Thiophene and furan appended pyrazoline based fluorescent chemosensors for detection of Al3+ ion

Article abstract of DOI:10.1016/j.inoche.2019.01.038

Thiophene and furan appended pyrazoline receptors R1 and R2 were designed and synthesized for selective detection of Al³⁺ ion. Their photophysical properties were studied by UV–visible and fluorescence spectra. Surprisingly, both receptors R1 a

Full text of DOI:10.1016/j.inoche.2019.01.038

Accepted Manuscript
Thiophene and furan appended pyrazoline based fluorescent
chemosensors for detection of Al3+ ion
Rangasamy Manjunath, Palaninathan Kannan
PII:
S1387-7003(18)31042-6
DOI:
Reference:
https://doi.org/10.1016/j.inoche.2019.01.038
INOCHE 7254
To appear in:
Inorganic Chemistry Communications
Received date:
Revised date:
Accepted date:
27 November 2018
24 January 2019
25 January 2019
Please cite this article as: R. Manjunath and P. Kannan, Thiophene and furan appended
pyrazoline based fluorescent chemosensors for detection of Al3+ ion, Inorganic Chemistry
Communications, https://doi.org/10.1016/j.inoche.2019.01.038
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Thiophene and furan appended pyrazoline based fluorescent chemosensors for detection of Al3+ ion
Rangasamy Manjunath and Palaninathan Kannan*
Department of Chemistry, Anna University, Chennai - 600 025, India.
Abstract
Thiophene and furan appended pyrazoline receptors R1 and R2 were designed and
synthesized for selective detection of Al3+ ion. Their photophysical properties were studied
by UV-visible and fluorescence spectra. Surprisingly, both receptors R1 and R2 were
displayed an excellent selective and sensitive response to Al3+ ion alone over other tested
metal ions. Both the receptor R1 and R2 displays 1:1 stoichiometry for [R1-Al3+] and [R2-
Al3+] complex and formed with an association constant of 1.84 × 104 M−1 and 1.92×104 M-1
respectively. The limit of detection were calculated for R1 and R2 since 8.92 × 10−8 M and
1.04 × 10−7 M by the fluorescence titration method. The thiophene based receptor R1
exhibited superior chemosensor characteristics such as high intensity absorption and emission
at longer wavelength as compered that of furan based receptor R2.
Keywords: Pyrazoline; Thiophene; Furan; Chemosensor; Aluminum (III) ion;
Corresponding author: Dr. P. Kannan
E-mail: pakannan@annauniv.edu , Tel: +91 44 2235 8666, Fax: +91 44 2220 0660

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Introduction
The design and synthesis of highly selective and sensitive fluorescent chemosensor
for detection of heavy and transition metals ions is still a hot research area in chemistry,
biochemistry, biology, material science and environmental science [1-6]. These metal ions
are entered into environment through natural and anthropogenic activities and biologically
and environmentally-important species. The presence of these metal ions is critical for human
life, ascribed to their toxicity.
The trivalent cation found in its ionic form in most kind of animals, plant tissues and
in natural waters everywhere [7,8]. Aluminum is the third most abundant element in the
earth’s crust and its compounds comprise about 8.8% by weight of the Earth's surface[9].
People are widely exposed to aluminum attributed to its widespread use in food additives,
pharmaceuticals, and cooking utensils, aluminum foil, vessels, trays and cosmetics [10,11].
Excessive exposure of human body to Al3+ ion can be more toxic to human
health[12]. The excess aluminum can cause damages to certain human tissues and cells,
resulting Alzheimer’s disease and Parkinson’s disease, aluminum accumulation in the body
leading to bone and brain damage [13-17]. Additionally, the concentration of aluminum has
been found to be crucial for fish, and also for agricultural production as it increases acidity of
the soil. Aluminum is toxic to plants at micro molar concentration, the phytotoxicity of
aluminum is rapid inhibition of root elongation of the plant, and aluminum may interact with
root cell walls, disrupt plasma membrane and inhibit transport process on the plasma
membrane[18].
A variety of analytical methods are available for determination of Al3+ ion, including,
photometric methods, flame or graphite furnace, atomic absorption spectroscopy (AAS),
inductively coupled plasma emission or mass spectrometry (ICP-ES, ICP-MS), anodic

