Highly selective fluorescent probe for sequential recognition of copper(II) and iodide ions

Article abstract of DOI:10.1016/j.tet.2017.06.034

A new highly selective fluorescent probe based on 2-(2′-aminolphenyl)-4-hydroxymethylthiazole (1) was developed for the detection of copper and iodide ions in ethanol. Probe 1 selectively detected Cu²⁺ through a “switch off” response, showing a

Full text of DOI:10.1016/j.tet.2017.06.034

Accepted Manuscript
Highly selective fluorescent probe for sequential recognition of copper(II) and iodide
ions
Hansol Seo, Miran An, Bo-Yeon Kim, Jun-Hyeak Choi, Aasif Helal, Hong-Seok Kim
PII:
S0040-4020(17)30667-1
10.1016/j.tet.2017.06.034
TET 28799
DOI:
Reference:
To appear in: Tetrahedron
Received Date: 27 April 2017
Revised Date: 7 June 2017
Accepted Date: 19 June 2017
Please cite this article as: Seo H, An M, Kim B-Y, Choi J-H, Helal A, Kim H-S, Highly selective
fluorescent probe for sequential recognition of copper(II) and iodide ions, Tetrahedron (2017), doi:
10.1016/j.tet.2017.06.034.
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Graphical Abstract
Highly selective fluorescent probe for sequential recognition of copper(II) and iodide ions
Hansol Seoa, Miran Ana, Bo-Yeon Kima, Jun-Hyeak Choi*, Aasif Helalb,*, and Hong-Seok Kima,*
S
(a)
800
700
600
500
400
300
200
100
0
432 nm
1
N
OH
1
NH2
Cu(ClO4)2
S
N
O
Cu2+
N
Turn-off
H2
S
3I- Cu2+
1+Cu2++I-
-
Cu+
I3
S
N
O
Cu+
N
Turn-on
H2
S
1+Cu2+, Br-, Cl-, ClO4-, CN-, F-, H2PO4-, HP2O7-, HSO4-, NO3-, OAc-
400
450
500
550
600
Wavelength (nm)
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Highly selective fluorescent probe for sequential recognition of copper(II) and
iodide ions
Hansol Seoa, Miran Ana, Bo-Yeon Kima, Jun-Hyeak Choia, Aasif Helalb,*, Hong-Seok Kima,*
a Department of Applied Chemistry, School of Applied Chemical Engineering,
Kyungpook National University, Daegu 41566, Republic of Korea
b Center of Research Excellence in Nanotechnology, King Fahd University of Petroleum and Minerals,
Dhahran 31261, Saudi Arabia
Corresponding authors:
Tel: +82-53-950-5588; Fax: +82-53-950-6594; E-mail address: kimhs@knu.ac.kr (H. Kim)
Tel: +966 013 860 7532; Aasifh@kfupm.edu.sa (A. Helal)
Abstract
A new highly selective fluorescent probe based on 2-(2’-aminolphenyl)-4-hydroxymethylthiazole (1) was
developed for the detection of copper and iodide ions in ethanol. Probe 1 selectively detected Cu2+ through a
“switch off” response, showing a good association constant (Ka = 1.43 × 105 M–1) and binding with Cu2+ ions in
a 1:1 stoichiometry. An in situ generated 1-Cu2+ (1:1) ensemble detected I– ions sequentially through a
“switch on” response. The “switch off” response occurred with selective complexation of Cu2+ ions due to
intramolecular charge transfer (ICT). An ensemble of 1 and copper(II) showed higher selectivity towards
iodide than towards any other anions due to the reduction of Cu2+ to Cu+ by I- and subsequent complexation
with 1.
Keywords: Fluorescence probe, Aminophenylthiazole, Sequential recognition, Copper ion-selective, Iodide.
