High solid-state efficiency fluorescent main chain liquid crystalline polytriazoles with aggregation-induced emission characteristics

Article abstract of DOI:10.1021/ma2021979

Fluorescent liquid crystals (LCs) have wide applications in optoelectronic devices. However, the synthesis of high solid-state emission efficiency LCs is difficult. On one hand, impeding the aggregation of traditional luminogens is the prerequisite for hi

Full text of DOI:10.1021/ma2021979

ARTICLE
pubs.acs.org/Macromolecules
High Solid-State Efficiency Fluorescent Main Chain Liquid Crystalline
Polytriazoles with Aggregation-Induced Emission Characteristics
Wang Zhang Yuan,†,‡ Zhen-Qiang Yu,|| Youhong Tang,† Jacky W. Y. Lam,† Ni Xie,† Ping Lu,†
Er-Qiang Chen,§ and Ben Zhong Tang*,†,^
†Department of Chemistry, The Hong Kong University of Science & Technology (HKUST), Clear Water Bay, Kowloon, Hong Kong,
China
‡School of Chemistry and Chemical engineering, Shanghai Jiao Tong University, Shanghai 200240, China
School of Chemistry and Chemical Engineering, Shenzhen University, Shenzhen 518060, China
§College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China
^State Key Laboratory of Molecular Neuroscience, HKUST, Clear Water Bay, Kowloon, Hong Kong, China
S Supporting Information
b
ABSTRACT: Fluorescent liquid crystals (LCs) have wide appli-
cations in optoelectronic devices. However, the synthesis of
high solid-state emission efficiency LCs is difficult. On one hand,
impeding the aggregation of traditional luminogens is the pre-
requisite for high efficiencies; on the other hand, self-organization
is an inherent attribute for LCs in the mesomorphic states. Because
of such irreconcilable conflict, new strategy toward efficient fluor-
escent LCs is highly desirable. Herein, a conceptually new approach
toward high efficiency fluorescent LCs is developed. Through
rational design and combination of biphenyl-containing diazides
and diynes carrying tetraphenylethylene units, soluble, regioregular,
and liquid crystalline polytriazoles with high solid-state emission
efficiencies (up to 63.7%) are obtained. The photophysical properties of the polymers are sensitive to their molecular structures and their
solid-state quantum yields decrease with increasing spacer length. The spacer length also impacts on the mesomorphic properties. While
polymers with rigid main chains exhibit nematicity, those with longer spacer lengths show better mesogenic packing and hence form smetcic
phases.
’ INTRODUCTION
use of polarizing sheets and absorbing color filters. The color and
brightness of the light emitted by the liquid crystalline lumino-
gens may be manipulated by external fields, which may lead to the
development of readily tunable electrochromic and optical switch-
ing systems. Such approach can simplify the device design and
substantially increase the device brightness, contrast, efficiency, and
viewing angle.3,18
Despite the promising prospects of high emission efficiency
LCs, their synthesis is intractable. In the mesophases, particularly
those formed by disk-like molecules,13c the chromophoric
mesogens are regularly packed and undergo strong intermo-
lecular interactions, which often quench their light emissions
due to the formation of detrimental species such as excimers
and exciplexes.13g,19ꢀ21 The light emission is often enhanced
with the sacrifice of molecular packing, thus making the synthesis
of efficient fluorescent LCs a daunting task.
Liquid crystals (LCs) have found a wide range of high-tech
applications due to their unique properties.1,2 For examples, they
are widely used in panel display devices because of their aniso-
tropic nature and responsiveness to electric fields.2,3 While
ferroelectric LCs are used as temperature4 and pressure sensors,5
and are promising nonlinear optical materials,6 liquid crystalline
semiconductors have favored potential applications in field-
effect transistors,7 photovoltaic cells,8 and light-emitting diodes.9
Furthermore, dynamic and anisotropic liquid crystalline struc-
tures can work as low-dimensional charge-,10 ion-,11 and mass-
transporting12 materials.
