Hydrogen-bond facilitated intramolecular proton transfer in excited state and fluorescence quenching mechanism of flavonoid compounds in aqueous solution

Journal Pre-proof Hydrogen-bond facilitated intramolecular proton transfer in excited state and fluorescence quenching mechanism of flavonoid compounds in aqueous solution

Yingqian Zhong, Yan Chen, Xia Feng, Yan Sun, Shen Cui, Xiaozeng Li, Xiaoning Jin, Guangjiu Zhao PII:

S0167-7322(19)35958-6

DOI:

https://doi.org/10.1016/j.molliq.2020.112562

Reference:

MOLLIQ 112562

To appear in:

Journal of Molecular Liquids

Received date:

28 October 2019

Revised date:

26 December 2019

Accepted date:

22 January 2020

Please cite this article as: Y. Zhong, Y. Chen, X. Feng, et al., Hydrogen-bond facilitated intramolecular proton transfer in excited state and fluorescence quenching mechanism of flavonoid compounds in aqueous solution, Journal of Molecular Liquids(2018), https://doi.org/10.1016/j.molliq.2020.112562

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© 2018 Published by Elsevier.

Journal Pre-proof Hydrogen-Bond Facilitated Intramolecular Proton Transfer in Excited State and Fluorescence Quenching Mechanism of Flavonoid Compounds in Aqueous Solution

Yingqian Zhong, Yan Chen, Xia Feng, Yan Sun, Shen Cui, Xiaozeng Li, Xiaoning Jin, Guangjiu Zhao*

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Tianjin Key Laboratory of Molecular Optoelectronic Sciences, Institute of Chemistry, National Demonstration Center for Experimental Chemistry & Chemical Engineering Education, National Virtual Simulation Experimental Teaching Center for Chemistry & Chemical Engineering Education, Department of Chemistry, School of Science, Tianjin University, Tianjin 300072, China

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*E-mail: [email protected]

Journal Pre-proof Abstract The density functional theory (DFT) and time-dependent density functional theory (TDDFT) were performed to investigate the ground state and excited state hydrogen-bonding dynamics of flavonoid in hydrogen donating aqueous solution. We demonstrated that the intermolecular hydrogen bond C=O•••H-O between flavonoid and water molecules is significantly strengthened in the electronically excited-state upon photoexcitation of the hydrogen-bonded complex. The radiationless deactivation via intramolecular proton transfer in excited state (ESIPT) can be facilitated by the

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hydrogen bond strengthening in the excited state. At the same time, quantum yields of

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the excited-state deactivation via fluorescence are correspondingly decreased. The total fluorescence of flavonoid in polar protic solvents can be drastically quenched by

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hydrogen bonding. As a consequence, we propose a fluorescence modulation

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mechanism by hydrogen bonding to explain fluorescence emissions of flavonoid in

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hydrogen- bonding solvents and no hydrogen-bonding solvents. Given that water molecules have anomalous properties that are different from other small organic

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molecules that form hydrogen bonds with flavonoids，this theoretical study has reference significance for the development of in vivo probes. Hopefully, the newly

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proposed mechanism can inspire experimentalists to develop and synthetic non-toxic high signal-to-noise ratios without the need to wash fluorescent probes for individual targets in vivo.

Keywords: Hydrogen-bond; Flavonoid; Aqueous solution; Intramolecular proton transfer; Excited state

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1. Introduction Solute-solvent interactions play a fundamental role in the photophysics and photochemistry of organic and biological chromophores in solution[1-4]. In addition to the nonspecific dielectric interactions between solute and solvent, the sitespecific intermolecular hydrogen bonding interaction between hydrogen donor and acceptor molecules is another important type of solute-solvent interaction and is central to the understanding of the microscopic structure and function in many molecular

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systems[5-7]. It has been found that many photochemical phenomena are affected by

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hydrogen bonding. For example, many geometric biochromic patterns emit fluorescence strongly in aprotic solvents, but fluorescence tends to quench in

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protonated solvents[8-10]. The discussion of how the electronically excited hydrogen

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bonds affect the photophysical photochemical properties of complex molecular

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systems provides a deeper understanding of the nature of hydrogen bonding and helps chemists to synthesize more stable and efficient luminescent materials. Research opens up new avenues for light damage and light protection in living organisms and

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makes a significant contribution to life sciences[11-16].

