The amplified circularly polarized luminescence emission response of chiral 1,1′-binaphthol-based polymers via Zn(II)-coordination fluorescence enhancement

Article abstract of DOI:10.1002/pola.29009

Two kinds of chiral 1,1′-binaphthol (BINOL)-based polymer enantiomers were designed and synthesized by the polymerization of 5,5′-((2,2′-bis (octyloxy)-[1,1′-binaphthalene]-3,3′-diyl)bis(ethyne-2,1-diyl))bis(2-hydroxybenzaldehyde) (M1) with alkyl diamine (M2) via nucleophilic addition–elimination reaction. The resulting chiral polymers can exhibit mirror image cotton effects either in the absence or in the presence of Zn²⁺ ion. Almost no fluorescence or circularly polarized luminescence (CPL) emission could be observed for two chiral BINOL-based polymer enantiomers in the absence of Zn²⁺. Interestingly, the chiral polymers can show strong fluorescence and CPL response signals upon the addition of Zn²⁺, which can be attributed to Zn²⁺-coordination fluorescence enhancement effect. This work can develop a new strategy on the design of the novel CPL materials via metal-coordination reaction.

Full text of DOI:10.1002/pola.29009

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The Amplified Circularly Polarized Luminescence Emission Response
of Chiral 1,10-Binaphthol-Based Polymers via Zn(II)-Coordination
Fluorescence Enhancement
Fandian Meng,1 Fei Li,2 Lan Yang,2 Yuxiang Wang,2 Yiwu Quan,1 Yixiang Cheng2
1Department of Polymer Science and Engineering, State Key Laboratory of Coordination Chemistry, School of Chemistry and
Chemical Engineering, Nanjing University, Nanjing, 210093, China
2Key Laboratory of Mesoscopic Chemistry of MOE, Collaborative Innovation Center of Chemistry for Life Sciences, School of
Chemistry and Chemical Engineering, Nanjing University, Nanjing, 210093, China
Correspondence to: Y. Quan (E-mail: quanyiwu@nju.eu.cn) or Y. Cheng (E-mail: yxcheng@nju.edu.cn)
Received 22 January 2018; accepted 7 March 2018; published online 00 Month 2018
DOI: 10.1002/pola.29009
ABSTRACT: Two kinds of chiral 1,10-binaphthol (BINOL)-based
polymer enantiomers were designed and synthesized by the
polymerization of 5,50-((2,20-bis (octyloxy)-[1,10-binaphthalene]-
3,30-diyl)bis(ethyne-2,1-diyl))bis(2-hydroxybenzaldehyde) (M1) with
alkyl diamine (M2) via nucleophilic addition–elimination reaction.
The resulting chiral polymers can exhibit mirror image cotton
effects either in the absence or in the presence of Zn21 ion. Almost
no fluorescence or circularly polarized luminescence (CPL) emis-
sion could be observed for two chiral BINOL-based polymer enan-
tiomers in the absence of Zn21. Interestingly, the chiral polymers
can show strong fluorescence and CPL response signals upon the
addition of Zn21, which can be attributed to Zn21-coordination
fluorescence enhancement effect. This work can develop a new
strategy on the design of the novel CPL materials via metal-
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coordination reaction. 2018 Wiley Periodicals, Inc. J. Polym. Sci.,
Part A: Polym. Chem. 2018, 00, 000–000
KEYWORDS: 1,10-binaphthol; circularly polarized luminescence;
coordination; fluorescence enhancement; polymer
right-handed CPL. Both circular dichroism (CD) and CPL signal
appeared reversable when the temperature was slightly modi-
fied, which is due to the Grandjean texture phase change
induced by the thermotropic N*-LC cell from the temperature
change.
INTRODUCTION Normally, circularly polarized luminescence
(CPL) can be regarded as the differential emission direction of
right- and left-handed circularly polarized light.1 Various CPL
materials with high dissymmetry factors (glum) have been suc-
cessfully used as an effective tool to investigate and characterize
the chiral features of natural and synthetic molecules.2 In the
past decade, many strategies have been developed to design
CPL emission materials based on chiral organic chromophores.3
In the recent years, the design on excellent CPL materials has
been mainly focused on chiral conjugated fluorescence poly-
mers,4 metal-containing chiral complexes,5 supramolecular self-
assembly,6 chiral emission nematic liquid crystal,7 and so on.
