Haloarene-Linked Unsymmetrically Substituted Triarylethenes: Small

Nov 16, 2017 - The possible mechanism underlying this AIE-behavior has also been addressed by the support of experimental/theoretical outcomes. Moreov...
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Cite This: J. Org. Chem. 2017, 82, 13359−13367

Haloarene-Linked Unsymmetrically Substituted Triarylethenes: Small AIEgens To Detect Nitroaromatics and Volatile Organic Compounds M. Zubair Khalid Baig,† Prabhat Kumar Sahu,‡ Moloy Sarkar,‡ and Manab Chakravarty*,† †

Department of Chemistry, Birla Institute of Technology and Science, Pilani-Hyderabad Campus, Jawahar nagar, Shamirpet Mandal, Hyderabad, Telangana 500078, India ‡ School of Chemical Sciences, National Institute of Science Education and Research-Bhubaneswar, Jatni, Odisha 752050, India S Supporting Information *

ABSTRACT: Unsymmetrically substituted triarylalkenes as aggregation-induced emission-active fluorogens (AIEgens) are sporadically explored by different researchers. In this Article, naphthalene, biphenyl, and haloarene-linked new triarylethenes are conveniently synthesized and presented as unsymmetrically substituted extensive π-conjugates to continue the discovery of small molecules as new AIEgens. Moreover, fluorophores attached to haloarenes are noteworthy, but such compounds are barely investigated as AIEgens. The possible mechanism underlying this AIE-behavior has also been addressed by the support of experimental/theoretical outcomes. Moreover, two of these small AIEgens are fruitfully employed for rapid sensing of nitroaromatic compounds (NACs) where picric acid (PA, as a model explosive) showed a strong quenching efficiency with the detection limit of 39 or 48 ppb along with other nitroaromatics such as p-nitrotoluene and p-nitrophenol. This quenching could be visualized by the naked eye under a UV (365 nm) lamp and performed almost in an aqueous medium. Such alkenes are also proved to exhibit very clean on/off fluorescence switching properties for polar volatile organic compounds (VOCs).

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INTRODUCTION Since the discovery of the AIE phenomena by Prof. Tang and others,1 there are persistent efforts by investigators from various disciplines to create numerous AIEgens.2 Despite several disputes on the mechanism of AIE activity, tempting applications using AIE-active molecules in the field of chemical/biosensors and organic electronics have drawn enormous attention to the scientific community.3 Molecules with a tetra-phenylethylene (TPE) core are quite well established as most successful and rich AIE-active luminogens due to the highly restricted intramolecular motions (RIM) that help to block the nonradiative channels by easing the radiative channels.2b,4 Further, new AIEgens were developed by replacing the phenyl of TPE with much bulkier naphthalene rings to afford comparatively better thermally stable and electroluminescent materials.4,5 In fact, naphthyl or biphenyl framework was earlier reported as a π-conjugated rotor to generate new symmetrically substituted TPE or triphenylethene-based AIEgens (Figure 1).6 In a few cases, triarylalkenes were linked to TPE to produce better AIEgens.4a Up to now, only a few triarylalkenes (such as CN-MBE,1b well-explored AIEgen) as exclusive AIEgens have been reported in the literature.6 Moreover, these reports dealt primarily with symmetrically substituted AIEgens; unsymmetri© 2017 American Chemical Society

Figure 1. A few reported biphenyl or naphthyl ring-linked symmetrically substituted triarylethenes as AIEgens.

cally substituted triarylalkenes as AIEgens are very much limited to our awareness. Because of the heavy atom effect and the presence of C−H···X (X = Br or Cl) interactions for πconjugated haloarenes, the luminescent behaviors in the aggregated or solid state turned out to be interesting; nevertheless, only few reports on such systems are recognized as effective AIEgens.4b,7 In this scenario, we describe new unsymmetrical triarylethenes as AIEgens where both naphthyl, biphenyl, and haloarene are installed as rotors for the ethylenic stator, and such a system is intended to have twisted molecular geometries that might enhance the fluorescence intensity when the molecules come under the vicinity of each other, that is, on Received: September 26, 2017 Published: November 16, 2017 13359

DOI: 10.1021/acs.joc.7b02438 J. Org. Chem. 2017, 82, 13359−13367

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The Journal of Organic Chemistry Scheme 1. Synthesis of Triarylalkenes 2a,b and 3

demonstrate naphthyl, biphenyl, and haloarene-linked unsymmetrically substituted small molecules as proficient AIEgens for the fast and sensitive detection of nitroaromatics and polar VOCs. The experimental outcomes are supported by fluorescence lifetime and dynamic light scattering studies. The fundamental understanding on detection of NACs is addressed by fluorescence-based titration studies.

