Novel salicylic acid-oriented thiourea-type receptors as colorimetric chemosensor: Synthesis, characterizations and selective naked-eye recognition properties

Article abstract of DOI:10.1016/j.saa.2012.04.102

Based on the salicylic acid backbone, three highly sensitive and selective colorimetric chemosensors with an acylthiourea binding unit have been designed, synthesized and characterized. These chemosensors have been utilized for selective recognition of fluoride anions in dry DMSO solution by typical spectroscopic titration techniques. Furthermore, the obtained chemosensors AR1-3 have shown naked-eye sensitivity for detection of biologically important fluoride ion over other anions in solution.

Full text of DOI:10.1016/j.saa.2012.04.102

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 96 (2012) 18–23
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Novel salicylic acid-oriented thiourea-type receptors as colorimetric
chemosensor: Synthesis, characterizations and selective naked-eye
recognition properties
a
b,
Shaowei Li a, Xiufang Cao a, , Changshui Chen , Shaoyong Ke
⇑
⇑
a College of Science, Huazhong Agricultural University, Wuhan 430070, People’s Republic of China
b National Biopesticide Engineering Research Center, Hubei Academy of Agricultural Science, Wuhan 430064, People’s Republic of China
g r a p h i c a l a b s t r a c t
h i g h l i g h t s
"
"
"
Receptors AR1–3 can be utilized as a
colorimetric chemosensor for
detection of FÀ anion.
Receptors AR1–3 are novel salicylic
acid-oriented thiourea-type
receptors.
Receptors AR1–3 showed excellent
selectivity toward FÀ over the other
competitive anions.
a r t i c l e i n f o
a b s t r a c t
Article history:
Based on the salicylic acid backbone, three highly sensitive and selective colorimetric chemosensors with
an acylthiourea binding unit have been designed, synthesized and characterized. These chemosensors
have been utilized for selective recognition of fluoride anions in dry DMSO solution by typical spectro-
scopic titration techniques. Furthermore, the obtained chemosensors AR1–3 have shown naked-eye
sensitivity for detection of biologically important fluoride ion over other anions in solution.
Ó 2012 Elsevier B.V. All rights reserved.
Received 22 February 2012
Received in revised form 17 April 2012
Accepted 28 April 2012
Available online 7 May 2012
Keywords:
Thiourea
Salicylic acid
Colorimetric sensor
Anion recognition
Naked-eye
Introduction
simulation, enzyme catalyst synthesis, etc. [4]. Up to now, there
are several structural types of synthetic receptors (Fig. 1) have been
In recent years, anions recognition has attracted considerable
attentions due to the increasing importance of anions in extension
areas such as the biology, environment and industry [1,2]. So the
development of novel anion receptors is of great interest and
significance in the field of host–guest chemistry [3]. Anion recogni-
tion by artificial receptors has represented a unique bioanalytical
application in anion sensor, membrane transmit carrier and
developed mainly included urea/thiourea-based receptors
(compounds 1–3) [5–7], amide-based receptors (compounds 4–6)
[8–10], and other neotype receptors (compounds 7–9) [11–13],
etc. In particular, colorimetric-based sensing is especially attractive
in these years, as it may allow naked-eye detection of the analyte
without resorting to any expensive equipment [7,9,14].
Meanwhile, as we know, salicylic acid is one of a wide variety of
phenolic compounds distributed in many organisms, and which
plays an important role in biochemical process [15–17] and coor-
dination chemistry [18,19]. Thus, keeping these perspectives
in mind, a novel series of salicylic acid-oriented thiourea-based
⇑
Corresponding authors. Tel.: +86 27 59101956; fax: +86 27 59101911.
E-mail addresses: caoxiufang@mail.hzau.edu.cn (X. Cao), keshaoyong@163.com
(S. Ke).
1386-1425/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.saa.2012.04.102

S. Li et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 96 (2012) 18–23
19
O
N
O
S
X
X
Y
CF3
NH HN
N
H
O
N
N
H
O2N
NH
HN
NO2
X
N
H
N
H
X=O and S
Compd. 3
O
Compd. 1
Compd. 2
O
NO2
N
O
O
O2N
NH HN
NH HN
N
NH HN
O
NH
HN
NO2
N
N
O
O
4
5
6
Compd.