Liu and coworkers6(f) reported a chiral gelator with weak CPL
signal via supramolecular self-assemble, but the amplified CPL
emission could be detected from blue to green color change
when this host gelator co-assembled into nanohelix with the
achrial guest BPEA chromophore due to intremolecular chirality
and energy transfer. Akagi and coworkers’ group7(a) reported a
chiral disubstituted liquid-crystalline polyacetylene (di-LCPA),
which can be dynamically switched and amplified from left- to
Chiral 1,10-binaphthol (BINOL) and its derivatives, as one of
the most important C2-symmetric compounds, can not only
exhibit the efficient and stable chiral configuration but also
high chiral induction and chiral discrimination.8 In the past
few years, much attention has been paid to the BINOL-based
dyes as CPL emission materials.9 Kawai and coworker10
reported a pair of bichromophoric perylene bisimide enan-
tiomers containing chiral binanphthane diamine unit with
weak CPL signal in dilute CHCl3 solution and the enhanced
CPL emission response behavior in the aggregation state due
to the effective integration of individual bichromophoric moi-
eties into helical nanofibers. Recently, Zhang et al. also found
strong CPL emission response signals of chiral BINOL-based
polymers induced by the effective chirality transfer from the
F. Meng and F. Li contributed equally to this article.
Additional Supporting Information may be found in the online version of this article.
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2018 Wiley Periodicals, Inc.
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chiral binaphthyl moiety to the conjugated polymer chain
NaHSO3 saturated solution. After removal of the solvent under
reduced pressure, the residue was extracted with ether (2 3
50 mL), and the combined organic layers were washed with
water. The residues were purified by silica gel column chroma-
tography, yielding light yellow powder in 44% yield.
backbone via the rigid p-conjugation bond linker.11
Salen, as a particular chelating Schiff base, can be synthesized
by the condensation of salicylaldehydes or salicylaldehyde
derivatives with 1,2-diamines. Salen-based ligands can form
stable complexes with various metal ions due to the strong
tetradentate N2O2 donor.12 Xu et al.13 reported Salen-based
polymer as fluorescent chemosensor which could exhibit
highly sensitive and selective enhancement response on Zn21
recognition. Recently, Nishihara and coworkers observed the
amplified CPL emission signals from bis(dipyrrinato)zinc(II)
complex chiroptical wires which could be exfoliated into single
strands, and glum could greatly increase as high as 5.9 times
larger than that of the corresponding monomer complex.9(b) In
this work, we found the strong CPL emission response behav-
ior of chiral BINOL-based polymers promoted by fluorescence
enhancement effect via Salen-Zn(II) coordination reaction.
This kind of in situ metal-coordination reaction can supply a
new design strategy on the novel CPL emission materials.
1H NMR (400 MHz, CDCl3): 8.54 (s, 2H), 7.78 (d, J 5 8.4 Hz,
2H), 7.16–7.45 (m, 6H), 4.81 (d, J 5 5.7 Hz, 2H), 4.70 (d, J 5 5.7
Hz, 2H,), 2.60 (s, 6H).
Synthesis of Compound R-2/S-2
R/S-3,30-diiodo-[1,10-binaphthalene]-2,20-diol (1.00 g, 1.86 mmol),
K2CO3 (0.77 g, 5.57 mmol), and 1-bromooctane (0.90 g, 4.65
mmol) were dissolved in 20 mL acetonitrile. After the reaction
mixture was refluxed overnight, the solvent was removed under
reduced pressure. The residue was purified by column chroma-
tography on silica gel (petroleum ether/ethyl acetate, 100/1, v/v)
to get R-2/S-2 as a yellow oil (1.20 g, 84.5%).
1H NMR (400 MHz, CDCl3): d 8.53 (s, 2H), 7.80 (d, J 5 6.0
Hz, 2H), 7.42(t, J 5 6.0 Hz, 2H), 7.28 (d, J 5 6.0 Hz, 2H), 7.15
(d, J 5 6.0 Hz, 2H), 3.89–3.82 (m, 2H), 3.38–3.80 (m, 2H),
1.37–0.57 (m, 30H); 13C NMR (100 MHz, CDCl3): d 154.1
139.6 134.0, 132.1, 126.9, 125.9, 125.7, 125.5, 93.2, 73.6,
31.8, 29.7, 29.2, 29.0, 25.5, 22.8, 14.3.