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1. Introduction
Fluorescent chemosensors are viable means for the nondestructive, facile monitoring of target ions with
high sensitivity, specificity, simplicity, low cost, and rapid tracking of analytes in biological, toxicological, and
environmental samples.1,2 Transition metals are of great concern, in spite of the fact that trace amounts of
most transition metals play crucial roles in living organisms, because they have an extremely remarkable
eco- and toxicological impact on both the environment and human beings due to bioaccumulation.3-6 In
particular, Cu2+ is the third most abundant essential trace element in the human body, and plays a pivotal
role in a variety of fundamental physiological processes such as cellular respiration, bone formation, and
connective tissue development. Copper ions also play critical roles as signal messengers and cofactors for a
variety of metalloenzymes, including superoxide dismutase, cytochrome C oxidase, and tyrosinase.7,8
However, high concentrations of free copper ions in living cells catalyzes the transformation of oxygen
molecules into reactive oxygen species that can cause oxidative damage to proteins, nucleic acids, and
lipids.9 Thus, excessive amounts of copper ions can cause aberrant oxidative and nitrosative stress events,
which may lead to diseases such as cancer,10 cardiovascular disorders,11 and neurodegenerative diseases,
such as Menkes, Wilson’s disease, and Alzheimer’s disease.12 Higher levels of Cu2+ are detected in tumors,
and are thought to play a role in the promotion of angiogenesis (new blood vessel growth). Furthermore,
copper can also be a significant environmental pollutant because of its widespread use in industrial and
agricultural practices. Hence, the U.S. Environmental Protection Agency has established the upper limit for
copper in drinking water as 1.3 ppm, and the allowable concentration of copper in bloods is limited to 100-
150 µg dL-1 (15.7-23.6 µM). Therefore, the effective and quantitative detection of Cu2+ is of great significance
in biology, chemistry, environmental science, and medicine.13-15 Current methods for copper screening,
including atomic absorption spectrometry (AAS), inductively coupled plasma mass spectrometry (ICP-MS),
and inductively coupled plasma atomic emission spectrometry or mass spectrometry (ICP-AES, ICP-MS), total
reflection X-Ray fluorimetry (TXRF), and anodic stripping voltammetry (ASV) often require expensive and
sophisticated instrumentation or complex sample preparation procedures.16-18 Many traditional methods for
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the detection of copper using small molecular chemosensors are based on Schiff bases,19-21 although sensors
based on hydrazone,22 oxadiazole,23 carbazole, 24 and acetamide25 moieties have also been reported.
Among biologically important anions, iodide is of particular interest, because it is an essential
micronutrient and plays fundamental physiological roles during all stages of human development.23 Iodide
also plays an essential role in thyroid gland function.26 Specifically, excessive iodine intake and deficiency can
lead to thyroid diseases; for example, low iodine intake results in hypothyroidism, cretinism, congenital
anomalies, endemic goiter, neurological disorders, and mental retardation.27-32 Iodine was also recently
found to display anti-inflammatory and anti-oxidative activities.33 However, excessive uptake of iodine leads
to adverse effects, such as degenerative, necrotic, and neoplastic lesions in the thyroid gland, stomach, and
salivary glands.34 Various methods have been used for the determination of iodide, including gas
chromatography with mass spectrometry detection, neutron activation analysis (NAA), electrostatic ion
chromatography, inductively coupled plasma-mass spectrometry (ICP-MS), capillary electrophoresis, indirect
atomic absorption spectrometry, and iodide-selective electrodes (ISEs).35-41 Among these methods, ICP-MS is
the most commonly used. However, ICP-MS requires sophisticated instrumentation, as well as laborious
detection procedures. On the contrary, fluorescence-based detection is highly sensitive, versatile, and is
amenable to high-throughput screenings. Some small-molecule fluorescence “turn-off” iodide probes
include bis-imidazolium and benzimidazole derivatives,42 as well as thiophene- and carbazole-containing
polymers.43 However, the major drawback of these probes is that they only exhibit quenched fluorescence
signals upon binding with iodide, due to the heavy atom effect of iodide.44
Moreover, the development of viable probes for iodide is challenging due to its large anionic radius, low
charge density, and low hydrogen-bonding ability.45 Cationic transition metal complexes attract anions
depending on the degree of ion-pair interactions. Some metal complexes, which exhibit a “turn on” response
upon detection of iodide, reported to date include Hg2+,46 Co2+,47 Cd2+,48 Ag+,49 and Cu2+.50 In this paper, we
report a highly selective copper sensor, which upon complexation with copper, produces a highly selective
“turn on” iodide sensor.
