Terphenyl based fluorescent chemosensor for Cu2+ and F - ions employing excited state intramolecular proton transfer

Article abstract of DOI:10.1016/j.tet.2010.11.083

A new terphenyl based bifunctional fluorescent chemosensor 3a has been synthesized, which demonstrates selective optical recognition of Cu²⁺ and F^- ions in two contrasting modes. The compound shows highly selective 'On-Off' switchabl

Full text of DOI:10.1016/j.tet.2010.11.083

Tetrahedron 67 (2011) 1266e1271
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Terphenyl based fluorescent chemosensor for Cu2þ and Fꢀ ions employing excited
state intramolecular proton transfer
*
*
Vandana Bhalla , Ruchi Tejpal, Manoj Kumar
Department of Chemistry, UGC sponsored-Centre of Advance Studies-I, Guru Nanak Dev University, Amritsar 143005, Punjab, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 19 October 2010
Received in revised form 19 November 2010
Accepted 24 November 2010
Available online 1 December 2010
A new terphenyl based bifunctional fluorescent chemosensor 3a has been synthesized, which demon-
strates selective optical recognition of Cu2þ and Fꢀ ions in two contrasting modes. The compound shows
highly selective ‘OneOff’ switchable behavior toward Cu2þ ions and ‘OneOffeOn’ behavior toward Fꢀ
ions among various cations and anions tested. The detection limits of chemosensor for Cu2þ and Fꢀ ions
are found to be 100 nM and 10 nM, respectively.
Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
The different kinds of signaling mechanisms including internal
charge transfer (ICT),14 photoinduced electron transfer (PET),15
The design and synthesis of chemosensors for both anions1 as
well as cations2 is an active area of research within the field of
supramolecular chemistry. Currently, there is an active effort to
develop molecular systems, which bind both cations as well as
anions. Among cations, the selective recognition and sensing of soft
metal ions is very important as these cations play an important role
in biology,3 chemistry,4 and environment.5 In particular, the se-
lective sensing of copper, which is third in abundance among the
essential transition metal ions in human body has gained attention
due to its significance in biological systems.6 Copper kills a variety
of potentially harmful pathogens and hence has antimicrobial ef-
fect against MRSA, Escherichia coli and other pathogens.7 But the
over accumulations of copper produce severe or lethal in-
toxications.8 On the other hand, among the biologically important
anions, fluoride has received significant interest due to its benefi-
cial effects9 (e.g., prevention of dental caries and treatment of os-
teoporosis) and detrimental (e.g., fluorosis) roles.10 Furthermore,
these ions are also associated with nerve gases, in the analysis of
drinking water, and the refinement of uranium used in nuclear
weapon manufacture. Thus, the diversity of their functions, both
beneficial and otherwise, makes the detection of copper and fluo-
ride ions important. The detection and monitoring of cations and
anions by methods, which allow the development of selective and
sensitive assays are in great demand. Fluorescence signaling is one
of the first choices due to its high detection sensitivity and sim-
plicity.11 Thus, designing fluorescent sensors for copper12 and
fluoride13 has recently drawn worldwide attention.
metal-to-ligand charge transfer (MLCT),16 excimer/exciplex for-
mation,17 and tuning proton transfer18 have been utilized for de-
signing fluorescent chemosensors. Nowadays, excited state
intramolecular proton transfer (ESIPT) has achieved a significant
interest in the fundamental investigation and the applications of
organic molecules because of their four level photophysical
scheme, spectral sensitivity to the surrounding medium and a large
Stokes’ shifted fluorescence.19 The ESIPT process involves a photo-
tautomerization reaction to yield an excited keto form in the sub-
picosecond time region, with the molecule changing from the
original enol form to the keto form on photoirradiation.20 The in-
tramolecularly hydrogen bonded chromophores generally exhibit
this process. We utilized this concept in our design and in-
corporated nitrophenolic moieties to the receptor as the electron
withdrawing nitro group is known to increase the acidity of hy-
droxyl groups and hence the hydrogen donor properties of the
receptor.
