Supramolecular Ensemble of a TICT-AIEE Active Pyrazine Derivative

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Supramolecular ensemble of 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 ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b00393 • Publication Date (Web): 03 May 2016 Downloaded from http://pubs.acs.org on May 6, 2016

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Supramolecular ensemble of TICT-AIEE active Pyrazine derivative and CuO NPs: A Potential Photocatalytic system for Sonogashira couplings Harnimarta Deol, Subhamay Pramanik, Manoj Kumar, Imran A. Khan, Vandana Bhalla* Department of Chemistry, UGC Sponsored Centre for Advanced Studies-II Guru Nanak Dev University, Amritsar 143005, and Punjab, India

ABSTRACT: A donor-acceptor system 4 having pyrazine scaffold as 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 “not quenched” probe for the detection of copper (II) ions. During this process, the aggregates of derivative 4 acted as reactors and stabilizers for generation of CuO NPs and themselves got oxidized to form polyamine derivative 6. Interestingly, the oxidized species 6 in combination with copper oxide nanoparticles served as light harvesting antenna and exhibited excellent photocatalytic efficiency in Sonogashira coupling under mild and ecofriendly conditions (room temperature, aqueous media, aerial condition and visible light irradiation).

KEYWORDS: pyrazine, TICT, AIEE, CuO NPs, photocatalyst, Sonogashira cross coupling

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INTRODUCTION 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 palladium catalyzed and requires harsh reaction conditions2-3 which restricts the large scale industrial applications of this reaction. Over the years, enormous efforts have been made to replace 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 effect on ecosystem. In this direction a variety of catalytic systems based on copper4, nickel5, silver6 and iron7 have been developed, however, most of these catalytic systems require the assistance of additional ligands and heating at high temperature for prolonged time to furnish the desired products in good yields. Recently, CuCl has been reported as photocatalyst to carry out palladium free Sonogashira coupling at room temperature under blue LED irradiation.8 The utilization of visible-light radiations9 for carrying out coupling is an economically viable approach and beneficial to environment, however, in the presence of this catalytic system all the reactions were carried out in organic media under inert atmosphere and completion of reaction required irradiation with blue LED for a longer period (15-27 h). Thus, the development of a novel catalytic system for carrying out Sonogashira coupling under mild and environmental 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 triazoles11, Beckmann rearrangement of aldoximes/ketoximes to primary/secondary amides12, Suzuki and


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Sonogashira couplings13. Recently, we developed supramolecular assemblies of AIEE active hexaphenylbenzene derivative which served as ‘not quenched’ reactors for preparation of αFe2O3 nanoparticles.10 The in situ generated α-Fe2O3 NPs exhibited high catalytic efficiency in Sonogashira coupling between alkyl halide 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 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 condition at room temperature. We envisioned that semiconductor nanoparticles in combination with dye stuff could serve as light harvesting antenna for carrying out Sonogashira coupling under photo catalytic condition. Copper oxide nanoparticles are the semiconductor material of our choice due to their known catalytic efficiency in forming carboncarbon bond.15 keeping this in view we envisaged to develop a supramolecular ensemble having copper oxide nanoparticles stabilized by assemblies of pyrazine derivative as light harvesting material.16 Pyrazine attached to rotors is a scaffold of our choice as dye stuff due to its absorption in visible region and its known AIEE characteristics.17 Pyrazine derivatives are also reported to exhibit stimuli dependent transition between local excited (LE) and twisted intramolecular charge transfer states (TICT) in aqueous media18. We planned to take advantage of this transformation between LE to TICT state and metal induced restriction to intramolecular rotation (RIR) for the development of pyrazine based aggregation induced emission enhancement (AIEE) active supramolecular assemblies. In this contest, we have designed and synthesized luminescent donor-acceptor system 4 having 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 transition from


