New regioisomeric naphthol-substituted thiazole based ratiometric fluorescence sensor for Zn2+ with a remarkable red shift in emission spectra

Article abstract of DOI:10.1016/j.tet.2011.10.106

Ratiometric fluorescent chemosensors based on the position of ring annulation of the naphthol-thiazole moiety for quantification of zinc ions in aqueous ethanol were synthesized and investigated. It was found that sensor 3 exhibited a remarkably large red shift of 140 nm in emission upon complexation with Zn²⁺. A TD-B3LYP/6-31G (d,p) calculation was performed to characterize the nature of the fluorescence behavior of sensor 3 upon Zn ²⁺ complexation. The combination of experimental and computational analyses provides a more complete understanding of the molecular level origin of these unique photophysical properties of this type of chemosensor.

Full text of DOI:10.1016/j.tet.2011.10.106

Tetrahedron 68 (2012) 647e653
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Tetrahedron
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New regioisomeric naphthol-substituted thiazole based ratiometric fluorescence
sensor for Zn2þ with a remarkable red shift in emission spectra
,
Aasif Helal a, Mohammad Harun Or Rashid b, Cheol-Ho Choi b, Hong-Seok Kim a
*
a Department of Applied Chemistry, Kyungpook National University, Daegu 702-701, Republic of Korea
b Department of 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:
Ratiometric fluorescent chemosensors based on the position of ring annulation of the naphtholethiazole
moiety for quantification of zinc ions in aqueous ethanol were synthesized and investigated. It was found
Received 27 September 2011
Received in revised form 29 October 2011
Accepted 30 October 2011
that sensor 3 exhibited a remarkably large red shift of 140 nm in emission upon complexation with Zn2þ
.
A TD-B3LYP/6-31G (d,p) calculation was performed to characterize the nature of the fluorescence be-
havior of sensor 3 upon Zn2þ complexation. The combination of experimental and computational
analyses provides a more complete understanding of the molecular level origin of these unique pho-
tophysical properties of this type of chemosensor.
Available online 6 November 2011
Keywords:
Ratiometric fluorescence
Ring annulation
Naphtholethiazole
Zinc ion
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
measurement. With the development of two-photon microscopy
imaging, which provides significant advantages over standard laser
Zn2þ is the second most abundant transition-metal ion in the
human body and its spatiotemporal detection by fluorescence
spectroscopy and imaging of biological molecules in living systems
has become a very instructive and productive topic in many fields
ranging from biological research to medical diagnosis.1 Zn2þ plays
a central role as a regulatory agent in cell metabolism and as a co-
factor in many enzymes, including those involved in gene expres-
sion, DNA repair, and regulation of neurological disorders such as
Parkinson’s disease, Alzheimer’s disease, amyotrophic lateral scle-
rosis, and epileptic seizures.2 However sensitivity and selectivity
alone are not sufficient to ensure that a probe will be valuable for
bioimaging analysis in vivo because the fluorescence emission in-
tensity depends on both the analyte and the probe concentration.
In a cellular environment, the sensor concentration depends on
many factors such as membrane permeability, incubation time and
temperature, or variations in the cell size and also photo-
transformation, which contributes to the fluorescence intensity
modulation of a system.3 Such limitations can be solved by using
ratiometric probes, which not only display a shift in peak emission
upon binding but consequently eliminates likely artifacts caused
due to extraneous factors by self-calibration of two emission
bands and can also increase the dynamic range of fluorescence
confocal approaches, a new generation of ratiometric probe is re-
quired, which exhibits large shifts of the emission energy upon
binding with the analyte.4
In particular, many fluorescent probes have been developed for
detection of intracellular Zn2þ, but most of them show enhance-
ment, turn on, or little spectral shift.5 Popular mechanisms
designed to produce ratiometric changes on Zn2þ binding include
(a) ICT (Intramolecular Charge Transfer),6 (b) FRET (Fluorescence
Resonance Energy Transfer),7 (c) CHEF (Chelation-Enhanced Fluo-
rescence),8 (d) the pyrene monomereexcimer transformation,9 (e)
the chemodosimeter approach through irreversible chemical re-
actions,10 (f) the two fluorophore approach,11 (g) conformational
restriction,12 (h) the self-assembly strategy,13 and (i) ESIPT (Excited
State Intramolecular Proton Transfer).14 When using these mecha-
nisms for designing certain sensors, the processes are laborious and
the molecules are too large. So there is a need for ratiometric probes
with a large shift in emission peak.15
The ICT process, which occurs from an electron donor to an
electron acceptor within a fluorophore15c and ESIPT, which is the
most common photophysical process occurring in benzazoles as
a potential switching mechanism, have been used to develop a new
class of ratiometric sensors. A cation interacting with the acceptor
in ICT enhances the electron withdrawing ability of this group
while inhibition of the ESIPT process by cation binding removes the
phenolic proton and disrupts the ESIPT with the emission of the
coordinated species occurring with the normal Stokes’ shift.4e,14
* Corresponding author. Tel.: þ82 53 950 5588; fax: þ82 53 950 6594; e-mail
address: kimhs@knu.ac.kr (H.-S. Kim).
