Supramolecular Ensemble of a TICT-AIEE Active Pyrazine Derivative and CuO NPs: A Potential Photocatalytic System for Sonogashira Couplings

Research Article pubs.acs.org/acscatalysis

Supramolecular Ensemble of a TICT-AIEE Active Pyrazine Derivative and CuO NPs: A Potential Photocatalytic System for Sonogashira Couplings Harnimarta Deol, Subhamay Pramanik, Manoj Kumar, Imran A. Khan, and Vandana Bhalla* Department of Chemistry, UGC Sponsored Centre for Advanced Studies-II, Guru Nanak Dev University, Amritsar 143005, Punjab, India S Supporting Information *

ABSTRACT: The donor−acceptor system 4 having pyrazine scaffold as an acceptor moiety coupled to donor amino groups through rotatable phenyl rings has been synthesized, which formed aggregates in aqueous media, exhibited copper induced restriction to intramolecular rotation, and served as a “not quenched” probe for the detection of copper(II) ions. During this process, the aggregates of derivative 4 acted as reactors and stabilizers for the generation of CuO NPs and themselves became oxidized to form polyamine derivative 6. Interestingly, the oxidized species 6 in combination with copper oxide nanoparticles served as light-harvesting antennas and exhibited excellent photocatalytic efficiency in Sonogashira coupling under mild and eco-friendly conditions (room temperature, aqueous media, aerial conditions, and visible light irradiation). KEYWORDS: pyrazine, TICT, AIEE, CuO NPs, photocatalyst, Sonogashira cross-coupling

1. INTRODUCTION

out Sonogashira coupling under mild and environmentally friendly condition is still a challenge. Our research work involves the development of supramolecular aggregates which serve as reactors for the preparation of different types of metal nanoparticles10 and their utilization for carrying out various types of organic transformations such as click synthesis of triazoles,11 Beckmann rearrangement of aldoximes/ketoximes to primary/secondary amides,12 and Suzuki and Sonogashira couplings.13 Recently, we developed supramolecular assemblies of aggregation-induced emission enhancement (AIEE) active hexaphenylbenzene derivatives which served as “not quenched” reactors for the preparation of α-Fe2O3 nanoparticles.10 The in situ generated α-Fe2O3 NPs exhibited high catalytic efficiency in Sonogashira coupling between alkyl halides and terminal alkynes.14 The reaction conditions required the presence of K2CO3 as a base, ethylene glycol as solvent, and heating at 80 °C under inert conditions. In a continuation of this work, we were then interested in the development of a new catalytic system which could harvest the solar energy to carry out Sonogashira coupling in aqueous media under aerial conditions at room temperature. We envisioned that semiconductor nanoparticles in combination with dyestuff could serve as light-harvesting antennas for carrying out Sonogashira coupling under photocatalytic

Sonogashira coupling is one of the most powerful carbon− carbon bond forming reactions for the preparation of many important intermediates of various materials, drugs, and natural products.1 Under conventional conditions, Sonogashira coupling is catalyzed by palladium and requires harsh reaction conditions,2,3 which restrict the large-scale industrial applications of this reaction. Over the years, enormous efforts have been made to replace the costly and toxic palladium-based catalytic system with relatively cheap and benign metal-based catalytic systems. Further, growing environmental concern has encouraged scientists to develop new synthetic approaches having minimal effects on the ecosystem. In this direction a variety of catalytic systems based on copper,4 nickel,5 silver,6 and iron7 have been developed; however, most of these systems require the assistance of additional ligands and heating at high temperature for prolonged times to furnish the desired products in good yields. Recently, CuCl has been reported as a photocatalyst to carry out palladium-free Sonogashira coupling at room temperature under blue LED irradiation.8 The utilization of visible-light radiation9 for carrying out coupling is an economically viable approach and is beneficial to the environment; however, in the presence of this catalytic system all reactions were carried out in organic media under an inert atmosphere and completion of these reactions required irradiation with a blue LED for a longer period (15−27 h). Thus, the development of a novel catalytic system for carrying © 2016 American Chemical Society

Received: February 6, 2016 Revised: April 27, 2016 Published: May 3, 2016 3771

DOI: 10.1021/acscatal.6b00393 ACS Catal. 2016, 6, 3771−3783

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ACS Catalysis

2. RESULTS AND DISCUSSION 2.1. Synthesis and Characterization of Pyrazine Derivative. Suzuki−Miyaura coupling of the dibromo derivative of pyrazine, 122 with the boronic ester of aniline 223 in THF furnished the yellow compound 4 in 85% yield (Scheme 1). The structure of compound 4 was elucidated by

conditions. Copper oxide nanoparticles are our semiconductor material of choice due to their known catalytic efficiency in forming carbon−carbon bonds.15 Keeping this in view, we envisaged developing a supramolecular ensemble having copper oxide nanoparticles stabilized by assemblies of pyrazine derivatives as light harvesting materials.16 Pyrazine attached to rotors is our scaffold of choice as dyestuff due to its absorption in the visible region and its known AIEE characteristics.17 Pyrazine derivatives are also reported to exhibit stimuli-dependent transitions between local excited (LE) and twisted intramolecular charge transfer states (TICT) in aqueous media.18 We planned to take advantage of this transformation from the LE to TICT state and metal-induced restriction to intramolecular rotation (RIR) for the development of pyrazine-based AIEE-active supramolecular assemblies. In this context, we have designed and synthesized the luminescent donor−acceptor system 4 having a pyrazine scaffold as the acceptor moiety coupled to donor amino groups through rotatable phenyl rings. The amino groups were incorporated as donor moieties because of their known interactions with soft metal ions.19 Derivative 4 exhibited transitions from the TICT to LE state in aqueous media. However, partial restriction to TICT resulted in the formation of weakly emissive spherical aggregates. These aggregates of derivative 4 exhibited copper-induced restriction to intramolecular rotation and formed highly emissive aggregates which served as “not quenched” reactors and stabilizers for the preparation of CuO NPs. To the best of our knowledge, this is the first report where switching between TICT and LE states in combination with restriction to intramolecular motions has been explored for the development of “not quenched” probes for copper ions. Further, this is the first report of pyrazinebased assemblies showing CuO NP induced emission enhancement characteristics in aqueous media. The literature reports show that, for the preparation of oxidized copper nanoparticles, annealing at high temperature is a prerequisite.20 In this context, the reductant-free wet chemical method reported in the present article for the preparation of CuO NPs at room temperature is better than the other reported methods (Table S1 in the Supporting Information). Interestingly, during the reduction process, aggregates of derivative 4 themselves become oxidized to form polymeric species 6 and supramolecular ensembles consisting of oxidized species 6 and CuO NPs working as light-harvesting systems which exhibited excellent photocatalytic efficiency in Sonogashira crosscoupling reactions (Tables 1−3). To the best of our knowledge, this is the first report where aggregates of pyrazine derivatives have been utilized for the preparation of CuO NPs and supramolecular ensembles of semiconductor CuO NPs and pyrazine-based dyestuff have been used as photocatalytic system for carbon−carbon bond formation. All reactions were carried out at room temperature in mixed aqueous media under aerial conditions, and only 3−9 h of irradiation was required for the completion of the reactions. Additionally, all of the products were purified by simple recrystallization and without column chromatography. Interestingly, the catalytic efficiency of supramolecular ensemble 6:CuO NPs was found to be better than that of the other catalytic systems (photochemical and thermal conditions) for Sonogashira cross-couplings reported in the literature (Tables S2 and S3 in the Supporting Information).21

