Concerted Mechanisms of Excited-State Proton Intramolecular

Article Cite This: J. Phys. Chem. A 2017, 121, 8217-8226

pubs.acs.org/JPCA

Concerted Mechanisms of Excited-State Proton Intramolecular Transfer for Bis-2,4-(2-benzoxazolyl)-hydroquinone and Its Derivatives Dongshuai Bao,† Meishan Wang,*,† Chuanlu Yang,† Yunfan Yang,‡ and Xiaoguang Ma† †

School of Physics and Optoelectronics Engineering, Ludong University, Yantai 264025, China State Key Lab of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China

‡

ABSTRACT: The concerted mechanisms of excited state intramolecular proton transfer (ESIPT) of bis-2,4-(2-benzoxazolyl)-hydroquinone (BBHQ′) and its derivatives (BBHQ′− and DHBO′) have been investigated using the density functional theory (DFT) and the time-dependent density functional theory (TDDFT). The calculated absorption and emission spectra of BBHQ′ and its derivatives are in good agreement with the experimental results. The calculated bond lengths, bond angles, and IR vibrational spectra linked with hydrogen bond of molecular BBHQ′ in the S0 and S1 states demonstrate that the hydrogen bond is strengthened in the S1 state. Compared to BBHQ′, BBHQ′− has a weak change of hydrogen bond between the S1 and S0 states. The calculation results show that there are three stable structures of BBHQ′ in the S1 state. We find that the structure corresponding to the 481 nm fluorescence spectrum corresponds to BBHQ′-A rather than BBHQ′−-K (Tetrahedron Lett., 2016, 57, 3518). The calculated frontier molecular orbitals (MOs) indicate the nature of the charge distribution and the trend of proton transfer of BBHQ′-A. The constructed potential energy surfaces of BBHQ′ and DBHO′ further elucidate the proposed mechanism that one-proton or two-proton transfer can happen (stepwise or synchronous) in the S1 states. The proposed ESIPT mechanism can provide a good explanation of the phenomenon of fluorescence quenching of BBHQ′ and its derivatives. Finally, the weak interaction types are discriminated through the reduced density gradient (RDG) analyses of BBHQ′ and BBHQ′−. rather limited.22−29 Upon photoexcitation, the molecule will be projected on an excited state potential energy surface and become unstable. The energy difference between the initial excited state and the relaxed excited state can provide the driving force for the transformations.30−32 The ultrafast ESIPT and a large Stokes shift make them avoid the self-absorption and be expected to be good candidates for fluorescent probes. In recent years, more and more attention has been paid to the study of dual proton transfer in biological and material systems.33−35 For instance, the excited state sequential proton transfer mechanism of [2,2′-bipyridyl]-3-3′-diol was reported by Lischka and his co-workers.31 A dual proton transfer reaction in the 7-azaindole dimer system was studied based on an ab initio potential energy surface and the corresponding empirical valence bond model by Ando et al.34 As the organic light emitting and fluorescent probing materials, 2-(2hydroxyphenyl) benzoxazole (HBO) and its derivatives have attracted much attention.36−40 In particular, due to its properties of the huge absorption spectral response and significant fluorescence enhancement, bis-2,5-(2-benzoxazolyl)-hydroquinone (BBHQ) composed of double HBO could be

1. INTRODUCTION Hydrogen bonds widely exist in nature and biological systems and play an indispensable role in our life. In the past decade, the investigation of hydrogen bonds has aroused extensive attention due to their special properties in photophysics, photochemistry, and biology.1−6 First, hydrogen bonds have a powerful ability to build supramolecules because of their collectively strong directional interaction, which is essential for the construction of fundamental building blocks of life. Second, hydrogen bonds can be used as the active site for the occurrence of many interactions due to their dynamic features.4 Therefore, the further study of hydrogen bond interaction will be critical in the crystal state, solutions, and living organisms. Especially in recent years, Han and his co-workers proposed a new mechanism to explain the changes and effects of hydrogen bond in the excited state, which made many photochemical and photophysical processes clearer, such as intra−molecular charge transfer (ICT), photo−induced electron transfer (PET) and ESIPT.7−18 As the most important elementary reaction in chemistry and biology, ESIPT was first studied by Weller et al. as early as about 1965.7,11,12,19 Since then, ESIPT has been widely investigated in experiments and theories.2,3,20,21 The ESIPT process is involved in fluorescence sensors, laser dyes, light-emitting diode (LEDs), UV filters, molecular switches, and so forth. However, knowledge of the ESIPT process is still © 2017 American Chemical Society

