A Fluorescent Chemosensor for Zn2+ Based on a C 3-Symmetrical and Pre-Organized 2,2′,2″-Nitrilotribenzoic Acid Material

Article abstract of DOI:10.1071/CH18308

A C₃-symmetrical 4,4″,4-nitrilotris(2′-methyl-[1,1′-biphenyl]-3-carboxylic acid) (4) derived from nitrilotriacetic acid (NTA) was found to selectively bind Zinc(ii) ions both in DMSO or MeOH. A synergistic effect of the anionic counter ion SO₄^2- on the sensing behaviour of 4 to metal ions was clearly observed in DMSO. Interestingly, 4 showed a rapid hypochromatic shift in emission ascribed to the deprotonation and the concomitant formation of a 4-metal complex upon the addition of Zn²⁺ ions, instead of the bathochromic shift and emission enhancement attributed to the SO₄^2--involved hydrogen-bonding interaction for Ni²⁺, Li⁺, Mg²⁺, and Na⁺ ions at ratios below 1:1 in DMSO. The observed sensing process of sulfate salts associated with the SO₄^2--involved hydrogen-bonding interaction, deprotonation, and the concomitant complexation can also be clearly monitored by titration methods utilising UV-vis, fluorescence, and NMR spectroscopy in solution. In comparison with 4, compound 1 showed an obvious difference in the binding interaction with zinc sulfate in MeOH, probably owing to the decreased acidity. Anion-induced hydrogen-bonding interactions and deprotonation of the COOH protons in the excited state also endowed 4 versatile spectroscopic properties. The addition of F^- and SO₄^2- anions resulted in a remarkable enhancement probably related with a rigidifying effect. 2,2′,2″-Nitrilotribenzoic acid can be utilised as a potential scaffold to build a series of conjugated fluorescent sensors by its chelation effect owing to the rigid cavity pre-organised by the triphenylamine moiety and the carboxylic groups and the conjugation extension in the 4,4′,4″ positions.

Full text of DOI:10.1071/CH18308

CSIRO PUBLISHING
Aust. J. Chem.
Full Paper
https://doi.org/10.1071/CH18308
A Fluorescent Chemosensor for Zn21 Based
on a C3-Symmetrical and Pre-Organized
2,29,200-Nitrilotribenzoic Acid Material
A B
,
A B
,
A B
,
A B C
, ,
A B
,
Wenguang Wei, Yao Jin, Tao Han, Bin Du,
Chaojun Wei,
Xiujuan Zhi,
A B
,
A B C
, ,
and Sichun Yuan
A
Beijing Key Laboratory of Agricultural Product Detection and Control of Spoilage
Organisms and Pesticide Residue, Beijing Laboratory of Food Quality and Safety,
Beijing 102206, China.
B
Faculty of Food Science and Engineering, Beijing University of Agriculture, Beijing
102206, China.
C
Corresponding authors. Email: Bindu80@bua.edu.cn; ysc@bua.edu.cn
A C3-symmetrical 4,400,40000-nitrilotris(20-methyl-[1,10-biphenyl]-3-carboxylic acid) (4) derived from nitrilotriacetic acid
(NTA) was found to selectively bind Zinc(II) ions both in DMSO or MeOH. A synergistic effect of the anionic counter ion
2ꢀ
SO4 on the sensing behaviour of 4 to metal ions was clearly observed in DMSO. Interestingly, 4 showed a rapid
hypochromatic shift in emission ascribed to the deprotonation and the concomitant formation of a 4–metal complex upon
the addition of Zn2þ ions, instead of the bathochromic shift and emission enhancement attributed to the SO42ꢀ-involved
hydrogen-bonding interaction for Ni2þ, Liþ, Mg2þ, and Naþ ions at ratios below 1:1 in DMSO. The observed sensing
process of sulfate salts associated with the SO42ꢀ-involved hydrogen-bonding interaction, deprotonation, and the
concomitant complexation can also be clearly monitored by titration methods utilising UV-vis, fluorescence, and NMR
spectroscopy in solution. In comparison with 4, compound 1 showed an obvious difference in the binding interaction with
zinc sulfate in MeOH, probably owing to the decreased acidity. Anion-induced hydrogen-bonding interactions and
deprotonation of the COOH protons in the excited state also endowed 4 versatile spectroscopic properties. The addition of
2ꢀ
Fꢀ and SO4 anions resulted in a remarkable enhancement probably related with a rigidifying effect. 2,20,200-
Nitrilotribenzoic acid can be utilised as a potential scaffold to build a series of conjugated fluorescent sensors by its
chelation effect owing to the rigid cavity pre-organised by the triphenylamine moiety and the carboxylic groups and the
conjugation extension in the 4,40,400 positions.
Manuscript received: 5 July 2018.
Manuscript accepted: 8 September 2018.
Published online: 11 October 2018.
Introduction
fluorescent sensors. The carboxylic groups and nitrogen atom
directly blended into the conjugation system can probably
endow them versatile photophysical properties when they
interact with ions, due to interweaving of the p-system and
recognition element.[11–16]
Over the past decades, acyclic N,O ligands such as iminodia-
cetic acid (IDA), nitrilotriacetic acid (NTA), bis(2-pyr-
idylmethyl)amine (BPA), tris(2-pyridylmethyl)amine (TPA), tri
(2-aminoethyl)amine (TREN), and their derivatives serving as
well known heavy metal chelators, have attracted much atten-
tion in molecular recognition.[1–4] Nitrilotriacetic acid has been
widely used as a chelate for more than 60 metal ions. Carbox-
ylate donor groups containing a central nitrogen atom can allow
tetradentate chelation, leading to the higher stability of the NTA
complexes in comparison with the corresponding IDA analo-
gues. In addition, the capability of their corresponding transition
metal complexes to selectively bind Lewis basic guest mole-
cules in competing solvents renders them as potential synthetic
receptors applicable in chemical biology or biotechnology.[5–10]
In recent years, many other NTA-based fluorescent sensors have
been developed by the attachment of the chromophores using
the unconjugated chain. However, as NTA analogues, 2,20,200-
nitrilotribenzoic acid derivatives (Chart 1; 2,20,200-NTBA)
haven’t been investigated for their potential application in
This article reports the synthesis and evaluation of the
fluorescent properties of C3-symmetrical 4,400,40000-nitrilotris
(20-methyl-[1,10-biphenyl]-3-carboxylic acid) (4) when inter-
acting with ions in solvent. Triphenylamine (TPA) introduced
as the electron donor and the three carboxylic acids used as
anion and cation binding sites, can pre-organise to form a
rigid cavity. The lone electron pair of the nitrogen atom on the
TPA can coordinate metal ions. Compound 4 demonstrated
versatile fluorimetric signals when exposed to anion- or
cation-containing solutions, due to conformational changes,
internal charge transfer (ICT), and the formation of metal
complexes attributed to the assistance of hydrogen-bonding
interactions, which makes it a promising skeleton for a
fluorescent sensor for rapid, simple, and low-cost methods
for ion detection.
Journal compilation ꢀ CSIRO 2018
www.publish.csiro.au/journals/ajc