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In the past years, researchers have discovered electronic excited-state hydrogen-bonding plays significant role on internal conversion (IC), electronic spectral shifts (ESS), photoinduced electron transfer (PET), fluorescence quenching (FQ), intramolecular charge transfer (ICT), metal-to-ligand charge transfer (MLCT) and so on[17-38]. In 2007, Zhao discovered that the intermolecular hydrogen bonds between coumarin 102 chromophore and hydrogen bonding solvents are significantly strengthened upon photoexcitation for the first time[36]. Besides, it has been demonstrated the radiationless deactivation via internal conversion (IC) from the fluorescent state to ground state for fluorenone system can be enhanced due to the hydrogen bond strengthening in the fluorescent state[37]. Moreover, the concerted hydrogen bond strengthening and weakening behaviors in electronic excited states of the thiocarbonyl chromophores in alcohols are reported for the first time. It is distinctly demonstrated that the photochemistry of thiocarbonyl chromophores in

Journal Pre-proof alcoholic solutions can be tuned by the concerted intermolecular hydrogen bond strengthening and weakening in electronically excited states[38]. Flavonoids are polyphenolic compounds of the benzopyrone structure that are widely present in plants and belong to secondary metabolites of plants. Flavonoids are considered to be indispensable components in various nutritional, pharmaceutical and cosmetic applications due to their antioxidant, anti-inflammatory, anti-mutagenic and anti-cancer properties and their physiological activities such as regulating key cellular enzyme functions[39-43]. As a plant-derived dye, flavonoids are widely developed for

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the detection of various disease targets and proteins[44-48].The fluorescence of

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chromophores in hydrogen-bonded surroundings is quenched or enhanced by hydrogen bonds. Hydrogen bonding effects on the excited-state intramolecular proton

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transfer (ESIPT) excited states of many functionalized molecular systems have been

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examined in recent years[49-53].

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Recent study shows that a large fluorescence turn-on can be induced when a weakly fluorescent flavonoid in aqueous solution is moved to the hydrophobic pocket

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of mitochondrial cells, thereby allowing the wash-free mitochondrial imaging[54]. It also has been found that a series of novel fluorescent probes for protein detection

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which response through tuning the polarity of the dye structure and optimize through interaction with the protein’s hydrophobic pocket[55]. As we all known, among the environmentally sensitive fluorophores, flavones have exhibited classical positive solvatochromism features. Because the highly polar water molecule is a strong H-bond donor, the fluorescence of flavones can be severely quenched by the intermolecular electron or proton transfer between dyes and water molecules[56]. The fluorescence of flavones could be efficiently switched on when they are incorporateded by proteins or lipid membranes into the nonpolar microenvironment in cells (Figure 1). It is said that the dipole moment increased dramatically upon excitation via photon irradiation because of the charge transfer from the D–A structure. An increase in the solvent polarity could relax the excited molecules more efficiently, thus decreasing the energy of the excited state of the fluorophore and resulting in a red-shifted emission[57]. However, the influence of intramolecular hydrogen bonding

Journal Pre-proof interactions between flavonoid and water molecules in aqueous solution was

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neglected in previous research.

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Figure 1. Illustration of the fluorescence turn-on for selective protein detection based on flavone

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In this work, the density functional theory (DFT), time-dependent density

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functional theory (TDDFT) are employed to systematically investigate the effect of hydrogen bonding on intramolecular proton transfer in excited state between

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flavonoid and water molecules . We calculated the ground and excited state minimum geometries, energetics, vibrational frequencies, IR intensities frontier molecular

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orbitals, and hydrogen bond binding energy of the isolated flavonoid and hydrogen-bonded flavonoid-(H2O)2 complexes. The most representative flavonoid is selected for research in consideration of experimental conditions.

