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A Tetraphenylethene-Naphthyridine-Based AIEgen TPEN with Dual Mechanochromic and Chemosensing Properties Shahida Umar,† Ajay Kumar Jha,† Deepak Purohit,† and Atul Goel*,†,‡ †

Fluorescent Chemistry Lab, Medicinal and Process Chemistry Division, CSIR-Central Drug Research Institute, Lucknow 226031, India ‡ Academy of Scientific and Innovative Research, New Delhi 110001, India S Supporting Information *

ABSTRACT: Synthesis of new tetraphenylethene (TPE) conjugates via an innocuous route led to the revelation of a unique TPE-based aggregation-induced emissive fluorogen 3 (TPEN), which showed an interesting mechanochromic property when the emission was changed from blue to green upon grinding and green to blue upon fuming. The mechanochromic property of TPEN has been explored to prepare ink-free rewritable paper for security documentation. A detailed photophysical investigation of the TPE-naphthyridine scaffold led to the discovery of its high sensitivity to silver ions (Ag+) over other metal ions with a detection limit of 0.25 μM in an aqueous system. The stoichiometry of the complex of TPEN and silver ion was established to be 2:1 (TPEN:Ag+) on the basis of photophysical studies, mass analysis, and high-resolution mass spectrometry analysis.

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INTRODUCTION Luminogens with reversible stimulus-responsive (pressing, grinding, crushing, or rubbing) emission switching in the solid state have attracted much attention because of their potential applications in optical information storage camouflaging, mechanical sensors, memory chips, and security papers.1−6 In the past few years, various aggregation-induced emission (AIE)-based mechanochromic (MC) materials have sparked intense enthusiasm among researchers for their successful applications in cutting-edge nanotechnologies.7−12 The unique feature exhibited by AIEgens, contrary to the conventional fluorophores, is that they are mostly nonemissive in the solution state but emit strongly in the aggregated state because of the restrictions of intramolecular rotations (RIR) or vibrations (RIV) upon aggregate formation.13−15 These fluorogens can be tailored to develop selective and sensitive biocompatible “off−on” probes that show weak fluorescence in aqueous media, with their fluorescence being turned on when they interact with target bioanalytes.16,17 Furthermore, AIE probes offer stronger resistance to photobleaching, thus leading to high stability and superior signal reliability relative to those of conventional fluorescent probes.18,19 Nonplanar π-conjugated tetraphenylethene (TPE)-based molecular rotors have played a pivotal role in the development of AIEgens for diverse applications.20−24 The chemical modulations in the TPE core via the locking of the phenyl ring by the introduction of a new bond, or by substitution of a heterocyclic group or functionalities, have led to the discovery of highly emissive materials.25−30 The literature © 2017 American Chemical Society

reveals that unsubstituted TPE does not exhibit mechanochromic behavior because of its intrinsic rapid crystallization and symmetrical structure.31−33 We envisaged that inhibition of the molecular symmetry of TPE or its substitution with πconjugated moieties may afford luminogens with interesting photophysical properties. Keeping these factors in mind, we orchestrated the synthesis of new TPE derivatives by increasing the level of π-conjugation or by substitution with N-heterocyclic moieties. An important thing that needs to be noted is that TPE-based molecules that are AIEgens and act as chemosensors are known, but to the best of our knowledge, the TPE-based AIEgens with dual mechanochromic and chemosensing properties are still unexplored. Herein, we report the synthesis and photophysical properties of new TPE conjugates and demonstrate a unique TPE-naphthyridine (TPEN) AIEgen with interesting dual mechanochromic and silver ion sensing properties.

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RESULTS AND DISCUSSION Synthesis. The synthetic strategy adopted for preparing TPE conjugates is outlined in Scheme 1. The ketone 1-[4(1,2,2-triphenylvinyl)phenyl]ethanone (1) was prepared from starting material benzophenone by adopting the McMurry coupling followed by its Friedel−Crafts acylation.34 Compound 1 was reacted with 2-aminonicotinaldehyde in alkaline medium at room temperature to afford TPE-naphthyridine 3 Received: February 25, 2017 Published: April 17, 2017 4766

DOI: 10.1021/acs.joc.7b00456 J. Org. Chem. 2017, 82, 4766−4773

Article

The Journal of Organic Chemistry Scheme 1. Synthetic Route to New TPE-Conjugated Molecules

