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The excited-state intramolecular proton transfer (ESIPT) process involving dual hydrogen bond acceptors (HBAs) is of great interest in research and has significant potential for applications due to its flexible adjustability. In this work, the solvent polarity-dependent changes in structures, photophysical properties and ESIPT processes of 2-(2-hydroxy-3-hydroxyiminomethyl-5-methylphenyl)-benzimidazole (HHMB) with two asymmetric HBAs are investigated. Firstly, the orientation of intramolecular hydrogen bond (IHB) under different solvent polarities are explored. The computational results indicate that in non-polar solvents, the central hydroxyl group predominantly forms an IHB with the imine of the oxime group, resulting in large Stokes and near-infrared emission peaks. Conversely, in polar solvents, the central hydroxyl group links the imidazole group to form a stronger IHB with improved fluorescence efficiency. Additionally, the quantum chemical calculations are used to simulate the absorption–emission spectra and plot the frontier molecular orbitals, as well as the potential energy curves about ESIPT in different solvents. In conclusion, we elucidate the rationale behind the preferential formation of the biased IHBs and examine its impact on the ESIPT process by manipulating the solvent polarity. Theoretical studies on the dual HBAs molecule, exhibiting specific selectivity, pave the way for the development of higher-performance fluorescence probes and luminescent materials. [ABSTRACT FROM AUTHOR]

The redox balance between HClO and H2S is essential for the physiological and pathological properties of organisms. Therefore, monitoring the redox process between HClO and H2S is of great significance. Shen et al. synthesised a reversible ratiometric fluorescence probe BT-Se based on excited state intramolecular proton transfer (ESIPT) reaction for the detection of HClO and H2S in previous experiments. When BT-Se detects HClO, it will be oxidised to a new substance which is named BT-SeO, and BT-SeO can be reduced to BT-Se by detecting H2S. BT-Se and BT-SeO emit different colours of fluorescence to realise detection. In this paper, the attribution of the fluorescence peak and the luminescence mechanism are studied. The bond parameters, infrared (IR) vibrational frequency, interaction region indicator (IRI) and the potential energy curve are analysed to demonstrate that the excited state is easier to conduct the ESIPT process. The combination of frontier molecular orbitals (FMOs) and hole–electron analysis reveals the charge excitation characteristics. This work suggests that the yellow and blue fluorescence observed in the experiment is emitted by the keto structures of BT-Se and BT-SeO, respectively. Our research will provide an important theoretical basis for the detection of photophysical phenomena in reversible ratiometric fluorescent probes. [ABSTRACT FROM AUTHOR]

[Display omitted] Recently, based on the mechanism of excited-state intramolecular proton transfer (ESIPT), a new fluorescent probe named 3-(benzo[d]thiazol-2-yl)-5-bromosalicylaldehyde-4N-phenyl thiosemicarbazone (BTT) was successfully synthesized [Analyst 146 (2021) 4348–4356.]. However, the importance of ESIPT processes of BTT probe and the mechanism of detecting Zn2+ ions have not been studied in detail. In this study, the photochemical behavior of ESIPT-chromophore and the photophysical changes of detecting Zn2+ ions were explained at the molecular level for the first time. The calculated spectral values were in agreement with the experiment. We not only confirmed the excited state hydrogen-bond strengthening by interaction region indicator (IRI), but also scanned the potential energy curves of BTT molecule in different electronic states, which confirmed that the hydrogen proton is easier to transfer in the first excited state. In addition, we had given the reasonable structure of the BTT-Zn2+ complex (L1) by comparing the binding free energies. The hole-electron distribution and interfragment charge transfer (IFCT) methods proved the excitation type of intraligand charge transfer (ILCT). Finally, the photophysical phenomenon of BTT for detecting Zn2+ ions is explained by calculating the electronic spectra and the energy gap (E gap) between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). [ABSTRACT FROM AUTHOR]

