Fluorescent Columnar Liquid-Crystalline Polymers: Synthesis, Mesomorphic Behaviors and Tunable Emission Wavelengths?

Article abstract of DOI:10.1002/cjoc.202100051

Fluorescent columnar liquid-crystalline polymers have attracted intensive attention due to their unique features that can be applied in many fields. However, the utilization of molecular engineering to achieve efficient fluorescence tuning has rarely been

Full text of DOI:10.1002/cjoc.202100051

Chinese Journal
of Chemistry
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中 国 化 学 - An International Journal
Accepted Article
Title: Fluorescent Columnar Liquid-Crystalline Polymers: Synthesis, Mesomorphic
Behaviors and Tunable Emission Wavelengths
Authors: Bin Mu, Zhelin Zhang, Yu Zhao, Xiangnan Hao, and Wei Tian*
This manuscript has been accepted and appears as an Accepted Article online.
This work may now be cited as: Chin. J. Chem. 2021, 39, 10.1002/cjoc.202100051.
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Cite this paper: Chin. J. Chem. 2021, 39, XXX—XXX. DOI: 10.1002/cjoc.202100XXX
Fluorescent Columnar Liquid-Crystalline Polymers: Synthesis,
Mesomorphic Behaviors and Tunable Emission Wavelengths
Bin Mu, Zhelin Zhang, Yu Zhao, Xiangnan Hao, and Wei Tian*
Shanxi Key Laboratory of Macromolecular Science and Technology, MOE Key Laboratory of Material Physics and Chemistry under Extraor-
dinary Conditions, School of Chemistry and Chemical Engineering, Northwestern Polytechnical University, Xi’an, 710129, China.
Keywords
Liquid crystals | Polymers | Columnar mesophases | Fluorescence
Main observation and conclusion
Fluorescent columnar liquid-crystalline polymers have attracted intensive attention due to their unique features that can be applied in
many fields. However, the utilization of molecular engineering to achieve efficient fluorescence tuning has rarely been resolved. Herein,
we report a series of liquid-crystalline polymers with α-cyanostilbene pendants attaching to polymethacrylate backbone by variant
length ethylene glycol spacers, that assemble into hexagonal columnar mesophases and exhibit efficient fluorescence emission. Both
the columnar ordering and the emission wavelength can be tuned just by altering the length of flexible spacers. With an elongation of
the spacer, the columnar ordering and dimensions increased, and the fluorescence emission blue-shifted gradually. With good correla-
tion of molecular design, columnar structure and fluorescence property, this study is expected to augment the understanding of spacer
length influence in liquid-crystalline polymers and provide a guidance for the design and development of novel functional polymer
materials.
Comprehensive Graphic Content
*E-mail: happytw_3000@nwpu.edu.cn
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Supporting Information
Chin. J. Chem. 2021, 39, XXX－XXX
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First author et al.
Report
Background and Originality Content
Columnar morphology is of fascinating interest for the construc-
tion of various intriguing materials including in optoelectronic, bio-
mimetic, and mechanical fields.[1-5] The self-organization of poly-
mers into columnar mesophase can share the common properties
of liquid crystals such as self-organization, self-healing, and the abil-
ity to respond to external stimuli.[6-10] Much of the effort has been
devoted to discotic liquid crystal-based polymers, as the stack of
discotic molecules can drive the formation of columnar phases that
are capable of promising optoelectronic applications.[11-15] Besides,
there are several other types of polymers that can form columnar
phase, including mesogen-jacketed liquid-crystalline polymers,[16,17]
dendronized polymers,[18,19] and hemiphasmid polymers,[20-25]
which make it possible to incorporate much more functions and ap-
plications.
Recently, thanks to the development of solid-state emission
materials based on the concept of aggregation-induced emis-
sion,[26,27] liquid-crystalline polymers bearing aggregation-induced
emission-active pendants have arisen considerable interest due to
the combination of liquid crystal property and fluorescence perfor-
mance that have shown great advantages on broad applications in
optoelectronics.[28-37] The most common efforts have been devoted
to incorporating tetraphenyl ethylene or α-cyanostilbene (CS) into
polymer system for the formation of fluorescent liquid-crystalline
polymers, which may assemble into various mesomorphic struc-
tures, however, in most cases lamellar mesophases were ob-
served.[31-37] As the chemical structures of both main-chain and
side-chain can be freely tailored, and subtle change may give rise to
significant structural transformation and thus fluorescence color al-
teration, it will provide an attractive strategy for the development
of smart optical materials. However, there are only a few examples
concerning fluorescent columnar liquid-crystalline polymers,[28-30]
especially the utilization of some key structural factors such as the
alteration of spacer length and molecular weight to achieve fluores-
cence tuning behavior remain unresolved.
