Hyperbranched poly(arylene ethynylene)s with triphenylamine core for polymer light-emitting diodes

Article abstract of DOI:10.1002/pola.24459

A series of light-emitting hyperbranched poly(arylene ethynylene)s (HB-PAEs) were prepared by the Sonogashira coupling from bisethynyl of carbazole, fluorene, or dialkoxybenzenes (A₂ type) and tris(4-iodophenyl)amine (B₃ type). For comparison, two linear polymers (L-PAEs) of the HB analogs were also synthesized. The polymers were characterized by Fourier transform infrared, NMR, and GPC. The HB polymers showed excellent solubility in chloroform, THF, and chlorobenzene when compared with their linear analogs. The number-average molecular weight (M_n) of the polymers determined from GPC was found to be in the range of 18,600-34,200. The polymers were thermally stable up to 298-330 °C with only 5% weight loss. The absorption maxima of the polymers were between 354 and 411 nm with optical band gap in the range of 2.5-2.9 eV. The HB polymers were found to be highly fluorescent with photoluminescence quantum yields around 33-42%. The highest occupied molecular orbital energy levels of the polymers calculated from onset oxidation potentials were found to be in the range from -5.83 to -6.20 eV. Electroluminescence (EL) properties of three HB-PAEs and one L-PAE were investigated with device configuration ITO/PEDOT:PSS/Polymer/LiF/Al. The EL maxima of HB-PAEs were found to be in the range of 507-558 nm with turn-on voltages around 7.5-10 V and maximum brightness values of 316-490 cd/m ². At the same time, linear analog of one HB-PAE was found to show a maximum brightness of 300 cd/m² at a turn-on voltage of 8.2 V.

Full text of DOI:10.1002/pola.24459

Hyperbranched Poly(arylene ethynylene)s with Triphenylamine
Core for Polymer Light-Emitting Diodes
AKSHAYA K. PALAI,1 SARADA P. MISHRA,1 AMIT KUMAR,2 RITU SRIVASTAVA,2
MODEEPARAMPIL N. KAMALASANAN,2 MANORANJAN PATRI1
1Naval Materials Research Laboratory, Shill-Badlapur Road, Ambernath 421506, India
2National Physical Laboratory, Dr. K. S. Krishnan Road, New Delhi 110012, India
Received 7 June 2010; accepted 22 October 2010
DOI: 10.1002/pola.24459
Published online 14 December 2010 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: A series of light-emitting hyperbranched poly(ary-
lene ethynylene)s (HB-PAEs) were prepared by the Sonoga-
shira coupling from bisethynyl of carbazole, fluorene, or
dialkoxybenzenes (A2 type) and tris(4-iodophenyl)amine
(B3 type). For comparison, two linear polymers (L-PAEs) of the
HB analogs were also synthesized. The polymers were charac-
terized by Fourier transform infrared, NMR, and GPC. The HB
polymers showed excellent solubility in chloroform, THF, and
chlorobenzene when compared with their linear analogs. The
number-average molecular weight (Mn) of the polymers deter-
mined from GPC was found to be in the range of 18,600–
34,200. The polymers were thermally stable up to 298–330 ꢀC
with only 5% weight loss. The absorption maxima of the poly-
mers were between 354 and 411 nm with optical band gap in
the range of 2.5–2.9 eV. The HB polymers were found to be
highly fluorescent with photoluminescence quantum yields
around 33–42%. The highest occupied molecular orbital energy
levels of the polymers calculated from onset oxidation poten-
tials were found to be in the range from ꢁ5.83 to ꢁ6.20 eV.
Electroluminescence (EL) properties of three HB-PAEs and one
L-PAE were investigated with device configuration ITO/
PEDOT:PSS/Polymer/LiF/Al. The EL maxima of HB-PAEs were
found to be in the range of 507–558 nm with turn-on voltages
around 7.5–10 V and maximum brightness values of 316–490
cd/m2. At the same time, linear analog of one HB-PAE was
found to show a maximum brightness of 300 cd/m2 at a turn-
C
V
on voltage of 8.2 V. 2010 Wiley Periodicals, Inc. J Polym Sci
Part A: Polym Chem 49: 832–841, 2011
KEYWORDS: electroluminescence; fluorescence; hyperbranched;
light emitting diodes (LED); poly(arylene ethynylene)s;
triphenylamine
INTRODUCTION Conjugated polymers have attracted lot of
interest in scientific and technological fields because of their
potential use as semiconductor and electroactive materials in
a variety of optoelectronic devices, such as organic light-
emitting diodes (OLEDs),1 field effect transistors,2 photovol-
taic cells,3 nonlinear optical devices,4 and electrochromic
cells.5 In particular, OLEDs using conjugated polymers have
drawn great attention because of easy processing, low oper-
ating voltages, faster response times, and facile color tuning
over the full visible range, which makes them suitable for
large-area flat panel display.6 Therefore, designing new poly-
meric materials with varied optoelectronic properties is of
the great choice to meet the requirements for the next gen-
eration of display technology.
