Preparation and property manipulation of high efficiency circularly polarized luminescent liquid crystal polypeptides

Article abstract of DOI:10.1039/d0tc03276j

Circularly polarized luminescent (CPL) materials show great potential applications in asymmetric synthesis, chemical sensors, biological probes, 3D displays and other fields. However, the preparation of high efficiency circularly polarized luminescent materials with a high solid fluorescence quantum yield and a large glum value is still a challenge. This article creatively combines the construction strategy of CPL materials and side-chain chiral liquid crystal polymer materials to engineer a series of new CPL-active side-chain chiral liquid crystal polymers (named PGAC-m, m is the spacer length, m = 4, 6, 10). The chiral poly(l-glutamate) with an α-helical secondary structure is devised as the backbone, and the cyanostilbene with aggregation-induced emission enhancement (AIEE) characteristics is applied as luminescent mesogen pendants, which is covalently linked to the backbone by different spacer lengths. The phase structure and the photophysical properties of PGAC-m were characterized in detail. The experimental results show that PGAC-m can self-assemble to form a lamellar-ordered chiral smectic C (SmC?) phase structure and show typical AIEE characteristics with high solid-state quantum yield. The chirality of polypeptides can be transferred to cyanostilbene, which makes PGAC-m emit circularly polarized light. Meanwhile, the chiral helix structure of PGAC-m can effectively enlarge the glum value, and therefore PGAC-m shows high solid quantum yield and a large glum value. Interestingly, the photophysical properties of PGAC-m can be regulated by changing the spacer length. The solid fluorescence quantum yield of PGAC-m decreases gradually with the increase of spacer length, while the glum value shows the opposite trend. The maximum solid fluorescence quantum yield and the glum value of PGAC-m can be 34.1% and 4.5 × 10-2, respectively. This journal is

Full text of DOI:10.1039/d0tc03276j

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Materials Chemistry C
Materials for optical, magnetic and electronic devices
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ISSN 2050-7526
PAPER
Nosang V. Myung, Yong-Ho Choa et al.
A
noble gas sensor platform: linear dense assemblies of
a multi-layered
single-walled carbon nanotubes (LACNTs) in
ceramic/metal electrode system (MLES)
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Page 1 of 11
J Po lue ran sael od fo Mn aot te rai ad l jsu Cs ht emm ai rs gt ri yn sC
View Article Online
DOI: 10.1039/D0TC03276J
ARTICLE
Preparation and Property Manipulation of High Efficiency
Circularly Polarized Luminescent Liquid Crystal Polypeptides
Received 00th January 20xx,
Accepted 00th January 20xx
Youde Chen, Peng Lu, Yongjie Yuan* and Hailiang Zhang*
DOI: 10.1039/x0xx00000x
Circularly polarized luminescence (CPL) materials show great potential applications in asymmetric synthesis, chemical
sensors, biological probes, 3D displays and other fields. However, the preparation of high efficiency circularly polarized
luminescent materials with high solid fluorescence quantum yield and large glum value is still a challenge. This article
creatively combines the construction strategy of CPL materials and side-chain chiral liquid crystal polymer materials to
engineer a series of new CPL-active side-chain chiral liquid crystal polymers (named PGAC-m, m is the spacer length, m = 4,
6
, 10). The chiral poly (L-glutamate) with α-helical secondary structure is devised as the backbone, and the cyanostilbene
with aggregation-induced emission enhancement (AIEE) characteristics is applied as luminescent mesogen pendants,
which is covalently linked to backbone by different spacer length. The phase structure and the photophysical properties of
PGAC-m were characterized in detail. The experimental results show that PGAC-m can self-assemble to form lamellar-
ordered chiral smectic C (SmC*) phase structure and show typical AIEE characteristics with high solid-state quantum yield.
The chirality of polypeptides can be transferred to cyanostilbene, which makes PGAC-m emit circularly polarized light.
Meanwhile, the chiral helix structure of PGAC-m can effectively enlarge the glum value, and therefore PGAC-m show high
solid quantum yield and large glum value. Interestingly, the photophysical properties of PGAC-m can be regulated by
changing the spacer length. The solid fluorescence quantum yield of PGAC-m decrease gradually with the increase of
spacer length, while the glum value show the opposite change rule. The maximum solid fluorescence quantum yield and glum
-2
value of PGAC-m can achieve 34.1% and 4.5 × 10 , respectively.
order to promote real applications, the researchers are being
Introduction
Circularly polarized luminescence (CPL) refers to the
devoted to develop CPL materials with high fluorescence
quantum yield and large glum value.
phenomenon that chiral luminescent materials emit different
Recently, considerable focuses have been attracted into
1
0-12
left-handed and right-handed circularly polarized light. The organic CPL-active materials.