2. Theoretical Methods

In the present work, the geometry optimizations of the isolated monomers and the hydrogen-bonded solute-solvent complexes considered here for the ground state were performed, using density functional theory with Becke’s three-parameter hybrid exchange function with Lee-Yang-Parr gradient-corrected correlation functional (B3-LYP functional)[58]. The 6-31++G (d,p) was chosen as basis set throughout. The excited-state electronic structures were calculated using DFT and TDDFT with B3-LYP hybrid functional and the 6-31++G (d,p) basis set. The calculation model consists of flavonoid molecule and two water molecules, where two water molecules

Journal Pre-proof constitute the basic hydrogen bond grid structure as the hydrogen-supplying aqueous environment while the flavonoid molecule serves as hydrogen acceptor. In the complex model, two water molecules form hydrogen bonds with flavonoid carbonyl and hydroxyl groups, respectively. In the initial model, the hydrogen bond O•••H-O bond length between water molecules and flavonoids is 2.5000 Å, and the bond angle is 180.00 °, which meet the conditions for hydrogen bond formation. Based on this model, configuration optimization and subsequent related calculations are performed. The solvent effects in all studied complexes were simulated using the conductor-like

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polarizable continuum model (CPCM). Water is chosen as a reference solvent for

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consistency with the experimental studies. Since only the inner solvent molecules can be attributed to the early time hydrogen bonding dynamics occurring in ultrafast time

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scale, the hydrogen-bonded complexes proposed here are good models for studying

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the ultrafast hydrogen-bonding dynamics in solutions. All the electronic structure

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3. Results and Discussion

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calculations were carried out using the Gaussian 09 program package.

3.1 Geometric Structures of Monomers and Hydrogen-Bonded Complexes

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To delineate the detailed aspects of hydrogen-bonding dynamics, we have been motivated to theoretically study the isolated flavonoid and hydrogen-bonded flavonoid-(H2O)2 complexes. In Figure 2, the geometric structures of the hydrogen-bonded flavonoid-(H2O)2 complexes in ground state are shown. The C=O group in flavonoid is the most important site that is responsible for hydrogen bond formation in hydrogen-donating environments. Thus, one can find that the hydrogen bond C=O···H-O can be formed between flavonoid and water molecules. In both the conformations, the hydrogen bond C=O···H-O remains in the plane of the flavonoid molecule.

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F(N)-H2O 1.2681

F(T)-H2O 1.2963

C3-C4(Å)

1.4570

1.4533

1.4415

1.4434

O7-C5-C3-O6(º)

4.21

3.11

0.00

1.54

C13-N8(Å)

1.3700

1.3683

1.3671

1.3650

O7-H9(Å)

2.0040

2.3477

1.8937

2.4250

O2-H10(Å) O11-H10···O2(Å) O7-H9···O12(Å) O11-H10-O2(º) O12-H9-O7(º) C1=O2(Å)

0.9810 — — — — F(N*) 1.2834

0.9857 2.9742 2.9712 145.56 158.38 F(N*)-H2O 1.2949

0.9944 — — — — F(T*) 1.2819

1.0057 2.9421 2.6463 175.85 177.92 F(T*)-H2O 1.2903

C3-C4(Å)

1.4418

1.4501

1.4264

1.4321

O7-C5-C3-O6(º)

0.02

3.64

0.00

4.32

C13-N8(Å)

1.3652

1.3663

1.3680

1.3669

O7-H9(Å)

1.8730

2.3611

1.9830

1.9536

O2-H10(Å) O11-H10···O2(Å) O7-H9···O12(Å) O11-H10-O2(º) O12-H9-O7(º)

0.9937 — — — —

0.9924 2.7508 2.6262 175.84 155.64

0.9850 — — — —

0.9872 2.9419 2.7810 173.69 178.80

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F(T) 1.2917

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C1=O2(Å)

F(N) 1.2563

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Figure 2. Geometric structures of hydrogen-bonded flavonoid-(H2O)2 complexes

Table 1. Calculated bond lengths (Å), bond angles (º) and dihedral angle (º) for the isolated flavonoid and hydrogen-bonded flavonoid-(H2O)2 complexes in the ground state.

Journal Pre-proof As for normal form (N) flavonoid, the length of the intermolecular hydrogen bond C=O•••H-O between O and O atom is 2.9742 Å and 2.9712 Å for the complexes in the ground state and reduce to 2.7508 Å and 2.6262 Å in the excited state which means hydrogen-bond is strengthen in the excited state. We can find that the lengths of the C=O bond is increased because of the formation of the hydrogen bond C=O•••H-O. The C=O bond length is increased to 1.2681 Å from 1.2563 Å for the hydrogen-bonded complexes. As for tautomeric form (T) flavonoid, the length of the intermolecular hydrogen

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bond C=O•••H-O between O and O atom is 2.9421 Å and 2.6463 Å for the complexes

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in the ground state and 2.9419 Å and 2.7801 Å in the excited state which means little change to hydrogen-bond. Therefore, the excited hydrogen-bond strengthening mainly

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occurs in normal form. Interestingly, the lengths of the intramolecular hydrogen bond

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C=O•••H-O between O and H atom bond is changed which implies water molecules

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participate in the process of proton transfer. The planar structure of the flavonoids is affected because of the intermolecular hydrogen bonds, and the dihedral angle

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between the substituted benzene ring and the chromone ring is increased.

