BODIPY-based ratiometric fluoroionophores with bidirectional spectral shifts for the selective recognition of heavy metal ions

Article abstract of DOI:10.1246/bcsj.20120235

Two novel asymmetric BODIPY fluoroionophores with dipicolylamine (BDP-DPA, dipicolylamine: bis(pyridylmethyl)) and terpyridine (BDP-TPY) are described. These fluoroionophores display opposite wavelength responses on complexation with heavy metal ions. Furthermore, the fluorescence spectra vary depending on the ionic species. In particular, BDP-DPA shows a high affinity toward Cr3+ and upon complexation, the fluorescence spectrum blue-shifts from 591 to 566 nm. In contrast, BDP-TPY preferentially binds to Zn2+ and the fluorescence spectra red-shifts from 539 to 567 nm. BDP-TPY is the first example of asymmetric BODIPY with a pyridyl receptor at the 3 position showing redshifted fluorescence by complexation with metal ions. The concentration of each metal ion was successfully determined by ratiometric measurement. The wavelength-responses characteristics of these fluoroionophores could be very useful in the development of novel ratiometric fluoroionophores for metal ions.

Full text of DOI:10.1246/bcsj.20120235

© 2013 The Chemical Society of Japan
Bull. Chem. Soc. Jpn. Vol. 86, No. 1, 37-44 (2013)
37
Selected Papers
BODIPY-Based Ratiometric Fluoroionophores with Bidirectional
Spectral Shifts for the Selective Recognition of Heavy Metal Ions
Akira Hafuka,1 Hiroki Taniyama,1 Sang-Hyun Son,2 Koji Yamada,2
Masahiro Takahashi,1 Satoshi Okabe,1 and Hisashi Satoh*1
1Division of Environmental Engineering, Faculty of Engineering, Hokkaido University,
North-13, West-8, Kita-ku, Sapporo, Hokkaido 060-8628
2Section of Materials Science, Faculty of Environmental Earth Science, Hokkaido University,
North-10, West-5, Kita-ku, Sapporo, Hokkaido 060-0810
Received August 30, 2012; E-mail: qsatoh@eng.hokudai.ac.jp
Two novel asymmetric BODIPY fluoroionophores with dipicolylamine (BDP-DPA, dipicolylamine: bis(pyridyl-
methyl)) and terpyridine (BDP-TPY) are described. These fluoroionophores display opposite wavelength responses on
complexation with heavy metal ions. Furthermore, the fluorescence spectra vary depending on the ionic species. In
particular, BDP-DPA shows a high affinity toward Cr3+ and upon complexation, the fluorescence spectrum blue-shifts
from 591 to 566 nm. In contrast, BDP-TPY preferentially binds to Zn2+ and the fluorescence spectra red-shifts from 539
to 567 nm. BDP-TPY is the first example of asymmetric BODIPY with a pyridyl receptor at the 3 position showing red-
shifted fluorescence by complexation with metal ions. The concentration of each metal ion was successfully determined
by ratiometric measurement. The wavelength-responses characteristics of these fluoroionophores could be very useful in
the development of novel ratiometric fluoroionophores for metal ions.
The conservation and protection of the natural environment
from trace contaminants are becoming increasingly important,
and in particular, heavy metals pose severe risks to human
health and to the environment because of their toxicity.1 For
instance, chromium (Cr) is a common heavy metal contami-
nant used throughout the world, and its compounds can have
genotoxic effects on humans and increase the risk of lung
cancer.2 It originates primarily from industrial uses, such as
electroplating, metal finishing, and leather tanning.3 Zinc (Zn)
is also one of the major heavy metal pollutants because of its
widespread industrial applications in chemical and alloyed
products, fabricated metal products, and paper products.4 Zn
compounds have hazardous effects not only on humans but
also on fish and plants.5 Currently, the most common analytical
methods for heavy metals are atomic absorption spectrometry
(AAS) and inductively coupled plasma (ICP) spectroscopy.6,7
Although precise, these analytical instruments are expensive
and often require complex sample preparation. In addition,
they are not applicable to continuous on-site monitoring and
measure only the total concentration of heavy metals. There-
fore, an inexpensive and simple method is needed for deter-
mining levels of heavy metal ions, which is appropriate for
monitoring industrial and environmental samples.
ions have been reported.9-11 In terms of sensitivity, fluoroiono-
phores that exhibit ratiometric spectral changes induced by
complexation with heavy metal ions are more favorable than
those exhibiting only fluorescence enhancement (“turn-on”) or
fluorescence quenching (“turn-off”). Ratiometric fluoroiono-
phores provide more detailed information about the analyte
in a sample, as well as allowing reliable measurements of
the analyte concentration, as the ratio of the fluorescence inten-
sities at two wavelengths is independent of fluctuations of the
source light intensity and sensitivity of the instrument.12 How-
ever, the development of ratiometric fluoroionophores is still
challenging.
