2-(4-Formylphenyl)phenanthroimidazole as a colorimetric and fluorometric probe for selective fluoride ion sensing

Article abstract of DOI:10.1007/s10895-011-0858-7

An easy-to-prepare chemosensor, 2-(4-formylphenyl) phenanthroimidazole 1, synthesized by facile onestep condensation under microwave irradiation, has been indicated to be a colorimetric and fluorometric probe for fluoride anion with good sensitivity and high selectivity. This probe shows obvious red shift and absorbance intensity changes in UV absorption and fluorescence spectra in the presence of fluoride anions. Such color and absorbance intensity changes are attributed to the deprotonation of N-H on the imidazole moiety with fluoride anion. Springer Science+Business Media, LLC 2009.

Full text of DOI:10.1007/s10895-011-0858-7

J Fluoresc (2011) 21:1331–1335
DOI 10.1007/s10895-011-0858-7
SHORT COMMUNICATION
2
-(4-Formylphenyl)phenanthroimidazole as a Colorimetric
and Fluorometric Probe for Selective Fluoride Ion Sensing
Fanpeng Kong & Qingjian Liu & Xue Wu & Zhen Wang &
Qing Chen & Lusheng Chen
Received: 23 December 2010 /Accepted: 13 February 2011 /Published online: 24 February 2011
#
Springer Science+Business Media, LLC 2011
Abstract An easy-to-prepare chemosensor, 2-(4-formyl-
phenyl)phenanthroimidazole 1, synthesized by facile one-
step condensation under microwave irradiation, has been
indicated to be a colorimetric and fluorometric probe for
fluoride anion with good sensitivity and high selectivity.
This probe shows obvious red shift and absorbance
intensity changes in UV absorption and fluorescence
spectra in the presence of fluoride anions. Such color
and absorbance intensity changes are attributed to the
deprotonation of N-H on the imidazole moiety with
fluoride anion.
fundamental role in a wide range of chemical, biolog-
ical, medical, and environmental processes [1–4]. Fluo-
ride ion, the smallest anion, has unique chemical
properties, and its recognition and detection are of
growing interest because it is associated with dental care,
the treatment of osteoporosis, nerve gases, the analysis of
drinking water, and the refinement of uranium used in
nuclear weapons manufacture. A number of synthetic
colorimetric [5–9], fluorometric [10–15], as well as dual
mode [16–25] probes have been designed and made for
this important analyte. All of these probes require
complicated synthesis. Easily and simply preparable
fluoride ion probes are therefore necessary and useful for
practical applications.
.
.
Keywords Phenanthroimidazole Fuoride anion
.
Chemosensor Colorimetric and fluorometric probe
The plane-structured molecule 1, 2-(4-formylphenyl)
phenanthroimidazole, synthesized by facile one-step con-
densation, has unique thermal stability and optical proper-
ties, and has been used as probes for copper ion [26],
cysteine and homocysteine [27]. Herein, we report the
studies of this compound as a highly selective fluoride ion
probe in both colorimetric and fluorometric analyses.
Introduction
The development of chemosensors for selective anion
recognition has received much attention, as anions play a
Experimental Section
F. Kong
Department of Chemistry, College of Science, Tianjin University,
Tianjin 300072, People’s Republic of China
Solvents and reagents were used as received. The NMR
spectra were determined on a Bruck 300 MH spectrometer
z
:
:
:
:
Q. Liu (*) X. Wu Z. Wang Q. Chen L. Chen
College of Chemistry, Chemical Engineering and Materials
Science, Key Laboratory of Molecular and Nano Probes,
Engineering Research Center of Pesticide and Medicine
Intermediate Clean Production, Ministry of Education, Shandong
Provincial Key Laboratory of Clean Production of Fine
Chemicals, Shandong Normal University,
using TMS as the internal standard in DMSO-d ; UV–vis
6
spectra were taken on a Pharmaspec TU-1901 spectropho-
tometer using quartz cells with path-length of 1 cm;
Fluorescence spectra were recorded on a Varian Cray
Eclipse fluorescence spectrophotometer with quartz cells
of path-length 1 cm. All measurements were performed
under ambient atmosphere at room temperature.
Jinan 250014, People’s Republic of China
e-mail: liuqj@sdnu.edu.cn

