Propargyl-functionalized single arm allied Anthracene based Schiff bases: Crystal structure, solvatochromism and selective recognition of Fe3+ ion

Article abstract of DOI:10.1016/j.molstruc.2020.129618

An organic moiety exhibit specific behavior in electronic absorption spectra while accommodating various solvents or metal ions and that specificity depends on particular guest solvent molecules or metal ions. Herein, we report about the photophysical pro

Full text of DOI:10.1016/j.molstruc.2020.129618

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Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstr
Propargyl-functionalized single arm allied Anthracene based Schiff
bases: Crystal structure, solvatochromism and selective recognition of
Fe3+ ion
Gurjaspreet Singha,∗, Pawana, Akshpreet Singhb, Shilpya, Dikshaa, Sumana,
Geetika Sharmab, Subash Chandra Sahooa, Amarjit Kaura,∗
a Department of Chemistry, Panjab University, Chandigarh 160014, India
b Department of Chemistry, GGDSD College, Chandigarh, 160032, India
a r t i c l e i n f o
a b s t r a c t
Article history:
An organic moiety exhibit specific behavior in electronic absorption spectra while accommodating vari-
ous solvents or metal ions and that specificity depends on particular guest solvent molecules or metal
ions. Herein, we report about the photophysical properties of newly synthesized Propargyl-functionalized
anthracene based Schiff bases (PASB). The formulated compounds have been thoroughly characterized by
elemental analysis, FT-IR, NMR (1H and 13 C) and UV-Vis spectroscopy, emphasized with Kamlet-Taft ap-
proach to explain the solvatochromic behavior of the synthesized compounds. PASBs were screened for
the sensorial aptitude towards various metal ions which were found to act as an excellent UV-Vis probe
for selective detection of Fe3+ ion with LOD 1.60 × 10−8M. Additionally, coordination behavior of ligand
with Fe3+ ion was examined through the change in magnetic property of Fe3+ ions by VSM (vibrating
sample magnetism) study. Moreover, complete structure elucidation of compound 4b was achieved via
X-ray crystallography. The binding sites of sensor conjugate 4b-Fe3+ are well supported by FT-IR spec-
troscopy and computational analysis following DFT approach. The reported contribution focused on the
factors determining the ability of propargyl appended Schiff base to present structure, mental picture of
solvatochromism and metal ion sensitivity.
Received 15 August 2020
Revised 5 November 2020
Accepted 6 November 2020
Available online xxx
Keywords:
Propargyl
Schiff base
Anthracene
Solvatochromism
Chemosensor
© 2020 Elsevier B.V. All rights reserved.
1. Introduction
solvatochromism is a type of fruitful interaction and has wide ac-
ceptance in theoretical and analytical chemistry.
The interactive nature of solute molecules with its surround-
ing solvent counterparts lies at the root of problems associated
with our fundamental understanding of reactivity in solution or in
physico-chemical nature of receptor binding sites. Hence the se-
lectivity of solvents towards the receptor molecules is highly in-
fluenced by the solvatochromic nature of receptor. Moreover, the
effect of solvent on the electronic absorption spectra of solute
observed with change in the environment of different solvents
has been a subject of interesting investigations.1 For any com-
pound, a solvent affects both ground and excited states differently.
A change in solvent is always accompanied by a change in polar-
ity and dielectric constant of the surrounding medium. The knowl-
edge of polarity of ground and electronically excited states pro-
vides an insight of photophysical properties of solute. Therefore,
Among various metal ions, iron (III) ion not only is an essen-
tial metal ion in living beings but also plays a key role in various
biological processes like oxygen transport, DNA and RNA synthe-
sis, cellular metabolism, electron transport, enzyme catalyses and
so on.2 In any organism both the deficiency and overload of iron
(III) ions can lead to serious disorders like anemia, parkinsons and
alzheimers, liver damage and hemochromatosis.3 Therefore selec-
tive and sensitive detection of iron (III) in biological and environ-
mental system is very essential.
