Because of its fundamental importance in many branches of science, hydrogen bonding is a subject of intense contemporary research interest. The physical and chemical properties of hydrogen bonds in the ground state have been widely studied both experimentally and theoretically by chemists, physicists, and biologists. However, hydrogen bonding in the electronic excited state, which plays an important role in many photophysical processes and photochemical reactions, has scarcely been investigated.Upon electronic excitation of hydrogen-bonded systems by light, the hydrogen donor and acceptor molecules must reorganize in the electronic excited state because of the significant charge distribution difference between the different electronic states. The electronic excited-state hydrogen-bonding dynamics, which are predominantly determined by the vibrational motions of the hydrogen donor and acceptor groups, generally occur on ultrafast time scales of hundreds of femtoseconds. As a result, state-of-the-art femtos...

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Hydrogen Bonding in the Electronic Excited State

In this work, we investigate the dual hydrogen bonded 1,4-bis-(3-hydroxy-4-oxo-4H-chromen-2-yl)-benzene (bisflavonol) system in detail. Via optimizing stable structures and constructing potential energy curves, we confirm that two primary structures (i.e., anti-bisflavonol and syn-bisflavonol) can coexist in the S0 state. Calculating the reduced density gradient (RDG) versus sign(λ2)ρ and gradient isosurfaces, we confirm the formation of double hydrogen bonds for both anti-bisflavonol and syn-bisflavonol. Comparing the primary geometrical parameters involved in hydrogen bonds, we verify that the double intramolecular hydrogen bonds should be strengthened in the S1 state. In view of the photo-excitation process, we find that the charge redistributions around the hydrogen bonded moieties of both anti-bisflavonol and syn-bisflavonol facilitate the excited state intramolecular proton transfer (ESIPT) reaction. Given the reaction paths for the ESIPT process, the S0-state and S1-state potential energy surfaces (PESs) are constructed for both anti-bisflavonol and syn-bisflavonol along with two hydrogen bonds to reveal the overall excited state dynamical behavior. Searching for the transition state (TS) structure and calculating the intrinsic reaction coordinate (IRC) energetic profile, we confirm the ESIPT reaction. Combining it with Born–Oppenheimer molecular dynamics (BOMD) simulations, we study the ESIPT dynamical behaviors in detail. We present that only the single proton transfer process occurs in the S1 state in aprotic solvents, which makes up for the deficiency of previous experiments. The theoretical electronic spectra further confirm our attribution. This work not only illustrates that anti-bisflavonol and syn-bisflavonol coexisting in the S0-state can promote the respective ESIPT reaction, but also makes a new attribution to previous experiments.

Exploring and elaborating the novel excited state dynamical behavior of a bisflavonol system

The effects of atomic electronegativity (O, S and Se atoms) on the excited-state intramolecular proton transfer (ESIPT) properties of three compounds (3-HF, 3-HTF and 3-HSeF) were systematically studied using the density functional theory (DFT) and time-dependent DFT (TD-DFT) methods. Furthermore, the antioxidant activities of the three compounds were evaluated using different parameters from the perspective of biological activities. The calculated results indicate that the generation of ESIPT is attributed to the excited-state intramolecular charge transfer owing to the change in electron density distribution on the frontier molecular orbitals and natural bond orbital (NBO) charge distribution of the O1 and O2 atoms at the S0 and S1 states. Moreover, 3-HTF shows the greatest red shift of the O1–H1 stretching peak and the lowest energy barrier at the S1 state among the three compounds, implying that 3-HTF would have the strongest intramolecular hydrogen bond at the S1 state, which is most beneficial for the occurrence of ESIPT. In addition, the decreased ionization potentials and energy gaps from 3-HF to 3-HTF and 3-HSeF means that the antioxidant activity of the compound would be enhanced along with the lowered atomic electronegativity. Interestingly, the lower ionization potentials and energy gaps of the compounds in keto forms suggest that the compound (3-HTF) easy to carry out ESIPT reaction would exhibit efficient antioxidant activity, which would establish the relationship between ESIPT reaction and antioxidant activity of the compound and provide a new notion for synthesizing more efficient antioxidants in experiments.