Our research work involves the design, synthesis, and evalua-
tion of artificial receptors for selective sensing of soft metal ions21
and anions22 of clinical and environmental interest. Recently we
reported new chemodosimeter for fluoride ions23 and ‘Turn On’
fluorescent sensors for mercury ions based on terphenyl deriva-
tives.21a,b Terphenyls have significant biological activities, e.g., po-
tent immunosuppressant, neuroprotective, antithrombotic,
anticoagulant, specific 5-lipoxygenase inhibitory, and cytotoxic
activities.24 These are also being used as key intermediates for the
synthesis of symmetrically and unsymmetrically substituted tri-
phenylenes, which have great potential for supramolecular and
material chemistry as their liquid crystalline behavior can be
modified by changing electronic properties of their substituents.
Thus, in continuation of our work on fluorescence sensing of soft
* Correspondingauthors. Tel.:þ91 (183) 2258802-9x3202; fax: þ91 (183) 2258820
(V.B.); tel.: þ91 183 2258802-9x3205; fax: þ91 (183) 2258820 (M.K.); e-mail
addresses: vanmanan@yahoo.co.in (V. Bhalla), mksharmaa@yahoo.co.in (M. Kumar).
0040-4020/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2010.11.083

V. Bhalla et al. / Tetrahedron 67 (2011) 1266e1271
1267
metal ions and anions, we have now synthesized a new receptor
based on terphenyl, which behaves as a dual chemosensor upon
chemical inputs of Cu2þ and Fꢀ ions. To the best of our knowledge,
this is the first dual mode fluorescent chemosensor based on ESIPT
with nitrophenolic groups appended to terphenyl skeleton.
2. Results and discussion
Condensation of diamine 121a with 2.2 mol equiv of 2-hydroxy-
5-nitrobenzaldehyde in ethanol furnished target compound 3a in
85% yield (Scheme 1). The structure of compound 3a was confirmed
from its spectroscopic and analytical data. The 1H NMR spectrum of
compound 3a showed two singlets (18H, 12H) for tert-butyldime-
thylsilyl (TBS) group, one singlet and one doublet for aromatic
protons of terphenyl moiety, two doublets and one singlet for
nitrosalicylaldehyde protons, one singlet for imino (N]CH) pro-
tons, and one singlet for hydroxyl protons (Fig. S2, Supplementary
data). The IR spectrum of compound 3a showed a C]N stretching
band at 1630 cmꢀ1. A molecular ion peak was observed at 819 (Mþ)
in the FAB mass spectrum, which corresponds to 1:2 condensation
product. These spectroscopic data corroborate the structure 3a for
this compound.
Fig. 1. UVevis spectra of compound 3a (10
in THF/H2O (9:1).
m
M) on addition of Cu2þ ions (0e10 equiv)
(Pb2þ, Hg2þ, Ba2þ, Cd2þ, Agþ, Zn2þ, Cu2þ, Ni2þ, Co2þ, Kþ, Mg2þ, Naþ,
and Liþ) tested, the addition of Cu2þ (10 equiv) ions only, resulted in
a slight redshift (9 nm) of band at 342 nm to 351 nm (Fig. 1). This is
presumably because of intramolecular charge transfer interactions
between phenolic OH and Cu2þ bound imino nitrogen.
In the fluorescence spectrum, the compound 3a (2.0
mM)
O
exhibited an emission band centered at 517 nm when excited at
360 nm, which can be attributed to a very fast enol-imine (3a) to
keto-amine (4) tautomerism involving the phenomenon of excited
state intramolecular proton transfer (ESIPT). To confirm the tau-
tomerism phenomenon, a reference compound 3c was synthesized
in which the phenolic hydroxyl groups of compound 3a were
protected by methyl groups. On excitation at 360 nm, the com-
pound 3c did not exhibit any band at 517 nm. This experiment
confirms the presence of ESIPT phenomenon in compound 3a.
