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Theory after experiment on sensing mechanism of a newly developed sensor molecule: Converging or diverging?
Accepted Manuscript Research paper Theory after experiment on sensing mechanism of a newly developed sensor molecule: Converging or diverging? Suvendu Paul, Monaj Karar, Biswajit Das, Arabinda Mallick, Tapas Majumdar PII: DOI: Reference:
S0009-2614(17)30920-X https://doi.org/10.1016/j.cplett.2017.10.008 CPLETT 35154
To appear in:
Chemical Physics Letters
Received Date: Accepted Date:
22 August 2017 2 October 2017
Please cite this article as: S. Paul, M. Karar, B. Das, A. Mallick, T. Majumdar, Theory after experiment on sensing mechanism of a newly developed sensor molecule: Converging or diverging?, Chemical Physics Letters (2017), doi: https://doi.org/10.1016/j.cplett.2017.10.008
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Theory after experiment on sensing mechanism of a newly developed sensor molecule: Converging or diverging? Suvendu Paula, Monaj Karara, Biswajit Dasb, Arabinda Mallickc*, Tapas Majumdara,*. a
Department of Chemistry, University of Kalyani, Nadia, West Bengal-741235, India,
E-mail: [email protected] b
Department of Chemistry, Sreegopal Banerjee College, Hooghly, West Bengal -712503,
India, E-mail: [email protected] c
Department of Chemistry, Kashipur Michael Madhusudan Mahavidyalaya, Purulia, West
Bengal -723132, India, E-mail: [email protected]
Abstract Fluoride ion sensing mechanism of 3,3'-bis(indolyl)-4-chlorophenylmethane has been analyzed with density functional and time-dependent density functional theories. Extensive theoretical calculations on molecular geometry & energy, charge distribution, orbital energies & electronic distribution, minima on potential energy surface confirmed strong hydrogen bonded sensor-anion complex with incomplete proton transfer in S0. In S1, strong hydrogen bonding extended towards complete ESDPT. The distinct and single minima on the PES of the sensor-anion complex for both ground and first singlet excited states confirmed the concerted proton transfer mechanism. Present study well reproduced the experimental spectroscopic data and provided ESDPT as probable fluoride sensing mechanism.
Introduction In order to accelerate the incorporation of emerging sensor materials for new applications and to regulate the efficiency of a sensor, it is immensely important to know the exact functioning mechanism of the sensor molecule. Proper understanding of the sensing mechanism may develop ideas about the controlling factors that govern the sensing ability. Also, by monitoring the key factors, synthetic chemists can monitor the sensor efficacy to detect any ion instantly without any intricate technique. A number of theoretical mechanism explorations basically by using density functional theory (DFT) [1-5] motivated us to perform our present assay. Keeping all these in mind, we investigated and developed the mechanism behind sensing of an excellent fluoride ion sensor experimentally reported in recent past [6].
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Excited-state proton transfer (ESPT) is one of the most elementary and important photochemical phenomenon in the field of biological and physicochemical processes [7]. In recent past, excited-state intramolecular and intermolecular proton transfer reactions became an attractive and significant field of research [8-19]. Sometimes, single proton transfer is inadequate to address biological processes where multiple proton transfer reactions play key roles [20]. In many cases, excited state double proton transfer (ESDPT) mechanism is the preliminary and essential step to follow about to investigate multiple proton transfer reactions. Receptors and biological sensors, sensing through double proton centers, became the field of growing interest [21-24]. Zhao et al. studied the ESDPT mechanism of 2,5-bis(benzoxazol-2-yl)thiophene-3,4-diol and reported a new mechanism regarding controlling the stepwise ESDPT mechanism via external electric field [20] & they also made competitive studies of bis-2,5-(2-benzoxazolyl)-hydroquinone and its derivatives between ESPT and ESDPT mechanisms [25]. As well, Li et al. accounted theoretical study of the PES’s for the double proton transfer reaction of in a model DNA base pair, 7-azaindole (7AI) dimer [26], Zhang et al. detailed theoretical investigation on excited state intramolecular proton transfer (ESIPT) process of pigment yellow 101 [27], Chi et al. detailed ESDPT Mechanism of 1,3-Bis(2-pyridylimino)-4,7dihydroxyisoindole using TD-DFT Study [28]. Hung et al. made theoretical calculations on the ground and exited state double proton transfer phenomena of 2-aminopyridine catalyzed by acetic acid [29].
Scheme 1: Structure of SH2. In this paper, we theoretically investigated proton transfer mechanism between 3,3'-bis(indolyl)-4chlorophenylmethane (hereafter SH2) and F– ions. Theoretical results were compared with experimental results. ESIPT process controlled most of the earlier reported ESDPT mechanisms [2529]. However, interestingly, here we reported ESDPT mechanism guided by intermolecular proton transfer. Recently, computational studies on chromophoric scaffolds gained significant attention to
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understand the mechanism and dynamics of artificial sensor molecules. Proton abstraction by F– ions was confirmed through NMR titration in the corresponding experimental report of the present work [6] by Mallick et al. However, the details of proton transfer including quantitative extent in different electronic states were lacking. In addition, these types of detailing were not possible through experiments. Inspired by these necessities, we initiated detailed computational studies on the functioning mechanism of the developed antenna molecule (SH2).
Computational details In this study, all the computational jobs were run on Gaussian 09W program package [30] devoid of any symmetrical constrains. GaussView 5.0.8 [31] was used for visual presentation and data analysis. DFT and TD-DFT methods were utilized for electronic ground and excited state calculations respectively, assembling B3LYP with 6-311++G** [31] basis set in Acetonitrile (ε = 35.688) solvent employing conductor polarizable continuum model (CPCM) [32]. At the beginning, we optimized the sensor and sensor-anion complexes in vacuum. Then, all were optimized in acetonitrile (ACN) with CPCM solvent effect. Afterwards, the stability of the molecule/anionic complexes was verified through global minima check. Subsequently, from the optimized geometries we calculated the IR frequencies and no negative frequency were observed. Further, the interaction energies of the SH2anion complexes were corrected by calculating the basis set superposition error (BSSE) only in vacuum using counterpoise method [33-35]. The NMR spectra of SH2 and all the SH2-anion complexes in CD3CN as well as in CDCl3 were calculated using the same 6-311++G** basis set with reference to tetramethylsilane (TMS) in B3LYP/6-311++G** SCF GIAO method [36]. The molecular electrostatic potential (MEP) 3D surface and 2D contour plots were generated from the check point files of geometry optimization. Potential energy surface (PES) of SH2-2F– complex were performed along the N-H and H-F coordinates with same basis set taking 20 steps each with step size 0.1 Å for S0 and S1 states in ACN. More detailed theoretical contextures can be found elsewhere [37].