Our concept for the ratiometric measurement of heavy metal
ions is based on the synthesis of an asymmetric 4,4-difluoro-4-
bora-3a,4a-diaza-s-indacene (BODIPY) fluorophore which has
an ion receptor at the 3 position. BODIPY fluorophores pos-
sess many valuable characteristics, such as sharp and intense
absorption and fluorescence bands, high fluorescence quantum
yields, high molar absorption coefficients, and good photo-
chemical stability.13 Furthermore, since BODIPY fluorophores
are amenable to structural modification, controlling the sub-
stituent pattern allows for changes in the wavelength of the
absorption and fluorescence spectrum.14 Many BODIPYs for
cations have been reported,15-22 usually containing an amino
group as a cation receptor at the 3 position of the BODIPY
core. However, there are no studies on asymmetric BODIPY,
which has a pyridyl ion receptor at the 3 position. In this study,
we present 2,2¤:6¤,2¤¤-terpyridine-substituted BODIPY (BDP-
Fluorescence spectroscopy is a desirable method for quanti-
fying heavy metal ions because of its high sensitivity, opera-
tional simplicity, and versatile instrumentation.8 The design
and development of novel fluoroionophores remain an active
area of research, and various fluoroionophores for heavy metal
Published on the web December 29, 2012; doi:10.1246/bcsj.20120235

38
Bull. Chem. Soc. Jpn. Vol. 86, No. 1 (2013)
Fluoroionophores for Heavy Metal Analysis
Scheme 1. Synthesis of BDP-DPA and BDP-TPY.
especially Zn2+ 26,27
In our synthetic scheme, the metal-ion
TPY) and di(2-picolyl)amine-substituted BODIPY (BDP-
DPA, di(2-picolyl): bis(pyridin-2-ylmethyl)) as fluoroiono-
phores for heavy metal ions. Zn2+ and Cr3+ ion concentrations
were successfully determined by ratiometric measurement
by using BDP-TPY and BDP-DPA, respectively. Interestingly,
these fluoroionophores show opposite wavelength responses
toward metal ions. Development of such diversified sensing
fluoroionophores allows for identification of several heavy
metal ions.
.
receptor was directly linked to the 3 position of the BODIPY
core using the Suzuki-Miyaura cross-coupling reaction. This
synthetic method facilitates the production of not only an
amino-group but also a pyridyl-group functional BODIPY
using the common compound 1.
Fluorescence and colorimetric titration experiments of BDP-
DPA with different metal ions were carried out to clarify its
ion sensing ability. Figure 1 shows the fluorescence spectra of
BDP-DPA (1 ¯M) measured with and without each respective
metal ion (500 equiv). Without the ions, BDP-DPA showed a
magenta fluorescence around 591 nm with a low fluorescence
quantum yield (Φ = 0.13). The possible influence of interfer-
Results and Discussion
BDP-DPA and BDP-TPY were synthesized according to the
sequences summarized in Scheme 1. We chose two types of
heavy metal ion receptors, namely di(2-picolyl)amine (DPA)
and 2,2¤:6¤,2¤¤-terpyridine (TPY), both of which are popular
metal-ion receptors. However, their chemical properties are
very different, that is, DPA is a strong electron-donating group
in intramolecular charge-transfer systems, while TPY is a
heterocyclic moiety in metal-to-ligand charge-transfer systems.
DPA is well known as a chelator of metal ions and is widely
used for the design of new fluoroionophores.10,17,20,23-25 The
terpyridine unit is an excellent general metal-ion binder. It is
known that the terpyridine unit is able to coordinate with
several kinds of metal ions with high binding constants
ing ions was also investigated. Upon addition of Cr3+, Fe2+
,
Fe3+, Zn2+, Cd2+, Hg2+, or Pb2+, the spectrum shifted to
shorter wavelengths. Interestingly, the extent of the spectral
shift could be divided into three patterns, and fluorescence
intensities varied depending on the ion species. Zn2+ and Cd2+
induced large hypsochromic shifts of the fluorescence band
from 591 to 547 nm for Zn2+ and to 549 nm for Cd2+. In
comparison, Cr3+, Fe2+, Fe3+, and Pb2+ showed relatively
small blue shifts from 591 to 566 nm. Hg2+ exhibited blue-
shifted fluorescence at 560 nm. The blue-shifted fluorescence
can be explained by an intramolecular charge-transfer (ICT)

A. Hafuka et al.
Bull. Chem. Soc. Jpn. Vol. 86, No. 1 (2013)
39
Cd2+
Zn2+
18
15
Cr3+
Fe2+
Pb2+
Fe3+
12
9
300 equiv
[Cr3+
]
0
6
3
Hg2+
0
-5
-4
-3
1x10
1x10
1x10
BDP-DPA
[Cr3+]
540
590
640
690
Wavelength/nm
540
590
640
690
Figure 1. Fluorescence spectra of BDP-DPA in the pres-
ence of different metal ions (500 ¯M) in aqueous aceto-
nitrile solution (acetonitrile/water = 9/1, v/v) with exci-
tation at 535 nm. The concentration of BDP-DPA was
1 ¯M.
Wavelength/nm
Figure 2. Changes in the fluorescence spectrum of BDP-
DPA with Cr3+ and plot of the fluorescence intensity ratio
(F566/F646) of BDP-DPA versus increasing Cr3+ concen-
tration. Spectra are for Cr3+ concentrations of 10, 20, 30,
33, 35, 36, 38, 39, 40, 44, 50, 70, 100, 200, and 300 ¯M.
Spectra were acquired in aqueous acetonitrile solution
(acetonitrile/water = 9/1, v/v) with excitation at 535 nm.
The concentration of BDP-DPA was 1 ¯M.
mechanism.28 When a fluoroionophore contains an electron-
donating group (e.g., amino group) linked to a fluorophore,
it undergoes ICT from the donor moiety to the fluorophore,
resulting in red-shifted fluorescence.12 Conversely, when
coordinated with an ion, the amino group loses its electron-
donating ability, and consequently ICT is inhibited and the
fluoroionophore exhibits blue-shifted fluorescence. The fluo-
rescence quantum yield of a fluoroionophore always increases
in this process. These spectral changes allow ratiometric fluo-
rescence measurements of the concentration of a metal ion.
