Differentiation of cis-and trans-isomers of the novel napthalene-aza receptor by naked-eye colorimetric anion sensing

Article abstract of DOI:10.1016/j.tetlet.2011.09.103

Novel chromofluorogenic receptors 1 (cis-isomer) and 2 (trans-isomer) were developed using an aza-nitro-phenyl group as a chromophore, urea moiety as a binding unit, and naphthalene as a fluoro-phore. The position of nitro-phenyl moiety in the chromophore

Full text of DOI:10.1016/j.tetlet.2011.09.103

Tetrahedron Letters 52 (2011) 6465–6469
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Tetrahedron Letters
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Differentiation of cis- and trans-isomers of the novel napthalene-aza receptor
by naked-eye colorimetric anion sensing
T. Daniel Thangadurai a,b, Gyusung Chung c, Ohyun Kwon d, Dongri Jin e, Yong-Ill Lee a,
⇑
a Anastro Laboratory, Department of Chemistry, Changwon National University, Changwon 641-773, Republic of Korea
b Department of Bio and Nano Chemistry, Kookmin University, 861-1 Jeongneung-dong, Seongbuk-gu, Seoul 136-702, Republic of Korea
c Department of Chemistry, Konyang University, Chungnam 320-711, Republic of Korea
d Samsung Mobile Display Co., Ltd, Yongin-Si, Gyeonggi 446-711, Republic of Korea
e Department of Chemistry, Yanbian University, Yanji 133002, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Novel chromofluorogenic receptors 1 (cis-isomer) and 2 (trans-isomer) were developed using an
aza-nitro-phenyl group as a chromophore, urea moiety as a binding unit, and naphthalene as a fluoro-
phore. The position of nitro-phenyl moiety in the chromophores influences the naked-eye colorimetric
anion sensing which differentiates the geometrical isomers. Receptor 2 showed sensitivity toward
Received 1 September 2011
Revised 15 September 2011
Accepted 22 September 2011
Available online 29 September 2011
F
ꢀ than HP2O73ꢀ and its fluorescence emission (kmax = 370 nm) was significantly ‘switched-off’ at an exci-
tation wavelength of 260 nm in CH3CN:DMSO (90:10, v/v) solution at 25 °C. The fluorescence titration
experimental results revealed that the receptor 2 binds strongly with Fꢀ than HP2O73ꢀ ions in 1:1 stoichi-
ometry. The quantum mechanical calculations through time dependant density functional theory
(TD-DFT) using basis set B3LYP/6-31G(d) supported our experimental findings agreeably.
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
Isomers
Urea
Anions
Colorimetric sensors
Fluorescence
Photoinduced electron transfer
Introduction
sensitivity at low analyte concentrations.5 Fluorosensors for a large
number of biotic and abiotic analytes have been designed in the
Anion recognition and sensing have received considerable
attention in recent years.1,2 The limited reports on colorimetric an-
ion sensing and selective affinity describe the complexity of the de-
sign and synthesis required to increase the intrinsic association
and ion specificity of a receptor.3 Targets such as pyrophosphate
and fluoride are of particular interest, as they possess important
biological and medicinal properties, and there are many reports
of their detection using fluorescent and colorimetric techniques.2
As they offer suitable binding sites for guests and stabilize com-
plexes by non-covalent interactions such as hydrogen and ionic
bonding, urea-based anion sensors are of utmost importance.4
Deprotonation of the more acidic urea proton and the subsequent
charge-transfer phenomenon were previously reported to occur
with the addition of [(n-Bu)4N+]OHꢀ, which produces a color
past decade by appending a fluorescent fragment to the envisaged
receptor framework: in all cases, an efficient mechanism has to be
provided for either quenching or reviving fluorescence, following
substrate recognition.5–10
Isomers play a fundamental role in science and are widely stud-
ied because of their significance in living systems.11 A number of
approaches have been used for quantitative and qualitative detec-
tion of isomers; the methods are based on either detection or spec-
troscopy characterization.12 The development of fluorescent
receptors with the properties of isomer recognition and optical
change, has attracted increasing attention because such receptors
have increased sensitivity and potential application in pharmaceu-
ticals, biology, and as catalysts.13 For these reasons, the design,
synthesis and structural activity relationships of enantioselective
receptors are still vital areas of research.14 Much attention has
been paid recently to the synthesis of molecular receptors with
the ability to recognize chiral molecules.13b,c Fluorescent sensors
are preferred because they are well-suited to meet the need for
in vivo probes, such as mapping the spatial and temporal distribu-
tion of the biological analyses.15
change similar to that of urea derivatives upon addition of Fꢀ/
HP2O3ꢀ
.
2d,3c,4e
7
Due to its simplicity and high sensitivity, fluorescence has be-
come increasingly important for chemical trace detection. Recep-
tors based on anion-induced changes in fluorescence are
particularly attractive, because they offer the potential for high
Herein, we report synthesis, characterization, binding studies,
theoretical calculations, and differentiation of new fluorescent
receptor isomers with dual purposes: (1) normal colorimetric
⇑
Corresponding author. Tel./fax: +82 55 213 3463/3469.
E-mail address: yilee@changwon.ac.kr (Y.-I. Lee).
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.09.103

