Ultrasound-assisted synthesis and photophysical investigation of a heterocyclic alkylated chalcone: a sensitive and selective fluorescent chemosensor for Fe3+ in aqueous media

Article abstract of DOI:10.1080/00958972.2020.1838490

Alkylated pyridine chalcone (AIPO) has been synthesized by reaction of 1-allyl-1H-indole-3-carbaldehyde with 2-acetyl pyridine. The chalcone structure was characterized by spectral and elemental analysis. The absorption and emission in ten solvents with different polarities were used to calculate the photophysical parameters for this compound. This heterocyclic fluorescent compound serves as a selective probe for recognition of Fe³⁺. A 1:1 bonding stoichiometry between AIPO and Fe³⁺ has been detected by Benesi–Hildebrand, Stern–Volmer and Job-plot methods. The binding stoichiometric ratio was further confirmed by the density functional theory (DFT) calculations.

Full text of DOI:10.1080/00958972.2020.1838490

Journal of Coordination Chemistry
ISSN: (Print) (Online) Journal homepage: https://www.tandfonline.com/loi/gcoo20
Ultrasound-assisted synthesis and photophysical
investigation of a heterocyclic alkylated chalcone:
a sensitive and selective fluorescent chemosensor
for Fe3+ in aqueous media
Abdullah M. Asiri , Mona Mohammad Al-Amari , Qasim Ullah & Salman A.
Khan
To cite this article: Abdullah M. Asiri , Mona Mohammad Al-Amari , Qasim Ullah & Salman A.
Khan (2020): Ultrasound-assisted synthesis and photophysical investigation of a heterocyclic
alkylated chalcone: a sensitive and selective fluorescent chemosensor for Fe3+ in aqueous media,
Journal of Coordination Chemistry, DOI: 10.1080/00958972.2020.1838490
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JOURNAL OF COORDINATION CHEMISTRY
https://doi.org/10.1080/00958972.2020.1838490
Ultrasound-assisted synthesis and photophysical
investigation of a heterocyclic alkylated chalcone:
a sensitive and selective fluorescent chemosensor
for Fe3þ in aqueous media
Abdullah M. Asiria,b, Mona Mohammad Al-Amaria, Qasim Ullahc and
Salman A. Khanc
b
aChemistry Department, Faculty of Science, King Abdulaziz University, Jeddah, Saudi Arabia; Center
of Excellence for Advanced Materials Research (CEAMR), King Abdulaziz University, Jeddah, Saudi
c
Arabia; Physical Sciences Section, School of Sciences, Maulana Azad National Urdu University,
Hyderabad, Telangana, India
ABSTRACT
ARTICLE HISTORY
Received 3 March 2019
Accepted 11 September 2020
Alkylated pyridine chalcone (AIPO) has been synthesized by reac-
tion of 1-allyl-1H-indole-3-carbaldehyde with 2-acetyl pyridine.
The chalcone structure was characterized by spectral and elemen-
tal analysis. The absorption and emission in ten solvents with dif-
ferent polarities were used to calculate the photophysical
parameters for this compound. This heterocyclic fluorescent com-
pound serves as a selective probe for recognition of Fe3þ. A 1:1
bonding stoichiometry between AIPO and Fe3þ has been
detected by Benesi–Hildebrand, Stern–Volmer and Job-plot meth-
ods. The binding stoichiometric ratio was further confirmed by
the density functional theory (DFT) calculations.
KEYWORDS
Chalcone; iron; pyridine;
sensor; Stokes shift
1. Introduction
Organic compounds containing electron donors and electron acceptors which can
assemble into long chain conjugated frameworks are donor–acceptor chromophores
with wide-ranging applications in light harvesting systems, optoelectronic devices [1],
thin film solar cells [2] and photofunctional devices [3] due to their charge transfer
properties within the molecule from electron donor to electron acceptor. These com-
pounds are also used as polarity probes, laser dyes, nonlinear optics, and memory
storage devices [4]. Photophysical parameters of the chromophores are important to
understand the photophysical properties of molecules [5]. a,b-Unsaturated ketones,
CONTACT Salman A. Khan
sahmad_phd@yahoo.co.in
ß 2020 Informa UK Limited, trading as Taylor & Francis Group

