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Mechanism of Excited-State Intramolecular Proton Transfer for 1,2Dihydroxyanthraquinone: Effect of Water on the ESIPT Yajing Peng,*,†,‡ Yuqing Ye,† Xianming Xiu,† and Shuang Sun† †

Department of Physics, Bohai University, JinZhou 121013, China State Key Laboratory of Molecular Reaction Dynamics Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China

‡

S Supporting Information *

ABSTRACT: Mechanisms of excited-state intramolecular proton transfer (ESIPT) of 1,2-dihydroxyanthraquinone (ALR) in ethanol solvent and binary solvent of water and ethanol are investigated using the density functional theory and time-dependent density functional theory. The intramolecular hydrogen bond is found to be reinforced in the excited state based on the bond lengths, bond angles, and infrared vibrational spectra of relevant group. The reinforcement of intramolecular hydrogen bond is attributed to the charge transfer in the excited state, which leads the ESIPT to form a keto isomer. The absorption and fluorescence spectra of ALR in binary solvent with different water percentage are obtained and demonstrate the inhibition effect of water on the ESIPT process, which are consistent with the experimentally observation. Furthermore, more water molecules are considered near the carbonyl group and hydroxyl group related to the intramolecular proton transfer to form intermolecular hydrated hydrogen bond with ALR for clarifying the block mechanism of water on ESIPT. The potential energy curves, frontier molecular orbitals, and NBO analysis are calculated for the several complexes in the ground and excited states. The results show that the interrupt role of water on the ESIPT originated from the forming of hydrated hydrogen bond between the carbonyl oxygen atom and the water molecule, which weakens the intramolecular hydrogen bond associated with proton transfer, increases the energy barrier of ESIPT, and thus precludes the transition of ALR-E to ALR-K in the excited state. In addition, the weakening of intramolecular hydrogen bonds is increased as the water molecule number increases. So the inhibitory effect is enhanced by the water quantity, which reasonably explains the experimental attenuating of keto emission spectra as the water percentage in binary solvent increases.

1. INTRODUCTION The excited-state intramolecular proton transfers (ESIPT) have been of great interest in physics, chemistry, and biology due to their wide application as laser dye, fluorescence chemosensors, and molecular switches.1−7 Understanding molecular dynamics mechanism related to the intramolecular proton-transfer reaction might provide important microinformation to cognize the chemical reaction dynamics of many chemical and biological systems.1,2 The hydrogen bond, one of the important weak interactions, has been indicated to play an important role in excited-state intramolecular proton transfer and thus has attracted more attention because its excited-state reinforcement can promote intramolecular proton transfer,8−13 which was proved first by Zhao and Han et al.14−18 1,2-Dihydroxyanthraquinone (Alizarin-ALR), containing three coplanar six-membered carbon rings, the central quinone ring and two peripheral ones with two hydroxyls in possible positions,19 is a typical molecule with intramolecular hydrogen bond and has numerous applications as a dye, anticancer agent, and chemical agent for data recording and storage material because of their distinctive activity.14 At present, experimental © 2017 American Chemical Society

spectroscopic studies have shown that ALR mainly exists in enol form, as shown in Scheme 1. An intramolecular proton Scheme 1. Enol Structure of ALR

transfer in the excited state arises easily in the enol-ALR to form a keto-isomer.19,20 Le Person et al.21 show that the intramolecular proton transfer of ALR is also present in the ground state by electronic spectroscopy and quantum-chemical calculations. Furthermore, some solvent polarity and pH effects on the transitions between ALR tautomer have been analyzed in spectroscopic experiments. Recently, Amat et al. 20 Received: April 25, 2017 Revised: July 8, 2017 Published: July 10, 2017 5625

DOI: 10.1021/acs.jpca.7b03877 J. Phys. Chem. A 2017, 121, 5625−5634

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The Journal of Physical Chemistry A

Scheme 2. Optimized Molecular Structures of ALR-E and Its Corresponding Keto-Isomer ALR-K Based on TDDFT(DFT)/ B3LYP/TZVP Theoretical Level