0040-4020/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2011.10.106

648
A. Helal et al. / Tetrahedron 68 (2012) 647e653
Moreover ESIPT is strongly influenced by the nature of the sub-
stituents and their attachment positions on the fluorophore
system, resulting in an absorption and emission peak shift.16 In
principle, the more p-orbitals participating in a -conjugated sys-
tem, the more discrete
-orbitals are involved in bonding.17 As
a consequence, the HOMO/LUMO energy gap generally decreases
with increasing size of a conjugated -system, an effect that has
The amphipathicity of a sensor is one of the main issues of
concern for its cellular applications because of the unique proper-
ties of the cellular membrane. A subtle equilibrium between the
hydrophilicity and lipophilicity of sensors is favorable for both cell
permeability and intracellular fluorescent imaging. All three sen-
sors were soluble in a mixture of polar organic solvents and water,
suggesting that they bear a proper amphipathicity. Therefore, all
the absorption and emission studies were carried out in an ethanol/
water system (3:1).23
p-
p
p
p
been utilized on structurally related 2-(20-hydroxyaryl)benzazole
ESIPT systems.18 Previously, we reported the synthesis and Zn2þ
sensing properties of several thiazole-based chemosensors.19
Among them, in chemosensor 119c it was also shown that in-
creasing the Zn2þ coordination number by introducing a pyridine at
the 4-position not only increased the zinc binding ability but also
defined the Zn2þ binding mode by making it ratiometric. This paper
investigates the effect of the relative position of the annulated ring
extension on Zn2þ sensing and the synthesis of a small-molecule
ratiometric Zn2þ probe.20
To explore the effect of different positions of annulations on the
photophysical properties of sensor 1 on binding with Zn2þ, three
derivatives 2, 3, and 4 (Fig. 1) were synthesized with a benzene ring
extension annulated to three different positions of the parent
structure (1).
2.1. Photophysical studies
The UVevis spectroscopic studies of 2, 3, and 4 were carried out
in 3:1 (v/v) water/ethanol buffered with 10 mM HEPES at pH 7.4 at
a concentration level of 20
behavior. Solutions of 2 and 4 produced absorption bands at 373
and 363 nm, respectively. These bands can be attributed to a
mM to understand their ground state
pꢀ
p
*
transition; that is, favored by the planar orientation enforced by the
intramolecular hydrogen bonding.24 The addition of Zn2þ to the
solution of 2 and 4 results in the formation of new red-shifted
bands at 421 and 409 nm, respectively (Fig. S-7), due to the dis-
ruption of the intramolecular hydrogen bond by deprotonation
S
S
S
S
N
N
N
N
N
N
N
OH
N
OH
OH
OH
1
2
3
4
Fig. 1. Structures of chemosensors.
2. Results and discussion
upon Zn2þ binding. However, different absorption changes were
found on the addition of Zn2þ to 3. The absorption peak at 314 nm
decreased concomitantly with the formation of a 110 nm red-
shifted peak at 424 nm and another peak at 328 nm (Fig. 2). The
peak at a higher wavelength (424 nm) with a low molar extinction
The precursors 5, 6, and 7 were synthesized according to the
published procedures21 from the respective hydroxy-naphthoic
acids first by protection of the hydroxyl group and the carboxylic
acid with dimethylsulfate, followed by deprotection of the acid by
alkaline hydrolysis, and subsequently converting it to acid chloride
and then to amides. Thionation of these amides with Lawesson’s
reagent gave thioamides 5, 6, and 7 in good yields. The thiazole ring
formation was carried out by Hantzsch condensation between
thiomide and 2-(2-bromoacetyl)pyridine22 in ethanol to give 8, 9,
and 10. Deprotection of the methoxy group with boron tribromide
gave 2, 3, and 4 in good yields, as shown in Scheme 1. The structures
of 2, 3, and 4 were confirmed by 1H NMR, 13C NMR, IR, HR-FAB mass,
and elemental analyses data. The 1H NMR spectra of 2, 3, and 4
coefficient (log
3
¼3.39, Table 1) is attributed to the deprotonation
of the naphthol proton that also acts as a donor atom in the Zn2þ
complex,4e while the maximal absorbance exhibited at 328 nm
with a high molar extinction coefficient (log
to the ICT from the naphthol to the pyridine moiety.
