Synthesis and selective recognition toward zinc ion of chiral poly(imine-triazole)

Article abstract of DOI:10.1002/pola.27223

A novel AB type of clickable monomer, (S)-2-[(2-azido-1-phenylethylimino) methyl]-5-propargyloxyphenol (AMPP) was designed and polymerized to yield a class of main-chain chiral poly(imine-triazole)s through the metal-free click reaction. With the thermally induced polymerization, the desired polytriazoles can be easily prepared in high yields by a stepwise heating-up process and have the number-average molecular masses ranging from 5.1 × 10³ to 58.1 × 10³ (polydispersity indices = 1.38-1.68). The polymers were characterized by Fourier Transform Infrared spectroscopy (FTIR), ¹H Nuclear Magnetic Resonance (NMR), and gel permeation chromatography, and their optical properties were studied by fluorescence and circular dichroism (CD) spectroscopies. As a chemosensor, these polymers exhibited a selective "turn-on" fluorescence enhancement response toward Zn²⁺ ion over other cations such as Na⁺, K ⁺, Mg²⁺, Ca²⁺, Ag⁺, Pb²⁺, Cd²⁺, Hg²⁺, Mn²⁺, and Ni²⁺ in dimethyl sulfoxide. However, the Zn²⁺-induced fluorescence signal was subject to serious interference by Al³⁺, Cu²⁺, Cr ³⁺, and Fe³⁺ ions. Interestingly, the chiral polymer showed distinctive changes in the CD spectra on complexation with Zn ²⁺, which allowed for the discrimination of this ion in the presence of other species tested including those interfering ions observed in the fluorescent detection.

Full text of DOI:10.1002/pola.27223

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Synthesis and Selective Recognition Toward Zinc Ion of Chiral
Poly(Imine-Triazole)
Jinting Zhou, Wei Lu, Fangyu Hu, Mengyu Zhang, Liming Jiang, Zhiquan Shen
MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering,
Zhejiang University, Hangzhou 310027, China
Correspondence to: L. Jiang (E-mail: cejlm@zju.edu.cn)
Received 3 December 2013; accepted 12 April 2014; published online 10 May 2014
DOI: 10.1002/pola.27223
ABSTRACT: A novel AB type of clickable monomer, (S)-2-[(2-
azido-1-phenylethylimino)methyl]-5-propargyloxyphenol (AMPP)
was designed and polymerized to yield a class of main-chain
chiral poly(imine-triazole)s through the metal-free click reaction.
With the thermally induced polymerization, the desired polytria-
zoles can be easily prepared in high yields by a stepwise
heating-up process and have the number-average molecular
masses ranging from 5.1 3 103 to 58.1 3 103 (polydispersity
indices 5 1.3821.68). The polymers were characterized by Fou-
rier Transform Infrared spectroscopy (FTIR), 1H Nuclear Mag-
netic Resonance (NMR), and gel permeation chromatography,
and their optical properties were studied by fluorescence and
circular dichroism (CD) spectroscopies. As a chemosensor, these
polymers exhibited a selective “turn-on” fluorescence enhance-
ment response toward Zn21 ion over other cations such as Na1,
K1, Mg21, Ca21, Ag1, Pb21, Cd21, Hg21, Mn21, and Ni21 in
dimethyl sulfoxide. However, the Zn21-induced fluorescence sig-
nal was subject to serious interference by Al31, Cu21, Cr31, and
Fe31 ions. Interestingly, the chiral polymer showed distinctive
changes in the CD spectra on complexation with Zn21, which
allowed for the discrimination of this ion in the presence of
other species tested including those interfering ions observed in
C
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the fluorescent detection.