Scheme 1. Synthesis of Pyrazine-Based Derivatives 4 and 5

spectroscopic methods (Figure S35A−E in the Supporting Information). The 1H NMR spectrum of compound 4 showed two quartets at 8.18 and 7.76 ppm (2H, 2H) and four doublets 7.62, 7.54, 7.46, and 6.76 ppm (4H, 4H, 4H, 4H) corresponding to aromatic protons and a broad singlet at 3.76 ppm (4H) corresponding to protons of amino groups. The ESI-MS mass spectrum of compound 4 showed a molecular ion peak at m/z 465.2049 [M + H]+. These spectroscopic data corroborate the structure for compound 4. 2.2. TICT and AIEE Behavior of Derivative 4. The UV− vis spectrum of derivative 4 in water exhibits two absorption bands at 340 and 410 nm, respectively (Figure S1 in the Supporting Information). The absorption band at 340 nm is attributed to the n−π* transitions of the pyrazine moiety, and the absorption band at 410 nm is assigned to an intramolecular charge transfer transition from the donor to the acceptor.24 The solvatochromic effect on the absorption behavior of derivative 4 was investigated by switching the solvent from a nonpolar, e.g. hexane, to a polar aprotic solvent, e.g. THF, which showed a small bathochromic shift (∼6 nm) in the absorption band at 410 nm, while the band at 340 nm remained unaffected (Figure S2 in the Supporting Information). On the other hand, the emission spectrum of derivative 4 in HEPES buffer shows the presence of two bands. The band at 445 nm is attributed to the local emission state (LE), and the band at 555 nm is due to the TICT state (vide infra). Interestingly, a THF solution of derivative 4 showed the presence of a single emission at 555 nm on excitation at 360 nm. To get insight into the origin of the emission band at longer wavelength, we carried out temperature-dependent emission studies. Interestingly, when the temperature was increased from 25 to 75 °C, the emission band at 555 nm was blue-shifted and its intensity increased (Figure S3 in the Supporting Information). Such behavior is generally observed in the case of compounds having a TICT state.25 Thus, these temperature-dependent emission studies suggest the presence of a TICT state in compound 4. The formation of the TICT state was further confirmed by viscositydependent fluorescence studies25 of derivative 4 in different fractions of glycerol and ethanol. When the glycerol fraction in an ethanol solution of derivative 4 was increased, the viscosity of the solution increased, which inhibited the intramolecular 3772

DOI: 10.1021/acscatal.6b00393 ACS Catal. 2016, 6, 3771−3783

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Table 1. Results Obtained (eV) for the Support of TICT with DFT-6-31g(d) and TDDFT 631g(d) Basis Sets for the DihedralAngle-Wise Differentiated State 1 2 3 4 5 6

state

LUMO

HOMO

dipole moment

Ehe

Eexcitation

ΔES1T‑P

Ego Eex o (3°) Eex o (0°) Eex o (30°) Eex o (60°) Eex o (90°)

−0.0753 −0.0837 −0.0838 −0.0848 −0.0865 0.0868

−0.1939 −0.1839 −0.1843 −0.1896 −0.2015 −0.2058

3.6336 6.8808 6.7278 5.1941 2.9319 2.8783

0.1185 0.1003 0.1006 0.1048 0.1149 0.2926

2.8485 2.3324 2.3791 2.4843 2.7262 2.8115

−0.5161 0 0.0467 0.1519 0.3938

exhibited two weak emission bands at 445 nm (Φ = 0.03) and at 555 nm (Φ = 0.04) corresponding to LE and TICT states, respectively (Figure S8 in the Supporting Information). Though the intensity of the emission band corresponding to the LE state was enhanced, the intensity of the emission band corresponding to the TICT state was not fully quenched. The SEM and TEM images of derivative 4 show the presence of spherical aggregates (Figure S9 in the Supporting Information). On the basis of all of these studies, we believe that derivative 4 undergoes aggregation in aqueous media; however, the hydrophobic environment and formation of aggregates could not restrict the TICT state completely and hence the system exhibited weak AIEE characteristics in aqueous media. To confirm that incomplete suppression of TICT was the main reason for the weak AIEE characteritices of derivative 4, we synthesized the model compound 530 having a pyrazine moiety as acceptor and rotatable phenyl groups but without amino groups (Scheme 1). The compound 5 upon photoexcitation did not exhibit the existence of a TICT state but was found to be strongly AIEE active in aqueous media (Figure S10 in the Supporting Information). These studies confirm that compound 4 serves as a TICT-AIEE-based dualemissive probe. 2.2.1. Theoretical Studies (DFT) of TICT State. To get further insight into the TICT-AIEE behavior of compound 4, we carried out density functional theory (DFT) and molecular dynamic aided DFT calculations. To examine the possibility of TICT and PICT (planar intramolecular charge transfer) behavior31 in derivative 4, we carried out DFT calculations using B3LYP/6-31g(d) as a basis set. The PICT and TICT could be defined and explained as

rotation of the rotor, resulting in restriction of the TICT state and enhanced emission intensity of the LE state (Figure S4 in the Supporting Information). We also studied the solvatochromic behavior of derivative 4 in different solvents. A significant Stokes shift of 165 nm in the emission band was observed on changing the solvents from a nonpolar, e.g. pentane (λem 390 nm), to a polar aprotic solvent, e.g. THF (λem 555 nm), on excitation at 360 nm (Figure S5 in the Supporting Information).24 This is the largest Stokes shift ever reported for any pyrazine-based TICT-active probe. In nonpolar solvents (hexane, pentane, and toluene) (Table S4, entries 1−3, in the Supporting Information) and in polar protic solvents (methanol, ethanol, water, and glycerol) (Table S4, entries 5−8, in the Supporting Information) only LE emission was observed. However, in polar aprotic solvents with a low dielectric constant such as THF, dioxane, CHCl3, DCM, and EtOAc, TICT emission dominates (Table S4, entries 9−13 in the Supporting Information). Nonpolar solvents with high hydrophobicity restrict the twisted intramolecular charge transfer (TICT), while in polar protic solvents TICT emission is deactivated26 due to the formation of hydrogen bonds between amino groups of molecule 4 and solvent molecules. Interestingly, in polar aprotic solvents (DMF, DMSO, and ACN) with high dielectric constants, molecule 4 showed only LE emission (Table S4, entries 14−16 in the Supporting Information). This may be attributed to partial excited state proton transfer from the molecule to DMSO, which enhanced the TICT deactivation of the singlet excited state.27 This strong solvent effect upon varying the polarity of the solvent suggests the existence of a TICT state. To investigate the aggregation behavior of derivative 4, we carried out absorption and emission studies of derivative 4 in different THF/H2O fractions. The UV−vis absorption studies of derivative 4 in different water/THF fractions (0−99%) showed a slight decrease in the absorption intensity of the bands at 340 and 410 nm; however, no shift (red/blue) in the absorption maxima was observed (Figure S6 in the Supporting Information). Further, the temperature-dependent UV−vis studies of derivative 4 in water showed only an increase in the absorbance band at 340 nm when the temperature was increased from 25 to 70 °C, but no significant change in the positions of absorption maxima was observed (Figure S7 in the Supporting Information). These results show that, in the aggregated state, molecules were not arranged in a J/H fashion.28 All of the above studies clearly support the formation of a TICT state.29 Further, we carried out fluorescence studies of derivative 4 in different THF/H2O fractions. When the water fractions (0−99%) in relation to the THF solution of derivative 4 were increased, the intensity of the TICT band drastically decreased and a new band corresponding to an LE state appeared at 445 nm. Finally, the fluorescence spectrum of derivative 4 in 100% aqueous media (HEPES solution)

