Thiazole-based chemosensor II: Synthesis and fluorescence sensing of fluoride ions based on inhibition of ESIPT

Article abstract of DOI:10.1007/s10847-009-9648-0

Novel chemosensors based on 2-(2′-hydroxyphenyl)-4-phenylthiazole were synthesized and their anion sensing behaviors were investigated. Sensors 1 and 2 show fluoride ion selective behaviors related to their absorption and emission spectra amongst F^-, CH₃CO₂^-, H₂PO₄^-, Cl^-, Br^-, I^-, ClO₄^-, NO₃^-, and HSO₄^- anions. Sensor 2 shows color change upon interaction with F^-. Interactions of 1, 2 and 3 with F^- cause a red-shift in UV-vis absorption and a large Stokes shift in fluorescence emission due to the inhibition of ESIPT induced by the deprotonation of phenolic proton by F^-. Springer Science+Business Media B.V. 2009.

Full text of DOI:10.1007/s10847-009-9648-0

J Incl Phenom Macrocycl Chem (2010) 66:87–94
DOI 10.1007/s10847-009-9648-0
ORIGINAL ARTICLE
Thiazole-based chemosensor II: synthesis and fluorescence sensing
of fluoride ions based on inhibition of ESIPT
Aasif Helal Æ Nugen Thi Thu Thao Æ
Soon W. Lee Æ Hong-Seok Kim
Received: 1 July 2009 / Accepted: 12 July 2009 / Published online: 1 August 2009
Ó Springer Science+Business Media B.V. 2009
Abstract Novel chemosensors based on 2-(20-hydroxy-
phenyl)-4-phenylthiazole were synthesized and their anion
sensing behaviors were investigated. Sensors 1 and 2 show
fluoride ion selective behaviors related to their absorption
and emission spectra amongst F-, CH3CO2-, H2PO4-, Cl-,
Br-, I-, ClO4-, NO3-, and HSO4- anions. Sensor 2 shows
color change upon interaction with F-. Interactions of 1, 2
and 3 with F- cause a red-shift in UV–vis absorption and a
large Stokes shift in fluorescence emission due to the inhi-
bition of ESIPT induced by the deprotonation of phenolic
proton by F-.
the sensors. However, emission intensity is also dependent
on other factors, such as bleaching, optical path length, and
illumination intensity. It is therefore desirable to eliminate
the effects of these factors by using a ratiometric sensor
that exhibits a spectral shift upon reaction or binding to the
analyte of interest, so that the ratio between the two
emission intensities can be used to evaluate the analyte
concentration. In this respect, dual fluorescence probes
exhibiting two well separated emission bands are of par-
ticular interest, since they provide a reliable ratiometric
signal independent of the probe concentration [9]. Excited
state intramolecular proton transfer (ESIPT) is very effec-
tive in the design of probes with dual fluorescence. ESIPT
results in the formation of two tautomeric forms when the
probe is in the excited state: N*-normal and T*-tautomer
forms [10, 11]. ESIPT is one of the most well known
photophysical processes occurring in o-hydroxy-benzox-
azole, benzimidazole and benzothiazole [12–14]. It is
expected that replacement of the oxygen atom in the oxa-
zole ring, with a sulfur atom to form a thiazole cycle, raises
molecular polarizabilities and introduces interesting novel
spectral-luminescent properties. Formation of an intramo-
lecular hydrogen bond in these compounds promotes
luminescence intensity and results in abnormally large
Stokes shifts that are essential for practical applications of
the luminophores [6].
Keywords Thiazole ꢀ Fluorescence ꢀ ESIPT ꢀ
Fluoride ions ꢀ Stokes’ shift
Introduction
Considerable attention has been focused on the design of
chemosensors for the detection of biologically relevant
anions [1–4]. In particular, selective sensing of fluoride ion
has gained attention due to its significance in clinical
treatment for osteoporosis and dental care [5, 6]. Recently a
large number of methods for the sensing of fluoride ion
have been reported [7, 8]. In biological systems fluores-
cence intensity is dependent on the local concentration of
With reference to binding groups, only a limited number
of reports are available which use –OH as a binding site
[15–19]. Understanding the nature of intramolecular
hydrogen bonding of phenol and fluoride ion is necessary
to develop a fluoride ion selective sensor [20]. Electron-
donating (CH3) and electron-withdrawing (Br) substituents
were incorporated on the para position of the phenol, and
fluoride ion selectivity was investigated. We herein report
F- selective chemosensors possessing thiazole and a
A. Helal ꢀ N. T. T. Thao ꢀ H.-S. Kim (&)
Department of Applied Chemistry, Kyungpook National
University, Daegu 702-701, Republic of Korea
e-mail: kimhs@knu.ac.kr
S. W. Lee
Department of Chemistry and School of Molecular Science,
Sungkyunkwan University, Suwon 440-746, Republic of Korea
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J Incl Phenom Macrocycl Chem (2010) 66:87–94
phenolic OH group which are able to bind fluoride ions via
H-bond interactions.
residue was purified using SiO2 column chromatography
(elution with EtOAc in hexane) to give compound 1.
