International Journal of Photonics and Optical Technology Vol ...Thus picric acid can be recognized photophysically by this BBIM-ethane salt due to RET based fluorescence quenching

International Journal of Photonics and Optical Technology Vol. 3, Iss. 3, pp: 4-12, Sept. 2017

Copyright © IJPOT, All Rights Reserved Page 4

Fluorescent Bis(benzimidazolium) ethaneborontetrafluoride: A ppb Level Sensor for Picric Acid Explosives Tandrima Chaudhuri1*, Chhanda Mukhopadhyay2, Sabari Ghosh2, Animesh Karmakar1 1Department of Chemistry, Dr. Bhupendranath Dutta Smriti Mahavidyalaya, Burdwan-713407, India 2Department of Chemistry, University of Calcutta, 92 APC Road, Kolkata-700009, India [email protected] (Received 24th August, 2017; Revised 04th September 2017; Accepted 21st September, 2017; Published: 30th September, 2017) Abstract: Highly photoluminescent white light emitting Bis(benzimidazolium)ethaneborontetrafluoride (BBIM-ethane salt) demonstrated as a selective sensory material for the trace detection of (20 ppb) picric acid (here after N4) among the other treated nitroaromatic and non-nitroaromatic compounds in acetonitrile medium via energy transfer based fluorescence quenching mechanism. 1H NMR study, Cyclic voltammetry, Monte Carlo simulation and FMO as well as NBO calculation reveal that nitroaromatic compounds (NACs) (N1 – N6, except N4) induce formation of H-bonded adduct with the BBIM-ethane salt causing fluorescence quenching of NAC. However, N4 can be recognized via deprotonation of strongly acidic phenolic -OH proton followed by anion exchange with the BBIM-ethane salt, giving a new emitting species that emits in longer wavelength region. Keywords: Picric acid sensor, ppb level, Monte Carlo, FMO, Explosive sensing. 1. INTRODUCTION The colour change in naked eye or upon UV irradiation due to strong H-bonding interaction or deprotonation followed by H-bonding interaction among organic molecules is a promising mode for the selective detection of various analytes [1]. Benzimidazolium salt based rotaxane or pseudo-rotaxane has a profound background of forming molecular machine via H-bonding with crown ether of appropriate cavity size [2]. Benzimidazolium salts are also used as good fluorophore for sensing of transition metal ions [3]. Picrate derivative of imidazole based fluorophores [4] are known but imidazolium salt based sensors of nitro aromatic compounds (NACs) especially of picric acid are scanty [5]. Picric acid sensors are mainly of two types based on mode of interaction with it through (a) the electron deficient phenyl ring [6] and (b) the acidic hydroxyl group [7]. But the former type of probes favour donor–acceptor complex formation for all nitro aromatics, thus lack good selectivity when picric acid is part of a mixed analyte containing other nitro aromatics. If those interfering entities do not have an acidic hydroxyl group, the latter type shows high selectivity for picric acid among nitro aromatic explosives. On the other hand, if other sensitive acidic components are present in the mixed analyte, their selectivity is severely affected. For example, nitrobenzoic acid or degradation of picric acid leads to possible acidic products,

such as dinitrophenol, nitrophenol, and phenol [8] which may affect the selective detection of picric acid. Therefore, probes that selectively identify picric acid from both electron deficient nitro aromatics and other acidic components are certainly advantageous. Fig. 1: Structure of BBIM-ethane salt and its non-mirror symmetric absorption-emission spectra in acetonitrile medium. Bis(benzimidazolium)ethaneborontetrafluoride (Figure 1) is a white light emitting fluorophore soluble only in acetonitrile medium form stable ground state isosbestic with six different NACs (Fig. S1) viz. 4-Nitrophenol (N1), 2,4-Dinitrophenol (N2), 2,5-Dinitrophenol (N3), picric acid (N4), 2,4-Dinitrotoluene (N5), 3,5-Dinitrobenzoic acid (N6) and three non-nitro aromatic compounds: 2-Iodo benzoic acid (7), 1-Bromo-4-chlorobenzene (8) and 3-Chloro-4-fluoro aniline (9) in this study. This BBIM-ethane salt was synthesized following our reported procedures [9]. All six NACs show massive quenching of fluorescence of BBIM-ethane while non-NACs demonstrate nominal quenching. However, addition of N4 to BBIM-ethane results in the appearance of a new peak in the longer wavelength region. Relative peak intensity of new peak increases with further addition of N4, though a quenching of fluorescence results in both peaks. Ultimately, N4 addition reflects > 96% fluorescence quenching of BBIM-ethane in acetonitrile. This dual emission clearly indicates simultaneous existence of two emitting

