ESIPT-based ratiometric probe for Zn2+ detection based on BINOL framework

Article abstract of DOI:10.1016/j.tetlet.2016.01.101

A ratiometric BINOL-based fluorescent probe for Zn²⁺ detection was rationally designed and synthesized. Based on the excited-state intramolecular proton transfer (ESIPT) mechanisms, the probe exhibited a significant variation on emission wavele

Full text of DOI:10.1016/j.tetlet.2016.01.101

Tetrahedron Letters 57 (2016) 1133–1137
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
ESIPT-based ratiometric probe for Zn2+ detection based on BINOL
framework
a
a,b,
Kewei Zhang a, Shengying Wu a, , Dahui Qu , Limin Wang
⇑
⇑
a Shanghai Key Laboratory of Functional Materials Chemistry, East China University of Science and Technology, Meilong Road, 130, Shanghai 200237, China
b Key Laboratory of Organofluorine Chemistry, Shanghai Institute of Organic Chemistry, The Chinese Academy of Sciences, 345 Lingling Road, Shanghai 200032, China
a r t i c l e i n f o
a b s t r a c t
A ratiometric BINOL-based fluorescent probe for Zn2+ detection was rationally designed and synthesized.
Based on the excited-state intramolecular proton transfer (ESIPT) mechanisms, the probe exhibited a sig-
nificant variation on emission wavelengths with shifts more than 100 nm after combining with Zn2+, with
emission colors changing from yellow to blue. Good selectivity and sensitivity of the probe toward Zn2+
could be found in aqueous solution. Sensing mechanism was proposed on the basis of fluorescence,
absorption, ESI mass spectrometry, IR spectra, and CD spectra.
Article history:
Received 23 November 2015
Revised 25 January 2016
Accepted 28 January 2016
Available online 28 January 2016
Keywords:
Zinc ion
Ó 2016 Elsevier Ltd. All rights reserved.
Fluorescent probe
ESIPT
Ratiometric
BINOL
Introduction
from auto-fluorescence for bioimaging applications.5 The fluores-
cence of ESIPT chromophores can be perturbed by many intermolec-
As the second most abundant transition metal ion in the human
body, Zn2+ plays a myriad of roles in various biochemical processes,
such as gene transcription, regulation of metalloenzymes, neural
signal transmission, and apoptosis.1 Zinc imbalance is closely
related to a number of pathological disorders, such as Alzheimer’s
disease, epilepsy, Parkinson’s disease, ischemic stroke, and infan-
tile diarrhea.2 Numerous previous work3 have demonstrated that
among various analytical methods available, the method of fluo-
rescent probe had become one of the best choices for detecting
and tracking Zn2+ in vitro and in vivo due to its simplicity, cost
effectiveness, and high sensitivity.4 However, among the reported
probes, many of them still have drawbacks, such as tedious organic
ular interactions, such as removal of the hydrogen involved in the
ESIPT process, thus paving way for sensing metal ions.
Optically active 1,10-bi-2-naphthol (BINOL) and its derivatives
have attracted tremendous research interests in asymmetric
catalysis and fluorescent enantioselective recognition due to their
backbone, which can be modified by strategic placement of func-
tional groups based on steric and electronic properties. However,
there are few reports on the recognition toward metal ions based
on BINOL.6
Herein, we report our research concerning the design, synthesis,
and properties of novel material 3,30-bis(4,5-diphenyl-1H-imida-
zol-2-yl)-[1,10-binaphthalene]-2,20-diol, which will be called 1
below. It is noteworthy that 1 can be utilized as a selective fluores-
cent Zn2+ probe based on the strategy of removal of the H-bond as
well as the resulting ESIPT modulated fluorescence off–on
response. The probe is composed of 4,5-diphenyl-1H-imidazol
dye and a BINOL skeleton as both the fluorophore and the recogni-
tion unit. The emission of 1 exhibited a pronounced red shift due to
the effective ESIPT process. The coordination of the OH group with
Zn2+ will turn off ESIPT, thus the probe mainly displays normal
emission, resulting in a substantial ratiometric response.
synthesis and poor selectivity,3b,f especially interference from Cu2+
Cd2+, and Hg2+, which possess very similar chemical property.
