ON–OFF Fluorescent Imidazole Derivative for Sensitive and Selective Detection of Copper(II) Ions

Article abstract of DOI:10.1134/S1070428020010248

A novel multichromophoric hybrid compound, 2-[1-(4-tert-butylphenyl)-4,5-diphenyl-1H-imidazol-2-yl]-6-(pyren-1-yl)quinoline (TDIPQ) has been synthesized as an ON-OFF fluorescent chemosensor for copper(II) ions. Colorless TDIPQ in acetonitrile—water (2:1, v/v) selectively turns yellow along with fluorescence quenching upon addition of copper(II) ions. The fluorescence quenching is directly proportional to the concentration of copper(II) ions. The interaction between TDIPQ and copper(II) was investigated with the aid of UV-Vis, fluorescence, ¹H NMR, and MALDI mass spectral techniques. The stoichiometry of the TDIPQ—Cu complex was determined to be 2:1 by Job’s Plot. Under similar experimental conditions, other competitive metal ions had negligible or no interference in the detection ability of TDIPQ. The detection and quantification limits of TDIPQ were estimated at 2 × 10^?6 M and 6.2 × 10^?6 M. respectively. This method showed an excellent precision of 0.98 ± 0.011 and recovery characteristic of 99.09±1.4%. It is applicable for the quantification of copper(II) in various samples such as drinking water, lab waste water, and soil. A mixture of TDIPQ with the BZA-Co-BZMA polymer can be cast as a film on a glass slide to be used as a sensor device to indicate the presence of copper. Polymer-coated TDIPQ chemosensing property was analyzed by SEM imaging.

Full text of DOI:10.1134/S1070428020010248

ISSN 1070-4280, Russian Journal of Organic Chemistry, 2020, Vol. 56, No. 1, pp. 158–168. © Pleiades Publishing, Ltd., 2020.
ON–OFF Fluorescent Imidazole Derivative
for Sensitive and Selective Detection of Copper(II) Ions
a
a
b,
c
a
K. G. Harsha , E. Appalanaidu , B. A. Rao *, T. R. Baggi , and V. J. Rao
a
Fluoro Agro Chemicals Division and AcSIR, CSIR-Indian Institute of Chemical Technology, Uppal Road Tanaka,
Hyderabad, 500007 India
b
Department of Chemistry, Osmania University, Hyderabad, 500007 India
c
Central Forensic Science Laboratory, Ramanthapur, Hyderabad, 500013 India
*
e-mail: anandiict05@gmail.com
Received August 16, 2019; revised October 6, 2019; accepted November 22, 2019
Abstract—A novel multichromophoric hybrid compound, 2-[1-(4-tert-butylphenyl)-4,5-diphenyl-1H-imidazol-
2
-yl]-6-(pyren-1-yl)quinoline (TDIPQ) has been synthesized as an ON-OFF fluorescent chemosensor for
copper(II) ions. Colorless TDIPQ in acetonitrile–water (2:1, v/v) selectively turns yellow along with fluores-
cence quenching upon addition of copper(II) ions. The fluorescence quenching is directly proportional to the
concentration of copper(II) ions. The interaction between TDIPQ and copper(II) was investigated with the aid of
1
UV-Vis, fluorescence, H NMR, and MALDI mass spectral techniques. The stoichiometry of the TDIPQ–Cu
complex was determined to be 2:1 by Job’s Plot. Under similar experimental conditions, other competitive
metal ions had negligible or no interference in the detection ability of TDIPQ. The detection and quantification
–
6
–6
limits of TDIPQ were estimated at 2×10 M and 6.2×10 M. respectively. This method showed an excellent
precision of 0.98±0.011 and recovery characteristic of 99.09±1.4%. It is applicable for the quantification of
copper(II) in various samples such as drinking water, lab waste water, and soil. A mixture of TDIPQ with the
BZA-Co-BZMA polymer can be cast as a film on a glass slide to be used as a sensor device to indicate the
presence of copper. Polymer-coated TDIPQ chemosensing property was analyzed by SEM imaging.
Keywords: imidazole–quinoline–pyrene conjugate, chemosensor, copper(II)-selective, polymer, fluorescence
quenching.
DOI: 10.1134/S1070428020010248
For many years researchers are keenly looking into
coordination compounds of metal salts and organic
ligands due to their potential application in homoge-
neous catalysis, bioinorganic applications, chemosens-
ing, etc. In recent years, the research has been focused
on developing selective and sensitive chemosensors.
There is a strong necessity for the development of
simple, cost-effective, selective, and sensitive analyt-
ical methods for the detection and determination of
transition metals due to increased demand for detection
of heavy metal ions [1]. Among transition metals,
copper has a vast utility in biotic and abiotic processes.