250 300 350 400 450 500

0.00.5

1.01.52.02.5

Normalised Fluorescence Inte

nsityWavelength (nm) 0

10002000300040005000

Normalised Absorbance



species those interact with picric acid. Relative peak ratio also indicates that the newly generated emitting species still exists after complete quenching of emission of BBIM-ethane. Thus BBIM-ethane can specifically as well selectively sense picric acid (N4) differentiating from other nitro aromatic and non-nitro aromatic compounds on the basis of deprotonation of strongly acidic phenolic -OH group followed by anion exchange from benzimidazolium salt to form picrate-adduct via RET based quenching. However, all other NACs interact with BBIM-ethane in same 1:2 ratio simply via H-bonding between oxygen centres of NO2-group with N-H protons of benzimidazolium unit. Thus picric acid can be recognized photophysically by this BBIM-ethane salt due to RET based fluorescence quenching as well as formation picrate adduct as new emitting species in acetonitrile. However, 1H NMR study, cyclic voltammetry, Monte Carlo simulation and NBO as well as FMO calculation divulge this fluorescence off is due to formation of multiple H-bonds between BBIM-ethane and other NACs but formation of simultaneously emitting picrate-BBIM-ethane adduct that emits in the longer wavelength region for N4. 2. EXPERIMENTAL A. MATERIALS AND METHODS Acetonitrile used as solvent was of HPLC grade. The six well known nitroaromatic compounds (NAC): 4-Nitrophenol (N1), 2,4-Dinitrophenol (N2), 2,5-Dinitrophenol (N3), Picric acid (N4), 2,4-Dinitrotoluene (N5), 3,5-Dinitrobenzoic acid (N6), and two non-nitro aromatic compounds: 2-Iodio benzoic acid (7), 1-Bromo-4-chlorobenzene (8) and 3-Chloro-4-fluoro-aniline (9) were purchased from Aldrich. The concentration of Bis(benzimidazolium)ethane salt (BBIM-ethane) was made in the range 10−4 M to10−5 M in all the spectral measurements. The concentration of NACs and non-NACs were of the order of ~10−4 M. B. INSTRUMENTS USED The absorption (UV-Vis) spectral measurements were performed with a Shimadzu UV 1800 spectrophotometer fitted with an electronic temperature controller unit (TCC –240 A). The steady state fluorescence emission and excitation spectra were recorded with a Hitachi F-4500 spectrofluorometer equipped with a temperature controlled cell holder. Temperature was controlled within ± 0.1 K by circulating water from a constant temperature bath (Heto Holten, Denmark). 1H NMR spectra were recorded on Bruker 300 MHz spectrometer at 298 K in CD3CN. The cyclic voltammograms were recorded with an electrochemical workstation of CH Instrument Inc. model CHI600D using a three electrode system. A platinum microelectrode was used as the working electrode. An Ag/AgCl (acetonitrile) was used as the reference electrode along with a platinum wire served as the counter electrode and as an electrolyte 0.1M TBA(HSO4) was used. C. MONTE CARLO SIMULATIONS Simulations were performed using Spartan’14 molecular modelling software from Wavefuntion Inc. (Irvine, CA, USA). Using Monte Carlo simulation [13b], global minima search for

