Chromogenic and fluorescent chemodosimeter for fluoride ion based on novel anion-catalyzed intramolecular hydrogen transfer

Article abstract of DOI:10.1039/b712554b

We have developed a chromogenic and fluorescent chemodosimeter 3, based on a novel anion-catalyzed intramolecular hydrogen transfer, which displayed drastic changes in UV-Vis absorption as well as fluorescence emission intensities showing selectively for

Full text of DOI:10.1039/b712554b

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PAPER
www.rsc.org/njc | New Journal of Chemistry
Chromogenic and fluorescent chemodosimeter for fluoride ion based on
novel anion-catalyzed intramolecular hydrogen transferwz
Chuanxiang Liu, Xuhong Qian,* Guangqiang Sun, Liwei Zhao and Zhong Li*
Received (in Durham, UK) 15th August 2007, Accepted 30th October 2007
First published as an Advance Article on the web 15th November 2007
DOI: 10.1039/b712554b
We have developed a chromogenic and fluorescent chemodosimeter 3, based on a novel anion-
catalyzed intramolecular hydrogen transfer, which displayed drastic changes in UV-Vis absorption
as well as fluorescence emission intensities showing selectively for Fꢀ over other anions.
length-responsive sensory polymer for the fluoride ion based
on fluoride induced Si–O bond cleavage.10 Tamao et al.
Introduction
The naked fluoride ion has potential as a very strong Lewis
reported colorful fluoride detection based on the reaction
base for use in catalysis and is of significant interest in
between fluoride and the boron atom of the receptors that
inorganic and organic synthesis.1 Many intriguing patterns
led to an interruption of the p-conjugation.11 Besides, Lam
et al. reported cyanide detection by chemodosimeters based on
the coordination of cyanide to metal complexes.12 Ahn et al.
and applications of the fluoride ion in the synthesis of organic
compounds,2 materials,3 and biological catalysis,4 have been
reported which clearly indicate its extraordinarily strong
reported N-acyl triazenes as tunable and selective chemodosi-
basicity and nucleophilicity. Therefore, conceptually, the find-
meters for cyanide ion detection on the basis of a displacement
reaction.13 And also, several chemodosimeters for cation
ing of novel patterns to more broadly realize applications is
(Hg2+, Cu2+) detection based on a specific reaction was also
still being intensively investigated.
The development of anion-sensing systems5 has attracted
reported.14 An exciting approach involving a catalytic reaction
reported earlier by Zhu and Anslyn,15 is used for signal
much attention in recent years as the important role of anions
in biological and chemical processes has become increasingly
amplification. Therefore, it is possible to find an anion cata-
understood. Especially, the sensing of the fluoride ion has
lytic reaction for development of a more elegant chemo-
attracted growing attention because of the importance of this
dosimeter for a specific anion.
anion in dental caries and in the treatment of osteoporosis.6
In our recent investigations to develop Fipronil16 derivatives
Most of the receptors for the fluoride ion described so far7
containing pyrazolopyridine (Scheme 1), the unexpected com-
are mainly based on the approach of a binding site–signalling
pound (Z)-1-(2,6-dichloro-4-(trifluoromethyl)phenyl)-6-phe-
subunit. The binding site usually explores Lewis acids, tri-
nyl-4-((E)-3-phenylallylidene)-4,7-dihydro-1H-pyrazolo[3,4-b]-
arylboranes, macrocycles, protonated polyammonium pyrroles,
pyridine-3-carbonitrile (3) was synthesized in moderate yield
guanidiniums, metalloreceptors, amides, (thio)ureas, or designed
(Scheme 1), which can be further transformed to fluorophore 4
involving an intramolecular hydrogen transfer17 in the pre-
hydrogen-bonding with the fluoride ion. The signalling subunit
based on a chromogenic and/or fluorogenic response upon
sence of naked fluoride ion as catalyst. In this paper, we
receptor–anion interaction appears particularly attractive.
present a novel dosimeter 3, which can be used as a fluorescent
While considerable progress has been made in sensing of
as well as chromogenic chemodosimeter for fluoride ion
fluoride, the chemodosimeter approach,8 which involves the
detection based on anion-catalyzed intramolecular17 hydrogen
use of specific chemical reactions induced by the presence of
transfer (Fig. 1).
target anions, has received much less attention comparatively.
