Excited-state intramolecular proton transfer mechanism for 2-(quinolin-2-yl)-3-hydroxychromone: A detailed time-dependent density functional theory study

Journal of Molecular Liquids 260 (2018) 447–457

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Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq

Excited-state intramolecular proton transfer mechanism for 2-(quinolin-2-yl)-3-hydroxychromone: A detailed time-dependent density functional theory study Yingying Yang a,1, Zhe Tang a,1, Panwang Zhou b, Yutai Qi a, Yi Wang a,⁎, Hongying Wang a,⁎ a b

School of Biological Engineering, Dalian Polytechnic University, Dalian 116034, China State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Science, Dalian 116023, China

a r t i c l e

i n f o

Article history: Received 20 December 2017 Received in revised form 15 March 2018 Accepted 22 March 2018 Available online 24 March 2018 Keywords: Hydrogen bonding ESIPT Mutual transformation Radiation transition

a b s t r a c t In this work, the Excited-state intramolecular proton transfer mechanism of 2-(quinolin-2-yl)-3hydroxychromone (Q3HC) in 1,2-dichloroethane solvent have been investigated via the density functional (DFT) and the time-dependent density functional theory (TDDFT) method. By calculation, we optimize three configurations (Q3HC-A, Q3HC-B and Q3HC-C) and find the phenomenon of converting between Q3HC-B and Q3HC-A in the S0 and S1 states. Based on comparing the primary bond lengths, bond angles and infrared vibrational spectra involved in the hydrogen bonds between S0 and S1 states, hydrogen bonds strengthening in the S1 state have been testified. The calculated absorption and fluorescence spectra of Q3HC-A, Q3HC-B and Q3HCC are agreement with the experimental data, and the results also show that the tautomer form of Q3HC-C has a fluorescence spectrum, which is different from the previous report (Svechkarev et al., Journal of Luminescence, 2011, 131, 253–261). Meanwhile, we also constructed the potential energy curves of S0 and S1 states, and further explained the ESIPT mechanism of Q3HC-A, Q3HC-B and Q3HC-C, respectively. These results have shown that the ESIPT of Q3HC-C is a barrierless process, which occurs more easily than that of Q3HC-A and Q3HC-B. Combination with reduced density gradient function, the hydrogen bond of Q3HC-C is strongest, compared with Q3HC-A and Q3HC-B. And it also can provide the real evidence that the ESIPT of Q3HC-C is more favorable than that of Q3HC-A and Q3HC-B. © 2018 Elsevier B.V. All rights reserved.

1. Introduction The hydrogen bonding is one of the most typical weak interactions and it exists widely in many biological systems [1–5]. As the construction of fundamental building blocks of life, it enables complex biomolecules to be set up, such as proteins, nucleic acids and so on [1–5]. In addition, the hydrogen bond also has dynamic features; it could act as an active site for the occurrence of a plethora of interactions [6]. Above all, the thorough investigations of hydrogen bonding interactions are vital to delve into the critical evaluation of many phenomena, which occurs in the crystal state, solutions and living organisms [7–9]. Recently, Han and co-workers have demonstrated that the intermolecular hydrogen bond between solvent and solute molecules is greatly strengthened in the excited state [10–18]. In fact, many different

⁎ Corresponding author. E-mail addresses: [email protected] (Y. Wang), [email protected] (H. Wang). 1 These authors contributed equally.

https://doi.org/10.1016/j.molliq.2018.03.094 0167-7322/© 2018 Elsevier B.V. All rights reserved.

