Thiazole-based Orange-emitting Excited-State Intramolecular Proton Transfer Chemosensors for Selective and Ratiometric Sensing of Zn2+ Ions

Full text of DOI:10.1002/bkcs.11872

Communication
DOI: 10.1002/bkcs.11872
BULLETIN OF THE
S. Kim et al.
KOREAN CHEMICAL SOCIETY
Thiazole-based Orange-emitting Excited-State Intramolecular Proton
Transfer Chemosensors for Selective and Ratiometric Sensing of Zn2+ Ions
Sangho Kim,†,§ Sivaraman Somasundaram,†,‡,§ and Sanghyuk Park†,
*
†Department of Chemistry, Kongju National University, Chungnam 32588, South Korea.
*E-mail: spark0920@kongju.ac.kr
‡Polymer & Organic Semiconductor Lab, Department of Chemistry, School of Natural Sciences, Ulsan
Institute of Science and Technology, Ulsan, South Korea
§These authors contributed equally to this work.
Received June 26, 2019, Accepted June 18, 2019, Published online September 16, 2019
Keywords: Chemosensor, Thiazole, Zn2+ ion, Excited-state intramolecular proton transfer, Specific binding
Investigation of highly sensitive and selective fluorescent
materials for transition metal ions has attracted significant
interest because of their important roles in biological and envi-
ronmental importances.1–3 Because the second most plentiful
transition metal ion in human body, Zn2+ plays various critical
roles in biological functions including neuronal signal trans-
mission, enzymatic regulations, influencing DNA synthesis,
and numerous cellular functions.4,5 In this respect, sensitive
and noninvasive technique to detect free zinc ions, which are
spectroscopically silent due to its 3d10 electron configurations,
is one of the important research areas.6–8 However, most of
the reported wavelengths of the chemosensors and their emis-
sions are still lies in blue or green region, and only a few mol-
ecules showed red or orange emissions, which are better
wavelengths in cellular bioimaging.9–11
To generate bathochromic shifted emission, we have
designed and synthesized novel class of excited-state intramo-
lecular proton transfer (ESIPT) fluorescent molecules,
3-(4,5-diphenylthiazol-2- yl)naphthalen-2-ol (DPTN) and
3-(5-phenyl-4-(pyridin-2-yl)thiazol-2-yl)naphthalen-2-ol (PPTN).
ESIPT is a fast enol (E)-to keto (K) phototautomerization which
occurs in the excited states of intramolecularly hydrogen (H)-
bonded molecules.12 Because they are more stable in E forms in
ground states and in K* forms in excited states, the ESIPT mole-
cules have the characteristic four-level cyclic proton transfer pro-
cesses (E ! E* ! K* ! K ! E) (see Scheme 1(a)).
Therefore, an unusually large Stokes’ shift without self-
absorption is observed in ESIPT molecules.13 Moreover, color
tuning of ESIPT molecules is possible by the strategy known as
a “nodal plane model”, which introduces an arbitrary nodal plane
to the molecule (see the dashed lines in Scheme 1(a)).13
normal phenyl group. The presence of pyridine in PPTN mole-
cule caused fluorescence enhancement at 585 nm because of
selective binding with zinc ion. All the prepared molecules
were soluble in a mixture of polar organic solvents and water,
suggesting that they have a proper amphiphilic property due to
the hydroxyl group as well as pyridine fragment.
The emission spectra of PPTN and DPTN, and their fluores-
cence evolutions, were recorded in EtOH/H2O (9:1, v/v)-
HEPES buffer solutions (20 mM, pH 7.4). In polar solvents, the
PPTN and DPTN showed dual emissions of approximately
430 and 550 nm, which are originated from the enol (E) form
and keto (K) form, respectively. As depicted in Figure 1(a),
upon addition of zinc ion (Zn2+) to the PPTN solution, emission
at 578 nm was increased while the emission at 418 and 558 nm
were decreased. However, other cations such as K+, Na+, Ba2+,
Cd2+, Hg2+, Ni2+, Ca2+, Cu2+, Co2+, Cr3+, NH4 , Fe2+ showed
+
no significant fluorescence enhancement at 578 nm compare to
the initial state. According to the screening test for the PPTN
molecule, the cations such as K+, Ca2+, Ni2+, Hg2+, Cu2+, Cr2+,
+
and NH4 quenched the fluorescence over approximately
553–578 nm region (see Figures S6 and S7). From the Job’s
plot analysis, PPTN displayed 1:1 stoichiometry with Zn2+ ion.
