A new Na+ sensor based on intramolecular fluorescence energy transfer derived from calix[4]arene

Article abstract of DOI:10.1039/a907457k

A new calix[4]arene having pyrene (as a donor) and anthroyloxy (as an acceptor) moieties at the lower rim has been synthesized as a fluorescent Na⁺ sensor based on intramolecular energy transfer.

Full text of DOI:10.1039/a907457k

A new Na+ sensor based on intramolecular fluorescence energy transfer
derived from calix[4]arene
Takashi Jin
Section of Intelligent Materials and Devices, Research Institute for Electronic Science, Hokkaido University, Sapporo
060-0812, Japan. E-mail: jin@imd.es.hokudai.ac.jp
Received (in Liverpool, UK) 14th September 1999, Accepted 1st November 1999
A new calix[4]arene having pyrene (as a donor) and
anthroyloxy (as an acceptor) moieties at the lower rim has
been synthesized as a fluorescent Na+ sensor based on
intramolecular energy transfer.
Calixarenes have been shown to be useful building blocks in the
design of fluorescent ion sensors for detecting metal ions and
anions.1–8 There are several designs of calixarene-based
fluorescent ion sensor in which ion complexation can be
monitored by intramolecular pyrene excimer formation,1,2
intramolecular fluorescence quenching3 and photoinduced
electron transfer.6–8 The objective of this work is to apply
fluorescence energy transfer to the metal ion sensing system
based on calixarenes. An advantage of the metal ion sensing
system using fluorescence energy transfer is that a variety of
fluorophore pairs can be chosen for the design of fluorescent ion
sensors. Herein we report a new Na+ sensor, based on
intramolecular fluorescence energy transfer, derived from
calix[4]arene.
Fluorescence energy transfer is the transfer of the excited
state energy from a donor (d) to an acceptor (a). This transfer
occurs as a result of transition dipole–dipole interactions
between a d–a pair.9 Thus, if a d–a pair is introduced to the
appropriate sites of ionophoric calixarenes,10 it is expected that
the ion complexed by the calixarenes can be monitored by the
change in the yield of fluorescence energy transfer between the
d–a pair. In view of this, we have introduced pyrene (as a donor)
and anthroyloxy (as an acceptor) moieties to the terminal
positions of the ion binding sites (RCO-)10 of ionophoric
calix[4]arenes.
Fluorescent calix[4]arene 3 was prepared in three steps from
the parent calix[4]arene. p-tert-Butylcalix[4]arene was di-
alkylated using 2 equiv. ethyl bromoacetate and 2 equiv. of
K2CO3 in THF. The remaining two phenolic groups were
functionalized firstly with 1.5 equiv. 1-pyrenemethyl iodoace-
tate and 2 equiv. K2CO3, and then with 1.5 equiv. p-
(9-anthroyloxy)phenacyl bromide and 2 equiv. K2CO3 in THF.
Mono-fluorophore calix[4]arenes 1 and 2 were prepared by the
reaction of the tris-ethoxycarbonylmethyl ether11 of p-tert-
butylcalix[4]arene with the fluorescent reagents described
1
above. All compounds were identified by H NMR and field
desorption mass spectroscopy.† The 1H NMR measurements of
the ArCH2Ar protons confirmed that the calixarenes 1–3 have
rigid cone conformations in CDCl3 solution.
Energy transfer from a donor to an acceptor depends on the
extent of overlap of the fluorescence spectrum of the donor and
the absorption spectrum of the acceptor.9 Fig. 1 shows the
fluorescence spectrum of 1 and the absorption spectrum of 2 in
THF. The fluorescence spectrum of 1 is characterized by the
pyrene monomer emission (ca. 385 nm). The absorption
spectrum of 2 results from the absorption of the anthroyloxy
group (lmax 363 nm). Thus, the overlap between the emission of
1 and the absorption of 2 suggests that intramolecular
fluorescence energy transfer from the pyrene to the anthroyloxy
group can take place in 3.
To verify the intramolecular energy transfer from the pyrene
to the anthroyloxy group in 3, we measured the fluorescence
spectra of 1, 2 and 3 at identical concentrations of 5.0 mmol
Fig. 1 Spectral overlap between the fluorescence spectrum of 1 and the
absorption spectrum of 2.
Chem. Commun., 1999, 2491–2492
This journal is © The Royal Society of Chemistry 1999
2491

