Bipolar π-conjugation interrupted host polymers by metal-free superacid-catalyzed polymerization for single-layer electrophosphorescent diodes

Article abstract of DOI:10.1039/c4ra06540a

Novel aromatic bipolar host polymers (P1 and P2) containing pyridine as an electron transporting unit and carbazole and fluorene as hole transporting units in the π-conjugation interrupted polymer backbone have been synthesized by a metal-free superacid-catalyzed polyhydroxyalkylation. The present polymers show good thermal stability, with high glass transition temperatures and decomposition temperatures. The conjugation lengths of the polymers are effectively confined into the repeating units due to the δ-C bond interrupted polymer backbone, giving rise to quite high triplet energy (2.79 eV for P1) and a wide band gap of around 3.33 eV, which make them promising hosts for phosphorescent OLEDs. The results suggest that the strategy of incorporating bipolar blocks into the π-conjugation interrupted polymer backbone can be a promising approach to obtain host polymers with high triplet level for green and even blue phosphorescent polymer light-emitting diodes on a simple device structure and using a solution-processed technique.

Full text of DOI:10.1039/c4ra06540a

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Bipolar p-conjugation interrupted host polymers
by metal-free superacid-catalyzed polymerization
for single-layer electrophosphorescent diodes†
Cite this: RSC Adv., 2014, 4, 50027
a
b
c
c
c
a
*
*
*
Bing Chen, Zhanhai Xiao, Yanhu Li, Lei Yu, Wei Yang and Jiwen Feng
Novel aromatic bipolar host polymers (P1 and P2) containing pyridine as an electron transporting unit and
carbazole and fluorene as hole transporting units in the p-conjugation interrupted polymer backbone have
been synthesized by a metal-free superacid-catalyzed polyhydroxyalkylation. The present polymers show
good thermal stability, with high glass transition temperatures and decomposition temperatures. The
conjugation lengths of the polymers are effectively confined into the repeating units due to the d-C
bond interrupted polymer backbone, giving rise to quite high triplet energy (2.79 eV for P1) and a wide
band gap of around 3.33 eV, which make them promising hosts for phosphorescent OLEDs. The results
suggest that the strategy of incorporating bipolar blocks into the p-conjugation interrupted polymer
backbone can be a promising approach to obtain host polymers with high triplet level for green and
even blue phosphorescent polymer light-emitting diodes on a simple device structure and using a
solution-processed technique.
Received 2nd July 2014
Accepted 17th September 2014
DOI: 10.1039/c4ra06540a
www.rsc.org/advances
phosphorescent blue emitters is poly(N-vinylcarbazole) (PVK),
which possesses a comparatively high ET level of 3.0 eV, which is
1 Introduction
nearly identical to that of the single carbazole unit, due to non-
conjugated polymer backbone.14 However, PVK-based devices
usually suffer from a high driving voltage, because of the
imbalanced carrier injection/transportation, since the hole
mobility of PVK is about three orders higher than that of the
electron.15–17
Blending an electron transported small molecule with PVK is
a good approach to balance the carrier transportation. For
example, the electrophilic small molecule PBD (2-tert-butyl-
phenyl-5-biphenyl-1,3,4-oxadiazole) is usually blended into PVK
to improve electron transportation.17–20 The added small mole-
cule has quite a lower glass transition temperature (Tg) than
PVK, possibly leading to phase separation under operation and
hence a decreased device performance. The covalent attach-
ment of the electron-transport small molecule into the host
polymer could be a good solution to suppress this phase
separation.
Organic light-emitting diodes (OLEDs) based on polymer
materials have attracted considerable attention in fabricating
exible and large area at-panel displays and lighting devices,
due to their low-cost spin-coating or use in ink-jet printing
technology.1,2 To effectively utilize the radiative recombination
of both singlet and triplet excitons, OLEDs usually dope a
phosphorescent emitter into a polymer matrix to form a host–
guest system and achieve nearly quantitative internal quantum
efficiency (IQE).3,4
Typical host polymers, such as polyuorenes, have a rela-
tively low triplet energy (ET) (<2.4 eV), due to the p-electron
delocalizations along the polymer backbone, and thus can only
host green and red phosphorescent emitters.5–9 For hosting blue
phosphorescent emitters, the object host polymer should
possess an appropriate ET (>2.65 eV), i.e., higher than that of the
phosphorescent emitter to prevent energy transfer back.10–13
One of the representative host polymers suitable for
Regarding the ET level and charge transport properties, non-
conjugated bipolar polymer hosts simultaneously incorporating
electron-rich and electron-decient moieties have been studied
recently.21,22 Yan et al. reported a polysiloxane-type host PCzMSi
containing a silicon–oxygen linkage in the backbone and
carbazole moieties in the side chain with a high ET of 3.0 eV.23
Ding et al. reported a bipolar poly(aryl ether) host containing
phosphine oxide in the main chain and carbazole in the side
chain with an ET of 2.96 eV.24 Ogino et al. reported a series of
random and block copolymers containing triphenylamine and
oxadiazole moieties as charge transportation layer via nitroxide-
aState Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics,
Wuhan Institute of Physics and Mathematics, Chinese Academy of Science, Wuhan
430071, People's Republic of China. E-mail: chenbing@wipm.ac.cn; jwfeng@wipm.