3
¼4.03, Table 1) is due
The absorption bands in all the three sensors linearly decreased
and increased, respectively, up to 1 equiv of zinc (Fig. 2 inset, S-7(a)
inset and S-7(b) inset), indicating the formation of a 1:1 complex
with a strong binding affinity.
To the solutions of 2, 3, and 4, 10 equiv of various biologically
and non-biologically relevant metal cations were added and their
complexation abilities were studied by fluorescence emission. In
the case of 2, the addition of Zn2þ produced a blue shift from
486 nm to 461 nm while in 4 there was a red shift to 475 nm from
378 nm with quenching (Fig. S-8); however, 3 produced a drastic
red shift of 140 nm with emission enhancement on binding with
Zn2þ (Fig. 3). None of the other cations except Zn2þ induced any
distinct emission shift on binding with any of the sensors, although
paramagnetic transition-metal cations Cu2þ, Ni2þ,Co2þ, Fe2þ, and
exhibited singlet protons at
assigned to thiazolyl-H, with the eOH protons occurring far
downfield ( 12.99, 11.39, and 11.07). Also in 2, 3, and 4 doublet
protons at 8.69, 8.66, and 8.67, respectively, were assigned to
d 8.39, d 8.40, and d 8.47, respectively,
d
d
d
d
d
d
the pyridyl-H nearest to the nitrogen. The isomer 3 can be easily
distinguished from the other two isomers 2 and 4 by the presence of
a downfield singlet at d 8.90 and an upfield singlet at d 7.41, assigned
to the proton of the naphthalene ring near to the thiazole and
phenolic eOH, respectively (Supplementary data Figs. S-1 to S-6).
c
b
c
c
S
S
O
BBr3
EtOH
+
Br
a
b
b
N
N
NH2
N
N
N
a
a
OCH3
OH
H3CO
S
5: a; 6: b; 7: c
2: a; 3: b; 4: c
8: a; 9: b; 10: c
Scheme 1. Synthesis of chemosensors 2, 3, and 4.

A. Helal et al. / Tetrahedron 68 (2012) 647e653
649
of a new peak at 561 nm, with an isoemission point at 513 nm,
producing a remarkable bathochromic shift of 140 nm with an
enhanced emission (Fig. 5).
Thus in sensor 3, on binding with Zn2þ, the peak at 421 nm was
completely switched off while the peak at 561 nm was switched on,
making it a potential ratiometric chemosensor. None of sensors 1, 2,
or 4 produced such a large emission shift (Fig. 6). The peaks at 561
and 421 nm show a linear enhancement and diminution, re-
spectively, with the increase of zinc ion concentration when the
ratio of Zn2þ and sensor 3 concentration was below or equal to 1:1.
However, when a ratio of 1:1 was reached, higher zinc ion con-
centration did not lead to any further emission enhancement (Fig. 5
inset). A fluorescence quantum yield of 0.15 for 3 was obtained at
Zn2þ saturation, and was twice the fluorescence quantum yield of
the free ligand (
F
¼0.07) under identical conditions. The fluores-
cence quantum yields of the other ligands in the free and the metal
bound states were found to be less than that in the 3eZn2þ com-
plex. This clearly indicates that the coordination of Zn2þ with 3
enhanced the charge transfer character of the ligand, inhibiting
major nonradiative decay pathways (Table 1).
Fig. 2. Changes in UVevis spectra of 3 (20 mM) upon addition of Zn(ClO4)2 in EtOH/
H2O (3:1) containing HEPES buffer (10 mM, pH 7.4). Inset: mol ratio plots of absor-
bance at 424 nm.
