Photoluminescent, liquid-crystalline, and electrochemical properties of para-phenylene-based alternating conjugated copolymers

Article abstract of DOI:10.1002/pola.23803

Novel liquid-crystalline alternating conjugated copolymers [P(P(6)CN-alt-Cz) and P(P(6)CN-alt-MeP)] with phenylene and carbazolylene or phenylene with methyl substitution onto the main chain have been synthesized through palladium-catalyzed Suzuki coupling reactions. The influence of the incorporation of carbazolylene and the substituted phenylene into the main chain on the thermal, mesomorphic, and luminescent properties has been investigated by Fourier transform infrared spectroscopy, thermogravimetry, differential scanning calorimetry, polarized optical microscopy, ultraviolet-visible spectroscopy, photoluminescence (PL), and cyclic voltammetry. These polymers show highly thermal stability, losing little of their weights when heated to 360 °C. The conjugated copolymers exhibit liquid crystallinity at elevated temperature. The existence of the chromophoric terphenyl core endows the copolymers with high PL and the polymer P(P(6)CN-alt-Cz containing carbazolylene unit can emit more pure blue light. All the copolymer films with low band gaps about 2.3-2.4 eV undergo reversible oxidation and reduction processes, significantly lower than the band gap of poly(pphenylene).

Full text of DOI:10.1002/pola.23803

Photoluminescent, Liquid-Crystalline, and Electrochemical Properties
of Para-Phenylene-Based Alternating Conjugated Copolymers
KAI YAO,1 YIWANG CHEN,1 LIE CHEN,1 HUANLING KONG,1 WEIHUA ZHOU,1 FAN LI,1 XIAOHUI HE,1 YEN WEI2
1Institute of Polymers/Department of Chemistry, Nanchang University, Xuefu Road 999, Nanchang 330031, China
2Department of Chemistry, Drexel University, Philadelphia, Pennsylvania 19104
Received 20 September 2009; accepted 28 October 2009
DOI: 10.1002/pola.23803
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: Novel liquid-crystalline alternating conjugated
copolymers [P(P(6)CN-alt-Cz) and P(P(6)CN-alt-MeP)] with
phenylene and carbazolylene or phenylene with methyl substi-
tution onto the main chain have been synthesized through
palladium-catalyzed Suzuki coupling reactions. The influence
of the incorporation of carbazolylene and the substituted phen-
ylene into the main chain on the thermal, mesomorphic, and
luminescent properties has been investigated by Fourier trans-
form infrared spectroscopy, thermogravimetry, differential
scanning calorimetry, polarized optical microscopy, ultravio-
let–visible spectroscopy, photoluminescence (PL), and cyclic
voltammetry. These polymers show highly thermal stability,
losing little of their weights when heated to 360 ꢀC. The conju-
gated copolymers exhibit liquid crystallinity at elevated tem-
perature. The existence of the chromophoric terphenyl core
endows the copolymers with high PL and the polymer
P(P(6)CN-alt-Cz containing carbazolylene unit can emit more
pure blue light. All the copolymer films with low band gaps
about 2.3–2.4 eV undergo reversible oxidation and reduction
processes, significantly lower than the band gap of poly(p-
C
V
phenylene).
2009 Wiley Periodicals, Inc. J Polym Sci Part A:
Polym Chem 48: 434–442, 2010
KEYWORDS: conjugated polymers; liquid-crystalline polymers
(LCP); liquid crystallinity; luminescence; photoluminescence;
poly(p-phenylene)
INTRODUCTION p-Conjugated aromatic polymers repre-
sented by poly(p-phenylene) (PPP), poly(p-phenylene-vinyl-
ene), and polythiophene and their copolymers are promising
candidates for advanced functional materials and devices,
such as light-emitting diodes (LEDs),1 electrochemical cells,2
and optoelectronic devices.3 Liquid crystals (LCs) find large-
scale applications in LC displays and are promising materials
for the induction of mesoor macroscale dynamics under mo-
lecular control, because they are spontaneously oriented and
furthermore macroscopically aligned by an external force,
such as shear stress, electric field, and magnetic force field,
giving rise to electrical and optical anisotropies and even a
linearly polarized luminescence.4,5 Side-chain liquid-crystal-
line polymers (LCPs), which prepared by introducing meso-
genic moieties into backbones of main chains, are of both
theoretical and practical interest because the side-chain sub-
stitution is essential to obtain solubility, at the same time, to
control the molecular orientation.6
ent derivatives offers the prospect of controlling polymer
properties, such as solubility, emission wavelengths, process-
ability, and molecular orientation.8,9 Thus, the LC conjugated
PPPs have been extensively investigated by attaching differ-
ent mesogenic units as the side chain.10,11 However, the
study on the LC conjugated copolymer based on PPPs and
other different aromatic moieties on the main chain is still
lacking.