6466
T. Daniel Thangadurai et al. / Tetrahedron Letters 52 (2011) 6465–6469
NO2
R =
H2N
N
N
NO2
N
N
+
dry THF
1
H
N
RT, stirring
H
N
R
O
N
N
NCO
NO2
2
Scheme 1. Syntheses of receptor 1 and 2.
naked-eye anion detection; (2) differentiation of isomers through
anion detection. To the best of our knowledge, we provide the first
example of differentiation of geometric isomers through anion rec-
ognition of new fluorescent receptors 1 (cis-isomer) and 2 (trans-
isomer) in less than a minute time.
and 2 (host:anion; 1:100 equiv). Upon the addition of anions,
receptor 1 solution did not show any significant color/absorption
spectral change, but the color of the receptor 2 solution changed
from yellow (kmax = 401 nm) to blue (599 nm) and green
(617 nm) upon addition of Fꢀ and HP2O37ꢀ ions, respectively, which
could be easily observed by the naked eye (Fig. 1; for correspond-
ing UV–vis spectrum, Supplementary data, Fig. S7). This is associ-
ated with deprotonation of more acidic urea proton and the
subsequent charge-transfer phenomenon.3b,c,16 As the receptor
Results and discussion
The reaction of a 1:1 molar ratio of 4-(4-nitrophenylazoe-
nyl)aniline and 1-naphthyl isocyanate in dry THF followed by fil-
tration afforded 1 (cis-isomer, 40%) as pale yellow and filtrate
afforded 2 (trans-isomer, 50%) as orange microcrystals (Scheme
1). The structures of 1 and 2 were elucidated by elemental, spectral
(UV–vis, 1H, and 13C NMR), and mass analyses (Supplementary
data, Figs. S1–S6).
The colorimetric selective sensing abilities of receptors 1 and 2
with anions in CH3CN:DMSO (90:10, v/v) were monitored by UV–
vis absorption and by ‘naked eye’ observation. In the absence of an-
ions, the UV–vis spectrum of 1 and 2 were characterized by two
peaks (316 and 409 nm for 1; 260 and 401 nm for 2). The anions
were added as tetrabutylammonium salts to the solutions of 1
(A)
(B)
Figure 1. Color changes of 0.075 mM hosts (A – cis isomer; B – trans isomer) upon
the addition of tetrabutylammonium salts of anions (7.5 mM) at a 1:100 equivalent
ratio in CH3CN:DMSO (90:10, v/v) mixture. (a) Host only; (b) H + Fꢀ; (c) H + Clꢀ; (d)
H + Brꢀ; (e) H + Iꢀ; (f) H þ HSO4ꢀ; (g) H þ H2POꢀ; (h) H + CH3COOꢀ; (i) H þ HP2O3ꢀ
;
Figure 2. Fluorescence emission spectral changes of receptor 2 (5
addition of tetrabutylammonium salts of anions (500 M) at 1:100 equivalent ratio
in CH3CN:DMSO (90:10, v/v) mixture (slit width = 3 nm; excitation = 260 nm).
lM) upon the
4
7
(j) H þ NOꢀ3 . Color pictures of trans-isomer H + Fꢀ and H þ HP2O3ꢀ were taken after
l
7
diluting twice with solvent in order to improve the clarity of the color.