The ratiometric measurement of the fluorescence intensities
at two appropriate fluorescence wavelengths provides a more
reliable measurement of the analyte concentration than the
measurement of the intensity at only a single wavelength. In
the absorption spectra, BDP-DPA showed a sharp and strong
absorption band around 549 nm (¾ = 56000) in the same
solvent system (Table S1). Upon interaction with 500 equiv of
metal ions, a distinct change in the absorption spectrum of
BDP-DPA was observed. Upon addition of heavy or transition-
metal ions, the spectra showed a hypsochromic shift. In par-
ticular, the absorption band of BDP-DPA at 549 nm underwent
a large hypsochromic shift to 517 nm upon addition of Cu2+
(Table S1).
increasing concentration of Cr3+, while the fluorescence inten-
sity at 646 nm was unchanged. Therefore, the ratio of the
fluorescence intensities at 566 to 646 nm (F566/F646) was
calculated for ratiometric analysis. The F566/F646 changed as
the Cr3+ concentration was varied. When the Cr3+ concen-
tration was 500 equiv, the F566/F646 increased to 17.0, and the
fluorescence color changed from magenta to yellow. As shown
in Figure 2, the sigmoidal plot of F566/F646 versus the Cr3+
concentration gives the useful range for quantitative determi-
nation of Cr3+. The LOD and LOQ were 3.2 © 10¹6 and 1.1 ©
10¹5 M for Cr3+ in this aqueous solvent system, respectively.
Moreover, the binding constant could be determined using
Benesi-Hildebrand plots (Figure S1).30 The result suggested
a 1:1 stoichiometry for the BDP-DPA/Cr3+ complex and
the binding constant to be 3.94 © 104 M for Cr3+, which is
¹1
relatively high compared to other Cr3+ fluoroionophores.31-35
The selectivity of BDP-DPA to Cr3+ was investigated with
some common ions. Environmentally important metal ions,
such as Na+, Mg2+, K+, Ca2+, and Mn2+ did not increase the
fluorescence intensity at 566 nm, which originates from the
complex of BDP-DPA with Cr (Figure 3). The signal increased
notably in other ion solutions. However, when Cr3+ was added
to these solutions, F566 increased further. This indicates that
BDP-DPA has a high affinity for Cr3+. Fe2+and Fe3+ are the
main competitive ions toward BDP-DPA as a Cr3+ fluoroiono-
phore because the fluorescence and absorption bands of these
ion solutions overlap with those of Cr3+. This result indicated
that interaction between Fe ion and lone pairs of nitrogen
atoms in a DPA unit may be similar to that of Cr3+. However,
the complex of Cr3+ and BDP-DPA showed slightly higher
fluorescence intensity than that of Fe2+ and Fe3+. Cu2+ showed
a fluorescence quenching effect on BDP-DPA, which has often
been found for other metal-ion fluoroionophores due to energy/
electron transfer (e.g., paramagnetic Cu2+).10,11,17,22,25,26,31,36
The fluorescence spectra of the concentration variable
titration of BDP-DPA with Cr3+ in an aqueous acetonitrile
solution are shown in Figure 2. Upon addition of Cr3+ to the
solution, the fluorescence band of BDP-DPA shifted hypso-
chromically by 25 nm (from 591 to 566 nm). The fluorescence
quantum yield increased from 0.13 to 0.48. An isoemission
point was found at 646 nm, which renders the fluoroiono-
phore useful for ratiometric measurements. To the best of our
knowledge, this is only the second example of ratiometric
sensing of Cr3+ 29
The spectral change was almost terminated
.
by the addition of 500 equiv of Cr3+. Similar changes were
observed in the absorption spectra. When Cr3+ was added to
the solution, the maximum absorbance shifted slightly from
549 to 537 nm with an isosbestic point at 542 nm (Table S1).
The fluorescent intensity at 566 nm significantly increased with

40
Bull. Chem. Soc. Jpn. Vol. 86, No. 1 (2013)
Fluoroionophores for Heavy Metal Analysis
500
400
300
200
100
0
8
6
4
2
0
0
[Zn2+
]
10 equiv
-7
-6
-5
1x10
1x10
[Zn2+]
1x10
Figure 3. Changes in the fluorescence intensity (F566) of
BDP-DPA upon the addition of different metal ions.
Spectra were acquired in aqueous acetonitrile solution
(acetonitrile/water = 9/1, v/v). White bars represent the
addition of an excess of the appropriate metal ion (1 mM
for Na+, Mg2+, K+, and Ca2+. 500 ¯M for all other ions) to
a 1 ¯M solution of BDP-DPA. Black bars represent the
subsequent addition of 500 ¯M of Cr3+ to the solution.
Excitation was provided at 535 nm.
500
550
600
650
Wavelength/nm
Figure 5. Changes in the fluorescence spectrum of BDP-
TPY with Zn2+ and plot of the fluorescence intensity ratio
(F567/F539) of BDP-TPY versus increasing Zn2+ concen-
tration. The spectra shown are for Zn2+ concentrations of
0, 0.1, 0.2, 0.3, 0.5, 0.7, 0.8, 1.0, 1.2, 1.5, 2.0, 3.0, 5.0, and
10.0 ¯M. Spectra were acquired in aqueous acetonitrile
solution (acetonitrile/water = 9/1, v/v) with excitation at
535 nm. The concentration of BDP-TPY was 1 ¯M.
BDP-TPY
Cd2+
Zn2+
the meso position showed fluorescence quenching toward
Zn
2+ 40 BDP-TPY is the first example of asymmetric BODIPY
.