The terphenyl, which is not only a strong light-emitting chro-
mophore but also a mesogenic core,12,13 has been introduc-
ing to the conjugated main chains as the mesogen pend-
ants.14,15 In our previous work, the PPP homopolymer
containing terphenyl mesogenic pendant, {A[C6H3ACOOA
(CH2)6AO-terphenylACN]nꢁ
, PP(6)CN}, has been synthe-
sized.16 The terphenyl mesogen pendant endows the conju-
gated copolymers with good LC and the energy could trans-
fer from the mesogen pendants to the main chain to favor
the photoluminescence (PL). To realize more about this type
of PPP bearing terphenyl mesogen, we move forward to pre-
pare a series of LC alternating copolymer with other aryl
moieties. In this article, we attempt to synthesis a novel
alternating conjugated copolymer P(P(6)CN-alt-MeP), where
the phenylene with methyl substitution units is inserted into
PPPs constitute a particularly important class of polymers in
that they emit in the blue and can act as hosts for downhill
energy transfer to generate green and red emitters in poly-
mer LEDs.7 Besides, compared with the instability and
intractability of PPP, PPP containing mesomorphic substitu-
Additional Supporting Information may be found in the online version of this article. Correspondence to: Y. Chen (E-mail: ywchen@ncu.edu.cn) or
L. Chen (E-mail: chenlienc@163.com)
C
V
Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 48, 434–442 (2010) 2009 Wiley Periodicals, Inc.
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ARTICLE
the repeated unit of PPPs containing terphenyl mesogenic
pendants. The steric crowding between the bulky terphenyl
mesogen pendants could be weakened by introducing phen-
ylene ring with methyl substituents, which probably lead the
copolymer with interesting properties. Besides, carbazole, an
aromatic heterocycle, which is a well-known hole-transport-
ing unit due to the electron-donating capabilities associated
with its nitrogen atom, enables higher current densities and
consequently higher luminance levels at lower voltages.17,18
Moreover, the carbazoles maintain a sufficiently high triplet
energy level due to the limited electronic conjugation of
biphenyl upon polymerization.19 For this reason, we also
incorporated carbazolylene onto PPP main chain, and the
properties of polymer P(P(6)CN-alt-Cz) containing terphenyl
mesogenic pendant are systemically studied.
Differential scanning calorimetry (DSC) was used to deter-
mine phase-transition temperatures on a Perkin-Elmer DSC
7 differential scanꢀning calorimeter with a constant heating/
cooling rate of 10 C/min. Texture observations by polarizing
optical microscopy (POM) were made with a Nikon E600POL
POM equipped with an Instec HS 400 heating and cooling
stage. The X-ray diffraction (XRD) study of the samples was
carried out on a Bruker D8 Focus X-ray diffractometer oper-
ating at 30 kV and 20 mA with a copper target (k ¼ 1.54 Å)
and at a scanning rate of 1ꢀ/min. Cyclic voltammetry (CV)
was carried out on a CHI660C electrochemical workstation
with platinum working electrode at a scan rate of 20 mV/s
against a Ag/AgNO3 (0.1 M) reference electrode with nitro-
gen-saturated solution of tetra-n-butylammonium tetrafluoro-
borate (0.1 M, TBAPF4) in acetonitrile (CH3CN) and platinum
wire as counter electrodes in a standard three-electrode cell.
Elemental analyses (EA) were characterized by means of EA
with Vario Elementar III.
EXPERIMENTAL
Materials
The monomer 2,5-bromo-1-[(6-(4-(40-cyano)terphenyloxy)-
hexyloxy)carbonyl]-benzene [P(6)CN] was synthesized
according to the literature,16 and the obtained product was
thoroughly purified before polymerization. Trimethyl borate,
n-butyllithium, thiophene-3-acetic acid, 2,5-dibromotoulene,
6-bromo-1-hexanol, 4-(4-bromophenyl)phenol, 4-bromoben-
zonitrile, N-ethyl-carbazole, 1,3-dicyclohexylcarbodiimine, 4-
(dimethylamino)pyridine, and tetrakis(triphenylphosphine)-
palladium were purchased from Alfa Aesar and used as
received without any further purification. Tetrahydrofuran
(THF) and CH2Cl2 were dried over sodium. Other chemicals
were obtained from Shanghai Reagent and used as received.
Synthesis of 1,4-Toluenediboronic Acid (1)
To a solution of 5 g 3,6-dibromo toluene (0.02 mol) in THF
ꢀ
(50 mL) at ꢁ100 C, 15 mL of n-butyllithium (2.87 M in hex-
ane, 0.043 mol) was added by syringe. The mixture was
stirred at below ꢁ78 ꢀC for 2 h. A solution of trimethyl
borate 10.4 mL in THF was added rapidly to the solution
and was stirred at ꢁ78 ꢀC for another 2 h. The resulting
mixture was warmed to room temperature and stirred for
24 h. The product was extracted into ether after the dilute
hydrochloric acid was added. The organic layer was washed
with brine and dried over anhydrous magnesium sulfate. The
solvent was removed under vacuum and the crude dissolved
in a small amount of THF and reprecipitated with hexane to
give a white solid with yield of 70%. IR (cmꢁ1): 3290, 2938,
1596, 1497, 792. 1H NMR (ppm, THF-d): 7.73 (m, aromatic,
3H) 2.25(m, CH3-Ar, 3H).