Compound 1
Experimental
Yield 84%; mp 103 °C (CH2Cl2-hexane); 1H NMR
(CDCl3) d 6.92 (dt, J = 7.6, 1.2 Hz, 1H), 7.12 (dd, J =
8.3, 1.0 Hz, 1H), 7.34 (dd, J = 8.3, 1.5 Hz, 1H), 7.38 (s,
1H), 7.40 (dt, J = 8.3, 1.2 Hz, 1H), 7.46 (dt, J = 8.1,
1.5 Hz, 2H), 7.65 (dd, J = 7.8, 1.5 Hz, 1H), 7.87 (dd,
J = 8.5, 1.5 Hz, 2H), 12.45 (bs, OH); 13C NMR (CDCl3) d
111.3, 117.1, 118.6, 119.6, 126.8, 127.6, 128.8, 129.1,
132.4, 133.2, 154.3, 157.2, 169.4; Anal. Calcd for
C15H11NOS: C, 71.12; H, 4.38; N, 5.53; S, 12.66, Found:
C, 70.84; H, 4.32; N, 5.46; S, 12.20.
Apparatus
Melting points were determined using a Thomas-Hoover
1
capillary melting point apparatus and are uncorrected. H
and 13C NMR spectra were recorded on a Bruker AM-400
spectrometer using Me4Si as the internal standard. UV–vis
absorption spectra were determined using a Shimadzu UV-
1650PC spectrophotometer. Fluorescence spectra were
measured using a Shimadzu RF-5301 fluorescence spec-
trometer equipped with a xenon discharge lamp, 1 cm
quartz cells. All of the measurements were carried out at
298 K.
Compound 2
1
Yield 95%; mp 112–114 °C (CH2Cl2-hexane); H NMR
X-ray structure determination
(CDCl3) d 2.2 (s, 3H), 6.87 (d, J = 8.4 Hz, 1H), 7.01 (dd,
J = 8.4, 2.4 Hz, 1H), 7.23 (s, 1H), 7.25-7.28 (m, 2H), 7.32
(t, J = 8.0 Hz, 2H), 7.73 (d, J = 8.0 Hz, 2H), 11.9 (bs,
OH); 13C NMR (CDCl3) d 20.8, 110.8, 116.7, 117.8, 119.6,
126.4, 127.3, 128.8, 129.0, 133.0, 133.4, 154.4, 154.9,
169.4; Anal. Calcd for C16H13NOS: C, 71.88; H, 4.90; N,
5.24; S, 11.99, Found C, 69.86; H, 4.87; N, 5.08; S, 11.36.
All X-ray data were collected with the use of a Siemens P4
diffractometer equipped with a Mo X-ray tube. The ori-
entation matrix and unit-cell parameters were determined
by least-squares analyses of the setting angles of 33
reflections in the range 10.0° \ 2h \ 25.0°. Intensity data
were empirically corrected for absorption with w-scan data.
All calculations were carried out with the use of SHELXTL
programs [21].
Compound 3
1
Yield 80%; mp 136–138 °C (CH2Cl2-hexane); H NMR
Chemicals
(CDCl3) d 6.83 (d, J = 9.0 Hz, 1H), 7.24–7.27 (m, 2H),
7.25 (s, 1H), 7.31 (t, J = 7.2 Hz, 2H), 7.55 (d, J = 2.5 Hz,
1H), 7.68 (d, J = 7.6 Hz, 2H), 12.12 (bs, OH); 13C NMR
(CDCl3) d 111.2, 111.6, 118.5, 119.9, 126.4, 129.0, 129.1,
129.4, 132.9, 134.6, 154.5, 156.4, 167.5; Anal. Calcd for
C15H10BrNOS: C, 54.23; H, 3.03; N, 4.22; S, 9.65, Found:
C, 53.88; H, 2.90; N, 4.17; S, 9.36.
