Solution, thermal, and optical properties of poly(pyridinium salt)s derived from an ambipolar diamine consisting of diphenylquinoline and triphenyl amine moieties

Article abstract of DOI:10.1002/pola.24250

Poly(pyridinium salt)s containing ambipolar diamine moiety exhibiting lyotropic LC phase, hand-drawn fibers, and aggregated entities.

Full text of DOI:10.1002/pola.24250

Solution, Thermal, and Optical Properties of Poly(pyridinium salt)s
Derived from an Ambipolar Diamine Consisting of Diphenylquinoline
and Triphenyl Amine Moieties*
ALEXI K. NEDELTCHEV, HAESOOK HAN, PRADIP K. BHOWMIK
Department of Chemistry, University of Nevada at Las Vegas, 4505 Maryland Parkway, Box 454003, Las Vegas, Nevada 89154
Received 4 June 2010; accepted 15 July 2010
DOI: 10.1002/pola.24250
Published online in Wiley Online Library (wileyonlinelibrary.com).
KEYWORDS: differential scanning calorimetry (DSC); gel permeation chromatography (GPC); light-emitting diodes (LEDs); liquid-
crystalline polymers; luminescence; poly(pyridinium salt)s
INTRODUCTION Organic semiconductors for fabrication of
various electronic and optoelectronic devices have been of
great interest in the scientific community.1–3 Molecular and
polymeric materials for both p-type1 and n-type2 semiconduc-
tors have been developed by introducing various electron-
donating and electron-withdrawing moieties. In organic light-
emitting diodes (OLEDs) there are three layers: hole-trans-
porting (p-type semiconductor), electron-transporting (n-type
semiconductor), and emissive, where the holes and electrons
are brought together to form excitons. During a recombina-
tion step, the process of light emission occurs.3 The emissive
layer should have an ambipolar character to bring together
the two charges. Ambipolar materials are based on a donor–
acceptor (D-A) arrangement; the recombination is achieved
through intermolecular charge transfer.4b Subsequently, D-A
architectures have been developed by introducing combina-
tions of HT and ET moieties in both molecular4 and poly-
meric5 systems as well as others based on alternating copoly-
mers, blends, and multilayers.6 They are advantageous over
single-layer semiconductors because they increase the proba-
bility of formation of excitons, and they can also run on both
AC and DC currents because of their dual polarity, which in
turn improves their stability by preventing reductive degrada-
tions. Additionally, they could be potential candidates for the
symmetrically configured alternating-current light emitting
devices.7 Moreover, they are independent of the work
functions of the electrodes, which make them even more ver-
satile.4–6 Besides providing layers for OLEDs, ambipolar
materials may find applications in organic electronics as pho-
tovoltaic cells5a based on organic materials and organic field-
effect transistors.5c,d Instead of using ordered materials such
as crystalline and liquid-crystalline (LC), it has been identified
that completely amorphous materials are the preferred choice
for several reasons: they prevent light scattering, they are
transparent, they are easier to process into thin films, and
they have homogeneous properties. Furthermore, high glass-
transition temperatures and adequate thermal stability are
required because Joules heating could potentially limit their
usage.1 Poly(pyridinium salt)s are a class of main-chain ionic
polymers that display both LC and light-emitting properties,
which have been studied quite extensively by Bhowmik and
co-workers.8 Additionally, they have high thermal stabilities
and good solubility in common organic solvents.8 The combi-
nation of functional properties makes them interesting mate-
rials for various applications. In this communication, we
describe the synthesis of two poly(pyridinium salt)s of tosyl-
ate and triflimide counterions having an ambipolar moiety
being incorporated into their backbones based on electron-
deficient diphenylquinoline and electron-rich triphenyl amine
moieties (Scheme 1).
Additionally, our foray was to explore their unique solution,
thermal, and optical properties in both the solution and solid
states by using a number of experimental techniques. Their
morphologies in thin film and fibrous states are also exam-
ined microscopically.
EXPERIMENTAL
Materials
2-Amino-5-nitrobenzophenone, acetophenone, 1-fluoro-4-
nitrobenzene, cesium fluoride, and 10% Pd/C were pur-
chased from Sigma-Aldrich (St. Louis, MO). The hydrazine
monohydrate was purchased from TCI (Long Beach, CA).