Copper is an essential trace element in all living
forms due to its involvement in several biochemical
processes. Copper-containing proteins act as redox
catalysts in biological processes where copper shifts
ingested in higher doses. Excess concentration of cop-
per can cause both Indian childhood cirrhosis (ICC) [3]
and non-Indian childhood cirrhosis (NICC) [4], as well
as Alzheimer’s [5] or Parkinson’s disease [6]. Accord-
ing to the World Health Organization (WHO), the
maximum limit of copper in drinking water is 2 mg/L
2
+
(30 mM) [7]. In aqueous solution Cu is more stable
1
+
than Cu since the energy required to remove one
1
+
2+
electron from Cu to Cu is compensated by high
2
+
hydration energy of Cu ion [8]. Copper detection is
an important aspect in forensic science due to copper
sulfate poisoning; in India it is commonly consumed
to commit suicidal; however, accidental poisonings are
also reported [9]. Current techniques used to detect
copper in toxicological and environmental samples in-
clude atomic absorption spectrometry [10], inductively
coupled plasma mass spectroscopy (ICP-MS) [11],
inductively coupled plasma atomic emission spectrom-
etry (ICP-AES) [12], and voltammetry [13]. Fluores-
cence-based techniques have gained much attention
1
+
2+
between the cuprous (Cu ) and cupric (Cu ) forms,
2
+
though the majority of copper exists in the Cu form
in the body [2]. Even though copper is an essential
trace element, it exhibits toxicity or causes illness when
1
58

ON–OFF FLUORESCENT IMIDAZOLE DERIVATIVE
159
due to their high sensitivity, selectivity, and speed and
low maintenance cost. In recent years, a number of
fluorescent probes for the detection of copper in vari-
ous biological and environmental samples have been
developed [14–19]. Imidazole-derived compounds
have engrossed the attention of researchers due to their
coordination properties, photoluminescence, and pH
sensing [20]. The electronic nature of imidazole as
N donor can be tuned by substitution of the active NH
proton [21]. Polymers provide a good matrix to cast
chemosensor molecules to obtain a device for the
detection of copper ions in field samples. Dispersion
polymerization consists of polymerization of mono-
mers that are moderately soluble in aqueous medium.
Benzyl acrylate and methyl methacrylate monomers
were chosen for this purpose due to their high refractive
indices (~1.53) and low cost.
During recent years, our research group has focused
on the design of new fluorimetric and colorimetric
chemosensors for the recognition of various cations
and anions [22–26]. Based upon this experience and
previously discussed considerations, in the present
work we have designed and synthesized a multichro-
mophoric hybrid TDIPQ molecule (Scheme 1), as
an ON–OFF fluorescence chemosensor for selective
detection of copper(II) ions in acetonitrile–water (2:1,
v/v). A TDIPQ solution exhibited a color change to
yellow, fluorescence quenching, and clear isosbestic
points in the UV-Vis absorption spectrum upon addi-
tion of Cu ions. The TDIPQ–Cu complex showed
a new red-shifted peak at λ ~382 nm in the UV-Vis
spectra. The MALDI mass spectrum along with Job’s
plot indicated 2:1 stoichiometric ratio for the TDIPQ
complex with copper(II). It is thermodynamically
2
+
2+
Scheme 1.
Bu-t
O
O
Br
NH2
i
+
+
Ph
Ph
N
N
N
CHO
t-Bu
N
Br
1
2
3
4
Me
Me
Me
Me
O
Br
B
O
ii
iii
5
6
7
Bu-t
iv
4
+ 7
N
N
N
TDIPQ
Reagents and conditions: i: NH Ac, AcOH, reflux overnight; ii: HBr, H O , MeOH–Et O (1:1), 0–5°C to room temp.;
4
2
2
2
iii: n-BuLi, THF, 4,4,5,5-tetramethyl-2(propan-2-yloxy)-1,3,2-dioxaborolane, –78°C to room temp.; iv: Pd(PPh ) , toluene,
3
4
Na CO H O, EtOH, reflux.
2
3,
2
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 56 No. 1 2020

160
HARSHA et al.
favored for a metal ion to form a chelate complex with
polydentate and bidentate ligands rather than a complex
with monodentate coordination mode due to the
atom of the quinoline ring appear at 1,4-position with
respect to each other, which makes the ligand more
favorable for the interaction with Cu ion. The tert-
butyl group improves the solubility of the ligand and
prevents intermolecular interactions through steric hin-
drance. TDIPQ alone (10 μM) and in presence of
2
+
1
entropy factor [27–29]. The H NMR, IR, and MALDI
2
+
mass spectrum of the 2TDIPQ–Cu complex predicted
the site of copper(II) binding to TDIPQ. Finally, the
behavior of a polymer-mixed colorless dye is described
with the observation of yellow colored complex as
a sensing response.
