Novel fluorescent carbazolyl-pyridinyl alternating copoloymers: Synthesis, characterization, and properties

Article abstract of DOI:10.1021/ma050425b

A novel series of AB-type polymers comprised of alternating carbazolyl and pyridinyl units were synthesized using palladium(0)-catalyzed Suzuki cross-coupling reaction in good to high yields. In this series of alternating copolymers, the electron-rich 2,7-(N-(2-ethylhexyl)carbazolyl) unit was used as a light-emitting unit, and the electron-deficient unit of pyridinyl was introduced to tune the wavelength of the emitting light and improve their electron transportation. These polymers were characterized by ¹H NMR and ¹³C NMR, gel permeation chromatography (GPC), thermal analysis, UV-vis, fluorescence spectroscopy, and cyclic voltammetry (CV). The glass transition temperature of these polymers ranged from 120 to 150°C, and the polymers showed high thermal stability with decomposition temperatures in the range 370-400°C in nitrogen. The different linkage pattern of pyridinyl units in the copolymer backbone has a significant effect on the electronic and optical properties in solution and in film phases. For pure blue emission and prevention of aggregation of polymer chain, the meta-linkage pattern (3,5-and 2,6-linkage) of pyridinyl units in the polymer backbone is more favorable than the para-linkage pattern (2,5-linkage) of the pyridinyl units. The cyclic voltammetry (CV) results indicate that these polymers with pyridinyl units possess lower LUMO energy levels for an easy electron injection from a cathode than poly[3,6-(N-octyl)carbazolyl] and its derivatives do.

Full text of DOI:10.1021/ma050425b

Macromolecules 2005, 38, 7629-7635
7629
Novel Fluorescent Carbazolyl-Pyridinyl Alternating Copoloymers:
Synthesis, Characterization, and Properties
Xiaoyong Pan,† Shouping Liu,‡ Hardy S. O. Chan,† and Siu-Choon Ng*,§
Department of Chemistry, National University of Singapore, 119260 Singapore; Singapore Eye
Research Institute, Singapore National Eye Center, 168751 Singapore; and Division of Chemical and
Biomolecular Engineering, Nanyang Technological University, 639798 Singapore
Received March 1, 2005; Revised Manuscript Received June 29, 2005
ABSTRACT: A novel series of AB-type polymers comprised of alternating carbazolyl and pyridinyl units
were synthesized using palladium(0)-catalyzed Suzuki cross-coupling reaction in good to high yields. In
this series of alternating copolymers, the electron-rich 2,7-(N-(2-ethylhexyl)carbazolyl) unit was used as
a light-emitting unit, and the electron-deficient unit of pyridinyl was introduced to tune the wavelength
of the emitting light and improve their electron transportation. These polymers were characterized by
1H NMR and 13C NMR, gel permeation chromatography (GPC), thermal analysis, UV-vis, fluorescence
spectroscopy, and cyclic voltammetry (CV). The glass transition temperature of these polymers ranged
from 120 to 150 °C, and the polymers showed high thermal stability with decomposition temperatures in
the range 370-400 °C in nitrogen. The different linkage pattern of pyridinyl units in the copolymer
backbone has a significant effect on the electronic and optical properties in solution and in film phases.
For pure blue emission and prevention of aggregation of polymer chain, the meta-linkage pattern (3,5-
and 2,6-linkage) of pyridinyl units in the polymer backbone is more favorable than the para-linkage pattern
(2,5-linkage) of the pyridinyl units. The cyclic voltammetry (CV) results indicate that these polymers
with pyridinyl units possess lower LUMO energy levels for an easy electron injection from a cathode
than poly[3,6-(N-octyl)carbazolyl] and its derivatives do.7a
Introduction
Poly(2,5-pyridinediyl) (PPy) and its derivatives,9 such
as poly(pyridine vinylene) (PPyV),9a,b with electron-
accepting nature is a family of promising conjugated
polymers because of their high luminescence,9f excellent
electron transporting behavior, and their general resis-
tance to oxidation.9d The application of pyridinyl as the
π-deficient moiety in this novel series of polymer is
driven by the consideration that polypyridinyl (PPy) was
used in blue-emitting devices10,11 and that polymers
based on pyridine have been demonstrated to be highly
luminescent.12 Therefore, the incorporation of a pyridi-
nyl unit into polymer backbone increases the electron
affinity of the polymers, which not only makes the
copolymers to be n-dopable and capable of better
electron transportation13 but also makes the polymers
more resistant to oxidation. These polymers with pyri-
dine units are expected to possess low LUMO energy
levels for an easy electron injection from a cathode.
