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Reconsideration of the Detection and Fluorescence Mechanism for a Pyrene Based Chemosensor for TNT Meiheng Lu, Panwang Zhou, Yinhua Ma, Zhe Tang, Yanqiang Yang, and Keli Han J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b11739 • Publication Date (Web): 16 Jan 2018 Downloaded from http://pubs.acs.org on January 16, 2018

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The Journal of Physical Chemistry

Reconsideration of the Detection and Fluorescence Mechanism for a Pyrene Based Chemosensor for TNT † Meiheng Lu,abc Panwang Zhou,a Yinhua Ma,ac Zhe Tang,a Yanqiang Yang,d Keli Han*a

a

State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of

Sciences, Dalian 116023, P. R. China b

c

College of Applied Chemistry, Shenyang University of Chemical Technology, Shenyang 110142, P. R. China

University of Chinese Academy of Sciences, Beijing 100049, P. R. China

d

National Key Laboratory of Shock Wave and Detonation Physics, Institute of Fluid Physics, China Academy of

Engineering Physics, Chengdu 610200, P. R. China

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] Author Contributions Meiheng Lu and Panwang Zhou contributed equally to this work.

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Abstract Rapid detection of chemical explosives is crucial for national security and public safety, and investigation of sensing mechanisms is important to design highly efficient chemosensors. This study theoretically investigates the detection and fluorescence mechanism of a newly synthesized pyrene based chemosensor for detection of trinitrotoluene (TNT) through density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods, suggesting a different interaction product of the probe and TNT from previously reported ones (Mosca, L.; Behzad, S. K.; Anzenbacher, P. Jr. J. Am. Chem. Soc. 2015, 137, 7967-7969). Instead of forming Meisenheimer complexes, the energies of which are beyond reactants, a low-energy product generated by the π–π stacking interaction is more rational and favorable. The fluorescence quenching property further confirms the π–π stacking product is the predicted one, rather than the luminescent Meisenheimer complexes. Frontier molecular orbitals (FMOs) analysis results show that photo-induced electron transfer (PET) is the mechanism underlying the luminescence quenching of the probe upon exposure to TNT.


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Introduction Rapid and efficient sensing of explosive nitroaromatic compounds is highly important because of increasing concerns over national security and public safety1-13. In recent years, fluorescent probes have received considerable attention in this field because of their high sensitivity, rapid response time, simple preparation, and potential for stand-off detection14. However, most of their sensing mechanisms remain unclear because experimental studies provide only indirect information about the photophysical and structural properties of fluorescent probes. Hence, theoretical simulations should be conducted to clarify the fundamental aspects concerning sensing mechanisms. Our group has conducted several studies in this field15-19. To the best of our knowledge, the first elaborate excited-state sensing mechanism theoretical study of a fluorescent probe was presented by our group15, which provided a basis for subsequent investigations of the sensing mechanism of fluorescent probes20-22. Recently, Mosca et al.23 synthesized a new small-molecule fluorescent probe, namely, N1,N1'(6-(pyren-1-yl)-1,3,5-triazine-2,4-diyl)bis(ethane-1,2-diamine) (PTBE), which displays a turn-on behavior upon exposure to RDX and turn-off fluorescence in the presence of TNT. They suggested that the fluorescence resonance energy transfer (FRET) mechanism was responsible for the luminescence quenching of TNT–amine Meisenheimer complexes24; these complexes are the predicted product of the detection reaction and are formed by nucleophilic attack between the nucleophilic probe and the electron-deficient TNT. However, the methyl moiety, as an electrondonating group, not only reduces the electrophilicity of TNT but also increases the steric


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hindrance, thereby obstructing the nucleophilic attack. So how difficult is it to form the Meisenheimer complex with the probe and TNT? Besides, there are two active sites for nucleophilic attack on TNT, which one is favored? We also noted that PTBE and TNT can form hydrogen bonds, will it be a possible interaction pattern of PTBE and TNT? Moreover, it is well known that pyrene, the conjugated backbone of the probe, is highly beneficial to bind aromatic rings through π–π interactions25-27, is it possible to form a π–π stacking product of PTBE and TNT which could quench the fluorescence as well? In this regard, the present study aims to further investigate the sensing mechanism of PTBE. Theoretical simulations are conducted to discuss the interaction of TNT with the probe and illustrate the fluorescence mechanism of this system. In this study, the geometric structures of PTBE, TNT, two Meisenheimer complexes and the π–π stacking product have been optimized. The Gibbs free energy profiles have been calculated to evaluate the possible products. The electronic spectra have been simulated to compare the fluorescent properties and prove the rationality of the current calculation level through comparison with the experimental data. Finally, the sensing mechanism has been illustrated on the basis of FMOs analysis.