Results and discussion Interaction energies To establish SH2 as a selective F– ion sensor among other anions, interaction energies of different anionic complexes were calculated and compared. The interaction energies (Eint.) of different anionic complexes were computed according to Equation (1) and tabulated in Table 1.
Eint Ecom ESH2 2EX – ............................. (1) Where, E denotes the optimized energy and the subscript denote the corresponding species.
3
Table 1 Ground state binding energies (kJ mol–1) for SH2-2X– (1:2) complexes in vacuum and in ACN/CPCM medium obtained using B3LYP/6-311++G** method (BSSE corrected binding energies are in parenthesis). System SH2-2F
–
SH2-2Cl – SH2-2Br – SH2-2AcO
–
Vacuum
Acetonitrile
–218.7 (–199.9)
–87.7
–53.4 (–48.6)
–28.4
–29.1 (–27.4)
–15.1
–72.9 (–67.2)
–53.1
In vacuum, the binding energy for SH2-2F– complex was –218.7 kJ mol–1 whereas for its nearest neighbor, SH2-2AcO– complex was –72.9 kJ mol–1. This clearly showed the selective affinity of SH2 towards F– ion among others. Also, in ACN medium, the binding energy of SH2-2F– complex was lowest (–87.7 kJ mol–1) indicating strongest interaction between SH2 & F–. Additionally, both in vacuum and in ACN, the binding energy decreased in the order SH2-2F– > SH2-2AcO– > SH2-2Cl– > SH2-2Br–. The BSSE corrected binding energies followed the same trend. We also calculated the binding energy of 1:1 SH2-F– (– 44.6 kJ mol–1) and SH2-Cl– (–14.8 kJ mol–1) complexes in ACN. The stabilization energies of the 1:1 complexes were found half of the corresponding 1:2 SH2-anion complexes. This strongly suggests 1:2 binding interaction between SH2 and F– ions. Also it was in nice agreement with the previous experimental reporting [6] of 1:2 binding between SH2 and F– ions.
Bond length variations
From optimized geometries, we calculated the N-H and H-X bond distances of SH2 and SH2-anion complexes and tabulated in Table 2. We reported only one N-H bond length because both the N-H bond lengths in SH2 and SH2-2X– complexes were equal. Introduction of F– ions elongated the N-H bond distance of SH2 from 1.00 Å to 1.06 Å also originated new H-F bond with bond distance 1.49 Å demonstrating a strong hydrogen bonding in ACN. Upon electronic excitation from S0 to S1, the N-H bond length was further extended from 1.06 Å to 1.15 Å and evolution of a more realistic H-F bond distance of 1.29 Å. Upon transition from S0 to S1 state, the N-H bond length was lengthened about 8 % and H-F bond distance was diminished about 14.5 % for SH2-2F– complex. In S1, the equally well stretched N-H bond length of 1.15 Å and H-F distance pretty close to natural HF bond distance for SH2-2F– at both pyrrolic nitrogen centers might be addressed as excited state double proton transfer (ESDPT). There was a little change in N-H bond length of SH2 upon interaction with other anions both in vacuum as well as in ACN. So, bond distances data clearly depicted immense selectivity of SH2 towards F–.
4
Table 2 N-H and H-X distances (Å) of SH2 and SH2-2X– complexes in vacuum and in ACN/CPCM calculated using the B3LYP/6-311++G**. System
Vacuum
Acetonitrile
N-H
H-X
N-H
H-X
1.01
---
1.00
---
1.33
1.10
1.06
1.49
1.04
2.13
1.02
2.26
SH2-2Br –
1.03
2.37
1.03
2.37
SH2-2AcO –
1.05
1.66
1.03
1.74
SH2 SH2-2F
–
SH2-2Cl
–
Figure 1: Optimized structures of SH2 and SH2-2X– complexes in ACN/CPCM solvent system with B3LYP/6-311++G**. [(a) SH2, (b) SH2-2F–, (c) SH2-2Cl–, (d) SH2-2Br–, (e) SH2-2AcO– complexes] (color key: "blue : nitrogen; gray : carbon; white : hydrogen; sky blue : fluorine; green : chlorine; brown : bromine; red : oxygen". We reported only one N-H bond length as both the N-H bond lengths were equal).
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IR studies
Vibrational frequency analyses lucidly explored the excited state dynamics of hydrogen bonding earlier [38-39]. Hence, extensive theoretical frequency calculations were performed on the optimized S0 and S1 state structures of the sensor molecule SH2 and SH2-2F– complex and presented in Figure 2. It was very easy to find out the N-H stretching frequency in IR spectra of the respective systems (SH2 and SH2-2F–). N-H stretching frequencies of SH2 were found at 3656 cm–1 in S0 state and at 3624 cm–1 in S1 state. But, upon interaction with F– ions, red shifted N-H stretching frequencies were observed at 2572 cm–1 in S0 state that further shifted to 1939 cm–1 in S1 state. If hydrogen bonds were formed between SH2 and F– ions, minor red shift of N-H stretching frequency would be observed. But, proton transfer from SH2 to F– ions would generate a huge red shift of N-H frequency or the peak may vanish depending upon the extent of proton transfer. Red shifts of the N-H frequency found upon formation of SH2-2F–. The red shifts were rather excessive in S1 than in S0. Consequently, the simulated IR spectra evolved as graphical evidence for the extensive H-bonding in S0 state and double proton transfer in S 1 state.
Figure 2: IR spectra of SH2 and SH2-2F– complex with characteristic N-H stretching frequencies computed using B3LYP/6-311++G** theoretical level in ACN/CPCM solvent model (Both the N-H stretching frequencies were equal; hence one N-H stretching frequency was reported). Spectral red shift of 1084 cm–1 (30 %) from SH2 to SH2-2F– complex in S0 state justified strong Hbonding of SH2 with F– ions. More interestingly, almost 47% red shift in the stretching frequencies was recorded on excited state binding (in S1) of SH2 with two F– ions. Moreover, the differences of N-
6
H frequencies between S0 to S1 states for SH2 and SH2-2F– complex were observed to be 32 cm-1 and 633 cm-1 respectively. These outstanding differences of N-H frequencies in S0 to S1 states (for SH2 and SH2-2F–) strongly recommend two different interaction types in two different electronic states. These observations made us believe that the interactions of SH2 with two F– ions were strong hydrogen bonding type in the S0 state and excited state double proton transfer (ESDPT) type in the S 1 state.
