Synthesis of indolo[3,2-b]carbazole-based new colorimetric receptor for anions: A unique color change for fluoride ions

Article abstract of DOI:10.3762/bjoc.6.12

A novel indolocarbazole-based chemosensor 1 containing hydrogen bond donor moieties has been established as a selective colorimetric and fluorometric sensor for F-in CH₃CN/H₂O (4:1 v/v). Upon the addition of a series of tetrabutylammonium salts to receptor 1 in aqueous CH₃CN, only when the counter ion was F-was a significant color change (from light violet to dark orange) observed.

Full text of DOI:10.3762/bjoc.6.12

Synthesis of indolo[3,2-b]carbazole-based
new colorimetric receptor for anions:
A unique color change for fluoride ions
Ajit Kumar Mahapatra*, Giridhari Hazra and Prithidipa Sahoo
Preliminary Communication
Open Access
Address:
Beilstein Journal of Organic Chemistry 2010, 6, No. 12.
Department of Chemistry, Bengal Engineering and Science
University, Shibpur, Howrah 711103, India, Tel.: +91 33 2668 4561;
Fax: +91 33 2668 4564
doi:10.3762/bjoc.6.12
Received: 01 October 2009
Accepted: 30 January 2010
Published: 08 February 2010
Email:
Ajit Kumar Mahapatra* - mahapatra574@gmail.com
Associate Editor: P. Skabara
* Corresponding author
© 2010 Mahapatra et al; licensee Beilstein-Institut.
License and terms: see end of document.
Keywords:
anion binding; colorimetry; fluorescence quenching; fluoride binding;
indolocarbazole
Abstract
A novel indolocarbazole-based chemosensor 1 containing hydrogen bond donor moieties has been established as a selective colori-
metric and fluorometric sensor for F− in CH3CN/H2O (4:1 v/v). Upon the addition of a series of tetrabutylammonium salts to
receptor 1 in aqueous CH3CN, only when the counter ion was F− was a significant color change (from light violet to dark orange)
observed.
Introduction
The design and synthesis of chromogenic receptors for biologi- diversity of function, both beneficial and otherwise, makes the
cally important and environmentally harmful anion pollutants problem of fluoride ion detection of considerable interest. In
has attracted considerable attention in supramolecular chem- this context, a colorimetric chemosensor is of particular interest
istry [1,2]. Most of the synthetic chemosensors generally due to its simplicity. Color changes that can be detected by the
involve covalent linking of an optical-signaling chromophoric naked eye are widely used as signals for detection of anions
fragment to a neutral anion receptor containing urea [3], without the need for any expensive equipment or even without
thiourea [4], amide [5], phenol [6,7], or pyrrole [8] subunits, the requirement of any equipment whatsoever [12,13].
which can provide one or more H-bond donor sites for selective
binding and sensing of certain anions, especially F−, AcO−, In the last few years, although some synthetic receptors have
H2PO4−, etc. In particular, the selective sensing of fluoride has become available for fluoride ions [14-28], there is a paucity of
gained attention due to its significant role in clinical treatments reports on selective naked-eye chemosensors for fluoride
e.g. dental care [9], osteoporosis [10] and for the detection of [29-31]. Nitrophenyl, nitronaphthalene urea [32-34], naph-
fluoride in bones as a result of over-accumulation [11]. This thalene triphenyl-phosphonium [35], benzimidazolyl pyridine
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Beilstein Journal of Organic Chemistry 2010, 6, No. 12.