Table 1. Calculated Primary Bond Lengths (Å) and Bond Angles (deg) Involved in Intramolecular Hydrogen Bonds of ALR-E and ALR-K in the Ground State (S0) and First Excited State (S1)a ALR-E

a

ALR-K

electronic state

S0

S1

S0

S1

O3−H7/O3···H7 O4···H7/O4−H7 δ(O3−H7···O4)/δ(O3···H7−O4) total energy

0.993 1.676 146.03° −839.342564

1.073 1.428 155.32° −839.254712

1.490 1.039 153.28° −839.334647

1.632 1.008 151.60° −839.259032

Corresponding total energy (Hartree) of every stationary point also is listed.

2. COMPUTATIONAL DETAILS All calculations were performed using DFT 22−26 and TDDFT27−29 with the Gaussian 09 program package.30 Becke’s three-parameter hybrid exchange function with the Lee−Yang− Parr gradient-corrected correlation functional (B3LYP) and the triple-ζ valence quality with one set of polarization functions (TZVP) basis set have been selected. The method and basis set have been confirmed to be appropriate to study the electronic excited-state hydrogen bond.31,32 There were no constraints on any of the atoms, bonds, angles, or dihedral angles during the geometric optimizations. To compare with the experimental results, the two solvent effects of single ethanol and binary mixtures of ethanol and water have been considered in the calculations. Strictly speaking, the explicit solvent model or Reference Interaction Site Model (RISM)33,34 are more suitable to study the system with intermolecular hydrogen bond, but the difficult points are the computational cost and complexity. Although the implicit solvent models have no specific intermolecular interactions, they are preferred in the study of excitation process and dynamics on excited-state potential energy surface because the electric excitation process is instantaneous and only the electron density of the solvent is able to quickly respond to the fast electron density change in the solute molecules.35 So the popular polarizable continuum model (PCM) using the integral equation formalism variant (IEFPCM)36−38 is chosen here to describe the solvent effects of hydrogen-bond systems because the PCM has been confirmed to be a useful approach to incorporate efficiently solvent effects and describe well the electric spectra of molecules in solution.39,40 The mixed solvent effects of ethanol and water are introduced in the PCM model by custom solvent function of Gaussian software. The average static and dynamic dielectric constants of the mixed solvents are estimated according to the different volume ratios of binary (GD formula).41,42 The ground- and excited-state potential energy curves have been scanned by constrained optimizations, keeping the O−H distance increased in fixed steps based on the optimized

theoretically investigated the emission properties involving an excited-state internal proton transfers and propose that the emission energy of ALR is sensitive to solvent polarity. Sasirekha et al.19 experimentally show that dual emission of ALR may be detected in nonpolar CCl4 solvent but not in methanol and ethanol solvents. Lee et al.1 investigated the ESIPT of ALR in ethanol solution and binary solvent of ethanol and water by femtosecond transient absorption spectroscopy and showed that the excited-state dynamic process in binary solvent is more complicated than that in ethanol solution. However, the ESIPT dynamic mechanisms of ALR in ethanol/ water solvent have not been clarified in detail. The changes in the dynamics caused by the water participation have not been revealed systematically. Therefore, it is necessary to understand the ESIPT mechanisms for ALR in single ethanol and ethanol/ water binary solutions to reveal the influence mechanism of water on the ESIPT. In the present work, the ESIPT process of the ALR isolated in the polar solvent of ethanol and binary mixtures of ethanol and water is investigated theoretically using density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods in view of the experimental measurement by Lee et al.1 to clarify the ESIPT mechanism of ALR in detail and understand the effect of water on the ESIPT of ALR. The molecular configurations of enol-ALR (ALR-E) and keto-ALR (ALR-K) in the ground and first excited states are optimized, and the infrared (IR) vibrational spectra are calculated for insight into the excited-state hydrogen-bond dynamics. Further vertical excitation energies, frontier molecular orbitals (MOs), natural bond orbital (NBO), and potential energy curves are analyzed to reveal the mechanism of ESIPT of ALR. The effects of more water molecules on the ESIPT are also examined to provide a reasonable explanation for the phenomena of absorption and emission spectra in corresponding solvent obtained by experiments. 5626