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KEYWORDS: click reaction; chiral; fluorescence; polytriazole;
sensors; zinc ion
tical applications.6 Nevertheless, there are fairly limited reports
on polymeric sensors incorporating the triazole moiety. Lately,
Yashima and coworkers7 developed a facile method to prepare
optically active helical poly(phenylacetylene)s via the click
polymer reaction of optically inactive azide-bound poly(pheny-
lacetylene)s with chiral acetylenes. The resulting polymers
formed a preferred-handed helical conformation and exhibited
characteristic induced circular dichroisms (CDs) whose Cotton
effect (CE) signs can be used to sense the chirality of the chiral
acetylenes. The Cheng group synthesized several conjugated
polymers containing triazole units in the main chain by using
Cu(I) catalyzed azide–alkyne cycloaddition.8 As fluorescence
sensors these polytriazoles showed a pronounced fluorescence
response for Ni21 or Hg21 with excellent selectivity over other
INTRODUCTION Click chemistry encompasses a class of reac-
tions that allow the coupling of two molecular fragments in an
extremely effective manner.1,2 The most widely utilized of
these efficient transformation reactions is the copper(I)-cata-
lyzed 1,3-dipolar cycloaddition of azides and alkynes, which
results in the formation of 1,4-disubstituted 1,2,3-triazoles in
high yields and with excellent regioselectivity.3 In the last 10
years, this simple click reaction has been creatively used to
facilitate the synthesis of complex-structured polymers and
modification of material surfaces.4 In the meantime, the prop-
erties of the functional 1,2,3-triazole motif have also begun to
attract a lot of research interest. The numerous studies on
supramolecular systems involving macrocyclic triazolophanes
and oligotriazoles demonstrated that the triazole unit exhibits
the ability to simultaneously play the role of hydrogen-bond
acceptor and hydrogen-bond donor, that is, the lone pair on
the middle nitrogen atom can act as a hydrogen-bond acceptor,
whereas the polarized CAH bond takes the role of a nonclassi-
cal hydrogen-bond donor.5 These marvellous features of tria-
zole ring make it an ideal building block not only in
supramolecular chemistry but also for the construction of poly-
meric materials for optoelectronic, biomedical, and pharmaceu-
transition metal ions, such as Cu21, Zn21, Co21, and Cd21
.
More recently, Qin and coworkers reported tetraphenylethene-
containing polytriazoles prepared through the metal-free click
polymerization.9 They found that the polymers exhibit the
aggregation-induced emission characteristics and the aggre-
gates formed in tetrahydrofuran (THF)/water mixture can
serve as a fluorescent probe for detecting explosives such as
picric acid.
Additional Supporting Information may be found in the online version of this article.
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SCHEME 1 Synthesis of the functional monomer AMPP and its click polymerization.
Over the course of our research on artificial chemosensors
for ions of biological importance, we developed an effective
method for Zn21 detection using a structurally simple Schiff
base derivative as a sensor.10 The probe molecule displayed
high affinity for zinc ion over other cations of interest includ-
ing Cd21 in unbuffered methanol. In general, compared to
the chemosensors based on small organic molecules, poly-
meric counterparts would offer some advantages such as sig-
nal amplification, enhanced binding efficiency and selectivity
for a specific analyte by virtue of the possible cooperative
effects of multiple recognition sites.11 In this aspect, we are
particularly interested in exploring chiral polymer-based flu-
orescent sensors, because such sensory systems would pro-
vide a chiroptical signal that indicates guest binding.12 As a
part of our continuing efforts in the field, this article deals
with the synthesis and structural characterization of a new
kind of main-chain chiral poly(imine-triazole)s as well as the
investigation of its luminescent and chiroptical properties on
exposure to targeted metal ions. The results revealed that
the polymer sensor exhibited differing spectral changes on
binding with Zn21, which can be used to effectively distin-
guish this ion from the common metal ions. To the best of
our knowledge, there are very few currently reported appli-
cations of chiral polymers in cation detection; especially,
main-chain chiral polytriazole-based sensors for selective
Zn21 recognition have scarcely been described so far.
applied for recording infrared spectroscopy (IR) spectra in
KBr pellets. Electrospray ionization mass spectrometry
(ESIMS) was measured on a Bruker Esquire3000 plus ion trap
mass spectrometer. The polymer molecular weights (Mw, Mn)
and polydispersity (Mw/Mn) were measured on a Water-
s2150C gel permeation chromatography system. Monodis-
perse poly(methyl methacrylate) was used as calibration
standards anꢀd DMF as the eluent at a flow rate of 1.0 mL
min21 at 60 C. Steady-state fluorescence spectra were meas-
ured on a PerkinElmer LS-55 fluorescence spectrometer in
the right-angle geometry (90ꢀ collecting optics). UV-vis and
CD spectra were obtained in DMSO at 25 ꢀC using a quartz
cell of 1 cm on MOS2450 (Biologic Company, France).