ES1 = EG + Egap + E he + E R

where ES1 is the relaxed first excited state energy, EG is the ground state energy obtained via a simple optimization at the B3LYP/6-31g(d) level. Egap is the HOMO−LUMO gap in ground state geometry, Ehe is the energy of the hole and electron interactions, obtained by subtracting the HOMO− LUMO gap energy (Egap) from the excitation energy calculated at the TD-B3LYP/6-31g(d) level at the ground state geometry, and ER is the relaxation energy of the excited state. The difference between TICT and PICT could be explained in terms of ΔES1T‐P = ΔEG T‐P + ΔEgap T‐P + ΔE he T‐P + ΔET‐P

where ΔES1T‑P is the ground state energy difference between the TICT and PICT states. The calculated value for ΔES1T‑P for compound 4 in the THF/water (0.5/99.5, v/v) system was found to be negative (−0.5161 eV). The negative value suggests that the twisted conformation is more stable in the excited state. The HOMO, HOMO-1 and LUMO, LUMO+1 energy gap also suggest that twisting of the H−Namine−C1−C2 3773

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to take place from a final quinoidal structure that resulted from the coupling of both local and charge-transfer excited states. The energy of the perpendicular conformation was found to be higher than that of the optimized geometry conformation by 0.04 eV in a THF/water (0.5/99.5, v/v) system. The formation of the stacked dimer was found to be unfavorable in the analysis carried out for the gas phase. The stacking of the monomer molecules in J and H patterns to form the n-mer was investigated through molecular dynamics aided DFT calculations via B3LYP-631g(d) basis sets. As expected, considerable aggregation of the probe molecules was observed in the larger simulation of four independent solute molecules. The coinlike stacking arrangement was found to be the predominant mode of the aggregates (Figures S11 and S12 in the Supporting Information).34 This stacking led to aggregates of various sizes, dynamically exchanging as monomers or larger aggregates added to the existing clusters, while other clusters broke up into smaller stacks or monomers. The molecules become oriented in such a way as to avoid the steric clashes between the arms of stacked molecules. These orientations were determined by changes in dipole vectors with variation of the angle. The potential steric clashes between the arms of stacked pairs of probe molecules led to a disordered 3fold symmetry with three probable relative positions for the DA. As might be expected, the stacking also perturbed the solvent conformation around the collective stack and the water molecules may be present between the proximate plane of each donor−acceptor. The simulation began with molecules randomly distributed in the cubic box. The formation of parallel, antiparallel, angular (due to π−π interactions), and Tshaped (due to CH−π interactions) conformation patterns within an intermolecular distance of 4.0 Å was observed.35 Initially, two dimers (in parallel and antiparallel patterns) were formed, one of which then grew to a trimer (21% of total simulation time in J and H stacking and the rest in random π−π stacking fashion) and then a tetramer (particularly formed due to monomer capture). These tetramers subsequently broke in two, giving a system of four possible combinations of dimers (in J (both in perpendicular, parallel, and T shaped) and H stacking patterns (as suggested by L torsion patterns in Figure S13A,B in the Supporting Information)) and two monomers. The system continued to exchange monomers in this fashion

dihedral angle from 0 to 90 °C increased the HOMO cloud at the twisted aromatic amino group, which favored the charge transfer.32,33 This conformation traces the twisting motion hypersurface and is transformed into the twisted conformation, which is a local minimum on the potential energy surface via a radiationless process. The calculated excitation energies for probe 4 in the twisted ground-state structure and their corresponding computed dipole moments are summarized in Table 1 (Table S5 in the Supporting Information). Molecular orbitals of the electronic ground state structure and that of the twisted geometry (90°) are displayed in Figure 1.

Figure 1. Contour MO plots for HOMO-1, HOMO, LUMO, and LUMO+1 obtained for probe 4 in a THF/water (0.5/99.5, v/v) system.

In its electronic ground state, the probe has an optimized geometry with C2v symmetry. In agreement with other theoretical results, the twisted S1 excited state (θ = 90°) was calculated to be higher than the optimized conformation (θ = 0°), by B3LYP-631g(d) methods. The fluorescence is proposed

Figure 2. (A) UV−vis spectra of derivative 4 (5 μM) upon addition of Cu2+ ions (0−50 equiv) in water. The inset shows 4-Cu complex formation at lower concentration (0−2 equiv). The appearance of the enlarged absorption band at 280 nm corresponds to CuO NPs. (B) Enlarged UV−vis spectra showing the bands at 280 nm. (C) NIR UV−vis spectrum showing a band at 750 nm corresponding to CuO NPs. 3774

