Colorimetric anion sensors based on positional effect of nitro group for recognition of biologically relevant anions in organic and aqueous medium, insight real-life application and DFT studies

Article abstract of DOI:10.1016/j.saa.2017.07.051

A new six colorimetric receptors A1-A6 were designed and synthesized, characterized by typical common spectroscopic techniques like FT-IR, UV–Visible, ¹H NMR, ¹³C NMR and ESI-MS. The receptor A1 and A2 exhibit a significant naked-eye response towards F^? and AcO^? ions in DMSO. Due to presences of the NO₂ group at para and ortho position with extended π-conjugation of naphthyl group carrying –OH as a binding site. Compared to receptor A2, A1 is extremely capable of detecting F^? and AcO^? ions present in the form of sodium salts in an aqueous medium. This is owed to the occurrence of –NO₂ group at para position induced in increasing the acidity of –OH proton. Consequently, it easily gets deprotonated in aqueous media. The detection limit of receptor A1 was turned out to be 0.40 and 0.35?ppm for F^? and AcO^? ions which is beneath WHO permission level (1.0?ppm). Receptor A1 shows a solitary property of solvatochromism in different aprotic solvents in presence of AcO^? ion. Receptor A1 depicts high selectivity towards AcO^? ion in DMSO: HEPES buffer (9:1, v/v). Receptor A1 proved itself for real life application by detecting anion in solution and solid state. The binding mechanism of receptor A1 with AcO^? and F^? ions was monitored from ¹HNMR titration and DFT study.

Full text of DOI:10.1016/j.saa.2017.07.051

Accepted Manuscript
Colorimetric anion sensors based on positional effect of nitro
group for recognition of biologically relevant anions in organic
and aqueous medium, insight real-life application and DFT
studies
Archana Singh, Suban K. Sahoo, Darshak R. Trivedi
PII:
DOI:
Reference:
S1386-1425(17)30610-8
doi: 10.1016/j.saa.2017.07.051
SAA 15338
Spectrochimica Acta Part A: Molecular and Biomolecular
Spectroscopy
To appear in:
Received date:
Revised date:
Accepted date:
6 June 2017
17 July 2017
28 July 2017
Please cite this article as: Archana Singh, Suban K. Sahoo, Darshak R. Trivedi ,
Colorimetric anion sensors based on positional effect of nitro group for recognition of
biologically relevant anions in organic and aqueous medium, insight real-life application
and DFT studies, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy
(2017), doi: 10.1016/j.saa.2017.07.051
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ACCEPTED MANUSCRIPT
Colorimetric anion sensors based on positional effect of nitro group for recognition of
biologically relevant anions in organic and aqueous medium, insight real -life application
and DFT studies
Archana Singha, Suban K. Sahoob, Darshak R. Trivedi*
aSupramolecular Chemistry Laboratory, Department of Chemistry, National Institute of
Technology Karnataka (NITK) Surathkal, Srinivasnagar - 575 025, Karnataka, India
bDepartment of Applied Chemistry, S.V. National Institute Technology Surat-395 007 Gujarat,
India
*Email: darshak_rtrivedi@yahoo.co.in, Phone No: +91-824-2473205, fax: +91 824 2474033.
Abstract
A new six colorimetric receptors A1-A6 were designed and synthesized, characterized by
typical common spectroscopic techniques like FT-IR, UV-Visible, 1H NMR, 13C NMR and ESI-
MS. The receptor A1 and A2 exhibit a significant naked-eye response towards F¯ and AcO¯ ions
in DMSO. Due to presences of the NO2 group at para and ortho position with extended π-
conjugation of naphthyl group carrying -OH as a binding site. Compared to receptor A2, A1 is
extremely capable of detecting F¯ and AcO¯ ions present in the form of sodium salts in an
aqueous medium. This is owed to the occurrence of –NO2 group at para position induced in
increasing the acidity of -OH proton. Consequently, it easily gets deprotonated in aqueous media.
The detection limit of receptor A1 was turned out to be 0.40 and 0.35 ppm for F¯ and AcO¯ ions
which is beneath WHO permission level (1.0 ppm). Receptor A1 shows a solitary property of
solvatochromism in different aprotic solvents in presence of AcO¯ion. Receptor A1 depicts high
selectivity towards AcO ¯ion in DMSO: HEPES buffer (9:1, v/v). Receptor A1 proved itself for
real life application by detecting anion in solution and solid state. The binding mechanism of
receptor A1 with AcO¯and F¯ ions was monitored from 1HNMR titration and DFT study.
Keywords: Fluoride; Acetate; Sensor; Coloimetric; DFT calculation
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INTRODUCTION
In recent decades, the scientific community has shown noticeable attention in the design
and development of organic receptors for the recognition of biologically important anions in
organic along with aqueous media, such as fluoride and acetate due to their imperative role
played in chemical, agricultural, industrial and biological processes [1]. Fluoride draws much
attention due to their importance in dental care, clinical remedy of osteoporosis and treatment of
drinking water. Moreover, fluoride ion used in anesthetics, hypnotic and psychiatric drugs,
military nerve gasses and in the refinement of uranium [2-6]. Accordingly, to world health
organization (WHO) and environmental protection agency standard level of fluoride in drinking
water should be 1.5 ppm [7] and toothpaste (1000-1500 ppm). In case, intake is exceeding the
limit, it may cause serious health issues in the human body such as fluorosis, neurological,
metabolic disorders and even lead to cancer like osteosarcoma [8-13].
Moreover, acetate ion plays an immense role in numerous metabolic processes such as acetyl
coenzyme. Additionally, it has been broadly used in the manufacturing of dyes, plastics, paper
and paint in nylon industry [14]. Sodium acetate is the main element present in food items such
as vinegar, medicine and cosmetic [15-17]. This anion widely investigated in prostate cancer and
its metastases [18-19].
To date, several methods have been reported for the recognition of fluoride and acetate
ions among which colorimetric chemosensors have gain lot of attention due to the unique
properties, simplicity, convenience, low cost, immediate response, easy naked-eye recognition of
target anion and also feasibility to offer both quantitative and quantity recognition of receptor-
anions interaction without employ to any expensive spectroscopic method [20-21]. In common,
the design of such chemosensor is based on the receptor-chromophore, which includes the
binding sites. However, the binding sites contains various functional groups such as urea [22-24],
thiourea [25-28], imidazoles/ benzimidazoles [29-37], amide/ diamides [38-40], azo phenol [41-
42], napthalimides [43-47], pyrrole [48-49], guanidium derivatives [50] etc. with chromophores
that is capable of signaling the binding of the receptor-anion gives rise to intramolecular charge
transfer (ICT) with a visible color change and give response towards UV-Vis /fluorescence.
Moreover, barely any reported kinds of literature are available using –OH as binding sites [51-
62]. The fluorescence also has gained substantial interest due to their high selectivity and easy
detection [63].
