FLUORANTHENE DERIVATIVES FOR DETECTION OF NITRO AROMATIC COMPOUNDS

FIELD OF INVENTION

The invention generally relates to the field of solid state chemistry and particularly to synthesis of fluoranthene derivatives for detection of nitro aromatic compounds.

BACKGROUND

Selective detection of nitro aromatic compounds including but not limited to trinitrotoluene (TNT), dinitrotoluene (DNT) and picric acid (PA), specifically in trace quantities has been a challenge. The role of chemical sensors for noninvasive detection of presence of nitroaromatics has been demonstrated by various methods known to exist in the prior art. More specifically, fluorescent conjugated polymers have been extensively used for detection of nitroaromatic compounds. Examples of fluorescent conjugated polymers include but are limited to poly(p-phenylenevinylenes), polysilanes, polycarbazoles and polymeric porphyrins. Further the fluorescent conjugate polymers have been adopted in techniques involving but not limited to detection in solid phase, aqueous phase, and vapor phase.

A significant disadvantage of the fluorescent conjugated polymers includes intrinsic interferences with electron acceptors such as Toluene, 1,2-dobromo benzene, pyridine, nitrobenzene and Quinones like benzoquinone, anthraquinone, 2,3-Dichloro-5,6-dicyano-1,4-benzoquinone. Further the process of synthesis is elaborate and expensive. Also the polymers are not thermally stable. Hence there is a need for a thermally stable polymer for detecting trace quantities of nitroaromatic compounds.

BRIEF DESCRIPTION OF DRAWINGS:

So that the manner in which the recited features of the invention can be understood in detail, some of the embodiments are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.

FIG.1 shows the reaction steps for obtaining S3, according to an embodiment of the invention.

FIG. 2 shows the absorption and fluorescent spectra of S3, according to an embodiment of the invention.

FIG.3a shows Fluorescence quenching of S3 upon addition of different concentrations of picric acid, according to an example of the invention.

FIG.3b shows extent of fluorescence quenching of S3 in the presence of various nitroaromatics, according to an embodiment of the invention.

FIG. 3c shows fluorescence lifetime measurements at a wavelength of 470 nm for different concentrations of picric acid, according to an example of the invention.

FIG.4 shows the absorption and fluorescent spectra of representative derivatives of S3, according to an embodiment of the invention.

FIG.5 shows the fluorescence quenching behaviour of representative derivatives of S3 for different concentrations of various nitro aromatics, according to an embodiment of the invention.

FIG.6 shows extent of fluorescence quenching of representative derivatives of S3 in the presence of various nitroaromatics, according to an embodiment of the invention.

FIG.7 shows the quenching efficiency of thin films obtained from representative derivatives of S3, when exposed to saturated vapors of plurality of nitroaromatics, according to an embodiment of the invention.

FIG. 8 shows the reversibility of the derivatives of S3 in thin film towards the exposure of saturated TNT vapors, according to an example of the invention.

FIG.9 a shows effect of exposure of plurality of nitroaromatics on derivatives of S3 coated onto thin layer silica chromatographic (TLC) plates, according to an embodiment of the invention.

FIG.9b shows the change in the emission spectral pattern of SV-58 and SV63 coated TLC plate against different concentrations of TNT according to an example of the invention.

FIG.9c shows the comparison of the fluorescence quenching efficiency of SV-58 and SV-63 polymers for different nitroaromatic analytes, according to an embodiment of the invention.

SUMMARY OF THE INVENTION

One aspect of the invention provides a method for obtaining a polymer of fluoranthene.

The method includes the steps of synthesizing a first moiety, synthesizing a second moiety and reacting the first moiety with the second moiety to obtain the polymer of fluoranthene.

Another aspect of the invention provides a polymer of fluoranthene and derivatives thereof.

Yet another aspect of the invention provides a method for detecting trace amounts of nitroaromatics. The method includes coating a surface with a polymer of fluoranthene; exposing the coated surface to saturated vapors of a nitroaromatic compound; detecting the fluorescence of the polymer in the presence of the nitroaromatic compound and estimating the amount of nitroaromatic present in the saturated vapour.