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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 state in combination with restriction to intramolecular motions have been explored for the development of ‘not quenched’ probe for copper ions. Further, this is the first report of pyrazine based assemblies showing CuO NPs 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 being 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 reduction process, aggregates of derivative 4 themselves get oxidized to form polymeric species 6 and supramolecular ensemble consisting of oxidized species 6 and CuO NPs worked as light harvesting system which exhibited excellent photocatalytic efficiency in the Sonogashira cross coupling reactions (Table 1-3). To the best of our knowledge, this is the first report where aggregates of pyrazine derivative have been utilized for the preparation of CuO NPs and supramolecular ensemble of semiconductor CuO NPs and pyrazine based dyestuff have been used as photocatalytic system for carbon-carbon bond formation. All the reactions were carried out at room temperature in mixed aqueous media under aerial condition and only 3-9 h of irradiation was required for the completion of the reactions. Additionally, all 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 the other


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catalytic systems (photochemical and thermal conditions) for Sonogashira cross couplings reported in the literature (Table S2-3 in the Supporting Information).21 2.

RESULTS AND DISCUSSION

(i) Pd(II)PPh3Cl2, THF (dry), K2CO3, 80°C,

Scheme 1. Synthesis of pyrazine based derivatives 4-5. 2.1.1. Synthesis and Characterization of Pyrazine Derivative. Suzuki Miyaura coupling of dibromo derivative of pyrazine, 122 with boronic ester of aniline, 223 in THF furnished yellow colored compound 4 in 85% yield (Scheme 1). The structure of compound 4 was elucidated by 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 465.2049 [M+H]+. These spectroscopic data corroborate the structure 4 for this compound. 2.1.2. 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 pyrazine moiety and the absorption band at 410 nm is assigned to intramolecular charge transfer


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transition from donor to acceptor.24 The solvatochromic effect on the absorption behavior of derivative 4 was investigated by switching the solvent from nonpolar e.g. hexane to polar aprotic solvent e.g. THF which showed small bathochromic shift (̴ 6 nm) in absorption band at 410 nm, while the band at 340 nm remained unaffected (Figure S2 in the Supporting Information). On the other hand, 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 band at 555 nm is due to the TICT state (vide infra). Interestingly, THF solution of derivative 4 showed the presence of a single emission at 555 nm when excited at 360 nm. To get insight into origin of emission band at longer wavelength, we carried out temperature dependent emission studies. Interestingly, upon increasing temperature 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 type of behavior is generally observed in case of compounds having TICT state.25 Thus, these temperature dependent emission studies suggest the presence of TICT state in compound 4. The formation of the TICT state was further confirmed by viscosity-dependent fluorescence studies25 of derivative 4 in different fractions of glycerol and ethanol. Upon increasing glycerol fraction in ethanol solution of derivative 4, viscosity of the solution increases which inhibits the intramolecular rotation of the rotor resulting in restriction of TICT state and enhanced emission intensity of LE state (Figure S4 in the Supporting Information). We also studied the solvatochromic behavior of derivative 4 in different solvents. A significant Stoke shift of 165 nm in the emission band was observed on changing the solvents from non-polar e.g. pentane (λem= 390 nm) to polar aprotic solvent e.g. THF (λem= 555 nm) when excited at 360 nm (Figure S5 in the Supporting Information)24. This is the largest ever Stoke shift reported for any pyrazine based TICT active probe. In non-polar solvents (hexane, pentane and toluene) (Table S4, entry 1-3 in the Supporting Information) and


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in polar protic solvents (methanol, ethanol, water and glycerol) (Table S4, entry 5-8 in the Supporting Information) only LE emission was observed. However, in polar aprotic solvents with low dielectric constant such as THF, dioxane, CHCl3, DCM and EtOAc, TICT emission dominates (Table S4, entry 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 formation of hydrogen bond between amino groups of molecule 4 and solvent molecules. Interestingly, in polar aprotic solvents (DMF, DMSO, ACN) with high dielectric constants; molecule 4 showed only LE emission (Table S4, entry 14-16 in the Supporting Information). This may be attributed to partial excited state proton transfer from molecule to DMSO, which enhanced the TICT deactivation of the singlet excited state27. This strong solvent effect upon varying the polarity the solvent suggests the existence of 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 slight decrease in the absorption intensity of 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 increase in absorbance band at 340 nm upon increasing the temperature from 25 to 70°C, but no significant change in the position of absorption maxima was observed (Figure S7 in the Supporting Information). These results show that in aggregated state, molecules were not arranged in J/H fashion.28 All the above studies clearly support the formation of TICT state29. Further, we carried out fluorescence studies of derivative 4 in different THF/H2O fractions. Upon increasing the water fractions (0-99%) to the


