A colorimetric and ratiometric fluorescent chemosensor with three emission changes: Fluoride ion sensing by a triarylborane-porphyrin conjugate

Article abstract of DOI:10.1002/anie.200250788

Perturbation in the energy-transfer pathway and of the π conjugation between the chromophores in a triarylborane-porphyrin conjugate on coordination of a fluoride ion to the boron center is assumed to account for the observation of the three emission bands (at 356, 670, and 692 nm). The system also displays a colorimetric (purple to green, see picture, and red to bluish emission) response to fluoride ions.

Full text of DOI:10.1002/anie.200250788

Communications
Fluorescent Chemosensor
visual sensing. We report herein that a triarylborane–por-
phyrin conjugate 1 displays colorimetric (purple to green and
red to bluish emission) and ratiometric fluorescent responses
to fluoride ions with three emission bands at 356, 670, and
692 nm.[5–9]
A Colorimetric and Ratiometric Fluorescent
Chemosensor with Three Emission Changes:
Fluoride Ion Sensing by a Triarylborane–
Porphyrin Conjugate**
Me2N
NMe2
Yohei Kubo, Masashi Yamamoto, Masato Ikeda,
Masayuki Takeuchi,* Seiji Shinkai,*
Shigehiro Yamaguchi, and Kohei Tamao
B
The design of chemosensors for a specific analyte is of great
importance for the facile monitoring of analytes.[1,2] Most
chemosensors developed so far are colorimetric and/or
fluorescent sensors, which efficiently change their photo-
physical properties (lmax shift of the UV/Vis spectrum, change
in the quantum yield or emission wavelength, etc) in the
presence of the analyte.[1,2] Ratiometric fluorescent probes, in
particular, have found widespread use in biological, poly-
meric, and sensory materials chemistry, where the two
emission bands from the covalently linked chromophores, or
a chromophore with a dual channel or with a dual signaling
pathway, were used to differentiate between the two signals.
In the former case, the analyte recognition site was connected
to two chromophores (an energy donor and an energy
acceptor or chromophores forming an excimer/exciplex)
through a covalent bond, and the analyte changes the
orientation and/or the dipolar interaction between the
chromophores.[2,3] In the latter cases, another signaling
channel or signaling pathway is activated in the analyte
binding site (which is frequently a part of a chromophore)
only when the analyte is bound to the chromophore.[3e,f] In
these systems, the chromophores were designed so that the
two emission changes would occur in the presence of the
analyte. It thus occurred to us that if the analyte induced
multiple emissions[4] with different colors from a chemo-
sensor, the perceived color change would be useful not only
for the ratiometric method of detection but also for rapid
N
HN
N
O
O
O
O
O
O
O
O
NH
1
Ar1
Ar2
NH
N
O
O
Ar1
B
B
N
HN
Ar2
Ar1
NMe2
Ar1=
Ar2 =
O
O
O
O
2
3
The triarylborane chromophore (an energy donor: D) and
the porphyrin chromophore (an energy acceptor: A) in 1
share a Lewis acidic boron junction that can bind a fluoride
ion with subsequent sp2-sp3 hybridization of the boron
atom.[10] The change in the hybridization of the boron atom
interrupts both the electronic communication and the dipolar
interaction between the triarylborane and porphyrin units as
well as the internal charge-transfer state between the two
chromophores so that each chromophore (D’ and A’) absorbs
and emits light independently only in the presence of fluoride
ions (Figure 1).[11] This situation could lead to changes in the
UV/Vis and fluorescence spectra, from which one could sense
the fluoride ion colorimetrically and ratiometrically.
[*] Dr. M. Takeuchi, Prof. S. Shinkai, Y. Kubo, Dr. M. Yamamoto,
Dr. M. Ikeda
Department of Chemistryand Biochemistry
Graduate School of Engineering, Kyushu University
Fukuoka 812-8581 (Japan)
Fax: (+81)92-642-3611
Compound 1 was prepared according to Scheme 1 and
identified by 1H NMR spectroscopy, MALDI-TOF mass
spectrometry, and elemental analysis. Compounds 2 and
3[10b] were synthesized for control experiments. The Soret
band of 1 ([1] = 3.00 mm) in THF appears at 436.5 nm, that is,
slightly red-shifted relative to that of 2 (430.0 nm), whereas
the absorption maxima of the triarylborane moiety (303.0,
322.5, and 392.5 nm) in 1 are identical with those of 3
(Figure 2A). No band attributable to electronic transitions
between mixed molecular levels of the triarylborane and
porphyrin in 1 was observed. The similarity between the UV/
Vis spectrum of 1 and the additive sum of 2 and 3 indicates
that the electronic coupling between the two chromophores in
the ground state is negligibly small.