Received: August 7, 2017 Revised: October 9, 2017 Published: October 10, 2017 8217

DOI: 10.1021/acs.jpca.7b07753 J. Phys. Chem. A 2017, 121, 8217−8226

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The Journal of Physical Chemistry A used for the dual-channel detection of anions.41 Only one proton transfer in the S1 state based on the nodal plane model proposed was first reported by Mordzinski et al.42 However, the proton transfer mechanism of BBHQ was still controversial. There were two main viewpoints based on experimental and theoretical researches until now: (1) only one proton could be transferred; (2) double protons could be transferred (stepwise or synchronous).30,41,43,44 As an important isomer of BBHQ, bis-2, 4-(2-benzoxazolyl)-hydroquinone(BBHQ′) was synthesized in 2016.45 Although BBHQ′ and BBHQ had the similar molecular structure, there existed the obvious differences between them. The fluorescence peak of BBHQ and BBHQ′ was at 597 nm (orange−red band) and 480 nm (blue band), respectively. In addition, it was BBHQ′ rather than BBHQ that had the ability to detect different stages of Zn2+ binding, which indicated that BBHQ′ could be used for the design of new fluorescent sensor.45 Benelhadj et al. synthesized 2, 4-bis (5tert-butyl-benzoxazol-2-yl) hydroquinone (DHBO′), which was also excellent chelates toward boron fragments such as BF2 or BAr2 leading to singlet emitters with a quantum yield in solution reaching up to 51%. Furthermore, the solid-state studies revealed intense fluorescence in the condensed matter DHBO′ and borate complexes. Hence, it expected to be the ratio type fluorescent probe for a variety of substance analyzes.44 Therefore, it is very important to fully understand the proton transfer mechanism of DHBO′, BBHQ′, and BBHQ′−. Although the absorption and emission spectra can be measured, it is difficult to understand the ESIPT process because the energy barriers of forward and reverse proton transfer cannot be observed in the experiment. The experiment observation can only provide the indirect information on photophysical properties of BBHQ′ and DHBO′. In order to study the ESIPT mechanism of BBHQ′ and DHBO′ in detail and the changes of substance in the solution, we employ the DFT and TDDFT methods to optimize the structure in the S0 and S1 states and find the minima in both states. The vertical excitation energies, frontier molecular orbitals (MOs), hydrogen bond energies, bond lengths, bond angles, and IR vibrational spectra of them are also calculated. Finally, we construct their potential energy surfaces in the S0 and S1 states with both O9−H10 and O11−H12 (shown in Figure 1) bond lengths at the DFT/ TDDFT/B3LYP theoretical level.