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The majority of the receptors is narrow to the recognition of the anion in a less polar
solvent such as (CH2Cl2, DMSO, and CH3CN etc.) [64]. There is a scarcity of reports are
available concerning the colorimetric recognition of the anion in an aqueous medium [65-71].
This interruption indicates the fact that anions are strongly solvated in water media, owing to
their highly hydration free energy that generates the recognition process of the anion in water
media much more complicated [72]. The selectivity and sensitivity of a receptor depend on the
acidity of the proton where the anion binds. In case, the acidity of theses proton is less than the
water then the anion gets solvated. However, this shortcoming can be solved to some extent by
designing a receptor where the anion binding proton are more acidic than water or by introducing
a base labile group such as hydroxyl (OH) functionality with the electron withdrawing –NO2
group to increase the acidity of the –OH proton which easily deprotonated with the relative basic
ion such as F¯ and AcO¯ ions [73-74] and induced color change noticeable by the naked eye in
the anion binding.
Keeping this concept in mind, in this research paper we have designed and synthesized
six colorimetric receptors A1-A6 for the detection of anions (Scheme 1). The presence of –NO2
group in receptor A1 and A2 at para and ortho position describes the initiation of electron
withdrawing and positional effect, linked to the conjugated system of naphthyl group possessing
–OH moiety as a binding site for the anion. The receptors A3-A4 were synthesized to study the
role of the π-conjunction effect of a naphthyl group. Similarly, the receptors A5-A6 were
designed with an emphasis on -OH group in anion binding. However, the para position of -NO2
group in the receptor A1 makes the -OH proton more acidity, which in turn, enhances the
detection ability, sensitivity and selectivity of the receptor towards F¯ and AcO¯ ions in organic
along with aqueous medium.
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NO2
NO2
NO2
NO2
NO2
NO2
N
N
N
N
N
N
N
N
N
N
N
N
OH
OH
OH
OH
A 1
A 3
A 5
A 6
A 2
A 4
Scheme 1. Designed colorimetric receptors A1-A6.
2. Experimental
2.1. Material and methods
The entire chemical was purchased from Sigma-Aldrich, Alfa Aesar or from
spectrochemical and used without further purification. All solvents were obtained from SD fine,
India with HPLC grade and used without further distillation. NMR spectra of the pure samples
were recorded using a Bruker Avance (500 MHz) spectrophotometer where tetramethylsilane
(TMS) is used as the internal standard and DMSO-d6 as the solvent. Resonance multiplicities are
explained as s (singlet), d (doublet), t (triplet), and m (multiplet). IR spectra were recorded on
Jasco FT-IR spectrometer. Mass spectra were analyzed on a Waters Micromass Q-Tofmicro
spectrometer with ESI source. UV-Vis spectra were recorded with an Analytikjena Specord S600
Spectrometer in standard 3.5 ml quartz cells with 1 cm path length and the fluorescence study
was monitored in a JASCO-FP-6200 Spectrometer. All the reactions were studied by TLC on
precoated silica gel 60 F254 plates. Melting point was recorded on struart-SMPS melting-point
apparatus in open capillaries.
S1: (4-nitro-benzylidene)-hydrazine: Yield 90%., m.p. 130ºC. FT-IR (KBr) (cm-1): 3285.29
(NH2), 3066.60 (Ar-H), 1630.48 (CH=N), 1576.35 (C=C), 1231.42-1403.42 (NO2).1H NMR
(500 MHz, (DMSO-d6): δ 7.544 (s,2H), 7.691 (d, J=8Hz, 2H), 7.750 (s, 1H), 8.177 (d, J=8Hz,
2H) . 13C NMR (125 MHz, (DMSO-d6): δ 146.20, 143.83, 135. 03, 125.85, 124.44. Mass (ESI):
m/z calculated for C7H7N3O2:165.15 Obtained: 166.0602 [M+H]+ .
S2: Synthesis of (2-nitro-benzylidene)-hydrazine: Yield 89%., m.p. 125ºC. FT-IR (KBr) (cm-1):
3318.02 (NH2), 3072.52 (Ar-H), 1600.25 (CH=N), 1568.15 (C=C), 1209.75-1443.10 (NO2). 1H
NMR (500 MHz, (DMSO-d6): δ 7.419- 7.447 (m, 1H), 7.524 (s, 2H), 7.623-7.654 (m, 1H),
13
7.961 (d, J= 8H, 1H), 7.9 (d, J=8H, 1H), 8.006 (s, 1H) . C NMR (125 MHz, (DMSO-d6): δ
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159.01, 149.37, 147.04, 134.38, 124.69. Mass (ESI): m/z calculated for C7H7N3O2:165.15
Obtained: 166.0602 [M+H]+.
Receptor A1:1-[4-nitro-benzylidene]-hydrazonomethyl]-naphthalen-2-ol: Yield 80%, m.p. 330
ºC. FT-IR (KBr) (cm-1): 3447.21 (OH), 2963.66-2872.64 (C-H), 3052.58 (Ar-H), 1284-1468.68
1
(NO2). H NMR (500 MHz, (DMSO-d6): δ 7.2495 (d, J= 8.5 Hz, 1H), 7.387-7.417 (m,1H),
7.556-7.589 (m,1H), 7.627-7.661 (m,1H), 7.734-7.764 (m,1H), 7.805 (d, J= 8.5 Hz, 1H), 7.8905
(d, J= 8.5 Hz, 1H), 8.094-8.113 (m,1H), 8.1545 (d, J= 9 Hz, 1H) 8.301-8.319 (m, 1H),9.165 (s,
1H), 9.752 (s, 1H), 13.106 (s, 1H). 107.99, 119.32, 119.96, 123.81, 124.84, 123.04, 128.23,
128.79, 129.26, 129.41, 129.59, 131.34, 132.90, 133.37, 135.30, 156.90, 161.81, 163.28. Mass
(ESI): m/z calculated for C18H13N3O3: 319.31 Obtained: 320.1073 [M+H]+ .
Receptor A2: 1-[2-nitro-benzylidene]-hydrazonomethyl]-naphthalen-2-ol: Yield 80%, m.p. 300
ºC. FT-IR (KBr) (cm-1): 3692.18 (OH), 3050.91 (Ar-H), 1623.11 (CH=N), 1569.98 (C=C),
1311.06-1346.18 (C-O), 1243.83-1479.55 (NO2). 1H NMR (500 MHz, (DMSO-d6): δ 7.246 (d,
J=12Hz, 1H), 7.4225 (d, J= 7.5Hz, 1H), 7.574-7.607 (m,1H), 7.811 (d, J= 8Hz, 1H), 7.9 (d, J=
9Hz, 1H), 8.052 (d, 9Hz, 2H), 8.1725 (d, J= 8.5Hz, 1H), 8.337 (d, J= 9Hz, 2H), 8.732 (s, 1H),
9.774 (s, 1H), 13.148 (s,1H). 13C NMR (125 MHz, (DMSO-d6): δ 117.18, 117.26, 117.32,
119.71, 124.73, 124.79, 128.71, 129.46, 129.59,131.48, 132.53, 133.42, 133.45, 157.78, 158.16,
159.87, 164.68, 166.72.Mass (ESI): m/z calculated for C18H13N3O3: 319.31 Obtained: 320.1073
[M+H]+ .