Synthesis of Monomer AMPP
To a solution of Boc-(S)-phenylglycinol (1.25 g, 5.2 mmol) in
20 mL anhydrous CH2Cl2 were added Et3N (1.45 mL, 10.4
mmol) and p-toluenesulfonyl chloride (1.25 g, 5.2 mmol;
Scheme 1). The reaction mixture was allowed to stir at room
temperature under N2 atmosphere for 24 h, and was then
quenched with aqueous 10% K2CO3 solution (20 mL). After
stirring for 2 h, the aqueous layer was separated and
extracted with CH2Cl2 (2 3 30 mL). The combined organic
extracts were washed with deionized water, dried over anhy-
drous MgSO4, filtered, and evaporated under reduced pres-
sure to give
a solid residue. The crude product was
recrystallized in EtOAc/n-hexane (1:2) to afford 1 as a white
EXPERIMENTAL
solid (yield 88%).
Materials
A mixture of 1 (0.5 g, 1.5 mmol) and NaN3 (0.13 g, 2 mmol)
in anhydrous DMF (10 mL) was stirred at 70 ꢀC under N2
atmosphere. After 24 h, the reaction mixture was poured
into deionized water (50 mL); the collected precipitate was
then recrystallized in DMF/water (1:5) give 2 as a white
powder (yield 80%).
Boc-(S)-phenylglycinol, sodium azide, 1-hexyne, triethylamine,
and N,N,N0,N00,N00-pentamethyldiethylenetriamine (PMDETA,
98%) were purchased from Sigma-Aldrich and used as
received without further purification. Dichloromethane
(CH2Cl2), N,N-dimethylformamide (DMF), dimethyl sulfoxide
(DMSO), and other organic solvents were commercially avail-
able (Sinopharm Chemical Reagent) and purified by standard
methods prior to use.
To a stirred solution of 2 (0.84 g, 4 mmol) in EtOAc (3 mL)
was added dropwise 6ꢀmL 2.5 M HCl/EtOAc at 0 ꢀC. After
stirring for 6 h from 0 C to room temperature, the reaction
mixture was concentrated via rotary evaporator and stirred
with 5 M NaOH aqueous solution (100 mL) for another 6 h,
and then extracted with CH2Cl2 (2 3 30 mL). The organic
extract was washed with deionized water, dried (anhydrous
Measurements
1H and 13C NMR spectra were recorded on a BrukerAvance
AMX2400 and 500 NMR spectrometer, respectively. A Bruker
Vector 22 Fourier Transform Infrared spectrophotometer was
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MgSO4), filtered, and evaporated under reduced pressure to
give 3 as a pale-yellow oil in near 100% yield.
Under N2 atmosphere and magnetic stirring, propargyl bro-
mide (0.6 g, 5 mmol) was added into the flask containing
2,4-dihydroxybenzaldehyde (0.7 g, 5 mmol) and K2CO3 (0.7
g) in anhydrous acetone (20 mL). The reaction mixture was
refluxed for 6 h. After filteration, the organic layer was
washed with deionized water, dried (MgSO4), and evaporated
under reduced pressure to yield a solid residue. The crude
product was purified by flash column chromatography
(CH2Cl2/n-hexane, 1/1) to afford 4 as white needle crystals
(yield 40%).
SCHEME 2 Synthetic route to the model compound 6.
3 (0.72 g, 4 mmol) and 4 (0.8 g, 4 mmol) were dissolved in
10-mL ethanol and then the reaction mixture was allowed to
stir at room temperature for 24 h. After removing the solvent
via rotary evaporator, the resulted residue was recrystallized
with ethanol to give the desired product (S)-2-[(2-azido-1-
phenylethylimino)methyl]-5-propargyloxyphenol (AMPP) as
20
½aꢁD 5 41.8ꢀ (c 0.5, DMSO); 1H NMR (CDCl3, 400 MHz): d
12.94 (s, 1H), 8.07 (s, 1H), 7.4427.30 (m, 6H), 7.11 (d,
1H), 7.07 (s, 1H), 6.97 (d, 1H), 6.85 (t, 1H), 4.86 (m, 2H),
4.58 (m, 1H), 2.58 (m, 2H), 1.43 (m, 2H), 1.16 (m, 2H),
0.78 (m, 3H). FTIR (KBr, cm21): 3435, 2927, 1630, 1277,
1061, 758.
