A highly selective fluorescence-based polymer sensor incorporating an (R,R)-salen moiety for Zn2+ detection

Article abstract of DOI:10.1002/chem.201001198

A chiral polymer incorporating an (R,R)-salen moiety was synthesized by the polymerization of (R,R)-1,2-diaminocyclohexane with 2,5-dibutoxy-1,4- di(salicyclaldehyde)-1,4-diethynyl-benzene by a nucleophilic additionelimination reaction. The fluorescence responses of the (R,R)-salen-based polymer toward various metal ions were investigated by fluorescence spectra. Compared with other cations, such as Na⁺, K⁺, Mg²⁺, Ca ²⁺, Mn²⁺, Fe²⁺, Fe³⁺, Co ²⁺, Ni²⁺, Cu²⁺, Ag⁺, Cd ²⁺, Hg²⁺, and Pb²⁺, Zn²⁺ can lead to a pronounced fluorescence enhancement as high as 7.8-fold together with an obvious blue-shift change of the chiral polymer. More importantly, the fluorescent color of the polymer changed to bright blue instead of weak yellow after addition of Zn²⁺, which can be easily detected by the naked eye. The results indicate that this kind of chiral polymer, incorporating an (R,R)-salen moiety as a receptor in the main chain backbone, can exhibit high sensitivity and selectivity for Zn²⁺ recognition.

Full text of DOI:10.1002/chem.201001198

DOI: 10.1002/chem.201001198
A Highly Selective Fluorescence-Based Polymer Sensor Incorporating an
(R,R)-Salen Moiety for Zn2+ Detection
Ying Xu,[a] Jie Meng,[a] Lingxing Meng,[a] Yu Dong,[a] Yixiang Cheng,*[a] and
Chengjian Zhu*[b]
Abstract: A chiral polymer incorporat-
ing an (R,R)-salen moiety was synthe-
sized by the polymerization of (R,R)-
1,2-diaminocyclohexane with 2,5-dibu-
toxy-1,4-di(salicyclaldehyde)-1,4-dieth-
ynyl-benzene by a nucleophilic addi-
tion–elimination reaction. The fluores-
cence responses of the (R,R)-salen-
based polymer toward various metal
ions were investigated by fluorescence
spectra. Compared with other cations,
such as Na+, K+, Mg2+, Ca2+, Mn2+
,
chiral polymer. More importantly, the
fluorescent color of the polymer
changed to bright blue instead of weak
yellow after addition of Zn2+, which
can be easily detected by the naked
eye. The results indicate that this kind
of chiral polymer, incorporating an
(R,R)-salen moiety as a receptor in the
main chain backbone, can exhibit high
sensitivity and selectivity for Zn2+ rec-
ognition.
Fe2+, Fe3+, Co2+, Ni2+, Cu2+, Ag+,
Cd2+, Hg2+, and Pb2+, Zn2+ can lead
to a pronounced fluorescence enhance-
ment as high as 7.8-fold together with
an obvious blue-shift change of the
Keywords: diaminocyclohexane
chiral polymer fluorescence
sensors · zinc detection
·
·
·
Introduction
rescence or UV/Vis spectroscopy arising from a Zn2+-in-
duced perturbation of a chromophore, is best suited for
Zn2+ detection in biological contexts or living systems. A
number of analytical techniques have been reported for
Zn2+ detection, such as electrochemical,[5] UV/Vis,[6] fluores-
cence,[7] and atomic absorption spectra methods.[8] Among
them, fluorescent sensors are of great practical value be-
cause of their advantages of simplicity, high sensitivity, and
low cost. A typical fluorescence sensor contains a receptor
(the recognition site) linked to a fluorophore (the signal
source), which translates the recognition event into the fluo-
rescence signal.[9] Therefore, an ideal fluorescence sensor
must meet two basic requirements. First, the receptor must
have the strongest affinity with species of interest (binding
selectivity), which is the central processing unit of a sensor.
Second, the fluorescence signal should not be perturbed by
the environment (signal selectivity). Investigation into the
development of highly sensitive and selective fluorescent
sensors has received much attention in recent years. In par-
ticular, numerous efforts have focused on the design of che-
mosensory systems with unique electrical and optical prop-
erties that are capable of detecting metal ions in both a real-
time and reversible fashion.