º
Receptor A3: 2-[(4-nitro-benzylidene)-hydrazonomethyl]-phenol: Yield 80%, m.p. 230 C. FT-
IR (KBr) (cm-1): 3684.22 (OH), 1621.78 (CH=N), 1523.23 (C=C), 1343.39-1384.67 (C-O),
1
1201.69-1485.01 (NO2). H NMR (500 MHz, (DMSO-d6): δ 6.958-6.988 (m, 1H), 7.029-7.056
(m, 1H), 7.354-7.413 (m, 3H), 7.627-7.660 (m, 1H), 8.0975 (d, J= 8Hz, 1H), 8.298 (d, J=8Hz,
1H), 8.719 (s, 1H), 9.114 (s, 1H), 11.373 (s,1H). 13C NMR (125 MHz, (DMSO-d6): δ 117.22,
117.36, 119.71, 124.06, 129.24, 132.17, 133.43, 139.43, 149.46, 156.90, 159.47, 160.19, 164.67,
166.97. Mass (ESI): m/z calculated for C14H11N3O3: 269.08 Obtained: 270.0922 [M+H]+.
º
Receptor A4: 2-[2-nitro-benzylidene]-hydrazonomethyl]-phenol: Yield 80%, m.p. 205 C. FT-
IR (KBr) (cm-1): 3427.58 (OH), 3058.51 (Ar-H), 1517.89 (C=C), 1339.65 (C-O), 1211.31-
144726 (NO2). 1H NMR (500 MHz, (DMSO-d6): δ 7.457-7.491 (m, 4H), 7.840-7.875 (m, 2H),
13
8.0155 (d, J= 8.5Hz, 2H), 8.309 (d, J=9Hz, 1H), 8.669-8.700 (m, 2H). C NMR (125 MHz,
(DMSO-d6): δ 108.09, 119.22, 119.98, 123.92, 124.08, 128.15, 128.25, 129.14, 129.28, 132.87,
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135.46, 158.52, 161.84, 163.46. Mass (ESI): m/z calculated for C14H11N3O3: 269.08 Obtained:
270.0958 [M+H]+.
º
Receptor A5: N-Benzylidene-N1-(4-nitro-benylidene)-hydrazine: Yield 80%, m.p. 180 C. FT-
IR (KBr) (cm-1): 1627.30 (CH=N), 1517.53 (C=C), 1212.67-1447.98 (NO2). 1H NMR (500 MHz,
(DMSO-d6): δ 7.455-7.497 (m, 1H), 7.594-7.630 (m, 2H), 7.594-7.757 (m, 2H), 8.088-8.106 (m,
2H), 8.289-8.306 (m, 2H), 9.114 (s, 2H). 13C NMR (125 MHz, (DMSO-d6): δ 118.30, 119.00,
124.55, 129.75, 129.82, 130.04, 1313, 137.42, 138.89, 139.92, 149.49, 150.89, 160.30, 161.91.
Mass (ESI): m/z calculated for C14H11N3O3: 253.09 Obtained: 254. 0993 [M+H]+.
º
Receptor A6: N-Benzylidene-N1-(2-nitro-benylidene)-hydrazine: Yield 80%, m.p. 160 C. FT-
IR (KBr) (cm-1): 1618.43 (CH=N), 1206.31-1442.85 (NO2), 1517.53 (C=C). 1H NMR (500 MHz,
(DMSO-d6): δ 7.489 (d, J=8Hz, 1H), 7.539-7.570 (m, 1H), 8.2125 (d, J=8.5Hz, 3H), 8.389 (d,
J=9Hz, 3H), 8.4495 (d, J=8.5Hz, 1H), 8.751 (s, 1H), 8.908 (s, 1H). 13C NMR (125 MHz,
(DMSO-d6): δ 124.09, 125.24, 126.82, 127.95, 128.29, 129.91, 131.02, 131.26,133.34, 134.38,
147.04, 149.37, 149.37, 159.01. Mass (ESI): m/z calculated for C14H11N3O3: 253.09 Obtained:
254. 0993 [M+H]+.
3. Results and discussion
3.1. Synthesis
The receptors S1, S2, R1, R2, R3, R4, R5 and R6 were synthesized by simple Schiff base
condensation reaction, by reacting with 4-nitro-benzaldehyde (0.344 g, 2 mmol) and hydrazine
hydrate (3 mL, 3 mmol) (S1), 2-nitro-benzaldehyde (0.344 g, 2 mmol) and hydrazine hydrate (3
mL, 3 mmol) (S2), (4-nitro-benzylidene)-hydrazine (0.374 g, 2 mmol) and 2-hydroxy-1-
naphthaldehyde (0.346 g, 2 mmol) (receptor A1), (2-nitro-benzylidene)-hydrazine (0.374 g, 2
mmol) and 2-hydroxy-1-naphthaldehyde (0.346 g, 2 mmol) (receptor A2), (4-nitro-benzylidene)-
hydrazine (0.374 g, 2 mmol) and 2-Hydroxy-benzaldehyde (0.244 g, 2 mmol) (receptor A3), (2-
nitro-benzylidene)-hydrazine (0.374 g, 2 mmol) and 2-hydroxy-benzaldehyde (0.244 g, 2 mmol)
(receptor A4), (4-nitro-benzylidene)-hydrazine (0.374 g, 2 mmol) and benzaldehyde (0.212 g, 2
mmol) (receptor A5), (2-nitro-benzylidene)-hydrazine (0.374 g, 2 mmol) and benzaldehyde
(0.212 g, 2 mmol) (receptor A6) in ethanolic medium by adding 3-4 drop of acetic acid. The
mixture was kept under stirring for 5hr. The yellow color solid product was collected by
filtration and washed with cold ethanol. The completion of the product was confirmed by thin
layer chromatography.
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3.2. Colorimetric responses of the receptors in DMSO
The receptors A1-A6 solutions (4.5 ×10-5M in dry DMSO) were a primary estimate for
naked-eye detection in the presence of several classes of anions such as (F−, Cl−, Br−, I−, NO3¯,
−
−
HSO4 , H2PO4 and AcO¯ in the form of tetrabutylammonium salts). On addition of 2 equiv. of
anions, a sharp color change was noticed for F− and AcO¯ ions from pale yellow to purple in the
case of receptor A1 and pink in the case of receptor A2 as illustrated in Fig. 1. The receptor A1
showed a sensational color change due to the existence of electron withdrawing -NO2 group in
the para position of the receptor which increases the hydrogen donating strength of the phenolic-
OH. The strong hydrogen bonding or deprotonation of the -OH proton changes the electronic
properties of the chromophores, as a result, give rise to huge color change [75].