Techniques
The infrared (IR) spectra were measured on a Shimadzu IR
Prestige-21 Fourier transform infrared spectrophotometer
with the KBr method. The nuclear magnetic resonance
(NMR) spectra were collected on a Bruker ARX 400 NMR
and Bruker AV 600 NMR spectrometer with deuterated chlo-
roform or THF or DMSO as the solvent and tet-
ramethylsilane(d ¼ 0) as the internal reference. The ultravio-
let–visible (UV) spectra of the samples were obtained with a
Hitachi UV-2300 spectrophotometer. Fluorescence measure-
ment for PL of the polymers in THF was carried out on a
Shimadzu RF-5301 PC, and fluorescence of solid state was
recorded on a Hitachi F-4500 spectrofluorophotometer with
a xenon lamp as the light source. The quantum yields were
obtained on a FLS-920 (Edinburgh Instruments) with an in-
tegral sphere. The gel-permeation chromatography, so-called
size-exclusion chromatography analysis, was conducted with
a Breeze Waters system equipped with a Rheodyne injector,
a 1515 Isocratic pump, and a Waters 2414 differential
refractometer using polystyrene as the standard and THF/
N,N-dimethylformamide (DMF) as the eluent at a flow rate of
Synthesis of Conjugated Copolymer P(P(6)CN-alt-MeP)
Monomer P(6)CN (0.63 g, 1 mmol) and 1 (0.185 g, 1.03
mmol) were dissolved in 15 mL of dry toluene under N2. To
the solution were added K2CO3 (aq) (2.0 M, 5 mL; N2
bubbled before use), Pd(PPh3)4 (0.12 g, 0.1 mmol), and 5
mL ethanol. After the mixture was stirred for 3 days at 80
ꢀC under an N2 atmosphere, the solvent was evaporated. The
resulting solid was dissolved in a small amount of THF and
reprecipitated in methanol for 24 h. After filtration, the poly-
mer was further washed with acetone and dried under vac-
uum to afford dark brown powder.
Polymer P(P(6)CN-alt-MeP)
Dark brown solid, Yield ¼ 65%. IR (KBr, cmꢁ1): 2933, 2854,
2218, 1723, 1592, 1481, 806. 1H NMR (ppm, THF-d):
8.10–8.00 (m, aromatic, 1H), 7.99–7.57 (m, aromatic, 15H),
6.98–6.90 (d, aromatic, 2H ortho to AOA), 4.30–4.25 (t,
2H, ACH2AOAAr), 4.01–3.98 (t, 2H, ACH2AOOCA),
1.99–1.92 (m, 3H, CH3-Ar), 1.60–1.51 (m, 4H, ACH2
ꢀ
1.0 mL/min and 40 C through a Styragel column set, Styra-
gel HT3 and HT4 (19 mm ꢂ 300 mm, 103 þ 104 Å) to sepa-
rate molecular weight (MW) ranging from 102 to 106. Ther-
CH2(CH2)2CH2CH2A),
1.51–1.37
(m,
4H,
ACH2
mogravimetric analysis (TGA) was performed on
a
CH2(CH2)2CH2CH2A). Anal. Calcd. for [C39H33O3N]n: C: 83.13;
PerkinElmer TGA 7 for thermogravimetry at a heating rate
of 20 C/min under nitrogen with a sample size of 8–10 mg.
H: 5.86; N: 2.48. Found: C: 83.75; H: 5.69; N: 2.39. Mn
17,860; Mw/Mn ¼ 2.13.
¼
ꢀ
PROPERTIES OF p-PHENYLENE-BASED COPOLYMERS, YAO ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
SCHEME 1 Illustration of proce-
dures for the synthesis of
P(P(6)CN-alt-MeP) and P(P(6)CN-
alt-Cz).
Synthesis of 3,6-Dibromo-N-ethyl-carbazole (2)
The organic layer was washed with brine and dried over an-
hydrous magnesium sulfate. The solvent was removed under
vacuum, and the residue was washed by large amount of n-
hexane to provide 3.0 g of the title product as a pale-white
solid in 65% yield.
N-Ethyl-carbazole (10 g, 70.3 mmol) was slowly added to a
solution of N-bromosuccinimide, NBS (30.2 g, 172 mmol), in
50 mL of DMF by a dropping funnel. After the addition, 10
mL of DMF was further added to the reaction mixture and
ꢀ
refluxed at 0 C for 20 h under an N2 atmosphere. The reac-
IR (cmꢁ1): 3290, 2938, 1596, 1497, 1272, 792. 1H NMR
(ppm, THF-d): 8.02 (m, aromatic, 2H); 7.64 (m, aromatic,
2H); 7.13 (t, aromatic, 2H); 3.89 (m, CH2AN, 2H); 1.51 (t,
CH3ACH2AN, 3H).
tion vessel was wrapped by aluminum foil to shield the reac-
tion from light. When the reaction completed, the reaction
mixture was allowed to warm to room temperature. The so-
lution was poured into a large amount of saturated sodium
sulfate cooled by ice water. The yellow precipitate was fil-
tered off and recrystallized from an ethanol/water mixture
to yield the product as white needle-like crystal. Yield:
63.0%. IR (cmꢁ1): 2943, 2864, 1280, 831, 743, 652, 482.