Computational details All theoretical calculations reported in this work were carried out with DFT and TDDFT methods in Gaussian 09 program suite28. The range-separated functional CAM-B3LYP29, which


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could provide accurate description of the charge transfer excited state properties30-32, and 631G(D,P)33-34 basis set have been selected to serve the geometry optimization and frequency analysis. The Grimme’s D3-dispersion correction35 was applied in the ground state simulations. The electronic spectra (UV absorption and emission spectra) were calculated at CAM-B3LYP/631+G(D,P) level. Solvation effects were included using the integral equation formalism36-37 (IEF) version of the Polarizable Continuum Model38-39 (PCM) with dichloromethane (eps=9.83, epsinf=2.03) as the solvent according to the experiment. Linear response scheme was chosen for IEFPCM to model excited states in solution. Solvent non-equilibrium scheme was applied for absorption spectra calculations, and solvent equilibrium scheme was used in emission spectra calculations. Geometric optimization was performed without any constrains for symmetry, bonds, angles or dihedral angles. In order to improve the performance, the 2-electron integral accuracy parameter was set to 12, and the default use of incremental Fock matrix formation and modest integral accuracy early in direct SCF has been prevented. All of the ground state and excited state optimized structures were confirmed to be local minima without imaginary mode in the vibrational analysis calculations. The gauge-independent atomic orbital (GIAO) method and B3LYP functional with 6-311+G(2d,p) basis set were employed in the

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C NMR calculations

with methylsilane as standard40. The RDG analysis was simulated with Multiwfn41.

Results and discussion Geometric structures


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The optimized structures of PTBE, TNT, two Meisenheimer complexes and the π–π stacking product on ground state are presented in Figure 1. Figure 1(a) and 1(b) show the optimized ground state structures of PTBE and TNT. Figure 1(c) and 1(d) represent the two kinds of Meisenheimer complexes (M1 and M2) respectively, which could be distinguished by the TNT binding site. For M1, it is located on C1 [Figure 1(b)], the C connected to the methyl group directly; and for M2, it is located on the meta position C3/C5. Notably, the methyl group or the hydrogen atom, which connects to the binding site, is out of the plane of the TNT skeleton with a small angle. This finding indicates that the binding site C becomes sp3 hybridization, rather than sp2 hybridization in TNT, thereby confirming the formation of Meisenheimer complexes. The primary bond lengths of the binding sites of M1 and M2 are marked in Figure 1(c) and 1(d), respectively. The bond length of M2 is shorter than that of M1, indicating the higher possibility of forming M2 in the nucleophilic attack reaction because of its less steric hindrance and stronger electrophilicity on the binding site than M1. Figure 1(e) and 1(f) show the front and side views of the π–π stacking product of the probe and TNT (PTBE-TNT). The benzene of TNT is located above the pyrene backbone of the probe. The distance between the two planes is 3.48 Å, confirming that PTBE-TNT is a typical π–π stacking configuration42-44. The geometric structures of M1, M2 are further confirmed by 13C NMR spectra45 (Figure S1). The signals of the binding site carbons C1 and C3/C5 originally at ~140 ppm in TNT are shifted to ~70 ppm in M1 and M2, which could be assigned to the signal of an amino group-connected carbon. Thus, the structure of the Meisenheimer complexes is confirmed to be correct. RDG


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analysis is carried out to identify the π–π interaction of PTBE-TNT46-47, shown in Figure S2. According to the position of the isosurface and the filled-in color which represent the location and the type of the weak interaction, evidently, a strong interaction exists between the TNT and the pyrene backbone of PTBE, and this interaction could be ascribed to a π–π interaction. Hence, PTBE-TNT is a π–π stacking product of PTBE and TNT. Energy profiles The Gibbs free energy profiles of the reaction process are calculated to elucidate the interaction process of PTBE and TNT17, 48. All energy data given in Figure 2 are relative energies based on the sum of reactant energies (the raw data are given in Table S1). The Gibbs free energies of M2 (9.53 kcal/mol) are slightly lower than that of M1 (11.17 kcal/mol). This finding is consistent with the result of geometry analysis, that is, M2 is more stable than M1. Both of the Gibbs free energies of M1 and M2 are higher than the energy sum of the reactants. Hence, M1 and M2 are slightly unstable, difficult to generate, and not the ideal interaction products of PTBE and TNT. Moreover, the intermediate complexes of M1 and M2 are hydrogen bond product of PTBE and TNT as shown in Figure 2, which decrease the system energy to approximately −1 kcal/mol. The hydrogen bonds are formed between the nitrogroup of TNT and the secondary amine group of PTBE and the corresponding bond lengths are around 2.3Å, manifesting they are typical hydrogen bond interactions. These hydrogen bond intermediates are more thermodynamically favorable configurations than M1 and M2 in view of their lower energies. The transition states have also been located and confirmed to connect the intermediate and the