T. Daniel Thangadurai et al. / Tetrahedron Letters 52 (2011) 6465–6469
6467
bound to the anions, hydrogen bonds were constructed to form
stable complexes, and the electron density in the supramolecular
system was increased. It promotes the charge-transfer phenome-
non between the electron-deficient urea-bound anion and the elec-
tron-rich aza-nitro-phenyl center of 2 in neutral/deprotonated
form, thus causing a color change in the host solution.17,4a On the
other hand, exposure ꢀof receptꢀor 2 to solutions of Clꢀ, Brꢀ, Iꢀ,
CH3COOꢀ, HSOꢀ, H2PO , and NO , did not lead to any conspicuous
were observed during the 1H NMR titration. Upon addition of
0.5 equiv. Fꢀ and HP2O3ꢀ ions, the urea moiety –NH signals (9.68
7
and 9.00 ppm) disappeared and the aromatic region signals broad-
ened, which implied deprotonation of one –NH fragment and inter-
action of the anions with the other –NH fragment of compound 2
through intermolecular hydrogen bonding.19 The signals of the urea
group disappeared completely with the further addition of anions
(e.g., 1.0 equiv of Fꢀ and HP2O3ꢀ), and additionally, the signals of
4
4
3
7
change in color, and no significant change in absorption spectra
was observed (Fig. S7), suggesting no/very very weak binding for
those anions to the receptor 2. This dramatic combination of an-
ion-specific response/non-response makes this system an effective
naked-eye-detectable anion sensor under solution-phase condi-
tions. Interestingly, addition of Fꢀ and HP2O73ꢀ ions to 1 and 2 solu-
tions resulted in differentiation of cis- and trans-isomers.
the aromatic protons exhibited slight upfield shifts from 8.44 and
8.15 ppm, indicating an increase in the electron density on the
phenyl ring owing to through-bond effects.20 The disappearance of
–NH signals may be understood in terms of the following equilibria:
LH þ Xꢀ $ ½L ꢁ H ꢁ Xꢂꢀ
ð1Þ
ð2Þ
ð3Þ
½L ꢁ H ꢁ Xꢂꢀ $ Lꢀ þ HX
½L ꢁ H ꢁ Xꢂꢀ þ Xꢀ $ Lꢀ þ ½HX2ꢂꢀ
The fluorescence emission changes of 2 upon the addition of
anions in the form of tetrabutylammonium salts at a 1:100 equiv
ratio in the CH3CN:DMSO (90:10, v/v) mixture is illustrated in
Figure 2 (for 1, Supplementary data, Fig. S9). Both 1 and 2 display
fluorescent quenching effects with all the anions via a photo-
induced electron transfer (PET) mechanism.18
However, it should be noted that we observed no signal for
[HX2]ꢀ in its 1H NMR spectral titrations even up to d 20 ppm, prob-
ably as the result of its instability in highly polar solvents such as
DMSO.21
We were unable to perform 1H NMR and UV–vis titrations of 2
with anions due to the disappearance of the urea –NH proton signals
after the addition of small equivalents of anion and the lack of a uni-
form decrease or increase in absorbance intensity (Supplementary
data, Figs. S13 and S8). Both shifts and substantial line broadening
To investigate the sensitivity of 2 toward biologically important
anions, we carried out fluorescence titrations using tetrabutyl
ammonium salts of Fꢀ and HP2O3ꢀ ions in a degassed dry
7
CH3CN:DMSO (90:10, v/v) mixture (Fig. 3). We found that fluores-
cence intensity was gradually decreased as the concentration of
Figure 3. Fluorescence emission spectral changes of receptor 2 with increasing concentrations of TBAF (A) and TBAPPi (B) in CH3CN:DMSO (90:10, v/v) mixture. For Fꢀ, 2
(2
l
M) (slit width = 5 nm; excitation = 260 nm); for HP2O3ꢀ, 2 (1
l
M) (slit width = 3 nm; excitation = 260 nm); (inset) Job plot analysis for 2ꢁFꢀ and 2ꢁHP2O73ꢀ complexes in
7
CH3CN:DMSO (90:10, v/v).