[36,37] and oxidized bis(indolyl)methane [38] as signal units be unstable at room temperature. Subsequent heating of 2 in
for fluoride have been reported as chromogenic chemosensors, CH3CN in the presence of I2 for 45 min afforded the desired
but a indolocarbazole ligand for the anion remains to be receptor 1 in 82% yield.
developed. Recently, Bhardwaj et al. reported a tripodal
receptor [39] bearing catechol groups [40] for the chromogenic To look into the orientation of hydrogen bond donors around
sensing of fluoride ions. Numerous bis(indolyl)methanes and the carbazole motif, we optimized the structure by the AM1
their derivatives exhibit important biological activities [41]. method [45] (Figure 2). It is evident from Figure 2 that the two
Therefore, there has been great interest in the synthesis of bisin- catechol units do not lie in the same plane as the carbazole unit.
dole compounds both naturally occurring and totally synthetic.
As an extension of our work [42] on supramolecular chemistry,
we now report a simple and new indolocarbazole-based
molecular receptor 1 for the selective sensing of anions by
investigating the effect of the addition of tetrabutylammonium
salts ([Bu4N]+X−, X = F−, Cl−, Br−, I−, AcO−, HSO4−, and
H2PO4−). Receptor 1 (Figure 1) was particularly important as a
chemosensor for fluoride owing to its noticeable color change
in the presence of F− ions.
Figure 2: The AM1 optimized structure of receptor 1 (heat of forma-
tion = −8.29 kcal/mol).
Interaction studies
UV–vis study
Figure 1: The structure of the indolocarbazole-based chemosensor 1.
The anion-binding properties of receptor 1 were investigated by
UV–vis, fluorescence and 1H NMR spectroscopic methods. The
Results and Discussion
sensing ability of chemosensor 1 with a series of tetrabutylam-
Synthesis
monium salts ([Bu4N]+X−, X = F−, Cl−, Br−, I−, AcO−, HSO4−,
Receptor 1 was synthesized [43,44] according to Scheme 1. and H2PO4−) in CH3CN/H2O (4:1 v/v) was monitored by
Condensation of indole with 3,4-dihydroxybenzaldehyde by the UV–vis absorption studies and by ‘naked-eye’ observation. The
reported procedure yielded intermediate 2, which was found to tetrabutylammonium salt (TBAX) under investigation was
Scheme 1: Synthesis of receptor 1.
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Beilstein Journal of Organic Chemistry 2010, 6, No. 12.
Figure 3: Color changes of receptor 1 (A) (c = 1.1 × 10−4 M) in CH3CN/H2O (4:1 v/v) on addition of tetrabutylammonium salt (TBAX), X = F− (B), Cl−
(C), Br− (D), I− (E), AcO− (F), HSO4− (G), and H2PO4− (H) (left side); green fluorescence observed on addition of F− ion to receptor 1 (right side).
added to a solution of receptor 1 (c = 1.1 × 10−4 M) in the spectroscopic titration (Figure 4). Receptor 1 itself displays two
above noted solvent mixture.
absorption bands at 283 and 338 nm in CH3CN/H2O (4:1 v/v).
Upon the gradual addition of F−, the absorbance increases by
In the naked-eye experiments, receptor 1 (c = 1.1 × 10−4 M) in different extents. On increasing the concentration of F−, two
CH3CN/H2O (4:1 v/v) showed distinct color changes from light new absorption bands appear at 408 and 491 nm, with the effect
violet to dark orange and pale pink, respectively, in the pres- that the solution instantaneously changes color from light violet
ence of two equivalent amounts of TBAF and TBAOAc to dark orange. These two new bands can be ascribed to the
(Figure 3). In the fluorescence study, the sky blue color of 1 deprotonated receptor. Figure 4 shows the F−-induced UV–vis
changed to a green color on the addition of TBAF. Importantly, spectral change of receptor 1 at different concentrations of
the receptor was found to be insensitive to the addition of large fluoride ion in CH3CN/H2O (4:1 v/v) (left side). A similar, but
excess of Cl−, Br−, I−, HSO4−, and H2PO4− (even up to 100 less remarkable spectral change, was observed upon addition of
equiv). The change in color was due to the deprotonation of AcO− (right side) where a color change from light violet to light
phenolic OH groups followed by hydrogen bonding with pink was achieved upon the addition of 10 equiv of AcO−.