Flexible spacer usually plays a vital role in determining the as-
sembly mechanism and supramolecular structure of liquid-crystal-
line polymers.[38-42] The spacer decoupling effect has been widely
adopted and applied in the study of calamitic mesogen based la-
mellarly arranged polymers, of which longer spacer would lead to
higher ordered smectic structures.[38-40] In contrast, in discotic co-
lumnar polymer system, we have proposed positive coupling be-
tween polymer backbone and discotic side groups is preferred for
the formation of well-organized columnar lattices.[12,13,41,42] In view
of above consideration, our intention here is to develop fluores-
cence columnar liquid-crystalline polymers and to regulate their su-
pramolecular assembly mechanisms and thus fluorescence proper-
ties through tuning the length of flexible spacers.
As shown in Figure 1, we herein report a series of hemiphasmid
polymers by adopting aggregation-induced emission-active α-cy-
anostilbene (CS)[43-47] as the calamitic mesogen with tapered do-
decyl minidendrons, which is connected to a polymethacrylate
main chain through variant length spacers. Hydrophilic oligo(eth-
ylene glycol) was designed as the flexible spacer to induce the for-
mation of columnar phase, possessing a core-shell structure as a
result of the nanosegregation within the column.[48-50] All the liquid-
crystalline polymers form hexagonal columnar mesophase, and no-
tably exhibit increased columnar dimensions, increased orderliness
and blue-shifted fluorescence emission with the increase of spacer
length from 0 to 4 ethylene glycol subunits. With good correlation
of molecular design, columnar structure and fluorescence property,
this finding is expected to augment the understanding of spacer
length influence in liquid-crystalline polymers and provide a guid-
ance for the design and development of novel functional polymer
materials.
Figure 1 Molecular structure of the fluorescent liquid-crystalline polymers
Pm and their assembly into hexagonal columnar mesophase (Colh), exhibit-
ing increased dimension, increased orderliness and blue-shifted fluores-
cence emission with an elongation of the flexible spacer from m = 0 to 4
ethylene glycol subunits.
Results and Discussion
Scheme 1 outlines the synthesis of the CS-based methacrylate
monomers Mm and their corresponding polymers Pm with differ-
ent length spacers. The synthesis of monomers was performed via
three synthetic routes, depending on the length of spacers, from an
easily available hydroxyl CS minidendron intermediate 2, which was
obtained according to our previous publication on the synthesis of
CS-based fluorescence liquid crystals.[51,52] Specifically, M0 without
flexible spacer was synthesized via a simple esterification of the hy-
droxyl functionalized CS 2 and methacryloyl chloride under a mild
condition. As for M1, an excess amount of 1,2-dibromoethane was
used to undergo etherification with 2, and the resulting 3 was sub-
sequently treated with methacrylic acid to afford the monomer
with only an ethylene glycol subunit spacer. The synthesis of Mm (m
= 2, 3, 4) was carried out by referring to the literature method[48] by
utilizing 4-toluenesulfonate functionalized oligo(ethylene glycol) to
introduce the spacers with varied number of ethylene glycol subu-
nits. Synthetic details and molecular characterization data are avail-
able in the Supporting Information. Figure 2 displayed nuclear mag-
netic resonance (NMR) and mass spectroscopy (MS) analysis for
representative M4 to illustrate the identified molecular structure of
thus obtained monomers. All resonance shifts together with their
protonic assignments and their corresponding integrations were in
good agreement with the expected chemical structure of M4 (Fig-
ure 2a). Mass spectrum (Figure 2b) also provided a strong support
for the identification of the monomer structure, with the appear-
ance of molecular ionic peaks arising from [M + H]+, [M + Na]+ and
[M + K]+.