Propargyl group is a very interesting functionality due to its
versatile reactivity which is utilized in many organic reactions
for various transformations and functionalizations by researchers
for decades.4 Propargyl reagents undergo “click reactions” i.e. 1,3-
dipolar cycloaddition reactions designed by Huisgen.5 One of the
very effective transformations to introduce propargyl functional
group is the well known Williamson reaction. Propargyl func-
tionalized compounds are of particular interest in the design of
pharmaceutically active drugs like rasagiline (N-propargyl-1-(R)-
∗
Corresponding authors: Gurjaspreet Singh, Professor, Amarjit Kaur, Assistant
Professor, Department of Chemistry, Panjab University, Chandigarh
E-mail addresses: gjpsingh@pu.ac.in (G. Singh), amarjitk@pu.ac.in (A. Kaur).
https://doi.org/10.1016/j.molstruc.2020.129618
0022-2860/© 2020 Elsevier B.V. All rights reserved.
Please cite this article as: G. Singh, Pawan, A. Singh et al., Propargyl-functionalized single arm allied Anthracene based Schiff bases:
crystal structure, solvatochromism and selective recognition of Fe3+ ion, Journal of Molecular Structure, https://doi.org/10.1016/j.molstruc.
2020.129618

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Scheme 1. Synthetic route for propargyl-functionalized anthracene based schiff bases (PASB)
cm−1 corresponding to C C and C C-H bonds. The C=N stretch-
aminoindan), selegiline (L-deprenyl), ladostigil (TV-3,326), agro-
chemicals like clodinafop-propargyl and in the field of analytical
and synthetic chemistry.6-11 Moreover, Schiff bases are enjoying a
special status in the field of chemistry which can be easily synthe-
sized through a simple synthetic procedure and have good solubil-
ity in common solvents.12–13
≡
≡
ing vibration of Schiff bases appears as a strong peak in the region
of 1635-1605 cm−1 15
.
The absorption band for the aromatic C=C
vibrations and C-H stretching appears in the region of 1500-1600
cm−1 and 3076-3060 cm−1 respectively. For aryl alkyl ether asym-
metric stretching band appears at 1271-1265 cm−1 and symmetric
stretching band appears at 1028-1100 cm−1. Alkyl C-H stretching
gives a medium absorption band around 2850 cm−1 and aromatic
C-H stretching gives absorption band in the region of 3076-3060
cm-1. The 1H NMR spectra of the synthesized compounds have
Usually solvatochromism have been studied for schiff bases
having tautomerism but in present study positive solvatochromism
is observed for synthesized compounds having no tautomerism
which may be due to the solute-solvent interactions by the for-
mation of hydrogen bonds between hydrogen-bond acceptor (HBA)
solvent and propargyl moiety. Till date only a few examples of
anthracene based Schiff bases are described.14 Nevertheless, to
the best of our knowledge no report is available on the study of
propargyl allied anthracene based schiff bases.
≡
demonstrated the symbolic peaks of allylic (-C C-H) protons in the
range of 2.55-2.50 ppm. Signals in the region of 7.60-6.40 ppm are
assigned for aryl ring protons. Peaks in the region of 4.40-4.80 ppm
are observed for alkyl protons (-O-CH2-). In all the compounds –
HC=N- protons are observed in the region of 8.40-8.80 ppm. In
13
Keeping the above perspectives in mind, herein we report the
synthesis, spectral characterization, crystal structure of propargyl
appended anthracene based Schiff bases (PASB) and focuses to elu-
cidates the solvatochromism and selectively detection of Fe3+ ions.
Additionally, metal ion and ligand coordination is also confirmed
by VSM (vibrating sample magnetism) studies followed by theo-
retical studies using DFT/B3LYP/6-31G level. The motive behind the
present work is to have a better visualization towards qualitative
solvents effect and selective metal ion sensitivity.