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THF solution of derivative 4, the intensity of TICT band drastically decreased and a new band corresponding to LE state appeared at 445 nm. Finally, the fluorescence spectrum of derivative 4 in 100% aqueous media (HEPES solution) exhibited two weak emission bands at 445 (Φ = 0.03) and at 555 nm (Φ = 0.04) corresponding to LE and TICT state, respectively (Figure S8 in the Supporting Information). Though intensity of emission band corresponding to LE state enhanced yet intensity of emission band corresponding to 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 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 as the main reason for weak AIEE characteritices of derivative 4, we synthesized a model compound 530 having pyrazine moiety as acceptor and rotatable phenyl groups but without

amino groups (scheme 1). The compound 5 upon photoexcitation did not exhibit

existence of 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 dual emissive probe. 2.1.2.1. 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) behaviour31 in derivative 4, we carried out the DFT calculations using B3LYP/631g(d) as a basic set. The PICT and TICT could be defined and explained as ES1= EG +Egap + Ehe + ER


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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/631g(d) level at the ground state geometry and ER is relaxation energy of excited state. The difference between TICT and PICT could be explained in terms of ∆ES1T-P = ∆EGT-P + ∆EgapT-P + ∆EheT-P + ∆ET-P Where ∆ES1T-P is ground state energy difference between TICT and PICT states. The calculated value for ∆ET–P S1 for compound 4 in THF:water (0.5:99.5, v/v) system was found to be negative (-0.0467 eV). The negative value suggests that 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 dihedral angle from 0 90oC increased the HOMO cloud at the twisted aromatic amino group which favoured 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 summarised in Table 1 (Table S5 in the Supporting Information). Molecular orbitals of the electronic ground state structure and that of twisted geometry (90°) are displayed in Figure 1.


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Table 1. The tabulated results obtained for the support of TICT at DFT-6-31g(d) and TDDFT 631g(d) basis set for the dihedral angle wise differentiated state. State 1 2 3 4 5 6

o

(3 ) 30°) 60°) 90°)

LUMO -0.0753 -0.0837 -0.0838 -0.0848 -0.0865 0.0868

HOMO -0.1939 -0.1839 -0.1843 -0.1896 -0.2015 -0.2058

Dipole moment

Ehe

Eexcitation

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

∆ET–P S1 (Kcal/mol) -0.5161 0 0.0467 0.1519 0.3938

Figure 1. Contour MO plots for HOMO-1, HOMO, LUMO qnd LUMO+1 obtained for probe 4 in THF:water (0.5:99.5, v/v) system. In its electronic ground state, probe has an optimized geometry with a 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°), with B3LYP-631g(d) methods. The


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fluorescence is proposed to take place from a final quinoidal structure that resulted from the coupling of both locally and charge-transfer excited states. The energy of the perpendicular conformation was found to be higher than the one of the optimized geometry conformation by 0.04 eV in THF:water (0.5:99.5, v/v) system. The formation of the stacked dimer was found unfavourable in the analysis carried out in gas phase. The stacking of the monomer molecules in J and H pattern to form the n-mer was investigated through molecular dynamic aided DFT calculation 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 coins like stacking arrangement was found to be the predominant mode of the aggregates (Figure S11-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 orientated in such a way to avoid the steric clashes between the arms of stacked molecules. These orientations were determined by change in dipole vectors with the variation of angle. The potential steric clashes between the arms of stacked pairs of probe molecules lead to a disordered 3-fold symmetry with three probable relative positions for the D-A. As might be expected, the stacking also perturbed the solvent conformation around the collective stack and the water molecules may be present in between the proximate plane of each donor-acceptor. The simulation began with molecules randomly distributed in the cubic box, The formation of parallel, antiparallel and angular (due to π-π interaction) and T shaped (due to CH-π interaction) conformation patterns within intermolecular distance of 4.0 Å were observed.35 Initially, two dimers (in parallel and antiparallel pattern) were formed, one of which then grew to a trimer (21% of total simulation time in J and H stacking and rest in random π-π stacking


