TDDFT Study on the Sensing Mechanism of a Fluorescent Probe

Jun 26, 2017 - Theoretical Insights Into the Excited State Double Proton Transfer Mechanism of Deep Red Pigment Alkannin. Jinfeng Zhao , Hao Dong , an...
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DFT/TDDFT Study on the Sensing Mechanism of a Fluorescent Probe for Hydrogen Sulfide: Excited State Intramolecular Proton Transfer Coupled Twisted Intramolecular Charge Transfer Yang Li†,§ and Tian-Shu Chu*,†,‡ †

State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, P. R. China ‡ Institute for Computational Sciences and Engineering, Laboratory of New Fiber Material and Modern Textile, the Growing Base for State Key Laboratory, Qingdao University, Qingdao 266071, P. R. China § University of Chinese Academy of Sciences, Beijing 100049, P. R. China ABSTRACT: By using density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods, the sensing mechanism of a fluorescent probe 2-(2hydroxyphenyl) benzothiazole (HBT) derivative HBTPP-S for hydrogen sulfide has been thoroughly studied. The thiolysis reaction has a moderate reaction barrier of 18.40 kcal mol−1, which indicates that the hydrogen sulfide sensing process has a favorable response speed. Because of the nonradiative donor-excited photoinduced electron transfer (d-PET, fluorophore as the electron donor) from the excited HBTPP group to the electron-withdrawing 2,4-dinitrophenyl group, as well as the inhibition of the proton transfer (PT) and the excited state intramolecular proton transfer (ESIPT) process by 2,4dinitrophenyl group, the probe HBTPP-S is essentially nonfluorescent. On the other hand, the added hydrogen sulfide induces the thiolysis of the 2,4-dinitrophenyl ether bond, and then the thiolysis product HBTPP comes into existence. The theoretically simulated potential energy surface demonstrates that without the electron-withdrawing 2,4-dinitrophenyl group, the thiolysis product HBTPP undergoes the excited state intramolecular proton transfer (ESIPT) coupled twisted intramolecular charge transfer (TICT) processes in the first excited state. The absence of the d-PET and the process mentioned above may explain the significant fluorescent turn-on response and large Stokes shift of the thiolysis product HBTPP.

1. INTRODUCTION In spite of its unpleasant smell and poisonous attribute, hydrogen sulfide (H2S) has been deeply researched due to its functions as the third gasotransmitter following nitric oxide (NO) and carbon monoxide (CO)1−4 in recent years. As an important endogenous antioxidant, H2S is of great significance to various pathophysiological processes, including regulation of cell growth, mediation of neurotransmission, and inhibition of insulin signaling.5−7 However, deregulation of H2S in cells will cause Alzheimer’s and Down’s syndrome, diabetes, and liver cirrhosis.8−11 Therefore, efficient detection of H2S in living system has become an important subject in chemical and biological field. Lately, scientists have reported several sorts of fluorescent probes, which are based on the H2S-triggered specific reactions, such as Cu2+ quencher removal,12−14 reduction of azide15−17 or nitro,18,19 nucleophilic reaction,20−22 thiolysis of dinitrophenyl ether23−25 and so on,26−28 to achieve fluorescent turn-on or ratiometric response. Among these fluorescent probes, most fluorophores show small Stokes shift, which restrains the potential of their biological applications. Considering this factor, it is highly desirable to design a fluorophore with large Stokes shift. © 2017 American Chemical Society

With this, Zeng and co-workers have synthesized 2-(2-hydroxyphenyl) benzothiazole (HBT) derivative HBTPP-S as a novel H2S probe based on thiolysis of 2,4-dinitrophenyl ether and the “turn-on” excited state intramolecular proton transfer (ESIPT) coupled intramolecular charge transfer (ICT) mechanism (see Scheme 1).29 The probe HBTPP-S is nonfluorescent and the thiolysis product HBTPP shows remarkably red keto emission (620 nm) and large Stokes shift (240 nm), which could be used for biological imaging in living cells.30,31 However, we are not clear about the mechanism of the fluorescence quenching of the probe HBTPP-S, and whether the proton transfer (PT) process would occur in the ground state of the thiolysis product HBTPP? Moreover, depending on the reaction time domain, the ESIPT and ICT can occur separately or concomitantly and can be classified into three categories (PT* and CT* are the excited state proton transfer form and the excited state charge transfer form, respectively): (1) PT* → CT* (the ESIPT takes prior to ICT). (2) CT* → PT* (the rate of ICT is faster than that of ESIPT), Received: March 20, 2017 Revised: June 20, 2017 Published: June 26, 2017 5245

