Azobenzene coupled chromogenic receptors for the selective detection of copper(II) and its application as a chemosensor kit

Article abstract of DOI:10.1016/j.tetlet.2006.11.066

Azobenzene-based receptors 1-4 as colorimetric sensing materials were synthesized and their sensing properties were examined. In solution, the proposed sensing materials give rise to a large cation-induced hypochromic shift for Cu²⁺ resulting i

Full text of DOI:10.1016/j.tetlet.2006.11.066

Tetrahedron Letters 48 (2007) 393–396
Azobenzene coupled chromogenic receptors for the selective
detection of copper(II) and its application as a chemosensor kit
Soo Jin Lee,a Shim Sung Lee,a Il Yun Jeong,b Jin Yong Leec and Jong Hwa Junga,*
aDepartment of Chemistry and Research Institute for Natural Science, Gyeongsang National University, Chinju 660-701, South Korea
bAdvanced Radiation Technology Institute, Korea Atomic Energy Research Institute, Jeongeup 580-185, South Korea
cDepartment of Chemistry and Institute of Basic Science, Sungkyunkwan University, Suwon 440-746, South Korea
Received 17 October 2006; revised 7 November 2006; accepted 13 November 2006
Available online 1 December 2006
Abstract—Azobenzene-based receptors 1–4 as colorimetric sensing materials were synthesized and their sensing properties were
examined. In solution, the proposed sensing materials give rise to a large cation-induced hypochromic shift for Cu2+ resulting in
a change from red to pale-yellow, whereas no significant color change was observed upon addition of other selected metal ions.
The use of the silica gel plate modified with immobilization of receptor 4 to detect Cu2+ was also reported.
ꢀ 2006 Elsevier Ltd. All rights reserved.
The development of artificial receptors for the sensing
and recognition of environmentally and biologically
important ionic species, especially transition-metal ions,
is currently of great interest.1–4 Due to its industrial
usage as a pollutant and an essential trace element in
biological systems, the chemosensors for copper(II)
based on the chromogenic probes which are expected
quickly, nondestructively, and sensitively to detect
copper has drawn much attention.5–8 Recently, we have
focused our efforts on the development of novel colori-
metric sensors for the ionic species,9–11 with the aim of
preparing the simple-to-use and naked-eye diagnostic
tools for the recognition of essential electrolytes and
molecules in serum for critical care analysis. We herein
report the synthesis and the spectroscopic evaluation
of a series of chromogenic receptors 1–4 (Schemes
S1–S3), which show selective metal-induced color
R
O
O
R
HN
NH
N
N
1: R = CH(CH3)NHCO(CH2)10CH3
2: R = CH(CH3)NHCOCH3
3: R = (CH2)10CH3
N
4: R = -NH(CH2)3Si(OEt)3
NO2
As expected, 1 showed a high selectivity and sensitivity
for Cu2+. To the best of our knowledge, 1 is a rare
example of azobenzene-based chemosensor for Cu2+
.
changes upon the addition of Cu2+
.
Synthesis of the receptors began with tosylation of
N-phenyldiethylene diamine to produce the N-phenyl-
diethylene ditosylate. Treatment with NaN3 to remove
the tosyl group gave N-phenyldiethylene diazide. Reduc-
tion with Pd/C yielded N-phenyldiethylene diamine as a
key precursor.12 The treatment with alanine-appended
fatty acid in ethyl acetate successfully produced the
N-phenylated alanine–fatty acid amide. Subsequently,
the diazonium salt generated in situ from the reaction
of p-nitroaniline with NaNO2 was added to a solution
of the corresponding N-phenylated fatty acid amide,
affording the desired product 1 as a red powder (see
Scheme S1). Receptors 2 and 3 were synthesized
Each chemosensor receptor (1–4) proposed contains
azobenzene moiety, in which one of the aromatic rings
also may act as an integrated part of the ion recognition.
Phenyl iminoethylenediamide and nitrobenzene moieties
were employed as electron donor and acceptor, respec-
tively. In case of 1, D-alanine moiety was introduced
to provide an extra chelating site especially for Cu2+
.