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Previously, we reported the cation (Zn2+,51-56 Cu2+,57 Al3+,58-59 Ga3+,60,61) and anion (F-,62 OAc-,63 CN-,64) sensing
properties of several thiazole-based chemosensors. Specifically, we showed that by changing the moiety at
the 4-position of the thiazole ring, the cation selectivity could be controlled. In most cases, we dealt with 2-
hydroxyphenyl type ESIPT (Excited State Intramolecular Proton Transfer) except in two examples,51,63 which
were chemically modified 2-tosylamidophenyl derivatives, and evaluated cations binding ability. However,
there are no reports which describe free NH2 groups and heteroazoles as part of a cation sensor. This is
probably due to the low acidity of the N-H protons, which produce weak intramolecular hydrogen bonds and
as a consequence little or no ESIPT.65 In this study, we used an aminophenyl thiazole moiety with a
hydroxymethyl group attached at the 4 position of the thiazole ring (1) to enhance the selectivity and binding
ability towards copper, and then used the 1-Cu2+ ensemble for the selective detection of iodide. Ethyl 2-(2′-
aminoyphenyl)thiazolyl-4-carboxylate (2) was also prepared to compare the effects of the hydroxylmethyl
group to those of the ester group in Cu2+ sensitivity and selectivity (Fig. 1).
S
S
O
N
N
OEt
OH
NH2
2
NH2
1
Fig. 1. Structures of probes 1 and 2.
2. Experimental section
2.1 Materials and methods
1
The H and 13C NMR spectra were recorded on a Bruker AM-400, and AM-500 spectrometer using Me4Si as
the internal standard. High-resolution fast atom bombardment (HR-FAB) mass spectra were recorded on a
Jeol JMS 700 High resolution mass spectrometer at the KBSI Daegu branch. UV–vis absorption spectra were
obtained on a Shimadzu UV-1650PC spectrophotometer, and fluorescence spectra were measured using a
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Shimadzu RF-5301 fluorescence spectrometer equipped with a xenon discharge lamp in 1 cm quartz cells. All
of the measurements were performed at 298 K. The fitting of the fluorescence titration data was performed
using gnuplot 4.6 software. Analytical grade ethanol was purchased from Merck (Germany). All other
materials were purchased from Aldrich Chemical Co. and used as received. The solutions of metal ions were
prepared from their perchlorate salts (which were of analytical grade); the anions were prepared from their
tetrabutylammonium (TBA) salts and the resulting solutions were subsequently diluted to prepare the
working solutions. Ethyl 2-(2’-nitrophenyl)thiazole-4-carboxylate was prepared according to the procedure in
the literature.51 The quantum yield (Φ) was calculated using the literature procedure.57
2.2.1. Synthesis of ethyl 2-(2’-nitrophenyl)thiazolyl-4-carboxylate (3)
A solution of 2-nitrothiobenzamide (200 mg, 1.10 mmol) and ethyl bromopyruvate (257 mg, 1.32 mmol) in
ethanol (30 mL) was heated under reflux for 2 h. The solvent was removed with an evaporator, 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 (SiO2, EtOAc:hexane = 1:4) to
give compound 3 (254 mg, 83%). 1H NMR (400 MHz, CDCl3) δ 1.40 (t, J = 7.2 Hz, 3H, CH3), 4.42 (q, J = 7.1 Hz,
2H, CH2), 7.61-7.73 (m, 3H, Ha,b,c), 7.98 (d, J = 7.8 Hz, 1H, Hd), 8.31 (s, 1H, He); 13C NMR (400 MHz, DMSO-
d6) δ 14.3, 61.7, 124.7, 127.8, 129.1, 131.0, 132.2, 132.7, 147.9, 148.6, 161.1, 163.0.