OR'
OTBS
TBSO
OTBS
TBSO
R
2a-c
ethanol, rt
N
N
H2N
NH2
OR'
R'O
R
R
1
3a-c
a; R = NO2, R' = H; 85%
b; R = H, R' = H; 86%
Upon addition of increasing amounts of Cu2þ ions (5.0
mM) to the
solution of compound 3a in mixed aqueous media (THF/H2O; 9:1),
the fluorescence intensity was found to be completely quenched
(Fig. 2) indicating the formation of 3aeCu2þ complex (Fig. 3). This
may be due to the coordination of Cu2þ ions to imino nitrogens
leading to the prohibition of ESIPT phenomenon and hence fluo-
rescence quenching.
c; R = NO2, R' = CH3; 80%
TBSO OTBS
OTBS
TBSO
ESIPT
N
N
NH
O
NH
OH
HO
O
O2N
NO2
O2N
NO2
3a
O
4
O
O
O
O
O
O
OH
O
O
O
O
O2N
2a
ethanol, rt
81%
N
N
H2N
NH2
OH
HO
O2N
NO2
5
6
Scheme 1.
Fig. 2. Changes in fluorescence emission spectra of 3a (2
ions (0e5
chiometry; L is the concentration of ligand.
m
M) upon addition of Cu2þ
M) in THF/H2O (9:1; lex¼360 nm). Inset: Job’s plot showing a 1:1 stoi-
m
The binding behavior of compound 3a toward different cations
(Pb2þ, Hg2þ, Ba2þ, Cd2þ, Agþ, Zn2þ, Cu2þ, Ni2þ, Co2þ, Kþ, Mg2þ, Naþ,
and Liþ) was investigated by UVevis and fluorescence spectroscopy.
The titration experiments were carried out in mixed aqueous media
(THF/H2O; 9:1). The UVevis spectrum of the compound 3a exhibits
absorption bands at 265 nm and 342 nm in THF/H2O (9:1) (Fig. 1).
TBSO OTBS
TBSO OTBS
Cu
N
N
N
N
Cu
OH
HO
O
The band at 265 nm is due to the transition between the
p
orbitals
O
H
H
O2N
NO2
localized on the imine (C]N) linkage. The band at 342 nm mayoccur
due tothe intramolecularchargetransfer (ICT)transitions within the
whole structure of the Schiff’s base.25 Among the various metal ions
O2N
NO2
3a
Fig. 3. Schematic representation of the complexation process for 3a with Cu2þ ions.

1268
V. Bhalla et al. / Tetrahedron 67 (2011) 1266e1271
Under the same conditions as used for Cu2þ ions, we also tested
(host/guest) was the most stable species in the solution with the
binding constant (log
(Job’s plot) was also used to prove the 1:1 stoichiometry (host/
guest) (Inset Fig. 2).
the fluorescence responseof compound 3a toother metal ions (Pb2þ
,
b
)¼5.40. The method of continuous variation
Hg2þ, Ba2þ, Cd2þ, Agþ, Zn2þ, Ni2þ, Co2þ, Kþ, Mg2þ, Naþ, and Liþ) and
as shown in Fig. 4 (series 1), no significant change in fluorescence
occurred in the presence of these metal ions. To test the practical
applicability of compound 3a as a Cu2þ selective sensor, competitive
Recently it has been reported that hydroxyl groups of serine and
tyrosine play an important role in anion binding pockets of bi-
ological systems like ClC chloride channels28 and halorhodopsin.29
Keeping in view the important role played by hydroxyl groups in
the anion binding pockets of the biological systems, the de-
velopment of receptors having hydroxyl groups for recognition of
anions is very important in supramolecular chemistry.30 Since our
receptor 3a has reasonably acidic phenolic moieties, we also in-
vestigated the sensing properties of compound 3a toward different
anions (Fꢀ, Clꢀ, Brꢀ, Iꢀ, HSO4ꢀ, NO3ꢀ, and H2POꢀ4 ) with tetrabuty-
lammonium as counter cation using UVevis, fluorescence, and
NMR experiments.
experiments were carried out in the presence of Cu2þ ions at 5
mM
mixed with the other metal ions at 150 mM and as shown in Fig. 4
(series 2), no significant variation in fluorescence intensity was
found by comparison with or without other metal ions.