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The Journal of Physical Chemistry A ground- and excited-state geometries, respectively. UV−vis spectra in ethanol and binary solvent are calculated using TDDFT method at the B3LYP/TZVP level on the basis of the optimized ground- and excited-state structures.43,44 The selfconsistent field (SCF) convergence thresholds of the energy are set to be 10−8 Hartree. All of the local minima have been confirmed by no imaginary frequencies mode in the vibrational analysis. A full NBO analysis is also performed using the same level of theory to obtain the charge distributions and bond orders in the isomers.

3. RESULTS AND DISCUSSION 3.1. Characteristics of Hydrogen Bond in Ground and Excited States. The optimized molecular structures of ALR-E and ALR-K in ethanol solution are shown in Scheme 2. The optimized atomic coordinates are shown in Figure S1 in the Supporting Information. The important structure parameters and energy for ALR-E and ALR-K in ground and excited states are listed in Table 1. It can be noted that the bond length O3− H7 of ALR-E is increased from 0.993 Å in the ground state (S0) to 1.073 Å in the first excited state (S1), while the length of hydrogen bond H7···O4 is decreased from 1.676 Å (S0) to 1.428 Å (S 1 ). The bond angle of δ(O 3 −H 7 ···O 4 ) also is correspondingly increased from 146.03° in the S0 state to 155.32° in the S1 state. These changes show that the hydrogen bond in ALR-E is reinforced in the first excited state. The hydrogen-bond reinforcement will greatly promote the proton transfer to realize the transition from the ALR-E to the isomer ALR-K in the excited state. As for the ALR-K, the optimized structure parameters show that the length of hydrogen bond H7···O3 is decreased from 1.632 Å (S1) to 1.490 Å (S0) and the O4−H7 bond length is increased from 1.008 Å (S1) to 1.039 Å (S0). Also, the bond angle δ(O3···H7−O4) is enlarged from 151.60° (S1) to 153.28° (S0). These data indicate that the intramolecular hydrogen bond is stronger in the S0 state for the ALR-K, which greatly increases the possibility of reverse ground-state proton transfer to form the isomer ALR-E. Furthermore, from the total energy of the four configurations given in Table 1, the energy of ALR-E is less than that of ALRK by ∼0.22 eV in the ground state, and it is higher than that of ALR-K by ∼0.12 eV in the first excited state. The single-point energy calculations for ALR-E and ALR-K in the S0 and S1 states using CAM-B3LYP method have been done to confirm the accuracy of B3LYP method. The calculated single energies are listed in the Supporting Information as Table S1. Both of the two methods show that the ALR-K is more stable in the excited state and the ALR-E is more stable in the ground state due to the role of intramolecular hydrogen bond. In addition, the electronic excited-state hydrogen-bond dynamics can be detected by spectral shift with special vibrational modes. The excited-state hydrogen-bond reinforcement in ALR-E can be indicated by the peak shift of infrared (IR) vibrational spectra.45−49 The ground- and excited-state IR vibrational spectra of ALR-E in ethanol solution at the spectral region of O3−H7 stretching band are shown in Figure 1. One can note that the ground-state vibrational mode of the O3−H7 group is located at 3231 cm−1, whereas the excited-state mode is located at 2100 cm−1. The stretching band has a red shift of 1131 cm−1 in the S1 state relative to the S0 state, which similarly indicates that the intramolecular hydrogen bond H7···O4 of ALR-E is reinforced in the S1 state. Therefore, the ESIPT mechanism of ALR-E should be ascribed to the excited-state reinforcement of hydrogen bond.

Figure 1. Calculated ground and first excited state IR spectra of ALR-E in ethanol solution at the spectral region of O3−H7 stretching band.