The receptors A3 and A4 showed a noticeable color change from colorless to light pink
and yellow in the presence of F¯ and AcO¯ ion as depicted in Fig. 1. On the other hand, it was
observed that the receptors A5 and A6 showed no noteworthy color change on addition F¯ and
AcO¯ ion. However, other competing anions are unable to produce any conspicuous color
change even with the addition of excessive.
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Fig. 1. Naked-eye visible color change of receptors A1-A6 (4.5 ×10-5M in dry DMSO) upon
addition of 2equiv. of several anions in the form of TBA salts (1 ×10-2M in dry DMSO).
3.3. UV-Vis spectroscopic responses of the receptors in DMSO
To further assess the selectivity of the receptors A1-A6 (4.5 ×10-5M in dry DMSO)
towards F¯ and AcO¯ ions against other anions, the UV-Vis absorbance experiments were
conducted by adding 2 equiv. of multiple anions in the form of tetrabutylammonium (TBA) salts
−
−
such as F−, Cl−, Br−, I−, NO3¯, HSO4 , H2PO4 and AcO¯(1×10¯2 M in DMSO) to the receptors
A1-A6 solutions. As shown in Fig. 2 (a) and (b) receptors A1 and A2 exhibit high selectivity
towards AcO¯ and F¯ ions over the other interference anions. Similarly, receptors A3 and A4
also display high affinity towards both AcO¯ and F ¯ions as depicted in Fig. 3 (a) and (b).
Moreover, receptor A5 and A6 are incapable to recognize neither F¯ nor AcO¯ ions even the
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addition of excessive anion due to the absence of -OH proton as represented in Fig. 4 (a) and (b).
The AcO¯ and F¯ ions with higher basicity and immense negative charge density [76] anticipate
in forming a strong hydrogen bond with receptors followed by deprotonation on addition of
excess amount of onions. In contrast, no shift in UV-Vis spectra was recorded for other anions
due to their weak basicity.
(a)
(b)
Fig. 2. UV-Vis absorption spectra change of (a) receptor A1 and (b) receptor A2 with the
addition of 2equiv of several anions in the form of TBA salts (1 ×10-2M in dry DMSO).
(a)
(b)
Fig. 3. Change in UV-Vis absorption spectra of (a) receptor A3 and (b) receptor A4 upon
addition of 2equiv of several anions in the form of TBA salts (1 ×10-2M in dry DMSO).
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(a)
(b)
Fig. 4. Change in UV-Vis absorption spectra of (a) receptor A5 and (b) receptor A6 on the
addition of 2equiv of various unions in the form of tetrabutylammonium salts (1 ×10-2M in dry
DMSO).
To obtain further insight into the sensitivity of the receptor A1 (4.5 ×10-5 M) with the
AcO¯ion, UV-Vis spectrophotometric titration was recorded in DMSO. The receptor A1
developed a strong characteristic band at 404 nm in DMSO, assigned to π π* electronic
transition of the chromophore [77]. With the successive addition of 0.1 equiv aliquot of AcO¯
ion to receptor A1 solution, progressively deduction in the intensity of the absorption band at 404
nm with simultaneous growth of a new band at 547 names attributed to π* transition [78]
Fig. 5. The bathochromic shift moving from 404 nm to 547 nm corresponding to the ICT and
accompanied by well define the isosbestic point at 456 nm suggests that only one active species
formation in the titration mixture. The hydrogen bond formation between –OH group in receptor
A1 and the added anions is aided by an intermolecular proton transfer (IPT), suggesting a proton
distrait from an OH group and create a negative charge on the oxygen atom [79-80]. The
increase in the ICT transition may be owed to a push-pull nature between the electron
withdrawing –NO2 moiety and extended π-conjunction with a naphthol group [81]. Similarly,
upon cumulative addition of 0.1 equiv. of F¯ ion to a receptor A1 solution, it presented a red
shift by a unit of 142 nm as depicted in Fig. S33 in the ESI†. Distinct isosbestic point centered at
455 nm represents the stable complex formation between receptor A1 and F¯ ion. The
stoichiometry of the anion complexation of the receptor A1 was analyzed from Benesi-
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Hildebrand (B-H) plot, linearity obtained from the grape confirmed 1:1 binding ratio for both the
AcO¯ and F¯ ions with receptor A1 as illustrated in Fig. S34 -Fig. S35 in the ESI†. To assure
that the detection of anions also follows the deprotonation on the addition of anion, receptor A1
carried out titration with TBAOH (1 ×10-2 M) in DMSO. Upon successive addition of 0.1 equiv
of TBAOH to receptor solution, the absorption band at 404 nm regularly decreases and a new
band at 550 nm increase steadily with increasing amount of TBAOH as described in Fig.S36 in
the ESI†. Evidently, the titration profile matches exactly as observed aforementioned ions,
corroborating the deprotonation of OH proton of receptor A1 with AcO¯ and F¯ ion. In fact, the
presence of NO2 at the para position increases the acidity of the hydrogen-bond donors, thus the
reason deprotonation of OH proton becomes easier by relativistic basic ions.
Fig. 5. UV-Vis absorption spectra of receptor A1 (4.5 ×10-5 M in DMSO) during the titration
process with incremental addition of 0.1 equiv. TBAAcO (1×10-2 M in DMSO). Insert showing
the equiv of AcO¯ ion vs. absorbance at selective wavelength 547 nm.
As shown in Fig. 6 the UV-Vis titration spectra of receptor A2 (4.5 ×10-5 M) with the
increase of AcO¯ ion concentration. The free receptor A2 in DMSO solution showed an
absorption band centered at 388 nm. On incremental addition of 0.1 equiv of AcO¯ ion, the
absorption band at 388 nm slightly decreases and at the same time, a new band gradually
increases at 510 nm with the accompanied a sharp isosbestic point at 426 nm. The consequent
bathochromic shift (red shift) of 122 nm was observed. Similarly, UV-Vis titration of receptor
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A2 (4.5 ×10-5 M) was performed with the successive addition of 0.1 equiv of F¯ ion (as
represented in Fig.S37 in the ESI†), which showed a red shift by 119 nm from 388 nm to 507 nm
with one sharp isosbestic point at 425 nm. The Benesi-Hildebrand confirmed to be 1:1 binding
ratio between the receptor A2 and added anions AcO¯ and F¯ (as illustrated in Fig.S38 and S39
in the ESI†).
Fig. 6. UV-Vis absorption spectra of receptor A2 (4.5 ×10-5 M in DMSO) during the titration
process with the stepwise addition of 0.1 equiv. TBAAcO (1×10-2 M in DMSO). Insert showing
the equiv of AcO¯ ion vs. absorbance at selective wavelength 510 nm.
For instance, receptor A3 and A4 were synthesized to understand the extended of the π-
conjunction effect of naphthol group. In UV-Vis titration, receptor A3 (4.5 ×10-5 M) in DMSO
exhibited a strong absorption band at 360 nm. Upon cumulative addition of 1 equiv of AcO¯ ions
to the receptor A3 solution, the absorption band at 360 nm showed declines tendency while
simultaneously new band started growing at 470 nm and showed a bathochromic shift of 110 nm
as illustrated in Fig.S40 in the ESI†. A distinct isosbestic point appeared at 390 nm reveal that
only one species is present at equilibrium over the series of the titration process. In a similar
way, UV-Vis titration was implemented with the F¯ ion on the progressive addition of 1 equiv.