In the UVevis spectrum of the compound 3a, the addition of
increasing amounts of Fꢀ ions from 0 to 3.0
mM resulted in a de-
crease in absorption at 265 nm and 342 nm along with the for-
mation of new red shifted band at 447 nm with a clear isosbestic
point at 379 nm (Fig. 6). Besides, a colorimetric change from col-
orless to yellow was also observed by the naked eye (Inset Fig. 6).
When the Fꢀ ions come in contact with compound 3a, the in-
termolecular proton transfer takes place between phenolic oxygens
and fluoride ions. The modulation in the electron donating abilities
of phenolic oxygen in the presence and absence of fluoride ions
directly influences the intramolecular charge transfer (ICT) from
the phenolic oxygen to the electron deficient p-nitrophenyl moi-
ety.31 In the absence of fluoride ions, ICT is inefficient while in the
presence of fluoride ions, ICT is facilitated by proton transfer from
phenolic oxygen to fluoride ions. Thus, we propose that the spectral
changes in Fig. 6 are due to the deprotonation of the phenolic
protons, which results in enhanced charge transfer interactions
between electron rich and electron deficient moieties resulting in
visible color change. Similar results were obtained when tetrabu-
tylammonium hydroxide was specifically employed. This further
confirms our doubt that the proton transfer between phenolic ox-
ygen and the fluoride ions is responsible for the observed change.
Similarly, UVevis experiments were carried out in the presence of
the other anions (Clꢀ, Brꢀ, Iꢀ, HSO4ꢀ, NOꢀ3 , and H2POꢀ4 ), besides Fꢀ
ion but no significant change in UVevis spectra of 3a was observed.
Fig. 4. Series 1 showing fluorescence quenching ratio (IoꢀI/Io)ꢁ100 of receptor 3a
(2
mM) at 517 nm upon addition of different metal ions (10 equiv, 10
mM) and series 2
showing competitive selectivity of receptor 3a (2
presence of other metal ions (150 M).
m mM) in the
M) toward Cu2þ (5
m
We also carried out a reversibility experiment, which proved
that Cu2þ binding to compound 3a is reversible. In the presence of
20 equiv of EDTA, Cu2þ ions formed complex with EDTA, resulting
in the decomplexation of receptoreCu2þ complex, as a result of
which the fluorescence emission of the receptor was revived again.
On adding Cu2þ ions again to the above solution, the fluorescence of
receptor 3a was quenched indicating the reversible complexation
of Cu2þ ions with the receptor 3a (Fig. 5).
Fig. 5. Fluorescence spectra showing reversibility of Cu2þ coordination to receptor 3a
by EDTA; blue line, free 3a (2
m
M), red line, 3aþ5 equiv Cu2þ, green line, 3aþ5 equiv
Cu2þþ20 equiv EDTA.
The detection limit26 of 3a as a fluorescent sensor for the analysis
of Cu2þ ions was determined from a plot of fluorescence intensity as
a function of the concentration of added metal ions and was found to
be 100 nM, which is sufficiently low for the detection of nanomolar
concentration range of Cu2þ ions found in many chemical systems.
Fitting the changes in fluorescence spectra of compound 3a with
Cu2þ ions, using the nonlinear regression analysis program SPEC-
FIT27 gave a good fit and demonstrated that 1:1 stoichiometry
Fig. 6. UVevis spectra of compound 3a (10 m
M) on addition of Fꢀ ions (0e3.0 equiv) in
THF; Inset. Showing the change in the color of receptor 3a (1ꢁ10ꢀ5 M) upon addition of
Fꢀ ions (3.0
mM).