ac.cn
bSchool of Materials Science and Engineering, Wuhan University of Technology, Wuhan
430070, People's Republic of China
cInstitute of Polymer Optoelectronic Materials and Devices, South China University of
Technology, Key Laboratory of Special Functional Materials, Ministry of Education,
Guangzhou 510640, People's Republic of China. E-mail: pswyang@scut.edu.cn
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra06540a
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mediated radical polymerization.25 Scherf et al. reported a host tetrahydrofuran (THF) as the eluent. Cyclic voltammetry (CV)
copolymer containing conjugated uorenetetrauorobenzene data were measured on a CHI660A electrochemical workstation,
building blocks in a non-conjugated backbone with an ET of using tetrabutylammonium hexauorophosphate (Bu4NPF6, 0.1
2.40 eV in the solid state.26
M in acetonitrile) as electrolyte at a scan rate of 50 mV sꢁ1 at
We previously reported an aromatic host polymer PICzFB room temperature under the protection of argon. A platinum
containing carbazole–tetrauorinebeneze–carbazole repeating electrode coated with a thin polymer lm was used as the
units in the p-conjugation interrupted backbone with a working electrode, Pt wire as the counter electrode, and Ag wire
considerably high ET level of 2.73 eV, and successfully applied as the reference electrode. At the end of the measurement, the
this in green and even in blue PLEDs.27 However, the p-electron ferrocene/ferrocium potential was measured and used as the
delocalization in the repeating units of PICzFB was not inter- reference.
rupted and it could be extended along the elongated molecular
axis (carbazole–tetrauorinebeneze–carbazole) due to the para
linkages of tetrauorinebeneze with carbazole, which leads to a
lowering of the ET level.28 To further interrupt the p-electron
delocalization, in this work, we use meta linkage carbazole–
2.2 Fabrication of PLEDs
The devices with the structure of ITO/PEDOT:PSS/LEL/CsF/Al
(ITO: iridium tin oxide, PEDOT:PSS: poly(3,4-ethyl-
enedioxythiophene):poly(styrene sulfonic acid), LEL: light-
emitting layer) were fabricated following a well-established
process: PEDOT:PSS was spin-coated onto the cleaned ITO-
coated glass substrate from aqueous solution and then heated
at 120 ꢀC for 20 min to remove the residual water solvent.
Polymer doped with phosphorescent emitters was used as the
light-emitting layer, which was spin-coated in chlorobenzene to
form a uniform 80 nm thickness lm on top of the PEDOT:PSS
pyridine–carbazole
and
uorene–pyridine–uorene
as
repeating units, respectively, in a d-C connection polymer
backbone to prepare two new bipolar host polymers (P1 and P2)
via superacid-catalyzed metal-free polyhydroxyalkylation. Such
metal-free copolymerization is essentially quite favorable for
avoiding the exciton quenching resulting from the heavy metal
catalyst residue in traditional cross-coupling copolymerization
reactions.29 The presented polymer P1 shows quite a high triplet
energy of 2.79 eV, due to the further all meta linkages in the
repeating units (carbazole–pyridine–carbazole) allowing the
conjugation between the carbazole and pyridyl units to be
interrupted. Furthermore, the simultaneous introduction of
both electron-decient pyridine and electron-rich carbazole in
each repeating unit should exhibit bipolar carrier trans-
portation characteristics.
