New diaminomaleonitrile derivatives containing aza-crown ether: Selective, sensitive and colorimetric chemosensors for Cu(II)

Article abstract of DOI:10.1016/j.dyepig.2013.01.018

Two novel soluble chemosensors based on Schiff-base diaminomaleonitrile derivatives which were modified by an aza-crown ether moiety have been synthesized. These chemosensors were fully characterized by ¹H NMR, ¹³C NMR spectroscopy, mass spectrometry, IR, DSC and single crystal X-ray diffraction. The recognition abilities of the sensors with a range of metal ions were evaluated and their photophysical properties have been systematically investigated. DFT theoretical calculations were employed to understand the behavior of the sensors toward the Cu(II). The sensing mechanism was derived through experimental and theoretical calculations. The results consistently indicated that the monoaza-10-crown-5 species was the ideal sensor, which can be utilized to monitor Cu(II) in solution over a wide pH range.

Full text of DOI:10.1016/j.dyepig.2013.01.018

Dyes and Pigments 98 (2013) 1e10
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
New diaminomaleonitrile derivatives containing aza-crown ether:
Selective, sensitive and colorimetric chemosensors for Cu(II)
,
a
a
a
a
Hongping Zhou a , Jianqing Wang , Yixin Chen , Wengang Xi , Zheng Zheng ,
*
Donglin Xu a, Yuanle Cao a, Gang Liu a, Weiju Zhu a, Jieying Wu a , Yupeng Tian
,
a,b,c
*
a College of Chemistry and Chemical Engineering, Anhui University, Key Laboratory of Functional Inorganic Materials Chemistry of Anhui Province, 230039 Hefei, PR China
b State Key Laboratory of Crystal Materials, Shandong University, 250100 Jinan, PR China
c State Key Laboratory of Coordination Chemistry, Nanjing University, 210093 Nanjing, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Two novel soluble chemosensors based on Schiff-base diaminomaleonitrile derivatives which were
modified by an aza-crown ether moiety have been synthesized. These chemosensors were fully
characterized by 1H NMR, 13C NMR spectroscopy, mass spectrometry, IR, DSC and single crystal X-ray
diffraction. The recognition abilities of the sensors with a range of metal ions were evaluated and their
photophysical properties have been systematically investigated. DFT theoretical calculations were
employed to understand the behavior of the sensors toward the Cu(II). The sensing mechanism was
derived through experimental and theoretical calculations. The results consistently indicated that the
monoaza-10-crown-5 species was the ideal sensor, which can be utilized to monitor Cu(II) in solution
over a wide pH range.
Received 30 November 2012
Received in revised form
23 January 2013
Accepted 27 January 2013
Available online 5 February 2013
Keywords:
Diaminomaleonitrile derivatives
Aza-crown ether
Colorimetry
Ó 2013 Elsevier Ltd. All rights reserved.
Sensors
Cu(II)
DFT theoretical calculations
1. Introduction
To date, a wide range of analytical methods have been employed
for chemoselective screening of Cu(II) ions in a variety of complex
Copper is one of a relatively small group of trace metal nutrients
that are essential to sustain normal human health [1]. Copper-
dependent enzymes act as catalysts to help a number of body
functions to provide energy for biochemical reactions, transform
melanin for pigmentation of the skin, assist the formation of cross-
links in collagen and elastin, and thereby maintain and repair
connective tissues [2]. However, copper in excessive amounts can
be toxic and cause oxidative stress and disorders associated with
neurodegenerative diseases including Alzheimer’s, Parkinson’s,
Menke’s, Wilson’s, and prion diseases [3]. Due to the extensive
application of copper (II) ions in life science, medicine, chemistry
and biotechnology, it has become one of the important environ-
mental pollutants. Therefore, it is important to design and develop
Cu(II) sensors with high sensitivity and selectivity. However there
are lots of impressive reports on fluorescent as well as colorimetric
sensors for Cu(II), most of them showed poor solubility in water [4],
low sensitivity [5] and poor selectivity over other metals [6].
environments, such as atomic absorption/emission spectroscopy,
inductively-coupled plasma mass spectroscopy, inductively-
coupled plasma atomic spectroscopy inductively coupled plasma
(ICP) spectroscopy, and voltammetric sensors [7]. Although all of
these conventional strategies are beneficial because of their supe-
rior sensitivity, they are limited by matrix interference, complicated
sample-pretreatment procedures, and analysis time [8]. Optical
signals based on changes of absorption wavelength are an area of
significant interest because of their low detection limits, high
sensitivity, simplistic operation and good selectivity. In this respect,
we have been interested in developing a selective colorimetric
sensor for the visual detection of metal ions due to their simplicity.