F¯ ion to receptor A3 and showed a bathochromic shift of 105 nm from 360 nm to 465 nm. An
isosbestic point makes out at 385 nm during titration indicating interaction with F¯ ion and
receptor A3 as observed in Fig.S41 in the ESI†. The Benesi-Hildebrand (B-H plot) imply 1:1
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binding ratio between receptor A3 with AcO¯ and F¯ anions as described in Fig.S42-S43 in the
ESI†.
Additionally, UV-Vis titration was recorded for receptor A4. Receptor A4 (4.5 ×10-5 M)
in DMSO solution showed the intrinsic maximum absorption band at 360 nm as given in. The
incremental addition of 1 equiv of AcO¯ ion to a receptor A4 solution cause decrease in
absorption band at 360 nm with a generation of a new band at 464 nm could be due to receptor-
anion interaction with an well defined isosbestic point at 390 nm and showed a red shift of 104
nm as represented in Fig S44 in the ESI†. Moreover, a similar procedure was repeated for F¯ ion
as a display in Fig S45 in the ESI†. In addition, of 1 equiv of F¯ ion to a receptor A4 solution
showed a red shift differing by a unit of 100 nm in comparison with the absorption band of free
receptor A4 with an isosbestic point at 387 nm. The definite stoichiometric ratio between
receptor A4-A5 and form mentioned anions was found to be 1:1 from UV-vis spectral change
which has been analyzed with the help of Benesi-Hildebrand (B-H) plot as represented in Fig
S46 –S47 in the ESI†.
The receptor A5 and A6 did not show any shift in UV-Vis spectral upon addition of
AcO¯ and F¯ ions. This emphasizes that the -OH proton is liable for the binding process of
anions. They showed a strong, intense band at 308 nm that corresponding to the π π* transition
of NO2 moiety present in receptors.
3.4. Change in absorption, binding constant and detection limit
Change in absorption (Δ휆max), binding constant and the detection limit of receptors A1-
A6 (4.5 ×10-5 M in DMSO solution) in the presence of F¯ and AcO¯ ion (2equiv.) is represented
in Table 1 and Table 2.
Table 1: Change in absorption (Δ휆max) of receptors A1-A6 (4.5 ×10-5 M in DMSO solution) in
the presence of F¯ and AcO¯ ion (2equiv.)
Receptor (R)
휆max
휆max
Δ휆max
Δ휆max
(R + TBAF)
546 nm
508 nm
465 nm
464 nm
308 nm
308 nm
(R + TBAAcO)
547 nm
510 nm
470 nm
460 nm
308 nm
308 nm
(R + TBAF)
142 nm
120 nm
105 nm
100 nm
0
(R + TBAAcO)
143 nm
122 nm
110 nm
104 nm
0
A1
A2
A3
A4
A5
A6
0
0
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Table 2: Binding constant and detection limit of receptors A1-A6 in the present of F¯ and AcO¯
ion
Receptor +Anion
Receptor A1+F¯
Receptor A1+AcO¯
Receptor A2+F¯
Receptor A2+AcO¯
Receptor A3+F¯
Receptor A3+AcO¯
Receptor A4+F¯
Receptor A4+AcO¯
Binding constant M-1
7.4 × 10 4
7.9 × 10 4
Detection limit ppm
2.8
1.7
5.0 × 10 4
5.2
5.5 × 10 4
3.3
2.1 × 10 2
2.9 × 10 2
12.3
7.9
1
× 10 2
15.4
11.3
2.17 × 10 2
A close look at the Table 1 and 2, result imply that receptor A1 shows a huge shift in
휆max along with the maximum binding constant and detection limits as compared to receptor A2,
its due to occurrence of –NO2 group at para position augments anion binding and also visible
sensing ability. Moreover, showed high selectively toward the Y- shaped AcO¯ ion probably
owing strong basicity and trigonal chemical structure which is facile to the formation of strong
multiple hydrogen bonding with the receptor. However, the receptor A3-A4 displays low binding
constant values for F¯ and AcO¯ ions compared to receptor A1 and A2 due to the absence of
extended π-conjunction effect.
3.5. Detection of inorganic anions using the receptors A1 and A2
Despite, anion recognition has wide application in the medical, environmental, biological
and industrial arenas which usually require an aqueous condition. Sodium salts of fluoride and
acetate have been major components of commercially available toothpaste, mouthwash and
vinegar respectively. Incase intake is beyond the certain limits anion can lead to a serious health
issue in the human body. Therefore, we have intent to check the anion binding ability in an
aqueous medium using sodium salts of fluoride and acetate. The binding and recognition ability
of receptor A1 and A2 (4.5 ×10-5 M in DMSO/H2O 9:1 v/v) was monitored by visual inspection
with naked eye detection and through UV-Vis studies. In the naked eye detection, a pale yellow
color solution of the receptor A1 turns to purple color upon addition of 2 equiv of NaAcO and
-2
NaF (10 M in H2O). Whereas, receptor A2 exhibits a discernible color change from pale
yellow to orange in the presences of 2 equiv. of NaAcO and NaF as shown in Fig. 7. The color
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change of the receptor solution both A1 and A2 implying the efficiency of the receptors to detect
the trace amount of anion in the organo-aqueous medium. In order to acquire quantitative insight
into receptor A1-anion interaction, UV-Vis spectrophotometric titration was carried out with the
consecutive addition of 0.1 equiv. of sodium salts of acetate and fluoride in receptor A1. As can
be seen in the Fig. 8 and Fig. 9 the intensity of the peak at 404 nm significantly decreases along
with increases in a new band at 540 nm and 538nm which assigned to intramolecular charge
transfer from the naphthyl –OH proton (donor) to the electron deficient -NO2 moiety (acceptor)
with a Δ휆max of 136 nm and 134 nm. A clear isosbestic point was observed at 457 nm and 454
nm, which determined that the formation of one stable species in titration solution. The Benesi-
Hildebrand (B-H) plot for the titration of the receptor A1 with NaAcO and NaF provided a
straight line, declare the formation of 1:1 bonding ratio with a binding constant of 6.9 × 10 3 M-1
3
and 6.6 × 10 M-1 as depicted in Fig S48 and Fig S49 in the ESI†. The detection limit
calculated in ppm, found to be 0.35 and 0.4 ppm for AcO¯ and F¯ ions. Surprisingly, a detection
limit of F¯ ion was much lower than the world health organization (WHO) standard level for
detection of fluoride in drinking water [82]. These revels, that receptor A1 is capable of detecting
sodium salts of AcO¯ and F¯ in aqueous medium and showed much lower detection limit in
comparison with previous reported literatures [83-86].