Among various LCs, fluorescent LCs are receiving increasing
attentions.9,13ꢀ15 The combination of intrinsic light-emitting
capability and spontaneous self-organization attribute within a
liquid crystalline phase is of crucial importance for optoelectronic
applications, such as anisotropic light-emitting diodes and emis-
sive liquid crystal displays.9,16,17 The fluorescent LCs may emit
linear or circular polarized light when aligned,9,13g,15,17 which
may be utilized for the construction of lighting and orientating
layers in liquid crystal optical display devices, thus obviating the
Received: September 30, 2011
Revised:
November 10, 2011
Published: November 22, 2011
r
2011 American Chemical Society
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Our group is interested in creating molecular and polymeric
LCs.22 We have synthesized a wide variety of side chain liquid
crystalline polyacetylenes carrying chromophoric and mesogenic
pendants. Some of which can emit intensely in the condensed
phase.23,24 Recently, we discovered a novel phenomenon of
aggregation-induced emission (AIE) in some propeller-shaped
molecules.25,26 Instead of quenching commonly observed in
“conventional” luminophors, aggregation has enhanced their
light emission, turning them from weak fluorophors into strong
emitters. Introduction of AIE-active dyes into liquid crystals
may solve such the above problem but such attempt has rarely
reported.
become a versatile synthetic tool with applicability in diverse
areas.29ꢀ31 In this paper we adopted such methodology for the
preparation of new polymers. All the synthesized polytriazoles
are both AIE-active and liquid crystalline. Their solid-state
quantum efficiencies range from 24.5 to 63.7%, which are much
higher than those of reported luminescent liquid crystals.15b,20
The effects of spacer length on their emission efficiency and
mesomorphic behaviors are also discussed.
’ EXPERIMENTAL SECTION
Materials. THF was distilled under normal pressure from sodium
benzophenone ketyl under nitrogen immediately prior to use. Dichlor-
omethane (DCM) was distilled under normal pressure over calcium
hydride under nitrogen before use. Triethylamine (TEA) was distilled
and dried over potassium hydroxide. p-Toluenesulfonic acid mono-
hydrate (TsOH), N,N0-dicyclohexylcarbodiimide (DCC), 4-dimethyl-
aminopyridine (DMAP), 6-bromohexanoic acid [7(5)], 11-bromo-
undecanoic acid [7(10)], 4,40-biphenol (8), 4-bromobenzophenone (10),
(trimethylsilyl)acetylene (11), 4-hydroxylbenzophenone (14), copper(I)
iodide (CuI), triphenylphosphine (PPh3), dichlorobis(triphenylphos-
phine)palladium(II) [Pd(PPh3)3Cl2], titanium tetrachloride (TiCl4),
zinc dust, 1 M tetrabutylammonium fluoride (TBAF) in THF (contain-
ing 5% water) were all purchased from Aldrich and used as received.
Catalysts Cu(PPh3)3Br and Cp*Ru(PPh3)2Cl were prepared according
to the literature methods.32
Tetraphenylethylene (TPE) is a typical AIE-active dye.26,27 It
is nonemissive in dilute solution because its active intramolecular
rotations effectively dissipates the energy of its excitons through
nonradiative pathways. On the contrary, it emits intensely in the
aggregate state due to the physical restriction of its intramole-
cular rotations. Polymers containing TPE units are thus expected
to show AIE phenomenone.28 Meanwhile, the polymers can be
imparted with mesomorphic properties through rational molec-
ular design. Herein, we present our effort on the creation of AIE-
active liquid crystalline polymers (AIE-LCPs). The copper-
catalyzed 1,3-dipolar cycloaddition of alkynes with azides is
a typical example of “click reaction”. Since the reaction enjoys
the advantages of mild reaction conditions, high efficiency and
regioselectivity, and simple purification procedures, it has
Instrumentations. 1H and 13C NMR spectra were measured on a
Bruker ARX 400 spectrometer using chloroform-d or DMSO-d6 as
solvent and tetramethylsilane (TMS) as internal standard. Matrix-
assisted laser desorption/ionization time-of-flight (MALDI-TOF)
high-resolution mass spectra (HRMS) were recorded on a GCT premier
CAB048 mass spectrometer. Absorption spectra were taken on a Milton
Roy Spectronic 3000 Array spectrometer. Emission spectra were taken
on a Perkin-Elmer spectrofluorometer LS 55. The quantum yields
(ΦF’s) of the polymers in THF solutions were estimated using quinine
sulfate (ΦF = 54% in 0.1 M H2SO4) as standard, while those of the solid
films were determined using an integrating sphere. The films were
prepared by casting the polymer solutions (5 mg/mL in chloroform,
THF, 1,2-dichloroethane or DMF) at ambient conditions. The films
Scheme 1
Scheme 2
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Scheme 3
Table 1. Click Polymerizations of Diazides with Diynesa
were stood overnight at room temperature before measurement. Molec-
ular weights (Mw and Mn) and polydispersity indexes (Mw/Mn) of the
polymers were estimated by a Waters Associates gel permeation
chromatography (GPC) system in THF. A set of monodisperse poly-
styrene standards covering molecular weight range of 103ꢀ107 was used
for the molecular weight calibration.
