How was the proton transfer process in bis-3, 6-(2- benzoxazolyl)-pyrocatechol, single or double proton transfer?

A theoretical analysis of proton transfer process for the symmetric systems with two intramolecular hydrogen bonds, bis-3,6-(2-benzoxazolyl)-pyrocatechol(BBPC) in hexane solvent, has been researched. In this study, we utilized ωB97X-D/ 6-311 + g (d,p) and B3LYP/6-31 + G(d) two procedures calculating the foremost bond length and bond angle, respectively. Our calculations demonstrate the two intramolecular hydrogen bonds were strengthened in S1 state, thus the proton transfer reaction can be facilitated. Furthermore, the calculated IR vibrational spectra confirmed hydrogen bonds were enhanced in S1 state. We found three local minima A B and C from the potential energy surfaces (PESs) on the S1 state, and the energy of B point and C point are identical. A new ESIPT mechanism has been proposed that was not equal to the previous conclusions. The new ESIPT mechanism elucidates that single proton transfer more likely occurs in the symmetric BBPC molecule in comparison with the double proton transfer reaction. And the frontier molecular orbitals(MOs) further illustrate the trend of ESIPT reaction.

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 bonds [1][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 on [8][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 methylsalicylate 21,22 . 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 photostabilizers [23][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 dedicated [27][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 protons [34][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 method 34 . Their calculation predict that the BBHQ undergoes single proton transfer process in S 1 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 state 35 . 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 reaction 36 . 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 S 1 state 37 . However, Wortmann et al. confirmed the single proton transfer happens in S 1 state of BBPC, which was confirmed by the absorption and emission electrooptical spectra 38 . 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 achievements [42][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 S 0 and S 1 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 S 0 and S 1 states were calculated in our investigation, we choose both ω B97X-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 S 0 state and S 1 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.  is more stable in the S 1 state than that in the ground (S 0 ) state. Moreover, for BBPC-PT, the double proton transferred form can not appear stable structure in S 0 state and the subsequent part of the potential surfaces will mention it.
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 state 1-7 . 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 ω B97X-D/ 6-311 + g (d,p) method, and the calculated stretching vibrational frequency of O-H is around 3469 cm −1 in S 0 state and shift to 2983 cm −1 in S 1 state. As the calculated O-H stretching vibrational frequency using B3LYP/6-31 + G(d) method is 3347 cm −1 in the S 0 state, nevertheless it changes to be 2950 cm −1 in the S 1 state, which are very close to the calculated results, using ω B97X-D/ 6-311 + g (d,p) method. This implies that the hydrogen bonds are reinforced in S 1 state.

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, respectively 48 . 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 /ω B97X-D/6-311 + g (d,p) , respectively, the TDDFT/ω B97X-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 4 shows MOs of BBPC in the solvent of hexane. Before we discuss the proton tranfer mechanism in S 1 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 S 1 state is attributed to the evident π π * 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

Figure 2. Calculated IR spectra of BBPC in the spectral region of both O-H stretching bands in the S 0 and S 1 states based on the B3LYP/6-31 + G(d)/IEF-PCM (hexane) theoretical level.
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.

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 optimi- 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 S 1 state in these minimum potential energies, the relationship are E A > E D > E B (E C ).
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 3.24 kcal/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 S 1 state. Consequently, we summarized the ESIPT process in BBPC as followed: BBPC exists in the S 0 state, after the

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 S 1 state and the ESIPT process be facilitated. The calculated MOs of BBPC also support the proton transfer in S 1 state. In order to exhaustive explain , we also calculated PESs of the S 0 state and S 1 state, based on constrained optimizations in keeping the O 1 -H 2 and O 4 -H 5 distances fixed in a serious of values. There are three local minimum in S 1 state, which the energy of B point and C point are identically. According to the calculated potential barriers among the minimum points in S 1 state, single proton transfers via the hydrogen bond.
A new ESIPT mechanism has been proposed that was not equal to the previous conclusions 41 . 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 S 0 and S 1 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 methods [48][49][50] . The ω B97X-D functional, has satisfactory accuracy for thermochemistry, kinetics, and non-covalent  interactions 51 . Therefore, we also calculated based on ω B97X-D functional and 6-311 + g (d,p) basis compared with the B3LYP functional [51][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 09 59 . The potential energy surface of BBPC in S 0 and S 1 state were constructed with the fixing the distance of O-H at a series of values. Secondly, the O-H bond length in S 0 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 S 0 state and S 1 state configuration optimization process, the self-consistent field (SCF) convergence criteria we adopt the default settings 10 −6 both in S 0 and S 1 state. The harmonic vibrational frequencies were determined by the Hessian diagonalization of 60 , and the infrared vibration intensity was determined by dipole moments 61 . 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.