It is proved to be an effective
study for CPL materials is conducive to reveal the excited state strategy to integrate chiral structure into fluorescent
1
3
structure information of chiral luminescent materials, and look molecules to construct organic CPL-active materials. From
insight into the chirality generation, transfer and amplification the perspective of molecular weight, organic CPL-active
1
mechanism. Owing to the multifaceted promising applications, materials can be divided into small-molecule CPL-active
2
such as circularly polarized organic light-emitting diodes,
organic materials and polymer-based CPL-active organic
3
4
12
information security encryption, asymmetric synthesis, CPL materials. At present, small-molecule CPL organic materials
switch5 and CPL sensing and probing, etc, CPL materials have have been widely studied and have harvested many excellent
become one of the research hotspots in the luminescence field. achievements. Polymer-based CPL organic materials possess
The luminescence dissymmetry factor (glum) and fluorescence the inherent advantages of polymers and exhibit better
quantum yield are important parameters for characterization processing capacity, film-forming performance as well as
of circularly polarized luminescent materials. The glum value is thermal stability, which dramatically expand the practical
-8
9
1
1
L R L R L R
acquired by glum = 2 (I -I ) / (I + I ), where I and I
are left- applications range of CPL materials.
handed and right-handed circularly polarized light intensity. In
Doping is an effective method to engender polymer-based
organic CPL materials. The conventional approach is to embed
achiral luminescent polymers guest into a chiral-nematic LC
1
4
a. Key Laboratory of Polymeric Materials and Application Technology of Hunan
matrix or embed chiral dopants guest into achiral-nematic
Province, Key Laboratory of Advanced Functional Polymer Materials of Colleges luminescent polymers matrix.1
5-17
The strong CPL signals can
and Universities of Hunan Province, College of Chemistry, Xiangtan University,
be induced by intermolecular interactions between achiral
fluorescence polymers and chiral moieties. Compared with the
host-guest doped polymer-based CPL materials, the organic
CPL materials fabricated from non-doped chiral luminescent
Xiangtan 411105, Hunan Province, China. E-mail: hailiangzhang@xtu.edu.cn.
yuanyongjie@xtu.edu.cn.
†Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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ARTICLE
Journal Name
polymers are unnecessary to consider the troubles of stability
and compatibility existing in doping systems, and thus it also
acquires extensive researches. At present, the research on the
non-doped organic CPL polymers is mainly focused on the
View Article Online
DOI: 10.1039/D0TC03276J
1
8-22
chiral conjugated polymer.
The conjugated polymer can
Scheme 1 Chemical structures of PGAC-m.
inherently emit fluorescence, and the introduction of chiral
building blocks into the conjugated polymers provides an
effective avenue to generate CPL-active polymers. However,
the majority of traditional rigid conjugated backbones only
emit fluorescence in dilute solutions. The fluorescence
intensity is greatly reduced or even unable to emit
fluorescence when aggregate owing to aggregation-caused
quenching (ACQ) effect, which is greatly restrict its application.
Recently, relevant researches concerning CPL-active chiral
influence of spacer length on the aggregation structure and
photophysical properties of PGAC-m will be also elucidated.
This work unfolds a new insight to harvest non-doped CPL
polymer materials with high solid quantum yield and large glum
value, and also provides theoretical guidance for the
preparation of new high-performance circularly polarized
luminescent materials.
conjugated polymers with AIE core have been gradually
reported,2
3-27
which overcomes the luminescence quenching
Experimental section
defects of traditional chiral conjugated polymers in solid state.
In 2018, Tang and Li et al prepared a chiral conjugated polymer Materials
with the backbone composed of TPE and chiral alanine by
All
reagents
including
4-hydroxyacetonitrile,
4-
2
7
azido-alkynyl click polymerization. The obtained polymer
showed typical aggregation-induced CPL emission with a solid-
cyanobenzaldehyde,
L-Glutamic acid (
L-Glu), ethylenediamine
tetraacetic acid (EDTA), triphosgene, triethylamine (TEA),
tetramethyl guanidine, 1, 4-dibromobutane, 1, 6-
dibromotexaneand as well as 1, 10-dibromodecane were
purchased from Aladdin Reagent Co. and utilized without
further purification. Glu-Cu was prepared in our laboratory.
THF was refined via sodium metal to remove trace amounts of
water. DMF was dried with molecular sieve activated under
high temperature before use. NaHCO , NaOH, copper acetate
3
and the other solvents were commercially available from
Aladdin Reagent Co.
–3
state glum value of 4.5 × 10 . At present, most of the reported
g
1
lum value of chiral conjugated polymers is in the range about
0-5 ~ 10 . It remains challenging to construct CPL-active
-3
polymer materials with larger glum value and higher solid
fluorescence quantum yield.
Chiral liquid crystal is a kind of liquid crystal with spiral-
2
8
stacking phase structure, which endows the chiral liquid
crystal with unique photophysical properties, such as selective
2
9-30
reflection, optical rotation, circular dichroism, etc.
Meanwhile, the spiral structure formed by the self-assembly of
chiral liquid crystal can achieve chiral amplification effect.