aggregation. Further, these molecules are an electronically rich system that can easily prefer interacting with the electrondeficient systems. Such alkenes can be considered as an atomeconomic and small AIEgens in comparison to well-reputed TPE-based AIEgens. Inspired by the reports on AIE-based chemosensors,3 new AIEgens synthesized herein are wellintentioned to explore in detecting NACs [especially picric acid (PA) as a model explosive] and polar VOCs. The significant development toward the detection of NACs was found by many researchers.8 Notably, the strategies and mechanisms of explosive detection using various fluorophores (small molecules, metal organic framework, polymers, AIEgens, etc.) are systematically presented by Prof. Lei.8a Further, a number of fluorescent molecules are exclusively reported to sense NACs such as p-nitrophenol (p-NP),8c p-nitrotoluene (p-NT),8d pnitroaniline (p-NA),8e and more importantly 2,4,6-trinitrophenol (picric acid; PA)8f−h as a part of the security and environmental safety concerns. Additionally, there are various alternative approaches for nitroaromatics sensing.9 However, significant reports on a simple fluorescence-based polar VOCs sensing are relatively few.10 Notably, photoluminescent polymers were effectively used to sense NACs by the research groups of Prof. Trogler11 and Prof. Swager.12 However, most of these reported probes and processes suffer from synthetic difficulties, high cost, and comfortability in terms of handling and accessibility. Therefore, availing small and easily reachable probes that are associated with fluorescence-based handy sensing techniques brings significant impact. Particularly, these AIEgens can be considered as purely organic electronrich small molecular systems that are synthesized through an economical route, and the fast/effective detection is carried out almost in an aqueous form. Moreover, such substances can easily be transformed in the form of a film with a given support. Thus, the AIEgens could enter into the field of explosive detections where organic small molecule AIEgens are of special interest. At the same time, sensing volatile organic polar solvents such as acetone, tetrahydrofuran (THF), dichloromethane, CHCl3, and acetonitrile (MeCN) in a simple manner is very much crucial due to the harmful nature of such solvents. Notably, such polar solvents having low vapor pressure at room temperature severely pollute the environment and are being produced and used in very large scale by industries.10 The existing technologies to sense VOC are also reported using metal−organic framework that needs to be activated for long duration under high temperature and vacuum.10c,d Thus, facile detection of VOCs using small fluorescent molecules is also meaningful. On the other hand, TPE-based AIEgens were reported for sensing PA as a model explosive being commercially available,3b,c,10d and the respective mechanism was studied at length by many researchers.8,11,12 Herein, we

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RESULTS AND DISCUSSION Synthesis of Fluorescent Probe and Structure Determination. With our continuing research on applications of organophosphonate/phosphates toward organic functional materials,13 particularly to develop small molecule AIEgens,13d compound 1 was utilized as a suitable precursor for the metalfree synthesis of biphenyl and naphthalene-linked triarylalkenes 2a,b (Scheme 1). The π-conjugation was extended by the Pd-catalyzed C−C bond-forming reaction of 2b to afford compound 3 as shown in Scheme 1. The most vital part of this work was to access the phosphonate 1 in 80% yield with very high regioselectivity (99:1).13a Precursor 1 was utilized to generate compounds 2a,b via Horner−Wadsworth−Emmons (HWE) reaction, but triarylalkenes 2a,b could be isolated with a very poor yield ( 2b > 3. This result insisted that we record the solid-state emission spectra for these compounds (see Figure S11) and illustrated compound 2a as more fluorescent than 2b and 3 in the solid state. In case of compound 3, a prompt fluorescence enhancement from f w > 50% was observed, and the maximum intensity was found at f w = 90% (Figure S9). On further increasing of f w, the fluorescence intensity started attenuating as observed in many other related studies reported in the literature.13d,14 The calculated αAIE parameter for 3 has appeared as 6.94, even weaker than compound 2b. Interestingly, in these AIE-studies, sharp bathochromic shifts in emission λmax were observed for all three compounds [for 2a, 21 nm; 2b, 38 nm; 3, 49 nm; Table S2] in their aggregated form, reflecting the stabilization of the excited energy states with the increase of the polarity. The larger red-shift for 3 could also arise due to the appropriate intermolecular arrangement through significant π−π stacking interaction between the molecules with more number of naphthyl rings favoring the electronic conjugation to the system. On the other hand, such an effect also reduces the fluorescence intensity in the