3.2. Electronic Spectra. 3.2.1. Single Ethanol Solvent. The calculated absorption and fluorescence spectra of ALR in ethanol solvent based on the TDDFT/B3LYP/TZVP calculated level are displayed in Figure 2. The optimized ground-

Figure 2. Calculated absorption and fluorescence spectra of ALR in ethanol solvent at the TDDFT/B3LYP/TZVP calculation level. The solid lines represent the calculated values, while the dotted lines represent the experimental values.

and excited-state geometries are used as the initial configurations to acquire the S0−S1 vertical excitation energy and electronic spectra. It can be noted that an absorption peak for ALR-E is located at 436 nm (2.85 eV), which is consistent with the experimental value 435 nm (2.86 eV).1,50 The calculated fluorescence emission peaks based on the optimized excitedstate molecular structures of ALR-E and ALR-K are, respectively, located at 574 nm (2.16 eV) and 673 nm (1.85 eV), which are close to the experimental data of 530 nm (2.34 eV) and 630 nm (1.97 eV).1 Obviously, the emission peak of ALR-E has a red shift of 0.69 eV (0.52 eV in experiment) relative to the absorption peak, which can be ascribed to the normal Stokes shift, while a large red shift of 1.00 eV (0.89 eV in experiment) appears for the ALR-K emission peak relative to the absorption peak. The double-emission peaks indicate that the two enol and keto isomers exist in the S1 state for the ALR and the ESIPT occurs to generate its isomer. The electronic 5627


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examined. The frontier MOs of ALR-E in ethanol solvent have been calculated and shown in Figure 5. Herein only the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are given because that the first excited state mainly involves the two orbitals,51 as shown in Table 2. It can be seen that a strong S0−S1 transition for ALR-E is predicted at ∼434 nm with large oscillator strength of 0.138 and the large composition of the S0−S1 transition of 69.67%. Although the composition of the S0−S2 transition from HOMO−2 to LUMO is 69.18%, its oscillator strength is only 0.0001 (dark state). So only the S0− S1transition is considered here. It can be found that the S1 state has a distinct ππ* feature due to the π character of HOMO and π* character of LUMO. It is worth noting that more electronic density distribution is located around the oxygen atom of hydroxyl group and less located around oxygen atom of carbonyl moiety in the HOMO orbital than that in the LUMO orbital. These conclusions are also obtained by the CAM-B3LYP method, as shown in Figure S4 in the Supporting Information. The NBO analysis is done to obtain the charge quantity assigned to the atoms related to intramolecular hydrogen bond, as shown in Table 3. Obviously, the negative charge assigned to the hydroxyl oxygen atom is decreased from −0.688 C in the S0 state to −0.649 C in the S1 state and the charge assigned to the O atom of H7···O4 moiety is increased from −0.600 C to −0.654 C. That means the charges are transferred from hydroxyl (O3−H7) to the hydrogen bond (H7···O4) group after the transition from HOMO to LUMO. So the intramolecular charge transfers arise for the ALR-E in the S1 state, which strengthen the interaction of intramolecular hydrogen bond H7···O4. The interaction is mainly responsible for the proton transfer. Therefore, the ESIPT mechanism can be ascribed to the excited-state intramolecular charge transfers. 3.3.2. Potential Energy Curves. To better reveal the ESIPT mechanism, the proton-transfer potential energy curves of ALR-E in ethanol solution in the S0 and S1 states are scanned using the TDDFT(DFT)/B3LYP/TVZP methods, which are reliable to obtain the shape of proton-transfer potential energy curves.52−55 The scans are based on the constrained optimizations based on the optimized geometries in their corresponding electronic states and constructed by varying the O3−H7 bond length in steps of fixed value, as shown in Figure 6. As can be seen, in the first excited state, the lowest energy point appears near 1.60 Å. It represents the stable configuration of ALR-K. The ALR-E in S1 state can be easily transferred to the ALR-K by crossing an almost negligible barrier (0.02 kcal/ mol). That means the proton transfer is likely to proceed in the S1 state. The proton transfer is relatively difficult for ALR-E in S0 state to convert to ALR-K due to the relatively high barrier of 5.85 kcal/mol and the endothermal process. However, the ground-state reverse proton transfer (RPT), that is the transition of ALR-K to the ALR-E, is an almost barrierless process. So the ALR-K exists less in the ground state due to its high energy or instability. Therefore, the enol-keto isomerization of ALR should be attributed to the excited-state proton transfer and ground-state reverse proton transfer. The enol-keto tautomerism of ALR can be described well by Figure 7. Under photoexcitation, ALR-E is first excited to the S1 Franck−Condon (FC) state, where the ALR is unstable and then relaxes to the minimum S1 state (ALR-E (S1)). After that, a part of ALR-E (S1) transitions back to ground state with a ∼574 nm fluorescence emission, and another part of ALR-E