Lithium triflimide, common solvents, and deuterated solvents
were procured from Fisher Scientific (Pittsburgh, PA). All of
the chemicals from various vendors were used as received.
*This article is dedicated to the late Professor Robert W. Lenz (1926–2010).
Additional Supporting Information may be found in the online version of this article. Correspondence to: P. K. Bhowmik (E-mail: pradip.bhowmik@
unlv.edu)
C
V
Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 48, 4611–4620 (2010) 2010 Wiley Periodicals, Inc.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
143.2, 139.0, 137.6, 132.8, 130.5, 130.2, 129.6, 129.3, 128.1,
127.0, 126.6, 126.3, 123.9, 122.5, 120.4, 120.2. Anal. Calcd.
for C33H22N4O4 (538.57): C, 73.60; H, 4.12; N, 10.40. Found:
C, 73.24; H, 4.52, N, 10.48.
Procedure for the N,N-Bis(4-aminophenyl)-
2,4-diphenylquinolin-6-amine (4)
An ethanol suspension (50 mL) of 3.0 g (5.6 mmol) of N,N-
bis(4-nitrophenyl)-2,4-diphenylquinolin-6-amine was heated
ꢀ
to 50 C in the presence of 0.1 g of Pd/C; 3 mL of hydrazine
monohydrate was added dropwise over a 30-min period. On
heating, the solution became homogeneous as the reaction
proceeded. To ensure complete reduction, the reaction was
kept at this temperature for an additional 12 h. At the end
of reaction, the compound 4 was precipitated out even at
reaction temperature. On cooling, the product was collected
and washed with distilled water and air-dried. On subse-
quent recrystallization from chloroform, it formed a highly
solvated solid product. The pure diamine 4 was prepared by
removing the solvent on drying in vacuo at 136 ꢀC. The re-
moval of solvent was monitored by thermogravimetric analy-
sis (TGA) measurement. It was 1.9 g (4.0 mmol, yield 71%)
ꢀ
of orange crystals. The diamine 4 showed a Tm ¼ 227 C in
ꢀ
the first heating cycle and a Tg ¼ 105 C at the second heat-
ing cycle as examined by differential scanning calorimetry
(DSC). 1H-NMR (400 MHz, d6-DMSO) d ppm 8.32 (s, 1H),
8.24 (dd, J ¼ 5.3, 3.3 Hz, 2H), 7.86 (d, J ¼ 9.3 Hz, 1H), 7.82
(s, 1H), 7.51 (ddd, J ¼ 8.3, 4.9, 2.4 Hz, 4H), 7.47–7.40 (m,
4H), 7.21 (dd, J ¼ 9.3, 2.7 Hz, 1H), 7.05 (d, J ¼ 2.7 Hz, 1H),
6.90–6.83 (m, 4H), 6.59–6.53 (m, 4H), 5.01 (d, J ¼ 8.5 Hz,
4H). 13C-NMR (100 MHz, d6-DMSO) d ppm 152.5, 148.3,
146.6, 146.3, 143.8, 139.7, 138.6, 135.9, 130.7, 130.0, 129.5,
129.4, 129.1, 128.9, 127.8, 127.4, 126.7, 123.5, 119.4, 115.4,
108.2. Anal. Calcd. for C33H26N4 (478.60): C, 82.82; H, 5.48;
N, 11.71. Found: C, 82.66; H, 5.68, N, 12.00.
Polymer Synthesis
Synthesis of Polymer I-1
The bis(pyrylium) salt was reacted with N,N-bis(4-amino-
phenyl)-2,4-diphenylquinolin-6-amine by a ring-transmuta-
tion reaction to yield the desired tosylate polymer (Scheme
1). Equal mole ratios of each of the monomers were placed
in a three-necked, round bottom flask equipped with a mag-
netic stirrer in DMSO. About 5 mL of toluene was added to
distill the water generated in the reaction through a tolu-
ene/water azeotrope; this was facilitated by using a Dean-
Stark and water condenser. The temperature was monitored
and adjusted to 130–135 ꢀC with a mercury thermometer.