all the optimized complexes neglecting solvent were performed using Merck molecular force-field calculations (MMFF). FMO and Wiberg NBO calculations were done in DFT/MPWPW91/6-31G single point calculation using the software Gaussian'09 Linax version. D. SYNTHESIS OF BBIM-ETHANE SALT The synthesis of bis-benzimidazole compounds have been previously reported by our group [9]. The salts were obtained directly from these bis-benzimidazoles by adding an equimolar amount of the corresponding aqueous fluoroboric acid and crystallization from acetonitrile-diethyl ether [2b]. 1H NMR (CD3CN): δ 7.87-7.83 (m, 4H, Ar H), 7.67-7.63 (m, 4H, Ar H) and 3.76 (s, 4H, CH2). 13C NMR (CD3CN): δ 150.6, 130.6, 126.8, 114.1 and 23.8. FT-IR (cmˉ1): 3309.1, 1628.0, 1572.9, 1462.8, 1377.2, 1301.5, 1263.9, 1226.3, 1091.6, 765.3, 670.4, 621.7 and 447.4. Anal. calcd. for C16H16B2F8N4 : C: 43.88; H: 3.68; N: 12.79 and found C: 43.88; H: 3.66; N: 12.80. GC-MS (EI+): 438, 399. Melting point (CH3CN-Et2O): 276 °C. Yield: 90 % (394 mg, colourless crystals). 3. RESULTS AND DISCUSSIONS A. PHOTOPHYSICAL STUDIES BBIM-ethane containing borontetrafluoride as counter anion (Fig.1) shows a non-mirror symmetric absorption-emission spectrum in acetonitrile. BBIM-ethane emits sharply at 300 nm with a broad hump centred at 375 nm on exciting at 260 nm. Thus the wavelength range covered under the emission spectra enters into visible region upto 500 nm. The interaction of NAC with BBIM-ethane is investigated photophysically. Incremental addition of BBIM-ethane upon all six NACs (N1 to N6) and non-NACs (7 to 9) form well established absorption isosbestic on interacting with low concentration of BBIM-ethane in acetonitrile [Table 1, Fig. 2(A-C)]. Table 1. Ground state photophysical parameters and excited state binding constants. Compounds interacting with BBIM-ethane Absorption isosbestic point at wavelength (nm) Binding constant (k) Binding sites(n) in NAC N1 224.2, 232.6, 280.6 1.17×105 1.21 N2 268.27, 280.1 2.13×107 1.67 N3 213.8, 232.1, 245.2 3.41×107 1.75 N4 267.5, 273.2, 274.1, 280.1 3.37×109 2.02 N5 274.4, 280.2 3.51×107 1.72 N6 213.1, 260.4, 284.8 1.61×107 1.68 7 208.3, 248.4, 282.4 - - 8 235, 218.5 9 282.8, 250.9 - -



With the decreasing absorbance of N1 (at 308.0 nm), absorption maxima of BBIM-ethane (at 269.5 nm) increases monotonously giving rise to a set of isosbestic at the juncture (Fig. 2A). Similarly for N6, absorbance at 228.5 nm decreases with increasing that of BBIM-ethane (shown in Fig. 2B inset). But on reversing the titration i.e., when analyte (any NACs or non-NACs) is added to BBIM-ethane solution a monotonous increment of absorbance throughout the range of BBIM-ethane was observed (Fig. 2[D] and Fig. S3a). On addition of N4, as it absorbs in visible region (400-500nm), colourless BBIM-ethane solution became yellow in colour (Fig. 2D inset). But this may be the colour of N4 itself. No other NAC addition to BBIM-ethane cause any visible colour change (Fig. S3b). Job’s plot (Fig. 2E) determined the mole ratio of BBIM-ethane/NAC adduct is 1:2 in the ground state.

