Antipyrine based Schiff's base as a reversible fluorescence turn “off-on-off” chemosensor for sequential recognition of Al3+ and F? ions: A theoretical and experimental perspective

Article abstract of DOI:10.1016/j.saa.2019.117596

A Schiff's base probe (L) based on antipyrine has been intended, synthesized and assessed as a turn “off-on-off” probe for successive recognition of Al³⁺ and F^?. The probe L act out as a turn “on” fluorescence probe towards Al³⁺

Full text of DOI:10.1016/j.saa.2019.117596

Journal Pre-proof
Antipyrine based Schiff's base as a reversible fluorescence turn “off-on-off”
3+
−
chemosensor for sequential recognition of Al and F ions: A theoretical and
experimental perspective
Priya Yadav, Mitlesh Kumari, Yachana Jain, Madhu Agarwal, Ragini Gupta
PII:
S1386-1425(19)30986-2
https://doi.org/10.1016/j.saa.2019.117596
SAA 117596
DOI:
Reference:
To appear in: Spectrochimica Acta Part A: Molecular and Biomolecular
Spectroscopy
Received Date: 1 July 2019
Revised Date: 2 October 2019
Accepted Date: 2 October 2019
Please cite this article as: P. Yadav, M. Kumari, Y. Jain, M. Agarwal, R. Gupta, Antipyrine based Schiff's
3
+
base as a reversible fluorescence turn “off-on-off” chemosensor for sequential recognition of Al
−
and F ions: A theoretical and experimental perspective, Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy (2019), doi: https://doi.org/10.1016/j.saa.2019.117596.
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Antipyrine based Schiff’s base as a reversible fluorescence turn “off-on-off” chemosensor
3
+
-
for sequential recognition of Al and F ions: A theoretical and experimental perspective
a
a
a
b
a,c*
Priya Yadav , Mitlesh Kumari , Yachana Jain , Madhu Agarwal and Ragini Gupta
a
Department of Chemistry, Malaviya National Institute of Technology Jaipur, Jaipur 302017,
India
b
Department of Chemical Engineering, Malaviya National Institute of Technology Jaipur, Jaipur
3
02017, India
c
Materials Research Centre, Malaviya National Institute of Technology Jaipur, Jaipur 302017,
India
*
E-mail: rgupta.chy@mnit.ac.in
Graphical Abstract

Antipyrine based Schiff’s base as a reversible fluorescence turn “off-on-off” chemosensor
3
+
-
for sequential recognition of Al and F ions: A theoretical and experimental perspective
a
a
a
b
a,c*
Priya Yadav , Mitlesh Kumari , Yachana Jain , Madhu Agarwal and Ragini Gupta
a
Department of Chemistry, Malaviya National Institute of Technology Jaipur, Jaipur 302017,
India
b
Department of Chemical Engineering, Malaviya National Institute of Technology Jaipur, Jaipur
3
02017, India
c
Materials Research Centre, Malaviya National Institute of Technology Jaipur, Jaipur 302017,
India
*
E-mail: rgupta.chy@mnit.ac.in
Abstract: A Schiff’s base probe (L) based on antipyrine has been intended, synthesized and
3
+
-
assessed as a turn “off-on-off” probe for successive recognition of Al and F . The probe L act
3
+
-
out as a turn “on” fluorescence probe towards Al in methanol at pH 6 which turned “off” by F
3
+
at 433 nm. The 1:1 binding stoichiometry of L+Al complex was revealed by Job's plot and
3
+
approved by ESI-HRMS data. The binding constant and limit of detection of probe L for Al
7
-1
−7
were found to be 2.951 × 10 M and 0.61×10 M respectively, which is lesser than the
−
7
acceptable limit (0.74 ×10 M) in drinking water. The proposed binding sites and the mode of
1
27
interaction of probe L was studied and validated by H NMR titration and Al NMR
spectroscopic studies. To get detailed vision into binding mechanism and optimized structure of
3
+
3+
-
receptor L and L+Al , L+Al + F complex, theoretical calculations using DFT/DND and
TDDFT method were performed. Furthermore, probe L can mimic INHIBIT logic function using
3
+
-
Al and F being logic inputs and examining the fluorescence maxima at 433 nm as output.
Keywords: Antipyrine based Schiff’s base; Spectroscopic studies; Reversible sensing; Detection
limit; INHIBIT logic function; Theoretical calculation.
1