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ACS Catalysis until a trimer and a pentamer combined to give a stacked octamer. Molecules of derivative 4 took 28 ns to form the octamer, which may be attributed to the slow diffusion rate of the solute molecules, and this octamer lasted for 1.0 ns. Thus, DFT and molecular dynamics studies suggest the formation of the TICT state and revealed no stacking of molecule in a J/H fashion. These studies further suggested that aggregation of molecules proceeded in an irregular way to form a globular pattern through π−π interactions between the aryl substrates. 2.3. Preparation of CuO Nanoparticles by Utilizing Aggregates of Derivative 4. The presence of amino groups in compound 4 prompted us to evaluate its molecular recognition behavior toward different metal ions, such as Ag+, Hg2+, Au3+, Zn2+, Cu2+, Fe2+, Fe3+, Co2+, Pb2+, Ni2+, Pd2+, and Mg2+, as their perchlorate/chloride salts by UV−vis and fluorescence spectroscopy. Upon gradual addition of Cu2+ ions (0−2 equiv) to the solution of 4 (5 μM) in H2O, the intensity of the absorption bands at 410 and 340 nm gradually decreased and a new band appeared at 310 nm with the appearance of two isosbestic points at 430 and 320 nm. This observation indicates the formation of a 4-Cu complex at lower concentration (Figure 2A and inset). Further addition of Cu2+ ions (3−50 equiv) resulted in the broadening of the absorption bands and the formation of new bands at 280 and 750 nm within 30 min (Figure 2B,C). The intensity of the absorption bands at 280 and 750 nm gradually increased with time (60 min). These spectral changes suggest the formation of CuO NPs.36 To get insight into the mechanism of CuO NP formation, we carried out the reaction between the aggregates of derivative 4 and Cu2+ ions under an inert atmosphere. The UV−vis spectrum of the reaction mixture after 1 h showed the appearance of a broad band at 600 nm which indicated the formation of Cu(0) nanoparticles.37 When the solution was kept for 10 min under aerial conditions, the band at 600 nm became broadened and two new bands appeared at 280 and 750 nm which suggested the formation of CuO NPs. These spectral changes were accompanied by a color change of the solution from blue to reddish and finally to black. On the basis of the above studies, we believe that initially the aggregates of derivative 4 reduce Cu(II) to Cu(0) NPs in aqueous solution (Figure S14 in the Supporting Information) and then these in situ generated NPs are oxidized to CuO NPs under aerial conditions. In the fluorescence spectrum, upon gradual addition of Cu2+ ions (0−50 equiv) to a solution of 4 (5 μM) in HEPES buffer, the emission intensity of the TICT band at 555 nm gradually decreased and a remarkable increase in emission intensity of a local excited band at 445 nm was observed (Φ = 0.36) (Figure 3). Further, we kept the solution as it was for 1 h and then recorded its emission spectrum, which showed a 42% decrease in the emission intensity of the band at 445 nm (vide infra) (Figure S15 in the Supporting Information). We also carried out time-resolved fluorescence studies of derivative 4 in the absence and in the presence of Cu2+ ions (Figure S16A in the Supporting Information). In the absence of Cu2+ ions, derivative 4 exhibited a single-exponential lifetime (100%, τ1 = 5.82 × 10−13 s) in HEPES buffer on measurement at 445 nm. On the other hand, in the presence of Cu2+ ions within 30 min, the fluorescence decay of derivative 4 was tripleexponential, which suggests the existence of three distinct species in the solution. The major fraction (76.84%) of the molecules was found to decay through the slower pathway (τ3). Further, a large decrease in nonradiative rate constant from

Figure 3. Fluorescence spectra of derivative 4 (5 μM) showing the ratiometric response upon addition of Cu2+ ions (0−50 equiv) in HEPES buffer with pH 7.05 and λex 358 nm.

166.3 × 109 to 0.116 × 109 s−1 was observed. On the other hand, when time-resolved fluorescence studies were carried out for emission at 555 nm in the presence of Cu2+ ions (Figure S16B in the Supporting Information), the major fraction (75.94%) of the molecules decayed through the faster pathway (τ3) and an increase in nonradiative rate constant from 0.329 × 109 to 5.08 × 109 s−1 was observed. These studies clearly indicate that the presence of Cu2+ ions accelerate the decrease in nonemissive rate constant of LE emission and increase in nonemissive rate constant of TICT emission, which is the main reason for the emission enhancement at 445 nm (Tables S6 and S7 in the Supporting Information). We believe that copper ions interact with aggregates of derivative 4 through amino groups and restrict the transition from the LE state to the TICT state. Further, in the presence of copper ions structural rigidification of the system was achieved, which is responsible for the emission enhancement of the supramolecular ensemble. Under the same conditions as used above for Cu2+ ions, we also tested the UV−vis and fluorescence response of aggregates of derivative 4 toward other metal ions such as Fe2+, Fe3+, Au3+, Co2+, Pb2+, Zn2+, Ni2+, Pd2+, Ag+, Hg2+, and Mg2+ as their chloride/perchlorate salts but no significant change in absorbance and emission behavior was observed in the presence of these metal ions (Figures S17A,B and 18A,B in the Supporting Information). 2.4. Characterization of CuO Nanoparticles. We also carried out scanning electron microscopic (SEM) and transmission electron microscopic (TEM) studies of aggregates of derivative 4 in the presence of copper ions to confirm the formation of CuO NPs. The SEM and TEM images clearly indicate the formation of CuO NPs on the surface of the spherical aggregates (Figure 4A,B). The SEM and TEM images of the oxidized species in H2O showed the formation of flakelike aggregates, thus suggesting the formation of polymeric species (Figure 4C,D). The HR-TEM image of derivative 4 in the presence of Cu2+ ions in aqueous media showed the presence of cupric oxide nanoparticles, and the interplanar spacing (Figure 4E−H) and size of the particles was found to be in range of 20−50 nm, as suggested by DLS studies (Figure S19 in the Supporting Information). The powder X-ray diffraction (XRD) studies of the precipitates (obtained by evaporating a water solution of derivative 4 and CuCl2) showed the presence of diffraction peaks located at 2θ values of 32.64, 35.62, 38.86, 49.06, 53.84, 58.60, 61.6, 66.4, 68.22, 72.26, and 75.0°, respectively, which supported the formation of CuO 3775

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Figure 4. (A, C) SEM images showing (A) aggregates of derivative 4 in the presence of Cu2+ ions and (C) oxidized species of derivative 4, i.e. polyamine 6. (B, D, E) TEM images showing (B) formation of CuO nanoparticles on the surface of aggregates of derivative 4, (D) oxidized species of derivative 4, i.e. polyamine 6, and (E) generated CuO NPs. (F−H) HRTEM images of CuO NPs showing interplanar spacing of planes (002), (110), and (111) respectively.

Scheme 2. Probable Representation of CuO NP Formation by Using Aggregates of Derivative 4 in Aqueous Media, Resulting in the Formation of the Supramolecular Ensemble 6:CuO NPs

days, precipitates were obtained, which were filtered and washed with THF. The 1H NMR spectrum of the residue so obtained after evaporation of a THF solution showed the broadening of all of the aromatic protons with the appearance of additional peaks at 7.04, 7.16, and 7.20 ppm and a broad peak at 7.8 ppm corresponding to the formation of polyamine species 6 (Figure S23 in the Supporting Information).39 Furthermore, we also carried out the reaction between derivative 4 and TBHP in THF/water (0.5/9.5, v/v) media under laboratory conditions. After the completion of the reaction (TLC), the oxidized product was isolated and its 1H NMR spectrum was recorded. Interestingly, the 1H NMR spectrum was found to be similar to that of the above residue obtained after the removal of CuO NPs (vide supra) (Table S8 in the Supporting Information). The FT-IR spectrum showed a broad stretching band at 3500 cm−1 corresponding to primary and secondary amines in the polymeric chain (Figure S24 in the Supporting Information).40 We believe that, upon addition of CuCl2 to the solution of aggregates of derivative 4, Cu2+ ions became reduced to Cu(0) NPs and, during this process, aggregates of 4 were oxidized to polyamine species 6.41 To get insight into the formation of polymeric oxidized species, we

nanoparticles38 (Figure S20 in the Supporting Information). The FT-IR spectrum of these nanoparticles exhibited five bands which confirmed the cupric oxide (Cu−O) architecture of the NPs (Figure S21 in the Supporting Information).20c On the basis of all these studies, we believe that the aggregates of derivative 4 acted as reactors to reduce Cu2+ ions to the Cu(0) state in the aqueous state and during this process they themselves became oxidized to polyamine 6, as shown in Scheme 2. 2.5. Characterization of Oxidized Species. To confirm the oxidation of aggregates of derivative 4, we studied the fluorescence behavior of aggregates of derivative 4 in the presence of tert-butyl hydroperoxide (TBHP, a strong oxidizing agent). Upon addition of TBHP (50 equiv) to a solution of aggregates of 4, an increase in the emission intensity at 445 nm was observed (Figure S22 in the Supporting Information). This fluorescence behavior is same as was observed for aggregates of derivative 4 in the presence of Cu2+ ions. These results suggest the oxidation of derivative 4 in the presence of Cu2+ ions which is responsible for the emission enhancement at 445 nm. To investigate further the structure of oxidized species, we slowly evaporated a solution of derivative 4 containing CuCl2. After 2 3776