2. COMPUTATIONAL DETAILS All theoretical calculations of BBHQ′, DHBO′, and BBHQ′− are accomplished by the DFT and TDDFT methods employing the Becke’s three-parameter hybrid exchange function with the Lee−Yang−Parr gradient-corrected correlation functional47−49 (B3LYP) by Gaussian 09 program.50 To be consistent with the experimental environment, dimethylformamide (DMF) and toluene solvents are chosen, along with the solvent effects of the polarizable continuum model (PCM) using the integral equation formalism variant (IEF), which was widely used successfully.51−56 For BBHQ′ and DHBO′, the 6-31+g(d,p) basis set is chosen to perform the geometry optimizations, vibrational frequencies, single point energy (SPE) and simulated spectra. Considering the computational cost, the somewhat smaller 6-31g(d,p) basis set is adopted for calculations of their (PESs). The potential energy surfaces (PESs) of BBHQ′ and DHBO′ are constructed by fixing the distance of bond O9−H10 and O11−H12 at a sequence of values based on relaxed scan. The simulated spectra based on vertical excitation energy and vertical emission energy by 6-31g(d,p) basis set are similar to the calculations result based on 631+g(d,p), indicating that the constructed PESs are reliable. For BBHQ′−, all calculations are completed with 6-31+g(d,p) basis set. The frequency calculations of all structures (BBHQ′, DHBO′, and BBHQ′−) are performed to confirm that each optimized structure is a real minimum (i.e., without imaginary vibrational frequencies). 3. RESULTS AND DISCUSSION 3.1. BBHQ′. The optimization of all geometry structures of BBHQ′ and its isomers show that they are stable (i.e., no imaginary frequencies). The hydrogen bond is presented in BBHQ′, single proton transfer structure (BBHQ′-A), and double proton transfer structure (BBHQ′-B), whereas the open form BBHQ′-O is hydrogen bond-free (shown in Figure 1). For the descriptive convenience, the atoms related to intramolecular hydrogen bond are marked. With the increasing of the length of O−H bond of BBHQ′ from 0.99 to1.00 Å, the bond length of N−H shorten from 1.76 to1.69 Å from the S0 state to the S1 state, which indicates that hydrogen bond is strengthened in the S1 state (listed in Table 1). Similarly, the changes of O−H···N angle are from 146.8° in the S0 state to 148.1° in the S1 state, which also demonstrate that the hydrogen bond of BBHQ′ is strengthened in the S1 state. Moreover, the bond length of O9−H10 and bond angle of O11··· H12−N35 for BBHQ′-A are 1.02 Å and 121.2° in the S1 state, while they change to be 0.99 Å and 131.2° in the S0 state. Therefore, we can draw a conclusion that the hydrogen bond of O11···H12 of BBHQ′-A is more stable in the S0 state, and the BBHQ′-A in the S1 state probably undergoes the process of radiative transition to the S0 state forming the stable intramolecular hydrogen bond H12−N35. There is also a possibility that another proton transfer will form the most stable structure of BBHQ′-B in the S1 state. The structure of BBHQ′-B can only be stable in the S1 state. The infrared vibrational spectra of BBHQ′ further illustrate the change of hydrogen bond in S0 and S1 state. It is worth noting that the calculated frequency of O−H stretching vibration of BBHQ′ is located at 3248 cm−1 in the S0 state, whereas 3010 cm−1 in the S1 state (shown in Figure 2). The 148 cm−1 red−shift of the frequency of the O−H stretching

Figure 1. Optimized structures of BBHQ′, BBHQ′-A, BBHQ′-B and BBHQ′-O at the B3LYP/6-31+g (d, p)/IEF-PCM (DMF) theoretical level. Red: O, Yellow: N, Blue: C, and White: H. 8218


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Table 1. Calculated Primary Bond Lengths (Å) and Angles of BBHQ′, BBHQ′-A, BBHQ′-B, and BBHQ′-O in the S0 and S1 States BBHQ′

BBHQ′-A

BBHQ′-B

BBHQ′-O

electronic state

S0

S1

S0

S1

S0

S1

S0

S1

R(O9−H10) R(H10−N37) R(O11−H12) R(H12−N35) δ(O9−H10−N37) δ(O11−H12−N35)

0.99 1.76 0.99 1.76 146.8 146.8

1.00 1.69 1.00 1.69 148.1 148.1

0.99 1.76 1.78 1.03 147.6 131.2

1.02 2.04 1.76 0.99 146.5 121.2

− − − − − −

2.06 1.01 2.06 1.01 120.5 120.4

0.97 3.76 0.97 3.76 161.97 161.96

0.97 3.73 0.97 3.73 14.73 14.73

vibration indicates that hydrogen bonding is enhanced in the S1 state.

BBHQ′-A is located at 482 nm, which is consistent with the experimental values of 481 nm (shown in Figure 3). However,

Figure 2. Calculated IR spectra of BBHQ′ in the spectral region of both O−H stretching bands in the S0 and S1 states based on the B3LYP/6-31+g (d, p)/IEF-PCM (DMF) theoretical level.