b
b
no.
solvent
time (h)
yield (%)
Mw
Mw/Mn
1(5) + 2
1
THF
DMF
THF
THF
12
12
6
85.1
94.1
40.0
94.0
11 800c
11 800c
3400
1.42
1.66
1.30
1.61
The thermal stability of the polymers was evaluated on a Perkin-
Elmer TGA 7 under nitrogen at a heating rate of 20 ꢀC/min. A Perkin-
Elmer DSC 7 was employed to measure the phase transition thermograms.
An OlympusBX 60 polarized opticalmicroscope (POM) equipped with a
Linkam TMS 92 hot stage was used to observe the anisotropic optical
textures. One-dimensional wide-angle X-ray diffraction (1D-WAXD)
powder experiments were performed on a Philips X’Pert Pro diffrac-
tometer with a 3 kW ceramic tube as the X-ray source (Cu Kα) and an
X’celerator detector. The sample stage was set horizontally and the
samples were protected by nitrogen gas during the measurements. The
reflection peak positions were calibrated with silicon powder (2θ > 15ꢀ)
and silver behenate (2θ < 10ꢀ). Background scattering was recorded and
subtracted from the sample patterns. A temperature control unit (Paar
Physica TCU 100) in conjunction with the diffractometer was utilized to
study the structure evolutions as a function of temperature.
2
3
4d
12
4900
1(5) + 3(3)
63.7
5
6
THF
DMF
12
12
8900
1.67
1.91
97.1
11 400
1(5) + 3(8)
75.2
7
8
THF
DMF
12
12
16 200
10 200
2.00
1.77
79.3
1(10) + 2
94.5
9
THF
DMF
12
12
14 800
14 600
1.48
1.47
10
94.8
1(10) + 3(3)
87.0
11
12
THF
DMF
12
12
12 900
12 900
1.72
1.76
’ RESULTS AND DISCUSSION
90.4
1(10) + 3(8)
63.6
Monomer Preparation. With a view to synthesize AIE-LCPs
by click reaction, we prepared biphenyl-containing diazides with
different aliphatic spacer lengths according to Scheme 1. We first
esterified 4,40-biphenol (8), with ω-bromoalkanoic acid 7(x) in
the presence of DCC, DMAP, and TsOH. The resulting com-
pounds 9(x), were then reacted with sodium azide, furnishing the
desirable products 1(x).
13
14
THF
DMF
12
12
11 900
12 900
1.61
1.62
85.4
a Carried out at 60 ꢀC under nitrogen using Cu(PPh3)3Br as catalyst
unless otherwise specified. b Estimated by GPC in THF on the basis of a
polystyrene calibration. c THF soluble fraction. d Cp*Ru(PPh3)2Cl was
used as catalyst.
We also synthesized TPE-functionalized diynes 2 and 3(y),
according to the synthetic routes shown in Scheme 2. To obtain
compound 2, we first prepared compound 12 by palladium-
catalyzed cross-coupling of 4-bromobenzophenone (10) with
trimethylsilylacetylene (11). McMurry coupling of 12 catalyzed
by TiCl4 and Zn afforded 13, which converted into 2 by cleavage
of its trimethylsilyl groups in basic medium. Diynes 3(y) are
structural congeners of 2 but their triple bond functionalities are
connected to the TPE core through alkyl chains and ester groups.
They were synthesized by McMurry coupling of 4-hydroxylben-
zophenone (14) followed by esterification with β-alkynoic acids
16(y). All the reactions proceeded smoothly and the desirable
monomers were obtained in high yields. The monomers were
characterized by standard spectroscopic methods, from which
satisfactory analysis data corresponding to their molecular struc-
tures were obtained (see Supporting Information for details).
Click Polymerization. We then carried out the polycycloaddi-
tion reactions of 1(x) with 2 and 3(y) according to Scheme 3.
We first tried to polymerize 1(5) and 2 by Cu(PPh3)3Br, an effec-
tive organosoluble catalyst for the 1,4-regioregular 1,3-dipolar
cycloaddition.29,31 Reaction conducted in THF at 60 ꢀC for 12 h
gives some THF-insoluble products, which can dissolve readily in
DMF. After precipitation of the solution into hexane/THF
mixture, P4(5) is isolated in a yield of 85.1% (Table 1, no. 1). The
Mw and Mw/Mn of its THF soluble fraction are 11 800 and 1.44,
respectively. The mixture remains homogeneous throughout the
polymerization process when the polymerization is carried out in
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Figure 1. IR spectra of (A) monomer 1(5), (B) monomer 2, and
(C) their polymer P4(5) (sample taken from Table 1, no. 2).