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fashion) and then a tetramer (particularly formed due to by monomer capture). These tetramers subsequently broke in two, giving a system of four possible combination of dimers [in J (both in perpendicular, parallel and T shaped) and H stacking pattern (as suggested by L-torsion patterns in Figure S13A-B in the Supporting Information)] and two monomers. The system continues exchanging monomers in this fashion 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 TICT state and revealed no stacking of molecule in J/H fashion. These studies further suggest that aggregation of molecules proceeded in irregular way to form globular pattern through π-π-interaction between the aryl substrates. 2.1.3. 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 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 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 NPs formation, we carried out the reaction between the aggregates of derivative 4 and Cu2+ ions under inert


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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 Upon keeping the solution for 10 min under aerial conditions the band at 600 nm got broadened and two new bands appeared at 280 and 750 nm which suggested the formation of CuO NPs. These spectral changes are accompanied by color change of the solution from blue to reddish and finally to black. On the basis of 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. (A)

(B) 2

50 Cu2+ (equiv.)

280 nm

0 410 nm

0 280 nm

The band at 280 nm due to CuO Nps

Cu2+ (equiv.)

310 nm

340 nm

(C) 750 nm Plasmon band of CuO NPs in visible region

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


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In the fluorescence spectrum, upon gradual addition of Cu2+ ions (0-50 equiv.) to solution of 4 (5 µM) in HEPES buffer, the emission intensity of TICT band at 555 nm gradually decreased and a remarkable increase in emission intensity of local excited band at 445 nm was observed (Φ = 0.36) (Figure 3). Further, we kept the solution as it is for 1 hr and then recorded its emission spectrum which showed 42% decrease in the emission intensity of the band at 445 nm (vide infra) (Figure S15 in the Supporting Information). 445 nm 50 Cu2+ (equiv.) 0 50 Cu2+ (equiv.)

555 nm 0

Figure 3. Florescence spectra of derivative 4 (5 µM) show ratiometric response upon addition of Cu2+ ions (0-50 equiv.) in HEPES buffer with (pH=7.05, λex= 358 nm). 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 when measured at 445 nm. On the other hand, in the presence of Cu2+ ions within 30 min, the fluorescence decay of derivative 4 was tri-exponential which suggests the existence of three distinct species in the solution. The major fraction (76.84%) of the molecules was found to be decaying through the slower pathway (τ3). Further, a large decrease in non-radiative rate constant from 166.3×109 s-1 to 0.116×109 s-1 was observed. On the other hand, when time resolved


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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 non-radiative rate constant from 0.329×109 s-1 to 5.08×109 s-1 was observed. These studies clearly indicate that the presence of Cu2+ ions accelerate the decrease in non-emissive 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 (Table S6-S7 in the Supporting Information). We believe that copper ions interact with aggregates of derivative 4 through amino groups and restrict the transition from LE state to TICT state. Further, in the presence of copper ions structural rigidification of the system was achieved which is responsible for the emission enhancement of 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 towards 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 (Figure S17A-B, S18A-B in the Supporting Information). 2.1.4. Characterization of CuO Nanoparticles. We also carried out scanning electron microscopic (SEM) and transmission electron microscopic (TEM) studies of aggregates of derivative 4 in 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 and 4B). The SEM and TEM images of the oxidized species in H2O showed the formation of flake like aggregates, thus, suggesting the formation of polymeric species (Figure 4C and 4D). The HR-TEM image of derivative 4 in the presence of Cu2+ ions in aqueous media showed the presence of cupric oxide nanoparticles with the inter-planar spacing


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(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 water solution of derivative 4 and CuCl2) showed the presence of diffraction peaks located at 2Ɵ 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 nanoparticles38 (Figure S20 in the Supporting Information). The FT-IR spectrum of these nanoparticles exhibited four bands which confirmed 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 Cu(0) state in aqueous state and during this process they themselves get oxidized to polyamine, 6 as shown in Scheme 2. (A)

(E)

(B)

(G) CuO (110)

200 nm 50 nm

(C)

(D)

(F)

500 nm

(H)

CuO (002)

CuO (111)