2
A. M. ASIRI ET AL.
known as chalcones, have been recognized for more than 100 years; the simplicity of
their synthesis with their structure adaptability attracts a new attention [6]. The use of
a chromophore having a ketone group in modern functional dyes has improved struc-
ture–property associations. Only recently have the photophysical properties of
a,b-unsaturated ketones been studied [7]. Chalcones having carbonyl groups and
donor groups such as OH, OCH3 and N-CH3 with the a,b-unsaturated ketones are
donor p-acceptor chromophores which can give better optical properties compared to
a,b-unsaturated ketones with applications in various physical sciences [8]. Chalcones
having donor and acceptor groups can be used as an off–on or on–off fluorescent
chemosensor for detection of toxic metals by the photo electron transfer (PET), transi-
tion charge transfer (CT), chelation-induced enhanced fluorescence (CHEF), excimer for-
mation and fluorescence resonance energy transfer [9]. Numerous heterocyclic
compounds were applied as chemosensors for recognition of metal ions. Iron has a
vital role in environment, biology and chemistry, an essential trace element for human
health [10, 11]. Since Fe3þ is a cofactor in many enzymatic reactions, it is crucial for
living cells functioning in humans with other roles, including specialization in protein
transport and storage [12]. The concentration of iron(III) has a major role in diseases,
including diabetes, anemia, liver damage, Parkinson’s disease, cancer, and hemo-
chromatosis [13]. Therefore, there is need for an emergent molecule which can be
used as a selective and sensitive fluorescent chemosensor that can differentiate Fe3þ
when present with different transition metal ions. The detection of iron ion was
achieved by many organic compounds, but synthetic hitches necessitate arduous mul-
tistep organic synthesis [14]. While there is need for selective and sensitive chemosen-
sors in Fe3þ recognition, there is desire to achieve a fluorescence probe with sensing
efficiency and time/cost-effectiveness. Herein, we describe the synthesis of a simple,
cost effective heterocyclic chalcone with photophysical, physicochemical studies for
this compound in detection of Fe3þ
.
2. Experimental
2.1. Chemical and reagents
Starting materials such as 1H-indole-3-carbaldehyde, 1-(pyridine-2-yl) ethan-1-one, 3-
bromoprop-1-ene and sodium hydroxide were obtained from Acros Organic.
Additional chemicals and solvents used in this project were of analytical grade and
used without purification. DMF was used to prepare the stock solution of AIPO. Metal
salts used in this paper, Co2þ, Hg2þ, Fe3þ, Cu2þ, Al3þ, Cd2þ, Sr2þ, Ni2þ, Cr3þ and
Mn2þ, were dissolved in double distilled water.
2.2. Instrumentation
Chalcone (AIPO) melting point was recorded by Stuart Scientific Co. Ltd. melting point
apparatus. A Perkin-Elmer 100 FT-IR spectrometer was used for FT-IR spectra and a
1
Bruker spectrometer was used to obtain H and 13C NMR spectra. Absorption spectra
ꢀ
of APIO were recorded at 25 C using a Shimadzu UV-16550PC UV/Vis spectrometer.