Therefore, chiral liquid crystal is an ideal medium to generate
CPL-active materials with high glum value. Relevant research
results prove that chiral luminescent LC exhibits great
advantages in the construction of CPL-active materials with
Measurements and characterizations
Chemical structure and purity information of monomers
MNCA-m and polymers PGAC-m were obtained by a Bruker
ARX400 MHz spectrometer. The molecular weight of small
molecule compound was measured by a Bruker Biflex III
MALDI-TOF instrument. GPC 120 instrument was applied to
measure the number-average molecular weight (Mn) and the
molecular weight distribution (PDI) of PGAC-m using DMF as
solvent. The flow rate is 1.0 mL min- at 80 ℃ . Therom
Scientific Nicolet 6700 FT-IR instrument was employed to
monitor the conversion degree of monomers and preliminarily
determine the spatial configuration of the PGAC-m. The
decomposition temperature of PGAC-m were measured by a
high glum value. 5
-6, 16, 31-34
At present, there are only a few kinds
of non-doped CPL chiral liquid crystal polymers, which are low
in solid fluorescence quantum yield and difficult to regulate in
properties. Non-doped CPL chiral liquid crystal polymers with
high solid quantum yield and large glum is rarely reported.
1
Based on the above fact, this article creatively combines the
construction strategy of CPL-active materials and side-chain
chiral liquid crystal polymer materials, and takes cyanostilbene
with aggregation-induced emission enhancement (AIEE)
TGA Q50 under N
2
atmosphere with a scanning rate of 10 ℃
-1
min . The liquid crystal textures of PGAC-m were observed by
a Mettler FP82 polarizing microscope (POM) with a hot stage.
The phase transition temperature of polymers PGAC-m was
measured by a TA Q10 DSC with the scanning rate of 10 ℃
3
5-40
characteristics as the luminescent mesogen,
glutamate) with α-helical conformation
chiral poly (
L-
4
1-42
as the backbone to
prepare a novel CPL-active side-chain chiral liquid crystal
polymers PGAC-m (the chemical structure is shown in Scheme
-1
min during the heating and cooling process. The phase
1
, m is the spacer length). The chirality of polypeptides can be
structure of polymers PGAC-m was confirmed by an Anton
Paar SAXSess. The preparation process of PGAC-m test
samples are described as follows: PGAC-m were heated to
transferred to cyanostilbene, which makes PGAC-m emit
circularly polarized light. The self-assembly process of chiral
liquid crystal polymers is used to amplify the glum value. The above their clear point temperature with a heating rate of 10 ℃
AIEE property of cyanostilbene is used to avoid the min-1, and then cooled to 30℃ with a cooling rate of 5 ℃ min-1
fluorescence quenching of PGAC-m in solid state. Therefore, to obtain the test samples. The UV-Vis absorption spectra of
circularly polarized luminescent chiral liquid crystals with high polymers PGAC-m were recorded on
a
Cary 100
solid quantum yield and large glum will be obtained. The
2
| J. Name., 2012, 00, 1-3
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ARTICLE
spectrophotometer instrument. The circular dichroism (CD)
spectra were recorded by a JASCO J-810 spectrophotometer.
The fluorescence spectra of polymers PGAC-m were measured
by a PTI Qm40 fluorescence spectroscopy. Circularly polarized
luminescence (CPL) tests were carried out on a JASCO CPL-300
instrument. The fluorescence quantum yields of PGAC-m in
solid state were measured by FLS980 fluorescence
spectroscopy with integral sphere. Zetasizer nano ZS90 particle
size analyzer was applied to gauge the average particle size.
View Article Online
DOI: 10.1039/D0TC03276J
Preparation of monomers and polymers
The synthetic process of monomers MNCA-m and polymers
PGAC-m with different spacer length (m = 4, 6, and 10) are
shown in Fig.1. The specific synthetic methods and structural
characterizations are elaborated by choosing MNCA-4 and
PGAC-4 as representative. The synthetic methods of MNCA-4
and PGAC-4 are as follows.
Fig.1 Synthetic routes of MNCA-m and PGAC-m.
hours. Lots of blue powder were gradually precipitated out
and purified by suction filter. The filter cake was washed
sequentially with deionized water, ethanol and ether, and then
dried in vacuum to obtain the blue solid powder product (22 g).
Yield: 91%.