Polymerization of the methacrylate monomers Mm was per-
formed in tetrahydrofuran using 2,2-azobis(isobutyronitrile) (AIBN)
as radical initiator (Scheme 1). Actually, attempts to synthesize the
target polymers in other solvents including 1,4-dioxane, toluene,
chlorobenzene and N,N-dimethylformamide have been carried out,
but it turned out to be unsuccessful and in most cases polymer
products were not obtained or only harvested in low yield. Alt-
hough the occurrence of severe volatilization loss of solvent tetra-
hydrofuran during polymerization could not be avoided and usually
a small amount of solvent remained after the completion of
polymerization, a very high polymerization efficiency can be ob-
served. The resulting polymers Pm were obtained in about 90%
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Running title
Chin. J. Chem.
yield (Table S1) even after purification by silica-gel column chroma-
tography (dichloromethane eluent to remove the residual mono-
mer). After polymerization, the characteristic vinyl proton at 5.59
and 6.15 ppm of the methacrylate monomer disappeared and the
whole resonance peaks became broadened and overlapped (Figure
2a), both suggesting the successful polymerization and pure poly-
mer sample was obtained. Gel permeation chromatography (GPC)
analysis (Figure 2c) of the obtained polymers showed relative num-
ber-average molecular weights around 30,000 g mol-1 and polydis-
persity indexes about 1.4 (Table S1). The successful preparation of
the series carefully designed liquid-crystalline polymers with rela-
tively high molecular weights offers an ideal platform for the sys-
tematical investigation of the spacer length dependent mesomor-
phic behaviors and fluorescence properties.
Scheme 1 Synthetic routes towards the methacrylate monomers (Mm) and their polymerization into corresponding polymers Pm with m = 0, 1, 2, 3, 4
denoting the ethylene glycol number of the spacer.
CN
CN
OC12H25
OC12H25
OC12H25
OC12H25
OC12H25
OC12H25
OH
HO
HO
O
O
C12H25Br
OH
OH
2
1
O
O
O
CN
OC12H25
Cl
CN
OC12H25
OC12H25
OC12H25
HO
OC12H25
OC12H25
2
M0
Br
Br
O
CN
OC12H25
OC12H25
OC12H25
CN
OC12H25
OC12H25
OC12H25
Br
O
OH
O
O
O
3
M1
O
H
O
m
S
O
O
O
CN
OC12H25
OC12H25
OC12H25
CN
OC12H25
OC12H25
HO
Cl
O
O
O
m
m
O
m
m
m
=
=
=
4
5
6
2)
3)
4)
m
(
(
(
M
=
OC12H25
m
4)
2, 3,
(
n
CN
OC12H25
CN
OC12H25
OC12H25
OC12H25
O
O
O
O
AIBN
m
m
OC12H25
OC12H25
O
O
m
m
M
P
m
m
=
=
4)
4)
0, 1, 2, 3,
0, 1, 2, 3,
(
(
Figure 2 (a) 1H NMR spectra comparison for representative monomer M4 and corresponding polymer P4 in CDCl3 solution. (b) Representative mass spec-
trum of monomer M4 obtained via electrospray ionization mass spectrometry. (c) Normalized GPC traces of thus obtained series polymers Pm with spacer
length ranging from m = 0 to 4.
Chin. J. Chem. 2021, 39, XXX－XXX
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First author et al.
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Table 1 Thermotropic properties, structural parameters, and fluorescence behaviors of the polymers Pm.