Although several PAEs were developed till date and widely
used as sensory materials,10 little effort has been paid to
PAEs as potential OLED materials.11 This is because of the
reduced emission efficiencies in the solid state, which has
been attributed to the formation of the aggregates.12 Besides,
the alkyne units in PAEs limit the redox properties of poly-
mers resulting in a large potential barrier to the injection
and transportation of holes.13 Hence, to use PAEs as efficient
OLED materials, one has to design the structure to minimize
the aggregation and enhance the charge-balancing properties.
One of the plausible solutions is to make hyperbranched
poly(arylene ethynylene) (HB-PAEs) with suitable choice of
the comonomers. HB polymers show good solubility, better
processibility, and more tunable emission color than linear
polymers.14 In addition, the HB polymers can improve the
morphologies of the thin films and prevent crystallization.15
Moreover, their three-dimensional (3D) structures help in
reducing the aggregation of the conjugated polymer chains
and in some cases, it improves their emission efficiency and
thermal stability.16
Among conjugated polymers, poly(arylene ethynylene)s
(PAEs) have unique properties such as photoluminescence,7
nonlinear optics,8 and exceptionally high-solution state PL
efficiencies9 in comparison to other conjugated polymers.
Such properties render these materials as another class of
prospective materials for various optoelectronic applications.
Additional Supporting Information may be found in the online version of this article. Correspondence to: M. Patri (E-mail: mrpatri@rediffmail.com)
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Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 49, 832–841 (2011) 2010 Wiley Periodicals, Inc.
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ARTICLE
Herein we report a series of PAEs with HB architecture syn-
thesized via A2 þ B3 approach. Several electron-donating
aromatic units have been introduced in the polymer backbone
with triphenylamine (TPA) as the core. Particular attention
was paid to the TPA moieties because of their ability for the
hole injection.17 TPA derivatives can be viewed as 3D sys-
tems, and amorphous character of these materials offers pos-
sibilities to develop active materials for OLEDs. On the other
hand, ethynylene units in the polymeric backbone would
provide an enhanced p-electron delocalization because of the
coplanar molecular structure. Hence, better charge-balancing
properties would be expected from these set of polymers.
standard. PLED devices were fabricated with ITO as a posi-
tive electrode, LiF/Al as a negative electrode and the film of
the polymer between them as emissive layer. The device con-
figuration was ITO/PEDOT:PSS/HB-PAE/LiF/Al structure.
Active area of all fabricated devices was 4 ꢂ 4 mm2 and all
electrical measurements were performed under ambient con-
dition without encapsulation. Electroluminescence (EL) spec-
tra of the devices were obtained using HR 2000 Ocean
Optics spectrometer, having a CCD array and fiber optic
probe. The current–voltage and luminescence–voltage char-
acteristics were studied using Keithley 617 electrometer and
luminance meter (LMT L1009).
We investigated thermal, electrochemical, and optoelectronic
properties of resulting polymers. The resultant polymers
have high molecular weights and high thermal stability and
can be used for OLEDs. The polymer light-emitting devices
(PLEDs) based on HB-PAE1, HB-PAE2, HB-PAE4, and L-PAE2
were fabricated and evaluated to investigate the light-emit-
ting properties.
Synthesis
Tri(4-iodophenyl)amine) (5)
TPA (3 g, 12.24 mmol), potassium iodate (2.88 g, 13.47 mmol),
and acetic acid (100 mL) were added to a 250-mL round-bottom
flask. To the mixture, potassium iodide (4.47 g, 26.93 mmol) was
added while stirring. The mixture was heated at 80 ꢀC for 12 h.
The reaction mixture was cooled and the precipitate formed
was collected by filtration. The residue was thoroughly washed
with water till neutral. The solid was dissolved in chloroform
(100 mL) and consecutively washed with water and NaHCO3.
The organic layer was dried over anhydrous Na2SO4 and
evaporated to obtain light gray solid. Purifications by column
chromatography (silica gel) using petroleum ether as eluent
isolated the compound as an off white solid. Yield: 87%. M.P.:
167 ꢀC. 1H NMR and 13C NMR data were well matched with lit-
erature values.19
EXPERIMENTAL
Materials
Anhydrous chlorobenzene was purchased from Sigma–Aldrich.
Diisopripylamine was dried over sodium ketyl radical.
