Cascade detection of fluoride and bisulphate ions by newly developed hydrazine functionalised Schiff bases

Article abstract of DOI:10.1016/j.molliq.2021.115293

Two hydrazine functionalized Schiff bases have been synthesized through the reaction between hydrazine and o-vanillin/salicylaldehyde compounds employing a green-chemical approach and characterized spectroscopically including XRD study. Crystal structure analysis reveals that both the chemosensors, N,N′-bis(o-vanilidine)hydrazine (P17) and N,N′-bis(salicylidene)hydrazine, (HARB) crystallize in monoclinic system with P2₁/n space group and exist in the locked forms through intramolecular H-bonding (~1.90 ?) between phenolic-OH and N atom of hydrazine. The chemosensors display excellent selectivity towards fluoride followed by bisulphate ions, over other potential competitor anions in acetonitrile. Binding stoichiometry of the individual chemosensor with F^? is confirmed to be 1:1 and assessed with absorption study and ¹H NMR analysis. Systematic DFT analysis reveals that the contribution of hydroxyl oxygen atoms to the HOMO increases sharply from the chemosensor to chemosensor–F^? adduct (17% to 28%) leading to deprotonation of one hydroxyl group and consequently involvement in conjugation impeding the C=N isomerisation. Thus, the hydroxyl proton captured by F^? restricts the C=N isomerisation as well as ICT character of both the chemosensors and confirms the cascade sensing mechanism for fluoride and bisulphate ions.

Full text of DOI:10.1016/j.molliq.2021.115293

Journal of Molecular Liquids 326 (2021) 115293
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Cascade detection of fluoride and bisulphate ions by newly developed
hydrazine functionalised Schiff bases
a
b
b
c
b
Suvojit Roy , Provakar Paul , Monaj Karar , Mayank Joshi , Suvendu Paul ,
c
a,
Angshuman Roy Choudhury , Bhaskar Biswas ⁎
Department of Chemistry, University of North Bengal, Darjeeling 734013, India
Department of Chemistry, University of Kalyani, Kalyani 741235, India
Department of Chemical Sciences, Indian Institute of Science Education and Research, Mohali, Sector 81, Knowledge City, S. A. S. Nagar, Manauli PO, Mohali, Punjab 140306, India
a
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Two hydrazine functionalized Schiff bases have been synthesized through the reaction between hydrazine and o-
vanillin/salicylaldehyde compounds employing a green-chemical approach and characterized spectroscopically
including XRD study. Crystal structure analysis reveals that both the chemosensors, N,N′-bis(o-vanilidine)hydra-
zine (P17) and N,N′-bis(salicylidene)hydrazine, (HARB) crystallize in monoclinic system with P2 /n space group
1
and exist in the locked forms through intramolecular H-bonding (~1.90 Å) between phenolic-OH and N atom of
Received 6 November 2020
Received in revised form 31 December 2020
Accepted 2 January 2021
Available online 8 January 2021
hydrazine. The chemosensors display excellent selectivity towards fluoride followed by bisulphate ions, over
Keywords:
−
other potential competitor anions in acetonitrile. Binding stoichiometry of the individual chemosensor with F
Fluoride and bisulphate sensing
Green-chemical synthesis
Naked eye colour change
Schiff base
Spectrophotometry
TD-DFT study
1
is confirmed to be 1:1 and assessed with absorption study and H NMR analysis. Systematic DFT analysis reveals
that the contribution of hydroxyl oxygen atoms to the HOMO increases sharply from the chemosensor to
−
chemosensor–F adduct (17% to 28%) leading to deprotonation of one hydroxyl group and consequently involve-
−
ment in conjugation impeding the C=N isomerisation. Thus, the hydroxyl proton captured by F restricts the C=
N isomerisation as well as ICT character of both the chemosensors and confirms the cascade sensing mechanism
for fluoride and bisulphate ions.
X-ray structure
©
2021 Elsevier B.V. All rights reserved.
1. Introduction
from this amphiphilic anion at high pH [10]. To remove such adverse ef-
fects, design of an efficient chemosensor for the selective detection of
−
Designing and development of receptors and chemosensors with the
HSO
4
is a promising challenge to the supramolecular chemists.
ability to recognize the anions with extreme selectivity are of high de-
mand in modern science as anions play fundamental roles in a varied
range of biological and chemical processes [1]. Fluoride (F ) is an ex-
tremely important anion in the living world. It is an essence to sustain
dental health and in curing osteoporosis [2–4]. On the flip side, excess
consumption of fluoride causes severe diseases like gastrointestinal
dysfunction, dental and bone fluorosis [5]. Focusing on this serious con-
cern, the World Health Organization (WHO), in a guideline has directed
Therefore, design and synthesis of environmentally and medicinally
important anion, selective probe is still an unsolved challenge. During
the last two decades, different scientific groups have synthesized fluo-
ride selective [11–22] as well as bisulphate selective [23–36] probes
for the selective and sensitive colorimetric detection of these two ions
individually. But, none of the probes was able to detect both the fluoride
and bisulphate anions in a cascade manner. Mallick and co-workers [37,
−
−
−
4
38] exploited the cascade detection of F and HSO ions employing a
to maintain fluoride concentration below 1.5 mg/mL in drinking water
single “Off-the-shelf” or commercially available probe molecule.