Hg2+
with a pyridyl receptor at the 3 position showing red-shifted
fluorescence by complexation with metal ions. It is known
that when electron-withdrawing groups (e.g., pyridyl group) are
part of the fluorophore ³-system and involved in ion binding,
the excited state is more stabilized than the ground state upon
complexation with metal ions, and therefore the energy gap
is reduced.41,42 In BDP-TPY, the terpyridine moiety could
interact with metal ions in this way, resulting in the observed
bathochromic shift of the fluorescence spectra. The red shift
also enables a ratiometric fluorescence measurement of the
metal ions. BDP-TPY showed a sharp and strong absorption
band around 515 nm (¾ = 51000) (Table S2). Upon addition
of heavy or transition-metal ions, the absorption spectra
showed bathochromic shifts. In particular, the absorption band
of BDP-TPY at 515 nm underwent a large shift to 543 nm
upon addition of Cu2+. The complex of BDP-TPY with Fe2+
and Fe3+ showed two absorption peaks around 513 and 640 nm
(Table S2).
The Zn2+-concentration-dependent fluorescence spectra of
BDP-TPY are shown in Figure 5. Fluorescence spectra of
BDP-TPY upon titration with Zn2+ displayed a bathochromic
fluorescence shift from 539 to 567 nm and a distinct ratiometric
change with a clear isoemission point at 551 nm. The quantum
yield changed from 0.91 to 0.66. The spectral change was
largely terminated by the addition of 10 equiv of Zn2+. Similar
changes were observed in the absorption spectra. The maxi-
mum absorbance shifted bathochromically from 515 to 534 nm
with an isosbestic point at 525 nm as the Zn2+ concentration
was increased in the solution (Table S2). Since F567 increased
while F539 simultaneously decreased with increasing concen-
tration of Zn2+, the ratio of fluorescence intensities at the 567
and 539 nm (F567/F539) increased in response to changes in the
Zn2+ concentration (Figure 5). When the Zn2+ concentration
500
550
600
650
Wavelength/nm
Figure 4. Fluorescence spectra of BDP-TPY in the pres-
ence of different metal ions (50 ¯M) in aqueous acetonitrile
solution (acetonitrile/water = 9/1, v/v) with excitation at
525 nm. The concentration of BDP-TPY was 1 ¯M.
The response of BDP-DPA to variations in pH was investi-
gated (Figure S2). The fluorescence bands around 591 nm
originating from BDP-DPA were stable at pH 5.0, 7.0, and
9.0. Fluorescence enhancement was observed at pH 3.0 with
a hypsochromic shift to 566 nm, which is possibly due to
protonation of BDP-DPA under low pH conditions.17,37,38
Figure 4 shows the fluorescence spectra of BDP-TPY in an
aqueous acetonitrile solution with and without the respective
metal ions (50 equiv). Without the ions, BDP-TPY showed
a greenish-yellow fluorescence at 539 nm with a high fluores-
cence quantum yield (Φ = 0.91) and a shoulder around 580
nm, which is typical of BODIPY fluorophores.39 In contrast
to BDP-DPA, the spectra of BDP-TPY were shifted to longer
wavelengths upon addition of Zn2+, Cd2+, and Hg2+. The
extent of the spectral shift depends on the ion species.
Complexes of BDP-TPY with Cd2+ and Hg2+ showed red-
shifted fluorescence bands at 563 and 561 nm, respectively,
while Zn2+ induced the largest bathochromic shift (from 539 to
567 nm) of the fluorescence band. Goze et al. reported that
BODIPY fluoroionophores functionalized with terpyridine at

A. Hafuka et al.
Bull. Chem. Soc. Jpn. Vol. 86, No. 1 (2013)
41
10
8
Only by changing the ion-receptor moiety could we modulate
the wavelength-response characteristics and ion selectivity of
the fluoroionophores, which is beneficial for the simultaneous
analysis of several metal ions. In a future study, we will
immobilize the fluoroionophores onto a solid support, which is
applicable as a fluorescence sensor for environmental aqueous
samples. In addition, Determination of Zn2+ in roadway
drainage by using BDP-TPY is now under way.
6
4
2
0
Experimental
Synthetic Materials and Methods.
Unless otherwise
Figure 6. Changes of the fluorescence intensity ratio (F567
/
stated, all reagents were purchased from commercial suppliers
and used without further purification. Reactions were monitored
using high-performance thin-layer chromatography (HPTLC;
silica gel 60 F254, Merck, Germany) or thin-layer chromatog-
raphy (TLC; aluminum oxide 60 F254, basic, Merck, Germany).
HPTLC plates were visualized in UV light and/or by staining
with p-methoxybenzaldehyde-H2SO4-MeOH (1:2:17, v/v)
followed by heating for a few minutes. Open-column chroma-
tography was performed using silica gel 60 (230-400 mesh)
F
539) of BDP-TPY upon addition of different metal ions.
Spectra were acquired in aqueous acetonitrile solution
(acetonitrile/water = 9/1, v/v). White bars represent the
addition of an excess of the appropriate metal ion (1 mM
for Na+, Mg2+, K+, and Ca2+. 10 ¯M for all other ions)
to a 1 ¯M solution of BDP-TPY. Black bars represent
the subsequent addition of 10 ¯M of Zn2+ to the solution.
Excitation was provided at 535 nm.
1
was 10 ¯M (i.e., 10 equiv), F567/F539 was 8.0. The fluorescence
color changed from greenish-yellow to orange. The sigmoidal
plot gives a quantitative determination of Zn2+. The LOD
and LOQ of BDP-TPY for Zn2+ were 5.1 © 10 and 1.7 ©
or aluminum oxide 90 active basic. H and 13C spectra were
1
recorded on a JEOL 400 (400 MHz H; 100 MHz 13C) spec-
trometer at room temperature. Chemical shifts are reported in
ppm relative to tetramethylsilane (TMS) as the internal stand-
¹9
1
10¹8 M, respectively. These values are sufficiently low for the
detection in the submicromolar concentration range of Zn2+
in many environmental, chemical, and biological systems.