Therefore, it remains significant challenge to develop
sensitive and selective fluorescent probe for Zn2+
,
a
a
.
A ratiometric sensor is highly desirable as it reduces or eliminates
the adverse effects of change of environment around the probe. The
excited state intramolecular proton transfer (ESIPT) strategy has
drawn much attention in the design of ratiometric fluorescent sen-
sors, because of its large Stokes shift, which offers an advantage that
it minimizes the self-absorption and eliminates the interference
Results and discussion
⇑
Corresponding authors. Tel./fax: +86 021 6425 3881.
E-mail addresses: wsy1986wsy@126.com (S. Wu), wanglimin@ecust.edu.cn
The synthesis of chemosensor 1 was outlined in Scheme 1.
3,30-Dialdehyde-BINOL (2) have been synthesized from
(L. Wang).
http://dx.doi.org/10.1016/j.tetlet.2016.01.101
0040-4039/Ó 2016 Elsevier Ltd. All rights reserved.

1134
K. Zhang et al. / Tetrahedron Letters 57 (2016) 1133–1137
1,10-bi-2-naphthol (BINOL) according to literature procedures
(Scheme S1),7 then 2 was refluxed with benzil and ammonium
acetate in acetic acid to afford 1 as the pale yellow solid in
82% yield. The structure of 1 was characterized by 1H NMR,
13C NMR, and a high resolution mass spectrometry (HRMS).
To get insight into the metal ion selectivity of 1, the fluorescence
600
500
400
300
200
100
0
Zn2+
changes of 1 solution (10 lM) to various metal ions in aqueous
solution (CH3OH/H2O, 9:1, v/v, HEPES 20 mM, pH 7.2) were investi-
gated. Free 1 solution displayed an emission band at 549 nm when
exited at 330 nm, which could be attributed to the relaxation of the
keto form to the ground state. Upon addition of 5.0 equiv of Zn2+ to
1 solution, the emission at 549 nm was decreased, accompanied
with the appearance of a strong emission band at 427 nm and a
remarkable hypochromatic shift of 122 nm. Whereas, the addition
1, 1+other metal ions
Hg2+ and Cu2+
of other cations such as Ag+, Pb2+, Ba2+, Cd2+, Ni2+, Co2+, Fe2+, Fe3+
,
400
500
600
700
Mn2+, Al3+, Cr3+, Ca2+, Mg2+, K+, and Na+ have caused less change
in the fluorescence of probe 1 except for Cu2+ and Hg2+ which led
to the fluorescence quenching of the ESIPT band (Fig. 1).
Wavelength (nm)
Figure 1. Fluorescence spectra of 1 solution (10
l
M, CH3OH/H2O, 9:1, v/v, HEPES
20 mM, pH 7.2) in the absence and presence of various metal ions (5.0 equiv of Zn2+
,
The dramatic fluorescence response to the binding of 1-Zn2+
may be ascribed to (i) the deprotonation of phenolic-OH and the
cause of restriction of excited-state induced proton transfer
(ESIPT), (ii) the prohibition of intramolecular electron-transfer pro-
cess, and (iii) the chelation-enhancement of fluorescence process
along with the lowering of vibrational loss due to the rigid molec-
ular structure of the complex.8 The UV–Vis absorption spectra of 1
in the presence of various metal ions were also studied (Fig. S9),
Hg2+, Cu2+, Ag+, Pb2+, Ba2+, Cd2+, Ni2+, Co2+, Fe2+, Fe3+, Mn2+, Al3+, Cr3+, Ca2+, Mg2+, K+
and Na+). kex = 330 nm.
0.8
1.0
0.8
while there was interference by the addition of Cu2+ and Hg2+
.