Indole-based TPE 9 displayed weak emission in the bluish green region with a maximum at 504 nm. Further investigation of the solid state fluorescence of synthesized TPE conjugates 3 (TPEN), 5a, 5b, 7, and 9 showed emission in the range of 474−593 nm (Figure 1b and Table 1). Surprisingly, TPE conjugate 3 (TPEN) showed a multifold higher emission intensity in powder form (quantum yield of 32%) than in a DMSO solution (nonemissive behavior). The high emission of TPEN prompted us to investigate the electronic properties in the ground state. Computational Study. To gain insight into the unusual behavior of 3 (TPEN), time-dependent density functional theory (TDDFT) calculations were performed with a Gaussian 09 pack.35 The geometries were optimized at the DFT/B3LYP level using a 6-31G(d,p) basis set, and TDDFT calculations were performed using a B3LYP/6-311++G(d,p) method. TDDFT studies revealed two prominent electronic transitions observed at 421 nm (HOMO to LUMO) and 351 nm (HOMO to LUMO+1) (Figure S2 and Table S1). The electron density of the HOMO was delocalized over the TPE core, while the LUMO was located on the 1,8-naphthyridine moiety (Figure 2). The electronic transition at 421 nm (HOMO to LUMO) corresponded to the effective intramolecular charge-transfer (ICT) band from the TPE donor to the 1,8-naphthyridine acceptor. The energies of the HOMO and LUMO levels and band gap of TPEN are listed in Table S1. AIE Properties. To check the possible AIE behavior of TPE conjugates [3 (TPEN), 5a, 5b, 7, and 9], the PL emission in the 90% water fraction was examined (Figures S3−S7). The study revealed that only TPEN has AIEgen characteristics as it emits brightly in the 90% water fraction while other TPE conjugates (5a, 5b, 7, and 9) showed weak or no fluorescence in the 90% water fraction (Table S2). This interesting observation may be explained on the basis of the restriction of intramolecular rotation of the planar naphthyridine moiety attached to the TPE scaffold in the 90% water

(TPEN). To tune the photophysical property by increasing the level of π-conjugation, different aromatic aldehydes 4a, 4b, and 8 were reacted with ketone 1 to furnish corresponding aldol products 5a, 5b, and 9, respectively, in good yields (75− 85%). Furthermore, derivative 5a was subjected to Suzuki coupling with N,N′-dimethylphenylboronic acid to give compound 7 in 83% yield. Photophysical Study. The absorbance and fluorescence studies of synthesized compounds 3 (TPEN), 5a, 5b, 7, and 9 were conducted in DMSO (2.5 × 10−5 M). All the compounds showed absorption peaks with maxima in the ranges of 258−302 and 321−430 nm, corresponding to the π−π* and n−π* transitions, respectively (Table 1 and Figure S1). Among them, TPE conjugates 3 (TPEN) and 5a showed no photoluminescence (PL) in a DMSO solution (Figure S1), while compound 5b showed fluorescence in the blue region (λmax = 476 nm). Similarly, π-conjugated TPE derivative 7 exhibited dual fluorescence with maxima at 590 and 735 nm. Table 1. Absorption (Abs) and Emission (Em) Properties of TPE Conjugates solution

solid Ema

Absa λmax (nm) 3 5a 5b 7 9

265, 353 260, 321 265, 330, 430 258, 302, 415 263, 395

λmax (nm)

Emb Imax (au)

λmax (nm)

Imax (au)

quantumc yield φf (%)

NF NF 476

NF NF 40

474 488 576

922 53 39

32 6.37 NDd

590, 735 504

11, 5

593

8

NDd

16

487

16

NDd

a

In DMSO. bIn the solid state. NF means nonfluorescent, and Imax means the intensity maximum. Excitation slit width of 2.5 nm and emission slit width of 5 nm. cSolid state quantum yield using the integrating sphere technique. dNot determined. 4767

DOI: 10.1021/acs.joc.7b00456 J. Org. Chem. 2017, 82, 4766−4773

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Figure 1. Fluorescence spectra of synthesized compounds 3 (TPEN), 5a, 5b, 7, and 9 (a) in a DMSO solution (2.5 × 10−5 M) and (b) in the solid state.

Figure 2. Computed molecular orbital energy diagrams and isodensity surface plots of TPEN as obtained from TDDFT calculations.