EXPERIMENTAL
Measures and Materials
All solvents and reagents were commercially available and
analytical reagent grade. THF was distilled from sodium. NMR
spectra were obtained from a Bruker 400 spectrometer at 400
MHz for 1H NMR and reported as parts per million (ppm) from
the internal standard TMS. UV-vis spectra were recorded on a
PerkinElmer Lambda 35 UV/VIS spectrometer. Fluorescence
spectra were obtained from an RF-5301PC spectrometer. FT-IR
spectra were obtained from a TENSOR27 FT-IR Spectrometer.
CD was recorded on a JASCO J-810 spectropolarimeter.
Thermogravimetric analyses (TGA) were performed using a
PerkinElmer Pyris-1 instrument under an N2 atmosphere.
Molecular weight was determined by gel permeation chroma-
tography (GPC) with a Waters 244 HPLC pump, and THF was
used as a solvent relative to polystyrene standards. CPL spec-
tra were obtained on a JASCO CPL-300 Spectrofluoropolarime-
ter. All optical measurements were investigated in THF with a
fixed concentration (1.0 3 1025 mol L21 corresponding to the
BINOL moiety). The magnitude of circular polarization in the
excited state is defined as glum 5 2(IL 2 IR)/(IL 1 IR), where IL
and IR indicate the output signals for left and right circularly
polarized light, respectively. Experimentally, the value of
glum is defined as 2DI/I 5 2[ellipticity/(32980/ln 10)]/
(unpolarized PL intensity) at the CPL extremum.
Synthesis of Compound R-M1/S-M1
R-2/S-2 (3.0 g, 4 mmol), 3 (1.31 g, 9 mmol), Pd(PPh3)2Cl2
(60 mg, 0.08 mmol, 2 mol %), and CuI (16 mg, 0.08 mmol,2
mol %) were dissolved in anhydrous THF (30 mL) and Et3N
(30 mL) under N2 atmosphere. After stirring at 80 8C for
36 h, the mixture were cooled to room temperature and
filtered. The filtrate was collected and the solvent was
removed under reduced pressure. The residue was purified
by chromatography on silica gel (petroleum ether/ethyl
acetate, 25/1, v/v) to get R-M1/S-M1 as a yellow oil (0.88 g,
27.5%).
1H NMR (400 MHz, CDCl3) d 11.13 (s, 1H), 9.90 (s, 2H), 8.18
(s, 2H), 7.86 (d, J 5 8.1 Hz, 2H), 7.78 (d, J 5 2.0 Hz, 2H), 7.70
(dd, J 5 8.7, 2.1 Hz, 2H), 7.46–7.36 (m, 2H), 7.33–7.24 (m, 2H),
7.19 (d, J 5 8.4 Hz, 2H), 4.08 (dt, J 5 8.9, 6.2 Hz, 2H), 3.72 (dt,
J 5 8.9, 6.3 Hz, 2H), 1.40–0.77 (m, 30H). 13C NMR (100 MHz,
CDCl3): d 196.1, 161.5, 155.1, 139.7, 136.6, 134.0, 133.8, 133.1,
127.8, 127.2, 125.9, 125.3, 125.2, 120.6, 117.5, 115.5, 91.5,
86.4, 73.9, 31.8, 30.1, 29.2, 29.1, 25.9, 25.6, 22.7, 14.1.
Synthesis of Polymer R-P1/S-P1
A Mixture of R-M1/S-M1 (160 mg, 0.2 mmol) and ethylene-
diamine (12.0 mg, 0.2 mmol dissolved in toluene) was dis-
solved in 6 mL of toluene. The solution was stirred at 95 8C
for 36 h. After removing the solvent under vacuum, a small
amount of dichloromethane was added, then dropped into
100 mL of methanol to precipitate the yellow solid R-P1/
S-P1. The resolution R-P1/S-P1 was filtered and washed
with methanol and dried in the yield of 63%.
