# New Ni-Anthracene Complex for Selective and Sensitive Detection of 2,4,6-Trinitrophenol

Authors: Kumbam Lingeshwar Reddy; Anabathula Manoj Kumar; Abhimanew Dhir; Venkata Krishnan
Journal: International Journal of Spectroscopy
 (2018)

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## Abstract

Selective and sensitive detection of explosive materials through a simple approach is an attractive area of research having implications on public safety and homeland security. Considering this implication in mind, a new Ni-anthracene complex was designed and synthesized and has been demonstrated as an efficient fluorescence chemosensor for the selective and sensitive detection of 2,4,6-trinitrophenol. Firstly, a fluorescent anthracene ligand (A) was synthesized by treating anthracene-9-carboxaldehyde with 1,3-diaminopropane in presence of a weak acid. To achieve superior selectivity and great quenching efficiency for 2,4,6-trinitrophenol (TNP), a Ni complex, namely, [Ni(μ2-L)(NO3)] (B), was synthesized via the reaction ofA with Ni(NO3)2·6H2O. ComplexB showed strong emission peak (λmax) at 412 nm and exhibited high selectivity towardsTNP among other nitroaromatics and anions. 100 equivalents ofTNP made 95% fluorescence quenching ofB and its detection limit forTNP was calculated as 2.8 μM.


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## Body

## 1. Introduction

Nitroaromatic compounds are generally used in many industries for the preparation of dyes, pharmaceuticals, rubber products, explosives, chemical fibres and pesticides, and so forth [1]. These nitroaromatic compounds were released into the environment as industrial wastage and are one of the main reasons for the pollution. Moreover, due to their poisonousness, carcinogenicity to living beings, and the risk to homeland safety, they cause serious intimidations to our lives [2]. 2,4,6-TNT; 2,4-DNT; 2,4-DNB; 2,6-DNT; and 2,4,6-TNP are the key constituents of environmental pollutants and explosive products [3]. Out of these compounds, TNP in particular creates health hazards and chronic, skin, and eye diseases and is used as a powerful explosive similar to TNT [4]. Furthermore, TNP is a more potent explosive than the more commonly used explosive TNT [5].Various detection techniques such as electrochemical sensing, cyclic voltammetry, Raman spectroscopy, Liquid chromatography-mass spectrometry (LC-MS), photoluminescence spectroscopy (PL), and several other techniques have been used to detect different types of explosives including TNP. Nevertheless, most of them are pretty sophisticated, time taking, and/or difficult to operate [6, 7]. Among these reported procedures, PL based method offers shorter response time and improved sensitivity and is economically worthwhile [7, 8]. Hence, with the above stated problems from TNP and the benefits of PL, TNP sensing through optical detection is highly needed.Till now, many fluorescent sensors have been developed for the detection of nitroaromatic explosives [7–10]. Nitroaromatics are electron deficient in nature; due to this reason some electron-rich fluorescent probes can always produce nonfluorescent Meisenheimer or π-stacking complexes with nitroaromatics [11, 12]. Selective detection of TNP is very difficult as it has strong electron affinity [13]. Through donor-acceptor electron transfer mechanism, TNP can quench the fluorescence emission of various molecules such as metal complexes, conjugated polymers, organic molecules, and metal-organic frameworks [14–17].Among various fluorophores used for designing of chemosensors, anthracene based compounds have been used for the detection of several analytes, due to their high chemical stability, richness ofπ-electrons and strong fluorescence [18, 19]. Mainly anthracene based compounds are widely used for the sensing of anions and cations. Nevertheless, there are very limited compounds reported for the detection of explosives/nitroaromatics [20–22]. In consequence, by considering the significance of the new photoluminescent material for TNP sensing, in this work, we synthesized a new anthracene based nickel complex with fluorescent activity for the detection of TNP. Solution phase spectral studies confirm the sensitivity of complex with TNP.


## 2. Materials and Methods

### 2.1. Materials

Anthracene-9-carboxaldehyde, 1,3-diaminopropane, nickel nitrate, tetra butyl ammonium salts of anions, Nitroaromatic compounds and other chemicals are purchased from Sigma-Aldrich, India and used without further purification.