Figure 3. 13C NMR spectra of (A) monomer 1(5), (B) monomer 2,
and (C) their polymer P4(5) (sample taken from Table 1, no. 2) in
DMSO-d6 at room temperature.
of 1(5) and 2, a soluble polymer is isolated in high yield but rather
low molecular weight (Table 1, no. 4).
The polymerizations of other monomer pairs are conducted
using Cu(PPh3)3Br as catalyst in THF or DMF. As depicted in
Table 1, nos. 5ꢀ14, all the polymers are obtained in high yields
(63.6ꢀ97.1%) with moderate Mw values of 8 900ꢀ16 200.
Generally, the polymerizations performed in DMF give much
higher yields than those conducted in THF. Thanks to their long
alkyl chains, P5(x) and P6(x) possess good solubility in THF,
DCM, and chloroform.
Structural Characterization. All the polymers are character-
ized by spectroscopic techniques and give satisfactory analysis
data corresponding to their expected structures. An example of
the IR spectrum of P4(5) is shown in Figure 1. The spectra of its
monomers 1(5) and 2 are also given in the same figure for com-
parison. Monomer 1(5) shows absorption band at 2096 cmꢀ1
due to the stretching vibration of its azido functionality. The
HCt and CtC stretching vibrations of monomer 2 are
observed at 3276 and 2107 cmꢀ1. After polymerization, all these
are absent in the spectrum of P4(5), indicating that all the azido
groups of 1(5) and triple bonds of 2 have been consumed by
polycycloaddition.
We further characterized the polymers by NMR spectroscopy.
Figure 2 shows the 1H NMR spectra of monomers 1(5) and 2,
and their corresponding polymer P4(5) in DMSO-d6. The
polymer gives no peaks corresponding the proton adjacent to
the azido group of 1(5) and acetylenic proton of 2 at δ ∼3.3 and
∼4.1, respectively (Figure 2, parts A and B). Meanwhile, new
resonances assigned to the absorptions of the olefin proton of
1,4-disubstituted triazole ring and its neighboring methylene
protons are emerged at δ ∼ 8.5 and δ ∼ 4.3. No olefin proton
resonance associated with 1,5-disubstituted is detected and all
the peaks can be readily assigned, suggesting that the polymeric
product is indeed P4(5) with molecular structure as shown in
Figure 2C.
Figure 2. 1H NMR spectra of (A) monomer 1(5), (B) monomer 2,
and (C) their polymer P4(5) (sample taken from Table 1, no. 2) in
DMSO-d6 at room temperature. The solvent peaks are marked with
asterisks.
DMF. Although a higher yield is obtained, the polymer is soluble
only in DMF and DMSO but dissolutes partially in THF, DCM,
and chloroform. The limited solubility of the polymer should be
ascribed to the rigidity of the polymer chain and its higher
molecular weight. We thus fine-tuned the molecular weight of
the polymer by lowering the reaction time to 6 h and obtained
completely soluble products albeit in a low yield. Previously, it is
found that ruthenium catalysts can produce polymers with better
solubility due to the formation of more twisted 1,5-triazole units.33
When Cp*Ru(PPh3)2Clwasusedascatalyst for the polymerization
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Figure 3 shows the 13C NMR spectrum of polymer P4(5)
along with those of monomer 1(5) and 2. While the methylene
carbon next to the azido group of 1(5) resonates at δ 50.31
(Figure 3A), the absorptions of the acetylenic carbons of 2 occur
at δ 83.14 and 80.88 (Figure 3B). The spectrum of P4(5) shows
no these absorptions but new peaks associated with the reso-
nances of the olefin carbons at δ 145.81 and 142.76. This result
testifies that the azido groups of 1(5) and triple bonds of 2 have
been cyclized into triazole rings in P4(5), which well agrees with
those from the IR and 1H NMR analyses.
