(R,R)-salen/salan-based polymer fluorescence sensors for Zn2+ detection

Article abstract of DOI:10.1016/j.polymer.2011.11.017

(R,R)-salen-based polymer fluorescence sensor P-1 could be synthesized by the polymerization of 5,5′-(isoquinoline-5,8-diylbis(ethyne-2,1-diyl))- bis(3-tert-butyl-2-hydroxybenzaldehyde) (M-1) with (R,R)-1,2-diaminocyclohexane (M-2) via nucleophilic addition-elimination reaction, and (R,R)-salan-based polymer sensor P-2 could be obtained by the reduction reaction of P-1 with NaBH₄. The fluorescence response behaviors of two chiral polymers P-1 and P-2 on Zn²⁺ were investigated by fluorescence spectra. The fluorescence intensities of P-1 and P-2 can exhibit gradual enhancement upon addition of Zn²⁺. Compared with other cations, such as Na ⁺, K⁺, Mg²⁺, Ca²⁺, Fe³⁺, Co²⁺, Ni²⁺, Cu²⁺, Ag⁺, Cd ²⁺, Cr³⁺ and Pb²⁺, Zn²⁺ can lead to the pronounced fluorescence enhancement as high as 22.8-fold for P-1 and 3.75-fold for P-2, respectively. The results show that P-1 and P-2 incorporating (R,R)-salen/salan moieties as receptors in the polymer main chain backbone can exhibit high sensitivity and selectivity for Zn²⁺ detection.

Full text of DOI:10.1016/j.polymer.2011.11.017

Polymer 52 (2011) 6029e6036
Contents lists available at SciVerse ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
(R,R)-salen/salan-based polymer fluorescence sensors for Zn2þ detection
,
,
a,
Fengyan Song a, Xiao Ma a, Jiali Hou a, Xiaobo Huang a b, Yixiang Cheng a , Chengjian Zhu
*
*
a Key Lab of Mesoscopic Chemistry of MOE, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, China
b College of Chemistry and Materials Engineering, Wenzhou University, Wenzhou 325027, China
a r t i c l e i n f o
a b s t r a c t
(R,R)-salen-based polymer fluorescence sensor P-1 could be synthesized by the polymerization of 5,50-
(isoquinoline-5,8-diylbis(ethyne-2,1-diyl))-bis(3-tert-butyl-2-hydroxybenzaldehyde) (M-1) with (R,R)-
1,2-diaminocyclohexane (M-2) via nucleophilic additioneelimination reaction, and (R,R)-salan-based
polymer sensor P-2 could be obtained by the reduction reaction of P-1 with NaBH4. The fluorescence
response behaviors of two chiral polymers P-1 and P-2 on Zn2þ were investigated by fluorescence
spectra. The fluorescence intensities of P-1 and P-2 can exhibit gradual enhancement upon addition of
Zn2þ. Compared with other cations, such as Naþ, Kþ, Mg2þ, Ca2þ, Fe3þ, Co2þ, Ni2þ, Cu2þ, Agþ, Cd2þ, Cr3þ
and Pb2þ, Zn2þ can lead to the pronounced fluorescence enhancement as high as 22.8-fold for P-1 and
3.75-fold for P-2, respectively. The results show that P-1 and P-2 incorporating (R,R)-salen/salan moieties
as receptors in the polymer main chain backbone can exhibit high sensitivity and selectivity for Zn2þ
detection.
Article history:
Received 20 April 2011
Received in revised form
2 November 2011
Accepted 9 November 2011
Available online 16 November 2011
Keywords:
Chiral fluorescence polymer
(R,R)-salen/salan
Zn2þ detection
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
molecules [17e27], only a few are polymer-based sensors [28e33].
In particular, fluorescence polymer sensors incorporating molec-
Fluorescent sensors have witnessed a tremendous rise in the
number of monitoring biologically relevant species such as metal
ions in the past few years [1]. Especially, optical imaging with
fluorescent sensors for Zn2þ detection has attracted great attention
due to the biological significance of zinc [2e9]. As the second most
abundant transition metal ion in the human body, Zn(II) ion plays
an essential role in cellular metabolism, gene expression, apoptosis
and neurotransmission [10e12]. The ability to track Zn(II) release in
cell requires the highly selective and sensitive detector in a real-
time and quantitative fashion [13,14]. Moreover, Zn(II) has no
optical spectroscopic signal due to its closed-shell 3d10 electronic
configuration, which limits the kinds of methods on its study and
detection. Optical detection, following changes in solution fluo-
rescence arising from a Zn(II)-induced perturbation of a chromo-
phore, is best suited for monitoring Zn(II) in either biological
contexts or living systems [2,15,16]. These problems have promoted
numerous efforts on the design of sensitive chemosensors that can
effectively detect zinc (II) ion without interference from other
ular recognition moieties are that they can make use of the high
sensitivity of conjugated polymers to external structural pertur-
bations and to electron density changes of the conjugated polymer
backbone, when they can interact and form complexes with tran-
sition metal ions [34,35]. Conjugated polyelectrolytes (CPEs) as
fluorescent chemosensors for metal ion detection based on the
photoinduced-electron-transfer and energy migration mechanism
have several advantages over small molecule sensors, such as
fluorescent enhancement associated with electronic interaction
between receptors and analytes within the conjugated
p-electronic
polymer main backbone. Swager reported that the delocalizable
p
-
electronic conjugated “molecular wire” polymer can greatly
amplify the fluorescence response signal due to facile energy
migration along the polymer backbone upon light excitations. As
a result, a single CP can provide the enhanced fluorescence
response behavior relative to one of its monomer units [36,37].
Therefore, the extended
p-conjugated polymer, coupled with the
chelation site, is expected to provide unique response toward
selected metal ions.
metal ions, especially Cd2þ
.
Recently, many fluorescence sensors for Zn(II) detection have
been reported, but most of them are based on organic small
Salen, as a well-known building block, has widely used as
a chelation-enhanced fluorescence sensor for transition metal ions
detection due to the potentially tetradentate N2O2 donor [38e42].