Therefore, these copolymers of a carbazolyl unit and
pyridinyl units are expected to possess low HOMO and
LUMO energy levels and will achieve a relatively
balanced charge injection.
As an extension of our research work concerning
polymeric blue light-emitting diodes,14 in the present
work, we first present the synthesis and characteriza-
tion of a novel series of blue-emissive alternating
copolymers of N-(2-ethylhexyl)carbazolyl unit with three
different pyridinyl units using the Suzuki cross-coupling
approach (Scheme 1). A 1:1 alternating copolymer
design was adopted because such arrangement would
generally give more consistent physical properties as
compared to random copolymers.
Ever since the first report of the polymeric light-
emitting diodes (PLED) device fabricated from poly(p-
phenylenevinylene)(PPV) in 1990,1 PLEDs have at-
tracted significant research attention from both academic
community and industry, particularly on account of
their potential applications in large-area, flat-panel
displays which can be operated at relatively low driving
voltages.1,2 For full-color display applications, the de-
velopment of red, green, and blue emitting materials
(either polymers or oligomers or small organic mol-
ecules) with high efficiency and stability is required.3-5
Among the three primary colors, only red and green
PLEDs have sufficient efficiencies and lifetimes to be
of commercial value. The pursuit of efficient and stable
blue PLEDs based on conjugated polymers remains a
challenge. This arises because it is hard to achieve a
balanced charge injection due to the large band gap
between the LUMO and HOMO energy levels.6
Recently, increasing interest has been paid to carba-
zole-based polymers because of their optical properties
and good hole-transporting ability in light-emitting
devices.7 Among the carbazole-derivatived polymers,
poly(N-alkyl-2,7-carbazolyl)s (PCs) have emerged as a
promising class of conjugated polymers, which can be
utilized as the blue light-emitting active layers in
PLEDs.8 However, poly(N-alkyl-2,7-carbazolyl) exhibits
additional emission bands in the long wavelength region
of the spectrum (453, 492 nm) in solid states, possibly
because of their tendency to form π-aggregates/excimers
in the solid state.8a
† National University of Singapore.
‡ Singapore National Eye Center.
§ Nanyang Technological University.
Experimental Part
Materials. The reagents 2-ethylhexyl bromide, 4,4′-dinitro-
2-azidobiphenyl, n-BuLi, tetrakis(triphenylphosphine)palladium-
(0) [(PPh3)4Pd(0)], 3,5-dibromopyridine, 2-isopropoxy-4,4,5,5-
* To
whom
correspondence
should
be
addressed.
E-mail: ngsc@ntu.edu.sg.
10.1021/ma050425b CCC: $30.25 © 2005 American Chemical Society
Published on Web 08/04/2005

7630 Pan et al.
Macromolecules, Vol. 38, No. 18, 2005
Scheme 1. Synthetic Approach to the Polymer PEHCP-26
tetramethyl-1,3,2-dioxaborolane, and tetrabutylammonium
percholate (Bu4NClO4) (AR) were obtained from Aldrich;
benzyltriethylammonium chloride, 2,5-dibromopyridine, and
2,6-dibromopyridine were obtained from Avocado Research
Chemicals Ltd. NaOH (AR), NaNO2 (AR), NaN3, SnCl2 (AR),
KI (AR), DMSO (AR), toluene (AR), acetonitrile, and all other
reagents were purchased from commercial sources and used
without further purification. The solvents diethyl ether,
chloroform, and THF were AR and were dried and distillated
prior to use.
ments of polymer solution and film were conducted on a
Shimadzu UV-1601 PC UV-vis spectrophotometer and a
Perkin-Elmer Instrument LS 55 luminescence spectrometer,
respectively. Cyclic voltammetry of polymer films was con-
ducted using a single-compartment, three-electrode cell com-
prising a platinum working electrode with the polymer spin-
coated on it, a platinum counter electrode, and a reference
electrode Ag/AgNO3 using a HB-105 Hokuto Denko Ltd.
arbitrary function generator and a HA-501 Hokuto Denko Ltd.
potentiostat. 0.1 M tetrabutylammonium percholate in aceto-
nitrile was used as the electrolyte solution.