B
W. Wei et al.
COOH
HO
O
O
OH
OH
OH
O
N
N
O
OH
O
N
HOOC
HO
O
COOH
NTA
2,2Ј2Љ-NTBA
4,4Ј4Љ-NTBA
Chart 1. The molecular structure of metal chelators.
Experimental
(d, 3H), 7.41–7.38 (dd, 3H), 7.27–7.22 (m, 15H), 3.47 (s, 9H),
2.32 (s, 9H). dC (100MHz, CDCl3) 167.56, 145.70, 140.35,
137.27, 135.32, 133.02, 131.67, 130.47, 129.69, 127.57,
127.16, 125.93, 125.85, 51.76, 20.53. m/z (ESI) 690.3
([M – H]ꢀ); calcd for C45H40NO6 690.2.
Materials Synthesis
All chemicals were used as received unless otherwise indicated.
All oxygen and moisture sensitive reactions were performed
under nitrogen atmosphere. Reagent grade tetrahydrofuran
(THF) was distilled over sodium and benzophenone.
Synthesis of the 4,400,40000-Nitrilotris(20-methyl-
[1,10-biphenyl]-3-carboxylic acid) (4)
Synthesis of the 2,20,200-Nitrilotribenzoic Acid (1)
To a solution of compound 3 (2.00 g, 2.90 mmol) in a mixture
of ethanol and water (150 mL) was added potassium hydroxide
(32.50 g, 580 mmol) and the solution refluxed overnight. The
reaction was terminated when the starting material was fully
consumed, which was detected by column chromatograph. The
acid was precipitated by the slow addition of 1 N HCl (aq) to the
aqueous phase. The white precipitate was filtered off and
thoroughly dissolved in ethyl acetate. The organic layer was
washed with deionized water and NaCl and dried over anhy-
drous MgSO4. After evaporating the solvent, a pale-yellow
powder was obtained (1.80 g, 95 %). dH (400 MHz, d6-DMSO)
12.75 (b, 3H), 7.71 (b, 3H), 7.53–7.51 (b, 3H), 7.33–7.26 (m,
12H), 6.98 (b, 3H), 2.28 (s, 9H). dC (100 MHz, d6-DMSO)
167.24, 146.07, 139.66, 136.98, 134.79, 133.44, 131.98, 130.49,
129.32, 127.59, 126.51, 126.10, 124.88, 20.15. m/z (HRMS ESI)
646.2232; calcd for C42H32NO6 [M – H]ꢀ 646.2235.
To a solution of trimethyl 2,20,200-nitrilotribenzoate (0.42 g,
1.0 mmol) in a mixture of ethanol and water (100 mL), was
added potassium hydroxide (3.25 g, 58 mmol) and the solution
refluxed overnight. The acid was precipitated by the slow
addition of 1 N HCl (aq) to the aqueous phase. The white
precipitate was filtered off and thoroughly dissolved in ethyl
acetate. The organic layer was washed with deionized water and
dried over anhydrous MgSO4. After evaporating the solvent, a
white powder was obtained (0.34 g, 90 %). dH (300 MHz,
methanol-d4) 7.84 (b, 3H), 7.44–7.41 (t, 3H), 7.23–7.20 (t,
3H), 6.83 (b, 3H). dC (75 MHz, methanol-d4) 168.45, 147.91,
132.84, 131.90, 126.53, 125.17, 124.17. m/z (ESI) 376.2
([M – H]ꢀ); calcd for C21H14NO6 376.1.
Synthesis of the Trimethyl 6,60,600-Nitrilotris
(3-bromobenzoate) (2)
Characterisation and Instruments
N-Bromosuccinimide (1.37 g, 7.89 mmol) was added to a
THF solution (40 mL) of trimethyl 2,20,200-nitrilotribenzoate
(1.00 g, 2.39mmol) and stirred overnight at room temperature.
After theaddition of deionized water and chloroform, the organic
layer was washed with water and sodium chloride. The organic
layer was dried over anhydrous MgSO4. After evaporating the
organic solvent, the residue was purified by column chromatog-
raphy (petroleum ether/ethyl acetate¼ 4 : 1) to afford a pale-
yellow product (1.38 g, 89 %). dH (400 MHz, CDCl3) 7.74–7.73
(d, 3H), 7.48–7.46 (dd, 3H), 6.92–6.89 (d, 3H), 3.45 (s, 9H). dC
(100 MHz, CDCl3) 165.73, 145.26, 135.34, 133.71, 129.15,
127.51, 116.82, 52.14. Anal. Calc. for C24H18Br3NO6: C
43.93, H 2.77, N 2.13. Found: C 43.72, H 2.57, N 2.23 %.
NMR spectra were obtained on a Bruker (300 or 400 MHz) using
CDCl3, methanol-d4, or DMSO-d6 as the solvent unless other-
wise noted. High resolution mass spectrometry (HRMS)
experiments were performed on an FT-ICR-MS spectrometer
(APEX II). The crystal data of compound 1 was collected using a
Bruker APEX II. Acetonitrile (HPLC reagent), THF (HPLC
reagent), DMSO (HPLC reagent), and methanol (HPLC
reagent) were purchased from Acros Reagent Corporation and
ꢀ
used as received. Sodium salts of Fꢀ, Clꢀ, Brꢀ, Iꢀ, NO3
,
SO32ꢀ, SO42ꢀ, AcOꢀ, S2ꢀ, and all metal sulfates were pur-
chased from Xiya Reagent Corporation and dissolved in deio-
nized water. UV-Vis spectra were determined on a Varian Cary
50 UV-vis spectrophotometer at room temperature. For a typical
titration experiment, an aqueous solution of metal salt was added
to a 5 mL solution of 4 using a pipette. Emission spectra were
measured on a Varian Cary Eclipse spectrofluorophotometer at
room temperature. All data were manipulated by using the
OriginLab software package.
Synthesis of the Trimethyl 4,400,40000-Nitrilotris(20-methyl-
[1,10-biphenyl]-3-carboxylate) (3)
A mixture of 2 (2.13 g, 3.26 mmol), o-tolylboronic acid
(2.67 g, 16.3 mmol), NaHCO3 (3.28 g, 39.10 mmol), Pd(PPh3)4
(0.19 g, 0.163 mmol) in THF (15 mL), and deionized water
(20 mL) were refluxed overnight under nitrogen atmosphere.
The mixture was extracted with ethyl acetate. The organic extract
was dried over MgSO4. After removal of solvents under reduced
pressure, the residue was purified by column chromatography
(petroleum ether/ethyl acetate¼ 100 : 8) to afford 3 as a pale-
yellow solid (2.03 g, 70 %). dH (400 MHz, CDCl3) 7.61–7.60
General Spectroscopic Methods and Theoretical
Calculations
To determine the stoichiometry of metal complexation, the
Job’s plot method was performed by keeping the total concen-
tration of 4 and the metal ions constant and by changing the mole