In this article, modified diaminomaleonitrile bearing aza-crown
ether in different size was synthesized and characterized (Fig. 1).
The diaminomaleonitrile (DMN) was chosen not only for its ability
to act as diamine and form simple schiff-base ligands but also for
the fact that the electron-withdrawing CN groups, which clearly
affect the coordinating capacity of DMN itself [9]. In addition, aza-
crown ether group which was easily dissolved in aqueous solution
and lipid solution was brought in to improve the solubility of the
* Corresponding authors. Tel.: þ86 551 5108151; fax: þ86 551 5107342.
E-mail addresses: zhpzhp@263.net, zhouhongping@ahu.edu.cn (H. Zhou).
0143-7208/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2013.01.018

2
H. Zhou et al. / Dyes and Pigments 98 (2013) 1e10
1H NMR and 13C NMR spectra were obtained on a Bruker Avance
400 MHz NMR spectrometer using CDCl3 as solvent. Chemical shifts
were reported in parts per million (ppm) relative to internal TMS
(0 ppm) and coupling constants in Hz. Splitting patterns were
described as singlet (s), doublet (d), triplet (t), quartet (q), or mul-
tiplet (m). The mass spectra were measured with an LTQ Orbitrap
XL and AXIMA-CFR plus MALDI-TOF MS. IR data were got from FT-IR
Nicolet NEXUS 870 FT-IR-NEXUS. Melting points were measured
with DSC Q2000 V24.9 build 121.
The one-photon absorption (OPA) spectra were recorded on the
UV-265 spectrophotometer. The one-photon excited fluorescence
(OPEF) spectra measurements were performed using the Hitachi F-
7000 fluorescence spectrophotometer. OPA and OPEF were meas-
ured in the concentration of 10
of 1 cm path length.
mM. The quartz cuvettes used were
2.2. Materials
Fig. 1. Structures of L1 (left) and L2 (right).
Aza-crown ether substituted benzaldehyde was synthesized in
high yield according to reported procedures [11]. Chemosensors
were synthesized through condensation of diaminomaleonitrile
and aza-crown ether substituted benzaldehyde (Scheme 1). Prod-
ucts were characterized by 1H NMR, 13C NMR, Mass Spectroscopy
and single-crystal X-ray diffraction. Single crystals of both L1 and L2
were obtained by diffusion of EtOH to solutions of L1 and L2 in
dichloromethane. Sensors L1 and L2 crystallize in the monoclinic
form with space group P2(1)/c. The single crystal structures of L1
and L2 are shown in Fig. 2 respectively, and the data are collected in
Table 1. A large planar can be seen between benzene and DMN in L1
from Fig. 2, for the bond lengths which are between the single bond
lengths and double single lengths, are quite conjugated, N(2)eC(17)
sensors and enlarge its extent of application, concurrently modu-
late the coordinating properties of molecules cooperatively with
DMN [10].
The sensor L1 is able to selectively detect Cu(II) over environ-
mentally relevant ions including Ag(I), Ba(II), Ca(II), Cd(II), Co(II),
Hg(II), Pd(II), Mg(II), Ni(II), Zn(II), Fe(III), Cr(III). The response upon
exposure to Cu(II) is fast and can be used to detect Cu(II) visually in
the concentration of 1 ꢀ 10ꢁ5 M in solution and the detection limit
is decreased down to 2 ꢀ 10ꢁ7 M by spectroscopy. By contrast L2
offered quiet poor selectivity to Cu(II), and its detection limit of
Cu(II) is much higher than that of L1. From the analysis of these
data, L1 was taken as the better sensor.