Figure 3. (a) Variation in the fluorescence intensity of TPEN with the gradually increasing water fraction [from 0 to 99% (v/v) DMSO/water]. (b) Images of TPEN in DMSO with the increasing water fraction taken under UV light illumination (λex = 365 nm). (c) Variation of the fluorescence intensity of TPEN with a gradually increasing viscosity in a methanol/ethylene glycol gradient [from 10 to 99% (v/v)].

fraction in DMSO, leading to an increase in emission via the radiative pathway. The other synthesized TPE derivatives (5a, 4768

DOI: 10.1021/acs.joc.7b00456 J. Org. Chem. 2017, 82, 4766−4773

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The Journal of Organic Chemistry

Figure 4. (a) Emission spectra of TPEN solids and their photographs taken under UV illumination. (b) Emission maxima of TPEN during the grinding (G) and fuming (F) cycles. “As” represents “as prepared”, and λex = 360 nm. (c) Photographs of paper coated with TPEN under normal and UV light (λex = 365 nm).

5b, 7, and 9) possess the flexible enone moiety, which does not become restricted in the water fraction, leading to weak or no emission via nonradiative decay. Thus, on the basis of these results, the photoluminescence of TPEN was further investigated in DMSO/water mixtures with varying water fractions (f w) of 0−99% (Figure 3). The emission intensity remained weak until the water fraction reached 50% and then increased rapidly (f w = 60−99%). When f w reached 99%, the system became highly emissive and the intensity of the solution was remarkably enhanced (up to 180-fold) relative to that of the pure DMSO solution. These data indicate that TPEN is an AIEgen that is brightly illuminated in the increased water fraction (Figure 3b) upon inhibition of the nonradiative emission via restriction of the intramolecular rotation of phenyl moieties. To gain insight into the role of intramolecular rotation in the emissive behavior of TPEN, the effect of solvent viscosity on the absorption and emission characteristics was studied. The absorption and emission maxima of TPEN were not affected significantly with a change in the viscosity of the solution, but the degree of AIE enhancement was appreciable [up to 10-fold (Figure 3c)]. AIEgen TPEN showed very weak fluorescence in a low-viscosity solution (10% ethylene glycol) (Figure 3c). As the viscosity of the solution was increased by the increase in the relative volume of ethylene glycol (up to 99%), a dramatic enhancement of the fluorescence intensity was observed because of the restriction in intramolecular rotations (RIR). In low-viscosity solvents (low percentage of ethylene glycol), the intramolecular rotations of phenyl rings are responsible for the nonradiative decay that leads to weak emission. Inhibition of these nonradiative motions in viscous solvents promotes the emissive pathways, resulting in increased PL intensities. These studies confirmed the AIE behavior of TPEN.

Mechanochromic Properties. Keeping in mind the reports8 and our design strategy that alteration in the symmetry of core TPE moiety may contribute to interesting mechanochromic behavior, we explored the mechanochromic properties of the TPE conjugates by gently grinding the powder. Once grinding had been performed, the emission of the AIEgen TPEN shifted by 21 nm from the blue region (λmax = 474 nm) to the green region [λmax = 495 nm (Figure 4a)], while other compounds (5a, 5b, 7, and 9) did not show mechanochromic behavior. After dichloromethane vapor had been applied for a few seconds, the original blue emission was regained. This mechanochromic switching between blue and green emissions was reversible, but some fatigue was observed with an increase in the number of cycles (Figure 4b). The powder X-ray diffraction (PXRD) study of TPEN showed distinct peaks in the pristine form. Once grinding had been performed, the peak intensity decreased with a slight blue shift, indicating the change in the morphology of the compound. The ground sample was further subjected to dichloromethane fumes, which resulted in the recovery of peaks close to the pristine state (Figure S8). Because of the intrinsic amorphous nature of TPEN in its pristine state, we did not observe any major changes in the diffraction pattern upon grinding. In addition, the mechanochromic behavior of TPEN prompted us to develop a simple, facile, and portable ink-free rewritable paper. The compound was dissolved in dichloromethane (DCM), and then the filter paper was coated with TPEN (Figure 4c). After drying, the coated paper showed blue emission under UV irradiation. An ink-free plastic tip was used to write “CDRI” on the TPENloaded security paper, which was visible in normal light without any change in the pattern or color, but UV irradiation clearly illuminated the “CDRI” pattern engraved with a green emissive appearance. This pattern was erased upon exposure 4769

