A highly selective bifunctional luminescence probe for potassium and fluoride ions

Article abstract of DOI:10.1021/ol200277n

Chemical equations presented. A novel bifunctional molecule 1 by a combination of 1,3-alternate calix[4]-crown-5 and triarylborane moieties through alkynyl linkers has been designed and synthesized. Compound 1 shows intense fluorescence with a photolumine

Full text of DOI:10.1021/ol200277n

ORGANIC
LETTERS
2011
Vol. 13, No. 9
2172–2175
A Highly Selective Bifunctional
Luminescence Probe for Potassium and
Fluoride Ions
Xiaoming He and Vivian Wing-Wah Yam*
Institute of Molecular Functional Materials and Department of Chemistry,
The University of Hong Kong, Pokfulam Road, Hong Kong, P. R. China
wwyam@hku.hk
Received February 7, 2011
ABSTRACT
A novel bifunctional molecule 1 by a combination of 1,3-alternate calix[4]-crown-5 and triarylborane moieties through alkynyl linkers has been
designed and synthesized. Compound 1 shows intense fluorescence with a photoluminescence quantum yield of 0.70 in CH2Cl2 solution and can
serve as a bifunctional luminescent probe for potassium and fluoride ions with high sensitivity and selectivity.
The sensitive and selective detection of potassium and
fluoride ions is essential because of their biological im-
portance. The Kþ ion is crucial for life, and a low potas-
sium ion level has been linked to the risk of stroke, heart
disease, arthritis, and infertility.1 The difficulty in measur-
ing accurately the concentration of potassium ions stems
from the presence of a large excess of sodium ions. There-
fore, a practical probe must have a great preference for Kþ
over Naþ. On the other hand, fluoride plays an important
role in preventing dental caries and in the treatment of
osteoporosis.2 High doses of this anion are, however,
dangerous and can lead to dental or skeletal fluorosis.
Currently, luminescence is widely used as the signal output
in the design of chemosensors due to its high sensitivity and
the simplicity of its equipment requirement.3 Although a
number of luminescent ion probes have been reported,
luminescence probes for potassium4ꢀ6 and fluoride7ꢀ9 ions
are still rather rare. In addition, most of the attention has
been focused on the design of a monofunctional lumines-
cent anion or cation sensors. To the best of our knowledge,
there is no report on the design of bifunctional fluorescent
(4) (a) Czarnik, A. W. Fluorescent Chemosensors for Ion and Molecule
Recognition; ACS: Washington, DC, 1993. (b) de Silva, A. P.; Gunaratne,
H. Q. N.; Gunnlaugsson, T.; Huxley, A. J. M.; McCoy, C. P.; Rademacher,
J. T.; Rice, T. E. Chem. Rev. 1997, 97, 1515.
(5) (a) Dietrich, B.; Lehn, J.-M.; Sauvage, J.-P.; Blanzat, J. Tetra-
hedron 1973, 29, 1629. (b) de Silva, A. P.; Gunaratne, H. Q. N.;
Sandanayake, K. R. A. S. Tetrahedron Lett. 1990, 31, 5193. (c) Crossley,
R.; Goolamali, Z.; Gosper, J. J.; Sammes, P. G. J. Chem. Soc., Perkin
Trans. 2 1994, 513. (d) Xia, W. S.; Schmehl, R. H.; Li, C. J. J. Am. Chem.
Soc. 1999, 121, 5599. (e) Helgeson, R. C.; Czech, B. P.; Chapoteau, E.;
Gebauer, C. R.; Kumar, A.; Cram, D. J. J. Am. Chem. Soc. 1989, 111,
6339. (f) Kim, J.; McQuade, D. T.; McHugh, S. K.; Swager, T. M.
Angew. Chem., Int. Ed. 2000, 39, 3868. (g) He, H. R.; Mortellaro, M. A.;
Leiner, M. J. P.; Fraatz, R. J.; Tusa, J. K. J. Am. Chem. Soc. 2003, 125,
1468. (h) Namkung, W.; Padmawar, P.; Mills, A. D.; Verkman, A. S.