ꢀ
ꢀ
ꢀ
(1.278 A), N(2)eC(19) (1.393 A), C(18)eC(19) (1.362 A), N(3)eC(18)
ꢀ
ꢀ
(1.351 A), C(14)eC(17) (1.436 A). And the same phenomenon can
been observed in L2 (Table 2). But the conformations of the whole
molecules of L1 and L2 were quiet distorted clearly shown in Fig. 2.
From the side view of the planar of benzene and DMN, obvious
differences between L1 and L2 were observed, L1 looking like
a couch while L2 resembling a key (Fig. 2, right).
2. Experimental section
2.1. Materials and instruments
Tetrahydrofuran (THF) was distilled under normal pressure from
sodium benzophenone ketyl under nitrogen immediately prior to
use. Dimethylformamide (DMF) was distilled and dried over po-
tassium hydroxide. Sodium hydride and 2,20-(phenylimino)bis-
ethanol were purchased from Aldrich. Diethylamine, triethylene
glycol, etracthylene glycol, 4-fluorobenzaldehyde, phosphorus
oxychloride and diaminomaleonitrile were available commercially.
DMSO, THF and CH3CN were spectrometric grade. Metal salts
LiNO3, NaNO3, KNO3, Mg(NO3)2, Ca(NO3)2, Ba(NO3)2, Cu(NO3)2,
Zn(NO3)2, Co(NO3)2, Ni(NO3)2, Cd(NO3)2, AgNO3, Hg(NO3)2,
Pd(NO3)2, Fe(NO3)3.9H2O, Cr(NO3)3 were purchased from Aldrich
and used as received.
2.3. Synthetic methods
2.3.1. Synthesis of 1-[4-(1, 4, 7, 10-tetraoxa-13-aza
cyclopentadecyl) benzyl]-2-aminomalonitrile (L1)
4-(1, 4, 7, 10-tetraoxa-13-aza cyclopentadecyl) benzaldehyde
(1.0 g, 3.0 mmol) was added to a benzene solution (100 mL), con-
taining diaminomaleonitrile (0.32 g, 3.0 mmol) and several drops of
piperidine, which had been vigorously stirred for 30 min. The
mixture was then refluxed by stirring for 8 h. Water was removed
using a DeaneStark trap. The reaction mixture was then cooled to
room temperature. The precipitated crude product was separated
Scheme 1. Synthetic route of sensors L1 and L2.

H. Zhou et al. / Dyes and Pigments 98 (2013) 1e10
3
Fig. 2. The front view of the single crystal structure of L1 (left top); the side view of the single crystal structure of L1 (right top); the front view of the single crystal structure of L2
(left bottom); the side view of the single crystal structure of L2 (right bottom).
Table 1
by filtration and washed with cold toluene, then recrystallized from
Single crystal data of L1 and L2.
ethane:dichloromethane (1:3). Reddish brown needle product was
obtained (0.95 g, 2.29 mmol) in 76.3% yield. 1H NMR (400 MHz,
Compound
L1
L2
Empirical formula
Formula weight
Crystal system
Space group
C43 H56 Cl2 N10 O8
911.88
Monoclinic
P2(1)/c
11.2247(2)
9.9154(2)
21.3063(5)
90.000
98.9120(10)
90.000
C23 H31 N5 O5
457.53
Monoclinic
P2(1)/c
10.362(5)
18.502(5)
13.007(5)
90.000(5)
97.346(5)
90.000(5)
2373.2(17)
4
CDCl3):
d
¼ 8.0941 (s, 1H), 7.8032 (J ¼ 8 Hz, d, 2H), 7.4719 (s, 2H),
6.7154 (J ¼ 8 Hz, d, 2H), 3.6678e3.5089 ppm (m, 20H). 13C NMR
(100 MHz, CDCl3):
d
¼ 158.6261, 151.1498, 131.3454, 122.8801,
ꢀ
121.9788, 114.3847, 112.9222, 111.5593, 109.7210, 71.2420, 70.2975,
69.9655, 68.2697, 52.8783 ppm. MALDI-TOF MS [M þ 1]þ: 414.17. IR
(selective peak): 3391.14 cmꢁ1, 3302.34 cmꢁ1 corresponds to
a [A]
ꢀ
b [A]
ꢀ
c [A]
a
b
g
[ꢂ]
[ꢂ]
[ꢂ]
amido-group NeH stretching vibration, 2228.20 cmꢁ1
,
2198.62 cmꢁ1 corresponds to cyano-group C^N stretching vibra-
tion. Mp: 162.628 ꢂC.