≡
C NMR spectra, peak corresponding to -C C- group in the region
of 75.06 to 85.07 ppm and for -C=N- group in the region of 160.02
to 162.07 ppm validate the product formation. Further, the most
upfield peak was observed for -OCH2 group in the region of 56.10
to 56.20 ppm. Moreover, the aromatic carbon peaks were seen in
the region of 156.29 to 115.55.
2.2. Crystal structure
The ORTEP diagram and atom numbering scheme for empirical
formula C24H17 NO (4b) are given in (Fig. 1). Crystallographic data
and structure determination parameters are summarized in (Table
S2). X-ray studies disclose that the compound 4b has comparable
structure, as is evident from the bond angles and bond parame-
ters. Compound 4b crystallized in triclinic crystal system contain-
ing two molecules per unit cell with P-1 space group. Antheracene
and Schiff base substituted aromatic ring are in parallel plane at
118.58(15)˚. The propargyl group attached to aromatic ring assume
to be lie above the plane at an angle of 124.89(15)˚ with an ex-
tended conformation which can be seen from the C19-O1-C22 tor-
sion angles of 118.25(13). The propargyl group in crystal structure
shows linear conformation which can be seen from the C22-C23-
C24 at an angle of 178.5(2)˚. The C1- N1 bond length is found to be
2. Results and Discussion
2.1. Synthesis and characterization
A two step synthetic route, shown in scheme 1, was employed
to synthesize PASBs (4a-4g). Firstly, 9-anthracenecarboxaldehyde
(1) was made to undergo condensation reaction with aromatic
amines (2a-2g) using ethanol as solvent to yield anthracene based
Schiff bases (3a-3g). The obtained Schiff bases were then stirred
in room temperature with propargyl bromide in DMF using potas-
sium carbonate as base for 16 hours to give desired PSABs (4a-4g).
The structure and purity of all the compounds were inferred
from their infrared and NMR (1H and 13C) spectra. The FT-IR spec-
tra of the newly synthesized compounds were recorded as neat
spectra in the range of 4000-400 cm−1. The absorption frequencies
recorded were found to be in close agreement with the structure
of the synthesized compounds. The typical bands observed for the
terminal alkynes appeared near 2123-2110 cm−1 and 3297-3180
˚
1.256(2) A which is closer to theoretical (C=N) double bond length
˚
(1.25A) than to C-N single bond length (1.47A˚).
In molecular lattice arrangement of compound 4b along a-axis,
molecules are packed via strong intermolecular hydrogen-bonding
˚
interactions (N•••H-C) 2.564(2) A and (O•••H-C) 2.630(2) involv-
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Moreover, high molar absorptivity of ortho-isomer may be the con-
sequence of high electron density. Slight increase in absorption of
compound 4c bearing methyl at meta-position is may be due to
electron releasing effect of methyl group while decrease in the ab-
sorption of compound 4d is may be due to Cl-atom with a slight
red shift. It is noteworthy to mention that substituent at o- and p-
position having oxygen atom shows more absorption compared to
substituent having sulfur atom which may be attributed to energy
difference in HOMO of sulfur and LUMO of conjugated backbone.
It is wide accepted that solvent can influence UV-Vis absorp-
tion spectra and can alter the shape, intensity and position of the
absorption band. Later the term “solvatochromism” introduced by
Hantzsch described this phenomenon, the effect of solvents polar-
ity and their interactions with the solute molecules.16 The study
of solvatochromism is important as it simply paves a way to study
the effect of environment on solute molecules under examination.