Cyanine dyes can be used as fluorescent probes to detect acidic hydrolytic acids in lysosomes. In this paper, the photophysical phenomena of two cyanine dyes (2-(2′-hydroxyphenyl) benzoxazole (HBO) and 2-(2′-hydroxyphenyl) benzothiazole (HBT)) and the intrinsic relation between the aromaticity and the excited state intramolecular proton transfer (ESIPT) process are studied. The harmonic oscillator measure of aromaticity (HOMA) method calculated that the two molecules has different aromaticity. Based on the theory of density functional and time-dependent density functional, the bond parameters and infrared vibration spectra of two cyanine dyes are analyzed in ground state (S0) and excited state (S1). The results show that the excited state hydrogen bonding interaction is enhanced, and the stronger the aromaticity of the five-membered ring associated with the hydrogen bond, the more significant the enhancement of the hydrogen bonding interaction of the molecule. The difference between the theoretically simulated fluorescence spectral values and the experimental spectral values is within the allowable error range. Frontier molecular orbitals (FMOs) and Mulliken charges are calculated at the microscopic level that the charge difference between the related atoms provides the driving force for the proton transfer process. Atoms in moleculs（AIM）topological analysis and core-valence bifurcation (CVB) index are calculated to estimate the hydrogen bond energies of the two molecules at different electronic states. Electron localization function (ELF) and localized orbital locator (LOL) graphs are conducted to analyze the hydrogen bond strength and components of hydrogen bonds and display the covalent components contained in hydrogen bonds graphically. The reduced density gradient (RDG) and potential energy curves furthue illustrate that the excited hydrogen bond interaction with strong aromaticity is more likely to trigger ESIPT process. This work suggests that molecular aromaticity can modulate the ESIPT process, which provides beneficial guidance for detection of human lysosomes. • Photophysical process of cyanine dyes are comprehensively investigated. • The aromaticity of the five-membered ring linked to the hydrogen bond controls the photophysical property of the fluorophore. • Aromaticity of a five-membered ring linked to hydrogen bond is positively correlated with the energy barrier in ESIPT process. • It provides a valuable reference for the subsequent application on fluorescence probes. [ABSTRACT FROM AUTHOR]

[Display omitted] • the molecules with stronger hydrogen bonds have higher IPs and EAs values. • The characteristics of molecular charge transfer are clarified and the contour map of electron cloud distribution is drawn. • The introduction of substituents promotes the ESIPT process. • The 2′-aminochalcone derivatives are promising materials for hole transport. This work investigates the different charge transfer characteristics and excited state intramolecular proton transfer process (ESIPT) of 2′-aminochalcones derivatives carrying different electron-withdrawing groups. Four new molecules are designed in the experiment and named as 2c, 3c, 4c and 5c, respectively. (Dyes and Pigments, 2022, 202.) Based on these four molecules, the effect of substituents on the ESIPT process and the charge transfer process are discussed in detail in our work. According to the study of the related parameters at the hydrogen bond site, infrared vibration spectrum, interaction region indicator isosurface (IRI) and scatter plots, it is concluded that the hydrogen bond interaction is enhanced under photoexcitation, and the descending order of the excited state hydrogen bond strength is 3c > 5c > 4c > 2c. The hydrogen bond energy is calculated by atoms in moleculs (AIM) topological analysis and core-valence bifurcation (CVB) index. The potential energy curve reveals the ESIPT mechanism. Frontier molecular orbital and electron-hole analyses explain the reasons for the changes in the ESIPT process at the electronic level. In addition, the ionization potentials (IPa and IPv), affinity energies (EAa and EAv) and reorganization energies are calculated to evaluate the potential application value of organic molecules in material transport field. [ABSTRACT FROM AUTHOR]

[Display omitted] • Increasing the number of conjugated C=C bonds can inhibit the occurrence of ESIPT reaction. • The ICT process can be promoted by increasing the number of conjugated C=C bonds. • The difficulty of the ESIPT reaction can be judged by the potential energy curves. The electronic structure of the molecule is significantly influenced by the number of conjugated C=C bonds. In this work, the influence of the conjugated C=C bonds of the SNCN derivatives on the excited state intramolecular proton transfer (ESIPT) and intramolecular charge transfer (ICT) properties are studied by density functional theory (DFT) and time-dependent density functional theory (TDDFT). The calculation level is proved to be reasonable by calculating electronic spectra. The hydrogen bond parameters, infrared vibrational frequency (IR), reduction density gradient (RDG) isosurface, topological analysis and potential energy curves of SNCN derivatives in ground state (S0) and the first excited state (S1) are analyzed. According to theoretical research results, ESIPT reaction has a higher likelihood of occurring in the S1 state. Moreover, the ESIPT reaction becomes more challenging to occur with the number of conjugated C=C bonds rising. Finally, the analyses of the frontier molecular orbitals (FMOs), dipole moment and charge transfer transition confirm that the ICT effect is aided by the increased number of conjugated C=C bonds. This work indicates that the number of conjugated C=C bonds can regulate the ESIPT and ICT processes, which provides guidance for the study of fluorescent groups with similar characteristics. [ABSTRACT FROM AUTHOR]