DOI: 10.1021/acs.jpca.7b02606 J. Phys. Chem. A 2017, 121, 5245−5256

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The Journal of Physical Chemistry A Scheme 1. Proposed Reaction Mechanism of the Probe HBTPP-S with H2S

Figure 1. Optimized geometries for the probe HBTPP-S and its thiolysis product HBTPP in acetonitrile.

is appended in the ortho-position of phenol through a carbon− carbon double bond, therefore, some of the issues worth studying include whether the structures of HBTPP-S and HBTPP are twisted by the rotating carbon−carbon double bond or not. Does the twisting motion affect the fluorescence emission or not? To answer the above questions to provide a clear idea about the sensing mechanism of this H2S probe, in this work, we carried out calculations based on the density functional theory (DFT)57 and time-dependent density functional theory (TDDFT).58 First, we studied the properties of the ground state and the first excited state of HBTPP-S, HBTPP, and other molecular structures involved in the sensing process. Then, based on these optimized structures, we theoretically simulated and reproduced the experimental absorption and emission spectra. Finally, we calculated the potential energy curves in the S0 and S1 states of HBTPP to explore the PT, ESIPT, and ICT processes and also analyzed the frontier molecular orbitals to explain the sensing mechanism. The DFT/TDDFT methods are suitable to study the mechanism involving PT, ESIPT, and ICT process.59−74 For instance, by using these calculation methods, our group has successfully proposed the mechanism for thiazolidinedione derivative in dimethylformamide solution that TICT could facilitate ESIPT initiated by intermolecular hydrogen-bond strengthening in the S1 state.62 Zhao’s group clearly demonstrated that for N,N-dimethylanilino-1,3-diketone (DMADK) chromophore, the ESIPT and TICT process were coupled in the S1 state rather than sequential ESIPT and TICT process,71 and these calculated results are consistent with those reported by Ciofini’s group.75

and (3) the rates of ESIPT and ICT events are about the same.32−35 Recently, the hydroxyphenylbenzazole family has been studied due to their ability to lead to ESIPT process, such as 2-(2′-hydroxyphenyl)benzoxazole (HBO)36−40 and 2-(2′-hydroxyphenyl)benzothiazole (HBT).41−49 Researchers have found that with the presence of an electron acceptor or electron donor substituent in the molecular framework, HBO and HBT show quite different mechanisms for ESIPT and ICT in the S1 state. When HBO and HBT have an electron acceptor substituent (−NO2, −COOH, −COOR or −RCN), the ESIPT reaction occurs prior to the ICT process.50,51 Nevertheless, when HBO and HBT have an electron donor group (−NH2, NR2, or −CH3), the ICT process is followed by the ESIPT reaction.52 Here, for the thiolysis product HBTPP, it is interesting to investigate whether the ESIPT and ICT processes occur separately or simultaneously? If the former, which comes first? Furthermore, among different ICT models, twisted ICT (TICT) and planar ICT (PICT) have attracted intensive interest and have been extensively reported.53−55 In the TICT state, the molecular configuration is twisted by rotating the acceptor group or donor group, whereas the acceptor and donor group are coplanar and conjugated in the PICT state. The molecule with donor and acceptor group linked by a π-conjugated bridging group may bear torsional relaxation in the excited ICT state, which could cause different degrees of charge separation depending on the torsional angle. More importantly, the fluorescence emission of such system is always influenced by the torsional relaxation process.56 Since, in the probe HBTPP-S and the product HBTPP designed by Zeng and co-workers,29 an electron acceptor group (methylpyridine) 5246