Figure 3. Calculated absorption spectra of BBHQ′ and BBHQ′-O forms and fluorescence of BBHQ′-A at the B3LYP/6-31+g(d,p)/IEFPCM (DMF) theoretical level. The red and black number represents the experimental and theoretical calculated values, respectively.

The calculated singlet transition energies (S0 → S1), (S0 → S2) and (S0 → S3) of BBHQ′ are at 346, 311, and 288 nm (list in Table 2), which are consistent with the experimental values

it is different from the explanation that the corresponding structure is BBHQ′−-K rather than BBHQ′-A in the experiment. Therefore, the explanation in the experiment is problematic. We will do the further proof in the following. The frontier MO analysis can provide information about the nature of the excited state conformations of charge distribution. The related calculations of BBHQ′ in DMF solvent are shown in Figure 4. The calculated electronic transition energies and the corresponding oscillator strengths of BBHQ′ are listed in Table 2. The dominant π−π*-type transition can be assigned with the 95.95% composition from HOMO to LUMO in the S1 state with the oscillator strength of 0.88. The changes of electron density around the atoms link with the hydrogen bond

Table 2. Electronic Transition Energy (nm), Corresponding Oscillator Strengths, and Compositions of the Low-Lying Singlet Excited States of BBHQ′ and BBHQ′−-C transition

λAbs (nm)

f

composition

CI (%)

BBHQ′

S0−S1 S0−S2 S0−S3

346 311 288

0.88 0.34 0.85

BBHQ′−-C

S0−S1

399.49

0.89

H→L H → L+1 H−1 → L+1 H → L+2 H→L

95.95 98.45 90.90 2.85 96.32

of 338−353 nm, 310 nm, 285 nm, respectively. The calculated emission maximum of the first singlet transition (S1 → S0) is at 402 nm. But one can know that it is very weak from the spectra shown in the experiment.45 Furthermore, it shows a relatively strong emission intensity in nonpolar solvent compared to several other polar solvents. Hence, the emission hump of BBHQ′ is ignored in experiment due to the inherent nature that the benzoxazoles in nonpolar solvents show a relatively strong emission of the excited dienol, while it can hardly be detected in polar solvents. However, it should be noted that the structure of BBHQ′ is stable on the present calculations. Moreover, the calculated results show that the emission peak of

Figure 4. Frontier molecular orbitals HOMO and LUMO of BBHQ′ based on the TDDFT/B3LYP/6-31+g(d,p)/IEF-PCM (DMF) calculations. 8219


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calculations prove that the method for the qualitative analysis of proton transfer process is still reliable enough.57−61 The potential energy surfaces of BBHQ′ in the S0 state and the S1 state are symmetrical due to the molecular symmetry. We mark the lowest points of energy, as shown in Figure 5a. The coordinates of three points are L (1.11 Å, 1.11 Å), M (2.11 Å, 1.11 Å) and N (2.11 Å, 2.11 Å) and they correspond to BBHQ′, BBHQ′-A, BBHQ′-B, respectively. The results show that the relationship of the energies of these minimum points is EL > EM > EN. It means that BBHQ′-B is the most stable structure in the S1 state. The potential barriers between them are also calculated to discuss whether proton transfer will occur. The energy barriers are 1.84 kcal/mol from L to M, 1.62 kcal/ mol from M to N, and 6.39 kcal/mol from L to N, which indicate that a single proton and two protons transfer are prone to occur in the S1 state. The potential barriers of the reverse process are also calculated. The potential barrier from M to N is 7.16 kcal/mol. The process from N to L or from N to M is very difficult to happen due to the potential barrier of 25.38 or 15.29 kcal/mol. Therefore, it proves that these three structures can exist simultaneously. We conclude that the transfer process of proton on the S1 surface is like this: a BBHQ′ molecule is excited to the S1 state. Then two possibilities exist: one possibility is that a proton is transferred from N to O in the direction of the hydrogen bond to form the BBHQ′-A structure; subsequently, another proton is transferred to form the BBHQ′-B structure or not. Another possibility is that both protons transfer along the direction of the hydrogen bond at the same time to form BBHQ′-B structure. Through the radiative transition process, the BBHQ′ structure can emit a 402 nm fluorescence hump (extremely weak in the experiment), and the BBHQ′-A structure can emit 481 nm fluorescence hump. The BBHQ′-B cannot emit fluorescence, because the corresponding oscillator strength of electronic transition energy (nm) for isomer B is 0.00 if the precision is accurate to second decimal places, as listed in Table 3. In