0.6
The mode of coordination of 1 with Zn2+ ions was initially inves-
tigated by UV–Vis titration. As shown in Figure 2. The free receptor
showed a maximum absorption wavelength at 373 nm. Upon the
addition of Zn2+ ions, the maximum absorbance exhibited an
obvious bathochromic shift from 373 to 388 nm, and the ratio of
absorbance (A388 nm/A373 nm) increased linearly from 0.23 to 0.92
by adding Zn2+ ions (Fig. 2). Meanwhile, the absorbance at
337 nm was decreased, while the absorbance at 250 nm and
290 nm were increased simultaneously. In addition, two clear isos-
bestic points were observed at 296 nm and 383 nm.
0.6
0.4
0.4
0.2
0
5
10
15
20
[Zn2+
]
0.2
0.0
Subsequently, fluorescence titration of 1 by increasing the
amounts of Zn2+ was examined (Fig. 3). Upon incremental addition
of Zn2+, ratiometric fluorescence changes were observed. The orig-
inal emission intensity at 549 nm was gradually decreased, and
concomitantly, the newly formed emission band at 427 nm was
250
300
350
400
450
500
Wavelength (nm)
Figure 2. Absorption spectra changes of 1 solution (10
lM, CH3OH/H2O, 9:1, v/v,
HEPES 20 mM, pH 7.2) upon the addition of different amounts of Zn2+ ions (0–
2.0 equiv). Inset: the linear relationship between the ratio of absorbance (A388nm
/
gradually increased. The ratio of fluorescence intensity (F427
/
nm
A373nm) and the concentration of Zn2+
.
F549 nm) increased from 0 to 43.8 with nonlinear relationship on
addition of Zn2+ ions (Fig. 3). The well-defined isoemissive point
appearing at 500 nm demonstrated the presence of a new complex
in equilibrium with the receptor.6c Notably, the fluorescence
intensity of 1 at both 427 nm and 549 nm has well fitted linear
600
427 nm
50
40
30
20
10
0
relationships with [Zn2+
]
in the range of 0.0–10.0 lM
(R2 = 0.99395 and 0.99784) (Fig. 4), which suggested that 1 could
450
300
150
be served as an efficient ratiometric fluorescent probe for Zn2+
549 nm
0
5
10
15
20
25
30
[Zn2+
]
HN
O
N
H
Benzil, ammonium acetate
OH
OH
OH
OH
0
Acetic acid, reflux
82% yield
400
500
600
700
H
N
Wavelength (nm)
HN
O
2
1
Figure 3. Fluorescence spectra changes of 1 solution (10 lM, CH3OH/H2O, 9:1, v/v,
HEPES 20 mM, pH 7.2) upon addition of different amounts of Zn2+ ions
(0–2.0 equiv). kex = 330 nm. Inset: the nonlinear relationship between the ratio of
Scheme 1. Synthetic route of 1.
fluorescence intensity (F427nm/F549nm) and the concentration of Zn2+
.

K. Zhang et al. / Tetrahedron Letters 57 (2016) 1133–1137
1135
ions. Moreover, according to IUPAC, the detection limit was
determined from three times the standard deviation of the blank
To determine the binding stoichiometry of 1-Zn2+ complex, the
Job’s plot was adapted based on the fluorescence titration. The
maximal fluorescence intensity appears at about 0.5 mol fraction
(Fig. 5a), indicating that 1-Zn2+ complex has 1:1 stoichiometry,
which is also further supported by mass spectrometric analysis
(Fig. S8). The peak at m/z 817.1 corresponding to [1+Zn2++-
MeOHꢀH]+ and 835.1 corresponding to [1+Zn2++MeOH+H2OꢀH]+
was found.
signal (3s) as 0.017 lM. This result provided support for 1 having
a high sensitivity to Zn2+ ions.
600
Intensity of 427 nm
Intensity of 549 nm
500
400
300
200
100
0
Benesi–Hildebrand analysis9 of the titration profiles based on
1:1 binding mode resulted in a nice linearity (Fig. 5b), indicating
the 1:1 binding stoichiometry of Zn2+ and 1, and the binding con-
stant was estimated to be 3.42 ꢁ 104 Mꢀ1. The observed high Ka
value clearly indicated the strong affinity of Zn2+ toward 1.