Synthesis of Conjugated Copolymer P(P(6)CN-alt-Cz)
Monomer P(6)CN (0.63 g, 1 mmol) and 3 (0.36 g, 1.02 mmol)
were dissolved in 15 mL of dry toluene under N2. Tetrakis(tri-
phenylphosphine)-palladium [Pd(PPh3)4] (0.12 g, 0.1 mmol)
was added into a mixture of toluene and 2 M potassium carbon-
1H NMR (ppm, THF-d): 8.10 (d, 2H); 7.59 (m, 2H); 7.27 (t,
2H); 3.91 (m, CH2AN, 2H); 1.53 (t, CH3ACH2AN, 3H).
ꢀ
ate aqueous solution. The mixture was stirred at 85–90 C for
72 h under the protection of N2. After the mixture was cooled
to the room temperature, it was poured into the stirred metha-
nol. A solid was obtained by filtration. The copolymer was fur-
ther purified by stirring in acetone for 24 h to remove oligom-
ers and catalyst residues. The resulting product was dried
under reduced pressure at room temperature.
Synthesis of N-Ethyl-carbazole-3,6-diboronic Acid (3)
To a solution of 5 g 3,6-dibromo-N-ethyl-carbazole (14.1
mmol) in THF (50 mL) at ꢁ78 ꢀC, 12 mL of n-butyllithium
(2.87 M in hexane, 0.034 mol) was added by syringe. The
ꢀ
mixture was stirred at ꢁ78 C for 2 h. Trimethyl borate (8.2
mL) was added rapidly to the solution, and the mixture was
ꢀ
stirred for 2 h at ꢁ78 C. The resulting mixture was warmed
Polymer P(P(6)CN-alt-Cz)
to room temperature and stirred for 24 h. Dilute hydrochlo-
ric acid was added and the mixture was extracted with ether.
Orange solid, Yield ¼ 69%. IR (KBr, cmꢁ1): 2933, 2854,
2218, 1723, 1592, 1480, 1268, 806. 1H NMR (ppm, DMSO-
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ARTICLE
ꢀ
d): 8.10–8.00 (m, aromatic, 1H), 7.99–7.57 (m, aromatic, 17H),
6.98–6.90 (d, aromatic, 2H ortho to AOA), 4.38–4.29 (t, 2H,
ACH2AOAAr), 4.09–4.01 (t, 2H, ACH2AOOCA), 3.49–3.42
(m, 2H, ACH2AN), 1.73–1.58 (m, 4H, ACH2 CH2(CH2)2
CH2CH2A), 1.51–1.24 (m, 7H, ACH2 CH2(CH2)2CH2CH2A and
CH3ACH2AN). Anal. Calcd. for [C46H38O3N2]n: C: 82.88; H:
5.71; N: 4.20. Found: C: 83.43; H: 5.51; N: 4.33. Mn ¼ 13,590;
Mw/Mn ¼ 2.43.
(ꢄ360 C), and the incorporation of terphenyl rings into the
main-chain structure has clearly enhanced the resistance of
the copolymer to thermolysis (see Supporting Information).
The ‘‘jacket effect’’ of the bulky terphenyl mesogenic appen-
dages well wraps the conjugated backbones and thus limits
their perturbations by heat and/or attack by the degradative
species. The DSC analysis further substantiates the thermal
stability of the copolymers: no irreversible peaks suspi-
ciously associated with copolymer degradation are observed
at the high temperatures during the cycles of repeated hea-
ting–cooling scans.
RESULTS AND DISCUSSION
Synthesis of the Conjugated Copolymer
The synthetic routes and structures of the copolymers are
shown in Scheme 1. Using tetrakis(triphenylphosphine) pal-
ladium(0) [Pd(PPh3)4] as catalyst, polymers P(P(6)CN-alt-
MeP) and P(P(6)CN-alt-Cz) were synthesized by Suzuki
coupling reaction of P(6)CN with 1,4-toluenediboronic acid
and N-ethyl-carbazole-3,6-diboronic acid, respectively. These
conjugated copolymers are readily dissolved in common or-
ganic solvents, such as DMF and DMSO, and polymer
P(P(6)CN-alt-MeP) even can dissolved in THF. Number-av-
erage (Mn) and weight-average (Mw) MWs of the polymers
are summarized in the Experimental section. All the interme-
diate and final products have been thoroughly purified and
fully characterized, and the chemical structures of the
Liquid Crystallinity
After checking the thermal stabilities of the copolymers, we
proceed to study their mesomorphic behaviors. Figure 2
shows the POM microphotographs of the textures of
P(P(6)CN-alt-MeP) and P(P(6)CN-alt-Cz). When copoly-
mers are cooled from its isotropic stage, bright colorful
mesogenic textures emerged from the homotropic dark
1
copolymers are confirmed by FTIR, H NMR, and EA.