can be seen clearly through the front molecular orbital. The electron density of the O atom decreases, while the electron density of the N atom increases after the transition from the HOMO to the LUMO. Additionally, the Natural Population Analysis (NPA) could be a quantitative analysis of the changes of atomic charge distribution from S0 state to S1 state, which have an effect on the process of proton transfer (suppression or promotion). The decrease of negative charge distribution on the O9 atom of the −O9H10 moiety is from −0.720 in the S0 state to 0.719 in the S1 state with the corresponding increase of the N37 atom from −0.550 to 0.555 based on the B3LYP/631+g(d,p) theoretical level. The changes of atomic charge distribution can facilitate the transfer process of a proton in the S1 state. In order to reveal the ESIPT mechanism of the BBHQ′ in detail and explain the fluorescence quenching, we construct the PESs of BBHQ in the S0 and S1 states as functions of the O9− H10 and O11−H12 bond lengths (ranging from 0.89 to 2.29 Å for O9−H10 and 0.91 to 2.31 Å for O11−H12), as shown in Figure 5. Although the potential energy surface of BBHQ′ constructed by the DFT/TDDFT is rough, lots of previous

Table 3. Electronic Transition Energy (nm), Corresponding Oscillator Strengths, and Compositions of the Low-Lying Singlet Excited States of BBHQ′-A, BBHQ′-B, BBHQ′−-C, and BBHQ′−-K BBHQ′-A BBHQ′-B BBHQ′−-C BBHQ′−-K

transition

λFlu (nm)

f

S0−S1 S0−S1 S0−S1 S0−S1

482 644 451 569

0.74 0.00 1.21 0.00

composition

CI (%)

→ → → →

99.16 99.62 98.61 99.25

H H H H

L L L L

addition, because of the barrierless processes from m to l, n to m, and n to l, BBHQ′-A structure and BBHQ′-B structure are very unstable on the S0 potential energy surface. It is easy to undergo a reverse proton transfer process to change back to BBHQ′ structure. In summary, both a single proton transfer process and a double proton transfer process are likely to happen in the S1 state of BBHQ′. 3.2. BBHQ′−. In order to study the effect of deprotonation on the transfer process of proton, the transfer process of proton for BBHQ′− in DMF solution is investigated. The parent BBHQ′− (BBHQ′) and optimized isomers of BBHQ′− are named BBHQ′−-closed (BBHQ′−-C), non-hydrogen bonded BBHQ′−-open (BBHQ′−-O), and BBHQ′−-keto (BBHQ′−-K) and are shown in Figure 6. The bond lengths and bond angles of BBHQ′−-C, BBHQ′−-O, BBHQ′−-K and BBHQ′ are listed

Figure 5. PESs of BBHQ′ in the S0 and S1 states as functions of the O9−H10 and O11−H12 lengths from 0.91 to 2.1 Å. (a) PES for S1 state. (b) PES for S0 state. The arrows indicate the direction of the proton transfer. 8220


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Figure 6. Optimized structures of BBHQ′, BBHQ′−-C, BBHQ′−-O and BBHQ′−-K at the B3LYP/6-31+g(d,p)/IEF-PCM (DMF) theoretical level. Red: O, yellow: N, blue: C, and white: H.