fluoride ions. The strong hydrogen bonding to, or deprotona-
tion/protonation of, the indolocarbazole moiety might modulate The spectral behavior indicated that deprotonation of the phen-
the electronic properties of chromophore [46] and give rise to olic OH as well as NH groups by F− (Scheme 2), and not
significant color changes.
hydrogen bonding to it, is responsible for the drastic color
change [47], as a result of a change in the optical properties of
The interaction of receptor 1 (c = 1.1 × 10−4 M) with F− was in- chromogenic indolocarbazole skeleton. This is in agreement
vestigated in aqueous CH3CN solvent in more detail by UV–vis with the NMR titration data. Such deprotonation was related to
Figure 4: UV spectral change of receptor 1 (c = 1.1 × 10−4 M) upon gradual addition of [Bu4N]+F− (left side) and [Bu4N]+AcO− (right side) in CH3CN/
H2O (4:1 v/v) (c = 1.1 × 10−4 M).
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Beilstein Journal of Organic Chemistry 2010, 6, No. 12.
Scheme 2: Schematic representation (the circles represent the indolocarbazole moiety) of the two-step process leading to receptor deprotonation
with basic fluoride anions.
the acidity of the H-bond donor site and the particular stability occurred at a slightly higher concentration of acetate than
of the [HF2]− ion. The stoichiometry of 1 with F− was deter- fluoride due to higher electronegativity, smaller size, and higher
mined to be 1:2 from the Job plot [48] (as shown in Figure 5). basicity of F− ions, which make them bind strongly with
receptor 1 [49]. The binding constants (Ka) of receptor 1
(Table 1) with fluoride and other ions were determined by
considering a hydrogen-bonded complex with the first two equi-
valents of anions in a 1:2 ratio of receptor and anion complex,
and that, subsequently, the second equivalent of anion (addition
of excess F− ion) abstracts a HF fragment to give [HF2]−.
Table 1: Association constantsa of receptor 1 (R1) with [Bu4N]+X−
salts (X = F−, Cl−, Br−, I−, AcO−, HSO4−, and H2PO4−) in CH3CN/H2O
(4:1 v/v) determined by UV–vis and fluorescence methods.
Guests
R1 (Ka M−1)
Fluorescence method
UV–vis method
F−
3.62 × 104
7.92 × 103
4.29 × 103
4.13 × 103
1.21 × 104
1.14 × 103
1.32 × 103
8.21 × 104
3.62 × 103
2.47 × 103
2.27 × 103
1.04 × 104
2.25 × 103
6.65 × 103
Cl−
Br−
Figure 5: The Job plot of 1 with fluoride ion from UV method in
CH3CN/H2O (4:1 v/v).
I−
AcO−
HSO4−
H2PO4−
Parallel investigations were carried out with a series of other
anions (Cl−, Br−, I−, AcO−, HSO4−, and H2PO4−). Similar
phenomena with negligible perturbations of UV–vis absorption
were observed with no noticeable change in color in the cases
aAll errors are ±0.8%.
of Cl−, Br−, I−, HSO4−, and H2PO4−, even at levels of up to 100 A higher association constant was observed for fluoride ion
equiv. Similar anion-sensing properties were also observed in than for other ions due to its strong hydrogen-bonding ability,
the more polar solvent DMSO (all UV–vis spectra in supporting small size, and better selectivity which resulted in a strong
information).
binding with receptor 1 [50].
Deprotonation of receptor 1 also took place with the basic anion Fluorescence study
AcO− and the development of a light pink color was observed Fluorescence spectroscopy studies were also carried out in order
after the addition of excess anion. No deprotonation took place to evaluate the ability of 1 as a fluorescent anion sensor. Signi-
in the presence of less basic anions such as Cl−, Br−, I−, HSO4−, ficant quenching of the fluorescence of 1 was observed upon
and H2PO4−. The receptor 1 is able to bind fluoride ion more addition of F− ions to the solution of 1 (Figure 6, left side). In
strongly than other anions, since the catechol moiety is particu- comparison, other anions, with the exception of AcO−
larly effective in binding smaller anions. The deprotonation (Figure 6, right side), hardly altered the emission of 1. A large
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Beilstein Journal of Organic Chemistry 2010, 6, No. 12.
