A ratiometric fluorescent probe for fluoride ion based on naphthoimidazolium receptor

Article abstract of DOI:10.1039/c4ra06062h

Three imidazolium derivatives 3-5 were designed and synthesized, in which naphthaimidazolium group acted as both fluorophore and anion receptor. Compound 3 exhibited high selectivity for F^- in CH₃CN solution over all the other anions and acted as a ratiometric fluorescent probe for F^- with an enhanced blue-shift in emission. However, the fluorescence of compound 4 and 5 displayed a quenched blue-shift in emission with fluoride ion and could be quenched by some other tested anions, where the degree of quenching depended on the characteristic of the anions. More importantly, only compound 3 could detect F^- in DMSO-water (955, v/v) aqueous solution ratiometrically. Based on the analysis of the results of ¹H-NMR and ¹⁹F-NMR, it was deduced that compound 3 bound with F^- mainly by the force of hydrogen bonding, while compound 4 and 5 coordinated with F^- through electrostatic interaction. This journal is

Full text of DOI:10.1039/c4ra06062h

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PAPER
A ratiometric fluorescent probe for fluoride ion
based on naphthoimidazolium receptor†
Cite this: RSC Adv., 2014, 4, 43746
Chunyan Zou,a Qinglong Qiao,ab Miao Zhao,b Deqi Mao,ab Danfeng Wang,a Lei Feng,a
a
ab
*
*
Jingnan Cui and Zhaochao Xu
Three imidazolium derivatives 3–5 were designed and synthesized, in which naphthaimidazolium group
acted as both fluorophore and anion receptor. Compound 3 exhibited high selectivity for Fꢀ in CH3CN
solution over all the other anions and acted as a ratiometric fluorescent probe for Fꢀ with an enhanced
blue-shift in emission. However, the fluorescence of compound 4 and 5 displayed a quenched blue-shift
in emission with fluoride ion and could be quenched by some other tested anions, where the degree of
quenching depended on the characteristic of the anions. More importantly, only compound 3 could
detect Fꢀ in DMSO–water (95 : 5, v/v) aqueous solution ratiometrically. Based on the analysis of the
Received 21st June 2014
Accepted 5th September 2014
DOI: 10.1039/c4ra06062h
1
results of H-NMR and 19F-NMR, it was deduced that compound 3 bound with Fꢀ mainly by the force of
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hydrogen bonding, while compound 4 and 5 coordinated with Fꢀ through electrostatic interaction.
been designed with uorescence quenching signals,2,4,8 which is
not propitious to quantitative detection.
Introduction
In the previous work,11,19,21 naphthaimidazolium group was
Fluorine is an essential trace element which is oen added to
toothpaste and drinking water because of its pivotal role in the
prevention of dental problems and treatment of osteoporosis.
However, excessive uoride causes skeletal uorosis and oste-
osarcoma.1 Therefore, development of detection methods for
uoride ion has received great attention. Many uorescent
probes toward uoride ion sensing have been reported in recent
years. Fluoride ion is the smallest radius unary anion with high
electron-negativity. According to this feature, a lot of Fꢀ recep-
tors have been designed based on different recognition mech-
anism, including hydrogen bonding,2–4 deprotonation,5–7
electrostatic interaction,2,8 specic reaction with uoride ion,9–12
special structure such as ring and cage,13–15 and anion–p
interactions.16–19 Many receptors have been reported combining
with multiple function units.7–9 Imidazolium is a useful func-
tional group in Fꢀ detecting because it is able to form a special
C–H/X hydrogen bonding through the 2-positon H atom. This
type of hydrogen bonding can be functioned as the (C–H)+/Xꢀ
type because the charge–charge electrostatic interaction domi-
nates.20,21 Although many imidazolium salts for Fꢀ detecting
have been reported so far, a lot of them are lack of effective
signal output, which is detected by NMR or electrochemistry
methods.13,15 In addition, a number of uorescent probes have
utilized as the signal source for the detection system, and also
as an anion binding receptor. As shown in Scheme 1, the cage
compounds 1 and 2 contained three naphthaimidazolium
groups inserted between two substituted benzene rings.21 With
the addition of 1 equiv. of Fꢀ, a strongly increased uorescent
emission of compound 2 centered at 385 nm appeared at the
expense of 474 nm. However, the addition of uoride quenched
the uorescence of compound 1. 19F and 1H NMR spectroscopy,
uorescence emission and theoretical calculations results
showed that three (C–H)+/Fꢀ-type ionic hydrogen bonds
formed and from this, the anion–p interaction between the two
electron-rich alkylbenzene rings and the uoride ion existed in
compound 2. For compound 1, the electrostatic interaction
between the imidazolium and uoride anion dominated in the
recognition process.21
aState Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian
116012, China. E-mail: jncui@dlut.edu.cn
bKey Laboratory of Separation Science for Analytical Chemistry, Dalian Institute of
Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China. E-mail:
zcxu@dicp.ac.cn
† Electronic supplementary information (ESI) available: Detailed characterization
results of synthesized compounds. See DOI: 10.1039/c4ra06062h
Scheme 1 Structures of compounds 1–5.