Structural Characterization
All the purified monomers and polymerization products give
satisfactory spectroscopic data corresponding to their
expected molecular structures (see Experimental section for
details). P(P(6)CN-alt-MeP) and P(P(6)CN-alt-Cz) exhibit
the similar IR distinct peak at 2933–2863, 2218, 1733, and
1250 cmꢁ1, which are due to stretching vibrations of
A(CH2)6A, CBN, C¼¼O, and Ar-O, respectively. There is no
obvious difference except the assigned peak to CAN at 1480
cmꢁ1 showed in the spectrum of P(P(6)CN-alt-Cz). The
AOH bending vibration at ꢃ3290 cmꢁ1 can be observed in
the monomers 1 and 3, whereas the bending vibration dis-
appears in the corresponding spectra of polymers P(P(6)CN-
alt-MeP) and P(P(6)CN-alt-Cz), indicating that the polymer-
izations were performed successfully by Pd-complex-cata-
lyzed polycondensation (for details, see Supporting Informa-
1
tion). The H NMR spectra of copolymers correspond well to
the proposed structure. Signals of aliphatic and aromatic
protons are observed in the ranges of d 1.2–4.4 and d
6.9–8.1, respectively. Except the peaks of solvent and water
remain in the spectra, no unexpected signals are observed in
the spectra of the copolymers, and all the resonance peaks
can be assigned to appropriate protons, as can be clearly
1
seen from Figure 1. The H NMR results are quite consistent
with the EA results.
Thermal Stability
As the formation of mesophases of a thermotropic LCP is
realized by the application of heat, the thermal stability of
the polymer is thus of primary importance. The thermal sta-
bility of the copolymers is evaluated by TGA under a nitro-
gen atmosphere. All the copolymers exhibit excellent thermal
stabilities with little decomposition at high temperatures
FIGURE 1 1H NMR spectra of polymers P(P(6)CN-alt-MeP) and
P(P(6)CN-alt-Cz).
PROPERTIES OF p-PHENYLENE-BASED COPOLYMERS, YAO ET AL.
437

JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
cooling and the second heating cycle, the DSC thermograms
recorded of the copolymers scan exhibit two peaks. Polymer
P(P(6)CN-alt-MeP) enters the SmA phase from its isotropic
ꢀ
state at 256.6 C. The mesophase is stable in a long tempera-
ꢀ
ture range over 82 C before P(P(6)CN-alt-MeP) finally sol-
idifies at 174.6 ꢀC. The associated g-SmA and SmA-i transi-
tions are observed at 179.2 and 283.9 ꢀC, respectively. The
mesomorphism is thus enantiotropic. The transition profiles
of P(P(6)CN-alt-Cz) are similar to those of P(P(6)CN-alt-
MeP), and the i-SmAd and SmAd-g transitions are found at
ꢀ
280.7 and 160.3 C, respectively. In the second heating scan,
its corresponding g-SmAd ꢀand SmAd-i transitions are
observed at 181.1 and 287.5 C, respectively. Compared with
homopolymer PP(6)CN with the temperature transition
range (ꢃ45 ꢀC), both of the copolymers possess a much
broader temperature range (ꢃ80 ꢀC) and higher transition
temperature, thanks for introducing the rigid aromatic rings
onto the main chain. The thermal transitions and their
FIGURE 2 Mesomorphic textures observed on cooling (a)
P(P(6)CN-alt-MeP) to 190 ꢀC and (b) P(P(6)CN-alt-Cz) to 220 ꢀC
from their isotropic melts.
background, forming an anisotropic texture, suggesting that
the terphenyl pendants with long flexible spacer A(CH2)6A
favors the mesogens to undergo thermal transitions in a rel-
atively independent fashion. The textures grow into bigger
dimension upon further cooling, but the exact nature of the
mesophases is difficult to identify. We repeatedly try to grow
the LCs with care but failed to obtain any readily identifiable
characteristic textures. With the aid of XRD measurements,
the textures of P(P(6)CN-alt-MeP) and P(P(6)CN-alt-Cz)
are identified to be associated with SmA and SmAd phases,
respectively (vide post). Reheating the copolymers, both of
them exhibit enantiotropic optical anisotropy with colorful
textures, suggesting that the mesomorphism is enantiotropic.
FIGURE 3 DSC thermograms of the copolymers recorded
under nitrogen during (a) the second heating and (b) first cool-
ing scans at a scan rate of 10 ꢀC/min.