As the second most abundant transition-metal ion in the
human body, the Zn2+ ion plays an essential role in cellular
metabolism, gene expression, apoptosis, and neurotransmis-
sion.[1] Disorder of Zn2+ homeostasis is involved in a
number of diseases, such as Alzheimerꢀs disease,[2] prostate
cancer,[3] and diabetes.[4] Consequently, the detection of
Zn2+ has attracted increasing interest in the areas of chemi-
cal and biological sciences. Zn2+ has no optical spectroscop-
ic signature due to its closed-shell 3d10 configuration, which
limits the kinds of methods that can be applied to its study
and detection. Optical detection, following changes of fluo-
[a] Y. Xu, J. Meng, L. Meng, Y. Dong, Prof. Dr. Y. Cheng
Key Lab of Mesoscopic Chemistry of MOE
School of Chemistry and Chemical Engineering
Nanjing University, Nanjing 210093 (P.R. China)
Fax : (+86)2583317761
E-mail: yxcheng@nju.edu.cn
[b] Prof. Dr. C. Zhu
State Key Laboratory of Coordination Chemistry
School of Chemistry and Chemical Engineering
Nanjing University, Nanjing 210093 (P.R. China)
Fax : (+86)2583317761
A number of fluorescence sensors containing quinoline,[10]
fluorescein,[11] and peptides[12] for Zn2+ detection have been
reported in the literature, but most of them are based on
small molecules, and reports of polymer-based chemosensors
E-mail: cjzhu@nju.edu.cn
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201001198.
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FULL PAPER
are few.[13] An advantage of fluorescent conjugated polymers
over small molecules is that signal amplification occurs from
electronic communication along the polymer backbone.
Swager and co-workers[14] reported that the delocalizable p-
electronic conjugated “molecular-wire” polymer can greatly
amplify the fluorescence responsive change due to facile
energy migration along the polymer backbone upon light ex-
citation. As a result, a single conjugated polymer provides
an enhanced optical response relative to one of its monomer
units, and conjugated polymers can be used as optical plat-
forms in highly sensitive chemical and biological sensors.
Additionally, to the best of our knowledge, some available
together with an obvious blue-shift change of the chiral
polymer. More importantly, the fluorescent color of the
polymer changed to bright blue instead of weak yellow after
addition of Zn2+, which can be easily detected by the naked
eye. The results indicate that this kind of chiral polymer in-
corporating (R,R)-salen moiety as a receptor in the main
chain backbone can exhibit high sensitivity and selectivity
for Zn2+ recognition.
Results and Discussion
Zn2+ sensors have difficulty in distinguishing Zn2+ and Cd2+ [15]
,
Syntheses and features of the chiral polymer sensor: 1,4-
Diiodo-2,5-dibutoxybenzene (2) was synthesized from the
starting material hydroquinone in a two-step reaction ac-
cording to reported procedures.[18,19] 5-Ethynylsalicylalde-
hyde (3) was synthesized in a two-step reaction from the
starting material 5-bromosalicylaldehyde.[20] Monomer, 2,5-
dibutoxy-1,4-di(salicyclaldehyde)-1,4-diethynylbenzene
(M1), was synthesized by a typical Sonogashira reaction
from 2 and 3. The chiral polymer incorporating the (R,R)-
salen moiety was obtained by Schiff base formation between
the dialdehyde M1 and (R,R)-1,2-diaminocyclohexane as a
yellow powder in 72.4% yield (Scheme 1). The specific rota-
tion value ([a]2D5) of the model compound 4 is À0.059 (c=
0.49, THF), and the [a]2D5 of the polymer is +10.62 (c=0.20,
THF). The molecular weight was determined by GPC by
using a Waters-244 HPLC pump with THF as the solvent
and relative to polystyrene standards. GPC analysis shows
the high molecular weight corresponding to approximately a
71 repeating unit (Mw=91080, Mn=41700, polydispersity
since Cd2+ is in the same group of the periodic table and
has similar properties to Zn2+. Therefore, the design of
highly selective and sensitive fluorescence sensor for Zn2+
detection without interference from other metal ions, espe-
cially Cd2+, is one of the most important objectives.