The reaction was kept under a nitrogen atmosphere for
48 h. At the end of polymerization reaction, the reaction mix-
ture was allowed to cool down to room temperature. The
solvent DMSO was subsequently reduced in vacuo. The poly-
mer was precipitated out by pouring into water, and ulti-
mately it washed a few times with boiling water. The result-
SCHEME 1 Synthesis of poly(pyridinium salt)s.
Monomer Synthesis
Procedure for the Synthesis of N,N-Bis
(4-nitrophenyl)-2,4-diphenylquinolin-6-amine (3)
Three grams (10.1 mmol) of 6-amino-2,4-diphenylquinoline,
3.0 g (21.3 mmol) of 1-fluoro-4-nitrobenzene, and 3.54 g
(23.3 mmol) of cesium fluoride in 40 mL of dimethyl sulfox-
ide (DMSO) were heated to 140 ꢀC for 18 h. The reaction
mixture was cooled down to room temperature and poured
in distilled water to precipitate out the crude product. It was
eluted in chloroform by using a column chromatography on
silica gel, and, subsequently, its chloroform solution was
poured in hexane to afford 3.4 g (6.3 mmol, yield 63%) of
1
ꢀ
orange compound. Tg ¼ 96 C. H-NMR (400 MHz, d6-DMSO)
d ppm 8.34 (td, J ¼ 12.8, 4.29 Hz, 2H), 8.28–8.22 (m, 1H),
8.22–8.17 (m, 4H), 8.08 (s, 1H), 7.66–7.50 (m, 7H), 7.46 (dd,
J ¼ 6.5, 2.7 Hz, 3H), 7.34 (m, J ¼ 9.2 Hz, 4H). 13C-NMR (100
MHz, d6-DMSO) d ppm 156.7, 152.0, 148.7, 147.2, 143.3,
ꢀ
ing polymer was dried in vacuo at 110 C for 72 h. Data for
polymer I-1: Anal. Calcd. for C75H55N3O6S2 (1158.41): C,
77.76; H, 4.79; N, 3.63; O, 8.29; S, 5.54. Found: C, 76.58; H,
5.86; N, 3.64; S, 5.99.
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unk denotes the unknown.9 To assess the molecular weight
of the polymer, gel permeation chromatography (GPC) was
run at 50 ꢀC with a flow rate of 1 mL/min. The GPC had a
Water 515 pump together with a Viscotek Model 301 Triple
Detector Array. The array contained a laser refractometer, a
differential viscometer, and a light scattering detector both
right-angle laser light scattering as well as low-angle laser
light scattering in a single instrument with a fixed interde-
tectꢀor and temperature control that can be regulated up to
80 C. The instrument was calibrated with a pullulan stand-
ard of P-50 obtained from Polymer Standard Services USA,
Inc. (Onley, MD). Separations were accomplished using Visco-
Gel I-MBHMW-3078 columns purchased from Viscotek. An
aliquot of 100–200 lL of 2 mg/mL polymer solution in
DMSO containing 0.1M LiBr was injected. The dn/dc values
were estimated by injecting different volumes of polymer so-
lution to assess the trend. All data analyses were performed
by using Viscotek TriSEC software. The X-ray scattering stud-
ies were performed on finely ground powdered samples at
room temperature with a PANalytical X’PERT Pro X-ray dif-
fraction spectrometer using Cu Ka radiation (k ¼ 1.5419 Å)
as an X-ray source operating at 45 kV and 40 mA. The scan-
ning electron microscopy (SEM) images of fibers drawn from
the polymer melts were taken with the JXA-8900 Super-
Probe. The transmission electron microscopy (TEM) studies of
polymers were performed by using a TECNAI-F30-Supertwin
TEM instrument operating at 300-kV field emission mode. For
this study, a drop of 2 wt % of polymer I-1 in methanol was
deposited on an electron microscopy copper grid coated with
carbon film and the organic solvent was evaporated at room
temperature. In the case of polymer I-2, a drop of a 5 wt %
acetonitrile solution was used for the deposition.
