New fluorescent chemosensor based on exciplex signaling mechanism

Article abstract of DOI:10.1021/ol0504648

(Chemical Equation Presented) A novel fluorescent chemosensor (compound 1) containing aminonaphthol, which selectively recognizes fluoride anion with high sensitivity, was synthesized. The fluorescence of compound 1 was quenched rapidly by fluoride ion, a

Full text of DOI:10.1021/ol0504648

ORGANIC
LETTERS
2
005
Vol. 7, No. 11
133-2136
New Fluorescent Chemosensor Based
on Exciplex Signaling Mechanism
2
†
,‡
†
†
,†
Jia-Sheng Wu, Jian-Hua Zhou, Peng-Fei Wang, Xiao-Hong Zhang,* and
Shi-Kang Wu*
,
†
Nano-Organic Photoelectronic Laboratory, Technical Institute of Physics and
Chemistry, Chinese Academy of Sciences, Beijing, 100101, P. R. China, and
Graduate School of Chinese Academy of Sciences, Beijing, China
zhangxh8@yahoo.com; wusk@mail.ipc.ac.cn
Received March 3, 2005
ABSTRACT
A novel fluorescent chemosensor (compound 1) containing aminonaphthol, which selectively recognizes fluoride anion with high sensitivity,
was synthesized. The fluorescence of compound 1 was quenched rapidly by fluoride ion, and a new peak at a longer wavelength emerged
concurrently, which constituted the signature for fluoride detection. The mechanism of exciplex formation was proposed for the interesting
observation.
Fluorescent chemosensors capable of selectively recognizing
anions are of current interest in supramolecular chemistry
because of their high selectivity, sensitivity, and simplicity.
species binds to a receptor, photophysical characteristics of
the receptor such as fluorescence intensity, emission wave-
length, and fluorescence lifetime change through various
mechanisms, and such changes provide a signal indicating
guest binding. Indeed, a variety of signaling mechanisms
1,2
Among the various anionic analytes, the biologically im-
portant fluoride anion is of particular interest due to its role
3
5
in dental care and treatment of osteoporosis. Although many
such as photoinduced electron transfer (PET), metal-ligand
6
7
examples are available for fluoride anion testing, the simplic-
ity and high sensitivity of the fluorescence method make it
increasingly important for applications in biological, envi-
charge transfer (MLCT), excimer formation, and intramo-
8
lecular charge transfer (ICT) have been developed so far.
4
ronmental, and microchemical detection. When a guest
(3) (a) Wu, F.-Y.; Li, Z.; When, Z.-C.; Zhou, N.; Zhao, Y.-F.; Jiang,
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Lee, J. Y.; Kim, S. K.; Yoon, J.; Nam, K. C. J. Am. Chem. Soc. 2003, 125,
12376
†
Technical Institute of Physics and Chemistry, Chinese Academy of
Sciences.
‡
Graduate School of Chinese Academy of Sciences.
(1) (a) Schmidtchen, F. P.; Berger, M. Chem. Rev. 1997, 97, 1609. (b)
Beer, P. D.; Gale, P. A. Angew. Chem., Int. Ed. 2001, 40, 486. (c) Martinez-
M a´ n˜ ez, R.; Sancen o´ n, F. Chem. Rev. 2003, 103, 4419. (d) Sessler, J. L.;
Davis, J. M. Acc. Chem. Res. 2001, 34, 989. (e) Gale, P. A. Coord. Chem.
Rev. 2001, 213, 79.
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K.; Dasgupta, S. J. Org. Chem. 2000, 65, 1907. (c) Hoffmann, R. W.;
Hettche, F.; Harms, K. Chem. Commun. 2002, 782. (d) Sun, S.-S.; Lees,
A. J. Chem. Commun. 2000, 1687. (e) Xie, H.-Z.; Yi, S.; Yang, X.; Wu,
S.-K. New J. Chem. 1999, 23, 1105. (f) Zhou, L.-L.; Sun, H.; Li, H.-P.;
Wang, H.; Zhang, X.-H.; Wu, S.-K.; Lee, S.-T. Org. Lett. 2004, 6, 1071.
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1
0.1021/ol0504648 CCC: $30.25
© 2005 American Chemical Society
Published on Web 04/29/2005