9-(2-Ethylhexyl)-3,6-diethynyl-9H-carbazole (1), 9,9-bis(2-eth-
ylhexyl)-2,7-diethynyl-9H-fluorene (2), 1-(2-ethylhexyloxy)-2,5-
diethynyl-4-methoxybenzene (3), 4-bis(2-ethylhexyloxy)-2,5-
diethynylbenzene (4), and 4-iodo-N-(4-iodophenyl)-N-phenyla-
niline (6) were synthesized following the literature procedure.18
General Method for Polymerization
Measurements
(for HB-PAE and L-PAE)
Thin layer chromatography was performed with Merck silica
gel 60 F254 aluminum sheets. Chromatographic separations
were carried out using Merck silica gel 100–200 mesh. Fou-
rier transform infrared (FTIR) spectra were obtained from a
Under nitrogen atmosphere, anhydrous chlorobenzene
(4 mL) and diisopropylamine (2 mL) were added by syringe
into
a thoroughly dried 25-mL Schlenk flask charged
with tris(4-iodophenyl)amine (5) (100 mg, 0.16 mmol),
(PPh3)2PdCl2 (2.24 mg, 0.0032 mmol), and CuI (1.22 mg,
0.0064 mmol). To the stirring mixture, bisethynylene monomer
(1, 2, 3, or 4; 0.24 mmol) in chlorobenzene (2 mL) was added
dropwise by syringe. The mixture was stirred for 72 h at room
temperature and excess bromobenzene was added. The solu-
tion was stirred for additional 6 h and poured into methanol
(50 mL). The precipitate was filtered and dried under vacuum
to obtain the polymer as yellow powder. The polymer was
stored in desiccators in absence of light for further use.
1
Nicolet 5700 FTIR spectrophotometer. H NMR spectra were
recorded using either a Brucker 500 or 300 MHz FT-NMR
spectrometer with chloroform-d as solvent. Chemical shifts
were reported in ppm units with tetramethylsilane as an in-
ternal standard. Molecular weights were determined by GPC
on Waters 2690 separations module apparatus using a
Waters 2410 refractive index detector. All the measurements
ꢀ
were carried out at 35 C using THF as solvent and polysty-
rene as standard. UV–vis and PL spectra were recorded using
Varian Carry 5000 scan and Spex Flurolog-3-11 spectrome-
ters. Thermogravimetric analysis (TGA; Hi Res TGA 2950, TA
Instruments) of the samples was carried out at a heating
rate of 10 ꢀC/min under nitrogen atmosphere. Differential
scanning calorimetric (DSC) measurements of the polymers
wereꢀperformed under nitrogen at a heating and cooling rate
of 5 C/min with a TA DSC Q100 instrument. Cyclic voltam-
metry (CV) was performed on an AUTOLAB/PGSTAT30
potentiostat with a three-electrode cell in a solution of
Et4NBF4 (0.10 M) in acetonitrile at a sweep rate of 50 mV/s.
Platinum electrode coated with polymer film was used as
working electrode. Platinum mesh electrode was used as
counter electrode and Ag/Agþ electrode was used as refer-
ence. The measurements were calibrated using ferrocene as
Similar procedure was adopted for synthesizing the linear
polymers. In this case, equimolar amount of 4-iodo-N-(4-
iodophenyl)-N-phenylaniline (6) and bisethynylene monomer
(1 or 2) were taken for polymerization.
HB-PAE1
9-(2-Ethylhexyl)-3,6-diethynyl-9H-carbazole (1) (78.48 mg,
0.24 mmol) was used. Yield: 82%. FTIR (cmꢁ1, KBr): 2954,
2926, 2868, 2360, 2338, 1597, 1505, 1481, 1315, 1281, 805.
1H NMR (CDCl3, 300 MHz), d (ppm): 8.22 (Ar-H, Carbazole),
7.56 (Ar-H, Carbazole), 7.46 (Ar-H, Carbazole), 7.04 (Ar-H,
TPA), 6.84 (Ar-H, TPA), 4.12 (NCH2), 2.00 (CH), 1.55–1.27
(CH2), 0.85 (CH3). 13C NMR (CDCl3, 125 MHz), d (ppm):
146.55, 141.44, 131.58, 129.47, 126.03, 123.95, 121.61,
PAE WITH TPA FOR POLYMER LIGHT-EMITTING DIODES, PALAI ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
119.90, 116.56, 111.20, 94.65, 85.59, 35.20, 33.52, 28.09,
26.92, 22.66, 14.03, 13.95, 10.53. Mn ¼ 19,000, Mw/Mn ¼ 2.4
(GPC, polystyrene calibration). Td (oC, 5% weight loss): 330.