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3.2 Molecular Electrostatic Potential Surface (MEPS) of Monomers and Hydrogen-Bonded Complexes Molecular electrostatic potential surface (MEPS) has been used extensively to understand sites for nucleophilic reactions and electrophilic attacks, such as intramolecular or intermolecular hydrogen-bonding interactions. A significant polarization distribution exists on the surface at the negative and positive electrostatic potential in S1 state, and the blue and red represent the region of the most positive and negative electrostatic potential respectively, as shown in Figure 3. The intramolecular proton transfer may occur between an electropositive H and an electronegative O.

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Figure 3. The MEPS of the N* form of isolated flavonoid and hydrogen-bonded

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flavonoid-(H2O)2 complexes.

S3 S4 S5

Flavonoid(T)

Flavonoid(T) -H2O

2.870 (0.802) H→L 3.891 (0.035) 4.036 (0.183) 4.172 (0.090) 4.181 (0.000)

2.806 (0.838) H→L 3.797 (0.042) 4.028 (0.188) 4.159 (0.041) 4.290 (0.000)

2.383 (0.874) H→L 2.273 (0.064) 3.545 (0.000) 3.839 (0.008) 3.941 (0.035)

2.457 (0.922) H→L 3.308 (0.032) 3.748 (0.000) 3.870 (0.027) 3.943 (0.021)

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Flavonoid(N) -H2O

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S2

Flavonoid(N)

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S1

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3.3 Nature of Low-Lying Excited States

Table 2. Calculated Electronic Excitation Energies (eV), Corresponding Oscillator Strengths, orbital transitions and absorption wavelength (nm) of the Low-Lying Electronically Excited States for Isolated flavonoids and Hydrogen-Bonded flavonoid-( H2O)2 Complexes.

Before we discuss the properties of excited states for flavonoids and its hydrogen-bonded complexes, it is useful to understand the nature of the low lying electronically excited states. The electronic excitation energies and corresponding oscillator strengths of the hydrogen bonded flavonoid-(H2O)2 complexes as well as the flavonoid monomer are presented in Table 1. The S1 state of the flavonoid

Journal Pre-proof monomer and flavonoid-(H2O)2 complexes has the largest oscillator strength. We can also find that the excitation energies for the electronic excited states of isolated flavonoid are correspondingly lowered in normal form but heightened in tautomeric form because of the influence of the intermolecular hydrogen-bonding interactions. Furthermore, the corresponding oscillator strengths are slightly increased. The S1 absorption peak is calculated to be at 429 and 440 nm for the isolated flavonoid and the flavonoid-(H2O)2 complex, respectively. It is in good agreement with the S1 absorption peaks of the non-hydrogen-bonded and hydrogen-bonded forms for

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flavonoid located at around 403 and 416 nm in the experimental absorption spectra.

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Moreover, we know that the two forms in the ground state are in equilibrium. To distinctly see the shape of the absorption spectra, we show the calculated absorption

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spectra from 250 to 700 nm in Figure 4. One can see that all the calculated absorption

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spectral features are in good agreement with the spectral results recorded in

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experiments. It is confirmed that the hydrogen-bonded flavonoid-(H2O)2 complex presented here can be viewed as a good model to simulate the solute-solvent

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bulk effect.

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interaction between the flavonoid in aqueous solution without consideration of the

Figure 4. Calculated absorption spectra of isolated flavonoid and the flavonoid-(H2O)2 complex. Green lines denote the corresponding peaks in experiments.