It has been known that solvent effect mostly depends on the vari-
ation of dipolar characteristics of any solute when promoted to ex-
cited state and the resulting interactions of the solvent depends
upon the change in the dipolar characteristics between the ground
and excited state of the solute molecules.17 The changes observed
with varying polarity of the solvents depends on the overall sol-
vation capability of the solvents which is the summative effect of
all solvent-solute interactions viz. relative polarities of ground and
excited states, H-bonding interactions and π-conjugated backbone
excluding some chemical reactions like oxidation, reduction, proto-
nation, complexation etc. leading to chemical changes in the solute
molecules.18 It is recognized that the propargyl moiety of all tem-
plates form an intermolecular hydrogen bond solvated molecular
complex with HBA (hydrogen bond acceptor) solvent (Scheme 2).
Herein we have attempted a comprehensive study for pos-
Fig. 1. ORTEP diagram of 4b with thermal ellipsoids at 40% probability, se-
˚
lected bond length (A) and angle (˚) O1-C19 = 1.3736(18), O1-C22 = 1.4218(19),
N1-C16
=
1.420(2), N1-C1
=
1.256(2), C19-C20 =1.377(2),C22-C23
=
1.465(3),
C23-C24
=
1.167(2), C19-O1-C22
=
118.25(13), C1-N1-C16 =118.58(15), O1-C19-
C20 =124.89(15), O1-C19-C18
C22 = 178.5(2).
=
115.46(14),O1-C22-C23
=
113.14(14), C24-C23-
Table 1
UV spectral data ῡmax (103 cm−1) of compounds (4a-4g).
Analyte ῡmax(103 cm−1
)
Solvent
4a
4b
4c
4d
4e
4f
4g
CH3CN
CHCl3
DMF
25. 44
25.00
25.12
25.64
25.38
24.93
25.25
25.38
25.18
25.38
25.00
25.44
25.51
24.69
25.00
25.51
25.38
25.00
25.12
25.90
25.44
25.00
25.18
25.31
25.25
25.00
25.00
25.83
25.44
25.00
25.00
25.18
25.25
25.00
25.06
25.38
25.06
25.00
25.25
25.31
25.83
25.00
25.25
26.10
26.04
25.00
25.00
25.06
25.18
25.18
27.79
25.25
25.06
27.17
27.47
27.77
sible interactions of PASB with
a series of polar aprotic sol-
MeOH
EtOH
vents viz. acetonitrile, chloroform, dimethyl sulfoxide, dimethyl for-
mamide, tetrahydrofuran, acetone and polar protic solvents viz.
methanol and ethanol by probing the corresponding absorption
spectra (Fig. 4). Concentration-dependent absorption behavior was
also investigated to avoid any interference with the results of sol-
vatochromism and its interperation and evidently did not show any
shift in UV-Vis absorption maxima value. Thus ensures that com-
pounds do not self- aggregate and can be investigated for their sol-
vatochromic properties.19
DMSO
THF
Acetone
ing imine group (N1) of one molecule and H24 of adjacent propar-
gyl moiety and furthermore, O1 of one propargyl moiety linked
with H18 of aromatic ring of adjacent molecule and overall pack-
ing looks like a staircase pattern (Fig. 2a). The pair of molecules
in unit cell are joined by non-classical interactions of (C-H•••O)
type between H18 of one propargyl moiety and O1 of another
A large number of non-equivalent scales have been developed
for achieving a correlation of experimental data with the empiri-
cal solvent parameters. Amongst them the multi parameters based
linear regression approach successfully enunciated by Kamlet and
Taft has been widely used for the quantitative treatment and esti-
mation of various solute-solvent interactions.20
˚
moiety with bond length of 3.168 A. The propargyl fragment pre-
vents close contact between molecules leading to an open struc-
ture packing, which shows that there are no intramolecular inter-
actions. Moreover, the propargyl fragment is clearly twisted out of
the plane with respect to the same fragment of the other molecule
in the unit cell (centrosymmetric dimer) due to head to head steric
hindrance represented in Fig. 2b and parallel anthracene units are
showing formation of layers with weak π- π interactions (Fig. 2c).