Synthesis of Compound R-1/S-1
2,20-Bis(methoxymethoxy)-1,10-binaphthalene (4.28 g, 11.4
mmol) was dissolved in 100 mL of anhydrous Et2O, 16 mL of
n-BuLi (2.5 mol L21 in hexane, 40 mmol) was added by syringe
injection at room temperature under N2 atmosphere. The solu-
tion was stirred for 2 h at room temperature, and then the
solution of iodine (12 g, 44.7 mmol) was slowly injected to the
mixed solution at 0 8C under N2 atmosphere. The mixture was
then stirred overnight while the temperature was gradually
warmed to room temperature. The reaction was quenched by
1H NMR (400 MHz, CDCl3): d 8.45–8.28 (m, 2H), 8.20–8.09
(m, 2H), 7.83 (d, J 5 8.0 Hz, 2H), 7.57–7.33 (m, 6H), 7.30–7.10
2
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SCHEME 1 Synthesis procedures of R-M1/S-M1 and two chiral polymers R-P1/S-P1 and R-P2/S-P2.
(m, 6H), 6.98–6.88 (m, 2H), 4.18–4.02 (m, 2H), 4.04–3.90
(s, 4H), 3.73 (tt, J 5 14.4, 7.2 Hz, 2H), 1.40–0.52 (m, 30H). GPC:
Mw 5 15,570, Mn 510,960, PDI 5 1.42. [a]D20 5 2292.0
(c 5 1.0 mg mL21, THF) for R-P1. IR(KBr): 3612(w), 3525(w),
3438(w), 3330(w), 2922(m), 2850(w), 2206(w), 1633(s,
C@N), 1488(vs), 1286(m), 1222(w), 1124(m), 889(m),
825(m), 750(m), 464(m).
ethylenediamine or butanediamine (M2) with chiral monomers
R-M1/S-M1 at about 65% yield. In all the H NMR spectra of
1
P1-P2 (Supporting Information † 1), the peak at 9.90 and 11.13
ppm from salicylaldehyde group almost disappeared, and a new
peak at 8.30–8.50 ppm from N@CAH of Schiff base appeared,
which could demonstrate the successful polymerization of the
target monomers. Both two polymers were soluble in common
organic solvents (THF, DCM, DMF, acetone, and toluene) due to
the flexible n-octyl group substituent in the polymer side chain.
The TGA curves reveal that the degradation temperature (Td) of
5% weight loss of all these polymers was above 370 8C, indicat-
ing that the polymers can provide considerable thermal prop-
erty for practical applications for optical materials.
Synthesis of Polymer R-P2/S-P2
A Mixture of R-M1/S-M1 (160 mg, 0.2 mmol) and 1,4-butanedi-
amine (17.6 mg, 0.2 mmol dissolved in toluene) was dissolved
in 6 mL of toluene. The solution was stirred at 95 8C for 36 h.
After removing the solvent under vacuum, a small amount of
dichloromethane was added, then dropped into 100 mL of
methanol to precipitate the yellow solid R-P2/S-P2. The resolu-
tion R-P2/S-P2 was filtered and washed with methanol and
dried in the yield of 67%.
UV-Vis and Fluorescence Behaviors of Polymers
In this article, we first investigated their UV-vis absorption and
fluorescence emission in THF at the fixed concentration (1.0 3
1025 mol L21 corresponding to the BINOL moiety). UV-vis spec-
tra of the polymers have similar absorption two bands situated
at about 320 and 336 nm in THF [Fig. 1(a)[. As shown in Figure
1(b) and Supporting Information Figure S6, the orginal absor-
bance two peaks of P1 disappear upon addition of Zn21 up to 1
equiv, and a new peak appears at 337 nm with a little reduction
and red shift. On the other hand, the shoulder absorption peak
at 370 nm of the long wavelength region shows obvious
enhancement at 1:1 molar ratio of Zn21, which can be regarded
as the conjugated structure between the binaphthyl moiety and
Salen-Zn(II) complex. As further increase in Zn21 molar ratio up
to 4.0 equiv, almost no change in UV-vis absorption can be
observed, which indicates the formation of 1:1 polymer-Zn(II)
complex. Meanwhile, P2 has similar change feature with P1
(Supporting Information Fig. S6).
