Theoretical Insights Into the Excited State Double Proton Transfer

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Theoretical Insights Into the Excited State Double Proton Transfer Mechanism of Deep Red Pigment Alkannin Jinfeng Zhao, Hao Dong, and Yujun Zheng J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b10492 • Publication Date (Web): 10 Jan 2018 Downloaded from http://pubs.acs.org on January 11, 2018

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The Journal of Physical Chemistry

Theoretical Insights Into the Excited State Double Proton Transfer Mechanism of Deep Red Pigment Alkannin Jinfeng Zhao, Hao Dong, and Yujun Zheng∗ School of Physics, Shandong University, Jinan 250100, China E-mail: [email protected]

∗ To

whom correspondence should be addressed

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Abstract 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 firstly 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 the first-step proton transfer (PT) process in the S1 state, and nonpolar solvents play important roles in lowering the potential energy barrier of the second-step PT reaction.

Introduction As one of the most fundamental reactions involved in biological and chemical reactions, proton transfer (PT) has attracted lots of attention in relevant fields. 1–4 Multifarious types of PT reactions have been identified based on acid-base neutralization, adiabatic versus non-adiabatic, weak as well as strong hydrogen bonds, and so forth. 5–8 Since the pioneering work reported the electronic excited state intramolecular proton transfer (ESIPT) reaction in methyl salicylate by Weller, 9,10 lots of investigations have focused on it due to its unique photophysical and photochemical properties. 11–15 Normally the ESIPT reaction occurs along with the pre-existing intra- or intermolecular hydrogen bond. And the energy gap between the Franck-Condon excited and relaxed excited states upon photo-excitation provides the driving force for the transformation. In most cases, such kind of reaction follows the cycle: absorption → ESIPT → emission → back ground-state PT. Probably the most useful consequence of the ESIPT systems is their dual fluorescence properties (i.e., the shorter wavelength emission derives from the initial normal structure and the longer wavelength 2


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stems from the photo-tautomer form in the excited state). Generally, the significant Stokes shift can be as large as 6000-12000 cm−1 . 16–22 Just due to the differences between these two tautomeric structures in the excited state, the ESIPT compounds have been recognized with optoelectronic applications, such as laser dyes and LEDs, molecular switches, fluorescence sensors, and so on. 23–30

Figure 1: Views of alkannin with relevant atom labels involved in two intramolecular hydrogen bonds. Herein, three kinds of PT structures of alkannin are also presented. Alkannin, as the major component of deep red pigments, can be easily extracted from roots of Lithospermun erythrorhizon and Alkanna tinctoria. 31–33 And it is said that use of alkannin as a kind of dyestuff for fabrics can almost date back centuries BC. 31 Furthermore, it could be also used as crude drugs for accelerating wound healing. 33,34 We show the corresponding chemical structures of alkannin in Fig. 1. Obviously, two intramolecular hydrogen bonds exist in alkannin dye, for convenience, we name them O1-H2· · ·O3 and O4-H5· · ·O6. Thus it probably refers to the excited state double proton transfer (ESDPT) reaction based on excitation. As far as we know, ESDPT process is very important in many biological fields since its intrinsic properties in mimicking mutation of DNA and RNA. 35–37 Therefore, alkannin may be also a model ESDPT compound to further explore its fundamental excited state behaviors, which could further promotes its applications in future. In 2002, Glazunov and co-workers firstly directed the infrared (IR) spectroscopy of alkannin and its derivatives and confirmed their chemical structures. 38 Based on 3