Keywords: Chromogenic; Selective detection; Copper(II); Chemosensor.
*
Corresponding author. Tel.: +82 55 751 6027; fax: +82 55 758
6027; e-mail: jonghwa@gnu.ac.kr
0040-4039/$ - see front matter ꢀ 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2006.11.066

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S. J. Lee et al. / Tetrahedron Letters 48 (2007) 393–396
similarly (Schemes S1 and S2). Compound 4 was synthe-
sized by Scheme 1. Treatment with 13 and 3-isocyanato-
propyl-triethoxysilane afforded the desired product 4 as
a red powder. The red products were analyzed by con-
ventional methods.13
as selective chemosensors for Cu2+ in medical diagnos-
tics such as serum studies.7
The titration of 1 with Cu(NO3)2 resulted in the 480 nm
absorption gradually decreasing, whereas the 320 nm
absorption gradually increased to give an isosbestic
point at 379 nm (Fig. 1b). Job’s plot for binding between
1 and Cu2+ shows a 1:1 stoichiometry (inset of Fig. 1b).
Furthermore, the existence of 1–Cu2+ complex
species (m/z 449) was confirmed in the FAB mass spec-
trum (Fig. S1). The logK values for the 1:1 complex for-
mation calculated through linear least squares analysis of
the titration data profiles by the Rose–Drago method14
were 4.41 for [Cu(1)]2+, 4.39 for [Cu(2)]2+, and 3.38 for
[Cu(3)]2+. The results indicate that 1–3 form relatively
stable complexes with Cu2+ in acetonitrile. Among
receptors 1–3, the logK value for [Cu(3)]2+ is smaller
than those of [Cu(1)]2+ and [Cu(2)]2+, implying that
donor atoms of alanine moieties play an important role
for complex formation. The logK values for Li+, Na+,
K+, NH4þ, Co2+, Cd2+, Pb2+, Fe3+, and Zn2+ (all as
nitrates) were too small to be determined by this method.
First, the metal-binding properties of 1–4 were examined
by UV–vis spectrophotometry with respect to color
changes (all as nitrates in acetonitrile, Fig. 1a). Receptor
1 exhibits an intense absorption at 480 nm (red). The
addition of Cu2+ resulted in the largest hypochromic
shift for 1 to 320 nm (Dk = 160 nm), changing its solu-
tion color from red to pale-yellow in only acetonitrile
(Fig. 2). However, no significant color change was
observed upon addition of excess amounts of Li+,
Na+, K+, NH4þ, Co2+, Cd2+, Pb2+, Zn2+, Hg2+, Fe3+
,
and Ag+, indicating that only Cu2+ forms a strong coor-
dination through the donor atoms of 1. Also, the color
change of receptor 1 was observed by the addition of
Cu2+ ion in multi-component system in the presence of
Li+, Na+, K+, NH4þ, Co2+, Cd2+, Pb2+, Zn2+, Hg2+
,
Fe3+, and Ag+. However, the color of receptor 1 was
changed from red to yellow by the addition of Cu2+
ion in the presence of other metal ions (Fig. S2). The
results also support that these receptors could be useful
The metal-induced color changes of 2, which possesses
methyl groups instead of long alkyl chains were similar
N3
N3
H2N
NH2
(EtO)3Si
Si(OEt)3
N
N
N
N
N3
N3
HN
HN
O
O
NH
NH
N
iii
i
ii
N
N
N
N
6
NO2
NO2
N
12
13
NO2
4
Scheme 1. Synthetic method. Reaction conditions: (i) diazonium salt, DMF, 0 ꢁC; (ii) Ph3P, THF/H2O; (iii) (3-isocyanatopropyl)triethoxy silane,
CH2Cl2. DMF = N,N0-dimethyl formamide, THF = tetrahydrofuran.
Figure 1. (a) UV–vis spectra of 1 (0.025 mM) in the presence of metal nitrates (5.0 equiv) in acetonitrile and (b) UV–vis titrations of 1 (0.025 mM)
with Cu(NO3)2 (0–5.0 equiv) in acetonitrile; (inset) Job’s plot.