aggregated state, and that could be the reason for being lesser AIE-active in comparison to 2a,b where the electronic conjugation effect is relatively weaker. As water concentration was increased gradually to the acetonitrile solution, and molecules must have formed aggregates, which might be responsible for the intense emission.2b Thus, all of these molecules are found to be AIE-active. Computational optimizations revealed that these compounds acquire twisted conformational structures due to the presence of much bulkier naphthyl/biphenyl rings. The dihedral angle measurements for 2b (see Figure 2, and for 2a and 3, see Figure S1) show that these aryl rotors are not in the plane of the alkene stator; however, the aryl rotors are twisted in such a way that they have not completely deviated from the planarity for the optimum electronic conjugations as shown in Figure 2. In fact, the halogen atom can also contribute to the C−H···π intermolecular interactions, and an edge-to-face orientation is probably involved in molecular packing for the aggregated state.4b,7a For such a system, intramolecular motions are restricted in the aggregated form, which block the nonradiative channels.5,6 However, the inter-/intramolecular energy transfer processes15 in the aggregated state could also be circumvented for such twisted molecular structure and result in an intensified emission. Further, particle size analyses using the dynamic light scattering (DLS) studies (Table S5) for 2a and 2b specified the average particle size as 77.89 and 105.8 nm, respectively, at f w = 90%, which further supports that the molecules aggregate into nanoparticles. The formation of nanoparticle upon aggregation was earlier reported to be responsible for the AIE phenomena of the organic compounds.1b In addition, the emission behaviors for both 2a,b were examined by the timeresolved fluorescence spectroscopic technique to investigate the excited-state lifetimes for these fluorophores [see the lifetime profiles for 2a, Figure S4; 2b, Figure S7; and 3, Figure S10 and Table S4]. This study disclosed16 the presence of two components (B1′ and B2′, Table S4) in the molecular form of 2a where the B1′ form is the major (98%) with fast decay (0.04 ns) and B2′ is the minor (only 2%) with comparatively slower decay (0.33 ns). The average excited-state lifetime for the compound 2a was found to be 0.05 ns in acetonitrile solution. On aggregate formation (90% f w), both of the channels were almost equally populated [B1′ (55%) and B2′ (45%)], and the lifetime for the excited states’ decaying was significantly enhanced for both of the channels [B1′ (0.25 ns) and B2′ (0.99 ns)]. Thus, the average lifetime of the excited state of aggregation became 0.58 ns, considerably higher in nanoaggregate state in comparison to molecular form 2a (0.05 ns). Compound 2b also relaxed through two decay channels in both molecular and aggregated forms with average lifetime 0.05 ns in solution and 0.30 ns in the aggregated state. Such a significant difference in the excited-state lifetime for 3 was not observed (0.43 ns in molecular form and 0.47 ns in aggregate form; see Table S4 for details). Being an important property for such materials in terms of device applications,4a,5 the thermal stabilities were measured for these alkenes, and the decomposition temperatures look to be comparable [Figure S12: 2a, 349.28 °C; 2b, 376.6 °C; and 3, 420 °C] with typical AIE molecules such as HPS (351 °C) and MPPS (361 °C).1a Detection of Nitroaromatic Compounds (NACs). Having been established as reasonably better AIEgens, 2a,b were selected to explore fluorescence-based detection of NACs. Noticeably, the quantum yield of 2a at f w = 90% (5.71%) and 13362

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Figure 5. Change in fluorescence intensity of (a) 2a and (b) 2b aggregates (f w = 99%) upon incremental addition of PA. Inset images of 2a and 2b aggregates ( f w = 99%) illuminated before and after addition of PA under 365 UV-lamp [2b (10 μM) and PA (5 μM)].

and p-NA (using expensive polymer).8d Thus, these 2a,b nanoaggregated probes can be used to detect particularly these three NACs proficiently. When anthraquinone was used as a quencher, the quenching was not as effective as compared to the nitroaromatics (see Figure S16a,b). Further, the efficiency was quantified by estimating the Stern−Volmer constant (Figure 6) by using the equation I0/I = 1 + KSV[Q] [all are with usual meaning].