Synthesis of Polymers I-2
Polymer I-2 was prepared by the metathesis reaction of
polymer I-1 with excess lithium triflimide salt in DMSO at
ꢀ
ꢁ60 C for 48 h (Scheme 1). On completion of the exchange
reaction, the solvent DMSO was removed under vacuum to
form a viscous solution, which was then poured into water,
resulting in the precipitation of polymer. The procedure was
repeated one or two times until all the tosylate counterions
were exchanged to triflimide counterions; this complete
exchange was monitored with the disappearance of aromatic
protons and methyl protons of tosylate groups by 1H-NMR
spectroscopy. At the end of final precipitation, it was washed
extensively with boiling water to remove any residual or-
ganic salts and entrapped DMSO. Finally, it was dried in
vacuo at 110 ꢀC for 72 h. Data for polymer I-2: Anal. Calcd.
for C65H41N5O8S4F12 (1376.30): 56.73; H, 3.00; N, 5.09; S,
9.32; O, 9.30; F, 16.56. Found: C, 57.42; H, 3.33; N, 5.19; S,
9.68.
Polymer Characterization
The 1H- and 13C-NMR spectra were recorded with a Varian
NMR 400 spectrometer equipped with three RF channels
operating at 400 and 100 MHz, respectively. Polymer solu-
tions were prepared by dissolving 35–40 mg of polymer per
mL of d6-DMSO with tetramethyl silane as an internal stand-
ard. Fourier transform infrared (FTIR) spectra were recorded
by using a Shimadzu spectrometer. Polymer samples were
prepared by coating NaCl plates with polymers and subse-
quently vacuum-dried at 70 ꢀC overnight. The phase-transi-
tion temperatures of polymers were measured using a ꢀTA
2100 DSC in nitrogen at heating and cooling rates of 10 C/
min. The temperature axis of the DSC thermogram was cali-
brated before use with reference standards of high-purity in-
dium and tin before use. Polymer samples of 8–10 mg were
used in these measurements. Thermal stability of each of the
polymers wereꢀanalyzed by TGA using TA 2100 instrument
at a rate of 10 C/min under nitrogen using samples not less
than 10 mg. The LC properties of both lyotropic and thermo-
tropic phases were assessed using a polarized optical micro-
scope (POM) Nikon Labophot 2 equipped with a pair of
crossed polarizers and hot stage. The UV-Vis absorption
spectra of polymer solutions in various organic solvents
were recorded using Varian Cary 50 Bio UV-Vis spectropho-
tometer in quartz cuvettes. Their photoluminescence proper-
ties in both solution and film states were analyzed using a
Perkin-Elmer LS-55 luminescence spectrophotometer. Quan-
tum yields were measured by adjusting the solution absorp-
tion using the UV-Vis to ꢁ0.05 at 350 nm wavelength. The
output was then measured using the luminescence spectrom-
eter at the same wavelength and was compared with known
9,10-diphenylanthracene standard using eq 1:
RESULTS AND DISCUSSION
The compound 6-nitro-2,4-diphenylquinoline 1 was prepared
by using a modified Friedla¨nder condensation reaction as
described in the literature.10 The 6-amino-2(4-aminophenyl)-
4-phenylquinoline 2 was also synthesized by using known
reductive hydrogenation over Pd/C.10 The identification of
chemical structures of these compounds as well as their
thermal and optical properties will be reported elsewhere.
The N,N-bis(4-nitrophenyl)-2,4-diphenylquinolin-6-amine
3
was prepared in good yield by a nucleophilic aromatic sub-
stitution reaction in accordance with the known procedure.11
The targeted diamine monomer N,N-bis(4-aminophenyl)-2,4-
diphenylquinolin-6-amine 4 was prepared by using a reduc-
tive hydrogenation similar to the procedure that was
adopted for the preparation of amine 2. Scheme 2 shows the
detailed four-step routes for the synthesis of 4. Figure 1
shows its 1H- and 13C-NMR spectra that include the assign-
ments of various proton and carbon signals, which confirms
the chemical structure of this novel diamine. Polymer I-1
was prepared by a ring-transmutation polymerization reac-
tion, whereas polymer I-2 was made by a metathesis reac-
tion by changing the counterion from tosylate to triflimide
as outlined in Scheme 1.8 The NMR and FTIR spectra for
both the monomer and the polymers are provided in the
ꢀ
ꢁꢀ
ꢁꢀ
ꢁ
2
Iunk
Astd
gunk
gstd
/
¼ /std
(1)
unk
Istd
Aunk
where / is the fluorescence quantum yield, I is the absorp-
tion of the excitation wavelength, A is the area under the
emission curve, and g is the refractive index of the solvents
used. Subscript std denotes the standard, whereas subscript
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
SCHEME 2 Synthesis of N,N-bis(4-aminophenyl)-2,4-diphenylquinolin-6-amine 4.