Synthesis of compound 1.
hydroxyphenylacetonitrile (20.0 g, 0.15 mol), K
.45 mol), catalytic amount of KI and 1, 4-dibromobutane (65.0
A
blend of 4-
2
CO (62.3 g,
3
0
g, 0.31 mol) were dissolved in 250 mL acetone. The mixtures
were heated to 70 ℃ and stirred for 10 hours. When the
reaction was almost complete, the insoluble solid was mmol), -Glu (1.4 g, 9.5 mmol), 1.5 mL deionized water and 8
Synthesis of compound 3. A mixture of Glu-Cu (2.3 g, 4.7
L
removed by suction filtration. The filtrate was evaporated by mL DMF were added into a 1000 mL round-bottomed flask.
rotary evaporator to get crude product. The crude product was Tetramethyl guanidine (2.5 mL) was slowly added and then the
further purified by column chromatography using petroleum mixtures were stirred at room temperature for 2 hours to
ether and CH
2
Cl
2
(v : v = 1 : 1) as eluent to get the target obtain a blue clear solution. Compound 2 (7.6 g, 20 mmol) was
product as a white solid (35.0 g). Yield: 87.5%. H NMR (δ, ppm, dissolved in 30 mL DMF and poured into the blue clear
1
CDCl
.94 (m, 2H, -OCH
CH Br); 1.92-1.76 (m, 4H, -CH
M] calcd for C12 14BrNO, 267.06; found 267.16.
3
): 7.23-7.21 (d, 2H, Ph-H); 6.90-6.87 (d, 2H, Ph-H); 3.97- solution. The mixture was heated at 40 ℃ for 48 hours in the
-); 3.68 (s, 2H, -CH CN); 3.45-3.41 (m, 2H, - dark. After that, 400 mL acetone was poured into the round-
-); Mass Spectrometry (MS) (m/z) bottomed flask and stirred for 30 minutes to fully dissolve
3
2
2
2
2
[
H
impurity. The insoluble was isolated via vacuum filter and
added into a newly prepared EDTA disodium salt solution (3.6
Synthesis of compound 2. A round-bottomed flask (500 mL)
were added with compound 1 (30.0 g, 0.12 mol), CH ONa (6.0
g, 0.11 mol) and 120 mL ethanol. 4-Cyanobenzaldehyde (14.6 g,
.11 mol) was dissolved in 120 mL ethanol and then handed
over into a 250 mL constant pressure drop funnel, which was
3 2
g EDTA, 2.1 g NaHCO , 30 mL H O), which was stirred for 4
3
hours to fully remove copper. Finally, a yellow-green solid was
obtained (4.5 g). Yield: 48%.
0
Synthesis of monomer MNCA-4. Compound 3 (2.0 g, 4.5
slowly added into the round-bottomed flask. After that, the mmol), triphosgene (0.9 g, 3.0 mmol) and 120 mL refined THF
mixtures were stirred at room temperature for 3 hours. were added into a 500 mL round-bottomed flask. The mixture
Abundant yellow solid powder precipitated out, and the was heated at 50℃ for 24 hours. With the process of reaction,
precipitate was separated by suction filter and washed three the reaction system gradually turned to a clear and
times with ethanol. The precipitate was collected and dried in transparent solution. After cooling, the mixture were slowly
vacuum to gain the pure product with pale yellow solid (31.5 g). added into the dry n-hexane to give a yellow solid, which was
Yield: 69%. 1 H NMR (δ, ppm, CDCl
.75-7.73 (d, 2H, Ph-H); 7.64-7.62 (d, 2H, Ph-H); 7.42 (s, 1H, - by column chromatography to give pure monomer MNCA-4 as
3
): 7.95-7.93 (d, 2H, Ph-H); then dissolved in a small amount of THF and further purified
7
1
CH=); 6.98-6.95 (d, 2H, Ph-H); 4.05-4.02 (m, 2H, -OCH
2
3
-); 3.51- a yellow solid (1.3 g). Yield: 65%. H NMR (δ, ppm, CDCl ): 7.98-
3
.49 (m, 2H, -CH -Br); 2.09-1.98 (m, 4H, -CH -); Mass 7.95 (d, 2H, Ph-H); 7.81-7.79 (d, 2H, Ph-H); 7.66-7.63 (d, 2H,
2
2
2
Spectrometry (MS) (m/z) [M] calcd for C20H17BrN O, 380.05; Ph-H); 7.46 (s, 1H, -CH=); 6.99-6.95 (d, 2H, Ph-H); 6.45 (s, 1H, -
found 380.16.
NH-); 4.41-4.38 (t, 1H, -CHNH-); 4.23-4.21 (m, 2H, -OCH -);
2
4
(
(
.23-4.21 (m, 2H, -OCH
m, 2H, -CH -); 1.94-1.90 (m, 4H, -CH
MS) (m/z) [M] calcd for C26 , 473.18; found 473.13.