99% (5.67%) did not differ much (0.04%), and hence fluorophore solution with f w = 99% was preferred for this study to minimize the involvement of organic solvent (acetonitrile) significantly. Thus, the experiment was performed almost in an aqueous medium (1980 μL of water in 20 μL of MeCN). Initially, we focused on the detection of PA as a classical and well-studied explosive.3b−e,8g In terms of the model explosive, trinitrotoluene (TNT) would have been more appropriate, but we restrict with commercially available PA due to the safety issues with TNT. However, a number of sensing studies with PA as a model explosive using large and expensive fluorophores have been reported in the literature as mentioned before.3e,8f−h,10,11 Consequently, small molecules 2a,b with a larger π-cloud were thought to be promising candidates for NACs detection. The broad absorption band with λmax ≈ 300 nm was observed in the UV−vis absorption spectra for the nanoaggregates 2a,b. Upon gradual addition of aqueous PA solution (0−10 μM) to 2a or 2b nanoaggregates (10 μM), the enhancement in the absorbance with ∼51 nm red-shift became prominent (Figure S13a). Further, to clarify if the spectral profiles shown in Figure S13a are not only a result of the increasing amounts of PA, UV−visible spectra of the fluorophore have also been obtained (Figure S13c) by subtracting the absorption spectra of only PA (Figure S13b) under similar conditions to support the complex formation. These hyperchromic and bathochromic features indicated the formation of ground-state charge transfer complex between the aggregates of 2a or 2b and electron-poor PA in the ground state. In the emission spectrum, the fluorescence intensity at maximum wavelength ∼450 nm for 2a or 2b nanoaggregates was gradually deteriorated and visualized by the rapid color change through the naked eye as shown in Figure 5. Next, a range of nitroaromatics were added independently to 2a or 2b aggregates, and the quenching of fluorescence intensity was observed, however, with different efficiencies as shown in Figures S14, S15, and S16c,d. Notably, these nitroaromatics are widely used in the mass production of explosive and organic-based dye, and some of the nitroaromatics such as p-NP, p-NT, and p-NA are water-soluble causing environmental pollution.8 The quenching efficiencies of p-NP and p-NT were also very much promising. Thus, along with PA, these fluorophores could also detect p-NP and p-NT satisfactorily. Thus, with the pleasing result with p-NT, we believe that TNT also can be successfully detected using this fluorophore. It is worth mentioning that there are many reports on discrete sensing of nitroaromatics (NA) such as p-NP (using MnO2 nanotubes),8a p-NT (using supramolecular strategy),8c,f

Figure 6. Stern−Volmer plot for different aromatics with 2a aggregate (10 μM, f w = 99%); [Q]: quencher, NACs and AQ. Please see the similar plot for 2b in Figure S17. (a) 1-Chloro-2,4-dinitrobenzene (ClDNB), (b) m-nitroacetophenone (m-NAP), (c) p-nitrobenzoic acid (p-NBAcid), (d) p-nitrobenzaldehyde (p-NB-CHO), (e) p-nitrophenol (p-NP), (f) nitrobenzene (NB), (g) o-nitrotoulene (o-NT), (h) p-nitroaniline (p-NA), and (i) p-nitrotoluene (p-NT). All of the KSV values are tabulated in Table S6.

The quenching of the fluorescence intensity was so rapid in case of PA addition in comparison to other NACs, that this fluorescence titration had to be performed cautiously with a very small fraction of PA because the fluorescence intensity drastically weakens by adding PA (see the close points for PA in Figure 6). The quenching constants (association constant) for compounds 2a and 2b toward PA were found to be 3.33 × 105 and 2.03 × 105 M−1 [calculated from KSV], respectively, which affirmed very strong quenching ability toward PA. The quenching constants for both 2a,b and the corresponding quenching efficiency in terms of a bar plot with different nitroaromatics are summarized in Figure 7 that further indicates the dissimilarity of quenching ability between the NACs. Thus, the chloro compound (2a) appears to be a relatively more effective quencher than 2b. The binding mechanism was further investigated by scrutinizing the fluorometric titration studies (Figure 5) to 13363

