The excited-state proton transfer mechanism in water-bridged 4-hydroxybenzoate: spectroscopy and DFT/TDDFT studies

Abstract

The excited-state proton transfer (ESPT) behavior of 4-hydroxybenzoic acid is studied by means of steady-state spectroscopy and theoretical calculations to obtain insight of the excited state dynamics. The large Stokes shift for 4-hydroxybenzoate (4HB) at the pH value of the microenvironment of 6.74 indicates that the ESPT process took place. The proton transfer dynamics of the water-bridged complexes, 4HB·(H₂O)_x, with two different water chain lengths is investigated using density functional theory and time-dependent density functional theory. The constructed potential energy curves among the optimized 4HB·(H₂O)_x (enol form) and 4KB·(H₂O)_x (keto form) geometries at the ground and the first singlet excited state indicate that the ESPT indeed occurs as the barrier is less than 10 kcal mol⁻¹. In addition, the driving force is confirmed by NBO population analysis.

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Introduction

Proton transfer (PT) reactions, caused by site-specific interactions such as hydrogen bonding, are of fundamental and practical interests in chemistry and biology.^1–4 They have attracted more and more research attention and led to thousands of publications on the relevant topics in the past few years.^5–10 PT reactions have been identified and classified into the following three categories.¹¹ In the first category, a five- or six-membered cycle is generated upon the formation of intramolecular hydrogen bonding between proton donor and acceptor such as 2-(2′-hydroxyphenyl)benzoxazole and salicylaldehyde.^9,11,12 In the second category, double PT occurs via a concerted process from one functional group to the other.^12–14 This kind of PT has been widely studied as the PT process can complete independently without other assistance.^2,12 The third category includes molecules such as 6-hydroxyquinoline, in which the two functional groups are far apart from each other and PT reaction is mediated by solvent molecules such as water.¹⁵ This kind of solvent-assisted PT reaction, which is generally called Grotthuss PT¹⁶ and has been widely studied in photochemical and biological processes,^1,3,17 is proposed to occur along a hydrogen-bonded “wire.” For example, Tanner et al. have established that the 3 molar equivalents of ammonia-bridged complex of 7-hydroxyquinoline (7-HQ)·(NH₃)₃ gives rise to the enol–keto tautomerization (7-HQ → 7-KQ) upon photo-excitation to the first optically allowed singlet state (S₁ ← S₀), as shown in Scheme 1.^18,19 The photoactivated PT process of green fluorescent protein (GFP), involving a proton wire, has also been documented.²⁰

Scheme 1 Solvent-assisted ESPT of 7-hydroxyquinoline ·(NH₃)₃.

4-Hydroxybenzoic acid (4-HBA, a in Scheme 2) is the simplest component of the hydroxybenzoic series of the polyphenols.²¹ In addition, based on its widely established antioxidative function in living organisms, especially in the human body, 4-HBA has great biological and bio-medical significance. For instance, it can be used to prevent and treat cell aging and other diseases like cardiovascular and carcinogenic diseases.^22,23 Typically, in neutral environment, its proton can be easily dissociated to yield 4-hydroxybenzoate (4-HB, b in Scheme 2). William et al. have used the ab initio method and the 6-3lG* basis set to calculate the optimized geometry of several conformers.²⁴ In their studies, the intermolecular hydrogen bond between solute and solvent molecules was taken into account, and one simulation with the most stable conformer of 4-HBA and the solute molecules was constructed. Altabef and co-workers then performed theoretical calculations and vibrational spectra measurements on both the monomers and dimers of 4-HBA and 4-HB. In order to get some physical insight into the PT reaction mechanism of this compound and its derivatives, more directive experiments and reliable theoretical calculations are necessary.²⁵

Scheme 2 The chemical structures of 4-hydroxybenzoic acid (a) and its three different dissociative products (b, c, d).

Because of the complicated environment, it is often not easy to ascertain the mechanism of these processes. In this work, we compared the fluorescence spectra of 4-HBA in different micro-environments. Note that the complicated micro-environment also makes our stimulation difficult. The best way to reduce this complexity is to construct a research object with a controlled number of solvent molecules such that a hydrogen-bonded water “wire” containing different numbers of water molecules is attached to the aromatic skeleton of 4-HB. The phenolic hydroxyl H–O and the carbonyl O=C groups, which are located at the two terminals of the phenyl ring, provide a space for constructing the wire. In order to further understand the ESPT mechanism, we adopted density functional theory (DFT) and time-dependent density functional theory (TD-DFT) to clarify fundamental aspects concerning the hydrogen bond formation in the S₀ and S₁ states, respectively. Then, we computed the required number of TDDFT excited state energies as solution of LR-TDDFT by using the M052X functional. In particular, we have focused our attention on the ESPT mechanism in both the ground state and excited state by means of two-dimensional potential energy curves (PECs) and NBO population analysis.

Experimental and theoretical methods

The experimental section

4-HBA was purchased from J&K Chemical and used as received. All solvents for spectroscopic measurements were of spectrograde quality and used without any further purification. The UV-vis absorption spectra were recorded on a HP 8453 spectrophotometer. The fluorescence spectra were recorded using a Horiba Jobin Yvon FluoroMax-4 spectrofluorometer. The deionized water was recorded as a function of the pH value at an ionic strength of 0.1 mol l⁻¹ (NaClO₄ at (25 ± 1) °C). The pH value was adjusted with a high-performance digital pH meter (PHS-3C, Rex Instrument Factory, Shanghai, China).

The theoretical calculation section

All the ground and excited state electronic structures of the monomer 4-HB, the clusters 4-HBA·(H₂O)₃ and 4-HBA·(H₂O)₅ were obtained by full geometry optimizations at DFT/TDDFT level using M052X functional in conjunction with 6-31+G(d, p) basis set.^26,27 In previous studies, it was shown that M052X functional can provide accurate geometric structures for weakly bonding intermolecular systems in both ground and excited states.^28,29 In order to extend the study to a more realistic environment, the solvent effect was taken into account by means of polarized continuum model (PCM).³⁰ Furthermore, the vertical excitation energy and the corresponding oscillator strengths of the low-lying singlet excited states were calculated at TD-M052X/6-31G+(d, p) with the linear response method based on the ground structure. All the NBO charges discussed were obtained from single-point calculations in the model solvent. We used the state-specific method to produce a better description of the vertical excited energy and the geometry of the first excited state.^31,32 Moreover, the calculated fluorescence spectra of different configurations were in good accordance with the experimental results, indicating that the selected method and basis set were appropriate for such matter. In addition, the ground state potential energy curves (GS-PECs) coordinates were calculated by using the minimum energy conformer of fully optimized geometry at fixed O1–H1 distances of the hydroxyphenyl ring. The PECs of the S₁ state were obtained by adding the Franck–Condon transition energies at fixed O1–H1 distances to the corresponding GS-PECs. The transition states (TS) were localized by potential energy scans along the reaction coordinates, followed by full optimization of TS geometries. These optimized reactants and products have no imaginary frequency, while all transition states have only one imaginary frequency. All the ground state (DFT) and excited state (TDDFT) calculations were carried out with Gaussian 09 suite of quantum chemistry programs.³³

Results and discussion

According to previous studies, 4-HBA is easily dissociated at the carboxylic acid group and phenolic hydroxyl group, respectively.^22,23 Fig. 1 shows the fluorescence spectra of 4-HBA as a function of pH value. In this work, in order to address the generation of different species, we measured the fluorescence spectra of 4-HBA in three different pH environments. It is observed that the fluorescence spectral profile is sensitive to the pH value. There is only one peak at around 315 nm when the pH value is 2.80, which can be ascribed to the emission from the undissociated structure a (Scheme 2). This is because 4-HBA cannot dissociate in an acidic environment. With increase in the pH value to 11.20, a new peak at around 330 nm, owing to the fluorescence of divalent anion (d), dominates the whole spectrum at the expense of the former peak. In strongly basic solution, the hydrogen atoms of both phenolic hydroxyl and carboxyl group are dissociated, generating the fluorescent divalent anion. More interestingly, when the solution is almost neutral with a pH of 6.74, three emission peaks can be detected at 294, 329 and 369 nm. In order to identify the origin of these emission peaks, TDDFT calculations were performed for three possible dissociated species of 4-HBA, i.e. carboxylic acid anion (4-HB) (b), divalent anion (d) and phenolic hydroxyl anion (4-KB) (c), and their fluorescence peaks are indicated by the black vertical lines in Fig. 1 (middle figure). The calculated emission peaks of species b, d, and c coincided with the three fluorescence peaks of 4-HBA that were obtained experimentally, indicating that the species b, d, and c should be responsible for these emission signals of 4-HBA at pH 6.74. Moreover, the dramatic Stokes shift of 119 nm indicates the presence of ESPT.

Fig. 1 The fluorescence spectra of 4-HBA in aqueous media at different pH values (λ_ex = 250 nm). In the middle figure (pH = 6.74), the red solid line represents the experimentally obtained fluorescence spectrum, and the three dotted lines are the deconvolution of the spectrum. The three blue vertical lines point to the peak of each deconvolution curve, and the three black vertical lines represent the theoretically calculated fluorescence peaks of structures b, c, and d.

To better understand the effect of solvents on the ESPT process, especially on intermolecular hydrogen bonding, different amounts of aprotic acetonitrile were added to the system while maintaining the pH at 6.74. As summarized in Table 1, at an acetonitrile : water volume ratio of 1 : 1, the PT is totally interdicted and the fluorescence of compound b dominates the spectra. It is clear that the emission intensity at 369 nm decreases with increase in the amount of acetonitrile. Thus, it can be deduced that the ESPT reaction can be hindered if the aqueous microenvironment is disturbed by a large amount of aprotic acetonitrile.

Table 1 Comparison of three fluorescence signal intensities at various acetonitrile : water volume ratios

λ/(nm)	0/100	1/80	1/30	1/10	1/3	1/1
294	0.282	0.144	1.863	1.862	0.187	∼0
329	1	1	1	1	1	1
369	0.388	0.341	0.322	0.321	0.257	∼0

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Theoretical calculations were performed to obtain insight into the ESPT process. It is obvious that the wire connecting the two terminals of 4-HB can be constructed by different numbers of water molecules. The calculated results indicate that many types of clusters between the 4-HB molecule and various numbers of water molecules possibly exist in the aqueous solution of 4-HB. Among them, only two kinds of clusters, 4-HB·(H₂O)₃ and 4-HB·(H₂O)₅, in which the water wires consist of three and five water molecules, are the most important, since all the water molecules on the wires are necessarily involved in the PT process. Fig. 2 and 3 show the structures of 4-HB·(H₂O)₃ and 4-HB·(H₂O)₅ before and after the PT process in the ground state. As shown in Fig. 2, at least three H₂O molecules are required to connect the phenolic group and the acceptor COO⁻. Our calculation result confirms that the 4-HB·(H₂O)₃ cluster is the most stable configuration, whereas the tautomeric 4-KB·(H₂O)₃ is less stable by 5.17 kcal mol⁻¹. It should be noted that the energy difference between the two isomers of 4-HB·(H₂O)₅ seems much smaller in the ground state than those of 4-HB·(H₂O)₃. Hence, from the point of view of the PT reaction, the most important parameters are the distances of the hydrogen bonds between acceptors and donors of the following types: between hydroxyl group and water, between the COO⁻ group and water, and among the water molecules. We label these three different kinds of hydrogen bonds that can identify the modification positions along the water wire as follows: B₁ = O1–H1, B₂ = H1–O2, B₃ = O2–H2, B₄ = H3–O3, B₅ = O3–H5, B₆ = H5–O4, B₇ = O4–H6, B₈ = H6–O5, B₉ = O5–H8, B₁₀ = H8–O6, B₁₁ = O6–H10, and B₁₂ = H10–O7. In addition, the ESPT process leads to conversion of strong single bonds (O–H) into weak intermolecular hydrogen bonds (O⋯H) and the reverse. Table 2 lists the details of bond parameters before and after ESPT. From our simulations, the length of single bonds is about 0.97 Å, and the intermolecular hydrogen bond is around 1.83 Å. It is clear that there exists some extraordinary bond-lengths in both enol and keto forms. We first pay particular attention to the system in which the wire consists of three water molecules. Before the ESPT process, the intermolecular hydrogen bond, B₈, between water and the acceptor is only 1.66 Å, which is much stronger than the other weak bonds. However, after the isomerization, the reformed intermolecular hydrogen bond B₁ seems much stronger (1.53 Å) than the normal bonds (∼1.8 Å). These anomalies may be caused by the strong negative charge on the benzene ring of the 4-HB, which is flat as indicated by the dihedral angle φ₁ of 0.9°. However, the sum (φ₂) of the two dihedral angles between the central phenyl ring and the two terminals of the molecules is larger than 10° in the enol form, confirming that the whole plane of 4-HB is bent by the wire. Upon photoexcitation, the bond length difference between intermolecular and intramolecular hydrogen bonds can be ignored for these two clusters in the S₁ state, as can be seen in Table 2. Despite the fact that all the hydrogen bonds are reformed after the reaction, the topology of the water wire in the S₁ state is different from that in the ground state. It turns out that after excitation, the simultaneous decrease of B₂ length in the S₁ state is favouring the proton exchanges along the water bridge.

Fig. 2 The optimized structures of the enol- and keto-form of 4-HB·(H₂O)₃ in the ground state. On the right side are the corresponding side views.

Fig. 3 The optimized structures of enol and keto forms of 4-HB·(H₂O)₅ in the ground state. On the right side are the corresponding side views.

Table 2 The calculated bond lengths B (Å) and dihedral angles φ (°) for four different configuration clusters in both S₀ and S₁ states

	4-HB·(H2O)3		4-KB·(H2O)3		4-HB·(H2O)5		4-KB·(H2O)5
	S0	S1	S0	S1	S0	S1	S0	S1
B1	0.98	0.99	1.54	1.77	0.99	0.98	1.53	1.73
B2	1.81	1.80	1.02	0.98	1.73	1.71	1.02	0.99
B3	0.98	0.98	1.78	1.81	0.98	0.98	1.74	1.79
B4	1.82	1.83	0.98	0.98	1.75	1.76	0.99	0.98
B5	0.98	0.98	1.82	1.82	0.98	0.98	1.87	1.85
B6	1.83	1.88	0.98	0.98	1.77	1.75	0.97	0.98
B7	1.00	0.98	1.74	1.90	0.98	1.78	1.80	1.79
B8	1.66	1.83	0.99	0.97	1.75	1.78	0.98	0.98
B9					0.99	0.98	1.77	1.79
B10					1.72	1.75	0.98	0.98
B11					1.00	0.99	1.71	1.79
B12					1.59	1.72	0.99	0.98
φ1	0.9	0.8	2.6	3.9	0.8	2.3	2.4	4.7
φ2	10.5	8.1	10.0	11.9	1.8	0.7	7.1	3.6

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In the case of the water wire consisting of five water molecules, before the ESPT process, the length of hydrogen bonds for the water wire are homogeneous and close to 1.75 Å, as shown in Fig. 3 and Table 2. The value of φ₂ is almost zero, indicating that the space for the water wire is abundant when compared with that in 4-HB·(H₂O)₃ and at the same time the aromatic nucleus would not be warped by the intermolecular hydrogen bonds. After being excited to the S₁ state, it maintains a similar structure to that in the S₀ state. However, the length of intermolecular hydrogen bonds for the two styles of water wires vary from 1.52 Å to 1.87 Å for the ESPT products (keto form), which may be induced by the different negative charge density located on different oxygen atoms of the water molecules. More interestingly, in the case of 4-HB·(H₂O)₅, a shorter wire consisting of three water molecules is generated together with the longer one containing five water molecules. The simultaneous observation of both 4-HB·(H₂O)₃ and 4-HB·(H₂O)₅ indicates that the PT process occurs through two channels, i.e. three water wires and five water wires.

Analysing the potential energy curves (PECs) of both the ground and excited states is essentially helpful to decipher the photophysical behaviour of 4HB as shown in Fig. 4 and 5.^3,34 Fig. 4 illustrates the PECs of 4-HB·(H₂O)₃. The energy barrier from 4-HB·(H₂O)₃ to its transition state (TS) is 18.03 kcal mol⁻¹ in the ground state, which is too large to permit proton conduction in the ground state. However, upon excitation from S₀ to S₁, the enol form of 4HB releases one proton into the water wire. Then, three successive protons get involved in the proton transfer process and generation of the 4 KB tautomer, as the energy barrier decreases to 9.5 kcal mol⁻¹. The ESPT process is completed as the length of B₈ decreased from 1.659 to 0.983 Å, and at the same time, length of B₁ increased from 0.980 to 1.541 Å. It is interesting to note that despite the completion of the PT between O1 and H1, no stable intermediate is formed. Our calculation results on PECs and TS structures seem to reveal that although the process starts with PT at the oxygen site of 4-HB, the reaction involves all three water molecules in a concerted way. The ESPT process of 4-HB·(H₂O)₅ is similar to that of 4-HB·(H₂O)₃ as shown in Fig. 5. In addition, the ESPT process seems much easier because the energy barrier along the PECs is smaller (8.29 kcal mol⁻¹). Above all, our calculation of the transfer energy barrier of the two different clusters proves that both two ESPT pathways are feasible and the later is more reasonable than the former one. According to the principle presented in literature by Datta,^35,36 isotope atoms will lead to various transition state energies, which finally results in different PT reaction rates. Therefore, it is possible to distinguish 4-HB·(H₂O)₅ and 4-HB·(H₂O)₃ by solvent kinetic isotope effects.

Fig. 4 The calculated potential energy curves on S₀ and S₁ state for 4-HB·(H₂O)₃. The energies of S₁ state were calculated using the geometries of the corresponding S₀ state.

Fig. 5 The calculated potential energy curves on S₀ and S₁ state for 4-HB·(H₂O)₅. The energies of S₁ state were calculated using the geometries of the corresponding S₀ state.

Since the driving force of the PT process is generally proportional to the charge density on the proton acceptor, we performed the NBO charge population analysis for the S₀ and S₁ states of the clusters 4-HB·(H₂O)₃ and 4-HB·(H₂O)₅.³⁷ The corresponding NBO charge densities are listed in Table 3. Take the 4-HB·(H₂O)₃ cluster as an example, the atomic charges of O1, O2, O3 and O4 are −0.78, −1.07, −1.07 and −1.09, respectively, while those on the two oxygen atoms of the carboxylate ion are −0.84 and −0.80. The weak acidity of the O1–H1 group and the weak basicity of the water molecules do not provide enough diving force for the tautomerization of the compound in the ground state.¹⁹ Consequently, 4-HB·(H₂O)₃ is energetically favoured over the 4 KB·(H₂O)₃ form by ∼5 kcal mol⁻¹. In contrast, in the S₁ state, the negative charge on O1 decreases to −0.69, and that on O2 increases to −1.08; thus, both acidity of O1–H1 group and the basicity of the water molecules become considerably enhanced to induce a fast enol → keto transfer. Hence, in the present system, H₂O wire should capture the proton from the phenolic group first, which gives rise to the conformation of a cationic hydrogen-bonded wire, and then the proton transfers along the water wire to the other side of the molecule. In summary, the deprotonation of the phenolic hydroxyl group is confirmed to be the initiating step in the ESPT process, which then induces a series of almost synchronous PT events.

Table 3 The calculated charge densities (NBO) on O and H atoms of cluster 4-HB·(H₂O)₃ and 4-HB·(H₂O)₅

	4-HB·(H2O)3		4-HB·(H2O)5
	S0	S1	S0	S1
H1	0.55	0.56	0.51	0.55
O1	−0.78	−0.69	−0.77	−0.67
O2	−1.07	−1.08	−1.07	−1.08
O3	−1.07	−1.08	−1.08	−1.08
O4	−1.09	−1.08	−1.07	−1.07
O5	−0.84	−0.87	−1.08	−1.08
O6	−0.80	−0.84	−1.09	−1.08
O7			−0.85	−0.88
O8			−0.80	−0.83

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Conclusions

The ESPT mechanism has been investigated for water-bridged 4-HB by spectroscopy and theoretical calculations. A large Stokes shift of 119 nm was observed in the fluorescence spectra of 4-HB in water solution, indicating the existence of ESPT. The obvious variation of the fluorescence spectral profile of 4-HB with changing water ratio in water–acetonitrile binary solvent systems confirmed the important contribution of water molecules to the luminescent behaviour. Theoretical calculations were performed to obtain insight into the spectroscopy results. It was observed that 4-HB can form two energetically stable clusters with water molecules in the ground state, 4-HB·(H₂O)₃ and 4-HB·(H₂O)₅, which is driven by the hydrogen bonding. The energy barrier in PECs of 4-HB·(H₂O)₃ and 4-HB·(H₂O)₅ are 9.49 kcal mol⁻¹ and 8.29 kcal mol⁻¹, respectively, suggesting that the ESPT process indeed occurs. In addition, the observed PT pathway involves the water bridges, which contain three and five water molecules, in which all protons are transferred simultaneously. This study provides a detailed description of the PT reaction mechanism and a practical strategy to reduce such a complex system into a simple research object with controlled numbers of solvent molecules for modelling.

Acknowledgements

We thank the National Natural Science Foundation of China (21274016 and 21374013), the Program for Changjiang Scholars and Innovative Research Team in University (IRT-13R06) and Program for DUT Innovative Research Team (DUT2013TB07), and the Fundamental Research Funds for the Central Universities (DUT13LK06 and DUT12ZD211) for financial support.

Notes and references

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