In the present work, we have explored the empirical solva-
tochromic scale developed by Kamlet and Taft.21
v¯max = v¯max,0 + sπ∗ + aα + bβ
Where ῡmax corresponds to UV-Vis absorption maxima wavelength
of a solute in a particular solvent, ῡmax,0, is the value of this prop-
erty for the same solute in a hypothetical condition for which
π∗ = α = β = 0. The parameter π∗, α and β reflects, respectively,
the general solvent dipolarity/Polarizability, specific H-bond do-
nating ability (HBD/acidity) and specific H-bond accepting ability
(HBA/basicity) which are collected in Table 2. Where regression co-
efficients s, a and b are the respective susceptibility constants. The
inclusive study of these analytical parameters is important as their
sign and magnitude are the measure of type of solvatochromism
exhibited by solute molecules.
2.3. UV-Vis spectral study
Preliminary investigations of the synthesized compounds un-
der UV-Visible spectroscopy were recorded in the range of 200-
600 nm in acetonitrile with 10 μM concentration of the com-
pounds (Fig. 3). The spectral data of the synthesized PASB are sum-
marized in Table 1. The absorbance in the range of 380-430 can
be attributed to π-π∗ transitions. Analysis of absorption spectra
shows appreciable difference with change in substitution position
and presence of different heteroatoms. While comparing spectral
shifts from ortho to para-isomer, a red shift was observed which
may be due to the planarity differences as Ortho-isomer (4a) be-
ing less planar then para-isomer (4b) in structural configuration.
The UV-Vis absorption spectra of all the compounds for both
polar protic and polar aprotic solvents has been depicted in Fig. 4,
while the absorption data of all the compounds in various solvents
is represented in (Table 1) and λmax and Ɛmax values of all the
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Fig. 2. (a) Molecular lattice arrangement for compound 4b along a-axis, (b) Pair of molecules joined by non-classical interactions of benzene ring H18 of one propargyl
moiety and O1 of another moiety, (c) Shows weak π interactions between two molecules along b- axis
Scheme 2. Possible Specific Interactions for PASB (4a-4g), Where α, β, and π∗
Stand for Acidity, Basicity and Polarity/Polarizability Parameters in the Kamlet−Taft
Solvent Scale.
esteingly, for compound 4g sign of ‘a’ comes out to be –ve, indicat-
Fig. 3. Absorption spectra of propargyl-functionalized Anthracene Schiff base
ing the bathochromic shift with increase in solvent hydrogen-bond
(PASB) (10μM) in CH3CN.
donor acidity and for compound 4a-4f ‘a’ comes out to be +ve
indicating hypsochromic shift with increasing solvent hydrogen-
bond donor acidity. Also, for compound 4g coefficient ‘b’ has +ve
compounds are summarized in Table S1. The data in Table 3 shows
the regression parameters of compounds 4a-4g as a function of
Kamlet–Taft solvent scale. The data depicts the –ve sign of s-value
for compounds 4a-4g except 4d and 4f, suggesting they have more
polar excited state as compared to ground state and hence these
compounds possess a positive solvatochromism. Moreover, further
information can be deduced from coefficients ‘a’ and ‘b’ values
which index HBD and HBA values of a solvent respectively. Inter-
value which can be attributed to its capability of forming strong H-
bonding with HBA solvent because of acidic hydrogen of propagyl
at p-position of aryl ring. Overall we can say that the percentage
contribution of the three solvatochromic parameters for the inves-
tigated PASB show that the solvent effect on UV-vis spectra is very
complex and depends on nature of substituents, position of substi-
tution and its electronic behaviour and hence a simple generaliza-
tion cannot be made (Table 4). Fig. 5 showing parcentage contribu-
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Fig. 4. Absorption spectra of PASB (4a-4g) in solvents of different polarity.
tion chart. Altogether we can say that exploring solvatochromism
helps to get insight of physical and chemical properties of solute
molecules.
acetonitrile (Fig. 6). Significant changes were observed in the ab-
sorption spectra only upon the addition of Fe3+ ions. The solution
of analyte 4b exhibited one absorption band at 393 nm. Upon ad-
dition of Fe3+ ions two strong bands were appeared at 355 nm and
480 nm. Also, color change was observed from pale yellow to or-
ange red which is evident from the fact that new peak at 480 nm
lies in visible range.