theoretical investigations, Liang et al. studied the conjugations and substituent effects of alkannin and its derivatives. 39,40 Combining with constructing coarse Franck-Condon ESIPT curves in gas state, they denied the fact that the ESIPT reaction of alkannin could be affected by the triple excited state. And they inferred the excited state single proton transfer process exists in the first excited (S1 ) state along with hydrogen bond O1-H2 · · ·O3. 40 Then as mentioned by Serdiuk and co-workers, 41 there are not enough subsequent reports about its excited state process, and people mainly focus on its dye applications and its derivatives. 42,43 In fact, it is well known that the Franck-Condon ESIPT curves are too rough to clarify excited state behavior. And previous papers on alkannin do not attach importance to differences of excited state mechanisms arising from solvent effects. In condensed dielectric media such as aqueous-based biological systems and liquid solvents, the charge redistribution not only affects the interactions with surrounding environment, but also presents different photochemical and photophysical properties. 44 In view of the interactions between solvent and solute, in this work, we mainly focus on the ESIPT mechanism of alkannin and on the influences stemming from solvents. Herein, to avoid the effects from intermolecular hydrogen bond between alkannin and surrounding solvents, we just choose the aprotic solvents (i.e., polar acetonitrile (CH3CN) and nonpolar cyclohexane (CYH)). The transition state (TS) search is started from an educated guess on the TS structure by placing the hydrogen atom being transferred between hydrogen donor and acceptor and by adjusting the bond lengths that are affected by the hydrogen transfer. Then, a Hessian calculation (second derivatives of energies with respect to the spatial coordinates) is performed at the guessed TS structure to obtain the force constants. On the basis of the force constants, the TS search can be carried out by Berny optimization method, 45 and we clarify the stepwise ESDPT mechanism of alkannin in detail. In addition, comparing gas and solvent states, we present that the increase of solvent polarity is in favor of occurring the first-step ESIPT process and further affects the second-step PT reaction. The remainder of this paper can be organized such that the next part describes the computational methodology. Then the following section shows and discusses our theoretical results about chemical structures, infrared (IR) vibrational spectra, charge redistributions, and lastly, the excited

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state dynamical processes and corresponding electronic spectra. A final section summarizes the conclusion of this work.

Computational methodology In the present work, all the electronic structural calculations are performed based on density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods using the exchange-correlation with Lee-Yang-Parr gradient-corrected B3LYP functional 46–48 in Gaussian 09 program. 49 After testing basis sets, we select the triple-ζ valence quality with one set of polarisation functions (TZVP) one. 50 Solvent effects are included in all this work using the integral equation formalism version of the Polarisable Continuum Model (IEFPCM) with the dielectric constant of CH3CN (ε = 35.67) and CYH (ε = 2.02). All the ground-state (S0 ) species are optimized without constraint with the vibrational frequencies analyses to confirm that these structures are the local minima on the S0 -state surface. The vertical excitation calculations are carried out from optimized S0 -state alkannin form using TDDFT method with the six low-lying absorption transitions. From the ground equilibrium configurations, the S1 -state geometries are optimized without constrain coupling with TDDFT/B3LYP/TZVP theoretical level. Also the vibrational frequencies are analyzed with no imaginary frequency. To explore the excited state behaviors and ESDPT mechanisms, potential energy surfaces (PESs) are constructed theoretically using TDDFT/B3LYP/TZVP theoretical level. All the stationary points along the reaction coordinates are scanned by constraining optimizations and frequency analyses to obtain the thermodynamic corrections in the corresponding electronic states. Further, we also adopt TDDFT/B3LYP/TZVP method to search the TS structures in the S1 state, which can be started from the educated guess with placing the hydrogen atom being transferred between hydrogen donor and acceptor and adjusting the corresponding bond lengths affected by PT reaction. The Hessian calculations are performed at the guessed TS forms to obtain the force constants. Based on Berny optimization method, 45 our theoretical TS structure is confirmed to be only one imaginary frequency, and its vibrational eigenvector points to

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the correct reaction direction. Thermal corrections and zero-point energy corrections to the Gibbs free energy are also carried out according to harmonic vibrational frequencies.

