A new indole incorporated chemosensor exhibiting selective colorimetric and fluorescence ratiometric signaling of fluoride

Article abstract of DOI:10.1016/j.tetlet.2011.11.139

We have synthesized an internal charge transfer based chemosensor, Q-1 utilizing π-rich indole as the H-donor and the cyano-quinazolinone ring as the π-acceptor. The probe is the first example of an indole incorporated receptor that exhibits both absorban

Full text of DOI:10.1016/j.tetlet.2011.11.139

Tetrahedron Letters 53 (2012) 765–768
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
A new indole incorporated chemosensor exhibiting selective colorimetric
and fluorescence ratiometric signaling of fluoride
⇑
Sabir H. Mashraqui , Sushil S. Ghorpade, Sapna Tripathi, Smita Britto
Department of Chemistry, University of Mumbai, Vidyanagari, Mumbai 400098, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 15 July 2011
Revised 29 November 2011
Accepted 30 November 2011
Available online 7 December 2011
We have synthesized an internal charge transfer based chemosensor, Q-1 utilizing p-rich indole as the H-
donor and the cyano-quinazolinone ring as the
p-acceptor. The probe is the first example of an indole
incorporated receptor that exhibits both absorbance and emission red shifts in the presence of fluoride,
offering dual naked eye detection (yellow to red) as well as fluorescence ratiometric capability. The 1H
NMR study supports the FÀ-induced deprotonation of the indole N–H bond, a process that significantly
perturbs both the ground as well as the excited states of the probe via the electronic charge shift. It is note-
worthy that in contrast to fluoride, the commonly competing AcOÀ and H2PO4À as well as HSO4À, ClÀ, BrÀ, IÀ,
NOÀ3 and SCNÀ induced none or relatively negligible optical perturbation even at significantly higher
concentrations.
Keywords:
Indole-incorporated chemosensor
Synthesis
Internal charge transfer
UV–visible
Ó 2011 Elsevier Ltd. All rights reserved.
Fluorescence
Ratiometric analysis
Fluoride selective
Clinically controlled doses, of fluoride is beneficial in the treat-
ment of dental cavities and osteoporosis.1 However, high oral in-
take of fluoride can cause fluorosis, immune system disruption,
thyroid disorder, kidney damage, and even cancer.2 In addition,
fluoride contamination in drinking water and the associated health
hazards are a matter of grave concern in many parts of the world,
not having access to fluoride-free drinking water.3 Furthermore,
high fluoride levels inhibit photosynthetic capacities and affect
biomass productivity.4 Accordingly, the trace recognition of fluo-
ride is a priority research area from biological and environmental
perspectives.
The highly basic fluoride anion is well-known to engage in
hydrogen-bond interactions with a variety of neutral and cationic
H-bond donors.5 Not surprisingly, FÀ chemosensors typically rely
upon the optical signal transduction triggered by FÀÁ Á ÁH interac-
tion with the H-acidic chemoreceptors. Though, colorimetric fluo-
ride sensors abound,6 comparatively fewer chemoreceptors
featuring the most reliable ratiometric fluorescence sensing capa-
bilities are known.7,8 The principal signaling mechanisms used to
design a few known ratiometric FÀ probes include either the pro-
ton transfer process9 with H-receptors or the Lewis acid–base
interaction on borane incorporated fluorophores.10 For example,
based on the proton transfer mechanism, Tian et al. developed a
diketopyrrolopyrrole system,8h while Peng et al. reported an
anthraquinone–benzimidazole conjugate8i to eliciting fluorescence
ratiometric signaling for fluoride detection. On the other hand,
Shinkai et al. reported a dual colorimetric and fluorescence ratio-
metric probe based on the complexation of fluoride with triaryl-
borane–porphyrin conjugate.10c
Of the many hetero-ring receptors investigated for anion sens-
ing, the easily accessible indole ring, carrying an acidic N–H bond
figures prominently in the design of fluoride selective optical sen-
sors. However, the indole incorporated probes reported to date of-
fer signaling by way of color or luminescence intensity variations
only.11 To our knowledge, the indole based chemosensors offering
dual colorimetric and fluorescence ratiometric responses have not
yet been described.
The key to realizing the wavelength ratiometric signaling re-
quires perturbations of both ground as well as the excited states un-
der external stimuli, causing absorbance as well as emission
spectral shifts. Donor–acceptor chromophores, which exhibit ana-
lyte induced internal charge transfer (ICT) processes are well suited
to deliver such optical modulations.12 Moreover, the acidity of the
H-bond donor within the ICT chromophores can be synthetically
manipulated to allow selective interaction with the basic fluoride,
while minimizing cross affinities from potentially interfering
anions.6a,13
Inspired by these considerations, we have presently designed
and synthesized a new ICT based indole receptor, Q-1 that signals
the presence of micromolar levels of fluoride by dual colorimetric
and fluorescence ratiometric responses. Synthesis of Q-1 was
achieved by the sequence depicted in Scheme 1. The known
N-acetyl-5-bromo anthranilic acid 1 was condensed with N,N-
⇑
Corresponding author. Tel.: +91 22 26526091; fax: +91 22 26528547.
E-mail address: sh_mashraqui@yahoo.com (S.H. Mashraqui).
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.11.139