sensing mechanisms have already involved hydrogen bond theory, like excited-state intramolecular proton transfer (ESIPT), intramolecular charge transfer (ICT) and fluorescence resonance energy transfer (FRET) [19–27]. The previous studies have shown that the hydrogen bonding interactions could provide the driving force for the ESIPT process [28–31]. Recently, the ESIPT has attracted special attention due to its remarkable applications, such as fluorescence sensors, laser dyes, LEDs, UV filters, molecular switches and so on [32–42]. Up to now, researches have investigated numbers of molecules exhibiting the ESIPT process, like 3Hydroxyflavone monomer [43], 3-Hydroxyisoquinoline [44] and so on. In last years, 3-Hydroxychromones (3-HC) seem to have become the most popular fluorescent sensing ESIPT molecules [45]. For example, the molecule of 2-(benzimidazol-2-yl)-3-Hydroxycromone has been discussed [46,47]. The highly environment sensitivity and dual-band fluorescent properties of 3-HC allow it to be applied in various biological analyses, such as fluorescent labeling of nucleic acid bases [48], fluorescence detection of apoptosis [50], the cell membranes studies [50–52], proteins structure investigations [52–54] and so on. Studies have indicated that 3-HC could exhibit dual-band fluorescence in solution thanks to the ESIPT reaction, which the short-wavelength band roots

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in the normal form and the long-wavelength band belongs to the phototautomer form of the ESIPT [55]. These works also found that for 3-HC, traditionally, the intramolecular H-bond of “flavonollike” type, which links from 3-OH to the oxygen atom of the 4 carbonyl group [56]. At the same, the derivatives of 3-HC exist the Hbond of alternative type, and the heteroatom of substituent also acts a role of a second proton accepting center [56]. Very recently, 2-(quinolin-2-yl)-3-Hydroxychromone (Q3HC) has been reported by Svechkarev et al., which belongs to the family of 3-HC [56]. They predicted the possibility of the two concurrent H-bond directions and ESIPT pathways (“flavonol-like” and alternative, see in Scheme 1), and they pointed that the structure with an alternative H-bond to nitrogen is more energetically favorable in solution. However, there is a lack of the detailed discussion on the ESIPT process for Q3HC molecules. They believed the intramolecular N⋯HO hydrogen bond type undergoing rapid radiation decreases the deactivation of the excited species, while the evidence is slightly inadequate. Although they provided the basis for the main components of Q3HC-C in solution, and provided three configurations for Q3HC-A, Q3HC-B and Q3HC-C (see in Fig. 1), they did not describe whether there was a relationship between the three. Therefore, in-depth investigation about ESIPT process of Q3HC molecule is necessary. In this paper, we will investigate the Q3HC in 1,2-dichloroethane solvent via the density functional (DFT) and the time-dependent density functional theory (TDDFT) method. We have optimized the three configurations of Q3HC-A, Q3HC-B and Q3HC-C in S0 and S 1 states and have analyzed their bond length, bond angle and infrared vibrational (IR) spectra to observe the trend of intramolecular hydrogen bonds. To distinguish the different types of interactions and compare the hydrogen-bonding interaction intensity, a reduced density gradient (RDG) function has been used. The frontier molecular orbitals (MOs), Mulliken's charge analyses and natural population analysis (NPA) methods understand the charge distribution, which provides the tendency for the ESIPT process. To further investigate the ESIPT process, potential energy curves for the S 0 and S1 state have been constructed by fixing the O\\H length to a series of values. 2. Computational details In this work, all the geometric and electronic structures presented were performed with the Gaussian 09 package [57]. The geometric optimizations for the S0 and S1 states were finished based on DFT and TDDFT methods with Becke's three-parameter hybrid exchange functional with Lee-Yang-Parr gradient-corrected correlation functional (B3LYP) [58–60]. The triple-ζvalence quality with one set of polarization functions (TZVP) [61,62] was selected as the basic set for the system. Considering the solvent effect [56], 1, 2dichloroethane had been selected as the solvent in the SCRF