Figure 6: Fluorescence change of receptor 1 (c = 4.475 × 10−5 M) upon gradual addition of [Bu4N]+F− (left side) and [Bu4N]+AcO− (right side) in
CH3CN/H2O (4:1 v/v) (c = 4.475 × 10−5 M) (λmax = 443 nm).
quenching of intensity with respect to other anions (Figure 7, Analogous investigation of fluorescence was carried out with
right side) was observed at 439 nm upon the addition of 2.0 other [Bu4N]+X− salts (X = Cl−, Br−, I−, HSO4−, and H2PO4−).
equiv of [Bu4N]+F−. These results indicate that formation of In all cases, only slight quenching occurs on the gradual addi-
hydrogen-bonded complex or deprotonation/protonation occurs tion of the anions (Cl−, Br−, I−, HSO4−, and H2PO4−) to
by forming the anion of receptor 1; the excited state was modi- receptor 1. The spectral variations observed for receptor 1 on
fied considerably leading to the quenching of fluorescence. A titrating with different anions are given in the supplementary
commonly accepted mechanism for the quenching phenomenon information.
involves an inversion between the strongly emissive ππ* and the
poorly emissive nπ* states of this fluorophore. Such a 1H NMR study
quenching results from a hydrogen bond interaction of phenolic The interaction of receptor 1 with F− was corroborated by 1H
OH with anions, which leads to the stabilization of the nπ* state NMR experiments carried out in DMSO-d6 (1 has only limited
with respect to the ππ* state and a subsequent decrease in the solubility in CD3CN). A partial 1H NMR spectrum of receptor 1
fluorescence emission intensity [51].
is shown in Figure 8. It was found that the aromatic proton
Figure 7: Binding constant calculation curves for receptor 1 vs F−, Cl−, Br−, I−, AcO−, HSO4−, and H2PO4− (left side); fluorescence spectra of 1 after
addition of 1:1 equivalent of receptor and anions (F−, Cl−, Br−, I−, AcO−, HSO4−, and H2PO4−) (right side).
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Beilstein Journal of Organic Chemistry 2010, 6, No. 12.
signals underwent upfield shifts with increasing F− concentra- complex with anions by multiple hydrogen-bonding interac-
tion. In the presence of equivalent amounts of [Bu4N]+F−, the tions. Among the anions, only receptor 1 has higher selectivity
signal for phenolic OH protons of 1 underwent large downfield for F− and leads to a distinct color change that can be observed
shift (Δδ = 1.34 ppm) and the proton signal was broadened. by the naked eye. The binding results with a series of anions
These observations further indicated that the first added F− suggest that receptor 1 efficiently binds F− as established by
establishes an H-bond interaction with the OH subunit of 1, UV–vis, fluorescence and 1H NMR spectroscopic methods. As
while an excess of F− induces the deprotonation of the catechol a colorimetric anion sensor, the indolocarbazole-based receptor
moieties and NH proton, which brings electron density onto the 1 displayed highly selective coloration for F− ion even in the
π-conjugated framework through bond propagation, thus presence of other anions.
causing a shielding effect and inducing upfield shift of aromatic
Experimental
protons. The above mentioned results indicate that receptor 1
exhibits selective sensing for F− (F− > AcO− >> other anions) in General details
an appropriate solvent.
All reactions were carried out under a nitrogen atmosphere.
