Highly sodium-selective fluoroionophore based on conformational restriction of oligoethyleneglycol-bridged biaryl boron-dipyrromethene

Article abstract of DOI:10.1021/ja042414o

A non-PET and non-PCT fluoroionophore has been designed and synthesized based on the combination of a biaryl boron-dipyrromethene fluorophore with an oligoethyleneglycol bridge. A specific red-shift response of absorption for NaClO₄ in acetonitrile was demonstrated based on conformational restriction triggered by cation recognition at the bridge. The concentration of Na⁺ can be determined accurately using fluorescence due to a high extinction coefficient (ε = 50000), a high fluorescence quantum yield (Φ_f = 0.68), and the sharpness of absorption and emission peaks of the boron-dipyrromethene derivative. Copyright

Full text of DOI:10.1021/ja042414o

Published on Web 04/26/2005
Highly Sodium-Selective Fluoroionophore Based on Conformational
Restriction of Oligoethyleneglycol-Bridged Biaryl Boron-Dipyrromethene
Koji Yamada,† Yuki Nomura,† Daniel Citterio,† Naoko Iwasawa,† and Koji Suzuki*,†,‡,§
Department of Applied Chemistry, Faculty of Science and Technology, Keio UniVersity, 3-14-1 Hiyoshi, Kohoku-ku,
Yokohama 223-8522, Japan, Kanagawa Academy of Science and Technology (KAST), KSP West 614, 3-2-1 Sakado,
Takatsu-ku, Kawasaki 213-0012, Japan, and Core Research for EVolutional Science and Technology (CREST),
JST Agency, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan
Received December 17, 2004; E-mail: suzuki@applc.keio.ac.jp
Fluoroionophores, by changing their optical properties upon
complexation with certain ions, have a great potential for practical
applications in the field of analytical chemistry, environmental
chemistry, life science, and other fields.1 Especially, fluoroiono-
phores for sodium ions have been extensively required to sensitively
monitor this important and widespread cation in nature. Among
the reported Na+ sensing molecules, crown ether derivatives2 that
form a rigid cavity around Na+ (e.g., N-(o-alkoxyphenyl)-aza-15-
crown-5 and 16-crown-5) and calix[4]arenes3 that have a flexible
Figure 1. Chemical structure of boron-dipyrromethene fluoroionophore
1.
cavity with bulky substituents have been used as a receptor.
absorption wavelength can also be changed by the orientation of
Fluorophores with a fluorescent switching property driven by
the aryl substituent. Utilizing the spectral shift property, 3,5-biaryl
photoinduced electron transfer (PET) are often employed to signal
boron-dipyrromethene with the dihedral angle controlled by
metal binding.4 However, the sensitivity and selectivity of the Na+
molecular recognition will be used as an optical sensor. As the
fluoroionophores with the receptor connected to a PET-type
first step toward the realization of a highly sensitive fluorescent
fluorophore are still inadequate due to their poor fluorescent
sensor based on a conformational restriction, we have designed and
switching and the interference of other ions, such K+. To improve
synthesized a fluoroionophore by combining the biaryl boron-
the selectivity and sensitivity to Na+ ions, we have been searching
dipyrromethene fluorophore with an oligoethyleneglycol bridge
for a new binding site with a different spectral change mechanism.
acting as a binding site for metal cations.
Finney et al. designed and synthesized a series of interesting
The chemical structure of the target fluoroionophore 1 is shown
fluoroionophores consisting of a rotatable biaryl fluorophore and a
in Figure 1. To convert the metal binding event into a change in a
bridged oligoethyleneglycol chain.5 On the basis of the conforma-
dihedral angle, the oligoethyleneglycol bridge is introduced between
tional restriction of the rotation triggered by cation recognition at
each of the m-position of the 3- and 5-phenyl rings. A branched
the chain, the fluorescence intensity varies with the ion species and
alkyl chain at the p-position of the 8-phenyl group is attached to
concentration. However, the sensitivity is impractical due to a poor
increase the solubility in organic solvents and the synthetic yield.8
extinction coefficient and fluorescence quantum yield. Assuming
Furthermore, methyl groups at the o-positions are introduced to
that the spectral change mechanism based on the conformational
enhance the florescence quantum yield.9