Literature survey indicates that different scientists have been utilized
hydrazine functionalized Schiff base to develop chemoreceptor colori-
−
[
6–8]. On the other hand, bisulphate (HSO
4
) is also a useful anion and
it is a constituent of many agricultural fertilizers, industrial raw mate-
rials, and nuclear fuel waste. Bisulphate has also an adverse effect on
the environment as a pollutant and in this context, sensing of this
anion over the entire group of oxo-anions is still an emerging area of
−
−
3
metrically for the detection of F , CH COO , and other anions in non-
aqueous solution [39–42]. Most of them have synthesized the
compounds without reporting their crystal structures. It is well docu-
mented that Ziolek et al. studied important spectroscopic and
photophysical properties of the conformers of different photochromic
Schiff base ligands in a number of differently interacting solvents and
micellar systems [43–45]. In this context, herein, we have designed
and synthesized two hydrazine functionalized Schiff bases (P17 and
modern research [9]. Irritation of the skin and eyes and even respiratory
paralysis is affected by the toxic sulphate ion (SO2
–
) which is produced
4
⁎
Corresponding author.
E-mail address: bhaskarbiswas@nbu.ac.in (B. Biswas).
https://doi.org/10.1016/j.molliq.2021.115293
167-7322/© 2021 Elsevier B.V. All rights reserved.
0

S. Roy, P. Paul, M. Karar et al.
Journal of Molecular Liquids 326 (2021) 115293
HARB) using a green-chemical approach. Subsequently, the chemos-
spectra were recorded on Hitachi F-7000 spectrofluorometer. High res-
olution mass spectra were recorded using a Q-tof-micro quadruple
mass spectrometer. The pH values of the solutions were measured by
Labman pH meter at room temperature. Elemental analyses were per-
formed on a Perkin Elmer 2400 CHN microanalyser.
−
−
ensors have been efficiently utilized in relay detection of F and HSO
4
anions in acetonitrile (MeCN) medium. Further, the binding aspects
1
are established through H NMR analysis and extensive density func-
tional theoretical (DFT) computations. The most attractive part of the
present report lies in the fact that structurally simple, easily synthes-
izable and cost-effective hydrazine functionalized simple Schiff bases
can be synthesized just in an undergraduate laboratory to detect both
2.4. Crystal structure determination and refinement
−
−
the F and HSO
4
anions with extreme selectivity and eminent effi-
Single crystal X-ray diffraction data of P17 and HARB were collected
using a Rigaku XtaLABmini diffractometer equipped with Mercury 375R
(2 × 2 bin mode) CCD detector. The data were collected with graphite
monochromated Mo-Kα radiation (λ = 0.71073 Å) at 100.0(2) K
using ω scans. The data were reduced using CrysAlisPro 1.171.38.46
[49] and the space group determination is done using Olex2. The struc-
ture is resolved by dual space method using SHELXT-2015 [50] and re-
fined by full-matrix least-squares procedures using the SHELXL-2015
−
ciency. Both of the chemosensors in presence of F display a pro-
nounced yellow colour in MeCN and the naked eye colour change may
be observed effortlessly with millimolar solutions.
2. Experimental
2.1. Chemicals, solvents and starting materials
[51] software package through OLEX2 suite.
Highly pure o-vanillin (Aldrich, USA), salicylaldehyde (SRL, India)
and hydrazine (TCI, Japan) were procured from the respective commer-
cial sources. All the anions in the form of tetrabutylammonium salts
2.5. Anion sensing experiments
(
TBAX) were purchased from Sigma-Aldrich and were used without
Stock solutions of 1.2 mM chemosensors, 10 mM TBAB, and 10 mM
TBAF in MeCN were prepared. Other stock solutions of similar anions
(tetrabutylammonium salts) were also prepared accordingly in MeCN.
About 2400 μL of solvent (pure MeCN) was taken first in a Teflon stop-
pered quartz cuvette, then 2 μL probe was added, stirred on a magnetic
stirrer, thermally equilibrated to 300 K. Afterward, appropriate volume
fractions of the respective tetrabutylammonium salt stock solutions of
bisulphate or/and fluoride or/and others were added as required to
the final mixture, stirred well and the absorbance spectra were re-
corded. These steps were repeated until spectral saturation.
further purification. All the solvents used in the present experiments
were spectroscopic grade from Spectrochem, India. All the reagents
and solvents were of commercially available A.R grade quality.
2
.2. Preparation of Schiff bases/chemosensors (P17 and HARB)
The Schiff bases were synthesized under solvent-free conditions
using a green-chemical approach following a reported procedure
46–48]. Both P17 and HARB were synthesized by mixing of o-vanillin
6.190 g, 40.1 mM or salicylaldehyde (4.820 g, 39.99 mM), respectively,
[
(
The chemosensory responses of the receptors were examined
through spectrophotometric spectral analyses upon the addition of
trace amounts of TBAX within MeCN medium. The selectivity for both
with hydrazine in (0.640 g, 19.98 mM) in a 250 mL bi-necked flat bot-
tom flask under the solvent-free condition at 40 °C with slow stirring
for ~4 h. Both the reaction produced bright yellow crystalline products.
−
−
4
the receptors towards F followed by HSO anions over other anions
The yellow crystalline compounds were isolated and dried over CaCl
2
.
of TBA salts was studied in detail [52,53]. All the spectrophotometric
analyses were carried out in triplet. The spectrofluorimetric responses
of the chemosensors were studied in the absence and presence of the
TBAX salts in MeCN medium. P17 was found as a fluorescence inactive
molecule both in the presence and absence of TBAX salts. Noteworthy,
HARB displayed poor fluorescent behaviour in MeCN. Although, in pres-
ence of TBAF, HARB exhibited improved fluorescence intensity.
TLC of the reactions was performed using DCM solution of the crystal-
line yellow products which strongly recommend the formation of a
single-phase pure products for both the reactions. The yields were ob-
tained as 6.13 g (89.7%) for P17 and 4.83 g (88.4%) for HARB. The single
crystals of the compounds P17 and HARB were obtained from the satu-
rated methanol solution of the compounds following the slow evapora-
tion technique at room temperature.
Anal cal. For C14
H
16
N
2
O
2
(P17): C, 63.99; H, 5.37; N, 9.33; Found: C,
2.6. Colorimetric test
−1
6
4.06; H, 5.41; N, 9.39. IR (KBr pellet, cm ): 3437(vOH), 1612 (vC=N);
−4
UV–Vis (1 × 10 M, λmax(abs), nm, MeCN): 227(0.419), 312(0.504),
The chemosensors, P17 (0.0340 g, 1 mmol) and HARB (0.030 g,
1 mmol) were dissolved in MeCN (10 mL) separately to get 0.1 mM so-
lution. The colourimetric tests were designed and developed by the ad-
dition of a drop of TBAF solution in acetonitrile (1 mM) to the
chemosensor solution in MeCN [54]. The colour change (instant
yellowish-red colour) was observed and recorded with a digital camera
in slow-motion mode. The TBAX salts containing different anions were
also dissolved in MeCN (10 mL) to set up the concentration to
0.1 mM. No changes of colouration were observed for other anions.
1
3
(
(
66(0.171); H NMR (CDCl
m, 6H), 3.93 (s, 3H) ppm; C NMR (300 MHz, DMSO‑d
HC=N); 149.82 (Ar-OCH ); 148.48 (Ar-OH); 124.19 (Ar-N=C);
24.19, 119.57, 117.48, 115.24, 114.40, 112.07 (Ar–C); 56.35 (-OCH );
HRMS (m/z): 300.09.
Anal cal. For C14
C, 70.03; H, 5.01; N, 11.69. IR (KBr pellet, cm ): 3421(vOH), 1630(vC=
3
) δ = 11.56 (s, 2H), 8.70 (s, 2H), 6.90–7.01
13
6
): 164.97
3
1
3
12 2 2
H N O (HARB): C, 69.99; H, 5.03; N, 11.66; Found:
−1
−
4
N
); UV–Vis (1 × 10
M, λmax(abs), nm, MeCN): 221(0.584), 293
1
(
6
0.663), 355(0.590); H NMR (CDCl
.9–7.5 (m, 8H) ppm; 13C NMR (300 MHz, DMSO‑d
N); 158.96 (Ar-OH); 132.80,131.60 (Ar-N=C); 119.23, 117.41, 116.38,
Ar–C); HRMS (m/z): 240.52.
3
) δ = 11.23 (s, 2H), 8.90 (s, 2H),
6
): 163.66 (HC=
2.7. Computational details
(
All mentioned computational calculations were executed using
Gaussian 09 W programme suite [55] ignoring symmetrical constraints
and GaussView 5.0 [55] was used as user-interface. The ground state
and excited state calculations were performed employing density func-
tional theory (DFT) and time-dependent density functional theory (TD-
DFT) method respectively. Moreover, widely used B3LYP computational
model was compiled with 6-311G basis set [56]. Initially, all the probes,
anions and probe-anionic complexes were optimized in the gas phase
(ε = 1.0) and then in MeCN (ε = 35.688) employing integral equation
formalism polar continuum model (IEFPCM) [57–60] solvent model.
2
.3. Physical measurements
Infra-red spectra of P17 and HARB were recorded with a FTIR-8400S
−
1
SHIMADZU spectrophotometer in the range of 400–3600 cm with
1
13
KBr pellet. H and C NMR spectra of the compounds were obtained
on a Bruker Advance 400 MHz spectrometer at 25 °C in CDCl . All the
3
ground state absorption and spectrophotometric titrations were re-
corded on a JASCO V-730 UV–Vis spectrophotometer. Fluorescence
2

S. Roy, P. Paul, M. Karar et al.
Journal of Molecular Liquids 326 (2021) 115293
appearance of strong peaks at 1621 and 1630 cm− indicated the pres-
1
ence of azomethine groups for P17 and HARB respectively. Other strong
−
1
−1
peaks at 1462 and 1256 cm for P17, and 1491 and 1280 cm for
HARB were attributed to phenoxo (–OPh) groups in the probes. These
characteristic peaks resemble well the reported values of the corre-
sponding functional groups [67,68].
The electronic spectra of P17 and HARB were also recorded in MeCN
in the 200–900 nm range. The UV–Vis spectrum of P17 displayed the ab-
sorption bands at 227, 312, and 366 nm while HARB showed the absorp-
tion bands at 221, 293, and 355 nm (Fig. S3). The observable electronic
bands within 320 nm may be attributed to intra-ligand π-π*/n-π* elec-
tronic transitions for both the probes. On the flip side, the optical bands
at 366 and 355 nm for P17 and HARB are originated from intra-ligand
charge transition. The reported electronic bands for P17 and HARB
were in good agreement with the reported values [69].
Scheme 1. Preparative route of Schiff bases, P17 and HARB under the solvent-free
condition.
The global minimum of each structure was confirmed through stability
calculation and IR frequency check-up with no imaginary frequency. For
binding energy calculations of the probe-anionoic complexes in vac-
uum, correction due to basis set superposition error (BSSE) was consid-
ered employing a counterpoise method [61–63]. The images of
molecular electrostatic potential (MEP) plots and frontier molecular or-
bitals (FMOs) were extracted from corresponding check point files. Fur-
ther details of computational methods could be found somewhere else
The NMR spectral analysis of the probes P17 and HARB was carried
3
out in CDCl using tetramethylsilane (TMS) as an internal standard.
1
13
Figs. S4-S7 displayed the H and C NMR spectra of P17 and HARB re-
spectively. It was observed that a sharp proton signal (singlet) assign-
able to phenolic-OH appeared at 11.56 and 11.21 ppm for P17 and
HARB respectively (Figs. S4,S6). Another characteristic singlet appeared
at 8.70 and 8.90 ppm for P17 and HARB which corresponds to the pro-
ton for azomethine group. Other characteristic signals for the
aromatic–H of both P17 and HARB correlated with the signals found in
6.90–7.01 and 6.9–7.5 ppm respectively. A singlet at 3.93 ppm was as-
signable to methoxy proton of P17 (Figs. S4,S6).
[
64,65]. GaussSum [66] program was used to calculate the fractional
contributions of various groups to each molecular orbital for the
complexes.
3. Results and discussion
The signal and splitting patterns in 13C NMR spectra (Figs. S5,S7) en-
sure that azomethine-C produced the signal at 164.97 and 163.66 ppm
for P17 and HARB respectively. Noteworthy, one signal appeared for
ipso carbon of P17 and HARB correspond to Ph–OH group at 148.48
and 158.96 ppm. Other characteristics signals assignable to aromatic
carbons in 13C spectra were observed in the range of 124.19–112.07
and 119.23–116.38 ppm. The signals appeared in proton and carbon
NMR spectra index well with the previously reported data [70].
High-Resolution (HR) mass spectral study of the synthesized probes
in MeCN was in good agreement with the theoretical molecular mass of
the compounds (Figs. S8, S9) and confirmed the solution stability of the
probes. P17 and HARB displayed the characteristic base peaks at m/z
300.0924 and 240.5225 a.m.u respectively.
3.1. Synthesis of the chemosensors (P17 and HARB)
The Schiff bases P17, and HARB were synthesized under the solvent-
free condition by the reaction between hydrazine and carbonyl com-
pounds (o-vanillin and salicylaldehyde) separately. The bright yellow
crystalline products were recrystallized in methanol using a slow evap-
oration technique. The synthetic route for the Schiff bases is shown in
Scheme 1.
3.2. Spectroscopic characterization of P17 and HARB
Different spectroscopic and analytical techniques were employed to
establish the structural compositions of the probes. The formulations of
the Schiff bases (P17 and HARB) were determined by infrared, ultra-
violet visible, nuclear magnetic resonance, and high-resolution mass
spectrometry spectral analysis. The probes were well soluble in acetoni-
trile, dichloromethane, etc.
3.3. Description of crystal structures of P17 and HARB
Single crystal X-ray diffraction study revealed that both the Schiff
The functional sites of P17 and HARB were examined by careful anal-
ysis of the stretching vibrations of the corresponding functional groups
through infrared spectra. The IR spectra of P17 and HARB (Fig. S1 and
S2) exhibited important broad peaks at 3411 and 3421 cm−1 which
were assignable to phenolic-OH groups in the probes respectively. The
1
bases, P17 and HARB crystallize in monoclinic crystal system with P2 /
n space group. The thermal ellipsoidal plot of P17 and HARB are
shown in Figs. 1 and 2. The structural refinement parameter is summa-
rized in Table 1. The bond distances and bond angles of P17 and HARB
are presented in Tables 2 and 3 respectively. Both the hydrazine
Fig. 1. Thermal ellipsoidal plot of P17.
3

S. Roy, P. Paul, M. Karar et al.
Journal of Molecular Liquids 326 (2021) 115293
Fig. 2. Thermal ellipsoidal plot of HARB.
Table 1
3.4. Spectrophotometric relay recognition of fluoride and bisulphate ions
Structural refinement parameter for P17 and HARB.
The binding propensity of the chemosensors P17, HARB towards the
anions of tetrabutylammonium salts (TBAX) were examined in pure
MeCN medium at room temperature. The binding properties were ex-
plored through detailed spectrophotometric investigation. The elec-
tronic spectra of P17 displayed the optical bands at 227, 312, and
Parameters
P17
HARB
Empirical formula
Formula weight
Temperature (K)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
C
16
H
16
N
2
O
4
14 12 2 2
C H N O
240.26
100.0(2)
Monoclinic
P21/n
8.3572(5)
6.2970(4)
11.6179(6)
90
106.749(4)
90
585.46(6)
2
300.31
100.0(2)
Monoclinic
P21/n
6.0167(7)
18.2464(17)
6.8349(7)
90
107.280
90
3
66 nm (Fig. S3) while the other analogue HARB exhibited the absorp-
tion bands at 221, 293, and 355 nm (Fig. S3). Upon gradual addition of
TBAF from 0 to 24.3 μM concentration within P17 solution in MeCN me-
dium, a new absorption band with increasing absorbance was initiated
to develop at 472 nm with the appearance of three distinct isobestic
points. Fig. 3a presents the absorption spectra of P17, treated with
TBAF in MeCN. With the progressive rise in the absorption band at
α
β
γ
Volume (Å3)
Z
ρ (gcm–3)
716.49(14)
2
1.392
4
72 nm, the resultant pale yellow solution of P17 became intensified
1.363
μ (mm–1)
F (000)
θ ranges (°)
Number of unique reflections
Total number of reflections
Final R indices
Largest peak and hole (eA°-3)
0.101
316
3.3–32.8
2531
8269
0.093
252
3.0–27.6
1453
3712
from yellow to reddish-yellow. In the course of spectrophotometric ti-
tration, the electronic band at 314 nm quenched drastically and another
band at 355 nm almost vanished after the completion of the titration
(Fig. 3a). The observed isobestic points at 282, 345, and 397 nm in the
0.0660, 0.2185
0.72, −0.29
0.050, 0.1167
0.29, −0.28
course of titration suggested that the interaction between P17 and
TBAF proceeded through a single component reaction.
On the flipside, HARB on interaction with TBAF in pure MeCN under
identical reaction conditions developed a new optical band at 462 nm
with a concurrent decrease of absorbance values of the existing elec-
tronic bands at 294 and 355 nm (Fig. 4a). Three distinct isobestic points
were also observed in the course of a spectrophotometric titration be-
tween HARB and TBAF within the concentration range of 0–29.0 μM.