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2. METHODS All calculations were carried out by using the Gaussian 09 program package.76 By applying DFT/TDDFT methods, optimal geometries were computed without any symmetry constraint of HBTPP-S and HBTPP in the ground electronic state and in the first excited electronic state. A number of functionals, including B3LYP,77−79 B3PW91,80 PBEPBE,81 MPW1PW91,82 and CAM-B3LYP83 were pretested before we carried out this study, and we found that the long-rangecorrected (LRC) CAM-B3LYP functional shows the most satisfactory agreement with the experimental results. Therefore, the CAM-B3LYP functional was chosen to study the properties of the ground state and the first excited state in the DFT/TDDFT method. The TZVP84,85 was also chosen as the basis set during both the ground state and the first excited state optimizations, and the subsequent frequency analysis ensured that our obtained structures had no imaginary vibration and were located at the local minima in the potential energy surfaces. The intermediate and the transition states were found by scanning the potential energy surface. The frequency analysis confirmed that the intermediate has no imaginary frequency and the transition state has only one imaginary frequency, and the intrinsic reaction coordinate (IRC) calculations make sure that the transition state connects the intermediates and the proper products. The free energy profile for the thiolysis reaction was computed with Gibbs free energy correction. Note that all Gibbs free energies computed were relative energies using the reagent energy as a benchmark. In all calculations, unless there is a special declaration, the solvent effects are included using the integral equation formalism (IEF)86,87 version of polarizable continuum model (PCM)88 and the dielectric constant of acetonitrile (ε = 35.69).

Figure 2. Potential energy curves of S1 state (red line) for the thiolysis product HBTPP calculated at the TDDFT/CAM-B3LYP/TZVP level as functions of the O−H bond length (ESIPT coordinate). The energies of S0 state (black line) were calculated under the geometries of the corresponding S1 state.

O−H bond length has been calculated to aid the study of the PT process. As shown in Figure 2, the transfer of the proton H from O to N in the S0 state exhibits a barrier of 4.24 kcal mol−1 with the O−H bond length increasing and the PT process of HBTPP is endothermic. This proves that, in the ground state, the proton of the thiolysis product HBTPP would not transfer from the proton donor (OH group) to the proton acceptor (neighboring nitrogen group). Considering that the absorption and fluorescence titration experiments were done in a mixed solution of NaSH (a commonly employed H2S donor) and CH3CN/PBS,29 the thiolysis reaction should take place between the probe HBTPP-S and HS−. For further comprehension of this thiolysis dynamics process, we computed the ground state potential energy curve of the probe HBTPP-S and HS− as a function of the S···C distance and displayed the free energy profile for the thiolysis reaction in acetonitrile solvent. As shown in Figure 3, the formation of the intermediate (a complex formed by HBTPP-S and HS−) is endergonic by 2.86 kcal mol−1. The reaction barrier for the S−C bond formation and the C−O bond cleavage is 18.40 kcal mol−1. The moderate reaction barrier (18.40 kcal mol−1) implies that the thiolysis reaction between HBTPP-S and HS− would have a favorable reaction rate, hence, after the formation of HBTPP−, HBTPP− would continue to capture one free H+ from the mixed solution to form the end product HBTPP of the thiolysis reaction.63 On the basis of our optimized ground-state structures, we calculated the low-lying singlet electronic transition energies and the corresponding oscillator strengths of HBTPP-S and HBTPP(E). Our calculated results are demonstrated in Table 1 and Figure 4b. For the probe HBTPP-S, the calculated absorption peak is located at 316 nm and an intense S0 → S1 transition for the thiolysis product HBTPP(E) is located at 347 nm. The calculated bathochromic shift (31 nm) agrees with the experimental results where upon addition of NaHS, the red-shifted new absorption peak is observed at 380 nm (in Figure 4a) and the initial absorption peak at 320 nm decreases.29 We also calculated the UV−vis absorption spectra of HBTPP−. The result displayed that the first singlet transition

