Ether based flexible bis Schiff base fluorescent colorimetric chemosensors for selective and sensitive detection of HF2 ? ion

Article abstract of DOI:10.1016/j.jphotochem.2019.112179

Two simple and low cost bis Schiff base fluorescent colorimetric chemosensors L1 and L2 have been designed and synthesized. These sensors could detect important anion HF₂ ^? by change in both absorption and fluorescence intensity in methanol tris HCl buffer (1:1 v/v) solution at physiological pH. The mechanism of selectivity and sensitivity of these receptors to HF₂ ^? has proposed to be on the basis of photo-induced electron transfer (PET), chelation enhanced fluoresence (CHEF) and hydrogen bonding mechanism, confirmed by FT-IR, ¹H-NMR and ESI-mass spectra. Interestingly, chelation of HF₂ ^? to the receptors takes place in 1:2 ratio, which has been confirmed by job plots analysis, ESI-mass spectra, DFT and TD-DFT studies. The detection limits of the probes L1 and L2 for HF₂ ^? were determined to be 6.1 μM for L1 and 3.94 μM for L2 respectively, on the basis of 3σ/K. Importantly, the low detection limits of L1 and L2 for HF₂ ^?suggest that L1 and L2 are sensitive sensors for detection of these ions in drinking water. The receptors L1 and L2 can successfully applied for detection and quantification of HF₂ ^? in real samples. To the best of our knowledge this is the third report of fluorescent colorimetric chemosensors for HF₂ ^? ions.

Full text of DOI:10.1016/j.jphotochem.2019.112179

Journal Pre-proof
Ether based flexible bis Schiff base fluorescent colorimetric
chemosensors for selective and sensitive detection of HF2- ion
Rukhmani Chandra, Meman Sahu, Amit Kumar Manna, Kalyani
Rout, Shubhamoy Chowdhury, Goutam K. Patra
PII:
S1010-6030(19)31359-0
DOI:
https://doi.org/10.1016/j.jphotochem.2019.112179
JPC 112179
Reference:
To appear in:
Journal of Photochemistry & Photobiology, A: Chemistry
Received Date:
Revised Date:
Accepted Date:
7 August 2019
11 October 2019
16 October 2019
Please cite this article as: Chandra R, Sahu M, Manna AK, Rout K, Chowdhury S, Patra GK,
Ether based flexible bis Schiff base fluorescent colorimetric chemosensors for selective and
sensitive detection of HF2- ion, Journal of Photochemistry and amp; Photobiology, A:
Chemistry (2019), doi: https://doi.org/10.1016/j.jphotochem.2019.112179
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Ether based flexible bis Schiff base fluorescent colorimetric
-
chemosensors for selective and sensitive detection of HF2 ion
Rukhmani Chandra,a Meman Sahu,a Amit Kumar Manna, a Kalyani Rout,a
Shubhamoy Chowdhuryb and Goutam K. Patraa*
aDepartment of Chemistry, Guru Ghasidas Vishwavidyalaya, Bilaspur (C.G)
bDepartment of Chemistry, Gour Banga University, Malda, West Bengal 732
103, India
Graphical Abstract
Two simple and low cost bis Schiff base fluorescent colorimetric chemosensors L1 and L2 have
-
been reported, could HF2 ion by change in both absorption and fluorescence intensity in
methanol tris HCl buffer solution at physiological pH. The detection limits of the probes were
determined to be 6.1 μM for L1 and 3.94 μM for L2 respectively. They can be successfully
-
applied for detection and quantification of HF2 in real samples.

Highlights

Two bis Schiff base fluorescent colorimetric chemosensors L1 and L2 have been
reported
-




L1 and L2 can detect HF2 ion by colorimetrically and fluorometrically
The detection limits are 6.1 μM for L1 and 3.94 μM for L2
-
The receptors can successfully detect and quantify of HF2 ion in real samples
-
this is the third report of fluorescent colorimetric chemosensors for HF2
Abstract
Two simple and low cost bis Schiff base fluorescent colorimetric chemosensors L1 and
-
L2 have been designed and synthesized. These sensors could detect important anion HF2 by
change in both absorption and fluorescence intensity in methanol tris HCl buffer (1:1 v/v)
solution at physiological pH. The mechanism of selectivity and sensitivity of these receptors to
-
HF2 has proposed to be on the basis of photo-induced electron transfer (PET), chelation
enhanced fluoresence (CHEF) and hydrogen bonding mechanism, confirmed by FT-IR, 1H-NMR
-
and ESI-mass spectra. Interestingly, chelation of HF2 to the receptors takes place in 1:2 ratio,
which has been confirmed by job plots analysis, ESI-mass spectra, DFT and TD-DFT studies.
-
The detection limits of the probes L1 and L2 for HF2 were determined to be 6.1 μM for L1 and
3.94 μM for L2 respectively, on the basis of 3σ/K. Importantly, the low detection limit of L1 and
-
L2 for HF2 suggest that L1 and L2 are sensitive sensors for detection of these ions in drinking
-
water. The receptors L1 and L2 can successfully applied for detection and quantification of HF2
in real samples. To the best of our knowledge this is the third report of fluorescent colorimetric
-
chemosensors for HF2 ions.
Keywords: ether-based bis Schiff base; fluorescent sensor; colorimetric sensor; anion sensing;
bifluoride sensor.
*Corresponding Author: Tel.: 91 7587312992, E-mail: patra29in@yahoo.co.in

