Carbazole based polymers as hosts for blue iridium emitters: Synthesis, photophysics and high efficiency PLEDs

Article abstract of DOI:10.1039/c3tc31441c

This article reports the synthesis of new carbazole based polymers and their application as hosts in sky-blue polymer light emitting devices (PLEDs) with a solution-processed emitting layer doped with a cyclometalated Ir(iii) complex. We systematically investigate their effect on the PLED performance. A current efficiency of 19.7 cd A^-1 and a brightness of 1850 cd m ^-2 were achieved with these polymers. The roll-off in electrophosphorescent quantum efficiency in PLEDs was shown to arise mainly from triplet-triplet annihilation between dopants in the hosts with tert-butyl groups. It has been shown that in the devices with hosts without tert-butyl groups the efficiency roll-off is additionally affected by electric field quenching. In these carbazole based polymers, triplet dimers are formed and tert-butyl groups do not limit the intermolecular interactions to prevent triplet dimer formation, nevertheless tert-butyl groups reduce charge transport.

Full text of DOI:10.1039/c3tc31441c

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Journal of Materials Chemistry C
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Journal Name
Cite this: DOI: 10.1039/c0xx00000x
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ARTICLE TYPE
Carbazole based polymers as hosts for blue iridium emitters: synthesis,
photophysics and high efficiency PLEDs
a
a
*b
c
Egle Stanislovaityte , Jurate Simokaitiene , Steponas Raisys , Hameed Al-Attar , Juozas
a
c
*c
V. Grazulevicius , Andy Monkman and Vygintas Jankus
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
This article reports the synthesis of new carbazole based polymers and their application as hosts in sky-
blue polymer light emitting devices (PLED) with solution-processed emitting layer doped with
cyclometalated Ir(III) Complex. We systematically investigate its effect on PLED performance. Current
2
1
0
efficiency of 19.7 cd/A and brightness of 1850 cd/m were achieved with these polymers. The roll off in
electrophosphorescent quantum efficiency in PLEDs was shown to arise mainly from triplet-triplet
annihilation between dopants in the hosts with tert-butyl groups. It has been showed that in the devices
with hosts without tert-butyl groups the efficiency roll-off is additionally affected by electric field
quenching. In these carbazole based polymers, triplet dimers are formed and tert-butyl groups do not limit
the intermolecular interactions to prevent triplet dimer formation, nevertheless tert-butyl groups reduce
charge transport.
1
5
Introduction
carbazole based hosts and its effects on PLED performance
hoping that this will be instructive for the future molecular design
of PLED hosts.
Polymer light emitting devices (PLED) with a soluble conjugated
polymer were first reported by Burroughes et al .
50
1
The
2
2
3
3
4
4
0
5
0
5
0
5
performance of PLEDs has increased enormously since then
especially with the introduction of phosphorescent heavy metal
Results and discussions
2
-3
complexes as emitting species. However, much work still needs
to be done to enhance the efficiency of phosphorescent PLEDs
for blue light production which is essential for lighting
Theory
In this article we explore various carbazole based polymer hosts
and their charge transport properties, triplet-triplet annihilation
and triplet-polaron annihilation. For this, we use the following
4
applications. A major issue for the improvement of the
5
5
efficiency of blue PLEDs is how to find suitable host polymers
for deep blue phosphorescent dopants, as these require high
triplet energy to confine triplet excitons on the phosphorescent
dopant. Poly(vinylcarbazole) (PVK) has been extensively used as
17,18
models previously derived for small molecule OLED studies.
Triplet-triplet annihilation (TTA) in PLED devices
5
-8
a hole transporting host for red or green emitting devices
60
Triplet-triplet annihilation (TTA) on the dopant follows a
9
1
7
reaching up to ~ 40 cd/A current efficiency and many people are
reaction:
1
0-15
turning to PVK as a host for blue phosphorescent dopants.
However PVK forms lower energy triplet dimers which can
3
*
3
*
kTT
3
*
M  M  M  M
,
(1)
1
6
quench blue dopants in the PLED. On the other hand, reducing
the dimer formation requires carbazole moiety isolation possibly
resulting in hole transport and charge imbalance. Clearly, a trade-
off needs to be made between dimer quenching and balanced
charge transport, therefore, new carbazole based polymer hosts
with high triplet energy for blue dopants were investigated in
PLEDs. The host polymer structure is varied by introducing tert-
butyl ligands in order to explore how it affects dimer formation
and charge transport properties. Furthermore, we investigate how
modification of carbazole based hosts affects triplet-triplet
3
*
here M is an excited triplet on a dopant molecule, M - is a
dopant molecule in the ground state, and kTT is the rate governing
the TTA reaction. We denote the concentration of triplets as
65
3
*
[
M ] which in devices will be dependent on the rate of triplet
generation (proportional to current density J) and the rates of
1
7
monoexcitonic (k ) and biexcitonic (k ) recombination:
m
TT
3
*
d[ M ]
1
J
3
*
3
*
 k [ M ] kTT [ M ]
m
dt
2
qd
,
(2)
1
7
annihilation between the dopant iridium complexes, whether
1
8
70 where q is electron charge, d– thickness of exciton formation
zone. From a steady state solution of this equation the quantum
efficiency of the device – EQE, can be derived :
triplet-polaron annihilation is relevant in carbazole based PLED
and if field induced quenching has an impact in these devices.
Thus, we systematically investigate the role of tert-butyl in
1
7
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AlCl3, t-BuCl
CH Cl2
2
KOH, Na SO4
2
EMK
EPKK: X =
OKK: X =
VKK: X =
*
*
*
NH SCN, C Cl N
3 3
4
3
THF
EPKK
TKK
Scheme 1. Synthesis of 3,6-di-tert-butyl-carbazole based
monomers.
EQE
J 
 1 J 1

0



EQE0
J
J0


(
3)
4)
5
with
30
Scheme 2. Cationic polymerization of the monomers.
4
qd
(
J0 
.