Moreover, very few salen-based fluorescence polymer sensors for
zinc ion detection have been reported [28,29,43e45]. To the best of
our knowledge, there has been no report on salan-based
* Corresponding authors. Tel.: þ86 25 83685199; fax: þ86 25 83317761.
E-mail addresses: yxcheng@nju.edu.cn (Y. Cheng), cjzhu@nju.edu.cn (C. Zhu).
0032-3861/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymer.2011.11.017

6030
F. Song et al. / Polymer 52 (2011) 6029e6036
fluorescence polymer sensors for metal ions detection so far. In this
paper, we reported the synthesis of two novel (R,R)-salen/salan-
2-tert-butylphenol and (R,R)-1,2-diaminocyclohexane were
purchased from Aldrich and directly used without purification. THF
and Et3N were purified by distillation from sodium in the presence
of benzophenone.
based polymers P-1 and P-2 as fluorescent sensors for Zn2þ
.
,
Compared with other cations, such as Naþ, Kþ, Mg2þ, Ca2þ, Fe3þ
Co2þ, Ni2þ, Cu2þ, Agþ, Cd2þ, Al3þ and Pb2þ, Zn2þ can lead to the
most pronounced fluorescence enhancement response without
interference from other metal ions, especially Cd2þ. More impor-
2.2. Preparation of 3-tert-butyl-2-hydroxy-5-iodobenzaldehyde (4)
tantly, P-1 and P-2 turn to be bright yellow after addition of Zn2þ
,
3-tert-butyl-2-hydroxybenzaldehyde (3) was synthesized
according to a reported method [46]. A mixture of 3-tert-butyl-2-
hydroxybenzaldehyde (2.6 g, 14.6 mmol) and iodine (5.56 g,
21.9 mmL) was dissolved in anhydrous THF (20 mL), a solution of
NaHCO3 (1.84 g, 21.9 mmol in 5 mL H2O) was added to the above
solution. The mixture was refluxed 12 h, and the reaction was
quenched by saturated NaS2SO3 solution (15 mL). Upon being
evaporated to dryness, the residue was extracted with ethyl acetate
(3 ꢀ 30 mL) and washed with aqueous Na2S2O3 and brine twice and
then dried over anhydrous Na2SO4. The solution was evaporated
under reduced pressure, and the crude product was recrystallized
by petroleum ether to afford a white solid 3-tert-butyl-2-hydroxy-
which can be easily detected by naked eyes. The results indicate
that two chiral polymers incorporating (R,R)-salen/salan moieties
as receptors in the main chain backbone can exhibit highly selective
and sensitive fluorescence responses for sensing zinc ion.
2. Experimental part
2.1. Measurements and materials
NMR spectra were obtained using a 300-Bruker spectrometer
300 MHz for 1H NMR and 75 MHz for 13C NMR and reported as
parts per million (ppm) from the internal standard TMS. FT-IR
spectra were taken on a Nexus 870 FT-IR spectrometer. UVevis
spectra were obtained from a PerkineElmer Lambda 25 spec-
trometer. Fluorescence spectra were obtained from an RF-5301PC
spectrometer. Thermogravimetric analyses (TGA) was performed
on a PerkineElmer Pyris-1 instrument under N2 atmosphere. MS
was determined on a Micromass GCT. C, H and N of elemental
analyses were performed on an Elementar Vario MICRO analyzer.
Circular dichroism (CD) spectrum was determined with a Jasco J-
810 spectropolarimeter. Molecular weight was determined by GPC
with Waters-244 HPLC pump and THF was used as solvent and
relative to polystyrene standards. All solvents and reagents were
commercially available and analytical-reagent-grade. Isoquinoline,
5-iodobenzaldehyde (4.2 g, 79%).1H NMR (300 Hz, CDCl3):
d 11.76 (s,
1H), 9.82 (s,1H), 7.73 (dd, J ¼ 9.0, 2.3 Hz, 2H),1.42 (s, 9H) (Scheme 1).
2.3. Preparation of the model compounds (5 and 6) (Scheme 1)
[47,48]
A
mixture of 3-tert-butyl-2-hydroxybenzaldehyde (3)
(712.9 mg, 4.0 mmol) and (R,R)-1,2-diaminocyclo- hexane
(228.4 mg, 2.0 mmol) was dissolved in 20 mL methanol. The ob-
tained yellow solution was heated at reflux for 1 h. After cooling to
room temperature, the solvent was removed by a rotary evaporator.
The crude product was recrystallized by the mixed solvents of ethyl
acetate and petroleum ether to afford a yellow solid 5 (692 mg, 80%
(1)
SiMe
Pd(OAc)2 PPh3 CuI/Et3N
Br
Br
t-Bu
+
H
H
NBS
H2SO4
(2)KOH
N
N
N
Et2O/MeOH
1
2
OH
OH
OH
t-Bu
CHO
(HCHO)n, MgCl2
Et3N ,THF
t-Bu
I2
CHO
NaHCO3, H2O, THF
I
4
3
OH
I
OHC
HO
t-Bu
CHO
OH
Bu-t
H
H
t-Bu
CHO
Pd(PPh3)4, CuI
Et3N
N
N
M-1
N
N
NH HN
H2N
NH2
NaBH4
M-2
OH HO
Bu-t
OH HO
Bu-t t-Bu
n
n
CHCl3
t-Bu
N
N
P-2
P-1
NH HN
OH HO
Bu-t t-Bu
OH
N
N
t-Bu
CH3OH
CHO
NaBH4
CH3OH
+
OH HO
t-Bu
Model compound 5
H2N
NH2
Bu-t
Model compound 6
Scheme 1. Synthesis procedures of the polymers P-1, P-2 and model compounds 5, 6.