Electronic Tansitions. Figure 4 shows the absorption spectra
of the monomers and polymers in THF. The biphenyl pendant of
1(5) absorbs at 257 nm, while the absorption of the TPE moiety
of monomer 2 occurs at 259 and 329 nm. After polymerization,
their corresponding polymer P4(5) absorbs at slightly longer
wavelengths of ∼265 and 336 nm. Clearly, the polymer is more
conjugated because of electronic communication between the
TPE unit and the newly formed triazole rings. Because of the
structural similarity, polymer P4(10) shows an absorption pro-
file identical to that of P4(5). The UV spectra of P5(x) and P6(x)
are similar, featuring two maximum at 250 and 315 nm. Their
spectra patterns are resembled to those of 3(y) because their
triazole rings are well separated from the TPE units by long alkyl
spacers, which have hampered their electronic interactions.
Aggregation Induced Emission. TPE is a well-known AIE
luminogen and it is thus anticipated that our TPE-containing
Figure 4. Absorption spectra of monomers and polymers in THF.
Concentration: 2 ꢁ 10ꢀ5 M (monomers); 0.008 mg/mL (polymers).
Soluble fraction of P4(5) in THF was used for the measurement.
Figure 5. Emission spectra of (A) P4(5) in DMF/H2O and (C) P6(5) in THF/H2O mixtures with varied water fractions. Plots of (I/I0 ꢀ 1) values
versus water fractions of the aqueous mixtures at (B) 493 and (D) 475 nm for P4(5) and P6(5), respectively. Photos of P4(5) and P6(5) in DMF and
THF with 0 and 90% water contents taken under UV light, respectively, are given in panels B and D. Concentration (mg/mL): 0.044 [P4(5)], 0.06
[P6(5)]. Excitation wavelength (nm): 350 [P4(5)]; 340 [P6(5)].
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Figure 7. TGA thermograms of P4(x), P5(x), and P6(x) measured
under nitrogen at a heating rate of 20 ꢀC/min.
slightly longer wavelengths (∼488 nm) than 2 (483 nm). The
emission maxima of P5(x) are located at wavelengths similar to
that of 3(3). Those of P6(x) are, however, blue-shifted from 3(8)
by 14 nm. The relative longer alkyl spacers in P6(x) may have
effectively hampered the interactions between the TPE units,
thus resulted in emission at the bluer region.
To have a quantitative comparison, the quantum yields (ΦF’s)
of the polymers in THF solutions and solid film states are
measured. The ΦF values in solutions are quite low. Those of
their solid films are, however, much higher and ranged from 24.5
to 63.7%. Closer inspection reveals that ΦF values of the mono-
mers and polymers are varied by the spacer length. For example,
when the methylene unit is progressively increased from P4(5)
to P5(5) and then P6(5), the quantum yield decreases from
63.7% to finally 24.5%. Thus, the incorporation of electronically
saturated alkyl chains in the monomers and polymers are harmful
to their light emissions, presumably due to the obstruction of the
TPE aggregation formation.
Thermal Stability. Formation of mesophases of thermotropic
liquid crystals is realized by the application of heat, the thermal
stability of the polymers is thus of primary concern. Figure 7 gives
the TGA thermograms of the polymers recorded under nitrogen.
All the polymers show high resistance to thermolysis, with 5%
weight loss (Td) occurring from 346 to 406 ꢀC. The Td values are
much higher than that of polystyrene (330 ꢀC), a stable
commodity polymer.34 The high thermal stability of the poly-
mers renders them useful for practical applications.
Mesomorphism. After checking the thermal stability of the
polymers, we investigated their mesomorphic properties. As mono-
mers 1(5) and 1(10) contain mesogenic biphenyl unit, we first
checked whether they are liquid crystals. Figure 8 shows the DSC
thermograms of 1(5) and 1(10) recorded during the first cooling
and second heating processes, while their POM textures observed
on cooling from their isotropic liquids are given in Figure 9.
During the first cooling cycle, 1(5) shows three exothermic
peaks at 88.6, 7.8, and 3.7 ꢀC. POM observation reveals that upon
cooling 1(5) from the isotropic state (i) to 94.2 ꢀC, typical mosaic
texture of smectic B phase (SB) with lancet35 emerges from the
homotropic dark background. More anisotropic domains appear
at 92.8 ꢀC and their sizes grow bigger when the temperature is
further lowered (Figure 9A). The transition peaks at 7.8 and
Figure 6. Emission spectra of thin films of monomers 2, 3(3), 3(8), and
polymers P4(x), P5(x), and P6(x) at room temperature. Films were cast
from corresponding CHCl3 solutions (5 mg/mL). Excitation wave-
length: 350 nm. Quantum yields of the polymers in THF: 0.67% [P4(5),
THF-soluble fraction], 0.51% [P5(5)], 0.28% [P6(5)], 0.19% [P4(10)],
0.20% [P5(10)], and 0.26% [P6(10)].