NMR studies
NMR spectral analysis may pretend an important role to elucidate the hydrogen bonding and proton transfer mechanism. Mallick et al. [6] performed NMR titration of SH2 with tetrabutylammonium bromide (TBAF) in CDCl3 and characteristic changes of the N-H proton signal in 1H NMR spectra were recorded exclusively. During experiment, a regular shift of NMR signal of the N-H proton towards higher δ value was observed with gradual addition of TBAF in SH2 leaving other proton signals almost unchanged. After complete titration (2.1 eqv TBAF), the N-H proton signal was shifted from 7.89 ppm to 9.93 ppm indicating strong hydrogen bonding between SH2 and F– ions. Theoretical NMR signals of SH2 and SH2-2F–complex were calculated in CDCl3 as well as in CD3CN (to compare with the experimental results). With introduction of F– ions, there must be deshielding of the N-H protons and the corresponding 1H NMR signal for SH2-2F– would show a minor lowfield shift for hydrogen bonding or a colossal lowfield shift for proton transfer. In our theoretical study, the 1
H NMR signal for N-H protons of SH2 were at 7.74 ppm in CDCl3 (7.89 ppm in CD3CN). The
theoretical results were very close to the experimental results and undoubtedly justified our theoretical model to be appropriate for the present system. As found in experimental results, a single 1H NMR peak was identified for both the N-H protons with double degeneracy (Figure 3). This single doubly degenerate proton peak at 7.74 ppm clarified that both the N-H protons in SH2 were chemically equivalent as stated in the experiment. For SH2-2F–, a doubly degenerate NMR signal was found at 16.68 ppm in CDCl3 (Figure 3) leaving the other 1H NMR signals almost unchanged (16.43 ppm in CD3CN). This comprehensive low field shift (8.84 ppm) of the N-H protons of SH2 on binding with two F– ions confirmed the strong dual hydrogen bonding between SH2 and two F– ions. Finally, our theoretical 1H NMR computation fully replicated the previous experimental observations [6].
7
Figure 3: Theoretical NMR: (a) NMR spectra of SH2 and (b) SH2-2F– complex following B3LYP/6311++G**, CDCl3 solvent, CPCM solvent model, SCF GIAO method, reference TMS (31.97 ppm), tolerance 0.05 (Both the N-H moieties were chemically equivalent; hence doubly degenerate single NH proton signal was observed).
UV-Vis spectral simulation
Highest Occupied Molecular Orbital (HOMO) and Lowest Unoccupied Molecular Orbital (LUMO) characterize chemical reactivity, global softness and hardness, molecule’s kinetic stability and interaction of a molecule with other species [40]. The electronic distribution for the HOMO, LUMO and their energy differences for SH2 and SH2-2F– complex were calculated and presented in Figure 4. There were minor changes in electronic distribution between the HOMO and LUMO of SH2 except the changes of the number and position of nodes. On the contrary, changes in electronic distribution between the HOMO and LUMO of SH2-2F– complex were extensive. In the HOMO of SH2-2F– complex, electronic charge cloud was exclusively located over the bis-indole moieties denoting dual
8
hydrogen bonding between SH2 and two F– ions. However, in the LUMO of SH2-2F–, electron cloud was exclusively located over the chlorophenyl moiety of SH2 leaving the F– ions with the N-H protons almost like two detached free H-F molecules.
Figure 4: HOMO and LUMO images of SH2 (left) and SH2-2F– (right) complex in ACN/CPCM medium computed with B3LYP/6-311++G** method, the values within the figure denotes corresponding HOMO-LUMO energy gaps. These observations revealed that the interactions of SH2 with two F– ions turned out to be dual hydrogen bonding in S0 state and excited state dual proton transfer (ESDPT) in S1 state. HOMOLUMO energy gap for SH2 was 3.36 eV whereas that for SH2-2F– complex was 2.84 eV. Almost 15.5 % lowering of HOMO-LUMO energy gap from SH2 to SH2-2F– complex supported charge transfer or proton transfer phenomena from SH2 to F– ions [41]. Electronic transitions were calculated theoretically for SH2 and SH2-2F– complex and compared with experimental data. The major experimental absorptions were in well agreement with the theoretical results. In theoretical UV-Vis spectra, major absorption band of SH2 was observed near 279 nm (280 nm in experiment) due to the π to π* transition. But, in presence of F– ions, the major absorption band at 279 nm red shifted to 437
9
nm (435 nm in experiment). This remarkable red shift was due to strong dual hydrogen bonding in S 0 state.
Table 3 Absorption wavelength (λabs) (nm), major contributing orbitals and oscillator strength (f) for SH2 and SH2-2F– complex in ACN/CPCM medium at the TD-DFT/B3LYP/6-311++G** level, Experimental data in ACN medium. System
SH2
SH2-2F–
Expt(nm)[6]
λabs (nm)
425
369
H
295
301
280 435 385
H–x
L+y
f
L
0.0300
H–1
L+2
0.0159
279
H–1
L+1
0.1396
437
H
L
0.0148
365
H
L+1
0.0175
361
H
L+2
0.0361
NBO charge analysis and 2D contour plots The natural bond orbital (NBO) charge distributions for SH2 and SH2-2F– complex in S0 & S1 states were theoretically calculated. The NBO charge analyses were adopted to aid the exploration of the mechanism behind F– sensing by SH2.
Figure 5: MEP 2D contour plots of SH2 (left) and SH2-2F– (right) complex using B3LYP/6311++G** model ACN/ CPCM medium. The NBO atomic charges on each N and pyrrolic H and each F– ions of SH2 were –0.540(–0.540); +0.422(+0.422) and –1.0(–1.0) unit respectively (the data within the parenthesis denotes the NBO charges in S1 state). But, the NBO atomic charges on each N, pyrrolic H and F of SH2-2F– complex were –0.575(–0.589), +0.463(+0.465) and –0.903(–0.829) unit respectively. So, we found a flow of
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negative charge from F– ions to SH2 in S0 state that extended further in S1. Calculated NBO group charge on [SH2], –0.033 (+0.021) unit, was very close to zero. However, The NBO group charge on [SH2-2F–] was –2.032 (–2.097) unit, pretty close to a di-negative charge. A free molecule like SH2 with almost cumulative zero NBO charge and SH2-2F– complex with di-negative NBO charge both in S0 and S1 state clearly legitimized our theoretical model for the present system exactly. The NBO group charges on [S] moiety (doubly deprotonated SH2) for SH2 was –0.877(–0.823) unit. But, upon interaction with two F– ions, this negative charge was negatively magnified to –1.152(–1.369) unit. This is clearly the flow of negative charge from F– ions to receptor moiety that undoubtedly indicated the beginning of dual proton transfer in S0 state that became most prominent in S1 state. The NBO charges on each free [HF] molecules were 0.0 unit in S0 and S1. Here, the charges on each [HF] moiety of SH2-2F– were –0.44 unit in S0 state. There was a tendency of each [HF] moiety to achieve a lesser negative charge signifying the beginning of proton transfer in S0 state that may better be called a dual hydrogen bonding in S0. The tendency to achieve lesser negative charge was more extensive with –0.364 unit NBO charge on each [HF] moiety significantly pointed towards the true dual proton transfer mechanism in S1 (ESDPT). The 2D contour plot was evenly distributed along with a red border around SH2 in its free form. But for anionic complex, the contour plot got separated into two distinct parts, one over the S (doubly deprotonated SH2) moiety and another over two H-F moieties. This cleavage formation indicated the tendency of separation of the [S] and two [HF] moieties as SH2 interacted with two F– ions in S0. Comparing the NBO charge analyses and 2D contour plots of SH2 and SH2-2F– in S0 raised clear possibility of beginning of proton transfer from SH2 to F– ions in S0 state that got severely enhanced in S1 state. Hence the NBO analysis established the strong dual hydrogen bonding in S0 and excited state dual proton transfer in S 1.