To examine whether probe 1 still retains sensitive toward Zn2+
under the potential competition from relevant analytes, fluores-
cence competition experiments were then conducted to further
validate the high selectivity of 1 to Zn2+ (Figs. 6 and S10). When
5.0 equiv of Zn2+ was added to 1 solution containing the same
amount of other metal ions, the solution emission intensities were
similar to that induced by Zn2+ only. Of note, Hg2+ and Cu2+ did not
cause any perturbance, though they quenched solution fluores-
cence when present alone with 1 in the previous selectivity exper-
iment. Thus, probe 1 could be particularly applicable to Zn2+ ions,
even in the presence of those relevant analytes.
y = 12.14+41.90x
y = 278.8-20.7x
0
5
10
Zn2+ [μM]
15
20
Figure 4. Fluorescence intensities of
1 at 427 and 549 nm versus different
concentrations of Zn2+ and their linear fits. The spectra were recorded in buffer
Circular dichroism (CD) spectra was also used to recognize the
zinc binding mode to optical probe (R)-1 (Fig. 7), which was
synthesized from optical (R)-BINOL. The product had been
solution (CH3OH/H2O, 9:1, v/v, HEPES 20 mM, pH 7.2). kex = 330 nm.
250
(a)
200
150
100
50
0
Figure 6. Fluorescence intensity ratio (F427nm/F549nm) of 1 solution (10 lM, CH3OH/
0.0
0.2
0.4
0.6
0.8
1.0
H2O, 9:1, v/v, HEPES 20 mM, pH 7.2) to various metal ions. The blue bars represent
the emission intensity of 1 solution on the addition of 5.0 equiv of different metal
ions; the red bars represent the emission intensity of the metal ion containing
solution upon further addition of 5.0 equiv of Zn2+. kex = 330 nm.
[Zn2+]/{[Zn2+]+[
1
]}
0.020
0.016
0.012
0.008
0.004
0.000
(b)
20
10
0
y = 6.326E-4+1.849E-8X
R2 = 0.999
Ka = 3.42x104 M-1
-10
-20
0
200000 400000 600000 800000 1000000
1/[Zn2+
250
300
350
400
450
]
Wavelength (nm)
Figure 5. (a) The Job’s plot evaluates from the fluorescence with a total concen-
tration of [1] + [Zn2+] as 10 M. (b) Benesi–Hildebrand plot of 1-Zn2+ complexes.
The spectra were recorded in buffer solution (CH3OH/H2O, 9:1, v/v, HEPES 20 mM,
l
Figure 7. Circular dichroism spectra changes of 1 solution (10
l
M, CH3OH/H2O, 9:1,
v/v, HEPES 20 mM, pH 7.2) upon the addition of different amounts of Zn2+ ions (0–
2.0 equiv).
pH 7.2). kex = 330 nm, kem = 426 nm.

1136
K. Zhang et al. / Tetrahedron Letters 57 (2016) 1133–1137
λ
λ
em = 427 nm
em = 549 nm
HN
HN
HN
N
N
N
Zn2+
N
N
H
H
ESIPT
H
O
O
O
O
O
O
Zn2+
H
N
HN
HN
HN
keto tautomer
enol tautomer
Scheme 2. Proposed sensing mechanism of 1 to Zn2+
.
29.1
recrystallized, with [
a
]
D
ꢀ344.1777 (c = 0.01, CHCl3). While the
gel (200–300 mesh). Yields were of purified products and were
not optimized. 1H NMR spectra were recorded with a Bruker
Advance 400 MHz using tetramethylsilane as the internal stan-
dard, 13C NMR spectra were recorded at 100 MHz. Mass spectra
(ESI-MS) were obtained on a Waters LCT Premier XE spectrometer.
Absorption spectra were carried out on a UV–Vis absorption Varian
Cray 500 spectrophotometer and fluorescence spectra were mea-
sured on a fluorescence Varian Cray Eclipse. Optical rotation was
obtained by Rudolph Research Analytical, Hackettstown, NJ, USA.
spectra properties of (R)-1 to Zn2+ ion were proved to be the same
as its racemate. The maximum CD wavelengths (kCD) of 1 appeared
at 375 nm, which was much longer than that of BINOL, indicating
that the whole molecules instead of only the BINOL moieties
become chiral.10 Upon the addition of Zn2+, CD signals at bands
270 nm, 350 nm, and 375 nm were gradually decreased, which
demonstrated the transformation of twisted angle between the
two naphthalene rings of 1, thus providing an indirect evidence
for the participation of phenolic–OH in coordinating with Zn2+
.