This is an attractive approach which usually shows high
Experimental section
selectivity, large spectroscopic shifts and different electronic
properties. Martınez-Manez et al. reported several chemodo-
´ ´
General information
simeters for fluoride detection based on the selective attack of
hydrofluoric acid on a silica matrix grafted with a certain dye.9
Kim and Swager reported a fluorescent self-amplifying wave-
Melting points were taken on a micro melting point apparatus
1
made in Beijing and were uncorrected. H NMR (400 MHz)
State Key Laboratory of Bioreactor Engineering and Shanghai Key
Lab of Chemical Biology, School of Pharmacy, East China University
of Science and Technology, P.O. Box 544, Shanghai 200237, PR
China. E-mail: xhqian@ecust.edu.cn. E-mail: lizhong@ecust.edu.cn;
Fax: +86-21-64252603; Tel: +86-21-64253589
w The HTML version of this article has been enhanced with colour
images.
z Electronic supplementary information (ESI) available: UV-Vis and
fluorescence emission titrations of 3, and 1H NMR and 13C NMR
spectra of compounds 2, 3, 4. See DOI: 10.1039/b712554b
Scheme 1 (a) Conc. HCl, acetonitrile, 50 1C.
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472 | New J. Chem., 2008, 32, 472–476

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mixture was heated at 50 1C for 6 h. The CH3CN was removed
in a vacuum and the residue purified by column chromato-
graphy on silica gel to give the pure products 2 (yield 35%)
and 3 (yield 15%). Spectral data for 2: mp 172.3–173.5 1C. IR
(KBr, v/cmꢀ1): 3071, 2918, 2239, 1597, 1319, 1128, 830. 1H
NMR (400 MHz, CDCl3) d: 7.45–7.50 (m, 3H), 7.85 (s, 2H),
7.93 (d, J = 8.5 Hz, 1H), 8.01 (m, 2H), 8.35 (d, J = 8.5 Hz,
1H). 13C NMR (100 MHz, CDCl3) d: 159.9, 151.1, 137.7,
136.4, 136.0, 134.3, 133.9, 130.5, 129.7, 129.0, 127.8, 126.1,
126.0, 121.0, 118.2, 115.2, 112.2. MS: m/z (relative intensity):
432 [M]+ (18.32), 397 (100.00), 344 (9.58), 310 (10.77), 213
(3.65), 178 (4.34), 166 (2.41), 143 (3.91),140 (12.60), 115
(13.98), 77 (5.65), 64 (4.09), 51 (4.02). HRMS calcd for
([M])+, 432.0156; found: 432.0156. For 3: mp 225.0–225.3
1C. IR (KBr, v/cmꢀ1): 3071, 2225, 1590, 1550, 1320, 1128, 830.
1H NMR (400 MHz, CDCl3) d: 5.44 (s, 1H), 6.87–6.90 (d, J =
15.2 Hz, 1H), 6.93–6.96 (d, J = 2.4 Hz, 1H), 7.02–7.07 (m,
1H), 7.28–7.40 (m, 10H), 7.77 (s, 2H), 7.85 (s, 1H). 13C NMR
(100 MHz, CDCl3) d: 164.2, 146.0, 143.8, 143.5, 136.1, 135.7,
132.5, 130.0, 129.4, 129.0, 127.9, 127.8, 127.6, 127.3, 125.9,
125.0, 113.0, 113.0, 112.5. MS: m/z (relative intensity): 548
[M]+, 457 (35.80), 421 (8.98), 409 (6.80), 92 (100), 77 (6.19).
HRMS calcd for ([M])+, 548.0782; found: 548.0828.
4-Cinnamyl-1-(2,6-dichloro-4-(trifluoromethyl)phenyl)-6-phe-
nyl-1H-pyrazolo[3,4-b]pyridine-3-carbonitrile (4). 3 (0.55 g, 1
mmol) and tetrabutylammonium fluoride, TBAF, (0.40 g, 1.5
mmol) were dissolved in 10 mL of CH2Cl2. The mixture was
stirred at room temperature for 24 h. The CH2Cl2 was
removed in a vacuum and the residue purified by column
chromatography on silica gel to yield 85% of the pure
product: mp 170.0–170.5 1C. IR (KBr, v/cmꢀ1): 3071, 2918,
Fig. 1 1H NMR spectra (CDCl3) of 3 (1.97 ꢂ 10ꢀ2 M) with different
amounts of added Fꢀ ions: (A) 0 equiv.; (B) 0.50 equiv.; (C) 0.65
equiv.; (D) 1.24 equiv.; (E) 1.24 equiv. (30 h); (F) 4 itself; NH and CH
(except phenyl–H) shifts are shown.