2,20,200-Nitrilotribenzoic Acid
C
COOCH3
N
COOCH3
N
Br
Br
Pd(PPh3)4
Na2CO3
NBS, THF/DMF
rt, 24 h, 89 %
H3COOC
COOCH3
H3COOC
COOCH3
THF/H2O
70 %
OH
B
2
Br
OH
90 % KOH
COOCH3
COOH
COOH
N
N
N
KOH
95 %
H3COOC
HOOC
COOCH3
COOH
HOOC
COOH
1
4
3
Scheme 1. The synthetic route of compound 1–4.
fraction of metal ions from 0 to 1. The mole fraction of metal
ions at which the maximum absorption or emission intensity was
observed in the plot indicated the stoichiometry of the complex.
The binding constant of the compound 4–Zn2þ interaction was
evaluated from the absorbance measurement and obtained using
the Benesi–Hildebrand equation.[13] The geometry was opti-
mized by density functional theory (DFT) calculations using the
B3LYP hybrid functional with a basis set limited to 6–31G(d).
The molecular orbital shapes and energies were those calculated
at optimized geometries with 6–311þg(d, p). Quantum chemi-
cal calculations were performed with the Gaussian03 package
and orbital pictures were prepared using Gaussview.[17]
H
O
N
Fig. 1. The single X-ray crystal structure of compound 1.
build a series of conjugated fluorescent sensors by conjugation
extension in the 4,40,400 positions. In addition, the fluorescent
properties of compound 1 and 4 can probably be affected by their
conjugation length when interacting with ions in a solvent,
owing to their different acidities.
Results and Discussion
Synthesis and Characterization
The synthetic approach to compounds 1–4 are shown in
Scheme 1. Subsequent bromination of trimethyl 2,20,200-nitri-
lotribenzoate afforded trimethyl 6,60,600-nitrilotris(3-bromo-
benzoate) (2) in the presence of NBS with 89 % yield. The
Suzuki coupling reaction between 2 and o-tolylboronic acid was
employed to produce trimethyl 4,400,40000-nitrilotris(20-methyl-
[1,10-biphenyl]-3-carboxylate) (3) in 70 % yield. Compound 3
underwent a NaOH-promoted hydrolysis to form 4,400,40000-
nitrilotris(20-methyl-[1,10-biphenyl]-3-carboxylic acid) (4) in
95 % yield. Compound 1 was synthesized according to the same
procedure to produce compound 4. All new compounds were
characterized by 1H and 13C NMR spectroscopy and mass
spectrometry.
Unlike 4,40,400-NTBA in Chart 1,[18] which has attracted
much attention because of its three-dimensional coordination
between metal ions and carboxy groups, compound 1[19,20] can
be pre-organised to form a rigid cavity with three carboxy
groups orientated in the same direction, which was confirmed
by its single crystal structure (Fig. 1). Thus, we assumed that the
2,20,200-nitrilotribenzoic acid derivatives have a multifunction-
ality: (i) the carboxy groups may donate 3-fold hydrogen
bonding to an anion and the hydrogen-bonding interaction
may increase their acidity; (ii) the pre-organised and tripodal
cavity can probably provide a good environment to capture their
target ions; (iii) they can be utilised as a potential scaffold to
The Binding Behaviours of 4 Towards Ions
Absorption and emission spectroscopic studies of 4 (3.5 ꢁ 10ꢀ5
M, Fig. 2) have been carried out in a series of protic and aprotic
solvents such as THF, DMSO, CH3CN, and MeOH. Compound
4 showed a broad absorption band centred at 329 nm and a
smaller shoulder at 301 nm in DMSO. Absorption spectra of 4
recorded in THF, CH3CN, and MeOH displayed a 2 nm blue-
shift, in comparison with the maxima in DMSO. The shoulder
was attributed to the p–p* transitions, whereas the former was
assigned to the ICT transition between the triphenylamine and
carboxylic acids. Upon excitation at 325 nm, an emission
maxima was exhibited at 464 nm in THF, which red-shifted to
474 nm in DMSO, and further shifted to 495 nm in MeOH.
When the polarity of the solvent increased, this solvatochromic
red-shift in emission suggested an excited state of 4 with
stronger polarity than that in the ground state, and also implied
that such absorption bands originated from p–p* transitions and
ICT transitions,[17] which can be further confirmed by theoret-
ical calculations (Fig. 3). The electron clouds of the highest
occupied frontier molecular orbital (HOMO) were mainly
localised over the TPA unit, whereas the lowest unoccupied
molecular orbital (LUMO) was partially localised in the car-
boxylic acid moiety, implying compound 4 as a D–p–A mole-
cule. Compound 4 can be pre-organised into a rigid cavity with