DETAIL DESCRIPTION OF THE INVENTION:

Various embodiments of the invention provide a method for synthesis of polymeric structures comprising fluoranthene unit. More specifically, embodiments of the invention provides a method for synthesizing a polymer of fluoranthene, namely, 7,10-bis(4-bromophenyl)-8,9-bis(4-(hexyloxy) phenyl) fluoranthene, referred to hereinafter as S3 and derivatives thereof. In one embodiment of the invention S3 is synthesized in a three-step process and is shown in FIG.1. Initially, a first moiety 1,2-bis(4-(hexyloxy) phenyl)ethyne is synthesized. In an example of the invention the synthesis of the first moiety is achieved by Sonogashira cross coupling reaction. Subsequent to synthesis of the first moiety, a second moiety 7,9-bis(4-bromophenyl)-8H-cyclopenta[a]acenaphthylen-8-one 2 is synthesized by refluxing acenaphthylene-1,2-dione (0.92g, 5.08 mmol) and 1,3-bis(4-bromophenyl)propan-2one(0.933 g, 3 mmol) in ethanol for 15 min. The synthesized first moiety and the second moiety are then subjected to a cyclo-addition reaction in the presence of 1,2,4-trichlorobenzene (2 mL) at 220°C in a sealed glass pressure tube to obtain the compound S3. In an example of the invention, the cyclo-addition reaction is performed using Diels-Alder reaction.

A plurality of derivatives of S3 can be obtained through various substitution reactions. In one embodiment of the invention, the polymer of fluoranthene S3 can be represented by a structure 7R1, 10R4-8R2, 9-R3 fluoranthene, where R1, R2, R3 and R4 are substitutions on the polymer of fluoranthene. In an embodiment of the invention, R1 was selected from a group comprising of aromatic compounds including but not limited to aromatic, aliphatic and halogen substitutes of benzene; R4 was selected from a group comprising of aromatic compounds including but not limited to aromatic, aliphatic and halogen substitutes of benzene; R2 was selected from a group comprising of benzene and hexyloxybenzene and R3 was selected from a group comprising of hydrogen, benzene and hexyloxybenzene. Two derivatives of S3, namely SV-58 and SV-63 are illustrated herein for demonstrating the detecting capabilities of derivatives for trace amounts of a plurality of nitroaromatics and these are not to be construed as limiting the scope of the invention. TABLE.1 herein below shows a generic structure for fluoranthene along with representative R group substitutions and derivatives obtained thereof. The table is provided herein only for the purpose of illustrating and should not be construed as limiting the scope of the invention. Numerous substitutions, other than the one illustrated herein are possible, as obvious to a person skilled in the art and all such substitutions and derivatives obtained therein should be construed to be within the scope of the invention.

Example 1: Synthesis of polymer SV-58: 0.23 mmol of compound S3 and 0.23mmol N-hexylcarbazole diboronic ester were dissolved in 10ml_ of toluene to obtain a S3 mixture.

Argon gas was purged for 5 minutes. 1.15mmol Anhydrous Na2CO3 was dissolved in 0.5mL distilled water and argon was purged into it for 1 minute and transferred to above solution. Subsequent to addition of the argon purged Na2CO3 solution to the S3 mixture, argon was purged again. The reaction mixture was heated at 70°C for 30 minutes. To the heated reaction mixture catalyst Pd(PPh3)4 and 1 drop of aliquot 336 was added. In an example of the invention, the aliquot 336 is a Starks' catalyst, a mixture of octyl (C8) and decyl (C10) chains with C8 predominating. The reaction mixture was refluxed at 110°C for 48h. After completion of the reaction, reaction mixture was cooled to the room temperature and extracted with chloroform, concentrated and dried over anhydrous Na2SO4(Concentration). A very concentrated solution in dichloromethane was made and polymer was precipitated in methanohhydrochloric acid (2N) mixture, centrifuged at 5500 rpm, washed at least twice with methanol. The derivative polymer was purified by soxhlet extraction.

Example 2: Synthesis of polymer SV-63: 0.23mmol of compound S3 and 0.23mmol of 9,9-dihexyl-2,7-dibromofluorene diboronic ester were dissolved in 10mL toluene. Argon gas was purged for 5 minutes. 1.15mmol of anhydrous Na2C03 was dissolved in 0.5mL distilled water and argon was purged into it for 1 minute and transferred to above solution Subsequent to addition of the argon purged Na2C03 solution to the S3 mixture, argon was purged again. The reaction mixture was heated at 70°C for 30 minutes. To the heated reaction mixture catalyst Pd(PPh3)4 and 1 drop of aliquot 336 was added. The reaction mixture was refluxed at 110°C for 48h. After completion of the reaction, reaction mixture was cooled to the room temperature and extracted with chloroform, concentrated and dried over anhydrous Na2SO4. A very concentrated solution in dichloromethane was made and polymer was precipitated in methanohhydrochloric acid (2N) mixture, centrifuged at 5500 rpm and washed at least twice with methanol. The derivative polymer was purified by soxhlet extraction.