3:2 molar ratio of the bis-ethynyl monomer (1, 2, 3, or 4)
and tri(4-iodophenyl)amine) (5) were allowed to react in
presence of catalytic amount of (PPh3)2PdCl2 and CuI as
shown in Scheme 1. Initially, the polymerization was carried
out following the procedure routinely used for the synthesis
of linear PAEs.20,11a In this procedure, A2 (0.24 mmol) and
B3 (0.16 mmol) monomers were dissolved in chlorobenzene
(4 mL) to which (PPh3)2PdCl2 (0.0032 mmol), CuI (0.0064
mmol), and diisopropylamine (2 mL) wꢀere added and the
mixture was heated under stirring at 65 C. However, in this
condition, the reaction mixture gelled within 30 min. There-
fore, to avoid gelation, the polymerization was carried out by
taking the B3 monomer along with catalyst and base and a
chlorobenzene solution of A2 was added slowly. For all the
polymerization, chlorobenzene was used as solvent for poly-
merization to obtain high molecular weight as well as more
soluble polymers.18 It was observed that by increasing the
reaction temperature above 50 ꢀC, the color of the reaction
mixture turned dark brown with the formation of insoluble
mass. Therefore, all the polymerization reactions were car-
ried out at room temperature. It was also observed that the
soluble polymers without end capping are found to become
insoluble after Soxhlet extraction and drying. This dark color
and insolubility of the polymers may be attributed to cross-
linking of the polymers. This type of behavior was also
observed by others and described the crosslinking of the
polymers due to the tendency of cyclo- and [4 þ 2] cyclodi-
merization of the remaining ethynyl groups with ene-ethynyl
moieties in the presence of Pd traces or possible Bergman
cyclization of the ene-dieyne moieties in the polymer.21
HB-PAE2
9,9-Bis(2-ethylhexyl)-2,7-diethynyl-9H-fluorene (2) (105.68
mg, 0.24 mmol) was used. Yield: 86%. FTIR (cmꢁ1, KBr):
2954, 2923, 2858, 2361, 2335, 1540, 1514, 1457, 820. 1H
NMR (CDCl3, 300 MHz), d (ppm): 7.65 (Ar-H, fluorene), 7.54
(Ar-H, fluorene), 7.07 (Ar-H, TPA), 6.86 (Ar-H, TPA), 2.17 (CH2),
2.00 (CH), 1.66–0.74 (CH2), 0.55 (CH3). 13C NMR (CDCl3, 125
MHz), d (ppm): 150.91, 146.55, 138.44, 132.74, 131.58, 130.47,
128.04, 127.00, 126.33, 123.35, 121.61, 120.12, 119.90, 94.15,
84.80, 44.54, 34.68, 33.52, 27.99, 26.92, 22.66, 14.03, 13.95,
10.23. Mn ¼ 22,700, Mw/Mn ¼ 3.3 (GPC, polystyrene calibra-
tion). Td (ꢀC, 5% weight loss): 325.
HB-PAE3
1-(2-Ethylhexyloxy)-2,5-diethynyl-4-methoxybenzene (3) (68.25
mg, 0.24 mmol) was used. Yield: 80%. FTIR (cmꢁ1, KBr): 2954,
2925, 2862, 2361, 2335, 1592, 1507, 1488, 1316, 1280, 1215,
1034, 828. 1H NMR (CDCl3, 300 MHz), d (ppm): 7.56 (Ar-H),
7.43 (Ar-H), 7.00 (Ar-H, TPA), 6.86 (Ar-H, TPA), 4.56–3.69
(OCH2), 3.22 (OMe), 1.79 (CH), 1.56–1.35 (CH2), 0.96 (CH3).
13C NMR (CDCl3, 125 MHz), d (ppm): 152.40, 150.91, 146.45,
131.58, 121.61, 119.12, 118.90, 117.60, 112.76, 95.15, 83.90,
70.23, 54.68, 33.56, 28.00, 26.93, 22.76, 14.03, 13.85, 10.21.
Mn ¼ 18,600, Mw/Mn ¼ 2.9 (GPC, polystyrene calibration). Td
(ꢀC, 5% weight loss): 302.