ꢀ
layer. All the lms were then annealed at 100 C on a hotplate
for 20 min. A Tencor Alpha-step 500 Surface Prolometer was
used to evaluate the thicknesses of the polymer lms. Finally,
1.5 nm of CsF, followed by 120 nm aluminum, were thermally
evaporated through a shadow mask at a base pressure of 3.0 ꢂ
10ꢁ4 Pa to form the cathode. The current density–luminance–
voltage (J–L–V) characteristics were measured in the nitrogen
dry-box using a keithley 236 source-measurement unit with a
calibrated silicon photodiode. The electroluminescence spectra
and CIE coordinates were recorded on a PR-705 SpectraScan
spectrophotometer (Photo Research).
2 Experimental part
2.1 Materials and measurements
Tetrakis(triphenylphosphine)palladium(0), triuoromethane-
sulfonic acid (TFSA), and N-methylisatin were purchased from
Sigma-Aldrich Chemical Industry. Toluene and dichloro-
2.3 Syntheses of monomers and polymers
9-Decyl-3-bromo-9H-carbazole (2). 40 mL dried THF in a 100
methane were distilled and dried overnight over 4A molecular mL three-neck ask was purged with nitrogen and stirred, then
sieves prior to use. Other commercially available reagents were 1.2 g (0.05 mol) NaH was added carefully. 9.88 g (0.04 mol) 3-
used without further purication.
bromocarbazole (1) was added slowly into the mixture. 11.05 g
1H NMR and 13C NMR spectra were recorded on a Bruker 300 (0.05 mol) 1-bromodecane diluted by 10 mL THF was added
MHz spectrometer in deuterated chloroform solution, with dropwise into the ask and reuxed for 12 h. The reactant was
tetramethylsilane as the internal reference. UV-visible absorp- cooled to room temperature and then the solvent was removed
tion spectra were recorded on a HP 8453 spectrophotometer. PL by evaporation. The residue was mixed with 60 mL H2O and
spectra were measured with a cooled charge coupled device extracted with methylene chloride. The extracted solution was
(CCD) coupled to a monochromator, using the 325 nm line of dried with anhydrous MgSO4 and then ltrated. Compound (2)
the He–Cd laser as the excitation. Elemental analyses were was isolated by silica gel column chromatography with petro-
performed on a Vario EL elemental analysis instrument (Ele- leum ether and ethyl acetic (20/1, v/v) as the eluent (yield 80%).
mentar Co.). Thermogravimetric analyses (TGA) were pꢁer1- 1H NMR (300 MHz, DMSO, d, ppm): 8.39 (s, 1H, ArH), 8.20 (d,
ꢀ
formed on a Netzsch TG 209 at a heating rate of 20 C min
.
1H, J ¼ 6.0 Hz, ArH), 7.56–7.62 (m, 3H, ArH), 7.48 (d, 1H, J ¼
DSC measurements were performed on a Netzsch DSC 204 7.20 Hz, ArH), 7.21 (t, 1H, J ¼ 7.92 Hz, ArH), 4.38 (t, 2H, J ¼ 7.02
under N2 ow, at a heating rate of 10 ꢀC minꢁ1. All the samples Hz), 1.72–1.78 (m, 2H), 1.17–1.25 (m, 14H), 0.81 (t, 3H, J ¼ 7.02
were typically subjected to a heating scan to about 250 ꢀC, a Hz). 13C NMR (75 MHz, CDCl3, d, ppm): 14.09, 22.64, 26.84,
cooling scan to room temperature (RT), and a re-heating scan. 27.31, 29.07, 29.24, 29.42, 29.49, 29.53, 31.83, 108.58, 109.11,
Liquid chromatograph-mass spectroscopy (LC-MS) was recor- 118.17, 118.78, 120.36, 122.87, 122.99, 125.61, 127.50, 133.71,
ded on a LCQ DECA XP spectrometer. The molecular weights of 139.79, 149.68; anal. calcd (%) for C22H28BrN: C, 68.39; H, 7.30;
the polymers were estimated by Waters GPC 2410, with N, 3.63. Found: C, 68.82; H, 7.41; N, 3.69; EIMS: m/z 386 (M + 1)+.