2
kTT /(km )
18
To test TTPA model the following equation was used:
EQE is the quantum efficiency in the absence of TTA taken to






0
EQE
qd
  k

kTT
TT
(6)


,
be the maximum value of the efficiency curve. J is the onset
2
0
EQE0 J /km
k
TT
current density at EQE=EQE /2. It can be determined
0
1
1
2
0
5
0
experimentally and also calculated by plugging in kTT found by
where
fitting equation (3) to EQE vs current density curve. J evaluation
0
1
/(l1)
will provide the test of validity of our fitting. km is evaluated
using photoluminescence (PL) transient measurements.
  k  k CJ
m
p
3
5
(7)
and
Triplet-polaron (TPA) and Triplet-triplet-polaron (TTPA)
annihilation in PLED devices
2
J
(8)


.
qd
For triplet-polaron annihilation and combined triplet-triplet and
triplet-polaron annihilation in PLEDs models outlined by Reineke
Synthesis
,6-Di-tert-butyl-9H-carbazole-based monomers with oxiranyl,
1
8
et al were used. Here the final equation of their TPA model is
3
given:
40
oxetanyl or vinyloxyethyl functional groups were synthesized by
two-step synthetic routes (Scheme 1). In the first step carbazole
was treated with 2-chloro-2-methylpropane in the presence of
aluminium chloride to yield 3,6-di-tert-butyl-9H-carbazole. Then
followed the reaction of 3,6-di-tert-butyl-9H-carbazole with an
excess of epichlorohydrin or 3-(bromomethyl)-3-methyloxetane,
or (2-chloroethyl)-vinylether under the basic conditions to yield
EPKK, OKK and VKK respectively. TKK was obtained from
EPKK under the action of ammonium thiocyanate and cyanuric
chloride (Scheme 1).
EQE
1

,
1
EQE0
(l1)
(1 k / k CJ
)
P
m
(5)
where k is the TPA rate, parameter C describes macroscopic
p
4
5
properties of the system that can only be estimated (mobility,
dielectric constant). Exciton recombination zone was considered
to be 50 nm – thickness of doped zone. C was considered to be
2
5
1
9
1/2
−6
2
1
0
cm/A , taking typical values of ε=3.5, μ=1·10 cm /Vs and
l=E /kT=1 (E depth of trap states) for organic semiconductors in
t
t
1
8
50
Polymers containing pendant carbazole and tert-butyl
carbazole moieties were prepared by cationic polymerization
line with Reineke et al evaluations.
2
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Journal of Materials Chemistry C
Table 1. Molecular weights, polydispersity indexes, number average
degree of polymerization of polymers and thermal characteristics of
monomers and polymers
1
50
00
PEPK
1
Oligomer PEPK PTPK PEPKK PTKK PVKK POKK
50
Mn
6910
2860
1230
1190
3320
8760
M
d
7460
1.1
12810
4.5
1370
1.1
3
1440
1.2
3
5070
1.5
9
21000
2.4
0
w
a
-
50
DP
b
3
1
12
24
c
o
T
g
, C
106
424
EPK
112
67
109
323
TPK
104
60
147
405
EPKK
158
116
32
149
330
TKK
137
-
146
392
VKK
101
-
186
444
OKK
176
-
-
100
d
T
ID
Monomer
0
.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
e
m
T , °C
e
+
E vs Ag/Ag , V
T
cr , °C
45
e
T
g
, °C
-
-
37
283
9
286
49
297
10
8
d
T
ID , °C
246
234
291
PEPKK
a
̅
̅̅̅ ̅̅̅̅
Polydispersity index determined by GPC; . ꢀꢁ ꢃꢁ .
ꢂ
ꢄ
6
b
̅
̅̅̅
5
Number average degree of polymerization determined by GPC; ꢅꢆꢀ
̅
̅̅̅
4
ꢁ
ꢃꢁ , where M
0
- the molecular weight of the monomer unit.
ꢄ
ꢇ
c
o
Determined by DSC from second heating scan: scan rate 20 C/min; N
2
2
atmosphere.
0
d
o
5% weight loss determined by TGA, heating rate 20 C/min; N
2
1
0
atmosphere.
-2
e
o
Determined by DSC: scan rate 20 C/min; N
2
atmosphere.
-
4
from the appropriate monomers using boron trifluoride diethyl
etherate as initiator (Scheme 2). Low-molecular-weight fractions
of the obtained products were removed by Soxhlet extraction
with methanol.
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
+
E vs Ag/Ag , V
Figure 1. Cyclic voltammograms of PEPK and PEPKK measured at scan
rate of 50 mVs vs Ag/Ag in a solution of TBAP (0.1 M) in CH C .
2 l2
-
1
+
1
2
2
3
3
4
5
0
5
0
5
0
The chemical structures of the synthesized compounds were
o
o
1
from 106 C to 186 C. Glass transition temperatures of the
polymers containing 3,6-di-tert-butyl substituted carbazolyl
groups are higher than those of the polymers containing
unsubstituted carbazolyl groups.
confirmed by IR and H NMR spectroscopies. The monomers
1
3
5
5
6
6
7
7
0
5
0
5
0
5
were also characterized by C NMR and mass spectrometries.
The molecular weights of the polymerization products
estimated by gel permeation chromatography (GPC) are given in
ꢈ
The thermal stability of the synthesized materials was
estimated by thermogravimetric analysis (TGA). The
temperatures of the onsets of the thermal degradation (T ) of the
monomers range from 246 C to 297 C while for the polymers
these values are considerably higher and range from 323 to
table 1. The number average molecular weights (ꢁ ) range from
ꢄ
1
190 to 8760 while the weight average molecular weights are in
the range of 1370-21000. PEPKK and PTKK polymers
containing tert-butyl-disubstituted carbazole moieties have
considerably lower molecular weights than the corresponding
polymers with un-substituted carbazole units and have to be
named as oligomers. The highest molecular weight is observed
for the oxetane polymer POKK.
The behaviour under heating of the synthesized monomers and
polymers was examined by differential scanning calorimetry
DSC). The results obtained are summarized in table 1. All
ID
o
o
o
4
44 C. The lower thermal stability of the monomers relative to
that of the corresponding polymers can apparently be explained
by the presence of the reactive functional groups and by the lower
intermolecular interaction. The monomers containing 3,6-di-tert-
butyl-9H-carbazole moieties exhibit higher 5 % weight loss
temperatures than the monomers having unsubstituted carbazolyl
groups, apparently, due to the enhanced intermolecular
interaction.
