Ratiometric and selective fluorescent sensor for F- based on intramolecular charge transfer (ICT)

Article abstract of DOI:10.1007/s10895-010-0681-6

A simple colorimetric and ratiometric fluorescent anion sensor 1, was rationally designed and synthesized by facile one-step condensation on basis of the mechanism of intramolecular charge transfer (ICT). The sensor 1 shows more highly and selectively abi

Full text of DOI:10.1007/s10895-010-0681-6

J Fluoresc (2010) 20:1299–1305
DOI 10.1007/s10895-010-0681-6
ORIGINAL PAPER
Ratiometric and Selective Fluorescent Sensor for F− Based
on Intramolecular Charge Transfer (ICT)
Yaping Li & Hai Lin & Huakuan Lin
Received: 10 March 2010 /Accepted: 26 May 2010 /Published online: 19 June 2010
#
Springer Science+Business Media, LLC 2010
Abstract A simple colorimetric and ratiometric fluorescent
anion sensor 1, was rationally designed and synthesized by
facile one-step condensation on basis of the mechanism of
intramolecular charge transfer (ICT). The sensor 1 shows
more highly and selectively ability to F− that induced the
color changes from little yellow to purple, absorption-
transferred to long wavelength and emission-transferred to
bones [6–8]. The carboxylate anions hold specific bio-
chemical behaviors in the enzymes and antibodies and are
critical components of numerous metabolic processes [9].
And phosphate is associated with a number of important
biomineralization processes such as bone formation as well
as pathological processes: the genesis of renal stones [10].
Nevertheless, these three kinds of anions can also cause
huge damages on environments, such as toxicwastes, water
eutrophication, and even in the production of nerve gases
[11]. As a consequence there is in need of sensitive and
easy testing methods for detecting these anions. Fortunate-
ly, fluorescent and colorimetric chemosensors for various
analytes have been developed for application in physiology
and medical diagnostics [12–15]. Therefore, the design and
synthesis of colorimetric and fluorescent anion chemo-
sensors is necessary for the workers. Meanwhile, the
interactions between the sensor and anions give birth to
color changes, absorption or fluorescence changes through
intermolecular hydrogen bonds. Indeed, the great variety of
hydrogen-bond donor (such as urea, thiourea, amide, and
pyrrole) has been employed to bind a target anion [16–20]
in this context. Accordingly, multiples (e.g. calixpyrroles)
and combinations of these groups, for instance, amidopyr-
roles have also been successfully used [21–23]. Work from
the groups of Jeong [24], Jurczak [25], Beer [26], Sessler
[27], Pfeffer [28] has shown that indoles, carbazoles,
indolocarbazoles and biindoles are effective components
of hydrogen bond donating anion receptors [29]. Our group
had reported one sensor, which can highly and effectively
recognize anions. However, there are only a few examples
of ratiometric fluorescent chemosensors for anions [30].
The major challenge to ratiometric fluorescence signaling is
developing a system with two emitting states having both
the wavelength dependent and substrate-dependent emis-
sion properties [31]. For example, Yi et al. studied a
1
short wavelength. Accordingly, H NMR analysis reveals
that the F−-induced colorimetric and fluorometric responses
of 1 are simply driven by hydrogen bonding interaction
between the NH protons and F− ions.
.
.
.
Keywords Colorimetric Ratiometric Fluorescence
.
Anion sensor Indole
Introduction
Recently, the recognition and detection of anions is a
rapidly maturing field that has received considerable
interest due to the fact that these species play a very
important role in many chemical and biological processes
[1–5]. For example, fluoride ion is drawing a special
attention due to its beneficial in the clinical treatment of
osteoporosis, the role it plays as an environmental pollutant,
and the diseases related to its over-accumulation in the
:
Y. Li H. Lin (*)
Department of Chemistry, Nankai University,
Tianjin 300071, People’s Republic of China
e-mail: hklin@nankai.edu.cn
H. Lin
Key Laboratory of Functional Polymer Materials of Ministry
of Education, Nankai University,
Tianjin 300071, People’s Republic of China

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J Fluoresc (2010) 20:1299–1305
switchable fluorescent organogel in which the fluorescent
property and gel state could be selectively controlled by F−
stimulus [32]. Accordingly, our group [33] reported a
simple and efficient colorimetric anion sensor based on a
thiourea group which can be highly detected fluoride anion
in DMSO and DMSO—water that may be have potential
application in real-life.