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T. Daniel Thangadurai et al. / Tetrahedron Letters 52 (2011) 6465–6469
Scheme 2. Proposed PET mechanism of fluorescence ‘on–off’ behavior of compound 2 upon the addition of anion.
the anions increased, due to PET behavior upon anion sensing.18 The
excited state of the fluorophore was not quenched, or was
quenched only to a minimal extent, by electron transfer (ET) from
the receptor to the fluorophore prior to the sensor-anion interac-
tions. However, upon interaction with anions, the reduction poten-
tial of sensor 2 was increased; in other words, electron transfer
from the electron-rich amide moiety bonded with the anion to
the electron-deficient naphthalene moiety became more feasible.
structure to a planar one, whereas the nonplanar cis-isomers are
scarcely affected by the addition of Fꢀ anion. This shows that the
trans-isomers have much larger
p-conjugation length than the
cis-isomers, which may lead to a favorable intramolecular charge
transfer in the trans-isomer upon addition of anions.
Conclusion
Upon further addition of Fꢀ and HP2O3ꢀ, it appeared that the
7
In summary, we developed novel colorimetric receptors 1 and 2
containing both chromogenic and fluorogenic signaling subunits
and urea as the binding site for anion sensing. Anion recognition
via hydrogen-bonding interactions can be easily monitored by an-
ion-complexation-induced changes in UV–vis absorption spectra
and with the naked eye. Theoretically calculated results support
our experimental findings that the receptor 2 binds with Fꢀ and
deprotonated species, which was more electron-rich than the
hydrogen-bonded complex with anion, activated the PET process
more efficiently and evidenced more profound quenching
(Scheme 2).22 The titration data were fitted to the 1:1 binding pro-
file with Fꢀ and HP2O3ꢀ (logKa = 6.50 for Fꢀ and 5.30 for HP2O3ꢀ
)
7
7
according to the standard equation.23 Based on the fluorescence
titration experimental result, the detection limit (LOD) of the sensor
HP2O3ꢀ ions in 1:1 stoichiometry. Furthermore, the observed chem-
2 is calculated to be ca. 20 and 80 nM for Fꢀ and HP2O3ꢀ, respec-
7
7
ical shifts upon complexation of these anions were also rationalized
by the calculated results. Fluorescence studies showed that the
complexation of 2 with Fꢀ and HP2O37ꢀ could quench the emission
intensity via PET mechanism. From the binding study experiments,
we concluded that the receptor 2 is flexible enough to orient itself
according to the size of the binding anion. In this Letter, we have
successfully demonstrated the differentiation of geometrical iso-
mers by naked-eye anion detection technique. Molecular sensors
of this kind are likely to be particularly useful in field tests and other
situations, where sophisticated instrumentation might be lacking.
tively (for receptor 1 fluorescence titrations, Supplementary data,
Figs. S10 and S11).
Optimized geometries and vibrational frequencies for possible
cis- and trans-isomers of hosts and anions were obtained at the
B3LYP level24 with the 6-31G(d) basis set. At the respective opti-
mized geometries, time-dependent DFT (TD-DFT) calculations,25
using the B3LYP/6-31G(d) level were carried out in order to obtain
the vertical excitation energies of singlet states, and UV-visible
spectra of anions were simulated. The simulated UV-visible spectra
of host molecules are well matched with the experimental ones.
TD-DFT calculations revealed that the origin of the newly devel-
oped spectral band upon complexation of 2 with Fꢀ and HP2O3ꢀ
Acknowledgments
7
ions around 600 nm is due to the contributions from intramolecu-
lar transitions (Supplementary data Fig. S14). The B3LYP optimized
structures of both cis- and trans-isomers of hosts show large
deviation from planar geometry. When Fꢀ anion is added to the
trans-isomer, the geometry changes drastically from a distorted
This work was supported by the Priority Research Centers Pro-
gram (NRF 2010-0029634) and the International Joint Research
Program (F01-2009-000-10082-0) through the National Research
Foundation of Korea.

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6469
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