1
332
J Fluoresc (2011) 21:1331–1335
Scheme 1 Synthesis of
-(4-formylphenyl)phenanthroi-
midazole 1
CHO
CHO
2
O
O
N
NH OAc
4
+
CHO
HOAc, MWI
N
H
1
Synthesis of the Probe 1 (Scheme 1)
phenyl unit, the electron rich phenanthroimidazole ring
acted as both a good fluorescent dye and a fairly strong
electron donor [29].
Fluorescent anion titration studies as shown in Fig. 1, the
probe exhibited greatly decrease of fluorescence intensity
as well as a large emission peak red-shifted (80 nm) upon
addition of fluoride anion, which is attributed to the
deprotonation with fluoride anion of the phenanthroimida-
zole moiety of 1, becoming a negatively charged core with
stronger electron donation.
Changes in the fluorescence spectra of probe 1 caused by
other halide anions were also investigated. With the
addition of 10 equivalents of halide anions for 10 min,
respectively, the fluorescence spectra of the solution 1 were
measured and presented in Fig. 2. It can be observed that
only fluoride ion give rise to dramatic changes in the
fluorescence spectra, while the other anions did not cause
obvious changes under the same conditions. Such result
implies that the selectivity of 1 for fluoride ion over the
other anions is remarkably high and efficient.
A mixture of phenanthraquinone 0.21 g (10 mmol),
ammonium acetate 0.39 g (50 mmol), and terephthalalde-
hyde 0.40 g (30 mmol) in 20 mL acetic acid was irradiated
in a microwave synthesizer for 10 min monitored with TLC
[
28]. The reaction mixture was neutralized with aqueous
ammonia and the crude product was collected and recrystal-
lized from nitrobenzene to afford the pure product, 71%
yield, and mp 315–317°C. IR (KBr pellet): ν
3442,
max
−
1 1
1
1
8
678 cm . H NMR (300 MHz, DMSO-d ): δ 13.77(s,
6
H
H), 8.82–9.10 (d, J=8.2 Hz, 2 H), 8.51–8.63 (m, 4 H),
.41–8.61 (d, J=8.2 Hz, 2 H), 7.81 (s, 2 H), 6.80 (s, 2 H)
ppm. C NMR (75 MHz, DMSO-d ): 122.39, 122.66,
1
3
6
1
1
1
24.27, 124.63, 125.98, 126.30, 126.90, 127.28, 127.68,
27.79, 128.19, 128.52, 128.80, 130.67, 135.87, 136.51,
37.86, 148.15, 193.02 ppm.
Results and Discussion
UV–Vis Anion Titration Studies
1
The resonance signal at 13.77 ppm in the H NMR
spectrum of 1 was assigned to the hydrogen of N-H in the
imidazole moiety of the molecule 1. The probe 1 is
composed of a phenanthroimidazole moiety and a formyl-
Titration of 1 as a function of fluoride ion (shown in Fig. 3)
gave a decreasing intensity with the formation of red-shifted
band at 440 nm. The fluoride ion induced colorimetric and
5
Fig. 1 Changes in emission spectra of 1 in Dichloromethane (DCM,
1
Fig. 2 Changes in absorption of 1 measured in DCM (1.0×10− M)
.0×10− M) upon addition of F (1–10 equiv) in (λex=370 nm)
5
−
upon addition of 10 equivalents of halide anions (as n-Bu
N salts)
+
4