766
S. H. Mashraqui et al. / Tetrahedron Letters 53 (2012) 765–768
diethyl-p-phenylenediamine 2 in refluxing POCl3/toluene to afford
quinazolinone 3. The corresponding cyano-substituted product 4
was obtained by treating 3 with CuCN in refluxing DMF under N2
atmosphere. Finally, the condensation of 4 with indole-3-aldehyde
in refluxing o-xylene, containing piperdine as a base afforded tar-
get Q-1 (Supplementary data).
0.6
0.4
0.2
2.5
2.0
1.5
1.0
0.5
0.0
0.0
0.0000
0.0004
0.0008
0.0012
Conc. of F-
The design of Q-1 belongs to the donor–p–acceptor formation
in which the -rich, indole ring can engage in the ICT process with
p
the conjugated cyano-quinazolinone ring. Additionally, the latter
ring can enhance the acidity of the indole N–H bond via the
push–pull interaction.14 The anticipated deprotonation by the
highly basic fluoride is expected to provoke enhanced ICT through
negative charge shift from the deprotonated indole to the p-conju-
gated quinazolinone ring. Accordingly, this phenomenon would
induce detectable photophysical perturbations of the sample, thus
forming a basis for the recognition and quantification of fluoride
anion.
300
400
500
600
The optical sensitivity of Q-1 toward FÀ, AcO À, H2PO4À, HSO4À,
ClÀ, IÀ, SCNÀ, NOÀ3 and BrÀ was first investigated spectrophotomet-
rically by adding a standard solution of anions as their tetrabutyl-
ammonium (TBA) salts to a fixed concentration of the probe in
DMSO solvent. The probe exhibited an absorption band at
412 nm, attributable to the ICT transition. Figure 1 shows the con-
centration dependent changes in the UV–vis profile of Q-1 during
the titration with fluoride ion. With incremental addition of TBAF,
the absorbance at 412 nm decreased gradually, while a red shifted
maximum at 524 nm appeared concurrently. At a saturating
10À3 M of TBAF, the original absorbance at 412 nm was fully re-
placed by a relatively more intense maximum at 524 nm with a
clear isosbestic point being observed at 457 nm. The stoichiometry
of the Q-1-fluoride complexation was deduced to be 1:1 from the
Jobs plot (Supplementary data). The remarkable bathochromic
shift of 112 nm implies the existence of a strong ground state
FÀÁ Á ÁH–N interaction. This phenomenon also results in instanta-
neous color change from yellow to red, an event that allows visible,
necked eye detection of fluoride. An image of visual color change
with increasing concentration of fluoride is given as Figure SI-14
in the Supplementary data. The absorbance ratiometric plot of
A524/A412 versus fluoride concentration revealed a near linear cor-
relation (inset, Fig. 1). Accordingly, it is possible to use the ratio
Wavelength (nm)
Figure 1. Spectrophotometric titration of Q-1 (2.83 Â 10À5 M) with TBAF (0–
1.1 Â 10À3 M) in DMSO solution. Inset: absorbance ratiometric plot of A524/A412
versus TBAF concentration.
The other fluoride source such as CsF also exhibited similar red
shift as observed with TBAF. The fluoride induced optical perturba-
tions were found to be fully reversible, since addition of protic sol-
vent such as water restored the absorbance profile due to the
native probe (Supplementary data). We also carried out spectro-
photometric experiment with tetrabutylammonium hydroxide, a
strong base capable of deprotonating the probe. In this case too,
we observed electronic perturbations similar to those seen with
TBAF or CsF (Supplementary data). Consequently, the optical per-
turbations induced by TBAF or CsF may be attributable to the
deprotonation of indole N–H bond by F-.
AsÀdepicted in Figure 2, seÀveral other anions, such as AcO À
,
H2PO4 , HSOÀ4 , ClÀ, IÀ, SCNÀ, NO3 and BrÀ failed to significantly im-
pact the absorption profile of the probe even in 10-fold higher con-
centrations than used for fluoride. Absence of significant changes
in the UV–vis of the probe implies none or minimal ground state
perturbation of the probe in the presence of these anions. By con-
trast to fluoride, no perceptible color changes were noticeable in
the presence of other anions.
A524/A412 for the determination of fluoride concentration by the
absorbance ratiometric analysis. The detection limit determined
by colorimetric data was found to be 1.096 Â 10À6 M (Supplemen-
tary data).
To gain an insight into the nature of the ground interaction be-
tween Q-1 and FÀ, we recorded the 1H NMR spectra of the probe
O
N
H2N
NH
OH
POCl3/Toluene
Reflux
+
N
Br
N
Br
O
O
N
2
3
1
CuCN/ DMF
Reflux/N2 Atm
H
N
H
N
N
O
N
O
H
N
NC
Xylene / Piperidine,
Reflux
N
NC
O
4
N
N
Q-I
Scheme 1. Synthesis of indole incorporated probe, Q-1.