Fig. 7. Naked-eye visible color change of receptors A1 and A2 solution in DMSO (4.5 × 10-5 M
in DMSO/H2O 9:1 v/v) before and after addition of 2 equiv. of NaAcO and NaF (10-2 M in H2O).
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Fig. 8. UV-Vis absorption spectra of receptor A1 (4.5 ×10-5 M in DMSO/H2O 9:1 v/v)
throughout the course of titration process with the stepwise addition of 0.1 equiv. NaAcO (1×10-
2 M in H2O). Inset shows the equiv of AcO¯ ion vs. absorbance at 540 nm.
Fig. 9. UV-Vis absorption spectra of receptor A1 (4.5 ×10-5 M in DMSO/H2O 9:1 v/v)
throughout the course of titration process with the stepwise addition of 0.1 equiv. NaF (1×10-2 M
in H2O). Insert absorbance at 538 nm versus equiv of F¯ added.
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Similarity, UV-Vis titration was recorded for receptor A2 (4.5 ×10-5 M in DMSO/H2O
9:1 v/v) with successive addition of 0.1 equiv. of AcO¯ and F¯ ions as sodium salts (1×10-2 M in
H2O), the band at 388 nm progressively decreases and accompanied invisible color change due
to the emergence of the new band at 506 nm and 503 nm with a redshift of 118 nm and 115 nm
as illustrated in Fig. 10 - Fig. 11. A distinct isosbestic point was noticed at 426 nm and at 427 nm
signifying the formation of a new complex between the receptor A2 and the AcO¯ and F¯ ions.
The binding ratio of 1:1 between the receptor A2 with AcO¯ and F¯ was deliberate through the
Benesi-Hildebrand (B-H) plot as shown in Fig. S50 and S51 in ESI†. The binding constant of the
3
3
receptor A2 towards AcO¯ and F¯ ions were obtained as 2.2 × 10 M-1 and 3.9 × 10 M-1,
respectively. The detection limit turned out to be 3 ppm and 4.3 ppm.
The titration of receptor A1 and A2 in aqueous-DMSO medium showed a slight less red
shift as compared with dry DMSO solvent with AcO¯ and F¯ ions, presumably due to the
solvation of anion in an aqueous medium. Moreover, the result demonstrated that the spectral
changed recorded was similar as noticed on titration of receptor A1 and A2 with TBAF and
TBAAcO in dry DMSO solvent. However, receptor A1 and A2 are capable of detecting AcO¯
and F¯ ions, both in dry DMSO and aqueous-DMSO medium.
The aforementioned results suggested, that upon addition of sodium fluoride to the
receptor A1 due to the existence of the para-NO2 group, the phenolic –OH becomes more acidic,
which readily get deprotonation to form corresponding sodium salts as depicted in scheme 2. and
showed high binding constant and detection limit compared to receptor A2.
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Fig. 10. UV-Vis absorption spectra of receptor A2 (4.5 ×10-5 M in DMSO/H2O 9:1 v/v)
throughout the course of titration process with the stepwise addition of 0.1 equiv. NaAcO (1×10-
2 M in H2O). Inset shows the equiv of AcO¯ ion vs. absorbance at 506 nm.
Fig. 11. UV-Vis absorption spectra of receptor A2 (4.5 ×10-5 M in DMSO/H2O 9:1 v/v)
throughout the course of titration process with the stepwise addition of 0.1 equiv. NaF (1×10-2 M
in H2O). Inset shows the binding isotherm at 503 nm.
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NO2
NO2
N
N
N
NaF
N
OH
O
Na
-HF
A 1
Scheme 2. Proposed binding mode of receptor A1 in aqueous medium
3.6. UV-Vis spectroscopic studies of receptor A1 in DMSO/H2O (7:3, v/v)
Sensing of anions in water media is one of the most challenging tasks in the chemosensor
field. Because protic solvent such as water can participate with anion for binding sites and
therefore interrupt the hydrogen bonded sites between the receptor and anion which lead to a
shift of color and spectral response [87]. Surprisingly, receptor A1 instead to detect AcO¯ and F¯
ion even in aqueous medium proved the formation of hydrogen bonding sequentially
deprotonation between OH proton, strongly influence the intermolecular proton transfer process.
This is possible due to the presence of para-NO2 group into the backbone of the receptor assist in
raising the acidity of active sites. Henceforth, receptor A1 is capable of competing with an
aqueous medium to detect anion. Initially, visual examination of the color change of receptor A1
(4.5 ×10-5 M in DMSO/H2O 7:3 v/v) and UV-Vis absorbance spectrum change of receptor A1
¯,
was monitored in the presence of several anions, namely F¯, Cl¯, Br¯, I¯, NO3¯, HSO4
¯
-2
H2PO4 and AcO¯ (1 × 10 M) as TBA salts in DMSO solution. In addition, of 3 equiv of a
different class of anions, presumably, the color change was discernible from brown to purple for
F¯ and AcO¯ ions as shown in Fig. 12. However, induce a redshift in absorbance spectra of
receptor A1 upon addition of AcO¯ and F¯ ions as depicted in Fig. 13. On the other hand, all the
other anions were unable to produce neither color change, nor a shift in UV-Vis absorbance
spectra.
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Fig. 12. Color change of receptor A1 (4.5 ×10-5 M in DMSO/H2O 7:3 v/v) with the addition of 3
equiv. of a stock solution of several anions in the form of TBA salts (1 ×10-2M in dry DMSO).
Fig. 13. Change in UV-Vis spectral of receptor A1 (4.5 ×10-5 M in DMSO/H2O 7:3 v/v) upon
addition of 3 equiv of a stock solution of several anions in the form of TBA salts (1 ×10-2M in
dry DMSO).
In order to expand a deep perception of the anion recognition ability of the receptor A1 in
competitive media, UV-Vis titration was conducted. The free receptor A1 displayed a strong
band at 404 nm. With stepwise addition of 1 equiv. of AcO¯ ion to receptor A1 remarkably
decline in intensity of absorption band at 404 nm. Concurrently, began to increase of a new band
at 535 nm and continuous addition of AcO¯ ion assists to the bathochromic shift of 131 nm from
404 nm to 535 nm. A sharp isosbestic point was noticed at 450 nm, which indicate that a stable
complex formation between the receptor A1-AcO¯ ion as given in Fig. 14. Similarly, UV-Vis
titration was recorded with F¯ ion. On cumulative addition of 1 equiv. of F¯ ion to receptor A1
solution showed the red shift of 129 nm as shown in Fig. S52 in ESI†.To reach a plateau required
3 equiv of AcO¯ and F¯ ions in aqueous medium, whereas in DMSO it requires 2 equiv of
20

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anions to complete the reaction. This may be due to the solvent interference. Further, the
stoichiometric ratios of 1:1 complexes between the receptor A1-AcO¯ and receptor A1----F¯
ions which have been analyzed with the help of UV-Vis titration data using Benesi- Hildebrand
method (B-H) plot as given in Fig. S53 and S54.