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lifetime, that is, 0.285 ns for 1.25 μM PA, 0.274 ns for 2.5 μM, and 0.271 ns for 5.0 μM (see Figure S22 and Table S7). These results (only 11 and 3 ps decrease) designate that the quenching is primarily due to static mechanism.16,17 In fact, the bathochromic shift in the absorbance spectra also supports the interactions between the fluorophore and NACs at the ground state, that is, static quenching. In collisional quenching, no change in the absorption spectra is expected as it disturbs the excited state of the fluorophore, not the ground state. On the basis of the lifetime profile obtained, the role of photoinduced electron transfer (PET) in this quenching process is insignificant. For the same reason, the quenching via resonance energy transfer (RET) is also irrelevant. Hence, the ground-state complex formation is the predominant reason for this quenching event. Interestingly, a spectral overlap (Figure S23a,b) between the emission and excitation spectra of the fluorophore (a probable donor) and the absorption of the PA (a possible acceptor) was observed, and that could play a significant role in fluorescence quenching as the emission frequency is absorbed by the quencher. However, this observation cannot be accounted due to RET, as mentioned earlier. Thus, the inner filter effect (IFE, more appropriately secondary IFE)17 is considered to be responsible for this fluorescence quenching. Considering this IFE effect, the SV plot has also been replotted (Figure S24). The KSV values varied from 3.33 × 105 to 3.23 × 105 M−1 for 2a with PA and from 2.03 × 105 to 1.95 × 105 M−1 for 2b upon correction using IFE. This indicates that the effect of IFE is present in quenching through ground-state charge transfer complex formation. Further, the exicitation spectra of 2a and 2b were measured upon gradual addition of PA (Figure S23c). The comparison of fluorescence excitation spectral profile indicates the formation of a complex with both 2a,b aggregates in the ground state. The observation therefore validates that the IFE is playing a significant role in quenching mechanism. With a concern on the quenching abilities of other aromatics, anisole, bromobenzene, catechol, and mesitylene were tested with 2b aggregates and found to be negligible even at a much higher concentration (Figure S25a−d). A poor quenching efficiency by using only catechol ruled out the substantial role of phenolic −OH toward this quenching behavior.8f Notably, the use of catechol as a proof that phenolic compounds do not interact with the receptor molecules is not valid because this phenol has a very different nature (the lack of nitro group). Therefore, a relatively electron-deficient phenol compound such as propyl p-hydroxybenzoate was used for this study, and the fluorescence intensity was not weakened by the gradual addition even at higher concentration (Figure S25e). Thus, phenolic −OH functionality might not play any role in this quenching process. Further, we have carried out a timedependent study for the aggregates of 2a and 2b upon continuous irradiation at the excitation wavelength λex = 295 nm (Figure S26). With time, no significant change in the fluorescent intensity has been observed, which shows the photostability of the aggregates. Volatile Organic Compounds Sensing. Additionally, motivated by the solvent vapor sensing studies using AIEgens,3c,6e and the solid-state fluorescent properties for these compounds, we primarily exposed the spot of 2b in a TLC plate (thin layer chromatography, silica gel) under a wide variety of solvent vapors. Before exposing under solvent vapors, an intense blue spot was observed under UV-light (365 nm) on the TLC plate (Figure 8), which disappeared under the contact

Figure 7. Fluorescence quenching efficiencies for 2a and 2b aggregates (10 μM) after the addition of different nitroaromatics and AQ (10 μM).

generate Job’s plot (Figure S18) that revealed 1:1 stoichiometry for both compounds 2a,b with PA forming the ground-state charge transfer complex. Considering 1:1 complex formation, we found an excellent fit for the Benesi−Hildebrand plot (Figure S19), which offered the binding constant of 3.24 × 105 and 3.16 × 105 M−1 for compounds 2a and 2b, respectively, which further support the strong interactions of 2a or 2b with PA. The detection limit calculations (Figure S20) revealed that PA could be identified until 39 ppb by compound 2a and 48 ppb by compound 2b in an almost aqueous medium. Furthermore, to find if this rapid quenching happens via a diffusion process, a kinetic study on fluorescence quenching was performed (Figure S21). When PA solution (2 μL of 10 mM in water) was progressively added to 2 mL of 2b (10 μM) aggregate, the fluorescence intensity was instantly weakened soon after the addition and remained unchanged for 10 min until another portion of PA (2 μL) solution was added, which further reduced the fluorescence intensity rapidly, that also remained almost the same for another 10 min. This study represents the occurrence of quick quenching as soon as the PA solution is added, and that means the nanoaggregates start interacting with PA without any diffusion procedure. Actually, such fluorescence quenching was earlier reported3c−e,8a,b,11,12 to happen through the photoinduced energy/electron transfer between the electron-rich fluorescent aggregates of 2a or 2b and the electron-poor nitroaromatics. Perhaps, in this case, the vicinal proximity of the aggregates with the NACs perturbs the twisting conformation of the molecule to facilitate the π−π stacking interaction and generates a nonemissive charge transfer ground-state complex, responsible for the reduction of fluorescence intensity. Thus, this quenching happens as soon as the PA solution is added, and that means the molecules adjacent to aggregates start interacting strongly with NACs ensuing the quenching of fluorescence intensities by reducing the number of available fluorophore molecules. Additionally, the Stern−Volmer plot was examined for both of the compounds to find the nature of quenching. The linear fit of the plot specifies the mode of quenching as either pure static or pure dynamic.16 As measuring lifetime is an important tool to distinguish the quenching mechanism through static or dynamics,16a the average lifetime of compound 2b aggregate (10 μM, 99% water in MeCN) was found to be 0.296 ns. After the gradual addition of PA, there is very little reduction in the 13364

