Fluorescence sensors based on chiral polymer for highly enantioselective recognition of phenylglycinol

Article abstract of DOI:10.1016/j.polymer.2010.01.038

Chiral polymer P-1 incorporating (R,R)-salen-type unit was synthesized by the polymerization of (R,R)-1,2-diaminocyclohexane with 2,5-dibutoxy-1,4-di(5-tert-butylsalicyclaldehyde)-phenylene (M-1) via nucleophilic addition-elimination reaction, and chiral polymer P-2 incorporating (R,R)-salan-type unit could be obtained by the reduction reaction of P-1 with NaBH₄. The fluorescence response of two chiral polymers P-1 and P-2 on (R)- or (S)-phenylglycinol were investigated by fluorescence spectra. The fluorescence intensities of two chiral polymers P-1 and P-2 show gradual enhancement upon addition of (R)- or (S)-phenylglycinol and keeps nearly linear correlation with the concentration molar ratios of (R)- or (S)-phenylglycinol. But both P-1 and P-2 exhibited more sensitive response signals for (S)-phenylglycinol. The values of enantiomeric fluorescence difference ratio (ef) are 1.84 and 2.05 for P-1 and P-2, respectively. The results also showed that two chiral polymers P-1 and P-2 can also be used as fluorescence sensors for enantiomer composition determination of phenylglycinol.

Full text of DOI:10.1016/j.polymer.2010.01.038

Polymer 51 (2010) 994–997
Contents lists available at ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Polymer Communication
Fluorescence sensors based on chiral polymer for highly enantioselective
recognition of phenylglycinol
,
b,
**
Ying Xu a, Lifei Zheng a, Xiaobo Huang a, 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 State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Chiral polymer P-1 incorporating (R,R)-salen-type unit was synthesized by the polymerization of (R,R)-1,2-
diaminocyclohexane with 2,5-dibutoxy-1,4-di(5-tert-butylsalicyclaldehyde)-phenylene (M-1) via nucleo-
philic addition–elimination reaction, and chiral polymer P-2 incorporating (R,R)-salan-type unit could be
obtained by the reduction reaction of P-1 with NaBH4. The fluorescence response of two chiral polymers P-1
and P-2 on (R)- or (S)-phenylglycinol were investigated by fluorescence spectra. The fluorescence intensities
of two chiral polymers P-1 and P-2 show gradual enhancement upon addition of (R)- or (S)-phenylglycinol
and keeps nearly linear correlationwith the concentration molar ratios of (R)- or (S)-phenylglycinol. But both
P-1 and P-2 exhibited more sensitive response signals for (S)-phenylglycinol. The values of enantiomeric
fluorescence difference ratio (ef) are 1.84 and 2.05 for P-1 and P-2, respectively. The results also showed that
two chiral polymers P-1 and P-2 can also be used as fluorescence sensors for enantiomer composition
determination of phenylglycinol.
Received 9 October 2009
Received in revised form
21 December 2009
Accepted 16 January 2010
Available online 25 January 2010
Keywords:
Chiral polymer
Fluorescence sensor
Molecular recognition
Ó 2010 Elsevier Ltd. All rights reserved.
Fluorescence-based enantioselective sensors are of great practical
value because of their high sensitivity and potential applications in
analytical, biological, and clinical biochemical environments [1–5].
They can effectively provide a real-time analytical tool for chiral
compound assay. Using these sensors can not only greatly facilitate
rapid determination of enantiomeric composition of chiral
compounds, but also allow a rapid screening of high-throughput
catalysts for their asymmetric synthesis [6–9]. To date, reports of
successful enantiodiscriminating sensors have included a variety of
chiral macrocycles (fluorophore-modified calixarenes, cyclodextrins,
and crown ethers), dendrimers, and oligomers [1,2,10–13].
Optically active 1,2-diaminocyclohexane is one of the most
important C2 symmetric compounds. The chiral salen/salan-based
ligands have been extensively used in asymmetric catalysis due to
the potentially tetradentate N2O2 donor with metal ions [14–16].
Recently, these molecules are getting increasing attention in chiral
recognition area. Pu and his coworkers reported that the bisbi-
naphthyl macrocycles containing chiral diamine were useful for the
enantioselective fluorescence recognition for amino acid deriva-
tives and R-hydroxycarboxylic acids [11,17]. Banerjee and his
showed highly enantioselective recognition of mandelic acid [18].
Many fluorescence sensors for enantioselective recognition of
amines, amino alcohols and hydroxycarboxylic acids have been
reported, but most of them are based on chiral small molecules, and
fluorescent polymer-based sensors are very few [19–23]. Chiral
polymers used as fluorescence-based enantioselective sensors for
chiral molecule recognition offer several advantages over small
molecule sensors, such as fluorescence efficiency enhancement and
possible cooperative effects of multiple chiral units [19]. Moreover,
these fluorescent chiral polymers can be systematically modified by
the introduction of the functional groups based on steric and
electronic property at well-defined molecular level.
In this paper, we first reported the synthesis of two novel (R,R)-
salen/salan-based polymers P-1 and P-2 used as fluorescence
sensors for chiral discrimination of phenylglycinol. The (R,R)-salen
or salan moieties can orient a well-defined spatial arrangement in
the regular polymer backbone. Both P-1 and P-2 exhibited highly
chiral discrimination of phenylglycinol. The results also indicated
that the two chiral polymers P-1 and P-2 can be used for enan-
tiomer composition determination of phenylglycinol.
coworkers synthesized
a
chiral Schiff-base compound which
The synthesis procedures of chiral polymers P-1 and P-2 are
shown on Scheme 1. 2,5-dibutoxy-1,4-phenylene diboronic acid was
synthesized from hydroquinone according to literature [24–27]. The
monomer 2,5-dibutoxy-1,4-di(5-tert-butylsalicyclaldehyde)pheny-
lene could be obtained from 2-tert-butylphenol by a 3-step reaction
according to reported literature [28–30] and needed to be kept in the
* Corresponding author. Tel.: þ86 25 83685199; fax: þ86 25 83317761.
** Corresponding author.
E-mail addresses: yxcheng@nju.edu.cn (Y. Cheng), cjzhu@nju.edu.cn (C. Zhu).
0032-3861/$ – see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymer.2010.01.038