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product by performing IRC calculations. Although the energy barriers from the intermediates to the transition states (17.01 and 15.64 kcal/mol for M1 and M2, respectively) are not considerably high to cross over, the reverse energy barriers from the Meisenheimer complexes to the transition states (4.44 and 5.14 kcal/mol for M1 and M2, respectively) are remarkably mild. Hence, the generated Meisenheimer complexes would prefer a reverse reaction process to decrease the energy of the system. By contrast, the π–π stacking product PTBE-TNT is exergonic. The combination of PTBE and TNT through the π–π interaction decreases the system energy by 7.21 kcal/mol, making it a spontaneous process. The Gibbs free energy profile manifests PTBE-TNT is a more rational interaction product for PTBE and TNT than the hydrogen bond products (the intermediate complexes mentioned above), which only decrease the system energy by about 1 kcal/mol. Though the hydrogen bond products are more thermodynamically favorable than Meisenheimer complexes, they are far less stable compared with PTBE-TNT, proving the hydrogen bond product is not an ideal product either, so we only compare PTBE-TNT with Meisenheimer complexes in the following parts. Therefore, instead of forming Meisenheimer complexes and hydrogen bond products, the energy-friendly molecule PTBE-TNT should be a rational and favored product of PTBE and TNT interaction. Electronic spectra and sensing mechanism The electronic emission spectra are recorded to study the fluorescence property of PTBE, M1, M2, and PTBE-TNT and to evaluate the current calculation levels49, displayed in Figure 3. The emission wavelengths of the lowest-lying singlet excited state for each molecule are marked, and


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the corresponding experimental values are given in the parenthesis. The emission peak of PTBE is centered at 428 nm, which ideally copies the experimental data (425 nm). This finding confirms the rationality of the current calculation level. Notably, the emission peaks of M1 and M2 are not quenched at all, which is inconsistent with the experimental phenomenon, the luminescence quenching. In contrary, the emission peak of PTBE-TNT is totally quenched, which is consistent with the experiment result. The electronic spectrum analysis further confirms the validity of PTBE-TNT, rather than the Meisenheimer complexes, as the interaction product of PTBE and TNT. The results also demonstrate the reliability of the sensing mechanism calculated in the following part. The sensing mechanism would no longer be a FRET process, as supposed by Mosca et al.,23 considering the new interaction product. Hence, a new sensing mechanism is investigated. Figure 4 shows the schematic of the sensing mechanism and the primary FMOs related to it. The detailed transition data are provided in Table 1. As shown in the left column of Figure 4, the main transition that contributed to the absorption and emission spectra of PTBE is the S0−S1 transition, which is composed of the transition between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) (Table 1). The electron densities of HOMO and LUMO are both mainly localized on the pyrene group, indicating a local excitation process with a distinct ππ* transition feature. Thus, PTBE exhibits strong fluorescence.


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The π–π stacking configuration PTBE-TNT undergoes an evident PET process, as shown in the right column of Figure 4. The electronic absorption transition for PTBE-TNT is S0−S4, which could be attributed to the local excitation of the PTBE moiety. The S0−S4 transition is mainly contributed by HOMO→LUMO+3, the electron densities of which are both distributed on the PTBE unit. The almost identical absorption bands (346.2 and 344.4 nm for PTBE and PTBETNT, respectively) also confirm that the S0−S4 transition could be mainly ascribed to the local excitation of PTBE. Upon excitation, PTBE-TNT undergoes ultrafast internal conversion to transfer electrons from the S4 state to the S1 state. The electron density of LUMO is localized on the TNT part, whereas the electron density of HOMO is on the PTBE part as shown, which contributes to the S1-S0 transition. Thus, the system prefers to be deactivated through a nonradiative transition process due to the complete charge separation characteristic of S1 state. The transfer of excited electrons from the donor PTBE to the acceptor TNT is a typical PET process, which leads to luminescence quenching. Therefore, the fluorescence mechanism of PTBE-TNT could be ascribed to a PET process.

Conclusions In summary, we have theoretically proposed a new sensing mechanism for the pyrene based probe PTBE for TNT detection. Based on its low Gibbs free energy, the π–π stacking product is a more rational and favorable product of PTBE and TNT than the Meisenheimer complexes, which have energy higher than those of the reactants. Furthermore, the luminescent M1 and M2


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could not explain the fluorescence quenching phenomenon, indicating that the nonluminous PTBE-TNT is a reasonable product. According to the FMOs analysis, the probe luminescence quenching upon exposure to TNT is ascribed to a PET process. Our results highlight the significance of π–π interactions in sensing nitroaromatics, providing an inspiration for designing new efficient chemosensors for explosives.

