A colorimetric and ratiometric fluorescent turn-on fluoride chemodosimeter and application in live cell imaging: High selectivity via specific SiO cleavage in semi aqueous media and prompt recovery of ESIPT along with the X-ray structures

Article abstract of DOI:10.1016/j.tetlet.2014.03.003

A ratiometric fluorescent turn-on probe for fluoride ion, based on modulation of the excited-state intramolecular proton transfer (ESIPT) process by chemodosimetric desilylation pathway is reported. The probe SNBT (silyl protected hydroxynaphthalene benzothiazole moiety) shows a significant increase of ratiometric absorption band at 440 nm and emission band at 477 nm by the deprotection of fluoride mediated silyl bond cleavage in CH₃CN-H ₂O (8/2, v/v, 25 °C). The test strips based on SNBT and F ^- are fabricated, which can act as a convenient and efficient F ^- test kits. Furthermore, the biological application shows that it can be very useful as a selective fluoride probe in the fluorescence imaging of living cells.

Full text of DOI:10.1016/j.tetlet.2014.03.003

Tetrahedron Letters 55 (2014) 2633–2638
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
A colorimetric and ratiometric fluorescent turn-on fluoride
chemodosimeter and application in live cell imaging: high selectivity
via specific SiAO cleavage in semi aqueous media and prompt
recovery of ESIPT along with the X-ray structures
a,
⇑
a
a
b
Shyamaprosad Goswami , Avijit Kumar Das , Abhishek Manna , Anup Kumar Maity ,
Hoong-Kun Fun
a
c,d,
, Ching Kheng Quah , Partha Saha b
c,
Department of Chemistry, Bengal Engineering and Science University, Shibpur, Howrah 711103, West Bengal, India
Crystallography and Molecular Biology Division, Saha Institute of Nuclear Physics, Kolkata 700064, West Bengal, India
X-ray Crystallography Unit, School of Physics, Universiti Sains Malaysia, 11800 USM, Penang, Malaysia
b
c
d
Department of Pharmaceutical Chemistry, College of Pharmacy, King Saud University, PO Box 2457, Riyadh 11451, Saudi Arabia
a r t i c l e i n f o
a b s t r a c t
Article history:
A ratiometric fluorescent turn-on probe for fluoride ion, based on modulation of the excited-state intra-
molecular proton transfer (ESIPT) process by chemodosimetric desilylation pathway is reported. The
probe SNBT (silyl protected hydroxynaphthalene benzothiazole moiety) shows a significant increase of
Received 1 January 2014
Revised 28 February 2014
Accepted 2 March 2014
Available online 14 March 2014
ratiometric absorption band at 440 nm and emission band at 477 nm by the deprotection of fluoride
À
mediated silyl bond cleavage in CH
3
CN–H
2
O (8/2, v/v, 25 °C). The test strips based on SNBT and F are
À
fabricated, which can act as a convenient and efficient F test kits. Furthermore, the biological application
shows that it can be very useful as a selective fluoride probe in the fluorescence imaging of living cells.
Ó 2014 Elsevier Ltd. All rights reserved.
Keywords:
Fluorimetric
Colorimetric
Benzothiazole
X-ray
Test trips
Cell-biology
Design and development of artificial molecular systems for
sensing anions in biologically relevant conditions is a challenging
task in supramolecular chemistry.1 In particular, sensing fluoride
anion has attracted increasing interest in the molecular recognition
community because of its pivotal importance in many areas of bio-
logical and chemical sciences.2 As the smallest anion, F has un-
ique chemical properties, its recognition and detection have
received considerable attention because fluoride ions are widely
used as an essential ingredient in toothpaste and pharmaceutical
agents and are even added to drinking water owing to their ten-
dency to prevent dental caries3 and enamel demineralization
resulting from wearing orthodontic appliances; they are also used
for the treatment of osteoporosis.4 In fact, the development of
receptor-based sensors for the detection of fluoride ions is chal-
lenging owing to the small size, high electronegativity, and high
hydration enthalpy of fluoride ion. Therefore, while most chemo-
sensors recognize fluoride ions in a tetrabutylammonium fluoride
5
(TBAF) salt in organic solvents, only a few can detect fluoride ions
6
in aqueous solutions. This is mainly because of complications such
as solubility, polarity, H-bonding, and ionic strength that arise in
aqueous conditions, resulting in large detection limits for fluoride
and thus the sensibility toward the receptor gradually diminishes.