Figure 3 shows the DSC thermograms of P(P(6)CN-alt-MeP)
and P(P(6)CN-alt-Cz) recorded under nitrogen. In the first
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TABLE 1 Thermal Transitions and Corresponding Thermodynamic Parameters of P(P(6)CN-alt-MeP) and P(P(6)CN-alt-Cz)
T, ꢀC [DH, kJ/mru; DS, J/(mru K)]a
Copolymers
Cooling
Heating
P(P(6)CN-alt-MeP)
P(P(6)CN-alt-Cz)
i 256.6 (ꢁ0.46; ꢁ0.86) SmA 174.6 (ꢁ4.81; ꢁ10.74) g
g 179.2 (5.35; 11.82) SmA 283.9 (0.50; 0.89) i
g 181.1 (0.34; 0.74) SmAd 287.5 (10.53; 18.79) i
i 280.7 (ꢁ11.01, ꢁ19.87) SmAd 160.3 (ꢁ0.20; ꢁ0.46) g
a
Sm, smectic phase; g, glassy state; i, isotropic liquid; mru, monomer
repeat unit.
Data taken from the DSC thermograms recorded under dry nitrogen in
the first cooling and second heating scans.
corresponding enthalpy and entropy changes of P(P(6)CN-
alt-MeP) and P(P(6)CN-alt-Cz) are summarized in Table 1.
The large DH and DS changes involved in the mesomorphic
transitions of P(P(6)CN-alt-MeP) and P(P(6)CN-alt-Cz) rule
out the possibility of nematic phase and further support the
assignment of smecticity to the mesophases of the poly-
phenylene derivates.20
positional order in the polymer again rules out the possibil-
ity of nematic classification.23 This confirms the SmA meso-
phase nature of P(P(6)CN-alt-MeP) and suggests that the
mesogens are packed in a monolayer structure. Similarly,
P(P(6)CN-alt-Cz) displays a sharp reflection at the low angle
of 2h ¼ 2.15ꢀ corresponds to a layer spacing of 40.04 Å,
which is longer than the molecular length (28.40 Å). Because
the d/l ratio is ꢃ1.40, the bilayer structure is thus an SmAd
type, in which the cyanoterphenyl mesogen is interdigitated
in an antiparallel fashion. The mesogens of the copolymers
are packing so well that the high-order secondary reflection
at a middle angle (2h ¼ 5.45ꢀ and 2h ¼ 5.87ꢀ, respectively)
was readily detected by the diffractometer.24
XRD analysis can provide useful information concerning
molecular arrangement, mode of packing, and type of order
in a mesophase of a polymeric LC. XRD patterns are obtained
from the mesogenic PPPs quenched with liquid nitrogen
from LC states, whereas the mesophases in the LC states are
frozen by the rapid quenching with liquid nitrogen. The XRD
diffractogram of a powdery sample can be generally divided
into the low-angle Bragg reflections at 2h ꢃ 3ꢀ correspond-
ing to the layer spacing of molecular orientational order and
the high-angle peaks at 2h ꢃ 20ꢀ associated with the two-
dimensional liquid-like intermesogenic organization within
the layers.21,22 The XRD diffractogram of the polymer
P(P(6)CN-alt-MeP) consists of several low-angle and high-
angle peaks (Fig. 4). The diffuse peak located at 2h ¼ 20ꢀ
gives the average distance of the shorter preferred spacing
(d3 ¼ 4.41 Å) occurring in the lateral packing arrangement
of the mesogenic pendants (Table 2). The layer spacing
derived from the Bragg reflection at 2h ¼ 2.86ꢀ (d1 ¼ 30.86
Å) is associated with molecular length at its most extended
conformation (l ¼ 28.40 Å). The existence of long-range
Photophysical Properties
Figure 5 shows UV spectra of the polymers P(P(6)CN-alt-
MeP) and P(P(6)CN-alt-Cz) in DMF solution. P(P(6)CN-alt-
Cz) shows a band at 306 nm and a shoulder at 368 nm,
which are assigned to p–p* transition of the terphenyl meso-
genic core of the side chain and p–p* transition of the conju-
gated main chain, respectively. The low absorption of the
main chain may be due to the carbazole ring twisting the
backbone, which results in a weak conjugation of the main
chain. When the carbazolylene is replaced by the para-link-
age phenylene with methyl substitution units, from the UV
spectrum of P(P(6)CN-alt-MeP), we can see that the inten-
sity of the absorption band of the main chain is dramatically
enhanced, which means that increase of para-linkage phenyl-
ene moiety in the conjugated skeleton seems to improve the
effective conjugate chain length.
A polymer with both LC and light-emitting properties may
find unique technological applications. As P(P(6)CN-alt-
MeP) and P(P(6)CN-alt-Cz) are PPPs derivatives bearing
chromophoric pendant groups and have different photoactive
materials such as phenylene and carbazolylene in the main
chain, it is of interest to check the effects of the structural
variables on their luminescence behaviors.25 Fluorescence
spectra of the copolymers in DMF solutions are depicted in
Figure 6. The copolymers solution (0.05 mM) is excited at
the two wavelengths of 320 nm (corresponding to the
absorption of the conjugated side chains) and 375 nm (cor-
responding to the absorption of the polymer main chain),
respectively. When the copolymers are excited at 320 nm,
polymer P(P(6)CN-alt-MeP) displays an emission kmax at
425 nm, even well extending to 600 nm, whereas the copoly-
mer shows an emission kmax at 470 nm with very low inten-
sity when excited at 375 nm. In comparison, monomer
FIGURE 4 X-Ray diffraction patterns of the copolymers
quenched from their liquid-crystalline states.