Fig. 2: Absorption isosbestic of the NACs and non-NACs with BBIM-ethane in Acetonitrile. Concentration of BBIM-ethane (µM): 0.00, 2.71, 5.18, 7.43, 9.50, 11.4, 13.1, 14.8, 16.3, 17.7, 19.0, 20.2, 21.4 and 22.45 at a fixed concentration of [A] N1 (166µM), [B] N6 (144.5µM) and [C] compound 7 (137µM) solution in Acetonitrile. [D] Ground state titration of BBIM-ethane (110µM) with N4 in acetonitrile. Concentration of N4 (µM): 0, 19, 38, 57, 75, 92 in acetonitrile. Inset is the colour change just after addition of excess equivalents of N4 to BBIM-ethane (1mM) in acetonitrile medium. [E] Job’s plot of BBIM-ethane/N5 system at 300 nm. Significant effects have been observed in the excited state interaction of N4 with BBIM-ethane. Enhancement of emission intensity at 300 nm (emission maxima of BBIM-ethane) was observed (in Fig. S4a) on addition of very low concentration (~ 10-8 mol.dm-3) of N4 to BBIM-ethane (~ 10-5 mol.dm-3). But for addition of same concentration of N5, nominal quenching of BBIM-ethane emission was observed (Fig. S4b). The fluorescence enhancement on addition of 10-8 mol.dm-3 N4 solution, clearly indicate formation of new adduct with BBIM-ethane in case of N4. Limit of detection [6f, 7d, 7g] of N4 by BBIM-ethane in excited state was 20 ppb (Fig.3a). The result indicates that the benzimidazolium-type compound BBIM-ethane has better sensitivity than most of the small organic functional compounds for picric acid (N4) reported till date [4,7, 10]. Excited state interaction of all six NACs (N) and non-NACs (7 to 9) with BBIM-ethane were studied at almost equimolar concentration range (~ 10-5mol.dm-3) also. Here also N4 interacts with BBIM-ethane salt in totally different way. BBIM-ethane reveals rapid quenching of fluorescence with all six NACs (One of which is shown in Fig. 3b). Picric acid (N4) among the six NACs diminishes the fluorescence of BBIM-ethane with highest efficiency (almost 97% for addition of same 13 equivalent NAC in 11µM BBIM-ethane)

[A] 0.00µM 22.45µM BBIM-ethane

[C] 0.00µM 22.45µM BBIM-ethane

[B] 0.0 5.0µ 10.0µ 15.0µ 20.0µ 25.0µ 30.0µ0.150.20

0.250.300.350.400.450.50

A 269.5 nm/A 228.5 nm

[BBIM-ethane] M

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.90.140.160.18

0.200.220.240.260.280.30

Absorbance

at 300 nmMole Ratio of (BBIM-ethane/N5) [E]

[D] BBIM-ethane BBIM-ethane/

N4 92 µM 0.0 µM N4



(Fig.3c). N5 shows ~90% quenching efficiency while non-NACs show negligible quenching (Fig. 3c). Thus the three non-NACs (7 to 9) used in this study show nominal quenching effect in excited state interaction though all of them contain heavy atoms. Among the all six NACs picric acid (N4) shows a profound highest quenching efficiency and highest excited state binding constant (3.37×109) (table-1, Fig. 3d). Beside that addition of picric acid (N4) upon BBIM-ethane, nature of emission spectra was totally changed. A dual emission peak of almost equal intensity appeared at 300 and 386 nm. The slightly most intense new peak at 386 nm regularly decreases along with that of 300 nm peak after each addition of N4 and shows a regular red shift. Ultimately with addition of large excess N4, emission peak at 300 nm is almost vanished and the red shifted longer wavelength peak appeared at 397nm. The relative fluorescence intensity ratio at 397 nm to 300 nm with increasing N4 concentration also increases (Fig. 3e inset). This may be due to formation of a new emitting species at the cost of interaction of BBIM-ethane with N4. Being most acidic among the NACs used in this study, picric acid easily forms picrate in presence of borontetrafluoride ion in medium. Consequently it is the picrate that interacts with BBIM-ethane salt. Thus BBIM-ethane can specifically and selectively sense N4 among all other treated NACs in acetonitrile via formation of longer wavelength emitting picrate derivative of BBIM-ethane.