Table 4 The NBO group charges on [SH2], [S], [SH2-2F–] and each [HF] of SH2 and SH2-2F– complex both in S0 and S1 states computed using B3LYP/6-311++G** model in ACN/ CPCM solvent system.
System
States
[SH2]
[S]
[SH2-2F–]
[HF]
SH2
S0
–0.033
–0.877
---
---
SH2
S1
+0.021
–0.823
---
---
SH2-2F–
S0
–0.226
–1.152
–2.032
–0.44
SH2-2F–
S1
– 0.439
–1.369
–2.097
–0.364
Molecular Electrostatic Potential (MEP) study
Molecular electrostatic potential 3D surface plots plays a central role for qualitative elucidation to assess the reactive sites (electrophilic and nucleophilic), intermolecular associations, physicochemical affairs, hydrogen bonding and proton transfer interactions [42, 43]. In the present work, the MEP 3D
11
surface plots of SH2 and SH2-2F– complex were generated from the checkpoint files of optimized geometries. The MEP plots of SH2 and SH2-2F– complex were presented in Figure 6. Molecular surface charge ranged from –7.265×10–2 to +7.265×10–2 for SH2 and –0.291 to +0.291 for SH2-2F– complex with deep red color indicating electron rich region and dark blue color denoting electron poor region respectively. A dark blue color over the two N-H proton regions of SH2 revealed their acidic nature and possibility of involvement in dual hydrogen bonding or dual proton transfer with F– ions. Conversely, in SH2-2F– complex, a faint yellow zone all over the SH2 moiety presenting as an electron rich zone that might be the result of proton transfer from SH2 to F– ions. Therefore, there was a huge probability of double proton transfer from SH2 to F– ions in S1 state (ESDPT).
Figure 6: MEP 3D surface plots of SH2 (left) and SH2-2F– complex (right) computed theoretically using B3LYP/6-311++G** in ACN/ CPCM medium.
Potential energy surfaces of S0 and S1 state In order to unfold the F– ion sensing mechanism clearly, potential energy surfaces (PES) were calculated with some predetermined and regular N-H and H-F distance variations ranging from 0.8 to 2.8 Å with step size 0.1 Å for SH2-2F– complex in S0 and S1 states (Figure 7) in ACN and presented in Figure 7. Construction of surfaces for S0 and S1 states were lengthy process due to high computational cost. Though DFT/B3LYP model for S0 state and TD-DFT/B3LYP model for S1 state were inadequate to produce précised data, previous calculations (using same model) made us confident enough to track the proton transfer mechanism [44-49]. The minimum energy was scaled as zero for SH2-2F– complex in S0 and S1 states to make our analysis easy and simple.
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Figure 7: Potential energy surface plots of SH2-2F– complex for S0 state (left) and for S1 state (right) extracted using B3LYP/6-311++G** model in ACN / CPCM medium.
The proton transfer mechanism was ensured to follow concerted pathway ignoring the stepwise mechanism from the single minima on both the PES plots. The minima for S0 surface was located at 1.09 Å N-H distance each and 1.42 Å H-F distance each whereas the same for S1 was located at 1.25 Å and 1.16 Å respectively. The potential energy barrier for S0 state was 30.58 kcal mol–1 that was relatively high in comparison to S1 state (17.12 kcal mol–1). The significant high potential energy barrier (30.58 kcal mol–1) in S0 declined the feasibility of ground state double proton transfer (GSDPT); rather this recommends the strong dual hydrogen bonding in S 0. Significant lowering of HF distances (1.42 Å to 1.16 Å) and increase of N-H distances (1.09 Å to 1.25 Å) at the minima of PES, along with considerable lowering of potential energy barrier (30.58 kcal mol–1 to 17.12 kcal mol–1) for the transition from S0 to S1 evolved as strong evidences supporting the ground state dual hydrogen bonding in S0 and excited state double proton transfer (ESDPT) in S1 as the interaction mechanism between SH2 and two F– ions in ground and excited states.
Scheme 2: Schematic presentation of the interaction of SH2 and two F– ions presenting dual hydrogen bonding in S0 and excited state double proton transfer (ESDPT) in S 1.
13
Conclusion In summary, we established theoretically the strong dual hydrogen bonding in S0 and ESDPT in S1 as parts of F– ion sensing mechanism of SH2. The theoretical 1H NMR and UV-Vis spectral studies defended the theoretical model to be apt and appropriate for the present system reproducing previous experimental data almost exactly. 1:2 (SH2:2F–) binding was confirmed from binding energy analysis, geometrical and bond parameters. Further, we established both the N-H centers of SH2 to be chemically equivalent as reported experimentally. Finally, the potential energy surface of SH2-2F– complex in S0 state indicated a little possibility of GSDPT mechanism but was rejected due to high potential energy barrier of 30.58 kcal mol–1. Rather, the ground state dual hydrogen bonding in S0 and ESDPT phenomena in S1 became most favorable interacting mechanisms between SH2 and F– ions.
Acknowledgement We sincerely acknowledge for the financial support from the Department of Science and Technology, DST, Govt. of India with project no. (DST YSS/2015/000904, dated 17-Nov-2015).
Keywords DFT, Fluoride ion sensor, HOMO-LUMO energy gap, Potential energy surface, Proton transfer.
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16
Graphical abstract
Highlights:
3,3’-bis(indolyl)-4-chlorophenylmethane (SH2) is a bis-indole type synthesized molecule. Two pyrrolic hydrogens of SH2 strongly and selectively interacts with the fluoride anion. The interaction is driven by pure dual hydrogen bonding in the ground state (S0). Theoretical Investigation revealed excited state double proton transfer (ESDPT) phenomena as a part of fluoride sensing mechanism (in S1).