Synthesis of Intermediate Compounds and Monomer.
The intermediate compounds of 4,4′-dinitro-2-azidobiphenyl
(II), 2,7-dinitrocarbazole (III), 2,7-diaminocarbazole (IV), 2,7-
diiodocarbazole (V), N-(2-ethylhexyl)-2,7-diiodocarbazole (VI),
and monomer 2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-
2-yl)-N-(2-ethylhexyl)carbazole (VII) were synthesized follow-
ing the procedure developed by Smith and Brown.15 All these
compounds have been characterized by NMR spectroscopy.
General Procedure for Polymerization by Suzuki
Cross-Coupling Reaction. Representative procedure for
polymerization by Suzuki cross-coupling reaction: 2,7-bis-
(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-N-(2-ethylhexyl)-
carbazole (VII) (0.2 g, 0.36 mmol), 2,6-dibromopyridine (0.084
g, 0.36 mmol), and (PPh3)4Pd(0) (8 mg, 0.007 mmol) (1 mol %
based on total monomers) were dissolved in a mixture of THF
and aqueous 2 M K2CO3(5 mL) (3/2 volume ratio) in a 25 mL
three-necked round-bottomed flask. The solution was stirred
under N2 and was heated at 95 °C with vigorous stirring for
48 h. The resulting mixture was then poured into 100 mL of
methanol. The precipitate was recovered by filtration and
washed with 2 M dilute HCl and methanol. The solid product
was extracted with acetone for 24 h in a Soxhlet apparatus to
Measurements. 1H NMR and 13C NMR spectra were
recorded on a Bruker ACF 300 FT-NMR spectrometer operat-
ing at 300 MHz. Deuterated chloroform was used as the
solvent, and tetramethylsilane (TMS) was used as the internal
standard. Weight-average molecular mass (Mw) and number-
average molecular mass (Mn) were determined by gel perme-
ation chromatography (GPC) using a Perkin-Elmer mode 200
HPLC system equipped with Phenogel MXL and MXM col-
umns using polystyrene as the standard and THF as the
eluant. The thermal properties of the polymers were investi-
gated by DSC and TGA under flowing nitrogen. Thermogravi-
metric analysis (TGA) was performed on a TA Instruments
with a TGA 2960 thermogravimetric analyzer module at a
heating rate of 20 °C min-1 with a nitrogen flow of 100 mL
min-1. The temperature regime was from room temperature
to 800 °C. Differential scanning calorimetry (DSC) of the
polymer powers was carried out using TA 2920 module at a
heating rate of 20 °C min-1 with a nitrogen flow of 70 mL
min-1. Dilute polymer solution (1 × 10-5 M) was prepared in
anhydrous spectrum-grade chloroform, and quinine sulfate (1
× 10-5 M in 0.1 M H2SO4) was used as the reference. The thin
polymer films were deposited onto quartz glass plates by spin-
coating. The absorption and fluorescence spectrum measure-

Macromolecules, Vol. 38, No. 18, 2005
Carbazolyl-Pyridinyl Alternating Copoloymers 7631
Table 1. Properties of the Polymers
c
d
polymer
yield(%)
color
Mwa (kDa)
PDI
DPb
Td
Tg
PEHCP-26
PEHCP-35
PEHCP-25
94
80
47
bellow-green
brown-yellow
yellow
4.9
4.3
1.4
1.56
1.35
1.22
14
12
4
400
392
372
145
128
122
a Weight-average molecular weights determined by GPC using polystyrene standards in THF. b Degree of polymerization. c Decomposition
temperature, determined by TGA in nitrogen, based on 5% weight loss. d Determined by DSC in nitrogen at a scan rate of 20 °C/min.
remove oligomers and catalyst residues; 0.125 g of yellow-green
poly[(2,7-(N-(2-ethylhexyl)carbazolyl)-alt-(2,6-pyridinyl)] (VIII)
was obtained; yield 94%. This polymer was designated as
PDEHCP-26, where the number 26 stands for the linkage
pattern of pyridinyl unit in the polymer backbone. The other
polymers were synthesized following a similar procedure.