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([1,1′-biphenyl]-4-yl(naphthalen-1-yl)methyl)phosphonate (1) was prepared in our lab following our recently developed method.12a All aldehydes and boronic acid were purchased from Aldrich and alfaaesar. Tetrakis(triphenylphosphine)palladium(0), potassium t-butoxide, and tetrabutylammonium bromide were purchased from Alfa aesar and used as received. THF was redistilled from sodium metal and benzophenone mixture. All other reagents were purchased from common suppliers and used without further purification. Column and flash chromatography was performed by using Dessica Silica gel 100− 200 mesh and 230−400 mesh, respectively. Reactions were monitored by thin-layer chromatography on precoated silica gel 60 F254 plates (Merck & Co.) and were visualized by UV (mainly 365 nm). The 1H, 13 C{1H} NMR spectra were recorded in CDCl3 solution using BrukerAvance DRX (400 and 500 MHz). The signals were referenced to TMS, and the solvent used was deuterated chloroform (7.26 ppm in 1 H NMR, 77.16 ppm in 13C{1H}- NMR). Chemical shifts are reported in ppm, and multiplicities are indicated by s (singlet), d (doublet), t (triplet), and dd (doublet of a doublet). CHN analysis was done on a Variomicro Elementar analyzer. The electronic absorption spectra were recorded with JASCO-650 V UV−vis scanning spectrophotometer. The fluorescence spectra were recorded on a JASCO-FP6300 spectrofluorimeter. APCI-LCMS was recorded in ShimadzuLCMS-2020. DLS analyses were carried out with a Zetasizer Nano S from Malvern Instruments at 25 °C. TGA was carried out on Shimadzu DTG-60 simultaneous DTA-TG apparatus with a rate of temperature rising 10 °C/min. Time-resolved measurements were performed by using a time-correlated single-photon counting (TCSPC) spectrometer (Edinburgh, OB920) with laser diode source (λexc. = 375 nm). A Dilute Ludox solution in water was used to measure the lamp profile. F900 decay analysis software was used to analyze the decay curves by using nonlinear least-squares iteration method. The quality of the fit was judged by the chi square (χ2) values. All data were plotted using DataGraph 4.2.1 or Origin-2016 software. Theoretical Section. All DFT calculations were performed in gas phase using Gaussian 09 software with the B3LYP exchange correlation functional. For the ground-state geometry optimization, all of the atoms were treated with 6-31 G(d,p) basis set, and later results were checked with 6-311 G(d,p) and 6-311++ G(d,p) basis set for convergence. The thermochemical data were calculated at T = 298.15 K and P = 101.325 kPa (1 atm, STP) as implemented in Gaussian 09 software (see the Supporting Information for details). Synthesis and Characterization. Compound 1 (0.3 g, 0.697 mmol) was subjected to vacuum in a 25 mL round-bottomed flask and was dissolved in dry THF (4 mL) under argon (a standard balloon) at 0 °C. Next, KOtBu (0.234 g, 2.092 mmol) was added and stirred for 4−5 min. 4-Chlorobenzaldehyde (0.088 g, 0.627 mmol) was cautiously added to the solution. The reaction was allowed to stir for 8 h at 25 °C, and completion of the reaction was monitored by TLC. The resulting reaction mixture was quenched with water, washed with brine, extracted with ethyl acetate (10 mL × 3), dried over anhydrous sodium sulfate, and concentrated under rotary evaporator. Compound 2a was purified by column chromatography using ethyl acetate and hexane (1% ethyl acetate in hexane). Compound 2b was synthesized in a manner similar to that of 2a. (Z)-1-(1-([1,1′-Biphenyl]-4-yl)-2-(4-chlorophenyl)vinyl)naphthalene (2a). Yield: 0.174 g, 60% yield. IR (ν cm−1, in KBr): 3422, 3026, 2923, 1598, 1484, 1393, 1071, 1008. 1H NMR (500 MHz, CDCl3): δ 8.23 (d, J = 8.2 Hz, 1H), 8.00−7.92 (m, 2H), 7.68 (d, J = 8.0 Hz, 2H), 7.60−7.54 (m, 5H), 7.51 (t, J = 7.5 Hz, 3H), 7.45−7.40 (m, 3H), 7.33−7.27 (m, 4H), 6.88 (s, 1H). 13C {1H} NMR (126 MHz, CDCl3): δ 142.0, 141.9, 140.5, 140.2, 139.6, 136.0, 134.2, 132.8, 132.0, 130.9, 130.4, 130.3, 128.9, 128.8, 128.5, 128.2, 127.6, 127.5, 127.2, 127.0, 126.3, 126.2, 125.9, 125.5. APCI-MS: 416 [M]+ and 418 [M + 2]+ (3:1). Anal. Calcd for C30H21Cl: C, 86.42; H, 5.08; Cl, 8.50. Found: C, 86.31; H, 5.12. (Z)-1-(1-([1,1′-Biphenyl]-4-yl)-2-(4-bromophenyl)vinyl)naphthalene (2b). This compound was isolated with 87:13 stereoselectivity. Yield: 0.209 g, 65% yield. IR (ν cm−1, in KBr): 3416, 3027, 2922, 1598, 1485, 1393, 1071, 1007. 1H NMR (500 MHz, CDCl3): δ 8.15 (d, J = 8.4 Hz, 1H), 7.92 (dd, J = 15.1, 7.7 Hz, 2H),