e
f
e
f
Polymer
code
P0
Phase behaviora
[°C(J/g)]
Lattice parameterb
[nm]
ξd
λmax,solid (w1/2
[nm]
)
Φsolid
[%]
4.2
4.6
4.4
3.9
3.8
λmax,solution (w1/2
)
Φsolution
[%]
μc
[nm]
35.3
40.1
62.0
67.8
73.5
[nm]
Cr -21(8.3) Colh 131(0.2) I
Cr -18(4.8) Colh 81(0.2) I
Cr -12(5.3) Colh 74(0.4) I
Cr 0(5.2) Colh 92(0.5) I
Cr 7(15.9) Colh 77(0.4) I
a = 4.74
6.23
7.02
7.24
7.31
7.58
467(100)
461(99)
454(97)
451(92)
446(89)
454(97)
445(86)
439(82)
438(78)
436(73)
0.6
P1
a = 5.16
0.8
P2
a = 5.37
0.7
P3
a = 5.52
0.5
P4
a = 5.75
0.6
a Thermal data were collected from DSC heating cycles at a rate of 10 °C min-1. b Lattice parameter (a) in the hexagonal columnar mesophase was determined
by X-ray scattering analysis. c μ is the number of monomer units per column stratum. d Two-dimensional inter-columnar correlation length (ξ) was evaluated
through Scherrer equation. e The emission maximum (λmax) in dichloromethane solution and solid states was determined by fluorescence measurement,
and w1/2 in bracket represents the full width at half maximum of the fluorescence spectra. f The fluorescence quantum yield (Φ) was obtained from dichloro-
methane solution and solid states.
state light emission.[43-47] Fluorescence images (Figure S1) taken un-
der 365 nm ultraviolet light illumination clearly demonstrated the
polymers exhibited fluorescence emission in their orderly assem-
bled solid state. Additionally, the emission color appeared to
slightly blue-shift from cyan to blue with an increase of the spacer
length. Relevant data regarding to their thermal behaviors, struc-
tural parameters, and photophysical properties in both solution
and solid states are summarized in Table 1.
To confirm the presence of mesomorphic phases and further
identify the detailed assembled structures for the series liquid-crys-
talline polymers, X-ray scattering experiments were performed
with the whole scattering curves and representative scattering pat-
terns presented in Figure 3a. A strong scattering peak at low angles
and a diffuse halo in the wide-angle range can be obviously seen,
both of which possibly pointed to the formation of columnar liquid
crystal mesophases. Specifically, the diffuse halo was approxi-
mately 14 nm-1 (~0.45 nm) as a result of the liquid-like arrangement
of peripheral tapered dodecyl minidendrons, while the strong peak
at low angles, associated with some small scattering shoulders, if
existed, were generated by the two-dimensional periodic array of
the columnar arrangement. The q-value for all the polymers with
different length spacers followed a ratio of 1:√3:√4:√7, typical for
hexagonally arranged columnar liquid crystal mesophases (Colh,
p6mm).
By a preliminary comparison, it is evident to notice that the or-
dering of the columnar mesophases revealed increase with the
longer spacer, which can be confirmed by the occurrence of higher
order scattering signals and also the increased correlation length
deduced by Scherrer equation (Figure 3b).[13] This is not unreason-
able by taking into consideration the conventional spacer decou-
Figure 3
(a) X-ray scattering patterns indicated with proposed indexing
pling effect where longer spacer usually induced higher ordered liq-
uid crystal structures in side-chain calamitic liquid-crystalline poly-
mers.[38-40] Besides, the nanosegration between the hydrophilic ol-
igo(ethylene glycol) spacer and hydrophobic aromatic and paraf-
finic segments may also contribute to the formation of more or-
dered columnar structure with a longer spacer.[48-50] The lattice pa-
rameters were calculated according to a = 2d(100)/√3 based on the
(100) scattering peak at small angles of the identified hexagonal co-
lumnar liquid crystal structure, which clearly indicated that the co-
lumnar dimensions underwent a significant increase with the elon-
gation of the flexible spacers (Figure 3b). However, the number of
monomer units per column stratum (μ), estimated by the literature
method adopted in dendronized polymers,[18,19] appeared to be
around 7 for the whole polymers with an exception of 6 for the pol-
ymer P0. This almost constant μ value irrespective of spacer length
is in sharp contrast to that reported in liquid-crystalline polymers
with side-chain azobenzene minidendrons where μ increased sig-
nificantly as the spacer became longer.[22] Such a significant differ-
ence was probably arising from more flexible ethylene glycol spacer
adopted rather than the common alkyl chain used in the literature.
It is just because the constant number of monomer units per col-
umn stratum that would facilitate the interaction between adjacent
of the series liquid-crystalline polymers with different length spacers. (b)
Spacer length dependent columnar dimensions and two-dimensional inter-
columnar correlation lengths, with insets showing column cross section and
right side depicting representative molecular arrangement in a column stra-
tum with different length spacers.