Salen, a particular chelating Schiff base, can be synthe-
sized by the condensation of salicylaldehydes or salicylalde-
hyde derivatives with 1,2-diamines. Salen-based ligands can
form stable complexes with various metal ions due to the
potentially tetradentate N2O2 donor.[16] In consideration of
the reported fluorescence properties of salen-Zn,[17] we de-
signed a novel optically active (R,R)-salen-based polymer
and used it as fluorescence sensor for Zn2+ recognition. In
this paper, the fluorescence response of the chiral polymer
toward various metal ions is fully investigated. Compared
with other cations, such as Na+, K+, Mg2+, Ca2+, Mn2+
Fe2+, Fe3+, Co2+, Ni2+, Cu2+, Ag+, Cd2+, Hg2+, and Pb2+
,
,
Zn2+ can lead to a pronounced fluorescence enhancement
Scheme 1. Synthesis procedures of the chiral polymer and the model compound 4.
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12899

Y. Cheng, C. Zhu et al.
index (PDI)=2.18). The chiral polymer is an air-stable
solid, yellow in color and shows good solubility in common
organic solvents, such as toluene, THF, CHCl3, and CH2Cl2,
which can be attributed to the nonplanarity of the twisted
polymer backbone and the flexible n-butoxy substituent.
According to thermal analysis of the polymer (Figure 1), it
Figure 2, the UV/Vis spectrum of compound 4 exhibits a
maximal absorption at 234 nm, the polymer shows a strong
and broad absorption in the region from 350 to 430 nm,
which is associated with a p–p* transition of the conjugated
segment. The chiral polymer has fluorescent emissions at
400, 420, and 530 nm upon excitation at 375 nm. Compared
with the model compound 4, the polymer shows a large red
shift due to the extended p-electronic structure in the main
chain backbone. Figure 3 illustrates the circular dichroism
Figure 1. TGA curve of the chiral polymer.
has a high thermal stability without loss of weight below
3908C. An apparently one-step degradation was observed at
temperatures ranging from 3908C to 7008C. The polymer
tends to complete decomposition at 8008C. A total loss of
about 46% was observed when it was heated to 8008C.
Therefore, the chiral polymer is stable and can provide de-
sirable thermal property for practical application as a fluo-
rescence sensor.
&
Figure 3. CD spectra of the model compound 4 ( ) and the polymer in
2+
~
!
the absence ( ) and presence of 1.0 equiv Zn
( ) in THF.
(CD) spectra of the model compound 4 and the chiral poly-
mer in the absence and presence of Zn2+ (1.0 equiv) in
THF. The molecular ellipticity of 4 is [q]l(max) =À6.06ꢂ105
(229.5 nm), 1.22ꢂ106 (243.0 nm), À1.37ꢂ105 (270.8 nm),
À6.56ꢂ104 (348.4 nm); and that of the chiral polymer is
[q]l(max) =À2.06ꢂ105 (224.7 nm), 1.62ꢂ105 (243.4 nm),
À2.36ꢂ104 (262.1 nm), 2.55ꢂ104 (297.6 nm), 8.04ꢂ104
(322.6 nm), À2.16ꢂ104 (343.4 nm), 1.95ꢂ105 (380.2 nm), re-
spectively. The model compound 4 and the chiral polymer
exhibit similar CD signals with negative and positive Cotton
effects at short wavelengths in their CD spectra. But the CD
intensity of the model compound 4 is much stronger than
that of the chiral polymer. On the contrary, the chiral poly-
mer has the longer wavelength CD effect at about 380 nm,
which is attributed to the extended conjugated structure in
the repeating unit. In addition, the CD spectrum of the
Zn2+ polymer exhibits little change except a 6 nm red shift
at long wavelength, which is consistent with the changing
features of the UV/Vis absorption spectra (Figure 4).