Compd.
Compd.
H
N N
NO2
O2N
N
HN
H
HN
O2N
N N
NO2
Ph3P
PPh3
Br- Br-
Compd.
O
7
8
9
Compd.
Compd.
Fig. 1. Representative structural types of various anion receptors.
Binding sites
III elemental analysis instrument. Analytical thin-layer chromatog-
raphy (TLC) was carried out on precoated plates, and spots were
visualized with ultraviolet light. The UV–vis spectra were recorded
R
NO2
COOH
O
O
S
on
a Shimadzu UV-2450 Spectrophotometer (Shimadzu 2.1
N
H
N
H
OH
Chromophore unit
Apparatus Corp., Kyoto, Japan) using a quartz cuvette (path
length = 1 cm) at 298.0 0.1 K. All the anions in the form of tetra-
butylammonium salts were purchased from Sigma–Aldrich
Chemical Co. or Sinopharm Chemical Reagent Co., and stored in a
vacuum desiccator containing self-indicating silica. Anhydrous sol-
vents like dimethylsulphoxide (DMSO) and acetonitrile (CH3CN)
were dried according to standard methods [20]. All other solvents
and reagents were analytical reagent and used directly without
purification.
Salicylic acid
Salicyl-Thiourea Receptors
Fig. 2. Design strategy of salicylic acid-oriented thioureas as anion receptors.
colorimetric anion receptors (Fig. 2) have been designed and syn-
thesized, and which have been elucidated to selectively recognize
for fluoride anions through naked-eye visible color changes. The
thiourea subunit in these receptors, speculated to be a hydrogen
bond-donor and suitable to interact with the anions, has been ap-
pended in between substituted-salicyl and 4-nitrophenylamine
units. The covalently linked 4-nitrophenylamine moiety intends
to act as a chromophore unit. The experimental results indicate
that these receptors are highly selective and sensitive to recognize
fluoride anion in dry DMSO and the processes of sensing can be
obviously seen through the visible color changes for naked-eye
recognition.
Syntheses of target receptors AR1–3
General synthetic procedure for the key intermediates 3
The key intermediates 3 can be obtained via three steps includ-
ing esterification, alkylation and saponification reactions. The gen-
eral procedures are as follows: (1) To a solution of prepared methyl
2-hydroxybenzoate (4.56 g, 30 mmol) and halides (30 mmol) in
MeCN (40 mL) was added K2CO3 (4.55 g, 33 mmol). The resultant
mixture was stirred at room temperature for 1.5 h and another
about 6–8 h at 50–60 °C, which was detected by TLC. The mixture
was cooled to ambient temperature, after filtration and concentra-
tion in vacuo, the residue was recrystallized from ethanol. (2) To a
solution of the newly prepared compound 2 (20 mmol) in MeOH
(5 mL) was added aqueous NaOH (20%, 1.5 eq.). The resulting mix-
ture was stirred at 45–50 °C for 4–6 h. After cooling the mixture to
room temperature, it was acidified with aqueous HCl (5.0 M) to pH
2–3 and the precipitates were filtered off by filter paper. After
washing (H2O and hexanes) and drying in vacuo, the intermediates
Experimental section
Materials and general methods
All melting points (m.p.) were obtained using a digital model
X-5 apparatus and are uncorrected. Infrared (IR) spectra in potas-
sium bromide (KBr) were recorded on a Thermo Nicolet FT-IR
Avatar 330 instrument. 1H NMR spectra were recorded on a
Brucker spectrometer at 400 MHz with DMSO-d6 as the solvent
and TMS as the internal standard. Chemical shifts are reported in
d (parts per million) values. Coupling constants nJ are reported in
Hz. Mass spectra were performed on a MicroMass Quattro micro™
API instrument. Elemental analyses were performed on a Vario EL
3 were obtained as a white powder. 3a (R1 = 2-chloro-5-methylpyr-
+
idine), yield 94%, m.p. 151–152 °C, ESI–MS: 264.5 (C13H11ClNO3
,
[M + H]+); 3b (R1 = 4-fluorobenzyl), yield 96%, m.p. 85–86 °C,
ESI–MS: 229.4 (C14H11FO3, [M-OH]+).