2.2.2. Synthesis of ethyl 2-(2’-aminohenyl)thiazolyl-4-carboxylate (2)
A mixture of 3 (200 mg, 0.72 mmol) and 5% Pd/C in ethanol (20 mL) was hydrogenated under H2 gas for 10
h. After removal of the solvent, CH2Cl2 was added and the mixture was filtered through a Celite pad to
remove the catalyst. The filtrate was concentrated and the residue was crystalized in CH2Cl2-hexane to give
1
amino compound 2 (159 mg, 89%). H NMR (400 MHz, DMSO-d6) δ 1.33 (t, J = 7.1 Hz, 3H, CH3), 4.33 (q, J =
7.1 Hz, 2H, CH2), 6.62 (ddd, J = 7.2, 6.9, 1.0 Hz, 1H, Hc), 6.83 (dd, J = 8.3, 1.0 Hz, 1H, Ha), 7.11 (s, 1H, NH), 7.18
(ddd, J = 7.1, 7.0, 1.4 Hz, 1H, Hb), 7.59 (dd, J = 7.9, 1.4 Hz, 1H, Hd), 8.47 (1H, He); 13C NMR (400 MHz, DMSO-
d6) δ 14.2, 60.9, 113.1, 115.7, 116.5, 126.7, 128.9, 131.4, 145.7, 146.7, 160.5, 169.3; HR-FAB mass: calcd. for
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C12H13N2O2S (M+H)+: 249.0698, Found: m/z 249.0699.
2.2.3. Synthesis of 2-(2’-aminophenyl)-4-hydroxymethylthiazole (1)
A solution of 2 (100 mg, 0.4 0mmol) and LiAlH4 (17 mg, 0.41 mmol) in dry THF (20 mL) was stirred for 1 h.
The solvent was removed by an evaporator, 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:hexane = 1:4) to give compound 1 (56 mg, 67%). 1H NMR (400 MHz, CD3CN)
δ 3.37 (t, J = 6.2 Hz, 1H, OH), 4.67 (dd, J = 6.2, 0.9 Hz, 2H, CH2OH), 6.40,(bs, 2H, NH2), 6.65 (ddd, J = 7.2, 7.2,
1.2 Hz, 1H, Hc), 6.79 (dd, J = 8.2, 1.0 Hz, 1H, Ha), 7.18 (ddd, J = 8.2, 7.2, 1.2 Hz, 1H, Hb), 7.18 (s, 1H, ThzH), 7.61
(dd, J = 7.9, 1.4 Hz, 1H, Hd); 1H NMR (500 MHz, CDCl3) δ 4.72 (d, J = 1.0 Hz, 2H, CH2OH), 6.64 (ddd, J = 8.0, 8.0,
1.5 Hz, 1H, Hc), 6.68 (dd, J = 8.0, 1.0 Hz, 1H, Ha), 7.01 (s, 1H, He), 7.10 (ddd, J = 7.0, 7.0, 1.5 Hz, 1H, Hb), 7.54
(dd, J = 7.5, 1.0 Hz, 1H, Hd); 13C NMR (125 MHz, CDCl3) δ 61.4, 112.6, 115.8, 116.9, 117.2, 129.2, 130.8, 145.4,
155.9, 170.1; HR-FAB mass: calcd. for C10H11N2OS (M+H)+: 207.0592, Found: m/z 207.0589.
2.3. UV-visible and fluorescence studies
A host solution (30 µM) was prepared in EtOH, and the guest solution (300 µM) was added to the host
solution for UV-visible study. For fluorescence titration, a host solution (10 µM in EtOH) was prepared, and
the guest solution (30 µM) was added to the host solution. In a typical titration experiment, the host solution
(2 mL) was transferred to a fluorescence cell, and the emission spectrum was recorded at a fixed wavelength.
The guest solution (20 µL) was added through a microsyringe, and the amount was increased until 10 equiv.
of guest. The fluorescence spectrum of each solution was recorded after each addition.
2.4. Association constant calculations
The association constants of probe 1 was calculated using Gnuplot ver. 4.6 software with the reduced chi-
square method (error bound within ± 10.0%) and the curve-fitting method (error bound ± 5.0%) from the
adjusted r-square method using fluorescence titration data. Solutions (0.2 × 10-6 M) of probe 1 were
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prepared in 50-mL volumetric flasks (± 0.025 mL) (see ESI). The limits of detection (LODs) were calculated
based on the results determined in the fluorescence titration studies.