To elucidate the energetic conditions for energy and electron
transfer in dilute solutions, the ionization potentials and electron
affinities were established by cyclic voltamperometry.
Voltammograms of PEPK and PTPK showed non-reversible
oxidation couple while polymers containing 3,6-di-tert-butyl
carbazolyl groups showed quasi-reversible oxidation, due to the
blocked electrochemically active C-3 and C-6 positions of
carbazole ring. Cyclic voltammograms of PEPK and PEPKK are
shown in figure 1. The electrochemical data are summarized in
table 2. Ionization potentials established by cyclic voltammetry
(
synthesized monomers were isolated after the synthesis as the
crystalline materials. They showed endothermic melting signals
in the first heating scans in the range of 101–176 ºC. EPK and
TPK could not be transformed into the glassy state by cooling
from the melt. Introduction of tert-butyl groups into carbazole
moieties decreased inclination to crystallization of the monomers.
EPKK, TKK, VKK, OKK formed glasses upon cooling from
the melts. In the second heating scans they showed glass
transitions in the range from 9 ºC to 49 ºC. The supercooled
liquid of EPKK was not morphologically stable and
o
recrystallized at 116 C during the second DSC heating scan.
DSC analysis confirmed that all the polymers were obtained as
amorphous substances. Their glass transition temperatures ranged
(
Ip(CV)) of PEPK and PTPK are comparable (5.55 eV and 5.50 eV,
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Table 2. Electrochemical and photoelectrical parameters of carbazole and
1
0
0
0
.0
.8
.6
.4
3
,6-di-tert-butyl carbazole based polymers
(
a)
PTPK
PEPK
PEPKK
PTKK
POKK
PVKK
ꢊꢎ
ꢉ
ꢊꢐꢍ
b
c
c
d
ꢊꢄꢋꢌꢍ
a
ꢉꢏ
,
I
p(CV)
,
EA(CV)
eV
,
p(EP)
I ,
eV
vs Fc,
V
eV
eV
PEPK
PTPK
0.86
0.91
0.83
0.84
0.82
0.85
3.51
3.51
3.44
3.46
3.43
3.48
5.50
5.55
5.48
5.48
5.46
5.49
-1.99
-2.04
-2.04
-2.02
-2.03
-2.01
5.95
5.91
5.91
5.90
5.88
5.89
PEPKK
PTKK
PVKK
POKK
0.2
0.0
a
The onset oxidation potential versus ferrocene (ꢉꢊꢎ vs Fc) is
ꢊꢄꢋꢌꢍ
measured by cyclic voltamperometry from the first redox cycle.
260
280
300
320
340
360
b
ꢊꢐꢍ
5
The optical band gap ꢉꢏ estimated from the edge of electronic
Wavelength, nm
absorption spectrum.
c
Ionization potential Ip (CV) and electron affinity EA(CV) were estimated by
1
.0
.8
.6
.4
ꢊꢎ
PTPK (b)
PEPK
PEPKK
PTKK
POKK
PVKK
cyclic voltamperometry data according to Ip(CV)=4.8 + ꢉ
EA(CV) = –(Ip(CV) –ꢉꢏ ).
vs Fc and
ꢊꢄꢋꢌꢍ
ꢊꢐꢍ
0
0
0
d
1
0
Ionization potential Ip(EP) was measured by electron photoemission in air
method from films.
respectively). Ip(CV) of polymers containing 3,6-di-tert-butyl
substituted carbazole moieties are a little lower (5.49–5.46 eV).
The electron affinity (EA(CV)) values of the polymers are close and
range from –2.04 eV to –1.99 eV. The ionization potentials
1
5
(
Ip(EP)) were also measured by electron photoemission in air
0.2
method from solid state samples of the polymers and their values
are given in table 2. The Ip(EP) values of all the polymers were
found to be comparable (5.88–5.95 eV).
0.0
350
400
450
500
550
600
Wavelength, nm
2
0
Photophysics
Luminescence and absorption spectra of polymer films are
depicted in figure 2a and 2b (peaks are given in table 3). Shape of
absorption spectra is almost identical for all polymers with first
peak varying from 346 nm to 351 nm. These slight absorption
shifts might reflect different steric environments in polymer films
since it is not expected that bulky substitutes would introduce
substantial change to a chromophore. This is confirmed by almost
equienergetic HOMO-LUMO levels (within error limits).
Luminescence spectra also have similarities, the highest peak
varying from 370 nm to 382 nm. Luminescence spectra of
polymers are similar to the spectrum of poly(N-vinylcarbazole)
1.0
Complex 1
(
c)
0.8
0.6
0.4
2
3
3
4
4
5
0
5
0
5
in PEPK PL
in Zeonex PL
in PEPK EL
0.2
0.0
1
9
(PVK) reported by Rippen et al. It is widely accepted that
singlet excimers and dimers are formed in carbazole based small
molecules and polymers, so PL spectra of polymers are most
450
500
550
600
650
Wavelength, nm
5
5
6
0
5
0
2
0-23
likely to arise from intramolecular excimers.
These slight
Figure 2. Absorption (a) and PL emission (b) from neat polymer host
films at room temperature. PL and EL from PEPK doped with Complex 1
and PL emission from Zeonex doped with Complex 1 (c), inset shows
chemical structure of Complex 1.
differences could be explained by different overlaps of carbazole
rings in films due to different moieties and differently constrained
steric environment. This can readily be appreciated by comparing
excimer formation studies performed by Burkhart et al on
hosts should not be rejected. It was previously shown that in
carbazole based PVK and Poly(N-Ethyl-2-Vinylcarbazole)
P2VK) lower energy dimers are formed and they could quench
excitons residing on blue phosphorescent dopants. This is also
observed in carbazole based hosts under investigation. Delayed
emission spectra (70 ms after excitation) from spincoated films at
different temperatures have been recorded (Fig. 3). At 15 K
delayed emission spectra from polymer films is similar to the
genuine phosphorescence spectrum from PVK or P2VK films.
2
4
Poly[N-((vinyloxy)carbonyl) carbazole] and by Rippen et al on
1
9
Poly(N-vinyl carbazole).