As we know, molecular orbitals (MOs) analysis can directly provide insight into the nature of the excited states. In Figure 5, we show the frontier molecular orbitals of

Journal Pre-proof the hydrogen bonded flavonoid-(H2O)2 complexes. It can be clearly seen that both the electron densities of LUMO and HOMO orbitals are entirely localized over the flavonoid molecule. The charge density of the HOMO orbital is mainly concentrated in the pyranone ring in the substituted benzene ring and the chromone ring, and the LUMO orbital has a density distribution throughout the flavonoid molecular region. After flavonoids are excited by light, the charge density is mainly transferred from the substituted benzene ring portion of the ground state to the chromone ring portion of the excited state (S1). From the TDDFT calculations of all the geometric

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conformations, the S1 state with the largest oscillator strength of the hydrogen-bonded

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complexes corresponds to the orbital transition from HOMO to LUMO. It is evident

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that the S1 state is of the π-π* character from Figure 5.

Figure 5. Frontier molecular orbitals (MOs) of hydrogen-bonded flavonoid-(H2O)2 complexes

3.4 Infrared Spectra of Ground and Excited States

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Figure 6. Calculated C=O and O=H stretching bands of isolated flavonoid and hydrogen-bonded flavonoid-(H2O)2 complexes in different electronic states.

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As we all known, the vibrational frequencies of the stretching vibrations of C=O and O-H groups involved in hydrogen bonds can provide a clear-cut signature of the hydrogen-bonding dynamics. Herein, all of the IR spectra of isolated molecules and the hydrogen-bonded complexes in both the ground state and the excited state are calculated. The calculation of the IR spectra in electronically excited states is difficult and very time-consuming. The calculated IR spectra in different electronic states at the spectral range from 1000 to 4000 cm-1 are shown in Figure 6. In Figure 6, as for normal flavonoid, the calculated vibrational absorption spectra of hydrogen-bonded complexes in the spectral region of the C=O stretching band are shown. The C=O stretching bands of flavonoid monomer in different electronic states are used here for comparison. The C=O stretching vibrational frequency of isolated flavonoid is downshifted by 6 cm-1 from 1580 cm-1 in ground state to 1574 cm-1 in the excited state. However, the C=O stretching vibrational frequency of flavonoid

Journal Pre-proof complex is downshifted by 11 cm-1 from 1550 cm-1 in ground state to 1539 cm-1 in the excited state. That is to say, the C=O stretching mode has a more red shift which is induced by the intermolecular hydrogen bond C=O···H-O between flavonoid and H2O in the excited state than in the ground state. The formation of hydrogen bond C=O···H-O between flavonoid and H2O induces a red shift of 149 cm-1 for the O-H stretching band from the free O-H stretching vibrational frequency of 3578 cm-1 for isolated flavonoid. Upon electronic excitation to the excited state of hydrogen-bonded complexes, the O-H stretching band downshifts from 3357 cm-1 to 3346 cm-1. This

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means that the intermolecular hydrogen-bonding interactions can induce a larger

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spectral redshift of the O-H stretching band in the excited state than that in the ground

3.5 Hydrogen Bond Binding Energy

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state.

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Due to the nature of the electronically excited S1 state of the hydrogen-bonded

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flavonoid-(H2O)2 complexes, the hydrogen bond binding energy in excited states can be easily calculated by the energy of the hydrogen-bonded complex in the S1 state

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minus the energy of isolated flavonoid in its S1 state and the energy of water molecules in its ground state. As for normal form flavonoid, it can be evidently

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concluded that the intermolecular hydrogen bond C=O•••H-O is significantly strengthened since the binding energy increases from 41.46 kJ/mol in ground state to 48.50 kJ/mol in the excited state. As for tautomeric form flavonoid, the binding energy decrease from 38.93 kJ/mol in ground state to 34.41 kJ/mol in the excited state which means weakened intermolecular hydrogen bond. Therefore, the relatively weak intermolecular hydrogen bond C=O•••H-O between normal form flavonoid and water molecules in ground state becomes a strong hydrogen bond upon electronic excitation to the S1 state. The results of hydrogen bond binding energy calculation are consistent with those of the length of the intermolecular hydrogen bond calculation in Table 1. All the calculated results suggest the hydrogen-bonding strengthening has been demonstrated upon photo excitation to the electronically excited state. From our calculated results, the strengthening of the intermolecular hydrogen bond C=O···H-O in the photo excited electronic state of the hydrogen-bonded flavonoid-(H2O)2