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stripping voltammetry (ASV), atomic emission spectrometry, etc are available for detection
of Al3+ ion [19–22]. But these methods require sophisticated analytical instruments. On the
other hand, the fluorescence detection method has concerned significant attention attributed
to its high signaling ability along with high sensitivity and selectivity, enable on-line and
field monitoring. Thus, emerged as useful alternative tool [23, 24].
Herein, we report the synthesis of thiophene and furan appended pyrazoline
derivatives R1 and R2. Both the receptors displayed an excellent selective and sensitive
response to Al3+ ion over other tested metal ions.
Experimental
Materials and Instruments
Deionized water was used throughout the experiment. All the chemicals for synthesis
were purchased from commercial suppliers and used after further purification. Perchlorate
salts of Na+, Mg2+, Al3+ K+, Ca2+, Mn2+, Fe3+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+ and Hg2+ were
purchased from Alfa Aesar, India. All the reagent-grade solvents were purified according to
the standard procedures and freshly distilled prior to use [25]. All the synthesis was
monitored by thin layer chromatography with detection by UV. Thin layer chromatography
(TLC) was performed on silica gel 60 F254 plates (Merck KGaA). Column chromatography
was executed on silica gel (60–120 mesh). FT-IR spectra were recorded with IR spectrometer
1
Perkin-Elmer FT-IR spectrometer using KBr pellets. H-NMR spectra were recorded on
Bruker AVIII 300 MHz spectrometer using tetramethylsilane (TMS) as an internal reference.
Electron spray mass spectra (ESI-MS) were carried out on a Bruker MaXis 10138 HRMS
instrument. UV-vis spectra were recorded in room temperature using a Shimadzu UV-

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1650PC spectrophotometer. Fluorescence spectra were recorded in room temperature using
Perkin Elmer LS-45 Fluorescence spectrophotometer.
Synthesis of receptor R1 and R2
The synthetic route of the prepared receptors R1 and R2 is depicted in Scheme 1. (E)-3-(3-
hydroxyphenyl)-1-(thiophen-2-yl)prop-2-en-1-one
and
(E)-1-(furan-2-yl)-3-(3-hydroxy
phenyl)prop-2-en-1-one were prepared according to the literature procedures[26- 27]. To a
stirred solution of chalcones (2) and (3), (1.0 mmol) and NaOH (3 mmol) in ethanol (15 mL)
2-hydrazinobenzothiazole (1.0 mmol) was added and reaction mixture were refluxed for 6 h
respectively. The progress of the reaction was monitored by TLC. After completion, the
reaction mixture was cooled to room temperature, resultant mixture quenched with ice water
and solution neutralized with dilute hydrochloric acid. The crude products were obtained as
pale yellow precipitate, filtered, washed with water and ethanol and recrystallized with
ethanol to give products with 80% yield. 3-(1-(benzo[d]thiazol-2-yl)-3-(thiophen-2-yl)-4,5-
-1
dihydro-1H-pyrazol-5-yl)phenol (R1) IR (Fig.S5) (γmax, KBr, cm ): 3437, 1600,
1
1532,1450,1236, 705; H NMR (Fig. S1) (300 MHz, CDCl3) δ (ppm): 10.01 (s, 1H), 7.82 (s,
1H),7.65 (s, 1H),7.43 (d, 2H), 6.97 (s, 1H),6.94 – 6.90 (m, 3H),6.85 (d, 2H),6.75 – 6.63 (s,
1H), 5.79 (s, H), 3.88 (s, 1H), 3.23 (s, 1H).; 13C NMR (Fig. S3) (100 MHz, DMSO–d6): δ
165.01, 156.46, 154.80, 152.00, 140.92, 138.35, 134.57, 130.60, 129.27, 127.37, 127.11,
126.08, 122.63, 122.41, 119.55, 119.22, 116.36, 116.21, 77.28, 77.03, 76.77, 67.15, 39.84.;
MS (ESI+): m/z (%): 377.48(100) (M + H)+ (Fig. S6)
3-(1-(benzo[d]thiazol-2-yl)-3-(furan-2-yl)-4,5-dihydro-1H-pyrazol-5-yl)phenol(R2) IR (Fig.
-1
1
S5) (γmax, KBr, cm ): 3427, 1606, 1532, 1456, 1242, 758; H NMR (Fig. S2) (300 MHz,