(
PL emission changes substantially when iridium complex
Complex 1) is doped into the polymers (Fig. 2c). This emission
16
(
is exactly the same as Complex 1 emission from zeonex (Fig. 2c)
and electroluminescence (EL) from PLED device. Clearly, most
of the emission is coming from phosphorescent dopant in the
PLED, but the possibility that excitons are quenched by polymer
4
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Journal of Materials Chemistry C
Table 3. Absorption and emission characteristics of polymer neat films.
1
0
0
.0
.8
.6
(a)
15 K
Absorption
peak, nm
Fluorescence
peak, nm
Delayed
Triplet
level,
eV
emisison peaks
recorded 70 ms
after excitation
at 15 K and at
1
PTPK
PEPK
PEPKK
POKK
PVKK
8
5 K, nm
PEPKK
349, 335,
377
430, 459,
485 (15 K);
2.88
2
98, 264
0.4
4
89 (85 K);
PEPK
PTPK
346, 332,
97, 263
346, 332,
97, 264
370, 381,
406, 432
356, 370,
404, 431
428, 488 (15 K);
530 (85 K);
425, 454,
2.90
2.92
0
.2
2
2
479 (15 K);
0.0
400
5
02 (85 K);
450
500
550
600
650
700
2
PTKK
347, 335,
98, 265
374
445, 470,
504 (15 K);
2.93
Wavelength, nm
2
4
4
23, 450,
74 (15 K in
1
.0
.8
(b)
85 K
zeonex);
45, 473, 509
85 K);
26 (150 K);
PTPK
PEPK
PEPKK
POKK
PVKK
4
(
0
5
PVKK
POKK
351, 337,
99, 266
382
431, 463,
488 (15 K);
4
427, 457,
478 (15 K);
4
5
2.88
2.90
0.6
2
85 (85 K);
0.4
0.2
0.0
348 335
98 266
362, 380
2
49, 476,
04 (85 K).
1
2
Determined taking the highest vibronic peak at 15 K.
400
450
500
550
600
650
700
We could not observe genuine triplet emission in film at 15 K.
Wavelength, nm
35
5
0
5
0
5
0
In analogy with PVK or P2VK we propose that at 15 K the
dominant component in host spectra is polymer phosphorescence
with a small contribution from triplet dimer emission formed
PTKK 15 K in zeonex
PTKK 15 K
PTKK 85 K
1
0
0
0
0
0
.0
.8
.6
.4
.2
.0
(
c)
PTKK 150 K
1
6
between carbazole moieties. After increase of temperature to
5 K, spectra in all films shift to the red. Stronger triplet dimer
component at 85 K indicates the population of lower energy
8
1
1
2
2
3
2
5,26
dimer sites due to increased triplet exciton hopping.
Only in
PTKK delayed emission spectrum recorded at 15 K is dominated
by triplet dimer emission, indicating very efficient triplet dimer
formation. Phosphorescence peaking at ~430 nm in PTKK can be
observed only when it is doped into zeonex (Figure 3c). Increase
of the temperature to 150 K leads to even more redshifted
spectrum observed between 500 nm and 550 nm in the PTKK.
This spectrum is very similar to triplet dimer spectrum recorded
4
00
450
500
Wavelength, nm
Figure 3. Delayed emission spectra of neat polymer films at 15 K (a) and
5 K (b). Delayed emission spectra for PTKK is shown in isolated zeonex
550
600
650
700
1
6
previously from PVK films. It has to be noted, that in PEPK
stronger emission from dimers at 15 K than in other polymers is
observed (at ~488 nm), however, unlike in PTKK a signature of
genuine phosphorescence emission peaks at 428 nm. We
observed that depending on preparation condition (solvent,
temperature of substrate and etc.) the ratio of genuine
phosphorescence and dimer emission changes slightly (not shown
here). Two types of delayed emission spectra (excluding
phosphorescence) are observed in these hosts - less redshifted, for
example, in POKK (peaks at 449 nm and 476 nm at 85 K) or
PTKK (445 nm and 473 nm at 85 K) and more redshifted for
other hosts including PTKK at lower temperatures peaking at ca.
8
matrix and at different temperatures in neat films (c).
4
4
5
0
5
0
might arise from a dimer with smaller overlap (only one phenyl
ring overlaps with another molecule’s phenyl ring) and more
redshifted from a dimer with greater overlap (both phenyl rings
2
7
overlap with two phenyl rings of another molecule).
2
2
Benten et al. also have performed calculation on bridged
carbazole pairs and suggested that the large red shift of the
monomer to dimer emission cannot be attributed only to excitonic
delocalization and charge resonance effects must contribute, with
large orbital overlap between the pairs. Thus, these triplet dimer
states will have significant charge transfer character and so be
subject to solvatochromic effects. This will lead to strong
broadening environmental effects in films resulting in broader
2
7
5
00 nm and 525 nm. Sainte Claire suggested that two types of
stacking is possible between carbazole rings in PVK. We thus
propose that in these polymers less redshifted emission
2
8
and less resolved dimer emission as expected classically.
This journal is © The Royal Society of Chemistry [year]
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5

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Journal of Materials Chemistry C
Page 6 of 12
Table 4. Photoluminescence decay lifetimes and rates of Complex 1
doped in polymer hosts used to fabricate PLEDs and in inert zeonex
matrix at 20% loading.
1.0 (a)
PTPK
PEPKK
PTKK
POKK
PVKK
-
1
2
0% Complex 1
Decay rate k
m
, ns
P
Lifetime τ , ns
-
1
doped in a host
error 0.00002 ns
ZEONEX
PEPKK
PEPK
0.00129
0.00125
0.00145
0.00139
0.00181
0.00143
0.00132
775
800
690
719
552
699
758
0.5
PTPK
PTKK
PVKK
POKK
5
0
5
We recorded PL transient luminescence of Complex 1 doped at
0.0
4
50
500
550
600
2
0% loading in each host and in an inert zeonex matrix exciting
2
9
Wavelength, nm
at 450 nm into the Complex 1 absorption band.
Fitted
monoexponential lifetimes and decay rates are given in Table 4.