50028 | RSC Adv., 2014, 4, 50027–50034
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9-Decyl-3-(4,4,5,5-tetramethyl-1,3,2-dioxaborolane-2-yl)-9H-
RSC Advances
MHz, CDCl3, d, ppm): 8.20 (d, 4H, J ¼ 9.48 Hz, ArH), 7.78–7.91
(m, 7H, ArH), 7.36–7.41 (m, 6H, ArH), 2.06 (d, 8H, J ¼ 8.76 Hz),
1.08–1.40 (m, 48H), 0.73–0.87 (m, 12H). 13C NMR (75 MHz,
CDCl3, d, ppm): 14.00, 22.56, 23.83, 29.23, 30.10, 31.77, 40.45,
55.18, 118.38, 119.80, 119.96, 121.40, 122.93, 125.93, 126.78,
127.23, 137.35, 138.38, 140.73, 142.12, 151.22, 151.35, 151.17;
anal. calcd (%) for C63H85N: C, 88.36; H, 10.00; N, 1.64. Found:
C, 87.30; H, 11.10; N, 1.56; EIMS: m/z 857 (M + 1)+.
carbazole (3). Compound (2) was dissolved in dry tetrahydro-
ꢀ
furan (THF, 150 mL) and cooled to ꢁ78 C, and then 1.6 M n-
BuLi solution in hexane (1.2 equiv.) was slowly dropped into it
under an argon atmosphere. The resulting solution was allowed
to stir for 2 hours at ꢁ78 ꢀC and 2-isopropoxy-4,4,5,5-tetra-
methyl-1,3,2-dioxaborolane (3 equiv.) was then added. The
mixture was warmed to RT and stirred for 24 h, then it was
poured into water and extracted with ethyl ether. The organic
layer was dried by anhydrous magnesium sulfate. Silica gel
column chromatography was performed with petroleum ether
and ethyl acetic (15/1, v/v) as the eluent to isolate compound 3
as a colorless oil (yield 63%).1H NMR (300 MHz, CDCl3, d, ppm):
8.62 (s, 1H, ArH), 8.15 (d, 1H, J ¼ 7.74 Hz, ArH), 7.93 (d, 1H, J ¼
8.25 Hz, ArH), 7.40–7.48 (m, 3H, ArH), 7.24 (s, 1H, ArH), 4.32 (t,
2H, J ¼ 7.08 Hz), 1.85–1.90 (m, 2H), 1.25–1.42 (m, 26H), 0.87 (t,
3H, J ¼ 6.27 Hz).
Syntheses of polymers. The polymers were prepared
according to the literature method26 as follows: tri-
uoromethanesulfonic acid (0.5 mL) was added to a mixture of
dichloromethane (3 mL), N-methylisatin (0.5 mmol), and M1/
M2 (0.5 mmol). The reaction mixture was stirred for 12 h at
room temperature under an argon atmosphere, before being
poured slowly into methanol. The resulting mixture was
neutralized with 2 M NaOH aqueous solution and extracted with
chloroform. The organic layer was subsequently washed with
brine and water, and dried over anhydrous Na2SO4. The solu-
tion was concentrated by rotary evaporation, and poured slowly
into methanol. The white polymers were ltered off, and then
washed with hot methanol and acetone, and subsequently
washed using Soxhlet extraction with methanol and acetone to
remove the oligomers and catalyst residues, before being nally
dried under vacuum to give white polymer bers.
2,6-Bis(9-decylcarbazole-3-yl)pyridine (M1). 2,6-Dibromopyr-
idine (2.7 g, 6.23 mmol), 9-decyl-3-(4,4,5,5-tetramethyl-1,3,2-
dioxaborolane-2-yl)-9H-carbazole (3) (0.68 g, 2.83 mmol), 2 M
Na2CO3 (6 mL), toluene (20 mL), and ethanol (8 mL) were added
in to a 100 mL three-neck ask, and degassed for 10 minutes,
then tetrakis(triphenylphosphino)Pd(0) (0.23 g, 0.1 mmol) was
added to the mixture, stirred, and reuxed for 24 h under an
argon atmosphere. The reaction was quenched by adding 10 mL
water, and then extracted by chloroform (100 mL). The organic
layer was subsequently washed with brine and water, and dried
over anhydrous Na2SO4. The solvents were removed by rotary
evaporation, and the residue was puried by column chroma-
tography over silica gel using a mixture of petroleum ether and
ethyl acetic (10/1, v/v) to give a white solid 1.17 g (yield, 70%). 1H
NMR (300 MHz, CDCl3, d, ppm): 8.94 (s, 2H, ArH), 8.41 (d, 2H, J
¼ 7.08 Hz, ArH), 8.26 (d, 2H, J ¼ 7.77 Hz, ArH), 7.80–7.88 (m, 3H,
ArH), 7.44–7.57 (m, 6H, ArH), 7.30–7.32 (m, 2H, ArH), 4.38 (t,
4H, J ¼ 7.02 Hz), 1.89–1.96 (m, 4H), 1.27–1.43 (m, 28H), 0.89 (t,
6H, J ¼ 6.18 Hz). 13C NMR (75 MHz, CDCl3, d, ppm): 14.08,
22.64, 27.32, 29.01, 29.07, 29.25, 29.42, 29.50, 29.52, 31.84,
108.77, 108.86, 117.47, 119.02, 119.18, 120.60, 123.24, 123.34,
125.17, 125.72, 130.89, 137.30, 141.01, 141.17, 157.72; anal.
