Introduction

Owing to the importance of hydrogen bond playing in nature, it has been researched and reported numerous publications on the relevant topics in the past few years. Particularly, Han and his partners put a new theory to explain the dynamic of excited-state hydrogen bonds1,2,3,4,5,6,7. The novel mechanism can well clarify some chemosensors via the interaction of inter- and intra- molecular hydrogen bonds, such as intramolecular charge transfer (ICT), excited state proton transfer (ESPT), photoinduced electron transfer (PET), fluorescence quenching, photoinduced electron transfer (PET), and so on8,9,10,11,12,13,14,15,16,17,18,19,20. The excited state intramolecular proton transfer (ESIPT) reaction belongs to the essential unimolecular processes in nature, as one of the fastest and quit complex processes in nature and now has been studied by several modern ultrafast technology. ESIPT reaction has been a popular research projects of photochemistry and photophysics up to now since 1956, firstly investigated by Weller et al. with the particular experiment of methylsalicylate21,22. The transferred tautomerization leads to strong and fast charge distribution restructuring, these molecules which with the characteristic are very fascinating towards the layout and application of fluorescence chemosensors, laser dyes and LEDs, ultraviolet ray (UV) filters and photostabilizers23,24,25,26. More and more spectroscopic techniques have been used to investigate ESIPT reaction in recent years. Even though the enormous volume of endeavor has been dedicated27,28,29,30,31,32,33,34,35,36,37,38,39,40, the investigations of ESIPT process still remains immense challenges primarily due to the intrinsic complicated processes of physical and chemical property, such as quantum nature, cleavage and formation of hydrogen bond, the change of the excited-state hydrogen bond, nuclear rearrangement process and so forth.

In recent years, a large number of ESIPT reactions literatures discussed the so-called double benzoxazoles containing two protons34,35,36,37,38,39,40. The best-known examples, bis-3,6-(2-benzoxazolyl)-pyrocatechol (BBPC) and bis-2, 5-(2-benzoxazolyl)-hydroquinon (BBHQ), are symmetrical system with double intramolecular hydrogen bonds. Both of them were published by Mordzinski et al. firstly, through the nodal plane model method34. Their calculation predict that the BBHQ undergoes single proton transfer process in S1 state, while BBPC do double proton transfer reaction. On the basis of vibronic spectra and wavepacket dynamics, Weiβ et al. considered that an existent double proton transfer process of BBHQ in the excited state35. How was the proton transfer of BBHQ, this problem caused many attention of researchers. In order to solve the problem, Zhao et al. put forward a new mechanism of BBHQ via the TDDFT method as the basic theory, that the BBHQ undergoes double proton transfer or consecutive single transfer reaction36. By contrast, the researches on the proton transfer mechanism in BBPC are very limited. Grabowska and his partners claimed that BBPC exist double proton transfer reaction in S1 state37. However, Wortmann et al. confirmed the single proton transfer happens in S1 state of BBPC, which was confirmed by the absorption and emission electrooptical spectra38. They proposed that the electrooptical measurements and quantum chemical calculations were the reliable tools for studying the mechanism of phototautomer. Whether single or double proton transfer undergoes in BBPC is worth to be revised.

In spite of the nodal plane model has been proved in a lot of former achievements42,43,44, the proton transfer mechanism perhaps may not be inferred from it merely. It is difficult to detect whether one proton transfers firstly, the second proton transfers subsequently according to the nodal plane model. Unsatisfactorily, we can only get some information about geometric configuration and physical properties of molecular via the spectrum technology. In this work, a theoretical calculation has been implemented to investigate ESIPT reaction mechanism of BBPC. In order to clarify the ESIPT process mechanism of BBPC in detail, DFT and TDDFT methods have been adopted to the calculations for ground and excited state, respectively. Herein, the optimization of geometric structures in S0 and S1 states have been done, vertical excitation energy were calculated, IR vibrational spectra, the frontier molecular orbitals (LUMOs and HOMOs) as well as the potential energy surfaces (PESs) in the S0 and S1 states were calculated in our investigation, we choose both ωB97X-D/ 6-311 + g (d,p) and B3LYP/6-31 + G(d) to calculate the foremost bond length and bond angle, so that ensure the accuracy of the calculation result. Similar calculation results were obtained by the two procedures and considering the practical factors only B3LYP/6-31 + G(d) method done the follow work.