+
2+
+
2+
various metal ions (260 μM) like Na , Mg , K , Ca ,
2
+
2+
2+
2+
2+
2+
2+
2+
Mn , Co , Ni , Zn , Pb , Cd , Ba , and Hg is
colorless in acetonitrile–water (2:1, v/v), but it selec-
tively turns to yellow along with fluorescence quench-
ing in presence of Cu ions. Selective color change
of TDIPQ for Cu ion enables naked-eye detection
of copper.
2
-[1-(4-tert-Butylphenyl)-4,5-diphenyl-1H-imid-
2
+
azol-2-yl]-6-(pyren-1-yl)quinoline (TDIPQ) was syn-
thesized as shown in Scheme 1 and was characterized
by spectral analyses. The TDIPQ molecule contains
a pyrene fragment as signaling moiety and quinoline-
linked imidazole as receptor moieties. The presence of
pyrene and quinoline moieties improves the quantum
2
+
Spectroscopic studies. The UV-Vis and fluores-
cence spectrophotometric data were used to ascertain
the interaction between copper(II) ions and TDIPQ
3
yield up to ~0.7. The imidazole N atom and nitrogen
in solution. TDIPQ in MeCN–H O (2:1, v/v) was
2
1
0
0
0
0
0
.0
.8
.6
.4
.2
.0
(a)
160
40
(b)
+
2+
TDIPQ + Na , Mg ,
1
+
2+
2+
2+
+
2+
K , Ca , Mn , Co ,
TDIPQ + Na , Mg ,
2+
2+
2+
2+
+
2+
2+
2+
Ni , Zn , Pb , Cd ,
K , Ca , Mn , Co ,
120
100
2
+
2+
2
+
2+
2+
2+
Ba , Hg
Ni , Zn , Pb , Cd ,
2
+
2+
Ba , Hg
8
6
4
2
0
0
0
0
0
TDIPQ + Cu2+
TDIPQ + Cu2+
2
50
300
350
400
450
λ, nm
400
450
500
550
λ, nm
Fig. 1. Metal ion (260 ꢀM) induced variations in the (a) absorption and (b) fluorescence spectra of TDIPQ (10 ꢀM); excitation
wavelength λ 364 nm.
1
60
(
a)
(b)
0
.5
140
120
λ 415 nm
λ 366 nm
140
120
0
0
0
0
.6
.4
.2
.0
0.4
1
00
80
0
0
.3
.2
6
0
40
1
00
2
0
0.1
.0
0.0000 0.0001 0.0002 0.0003 0.0004
8
6
4
2
0
0
0
0
0
0
0
.0000 0.0001 0.0002 0.0003 0.0004
3
00
350
400
450
λ, nm
400
450
500
550
λ, nm
Fig. 2. Copper(II) concentration-dependent variations in the (a) absorption and (b) fluorescence spectra of TDIPQ.
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 56 No. 1 2020

ON–OFF FLUORESCENT IMIDAZOLE DERIVATIVE
Table 1. Detection of copper(II) using various chemosensorsa
161
Signaling
moiety
LOD
( μM)
Solvent
Stoichiometry NMR data MS data Color change
Remarks Reference
Naphthalene
DMSO/H O
0.65
1:1
1:1
1:1
1:1
1:1
2:1
1:1
1:1
1:1
2:1
NA
NA
NA
NA
On/off
On/off
[14]
[15]
2
(
1:1,v/v)
MeCN/HEPES 5.80
4/6, v/v)
MeCN/H O
Coumarin
Present
NA
Green
to yellow
Red
to yellow
Light yellow
to red
(
Quinazolinone
Coumarin
37.5
NA
Present
NA
Colorimetric
detection
On/off
[19]
2
(9:1, v/v)
DMSO/H O
Present
Present
Present
Present
Present
NA
[31]
2
(1:1,v/v)
Quinoline
DMSO/H O
0.53
Present
NA
Colorless
to pink
Colorimetric
detection
On/off
[32]
2
(2:1,v/v)
Calix[4]arene
CH Cl /MeCN NA
NA
[33]
2
2
(
1000:1, v/v)
Rhodamine B Tris–HCl/MeCN 0.06
1:1, v/v)
DMSO/H O
Present
Present
NA
Colorless
to yellow
NA
Off/on
On/off
On/off
On/off
[34]
(
Benzimidazole
Quinoline
Pyrene
9.76
[35]
2
(
7:3, v/v)
MeCN/HEPES 1.86
9:1, v/v)
MeCN/H O
Yellow
to pink
Colorless
to yellow
[36]
(
2
Present
Present
This work
2
(2:1,v/v)
a
NA stands for not applicable.
screened against various metal ions. The electronic
absorption spectrum of TDIPQ showed a negligible
effect in the presence of various metal ions, and
a similar trend was reflected in the fluorescence
characteristics of TDIPQ. The absorption maximum of
values of various chemosensors for Cu2+ ions [25, 26,
30–36], including TDIPQ, are compared in Table 1.