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CDCl3) δ (ppm): 10.01 (s), 7.62 (s, 1H),7.59 (s, 1H),7.26 (s, 1H), 7.17 (d, J = 3.1 Hz, 2H)
7.05(s, 1H),6.81 (d, J = 11.6 Hz, 2H),6.68(s, 1H),6.59 (s, 1H), 6.54 (s, 1H), 5.95 (s, 1H), 3.88
13
(s, 1H), 3.26 (s, 1H).; C NMR (Fig. S4) (100 MHz, DMSO–d6): δ 165.01, 156.46, 154.80,
152.00, 140.92, 138.35, 134.57, 130.60, 129.27, 127.37, 127.11, 126.08, 122.63, 122.41,
119.55, 119.22, 116.36, 116.21, 77.28, 77.03, 76.77, 67.15, 39.84.; MS (ESI+): m/z (%):
361.42 (100) (M + H)+ (Fig. S7).
General procedure for spectral detection
The stock solutions of R1 and R2 (1.0 x10-5 M) were prepared by dissolving in
acetonitrile. The cationic stocks were all dissolved in deionized water with concentration of
10−3 M for UV–Vis absorption and fluorescence spectral analysis. UV-visible and
fluorescence titration experiments were performed using 1.0 × 10−5M and 10−5M of R1 and
R2 in HEPES buffer (20 mM HEPES, pH = 7.2, 50% (v/v) CH3CN), respectively. The
sensing studies were carried out by monitoring absorption and fluorescence spectral analyses,
3 mL solution of R1 and R2 were filled in a quartz cell of 1 cm optical path length, and
gradually increased Al3+ ion concentration using a micro-pipette every time. They were kept
1 min for incubation after each addition of Al3+ ion.
Job’s plot analyses
The stoichiometric behavior of receptors R1 and R2 towards Al3+ ion were
determined by Job’s plot method using fluorescence titration data [28]. The total
concentration of receptor and Al3+ ion kept as constant and mole fraction of Al3+ ion altering
from 0 to 1. Fluorescence spectra were recorded for all portions. The relative fluorescence
intensity values was plotted against [Al3+]/ {[Al3+]+[R]}. The mole fraction of Al3+ at which
the emission intensity is at maximum gives the stoichiometry of complex from the plot.

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Determination of association constant
The association constant can be estimated by fluorescence titration according to
Benesi-Hildebrand equation [29]
1
1
1
=
+
3+
(
)
(∆퐹) 퐾푎 ∆퐹 [Al ] ∆퐹
푠푎푡
푠푎푡
∆F is the fluorescence difference between the presence and absence of Al3+ with
receptor, Fsat is fluorescence difference at saturated, Ka is association constant and was
determined from slope of linear plot, [Al3+] is concentration of Al3+ ion added during
titration studies. The goodness of linear fit of B-H plot of 1/∆F verses 1/[Al3+] for 1:1
complex formation confirms binding stoichiometry between receptor and Al3+.
Determination of quantum yield
Fluorescence quantum yield was determined using quinine sulfate dehydrate (≥ 99.0
%) in 0.1 N H2SO4 as the main standard. The corrected emission spectra were measured for
quinine sulfate dehydrate standard (λex=330 nm; A (Absorption) <0.01; ΦF= 0.560). The
general equation used in determination of relative quantum yield of samples was determined
according to the following equation. [30]
퐹푢 퐴푠 (ƞ푢)2
퐹푠 퐴푢 ((ƞ푠)2
∅u = ∅푠.
.
.
Where Φu and Φs are fluorescence quantum yield of sample and standard, Fu and Fs are the
integrated fluorescence intensities of sample and standard, Au and As are absorbance of
sample and standard and ηu and ηs are refractive index of sample and standard solutions
respectively.