DOI: 10.1021/acs.joc.7b00456 J. Org. Chem. 2017, 82, 4766−4773

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Figure 5. Selectivity studies of TPEN (2.5 × 10−5 M) in the absence or presence of a series of perchlorate salts of metal ions (2.5 × 10−4 M) in a 9:1 (v/v) HEPES/DMSO mixture (pH 7.2), where λex = 360 nm. Blue bars denote the intensity difference (I0 − IM) in the absence or presence of a series of metal ions. Green bars denote the intensity difference (I0 − IM) of TPEN in the presence of metal ions and Ag+ ion. I0 is the intensity of TPEN only, and IM is the intensity of TPEN in the presence of metal ions.

Figure 6. (a) Normalized absorbance and (b) normalized fluorescence spectra of TPEN (2.5 × 10−5 M) at different concentrations of Ag+ ions (0−5.0 × 10−5 M) in a 9:1 (v/v) HEPES/DMSO mixture (pH 7.2). λex = 360 nm.

Figure 7. (a) Variation of the fluorescence intensity at 492 nm of TPEN with a gradual increase in the concentrations of Ag+ ions. (b) Variation of the intensity ratio of TPEN (8 × 10−7 M) with an increase in the concentration of Ag+ for the determination of the detection limit, where Io, Ic, and If are the intensities at 492 nm when the metal ion concentration is zero, the intensity at each metal ion concentration tested during titration, and the final intensity, respectively. λex = 360 nm.

Metal Ion Sensing Studies. Considering the metal chelating ability of the naphthyridine moiety36−42 toward metal cations, we examined the absorption and emission characteristics of TPEN in the presence of various metal ions in a 9:1 (v/v) HEPES/DMSO mixture at pH 7.2. TPEN showed significant changes in the absorption and emission characteristics in the presence of Ag+ ions, while other metal

to DCM vapor with the reappearance of blue emission under UV light. This process may be repeated efficiently for several cycles. Thus, we have so far observed that the introduction of a naphthyridine ring has altered the intrinsic property and symmetry of TPE, which accounted for the mechanochromic behavior of TPEN. 4770

DOI: 10.1021/acs.joc.7b00456 J. Org. Chem. 2017, 82, 4766−4773

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Figure 8. (a) Job’s plot showing 2:1 binding of TPEN with Ag+. (b) Schematic representation showing the 2:1 binding of TPEN with Ag+.

ions (perchlorate salts of Li+, Na+, K+, Cs+, Mg2+, Ca2+, Ba2+, Cr3+, Co2+, Mn2+, Fe2+, Fe3+, Cu+, Cu2+, Al3+, Zn2+, Pb2+, and Cd2+) remained silent or produced no considerable change (Figure 5). In the absence of metal ions, the buffered solution of TPEN showed absorption maxima at 357 nm, which gradually shifted to 365 nm with an increase in the concentration of Ag+ ions (Figure 6a,b). The naphthyridine-linked TPE conjugate (TPEN) displayed a significant bathochromic shift in its fluorescence maximum from 492 to 525 nm (blue to yellowish green) after addition of different concentrations of Ag+ ions (Figure 6b). Fluorescent chemosensors for silver ions have attracted considerable attention of researchers in the past few years.43−48 Thus, the unique property of TPEN binding Ag+ ions selectively prompted us to determine its sensitivity and binding affinity. Titration studies with TPEN revealed a good linearity between the fluorescence intensity and Ag+ concentration with a detection limit of approximately 0.25 μM in the aqueous medium (Figure 7). The detection limit of TPEN was below the U.S. Environmental Protection Agency (EPA) standard, which has set a secondary maximum contaminant level (SMCL) for silver metal at 0.1 mg L−1 (0.93 μM).49,50 Furthermore, Job’s plot established a 2:1 binding stoichiometry of TPEN with Ag+ (Figure 8a), which was supported by mass and high-resolution mass spectrometric (HRMS) analysis (Figure 8b and Figures S9 and S10). The binding constant (log β) of TPEN with Ag+ ions was found to be 12.2 ± 0.7 by the nonlinear least-square fit of the absorbance titration data for the 2:1 model using Hypspec51 (Figure S11). Thus, the studies described above showed that TPEN is a new mechanochromic AIEgen that is useful for the development of rewritable paper with additional capability for sensing silver ions in aqueous systems.

has shown great potential as a smart AIEgen with dual mechanochromic and chemosensing properties.