The LOD of the TDIPQ is well in range to detect
maximum limit of copper in drinking water that is
–
5
3×10 M (2 mg/L; according to the WHO). The limit
2
+
–6
TDIPQ was red shifted in the presence of Cu ions
Fig. 1a), and fluorescence quenching of TDIPQ was
of quantification was found to be 6.2×10 M. The
(
LOD was calculated as per the reported procedures
observed (Fig. 1b). These selective changes in the
absorption and emission characteristics of TDIPQ
indicated its ability to detect Cu ions. TDIPQ exhibits
1
1
1
40
20
00
2
+
an absorption maximum at λ 364 nm (ε = 5.25×
4
–1
–1
2+
1
0 L mol cm ). Upon incremental addition of Cu
(
260 μM), the absorption maximum of TDIPQ shifted
8
0
red to λ 382 nm with an isosbestic point at λ 386 nm.
This shift in the absorption wavelength was saturated
60
2
+
when the concentration of Cu reached 240 μM
Fig. 2a). Under similar conditions, TDIPQ (10 μM)
4
2
0
0
0
(
showed an emission maximum at λ 467 nm upon
excitation at λ 364 nm with a fluorescence quantum
yield of 0.69. TDIPQ selectively responded to the ad-
dition of Cu2 ions by quenching its emission (fluores-
cence quantum yield 0.01), and the emission quenching
+
2
+
was saturated at 240 μM of Cu ions (Fig. 2b).
The limit of detection (LOD) and limit of quan-
Fig. 3. Metal-ion selectivity of TDIPQ (10 ꢀM) in MeCN–
2
+
H O (2:1). Red bars represent the fluorescence emission in-
tification (LOQ) of TDIPQ for Cu were calculated
2
using the Cu2+ ion concentration-dependent variation
tensity of solutions containing TDIPQ (10 ꢀM) and 20 equiv
of the corresponding cation; gray bars show the fluorescence
in the fluorescence intensity of TDIPQ. The minimum
2+
intensity after addition of 20 equiv of Cu to the solution
–
6
detectable limit was found to be 2×10 M. The LOD
containing TDIPQ (10 ꢀM) and the cation (200 ꢀM).
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 56 No. 1 2020

162
HARSHA et al.
+
2+
+
2+
2+
2+
metal ions (200 μM; Na , Mg , K , Ca , Mn , Co ,
2
+
2+
2+
2+
2+
2+
0
.12
.10
.08
.06
.04
.02
.00
Ni , Zn , Pb , Cd , Ba , and Hg ), competition
experiment was conducted, and the resultant emission
maximum at λ 467 nm confirmed that TDIPQ can
0
0
0
0
0
0
2+
recognize Cu ions with its characteristic fluorescence
quenching even under coexistence with other metal
ions (Fig. 3).
Job’s plot. A binding assay was carried out using
Job’s plot. The absorbance at λ 426 nm was plotted
against the mole fraction of TDIPQ to get the stoi-
2
+
–
–
0.02
0.04
chiometric ratio. For TDIPQ and Cu ion, the maxi-
mum of Job’s plot was observed at a 0.6 mole fraction
of the ligand (Fig 4), which strongly suggests 2:1
complex formation. The 2:1 stoichiometry of the com-
plex was confirmed by the MALDI mass spectrum
which showed a peak at m/z 1422.86 corresponding
to 2TDIPQ·Cu species. This is a direct evidence of
complex formation showing changes in the absorption
and emission spectra of TDIPQ in the presence of
copper(II) ions.
0
.0
0.2
Mole fraction [Cu ]/[Cu + TDIPQ]
Fig. 4. Job’s plot indicating the 2:1 stoichiometry for the
0.4
0.6
0.8
1.0
2+
2+
2+
complex TDIPQ–Cu .
using LOD = 3σ/S, where σ is the standard deviation
0.075) and S is the slope of the linearity curve [37],
and the LOQ was determined as 3×LOD. The LOD and
(
LOQ values indicated the high sensitivity of TDIPQ
1H NMR study. To complement spectrophotometric
studies and further explicate the coordination mode of
TDIPQ with the metal ion, we performed H NMR
analysis of the complex formation by adding Cu
0.6 mM) to a solution of TDIPQ. The H NMR spec-
trum of the free receptor has doublets at δ 8.38 and
.56 ppm corresponding to the quinoline protons. Upon
addition of Cu ion, these signals shift to δ 8.36 and
7.57 ppm, respectively, with a decrease in their inten-
sity, indicating complex formation (Fig. 5). These find-
ings suggest coordination of copper(II) to the quinoline
and imidazole nitrogen atoms.
toward Cu2+ ions. Next, precision, accuracy, and
recovery of Cu2+ added were estimated for the present
method, and the results indicated that TDIPQ is
an efficient fluorescent chemosensor for both qualita-
1
2+
1
(
2
+
tive and quantitative analysis of Cu ions.