J. Am. Chem. Soc. 2008, 130, 7794. (i) Carpenter, R. D.; Verkman, A. S.
Org. Lett. 2010, 12, 1160.
(1) (a) Kager, H.; Wadman, W. J.; Somjen, G. G. J. Neurophysiol.
2000, 84, 495. (b) Frolov, R. V.; Brelim, I. G.; Saptal, S. J. Biol. Chem.
2008, 283, 1518.
(2) (a) Holland, M. A.; Kozlowski, L. M. Clin. Pharm. 1986, 5, 737.
(b) Carton, R. J. Fluoride 2006, 39, 163.
(3) Lakowicz, J. R. Principles of Fluorescence Spectroscopy; Springer:
New York, 2006.
(6) (a) Thibon, A.; Pierre, V. C. J. Am. Chem. Soc. 2009, 131, 434. (b) Li,
C.; Law, G. L.; Wong, W. T. Org. Lett. 2004, 6, 4841. (c) Gunnlaugsson, T.;
Leonard, J. P. Chem. Commun. 2003, 2424. (d) Gunnlaugsson, T.; Leonard,
J. P. J. Chem. Soc., Dalton Trans. 2005, 3204.
r
10.1021/ol200277n
Published on Web 04/12/2011
2011 American Chemical Society

sensors which could selectively detect potassium and
fluoride ions.
Scheme 1. Synthetic Route of 1
Herein, we report a novel bifunctional luminescence
probe for potassium and fluoride ions with high sensitivity
and selectivity, by a combination of 1,3-alternate calix[4]-
crown-5 and triarylborane moieties through alkynyl lin-
kers. In previous studies, the 1,3-alternate calix[4]-crown-5
platform has been proven to bind a potassium ion selec-
tively through the complexation of the crown ether ring
and additional binding by cationꢀπ interactions between
the two phenyl rings.10,11 A recent study showed that
triarylborane compounds containing sterically hindered
groups such as dimesityl and triduryl moieties showed
enhanced size selectivity toward fluoride ions by utilizing
the empty pπ orbital on the boron center.8 In addition, they
have a tendency to display intense intramolecular charge-
transfer transitions when an appropriate electron donor is
present. The alkynyl group could serve as a very good
connector for electron communication which produces
intense intramolecular charge transfer from the oxygen
to the boron center.
The synthetic procedure of 1 is summarized in Scheme 1.
Iodination of dipropoxylcalixarene nPrO-Calix followed by a
Sonagashira cross-coupling reaction gave the bis(trimethyl-
silylethynyl)-substituted calixarene nPrO-Calix-CtCTMS.
A subsequent reaction of nPrO-Calix-CtCTMS with penta-
ethylene glycol di-p-toluenesulfonate in the presence of
Cs2CO3 gave 1,3-alternate nPrO-Calixcrown[5]-CtCH.
A further Sonagashira cross-coupling reaction of
nPrO-Calixcrown[5]-CtCH and 1-iodo-4-dimesitylboryl-
benzene afforded the desired compound 1. The identity
of 1 was established by 1H NMR, 13C NMR, mass spectro-
metry, and satisfactory elemental analysis.
fluorescence (λem = 430 nm, φem = 0.70) in CH2Cl2
solution. The emission is assigned to originate from an
excited state derived from the intramolecular charge trans-
fer transition from the phenol O atom to the boron center.
In more polar solvents like DMF, a significant red shift in
the emission maximum of ca. 1830 cmꢀ1 was observed,
characteristic of charge transfer transitions.