20
ꢀ
yellow crystals (1.3 g, yield 85%). M.p. 59.0259.2 C; ½aꢁD
5
274.4ꢀ (c 0.5, DMSO); 1H NMR (DMSO2d6, 400 MHz):
d 13.55 (s, 1H), 8.63 (s, 1H), 7.5627.23 (m, 4H), 6.5726.36
(m, 2H), 4.84 (2H, J 5 2.3Hz), 4.66 (dd, 1H, J 5 8.6Hz), 3.83
(1H, dd, J 5 8.6 Hz), 3.71 (dd,1 H, J 5 8.5 Hz), 3.61 (s, 1H,
J 5 2.3 Hz); 13C NMR (DMSO2d6, 125 MHz): d 166.6, 163.2,
161.5, 140.5, 133.8, 129.2, 128.3, 127.5, 113.3, 107.3, 102.3,
79.3, 79.0, 71.7, 56.5, 56.1; FTIR (KBr, cm21): 3442, 3292,
2929, 2104, 1630, 1298, 1144, 822. MS [ESI, m/z([M1H]1)]:
Calculated 321.1, Found 321.1.
General Procedure for Metal Ion Recognition
Metal ion titration experiments were performed with a 2.0
mL DMSO solution of the polymer by adding aliquots of a
solution of the selected salt at room temperature. Mercury
perchlorate salt and other various metal nitrate salts (aque-
ous solution of 1.0 3 1022 mol L21) were used in the titra-
tion. The polymer concentration (based on monomeric units)
was 1.0 3 1024 mol L21 for UV-vis and CD measurements
and 1.0 3 1025 mol L21 for fluorescence detection. Before
each measurement, the sample solution containing the poly-
mer and metal ions with different molar ratios was allowed
to stand at ambient temperature for 2 h with the view of
insuring complete host-guest complexation.
Preparation of Poly(Imine-Triazole)s by Bulk
Polymerization
Typically, 0.1 g of AMPP was placed in a 10 mL ampoule and
degassed by being subjected to the freeze-thaw cycle three
times. Then, the ampoule was sealed under vacuum and
immersed in a constant-temperature oil bath. After being
heated for a definite period of time, the reaction was termi-
nated by putting the ampoules into liquid nitrogen. The
crude product was dissolved in DMF and precipitated from
methanol. Further purification could be conducted by repeat-
ing this process twice. The resulting polymer (PAMPP) was
dried to a constant weight in vacuum at 40 ꢀC. The same
method was used for the solution polymerizations in DMF.
RESULTS AND DISCUSSION
Monomer Synthesis and Thermal 1,3-Dipolar
Cycloaddition Reaction
The synthesis procedures for the monomer (S)-2-[(2-azido-1-
phenylethylimino)methyl]-5-propargyloxyphenol (AMPP) and
the corresponding polymer (PAMPP) are shown in Scheme 1.
The monomer is a chiral Schiff base derivative of salicylalde-
hyde containing alkyne and azide groups, which was readily
prepared from commercially available starting materials by
simple functional-group transformations. The well-defined
chemical structure of AMPP was confirmed by various spec-
troscopic techniques, such as FTIR, NMR, and ESIMS (see
Experimental part and Supporting Information Figs. S1–S4).
Synthesis of Model Compound 6
A mixture of 3 (0.2 g, 1.2 mmol), 1-hexyne (0.1 g, 1.2
mmol), CuBr (0.09 g, 0.6 mmol), and PMDETA (0.107 g, 0.6
mmol) in 15 mL DMF was stirred at ambient temperature
under N2 atmosphere (Scheme 2). After stirring for 8 h, the
solution was passed through neutral alumina to remove the
copper catalyst. The elute was concentrated, and the residue
was recrystallized in ethyl acetate/n-hexane (1:1) to give 5
as a white solid (0.26 g). Then, the intermediate was dis-
solved in 20 mL of ethanol together with 0.13 g of salicylal-
dehyde. The reaction mixture was refluxed for 12 h, and the
solvent was removed under vacuum. The resulting residue
was recrystallized in ethanol to afford 6 as a pale yellow
solid in a total yield of 72% (0.32 g). M.p. 99.52100.5ꢀC;
Knowing that the CuI-catalyzed azide–alkyne cycloaddition
has been developed as a powerful technique in polymer syn-
theses,4 we first tried to use it to polymerize AMPP in DMF.