S. J. Lee et al. / Tetrahedron Letters 48 (2007) 393–396
395
Figure 2. Pictures of 1 (0.025 mM) upon addition of various metal ions
(5.0 equiv) in acetonitrile.
Figure 3. Pictures of silica gel plates: (a) unmodified, (b) modified with
4, (c) plate b after immersion in Cu2+ solution, and (d) plate b after
immersion in Li+, Na+, K+, NH4þ, Co2+, Cd2+, Pb2+, Fe3+ or Zn2+
solution.
to those observed for 1 (Fig. S3), suggesting that the
lengths of alkyl chains are not important for selective
coloration. Receptor 3 with no alanine moiety produced
a smaller hyphochromic shift to 360 nm upon addition
of Cu2+, resulting in a color change to yellow
(Fig. S4). Once again, this result suggests that the
alanine units play an important role in forming
portable sensor kit for the detection of Cu2+ in the
environmental field.
the complex with Cu2+
.
In conclusion, we have developed highly selective azo-
benzene-based colorimetric chemosensors 1–4 for the
detection of Cu2+ in solution. The recognition of Cu2+
gave rise to major color changes from red to pale-yellow
that was clearly visible to the naked eye. The receptor
immobilized silica gel plate was also developed for
Cu2+ detection in a low level. Such Cu2+ colorimetric
chemosensors could be of great importance in medical
diagnostics such as serum studies. In particular, the
anchoring of molecular receptors onto suitable solid
substrates can be a promising tuning tool for selectivity
enhancement which may, in principle, be applied to the
design of new chemosensors for a broad range of target
species.
Our repeated efforts to obtain crystal structures to eluci-
date the coordination behavior between 1 or 2 and Cu2+
were not successful. Also, NMR study for complexation
was not available due to the paramagnetic property of
Cu2+. Instead we measured IR spectra of 1 and its com-
plex to examine the binding site. The characteristic
peaks due to –C@O and –N–C groups in 1 at 1639
and 1338 cmꢀ1 were observed to shift to 1628 and
1327 cmꢀ1, respectively, in its Cu2+ complex (Fig. S5).
The resulting shifts to shorter wavelengths are attributed
to the coordination of the Cu2+ to the carbonyl amides,
and to the amino moiety (Fig. S6).15,16
Based on the successful color changes for the receptors
obtained in solution state, we also prepared the portable
chemosensor kit by immobilizing receptor 4 onto the
simple silica gel plate. Immobilization of 4 onto the
silica gel plate was conducted in toluene under reflux
condition for 24 h. In this process,17 the triethoxylsilyl
group in 4 undergoes hydrolysis and was attached cova-
lently to the surface of the silica gel. In the final step, the
red colored silica gel plate was washed with THF and
toluene to remove physically adsorbed receptor 4, and
then the plate was dried. As shown in Figure 3, the color
of the silica gel plate was changed from white to red
after immobilization of 4, indicating that 4 was cova-
lently attached by the sol–gel reaction.
Acknowledgments
This work was supported by KRF (KRF-2005-070-
C00068) and KOSEF (R01-2005-000-10229-0). Also,
this work was supported in part by the Gyeongsang
National University.
Supplementary data
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.tetlet.2006.
11.066.
The content of incorporated chromogenic receptor 4
was determined by TGA and elemental analysis, while
UV–vis spectroscopy was used for chromophore deter-
mination. The silica gel plate contains approximately
8.2 wt % of the receptor 1 (Fig. S7). The solid UV–vis
spectrum of silica gel plate immobilized with 4 exhibits
max absorption at 480 nm (Fig. S8). Figure 3c clearly
shows that the red color of the modified silica gel plate
with 4 was changed to yellow after immersion in Cu2+
solution, whereas no significant color change was
observed by other selected metal ions (Fig. 3d). When
the modified silica gel plate with 4 was employed, it
was possible to detect Cu2+ content up to 0.01 mM with
the naked eye. Hence, the result implies that the pro-
posed receptor-modified plate can be applicable as a
References and notes
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Compound 13. Compound 12 (500 mg, 1.31 mmol) was
dissolved in THF (10 mL) and triphenylphosphane
(758 mg, 2.9 mmol) in THF (5 mL) was added to the
stirred, ice-cooled solution under an N2 atmosphere.