1. Introduction
The development of sensitive and selective chemosensors, which is capable of
recognition of various biologically and environmentally important anionshas received
considerable attention in recent research [1]. Among the various types of anion sensors,
fluorescent colorimetric chemosensors have attracted much attention due to their high sensitivity,
simplicity, and real time monitoring with rapid response time and without any use of
sophisticated instruments [2, 3]. Furthermore, colorimetric methods are also extremely attractive
because they allow naked-eye detection of showing visible color change [4].
2-
2-
-
−
In the presence of various anion e.g. SO4 , SO3 , HSO3 , F−, OAc−, Cl−, Br−, I−, H2PO4 ,
−
-
−
−
-
HCO3 , HF2 , N3 , NO3 , CN- and CO32- to recognize, sensing and quantifying HF2 is a most
challenging job and it draws much attention of the current researchers [5]. The hydrogen
-
bifluoride anion, HF2 is of considerable structural and theoretical importance. It is a typical
example of three-centre, four-electron bonding, semi-ionic and displayed the strongest known
hydrogen bonds [6]. The hydrogen bifluoride is used in etching borosilicate glasses [7], as
insecticides [8], understanding supramolecular interaction [9], synthesizing borane derivatives
[10], preparation of molten salts [11] and fluorescence detection of beryllium in occupational
hygiene samples [12] etc. There are several number of chemosensors reported in literature for
2-
2-
-
−
−
−
−
various anions like SO4 , SO3 , HSO3 , F−, OAc−, Cl−, Br−, I−, H2PO4 , HCO3 , N3 , NO3 , CN-
-
and CO32- etc. [13]; but for sensing of HF2 only a few number of fluorescence/colorimetric
chemosensors have been reported. To the best of our knowledge there are only two fluorescent-
colorimetric reports on sensing of biflouride ion. Recently, Lakshminarayanan et.al have
reported turn-on colorimetric bifluoride sensor, based on significant colour change and
fluorescent enhancement [14]. Das et. al reported prompt fluorescent detection technique of
bifluoride ion using N1,N3-di(naphthalen-1-yl)isophthalamide [15]. Still, there is an acute
scarcity of bifluoride sensor. So it is of urgent need to design suitable simple and sensitive
bifluoride sensors which can provide real time, onsite analysis of bifluoride ions with high
selectivity and sensitivity.
As a part of our on-going research work [16,17] in the designing and synthesis of
chemosensors for anions, cations and neutral molecules, herein, we report Bis[4-(2-hydroxy-3-
methoxybenzylideneamino)phenyl] ether) (L1) and (Di[(4-phenylimino) 4-diethylsalicyl-
aldehyde] ether) (L2) which are 1:2 condensation products of 4-(4-aminophenoxy) benzenamine
with
2-hydroxy-3-methoxybenzaldehyde
and
4-(diethylamino)-2-hydroxybenzaldehyde

-
respectively. L1 and L2 can act as a fluorescent colorimetric chemosensors for HF2 over a
-
number of anions and recognize HF2 by exhibiting instant change in colour from yellow to
colourless in methanol-tris-HCl buffer (1:1 v/v, 10 mM, pH 7.2) and change in fluorescence
intensity. The strong selectivity and in-field application possibility of the developed
chemosensors have also been verified in this manuscript. In addition, the theoretical calculation
supported the experimental spectroscopic results and the sensing mechanism.
2. Experimental
2.1. General Information
All of the materials used for synthesis were obtained from Sigma Aldrich and used
without further purification. The analytical grade solvents were used for the overall experiments
and freshly prepared double deionized water was used for dilution purpose and preparing
methanol tris HCl buffer (10 µM, pH=7.2) solution. 1HNMR spectra were recorded on a Bruker
DRX spectrometer operating at 400 MHz in CDCl3 and chemical shift were recorded in ppm
relative to TMS. UV–Visible spectra were recorded on a Shimadzu UV 1800 spectophotometer
using 10 mm path length quartz cuvettes with the wavelength in the range of 200–800 nm. The
emission spectra were recorded on a Perkin Elmer LS-55 Fluorescence Spectrometer. Mass
spectra were recorded on a Waters mass spectrometer using mixed solvent methanol and triple
distilled water which was equipped with an ESI source. The pH measurements carried out using
a digital pH meter (Merck). All the chemicals and metal salts were purchased from Merck. All
the anions are of sodium salts and re-crystallized from water (Millipore) before use. Solutions of
the receptors L1 and L2 (5x10-5 M) and anions (5x10-4 M) were prepared in methanol-tris-HCl
buffer (1:1 v/v, pH 7.2) and deionized water respectively.
2.2. X-ray data collection and structure determination
The crystal data for L1 has been collected on a Bruker SMART CCD diffractometer (Mo-
Kα radiation, λ = 0.71073 Å). The program SMART [18] was used for collecting frames of data,
indexing reflections, and determining lattice parameters, SAINT[18] for integration of the
intensity of reflections and scaling, SADABS [19] for absorption correction, and SHELXTL [20]
for space group and structure determination and least-squares refinements on F2. The crystal
structure was solved and refined by full-matrix least-squares methods against F2 by using the
program SHELXL-2018 [21] using Olex-2 software [22]. All the non-hydrogen atoms were
refined with anisotropic displacement parameters. Hydrogen positions were fixed at calculated