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EXPERIMENTAL SECTION

General Information. 1H NMR spectra were recorded at 400 MHz, and 13C NMR spectra were recorded at 100 MHz. Chemical shifts are reported in parts per million shift (δ value) from Me4Si (δ 0 for 1H) or based on the middle peak of the solvent (CDCl3) (δ 7.26 for 1H NMR and δ 77.00 for 13C NMR) or DMSO-d6 (δ 2.50 for 1H NMR and δ 40.00 for 13C NMR) as an internal standard. Signal patterns are indicated as s for singlet, d for doublet, t for triplet, and m for multiplet. Coupling constants (J) are given in hertz. Infrared (IR) spectra were recorded in KBr disks and reported in wavenumbers (cm−1). The ESI-MS spectra were recorded on a MICROMASS Quadro-II LCMS system. The HRMS spectra were recorded on a Quadrupole-TOF mass analyzer system. All the reactions were monitored by TLC, and visualization was achieved with UV light (254 nm). The solid state quantum yield was measured using a model F-3029, Quanta-Phi 6″ integrating sphere connected to a Horiba JobinYvon Fluorolog 3 spectrophotometer and slit widths for excitation and emission measurements kept at 1 nm. The powder X-ray diffraction of TPEN was recorded with a Rigaku MiniFlex 300 instrument at 25 °C, 40 kV, and 15 mA at a scan rate of 10° (2θ)/min (scan range of 5−80°). Synthesis. 2-[4-(1,2,2-Triphenylvinyl)phenyl]-1,8-naphthyridine (3). A mixture of 1-[4-(1,2,2-triphenylvinyl)phenyl]ethanone (374 mg, 1 mmol, 1 equiv), 2-aminonicotinaldehyde (122 mg, 1 mmol, 1 equiv), and 10% aqueous NaOH (2 mL) in ethanol was stirred at room temperature (25 °C) for 30 h. The progress of the reaction was monitored by TLC, and on completion, the reaction mixture was filtered and the residue purified on a silica gel column with 1% methanol in chloroform as the eluent to afford 345 mg (75%) as a white solid: Rf = 0.37 [98:2 (v/v) chloroform/methanol]; mp (chloroform/methanol) 206−208 °C; IR (KBr) ν 1703, 2209 (s) cm−1; 1H NMR (400 MHz, CDCl3) δ 6.94−7.31 (m, 17H), 7.34− 7.56 (m, 1H), 7.83−8.28 (m, 5H), 9.00−9.21 (m, 1H); 13C NMR (100.6 MHz, CDCl3) δ 119.6, 121.6, 125.8, 126.5, 126.6, 126.7, 127.2, 127.6, 127.7, 127.8, 128.5, 131.3, 131.4, 131.8, 136.3, 136.6, 137.5, 140.3, 141.8, 143.4, 143.4, 143.5, 145.9, 153.7, 156.1, 160.0; MS (ESI) 461 [M + H]+; HRMS calcd for C34H25N2 [M + H]+ 461.2018, found 461.1994. (E)-3-(4-Bromophenyl)-1-[4-(1,2,2-triphenylvinyl)phenyl]prop-2en-1-one (5a). A mixture of 1-[4-(1,2,2-triphenylvinyl)phenyl]ethanone (374 mg, 1 mmol, 1 equiv), 4-bromobenzaldehyde (185 mg, 1 mmol, 1 equiv), and 10% aqueous NaOH (2 mL) in ethanol was stirred at room temperature (25 °C) for 24 h. The progress of the reaction was monitored by TLC, and on completion, the reaction mixture was filtered and the residue purified on a silica gel column with chloroform as the eluent to afford 444 mg (82%) as a white solid: Rf = 0.40 [99:1 (v/v) chloroform/methanol]; mp (chloroform/ methanol) 168−170 °C; IR (KBr) ν 1606, 1680, 975 cm−1; 1H

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CONCLUSION In conclusion, we have synthesized a new TPE-based AIEgen TPEN, which exhibited an interesting mechanochromic property with a change in its emission from blue to green upon grinding and green to blue upon fuming. This property of TPEN was successfully demonstrated for the preparation of ink-free rewritable papers for security documentation purposes. Furthermore, the fluorescent probe TPEN showed high selectivity and strong affinity for silver ions in an aqueous system with a detection limit of 0.25 μM, which is lower than the permissible limit set by the EPA. Thus, TPEN 4771