2.5. Theoretical calculations
Theoretical DFT calculations were carried out according to the methods reported in the literatures.65,66
3. Results and discussion
3.1. Synthesis
Probe 1 was prepared in good yield by reduction of compound 2 with LiAlH4 in THF (Scheme 1); its
1
structure was confirmed by H and 13C NMR and HR-mass spectroscopic methods. The CH2OH moiety in 1
was observed as a singlet at δ 3.37, corresponding to the OH and a doublet of doublets at δ 4.67
corresponding to the CH2 in the 1H NMR spectrum of CD3CN solvent. A singlet at δ 6.40 corresponded to the
NH2 group, while the thiazole-H proton appeared at δ 7.18 as a singlet. 13C NMR spectrum of 1 in CDCl3
showed CH2OH at δ 61.4. The high-resolution mass spectrum (HRMS) of probe 1 showed a molecular ion
peak for (M+H)+ at m/z = 207.0589 (see ESI).
Scheme 1. Synthesis of probe 1.
S
S
S
O
O
H2, Pd/C
EtOH
LiAlH4
THF
N
N
N
OH
OEt
OEt
NH2
1
NO2
3
NH2
2
3.2. UV-visible and fluorescence studies of probes
Initial studies on the UV-vis absorption and fluorescent emission of probe 1 were carried out in EtOH. The
binding behavior of 1 towards Cu2+ was examined using UV-vis spectroscopy. As shown in Fig. 2a, in the
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absence of copper (II), 1 (10 µM) showed absorption bands at 287 (log ε = 3.83) and 359 nm (log ε = 3.87),
which were ascribed to π−π* transitions.67 Upon the addition of Cu2+ to 1, the absorption bands were red-
shifted to 300 (log ε = 4.15) and 445 nm (log ε = 2.82). This was attributed to the intramolecular charge
transfer from the ligand to the copper ion. Probe 1 showed a strong fluorescence emission peak at 432 nm
with a high quantum yield (Φ = 0.1682). The emission spectrum was a mirror image of the absorption
spectrum, with a small Stokes shift. This suggested that the emission was from the relaxed Franck−Condon
excited state. In the presence of Cu2+, the emission band at 432 nm was nearly quenched, due to the
intramolecular charge transfer from the ligand to the paramagnetic copper ion upon binding (Fig. 2b). The
quenching results obtained upon addition of Cu2+ suggested that the open shell d-orbitals of Cu2+ provided a
very fast and efficient non-radiative decay of the excited states, due to the electron or energy transfer
between the metal cation and probe 1. Similar UV-vis and fluorescence studies of probe 2 did not reveal any
significant selectivity or sensitivity towards various metal ions (Fig. S7).
0.5
0.4
0.3
0.2
0.1
0.0
500
400
300
200
100
0
(b)
(a)
432 nm
1
300 nm
1+Cu2+
359 nm
1
287 nm
328 nm
445 nm
1+Cu2+
300
350
400
450
500
400
450
500
550
600
Wavelength (nm)
Wavelength (nm)
Fig. 2. (a) UV-vis spectra of probe 1 (30 µM), and (b) fluorescence spectra of probe 1 (10 µM) in the presence
of 10 eq. of Cu(ClO4)2 in EtOH (λex = 328 nm).
UV-vis and fluorescence titrations of 1 were carried out in EtOH. Upon slow addition of Cu2+, increases at
287 and 445 nm and a decrease at 359 nm in the UV-vis (Fig. 3a), as well as a constant decrease of the peak
at 432 nm in the emission spectra (Fig. 3b) were observed until the concentrations of 1 and Cu2+ were below
or equal to 1:1. When the 1:1 ratio was reached, however, higher Cu2+ concentrations did not lead to any
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further quenching (Fig. 3b, inset). The emission band of 1 linearly decreased upon addition of up to 1 eq. of
Cu2+ (Fig. 3b). The binding constant between probe 1 and Cu2+ was determined by non-linear fitting of the
corresponding fluorescence titration data and yielded a value of 1.43 × 105 M-1. From the linear plot of I432
versus [Cu2+], the minimum detection limit for Cu2+ was found to be 1 µM (R2 = 0.997) (Fig. S8).
0.5
0.4
0.3
0.2
0.1
0.0
900
800
700
600
500
400
300
200
100
0
0.25
0.20
0.15
0.10
0.05
0.00
432 nm
0~10 eq.
800
600
400
200
0
(b)
(a)
0~10 eq.