The results showed that the monomer seemed to be consid-
erably reactive and the expected polymerization occurred
ꢀ
smoothly at 50 C. However, the purification of the resultant
products was very difficult so that we failed to get the
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TABLE 1 Results on the Metal-Free Click Polymerization of AMPP
Reaction conditions
Entry
Polymers
20
d
Temp. (ꢀC)/Time (h)
[AMPP] (mol L21 a
)
Code
Yield (%)
Mn (103)
PDIb
1.38
1.40
1.43
1.48
1.66
1.68
1.89
1.27
1.65
F1,4 (%)
½aꢁD
b
c
1
2
3
4
5
6
7
8
9
80/12
Bulk
Bulk
Bulk
Bulk
Bulk
Bulk
Bulk
2.0
PAMPP-1
PAMPP-2
PAMPP-3
PAMPP-4
PAMPP-5
PAMPP-6
PAMPP-7
PAMPP-8
PAMPP-9
89
92
90
94
94
95
93
45
67
5.1
5.6
63.0
62.6
62.4
62.1
63.0
62.3
62.5
62.6
61.9
17.2
80/24
18.3
80/36
5.8
110.8
18.7
80/121100/12
80/121120/12
80/121120/61150/6
150/6
7.3
13.7
58.1
57.4
4.3
121.2
125.1
124.2
110.7
117.6
80/12
80/121120/12
2.0
23.7
a
c
DMF was used as solvent;
Fraction of 1,4-regioisomic units determined by 1H NMR;
b
d
Taken from GPC in DMF solution and calibrated against standard pol-
The optical rotation values were measured in DMSO (c 5 0.5).
ymethylmethacrylate, PDI 5 Mw/Mn;
desired polymer samples for structural characterizations.13
The reason is probably related to the formation of Cu(II)-
containing polymer complexes due to the strong affinity of
both imine and triazole units toward the metal residue.
Thus, we turned our attention to the metal-free click
approach for production of poly(imine-triazole)s, which
might be a more desirable alternative for developing fluores-
cent sensors because the residual copper ions would serve
as a fluorescence quencher.14
seems to be fairly common in the click polymeriza-
tions.4f2h,8,9a,b These polymers are only soluble in organic
solvents with rather high polarity, such as DMF and DMSO.
The specific rotation (½aꢁ2D0) obtained in DMSO (see Table 1)
for the polymers always shows the opposite sign to the one
20
for the corresponding monomer (½aꢁD 5 274.4ꢀ, c 5 5 mg
20
mL21, DMSO), and the ½aꢁD value increases roughly with the
increasing of Mn.
Structural Characterization
The thermally induced polyaddition of AMPP was exploited
in both bulk and solution states. As listed in Table 1, the
bulk polymerizations conducted at 80 C showed high levels
IR spectra of AMPP and the polymer samples with an inten-
tionally low and high molar mass are given in Figure 1. The
monomer shows the characteristic absorption band at 3291
cm21 due to BCAH stretching vibration, while the absorp-
tion of CBC nearly merged with that of azide groups around
2104 cm21. It can be seen that these absorption bands
become much weaker for PAMPP-1 (Mn 5 5.1 3 103), even
ꢀ
of monomer conversion (89–92%) and yielded the polymers
with lower molar masses (Mn 5 5.1 3 103 2 5.8 3 103,
Entries 1–3). At this temperature extending the reaction
time had only a small effect on the Mn value of resulting
polymers. However, when the temperature was increased in
steps up until 150 ꢀC, significantly higher molar mass could
be realized by longer reaction time (Entries 4–6). If perform-
ing the polyaddition directly at 150 ꢀC, the molar mass of
the polymer is comparable to that of PAMPP-5 obtained in
the gradual heating-up mode, but a relatively broad polydis-
persity (Mw/Mn 5 1.89) is observed (Entries 6, 7). The rea-
son for this is likely associated with the greatly increased
viscosity of the bulk polymerization system. Indeed, in the
case of rapid elevation of polymerization temperature to 150
ꢀC, a rather viscose reaction medium occurred at the early
stage of polymerization. The high viscosity may decrease the
homogeneity of the reaction system and thereby enlarge the
molecular weight distribution to a certain extent. Conversely,
the polymerization can also proceed in DMF with a moderate
yield, and similarly, higher reaction temperatures are benefi-
cial to the enhancement of polymer molar masses (Entries 8
and 9). The poly(imine-triazole)s formed in all cases have
lower polydispersity indices (PDIs) than the theoretical value
(2.0) typical of a step growth polymerization process, which
FIGURE 1 Stacked FTIR spectra of AMPP and the polymers
obtained under different reaction conditions. PAMPP-1 (Mn
5.1 3 103), PAMPP-6 (Mn 5 58.1 3 103), see: Table 1.