While for band 270 nm, CD signal was increased toward other side
instead of decreasing to none accompanied with generation of new
bands at 340 nm and 394 nm, which was corresponding to the UV–
Vis LMCT (ligand-to-metal charge transfer) transition, highlighted
a chiral induction to the metal centered complex and therefore chi-
rality of the supramolecular complex as a whole.11
Synthesis of compound 1
Benzil (210 mg, 1.0 mmol) and 3 (171 mg, 0.5 mmol) were dis-
solved in glacial acetic acid (5 mL) at room temperature. After the
addition of ammonium acetate (2.31 g, 30 mmol) the reaction mix-
ture was heated at 110 °C for 24 h under an argon atmosphere.
After cooling to room temperature, the reaction mixture was
poured into the ice-water. The precipitate was filtered to give a
light yellow solid. The solid was further purified by column
chromatography with a petroleum ether/ dichloromethane (1/1)
mixture on silica gel. Yield: 82%. 1H NMR (400 MHz, DMSO-d6) d
13.50 (s, 2H), 13.21 (s, 2H), 8.84 (s, 2H), 7.94 (d, J = 8 Hz, 2H),
7.65–7.63 (m, 4H), 7.56–7.49 (m, 10H), 7.38–7.34 (m, 2H),
7.31–7.27 (m, 6H), 7.25–7.23 (m, 2H), 7.11–7.08 (d, J = 8.2 Hz,
2H). 13C NMR (100 MHz, CDCl3) d 171.9, 151.9, 145.8, 134.1,
128.5, 128.2, 127.3, 126.9, 124.0, 124.7, 123.6, 117.5, 115.2. EI-MS,
m/z: [M+H]+ calcd for C50H34N2O4, 723.2760, found 723.2772.
In the IR spectra of 1 (Fig. S7), a broad sharp band (3414 cmꢀ1
)
corresponding to the
m (O–H) as well as m (N–H) vibration mode
was present. Upon the addition of Zn2+ (Fig. S8), the sharp band
was weakened to a broad band, providing an evidence for the
disappearance of phenolic–OH. In addition, the band
m (C@N) at
1628 cmꢀ1, from the imidazole ligand, was shifted to 1639 cmꢀ1
,
indicating that the metal ion was coordinated through the
imidazolic nitrogen atom. Moreover, the sharp strong band
(1117–1126 cmꢀ1) assigned to
m
(C–O) was also weakened due to
the combination of 1 with Zn2+, accompanied with an obvious shift
to 1167–1196 cmꢀ1
.
The sensing mechanism of 1 to Zn2+ was proposed as shown in
Scheme 2. Free 1 exhibited keto emission in solution with negligi-
ble enol emission12, thus yellow fluorescence was observed. The
coordination with zinc ion perturbed ESIPT mechanism and hin-
dered the generation of keto tautomer, thus probe 1 exhibited enol
emission with blue fluorescence.
Acknowledgments
This research was financially supported by the National Nature
Science Foundation of China (NSFC, 21272069, 20672035) and the
Fundamental Research Funds for the Central Universities and Key
Laboratory of Organofluorine Chemistry, Shanghai Institute of
Organic Chemistry, Chinese Academy of Sciences.
Conclusions
In summary, we have developed a new 1,10-BINOL skeleton
bearing 4,5-diphenyl-1H-imidazol fluorescent probe 1 for sensing
Zn2+ with a high sensitivity and selectivity. The combined results
showed that the probe 1 could detect for Zn2+ ions by two signal
channels and minor interruption could be observed from other
representative interferences. In addition, detecting mechanism
was proposed on the basis of fluorescence, absorption, ESI mass
spectrometry, IR spectra, and CD spectrum.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2016.01.
101.
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