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Synthesis of compound 11
Under N2 gas, 8 (0.22 g, 1.3 mmol) was dissolved in 15 mL THF
and NaH (petroleum ether 60%, 0.065 g, 1.6 mmol) was added
under ice bath. The mixture was stirred for 0.5 h, and benzyl
bromide (0.18 mL, 1.5 mmol) in THF (5 mL) was added drop-
wise. Then the mixture was kept for 2 h under ambient
temperature, and water (25 mL) was added to nish the reac-
tion. Multiple extraction with CH2Cl2 combined with drying by
MgSO4. The ltrate was concentrated and puried by ash
column chromatography (CH2Cl2–CH3OH, 100 : 1, Rf ¼ 0.5) to
obtain the compound 11 (0.26 g, yield: 75%). 1H NMR (400 MHz,
CDCl3, d ppm) 8.32 (s, 1H), 8.12 (s, 1H), 8.01 (d, J ¼ 5.9 Hz, 1H),
7.87 (d, J ¼ 5.9 Hz, 1H), 7.68 (s, 1H), 7.40 (d, J ¼ 5.0 Hz, 2H), 7.34
(d, J ¼ 7.8 Hz, 3H), 7.23 (d, J ¼ 6.4 Hz, 2H), 5.43 (s, 2H), 1.55–
1.53 (m, 1H). MS (ES API+) m/z [C18H14N2 + H]+ calcd 259.3,
found: 259.2.
Scheme 2 Synthesis of compounds 3–5.
Synthesis of compound 12
Starting from compound 8 (0.20 g, 1.2 mmol) and penta-uoro-
benzyl bromide (0.21 mL, 1.4 mmol), compound 12 was
prepared according to the same procedure of compound 11 and
was puried by ash column chromatography (CH2Cl2–CH3OH,
100 : 1, Rf ¼ 0.4) (0.28 g, yield: 68%). 1H NMR (400 MHz, CDCl3,
d ppm) 8.32 (s, 1H), 8.12 (s, 1H), 8.01 (d, J ¼ 5.9 Hz, 1H), 7.87 (d, J
¼ 5.9 Hz, 1H), 7.68 (s, 1H), 7.40 (d, J ¼ 5.0 Hz, 2H), 7.34 (d, J ¼
7.8 Hz, 3H), 7.27–7.17 (m, 2H), 5.43 (s, 2H), 1.31 (t, J ¼ 231.6 Hz,
1H). MS (ES API+) m/z [C18H9N2 + H]+ calcd 349.3, found: 349.1.
In this study, a two benzyl naphthaimidazolium salts
(compound 3), an approximate “half open structure” of
compound 1, was designed and synthesized to further study the
mechanism. By the change of the substitutions in the 1,3-
position of imidazole, another two reference compounds 4 and
5, with one or two electropositive pentauoro-substituted
benzyl groups, were synthesized. (Scheme 2).
Synthesis of compound 3
Experimental
Under N2 gas, benzyl bromide (55 mL, 0.47 mmol) was added to
the solution of compound 11 (0.10 g, 0.39 mmol) in 15 mL
dioxane, which was heated to reux and kept for 24 h. Aer
cooling to room temperature, the yellowish precipitate was
collected by ltration and then washed with some cold CH2Cl2
to give the bromated. Then it was dissolved in DMF, and
dropped saturated KPF6 aqueous solution to precipitate the
product 3, which was puried by ash column chromatography
(CH2Cl2–CH3OH, 20 : 1, Rf ¼ 0.4) to give light gray white powder
(0.11 g, yield: 60%). 3: 1H NMR (400 MHz, CD3CN, d ppm) 9.19
(s, 1H), 8.31 (s, 2H), 8.10 (dd, J ¼ 6.4, 3.3 Hz, 2H), 7.64 (dd, J ¼
6.5, 3.2 Hz, 2H), 7.52 (dd, J ¼ 7.8, 1.5 Hz, 4H), 7.46 (m, J ¼ 7.2
Hz, 6H), 5.70 (s, 4H). 13C NMR (100 MHz, CD3CN, d ppm)
147.24, 134.37, 131.38, 130.64, 129.50, 129.22, 128.83, 128.71,
127.26, 111.90, 50.61. HRMS (TOF LD+) m/z [C25H21N2+] calcd
349.1699, found: 349.1728.