Analytical grade acetonitrile was purchased from Merck
and dried with CaH2 before use. All other materials used
for synthesis were purchased from Aldrich Chemical Co
and used without further purification. 2-Hydroxythioben-
zamide, 5-methyl-2-hydroxythiobenzamide, and 5-bromo-
2-hydroxythiobenzamide were prepared by thionation of
the corresponding salicylamide with Lawesson’s reagent.
In the titration experiments, all the anions were added in
the form of tetrabutylammonium (TBA) salts, which were
purchased from Aldrich Chemical Co., stored in a vacuum
desiccator and dried fully before diluting to prepare
working solutions.
Results and discussion
Synthesis
The chemosensors 1, 2, and 3 were synthesized in good
yields by Hantzsch thiazole synthesis from the reaction of
the corresponding 2-hydroxythiobenzamide and 2-bromo-
acetophenone as shown in Scheme 1. Initial attempts using
the Hantzsch method by Bach failed to provide 1 but we
succeeded in preparing the desired compounds via a simple
condensation reaction [22]. The structures of chemosensors
1–3 were confirmed by 1H, 13C NMR, and elemental
analyses data.
Syntheses of compounds 1, 2 and 3
A solution of 2-hydroxythiobenzamide (1 mmol) and
2-bromoacetophenone (1 mmol) in dry EtOH (20 mL) was
stirred at room temperature for 5 h. After the solvent
was removed by evaporator, the mixture was extracted with
CH2Cl2, dried over anhydrous Na2SO4 and evaporated. The
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J Incl Phenom Macrocycl Chem (2010) 66:87–94
89
X
orientation enforced by intramolecular hydrogen bonding
[23].
X
O
Br
+
EtOH
Titration of 1, as a function of F-, revealed that the
intensity of the absorption band at 332 nm decreased and a
new absorption band at 400 nm appeared gradually, with
an isobestic point at 353 nm (Fig. 2d). The intensity of this
peak reached its maximum after the addition of 6 equiv of
F- (Fig. 2d, inset). When an excess of tetrabutylammo-
nium salts of anions were added to sensor 1, only F- led to
a change in kmax and absorption intensity, although the
NH2
S
OH
N
OH
S
1. X= H
2. X= CH3
3. X= Br
Scheme 1 Synthesis of chemosensors 1, 2 and 3
Crystal structure of 1
-
addition of CH3COO- and H2PO4 produced changes in
Crystal of 1, suitable for single crystal X-ray diffraction
analysis, was obtained from dichloromethane-hexane. The
molecular structure of 1 with the atom-numbering scheme is
shown in Fig. 1. The overall structure is essentially planar,
with dihedral angles between the central thiazole ring and
the planar adjacent aryl rings, in the range of 2.3(1)–6.4(1)°.
There is an intramolecular O–HꢀꢀꢀN hydrogen bonding
k
max, their absorption intensities were very low, whereas
-
the addition of other anions such as Cl-, Br-, I-, ClO4
,
NO3-, and HSO4 resulted in insignificant changes in the
absorption spectra. In the case of sensor 2, when excess of
various anions were added to the solution (Fig. 2b), only
F- causes a red shift to 410 nm and the color of the
solution changed from colorless to yellow (Fig. 3).
Whereas the addition of other anions including CH3COO-,
-
˚
˚
˚
(O1–HO1: 0.89 A; HO1ꢀꢀꢀN1: 1.83 A; O1ꢀꢀꢀN1: 2.65 A;
O1–HO1ꢀꢀꢀN1 153°), whose bonding parameters indicate
relatively strong hydrogen bonding.
H2PO4-, Cl-, Br-, I-, ClO4-, NO3-, and HSO4 resulted
-
in negligible changes in the absorption spectra. The color
change of 2 in the CH3CN solution can be used for naked-
eye detection of F- in the presence of other anions.
In sensor 3, when excess of anions were added (Fig. 2c),
UV–vis responses toward anions
F-, CH3COO- and H2PO4 caused a significant red shift
-
Initial studies of the UV–vis absorption showed that 1
exhibited selectivity toward fluoride ion in CH3CN. Fig-
ure 2a, shows the absorption spectra of 1 in CH3CN
(30 lM) upon addition of 10 equiv of tetrabutylammonium
salts of these anions: F-, CH3CO2-, H2PO4-, Cl-, Br-, I-,
ClO4-, NO3-, and HSO4-. Only F- and CH3CO2- showed
significant changes over other anions. Upon addition of
fluoride ion into the 1 solution a significant red-shift
(Dk = 68 nm) occurred with enhancement of absorbance
in the UV–vis absorption spectrum. Sensor 1 exhibited an
absorption band at 332 nm (Fig. 2a), sensors 2 (Fig. 2b)
and 3 (Fig. 2c) showed absorption bands at 338 and
340 nm, respectively. This may be attributed to a p–p*
charge-transfer state; which is favored by the planar
to 410 nm whereas the addition of other anions such as
Cl-, Br-, I-, ClO4-, NO3 and HSO4 did not produce
any change. These photophysical data from the chemo-
sensors are summarized in Table 1.
