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Article
Sequential Sensing of Copper(II) and Iodide Ions Based on Thiazole
KOREAN CHEMICAL SOCIETY
To investigate the significance of the dansyl unit for the
geometries, respectively.3a It is well known that iodide can
reduce Cu2+ to Cu+ at ambient temperatures, and this iodo-
metric method has been widely used for Cu2+ determina-
tion.33,34 Accordingly, the rationally designed ESIPT
dansyl-thiazole hybrid probe can be used for the identifica-
tion of Cu2+ redox chemistry as a “turn on” fluorescent
probe for iodide. The fluorescent “on” state “turns off”
upon coordination with Cu2+. However, upon further treat-
ment of iodide with 1-Cu2+, Cu2+ is reduced to Cu+, which
binds with 1, thereby enhancing the fluorescence, and a
new peak is formed at 544 nm. In addition, after reduction,
free I2 molecules could be generated, which turn the reac-
tion solution yellow. The iodinated solution typically shows
absorption at 420–480 nm in under emission spectra of 1,
and resonance energy transfer (RET) from donor probe 1 to
I2 (solution) is the main reason for the incomplete fluores-
cence restoration. In addition, excess iodide in solution can
stabilize diamagnetic Cu+ ions in square planar geometry,
which is another reason for the incomplete restoration of
the red-shifted emission intensity at 544 nm.
Non-linear curve fitting of the fluorescence titration data
at λem = 512 nm (Figure 7(a)) yielded an appreciably high
binding constant of 1-Cu2+ to I− (Ka = 4.76 × 105). The
emission intensity of 1-Cu2+ at 512 nm steadily increased
until 100 equiv. of I− was added. Job’s plot showed a 1:1
stoichiometry between 1-Cu2+ and I− (Figure 7(b)), indicat-
ing the formation of reduced Cu+ bound with I− ions in a
square planar geometry. Additionally, excess I− can reduce
the excess Cu2+, generating more I2 in solution, resulting in
yellow color.
highly selective sensing of Cu2+, model compound 2 was
analyzed via UV–Vis and fluorescence spectroscopy. Here,
the 2 tosyl-sulfonamide can act as a proton source for
ESIPT for the thiazole unit. Under similar experimental
conditions as those for probe 1, UV–Vis and fluorescence
studies were performed (Figures S4 and S5). The obtained
results showed that only the dansyl-sulfonamide unit conju-
gated with the thiazole moiety was able to recognize Cu2+
selectively.
Anions are a vital factor in the determination of metal
complex stability, and various reports have demonstrated
anion sensing using metal chelates, in situ generated
ensembles, and metal organic frameworks (MOFs) that rec-
ognize anions either through displacement, ligand
exchange, or encapsulation.31,32 To explore the possibility
of using the 1-Cu2+ ensemble as a practical ion-selective
fluorescent probe for anions, competition experiments were
performed, where 1 (10 μM) was mixed with excess Cu2+
(~10 equiv), followed by the addition of 10 equiv of com-
peting anions. To validate the redox process with probe
1 and excess Cu2+ ions, the Cu2+ and I− ions were added in
excess, allowing for the differentiation of free Cu2+ and 1-
chelated Cu2+ through different optical responses.
The 1-Cu2+ ensemble (10 μM) was treated with 10 equiv
of various anions, including I−, F−, Cl−, Br−, CN−, HSO4
,
−
NO3−, OAc−, ClO4−, and H2PO4−. As shown in Figure 6,
the addition of I− resulted in remarkable fluorescence
enhancement, whereas the addition of CN−, HSO4−, and
−
NO3 showed only a minor change in emission intensity.
This indicates that the 1-Cu2+ complex shows significant
and selective interactions with I−. Upon gradual addition of
I− (0 to 100 equiv), a concomitant increase in the emission
signal centered at 544 nm was observed, which was red-
shifted from the original signal at 512 nm. Further addition
of I− did not result in significant shifts, indicating that com-
plete saturation was achieved. The high selectivity of the
copper complex for iodide is due to the different binding
behaviors of 1 toward paramagnetic Cu2+ and diamagnetic
Cu+, as their complexes prefer square planar and tetrahedral
To investigate the sensing mechanism between probe
1 and Cu2+ and its complex with tetrabutylammonium
1
iodide (TBAI), H NMR spectroscopy was performed in
CD3CN. The addition of
1 a
equiv of Cu2+ to
4.5 × 10−3 M solution of 1 resulted in complete quenching
of the proton resonances except for the downfield shift of
the N-CH3 group from δ 2.140 to 2.214 ppm, owing to the
characteristic paramagnetic properties of Cu2+. Subsequent
addition of 1 to 2 equiv of I− to 1-Cu2+ resulted in the
appearance of new proton signals, and upon the addition of
another equiv of I− to the solution (3 equiv), the free probe
1 was recovered from complex. Further addition (4 equiv)
Figure 6. (a) Fluorescence spectra of 1 (10 μM) upon addition of
Cu2+ (10 equiv) and subsequent addition of I− (10 equiv) at room
temperature, (b) fluorescence responses of 1-Cu2+ in the presence
of interfering anions (10 equiv) in CH3CN. The bars represent the
emission upon the subsequent addition of 10 equiv of the interfer-
ing anions to a 10 μM solution of 1-Cu2+ (λex = 333 nm,
λem = 533 nm).
Figure 7. (a) Fluorescence changes of 1-cu2+ (10 μM) titrated by
TBA+I− from 0 to 10 equiv in CH3CN. Inset plot: Dependence of
the fluorescence intensity at λex = 333, λem = 533 nm on the
amount of TBA+I− added. (b) Job’s plot of 1 (20 μM) with
Cu(ClO4)2 (20 μM) in CH3CN (λex = 333 nm, λem = 544 nm).
Bull. Korean Chem. Soc. 2019
© 2019 Korean Chemical Society, Seoul & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Kim, Bo-Yeon