F. Song et al. / Polymer 52 (2011) 6029e6036
6031
yield). ½a 2D5
ꢁ
¼ ꢂ431.00 (c 0.29, THF). 1H NMR (300 Hz, CDCl3):
anhydrous Na2SO4, and evaporated under reduced pressure to
afford P-2 as a yellow powder (85 mg, 85.3% yield). GPC:
d
13.87 (br, 2H), 8.32 (s, 2H), 7.27 (d, 2H), 7.03 (d, J ¼ 7.5 Hz, 2H), 6.74
(t, J ¼ 7.7 Hz, 2H), 3.36e3.38 (m, 2H), 1.06e2.02 (m, 6H), 1.49e1.53
(m, 2H), 1.46 (s, 18H). FT-IR (KBr, cmꢂ1): 3292, 3058, 2997, 2945,
2860, 1631, 1486, 1438, 1269, 1139, 1087, 852, 754, 680.
Mw ¼ 8980; Mn ¼ 5060; PDI ¼ 1.7. ½a D25
ꢁ
¼ þ178.36 (c 0.21, THF). 1H
NMR (300 Hz, CDCl3):
d
¼ 9.85 (s, 1H), 8.67 (d, J ¼ 5.1 Hz, 1H), 8.20
(d, J ¼ 5.4 Hz, 1H), 7.83 (s, 1H), 7.74 (s, 1H), 7.50 (s, 3H), 7.23 (d,
J ¼ 9.6 Hz 1H), 4.05 (dd, J ¼ 29.7, 11.7 Hz 4H), 2.53e2.66 (m, 2H),
2.21 (s, 2H), 1.75 (s, 4H), 1.42 (s, 18H), 1.37 (s, 2H). FT-IR (KBr, cmꢂ1):
3427, 2947, 2860, 2198, 1598, 1556, 1471, 1257, 1139,1076, 883, 840,
786. Anal. Calcd for C42H49N3O2: C, 80.34; H, 7.87; N, 6.69; O, 5.10
Found: C, 80.45; H, 7.91; N, 6.66; O, 4.98.
The model compound (5) (200 mg, 0.46 mmol) was dissolved in
the solvent of 20 mL MeOH, and then NaBH4 (44.0 mg, 1.16 mmol)
was added portionwise at room temperature over a period of
30 min to the above solution. The reaction mixture was stirred at
room temperature until the yellow color disappeared. The colorless
solution was stirred for another 30 min, and then 10 mL water was
added to stop the reduction reaction. The mixture was extracted
with CH2Cl2 (3 ꢀ 20 mL). The combined organic layers were dried
with anhydrous Na2SO4 and evaporated under re cduced pressure
to yield the model compound (6) (200 mg, 100% yield) as a white
2.6. Metal ion titration
Each metal ion titration experiment was started with a 3.0 mL
polymer in THF solution with
a
known concentration
solid. ½a 2D5
ꢁ
¼ ꢂ46.77 (c 0.29, THF). 1H NMR (300 Hz, CDCl3):
d
7.21
(1.0 ꢀ 10ꢂ5 mol Lꢂ1 corresponding to (R,R)-salen or (R,R)-salan
moiety). Zn(NO3)2 salt and other various metal salts (nitrate,
1.0 ꢀ 10ꢂ3 mol Lꢂ1, H2O) were used for the titration. Polymer-metal
complexes were produced by adding aliquots of a solution of the
selected metal salt to a THF solution of the polymer. All kinds of
measurements were monitored 30 min after addition of the metal
salt to the polymer solutions.
(d, J ¼ 8.1 Hz, 2H), 6.89 (d, J ¼ 7.5 Hz, 2H), 6,74 (t, J ¼ 7.5 Hz, 2H). 4.03
(dd, J ¼ 36.6, 13.5 Hz, 4H), 2.46 (t, J ¼ 4.2 Hz, 2H), 2.20 (d, J ¼ 12 Hz,
2H), 1.73 (d, J ¼ 4.2 Hz, 2H), 1.393e1.470 (m, 20H), 1.25 (d, J ¼ 8.1 Hz,
2H). FT-IR (KBr, cmꢂ1): 3298, 3051, 2997, 2945, 2858, 1591, 1440,
1240, 1089, 781, 746.
2.4. Preparation of M-1 (Scheme 1)
3. Results and discussion
To a mixture of 5,8-bis(ethynyl)isoquinoline (2) (177.2 mg,
1.0 mmol), 3-tert-buty-2-hydroxy-5- iodobenzaldehyde (4)
(608.2 mg, 2.0 mmol), Pd(PPh3)4 (74.7 mg, 0.06 mmol), and CuI
(11.4 mg, 0.06 mmol), 30 mL Et3N was added. The reaction mixture
was heated to 45 ꢃC for 4 h under a N2 atmosphere. After cooled to
room temperature, the solvent was removed by a rotary evaporator.
The residue was purified by silica gel column chromatography to
yield 460 mg (87%) of the product as a yellow powder after removal
3.1. Synthesis and structure feature of the polymers
The synthesis procedures of the monomer (M-1), model
compounds (5, 6) and the chiral polymers P-1, P-2 are outlined in
Scheme 1. 5,8-Bis(ethynyl)isoquinoline (2) was synthesized by
a three-step reaction from the starting product isoquinoline
according to the literature [49]. 3-Tert-butyl-2-hydroxy-5-
iodobenzaldehyde (4) could be synthesized from 2-tert-butylphe-
nol [46]. Monomer M-1, 5,50-(isoquinoline-5,8-diylbis(ethyne-2,1-
diyl))bis(3-tert-butyl-2-hydroxybenzaldehyde), was synthesized
by the reaction of 2 with 4 via a typical Pd-catalyzed Sonogashira
cross-coupling reaction. Chiral salen-based polymer P-1 was ob-
tained by Schiff base formation between M-1 and (R,R)-1,2- dia-
minocyclohexane (M-2) as a yellow powder in 75% yield, and chiral
salan-based polymer P-2 could be obtained by reduction of P-1
with NaBH4 in 85.3% yield. (R,R)-salen/salan moieties as the metal
chelating ligands can orient in a well-defined spatial arrangement
in the chiral polymer main chain backbones and also directly
coordinate with different transition metal ions to form the corre-
sponding metalepolymer complexes. Furthermore, we also chose
an electron-deficient heterocyclic unit 5,8-bis(ethynyl)isoquinoline
as the conjugated molecular bridge linker into the polymer main
chain. Quinoline, isoquinoline, quinoxaline and oxadiazole deriva-
tives have been widely used as electron transporting/hole blocking
materials in LED devices and LED blends because these units have
many excellent properties on the better chromophore, high elec-
tron affinity, high thermal and oxidative stability, and the good
charge injection and transporting building blocks [49e61]. 5,8-
Bis(ethynyl)isoquinoline can maintain conjugation between the
isoquinoline and salen/salan moieties so that the electron trans-
porting properties of the chiral polymers can be improved. In
addition, the ethynyl linker can reduce steric hindrance between
isoquinoline and phenyl groups and also have a beneficial influence
on the stability of the resulting chiral polymers.