calcd (%) for C49H59N3: C, 85.34; H, 8.56; N, 6.09. Found: C,
85.30; H, 8.34; N, 6.08; EIMS: m/z 690 (M + 1)+.
Poly{[2,6-bis(9-decylcarbazole-3-yl)pyridine-6,60-diyl]-alt-[N-
1
methylisatin-2-one-3,3-diyl]} (P1). 260 mg, yield: 90%. H NMR
(300 MHz, CDCl3, d, ppm): 8.64 (s, 2H, ArH), 8.46 (d, 2H, J ¼ 9
Hz, ArH), 8.13 (s, 2H, ArH), 7.73 (s, 3H, ArH), 7.47–7.54 (m, 5H,
ArH), 7.34–7.38 (m, 3H, ArH), 7.17 (t, 1H, J ¼ 7.32 Hz, ArH), 7.02
(d, 1H J ¼ 7.62 Hz, ArH), 4.3 (s, 4H), 3.41 (s, 3H), 1.87 (s, 4H),
1.23–1.34 (m, 28H), 0.85 (t, 6H, J ¼ 6.06 Hz). 13C NMR (75 MHz,
CDCl3, d, ppm): 14.10, 22.64, 26.84, 27.31, 29.07, 29.24, 29.42,
29.49, 29.53, 31.83, 62.69, 108.91, 120.17, 123.04, 126.87,
134.34, 140.11, 141.47, 157.56, 178.86.
Poly{[2,6-bis(9,9-dioctyluoren-2-yl)pyridine-7,70-diyl]-alt-[N-
1
methylisatin-2-one-3,3-diyl]} (P2). 290 mg, yield: 89%. H NMR
(300 MHz, CDCl3, d, ppm): 8.20 (d, 2H, J ¼ 8.76 Hz, ArH), 8.12 (s,
2H, ArH), 7.74–7.87 (m, 6H, ArH), 7.66 (d, 2H, J ¼ 8.43 Hz, ArH),
7.28–7.39 (m, 5H, ArH), 7.11 (t, 1H, J ¼ 7.47 Hz, ArH), 6.99 (d,
1H, J ¼ 7.8 Hz, ArH), 3.39 (s, 3H), 1.87–1.98 (m, 8H), 1.06–1.45
(m, 48H), 0.72–0.85 (m, 12H). 13C NMR (75 MHz, CDCl3, d,
ppm): 14.10, 22.61, 22.62, 24.00, 26.76, 29.28, 29.35, 30.10,
31.83, 40.12, 55.24, 62.94, 118.41, 119.93, 121.32, 123.06,
126.05, 127.27, 128.23, 138.35, 139.83, 141.33, 141.69, 143.18,
151.41, 151.52, 157.14, 177.61.
2-Bromo-9,9-dioctyluorene (5). The synthesis of compound
5 was similar to that of compound 2, except that the 3-bromo-
1
carbazole (1) was replaced with 2-bromouorene (4). H NMR
(300 MHz, CDCl3, d, ppm): 7.66–7.70 (m, 1H, ArH), 7.58 (d, 1H, J
¼ 6.0 Hz, ArH), 7.47 (s, 1H, ArH), 7.45 (s, 1H, ArH), 7.33–7.36 (m,
3H, ArH), 1.98 (t, 4H, J ¼ 9.18 Hz), 1.06–1.30 (m, 20H), 0.88 (t,
6H, J ¼ 6.99 Hz), 0.59–0.63 (m, 4H).
3 Results and discussion
3.1 Synthesis of monomers and polymers
9,9-Dioctyluorene-2-(4,4,5,5-tetramethyl-1,3,2-dioxabor-
olane) (6). The synthesis of compound 6 was similar to that of
compound 3 except that the 9-decyl-3-bromo-9H-carbazole (2)
was replaced with 2-bromo-9,9-dioctyluorene (5).