However, none of the fluoride chemosensors meet the require-
ments for biological applications, which are as follows; (i) to be
able to selectively detect fluoride ions in 100% water, (ii) to be
capable of permeating the cell membrane, (iii) to be non-toxic
and display fluorescence upon the detection of fluoride ions in cel-
lular systems. In fact, numerous fluorescent and chromogenic sen-
À
À
sors have been reported to selectively detect F , which contain
7
receptors based on Lewis-acidic boron and sensors using hydro-
8
,9
gen bonding interaction with the guest. Compared with these
sensors, reaction-based sensors exhibit higher selectivity and sta-
1
0,11
À
bility.
Tamao found that F has a unique chemical reactivity
1
0a
with silicon, and then Martínez-Máñez developed a new method
for the determination of F in water samples based on the specific
reaction between F and silica. Considering all of the relevant
⇑
Corresponding author. Tel.: +91 3326684561–3; fax: +91 3326682916.
E-mail address: spgoswamical@yahoo.com (S. Goswami).
Fax: +60 4 6579150.
À
À
12
http://dx.doi.org/10.1016/j.tetlet.2014.03.003
040-4039/Ó 2014 Elsevier Ltd. All rights reserved.
0

2
634
S. Goswami et al. / Tetrahedron Letters 55 (2014) 2633–2638
0
.35
issues, new systems for fluoride anion detection should be consid-
ered and a well-established strategy for sensing fluoride anions,
based on the chemical affinity between silicon and fluoride, has re-
SNBT
5 µM F
-
+
-
-
0.28
0.21
0.14
0.07
0.00
0
0
.70
.56
+ 10 µM F
+
+
+
15 µM F -
20 µM F -
25 µM F
-
13–16
+ 30 µM F -
cently undergone a quiet revolution.
However, in most cases
+ 35 µM F
-
-
+
40 µM F -
the naked eye color detection and the fluorescence changes can
only be observed in non-aqueous solvent and there are relatively
few chemosensors which work in aqueous media, which greatly
limit their analytical application in real samples. For this reason
we use the SiAO bond cleavage technique in mixed aqueous media
0.42
.28
+ 45 µM F
+
+
+
50 µM F -
55 µM F
-
60 µM F -
0
17
34
51
[F ]/(μΜ)
68
85
0
+ 65 µM F -
-
+ 70 µM F
-
+
+
+
+
75 µM F -
80 µM F
0.14
90 µM F
-
-
100 µM F -
+ 110 µM F
À
0.00
by F ion and apply this effectively in biological system also. Here
2
50 300 350 400 450 500 550
we have developed a colorimetric and ratiometric fluorescent
chemosensor SNBT based on chemical reaction of an organosilicon
compound. The sensor SNBT was prepared in 70% overall yield by
silyl protection of compound 1 with tert-butyldimethylsilyl chlo-
Wavelength(nm)
À5
Figure 1. UV–vis absorption spectra of SNBT (c = 4.0 Â 10 M) in CH
3 2
CN–H O (8:2,
v/v, 25 °C) at pH 7.4 by using 10 mM HEPES buffer solution upon titration with 0–
À4
100
l
M
of tetrabutyl ammonium fluoride (c = 2 Â 10 M). The arrows show
17
ride (TBDMSCl) and imidazole in CH
2
Cl
2
.
The compound 1 was
À
changes due to the increased concentration of F . Inset: binding isotherms were
À
synthesized by the condensation of 2-hydroxy-1-napthaldehyde
with 2-aminothiophenol in dry ethanol by using a catalytic amount
recorded at 440 nm with the change of the concentration of F . The solid line is
global least-squares fit to the experimental data.