Figure 1. Molecular structure of compound 1.
However, exciplex formation, to the best of our knowledge,
is not usually used as a signaling mechanism for anion
recognition, though it has been documented previously.
Figure 2. Fluorescence spectra of compound 1 (10 µM) in
9
-
-
2 4
acetonitrile upon addition of (A) different anions (F , H PO ,
Herein, we report a new fluorescent chemosensor consist-
ing of aminonaphthol, which can be used for the selective
detection of fluoride anion. The signal reporting of such a
sensor derives from the fact that with addition of fluoride
anion species, the fluorescence of compound 1 is quenched
gradually and a new peak at a longer wavelength clearly
appears simultaneously. The mechanism of the new peak
formation in the fluorescence spectrum is also discussed.
-
-
-
-
-
-
4 3
AcO , HSO , NO , Cl , Br , I ) (100 µM) and (B) different
-
concentrations of F . Excitation wavelength: 320 nm.
in acetonitrile. Figure 2B shows that with increasing con-
centration of fluoride anion, the fluorescence intensity of
compound 1 at 360 nm decreases and a new broad emission
band peaked at 455 nm emerges with increasing intensity.
Meanwhile, a distinct isoemissive point at 405 nm is also
observed. This implies that a 1:1 complex is formed and the
increase of fluorescent intensity at 455 nm is closely
correlated with the decrease of the intensity at 360 nm,
revealing a definite relationship between peaks at 360 and
1
Compound 1 was synthesized and characterized by H
NMR, MS, and elemental analysis (Supporting Information).
The molecular structure was shown in Figure 1. It was
designed as an efficient fluoride-selective fluorescent
chemosensor on the basis of consideration that a fluoride
anion can strongly interact with a hydrogen-donating group
such as hydroxyl or amide through hydrogen bonding
interaction to form HF. Therefore, it is necessary and
4
55 nm. On the basis of the spectral results and possible
structural change of compound 1 upon addition of fluoride
anion, the luminescence at 455 nm can be reasonably
attributed to the exciplex formation by a photoinduced
electron-transfer process between the naphthoxy moiety and
antipyrine moiety of the molecule. As noted above, in the
absence of fluoride anion, compound 1 contains an intramo-
lecular hydrogen bonding structure and possesses a rigid
structure that is difficult to fold. However, when fluoride
anion is added, the original intramolecular hydrogen bonding
is broken due to the stronger interaction between fluoride
anion and hydroxyl group. As a result, the whole molecular
structure becomes flexible, which makes it easy for the two
parts of compound 1 to fold. Consequently, a photoinduced
electron-transfer process may induce the interaction of the
naphthoxy with the antipyrine, leading to exciplex formation,
as shown in Scheme 1.
important that the -OH or/and -NH
2
groups of compound
1
serve as a part of the receptor. On the other hand, to
improve the sensitivity of the chemosensor with PET
reporting system, it is better to have a competitive mechanism
of hydrogen bonding formation introduced during the
recognition process. For instance, the formation of an
intramolecular hydrogen bond makes the structure of com-
pound 1 rigid when the fluoride is absent, whereas replace-
ment of the hydrogen bonding by the fluoride anion will
change the structure of compound 1 from rigid to flexible.
As a result, the presence of fluoride anion will induce an
evident change in the fluorescence spectrum of compound
1.
Figure 2A shows variations of the fluorescence spectra of
The stability constant of the complex was calculated by
compound 1 in acetonitrile in the presence of different anions
1
0
-
-
-
-
-
-
-
the linear Benesi-Hildebrand expression:
such as F , H
PO
2 4
, AcO , HSO
4
, NO
3
, Cl , Br , and
-
I . It is clear that compound 1 can recognize fluoride anion
1
1
1
1
with excellent sensitivity and selectivity over other anions
)
+
∆I
[1]∆X K [1]∆X [F]
ass
(7) (a) Nishizawa, S.; Kaneda, H.; Uchida, T.; Teramae, N. J. Chem.
Soc., Perkin Trans. 2 1998, 2325. (b) Nishizawa, S.; Kato, Y.; Teramae,
N. J. Am. Chem. Soc. 1999, 121, 9463. (c) Albelda, M. T.; Garcia-Espana,
E.; Gil, L.; Lima, J. C.; Lodeiro, C.; Seixas de Melo, J.; Melo, M. J.; Parola,
A. J.; Pina, F.; Soriano, C. J. Phys. Chem. B. 2003, 107, 6573. (d) Thomas,
A.; Polarz, S.; Antonietti, M. J. Phys. Chem. B. 2003, 107, 5081. (e)
Reichwagen, J.; Hopf, H.; Del Guerzo, A.; Desvergne, J.-P.; Bouas-Laurent,
H. Org. Lett. 2004, 6, 1899.
where, ∆I is the change in the fluorescence intensity at 360
nm; Kass is the stability constant; ∆X is the difference of
fluorescence quantum yields between the complex and
compound 1; and [1] and [F] are the concentrations of
compound 1 and fluoride anion, respectively. On the basis
(
8) (a) Xu, Z.; Xiao, Y.; Qian, X.; Cui, J.; Cui, D. Org. Lett. 2005, 7,
89. (b) Wu, F.-Y.; Jiang, Y.-B. Chem. Phys. Lett. 2002, 355, 438.
9) (a) Kawai, T.; Ikegami, M.; Arai, T. Chem. Commun. 2004, 824. (b)
8
(
(10) (a) Benesi, H. A.; Hildebrand, J. H. J. Am. Chem. Soc. 1949, 71,
2703. (b) Barra, M.; Bohne, C.; Scaiano, J. C. J. Am. Chem. Soc. 1990,
112, 8075
Bencini, A.; Bianchi, A.; Lodeiro, C.; Masotti, A.; Parola, A. J.; Pina, F.;
Seixas de Melo, J.; Valtancoli, B. Chem. Commun. 2000, 1639.
2134
Org. Lett., Vol. 7, No. 11, 2005