The M1/M2 was synthesized by the Suzuki-type aryl–aryl cross-
coupling between the boronic ester (3 or 6) and the aryl halides.
The polymers were prepared by a metal-free, superacid-cata-
lyzed polyhydroxyalkylation (Scheme 1). N-Methylisatin was
used as the hydroxymethylation component for coupling with
the bifunctional aromatic monomers at the electron-rich posi-
tions (3,6 positions for carbazole (M1), 2,7 positions for uorene
(M2)), in the presence of TFSA, to give linear, high molecular
2,6-Bis(9,9-dioctyluorene-2-yl)pyridine (M2). The synthesis
of M2 was similar to that of M1, except that the 9-decylcarbazole
boronic ester (3) was replaced with 9,9-dioctyluoren boronic
1
ester (6) to give a white solid 0.5 g (yield, 70%). H NMR (300
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Scheme 1 Synthetic routes of the monomers and polymers.
weight polymers. The number average molecular weights (Mn) thermal stabilities, with comparatively high glass transition
ꢀ
ꢀ
of P1 and P2 were estimated by gel permeation chromatography temperatures (Tg) (185 C for P1, 110 C for P2) and decompo-
using tetrahydrofuran as the eluent (Table 1). The evidence of sition temperatures (Td corresponding to 5 wt% loss) (414 ꢀC for
the polymerization includes the emergence of a new signal with P1, 405 ꢀC for P2). The high Tg and Td are important for the
d ¼ 3.40 ppm in the 1H NMR spectrum of P1/P2, as can be practical PLED application of polymer materials, and could be
ascribed to the methyl group in the N-methylisatin unit, indi- expected to prevent device degradation during the long-term
cating the connection of N-methylisatin unit with the M1/M2 operation.
unit (Fig. S3–S6†).
3.3 Electrochemical properties
3.2 Thermal properties
The electrochemical properties of the polymers were investi-
The thermal properties were investigated by thermogravimetric gated by cyclic voltammetry (Fig. 2), and are summarized in
analyses (TGA) (Fig. 1) and differential scan calorimetry (DSC), Table 1. The onset of the oxidation potentials of P1 and P2 are
and the data are shown in Table 1. The polymers exhibit good 0.92 and 1.11 V versus ferrocene/ferrocenium (Fc/Fc+). The
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Table 1 Physical and electrochemical properties of the polymers
Polymer
Mn (ꢂ 103)
PDI
Tg (ꢀC)
HOMO (eV)/Eox
LUMOb (eV)
Eg (eV)
ET (eV)
a
Td (ꢀC)
c
c
P1
P2
49.9
76.5
1.65
1.86
414
405
185
110
ꢁ5.72 (0.92V)
ꢁ5.91 (1.11V)
ꢁ2.39
ꢁ2.57
3.33
3.34
2.79
2.43
a
b
c
Corresponding to 5 wt% loss. Estimated from the HOMO levels and the optical gaps. Calculated from the onset of the spectra.
3.4 Photophysical properties
The absorption spectra of P1 and P2 were measured in dilute
CH2Cl2 solution and in the lm state (Fig. 3). In solution, P1
shows absorption peaks at 252 nm and 299 nm, possibly cor-
responding to n–p* and p–p* transitions, respectively, with a
weak absorption at ca. 330 nm, corresponding to the intra-
molecular charge transfer (ICT) with regard to the interaction
between the electron-donor carbazole and the electron-with-
drawing pyridine. In the case of P2, it shows a p–p* transition
absorption band centered at 340 nm, and no obvious ICT
transition can be observed, due to the weaker electron-donor
nature of uorene. Compared to solution absorption, P1 and P2
lms show similar proles, but broader absorption bands,
ascribed to the molecular aggregation in the solid state. The
optical gaps of P1 and P2 are 3.33 eV and 3.34 eV, respectively
(Table 1), as estimated from the onset of the absorption in lm.
Fig. 1 TGA curves of P1 and P2.
To further study the effectiveness of the conjugation
breaking in the polymer backbone, both the photo-
luminescence (PL) spectra and the absorption spectra of M1/P1
and M2/P2 were measured in CH2Cl2 solutions (10ꢁ5 M) (Fig. 4
and S2†). The PL spectra of P1/P2 in solution show a slight red-
shi (only about 5 nm) compared with that of M1/M2, as well as
in their absorption spectra (Fig. S2† and inset of Fig. 3), indi-
cating that the conjugation lengths of the polymers are
successfully conned into the repeating units because of the
non-conjugated d-C bond connection in the polymer backbone.