0
1
2
3
2+
4
5
359 nm
0
1
2
3
4
5
Eq. of Cu
2+
Eq. of Cu
287 nm
445 nm
300
350
400
450
500
400
450
500
550
600
Wavelength (nm)
Wavelength (nm)
Fig. 3. (a) UV-vis titration of probe 1 (30 µM) with Cu(ClO4)2 in EtOH. Inset represents the dependence of the
absorbance intensity at 359 nm on the addition of Cu(ClO4)2. (b) Fluorescence titration of probe 1 (10 µM)
with Cu(ClO4)2 in EtOH, λex = 328 nm. Inset represents the dependence of the fluorescence intensity at 432
nm on the addition of Cu(ClO4)2.
To further quantify the complexation ratio of probe 1 with Cu2+, a Job’s plot was employed, in which the
emission changes at λem = 432 nm as a function of the molar fraction of Cu2+ was considered. The maximum
emission was observed when the molar fraction of Cu2+ reached 0.5, which indicated a 1:1 stoichiometry in
the complex formed between probe 1 and Cu2+ (Fig. S9).
One of the most important criteria for potential applications of a cation sensor is the selectivity. Therefore,
10 eq. of various biologically and non-biologically relevant metal cations were added to a solution of 1 (10
µM), and the complexation was studied using fluorescence emission spectroscopy. None of the cations,
except Cu2+, led to prominent fluorescence quenching of the peak at 432 nm upon binding with 1 (Fig. 4).
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500
400
300
200
100
0
(b)
(a)
432 nm
100
1, Ag+, Ca2+, Cd2+, Co2+, Cs+, Fe2+, Ga3+
In3+, K+, Mg2+, Na+, Ni2+, Pb2+, Rb2+, Zn2+
,
80
60
40
20
0
Al3+, Fe3+
Mn2+
Hg2+
Ni2+
Cu2+
350
400
450
500
550
600
Wavelength (nm)
Fig. 4. (a) Fluorescence spectra of probe 1 (10 µM) with 10 eq. of various cations in EtOH (λex = 328 nm; λem
432 nm). (b) Relative fluorescence responses of probe 1 in the presence of various cations (λem = 432 nm).
=
To explore the possibility of using 1 as a practical ion-selective fluorescent probe for Cu2+, competition
experiments were carried out, in which 1 (10 μM) was first mixed with 10 eq. of Cu2+ followed by addition of
10 eq. of various competing metal ions. Fluorescence studies showed that a wide variety of common metal
ions exhibited no obvious interference in the detection of Cu2+ (Fig. S10). This high selectivity was because of
the fact that although transition metals do not differ too much in size, they can establish coordinative
interactions at very different energies, which can be used for discriminative purposes, especially in
fluorescent sensing. This phenomenon is consistent with copper, which is the highest in the Irving–Williams
series.69 Copper(II) has a particularly high thermodynamic affinity for the imino nitrogen of the thiazole ring,
as well as the amino nitrogen and oxygen of the hydroxyl group during complex formation, with fast metal-
to-ligand binding kinetics that are not possible with other transition metal ions.
It is known that fluorescent metal complexes can be used to detect the various anions through counter ion
studies.1,19-21,57,61,62 Such studies will reveal the complex stability in presence of counter anions in such a way
that can extrude the metal ions from the fluorescent ligand or co-operatively bind resulted in the different
fluorescent emission behavior. In that sense, in-situ generated 1-Cu2+ (1:1) ensemble (10 µM) was utilized to
detect the various counter anions in EtOH. The fluorescence of the 1-Cu2+ complex (10 µM) was evaluated in
-
-
-
3-
-
the presence of various anions (10 eq.) such as F-, Cl-, Br-, I-, ClO4 , CN-, HSO4 , NO3 , OAc-, HP2O7 , and H2PO4
in EtOH (Fig. 5). Only I- ions caused a remarkable fluorescence enhancement, whereas other anions did not
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-
-
result in a significant change; OAc-, H2PO4 , and NO3 resulted in a very small enhancement (Fig. 5b). Figure 5
clearly depicts that the 1-Cu2+ complex exhibited high selectivity and sensitivity towards I- ions. During
titration of 1-Cu2+ with a solution of I- ions, it was found that the gradual addition of I- led to the revival of the
fluorescence intensity at 432 nm, which was quenched upon addition of Cu2+ to 1; the fluorescence gradually
increased until 2 eq. of I- was added. The high selectivity of the copper complex towards iodide was due to
the binding of probe 1 with paramagnetic Cu2+.67 It is known that iodide anions can reduce Cu2+ into Cu+ at
room temperature. This iodometric method has been broadly used for the determination of Cu2+. 70,71 Here,
this redox reaction was utilized to develop a “turn on” fluorescent probe for iodide. As shown in Fig 5,
unbound probe 1 is in the fluorescence “on” state, and it “turn off” upon coordination with Cu2+. However,
upon treatment of 1-Cu2+ with iodide, Cu2+ is reduced to Cu+, which binds with 1; because Cu+ is diamagnetic,
an enhancement in the fluorescence at 432 nm is observed.