3
ꢀ
V [A ]
2342.71(8)
2
Z
T [K]
296(2)
1.290
0.2
1.84e25.05
15,932
298(2)
1.229
0.088
1.92e25
17,432
Dcalcd [Mg mꢁ3
]
m
[mmꢁ1
]
qrange [ꢂ]
Total no. data
No. unique data
4142
4352
No. params refined
299
298
R1
0.0789
0.057
wR2
GOF
0.2572
1.106
0.1496
1.008
Table 2
ꢀ
Selected bond lengths of L1 and L2 (A).
L1
L2
N(2)eC(17)
N(2)eC(19)
C(18)eC(19)
N(3)eC(18)
C(14)eC(17)
1.278 (4)
1.393 (4)
1.362 (4)
1.351 (4)
1.436 (4)
N(2)eC(19)
N(2)eC(20)
C(20)eC(22)
N(3)eC(22)
C(16)eC(19)
1.291 (3)
1.394 (3)
1.361 (4)
1.339 (3)
1.443 (4)
Fig. 3. Absorption of L1 in different pH values at 420 nm.

4
H. Zhou et al. / Dyes and Pigments 98 (2013) 1e10
Fig. 4. (a) Solutions of L1 upon addition of different metal ions. (b) Absorption changes of L1 in CH3CN (1.0 ꢀ 10ꢁ5 M) upon addition of 1 equiv of different nitrate salts
(1.0 ꢀ 10ꢁ5 M). (c) Absorption of L1 at 336 nm upon addition of different cations.
2.3.2. Synthesis of 1-[4-(1, 4, 7, 10, 13-pentaoxa-16-aza
cycloctadecyl) benzyl]-2-aminomalonitrile (L2)
Compound L2 was obtained as orange powders in 61.4% by
following a procedure similar to that of L1. 1H NMR (400 MHz,
CDCl3):
d
¼ 8.2512(s, 1H), 7.6639 (d, 2H), 6.7058 (J ¼ 12 Hz, t, 2H),
5.1402 (J ¼ 12 Hz, d, 2H), 3.7218e3.6580 ppm (m, 24H). 13C NMR
(100 MHz, CDCl3):
d
¼ 158.2597, 151.3713, 131.3140, 122.9994,
122.9733, 122.3487, 114.4846, 113.0713, 111.4838, 70.6253,
68.4661, 51.0916 ppm. MS (ESI): [M þ 1]þ 458.23. IR (selective
peak): 3391.14 cmꢁ1, 3302.34 cmꢁ1 corresponds to amido-group
N-H stretching vibration, 2228.20 cmꢁ1, 2198.62 cmꢁ1 corre-
sponds to cyano-group C^N stretching vibration. Mp:
142.979 ꢂC.
Fig. 5. Top: UVevis titration curves of L1 in CH3CN (1.0 ꢀ 10ꢁ5 M) in the presence of
different amounts of Cu(II) (0, 0.02, 0.04, 0.06, 0.08, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8,
0.9, 1.0, 1.1, 1.2 ꢀ 10ꢁ5 M), bottom: FL titration curves of L1 in CH3CN (1.0 ꢀ 10ꢁ5 M) in
the presence of different amounts of Cu(II).
Fig. 6. Job’s plot of L1 and Cu(II) (lex ¼ 300 nm). The total concentrations of L1 and
Cu(II) are 1 mM. The experiments were measured at room temperature in CH3CN.

H. Zhou et al. / Dyes and Pigments 98 (2013) 1e10
5
3. Results and discussion
3.1. UVevis spectroscopic recognition of L1 and L2 toward metal
ions
The visible spectra of the chemosensors L1 and L2 in acetonitrile
solutions (1 ꢀ10ꢁ5 M) were characterized by two bands at 290 nm
and 420 nm, respectively. The band located at 290 nm corresponds
to the
pe
p* absorbance of diaminomaleonitrile and the band
centered at about 420 nm can tentatively be assigned to a charge-
transfer band from the donor to the acceptor [12].