20
S. Li et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 96 (2012) 18–23
General synthetic procedure for receptors AR1–3
Adsorption titration studies
All the receptors AR1–3 were easily prepared according to re-
ported conventional method via chlorination, nucleophilic
substitution and subsequent nucleophilic addition reaction. The
general procedure are as follows: The newly prepared acylchloride
were treated with dry potassium thiocyanate in the presence of
anhydrous acetonitrile for about 1–2 h, which were filtered and
the filtrate were used directly for the next reaction with nitroani-
line at 60–70 °C for 0.5–3 h. The precipitate were filtered and
recrystallized from alcohol to afford the target compounds AR1–
3. Their physico–chemical properties and the spectra data are as
follows:
A 2.0 Â 10À3 M solution of the receptor AR1–3 in DMSO was pre-
pared and stored in the dry atmosphere. Solutions of 1.0 Â 10À2 and
1.0 Â 10À1 M tetrabutylammonium salt of the respective anionwere
prepared in dried and distilled DMSO and were stored under a dry
atmosphere. The binding stability of receptor or chemosensor for
several anions (as tetrabutylammonium salts) was investigated by
UV–vis absorption spectroscopy in DMSO solution keeping a
constant host concentration (4 Â 10À5 M) and increasing concentra-
tions of anions by stepwise addition of different equivalents.
Results and discussion
2-((6-Chloropyridin-3-yl)methoxy)-N-(4-nitrophenylcarbamothioyl)
benzamide AR1. This compound was obtained following the
above-described method as light yellow powder, yield 95%, m.p.
Synthesis of receptors AR1–3
229–230 °C; IR (m
max, KBr, cmÀ1): 3303.80 (NH), 1673.44 (NHC@O),
In the present study, three novel salicylic acid-oriented thiourea
derivatives were designed and synthesized as artificial anion
receptors. The general method for the preparation of target recep-
tor molecules AR1–3 are outlined in Scheme 1.
1291.31 (C@S); 1H NMR (400 MHz, DMSO-d6): dH = 8.64 (s, 1H),
8.26 (d, J = 8.0 Hz, 2H), 8.16 (d, J = 8.0 Hz, 1H), 8.03 (d, J = 8.0 Hz,
2H), 7.86 (d, J = 8.0 Hz, 1H), 7.68 (t, J = 8.0 Hz, 1H), 7.56
(d, J = 8.0 Hz, 3H), 7.39 (d, J = 8.0 Hz, 1H), 7.19 (t, J = 8.0 Hz, 1H),
5.35 (s, 2H); MS (ESI) m/z 443.8 (M + H)+, calcd. for C20H15ClN4O4S
m/z = 442.1; Anal. Calcd for C20H15ClN4O4S: C, 54.24; H, 3.41; N,
12.65. Found: C, 54.37; H, 3.49; N, 12.53.
The low-cost salicylic acid was selected as starting materials,
which were routinely transferred to the corresponding methyl
salicylate by general esterification reaction. The following alkyl-
ation reaction of methyl salicylate was treated with selective ha-
lides in the presence of potassium carbonate, and the key
intermediate 2 can be conveniently separated as colorless crystal
with good yield. The ester 2 was saponified with aqueous metha-
nolic sodium hydroxide solution to substituted acid 3. Activation
of the carboxylic acid 3 with thionyl chloride provided the corre-
sponding acid chloride, which was further used to react with
potassium thiocyanate to give acylisothiocyanate 4a–c. The target
receptor molecules AR1 and AR2 can be easily obtained via nucle-
ophilic addition reaction of intermediate 4a–b with 4-nitroaniline.
The receptor AR3 was prepared using similar method starting from
2-methoxybenzoic acid 3c. All the synthesized compounds AR1–3
gave satisfactory chemical analyses. The IR, 1H NMR, and ESI-MS
spectra were consistent with the assigned structures.