Figure 8. Images of 2b on TLC plate under UV-lamp (365 nm). (a) Fluorescent thin film of aggregate, (b) film exposed to VOC vapors leading to quenching, and (c) fluorescence regained after evaporation of solvents.

of selected solvent vapors and reappeared on evaporation of the solvents within a few seconds at room temperature. Such effect was observed for the VOCs such as acetone (MeCOMe), tetrahydrofuran (THF), dichloromethane (DCM), chloroform (CHCl3), and acetonitrile (MeCN), used under investigations. This strategy was quite efficient and reversible. The same TLC spot can be used repeatedly for these solvents. This fact can be attributed to the solute−solvent interaction that leads to disruption of aggregated structures and finally decreases the fluorescence efficiency.6e Later, the blue fluorescence reappeared on evaporation of solvents, facilitating the aggregate formation. Thus, a visual detection of polar VOCs offers an indication of the potential application of such molecular aggregates. The same experiment could be also performed using compound 2a. These AIEs were not tested as solid sensors due to the lower vapor pressure for these NACs. In fact, when we exposed the aggregate spot in the TLC plate under some other NACs (such as nitrophenol, nitrotoluene) for almost 24 h at room temperature in a closed beaker, no change in fluorescence intensity was observed. The very low vapor pressures of these NACs demotivated us to explore this application further. The quantification and selectivity studies of aggregates 2a,b towards VOCs have also been carried out (Figures S27, S28) in the solution state where both these aggregates were found to be more selective towards acetone and chloroform through fluorescence quenching. The detection limit calculations (Figure S29) reveal that these solvents can be detected in the range of 0.1−0.55 mm (Table S8) using these fluorophores. In conclusion, we have successfully synthesized new unsymmetrically substituted naphthyl, biphenyl, and haloarene-linked triarylalkenes with excellent stereoselectivity. The optimized geometries for these molecules were obtained through DFT studies and analyzed. All of these alkenes are shown to be AIE-active, and the enhanced emission has been interpreted with the support of lifetime and DLS studies. The twisted molecular structure was described from the optimized structures for these compounds and responsible for such AIE activity. In addition, these easily accessible AIEgens 2a,b are used to detect nitroaromatics through fluorescence quenching in almost aqueous solution with a detection limit of 39 ppb for PA. The fluorescence-based simple and effective detection of polar VOCs using such AIEgens is also noteworthy. Hence, we could develop small molecule-based AIEgens that can be used as promising fluorescence-based chemo detectors.

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EXPERIMENTAL SECTION

General Comments. All experiments were carried out in hot air oven-dried glassware under nitrogen and argon atmosphere. Diethyl 13365

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The Journal of Organic Chemistry

to a closed Petri-dish chamber that was partially filled with volatile organic solvent vapor. The fluorescence intensity of 2b was slowly reduced, and the intensity reverted back when the TLC plate was taken out of the chamber.