Y. Xu et al. / Polymer 51 (2010) 994–997
995
OBu-n
B(OH)2
OH
OH
(HO)2B
t-Bu
HO
OHC
OBu-n
Bu-t
OH
CHO
OHC
Bu-t
Bu-t
OBu-n
(1) CHCl3, NaOH
(2) Br2
Pd(PPh)4, Na2CO3
n-BuO
Br
N
OBu-n
N
OBu-n
NH HN
OH HO
H2N
NH2
NaBH4
THF
OH HO
Bu-t t-Bu
CHCl3
n
n
n-BuO
n-BuO
Bu-t
t-Bu
Host molecule P-1
Host molecule P-2
NH2
OH
NH2
OH
Guest molecule (R)-phenylglycinol
Guest molecule (S)-phenylglycinol
Scheme 1. Synthesis procedures of chiral polymers P-1 and P-2.
dark at ꢀ4 ꢁC before using. Chiral polymer P-1 incorporating
(R,R)-salen-type unit was synthesized by the polymerization of
(R,R)-1,2-diaminocyclohexane with the monomer 2,5-dibutoxy-1,4-
di(5-tert-butylsalicyclaldehyde)phenylene via nucleophilic addi-
tion–elimination reaction, and chiral polymer P-2 could be obtained
by reduction of P-1 with NaBH4. The GPC results of two chiral
polymers show moderate molecular weight (Table 1). The two
polymers 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 n-butoxy substitutents. The fluorescence response
behavior of two chiral polymers P-1 and P-2 on (R)- or (S)-phenyl-
glycinol have been investigated by fluorescence spectra. Fig. 1 shows
the fluorescence spectra of the chiral polymers P-1 and P-2
(1.0 ꢂ 10ꢀ5 mol Lꢀ1 corresponding to salen-based or salan-based
unit in CHCl3 solution) upon addition of (R)- or (S)-phenylglycinol
(0.1 mol Lꢀ1 in CHCl3) at 1:800 M ratio. Remarkable differences in
fluorescence enhancement were observed as demonstrated in Fig. 1,
(R)-phenylglycinol has little effect on the fluorescence of P-1 or P-2.
But (S)-phenylglycinol causes a large increase in the fluorescence
intensity of P-1 or P-2 under the same conditions. As shown in Fig. 1,
highly enantioselective fluorescence differences were observed as
expected when P-1 and P-2 were treated with (R)- or (S)-phenyl-
glycinol, respectively. Herein, remarkable fluorescence differences
indicate that (R)-phenylglycinol caused little change on the fluo-
rescence enhancement of P-1 and P-2, on the contrary, P-1 and P-2
can show more pronounced fluorescence response for (S)-phenyl-
glycinol under the same conditions. The selective recognition effect
on the guest of the chiral molecular isomers is related to the enan-
tiomeric fluorescence difference ratio, ef [ef ¼ (IS ꢀ I0)/(IR ꢀ I0)]. I0
represents the fluorescence emission intensity in the absence of
the chiral substrate, IS and IR are the fluorescence intensities in the
presence of (S)-substrate and (R)- substrate, respectively [31].
The values of ef are 1.84 and 2.05 for P-1 and P-2, which indicates
that P-1 and P-2 can exhibit highly enantioselective response toward
(S)-phenylglycinol, and P-2 incorporating (R,R)-salan-type receptors
shows more sensitive effect than P-1 incorporating (R,R)-salen-type
receptors. The reason may be attributed to an inherent chiral
recognition based on the steric repulsion of (R,R)-salen or salan
precursor for S-phenylglycinol. The building block of (R,R)-salen or
salan receptor can well fit for the formation of a more stable complex
of R–S complex as compared to the R–R diastereomeric complex. In
addition, the interaction of P-1 and P-2 with phenylglycinol was
studied at a much broader concentration range of the substrate. In
regard to the fluorescence signal changes of the chiral polymers P-1
and P-2 on (R)- or (S)-phenylglycinol, the fluorescence intensities of
both P-1 and P-2 appear the obvious gradual enhancement upon the
addition of (R)- and (S)-phenylglycinol from the molar ratios of 1:50
to 1:800. It can also be found that the addition curves of both P-1 and
P-2 keep nearly linear correlation with the molar ratio of (R)- and
(S)-phenylglycinol (Fig. 2). The obvious fluorescence enhancement
can be attributed to suppressed PET (photoinduced-electron-trans-
fer) quenching [32–35] when the protons of phenylglycinol interacts
with the nitrogen atoms of (R,R)-salen/salan-based moieties in the
chiral polymer main chain. On complexation, the lone pair of elec-
trons on the nitrogen atom is no longer available for PET, leading to
the fluorescence enhancement. In the same way, salen-based poly-
mer P-1 should show the weaker H-bonding interaction with phe-
nylglycinol than salan-based polymer P-2, and lead to the reduced
enantioselective fluorescence response due to the p-p conjugation of
the lone pair electrons of the nitrogen atoms in salen moieties. In
a set of comparable experiments, we also studied the interaction of
these two polymers P-1 and P-2 with (R)-/(S)-2-amino-1-propanol
and (R)-/(S)-mandelic acid, but no enantioselective fluorescence
responses were observed.
Table 1
Polymerization results and characterization of P-1 and P-2.
a
a
b
In this paper, we further investigated the fluorescence response
of the chiral polymers P-1 and P-2 on different enantiomeric
compositions of phenylglycinol. The fluorescence intensities of
both P-1 and P-2 based on various molar ratios of (R)- and
(S)-phenylglycinol revealed a fair linear relationship between I/I0
and the percent of the (S)-phenylglycinol component (Fig. 3). This
Yield (%)
Mw
Mn
PDI
[a]
D
P-1
P-2
72.4
85.3
9860
13,530
4100
5880
2.4
2.3
þ220.0
þ63.5
a
Mw, Mn and PDI of P-1 and P-2 were determined by gel permeation chroma-
tography using polystyrene standards in THF.
b
Temperature at 25 ꢁC and solvent in CHCl3.