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ACS Catalysis carried out the reaction between derivative 4 and Cu2+ ions in a THF/water (0.5/9.5, v/v) fraction under laboratory conditions. The progress of the reaction was monitored by thin-layer chromatography (TLC), and after 10 min formation of two spots was observed. The reaction was quenched immediately, and two products were isolated by flash column chromatography. The 1H NMR and ESI-MS spectroscopic data of these two compounds corroborate the dimeric and trimeric polyamine species 6d and 6t (please see Figures S25−S27 in the Supporting Information).42 To further understand whether the oxidation product of derivative 4, i.e. oxidized polyamine species 6, plays any role in the formation of CuO NPs, we carried out the UV−vis studies of polyamine species 6 in the presence and in the absence of Cu2+ ions (prepared by adding TBHP to the solution of aggregates of derivative 4). The solution of oxidized species 6 in aqueous media showed absorption in the visible region; however, upon addition of Cu2+ ions no absorption band corresponding to CuO NPs was observed in the UV−vis spectrum (Figure S28 in the Supporting Information). This study showed that oxidized species 6 did not play any role in the formation of CuO NPs but stabilized the in situ generated CuO NPs. The fluorescence spectrum of oxidized species 6 (10 μM) in H2O solution exhibited an emission band at 460 nm on excitation at 360 nm. Upon addition of an aqueous dispersion of CuO NPs (100 μL) to this solution, 55% quenching of the emission was observed (Figure S29A in the Supporting Information). We also carried out time-resolved fluorescence studies of oxidized species 6 in the absence and presence of an aqueous dispersion of CuO NPs. The solution of oxidized species in H2O showed a decay time of 0.92 ns; however, in the presence of CuO NPs (100 μL dispersion in water) the decay time decreased to 0.12 ns (Figure S29B in the Supporting Information). Furthermore, significant overlap was observed between the emission spectrum of oxidized species 6 and the absorption spectrum of CuO NPs which indicated energy transfer from the polyamine (oxidized form, 6) derivative to CuO NPs (Figure S30 in the Supporting Information).43 The above studies support the energy transfer from pyrazine-based dyestuff to CuO NPs. As we know that CuO NPs serve as a semiconducting material and absorb visible light irradiation, we have thus calculated the band gap of CuO NPs from the absorption spectrum. The band gap was found to be 1.65 eV (λab 750 nm), which lies in the reported range (1.2−1.9 eV) of the indirect band gap of intrinsic CuO NPs p-type semiconductors and is narrow enough to facilitate the photocatalytic reaction.44 2.6. Photocatalytic Activity of CuO NPs for Sonogashira Coupling. The energy transfer from pyrazine to CuO NPs encouraged us to examine the catalytic activity of the in situ generated supramolecular ensemble 6:CuO NPs in the Sonogashira−Hagihara cross-coupling in the presence of visible light, as CuO NPs are known to catalyze C−C bond forming reactions. We carried out a reaction between iodobenzene (7a) and phenylacetylene (8) as a model reaction using K2CO3 (1 equiv) as a base and in situ generated supramolecular ensemble 6:CuO NPs (0.5 mol %) under aerial and visible light irradiation in H2O/EtOH (8/2) medium (Scheme 3). A 100 W tungsten filament bulb was used as the irradiation source, and the round-bottom flask was immersed in a water bath to inhibit the photoheating effect. To our pleasure, the reaction was

Scheme 3. Sonogashira Cross-Coupling between Iodobenzene (7a) and Phenylacetylene (8) Catalyzed by in Situ Generated Supramolecular Ensemble 6:CuO NPs in the Presence of Visible Light

complete in 3 h and the desired product 9a was obtained in 89% yield (Table 2, entry 4). We also studied the effect of different solvents such as triethylene glycol (TEG), EtOH, water, and an EtOH/H2O mixture (Table 2, entries 1−5). The desired product was obtained in comparable yields in the case of EtOH and H2O/EtOH (1/1), and H2O/EtOH (8:2) solvent mixtures. Thus, we choose H2O/EtOH (8/2) as the reaction medium for carrying out further reactions. We also performed the model reaction under dark conditions and the coupled product was obtained in lower yield, 30% (Table 2, entry 6). The above results suggest that the active phase of CuO NPs along with light-harvesting polyamine species are necessary for Sonogashira coupling under photocatalytic conditions. We also investigated the effect of base in photocatalytic Sonogashira coupling in this model reaction. Interestingly, the presence of K2CO3 in higher concentration (2 equiv) had no effect on the yield of the reaction. However, in the absence of K2CO3, the desired product was not obtained, which suggests that a basic medium is a prerequisite for reaction (Table 2, entries 7 and 8). To rule out the possibility of contamination of palladium in K2CO3, we also carried out a model reaction in the presence of Cs2CO3 as a base and the desired product was obtained in 89% yield (Table 2, entry 9). Further, in the absence of supramolecular ensemble 6:CuO NPs the reaction did not proceed at all (Table 2, entry 10). Furthermore, in the presence of only oxidized species 6, the model reaction did not proceed (Table 2, entry 11). To understand the role of aggregates of derivative 4/oxidized species 6 along with CuO NPs in Sonogashira coupling reactions, we prepared bare CuO NPs of sizes in the range of 30−60 nm by a hydrothermal method (Figure S31 and S32 in the Supporting Information).20c We evaluated the catalytic efficiency of these bare CuO NPs in the Sonogashira cross-coupling of iodobenzene with phenylacetylene under visible light. The reaction was complete within 12 h to furnish the desired product in 48% yield (Table 2, entry 12). Interestingly, the addition of aggregates of derivative 4 to this reaction mixture did not affect the yield of the reaction (Table 2, entry 13), while upon addition of oxidized species of derivative 4 (residue obtained after filtrate of CuO NPs) the desired product was obtained in 86% yield (Table 2, entry 14). These results highlight the importance of supramolecular ensemble 6:CuO NPs in Sonogashira cross-coupling reactions under visible light. In the next part of our investigation, we prepared several mixtures of photocatalytic supramolecular ensemble by mixing oxidized species 6 and CuO NPs in different ratios such as 1:1, 2:1, 1:2, 3:1, 1:3, 4:1, 1:4, 5:1, and 1:5 (Table S9 in the Supporting Information). The reaction between iodobenzene (7a) and phenylacetylene (8) was chosen as a model reaction. When the ratio of oxidized species and CuO NPs was 1:1, the alkynylated product diphenylacetylene (9a) was formed in 86% yield. Upon changing this ratio from 1:1 to 2:1, the yield of the product increased from 86 to 90% (Table S9, entries 1 and 2, in the Supporting Information). 3777