Herein, we report that one simple-structured molecule 1,
contains the NH moiety acted as an anion binding site and
one −NO2 groups were an electron-withdrawing group and
a chromophore, which showed ratiometric response to-
wards anions based on the ICT mechanism. Particularly, in
this molecule, an electron-donating group (−NH as an anion
binding site) was conjugated to an electron withdrawing
group. Accordingly, ratiometric sensing is possible due to
altered ground and excited state energetics of the ICT probe
upon anion complexation. Actually, the receptor 1 synthe-
sized by facile one step condensation (Scheme 1), exhibits a
highly selective F− ions in both colorimetric and fluoro-
metric analyses. The probe 1 shows F−-induced color
changes from little yellow to purple and the responses of
the colorimetric and fluorometric are ascribed to the anions-
induced by hydrogen bonding interaction between NH
protons and F− anions.
Apparatus
1H NMR spectra were obtained on a Varian UNITY Plus-
400 MHZ Spectrometer. ESI-MS performed with a
MARINER apparatus. C, H, N elemental analyses were
made on an elementar varuo EL. UV–vis spectra were
recorded on a Shimadzu UV-2450 Spectrophotometer with
a quartz cuvette (path length=1 cm) at 298.2 0.1 K and
the width of the slits used is 10 nm.
General method
All experiments were carried out at 298.2 0.1 K, unless
otherwise mentioned. UV–vis spectra were measured
using an ultraviolet-visible spectrophotometer, UV-2450
(Shimadzu Corp., Kyoto, Japan). A 5.0×10−4 M solution
of the compound 1 in DMSO was prepared and stored in
the dry atmosphere. This solution was used for all
spectroscopic studies after appropriate dilution. Solutions
of 1.0×10−3 M tetrabutylammonium salt of the respective
anion were prepared in dried and distilled DMSO and
were stored under a dry atmosphere.
1H NMR titration experiments were carried out in the
DMSO-d6 solution (TMS as an internal standard). Certain
amount of 0.01 M solution of the compound 1 in the
DMSO-d6 was prepared, into which the increased amount
1
Experiment
of acetate anion (1.0 M in DMSO-d6) was added and H
NMR of the host–guest system was recorded.
Materials
Synthesis of 1H-Indole-3-carboxaldehyde
All reagents for synthesis obtained commercially were
used without further purification. In the titration experi-
ments, all the anions were added in the form of
tetrabutylammonium (TBA) salts, which were purchased
from Sigma-Aldrich Chemical, stored in a vacuum
desiccator containing self-indicating silica and dried fully
before using. DMSO was dried with CaH2 and then
distilled in reduced pressure.
p-nitrophenylhydrazone
The structures and synthesis of receptor 1 are shown in
Scheme 1. 0.29 g (2 mmol) 1H-Indole-3-carboxaldehyde,
0.306 g p-nitrophenylhydrazine (2 mmol) and four drops of
acetic acid were dissolved in 40 mL CH3CH2OH and then
the resulting solution was heated and refluxed 2 h. The
mixture was cooled at room temperature after reaction.
Scheme 1 The synthesis route
of receptor 1
H
N
NO2
H
N
CH3CH2OH
+
AcOH Reflex 2h
N
HN
HN
O
NH2
NO2

J Fluoresc (2010) 20:1299–1305
1301
Precipitate formed was filtered, and 0.448 g pure red solid
was obtained after recrystallization by CH3CN. Yeild=
1
80%. H NMR (DMSO-d6, 300 MHz) δH 7.13 (s, 1H, Ar-
H), 7.22 (d, 2H Ar-H), 7.46 (s, 1H, −CH = N), 7.82 (s, 1H,
Ar-H), 8.16 (s, 2H, Ar-H), 8.28 (t, 3H, Ar-H), 11.04 (s, 1H,
amide NH2), 11.59 (s, 1H, NH1), Elemental analysis calcd
for C15H12N4O2 (280.1): C: 64.28, H: 4.23, N: 19.99,
Found: C: 64.26, H: 4.23, N: 19.98.