3. RESULTS AND DISCUSSION 3.1. The Ground State Structures and Absorption Spectra. The equilibrium geometric structures of the probe HBTPP-S and the thiolysis product HBTPP were displayed in Figure 1. For HBTPP-S and HBTPP, the dihedral angle C1−C2−C3−C4 is respectively 136.21° and 140.88° in the S0 state, which indicates that both HBTPP-S and HBTPP have the twisted geometry by rotating the ethylenic double bond and such torsional motion is usually accompanied by the ICT process. As illustrated, for the probe HBTPP-S, the phenolic hydroxyl group is masked by 2,4-dinitrophenyl group and is converted to the ether group, which induce that the proton transfer process cannot proceed in the ground state and the first excited state. After formation of the product HBTPP through thiolysis of 2,4-dinitrophenyl ether bond, the O−H bond lengths of the phenolic hydroxyl group and the intramolecular hydrogen bond H···N in the ground state are 0.99 and 1.68 Å. To examine whether the thiolysis product HBTPP involves the PT process (via intramolecular hydrogen bond from O to N) or not, we further optimized the proton transferred structure and compared the energy between the enol form (here referred to as HBTPP(E)) and keto form (here referred to as HBTPP(K)). In HBTPP(K), the dihedral angle C1−C2− C3−C4 is 146.06° in the S0 state and the O···H bond is lengthened to 1.64 Å, while the H−N bond is shortened to 1.05 Å. Correspondingly, the optimized SCF energy increases from −1393.59 a.u. to −1393.04 a.u., implying that the enol form structure is a more stable configuration. Meanwhile, the S0 state potential energy curve of HBTPP as a function of the 5247

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Figure 3. Free energy profiles for the thiolysis reaction. R reactant, IM intermediate, TS transition state, P product. All free energies are in kcal mol−1; all bond lengths are in Å. The inset shows the ground state potential energy curve of the probe HBTPP-S and HS− as a function of the S···C distance.

Table 1. Comparison of Experimental and Calculated UV−Vis Absorption and Fluorescence Emission for the Probe HBTPP-S and Its Thiolysis Product HBTPP in Acetonitrile electronic transition HBTPP-S HBTPP

absorption emission absorption emission

S0 S1 S0 S1

→ → → →

S1 S0 S1 S0

energy (nm (eV)) 316 691 347 571

(3.91) (1.79) (3.57) (2.17)

fa

contribb

expt (nm)c

0.0175 0.0007 0.9940 1.2880

H→L+1 L→H−5 H→L L→H

320 380 620

a

Oscillator strength. bH, HOMO (highest occupied molecular orbital) and L, LUMO (lowest unoccupied molecular orbital). cThe experimental absorbance and emission band taken from ref29.

Figure 4. Experimental absorption/fluorescence spectra (a) (the data were taken from ref 29) and the calculated absorption/fluorescence spectra (b) of the thiolysis product HBTPP in acetonitrile.

(S0 → S1) is located at 489 nm, which is not in agreement with the experimental absorption spectra of the product (380 nm). Hence, we further make sure that the end product of the thiolysis reaction should be HBTPP.

3.2. The Frontier Molecular Orbitals of HBTPP-S and HBTPP(E) and the Two Indexes of DCT and ξCT. The frontier molecular orbitals of HBTPP-S and HBTPP(E) are shown in Figure 5. The first singlet transition for HBTPP-S is 5248

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Figure 5. Frontier molecular orbitals (MOs) of the probe HBTPP-S (left column) and its thiolysis product HBTPP (right column) in acetonitrile. E and K are the enol and keto form, respectively. The asterisk indicates the first excited state. d-PET stands for donor-excited photoinduced electron transfer, ESIPT stands for excited state intramolecular proton transfer, and TICT stands for twisted intramolecular charge transfer.

Figure 6. Divided part 1 and part 2 of the probe HBTPP-S and its thiolysis product HBTPP.

Table 2. Contribution of Part 1 to Frontier Molecular Orbitals for HBTPP-S and HBTPP Using the Ground State and the First Excited State Orbitals by C-Squared Population Analysisa first excited state orbitals

ground state orbitals HBTPP-S (%) HOMO LUMO+1

7.00 12.44

HBTPP(E) (%) HOMO LUMO

9.82 36.64

HBTPP(K*) (%) HOMO LUMO

14.74 30.49

a

E and K are the enol and keto form of HBTPP, respectively. The asterisk (∗) indicates the first excited state.