ASSOCIATED CONTENT Supporting Information. 13

C NMR spectra, RDG analysis, raw data of Gibbs free energies.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] Author Contributions Meiheng Lu and Panwang Zhou contributed equally to this work. Notes The authors declare no competing financial interest. ACKNOWLEDGEMENT This work is supported by the Science Challenging Program (JCKY2016212A501) and the National Natural Science Foundation of China (Grant No. 21533010)


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(41) Lu, T.; Chen, F. W. Multiwfn: A Multifunctional Wavefunction Analyzer. J. Comput. Chem. 2012, 33, 580-592. (42) Hunter, C. A.; Sanders, J. K. M. The Nature of Pi-Pi Interactions. J. Am. Chem. Soc. 1990, 112, 5525-5534. (43) Kim, Y.; Jun, T.; Mulay, S. V.; Manjare, S. T.; Kwak, J.; Lee, Y.; Churchill, D. G. Novel Intramolecular Pi-Pi-Interaction in a Bodipy System by Oxidation of a Single Selenium Center: Geometrical Stamping and Spectroscopic and Spectrometric Distinctions. Dalton Trans. 2017, 46, 4111-4117. (44) Yuan, K.; Zhao, R.-S.; Zheng, J.-J.; Zheng, H.; Nagase, S.; Zhao, S.-D.; Liu, Y.-Z.; Zhao, X. Van Der Waals Heterogeneous Layer-Layer Carbon Nanostructures Involving Pi···H-C-CH···Pi···H-C-C-H Stacking Based on Graphene and Graphane Sheets. J. Comput. Chem. 2017, 38, 730-739. (45) Fant, F.; De Sloovere, A.; Matthijsen, K.; Marle, C.; El Fantroussi, S.; Verstraete, W. The Use of Amino Compounds for Binding 2,4,6-Trinitrotoluene in Water. Environ. Pollut. 2001, 111, 503-507. (46) Johnson, E. R.; Keinan, S.; Mori-Sanchez, P.; Contreras-Garcia, J.; Cohen, A. J.; Yang, W. T. Revealing Noncovalent Interactions. J. Am. Chem. Soc. 2010, 132, 6498-6506. (47) Contreras-Garcia, J.; Johnson, E. R.; Keinan, S.; Chaudret, R.; Piquemal, J. P.; Beratan, D. N.; Yang, W. T. Nciplot: A Program for Plotting Noncovalent Interaction Regions. J. Chem. Theory Comput. 2011, 7, 625-632. (48) Zhou, P. W.; Hoffmann, M. R.; Han, K. L.; He, G. Z. New Insights into the Dual Fluorescence of Methyl Salicylate: Effects of Intermolecular Hydrogen Bonding and Solvation. Journal of Physical Chemistry B. 2015, 119, 2125-2131. (49) Lu, M. H.; Yang, Y. F.; Chu, T. S. Insight into the Excited-State Intramolecular DoubleProton Transfer of the 2,5-Bis(Benzoxazol-2-Yl)Thiophene-3,4-Diol: One-Step or Stepwise Mechanism? Theoretical Chemistry Accounts. 2017, 136, 7.


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Table Captions

Table 1. The detailed theoretical and experimental electronic spectra data for PTBE and PTBETNT.

Figure Captions

Figure 1. Geometric structures of optimized compounds (a) PTBE, (b) TNT, (c) M1, (d) M2, (e) front view of PTBE-TNT and (f) side view of PTBE-TNT on ground state with tinted atoms. Black: C, white: H, red: O, blue: N. The primary atoms, bond lengths and angles are marked in the figure.

Figure 2. Gibbs free energy profiles (kcal/mol) for the forming mechanism of Meisenheimer complexes M1, M2 and π-π stacking product PTBE-TNT. Primary bond lengths and angles are also labeled. IM: intermediate, TS: transition state.

Figure 3. The calculated electronic spectra of PTBE, M1, M2 and PTBE-TNT. The emission wavelengths (nm) of the lowest-lying excited state are marked and corresponding experimental values are given in the parenthesis.

Figure 4. Scheme of the different sensing mechanisms and relevant frontier molecular orbitals for PTBE (left) and PTBE-TNT (right).


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Table 1 Electronic transition

Wave length (nm)

Energy (ev)

fa

Contribb

CIc

Absorption

S0→S1

346.20

3.5813

0.7835

H→L

0.95

Emission

S1→S0

427.68

2.8990

1.1725

L→H

0.98

S0→S4

344.47

3.5993

0.3312

H→L+3

0.70

expd (nm)

PTBE

~425

PTBE-TNT Absorption a

Oscillator strength. b H, highest occupied molecular orbital (HOMO) and L, lowest unoccupied

molecular orbital (LUMO).

c

The CI coefficients are in absolute values.

d

The experimental

electronic spectra data are from Ref.23.


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Fig. 1.


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Fig. 2.


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Fig. 3.


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Fig. 4.


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TOC Graphic


22

Reconsideration of the Detection and Fluorescence Mechanism of a

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