H. M. Chawla et al. / Tetrahedron Letters 56 (2015) 682–685
683
comprehend the Cu2+ binding nature of 4, quantitative UV–Vis
titration of 4 (20 M) was carried out with increasing equivalents
of Cu ions (0–1.45 equiv). It was determined that gradual addi-
tion of Cu ions to the receptor 4 led to a decrease and a shift in
absorbance at 405 nm to 430 nm (Fig. 1) accompanied by a color
change of the solution of receptor.
O
NNH2
OH
H
l
2
+
OH
NH NH .2H O
2
2
2
2
+
EtOH
Stir
Fluorescence emission spectra of 4 (20
3
lm) in CH CN was
recorded after excitation at 337 nm. It was determined that on
2
+
addition of Cu ions a prompt change in the intensity of fluores-
cence emission of 4 was observed to yield more than 90% quench-
ing of the emission maxima at 505 nm. To learn more about the
1
,3 dibromopropane
CO /ACN
reflux
O
OHOH
O
K
2
3
OH
OHOH
OH
2
+
properties of 4–Cu complex formation, a titration of the receptor
(
1)
(2)
2+
Br
Br
was performed with increasing concentration of Cu ions. The
fluorescence intensity of a 20 M solution of 4 decreased with an
increase in the concentration of copper perchlorate to reach satu-
l
4
K
-hydroxy benzaldehyde
2
CO
3
3
/ CH CN
2
+
reflux
ration levels after addition of 1.5 equiv of Cu ions (Fig. 2). Though
analysis of the vibronic spectrum is outside the scope of this study,
O
O
O
3
O
H
(
H
2
C)
H
(
CH
2
)
3
it appears that the fluorophore gets excited to its S
absence of metal ions to result in higher intensity emission band
due to S ?S state. However in the presence of copper ions, spe-
cific complexation leads to intersystem cross over from S to T
1
1
state in the
O
O
NNH
2
1
0
OH
O
O HO H
O
O
1
CH
N
CH
N
state to usher a much decreased intensity of fluorescence and
deactivation by bimolecular non radiative processes. Analysis of
the infrared spectrum of the compound and its complex with
EtOH/AcOH
Reflux
N
N
O
OH
HO
2+
ꢁ1
Cu reveals that a significant shift (26 cm ) in
m(OH) of the
(4)
CHO
CHO
(
3)
1
1
1
1
0
0
0
0
0
.3
.2
.1
.0
.9
.8
.7
.6
.5
Scheme 1. Synthesis of molecular receptor 4.
refluxed with 2-hydroxy-1-naphthaldehyde hydrazone in the pres-
ence of glacial acetic acid in ethanol to yield a product which when
washed with methanol gave a novel calixarene derivative 4 in good
yield (83%). The identity of all the intermediates and the target
0.4
0.3
1
13
molecular receptor 4 was established by IR, H and C NMR as well
0
.2
0.1
0.03
as by ESI-MS analysis (ESI, Fig. S1). The synthesized molecular
ꢁ
1
receptor 4 showed a C@NA signal at 1603 cm and a sharp pair
of doublets at a d 3.36 and d 4.29 for axial and equatorial protons,
-
2
00
250 300 350 400 450 500 550 600 650 700
1
respectively, in the H NMR spectrum. A distinct signal at d 31.02
nm
for the methylene carbons in its 13C NMR spectrum revealed its
symmetric cone conformation for the calix[4]arene scaffold. It
2+
Figure 1. Change in the UV–Vis spectra of 4 (20
M) in CH CN; inset: color change upon addition of Cu to the solution of
molecular receptor 4.
lM) upon addition of Cu (0–
2+
28
l
3
2
was further confirmed by observing D O exchangeable singlets at
d 13.03 which could be assigned to the –OH protons. Non deuter-
able singlets at d 9.69 and 8.29 for the azo-methine proton
(
–N@CH) confirmed the depicted structure for 4 (Scheme 1).
The sensing ability of naphthalene based molecular receptor 4
400
3
for cations was examined in CH CN by naked eye color change,
UV–Vis absorption and fluorescence spectroscopy.
+
+
+
+
On gradual addition of metal cations such as Li , Na , K , Cs ,
3
2
1
00
00
00
0
2+
[Cu ]
+
2+
2+
2+
2+
2+
2+
2+
2+
2+
Ag , Ca , Mn , Co , Cd , Pb , Fe , Ni , Zn , and Cu as their
perchlorate salts in CH CN, a color change from light yellow to dark
3
yellow was observed only in the presence of copper ions. ESI-MS of
+
the 4 showed a molecular–ion peak [M+Na] at m/z 1331.5334 and
2
+
at m/z 1371.7741 (ESI, Fig. S5) due to the presence of [4+Cu ].
Other metal ions did not produce any noticeable change in color
even on adding higher concentrations.
Following our observations based on color change, cation sens-
3
ing ability of receptor 4 in CH CN was further monitored by UV–Vis
absorption spectroscopy. UV–Vis spectra of 4 was characterized by
4
ꢁ1
ꢁ1
two peaks (337 nm and 405 nm) with
e
= 6.4 ꢂ 10 M cm
.
4
50
500
550
Wavelength
600
650
Addition of different cations as perchlorates, led to no change in
the absorption spectra of 4 except copper perchlorate.
2
+
On addition of Cu ions, there was considerable change in the
absorption spectra of synthesized calixarene derivative. To further
3
Figure 2. Quenching in the fluorescence intensity of 4 (20 lM) in CH CN in the
presence of Cu2 (0–29
lM).
+