Optical properties: Figure 2 illustrates the UV/Vis absorp-
tion spectra and fluorescence spectra of the model com-
pound 4 and the chiral polymer (1.0ꢂ10À5 molLÀ1 corre-
sponding to the (R,R)-salen moiety in THF). As shown in
UV/Vis tiration of the chiral polymer on Zn2+: As shown in
Figure 4, the UV/Vis spectra of the polymer exhibit a maxi-
mal absorption at 240 nm and two broad bands around 337
and 391 nm before titrations in THF. Upon addition of Zn2+
(0–1.2 equiv), the absorbance peaks at 240 and 337 nm show
a little reduction with increasing amounts of Zn2+. On the
contrary, the strongest broad-band absorption peak at
395 nm arising from the conjugated structure exhibits a
Figure 2. UV/Vis spectra and fluorescence spectra of the model com-
pound 4 (1.0ꢂ10À5 molLÀ1) and the chiral polymer (1.0ꢂ10À5 molLÀ1) in
~
~
*
THF. =UV (model compound 4), =UV (polymer), =PL (model
*
compound 4, lex =324 nm), =PL (polymer, lex =375 nm).
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Fluorescence-Based Polymer Sensor for Zn2+ Detection
FULL PAPER
Figure 4. UV/Vis spectra of the polymer (1.0ꢂ10À5 molLÀ1) in THF with
Figure 6. Plot of the concentration of Zn2+ versus IÀI0/I0, in which I=the
fluorescence intensity of the polymer (1.0ꢂ10À5 molLÀ1) with addition of
Zn2+ at lem =480 nm and I0 =the fluorescence intensity of the polymer
without Zn2+ at lem =530 nm.
increasing amounts of Zn2+ (Polymer/Zn2+ =1:0 ( ), 1:0.1 ( ), 1:0.2 ( ),
~
&
*
!
~
^
1:0.3 ( ), 1:0.4 (3), 1:0.6 ("), 1:0.8 ( ), 1:1.0 ( ), 1:1.2 (3)).
gradual enhancement and an obvious bathochromic shift.
Meanwhile, a new absorption peak at 284 nm can be ob-
served. Moreover, there are four isosbestic points at 298,
343, 362, and 376 nm, which indicate the formation of the
UV active zinc complex.[15b]
tion with the concentration molar ratio of Zn2+ from 0.2 to
0.6 (Figure 6). Moreover, the maximum emission wavelength
of the polymer was remarkably blue shifted from 530 to
480 nm along with a dramatic enhancement of fluorescence
intensity. The Zn2+-polymer solution can emit blue fluores-
cence and was easily detected by the naked eye (Inset;
Figure 5). The obvious fluorescent enhancement can be at-
tributed to two reasons.[23] One is suppressed PET (photoin-
duced-electron-transfer) quenching when Zn2+ coordinates
with the nitrogen atoms of the (R,R)-salen-based moiety in
the chiral polymer main chain. Upon complexation, the lone
pair of electrons on the nitrogen atom is no longer available
for PET, thus leading to an enhancement of the emission.
On the other hand, the formation of Zn2+-polymer can en-
hance the planarity and rigidity of the conjugated segment
of the chiral polymer, which can reduce the nonradiative
decay of the excited state and lead to the pronounced fluo-
rescence enhancement. As shown in Figure 5, we also found
that when the salen-based N2O2 receptor in the main chain
backbone of the chiral polymer coordinates with Zn2+, a
large blue shift of about 50 nm for the emission spectrum
peak of the polymer is observed, which may be attributed to
a decrease of the HOMO of the conjugated segment of the
chiral polymer.[24]
The selective and sensitive recognition of the chiral polymer
on Zn2+: The fluorescence titration of the polymer with
Zn2+ was presented in Figure 5. The polymer displayed a
The above studies prompted us to select the polymer for
further evaluation aimed at determining its selectivity. The
fluorescence titration of the polymer with various metal ions
exhibits high selectivity to Zn2+ (Figure 7). Addition of
1.0 equiv of Na+, K+, Mg2+, Ca2+, Ag+, Cd2+, and Pb2+
Figure 5. Fluorescence spectra of the polymer (1.0ꢂ10À5 molLÀ1) in THF
with increasing amounts of Zn2+ (0, 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 8.0, 10.0,
11.0, 12.0ꢂ10À6 molLÀ1) (lex =375 nm).