1
. Introduction
Aluminium is one of the most abundant element found in the earth’s crust [1]. Various sources
from where it enters the environment are raising agents (aluminium ammonium sulphate) and
anti-caking agents (sodium aluminosilicate, aluminium silicate) in food additives [2], alum in
paper industry [3], antacids in pharmaceuticals [4], cans and foils in food packaging, kitchen
utensils, coagulant (alum) in water treatment [5]. In soil, level of aluminium upsurges by virtue
of acid rain, as the solubility of aluminium increases in low pH that limits plant growth and
development [6-8]. In humans it enters into the blood stream and gets accumulated in the organs
causing Alzheimer’s disease, persistent infection in intestine, Parkinson’s disease and reduces
3
+
growth of bone [9, 10]. According to WHO, the average acceptable limit of Al ions that a
human can intake in weekly diet is about 7 mg/kg body weight [11] while in drinking water the
−6
permissible limit is 7.4 ×10 M. Ill-effects of aluminium on human health and environment
compels its precise detection and monitoring. Numerous approaches such as flame atomic
absorption spectrometry [12], voltammetry [13], inductively coupled plasma emission
spectrometry [14], ion selective electrodes analysis [15], graphite furnace atomic absorption
spectrometry [16, 17] and electrochemical stripping analysis [18], etc. have been developed for
3
+
detecting Al ions in solution. But the major shortcomings of these methods is that they lack
selectivity due to interference from other metal ions, require expensive and technically
demanding sophisticated instruments which needs skilled personnel for operating them with
tiresome sample preparation methods. Development of colorimetric and fluorescent sensor is an
alternative to the traditional analytical instruments, which have not only the benefit of being
selective and inexpensive but also provide reliable results and simple ways of detecting ions in
solution with quick response time [19-27].
The detection of fluoride ions has fascinated significant interest [28-32] due to its duplicitous
nature [33, 34]. Fluoride ion is the biologically active ion and appropriate amount of fluorine can
prevent dental caries and senior osteoporosis [35]. However, excess exposure of fluoride ion can
causes various diseases like fluorosis, kidney failure and bone disorders [36, 37].
To address this issue, various chemosensors having capability of successive recognition of both
metal ion and anion are gaining attention [38-43]. Recently, Dwivedi et al. synthesized a
chemosensor based on Schiff’s base from 2-thiophene carboxylic acid hydrazide that
2

3
+
-
successively detects Al and F [44]. Erdemir and coworkers designed a fluorescent sensor
3
+
-
2
centered on triazole that detects Al in MeCN–H O solution and F in MeCN [45]. Mi et al.
3
+
synthesized a rhodamine-quinoline centered receptor that displays high selectivity toward Al
-
and successively detects F in ethanol–water solution [46]. However, many of them have
encountered interference caused by other metal ions/ anions, multiple synthetic routes, high cost
and use of other harmful organic solvents.
Thus, considering the present interest of exploration to the design and synthesis of effective
chemosensors that could sequentially recognize both metal ion and anion, and in extension to our
-
earlier work on F sensor [47-51], we hereby describe the synthesis, characterization and sensing
behavior of antipyrine based schiff’s base L (Scheme 1), which exhibits naked eye color change
from colorless to transient yellow and an increase in emission intensity in methanol upon
3
+
3+
addition of Al . The complex developed between chemosensor L and Al is completely
reversible with fluoride ions and it could act as a dual probe which has the potential to sense
3
+
-
successively Al and F by fluorescence switchable signaling reaction for at least 6 cycles.
3
+
Anions breakdown the interaction between chemosensor L and Al ion and undergo
displacement reaction. Logic behavior of probe L was also explored using optical activities of
-
3+
probe L with F and Al ions as inputs and change in emission intensity at 433 nm as output.
Further, DFT and TDDFT calculations were carried out to attain thorough understanding into the
3+
3+
-
binding process and optimized structure of L and L + Al , L + Al + F complex. The limit of
−
7
detection of probe L for aluminium ions was obtained as 0.61×10 M which is lesser than the
−
7
permissible limit (0.74 ×10 M) in drinking water.
2
2
. Experimental
.1. Material and methods
All chemicals and solvents purchased are of analytical grade and used without further
purification. Bruker Fourier transform infrared spectrophotometer (FTIR) (Alpha) was used for
IR spectral analysis with pressed KBr pellets. High-resolution mass spectra were recorded on a
Xevo G2-S Q-Tof (Waters, USA) in the ESI (Electrospray Ionization) mode. Shimadzu UV-
1
800 Spectrophotometer and Perkin Elmer LS 55 fluorescence spectrophotometer were used for
collecting UV-vis absorption spectral and emission data respectively, at room temperature (298
1
13
K). Jeol ECS 400 multinuclear spectrophotometer was utilized for collecting H NMR and C
NMR data.
3