Poly[(2,7-(N-(2-ethylhexyl)carbazolyl)-alt-(2,6-pyridi-
kDa) is lower than that (5 kDa) of one very similar
derivative, poly(N-(2-ethylhexyl)-2,7-carbazole-alt-4-
heptyl-2,5-pyridine) (PCPy).8b The main reason for the
relatively lower molecular weight of this series of
carbazloyl-pyridinyl-based alterning copolymers might
due to the lower activity of 2,7-bis(4,4,5,5-tetramethyl-
1,3,2-dioxaborolan-2-yl)-N-(2-ethylhexyl)carbazole (VII)
in Suzuki coupling in comparison with other monomer
system, such as 2,7-bis(4,4,5,5-tetramethyl-1,3,2-diox-
aborolan-2-yl)-9,9-dihexylfluorene.14f For example, Mw
(66.3 kDa) of poly[2,7-(9,9-dihexylfluorenyl)-alt-(2,5-
pyridinyl)] (PDHFP-25) with a DP of 16214f is much
higher than that (1.4 kDa) of PEHCP-25 (VIII) with a
DP of 4. The partial reason for the low value of DP of
might be the unstable reaction condition (e.g., concen-
tration) for Suzuki coupling because of the evaporation
loss of solvent under nitrogen.
1
nyl)] (PEHCP-26) (VIII). Yield 94%; yellow-green solid. H
NMR (300 MHz, CDCl3): δ (ppm) 7.86-8.32 (m, 9H, proton
of pyridinyl unit and carbazolyl unit), 4.42 (m, 2H), 2.28 (m,
1H), 0.83-1.50 (m, 14H).
Poly[(2,7-(N-(2-ethylhexyl)carbazolyl)-alt-(3,5-pyridi-
1
nyl)] (PEHCP-35). Yield 80%; brown-yellow solid. H NMR
(300 MHz, CDCl3): δ (ppm) 9.0 (s,1H), 8.28 (dd, 2H), 7.57-
7.70 (m, 6H), 4.33 (m, 2H), 2.20 (m, 1H), 0.82-1.46 (m, 14H).
Poly[(2,7-(N-(2-ethylhexyl)carbazolyl)-alt-(2,5-pyridi-
nyl)] (PEHCP-25) Yield 47%; yellow solid. 1H NMR (300
MHz, CDCl3): δ (ppm) 7.57-9.12 (m, 9H, proton of pyridinyl
unit and carbazolyl unit), 4.34 (m, 2H), 2.21 (m, 1H), 0.90-
1.44 (m, 14H).
The thermal stabilities of polymers were evaluated
by thermogravimetric analyses (TGA) in nitrogen, and
the results are also summarized in Table 1. It is
apparent that all the polymers exhibited excellent
thermal stability with an onset of decomposition in the
range 370-400 °C. Thermally induced phase transition
behavior of the polymers was investigated with dif-
ferential scanning calorimetry (DSC) under a nitrogen
atmosphere. The glass transition temperatures (Tg) of
polymers are also summarized in Table 1. Tg of the
polymers was in the range 120-150 °C. Tg (122 °C) of
PEHCP-25 is much higher than that (66 °C) of its very
similar derivative, poly(N-(2-ethylhexyl)-2,7-carbazole-
alt-4-heptyl-2,5-pyridine) (PCPy).8b The reason for large
difference between Tg of PEHCP-25 and PCPy was
mainly due to incorporation of long side chain heptyl
into pyridinyl units leading to an obvious decrease of
Tg of PCPy. Tg (122 °C) of PEHCP-25 oligomer is very
close to that (128 °C) of PEHCP-35; the reason for this
is due to the balance contribution of relatively low Mw
of PEHCO-25 (low Mw, leading to increase of Tg) and
more free movement of PEHCP-25 main chain (the less
steric hindrance with p-pyridinyl linkage, leading to
decrease of Tg). The relatively high glass transition
temperature of fluorescent polymer is essential for its
potential application, such as in PLEDs as emissive
materials.17 In all, the PEHCP series of polymers have
excellent thermal stability and high Tg.
Optical Property. The UV-vis absorption and
fluorescence emission spectra of the polymer solution
in CHCl3 and the polymers films, which were spin-cast
from CHCl3 solutions, were recorded at room temper-
ature. The optical data are summarized in Table 2.