7.64 (d, J = 7.9 Hz, 2H), 7.52 (d, J = 7.1 Hz, 4H), 7.50−7.43 (m, 4H), 7.40 (t, J = 9.4 Hz, 3H), 7.38−7.35 (m, 2H), 7.18 (d, J = 8.2 Hz, 2H), 6.81 (s, 1H). 13C {1H} NMR (126 MHz, CDCl3): δ 142.0, 141.8, 140.5, 140.2, 139.5, 136.4, 134.1, 131.9, 131.4, 131.1, 130.4, 130.2, 128.8, 128.4, 128.2, 127.5, 127.4, 127.1, 126.9, 126.2, 126.1, 125.8, 125.2, 120.9. APCI-MS: 461 [M]+ and 463 [M + 2]+ (1:1). Anal. Calcd for C30H21Br: C, 78.09; H, 4.59; Br, 17.32. Found: C, 78.16; H, 4.52. Synthesis of (Z)-1-(1-([1,1′-Biphenyl]-4-yl)-2-(4-(naphthalen-1yl)phenyl)vinyl)naphthalene (3). Compound 2b (0.3 g, 0.65 mmol) was dried under vacuum and was dissolved in 3.3 mL of a solution of 1,4-dioxane and water in 10:1 ratio. Next, K2CO3 (0.269 g, 1.95 mmol) and 1-naphthaleneboronic acid (0.145 g, 0.84 mmol) were added successively to the above solution and allowed to stir for 2−3 min. Tetra-n-butylammonium bromide (TBABr) (0.021 g, 0.065 mmol) and tetrakis(triphenylphosphine)palladium(0) [Pd(PPh3)4] (0.069 g, 0.065 mmol) were added to the above solution and allowed to stir overnight at 90 °C. Completion of the reaction was monitored by TLC. The resulting reaction mixture was quenched with water, extracted with ethyl acetate (20 mL × 2), washed with brine, dried over anhydrous sodium sulfate, and concentrated. Compound 4 was purified by column chromatography using ethyl acetate and hexane (2% ethyl acetate in hexane), 0.199 mg, 60% yield. IR (ν cm−1, in KBr): 3027, 2955, 2923, 2853, 1737, 1590, 1485, 1393, 1261, 1186, 1080, 1018. 1H NMR (400 MHz, CDCl3): δ 8.24 (d, J = 8.3 Hz, 1H), 8.02 (d, J = 8.3 Hz, 1H), 7.99−7.92 (m, 3H), 7.92−7.81 (m, 3H), 7.65−7.64 (m, 1H), 7.63−7.62 (m, 1H), 7.56 (s, 2H), 7.55 (d, J = 1.2 Hz, 1H), 7.54 (s, 1H), 7.52 (s, 1H), 7.49 (d, J = 4.5 Hz, 2H), 7.47 (d, J = 2.5 Hz, 2H), 7.45 (d, J = 2.1 Hz, 2H), 7.43 (s, 3H), 7.37 (d, J = 7.5 Hz, 2H), 7.21−7.02 (m, 1H), 6.96 (s, 1H). 13C {1H} NMR (101 MHz, CDCl3): δ 142.2, 141.1, 140.6, 140.0, 139.97, 139.91, 139.4, 136.3, 134.1, 133.9, 132.0, 131.5, 131.4, 130.2, 129.9, 129.4, 128.8, 128.8, 128.4, 128.3, 128.0, 127.7, 127.5, 127.3, 127.1, 126.9, 126.3, 126.1, 126.0, 125.9, 125.8, 125.7, 125.5, 125.4. APCI-MS: 509 [MH]+. Anal. Calcd for C40H28: C, 94.45; H, 5.55. Found: C, 94.32; H, 5.61. AIE Studies. A stock solution of 2a in MeCN (10−3 M) was prepared and further diluted to 10−5 M [1980 μL (1.98 mL) of MeCN was added to 20 μL of stock solution] for the photophysical studies. All of the following solutions with different f w values were prepared by injecting water in MeCN solution (see Table S3 for details). The absorption and emission spectra were measured for all of these solutions. The solutions of 2b and 3 were prepared for the AIE studies in a similar manner. NACs Quenching Studies. Absorption and Emission Studies. 0.5 μL of PA (10−3 M) was added progressively to a 2 mL aggregated (99%) solution of 2a or 2b (10−5 M), and the absorption spectrum was recorded for each case. With NACs: 2 μL of nitro-aromatics (ClDNB, m-NAP, p-NBAcid, p-NB-CHO = 10−2 M) or 0.5 μL of nitroaromatics (p-NP, NB, p-NT, o-NT, p-NA: 10−2 M) was added to a 2 mL aggregated (99%) solution of 2a or 2b (10 μM) incrementally, and the emission spectra were recorded. With non-NACs: 1 μL of nonNACs (10−2 M) was added to a 2 mL aggregated (99%) solution of 2b (10 μM) incrementally, showing no quench of the fluorescence in the emission spectra. Time-Dependent Fluorescence Quenching Study. Two microliters of PA (10−3 M) was added to a 2 mL aggregated ( f w = 99%) solution of 2b (10−5 M), and the emission intensities of the solution at 450 nm were measured at a certain interval (1 min) for 10 min, and further the same amount of PA was added and the intensity was measured every 1 min for another 10 min. Detection Limit Calculation.11c A series of titrations were carried out for the detection of PA using 2a or 2b. A graph is plotted taking the log of quencher concentration on the X-axis and the ratio of fluorescence intensities after addition of quencher and maximum fluorescence intensity quenched on the Y-axis. The antilog of the Xintercept is considered to be the detection limit. VOC Sensing Procedure. A TLC plate (Merck, Silica gel 60 F254) with a dimension of 2 × 2 cm2 was taken, and a 99%-aggregated solution of 2b (10−5 M) was spotted and allowed to dry. The plate with an intense blue spot under UV lamp (365 nm) then was brought

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/joc.7b02438.

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Spectral data for all compounds (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Moloy Sarkar: 0000-0002-8426-5115 Manab Chakravarty: 0000-0001-8604-7886 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank DST-SERB (SB/S1/IC-07/2013) for financial support. We thank Dr. Subhas and Mr. Santosh for theoretical studies and Mr. Sandeep for his help. The DST-FIST facility is also partially acknowledged. We acknowledge reviewers for helpful suggestions.

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REFERENCES

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