A highly selective fluorescent turn-on probe for Al3+ via Al3+-promoted hydrolysis of ester

Article abstract of DOI:10.1016/j.tet.2013.05.062

A new reactive and highly selective fluorescent chemosensor (1) based on thiazole was synthesized for the quantification of aluminum ions in ethanol. The mechanism of fluorescence was based on the aluminum-promoted hydrolysis of the ester moiety and subse

Full text of DOI:10.1016/j.tet.2013.05.062

Tetrahedron 69 (2013) 6095e6099
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Tetrahedron
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A highly selective fluorescent turn-on probe for Al3þ via
Al3þ-promoted hydrolysis of ester
*
Aasif Helal, Sang Hyun Kim, Hong-Seok Kim
Department of Applied Chemistry, Kyungpook National University, Daegu 702-701, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 2 April 2013
Received in revised form 15 May 2013
Accepted 17 May 2013
Available online 23 May 2013
A new reactive and highly selective fluorescent chemosensor (1) based on thiazole was synthesized for
the quantification of aluminum ions in ethanol. The mechanism of fluorescence was based on the
aluminum-promoted hydrolysis of the ester moiety and subsequent complexation.
Ó 2013 Elsevier Ltd. All rights reserved.
Keywords:
Al3þ-selective
Fluorescent probe
Turn-on
Al3þ-promoted
Hydrolysis
1. Introduction
S
S
O
O
The developments of turn-on fluorescent probes that operate
through chemical reactions triggered by the target analytes have
attracted attention.1e3 A range of chemical reactions have been
implemented in the development of many reactive probes.
This approach involves the use of highly selective and normally
irreversible chemical reactions coupled with signal transduction
induced by analytes. These reactive probes provide higher selec-
tivity with larger spectroscopic changes than chemical probes.
Previously, the synthesis and Zn2þ or Cu2þ sensing properties of
several thiazole based chemosensors were reported.4e7 Among
them, the selectivity for cations can be controlled by changing the
moiety at position-4 of the thiazole ring. Schiff base fluorescent
sensors have attracted special attention owing to their versatile
organic blockers possessing comparable accessibility and structure
variable. On the other hand, few studies have examined Schiff base
chemosensors for the detection of Al3þ ion due to its weak co-
ordination ability compared to transition metal ions8e16 Therefore,
considering the Schiff base and functional carboxylic moiety17
affinity to Al3þ, probe 1 (Fig. 1) was prepared by introducing an
ester and phenol group at positions-4 and -2 of the thiazole ring,
respectively. This provided harder donors, such as oxygen and
N
N
OR
O
2
R
O
Al
1
O
H
1: R = OH, R = CH CH
3
1
1
1
2
2
2
2
H
2: R = OH, R = H
+
3+
1- Al
3: R = H, R = CH CH
3
2
Fig. 1. Fluorescent thiazole probes, 1e3 synthesized in this study and aluminum
complex of 1.
Moreover, the hydroxyl group and ternary N atom as coordinating
sites act as a bidentate ligand and show the same coordinating
motif as the family of 8-hydroxyquinoline ligands that are used
extensively in analytical chemistry for the detection of Al3þ 18
.
Probes 2 and 3 were also prepared to understand the critical role
of phenol at the 2-position of the thiazole rings and the mode of
complexation with Al3þ
.
2. Results and discussion
nitrogen atoms, in the molecular skeleton for the hard acid, Al3þ
.
Probes 1 and 3 were obtained in good yield by a Hantzsch
condensation reaction of 2-hydroxyphenylthiobenzamide and
thiobenzamide with ethyl bromopyruvate in refluxing ethanol,
and probe 2 was prepared in good yield by the hydrolysis of probe 1.
* Corresponding author. Tel.: þ82 53 950 5588; fax: þ82 53 950 6594; e-mail
addresses: kimhs@knu.ac.kr, kimhs053@gmail.com (H.-S. Kim).