Fig. 3 Na+ titration of the fluorescence spectra of 3 (5.0 mmol dm23) in
MeOH–THF (15+1 v/v) at 25 °C: a, [NaSCN] = 0; b, [NaSCN] = 5.8; c,
[NaSCN] = 17.4; d, [NaSCN] = 46.3; e, [NaSCN] = 104; f, [NaSCN] =
218 mmol dm23
.
Fig. 2 Fluorescence spectra (ex. 330 nm) of 1, 2 and 3 in THF at 25 °C.
calix[4]arene-based Na+ sensor which can be excited by visible
light along the same design principles. From the standpoint of
the practical application of fluorescent ion sensors, the visible-
light excitation is desirable to avoid a breaching of fluorescent
dyes introduced to sensor compounds. We believe that a variety
of donor–acceptor fluorophore pairs can be used for the design
of calixarene sensors based on fluorescence energy transfer.
dm23 (Fig. 2). At this concentration, 1 shows only the pyrene
monomer emission; the intermolecular excimer emission is
negligible. The fluorescence spectrum of 2 shows a broad,
structureless band (lmax 480 nm), which does not have an
anthracene monomer-like structure.12 It is well known that the
9-anthroyloxy group affords a large Stokes’ shift of the broad
emission which is a consequence of an excited-state rotation of
the carboxyl group into the plane of the anthracene ring.12 If
energy transfer takes place in 3, the emission of the anthroyloxy
group (as an acceptor) should be enhanced compared with that
of 2, and the emission of the pyrene monomer (as a donor)
should be depressed compared with that of 1. As expected, the
emission intensity of the anthroyloxy group of 3 increased by a
factor of ca. 2.5 compared with 2, while the emission intensity
of the pyrene monomer of 3 decreased by a factor of ca. 10
compared with that of 1. It should be noted that the emission of
the anthroyloxy group of 3 has its maximum at the same
wavelength (lmax 480 nm) as that due to this group in 2. This
provides evidence that the anthroyloxy group is not forming an
intramolecular exciplex with the pyrene in 3.
1
We thank Mr E. Yamada for the measurement of H NMR
spectra.
Notes and references
† Selected data for 3: 1H NMR (400 MHz, CDCl3), d 0.98 (s, 18H, CMe3),
1.08 (t, 6H, CH2CH3), 1.17 and 1.19 (s, 9H, CMe3), 3.14, 3.26, 4.85 and
5.02 (d, 2H, ArCH2Ar, J 13.0 Hz), 4.05 (m, 4H, CH2CH3), 4.73 (s, 4H,
OCH2O), 5.09 (s, 2H, OCH2OCH2Py), 5.83 (s, 2H, CH2COPh), 5.87 (s, 2H,
CH2Py), 6.62 and 6.69 (d, 2H each, ArH, J 2.4 Hz), 6.86 and 6.95 (s, 2H,
ArH), 7.45–8.62 (m, 18H, anthracene and pyrene); field desorption mass
spectrum, m/z 1431 (M+).
‡ Since the absorption spectrum of 3 did not change upon addition of Na+
and K+ ions, a simple relationship between the intensity change (DF) in the
anthroyloxy emission after complexation with metal ions and the concentra-
tion ([M+]) of the metal ion can be derived as follows: 1/DF = c + cKd/
[M+], where Kd and c represent the dissociation constants of the metal ion
complexes and a constant including terms of the quantum yields of free and
complexing species, respectively.
The effects of addition of NaSCN to 1 and 2 were first
examined in MeOH–THF (15+1 v/v) solution. The emissions of
1 and 2 were only slightly affected by the presence of Na+ ions.
The emission of the pyrene monomer of 1 (5.0 mmol dm23) at
385 nm was enhanced by a factor of 8% in the presence of
excess amounts of NaSCN (454 mmol dm23). In the case of 2
(5.0 mmol dm23), the emission of the anthroyloxy group at 480
nm was depressed by a factor of 3% under the same NaSCN
concentration. The fluorescence responses of these calix[4]ar-
enes toward Na+ ions are very poor and they have no potential
as Na+ sensors. In contrast, the fluorescence spectra of 3 showed
a significant change in the presence of Na+ and K+ ions. Fig. 3
shows Na+ titrations of the fluorescence spectra of 3 in MeOH–
THF (15+1 v/v). When NaSCN was added to the solution of 3,
the fluorescence intensity of the anthroyloxy group increased
significantly compared with that of the pyrene monomer. Such
a fluorescence change was also observed in the case of KSCN.
In contrast, the addition of other alkali metal ions such as Li+,
Rb+ and Cs+ did not cause fluorescence changes at concentra-
tions as high as 50 mmol dm23. From the Na+ titration data, the
dissociation constant of the Na+–3 complex was determined to
be 15 mmol dm23.‡ The Na+/K+ selectivity evaluated from the
dissociation constants was found to be ca. 59. For Li+, Rb+ and
Cs+, the affinities were too low to be determined accurately and
the dissociation constants were estimated to be greater than 100
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mmol dm23
.
In conclusion, we have synthesized a first example of a Na+
sensor based on fluorescence energy transfer. The next step of
this investigation will be the synthesis of a fluorescent
Communication 9/07457K
2492
Chem. Commun., 1999, 2491–2492