HB-PAE4
4-Bis(2-ethylhexyloxy)-2,5-diethynylbenzene (4) (91.82 mg,
0.24 mmol) was used. Yield: 84%. FTIR (cmꢁ1, KBr): 2955,
2926, 2862, 2361, 2335, 1509, 1487, 1316, 1271, 829. 1H
NMR (CDCl3, 300 MHz), d (ppm): 7.54 (Ar-H), 7.02 (Ar-H,
TPA), 6.85 (Ar-H, TPA), 4.46–3.65 (OCH2), 1.79 (CH), 1.56–
1.35 (CH2), 0.96 (CH3). 13C NMR (CDCl3, 125 MHz), d (ppm):
151.48, 146.05, 131.58, 122.61, 118.90, 117.60, 112.56,
95.95, 83.90, 72.23, 41.68, 32.56, 28.96, 23.93, 22.76, 13.85,
11.21. Mn ¼ 19,500, Mw/Mn ¼ 3.4 (GPC, polystyrene calibra-
tion). Td (ꢀC, 5% weight loss): 298.
To compare with the linear analog, two linear polymers were
synthesized as shown in Scheme 1 (part 2). The polymeriza-
tions were carried out by reacting equimolar ratio of 4-iodo-
N-(4-iodophenyl)-N-phenylaniline (6) and bisethynylene
monomers (1 or 2) in similar conditions as for HB polymers.
After the successive polymerization and end capping, it was
found that the fluorene-containing polymer (L-PAE2) was
soluble in most of the organic solvents where as the carba-
zole-containing polymer (L-PAE1) was insoluble. This could
be because of the more rigid structure of the linear PAEs
when compared with its HB analog. The higher solubility of
L-PAE2 when compared with L-PAE1 is because of the pres-
ence of two 2-ethylhexyl group which enhances the solubil-
ity. As L-PAE1 was insoluble, no characterizations of the
polymer could carry out.
L-PAE2
4-Iodo-N-(4-iodophenyl)-N-phenylaniline (6) (100 mg, 0.20
mmol) and 9,9-bis(2-ethylhexyl)-2,7-diethynyl-9H-fluorene
(2) (88.13 mg, 0.20 mmol) were used. Yield: 79%. 1H NMR
(CDCl3, 300 MHz), d (ppm): 7.67 (Ar-H, fluorene), 7.54 (Ar-H,
fluorene), 7.48 (Ar-H, fluorene), 7.36 (Ar-H, TPA), 7.19 (Ar-H,
TPA), 7.12 (Ar-H, TPA), 6.93, 6.86 (Ar-H, TPA), 2.08 (CH2), 1.58
(CH), 1.26–0.74 (CH2), 0.55 (CH3). 13C NMR (CDCl3, 125 MHz),
d (ppm): 150.94, 147.11, 138.24, 132.60, 129.52, 126.07,
125.39, 125.11, 124.00, 123.35, 122.94, 119.76, 44.54, 34.62,
33.51, 27.98, 26.98, 22.67, 14.03, 10.29. Mn ¼ 34,200, Mw/Mn
¼ 3.8 (GPC, polystyrene calibration). Td (ꢀC, 5% weight loss):
314.
The number-average molecular weights (Mn) of the polymers
estimated by GPC were in the range of 18,600–34,200 with
polydispersity index of 2.4–3.8. The yield and molecular
weights of the polymers are summarized in Table 1. In HB
polymer series, the polymers HB-PAE2 and HB-PAE4 had
higher molecular weight when compared with HB-PAE1 and
HB-PAE3. This could be attributed to the presence of the
two 2-ethylhexyl groups which enhance the solubility. How-
ever, comparing with the HB polymers, the linear polymer
was found to possess much higher molecular weight in GPC
probably because of larger hydrodynamic volume. All the
synthesized polymers were analyzed by FTIR and 1H NMR
RESULTS AND DISCUSSION
Synthesis and Characterization
The HB polymers were synthesized by A2 þ B3 approch
using Sonagashira polycondensation reaction. In this method,
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SCHEME 1 Synthesis of polymers.
spectroscopy. In IR spectra, the presence of two bands (due
to Fermi resonance) around 2300 cmꢁ1 assignable to disub-
stituted ethynylene moiety at the main chain and the ab-
sence of absorption peaks attributed to monosubstituted
ethynylenes (CAH stretching vibration around 3300 cmꢁ1
and CBC stretching vibration around 2100 cmꢁ1) support
the formation of polymers. The 1H NMR spectra of a repre-
sentative polymer HB-PAE1 is shown in Figure 1. In H NMR
spectra, the aromatic protons show broader resonance peak
ranging from 8.25 to 6.85 ppm (peaks have been assigned in
Fig. 1) because of the possible aggregation of polymer in so-
lution. The signal of the methylene group adjacent to N-atom
appears around 4 ppm. For all polymers, the characteristic
resonance peaks of 2-ethylhexyl chains appear in the range
2.20–0.57 ppm. The absence of terminal acetylene proton in
1H NMR spectra indicated that the end-capping reaction was
completed. Because of the unavailability of distinguishable
1
1
dendritic, linear and terminal signals in H NMR spectra, the
degree of branching of the polymers could not determined.22
To confirm the HB structure, 13C NMR analysis of both linear
PAE WITH TPA FOR POLYMER LIGHT-EMITTING DIODES, PALAI ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
TABLE 1 Molecular Weight Distribution and Thermal Stability
of Polymers
a
Polymer
Yield (%)
Mn
PDI (Mw/Mn)
Td (ꢀC)
HB-PAE1
HB-PAE2
HB-PAE3
HB-PAE4
L-PAE2
a
82
86
80
84
79
19,000
22,700
18,600
19,500
34,200
2.4
3.3
2.9
3.4
3.8
301b (330)c
287b (325)c
260b (302)c
263b (298)c
270b (314)c
Determined by GPC relative to polystyrene standards.