Photophysical properties of the synthesized compound S3 and derivatives thereof were determined through spectroscopic measurements. FIG. 2 shows the absorption and fluorescence spectra of S3 when taken in solution and deposited as a thin film on quartz substrate, according to an embodiment of the invention. . The absorption spectra of S3 in chloroform exhibits two major bands with absorption maxima at 303 and 374 nm which arises from the TF→TT* transitions. The emission spectra show strong blue fluorescence at 475 nm with Stoke's shift of 101 nm. The absorption and emission spectra of S3 in thin film is quite similar to that of solution spectra indicating that the arrangement of the molecules in the thin film is quite similar to the arrangement of the molecules in the solid state packing configuration. The optical band gaps were calculated to be 3.084 and 3.017eV in solution and thin film. The fluorescence lifetime of S3 in chloroform is found to be ~11.37 ns by Time correlated single photon counting (TCSPC) method.

Another embodiment of the invention provides a method for detecting trace amounts of nitroaromatics. The method includes coating a surface with a polymer of fluoranthene, exposing the coated surface to saturated vapors of at least one nitroaromatic compound, detecting the fluorescence of the polymer in the presence of the nitroaromatic compound and estimating the amount of nitroaromatic present in the saturated vapour. In an embodiment of the invention, the synthesized S3 was employed for the detection of nitroaromatics including but not limited to PA, TNT, 2,4-dinitrotoulene (DNT), 2,4-dinitrophenol (DNP) and common interfering agents like 1,4-dichlorobenzene (DCB), benzoquinone (BQ) and other analytes. The detection of nitroaromatics was determined by fluorescence titration experiment in ethanol medium. FIG.3 shows the fluorescence quenching behaviour of S3 upon gradual addition of 0.1 mM solution of PA. The
fluorescence intensity was found to be decreased significantly as a function of concentration and 80 % of quenching was observed for addition of 10μM of 0.1 mM picric acid. Further, fluorescence quenching was analyzed by Stern-Volmer (SV) equation lo/l=1+Ksv[PA], where lo and I are the fluorescence intensities of S3 in presence and absence of PA respectively and Ksv is the Stern-Volmer constant.

FIG.4 shows the absorption and emission spectra of the derivatives SV-58 and SV-63 when taken in solution and deposited as a thin film on quartz substrate, according to an embodiment of the invention. The absorption spectra of of SV-58 in chloroform exhibits two major bands with absorption maxima at 273 and 340 nm and SV-63 shows 306 and 343 nm which arises from the TT->TT* transitions. The emission spectra show strong blue fluorescence for both the polymers at ~ 475±2 nm with Stoke's shift of ~ 132 nm.

The absorption and emission spectra of in thin film is quite similar to that of solution spectra. The optical band gaps were calculated to be 3.13 and 3.03eV in solution and thin film for SV-58 and 3.16 eV for SV-63 in solution and in solid state. Based on the spectral pattern of the absorption and fluorescence, we can infer that the polymers exhibits weak TT-TT stacking interactions and the arrangement of the molecules in the thin film is quite similar to solid state packing. The fluorescence lifetime in chloroform is found to be -13.57 ns by Time correlated single photon counting method.

Solution fluorescence quenching: To explore the potential applications of the polymers as sensory material for the detection of NACs such as PA, TNT, 2,4-dinitrotoulene (DNT), 2,4-dinitrophenol (DNP) and common interfering agents like 1,4-dichlorobenzene (DCB), benzoquinone (BQ) and other analytes were studied by fluorescence titration experiment in ethanol medium. FIG.5 shows the fluorescence quenching behaviour of derivatives of S3 for different concentrations of various nitro aromatics. The fluorescence intensity was found to be decreased with respect to different nitroaromatic used. 80 % of fluorescence quenching was observed for addition of 10uM of 0.1 mM picric acid (2-20 ppb) for SV-58 and 63 % quenching observed for TNT. While SV-63 shows 70 % and 52% of quenching observed for PA and TNT. This suggests that SV-58 has better quenching behaviour than SV-63.