J Fluoresc (2011) 21:1331–1335
1333
Fig. 5 Plot of the absorbance ratio of 1 DCM(1.0×10− M) at 387 nm
5
Fig. 3 Changes in absorption spectra of 1 in DCM(1.0×10−5 M) upon
addition of tetrabutylammonium fluoride
−
and 440 nm (A440/A387) vs. concentration of F in DCM
fluorometric responses of 1 are simply driven by the
intermolecular proton transfer between the imidazole ring
and the fluoride anion. The spectral changes are due to the
deprotonation [30–32] of the phenanthroimidazole moiety by
fluoride ion. This would suggest that a negatively charged
phenanthroimidazole has formed, which causes a significant
increase in the charge density on the imidazole moiety
associated with enhancement in the push-pull effect of the
intermolecular charge transfer (ICT) transition [33]. As a
result, when the imidazole moieties are deprotonated, charge
redistribution takes place within the phenanthroimidazole
molecules, the deprotonated push-pull chromospheres are
responsible for the shift in both absorption (changes from
colorless to yellow) and fluorescence (changes of fluores-
cence from cyan to yellow) (Fig. 4). The perceived color
changes would be useful not only for the ratiometric
detection but also for the rapid visual sensing.
Figure 5 shows a correlation between intensity ratios of
absorption at 440 nm and 387 nm (A440/A387) and
concentration of fluoride ion in dichloromethane. This
demonstrates the potential utility of probe 1 for calibrating
and determining fluoride ion concentration in dichloro-
methane in the presence of other halide ions, which did not
cause any changes of the absorption of 1. Figure 6 reveals
that the changes in spectral absorption of 1 measured in
DCM upon addition 10 equivalents halide anions, respec-
tively. Without fluoride anion, 1 exhibits a characteristic
absorption band centered at 387 nm, but the addition of
fluoride ion leads to a sharp decrease in this absorption and
a development of red-shifted absorption centered at
Fig. 4 Color (left) and fluores-
cence (right, irradiation at
3
65 nm with UV lamp) changes
−5
of 1 in DCM (1.0×10 M)
upon addition of 10 equivalents
of tetrabutylammonium fluoride

1
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J Fluoresc (2011) 21:1331–1335
Analytical Applications
Experimental results given show the performance of probe
−
1
with high selectivity for F anion. Color changes were
observed obviously, turning from cyan to yellow upon
−
−5
addition of F to the solution of 1 (1.0×10 M). However,
no detectable color responses were observed while adding
−
−
−
other halide anions such as Cl , Br and I ions.
Conclusion
In summary, we have presented a chemosensor for fluoride
anion, 2-(4-formylphenyl)phenanthroimidazole 1, synthe-
sized by facile one-step condensation under microwave
irradiation, behaving as a highly sensitive fluoride anion
probe in both colorimetric and fluorometric analyses. The
sharp changes in UV absorption and fluorescence spectra
are simply driven by the interaction between the imidazole
N-H proton and fluoride ion. The remarkable photophysical
properties of the probe would help to extend the design and
development of colorimetric and ratiometric fluorescent
chemosensors for fluoride anion.
Fig. 6 Changes in absorption of 1DCM (1.0×10−5 M) measured in
+
DCM upon addition of 10 equivalents of halide anions (as n-Bu
4
N salts)
4
40 nm. In contrast, the addition of other halide anions
does not produce any spectral change.
1
H NMR Titrations
To further elucidate the nature of the intermolecular
Acknowledgments Tianjin University and College of Chemistry,
Chemical Engineering and Materials Science, Key Laboratory of
Molecular and Nano Probes, Engineering Research Center of Pesticide
and Medicine Intermediate Clean Production, Ministry of Education,
Shandong Provincial Key Laboratory of Clean Production of Fine
Chemicals, Shandong Normal University are acknowledged.
interactions between anions and probe 1, we carried out
1
the H NMR analysis of the probe in the presence of TBAF
in DMSO-d (Fig. 7). Obviously, the resonance signal at
6
1
3.77 ppm, which was assigned to the hydrogen of N-H
group on the imidazole moiety of the molecule 1, can be
observed in the absence of fluoride anion. The disappear-
ance of the NH signal in the presence of TBAF confirms
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