2
1
.2. Synthesis of the receptor 4-((2,4-dihydroxybenzylidene)amino)-1,5-dimethyl-2-phenyl-
H-pyrazol-3(2H)-one (L)
The Schiff’s base ligand (L) was prepared by previously reported method [52-59]. 2,4-
dihydroxybenzaldehyde (1 mmol, 0.1381g) and 4-aminoantipyrine (1 mmol, 0.2032g) were
added to round bottom flask containing ethanol (15 ml). Then 2 drops of glacial acetic acid was
added which was then refluxed in an oil bath for 3 h. The obtained solid was filtered, washed
with cold ethanol and later recrystallized from ethanol (Scheme 1).
Analytical data: Shiny yellow color solid, yield: 91%, M.P. found: 231-234 ℃, reported: 232 ℃
-1
1
[
60]. IR (KBr, cm ): v(>C=O), 1622; v(>C=N), 1518; v(O-H), 3448. H NMR (δ, ppm, DMSO-
d6, 400 MHz): 9.51 (s, -N=CH-, 1H); 7.53 - 6.31 (m, Ar-H, 7H); 6.22 (s, Ar-H, 1H); 3.14 (s, -N-
1
3
CH , 3H); 2.31 (s, =C-CH , 3H). C NMR (δ, ppm, DMSO-d , 100 MHz): 10.31, 35.83, 102.97,
3
3
6
1
08.52, 113.43, 115.53, 125.34, 127.50, 129.60, 133.72, 150.19, 158.97, 159.98, 161.73, 162.80.
+
HRMS (ESI) (C H N O ) [M+H] , m/z calc. 324.1343 found: 324.1342.
1
8
17
3
3
Scheme 1: Synthesis of receptor L.
2
.3. General process for UV–vis and emission studies
-2
-2
Stock solutions of chemosensor L (1× 10 M), nitrate salts of metal ions (1× 10 M) and sodium
-2
salts of anions (1× 10 M) were prepared in methanol using buffer solution at pH = 6. The above
4

stock solutions were then later diluted. Absorption and emission spectra were determined in 1.0
cm path length quartz cuvettes. Emissions were recorded from 360 to 680 nm with excitation at
3
47 nm. Binding constant for receptor L was determined by outlining a graph between 1/(I-Io)
x+
and 1/M obtained through fluorescence titration. Benesi-Hildebrand equation (Eq. (1)) was
used for calculating the value of binding constant.
a+
I /I-I
o
o
=(x/y-x)(1/K
b
[M ]+1)
(1)
where, I is the emission intensities of receptor L at 433 nm and I is the emission intensities of
o
3
+
a+
3+
receptor L upon addition of Al ; x, y are constants; M is the concentration of metal ion (Al ).
3
+
The detection limit of receptor L for the examination of Al was calculated with the aid of blank
standard method (Eq. (2)).
Detection limit = 3(σ)/slope
(2)
where, σ is the standard deviation; slope is obtained from the calibration plot of fluorescence
3
+
intensity and concentration of Al .
1
-2
3+
-1
3
H NMR titration was done by using receptor L (1×10 M in CD OD) with Al (1×10 M in
3
+
CD OD) on adding Al (0.25, 0.5, 1.0 equivalents).
3
2
.4. General process for fluorescence quantum yield calculations
3
+
The fluorescence quantum yield value (Ф ) of the chemosensor L with Al ion was calculated
S
using the comparative William’s method which uses well-characterized standards with known
quantum yield value (Ф ) [61]. For the calculation of quantum yield, the electronic spectra and
S
fluorescence spectra of standard reference i.e. quinine sulfate in 0.5 M H
2
4
SO , chemosensor L,
3
+
3+
-
L+Al , L+Al +F were recorded. The integrated fluorescence intensities were plotted against
3+
3+
-
absorbance for the standard reference, chemosensor L with Al and L+Al +F . The gradients
of the plots are proportional to the quantity of the quantum yield of the studied molecules. The
quantum yield (Ф ) value was calculated from the data obtained as per the following Eq. (3)
S
2
IS GradS
* *
∩
∩
S
r
φS
=
φr
*
2
Ir Gradr
(3)
where “r” and “s” denote reference and sample respectively, “Grad” is the gradient from the plot
and “∩” is the refractive index of the solvent.
5