In the fluorescence spectrum, addition of Fꢀ ions (5
mM) to the
solution of compound 3a in THF leads to a significant decrease in
the emission band at 517 nm in THF. On further addition of Fꢀ ions
(350 mM), a new blue shifted band centered at 478 nm appeared

V. Bhalla et al. / Tetrahedron 67 (2011) 1266e1271
1269
(Fig. 7). We propose that this fluorescence response of compound 3a
is due to the modulation of the existing ESIPT state (4) by the in-
teraction of Fꢀ ions along with the simultaneous desilylation reaction
of compound 3a in the presence of Fꢀ ions in THF. The quenching of
emission band of 3a at 517 nm on addition of Fꢀ ions indicates that
the presence of Fꢀ ions completely inhibits ESIPT phenomenon. This
inhibition of ESIPT phenomenon is ascribed to the intermolecular
proton transfer from phenolic oxygen to the Fꢀ ions. The formation of
new band at 478 nm is due to the cleavage of SieO bond, which
results in increased negative charge on oxygen atoms leading to the
charge delocalization of the system (Fig. S4, Supplementary data). To
confirm this assumption and evaluate the intermolecular in-
teractions between the compound 3a and Fꢀ ions, we carried out
NMR studies in CDCl3. It was found that on addition of tetrabuty-
lammonium fluoride to the solution of compound 3a in CDCl3, signal
fluorescence intensity of receptor 6 got quenched, however, no new
band was formed at 478 nm (Fig. 9). This indicates that OTBS groups
in compound 3a are involved in the formation of new band at
478 nm in the presence of Fꢀ ions (Fig. 7).
of phenolic hydroxyl groups at
d 14.4 disappeared, which indicates
that the deprotonation occurred on addition of Fꢀ ions (Fig. S3,
Supplementary data). The signals due to OTBS group also dis-
appeared in the presence of Fꢀ ions indicating that desilylation has
taken place on addition of Fꢀ ions (Fig. S3, Supplementary data).
Fig. 9. Changes in fluorescence emission spectra of 6 (2
(0e10
M) in THF (lex¼360 nm).
m
M) upon addition of Fꢀ ions
m
On the other hand, in compound 3b, which lacks crown-5 ring
and nitro groups (NO2) but have OTBS groups showed very weak
emission indicating that PET from imino nitrogen to phenol pre-
dominates. The addition of Fꢀ ions to the solution of 3b (2
mM) in
THF resulted in the formation of a new band at 478 nm as was
observed in 3a, which confirms that the desilylation is responsible
for fluorescence enhancement (Fig. 10).
100
80
400 µM
60
F-
Fig. 7. Changes in fluorescence emission spectra of 3a (2
(0e350
M) in THF (lex¼360 nm).
m
M) upon addition of Fꢀ ions
40
m
0 µM
20
0
To further investigate the binding mechanism of receptor 3a
toward Cu2þ and Fꢀ ions, we also synthesized model compounds 3b
and 6. The compound 3b contains relatively weak acidic phenolic
protons, whereas in compound 6 OTBS groups were replaced by
crown-5 ring.
400
450
500
550
600
650
Wavelength
Compound 6 showed similar fluorescence quenching on addi-
tion of Cu2þ ions as was found in compound 3a in the presence of
Cu2þ ions (Fig. 8) (vide supra). Whereas, on addition of Fꢀ ions, the
Fig. 10. Changes in fluorescence emission spectra of 3b (2
(0e400
M) in THF (lex¼360 nm).
m
M) upon addition of Fꢀ ions
m
Thus from above two control experiments we conclude that the
presence of nitro groups in receptor 3a makes the phenolic protons
relatively more acidic, which is responsible for emission at 517 nm.
Thebehaviorofmodelcompounds6and 3b illustratestheimportance
of OTBS groups in compound 3a responsible for blue shifted band.
Under the same conditions as used for Fꢀ ions, we also tested
the fluorescence response of compound 3a to other anions (Clꢀ,
Brꢀ, Iꢀ, HSO4ꢀ, NOꢀ3 , and H2POꢀ4 ) and as shown in Fig.11 (series 1), no
significant fluorescence change was observed in the presence of
these anions. The practical applicability of compound 3a as a Fꢀ
selective fluorescence sensor was tested by carrying out competi-
tive experiments in the presence of Fꢀ ions mixed with Clꢀ, Brꢀ, Iꢀ,
HSOꢀ4 , NOꢀ3 , and H2POꢀ4 ions. No significant variation in fluores-
cence intensity change was found by comparison with or without
other anions besides, Fꢀ ions (Fig. 11, series 2).