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ACS Catalysis Table 2. Sonogashira Cross-Coupling between Iodobenzene and Phenylacetylene under Different Conditions entry

conditions

time (h)

base (K2CO3) (equiv)

1 2 3 4 5 6 7 8 9 10 11 12 13 14

in situ generated supramolecular ensemble 6:CuO NPs in situ generated supramolecular ensemble 6:CuO NPs in situ generated supramolecular ensemble 6:CuO NPs in situ generated supramolecular ensemble 6:CuO NPs in situ generated supramolecular ensemble 6:CuO NPs reaction in dark (in situ generated supramolecular ensemble 6:CuO NPs) in situ generated supramolecular ensemble 6:CuO NPs in situ generated supramolecular ensemble 6:CuO NPs in situ generated supramolecular ensemble 6:CuO NPs without (6:CuO NPs) catalyst only in presence of oxidized species 6 bare CuO NPs bare CuO NPs + derivative 4 bare CuO NPs + oxidized species 6 (1/1)

8 3 3 3 7 12 12 3 3 12 24 12 12 3

1 1 1 1 1 1 0 2 1 (Cs2CO3) 1 1 1 1 1

solvent TEG EtOH EtOH/H2O H2O/EtOH H2O H2O/EtOH H2O/EtOH H2O/EtOH H2O/EtOH H2O/EtOH H2O/EtOH H2O/EtOH H2O/EtOH H2O/EtOH

isolated yield (%)

(1/1) (8/2) (8/2) (8/2) (8/2) (8/2) (8/2) (8/2) (8/2) (8/2) (8/2)

68 90 88 89 60 30 0 89 89 0 0 48 50 86

Table 3. Effect of Halide Group in Sonogashira Cross-Coupling between Alkyl Halide and Phenylacetylene in the Presence of Visible Light by Using in Situ Generated Supramolecular Ensemble 6:CuO NPs as Catalysts

this study, we believe that oxidized species has an important role in the photocatalytic efficiency of supramolecular ensemble 6:CuO NPs in Sonogashira coupling reactions. To check the scope of reaction with respect to aryl halides, we carried out Sonogashira cross-couplings between different aryl halides and phenylacetylene in the presence of supramolecular ensemble 6:CuO NPs (Table 3). Interestingly, in the presence of supramolecular ensemble 6:CuO NPs as a catalyst, Sonogashira cross couplings involving bromobenzene, chlorobenzene, p-chloroaniline went smoothly to yield the products 9a,b in good to moderate yield (Table 3, entries 2−4). Thus, supramolecular ensemble 6:CuO NPs are capable of catalyzing Sonogashira coupling of alkyne and aryl halide having chloride as the leaving group. In the presence of supramolecular ensemble 6:CuO NPs, 4-bromo-1-iodobenzene reacted with phenylacetylene to furnish the 4-bromo diphenylacetylene 9c in 72% yield (Table 3, entry 5). These studies highlight the

Interestingly, upon changing the ratio of 6:CuO NPs from 1:1 to 1:2, the yield of the product decreased from 86% to 80% (Table S9, entries 1 and 3, in the Supporting Information). A further increase in the amount of polyamine species up to 5:1 had no significant effect on the yield of the product, while upon a decrease in the amount of polyamine species 6 in the supramolecular ensemble by changing the ratio from 1:1 to 1:5 (6:CuO NPs), the yield (60%) of the target product significantly decreased (Table S9, entries 8 and 9, in the Supporting Information). To understand the reason behind the decrease in the yield of the product, we carried out TEM studies of a sample having oxidized species and CuO NPs in 1:2 ratio (6:CuO NPs) (Figure S33 in the Supporting Information). The TEM images indicated that metal nanoparticles accumulate when the polyamine species is present in lower concentration (6:CuO = 1:2); hence, the catalytic activity is reduced. On the basis of 3778

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Table 4. Sonogashira Cross-Coupling between Aryl Halides 7f−r and Phenylacetylene (8) in the Presence of Visible Light Catalyzed by in Situ Generated Supramolecular Ensemble 6:CuO NPs

Further, we investigated the scope of the catalytic system with regard to aryl iodides having different substituents (Table 4). Interestingly, aryl iodides 7f−l bearing electron-donating

excellent catalytic efficiency and high selectivity of in situ supramolecular ensemble 6:CuO NPs in the Sonogashira coupling under visible light. 3779

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ACS Catalysis Scheme 4. Proposed Mechanism of Photocatalytic Sonogashira Cross-Coupling Reaction by Using 6:CuO

the intermediate D to furnish the desired alkynylated products, as shown in Scheme 4. To check the generation of free radical during the reaction process, we carried out a reaction between iodobenzene (7a) and phenylacetylene (8) in the presence of the well-known radical scavenger 2,2,6,6-tetramethyl-1-piperidinoxyl (TEMPO; 1 equiv).46 Interestingly, the reaction did not proceed, which supports our assumption that the reaction proceeds through a free radical pathway. This mechanism also supports that aryl halides bearing electron-donating substituents give higher yields in comparison to those with electronwithdrawing groups. 2.8. Recyclability and Reusability of in Situ Generated CuO NPs. We evaluated the efficiency of in situ generated CuO NPs for the reaction between aryl iodide and phenylacetylene using various amounts of catalyst (Table 5). When 5000 ppm of catalyst was used, the desired product was obtained in 89% yield (Table 5, entry 1) in 3 h and 0% when the reaction was carried out without catalyst. When the catalyst amount was reduced down to 1 ppm, then 28 h was required to furnish the desired product in 78% yield (Table 5, entry 8). Such an extremely low quantity of photocatalyst has never been successfully used for Sonogashira couplings before the present study. Further, we chose the reaction between iodobenzene (7a) and phenylacetylene (8) as a model reaction to determine the recyclability of the in situ supramolecular ensemble 6:CuO NP

substituents undergo Sonogashira coupling smoothly with phenylacetylene to furnish the desired products 9b,d−i in higher yields (Table 4, entries 1−7). The reaction conditions could also tolerate −CHO and −NO2 functionalities (Table 4, entries 8 and 9). However, in case of nitro-substituted aryl halide, the reaction took longer time (12 h) to furnish the desired product in 45% yield. Furthermore, in the presence of in situ supramolecular ensemble 6:CuO NPs, 2-thiophenyl bromide and 3-pyridinyl bromide reacted with phenylacetylene to furnish the arylated alkynes 9l−m in good yields (Table 4, entries 10 and 11). To check the practical applications of in situ generated supramolecular ensemble 6:CuO NPs as photocatalysts, we carried out the reaction between a bromo derivative of triphenylamine (7q) and hexaphenylbenzene (7r) with phenylacetylene to furnish the desired products 9n,o in 72 and 58% yield, respectively (Table 4, entries 12 and 13). 2.7. Mechanism of Photocatalytic Sonogashira Coupling by Supramolecular Ensemble 6:CuO NPs. We believe that the reaction follows a single-electron transfer (SET)45 mechanism and photoexcited electrons generated in the CuO NPs activate the C−H bond of phenylacetylene to form the CuO−phenylacetylide adduct A, in basic media. In the next step, this adduct A acts as a catalytic photosensitizer and generates the aryl radical C, forming complex B. Finally, the aryl radical C attacks the copper acetylide complex B to form 3780