4
of KHSO (Scheme 1).
The sensor SNBT in solution phase undergoes ‘naked eye’ color
0
0
0
0
0
0
.70
.56
.42
.28
.14
.00
changes from colorless to green as well as fluorescence changes
À
from blue to green color when reacting with F . Thus we can detect
the fluoride ion utilizing both the absorption and emission spectra.
-
In most of the earlier reports,16 the desilylation product of the sen-
F
SNBT, AcO
-
-
-
Cl, Br,
sory systems exhibits spectroscopic properties by the normal host–
I , ATP,
-
À
KDHP, PO-3
,
4
guest interaction with F . However, in this Letter, the SNBT dis-
-
2
-
NO-2
, NO ,
3
plays entirely different optical behaviors from the desilylation
product after the reaction, which can be attributed to the ex-
cited-state intramolecular proton transfer (ESIPT) mechanism.
The sensing performance of SNBT toward fluoride anion was stud-
-
S03 , BzO
3 2
ied in CH CN–H O (8:2, v/v, 25 °C) and monitored using UV–vis,
300
350
400
450
500
1
fluorescence, and H NMR spectroscopic techniques. The UV–vis
Wavele nn gth (nm)
spectrum of SNBT shows moderately strong absorption band at
À5
2
93 and 330 nm when the absorption study of SNBT solution
Figure 2. UV–vis absorption spectra of SNBT (c = 4 Â 10 M) in CH
3 2
CN–H O (8:2,
À5
À
À4
v/v, 25 °C) at pH 7.4 by using 10 mM HEPES buffer solution upon titration with
(c = 4 Â 10 M) is carried out with F ion (c = 2 Â 10 M) in CH3-
À4
1
0.0 equiv. each of the different guest anions (c = 2 Â 10 M).
CN–H O (8:2, v/v) at pH 7.4 by using 10 mM HEPES buffer solution.
2
Addition of gradually increasing concentrations of fluoride (as its
tetrabutylammonium salt) resulted in dramatic color development
from colorless to green.
The color development phenomenon is associated with a grad-
ual decrease in the absorption band at 293 nm and simultaneous
small growth of band at 330 nm along with the large increase of
a new absorption band at 440 nm. The cleavage reaction of ‘SiAO’
bond establishes two clear isosbestic points at 285 and 304 nm
represent the sensibility toward competitive anions which remain
innocent toward sensing (Fig. 3).
The breaking of SiAO bond of SNBT results in the formation of
hydroxynaphthyl intermediate in which the acidity of –OH proton
18
is drastically enhanced by photoexcitation and the excited state
intramolecular channel might be opened upon anion binding.
ESIPT could enhance
reduce the energy gap between
P
delocalization which was expected to
(Fig. 1) in absorption spectra. The absorbance increases 17 fold at
⁄
p
and
p
transitions and therefore
4
40 nm which is also responsible for the generation of yellowish
accounts for the appearance of a new absorption band near 440 nm
green color after addition of fluoride into the solution of the recep-
tor. After addition of 10.0 equiv. of fluoride, it reaches a saturation
19
resulting in the formation of a ‘naked-eye’ yellowish green color.
À
4
The molar absorption coefficient of the probe for F is 1.63 Â 10
À
À
À
À
À
level. The various anions used are AcO , Cl , Br , I , C
6
H
5
COO ,
À1
À1
M
cm at 440 nm. On the other hand, the emission spectrum
À
À
2À
3À
4
ATP, DHP, NO
3
, NO
2
, SO
3
, and PO
as their tetra butyl ammo-
À
also exhibits remarkable changes upon interaction with
F
nium salts but there is no interference by addition of these anions
which is shown in Figure 2.