Competition study. A chemosensor should be
selective for the target metal ion but should have mini-
mal response toward coexistent metal ions normally
present in various matrices. To understand the ability
7
2
+
of TDIPQ (MeCN:H O (2:1 v/v)) to selectively re-
2
spond to Cu2 ions (200 μM) in the presence of other
+
δ, ppm
1
2+
Fig. 5. Variations in the H NMR spectra of TDIPQ upon addition of 0.6 mM of Cu ions.
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 56 No. 1 2020

ON–OFF FLUORESCENT IMIDAZOLE DERIVATIVE
163
Scheme 2.
Bu-t
Ph
Ph
Bu-t
N
N
N
Cu(II)
Cu(II)
Ph
Ph
N
N
Ph
Ph
N
N
N
N
TDIPQ
t-Bu
TDIPQ–copper(II) complex
Plausible mechanism of Cu2+ sensing. Conclu-
ment, indicating that TDIPQ is suitable to detect Cu2+
ions present in samples (Table 2). Moreover, advan-
tages like high sensitivity, good selectivity, speed, low
cost, and simple sample preparation procedure asso-
ciated with spectrophotometer make this method more
preferable that conventional ICP-OES method.
sions from the preceding discussions put forward
2
+
a binding mode of TDIPQ with Cu ion as shown in
Scheme 2. The interaction involves nitrogen atoms of
the quinoline and imidazole rings of two TDIPQ mole-
2
+
cules and one Cu ion, as is well evident from the
MALDI spectrum and Job’s plot. The interaction be-
Instant sensing of Cu2+ ion on a polymer (BZA-
2
+
tween TDIPQ and Cu ions brings about a change in
color to yellow and fluorescence quenching of TDIPQ.
Co-BZMA) film containing TDIPQ. To make the
method more feasible and readily available, polymer-
based thin films were cast on a glass slide doped with
TDIPQ. The polymer (BZA-Co-BZMA) and TDIPQ
were taken into a round-bottom flask and dissolved in
chloroform. After addition of copper(II) to the polymer
Analytical applications. To understand the prac-
2
+
tical utility of TDIPQ for detection of Cu ions in
environmental samples such as drinking water, soil,
and lab wastewater, experiments have been conducted
(BZA-Co-BZMA)–TDIPQ mixture, the color of the
by spiking the samples with CuCl . The samples were
2
2
+
solution changed to yellow and was retained even after
solidification of the mixture. On exposure to UV light,
the (BZA-Co-BZMA)–TDIPQ film showed blue fluo-
rescence, whereas the (BZA-Co-BZMA)–TDIPQ–Cu2
film showed fluorescence quenching (Fig. 6).
then analyzed for Cu ions using the proposed method.
Control experiments were performed using ICP-OES
to correlate the results and to estimate the reliability
of the proposed method. The detailed procedure for
sample preparation is provided in Experimental. The
results obtained by spectrophotometry employing
TDIPQ as an indicator and ICP-OES were in agree-
+
The morphology of the (BZA-Co-BZMA)–TDIPQ
2
+
and (BZA-Co-BZMA)–TDIPQ–Cu films was inves-
Table 2. Estimation of Cu2+ concentration (μg/mL) in various samples using TDIPQ and ICP-OES methods
2
+
2+
a
2+
Sample
[Cu ] added
0.13
[Cu ] found using TDIPQ
[Cu ] found by ICP-OES
Drinking water
0.13
0.32
0.12
0.32
0.10
0.31
0.13
0.32
0.13
0.31
0.12
0.32
0
.32
0.13
.32
0.13
.32
Laboratory wastewater
Soil
0
0
a
Mean value of three experiments.
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 56 No. 1 2020

164
HARSHA et al.
responds to Cu2+ ion through color change to yellow
along with fluorescence quenching, and the detection
2
+
ability of TDIPQ for Cu ion remains unaffected even
in the presence of common coexisting metal ions. The
method is characterized by limit of detection and limit
–
6
–6
of quantification values of 2×10 and 6.2×10 M,
respectively. The stoichiometry of the complex formed
2
+
between TDIPQ and Cu was determined to be 2:1
using Job’s plot. A polymer–TDIPQ mixture cast on
a glass slide can be used to detect copper ion in
solution. Practical application of the proposed method,
simple synthesis of TDIPQ, uncomplicated operation
procedures, and cost effectiveness demonstrate the
potential utility of TDIPQ for monitoring copper in
various samples.