Results and discussion
Fig. 2 Color changes of the receptor 1 in DMSO (5.0×10−4 M) in
absence and presence of 10 equiv of anions (from the left to the right: 1
UV-vis spectral titrations
−
only, 1+F− (the color changes of the AcO− and H2PO4 is similar with
F−), 1+ Cl− (the color changes of the I− and Br− is similar with Cl−)
The qualitative evaluation of the anion sensing capability of
1 was performed in DMSO solutions containing a series of
anions, namely, F−, AcO−, H2PO4 , Cl−, Br− and I−.
the results of absorption titration of 1 with F−. The
gradually added the F− leads to the band centered at
447 nm decrease, while the bond centered at 570 nm
increase. The spectra change almost stops upon addition of
40 equiv. of F−, which were ascribed to the intramolecular
charge transfer (ICT) between the electron rich −NH moiety
and the electron deficient −NO2 moiety [34, 35]. Therefore,
the results testified the host interacted with anionic guests
through hydrogen bond, which would affected the elec-
tronic properties of the chromophore so that a new charge
transfer mechanism established between the electron rich
−NH moiety and the electron deficient −NO2 moiety along
with a color change. The clear isosbedtic points at 486 nm
and 384 nm indicate that a single component is produced in
response to the interaction between 1 and F−. In addition,
−
Figure 1 shows changes in UV-vis absorption spectra of 1
(1.0×10−5 M) upon addition of 8 equiv. of respective
anions as the n-tetrabutylammonium (n-bu4N+) salt. As
shown in Fig. 1, without anions, 1 exhibits a strong band
centered at 447 nm, as the concentration of F− increased
stepwise, leads to a gradually decrease in this absorption at
447 nm, along with a new broadly band centered at
570 nm. As seen from Fig. 2, the solution color changes
from little yellow to purple upon addition of F− ions.
Meanwhile, on addition equivalent anions of AcO− and
−
H2PO4 , the solution induces less color changes than of F−.
In contrast, addition of any other halide anions does not
show any spectra and color changes.
To further evaluate anion binding behavior of the
receptor 1, the UV-vis titrations were carried out in DMSO.
A solution of the host 1 (1.0×10−5 M) was added with
various kinds of anions such as fluoride, chloride, bromide,
iodide, dihydrogen phosphate, and acetate. Figure 3 shows
the spectral responses of 1 toward AcO− and H2PO4
−
exhibited similar changes as with F− while the changes is
smaller than that of fluoride ions (see Fig. 3a and b); on the
contrary, the spectral absorption hardly changed upon
addition of a large amount of Cl−, Br− and I− anions.
Consequently, the receptor 1 may be highly selective with
the fluoride anions.
0.6
1, 1 + Cl-, Br-, I-
0.5
Fluorescent titrations
0.4
0.3
The interactions of the anions with the receptor 1 (1.0×
10−5 M) were determined by the fluorescent spectroscopic
titrations in DMSO solution. Figure 4 shows the results of
fluorescence titration (Iex=428 nm) of 1 with F−. Upon
addition of F− leads to continuous decrease in the band
centered at 626 nm, along with an increase in the 515 nm
band and the clear isoemission points at 591 nm indicate
that a component is formed in response to the interaction
between 1 and F−, (See Fig. 4). Interesting, such a
significant blue-shift 0 ∼110 nm emission band shift from
626 to 514 nm was rarely seen in the ever-reported ICT
anion sensors. Meanwhile, the band centered at 514 nm is
1 + 8 equiv.F-
0.2
1 + 8 equiv. AcO-
1 + 8 equiv. H2PO-4
0.1
0.0
300
400
500
600
700
800
Wavelength/nm
Fig. 1 UV-vis spectra spectra of 1 (1×10−5 M) in DMSO in the
presence of 8 equiv of F−, AcO−, H2PO4−, Cl−, Br− and I−

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J Fluoresc (2010) 20:1299–1305
50
40
30
20
10
0
0.7
0 equiv F-
8 equiv F-
0.6
0.5
0.4
0.3
0.2
0.1
0.0
0 equiv. F-
40 equiv.F-
300
400
500
600
700
Wavelength/nm
450
500
550
600
650
700
750
800
a
Wavelength/nm
0.6
0.5
0.4
0.3
0.2
0.1
0.0
Fig. 4 Fluorescent changes of the sensor 1 in DMSO (1×10−5 M)
0 equiv AcO-
100 equiv AcO-
upon addition of F− ions
the anion interacted with the binding site, the excited state
was more strongly destabilized by the anion than the
ground state, and consequently, a blue-shift of the emission
spectra was expected. In addition, the presence of AcO−
−
and H2PO4 induced similar changes in fluorescent spec-
trum with F−, but addition of excess equiv. Cl−, Br− and I−
ions had slight effects on fluorescence intensity.
