280
(b)
only Cu2+ ion by differing absorption spectra and visual color
change even in of the presence of other metal ions.
2.8
2.1
1.4
0.7
0.0
1
4.0
(A − A )
(a)
(i)
0 3.2
The selectivity of the sensor 1 toward the Cu2+ ion
was further established by ion-competition experiments
(Figure S4).13 Addition of a mixture of ions to the ACN
solution of chemosensor 1 results in almost similar response as
Cu2+ ion itself. Only a little interference is observed due to the
presence of other metal ions. A distinct color (lemon yellow to
red, Figure S4b)13 was observed when a mixture of ions is added
to the ACN solution of sensor 1. All the titration spectra (except
in the case of Co2+ ion) show three clear isosbestic points at 253,
349, and 450 nm with decreasing the initial band of the sensor
1 at 415 and 436 nm, which indicates strong equilibrium
between the complexed and uncomplexed form of the sensor 1
(Figures 3, S2,13 and S4a13).3
Cu2+
2.4
1.6
0.04 0.08 0.12
(1/[Cu2+]) × 106/M−1
(vi)
200
300
400
500
600
Wavelength/nm
Figure 3. (a) Titration of sensor 1 with Cu(ClO4)2¢6H2O salt
in acetonitrile solvent, where (i) to (vi) [Cu2+] = 0, 9.04, 13.55,
18.05, 25.55, and 33.03 ¯M respectively. [1] = 1.6 ¯M. (b)
Ratiometric titration curve, optical density as a funtion of Cu2+
concentrations (B-H plot) at 538 nm.
Finally we have correlated the experimental findings with
theoretical calculations. Structural calculations for sensor 1 and
its Cu2+ complex were preformed using Gaussian 03 software at
the density functional theory (DFT) level (SI13). Calculation
shows that Conformer 1 (s-trans) is more stable than deformed
s-trans conformer (Conformer 2, Figures S5, S6, and Table S3)13
and it is expected that only Conformer 1 exists at room
temperature. The absorption band of sensor 1 in ACN solvent
was observed at 415 nm which is similar to that of calculated
value (at 403.68 nm in vacuum). During the complexation
process it is found that Cu2+ can form stable 1:1 complex when
the two -OH groups are in the same side i.e., transformation
from s-trans to high energy deformed s-trans form and generates
cyclic six- and seven-member rings (Figure S7).13 In the
calculated structure of the complex, the Cu2+ ion is tetracoordi-
nated and the binding sites are two O atom, one nitrogen lone
pair, and water.12 So in the presence of metal ion (especially
Cu2+ ion) we can trap unstable high energy conformer from
stable low energy conformer (sensor 1).
In conclusion, we have reported simple Schiff base chemo-
sensor N,N¤-bis(4-diethylaminosalicylidene)hydrazine which
can selectively detect Cu2+ ion in the ¯M (detection limit
2.4 © 10¹6 M) range irrespective of the presence of other metal
ions. Structurally the sensor 1 is of low energy stable s-trans
geometry, but it transfers to high energy deformed s-trans
geometry upon binding with metal i.e., metal-mediated trapping
of high energy conformer. An excellent correlation has been
observed between experimental results and theoretical calcula-
tions. A simple synthesis of sensor 1 also has significant
advantage in practical applications because of its distinct
“naked-eye” color change in the presence of Cu2+ ion itself
(purple) and also in the presence of other metal ions (red).
(Figure S2).13 The only band at 487 nm with appreciable
intensity is observed in the case of addition of Fe2+, Zn2+
,
Cr3+, and Co2+ ions. As seen in Figure 2, other metal ions such
as Na+, Mn2+, Co2+, Ni2+, Cd2+, and Hg2+ ions have almost no
effect on the absorption peak of the bare sensor.
The titration curve of sensor 1 with Cu2+ ion is shown in
Figure 3. The band at 584 nm can be attributed to the d-d
transition of sensor 1-Cu2+ complex.4,9 The binding constants
for sensor 1-metal complexes have been determined from
Figure 3b using the Benesi-Hildebrand (B-H) relation which
indicate the formation of 1:1 complex between sensor 1 and
Cu2+ ion with binding constant 1.38 © 104 M (Table S213).
¹1
The peak at 538 nm arises due to formation of strongly chelating
complex between sensor 1 and Cu2+ ion by deprotonation of two
adjacent phenolic -OH groups6,10 which is further confirmed by
the mass spectra (Figure S1).13 Cu2+ ion easily forms a strong
internal-charge-transfer (ICT) complex6 by deprotonation which
causes an ICT in the metal complex and generates a new band at
538 nm.5,11 The red-shifted band at 487 nm arises by protonation
of sensor 1 due to the acidic nature of perchlorate salt (or HClO4
formed by hydrolysis) or may be due to the similar binding
nature of metal ions to that of proton (H+) (Figure S2).13 The
bathochromic shift (72 nm) of the protonated species suggests
that protonation takes place at the nitrogen present in the azine
bridge or nitrogen in one of the diethyl amino (-NEt2) groups.
Protonation or metal binding favors the ICT or metal-induced
charge-transfer (MICT) mechanism10 which is responsible for
the color change6,10,11 from lemon yellow to pale yellow
(Figure S4b).13 Addition of individual metal ions such as Zn2+
,
This work is supported by grants from DST, India (Project
No. SR/S1/PC/26/2008) to NG. S.D would like to acknowl-
edge UGC, Utpal Rana and Saptarshi Biswas for their grateful
contribution.
Co2+, Fe2+, and Cr3+ ion results in similar observation as acid
effect (Figure S2).13 This indicates 1:1 complexation between
sensor 1 and any of the studied metal ions.
The titration curves with Fe2+ and Cr3+ ion (Figures S3a
and S3b)13 predict weaker complex formation than Cu2+ ion
(Table S213). Addition of Na+, Mn2+, Ni2+, Cd2+, and Hg2+
ions result in no significant change in the absorption spectra,
which indicates the absence of binding of these metal ions with
the sensor 1. Due to stronger binding ability of Cu2+ ion and
larger binding constant as well as greater free energy change
(¦G) (Table S213), the chemosensor 1 can selectively detect
References and Notes
1
2
Chem. Lett. 2011, 40, 279-281
© 2011 The Chemical Society of Japan
Dalapati, Sasanka