Our paper is logical as follows part introduces the calculation of the details. Part3 describes the results and discussion, and by considering the geometry, electronic spectra and the potential energy surface. The last part summarizes the conclusions of the research.

Results

Geometric Structures of BBPC

The DFT and TDDFT methods have been adopted to optimize the structures for BBPC in S0 state and S1 state, respectively. To test for the optimization of the structure is the most stable configuration, we also analyzed the frequency. Considering the solvent effects, hexane has been selected as the reaction solvent in the IEFPCM model. There are three structures of stable isomers BBPC, BBPC-A and BBPC-PT calculated display in Fig. 1. To make sure the description of bond lengths and bond angles more clearly and concisely, the hydrogen bonded atoms have been numbered. Herein, BBPC-A is the single proton transfer form, BBPC-PT is the double proton transfer construction. We listed the most important structural parameters in Table 1, which were calculated by the B3LYP/ 6-31 + G (d) procedure that is related to the hydrogen bonds. Based on our calculated results, it’s worth noting that the bond lengths of O1-H2, H2-N3, O4-H5 and H5-N6 of BBPC structure are 0.99 Ǻ, 1.81 Ǻ, 0.99 Ǻ and 1.81 Ǻ in the ground (S0) state, respectively. However, after being excited to S1 state, the bond lengths are 1.01 Ǻ, 1.71 Ǻ, 1.01 Ǻ and 1.71 Ǻ, respectively. Meanwhile, the O-H-N bond angle varies from 145.1° in S0 state to 148.1° in the S1 state. For comparison, the bond lengths of H2-N3 and H5-N6 of BBPC are shorten from 1.80 Ǻ in the ground (S0) state to 1.68 Ǻ in the S1 state by ωB97X-D/ 6-311 + g (d,p). Significantly, both methods show that the bond lengths of O1-H2 and O4-H5 are longer as well as H2-N3 and H5-N6 are shorten in the excited S1 state, which indicates these two intramolecular hydrogen bonds are simultaneously enhanced in the excited S1 state. For BBPC-A structure, the bond length of H2-N3 decreased from 1.80 Ǻ in the ground (S0) state to 1.77 Ǻ in the excited S1 state and the concomitant enlargement of O1-H2-N3 bond angle from 145.1°–147.0°. It indicated that the intramolecular hydrogen bond O1-H2···N3 is more stable in the S1 state than that in the ground (S0) state. Moreover, for BBPC-PT, the double proton transferred form can not appear stable structure in S0 state and the subsequent part of the potential surfaces will mention it.

Figure 1: Optimized structures of BBPC, BBPC-A and the BBPC-PT at the B3LYP/6-31 + G(d)/ IEF-PCM (benzene) theoretical level.
figure 1

Red: O; Gray: H; Blue: N; Black: C.

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Table 1 The calculated bond lengths (Å) and bond angles (°) of BBPC in the S0 and S1 states.
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By observing and analysising the infrared vibration spectrum of related chemical bonds, it can be further used to indicate the hydrogen bond in excited state is different from ground state1,2,3,4,5,6,7. Figure 2 provided the O-H stretching vibrational modes of BBPC by B3LYP/6-31 + G(d). Our calculated results for comparison are computed via ωB97X-D/ 6-311 + g (d,p) method, and the calculated stretching vibrational frequency of O-H is around 3469 cm−1 in S0 state and shift to 2983 cm−1 in S1 state. As the calculated O-H stretching vibrational frequency using B3LYP/6-31 + G(d) method is 3347 cm−1 in the S0 state, nevertheless it changes to be 2950 cm−1 in the S1 state, which are very close to the calculated results, using ωB97X-D/ 6-311 + g (d,p) method. This implies that the hydrogen bonds are reinforced in S1 state.

Figure 2
figure 2

Calculated IR spectra of BBPC in the spectral region of both O-H stretching bands in the S0 and S1 states based on the B3LYP/6-31 + G(d)/IEF-PCM (hexane) theoretical level.