Fig. 3: Excited state interaction of different NACs with BBIM-ethane in Acetonitrile medium. (a) Fluorescence intensity of BBIM-ethane (11µM) at 300nm as a function of N4 concentration. Inset: showing the calculation of detection limit of N4. (b) Concentration of N5 (µM): 0.00, 17, 33.5, 49.6, 65.4, 80.8, 95.8, 110.5,125, 139, 152.5 and 166 at a fixed concentration (11µM) of the BBIM-ethane solution in Acetonitrile. (c) Quenching efficiency bar diagram comparing all six NACs and three non-NACs for fixed concentration of BBIM-ethane (11µM) and 13 equiv. analyte in each case at λem = 300nm, λex = 260 nm. (d) Stern-Volmer plot of BBIM-ethane/N4 system. Inset: Binding constant plot. (e) Addition of N4 (legend shows the concentration) at a fixed concentration (11µM) of the BBIM-ethane solution in Acetonitrile. λex = 260 nm.

0.0000 0.0005 0.0010 0.0015 0.0020 0.0025 0.0030 0.00353300

36003900420045004800 I 300n

m

N4 concentration in equivalents

(a) DL= 1.1×10-5 mol. L-1 ×0.0018 equiv. = 0.00198×10-5 mol. L-1 = 1.98×10-8 mol. L-1 = 20ppb

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150020002500300035004000

Fluorescence intensity

Wavelength (nm)

(b) [N5] 0.00µM 166µM

BBIM-ethane

N1 N2 N3 N4 N5 N6 7 8 90.00.1

0.20.30.40.5

(F o- F)/F 0 Analytes

(c)

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304050607080

F 0 /F[N4] µM

(d) -4.6 -4.4 -4.2 -4.0 -3.8 -3.60.00.51.01.52.0

log [(F 0-F)/F] log[N4](e)

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Fluorescence intensity

Wavelength (nm)

11µM BBIM-ethane +19µM N4 +38µM N4 +57µM N4 +75µM N4 +92µM N4 +110µM N4 +126µM N4 +143µM N4 +159µM N4 +175µM N4 +190µM N4375 397 0 25 50 75 100 125 150 175 20001

23456

F 397nm /F 300 nm [N4] µM300

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Fig. 4: (a) Spectral overlap of the absorption spectra of N4 emission spectrum of BBIM-ethane (blue line). (b) Fluoresof BBIM-ethane for N4 under different λex. (c) Visual color upon subjecting a 1mM BBIM-ethane solution in acetonitri20 equiv of six different NACs (N1 to N6) in acetonitrile in roand under UV irradiation of 366nm (bottom). B. FLUORESCENCE QUENCHING MECHANISThe good selectivity and high sensitivity of for the detection of N4 may be related to the quenching mechanism. The observed fluorescencfor N4 is attributed to the energy transfer from pπ-electron rich BBIM-ethane to ground-sta

(a)

200 220 240 260 280020

406080100

Quenching efficiency (%)

Excitation wavelength (nm)(b)

(c) BBIM- N4 N5 N6 N1 N2 N3 BBIM- N4 N5 N6 N1 N2 N3

International Journal of Photonics andVol. 3, Iss. 3,

(red line) with the luorescence quenching color change observed tonitrile to addition of le in room light (upper) NISM. y of BBIM-ethane to the fluorescence rescence quenching from photo excited state electron-