Determination quenching rate constants through Stern volmer equation: The decrease in the fluorescence intensity after addition of different nitroaromatic analytes is analyzed by Stern-Volmer (SV) equation lo/l=1+Ksv[Q], where lo and I are the fluorescence intensities of polymers in presence and absence of quencher (Q) respectively and Ksv is the Stern-Volmer constant. On the basis of quenching, a linear Stern-Volmer (SV) plot was obtained for all nitroaromatics. SV-58 shows a rate constant of 18203 M-1 for PA and 5493 M-1 for TNT while SV-63 shows 6789 M-1 for PA 4164 M-1 for TNT. On the other hand, there were no significant changes in fluorescence quenching behaviour for other NACs at this concentration levels. Based on the SV plots (FIG.6), the polymer SV-58 shows selective detection of picric acid in solution and the quenching rate is three times higher than the SV-63. The appearance of linear SV plot for all analytes indicates the quenching mechanism is static in nature Fluorescence quenching behaviour of polymer thin films with saturated vapors of nitroaromatics: The solution fluorescence quenching study demonstrates that both the polymers selectively detect picric acid at ppb levels. To make ideal sensing devices for onsite real time applications polymer thin films are prepared by a spin coating technique on a quartz substrate with a rate of 2000 rpm and a concentration of 1 mg per 0.2 mL in chloroform solution. The film is annealed at 50° C for 2 hours. The fluorescence spectrum is collected immediately after exposing the film to saturated vapors of different nitroaromatics as a function of time. The fluorescence intensity is found to decrease over elapsed time of exposure. Fig. 7 shows the fluorescent spectra of thin film exposed to saturated vapours of different nitroaromatics

The fluorescence intensity of SV-58 is significantly reduced by 93% for 120 sec exposure to the saturated vapors of TNT. The quenching efficiency increases with increase in the electron affinity strength of the analyte. For example, thin film of SV58 exposed to saturated vapors of NT, 86% of quenching observed in 360 seconds of exposure time while DNT shows 160 seconds. On the other hand, nitro substituted derivatives of phenols show weak response compared with others. Picric acid show very weak quenching behavior and show 48% of quenching in 30 minutes of exposure time. This indicates that SV-58 film exhibits fast response for detection of TNT in ppb levels.

FIG.7 shows the quenching efficiency of thin films obtained from representative derivatives of S3, when exposed to saturated vapours of plurality of nitroaromatics, according to an embodiment of the invention. The variations in the quenching behavior arises due to difference in redox potential of the analytes, electron accepting capability of the SV-58 and adsorptive affinity of film to the NACs. The trapping of TNT vapors into the cavities created by phenyl rings and formation of strong intermolecular interaction with alkyl chains of results in high quenching performance. In case of SV-63 polymer, DNT shows greatest quenching behavior compared with the other nitroaromatics. TNT shows 49% of quenching efficiency in 160 seconds of exposure time.

FIG. 8 shows the reversibility of the both polymer thin film towards the exposure of saturated TNT vapors. The films were exposed to saturated vapors of TNT at room temperature for 120 sec and the emission spectrum was recorded. The film was washed with ethanol and dried at 45 °C under vacuum and the emission spectrum was recorded again and the whole process was repeated. The results show that the initial fluorescence intensity was significantly retained after several washings indicating high 0 photo stability of the film. Quenching efficiency of 85% was observed even after 6 cycles of exposure to TNT vapors Since, the vapor pressures of nitroaromatics are extremely low to sense the proximity of explosives. Therefore we have carried out surface sensing approach to test the viability by coating the polymers on thin layer silica chromatographic plates (200 urn thickness). The visual change in the fluorescence intensity will provide the relevance for the onsite instant detection of explosives

Fig.9 a shows effect of exposure of plurality of nitroaromatics on derivatives of S3 coated onto thin layer silica chromatographic (TLC) plates, according to an embodiment of the invention. The effect of exposure of nitroaromatics is determined by taking a photograph of SV-58 and SV-63 coated TLC plates before and after addition of different concentrations of various nitroaromatics. The nitroaromatics are added by contact mode with spot area of ~ 0.2 cm2 and photograph obtained upon exposure to the UV lamp at 365 nm.

