1212 Bull. Chem. Soc. Jpn. Vol. 83, No. 10 (2010)
Zn2+ Specific Colorimetric Receptor Based on Coumarin
O
O
O
O
OH
a
S
b
N
O
N
NH2
CHO
CH3
CH3
H
CH3
3
2
H
N
O
O
N
S
N
c
N
S
N
N
O
O
CH3
H
Receptor 1
Scheme 1. Synthetic route of receptor 1. Reagents and conditions: (a) Ethylacetoacetate, piperidine, EtOH, 0-5 °C; (b)
thiosemicarbazide, EtOH, reflux 4 h; (c) 1,4-dibromobuta-2,3-dione, EtOH, stirring, 60 °C, 2 h.
3-acetylcoumarinthiosemicarbazone (0.261 g, 1.0 mmol) in
20 mL methanol, a solution of 1,4-dibromobutane-2,3-dione
(0.121 g, 0.5 mmol) in 10 mL methanol was slowly added with
constant stirring. The stirring along with mild heating ca. 60 °C
was further continued for 2 h resulting into yellow solid which
(a)
was filtered and dried over a pump. The above product was
basified with aqueous NaHCO3 leading to formation of free
base i.e., receptor 1 followed by its recrystallization from
DMSO-H2O mixture (50:50, v/v). Receptor 1 was character-
1
ized through IR, H and 13C NMR spectral studies along with
(b)
its mass determination through LC-MS and HRMS (see ESI,
Figures S1-S4). IR ¯/cm¹1: 3443, 2924, 1716, 1609, 1566,
1454, 1369, 1300, 1234, 1134, 1028, 964, 757, 633, 462;
1H NMR (400 MHz, DMSO-d6): ¤ 2.28 (s, 6H, CH3), 7.04 (s,
2H, H-Ar), 7.37-7.45 (m, 4H, H-Ar), 7.63-7.67 (t, 2H, H-Ar),
7.84-7.86 (d, 2H, H-Ar), 8.17 (s, 2H, H-Ar), 11.47 (s, 2H,
-NH); 13C NMR (75 MHz, DMSO-d6): ¤ 16.26, 105.17,
116.02, 118.95, 124.82, 126.57, 129.11, 132.28, 140.80,
145.30, 153.32, 159.21, 169.48; MS m/z (ESI) 569.00, Calcd
for C28H20N6O4S2: 568.10; HRMS (C28H20N6O4S2) Theoretical
569.1066, Experimental 569.1065.
HOMO
LUMO
Figure 1. (a) Energy minimized structure of receptor 1
through DFT using B3LYP/6-31G** basis set. (b) DFT-
computed HOMO, LUMO orbitals of receptor 1 with the
correlation functional B3LYP and a 6-31G** basis set.
transfer (ICT) probe of acceptor-donor-acceptor (A-D-A)
type.
Zn2+ Binding Studies. The colorimetric recognition of
Zn2+ by receptor 1 was studied through UV-vis spectrophoto-
metric studies (Figure 2) involving concomitant additions of
Zn2+ as its chloride salt to a 5 © 10¹5 M DMSO solution of
receptor 1. Receptor 1 itself exhibited two absorption bands
with their maxima at 278 and 375 nm due to ³-³* and n-³*
transitions respectively.33
The synthesis of the receptor 1 may be presented conven-
iently through Scheme 1 as follows. Compounds 2 and 3 were
synthesized according to reported methods.30,31
Results and Discussion
DFT Studies on Receptor 1. The quantum mechanical
calculations at the DFT level using B3LYP/6-31G** basis set
through Gaussian 03 package32 were performed on receptor 1
in order to support its structural characterization through
spectroscopic studies along with its mass determination as
described above. The resulting energy minimized structure
along with its HOMO-LUMO orbitals have been shown in
Figures 1a and 1b. As can be seen, the receptor 1 is a
symmetric molecule with two identical halves positioned in
antagonistic way having one coumarin and one thiazole in each
half. The highest occupied molecular orbital i.e., HOMO and
lowest unoccupied molecular orbital i.e., LUMO of receptor 1
are localized mainly on the thiazole and coumarin moieties
making them electron-rich and -deficient centers respectively.
By virtue of incorporating the electron-rich and -deficient
centers within the same molecule receptor 1 is a good example
of a push-pull system and is a potential intramolecular charge
On gradual additions of Zn2+ to the above receptor 1
solution, there was almost no effect on the visual and spectral
characteristics of receptor 1 until the level of addition reached
forty equivalents resulting in bathochromic shifting of 278 and
375 nm bands to 325 and 495 nm respectively. Simultaneously
the visual appearance of the DMSO solution of receptor 1 at
this stage changed from yellow to red. The new bands at 325
and 495 nm were assigned as metal-induced intramolecular
charge transfer (MICT)34 due to complexation of receptor 1
with Zn2+. The visual and spectral changes in receptor 1 kept
on intensifying further up to addition of 100 equivalents of
Zn2+. Further additions of the same did not affect the visual
intensity but the spectral intensity of receptor 1 was affected to
a minor extent. Finally a beautiful titration pattern as shown
above in Figure 2 with three well-defined isosbestic points
at 305, 350, and 423 nm indicating a neat interconversion