S. H. Mashraqui et al. / Tetrahedron Letters 53 (2012) 765–768
767
6
5
4
3
2
1
0
2.5
2.0
1.5
1.0
0.5
0.0
F-
55
QI-1,Cl-, Br-,
I-, HSO4-,
H2PO4-, No3-,
SCN-, AcO-.
50
45
40
35
30
25
20
15
10
5
*10-04
0
2
4
8
10
[F-]6
0
480 500 520 540 560 580 600 620 640
Wavelength
300
400
500
600
700
Wavelength (nm)
Figure 4. Fluorescence spectrum of Q-1 (5 Â 10À6 M) on titration with TBAF (0–
8.8 Â 10À4 M) in DMSO solution, kex = 457 nm. Inset: Emission ratiometric plot of
I551/I489 versus TBAF concentration.
Figure 2. Absorption spectra of Q-1 (2.83 Â 10À5 M) without and with added FÀ
(10À3 M), AcOÀ, H2POÀ, BrÀ, ClÀ, IÀ, HSO4À, NO3À and SCNÀ (10À2 M each) in DMSO.
4
emission at 489 nm was fully replaced by the new red shifted
emission. The bathochromic shift in the emission spectra is consis-
tent with the shift of similar nature also observed in the UV–vis of
the probe when exposed to FÀ. An isoemissive point at 505 nm sug-
gests that a well-defined equilibrium F–Q-1 complex exists in the
excited state as well. With gradual increase in the fluoride concen-
tration, the emission intensity ratio, I551/I489, increased linearly
with ca. 18-fold emission ratio change. This experiment demon-
strates that Q-1 can be effectively used to detect fluoride ratiomet-
rically (Supplementary data).
without and with added TBAF in DMSO-d6. As shown in Figure 3
addition of just 1 equiv of TBAF resulted in the complete disappear-
ance of the NH signal of the probe at d 11.7, while a new broad sig-
nal attributable to the formation of F–H–FÀ was observed at
15.74 ppm.15 The shielding effect of the negatively charged indole
ring is evident from the upfield shifts of the indole C1–H, the ole-
finic C4–H proximate to the quinazolinone ring, and the C9–H of
the quinazolinone ring by d 0.16, 0.42, and 0.19, respectively. In
addition, the negative charge shift also promotes upfield shift of
the indole phenyl ring protons in the range of d 0.1–0.2. We also
recorded 19F NMR of CsF, a non-hygroscopic salt before and after
adding the probe. The 19F signal of CsF in DMSO-d6 appeared at
À142 ppm, which was shifted to À137.4 ppm after addition of ex-
cess of Q-1 (Supplementary data). The downfield shift of 19F reso-
nance and the observation of F–H–FÀ in the 1H NMR in the
presence of the probe could be attributed to the direct FÀÁ Á ÁH–N
bond interaction. These findings support the FÀ induced deproto-
nation of the indole N–H bond of the probe.
Whereas, FÀ at 8.8 Â 10À4
M
exhibited dramatic emÀission
changes, in contrast AcO À, H2POÀ, HSOÀ, ClÀ, IÀ SCNÀ, NO3 and
,
4
4
BrÀ even at 10-fold higher concentration (8.8 Â 10À3 M) caused
emission enhancements just by 5–25% and exhibited virtually no
emission shift at all (Fig. 5). On the other hand, addition of AcOÀ
at 8.8 Â 10À3 M did exhibit emission band shift from 489 nm to
512 nm, however unlike the fluoride at 8.8 Â 10À4 M, the process
failed to give rise to any isoemissive point. These results clearly
demonstrate that among the tested anions, only FÀ is capable of
inducing relatively more emission changes at significantly lower
concentration.
The sensitivity of Q-1 toward anions was next evaluated by
fluorimetric titration in DMSO. Excitation of Q-1 (5 lM) at the isos-
bestic point 457 nm resulted in a weakly emission band at 489 nm
with a quantum yield (UFÀ) of 0.068, measured with respect to cou-
marin-153 (UFÀ = 0.89).10 Of the different anions examined, only FÀ
induced remarkable changes in the emission profile of the probe.
As shown in Figure 4, with increasing TBAF, the probe’s emission
band at 489 nm was progressively red shifted to a new, more in-
tense band at 551 nm. At a limiting 8.8 Â 10À4 M of FÀ, the probe’s
The pKa of the Q-1 and that of the plain indole were found to be
8.8 and 9.7, respectively (Supplementary data). An order of magni-
tude lower pKa of the probe implies markedly higher acidity of the
indole nucleus in the probe compared to indole itself. Clearly, the
participation of the
p-deficient cyano-quinazolinone ring in the
push-pull interaction sufficiently polarizes the indole N–H bond
Figure 3. 1H NMR spectra of (a) Q-1 and (b) Q-1 + 1 equiv of TBAF in DMSO-d6.