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Scheme1. Synthesis of R1 and R2.
Results and discussion
The receptors R1 and R2 were facile synthesized by the reaction of chalcone 1 and 2
with 2-hydrazinobenzothiazole in ethanol under reflux in 80% yield (Scheme 1). The
structure of R1 and R2 was confirmed by 1H-NMR, 13C-NMR and ESI-MS spectra.
Absorption and emission behavior of R1 and R2
The receptors R1 and R2 exhibits a broad absorption bond at 358 nm and 347 nm
respectively (Fig.1), which could be assigned to an allowed π-π* transition and intra charge
transfer (ICT) of conjugated localized pyrazoline ring system, The emission spectra of
receptors R1 and R2 are depicted in Fig.2. Receptor R1 and R2 exhibited a bright blue
emission with peak at 447 nm and 436 nm as similar to pyrazoline family [31, 32]. Thiophene
and furan substituents present in the pyrazoline displays different emission peak range.

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Among the two receptors, thiophene appended receptor R1 exhibit better photophysical
properties compared to furan appended receptor R2. It reveals that pyrazoline derivatives
could exhibit different emission with respect to changing substitution in pyrazoline structure.
Fig. 1. UV-Vis absorption spectra of R1 and R2
Fig. 2. Fluorescence spectra of R1 and R2

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UV–Vis absorption spectral responses of R1 and R2 towards metal ions
Binding affinity of receptor R1 and R2 towards various metal ions was estimated by
UV–Vis spectroscopy measurements. Addition of various metal ions to the solution of R1
and R2, absorption spectrum changes differently as evidenced in Fig. 3 and 4. In the case of
Na+, Mg2+, K+, Mn2+, Fe3+, Ni2+, Cu2+, Zn2+, Cd2+, Pb2+ and Hg2+ additions, there is no
significant change was observed in absorption spectra of R1 and R2, whereas in the presence
of Al3+ ion the wavelength absorption peak of R1 and R2 at 358 nm and 347 nm was shifts
hypsochromically to 349 nm and 341 nm, respectively, attributed to the formation of new
complex between receptor and Al3+ ion.
To determine the binding behavior of receptor R1 and R2 towards Al3+ ion, the UV-Vis
absorption titration experiments were carried out in HEPES buffer solution (20 mM HEPES,
pH = 7.2, 50% (v/v) (CH3CN; Fig.S8 and S9). The progressive addition of Al3+ion to solution
of receptor R1 resulted in blue shift in absorption spectra, the absorption peak at 358 nm
gradually hypsochromically shifted to 349 nm and its intensity gradually increased with
increasing concentration of Al3+ ion, the absorption intensity at 266 nm was gradually
decreased with isobestic point at 328 nm was ascribed to equilibrium between receptor R1
and Al3+ throughout the titration process, similarly addition of Al3+ ion to solution of receptor
R2, absorbance peak at 347nm gradually blue shifted to 341 nm. This absorption peak is
presumably attributed to coordination of receptor R2 with Al3+ion.

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Fig.3. UV–Vis absorption spectra of R1 (1 ×10−5 M) in HEPES buffer solution (20
mM HEPES), pH = 7.2, 50% (v/v; CH3CN) in the presence of various metal ions (3 ×
10−5 M)
Fig.4. UV–Vis absorption spectra of R2 (1 ×10−5 M) in HEPES buffer solution (20
mM HEPES), pH = 7.2, 50% (v/v; CH3CN) in presence of various metal ions (3 × 10−5 M)