E-mail: taketcm@mbox.nc.kyushu-u.ac.jp
seijitcm@mbox.nc.kyushu-u.ac.jp
Dr. S. Yamaguchi, Prof. K. Tamao
Institute for Chemical Research, Kyoto University
Gokasho, Uji, Kyoto 611-0011 (Japan)
[**] Y.K. and M.Y contributed equallyto this work. We thank Dr. Midori
Watanabe and Ms. Hongyue Li at Kyushu University for 1H NMR
measurements. M.T. and Y.K. thank prof. H. Yonemura for helpful
discussions. M.I. thanks the JSPS Research Fellowship for Young
Scientists for financial support. This work was partiallysupported by
a grant-in-aid for COE research (“Design and Control of Advanced
Molecular AssemblySystems”) from the Ministryof Education,
Science, and Culture, Japan (no. 08CE2005).
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
2036
ꢀ 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/anie.200250788
Angew. Chem. Int. Ed. 2003, 42, 2036 – 2040

Angewandte
Chemie
Figure 1. Schematic representation of the different emission processes
occurring in 1 on coordination of an analyte.
Compound 1 was titrated with fluoride ions (as a
tetrabutylammonium salt, TBAF) in THF at 258C and
monitored by UV/Vis absorption spectroscopy. As shown in
Figure 2B, the Soret band shifted to a longer wavelength
(ca. 480.0 nm). A significant hypsochromic effect was seen
upon increasing the concentration of TBAF and this was
accompanied by a bathochromic shift (13–20 nm) of the
Q band.[12] The hypsochromic and bathochromic shifts of the
Soret and Q bands, respectively, were also observed when
control compound 2 was titrated with TBAF in THF.
Recently, Crossley and Johnston[13] reported that a change
in the conjugation in a laterally expanded porphyrin through a
redox-induced reaction of a linker results in a similar
significant hypsochromic effect of the Soret band together
with the bathochromic shift of the Q band. In the case of
compound 1, coordination of a fluoride ion, which generates
anionic charge on the boron atom, should change the
conjugation or the dipole moment of the durylborane-
Figure 2. A) UV/Vis absorption spectra of compounds 1–3 (3.0 mm) in
THF and B) dependence of the UV/Vis spectra on the concentration of
fluoride ions; [1]=3.0 mm, [TBAF]=0–0.10 mm, in THF at 258C.
Ar1
Ar1
Ar1
B
I
c
b
a
B
I
B
B
H
3
OH
+
Ar1
H
Ar1
Ar1
Ar1
Ar2
4
5
6
Ar2
NH
Ar2
N
N
H
g
N
H
N
N
N
HN
N
N
f
N
N
d, e
I
Zn
8
Zn
N
N
+
Ar2CHO
Ar2
Ar2
Ar2
7
9
Ar1
Ar2
a, h
NH
N
N
6 + 9
B
HN
Ar1=
Ar2
NMe2
Ar2
Ar1
O
O
O
O
1
Scheme 1. Reaction and conditons: a) [Pd(PPh3)Cl2], CuI, THF, (iPr)2NH; b) 3-methyl-1-butyn-3-ol, [Pd(PPh3)2Cl2], CuI, THF, (iPr)2NH; c) NaOH,
toluene; d) TFA, CH2Cl2; e) DDQ, CH2Cl2; f) Zn(OAc)2, CHCl3, MeOH, g) AgPF6, I2, CHCl3, pyridine; h) TFA, CH2Cl2. DDQ=2,3-dichloro-5,6-
dicyano-1,4-benzoquinone, TFA=trifluoroacetic acid.
Angew. Chem. Int. Ed. 2003, 42, 2036 – 2040
www.angewandte.org
ꢀ 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2037

Communications
linked porphyrin moiety. In contrast to the porphyrin moiety,
fluorescence spectrum for an equimolar mixture (0.5 mm
each) of 2 and 3 was equivalent to the sum of the two
individual fluorescence spectra. Furthermore, the excitation
spectrum of 1 shows an intense triarylborane absorption band
as well as the porphyrin band. The findings clearly show that
efficient transfer of energy from the triarylborane to the
porphyrin chromophore takes place in the absence of fluoride
ions.
