A BODIPY-indole conjugate as a colorimetric and fluorometric probe for selective fluoride anion detection

Article abstract of DOI:10.1016/j.tetlet.2009.05.018

A BODIPY-indole conjugate, 1, behaves as a colorimetric and fluorometric probe for selective and sensitive detection of F^-. Compound 1 interacts with F^- in a 1:1 stoichiometry via a hydrogen bonding interaction between the indolic NH

Full text of DOI:10.1016/j.tetlet.2009.05.018

Tetrahedron Letters 50 (2009) 4293–4296
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Tetrahedron Letters
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A BODIPY–indole conjugate as a colorimetric and fluorometric probe
for selective fluoride anion detection
*
Yasuhiro Shiraishi , Hajime Maehara, Takahiro Sugii, Dongping Wang, Takayuki Hirai
Research Center for Solar Energy Chemistry, and Division of Chemical Engineering, Graduate School of Engineering Science, Osaka University, Toyonaka 560-8531, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
A BODIPY–indole conjugate, 1, behaves as a colorimetric and fluorometric probe for selective and sensi-
tive detection of Fꢀ. Compound 1 interacts with Fꢀ in a 1:1 stoichiometry via a hydrogen bonding inter-
action between the indolic NH proton and Fꢀ, leading to clear color change from blue to green and
quenching of orange fluorescence.
Received 25 April 2009
Revised 8 May 2009
Accepted 11 May 2009
Available online 13 May 2009
Ó 2009 Elsevier Ltd. All rights reserved.
Design and development of efficient probes for selective anion
sensing have attracted a great deal of attention, since anions play
fundamental and important role in chemical, biological, medical,
and environmental processes.1 Of particular interest is the design
of probes for selective fluoride anion (Fꢀ) detection because of its
important roles in dental care2 and in the treatment of osteoporo-
sis.3 Various colorimetric4 or fluorometric5 Fꢀ probes have been
proposed. For practical application, probes that can detect Fꢀ by
both colorimetric and fluorometric analyses are favorable due to
their ease of use. Several dual-mode Fꢀ probes have therefore been
proposed so far,6 and intensive researches have still been made by
many researchers.
derivative, 2,8d with indole-3-carbaldehyde in a Dean–Stark appa-
ratus (yield 34%).12 The purity of 1 was confirmed by 1H, 13C NMR,
and FAB-MS analyses (Supplementary data, Figs. S5–S7).
Figure
(kex = 605 nm) spectra of 1 (5
tive anions as a n-Bu4N+ salt (200 equiv). As shown in Figure 1a,
without anions, exhibits an absorption band centered at
599 nm (
= 57,600 Mꢀ1 cmꢀ1). Fꢀ addition leads to a decrease in
the 599 nm absorption, along with an appearance of red-shifted
band at 718 nm (
= 58,700 Mꢀ1 cmꢀ1). Accordingly, the solution
1
shows change in absorption and fluorescence
lM) measured in MeCN with respec-
1
e
e
color changes drastically from blue to green (Fig. 2a). In contrast,
addition of other anions (Brꢀ, Clꢀ, ClO4ꢀ, H2PO4ꢀ, HSO4ꢀ, Iꢀ,
NO3ꢀ, SCNꢀ, and AcOꢀ) does not show change in absorption spectra
(Fig. 1a).
As shown in Figure 1b, without anions, 1 shows strong fluores-
cence at 575–720 nm (fluorescence quantum yield: UF = 0.353).13
Addition of 200 equiv of Fꢀ, however, leads to complete quenching
of this fluorescence (UF < 0.001). As shown in Figure 2b, bright or-
ange fluorescence disappears completely by Fꢀ addition. As shown
in Figure 1c, ratio of the fluorescence intensity (FI0/FI) measured at
624 nm with and without Fꢀ is determined to be 378. This value is
much higher than that obtained with early-reported fluorescent Fꢀ
probes.6,8f,g In contrast, addition of other anions shows almost no
Boradiazaindacene (BODIPY)7 is a dye being studied extensively
because of its excellent photophysical properties, such as high fluo-
rescence quantum yield, large extinction coefficient, high photo-
stability, long absorption and fluorescence wavelengths. Recently,
application of BODIPY to optical chemosensors,8 fluorescent biola-
beling reagents,9 light harvesting materials,10 and photodynamic
therapy reagents,11 has been studied extensively. In particular, me-
tal cation sensors have attracted a great deal of attention.8a–e How-
ever, there are only
a few reports of BODIPY-based anion
sensors,8f–k where only two reports describe Fꢀ probe.8f,g
Herein, we report that a new BODIPY-based probe (1) contain-
ing an indole moiety allows highly selective and sensitive Fꢀ detec-
tion by both colorimetric and fluorometric analyses. The probe 1
shows Fꢀ-induced clear color change from blue to green and
quenching of orange fluorescence. We describe that the Fꢀ-induced
colorimetric and fluorometric responses of 1 are simply driven by
hydrogen bonding interaction between the indolic NH proton of 1
and Fꢀ.
OHC
N
N
F
N
F
H
B
Synthesis route of 1 is depicted in Scheme 1. Compound 1 is
easily obtained by Knoevenagel-type condensation of a BODIPY
AcOH/piperidine
N
F
N
F
B
benzene, reflux, 12 h
N
1
2
H
* Corresponding author. Tel.: +81 6 6850 6271; fax: +81 6 6850 6273.
E-mail address: shiraish@cheng.es.osaka-u.ac.jp (Y. Shiraishi).
Scheme 1. Synthesis of BODIPY–indole conjugate, 1.
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.05.018