(Fig. 4a). The SNBT shows moderate blue fluorescence in
The selectivity and sensibility of SNBT are further explained by
the bar diagram in which green bar represents the extent of
sensibility of the probe toward fluoride anion and the other bars
S
S
N
N
CHO
Si
O
OH
OH
(
a)
(b)
À5
Figure 3. (A À A
0
)/A
0
ratios of SNBT (c = 4 Â 10 M) after the addition of
À4
Compound 1
SNBT
10.0 equiv. each of the various anions (c = 2 Â 10 M) in acetonitrile/water (8:2,
v/v, 25 °C) at pH 7.4 by using 10 mM HEPES buffer solution. Inset: Color changes of
À5
Scheme 1. Reagents and conditions: (a) 2-Amino thiophenol, KHSO
overnight. (b) tert-Butyl dimethylsilyl chloride, imidazole, dry DCM, O °C.
4
,
reflux,
receptor SNBT (c = 4 Â 10 M) upon addition of 10.0 equiv. each of the different
À4
guest anions (c = 2 Â 10 M).

S. Goswami et al. / Tetrahedron Letters 55 (2014) 2633–2638
2635
SNBT
5 µM F
+ 10 µM F
-
1000
+
1
000
-
-
(
a)
1000 (b)
F
-
+
+
+
+
15 µM F-
20 µM F-
25 µM F
I477
800
600
400
200
0
-
8
6
4
2
00
00
00
00
0
SN B T, A cO
- -
800
600
400
200
0
-
30 µM F-
C l , B r ,
+ 35 µM F
-
-
I , A TP,
+
+
40 µM F
45 µM F-
-
-3
K D H P, PO
,
+50 µM F
4
-
-
-
+
+
+
+
55 µM F-
N O 2 , N O 3
,
60 µM F
-
-2
-
65 µM F-
70 µM F-
S0 , B zO
3
+75 µM F
I407
-
+80 µM F
3
50 400 450 500 550 600 650
0
10 20 30 40 50 60 70
-
[
F ] /μΜ
Wavelength(nm)
350
400
450
500
550
600
Wavelength(nm)
À5
Figure 4. (a) Fluorescence spectra of the receptor SNBT (c = 4 Â 10 M) with 0–
À4
À5
8
2
0
l
M of tetrabutylammonium fluoride (c = 2 Â 10 M) in CH
3
CN/H
2
O (8:2, v/v,
Figure 5. Fluorescence spectra of receptor SNBT(c = 4 Â 10 M) in CH
3 2
CN/H O
5 °C) at pH 7.4 by using 10 mM HEPES buffer solution. (b) Binding isotherms were
(8:2, v/v, 25 °C) at pH 7.4 by using 10 mM HEPES buffer upon titration with
À
À4
recorded at 407 and 477 nm with the change of the concentration of F .
8.0 equiv. of each of the different guest anions (c = 2 Â 10 M).
acetonitrile/water (8:2, v/v, 25 °C) at pH 7.4 by using 10 mM HEPES
buffer solution in the absence of F for the short-wavelength weak
that within this family of anions, fluoride is the only one that sig-
À
nificantly causes the cleavage of SiAO bond. So, the probe SNBT can
À
emission band at 407 nm.
be used to detect F with good selectivity and sensitivity over
À
However, after addition of F , it immediately exhibits strong
other anions in the emission spectra (Fig. 5). In the bar representa-
tion in Figure 6, it is shown that the receptor SNBT selectively
binds with anion fluoride which is shown as the red bar and it re-
mains silent toward the other interfering anions in the emission
spectra.