slightly enhanced fluorescence (I/I0À1<0.5). And Fe2+
,
Ni2+, and Hg2+ caused limited quenching of the fluores-
cence intensity under the same conditions. Meanwhile, Fe3+
and Co2+ can effectively quench fluorescence of the chiral
polymer, but Mn2+ and Cu2+ lead to almost complete
quenching of the chiral polymer (Figure 7). Most important-
ly, based on the fluorescence image of the chiral polymer in
the absence and presence of 1.0 equiv of a metal ion, we can
weak and broad emission band situated at 530 nm due to
the extended p-electronic structure. The fluorescence inten-
sities of the Zn2+-containing polymer complex show gradual
enhancement, as high as 7.8-fold, upon the concentration
molar ratio addition of Zn2+ from 0.1 to 1.2. It can also be
found that the addition curve keeps a nearly linear correla-
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Y. Cheng, C. Zhu et al.
Experimental Section
Measurements and materials: All solvents and reagents were commer-
cially available and analytical reagent grade. THF and Et3N were purified
by distillation from sodium in the presence of benzophenone. NMR spec-
tra were obtained by using a 300-Bruker spectrometer 300 MHz for
1H NMR spectroscopy and 75 MHz for 13C NMR spectroscopy and are
reported as parts per million (ppm) from the internal standard TMS.
FTIR spectra were taken on a Nexus 870 FTIR spectrometer. Fluores-
cence spectra were obtained from an RF-5301PC spectrometer. UV/Vis
spectra were obtained by using a Perkin–Elmer Lambda 25 spectropho-
tometer. Specific rotation was determined with a Ruololph Research An-
alyfical Autopol I. MS was determined on a Micromass GCT. Elemental
analyses for C, H, and N were performed on an Elementar Vario
MICRO analyzer. Molecular weight was determined by GPC with
Waters-244 HPLC pump and THF was used as solvent and relative to
polystyrene standards.
Metal ion titration: Each metal ion titration experiment was started with
polymer (3 mL) of known concentration (1.0ꢂ10À5 molLÀ1 with respect
to the (R,R)-salen moiety in THF solution). ZnACTHNUTGRNEG(UN NO3)2 salt and other var-
ious metal salts (nitrate, 3.0 ꢂ10À4 molLÀ1 in H2O) were used for the ti-
tration. Polymer–metal complexes were produced by adding aliquots of a
solution of the selected metal salt to a THF solution of the chiral poly-
mer. All types of measurement were monitored 5 min after addition of
the metal salt to the polymer solutions.
Figure 7. The selectivity of the polymer toward Zn2+ and other metal
ions. In these experiments, the fluorescence measurement was taken at
lex =375 nm from 10 mm of the polymer in THF at room temperature and
in the absence and presence of 1.0 equiv of a metal ion. The fluorescence
intensity at lem =530 nm is plotted against the analyte. Bottom: The fluo-
rescence image of
1.0 equiv metal ion excited by a commercially available UV lamp (l=
365 nm).
Preparation of 5-ethynylsalicylaldehyde (3): 1,4-diiodo-2,5-dibutoxyben-
zene (2) was synthesized from hydroquinone in a two-step reaction ac-
cording to the reported procedure.[18,19] Compound 3 was prepared ac-
a ) plus
solution of the polymer (1ꢂ10À5 molLÀ1
cording to
124.3 mmol) was added to a mixture of 5-bromosalicylaldehyde (5.00 g,
24.87 mmol), [PdCl2A(PPh3)2] (873 mg, 1.24 mmol), and CuI (474 mg,
a
reported method.[20] Trimethylsilylacetylene (17.7 mL,
CTHUNGTRENNUNG
2.48 mmol) in Et3N (80 mL). The mixture was stirred for 12 h at 808C.
