Supramolecular ensemble of a TICT-AIEE active pyrazine derivative and CuO NPs: A potential photocatalytic system for sonogashira couplings

Article abstract of DOI:10.1021/acscatal.6b00393

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).

Full text of DOI:10.1021/acscatal.6b00393

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Article
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
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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
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Sonogashira coupling is one of the most powerful carbon-carbon bond forming reactions for
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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.
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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 α-
Fe2O3 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 carbon-
carbon 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
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2.
RESULTS AND DISCUSSION
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(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
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spectroscopic methods (Figure S35A-E in the Supporting Information). The H 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
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derivative 4 was investigated by switching the solvent from nonpolar e.g. hexane to polar aprotic
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solvent e.g. THF which showed small bathochromic shift (̴ 6 nm) in absorption band at 410 nm,
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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.
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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.
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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/6-
31g(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/6-
31g(d) level at the ground state geometry and ER is relaxation energy of excited state.
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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
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631g(d) basis set for the dihedral angle wise differentiated state.
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∆ET–P
(Kcal/mol)
S1
Dipole moment
Ehe
Eexcitation
State
(3o)
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
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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
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-0.5161
0
0.0467
0.1519
0.3938
30°)
60°)
90°)
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.
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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 ꢀM) 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.
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(A)
(B)
2
The band at 280 nm
due to CuO Nps
Cu2+
(equiv.)
310 nm
340 nm
50
0
Cu2+
(equiv.)
280 nm
0
410 nm
280 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
ꢀM) 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).
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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 non-
emissive 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).
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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
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2.
(E)
(A)
(B)
(G)
CuO
(110)
200 nm
50 nm
(D)
(C)
(F)
(H)
CuO
(111)
CuO
(002)
500 nm
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) HR-
TEM 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
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1
washed with THF. The H 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
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 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
41
oxidized to polyamine species 6. 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
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60 min
TICT active 4
Supramolecular ensemble 6:CuO Nps
Complexation of
aggregates of 4
Random Aggregates of
4 (TICT-AIEE)
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
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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.
9a
7a
8
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.
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Table 2. Sonogashira cross coupling between iodo benzene and phenyl acetylene under different
5
6
conditions.
7
8
9
Entry
Conditions
Time (h)
Base
(K2CO3)
Equiv.
1
Solvent
Isolated
yield (%)
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in situ generated supramolecular
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in situ generated supramolecular
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EtOH
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in situ generated supramolecular
ensemble 6:CuO NPs
in situ generated supramolecular
ensemble 6:CuO NPs
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5
in situ generated supramolecular
ensemble 6:CuO NPs
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H2O
60
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Reaction in dark (in situ generated
supramolecular ensemble 6:CuO NPs)
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(8:2)
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in situ generated supramolecular
ensemble 6:CuO NPs
H2O:EtOH
(8:2)
in situ generated supramolecular
ensemble 6:CuO NPs
H2O:EtOH
(8:2)
89
9
in situ generated supramolecular
ensemble 6:CuO NPs
Without (6:CuO NPs) catalyst
3
1 equiv. of
Cs2CO3
1
H2O:EtOH
(8:2)
H2O:EtOH
(8:2)
H2O:EtOH
(8:2)
H2O:EtOH
(8:2)
89
0
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Only in presence of Oxidized species 6
Bare CuO NPs
1
1
1
0
48
50
Bare CuO NPs + derivative 4
H2O:EtOH
(8:2)
14
Bare CuO NPs + Oxidised spices 6 (1:1)
3
1
H2O:EtOH
(8:2)
86
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).
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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.
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Entry
ArX
Alkyne
Time
[h]
Product
Isolated
yield
[%]
1
3
3
4
89
80
60
50
8
8
8
8
8
7a
7b
7c
7d
7e
9a
9a
2
3
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9b
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72
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.
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53
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58
59
60
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]
5
Product
Isolated
yield [%]
78
1
7f
7g
7h
9b
9d
9e
4
5
5
75
70
75
2
3
4
9f
7i
7j
5
6
72
70
5
6
9g
7k
9h
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7
8
7
8
6
12
9
68
45
65
78
7l
7m
7n
9i
9j
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
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53
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59
60
9
9k
10
7
7o
7p
9l
11
12
8
8
64
72
9m
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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8
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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 9l-
m in good yields (Table 4, entries 10-11).
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53
54
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58
59
60
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-1-
piperidinoxyl (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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2
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4
5
6
H
*
CuO
K2CO3
7
-H
8
9
R
10
11
12
13
14
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R
*
CuO
CuO
A
D
-X
CuO
R
X
R
X
C
B
Energy
transfer
hv
NH2
H
N
N
CuO
N
-
-
-
n
H
N
N
NH2
e e e
CuO
N
CB
N
CuO
NH2
NH2
NH2
N
*
H
N
N
N
H
CuO
NH2
CuO
N
N
N
n
N
N
CuO
CuO
NH2
VB
+
+
+
NH2
H
NH2
CuO
NH2
h h h
H
N
N
N
N
N
n
N
N
N
CuO
NH2
CuO
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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5
6
7
8
9
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.
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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)
5000
4000
3000
1500
1000
100
3
5
89
89
86
84
84
82
80
78
0
17.8
22.25
28.66
56
5.93
4.45
3.58
4.66
5.25
41
1
2
3
4
5
6
7
8
9
8
12
16
20
24
28
0
84
820
10
8000
78000
0
333.3
2785.71
0
1
0
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3. CONCLUSIONS
5
6
7
8
9
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).
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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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