The longest emission lifetime of Complex 1 has been observed in
PEPKK and zeonex (800 ns and 750 ns), whereas the shortest
was found in PTKK (552 ns). The shortest lifetime of PTKK also
correlates with the fact that a PLED with PTKK host displayed
the smallest efficiency (see below).
There might be several reasons why the Complex 1 lifetime is
different from host to host and we can gain insight into this
matter by expressing the recorded decay in the following way:
(
b)
30
20
10
0
PVKK
1
K
K
K
TP
P
PEPKK
O
P
1
PEPK
TKK
P
1
/  k  k  k  k
P r nr q d
(9)
where k is the radiative lifetime, k is non-radiative constant, k -
r
nr
q
concentration quenching constant, k - quenching due to host
d
5
6
7
8
9
10
11
12
2
2
3
3
4
4
0
5
0
5
0
5
dimers (i.e. triplet transfer from dopant to the host triplet dimer
which are weakly emissive).
The observed different lifetimes for the same dopant in
different hosts indicate the change of rates in eq. 9. Although it is
Voltage, V
5
0
1
6
2
0
PEPK
(c)
18
usually accepted that k does not change for the same molecule
16
14
r
and is independent on environment, however, it has been shown
3
0
by Yersin et al. that in FIrpic (a prototype molecule of our
1
2
0
8
6
4
2
0
dopant) k might be affected by the matrix cage and the host
r
PO
KK
1
might change the radiative rate of the complex due to the high
sensitivity of the MLCT states to matrix effects. This has also
PVKK
PEPKK
3
1
been observed in another iridum complex Ir(btp)(2)(acac).
According to Yersin et al. the host cage can alter the complex
PTPK
30
geometry leading to different zero field splitting of heavy metal
PTKK
15
3
0,31
complex triplet level and changed spin orbit coupling.
This
can result in a decrease or increase of radiative decay rate of the
complex. It is also accepted that knr can depend on the matrix
0
5
10
20
25
2
35
Current density, mA/cm
cage. k - the quenching rate of the blue complex by dimer traps,
d
Figure 4. PLED device (Glass / ITO / PEDOT 57 nm / host: 20%
Complex 1 50 nm / TPBi 33 nm / LiF 0.7 nm / Al 100 nm.) characteristics
with newly synthesized hosts: electroluminescence spectra (a), current
density - voltage characteristics (b) and efficiency cd/A - current density
characteristics (c).
shown to be present in these hosts will be dependent on a
particular host. Only kq can be considered constant in this
equation as the concentration of the dopant is the same. Hence it
is difficult to interpret lifetime changes with a change of host, as
three rates in eq. 9 can be responsible for an overall change of
lifetime. However, we propose that in the PTKK case, where
triplet dimer emission dominates over phosphorescence, even at
55
(2,1,3-b-(2,1,3-benzothiadiazole)] crystallizes less than
poly[(4,40-bis(2-ethylhexyl)dithieno[3,2-b:20,30-d]silole)-2,6-
diyl-alt-(4,7-bis(2-thienyl)-2,1,3-benzothiadiazol-2-thienyl)-
1
5 K, k , quenching due to dimer formation, controls lifetime 60 2,1,3-benzothiadiazole)-5,50-diyl] and the only difference
d
3
2
decrease. This shows that tert-butyl groups attached to carbazoles
do not necessarily decrease triplet dimer formation. Also, it has
been observed previously that poly[2,6-(4,4-bis-(2-ethylhexyl)-
between them is the exchanged Si atom instead of C. Hence it is
reasonable to expect that replacement of carbon atom in the chain
by sulphur could result in stronger dimer formation.
4
H-cyclopenta[2,1-b;3,4-b0]-dithiophene)-alt-4,7-
6
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Journal of Materials Chemistry C
Table 5. PLED device characteristics.
-
10
2
0%
Current
onset
voltage at
0.1 mA/cm
6.2
Brightness
Max.
Max.
power
efficiency,
%
Max.
luminous
efficacy,
lm/W
Max.
current
efficiency,
cd/A
Slope
k
tt, 10
3
J
0
0
J predicted
-1
Complex 1
doped in a
host
onset voltage
brightness,
cm s
Measured,
mA/cm
by using
eq. 3,
2
2
2
at 10 cd/m ,
cd/m
2
2
V
6.1
mA/cm
PEPKK
2130
1850
810
3.28
7.1
15.3
0.017
0.020
0.037
0.040
0.016
0.022
2.7
20.4
21.3
1
1
1
PEPK
PTPK
PTKK
PVKK
POKK
5.7
4.8
6.6
5.9
6.2
5.5
4.6
6.9
5.8
6.0
3.69
3.77
0.98
2.60
3.64
10.2
11.0
2.3
19.7
17.9
5.1
---
---
---
1
1
1
---
---
---
230
0.3
3.3
2.5
4.2
3.5
1960
2360
6.8
13.9
16.9
20.9
21.0
22.1
20.9
8.1
1
For devices with PEPK and PTPK host TTA model (eq. 3) did not fit well, see figure 7 for quality of fitting.
Current density voltage characteristics displayed in Fig. 4b
show that current onset is highest in PTPK and PEPK polymers
without tert-butyl groups and demonstrates better charge
transport properties in these hosts. Better exciton formation is
also observed in these polymers reflected in smallest brightness
3
4
4
5
5
6
6
7
7
5
0
5
0
5
0
5
0
5
2
onset voltages (at 10 cd/m ) for PTPK and PEPK PLEDs (4.6 V
and 5.5 V appropriately). We therefore propose that tert-butyl
groups reduces hole mobility leading to charge unbalance. The
2
highest brightness was achieved using POKK (2360 cd/m ) and
2
PEPKK (2130 cd/m ) as hosts, whereas the smallest - using
2
PTKK (230 cd/m ) as a host. Hosts having oxygen atom in the
2
chain demonstrated at least twice higher brightness (1850 cd/m
2
2
to 2360 cd/m ) than those having sulphur atom (230 cd/m to
10 cd/m ). The highest efficiency is recorded from PLEDs with
2
8
5
PEPK (19.7 cd/A), one of the highest reported so far in literature.
The smallest efficiency is recorded from PLEDs with PTKK
(5.1 cd/A). We suspect that this is due to more efficient formation
of triplet dimers in PTKK (observed even at 15 K) and increase
of dopant quenching by triplet dimer traps (increase of kd).