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


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2.1.5. 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 TBHP (50 equiv. to the 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 presence of Cu2+ ions. These results suggest the oxidation of derivative 4 in presence of Cu2+ ions which is responsible for the emission enhancement at 445 nm. Further to investigate the structure of oxidized species, we slowly evaporated the solution of derivative 4 containing CuCl2. After 2 days, precipitates were obtained which were filtered and washed with THF. The 1H NMR spectrum of the residue so obtained after evaporation of THF solution showed the broadening of all the aromatic protons with the appearance of additional peaks at 7.04, 7.16, and 7.20 ppm and a board peak at 7.8 ppm corresponding to 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 lab conditions. After the completion of the reaction (TLC), the oxidized product was isolated and its 1

H 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 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 get reduced to Cu(0)NPs and during this process, aggregates of 4 are oxidized to polyamine species 6.

41

To get insight into the formation of polymeric oxidized

species, we carried out the reaction between derivative 4 and Cu2+ ions in THF/water (0.5:9.5,


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v/v) fraction under lab condition. The progresses of the reaction was monitored by thin layer chromatography (TLC) and after 10 minutes formation of two spots were 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 with dimeric and trimeric polyamine species 6d and 6t (please see Figure S25-27 in the supporting information).42

60 min

TICT active 4

Random Aggregates of 4 (TICT-AIEE)

Complexation of aggregates of 4

Supramolecular ensemble 6:CuO Nps

Scheme 2. Probable schematic representation of CuO NPs formation by using aggregates of derivative 4 in aqueous media and resulting in the formation of supramolecular ensemble 6:CuO NPs. Further, To 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 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.


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The fluorescence spectrum of oxidized species 6 (10 µM) in H2O solution exhibited an emission band at 460 nm when excited at 360 nm. Upon addition of 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 aqueous dispersion of CuO NPs. The solution of oxidized species in H2O showed a decay time of 0.92 ns, however, in 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 absorption spectrum of CuO NPs which indicate energy transfer from 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 dye stuff to CuO NPs. As we know that CuO NPs serve as semiconducting material and absorb visible light irradiation, thus, we have 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 indirect band gap of intrinsic CuO NPs p-type semiconductor and narrow enough to facilitate the photocatalytic reaction.44 2.2. 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 the C-C bond forming reactions.

7a

8

9a

Scheme 3. Sonogashira cross coupling between iodobenzene 7a and phenyl acetylene 8 catalyzed by in situ generated supramolecular ensemble 6:CuO NPs in the presence of visible light. ACS Paragon Plus Environment

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Table 2. Sonogashira cross coupling between iodo benzene and phenyl acetylene under different conditions. Entry

Conditions

Time (h)

Solvent

Isolated yield (%)

8

Base (K2CO3) Equiv. 1

1

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

TEG

68

3

1

EtOH

90

3

1

88

3

1

EtOH:H2O (1:1) H2O:EtOH (8:2)

5

in situ generated supramolecular ensemble 6:CuO NPs

7

1

H 2O

60

6

Reaction in dark (in situ generated supramolecular ensemble 6:CuO NPs)

12

1

H2O:EtOH (8:2)

30

7


12

0

H2O:EtOH (8:2)

0

8


3

2

H2O:EtOH (8:2)

89

9

3 12

1 equiv. of Cs2CO3 1

11

Only in presence of Oxidized species 6

24

1

12

Bare CuO NPs

12

1

13

Bare CuO NPs + derivative 4

12

1

H2O:EtOH (8:2) H2O:EtOH (8:2) H2O:EtOH (8:2) H2O:EtOH (8:2) H2O:EtOH (8:2)

89

10

in situ generated supramolecular ensemble 6:CuO NPs Without (6:CuO NPs) catalyst

14

Bare CuO NPs + Oxidised spices 6 (1:1)

3

1

H2O:EtOH (8:2)

86

2 3 4

89

0 0 48 50

We carried out 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 100W tungsten filament bulb was used as the irradiation source and the round bottom flask was immersed in a water bath to inhibit the photo-heating effect. To our pleasure, the reaction was complete in 3 h and 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