Fig. 5. (a) Fluorescence spectra of probe 1 (10 µM) upon addition of Cu2+ (10 eq.) and subsequent addition of
anions (10 eq.). (b) Fluorescence responses of 1-Cu2+ in the presence of various anions (10 eq.) in EtOH. The
white bars represent the addition of 10 eq. of Cu2+ to the solution of probe 1 (10 µM). The bars filled with
dashed lines represent the change in emission that occurred upon subsequent addition of 10 eq. of various
anions to a 10 µM solution of 1-Cu2+ (λex = 328 nm; λem = 432 nm).
The binding constant between 1-Cu2+ and I- (1.74 × 105 M-1) was determined by fluorescence titrations, as
shown in Fig. 6 and summarized in Table 1. The emission intensity of 1-Cu2+ at 432 nm steadily increased until
2 eq. of I- was added. The Job’s plot showed a 1:2 stoichiometry between 1-Cu2+ and I- (Fig. 6b). From the
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linear plot between I432 versus [I-], the minimum detection limit for I- was found to be 1 µM (R2 = 0.980), Fig.
S11). However, fluorescence studies of 2-Cu2+ (10 µM) with 10 eq. of various anions did not show any
significant changes due to the poor complexation of 2 towards Cu2+ (Fig. S12). The binding affinity of 2
towards Cu2+ is less than that of probe 1 due to the ester group in probe 2, which is less efficient than the
hydroxyl group in 1. Thus, 2-Cu2+ exhibited poor complexation.
1.4x10-3
400
(a)
(b)
300
250
200
150
100
1.2x10-3
1.0x10-3
8.0x10-4
6.0x10-4
4.0x10-4
0~10 eq.
300
200
100
0
0
2
4
6
8
10
equiv. of I-
2.0x10-4
0.0
400
450
500
550
600
0.0
0.2
0.4
0.6
0.8
1.0
Equiv. of I-
Wavelength (nm)
Fig. 6. Fluorescence changes in the 1-Cu2+ complex (10 µM) titrated with TBA+I- (10 eq.) in EtOH. Inset
represents the dependence of the fluorescence intensity at 432 nm upon addition of TBA+I-. (b) Job’s plot of
probe 1-Cu2+ (10 µM) with TBA+I- (10 µM) in EtOH (λex = 328 nm; λem = 432 nm).
Table 1 Association constants and quantum yields of probe 1 with Cu2+ and I-
Association constant Ka (M-1)
Quantum yield
0.1682
1
-
1+Cu2+
1.43
1.74
x
x
105
105
0.0148
1-Cu2++I-
0.1235
3.3. 1H NMR study of probe 1
To further investigate the nature of the interaction and the mode of complexation between Cu2+ and I- and
1
probe 1, H NMR titrations of 1 with Cu2+ and I- were carried out. The addition of 1 eq. of Cu(ClO4)2 to a
solution of probe 1 in CD3CN led to the complete quenching of the proton signals, owing to the characteristic
paramagnetic properties of Cu2+; subsequent addition of 2 eq. of I- to the 1-Cu2+ solution led to the
appearance of new proton signals as shown in Fig. 7.
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Fig. 7. Partial 1H NMR spectra (CD3CN) of (i) probe 1, (ii) 1 + 1 eq. Cu2+, (iii) 1 + 1 eq. Cu2+ + 1 eq. I-, (iv) 1 + 1 eq.
Cu2+ + 2 eq. I-, and (v) 1 + 1 eq. Cu2+ + 3 eq. I-.