Also efficiency roll-off is different for each host. We
characterized it by plotting efficiency in log-lin scale and used
linear fit to determine slopes (table 5). The largest roll-off has
been observed in PTKK and PTPK devices (~0.04), carbazole
derivatives containing sulphur atom. Other hosts, containing
oxygen atoms, had a slope of ~0.02.
Figure 5. Schematic energy level diagram for the blue OLEDs with
different host material.
PLED devices
Synthesized polymers doped with
blue iridium complex
1
1
2
2
3
0
5
0
5
0
Iridium(III)bis[2-(2,4-difluorophenyl)-4-(2,4,6-
2
′
trimethylphenyl)pyridinato-N,C ]picolinate
(Complex 1)
2
9
reported recently were used as emissive layer in PLEDs. The
only parameter changed in these PLEDs is the host, whereas
other elements were kept the same – Glass / ITO / PEDOT:PSS
57 nm / host: 20% Complex 1 50 nm / TPBi 33 nm / LiF 0.7 nm /
Al 100 nm. Our previous study has showed that Complex 1 do
not aggregate up to high concentrations due to incorporated
mesityl group and therefore we use 20% load of Complex 1
giving sufficient electron transport. Figure 5 shows schematic
HOMO-LUMO levels diagram of blue OLEDs studied in this
paper (adopted from ref.33). Energy levels for the Complex 1 are
considered to be similar to well know iridium complex FIrpic
Here we would like to point out that although there are reports
17,18,35-37
on roll-off in small molecule OLEDs
including blue
38
PhOLEDs, to the best of our knowledge roll-off in blue PLEDs
has not been investigated diligently yet. Hence to explore roll-off
in these PLEDs we used equation (3), (5) and (6) and fitted them
to the EQE vs current density characteristics to determine which
mechanism (TTA, TPA, TTPA appropriately, Fig. 6) is
responsible for the efficiency roll-off in PLEDs. Previously for
small molecule phosphorescent OLEDs it was shown that the
2
9
because of similar chemical structure. From the energy level
diagram we conclude that TPBi layer serves as effective electron
infection and hole blocking layer due to its rather low HOMO
level (6.2 eV). Electrons are probably transported in the emissive
layer via the iridium complex. PLED device characteristics are
shown in figure 4 and main characteristics can be found in
table 5. Electroluminescence spectra (Fig. 4a) is peaking at
475 nm and is arising from dopant, compare with PL of
Complex 1 in inert zeonex matrix in Fig. 2c. PL from polymer
17
efficiency roll-off is mainly influenced by TTA . Reineke et al,
however, have shown that both processes TTA and TPA
18,39
influence efficiency roll-off.
Nevertheless, in some OLEDs,
TPA could be the only source for efficiency roll-off, which was
40
shown by Song et al. In polymer devices presented here we
observed that POKK, PEPK, PVKK and PTKK roll-off best fits
to the TTA model (eq. 3). Effectively only one parameter kTT was
fitted, as k was determined experimentally (table 4).
m
3
4
host film (Fig. 2b) and TPBi is very different and blueshifted in
comparison with that of Complex 1 (Fig. 2).
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Journal of Materials Chemistry C
Page 8 of 12
1
0
9
8
7
6
5
4
3
2
1
0
10
TTA model fit (dashed lines) (a)
(a)
PEPK
9
8
7
6
5
4
3
2
1
0
PEPKK
PTPK
POKK
PVKK
PTKK
TTA fit (dashed lines)
-
1
0
1
0
.1
1
Current density, mA/cm
10
10
10
10
2
2
Current density, mA/cm
1
0
1
0
9
8
7
6
5
4
3
2
1
0
(b)
PEPK
(b)
9
8
7
6
5
4
3
2
POKK
TTPA fit to
PEPK with
-
10
PVKK
ktt=4.15*10
-
16
kp=10
-13
TTA (gray dashed lines)
vs
TPA (gray straight line)
model fits
kp=10
-
12
kp=10
-11
kp=10
-10
kp=10
-2
10
0.1
1
10
-
3
-1
0
1
2
3
2
1
0
10
10
10
10
10
Current density, mA/cm
2
Current density, mA/cm
Figure 6. POKK, PVKK, PEKK and PTKK fitting, TTA model fits
equation 3) much better than TPA model (eq. 5).
(
2
.5
.0
(
c)
5
0
5
0
5
0
J0 values predicted from the model are comparable to those
determined experimentally (see table 5 and eq. 4). Triplet
annihilation constants are provided in table 5 and are in the range
2
PTPK
PEPK
PEPKK
PTKK
POKK
PVKK
-
10
3 -1
of 10 cm s . k are very similar for PVKK, PEPKK and
TT
1.5
1.0
0.5
-
10
3
-1
-10
3 -1
POKK PLEDs ranging from 2.7∙10 cm s to 3.2∙10 cm s
and kTT for PLEDs with PTKK belong to a smaller order of
magnitude (0.2∙10
determined for small molecule devices by Baldo et al (in the
range of 10 – 10 cm s ), by Reineke et al (in the range of
1
1
2
2
3
-
10
3 -1
cm s ). kTT are greater than those
1
7
-
12
-14
3
-1
18
-
12
3
-1
41
-12
3 -1
1
0
cm s ) and Wang et al
(in the range of 10 cm s ).
However their devices were prepared using molecular
0.0
4
2
4
6
8
10
12
evaporation technique, whereas Holzer et al determined kTT to
-
10
3 -1
be in the range of 10 cm s for Ir(ppy) :CBP, more in line with
35
Voltage, V
3
our results, and, as in our case, Holzer et al samples were
prepared using spincoating technique. Thus we conclude that
triplet-triplet annihilation is more dominant in samples prepared
using spincoating, probably due to stronger aggregation or poorer
charge mobilities.
The fit of the TPA model (eq. 5) is much worse for POKK,
PVKK, PTKK and PEPKK. Example of TPA and TTA fits to
PVKK and POKK EQE vs J characteristics is also provided in
Fig. 6b. We also fitted TTPA model (eq. 6) to POKK, PVKK,
Figure 7. TTA model (eq. 5) does not fit PEPK and PTKK PLEDs (a).