Table 1
Absorption, emission maximum, and binding constant of sensors 1, 2, 3, and 4 in EtOH/H2O (3:1)
Sensor
Absorption max. nm (log
3
Dl (nm)
Emission max. (nm)
Dl (nm)
Binding constant
Quantum yielda
(F)
Ligand
LigandþZn2þ
384 (3.79)
421 (3.99)
Ligand
LigandþZn2þ
461
461
Ligand
LigandþZn2þ
0.04
0.07
1
2
329 (3.74)
357 (3.82)
373 (3.82)
314 (3.98)
55
64
48
14
110
46
502
488
ꢀ41
ꢀ27
2.1ꢁ104
3.3ꢁ104
0.05
0.11
3
328 (4.03)
424 (3.39)
409 (3.79)
421
378
561
475
þ140
þ97
2.4ꢁ105
4.1ꢁ103
0.07
0.10
0.15
0.06
4
363 (3.57)
a
Fluorescence quantum yields were obtained using quinine sulfate in 0.5 M H2SO4 as the standard.
250
To quantify the complexation ratio between 3 and Zn2þ ions, the
absorption and fluorescence Job’s plot measurements were con-
ducted by varying the concentration of both the sensor and the
cation (Fig. S-10). The maximum point appears at the mole fraction
of 0.5 or a 1:1 ligand to metal complex. The UV and fluorescence
titration profiles also suggest a 1:1 Zn2þ binding mode with sensor
3. From the fluorescence titration, the binding constant for zinc ion
was observed to be 2.4ꢁ105 Mꢀ1 (Error estimated to be ꢂ10%)
200
150
100
50
2+
Zn
+
2+
+
+
3, Ag , Ca , K , Na ,
+
+
2+
2+
(Table 1).25 The ratio of fluorescence intensities (F561/F421
)
Cs , Rb , Hg , Mg
.
(
lex¼375 nm) changed from 0.2 to 10.5 (R¼51-fold) with a de-
tection limit of 1.12
m
M.26
The Zn2þ specific ratiometric response of sensor 3 was further
confirmed by a competitive binding experiment in the presence of
biologically relevant cations, such as Naþ, Kþ, Mg2þ, and Ca2þ, and
other biologically non-relevant cations. These cations did not in-
terfere with the ratiometric response to Zn2þ, even though their
concentrations were 100-fold greater than the Zn2þ concentration,
as illustrated in Fig. S-11, except in the cases of Cu2þ, Co2þ, Fe2þ, and
Fe3þ where the enhancement was less than in others.
2+
Cd
2+
Pb
2+
2+
2+
2+
3+
Co
,
Cu
,
Ni , Fe , Fe
0
400
450
500
Wavelength (nm)
550
600
650
Fig. 3. Fluorescence spectra of 3 (20
(10 mM, pH 7.4) upon addition of various metal cations (each concentration was
200 M) with an excitation wavelength of 375 nm.
mM) in EtOH/H2O (3:1) containing HEPES buffer
We prepared the complex of 3 with Zn2þ in EtOH/H2O (3:1) and
characterized it by HR-FAB mass (Fig. S-12). The mass spectra of
3eZn2þ shows a 1:1 stoichiometry (between 3 and Zn2þ) with the
molecular ion peak at m/z 366.9880, which corresponds to [3-
HþZn2þ].
m
Fe3þ produced complete quenching on binding with the sensor 3
(Fig. 4).
To examine the reversibility of 3 toward Zn2þ, an EtOH/H2O
The titrations of all sensors were carried out with Zn2þ in EtOH/
H2O (3:1) containing a HEPES buffer (10 mM, pH 7.4). In 2 and 4
titrations with Zn2þ produced a blue and red shift with quenching,
respectively (Fig. S-9). Sensor 3 shows an emission peak at 421 nm
with a small shoulder at 517 nm. On slow addition of Zn2þ, the
emission peak of 3 decreased synchronously with the development
(3:1) solution of EDTA (200
the complexed solution of 3 (20
pected, an absorption peak at 314 nm and a fluorescence signal at
421 nm were completely recovered, demonstrating the binding
was chemically reversible and not a cation-catalyzed reaction
(Scheme 2).27
m
M), a metal ion chelator, was added to
m
M) and Zn2þ (200
mM). As ex-

650
A. Helal et al. / Tetrahedron 68 (2012) 647e653
Fig. 4. Fluorogenic changes of a 20
mM solution of 3 in EtOH/H2O (3:1) containing HEPES buffer (10 mM, pH 7.4) in the presence of 10 equiv of each cation upon illumination using
UV-lamp at 365 nm.