Results and discussion Structural analyses As shown in Fig. 1, we find four stable structures in S0 and S1 states (i.e., alkannin, alkanninPT1 (single-proton transfer form along with O1-H2· · ·O3), alkannin-PT2 (single-proton transfer form along with O4-H5· · ·O6) and alkannin-DPT (dual-proton transfer form of alkannin)). As mentioned above, vibrational frequency analyses are performed to ensure the validity of these stable points. Since Liang et al. studied relevant structures, 40 as a comparison, we also calculate these structures in gas state. In our calculated results, no matter in gas, CYH and CH3CN, we confirm that the alkannin form is the most stable structure in both S0 and S1 states. To reveal the intramolecular interactions in the real space, we calculate and display the reduced density gradient (RDG) versus sign(λ2 )ρ and gradient isosurfaces of alkannin in Fig. 2, and those of other structures are shown in Figure S1, ESI†. As reported by Cohen et al., 51 multiple weak interactions can be clearly presented, namely, negative values of sign(λ2 )ρ refer to hydrogen bonding effects, positive sign(λ2 )ρ stands for steric effect and values near zero exhibit the Van der Waals (VDW) interactions. Obviously, the spikes located around -0.02 a.u. for alkannin in both S0 and S1 states indicate the hydrogen bonding interactions of H2· · ·O3 and H5· · ·O6. In addition, one thing should be noticed that the interactions between O-H moiety of side chain and O6 atom are not in the realm of hydrogen bonding effects in both S0 and S1 states, thus we do not consider this O-H moiety in this work. The most important structural parameters involved in two intramolecular hydrogen bonds (O1-H2· · ·O3 and O4-H5· · ·O6) in gas state are listed in Tab. 1. For CYH and CH3CN solvents, the calculated results are listed in Table S1 and Table S2, ESI†. It is worth mentioning that the bond lengths of O1-H2 and O4-H5 of alkannin are elongated in the S1 state, and those of hydrogen bonds (H2· · ·O3 and H5· · ·O6) are shortened with the concomitant increasement 6


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of bond angles δ (O1-H2· · ·O3) and δ (O4-H5· · ·O6). In other words, these two intramolecular hydrogen bonds can be strengthened in the S1 state. 52–58 In addition, one thing should be noticed that the hydrogen bond length H2· · ·O3 owns larger changes than the H5· · ·O6 one along with the enlargement of solvent polarity. That is to say, hydrogen bond O1-H2· · ·O3 might strengthen larger than O4-H5· · ·O6.

Figure 2: The RDG (Function 2) vs. sign(λ2 )ρ (Function 1) and lower panel low-gradient (s=0.5 a.u.) isosurfaces for alkannin in both S0 (a) and S1 (b) states. Herein, the interactions are shown below corresponding RDG (Red: Steric effect; Blue: Hydrogen bonding effect; Green: VDW effect). Table 1: The primary bond lengths (Å) and bond angles δ (◦ ) of alkannin, alkannin-PT1, alkannin-PT2 and alkannin-DPT structures in gas state.

O1-H2 H2-O3 O4-H5 H5-O6 δ (O1-H2-O3) δ (O4-H5-O6)

alkannin alkannin-PT1 S0 S1 S0 S1 0.993 1.006 1.613 1.587 1.694 1.633 1.010 1.018 0.991 1.001 1.009 1.016 1.691 1.659 1.607 1.593 146.7 149.8 149.5 151.1 146.2 148.7 149.1 150.8

alkannin-PT2 S0 S1 1.010 1.024 1.617 1.572 1.534 1.520 1.026 1.036 149.4 151.8 151.8 153.4

alkannin-DPT S0 S1 1.705 1.658 0.991 1.002 1.650 1.605 0.997 1.011 146.5 149.0 148.2 150.8

Given another effective manner to investigate excited state hydrogen bond, we also calculate 7



Figure 3: The theoretical IR spectra of the alkannin structure in the spectral region of the O1-H2 and O4-H5 stretching bonds in both S0 and S1 states. (a) gas; (b) CYH; (c) CH3CN. the IR vibrational spectra in this work since monitoring the spectral shifts of some characteristic vibrational modes involved in hydrogen bonds is the acknowledged method. 57–62 In Fig. 3, we present our theoretical results about O1-H2 and O4-H5 stretching modes of the alkannin in both S0 and S1 states. It can be clearly found that the O1-H2 vibrational frequencies in the S0 state are located at 3249, 3244 and 3235 cm−1 , which are red shift to the S1 -state 3005, 2980 and 2936 cm−1 in gas, CYH and CH3CN, respectively. That is to say, the strong red-shift of 244, 264 and 299 cm−1 for O1-H2 stretching band could be induced by the strengthening hydrogen bond O1H2· · ·O3 in the S1 state. In the similar way, O4-H5 vibrational frequencies are also red shift in the S1 state. However, comparing with O1-H2 vibrational modes, O4-H5 owns relatively weaker redshift of 171, 195 and 239 cm−1 in gas, CYH and CH3CN, respectively. This results are consistent with the analyses of bond lengths and bond angles discussed above. Therefore, we have reasons to believe that the strengthening hydrogen bonds O1-H2· · ·O3 and O4-H5· · ·O6 may facilitate the ESDPT reaction in the S1 state. And we predict that the O1-H2· · ·O3 one may be more affected upon the photo-excitation in polar aprotic solvents.