1H NMR (400 MHz, CDCl3) d 8.31 (d, 2H), 8.16 (d, 2H), 7.81
(t, 2H), 7.59–7.42 (m, 4H), 7.38 (t, 2H), 7.31–7.20 (m, 4H), 7.17
(d, 2H), 7.03–6.87 (m, 2H), 4.08 (m, 4H), 3.84–3.55 (m, 6H),
1.83 (s, 4H), 1.40–0.51 (m, 28H). GPC: Mw 511,220, Mn 58090,
PDI 5 1.38. [a]D20 5 2310.0 (c 5 1.0 mg mL21, THF) for R-P2.
IR(KBr): 3525(w), 3442(w), 2929(m), 2850(w), 2407(w),
2206(w), 1633(s, C@N), 1488(vs), 1286(m), 1222(m), 1126(m),
887(m), 827(m), 750(m), 460(m).
RESULTS AND DISCUSSION
The synthesis procedures of monomers and chiral polymers are
outlined in Scheme 1. The compounds 1-–3 and chiral mono-
mers R-M1/S-M1 could be synthesized according to the
reported literatures.14 Two chiral BINOL-based polymer enan-
tiomers could be synthesized by nucleophilic addition-
elimination polymerization reaction of the linker alkyl diamine
Almost no fluorescent emissions could be detected for P1–
P2 in THF solution. Interestingly, obvious fluorescent signals
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the Schiff-base ligand since the lone pair of electrons from
ACH@NA group is no longer available for PET upon the for-
mation of Salen-Zn(II) complex. On the other hand, the for-
mation of Zn21-polymer can enhance the planarity and
rigidity of the conjugated segment of the chiral polymer,
which can prevent the free rotation of alkyl amine chain and
reduce the nonradiative decay of the excited state.13,15 Inter-
estingly, the fluorescence of 1:1 Zn21-coordinated polymer
complex almost turned off upon the addition of EDTA (Sup-
porting Information Fig. S7), which can be attributed to the
release of Zn21 ion from Zn21-coordinated polymer complex
since EDTA shows stronger binding ability for Zn21 than
Salen ligand.
CD and CPL Behaviors of Polymers
The CD spectra of polymers P1 and P2 were recorded using
a JASCO J-810 spectropolarimeter in THF solution at the
fixed concentration 1.0 3 1025 mol L21). As is evident from
Figure 3(a), both the resulting chiral polymers P1 in THF
solution can exhibit the same CD response signals as P2
FIGURE 1 (a) UV-vis spectra of chiral polymers P1 and P2 in the
absence of Zn21. (b) UV-vis spectra of the chiral polymers P1 in
THF (1.0 3 1025 mol L21) in the presence of Zn21 (from 0.1 to 4
equiv). [Color figure can be viewed at wileyonlinelibrary.com]
at about 466 nm can be observed for both two polymers P1
and P2 upon the addition of Zn21 as shown in Figure 2. As
further increase in Zn21 up to 1:1 molar ratio, the fluores-
cent intensity of Zn21-containing polymer complex shows
gradual enhancement as high as 6.71-fold for P1 and 8.94-
fold for P2 [Inset; Fig. 2(a,b)]. The Zn21-polymer solution
can emit bright blue fluorescence, which can be visually
observed by the naked eye (Inset; Figure 2(a,b), left: in the
absence of Zn21, right: in the presence of Zn21 of 1.0 equiv).
The fluorescence curve of the polymer sensor with Zn21
reveals that the fluorescent enhancement value (I/I0 2 1) can
exhibit the good linear relationship on the Zn21 molar ratio
versus chiral polymers from 0.1 to 1.0. Only a little increase
can be detected from 1.0 to 4.0 equiv, which further demon-
strates that both two chiral polymers P1 and P2 can form a
1:1 Zn21-coordinated polymer complex. The fluorescence
enhancement phenomenon can be attributed to two reasons.
On the one hand, the suppressed photoinduced electron
transfer (PET) effect can lead to fluorescence enhancement
response while Zn21 coordinates with the nitrogen atoms of
FIGURE 2 Fluorescence spectra of the chiral polymers P1 (a)
and P2 (b) in THF (1.0 3 1025 mol L21) in the presence of Zn21
(from 0.1 to 4.0 equiv) at the excitation wavelength of 365 nm.