TTPA also does not fit PEPK and PTKK (b). Straight black lines are
fittings with different k
fit is achieved when k ~10 which is 3 orders of magnitude smaller than
ktt. Brightness vs voltage characteristics are depicted in (c).
3
-1
p
while keeping ktt fixed (units in cm s ). The best
-13
p
40
2
in their devices up to 200 mA/cm and current densities in our
devices only reach 30 mA/cm . This shows that there will be
more charge available in their device than ours resulting in more
efficient TPA. We conclude that in devices with POKK, PVKK,
2
PTKK and PEPKK devices but the fit was not significantly better 45 PTKK and PEPKK hosts TTA is most dominant efficiency roll-
than that of the TTA model and to achieve good fit k had to be in
off mechanism.
p
-
13
-16
3 -1
the order 10 -10 cm s , at least three orders of magnitudes
However, we do not get good TTA fit on PEPK and PTPK
devices (Fig. 6a). TTPA as well as TPA models do not fit the
roll-off well either (Fig. 7b). The question arises whether another
50 mechanism could be responsible for the EQE roll-off in PEPK
smaller than k , confirming that TPA is negligible.
TT
1
8
Reineke et al concluded that in their small molecule OLEDs
TPA is important but the current densities are significantly higher
8
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Journal of Materials Chemistry C
and PTPK. After comparing PEPK and PTPK devices with
fittings of both, TTA and TTPA, models we observe that at low
prevent triplet dimer formation. Furthermore, in certain cases,
adding tert-butyl groups to pendant carbazole units was shown to
current densities the experimental values are higher than the 60 increase dimer formation, especially in the case of PTKK. This is
model curves for both models and at higher current densities,
experimental values roll-off quicker than models predict
reflected in low temperature delayed emission spectra, decreased
dopant lifetime in PTKK and low efficiency of devices.
Moreover, it was shown that tert-butyl groups reduce charge
5
0
5
0
5
0
5
0
(
Fig. 7a). Another possible mechanism which could induce such
1
8
roll-off is electric field quenching. Although Reineke et al and
Baldo et al
transport. However the absence of tert-butyl groups (PEPK and
1
7
found that in small molecule OLEDs this 65 PTPK) leads to high charge densities causing strong electric field
mechanism is negligible, but much smaller voltages are used to
quenching. Overall we conclude that no simple recommendation
on how to synthesize high triplet energy based on carbazole could
be formulated and no simple molecular structure - PLED
performance relationship can be derived. Addition of groups, as
1
1
2
2
3
3
4
run their devices (up to 6 V in Reineke et al case), however, in
polymer devices, higher voltages are required to achieve the same
emission. Thus it is more likely that electric field quenching of
excitons can occur. This is confirmed in PEPK and PTPK devices 70 simple tert-butyl, can affect many parameters in device
brightness vs voltage characteristics (Fig. 7c). Only PEPK and
PTPK devices exhibit clear brightness roll off (10 V and 8 V
appropriately) versus voltage. In other devices, brightness
increases in a more stable manner with an increase of voltage.
Previously, it has been observed that above 1 MV/cm electric
performance, for example, increase charge transfer properties but
at the same time increase electric field quenching, and diligent
studies must be performed in order to elucidate the physical
impact of even the smallest structural changes.
4
3
field, Ir(ppy)3 excitons are quenched by an electric field.
75 Experimental
4
4
Exciton quenching also has been observed when Ir(ppy) or
dyes were doped into PVK. 10 V is equivalent to 2∙10 V/cm
and 8 V is 1.6∙10 V/cm in our PEPK and PTPK devices and in
Materials
4
5
6
6
9-(2-Oxiranylmethyl)carbazole (EPK) was purchased from Biolar
(Latvia) and recrystallized five times from 2-propanol. 9H-
carbazole (Reakhim), 2-chloro-2-methylpropane (Fluka),
these conditions it is very likely that excitons are quenched by the
field. However, in other devices, with POKK host, for example,
at these voltages exciton quenching due to electric field is not 80 epichlorohydrin (Fluka), 3-(bromomethyl)-3-methyloxetane
observed. We propose that this is due to the fact that in PEPK and
PTPK devices at the same voltage, the current density is
significantly higher (Fig. 4a) than in other devices. This saturates
the dopants and leads to an accumulation of charges at the
(Chemada),
(2-chloroethyl)-vinylether
(Aldrich),
tetrabutylammonium hydrogensulfate (Aldrich), cyanuric
chloride (Aldrich), boron trifluoride diethyl etherate
(BF O(C H ) ) (Aldrich) were purchased and used as received. 9-
3
2
5 2
interfaces. The result is higher internal electric fields and higher 85 (2-Thiiranylmetyl)carbazole (TPK) (m.p.: 100-101°C, lit.:
4
9
electric field quenching of excitons. It has been shown using
simulations and experiments that in small molecule OLEDs
positive and negative charges accumulate respectively at hole and
electron transport layers, which leads to higher internal electric
104°C) , 3,6-di-tert-butyl-9H-carbazole (m.p.: 225-227 °C, lit.:
5
0
228-233°C) were synthesized according to the reference
procedures.
Characterization
4
6
fields giving more efficient exciton quenching by electric field.
1
13
In addition, it is possible that the reduction of the device 90 H NMR and C NMR spectra were recorded using Varian Unity
1
13
efficiency at high current is due to a thermal effect. The thermal
Inova [300 MHz ( H), 75.4 MHz ( C)] apparatus. All the data
are given as chemical shifts δ (ppm) downfield from Si(CH ) .
3 4
4
7
conductivity of thin aluminium layer (~100 nm) is ~94 W/mK
whereas ITO thermal conductivity for 100 nm thickness film is
Infrared (IR) spectra were recorded using Perkin Elmer Spectrum
GX II FT-IR System. The spectra of solid compounds were
-
3
48
very small ~3x10 W/mK . Therefore the heat dissipation will
take place only through the aluminum cathode. We have shown 95 performed in KBr pellets. Mass spectra (MS) were obtained on a
(data will be published soon) that a temperature rise > 23°C can
Bruker maxis 4G. Melting points (m.p.) of the materials were
be reached in an isolated pixel for current densities higher than
determined using Electrothermal Mel-Temp apparatus.