and 13C NMR (100 MHz) spectra were recorded on a Bruker
AVANCE 400 MHz spectrometer using CDCl3 as solvent at
298 K and Me4Si as an internal standard. IR spectra were
recorded on a Nicolet Magna-IR 550 instrument using KBr
pellets. High resolution mass spectra (HRMS) were obtained
on a MicroMass GCT CA 055 spectrometer. Absorption
spectra were determined on a TU-1901 UV-Vis spectrophot-
ometer. Analytical thin-layer chromatography (TLC) was
carried out on precoated plates (silica gel 60F254), and spots
were visualized with ultraviolet light. All chemicals or reagents
1
2229, 1569, 1310, 1132, 830. H NMR (400 MHz, CDCl3) d:
3.64 (d, J = 5.2 Hz, 2H), 6.25 (m, 2H), 7.20–7.62 (m, 10H),
7.85 (s, 2H), 8.66 (s, 1H). 13C NMR (100 MHz, CDCl3) d:
153.9, 149.8, 143.7, 136.8, 136.5, 135.9, 134.5, 134.2, 132.9,
132.2, 130.2, 129.7, 129.0, 128.7, 128.6, 127.6, 126.1, 123.5,
120.9, 116.1, 111.7, 33.4. MS: m/z (relative intensity): 548
[M]+ (18.35), 513 (5.25), 457 (100), 421 (20.24), 409 (15.13),
92 (7.51). HRMS calcd for ([M])+, 548.0782; found, 548.0772.
1
were purchased from standard commercial suppliers. For H
NMR titrations a solution of receptor (1.97 ꢂ 10ꢀ2 mol Lꢀ1
)
Results and discussion
in CDCl3 was prepared in an NMR tube. For UV-Vis and
fluorometric titrations a solution of receptor (2 ꢂ 10ꢀ5 mol
Mechanism analysis from NMR study
L
ꢀ1) in CH2Cl2 (or CH3CN, CH3CN–H2O) was titrated with
1
The H NMR spectra in CDCl3 show dramatic changes for
incremental amounts of appropriate anion (0.001–0.1 mol
ꢀ1). A 10 mM aqueous HEPES buffer solution (pH 7.4)
receptor 3 upon addition of increasing amounts of fluoride
ions. A new set of peaks immediately appeared along with
decreasing peak intensity from 3, the internal –NH signal
(s, 1H, d = 7.85 ppm in the free receptor, Fig. 1A) disappears
slowly and the double bond Hb–d signals also reduce gradually,
which clearly indicate that the p systems are becoming re-
organized involving a change in the proton above. Meantime,
L
was used in a mixed solvent system. The anions (X) used in this
study are all as [Bu4N]X salts.
Synthesis
1-(2,6-Dichloro-4-(trifluoromethyl)phenyl)-6-phenyl-1H-pyr-
azolo[3,4-b]pyridine-3-carbonitrile (2) and (Z)-1-(2,6-dichloro-
4-(trifluoromethyl)phenyl)-6-phenyl-4-((E)-3-phenylallylidene)-
0
a new set of peaks (Hb –e ) appears with a finer proportional
0
0
0
0
relationship (Hb = 2Hc = 2He ), which could be assigned to
a new compound obtained in this fluoride–receptor interaction
system (Fig. 1B, 1C and 1D). On extending the reaction time,
legible signals are shown in Fig. 1E, which intensively indicate
that receptor 3 completely disappeared and only the new set of
peaks was observed. Further, the new set of peaks was
4,7-dihydro-1H-pyrazolo[3,4-b]pyridine-3-carbonitrile
(3).
5-Amino-1-(2,6-dichloro-4-(trifluoromethyl)phenyl)-1H-pyra-
zole-3-carbonitrile18 1 (0.64 g, 2.0 mmol) and cinnamaldehyde
(0.53 g, 4.0 mmol) were dissolved in 10 mL of CH3CN. HCl (3
drops) was added to the stirred solution and the reaction
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New J. Chem., 2008, 32, 472–476 | 473

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coincident with that of compound 4 (Fig. 1F). The comparison
of 1H-NMR spectra in detail, is shown in Fig. 1. Thus,
the fluoride-catalyzed intramolecular hydrogen transfer was
confirmed.