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S. H. Mashraqui et al. / Tetrahedron Letters 53 (2012) 765–768
60
40
20
0
References and notes
Q-1+ F-
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500
550
Wavelength (nm)
600
650
Figure 5. Fluorescence spectra of Q-1 (5 Â 10À6 M) in the absence and presence of
8.8 Â 10À3
M
each AcOÀ, H2POÀ
,
BrÀ, ClÀ, IÀ, HSOÀ4
,
NOÀ3 and SCNÀ and FÀ
4
(8.8 Â 10À4 M) in DMSO.
to facilitate its selective interaction with the highly basic fluoride
over other anions examined under the condition of measurements.
The absorption and fluorescence spectral shifts in the presence of
FÀ are fully consistent with the increase in the ICT interaction upon
deprotonation of the indole. Though, the native probe exhibits
weak emission (Fig. 4), the new emission band generated upon
exposure to fluoride fluoresces over fourfold more intensely. We
attribute the weakly emissive nature of Q-1 to the thermally deac-
tivating free rotation around the C–C single bonds connecting in-
dole with the quinozolinone ring. However, in the deprotonated
form, the Àve charge generated on the indole ring of the probe
would be delocalized over the entire framework, conferring partial
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double bond character to the p-conjugated system. Consequently,
the C–C bond rotation in the deprotonated form would be re-
stricted relative to that existing in the free probe. This phenome-
non would relax the excited state as well as accelerate the
radiative channels, leading to the fluorescence switch-on and the
emission band shift in the presence of fluoride.16
In conclusion, we have successfully designed, what appears to
be the first example of an indole-incorporated sensor that responds
to fluoride via visual color change as well as an efficient fluores-
cence ratiometric signaling. The sensing mechanism is attributable
to the deprotonation of the indole N–H by fluoride, an event that
modulates the ICT interaction, resulting in high optical spectral
shifts. It is noteworthy that the high selectivity for fluoride is evi-
dent from the fact that even at much higher loading, several other
anions, including the frequently interfering AcOÀ and HSO4À impose
none or much truncated optical variations.
Acknowledgments
We are thankful to the BRNS, Government of India for the finan-
cial assistance. Thanks to the UGC, India for providing fellowship to
S.G., S.T. and S.B.
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Supplementary data
Supplementary data (synthesis of Q-1, spectral data, Job’s plot
analysis, colorimetric plot of Q-1 with CsF, binding reversibility
study and the determination of detection limit) associated with
this article can be found, in the online version, at doi:10.1016/
j.tetlet.2011.11.139.
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