D
W. Wei et al.
(a)
(b)
1.2
1.0
0.8
0.6
0.4
0.2
0
1.2
1.0
0.8
0.6
0.4
0.2
0
DMSO
THF
DMSO
THF
CH3CN
CH3CN
MeOH
MeOH
300
350
400
450
350
400
450
500
550
600
650
Wavelength [nm]
Wavelength [nm]
Fig. 2. Absorption and emission spectrum (lex 325 nm) of 4 in different solvents.
(a)
(b)
474 nm was completely quenched, which is likely ascribed to a
strong interaction between transition metal ions and the fluor-
ophore.[22] Interestingly, Mg2þ ions (as their chlorate or nitrate
salt) had a negligible effect on the fluorescence and absorption
in DMSO solvent, implying the existence of a synergy effect of
anionic counter ions on the sensing behaviour of 4 to metal ions
(Fig. S1, Supplementary Material). In addition, upon addition of
Zn2þ ions, the complex formation induced an obvious hypso-
chromic shift of the long-wavelength absorption band, in a
4 ꢁ 10ꢀ5 M protic solution (MeOH) of 4 (Fig. 4c). The emission
peak at 495 nm was markedly blue-shifted to 445 nm upon
addition of Zn2þ. It is noteworthy that there were no obvious
changes in the absorbance responses of 4 to Mg2þ, Mn2þ, and
Liþ ions, in comparison with their obvious change of absorption
in DMSO. Upon addition of the same amount of Ni2þ ions, the
fluorescence intensity was moderately quenched with the
enhancement of the absorbance band at 300 nm, which was
different in comparison with those in DMSO solvent. The
absorption peak at 327 nm was acutely blue-shifted after the
addition of copper sulfate, which is similar to the capture of Ni2þ
by 4 in DMSO. Metal ion (as sulfate salts) binding studies of 4 in
THF solvent showed behaviours different to those observed in
DMSO and MeOH (Fig. S2, Supplementary Material). Among
the investigated metal ions, the same amount of sulfate salts
induced a negligible change in fluorescence and absorbance,
suggesting that the polarity of the solvent played a major role in
the metal ion interaction of the receptor. More interestingly, the
same amount of sodium and ammonium sulfate salts uniquely
induced an enhancement and red-shift in fluorescence and
HOOC
COOH
N
HOOC
(c)
(d)
HOMO
LUMO
Fig. 3. The structure of 4 (a), optimized molecular structure (b), and orbital
plots of HOMO and LUMO energy levels (c and d).
three carboxy groups orientated in the same direction, which
was also confirmed by its optimized structure from theoretical
calculations.
To explore the ionic sensing behaviours of 4 to metal ions,
absorption and fluorescence responses of 4 to different sulfate
salts were examined in solution. In the cases of Zn2þ, Mg2þ
,
Mn2þ, and Liþ ions, the complex formation respectively
induced a hypsochromic shift (Dl value) of 25, 32, 20, and
18 nm in DMSO (Fig. 4). The emission peak at 474 nm was
markedly blue-shifted to 413, 420, and 421 nm upon addition of
the Zn2þ, Mg2þ, and Liþ ions, respectively, with a concomitant
fluorescence enhancement at the excitation wavelength of
325 nm. The obvious blue-shift in absorption and emission can
be attributed to the interaction between the metal cation and the
heteroatom of the electron-donating part of the chromophore on
the basis of the ICT mechanism widely exploited for cation
sensing.[21] However, upon addition of the same amount of
Ni2þ, Fe2þ, Mn2þ, and Cu2þ ions, the fluorescence intensity at
absorbance. The observed spectroscopic changes of 4 in the
2ꢀ
presence of metal sulfate possibly originated from SO4
involved hydrogen-bonding interactions or a simple deprotona-
-
2ꢀ
tion reaction. SO4 possibly increased the receptor’s affinity
towards ionic guests because the hydrogen bonding can lead to a
higher acidity of the benzoic acid group in the excited state, with
the formation of contact and/or solvent-separated ion pairs in
polar solvents.[23]
To further gauge selectivity for cations, we also examined
metal-coexisting systems in solution. By monitoring the emis-
sion peak, Zn2þ ions could be distinguished in multimetallic
solutions. After the respective addition of Mg2þ, Cu2þ, Liþ,

2,20,200-Nitrilotribenzoic Acid
E
(a)
1.2
(b)
600
4
4
MgSO4
MgSO4
Na2SO4
Li2SO4
1.0
0.8
0.6
0.4
0.2
Na2SO4
Li2SO4
500
400
300
200
100
0
FeSO4
MnSO4
CuSO4
ZnSO4
(NH4)2SO4
NiSO4
FeSO4
MnSO4
CuSO4
ZnSO4
(NH4)2SO4
NiSO4
0
300
350
400
450
500
350
400
450
500
550
600
Wavelength [nm]
Wavelength [nm]
(c)
1.2
(d)
300
250
200
150
100
50
4
4
MgSO4
MgSO4
Na2SO4
Li2SO4
FeSO4
MnSO4
CuSO4
ZnSO4
(NH4)2SO4
NiSO4
Na2SO4
Li2SO4
1.0
0.8
0.6
0.4
0.2
FeSO4
MnSO4
CuSO4
ZnSO4
(NH4)2SO4
NiSO4
0
0
350
300
350
400
450
500
400
450
500
550
600
Wavelength [nm]
Wavelength [nm]
Fig. 4. Absorption and emission spectrum (lex 325 nm) of 4 in DMSO (3.5 ꢁ 10ꢀ5 M, a and b) and MeOH (4 ꢁ 10ꢀ5 M, c and d) after addition
of metal sulfate with the ratio of [metal]/[4] (30 : 1).
Ni2þ, Fe2þ, Mn2þ, NH4þ, and Naþ into a solution of 4 in the
presence of Zn2þ, a close similarity of emission spectrum with
those containing only Zn2þ was observed in DMSO and MeOH,
except for a slight quenching in the cases of Cu2þ and Fe2þ
(Fig. 5 and Fig. S3, Supplementary Material). In MeOH, the
emission peaks almost located at ,445 nm, which were
assigned to the emission of the 4–Zn complex. The fluorescence
selectivity.[2] To further gauge selectivity in distinguishing
Zn2þ and Cd2þ, we conducted competitive experiments by
adding Zn2þ into the MeOH solution of 4 with Cd2þ and adding
Cd2þ into the solution of 4 containing Zn2þ. As shown in Fig. S5
(Supplementary Material), the absorbance and emission spec-
trum of 4 in the presence of both Zn2þ and Cd2þ showed a close
resemblance to those of 4 containing only ZnSO4 salt, indicating
emission spectra of 4 in the presence of various cations (Zn2þ
,
that 4 can detect Zn2þ in the presence of Cd2þ
.
Mg2þ, Cu2þ, Liþ, Ni2þ, Fe2þ, Mn2þ, NH4þ, and Naþ, as their
sulfate salts) in MeOH and DMSO also showed a close resem-
blance of the emission of those of 4 containing only ZnSO4 salt
(Fig. 6). The results imply that 4 has a high selectivity for Zn2þ
in the presence of other competitive metal ions. The binding
constant of 4–Zn2þ was evaluated as 3.4 ꢁ 104 Mꢀ1 from the
absorbance measurement in MeOH (Fig. S4, Supplementary
Material) using the Benesi–Hildebrand equation, and the detec-
tion limit of 4 to Zn2þ was calculated to be 3.01 ꢁ 10ꢀ6 M,
according to a previously reported procedure.[13] Although
many fluorescent sensors for the detection of Zn2þ and Cd2þ
have been reported, most of them have so far suffered from poor
The Proposed Binding Mechanism
To investigate the synergic effect of SO42ꢀ on the spectra of 4 in
more detail, we performed a titration in DMSO by adding Liþ,
Ni2þ, Naþ, and Mg2þ ions. The titration of Liþ into 4 gave a red-
shifted emission centred at 486 nm when the ratio of [salt]/
[COOH] is equal to 2 : 1, with a 4.5-fold enhancement of fluo-
rescence. When the ratio exceeded 2.25 : 1, however, the
intensity gradually decreased in emission, with the concomitant
appearance of a new band centred at 421 nm and a clear iso-
sbestic point at 447 nm (Fig. 7). On the other hand, the absorp-
tion of 4 varied remarkably from a 2 : 1 to 4 : 1 ratio, with the