Job’s plot43 suggested a 1:1 complex of BDP-TPY with Zn2+
(Figure S3). In addition, the observation of the isoemissive
point in Figure 5 also indicates the formation of only one type
of complex in this system. Therefore, Benesi-Hildebrand plots
were used to determine a binding constant, which was
ard (residual CHCl3; H NMR 7.26 ppm, 13C NMR 77.2 ppm).
Coupling constants (J) were reported in hertz. Splitting patterns
are indicated as s, singlet; d, doublet; t, triplet; q, quartet;
1
m, multiplet; brs, broad singlet for H NMR data. ESI high-
resolution mass spectra (HRMS) were recorded on a Thermo
Scientific Exactive spectrometer or JMS-T100LP spectrometer.
FD HRMS spectra were recorded on JEOL JMS-T100GCv
spectrometer.
4.64 © 105 M for Zn2+ in the solution (Figure S3).
(4-Iodophenyl)bis(pyridin-2-ylmethyl)amine. 4-Iodoani-
line (500 mg, 2.28 mmol), 2-(bromomethyl)pyridine hydrobro-
mide (1.73 g, 6.85 mmol), and potassium carbonate (947 mg,
6.85 mmol) were dissolved in DMF (12 mL) purged with N2
in a 25-mL round flask. The reaction mixture was stirred at
70 °C for 3 h. After being cooled to room temperature, CH2Cl2
was added and the mixture was washed with H2O, saturated
aqueous (st. aq.) NaHCO3, and brine, then dried (Na2SO4),
and concentrated. The crude product was purified by column
chromatography on silica gel (CH2Cl2/acetone = 5/1) to yield
302 mg (0.75 mmol, 33%) of (4-iodophenyl)bis(pyridin-2-
ylmethyl)amine as a light brown solid. 1H NMR (400 MHz,
CDCl3): ¤ 8.59 (d, 2H, J = 4.8 Hz), 7.63 (td, 2H, J = 7.7,
1.8 Hz), 7.39 (d, 2H, J = 9.2 Hz), 7.22-7.16 (m, 4H), 6.48
(d, 2H, J = 9.1 Hz), 3.98 (s, 4H); 13C NMR (100 MHz, CDCl3):
¤ 158.0, 149.6, 149.5, 137.6, 136.7, 122.0, 120.6, 114.7, 78.2,
57.3; HRMS (ESI) m/z calcd for [M + H]+ C18H16N3IH
402.0462; found 402.0461.
4-(2-Butyloctyloxy)-1,5-dihydropyrrol-2-one (3). Com-
pound 245 (2.0 g, 17.7 mmol) was dissolved in 2-butyl-1-
octanol (20 mL, 89.4 mmol) and the solution was warmed to
80 °C. Methanesulfonic acid (0.1 mL, 1.8 mmol) was slowly
added to the solution, and then the mixture was stirred at this
temperature for 24 h under vacuum. After being cooled to
room temperature, the solution was filtered and then diluted
with CH2Cl2. The filtrate was successively washed with H2O,
saturated aqueous (sat. aq.) NaHCO3, and brine, and then dried
¹1
Competition experiments of Zn2+ with some common ions
were conducted to evaluate the selectivity of BDP-TPY toward
Zn2+ (Figure 6). The fluorescence intensity ratio (F567/F539
)
,
did not change upon addition of Na+, Mg2+, K+, Ca2+, Cr3+
Mn2+, and Fe3+ to the solution of BDP-TPY. The signal
increased slightly for Hg2+ and Pb2+ solutions, whereas it
increased up to 5.2 for the Cd2+ solution. Further addition of
Zn2+ to the solutions resulted in further increases in the signal.
This result demonstrated that BDP-TPY preferentially binds to
Zn2+ rather than Cd2+ or Hg2+. Other research also reported
that terpyridine shows good affinity for Zn2+ 11,26,27,40
.
The pH dependency of BDP-TPY was also investigated. In
the pH range between 5.0 and 9.0, the fluorescence spectra
of BDP-TPY were unchanged while the fluorescence intensity
declined, with no spectral shift at pH 3 (Figure S4), and this
might be attributed to diprotonation of the terpyridine.11,44
Conclusion
In this study, two types of metal-ion receptor were directly
linked to the 3 position of the BODIPY core using the Suzuki-
Miyaura cross-coupling reaction. The fluoroionophores showed
opposite wavelength responses toward heavy metal ions. The
fluoroionophores responded to several heavy metal ions with
specific fluorescence spectra that depend on the nature of the
ionic species. Concentrations of Cr3+ and Zn2+ were success-
fully determined using a ratiometric fluorescence method.

42
Bull. Chem. Soc. Jpn. Vol. 86, No. 1 (2013)
Fluoroionophores for Heavy Metal Analysis
(Na2SO4), and concentrated. The crude product was purified by
column chromatography on silica gel (CHCl3/EtOAc = 3/1)
to yield 3.9 g (14.6 mmol, 82%) of 3 as a white solid. 1H NMR
(400 MHz, CDCl3): ¤ 6.42 (brs, 1H), 5.02 (s, 1H), 3.92 (s, 2H),
3.82 (d, 2H, J = 5.6 Hz), 1.78-1.71 (m, 1H), 1.34-1.28 (m,
16H), 0.92-0.87 (m, 6H); 13C NMR (100 MHz, CDCl3): ¤
175.8, 175.1, 94.0, 74.3, 46.9, 37.4, 31.8, 31.1, 30.8, 29.5,
28.9, 26.7, 22.9, 22.6, 14.1, 14.0; HRMS (ESI) m/z calcd for
[M + Na]+ C16H29NO2Na 290.2096; found 290.2092.