Supporting Information. Polymers I-1 and I-2 had intrinsic
viscosities of 0.10 and 0.13 dL/g, respectively. Their number-
average molecular weight (Mn) was 41,000 for the tosylate
polymer and 50,000 for the triflimide one. They had weight-
average molecular weights (Mw) of 59,000 and 64,000,
respectively. Polymer I-2 had a slightly narrower polydisper-
sity (Mn/Mw) of 1.27 than that of 1.44 for the polymer I-1.
These polymers had reasonably high molecular weights com-
pared with those of other previously reported poly(pyridi-
nium salt)s,8a,f which would enable one to study their prop-
erties without concerns the effect of molecular weight on
these properties. Figure 2 shows the GPC plots of polymers
I-1 and I-2 that are suggestive of rather expected distribu-
tions of molecular weights considering the method of poly-
condensation reaction used and of monomodal type.
acts as a nucleophile to cause the degradation at higher tem-
perature. The former statement is supported by the fact that
the decomposition products generated from poly(pyridinium
salt)s with tetrafluoroborate counterions wherein the coun-
terion was the first to degrade.12 Overall, this result is in
excellent agreement with the decomposition temperatures of
analogus poly(pyridinium salt)s.8 The DSC studies reveal
that these two polymers were completely amorphous
because no other thermal transitions were detected in their
thermograms as shown in Figure 4, which is a highly desira-
ble property of many materials for various optoelectronic
applications.1,4 Polymer I-1 showed an endotherm at 104 C,
ꢀ
which is attributed to a loss of entrapped solvent molecules,
which is usually associated with many ionic polymers in gen-
eral. This observation was supported by its TGA thermogram
ꢀ
as shown in Figure 3. At 266 C, a distinct Tg was observed,
An introduction of heterocyclic moiety has been shown to
improve the thermal stability in various polymeric5 and mo-
lecular6 materials. The poly(pyridinium salt)s in this study
also exhibited excellent thermal stability of 379 ꢀC for poly-
mer I-1 and 454 ꢀC for polymer I-2 (Fig. 3). Polymer I-2
was more stable than polymer I-1 to a significant extent of
75 ꢀC. This result may be explained by the following two
reasons. First, sodium tosylate salt (155 ꢀC) has a lower
which recurred in the subsequent cooling and heating cycles.
Polymer I-2 had significantly less entrapment of solvent mol-
ecules; as a result it showed a small endotherm at 89 ꢀC.
This ionic polymer showed a Tg at 210 ꢀC. The exchange
from tosylate to triflimide counterion resulted in lowering
ꢀ
the Tg significantly by 56 C. This decrease in Tg temperature
was presumably because of the size of the triflimide counter-
ion. One can deduce that the counterions have a significant
impact on the thermal transitions of these polymers. This
phenomenon is in excellent agreement with the reports of
other poly(pyridinium salt)s8 as well as ionic liquids.13
ꢀ
thermal stability than lithium triflimide (360 C). Second, tri-
flimide counterion has a significantly weaker nucleophilic
character compared with tosylate couterion and, therefore,
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FIGURE 1 (a) 1H- and (b) 13C-NMR
spectra of N,N-bis(4-aminophenyl)-2,4-
diphenylquinolin-6-amine.
Additionally, the amount of solvent molecules entrapped in
an ionic polymer depends on the nature of counterions
because even after extensive drying in vacuo, polymer I-1
with tosylate counterion entrapped significantly more sol-
vent molecules compared with the triflimide polymer I-2.
Figure 5 shows the X-ray powder diffraction plot of polymer
I-2 recorded at room temperature. It shows broad diffraction
peaks with relatively low intensities at both small and wide
angles, which are the characteristic features of glassy phase
of this ionic polymer. This result is in agreement with the
DSC thermograms. These peaks can be assigned to the
intermolecular short-range interactions that are parallel and
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE 2 GPC plots of polymers I-1 and I-2 obtained from (a) RI, (b) IV, (c) right-angle laser light scattering, and (d) low-angle
laser light scattering detectors, respectively. [Color figure can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
perpendicular to the long axes of the polymer chain. The
broad wide-angle diffraction peaks at around 2h ¼ 19ꢀ that
corresponded to the d-spacings of 4.6 Å are the distances of
two aromatic planes of the polymer chains. The relative
sharp, wide-angle diffraction peaks at around 2h ¼ 10ꢀ that
corresponded to the d-spacings of 8.6 Å are related to long-
range order between the polymer chains.
ment of a birefringence and dark domains indicative of a
biphasic solution. However, a further increase of its concen-
tration did not yield a fully grown LC phase in this solvent.