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EtOH:H2O mixture (Table 2, entry 1-5). The desired product was obtained in comparable yields in case of EtOH, H2O:EtOH (1:1) and H2O:EtOH (8:2) solvent mixture. Thus, we choose H2O:EtOH (8:2) as 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, presence of K2CO3 in higher concentration (2 equiv.) has no effect on the yield of the reaction. However, in the absence of K2CO3, the desired product was not obtained which suggests that basic medium is a prerequisite for reaction (Table 2, entries 7-8). To rule out the possibility of contamination of palladium in K2CO3, we also carried out 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, entries 10). Furthermore, in 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 reaction, we prepared bare CuO NPs of size in the range of 30-60 nm by hydrothermal method (Figure S31 and S32 in the Supporting Information).20c We evaluated the catalytic efficiency of these bare CuO NPs in 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 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%


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yield (Table 2, entry 14). These results highlight the importance of supramolecular ensemble 6:CuO NPs in Sonogashira cross coupling reaction 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 diphenylaceteylene (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, entry 1-2 in the Supporting Information). 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, entry 1 and 3 in the Supporting Information). Further increase in the amount of polyamine species up to 5:1 had no significant effect on the yield of the product while upon decreasing 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, entry 8-9 in the Supporting Information). To understand the reason behind the decrease in the yield of the product, we carried out TEM studies of 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 get accumulated when polyamine species is present in lower concentration (6:CuO = 1:2); hence, the catalytic activity is reduced. On the basis of this study, we believe that oxidized species has important role in the photocatalytic efficiency of supramolecular ensemble 6:CuO NPs in Sonogashira coupling reactions.


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Table 3: Effect of halide group in Sonogashira cross coupling between alkyl halide and phenyl acetylene in the presence of visible light by using in situ generated supramolecular ensemble 6:CuO NPs as catalyst. Entry

ArX

Alkyne

1

Time [h]

Product

3

7a

8

2

89

9a

3

7b

8

3

80

9a

4

7c

8

4

60

9a

6

7d

Isolated yield [%]

50 9b

8 7

5

72 7e

8

9c

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


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6:CuO NPs as a catalyst, Sonogashira cross couplings involving bromobenzene, chlorobenzene, p-chloro aniline went smoothly to yield the products 9a-b in good to moderate yield (Table 3, entries 2-4). Thus, supramolecular ensemble 6:CuO NPs is 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 excellent catalytic efficiency and high selectivity of in situ supramolecular ensemble 6:CuO NPs in the Sonogashira coupling under visible light. Table 4. Sonogashira cross coupling between aryl halides 7f-r and phenyl acetylene 8 in the presence of visible light catalyzed by in situ generated supramolecular ensemble 6:CuO NPs.

Entry

ArX

Time [h]

Product

5

1

7f

78

9b 4

2

7g

75

9d 5

3

7h

70

9e 5

4

75

9f

7i 5

5

72

9g

7j 6

6

7k

Isolated yield [%]

70

9h


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7

6

68

7l

9i

8

12

45

7m

9j

9

9

65

7n

9k

10

7

78

7o

9l

11

8

64

9m

7p 12

8

72

N

N Br

7q 9n 13

10

58

Br

Br

7r

9o

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 substituents undergo Sonogashira coupling smoothly with phenylacetylene to furnish the desired products 9b, 9d-i in higher yields (Table 4, entries 1-7). The reaction conditions could also


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tolerate -CHO and -NO2 functionalities (Table 4, entries 8-9). However, in case of nitro substituted aryl halide, reaction took longer time (12h) 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 9lm in good yields (Table 4, entries 10-11). To check the practical applications of in situ generated supramolecular ensemble 6:CuO NPs as a photocatalyst, we carried out reaction between bromo derivative of triphenylamine (7q) and hexaphenylbenzene (7r) with phenylacetylene to furnish the desired products 9n and 9o in 72 and 58% yield, respectively (Table 4, entries 12-13). 2.2.1 Mechanism of photocatalytic Sonogashira coupling by supramolecular ensemble 6:CuO NPs. We believe that the reaction follows 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 photosensitizers and generates the aryl radical C, and formed complex B. Finally, the aryl radical C attacks onto the copper acetylide complex B to form 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 the reaction between iodobenzene (7a) and phenylacetylene (8) in presence of well-known radical scavenger 2,2,6,6-tetramethyl-1piperidinoxyl (TEMPO 1 equiv.).46 Interestingly, the reaction did not proceed which supports our assumption that reaction proceeds through the free radical pathway. This above mechanism also supports that the aryl halides bearing electron donating substituents give higher yield rather than electron withdrawing groups.