0040-4020/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2013.05.062

6096
A. Helal et al. / Tetrahedron 69 (2013) 6095e6099
The structures of probes 1, 2, and 3 were confirmed by 1H and 13C
NMR spectroscopy, and elemental analyses.
UVevis study was carried out in ethanol at a concentration of
20
the guest to ensure complete complexation. Probe 1 displayed an
obvious absorption band at 325 nm. This was attributed to a pep
m
M. The host solution was heated to 50 ꢀC for 1 h after adding
transition, which is favored by the planar orientation enforced by
intramolecular hydrogen bonding.19 On the other hand, upon the
addition of aluminum ions to a solution of probe 1, the absorption
band at 364 nm was enhanced gradually with a concomitant de-
crease in the absorption band at 325 nm, as shown in Fig. 2 with an
isosbestic point at 339 nm. These findings are characteristic of
phenolethiazole derivatives when a complexation process is ac-
companied by the deprotonation of a hydroxyl group.6,7 Moreover,
there are three clear isosbestic points at 339, 304, and 286 nm,
which clearly suggests the conversion of free chemosensor 1 to an
Al3þ complex of only one stoichiometry.20,21
Fig. 3. Fluorescence spectra of probe 1 (2 m
M) in EtOH upon heating at 50 ꢀC for 1 h on
0.4
the addition of 10 equiv of various cations. lex¼364 nm.
364 nm
0.3
Al3þ reached 0.5, which is indicative of 1:1 complexation between
probe 1 and Al3þ. The binding constant for the complex between
probe 1 and Al3þ was determined by a non-linear fitting of the
corresponding fluorescence titration data (Fig. 5) as 2.1ꢂ106 Mꢃ1
with a satisfactory correlation coefficient value (R¼0.9983) (Error
estimated to be ꢁ10%).22,23
325 nm
0-10 eq
0-10 eq
Equivalent of Al
364 nm
325 nm
0.2
A competitive binding experiment with different cations and
Al3þ also shows that probe 1 is highly selective to Al3þ (Fig. S-1). It
0.1
0.0
was also found that probe 1 shows 0.39 mM of detection limit to
sufficiently sense Al3þ (Fig. S-2).24 This unique selectivity of probe 1
toward Al3þ could be interpreted in terms of the mechanism in
Scheme 1. Aluminum is a well known Lewis acid25 that is capable of
hydrolyzing probe 1 and generating probe 2 that binds with Al3þ
resulting in the liberation of ethanol and emission peak at 434 nm
due to a chelation effect.26e28 At the initial stages, deprotonation of
the phenolic proton occurs by the addition of Al(ClO4)3, which
subsequently acts as a Lewis acid hydrolyzing the ester to generate
acid 2. This acid provides a better additional binding site for the
chelation of Al3þ and increases the fluorescence dramatically
through a highly efficient chelation-enhanced fluorescence (CHEF)
effect. Although there are some examples of Cu2þ-facilitated hy-
drolysis of an ester bond, there have been no reports of Al3þ-pro-
moted ester hydrolysis, which is utilized here to improve the
sensitivity and broaden the methodologies for designing a range of
fluorescent probes.29e31 This mechanism also helps explain why
300
350
Wavelength (nm)
400
450
Fig. 2. UVevis titration spectra of probe 1 (20 mM) upon the addition of 10 equiv of
Al(ClO4)3 in EtOH on heating at 50 ꢀC for 1 h.
The emission studies were carried in a similar manner at
a concentration of 2 M in ethanol. As a cation sensor, one of the
m
important criteria for potential applications is the selectivity. To
solutions of probe 1, 10 equiv of a range of biologically and non-
biologically relevant metal cations were added, and their com-
plexation abilities were examined by fluorescence emission. None
of the other cations except for Al3þ, which produced a prominent
fluorescence peak at 434 nm, induced a distinct emission shift with
probe 1 (Figs. 3 and 4). Even the addition of Ga3þ, which exhibits
similar coordination behavior to Al3þ, and Fe3þ did not produce any
changes in the emission spectra. Fig. 5 shows the fluorescence
spectrum of the free probe 1 and those in the presence of an in-
a solution of probe 1 (2 mM) in the presence of 20 m
M of Al3þ when
allowed to stand at room temperature (25 ꢀC), shows blue fluo-
rescence that gradually grows more intense before reaching
a maximum within 72 h, whereas an elevated temperature (50 ꢀC)
could promote the generation of fluorescence within 1 h.