The isobestic points at 283, 319, and 387 nm indicated a dynamic equi-
librium was also established between the chemosensor, HARB, and its
functionalized Schiff bases exist in a locked state with strong intramo-
lecular H-bonded interactions and adopt anti conformation. In P17,
the azomethine bond distance between C8 and N1 atoms was found
as 1.289 Å while N1-N1a bond distance value was noted as 1.403 Å.
The bond distance values correspond to a pure double and single bond
character for the bonds C8-N1 and N1-N1a in P17 (Tables 2). The
same structural parameter for C4-N2 and N2-N2a in HARB was found
as 1.288 Å and 1.4006 Å respectively (Tables 3). The presence of an ad-
ditional methoxy group in P17 makes a little deviation in bond distances
and bond angles between P17 and HARB.
−
HARB-F complex. These ratiometric spectral changes might be utilized
−
qualitatively and quantitatively to detect F in the real samples. In the
MeCN medium, the naked eye colour change from yellow to reddish-
yellow, upon addition of TBAF within HARB was also observed.
Table 2
Selected bond distances (Å) and angles (°) for P17.
XRD
DFT
XRD
DFT
XRD
DFT
Bond distances (Å)
C1-O1
C7-O1
C2-O2
C8-N1
1.3708(19)
1.436(2)
1.3568(18)
1.289(2)
1.3860
1.4511
1.3632
1.3062
N1-N1a
C1-C6
C1-C2
C2-C3
1.4036(18)
1.384(2)
1.410(2)
1.406(2)
1.4053
1.3907
1.4146
1.4167
C3-C4
C3-C8
C4-C5
C5-C6
1.410(2)
1.456(3)
1.380(3)
1.401(3)
1.4172
1.4442
1.3815
1.4081
Bond angles (°)
C1-O1-C7
N1a-N1-C8
O1-C1-C6
C2-C1-C6
C4-C5-C6
115.95(13)
112.98(14)
124.87(16)
120.25(14)
120.38(18)
118.74
115.17
124.86
119.85
120.02
O1-C1-C2
O2-C2-C3
C1-C2-C3
O2-C2-C1
C1-C6-C5
114.88(12)
122.85(15)
119.11(13)
118.04(13)
120.28(17)
115.28
122.23
119.48
118.28
120.73
C2-C3-C8
C4-C3-C8
C2-C3-C4
C3-C4-C5
N1-C8-C3
121.54(14)
118.46(15)
120.00(16)
119.90(17)
121.88(16)
121.15
119.37
119.46
120.44
122.09
4

S. Roy, P. Paul, M. Karar et al.
Journal of Molecular Liquids 326 (2021) 115293
Table 3
Selected bond distances (Å) and angles (°) for HARB.
XRD
DFT
XRD
DFT
XRD
DFT
Bond lengths (Å)
C5-O1
C4-N2
N2-N2 a
C3-C5
1.354(2)
1.288(2)
1.4006(18)
1.415(2)
1.368
1.306
1.405
1.422
C3-C6
C3-C4
C5-C8
C6-C7
1.398(3)
1.446(3)
1.390(3)
1.379(3)
1.413
1.443
1.399
1.386
C7-C9
C8-C9
1.396(3)
1.377(3)
1.405
1.390
Bond angles (°)
N2a-N2-C4
C4-C3-C5
C5-C3-C6
C5-C8-C9
113.25(14)
121.87(16)
118.20(16)
120.33(16)
115.09
121.68
118.49
120.80
N2-C4-C3
O1-C5-C8
O1-C5-C3
C7-C9-C8
120.88(15)
118.14(15)
121.86(16)
120.60(17)
121.85
118.16
121.78
120.79
C3-C5-C8
C3-C6-C7
C6-C7-C9
120.00(16)
121.58(16)
119.28(17)
120.04
121.21
119.37
Most captivatingly, upon addition of other competitive anions to
the chemosensors, P17 and HARB in the MeCN medium didn't
display any noticeable changes in absorption spectra. Further
expansion of our chemosensing approach with the addition of
tetrabutylammonium bisulphate (TBAB) to these fluorinated com-
HARB, 1H NMR titration with TBAF was carried out in CDCl
3
. It was
−
evident that upon addition of 1 equivalent of F , the proton signal
corresponds to the phenolic-OH of P17 and HARB disappeared
−
completely (Fig. 5). This experiment ensured that F makes an effec-
tive interaction with phenolic O–H protons of P17 and HARB. In con-
trast, other proton signals shifted upfield due to hydrogen bonding
interaction increasing the electron density of the o-vanillin/
salicylaldehyde moieties (through-bond effect) [73–75]. The chemi-
cal shift of phenolic O–H protons in P17 was at 11.56 ppm which cor-
responds to the formation of intramolecular hydrogen bond
between phenolic O–H protons and imine. The phenolic O–H protons
−
plexes (P17–F and HARB-F ) result in promising aspects in dual-
ion recognition. Interestingly, with selective addition of TBAB to
the MeCN solution of these TBAF treated chemosensors, the newly
developed absorption bands at 472 and 462 nm for P17–F and
−
HARB-F systems, respectively, started to decrease and ultimately
disappeared to lead the original absorption spectra for the pure
−
−
P17 and HARB (Fig. 3b and 4b). Thus, upon the addition of HSO
4
disappeared completely upon the addition of F anion, and the other
ion within the fluorinated chemosensors, pure states of P17 and
HARB was restored.