green fluorescence which results in the emission intensity at
4
07 nm which gradually decreases and a new red shifted peak
(
D
k = 70 nm) at 477 nm appears with increasing concentration of
À
F . A clear isoemission point at 410 nm and the changes of two
emission intensities with the concentration of F ion (Fig. 4b) show
À
The competition experiment was also carried out by adding
fluoride to the solution of SNBT in the presence of commonly em-
ployed interfering anions. The selectivity profile diagram (Fig. 7)
its ratiometric behavior toward the fluoride ion. Upon the addition
À
of F , the fluorescence emission intensity of SNBT increases drasti-
À
À
cally 20-fold at 477 nm and is saturated with 8.0 equiv. of F . The
reveals that F induced absorbance and fluorescence enhancement
emission peak at 407 nm is for the silyl protected form that is for
the SNBT itself (without ESIPT). After addition of F ion, fluores-
remain unperturbed and the coexistence of other interfering an-
ions remains innocent toward sensing of fluoride. These results
are indicative of the excellent selectivity of SNBT toward F over
À
À
cence quantum yield increases 55-fold (U/U
0
= 0.273/0.005 = 55,
kmax (em) = 477 nm). The rise of a new ratiometric emission band
other competitive anions by this chemodosimeter approach for
at 477 nm for the cleavage of SiAO bond was observed for the exis-
de-silylation of SNBT moiety.
tence of a new keto form which is generated from enol intermedi-
ate of SNBT by the addition of fluoride ion (Scheme 2).
À
12
It is revealed that minimum 8.4
using 40
l
M of F can be detected by
1
0
8
6
4
2
0
l
M of SNBT solution in UV–vis titration and from the
fluorescence titration experiments, the detection limit of SNBT
À
for
F
is determined to be 6.5 lM using the equation
DL = K Â Sb1/S, where K = 3, Sb1 is the standard deviation of the
blank solution and S is the slope of the calibration curve (ESI).
The changes of absorbance and fluorescence at different time
intervals increase with an increase in concentration of fluoride.
By using these optical properties at different time intervals and
also the first order rate equation, the fluoride mediated desilylation
Figure 6. Change of emission intensity in bar representation of SNBT
À5
(
(
c = 4 Â 10 M) after adding 8.0 equiv. of each of the guest anions
rate was calculated from the time versus absorbance plot (Fig. S11)
À4
c = 2 Â 10 M) at 477 nm wavelength. Inset: Fluorescence changes of receptor
À2 À1
À5
at fixed wavelength at 440 mm (rate constant = 1.19 Â 10
s
)
SNBT (c = 4 Â 10 M) upon addition of 8.0 equiv. of each of the different guest
À4
and time versus fluorescence intensity plot (Fig. S12) at fixed
anions (c = 2 Â 10 M).
wavelength at 477 mm the rate constant is found (rate
À2 À1
constant = 2.76 Â 10
s ).
To evaluate the selectivity of SNBT toward fluoride, other typi-
À
À
À
À
À
cally encountered anions including AcO , Cl , Br , I , C
6
H
5
COO ,
as their tetra butyl ammo-
nium salts were added to the receptor solution and it was observed
À
À
2À
3À
4
ATP, DHP, NO
3
, NO
2
, SO
3
, and PO
ESIPT incapable
ESIPT capable
S
N
N
S
H
N
F
Si
S
H
O
O
Si
O
TBAF
CH CN:H
Figure 7. Anion selectivity profile of the sensor SNBT: (a) (Brown bars) change of
3
2
O
À5
absorbance of SNBT (c = 4 Â 10 M) + 10.0 equiv. of different anions
Enol Form
+
À4
Keto Form
(c = 2 Â 10 M); (pink bars) change of absorbance of sensor + 10.0 equiv. of
À
Si
F
different anions, followed by 10.0 equiv. F at 440 nm. (b) (Brown bars) change of
À5
emission intensity of SNBT (c = 4 Â 10 M) + 8.0 equiv. of different anions
À4
(
c = 2 Â 10 M); (pink bars) change of emission intensity of sensor + 8.0 equiv. of
À
Scheme 2. Possible fluoride-selective signaling mechanism in solution phase.
different anions, followed by 8.0 equiv. F at 477 nm.

2
636
S. Goswami et al. / Tetrahedron Letters 55 (2014) 2633–2638
0.8
0.6
0.4
0.2
0.0
900
7
6
4
50
00
50
-
S N B T +
F
300
50
600
S N B T
1
300
350
400
450
500
550
Wavelength(nm)
Figure 9. The absorption and emission spectra SNBT showing the large difference
on excitation and emission wavelength in CH CN/H O (8:2, v/v, 25 °C) at pH 7.4 by
3
2
using 10 mM HEPES buffer. Solid line (absorption spectra) Dashed line (emission
spectra).