In DMSO, polymer I-1 was also very soluble and retained an
To process materials into thin films and fibers, a sufficient
solubility in common organic solvents provides a convenient
method for solution processing. Most highly conjugated poly-
mers are difficult to dissolve because of their efficient p-p
stacking.3 Despite the presence of multiple aromatic moieties
in the polymer chains, their solubility in solvents like metha-
nol, acetonitrile, and DMSO were excellent because of the
presence of ionic charges in their backbones in conjunction
with the diphenylquinoline and triphenyl amine moieties in
their chemical structures. This solution property motivated
us to extensively examine their lyotropic LC properties in
various organic solvents. Polymer I-1 had very good solubil-
ity in methanol and retained its isotropic solution up to 20
wt %. On increasing its concentration, there was a develop-
FIGURE 3 TGA thermograms of polymers I-1 and I-2 obtained
at a heating rate of 10 ꢀC/min in nitrogen.
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RAPID COMMUNICATION
cal concentration (*C) to form a biphasic solution. Further, the
increase of its concentration in this solvent resulted in a fully
grown lyotropic LC phase at 39 wt % as examined with POM
studies. A photomicrograph of such a lyotropic solution of this
polymer is displayed in Figure 6. It also formed a lyotropic
phase as high as up to 51 wt % in this solvent. Similar to poly-
mer I-1, in DMSO, polymer I-2 also formed an isotropic solu-
tion only. Overall, the exchange of the counterion from tosylate
to triflimide affected both the solubility and the formation of
the LC phase in this ionic polymer.
Development of novel light-emitting materials has been of
particular interest for applications in optoelectronics. To
achieve this goal, many neutral light-emitting polymers have
been developed,3 but there are a few examples that have a
D-A chemical architecture.4b,14 Here, we demonstrated how a
D-A arrangement based on ambipolar moiety affects the opti-
cal properties of poly(pyridinium salt)s. The fluorescence of
polymer solutions in neutral condition was too low to mea-
sure. However, similar to some neutral heterocyclic poly-
mers, we were able to observe light emission in both solu-
tion and solid states of these polymers after protonation by
adding drops of concentrated HCl.15 Their photolumines-
cence spectra in various solvents in acid media are displayed
in Figure 7(a).
They exhibited kem peaks in the narrow range of 602–605
nm light in various organic solvents having a wide range of
polarities. Both polymers showed essentially an identical
kmax peak at 335 nm as detected in their UV-Vis spectra
recorded in methanol. The optical band gaps values (Eg)
were estimated from the onsets of their UV-Vis absorption
spectra and were found to be essentially identical value of
3.28 eV. This value is large for neutral light-emitting poly-
mers3 but similar to those of previously reported poly(pyri-
dinium salt)s.8 Their full-width at half-maximum values were
greater than 100 nm, implying that their light emission spec-
tra stemmed from a number of chromophoric species. To
estimate their potential for optoelectronic devices, we
FIGURE 4 DSC thermograms of polymers I-1 and I-2 obtained
at heating and cooling rates of 10 ꢀC/min.
isotropic solution at relatively high concentrations of 41 wt %.
Similarly, polymer I-2 formed a biphasic solution in methanol
at 20 wt %. In contrast to polymer I-1, polymer I-2 was solu-
ble in acetonitrile; at 20 wt % its solubility exceeded its criti-
FIGURE 6 Photomicrograph of polymer I-2 at 40 wt % CH3CN
under crossed polarizers exhibiting lyotropic LC phase (magni-
fication, ꢂ400). [Color figure can be viewed in the online issue,
which is available at wileyonlinelibrary.com.]