### 2.2. Synthesis ofA

Anthracene-9-carboxaldehyde (0.2 g, 0.97 mM) was dissolved in 10 mL of methanol. 1,3-Diaminopropane (0.036 g, 0.485 mM) and acetic acid (27.8μL, 0.485 mM) are added to the above solution while stirring. The yellow colored solution mixture was refluxed for 12 h. Upon cooling the yellow precipitate,(7E,19E)-N-((anthracen-10-yl)methylene)-N′-((anthracen-9-yl)methylene)propane-1,3-diamine,A, was separated by filtration under reduced pressure and then washed with excess methanol. The product was recrystallized from methanol. Yield: 85.32%. 1H NMR (500 MHz, CDCl3, Me4Si) δ 2.57–2.54 (t, 2H); δ 4.23–4.21 (m, 4H); δ 7.51–7.48 (m, 8H); δ 8.04–8.02 (d, 4H); δ 8.51 (s, 2H); δ 8.58–8.56 (d, 4H); δ 9.56 (s, 2H) (see supplementary Figure S1); 13C NMR (CDCl3): δ 160.74, 131.32, 130.05, 129.31, 128.88, 128.24, 126.71, 125.25, 124.80, 60.89, 32.48 (see supplementary Figure S2); HR-MS Data: [M + 1]+ = 450.86.


### 2.3. Synthesis ofB

To the solution ofA (100 mg, 0.22 mM in THF), nickel nitrate [Ni(NO3)2·6H2O, 0.56 g, 1.93 mM] was added and stirred for two hours at room temperature. The yellowish green precipitate was separated out under reduced pressure. Yield: 83.96%. HR-MS [M + 1]+ found 1082.34.


### 2.4. Characterizations

The1H and 13C NMR spectra of theA have been recorded on a Bruker 400 MHz NMR instrument. Mass spectra of theB have been recorded on Kratos PC Axima HR-Mass spectrometer in linear mode. The fluorescence quenching measurements were performed using Cary Eclipse Fluorescence spectrophotometer. The excitation and emission slit widths (each 10 nm) and scan rate (500 nm min−1) were kept constant for all the measurements. The optical absorption spectral measurements were recorded using a Shimadzu UV-2450 spectrophotometer with a quartz cuvette (path length, 1 cm). Energy dispersive X-ray spectra (EDAX) were obtained using scanning electron microscope (SEM), FEI Nova Nano SEM-450.


## 2.1. Materials

Anthracene-9-carboxaldehyde, 1,3-diaminopropane, nickel nitrate, tetra butyl ammonium salts of anions, Nitroaromatic compounds and other chemicals are purchased from Sigma-Aldrich, India and used without further purification.


## 2.2. Synthesis ofA

Anthracene-9-carboxaldehyde (0.2 g, 0.97 mM) was dissolved in 10 mL of methanol. 1,3-Diaminopropane (0.036 g, 0.485 mM) and acetic acid (27.8μL, 0.485 mM) are added to the above solution while stirring. The yellow colored solution mixture was refluxed for 12 h. Upon cooling the yellow precipitate,(7E,19E)-N-((anthracen-10-yl)methylene)-N′-((anthracen-9-yl)methylene)propane-1,3-diamine,A, was separated by filtration under reduced pressure and then washed with excess methanol. The product was recrystallized from methanol. Yield: 85.32%. 1H NMR (500 MHz, CDCl3, Me4Si) δ 2.57–2.54 (t, 2H); δ 4.23–4.21 (m, 4H); δ 7.51–7.48 (m, 8H); δ 8.04–8.02 (d, 4H); δ 8.51 (s, 2H); δ 8.58–8.56 (d, 4H); δ 9.56 (s, 2H) (see supplementary Figure S1); 13C NMR (CDCl3): δ 160.74, 131.32, 130.05, 129.31, 128.88, 128.24, 126.71, 125.25, 124.80, 60.89, 32.48 (see supplementary Figure S2); HR-MS Data: [M + 1]+ = 450.86.


## 2.3. Synthesis ofB

To the solution ofA (100 mg, 0.22 mM in THF), nickel nitrate [Ni(NO3)2·6H2O, 0.56 g, 1.93 mM] was added and stirred for two hours at room temperature. The yellowish green precipitate was separated out under reduced pressure. Yield: 83.96%. HR-MS [M + 1]+ found 1082.34.


## 2.4. Characterizations

The1H and 13C NMR spectra of theA have been recorded on a Bruker 400 MHz NMR instrument. Mass spectra of theB have been recorded on Kratos PC Axima HR-Mass spectrometer in linear mode. The fluorescence quenching measurements were performed using Cary Eclipse Fluorescence spectrophotometer. The excitation and emission slit widths (each 10 nm) and scan rate (500 nm min−1) were kept constant for all the measurements. The optical absorption spectral measurements were recorded using a Shimadzu UV-2450 spectrophotometer with a quartz cuvette (path length, 1 cm). Energy dispersive X-ray spectra (EDAX) were obtained using scanning electron microscope (SEM), FEI Nova Nano SEM-450.