Figure 7. Calculated absorption and fluorescence spectra of BBHQ′−C at the B3LYP/6-31+g(d,p)/IEF-PCM (DMF) theoretical level, the blue and black numbers indicate the experimental and theoretical calculated values, respectively.

in Table 4. The length of the N···H bond in BBHQ′−-C decreases from 1.76 Å (S0) to 1.72 Å (S1) with the increasing of the O−H···N angle from 148.61° (S0) to 148.67°(S1), which indicates that the hydrogen bond is strengthened in the S1 state. The calculated absorption and emission peaks of BBHQ′−−C are located at 402 and 451 nm respectively, which are consistent with the experimental values of 388 and 439 nm (shown in Figure 7). However, our calculations show that BBHQ′−-K structure cannot emit fluorescence. Because the corresponding oscillator strength of electronic transition energy (nm) for BBHQ′−−K is 0.00 if the precision is accurate to second decimal places, as listed in Table 3. Therefore, combined with the calculation results of BBHQ′, it is confirmed that the emission spectrum at 481 nm corresponds to BBHQ′A rather than BBHQ′−-K reported by Abeywickrama et al.45 The energy of hydrogen bond is another indicator of the strength of hydrogen bond. Therefore, the energy of hydrogen bond of S0 and S1 states are calculated, which can be obtained by the energy difference between the BBHQ′−-C and the BBHQ′−-O. The potential energy curves of BBHQ′− (shown in Figure 8) are constructed by artificially twisting the hydrogen bond about 180°. The calculated energy of hydrogen bond is 7.52 kcal/mol in the S0 state and 8.28 kcal/mol in the S1 state, which indicates that the intramolecular hydrogen bond is strengthened in the S1 state. Although the effect of the excited state on hydrogen bonding in BBHQ′− is not significant, we can still conclude that the hydrogen bond of BBHQ′− strengthens in the excited state S1. To further explore the cause of proton transfer, the frontier molecular orbitals (MOs) of BBHQ′− are calculated (shown in Figure 9). The frontier MOs of the BBHQ′−−C show a π−π* type transition from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital

Figure 8. Energy variation of BBHQ′− as a function of dihedral angle θ for the transformation of the intramolecular hydrogen-bonded BBHQ′−-C form to the non-hydrogen-bonded BBHQ′−-O form as obtained at the B3LYP/6-31+g(d,p)/IEF-PCM (DMF) theoretical level.

(LUMO). The increasing electron density around N can facilitate the process of proton transfer. In order to further study the mechanism of proton transfer of BBHQ′−, the potential energy curves of BBHQ′− (shown in Figure 10) in the S0 and S1 states are constructed as a function of O−H bond from 0.99 to 2.39 Å in the increments of 0.07 Å. As shown in Figure 10, BBHQ′−−K is not stable in the S0 state, while it is stable in the S1 state. The energy barriers for the forward and backward proton transfer between BBHQ′−−C

Table 4. Calculated Bond Lengths (Å) and Angles (deg) of BBHQ′, BBHQ′−-C, BBHQ′−-K, and BBHQ′−-O Forms in the S0 and S1 States BBHQ′−-C

BBHQ′

BBHQ′−-K

BBHQ′−-O

electronic state

S0

S1

S0

S1

S0

S1

S0

S1

R(O9−H10) R(H10−N37) δ(O9−H10−N37) δ(O11−H12−N35)

0.99 1.76 146.8 146.8

1.00 1.69 148.1 148.1

0.99 1.76 148.6 −

0.99 1.72 148.7 −

− − − −

2.03 1.01 122.02 −

0.97 0.97 13.46 13.46

− − − −

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Figure 9. Calculated frontier molecular orbitals HOMO and LUMO of the BBHQ′− structure.

Figure 11. Optimized structures of DHBO′, DHBO′-A (single proton transfer) and DHBO′-B (double proton transfer) at the B3LYP/631+g(d,p)/IEF-PCM (toluene) theoretical level. Red: O, Yellow: N, Blue: C, and White: H.