restriction is applied to high-performance fluorophores, the sensitiv-
ity would be favorable.
Boron-dipyrromethene derivatives are useful fluorophores be-
cause of their advantageous characteristics, such as sharp absorption
and fluorescence bands, high extinction coefficients, high fluores-
cence quantum yields, and high stability against light and chemical
The absorption and emission spectra were recorded in acetoni-
trile. The ion-free fluoroionophore 1 has sharp absorption and
emission bands with a maximum at 551 and 589 nm, respectively.
The wavelengths are almost the same as those of the 3,5-diphenyl
boron-dipyrromethene derivatives.9 It suggests that the electronic
effect of the substituent group at the m-positions scarcely affects
reactions.6 In addition, they have the advantage that the absorption
the spectra, and the oligoethyleneglycol bridge hardly alters the
and emission wavelengths are adjustable by replacing appropriate
dihedral angle. The spectral response to the addition of alkali ions
substituents. However, their wavelengths are not only modulated
was measured using the corresponding perchlorate salt. The
by the electronegativity of the substituents and the extension of
absorption spectra of 1 (5.0 × 10-5 M) with 1000 equiv of added
the π-system. Among the various 3,5-substituted boron-dipyr-
romethene derivatives reported by Burgess et al., we have noted
salt are shown in Figure 2. The addition of NaClO4 leads to a
characteristic shift in the absorption maximum (λmax) to a longer
that the absorption maxima of the 3,5-bis(2-methoxyphenyl) and
wavelength by 12 nm. As for the other alkali ions, the more their
3,5-bis(1-naphthyl) derivatives are shifted to shorter wavelengths,
ionic radius differs from the one of Na+, the lesser the red shift. In
regardless of the incorporation of electron-donating groups and the
titration experiments, the absorption spectra of 1 with various ion
concentrations showed an isosbestic point, and their data were
linearly fit using Benesi-Hildebrand plots.10 The results indicate
extension of the π-system, respectively.7 The reason is that the
dihedral angles between the dipyrromethene plane and the aryl
planes are greater due to steric hindrance so that the orbital
interaction becomes weaker. However, as mentioned above, the
that the fluoroionophore 1 selectively forms a 1:1 complex with
the alkali ion. The binding constant of 1 with Na+ was estimated
to be 1560 mol-1 dm3, while that with K+, which is the main
† Keio University.
interference in biological measurements, was found to be 280 mol-1
‡ KAST.
§ CREST, JST Agency.
dm3. Although the selectivity of 1 for Na+ over K+ is low, only
9
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10.1021/ja042414o CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
In conclusion, we have revealed that the fluoroionophore 1 shows
a specific red-shift response for the Na+ ion. The photophysical
properties of 1, which are the sharpness of the absorbance band,
the high extinction coefficient, and the high fluorescence quantum
yield, have been found to be advantageous for quantitative analyses.
We believe that the conformational restriction approach can be
extended to optical sensors for neutral molecules because the push-
pull interactions between the host and guest are unnecessary for
signal transduction, unlike that for PET and intramolecular charge
transfer (ICT). Unfortunately, the fluoroionophore 1 is hardly
soluble in water, as well as most of fluoroionophores, including
Finney’s molecules. However, a hydrophilic substituent in the place
of the branched alkyl chain at the 8-position may render the sensor
molecules useful in aqueous solution.11 The next step of this
investigation will be to enhance the wavelength shift using rigid
recognition sites, as well as the application for anions and neutral
molecules.
Figure 2. Response of absorption to added alkali salt.
Acknowledgment. We thank Prof. T. Sugawara and H. Segawa
for the fluorescence quantum yield measurements.
Supporting Information Available: Preparative procedures and
analytical data for compound 1 and its intermediates; Benesi-
Hildebrand plots of 1 with Na+ and K+ (PDF). This material is available
free of charge via the Internet at http://pubs.acs.org.
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