calculations based on the polarizable continuum model (PCM) model using the integral equation formalism variant (IEFPCM) [63–66]. All the geometries in the S 0 and S1 states were optimized without the constraint of bonds, angles and dihedral angles. The vibrational frequency calculations confirmed that all the optimized structures were at the local minima on the S0 and S1 (no imaginary frequency). The potential energy curves were scanned in the S0 and S1 states, increased the O\\H bond lengths fixed step size [67–69]. The weak interaction types had been discussed via the RDG function; the results were calculated and plotted by the Multiwfn software [70] and the VMD progress [71]. 3. Result and discuss 3.1. The optimization of conformations In this work, we have optimized three configurations based on the B3LYP functional and TZVP basis set in the 1.2-dichlorethane solvent, which denoted as Q3HC-A, Q3HC-B and Q3HC-C (see in Fig. 1). As shown in Fig. 1, the direction H-bonds of Q3HC-A and Q3HC-B are “flavonol-like” type and that of Q3HC-C is alternative type. The calculated primary bond lengths (Å) and angles (°) of these stable structures for both the S 0 and S 1 states are listed in Table 1. For Q3HC-A and Q3HC-B, the O 1\\H 2 bond lengths are both 0.979 Å in the S 0 state, while increased to 0.995 Å and 0.994 Å in the S1 state, growing 0.016 Å and 0.015 Å, respectively. Meanwhile, it should be noted that the shorting of the H 2\\O 3 bonds from 2.005 Å and 1.998 Å in the S0 state to 1.889 Å and 1.886 Å in the S 1 state, decreasing 0.116 Å and 0.112 Å, respectively. The bond angles of (O 1\\H 2 ⋯O 3 ) change from 118.8° and 119.2° in the S0 state to 122.0° and 122.5° in the S1 state, which the extension are 3.2° and 3.3°, respectively. The above analysis results show that the intramolecular hydrogen bonds (O1\\H2⋯O3) are strengthened in the S1 state. Upon photo-excitation, the ultrafast ESIPT reaction occurs usually tautomerism in the proton acceptor groups, such as enol transfer to keto tautomerization in the excited state (see in Fig. 1). For tautomer forms of Q3HC-A and Q3HC-B, the bond angle of (O 3\\H 2 ⋯O 1 ) changes from 116.2° and 115.9° in the S 1 state to 123.4° and 125.1° in the S0 state, increasing 7.2° and 9.2°, respectively. The bond lengths of H 2\\O 1 decrease from 2.047 Å and 2.058 Å in the S 1 state to 1.839 Å and 1.786 Å in the S 0 state, the shorting are 0.208 Å and 0.272 Å, respectively. While the bond lengths of O 3\\H 2 increase from 0.979 Å and 0.979 Å in the S1 state to 0.999 Å and 1.006 Å in the S0 state, which the variation are 0.020 Å and 0.027 Å, respectively. It can be clearly that the intramolecular hydrogen bonds (O3\\H2⋯O1) are more stable in the S0 state. We could not find the stable normal form of Q3HC-C in the S1 state. Presumably, the ESIPT reaction of Q3HC-C could occur rapidly with lower potential barriers in the S1 state; and the proton transfer process

Scheme 1. H-bond and ESIPT directions in Q3HC: “flavonol-like” (left) and alternative (right) [56].

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Fig. 1. The optimized geometrical configurations of the Q3HC molecule. A: the normal form of Q3HC-A, B: the normal form of Q3HC-B, C: the normal form of Q3HC-C. a: the tautomer form Q3HC-A, b: the tautomer form of Q3HC-B, c: the tautomer form of Q3HC-C. The gray: C, the blue: H, the red: O, the pink: N. The dash line refers to the intramolecular hydrogen bond. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

of Q3HC-C could happen in the S0 state. The H-bond type of Q3HC-C is alternative, which linked from 3-OH to N atom of the substituent's (see in Fig. 1). For the tautomer form of Q3HC-C, the bond lengths of H2\\O1 decreased from 1.816 Å in the S1 state to 1.709 Å in the S0 state, the shorting is 0.107 Å. While the bond lengths of N4\\H2 increased from 1.028 Å in the S1 state to 1.039 Å in the S0 state, which the extension is 0.011 Å. The bond angle of (N4\\H2⋯O1) is enlarged from 137.8° in the S1 state to 139.9° increasing 2.1°. The results show

that the intramolecular hydrogen bond (N4\\H2⋯O1) is more stable in the S0 state. 3.2. The analysis of infrared (IR) vibrational spectra The hydrogen bond strengthening or weakening could be revealed by means of the IR vibrational spectra analysis [72]. For Q3HC-A and Q3HC-B, the O1\\H2 stretching vibrational frequencies are both located

Table 1 The calculation primary lengths (nm) and angles (°) of Q3HC-A, Q3HC-B and Q3HC-C in the S0 state and S1 state. Electronic state