H signals moved upfield significantly which may point to the
through-bond electronic propagation effect induced by the hydro-
1
The association constant of the chemosensors, P17 and HARB
gen bonding (Fig. 5). From the UV–Vis and H NMR titrations spectra,
with F ion was determined to be 2.54 × 104
−
M
−1
and 2.56 ×
possible binding models of chemosensors P17 and HARB with F
was proposed, which was shown in Scheme 2. Upon binding of F
−
4
M−1, respectively, using the Benesi–Hildebrand method
−
1
0
[
71,72] considering a 1:1 stoichiometry (Fig. 3a inset and Fig. 4a
inset, respectively). To check the reversibility of the responses, a
protic solvent like H O was used to make the solution of the
chemosensors. Interestingly, when a small amount of water (1%, v
) was added, the colour of the solutions of the fluorinated receptors
anions, the -C=N- isomerization of the chemosensors P17 and
HARB were restricted and the ICT (intramolecular charge transfer)
interaction between electron-rich imine (C=N–) and electron-
deficient fragments was enhanced, which might be responsible for
the observed spectral behaviours. The populations of electrons on
the whole conjugate of the chemosensors may result in fluorescence
2
%
P17 and HARB changed back to colorless which suggested that the
protic solvents destroyed the hydrogen bonding between
chemosensors P17 and HARB and F anion. The above results indi-
cated that the interaction between the chemosensors, P17 and
HARB and F anion could be hydrogen bonding interaction. To un-
−
enhancement. As for chemosensors HARB, about 10 equiv. of F
−
needed to destroy the internal hydrogen bond between phenolic
OH protons and imine nitrogen which induced the conformational
switching of C=N that might result in the sensitive fluorescence re-
sponse of the receptor, HARB (Fig. S10).
−
derstand the recognition properties of the chemosensors, P17 and
Fig. 3. (a). Changes in absorption spectra of P17 upon gradual addition of TBAF salt, [P17] = 50 μM, at 298 K [Inset: B \\ H plot of P17 with addition of TBAF considering 1:1 binding
stoichiometry]; (b). Changes in UV–Vis spectra of the TBAF-treated P17 upon gradual addition of TBAB in MeCN.
5

S. Roy, P. Paul, M. Karar et al.
Journal of Molecular Liquids 326 (2021) 115293
Fig. 4. (a). Changes in absorption spectra of HARB upon gradual addition of TBAF salt, [HARB] = 50 μM, at 298 K [Inset: B \\ H plot of HARB with addition of TBAF considering 1:1 binding
stoichiometry]; (b). Absorption spectra of the mixture of HARB-TBAF upon gradual addition of TBAB in MeCN.
−
−
−
3−
−
3.5. Selectivity studies
ineffective to brought significant spectral changes of both the
The selectivity of P17 and HARB was verified towards F− ion over
chemosensors, P17 and HARB. In the contrast, with the immediate addi-
−
other competitive anions. The ground state absorption was measured
separately under identical reaction conditions (Fig. S11a and S11b). Re-
ports on the individual selectivity of the receptors towards different
ions were many. However, the selectivity and sensitivity of the receptor
molecule are well justified if it can detect a particular anion with differ-
ent harsh chemical environments. For the real purpose, the receptor
molecule must pose the ability to detect a particular ion in presence of
other ionic components within the same mixture. Keeping in mind
tion of F ion to the respective mixture of anions, new absorption spec-
tra were generated (Fig. S11a and S11b). Therefore, from the selectivity
studies of P17 (Fig. 6) and HARB (Fig. S12) towards TBAF, the
chemosensors may be assigned as excellent selective and sensitive col-
−
orimetric chemosensors for F ion.
3.6. Computational study on binding aspects of fluoride
−
this fact, we studied the selectivity of P17 and HARB towards F , in pres-
3.6.1. Potential energy scan study
ence of all other competitive anions. Without any uncertainty from the
designed spectroscopic selectivity studies (Fig. 6 and S12), it was quite
The molecular structures of the chemosensors were optimized theo-
retically in vacuum and MeCN to understand the trans orientation of the
Fig. 5. 1H NMR titration represents gradual addition of TBAF to P17 in CDCl
medium [P17] = 49 μM; [X]Final = 25 μM].
3
6

S. Roy, P. Paul, M. Karar et al.
Journal of Molecular Liquids 326 (2021) 115293
Fig. 6. Selectivity plot of P17 towards F− over other competitive anions and their mixture
in MeCN medium at 298 K; [P17] = 49 μM; [X]Final = 25 μM]
Fig. 8. Potential energy scan of P17 along potential energy (au) vs dihedral angle (Å),
obtained using DFT/B3LYP/6-311G theoretical model and IEFPCM/MeCN solvent system.
two –OH bonds for P17 and HARB. The optimized structure of P17
(
Fig. 7) and HARB (Fig. S13, ESI) are presented in the respective section.
3
.6.2. Binding energy
The optimized bond lengths and bond angles well reproduced the XRD
parameters which provide additional support in favour of trans orienta-
tion of two –OH groups (Table 2 and Table 3). Thereafter, the potential
energy scan study was performed theoretically to find out the origin be-
hind the existence of specific trans orientation of P17 and HARB. This
study was performed by varying the dihedral angle (C11-N13-14 N-
In order to verify the selectivity of P17 and HARB towards F− ion
over other anions, binding energy analysis was also carried out. Binding
energy calculations of different chemosensors-anionic complexes were
performed initially in the gas phase (along with BSSE correction) and
then in MeCN medium by following Eq. (1).