then EtOAc was added. The organic layer was separated. The
aqueous layer was washed with EtOAc twice. The combined
organic layer was successively washed with H2O and brine,
then dried (Na2SO4), and concentrated. The crude product was
purified by column chromatography on aluminum oxide basic
(CHCl3/EtOAc = 15/1) to yield 180 mg (0.46 mmol, 63%)
1
of 6 as a brown solid. H NMR (400 MHz, CDCl3): ¤ 8.58 (d,
2H, J = 4.1 Hz), 7.62-7.58 (m, 4H), 7.23 (d, 2H, J = 7.9 Hz),
7.16 (dd, 2H, J = 7.4, 4.9 Hz), 6.68 (d, 2H, J = 8.8 Hz), 4.84
(s, 4H), 3.70 (s, 4H), 0.98 (s, 6H); 13C NMR (100 MHz,
CDCl3): ¤ 158.3, 149.9, 149.5, 136.6, 135.2, 121.9, 120.6,
111.4, 72.1, 56.9, 31.8, 21.9; HRMS (FD) m/z calcd for [M]+
C23H26BN3O2 387.2118; found 387.2260.
4-(2-Butyloctyloxy)-5-(3,5-dimethyl-1H-pyrrol-2-ylmeth-
ylene)-1,5-dihydropyrrol-2-one (5).
Compound 3 (1.6 g,
6.1 mmol) and compound 446 (0.5 g, 4.1 mmol) were dissolved
in hexamethylphosphoric triamide (8 mL) and the solution
was warmed to 70 °C. A 3 M NaOH (10 mL) solution was
slowly added to the reaction mixture and stirred for 12 h. After
being cooled to room temperature, the mixture was diluted
with EtOAc and successively washed with H2O, then dried
(Na2SO4), and concentrated. The crude product was purified by
column chromatography on silica gel (CH2Cl2/EtOAc = 10/1)
to yield 0.9 g (2.5 mmol, 62%) of 5 as a yellow solid. 1H NMR
(400 MHz, CDCl3): ¤ 10.93 (s, 1H), 10.37 (s, 1H), 6.35 (s,
1H), 5.81 (s, 1H), 5.05 (s, 1H), 3.90 (d, 2H, J = 5.8 Hz), 2.39
(s, 3H), 2.16 (s, 3H), 1.86-1.82 (m, 1H), 1.40-1.29 (m,
16H), 0.91-0.86 (m, 6H); 13C NMR (100 MHz, CDCl3): ¤
173.2, 167.0, 134.5, 126.7, 122.4, 121.9, 109.9, 100.0, 89.6,
74.3, 37.5, 31.8, 31.5, 31.2, 29.6, 29.0, 26.8, 23.0, 22.7, 14.1,
14.0, 13.1, 11.3; HRMS (ESI) m/z calcd for [M + H]+
C23H36N2O2H 373.2855; found 373.2861.
4-[4-(2-Butyloctyloxy)-5-(3,5-dimethyl-1H-pyrrol-2-yl-
methylene)-5H-pyrrol-2-yl]phenylbis(pyridin-2-ylmethyl)-
amine (7). Compound 1 (116 mg, 0.23 mmol), compound 6
(89 mg, 0.23 mmol), sodium carbonate (73 mg, 0.69 mmol), and
tetrakis(triphenylphosphine)palladium(0) (8 mg, 0.01 mmol)
were dissolved in toluene (8 mL) and MeOH (5 mL) purged
with N2 in a 25-mL round flask. The reaction mixture was
refluxed for 17 h with stirring. After being cooled to room
temperature, the reaction mixture was quenched by addition
of H2O, and then EtOAc was added. The organic layer was
separated. The aqueous layer was washed with EtOAc twice.
The combined organic layer was successively washed with
H2O and brine, then dried (Na2SO4), and concentrated. The
crude product was purified by column chromatography on
silica gel (CHCl3/EtOAc = 30/1) to yield 69 mg (0.11 mmol,
1
Trifluoromethanesulfonic Acid 4-(2-Butyloctyloxy)-5-
(3,5-dimethyl-1H-pyrrol-2-ylmethylene)-5H-pyrrol-2-yl Es-
ter (1). CH2Cl2 (15 mL) was stirred into a solution of 5
(500 mg, 1.34 mmol) and triethylamine (0.56 mL, 4.03 mmol)
at room temperature for 30 min under a nitrogen atmosphere.
The reaction mixture was cooled to ¹78 °C, followed by
addition of trifluoromethanesulfonic anhydride (0.68 mL, 4.03
mmol), and stirred at this temperature for 1 h. The reaction
mixture was diluted with CH2Cl2 and successively washed with
sat. aq. NaHCO3 and brine, then dried (Na2SO4), and concen-
trated. The residue was purified by column chromatography on
silica gel (hexane/EtOAc = 30:1) to yield 583 mg (1.15 mmol,
86%) of 1 as a brown oil. 1H NMR (400 MHz, CDCl3): ¤ 10.68
(brs, 1H), 7.03 (s, 1H), 5.87 (s, 1H), 5.37 (s, 1H), 3.88 (d, 2H,
J = 5.8 Hz), 2.33 (s, 3H), 2.22 (s, 3H), 1.85-1.79 (m, 1H),
1.40-1.29 (m, 16H), 0.93-0.87 (m, 6H); 13C NMR (100 MHz,
CDCl3): ¤ 166.6, 160.2, 140.0, 134.3, 131.0, 126.7, 123.4,
120.2, 119.0, 117.0, 113.8, 112.3, 86.6, 74.9, 37.6, 31.8, 31.4,
31.1, 29.6, 29.0, 26.8, 23.0, 22.7, 14.1, 14.1, 13.8, 11.3; HRMS
(ESI) m/z calcd for [M + H]+ C24H35F3N2O4SH 505.2348;
found 505.2346.