The addition of fluoride ions to a solution of 1 in THF
results in a substantial increase in the intensity of the two
emission bands at 356 nm and 692 nm along with a decrease in
the intensity of the emission band at 670 nm (Figure 4). The
a decrease (to 392.5 nm) and an increase (to 323.0 and
343.4 nm) in the intensities of the triarylborane absorption
bands in 1 were observed. These trends are the same as those
observed for 3, and the result is interpreted as a loss of
p conjugation through the boron atom.[9] These results sup-
port the view that the two chromophores, the aryl borane (D’)
and the porphyrin (A’), generate their absorption bands
independently in the presence of fluoride ions. The change in
the absorption spectra resulted in a change in the color of the
solution from purple to green (Figure 3). The stoichiometry in
the reaction between 1 and TBAF was confirmed to be 1:1
Figure 3. Color changes observed in samples of 1 and TBAF in THF. Left
to right: 1, 1·TBAF, visible emission of 1, and emission of 1·TBAF.
Figure 4. Changes in the emission spectra upon addition of TBAF to
solutions of THF at 258C; excitation wavelength 294 nm.
from a plot of the mole ratio versus absorption. A standard
curve-fitting method was utilized for the A436.5 versus [TBAF]
plot to evaluate the association constant for the 1:1 complex,
which was estimated to be 99700mÀ1. This value is compara-
ble with those previously determined for triarylboranes and
fluoride ions.[10] No change in the Soret band, Q band, or the
tridurylborane absorption was observed upon exposure of 1
to larger halides, acetate, and hydroxide ions. Apparently,
only the smaller anion, the fluoride ion, can fit in the space
around the triduryl-surrounded Lewis acidic boron atom.
The coordination of the fluoride ion to the boron center
was confirmed by NMR spectroscopy. A meso-H proton
signal and pyrrole b-proton signals of the porphyrin unit as
well as methyl proton signals of the duryl group are shifted to
higher magnetic fields in the 1H NMR spectra of
[1]:[TBAF] = 1:0 and 1:2 in [D8]THF at 258C ([1] =
1.00 mm; see the Supporting Information). A 19F signal was
shifted to a lower magnetic field (Dd = 8.40 ppm) relative to
free TBAF in the 19F NMR spectra as a result of its
coordination to the boron center (see the Supporting
Information).
The fluorescence spectrum of 1 (0.5 mm) in THF was
recorded with excitation at an isosbestic point (294 nm, see
the Supporting Information) where the selectivity of the
triarylborane absorption over that of the porphyrin ring was
3:1, as estimated from the absorbance of compounds 2 and 3.
Compound 1 gives the porphyrin emission band (670 nm)
without the triarylborane emission band that should appear at
515 nm, as seen in the emission spectrum of 3 (see the
Supporting Information). The same spectrum was obtained in
a nonpolar solvent (benzene). The intermolecular energy
transfer is negligible at such a low concentration, since the
appearance of the emission band at 356 nm was the same
response to fluoride ions as that of 3. The longer wavelength
shift of 22 nm in the porphyrin emission can be attributed to
the lower energy of the Q band in the ground state. A visible
change in the emission from a red to bluish color was also
observed (Figure 3). Compound 1 showed a ratiometric
response to fluoride ions at two different wavelengths (356
and 692 nm) when normalized against the emission at 670 nm
(see the Supporting Information). The excitation spectra of 1
with added fluoride ions revealed that the absorbance of the
aryl borane still participates in the appearance of the
porphyrin emission at 692 nm. This observation means that
the energy transfer from the aryl borane moiety to the
porphyrin unit occurs even after complexation with a fluoride
ion.[14] The fluorescence decay of the emission band of the aryl
borane moiety in 1 was evaluated to be shorter than 100 ps
(515 nm) and 0.53 ns (380 nm) without and with the guest
fluoride ion, respectively. A shorter lifetime at the 515 nm
emission without fluoride ions than that of 3 (4.52 ns) could
be attributed to an efficient Dexter-type energy transfer from
the aryl borane to the porphyrin moiety in 1. In the presence
of fluoride ions, the lifetime of 0.53 ns is similar to that of the
3·fluoride ion complex(monitored at 380 nm, 0.39 ns). This
finding together with the excitation spectrum of the 1·fluoride
ion complexenables one to suppose that energy transfer from
the aryl borane moiety to the porphyrin unit in 1 takes place
to some extent in the presence of fluoride ions. The result
suggests, therefore, that the guest fluoride ion can switch the
energy-transfer pathway to make it possible to monitor the
2038
ꢀ 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
Angew. Chem. Int. Ed. 2003, 42, 2036 – 2040

Angewandte
Chemie
[3] For selected recent examples, see a) S. Maruyama, K. Kikuchi, T.
binding event through the changes in the three emissions
(Figure 5).