Q3HC-A

Q3HC-B

Normal form

O1\ \H2 \O3 H2\ \N4 H2\ \H2\ \O3) δ(O1\ \H2\ \N4) δ(O1\

Tautomer form

Normal form

Q3HC-C Tautomer form

Normal form

Tautomer form

S0

S1

S0

S1

S0

S1

S0

S1

S0

S1

S0

S1

0.979 2.005 – 118.8 –

0.995 1.889 – 122.0 –

1.839 0.999 – 123.4 –

2.047 0.979 – 116.2 –

0.979 1.998 – 119.2 –

0.994 1.886 – 122.5 –

1.786 1.006 – 125.1 –

2.058 0.979 – 115.9 –

1.001 – 1.669 – 147.3

– – – – –

1.709 – 1.039 – 139.9

1.816 – 1.028 – 137.8

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Fig. 2. The calculated IR vibrational spectra of the hydrogen bond groups O\ \H stretching absorption band in the S0 and S1 state. The IR vibrational spectra of the normal form, a and b; the IR vibrational spectra of the tautomer form, c, d and e.

at 3560 cm−1 in the S0 state, while they are located at 3305 cm−1 and 3314 cm−1 in the S1 state (in Fig. 2(a) and (b)). The 255 cm−1 and 246 cm−1 red-shift of O1\\H2 stretching frequencies show the intramolecular bonds (O1\\H2⋯O3) are strengthened in the S1 state. In addition, Fig. 2(c) and (d) provide the vibrational frequencies of tautomer forms of Q3HC-A and Q3HC-B. It should be noted that the O3\\H2 stretching frequencies are located at 3572 cm−1 and 3575 cm−1 in the S1 state, decreased to 3237 cm−1 and 3133 cm−1 in the S0 state. The 335 cm−1 and 442 cm−1 blue-shift of O3\\H2 stretching vibrational frequencies show the intramolecular hydrogen bonds (O3\\H2⋯O1) are weakened in the S1 state. The IR vibrational spectra of hydrogen bond group of the Q3HC-C tautomer form have been shown in Fig. 2(e). The H 2\\N 4 stretching vibrational frequency is located at 3270 cm −1

in the S 1 state, decreased to 3082 cm −1 in the S 0 state. The 188 cm−1 blue-shift of H2\\N4 stretching frequency demonstrates the intramolecular hydrogen bond (N 4\\H2 ⋯O 1 ) is strengthened in the S 0 state. The IR vibrational spectra of the Q3HC-C normal form in the S1 state has not been displayed, since its stable structure in S 1 state has not been optimized, and we will provide the explanation later. 3.3. The analysis of absorption and fluorescence spectra Based on the DFT and TDDFT method, we have calculated the absorption and fluorescence spectra of Q3HC molecule and the results have been displayed in Fig. 3. It can be seen that the peaks of three normal forms are 367 nm, 369 nm and 389 nm, which are all in agreement

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Fig. 3. The calculated absorption and fluorescence spectra of Q3HC. a: Q3HC-A; b: Q3HC-B; c: Q3HC-C. N: normal form; T: tautomer form.

with the experiment data [56]. Upon photo-excitation, the fluorescence emission peaks of Q3HC-A and Q3HC-B normal forms are 437 nm and 442 nm, which are close to the experimental data [56]. Moreover, the emission leaks of the three tautomer forms are located at 593 nm, 610 nm, and 573 nm, which agree well with the experimental value [56]. It is worth finding that the large Stokes shift values are 226 m, 241 nm and 184 nm, which between the three normal forms and tautomer forms, respectively. As we all know, the tautomerism is an important feature of the ESIPT process, which leads to the large Stokes Shift.

in Fig. 4(a), from Q3HC-B to Q3HC-A in S0 state, the barrier is 2.51 kcal/ mol, which shows the transformation could occur. In Fig. 4(b), from Q3HC-B to Q3HC-A in S1 state, the barrier of 12.23 kcal/mol demonstrates the occurrence of the transformation. Therefore, it can draw conclusion that the phenomenon of converting exists between Q3HC-B and Q3HC-A in the S0 and S1 states. In Fig. 4(c), from Q3HC-B to Q3HC-C in the S0 state, the barrier of 8.16 kcal/mol, thus, it is too high to transform. However, Q3HC-C is nonexistent in the S1 state, the transformation is also not. 3.5. The analysis Frontier orbital (MOs)