However, the polymers in the lm show a broader and more red-
Fig. 2 Cyclic voltammograms of P1 and P2 in an acetonitrile solution.
reduction processes of the polymers cannot be observed, and
the highest occupied molecular orbital (HOMO) and the lowest
unoccupied molecular orbital (LUMO) are calculated according
to the equation: EHOMO ¼ ꢁe(Eox,onset + 4.80) eV, ELUMO ¼ Eg ꢁ
EHOMO, with regard to the energy level of ferrocene 4.8 eV below
the vacuum. The calculated HOMO/LUMO of P1 and P2 are
ꢁ5.72/ꢁ2.39 eV and ꢁ5.91/ꢁ2.57 eV, respectively. The higher
HOMO of P1 can be attributed to the electron-rich nature of the
carbazole units, which increases the electron density of the
Fig. 3 Absorption spectra of P1 and P2 in solution and in the film state
polymer.
(inset: absorption spectra of M1 and P1 in dilute solution).
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Fig. 6 PL spectra of P1 and P2 and absorption of Ir(mppy)3.
Fig. 4 PL spectra of the monomers and polymers in film and in a
diluted solution (10ꢁ5 M).
mentioning that the ET of P1 is also higher than for some
copolymers published in literature, such as poly(9,9-dialkyl-3,6-
dibenzosilole) (2.55 eV),31,32 and poly(9,9-dioctyluorene-2,7-
diyl-co-2,8-dihexyldibenzo-thiophene-S,S-dioxide-3,7-diyl) (2.46
eV),33 and also higher than that of the widely used green-emit-
ting phosphorescent dopant tris(2-(4-tolyl)phenylpyridine)
iridium (Ir(mppy)3) (2.38 eV),34 as well as the blue-emitting iri-
shied emission than in solution, probably due to the severe
molecular aggregation in the solid state.
The phosphorescence spectra of P1/P2 were measured at 77
K in lm, according to the literature method30 (Fig. 5). The
triplet energy levels (ET) of P1 and P2 are located at 2.79 eV and
2.43 eV respectively, relative to the onset of the phosphores-
cence spectra. The lower ET of P2 can be ascribed to the uorene
unit, which itself has lower triplet energy levels than the
carbazole unit. In addition, the para linkages of uorene with
pyridine in P2 extend the p-electron delocalizations along the
uorene–pyridine ring, allowing a lower ET. Compared to
PICzFB (ET ¼ 2.73 eV), which we published earlier,27 the slightly
higher ET of P1 (2.79 eV) could be ascribed to the all meta
linkages between the carbazole and pyridine units, which
possibly further interrupt the molecular conjugation along the
repeating units and increase the triplet energy levels. It is worth
dium(III)
(FIrpic) (2.65 eV).35–37
[bis(4,6-diuorophenyl)pyridinato-N,C20]-picolinate
When considering that a high ET is an important prerequi-
site for host materials, to effectively prevent energy back
transfer from the phosphorescent complex to the polymer host,
it is thus expected that the present high-ET copolymer will have
great potential for application as a host for both blue and green
phosphorescent emitters.
To evaluate the potential of the polymers as a host for green
phosphorescent PLEDs, the absorption of Ir(mppy)3 (tris(2-(4-
tolyl)phenylpyridine)iridium) and the emission of the polymers
were investigated to conrm the energy transfer between P1/P2
Fig. 5 Phosphorescence spectra of P1 and P2 in 77 K in the film state. Fig. 7 PL spectra of P1 and P2 films doped with Ir(mppy)3 (5 wt%).
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minor contribution at 380 nm from the polymer. This indi-
cates that an efficient energy transfer occurs from the poly-
mers to Ir(mppy)3 and that the back energy-transfers do not
occur to a substantial degree. These results suggest that the
polymers can be used as a host for green phosphorescent
PLEDs.
3.5 Electroluminescence properties
Single-layer PLEDs based on P1/P2 as the host and Ir(mppy)3 as
the guest with the devices conguration of ITO/PEDOT:PSS/
LEL/CsF/Al were fabricated (LEL denotes P1/P2 lms doped
with 5 wt% Ir(mppy)3). In contrast with the PL spectra, the EL
(electroluminescence) spectra of both P1 and P2 exhibit only a
green emission peaked at 515 nm derived from Ir(mppy)3 and
no emission can be observed from the polymers (Fig. 8), sug-
gesting that both energy transfer and direct charge trapping/
recombination at Ir(mppy)3 were responsible for the EL
mechanism.