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resulting solution was stirred at room temperature for 60 min, and formation of black CuO nanoparticles took place. A 200 μL portion of this solution (0.5 mol %) was used as such in each catalytic experiment. 4.4. Synthesis of Pyrazine Derivative 4. To a solution of 2,3-bis(4-bromophenyl)quinoxaline (1; 0.4 g, 0.909 mmol) and 4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)aniline (2; 0.5 g, 2.27 mmol) in 20 mL of THF was added 2 mL of an aqueous solution of K2CO3 (1g, 7.72 mmol) followed by addition of [Pd(PPh3)2Cl2] (0.383g, 0.545 mmol) as a catalyst under an N2 atmosphere. The reaction mixture was refluxed overnight and thereafter cooled to room temperature and treated with water. The aqueous layer was extracted with CHCl3 (3 × 10 mL), and the combined organic layers were dried over anhydrous sodium sulfate and then distilled under reduced pressure to give a solid residue. The desired product was isolated by column chromatography using 80/20 CHCl3/hexane as an eluent followed by recrystallization with a 5/1 CHCl3/MeOH mixture to give 0.4 g (85%) of compound 4 as an orange solid: mp >260 °C (Scheme 1). 1H NMR (500 MHz, CDCl3): δ 8.18 (dd, J = 5 Hz, 2H), 7.76 (dd, J = 5 Hz, 2H), 7.62 (d, J = 10 Hz, 4H), 7.54 (d, J = 10 Hz, 4H), 7.46 (d, J = 5 Hz, 4H), 6.76 (d, J = 10 Hz, 4H), 3.76 (s, 4H, NH2) ppm. 13C NMR (125 MHz, CDCl3): δ 153.3, 143.7, 140.9, 139.3, 137.0, 132.3, 130.6, 130.2, 129.8, 129.2, 128.0, 126.1, 115.4 ppm. ESI-MS found for compound 4 (m/z): 465.2049 [M + H]+ and 522.3110 [M + H2O + K]+. The FT-IR spectrum showed bands at 3315, 1596, and 1190 cm−1 due to N−H stretching, CN bonding, and C−N bond stretching, respectively. Anal. Calcd for 4 for C32H24N4: C, 82.73; H, 5.21; N, 12.06. Found: C, 82.74; H, 5.20; N, 12.06. 4.5. General Procedure for Photocatalytic Sonogashira-Hagihara Cross-Coupling Catalyzed by Supramolecular Ensemble 6:CuO NPs. A mixture of aryl iodide 7a−r (1 mmol), phenylacetylene (8; 100 mg, 1 mmol; in the case of 7r, 200 mg, 2 mmol), and K2CO3 (276 mg, 2 mmol) in H2O/EtOH (8/2) in the presence of 6:CuO NP (0.5 mol %) as a photocatalyst was stirred for 3−12 h at room temperature under visible light (Tables 2−4) (only in the case of 7r ethanol used as a solvent). After the completion of the reaction (TLC), the reaction mixture was treated with water and extracted with CHCl3 and the combined organic layers were dried over anhydrous sodium sulfate and distilled under reduced pressure to give a solid residue. The desired products 9a−o were purified by recrystallization in a methanol/chloroform (5/1) mixture. The alkynylated compounds 9a−o were found in good yield and were confirmed from their spectroscopic and analytical data (Figures S36−S50 in the Supporting Information). 4.5.1. Compound 9a.14 1H NMR (500 MHz, CDCl3): δ 7.55−7.51 (m, 4H), 7.37−7.30 (m, 6H) ppm. 4.5.2. Compound 9b.48 1H NMR (500 MHz, CDCl3): δ 7.86 (d, J = 10 Hz, 1H), 7.49 (d, J = 5 Hz, 1H), 7.41 (d, J = 5 Hz, 2H), 7.35−7.30 (m, 2H), 6.62 (d, J = 10 Hz, 1H), 6.46 (d, J = 10 Hz, 2H), 3.49 (s, 2H, NH2) ppm. 4.5.3. Compound 9c.14 1H NMR (300 MHz, CDCl3): δ 7.57−7.55 (m, 2H), 7.51 (d, J = 5 Hz, 2H), 7.42 (d, J = 10 Hz, 2H), 7.39−7.38 (m, 3H) ppm. 4.5.4. Compound 9d.14 1H NMR (500 MHz, CDCl3): δ 7.55 (d, J = 10 Hz, 2H), 7.51 (d, J = 5 Hz, 1H), 7.47 (d, J = 10 Hz, 1H), 7.33 (dd, J = 10 Hz, 1H), 6.88 (d, J = 10 Hz, 1H), 6.67 (t, J = 7.5 Hz, 3H), 3.48 (s, 3H, OMe) ppm.

Table 5. Sonogashira Coupling Reaction of 1a and 2 by Using Various Amount of in Situ Generated Supramolecular Ensemble 6:CuO NPs entry

CuO NP (ppm)

time (h)

yield (%)

TON

TOF

1 2 3 4 5 6 7 8 9

5000 4000 3000 1500 1000 100 10 1 0

3 5 8 12 16 20 24 28 0

89 89 86 84 84 82 80 78 0

17.8 22.25 28.66 56 84 820 8000 78000 0

5.93 4.45 3.58 4.66 5.25 41 333.3 2785.71 0

catalyst. After the completion of the reaction, the product was extracted using organic solvent (CHCl3) and the aqueous layer having the catalyst was used as such in the next cycle of the reaction. The product yield remained quantitative even after six cycles of the reaction without any change in catalytic activity; the TEM images of recycled supramolecular ensemble 6:CuO NPs show no change in size and morphology (Figure S34 in the Supporting Information).

3. CONCLUSIONS In conclusion, we synthesized the pyrazine derivative 4, which formed weakly fluorescent aggregates in aqueous media. The aggregates of derivative 4 exhibited a “not quenched” response toward copper ions and served as reactors for the generation of CuO NPs in aqueous media. During this process, the aggregates of derivative 4 themselves became oxidized to form polyamine species 6. Interestingly, these in situ generated supramolecular ensembles of oxidized species 6:CuO NPs exhibited excellent catalytic efficiency in Sonogashira couplings under mild conditions with a wide variety of substrates such as aryl halides including iodide, bromide, and chloride (room temperature, aqueous media, aerial conditions, and visible light). 4. EXPERIMENTAL SECTION 4.1. General Experimental Methods and Materials.47 The general experimental methods, quantum yield calculations, and materials used are same as those reported earlier by us.47 The TEM images were recorded using an HR-TEM-JEM 2100 microscope. The FT-IR spectra were recorded using a Varian 660 IR spectrometer. 4.2. UV−Vis and Fluorescence Titrations.47 A 10−3 M stock solution of compound 4 was prepared by dissolving 4.64 mg of compound 4 in 10.0 mL of dry THF. A 15 μL portion of this stock solution was further diluted with 2985 μL of water/ HEPES buffer (0.05 M, pH 7.05) to prepare a 3.0 mL solution of derivative 4 (5.0 μM), and this solution was used for each UV−vis and fluorescence experiment. The aliquots of freshly prepared standard solutions of metal perchlorates (M(ClO4)x: M = Ag+, Hg2+, Au3+, Zn2+, Cu2+, Fe2+, Fe3+, Co2+, Pb2+, Ni2+, Pd2+, Mg2+; x = 1−3), chlorides (MCly: y = 1−3), and CuCl2 (10−1−10−3 M) in distilled water were added to a 3 mL solution of compound 4 placed in a quartz cuvette, and spectra were recorded. 4.3. Synthesis of Copper Oxide Nanoparticles (CuO NPs). A 1 mL portion of an aqueous solution of 0.1 M CuCl2 was added to 10 mL of a 0.01 M solution of compound 4 in distilled water (15 μL of THF was added to dissolve). The 3781

