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  Self-Assembled Pentacenequinone Derivative for Trace Detection of Picric Acid  Vandana Bhalla, *  , †  Ankush Gupta, † Manoj Kumar, † D. S. Shankar Rao, ‡  , § and S. Krishna Prasad ‡  , § † Department of Chemistry, UGC Sponsored-Centre for Advanced Studies-I, Guru Nanak Dev University, Amritsar-143005, Punjab,India ‡ Centre for Soft Matter Research, Jalahalli, Bangalore-560013, India * S  Supporting Information  ABSTRACT:  Pentacenequinone derivative 3 forms lumines-cent supramolecular aggregates both in bulk as well as insolution phase. In bulk phase at high temperature, long-rangestacking of columns leads to formation of stable and orderedcolumnar mesophase. Further, derivative 3 works as sensitivechemosensor for picric acid (PA) and gel-coated paper stripsdetect PA at nanomolar level and provide a simple, portable,and low-cost method for detection of PA in aqueous solution, vapor phase, and in contact mode. KEYWORDS:  pentacenequinone, liquid crystal, organogel, supramolecular aggregates, columnar mesophase, chemosensor  ■  INTRODUCTION Nitroaromatic compounds (NAC) such as trinitrotoluene(TNT), dinitrotoluene (DNT), and picric acid (PA) are well-known primary constituents of many unexploded land mines worldwide. Further, soil and groundwater of war zone andmilitary facilities can contain toxic levels of these compoundsand their degradation products. Thus, these compounds areconsidered as environmental contaminants and toxic to livingorganisms. 1 − 3 The concern over the adverse e ff  ects of nitroaromatics on environment and health provide thesu ffi cient impetus to develop cost e ffi cient, selective, porta ble, fast, and sensitive method for detection of nitroaromatics. 4 − 6  Among various techniques used for the detection of nitro-aromatics,  󿬂 uorescence signaling is one of the  󿬁 rst choices because of its high detection sensitivity and selectivity. To date, various  󿬂 uorescent chemosensors,  󿬂 uorescent polymers, andnanoparticles have been developed for detection of nitro- aromatics. 7 − 22 Out of which,  󿬂 uorescent conjugated polymersare considered to be more advantageous as compared to small 󿬂 uorescent molecules because of their excellent molarabsorptivity, high quantum yields and ampli 󿬁 ed sensory responses. 23 However, real time monitoring of the polymericmaterials is limited becaues of their multi step conventionalcovalent synthesis. Recently, several  󿬂 uorescent nano 󿬁  bersobtained by molecular self-assembly have been reported assensing materials for detection of nitroaromatic explosive-s. 24 ,25,26a Fluorescent molecular assemblies have a large planarsurface because of e ff  ective  π  − π   stacking between themolecules. 25 These  󿬂 uorescent molecular assemblies are thee ffi cient sca ff  old for the construction of light harvesting, whitelight emitting,  󿬂 uorescence imaging materials and for thedetection of various analytes. 27 The one-dimensional  π  − π  stacking provide ordered molecular organization, thus, enablinglong-range exciton migration and their quick annihilation by the explosive quenchers. Further, the network formed by interdigitated nano 󿬁  bers produces multiscale porosity, hencemaking them an ideal material for sensing of nitro- aromatics. 28 − 30 In several cases,  󿬂 uorescent nano 󿬁  bers have been reported to be good sensing materials for the detection of  TNT and DNT. 24 − 26 However, detection of PA at ppb levelusing molecular self-assemblies remains a challenge. Develop-ment of quick and sensitive chemosensors for PA is importantas it is w idel y used in the manufacture of rocket fuels and 󿬁 reworks. 31,32 Further, explosive power of PA is superior to thatof TNT. Recently, from our laboratory, we reported apentacenequinone derivative that forms  󿬂 uorescent nano-aggregates in aqueous media that selectively sense picric acidin solution phase. 33a But these nanoaggregates could not detectpicric acid in vapor phase and contact mode. However,detection of nitroaromatics is essential both in vapor andaqueous phases for security screening, environmental quality monitoring and industrial process controlling. 33b Further,moderate vapor pressure of picric acid makes its vapor phasedetection more challenging. 