FIGURE 5 X-ray powder diffraction plot of powdered sample of
polymer I-1 recorded at room temperature.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
a very viscous melt on heating above its Tg. As a result, thin
films or fibers could not be produced. On the other hand,
polymer I-2 efficiently formed a uniform thin film when
heated above its glass transition temperature (not shown).
There was no development of birefringence texture indicat-
ing that this film was isotropic with no specific order even
after annealing it for 24 h. From the melt of this polymer, we
were able to draw fibers at a thickness of ꢁ100–150 lm
with a pair of tweezers. Figure 8(a) shows a POM photomi-
crograph of this fibrous structure taken by using a red quar-
ter plate. The surface of the fiber from polymer I-2 appeared
relatively smooth, but there was some evidence of texture in
the form of small furrows along its long axis. For additional
elucidation of its morphology, we used SEM studies to
observe the fiber morphology as shown in Figure 8(b). Simi-
lar to the POM photomicrograph, the SEM image showed
that this fiber had a very smooth surface. Even though it is
not clearly observed in Figure 8(b), the cross-section of the
fiber appeared to be hollow. In general, the formation of
fibers in polymeric and molecular materials occurs by self-
assembly through various intermolecular interactions such
FIGURE 7 Emission spectra of polymers I-1 and I-2 in (a) solu-
tions of various organic solvents after protonation with HCl
and (b) films cast from methanol after protonation with HCl at
various excitation wavelengths.
studied the light-emitting properties in their thin film states.
Figure 7(b) shows the photoluminescence spectra of poly-
mers I-1 and I-2 in the film cast from methanol. They exhib-
ited kem peaks at 587 nm and 586 nm when excited with a
335 nm wavelength of light. Interestingly, there were hypso-
chromic shifts of 15 and 19 nm in the solid states compared
with those of their solution spectra. The hypsochromic shifts
in their solid states, as opposed to bathochromic shifts of
p-conjugated polymers in ordered structures, may be related
to the fact that the extensive p-p stacking phenomena of aro-
matic moieties are not being operative presumably because
of the presence of repulsive interactions of charges in these
ionic polymers. Unlike other poly(pyridinium salt)s that nor-
mally emit blue or green light, these polymers exhibited or-
ange light emission in both solution and film states.
Preparation of thin films and fibers from the melts of poly-
mers is a common approach to process them into functional
devices. We examined the morphology of the poly(pyridi-
nium salt)s in this study at these states. Polymer I-1 formed
FIGURE 8 Images of drawn fiber of polymer I-2 from melt
taken with (a) POM (magnification, ꢂ400) and (b) SEM.
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RAPID COMMUNICATION
FIGURE 9 TEM images of (a) polymer I-1 (ꢂ5000) from 2 wt % CH3OH solution and (b) polymer I-2 (ꢂ1400) from 5 wt % CH3CN
solution.
as hydrogen bonding, hydrophobic, electrostatic, and van der
Waals interactions. Both the mechanisms for the preparation
of fibers from the melt of polymer I-2 and for the orientation
the polymer chains inside the fibrous structures of this ionic
polymer remain unknown at the present time. However, a
combination of intermolecular interactions is presumably
involved, but we cannot pinpoint the exact relationship of
these interactions, which remains to be further studied.
Next, we examined the morphology by using 2 wt % of poly-
mer I-1 in methanol by TEM studies. Similarly, morphology
of polymer I-2 by using 5 wt % acetonitrile solution was
also examined. Figure 9 shows TEM images for both the
ionic polymers. Even though the poly(pyridinium salt)s in
this study possessed the identical backbone of the polymer
chain, they exhibited markedly different morphologies as the
counterion was exchanged from tosylate to triflimide. Figure
9(a) shows that polymer I-1 formed well-defined spheres of
essentially identical sizes. In contrast, polymer I-2 formed an
irregular, thin amorphous film as displayed in Figure 9(b).
The formation of spherical objects of this ionic polymer is
presumably related to the aggregation phenomenon, which
may also be related to the formation of lyotropic phase of
these polymers. The aggregation phenomena of ionic poly-
mers, irrespective of the nature of polyions, either rigid-rod
or flexible, in water or in organic solvents are quite remark-
able in general because the macroions approach one another,
leading to the formation of aggregates instead of repulsion
between like charges. The mechanism of this aggregation
process remains unknown to date, but aggregation processes
do occur in both flexible and rod-like polyelectrolytes as
detected by various experimental techniques, including light
microscopy, dynamic and static light scattering, X-ray scatter-
ing, scanning force microscopy, and TEM, among others.16
Overall, the morphology of these ionic polymers revealed
some interesting properties with regard to their aggregation
process that can be used as guidelines when incorporating
them into useful high-performance devices.