DOI: 10.1021/acs.joc.7b00456 J. Org. Chem. 2017, 82, 4766−4773

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experiments were recorded after addition of an analyte at 25 °C under 405 nm excitation in a DMSO/HEPES mixture [9:1 (v/v), 20 mM, pH 7.2]. TDDFT Study of TPEN. To study the electronic behavior of TPEN, time-dependent density functional theory (TDDFT) calculations were performed with a Gaussian 09 package.35 The geometries were optimized at the DFT/B3LYP level using a 631G(d,p) basis set. TDDFT calculations were performed using a B3LYP/6-311++G(d,p) method. Silver Ion Sensing. Parent stock solutions (2.5 mM) of the perchlorate salts of all metal ions and synthesized TPEN (2.5 mM) were prepared in analytical grade DMSO. The absorption and fluorescence spectra for metal sensing experiments were recorded after addition of the analyte at 25 °C (λex = 360 nm) in a HEPES/ DMSO mixture [9:1 (v/v), 20 mM, pH 7.2]. Determination of the Detection Limit. The detection limit was calculated on the basis of the fluorescence titration. The fluorescence emission spectrum of TPEN (8 × 10−7 M) was measured at different concentrations of Ag+ ions in a HEPES/DMSO mixture [9:1 (v/v), 20 mM, pH 7.2] for the determination of the detection limit (λex = 360 nm), and a linear regression curve was fitted.52,53