Water (2 mL) was added and the mixture was stirred at
room temperature for 12 h and for a further 6 h at 50 ꢁC.
The solution was cooled and then neutralized by addition
of concd HCl and then concentrated under reduced
pressure. The residue was partitioned between CH2Cl2
(40 mL) and 0.5 M HCl (40 mL), and the aqueous phase
was extracted twice with CH2Cl2 (30 mL each). The
aqueous phase was made alkaline (pH = 9–10) with
NaOH and the product re-extracted with CH2Cl2
(4 · 30 mL). The organic layer was dried (Na2SO4) and
concentrated to afford compound 13 (211 mg, 0.66 mmol)
as a red solid, and was used directly in the next step.
Compound 4. (3-Isocyanatopropyl)triethoxysilane (100 mg,
0.30 mmol) was slowly added to the solution of compound
13 (165 mg, 0.67 mmol) in dry CH2Cl2 (5 mL) at 0 ꢁC.
After the mixture was stirred for overnight, the solvent was
removed in vacuo to yield an off-red solid. Unreacted (3-
isocyanatopropyl)triethoxysilane was removed by washing
with dry n-pentane. After drying under vacuum, com-
pound 4 was obtained as a reddish solid. Yield: 5.3 g, 80%.
12. Lee, S. J.; Lee, S. S.; Kim, J. S.; Lee, J. Y.; Jung, J. H.
Chem. Mater. 2005, 17, 6517–6520.
13. Compound 1. Mp 240–242 ꢁC. Yield = 30% 1H NMR
(300 MHz, DMSO-d6, 25 ꢁC) d 8.33 (d, 2H, J = 7.50 Hz),
7.92 (m, 2H), 7.83 (m, 2H), 7.02 (d, 2H, J = 7.80 Hz), 4.28
(m, 2H), 3.88 (t, 4H, J = 7.68 Hz), 3.38 (t, 4H,
J = 7.68 Hz), 2.13 (t, 4H, J = 7.14 Hz), 1.53 (m, 8H),
1.28 (m, 36H), 0.89 (t, 6H, J = 2.76 Hz); MS(FAB) m/z
835 (M+H)+. Anal. Calcd for C46H74N8O6: C, 66.16; H,
8.93; N, 13.42. Found: C, 66.50; H, 8.50; N, 13.11. IR
~
(KBr, pellet): v ¼ 3284, 2955, 2920, 285, 1644, 1602, 1553,
1516, 1337, 1248, 688 cmꢀ1
.
Compound 2. Mp 221–223 ꢁC. Yield = 40% 1H NMR
(300 MHz, DMSO-d6, 25 ꢁC) d 8.31 (d, 2H, J = 7.80 Hz),
7.88 (m, 2H), 7.84 (m, 2H), 7.01 (d, 2H, J = 7.50 Hz), 4.28
(m, 2H), 3.88 (t, 4H, J = 7.68 Hz), 3.38 (t, 4H,
J = 7.68 Hz), 2.14 (s, 6H), 1.53 (s, 6H); MS(FAB) m/z
835 (M+H)+. Anal. Calcd for C46H74N8O6: C, 66.16; H,
8.93; N, 13.42. Found: C, 66.50; H, 8.50; N, 13.11.
Compound 3. Mp 198–200 ꢁC. Yield = 43% 1H NMR
(300 MHz, CDCl3, 25 ꢁC) d 8.34 (d, 2H, J = 7.2 Hz), 7.97
(m, 4H), 6.84 (d, 2H, J = 9.3 Hz), 5.99 (s, 2H), 3.64 (t, 4H,
J = 6.00 Hz), 3.53 (d, 4H, J = 6.6 Hz), 2.17 (d, 4H,
J = 7.8 Hz), 1.61 (m, 4H), 1.26 (m, 32H), 0.90 (t, 6H,
J = 6.90 Hz); 13C NMR (75 MHz, CDCl3, 25 ꢁC) d 174,
157, 152, 144, 129, 126, 123, 113, 112, 62, 38, 37, 29, 26, 23,
20, 14; MS(FAB) m/z 694 (M+H)+. Anal. Calcd for
C40H64N6O4: C, 69.33; H, 9.31; N, 12.13. Found: C, 68.89;
H, 9.20; N, 12.02.