1
5C) from −220° to +220° and plotting against the potential energy
EBind ¼ EComplex−ðEProbeþEAnion
Þ
ð1Þ
of the chemosensors in MeCN medium. The plot of variation of potential
energy with dihedral angle for P17 (Fig. 8) and HARB (Fig. S14, ESI)
molecules displayed a similar type of variation. The plots displayed
that potential energy becomes minimum when the dihedral angle
along C11-N13-14 N-15C are −180 and + 180. Also, there were two
saddle points when C11-N13-14 N-15C dihedral angles are −90
and + 90. Also, the stability of both the probes was maximum when
the dihedral angle (C11-N13-14 N-15C) was zero. This is because in
both the molecules the two hydrogen atoms of –OH group involve in in-
tramolecular hydrogen bonding with two nitrogen atoms. The extent of
hydrogen bonding becomes maximum when the two –OH moieties re-
main in trans position. At trans position due to maximum co-planarity,
the extent of conjugation is maximum. This promotes the hydrogen
bonding interaction to be maximum.
where EBind denotes the binding energy of the probe-anion complex,
E
E
Complex denotes the optimized energy of the probe-anion complex,
Probe denotes the optimized energy of the probe and EAnion denotes
the optimized energy of the anion. The calculated binding energy values
for P17 and HARB with different anions are presented in Tables 4. The
−
BSSE corrected binding energy values of P17–F and HARB-F complexes
in gas phase were remarkably high than other probe-anionic com-
plexes. This fact theoretically established the selectivity of both P17
and HARB towards fluoride ion over other competitive anions. The
−
BSSE corrected binding energy values of P17–F and HARB-F complexes
−1
in gas phase were − 232.7 and − 230.1 kJ mol respectively. Now, for
−
1:2 binding between the probe and F
,
it is quite expected to show al-
most double binding energy in comparison to the binding energy of
Fig. 7. Optimized geometry of P17 obtained using DFT/B3LYP/6-311G theoretical model and IEFPCM/MeCN solvent system (The dotted lines denote the hydrogen bonds, each atoms are
marked with serial number and symbol).
7

S. Roy, P. Paul, M. Karar et al.
Journal of Molecular Liquids 326 (2021) 115293
Table 4
3.6.3. Bond length variation
Interaction energies (kJ mol−1) of P17-anionic and HARB-anionic complexes in vacuum
and acetonitrile medium obtained using the B3LYP/6-311G method (BSSE corrected ener-
gies are in the parenthesis).
Bond length variation is an eminent tool to study the hydrogen bond-
ing interaction or proton transfer process and both hydrogen bonding
and proton transfer as literally both of them give similar type of bond
length variation but differ in the extent of variation. The bond length var-
iations of P17 and HARB are presented in Table 5 and Table S1. The –OH
bond length of P17 and HARB will be stretched upon interaction with an-
ions and the highest interacting anion will show the maximum shift. On
the other side, the strongest interacting anion will construct the shortest
Complex
Vacuum
MeCN
Complex
Vacuum
MeCN
Binding energy (kJ mol−1)
P17-F
−
−119.6 HARB-F−
−308.5
−232.7)
−67.4 (−63.4)
−66.2(−57.9)
−299.3
(−230.1)
−113.6
(
P17-Cl−
−1.6
−3.5
HARB-Cl−
−64.7 (−60.8)
−63.2 (−55.3)
−114.5 (−85.7) −32.4
−60.5 (−47.2) −15.4
−81.6 (−60. 4) −20.4
−35.9 (−33.2) −28.5
−1.4
−3.3
−
−
P17-Br
HARB-Br
−
P17-AcO−
−131.3 (−92.5) −31.1
HARB-AcO−
H-X distance (X = F, Cl, Br, etc). The –OH bond length of P17 (HARB) in
−
−
P17-HSO
P17-H
P17-PO
4
−36.9 (−50.7)
−85.5 (−64.1)
−48.8 (−44.2)
−24.4
−20.8
−34.6
HARB-HSO
4
MeCN was 1.006 Å (1.003 Å) that became maximumly elongated upon
−
−
2
PO
4
HARB-H
2
PO
4
−
interaction with F ion to 1.441 Å (1.426 Å). What's more, the H–F dis-
3
−
3−
4
HARB-PO
4
tance squeezed to almost a real H–F bond to 1.020 Å (1.027 Å). Jeffery
et al. [77] documented that hydrogen bond distance between 1.2 and
1
.5 Å denote strong hydrogen bonding and hydrogen bond length
below 1.2 Å indicate the possibility of proton transfer. Hence, the bond
length variation data clearly released the possibility of proton transfer
from P17/HARB to F in the ground state.
1
:1 probe-anion complex [76]. However, the BSSE corrected binding en-
−
−
ergy values of P17–2F and HARB-2F complexes in gas phase
−1
were − 272.3 and − 268.2 kJ mol respectively. These data clearly in-
dicated the 1:1 binding stoichiometry is more favourable over 1:2.
Moreover, the binding energy value of P17–F was found to be larger
than HARB-F complex denoting better binding interaction between
P17 and F over HARB and F .
3.6.4. Frontier molecular orbital analysis
−
Thereafter, the frontier molecular orbital (FMO) images of P17, HARB,
−
−
and their fluoride complexes (Fig. 9 and Fig. S15, ESI) were depicted to
understand the binding interaction. The energy of HOMO-LUMO energy
gap of P17 and HARB were 3.55 and 3.72 eV respectively. The HOMO-
−
LUMO energy gap of P17–F and HARB-F complexes were 3.17 and
3
.20 eV respectively. Such decrease of HOMO-LUMO energy gap obvi-
Table 5
−
ously denotes strong binding interaction between the probe and F ion.
Also, the decrease of HOMO-LUMO energy gap clearly justified the red
shift of absorption maxima observed in the experimental findings.
Moreover from Table 6 and Fig. 9, it was observed that the contribu-
tion of hydroxyl oxygen atoms to HOMO sharply increased from probe
O \\ H and H-X bond length variations of P17 and their anionic complexes in gas phase and
MeCN.