3.4. The relationship between normal form of Q3HC-A, Q3HC-B and Q3HC-C Although there are three configurations of Q3HC, the relationships among them have not been described clearly [56]. In our calculation, the stability order follows the trend: Q3HC-C N Q3HC-A N Q3HC-B (energies of the three configurations have been listed in Table 2). As shown

Table 2 The energies (hartree) of these stable structures of normal forms on in the S0 and S1 states based on the DFT and TDDFT methods, respectively. Q3HC-A

Energy

Q3HC-B

Q3HC-C

S0

S1

S0

S1

S0

S1

−973.3023

−973.1902

−973.3011

−973.1898

−973.3098

–

As far as we know, the direct evidence of the nature of the excited states could be provided by the Frontier orbital (MOs) [13,15,17,21,73–75]. Therefore, we have calculated the MOs of the molecule Q3HC to reveal the molecule characteristics and transfer mechanism. Table 3 lists the calculated electronic transition energies and relative oscillator strengths as well as the compositions. Obviously, the first excited singlet has the ππ*-type character from the highest occupied molecular orbital (HOMO) orbital to the lowest occupied molecular orbital (LUMO) orbital with oscillator strength of 0.5713, 0.5168 and 0.4171, corresponding to Q3HC-A, Q3HC-B and Q3HC-C, respectively. For this reason, we only show the HOMO and the LUMO in Fig. 5. It should be noted that the electron density of the O 1 atoms of hydrogen moieties decrease, while the O 3 atoms (for Q3HC-A and Q3HC-B) and the N4 atom (for Q3HC-C) increase after the transition from HOMO to LUMO. In order to investigation the charge distribution over the atoms involved in intramolecular hydrogen bond, we use Mulliken's

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Fig. 4. Variation of energy of Q3HC as a function of rotation of the torsional angle (θ) for the transformation of three conformations, a: from Q3HC-B to Q3HC-C in the S0 state, b: from Q3HCB to Q3HC-C in the S1 state, c: from Q3HC-B to Q3HC-C in the S0 state.

charge distribution to analysis (see in Table 4). For Q3HC-A and Q3HCB, the results show that the negative charge of O1 atoms of the\\OH moieties decrease from −0.291 and −0.254 in the S0 state to −0.273 and −0.227 in the S1 state, while increased on the O 3 atoms from −0.411 and −0.409 to −0.427 and −0.423. Another, to make our calculation results more reliable, we also use NPA analysis methods to discuss the charge distribution. All the data have been displayed in Table 4. The analogous results show that the charge of O1 atoms decrease from −0.645 and −0.624 to −0.638 and −0.609. On the contrary, the charge of O3 atoms increase from −0.621 and −0.620 to −0.635 and −0.634. The trend of above two kinds of charge distribution analysis is consistent. Above all, the enhanced two intramolecular hydrogen bonds and the intramolecular charge transfer further trigger the ESIPT process.

Table 3 Electronic excited energy (nm), corresponding oscillator strengths and the corresponding compositions for Q3HC compound based on the TDDFT method.

Q3HC-A Q3HC-B Q3HC-C

Transition

λ (nm)

ƒ

Composition

CI (%)