of the solvent. 1H NMR (300 Hz, CDCl3):
d 12.05 (s, 1H), 12.04 (s, 1H),
9.95 (s, 2H), 9.87 (s, 1H), 8.74 (d, J ¼ 5.0 Hz 1H), 8.24 (d, J ¼ 5.7 Hz
1H), 7.94 (d, J ¼ 7.5 Hz 1H), 7.83 (d, J ¼ 7.8 Hz 1H), 7.75e7.79 (m, 4H),
1.49 (s, 18H). 13C NMR (75 Hz, CDCl3):
d 196.5, 161.7, 161.6, 151.4,
144.5, 139.2, 136.9, 135.9, 135.5, 135.3, 133.3, 130.6, 121.6, 120.5,
120.2, 113.6, 113.4, 96.6, 96.1, 84.9, 84.6, 35.0, 29.0. FT-IR (KBr,
cmꢂ1): 3437, 2958, 2871, 2210, 1652, 1598, 1450, 1407, 1269, 1151,
765, 713. MS (FAB): m/z 528.3 (Mꢂꢂ1). Anal. Calcd for C35H31NO4: C,
79.37; H, 5.90; N, 2.64; O, 12.08. Found: C, 79.31; H, 5.88; N, 2.66; O,
12.15.
2.5. Preparation of the polymers P-1 and P-2 (Scheme 1)
A
mixture of M-1 (0.11 g, 0.21 mmol) and (R,R)-1,2-
diaminocyclohexane (23.7 mg, 0.21 mmol) was dissolved in
10 mL of chloroform. The obtained solution was stirred at 40 ꢃC for
4 h. Methanol was added precipitate the yellow (R,R)-salen-based
polymer. The resulting polymer was filtrated and washed with
methanol several times, and then dried in the yield of 75% (95 mg).
GPC: Mw ¼ 10840, Mn ¼ 6300, PDI ¼ 1.7. ½a D25
¼ þ917.03 (c 0.21,
ꢁ
THF). 1H NMR (300 Hz, CDCl3):
d 14.33 (br, 2H), 9.79 (s, 1H), 8.66 (d,
J ¼ 5.8 Hz, 1H), 8.33 (s, 2 H), 8.14 (d, J ¼ 5.7 Hz, 1H), 7.82 (d,
J ¼ 7.6 Hz,1H), 7.71 (d, J ¼ 7.6 Hz,1H), 7.52 (d, J ¼ 4.7 Hz, 4H), 7.35 (d,
J ¼ 8.4 Hz, 2H), 1.80e2.08 (m, 8H), 1.46 (d, J ¼ 2.1 Hz, 18H). FT-IR
(KBr, cmꢂ1): 3439, 2935, 2860, 2198, 1631, 1450, 1386, 1363, 1278,
1203, 1161, 1001, 563, 487. Anal. Calcd for C42H45N3O2: C, 80.86; H,
7.27; N, 6.74; O, 5.13. Found: C, 79.79; H, 7.32; N, 6.78; O, 6.11.
100 mg polymer P-1 was dissolved in the mixed solvents of
10 mL THF and 10 mL MeOH, and then NaBH4 was added in batches
to the above solution. The reaction mixture was stirred at room
temperature for 2 h, and 10 mL water was added to stop the
reduction reaction. The mixture was extracted with CH2Cl2
(3 ꢀ 20 mL). The combined organic layers were dried with
The GPC results of two chiral polymers P-1 and P-2 show
moderate molecular weights. P-1 and P-2 are air stable solid and
show good solubility in common solvents, such as toluene, THF,
CHCl3 and CH2Cl2, which can be attributed to the nonplanarity of
the twisted polymer backbone and the flexible t-butoxy substitu-
ents on phenyl units as side chains of the polymers. TGA of P-1 and

6032
F. Song et al. / Polymer 52 (2011) 6029e6036
Table 1
CD spectra data of compounds 5 and 6, P-1 and P-2 in THF.
5 ꢀ 105
6 ꢀ 105
P-1 ꢀ 105
P-2 ꢀ 105
[q
](lmax
in nm)
ꢂ0.45 (222.0)
þ0.71 (231.4)
þ0.86 (250.8)
ꢂ0.83 (268.2)
ꢂ0.67 (330.5)
þ0.016 (219.1)
ꢂ1.95 (228.8)
þ0.81 (260.4)
ꢂ0.69 (361.0)
þ1.87 (413.6)
þ0.80 (218.6)
ꢂ0.64 (222.5)
þ0.40 (416.8)
ꢂ0.075 (221.0)
positive Cotton effects at short wavelengths in their CD spectra. The
reason may be contributed to the (R,R)-salen moietie can orient
a well-defined spatial arrangement in the regular polymer back-
bone. That is, each unit in the polymer P-1 acts independently with
an organized helical chain structure. The polymer P-2 exhibits
relatively weak positive and negative Cotton effect at the wave-
length less than 230 nm and one strong positive Cotton effect band
centered at 417 nm, respectively. However, the model compound 6
almost appears no Cotton effect signal in the CD spectrum (Table 1).