After cooling, the resulting ammonium salt was filtered off, and the resi-
due was purified by chromatography on silica gel with petroleum ether
as eluent. Removal of solvent under vacuum afforded a yellow powder,
and the product was identified as 5-trimethylethynylsilylsalicylaldehyde
easily distinguish Zn2+ by the distinctive bright blue fluores-
cence from use of a UV lamp (l=365 nm) with the naked
eye. In this paper, we further investigated the selectivity of
the polymer for Zn2+ using a solution of Zn–polymer com-
plex treated with other metal ions. The fluorescence intensi-
ties of the chiral polymer do not appear to have obvious dif-
ferences in the presence of other metal ions, or the mixture
of the chosen metal ions even at a higher concentration
1
(4.0 g, 74%). H NMR (300 MHz, CDCl3): d=11.11 (s, 1H), 9.85 (s, 1H),
7.70 (d, J=2.1 Hz, 1H), 7.60 (dd, J=8.7, 2.1 Hz, 1H), 6.94 (d, J=8.7 Hz,
1H), 0.25 ppm (s, 9H). 5-Trimethylethynylsilylsalicylaldehyde (2.18 g,
10.0 mmol) was dissolved in CH2Cl2 (10 mL). KOH (561 mg, 10.0 mmol)
was dissolved in MeOH (5 mL) and added to the CH2Cl2 solution. The
reaction mixture was stirred at room temperature for 4 h, and then the
solvent was concentrated under reduced pressure. A mixture of H2O
(20 mL) and CH2Cl2 (20 mL) was added to the residue to afford a two-
phase solution. The aqueous layer was extracted with CH2Cl2 (2ꢂ20 mL),
and the combined CH2Cl2 solutions were washed with H2O and dried
over MgSO4. The solution was filtered, and the solvent was removed by
rotary evaporation to obtain a light yellow powder identified as 5-ethy-
nylsalicylaldehyde (3) (1.3 g, 88%). 1H NMR (300 MHz, CDCl3): d=
11.15 (s, 1H), 9.89 (s, 1H), 7.75 (d, J=2.0 Hz, 1H), 7.65 (dd, J=8.6,
2.1 Hz, 1H), 6.98 (d, J=8.7 Hz, 1H), 3.06 ppm (s, 1H).
except that the coexisting ions (Co2+, Cu2+, and Fe3+
)
showed interference in Zn2+ detection to some extent. The
results indicate that the resulting chiral polymer can be used
as Zn2+ selective probe that was hardly affected by coexist-
ing ions (see the Supporting Information, Figures S7 and
S8).
Preparation of 2,5-dibutoxy-1,4-di(salicyclaldehyde)-1,4-diethynylbenzene
(M1):[21] The mixture of 5-ethynylsalicylaldehyde (438.3 mg, 3.0 mmol),
1,4-bis-dodecyloxy-2,5-diiodobenzene (474.1 mg, 1.0 mmol), [PdCl2-
Conclusion
A chiral polymer incorporating an (R,R)-salen moiety was
synthesized, which exhibits excellent fluorescent sensor
properties for the detection of Zn2+. Compared with other
cations, such as Na+, K+, Mg2+, Ca2+, Mn2+, Fe2+, Fe3,
Co2+, Ni2+, Cu2+, Ag+, Cd2+, Hg2+, and Pb2+, Zn2+ can
produce a pronounced fluorescence enhancement as well as
a large blue shift of the polymer fluorescence. Most impor-
tantly, we can identify Zn2+ with the naked eye by using a
commercially available UV lamp (l=365 nm). This work
can be applied to the detection of Zn2+ by a simple, rapid,
sensitive, and selective method.
AHCTUNTGREGUN(NN PPh3)2] (70.6 mg, 0.1 mmol), and CuI (37.5 mg, 0.2 mmol) was dissolved
in THF (10 mL), followed by addition of Et3N (30 mL). The reaction
mixture was heated to 708C overnight. After cooling to room tempera-
ture, the solvent was removed by a rotary evaporator. The residue was
purified by flash chromatography on silica gel (ethyl acetate/petroleum
ether, 1:3, v/v), then with ethyl acetate as eluent on a short plug of silica
gel. The solvent was removed, and then washed by methanol to afford
2,5-dibutoxy-1,4-di(salicyclaldehyde)-1,4-diethynyl- benzene (M1) as
a
brown powder (294 mg, 57%). 1H NMR (300 MHz, CDCl3): d=11.15 (s,
2H), 9.91 (s, 2H), 7.77 (d, J=2.0 Hz, 2H), 7.69 (dd, J=8.6, 2.0 Hz, 2H),
7.02 (t, J=4.3 Hz, 4H), 4.06 (t, J=6.4 Hz, 4H), 1.96–1.78 (m, 4H), 1.71–
1.49 (m, 4H), 1.03 ppm (t, J=7.4 Hz, 6H); 13C NMR (75 Hz, CDCl3): d=
196.0, 161.4, 153.5, 139.7, 136.7, 120.5, 118.1, 116.7, 115.3, 113.6, 93.0,
85.4, 69.2, 31.3, 19.2, 13.8 ppm; MS (FAB): m/z: 510.3 [M+1]+.