spectra have also been calculated using the CAM-B3LYP method with TZVP basis set, and the calculation results are shown in Figure S2 in the Supporting Information. The comparisons of results from CAM-B3LYP method with that from B3LYP are provided in the Supporting Information. 3.2.2. Binary Solvent of Ethanol and Water. To examine the effect of binary solvent of ethanol and water on the absorption spectra and emission spectra of ALR, three types of water content, 10, 30, and 50%, are considered, respectively, in the mixed solvent based on the previous experiment.1 The calculated results are shown in Figures 3 and 4. It should be

Figure 3. Calculated absorption spectra of ALR in binary solvent of ethanol and water with different water content (10, 30, and 50%). The arrow direction represents a gradual increase in water content.

Figure 4. Calculated dual emission spectra of ALR in binary solvent with different water content (10, 30, and 50%). The arrow direction represents a gradual increase in water content.

noted that the absorption and emission intensities of ALR-E increase as the increased water content, whereas the emission intensity of ALR-K decreases. These are consistent with the results of experiment.1 It indicates that the transition from ALR-E in the first excited state to the keto-isomer is precluded when water exists. 3.3. Mechanism of ESIPT of ALR. 3.3.1. Frontier Molecular Orbitals. To explore the mechanism of hydrogen bond dynamics or ESIPT dynamics further, the properties of charge distributions and charge transfers in the excited state are 5628


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Figure 5. Frontier molecular orbitals of ALR-E in ethanol solvent.

Table 2. Electronic Excitation Energy (nm), corresponding Oscillator Strengths and the Corresponding Orbital Transition Contributions for ALR-E in Ethanol Solventa transitions

electronic excitation energy (nm)

corresponding oscillator strengths (OS)

orbital contributions (OT)

S0−S1 S0−S2

433.75 404.65

0.1377 0.0001

H → L 69.67% H-2 → L 69.18%

a

H represents HOMO and L refers to LUMO.

Table 3. Natural Population Analysis (NPA) Charge Distributions on Hydroxyl Oxygen and Hydrogen Bond Oxygen Atoms of ALR-E in Ethanol Solvent in the S0 and S1 States based on DFT and TDDFT Calculations NPA charges (C) states

O of O3−H7

O of H7···O4

S0 S1

−0.689 −0.650

−0.600 −0.654

Figure 7. Enol-keto tautomerism mechanisms for ALR.

increased water content. It means that the addition of water precludes the excited-state transition of ALR-E to ALR-K. To clarify the inhibition effect of water on the ESIPT process, the formation of hydrated hydrogen bond between water and ALRE is considered because water is a system that easily forms hydrogen bonds. The carbonyl group (C7O4) and hydroxyl group (O3−H7 and O2−H8) are important sites to form intermolecular hydrogen bond with the water molecule,56 which might affect greatly the intramolecular proton transfer. The optimized configurations containing a hydrated hydrogen bond formed by carbonyl oxygen O4 with water molecule (ALR-E-H2O) and by hydroxyl hydrogen H8 with water molecule (ALR-E-H2O-(a)) are shown in Scheme 3. The corresponding atomic coordinates are shown in Figure S1 in the Supporting Information. The energy of the ALR-E-H2O (−915.796070 hartree) is slight lower than that of the ALR-EH2O-(a) (−915.796028 hartree) because that the formation of complex ALR-E-H 2 O-(a) needs to break the original interaction of O3 and H8. Therefore, compared with the hydroxyl oxygen (O3), intermolecular hydrated hydrogen bonds are more easily formed at carbonyl oxygen (O4) site. Figure 8a,b give the scanned potential energy curves of proton-transfer reaction occurring, respectively, in the ALR-EH2O and ALR-E-H2O-(a) based on the optimized ground- and excited-state geometries. The excited-state atomic coordinates for them also are given in Figure S1 in the Supporting Information. It can be noted that the potential barriers of intramolecular proton transfer for the ALR-E-H2O are increased from little barrier (0.02 kcal/mol) to 0.58 kcal/mol in the S1 state and from 5.85 to 6.77 kcal/mol in the S0 state,

Figure 6. Potential energy curves of ALR-E in ethanol along with O3− H7 bond length in the S0 and S1 states.

(S1) increases the ESIPT due to the reinforcement of intramolecular hydrogen bond to form the ALR-K (S1). Then, the first excited-state ALR-K regresses to the ground state via ∼673 nm radiative transition. Finally, the ground-state ALR-K quickly returns to the enol isomer by the ground-state reverse proton transfer with little barrier. 3.4. Effects of Water on the ESIPT. As shown by our calculations and previous experiment,1 the emission intensity from ALR-K is reduced in the ethanol/water solvent as 5629


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Scheme 3. Optimized Structures of ALR with a Hydrated Hydrogen Bond Formed by Carbonyl Oxygen and Water Molecule (ALR-E-H2O) as Well as by Hydroxyl Hydrogen and Water Molecule (ALR-E-H2O-(a)) at the DFT/B3LYP/TZVP Level

Figure S2 in the Supporting Information. The transition barrier of ESIPT obtained by CIS and TDDFT methods is listed in Table 4 for comparison. As can be seen, the transition barrier of Table 4. Transition Barriers of ESIPT for ALR-E, ALR-EH2O, and ALR-E-H2O-(a) Calculated by Different Methods at the B3LYP/TZVP Level configurations transition barriers of ESIPT

ALR-E TDDFT CIS

0.02 0.09

ALR-E-H2O ALR-E-H2O-(a) 0.58 1.90

0.00 0.00

ESIPT given by CIS method for ALR-E-H2O is increased to 1.90 kcal/mol, which is still higher than that of ALR-E (0.09 kcal/mol). The absence of a barrier for the ESIPT process of ALR-E-H2O-(a) is also supported by CIS studies. Therefore, although the CIS method gives slightly higher energy barrier than the TDDFT method, the inhibitory effect of water on ESIPT for ALR-E-H2O due to the formation of hydrated hydrogen bond between carbonyl oxygen and water molecule can still be demonstrated. Therefore, the intermolecular hydrogen bond between carbonyl oxygen and water molecule should be formed when water exists, which increases the ESIPT barrier height and precludes the transition of ALR-E to ALR-K in the S1 state. To further confirm our conclusion, two and three water molecules are considered, respectively, near the carbonyl group (C7O4) and hydroxyl groups (O3−H7 and O2−H8). For the two H2O case, water molecules are placed, respectively, near the C7O4 and O3−H7, while for the three water molecules, they are placed near C7O4, O3−H7, and O2−H8, respectively. The optimized ground- and excited-state geometries for the above two cases (ALR-E-2H2O and ALR-E-3H2O) are shown in Scheme 4. The corresponding atomic coordinates are listed in Figure S1 in the Supporting Information. During the optimization of ALR-E-2H2O, we find that it is difficult for the water molecule to stay near hydroxyl O3−H7, whether through hydrogen bond O5···H7 or H9/H10···O3, due to the strong intramolecular hydrogen bond (O4···H7), especially in the S1 state. Eventually, the water molecule will link to another water molecule via intermolecular hydrogen bond to form a stable chain structure, which is linked on the carbonyl oxygen O4. Therefore, intermolecular proton transfer is almost impossible via intermolecular hydrated hydrogen bond to complete the transition from ALR-E to ALR-K. For the ALR-E-3H2O, the chain structure of three water molecules is formed between carbonyl oxygen O 4 and hydroxyl hydrogen H 8 via

Figure 8. Potential energy curves of proton transfer for ALR-E-H2O and ALR-E-H2O-(a) in binary solvent along with O3−H7 bond length in the S0 and S1 states.

while the corresponding barrier heights for the ALR-E-H2O-(a) are decreased from 0.02 kcal/mol to no barrier in the S1 state and from 5.85 to 5.21 kcal/mol in the S0 state because of the hydrogen bond (O3···H9) action. The single-excitation CI (CIS) method is used to scan the excited-state potential energy curves for ALR-E, ALR-E-H2O and ALR-E-H2O-(a) compounds to demonstrate the reliability of our TDDFT results.57,58 The scanned potential energy curves are shown in 5630


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The Journal of Physical Chemistry A Scheme 4. Optimized Structures of AlR-E-2H2O and ALR-E3H2O in the S0 and S1 States

Figure 10. Ground- and excited-state potential energy curves of proton transfer for the ALR-E linked with three H2O molecules calculated at TDDFT(DFT)/B3LYP/TZVP level.

Table 5. Transition Barriers of Intramolecular Proton Transfer for Several Configurations in the S0 and S1 States Calculated at TDDFT(DFT)/B3LYP/TZVP Level barrier height of intramolecular proton transfer

intermolecular hydrogen bonds. As can be seen, in any case, the carbonyl oxygen O4 easily forms an intermolecular hydrogen bond with the water molecule. In addition, the intramolecular hydrogen bond O4···H7 is stable and is enhanced in the S1 state because the length of the hydrogen bond O4···H7 is decreased from 1.692 Å (S0) to 1.525 Å (S1) for ALR-E-2H2O and from 1.664 Å (S0) to 1.554 Å (S1) for ALR-E-3H2O. The scanned potential energy curves are shown in Figures 9 and 10 for ALR-E-2H2O and ALR-E-3H2O, respectively. The obtained ground- and excited-state reaction barriers of intramolecular proton transfer for the two complexes are listed in the Table 5 together with those of the compounds ALR-E and ALR-E-H2O in the S0 and S1 states for comparison. As can be seen, in a water environment, the ground- and excited-state

electronic state

ALR-E

ALR-E-H2O (a)

ALR-E-2H2O

ALR-E-3H2O

S1 S0

0.02 5.85

0.58 6.77

0.79 6.98

1.60 7.58

barrier heights of intramolecular proton transfer are all increased compared with that of ALR-E. This conformably shows that the intermolecular hydrogen bond formed by carbonyl oxygen with water molecules impedes the ESIPT. In addition, interestingly, we found that the ESIPT barrier height increases with the increase in number of water molecules. That means the transition from ALR-E to ALR-K will become difficult with increase in water. This successfully explains the attenuation of keto emission spectra in experiments with the increase in water content in binary solvent.1 To examine the effect of hydrated hydrogen bond linked to the carbonyl oxygen on the potential barrier increase of ESIPT, the frontier MOs and NBO analysis are conducted for the ALRE-H2O in binary solvent of water and ethanol. Figure 11 shows the corresponding charge distributions in HOMO and LUMO orbitals. It can be seen that the charges on hydroxyl (O3−H7) oxygen atom O3 in the ground state are still transferred to the

Figure 9. Ground- and excited-state potential energy curves of proton transfer for the ALR-E linked with two H2O molecules via the hydrated hydrogen bond on the carbonyl oxygen calculated at TDDFT(DFT)/B3LYP/TZVP level.

Figure 11. Frontier molecular orbitals of ALR-E with hydrated hydrogen bond formed between carbonyl oxygen and water molecule in binary solvent of ethanol and water calculated at TDDFT/B3LYP/ TZVP level. 5631


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the excited state. What’s more, the intramolecular hydrogen bond attenuates with the increase in water molecules, which can explain the experimental reduction of emission intensity as the water proportion in binary solvent increases.

carbonyl oxygen atom O4 in the excited state. It indicates that the intramolecular charge transfer is still the main mechanism of ESIPT for ALR-E in mixed solvent. The bond orders of hydroxyl O3−H7 and the intramolecular hydrogen bond H7···O4 in the ALR-E, ALR-E-H2O, ALR-E2H2O, and ALR-E-3H2O in the S0 and S1 states are shown in Table 6. It can be noted that the bond orders of hydrogen bond