positions and refined isotropically. The crystallographic data of L1 has been listed in Table S1.
Structural information of L1 has been deposited at the Cambridge Crystallographic Data Center
(CCDC number 1893984).
2.3. Synthesis of L1
0.20 g (1 mmol) of 4-(4-aminophenoxy)benzenamine dissolved in 30 mL dehydrated
methanol and to this solution 2-hydroxy-3-methoxy benzaldehyde (0.30 g, 2 mmol, in 10 mL
dehydrated methanol) was added drop-wise with constant stirring. The yellow coloured reaction
mixture was refluxed for 6 h at 40oC, maintaining dry conditions. Then the solvent was
evaporated at room temperature. A light yellow precipitate was obtained. It was filtered off and
washed several times with n-hexane and then finally re-crystallized from methanol and dried in
1
air. The solid compound was obtained with 0.37 g (80%) yield; m.p.1150C. H NMR δ (CDCl3,
ppm): 13.61 (s, 2H, -OH); 8.56 (s, 2H, -CH=N); 7.20 (t, 4H, -HAr); 7.02 (d, 4H, -HAr); 6.94 (m,
13
4H, -HAr); 6.81 (t, 2H, -HAr) and 3.87 (s, 6H, -OCH3) (Fig. S1). C NMR (CDCl3, δ ppm,
TMS): δ 161.74, 156.12, 151.17, 148.38, 143.59, 123.62, 122.57, 119.62, 119.04, 118.54,
114.52, 56.11 (Fig. S2). ). ESI-MS (m/z): 468.37 (M+) (Fig. S3). FTIR/cm-1 (KBr): 3516 (s),
2982 (w), 2870 (m), 1621 (vs), 1600 (vs), 1510 (vs), 1500 (vs), 1435 (s), 1386 (m), 1182 (m),
1028 (vs), 971 (m), 746 (s), 613 (s) (Fig. S4). Anal. Calcd. (%) C28H24N2O5: C, 71.71; H, 5.14;
N, 5.93; found: C, 71.78; H, 5.16; N, 5.98.
2.4. Synthesis of L2
To a dehydrated methanol solution (50 mL) of 4-(4-aminophenoxy)benzenamine (0.20 g,
1 mmol), 10 mL of methanol solution of 4-(diethylamino)-2-hydroxybenzaldehyde (0.39 g, 2
mmol) was added. Then the mixture was refluxed for 6 h at 45oC, maintaining dry conditions. A
light yellow precipitate was obtained. The solid compound was obtained with 0.41 g (75%)
yield.; m.p. 1500C. 1H NMR δ (CDCl3, ppm): 13.62 (s, 2H, -OH), 8.41 (s, 2H, -CH=N); 7.22 (m,
4H, -HAr); 7.16 (m, 4H, -HAr); 7.03 (m, 4H, -HAr); 6.25 (d, 2H, -HAr); 6.19 (s, 2H, -OH); 3.41
13
(q, 8H, -CH2) and 1.22 (m, 12H, -CH3) (Fig. S5). C NMR δ (CDCl3, ppm): δ 163.86, 159.90,
155.18, 151.62, 144.38, 143.59, 133.59, 121.95, 119.47, 109.01, 103.64, 97.70, 44.57 and 12.67
(Fig. S6). ESI-MS (m/z): 551.21 (MH+) (Fig. S7). FTIR/cm-1 (KBr): 3434 (m, -OH), 2970 (s),
2687 (w, br) 1708 (s), 1632 (vs, C=N), 1493 (s), 1354 (vs), 1242 (s), 1190 (s), 1132 (s), 839 (s),
786 (s), 530 (s) (Fig. S8). Anal. Calcd. (%) C34H38N4O3: C, 74.29; H, 6.91; N, 10.13; found: C,
74.15; H, 6.96; N, 10.17.

2.5. UV-Vis titrations
L1 (4.70 mg, 0.01 mmol) was dissolved in methanol-tris-HCl buffer (1:1 v/v, pH 7.2, 10
mL) and 30 μL of it was diluted to 3 mL with the solvent mixture to make a final concentration
of 10μM. NaHF2 (6.2 mg, 0.1 mmol) was dissolved in10 mL of triple distilled water and 1.5 – 90
μL of the bifluoride ion solution (10 mM) were transferred to the solution of L (10 μM) prepared
above. After mixing them for a few seconds, UV-Vis spectra were obtained at room temperature.
Similar procedure was performed for L2, taking 5.50 mg (0.01 mmol).
2.6. Fluorescence Titrations
L1 (4.70 mg, 0.01 mmol) was dissolved in 10 mL of methanol-tris-HCl buffer (1:1 v/v)
to make a solution of 1 × 10−3 M and 30 μL of this solution were diluted with 2.97 mL of solvent
mixture to make the final concentration of 10 μM. NaHF2 (6.20 mg, 0.1 mmol) was dissolved in
triple distilled water (10 mL) and 1.5 – 60 μL of this bifluoride ion solution (10 mM) were
transferred to each receptor solution (10 μM) to give 0.5 – 20 equiv. After mixing them for a few
seconds, fluorescence spectra were obtained at room temperature.
Similar procedure was performed for L2, taking 5.50 mg (0.01 mmol) of the probe.
2.7. Competition with other anions
L1 (4.70 mg, 0.01 mmol) was dissolved in afore-mentioned buffer solution (10 mL) and
30 μL of it was diluted to 3 mL with the solvent mixture to make a final concentration of 10 μM.
−
−
−
−
2-
2-
2-
Sodium salts of F−, Cl−, Br−, I−, H2PO4 , NO3 , OAc−, HCO3 , HSO3 , CO3 , SO4 , SO3 , CN-,
-
-
S2 and HF2 (0.1 mmol) were dissolved in 10 mL of triple distilled water and 6 μL of each
solution (10 mM) were added to 3 mL of the solution of receptor L1 (10 μM) to give 2 equiv.of
-
anion conc. Then, 6 μL of HF2 solution (10 mM) were added to the mixed solution of each anion
and L1 to make 2 equiv. After mixing them for few seconds, absorbance spectra were obtained at
room temperature.
Similar procedure was performed for L2, taking 5.50 mg (0.01 mmol) of the probe.
2.8. Job plot measurements
L1 (4.70 mg, 0.01 mmol) was dissolved in afore-mentioned buffer solution (10 mL).100,
90, 80, 70, 60, 50, 40, 30, 20,10 and 0 μL of the L1 solution were taken and transferred to vials.
Each vial was diluted with 2.9 mL of a mixed solvent. NaHF2 (0.01 mmol) was dissolved in
triple distilled water (10 mL). 0, 10, 20, 30, 40, 50, 60, 70, 80, 90 and 100 μL of the bifluoride
ion solution were added to each diluted L1 solution. Each vial had a total volume of 3 mL. After
shaking them for one minute, UV-Vis spectra were obtained at room temperature.
Similar procedure was performed for L2, taking 5.50 mg (0.01 mmol) of the probe.