All chemicals were analytical pure and used without further
purication. 1H-NMR and 13C-NMR spectra were recorded on a
Bruker AV-400 spectrometer (TMS as an internal standard).
Mass spectrometry data were obtained with a HP1100LC/MSD
and MALDI micro MX mass spectrometer. Fluorescence
measurements were carried out on a JASCO FP-6500 uores-
cence spectrophotometer. Absorption spectra were collected on
a Hewlett-Packard HP-8453 UV-Vis spectrophotometer.
Synthesis of compound 8
Under N2 gas, 2,3-diaminonaphthalene (1.0 g, 6.3 mmol) was
dissolved in 10 mL formic acid. The solution was heated to
reux and kept for 5 h. Aer cooling to room temperature, the
mixture was concentrated to remove formic acid. The residue
was treated with 30 mL boiling water to dissolve, reuxed for 0.5
Synthesis of compound 4 and 5
h with some activated charcoal, and then ltered while hot and Starting from compound 12 (0.20 g, 0.57 mmol), compound 4
added stronger ammonia water to the ltrate dropwise until the (0.21 g, yield: 64%) and 5 (0.20 g, yield: 59%) were obtained
product precipitated completely. The grey white precipitate was according to the synthetic procedure of 3.
collected by ltration and then dried in a vacuum oven to give 8
Compound 4. 1H NMR (400 MHz, CD3CN, d ppm) 9.30 (s,
(0.75 g, yield: 71%). 1H NMR (400 MHz, CDCl3, d ppm): 8.20 (s, 1H), 8.36 (s, 1H), 8.32 (s, 1H), 8.22–8.17 (m, 1H), 8.13–8.08 (m,
1H), 8.09 (s, 2H), 7.95 (d, J ¼ 3.0 Hz, 2H), 7.41 (d, J ¼ 3.1 Hz, 2H), 1H), 7.71–7.64 (m, 2H), 7.52 (dd, J ¼ 7.7, 1.6 Hz, 2H), 7.47 (dd, J
3.10 (s, 1H). MS (ES API+): m/z [C11H8N2 + H]+ calcd 169.19, ¼ 10.4, 5.1 Hz, 3H), 5.83 (s, 2H), 5.72 (s, 2H). 13C NMR (100 MHz,
found: 169.1.
CD3CN, d ppm) 146.57, 133.56, 132.65, 132.54, 131.08, 30.26,
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129.47, 129.37, 129.18, 128.36, 128.32, 118.27, 112.71, 111.81,
111.79, 111.77, 52.13, 39.59. HRMS (TOF LD+) m/z
[C25H16F11N2+] calcd 439.1228, found: 439.1198.
Compound 5. 1H NMR (400 MHz, CD3CN, d ppm) 9.40 (s,
1H), 8.35 (s, 2H), 8.21 (dd, J ¼ 5.9, 2.9 Hz, 2H), 7.70 (dd, J ¼ 6.3,
2.8 Hz, 2H), 5.85 (s, 4H). 13C NMR (100 MHz, CD3CN, d ppm)
148.13, 147.08, 145.69, 137.67, 132.64, 130.68, 129.39, 128.50,
112.01, 107.79, 39.83. HRMS (TOF LD+) m/z [C25H21N2+] calcd
529.0757, found: 529.0752.