F
W. Wei et al.
(a)
(b)
250
200
150
100
50
1.0
0.8
0.6
0.4
0.2
0
4
4
ZnSO -4
ZnSO -4
4
4
ZnSO -LiSO
4
ZnSO -LiSO
4
4
4
ZnSO -MgSO
4
ZnSO -MgSO
4
4
4
ZnSO -NiSO
4
4
ZnSO -NiSO
4
4
ZnSO -Na SO
4
2
4
ZnSO -Na SO
4 2 4
ZnSO -FeSO
4
4
ZnSO -FeSO
4 4
ZnSO -MnSO
4
4
ZnSO -MnSO
4
4
ZnSO -CuSO
ZnSO -CuSO
4
4
4
4
ZnSO -(NH ) SO
ZnSO -(NH ) SO
4
4
2
4
4
4
2
4
0
350
300
350
400
400
450
500
550
600
650
Wavelength [nm]
Wavelength [nm]
Fig. 5. Absorbance and fluorescence spectrum of 4 (in MeOH, 4 ꢁ 10ꢀ5 M, lex 325 nm), 4 containing 12 equiv. of ZnSO4 and the spectrum
change of 4 containing 12 equiv. of other metal ions with subsequent addition of 12 equiv. of ZnSO4.
1.2
ratio (Fig. S6, Supplementary Material). The fluorescence
titration with Ni2þ was recorded in a DMSO solution (Fig. S7,
Supplementary Material). When Ni2þ was added to the solution
of 4, the fluorescence intensity peaked at 486 nm and was found
to be highest at a ,1 : 1 mole ratio. As the Ni2þ content
increased beyond the ratio of 1 : 1, the fluorescence gradually
decreased and completely quenched at a ,3 : 1 ratio. From the
above description, Liþ, Ni2þ, and Mg2þ sulfate induced the
most significant absorbance and emission spectroscopic chan-
ges when interacting with 4 in DMSO. Upon the addition of 1
equiv. of metal sulfate, the emission intensity was markedly
enhanced with a slight bathochromic shift. Upon further addi-
tion of the metal sulfate, the emission was obviously blue-
shifted for Liþ and Mg2þ sulfate, and quenched for Ni2þ sulfate.
The two-stage absorbance and emission spectroscopic changes
signified the presence of two distinctly different equilibrium
processes. The enhancement of molecular rigidity and change of
electron density ascribed to SO42ꢀ-involved hydrogen-bonding
interactions led to the increase of fluorescent intensity and a
spectroscopic red-shift in the first stage with a mole ratio below
1 : 1. In the second stage, a successive blue-shift in emission and
absorption were attributed to deprotonation and the concomitant
formation of a 4/metal complex, which will lead to the change in
molecular conformation and ICT interaction (Scheme 2).
More interestingly, for the emission spectrum of 4 upon the
addition of Zn2þ in DMSO (Fig. S8, Supplementary Material),
the increase of fluorescence intensity and spectroscopic red-
shift, which was attributed to SO42ꢀ-involved hydrogen-bond-
ing interactions in the early stage, was not observed, instead a
rapid deprotonation and complexation was seen, implying the
stronger ability of 4 to complex Zn2þ over Liþ, Ni2þ, and Mg2þ
ions. The titration of 4 with ZnSO4 salt was also monitored via
changes in the 1H NMR spectrum in d6-DMSO. The 1H NMR
spectrum showed proton signals (Ha, Hb and Hc) of 4 at 7.71,
7.55, and 6.98 ppm. With the addition of ZnSO4 salt, the proton
signals (Ha, Hb and Hc) shifted upfield gradually, as shown in
Fig. 9. At 1 equiv. of Zn2þ, the Hb and Hc signals were split into
two, two of them shifted upfield to 7.31 and 6.67 ppm, and the
rest remained at the same position. At 3 equiv., the Ha, Hb, and
Hc signals completely shifted upfield to 7.69, 7.31, and
4 DMSO
a
b
1.0
0.8
0.6
0.4
0.2
0
4 MeOH
c
d
350
400
450
500
550
600
650
Wavelength [nm]
Fig. 6. Normalized fluorescence of 4 (DMSO, 3.5 ꢁ 10ꢀ5 M and MeOH,
4 ꢁ 10ꢀ5 M, lex 325 nm) exposed to 12 equiv. of various metal ions. (a, c)
Normalized
emission
of
4
containing
mixed
ions
(mix ¼ Zn2þ þMg2þ þCu2þ þLiþ þNi2þ þFe2þ þMn2þ þNH4þ þNaþ)
in DMSO and MeOH, respectively. (b, d) Normalized emission of 4
containing only Zn2þ in DMSO and MeOH, respectively.
concomitant appearance of a new blue-shifted band centred at
313 nm and a clear isoabsorbance point at 321 nm. A Job plot
analysis showed that the maximum intensity of fluorescence
peaked at 486 nm and appeared at a mole fraction of 0.5, which
corresponded to a 1 : 1 ratio ([Li2SO4]/[4]), under the condition
of an invariant total concentration (Fig. 8b). When the ratio
exceeded 1 : 1, the intensity gradually increased in emission
centred at 421 nm and reached a maximum at ,3 : 1. Likewise,
the absorption peak was red-shifted to a maximum for a 1 : 1
ratio and then blue-shifted to 313 nm for a 3 : 1 ratio (Fig. 8a).
Again, the Job plot analysis of Mg2þ also showed that the
maximum intensity of fluorescence peaked at 486 nm and
appeared at a mole fraction of 0.5, and the emission intensity
attributed to the metal complexes reached a maximum at a 3 : 1