polymers are also AIE-active. To confirm this, we investigate
their emission behaviors in solvent/nonsolvent mixtures. As
shown in Figure 5A, polymer P4(5) is almost nonluminescent in
pure DMF. Addition of water into its DMF solution, has, however,
enhanced its light emission. The higher the water content, the
stronger is the light emission. In 90% aqueous mixture, the
emission intensity is >135-fold higher than that in the pure DMF
solution, exhibiting a bright emission with its maximum at
493 nm (Figure 5B). Since biphenyl emits in the UV region,
the visible emission of P4(5) should be, in most cases, originated
from its TPE chromophoric units. The photographs depicted
in the inset exemplify the nonemissive and strongly luminescent
nature of the DMF solution and 90% aqueous mixtures. Clearly,
the AIE characteristics of the TPE units are preserved when
they are incorporated into polymer. Polymer P6(5) also exhibits
typical AIE behavior. As can be seen in Figures 5C and 5D, the
polymer emits no light in THF but gives a strong sky-blue emission at
475 nm when aggregated in the aqueous mixture. The emission is
gradually intensified with increasing water content in the THF/water
mixture. At 90% water content, the intensity is more than 159-fold
higher than that in pure THF solution. Similar phenomenon are also
observed in P4(10), P5(x), andP6(10) (Figures S1ꢀS4,Supporting
Information). In their 90% aqueous mixtures, the emission intensities
are >150-fold higher than those in pure THF.
We also investigated their photophysical properties in the
solid state. Thanks to the AIE effect of the TPE units, the bulk
solids of the P4(x), P5(x), and P6(x) are highly emissive under
UV light illumination (Figure S5, Supporting Information). The
polymer films are also capable of emitting light intensely. Figure 6
depicts their emission spectra; for comparison, the spectra of
their monomers 2 and 3(y) are also given in the same figure.
Since P4(5) and P4(10) possess higher conjugation, they emit at
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Figure 8. DSC thermograms of monomers 1(5) and 1(10) recorded
under nitrogen during the first cooling and second heating cycles with a
scan rate of 10 ꢀC/min.
Figure 10. X-ray diffraction patterns of 1(5) and 1(10) obtained on
cooling from their isotropic liquids.
Figure 9. Mesomorphic textures observed on cooling (A) 1(5) to
92.8 ꢀC and (B) 1(10) to 95.4 ꢀC from their isotropic states at a cooling
rate of 0.5 and 0.2 ꢀC/min, respectively.
3.7 ꢀC are ascribed to SBꢀcrystal (k) and kꢀk transitions, respec-
tively. The DSC curve recorded in the second heating cycle is the
mirror image of that obtained in the first cooling scan, three
endothermic peaks corresponding to the kꢀk, kꢀSB, and SBꢀi
transitions are detected at 29.8, 34.2, and 91.5 ꢀC, respectively.
1(10) is the congener of 1(5) with much longer flexible
spacers and it is of interest to study how it behaves mesomor-
phically. As depicted in Figure 8, during the first cooling cycle,
1(10) shows two distinct transition peaks at 93.6 and 51.0 ꢀC.
When it is cooled from its isotropic state to 95.4 ꢀC, mosaic texture
of SB mesophase with lancet similar to that of 1(5) is observed. The
second heating scan detects two endothermic peaks at 57.8 and
95.4 ꢀC, which are associated with the kꢀSB and SBꢀi transitions,
respectively.
To gain more information on the monomer packing arrange-
ments in the mesophases, 1D-WAXD measurements were
carried out. The diffractograms of 1(5) and 1(10) obtained at
100 ꢀC display only diffuse halos at high-angle region, indicative
of their isotropic feature (Figure 10). When 1(5) is cooled to
90 ꢀC, two sharp peaks at 2θ = 2.99 and 19.83ꢀ are recorded.
The d-spacing derived from the former peak is 29.53 Å, which
is comparable to the fully extended molecular length of 1(5)
(l = 31.63 Å). The peak at 2θ = 19.83ꢀ has quite a narrow
full width at half-maximum of 0.326ꢀ. Such results suggest that
long-range order is present within the smectic layer, which is
typical characteristic of the SB mesomorphic phase.
Figure 11. DSC thermograms of the polymers recorded under nitrogen
during the first cooling (solid line) and second heating (dash line) cycles
at a scan rate of 10 ꢀC/min.