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Fluorescence-Based Polymer Sensor for Zn2+ Detection
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Preparation of the model compound 4 (Scheme 1): A mixture of 5-ethy-
nylsalicylaldehyde (584.6 mg, 4.0 mmol) and (R,R)-1,2-diaminocyclohex-
ane (228.4 mg, 2.0 mmol) was dissolved in methanol (20 mL). The ob-
tained solution was stirred at 408C for 12 h. After cooling to room tem-
perature, the solvent was removed by a rotary evaporator. The crude
product was recrystallized from ethyl acetate and petroleum ether to
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afford a yellow solid 4 (592 mg, 80% yield). Mp: 146–1488C; [a]D25
=
À0.059 (c=0.49, THF); 1H NMR (300 Hz, CDCl3): d=11.16 (br, 2H),
8.23 (s, 2H), 7.39 (dd, J=8.4, 1.8 Hz, 2H), 7.34 (d, J=1.8 Hz, 2H), 6.87
(d, J=8.4 Hz, 2H), 3.34–3.37 (m, 2H), 2.97 (s, 2H), 1.45–1.97 ppm (m,
8H);13C NMR (75 Hz, CDCl3): d=163.9, 161.5, 135.9, 135.3, 118.2, 117.3,
112.2, 83.0, 75.7, 72.4, 32.8, 24.0 ppm; FTIR (KBr): n˜ =3299, 3281, 2941,
2863, 1633, 1587, 1486, 1286, 826,585 cmÀ1
; MS (FAB): m/z: 370.2
[M+1]+; elemental analysis calcd (%) for C24H22N2O2: C 77.81, H 5.99, N
7.56; found: C 77.81, H 6.01, N 7.58.
Preparation of the chiral polymer (Scheme 1):[22] A mixture of M1 (0.1 g,
0.17 mmol) and (R,R)-1,2-diaminocyclohexane (20 mg, 0.17 mmol) was
dissolved in chloroform (4 mL). The obtained solution was stirred at
408C for 4 h. Methanol was added to precipitate the yellow (R,R)-salen
polymer. The resulting polymer was filtered and washed with methanol
several times, and then dried resulting in a yield of 72.4% (70 mg). GPC
results: Mw=91080, Mn=41700, PDI=2.18; [a]2D5 =+10.62 (c=0.20,
THF); 1H NMR (300 Hz, CDCl3): d=13.59 (br, 2H), 8.26 (s, 2H), 7.42
(dd, J=8.7, 1.8 Hz, 2H), 7.38 (d, J=1.8 Hz, 2H), 6.97 (s, 2H), 6.88 (d,
J=8.7 Hz, 2H), 4.01 (t, J=6.3 Hz, 2H), 3.36 (br, 2H), 1.75–1.92 (m,
10H), 1.46–1.62 (m, 8H), 0.99 ppm (t, J=7.5 Hz, 6H); FTIR (KBr): n˜ =
3441, 2955, 2931, 2863, 1632, 1504, 1486, 1287, 1214 cmÀ1; elemental anal-
ysis calcd (%) for C38H40N2O4: C 77.52, H 6.85, N 4.76; found: C 77.48, H
6.73, N 4.59.
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Acknowledgements
This work was supported by the National Natural Science Foundation of
China (nos.: 20774042, 20832001) and the National Basic Research Pro-
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Received: May 5, 2010
Published online: September 28, 2010
Chem. Eur. J. 2010, 16, 12898 – 12903
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
12903