4294
Y. Shiraishi et al. / Tetrahedron Letters 50 (2009) 4293–4296
–
–
–
–
0.3
0
none, Br , Cl , ClO , H PO
4
4
2
0.4
–
–
–
–
HSO , I , NO , SCN
4
3
–
F
0.5
0
100
200
–
–
AcO
[F ]0/[1]
0
500
600
700
800
Wavelength / nm
none, others
300
400
–
AcO
0
500
600
700
800
Wavelength / nm
–
F
Figure 3. Change in absorption spectra of 1 (5 lM) in MeCN upon addition of 0, 5,
0
10, 20, 30, 40, 50, 70, 100, 150, and 200 equiv of Fꢀ as a n-Bu4N+ salt. (Inset) Change
600
650
700
750
Wavelength / nm
in absorbance monitored at 718 nm.
0
ꢀ
ꢁ
1
1
1
¼
þ 1
ð1Þ
A ꢀ A0
A
1 ꢀ A0 K½Fꢀꢁ0
A0 is the absorbance of free 1, A is the absorbance measured with
Figure 1. (a) Absorption and (b) fluorescence (kex = 605 nm) spectra of 1 (5 lM)
1
measured in MeCN with 200 equiv of respective anions as a n-Bu4N+ salt. (c) The
ratio of the fluorescence intensity (FI0/FI) of 1, where FI and FI0 are the fluorescence
intensity monitored at 624 nm with and without anions.
excess amount of Fꢀ, A is the absorbance measured with Fꢀ, K is the
association constant (Mꢀ1), and [Fꢀ]0 is the concentration of Fꢀ
added (M). As shown in Figure 5, the plot of 1/(A ꢀ A0) against 1/
[Fꢀ]0 shows a linear relationship (R = 0.998), indicating that 1 in-
deed associates with Fꢀ in a 1:1 stoichiometry. The association con-
stant, K, between 1 and Fꢀ, is determined from the ratio of
change in fluorescence spectra (Fig. 1b). These findings suggest
that 1 allows selective and sensitive Fꢀ detection by both colori-
metric and fluorometric analyses. It must be noted that the color-
imetric and fluorometric responses of 1 to Fꢀ are unaffected by
other anions (Supplementary data, Fig. S1), indicating that 1 can
detect Fꢀ selectively even in the presence of other anions
intercept/slope to be 7.8 ꢂ 102 Mꢀ1
.
The probe 1 associates with Fꢀ via a hydrogen bonding interac-
tion between the indolic NH proton of 1 and Fꢀ. This is confirmed
by 1H NMR analysis. Figure 6 shows the results of 1H NMR titration
of 1 with Fꢀ in CD3CN. Upon Fꢀ addition, the indolic NH proton of 1
Figure 3 shows the results of absorption titration of 1 with Fꢀ.
Addition of Fꢀ leads to decrease in the 599 nm absorption, along
with an increase in the 718 nm band. The spectral change almost
stops upon addition of 150 equiv of Fꢀ. The clear isosbestic points
at 504 and 625 nm indicate that a single component is produced in
response to the interaction between 1 and Fꢀ. Figure 4 shows the
results of fluorescence titration of 1 with Fꢀ. Addition of Fꢀ leads
to continuous decrease in the 575–720 nm fluorescence. Complete
fluorescence quenching takes place upon addition of 150 equiv of
Fꢀ, which is similar to the change in absorption spectra (Fig. 3).
Based on the change in fluorescence intensity at 624 nm, the detec-
(d = 9.66 ppm) shows drastic downfield shift (Dd = 0.63 ppm), indi-
cating that Fꢀ indeed interacts with the indolic NH proton. In con-
trast, the Hb–He protons on the indole moiety shift upfield. This is
because the hydrogen bonding interaction leads to an increase in
the electron density of the indole moiety (through-band effec-
t).4e,6k In contrast, Ha proton of 1 shifts downfield. This is because
the hydrogen bonding interaction leads to a polarization of the
adjacent CH moiety and, hence, creates a partial positive charge
tion limit for Fꢀ is determined to be 36 M.14
l
The probe 1 associates with Fꢀ in a 1:1 stoichiometry. This is
confirmed by the Benesi–Hildebrand analysis.8d,15 When assuming
a 1:1 association between 1 and Fꢀ, the Benesi–Hildebrand equa-
tion is given as follows:
300
300
0
0
100
200
–
[F ]0/[1]
0
600
650
700
750
Wavelength / nm
Figure 4. Change in fluorescence (kex = 605 nm) spectra of 1 (5
l
M) in MeCN upon
Figure 2. Change in (a) color and (b) fluorescence color of
containing 1 upon addition of Fꢀ.
a
MeCN solution
addition of 0, 5, 10, 20, 30, 40, 50, 70, 100, 150, and 200 equiv of Fꢀ as a n-Bu4N+ salt.
(Inset) Fluorescence intensity monitored at 624 nm.