2,20,200-Nitrilotribenzoic Acid
G
(a)
1.2
(b)
600
4
4
4:1
4:1
3.75:1
3.5:1
3.25:1
3:1
2.75:1
2.5:1
2.25:1
2:1
1.0
0.8
0.6
0.4
0.2
0
500
400
300
200
100
0
3.75:1
3.5:1
3.25:1
3:1
2.75:1
2.5:1
2.25:1
2:1
300
350
400
350
400
450
500
550
600
650
Wavelength [nm]
Wavelength [nm]
Fig. 7. Absorption and emission spectrum of 4 (3.5 ꢁ 10ꢀ5 M, lex 325 nm) in DMSO after addition of lithium sulfate with the different
ratios of [salt] to [COOH]
(a)
(b)
2.5
700
10:0
9:1
5:5
10:0
9:1
8:2
7:3
6:4
blank
5:5
8:2
7:3
6:4
5:5
4:6
3.5:6.5
3:7
2.8:7.2
2.5:7.5
2.3:7.7
2:8
600
500
400
300
200
100
0
2.5:7.5
2.0
1.5
1.0
0.5
0
5:5
4:6
blank
2.5:7.5
3.5:6.5
3:7
2.8:7.2
2.5:7.5
2.3:7.7
2:8
1:9
0:10
1:9
0:10
300
350
400
350
400
450
500
550
600
650
Wavelength [nm]
Wavelength [nm]
(c)
(d)
700
350
600
500
400
300
200
100
0
300
250
200
150
100
50
0
0
0.2
0.4
0.6
0.8
1.0
0
0.2
0.4
0.6
0.8
1.0
Fraction of lithium sulfate
Fraction of 4
Fig. 8. Absorption and emission spectrum (a and b, lex 325 nm) of 4 in DMSO after addition of lithium sulfate with different ratios of [salt] to
[4], under the conditions of an invariant total concentration (5 ꢁ 10ꢀ5 M). A Job plot analysis (c and d) between 4 and lithium sulfate.

H
W. Wei et al.
R
N
O
O
O
O
R
R
N
M
Metal ions
S
R
Mg2ϩ Liϩ Ni2ϩ Sulfate
O
O
O
slow deprotonation and
complexation
H
O
O
O
O
O
OH
O
O
H-bonding interaction
below 1 equiv.
H
H
H
OH
O
N
M
1–3 equiv.
First stage
Second stage
H
O
R
O
HO
R
H
R
R
R
R
O
O
Zn2ϩ Sulfate
rapid deprotonation and complexation
Scheme 2. Proposed mechanism of the interaction between 4 and metal sulfate in DMSO.
R
R
a
aЈ
bЈ
cЈ
b
c
O
O
Zn2ϩ
OH
O
O
OH
O
O
N
Zn
N
O
HO
R
R
R
R
O
O
4
4
b
COOH
a
c
14.5 14.0 13.5 13.0 12.5 12.0 11.5 11.0 10.5 10.0 9.5 9.0 8.5 8.0 7.5 7.0 6.5
ppm
4:ZnSO4
1:1
bЈ
aЈ
a
b
cЈ
c
14.5 14.0 13.5 13.0 12.5 12.0 11.5 11.0 10.5 10.0 9.5
9.0
8.5
8.0
7.5
7.0
6.5
ppm
4:ZnSO4
bЈ
1:2
aЈ
a
cЈ
b
c
14.5 14.0 13.5 13.0 12.5 12.0 11.5 11.0 10.5 10.0 9.5 9.0 8.5 8.0 7.5 7.0 6.5 6.0
ppm
4:ZnSO4
1:3
aЈ
bЈ
cЈ
15.0 14.5 14.0 13.5 13.0 12.5 12.0 11.5 11.0 10.5 10.0 9.5 9.0 8.5 8.0 7.5 7.0 6.5 6.0
ppm
Fig. 9. The 1H NMR spectrum change of compound 4 (300 MHz, d6-DMSO) with the addition of ZnSO4 at different ratios.
6.67 ppm. The NMR and spectrophotometric tritration clearly
showed that compound 4 can show rapid deprotonation and
complexation at a low ZnSO4 concentration below a 1 : 1 ratio.
In comparison with the presence of two distinctly different
equilibrium processes with Liþ, Ni2þ, and Mg2þ sulfate, the
rapid depotonation and complexation was probably the reason
for the high selectivity for Zn2þ in the mixed-metal systems. In
addition, the binding interaction of compound 1 to Zn2þ was
also investigated in DMSO, and the emission intensity attributed
to the Zn complexes reached its maximum at a 3 : 1 ratio (Fig.
S9, Supplementary Material) in the Job plot analysis, which is
the same as the 3 : 1 ratio of compound 4.