PROPERTIES OF p-PHENYLENE-BASED COPOLYMERS, YAO ET AL.
439

JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
TABLE 2 X-Ray Diffraction Analysis Data of P(P(6)CN-alt-MeP) and P(P(6)CN-alt-Cz)a
Molecular
Length (l, —)b
Copolymers
T (ꢀC)
d1 (—)
d2 (—)
d3 (—)
Ratio d1/l
Phase
P(P(6)CN-alt-MeP)
200
220
30.86
40.04
16.06
15.07
4.41
4.41
28.40
28.40
1.09
1.40
SmA
P(P(6)CN-alt-Cz)
SmAd
a
b
The mesophases in the liquid-crystal states at the given temperatures
were frozen by the rapid quenching with liquid nitrogen.
Calculated from mesogenic pendants in their fully extended
conformation.
P(6)CN exhibits a PL maximum at 399 nm under the excita-
tion at 320 nm. Obviously, the emitting center is both the
mesogenic terphenyl pendant and the backbone, and the PL
peak at the long wavelength should come from the main
chain of the copolymer. The relative intensity of the two PL
peaks emitted from conjugated side chain (399 nm) and
main chain (470 nm) is quit different. The PL peak of poly-
mer P(P(6)CN-alt-MeP) at 399 nm is much weaker than
that of monomer P(6)CN, whereas its PL peak at 470 nm is
much stronger than that excited at 375 nm. It suggests that
the energy transfer from the terphenyl pendant to the back-
bone favors the stronger light emitting of backbone.14,16
Therefore, although the 320 nm excitation pumps its ter-
phenyl chromophore to the excited state, the UV light emit-
ted from the pendant is reabsorbed by the PPP main chain,
hence the emission of its backbone.24 The same energy
transfer can be observed in the case of polymer P(P(6)CN-
alt-Cz). Different from P(P(6)CN-alt-MeP), the emission
kmax of P(P(6)CN-alt-Cz) is blue-shifted to 412 nm after the
introduction of carbazole unit into the main chain. The blue-
shift of the absorption maxima may be ascribed to two rea-
sons. One is that the backbone of the P(P(6)CN-alt-Cz) has
the less coplanarity than PPP and induces the distortion of
the backbone, due to the linkage manner via the 3,6-position
for the carbazole.26 Another reason is that the carbazole can
act as electron-withdrawing group on the copolymer back-
bone, which decrease the electron density of the copolymer
backbone and thus shift the absorption maximum.27 Besides,
another interesting result is that, compared with P(P(6)CN-
alt-MeP), the introduction of the carbazole obviously
depresses the full width (from 85 to 68 nm) at half-maxi-
mum. It indicates that introducing the carbazole onto the
backbone can get more pure blue light emitting. The quan-
tum yields (UF) of the copolymers are also evaluated, and UF
of P(P(6)CN-alt-MeP) and P(P(6)CN-alt-Cz) is 43 and 37%,
respectively.
FIGURE 6 Fluorescence spectral of the monomer P(6)CN (kex
¼
FIGURE 5 UV spectra of the polymers P(P(6)CN-alt-MeP) and
P(P(6)CN-alt-Cz) (0.05 mM) in DMF solutions.
320 nm) and of (a) P(P(6)CN-alt-MeP) and (b) P(P(6)CN-alt-Cz) at
two excitation wavelengths 320 and 375 nm in DMF (0.05 mM).
440
INTERSCIENCE.WILEY.COM/JOURNAL/JPOLA

ARTICLE
do not form interchain aggregates in solutions.30 We further
fabricated the copolymers into films. Casting films give rise to
a broadening of band, as shown in Figure 7, spreading from
350 to 600 nm with an intense blue luminescence when
excited at 320 nm. The film of P(P(6)CN-alt-MeP) has a
decreased maximum band at 455 nm, which is red-shift by 25
nm from the case of DMF solution, and the red-shift of
P(P(6)CN-alt-Cz) is also about 25 nm. This slight shift is due
to an increase of the interchain interaction of the stacked main
chain and is suggestive of small amount of excimer absorption
and emission of the backbone.