To achieve the best sensing performance, ionochromic stud-
ies were performed at room temperature and sufficient time was
provided for the solutions to become consistent before recording
the spectra. The sensing ability of compounds were investigated
with the help of UV-visible experiments in the presence of various
To gain an insight into the sensing properties, a titration exper-
iment was carried out with consequent addition of Fe3+ ions (0-1.6
equiv.) to 10μM solution of analyte 4b in acetonitrile. As depicted
metal cations Na+, K+, Rb+, Ca2++, Mg2+, Ni2+, Fe3+, Co2+, Zn2+
,
Ba2+, Hg2+, Ce2+, La2+ and NH4 and as their chlorides in 10μM
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Table 2
Kamlet-Taft parameters and physical properties of the em-
ployed solvents.
Solvent
parameters
Solvent
Ɛ
n
A
β
π∗
μaD
CH3CN
CHCl3
DMF
35.94
4.81
1.34
1.44
1.43
1.33
1.36
1.47
1.40
1.35
0.19
0.20
0.00
0.98
0.86
0.00
0.00
0.08
0.40
0.10
0.69
0.66
0.75
0.76
0.55
0.48
0.75
0.58
0.88
0.60
0.54
1.00
0.58
0.71
3.53
1.15
3.24
2.87
1.66
4.06
1.75
2.69
36.7
MeOH
EtOH
32.7
24.5
DMSO
THF
46.45
7.58
Acetone
20.56
Ɛ= relative permittivity; n= refractive index; α= solvents HBD
acidity; β= solvents HBA basicity; π∗= solvents polarizabil-
ity/dipolarity, μaD = dipole moment
Fig. 7. Change in UV–vis absorption spectrum of 4b upon addition of increasing
amounts of Fe+3 (0.2,0.4,0.6,0.8,1.0,1.2,1.4,1.6 equiv.,respectively) in CH3CN solution.
Insets: the LOD plots for Fe3+ ions
in Fig. 7 gradual increase in the absorption bands were observed
centered at 355nm and 480 nm with increase in the concentration
of Fe3+ ions. However, no prominent changes were observed in
the absorbance for metal ion concentration greater than 1.6 equiv.
Furthermore, Job’s continuous variation method was employed to
evaluate the stoichiometry of the complex. The stoichiometry of
the complex formed between 4b and Fe3+ would be 1:1 as evi-
dent from the maxima at 0.5 (see Fig. 8a). To further explore the
Fig. 5. Percentage Contribution chart of the Solvatochromic Effects for (4a-4g)
Table 3
ῡ0, a, s and b (in cm−1) and correlation coefficient (R2) obtained from the Kamlet-Taft fitting of
the absorption data.
Analyte
ῡ0 (103cm−1
)
s (103cm−1
)
a (103cm−1
)
b (103cm−1
)
R2
F-statics
a
b
c
d
e
f
25.38
25.77
25.09
24.81
25.34
24.66
27.94
-0.396
-0.602
-0.050
0.280
0.286
0.397
0.586
0.762
0.012
1.208
-3.435
0.147
-0.449
0.120
-0.040
0.962
-0.015
3.952
0.479
0.694
0.688
0.856
0.130
0.766
0.780
1.22
3.03
4.24
7.95
0.20
6.33
4.96
-0.334
0.524
g
-2.498
Fig. 6. (a) absorption spectra of 4b (10μM) in the presence of various metal ions (10μM) in acetonitrile; (b) Bar diagram showing the change in the absorbance of the
analyte 4b at λmax = 393 nm.