2-(4-Fluorobenzyloxy)-N-(4-nitrophenylcarbamothioyl)benzamide
AR2. This compound was obtained following the above-described
method as light yellow powder, yield 84%, m.p. 201–203 °C; IR
(m
max, KBr, cmÀ1): 3285.05 (NH), 1660.48 (NHC@O), 1223.69
(C@S); 1H NMR (400 MHz, DMSO-d6): dH = 12.85 (s, 1H), 11.42 (s,
1H), 8.28 (d, J = 9.2 Hz, 2H), 8.08 (d, J = 8.8 Hz, 2H), 7.89
(d, J = 7.6 Hz, 1H), 7.67 (t, J = 8 Hz, 3H), 7.40 (d, J = 8.4 Hz, 1H),
7.25-7.17 (m, 3H), 5.32 (s, 2H); MS (ESI) m/z 426.7 (M + H)+, calcd.
for C21H16FN3O4S m/z = 425.1; Anal. Calcd for C21H16FN3O4S: C,
59.29; H, 3.79; N, 9.88. Found: C, 59.42; H, 3.72; N, 9.76.
2-Methoxy-N-(4-nitrophenylcarbamothioyl)benzamide
compound was obtained following the above-described method
as light yellow powder, yield 88%, m.p. 219–221 °C; IR ( max, KBr,
AR3. This
m
cmÀ1): 3282.36 (NH), 1662.74 (NHC@O), 1262.61 (C@S); 1H NMR
(400 MHz, DMSO-d6): dH = 8.28 (d, J = 12 Hz, 2H), 8.03 (d,
J = 8.0 Hz, 2H), 7.89 (t, J = 4.0 Hz, 1H), 7.66 (t, J = 8.0 Hz, 1H), 7.28
(d, J = 8.0 Hz, 1H), 7.16 (t, J = 8.0 Hz, 1H), 4.00 (s, 3H); MS (ESI)
m/z 352.5 (M + Na)+, calcd. for C15H13N3O4S m/z = 331.1; Anal.
Calcd for C15H13N3O4S: C, 54.37; H, 3.95; N, 12.68. Found: C,
54.50; H, 4.04; N, 12.62.
Spectroscopic studies on receptors
The anion binding affinity of present thiourea-base receptors
AR1–3 were determined by UV–vis absorption spectra in the ab-
sence and presence of different anions such as FÀ, ClÀ, BrÀ, IÀ,
À
HSO4À, ClO4 (as tetrabutylammonium salts). The experiment
was performed by preparing 2 Â 10À5 M solution of receptor in
R1
O
R1
R1
O
O
O
a and b
O
d and e
O
c
O
OH
NCS
O
OH
OH
1
2a-c
3a-c
AR1 R1 =
4a-c
, R2 = 4-NO2
R1
O
O
S
R2
f
Cl
N
N
H
N
H
AR2 R1 =
, R2 = 4-NO2
F
AR1-3
R1 = Me, R2 = 4-NO2
AR3
Scheme 1. Synthesis of designed molecules. Reagents and conditions: (a) MeOH, Conc. H2SO4; (b) halides, K2CO3, MeCN, r.t. to reflux for 10–12 h; (c) NaOH, MeOH/H2O, r.t. to
45 °C for 4–6 h; (d) SOCl2, reflux for 3–6 h; (e) potassium thiocyanate, MeCN, 60–70 °C for 1–2 h and (f) nitroaniline, MeCN, 60–70 °C for 0.5–3 h.

S. Li et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 96 (2012) 18–23
21
AR1-F-
(A)
(B)
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
0.5
AR1
AR1-F-
AR1-Cl-
AR1-Br-
AR1-I-
0.4
0.3
0.2
0.1
0.0
AR1-HSO4-
AR1-ClO4-
AR1-Br-
AR1-Cl-
-
AR1-ClO4
-
AR1
AR1-HSO4
AR1-I-
300
350
400
450
500
550
410nm
nm
Fig. 3. (A) Change in UV–vis spectrum of receptor AR1 (4 Â 10À5 M) in DMSO solution with addition of 10 times of different anions (FÀ, ClÀ, BrÀ, IÀ, HSO4À, ClO4À) as their
tetrabutylammonium salt. (B) Absorbance of receptor AR1 (4 Â 10À5 M) at 410 nm with addition of various anions (1 Â 10À3 M) in DMSO solution.