-
-
The resonance structures of the deprotonated form of 1
show that the electron density of phenyl at the para posi-
tion is increased upon deprotonation as shown in
Scheme 2. This indicates that the acidity in this kind of
sensor can be tuned by changing the electronic property of
the substituent on the para position. The introduction of an
electron donating methyl group at the para position
decreases the acidity of the proton, and thus the hydrogen
bonding ability is reduced. Sensor 2 can only interact with
Fig. 1 ORTEP drawing of 1
with 50% probability thermal
ellipsoids
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Fig. 2 UV–vis spectra of (a) 1
(b) 2 (c) 3 in CH3CN (30 lM)
in the presence of 10 equiv of
various anions. (d) Changes in
UV–vis spectra of 1 upon
addition of [(Bu)4N]F in
CH3CN. Inset: mol ratio plots of
absorbance at 332 and 400 nm
0.8
0.6
0.4
0.2
(a)
(b)
Host 1 only, Cl- , Br-, I-, ClO4-,
NO3-, HSO4-
Host 1 only, Cl- , Br-, I-, ClO4-,
NO3-, HSO4-, H2PO4-
F-
F-
OAc-
OAc-
H2PO4-
0.0
0.8
300
350
400
450 300
350
400
450
Wavelength (nm)
0.4
0.3
0.2
0.1
0.0
400 nm
(c)
(d)
332 nm
0.6
0.4
0.2
0.0
0
2
4
6
8
10
Host 3 only, Cl-, Br-, I-, ClO4-,
NO3-, HSO4-.
Equivalent of Fluoride ion
0-10 eq
332 nm
F-
0-10 eq
400 nm
OAc-
H2PO4-
353 nm
300
350
400
450 300
350
400
450
Wavelength (nm)
Fig. 3 Color changes of 2 in
CH3CN (50 lM) after addition
of 10 equiv of representative
anions
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91
Table 1 The photophysical
Sensor
Absorption max. (nm, (log e))
Emission max. (nm)
Free (nm)
data of 1, 2 and 3 in CH3CN
Free
With F-
Dk (nm)
With F-
Dk (nm)
N*
T*
1
2
332 (3.85)
338 (3.76)
340 (3.76)
400 (3.75)
410 (3.77)
410 (3.89)
68
78
78
369
373
378
506
525
513
453
469
457
53, 84
56, 96
56, 79
N* normal emission,
T* tautomer emission
3
Scheme 2 Resonance
structures of 1
S
-H
S
S
S
O
OH
N
N
O
N
+H
O
N
fluoride ions due to their higher electronegativity and their
smaller size compared to the other halides. The presence of
an electron withdrawing bromide group increases the
acidity of the proton and increases the hydrogen bonding
ability. Thus sensor 3 loses its selectivity toward anions.
The deprotonation of the phenolic proton induced a
great fluorescence enhancement due to the inhibition of
ESIPT in the case of chemosensors 1, 2, and 3. The
emission of the deprotonated form lies just between the
normal and tautomer emissions. In the case of 2, due to
the presence of the electron donating group, the normal
form is more stable than the tautomer, while in sensor 3
the electron withdrawing group makes the tautomer form
more stable than the normal form, but its selectivity toward
anions is lost.
Fluorescent responses toward anions
The fluorescence studies of 1, 2 and 3 were performed in
CH3CN (Fig. 4). Intensities of the normal and the tautomer
emissions of 1, 2 and 3 change with the substituents at the
para of the phenyl ring. Sensor 1, with excitation at kmax
332 nm, exhibits two emission bands at kmax 369 and
506 nm. The longer wavelength emission is a typical
ESIPT emission band [17]. On addition of F- to the
solution, a ratiometric change (spectral shift) occurs with
the formation of a new emission band at 453 nm with the
disappearance of the two emission bands of the sensor, but
1H NMR titration
To examine the ground state behavior of sensor 1 with
fluoride ion, 1H NMR experiments involving 1 with
TBA?F- were performed in CD3CN. Partial 1H NMR
spectra of 1, with different amounts of F-, is shown in Fig. 5.