17
S0009-2614(17)30920-X https://doi.org/10.1016/j.cplett.2017.10.008 CPLETT 35154
To appear in:
Chemical Physics Letters
Received Date: Accepted Date:
22 August 2017 2 October 2017
Please cite this article as: S. Paul, M. Karar, B. Das, A. Mallick, T. Majumdar, Theory after experiment on sensing mechanism of a newly developed sensor molecule: Converging or diverging?, Chemical Physics Letters (2017), doi: https://doi.org/10.1016/j.cplett.2017.10.008
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Theory after experiment on sensing mechanism of a newly developed sensor molecule: Converging or diverging? Suvendu Paula, Monaj Karara, Biswajit Dasb, Arabinda Mallickc*, Tapas Majumdara,*. a
Department of Chemistry, University of Kalyani, Nadia, West Bengal-741235, India,
E-mail: [email protected] b
Department of Chemistry, Sreegopal Banerjee College, Hooghly, West Bengal -712503,
India, E-mail: [email protected] c
Department of Chemistry, Kashipur Michael Madhusudan Mahavidyalaya, Purulia, West
Bengal -723132, India, E-mail: [email protected]
Abstract Fluoride ion sensing mechanism of 3,3'-bis(indolyl)-4-chlorophenylmethane has been analyzed with density functional and time-dependent density functional theories. Extensive theoretical calculations on molecular geometry & energy, charge distribution, orbital energies & electronic distribution, minima on potential energy surface confirmed strong hydrogen bonded sensor-anion complex with incomplete proton transfer in S0. In S1, strong hydrogen bonding extended towards complete ESDPT. The distinct and single minima on the PES of the sensor-anion complex for both ground and first singlet excited states confirmed the concerted proton transfer mechanism. Present study well reproduced the experimental spectroscopic data and provided ESDPT as probable fluoride sensing mechanism.
Introduction In order to accelerate the incorporation of emerging sensor materials for new applications and to regulate the efficiency of a sensor, it is immensely important to know the exact functioning mechanism of the sensor molecule. Proper understanding of the sensing mechanism may develop ideas about the controlling factors that govern the sensing ability. Also, by monitoring the key factors, synthetic chemists can monitor the sensor efficacy to detect any ion instantly without any intricate technique. A number of theoretical mechanism explorations basically by using density functional theory (DFT) [1-5] motivated us to perform our present assay. Keeping all these in mind, we investigated and developed the mechanism behind sensing of an excellent fluoride ion sensor experimentally reported in recent past [6].
1
Excited-state proton transfer (ESPT) is one of the most elementary and important photochemical phenomenon in the field of biological and physicochemical processes [7]. In recent past, excited-state intramolecular and intermolecular proton transfer reactions became an attractive and significant field of research [8-19]. Sometimes, single proton transfer is inadequate to address biological processes where multiple proton transfer reactions play key roles [20]. In many cases, excited state double proton transfer (ESDPT) mechanism is the preliminary and essential step to follow about to investigate multiple proton transfer reactions. Receptors and biological sensors, sensing through double proton centers, became the field of growing interest [21-24]. Zhao et al. studied the ESDPT mechanism of 2,5-bis(benzoxazol-2-yl)thiophene-3,4-diol and reported a new mechanism regarding controlling the stepwise ESDPT mechanism via external electric field [20] & they also made competitive studies of bis-2,5-(2-benzoxazolyl)-hydroquinone and its derivatives between ESPT and ESDPT mechanisms [25]. As well, Li et al. accounted theoretical study of the PES’s for the double proton transfer reaction of in a model DNA base pair, 7-azaindole (7AI) dimer [26], Zhang et al. detailed theoretical investigation on excited state intramolecular proton transfer (ESIPT) process of pigment yellow 101 [27], Chi et al. detailed ESDPT Mechanism of 1,3-Bis(2-pyridylimino)-4,7dihydroxyisoindole using TD-DFT Study [28]. Hung et al. made theoretical calculations on the ground and exited state double proton transfer phenomena of 2-aminopyridine catalyzed by acetic acid [29].
Scheme 1: Structure of SH2. In this paper, we theoretically investigated proton transfer mechanism between 3,3'-bis(indolyl)-4chlorophenylmethane (hereafter SH2) and F– ions. Theoretical results were compared with experimental results. ESIPT process controlled most of the earlier reported ESDPT mechanisms [2529]. However, interestingly, here we reported ESDPT mechanism guided by intermolecular proton transfer. Recently, computational studies on chromophoric scaffolds gained significant attention to
2
understand the mechanism and dynamics of artificial sensor molecules. Proton abstraction by F– ions was confirmed through NMR titration in the corresponding experimental report of the present work [6] by Mallick et al. However, the details of proton transfer including quantitative extent in different electronic states were lacking. In addition, these types of detailing were not possible through experiments. Inspired by these necessities, we initiated detailed computational studies on the functioning mechanism of the developed antenna molecule (SH2).
Computational details In this study, all the computational jobs were run on Gaussian 09W program package [30] devoid of any symmetrical constrains. GaussView 5.0.8 [31] was used for visual presentation and data analysis. DFT and TD-DFT methods were utilized for electronic ground and excited state calculations respectively, assembling B3LYP with 6-311++G** [31] basis set in Acetonitrile (ε = 35.688) solvent employing conductor polarizable continuum model (CPCM) [32]. At the beginning, we optimized the sensor and sensor-anion complexes in vacuum. Then, all were optimized in acetonitrile (ACN) with CPCM solvent effect. Afterwards, the stability of the molecule/anionic complexes was verified through global minima check. Subsequently, from the optimized geometries we calculated the IR frequencies and no negative frequency were observed. Further, the interaction energies of the SH2anion complexes were corrected by calculating the basis set superposition error (BSSE) only in vacuum using counterpoise method [33-35]. The NMR spectra of SH2 and all the SH2-anion complexes in CD3CN as well as in CDCl3 were calculated using the same 6-311++G** basis set with reference to tetramethylsilane (TMS) in B3LYP/6-311++G** SCF GIAO method [36]. The molecular electrostatic potential (MEP) 3D surface and 2D contour plots were generated from the check point files of geometry optimization. Potential energy surface (PES) of SH2-2F– complex were performed along the N-H and H-F coordinates with same basis set taking 20 steps each with step size 0.1 Å for S0 and S1 states in ACN. More detailed theoretical contextures can be found elsewhere [37].