Fig. 14. UV-Vis absorption spectra of receptor A1 4.5 × 10–5 M in (7:3, DMSO/H2O) throughout
the course of titration process with the stepwise addition of (1-3 equiv.) of a stock solution of
TBAAcO (1 × 10–2 M). Insert showing the binding isotherm at 535 nm.
3.7. UV-Vis spectroscopic studies of receptor A1 in buffer solution
The anion is more pH sensitivity. Therefore, anion binding properties were a probe in
buffer medium with receptor A1 (4.5×10¯5 M, DMSO: HEPES buffer; 9:1, v/v, pH 7.4). The
receptor A1 showed the excellent affinity towards AcO¯ ion. This could be credited due to the
following reasons 1. The shape complementary of the receptor A1 matches well with the AcO¯
ion 2. Alkalescence of the AcO¯ ion is higher than the other competitive anions. 3. Basicity of
receptor. These are the reasons for AcO¯ ions bind strongly with the receptor A1 [88].
UV-Vis absorbance of the receptor was initially carried out in the presence of various
¯
-2
interference anions, including (F¯, Cl¯, Br¯, I¯, NO3 , HSO4¯, H2PO4¯ and AcO¯ (1 × 10 M in
dry DMSO) in form of TBA salts. Upon addition of 3 equiv. of aliquot anions, receptor A1
showed a perceptible color change from pale yellow to pink as represented in Fig. 15. and a
considerable red shift in the UV-Vis absorption band for AcO¯ ion as shown in Fig. 16. In
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contrast, no significant shift was observed in the absorption spectrum with the addition of other
anions. To further enlighten the nature of the interaction between receptor A1 and AcO¯ ion,
UV-Vis titration studies were performed. Free receptor A1 showed an intrinsic maximum
absorption band at 404 nm attribute to the π π* transition. Upon successive addition of 1 equiv
of a standard solution of TABACO to receptor A1 solution, the diminution in the intensity of
absorption band at 404 nm and the parallel increase new band at 513 nm with a sharp isosbestic
point at 453 nm, which indicate the one stable complex formation between receptor A1---AcO¯
ion as given in Fig. 17. Bathochromic shift of 109 nm was noticed. This may be due to charge
transfer between the anion bond -OH protons and the electron deficient NO2 group in the para
position. The binding ratio 1:1 obtained between receptor A1 and AcO ion has been investigated
with Benesi-Hildebrand (B-H) plot as represented in Fig S55 in ESI†. Under the similar
condition, UV-Vis titration spectra were recorded with TBAOH showed new absorption band at
513 nm well matched with the absorption band in presence of AcO¯ ion as depicted in Fig. S56
in ESI†. Hence, result discloses the acid-base process to give rise the deprotonation phenomenon
of receptor A1 upon addition of AcO¯ ion at pH 7.4 [89].
Fig. 15. Color change of receptor A1 (4.5×10–5 M, DMSO: HEPES buffer; 9:1, v/v, pH 7.4) after
addition of 3 equiv. Of the various anions in form of TBA salts
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Fig. 16. UV-Vis absorption spectra change of receptor A1 (4.5×10–5 M, DMSO: HEPES buffer;
9:1, v/v, pH 7.4) upon addition of 3 equiv of TBA salts of anions
Fig. 17. UV-Vis titration spectral of receptor A1 (4.5×10–5 M, DMSO: HEPES buffer; 9:1, v/v,
pH 7.4) upon step-wise addition of 1 equiv of TBAAcO (1×10–2 M). Inset shows the binding
isotherm at 513 nm.
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3.8. Fluorescence response studies of receptors A1 in DMSO
The anion binding behavior of receptors A1-A4 (4.5×10–5 M) was also investigated by
fluorescence titration in DMSO solvent. Receptors (A1-A4) were more active towards AcO¯ and
F¯ ions over the other interference anions due to high basicity.
Receptor A1 showed fluorescence emission at 470 nm (excited at 430 nm) in DMSO. Upon
subsequent addition of 0.1 equiv of AcO¯ ion to the solution of receptor A1 the fluorescence got
enhanced due to the complex formation between the receptors and added anions as represented in
Fig. 18. Under the similar condition fluorescence titration was carried out with F¯ ion, showed
fluorescence emission at 465 nm (excited at 425 nm) as shown in Fig.S57 in ESI†.
Moreover, receptor A2 displayed fluorescence emission at 437 nm (excited at 373 nm) in
DMSO. Upon successive addition of 0.1 equiv of AcO¯ ion to receptor A2 the enhancements in
the fluorescence intensity as represented in Fig.S58 in ESI†. Similar fluorescence titration was
performed with F¯ ion, showed fluorescence emission at 433 nm (excited at 360 nm) as shown in
Fig.S59 in ESI†. Receptor A3 exhibited fluorescence emission at 357 nm (excited at 305 nm) in
DMSO. Upon consecutive addition of 0.1 equiv of AcO¯ ion to solution of receptor A3 showed
the enhancements in the fluorescence intensity as represented in Fig.S60 in ESI† parallel
fluorescence titration was also performed with F¯ ion and showed fluorescence emission at 337
nm (excited at 300 nm) as shown in Fig.S61 in ESI†. Correspondingly receptor A4, fluorescence
titration carried out with AcO¯ and F¯ ions showed fluorescence emission at 279 nm (excited at
235 nm) in DMSO. Upon incremental addition of 0.1 equiv of AcO¯ ion to solution of receptor
A4 showed the enrichment in the fluorescence intensity as represented in Fig.S62 in ESI†.
Similar fluorescence titration was performed with F¯ ion, showed fluorescence emission at 255
nm (excited at 180 nm) as shown in Fig.S63 in ESI†.
The fluorescence titration spectra proved that receptor A1 showed a higher fluorescence
enhancement in addition of anions when compared with other receptors probability due to the
presence of NO2 at a para position which easily induced electron transfer mechanism. However,
upon interaction with anions, the electron transfer from the electron rich –OH moiety bonded
with anion to the electron deficient –NO2 became more feasible and hence fluorescence
enhancement was observed [91]. Upon further addition of anion it appeared that the deprotonated
24

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species being more electron rich compared to the hydrogen bonded complex with acetate or
fluoride ions, undergo very fast electron transfer showed up greater fluorescence enhancement.
Fig. 18. Changes in the emission spectra of receptor A1 upon addition of TBAAcO solutions in
DMSO (휆exc= 430 nm).
3.9. Solvatochromic study
Solvent effect plays an imperative role in anion binding and selectivity. Hence, the
sensing behavior of the receptor was studied in a different solvent. The absorption bands of a
receptor change with the solvent polarity they are called solvatochromic. To our surprise,
receptor A1 shows a solvatochromism behavior with different solvent upon addition of AcO¯
ion. To gain an insight into solvatochromic effect, the receptor A1 (4.5 × 10¯5 M) was studied in
seven distinct organic solvents with a different polarity such as DMSO, ACN, THF, Acetone,
DCM, DMF, and 1,4-dioxane respectively. Upon addition of 2 equiv. AcO¯ ion a discernible
color change was observed from pale yellow to purple, pink, dark blue, light purple, brown, blue,
and lavender, respectively as shown in Fig. 22 and showed a considerable shift in the absorbance
band as depicted in fig. S64 in ESI†.