5
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FIGURE 2 1H NMR spectra of AMPP and the corresponding polymers PAMPP-1 and PAMPP-6 in DMSO-d6.The solvent peaks are
marked with asterisks. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
almost completely disappear for PAMPP-6 (Mn 5 58.1 3
103). This indicates that the azide and alkyne groups of the
monomers have been efficiently transformed into the triazole
rings of the polymer by the metal-free promoted click
reaction.
former is likely from the 1,4-isomeric unit while the latter
from the 1,5-isomeric unit.15 As a result, the fractions of 1,4-
substituted triazole (F1,4) could be calculated according to
their integrals.16 For all the polymers, the F1,4 values are in
the range of 61.9–63.0% (see Table 1), that is, the variation
in reaction conditions does not seem to have a large influ-
ence on the regioselectivity of the 1,4-triazole linkers.
Figure 2 displays 1H NMR spectra of AMPP and the corre-
sponding polymers PAMPP-1 and PAMPP-6. The signals due
to alkyne group at 3.60 as well as those due to the methyl-
ene protons (e) adjacent to the azido group of AMPP at 3.76
ppm decrease dramatically after polymerization, concomitant
with appearance of the new peaks at d 8.11 (a) and 7.73
(a0). Meanwhile, the azomethine proton signal (f) in the poly-
mer main chain shifts upfield and splits into multiple peaks.
Owing to having a lower molecular weight, PAMPP-1 exhibits
the characteristic signals of terminal monomeric residues at
13.55, 8.63, 4.66, 3.83, 3.71, and 3.61 ppm, as marked with a
cross. However, all of which nearly vanish in the spectrum of
higher-molar-mass sample PAMPP-6, suggesting that most of
ethynyl and azido groups have been turned into the triazole
rings with the increasing polymerization extent. These obser-
vations provide further evidence for the conclusion drawn
from the IR spectra analysis. Furthermore, it was found that
the relative integrated area of both a and a0 peaks for all
polymers is always a unity irrespective of their molecular
weights. Apparently, the two peaks are associated with the
resonance signals of the triazole methine protons, and the
Florescence Sensing Ability of PAMPPs to Metal Ions
The binding behavior of PAMPPs toward various metal ions
was first examined by means of fluorescence spectroscopy in
DMSO. Figure 3 shows the fluorescence change of PAMPP-4 as
an example on addition of cations. When excited with the max-
imum absorption at 310 nm, almost no emission band was
observed in the region beyond 350 nm for the free polymer;
in other words, the polytriazole is virtually fluorescence silent.
However, the addition of zinc ions can significantly provoke its
fluorescence emission. In response to this metal ion, the emis-
sion intensity around 430 nm increased over 17-fold when 2
equiv of Zn21 was added, but other ions tested (i.e., Na1, K1,
Ag1, Ca21, Mg21, Ni21, Mn21, Pb21, Cd21, Hg21, Cu21, Al31
Cr31, and Fe31) caused no obvious luminescence (Fig. 3).
,
To gain some insight into the possible interaction patterns
between the polymer host and Zn21 ion, we designed and
synthesized the N-salicylidene derivative of (S)-1-phenyl-2-(4-
butyl-[1,2,3]triazol-1-yl)ethylamine (6) as the corresponding
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FIGURE 5 Plot of FE of PAMPPs (10 lM) in DMSO as a function
of Mn on addition of 2 equiv Zn(NO3)2. I0 is the fluorescence
intensity at 430 nm for free polymer, and I is the fluorescence
intensity after adding Zn21; kex 5 310 nm. For model com-
pound 6, I/I0 represents the FE at 456 nm on addition of 2 equiv
Zn(NO3)2 with an excitation at 320 nm. [Color figure can be
viewed in the online issue, which is available at wileyonlineli-
brary.com.]
FIGURE 3 The relative fluorescence intensity of PAMPP-4 (10
lM, kex 5 310 nm) in DMSO in the presence of various metal
ions of interest (20 lM). In the titration, Hg21 as mercury per-
chlorate and other metal ions as their nitrate salts were used.
Inset: the corresponding fluorescence spectra of PAMPP-4 in
the absence and presence of various metal ions. [Color figure
can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
under the same experimental conditions except for excitation
at 310 nm (Supporting Information Figs. S9 and S10). There-
fore, the model compound as a Zn21 receptor exhibits a
higher sensitivity than AMPP in terms of their fluorescence
responsiveness, which may be attributed to the fact that the
former contains the triazole moiety acting as additional coor-
dination sites for the metal ion.
low molecular weight model compound (Scheme 2 and Sup-
porting Information Fig. S5). The compound is structurally
very close to the repeating unit of poly(imine-triazole)s. As
expected, the addition of zinc ion to the DMSO solution of 6
also caused a dramatic luminescence enhancement with the
emission wavelength shift from 430 to 456 nm (kex 5 320
nm, Supporting Information Fig. S6). An ꢂ12-fold enhance-
ment in the fluorescence intensity was observed on addition
of 2 equiv Zn21, while other cations produced little or no
effect. However, Zn21 binding of the monomer AMPP leads to
an approximately sevenfold increase in the 437 nm emission
On the other hand, fluorescence titrations of 6 were also inves-
tigated with an excitation at 320 nm (Supporting Information
Fig. S7). The peak at 456 nm showed a linear increase with
the increase of Zn21 ion concentration when the ratio of Zn21
and 6 concentrations is below or equal to the ratio of 1:1.