Orbital contribution and charge distribution As far as we know, the charge distribution over a molecule could be changed upon photo-excitation process, which can effectively affect its excited state dynamical process. 52–58 Thus the frontier molecular orbitals (MOs) are calculated in this work to investigate the information about the na-

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ture of excited state structures. Our calculated electronic transition energies and relative oscillator strengths show that the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) mainly corresponds to the S0 → S1 transition with around 99.97% composition in gas, CYH and CH3CN. Therefore, we just provide the HOMO and LUMO orbital in Fig. 4. Obviously, the transition of HOMO to LUMO is a classic ππ ∗ -type transition. Herein, we just concentrate on the charge redistribution around hydrogen bonding moieties, which can seriously affect excited state hydrogen bonding dynamics. It can be noticed that the electron densities of O1 and O4 of hydroxy moieties decrease, whereas those of O3 and O6 atoms increase after transition from HOMO to LUMO. No matter gas, CYH and CH3CN surroundings, the changes about electron density are almost the same. So we just show the quantificational results in CH3CN solvent as an example. The contributions of O1 and O4 sharply drop from 7.28% and 7.04% to 3.94% and 4.11%, whereas those of O3 and O6 increase from 3.87% and 3.95% to 6.84% and 6.27%, 63 respectively. The incremental electron densities of O3 and O6 moieties could strengthen the intramolecular hydrogen bonds O1-H2· · ·O3 and O4-H5· · ·O6 in the S1 state, which can facilitate the PT process. Peculiarly, O3 atom moiety owns larger augment of composition than O6 moiety, which is also in good agreement with the discussions of bond lengths and IR analyses above. So we predict the excited state dynamical behaviors of alkannin are more affected by O1-H2· · ·O3 one.

Mechanism discussions It cannot be denied that the excited state properties of alkannin could be influenced by the charge redistributions as mentioned above. In order to further study the detailed ESDPT processes and present the different excited state behaviors in gas, CYH and CH3CN surroundings, we optimize all the S0 -state and S1 -state geometrical structures with fixing both O1-H2 and O4-H5 bond lengths from 0.9 to 2.1 Å in step of 0.1 Å based on TDDFT B3LYP/TZVP method. The S0 -state PESs in gas, CYH and CH3CN surroundings are shown in Figure S2, ESI†. In view of the relationships among the stable structures on S0 -state PESs, our theoretical results show that the most stable one 9



Figure 4: View of HOMO and LUMO orbitals for the alkannin structure. is alkannin itself, and the next most stable tautomer is alkannin-DPT, then is alkannin-PT1, and the last one is alkannin-PT2 in all these three surroundings. And this result is consistent with previous work by Liang et al. 39,40 Due to the similar conformations of the S1 -state PESs in gas, CYH and CH3CN solvents, we just present the PES of CH3CN solvent in Fig. 5 (a), and S1 -state PESs of CYH and gas are shown in Figure S3, ESI†. Clearly, it can be found that the gaps between S0 state and S1 -state PESs are all larger than 0.0637 a.u. (around 40 kcal/mol) in these three kinds of surroundings. Therefore, it can be confirmed that the ESPT reaction of alkannin system could not be influenced by intersections between S0 and S1 states. Since we mainly focus on the excited state process, for convenience, the projective plane of Fig. 5 (a) is shown in Fig. 5 (b). Obviously, four stable structures can be located (i.e., I stands for the alkannin; II stands for alkannin-PT1; III means alkannin-PT2; IV means alkannin-DPT), since alkannin itself is not a symmetrical structure. As mentioned above, the PESs of gas, CYH and CH3CN solvents are almost the same, the only difference is the potential energy barriers among the excited state paths. From Fig. 5 (a), it can be obviously seen that there is a high barrier along with the diagonal path I → IV, namely, 7.817 kcal/mol in gas; 7.209 kcal/mol in CYH; 6.532 kcal/mol in CH3CN. Accordingly, the concerted 10