2
-); 2.58-2.54 (m, 2H, -CH
2
-); 2.27-2.12
Synthesis of Glu-Cu. A mixture of -Glu (14.7 g, 100 mmol)
L
2
2
-); Mass Spectrometry
and 375 mL deionized water were added into a 1000 mL
round-bottomed flask. The mixtures were stirred at 70℃ for 2
hours to obtain an uniform clear solution. Copper acetate
23 3 6
H N O
Synthesis of PGAC-4. The polymer PGAC-4 was obtained by
(18.6 g, 103.0 mmol) was dissolved in a beaker containing 375 ring opening polymerization of N-Carboxyanhydride (NCA)
mL deionized water, and then was slowly added into the
above-mentioned 1000 mL round-bottomed flask. After that,
the reaction mixtures were stirred at room temperature for 48
using TEA as the initiator. The specific operation steps were
described as follows: MNCA-4 (0.5 g, 1.2 mmol), refined DMF
This journal is © The Royal Society of Chemistry 20xx
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4 mL) and TEA (1.3 mg, 0.012 mmol) were successively added
Journal Name
(
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DOI: 10.1039/D0TC03276J
into a glass polymerization tube. The polymerization tube was
frozen by liquid nitrogen and then was vacuumed for 5 min to
remove oxygen. After that, nitrogen was injected. After three
cycles, the polymerization tube was sealed and reacted at 30°C
for 24 hours with vigorously stirring. When the reaction was
completed, the glass polymerization tube was opened and the
reaction mixtures were slowly added into 300 mL methanol
through a disposable syringe. The yellow powder solid
precipitate was collected and dried in vacuum to gain the
desired polymer PGAC-4 with a yield of 71%. The similar
method was adopted to prepare PGAC-6 and PGAC-10
according to preparation route elaborated in Fig.1.
Results and discussion
Structural characterization of MNCA-m and PGAC-m
1
Fig.2 (a) The H NMR spectrum of MNCA-4 in CDCl
MNCA-4 in CDCl
; (c) The 13C NMR of MNCA-4 in DMSO-d
spectrum of PGAC-4 in DMSO-d
3
; (b) The mass spectrum of
Monomers MNCA-m and polymers PGAC-m were obtained
according to the synthetic route showed in Fig.1. The chemical
; (d) The 1H NMR
3
6
6
.
1
structures of MNCA-m were confirmed by H NMR, mass
spectrum and 13C NMR. Fig.2 (a) displays the H NMR spectrum
of MNCA-4. The spectrum showed that all hydrogen atoms in
the molecular structure can be found the corresponding
chemical shift peaks. Moreover, the area ratio of the peaks is
basically consistent with the number ratio of the
corresponding hydrogen atoms, which indicated that the pure
MNCA-4 is obtained. Mass spectrum (Fig.2 (b)) and 13C NMR
1
(
Fig.2 (c)) also confirmed the purity of MNCA-4. Fig.2 (d) shows
1
the H NMR spectrum of PGAC-4. Compared with MNCA-4, the
corresponding proton peaks of PGAC-4 became wider due to
the decreased proton activity after polymerization. Besides,
the chemical shift peak of -NH on the N-carbonyl anhydride of
MNCA-4 is found to have thoroughly disappeared, indicating
that TEA successfully initiates NCA ring opening polymerization
of MNCA-4. The chemical structure of the other monomers
Fig.3 FT-IR spectra of MNCA-4 and PGAC-4 in solid state.
GPC instrument was applied to measure the M and PDI of
n
and polymers were also confirmed by similar methods, and the PGAC-m. The results are elaborated in Table 1 and
characterization results are shown in Fig. S1.
n
corresponding GPC curves are shown in Fig.S3. The M of all
polymers is more than 3.5 × 10 g mol to reduce the influence
4
-1
FT-IR spectrometer was applied to monitor whether the
conversion of MNCA-4 is complete after the polymerization,
and confirm the secondary structure of PGAC-m. Fig.3 shows
the FT-IR spectra of MGAC-4 and PGAC-4. After polymerization,
the stretching vibration peaks of -C=O on N-carbonyl
n
of M on the phase transition temperature and phase
structure. The GPC curves are symmetrical single peak with
narrow distribution, indicating that the target polymers have
been successfully prepared.
-1
-1
anhydride located at 1850 cm and 1780 cm are completely
disappeared. The stretching vibration peak (amide I) located at
Table 1 GPC, TGA and DSC test results of PGAC-m
Polymers
M
n
(×104)a
PDIa
T
i
(℃)b
Tg (℃)b
127
T
d
(℃)c
657 cm-1 and the deformation vibration peak (amide II)
1
-1
located at 1548 cm belong to polymer PGAC-4, indicating that
MNCA-4 have been unreservedly converted into PGAC-4. In
addition, it can be concluded that PGAC-4 present thoroughly
α-helical conformation in solid state according to the position
of the amide I and amide II.43 The FT-IR spectra of PGAC-6 and
PGAC-10 are exhibited in Fig.S2. The results indicate that
PGAC-6 and PGAC-10 also exhibit α-helical conformation in
solid state.
PGAC-4
PGAC-6
PGAC-10
3.7
4.7
5.2
1.20
1.19
1.28
143
140
137
341
121
331
349
104
a
b
C
Obtained from the GPC instrument.
Measured by DSC instrument.
Obtained by TGA Q50 instrument.
4
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polymers showed obvious peaks. For PGAC-4 and PGAC-6, the
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DOI: 10.1039/D0TC03276J
scattering vectors ratio of the two peaks (q1 and q2) is 1 : 2.