2.2. Quantum chemical calculations
The remarkable bathochromic shift in sensor 3 upon binding
with Zn2þ motivated us to get a comparative view with sensors 1, 2,
and 4 as well as elucidate the complexation mechanism using TD-
B3LYP/6-31G (d,p) calculations. From the excited state optimization
calculations of sensors 2, 3, and 4, and the energy and/or charge
transfer model we find that the photophysical properties depend
not only on the size of the
p-conjugated system, but also on the
relative position of the annulated ring extension. Fig 7 shows the
calculated HOMO and LUMO energy level differences of all the four
compounds in the excited state. It can be seen that the orientation
of the naphthol ring largely affects the HOMO/LUMO energy dif-
ferences. In the Zn2þ complex of 3 and 4, the HOMO/LUMO energy
differences is smaller than the free ligands, while in the Zn2þ
complex of 1 and 2 the HOMO/LUMO energy differences are larger
than in the free ligands. This phenomenon is explained by the
frontier molecular orbital calculations. Table 2 shows the most
representative molecular frontier orbitals in the excited states with
the highest oscillatory strengths. It is important to note here for
generalization all the transition energies will be denoted as HOMO/
LUMO energy differences.
Fig. 5. Changes in fluorescence spectra of 3 (20 mM) in EtOH/H2O (3:1) containing
HEPES buffer (10 mM, pH 7.4) upon addition of Zn(ClO4)2
ratio plots of emissions at 421 and 561 nm.
(
lex¼375 nm). Inset: mol
Fig. 6. Fluorescence spectra of sensors (1) 1, (2) 2, (3) 3, and (4) 4 (20 mM) upon the
addition of 10 equiv of Zn2þ ions in EtOH/H2O (3:1) containing HEPES buffer (10 mM,
pH 7.4).
Fig. 7. The calculated HOMO/LUMO energy differences (ꢀ
DEHOMO/LUMO) in eV of sen-
sors 1, 2, 3, 4, and their Zn2þ complexes by B3LYP/6-31G(d,p) level.
S
S
Zn(ClO4)2
EDTA
N
N
N
N
Zn2+
OH
O
Scheme 2.

A. Helal et al. / Tetrahedron 68 (2012) 647e653
651
Table 2
Electronic transitions, major coefficients (0.5 and above), and HOMO/LUMO energy
EHOMO/LUMO) of free and Zn2þ complex ligand (considering the higher
differences (ꢀ
D
oscillator strength) at the excited state geometry calculated with the B3LYP/6-31G
(d,p) method
Ligands/Zn2þ complex
State
Emission
Coef
ꢀDEHOMO/LUMO
1
2
3
S1
S1
S3
H/L
H/L
0.98
0.97
0.70
0.57
0.89
0.98
0.77
0.54
0.87
0.74
0.52
2.178
2.205
3.729
H-1/L
H/Lþ1
H/L
H/Lþ1
H/Lþ3
H-1/L
H-1/L
H/Lþ2
H/Lþ3
4
S1
S2
S5
3.348
2.913
3.267
1þZn2þ
2þZn2þ
3þZn2þ
4þZn2þ
S3
S5
2.668
3.212
Fig. 9. Optimized structure (B3LYP/6-31G) of 3eZn2þ complex.
The calculated frontier molecular orbitals of 3 and 3eZn2þ
show that there are no ESIPT. In ligand 3, transitions are pre-
dominately characterized by HOMO-1/LUMO at the excited
state geometry. HOMO-1 is localized only on the naphthol and
thiazole moiety and after excitation electron is excited from the
H: HOMO (highest occupied molecular orbital); L: LUMO (lowest unoccupied mo-
lecular orbital); S1, S2, S3, and S5: different excited states.