2.9. pH effect test
The charge distribution of the free receptor L1 and its metal complex solutions were
affected with variation of pH which is responsible for difference absorption behavior of solution.
For this purpose a series of buffers with pH values ranging from 2 to 11 were prepared using HCl
and NaOH solution. After a solution with the desired pH was achieved, receptor L1 (4.70 mg,
0.01 mmol) was dissolved in methanol (10 mL), and then 30 µL of this solution (1 mM) was
diluted to 3 mL with the above mentioned buffers to make the final concentration of 10 μM
-
NaHF2 (6.2 mg, 0.1 mmol). 30 µL of the HF2 solution (10 mM) was transferred to each receptor
solution (10 µM) prepared above. After few seconds of mixing, fluorescence spectra were
obtained at room temperature.
Similar procedure was performed for L2, taking 5.50 mg (0.01 mmol) of the probe.
2.10. Colorimetric test kit
Receptor L1 (4.70 mg, 0.01 mmol) was dissolved in methanol-tris-HCl buffer (1:1 v/v,
1o ml) to get 1 mM solution. Receptor L1-test kits were prepared by immersing filter papers into
2-
receptor L1 solution (1 mM), and then dried in air to get rid of the solvent. Sodium salts (SO4 ,
2-
-
−
−
-
−
−
2-
SO3 , HSO3 , F−, OAc−, Cl−, Br−, I−, H2PO4 , HCO3 , HF2 , N3 , NO3 , CN- and CO3 ; 0.001
mmol) was dissolved in distilled water (10 mL), respectively. The test kits prepared above were
added into different anion solutions (10 mL), and then dried at room temperature.
Similar procedure was performed for L2, taking 5.50 mg (0.01 mmol) of the probe.
2.11. Computational details
The GAUSSIAN-09 Revision C.01 program package was used for all calculations [23].
The gas phase geometries of the molecule L1, L2, L1+4HF and L2+4HF were optimized fully
unrestricting symmetry in singlet ground states with PBEPBE functional [24]. The basis set 6-
31++G(d, p) was employed for all the atoms [25]. The electronic spectrums of the molecules
were calculated with the TD-DFT method and the solvent effect (in methanol) was simulated
using the polarizing continuum model with the integral equation formalism (C-PCM) [26, 27].
3. Results and discussion
3.1. Synthesis and structures of L1 and L2
The chemosensors L1 and L2 were synthesized in single step in more than 75% yield as
yellow crystalline solids via condensation reaction of 4-(4-aminophenoxy)benzenamine with 2-

hydroxy-3-methoxybenzaldehyde and 4-(diethylamino)-2-hydroxy benzaldehyde respectively
(Scheme 1).
Scheme 1. Synthetic procedures of the receptors L1 and L2.
Geometry optimizations starting from the X-ray crystal structures of L1 (Fig. 1) and L2
(Fig.S9) led to a global minimum [28] as stationary point. Selected geometry optimized
parameters are listed in Table S2. The general drift observed in the crystal structure data is well
reproduced. The calculated bond lengths as well as bond angles are in good agreement with the
experimental crystal structures data. In general, a little deviation of the calculated bond distances
and angles from their respective experimental data is observed. The calculated bond lengths of
L1 and L2 deviate within range of 2% from the respective observed crystal data, while the bond
angles are vary within the range of ±3%. Since the optimizations L1 and L2 are conceived under
gas phase model [29].
Fig. 1. ORTEP diagram of L1 showing the atom-labeling scheme and 30% thermal ellipsoids.

The single crystals of the probe L1 (Bis[4-(2-hydroxy-3-methoxybenzylideneamino)phenyl] ether)
were grown by slow evaporation of the methanol solution at room temperature. L1 was
crystallized as monoclinic C2/c space group. The molecule of L1 lies on a twofold rotation axis,
and each half exhibits an imine E configuration and an O—H···N hydrogen bond (Fig. 1). The
dihedral angle between the two benzene rings attached to the central O atom is 69°, and that
between each of these rings and the other benzene ring in the same half of the molecule is about
24°, which makes the molecule flexible. Recently Kaabi et al. reported the single crystal structure
of L2 [28]. The ligand L2 is having a phenol–imine tautomer with eight molecules in each unit
cell.The bond lengths of C-O (phenol) are 1.358 Å (2) and 1.354 Å respectively, which lie in
usual ranges [30].
-
3.2. Colorimetric and spectral response of L1 and L2 towards HF2
Initially the spectroscopic properties of receptors L1 and L2 were recorded in different
solvents of varied polarity at room temperature. As shown in Fig. S10 and summarized in
Table 1, the absorption spectra of L1 and L2 featured a marginal red-shift of absorption maxima
(Δλabs = 10 nm), indicating an apparent solvent dependence of the absorption bands. However all
the sensing studies were performed in methanol-tris-HCl buffer (1:1 v/v) as both the receptors
are stable more than 3 days in methanol-tris-HCl buffer (1:1 v/v).
Table 1. max values of the probes L1 and L2 in different solvents