Results and discussion
The selectivity of 3–5 for uoride ion detection was rst inves-
tigated through UV-Vis spꢀectra by adding various anions (Fꢀꢀ,
ꢀ
Clꢀ, Brꢀ, Iꢀ, AcOꢀ, H2PO4 and HSO4 as n-Bu4N+ salts; NO3
as sodium salt) to the solution of compound 3–5 in CH3CN
(Fig. S1†). The free compounds 3–5 displayed a similar
absorption band peaked at 325 nm due to the absorption of
naphthalene part. Upon addition of 5 equiv. of Fꢀ, the
absorption peak of compound 3–5 approximately shied to 347
nm (Fig. 1 and S1†), which indicated the strong interactions
between compound 3–5 and Fꢀ in stationary state. With addi-
ꢀ
tion of 5 equiv. of other anions (Clꢀ, Brꢀ, Iꢀ, AcOꢀ, H2PO4
,
NO3ꢀ, and HSO4ꢀ), the UV-Vis spectra of compound 3 exhibited
little change, whereas the absorption spectra of compound 4
and 5 showed different levels of redshi on the addition of 5
ꢀ
equiv. of AcOꢀ and H2PO4
.
The UV-Vis titration experiments were performed in CH3CN
solution to investigate the behaviour of compound 3–5 upon
progressive addition of Fꢀ, as shown in Fig. 1. When Fꢀ (0–40
equiv.) was progressively added to a solution of 3, a decrease of
325 nm was observed with the appearance of a red-shied and
increased absorption centered at 347 nm.
It suggested the hydrogen bond formed between compound
3 and Fꢀ reduced the transfer energy of excited stated charge to
redshi the absorption spectra. As for compound 5, the rst 4
equiv. Fꢀ induced the similar UV-Vis response compared to
compound 3, but more Fꢀ addition (4–40 equiv.) decreased the
intensity at 347 nm while a new absorption at 360 nm appeared
(Fig. S2†). Compound 4 displayed the same responses as
compound 5 when Fꢀ was added gradually. It showed that
compound 4 and 5 may experience two steps of complexation
with Fꢀ.
Fig. 1 Changes in UV-Vis spectra of compound 3–5 (10 mM) upon the
addition of 0–40 equiv. of Fꢀ in CH3CN. (a) Compound 3, (b)
compound 4 and (c) compound 5.
The uorescence emission spectroscopy and titration
experiments were displayed in Fig. S3† and 2, respectively. The
emission spectrum of free 3 displays a broad band with a
maximum at 438 nm when it was excited at 331 nm. When Fꢀ
was added progressively to a solution of 3, a signicant undergo internal charge transfer (ICT) from naphthalene to
increasing uorescent emission in the 373 nm was observed at imidazolium upon excitation by light, with imidazolium acting
the expense of the uorescent emission centered at 438 nm, as an acceptor. When Fꢀ bonds with imidazolium, the electron-
which behaved differently from compound 1. A clear iso- withdrawing ability of imidazolium would be weakened and an
emission point appeared at 407 nm, and no obvious changes anion-induced blueshi in emission would occur.19
were observed with the addition of other anions. Hence, 3 is a
As for compound 4 and 5, a strong uorescence quenching
ratiometric uorescent probe for Fꢀ, and the dependence of the were observed with the addition of Fꢀ, and the uorescence
ratio of the emission intensities at 373 and 438 nm (I373/I438) on decreased to the minimum with about 4 equiv. Adding more Fꢀ
the concentration of Fꢀ was shown in the inset of Fig. 2(a). induced a low uorescence recovery peaked at 396 nm, which
Naphthoimidazolium is a donor–acceptor system and can was in accordance with the UV-Vis titration results of
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naphthoimidazolium part under the force of static electricity for
the steric hindrance of the pentauoro benzyl group, which
induced a PET effect from Fꢀ to naphthalene to quench the
uorescence, and then small uorescence recovered until a
comparable stable hydrogen bond formed aer more than 4
equiv. Fꢀ addition. While 3 was attacked by Fꢀ from the direc-
tion of imidazolium–C2–H more easily for the less hindrance,
which would not quench the uorescence.