After the addition of 2 eq. of I- to 1-Cu2+ (1:1), a significant downfield shift of the phenyl protons (labeled c in
Fig. 7) from δ 6.65 to δ 6.90 was observed. Signals corresponding to protons a, b, d, e, and f shifted
downfield from δ 6.79, 7.15, 7.61, 7.18, and 4.67 ppm to δ 6.99, 7.27, 7.71, 7.25, and 4.69 ppm, respectively.
Upon addition of 1 more eq. of I- to this solution, no further shift in the proton signals was observed, which
indicated 2 eq. of I- ions was consumed to form the 1-Cu+ complex. Large changes in the chemical shifts of
the labeled protons Ha, Hb, Hc and Hd and in the multiplicity of Hf (from doublet to singlet) confirmed that the
nitrogen of the amino group and the oxygen of the hydroxyl group were involved in the coordination with
1
Cu+; the proton signals of the 1-Cu+ complex appeared in the H NMR spectrum due to the diamagnetic
properties of Cu+, and differed from those of probe 1. These results implied that Cu2+ was reduced to Cu+ by I-
-
anions, and subsequently bound to probe 1 and I3 anions (Scheme 2).
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Scheme 2. Plausible mechanism of the complexation between probe 1, Cu2+, and I-.
S
S
S
-
Cu2+
3I-
I3
N
N
N
OH
O
O
EtOH
Cu+
EtOH
Cu2+
Cu2+
Cu+
NH2
1
N
H2
1 Cu2+
N
H2
1 Cu+
S
S
"Turn-On"
"Turn-Off"
"Turn-On"
3.4. Theoretical calculations
To understand the interactions and complexation mode of probe 1, DFT calculations were performed
using B3LYP/6-31G* as the basis set.66,67 The energy-minimized structure of 1 with Cu2+ is shown in Fig. 8a.
DFT calculations revealed that the Cu2+ ion essentially coordinated with the nitrogen of the –NH2 group
(d(Cu-N) = 2.02 Å, labeled 1, in Fig. 8) and thiazole (d(Cu-N) = 1.92 Å, labeled 2). The oxygen in the CH2OH
moiety could also bind with Cu2+ (d(Cu-O) = 1.87 Å, labeled 3). Similarly, the energy-minimized structure of
1·Cu+ is shown in Fig. 8b. The distances between the amino nitrogen, thiazole nitrogen, and oxygen and the
Cu+ ion (labeled 5, 6, and 7) were calculated to be 1.92, 1.99 and 1.85 Å, respectively. The differences in the
distances between the amino nitrogen and Cu2+ and Cu+ could determine the extent of charge transfer,
resulting in extensive variations in fluorescence emission after complexation.
Fig. 8. Energy minimized structures of probe 1 with (a) Cu2+ and (b) Cu+.
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Furthermore different HOMO-LUMO distributions were observed for 1·Cu2+ and 1·Cu+ (Fig. 9), suggesting
variations in the electron density caused the observable changes in the fluorescence spectra, which were
mainly due to the different points of interaction with the respective analytes. The different fluorescence
responses observed with Cu2+ and Cu+ ions suggested different extents of charge transfer with different
binding modes. The band gap energy (–∆E = EHOMO–ELUMO) for 1·Cu+ (2.97 eV) was smaller than that of 1·Cu2+
(4.29 eV). This result further supported that the complexation was thermodynamically stable and explained
the red shift observed in the UV-vis spectrum of 1·Cu2+ (Fig. 2a).
Fig. 9. HOMO, LUMO and band gap energies (–∆E = EHOMO–ELUMO) of 1, 1·Cu2+, and 1·Cu+ calculated using the
B3LYP/6-31G* set.
4. Conclusion
In conclusion, we synthesized 2-(2’-aminophenyl)-4-hydroxtmethylthiazole, probe 1, which was found to be
highly selective towards copper(II) ions. Probe 1 demonstrated a highly selective ‘‘on-off–on’’ type
fluoroionophoric switching property. The 1-Cu2+ complex demonstrated high selectivity and sensitivity
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towards I- ions; upon binding I-, the fluorescence of the complex was enhanced as Cu2+ was reduced to Cu+ in
the presence of I-.
Acknowledgement
This research was supported by the Basic Science Research Program through the National Research
Foundation (NRF) funded by Republic of Korea (2016R1D1A1B03931989).
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