48%) of 7 as a red solid. H NMR (400 MHz, CDCl3): ¤ 9.09
(br, 1H), 8.61 (d, 2H, J = 4.0 Hz), 7.82 (d, 2H, J = 8.9 Hz),
7.63 (td, 2H, J = 7.7, 1.8 Hz), 7.26 (d, 2H, J = 7.9 Hz), 7.20-
7.17 (m, 2H), 6.81 (s, 1H), 6.77 (d, 2H, J = 9.0 Hz), 5.95 (s,
1H), 5.81 (s, 1H), 4.90 (s, 4H), 3.90 (d, 2H, J = 5.9 Hz), 2.33
(s, 3H), 2.21 (s, 3H), 1.88-1.82 (m, 1H), 1.41-1.29 (m, 16H),
0.93-0.86 (m, 6H); 13C NMR (100 MHz, CDCl3): ¤ 167.0,
164.6, 158.1, 149.6, 149.0, 140.2, 136.7, 136.5, 129.8, 128.1,
128.0, 124.4, 122.0, 120.7, 113.1, 112.2, 111.1, 94.1, 74.2,
57.2, 37.6, 31.8, 31.5, 31.2, 29.6, 29.0, 26.8, 23.0, 22.6, 14.1,
14.1, 13.9, 11.2; HRMS (ESI) m/z calcd for [M + H]+
C41H51N5OH 630.4166; found 630.4166.
1-(2-Butyloctyloxy)-5,7-dimethyl-3-[4-bis(pyridin-2-ylmeth-
yl)aminophenyl]-4,4-difluoro-3a,4a-diaza-4-bora-s-indacene
(BDP-DPA). A solution of compound 7 (72 mg, 0.11 mmol),
boron trifluoride etherate (72 ¯L, 0.57 mmol), and triethylamine
(48 ¯L, 0.34 mmol) was stirred in anhydrous toluene (30 mL)
at room temperature under a nitrogen atmosphere. The reaction
mixture was warmed to 115 °C and stirred for 19 h. After being
cooled to room temperature, CH2Cl2 was added and the mixture
was washed with H2O, sat. aq. NaHCO3, and brine, then dried
(Na2SO4), and concentrated. The crude product was purified by
column chromatography on silica gel (hexane/EtOAc = 1/1)
to yield 63 mg (0.09 mmol, 81%) of BDP-DPA as a purple
solid. 1H NMR (400 MHz, CDCl3): ¤ 8.60 (d, 2H, J = 4.0 Hz),
7.88 (d, 2H, J = 9.9 Hz), 7.64 (td, 2H, J = 7.7, 1.8 Hz), 7.27 (d,
2H, J = 7.4 Hz), 7.20-7.17 (m, 2H), 7.10 (s, 1H), 6.77 (d, 2H,
J = 9.0 Hz), 5.99 (s, 1H), 5.91 (s, 1H), 4.88 (s, 4H), 3.95 (d,
2H, J = 5.8 Hz), 2.47 (s, 3H), 2.23 (s, 3H), 1.86-1.80 (m, 1H),
1.41-1.29 (m, 16H), 0.93-0.86 (m, 6H); 13C NMR (100 MHz,
CDCl3): ¤ 162.4, 158.2, 157.9, 152.2, 149.6, 149.4, 137.2,
[4-(5,5-Dimethyl-1,3,2-dioxaboran-2-yl)phenyl]bis(pyri-
din-2-ylmethyl)amine (6).
(4-Iodophenyl)bis(pyridin-2-
ylmethyl)amine (297 mg, 0.74 mmol), bis(2,2-dimethyl-1,3-
propylidenedioxy)diborane (170 mg, 0.75 mmol), potassium
acetate (218 mg, 2.22 mmol), and [1,1¤-Bis(diphenylphos-
phino)ferrocene]palladium(II) dichloride dichloromethane ad-
duct (18 mg, 0.02 mmol) were dissolved in DMF (6 mL) purged
with N2 in a 25-mL round flask. The reaction mixture was
stirred at 80 °C for 5 h. After being cooled to room temperature,
the reaction mixture was quenched by addition of H2O, and

A. Hafuka et al.
Bull. Chem. Soc. Jpn. Vol. 86, No. 1 (2013)
43
136.7, 131.1, 130.8, 126.9, 122.0, 120.9, 120.6, 117.6, 116.3,
111.9, 98.4, 74.3, 56.9, 37.6, 31.8, 31.3, 31.0, 29.6, 29.0, 26.7,
23.0, 22.6, 14.6, 14.1, 14.1, 11.2; HRMS (ESI) m/z calcd for
[M + Na]+ C41H50BF2N5ONa 699.4005; found 699.4017.
4¤-[4-(2-Butyloctyloxy)-5-(3,5-dimethyl-1H-pyrrol-2-yl-
grade perchlorate salts in a Tris-HCl buffer (0.01 M). Mill-Q
water (18.25 M³ cm) was used to prepare all aqueous solu-
tions. Quartz cells (cross section of 1 cm © 1 cm) were used
for fluorescence and absorption measurements. Fluorescence
and absorption spectra were obtained on a JASCO FP-6600
spectrofluorometer and a JASCO V-630 spectrophotometer,
respectively. The excitation and fluorescence slit widths were
5.0 and 6.0 nm, respectively. A detection limit (LOD, 3·/
slope) and a quantification limit (LOQ, 10·/slope) for each
metal ion were determined based on the standard deviation (·)
of 11 blank solutions.48
Determination of the Fluorescence Quantum Yield. The
relative quantum yields of the samples were obtained by
comparing the area under the corrected fluorescence spectrum
of the test sample with that of a solution of Rhodamine
6G in H2O, which has a reported quantum yield of 0.76.49
The quantum yields of fluorescence (ΦS) were obtained from
multiple measurements (N = 3) with the following equation:
methylene)-5H-pyrrol-2-yl]-2,2¤:6¤,2¤¤-terpyridine
(9).