Complex
Vacuum
O-H
MeCN
O-H
H-X
H-X
−
Bond length (Å)
P17
P17-F
to probe-F complex (17% to 27% for P17 and 17% to 28% for HARB)
1.002
1.561
1.005
1.004
1.495
1.006
1.004
1.122
–
1.006
1.441
1.007
1.006
1.085
0.999
1.009
1.190
–
denoting deprotonation of one hydroxyl group and involvement in con-
jugation impeding the C=N isomerisation. Thus, from frontier molecu-
lar orbital analysis, it could be stated that the strongly basic F− ion
−
0.994
4.076
4.008
1.043
2.155
1.787
1.950
1.020
4.147
4.115
1.393
2.422
3.264
1.987
−
P17-Cl
−
P17-Br
P17-AcO−
captured acidic hydroxyl proton and restricted the C=N isomerisation
−
as well as ICT character. Consequently, upon binding with F− ion, the
P17-HSO
P17-H PO
2 4
P17-PO
4
−
probes displayed a sharp red shift of absorption maxima. Thereafter,
the addition of HSO
3
−
4
−
4
facilitates the reprotonation of the probe with
Fig. 9. The HOMO-LUMO images of P17 and P17-F− complex with corresponding energy values calculated using MeCN/IEFPCM solvent model.
8

S. Roy, P. Paul, M. Karar et al.
Journal of Molecular Liquids 326 (2021) 115293
Table 6
recovering the absorption band of the pure probe. Based on the experi-
mental observations and computational results, we could propose the
following mechanism (Scheme 2). Here, actually, the switching of the
probes from neutral to deprotonated forms was monitored by fluoride
and bisulphate ions and resulting corresponding sensing phenomenon.
Energy and % of composition of different group of atoms in FMO's for P17, P17 \\ F, HARB
and HARB-F− complexes in MeCN.
Compound FMO's
Energy
eV)
% Composition
(
N
Hydroxyl
O
Hydroxyl
H
C
Other
Atoms
F
4
. Conclusions
P17
LUMO
−0.15
−0.30
−1.22
0
3
3
3
0
0
0
95
94
65
1
3
0
–
–
–
+3
LUMO
0
In summary, we report a green-chemical method for the synthesis of
+2
hydrazine functionalised Schiff base chemosensors (P17, HARB). Both the
structurally simple, easily synthesizable and cost-effective chemosensors
consecutively detected F followed by HSO ions with excellent selectiv-
4
ity over various individuals and a mixture of anions in MeCN medium.
The chemosensors display a visible absorption profile with pronounced
changes of colour from colorless to reddish-yellow at room temperature.
Spectrofluorimetry study suggests that HARB exhibits turn-on fluores-
cence properties in presence of TBAF perhaps P17 remains fluorescent si-
lent in the presence and absence of TBAB. Most amusingly, the TBAF
LUMO
32
+1
−
−
LUMO
HOMO
HOMO–1 −6.37
HOMO–2 −6.61
HOMO–3 −7.34
−2.57
−6.12
14
3
7
11
51 13
4
0
0
0
0
3
0
82
78
76
85
0
2
1
2
–
–
–
–
–
0
17
16
2
25 11
P17-F−
LUMO
+0.45
+0.03
−0.61
1
7
3
3
86
93
67
6
4
1
+3
LUMO
0
0
0
0
0
+2
treated chemosensors displayed a reversible response in presence of
LUMO
29
TBAB. Extensive DFT calculations and 1H NMR spectral analysis
+1
established the mechanistic aspects of sensing and suggest that the
changes in absorption spectra were driven by the hydrogen bonding be-
tween the OH and fluoride anions. The hydroxyl proton captured by F
restricts the C=N isomerisation as well as ICT character for both P17
and HARB, and confirm the cascade sensing mechanism for fluoride and
bisulphate ions. This strategy for the design of the molecules employing
a green-chemical synthetic approach may provide a simple way to de-
velop more efficient dual-channel anion chemosensors.
LUMO
HOMO
HOMO–1 −5.91
HOMO–2 −6.06
HOMO–3 −6.13
−1.96
−5.13
15
8
5
32 35
15
1
5
27
16
0
0
0
1
0
0
79
61
64 15
20 11
65 14
95
96
67
1
4
0
0
0
1
0
–
−
6
4
HARB
LUMO
−0.32
−0.32
−1.21
0
0
0
+3
LUMO
0
4
2
4
0
0
–
–
+2
LUMO
31
14
15 17
1
9
+1
Declaration of Competing Interest
LUMO
HOMO
HOMO–1 −6.65
HOMO–2 −7.00
HOMO–3 −7.33
−2.55
−6.27
0
0
0
0
3
0
82
68
76
84
9
0
0
0
0
9
0
–
–
–
–
–
0
23
7
• All authors have participated in (a) conception and design, or analysis and
interpretation of the data; (b) drafting the article or revising it critically for
important intellectual content; and (c) approval of the final version.
•
•
•
65 14
1
HARB -F−
LUMO
+0.45
−0.13
−0.62
8
4
2
4
91
This manuscript has not been submitted to, nor is under review at, an-
other journal or other publishing venue.
+3
LUMO
1
0
0
95
71
0
0
0
0
+2
The authors have no affiliation with any organization with a direct or in-
direct financial interest in the subject matter discussed in the manuscript
The following authors have affiliations with organizations with direct or
indirect financial interest in the subject matter discussed in the
manuscript:
LUMO
27
+1
LUMO
HOMO
HOMO–1 −6.07
HOMO–2 −6.13
HOMO–3 −6.57
−1.96
−5.15
15
0
0
2
0
0
81
62
13
72
82
0
0
6
0
0
0
0
2
0
0
10 28
38 39
13 15
5
13
Scheme 2. Proposed binding mechanism of P17 with fluoride and subsequently bisulphate ions.

S. Roy, P. Paul, M. Karar et al.
Journal of Molecular Liquids 326 (2021) 115293
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Acknowledgments
BB sincerely thanks Science and Engineering Research Board (SERB),
India for financial support under the TEACHERS’ ASSOCIATESHIP FOR
RESEARCH EXCELLENCE (TAR/2018/000473). The authors also sincerely
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