2,20,200-Nitrilotribenzoic Acid
I
(a)
(b)
1.5
1.0
0.5
0
700
600
500
400
300
200
100
0
10:0
9:1
8:2
7:3
5:5
4.6:5.4
4:6
3:7
2:8
1:9
10:0
9:1
8:2
7:3
5:5
4.6:5.4
4:6
3:7
2:8
1:9
0:10
0:10
300
350
400
450
350
400
450
500
550
600
650
Wavelength [nm]
Wavelength [nm]
Fig. 10. Absorption and emission spectra of 4 in DMSO after addition of sodium sulfate with different ratios of [salt]/[4], under the conditions of an
invariant total concentration (5 ꢁ 10ꢀ5 M, lex 325 nm).
The titration of Naþ into 4 gave a red-shifted emission
centred at 486 nm which showed a 6-fold enhancement when
the ratio of [SO42ꢀ] to [COOH] was equal to 1.67 : 1 (Fig. S10,
Supplementary Material). When the ratio exceeded 1 : 1.25, the
emission intensity gradually increased at 486 nm. The absorp-
tion of 4 varied remarkably from a 1 : 1 to 1.67 : 1 ratio, with the
concomitant appearance of a clear isoabsorbance point at
310 nm. The Job plot analysis of Naþ showed that the maximum
intensity of fluorescence peaked at 486 nm and appeared at a
mole fraction of 0.5. When the ratio exceeded 1 : 1, the remark-
able emission at 421 nm was not observed, in comparison with
the Liþ and Mg2þ ions, which was possibly attributed to the
different ability of 4 to capture metal cations under the existence
of SO42ꢀ-involved hydrogen-bonding interactions in the excited
state. The absorption peak red-shifted to its maximum at 331 nm
at a mole fraction of 0.5 (Fig. 10). With the addition of Na2SO4,
the proton signals (Ha, Hb, and Hc; Fig. S11, Supplementary
Material) slightly shifted upfield. At 3 equiv., the Ha and Hb
shifted respectively from 7.71 and 7.55 to 7.63 and 7.48 ppm and
the Hc remained at the same position. The results are probably
due to the increase of electronegativity caused by the hydrogen-
Supplementary Material). In addition, the singly charged
[4 þ Cu – 3H]ꢀ complex was also observed by HRMS analysis
(Fig. S14, Supplementary Material) when 4 interacted with
Cu2þ in MeOH. The increase of fluorescent intensity and
spectroscopic red-shift, which was attributed to SO42ꢀ-involved
hydrogen-bonding interactions in the early stage, was also
not observed in MeOH when the Cu2þ and Zn2þ were added,
instead of the rapid deprotonation and complexation. More
interestingly, the emission intensity attributed to the metal
Zn-complexes reached its maximum at a 3:1 ratio (Fig. 12) in
the Job plot analysis of compound 1, which is different from the
1 : 1 ratio of 4. This type of conjugation-dependent difference in
binding interaction was probably due to the decreased acidity,
which would weaken the deprotonation capability of 1. Com-
pared with DMSO , the protic solvent MeOH can probably lead
to a higher acidity of 4 to improve the Zn–metal complexation in
the excited state, owing to the formation of contact and/or
solvent-separated ion pairs.[23] The results showed that the
acidity of nitrilotribenzoic acid derivatives and the solvent
environment are closely linked with the binding mode.
To further observe the interactions of 4 for other anions,
aqueous solutions containing representative anions such as Fꢀ,
Clꢀ, Brꢀ, Iꢀ, ClO4ꢀ, NO3ꢀ, SO32ꢀ, SO42ꢀ, PO43ꢀ, AcOꢀ, and
S2ꢀ (as their sodium salts) were also added respectively to a
3.5 ꢁ 10ꢀ5 M solution (DMSO) of 4 at room temperature
(Fig. 13a, b). The original absorption peak at 329 nm was bareꢀly
2ꢀ
bonding interaction between SO4 and the carboxy units. In
addition, the results also evidence the weak ability of 4 to
capture Naþ to form a chelate. Electrospray ionization mass
spectrometry was also utilised to detect labile supramolecules in
solution. Mass experiments were performed by adding 30 equiv.
of metal sulfate (Mg2þ, Cu2þ, Fe2þ, and Zn2þ; Fig. S12,
Supplementary Material) in the presence of 4 in DMSO and
the mass spectra were dominated by a singly charged peak of
744, which is likely assigned to the adduct [4(SO42ꢀ)(Hþ)]ꢀ,
further evidencing that strong SO42ꢀ-involved hydrogen-bond-
ing interactions exist.
In the case of Zn2þ in methanol, a Job plot analysis based on
the emission data showed that the maximum intensity peaked at
448 nm and appeared at a mole fraction of 0.5, which corre-
sponded to a 1 : 1 4–Zn complex, under the condition of an
invariant total concentration in MeOH (Fig. 11). In accordance
with this result, the HRMS analysis revealed a singly charged
[4 þ Zn – 3H]ꢀ complex, indicating that the deprotonation and
coordination process occurred after addition of Zn2þ (Fig. S13,
changed upon addition of Clꢀ, Brꢀ, Iꢀ, NO3ꢀ, and ClO4
However, upon addition of the stronger basicity anions SO3
.
,
2ꢀ
PO43ꢀ, AcOꢀ, and S2ꢀ, a noticeable enhancement of absorption
intensity centred at ,331 nm was observed with a concomitant
decrease at ,301 nm, demonstrating a strong hydrogen-bonding
interaction between 4 and these anions. In addition, the emission
peaks red-shifted from 474 to 486, 481, and 486 nm, with an
3ꢀ
obvious enhancement of intensity, after addition of Fꢀ, PO4
,
and SO42ꢀ. The emission intensity enhancement was probably
related to a rigidifying effect which decreased the probability of
2ꢀ
a non-radiative decay upon addition of Fꢀ, PO43ꢀ, and SO4
.
Furthermore, the binding of Naþ at the carbonyl group and the
hydrogen-bonding interaction can increase the ICT character of
the electronic transition, as evidenced by the red-shift of

J
W. Wei et al.
(a)
(b)
250
200
150
100
50
250
200
150
100
50
10:0
9:1
8:2
7:3
6:4
5:5
4:6
3:7
2:8
1:9
0:10
5:5
blank
0
0
400
450
500
550
600
0
0.2
0.4
0.6
0.8
1.0
Wavelength [nm]
Fraction of 4
Fig. 11. Emission spectrum (a) of 4 in MeOH after addition of Zn2þ with different ratios of [salt]/[4], under the conditions of an invariant total
concentration (5 ꢁ 10ꢀ5 M, lex 325 nm). A Job plot analysis (b) between 4 and zinc sulfate.
(a)
(b)
10:00
9:1
8:2
7:3
6:4
5:5
4:6
3:7
2.5:7.5
2:8
10:00
9:1
8:2
7:3
6:4
5:5
4:6
3:7
2.5:7.5
2.4
2.2
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
700
600
500
400
300
200
100
0
2:8
1:9
0:10
1:9
0:10
300
350
400
350
400
450
500
550
600
650
Wavelength [nm]
Wavelength [nm]
Fig. 12. Absorbance and emission spectrum of 1 in MeOH after addition of Zn2þ with different ratios of [1]/[salt], under the conditions of an invariant total
concentration (3.5 ꢁ 10ꢀ4 M, lex 325 nm).
2ꢀ
absorption and emission after addition of Fꢀ and SO4
of SO32ꢀ, PO43ꢀ, and S2ꢀ, the absorption peak at 326 nm was
slightly blue-shifted to ,322 nm and the emission peak was
obviously blue-shifted from 495 nm to 440, 436, and 441 nm,
anions.[24–27] Significantly, the emission peak at 474 nm was
markedly blue-shifted respectively to 422 and 426 nm upon
addition of the SO32ꢀ and S2ꢀ anions, which can be attributed to
the strong interaction between carboxylic acids and anions.
respectively. In addition, the close resemblance of the absorp-
2ꢀ
tion and emission spectrum of 4 in the presence of SO3
,
2ꢀ
SO3 and S2ꢀ anions can possibly lead to strong hydrogen
PO43ꢀ, and S2ꢀ with those of the strong basicity anion OHꢀ in
MeOH were consistent with the deprotonation of COOH units.
However, the spectroscopic patterns for 4 with OHꢀ failed to
bonds in the ground state and the deprotonation of COOH and
formation of carboxylate in the excited state, which probably
weakened the ICT interaction from the HOMO mainly localised
over the TPA unit to the LUMO localised in the carboxylate
moieties, leading to the hypochromatic shift in emission.[28]
When anions were added to a 4 ꢁ 10ꢀ5 M protic solution
(MeOH) of 4, as shown in Fig. 13c, d, the absorption and
emission peaks were negligibly changed upon addition of Fꢀ,
Clꢀ, Brꢀ, Iꢀ, NO3ꢀ, ClO4ꢀ, and SO42ꢀ. However, upon addition
2ꢀ
have a close resemblance to the spectra in the presence of SO3
and S2ꢀ in DMSO (Fig. S15, Supplementary Material). The
spectroscopic profile with NaOH experienced a slight blue-shift
in absorption, whereas with SO32ꢀ and S2ꢀ a slight red-shift was
2ꢀ
observed, indicating the absence of the deprotonation by SO3
and S2ꢀ anions in the ground state,[29] and anion-induced
hydrogen-bonding interactions and deprotonation of the COOH