deficient N4, which is further confirmoverlap of the emission spectrum ofabsorption spectrum of N4 in the range4a). Notably, the fluorescence titratiethane by addition of 13 equiv of wavelengths (200−280 nm) are investigThe quenching efficiency value ofchange (Fig 4b). This means that the dintensity of BBIM-ethane is not due tenergy transfer mechanism proposed aresults. Because there is no spectral overlap of N5 and the emission of BBIM-ethmain quenching mechanism for N5 shdue to H-bonding based equilibrium foThis is confirmed by the result of UV tiBBIM-ethane salt solution as wmeasurement at different concentrationwell-known that the energy transfer is a11]. N4 can interact with the BBIsurrounding it and forming a picrate deBBIM-ethane and its picrate derivativsimultaneously giving dual emission (NACs only can quench the emission ointeracting with it. Thus, compared to selective as well as specific to quench ethane. C. VISUAL DETECTION OF PA IN SLIGHT. The visual detection of picric acid UV light is carried out and shown in Fthe solution of fluorophore BBIMchanges significantly on addition of NACs is added, the fluorescence completely quenched. But for N4, thei.e. almost nothing is emitted, whichabove results. For the highly selective detection oonly a few reports about the DL valuesame detection level as reported hereand the sensitivity here is better than t[4, 5, 7, 10] Therefore, acetonitrile BBIM-ethane can be used to visuallyof picric acid with high sensitivity. D. 1H NMR STUDY Further, to investigate the interactiowith NACs and non-NACs, we dimentional 1H NMR studies of BBIMethane/7 adducts (1:2) in CD3CN. Roy1H NMR shift of a tris-imidazoliuminteracting with picric acid (N4) wheinstead of picric acid. But as fluorophowater soluble wet N4 cannot be usedhas to be stored always under water)mode of interaction of NACs with BB(very similar to N4 and also soluble in The 1H NMR spectrum of BBIM-ethwell separated peaks for different pro

(a.u.) 280

N3 N3

tonics and Optical Technology l. 3, Iss. 3, pp: 4-12, Sept. 2017

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firmed by a large spectral m of BBIM-ethane with the range of 270 to 500 nm (Fig. titrations of 11µM BBIM- of N4 at various excitation vestigated. lue of N4 has no significant t the decrease in fluorescence t due to masking by N4. The osed above is proved by the verlap between the absorption ethane (Figure S4c), the should be static and that um formation in ground state. UV titration of N5 toward. as well as by life time tration of N5 (Fig. S4D). It is fer is a long-range process [7, BBIM-ethane fluorophore derivative. Then both free erivative emit in the medium ssion (Fig. 3e). But all other sion of BBIM-ethane salt on ed to other NACs, N4 is more uench the emission of BBIM-IN SOLUTION UNDER UV c acid (N4) in solution under n in Figure 4c. The colour of BIM-ethane in acetonitrile n of N4. When 20 equiv of cence of BBIM-ethane is , the solution become black which is consistent with the ction of picric acid, there are L value around 20 ppb. The here is unparallel [4, 7, 10] than that reported previously itrile solution of compound sually detect instant presence eraction of the BBIM-ethane we carried out the one-BIM-ethane/N5 and BBIM-. Roy et al [5] demonstrated olium based fluorophore on where N4 exists as picrate rophore BBIM-ethane is not used for this NMR study (N4 water), rather to identify the BBIM-ethane, N5 system ble in acetonitrile) is selected. ethane showed clean and nt protons (Fig. 5 and S6a).



The methyl group of N5 shifted downfield from δ 2.69 to δ 2.7. Two aromatic protons of N5 shifted from δ 7.71 to δ 7.714 and from δ 8.41- δ 8.38 to δ 8.42 - δ 8.39 (shown in Fig. S6b and S6c) while the aromatic proton between the two nitro groups remained constant at δ 8.75. All protons of the bis-benzimidazolium salt (BBIM-ethane) remain fixed at δ 7.87-7.83, δ 7.67-7.63 and δ 3.76 on interacting with N5. Only the peak for CH2-protons at δ 3.76 get splitted (Fig. 5 inset) on interacting with N5 indicating definite interactions between BBIM-ethane and N5. It seems that the two methylene protons of the CH2 group lie in two different environments leading to such splitting in adduct. After interaction of compound 7 and the bis-benzimidazolium salt (BBIM-ethane) all the aromatic protons of 7 shifts upfield (shown in Figure S6d and S6e). The protons of the BBIM-ethane at δ 7.87-7.83, δ 7.67-7.63 shifted upfield to δ 7.86-7.81, δ 7.66-7.62 and the methylene protons at δ 3.76 remain unchanged at δ 3.76 in presence of compound 7. Fig. 5: 1H NMR spectra of BBIM-ethane, merged with BBIM-ethane/N5 (1:2) adduct in CD3CN at 298K. The behaviour of BBIM-ethane/N5 system varies considerably from the BBIM-ethane/7 system as observed from their 1H NMR behaviour. The former system gets most of the protons to shift downfield while the opposite happens when compound 7 is present. The presence of three sets of lone pairs of electrons on the iodine atom in compound 7 makes a very different set of environment as compared to N5. In these systems, charge transfer complexes do not occur since the BBIM-ethane system is an electron deficient species as also N5. However, N5 is also electron deficient due to the presence of two nitro groups. E. CYCLIC VOLTAMOMETRY The electrochemical studies using cyclic voltammetric technique were performed to evaluate the redox potentials of the NAC, N3, BBIM-ethane and their adduct in acetonitrile containing 0.1M TBA(HSO4) as electrolyte. The data is used to verify the possibility of change in reduction potential of NAC on interacting with BBIM-ethane in the mixture. The current-potential curve for BBIM-ethane and N3 are shown in Fig. S7a and S7b respectively.