Fig. 6. Films of polymer (BZA-Co-BZMA), (BZA-Co-
BZMA)–TDIPQ, and (BZA-Co-BZMA)–TDIPQ–Cu2+
under sunlight and UV light.
tigated by scanning electron microscopy. The SEM
images (Fig. 7) revealed tightly packed spherically
shaped arrangement of the (BZA-Co-BZMA)–TDIPQ
film. The size of the spheres varied from ~0.5 μm to
EXPERIMENTAL
~
1 μm. In the presence of copper, the size of the
All chemicals used were obtained from Aldrich,
Alfa Aesar, SD Fine Chem Ltd., and Merck. All reac-
tions were carried out under nitrogen atmosphere, and
the synthesized compounds were purified by silica gel
column chromatography. Redistilled organic solvents
were used. Metal salt stock solutions were prepared in
Millipore water. The H and C NMR spectra were
recorded on 400- and 500-MHz Bruker UltraShield
Plus spectrometers using tetramethylsilane as internal
spheres increased to ~6.6 μm, and the spheres were
surrounded by satellite ovals. The difference in the size
and morphology of the films may be due to the forma-
tion of copper complex with two molecules of TDIPQ.
The above (BZA-Co-BZMA)–TDIPQ mixture was
used to cast a film on a glass slide. After solidification
of the polymer on the glass slide, it was used as
a device to indicate the presence of copper ion in
solution. When a drop of a 1×10 M Cu solution was
applied onto the coated glass slide, it showed a color
change to yellow along with fluorescence quenching
1
13
–
5
2+
standard and CDCl as solvent. The UV-Vis absorption
3
spectra were measured on an Agilent Cary 5000 UV-
Vis-NIR spectrophotometer at room temperature using
(Fig. 8).
1
0-mm quartz cells. Fluorescence analysis was done
In summary, we have developed a method for
at room temperature on a Cary Eclipse fluorescence
spectrophotometer with a slit width of 2.5 nm (10-mm
quartz cell). The mass spectra (positive electrospray
ionization) were recorded on a Thermo Scientific mass
qualitative detection and quantification of copper(II)
ions in various samples by using a novel multichromo-
phoric hybrid molecule, TDIPQ. TDIPQ selectively
(a)
(b)
Fig. 7. SEM images of (a) poly(BZA-Co-BZMA) and (b) poly(BZA-Co-BZMA)–TDIPQ.
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 56 No. 1 2020

ON–OFF FLUORESCENT IMIDAZOLE DERIVATIVE
165
spectrometer (Waltham, MA, USA). The high-resolu-
tion mass spectra were obtained with an orbitrap mass
analyzer. The IR spectra were recorded on a Bruker
Alpha FTIR spectrophotometer. A Thermo Fisher
Scientific UK iCAP 6500 DUO inductively coupled
plasma–optical emission spectrophotometer (ICP-
OES) was used to analyze copper in real/simulated
samples. Scanning electron microscopy (SEM) images
were obtained using a Hitachi S-3000N scanning elec-
tron microscope instrument. A stock solution of TDIPQ
Spot formed after addition
of a solution containing Cu
ions on a polymer–TDIPQ
film
–
2
2+
with a concentration of 1×10 M was prepared using
HPLC-grade chloroform. Metal ion stock solutions
1×10–2 M) were prepared from the corresponding
(
metal chlorides using Millipore water as a solvent.
–
5
A working solution of TDIPQ (1×10 M) was prepared
by adding 3 μL of the stock solution to HPLC-grade
acetonitrile–water (2:1, v/v), followed by adjusting the
volume to 3 mL. All experiments were carried out in
acetonitrile–water (2:1, v/v).
Fig. 8. Polymer (BZA-Co-BZMA)–TDIPQ film indicating
the presence of copper as a yellow sport under sunlight and
fluorescence quenching under UV light.