2,20,200-Nitrilotribenzoic Acid
K
(a)
(b)
1.2
1.0
0.8
0.6
0.4
0.2
0
600
500
400
300
200
100
0
4
4
FϪ
ClϪ
BrϪ
IϪ
FϪ
ClϪ
BrϪ
IϪ
Ϫ
Ϫ
NO32Ϫ
PO42Ϫ
SO42Ϫ
SO3
NO32Ϫ
PO42Ϫ
SO42Ϫ
SO3
Ϫ
Ϫ
ClO4
ClO4
S2Ϫ
S2Ϫ
AcO2Ϫ
AcO2Ϫ
280
300
320
340
360
380
400
400
450
500
550
600
Wavelength [nm]
Wavelength [nm]
(c)
(d)
1.0
0.8
0.6
0.4
0.2
0
300
250
200
150
100
50
4
4
FϪ
ClϪ
BrϪ
IϪ
FϪ
ClϪ
BrϪ
IϪ
Ϫ
Ϫ
NO32Ϫ
NO32Ϫ
PO42Ϫ
SO42Ϫ
SO3
PO42Ϫ
SO42Ϫ
SO3
Ϫ
Ϫ
ClO4
ClO4
S2Ϫ
S2Ϫ
AcO2Ϫ
AcO2Ϫ
0
400
450
500
550
600
280
300
320
340
360
380
400
420
Wavelength [nm]
Wavelength [nm]
Fig. 13. Absorption and emission spectrum of 4 in DMSO (3.5 ꢁ 10ꢀ5 M, a and b, lex 325 nm) and MeOH (4 ꢁ 10ꢀ5 M, c and d) after addition of anions
with a 30 : 1 ratio of [anion] to [4].
protons of the receptors both existing in the excited state. The
results showed that the basicity and geometrical configuration of
anions and the solvent environment have an important effect on
hydrogen-bonding interactions and deprotonation. Compound 4
can probably accommodate anions selectively through the
formation of hydrogen bonds with the COOH moieties, which
induces a change in the constructional conformation and rigid-
ity. The above-mentioned different absorption and fluorescence
outputs after addition of different anions in different solvent
may be highly related to the acidity of 4, the basicity of anions,
the charge numbers, and the shape of the match between the
hydrogen-bonding donor and acceptor groups.[29]
In the past decades, several organic compounds containing
pyrrole, imidazole, urea, or thiourea moieties that are capable of
providing an acidic NH group have been widely reported to
exhibit a strong affinity and selectivity towards certain
anions.[30] However, the application of 2,20,200-nitrilotribenzoic
acid derivatives to molecular sensing has not been reported. In
addition, site selective binding to polyhistidine through transi-
tion metal ions (often Cu2þ, Ni2þ, and Zn2þ) complexed by
NTA attached to fluorophores, have been widely applied in
molecular recognition and imaging in live cells under mild
conditions.[1] In our study, the 2,20,200’-nitrilotribenzoic acid
ligand has a similarly tetradentate NO3 donor to form complexes
and can be potentially applied as an alternative to the NTA
ligand. The readily modifiable C3 symmetry can be utilised to
build a series of water-soluble conjugated fluorescent sensors by
molecular extension in the 4,40,400 positions. The efforts in
developing 2,20,200-nitrilotribenzoic acid derivatives and their
metal complexes for anion sensing are in progress and will be
reported in a forthcoming paper.
Conclusions
In summary, a C3-symmetrical 4,400,40000-nitrilotris(20-methyl-
[1,10-biphenyl]-3-carboxylic acid) (4) has been synthesized for
ion recognition. Nitrilotribenzoic acid, comprised of a triphe-
nylamine backbone and three carboxylic units in the 2,20,200-
positions, can pre-organise to form a cavity for ion capture. The
observed sensing process of sulfate salts associated with the
SO42ꢀ-involved hydrogen-bonding interactions, deprotonation,
and concomitant complexation can also be clearly monitored by
titration methods using UV-vis, fluorescence, and NMR spec-
troscopy in solution. The results show that 4 can bear high
selectivity and sensitivity for Zn2þ ions in the presence of other
competitive metal ions with a 1 : 1 binding stoichiometry.
Anion-induced hydrogen-bonding interactions and deprotona-
tion of the COOH groups in the excited state also endows 4
versatile spectroscopic properties. The 2,20,200-nitrilotribenzoic

L
W. Wei et al.
acid can be utilised as a potential scaffold to build a series of
conjugated fluorescent sensors by its chelation effect owing to
the rigid cavity pre-organised by the triphenylamine moiety and
the carboxylic groups and conjugation extension in the 4,40,400
positions.
[11] A. J. Zucchero, J. N. Wilson, U. H. F. Bunz, J. Am. Chem. Soc. 2006,
128, 11872. doi:10.1021/JA061112E
[12] W. F. Li, H. C. Ma, Z. Q. Lei, RSC Adv. 2014, 4, 39351. doi:10.1039/
C4RA05843G
[13] Y. D. Hang, J. Wang, T. Jiang, N. N. Lu, J. L. Hua, Anal. Chem. 2016,
88, 1696. doi:10.1021/ACS.ANALCHEM.5B03715
[14] K. Ghosh, G. Masanta, R. Fro¨hlich, I. D. Petsalakis, G.
Theodorakopoulos, J. Phys. Chem. B 2009, 113, 7800. doi:10.1021/
JP901151W
[15] J. J. Yao, Y. Y. Fu, W. Xu, T. C. Fan, Y. X. Gao, Q. Q. He, D. F. Zhu, H.
M. Cao, J. G. Cheng, Anal. Chem. 2016, 88, 2497. doi:10.1021/ACS.
ANALCHEM.5B04777
Supplementary Material
1
The H-NMR and 13C-NMR spectra, the ESI spectrum, and
absorption and emission spectra of 4 after addition of salts are
available on the Journal’s website.
[16] A. Chowdhury, P. S. Mukherjee, J. Org. Chem. 2015, 80, 4064. doi:10.
1021/ACS.JOC.5B00348
Conflicts of Interest
The authors declare no conflicts of interest.
[17] H. C. Ma, Z. W. Zhang, Y. Y. Jin, L. J. Zha, C. X. Qi, H. Y. Cao, Z. M.
Yang, Z. W. Yang, Z. Q. Lei, RSC Adv. 2015, 5, 87157. doi:10.1039/
C5RA12154J
Acknowledgements
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This work is financially supported by the National Natural Science Foun-
dation of China (21772014 and 21372029).
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