changed from yellowish to brownish yellow (Fig. 4). However, no
detectable color responses were observed when adding ClÀ, BrÀ,
IÀ, HSO4À, and ClO4À ions, even in a large amount (up to 100 times)
to the solution. These results suggest that receptor AR1 can distin-
guish FÀ from many other anions. From Fig. 3A, in the absence of
anions, the spectrum of chemosensor AR1 showed two characteris-
tic absorption peaks at 270 and 345 nm. Upon the addition of FÀ to
AR1, a prominent change was observed in UV–vis absorption spec-
tra due to complexation between host–guest molecules. The ICT
band at 345 nm as shown in Fig. 3A, disappeared with the
Fig. 4. Color changes of chemosensor AR1 in DMSO solution (4 Â 10À5 M) after
formation of a new peak in yellow region of spectra at 410 nm.
addition of 10 equiv. of different anions. From left to right: AR1, AR1 + FÀ, AR1 + ClÀ,
The Fig. 3B showed the absorbance of receptor AR1 (4 Â 10À5 M)
AR1 + BrÀ, AR1 + IÀ, AR1 + HSO4À, AR1 + ClO4
.
À
at 410 nm with addition of various anions (1 Â 10À3 M) in DMSO
solution.
dimethylsulphoxide (DMSO) solution followed by addition of par-
ticular tetrabutylammonium anionic salts separately.
Similarly, the colorimetric properties of these receptors were
explored to establish if the changes in absorbance could be ob-
servable through the ‘naked eye’ and the findings are presented
in Fig. 4. It was observed that receptor AR1 showed selectivity
Obviously spectral changes were observed after adding FÀ to
the solution (Fig. 3). Moreover the color of solution of AR1 was
0.8
(A)
(B)
0.5
0.4
0.7
0.3
0.2
0.6
0.1
0
2
4
6
8
10
0.5
0.4
0.3
0.2
0.1
0.0
[F]/[AR-5]
300
350
400
450
500
550
nm
Fig. 5. (A) Changes in UV–vis spectra for the receptor AR1 (4 Â 10À5 M) in DMSO solution with sequential addition of [Bu4N]F from 0 to 10 equiv. Inset: The observed changes
(the black square) at 410 nm at room temperature and the result (Red solid line) of the fitted data using Nonlinear curve fitting. (B) Color changes of chemosensor AR1 in
DMSO solution (4 Â 10À5 M) after addition of different equivalent of fluoride anions (From left to right: AR1, AR1 + 10 eq. FÀ, AR1 + 21 eq. FÀ, AR1 + 44 eq. FÀ, AR1 + 76 eq. FÀ).
(For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

22
S. Li et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 96 (2012) 18–23
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
-0.05
toward the various anions and the color changes were observed
by ‘naked eye’ experiments in which the solution changed from
pale yellow to orange-yellow. It was noted that it required
about 20 equiv. of fluoride to produce the orange-red (Fig. 5B).
Similar experiments [21] were carried out for receptor AR2 and
AR3, and the similar color changes and test results were also
obtained.
Subsequently, in order to further evaluate the specific concen-
tration for selective anions and its associated colorimetric
changes, receptor AR1 was titrated by successive increment of
number of equivalents for fluoride ion separately and monitored
the UV–vis absorption spectra. From UV–vis titration spectra
(Fig. 5A), it can be seen that as the number of equivalents of
FÀ increases, the absorption bands appeared at 270 and
345 nm wavelengths gradually reduced and new bands appeared
at 315 and 410 nm wavelengths. The formation of isosbestic
points indicated the presence of two ionic species in the med-
ium. In titration experiment, the small change at high energy
band was observed, which is attributed to intramolecular charge
transfer (ICT) towards electron deficient ANO2 group of
0.0
0.2
0.4
0.6
0.8
1.0
[F-]/[F-]+[AR1]
Fig. 6. Job’s plot between receptor AR1 and fluoride anion in DMSO solution at
25 °C.