Temperature at 2% weight loss.
b
c
Temperature at 5% weight loss.
and HB polymers were performed and were compared side by
side. In 13C spectra, the peak at 125.39 ppm assigned to the
unsubstituted phenyl ring in triphenyl amine core indicated the
formation of linear polymer (L-PAE2) where as the absence of
that peak confirmed the formation of HB structure (Figs. S1
and S2, Supporting Information). Additionally, two peaks at
147.11 ppm for linear polymer (L-PAE2) assigned to three car-
bon atoms attached to N-atom of TPA indicate the formation of
linear polymer where as a single peak at 146.55 ppm for HB-
PAE2 assigned to three carbon atoms attached to N-atom of
TPA confirmed the formation of HB polymer (Figs. S1 and S2,
Supporting Information, indicated by arrow).
FIGURE 2 UV–vis absorption spectra of polymers in dilute
CHCl3 solution.
polymers were studied by UV–vis and PL spectroscopies. The
UV–vis spectra of all the polymers were recorded in solution
as well as in thin film. Figure 2 shows the normalized UV–
vis spectra of polymers measured in THF. The absorption
maxima of the solutions are summarized in Table 2. The HB
polymers showed solution absorption maxima in the wave-
length region between 353 and 418 nm where as the linear
polymer L-PAE2 showed absorption maxima at 388 nm. The
Optical Properties
As the optical spectra are usually indicative of the structural
features of conjugated polymers, the properties of these new
FIGURE 1 1H NMR spectrum of HB-PAE1 in CDCl3.
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TABLE 2 Optoelectronic Properties of the Polymers
Solution (Optical)
Thin Film (Optical)
Oxidation (Electrochemical)
Abs kmax
PL kmax
Abs kmax
PL kmax
HOMO
(eV)b
LUMO
(eV)c
Polymer
(nm)
(nm)
(nm)
(nm)
UPL (%)
Eg (eV)a
Eonset (V)
HB-PAE1
HB-PAE2
HB-PAE3
HB-PAE4
L-PAE2
a
353
374
408
411
388
434
438
469
473
442
354
375
410
411
401
435, 483
471, 507
540
33
42
36
39
30
2.91
2.66
2.58
2.53
2.80
1.03
1.20
1.36
1.40
1.10
ꢁ5.83
ꢁ6.00
ꢁ6.16
ꢁ6.20
ꢁ5.90
ꢁ2.92
ꢁ3.34
ꢁ3.58
ꢁ3.67
ꢁ3.10
545
508
c
Optical band gap.
Calculated from the HOMO energy level and the optical band gap.
b
Calculated from onset oxidation potential.
absorption peak of L-PAE2 is red shifted 14 nm relative to
that of HB-PAE2, which is attributed to its longer effective
conjugation length because of higher molecular weight. Nor-
malized thin film absorption spectra of the polymers are
depicted in Figure 3 and the corresponding optical data are
given in Table 2. The absorption maxima of the HB polymer
thin films occur between 354 and 411 nm where as for lin-
ear L-PAE2, absorption maxima occur at 401 nm. It is impor-
tant to note that in the absorption spectra, red shift of
L-PAE2 (13 nm) in going from solution to film state is higher
than that of HB-PAE2 (01 nm). This is attributed to higher
interchain interaction in L-PAE2, which is facilitated by its
linear structure. However, the smaller red shift of HB-PAE2
is mainly because of its globular structure.23 The onset of
the thin film absorption spectra were used to determine the
band gap of the polymers. The calculated band gaps of poly-
mers are in the range of 2.5–2.9 eV and are summarized in
Table 2.