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ACS Catalysis

H

* CuO

K2CO3 -H

R R

*

CuO CuO

A

D

-X

CuO

X

R C

R

X

B Energy transfer

hv NH2 H N N

CuO

NH2

N

N

N N

n

* VB

NH 2 NH 2 H N N N

H N N N

NH2 n

CuO NH 2

-

NH2

CuO

CuO

-

eee

CuO

CuO

N

N N

CB NH 2

H N H N

-

NH 2

CuO

N

NH2

CuO

N

n

H N N

N

N N

+

+

+

h h h

CuO

CuO NH 2

NH2

Scheme 4: Proposed mechanism of photocatalytic Sonogashira cross-coupling reaction by using 6:CuO. 2.2.3 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


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carried out without catalyst. When catalyst amount was reduced upto 1 ppm, then 28 h were 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 reaction between iodobenzene, 7a and phenylacetylene, 8 as model reaction to determine the recyclability of the in situ supramolecular ensemble 6:CuO NPs catalyst. After the completion of the reaction, product was extracted using organic solvent (CHCl3) and aqueous layer having 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 shows no change in its size and morphology (Fig. S34, Supporting Information). Table 5. Sonogashira coupling reaction of 1a and 2 by using various amount of in situ generated supramolecular ensemble 6:CuO NPs. S.No

CuO NPs

Time (h)

Yield (%)

TON

TOF

(ppm) 1

5000

3

89

17.8

5.93

2

4000

5

89

22.25

4.45

3

3000

8

86

28.66

3.58

4

1500

12

84

56

4.66

5

1000

16

84

84

5.25

6

100

20

82

820

41

7

10

24

80

8000

333.3

8

1

28

78

78000

2785.71

9

0

0

0

0

0


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ACS Catalysis

3. CONCLUSIONS In conclusions, we synthesized a pyrazine derivative 4 which formed weakly fluorescent aggregates in aqueous media. The aggregates of derivative 4 exhibited ‘not quenched’ response towards copper ions and served as reactors for generation of CuO NPs in aqueous media. During this process, the aggregates of derivative 4 themselves got oxidized to form polyamine species 6. Interestingly, these in situ generated supramolecular ensemble of oxidized species 6:CuO NPs exhibited excellent catalytic efficiency in Sonogashira couplings under mild conditions with a wide variety of substrates like 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 Materials47. The general experimental methods,

quantum yield calculations and materials used are same as reported earlier by us.47 The TEM images were recorded using HR-TEM-JEM 2100 microscope. The FT-IR spectra were recorded using VARIAN 660 IR Spectrometer. 4.2. UV-vis and Fluorescence Titrations47. 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. 15 µl of this stock solution was further diluted with 2985 µl water/HEPES buffer (0.05 M, pH = 7.05) to prepare 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+ and Mg2+; X = 1-3], chlorides (MClY; Y = 1-3) and CuCl2 (10-1M to 10-3M) in distilled water were added to 3 ml solution of compound 4 taken in quartz cuvette and spectra were recorded. 4.3. Synthesis of copper oxide nanoparticles (CuO NPs).


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1 ml aqueous solution of 0.1 M CuCl2 was added to a 10 ml 0.01 M solution of compound 4 in distilled water (15 µl THF was added to dissolve). The resulting solution was kept under stirring at room temperature for 60 min and formations of black colored CuO nanoparticles take place. 200 µl 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-(4,4,5,5tetramethyl-1,3,2-dioxaborolan-2-yl) aniline 2 (0.5g, 2.27 mmol) in 20 ml THF, was added 2 ml aqueous solution of K2CO3 (1g, 7.72 mmol) followed by addition of [Pd(PPh3)2Cl2] (0.383g, 0.545 mmol) as a catalyst under N2 atm. 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 layer was dried over anhydrous sodium sulphate 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 5:1 CHCl3:MeOH mixture to give 0.4 g (85%) of compound 4 as 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 3315, 1596 and 1190 cm-1 due to N-H stretch, C=N bond and C-N bond stretching respectively. Elemental Analysis of 4 for C32H24N4: calculated C, 82.73; H, 5.21; N, 12.06; experimentally found: C, 82.74; H, 5.20; N, 12.06.