dominated by the HOMO → LUMO+1 transition, while for HBTPP(E) the first singlet transition corresponds to the transition of HOMO → LUMO. For HBTPP-S or HBTPP(E), the electron density of methylpyridine moiety increases after the transition from HOMO to LUMO+1 or to LUMO, suggesting that the S1 states of HBTPP-S and HBTPP(E) are both TICT states. To describe the TICT character more accurately, we further divided HBTPP-S and HBTPP(E) into two parts and calculated the contribution of part 1 to the HOMO and LUMO+1/LUMO using the ground state orbitals of HBTPP-S and HBTPP(E) (see Figure 6 and Table 2) by C-squared population analysis.89 For HBTPP-S, there is 7.00% of the electron density residing on the part 1 for HOMO and 12.44% for LUMO+1, while for HBTPP(E), 9.82% of the electron

Figure 7. Constructed two-dimensional potential energy surface of the thiolysis product HBTPP in the S1 state as functions of the methylpyridine twisting angle ranging from 140.88° to 175° and the O−H bond length ranging from 0.95 to1.9 Å, calculated at the TDDFT/ CAM-B3LYP/TZVP level. The white arrow indicates the direction of both ESIPT and twisting process in the S1 state. The structures corresponding to HBTPP(E) and HBTPP(K*) are also shown. The asterisk (∗) indicates the first excited state.

density resides on the part 1 for HOMO and 36.44% for LUMO. Therefore, the S1 state of HBTPP(E) shows more obvious TICT character than HBTPP-S, which may give rise to the red shift in the absorbance spectra that is observed in the experimental UV−vis absorbance.29 5249

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The Journal of Physical Chemistry A Table 3. Comparison of the Calculated S0 and S1 States Geometric Parameters of HBTPP(E) and HBTPP(K*) in Gas Phase and in Acetonitrile Solution ground state

first excited state

molecular parameters

gas phase

acetonitrile

gas phase

acetonitrile

O−H bond (Å) C1−C2−C3−C4 (deg)

1.00 145.95

0.99 140.88

1.80 159.14

1.85 169.35

Figure 10. Values of the DCT index calculated for the first excited state on the S1 relaxed geometries along the O−H bond length of HBTPP at the TDDFT/CAM-B3LYP/TZVP level. The energies of the S0 state were calculated under the geometries of the corresponding S1 state.

depletion and increase regions produced upon excitation, and ξCT means the dihedral angle formed between the planes defined by the two barycenters of charges and two carbon atoms belonging to the ethylene bridge.75,90−93 We also calculated the DCT and ξCT indexes using the Multiwfn software,94 and we got the DCT value of 2.44 Å (0.47 Å) and the ξCT value of 17.75° (1.17°) for HBTPP(E) (HBTPP-S), which demonstrated more obvious TICT character in HBTPP(E), in accordance with the picture of the frontier molecule orbitals. 3.3. The First Excited State Structures, the S1-State Potential Energy Surface/Curves, and the Mechanism of Coupled ESIPT and TICT in HBTPP. The geometries of the S1 state of HBTPP-S and the enol form (HBTPP(E*), the asterisk indicates the first excited state) as well as the keto form (HBTPP(K*)) of HBTPP were also presented in Figure 1.

Figure 8. Potential energy curves of the thiolysis product HBTPP(K), as a function of the twisted dihedral angle (C1−C2−C3−C4) in the S0 and S1 states (g, in the gas phase; s, in acetonitrile solution), calculated at the TDDFT/CAM-B3LYP/TZVP level. The small circles mean the structures of HBTPP(K*) before and after the TICT process. The asterisk (∗) indicates the first excited state.

The DCT and ξCT indexes are also suitable for describing the degree of the TICT character (that is, the larger the DCT and ξCT values, the larger the TICT character). Here, DCT means the distance between the barycenter of charges of the density

Figure 9. Schematic illustration of d-PET from the excited HBTPP to 2,4-dinitrophenyl group in the unreacted probe HBTPP-S, but d-PET lacks in the keto form of the thiolysis product HBTPP. 5250

DOI: 10.1021/acs.jpca.7b02606 J. Phys. Chem. A 2017, 121, 5245−5256

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The Journal of Physical Chemistry A Scheme 2. Four-Level Model for the ESIPT Process of HBTPPa

a

E and K are the enol and keto forms, respectively. The asterisk indicates the first excited state.