Figure 10. Potential energy curves in the S0 and S1 states of BBHQ′− as a function of O9−H10 bond length.

and BBHQ′−−K in the S1 state are 7.96 and 6.62 kcal/mol, respectively. It indicates that BBHQ′−−C and BBHQ′−−K can simultaneously exist in the S1 state, and the process of proton transfer of the BBHQ′− only occurs in the S1 state. 3.3. DHBO′. As an important derivative of BBHQ′ and an electron donative group, DHBO′ with tBu group (DHBO′) arouses our interest, and we are curious about whether this feature will have an impact on the proton transfer. Therefore, the DHBO′ with tBu group is chosen. It is also studied by the same theoretical method as previously described in toluene. There are four stable geometry structures. Three of them contain a hydrogen bond and can be named as DHBO′, DHBO′-A, and DHBO′-B, respectively (Figure 11). The calculated results of the vibrational frequency have confirmed that these structures are stable structures. (i.e., no imaginary frequencies). For DHBO′, the absorption peak is calculated to be at 351 nm, which is consistent with the experimental values of 369 nm. However, the calculated fluorescence hump of DHBO′ is at 400 nm, which is not reported in the experiment.46 (Based on the same reason, the emission peak was not observed in the experiment.) The calculated emission peak of DHBO′−A is at 496 nm, which is consistent with 492 nm emission hump in the experiment, as shown in Figure 12. The front molecular orbital of DHBO′ is still a π−π* type transition from HOMO to LUMO, as shown in Figure 13. It is found that the tBu substituents do not have much effect on the front molecular orbital. The electrons of front molecular orbital are prone to transition transfer from HOMO to LUMO and promote proton transfer. Since tBu substituents do not have much effect on the front molecular orbital, therefore, one can

Figure 12. Calculated absorption and fluorescence spectra of DHBO′ forms at the B3LYP/6-31+g(d,p)/IEF-PCM (DMF) theoretical level. The red and black numbers represent the theoretical calculated and experimental results, respectively.

Figure 13. Calculated frontier molecular orbitals HOMO and LUMO of the DHBO′ structure.

conclude that the tBu substituents have little impact on the proton transfer of DHBO′. Our calculations prove that DHBO′ is stable in the S1 state, which contrasts with the conclusion of Benelhadj et al.46 We investigate the proton transfer mechanism of the DHBO′ by constructing PESs of the S0 8222


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The Journal of Physical Chemistry A and S1 states for DHBO′ (see Figure 14). There are three stable points of the excited potential surface labeled as L (1.10 Å, 1.10

RDG(r ) =

|∇ρ(r )| 1 2(3π 2)1/3 ρ(r )4/3

(1)

where ρ(r) is the total electron density, and the RDG (r) is the reduced density gradient of the exchange contribution. According to Bader’s Atoms in Molecules (AIM) theory, the relationship to the second largest eigenvalue λ2 of the Hessian matrix of electron density and ρ(r) can be written in the form. Ω(r ) = sign(λ 2(r ))ρ(r )

(2)

This means that the weak interaction is related to ρ and λ2. Therefore, the sign λ2 has been further analyzed via plotting the scatter diagram of the function 1 (RDG) value versus the function 2 (Ω(r)) value and the contour value being set as 0.5 is shown in Figure 15). We use the different colors to stand for

Figure 14. PESs of DHBO′ in the S0 and S1 states as a function of O9−H10 and O11−H12 bond lengths ranging from 0.79 to 2.09 Å. (a) PES for S1 state. (b) PES for S0 state. The arrows indicate the direction of the proton transfer.

Å), M (2.10 Å, 1.10 Å), and N (2.10 Å, 2.10 Å), and they correspond to DHBO′, DHBO′-A, DHBO′-B, respectively. The potential energies relationship among them is EL > EM > EN. Moreover, the calculated results show that the energy from L to M is 1.79 kcal/mol, 3.42 kcal/mol energy from L to N, and a barrierless process from M to N. The reverse energy barriers are also calculated. They are 8.55 kcal/mol from M to L, 21.38 kcal/mol from N to L, and 10.14 kcal/mol from N to M. The potential barrier of 21.38 kcal/mol is difficult to cross, while the 8.55 and 10.14 kcal/mol energy barriers are enough low to cross. It means that DHBO′ follows the same proton transfer mechanism that single proton transfer or both protons transfer (stepwise or synchronous) can occur in the S1 state. The ESIPT mechanism we proposed is a good explanation of the phenomenon of fluorescence quenching. 3.4. Reduced Density Gradient (RDG) Isosurfaces. We use the RDG function to distinguish the different types of interaction, which can be expressed as