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ESDPT reaction mechanism can be excluded for alkannin system. In effect, from Fig. 5 (b), it can be found that the stepwise ESDPT paths are much more supportive than the concerted one. To check the rationality of B3LYP functional, we also calculate the potential energy curves using other two exchange-correlation functionals B3P86 64 and MPW1PW91. 65 Along with the stepwise ESDPT path (I → II → IV), the S1 -state potential energy curves are constructed with fixing corresponding bond distances in step of 0.1 Å based on based on B3LYP, B3P86 and MPW1PW91 functionals. Taking the results of CH3CN solvent as an example, the potential energy curves are displayed in Fig. 6. And relative results of gas and CYH solvent are shown in Figure S4, ESI†. Obviously, the conformations of potential energy curves are almost the same under these three kinds of functionals, respectively. Thus there is no need to doubt the feasibility of the B3LYP one adopted in this work.

Figure 5: (a) The S1 -state PES of alkannin as functions of O1-H2 and O4-H5 bond lengths for all the relative structures. (b) The S1 -state projective plane with four stable points (I, II, III and IV). The energy between every contour is 0.6275 kcal/mol. Herein, transition states (TS1, TS2 and TS3) along with stepwise paths are marked. To calculate more accurate potential energy barriers, by Berny optimization method, 45 we search the transition state structures (TS1 for I → II; TS2 for II → IV and TS3 for I → III) under TDDFT/B3LYP/TZVP method in Figure S5, ESI†. All the TS structures are confirmed to

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Figure 6: The S1 -state potential energy curves along with O1-H2 (a-c) and O4-H5 (d-f) bond distances in CH3CN solvent, respectively. Herein, (a) and (d) are obtained under B3LYP functional; (b) and (e) are under B3P86 functional; (c) and (f) are under MPW1PW91 functional.

Figure 7: The vibrational eigenvector orientations of TS1, TS2 and TS3 structures on the S1 -state PES. be only one imaginary frequency, and its vibrational eigenvector points to the reaction directions in Fig. 7. The corresponding imaginary frequencies of TS1, TS2 and TS3 are listed in Table S3, ESI†. It is worthwhile arguing that the vibrational eigenvector orientations of TS1 and TS2 point to the stepwise ESDPT reaction (I → II → IV), while the that of TS3 points to the reverse

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orientation (III → I). In other words, the proton transfer reaction could not occur along with I → III path in the S1 state. Thus the III → IV path is not considered in this paper. Furthermore, the excited state reaction path is also constructed by integrating the intrinsic reaction coordinate (IRC) at the same level of theory. This scheme of integration over the S1 -state PESs are obtained by combining the first-order Euler predictor approach with a modified Bulrisch-Stoer integrator for the corrector algorithm. 66–69 Beginning from the TS1 and TS2 in CH3CN, CYH and gas, two minima (i.e., reactants and products) have been searched according to the direction of energy decrease, respectively. The relative IRC curves have been shown in Figure S6, ESI†, which further verifies the reliability of stepwise ESDPT path.

Figure 8: The potential energy barriers (kcal/mol) of stepwise ESDPT reaction paths (alkannin → alkannin-PT1 and alkannin-PT1 → alkannin-DPT) in Gas, CYH and CH3CN surroundings in the S1 state. Given the solvent effects, the potential energy barriers along with I → II and II → IV are displayed in Fig. 8. The barrier of first-step ESIPT reaction (I → II) decreases along with the increasement of polarity, while that of the second-step ESIPT process (II → IV) rises more or less. Although the barriers of I → III are also depressed (i.e., 4.594 kcal/mol in gas, 3.924 kcal/mol in CYH and 3.532 kcal/mol in CH3CN), aforementioned vibrational eigenvector orientation of TS3 hinders this reaction. In addition, paying attention to the bond lengths of TS structures in Figure S5, ESI†, we find the O1-H2 length of TS1 is shortened from gas to CH3CN, which indicates 13