33c In continuation of this work, we were interested in synthesis of new materials with more de 󿬁 nedmorphologies for selective and sensitive detection of picric acidin solution, solid and vapor phase. In the present investigation, we designed and synthesized a new pentacenequinone Received:  September 27, 2012  Accepted:  January 14, 2013 Published:  January 14, 2013 Research Articlewww.acsami.org © 2013 American Chemical Society  672  dx.doi.org/10.1021/am302132h  |  ACS Appl. Mater. Interfaces  2013, 5, 672 − 679  derivative which forms  󿬂 uorescent nano 󿬁  bers which selectively sense picric acid among various nitroaromatics tested. We havechosen pentacenequinone moiety as the motif for preparationof luminescent supramolecular aggregates because of thetendency of 6,13-pentacenequinones to form ordered thin 󿬁 lms which make them good candidates for preparation of  organic electronic devices. 34 − 36 Besides, pentacenequinonederivatives are important precursors for the design andsynthesis of solution-processable pentacene derivatives 37  butthe potential of these derivatives as self-assembling materials isstill unexplored. We envisioned that rigid pentacenequinonemotif bearing  󿬂 exible alkyl chains and having 1,2,3-triazolegroups as connecting units may form luminescent supra-molecular assemblies. Interestingly, pentacenequinone deriva-tive  3  bearing 1,2,3-triazole groups forms supramolecularaggregates in bulk as well as in solution phase and during theself-assembly in bulk phase formation of stable columnar phaseon heating is observed. Interestingly, derivative  3  work assensitive chemosensor for PA. We also prepared gel-coatedpaper strips that serve as a simple, portable, fast, and low-costmethod for detection of PA in aqueous solution at the parts per billion level. To the best of our knowledge, this is the  󿬁 rstreport where a pentacenequinone derivative forms supra-molecular assemblies both in bulk as well as in solution phaseand works as sensitive chemosensor for PA. In addition, thepresent study shows the transformation of short-rangecolumnar stacking obtained in the bulk phase to long-rangecolumnar stacking, which is of signi 󿬁 cant interest for thepreparation of supramolecular electronic materials. Scheme 1. Synthesis of Pentacenequinone-Based Compound 3 a a Key: (i) CuI, DMF (Dry), 70 − 80  ° C; (ii) Pd(PPh 3 ) 4  , K  2 CO 3  (2 M), 1,4-dioxane, 80 − 90  ° C. Figure 1.  (A) Powder X-ray di ff  raction pattern of the columnar mesophase of compound  3  at 204  ° C. (B) Polarized optical microscopic image of compound  3  at 204  ° C. ACS Applied Materials & Interfaces  Research Article dx.doi.org/10.1021/am302132h  |  ACS Appl. Mater. Interfaces  2013, 5, 672 − 679 673  ■  RESULTS AND DISCUSSION Pentacenequinone derivative  3  was synthesized by Cu (I)catalyzed  “ click reaction ”  of compound 1 38  with hexylazide  2 (Scheme 1 ,  Path 1 ) in 25% yield. We also prepared compound 3  via Suzuki-Miyaura coupling of 2,3,9,10-tetrabromopentace-nequinone  1a  with boronic ester  2a  (see pS4 in the SupportingInformation) in 50% yield (Scheme 1 ,  Path 2 ).The structure of compound  3  was characterized by   1 H NMR, 13 C NMR, mass, and elemental analysis. The  1 H NMR spectrum of compound  3  showed nine singlets (4H, 4H, 4H,4H, 4H, 8H, 8H, 16H, 48H), two doublets (4H, 4H), onetriplet (24H) and one multiplet (16H) (see Figure S28 in theSupporting Information). The mass spectrum of compound  3 showed a parent ion peak at  m /  z  2062.317 (M+1) + (see Figure30 in the Supporting Information). These spectroscopic datacorroborate the structure  3  for this compound.Polarized optical microscopy (POM) analysis and powder X-ray di ff  raction studies (Figure 1) of derivative  3  show presenceof isotropic phase at 233  ° C and columnar mesophase in thetemperature range 187 to 206  ° C. The hexagonal columnarstructure is assigned on the basis of ratio 1:0.58:0.5:0.38 for thepositions of strong re 󿬂 ections observed in the powder X-ray scans. Further, absence of di ff  raction peak in the range expectedfor the  π  − π   stacking corresponding to a distance of 3.5 Å             ́  ,suggests short-range columnar stacking. Di ff  erential scanningcalorimetry (DSC) analysis of derivative  3  provided no clearthermal information.However, interesting results were obtained by gradually cooling isotropic melt of the compound  3  several times. DSCanalysis of this sample shows appearance of mesophase at 205 ° C and absence of any phase transition in the temperaturerange 30 to 205  ° C (see Figure S2 in the SupportingInformation). POM analysis (inset of Figure 2 , Figure S3, Supporting Information) and temperature dependent powder X-ray di ff  raction studies exhibit columnar mesophase whichdoes not change over the temperature range 30 to 205  ° C(Figure 2 , Figure S1, Supporting Information). The assignment of hexagonal columnar structure is based on the powder X-ray di ff  raction scans which show the ratio of 1:0.58:0.5:0.38 for thepositions of strong re 󿬂 ections. The presence of wide range of columnar mesophase over the temperature range 30 to 205  ° Calong with the presence of di ff  raction peak at 3.56 Å             ́  , in therange expected for the  π  − π   stacking indicate long-rangecolumnar stacking after heating and cooling cycles. We believe that reheating and cooling cycles in bulk phasetransform short-range columnar stacking to long-rangecolumnar stacking. Such behavior is rarely observed. 