To understand the mechanism by which probe 1 was hydrolyzed
in the presence of Al3þ a 1H NMR titration of probe 1 with Al(ClO4)3
was checked in CD3OD as shown in Fig. 7. The 1H NMR titration
spectra showed the concomitant development of a CH3CH2 peak for
ethanol and the disappearance of a CH3CH2 signal of the ester
within 24 h. This supports the assumption that the fluorescence of
complex 1-Al3þ is generated by ester hydrolysis 1 with Al3þ as
a promoter.
cremental amount of Al3þ
.
Probe 1 showed no emission peak when excited at 364 nm and
the slow addition of Al3þ elicited a large fluorescence peak at
434 nm. Therefore, in probe 1, the fluorescence is turned on by
binding with Al3þ. The inset in Fig. 5 shows the dependence of the
normalized emission intensity ratio at 434 nm on the Al3þ con-
centration. The linear enhancement of fluorescence was observed
with increasing Al3þ ion concentration when the Al3þ to the probe
1 concentration ratio was ꢁ1:1. On the other hand, higher Al3þ
concentrations did not lead to any further emission enhancement.
To determine the stoichiometry of probe 1 and Al3þ in the complex,
Job’s method was employed using the emission changes at 434 nm
as a function of the molar fraction of Al3þ. As shown in Fig. 6
maximum emission was observed when the molar fraction of
The above mechanistic proposal is further supported by the
fluorescent and non-fluorescent behaviors of the control probes 2
and 3 in the presence of Al3þ (Fig. S-4). Probe 2 produce identical
changes in the photophysical properties with Al3þ as probe 1 except
that it showed emission selectivity toward Ga3þ, Zn2þ and Cd2þ
along with Al3þ (Fig. 8). On the other hand, probe 3 did not produce

A. Helal et al. / Tetrahedron 69 (2013) 6095e6099
6097
Fig. 4. Fluorogenic changes of a 2
mM solution of probe 1 in EtOH in the presence of 10 equiv of different cations upon illumination at 365 nm.
3. Conclusions
0-10 eq
600
500
400
300
200
100
0
In summary, a thiazole based ‘turn-on’ fluorescence probe (1)
for Al3þ was prepared. For the first time, such high selectivity to
Al3þ over other cations was observed due to the Al3þ-promoted
hydrolysis of the ester to give an acid that binds to Al3þ resulting in
a dramatic change in fluorescence by CHEF.
Equivalent of Al ion
4. Experimental section
4.1. General
The melting points were determined using a Thomas-Hoover
capillary melting point apparatus and were uncorrected. The 1H
and 13C NMR spectra were recorded on a Bruker AM-400 spec-
trometer using Me4Si as the internal standard. The FAB mass was
determined at KBSI Daegu branch. Infrared spectra were recorded
using a Shimadzu Prestige-21 spectrometer on a KBr pellet. The
400
450
500
550
Wavelength (nm)
600
650
UVevis absorption spectra were determined on
a Shimadzu
Fig. 5. Fluorescence titration spectra of probe 1 (2
m
M) with Al(ClO4)3 in EtOH upon
heating at 50 ꢀC. lex¼364 nm. Inset: mol ratio plots of emission at 434 nm.
UV-1650PC spectrophotometer. The fluorescence spectra were
measured on a Shimadzu RF-5301 fluorescence spectrometer
equipped with a xenon discharge lamp and 1 cm quartz cells with
slit 3. All the measurements were taken at 298 K. Analytical grade
ethanol was purchased from Merck. All other materials for synthe-
ses were purchased from Aldrich Chemical Co. and used as received.