The UV-vis absorption and photoluminescence (PL)
spectra of the PDEHCP series of polymers solutions are
depicted in Figure 1. It is evident that the linkage
pattern of the pyridinyl moiety in the polymer backbone
has a major influence on the absorption maximum of
polymer. The absorption maximum of PEHCP-25 with
para-linkage of pyridinyl unit at 365 nm is blue-shifted
in comparison with that of homopolymers poly(N-octyl-
2,7-carbazole),8a poly(N-(2-ethylhexyl)-2,7-carbazole)
Result and Discussion
Synthesis. The synthetic route for monomer VII and
this series of alternating copolymers is outlined in
Scheme 1. The compound N-(2-ethylhexyl)-2,7-diiodocar-
bazole (VI) was prepared from 4,4′-dinitro-2-azidobi-
phenyl (I) with a total yield of 18% in five steps
according to refs 8a and 15. The monomer compound
VII was synthesized with a yield of 42.4% using N-(2-
ethylhexyl)-2,7-diiodocarbazole (VI) as the starting
material following a similar procedure reported in the
literature.14f,g,16 The polymerization is based on the
Suzuki cross-coupling reaction. Poly[(2,7-(N-(2-ethyl-
hexyl)carbazolyl)-alt-(2,5-pyridinyl)] (PEHCP-25) (VIII)
was obtained in a moderate yield of 47%; poly[(2,7-(N-
(2-ethylhexyl)carbazolyl)-alt-(3,5-pyridinyl)] (PEHCP-
35) and poly[(2,7-(N-(2-ethylhexyl)carbazolyl)-alt-(2,6-
pyridinyl)] (PEHCP-26) were obtained in high yields of
80 and 94%, respectively. All three polymers were easily
soluble in chloroform and THF. The polymerization
results are summarized in Table 1.
All the polymers were characterized by 1H NMR and
1
GPC. From the H NMR spectrum and hydrogen as-
signment analysis (the latter mentioned above), the
singals of carbazolyl, 2-ethylhexyl, and pyridinyl can be
clearly seen. For example, PEHCP-25: δ (ppm) 7.57-
9.12 (m, 9H, proton of pyridinyl unit and carbazolyl
unit), 4.34 (m, 2H), 2.21 (m, 1H), 0.90-1.44 (m, 14H).
The intergration indicates that the ratio of carbazolyl
unit and pyridinyl unit in the polymer backbone is 1:1;
1
therefore, H NMR confirmed the proposed structures
of polymer. The molecular weights of polymers were
determined by GPC using THF as the eluent and
polystyrene as the standard. The results of GPC are
also shown in Table 1. PEHCP-26 and PEHCP-35 were
found to be polymers with relatively low weight-average
molecular weight (Mw) of ca. 4900 and 4300, corre-
sponding to a polymerization degree of approxi-
mately 14 and 12, and a polydispersity index of 1.56
and 1.35, respectively. PEHCP-25 was actually an
oligomer with a polymerization degree of 4; its Mn (1.1

7632 Pan et al.
Macromolecules, Vol. 38, No. 18, 2005
Table 2. Spectra Properties and Quantum Yield
solutiona
filmb
polymer
λmaxc (nm)
λplc (nm)
Φd
λmaxc (nm)
λplc (nm)
optical band gap (eV)
PEHCP-26
PEHCP-35
PEHCP-25
355
337
365
411, 428
403 (418)
420 (438)
0.395
0.458
0.376
361
344
376
436
2.97
3.02
2.77
431
463 (488)
a 1 × 10-5 M in anhydrous chloroform. b Spin-coating on quartz glass from anhydrous chloroform solution. c Wavelength of maximum
absorbance or emission, wavelength of the shoulder peak is in parentheses. d Relative to 1 × 10-5 M quinine sulfate in 0.1 M H2SO4.
Figure 1. UV-vis absorption (abs) and photoluminescence (PL) spectra of PEHCP series polymers in CHCl3 solution.