For PAGC-10, the scattering vectors ratio of the five peaks is 1 :
2
: 3 : 4 : 5. The results confirm that PGAC-m with different
spacer length can form lamellar-ordered smectic structure,
which is consistent with POM test results. The layer spacing
values are also calculated according to 2π / q. For PGAC-4, its
-1
first-order diffraction peak is located at 2.25 nm , and the
corresponding layer spacing is 2.79 nm. The theoretical
molecular length obtained from Material Studio is about 2.42
nm. Therefore, there should be an inclination angle between
the mesogenic unit and the bedding planes, implying that
PGAC-4 adopt a double-layer smectic C molecular packing.
According to similar calculation method, we presume that
polymer PGAC-m can also self-assemble into double-layer
smectic C phase structure. Moreover, with the increase of
spacer length, the inclination angles decrease gradually.
Comprehensive consideration of POM, DSC and SAXS test
results, we speculate that the possible molecular stacking
arrangements of PGAC-m are as presented in Fig.8.
Fig.4 Thermal decomposition curves of PGAC-m.
Thermal stability of PGAC-m
The thermal stability of PGAC-m was characterized by TGA
instrument. The thermal decomposition curves are recorded in
Fig.4 and the detailed data is listed in Table 1. As can be seen
from the TGA curves, the thermal decomposition temperature
defined as the temperature at which the mass fraction is
reduced by 5 wt%) of all polymers is above 330 ℃ , which
(
indicates that PGAC-m exhibit good thermal stability.
Phase structure and phase transition behavior of PGAC-m
The phase structure and phase transition behavior of PGAC-
m were examined in detail. The phase textures can be visually
observed through POM instrument. To remove thermal history,
all polymers were heated to 160℃, and then were cooled to
-1
2
0℃. The heating and cooling rate were set to 10 ℃ min .
POM images of PGAC-m were captured during the cooling
process. The typical images are shown in Fig.5. Obvious
birefringence phenomenon and typical smectic phase textures
appeared during the cooling process, which directly indicates
that PGAC-m can self-assemble into smectic phase structure.
The differential scanning calorimetry (DSC) was employed to
measure the glass transition temperature (T
g
) and clearing
point temperature (T ) of polymers PGAC-m. For consistency
i
with POM test conditions, all polymers were heated to 160℃
and then cooled to 20℃. After that, all polymers were heated
Fig.5 POM images of PGAC-4 obtained at 130℃ (a), PGAC-6 obtained at 125℃
to 160℃ again. The heating and cooling rate were set to 10 ℃ (b), PGAC-10 obtained at 120℃ (c). (Magnification: ×200).
-1
min . The second DSC heating curves were recorded in Fig.6
and the test results are exhibited in Table 1. The obvious
endothermic peaks at 143℃, 140℃ and 137℃ are observed,
which belong to the phase transition temperature from liquid
crystal phase to isotropic phase of PGAC-4, PGAC-6, and PGAC-
1
0, respectively. The glass transition temperature and clearing
point temperature decreased with the increase of spacer
length, implying that longer spacer is beneficial to the
movement of polymer segments.37
To better understand the phase structures of the resulting
polymers, small-angle X-ray scattering (SAXS) test was
manipulated. The test samples were heated to above their
clearing point temperature, and then cooled to 30℃ with the
rate of 5 ℃ min . The SAXS patterns were recorded at 30℃
and the test results are exhibited in Fig. 7. As observed, all
Fig.6 DSC curves of PGAC-m during the second heating process with a heating rate of
-1
-1
1
0 ℃ min .
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DOI: 10.1039/D0TC03276J
Fig.7 SAXS patterns of PGAC-m recorded at 30℃.
Fig.9 UV-Vis absorption spectra of PGAC-m in the film state (a) and in DMF
solution (b) (Concentration = 1 × 10-5 mol L-1).
PGAC-4 shifts from 462 nm to 493 nm. Subsequently, the
wavelength corresponding to the fluorescence emission peak
remains basically unchanged. In the whole process, the
fluorescence intensity increases with the increase of water
fraction. When the water fraction is 97%, the fluorescence
intensity reaches the maximum value. At this time, the
Fig.8 The schematic illustration of possible molecular stacking arrangements of
PGAC-m.