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4.5.5. Compound 9e.14 1H NMR (500 MHz, CDCl3): δ 7.53 (d, J = 10 Hz, 2H), 7.43 (d, J = 5 Hz, 1H), 7.37−7.31 (m, 4H), 7.16 (t, J = 7.5 Hz, 2H), 2.37 (s, 3H, Me) ppm. 4.5.6. Compound 9f. 1H NMR (500 MHz, CDCl3): δ 7.74− 7.65 (m, 1H), 7.55 (d, J = 7.5 Hz, 2H), 7.39−7.35 (m, 2H), 6.94 (d, J = 5 Hz, 2H), 6.52 (s, 1H), 3.45 (s, 4H, NH2) ppm. 13 C NMR (125 MHz, CDCl3): δ 136.4, 132.5, 129.2, 128.5, 128.4, 128.2, 122.6, 121.8, 119.1, 117.9, 81.6, 74.0 ppm. The ESI-MS mass spectrum of compound 9f showed a base peak at m/z 231.185 [M1+Na]+. Anal. Calcd for 9f, C14H12N2: C, 80.74; H, 5.81; N, 13.45. Found: C, 80.73; H, 5.81; N, 13.46. 4.5.7. Compound 9g.49 1H NMR (300 MHz, CDCl3): δ 7.51−7.44 (m, 2H), 7.31 (t, J = 6 Hz, 3H), 7.03−6.94 (m, 2H), 6.70 (d, J = 9 Hz, 1H), 3.83 (s, 3H, OMe), 3.82 (s, 3H, OMe) ppm. 4.5.8. Compound 9h.50 1H NMR (300 MHz, CDCl3): δ 7.65−7.62 (m, 2H), 7.46 (d, J = 9 Hz, 3H), 7.37 (s, 1H), 7.03 (s, 2H), 2.41 (s, 6H, Me) ppm. 4.5.9. Compound 9i.14 1H NMR (500 MHz, CDCl3): δ 9.92 (s, 1H, OH), 7.91 (t,, J = 12.5 Hz, 1H), 7.54 (t,, J = 12.5 Hz, 2H), 7.38−7.33 (m, 1H), 7.22 (t, J = 7.5 Hz, 1H), 7.07 (t, J = 10 Hz, 1H), 6.93 (t, J = 7.5 Hz, 1H), 6.80 (d, J = 10 Hz, 1H), 6.69 (d, J = 10 Hz, 1H) ppm. 4.5.10. Compound 9j.14 1H NMR (500 MHz, CDCl3): δ 8.24 (d, J = 10 Hz, 2H), 7.69 (d, J = 5 Hz, 2H), 7.59−7.57 (m, 2H), 7.43−7.42 (m, 2H), 7.41−7.40 (m, 1H) ppm. 4.5.11. Compound 9k.14 1H NMR (300 MHz, CDCl3): δ 10.07 (s, 1H, CHO), 8.21 (d, J = 9 Hz, 2H), 7.65 (d, J = 9 Hz, 2H), 7.55 (d, J = 5 Hz, 2H), 7.39 (d, J = 1.6 Hz, 3H) ppm. 4.5.12. Compound 9l.14 1H NMR (500 MHz, CDCl3): δ 7.56 (dd, J = 7.5 Hz, 2H), 7.43 (d, J = 10 Hz, 1H), 7.40−7.35 (m, 2H), 6.71 (d, J = 10 Hz, 2H), 6.49 (d, J = 10 Hz, 1H) ppm. 4.5.13. Compound 9m.14 1H NMR (500 MHz, CDCl3): δ 8.78 (s, 1H), 8.57 (d, J = 9 Hz, 1H), 7.82 (d, J = 6, 1H), 7.55− 7.49 (m, 2H), 7.38−7.34 (m, 4H) ppm. 4.5.14. Compound 9n.51 1H NMR (300 MHz, CDCl3): δ 7.22 (d, J = 9 Hz, 2H), 7.15 (t, J = 9 Hz, 6H), 6.94 (dd, J = 9 Hz, 7H), 6.84 (d, J = 9 Hz, 4H) ppm. 4.5.15. Compound 9o. 1H NMR (500 MHz, CDCl3): δ 7.73 (dd, J = 5 Hz, 2H), 7.56 (d, J = 10 Hz, 2H), 7.38 (t, J = 5 Hz, 2H), 7.07 (d, J = 10 Hz, 2H), 7.01 (d, J = 5 Hz, 2H), 6.94 (d, J = 10 Hz, 6H), 6.89−6.80 (m, 10H), 6.76−6.72 (m, 6H), 6.58 (d, J = 5 Hz, 2H), 6.54 (d, J = 5 Hz, 2H), 6.36 (d, J = 20 Hz, 2H) ppm. 13C NMR (125 MHz, CDCl3): δ 147.1, 140.6, 140.3, 138.8, 134.7, 134.5, 133.1, 131.7, 131.5, 131.3, 131.2, 130.3, 129.8, 129.7, 126.7, 126.6, 125.5, 125.1, 88.4, 84.2 ppm. The ESI-MS mass spectrum of compound 9o showed a molecular ion peak at m/z 773.5070 [M2 + K]+. Anal. Calcd for 9o, C58H38: C, 94.79; H, 5.21. Found: C, 94.77; H, 5.23.

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AUTHOR INFORMATION

Corresponding Author

*E-mail for V.B.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS V.B. is thankful to the SERB, New Delhi, India (ref. no. EMR/ 2014/000149), for financial support. We are thankful to Mr. Ravinder Singh for the TEM study. We are also thankful to the UGC (New Delhi) for the ‘‘University with Potential for Excellence’’ (UPE) project. H.D. is thankful to the UGC-BSR (New Delhi) for a fellowship. S.P. is thankful to the UGC (New Delhi) for a Senior Research Fellowship (SRF).

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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/acscatal.6b00393. 1 H and 13C NMR, mass, and IR spectra of compounds 4−6 and 9a−o, UV−vis and fluorescence studies, detection limits, SEM and TEM images, powder XRD analysis, DLS studies, comparison of present data with those in previous reports (PDF) 3782

DOI: 10.1021/acscatal.6b00393 ACS Catal. 2016, 6, 3771−3783

Research Article

ACS Catalysis

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DOI: 10.1021/acscatal.6b00393 ACS Catal. 2016, 6, 3771−3783