Fig. 8. Changes in fluorescence emission spectra of 6 (2
ions (0e8.0
M) in THF/H2O (9:1); lex¼360 nm.
m
M) upon addition of Cu2þ
The detection limit of compound 3a as a fluorescence sensor for
m
the analysis of Fꢀ ions was found to be 10 nM, which is sufficiently

1270
V. Bhalla et al. / Tetrahedron 67 (2011) 1266e1271
(CH2Cl2) 0.54; 1H NMR (300 MHz, CDCl3):
d 0.28 (12H, s, Si(CH3)2),
1.04 (18H, s, C(CH3)3), 6.92 (2H, s, ArH), 7.08 (2H, d, J 9.3 Hz, ArH),
7.24 (8H, s, ArH), 8.27 (2H, d, J 9.0 Hz, ArH), 8.37 (2H, s, ArH), 8.72
(2H, s, N]CH), 14.48 (2H, s, OH). 13C NMR (75 MHz, CDCl3): 160.3,
154.9, 148.7, 148.0, 146.3, 140.3, 140.2, 136.6, 133.1, 130.8, 129.9,
129.7, 128.9, 127.7, 127.6, 123.1, 121.0, 118.7, 25.9, 18.5, 3.9; IR nmax
(KBr, cmꢀ1): 1630 (C]N). MS (FAB): 819 (Mþ).
Compound 3b: Yield: 86%; mp: 370 ꢂC; [found: C, 72.80; H, 7.30;
N, 3.67. C44H52N2O4Si2 requires C, 72.49; H, 7.19; N, 3.84]; Rf (80%
CH2Cl2/hexane) 0.7; 1H NMR (300 MHz, CDCl3):
d 0.11 (12H, s, Si
(CH3)2), 1.04 (18H, s, C(CH3)3), 6.92 (2H, s, ArH), 7.01 (2H, d, J 8.1 Hz,
ArH), 7.16 (10H, s, ArH), 7.34e7.39 (4H, m, ArH), 8.64 (2H, s, N]CH),
13.31 (2H, s, OH). 13C NMR (75 MHz, CDCl3): 146.4, 140.1, 133.0,
132.9, 130.8, 123.1, 120.9, 119.3, 117.2, 26.0, 18.5, 4.0; IR nmax (KBr,
cmꢀ1): 1630 (C]N). MS (FAB): 730 (Mþ).
Fig. 11. Series 1 (front) showing fluorescence quenching ratio (IoꢀI/Io)ꢁ100 of receptor
3a (2
mM) at 517 nm upon addition of different anions in THF and series 2 (back)
showing competitive selectivity of receptor 3a toward Fꢀ ions in the presence of other
anions.
low for the detection of nanomolar concentration range of Fꢀ ions
found in many chemical systems.
Compound 3c: Yield: 80%; mp: >280 ꢂC; [found: C, 64.96; H,
6.62; N, 6.49. C46H54N4O8Si2 requires C, 65.22; H, 6.43; N, 6.61]; Rf
(CH2Cl2) 0.41; 1H NMR (300 MHz, CDCl3):
d 0.28 (12H, s, Si(CH3)2),
In conclusion, we have synthesized a highly selective fluores-
cent chemosensor for Cu2þ and Fꢀ ions based on terphenyl as
a scaffold employing ESIPT. The recognition of Cu2þ ions give rise to
the quenched fluorescence of receptor 3a, whereas Fꢀ ions recog-
nition leads to the appearance of a blue shifted band at 478 nm. The
detection limits for Cu2þ and Fꢀ ions were found to be 100 nM and
10 nM, respectively. Thus receptor 3a may be considered as a po-
tential bifunctional fluorescent chemosensor for Cu2þ and Fꢀ ions.