0.8
(A)
(B)
AR1-F-
1:9(V/V)
2:9(V/V)
3:9(V/V)
AR1
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
300
350
400
450
500
nm
Fig. 7. (A) Change in UV–vis spectrum of receptor AR1 (4 Â 10À5 M) and fluoride anion (4 Â 10À4 M) in DMSO solution with addition of protonic solvent such as methanol
(VMeOH:VAR1+F = 1:9, 2:9, 3:9). With CH3OH was dropped in the DMSO solution of complextion of AR1 and [(Bu)4N]F, the color of the complextion was fade when
V
MeOH:VAR1+F = 1:9, but it will recover when the DMSO was dropped in the complextion again. When VMeOH:VAR1+F = 3:9, the color of the complextion will fade forever no
matter how much DMSO was added in. (B) Color changes of chemosensor AR1 and fluoride anion in DMSO solution (4 Â 10À5 M) after addition of methanol. From left to right:
AR1, AR1 + FÀ, AR1 + FÀ + MeOH.
F-
H
N
H
N
H
N
H
N
Cl
O
F-
Cl
Cl
O
O
N
O
S
N
N
O
O
S
NO2
NO2
AR1
Cl
O
N
N
N
N
ICT
N
O
S
O
S
O
N
O
N
O
Scheme 2. Proposed deprotonation mechanism for thiourea-based chemosensor AR1 induced by FÀ ion.

S. Li et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 96 (2012) 18–23
23
phenylamine moiety from anionic ANH fragment of receptor in
the influence of FÀ anion. It can be also seen from Fig. 5B that
as the number of equivalents of FÀ anion increases, the color
changes were observed by ‘naked eye’ experiments in which
the solution changed from pale yellow to orange-red.
chemosensors for selective and sensitive naked-eye recognition
for biologically important anions such as the fluoride ion. Further
application and properties about the designed novel thiourea
derivatives are well ongoing in our laboratory.
Acknowledgments
Binding mechanism and stoichiometry
This work was financially supported by the Fundamental Re-
search Funds for the Central Universities (2010JC001), and the
authors also gratefully acknowledge the partial support from the
National Natural Science Foundation of China (31101467).
We set AR1 as an example to study the recognition mechanism
and combination mode between host and guest. Firstly, Job’s plot
(Fig. 6) has been drawn to know the stoichiometry between the
guest (FÀ) and host (receptor AR1) molecules in DMSO solution.
Secondly, based on the result of this experiment, we can see
that with the increasing concentration of fluoride anion, the
absorption value at 410 nm was enhanced gradually while weak-
ened at 315 nm, with one isosbestic point at 360 nm all the time.
From the titration curve of Job–Plot, we also know that receptor
AR1 can form a stable 1:1 complexation with fluoride. The obvious
color fade away (Fig. 7B) when methanol, the competitive hydro-
gen-bonding solvent, was added in the mixed solution of AR1
and fluoride, which indicated that the hydrogen bond between
guest and host molecules was formed [22,23] in this recognition
process with the form of ANAHÁ Á ÁF. We consider this phenomenon
may be caused by the strong electron-withdrawing effect of ANO2
group, which enhanced the acidity of ANH of the thiourea group
and made it was more easy to deprotonation so as to form hydro-
gen bond with anion easily.
There was another phenomenon as that when the fluoride was
increased to a certain degree, the titration curve at 410 nm was al-
most no change (see Fig. 5A), this may attributed to the firstly
formed hydrogen bond between FÀ and ANH of ANHC@O turned
into the form of FÀ and ANH of ANHC@S due to electronegativity
of fluoride anion. Based on our understanding of recognition effect
between thiourea-based host and anions [24–26], we consider the
combination mode between AR1 and fluoride as what Scheme 2
shown, and we think it is also suit for AR2 and AR3 when affected
with fluoride anion.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.saa.2012.04.102.
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In summary, we have described the molecular design, synthesis,
and colorimetric recognition properties of a series of novel salicylic
acid-oriented thiourea receptors AR1–3. These ligands have been
conveniently synthesized, and characterized by IR, 1H NMR, ESI-
MS spectra. To our best knowledge, this is the first report about
the syntheses of salicylic acid-oriented thioureas receptors, which
are promising potential compounds for developing novel