Compared to the rigid chain backbone structure P-1, the flexible
chain backbone P-2 shows weaker Cotton effect although P-2
should generate chiral secondary helical structure. It could be
contributed to another reason that the chiral (R,R)-1,2-
diaminocyclohexane unit is no longer available for the conjugated
system of model compound 6 and P-2. It can also be found that P-1
and P-2 have strong Cotton effect at long wavelengths of 414 nm
and 417 nm, which can be regarded as the extended conjugated
structure [43,49,69e71].
Fig. 1. TGA curve of the chiral polymers.
P-2 were carried out under a N2 atmosphere at a heating rate of
10 ꢃC/min. As shown in Fig. 1, P-1 show an apparently one-step
degradation at temperature ranging from 350 to 550 ꢃC. P-2
exhibits a two-step degradation process: the first step degradation
is observed at temperature ranging from 200 to 300 ꢃC, and the
second step degradation appears at temperature ranging from
360 ꢃC to 700 ꢃC. There is a total loss of about 50%, 45% for P-1, P-2
when heated to 700 ꢃC. Therefore, the herein described materials
can provide desirable thermal properties for practical applications
as fluorescent chemosensors.
3.3. UVevis titration of the chiral polymers on Zn2þ
Fig. 3 illustrates the UVevis absorption spectra of model
compounds 5, 6, and P-1, P-2 (1.0 ꢀ 10ꢂ5 mol Lꢂ1) in THF. UVevis
spectra of the polymers P-1, P-2 are similar due to the similar
repeating units of the conjugated polymer backbone. UVevis
absorption maxima lmax of model compounds 5 and 6 appear at
331 nm and 292 nm, respectively. But the conjugated polymers P-1
and P-2 display great red shifts and show the strong and broad
absorption at the region from 325 to 450 nm. The strongest
absorption wavelengths lmax of P-1 and P-2 and appear at 389 nm
and 394 nm. A large red shifts in the electronic absorptions of the
3.2. CD spectra
The specific rotation values (½a D
ꢁ
25) of the model compounds 5
and 6 are ꢂ431.00 (c 0.29, THF) and ꢂ46.77 (c 0.29, THF). And ½a D25
ꢁ
of the chiral polymers P-1 and P-2 are þ917.03 (c 0.21, THF)
and þ178.36 (c 0.21, THF), respectively. But the absolute values of
the specific rotation (½a D
ꢁ
25) of two polymers are much larger than
their corresponding model compounds and have opposite specific
rotation signal, which can be attributed to the counteracting result
of the repeating chiral unit upon the formation of helical chiral
polymer configuration [62e68]. Fig. 2 illustrates the CD spectra of
the model compound 5, compound 6 and the chiral polymers P-1
and P-2 (1.0 ꢀ 10ꢂ5 mol/L) in THF. The model compound 5 and the
chiral polymer P-1 exhibit different CD signals with negative and
conjugated polymers can be attributed to the effective
p-p*
conjugated segments of the polymer main chain backbone [49,71].
The UVevis responses of the chiral polymers P-1 and P-2 on
transition Zn2þ ions were tested in THF solution (Fig. 4). Upon
Model compound 5
Model compound 6
P-1
P-2
model compound 5
0.6
200000
model compound 6
150000
100000
50000
P-1
P-2
0.5
0.4
0.3
0.2
0.1
0.0
0
-50000
-100000
-150000
-200000
-250000
250
300
350
400
450
250
300
350
400
450
Wavelength(nm)
Wavelength(nm)
Fig. 2. CD spectra of the model compounds 5, 6, P-1 and P-2 in THF.
Fig. 3. UVevis spectra of model compounds 5, 6, P-1 and P-2 in THF.

F. Song et al. / Polymer 52 (2011) 6029e6036
6033
a
b
0
0.6
0.5
0.4
0.3
0.2
0.1
0.0
0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
0.4
0.2
0.4
0.6
0.3
0.8
1.0
1.2
1.4
0.2
1.6
1.8
2.0
0.1
0.0
250
300
350
400
450
500
550
250
300
350
400
450
500
550
Wavelength(nm)
Wavelength(nm)
Fig. 4. UVevis spectra of P-1 (a) and P-2 (b) with increasing amounts of Zn2þ (0e2.0 equiv).
observed with increasing addition of Zn2þ (Fig. 4b). In addition, the
weak absorption band of the polymer P-2 at 275 nm shows a little
enhancement. There are three isosbestic points located at 281, 340
and 414 nm, which also indicate the formation of the UV active zinc
complex [72].
90
80
70
60
50
40
30
20
10
0
model compound 5
model compound 6
P-1
P-2
3.4. The selective and sensitive recognition of the polymer sensors
on Zn2þ
Fluorescence spectra of model compounds 5, 6 and P-1, P-2 are
shown in Fig. 5. The two chiral polymers P-1 and P-2 can emit
yellow light under ultraviolet light (361 nm) in low concentration
(1 ꢀ 10ꢂ5 mol Lꢂ1). On excitation at 280 nm, fluorescence maxima
lmax of model compounds 5, 6 appear at 304 and 322 nm. On the
contrary, the emission wavelengths lmax of P-1, P-2 (lex ¼ 420 nm)
exhibit large red shifts as high as 234 nm and 237 nm due to the
300 350 400 450 500 550 600 650 700
Wavelength(nm)
extended
[69,73e75].
p-electronic structure in the main chain backbone
Fig. 5. Fluorescence spectra of model compound 5, model compound 6, P-1 and P-2 in
THF.