In conclusion, we have demonstrated that 1 displays a
colorimetric (purple to green and red to bluish emission) and
a ratiometric response (by utilizing changes in three emis-
Hirano, Y. Urano, T. Nagano, J. Am. Chem. Soc. 2002, 124,
10650; b) J. Raker, T. E. Glass, J. Org. Chem. 2002, 67, 6113; c) F.
Wꢀrthner, S. Ahmed, C. Thalacker, T. Debaerdemaeker, Chem.
Eur. J. 2002, 8, 4742; d) J. Kim, D. T. McQuade, A. Rose, Z. Zhu,
T. M. Swager, J. Am. Chem. Soc. 2001, 123, 11488; e) J. V. Mello,
N. S. Finney, Angew. Chem. 2001, 113, 1584; Angew. Chem. Int.
Ed. 2001, 40, 1536; f) K. Choi, A. D. Hamilton, Angew. Chem.
2001, 113, 4030; Angew. Chem. Int. Ed. 2001, 40, 3912; g) J.-S.
Yang, C.-S. Lin, C.-Y. Hwang, Org. Lett. 2001, 3, 889; h) S. Deo,
H. A. Godwin, J. Am. Chem. Soc. 2000, 122, 174; i) A. P. de Silva,
J. Eilers, G. Zlokarnik, Proc. Natl. Acad. Sci. USA 1999, 96, 8336.
[4] a) Y.-A. Lee, J. E. McGarrah, R. J. Lachicotte, R. Eisenberg, J.
Am. Chem. Soc. 2002, 124, 10662; b) H. Fu, B. H. Loo, D. Xiao,
R. Xie, X. Ji, J. Yao, B. Zhang, L. Zhang, Angew. Chem. 2002,
114, 1004; Angew. Chem. Int. Ed. 2002, 41, 962.
[5] For reviews on anion receptors, see a) P. D. Beer, P. A. Gale,
Angew. Chem. 2001, 113, 502; Angew. Chem. Int. Ed. 2001, 40,
486; b) P. A. Gale, Coord. Chem. Rev. 2000, 199, 181; c) T. S.
Snoeden, E. V. Anslyn, Curr. Opin. Chem. Biol. 1999, 3, 740;
d) P. D. Beer, Acc. Chem. Res. 1998, 31, 71; d) F. P. Schmidtchen,
M. Berger, Chem. Rev. 1997, 97, 1609; e) J. Scheerder, J. F. J.
Engbersen, D. N. Reinhoudt, Recl. Trav. Chim. Pays-Bas 1996,
115, 307.
Figure 5. Effects of the coordination of a fluoride ion on the emissions
from 1 on excitation at 294 nm.
[6] For recent anion sensors, see a) P. Piatek, J. Jurczak, Chem.
Commun. 2002, 2450; b) J.-H. Liao, C.-Y. Chen, J.-M. Fang, Org.
Lett. 2002, 4, 561; c) A. Ojida, Y. Mito-oka, M. Inoue, I.
Hamachi, J. Am. Chem. Soc. 2002, 124, 6256; d) J. R. Blas, M.
Marquez, J. L. Sessler, F. J. Luque, M. Orozco, J. Am. Chem. Soc.
2002, 124, 12796; e) J. L. Sessler, H. Maeda, T. Mizuno, V. M.
Lynch, H. Furuta, Chem. Commun. 2002, 862; f) K. Choi, A. D.
Hamilton, J. Am. Chem. Soc. 2001, 123, 2456; g) D. H. Lee, K. H.
Lee, J. I. Hong, Org. Lett. 2001, 3, 5; h) P. D. Beer, N. Berry,
M. G. B. Drew, O. D. Fox, M. E. Padilla-Tosta, S. Patell, Chem.
Commun. 2001, 199; i) M. J. Deetz, M. Shang, B. D. Smith, J.
Am. Chem. Soc. 2000, 122, 6201; j) Y. Kubo, M. Tsukahara, S.
Ishihara, S. Tokita, Chem. Commun. 2000, 653; k) S. Sasaki, M.
Mizuno, K. Naemura, Y. Tobe, J. Org. Chem. 2000, 65, 275; l) H.
Miyaji, P. Anzenbachlar, Jr., J. L. Sesslar, E. R. Bleasdale, P. A.
Gale, Chem. Commun. 1999, 1723.
sions) that are useful for the detection of fluoride ions. The
mechanism for the increase in the two emission bands and not
the third emission band was assumed by the finding that the
coordination of a fluoride ion to the boron center in 1 caused
perturbation in the energy transfer pathway and the p conju-
gation. Thus, two chromophores sharing a recognition unit
linked though a conjugate spacer would become a versatile
molecular design scheme to achieve the monitoring of
multiple emissions useful for signal differentiation.