S0 → S1 S0 → S2 S0 → S1 S0 → S2 S0 → S1 S0 → S2

367 340 369 334 387 347

0.5713 0.0089 0.5168 0.0236 0.4171 0.0000

H→L H-1 → L H→L H-2 → L H→L H-1 → L

96.78% 91.94% 97.46% 86.06% 97.18% 89.42%

3.6. The analysis of potential-energy curves To further study the ESIPT mechanism of Q3HC, we construct the potential-energy curves. All the points of the potential-energy curves have been optimized at the TDDFT/B3LYP/TZVP level and displayed in Fig. 6. The ESIPT pathways of Q3HC-A and Q3HC-B are “flavonol-like” and that of Q3HC-C is alternative. For Q3HC-A and Q3HC-B, the potential energy barrier are 3.56 kcal/mol and 4.39 kcal/mol in the S1 state (see in Fig. 6(a) and (b)). In Fig. 6(c), the ESIPT reaction of Q3HC-C is a barrierless process. In other words, this ESIPT pathway can occur spontaneously in the S1 state. The phenomenon also provides a reason that the normal form of Q3HC-C is nonexistent in the S1 state. The results indicate that the alternative pathway is easier than “flavonol-like” pathway for Q3HC molecule. In Fig. 6, it can be seen that the tautomer forms of Q3HC-A, Q3HC-B and Q3HC-C are unstable in the S0 state and the energy barrier is lower, which provides the real evidence for reversing proton transfer. It is worth noting that Q3HC-C involves the intramolecular hydrogen bonds O\\H⋯N (normal form) and N\\H⋯O (tautomer form), and it is necessary to discuss in terms of the LefferHammond postulate [76–78]. For Q3HC-C, the normal form with the intramolecular hydrogen bond (O1\\H2⋯N4) is more stable than the tautomer form with the (N4\\H2⋯O1). That is to say, the tautomer form has higher energy than the normal form, and the hydrogen bond (N4\\H2⋯O1) is stronger than (O1\\H2⋯N4). According to the LefferHammond postulate [76–78], for exothermic reaction, the energy difference between the transition state and the reactants is smaller than that between the transition state and the product. Therefore, the transition

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Fig. 5. The calculation frontier molecular orbitals (HOMO and LUMO) of Q3HC complex at TDDFT/B3LYP/TZVP level.

state is closer to the reactants than to the products. On the contrary, for endothermic reaction, the transition state is closer to the products than to the reactants. Thus, for the reverse proton transfer of Q3HC-C in the S0 state, the transition state is closer to the tautomer form with the (N4\\H2⋯O1) than to the normal form with the (O1\\H2⋯N4). Fig. 7 displays the curve of Intrinsic Reaction Coordinate of Q3HC-C. It can be seen that the calculation result is consistent with the Leffer-Hammond hypothesis, and it also further proves the rationality of the calculation method in this paper. Thus, the ESIPT mechanism of Q3HC could be summarized as follow: the normal form exists in S0 state, which forms the intramolecular hydrogen bond. Upon photo-excited, the ESIPT reaction occurs to form the stable tautomer forms in the S1 state. Undergoing radiative transition, the molecule regresses to the S0 state with a long fluorescence emission. In the S0 state, the tautomer form is unstable and collapses back to the normal form. It notes that the tautomer form of Q3HC-C in the S1 state is likely to radiate back to the S0 state

forming the stable structure, which is different from the previous report [56]. 3.7. The discriminate of weak interaction types by RDG isosurfaces To compare the different types of hydrogen bonding interactions, we have used the reduced density gradient (RDG) [79]. It can be defined as this equation RDGðr Þ ¼

j∇ρðr Þj

1 1=3

2ð3π 2 Þ

ρðrÞ4=3

In this equation, the RDG(r) is the reduced density gradient of the exchange contribution, the ρ(r) is the total electron density. In Bader's AIM theory [80], the relative to the second largest eigenvalue of Hessian matrix of electron density, which is called λ2, and the ρ(r) of the total electron density can be expressed as Ωðr Þ ¼ Signðλ2 ðr ÞÞρðrÞ

Table 4 Calculated natural population analysis and Mulliken's charge of O1, O3 and N atoms. Natural population analysis

Q3HC-A Q3HC-B Q3HC-C

O1 O3 O1 O3 O1 N4

Mulliken's charge

S0

S1

S0

S1

−0.645 −0.621 −0.624 −0.620 −0.642 −0.457

−0.638 −0.635 −0.609 −0.634 – –

−0.292 −0.411 −0.254 −0.409 −0.264 −0.180

−0.273 −0.427 −0.227 −0.423 – –

ð1Þ

ð2Þ

The calculation results of the weak interaction depend on the ρ of electron density and the λ2 of eigenvalue together. The λ2 value has been used to discriminate the types of interaction. It is greater than zero represents bonding, whereas it is antibonding. Specifically, the large negative sign (λ2)ρ means the attractive interactions, such as hydrogen bonding, while the large positive values of sign (λ2)ρ are indicative of nonbonding interaction, usually strong repulsion or steric effect in a ring or case. When the value is near zero, it shows the van der Waals interactions [79]. To clearly display the different types of the interactions, the color gradient has been used to stand for the ρ(r) and λ2