Thermal analysis by a combination of polarized optical micros-
copy (POM), thermal gravimetric analysis (TGA), differential scan-
ning calorimetry (DSC), and X-ray scattering measurements demon-
strated the whole polymers can transform into birefringent and
shearable liquid crystal mesophases with a relatively broad temper-
ature range that spans across the room temperature. POM obser-
vation (Figure S1) for representative P0, P2, and P4 showed obvious
pseudo focal-conic fan-shaped textures, typical for columnar liquid
crystal mesophases. TGA (Figure S2) demonstrated good thermal
stability over 300 °C. Then DSC was utilized to examine their ther-
motropic properties (Figure S3), which displayed a transition peak
from crystalline (Cr) to liquid-crystalline phase at low temperature
and a small isotropization signal at high temperature for the whole
polymers. As expected, the CS containing polymers exhibited ag-
gregation-induced emission property (Figure S4) and thus solid-
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Running title
Chin. J. Chem.
CS moieties to be controlled by the spacer length. The smaller co-
lumnar dimensions would lead to stronger interaction between ad-
jacent CS mesogens as illustrated in the right side of Figure 3b,
which accordingly would give rise to increased electronic coupling
and thus red-shifted fluorescence emission as discussed later.
of this kind of CS-containing polymers. Notably, it is surprising to
find that the emission maxima underwent a gradual red-shift and
broadening with the decrease of the spacer lengths in both solution
and solid states, as revealed in Figure 5c and the full width at half
maximum (w1/2) of the fluorescence spectra (Table 1), respectively.
Such a red-shift with the shortening of the spacer length was sup-
posed to be arising from the increased interaction (or electronic
coupling) between adjacent CS moieties by making a comparison
with the emission maximum at 436 nm of molecularly dissolved CS
monomers (Figure S6), agreeing well with the result derived from
the X-ray scattering analysis. This assumption could also be sup-
ported by the time-resolved fluorescence lifetime experiment (Fig-
ure S7), where the more degree of red-shift in solid state was asso-
ciated with a slightly longer fluorescence lifetime (Figure 5d), while
the lifetime was too short to be resolved in their solution state.
Figure 4
Dependence of phase transition temperatures versus the length
of the spacer m of the columnar liquid-crystalline polymers Pm determined
from DSC with a heating rate of 10 °C min-1.
Figure 4 shows the phase transition temperatures during heat-
ing process and their change trends as a function of the spacer
length (m). First, it is very interesting to notice that the melting tem-
peratures from crystalline to liquid-crystalline transition were mon-
otonically increasing with an elongation of the spacers. This was
probably arising from the decreased coupling effect with the elon-
gation of flexible spacers that facilitated the formation of more sta-
ble crystalline phases of CS mesogens with peripheral aliphatic
minidendrons, which showed great accordance to the X-ray scat-
tering results where longer spacers induced more ordered colum-
nar mesophases. In contrast, the isotropization temperatures ex-
hibited a trend of decreasing with the increase of spacer length,
suggesting a decrease of the columnar stability. It can be rational-
ized based on the core-shell columnar structures as shorter spacer
would lead to stronger jacketing effect of the tapered CS miniden-
drons around polymethacrylate main chain, which accordingly gave
rise to more stable polymer columnar morphology against temper-
atures. This was reminiscent of the fact found in other types of co-
lumnar polymer materials such as positive coupling between the
polymer backbone and side-chain discotics,[12] and the concept of
mesogen-jacketed liquid-crystalline polymers,[16,17] as they usually
contributed to the formation of columnar mesophases with higher
isotropization temperatures. The unexpected elevation of the iso-
tropization temperature from P2 to P3 was supposed to be caused
by the competition between the crystallinity of the CS miniden-
drons and the forming ability of polymer columnar phases.