CONCLUSIONS
In summary, two novel poly(pyridinium salt)s with tosylate
and triflimide counterion were synthesized by either ring-
transmutation polymerization or metathesis reaction. They
contain a D-A system of electron-rich triphenyl amine moiety
in conjunction with electron-deficient diphenylquinoline moi-
ety. Their structures were characterized spectroscopically
using FTIR, NMR, and elemental analysis. They have excellent
thermal stability and high glass-transition temperatures. In
addition, they were soluble in common organic solvents and,
in some cases, exceeded critical concentration and produced
lyotropic LC phases. Additionally, the morphologies of the
thin film states and melt-drawn fibers of these polymers
were examined with POM, SEM, and TEM. They emitted light
in both the solution and solid state on protonation with HCl.
A combination of high thermal stability, high glass-transition
temperatures, solubility in common organic solvents, lyo-
tropic LC properties, and light emission in both solution and
solid state makes these polymers as potential materials in
opto- and microelectronics. More specifically, they could be
used in light-emitting electrochemical cells17 because they
carry ionic charges. They could also be used as an emissive
layer in OLEDs4–6 because they possess ambipolar character.
RAPID COMMUNICATIONS, NEDELTCHEV, HAN, AND BHOWMIK
4619

JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
Lai, J.-Y.; Chan, L.-H.; Chen, C.-T. J Polym Sci Part A: Polym
Chem 2009, 47, 6231–6245.
Finally, they could be used as polycations in conjunction
with the appropriate polyanions to build multilayered nano-
composites on both substrates and free-standing films.18
6 (a) Champion, R. D.; Cheng, K.-F.; Pai, C.-L.; Chen, W.-C.;
Jenekhe, S. A. Macromol Rapid Commun 2005, 26, 1835–1840;
(b) Babel, A.; Zhu, Y.; Cheng, K. F.; Chen, W. C.; Jenekhe, S. A.
Adv Funct Mater 2007, 17, 2542–2549.
P. K. Bhowmik acknowledges the University of Nevada Las
Vegas (UNLV) for New Investigation Award (NIA), Planning Ini-
tiative Award (PIA), and Applied Research Initiative (ARI)
grants, and the donors of the Petroleum Research Fund, admin-
istered by the American Chemical Society, and an award
(CCSA# CC5589) from Research Corp. for the support of this
research. A.K.N. acknowledges the Graduate College (UNLV) for
providing him a Nevada Stars Graduate Assistantship for the
period of 2006–2008. The authors also acknowledge Dr. Long-
zhou Ma for his expertise in the analysis of transmission elec-
tron microscopy studies.
7 Wang, Y. Z.; Gebler, D. D.; Lin, L. B.; Blatchford, J. W.; Jes-
sen, S. W.; Wang, H. L.; Epstein, A. J. Appl Phys Lett 1996, 68,
894–896.
8 (a) Bhowmik, P. K.; Burchett, R. A.; Han, H.; Cebe, J. J.
J Polym Sci Part A: Polym Chem 2001, 39, 2710–2715; (b)
Bhowmik, P. K.; Burchett, R. A.; Han, H.; Cebe, J. J. Macromo-
lecules 2001, 34, 7579–7581; (c) Bhowmik, P. K.; Han, H.; Cebe,
J. J.; Nedeltchev, I. K.; Kang, S.-W.; Kumar, S. Macromolecules
2004, 37, 2688–2694; (d) Bhowmik, P. K.; Han, H.; Nedeltchev,
A. K. J Polym Sci Part A: Polym Chem 2006, 44, 1028–1041;
(e) Bhowmik, P. K.; Kamatam, S.; Han, H.; Nedeltchev, A. K.
Polymer 2008, 49, 1748–1760; (f) Bhowmik, P. K.; Han, H.;
Nedeltchev, A. K.; Mandal, H. D.; Jimenez-Hernandez, J. A.;
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