Compound 1 displays an intense electronic absorption
band centered at 342 nm (ε = 77000) and a bright blue
Figure 1 shows the UVꢀvis and emission spectral
changes of 1 in CH2Cl2/CH3OH (1:1, v/v) upon addition
of Kþ ions. The addition of Kþ ions to a solution of 1 in
CH2Cl2/CH3OH resulted in an increase in the absorbance
and a small blue shift in the absorption energy of the low-
energy band. Two well-defined isosbestic points at 280 and
348 nm are observed, indicative of a clean conversion of 1
to the ion-bound adduct. In addition, the luminescence
response of 1 toward Kþ was found to be more pro-
nounced. Significant emission quenching with a concomi-
tant blue shift of ca. 20 nm was observed, which was
consistent with the blue shift observed in the UVꢀvis
absorption spectra. The emission quenching and blue shift
in emission energies upon Kþ inclusion could be rationa-
lized by the reduced electron-donating ability of the
diethynylcalixcrown moiety upon inclusion of the guest
metal ion. Although ion-binding may lead to an increase in
the rigidity of the calixcrown complex and the inhibition of
photoinduced electron transfer and therefore may enhance
the emission intensity, the significant changes in the emis-
sion intensity may be a result of the high sensitivity of the
intramolecular charge transfer emission to the binding
event. Moreover, the sensor showed a high sensitivity
ꢀ~
ꢀ
(7) (a) Martınez-Manez, R.; Sancenon, F. Chem. Rev. 2003, 103,
4419. (b) Beer, P. D.; Gale, P. A. Angew. Chem., Int. Ed. 2001, 40, 486.
ꢀ
(c) Amendola, V.; Esteban-Gomez, D.; Fabbrizzi, L.; Licchelli, M. Acc.
Chem. Res. 2006, 39, 343. (d) Sessler, J. L.; Gale, P. A.; Cho, W. S. Anion
Receptor Chemistry; RSC: Cambridge, 2006.
(8) (a) Wade, C. R.; Broomsgrove, A. E. J.; Aldridge, S.; Gabbaı,
F. P. Chem. Rev. 2010, 110, 3958. (b) Hudson, Z. M.; Wang, S. Acc.
Chem. Res. 2009, 42, 1584. (c) You, Y.; Park, S. Y. Adv. Mater. 2008, 20,
3820. (d) Yamaguchi, S.; Akiyama, S.; Tamao, K. J. Am. Chem. Soc.
2001, 123, 11372. (e) Lam, S. T.; Zhu, N.; Yam, V. W. W. Inorg. Chem.
2009, 48, 9664.
(9) (a) Hu, R.; Feng, J.; Hu, D.; Wang, S.; Li, S.; Li, Y.; Yang, G.
Angew. Chem., Int. Ed. 2010, 49, 4915. (b) Kim, S. Y.; Park., J.; Koh, M.;
Park, S. B.; Hong, J. I. Chem. Commun. 2009, 4735. (c) Kim, T. H.;
Swager, T. M. Angew. Chem., Int. Ed.. 2003, 42, 4803. (d) Jiang, X.;
Vieweger, M. C.; Bollinger, J. C.; Dragnea, B.; Lee, D. Org. Lett. 2007, 9,
3579. (e) Black, C. B.; Andrioletti, B.; Try, A. C.; Ruiperez, C.; Sessler,
J. L. J. Am. Chem. Soc. 1999, 121, 10438. (f) Cho, E. J.; Moon, J. W.; Ko,
S. W.; Lee, J. Y.; Kim, S. K.; Yoon, J.; Nam, K. C. J. Am. Chem. Soc.
2003, 125, 12376. (g) Liu, X. Y.; Bai, D. R.; Wang, S. Angew. Chem., Int.
Ed. 2006, 45, 5475. (h) Reetz, M. T.; Niemeyer, C. M.; Harms, K. Angew.
Chem., Int. Ed. 1991, 30, 1472.
(10) (a) Kim, J. S.;Quang, D. T. Chem. Rev. 2007, 107, 3780. (b)Casnati,
A.; Pochini, A.; Ungano, R.; Bocchi, C.; Ugozzoli, F.; Egberink, R. J. M.;
Struijk, H.; Lugtenberg, R.; Jong, F.; de, Reinhoudt, D. N. Chem.;Eur. J.
1996, 2, 436.