2-Hydroxyphenylthiobenzamide was prepared by the procedure
reported in the literature.6 Solutions of metal ions were prepared
from their perchlorate salts of analytical grade, and subsequently
1000
800
600
400
200
0
diluted to prepare the working solutions. The quantum yield (
F) was
calculated using the procedure reported in the literature.4e7
4.2. Synthesis of probe
4.2.1. Ethyl 2-(2-hydroxyphenyl)thiazole-4-carboxylate (1). A solu-
tion of 2-hydroxyphenyhlthiobenzamide 4 (300 mg, 1.96 mmol) and
ethyl bromopyruvate (458 mg, 2.35 mmol) in ethanol (15 mL) was
heated under reflux for 2 h. The reaction progress was monitored by
TLC analysis. The solvent was removed in vacuo, and the residue was
washed with water and extracted with EtOAc. The organic layer was
dried over anhydrous Na2SO4 and concentrated. The residue was
purified by column chromatography (EtOAc:hexane¼1:4) to give
compound 1 as a colorless needle shaped solid (350 mg, 1.40 mmol)
in 71% yield. Mp 109e111 ꢀC (CH2Cl2ehexane); IR (KBr, cmꢃ1) 3140,
0.0
0.2
0.4
0.6
0.8
1.0
3+
Mole fraction of Al
Fig. 6. Job’s plot for probe 1 (20
50 ꢀC for 1 h. lex¼364 nm.
mM) with Al(ClO4)3 (20 mM) in EtOH upon heating at
any change in absorption or emission upon the addition of 10 equiv
of Al3þ due to the absence of phenoliceOH because it is unable to
exhibit three ways coordination with Al3þ on a similar line of that
observed with probes 1 and 2 (Fig. S-4). This suggests that Al3þ first
binds to the eOH of the phenol and causes the hydrolysis of the
ester. A complex of probe 1 with Al3þ in EtOH was also prepared
and characterized by HR-FAB mass. The mass spectra of probe
1-Al3þ showed 1:1 stoichiometry (between probe 1 and Al3þ) with
the molecular ion peak at m/z, 263.9903, which corresponds to
(probe 1þAl3þeEtOHþH2OeH)þ (Fig. S-5). This also provides evi-
dence that the ester is hydrolyzed to an acid by Al3þ followed by
complexation.
2978, 1720, 1474, 1211; 1H NMR (400 MHz, CDCl3)
d 8.09 (s, 1H), 7.60
(dd, J¼7.8, 8.8 Hz, 1H), 7.35 (m, 1H), 7.08 (dd, J¼8.4, 8.4 Hz, 1H), 6.92
(m, 1H), 4.42 (q, J¼7.1 Hz, 2H), 1.41 (t, J¼7.4 Hz, 3H); 13C NMR
(100 MHz, CDCl3) d 169.8, 160.9, 157.4, 146.5, 132.9, 127.7, 125.4, 119.9,
118.5, 116.7, 62.0, 14.7; Anal. Calcd for C12H11NO2S: C, 57.82; H, 4.45;
N, 5.62; S, 12.86, found: C, 57.78; H, 4.36; N, 5.66; S, 13.12. HR-mass
Calcd for: C12H11NO3S [MþH]þ: 249.0460; found: m/z 249.0457.
4.2.2. 2-(2-Hydroxyphenyl)thiazole-4-carboxyic
acid
(2).
Compound 1 (100 mg, 0.65 mmol) and 5 mL of 1 N KOH in ethanol
(25 mL) was heated under reflux for 2 h. The reaction progress was
monitored by TLC. The solvent was removed in vacuo, and the

6098
A. Helal et al. / Tetrahedron 69 (2013) 6095e6099
OEt
S
H
S
O
O
-HClO
-EtOH
Al(ClO )
H
4
N
H
4 3
N
OEt
O
o
EtOH, 50 C
O
O
Al ClO
4
1
ClO
4
S
O
O
S
O
S
H O
H O
2
2
N
N
N
O
OH
O
Al(ClO )
4 3
Al
Al
O
OH
O
H
ClO
4
O
H
+3 +
]
[1-Al
2
Scheme 1. Al3þ-promoted hydrolysis of probe 1.
found: C, 54.57; H, 3.20; N, 6.45; S, 14.40. HR-mass Calcd for:
C10H7NO3S [MþH]þ: 222.0225; found: m/z 222.0221.