Electrochemical Properties
CV in CH3CN solutions of 0.1 M TBAPF4 was used to evalu-
ate the ionization potentials and the redox stability of the
copolymers synthesized. Using a Pt wire as the counter elec-
trode and a Ag/AgNO3 (0.1 M) electrode as the reference
electrode, which was calibrated using the ferrocene/ferroce-
nium redox couple.31 The oxidation and reduction potentials
can be used as a surrogate for HOMO energy levels and
LUMO energy levels, which were calculated using the inter-
nal standard ferrocene value of ꢁ4.8 eV with respect to the
vacuum level. Hence, the HOMO and LUMO energy levels of
the copolymers can be estimated using the equations of
EHOMO ¼ ꢁ(Eox þ 4.8) eV and ELUMO ¼ ꢁ(Ered þ 4.8) eV,
respectively, where Eox and Ered are the onset potentials for
oxidation and reduction, respectively,.32 The band gap of the
copolymers derives from the difference between the HOMO
and the LUMO energy levels. Cyclic voltammograms of the
copolymer thin films and the obtained electrochemical
results are summarized in Table 3 (for details, see Support-
ing Information). The band gap of polymers P(P(6)CN-alt-
Cz) and P(P(6)CN-alt-MeP) is 2.33 and 2.37, respectively,
significantly lower than the band gap of PPP without substi-
tution, 3.5 eV, in the undoped state.
FIGURE 7 Concentration dependence of fluorescence spectra in
DMF of (a) P(P(6)CN-alt-MeP) and (b) P(P(6)CN-alt-Cz). Excita-
tion wavelength: 320 nm.
CONCLUSIONS
In this work, we succeeded in polymerizing novel LC poly-
phenylene derivates containing terphenylene mesogenic
pendants, based on phenylene and carbazolylene (P(P(6)CN-
alt-Cz) or phenylene with methyl substitution P(P(6)CN-alt-
MeP), by using Suzuki coupling reaction. The copolymers
possess high heat stabilities and exhibit enantiotropic smec-
ticity mesogenic phase. Thanks for introducing the rigid aro-
matic rings onto the main chain, both of the copolymers
Most conjugated copolymers emit intensely in diluted solution
become weak emitters when concentrated or fabricated into
films,28,29 because of the strong interchain interaction. The
polymer strands aggregate together to form less luminescent
species such as excimers, leading to red-shifted emissions
with low efficiencies. Thus, the effect of the concentration of
the copolymers on the PL is investigated (Fig. 7). The fluores-
cence spectral shapes of all copolymers are observed in the
similar spectral profiles, even increasing the concentration
from 0.05 to 0.2 and 0.5 mM, indicating that the copolymers
show
a much broader mesophase temperature range
(ꢃ80 ꢀC) and higher transition temperature than homopoly-
mer PP(6)CN. The copolymers are of strong electronic
absorption and high PL and they can emit strong blue light.
TABLE 3 Electrochemical Properties and Calculated Energy Differences of the Alternating Conjugated Copolymers
a
b
a
b
Copolymers
Eonset/ox (V)
Ef/ox (V)
Eonset/red (V)
Ef/red (V)
HOMO (eV)
LUMO (eV)
Band Gapc (eV)
P(P(6)CN-alt-MeP)
0.65
0.64
0.77
0.76
ꢁ1.72
ꢁ1.69
ꢁ1.51
ꢁ1.47
ꢁ5.45
ꢁ5.44
ꢁ3.08
ꢁ3.11
2.37
2.33
P(P(6)CN-alt-Cz)
a
c
Onset voltage for oxidation versus SCE.
Formal oxidation potential.
Cyclic voltammetric band gap derived from the difference between
HOMO and LUMO energy level.
b
PROPERTIES OF p-PHENYLENE-BASED COPOLYMERS, YAO ET AL.
441

JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
14 Zhou, D.; Chen, Y. W.; Chen, L.; Zhou, W. H.; He, X. H. Macromole-
cules 2009, 42, 1454–1461.
The energy transfer from the terphenyl pendant to the back-
bone favors the stronger light emitting of backbone. The car-
bazolylene lead the light emission band of the copolymer to
be blue-shifted with more pure blue light emitting. The
mixed aromatic rings onto the polyphenylene main chain
exert great influence on the electrochemical properties of the
copolymers. The band gaps of copolymers are much lower
than the band gap of PPP without substitution.
15 Chen, L.; Chen, Y. W.; Yao, K.; Zhou, W. H.; Li, F.; Chen, L. P.; Hu, R.
R.; Tang, B. Z. J Polym Sci Part A: Polym Chem 2009, 47, 4723–4735.
16 Chen, L.; Chen, Y. W.; Yao, K.; Zhou, W. H.; Li, F.; Chen, L. P.; Hu, R.
R.; Tang, B. Z. Macromolecules 2009, 42, 5053–5061.
17 Morin, J.-F.; Leclerc, M.; Ade’s, D.; Siove, A. Macromol Rapid Com-
mun 2005, 26, 761–778.
Financial support for this work was provided by the National
Natural Science Foundation of China (50773029 and
50902067), the Natural Science Foundation of Jiangxi Province
(2007GZC1727 and 2008GQH0046), and Program for Innova-
tive Research Team in University of Jiangxi Province.
18 Van Dijken, A.; Bastiaansen, J. J. A. M.; Kiggen, N. M. M.; Langeveld,
B. M. W.; Rothe, C.; Monkman, A.; Bach, I.; Stossel, P.; Brunner, K. J Am
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