(PEHC),8b and poly(pyridine-2,5-diyl) (PPy),13 being at
380, 380, and 373 nm, respectively, but slightly red-
shifted in comparison with that of its very similar
derivative, poly(N-(2-ethylhexyl)-2,7-carbazole-alt-4-
heptyl-2,5-pyridine) (PCPy)8b at 354 nm because of the
effect of incorporation of incorporation of long side chain
heptyl into pyridinyl units. The absorption maximum
of PEHCP-26 and PEHCP-35 with the para-linkage of
the pyridinyl unit, being at 355 and 337 nm, respec-
tively, is much blue-shifted in comparison with that of
poly(N-octyl-2,7-carbazole),8a poly(N-(2-ethylhexyl)-2,7-
carbazole) (PEHC),8b and poly(pyridine-2,5-diyl) (ab-
sorption maxima of poly(pyridine-2,6-diyl) and poly-
(pyridine-3,5-diyl) were unavailable). The difference
among the absorption maximum of the PEHCP series
of copolymers is attributed to the lesser extent of
π-electron delocalization in the polymer backbone aris-
ing from meta-linkages, causing a reduction in the
π-conjugation lengths. The effect of different linkage
position of pyridinyl units on the absorption maximum
of polymer in this series of polymers is similar to that
one in our previous report.14a,f,g
2,7-carbazole) (PEHC)8b at 417 and 420 nm, respec-
tively, but also slightly red-shifted in comparison with
that of PCPy)8b at 407 nm because of the same reason
interpreted above, for PEHCP-26 and PEHCP-35, while
the emission maximum of copolymers with the meta-
linkage of the pyridinyl unit in the polymer backbone
at 411 and 428 nm and 403 and 418 nm, respectively,
is slightly violet-shifted as compared to that of poly(N-
octyl-2,7-carbazole)8a and poly(N-(2-ethylhexyl)-2,7-car-
bazole) (PEHC).8b
The photoluminescence quantum yields in chloroform
solution were measured according to the method de-
scribed by Davey et al.19 relative to 10-5 M quinine
sulfate in 0.1 M H2SO4. The results are summarized in
Table 2. High quantum yields of all the polymers are
obtained, being in the range 35-50%.
Figure 2 shows the UV-vis absorption and photolu-
minescence (PL) spectra of the PEHCP series of poly-
mers in thin films. As compared to absorption maxima
of poly(N-octyl-2,7-carbazole)8a and poly(N-(2-ethylhexyl)-
2,7-carbazole) (PEHC)8b at 382 and 383 nm, respec-
tively, this series of pyridinyl-containing copolymers
show blue-shifted absorption maxima in the range 344-
476 nm. The absorption maximum of PEHCP-25 at 376
nm is also slightly red-shifted in comparison with that
of poly(N-(2-ethylhexyl)-2,7-carbazole-alt-4-heptyl-2,5-
pyridine) (PCPy)8b at 362 nm, which is as the same as
in the solution discussed above. For PEHCP-25, in
which the pyridinyl unit is para-linked in the polymer
main chain, its absorption maximum in film at 376 nm
The photoluminescence (PL) spectra, which were
attributed to fluorescence on account of the short
lifetime of the excited state,18 were obtained by irradia-
tive excitation at the wavelength of the absorption
maximum. The blue emission maximum of PEHCP-25
with para-linkage of pyridinyl unit in the polymer
backbone at 420 nm is almost equivalent with that of
poly(N-octyl-2,7-carbazole)8a and poly(N-(2-ethylhexyl)-

Macromolecules, Vol. 38, No. 18, 2005
Carbazolyl-Pyridinyl Alternating Copoloymers 7633
Figure 2. UV-vis absorption (abs) and photoluminescence (PL) spectra of PEHCP series polymers in thin films.
is red-shifted in comparison with the absorption maxima
(at 361 and 344 nm, respectively) of PEHCP-26 and
PEHCP-35 with meta-linkages of the pyridinyl units.
This is attributed to the lower extent of π-electron
delocalization of the polymer main chain arising from
meta-linkages of the pyridinyl moiety, resulting in a
reduction in the π-conjugation length.
Electrochemical Property. The electrochemical
behavior of the polymers was investigated by cyclic
voltammetry (CV). CV was performed in a solution of
Bu4NClO4(0.10 M) in acetonitrile at a scan rate of 50
mV/s at room temperature under the protection of
nitrogen. A platinum electrode coated with a thin
polymer film was used as the working electrode. A Pt
wire was used as the counter electrode, and an Ag/
AgNO3(0.01 M) electrode was used as the reference
electrode. The resulting cyclic voltammograms are
outlined in Figure 3.
In the film state, the blue emission peaks appear at
436, 431, and 463 nm (488 nm, shoulder peak) for
PEHCP-26, PEHCP-35, and PEHCP-25, respectively.