The fluorescence response behaviors of two chiral polymers P-1
and P-2 (1.0 ꢀ 10ꢂ5 mol Lꢂ1 in THF) on Zn2þ have been investigated
by fluorescence spectra with the common excitation at 420 nm
(Fig. 6a and b). As is evident from Fig. 6, obvious fluorescence
enhancement could be observed upon the addition of Zn2þ. Zn2þ
can lead to fluorescence enhancement as high as 22.8-fold for P-1
and 3.75-fold for P-2 at a concentration of 1:1 M ratio. More
importantly, P-1 and P-2 turns to be bright yellow after addition of
Zn2þ, which can be easily detected by naked eyes (Seen the inset
figure of Fig. 6). It can also be found that the addition curves keep
nearly linear correlation with the concentration molar ratio of Zn2þ
from 0.1 to 0.9 for P-1 and P-2 (Fig. 7). Normally, the fluorescent
addition of Zn2þ (0e2.0 equiv), the main absorbance peak of the
chiral polymer P-1 at 389 nm appears the obvious reduction,
meanwhile a shoulder peak in the wavelength region at 453 nm can
be observed and show gradual enhancement with increasing
amounts of Zn2þ. However, the crest band of the polymer P-1 at
306 nm shows a little enhancement (Fig. 4a). Moreover, there are
two isosbestic points at 338 and 411 nm, which indicate the
formation of the UV active zinc complex [72]. In view of P-2, the
main absorbance peak of P-2 also appears the obvious decrease,
and the gradual enhancement of a shoulder peak at 454 nm can be
Fig. 6. Fluorescence spectra of P-1 (a) and P-2 (b) with increasing amounts of Zn2þ
(
lex ¼ 420 nm).

6034
F. Song et al. / Polymer 52 (2011) 6029e6036
4.0
a
b
25
20
15
10
5
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8
2+ -5
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8
2+ -5
Zn /10
M
Zn /10
M
Fig. 7. Fluorescence enhancement values (I/I0-1) vs the increasing concentration of Zn2þ for P-1 (a) and P-2 (b).
enhancement for salen/salan-based polymers can be attributed to
two reasons [19,43,76,77]. One is suppressed PET (photoinduced-
electron-transfer) quenching when Zn2þ coordinates with the
nitrogen and oxygen atoms of (R,R)-salen/(R,R)-salan-based moie-
ties in the chiral polymer main chain. On complexation, the lone
pair of electrons on the nitrogen and oxygen atom is no longer
available for ICT (intramolecular charge transfer) or PET (photoin-
duced-electron-transfer), thus leading to an enhancement of the
emission. On the other hand, the formation of Zn2þ-polymer
complexes can enhance the planarity and rigidity of the conjugated
segment of the chiral polymer, which can reduce the non-radiative
decay of the excited state and lead to the pronounced fluorescence
enhancement. However, we found that P-1 bearing salen unit can
exhibit more pronounced fluorescence response on Zn2þ than
salan-based polymer P-2. One reason may result from the steric
hindrance on the complex formation of the flexible salan-based
unit with Zn2þ. The other reason is due to the fact that the lone
pair of electrons on the nitrogen atom is no longer available for
conjugated system of Zn2þ-P-2 complex, leading to the dis-
appearence of suppressed PET quenching. As shown in Fig. 6, we
also found that a short red shift of about 10 nm is observed when
salen-based receptor in the main chain backbone of the polymer P-
1 coordinates with Zn2þ, while salan-based P-2 shows a larger blue
shift as high as 20 nm, which may be attributed to the changes of
HOMO of the conjugated segment of the chiral polymer [78,79].
In this paper we further investigated the fluorescence response
behaviors of the chiral polymers on other various metal ions,
including Naþ, Kþ, Mg2þ, Ca2þ, Fe3þ, Co2þ, Ni2þ, Cu2þ, Agþ, Cd2þ
,
Fig. 8. The selectivity of the polymers toward Zn2þ and other metal ions. (a) Fluorescence spectra of P-1 (1.0 ꢀ 10ꢂ5 mol Lꢂ1) in THF with 1.0 equiv of a metal ion. (b) Relative
fluorescence intensity of P-1-metal ions complex. (c) Fluorescence spectra of P-2 (1.0 ꢀ 10ꢂ5 mol Lꢂ1) in THF with 1.0 equiv of a metal ion. (d) Relative fluorescence intensity of P-2-
metal ions complex.

F. Song et al. / Polymer 52 (2011) 6029e6036
6035
Cr3þ and Pb2þ. In these experiments, the fluorescence spectra of
two polymers (1.0 ꢀ 10ꢂ5 mol Lꢂ1 in THF) were determined at room
temperature in the absence and presence of 1.0 equiv. of a metal ion
with the excitation at 420 nm. As is evident from Fig. 8a and c,
addition of Zn2þ ion to a THF solution of P-1 or P-2 can induce
a remarked fluorescence enhancement. By contrast, the fluorescent
enhancement response of the salan-based polymer P-1 are not
almost affected by the addition of most of the other metal ions,
especially Cd2þ (Fig. 8d). But addition of 1.0 equiv of Agþ and Cd2þ
can slightly enhance fluorescence for polymer P-1 (I/I0-1 ¼ 0.850,
0.456), and Naþ, Kþ, Mg2þ, Ca2þ, Cr3þ, Fe3þ, Al3þ and Pb2þ caused
limited quenching of the fluorescence intensity on the same
condition. Meanwhile, Cu2þ and Ni2þ can effectively quench fluo-
rescence of the chiral polymer (I/I0-1 ¼ ꢂ0.848, ꢂ0.868), but Co2þ
can lead to almost complete quenching of the chiral polymer (I/I0-
1 ¼ ꢂ0.926). In view of P-2, the similar effect was observed (Fig. 8d).
Addition of 1.0 equiv of Naþ, Ni2þ, Agþ, Pb2þ slightly enhanced
fluorescence (I/I0-1 ¼ 0.297, 0.297, 0.197, 0.719) for polymer P-2.