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Table 4
% Contribution of solvatochromic parameter for analyte 4a-4g.
Ɛmax × 104 L mol−1
% Contribution of solvatochromic parameter
Compound
ꢀmax (nm)
cm−1
P
P
P
π
α
β
A
B
C
D
E
400
394
400
400
400
400
397
4.75
3.0
47.76
41.57
06.61
25.87
25.53
29.99
25.27
34.49
27.41
77.51
70.42
9.35
17.73
31.00
15.87
03.69
73.54
08.58
39.97
2.86
0.99
7.48
4.85
2.04
F
69.14
34.74
G
Fig. 8. (a) Job’s plot for the complexation of 4b with Fe3+ showing 1:1 stoichiometry in CH3CN at nm; (b) B-H plot for 1:1 complexation.
icant value of Ka was obtained 2.6 × 108
M
−1, indicating strong
concentration limit that the compound 4b can detect, the limit of
binding ability of probe with Fe3+ ion.
detection (LOD) was calculated from UV-Vis titration of 4b with
Fe3+ ions. By employing the equation LOD = 3σs−1, where σ is
standard deviation of the response and s is slope of the curve ob-
tained and the value comes out to be 1.60 × 10−8M (see inset
Fig. 7). The curve exhibited good linear relationship between the
absorbance or 4b and Fe3+ ion concentration with correlation co-
efficient of R2 = 0.975.
2.4. Vibrating sample magnetism (VSM)
An attempt was made to further explore the binding mecha-
nism, by applying Benesi-Hildebrand (B-H) method, to calculate as-
sociation constant (Ka) and stoichiometry of complexation assum-
ing a 1: n complex formation.22 The equilibrium is given by the
Equation 1.
Additionally, we have also investigated the magnetic behavior
(M) of bare ligand 4b, Fe3+ ion and complex [4b+Fe3+] as a func-
tion of applied field H (hysteresis loop) at room temperature in
order to analyze the possible contribution on magnetic properties
of the complex. The values of coercivity (Hc) and remanent mag-
netization (Mr) for ligand 4b was found to be in the range of 150
Oe and 5 × 10−4 emu/g respectively. Further very high values of
saturation magnetization (Ms) were obtained. The results obtained
indicated that bare ligand 4b shows clear signature of ferromag-
netic nature at room temperature (Fig. 9a). The slope of M-H curve
is a measure of magnetic permeability (μ), where μ = M/H, and
has a finite value for free space, μ0 = 4π × 10−7 or 1.257 × 10−6
Hm−1. In case of Fe3+ ion magnetization moment increases lin-
early with applied field and it is found that slope >1 which is
a indicative feature of paramagnetism but in the case of complex
[4b + Fe3+] value of slope is <1 indicating its diamagnetic behav-
ior (see Fig. 9b). Hence, change in magnetic behavior provides a
clear evidence that ligand 4b is responsible for pairing electrons in
Fe3+ ion and coordinate with metal ion to form complex.
ꢀ
ꢁ
4b + n Fe3+
ꢀ
4b • Fe3+
(1)
n
ꢀ
ꢁ
4b Fe3+
Ka
n
n
4b Fe3+
[
][
]
The estimation of UV-Vis titration plot is made in a way to have
linearity of regression in the plot of [A-A0]−1 vs [Fe3+]n, where [A-
A0] is the change in the intensity of absorbance of compound in
the presence of metal ions recorded at particular wavelength. The
n
graph of [A-A0]−1 vs [Fe3+
]
comes out to be linear with a high
regression coefficient (R = 0.997), which illustrates that the stoi-
chiometry of the complexation of 4b with Fe3+ ion is 1:1. The asso-
ciation constant (Ka), a measure of strength of interaction between
the probe and Fe3+, was determined from the B-H plot. A signif-
7

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G. Singh, Pawan, A. Singh et al.