6
-Bromo-2-[1-(4-tert-butylphenyl)-4,5-diphenyl-
1
4
6
H-imidazol-2-yl]quinoline (4). Benzil (1.00 g,
.75 mmol), ammonium acetate (3.66 g, 47.50 mmol),
-bromoquinoline-2-carbaldehyde (1.12 g, 4.75 mmol),
mixture was diluted with water (50 mL) and extracted
with methylene chloride (3×100 mL). The combined
extracts were dried over anhydrous Na SO , filtered,
and 4-tert-butylaniline (663 mg, 7.12 mmol) were
mixed in glacial acetic acid under stirring, and the
mixture was refluxed for 12 h under nitrogen. The
mixture was cooled, and the precipitate was filtered off
and washed with water (5×10 mL). The filtrate was
dissolved in ethyl acetate, dried over Na SO , and
2
4
and evaporated under reduced pressure, and the residue
was subjected to column chromatography using hexane
1
as eluent. Yield 90%, mp 81–83°C. H NMR spectrum
(300 MHz), δ, ppm: 8.43 d (1H, J = 9.8 Hz), 8.25–
2
4
purified on a silica gel column using hexane–ethyl
acetate (10:2, v/v) as eluent. Yield 60%, white solid.
H NMR spectrum (400 MHz), δ, ppm: 8.30 d (1H, J =
8.13 m (4H), 8.11–7.98 m (4H).
4
,4,5,5-Tetramethyl-2-(pyren-1-yl)-1,3,2-
dioxaborolane (7). A solution of compound 6 (6.0 g,
1.4 mmol) in anhydrous THF (100 mL) was cooled
to –78°C, and a 2.0 M solution of n-BuLi in hexane
11.7 mL, 23.5 mmol) was slowly added under nitro-
gen atmosphere. The mixture was stirred for 1 h,
,4,5,5-tetramethyl-2-(propan-2-yloxy)-1,3,2-dioxa-
1
8
2
7
.7 Hz), 8.00 d (1H, J = 8.7 Hz), 7.87 d (1H, J =
.1 Hz), 7.65–7.61 m (2H), 7.58–7.55 m (1H), 7.30–
.18 m (11H), 7.10–7.07 m (2H), 1.31 s (9H).
2
(
1
3
C NMR spectrum (125 MHz), δ , ppm; 151.01,
C
1
1
1
1
49.57, 145.61, 144.92, 138.87, 135.53, 134.70,
34.33, 133.06, 132.59, 131.08, 131.03, 130.27,
29.38, 128.25, 128.15, 128.09, 128.04, 127.39,
4
borolane (6.4 g, 34.2 mmol) was added, and the mix-
ture was stirred at room temperature for 3 h. After
completion of the reaction (TLC), water (3× 10 mL)
was added, and the mixture was extracted with methy-
lene chloride (3×60 mL). The combined extracts were
dried over anhydrous Na SO and evaporated under
26.72, 125.13, 121.84, 120.17, 34.64, 31.34. Mass
+
spectrum: m/z 560.15161 [M + H] .
1
-Bromopyrene (6). 1-Bromopyrene was prepared
according to the procedure described in [30]. A solu-
tion of pyrene (10 g, 49.5 mmol) in methanol–diethyl
ether (60 mL, 1:1, v/v) was cooled to 5–10°C, and
a mixture of 48% aqueous HBr (9.2 mL, 54.4 mmol)
and 30% aqueous hydrogen peroxide (5.6 mL,
2
4
reduced pressure, and the residue was subjected to
column chromatography on silica gel (60–120 mesh)
using hexane–ethyl acetate (98 : 2) as eluent. Yield
1
70%. H NMR spectrum (300 MHz), δ, ppm: 9.07 d
4
9.5 mmol) was added over a period of 15 min with
(1H, J = 9.15 Hz), 8.53 d (1H, J = 7.62 Hz), 8.23–
stirring. The mixture was stirred at room temperature
for 12 h, and the progress of the reaction was moni-
tored by TLC. After completion of the reaction, the
8.10 m (5H), 8.08–8.05 m (1H), 8.02–7.98 m (1H),
1
3
1.49 s (12H). C NMR spectrum (75 MHz), δ , ppm:
C
136.37, 133.81, 133.38, 131.02, 130.69, 128.47, 127.95,
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 56 No. 1 2020

166
HARSHA et al.
1
1
27.73, 127.41, 125.62, 125.28, 125.13, 124.53,
24.31, 124.02, 83.82, 25.00. Mass spectrum:
cision values. Recovery studies were also carried out to
illustrate the accuracy of the method. All experiments
were performed in triplicate and the mean value for
each concentration was plotted to get recovery values.
+
10
m/z 328.1749 [M + H] ( B).