In the first excited state, the dihedral angle C1−C2−C3−C4 of HBTPP-S is 134.57° and HBTPP-S has a similar structure to that in the ground state. In HBTPP(E*) (see Figure 1), the bond length of O−H is lengthened from 0.99 Å in the S0 state to 1.03 Å in the S1 state and the intramolecular hydrogen bond H···N is shortened from 1.68 Å in the S0 state to 1.56 Å in the S1 state. The strengthening of the S1 state intramolecular hydrogen bond implies that the ESIPT process would take place via intramolecular hydrogen bond from O to N. We also note that the energy of HBTPP(K*) is lower than that of HBTPP(E*), thus, HBTPP(K*) is a more stable structure than HBTPP(E*). For HBTPP, the dihedral angle C1−C2− C3−C4 changes from 140.88° in the S0 state (HBTPP(E)) to 169.35° in the S1 state (HBTPP(K*)). After excitation to the S1 state, HBTPP undergoes the ESIPT process and the torsional relaxation by twisting the methylpyridine group. The torsional angle (C1−C2−C3−C4) changes from 140.88° in the S0 state to 169.35° in the S1 state, suggesting that the product HBTPP is more coplanar in the S1 state than that in the S0 state. Meanwhile, the dipole moment for HBTPP decreases from 18.88 D in the S0 state to 11.29 D in the S1

state, and the larger dipole moment of HBTPP(E) may be owing to greater charge transfer character. Figure 2 shows the S1-state potential energy curve of HBTPP as a function of the O−H bond length (the S0-state curve of HBTPP is also presented in the figure). To obtain this curve, structure optimizations were done by fixing the O−H bond while relaxing other parameters. The S1-state curve exhibits a very low barrier of 0.65 kcal mol−1, after overcoming the barrier, the energy of S1 state decreases along the ESIPT coordinate. The energy of the keto form in the S1 state is lower than that of enol form, proving that the ESIPT process of HBTPP is exothermic and formation of the keto form is thermodynamically allowed. Consequently, the transfer of the proton H from O to N via the intramolecular hydrogen bond of HBTPP in the S1 state is energetically favorable and ESIPT process can proceed to form the stable keto form structure. To sum up, in the ground state, the ESIPT process is inhibited because phenolic hydroxyl group is masked by 2,4-dinitrophenyl group, however, after thiolysis of 2,4-dinitrophenyl ether by H2S and upon photoexcitation, the proton of phenolic hydroxyl group can transfer to the 5251