When the ratio reached 1:1, however, higher zinc ion concen-
tration did not lead to any further emission enhancement (the
FIGURE 4 Fluorescence titration profiles of PAMPP-4 (10 lM)
in DMSO with Zn(NO3)2 (0–6 equiv). Inset shows plots of the
relative emission intensity (I/I0) as a function of Zn21 concen-
tration (0–60 lM); the detection limit 5 1 lM based on the S/B
criteria. kex 5 310 nm. [Color figure can be viewed in the online
issue, which is available at wileyonlinelibrary.com.]
FIGURE 6 Competitive selectivity of PAMPP-4 (10 lM) toward
Zn21 (20 lM) in the presence of other metal ions (20 lM) in
DMSO solution. Hg21 as mercury perchlorate and other various
metal ions as their nitrate salts were used in the detection.
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FIGURE 7 CD (a) and UV-vis (b) responses of PAMPP-4 (100 lM) to various metal ions (2 equiv.) in DMSO. Hg21 as mercury per-
chlorate and other various metal ions as their nitrate salts were used in the detection. [Color figure can be viewed in the online
issue, which is available at wileyonlinelibrary.com.]
inset of Supporting Information Fig. S7). The binding stoichi-
ometry was supported by the Job’s plot of 6 with Zn21 (Sup-
porting Information Fig. S8). From the spectral titration data,
the association constant (Kass) and the detection limit was
determined to be 8.27
respectively.
with many fluorescent Zn21 sensors.10,14,17 Interestingly, the
magnitude of fluorescence enhancements (FE) is positively
correlated with the polymer molar masses in the range 5.1
3 103 g mol21 < Mn < 7.3 3 103 g mol21, as seen in Figure
5. On addition of Zn21 (2 equiv) the FE value for PAMPP-4
(Mn 5 7.3 3 103) is roughly 1.2 and 1.6 times as great as
for PAMPP-1 (Mn 5 5.1 3 103) and the model compound 6,
respectively. However, a further increase in the molar mass
does not significantly affect the fluorescence responsiveness.
The comparison of the detection limits of 6 and of PAMPP-4
(5.0 lM vs. 1.0 lM; also see Fig. 4 and Supporting Informa-
tion Fig. S7 inset) in combination with the observed Mn – FE
relationship demonstrates that these poly(imine-triazole)s
are more effective as a fluorescent sensor for selective Zn21
recognition compared to its small molecule counterparts,
3
104 M21 and about 5 lM,
Figure 4 depicts the results of fluorescence titration of
PAMPP-4 in DMSO with Zn21 under UV irradiation. In the
titration experiments using the polymer of 10 lM, the Zn21
addition leads to an increase in the 430 nm emission and
the increase is saturated with 2 equiv of Zn21. Minimum 1.0
lM concentration of Zn21 can be easily detected by a three-
fold enhancement in the fluorescence intensity. The detection
limit value is lower than or comparable to those observed
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FIGURE 8 UV-vis (a) and CD (b) titration profiles of PAMPP-4 (100 lM) with Zn(NO3)2 (023 equiv) in DMSO. Inset displays plots of
the CE intensity at 370 nm as a function of Zn21 concentration (023 equiv). [Color figure can be viewed in the online issue, which
is available at wileyonlinelibrary.com.]