Scheme 1. Schematic Illustration of Breaking of
Intramolecular Hydrogen Bonding upon Addition of Fluoride
Ion: The Molecular Structure Changes from Rigid to Flexible
Table 1. Distance of Compound 1 between Atoms with and
without Fluoride Anion
distance between atoms
without F- (Å)
distance between atoms
with F- (Å)
atoms
1
2
3
1
2
-2
-3
-5
-5
-6
1.809
0.988
3.358
3.119
4.590
1.459
3.135
2.888
0.984
simulate the optimized molecular configuration of compound
with and without fluoride anion. The results are depicted
1
and listed in the Abstract Graphic and Table 1, respectively.
As shown in Table 1, the distance between atoms 1 and 2,
which corresponds to the hydrogen bond length between the
naphthol and carbonyl of antipyrine, changes to 4.590 Å in
the presence of fluoride anion from its original value 1.809
Å. The distance between atoms 2 and 3, which is the bond
length of hydroxyl of naphthol, is 0.988 and 1.459 Å without
and with fluoride, respectively. This thus shows that intro-
duction of fluoride anion leads to an increase in the distance
between atom H and atom O. Thus, quantum chemical
calculation supports the experimental observation that the
intramolecular hydrogen bonding of compound 1 is broken
upon fluoride addition.
of the plot of 1/∆I and 1/[F], the stability constant was
5
-1
determined to be 1.8 × 10
M
with a good linear
relationship (R ) 0.997), which also indicates that the
formation of the complex occurs with 1:1 stoichiometery.
To confirm the mechanism proposed above, several
experiments have been carried out as follows. First, the
formation and breaking of intramolecular hydrogen bonding
in the molecule of compound 1 in acetonitrile are supported
1
by H NMR measurement (Figure 3). It was found that two
1
peaks at δ 13.19 and 10.88 were present in the H NMR
Third, the fluorescence decay process of compound 1 in
acetonitrile was measured by a single-photon counting
method, and the resulting decay curves were found to display
either a good single- or double-exponential decay process
spectrum of compound 1 in acetonitrile without fluoride
anion. However, when fluoride anion was added, the distinct
intensity at δ 10.88 decreased dramatically, while that at δ
13.19 almost completely disappeared. It is well recognized
2
with ø typically below 1.2. The data are listed in Table 2.
that the downfield chemical shifts are related to hydrogen
bonding formation; therefore, the result is consistent with
the breaking of hydrogen bonding when fluoride anion was
added.
The fluorescence lifetime of compound 1 in acetonitrile
without fluoride anion is 7.46 ns with a single-exponential
decay process (monitored wavelength: 360 nm). Upon
addition of 10 equiv of fluoride anion into the solution of
compound 1, two lifetimes (7.07 and 1.54 ns) were observed
in the decay process with a double-exponential decay at the
same monitored wavelength. However, when the monitored
wavelength was changed to 455 nm, the lifetime with a
single-exponential decay came to be 8.12 ns, which is slightly
longer than that of compound 1. This infers that a new
species that appears only upon addition of fluoride anion
could be assigned to the exciplex formed between the excited
state of naphthoxy and the ground state of antipyrine. The
decay process is shown in Scheme 2.
Table 2. Fluorescence Lifetime of Compound 1 in CH
with and without F Anion
3
CN
-
lifetime (ns)
2
λ
em
360 nm
)
λ
em
450 nm
)
ø
Figure 3. 1H NMR (300 MHz) spectra of compound 1 in CD
3
CN
samples
value
0.95
in the presence of 0 (a) and 10 equiv (b) of tetrabutylammonium
fluoride.
compound 1
7.46 (100%)
single-exponential
compound 1
7.07 (95%)
1.12
-
+
F
(10 equiv)
double-exponential
.54 (5%)
1
double-exponential
Second, the breaking process of intramolecular hydrogen
bonding was investigated by quantum chemical calculation
using the Gaussian 98 program at B3LYP/6-31G* level to
8
.12
single-exponential
1.18
Org. Lett., Vol. 7, No. 11, 2005
2135