The PL emission of polymer in the solid state is red-
shifted in comparison with that of polymer in solution,
as PL emission was obtained by irradiative excitation
at longer wavelength of the absorption maximum in the
solid sate. From photoluminescence (PL) spectra of
PEHCP-35 and PEHCP-26 (Figure 2), it can be seen
that the two polymers can emit purer blue light without
tailing phenomena and an additional red-shifted peak
in the long wavelength region of the spectrum as
compared with poly(N-octyl-2,7-carbazole),8a poly(N-
octyl-3,6-carbazole),7a and PEHCP-25. The tailing phe-
nomena and additional red-shifted peak in the long
wavelength region of the spectrum often exist in car-
bazole-based conjugated polymers, such as peaks at 453
and 492 nm for poly(N-octyl-2,7-carbazole),8a a peak at
500-520 nm for poly(N-octyl-3,6-carbazole),7a and a
peak at 488 nm for PEHCP-25. Therefore, PEHCP-35
and PEHCP-26 can emit purer blue light without any
evidence of π-aggregates/excimers in the solid state as
compared to the PL of poly(N-octyl-2,7-carbazole)8a and
poly(N-octyl-3,6-carbazole).7a The emission of PEHCP-
25 at 463 nm is obviously red-shifted in comparison with
that of poly(N-(2-ethylhexyl)-2,7-carbazole-alt-4-heptyl-
2,5-pyridine) (PCPy)8b at 430 nm; the reason may be
due to the effect of long side chain heptyl into pyridinyl
units. The results of PL in film states also show that
optical properties of carbazole polymers can be well
tuned by properly rational design of polymer architec-
tures, such as the incorporation of different pyridinyl
moieties into a polymer backbone.
The onset potentials for oxidation were observed to
be 2.09, 2.01, and 1.86 V for PEHCP-26, PEHCP-35, and
PEHCP-25, respectively. On the other hand, the onset
potentials for reduction were -0.72, -0.55, and -1.26,
respectively. From the onset potentials of the oxidation
and reduction processes, the band gaps of the polymers
were estimated to be 2.81, 2.56, and 3.12 eV for PEHCP-
26, PEHCP-35, and PEHCP-25, respectively. According
ox
to the equations20,21 IP ) -([Eonset
]
+ 4.4) eV and EA
red
ox
red
) -([Eonset
]
+ 4.4) eV, where [Eonset
]
and [Eonset]
are the onset potentials for the oxidation and reduction
of the polymers, the HOMO and LUMO of the polymers
were estimated to be -6.49, -6.41, and -6.26 eV and
-3.68, -3.85, and -3.14 eV for PEHCP-26, PEHCP-
35, and PEHCP-25, respectively. The corresponding
electrochemical data of the polymers are summarized
in Table 3. The HOMO (-6.26 to -6.49) and LUMO
(-3.14 to -3.85) of this series of polymers are much
lower than those of polymers derived from carbozole and
benzothiadiazole; the HOMO and LUMO of the latter
were in the range of -5.25 to -5.40 and -2.00 to -3.12,
respectively.7a
From Table 3, we can see that these polymers with
pyridinyl units possess very low LUMO energy levels
for an easy electron injection from a cathode, which is
consistent with our expectation. We can also find that
the HOMO and LUMO energy levels of PEHCP-26 and
PEHCP-35 are quite similar. As for PEHCP-25, its
HOMO is close to those of the other two polymers while

7634 Pan et al.
Macromolecules, Vol. 38, No. 18, 2005
Figure 3. Cyclic voltammograms of PEHCP series polymer films coated on platinum plate electrodes in acetonitrile containing
0.1 M Bu4NClO4.
Table 3. Electrochemical Properties of the Polymers
Dr. Shouping Liu gratefully acknowledges Singapore’s
National Science and Technology Board (NSTB) for the
award of a postdoctoral research fellowship.
energy levels (eV)
Eonset(V)
Eonset(V)
polymer
(p-doping) (n-doping) HOMO LUMO band gap
Supporting Information Available: Experimental de-
tails. This material is available free of charge via the Internet
at http://pubs.acs.org.
PEHCP-26
PEHCP-35
PEHCP-25
2.09
2.01
1.86
-0.72
-0.55
-1.26
-6.49 -3.68
-6.41 -3.85
-6.26 -3.14
2.81
2.56
3.12
References and Notes
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