(11) (a) Yam, V. W. W.; Yip, S. K.; Yuan, L. H.; Cheung, K. L.; Zhu,
N.; Cheung, K. K. Organometallics 2003, 22, 2630. (b) Lo, H. S.; Yip,
S. K.; Wong, K. M. C.; Zhu, N.; Yam, V. W. W. Organometallics 2006,
25, 3537. (c) Yip, S. K.; Cheng, E. C. C.; Yuan, L. H.; Zhu, N.; Yam,
V. W. W. Angew. Chem., Int. Ed. 2004, 43, 4954.
Org. Lett., Vol. 13, No. 9, 2011
2173

the two phenyl rings. The downfield shift can be readily
rationalized by the electron-withdrawing effect of the
cation. In addition, the hostꢀguest complexation equili-
brium has a very slow exchange rate compared to that of
the NMR time scale. The signals that are assigned to both
the complex [1 K]þ and the free host 1 could be observed
3
in the same NMR spectrum in the presence of 0ꢀ1 equiv of
Kþ. The low exchange rate suggests the very strong bind-
1
ing affinity of 1 for Kþ. The H NMR spectra reach
saturation upon addition of 1 equiv of Kþ, suggesting a
1:1 binding stoichiometry with a high binding constant. A
very intense ion cluster at m/z 1403, corresponding to
the [1 K]þ complexation upon addition of Kþ ion to 1,
3
is observed in the positive-ion ESI mass spectrum
(Figure S1).
To examine the selectivity of 1 toward various alkali and
alkaline earth metal ions, the luminescence response upon
the addition of Liþ, Naþ, Kþ, Mg2þ, Ca2þ, and Ba2þ has
been investigated. As shown in Figure 3, upon addition of
50 equiv of various metal ions, only a drastic spectral
change is shown by Kþ ion binding, indicating the high
selectivity of 1 toward Kþ. In addition, the log Ks values of
7.40 ( 0.14 and 2.38 ( 0.02 of 1 for Kþ and Naþ ions were
obtained from a nonlinear least-squares fit for 1:1 binding.
The close agreement of the data to the theoretical fit is
further supportive of a 1:1 binding mode. The high Kþ/
Naþ selectivityofca. 105 ishigherthanthatofthe naturally
occurring valinomycin, suggesting that 1 would serve as a
promising candidate for application as a selective Kþ ion
chemosensor.
Figure 1. (a) UVꢀvis and (b) emission spectral changes of com-
pound 1 (1.18 ꢁ 10ꢀ5 M) in CH2Cl2/MeOH (1:1, v/v; 0.1 M
nBu4NPF6) upon addition of KPF6. Inset of (a): Plot of absor-
bance at 320 nm as a function of Kþ concentration and its theo-
retical fit for the 1:1 binding of complex 1 with Kþ. Inset of (b):
Emission color change upon addition of KPF6 to 1 (1.74 ꢁ 10ꢀ6
M) in CH2Cl2/CH3OH (1:1, v/v; 0.1 M nBu4NPF6).
toward Kþ and a pronounced intensity quenching was
observed even with a Kþ concentration of 1 ꢁ 10ꢀ8 M.
Figure 2 shows the 1H NMR spectral changes of 1 upon
addition of Kþ in CDCl3/CD3OD (1:1, v/v). Large down-
field shifts of the proton resonances of the crown moiety
(HhꢀHk) and the calixarene moiety (Hd, Hg, Hf) were
observed upon addition of Kþ, confirming the binding
mode through the complexation of the crown ether ring
and additional binding by cationꢀπ interactions between
Figure 3. Responses of 1 (1.0 ꢁ 10ꢀ6 M) in CH2Cl2/CH3OH
(1:1 v/v; 0.1 M nBu4NPF6) upon addition of 50 equiv of different
metal ions as ClO4ꢀ salts. Excitation was at 350 nm, and the
emission was monitored at 450 nm.
The anion-binding properties of 1 were also examined
and 1 was found to be highly selective and sensitive to Fꢀ,
as revealed by the large UVꢀvis and emission spectral
changes, while the addition of other anions, such as Clꢀ,
Brꢀ, Iꢀ, H2PO4ꢀ, and AcOꢀ led to negligible changes.
Figure 4 shows the electronic absorption and emission
spectral changes upon addition of Fꢀ in CH2Cl2 solution.