Photophysical properties of PGAC-m
The photophysical properties of PGAC-m were also fluorescence intensity is 4 times of that in pure DMF solution
characterized in details. Fig.9 (a) and Fig.9 (b) display the UV (Fig.10 (b)). To better understand this enhanced emission
absorption spectra of PGAC-m in thin film state and in dilute behavior, dynamic light scattering (DLS) measurement of
DMF solution, respectively. As shown in Fig.9 (a), PGAC-m with PGAC-4 was carried out (Fig.S4(e)). The average diameter
different spacer length shows the similar UV absorption increases significantly from 16 nm (f
w w
= 0%) to 847 nm (f =
spectra. The absorption peaks are located at about 350 nm, 90%), indicating that PGAC-4 gradually aggregate to form
which belongs to the π-π* transition of cyanostilbene moiety. nanoparticle suspensions with the addition of water, which
In DMF solution, the PGAC-m also shows the similar UV results in the molecular configuration planarization from
absorption spectra. However, the absorption peaks shift to twisted structure to coplanar structure. This planarization
3
43 nm (Fig.9 (b)), indicating that cyanostilbene shows twisted configuration is in favour of J-aggregation formation and
conformation in dilute DMF solution.
restrains non-radiative transition, leading to the enhanced
emission behavior.36, 44 The UV-visible absorption spectra of
PGAC-4 with different water fraction also reveal aggregation
process (Fig.S4(f)). The illustrations in Fig.10 (b) are the
luminescent pictures of the PGAC-4 in the pure DMF solution,
Cyanostilbene is a typical AIEE-active group, and the
prepared polymers PGAC-m are anticipated to retain the AIEE
properties. As verification, the fluorescence spectrometer was
performed to investigate the emission behavior of PGAC-m in
2
in the mixed DMF-H O solutions with water fraction of 20%
DMF-H
selected as an example to describe this process. Fig.10 (a)
shows the emission spectra of PGAC-4 in DMF-H O mixtures
2 w
O mixtures with different water fraction (f ). PGAC-4 is
and 97%, and in solid state, respectively. All the pictures were
taken under 365 nm UV light. These illustrations further
confirm the enhanced emission behavior and red shift
phenomenon. The fluorescence test results suggest that PGAC-
2
with different water fraction. Firstly, weak luminescence
intensity is observed in pure DMF, and the fluorescence
emission peak is located in at 462 nm. When the water
fraction increases to 20%, the fluorescence emission peak of
4
exhibits AIEE properties. Similarly, PGAC-6 and PGAC-10 are
also successfully endowed with AIEE properties and their
emission spectra are displayed in Fig.S4. The luminescence
6
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resulting polymers exhibit striking cotton effects at around 350
View Article Online
DOI: 10.1039/D0TC03276J
nm (Fig.11 (b), Fig.11 (c) and Fig.11 (d)), which belongs to the
absorption of cyanostilbene moiety (Fig.11 (a)). The test
results show that the chirality of poly ( -glutamate) is
L
successfully transferred to cyanostilbene and induced the
formation of preferred-handed screw conformation. Taking
comprehensive into account CD and SAXS test results, we
deem that PGAC-m form helix-arranged chiral smectic C (SmC*)
phase structure, which is well maintained in the glassy state.
2
Fig.10 (a) Emission spectra of PGAC-4 in DMF-H O mixtures with different water
fraction; (b) Plots of I/I values versus water fraction of PGAC-4; (c) Fluorescence
0
spectrum of PGAC-4 in thin film state. Excitation wavelength = 365 nm.
Concentration = 1 × 10-5 mol L . The inserted pictures were taken under 365 nm
UV light.
-1
emission spectrum of PGAC-4 in thin film state was also
measured. As observed from Fig.10 (c), the emission peak is
located at approximately 478 nm.
Chiral optical properties of PGAC-m
What is known to us is that circular dichroism (CD),
reflecting the absorption differences of left-handed and right-
handed light, can be applied to investigate structural
information concerning ground electronic state of chiral
fluorescent materials. To judge whether the chirality of poly ( -
L
glutamate) backbone has been transferred to cyanostilbene
pendants, we investigated the CD spectra of PGAC-m in two
states at room temperature, including in pure DMF solution
and in glassy liquid crystal film. After heating the polymer
sample to a temperature above its clearing point temperature,
the film is uniformly pressed, and then slowly cooled to room
temperature to obtain the glassy liquid crystal film. No CD
signals were detected in pure DMF solution for all polymers
(
Fig.S5), implying that cyanostilbene may adopt random Fig.11 (a) The UV absorption spectra of PGAC-m in glassy liquid crystal film; (b) CD
orientation conformation, causing racemization in DMF spectrum of PGAC-4, (c) CD spectrum of PGAC-6 and (d) CD spectrum of PGAC-10 in
solution.33,
45
glassy liquid crystal film state.