Determination of affinity constants
300
400
500
600
700
Wavelength/nm
b
To determine the stoichiometry of the host-guest com-
plex, Job plots were obtained according to the method
reported by K.A. Connors [39]. As an example (for F−),
Fig. 5 demonstrates the formation of 1:1 stoichiometry
host-fluoride complex. The association constants of the
host 1 for anionic species, which are shown in Table 1,
0.6
0.5
0.4
0.3
0.2
0.1
0.0
0 equiv H2PO4-
200 equiv H2PO-4
0.50
0.45
0.40
0.35
0.30
300
400
500
600
700
Wavelength/nm
Fig. 3 UV-vis spectral changes of receptor 1 (1.0×10−5 M) upon
addition of fluoride ion (0 to 40 equiv) in DMSO at 298.2 0.1 K. a
UV-vis spectral changes of receptor 1 (1.0×10−5 M) upon addition of
acetate ion (0 to 100 equiv) in DMSO at 298.2 0.1 K. b UV-vis
spectral changes of receptor 1 (1.0×10−5 M) upon addition of
H2PO4−ion (0 to 200 equiv) in DMSO at 298.2 0.1 K
1:9
2:8
3:7
4:5
5:5
6:4
7:3
8:2
9:1
--
shown in the UV-vis absorption spectra of 1 with addition
of the F−. This would indicate that the blue-shift of
fluorescence spectra was caused by a change of the charge
transfer character of the emissive species [36–38]. When
[Anion]/([Host]+[Anion])
Fig. 5 The stoichiometry analysis of complex 1•F− by Job plot
analysis

J Fluoresc (2010) 20:1299–1305
1303
Table 1 Association constants for various anions toward receptor 1 in DMSO at 298.2 0.1 K, respectively
Anionsa
Kass(M−1 b
AcO−
1.09( 0.71)×103
H2PO4
F−
Cl−
NDd
Br−
ND
I−
−
)
0.78( 0.16)×103
3.89( 0.31)×103
ND
a The anions were added as their tetrabutylammonium salts
b Kass was determined in dry DMSO
d ND indicated that the spectra showed little or no change with the addition of anion so that the association constants can not be determined using the
spectra
were determined by the non-linear fitting analyses of the
titration curves according to Eq. 1, which is applied to the
1:1 host-guest complexation and reported by B. Valeur
[40],
global and the smallest atoms radius might be the best to
fit for the sensor 1 with hydrogen-bonding interactions
(Scheme 2) that the receptor 1 has better selectivity and
higher affinity constants with fluoride anions than others
examined by present work.
A ¼ A0 þ ðAlim ꢀ A0Þ
ꢀ
ꢁ
h
i
1=2
2
1H NMR titrations
ꢁ
cH þ cG þ 1=K ꢀ ðcH þ cG þ 1=KÞ ꢀ 4cHcG
=2cH
ð1Þ
1
To further study the nature of host-guest interactions, H
Where, cG and cH are the concentrations of guest and host,
respectively, and A is the intensity of absorbance at the
monitored wavelength at certain concentrations of the host
and the guest. A0 is the intensity of absorbance of host
only and Alim is the maximum intensity of absorbance of
host when gust is added. K is the affinity constant. As
clearly shown from Table 1, the order of selectivity or the
binding affinity of anions for 1 in DMSO is F− > AcO− >
NMR spectra changes upon addition of F− as their
tetrabutylammonium salts to the DMSO-d6 solution of 1
(1×10−2 mol L−1) were investigated. Obviously, the proton
signals at 11.58 ppm and 11.04 ppm, which were assigned
to the −NH moiety of the Indole-3-carboxaldehyde and the
amide of the p-nitrophenylhydrazine, respectively, were
observed in the absence of the fluoride ions (see Fig. 5).
