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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]

Excited state double proton transfer (ESDPT) has attracted great scientific interest because of its excellent luminescent properties. However, the complex process of ESDPT has plagued theoretical and experimental scientists for a long time and has become a hot issue. In this work, the ESDPT process of 2,2′-bipyridine-3,3′-diol-5,5′-dicarboxylic acid ethyl ester (BP(OH)2DCEt2) is systematically studied and the regulation of the ESDPT process is further realized. The potential energy curves indicate that BP(OH)2DCEt2 shows the characteristics of stepwise ESDPT in different polar solvents. The increase in solvent polarity will be beneficial to the stepwise ESDPT reaction. Regrettably, it is not possible to distinguish the specific stepwise transfer path of the BP(OH)2DCEt2 molecule due to the symmetry of the potential energy surface along the diagonal. On this basis, we proposed a method to control and regulate the stepwise ESDPT path using an external electric field. The results show that the increase of external electric field intensity is favorable to stepwise ESDPT. It is interesting to note that applying an external electric field in a specific direction will effectively distinguish stepwise ESDPT reaction paths. Therefore, this work not only helps to understand the mechanism of ESDPT, but also contributes to regulation and design of new luminescent materials with excellent luminescent properties. [ABSTRACT FROM AUTHOR]