Scheme 2 Fluorescence Decay Process of Compound 1 in
Acetonitrile with Two Lifetimes at a Monitored Wavelength of
3
60 nm
Figure 4. Fluorescence quenching of (A) 1-naphthol (10 µM) in
acetonitrile upon addition of 4-aminoantipyrine (corrected concen-
trations: 0, 5, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 120, 140,
Fourth, to further verify that the new species shown in
Figure 2B is an exciplex formed between the naphthoxy and
antipyrine, control experiments were carried out. 1-Naphthol
and its sodium salt were used separately with 4-amino-
antipyrine to clarify which of them participates in the
formation of the exciplex. The fluorescence quenching
spectra of 1-naphthol and its sodium salt due to 4-amino-
antipyrine are given in Figures 4A and 4B, respectively. For
the combination of 1-naphthol sodium salt and 4-amino-
antipyrine, Figure 4B shows that as the naphthoxy emission
at 342 nm is quenched, a new emission peak at 445 nm and
a clear isoemissive point at 410 nm emerge simultaneously.
In contrast, for the combination of 1-naphthol and the same
quencher, Figure 4A shows no new peak at a longer
wavelength, except for a gradual intensity decrease of the
fluorescence peak at 342 nm (Figure 4A). These results show
that naphthoxy but not 1-naphthol is involved in the exciplex
formation with antipyrine after addition of fluoride anion
into the solution.
1
60, 180, and 200 µM) and (B) 1-naphthyloxy sodium (10 µM)
upon addition of different concentrations of 4-aminoantipyrine
corrected concentrations: 0, 5, 10, 20, 30, 40, 50, 60, 70, 80, 90,
00, and 110 µM). Excitation wavelength: 300 nm.
(
1
excitation. In other words, the fluorescence peak at 455 nm
can be attributed to the emission of an exciplex.
In summary, compound 1 exhibits excellent selectivity and
sensitivity for fluoride detection. Exciplex formation was
responsible for the recognition process. This exciplex mech-
anism not only increases the sensitivity of the present sensor
but also provides a new concept for development of other
novel chemosensors.
Acknowledgment. We thank the Major State Research
Development Program of China (Grant G2000078100) and
the Chinese Academy of Sciences for financial support.
Finally, when the excitation spectra of compound 1 in
acetonitrile were recorded at the emission wavelengths of
Supporting Information Available: Synthesis of com-
pound 1. This material is available free of charge via the
Internet at http://pubs.acs.org.
360 and 455 nm, both spectra had an identical peak at 280
nm. This suggests that two fluorescence peaks observed in
Figure 2B should be generated from the same source of
OL0504648
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Org. Lett., Vol. 7, No. 11, 2005