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4.5. General procedure for Photocatalytic Sonogashira-Hagihara cross coupling catalyzed by supramolecular ensemble 6:CuO NPs. The mixture of aryl-iodide (7a-r, 1 mmol), phenylacetylene, 8 (100 mg, 1 mmol; except in case of 7r, 200 mg, 2 mmol) and K2CO3 (276 mg, 2 mmol) in H2O:EtOH (8:2) in presence of 6:CuO NPs (0.5 mol%) as a photo catalyst was stirred for 3-12 h at room temperature under visible light (Table 2-4) (Except: only in case of 7r, ethanol has been used as solvent). After the completion of the reaction (TLC), the reaction mixture was treated with water, extracted with CHCl3, and the combined organic layer was dried over anhydrous sodium sulphate and distilled under reduced pressure to give a solid residue. The desired products (9a-o) were purified by recrystallization in methanol: chloroform (5:1) mixture. The alkynylated compounds 9a-o were found in good yield and confirmed from its spectroscopic and analytical data (Figure S36-S50 in the supporting information). Compound 9a14. The 1H NMR (500 MHz, CDCl3) δ = 7.55-7.51 (m, 4H), 7.37-7.30 (m, 6H) ppm. Compound 9b48. 1H NMR (500 MHz, CDCl3) δ = 7.86 (d, J = 10 Hz, 1H), 7.49 (d, J = 5 Hz, 1H), 7.41 (d, J = 5Hz, 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. Compound 9c14. 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. Compound 9d14. 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.


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Compound 9e14. 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. 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. 13C 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, m/z = 231.185 [M1+Na]+. Elemental Analysis of 9f for C14H12N2: calculated: C, 80.74; H, 5.81; N, 13.45; experimentally found: C, 80.73; H, 5.81; N, 13.46. Compound 9g49. 1H NMR (300 MHz, CDCl3) δ 7.51-7.44 (m, 2H), 7.31 (t, J = 6 Hz, 3H), 7.036.94 (m, 2H), 6.70 (d, J = 9 Hz, 1H), 3.83 (s, 3H, OMe), 3.82 (s, 3H, OMe) ppm. Compound 9h50. 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. Compound 9i14. 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. Compound 9j14. 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. Compound 9k14. 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. Compound 9l14. 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. Compound 9m14. 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.


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Compound 9n51. 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. 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.

13

C 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, m/z = 773.5070 [M2+K]+. Elemental Analysis of 9o is calculated for C58H38: C, 94.79; H, 5.21. Found: C, 94.77; H, 5.23. ASSOCIATED CONTENT Supporting Information. The contents of the SI section include 1H,

13

C, mass spectra and IR

spectrum of compounds 4, 5, 6 and 9a-o; UV-vis and fluorescence studies; detection limits; SEM, TEM images; powder XRD analysis; DLS studies, IR and table of comparison of present manuscript with previous reports. This material is available free of charge via the Internet at http://pubs.acs.org AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]

Notes Authors declare no competing financial interest. ACKNOWLEDGMENT


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V.B. is thankful to SERB, New Delhi (ref. no. EMR/2014/000149) for financial support. We are thankful to Mr. Ravinder Singh for TEM study. We are also thankful to UGC (New Delhi) for ‘‘University with Potential for Excellence’’ (UPE) project. H.D is thankful to UGC-BSR (New Delhi) for fellowship. S.P. is thankful to UGC (New Delhi) for Senior Research Fellowship (SRF). REFERENCES (1)

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Graphic for Manuscript

Energy transfer hv

*Aqueous media *Aerial conditions *Room temperature *Visible light

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CB

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Supramolecular Ensemble of a TICT-AIEE Active Pyrazine Derivative

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