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Fig. 7. The curve of Intrinsic Reaction Coordinate (IRC) of Q3HC-C in S0 state. RC: reactant; TS: transition state; Pro: product.

interactions and steric effect in the Q3HC-A, Q3HC-B and Q3HC-C have been marked by the blue, green and red circles, respectively. The intramolecular hydrogen bond (O1\\H2⋯N4) of Q3HC-C is stronger than (O1\\H2⋯O3) of Q3HC-A and Q3HC-B. These intramolecular attractive interactions are also visualized and shown in the right of Fig. 7. The contour value is set to 0.5 and the RDG isosurface ranges from −0.04 to 0.02. The blue, green and red in these structures correspond to the hydrogen bond interactions, van der Waals interactions and steric effect, respectively. We denote the H-bonds by blue circles particularly. In isosurface plots, the isosurface between hydrogen group and carbonyl oxygen (for Q3HC-A and Q3HC-B) appear as small light blue part in the center, while the isosurface between hydrogen group and nitrogen atom (for Q3HC-C) changes to dark blue. The results again illustrate that the intramolecular hydrogen bond (O1\\H2⋯N4) is stronger than (O1\\H2⋯O3). Further, the analysis provides the real evidence for the ESIPT process of Q3HC-C is earlier to occur. 4. Conclusion

Fig. 6. Potential energy curves of S0 and S1 states for Q3HC complex along with O\ \H bond length of Q3HC. The inset shows the corresponding optimized configuration. a: Q3HC-A; b: Q3HC-B; c: Q3HC-C.

In summary, we have investigated the ESIPT mechanisms of Q3HC via the DFT and TDDFT method. We optimize three configurations (Q3HC-A, Q3HC-B and Q3HC-C) and declare the relationship among them, which the Q3HC-A and Q3HC-B could mutual transformation in the S0 and S1 states. The analysis of bond lengths, bond angles and IR vibrational spectra involved hydrogen bonds have shown that the intramolecular hydrogen bonds are strengthened in S1 state. The results of the frontier molecular orbits (MOs), charge distribution using Mulliken's charge distribution analysis and NPA analysis method reveal that the intramolecular charge transfer further triggers the ESIPT processes. The potential energy curves have shown that the ESIPT of Q3HC-C can be spontaneous in the S1 state, while the Q3HC-A and Q3HC-B can also happen due to the lower potential barrier. Combination with the RDG function, we further illustrate that the ESIPT process of Q3HC-C is easier than that of Q3HC-A and Q3HC-B. In particular, we find that tautomer form of Q3HC-C in the S1 state is likely to undergo the radiative transition to the S0 state forming the stable structure. Acknowledgements

value. In addition, we have shown the visual graph by software VMD [71,81]. Fig. 8 displays the RDG scatter plots (left) and isosurfaces (right) for Q3HC-A, Q3HC-B and Q3HC-C, the main concern is the intramolecular hydrogen bonds. In scatter plots, the hydrogen bond, van der Waals

This work was supported by the Open Project of SKLMRD-K2018 (Open Project of State Key Laboratory of Molecular Reaction Dynamics) and National Natural Science Foundation of China (Grant 21773238). The results of quantum chemical calculations described in this paper were obtained on the homemade Linux cluster of group 1101, Dalian Institute of Chemical Physics.

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Fig. 8. RDG scatter plots (left) and isosurfaces (right) for a Q3HC-A, b Q3HC-B and c Q3HC-C. The isosurfaces are colored on a blue-green-red scale according to values of sign (λ2)ρ, which ranges from −0.04 to 0.02 a.u. Blue indicates strong attractive interactions, green indicates van der Waals interaction, and red indicates steric effect in ring and cage. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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