To understand the interaction of compound 3–5 with Fꢀ, 1H
NMR and 19F NMR tests were carried out in CD3CN and the
results are shown in Fig. 3, 4 and S4.† As shown in Fig. 3(a),
when 1 equiv. of Fꢀ was added to the solution of 3, the peak of
imidazole–C2–H shied downeld (d ¼ 9.19 to 9.50 ppm), which
indicates a hydrogen bond was formed between the proton and
Fꢀ. Upon the addition of Fꢀ $ 2 equiv., the signal of imidazole–
C2–H of compound 3 disappeared, which might be due to the
partial deprotonation of imidazole–C–H and/or the formation
of imidazole–C2–F complex.19 The aromatic hydrogen almost
shied higheld due to the electrostatic interaction between
compound and anion. In the corresponding 19F NMR of
ꢀ
compound 3 as shown in Fig. 3(b), the resonance of PF6
appeared as a doublet at d ¼ 72 and 74 ppm. The sharp singlet at
d ¼ 152 ppm may result from C2–H–Fꢀ, and the signal increased
Fig. 2 Fluorescence titration spectra of 3–5 (a–c) in the presence of
different amounts of Fꢀ (0–40 equiv.) in CH3CN. [3] ¼ [4] ¼ [5] ¼ 10
mM, lex ¼ 331 nm. Inset: ratiometric calibration curve I373/I438 of
compound 3 as a function of the Fꢀ concentration.
ꢀ
compound 4 and 5. Adding of AcOꢀ and H2PO4 caused little
emission changes between 4 and 5, and negligible changes
occurred before and aer adding of other anions. The uores-
cence responses of compounds 4 and 5 were quenched by Fꢀ
while compound
3 behaved as a selective uorescence
increased probe for Fꢀ. Based on the former work,11,18 we
preliminarily concluded that most amount of Fꢀ approached
Fig. 3 1H NMR (a) and 19F NMR (b) spectra of 3 with fluoride in CD3CN
compounds
4 and 5 from the direction close to the
at room temperature. [3] ¼ 2 ꢁ 10ꢀ2 mol Lꢀ1
.
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Scheme
3
Proposed recognition process that occurred when
excess Fꢀ was added to compounds 3–5.
Fig. 4 1H NMR (a) and 19F NMR (b) spectra of 5 with fluoride in CD3CN
at room temperature. [5] ¼ 1 ꢁ 10ꢀ2 mol Lꢀ1
.
with the amount of Fꢀ. As for compound 4 and 5, although
different responses observed in UV-Vis and uorescence emis-
1
sion, the H NMR and 19F NMR results displayed the similar
trends as compound 3: the signal of imidazole–C2–H dis-
appeared with the progressive addition of Fꢀ, meanwhile the
aromatic hydrogen shied higheld and the sharp singlet at d ¼
152 ppm in 19F NMR increased with the amount of Fꢀ. These
results indicated that interaction of hydrogen bond and elec-
trostatic force also existed in compound 4 and 5. The sharp
singlet appeared at near d 125 ppm corresponds to the free Fꢀ
which appeared in the 19F NMR results of compound 4 and 5.
Besides, a board singlet at about 150 ppm was almost due to the
formation of C2–F bond,19,22 and the MALDI-TOF mass spectra
(Fig. 5, S5 and S6†) were examined to prove the mechanism. For
Fig. 6 Fluorescence responses of 3 (a), 4 (b) and 5 (c) to various anions
(50 mM) in DMSO–water (95 : 5, v/v). [3] ¼ [4] ¼ [5] ¼ 10 mM, lex
¼
Fig. 5 MALDI-TOF (LD+) spectrum of compound 5 with 5 equiv. Fꢀ
331 nm.
addition.
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example, the peak at m/z 529.07 was assigned to M+ of
compound 5, and the peak at m/z 548 was uorine adduct of 5 as
shown in the inset of Fig. 5. Similar results were observed for
compound 3 and 4 (see Fig. S5 and S6†). In addition, the
aromatic uorine of compound 4 and 5 shied higheld which
may result from the interaction between Fꢀ and the
pentauoro-benzyl groups, such as anion–p interaction for the
positive quadrupole moment in the pentauoro-substituted
benzyl groups.
Notes and references
¨
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Based on the above discussion, we know that compound 3–5
bound with Fꢀ by hydrogen bond and electrostatic force, and
excess Fꢀ induced deprotonation and addition (Scheme 3).
In addition, the Fꢀ binding ability of compound 3–5 in
aqueous solution was examined by the uorescence responses
in the presence of Fꢀ in DMSO–water (95 : 5, v/v). Fig. 6 shows
the uorescence emission of compound 3–5 with 5 equiv. of
various anions. It should be noticed that compound 3 behaved
similarly in DMSO–water (95 : 5, v/v) as in CH3CN, while only
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We thank nancial supports from the National Natural Science
Foundation of China (21276251), Ministry of Human Resources
and Social Security of PRC, the 100 talents program funded by
Chinese Academy of Sciences, and State Key Laboratory of Fine
Chemicals of China (KF1105).
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