Compound 1 (128 mg, 0.25 mmol), compound 847 (92 mg,
0.27 mmol), cesium fluoride (116 mg, 0.76 mmol), and tetra-
kis(triphenylphosphine)palladium(0) (29 mg, 0.03 mmol) were
dissolved in 1,4-dioxane (8 mL) purged with N2 in a 25-mL
round flask. The reaction mixture was refluxed for 1 h with
stirring. After being cooled to room temperature, the reaction
mixture was quenched by addition of H2O, and then CH2Cl2
was added. The organic layer was separated. The aqueous
layer was washed with CH2Cl2 twice. The combined organic
layer was successively washed with H2O and brine, then dried
(Na2SO4), and concentrated. The crude product was purified
by column chromatography on aluminum oxide basic (hexane/
EtOAc = 7/1) to yield 18 mg (0.03 mmol, 12%) of 9 as an
orange solid. 1H NMR (400 MHz, CDCl3): ¤ 8.99 (s, 2H), 8.74
(d, 2H, J = 3.9 Hz), 8.66 (d, 2H, J = 7.9 Hz), 7.87 (td, 2H,
J = 7.7, 1.8 Hz), 7.36-7.32 (m, 2H), 7.05 (s, 1H), 6.33 (s, 1H),
5.91 (s, 1H), 3.98 (d, 2H, J = 5.9 Hz), 2.47 (s, 3H), 2.27
(s, 3H), 1.90-1.86 (m, 1H), 1.43-1.31 (m, 16H), 0.96-0.88
(m, 6H); 13C NMR (100 MHz, CDCl3): ¤ 166.7, 160.9, 156.2,
155.6, 148.9, 144.1, 141.0, 138.8, 136.6, 134.0, 129.1, 123.5,
121.1, 117.8, 117.4, 112.6, 94.9, 74.5, 37.7, 31.8, 31.5, 31.2,
29.7, 29.1, 26.8, 23.0, 22.7, 14.2, 14.1, 14.1, 11.4; HRMS (ESI)
m/z calcd for [M + H]+ C38H45N5OH 588.3697; found
588.3700.
2
ꢀS ¼ ꢀR ꢀ SS=SR ꢀ AR=AS ꢀ ð©S=©RÞ
ð1Þ
where Φ is the quantum yield, S is the integrated area of the
corresponding fluorescence spectrum, A is the absorbance at the
excitation wavelength, © is the refractive index of the solvent
used, and the subscripts S and R refer to the sample and the
reference fluorophore, respectively.
This research was financially supported by Core Research of
Evolutional Science & Technology (CREST) for “Innovative
Technology and Systems for Sustainable Water Use” from
Japan Science and Technology Agency (JST). This study was
also partially supported by Grants-in-Aid for Scientific Re-
search (No. 23686074) from Japan Society for the Promotion
of Science.
1-(2-Butyloctyloxy)-5,7-dimethyl-3-(4¤-2,2¤:6¤,2¤¤-terpyri-
dinyl)-4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BDP-
TPY).
A solution of compound 9 (109 mg, 0.19 mmol),
boron trifluoride etherate (117 ¯L, 0.93 mmol), and trieth-
ylamine (77 ¯L, 0.56 mmol) was stirred in anhydrous toluene
(30 mL) at room temperature under a nitrogen atmosphere.
The reaction mixture was warmed to 130 °C with stirring for
3 h under nitrogen. After being cooled to room temperature,
CH2Cl2 was added and the mixture was washed with H2O, sat.
aq. NaHCO3, and brine, then dried (Na2SO4), and concentrated.
The crude product was purified by column chromatography on
aluminum oxide basic (hexane/EtOAc = 6/1) to yield 51 mg
(2.5 mmol, 62%) of BDP-TPY as a dark purple solid. 1H NMR
(400 MHz, CDCl3): ¤ 8.89 (s, 2H), 8.74 (d, 2H, J = 4.2 Hz),
8.62 (d, 2H, J = 8.1 Hz), 7.86 (td, 2H, J = 7.7, 1.8 Hz), 7.35-
7.31 (m, 2H), 7.29 (s, 1H), 6.21 (s, 1H), 6.07 (s, 1H), 4.03 (d,
2H, J = 5.9 Hz), 2.49 (s, 3H), 2.29 (s, 3H), 1.90-1.84 (m, 1H),
1.46-1.31 (m, 16H), 0.96-0.88 (m, 6H); 13C NMR (100 MHz,
CDCl3): ¤ 161.5, 157.5, 156.1, 155.5, 153.1, 149.1, 142.2,
141.3, 136.6, 133.1, 125.6, 123.5, 121.4, 120.6, 119.5, 119.0,
100.1, 74.7, 37.7, 31.8, 31.3, 31.0, 29.6, 29.0, 26.8, 23.0, 22.7,
15.0, 14.1, 14.1, 11.3; HRMS (ESI) m/z calcd for [M + Na]+
C38H44BF2N5ONa 658.3499; found 658.3509.
Supporting Information
Additional fluorescence and absorption spectra of fluoro-
1
ionophores, H NMR, 13C NMR, and mass spectra of all new
compounds. This material is available free of charge on the
Web at: http://www.csj.jp/journals/bcsj/.
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