3
3
. Results and discussion
.1. Metal ions sensing studies of Receptor L
Figure 1: UV-visible spectra of chemosensor L (50 µM) in the presence of different metal ions
10 equiv.) in methanol at pH = 6.
(
3
+
Figure 2: Absorbance spectral changes of chemosensor L (50 µM) upon gradual addition of Al
ion (0 to 100 µM) in methanol.
6

To examine the binding behavior of chemosensor L (50 µM) towards various metals (10 equiv.),
UV–vis absorption spectral studies were performed in methanol. The free chemosensor L shows
two strong absorption band centered at 245 nm, 347 nm and weak bands in the wavelength range
from 283 to 309 nm corresponding to π–π* and n–π* and charge transfer transitions. No
3
+
noteworthy alteration in absorption bands were noted upon addition of metal ions except Al
3
+
(
Fig. 1). Continuous addition of Al (0 to 100 µM) displays a lessening in absorbance intensity
at 245 nm and 347 nm accompanied by occurrence of absorbance band at 385 nm. Distinctive
3+
isosbestic points were appears at 257 nm and 373 nm with raising the concentration of Al
3
+
indicates the formation of L+Al complex (Fig. 2). These variations in the absorption spectrum
3
+
supported the visual change in color from colorless to transient yellow in the presence of Al
ions. Further, fluorescence behavior of chemosensor L (50 µM) on adding up numerous metal
ions (50 µM) has been examined in methanol. Free probe L shows a weak emission band at 433
nm with an excitation of 347 nm; however adding up of various metal ions shows no noteworthy
3
+
changes (Fig. 3). The emission intensity at 433 nm upsurges with the add-on of Al that justifies
the appearance of blue fluorescence with chemosensor L, however with other metal ions
3
+
chemosensor remains turn off (Fig. S11). Furthermore, from titration of Al (0–100 µM) with
chemosensor L shows increment in fluorescence intensity at 433 nm, this shows the fluorescence
3+
turn “off-on” behavior of chemosensor L with Al (Fig. 4 (a)), which can be justified from PET
photo induced transition) mechanism (Scheme 2). For further applicability of chemosensor L to
(
3
+
real samples, fluorescence titration of Al (0–100 µM) in water with chemosensor L was
3
+
performed and receptor shows turn “off-on” behavior towards Al in water (Fig. 4 (b)) . The L-
3
+
Al complex gives a quantum yield value of (Ф ) = 0.0207 which is higher as compared to
S
3
+
chemosensor L (Ф ) = 0.0013. Binding constant for L+Al complex was measured by Benesi-
S
7
-1
Hildebrand equation and the calculated value was found to be 2.951 × 10 M (Fig. S7). Further,
−7
the limit of detection was revealed by IUPAC standard method and was found to be 0.61×10 M
−
7
and 0.61×10 M in methanol (Fig. S6) and water (Fig. S15) respectively.
7

Figure 3: Fluorescence spectra of receptor L (50 µM) upon addition of various metal ions (1
equiv.) in methanol (pH = 6) with an excitation of 347 nm.
3
+
Figure 4: Fluorescence emission spectra of L (50 µM) upon addition of (a) Al ion (0 to 100
3
+
µM) in methanol; (b) Al ion (0 to 100 µM) in water with an excitation of 347 nm.
3
.2. Interference study with metal ions
3
+
The selectivity of chemosensor L with respect to Al was inspected by carrying out interference
study in the presence of different metal ions. No noteworthy variations in emission intensity of
3
+
L+Al complex were noted, showing that added metal ions not meddle in the recognition of
3
+
3+
Al (Fig. 5). It can depicted from small ionic radius and high charge density of Al [62] as well
8

3
+
as the appropriate coordination geometry of chemosensor L by offering hard donor sites to Al
3
+
(
hard acid) also act as vital factor for acting as selective chemosensor for Al .
3
+
Figure 5: Selectivity of chemosensor L (50 µM) towards Al (1 equiv.) in presence of other
metal ions (1 equiv.) at 433 nm in methanol (pH=6) at room temperature. The orange bars
represent the fluorescence intensities of chemosensor L in the presence of various metal ions.
The blue bars represent the change in fluorescence intensity that occurred upon subsequent
3
+
addition of Al ion to the above mentioned solution.
3
+
3
.3. Stoichiometry of chemosensor L with Al ion
-4
3+
The binding stoichiometry of metal ion complex between chemosensor L (1× 10 M) and Al
-4
(
1× 10 M) was accomplished from job’s plot centered on absorbance. For job’s plot various
3
+
solutions were prepared by keeping constant the total concentration of the L+Al solution (100
µM). Plot between absorbance and mole fraction of chemosensor L shows that absorbance
intensity at 385 nm reached to maximum at 0.5 mole fraction, representing 1:1 binding
3
+
stoichiometry of receptor L with Al (Fig. S5). Further, binding stoichiometry was approved by
3
+
performing ESI mass spectra, chemosensor L with Al shows m/z at 411.0889 agreeing to
3+
+
[
L+Al + NO - H ] (calculated: 411.0885).
3
3
.4. Effect of solvent
3
+
The optical behavior of chemosensor L towards Al was analyzed in various solvents like
methanol, water, THF, ethanol, DMSO, acetonitrile and hexane. The sensing property of
9

3
+
chemosensor L towards Al was found to be reliant on the type of solvent and show maximum
fluorescence intensity in methanol at 433 nm (Fig. 6).
3
+
Figure 6: Solvent effect on fluorescence intensity of chemosensor L with Al in different
solvents at 433 nm.
3
.5. pH effect
3
+
pH effect on the optical behavior of chemosensor L towards Al was then further examined at
3
+
different pH varying from 2 to 10. The emission changes of probe L (50 µM) with Al (1.0 eq.)
was analyzed in methanol at 433 nm. No change in fluorescence intensity was witnessed in both
acidic and neutral pH range whereas in basic pH range (pH >7) decrease in fluorescence
intensity was obtained which may be attributed to the origination of free chemosensor by the
3
+
deletion of Al ion as Al(OH) (Fig. 7).
3
1
0

Figure 7: The variation in fluorescence intensity with the pH of the chemosensor L (50 µM)
3
+
with Al (1.0 eq.) at excitation wavelength = 347 nm and emission wavelength = 433 nm.
3
.6. Spectroscopic studies
1
27
3
.6.1. H NMR titration and Al NMR
1
3+
H NMR titrations were conducted in the presence of Al (0.0–1.0 eq.) using CD OD as a
3
solvent. The proton signal of HC=N of chemosensor L at 9.51 ppm was shifted upfield at 8.69
3
+
ppm upon addition of Al , indicating the disruption of intramolecular H-bonding between
phenolic -OH group and nitrogen in imine group [63]. Imine proton signal of chemosensor L at
3
+
9
.51 ppm totally vanished with the add-on of 1 eq. of Al , indicating that receptor L form
3
+
complex with Al with 1:1 stoichiometry (Fig. 8). At the same time aromatic proton signal of
3
+
phenylene were moved to downfield with the add-on of Al (0.0–1.0 eq.), which specifies that
3
+
1
upon binding with Al structure of chemosensor becomes rigid. These changes in the H NMR
spectra reveal that oxygen ( phenolic –OH group ) and nitrogen (HC=N group) contributes in
3
+
binding with Al ions.
2
7
3+
The Al NMR of L+Al complex was recorded in CD OD, as shown in Fig. S10 complex
3
exhibit broad signal at 68.93 ppm which is indicating the four coordination around the
3
+
+
aluminium atom corresponding to [L+Al + NO
3
- H ], whereas signal at 5.03 ppm attributed to
3−n
different aluminium-nitro-methanol species, [Al(NO ) (MeOH) ] [64, 65].
3
n
6−n
1
1

1
3+
Figure 8: H NMR (400 MHz) spectra of chemosensor (L) with Al (0.0 to 1.0 eq.) in CD OD.
3
3
+
3
.7. Anion sensing studies of L+Al complex
3
+
Figure 9: UV-visible spectra of L-Al complex (50 µM) upon addition of 3 equivalents of
different anions in methanol (pH=6).
3
+
Anion sensing studies of L+Al complex were performed to examine the effect of anions on the
3
+
reversibility of Al with chemosensor L. From UV-vis spectroscopic studies, it was noted that
-
-
2-
2-
2-
-
-
addition of other anions such as I , Br , SO , CO , HPO , Cl , NO does not show any
4
3
4
3
1
2

-
variations in the UV-vis spectra. It is clear from the Fig. 9 that add-on of F (3 equiv.) to the
3
+
L+Al complex (50 µM) formed in situ brings out changes in the absorbance spectra. However,
-
3+
small changes were also observed with AcO ion. The absorption bands appeared in L+Al
-
complex becomes diminished upon addition of F along with the restoration of absorption bands
of free chemosensor L.
From emission spectroscopic studies, we examined the fluorescence turn “on-off” behavior of
3
+
-
L+Al complex with F . Fluorescence intensity at 433 nm gradually diminished with the add-on
of F showing that the solution becomes turn off (Fig. 10). From the above results, it is indicated
-
3+
that the chemosensor L become free with the add-on of F to the L+Al complex (Scheme 2). As
-
3+
F has high binding affinity towards Al they break the coordination bond between chemosensor
3
+
3+
L and Al and undergoes displacement reaction. Further, titration experiments of L+Al
-
complex with F were carried out and they offer an evidence for regeneration of chemosensor L
by showing bands in reverse trend with the bands obtained from the titration of probe L with
3
+
3+
-
Al ions in the emission spectra (Fig. 11) and L+Al +F complex gives a quantum yield value
-
of (Ф ) = 0.0015 . IUPAC standard method was used to determine detection limit of F ion, and
S
−
6
found as 4.06×10 M (Fig. S13).
3
+
Figure 10: Change in emission spectrum of L-Al (50 µM) upon gradual addition of different
anions (3 equivalents) in methanol (pH=6).
1
3

3
+
Figure 11: Fluorescence emission spectra of L-Al (100 µM) in methanol (pH=6) upon addition
-
of F ion (0 to 500 µM) at 433 nm with an excitation of 347 nm.
3
+
-
Scheme 2: Proposed mechanism for the sequential detection of Al and F ions.
3
.8. Reversible Sensing nature
Reversible sensing property and regeneration of chemosensor are principal features of molecular
3
+
−
sensor. Alternative addition of Al and F to chemosensor L results into switchable variation in
-
3+
the emission intensity at 433 nm (Fig. 12). As F ion has high binding affinity towards Al , it
3
+
dissociates L+Al complex and forms AlF . The free chemosensor L could be easily revived
3
3
+
−
and reused for Al and F sensing up to 6 cycles. Thus, the chemosensor L can act as a reversible
3
+
−
probe for Al and F .
1
4

-4
Figure 12: Reversible switching of the emission of L (1×10 M) at 433 nm when excited at 347
3
+
-3
-
-3
nm, upon alternate addition of Al (1×10 M) and F (3×10 M).
3
.9. Application as logic function
3
+
Molecular logic function was built on the basis of optical behavior of chemosensor L with Al
-
and F as inputs. The output corresponds to the emission intensities of these systems at 433 nm.
To construct logic gate, output logic 0 and logic 1 were assigned to turn “off” and turn “on”
fluorescence respectively. Chemosensor L remains turn “off” in the absence of the inputs (i.e.
3
+
-
3+
Al and F ). When Al alone was added to the chemosensor L, an increase in emission intensity
-
3+
at 433 nm was noted resulting in output logic 1. Addition of F to the L+Al solution makes the
system turn “off” resulting in output 0. Thus based on emission intensities of these systems at
4
33 nm, truth table and INHIBIT logic function was developed (Fig. 13).
1
5

Figure 13: Truth table and logic gate representation based on fluorescence response of
3
+
-
chemosensor L towards Al and F as chemical inputs and emission mode as output.
3
.10. Electronic spectra and DFT calculation
Figure 14: Diagram showing HOMO and LUMO energy levels and the interfacial plots of the
3
+
3+
-
orbitals for free chemosensor L, L+Al , L+Al + F complex.
The theoretical studies were achieved to comprehend the electronic transitions, proposed binding
3+
3+
-
sites and mode of interaction of receptor L, L+Al , L+Al + F complex by DFT/TDDFT
3
calculation in DMoL module, PWC (Perdew Wang Correlations) local DFT functional and
1
6

DND (4.4) basis set in methanol using Materials studio (version 8.0) program suite [66, 67]. The
3
+
optimized structures of free chemosensor L and L+Al complex with their molecular orbital
(HOMO and LUMO) are represented in Fig. 14, which depicts that receptor L act as a tridendate
3
+
ligand towards Al with O, N, O donor sites. The ∆ EHOMO-LUMO energy gap for free
3
+
chemosensor L and L+Al complex were found to be 0.368 eV and 0.085 eV respectively. This
3
+
decrease in band gap in L+Al complex confirms the formation of stable complex. However, ∆
n+
EHOMO-LUMO energy gap of other L+M complexes were much higher when compared with
3
+
L+Al complex (Table 1). Above results delivers good understanding about the coordination
3
+
behavior of chemosensor L towards Al and gives a plausible explanation for the blockage of
3
+
PET phenomenon from chemosensor L (nitrogen atom) to fluorophore (phenyl ring) in L+Al
complex (Scheme 2).
Table 1: ∆ EHOMO-LUMO (eV) energy gap of chemosensor L in the presence of various metal ions.
n+
S. No.
L+M complex
∆ EHOMO-LUMO (eV)
0.085 eV
0.498 eV
0.510 eV
0.456 eV
0.507 eV
0.506 eV
0.547 eV
0.589 eV
0.588 eV
0.493 eV
0.580 eV
0.469 eV
1.41 eV
3
+
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
.
L+Al
2
+
+
.
L+Ba
L+Ca
L+Co
L+Cu
2
.
2
2
+
+
.
.
2
+
.
L+Sr
3
+
.
L+Cr
2
+
.
L+Mn
3
+
.
L+Fe
L+Ni
2
+
0.
1.
2.
3.
4.
5.
2
+
L+Zn
2
+
L+Pb
+
L+Na
+
L+K
0.471 eV
0.588 eV
2
+
L+Mg
1
7

-
3+
-
The optimized geometries of F ions with L-Al complex reflects common feature that F ions
3
+
were interacting with Al by showing elongation of bond between Al-N, Al-O (keto group), Al-
O (hydroxyl group) from 1.542, 1.753, 1.602 Å to 1.909, 1.865, 1.758 Å (Fig. S14). These
3
+
elongations result in weakening of bonds, thereby decreasing the binding of Al with
chemosensor L. It is interesting to note that the ∆ EHOMO-LUMO energy gap has increased from
0
.085 eV to 0.305 eV, which is comparable to ∆ EHOMO-LUMO energy gap of free ligand L slight
3
+
discrepancy, may be due to the reason that some receptor is still bonded with Al .
Further, TDDFT calculations were performed to get deeper understanding of electronic transition
processes. The intense peak at 347 nm for probe L matches to HOMO→LUMO transition. The
high energy band at 245 nm matches to HOMO→LUMO+1/HOMO-2→LUMO transitions. In
3
+
L+Al complex, peaks at 330, 307, 385 nm matches to HOMO→LUMO+1; HOMO-1→LUMO
3
+
and HOMO→LUMO transition respectively (Table 2). TDDFT calculations of L-Al complex
show that nearly all transitions are metal centered π–π* or LMCT (Fig. 15).
3+
Table 2: UV-visible transitions in probe L and L+Al complex calculated by TDDFT method.
Probe L
λmax (exp.)(nm) Oscillator strength (f) λmax (Calc.)(nm) Transition
3
2
2
47
83
45
0.076703
0.266434
0.103263
328
293
282
276
HOMO→LUMO
HOMO-1→LUMO
HOMO→LUMO+1
HOMO-2→LUMO
0
.068986
3
+
L+Al complex
λ
max (exp.)(nm) Oscillator strength (f)
λ
max (Calc.)(nm) Transition
3
85
0.031564
379
HOMO→LUMO
LMCT)
(
3
3
30
07
0.011445
0.010599
322
307
HOMO-1→LUMO
HOMO→LUMO+1
1
8

3
+
Figure 15: Diagram showing low energy electronic transitions (a) Receptor L (b) L+Al
complex.
4
. Conclusion
1
9

In conclusion, antipyrine based schiff base has been synthesized following high yielding and
3
+
uncomplicated synthetic procedure that act as a turn on fluorescence probe towards Al without
3
+
any interference from other metal ions. Upon addition of Al , chemosensor L shows increase in
−7
fluorescence intensity because of the blockage of PET process with detection limit of 0.61×10
3
+
M. Further, L+Al complex were then investigated for anion sensing ability via UV-vis,
-
3+
fluorescence and computational studies. Addition of F ions causes dissociation of L+Al
complex leading to fluorescence turn “off”. Chemosensor L displays an INHIBIT logic function
3
+
-
using Al and F being logic inputs and examining the emission maxima at 433 nm as output
3
+
mode. DFT studies resulted that binding of Al to chemosensor L reduced the ∆ EHOMO-LUMO
-
3+
energy gap further, adding up F ions to L+Al complex increases the ∆ E
energy gap
HOMO-LUMO
gives an evidence for fluorescence turn “off-on-off” response of chemosensor L for successive
3
+
-
recognition of Al and F . Thus, chemosensor L can act as a dual receptor for the detection of
3
+
-
Al and F in environmental and biological samples.
Acknowledgements
P. Yadav thanks UGC-CSIR for Junior Research Fellowship Grant No. F.16-6(DEC.
2
016)/2017(NET). M. Kumari acknowledges DST Inspire doctoral fellowship (IF140506). Y.
Jain is thankful to MNIT Jaipur for doctoral fellowship. Authors are grateful to Materials
Research Centre, MNIT Jaipur, for granting characterization facilities.
2
0

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[
2
6

Antipyrine based Schiff’s base as a reversible fluorescence turn “off-on-off” chemosensor
3
+
-
for sequential recognition of Al and F ions: A theoretical and experimental perspective
a
a
a
b
a,c*
Priya Yadav , Mitlesh Kumari , Yachana Jain , Madhu Agarwal and Ragini Gupta
a
Department of Chemistry, Malaviya National Institute of Technology Jaipur, Jaipur 302017,
India
b
Department of Chemical Engineering, Malaviya National Institute of Technology Jaipur, Jaipur
3
02017, India
c
Materials Research Centre, Malaviya National Institute of Technology Jaipur, Jaipur 302017,
India
*
E-mail: rgupta.chy@mnit.ac.in
Highlights
•
•
•
Development of antipyrine based schiff’s base (L) as reversible fluorescence turn “off-
3
+
-
on-off” sensor for sequential detection of Al and F ions.
1
27
The sensing property is monitored by UV-vis, fluorescence, H NMR and Al NMR
spectroscopic studies.
Theoretical calculations using DFT and TDDFT method were performed to get detailed
3+
insight into the sensing mechanism and electronic structure of receptor L and L+Al ,
3
+
-
L+Al + F complex.
3
+
-
•
Probe L can mimic INHIBIT logic function with Al and F as inputs and monitoring the
emission maxima at 433 nm as output.

Declaration of interests
V The authors declare that they have no known competing financial interests or personal
relationships that could have appeared to influence the work reported in this paper.
V The authors declare the following financial interests/personal relationships which may be
considered as potential competing interests:
None