Fig. 3 shows that L1 is stable within a wide pH range of 5.5e10.5,
and the following tests are in the range of pH. The response of
solutions of L1 to a series of individual metallic cations was
investigated. Addition of equimolar quantities of Ag(I), Ba(II), Ca(II),
Cd(II), Co(II), Cu(II), K(I), Hg(II), Pd(II), Mg(II), Li(I), Na(I), Ni(II),
Zn(II), Fe(III), Cr(III) to solutions of L1 respectively, it was found that
both Cu(II) and Hg(II) induced a rapid color change, but obvious
difference in color between the L1eCu(II) and L1eHg(II) still can be
found clearly. The solution of L1eCu(II) turned to colorless while
that of L1eHg(II) turned to pale yellow. The changes attributed to
metal coordination with the donor nitrogen atom of aniline group
[13] and the difference between Cu(II) and Hg(II) owed to the dif-
ferent binding affinity with L1 [20]. The other ions failed to show
any significant interference as shown in Fig. 4a. Their correspond-
ing UVevis spectra are shown in Fig. 4b and c.
Fig. 7. BenesieHildebrand plot of the Cu(II)eL1 complex in acetonitrile solution.
To elicit the interactions between L1 and Cu(II), UVevis ab-
sorption spectrum of L1 (1 ꢀ 10ꢁ5 M) in acetonitrile solution was
titrated with Cu(II) from 0 to 1.2 ꢀ 10ꢁ5 M. As shown in Fig. 5, the
maximum absorbance at 420 nm gradually decreased, and a new
band centered at 336 nm was increased. An isosbestic point was
clearly observed at 366 nm. The presence of this isosbestic point is
indicative of the formation of only one complex [14]. Job’s plot
(Fig. 6) analysis indicated that when the Cu(II) molar fraction rea-
ches 0.5, the maximum absorbance is obtained, indicating a 1:1
binding model between L1 and Cu(II). On the basis of 1:1 stoichi-
ometry, the stability constant of the complex between L1 and Cu(II)
Fig. 8. Absorption of L2 in different pH values at 425 nm.
Fig. 9. (a) Solutions of L2 upon addition of different metal ions. (b) Absorption changes of L2 in CH3CN (1.0 ꢀ 10ꢁ5 M) upon addition of 1 equiv of different nitrate salts
(1.0 ꢀ 10ꢁ5 M). (c) Absorption of L2 at 360 nm upon addition of different cations.

6
H. Zhou et al. / Dyes and Pigments 98 (2013) 1e10
gradually with a new isosbestic point at 297 nm. The dramatic
difference in results of the stepwise UVevis changes may be due to
the competition between aza-crown ether and DMN which was
demonstrated by theoretical calculations. The limit detected con-
centration of Cu(II) is about 1 ꢀ 10ꢁ6 M in acetonitrile, much lower
than the value of L1.
From the results, L1 and L2 showed dramatically different
selectivity and sensitivity to Cu(II), which arises as a consequence of
their structures (cavity size). From L1, only one isosbestic point was
clearly seen. But to L2, there were three isosbestic points formed.
This may because in L1, Cu(II) can only combine with DMN, while in
L2 it can combine with DMN and the large cavitied aza-18-crown-
6-ether.
The correct combination of ring and cation size would result in
a suitable coordination to the nitrogen atom of the macrocycle
group, thus inducing the intensity of the charge-transfer band
decreasing [15]. The data demonstrated that the two binding sites
in L2 have more fierce competition in capturing cations while L1
has a stricter standard and a better specify in selecting the cations
based on the two bonding sites cooperating operation.
Fig. 10. Titration curves of L2 in CH3CN (1.0 ꢀ 10ꢁ5 M) in the presence of different
amounts of Cu(II) (0, 0.1, 0.2, 0.4, 0.6, 0.8, 1.0, 1.2, 1.4, 1.6, 1.8, 2.0, 2.2, 2.4, 2.6, 2.8,
3.0 ꢀ 10ꢁ5 M). Insert: Titration curves of L1 in CH3CN (1.0 ꢀ 10ꢁ5 M).