À4
Figure 8. Photograph of SNBT (c = 4 Â 10 M) in various concentrations of
À3
tetrabutyl ammonium fluoride (c = 2 Â 10 M) on test papers. Top panel: UV–vis
naked eye color change on addition of (a) SNBT itself (b) after immersion into
À
3
CH CN–H
2
O (8:2, v/v, pH-7.4) with 10.0 equiv. of F . (c) SNBT + 10.0 equiv. of other
À
anions. (d) SNBT + 10.0 equiv. of F followed by 10.0 equiv. of other anions. Bottom
panel: Fluorescence changes on addition of (e) SNBT itself (f) after immersion into
À
3
CH CN–H
2
O (8:2, v/v, pH-7.4) solutions with 8.0 equiv. of F . (g) SNBT + 8.0 equiv.
À
of other anions. (h) SNBT + 8.0 equiv. of F followed by 8.0 equiv. of other anions.
To investigate the practical application of sensor SNBT, test
strips were prepared by immersing the TLC plates into the receptor
À4
solution (c = 4 Â 10 M) in CH
3 2
CN–H O (8:2, v/v) by using 10 mM
HEPES buffer solution at pH 7.4. The test strips containing SNBT
were utilized to sense F at different conditions. As shown in Fig-
ure 8, when F and the other anions were added on the test kits
Figure 10. Molecular view of the (a) compound 1 and (b) SNBT with atomic
numbering schemes, showing 30% probability displacement ellipsoids for non-H
atoms. Intramolecular hydrogen bonds are shown as dashed lines. The crystallo-
graphic information files for compound 1 and SNBT have been deposited in the
Cambridge Crystallographic Data Center as CCDC 943908 and CCDC 942746,
respectively.
À
À
respectively, the obvious yellowish green color was observed only
with F and also the sensing property remains unperturbed in the
presence of other interfering anions in absorption (Top panel,
À
Fig. 8). In emission spectroscopy, the blue color for SNBT changes
À
to green after addition of F and its green color remains unchanged
is compound 1 can undergo an ESIPT from the acidic hydroxyl pro-
ton to the basic site (aromatic nitrogen) when photo-excitation
changes their charge density distribution which is probably fol-
lowed by hydrogen bonding with fluorine attached with TBDMS
as TBDMSF possibly due to strong electronegativity of F (scheme
2). The intramolecular OAHÁ Á ÁN hydrogen bond in X-ray crystal
structure of enol form arises from the acidic hydroxyl group to ba-
sic nitrogen of benzothiazole moiety. From the NMR titration curve
the addition of excess fluoride, slight appearance of a small peak at
d 4.5 ppm indicates the formation of –NH signal response due to
ESIPT via fluoride mediated desilylation (Fig. S10).
in the presence of other anions (bottom panel, Fig. 8). Potentially
the competitive anions exerted no influence on the detection of
À
F
by the test strips and therefore the test strips could conve-
À
niently detect F in solutions for both absorption as well as emis-
sion spectra. The development of such dipsticks or test strips is
useful as instant qualitative information is obtained without
resorting to the instrumental analysis.
The probable binding mode in solution phase may be explained
as shown in Scheme 2 and also the orientation of the key function-
ality in the X-ray crystal structure. In these structures we see that
the –OH group is in the same side to the benzothiazole nitrogen by
the intramolecular hydrogen bonding which is shown in Figure 10.