Results and discussion Interaction energies To establish SH2 as a selective F– ion sensor among other anions, interaction energies of different anionic complexes were calculated and compared. The interaction energies (Eint.) of different anionic complexes were computed according to Equation (1) and tabulated in Table 1.
Eint Ecom ESH2 2EX – ............................. (1) Where, E denotes the optimized energy and the subscript denote the corresponding species.
3
Table 1 Ground state binding energies (kJ mol–1) for SH2-2X– (1:2) complexes in vacuum and in ACN/CPCM medium obtained using B3LYP/6-311++G** method (BSSE corrected binding energies are in parenthesis). System SH2-2F
–
SH2-2Cl – SH2-2Br – SH2-2AcO
–
Vacuum
Acetonitrile
–218.7 (–199.9)
–87.7
–53.4 (–48.6)
–28.4
–29.1 (–27.4)
–15.1
–72.9 (–67.2)
–53.1
In vacuum, the binding energy for SH2-2F– complex was –218.7 kJ mol–1 whereas for its nearest neighbor, SH2-2AcO– complex was –72.9 kJ mol–1. This clearly showed the selective affinity of SH2 towards F– ion among others. Also, in ACN medium, the binding energy of SH2-2F– complex was lowest (–87.7 kJ mol–1) indicating strongest interaction between SH2 & F–. Additionally, both in vacuum and in ACN, the binding energy decreased in the order SH2-2F– > SH2-2AcO– > SH2-2Cl– > SH2-2Br–. The BSSE corrected binding energies followed the same trend. We also calculated the binding energy of 1:1 SH2-F– (– 44.6 kJ mol–1) and SH2-Cl– (–14.8 kJ mol–1) complexes in ACN. The stabilization energies of the 1:1 complexes were found half of the corresponding 1:2 SH2-anion complexes. This strongly suggests 1:2 binding interaction between SH2 and F– ions. Also it was in nice agreement with the previous experimental reporting [6] of 1:2 binding between SH2 and F– ions.
Bond length variations
From optimized geometries, we calculated the N-H and H-X bond distances of SH2 and SH2-anion complexes and tabulated in Table 2. We reported only one N-H bond length because both the N-H bond lengths in SH2 and SH2-2X– complexes were equal. Introduction of F– ions elongated the N-H bond distance of SH2 from 1.00 Å to 1.06 Å also originated new H-F bond with bond distance 1.49 Å demonstrating a strong hydrogen bonding in ACN. Upon electronic excitation from S0 to S1, the N-H bond length was further extended from 1.06 Å to 1.15 Å and evolution of a more realistic H-F bond distance of 1.29 Å. Upon transition from S0 to S1 state, the N-H bond length was lengthened about 8 % and H-F bond distance was diminished about 14.5 % for SH2-2F– complex. In S1, the equally well stretched N-H bond length of 1.15 Å and H-F distance pretty close to natural HF bond distance for SH2-2F– at both pyrrolic nitrogen centers might be addressed as excited state double proton transfer (ESDPT). There was a little change in N-H bond length of SH2 upon interaction with other anions both in vacuum as well as in ACN. So, bond distances data clearly depicted immense selectivity of SH2 towards F–.
4
Table 2 N-H and H-X distances (Å) of SH2 and SH2-2X– complexes in vacuum and in ACN/CPCM calculated using the B3LYP/6-311++G**. System
Vacuum
Acetonitrile
N-H
H-X
N-H
H-X
1.01
---
1.00
---
1.33
1.10
1.06
1.49
1.04
2.13
1.02
2.26
SH2-2Br –
1.03
2.37
1.03
2.37
SH2-2AcO –
1.05
1.66
1.03
1.74
SH2 SH2-2F
–
SH2-2Cl
–
Figure 1: Optimized structures of SH2 and SH2-2X– complexes in ACN/CPCM solvent system with B3LYP/6-311++G**. [(a) SH2, (b) SH2-2F–, (c) SH2-2Cl–, (d) SH2-2Br–, (e) SH2-2AcO– complexes] (color key: "blue : nitrogen; gray : carbon; white : hydrogen; sky blue : fluorine; green : chlorine; brown : bromine; red : oxygen". We reported only one N-H bond length as both the N-H bond lengths were equal).
5
IR studies
Vibrational frequency analyses lucidly explored the excited state dynamics of hydrogen bonding earlier [38-39]. Hence, extensive theoretical frequency calculations were performed on the optimized S0 and S1 state structures of the sensor molecule SH2 and SH2-2F– complex and presented in Figure 2. It was very easy to find out the N-H stretching frequency in IR spectra of the respective systems (SH2 and SH2-2F–). N-H stretching frequencies of SH2 were found at 3656 cm–1 in S0 state and at 3624 cm–1 in S1 state. But, upon interaction with F– ions, red shifted N-H stretching frequencies were observed at 2572 cm–1 in S0 state that further shifted to 1939 cm–1 in S1 state. If hydrogen bonds were formed between SH2 and F– ions, minor red shift of N-H stretching frequency would be observed. But, proton transfer from SH2 to F– ions would generate a huge red shift of N-H frequency or the peak may vanish depending upon the extent of proton transfer. Red shifts of the N-H frequency found upon formation of SH2-2F–. The red shifts were rather excessive in S1 than in S0. Consequently, the simulated IR spectra evolved as graphical evidence for the extensive H-bonding in S0 state and double proton transfer in S 1 state.
Figure 2: IR spectra of SH2 and SH2-2F– complex with characteristic N-H stretching frequencies computed using B3LYP/6-311++G** theoretical level in ACN/CPCM solvent model (Both the N-H stretching frequencies were equal; hence one N-H stretching frequency was reported). Spectral red shift of 1084 cm–1 (30 %) from SH2 to SH2-2F– complex in S0 state justified strong Hbonding of SH2 with F– ions. More interestingly, almost 47% red shift in the stretching frequencies was recorded on excited state binding (in S1) of SH2 with two F– ions. Moreover, the differences of N-
6
H frequencies between S0 to S1 states for SH2 and SH2-2F– complex were observed to be 32 cm-1 and 633 cm-1 respectively. These outstanding differences of N-H frequencies in S0 to S1 states (for SH2 and SH2-2F–) strongly recommend two different interaction types in two different electronic states. These observations made us believe that the interactions of SH2 with two F– ions were strong hydrogen bonding type in the S0 state and excited state double proton transfer (ESDPT) type in the S 1 state.