[Color figure can be viewed at wileyonlinelibrary.com]
4
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FIGURE 3 (a) CD spectra of R-P1/S-P1 and R-P2/S-P2 in THF
solution (1.0 3 1025 mol L21) in the absence of Zn21. (b) CD
spectra of R-P1 in THF solution (1.0 3 1025 mol L21) in the
presence of Zn21 (from 0 to 2.0 equiv). [Color figure can be
viewed at wileyonlinelibrary.com]
with clear mirror image due to similar polymer structure.
The strong Cotton effects situated at 255 and 285 nm can be
assigned to chiral BINOL moiety. Meanwhile, the long wave-
length CD absorption peak appears at 335 nm, which can be
regarded as the extended conjugated structure between chi-
ral binaphthyl moiety and Salen group. Upon addition of
Zn21, the Cotton effect of R-P1 at 285 nm shows obvious
decrease, but a new peak appears at 365 nm of long wave-
length and gradually enhances as the increase of Zn21 molar
ratio up to 1:1 [Fig. 3(b); Supporting Information Fig. S8],
which can be attributed to the formation of the p-conjugated
structure system between the binaphthyl moiety and
Salen-Zn(II) complex and the effective chiral transfer from
the binaphthyl moiety to Salen-Zn(II) complex. The gabs
(365 nm) values can reach as high as 8.3 3 1024 for P1 and
FIGURE 4 (a) CPL spectra of R-P1/S-P1 and R-P2/S-P2 in THF
(1.0 3 1025 mol L21) in the presence of Zn21 (1 equiv), (b,c)
CPL spectra of R-P1 and R-P2 in THF (1.0 3 1025 mol L21) in
the presence of Zn21 (from 0.3 to 2.0 equiv). [Color figure can
be viewed at wileyonlinelibrary.com]
1.0 3 1023 for P2 at 1:1 molar ratio of Zn21
.
to further investigate their CPL behaviors. As the essential
parameter of CPL, the optical dissymmetry factors (glum) can
be obtained from glum 5 2(IL 2 IR)/(IL 1 IR), where IL and IR
are the emission intensities of left and right circularly polar-
ized luminescence, respectively. We found that almost no
In situ Zn(II)-containing chiral polymer complexes could
exhibit strong fluorescence enhancement emission and CD
absorption in the long wavelength region, which inspired us
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H. Sakai, S. Shinto, J. Kumar, Y. Arak, T. Sakanoue, T.
Takenobu, T. Wada, T. Kawai, T. Hasobe, J. Phys. Chem. C
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CPL emission signal could be detected for either P1 or P2 in
the absence of Zn21 and lower molar ratios at 1:0.1 or 1:0.2
due to very weak fluorescence emission. But we found
that the strong CPL emission with clear mirror image can be
detected as the increase of Zn21 molar ratio up to 1:0.3, and
CPL emission wavelengths at about 465 nm are similar with
that of their fluorescence. We assume that the CPL response
behavior of chiral BINOL-based polymers can be ampified by
fluorescence enhancement effect of Salen-Zn(II) coordination
reaction. The glum can reach as high as 8.5 3 1023 for P1
and 8.0 3 1023 for P2. No obvious glum enhancement can be
observed as the increase in Zn21 molar ratio from 0.3 to 4.0
equiv as shown in Figure 4(a,b). Meanwhile, we also found
that CPL emission signal almost disappears upon the addi-
tion of EDTA due to the degradation of Salen-Zn(II) complex
unit and fluorescence quenching. This research work could
provide a strategy to develop a higher glum value of CPL-
active polymer materials by metal-coordination fluorescence
enhancement.
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CONCLUSIONS
Two chiral BINOL-based polymer enantiomers incorporating
Salen moiety in the main chain backbone can exhibit “off-on”
CPL response signals only in the present of Zn21 ion, which
can be attributed to the fluorescence enhancement effect of
Salen-Zn(II) coordination reaction and the effective chiral
transfer from the binaphthyl moiety to Salen-Zn(II) complex.
This work can provide a new design strategy on the ampli-
fied CPL materials promoted by metal-coordination chiral
complexes.
7 (a) B. A. S. Jose, J. Yan, K. Akagi, Angew. Chem. 2014, 126,
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8 (a) J. F. Cui, M. Ko, K. P. Shing, J. R. Deng, N. C. H. Lai, M.
K. Wong, Angew. Chem. Int. Ed. 2017, 56, 3074; (b) G. Mirri, S.
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