In SNBT the intramolecular OAHÁ Á ÁN hydrogen bonding does not
arise. In Scheme 2 when the hydroxyl group was protected by
the TBDMS (tert-butyldimethyl silyl) group, the ESIPT process of
SNBT was switched off and the probe shows absorption band at
93, 330 nm and emission maximum at 407 nm. Because of the
high affinity of fluoride for silicon, silyl ethers are easily cleaved
by fluoride. Upon introducing fluoride in CH CN/H O (8:2, v/v,
5 °C) the TBDMS group of the probe was removed readily through
a fluoride-induced SiAO bond cleavage (strong bond energy of
SiAF bond) creating the enol intermediate which may undergo
ultrafast photo-induced tautomerization process via ESIPT (ex-
cited-state intramolecular proton transfer) because the transfer-
ring proton is already next to the proton acceptor at the moment
of excitation and the ESIPT process of the probe was switched on.
It is observed that 1-(benzo[d] thiazol-2-yl) naphthalene-2-ol that
As a result the absorption band at 293 nm diminishes gradually
and the peak at 330 nm increases slightly with the large stokes
shift to a new band appearance at 440 nm (
keto form.
Dk = 147 nm) for the
In the fluorescence spectra the emission band at 407 nm stea-
dily decreases with the appearance of the ratiometric emission
band at 477 nm with a clear isoemission point at 410 nm and large
red shift of 70 nm. The large difference of wavelength (SNBT itself
2
À
3
2
and after addition of F ) that is 147 nm for absorption and 70 nm
2
for emission indicates ESIPT phenomena after de-silylation of
OTBDMS moiety of the sensor (Fig. 9).
In compound 1, the crystal structure is stabilized by an intramo-
lecular
OAHÁ Á ÁN
hydrogen
bond
[O1Á Á ÁN1 = 2.519 Å,
H1O1Á Á ÁN1 = 1.740 Å and O1AH1O1Á Á ÁN1 = 146.0°], forming S(6)
ring motif (Fig. 11a), and the dihedral angle between the least-
square plane (all atoms) of benzothiazole [r.m.s. devia-
tion = 0.014 Å] and naphthalene [r.m.s. deviation = 0.054 Å] ring

S. Goswami et al. / Tetrahedron Letters 55 (2014) 2633–2638
2637
systems is 18.57(9)°. This structural orientation favors the excited
state intramolecular proton transfer (ESIPT) to the enol intermedi-
ate from naphthyl –OH moiety to the benzothiazole nitrogen after
desilylation of SNBT. But, when the tert-butyldimethylsilane moi-
ety is present and the intramolecular OAHÁ Á ÁN hydrogen bonding
is not present, the benzothiazole ring system rotates slightly. The
corresponding dihedral angle in SNBT is 68.56(8)°. In the SNBT, a
weak intramolecular CAHÁ Á ÁN hydrogen bond [C21Á Á ÁN1 =
and emission spectroscopy. We believe the test strips can act as
convenient and efficient F test kits.
À
Acknowledgements
Authors thank the DST and CSIR (Govt. of India) for financial
support. A.K.D., A.M. and A.K.M. acknowledge the CSIR for provid-
ing fellowships. The authors extend their appreciation to The
Deanship of Scientific Research at King Saud University for funding
the work through research group project no. RGP-VPP-207.
3
.893 Å, H21CÁ Á ÁN1 = 2.945 Å and C21AH21CÁ Á ÁN1 = 169.8°] be-
tween the nitrogen atom of the benzothiazole ring system and
one of the carbon atoms of the methylpropane group ensures a
favorable orientation that the nitrogen atom and the OTMS are
on the same side (Fig. 10b). There are no significant intermolecular
hydrogen bonds in the crystal structures of compound 1 and SNBT.
Supplementary data
Supplementary data (detailed characterization of the com-
pound 1 and SNBT along with the intermediates, additional spec-
troscopic details, spectra and crystallographic analysis are
provided in supplementary information) associated with this arti-
cle can be found, in the online version, at http://dx.doi.org/
10.1016/j.tetlet.2014.03.003.
Cell imaging
In order to determine the membrane permeability of the recep-
tor (SNBT) and its ability to specifically bind to the fluoride ion in
living cells, HeLa cells were first incubated with the fluoride ion
molecules followed by the addition of the receptor. A control
experiment, that is, incubation with the receptor (SNBT) only
was also carried out.
References and notes
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