The excited state intramolecular proton transfer (ESIPT) process
involves the presence of an intramolecular hydrogen bond between
the acidic group (phenol or napthol) and the basic group (thiazole
nitrogen), which are in close proximity in the ground state. Upon
excitation to the excited state (HOMO/LUMO), due to ICT from the
phenol (1) or naphthol (2) ring to the thiazole and pyridine rings
(Figs. S-13a and S-13c for 1, and Figs. S-14a and S-14c for 2) there
occurs a change in the charge density that increases the acidity of
the acidic center and the basicity of the basic center.14c This leads to
migration of proton to give phototautomer at 502 nm in 1 and
486 nm in 2. In 1eZn2þ complex, ESIPT is disrupted on cation
binding, which makes the HOMO/LUMO energy difference larger
than the free ligand (Figs. S-13b and S-13d). In coordination with
the Zn2þ in 2 the ESIPT is disrupted due to deprotonation of the
hydroxyl proton with the emission of the coordinated species oc-
curring with normal Stoke shift. Moreover the planarity of the
complex is distorted (Fig. 8 and Table 3, Fig. 9) where the thiazole
ring is 23o above the plane (<C13eC12eC10eC9) and Zn2þ deviates
in a 14o (<O18eZn19eN2) angle down from the main plane of the
molecule. Excitation energy calculations indicate the highest os-
cillator strength bearing transition is predominately characterized
by HOMO/LUMOþ3. The excited state shows that there is a partial
charge transfer and the electron density of the LUMOþ3 is delo-
calized on the whole backbone of the molecule making the LUMO
destabilized and increasing the HOMO/LUMO energy difference on
metal complexation (Figs. S-14b and S-14d).
occupied
p
into an unoccupied
*
p orbital giving the local exci-
tation emission which produces a normal emission peak and
a large HOMO/LUMO energy gap (Fig. 10a and c). On complexa-
tion with Zn2þ, the electron density of HOMO-1 is mainly local-
ized on the naphthol moiety while the electron density of the
LUMO is localized on the pyridine (Fig. 10b and d) indicating that
the asymmetrical charge distribution of the free ligand gains an
ICT character on binding with Zn2þ. Thus, a planar structure of the
Zn2þ complex with the ligand (Fig. 8 and Table 3) results in
a complete ICT from the naphthol ring (donor) to the pyridine
ring (acceptor) with the latter’s electron withdrawing ability en-
hanced by the metal binding. This results in the smallest HOMO/
LUMO gap and the lowest LUMO level among all the four Zn2þ
ligand complexes and a large bathochromic shift (561 nm) in the
emission spectrum.
1-Zn2+
3-Zn2+
2-Zn2+
4-Zn2+
Fig. 8. Excited state optimized structure of zinc complexed ligand 1, 2, 3, and 4 by
B3LYP/6-31G (d,p) level.
Table 3
The calculated angle distribution of excited state geometries for Zn2þ-complexed 1,
2, 3, and 4 by B3LYP/6-31G (d,p) level (see Fig. 9)
Fig. 10. B3LYP/6-31G (d,p) calculated molecular orbitals of 3 and 3eZn2þ
.
Parameter
1eZn2þ
2eZn2þ
3eZn2þ
4eZn2þ
<
177.1
177.4
177.6
5.4
1.2
ꢀ0.8
166
172.7
177.3
176.3
10.2
1.9
ꢀ0.9
ꢀ159.6
160.7
166.4
ꢀ50.5
ꢀ9.9
O18eZn19eN2
<
166.8
167.9
23.0
7.1
C12eC10eN11eC7
In the case of 4 and its Zn2þ complex, the frontier orbital cal-
culations show that there are no ESIPT. The emission without the
metal arises due to the local excitation and molecular orbital cal-
culation shows electronic transition is characterized by
<
C1eC7eN11eC10
<
C13eC12eC10eC9
<
C1eC7eC8eH24
<
1.5
ꢀ3.9
C10eC12eC13eH25

652
A. Helal et al. / Tetrahedron 68 (2012) 647e653
HOMO/LUMO. On binding with the Zn2þ the excited state ge-
ometry shows HOMO is localized on naphthol ring and LUMOþ2 is
spread over the entire structure resulting in partial ICT (Fig. S-15).
Moreover the planarity of the molecule is completely distorted
where the thiazole ring is 50.5ꢃ down the plane
(511 mg, 2.04 mmol) in dry CH2Cl2 (15 mL) was slowly added to
a solution of 8 (100 mg, 0.31 mmol) in dry CH2Cl2 (15 mL) at ꢀ78 ꢃC
for 1 h. The cold bath was removed and the mixture was stirred at
room temperature for 5 h, and then poured into water and stirred
for an additional hour. An aqueous layer was extracted with CH2Cl2,
the organic layer was washed with brine, dried over anhydrous
Na2SO4, and evaporated. The residue was purified by SiO2 column
chromatography (elution with EtOAc/hexane 2:1) to give 2 in 92%
(<C13eC12eC10eC9) and Zn2þ deviates in
a
20.4ꢃ
(<O18eZn19eN2) angle upwards from the plane of the molecule,
which consequently inhibits the charge transfer, and makes the
product unstable (Fig. 8 and Table 3). This consequently raises the
excitation energy as well as the HOMO/LUMO energy differences.