## 3. Results and Discussion

Condensation of 1,3-diaminoprone2 with anthracene-9-carboxaldehyde1 produced a fluorescent ligandA, which gave a nickel complexB with the addition of Ni(NO3)2·6H2O at RT (Scheme 1). The as-synthesized complex was characterized by mass spectrometry, where the parent ion (m/z) peak appeared at 1082 corresponding to the species [2A·Ni(NO3)2] (see supplementary Figure S3). The presence of all the elements in complexB is confirmed through elemental analysis (see supplementary Figure S4). As we are developing a chemosensor forTNP, the main motivation behind the synthesis of compoundA is that it was expected to become a nonfluorescent moiety as photoinduced electron transfer (PET) process can take place due to presence of imino nitrogen atom [23]. The binding of nickel ions to the imino nitrogen of compoundA will prevent the PET process and it eventually results in the perseverance of fluorescence. Consequently,TNP can perhaps quench the fluorescence emission ofB with electron transfer or energy transfer mechanism and could be sensed [24].Scheme 1
Synthetic route of nickel complexB.ComplexB (1 μM) showed reasonably good emission intensity of anthracene monomeric species at 412 nm in THF : HEPES (9.5 : 0.5) when excited at λex = 365 nm [25]. On addition ofTNP (150 μM) to the solution ofB (1 μM) in THF : HEPES (9.5 : 0.5), the emission band shows quenching of emission band (Figure 1). To observe the selectivity ofBtowardsTNP, we carried out the fluorescence titration experiments with similar amount of 150 μM other nitroderivatives, namely, nitrotoluene (NT), 2,4-dinitrotoluene (2,4 DNT), and 1,4-dinitrobenzene (1,4 DNB) and anions (F−, Cl−, Br−, I−, OH−, HSO3-, CN−, NO3-, CH3COO−, ClO4-, and PO4-) with tetrabutyl ammonium as counter ion, and observed no significant change in the emission spectrum ofB depicting the selectivity ofB (Figure 2). Detection limit (DL) was calculated using the equation: DL=3α/K, where α is the standard deviation calculated by measuring the fluorescence intensity of complexB for more than 15 times and K presents the slope of fitting curve between variation of the fluorescence intensity (I0-I) and the added amount ofTNP at low concentration. According to the linear fitting curve (Figure S5 shown in supporting information), the detection limit of complexB for sensing ofTNP was found to be 2.857 μM [26]. The complexB DL in sensing ofTNP is as comparable with the existing materials (see Table S1 in supporting information).Figure 1
Change in emission spectrum ofB (1 μM) on addition ofTNP (150 μM) in THF : HEPES (9.5 : 0.5) (ν/ν).Figure 2
Fluorescence response ofB (1 μM) in THF : HEPES (9.5 : 0.5) (ν/ν) among various nitroaromatics and anions. All analytes are tested in the same molar ratio toB.We also examined the meddling of other anions and nitroaromatics towards the sensing ofTNP (Figure 3) and it was confirmed that theB was detectingTNP even in the presence of other potential analytes. The emission plot ofB in the presence of TNP was studied using Stern-Volmer relationship. Stern-Volmer plot seemed like a hyperbolic curve (see supplementary Figure S6) which may be accredited to the combination of static and dynamic (collision) quenching [27].Figure 3
Fluorescence quenching inB with the addition ofTNP alone andTNP in the presence of other analytes. The molar ratio toB used in each case is 100 equivalents.We have also detected these variations with the assistance of UV-vis analysis. Nickel complexB (1 μM) in THF : HEPES (9.5 : 0.5) showed an absorbance band at 260 nm (Figure 4). On addition ofTNP (120 μM) along with the formation of band at 340 nm corresponding toTNP(Figure 4), there was subsequent increase in the band at 260 nm presenting that there is some photochemical process happening betweenB andTNP. To have a deep insight into the mechanism, we made plot between the absorbance spectrum ofTNP and emission spectrum ofB (Figure 5). The overlap between the two plots suggests an energy transfer mechanism from photoexcited π-electron-rich derivativeB to ground state electron-deficientTNP. Thus, the changes occurring in the optical properties ofB on addition ofTNP are attributed to the energy transfer process.Figure 4
Change in absorption spectra of4 (1 μM) with the addition ofTNP in a mixed aqueous medium of THF : HEPES (9.5 : 0.5) (ν/ν).Figure 5
Spectral overlap of the absorption ofTNP (red line) with the emission spectrum ofB (blue line).


## 4. Conclusions

In conclusion, we have designed and synthesized a new photoluminescent Ni-anthracene complex. The luminescence behaviour of the complex was assessed towards different nitroaromatics and other analytes. The synthesized Ni-anthracene complex shows selective and sensitive detection ofTNP and could be a helpful tool for the people working in forensic sciences.



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*Source: 1321427-2018-02-06.xml*