JOURNAL OF COORDINATION CHEMISTRY
3
Fluorescence emission data were obtained from a Shimadzu RF 5301PC fluorescence
spectrometer.
2.3. Synthesis of 1-allyl-1H-indole-3-carbaldehyde
At room temperature KOH pellets (1.15 g, 0.20 mol) were added to a stirred mixture of
indole-3-carbaldehde (2.5 g, 0.17 mol) in 25 mL of ethanol to achieve complete dissol-
ution. Then by vacuum filtration we eliminate ethanol from the crude product and
adding 25 mL of acetone to the crude mixture, 3-bromoprop-1-ene (1.45 mL, 0.17 mol)
was added to the solution. The 3-bromoprop-1-ene precipitate was achieved instant-
aneously. The product solid was separated by vacuum and the solution was concen-
trated. Crystallization from ethanol gave 1-allyl-1H-indole-3-carbaldehyde [15].
2.4. Synthesis of (E)-3-(1-allyl-1H-indol-3-yl)-1-(pyridin-2-yl) prop-2-en-1-
one (AIPO)
Allyl-1H-indole-3-carbaldehyde and 1-(pyridine-2-yl) ethan-1-one were dissolved in
50 mL ethanolic solution of NaOH in a 100 mL Erlenmeyer flask. The reaction mixture
was exposed to the ultrasonic generator in a water bath at 40 ꢀC for 12 min.
Completion of the reaction was monitored by TLC, then the mixture was poured into
crushed ice in a 200 mL beaker and the pH adjusted to ꢁ2 by the addition of HCl.
Yellow solid was obtained, filtered by suction, washed by water, dried and crystallized
in chloroform and ethanol (8:2) [16].
Brown crystals, %yield: 88.76%; m.p: 117 ꢀC; FT-IR (cmꢂ1): ꢀ: 3097 (C ¼ CHstr.), 2915
(CHstr), 1655 (C ¼ Ostr, conjugated ketone), 1572 (C ¼ Caromatic Str), 1379 (C-
Naromatic str.), 740 (1,2 disubstituted (z) CH ¼ CH out of plane); 1H NMR (600 MHz
CDCl3) d: 8.79 (1H2, dd, J ¼ 4.6), 7.49 (1H3, td, J ¼ 4.7, 7.4), 8.23 (1H4, dt, J ¼ 1.0, 7.7),
7.89 (1H5, td, J ¼ 1.7, 7.6), 8.27 (2H8-9, d, J ¼ 15), 7.63 (1H11, s), 8.15 (1H13, dd, J ¼ 2.0,
5.4), 7.33–7.34 (2H14-15, m), 7.39 (1H16, dd, J ¼ 2.0, 5.8), 4.78 (2H19, d, J ¼ 5.4), 6.02
(1H20, m, J ¼ 5.4, 10.8, 16.8), 5.18 (1H21, dd, J ¼ 0.6, 17.4), 5.30 (1H210, dd, J ¼ 0.6,
10.2); 13C NMR (600 MHz, CDCl3) d: 148.7, 121.1, 136.8, 123.1, 155.0 (pyridine moiety),
189.5 (C ¼ O), 126.3, 138.4 (alkene moiety), 113.8, 132.2, 137.5, 126.6, 121.6, 122.7,
118.4, 116.0 (indole moiety), 49.2, 133.3, 110.3 (propene moiety); Anal. Calcd. for
C19H16N2O: C, 79.14; H, 5.59; N, 9.72. Found: C, 79.12; H, 5.55; N, 7.88.
3. Results and discussion
3.1. Chemistry
Alkylated heterocyclic chalcone (E)-3-(1-allyl-1H-indol-3-yl)-1-(pyridin-2-yl) prop-2-en-1-
one (AIPO) was prepared by a two-step reaction, first alkylation of indole-3-carbalde-
hyde by reaction of 3-bromoprop-1-ene and then 1-allyl-1H-indole-3-carbaldehyde
reacted with 2-acetyl pyridine to give AIPO (Scheme 1). The AIPO structure has been
1
confirmed by FT-IR, H NMR and 13C NMR spectral analysis. FT-IR spectra of AIPO gave
absorption bands at 1655 and 1562 cmꢂ1 due to the conjugated C ¼ O and conju-
gated C ¼ C groups, respectively, which represents the characteristic peaks of AIPO.

4
A. M. ASIRI ET AL.
Scheme 1. Synthesis of AIPO.
Absences of a sharp peak at 3451 cmꢂ1 indicates indole is converted to allyl-1H-indole.
1
1H NMR spectra further confirmed the chalcone structure. H NMR spectra of AIPO in
CDCl3 displayed characteristic peaks for a,b-unsaturated ketone in the form of a doub-
let at 8.27 ppm with coupling constant (J) 15 Hz, indicating the trans configuration of
the alkene moiety of a,b-unsaturated ketone. Nine aromatic protons of AIPO were a
singlet doublet and doublet of doublets from 7.63 to 8.79 ppm. 13C NMR spectral data
further confirm and identify the compound. The carbonyl carbon was at 189.5 ppm.
The aromatic signals were located at 116.0–148.7 ppm and alkylated propene gave
three signals at 49.2, 133.3 and 110.3 ppm for N–CH2–CH ¼ CH2 as shown in Figure 1.
3.2. Photophysical properties of AIPO
AIPO is soluble in many solvents. Its Uv/vis spectra were recorded in ten solvents
(polar to non-polar, protic, and aprotic), DMSO, DMF, ethanol, methanol, CHCl3, CCl4,
CH3CN, dioxane, THF, and n-hexane (Figures 2 and 3). In all the tested solvents the
absorption spectra are mainly characterized by the band at 310–500 nm (Figure 2)
which may be assigned S1 S0 (Table 1).

JOURNAL OF COORDINATION CHEMISTRY
5
Figure 1. Structure of AIPO with numbering of hydrogen atoms.
Figure 2. UV-absorbance spectra of 1 ꢄ 10ꢂ5 M of AIPO in different solvents.
The properties of solvent noticeably affected the absorption maxima of AIPO by the
bathochromic shift or red shift of 25 nm when polarity of the solvents increases from
n-hexane to DMSO, indicating the AIPO ground state has polar nature. Red shift is due
to intramolecular charge transfer from donor group (1-allyl-1H-indole) to the acceptor
group (pyridine ring) [17]. The AIPO emission spectra were intensely associated with
solvent polarity upon excitation at 350 nm. AIPO chromophore shows one broad band
(Figure 3) which indicates electronic transition S1!S0.
The fluorescence maxima shift to longer wavelength by increasing the solvent
polarity 91 nm from n-hexane (402 nm) to DMSO (493 nm), implying that solvent polar-
ity affected excited state energy of the AIPO chromophore more than the ground
ꢃ
state, owing to a p–p transition [18].
The absorbance energy (Ea) and emission energy (Ef) of AIPO in various solvents
correlate with the empirical Dimroth polarity parameters ET(30) of the different

6
A. M. ASIRI ET AL.
Figure 3. Fluorescence spectra of 1 ꢄ 10ꢂ5 M of AIPO in different solvents.
Table 1. Photophysical properties and fluorescence quantum yields of AIPO in various solvents.
ET(30)
kab
(nm)
kem
(nm)
l12
Debye
Dꢀst
Solvent
Df
ETN
Kcal mol-1
e M-1 cm-1
f
(cm-1)
Uf
DMSO
DMF
EtOH
MeOH
CHCl3
CCl4
CH3CN
Dioxane
THF
0.263
0.274
0.305
0.308
0.217
0.0245
0.274
0.148
0.209
0.0014
0.441
0.404
0.654
0.762
0.259
0.472
0.164
0.210
0.006
0.0002
45.1
43.8
51.9
55.4
39.1
32.4
45.6
36.0
35.1
31.1
398
395
400
404
397
384
383
386
384
373
493
488
508
511
484
439
468
445
438
402
13,109
12,203
11,930
12,440
11,930
13,150
13,050
12,820
13,100
12,150
0. 25
0.23
0.25
0.26
0.22
0.17
0.24
0.18
0.17
0.10
4.58
4.35
4.58
4.69
4.24
3.74
4.35
3.74
3.60
2.81
4842
4825
5315
5183
4527
3262
4742
3435
3210
1934
0.52
0.50
0.13
0.099
0.33
0.022
0.45
0.14
0.11
0.077
n-Hexane
solvents as represented in Figure 4. A single dimension correlation between solvent
polarity and energy of the absorption and emission was established by using the fol-
lowing equations [19].
ð
Þ
Þ
Ea ¼ 80:82–0:018 ꢄ ET 30
(1)
(2)
ð
Ef ¼ 79:30–0:044 ꢄ ET 30
3.3. Transition diploe moments and oscillator strength
From absorption and emission spectra of AIPO in different solvents we obtain the dif-
ferent dipole moments between singlet excited state and ground state Dl ¼ le – lg.
Photophysical parameters of AIPO in different solvents (Table 1) have been
obtained by using the Lippert–Mataga equation [20]:

JOURNAL OF COORDINATION CHEMISTRY
7
Figure 4. Plot of energy of absorption (Ea) and emission (Ef) versus ET(30) of different solvents.
2Dl2
Dꢀꢀst ¼ ꢀabsꢂꢀem
¼
Df þ Const:
(3)
hca3
By increasing solvent polarity (n-hexane to DMSO) the Stoke shift value increases,
implying that the more polar the solvent the more it stabilizes the excited state, h is
plank’s constant, the velocity of light is c, a is the radius cavity, and positioning polar-
izability of various solvents Df are represented by Equation 4 [21],
eꢂ1
g2ꢂ1
Df ¼
ꢂ
(4)
2e þ 1 2g2 þ 1
where e is the dielectric constant and g is refractive index of the solvents.
Figure 5 shows a linear relationship between the orientation polarizability and
Stokes shift, which shows dependence of Stokes shift on the polarity of
the solvent.
Lippert–Mataga was used to calculate the dipole moments (Dl) of AIPO (Equation
3) and a radius cavity of AIPO was calculated by Equation 5 [22],
ꢀ
ꢁ
1=3
3M
4Npd
a ¼
(5)
where M, N, and d are molecular mass, Avogadro number, and the density of the
fluorescent dye of 1 g/cm3, respectively. Therefore, AIPO gives dipole moment of 0.81
Debye as presented in Figure 5. The positive value specified that the excited state is
more polar than ground state.
Transition dipole moments of the fluorescent dye between excited state and
ground state in various solvents have been calculated as Equation 6. The dipole
moment values increase with increasing the solvent polarity, as shown in Table 1 [23].

8
A. M. ASIRI ET AL.
Figure 5. Plot of Df versus Stokes shift.
f
l2 ¼
(6)
ꢂ7
4:72 ꢄ 10
E
max
Emax is the maximum energy of the absorption in cmꢂ1, f is the oscillator strength,
and l is the transition dipole moment.
Equation 7 has been used to calculate the oscillator strength f of the fluorescent
dye AIPO in various solvents,
ð
f ¼ 4:32 ꢄ 10ꢂ9 eðꢀꢀÞdꢀꢀ
(7)
where ꢀꢀ and e are the wavenumber and the dielectric constant (Mꢂ1 cmꢂ1). The oscil-
lator strength value of AIPO calculated in different solvents is listed in Table 1, and
value of oscillator strength of AIPO increases by increasing solvent polarity from n-hex-
ane to DMSO.
3.4. Fluorescence quantum yields of AIPO
The standard dye quinine sulfate (ɸr ¼ 0.55 in 0.1 M H2SO4 solution) was used as refer-
ence to calculate the fluorescence quantum yields of the heterocyclic chalcone (AIPO)
in various solvents by using Equation 8 [24],
I ꢄ Ar ꢄ n2
Uf ¼ Ur
(8)
Ir ꢄ A ꢄ n2r
where ɸr is the fluorescence quantum yield of quinine sulfate and ɸf is the AIPO calcu-
lated fluorescence quantum yield in the solvents. The absorbance of AIPO and refer-
ence dye at the excited wavelength are represented by A, I is the integrated emission
intensity and n is the solvent refractive index (Table 1). The calculated value of AIPO

JOURNAL OF COORDINATION CHEMISTRY
9
Figure 6. Plot of ɸf versus ET(30) of AIPO in different solvents.
fluorescence quantum yield demonstrates that the value depends on solvent polarities
since the emission spectra were affected by solvent polarity. Figure 6 demonstrates
the correlation between AIPO fluorescence quantum yield in various solvents and
ET(30) of those solvents [25]; the solvent polarity parameter represented by ET(30) was
identified by Reichardt. The data show increasing solvent polarity increased in ɸf due
to negative solvatokinetic effects. Another factor affecting the ɸf values is hydrogen
bonding between the allyl of AIPO and the solvent. This hydrogen bond is responsible
for the decrease of ɸf in polar protic solvents like MeOH due to radiation-less proc-
esses. This positive solvatokinetic effect from the DMSO to polar protic solvents is
from strong ICT interaction between solvent and excited chromophore [26].
3.5. Selective fluorescent chemosensor of AIPO for metal ion
The AIPO chromophore has been applied as a fluorescent chemosensor for metal ion
recognition. AIPO was examined for its selectivity toward ten metal ions by emission
spectra. Stock solution (1 ꢄ 10ꢂ5 M) in DMF/water (9:1) (v/v) of Co2þ, Hg2þ, Fe3þ, Cu2þ
,
Al3þ, Cd2þ, Sr2þ, Ni2þ, Cr3þ and Mn2þ were prepared. When excited at 385 nm, the
emission spectra of AIPO exhibited exceptionally strong peak and the intensity was
maintained even after addition of nine equiv. of Co2þ, Hg2þ, Cu2þ, Al3þ, Cd2þ, Sr2þ
,
Ni2þ, Cr3þ and Mn2þ (Figure 7). However, the fluorescence intensity of AIPO decreased
eight-fold after addition of the same amount of Fe3þ, an observed quenching in emis-
sion intensity up to 80% with clear color change from yellow to dark brown (Figure 8).
AIPO (ɸf) in absence and presence of Fe3þ is 0.52 and 0.099, respectively; the yield is
low after adding Fe3þ. This quenching in fluorescent intensity of AIPO following the
addition of Fe3þ is due to complex formation between AIPO and Fe3þ. AIPO photo-
physical properties show sensitivity toward interacting molecule due to AIPO having

10
A. M. ASIRI ET AL.
Figure 7. Emission spectra of 1 ꢄ 10ꢂ5 M of AIPO upon addition of 5.0ꢄ 10ꢂ5 M of Co2þ, Hg2þ
,
Fe3þ, Cu2þ, Al3þ, Cd2þ, Sr2þ, Ni2þ, Cr3þ, and Mn2þ
.
Figure 8. Emission of AIPO in the presence of different metal ion.
charge transfer (CT) structure. The nitrogen of the pyridine and the oxygen of carbonyl
are the binding site between AIPO and Fe3þ which induce the ICT from the electron
donor’s nitrile and oxygen to Fe3þ. This complexation led to the ligand to metal
charge transfer (LMCT), accountable for the quenching in fluorescence intensity
(Scheme 2). Failure of the other nine metal ions to achieve this complexation with the
chalcone can be explained by unsuitable ion radius [27].

JOURNAL OF COORDINATION CHEMISTRY
11
Scheme 2. Fluorescent chemosensor behavior of AIPO with Fe3þ
.
Figure 9. Competitive experiments in the AIPO þ Fe3þ system with interfering metal ion.
Testing the selectivity of an indicator is important for the practical applicability of
sensors and can be examined by the anti-Jamming ability. The possibility of interfer-
ence of Co2þ, Hg2þ, Cu2þ, Al3þ, Cd2þ, Sr2þ, Ni2þ, Cr3þ and Mn2þ ions with Fe3þ ion
was tested through a competition experiment. Figure 9 shows the result when exam-
ining the intensity change of AIPO (1 ꢄ 10ꢂ5 M) at 385 nm when adding 5 ꢄ 10ꢂ5 M of
Fe3þ and (1 ꢄ 10ꢂ5 M) of the other ions. The results imply that AIPO has highly sens-
ing selectivity toward Fe3þ because the competitive metal ion present did not inter-
fere with sensing the Fe3þ. Thus, the heterocyclic chalcone (AIPO) is a fluorescent
chemosensor for Fe3þ with high selectivity.

12
A. M. ASIRI ET AL.
Figure 10. Emission spectra of AIPO (1ꢄ 10ꢂ5 M) exposed to various concentrations of Fe3þ in
DMF:water (9:1) v/v.
Figure 11. Fluorescence titration curve of AIPO (1ꢄ 10ꢂ5 M) with Fe3þ in aqueous solution.
By gradual titration, the fluorescence quenching behavior of AIPO with Fe3þ shows
decreased intensity by increasing Fe3þ concentration, and when Fe3þ is about
9 ꢄ 10ꢂ5 M the fluorescence intensity of AIPO reached the minimum values, owing to
AIPO complexation with Fe3þ (Figure 10). Figure 11 illustrates that by decreasing the
Fe3þ concentration to 4 to 9 ꢄ 10ꢂ5 M the emission intensity of the probe AIPO was

JOURNAL OF COORDINATION CHEMISTRY
13
Figure 12. Benesi–Hildebrand plot for AIPO (1 ꢄ 10ꢂ5 M) at various concentrations of Fe3þ
.
quickly reduced; the good linear response (R ¼ 0.99) allows quantitative evaluation of
coordination between the probe and Fe3þ
.
Equation 9 represents the Benesi–Hildebrand plot of the binding stoichiometry and
the association constant K between the chemosensor and Fe3þ [28].
1
1
1
1
¼
þ
ꢄ
(9)
Fo ꢂ F0 Fo ꢂ F0 Kapp
3þ
o
0
½
ꢅ
ðF ꢂ F Þ Fe
Kapp is the association constant, F and Fo are the intensity of fluorescence for AIPO in
the absence and presence of Fe3þ, respectively. When plotting 1/(Fo - F) vs. 1/Fe3þ the
linearity proved that the complexation between metal–ligand is 1:1 stoichiometric
ratio as given in Figure 12. The 1:1 binding stoichiometric of metal–ligand was
strongly supported by linear fitting of the experiment plot with a coefficient over 0.98;
the fitted curve is almost superimposed over the experimental plot. The estimated
value of the binding constant between metal–ligand is 8.92 ꢄ 10ꢂ6 Mꢂ1
.
The Stern–Volmer relationship was used to help understand the fluorescence
quenching behavior of AIPO sensor with Fe3þ; the Stern–Volmer analysis of intensity
quenching of the sensor is given by Equation 10 [29],
Io
I
3þ
½
ꢅ
¼ 1 þ Ksv Fe
(10)
where Io is the intensity of the fluorescence from AIPO without the presence of Fe3þ
ion and I is the intensity of the fluorescence from AIPO when Fe3þ is present, and Ksv
is Stern–Volmer constant. Figure 13 shows the Stern–Volmer plot with Ksv of
5.13 ꢄ 10ꢂ4 Mꢂ1, R2 equal 0.98. The Stern–Volmer constant value indicates that the
interaction between AIPO and Fe3þ is efficient.

14
A. M. ASIRI ET AL.
Figure 13. Stern–Volmer plot for AIPO (1 ꢄ 10ꢂ5 M) at different concentrations of Fe3þ
.
Figure 14. Job’s plot of fluorescence intensity versus mole fraction of Fe3þ
.
The stoichiometry of AIPO and Fe3þ was estimated by Job’s plot [30]. The Job’s
plot was drawn by plotting mole fraction of Fe3þ versus fluorescence intensity as pre-
sented in Figure 14. When the mole fraction of Fe3þ was 0.5 the plot began to
decrease, indicating that complex stoichiometry of Fe3þ and AIPO is 1:1. This behavior
was further supported by density functional theory (DFT) calculations using 09 pro-
gram on AIPO and the iron 1:1 complex; their corresponding energies are

JOURNAL OF COORDINATION CHEMISTRY
15
Figure 15. Conformation of AIPO (left) and AIPOꢆFe3þ (right) optimized by density functional the-
ory calculations.
ꢂ917.9156893 AU and ꢂ1041.2032538 AU, respectively, as shown in Figure 15, sug-
gesting that the 1:1 stoichiometry is stable.
4. Conclusion
The reaction of 1-allyl-1H-indole-3-carbaldehyde with 2-acetyl pyridine produced the
heterocyclic chalcone (E)-3-(1-allyl-1H-indol-3-yl)-1-(pyridin-2-yl) prop-2-en-1-one
(AIPO). The oscillator strength, diploe moment, Stokes shift, and fluorescence quantum
yields have been calculated in ten solvents on the basis of the polarity of the solvent
by the absorption and emission, giving information about the interaction between the
chromophore with different polarity of solvents. The heterocyclic chalcone is a turn-off
fluorescent chemosensor for Fe3þ, with much higher selectivity over competitive metal
ions. The Benesi–Hildebrand, Stern–Volmer and Job’s plots have been used to explain
the complexation between AIPO and Fe3þ in 1:1 ratio. Binding stoichiometric ratio
was further conformed by the density functional theory (DFT) calculations.
Disclosure statement
No potential conflict of interest was reported by the authors.
Funding
This project was funded by the Deanship of Scientific Research (DSR) at King Abdulaziz
University, Jeddah, Saudi Arabia under grant no. KEP-PhD-19-130-41. The authors, therefore,
acknowledge with thanks DSR for technical and financial support.
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