3.2. Theoretical calculation study
In order to further understand the behavior of L1 and L2 with
the Cu(II), DTF calculations have been carried out. As Cu(II) is
known for its 4-coordination complex, the input structures were
designed in a way to have bidentate coordination with the DMN,
whereas the other two coordination sites were occupied by the
solvent molecule (Fig. 11). The structural optimization was done
using the Gaussian 03 package adopting the B3LYP method with 6e
31G as basic set [16]. TD-DFT calculations were carried out for the
first 25 excitation to investigate the change of the absorption
spectra upon addition of Cu(II). Electron density of HOMO and
LUMO of both L1 and L2 lies on the electron-withdrawing part.
From the energy level diagram (Fig. 12), it can be seen that addition
of the metal ions leads to the stabilization of the HOMO and LUMO
of the sensors (Table 3).
From the calculated results, energy levels of HOMO and LUMO of
L1 decreased more dramatically than that of L2 after addition of
Cu(II), namely L1 was more selective and sensitive to Cu(II) [17],
which was in line with the experimental results. From Table 4, we
see nitrogen atoms in L1 have higher electron density than that of
L2, which indicated that L1 was more sensitive to Cu(II) [18]. And
this result confirmed that L1 was the better sensor for Cu(II) again.
was estimated to be 1.04 ꢀ 104 Mꢁ1 by plotting 1/
DA against 1/
[Cu(II)] according to the BenesieHilderbrand equation (Fig. 7). The
limit detected concentration of Cu(II) is about 2 ꢀ 10ꢁ7 M in
acetonitrile.
In contrast L2, with a larger cavity, is also stable within a pH
range of 5.5e10.5 (Fig. 8). When an equimolar quantity of Ag(I),
Ba(II), Ca(II), Cd(II), Co(II), Cu(II), K(I), Hg(II), Pd(II), Mg(II), Li(I),
Na(I), Ni(II), Zn(II), Fe(III), Cr(III) was added to the solution of L2
respectively, it shows three different behaviors (Fig. 9), a slight
intensity enhancement of the band at 420 nm in the presence of
Ca(II), appearance of a new band at about 360 nm and obvious
intensity decrease of the band centered at 420 nm in the presence
of Ba(II), Cu(II), Hg(II), Fe(III), Cr(III) respectively, and no changes
upon addition of the other ions. To investigate the difference be-
tween L1 and L2, L2 was titrated with Cu(II) from 0 to 3.0 ꢀ 10ꢁ5 M.
With progressive addition of Cu(II) (Fig.10), the UVevis absorptions
of L2 at 300 and 420 nm were diminished. The well located iso-
sbestic points at 373 nm and 316 nm indicated new compounds
produced. After more than 0.8 equivalents of Cu(II) were added,
however, the absorption at 260 nm emerged and intensified
Fig. 11. Optimized structures of L1 with Cu(II) (left), L2 with Cu(II) (right).

H. Zhou et al. / Dyes and Pigments 98 (2013) 1e10
7
Fig. 12. (a) Energy levels of various HOMO and LUMO’s of L1 in the absence and the presence of Cu(II); (b) Energy levels of various HOMO and LUMO’s of L2 and compound
coordinated with DMN; (c) Energy levels of various HOMO and LUMO’s of L2 and compound coordinated with crown ether.
Table 5 shows the electronic density assigned in oxygen atoms
and nitrogen atoms was merely equal in L2. So we calculated the
energy of the complex Cu(II) coordinated with the crown ether (L2-
Cu-E) to realize the possible reasons of the multi-isosbestic points.
From Fig. 12(b, c), we found HOMO orbital of the complex coordi-
nated with DMN (L2-Cu-N) had much lower energy. But the energy
of (HOMO-1) of L2-Cu-E was next to the HOMO of L2-Cu-N, as well
as the LUMO orbital of the two complexes. Because the energies
have little difference, we speculated that two formations of com-
plexes may change from one form to another.
3.3. Solvatochromin and cations interference of L1 reacting with
Cu(II)
In order to research sensor L1 more thoroughly the response of L
toward Cu(II) in different solvents e.g. THF and THF/H2O (20:80)
was studied. The titrations were done to find out the limit sensi-
tivity concentration (Figs. 13 and 14). The results indicated that the
highest sensitivity with the value of 2 ꢀ 10ꢁ7 M and binding con-
stant 4.97 ꢀ 106 M were obtained in THF/H2O (20:80) solution. This
result can be explained by the possible reasons. As reported, the
main driving forces to construct these complexes including
All the results consistently indicate that L1 is the ideal chemo-
sensor for Cu(II) which has higher sensitivity and selectivity.
hydrogen bond, pep stacking and charge transfer interaction [19].