yield. Mp 165 ꢃC (CH2Cl2/hexane); 1H NMR (DMSO-d6)
d 7.43 (t,
J¼5.8 Hz, 1H), 7.53 (d, J¼8.6 Hz, 1H), 7.63e7.60 (m, 2H), 7.93e7.90
(m, 2H), 7.99 (dt, J¼1.8, 7.8 Hz, 1H), 8.13 (d, J¼7.8 Hz, 1H), 8.37 (d,
J¼2.8 Hz, 1H), 8.39 (s, 1H), 8.69 (d, J¼4.0 Hz, 1H), 12.99 (s, 1H, OH);
3. Conclusion
13C NMR (DMSO-d6)
d 110.9, 117.1, 119.7, 120.6, 122.9, 123.5, 123.9,
In conclusion, we have investigated the effect of the position of
ring annulations in sensor 1. Among the sensors 2, 3, and 4, it was
found that 3 produced a remarkably large (140 nm) bathochromic
124.7,126.2,127.8, 128.1,134.8,137.6, 149.8,150.8, 152.7,153.1, 167.9;
IR (KBr) 3358, 3255, 3210, 3197, 3145, 1810, 1760, 1702, 1653, 1348,
1153, 832 cmꢀ1; HR-FAB mass: calcd for C18H13N2OS (Mþ1)þ:
305.0749, found: 305.0751. Anal. Calcd for C18H12N2OS: C, 71.03; H,
3.97; N, 9.20; S, 10.54. Found: C, 70.77; H, 4.13; N, 8.90; S, 10.23.
shift and emission enhancement upon complexation with Zn2þ
,
with an ‘oneoff’ type fluoroionophoric switching property, while
other sensors produced little ratiometric change and quenching.
This large reversible ratiometric shift of sensor 3 with respect to
other sensors was investigated by quantum calculations. The
frontier orbital calculations and the optimized structures showed
that the planar nature of the 3eZn2þ complex enhanced the ICT
character, leading to the decrease in the HOMO/LUMO energy dif-
ference and the large red shift.
4.1.2. Chemosensor 3. This compound was obtained from the
deprotection of 9, which was prepared from the reaction of 6
(90 mg, 0.42 mmol) with 2-(2-bromoacetyl)pyridine (140 mg,
0.50 mmol), with BBr3. Work-up was as in the above procedure for
the preparation of 2. The residue was purified using SiO2 column
chromatography (EtOAc/hexane 1:1) to give 3 in 95% yield. Mp
208 ꢃC (CH2Cl2/hexane); 1H NMR (DMSO-d6)
d 7.40e7.33 (m, 2H),
4. Experimental section
4.1. General methods
7.41 (s, 1H), 7.46 (t, J¼7.2 Hz, 1H), 7.76 (d, J¼8.3 Hz, 1H), 8.00e7.93
(m, 2H), 8.31 (d, J¼7.8 Hz, 1H), 8.40 (s, 1H), 8.66 (d, J¼3.8 Hz, 1H),
8.90 (s, 1H), 11.39 (s, 1H, OH); 13C NMR (DMSO-d6)
d 110.1, 119.3,
120.8, 121.6, 123.2, 123.8, 125.8, 127.4, 127.5, 127.6, 128.6, 134.8,
137.5, 149.6, 151.9, 152.4, 153.6, 163.1; IR (KBr) 3414, 3252, 3209,
3198, 3146, 1809, 1703, 1669, 1609, 1369, 1064, 835 cmꢀ1; HR-FAB
mass: calcd for C18H13N2OS (Mþ1)þ: 305.0749, found: 305.0746.
Anal. Calcd for C18H12N2OS: C, 71.03; H, 3.97; N, 9.20; S, 10.54.
Found C, 70.92; H, 3.98; N, 9.11; S, 10.24.