25

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-5
Fig. 22. Solvatochromic effect of receptor A1 (4.5×10 M) in various polar aprotic solvents
with the addition of 2 equiv. of TBAAcO (1×10 -2 M).
To sense deeply about the interaction of receptor A1 with various aprotic solvents UV-
Vis titration was performed. UV-Vis titration of receptor A1 with TABACO in THF exhibits a
bathochromic shift of 160 nm from 404 nm to 564 nm. Similarly DMF, the considerable
bathochromic shift of 156 nm from 404 nm to 560 nm relatively more than DMSO as illustrated
in fig.S65 ESI†. However, in case of ACN, Acetone, DCM and 1,4-Dioxane UV-Vis titration
showed a shift of 130 nm (휆1=400 nm and 휆2= 530 nm) ,124 nm (휆1=400 nm and 휆2= 524 nm) as
depicted in Fig.S66. in ESI†,120 nm (휆1=400 nm and 휆2= 520 nm) and 115 nm (휆1=400 nm and
휆2= 515 nm) respectively, as represented in Fig.S67 in ESI†. This result enunciates that [receptor
A1---AcO¯] complex is more stable in THF followed by DMF and DMSO among other protic
solvent.
3.10. DFT Calculations
The molecular modeling calculations of the receptors (A1 and A2) in the absence and
presence of the selective anions (F- and AcO-) were performed by applying the B3LYP
exchange-correlation functional and the basis set 6-31G* in the DMSO medium. The solvent
effect on the model structure was applied by the conductor-like polarizable continuum model
(CPCM) solvation model. All calculations were done by using the computational code Gaussian
09W [91]. The optimized structure of the receptors A1 and A2 was shown in Fig. 23 with the
intramolecular hydrogen bond of length 1.693 and 1.713 Å. The nitro substitution at the ortho
position of A2 resulted in a less planar structure in compared to A1, which possibly restricted the
internal change transfer process. Also, the receptor A1 is energetically -9.68 kcal/mol more
stable than A2. Analysis of Mulliken’s atomic charges indicates that the naphthol proton
26

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possessed the maximum positive charge of 0.489 (A1) and 0.444 (A2) to make possible
interaction with the incoming anions. Upon interaction with the anions, the O-H bond length of
A1 (1.004 Å) and A2 (0.997 Å) increases to ~1.65 Å which supported that the anionic analytes
have the potential to form an internal hydrogen bond and then to abstract the naphthol-OH
towards itself. On complexation of A1 with F- and AcO- resulted in the lowering of the
interaction energy (Eint = Ecomplex-Ereceptor-EAcO-/F-) by -0.40 kcal/mol and -10.90 kcal/mol, which
indicates the affinity between the receptor and the anions along with the formation of a stable
complex. Similar lowering in the interaction energy by -47.80 kcal/mol and -15.00 kcal/mol was
noted when A2 interacted with F- and AcO-, respectively.
Fig. 23. Optimized structure of receptors (a) A1 (b) A2 and their complexes with F¯ and AcO¯
at B3LYP/6-31G* level in DMSO.
As proposed, the possible intramolecular change transfer in the receptors upon interaction
with the anions resulted in the red shift in the absorption band in the visible region resulting in
the intense color change which can be easily visually detected. To complement the experimental
evidence, the HOMO and LUMO diagrams of the receptors A1 and A2 was compared with their
complexes formed with the F- and AcO- anions. As shown in Fig. 24, the electron density of the
HOMO of A1 mainly located over the nitrophenol unit, but the LUMO electron density was
distributed over naphthol unit. Upon interaction with the anions F- and AcO-, the HOMO
electron density was observed at naphthol unit, whereas the LUMO electron density at
nitrophenol units. This observation supported the possible intramolecular change transfer
occurred with the receptor A1 upon interaction with the anions. Also, the calculated band gap ΔE
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(ΔE=ELUMO-EHOMO) of A1 was decreased from 0.10363 eV to 0.07682 eV and 0.10159 eV in the
presence of F- and AcO- respectively.
The TD-DFT calculations of A1-F and A1-AcO- resulted in an absorption band in the
visible region at 657.89 (oscillatory strength, f = 0.7013) and 648.28 (f = 0.7182), which
corroborates well with the experimental evidence as depicted in Fig.S68 in ESI†. . The decrease
in band gap is due to ICT mechanism involve the deprotonation of OH proton due to the
presence of NO2 group at a para position which makes OH proton more acidic for easy
deprotonation upon interaction with the F¯ ion Similarly, in the case of receptor A2, the HOMO-
LUMO energy gap was found to be ΔE = 0.10971 before binding decreased to 0.08098 EV and
0.07933 eV after complication with the F¯ and AcO¯ as illustrated in Fig.S69 in ESI†. Also, TD-
DFT calculations supported the absorption in the visible region 678.62 nm (f = 0.3151) and
665.74 nm (f = 0.3113) for the A2-F¯ and A2-AcO¯ complexes, respectively as represented in
Fig.S70 in ESI†.
Fig. 24. HOMO’s and LUMO’s diagrams for (a) receptor A1, (b) receptor A1-F¯ and (c)
receptor A1-AcO¯ obtained by B3LYP/6-31G* method.
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3.11. 1H NMR study
In order to deeply validate into the sensing mechanism. 1H NMR titration experiment was
conducted in a DMSO-d6 solvent. 1H NMR titration of receptor A1 was evaluated in the absence
and presence of TBAAcO (1×10¯2 M in dry DMSO) as depicted in Fig. 25. The free receptor
showed a peak at 12.8 ppm imply to OH proton attached to the naphthalene unit. In addition, of
0.5 equiv of aliquots of AcO¯, the peak at 12.8 ppm exhibited a downfield shift to 13.2 ppm.
This indicates the AcO¯ ion indeed interacted with –OH proton through hydrogen bonding
[92].With the increasing the concentration of AcO¯ ion to 1 equiv.complete vanish –OH proton
through deprotonation. On further addition of 2 equiv. AcO¯ ion no chemical shift was observed,
denoting the 1:1 binding ratio between receptor A1 and AcO¯ ion which proved by B-H plot.
1
The above HNMR result precisely reflects that binding process involved two step (1) On
addition of 0.5 equiv. AcO¯ ion induced a strong hydrogen bonding interaction between
AcO¯……. O-H proton of receptor A1 (2) Further, the addition of 1 equiv. of AcO¯ ion exhibits
deprotonation of -OH proton. However, the binding mechanism of receptor A1 and AcO¯ ion is
represented in scheme 3.
Fig. 25. 1H NMR titration of receptor A1 in a DMSO-d6 solvent with an incremental addition of
(0-2 eq.) of TBAAcO.
29