Fig. 8 EL spectra of the device ITO/PEDOT/polymer: Ir(mppy)3 (5
wt%)/CsF/Al.
Luminance–current density–voltage characteristics are
shown in Fig. 9 and the results summarized in Table 2. The P1-
based device shows best EL properties with maximum external
quantum efficiency of 1.26%, luminance efficiency of 4.4 cd Aꢁ1
and brightness of 4493 cd mꢁ2 in such a preliminary device
conguration, compared to the P2-based device and our previ-
ously reported PICzFB-based device. The higher efficiency for
P1-device could be ascribed to higher triplet energy as well as
the balanced charge transportation/injection due to the excel-
lent hole-transport carbazole and electron-transport pyridine
units incorporated in the polymer backbone. According to the
energy levels, P1 has a higher HOMO (ꢁ5.72 eV) than PICzFB
(ꢁ5.82 eV) and P2 (ꢁ5.91 eV). The high HOMO facilitates the
holes injection from PEDOT to the LEL and leads to lower turn
on voltage (Table 2).
Although the efficiencies of the P1-based devices are lower
than those of the multilayer devices reported in previous
literature,38,39 the results here were obtained in a very simple
device structure; i.e., a single-layer device without blending
additional electron/hole-transporting materials into the host,
which would be advantageous in future markets. The pre-
sented results suggest that P1 actually has high triplet energy,
as well as balanced carrier transportation, and could be a
promising host polymer for green-emitting PLEDs. The rele-
vant work into the P1-host for blue-emitting PLEDs is also in
progress.
Fig. 9 Luminance–current density–voltage properties of the devices
structure of ITO/PEDOT/polymer: Ir(mppy)3 (5 wt%)/CsF/Al.
and Ir(mppy)3 (Fig. 6). As seen from Fig. 6, Ir(mppy)3 exhibits a
strong absorption band centered at 370 nm, which overlaps well
with the emission of polymers P1 and P2, implying a possibly
effective energy transfer from the polymers to Ir(mppy)3.
The PL spectra of P1 and P2 doped with Ir(mppy)3 were
investigated (Fig. 7). The blended lms exhibit a strong
emission at 516 nm arising from Ir(mppy)3, together with a
Table 2 Device performance with P1/P2 as the host and Ir(mppy)3 as the guest (device structure: ITO/PEDOT/polymer:Ir(mppy)3 (5 wt%)/CsF/Al)
Brightness ¼ 100 cd mꢁ2
Polymera
Von (V)
Lmax (cd mꢁ2
)
LEmax (cd Aꢁ1
)
EQEmax (%)
V (V)
J (mA cmꢁ2
)
LE (cd Aꢁ1
)
EQE (%)
CIEc
b
P1
P2
PICzFB
10
11
15
4493
552
1242
4.4
0.98
4.2
1.26
0.39
1.18
13
18
22
2.6
12
3.2
4.16
0.81
3.7
1.2
0.32
1.1
(0.31, 0.61)
(0.34, 0.59)
(0.32, 0.61)
a
c
Doped with 5 wt% Ir(mppy)3. b Corresponding to a luminance of 1 cd mꢁ2
. .
Measured at 12 mA mꢁ2
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4 Conclusions
In summary, two novel bipolar host polymers based on carba-
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zole–pyridine–carbazole
and
uorene–pyridine–uorene
repeating units have been prepared, respectively, by metal-free
superacid-catalyzed polyhydroxyalkylation. The polymers show
goodꢀthermal stability, with high glass transition temperatures
ꢀ
(185 C for P1, and 110 C for P2), a wide band gap about 3.33
eV, and considerably high triplet energy (2.79 eV for P1). Effi-
cient single-layer green phosphorescent PLEDs have been real-
ized using P1 as the host and Ir(mppy)3 as the guest without
blending any electron/hole-transporting materials. The supe-
rior results can be ascribed to the all meta linkages in the
repeating units, as well as to the conjugation-conned main
chain due to the d-C bond interrupted polymer backbone.
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Acknowledgements
The authors are grateful for nancial support from the National
Natural Science Foundation of China (Grant nos 21303256,
21221064 and 10974223).
´
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