Notably, when 1 equiv of F- was added, disappearance of
phenolic proton and upfield shifts of phenyl and thiazole
protons were observed. The signal of Hd shows a small
upfield shift (Dd = 0.01 ppm) while the signals of Ha, Hb, Hc
and He were shifted upfield significantly (Dd = 0.08–
0.19 ppm) due to the shielding effect. Due to the deproto-
nation of phenolic proton, the OH proton disappeared and the
para proton (Hc, Dd = 0.19 ppm) shifts upfield substan-
tially, compared to the other phenyl protons (Ha, Hb, Hd).
Theses results clearly imply that for 1, phenolic proton
strongly binds with F- via H-bond interactions.
addition of Cl-, Br-, I-, ClO4-, NO3 and HSO4- results
-
in insignificant changes in the fluorescent spectra. In the
-
-
case of CH3CO2 and H2PO4 anions, two new emission
bands appear at 444 and 446 nm, respectively, but the
intensity of the CH3CO2--induced band is 4 times smaller
than that of F- and the H2PO4--induced band is 15 times
smaller.
The titration of sensor 1 with F- revealed that the
fluorescence intensity reached its maximum after addition
of 6 equiv of F- (Fig. 4a, d). In sensor 2 (Fig. 4b), the
normal emission band appears at 373 nm and upon addi-
tion of F- the emission band shifted to 469 nm and the
intensity of the emission band decreased as compared to 1.
Selectivity of 2 toward F- is the same as that of 1. The
emission band in 3 (Fig. 4c) shifted to 457 nm from the
tautomer emission band (513 nm) with a sharp isoemission
point at 501 nm. However its selectivity decreased as
compared to 1 and 2.
Binding constants
The UV and fluorescence titration studies involving the
sensors indicate that the addition of 6 equiv of F- result in
the intensities of the absorption and emission bands
reaching their maxima. This could be assigned as evidence
for the occurrence of hydrogen bond interactions between
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Fig. 4 Fluorescence spectra of
(a) 1, (b) 2, (c) 3 in CH3CN
(30 lM) in the presence of 10
equiv of various anions with an
excitation wavelength of 332,
338 and 340 nm, respectively.
(d) Changes in fluorescent
spectra of 1 in CH3CN (30 lM)
upon addition of [(Bu)4N]F
excited at 332 nm
800
600
400
200
0
(a)
(b)
Host 1 only, Cl-,
Br-, I-, ClO4-,
NO3-, HSO4-
F-
Host 1 only, Cl-,
Br-, I-, ClO4-,
F-
NO3-, HSO4-
OAc-
OAc-
H2PO4-
H2PO4-
350
400
450
500
550
36500
400
450
500
550
600
Wavelength (nm)
800
600
400
200
0
(c)
(d)
0-10 eq
453 nm
Host 3 only, Cl-,
Br-, I-, ClO4-,
NO3-, HSO4-.
F-
OAc-
H2PO4-
369 nm
506 nm
350
400
450
500
550
60
350
400
450
500
550
600
Wavelength (nm)
OH and F- as shown in Scheme 3. Fluorescence based
Job’s plot for the complexation of 1-F- shows a 1:2 stoi-
chiometry (Fig. 6).
to 1 and 2. Moreover the hydrogen bonding of F-, rein-
forces the electron-withdrawing character of the bromo
group, thus increasing the extinction coefficient. Sensor 3
The binding constants for the sensor-F- complex were
obtained from the variation in fluorescent intensity at an
appropriate wavelength. The binding constants (Ka) for 1,
2, and 3 with F- were calculated to be 5.8 9 103,
2.1 9 103, and 8.2 9 103 M-1 L, respectively (Error
estimated to be B10%). Sensor 3 shows the highest binding
constant among the tested sensors. This may be due to the
presence of the electron-withdrawing group, which results
in strong hydrogen bond interactions with F- as compared
also showed binding affinity toward OAc- and H2PO4
-
with binding constants 2.0 9 103 and 1.2 9 103 M-1 L,
respectively (Error estimated to be B10%).
Conclusion
In summary, new ratiometric ESIPT-chemosensors based
on 2-hydroxyphenyl-thiazole have been developed for
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J Incl Phenom Macrocycl Chem (2010) 66:87–94
93
1
Fig. 5 Partial H NMR
(400 MHz) spectra of 1 with
[(Bu)4N]F in CD3CN
phenyl ring. Sensor 2 showed a color change upon
addition of fluoride ion and sensor 3 has a higher binding
affinity than 1 and 2.
S
S
S
F
F
OH
N
O
N
O
N
H
+
F
HF2
References
Scheme 3 The mechanism of the interaction of fluoride ions with 1
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