Journal of Molecular Structure xxx (xxxx) xxx
Fig. 9. (a) Magnetization curves recorded at room temperature: (a) the magnetization curve of 4b (b) Fe3+ and [4b + Fe3+] complex.
Fig. 10. Comparison of IR spectra of 4b (black spectra) and 4b with 1 equiv. Fe3+ ions (red spectra).
2.5. Mode of binding
The mode of binding was confirmed through FT-IR spec-
troscopy. The IR spectra of 4b clearly reveals that the peak at
1028 cm−1, the characteristic frequency for the C-O-C group, shifts
to 1008 cm−1 on coordination to the Fe3+ ions, in presence of
1.0 equiv. of metal ions. Moreover, downward shift of the band
from 3187 cm-1to 2980 cm-1 implies the participation of propar-
gyl group in the bonding. Bands at 1617 cm-1 corresponding to
Schiff base (-C=N-) remains intact after complexation, suggesting
no contribution to bonding (Fig. 10).and possible mode of interac-
tion shown in Fig. 11.
2.6. Theoretical study and computational details
Quantum chemical calculations at DFT level were studied for
ligand 4b and its complex with Fe3+ ion. Energy optimization for
Fig. 11. plausible mode of intraction
8

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G. Singh, Pawan, A. Singh et al.
Journal of Molecular Structure xxx (xxxx) xxx
putational study following DFT approach pointed out that propar-
gyl group of PASB is the main binding site Schiff base (-C=N-)
made no contribution towards bonding with metal ion. In brief,
the present contribution opens a promising approach to reliably
explain the surrounding environment effect on electronic absorp-
tion spectra of solute and design propargyl based sensor for Fe3+
ion. This will probably be useful under more practical condition
and provides a wider scope in the future.
Declaration of Competing Interest
There are no conflict to declare.
CRediT authorship contribution statement
Gurjaspreet Singh: Supervision, Conceptualization. Pawan: In-
vestigation, Writing - original draft. Akshpreet Singh: Method-
ology.
Shilpy: Writing
-
review
&
editing.
Diksha: Re-
sources.
Suman: Data curation. Geetika Sharma: Validation.
Subash Chandra Sahoo: Investigation. Amarjit Kaur: Supervision,
Project administration.
Fig. 12. Energy diagram of HOMO and LUMO orbitals of 4b and 4b +Fe3+ complex
Acknowledgement
calculated on the DFT level using a B3LYP/6-31G∗ basis set.
The authors are thankful to CSIR (01(2950)/18/EMR-11) and
DST-PURSE for providing financial support and University Grant
Commission (UGC), New Delhi, for JRF.
4b+Fe3+ complex was done with B3LYP in combination with 6-
31+G(d,p) basis set for H,C,N and O atoms and LANL2DZ basis set
for Fe atom. The energy minimized structure clearly evident that
ligand 4b afforded a suitable cavity resulting from preorganiza-
Supplementary materials
≡
tion of ligating sites i.e electronically rich O atom and C C moiety
Supplementary material associated with this article can be
found, in the online version, at doi:10.1016/j.molstruc.2020.129618.
of propargyl group (Fig. S18). At the same level of theory, fron-
tier molecular orbitals were also evaluated and it is found that
in ligand 4b the electron density at HOMO is distributed all over
the ligand but in case of LUMO it is located more over the an-
thracene part. Moreover in 4b+Fe3+ complex, the HOMO is dis-
tributed over anthracene and imine part.Whereas LUMO is located
over Fe3+ complex. This indicates that this is a ligand to metal
electron transfer Fig. 12. Furthermore, from the computed result,
it can be clearly seen that the energy gap between HOMO and
LUMO in case of complex is lowered by 0.049 eV as compared to
bare ligand this pointed out the greater stability of the complex as
compared to ligand. This lowering of energy band gap is corrobo-
rated with the bathochromic shift in UV–vis absorption spectra as
observed by sequential addition of Fe3+ and publicizes the higher
stability of complex generated.
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