1.04 (18H, s, C(CH3)3), 4.05 (6H, s, OCH3), 6.92 (2H, s, ArH), 7.04 (2H,
d, J 9.3 Hz, ArH), 7.14 (8H, s, ArH), 8.32 (2H, d, J 9.0 Hz, ArH), 8.88
(2H, s, ArH), 9.04 (2H, s, N]CH). 13C NMR (75 MHz, CDCl3): 160.3,
156.9, 154.9, 148.7, 148.0, 146.3, 140.3, 140.2, 136.6, 133.1, 130.8,
129.9, 129.7, 128.9, 127.8, 127.7, 123.2, 121.1, 118.6, 25.9, 18.6, 4.0; IR
nmax (KBr, cmꢀ1): 1631 (C]N).
Compound 6: Yield: 81%; mp: 280 ꢂC; [found: C, 64.56; H, 5.06;
N, 7.09. C40H36N4O11 requires C, 64.17; H, 4.85; N, 7.48]; Rf (8%
CH3OH/EtOAc) 0.54; 1H NMR (300 MHz, CDCl3):
d 3.80 (8H, s, crown
3. Experimental
3.1. General
H), 3.96 (4H, t, J 4.2 Hz, crown H), 4.25 (4H, t, J 4.2 Hz, crown H), 6.97
(2H, s, ArH), 7.08 (2H, d, J 9.0 Hz, ArH), 7.24 (8H, s, ArH), 8.26 (2H, d, J
6.3 Hz, ArH), 8.37 (2H, s, ArH), 8.71 (2H, s, N]CH), 14.44 (2H, s, OH).
13C NMR (75 MHz, CDCl3): 160.3, 154.9, 148.7, 148.0, 146.3, 140.3,
136.6, 133.1, 130.8, 129.9, 129.7, 128.9, 127.7, 127.6, 123.1, 121.0, 118.7,
25.9,18.5, 3.9; IR nmax (KBr, cmꢀ1): 1632 (C]N). MS (FAB): 748 (Mþ).
Palladium chloride, 2-hydroxy-5-nitrobenzaldehyde, 2-hydrox-
ybenzaldehyde, all metal perchlorates, and tetrabutylammonium salts
of anions were purchased from Aldrich and were used without further
purification. Potassium carbonate, ethanol, and 1, 4-dioxane (99%)
were purchased from S.d. fine chemicals. THF was dried over sodium
metal and benzophenone before it was used for analytical studies.
All the fluorescence spectra were recorded on Shimadzu RF-
5301PC spectrofluorophotometer. UV spectra were recorded on
Shimadzu UV-2450PC spectrophotometer with a quartz cuvette
(path length: 1 cm). The cell holder was thermostatted at 25 ꢂC. IR
spectra were recorded with Shimadzu FTIR 8400S IR spectropho-
tometer by using KBr as medium. Elemental analysis was done in
department of Chemistry, Guru Nanak Dev University, Amritsar
using Flash EA 1112 CHNSeO analyzer of Thermo Electron Corpo-
ration. 1H and 13C NMR spectra were recorded on JEOL-FT NMR-AL
300 MHz spectrophotometer using CDCl3 as solvent and TMS as
internal standards. Data are reported as follows: chemical shifts in
Acknowledgements
V.B. is thankful to DST for financial support (Ref. No. SR/FT/CS/
10-2006). R.T. is thankful to CSIR. We are also thankful to Central
Drug Research Institute (CDRI), Lucknow, for FAB mass spectra and
UGC for SAP program.
Supplementary data
Supplementary data related to this article can be found online at
doi:10.1016/j.tet.2010.11.083.
References and notes
parts per million
(
d
), multiplicity (s¼singlet, d¼doublet,
m¼multiplet), coupling constants (Hz), integration, and in-
terpretation. All spectrophotometric titration curves were fitted
with SPECFIT 32 software.
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Compound 3a: Yield: 85%; mp: >280 ꢂC; [found: C, 64.65; H,
6.05; N, 6.91. C44H50N4O8Si2 requires C, 64.52; H, 6.15; N, 6.84%]; Rf

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