which could be explained by the cooperative effects of chro-
mophores connected to the polymer main chain.11,12
caused fluorescence quenching of ca. 30, 60, and 85%,
respectively. The metal cations including Cu21, Cr31, and
Fe31 severely interfered the detection of Zn21 probably due
to the competing complexation of imine-triazole moieties
with these metals. In general, these transition metal ions are
considered as fluorescence quenchers, like other heavy metals
with partially filled d-shells.14,19
As inferred from the competitive experiments (Fig. 6), the pol-
y(imine-triazole) sensor exhibited a strong preference for
Zn21 over other metal cations including Cd21 as in the case
of the previously reported Schiff base receptor.10 It is note-
worthy that such a chemosensor capable of distinguishing
between Zn21 and Cd21 is highly desirable, because
recognization of the two metal ions with similar chemical
properties is still challenging.18 Addition of Ag1, Mg21, or
CD and UV-vis Response Behavior of PAMPPs Toward
Metal Ions
Chiral polymers would adopt different asymmetric conforma-
tions or aggregates on complexation with possible target
Ca21 promoted
a
small increase of the Zn21-induced
luminescence, whereas the presence of Pb21, Hg21, and Al31
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species. Therefore, one attractive feature of chiral chemosen-
sors is their capacity of translating a recognition event into
the chiroptical signal. As an example, the CD and UV-vis
spectra of PAMPP-4 as well as its specific rotation were
measured in the absence and presence of metal ions in
DMSO. As shown in Figure 7, the free polymer presented a
noticeable CD signal at 2502450 nm that exhibits a single
negative CE around 306 nm. The two bands of the UV-vis
spectrum in this region are assignable to the triazole and
salicylidene chromophores of the polymer backbone. After
the addition of Zn21 (2 equiv), a sharp spectral change was
observed for the polymer solution, wherein the 306-nm CE
disappeared while one negative and two positive CEs
emerged at 370, 343, and 288 nm, respectively. On the con-
Information Fig. S12), indicating that the chiral polymers as
a sole Zn21 probe offer distinctive advantages over these
small molecule analogs.
CONCLUSIONS
In summary, we designed a AB type of clickable monomer
(S)-2-[(2-azido-1-phenylethylimino)methyl]-5-propargyloxy-
phenol (AMPP) and conducted the metal-free click polymer-
ization to synthesize the corresponding poly(imine-triazole)s.
By using a stepwise heating-up process, the thermally
induced polyaddition produced the desired main-chain chiral
polymers with controlled molar masses (Mn 5 5.1 3 103
2
58.1 3 103) and narrower polydispersity (PDI 5 1.38 2
1.68). It was demonstrated that these chiral polytriazoles
can be employed as a novel sensor for dual-mode fluorescent
and CD detection of zinc ions in DMSO. The polymer probe
displays a moderate fluorescent selectivity for Zn21 with a
large turn-on emission. Although the metal cations including
Al31, Cu21, Cr31, and Fe31 would interfer the detection of
Zn21 due to their fluorescence quenching, discrimination of
Zn21 from these metal ions can be facilely achieved by the
chiral sensor because the Zn21-induced CD spectral change
is not only noticeable but also significantly different from
those with other ions tested.
trary, other cations tested including Na1, K1, Ag1, Pb21
,
Cd21, Mg21, Ca21, Ni21, Mn21, Al31, Cr31, and Fe31 did not
induce any noticeable variation in the CD profile of the chiral
polymer. Although the addition of Cu21 disturbed the poly-
mer’s CD pattern, the spectral changes induced by this ion
are totally different from that found in the case of Zn21. In
this way, the selective CD responses exhibited by the poly-
mer sensor allowed for the discrimination of Zn21 from
Al31, Cu21, Cr31, and Fe31 ions that would produce serious
interference in the fluorescence detection. The fact that
PAMPP-4 give rise to a strong induced CD signal suggests
the formation of a Zn(II)-containing polymer complex with
an ordered structure, but it does not seem to exist in a heli-
20
ACKNOWLEDGMENT
cal conformation as judged from the small ½aꢁD change (e.g.,
from 18.7 to 137.1 for PAMPP-4 on Zn21 binding).
We gratefully acknowledge the support from the Natural Sci-
ence Foundation of China (Grant No. 21074107).
Figure 8 plots the changes in the absorption and CD spectra
of PAMPP-4 in DMSO in the presence of different concentra-
tion of Zn21 cations. During the course of the titration, a sig-
nificant decrease in the band at 310 nm and the emergence
of a new band centered at 358 nm were observed with two
clear isosbestic points at 330 and 304 nm, respectively [Fig.
8(a)]. Correspondingly, the polymer bound Zn21 ion gave
typical exciton-coupled CD spectra [Fig. 8(b)]. Such an
induced CD couplet should arise from the enhanced interac-
tion between nearby chromophores embedded in a chiral
environment owing to the host–guest complexation. Also, the
amplitude of the induced CEs is linearly dependent on the
Zn21 concentration (20–200 lM), as can be seen in Figure
8(b) inset, which would allow the quantitative determination
in a broader concentation range.
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