NMR studies
NMR spectral analysis may pretend an important role to elucidate the hydrogen bonding and proton transfer mechanism. Mallick et al. [6] performed NMR titration of SH2 with tetrabutylammonium bromide (TBAF) in CDCl3 and characteristic changes of the N-H proton signal in 1H NMR spectra were recorded exclusively. During experiment, a regular shift of NMR signal of the N-H proton towards higher δ value was observed with gradual addition of TBAF in SH2 leaving other proton signals almost unchanged. After complete titration (2.1 eqv TBAF), the N-H proton signal was shifted from 7.89 ppm to 9.93 ppm indicating strong hydrogen bonding between SH2 and F– ions. Theoretical NMR signals of SH2 and SH2-2F–complex were calculated in CDCl3 as well as in CD3CN (to compare with the experimental results). With introduction of F– ions, there must be deshielding of the N-H protons and the corresponding 1H NMR signal for SH2-2F– would show a minor lowfield shift for hydrogen bonding or a colossal lowfield shift for proton transfer. In our theoretical study, the 1
H NMR signal for N-H protons of SH2 were at 7.74 ppm in CDCl3 (7.89 ppm in CD3CN). The
theoretical results were very close to the experimental results and undoubtedly justified our theoretical model to be appropriate for the present system. As found in experimental results, a single 1H NMR peak was identified for both the N-H protons with double degeneracy (Figure 3). This single doubly degenerate proton peak at 7.74 ppm clarified that both the N-H protons in SH2 were chemically equivalent as stated in the experiment. For SH2-2F–, a doubly degenerate NMR signal was found at 16.68 ppm in CDCl3 (Figure 3) leaving the other 1H NMR signals almost unchanged (16.43 ppm in CD3CN). This comprehensive low field shift (8.84 ppm) of the N-H protons of SH2 on binding with two F– ions confirmed the strong dual hydrogen bonding between SH2 and two F– ions. Finally, our theoretical 1H NMR computation fully replicated the previous experimental observations [6].
7
Figure 3: Theoretical NMR: (a) NMR spectra of SH2 and (b) SH2-2F– complex following B3LYP/6311++G**, CDCl3 solvent, CPCM solvent model, SCF GIAO method, reference TMS (31.97 ppm), tolerance 0.05 (Both the N-H moieties were chemically equivalent; hence doubly degenerate single NH proton signal was observed).
UV-Vis spectral simulation
Highest Occupied Molecular Orbital (HOMO) and Lowest Unoccupied Molecular Orbital (LUMO) characterize chemical reactivity, global softness and hardness, molecule’s kinetic stability and interaction of a molecule with other species [40]. The electronic distribution for the HOMO, LUMO and their energy differences for SH2 and SH2-2F– complex were calculated and presented in Figure 4. There were minor changes in electronic distribution between the HOMO and LUMO of SH2 except the changes of the number and position of nodes. On the contrary, changes in electronic distribution between the HOMO and LUMO of SH2-2F– complex were extensive. In the HOMO of SH2-2F– complex, electronic charge cloud was exclusively located over the bis-indole moieties denoting dual
8
hydrogen bonding between SH2 and two F– ions. However, in the LUMO of SH2-2F–, electron cloud was exclusively located over the chlorophenyl moiety of SH2 leaving the F– ions with the N-H protons almost like two detached free H-F molecules.
Figure 4: HOMO and LUMO images of SH2 (left) and SH2-2F– (right) complex in ACN/CPCM medium computed with B3LYP/6-311++G** method, the values within the figure denotes corresponding HOMO-LUMO energy gaps. These observations revealed that the interactions of SH2 with two F– ions turned out to be dual hydrogen bonding in S0 state and excited state dual proton transfer (ESDPT) in S1 state. HOMOLUMO energy gap for SH2 was 3.36 eV whereas that for SH2-2F– complex was 2.84 eV. Almost 15.5 % lowering of HOMO-LUMO energy gap from SH2 to SH2-2F– complex supported charge transfer or proton transfer phenomena from SH2 to F– ions [41]. Electronic transitions were calculated theoretically for SH2 and SH2-2F– complex and compared with experimental data. The major experimental absorptions were in well agreement with the theoretical results. In theoretical UV-Vis spectra, major absorption band of SH2 was observed near 279 nm (280 nm in experiment) due to the π to π* transition. But, in presence of F– ions, the major absorption band at 279 nm red shifted to 437
9
nm (435 nm in experiment). This remarkable red shift was due to strong dual hydrogen bonding in S 0 state.
Table 3 Absorption wavelength (λabs) (nm), major contributing orbitals and oscillator strength (f) for SH2 and SH2-2F– complex in ACN/CPCM medium at the TD-DFT/B3LYP/6-311++G** level, Experimental data in ACN medium. System
SH2
SH2-2F–
Expt(nm)[6]
λabs (nm)
425
369
H
295
301
280 435 385
H–x
L+y
f
L
0.0300
H–1
L+2
0.0159
279
H–1
L+1
0.1396
437
H
L
0.0148
365
H
L+1
0.0175
361
H
L+2
0.0361
NBO charge analysis and 2D contour plots The natural bond orbital (NBO) charge distributions for SH2 and SH2-2F– complex in S0 & S1 states were theoretically calculated. The NBO charge analyses were adopted to aid the exploration of the mechanism behind F– sensing by SH2.
Figure 5: MEP 2D contour plots of SH2 (left) and SH2-2F– (right) complex using B3LYP/6311++G** model ACN/ CPCM medium. The NBO atomic charges on each N and pyrrolic H and each F– ions of SH2 were –0.540(–0.540); +0.422(+0.422) and –1.0(–1.0) unit respectively (the data within the parenthesis denotes the NBO charges in S1 state). But, the NBO atomic charges on each N, pyrrolic H and F of SH2-2F– complex were –0.575(–0.589), +0.463(+0.465) and –0.903(–0.829) unit respectively. So, we found a flow of
10
negative charge from F– ions to SH2 in S0 state that extended further in S1. Calculated NBO group charge on [SH2], –0.033 (+0.021) unit, was very close to zero. However, The NBO group charge on [SH2-2F–] was –2.032 (–2.097) unit, pretty close to a di-negative charge. A free molecule like SH2 with almost cumulative zero NBO charge and SH2-2F– complex with di-negative NBO charge both in S0 and S1 state clearly legitimized our theoretical model for the present system exactly. The NBO group charges on [S] moiety (doubly deprotonated SH2) for SH2 was –0.877(–0.823) unit. But, upon interaction with two F– ions, this negative charge was negatively magnified to –1.152(–1.369) unit. This is clearly the flow of negative charge from F– ions to receptor moiety that undoubtedly indicated the beginning of dual proton transfer in S0 state that became most prominent in S1 state. The NBO charges on each free [HF] molecules were 0.0 unit in S0 and S1. Here, the charges on each [HF] moiety of SH2-2F– were –0.44 unit in S0 state. There was a tendency of each [HF] moiety to achieve a lesser negative charge signifying the beginning of proton transfer in S0 state that may better be called a dual hydrogen bonding in S0. The tendency to achieve lesser negative charge was more extensive with –0.364 unit NBO charge on each [HF] moiety significantly pointed towards the true dual proton transfer mechanism in S1 (ESDPT). The 2D contour plot was evenly distributed along with a red border around SH2 in its free form. But for anionic complex, the contour plot got separated into two distinct parts, one over the S (doubly deprotonated SH2) moiety and another over two H-F moieties. This cleavage formation indicated the tendency of separation of the [S] and two [HF] moieties as SH2 interacted with two F– ions in S0. Comparing the NBO charge analyses and 2D contour plots of SH2 and SH2-2F– in S0 raised clear possibility of beginning of proton transfer from SH2 to F– ions in S0 state that got severely enhanced in S1 state. Hence the NBO analysis established the strong dual hydrogen bonding in S0 and excited state dual proton transfer in S 1.