max in nm in
MeOH

max in nm in
CH3CN

max in nm in
DMSO

max in nm in
DMF
Probe L1
Probe L2
338
434
344
437
349
444
348
442
The spectroscopic properties of receptors L1 and L2 towards various anions as F-, Cl-, Br-,
-
-
-
-
-
-
2-
2-
-
I-, N3 , H2PO4 , NO3 , OAc-, HCO3 , HSO3 , CO3 , SO4 , SO3 , CN-, S2- and HF2 (as their sodium
salts) were investigated in methanol-tris-HCl buffer (1:1 v/v) as shown in Fig. 2. Here, the
receptor L1 and L2 exhibited three absorption bands at 234 nm, 287 nm and 341 nm for L1 and
368, 436 and 470 nm for L1. After the addition of number of test anions, low energy absorbance
band at 341 nm for L1 and 436 nm for L2 exhibited non-significant changes. However, in case of
-
addition of HF2 in both the chromophores L1 and L2 distinctive absorption shifts were observed.

Fig. 2. Absorbance spectrum of L1 and L2 (10 µM) in methanol-tris-HCl buffer (1:1 v/v) in
-
-
-
-
presence of different anions (10 µM) such as F-, Cl-, Br-, I-, N3 , H2PO4 , NO3 , OAc-, HCO3 ,
-
-
2-
2-
-
HSO3 , CO3 , SO4 , SO3 , CN-, S2- and HF2 .
-
Upon addition of HF2 (0 - 5 equiv.) to the receptors L1 and L2 individually, absorption
band of L1 at 341 nm was gradually shifted to shorter wavelength (blue shift) at 311 nm and
absorption band of L2 at 436 nm was gradually shifted to longer wavelength (red shift) at 448
nm. These changes in absorption spectra are also supported by visible changes of both the
-
receptors from yellow to colorless (Fig. 3). Another anion HSO3 may also exhibited some
binding with the receptors, which accompanied by hypsochromic shift of the absorption maxima
at 341 nm for L1 and at 436 nm for L2 and this anion shows very minute colour change from
yellow to light yellow.
L1/
L2
-
-
2-
-
-
-
-
2-
-
-
HCO3 H2PO4 CI-
I-
CO3
Br
NO3 OAc- F-
HF2
N3
SO4 SO32- HSO3 CN- S2
Fig. 3. The colour changes of L1 and L2 upon addition of various anions (up to 5 equiv.) in
methanol-tris-HCl buffer (1:1 v/v).
The sensitivities were determined from the absorption titration method. In the titration
-
experiment, upon increasing the concentration of HF2 separately to L1 and L2, the absorbance
bands at 287 and 341 nm of L1 get decreased with concomitant increase in absorbance band at

234 nm causing well-defined isosbestic points at 275, 245 and 235 nm respectively (Fig. 4A). In
case of L2, the absorbance band at 436 nm gets decreased with concomitant increase in
absorbance band at 368 nm causing well-defined isosbestic points at 473 nm and 376 nm
respectively (Fig. 4B). As a result yellow colour of both the receptors solution L1 and L2
become colourless and 5 equiv. of guest molecule was enough to reach the saturation of
absorption change.
(B)
(A)
Fig. 4. Absorption spectral changes of (A) L1 and (B) L2 (10 µM) in the presence of different
-
concentrations of HF2 ( in ranges 0-5 equiv).
-
3.3. Fluorescence spectroscopic studies of L1 and L2 towards HF2
We have investigated the emission studies of anion binding properties of receptor L1 and
L2 towards different anions (10 equiv.) methanol-tris-HCl buffer (1:1 v/v) (Fig. 5a). The free
receptor L1 showed a broad band emission spectrum with a maximum peak centered at 530 nm.
-
-
-
-
-
-
Among the various anions as a F-, Cl-, Br-, I-, N3 , H2PO4 , NO3 , OAc-, HCO3 , HSO3 , CO3 ,
2-
2-
-
SO4 , SO3 , CN- and S2- examined, L2 showed high selectively towards HF2 anion (Fig. 5b). In
-
the proposed sensing process the HSO3 ion can influence the absorption and fluorescence
behaviour of both the probes L1 and L2. But those were not enough for clear sensing studied
even in 2 fold higher concentration levels.

Fig. 5. Fluorescence spectra of (a) L1 and (b) L2 (50 μM) before and after addition of 10 equiv.
of various anions in methanol-tris-HCl buffer (1:1 v/v).
-
Subsequently, fluorescence titration of L1 (10 μM) by different amount of HF2 (0-10
-
equiv.) was then investigated. With increasing the concentration of HF2 , the original strong
-
fluorescence emission of L1 was gradually reduced. When 10 equiv. of HF2 was added to
methanol-tris-HCl buffer (1:1 v/v) solution of L1, the fluorescence intensity was almost
-
completely quenched at 530 nm. Further increasing the concentration of HF2 did not affect
-
significantly towards the fluorescence emission of L1 (Fig. 6A). In the absence of HF2 , L2 (10
μM) is characterized by weak emission bands centered at 497 nm. This weak emission band of
L2 might be owing to the photo-induced electron transfer (PET), excited state intramolecular
proton transfer (ESIPT) from phenolic –OH to azomethaine-N, and rotation across the C=N bond
-
(cis/trans isomerization) (Fig. 6B) [31]. In the presence of a set of anions (F-, Cl-, Br-, I-, N3 ,
-
-
-
-
-
2-
2-
-
H2PO4 , NO3 , OAc-, HCO3 , HSO3 , CO3 , SO4 , SO3 , CN-, S2- and HF2 ) in methanol-tris-HCl
buffer, a remarkable enhancement in the emission spectra of L2 was observed only in case of
-
-
-
addition of HF2 (0-10 equiv.) at 490 nm. Here, HF2 and HSO3 are degradation sensor of L1 and
-
-
L2. On addition of HF2 and HSO3 anions decreasing in the fluorescence takes place in case of
L1. However, the fluorescence of L2 increases, probably because the absorption of L2 goes until
500 nm. If it decreases, between 400 nm up to 500 nm, the fluorescence that is covered in part
for the absorption, will increase, initially. Titration experiments were also performed with
-
concomitant additions of HF2 (0-10 equiv) to the solution of L2. An approximate 25-fold
enhancement in the fluorescence emission intensity was observed.