And Kþ, Mg2þ, Ca2þ, Cr3þ, Fe3þ and Al3þ caused limited quenching
of the fluorescence intensity on the same condition. Meanwhile,
Co2þ, Cu2þ and Cd2þ can effectively quench fluorescence of the
chiral polymer (I/I0-1 ¼ ꢂ0.965, ꢂ0.965, ꢂ0.928). These results
confirmed the polymer P-1 and P-2 can exhibit high sensitivity and
selectivity fluorescence enhancement response to Zn2þ. In addition,
the polymer chemosensors can also exhibit obvious fluorescence
quenching response to Co2þ, Ni2þ and Cu2þ, which can be attrib-
uted to photoinduced charge transfer (PCT) between the polymer
backbones and binding metal complexes. Herein, metal ions with
an open shell electronic structure, which generally show the strong
metal-ligand orbital interaction to form low-lying ligand-to-metal
(LM) charge transfer state, may exhibit a highly quenching response
as in the case of Co2þ with 3d7, Ni2þ with 3d8 and Cu2þ with 3d9
electronic structure.
[2] Que EL, Domaille DW, Chang CJ. Chem Rev 2008;108:1517e49.
[3] Nolan EM, Lippard SJ. Acc Chem Rev 2009;42:193e203.
[4] Dai Z, Canary JW. New J Chem 2007;31:1708e18.
[5] Jiang P, Guo J. Coord Chem Rev 2004;248:205e29.
[6] Carol P, Sreejith S, Ajayaghosh A. Chem Asian J 2007;2:338e48.
[7] Lim NC, Freake HC, Bru’ckner C. Chem Eur J 2005;11:38e49.
[8] Kimura E, Aoki S. Biometals 2001;14:191e204.
[9] Domaille DW, Que EL, Chang CJ. Nat Chem Biol 2008;4:168e75.
[10] Vallee BL, Falchuk KH. Physiol Rev 1993;73:79e118.
[11] Coleman JE. Annu Rev Biochem 1992;61:897e946.
[12] Williams RJP, Frau0sto da Silva JJR. Coord Chem Rev 2000;200e202:247e348.
[13] Woodroofe CC, Lippard SJ. J Am Chem Soc 2003;125:11458e9.
[14] De Silva AP, Gunaratne HQN, Gunnlaugsson T, Huxley AJM, McCoy CP,
Rademacher JT, et al. Chem Rev 1997;97:1515e66.
[15] Maret W, Li Y. Chem Rev 2009;109:4682e707.
[16] Hosseinia M, Vaezib Z, Ganjalic MR, Faridbodd F, Abkenara SD, Alizadehe KH.
Anal Chim Acta 2002;460:177e83.
[17] Wang H, Gan Q, Wang X, Xue L, Liu S, Jiang H. Org Lett 2007;9:4995e8.
[18] Zhang Y, Guo X, Si W, Jia L, Qian X. Org Lett 2008;10:473e6.
[19] Xu ZC, Baek KH, Kim HN, Cui JN, Qian XH, Spring DR, Shin I, Yoon J. J Am Chem
Soc 2010;132:601e10.
[20] You Y, Tomat E, Hwang K, Atanasijevic T, Nam W, Jasanoffc AP, et al. Chem
Commun 2010;46:4139e41.
[21] Hanaoka K, Muramatsu Y, Urano Y, Terai T, Nagano T. Chem Eur J 2010;16:
568e72.
[22] Esqueda AC, López JA, Andreu-de-Riquer G, Alvarado-Monzón JC, Ratnakar J,
Lubag AJM, et al. J Am Chem Soc 2009;131:11387e91.
[23] Dennis AE, Smith RC. Chem Common; 2007:4641e3.
[24] Zhang L, Chark RJ, Zhu L. Chem Eur J 2008;14:2894e903.
[25] Zhu Y, Kim HN, Yoon J. Bioorg Med Chem Lett 2010;20:125e8.
[26] Li L, Dang YQ, Li HW, Wang B, Wu YQ. Tetrahedron Lett 2010;51:618e21.
[27] Zhu LN, Gong SL, Gong SL, Yang CL, Qin JG. Chin J Chem 2008;26:1424e30.
[28] Grabchev I, Sali S, Betcheva R, Gregoriou V. Eur Poly J 2007;43:4297e305.
[29] He S, Iacono ST, Budy SM, Dennis AE, Smith Jr DW, Smith RC. J Mater Chem
2008;18:1970e6.
[30] Kim HN, Guo ZQ, Zhu WH, Yoon J, Tian H. Chem Soc Rev 2011;40:79e93.
[31] Zhaochao X, Juyoung Y, David RS. Chem Soc Rev 2010;39:1996e2006.
[32] Sarkar K, Dhara K, Nandi M, Roy P, Bhaumik A, Banerjee P. Adv Funct Mater
2009;19:223.
[33] Jose AD, König B. Org Biomol Chem 2010;8:655.
[34] Chen Y, Pu KY, Fan QY, Huang YQ, Lu XM, Huang W. J Polym Sci A Polym Chem
2009;47:5057e67.
[35] Liu XQ, Zhou X, Shu X, Zhu J. Macromolecules 2009;42:7634e7.
[36] McQuade DT, Pullen AE, Swager TM. Chem Rev 2000;100:2537e74.
[37] Thomas SW, Joly GD, Swager TM. Chem Rev 2007;107:1339e86.
[38] Cozzi PG. Chem Soc Rev 2004;33:410e21.
4. Conclusion
[39] Chu ZL, Huang W, Wang L, Gou SH. Polyhedron 2008;27:1079e92.
[40] Pucci D, Aiello I, Bellusci A, Crispini A, Ghedini M, Deda ML. Eur J Inorg Chem;
2009:4274e81.
[41] Hosseini M, Vaezi Z, Ganjali MR, Faridbod F, Abkenar SD, Alizadeh K, et al.
Spectrochim Acta A; 2009:1386e425.
[42] Singh N, Kaur N, McCaughan B, Callan JF. Tetrahedron Lett 2010;51:3385e7.
[43] XuY, Meng J, MengLX, DongY,ChengYX, Zhu CJ. ChemEurJ 2010;16:12898e903.
[44] Xu Y, Zheng LF, Huang XB, Cheng YX, Zhu CJ. Polymer 2010;51:994.
[45] Leung ACW, MacLachlan MJ. J Mater Chem 2007;17:1923e32.