Table 4 The NBO group charges on [SH2], [S], [SH2-2F–] and each [HF] of SH2 and SH2-2F– complex both in S0 and S1 states computed using B3LYP/6-311++G** model in ACN/ CPCM solvent system.
System
States
[SH2]
[S]
[SH2-2F–]
[HF]
SH2
S0
–0.033
–0.877
---
---
SH2
S1
+0.021
–0.823
---
---
SH2-2F–
S0
–0.226
–1.152
–2.032
–0.44
SH2-2F–
S1
– 0.439
–1.369
–2.097
–0.364
Molecular Electrostatic Potential (MEP) study
Molecular electrostatic potential 3D surface plots plays a central role for qualitative elucidation to assess the reactive sites (electrophilic and nucleophilic), intermolecular associations, physicochemical affairs, hydrogen bonding and proton transfer interactions [42, 43]. In the present work, the MEP 3D
11
surface plots of SH2 and SH2-2F– complex were generated from the checkpoint files of optimized geometries. The MEP plots of SH2 and SH2-2F– complex were presented in Figure 6. Molecular surface charge ranged from –7.265×10–2 to +7.265×10–2 for SH2 and –0.291 to +0.291 for SH2-2F– complex with deep red color indicating electron rich region and dark blue color denoting electron poor region respectively. A dark blue color over the two N-H proton regions of SH2 revealed their acidic nature and possibility of involvement in dual hydrogen bonding or dual proton transfer with F– ions. Conversely, in SH2-2F– complex, a faint yellow zone all over the SH2 moiety presenting as an electron rich zone that might be the result of proton transfer from SH2 to F– ions. Therefore, there was a huge probability of double proton transfer from SH2 to F– ions in S1 state (ESDPT).
Figure 6: MEP 3D surface plots of SH2 (left) and SH2-2F– complex (right) computed theoretically using B3LYP/6-311++G** in ACN/ CPCM medium.
Potential energy surfaces of S0 and S1 state In order to unfold the F– ion sensing mechanism clearly, potential energy surfaces (PES) were calculated with some predetermined and regular N-H and H-F distance variations ranging from 0.8 to 2.8 Å with step size 0.1 Å for SH2-2F– complex in S0 and S1 states (Figure 7) in ACN and presented in Figure 7. Construction of surfaces for S0 and S1 states were lengthy process due to high computational cost. Though DFT/B3LYP model for S0 state and TD-DFT/B3LYP model for S1 state were inadequate to produce précised data, previous calculations (using same model) made us confident enough to track the proton transfer mechanism [44-49]. The minimum energy was scaled as zero for SH2-2F– complex in S0 and S1 states to make our analysis easy and simple.
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Figure 7: Potential energy surface plots of SH2-2F– complex for S0 state (left) and for S1 state (right) extracted using B3LYP/6-311++G** model in ACN / CPCM medium.
The proton transfer mechanism was ensured to follow concerted pathway ignoring the stepwise mechanism from the single minima on both the PES plots. The minima for S0 surface was located at 1.09 Å N-H distance each and 1.42 Å H-F distance each whereas the same for S1 was located at 1.25 Å and 1.16 Å respectively. The potential energy barrier for S0 state was 30.58 kcal mol–1 that was relatively high in comparison to S1 state (17.12 kcal mol–1). The significant high potential energy barrier (30.58 kcal mol–1) in S0 declined the feasibility of ground state double proton transfer (GSDPT); rather this recommends the strong dual hydrogen bonding in S 0. Significant lowering of HF distances (1.42 Å to 1.16 Å) and increase of N-H distances (1.09 Å to 1.25 Å) at the minima of PES, along with considerable lowering of potential energy barrier (30.58 kcal mol–1 to 17.12 kcal mol–1) for the transition from S0 to S1 evolved as strong evidences supporting the ground state dual hydrogen bonding in S0 and excited state double proton transfer (ESDPT) in S1 as the interaction mechanism between SH2 and two F– ions in ground and excited states.
Scheme 2: Schematic presentation of the interaction of SH2 and two F– ions presenting dual hydrogen bonding in S0 and excited state double proton transfer (ESDPT) in S 1.
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Conclusion In summary, we established theoretically the strong dual hydrogen bonding in S0 and ESDPT in S1 as parts of F– ion sensing mechanism of SH2. The theoretical 1H NMR and UV-Vis spectral studies defended the theoretical model to be apt and appropriate for the present system reproducing previous experimental data almost exactly. 1:2 (SH2:2F–) binding was confirmed from binding energy analysis, geometrical and bond parameters. Further, we established both the N-H centers of SH2 to be chemically equivalent as reported experimentally. Finally, the potential energy surface of SH2-2F– complex in S0 state indicated a little possibility of GSDPT mechanism but was rejected due to high potential energy barrier of 30.58 kcal mol–1. Rather, the ground state dual hydrogen bonding in S0 and ESDPT phenomena in S1 became most favorable interacting mechanisms between SH2 and F– ions.
Acknowledgement We sincerely acknowledge for the financial support from the Department of Science and Technology, DST, Govt. of India with project no. (DST YSS/2015/000904, dated 17-Nov-2015).
Keywords DFT, Fluoride ion sensor, HOMO-LUMO energy gap, Potential energy surface, Proton transfer.
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Graphical abstract
Highlights:
3,3’-bis(indolyl)-4-chlorophenylmethane (SH2) is a bis-indole type synthesized molecule. Two pyrrolic hydrogens of SH2 strongly and selectively interacts with the fluoride anion. The interaction is driven by pure dual hydrogen bonding in the ground state (S0). Theoretical Investigation revealed excited state double proton transfer (ESDPT) phenomena as a part of fluoride sensing mechanism (in S1).
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