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.7b03877. Optimized atomic coordinates of some primary compounds in ground and excited states in ethanol and binary solvents at the TDDFT(DFT)/B3LYP/TZVP calculation level (Figure S1). Computed potential energy surface of intramolecular proton transfer for the ALR-E, ALR-E-H2O, and ALR-E-H2O-(a) in ethanol and binary solvents using CIS method at the B3LYP/TZVP basis set level (Figure S2). Comparison of results of CAM-B3LYP single point energy calculations with that of B3LYP optimization calculations for ALR-E and ALR-K in the ground and excited states (Table S1). The frontier MOs calculated at TDDFT/CAM-B3LYP/TZVP level (Figure S3). The calculated absorption and emission spectra of ALR in ethanol solvent at the TDDFT/CAM-B3LYP/ TZVP calculation level (Figure S4). (PDF)

Table 6. NBO Bond Orders of O3−H7 and H7···O4 groups of ALR-E, ALR-E-H2O, ALR-E-2H2O, and ALR-E-3H2O Compounds in the S0 and S1 States Based on TDDFT(DFT)/B3LYP/TZVP Calculation Level bond orders configurations ALR-E ALR-E-H2O ALR-E-2H2O ALR-E-3H2O

electronic states

O3−H7

H7···O4

S0 S1 S0 S1 S0 S1 S0 S1

0.6415 0.5450 0.6477 0.5882 0.6496 0.5954 0.6523 0.6193

0.0832 0.1957 0.0744 0.1417 0.0720 0.1326 0.0800 0.1194

ASSOCIATED CONTENT

S Supporting Information *

H7···O4 are decreased and the bond orders of hydroxyl O3−H7 are increased in the S1 state with the increase of water molecule numbers. That means the O3−H7 is strengthened, while the H7···O4 is weakened by the hydrated hydrogen bond formed on carbonyl oxygen atom. That is the essential reason for the inhibition of water on ESIPT of ALR. Moreover, the weakening of intramolecular hydrogen bond is increased as the number of water molecules increases. This is also why a regression of keto emission spectra is observed with the increase in water percentage in binary solvent in experiment.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Yajing Peng: 0000-0002-3760-0179 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported partly by the Natural fund of Liaoning Province (2015020248) and the National Natural Science Foundation of China (Grant No. 21203012 and 11604333). We also thank the reviewers for their comments and suggestions, which helped to improve the manuscript significantly.

4. CONCLUSIONS The ESIPT mechanisms of alizarin in ethanol solvent and binary solvent of ethanol and water with different proportion have been investigated based on DFT and TDDFT methods. The ground- and excited-state geometric structures, IR vibrational spectra, electronic spectra, frontier MOs, NBO, and the potential energy curves are examined to explore the ESIPT mechanism. The results show that the hydrogen bond is strengthened in the first excited state due to the decrease in hydrogen bond length and the red shift of IR vibrational spectra of group involved in the formation of hydrogen bond, which would facilitate the excited-state intramolecular proton transfer. The calculations of frontier MOs and NBO analysis reveal that the intramolecular hydrogen bond reinforcement is ascribed to the characteristics of intramolecular charge transfer in the excited state. Therefore, the excited-state intramolecular charge transfer is the underlying mechanism of ESIPT, and the ESIPT is the primary isomerization pathway from ALR-E to ALR-K. The effects of water on the ESIPT mechanism of ALR are examined in detail by considering more water molecules to be linked to the ALR-E molecule via intermolecular hydrated hydrogen bonds. The molecular structure, potential energy curves, frontier MOs and NBO analysis are calculated for the several complexes. It can be concluded that the formation of a hydrated hydrogen bond between the carbonyl oxygen and a water molecule weakens the intramolecular hydrogen bond associated with proton transfer, increases the barrier height of ESIPT, and thus hinders the transition of ALR-E to ALR-K in

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