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Fluorescence spectral responses of R1 and R2 towards metal ions
The fluorescence spectrum changes of receptor R1 and R2 in HEPES buffer solution
(20 mM HEPES, pH = 7.2, 50% (v/v; CH3CN), before and after addition of different metal
ions were recorded to examine the selectivity. The free receptors R1 and R2 exhibited a
strong fluorescence emission at 447nm and 436nm respectively in the absence of various
metal ions as depicted in Fig. 5 and 6. Interestingly, addition of 3 equiv. of various metal ions
(Na+, Mg2+, Al3+ , K+, Mn2+, Fe3+, Ni2+, Cu2+, Zn2+, Cd2+, Pb2+ and Hg2+) to the solution of
receptor R1 and R2, Al3+ ion alone induced remarkable quenching of fluorescence intensity at
447nm and 436nm respectively and other tested metal ions did not exhibit any significant
change in emission, which reveals that receptor R1 and R2 has a high selectivity towards Al3+
ion.
Fig. 5. Fluorescence spectra of R1 (1 ×10−5 M) in HEPES buffer solution (20 mM HEPES),
pH = 7.2, 50% (v/v; CH3CN) upon additions of various metal ions (3 × 10−5 M)

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Fig. 6. Fluorescence spectra of R2 (1 ×10−5 M) in HEPES buffer solution (20 mM HEPES),
pH = 7.2, 50% (v/v; CH3CN) upon additions of various metal ions (3 × 10−5 M)
To further investigate the interaction of receptor R1 and R2 towards Al3+ ion, a
fluorescence titration experiment was carried out, as presented in Fig. 7 and Fig.8. The free
receptor R1 and R2 exhibits emission peak centered at 447nm and 436nm respectively. The
fluorescence quantum yield of receptor R1 and R2 in the absence of Al3+ were calculated to
be 0.50 and 0.47 with respect to quinine sulphate in 0.1 N H2SO4 solution (∅푠 = 0.54). The
fluorescence intensity of receptors R1 and R2 was linearly decreased with incremental
addition of Al3+ ion, it was quenched almost completely after addition of one equivalent of
Al3+ ion and now the quantum yield was calculated to be 0.037 and 0.039 for receptor R1and
R2 respectively. In the absence of Al3+ ion, the receptors R1 and R2 displays bright blue

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emission under the irradiation at long UV light, while addition of Al3+ ion, the emission was
quenched and produces almost nil-fluorescence ( Fig.7, Fig.8 insert).
Fig.7. Fluorescence spectra of R1 with increasing concentration of Al3+ ion in HEPES buffer
solution (20 mM HEPES), pH = 7.2, 50% (v/v; CH3CN).

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Fig.8. Fluorescence spectra of R2 with increasing concentration of Al3+ in HEPES buffer
solution (20 mM HEPES), pH = 7.2, 50% (v/v) CH3CN).
Competition experiment
To further investigate the competitive behavior of receptors R1 and R2 towards Al3+
ion with other coexist metal ions, the competitive experiment was carried out, the
interference of coexist metal ions to Al3+ detection was investigated by fluorescence spectra
as exhibited in Fig.9 and 10. Receptor R1 and R2 displays excellent selectivity for Al3+ ion
over other tested metal ions. Additionally, competitive metal ions did not cause any
significant changes in fluorescence intensity of receptor R1 and R2 in presence of Al3+ ion.
These results suggested that receptor R1 and R2 could be used as Al3+ selective fluorescent
turn-off chemosensor.

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Fig. 9. Fluorescence intensity of R1 (10 μmol/L) to various metal ions. The first row
represents intensity of R1 in the presence of 5 equiv. of various metal ions; the second
row represents intensity of above solution upon addition of 5 equiv. of Al3+ ion.
Fig. 10. Fluorescence intensity of R2 (10 μmol/L) to various metal ions. The first row
represents intensity of R2 in the presence of 5 equiv. of various metal ions; the second
row represents intensity of above solution upon addition of 5 equiv. of Al3+ ion.
. Binding stoichiometry and association constant
The stoichiometric behavior of receptors R1 and R2 towards Al3+ ion were
determined by Job’s plot method by measuring fluorescence intensity at different mole