2
-[1-(4-tert-Butylphenyl)-4,5-diphenyl-1H-imid-
azol-2-yl]-6-(pyren-1-yl)quinoline (TDIPQ). Com-
pounds 4 (1 g, 1.99 mmol) and 7 (0.784 g,
Quantum yield (Φ) determination. The fluores-
cence quantum yield was determined by integrating the
area under the curve of the sample and fluorescence
standard using the equation
2
.390 mmol) and Pd(PPh ) (5 mol %) were added to
3 4
toluene (40 mL) while stirring. A solution of sodium
carbonate (2.11 g, 19.92 mmol) in a mixture of water
2
2
(16 mL) and ethanol (8 mL) was added with stirring,
Φ = ΦR(IDRn )/(IRDnR),
and the mixture was refluxed at 110°C overnight. The
mixture was then cooled to room temperature, 50 mL
of water was added, and the mixture was extracted
with 100 mL of chloroform. The extract was evaporat-
ed, and the residue was purified by silica gel column
chromatography using hexane–ethyl acetate (95:5, v/v)
where Φ is the quantum yield, I is the integrated
intensity, n is the refractive index, D is the optical
density, and the subscript R refers to the standard
(diphenylanthracene in cyclohexane) [39]. The fluores-
cence quantum yields of TDIPQ were calculated in the
2
+
–
1
presence and in the absence of Cu ions.
as eluent. Yield 60%. IR spectrum, ν, cm : 3041
(
C–Harom), 1596 (C=C), 1474, 1413, 1377 (C–N).
H NMR spectrum (400 MHz), δ, ppm: 8.38 d (1H, J =
Analytical applications. The proposed method was
employed to determine Cu ions in drinking water,
1
2+
8
.5 Hz), 8.26–8.10 m (7H), 8.04–7.99 m (3H), 7.97 s
soil, and lab wastewater. The samples were spiked with
(1H), 7.84–7.80 m (1H), 7.70–7.65 m (2H), 7.56 d
CuCl and filtered to remove solid impurities. The
2
(
1H, J = 8.5 Hz), 7.36–7.16 m (12H), 1.35 s (9H).
amount of copper present in all samples was analyzed
by ICP-OES and by the proposed method. Copper(II)
chloride, 6.7 mg, was added to 5 mL of a sample to get
13
C NMR spectrum (100 MHz), δ , ppm: 151.01,
C
1
1
1
1
1
1
1
49.46, 146.38, 145.35, 139.40, 138.81, 136.81,
35.87, 135.71, 134.47, 132.98, 132.22, 131.45,
31.17, 130.91, 130.79, 130.44, 129.28, 128.67,
28.55, 128.26, 128.24, 128.17, 128.01, 127.75,
27.70, 127.61, 127.46, 127.38, 127.08, 126.69,
26.08, 125.26, 125.20, 124.96, 124.66, 124.83,
–
2
a concentration of 1×10 M. Soil sample spiked with
6.7 mg of CuCl was dissolved in Millipore deionized
2
water (5 mL), and the solution filtered to remove
insoluble impurities. A portion from all samples was
–
5
–5
diluted to concentrations of 2×10 M and 5×10
M
21.54, 34.68, 31.38. Mass spectrum: m/z 680.30569
for the ICP-OES analysis and the proposed method,
respectively. Available polymer (BZA-Co-BZMA)
along with TDIPQ was used to cast a device as a spot
+
[
M + H] .
Job’s plot. A binding assay was carried out by
constructing Job’s plot [38] using a series of TDIPQ–
2
+
test for Cu ions. For this purpose, 150 mg of BZA-
Co-BZMA was dissolved in 5 mL of chloroform, and
2+
Cu solutions (10 μM in total). Solutions were pre-
pared by mixing different ligand/metal concentration
ratios and diluting to a constant volume so that the total
concentration of the mixture remained constant, fol-
lowed by measuring their emission profile. The inten-
sity of the emission maximum was plotted against
mole fraction of TDIPQ to get the stoichiometric ratio.
3
40 μg of TDIPQ was added. The mixture was stirred
at room temperature for 30 min and transferred to
a round-bottom flask, 5 mL of chloroform was added,
2
+
–5
a solution containing Cu ions (1×10 M) was slowly
added, and the mixture was stirred at room temperature
for 30 min.
1
H NMR study of the complex formation of
ACKNOWLEDGMENTS
TDIPQ with copper(II). A 0.6 mM solution of Cu2+
was added to a solution of TDIPQ (12.3 mM) in
K.G. Harsha and E. Appalanaidu thank UGC-New Delhi
and CSIR-Delhi, respectively, for the financial assistance.
B.A. Rao thank CSIR-Delhi for fellowship as CSIR-RA.
V.J. Rao thanks CSIR-Delhi for Emeritus Scientist honor.
CDCl , and the resulting solution was subjected to
3
1
H NMR analysis.
Precision and recovery studies. The precision of
the proposed method was experimentally determined
by estimating the corresponding spectrophotometric
responses for five repeated analysis of a known quan-
tity of the Cu2 ion from working solution. The result-
ing five concentration values were plotted to get pre-
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