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The Journal of Physical Chemistry A neighboring nitrogen group and HBTPP(E) is converted into the stable HBTPP(K*). On the other hand, after excitation, the product HBTPP may undergo TICT accompanied by the internal rotational motion of the methylpyridine group. To investigate such a mechanism of ESIPT and TICT in the S1 state, we constructed the two-dimensional S1-state potential energy surface (PES) of the thiolysis product HBTPP, as a function of methylpyridine twisting angle C1−C2−C3−C4 (ranging from 140.88° to 175°) and the distance between O and H (ranging from 0.95 to 1.9 Å), by going through a time-consuming process. It is necessary to explain that the bond length at 0.99 and 1.85 Å corresponds to the O−H bond length of HBTPP(E) and HBTPP(K*), and the angles of 140.88° and 169.35° correspond to the dihedral angle of HBTPP(E) and HBTPP(K*). The two-dimensional PES are shown in Figure 7, together with the two critical points of HBTPP(E) and HBTPP(K*). From the PES, it can be seen that HBTPP first undergoes the TICT process by rotating the methylpyridine moiety upon photoexcitation. Then, the concomitant action of ESIPT and methylpyridine rotational motion comes into existence when the twisting angle of the enol form is larger than ∼155° (the coupling of ESIPT and TICT processes appears in the region of the twisting angle ranging from 155° to 169.35°). Afterward, the energy drops markedly with the increase in the O−H bond and reaches a stable point at the O−H bond length of ∼1.85 Å. Here the ESIPT process is very likely to play the main part in the sensing process, due to only after undergoing the coupled ESIPT and TICT processes, the stable near-planar HBTPP(K*) can be formed eventually. To reveal the solvent effect on the torsional motion of HBTPP after ESIPT, Table 3 compares the geometric parameters of HBTPP(E) and HBTPP(K*) in gas phase and in the acetonitrile solution. In the gas phase, the torsional angle (C1−C2−C3−C4) changes from 145.95° (S0) to 159.14° (S1). Figure 8 compares the S1-state potential energy curves of HBTPP(K*) in the gas phase and in acetonitrile solution, as a function of the twisted dihedral angle C1−C2−C3−C4 and at the fixed O−H bond lengths of 1.80 and 1.85 Å, respectively. The energies of S0 state calculated under the geometries of the corresponding S1 state are also shown in the figure. The two barrierless excited-state potential energy curves might demonstrate that TICT occurs easily in both gas phase and acetonitrile solution. The whole picture of the potential curves shows that, after the ESIPT, there probably is no intersystem crossing or conical intersection (CI) between the S0 and S1 states of HBTPP, which might exclude the competitive nonradiative channel of CI for HBTPP. Additionally, the energy difference before and after the TICT process in the S1 state is rather small in both cases, suggesting that the rotational motion lowers little energy for HBTPP in the S1 state in both gas phase and acetonitrile solution. So the internal rotational motion of methylpyridine group seems to play the minor part in the sensing process of the thiolysis product HBTPP. 3.4. Fluorescence Spectra, d-PET Process, ESIPTCoupled TICT, and Sensing Mechanism. Table 1 shows the calculated S1 → S0 transition for HBTPP-S* and HBTPP(K*). It is found that for the probe HBTPP-S the oscillator strength (f = 0.0007) of this transition approaches zero, thus, the S1 state of HBTPP-S is a dark state which decays to the ground state via the nonradiative channels, causing the quenched fluorescence of HBTPP-S. On the other hand, after the thiolysis of 2,4-dinitrophenyl ether bond, the S1 → S0

vertical excitation energy for HBTPP(K*) is predicted at 571 nm with the oscillator strength of 1.2880. Here, the large Stokes shift (224 nm) could be ascribed to the ESIPT process. All these theoretical results agree well with the previous experimental observations that the probe HBTPP-S is nonfluorescence and the thiolysis product HBTPP shows remarkably red keto emission (620 nm) and large Stokes shift (240 nm)29 (see Figure 4). On the basis of the molecular orbitals in Figure 5, for HBTPP-S, the π electrons on the LUMO+1 are primarily resided on the HBTPP group while the electrons on LUMO is delocalized over the electron-withdrawing 2,4-dinitrophenyl moiety. Therefore, upon photoexcitation, the nonradiative donor-excited photoinduced electron transfer (d-PET) can take place from the excited HBTPP group to the electronwithdrawing 2,4-dinitrophenyl group. Because of such efficient d-PET process (see Figure 9) and the notorious fluorescence quenching effect of 2,4-dinitrophenyl group,23−25,95,96 the probe HBTPP-S is essentially nonfluorescent. In addition to the d-PET process, other decay paths such as the nonradiative decay via nonadiabatic CI could also contribute to the fluorescence quenching of HBTPP-S. Because of the present TDDFT method is unable to locate the CI in HBTPP-S, the search of these other decay paths is temporally beyond the scope of this study. Thiolysis of the 2,4-dinitrophenyl ether bond by H2S has converted the nonfluorescent HBTPP-S into the fluorescent HBTPP, which lacks the d-PET process (see Figure 9) and undergoes the ESIPT coupled the TICT at the first excited state leading to the overlap of electrons on the transition orbitals of HBTPP(K*) (Figure 5). Further evidence can be seen from the charge transfer characters of HBTPP in Table 2. The values in this table were calculated by C-squared population analysis using the ground state orbital of HBTPP(E) and the first excited state orbital of HBTPP(K). Seen from the table, there is 9.82% of the electron density residing on part 1 for HOMO and 36.64% for LUMO using the ground state orbitals in the calculation, while there is 14.74% of the electron density residing on the part 1 for HOMO and 30.49% for LUMO using the first excited state orbitals in the calculation, demonstrating the ESIPT process (coupled to the TICT process) leads to the more overlap of electrons on the transition orbitals. The charge redistribution upon photoexcitation gives rise to that HBTPP(K*) possesses relatively weak ICT character accounting also for its fluorescent nature. Figure 10 shows our calculated DCT index values along the ESIPT coordinate of the O−H distance where the highest DCT value is found at the O−H distance of 1.03 Å corresponding to the structure of HBTPP(E*). The value decreases to the end of the O−H bond length at 1.85 Å corresponding to the structure of HBTPP(K*). Our analysis of the DCT index further confirmed the weak ICT character of HBTPP(K*). Some of these calculated results agree well with the previous studies about the fluorescent turn-on probe based on the H2S-promoted thiolysis of dinitrophenyl ether.23−25,95,96 According to previous literature work,97−99 ESIPT can be viewed as a very fast photo tautomerism process taking place along an intramolecular hydrogen bond, based on a four-level model (E → E* → K* → K → E). Scheme 2 presents the four-level model deduced from our calculation results for the ESIPT process in HBTPP. As illustrated, the energy level of HBTPP(K*) is lower than that of HBTPP(E*) while the energy level of HBTPP(K) is higher than that of HBTPP(E). 5252