Similar WAXD pattern is observed in 1(10) when it is cooled
to 80 ꢀC. The reflection at 2θ = 20.20ꢀ gives the average distance
of the shorter preferred spacing (d = 4.39 Å) occurring in the
lateral packing arrangement of the mesogens. The layer spacing
derived from the peak at 2.11ꢀ is 41.84 Å, which well approaches to
the molecular length of 1(10) (l = 44.07 Å) at its most extended
conformation. All these features confirm the formation of SB phase.
We then continued our study on the mesomorphic properties
of the polymers. We first checked the thermal transitions of the
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Figure 12. Mesomorphic textures observed on cooling (A) P4(5) to
159.9 ꢀC, (B) P5(5) to 89.9 ꢀC, (C) P6(5) to 94.9 ꢀC, (D) P4(10) to
250.6 ꢀC, (E) P5(10) to 69.8 ꢀC, and (F) P6(10) to 114.9 ꢀC from their
melting states at a cooling rate of 1 ꢀC/min. Photos in parts A, B, D, and
E are taken after application of a shearing force.
Figure 13. X-ray diffractograms of P4(10) at different temperatures
obtained during the (A) heating and (B) cooling cycles.
polymers by DSC. In the first cooling cycle, P4(5), P4(10), and
P5(5) exhibit discontinuities in the heat capacity at 152.2, 102.4,
and 67.2 ꢀC (Figure 11), respectively, associated with the glass
transitions of the polymer chains. The corresponding transitions
in the second heating scan are observed at 155.4, 106.9, and 69.2 ꢀC,
respectively. For P5(10), while no obvious peak is observed in the
first cooling process, a weak diffusion transition peaked at 104.8 ꢀC is
observed, which may be resulted from a close overlap of an
exothermic peak of chain rearrangement and an endothermic peak
of a LCꢀi phase transition.
o
broad peak at 2θ = 6.27 and a halo at 18.91ꢀ. The spectral
profile keeps almost unchanged when the polymers is heated to
temperatures up to 210 ꢀC, which is much higher than the glass
transition temperature (Tg). The diffractogram also changes little
upon cooling (Figure 13B). It indicates that the film formed from
the solution is liquid crystalline and the mesophase is stable
during both heating and cooling processes. To some degree, this
result is similar to those observed in Zhou’s mesogen-jacketed
liquid-crystalline polymers, in which the mesomorphic phases are
stable at both elevated and low temperatures.36 It is in some sense
understandable when we take the high stiffness of the polymer
main chain into consideration. The rigid polymer strand hampers
the packing of the mesogenic units but on the other hand,
stabilizes and fixes their alignment. The d-spacing associated with
the diffuse halo changes at temperatures higher than Tg, which is
commonly observed in LCs exhibiting nematicity.37 According
to its poor packing feature and temperature-dependent d-spacing
phenomenon, we can assign the mesomorphic phase of P4(10)
to be nematic.
Unlike to those observed for P4(x) and P5(x), the thermo-
grams of P6(x) show distinct sharp peaks, which are usually
associated with first order phase transitions. As depicted in
Figure 11, the thermogram of P6(5) recorded in the first cooling
cycle is basically a flat line parallel to the abscissa. On the
contrary, during the second heating cycle, two peaks related to
chain rearrangement and LC-i transition are observed at 77.1 and
118.8 ꢀC, respectively. For those of P6(10), sharp peaks at 90.2
and 119.7 ꢀC are recorded during the first cooling and second
heating circles, which are corresponded to the i-LC and LC-i
transitions, respectively.
When three more methylene units are introduced into P4(10),
the resultant polymer P5(10) shows better mesogenic pack-
ing. Its X-ray diffractograms measured at different temperatures
during the heating and cooling cycles are depicted in Figure 14.
At 40 ꢀC, the diffractogram shows three reflection peaks at 2θ =
3.93, 8.30, 20.48 and a diffuse halo at 18.40ꢀ, from which d-
spacings of 22.46, 10.64, 4.32, and 4.82 Å are derived, respectively
(Figure 14A). The d-spacings of the former two peaks is close to
2:1, meanwhile the peak in the high angle region is relatively
narrow, thus indicating the formation of smectic phase and
relatively ordered molecular packing within the layer. The peaks
in low angle region become weaker at elevated temperatures and
disappear completely at 110 ꢀC, which is associated with the LC-i
transition and consistent with the DSC result. Meanwhile, in the
high angle region, the peaks become broader and weaker and
only the diffuse halo remained. In the subsequent cooling process
from 150 to 40 ꢀC, no observable change is observed in the
diffractograms. This indicates that the ordered mesogenic pack-
ing lost in the heating scan cannot be recovered by subsequent
cooling process due to the low chain mobility caused by the still
rigid polymer strand.