996
Y. Xu et al. / Polymer 51 (2010) 994–997
1000
800
600
400
200
0
P-2
b
P-1
a
1000
P-2+(R)-phenylglycinol
P-2+(S)-phenylglycinol
P-1+(R)-phenylglycinol
P-1+(S)-phenylglycinol
800
600
400
200
0
400
450
500
550
600
400
450
500
550
600
Wavelength(nm)
Wavelength(nm)
Fig. 1. Fluorescence spectra of P-1 (a) and P-2 (b) (1.0 ꢂ 10ꢀ5 mol Lꢀ1 corresponding to salen-based or salan-based unit) both with and without (R)- and (S)-phenylglycinol at
1:800 M ratio (lex ¼ 367 nm) in CHCl3.
3.2
3.0
2.8
2.6
2.4
2.2
2.0
1.8
1.6
1.4
1.2
1.0
3.2
3.0
2.8
2.6
2.4
2.2
2.0
1.8
1.6
1.4
1.2
1.0
a
b
P-2+(S)-phenylglycinol
P-1+(S)-phenylglycinol
P-1+(R)-phenylglycinol
P-2+(R)-phenylglycinol
0
10 20 30 40 50 60 70 80 90
0
10 20 30 40 50 60 70 80 90
-4
-4
[phenylglycinol]x10 M
[phenylglycinol]x10 M
Fig. 2. Fluorescence enhancement of P-1 (a) and P-2 (b) (1.0 ꢂ 10ꢀ5 mol Lꢀ1 in CHCl3) vs molar ratios of (R)- and (S)-phenylglycinol from 1:50 to 1:800.
3.2
3.0
2.8
2.6
2.4
2.2
2.0
1.8
3.2
3.0
2.8
2.6
2.4
2.2
2.0
1.8
b
a
P-2 + phenylglycinol
P-1 + phenylglycinol
0
20
40
60
80
100
0
20
40
60
80
100
(S)-Enantiomer (%)
(S)-Enantiomer (%)
Fig. 3. Fluorescence enhancement of P-1 (a) and P-2 (b) (1.0 ꢂ 10ꢀ5 mol Lꢀ1) vs the enantiomeric composition of phenylglycinol (8.0 ꢂ 10ꢀ3 mol Lꢀ1) in CHCl3.
indicated that the enantioselective fluorescence sensors P-1 and
P-2 can be effectively applied for enantiomer composition deter-
mination of phenylglycinol.
In summary, two fluorescence-based polymers P-1 and P-2
incorporating (R,R)-salen and salan moieties as chiral receptors can
exhibit as excellent fluorescence sensor for enantioselective recog-
nition of (S)-phenylglycinol, and can also be used in ascertaining the
enantiomeric composition of (R)- and (S)-phenylglycinol.
Appendix. Supplementary data
The supplementary data associated with this article can be
found, in the online version at doi:10.1016/j.polymer.2010.01.038.
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