Journal Pre-proof complexes can be strongly supported. 3.6 Fluorescence Quenching A schematic view of the excited state deactivation process of the hydrogen-bonded complexes formed by flavonoid and its aqueous solvent. The figure 7 mainly involves several processes of absorption (Abs), fluorescence emission (Flu), excited state intramolecular proton transfer (ESIPT), and internal conversion (IC). We know that the fluorescence of flavonoid is mainly in competition with the intramolecular proton transfer between normal form flavonoid and tautomeric form

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flavonoid and internal conversion process. The energy levels of the ground state and

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the excited state are correspondingly reduced due to the formation of intermolecular hydrogen bonds C=O···H-O in a protic solvent such as aqueous solvent. The degree of

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decline in the energy levels of different configurations and electronic states is

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different, which is caused by the strength of hydrogen bonding. The stronger the

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hydrogen bond is, the lower the corresponding electronic state energy level is. We have previously confirmed that hydrogen bond in the excited state of the normal form

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flavonoid and the water molecules is much stronger than the ground state, and the hydrogen bond of the tautomeric form is not significantly enhanced. Compared with

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the flavonoid monomer, the spacing between the various energy levels of the composite has changed which promotes the excited state intramolecular proton transfer process and the internal conversion process. Here, we explain that the enhancement of the excited state hydrogen bond promotes the intramolecular proton transfer process and the reason for fluorescence quenching.

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Figure 7. Schematic view of the excited state intramolecular proton transfer (ESIPT) and internal

4. Conclusions

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conversion (IC) processes in different electronic states for free and hydrogen-bonded flavonoid.

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The mechanism of the hydrogen bonding on intramolecular proton transfer in excited state of flavonoid is deeply described for the first time in this paper. The

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excited-state hydrogen-bonding dynamics of flavonoid in hydrogen donating aqueous solvents was investigated using the time-dependent density functional theory (TDDFT) method. The geometric structures and energetics of the hydrogen-bonded flavonoid-(H2O)2 complexes as well as the isolated flavonoid in ground state and the S1 state were discussed. All the calculated spectral features are in good agreement with the spectral results recorded in experiments. By monitoring the spectral shift of the stretching vibrational mode for the hydrogen-bonded O-H group and C=O group in different electronic states, it is demonstrated that the intermolecular hydrogen bond C=O•••H-O between flavonoid and water molecules is significantly strengthened in the electronically excited-state upon photoexcitation. Moreover, it was proposed that the radiationless deactivation of the fluorescent state via ESIPT can be facilitated by the hydrogen bond strengthening in the excited state. As a result, the hydrogen bond strengthening plays an important role for increasing the ESIPT process. Compared

Journal Pre-proof with other organic molecules that can form hydrogen bonds with flavonoid , the small size of water molecules forms a strong hydrogen bond, and there are also anomalies in other properties' studies, which makes the research in aqueous solution systems more complicated. Considering the characteristics of commonality or personality, the subsequent effects of studying the other proton solution on the fluorescence with excited intramolecular proton transfer types can be increased. Other interactions between mitochondria and flavonoid probes still need to be explored. The theoretical research in this paper is significance for the development of fluorescent probes in vivo.

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The experimenter can try to modify and develop other probes with excellent

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performance based on this theoretical study. Acknowledgment

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This work was supported by the National Natural Science Foundation of China

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(Nos. 21873068, 21573229 and 21422309). We also thank the financial support from

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the Frontier Science Project of the Knowledge Innovation Program of Chinese Academy of Sciences (CAS), Project for Excellent Member of CAS Youth Innovation

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Promotion Association, the Open Research Funds of State Key Laboratory of Bioelectronics (Southeast University) and Double First-Rate and Peiyang Scholar

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Projects of Tianjin University. References and Notes

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Author Statement

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Yingqian Zhong: Methodology; Software; Writing - Original Draft; Writing - Review & Editing; Visualization Yan Chen: Validation; Writing - Review & Editing; Visualization Xia Feng: Formal analysis Yan Sun: Resources Shen Cui: Data Curation Xiaozeng Li: Writing - Review & Editing ; Xiaoning Jin: Supervision ; Guangjiu Zhao: Conceptualization ; Supervision ;Project administration; Funding acquisition

Journal Pre-proof Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

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Graphical Abstract

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Highlight  Hydrogen bond is strengthened from S0 to S1 in aqueous solution.  Strengthening hydrogen bond facilitate ESIPT.  A stronger hydrogen bond makes a lower electronic state energy level.  The enhancement of ESIPT promotes fluorescence quenching.