Upon addition of 0.5 equiv. of F−ions, the signals of the NH
moiety were broadened and the signals on phenyl rings
changed slightly. This result indicated that a hydrogen-
bonding complex was formed at this stage. With further
addition of F− ions, the phenyl protons especially for H1
shifted downfieds significantly, due to the increase of the
electron density on the phenyl rings by the through-bond
effect. At the same time, the signals from binding sites of
the receptor 1 downshifted and the proton of the p-
−
H2PO4 >> Cl− ∼ Br− ∼ I−. The main reasons can be
ascribed to the basicity of the guest molecule and the
shape complementarity between the host and the anionic
guests [41]. Particularly, the presence of multiple hydro-
gen bonding interactions are necessary to generate the
high-affinity anion binding sites. Among the anions tested,
−
the basicity of AcO−, F− and H2PO4 anions are stronger
than other anions. Furthermore, the fluoride anions are
Scheme 2 The proposed host–
guest binding mode in
solution

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J Fluoresc (2010) 20:1299–1305
real-life application, respectively. Although, the sensor 1
didn’t induce the UV-vis spectrum, a sample that contained
commercially available toothpaste and F− (as tetrabutylam-
monium salts) was prepared in DMSO solution. Adding
proper amount of the sample to the solution of the receptor
1 in DMSO forming mixture solutions, which were made
up of 0.1 mgmL−1 toothpaste, 4×10−4 mol L−1 F− ions, was
obtained. Then, the UV-vis absorbance method for the
mixture solutions were measured and compared to a
toothpaste-free F− solution. As shown in the Fig. 7, the
intensity containing toothpaste was stronger than that
without it, which is ascribed to fluoride of the toothpaste
so that the receptor 1 may be used in qualitative detecting
fluoride in toothpaste. But detecting fluoride in the protic
solution is called for further research.
3 equiv
2.5 equiv
2 equiv
1.5 equiv
0.5 equiv
0 equiv
H1
12
H2
10
ppm
8
1.5 equiv
Conclusions
1 equiv
In summary, we have successfully designed and synthe-
sized a colorimetric and ratiometric fluorescent anion
sensor which recognizes anions through hydrogen
bonding between −NH protons and F−, AcO− and
0.5 equiv
0 equiv
H2PO4 shows more selective to F− in DMSO solution
−
NH
based on an intermolecular charge transfer mechanism.
The changes in absorption and fluorescence spectra upon
addition F−, AcO− and H2PO4 anions were attributed to
−
13.0
12.5
12.0
11.5
ppm
11.0
10.5
10.0
the conjugation of the host and the guest. For practical
application, the ratiometric sensor displays distinct color
changes for fast and efficient sensing. The anion recogni-
tion and sensing processes are supported by UV–vis
spectral titration, fluorescent titration and 1H NMR
titration. More important, the remarkable photophysical
properties of the sensor would help to extend the
development of fluorescent sensors for biologically inor-
ganic anions.
Fig. 6 1H NMR spectra of the receptor 1 in DMSO-d6 (1×10−2) upon
addition of molar equiv. of F−
nitrophenylhydrazine eventually disappeared with the F− up
to 1.5 equiv., while the −NH proton of the indole shifted
downfields and the peaks broadened, which was ascribed to
the deprotonation of the amide proton that induced the
increase of the electron density on the indole rings through
bond effect. So, the process of anion recognition had two
steps, firstly, the NH binding parts of the p-nitrophenylhy-
drazine formed supermolecule through hydrogen bond,
secondly, the NH proton of the p-nitrophenylhydrazine
was deprotonated by adding fluoride ions. Consequently,
0.1
0.09
0.08
0.07
0.06
0.05
0.04
0.03
0.02
0.01
0
1
the results of the UV-vis titrations and H NMR titrations
have testified the above supposition of the H-bond
interactions between the host and the F− ions (Fig. 6).
Analytical applications
1
2
As a good anion sensor, the high selectivity for anions in
the analyte has great interest over the practical analytical.
Recently, inspired by Koide and coworkers [42, 43] who
reported qualitative detection of palladium and fluoride in
Fig. 7 The proof of concept for fluoride detection in toothpaste (1)
sensor + tetrabutylammonium fluoride; (2) sensor + tetrabutylammo-
nium fluoride + toothpaste (_A = Ahost + guest − Ahost only)

J Fluoresc (2010) 20:1299–1305
1305
Acknowledgments This project was supported by the National
22. Gale PA (2005) Chem Commun 3761–3772
Natural Science Foundation of China (20371028, 20671052).