[46] Hansena TV, Skatteb L. Tetrahedron Lett 2005;46:3829e30.
[47] Sakthivel S, Punniyamurthy T. Tetrahedron Asymmetry 2010;21:2834e40.
[48] Dai ZY, Shao MR, Hou XS, Zhu CJ, Zhu YH, Pan Y. Appl Organometal Chem
2005;19:898e920.
[49] Cheng YX, Song JF, Zou XW, Zhang SW, Liu Y, Huang H. Polymer 2006;47:
6598e605.
[50] Zhu JF, Yuan H, Chan WH, Lee AWM. Org Biomol Chem 2010;8:3957e64.
[51] Mikata Y, Yamanaka A, Yamashita A, Yano S. Inorg Chem 2008;47:7295e301.
[52] Singh NJ, Jun EJ, Chellappan K, Thangadurai D, Chandran RP, Hwang IC, et al.
Org Lett 2007;9:485e8.
Chiral polymer P-1 incorporating (R,R)-salen-type unit was
synthesized via nucleophilic additioneelimination reaction, and
chiral polymer P-2 incorporating (R,R)-salan-type unit could be
obtained by the reduction reaction of P-1 with NaBH4. Compared
with other cations, such as Naþ, Kþ, Mg2þ, Ca2þ, Fe3þ, Co2þ, Ni2þ
,
Cu2þ, Agþ, Cd2þ, Cr3þ and Pb2þ, Zn2þ can lead to the pronounced
fluorescence enhancement as high as 22.8-fold for P-1 and 3.75-
fold for P-2. The two chiral polymers P-1 and P-2 incorporating
(R,R)-salen/salan as a receptor in the main chain backbone can
exhibit high sensitivity and selectivity for Zn2þ. And P-1 exhibited
more sensitive response signals for Zn2þ than P-2.
Acknowledgments
[53] Ambrosi G, Formica M, Fusi V, Giorg L, Macedi E, Micheloni M, et al. Inorg
Chem 2010;49:9940e8.
This work was supported by the National Natural Science
Foundation of China (No. 21074054, 51173078), National Basic
Research Program of China (2010CB923303), the Bureau of Science
and Technology of Wenzhou (No. S20100007) and the Fundamental
Research Funds for the Central Universities (1082020502).
[54] Kulkarni AP, Zhu Y, Jenekhe SA. Macromolecules 2005;38:1553e63.
[55] Zhan XW, Liu YQ, Wu X, Wang S, Zhu DB. Macromolecules 2002;35:2529e37.
[56] Peng ZH, Bao ZN, Galvin ME. Adv Mater 1998;10:680e3.
[57] Tonzola CJ, Alam MM, Jenekhe SA. Macromolecules 2005;38:9539e47.
[58] Tonzola CJ, Alam MM, Bean BA, Jenekhe SA. Macromolecules 2004;37:
3554e63.
[59] Chen XLD, Jenekhe SA. Macromolecules 2001;34:3607e14.
[60] Jonforsen M, Johansson T, Inganas O, Andersson MR. Macromolecules 2002;
35:1638e43.
[61] Bangcuyo CG, Rampey-Vaughn ME, Quan LT, Angel SM, Smith MD, Bunz UHF.
Macromolecules 2002;35:1563e8.
[62] Zheng YJ, Cui JX, Zheng J, Wan XH. J Mater Chem 2010;20:5915e22.
[63] Cui JX, Lu XC, Liu AH, Wan XH, Zhou QF. Macromolecules 2009;42:7678e88.
[64] Zhi JG, Guan Y, Cui JX, Liu AH, Zhu ZG, Wan XH, et al. J Polym Sci A Polym
Chem 2009;47:2408e21.
Appendix. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.polymer.2011.11.017.
References
[65] Cui JX, Liu AH, Zhi JG, Zhu ZG, Guan Y, Wan XH, et al. Macromolecules 2008;
41:5245e54.
[66] Zhi JG, Zhu ZG, Cui JX, Wan XH, Zhou QF. Macromolecules 2008;41:1594e7.
[1] Tsien RY. In: Czarnik AW, editor. Fluorescencent and photochemical probes of
dynamic biochemical signal inside living cells. Washington, DC: Americal
Society; 1993. p. 130e60.

6036
F. Song et al. / Polymer 52 (2011) 6029e6036
[67] Liu AH, Zhi JG, Cui JX, Wan XH, Zhou QF. Macromolecules 2007;40:8233e43.
[68] Zhang J, Cao HQ, Wan XH, Zhou QF. Langmuir 2006;22:6587e92.
[69] Huang XB, Meng J, Dong Y, Cheng YX, Zhu CJ. J Polym Sci A Polym Chem 2010;
48:997e1006.
[73] Li J, Meng J, Huang XB, Cheng YX, Zhu CJ. Polymer 2010;51:3425e30.
[74] Huang XB, Meng J, Dong Y, Cheng YX, Zhu CJ. Polymer 2010;51:3064e7.
[75] Huang XB, Ying X, Zheng LF, Meng J, Cheng YX, Zhu CJ. Polymer 2009;50:
5996e6000.
[70] Liu Y, Zhang SW, Miao Q, Zheng LF, Zong LL, Cheng YX. Macromolecules 2007;
40:4839e47.
[71] Song JF, Cheng YX, Chen LW, Zou XW, Wang ZL. Euro Polym J 2006;42:663.
[72] Hung CH, Chang GF, Kumar A, Lin GF, Luo LY, Ching WM, et al. Chem Com-
mun; 2008:978e80.
[76] Pina F, Bernardo MA, GarcÍa-España E. Eur J Inorg Chem; 2000:2143e57.
[77] Ngwendson JN, Banerjee A. Tetrahedron Lett 2007;48:7316e9.
[78] Wang YW, Yu MX, Yu YH, Bai ZP, Shen Z, Li FY, et al. Tetrahedron Lett 2009;
50:6169e72.
[79] Tomat E, Lippard SJ. Curr Opin Chem Bio 2010;14:225e30.