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Calculated Frontier Molecular Orbitals (MOs) and Electronic Spectra of BBPC

The electronic absorption spectra, calculated using TDDFT/B3LYP/6-31 + G (d) theoretical method and the IEF-PCM solvent model, have been shown in Fig. 3 with the range of λ = 200~600 nm. The absorption peak and emission peak of BBPC are at 361.1 nm and 430.4 nm, respectively48. We can clear see our calculated results has very constant with the experimental results from Fig. 1. Besides, the value only 30.8 nm and 50.1 nm bigger than the results obtained from TDDFT /ωB97X-D/6-311 + g (d,p), respectively, the TDDFT/ωB97X-D/6-311 + g (d,p) results are 330.3 nm and 380.3 nm.Therefore, it demonstrates that the TDDFT/B3LYP/6-31 + G (d) method is feasible and satisfactory. We have tested the two programs and found them that they produce essentially same results in our case. Therefore, we only used the first method. Moreover, the emission peak of BBPC-A is around 521.6 nm, and the 564.1 nm fluorescence hump of BBPC-PT sturcture was also found based on our calculated method.

Figure 3: The calculated absorption and fluorescence spectra of BBPC and BBPC-A forms at the B3LYP/6-31 + G(d)/IEF-PCM (hexane) theoretical level.
figure 3

The croci vertical lines show the experimental results.

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Figure 4 shows MOs of BBPC in the solvent of hexane. Before we discuss the proton tranfer mechanism in S1 state, it’s should be qualitatively analysis the nature of charge distribution and charge transfer. We only display HOMO-1 and LUMO orbitals in Fig. 4 that was because mainly involves the two orbitals in the first excited state(HOMO-1 → LUMO: 97.51%). Obviously, it can be seen that HOMO-1show the π character yet π* character for LUMO, which is defined that the S1 state is attributed to the evident ππ* feature. It is worth to pay attention that the electron distribution of HOMO-1 and LUMO orbit for BBPC molecule are different. From HOMO-1 transfer to LUMO, the charge density of hydroxyl moiety were decreased yet for N atom was increased. Moreover, the increased charge density of N atoms make the hydrogen bond enhanced and promoting the ESIPT process.

Figure 4
figure 4

Frontier molecular orbitals, HOMO-1 and LUMO, for the BBPC chromophore based on TDDFT/B3LYP/6-31 + G(d)/IEF-PCM (hexane) calculations.

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Potential Energy Surfaces (PESs)

Construction of potential energy surface is an effective method to researching the molecular properties and reactions, and it can be more intuitive understanding the intramolecular proton transfer reaction process. Therefore, the calculation of potential energy surfaces are necessary to clarifying the process of the ESIPT in the BBPC. Potential energy surfaces are using the constrained optimizations in the relative electronic states along with fixing the O1-H2 and O4-H5 distances in a serious of values, respectively. The bond lengths of O1-H2 and O4-H5 are fixed in the S0 state and S1 state geometrical structures. Even though the DFT/TDDFT method may not be fully accurate to reproduce the proton transfer process from the surfaces,former works has proved this method to qualitatively analyze the status of excitation energy surface is very reliable and effective, that can provide the proton transfer process with precise proton transfer path45,46,47. The constructed PESs of the S0 and S1 state of the O1-H2 and O4-H5 bond lengths (vary from 0.81–2.11 Å in the S1 state and 0.89–2.19 Å in the S0 state) are in Fig. 5. The symmetrical PES of the S1 state has been displayed in Fig. 5(a) with minima signed. The coordinates are: A (1.01 Å, 1.01 Å), B (1.91 Å, 1.01 Å), C (1.01 Å, 1.91 Å) and D (1.91 Å, 1.91 Å). Herein, the energy of B and C points are the same and symmetrical by the diagonal AD. From the calculated results, we know that the BBPC-A is the most firm structure of the S1 state in these minimum potential energies, the relationship are EA > ED > EB (EC).

