view of the photoexcitation, we find that the charge redistribution around hydroxyl moiety plays important roles in providing driving force for ESIPT. And the constructed potential energy curves further verify the ESIPT process of HBPMM should be ultrafast. That is the reason why the normal HBPMM fluorescence cannot be detected in previous experiment. Further, with the addition of fluoride anions, the exothermal deprotonation process occurs spontaneously along with the intermolecular hydrogen bond O-H•••F. It reveals the uniqueness of detecting fluoride anion using HBPMM molecule. As a whole, the fluoride anion inhibits the initial ESIPT process of HBPMM, which results in different fluorescence behaviors. This work presents the clear ESIPT process and fluoride anion sensing mechanism for the novel HBPMM chemosensor.

Two N-H type excited state intramolecular proton transfer (ESIPT) systems (i.e., 2-amino-3-(2’-benzoxazolyl)quinoline (ABO) and 2-amino-3-(2’-benzothiazolyl)-quinoline (ABT)) have been investigated. Adopting DFT and TDDFT methods coupling with B3LYP functional and TZVP basis set, our simulations about ABO and ABT molecules have successfully reappeared experimental results, based on which the rationality of our calculations is confirmed. Using Atoms in Molecules (AIM) analytical method, we firstly explore the interactions about chemical bond and verify the formation of hydrogen bond N-H•••N for ABO and ABT in the $S_0$ state. Investigating the primary geometrical parameters involved in N-H•••N, we find it should be strengthened in the $S_1$ state. Upon photoexcitation, charge transfer phenomenon is found via frontier molecular orbitals (MOs), and charge redistribution provides the tendency of ESIPT reaction for ABO and ABT. According to our constructed potential energy curves of both $S_0$ and $S_1$ states for ABO and ABT using two kinds of methods (i.e., the elongation of N-H single bond and the weakening of H•••N hydrogen bond), we clarify the ESIPT mechanisms and explain the recovery of four-level reaction cycle. Our searching transition state (TS) structures and simulated intrinsic reaction coordinate (IRC) path further confirm the ESIPT reaction.

The effect of propenyl group on the antioxidant activity of p-cresol and its derivatives were investigated in gas and solvent phases by using density functional theory (DFT) method. Three accepted antioxidant mechanisms, including hydrogen atom transfer (HAT), single-electron transfer followed by proton transfer (SET-PT) and sequential proton loss electron transfer (SPLET), were considered to explore which mechanism p-cresol and its derivatives preferred to in different environments. The bond dissociation enthalpy (BDE), ionization potential (IP), proton dissociation enthalpy (PDE), proton affinity (PA) and electron transfer enthalpy (ETE), closely related to the above three antioxidant mechanisms, were calculated systematically. The obtained results indicate that the antioxidant process of studied compounds prefers to the SET-PT and SPLET mechanism in gas phase and polar solvents, respectively. Meaningfully, the introduction of the propenyl group into the molecular structure can reduce the PA values and then improve the antioxidant activity of the studied compounds, which can provide theoretical guidance for the synthesis of novel antioxidants in the experiment.

At the B3LYP/6-311+G(d, p)/IEFPCM (in dichloromethane) theory level, the N-H-based excited-state intramolecular proton transfer (N-H-based ESIPT) process of 2-(2’-aminophenyl)benzothiazole (PBT-NH2) and its three derivatives 2-(2’-methylaminophenyl) benzothiazole(PBT-NHMe), 2-(2’-acetylaminophenyl)benzothiazole (PBT-NHAc) and 2-(2’-tosylaminophenyl) benzothiazole (PBT-NHTs) have been explored by the time-dependent density functional theory (TD-DFT) method. Our calculated hydrogen bond lengths and angles sufficiently confirm that the intramolecular hydrogen bonds $N_1-H$•••$N_2$ formed at the $S_0$ states of the four compounds should be significantly strengthened in the $S_1$ state, which are further supported by the results obtained based on the analyses of infrared spectra shifts. The scanned potential energy curves reveal that the energy barriers of the first singlet excited state of the four titled compounds along the ESIPT reactions are predicted at 8.74, 8.98, 6.72 and 1.69 kcal/mol, respectively, suggesting that the inclusion of a strong electron-withdrawing tosyl (Ts) group can remarkably facilitate the occurrence of the ESIPT reaction, while the involvement of an electron-donating methyl group has slight opposite effect on the ESIPT process of the amino-type hydrogen-bonding system.