the first-step ESIPT reaction is more likely to occur in polar solvent consistent with the potential barriers in Fig. 8. In view of the O4-H5 length of TS3 in Figure S5, ESI†, it can be found that O4H5 shortens with the increase of solvent polarities, which further conduces to the reverse reaction (III → I). Till now, we confirm that alkannin should proceed a stepwise ESDPT reaction along with the I → II → IV reaction path. High polar aprotic solvents can effectively promote the first-step PT reaction in the S1 state, and nonpolar aprotic solvents can facilitate the second-step PT process.

Figure 9: The theoretical electronic spectra of alkannin, alkannin-PT1 and alkannin-DPT forms in (a) gas, (b) CYH and (c) CH3CN. Our theoretical absorption and emission peaks of the corresponding structures involved in the stepwise ESDPT process about alkannin system are shown in Fig. 9. It can be found that the absorption peaks of alkannin are not very sensitive to the solvent polarity. Upon excited to the S1 state, the normal fluorescence peak of alkannin form generates bathochromic shift from 581 nm to 620 nm (Fig. 9 (a) and Fig. 9 (c)). Along with the stepwise ESDPT reaction, we find that the emission peaks of alkannin-PT1 and alkannin-DPT are also red shift in these three surroundings from Fig. 9 (a) to Fig. 9 (c). In addition, the emission peaks of alkannin, alkannin-PT1 and alkannin-DPT are very close, which may be difficult to be distinguished via steady-state spectrum measurement. Maybe that is why Ordoudi et al. did not obverse fantastic phenomena about their visible spectra of alkannin in phosphate-buffered saline (PBS). 42 The clearer excited state dynamical processes about alkannin, such as lifetime and reaction rates in the S1 state, should depend on time-resolved spectroscopic measurements experimentally, while it is beyond the ability of our group.

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Conclusions In the present work, we theoretically study and clarify the stepwise ESDPT process of alkannin system with DFT and TDDFT methods. Upon the photo-excitation, two intramolecular hydrogen bonds of alkannin are testified to be strengthened in the S1 state in three kinds of surroundings (gas phase, CYH and CH3CN solvents), which provides the possibility for the ESDPT process. The redistribution of charge further reveals ESDPT tendency of alkannin. It is worth mentioning that the O1-H2· · ·O3 hydrogen bond is more affected upon excitation. Exploring the S1 -state PESs of alkannin in three different surroundings, we verify that the concerted ESDPT path can be excluded due to high potential energy barriers. Via searching TS structures on the S1 -state PESs, we confirm the excited state reaction path (i.e., I → II → IV) since the vibrational eigenvector orientation of TS3 points to the reverse reaction. Analyzing the relationships of potential barriers along with stepwise ESDPT path, we present that the first-step ESIPT (I → II) reaction can be controlled by solvent polarity and the nonpolar solvents can facilitate the second-step PT (II → IV) process. We sincerely wish that this work not only clarifies the ESDPT process of alkannin in aprotic surroundings, but also might promote the relevant applications and developments about alkannin pigment in future.

Supporting Information The comparisons of the primary bond lengths and bond angles of alkannin, alkannin-PT1, alkanninPT2 and alkannin-DPT structures in CYH (Table S1) and CH3CN (Table S2) solvents, the calculated imaginary frequencies for TS1, TS2 and TS3 forms along with stepwise ESDPT path (Table S3), the RDG versus sign(λ2 )ρ of alkannin-PT1, alkannin-PT2 and alkannin-DPT in both S0 and S1 states (Figure S1), the S0 -state PESs of alkannin in three kinds of surroundings (Figure S2), the S1 -state PESs of alkannin in gas and CYH (Figure S3), the potential energy curves under three functionals in gas and CYH (Figure S4), the TS1, TS2 and TS3 structures on the S1 -state PESs (Figure S5), and the IRC curves for the stepwise ESDPT path for alkannin (Figure S6) 15



Acknowledgments This work was supported by the National Natural Science Foundation (No. 11674196 and 11604333) and the National Basic Research Program of China (No. 2015CB921004).

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