Figure 5
figure 5

PESs of the S0 and S1 states of BBPC as functions of the O1-H2 and O4-H5 lengths ranging from 0.81–2.11 Å in the S1 state and 0.89–2.19 Å in the S0 state (a) S1 state PES; (b) S0 state PES.

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The minimum potential energies are given in Table 2, calculated by B3LYP/6-31 + G (d), for the sake of clarifying the potential energies definitely. Meanwhile, we also calculated the potential barriers among these minimum energy points, the results show that: there is 3.24kcal/mol potential barrier between A and B (C); the potential barrier between A and D is 8.28 kcal/mol; from B (C) need to cross the 6.66 kcal/mol barrier to reach D point. Furthermore, after the radiative transition, single proton forming B (C) structure is the most stable of the three points due to the barrierless process that has shown in Fig. 5(b). Although the potential barrier separates point B (C) from point D is not large, only single proton transfer reaction more likely happens in the S1 state. Consequently, we summarized the ESIPT process in BBPC as followed: BBPC exists in the S0 state, after the photoexcited to the S1 state, it changed be the structure at point A. Then a proton transfers along the hydrogen bond from hydroxyl O to the N atom formed BBPC-A structure.

Table 2 The potential energies (Hartree) of stable structures on PESs of the S0 state the S1 state for BBPC, BBPC-A and BBPC-PT.
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Discussion

Summing up, theoretical to investigate the proton transfer reaction process of BBPC chemosensor has been performed by quantum chemical method and IEF-PCM to evaluate the solvent effect. Through analyse the calculated result, the variable bond length, angles and IR vibrational spectra, it can be illustrate that the hydrogen bond enhanced in S1 state and the ESIPT process be facilitated. The calculated MOs of BBPC also support the proton transfer in S1 state. In order to exhaustive explain, we also calculated PESs of the S0 state and S1 state, based on constrained optimizations in keeping the O1-H2 and O4-H5 distances fixed in a serious of values. There are three local minimum in S1 state, which the energy of B point and C point are identically. According to the calculated potential barriers among the minimum points in S1 state, single proton transfers via the hydrogen bond.

A new ESIPT mechanism has been proposed that was not equal to the previous conclusions41. The new ESIPT mechanism elucidates that single proton transfer more likely occurs in symmetric molecule of BBPC is essentially two equivalent single proton transfer reaction in comparison with the mechanism of double proton transfer.

Methods

In this study, the S0 and S1 state configuration of BBPC molecule was optimized by the DFT and TDDFT method, respectively. The B3LYP functional, and the 6-31 + G (d) basis set was used in both the DFT and TDDFT methods48,49,50. The ωB97X-D functional, has satisfactory accuracy for thermochemistry, kinetics, and non-covalent interactions51. Therefore, we also calculated based on ωB97X-D functional and 6-311 + g (d,p) basis compared with the B3LYP functional51,52,53,54. Combining the integral equation formalism variant (IEFPCM) and the model Polarizable Continuum Model (PCM)55,56,57,58, the calculations were performed in hexane solvent. The geometric optimization of the atoms, bonds and angles with no constraints. After analysis of the calculated vibration, all the local minima were demonstrated that has none of an imaginary mode. All theoretical calculations were performed by the Gaussian 0959.

The potential energy surface of BBPC in S0 and S1 state were constructed with the fixing the distance of O-H at a series of values. Secondly, the O-H bond length in S0 state variable range is from 0.89 Å to 2.19 Å and changed from 0.81 Å to 2.11 Å in S1 state. In S0 state and S1 state configuration optimization process, the self-consistent field (SCF) convergence criteria we adopt the default settings 10−6 both in S0 and S1 state. The harmonic vibrational frequencies were determined by the Hessian diagonalization of 60, and the infrared vibration intensity was determined by dipole moments61. The excited-state Hessian matrix is based on the analytical gradients of numerical difference and the method we adopted is the central difference method, set the step length as 0.02 Bohr.

Additional Information

How to cite this article: Zhang, Y. et al. How was the proton transfer process in bis-3, 6-(2-benzoxazolyl)-pyrocatechol, single or double proton transfer? Sci. Rep. 6, 25568; doi: 10.1038/srep25568 (2016).