[7] Receptors for fluoride ions are of special importance for
monitoring fluoride metabolism in nature, the analysis of
drinking water, and the detection of chemical warfare agents.[9]
[8] For recent fluoride chemosensors, see a) J. L. Sessler, H. Maeda,
T. Mizuno, V. M. Lynch, H. Furuta, J. Am. Chem. Soc. 2002, 124,
13474; b) S. Aldridge, C. Bresner, I. A. Fallis, S. J. Colesb, M. B.
Hursthouseb, Chem. Commun. 2002, 740; c) S. K. Kim, J. Yoon,
Chem. Commun. 2002, 770; d) T. Mizuno, W.-H. Wei, L. R. Eller,
J. L. Sesslar, J. Am. Chem. Soc. 2002, 124, 1134; e) T. Gunn-
laugsson, A. P. Davis, M. Glynn, Chem. Commun. 2001, 2556;
f) K. H. Lee, H.-Y. Lee, D. H. Lee, J.-I. Hong, Tetrahedron Lett.
2001, 42, 5447; g) H. Furuta, H. Maeda, A. Osuka, J. Am. Chem.
Soc. 2001, 123, 6435; h) M. Takeuchi, T. Shioya, T. M. Swager,
Angew. Chem. 2001, 113, 3476; Angew. Chem. Int. Ed. 2001, 40,
3372; i) C. R. Cooper, N. Spencer, T. D. James, Chem. Commun.
1998, 1365.
Experimental Section
All starting materials and solvents were purchased from Tokyo Kasei
1
Chemicals or Wako Chemicals, and used as received. The H NMR
spectra were recorded either on a Brucker AC 250 (250 MHz) or
Brucker DRX 600 (600 MHz) spectrometer. Chemical shifts are
reported in ppm downfield from tetramethylsilane as the internal
standard. Mass spectral data were obtained by using a Perseptive
Voyager RP MALDI-TOF spectrometer. UV/Vis and fluorescent
spectra were recorded with a Shimadzu UV-2500 PC and Hitachi F-
4500 spectrophotometer.
Received: December 17, 2002 [Z50788]
Keywords: fluorescent probes · fluorides · molecular
.
recognition · porphyrinoids · sensors
[9] H. Sohn, S. Lꢁtant, M. J. Sailor, W. C. Trogler, J. Am. Chem. Soc.
2000, 122, 5399.
[10] a) S. Yamaguchi, S. Akiyama, K. Tamao, J. Am. Chem. Soc. 2000,
122, 6335; b) S. Yamaguchi, T. Shirasaka, K. Tamao, Org. Lett.
2000, 2, 4129; c) S. Yamaguchi, S. Akiyama, K. Tamao, J. Am.
Chem. Soc. 2001, 123, 11372; d) S. Yamaguchi, T. Shirasaka, S.
Akiyama, K. Tamao, J. Am. Chem. Soc. 2002, 124, 8816.
[11] The change in electron-transfer efficiency caused by the binding
of the fluoride ion to a boronate ester has already been reported,
see H. Shiratori, T. Ohno, K. Nozaki, A. Osuka, Chem.
Commun. 1999, 2181.
[1] a) A. P. de Silva, D. B. Fox, A. J. M. Huxley, T. S. Moody, Coord.
Chem. Rev. 2000, 205, 41; b) L. Prodi, F. Bolletta, M. Montaliti,
N. Zaccheroni, Coord. Chem. Rev. 2000, 205, 59; c) V. Amen-
dola, L. Fabbrizzi, M. Licchelli, C. Mangano, P. Pallavicini, L.
Parodi, A. Poggi, Coord. Chem. Rev. 2000, 192, 649; d) J. P.
Desvergne, A. W. Czarnik, NATO ASI Ser. Ser. C 1997, 492.
[2] J. R. Lakowicz, Priciples of Fluorescence Spectroscopy, 2nd ed.,
Plenum, New York, 1999.
Angew. Chem. Int. Ed. 2003, 42, 2036 – 2040
www.angewandte.org
ꢀ 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2039

Communications
[12] It was confirmed by 1H NMR spectroscopy that 1 does not
aggregate in THF both in the presence and in the absence of
TBAF.
[13] M. J. Crossley, L. A. Johnston, Chem. Commun. 2002, 1122.
[14] The emisson intensity of 1 excited at 294 nm is 13 (at 670 nm)
and 16 (at 692 nm) times larger than that of 2 in the absence and
in the presence of fluoride ions, respectively, if it is assumed that
the energy transfer occurs from the aryl borane moiety to the
porphyrin ring.
2040 ꢀ 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
Angew. Chem. Int. Ed. 2003, 42, 2036 – 2040