39 Theabove results also demonstrate the thermal stability of thecompound at high temperature followed by cooling andreheating cycles.The gelation ability of derivative  3  in di ff  erent solvents wascon 󿬁 rmed by the  “ stable-to-inversion protocol of a test tube ” method.It is freely soluble in CH 2 Cl 2  , CHCl 3  and THF and formsstable opaque gel (see Figure S4 in the SupportingInformation) in mixture of toluene/DCM (8:2), benzene/DCM (8:2) and o-xylene/DCM (8:2) and weak gel in p-xylene(see Figure S5A in the Supporting Information). Weinvestigated the gel state for di ff  erent portions of toluene/DCM and found that optimum portion to form gel state is 8:2and lowest concentration for the gel formation in this ratio is1.25 wt %/vol (see Figure S5C in the Supporting Information).The organogels are thermoreversible and stable for severalmonths (Figure 3). The POM image of the organogel of compound  3  in toluene/DCM shows birefringence at roomtemperature (Figure 4 A), thus, indicating ordered morphology in solution phase. Thermal sta bility of the gel was measured by the dropping ball method. 40a The sol − gel transition temper-ature ( T  gel ), the required temperature for the organogel tocollapse, increases with increase in concentration of gelator as isclear from the plot of the gel to sol melting temperature,  T  gel  ,against the concentration of compound  3  (Figure 4B). Atconcentration  ≥ 6.25% wt/vol precipitation takes place (seeFigure S5B in the Supporting Information). Figure 2.  Powder X-ray di ff  raction pattern of the compound  3  at 30and 205  ° C obtained after reheating and cooling cycle. Inset polarizedoptical microscopic image of compound  3  at 30  ° C. Figure 3.  Photographs of gel (under 365 nm UV light) of compound  3 formed in toluene/DCM (8:2) solvent implement sol phase transition by heating − cooling. Figure 4.  (A) Polarized optical microscope image gelator  3  formed intoluene/DCM at room temperature through crossed polarizing  󿬁 lters.(B) Variation of   T  gel  with increasing concentration of the gelator  3  intoluene/DCM (8:2). ACS Applied Materials & Interfaces  Research Article dx.doi.org/10.1021/am302132h  |  ACS Appl. Mater. Interfaces  2013, 5, 672 − 679 674  The gel stability is qualitatively in agreement with themorphology exhibited by transmission electron microscopy (TEM) studies. The TEM images of compound  3  showed thepresence of numerous  󿬁  bers entangled with one another to givethe extended  󿬁  brillar network structure, thus indicatingtendency of the molecule to self-assemble into 1D  󿬁  bers(Figure 5). We believe that in addition to e ff  ective  π  − π   stacking between pentacenequinone molecules the extended  󿬁  brillarnetwork structure of compound  3  could result from thehydrophobic interactions between  󿬂 exible alkyl chains attachedto triazole rings. 40b This extended network of interlocked  󿬁  bersis responsible for the immobilization of the solvent and gelformation. Further, the more entangled  󿬁  ber morphology observed in gel of compound  3  indicates stronger interactions between individual  󿬁  bers. We also prepared a model compound  4 41  without 1,2,3-triazole groups (Scheme 2) that exhibits liquid crystalline behavior but displays no gelation abilities, thus suggesting thatdipole − dipole and  π  − π   interactions between 1,2,3-triazolesplayed a synergic e ff  ect in formation of the gel. Figure 5.  TEM images of toluene/DCM organogelator  3  scale bar (A) 5, (B) 2, and (C) 1  μ m. Scheme 2. Synthesis of Pentacenequinone-Based Compound 4 a a Key: (i) Pd(PPh 3 ) 4  , K  2 CO 3  (2 M), 1,4-dioxane, 80 − 90  ° C. Figure 6.  (A) Change in the  󿬂 uorescence spectra of compound  3  (10  μ M) with the addition of PA in toluene/DCM (8:2) solution,  λ ex   = 330 nm(B) Stern −  Volmer plot in response to PA and inset  󿬁 gure shows the Stern −  Volmer plot obtained below 200  μ M concentration of PA. (C)Comparison of   󿬂 uorescence quenching of   3  (10  μ M) in toluene/DCM (8:2) after the addition of 30 equiv of various nitroderivatives. ACS Applied Materials & Interfaces  Research Article dx.doi.org/10.1021/am302132h  |  ACS Appl. Mater. Interfaces  2013, 5, 672 − 679 675
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