(A)
(B)
(A)
Fig. 6. Fluorescence titration of (A) L1 and (B) L2 (10 μM, λex = 330 nm) after addition of
increasing amounts of bifluoride (up to 10 equiv.) in methanol-tris-HCl buffer (1:1 v/v) at room
temperature.
3.4. Determination of the Association Constants and Detection Limits
From the fluorescence titration curve, to calculate the binding constants and the
-
stoichiometries of binding, log (I–I0/Imax–I) were plotted against log [HF2 ] using modified Hill’s
plot. The plots were found to be linear with good linear relationship (R=0.97385 for L1 and
R=0.9906 for L2) and have been shown in Fig. 7 The binding constants were calculated to be
-
2.046×106 M-2 and 4.797×106 M-2 for HF2 -adducts of receptor L1 and L2 respectively. The
obtained slopes of the linear fitting curves in L1 and L2 determined the number of guests
attached to the selective hosts; which confirmed the 1:2 binding of the receptors L1 and L2
-
towards HF2 .

-
-
Fig. 7. Stability constants of (a) L1-HF2 and (b) L2-HF2 -adducts
-
By using the fluorescence titration results, the detection limits of L1 and L2 towards HF2 were
determined to be 6.11 μM and 3.94 μM respectively on the basis of 3σ/K (Fig. 8). Importantly,
-
the low detection limit of L1 and L2 for HF2 suggests that L1 and L2 would act as sensitive
-
sensors for detection of HF2 in drinking water.
-
Fig. 8. Detection limits of (a) L1 and (b) L2 for HF2 .
-
3.5. Binding stoichiometries of L1 and L2 towards HF2
-
To investigate the binding stoichiometries of L1 and L2 towards HF2 , Job’s titration
method based on UV-vis were performed, in which the absorbance intensity reached to a
-
maximum at the 0.7 molar ratio for both the cases, indicated that L1 and L2 bound to HF2 with
1:2 ratio (Fig. S11). This was further confirmed by Hill’s plot analysis (vide supra) and ESI mass

-
-
spectra. In the mass spectra of L1-HF2 and L2-HF2 adducts, the base peaks obtained at m/z =
-
-
545.21 and 628.65 correspond to L1+2HF2 and L2+2HF2 respectively (Figs. S12 and S13).
3.6. Competitive experiments
-
To further examine the selectivity of L1 and L2 for HF2 ion, we measured the
-
fluorescence intensity of L1 and L2 in the presence of HF2 ion mixed with higher concentration
of various anions in methanol-tris-HCl buffer (1:1 v/v) (Fig. 9). In result, the emission intensity
-
-
of L1-2HF2 and L2-2HF2 adducts remained unchanged in the presence of 10 equiv. of other
competing ions, indicating that the receptors L1 and L2 can be used potentially applicable to
-
-
detect HF2 ion over the competing anions. Although HSO3 has some influence on the emission
behavior of the probes L1 and L2, there is much potentiality of the probes for the detection of
-
-.
HF2 in presence of HSO3
(A)
(B)
(A)
-
-
Fig. 9. Competitive selectivity of L1 and L2 towards HF2 / HSO3 in the presence of other anion
(10 equiv.) with an fluorescence intensity at 530 nm and 496 nm in methanol-tris-HCl buffer
(1:1 v/v) respectively.
3.7. pH effect test
For realistic applications, it is very necessary whether the sensors are active or not at the
suitable pH conditions for successful operation. Therefore, we measured the fluorescence
-
-
intensity of L1, L2, L1-2HF2 and L2-2HF2 adducts between pH 2 to 13. In very acidic medium
H+ can efficiently bind with multiple “N”, “O” center and there was a chance of imine breaking
which lowers the fluorescence intensity. With the increase of pH, the fluorescence intensity at
-
530 nm for both L1 and L1-2HF2 adduct increased up to 7.0, and remained constant through the
basic medium. In case of L2, fluorescence intensity at 490 nm decrease with a pH range of 2.0 to

6.0, and then increase till pH 8.0 and remained constant till pH 12.0. But in the presence of 10
-
-
equiv of HF2 , the fluorescence intensity of L2-2HF2 found to increase from pH 2.0 to 6.0 and
remained constant thereafter till pH 12 indicating that strong H-bonding ability and retardation of
ESIPT in basic medium. Hence, the emission intensity is stable over this wide range of pH (6.0-
10.0) and well-suitable for applications under physiological pH conditions (Fig. S14).
3.8. Time evolution and temperature effect test
-
The time evolution of receptors L1 and L2 in the presence of 2.0 equiv. of HF2 in
methanol-tris-HCl buffer (1:1 v/v) was investigated as shown in Fig. 10, the recognition
-
interaction gets almost completed just after the addition of HF2 ions and the fluorescence
intensities remain almost the same up to 10 min. These ensure the receptor L1 and L2 to be
sensitive sensors which can be applied in environmental analysis.
-
Similarly, temperature effect of receptors L1 and L2 in the presence of 2.0 equiv. of HF2
in methanol-tris-HCl buffer were investigated as shown in Fig. S15, which indicate the
fluorescence intensities remain almost the same up to 600C temperature.
Fig. 10. Time evolution of L1 and L2.
3.9. Plausible Sensing mechanism
-
The anion HF2 recognition model is mainly based on H-bonding induced mechanism. As
bifluoride has the strongest aptitude of forming H-bonds among all common anions, it forms
firm association with the hydrogen atoms of the hydroxyl groups of the salicylaldehyde moietes.
-
In order to check the sensing mechanism FT-IR spectra of both L1 and L2 and their HF2 adducts
were taken (Fig. 11). In the IR spectra, the receptor L1 showed sharp band at 3516 cm-1 and
1622 cm-1 due to O-H and C=N stretching vibrations. Whereas the L2 showed 1631 cm-1 due to