Figure 15. (A) Scatter plot of the reduced density gradient (RDG(r)) versus Ω (r) are expressed as Function value 1 and Function value 2. The visual diagram of RDG isosurfaces. (B) Weak interaction of BBHQ′ and BBHQ′−. X is the color gradient corresponding to the different types of the interaction; Y is the weak interaction for (a) S1 and (b) S0 in BBHQ′, and (c) S1 and (d) for S0 in BBHQ′−.

the different ρ(r) and λ2 values, and fill in the RDG isosurfaces.62 The value range of RDG isosurfaces is set as −0.04 to 0.02 (shown in Figure 15B(X)). The weak interaction of BBHQ′ and BBHQ′− including the hydrogen bond interactions (the blue areas) calculated and plotted used the Multiwfn63 and VMD64 program is shown in Figure 15B(Y). A blue disc exists between N and H atoms in Figure 15B(Y)b, which demonstrates the presence of hydrogen bond in the S0 state. As shown in Figure 15B(Y)a,b, the 8223


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hydrogen bond has undergone great changes in the S1 state comparing with the hydrogen bond in the S0 state. The blue disc turns into a blue ring, and the color is slightly deepened in Figure 15B(Y)b, which indicates that the hydrogen bond is strengthened in the S1 state. The weak interactions of BBHQ′− are shown in Figure 15B(Y)c,d. The change of hydrogen bond of BBHQ′− is very small from the S0 state to the S1 state.

4. CONCLUSION We use the DFT and TDDFT methods and IEF−PCM solvation model to optimize the BBHQ′ and its derivatives based on 6-31g+(d,p) basis sets. Several key parameters of the hydrogen bond including bond lengths, bond angles, IR vibrational spectra, and energies of hydrogen bond are calculated. The changes of these parameters indicate that the hydrogen bond strengthen in the S1 state and facilitate the process of proton transfer. Through the analysis of forward and backward energy barriers on the PESs of S0 and S1 state for BBHQ′, three stable structures of BBHQ′, BBHQ′-A, and BBHQ′-B are found on the potential energy surface of S1 state. Our calculations show that the measured emission spectrum at 481 nm corresponds to BBHQ′-A rather than BBHQ′−-K. In the concerted mechanism of proton transfer of BBHQ′, single proton transfer or both protons transfer (stepwise or synchronous) can occur in the S1 state. The frontier molecular orbitals of BBHQ′ show the electron redistribution, which will facilitate the ESIPT process. For the BBHQ′−, we also do a similar calculation, the results show that the hydrogen bond strengthen in the S1 state and facilitate the proton transfer. The difference between BBHQ′ and BBHQ′− is that the effect of the excited state on BBHQ′− is relatively smaller than that of BBHQ′. The RDG isosurfaces not only show hydrogen bond and other types of weak interactions, but also reveal the changes of hydrogen bond in the S1 state compared to the S0 state. What’s more, it further confirms that the hydrogen bond strengthens in the excited state and facilitates the process of proton transfer.

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AUTHOR INFORMATION

Corresponding Author

*Tel./fax: +86 535 6672142. E-mail address: mswang1971@ 163.com. ORCID

Dongshuai Bao: 0000-0002-2823-308X Meishan Wang: 0000-0001-8085-2657 Xiaoguang Ma: 0000-0002-7935-499X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant No. 11474142), the Natural Science Foundation of Shandong Province (ZR2014AM005), and the Taishan Scholarship Project of Shandong Province (ts201511055). All calculation data were carried out in the Tiansuo Super Computer Center (TSCC) of Ludong University.

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REFERENCES

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