Anion-recognition studies
The changes observed in the absorption spectrum of receptor 3
in CH2Cl2 upon the addition of different anions are shown in
Fig. 2. The spectrum of receptor 3 with tetrabutylammonium
fluoride (TBAF) is observed with two new peaks at l = 256
and 310 nm. Compared to 3 itself (l = 386 nm), the larger
blue-shift of 3 binding with TBAF, might be ascribed to the
presence of a reduced p-conjugation network involving the
phenylallylidene moiety and the 1H-pyrazolo[3,4-b]pyridine.
A complete color change from yellow to colorless, was ob-
served after the addition of ten equivalents of fluoride ions.
The addition of acetate ions induced a less dramatic color
change. Greater amounts of acetate ions were required to
effect a commensurate change. On the other hand, exposure to
Clꢀ, Brꢀ, Iꢀ, NO3ꢀ and HSO4ꢀ (as their tetrabutylammonium
salts), did not lead to any obvious change in color. This
indicates that the sensor shows a very typical response based
on anion basicity.19 Fluoride gives the biggest response mainly
because it is more basic20 than acetate. This dramatic chromo-
genic response or nonresponse for a specific anion can make
receptor 3 a potential anion sensor. The changes observed
when receptor 3 was treated with increasing quantities of
TBAF in CH2Cl2 were also recorded (Fig. 3). The intensity
of the peak at 386 nm reduced and that of two new peaks at
l = 256 and 310 nm increased upon addition of TBAF with a
clear isosbestic point at 324 nm, and the absorbance intensity
of dosimeter 3 was saturated with 40 equiv. of Fꢀ. Meanwhile,
a color change of dosimeter 3 was observed from yellow to
colorless.
Fig. 3 UV-Vis titration of 3 (20 mM) with Bu4N+Fꢀ in CH2Cl2.
Arrows show changes due to increasing concentration of Fꢀ. The inset
shows the absorbance at 386 nm as a function of [Fꢀ].
enough to lose the excited energy easily through vibration or
migration. However, upon the addition of fluoride ions, the
emission spectrum displays an increasing strong band (lmax
=
400 nm, Fig. 4). This may be ascribed to the rigid pyrazolo-
pyridine fluorophore21 coming into being. Therefore, this
process clearly demonstrates that receptor 3 can selectively
detect fluoride ion by a fluorogenic ‘‘off–on’’ response.
In view of the fact that the detection method described
above is dependent on the fluoride-catalyzed reaction relating
to intramolecular hydrogen transfer, its operation in a mixed
solvent system, MeCN–water (19 : 1), was also carried out in
similar titrations as above. In this medium, dosimeter 3
exhibited a similar color change in UV-Vis absorption
(Fig. 5) and fluorescence enhancement (Fig. 6). But it required
greater amounts of fluoride ions (20 000 equiv.) to effect a
complete color change; this indicates that the fluoride ions
show extraordinarily strong basicity in media of low polarity22
(the titrations in CH3CN solvent are shown in ESIz). It should
be noted that this is a novel approach to selectively recognize
Fluorescence, on account of its simplicity and high sensi-
tivity, is becoming of increasing importance for chemical trace
detection. Therefore, fluorescent titrations of receptor 3 with
TBAF were also recorded to evaluate it as a selective fluor-
escent receptor for the fluoride ion. In CH2Cl2 solvent, re-
ceptor 3 shows very weak fluorescence. Though it was
donor–acceptor system, the hydrogen atom might be flexible
Fig. 4 Increase in fluorescence emission intensity (lex = 324 nm)
when sensor 3 (20 mM in CH2Cl2) is titrated with increasing concen-
tration of Fꢀ. Arrows show changes due to the increasing concentra-
tion of Fꢀ. The inset shows the emission intensity at 400 nm as a
function of [Fꢀ].
Fig. 2 UV-Vis spectra of 3 (10 mM in CH2Cl2) with different anions
(ca. 10 equiv.). The inset shows color changes from yellow to colorless.
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´
´
´
In summary, we have demonstrated above (1) a previously
undescribed organic reaction resulting in dosimetric fluoride
ion determination; (2) obvious features such as high selectivity
for Fꢀ over other anions, fluorescence enhancement as well as
dramatic color changes. A further study on the development of
chemosensors/chemodosimeters based on analyte-specific re-
actions is in progress in our laboratory.
´
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ꢁc
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2008
476 | New J. Chem., 2008, 32, 472–476