NMR (400 MHz, CDCl3) δ 6.89−7.25 (m, 22H), 7.36−7.63 (m, 2H), 7.66−7.87 (m, 1H); 13C NMR (100.6 MHz, CDCl3) δ 56.3, 122.4, 124.7, 125.8, 126.4, 126.8, 126.9, 127.6, 127.7, 127.8, 127.9, 128.0, 128.1, 128.5, 129.7, 131.2, 131.3, 131.6, 132.2, 133.9, 135.7, 139.9, 142.7, 142.9, 143.0, 143.1, 143.2, 143.4, 149.0, 189.4; MS (ESI) 541 [M + H]+, 543 [M + 2 + H]+; HRMS calcd for C35H26BrO [M + H]+ 541.1167, found 541.1173, [M + 2 + H]+ 543.1147, found 543.1156. (E)-3-[4-(Dimethylamino)phenyl]-1-[4-(1,2,2-triphenylvinyl)phenyl]prop-2-en-1-one (5b). A mixture of 1-[4-(1,2,2triphenylvinyl)phenyl]ethanone (374 mg, 1 mmol, 1 equiv), 4,4(dimethylamino)benzaldehyde (149 mg, 1 mmol, 1 equiv), and 10% aqueous NaOH (2 mL) in ethanol was stirred at room temperature (25 °C) for 24 h. The progress of the reaction was monitored by TLC, and on completion, the reaction mixture was filtered and the residue purified on a silica gel column with 1% methanol in chloroform as the eluent to afford 353 mg (70%) as an orange solid: Rf = 0.37 [99:1 (v/v) chloroform/methanol]; mp (chloroform/ methanol) 180−182 °C; 1H NMR (400 MHz, CDCl3) δ 3.04 (s, 6H), 6.68 (d, J = 8.9 Hz, 2H), 6.92−7.25 (m, 18H), 7.52 (d, J = 8.9 Hz, 2H), 7.63−7.87 (m, 3H); 13C NMR (100.6 MHz, CDCl3) δ 40.1, 111.8, 116.8, 122.8, 126.7, 126.8, 127.7, 127.8, 127.9, 130.3, 131.3, 131.4, 136.8, 140.1, 142.3, 143.2, 143.2, 143.3, 145.4, 148.1, 152.0, 189.9; MS (ESI) 506 [M + H]+; HRMS calcd for C37H32NO [M + H]+ 506.2484, found 506.2476. (E)-3-[4′-(Dimethylamino)biphenyl-4-yl]-1-[4-(1,2,2triphenylvinyl)phenyl]prop-2-en-1-one (7). A solution of (E)-3-(4bromophenyl)-1-[4-(1,2,2-triphenylvinyl)phenyl]prop-2-en-1-one (541 mg, 1 mmol, 1 equiv) and N,N′-dimethylphenylboronic acid (268 mg, 2.2 mmol) in DMF (8 mL) was degassed with nitrogen for 20 min followed by addition of Na2CO3 (3.0 mL, 2 M) under a continuous flow of nitrogen. PdCl2(PPh3)2 (280 mg, 0.40 mmol) was added to the reaction mixture under a nitrogen atmosphere. The reaction mixture was stirred at 80 °C for 4 h. On completion, the reaction mixture was diluted with H2O (50 mL) and then extracted four times with chloroform (10 mL). The combined organic layer was dried over Na2SO4, and the solvent was vacuum evaporated to obtain the crude compound. The crude product was purified on a silica gel column using 1% methanol in chloroform to afford 484 mg (83%) as an orange solid: Rf = 0.38 [99:1 (v/v) chloroform/ methanol]; mp (chloroform/methanol) 184−186 °C; IR (KBr) ν 1600, 1655, 990 cm−1; 1H NMR (400 MHz, CDCl3) δ 3.01 (s, 6H), 6.80 (d, J = 8.9 Hz, 2H), 6.92−7.23 (m, 17H), 7.36−7.70 (m, 7H), 7.75−7.93 (m, 3H); 13C NMR (100.6 MHz, CDCl3) δ 40.4, 112.6, 120.9, 126.4, 126.7, 126.9, 127.7, 127.8, 127.9, 128.0, 129.0, 131.3, 131.5, 132.5, 136.2, 140.0, 142.5, 143.1, 143.2, 143.3, 143.3, 144.4, 148.6, 150.4, 189.8; MS (ESI) 582 [M + H]+; HRMS calcd for C43H36NO [M + H]+ 582.2797, found 582.2792. (E)-3-(1H-Indol-3-yl)-1-[4-(1,2,2-triphenylvinyl)phenyl]prop-2-en1-one (9). A mixture of 1-[4-(1,2,2-triphenylvinyl)phenyl]ethanone (374 mg, 1 mmol, 1 equiv), 1H-indole-3-carbaldehyde (145 mg, 1 mmol, 1 equiv), and 10% aqueous NaOH (2 mL) in ethanol was stirred at room temperature (25 °C) for 35 h. The progress of the reaction was monitored by TLC, and on completion, the reaction mixture was filtered and the residue purified on a silica gel column with 1% methanol in chloroform as the eluent to afford 340 mg (68%) as a white solid: Rf = 0.39 [20:1 (v/v) chloroform/methanol]; mp (chloroform/methanol) 220−224 °C; 1H NMR (400 MHz, CDCl3) δ 6.93−7.69 (m, 22H), 7.71−8.19 (m, 4H), 8.6 (b, 1H); 13 C NMR (100.6 MHz, DMSO-d6) δ 79.7, 113.2, 113.3, 115.0, 120.8, 121.5, 123.0, 125.9, 126.1, 126.9, 127.3, 127.4, 128.2, 128.3, 128.4, 128.5, 128.7, 130.0, 131.1, 131.2, 131.4, 134.7, 136.9, 138.8, 139.4, 140.4, 142.3, 143.3, 143.4, 143.8, 147.9, 188.3; MS (ESI) 502 [M + H]+; HRMS calcd for C37H27NO [M + H]+ 502.2171, found 502.2186. Photophysical Properties of Synthesized TPE Conjugates 3 (TPEN), 5a, 5b, 7, and 9. Absorption and emission spectra were recorded with a UV−vis and fluorescence spectrophotometer. Samples were prepared in DMSO at a final concentration of 2.5 × 10−5 M. The absorption and fluorescence spectra for metal sensing

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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/acs.joc.7b00456. Spectroscopic data and 1H NMR, 13C NMR, and UV− FL spectra of compounds 3, 5a, 5b, 7, and 9 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Fax: +91(522)2771941. E-mail: [email protected]. ORCID

Atul Goel: 0000-0003-2758-2461 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS A.G. thanks the Department of Atomic Energy (DAE-SRC), India, for an Outstanding Investigator Award (21/13/2015BRNS/35029). The authors acknowledge SAIF-CDRI for providing spectral data, Prof. Rajiv Prakash from IIT, BHU (Varanasi, India), for providing XRD data, and Dr. S. K. Asha from NCL (Pune, India) for solid state quantum yield measurements. This is institutional communication 9481.

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