solutions and thin films are summarized in Table 2. The
maxima of emission in solution are centered around 434–
473 nm (Fig. 4). While comparing the emission maxima with
the polymeric structure, it was observed that the alkoxy-
phenyl-containing polymers (HB-PAE3 and HB-PAE4) showed
a bathochromic shift when compared with the others (HB-
PAEP1 and HB-PAE2). This indicates the better packing of
the polymeric main chain in case of HB-PAE3 and HB-PAE4
when compared with rest of the HB polymers. This could be
because of the formation of symmetric acetylene linkage
between the TPA unit and the alkoxyphenyl units which
helps the polymer chains for better packing. The PL emission
in thin film of HB-PAEs and L-PAE2 shows the emission max-
ima between 435 and 545 nm (Fig. 5). Compared with solu-
tion emission, the PL emission of the polymer films shows a
red shift, probably because of the aggregation of the polymer
chains in the solid state. The PL spectra of L-PAE2 was found
to be red shifted both in solid and in solution state as com-
pared the HB analog HB-PAE2 probably due to the better
packing of the former polymeric main chain. Fluorescence
The PL spectra of all the polymers were also recorded in
THF solution as well as in film. The emission maxima of the
FIGURE 3 UV–vis absorption spectra of polymer thin films.
[Color figure can be viewed in the online issue, which is avail-
able at wileyonlinelibrary.com.]
FIGURE 4 PL spectra of the studied polymers in dilute CHCl3
solution. [Color figure can be viewed in the online issue, which
is available at wileyonlinelibrary.com.]
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE 6 TGA traces of the polymers. [Color figure can be
viewed in the online issue, which is available at
wileyonlinelibrary.com.]
FIGURE 5 PL spectra of the polymer thin films. [Color figure
can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
Thermal Properties
quantum yield (UPL) of the polymers was determined follow-
ing our earlier reported procedure24 and the results are sum-
marized in Table 2. A moderately PL quantum yield was
obtained from all HB polymers where as a weak PL quantum
yield was obtained for L-PAE2. The low PL quantum yield of L-
PAE2 could be because of its more aggregation nature thereby
quenching the PL quantum yield. Compared with TPA–carba-
zole copolymer HB-PAE1, TPA–fluorene copolymer HB-PAE2
had higher quantum yield. The introduction of carbazole units
into TPA polymers reduces the quantum yield of emission.
The thermal properties of the synthesized polymers were
investigated using TGA and DSC. The carbazole-containing
HB polymer HB-PAE1 was the most stable one among these
polymers, which shows 2% decomposition at 300 ꢀC under
nitrogen (Fig. 6). This can be attributed to the presence of
additional nitrogen atom in the polymer. The other polymers
HB-PAE2, HB-PAE3, and HB-PAE4 show 2% weight loss at
287, 260, and 268ꢀꢀC, respectively. The L-PAE2 shows 2%
weight loss at 270 C. DSC analysis of the polymer indicated
FIGURE 7 Cyclic voltammograms
of the polymers.
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ARTICLE
pared with the dialkoxy phenyl units (in HB-PAE3 and
HB-PAE4). The onset oxidation potential of linear polymer
(L-PAE2) was found to be 1.10 V. From the onset oxidation
potentials, the HOMO energy levels of the polymers were cal-
culated according to the equation: HOMO [ionization poten-
tial (IP)] ¼ ꢁe(Eox þ 4.8 eV), assuming that the IP of ferro-
cene is ꢁ4.8 eV for the Fc/Fcþ system.26 The HOMO energy
levels of the HB polymers were found to be in the order:
HB-PAE4 (ꢁ6.20 eV) < HB-PAE3 (ꢁ6.16 eV) < HB-PAE2
(ꢁ6.00 eV) < HB-PAE1 (ꢁ5.83 eV). The higher IP of alkoxy
phenyl-containing polymers (HB-PAE3 and HB-PAE4) sug-
gested their better stability when compared with more elec-
tron-rich carbazole-(HB-PAE1) and fluorene- (HB-PAE2) con-
taining polymers. The HOMO energy level of L-PAE2 was
found to ꢁ5.80 eV. The LUMO energy levels of polymers, cal-
culated from optical band gap and the HOMO energy level
were in the range ꢁ2.92 to ꢁ3.67 eV.
Electroluminescence Properties
FIGURE 8 Electroluminescence spectra of the studied poly-
mers. [Color figure can be viewed in the online issue, which is
available at wileyonlinelibrary.com.]
To test the performances of the polymers, EL devices were
fabricated using HB polymers HB-PAE1, HB-PAE2, HB-PAE4,
and the linear polymer L-PAE2 as an emissive layer. The fab-
rication of the devices followed our earlier reported proce-
dure.22 The concentration of the polymers in chlorobenzene
was kept at 10 mg/mL. Typical EL spectra of the polymers
were shown in Figure 8. The EL maxima of HB-PAE1, HB-
PAE2, HB-PAE4, and L-PAE2 devices at 10 V are centered at
506, 535, 558, and 540 nm with Commission Internationale
De L’Eclairage (CIE) coordinates at (0.21, 0.46), (0.35, 0.52),
(0.47, 0.50), and (0.41, 0.55), respectively (Table 3). The EL
of all polymers is red shifted with respect to the solid state
PL. Similar red shift has been observed in other PAE sys-
tems, which is probably because of increased radiative decay
from the longer conjugated segments during EL.1a
the absence of glass transition in the range of 0–200 ꢀC.