Melting points were determined using a ThomaseHoover cap-
illary melting point apparatus and are uncorrected. 1H and 13C NMR
spectra were recorded on a Bruker AM-400 spectrometer using
Me4Si as the internal standard. IR spectra were recoded on a Shi-
madzu IR Prestige-21 FTIR spectrophotometer. UVevis absorption
spectra were determined on a Shimadzu UV-1650PC spectropho-
tometer. Fluorescence spectra were measured on a Shimadzu RF-
5301 fluorescence spectrometer equipped with a xenon discharge
lamp,1 cm quartz cells. All of the measurements were carried out at
298 K.
Analytical grade absolute ethanol was purchased from Merck.
Deionized water (double distilled) was used throughout the ex-
periment as an aqueous layer. All other materials used for synthesis
were purchased from Aldrich Chemical Co. and used without further
purification. Compounds 119c and 5, 6, 7,21 and 2-(2-bromoacetyl)
pyridine22 were prepared in accordance with the procedure in the
literature. The solutions of metal ions were prepared from their
perchlorate salts of analytical grade, and then subsequently diluted
to prepare working solutions. HEPES buffer solutions of different pH
were prepared using proper amount of HEPES and KOH (all of ana-
lytical grade) under adjustment by a pH meter.
4.1.3. Chemosensor 4. This compound was obtained from the
deprotection of 10, which was prepared from the reaction of 7
(100 mg, 0.47 mmol) with 2-(2-bromoacetyl)pyridine (158 mg,
0.56 mmol) with BBr3. Work-up was as in the above procedure for
the preparation of 2. The residue was purified using SiO2 column
chromatography (elution with EtOAc/hexane 2:1) to give 4 in 80%
yield. Mp 174 ꢃC (CH2Cl2/hexane); 1H NMR (DMSO-d6)
d 7.39e7.33
(m, 3H), 7.52 (t, J¼8.4 Hz, 1H), 7.89 (d, J¼8.3 Hz, 1H), 7.96e7.91 (m,
2H), 8.15 (d, J¼7.3 Hz, 1H), 8.47 (s, 1H), 8.54 (d, J¼8.6 Hz, 1H), 8.67
(d, J¼4.8 Hz, 1H), 11.07 (s, 1H, OH); 13C NMR (DMSO-d6)
d 112.0,
118.2, 118.9, 120.6, 123.1, 123.3, 124.2, 127.7, 128.1, 128.4, 131.9, 132.3,
137.5, 149.7, 152.2, 153.7, 154.5, 162.9; IR (KBr) 3438, 3252, 3210,
3197, 3145, 1810, 1783, 1670, 1504, 1470, 888, 834 cmꢀ1; HR-FAB
mass: calcd for C18H13N2OS (Mþ1)þ: 305.0749, found: 305.0752.
Anal. Calcd for C18H12N2OS: C, 71.03; H, 3.97; N, 9.20; S, 10.54.
Found: C, 70.77; H, 4.08; N, 8.97; S, 10.21.
Density functional optimizations were carried out using
Gamess.28 The geometries of each of the compounds were opti-
mized using density functional theory (DFT) with B3LYP func-
tional29 at the 6-31G (d,p) basis set. The excited state electronic
structures were optimized using time-dependent density func-
tional theory (TD-DFT)30 with the same functional and basis set
using a suite of Gaussian 03 programs.31
4.1.4. 3eZn2þ complex. A mixture of 3 (100 mg, 0.33 mmol) and
Zn(ClO4)2$6H2O (147 mg, 0.39 mmol) in ethanol/H2O (v/v 3:1,
10 mL) was refluxed for 12 h. The mixture was cooled to room
temperature and the precipitated complex was filtered. The filtered
cake was washed thoroughly with water, ethanol, and dried under
vacuum to provide the complex (100 mg, 82% yield). HR-FAB mass:
calcd for (C18H11N2OSZn) 366.9884; found: 366.9880.
4.1.1. Chemosensor 2. A solution of 5 (100 mg, 0.47 mmol) and 2-
(2-bromoacetyl)pyridine (158 mg, 0.56 mmol) in ethanol (10 mL)
was refluxed for 5 h. The solvent was removed under vacuum and
the residue was diluted with water and extracted with CH2Cl2. The
organic layer was dried over anhydrous Na2SO4 and concentrated.
The residue was purified by SiO2 column chromatography (elution
with EtOAc/hexane 1:2) to give 8 in 87% yield. A solution of BBr3
Acknowledgements
This research was supported by the NRF grant funded by MEST
in Korea (NRF-2010-0010070). The Research Institute of Industrial
Technology at Kyungpook National University also supported A.H.

A. Helal et al. / Tetrahedron 68 (2012) 647e653
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