No birefringences are observed when P4(x) and P5(x) are
cooled from their melted states. Anisotropic domains are, how-
ever, emerged under shearing (Figure 12, parts A, B, D, and E).
Thus, all the polymers are mesomorphic. The liquid crystalline
domains of the polymers are too small to be observed without
external stimuli. Such phenomenon should be caused by the high
rigidity of the main chain, which impedes the packing arrange-
ments of the mesogenic units. When a shear force is applied, the
small domains merge together and aligned along the shear
direction, thus them observable under POM.
Thanks to the relative longer flexible spacers in P6(5) and
P6(10) which render the mesogenic units more freedom to pack,
small entities are emerged from the homotropic dark background
when their isotropic liquids are cooled. The entities grow bigger
upon cooling but their development into a typical texture seems
to be a difficult task. We attempted to grow such bigger LC
domains with care but failed to obtain any characteristic texture.
We carried out 1D-WAXD experiments of the polymers at
different temperatures in order to gain more information con-
cerning their molecular arrangements, and modes of packing in
the mesophases. As shown in Figure 13A, the diffractogram of
thin film of P4(10) cast from its THF solution at 40 ꢀC shows a
Polymer P6(10) has even longer flexible spacers than that of
P5(10), and its temperature-variable WAXD diagrams are shown
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ARTICLE
soluble polytriazoles in high yields (up to 97.1%) narrow
molecular weight distributions.
All the polymers are AIE-active. While they emit weakly in
solutions with ΦF values not more than 0.67%, they become strong
emitters in the aggregated states with ΦF values up to 63.7%. The
ΦF value is sensitive to the spacer length and decreases when the
ethylene unit in the polymers becomes higher.
All the polymers enjoy high thermal stability and are liquid
crystalline. While polymers P4(x) possess rigid main chains and
thus exihibit nematicity, their counterparts with longer spacer
lengths, i.e., P5(x) and P6(x), show better mesogenic packing
and may form smectic phases. Our results have shown that
through rational design, it is feasible to create liquid crystals with
strong light emissions. We are currently working on the synthesis
of new mesomorphic polymers and studied their polarized
emission. Details will be published in a separate paper.
Figure 14. X-ray diffractograms of P5(10) at different temperatures
obtained during the (A) heating and (B) cooling cycles.
’ ASSOCIATED CONTENT
S
Supporting Information. Synthetic procedures and char-
b
acterization data of the polymers and monomers, emission spectra
of P4(10), P5(5), P5(10), and P6(10) in THF/water mixtures,
photographs of the polymer powders taken under normal room
and 365 nm UV illumination, schematic illustration of the molec-
ular length of 1(x) in their fully extended conformations, XRD
diffractograms of P4(5), P5(5), and P6(5), and emission spectra of
different P6(5) and P6(10) films. This material is available free of
charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*Telephone: +852-2358-7375. Fax +852-2358-1594. E-mail tangbenz@
ust.hk.
’ ACKNOWLEDGMENT
We thank the support from the Research Grants Council of
Hong Kong (603509, HKUST2/CRF/10, and 604711), the
Innovation and Technology Commission (ITP/008/09NP), the
University Grants Committee of Hong Kong (AoE/P-03/08),
the NSFC/RGC grant (N_HKUST620/11), and the National
Science Foundation of China (21104044, 20974028, 21074073).
Figure 15. X-ray diffractograms of P6(10) at different temperatures
obtained during the heating and cooling cycles.
in Figure 15. At 30 ꢀC, three peaks and one diffuse halo at 2θ =
2.63, 5.47, 7.77, and 20.76ꢀ are observed. When the sample is
heated to 130 ꢀC, the former three reflections disappear, reveal-
ing that the isotropic state is reached and the mesogens are now
randomly orientated. These peaks, are, however, emerged again
upon cooling (Figure 15B), thanks to the relative more flexible
chain of P6(10), which provides little constraint for the meso-
gens to pack in the mesophase. From the XRD profile, it is
reasonable to assign its mesogenic phase as smectic. For detailed
phase identification, further investigation on its molecular packing
is required and is actively undergone in our laboratory. Other
WAXD measurement result for P4(5), P5(5), and P6(5) are
given in Figures S6ꢀS8, Supporting Information. No obvious
signals at low angles are recorded, suggesting that all the polymers
exhibit nematicity at high temperature.
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