1
Mp 202–204 ꢁC; H NMR (300 MHz, CDCl3) d 8.35 (d,
2H, J = 9 Hz), 7.96 (d, 4H, J = 9.3 Hz), 6.91 (d, 2H,
J = 9 Hz), 3.81 (q, 12H), 3.75 (t, 4H, J = 6 Hz), 3.64 (t, 4H,
J = 5.7 Hz), 3.34 (t, 4H, J = 5.1 Hz), 1.62 (m, 4H),1.26
(m, 18H), 0.66 (m, 4H) 13C NMR (75 MHz, CDCl3) d
158.84, 58.37, 42.85, 40.33, 30.27, 29.29, 29.20, 29.14,
26.76, 23.69, 18.26, 7.62 ppm. HRMS calcd for
C36H62N8O10Si2 [M]+ 822.4127. Found: 822.4120; IR
(KBr, pellet): ~v ¼ 3332, 2973, 2927, 2885, 1627, 1597,
1514, 1138, 1103, 1077 cmꢀ1
.
14. (a) Rose, N. J.; Drago, R. S. J. Am. Chem. Soc. 1959, 81,
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17. Immobilization of 4 onto the Silica Gel Plate. Compound
4 (50 mg) was dissolved in toluene (10 mL). Silica gel plate
(15 mm · 50 mm) was added to the solution of compound
4. The solution was refluxed for 24 h in toluene. After
cooling to room temperature, silica gel plate was taken out
from toluene. The red color silica gel plate was washed
with THF and toluene to remove physically adsorbed 4
onto the silica gel plate, and then dried. IR (KBr, pellet):
~v ¼ 3406, 2959, 2926, 2854, 1635, 1522, 1382, 796, 576,
457 cmꢀ1; UV–vis (solid): kmax = 480 nm. Anal. Calcd for
C24H114N8O190Si93: C, 4.67; H, 1.86; N, 1.82. Found: C,
4.58; H, 1.48; N, 1.79.
Compound 12. A diazonium salt of 4-nitroaniline (155 g,
1.12 mmol) was prepared by adding an aqueous solution
of NaNO2 (78 mg, 1.12 mmol) dropwise into a homo-
geneous mixture of 1.6 mL of sulfuric acid and 4.2 mL of
glacial acetic acid. The mixture was stirred at 0 ꢁC for
5 min. The diazonium salt solution was added dropwise
into a solution of compound 6 (200 mg, 0.86 mmol) in
DMF(10 mL) at 0 ꢁC. The reaction mixture was stirred for
an additional 12 h at 0 ꢁC, treated with H2O (150 mL) and
extracted with CHCl3 (200 mL). The organic layer was
dried over MgSO4. Removal of the organic solvent in
vacuo was afforded a reddish oil. Column chromato-
graphy on silica gel with CH2Cl2/MeOH/TEA, 90/10/0.1
as the eluent provided 12 as a reddish crystalline solid in
75% yield. Mp 93–94 ꢁC. 1H NMR (300 MHz, CDCl3,
25 ꢁC) d 8.35 (d, 2H, J = 8.7 Hz), 7.96 (d, 4H, J = 9 Hz),
6.84 (d, 2H, J = 9 Hz), 3.74 (t, 4H, J = 6 Hz), 3.62 (t, 4H,
J = 5.7 Hz); 13C NMR (75 MHz, CDCl3, 25 ꢁC) d 154,
150, 148, 147, 141, 122, 114, 59, 55, 47, 40, 25, 18, 7.
HRMS calcd for C16H16N10O2 [M]+ 380.1458. Found: