Mechanism for the Excited-State Multiple Proton Transfer Process of

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Mechanism for the Excited-State Multiple Proton Transfer Process of Dihydroxyanthraquinone Chromophores Qiao Zhou,† Can Du,† Li Yang,† Meiyu Zhao,‡ Yumei Dai,§ and Peng Song*,† †

College of Physics, Liaoning University, Shenyang 110036, P. R. China School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin 150080, P. R. China § Normal College, Shenyang University, Shenyang 110044, P. R. China ‡

ABSTRACT: The single and dual cooperated proton transfer dynamic process in the excited state of 1,5-dihydroxyanthraquinone (1,5-DHAQ) was theoretically investigated, taking solvent effects (ethanol) into account. The absorption and fluorescence spectra were simulated, and dual fluorescence exhibited, which is consistent with previous experiments. Analysis of the calculated IR and Raman vibration spectra reveals that the intramolecular hydrogen bonding interactions (O20−H21···O24 and O22−H23···O25) are strengthened following the excited proton transfer process. Finally, by constructing the potential energy surfaces of the ground state, first excited singlet state, and triplet state, the mechanism of the intramolecular proton transfer of 1,5-DHAQ can be revealed.

1. INTRODUCTION The proton transfer (PT) is a fundamental process in physics, chemistry, and biological systems.1,2 The excited-state interand/or intramolecular proton transfer (ESIPT) reaction have been found to play important roles in determining the macroscopic performance of functional devices.3−12 Especially, the excited-state double PT reaction and its mechanism analysis in multiple hydrogen-bonded complexes has attached particular interest recently.13−22 Such as 1,5-dihydroxyanthraquinone (1,5-DHAQ) studied here, it is the hydroxyanthraquinone (HAQ) derivative belonging to anthraquinone that exhibits two intramolecular hydrogen bonds. In particular, HAQ derivatives, for instance, 1,4-DHAQ, 23−25 1,5-DHAQ, 26−28 1,8DHAQ,29−31 and so on, were widely investigated, and they have been successfully used in dyes, paper manufacturing industries, and as anticancer drugs, owing to these compounds being provided with a larger conjugated system with planar construction. 1,5-DHAQ has been of considerable interest, with a number of studies having been performed. As early as in the last century in 1960s, its molecular structure was determined experimentally by Hall.32 Marasinghe and co-workers redetermined its structure, and found that the intramolecular hydrogen bond is asymmetrically placed with both O20−H21 and O22−H23 (labeled in this work) and is 0.94 Å in length.33 Gillispie et al. proposed that 1,5-DHAQ undergoes a rapid ESIPT reaction from the first excited singlet state (S1) to ground state (S0) state and presented a marked geometry difference between S0 to S1 with a large Stokes shift, as a result of dual fluorescence.34 It is also found that the Stokes shift is insensitive to the solvent polarity and that the ESIPT reaction plays a key aspect in photodynamics for 1,5-DHAQ.35 In addition, Mario and co© 2017 American Chemical Society

workers investigated the ESIPT reaction in HAQs, drawing a conclusion that its dual emission associated with the ESIPT reaction.26 Recently, the related study demonstrated that the most striking effect for 1,5-DHAQ is the ESIPT process in solution, using time-resolved infrared vibrational spectroscopic analysis.36 And it is found that with the formation of the encounter complex, 1O2 quenching activity took place, which indicates 1,5-DHAQ is susceptible to the ESIPT reaction, and it also exhibits dual fluorescence.37 In 2015, Mohandoss synthesized β-CD:1,5-DHAQ as a new dual functional probe with high selectivity and sensitivity to Fe3+.38 Alternatively, Ferreiro studied two types of the proton transfer reaction mechanism of 1,5-DHAQ, based the fact that the dual proton transfer reaction induced the tautomer with high instability, using the Hartree− Fock ab initio method.39 Furthermore, from the point view of molecular orbital theory, the intramolecular hydrogen bonding influence on molecular structure, electronic spectra, and thermodynamical function of anthraquinone and its hydroxyl derivatives were also investigated by adopting density functional theory (DFT) method.40 However, there still has been limited theoretical research in field of detailed dual ESIPT mechanism for simple 1,5-DHAQ, especially the solvation effect is simultaneously involved. The relevant geometric constructions were optimized and shown in Figure 1. The IR and Raman vibration spectra, frontier molecular orbital (MOs) in according with hydrogen bond Received: April 29, 2017 Revised: May 29, 2017 Published: May 31, 2017 4645

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Figure 1. Optimized structures for 1,5-DHAQ. (a) Enol configuration, (b) keto configuration, (c) enol−keto I configuration, and (d) enol−keto II configuration.

energies, and potential energy surfaces (PESs) were calculated for analyzing the ESIPT process.

tion, according to the DFT/TDDFT and B3LYP/TZVP calculated level are presented in Table 1 and Figure 2,

2. COMPUTATIONAL DETAILS Density functional theory (DFT) and time-dependent density functional theory (TDDFT) were used to accomplish all the theoretical calculations presented in this work in combination with the Becke three-parameter hybrid exchange functional and the Lee−Yang−Parr gradient-corrected correlation functional (B3LYP),41−46 and the basis set of triple-ζ valence quality with one set of polarization function (TZVP)47 in Gaussian 09 program.48 In addition, according to the previous experimental investigation of 1,5-DHAQ, the polarizable continuum model using the integral equation formalism variant (IEF-PCM) with the ethanol solvent was considered when theoretically calculating.49,50 The vibrational frequencies and the structure parameter of 1,5-DHAQ were provided without constraints in DFT/TDDFT calculation, even though the calculations based on TDDFT method are very time-consuming and computationally expensive.51−55 Vertical excitation energies were calculated based on the optimized geometry in S0 using TDDFT. Moreover, the intramolecular interactions have been studied with calculated infrared (IR) and Raman vibration spectra.56−58 Ultimately, to investigate the PT process in ethanol, the corresponding PESs of the S0 and S1 states and triplet state (T1) were also constructed using DFT/TDDFT without geometric constraints.

Table 1. Primary Bond Lengths (Å) and Bond Angles (deg) of 1,5-DHAQ Forms in S0 and S1 States for Enol Configurationa electronic state O20−H21 H21−O24 δ(O20−H21− O24) a

3. RESULTS AND DISCUSSION 3.1. Structural Analysis and Electronic Spectra. The primary structure parameter of 1,5-DHAQ and its excited-state properties in ethanol, based on stable equilibrium conforma-

S0

S1

0.99 1.67 147.34

1.03 1.53 153.26

electronic state O22−H23 H23−O25 δ(O22−H23− O25)

S0

S1

0.99 1.67 147.34

1.03 1.53 153.26

On the basis of the DFT/TDDFT methods in ethanol.

Figure 2. Calculated absorption and fluorescence spectra of 1,5DHAQ in ethanol based on the B3LYP/TZVP theoretical level. 4646

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transfer dynamics, as shown in Figure 4. As displayed in Figure 4a, a Raman peak at 3258.69 cm−1 for the enol form in S0 state

respectively. According to the calculated results, the calculated bond lengths of O20−H21, H21···O24, O22−H23, and H23···O25 for the enol form in the S0 state change from 0.99, 1.67, 0.99, and 1.67 Å, respectively, to 1.03, 1.53, 1.03, and 1.53 Å in the S1 state following with photoexcitation. On the one hand, obviously, the hydrogen bonds (H21···O24 and H23···O25) are shortened. On the other hand, the bond angle δ(O20−H21− O24) and δ(O22−H23−O25) elongates from 147.34° to 153.26° from S0 to S1 state. Clearly, the primary structure parameter of 1,5-DHAQ indicated that, in the S1 state, the intramolecular hydrogen bonds are strengthened. From the electronic spectra in Figure 2, evidently there exists one absorption peak located at 431.04 nm, and the fluorescence peak, calculated at 512.85 nm for the enol form of 1,5-DHAQ, displays a Stokes shift of 81.81 nm. Moreover, another fluorescence peak appears at 619.09 nm. So, it reveals that the calculated 1,5-DHAQ own dual fluorescence, in accordance with experimental results.34,35 This is expected to accelerate the interaction of the hydrogen bond. Also, the above analysis indicates that our selected calculation method is rational. Note that 1,5-DHAQ has been experimentally proved to be extraordinarily insensitive to the solvent characters, such as whether the solvent is nonpolar (hexane), polar (Acetonitrile (ACN) and ethanol), hydrogen-bond donating (ethanol), or hydrogen-bond accepting (2-methyl tetra-hydrofuran (2MTHF)).35 And our theoretically calculated absorption peaks at 430.76, 430.84, and 431.04 nm in acetonitrile, nhexane, and ethanol, respectively, agree well with the experimental result. 3.2. Infrared and Raman Vibration Spectra. As known, the intramolecular hydrogen bond interaction is directly related to the O−H stretching vibration involved, which can be wellcharacterized by their IR vibrational spectra. In detail, the valid stationary points in PESs can be ensured by the relative vibrational frequency analysis, which provides terse information on the hydrogen-bonding dynamics.51−55 Figure 3 depicts the

Figure 4. Calculated Raman spectra of the (a) enol and (b) keto forms in ethanol based on the B3LYP/TZVP theoretical level.

is clearly displayed, while for the S1 state, it changes to 2677.49 cm−1, revealing an obvious red shift with 581.2 cm−1. This might be a result of the strong intramolecular interaction upon the light excitation. By comparing Figure 4a,b, it could be clearly found that the Raman peaks of the reactant and product of PT process are significantly different, which should be assigned to different vibrational modes. Similarly, for the keto form (the product of ESIPT process), it changed from 3768.29 to 3780.47 cm−1 with a minute blue shift between S0 to S1 state, as shown in Figure 4b. Thus, these results lead to the conclusion that the strengthened intramolecular interaction takes place in the S1 state and the dual PT may be concerted during the ESIPT mechanism. 3.3. Charge Distributions and Bond Energy. The electronic excitation energies, corresponding oscillator strengths ( f), and relevant composition for the first two lowlying single excited states are provided in Table 2. It is shown that the electronic excitation energy of the S0→S1 transition for enol is 2.8764 eV with a large oscillator strength of 0.2866, as well as a Configuration Interaction (CI) of 98.85%. Though the CI of H-1-L for the S0→S2 transition is 95.85%, the corresponding oscillator strength (f) is zero. As a consequence, the S0→S1 transition for enol form plays a dominant role upon light excitation. The simulated frontier molecular orbitals are shown in Figure 5, which can provide the charge distribution information on electronic transition qualitatively. Evidently, there is π property for the highest occupied molecular orbital

Figure 3. Calculated IR spectra of the enol form in ethanol based on the B3LYP/TZVP theoretical level.

simulated IR spectrum of the enol form in ethanol solvent. For S0 state, the O−H stretches with frequency located at 3262.14 cm−1, while in the S1 state, it changes to 2562.91 cm−1. The distinct red shift of 699.23 cm−1 indicates the synchronous formation of intramolecular hydrogen bonds O20−H21···O24 and O22−H23···O25. In addition, Raman spectra for its enol and keto configurations in the S0 and S1 states were also calculated to further illuminate the hydrogen bonding induced proton 4647

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Table 3. Bond Energies (kcal/mol) of the Stable Structuresa on the S0, S1, and T1 States Based on DFT and TDDFT, Respectively

Table 2. Calculated Electronic Excitation Energy (eV), Corresponding Oscillator Strengths, and the Corresponding Compositions of the Low-Lying Singlet Excited States for 1,5-DHAQ enol

a

S0

S1

T1

transition

energy (eV)

f

compositiona

CI (%)

enol

keto

enol

keto

enol

keto

S0→S1 S0→S2

2.8764 3.1008

0.2866 0.0000

H-L H-2-L H-1-L

98.85% 2.54% 95.85%

0.00

48.39

4.15

51.01

37.80

65.23

a

The most stable structure S0 of the enol structure was selected as the zero-point energy.

H stands for HOMO and L stands for LUMO, respectively.

and O22−H23 lengths were optimized by the DFT/TDDFT level combined with B3LYP/TZVP in ethanol, respectively. On the basis of these stable conformations, we constructed PESs in the S0, S1, and T1 states along with O20−H21 ···O24 and O22− H23···O25. As can be clearly found in Figure 6 that the dual proton transfer has taken place, and this process is synergy as well. First, the ground-state intramolecular proton transfer (GSIPT) process can be excluded due to the calculated PT reaction energy barrier is 25.55 kcal/mol from a point to d point (as presented in Figure 6a). For the S1 state, see Figure 6b, the PESs have three substable configurations, in detail d point with O20−H21/O22−H23 lengths 1.02/1.54 Å, e point with O20−H21/O22−H23 lengths 1.54/1.02 Å, and f point with O20−H21/O22−H23 lengths 1.54/ 1.54 Å, respectively. Therefore, there are two step-to-step single ESIPT pathways depicted. One is c→e→f, and the other is c→ d→f with reaction energy barrier 0.38 kcal/mol (c→e/c→d) and 3.16 kcal/mol (e→f/d→f), respectively. Alternatively, the coordinated dual ESIPT pathways (c→f) also has relative lower reaction energy barrier 3.02 kcal/mol. This means that both the single and dual proton transfer processes can occur in the S1 state, due to the small reaction energy barrier is so small to easily overcome. However, further PT process is prevented, because both d/e→g and f→g have larger energy barrier, 11.32 and 9.14 kcal/mol, respectively. Similarly, there are also three stable configurations in the PES of T1 state, that is j point (0.99/1.67 Å), k point (1.67/0.99 Å), and n point (1.67/1.67 Å) for O20−H21/O22−H23 lengths, respectively, as can be seen from Figure 6c. Likewise, the proton transfer pathways, though h→k/j→n or h→n, are both thermodynamically favored, especially for the coordinated dual proton transfer pathways (h→n). However, it is more reasonable to believe that the PT dynamic process should start from S1 state, possibly single and/ or dual proton transfer, following which the proven internal conversion takes place; as the result, the proton then transfers from m point with O20−H21/O22−H23 lengths 1.54/1.54 Å to n point with O20−H21/O22−H23 lengths 1.67/1.67 Å. Thus, the conclusion is that the excited-state single and dual proton transfer processes simultaneously occur in the S1 state. Furthermore, the single proton transfer process takes place in the T1 state.

Figure 5. HOMO and LUMO orbitals for 1,5-DHAQ in ethanol.

(HOMO), and the lowest unoccupied molecular orbital (LUMO) possesses π* character. Obviously, the S0→S1 transition can be assigned to the π→π* transition from H to L with a percentage of 98.85%. Moreover, the O20 and O22 atoms are deprotonated with the electron density decreasing, while it is a protonation for the O24 and O25 atoms as the electron density increasing based on the transition from H to L following light excitation. From the point view of charge distribution will influence the intramolecular interaction, the above calculated results informs that increasing the electron density on O24 and O25 atoms, accompanied by decreasing electron density on O20 and O22 atoms, are propitious to strengthen intramolecular hydrogen bonding and promote the ESIPT process. According to the previous experimental study, the ESIPT process of 1,5-DHAQ in solvents also possibly takes place in the triplet states though the rapid internal conversion and results in forming keto form as well as in the singlet state.35,59 Thus, to further investigate the intramolecular hydrogen bonding associated ESIPT dynamics, the total bond energies of O20−H21···O24 and O22−H23···O25 in the S0, S1, and T1 states are calculated and shown in Table 3. It is found that the enol configuration is most stable in S0 state. Most importantly, the total bond energies for the keto configuration in S1 and T1 states are both larger than that of the enol form. This means that both S1 and T1 states provide reaction pathways for ESIPT process carrying though possibly. 3.4. Excited-State Dual Proton Transfer Mechanism. To evaluate the proton transfer dynamic process of 1,5-DHAQ, the S0, S1, and T1 state equilibrium structures by fixing O20−H21

4. CONCLUSION In summary, we theoretically studied the excited-state proton transfer process of 1,5-DHAQ using DFT/TDDFT with the IEFPCM model. The simulated dual fluorescence spectra agree well with the experimental results. The calculated IR and Raman vibration spectra demonstrate that the two intramolecular hydrogen bonds, that is, O20−H21···O24 and O22− H23···O25, are strengthened in the first excited state. By further combining with the frontier MOs analysis, it can confirm the ESIPT process taking place. Finally, the PESs in the S0, S1, and T1 states along with O20−H21···O24 and O22−H23···O25 were 4648

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Figure 6. Constructed PESs of the (a) S0, (b) S1, and (c) T1 states of 1,5-DHAQ.

constructed. The results inform that single and dual ESIPT processes cooperate with each other in S1 state. Following with internal conversion process, the single proton transfer takes place in the T1 state.

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ACKNOWLEDGMENTS

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REFERENCES

This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 11304135 and 11404080) and the Shenyang Natural Science Foundation of China (F15-199-1-04). Liaoning Provincial Department of Education Project (Grant No. L2015200). The Natural Science Foundation of Liaoning Province (Grant No. 201602345).

AUTHOR INFORMATION

Corresponding Author

*Phone: +86 24 62202306. Fax: +86 24 62202304. E-mail: [email protected]. ORCID

Peng Song: 0000-0003-3093-0068

(1) Beens, H.; Grellmann, K. H.; Gurr, M.; Weller, A. H. Effect of Solvent and Temperature on Proton Transfer Reactions of Excited Molecules. Discuss. Faraday Soc. 1965, 39, 183−193.

Notes

The authors declare no competing financial interest. 4649

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DOI: 10.1021/acs.jpca.7b04051 J. Phys. Chem. A 2017, 121, 4645−4651

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DOI: 10.1021/acs.jpca.7b04051 J. Phys. Chem. A 2017, 121, 4645−4651