To gain further insight into their spectroscopic properties, all
polymers were investigated by ultraviolet visible (UV-vis) absorp-
tion and fluorescence emission experiments. The absorption max-
ima in both solution and solid states (Figure S5) seemed to be al-
most the same irrespective of the length of spacers, suggesting a
simple intermolecular packing rather than the H-/J-type aggrega-
tion that was usually observed in CS-based liquid crystals.[30-32,43-47]
However, the solid-state emission spectra exhibited red-shift of
more than ten nanometers with respect to that in solution (Figure
5a,b), possibly suggesting the presence of electronic coupling be-
tween the CS chromophores in the excited state. Besides, the fluo-
rescence quantum yields collected from solid-state were around
4.2%, showing several times larger than that in solution (Table 1),
which further confirmed the aggregation-induced emission feature
Figure 5 Fluorescence emission spectra of the series polymers (a) in di-
chloromethane solution and (b) in assembled solid state with the excitation
wavelength at 350 nm of the maximum UV absorption. Dependence of (c)
fluorescence emission maximum and (d) average lifetime versus the spacer
length (m) of the polymers Pm in both solution and solid states.
Therefore, a good correlation can be obtained between the mo-
lecular design especially the spacer lengths, assembled liquid crys-
tal structures, and fluorescence emission properties. Although flu-
orescent liquid-crystalline polymers have been reported in some
cases,[28-37] the utilization of molecular engineering strategy to
achieve efficient tuning of their emission remain a challenge, espe-
cially lacking systematical and well-regulated investigations. Thus
the finding herein allowing a precise control on the emission wave-
length via tuning the spacer length of this kind of fluorescent co-
lumnar liquid-crystalline polymers, will provide a guidance for the
design and development of novel functional polymers that will have
potential applications in optoelectronic devices. Besides, the
spacer length influence, a key question in liquid-crystalline poly-
mers,[38-41] can be better understood by quantitatively comparing
the sensitive change of fluorescence in response to the subtle dif-
ference of molecular arrangement.
Conclusions
In conclusion, we have reported here a series of fluorescent liq-
uid-crystalline polymers by attaching aggregation-induced emis-
sion-active CS minidendrons to polymethacrylate main chains with
Chin. J. Chem. 2021, 39, XXX－XXX
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First author et al.
Report
variant length ethylene glycol spacers, which assembled into hex-
agonal columnar mesophases. Although the columnar dimensions
underwent a significant increase with the elongation of the flexible
spacers, the number of monomer units per column stratum (μ) ap-
peared to be around 7 for the whole polymers. Such an almost con-
stant μ value irrespective of the spacer length facilitated the inter-
action (or electronic coupling) between adjacent CS moieties to be
controlled by the spacer length. The fluorescence emission dis-
played a gradual blue-shift from cyan to blue with an increase of
the spacer length from 0 to 4 ethylene glycol subunits. The study
provides an efficient strategy to get better understanding towards
the influence of spacer lengths in liquid-crystalline polymers, and
contributes to the design and development of novel functional pol-
ymer materials.
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Zhu, Y.-F.; Guan, X.-L.; Shen, Z.; Fan, X.-H.; Zhou, Q.-F. Competition and
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Supporting Information
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The supporting information for this article is available on the
WWW under https://doi.org/10.1002/cjoc.2021xxxxx.
Acknowledgement
This work was supported by the National Natural Science Foun-
dation of China (21805228, 22022107 and 22071197), China Post-
doctoral Science Foundation (2018M633563) and Natural Science
Basic Research Plan in Shaanxi Province of China (2020JC-20). We
would also like to thank the Analytical & Testing Center of North-
western Polytechnical University for DSC measurement.
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(The following will be filled in by the editorial staff)
Manuscript received: XXXX, 2021
Manuscript revised: XXXX, 2021
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Manuscript accepted: XXXX, 2021
Accepted manuscript online: XXXX, 2021
Version of record online: XXXX, 2021
Chin. J. Chem. 2021, 39, XXX－XXX
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Fluorescent Columnar Liquid-Crystalline Polymers: Synthesis, Mesomorphic Behaviors and Tunable Emission Wavelengths
Bin Mu, Zhelin Zhang, Yu Zhao, Xiangnan Hao, Wei Tian*
Chin. J. Chem. 2021, 39, XXX—XXX. DOI: 10.1002/cjoc.202100XXX
A series of fluorescent polymers in hexagonal columnar liquid crystals were prepared, and their mesomorphic behaviors and emission wavelengths were
tuned by the lengths of flexible spacers.
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© 2021 SIOC, CAS, Shanghai, & WILEY-VCH GmbH
Chin. J. Chem. 2021, 39, XXX－XXX