Upon complexation with Fꢀ, the characteristic strong
absorption band of 1 at 350 nm completely disappeared,
and a new band centered at 300 nm gradually appeared.
The disappearance of the low-energy band could be
ascribed to the interruption of the π-conjugation extended
Figure 2. 1H NMR spectral changes of 1 (2.06 mM) upon addition
of KPF6 in CDCl3/CD3OD (1:1, v/v) at 298 K.
2174
Org. Lett., Vol. 13, No. 9, 2011

correspond to the unbound and bound Fꢀ ions, respectively,
and the latter is comparable to those observed in other
triarylfluoroborate systems.8d,e
Figure 5. 1H NMR spectral changes of 1 (2.56 mM) upon addition
of nBu4NF in CD2Cl2 at 298 K.
In summary, we have designed and synthesized a bifunc-
tional luminescent receptor 1 by a combination of 1,3-
alternate calix[4]-crown-5 and triarylborane moieties
through alkynyl linkers. Compound 1 shows high selectiv-
ity and sensitivity toward potassium and fluoride ions with
large UVꢀvis and emission changes. In addition, the
attachment of the triarylborane moiety to the calix[4]arene
provides a novel route to the design of luminescent bifunc-
tional sensors.
Figure 4. (a) UVꢀvis and (b) emission spectral changes of com-
pound 1 (1.18 ꢁ 10ꢀ5 M) in CH2Cl2 (0.1 M nBu4NPF6) upon
addition of nBu4NF at 298 K. Inset of (b): Emission color change
upon addition of nBu4NF to 1 (1.74 ꢁ 10ꢀ6 M) in CH2Cl2 (0.1 M
nBu4NPF6).
through the boron center upon complexation of Fꢀ to the
boron atom. In addition, significant emission quenching
was also observed upon addition of Fꢀ to the CH2Cl2
solution of 1, which can be attributed to the destruction of
the intramolecular charge transfer. However, in CH2Cl2/
CH3OH mixed solvents, no apparent UVꢀvis and fluor-
escence changes were observed upon addition of fluoride.
Thismay beattributedtothe formation of hydrogen bonds
between fluoride ions and methanol, which compete with
the binding to the boron center. The binding constant
(log Ks) of 1 for a fluoride ion was determined to be 4.46 (
0.10 in CH2Cl2, which is comparable to those of com-
pounds reported previously.8
Acknowledgment. V.W.-W.Y. acknowledges the sup-
port from The University of Hong Kong under the Dis-
tinguished Research Achievement Award Scheme, and X.
H. the receipt of a postgraduate studentship from The
University of Hong Kong. This work is supported by the
University Grants Committee Areas of Excellence Scheme
(AoE/P-03/08) (Institute of Molecular Functional Materi-
als, University of Hong Kong) and General Research
Fund (GRF) grant from the Research Grants Council of
Hong Kong Special Administrative Region, P. R. China
(HKU 7050/08P). We thank Dr. A. Y. Y. Tam for his
technical assistance in the latter stage of the work.
The binding of fluoride ions to 1 was also verified by 1H,
19F, and 11B NMR titration experiments, as shown in
Figures 5 and S2. Upon addition of Fꢀ, only the protons
of the triarylborane moiety (Ha, Hb, Hc) and the 11B signal
underwent an upfield shift, due to the increased electron den-
sity as a result of Fꢀ-binding to the boron center. The addi-
tion of excess Fꢀ ions to 1 in CD2Cl2 resulted in two distinct
characteristic 19F signals at δꢀ119.5 and ꢀ174.1 ppm, which
Supporting Information Available. Detailed synthetic
procedure and characterization data for 1; ESI mass spectra
for the binding of 1 with Kþ; 11B and 19F NMR titration for
the binding of 1 with Fꢀ; emission spectra of 1 in different
solvents; electronic absorption and emission spectral traces
of 1 upon addition of NaClO4; 1H and 13C NMR spectra of
1; EI mass spectrum of 1. This material is available free of
charge via the Internet at http://pubs.acs.org.
Org. Lett., Vol. 13, No. 9, 2011
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