During the cathodic excursion of the potential, one reduction within the accessible potential window of the solvent is observed for N3. The reduction is found to be irreversible [12] as peak-to-peak separation, ∆Epp = 1560 mV and the cathodic-to-anodic peak current ratio, ipc/ipa is not unity (Fig. S7b). BBIM-ethane is also electron deficient having reduction potential -0.97 V (Figure S6a). Now the adduct BBIM-ethane/N3 is also an electron seeking complex and having smaller reduction potential of – 0.82 V (Figure S7c), compared to that both of N3 (-0.87 V, Figure S7b) and of BBIM-ethane (-0.97 V, Figure S7a). F. Monte Carlo Simulation A rigorous Monte Carlo (MC) conformational search protocol [13] is employed for better understanding of these associated complexes [1b, 2a]. Fig. 6 gives the MC simulated global minimum conformers of the three complexes. All six NAC compounds (N) except picric acid (N4) interact with BBIM-ethane salt in a similar fashion is giving 12 conformers each. The global minimum conformers of BBIM-ethane/N in each case form four H-bonds with four benzimidazolium protons of BBIM-ethane salt and oxygen centres nitro groups of N (shown in Fig. 6A – 6B). H-bond distances are within (1.69 to 1.84) Å in all six cases of NAC adduct of BBIM-ethane (Fig. S7). 2-Iodio benzoic acid (compound 7) interacts with the BBIM-ethane salt in a totally different way. Here BBIM-ethane salt gets folded completely and is also forming four H-bonds between carbonyl oxygen (of compound 7) and benimidazolium protons (of BBIM-ethane) of length (1.62 – 1.66) Å. BBIM-ethane/7 adduct has 24 conformers in gas phase and the global minimum one is shown in Fig. 6[C]. As observed from Monte Carlo simulation H-bonding interactions take place in the BBIM-ethane/N5 system while some lone-pair interactions coupled with H-bonding is probably present in the BBIM-ethane/7 system, so 1H NMR shift show entirely opposite effects for N5 and compound 7 to interact with BBIM-ethane salt in CD3CN. The global minimum conformer of picrate-BBIM-ethane adduct shown in Fig.6[D] was more stable compared to the corresponding picric acid (N4) adduct in respect to H-bond distances as well as calculated values of Wiberg NBO bond indices (Table 2). Values of H-bond indices are in well agreement with the calculated H–bond distances of the picric acid-BBIM-ethane adduct and picrate adduct. The reasonable high value of H-bond indexes of picrate adduct compared to all other NAC adducts was found. Thus picrate-BBIM-ethane has greater H-bond indexes, indicating stronger H-bonding interaction as compared to N4-BBIM-ethane adduct. The frontier molecular orbital energy profile diagram in Figure 6E indicates that the ground-state charge transfer occurs from picrate anions to the positively charged sensor molecules in the gas phase as evident from the DFT calculations. The DFT studies revealed that the energy of the highest occupied molecular orbital (HOMO) of the free sensor BBIM-ethane is −13.47 eV and the lowest occupied molecular orbital (LUMO) is -8.47 eV, while the energies of HOMO and LUMO of picrate anion are −3.13 eV and +0.63 eV, respectively. Hence, the ground-state electronic charge transfer takes place from the HOMO of picrate anion to the LUMO of sensor BBIM-ethane