Solvents were dried before use. Solvents for spectroscopic
measurements were of spectroscopic or HPLC grade. The
1H NMR spectra were recorded on a Bruker AM-500 spectro-
meter. The 1H NMR chemical shift values are expressed in ppm
(δ). UV–visible and fluorescence spectra measurements were
performed on a JASCO V530 and a PerkinElmer LS-55 spec-
trofluorimeter, respectively. Receptor 1 and guest anions were
dissolved in UV-grade acetonitrile and water (4:1 v/v). The cor-
responding absorbance values for receptor 1 were noted during
titration and used for the determination of binding constant
values. Binding constants were determined by using the expres-
sion A0/A − A0 = [εM/(εM − εC)](Ka−1 Cg−1 + 1), where εM and
εC are molar extinction coefficients for receptor and the
hydrogen-bonding complex, respectively, at selected
wavelengths, A0 denotes the absorbance of the free receptors at
the specific wavelength, and Cg is the concentration of
[Bu4N]+X− (X = F−, Cl−, Br−, I−, AcO−, HSO4−, and H2PO4−
anions). The measured absorbance A0/A − A0 as a function of
the inverse of the guest anion concentration fits a linear rela-
tionship, indicating a 1:2 complexation of the receptor and
anions. The ratio of the intercepts to the slope was used to
determine the binding constant Ka.
Figure 8: 1H NMR spectra of receptor 1 (bottom), 1 with [Bu4N]+F− 1:2
[receptor 1:(Bu4N)+F−] (middle) and excess [Bu4N]+F− (top).
Geometric optimization of their stable conformation of receptor
The fluoride- and acetate-induced deprotonation process is 1 at the AM1 level was carried out using the minimal valence
reversible as evidenced from the addition of CH3OH. The addi- basis as STO 3G in ArgusLab 4.0.1 software suite. We have
tion of the polar protic solvent methanol results in a gradual refrained from citing calculated total energy value, the calcula-
decrease of absorbance in UV–vis studies. This is presumably tion being for molecule only in the gas phase.
because the presence of a relatively high amount of protic
solvent disfavors the formation of the deprotonated receptor. Receptor 1
However, in the water–acetonitrile system, no color changes 3,3′-Bis(indolyl)-3,4-dihydroxyphenylmethane (2, 0.5 g, 1.41
were observed upon addition of organic bases such as triethyl- mmol) in a round-bottom flask containing dry acetonitrile (5
amine whilst the addition of excess [Bu4N]+OH− can depro- mL), I2 (2 mol %) was added and the mixture refluxed for 45
tonate receptor 1 and induce a color change.
min. The solid obtained was filtered, dried and recrystallized
from a mixture of DMF–CHCl3. Yield 82%, mp 258 °C; 1H
NMR (500 MHz, DMSO-d6): δ (ppm) 10.53 (s, 2H), 8.63 (bs,
Conclusion
In conclusion, a new colorimetric receptor 1 based on indolo- 4H), 7.24 (d, J = 8 Hz, 2H), 7.12 (d, J = 8 Hz, 2H), 6.94 (t, J =
carbazole was synthesized in high yield, which can form 1:2 7.5 Hz, 2H), 6.80 (t, J = 7.5 Hz, 2H), 6.68 (d, J = 8 Hz, 2H),
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Beilstein Journal of Organic Chemistry 2010, 6, No. 12.
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Supporting Information
Supporting Information File 1
13C NMR and mass spectra of the synthesized compound
R1 and its UV–vis and fluorescence spectra in the presence
of different anions (Cl−, Br−, I−, HSO4−, and H2PO4−).
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-6-12-S1.pdf]
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Hu, C.-W.; Yu, X.-Q. Org. Lett. 2009, 11, 669–672.
doi:10.1021/ol8027303
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Acknowledgements
We thank DST [W.B. Government, project no. 694 (sanc.)/ST/
P/S&T/6G-6/2007] for financial support. GH thanks CSIR,
New Delhi for a fellowship during the course of work. Also
thanks are due to Prof. S. P. Goswami for providing some
instrumental facilities.
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