In the structures of THF and H2O, there is an oxygen atom with
a pair of lone electrons respectively. Nitrogen and oxygen will
compete to obtain the hydrogen ions. The pKa of cyanoamide
protons is in the range of 9.95e11.0, and the pKa values of H2O and
THF are 15.7 and 3.70 respectively. From the data, we speculated
that the hydrogen of cyanoamine could be taken by oxygen atom
easily. As a result, the sensor L1 selected Cu(II) more sensitively and
bound with it more tightly.
Table 3
Energies of HOMO and LUMO of the ligands and complexes (a.u.).
L1
L1 þ Cu(II)
ꢁ7.272
ꢁ4.092
3.180
L2
L2 þ Cu(II)
ꢁ5.040
ꢁ2.056
2.985
HOMO
LUMO
D
ꢁ3.702
ꢁ1.935
1.767
ꢁ4.10
ꢁ1.837
2.261
Table 4
Table 5
Electronic density of nitrogen atoms in L1 and L2.
Electronic density of nitrogen and oxygen atoms in L2.
L1
L2
L2
N1
N2
N11
N23
N41
ꢁ0.4541
ꢁ0.5184
ꢁ0.4912
ꢁ0.6715
ꢁ0.5403
N4
ꢁ0.2482
ꢁ0.2083
ꢁ0.4984
ꢁ0.2366
ꢁ0.2048
O1
O2
O3
O6
O64
ꢁ0.2186
ꢁ0.2172
ꢁ0.2181
ꢁ0.2170
ꢁ0.2542
N4
ꢁ0.2482
ꢁ0.2083
ꢁ0.4984
ꢁ0.2366
ꢁ0.2048
N11
N20
N32
N48
N11
N20
N32
N48

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H. Zhou et al. / Dyes and Pigments 98 (2013) 1e10
Fig. 13. (a) Titration curves of L1 in THF (1.0 ꢀ 10ꢁ5 M) in the presence of different amounts of Cu(II) (0, 0.01, 0.04, 0.06, 0.08, 0.1, 0.2, 0.4, 0.6, 0.8, 1.0, 1.4, 1.8, 2.0 ꢀ 10ꢁ5 M); (b)
BenesieHildebrand plot of the Cu(II)eL1 complexes in THF.
Fig. 14. (a)Titration curves of L1 in THF/H2O (1/4) in the presence of different amounts of Cu(II) (0, 0.001, 0.002, 0.006, 0.01, 0.02, 0.04, 0.06, 0.08, 0.1, 0.2, 0.4, 0.6, 0.8, 1.0, 1.4, 1.8, 2.2,
2.6, 3.0, 3.4, 3.8, 4.2, 4.6 ꢀ 10ꢁ5 M); (b) BenesieHildebrand plot of the Cu(II)eL1 complexes in THF/H2O (1/4).
Fig. 15. Absorbance of L1eCu(II) at the new band upon addition of various cations. The low bars represent L1 (1.0 ꢀ 10ꢁ5 M) with cations (5.0 ꢀ 10ꢁ5 M) without Cu(II); the high bars
represent L1 (1.0 ꢀ 10ꢁ5 M) with cations (5.0 ꢀ 10ꢁ5 M) upon the subsequent addition of Cu(II) (1.0 ꢀ 10ꢁ5 M). (a), (b), (c) is the diagram of CH3CN, THF/H2O (20:80), THF, respectively.

H. Zhou et al. / Dyes and Pigments 98 (2013) 1e10
9
Acknowledgments
This work was supported by Program for New Century Excellent
Talents in University (China), Doctoral Program Foundation of
Ministry of Education of China (20113401110004), National Natural
Science Foundation of China (21271003, 21071001, 51142011), the
211 Project of Anhui University, Education of committee of Anhui
province (KJ2012A024), the Natural Science Foundation of Anhui
Province (1208085MB22), Ministry of Education Funded Projects
Focus on Returned Overseas Scholar.
Appendix A. Supplementary material
Supplementary material associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/j.dyepig.
2013.01.018.
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