However, in glassy liquid crystal film, the
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glassy liquid crystal film (Fig.12 (a)). The corresponding glum
View Article Online
value can achieve 4.5 × 10-2 (Fig.12 (d)), which is rarely
reported. The glum value is larger than that of most reported
non-doped CPL-active polymer materials, the comparison data
of glum values and quantum yields with those reported in
literatures are given in Table S1. This large glum value of PGAC-4
is acquired by the self-assembly of chiral liquid crystals, which
shows amplification effect. Although the amplification effect is
not prominent in comparison with the glum value of reported
emissive chiral nematic liquid crystal systems, it shows a great
superiority in fluorescence quantum yield (which will be
discussed in the following part). Similarly, PGAC-6 and PGAC-
DOI: 10.1039/D0TC03276J
1
0 also exhibit silent CPL signals in DMF solution (Fig.S7 and
Fig.S8), while evident CPL signals are detected in glassy liquid
crystal film (Fig.12 (b) and Fig.12 (c)). Impressively, the CPL
properties can be manipulated by changing spacer length. The
g
lum value decreases with the increase of spacer length (Fig.12
d)), which is attributed to the “decoupled effect” between the
chiral poly ( -glutamate) backbone and cyanostilbene side
group. The shorter the spacer length, the more efficient the
(
L
chiral transfer from poly (L-glutamate) to cyanostilbene, and
the larger the glum value.
The fluorescence quantum yield in solid state
The fluorescence quantum yield is an important indicator for
evaluating CPL property, and the high solid quantum yield of
luminescence is conducive to promote the practical application
of CPL materials. Therefore, we measured the solid quantum
yield of PGAC-m, and also explored the influence of spacer
length on the solid fluorescence quantum yield of PGAC-m. As
observed from Fig.13, all the resulting polymers possess
relatively high solid quantum yields, and the solid quantum
yield increases with the increase of spacer length. The
fluorescence quantum yields of PGAC-4, PGAC-6 and PGAC-10
are 30.2%, 32.8% and 34.1%, respectively. This enhanced solid
fluorescence quantum yield is mainly caused by decoupling
effect,37 which means that the long space is conducive to the
aggregation of cyanostilbene group, and extend the
conjugation of α-cyanostilbene to improve the solid quantum
yield of PGAC-m.
Fig.12 CPL spectra of PGAC-m in glassy liquid crystal film (a) PGAC-4; (b) PGAC-6;
(c) PGAC-10; (d) Corresponding luminescence dissymmetry factor (glum) spectra.
Excitation wavelength = 350 nm.
Considering that the resulting polymers can not only form
spiral-stacked SmC*, but also possess high emission efficiency,
which create a suitable CPL emission environment. Reasonably,
PGAC-m is expected to generate CPL emission in glassy liquid
crystal film. Inspired by this, we investigated the CPL
properties of PGAC-m in DMF solution and in glassy liquid
crystal film by JASCO-CPL-300 instrument. PGAC-4 is selected
as an example to illustrate the CPL properties. Similar to CD
test results, the CPL signal is also silent in DMF solution (Fig.S6).
It's inspiring that strong CPL signal at 458 nm is detected in
Fig.13 The solid fluorescence quantum yields of PGAC-m (excitation wavelength = 365
nm).
8
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6
5
J. Qiao, S. Lin, J. Li, J. Tian and J. Guo, Chem. Commun., 2019,
5, 14590-14593.
Conclusions
View Article Online
DOI: 10.1039/D0TC03276J
In this article, we have creatively constructed a series of
novel CPL-active side-chain chiral liquid crystal polymers PGAC-
7
2
S. Lin, H. Sun, J. Qiao, X. Ding and J. Guo, Adv. Opt. Mater.,
020, 2000107.
m
by integrating poly
( -glutamate) backbone and
L
cyanostilbene pendants through different spacer length. The 8 J. Li, H. K. Bisoyi, S. Lin, J. Guo and Q. Li, Angew. Chem. Int.
Ed., 2019, 131, 16198-16202.
influence of spacer length on the phase structure and the
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m can self-assemble to form helix-arranged chiral smectic C
1
0 F. Song, Z. Zhao, Z. Liu, J. W. Lam and B. Z. Tang, J. Mater.
(
SmC*) phase. The clearing point temperature of PGAC-m
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decreases with the increase of spacer length. All polymers
exhibited typical AIEE characteristics, and the solid
fluorescence quantum yields of PGAC-m increases with the
increase of spacer length. The solid fluorescence quantum
yield solid-state of PGAC-10 can achieve 34.1%. The chirality of
1
1
poly (
-glutamate) is successfully transferred to cyanostilbene 13 J. Liu, H. Su, L. Meng, Y. Zhao, C. Deng, J. C. Ng, Ping. Lu, M.
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-
2
PGAC-4 can achieve 4.5 × 10 . To the best knowledge, this is
the first report concerning non-doped CPL-active polymer
materials fabricated from side-chain chiral luminescent liquid
crystal polymer. This work unfolds a new insight to harvest
non-doped CPL polymer materials with high solid quantum
yield and large glum value, and also provides theoretical
guidance for the preparation of new high-performance
circularly polarized luminescent materials.
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Conflicts of interest
There are no conflicts to declare.
2
2
Acknowledgements
This work was supported by National Natural Science Foundation of
China (No. 51973181, 51573152) and Research Foundation of
Education Bureau of Hunan Province, China (Grant No.19C1784).
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Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/D0TC03276J
A series of novel high efficiency CPL-active side-chain chiral liquid
crystal polypeptides were constructed.