However, because of the very rigid nature of these polymers,
the possibility of a glass transition temperature higher than
ꢀ
200 C cannot be completely excluded. In all cases, no crys-
tallizing and melting peaks were detected, suggesting that
these polymers are amorphous.25
Electrochemical Properties
To investigate the energy levels of their highest occupied
molecular orbital (HOMO) and lowest unoccupied molecular
orbital (LUMO), the electrochemical properties of polymers
were investigated by CV. The measurements were calibrated
using ferrocene as the standard. The results of electrochemi-
cal measurements are listed in Table 2. Figure 7 shows the
oxidation waves of the polymers as a result of the p-type
doping. The polymers exhibit irreversible p-doping/dedopin-
g(oxidation/rereduction) processes at positive potentials
with onset of potentials between 1.03 and 1.40 V versus
Ag/Agþ. Among the four HB polymers, HB-PAE1 possesses
the lowest onset oxidation potential and HB-PAE4 possesses
the highest onset oxidation potential. This could be because
of the better electron-donating properties of carbazole (in
HB-PAE1) and fluorene (in HB-PAE2) moieties when com-
Figure 9 shows the voltage–luminance characteristics (inset)
of devices along with current density as a function of volt-
age. These data show that the turn-on voltages were 7.5,
10, 10, and 8.2 V for the HB-PAE1, HB-PAE2, HB-PAE4, and
L-PAE2 devices, respectively. The low turn-on voltage of
L-PAE2 device when compared with its HB analogs could be
because of the better conjugation of rigid polymeric struc-
ture. A brightness of 490, 349, and 316 cd/m2 could be
achieved at a bias of 13, 15, and 15 V for HB-PAE1, HB-
PAE2, and HB-PAE4 devices, respectively. Where as a bright-
ness of 300 cd/m2 could be achieved for L-PAE2 device at a
TABLE 3 Electroluminescent Properties of the Devices
Emitting
Layera
Turn-On
Voltage (V)b
Lmax
(cd/m2)c
LEmax
(cd/A)d
Emission
CIE Coordinates
(x, y)e
(kem, nm)
HB-PAE1
HB-PAE2
HB-PAE4
L-PAE2
a
7.5
10
490
349
316
300
0.14
0.08
0.11
0.075
c
506
(0.21, 0.46)
(0.35, 0.52)
(0.47, 0.50)
(0.41, 0.55)
535, 558
558
10
8.2
540, 570
Device configuration: ITO/PEDOT:PSS/emitting
polymer/LiF/Al.
Maximum luminance.
d
e
Maximum current efficiency.
Turn-on voltage at 2 cd/m2.
The 1993 CIE coordinates at 10 V.
b
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE 9 Voltage–current den-
sity and voltage–luminance
(inset) characteristics of the
studied devices. [Color figure
can be viewed in the online
issue, which is available at
wileyonlinelibrary.com.]
bias of 14 V. The poor device performance of L-PAE2 could
be because of the strong intermolecular interactions and ag-
gregate formation of the linear polymers thereby quenching
the fluorescence. Figure 10 shows the intensity of the emit-
ted light as a function of wavelength and applied voltage for
the HB-PAE1 device. As the driving voltage increases, the EL
peak position and the CIE coordinates of the device were not
found changed significantly. The EL emission stayed at 506
nm with the CIE color coordinates at (0.21, 0.46). As shown
in Figures 11 and 12, a maximum current efficiency of 0.14
cd/A and power efficiency of 0.041 lm/W for the device
ITO/PEDOT:PSS/HB-PAE1/LiF/Al was realized.
CONCLUSIONS
In summary, we successfully synthesized a series of PAEs
with HB architecture. These polymers with TPA as core were
found to be soluble in common organic solvents and show
excellent film-forming properties. The polymers were found
FIGURE 10 EL spectra of HB-PAE1 at different applied voltages.
[Color figure can be viewed in the online issue, which is avail-
able at wileyonlinelibrary.com.]
FIGURE 11 Current efficiency–current density characteristics of
the studied devices. [Color figure can be viewed in the online
issue, which is available at wileyonlinelibrary.com.]
840
WILEYONLINELIBRARY.COM/JOURNAL/JPOLA

ARTICLE
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