The same metal cation screening test was performed
using DPTN at same condition. However, there was no sig-
nificant change in fluorescence spectra of DPTN in contrast
to the PPTN which showed bathochromic shifted emission
from 558 to 578 nm. As shown in Figure 1(b), almost all of
the fluorescence were similar at approximately 548 nm
without any bathochromic or hypsochromic shifts compare
According to the “nodal plane” strategy, in this work,
thiazole-based novel ESIPT molecules, DPTN and PPTN,
were designed and synthesized (Scheme 1(b)). The molecu-
1
lar structures were fully confirmed by H NMR, 13C NMR,
and elemental analyses. A brief synthetic scheme was
shown in Scheme 2 and the detailed experimental proce-
dures were included in File S1 (Supporting Information).
Both of the prepared molecules showed orange ESIPT emis-
sion at λmax = 578 and 585 nm, respectively. For the selective
sensing of the cation, pyridine was substituted instead of
Scheme 1. (a) Schematic representation of the ESIPT photocycle.
(b) Molecular structure of DPTN and PPTN.
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O
OH
NH2
S
Br
B
O
OH
O
O
H
N
N
S
X
iv)
i)
ii)
iii)
N
S
N
S
X
X
X
X
Scheme 2. Synthetic routes for the ESIPT-based chemosensor
molecules.
Figure 2. (a) Absorption change and (b) emission titrations of
PPTN (20 μM) with Zn2+ in EtOH/H2O(9:1.v/v) containing
HEPES buffer (20 mM, pH 7.4) (λex = 360 nm).
fluorescence (λmax,ems = 585 nm) due to the ESIPT process as
well as extended conjugation by introduced naphthalene unit,
which is known as nodal plane. Due to the deliberately intro-
duced pyridine group, synthesized PPTN senses Zn2+ ion in
aqueous ethanol system at pH 7.4, while the molecule without
Figure 1. Emission spectra of (a) PPTN and (b) DPTN (10 μM)
with various metal ions (100 μM) in EtOH/H2O (9:1, v/v)-HEPES
buffer (0.02 M, pH 7.4).
pyridine
unit,
3-(4,5-diphenylthiazol-2-yl)naphthalen-2-ol
(DPTN), shows no significant detection of Zn2+ under same
conditions. The selective detection of Zn2+ ion using ESIPT-
active fluorescent materials can provide a potential guideline in
environmental or biological application-related sensor research.
to the initial emission, which implies the metal cation binding
is not effective with DPTN molecule and the pyridine unit
in PPTN plays a critical role in Zinc ion binding. The
characteristic fluorescence changes to the binding of Zn2+
can be ascribed to (a) deprotonation of naphtholic –OH and
successive restriction of the ESIPT process, (b) prohibition
of intramolecular charge transfer process, and (c) chelation-
enhancement of fluorescence as a result of vibrational loss
due to the rigid molecular conformation of the complex.7,9
Figure 2 shows absorption and emission spectra of PPTN dur-
ing their fluorescence titrations. The titration was performed with
an excitation wavelength of 360 nm in EtOH/H2O (9:1) con-
taining HEPES buffer (20 mM, pH 7.4) upon addition of Zn2+
ion at room temperature. PPTN showed absorption λmax,abs at
315 nm and fluorescent emission λmax,ems at 558 nm in the
absence of Zn2+, whereas in the presence of Zn2+ it revealed the
absorption and emission bands at 244 and 578 nm, respectively.
As a consequence of complexation with zinc ion, PPTN showed
hypsochromic shift in absorption and bathochromic shift
in emission spectrum. From the fluorescence titrations, the associ-
ation constants were calculated as 4.7 × 105 M−1 for Zn2+.
Moreover, PPTN showed ratiometric fluorescence change
upon addition of zinc ion into the solution, buffered at
pH 7.4. Also, due to the presence of pyridine ring, PPTN
has significant changes in absorption and fluorescence emis-
sions than DPTN upon addition of zinc ion. The colorimetric
experiment was carried out with PPTN visualized by irradia-
tion at wavelength 360 nm UV light and the optical response
was measured by UV–Visible spectroscopy. Under the UV
irradiation, the probe PPTN showed dramatic color change
from colorless to fluorescent yellow orange according to
addition of Zn2+ ion whereas the other metal ions did not
show any significant color changes (Figures S5 and S7).
In summary, a novel ESIPT-based selective and ratiometric
chemosensor for Zn2+ ion was developed in this work. The
synthesized molecule, PPTN showed bathochromic shifted
Acknowledgments. This work was supported by the grant
from Kongju National University, South Korea.
Supporting Information. Additional supporting informa-
tion is available in the online version of this article.
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