Y. Shiraishi et al. / Tetrahedron Letters 50 (2009) 4293–4296
4295
40
20
0
–
740
1–F
A
a
b
b
730
A
c
720
610
0
4000
1/[F ]0
8000
a
–
1
600
Figure 5. Benesi–Hildebrand plot (718 nm) using Eq. 1, assuming 1:1 stoichiometry
for association between 1 and Fꢀ.
c
0
20
Dielectric constant, ε
40
on the Ha proton (through-space effect).4e,6k These 1H NMR titra-
tion results clearly suggest that 1 associates with Fꢀ via a hydrogen
bonding interaction between the indolic NH proton and Fꢀ.
The Fꢀ-induced shift of the absorption spectra is due to the
charge-transfer character of the 1–Fꢀ complex. This is confirmed
by absorption spectra of 1 and 1–Fꢀ complex measured in different
solvents. Figure 7 shows the relationship between kmax of the
Figure 7. Relationship between kmax of the absorption spectra of 1 (5
l
M) obtained
, of the
without (open keys) and with (closed keys) Fꢀ and dielectric constants,
e
solvents. The solvents are (a) THF, (b) acetone, and (c) MeCN, respectively. The
absorption spectra of the respective samples are summarized in Figure S2
(Supplementary data).
absorption spectra and dielectric constants, e 16
of the solvents.
,
The 1–Fꢀ complex does not show fluorescence upon photoexci-
tation at 600–800 nm (Fig. S3, Supplementary data). As repor-
ted,17b excited state molecules with charge-transfer nature
undergo nonradiative decay due to the acceleration of internal con-
version. The no fluorescence of 1–Fꢀ complex is probably due to its
charge-transferred excited state. As a result of this, the fluores-
cence intensity of the solution just depends on the absorbance of
free 1 at the excitation wavelength (605 nm). This therefore results
in fluorescence intensity decrease (Fig. 4) similar to the decrease in
the absorbance of free 1 (Fig. 3).19
In summary, we found that a new BODIPY derivative, 1, behaves
as a highly selective and sensitive20 Fꢀ probe in both colorimetric
and fluorometric analyses. The drastic color change and strong
fluorescence quenching of 1 are simply driven by hydrogen bond-
ing interaction between the indolic NH proton and Fꢀ. The simple
probe design presented here may contribute to the development of
more efficient and more useful dual-mode anion probes.
In the case of 1, kmax shows minor solvent-dependent shift. This
indicates that dipole moments of the ground and excited states
of 1 are similar and both states have minor charge-transfer charac-
ter.17 In contrast, kmax of 1–Fꢀ shows stronger solvent dependence.
This indicates that relatively large difference in the dipole moment
exists between the ground and excited states of 1–Fꢀ, and the elec-
tronic excitation has a charge-transfer character.18 The charge-
transfer character of 1–Fꢀ is probably due to the positive charge
on the indole moiety by the hydrogen bonding interaction. The
dipole moment difference between the ground and excited states
of 1–Fꢀ may therefore lead to a decrease in electronic transition
energy, resulting in a red-shift of the absorption spectra (Fig. 3).
j
i
i
h
h
d
b
e
f
N
c
N B
F
g
F
Acknowledgments
a
N
H
We are grateful for financial support by Grants-in-Aid for Scien-
tific Research (No. 21760619) from the Ministry of Education, Cul-
ture, Sports, Science and Technology, Japan (MEXT).
b,i,j
a,g
h
c,d
e
f
NH
Supplementary data
× 5
Supplementary data (methods and Figs. S1–S7) associated with
this article can be found, in the online version, at doi:10.1016/
j.tetlet.2009.05.018.
× 5
× 5
× 5
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19. As shown in Figure S4 (Supplementary data), plots of the fluorescence intensity
(kem = 718 nm) and the absorbance of free 1 at the excitation wavelength
(605 nm) clearly show linear relationship.
20. A referee pointed out that the detection limit for Fꢀ by 1 (36
lM) is relatively
high as compared to that of the previously reported Fꢀ probes (Refs. 6,8f,g).
However, as shown in Figure 1c, the change in fluorescence intensity of 1 upon
Fꢀ addition (FI0/FI = 378) is much larger than that of the previously reported
probes (Refs. 6,8f,g). This suggests that 1 shows high ‘sensitivity’ to Fꢀ.