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Therefore, the energy gap between HBTPP(K*) and HBTPP(K) is smaller than that between HBTPP(E*) and HBTPP(E), demonstrating that HBTPP(K*) emits long-wavelength fluorescence. Therefore, the large Stokes shift observed in HBTPP is due to the ESIPT after the UV-excitation. Overall, our theoretical results confirmed that the nonradiative d-PET process can lead to the fluorescence quenching of the probe HBTPP-S, while thiolysis of the ether bond by H2S, the lack of d-PET process, and the coupled ESIPT and TICT processes are all responsible for the strong fluorescence emission and large Stokes shift of the thiolysis product HBTPP. As a results, an intensive fluorescence emitted by HBTPP(K*) can be easily observed by the naked eye and the probe HBTPP-S can serve as an efficient fluorescent turn-on probe for the detection of the hydrogen sulfide.

REFERENCES

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4. CONCLUSIONS DFT and TDDFT methods have been applied to elucidate the sensing mechanism of the hydrogen sulfide probe HBTPP-S based on the thiolysis of 2,4-dinitrophenyl ether bond. The free energy profile displays that the thiolysis reaction has a moderate barrier (18.40 kcal mol−1) that can lead to a favorable response speed of the probe HBTPP-S for HS−. The red shift in the absorbance spectra, the strong fluorescence emission and the large Stokes shift, all coincide with the experimental observations. The 2,4-dinitrophenyl group in the probe HBTPP-S efficiently quenches the fluorescence of the HBTPP fragment through the d-PET process, and it also masks the phenolic hydroxyl group to inhibit the PT and ESIPT processes. Accordingly, the probe HBTPP-S is essentially nonfluorescent. After removing the masking 2,4-dinitrophenyl group through the H2S-promoted thiolysis, the twisted enol form of thiolysis product HBTPP(E) is formed. Upon photoexcitation, HBTPP(E*), first undergoes the TICT process accompanied by the torsional relaxation, and meanwhile, the phenol proton of HBTPP(E*) is transferred to the neighboring nitrogen group through the ESIPT process. In the end, the HBTPP(E*) is converted into the stable near-planar keto form HBTPP(K*) which can emit strong fluorescent and show large Stokes shift. Here, the H2S-promoted thiolysis, the absence of d-PET process and the ESIPT-coupled TICT processes all account for the dramatic change in fluorescent properties of HBTPP such as strong fluorescent emission and large Stokes shift. In conclusion, the thiolysis of the ether bond, the nonradiative d-PET process and the coupling of ESIPT with TICT processes play the decisive roles during the sensing processes of the hydrogensulfide probe HBTPP-S.

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Article

AUTHOR INFORMATION

Corresponding Author

*(T.-S.C.) E-mail: [email protected]; [email protected]. ORCID

Tian-Shu Chu: 0000-0002-3519-8737 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (Grant No. 21273234), the Shandong Provincial Natural Science Foundation of China (Grant No. ZR2014AM025), and the National Basic Research Program of China (Grant No. 2013CB834604). 5253

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DOI: 10.1021/acs.jpca.7b02606 J. Phys. Chem. A 2017, 121, 5245−5256