Tunable ESIPT reaction and antioxidant activities of 3-hydroxyflavone and its derivatives by altering atomic electronegativity

In this review, we present a systematic and comprehensive summary of the recent development and applications of excited‐state intramolecular proton transfer‐based (ESIPT‐based) aggregation‐induced emission luminogens (AIEgens), a type of promising materials that inherit the advantages of ESIPT and AIE, such as large Stokes shift, excellent photostability, and low self‐quenching. We first summarize the backbones that have been used to construct the ESIPT‐based AIEgens and classify the constructed ones based on the relation between ESIPT and AIE unit. According to the sensing mechanisms and design strategies, we have reviewed the applications of ESIPT‐based AIEgens in bioimaging, drug delivery systems, organic light‐emitting diodes, photo‐patterning, liquid crystal, and the detection of metal cations, anions, small molecules, biothiols, biological enzymes, latent fingerprinting, and so on. We have also reviewed the recent advances in the development of new theoretical methods for investigating molecular photochemistry in crystals and their applications in ESIPT‐based AIEgens. We discussed the remaining challenges in this field and the issues that need to be addressed. We anticipate that this review can provide a comprehensive picture of the current condition of research in this field, and promote researchers to make more efforts to develop novel ESIPT‐based AIEgens with new applications.

https://onlinelibrary.wiley.com/doi/pdfdirect/10.1002/agt2.160

ESIPT‐based AIE luminogens: Design strategies, applications, and mechanisms

The triple hydrogen-bonded triquinolonobenzene (TQB) molecule is investigated for its excited-state dynamics and proton transfer (ESIPT) mechanism in different solvents in this work. Through insigh...

Solvent-Polarity-Dependent Excited-State Behavior and Thermally Active Delayed Fluorescence for Triquinolonobenzene

As the most important component of deep red pigments, alkannin is investigated theoretically in detail based on time-dependent density functional theory (TDDFT) method. Exploring the dual intramolecular hydrogen bonds (O1–H2···O3 and O4–H5···O6) of alkannin, we confirm the O1–H2···O3 may play a more important role in the first excited state than the O4–H5···O6 one. Infrared (IR) vibrational analyses and subsequent charge redistribution also support this viewpoint. Via constructing the S1-state potential energy surface (PES) and searching transition state (TS) structures, we illuminate the excited state double proton transfer (ESDPT) mechanism of alkannin is the stepwise process that can be first launched by the O1–H2···O3 hydrogen bond wire in gas state, acetonitrile (CH3CN) and cyclohexane (CYH) solvents. We present a novel mechanism that polar aprotic solvents can contribute to the first-step proton transfer (PT) process in the S1 state, and nonpolar solvents play important roles in lowering the potenti...

Theoretical Insights Into the Excited State Double Proton Transfer Mechanism of Deep Red Pigment Alkannin.

Elaborating the excited state multiple proton transfer mechanism for 9H-pyrido[3,4-b]indole

Excited-state intramolecular proton transfer (ESIPT) coupled to aggregation-induced enhanced emission (AIEE) offers a better route to design more efficient highly emissive materials. These material...

Hao Dong

Papers

Theoretical Insights Into the Excited State Double Proton Transfer Mechanism of Deep Red Pigment Alkannin.

Exploring and elaborating the novel excited state dynamical behavior of a bisflavonol system

Elaborating the excited state multiple proton transfer mechanism for 9H-pyrido[3,4-b]indole

Solvent-Polarity-Dependent Excited-State Behavior and Thermally Active Delayed Fluorescence for Triquinolonobenzene

Aggregation Promotes Excited-State Intramolecular Proton Transfer for Benzothiazole-Substituted Tetraphenylethylene Compound