Fig.9b shows the change in the emission spectral pattern of SV-58 and SV-63 coated TLC plate (Aex =340 nm) against different concentrations of TNT (1x10"15 Mx10"3 M) according to an example of the invention. Different concentrations of various nitroaromatics were prepared. 5uL of each of the nitroaromatic solution was spotted on to the polymer coated thin layer silica plates to give a spot area of - 0.2 cm2. The visual fluorescence response for different analytes at different concentration levels is shown in the Figure 7a. The fluorescence intensity was found to decrease as a function of concentration and the minimum amount of TNT detectable by naked eye is as low as 5 uL of 1x10"13 M solution with a detection limit of -1-2 fg/cm2. Figure 7b shows the fluorescence quenching pattern of polymers coated TLC plates for different concentrations of TNT solution. For SV58, we have observed 22 % of quenching for 1x1013 M solutions of TNT and SV-63 shows 15% of quenching efficiency. A comparison of the fluorescence quenching efficiency for different nitroaromatic analytes are shown in the Figure 7c. The quenching effect is found to be higher for TNT for SV-58 compared with SV-63 polymer.

The invention as described herein above provides a method of obtaining a novel fluoranthene derivative S3. The invention also provides method of detecting nitroaromatics through S3. Further the invention also provides method for obtaining derivates of S3, namely SV-58 coupled in conjugation with carbazole and SV-63 coupled in conjugation with fluorine. The invention also demonstrates potential utility of derivatives of S3 in ultra-low level detection of explosive nitroaromatics such as DNT, TNT and PA in solution, solid state and contact mode approach. The present invention has particularly advantage in that the analyte molecule forms strong TT-TT interaction with the luminescent polymers resulting in photoinduced electron transfer from donor nitroaromatics to fluorescent polymers. The hybrid polymers can be readily synthesized as described in the Scheme 1 and these polymers were found to be highly photo-stable.

The foregoing description of the invention has been set for merely to illustrate the invention and is not intended to be limiting. Since modifications of the disclosed embodiments incorporating the spirit and substance of the invention may occur to person skilled in the art, the invention should be construed to include everything within the scope of the appended claims and equivalents thereof.

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We Claim:
1. A method for obtaining a polymer of fluoranthene, the method comprising the steps of:

a. Synthesizing a first moiety;

b. Synthesizing a second moiety;

c. Reacting the first moiety with the second moiety to obtain the polymer of fluoranthene

2. The method according to claim 1, wherein the reaction step includes cyclo addition of the first moiety with the second moiety in presence of 1, 2, 4 tirchlorobenzene.

3. A polymer of fluoranthene having the structure 7R1, 10R4-8R2, 9-R3 fluoranthene.

4. The polymer according to claim 3, wherein R1 is selected from a group comprising of aromatic compounds, wherein the aromatic compounds comprises of aromatic, aliphatic and halogen substitutes of benzene.

5. The polymer according to claim 3, wherein R4 is selected from a group comprising of aromatic compounds, wherein the aromatic compounds comprises of aromatic, aliphatic and halogen substitutes of benzene.

6. The polymer according to claim 3, wherein R2 is selected from a group comprising of benzene and hexyloxybenzene.

7. The polymer according to claim 3, wherein R3 is selected from a group comprising of hydrogen, benzene and hexyloxybenzene.

8. A method for detecting trace amounts of nitroaromatics wherein the method comprises of coating a surface with a polymer of fluoranthene; exposing the coated surface to saturated vapors of at least one nitroaromatic compound; detecting the fluorescence of the polymer in the presence of the nitroaromatic compound; and estimating the amount of nitroaromatic present in the saturated vapour.

9. The method according to claim 8, wherein the polymer of fluoranthene is 7R1, 10R4-8R2, 9-R3 fluoranthene.

10. The polymer according to claim 9, wherein R1 is selected from a group comprising of aromatic compounds, wherein the aromatic compounds comprises of aromatic, aliphatic and halogen substitutes of benzene.

11 .The polymer according to claim 9, wherein R4 is selected from a group comprising of aromatic compounds, wherein the aromatic compounds comprises of aromatic, aliphatic and halogen substitutes of benzene.

12. The polymer according to claim 9, wherein R2 is selected from a group comprising of benzene and hexyloxybenzene.

13. The polymer according to claim 9, wherein R3 is selected from a group comprising of hydrogen, benzene and hexyloxybenzene.

14. The method according to claim 8, wherein the surface is at least one selected from the group comprising silica, polyvinyl and Teflon.

15. The method according to claim 8, wherein the nitroaromatic compound is at least one selected from the group comprising of picric acid, dinitrotoluene, trinitrotoluene, dinitrobenzene and nitroglycerine.

16. The method according to claim 8, wherein the detection of the nitroaromatics is in the range of 1 parts per billion to 2 parts per billion.