23. Pfeffer FM, Gunnlaugsson T, Jensen P, Kruger PE (2005) Org Lett
7:5357
24. Suk J-M, Chae MK, Kim N-K, Kim U-L, Jeong K-S (2008) Pure
Appl Chem 80:599–608
25. Zielínski T, Dydio P, Jurczak J (2007) Tetrahedron 64:568–
574
References
1. Beer PD, Gale PA (2001) Angew Chem 113:502–32
2. Beer PD, Gale PA (2001) Angew Chem Int Ed 40:486–516
3. Martínez-Mañez R, Sancenón F (2003) Chem Rev 103:4419–76
4. Kubik S, Reyheller C, Stüwe S (2005) J Incl Phenom Macrocycl
Chem 52:137–87
26. Chmielewski MJ, Zhao L, Brown A, Curiel D, Sambrook MR,
Thompson AL, Santos SM, Felix V, Davis JJ, Beer PD (2008)
Chem Commun 3154–3156
27. Piatek P, Lynch VM, Sessler JL (2004) J Am Chem Soc
126:16073–16076
5. Gunnlaugsson T, Glynn M, Tocci GM, Kruger PE, Pfeffer FM
(2006) Coord Chem Rev 250:3094–117
28. Pfeffer FM, Lim KF, Sedgwick KJ (2007) Org Biomol Chem
5:1795–1799
6. Shao J, Yu M, Lin H, Lin HK (2008) Spectrochim Acta A Mol
Biomol Spectrosc 70:1217–1221
7. Ayoob S, Gupta AK (2006) Crit Rev Environ Sci Technol
36:433–487
8. Liu B, Tian H (2005) J Mater Chem 15:2681–2686
9. Gunnlaugsson T, Davis AP, O’Brien JE, Glynn M (2002) Org Lett
4:2449–2452
10. Grases F, March JG (1990) Anal Chim Acta 229:249–254
11. Baird C, Cann M (2005) Environmental chemistry. Freeman, New
York
29. Gale PA (2008) Chem Commun 4525–4540
30. Peng X, Wu Y, Fan J, Tian M, Han KJ (2005) Org Chem
70:10524–10531
31. Zhang Y, Guo X, Si W, Jia L, Qian X (2008) Org Lett 10:473–476
32. Yang H, Yi T, Zhou Z, Zhou Y, Wu J, Xu M, Li F, Huang C
(2007) Langmuir 23:8224–8230
33. Shao J, Lin H, Lin HK (2008) Talanta 75:1015–1020
34. Shao J, Lin H, Yu M, Lin HK (2008) Talanta 75:551–555
35. Shao J, Lin H, Shang XF, Chen HM, Lin HK (2007) J Incl
Phenom Macrocycl Chem 59:371–375
12. Pu L (2004) Chem Rev 104:1687–1716
36. Xu Z, Xiao Y, Qian X, Cui J, Cui D (2005) Org Lett 889–892
37. Valeur B (2001) Molecular fluorescence: principles and applica-
tions. Weinheim, NewYork, pp 298–299
38. Shao J, Lin H, Lin HK (2008) Talanta 77:273–277
39. Connors KA (1987) Binding constants: the measurement of
molecular complex stability. Wiley, New York, pp 25–28
40. Bourson J, Pouget J, Valeur B (1993) J Phys Chem 97:4552–4556
41. Korendovych IV, Cho M, Butler PL, Staples RJ, Rybak-Akimova
EV (2006) Org Lett 8:3171–3174
42. Palacios MA, Nishiyabu R, Marquez M, Anzenbacher P Jr (2007)
J Am Chem Soc 129:7538–7544
43. Song F, Garner AL, Koide K (2007) J Am Chem Soc 129:12354–
12355
13. de Silva AP, Gunarame HQN, Habib-Jiwan J, McCoy CP, Rice
TE, Soumillion J (1995) Angew Chem Int Ed 34:1728–1731
14. Roland K (1998) Angew Chem Int Ed 37:772–773
15. Gunnlaugsson T, Leonard JP, Murray NS (2004) Org Lett 6:1557–
1560
16. Bondy CR, Loeb SJ (2003) Coord Chem Rev 240:77–99
17. Chmielewski MJ, Jurczak J (2005) Chem Eur J 11:6094
18. Sessler JL, Camiolo S, Gale PA (2003) Coord Chem Rev 240:17–55
19. Kang SO, Llinares JM, Powell D, VanderVelde D, Bowman-
James K (2003) J Am Chem Soc 125:10152–10153
20. Beer PD, Stokes SE (1995) Polyhedron 14:873–879
21. Brooks SJ, Gale PA, Light ME (2006) Chem Commun 4344–4346