imine moiety and -OH peak appeared very less intense due to presence of electron donating
diethyl amino groups. Now in presence of HF-2 ions, the intensity of these peak become
decreases and somewhat shifted indicating the H-bonding based mechanism. These have also
1
1
been reflected in the HNMR- titration experiments (Fig. 12). In the HNMR spectra the –OH
protons of both the ligands (around  =13.6 ppm) become broad in presence of 2 equivalent of
HF-2 ions. The other aromatic protons shifted to the higher chemical shift values due to decrease
in electron density of the aromatic rings.
-
-
Fig. 11. IR spectra of (a) L1 and L1 + 2HF2 adduct; (b) L2 and L2 + 2HF2 adduct.
1
-
Fig. 12. Partial H-NMR spectra of (A) L1 and (B) L2 without and with HF2 .
3.10. Theoretical studies
The hydrogen bonding recognition was also well documented by TD-DFT and DFT
studies. Bifluoride ion can form hydrogen bonding with the probes L1 and L2 in two hydroxyl
-
moieties attached to the phenyl rings. The absorption spectra of L1 and L1+2HF2 are simulated
in C-PCM model employing the TD-DFT methods with the same hybrid basis set and functional
as used in geometry optimization calculations. The outcomes of electronic transitions data are in
good agreement with our experimentally observed electronic spectra (Fig. 13). Calculated spin-

-
allowed electronic transitions with the experimentally observed data for L1 and L1+2HF2 in
-
methanol are listed in Table 2. The selected transitions of L1 and L1+2HF2 having moderate
oscillator strengths are included. The screening the transitions only with orbital contributions
larger than 5% are taken into account for molecules. A schematic representation of the energy of
-
MOs and contours of selected orbitals of L1 and L1+2HF2 are presented in Figs. S16 and 14
-
respectively. The HOMO to LUMO energy difference for L1 and L1+2HF2 are 3.888 and 3.926
eV respectively. The energy transition was calculated at 341.10, 288.99 and 231.16 nm for L1
and the transitions can be assigned as (H-2→L, H-1→L+1, H-3→L+1), (H-3→L+2, H-5→L+2,
H-9→L+1) and (H→L+7, H-1→L+4, H-3→L+6) respectively. The characters of the all HOMO
and majority of LUMO levels are mixed type of π and nonbonded orbitals of nitrogen and/or
oxygen atom. On basis of molecular orbital analysis we can conclude the calculated transitions
are mixed transition of nπ to (nπ)*. The calculated energy transitions at 341.10, 288.99 and
231.16 nm are in good agreement with experimental λmax of 340, 285, and 230 nm respectively.
-
Fig. 13. The experimental absorbance spectra of L1 (blue) and L1+2HF2 (red) and the
-
calculated electronic transition L1 (blue line) and L1+2HF2 (red line) are displayed.
Table 2 The energy and molar absorption coefficients of experimental absorption bands and the
-
electronic transitions calculated with the TD-DFT method for L1 and L1+2HF2 .

-
L1
L1+2HF2
Most important orbital
excitations
Expt. λ
Most important
orbital excitations
H→L, H-1→L, H-
1→L+1
Expt. λ
λ [nm]
f
λ [nm]
f
[nm]
[nm]
H-1→L+1, H→L
368.79
0.0505
363.44
341.10
332.22
0.12035
H-2→L, H-1→L+1, H-
3→L+1
H-1→L, H→L+1, H- 353.79
4→L+1
0.1709 350 sh
0.3902 310
0.2576
0.45794 340
0.05683
H→L, H-1→L, H-
2→L+1
H-2→L, H-4→L, H- 311.65
3→L+1
H-2→L, H-1→L+1,
H→L
H-3→L+1, H-4→L,
H-3→L+1, H-
285.85
325.65
315.91
0.13945
0.1275
5→L+1
H-2→L+1, H-3→L, H-
3→L, H-1→L
H-2→L+1, H-
283.32
0.3042 280
3→L+1, H-5→L, H-
3→L, H-4→L+1
H-4→L, H-5→L+1
H-3→L, H-7→L+1, H-
3→L, H-5→L
266.23
235.34
233.60
0.0722
301.48
288.99
277.29
277.03
231.16
228.14
224.93
0.14327
H-3→L+2, H-5→L+2,
H-9→L+1
H-3→L+2, H-
0.4543
0.27999 285
0.10453
5→L+2, H-9→L+1
H-7→L, H-8→L+1,
H-6→L+1
H-3→L+1, H-4→L, H-
5→L+1, H-9→L+1
H-7→L+1, H-5→L, H-
4→L+1, H-8→L
0.3627 230
0.10198
H→L+7, H-1→L+4, H-
3→L+6
0.18322 230
0.3837
H-3→L+2, H-2→L+3,
H-6→L+1
H-2→L+4, H-8→L+1,
H→L+4, H-9→L+1
0.06567
f – oscillator strength; H – highest occupied molecular orbital; L – lowest unoccupied molecular
orbital.