4.2.3. Ethyl 2-(2-phenyl)thiazole-4-carboxylate (3). This compound
was obtained by the same procedure for synthesizing compound 1
using thiobenzamide (200 mg, 1.46 mmol) and ethyl bromopyr-
uvate (341 mg, 1.75 mmol) in ethanol (15 mL). The product was
purified by column chromatography (10% EtOAc in hexane) to give
compound 3 as a colorless needle shaped solid in 70% yield. Mp
Fig. 7. 1H NMR spectrum of probe 1 (0.04 mM) in the presence of 1.0 equiv of Al3þ ion
120e121 ꢀC (CH2Cl2ehexane); 1H NMR (400 MHz, CDCl3)
d 7.78 (d,
after 24 h at 25 ꢀC in CD3OD (detail titration in Fig. S-3).
J¼8.0 Hz, 2H), 7.77 (s, 1H), 7.31 (t, J¼8.0 Hz, 3H), 4.34 (q, J¼8.0 Hz,
2H), 1.34 (t, J¼7.1 Hz, 3H); 13C NMR (100 MHz, CDCl3)
d 169.5, 162.0,
148.3, 133.0, 129.4, 129.13, 128.4, 127.8, 127.7, 127.3, 62.1, 14.8; Anal.
Calcd for C12H11NO2S: C, 61.78; H, 4.75; N, 6.00; S, 13.74; found: C,
61.72; H, 4.69; N, 6.03; S, 13.67.
1000
2+
Zn
3+
Ga
4.2.4. 1-Al3þ complex. A mixture of compound
1 (100 mg,
800
0.45 mmol) and Al(ClO4)3$9H2O (263 mg, 0.54 mmol) in ethanol
(10 mL) was heated under reflux for 1 h. The mixture was cooled to
room temperature, and the ethanol was evaporated and the residue
was dried under vacuum to provide 1-Al3þ complex as a white solid
extremely hygroscopic in nature (110 mg, 92% yield). HR-FAB Mass:
Calcd for (C10H7NO4S.Al) 263.9911; found: 263.9903.
3+
600
Al
400
200
Acknowledgements
2+
Cd
This research was supported by the NRF grant funded by MEST
in Korea (NRF-2010-0010070).
+ + + + 2+
Na , K ,Cs , Rb , Mg , Cu
2+
,
Ag , Hg , Pb , Ca , Fe , Fe
2+
Ni , Co
2+
2+
2
,
+
2+ 2+ 2+
3+
0
400
450
500
Wavelength (nm)
550
600
650
Supplementary data
Figures of competitive binding of probe 1-Al3þ, method of de-
termination of detection limit of Al3þ by probe 1, 1H NMR experi-
ments spectra of probe 1, absorbance (UVevis), fluorescence
spectra of probes 1e3, and HR-FAB mass of probe 1-Al3þ complex
are available. Supplementary data related to this article can be
found at http://dx.doi.org/10.1016/j.tet.2013.05.062.
Fig. 8. Fluorescent spectra of probe 2 (2
cations in EtOH. lex¼364 nm.
mM) upon the addition of 10 equiv of various
residue was washed with water, acidified with 10 mL of 1 N HCl and
extracted with CH2Cl2. The organic layer was dried over anhydrous
Na2SO4 and concentrated. The residue was purified by column
chromatography (10% MeOH in CH2Cl2) to give compound 2 as
a white solid (82 mg, 0.37 mmol) in 90% yield. Mp 249e251 ꢀC
(CH2Cl2ehexane); IR (KBr, cmꢃ1) 3450, 3125e2577, 1690, 1481,
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d
6.98 (dd, J¼7.8, 7.3 Hz, 1H),
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