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fractions of Al3+ ion (0-1). The relative fluorescence intensity values was plotted against
[Al3+]/ {[Al3+] +[R]}. A maximum emission was observed when molar fraction of Al3+ ion
reached 0.5 (Fig.10 and Fig.S10) indicating that Al3+ ion form a 1:1 binding stoichiometry for
[R1-Al3+] and [R2-Al3+]. The association constant for formation of respective complexes
[R1-Al3+] and [R2-Al3+] were estimated graphically by plotting 1/∆F against 1/[ Al3+ ] (Fig.11
and S11). The data were linearly fitted to Benesi–Hildebrand equation and Ka values were
obtained from slope of linear plot. The association constant for Al3+ ion binding in R1 and
R2 were determined to be 1.84 × 104 M−1 and 1.92×104 M-1 respectively. Sensitivity of R1
and R2 towards Al3+ ion has been checked by determining limit of detection (LOD) value. By
using the fluorescence titration results, the detection limits for Al3+ were found to be 8.92 ×
10−8 M and 1.04 × 10−7 M on the basis of 3σ/m (Fig.S12 and Fig.S13). where σ is the
standard deviation of the blank signals and m is the slope of the linear calibration plot (Fig.
S6). According to World Health Organization (WHO) standard the concentration of Al3+
should be lower than 7.41 mM in drinking water [33, 34], which means that the sensors R1
and R2 are sensitive enough to monitor the water quality of drinking water. In Table S1†
some recently reported chemosensors for selective detection of Al3+ ion has been described
along with their LOD values. Those have some advantages over the others though it has some
draw backs too. Our probe is synthesized by easy single step Schiff base condensation
process. LOD value is significantly low but not the lowest among all reported.

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Fig. 11. Job plot for 1:1 complex of R1 and Al3+ ion at 447 nm.

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Fig. 12. Benesi–Hildebrand plot (at 447nm) for complexation of R1 with Al3+ ion.
Conclusion
Thiophene and furan appended pyrazoline based receptors R1 and R2 have been
synthesized and their photophysical properties examined by UV-visible and fluorescence
spectrophotometers. The receptors R1 and R2 demonstrate an excellent selective and
sensitive response to Al3+ ion over other metal ions. Among them, receptor R1 exhibit
absorption and emission bond centered at longer wavelength and high fluorescence intensity
compared to receptor R2. The fluorescence nature of receptors gradually decreased upon the
addition of Al3+ ion. The chemosensor R1 and R2 displayed a 1:1 complex formation with
Al3+ ion with an association constant of 1.84 × 104 M−1 and 1.92×104 M-1 respectively. Both
probes have good detection limits for Al3+ ions since 8.92 × 10−8 M (R1) and 1.04 × 10−7
M
(R2). Receptors R1 and R2 can be used as selective ‘Turn-Off’ sensor for Al3+ ion.

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Acknowledgements:
The author R.M. thanks the Anna Centenary Research Fellowship (ACRF), Anna University,
Chennai (CR/ACRF/2013/38) for financial support. P.K. is thankful to UGC, New Delhi for
the award of UGC-BSR Faculty Fellowship [F.18-1/2011(BSR) dated 30.12.2016].
Instrumentation facility provided under FIST-DST and UGC-DRS to Department of
Chemistry, Anna University are sincerely acknowledged.
Appendix A. Supplementary data
Characterization data and additional spectroscopic data are available as
supplementary data.
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Graphical Abstract

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Highlights
 A selective fluorescent chemosensor for Al3+ was synthesized.
 The receptors R1 and R2 were displays an excellent selective and sensitive response
to Al3+ ion over other metal ions.
 Job’s plot indicated 1:1 binding- stoichiometry for receptor R1 and R2 with Al3+ ion.
 Association constant of receptor R1 and R2 with Al3+ is determined on the basis of
Bensi-Hilderbrand as Ka = 1.84 × 104 M−1 and Ka =1.92×104 M-1 respectively.
 The thiophene based receptor R1 exhibit high intensity absorption and emission bond
at longer wavelength compered to furan based receptor R2.

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