LUMO
HOMO
LUMO
HOMO
L+1
H-1
L+1
H-1
L+2
H-2
L+2
L+3
H-3
L+3
H-3
L1
L1+2
-
HF2
H-2
-
Fig. 14 Selected HOMOs and LUMOs of L1 and L1+2HF2 . Positive values of the orbital
contour are represented in yellow (0.03 au) and negative values in blue (−0.03 au).
-
-
The probes L1 and L2 are yellow in color in contrast L1+2HF2 and L2+2HF2 are
colorless. The energy gap between the highest occupied molecular orbitals and lowest
-
-
unoccupied molecular orbitals of L1, L1+2HF2 , L2 and L2+2HF2 are 3.888, 3.926, 2.506 and
-
3.428 eV are respectively (Fig. S17). Calculated HOMO-LUMO energy gap of L1+2HF2 and
-
L2+2HF2 higher compared to the L1 and L2 molecules respectively. This observation has been
-
-
reflected on absorption spectra of L1+2HF2 and L2+2HF2 ; which are colorless.
-
-
The geometry optimized structures of the molecules L1+2HF2 and L2+2HF2 are
-
displayed in Fig. S18. The molecule L1+2HF2 is revealed a zigzag hydrogen bond with oxygen
-
atom of L1 and the excited state of L1+2HF2 is probably dissipating energy through hydrogen
-
bond results decrease of fluorescence compared to the probe L1. In contrary the L2+2HF2
molecule observed a cyclic hydrogen bond with oxygen atom of L2. Consequently the excited
-
state of L2+2HF2 molecule is getting extra stability or non dissipating energy probably and the
molecule exhibits enhance fluorescence compared to the L2 molecule [32].
3.9. Application of chemosensors L1 and L2 in real samples

In order to consider the practical feasibility of the sensor L1 and L2 in water samples for
-
the determination of HF2 ion. Tap and drinking water were collected. Each sample was
examined for three replicates. The samples were spiked with three different concentrations of
-
HF2 ion. The results are also summarized in Tables 3 and 4, which shows suitable recovery and
R.S.D. values for water samples.
-
Table 3. Determination of HF2 in water samples by the probe L1
Table3
Spiked amount
(μM)
RSD%
Value
Sample
Recovered amount (μM) Recovery(%)
Tap Water
10
20
30
9.87
19.75
30.35
98.7
98.75
101.16
1.7
Drinking
Water
10
20
30
9.9
99
20.11
29.56
100.55
98.53
1.8
Table 4
Spiked amount
(μM)
RSD%
Value
Sample
Recovered amount (μM) Recovery(%)
Tap Water
10
20
30
9.92
20.25
30.15
99.2
101.25
100.5
1.54
Drinking
Water
10
20
30
10.11
20.01
30.06
101.1
100.05
100.2
0.75
3.10 Test kits application of chemosensors L1 and L2
For the practical application of the receptors L1 and L2, test kits wereprepared by
immersing filter papers in methanol tris HCl buffer (10 µM, pH=7.2) solution of L1/L2 (1 mM)
-
and then drying in air. These test kits were utilized tosense HF2 among different anions. As
shown in Fig. S19, when the test kits coated with L1/L2 were added to different anion solutions,

-
the obvious colour change from yellow to colourless were observed only with HF2 ion. The color
-
changes of L1 and L2 upon the treatment of HF2 and other anions in the test strips under UV
light have been shown in Fig. 15. Therefore, the test kits coated with thereceptor L1/L2 solution
-
would be convenient for the detection HF2 . These results showed that both the receptors L1/L2
-
could be valuable practical sensors for environmental analyses of HF2 .
-
Fig. 15. The change in colour of L1 and L2 upon the treatment of HF2 and other anions in the
test strips under UV light.
4. Conclusion
In conclusion, we report here two simple bis Schiff base fluorescent colorimetric
-
chemosensors L1 and L2 for easy and convenient detection of HF2 , exhibiting the change of
both absorption and fluorescence intensity in methanol tris HCl buffer solution at physiological
-
pH. The mechanism of selectivity and sensitivity of the receptors L1 and L2 towards HF2 has
been proposed to be on the basis of photo-induced electron transfer (PET), chelation enhanced
-
fluoresence (CHEF) and hydrogen bonding mechanism. Chelation of HF2 to the receptors L1
and L2 takes place in 1:2 ratio, which has further been confirmed by Job’s plots analysis, 1H-
NMR, IR spectra, ESI-mass spectra and DFT studies. The detection limits of the chemosensors
-
for HF2 were determined to be 6.11 μM for L1 and 3.94 μM for L2, which are far lower than
those recommended by the WHO guidelines for drinking water. The receptors L1 and L2 can
-
successfully been applied for detection and quantification of HF2 in real samples. To the best of
-
our knowledge this is the third report of fluorescent colorimetric chemosensors for HF2 .
Conflict of Interest: The authors declare no conflict of interest.
Acknowledgements
G.K.P would like to thank the Department of Science and Technology (SR/FST/CSI-264/2014)
and Department of Biotechnology, Government of India, New Delhi for financial support.

A.K.M. thanks the CSIR, and KR thanks DST, Government of India (No. PDF/2017/001365) for
financial support in the form of research fellowships.
Supporting Information Summary
CCDC number 1893984 contains the supplementary crystallographic data for the probe L1. The
data can be obtained free of charge from the Cambridge Crytallographic Data Centre, 12 Union
Road, Cambridge CB21EZ, UK; Fax: (+44)1223-336-033; or email: deposit@ccdc.cam.ac.uk:
Supplementary data containing Figs. S1-S19 of this article can be found online
at…………………………..
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