BBIM-ethane BBIM-ethane/N5



Fig. 6: Wireframe structure of Monte Carlo global minimum onformers of [A] BBIM-ethane/N4 adducts, [B] BBIM- ethane/N5 adduct, [C] BBIM-ethane/7 adduct and [D] BBIM-ethane-picrate adduct in gas phase. [E] Energy profile diagram of FMOs of N4, BBIM-ethane, picrate and BBIM-ethane-picrate adduct describing charge transfer phenomena.

N4 BBIM-ethane Picrate Picrate adduct-14-12-10

-8-6-4-202

FMO energy (eV)

3.65eV 5.24eV

[E]

[A]

[B]

[C]

[D]



Table 2. Wiberg NBO indexes and distances of H-bonds of the MC global minimum conformers of NAC adducts. BBIM-ethane adduct H-bond distances (Å) Wiberg NBO H-bond index NO2(O)... H-N NO2(O)... H-N NO2(O)... H-N NO2(O)... H-N N1 1.706 1.996 0.1860 -0.0672 N2 1.701 1.750 0.1171 0.0653 N4 1.704 1.764 0.1332 0.0620 N5 1.707 1.713 0.1291 0.0700 N6 1.696 1.808 0.1256 0.0660 Picrate NO2(O)... H-N Ph-O-... H-N NO2(O)... H-N Ph-O-... H-N 1.593 1.618 0.1518 0.1251 cause quenching. Band gap of BBIM-ethane salt (5.24eV, Fig. 6E) on forming picrate-adduct was decreased to 3.65eV, resulting a longer wavelength emission compared to that of free BBIM-ethane. Though N4 and N5 have similar reduction potentials (RNO2/RNO2- = -0.39V for Picric acid and -0.40V for 2,4-DNT) [14], selectivity and specificity of picric acid among all other NACs can be attributed to deprotonation of strongly acidic phenolic -OH group followed by anion exchange to interact with the sensor molecule. Ray et. al [5] reported water soluble tris imidazolium based picric acid sensor of DL 354 ppb, but there also N4 exists as picrate. However as that sensor does not has any imidazolium proton in it so no chance of H-bonding was there. Consequently, simple quenching is observed. In this study picrate form stable differently emitting adduct with acetonitrile soluble bis-benzimidazolium based sensor BBIM-ethane having DL 20 ppb. 4. CONCLUSION In summary, a new simple bis-benzimidazolium based salt was designed for detection of picric acid on the basis of long-range energy transfer mechanism. BBIM-ethane salt shows good selectivity and high sensitivity of picric acid, the DL is 20 ppb and binding constant is 3.39×109 M-1. The fluorescence turn off based visual detection is developed in acetonitrile medium. Thus, simple bis-benzimidazolium salts can also be viewed as a suitable class of compounds in the development of efficient sensor for the trace detection of picric acid. The recognition can be done as picrate derivative in solution by a noteworthy colour change for the first time. ACKNOWLEDGEMENTS Author TC acknowledges the grant received from UGC funded Major project, F.42-390/2013(SR)/dated 25/03/2013. REFERENCES [1] (a) X. He et al., “Oxidized bis (indolyl) methane: a simple and efficient chromogenic-sensing molecule based on the proton transfer signalling mode,” Org. Lett., vol. 8, pp. 333-336, 2006. (b) A. Karmakar et al., “Quinone–Bodipy H-bonding interaction over π-stacking in toluene,” Photochem. Photobiol. Sci., vol. 14, pp. 1207- 1212, 2015. (c) A. Karmakar et al., “Recognition of steric factor in external association of xanthenocrown-5 and bis-napthalenocrown-6 with

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International Journal of Photonics and Optical Technology Vol ...Thus picric acid can be recognized photophysically by this BBIM-ethane salt due to RET based fluorescence quenching

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