Ultraviolet (UV) absorption spectra of 10 M solutions of
2
-5
1
6 mA/cm leading to an efficiency drop at high currents.
synthesized compounds in tetrahydrofuran were recorded with
00 Perkin Elmer Lambda 35 spectrometer using 1 mm internal width
microcell (Shimadzu UV-3600 UV/Vis spectrometer). Steady
state fluorescence (FL) spectra were recorded with Perkin Elmer
LS55 spectrometer or Jobin-Yvon Horiba Spex Fluoromax 3
Spectrometer.
1
1
1
4
5
Conclusions
2
Current efficiency of 19.7 cd/A, brightness of 1850 cd/m and
EQE of 9.2% were obtained for devices with PEPK host doped
with 20% Complex 1 and TPBi electron transport layer showing
that these polymers are promising candidates for high efficiency
PLEDS. The roll off in electrophosphorescent quantum efficiency
in PLEDs was shown to arise mainly from triplet-triplet
annihilation between dopants in POKK, PVKK, PEPKK and
PTKK hosts. The efficiency roll-off in devices with PEPK and
PTPK hosts was shown to be additionally affected by electric
field quenching. It has also been shown that in these carbazole
based polymers, triplet dimers are formed as it was observed
previously for PVK and P2VK and that tert-butyl group does not
05
Differential scanning calorimetry (DSC) measurements were
carried out using TA Instruments Q10. Thermogravimetric
analysis (TGA) was performed on Perkin Elmer TGA 4000. DSC
5
0
and TGA measurements were performed in
atmosphere at a heating rate of 20ºC/min.
a nitrogen
10
The molecular weights of polymerization products were
estimated by a gel permeation chromatography (GPC) system
including Viscotech GMHHR-M and Viscotech G2500HHR grade
columns and WATERS Lambda max 481 HPLC UV/VIS
5
5
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Journal of Materials Chemistry C
Page 10 of 12
detector. Polystyrene standards were used for calibration of the
4.32 (dd, 1H, J = 4.7 Hz, J = 15.7 Hz, 1H of CH N), 4.51 (dd,
2
columns and tetrahydrofuran was used as an eluent. The elution 60 1H, J = 3.5 Hz, J = 15.7 Hz, 1H of CH N), 7.33 (d, 2H, J = 8.6
2
rate was 1 ml/min.
Hz, Ar), 7.50 (dd, 2H, J = 1.9 Hz, J = 8.6 Hz, Ar), 8.09 (d, 2H, J
Cyclic voltammetry (CV) measurements were carried out by a
three electrode assembly cell from Bio-Logic SAS and a micro-
AUTOLAB Type III potentiostat-galvanostat. The measurements
= 1.9 Hz, Ar).
1
3
5
0
5
0
5
0
5
0
C NMR (75.4 MHz, CDCl ), δ (ppm): 32.3 (6×-CH ), 35.0
3 3
(2×-C-(CH ) ), 44.9 (O-CH -), 45.7 (O-CH-), 50.9 (N-CH ),
3
3
2
2
were carried out with a glassy carbon electrode, Ag/AgNO as the 65 108.4, 116.6, 123.2, 123.8, 139.5, 142.4.
3
-
1
reference electrode and a Pt wire as counter electrode in
dichloromethane solutions containing 0.1 M tetrabutylammonium
perchlorate as the electrolyte. The EHOMO energy values of the
compounds were determined from oxidation potentials, taking the
value of -4.8 eV as the EHOMO energy level for ferrocene (Fc)
IR (KBr, cm ): 3056  (C-H arene); 2960, 2866  (C-H
aliphatic); 1632, 1607, 1493, 1478  (C=C arene); 1364, 1298,
1260  (C=C, C-N arene); 922  (C-O-C oxirane ring); 878, 810,
614 γ (C-H arene).
1
1
2
2
3
3
4
+
+
+
70
MS (APCl ), m/z: 280 ([M-C H ] ), 224([M-C H ] ), 180
4 9 8 18
5
1
+
with respect to zero vacuum level . Ionization potentials (I ) of
([M-C H O] ).
p
10 21
the films of the synthesized compounds were measured by
3,6-Di-tert-butyl-9-(thiiran-2-ylmethyl)-9H-carbazole (TKK).
3,6-di-tert-butyl-9H-carbazole (1 g, 10 mmol) was dissolved in
tetrahydrofuran (5 ml). Ammonium thiocyanate (0.45 ml,
20 mmol) and cyanuric chloride (0.02 g, 2 wt% from carbazole)
were added stepwise and stirred at room temperature for 12 h.
The end of the reaction was detected by TLC (eluent:
hexane/ethylacetate, 6:1). The reaction mixture was extracted
with ethylacetate, the crude product was purified by column
chromatography using an eluent mixture of hexane and
ethylacetate in volume ratio of 10:1 and crystallized from the
eluent mixture of solvents. The yield was 70% (0.7 g) of white
5
2
electron photoemission in air method as described before .
Photophysical data were obtained using freshly prepared films
of the hosts or hosts with iridium complexes. Films were
prepared by dissolving the polymers in chlorobenzene at
75
2
0 mg/ml or 4 mg/ml concentration, then heating at 60 ºC for 5
min and spin coating at 870 rpm or dropcasting (to get higher
absorption for delayed emission measurements) on sapphire
substrates. For delayed emission, phosphorescence and decay
measurements, samples were subsequently mounted in a displex
cryostat and vacuumed with turbomolecular pump. Samples were
excited with 3rd harmonics of YAG laser (355 nm) from
EKSPLA at 45° angle to the substrate plane and the energy of
80
crystals (FW = 351 g/mol, m.p.: 130-132°C).
1
H NMR (300 MHz, CDCl
S), 3.23-3.31 (m, 1H, CHS), 4.31 (dd, 1H,
J = 6.9 Hz, J = 15.3 Hz, 1H of CH N), 4.60 (dd, 1H, J = 5.3 Hz, J
), δ (ppm): 1.46 (s, 18H, 6×CH ),
3
3
each pulse was ca. 40 µJ per pulse. Emission was focused onto a 85 2.41-2.45 (m, 2H, CH
spectrograph and detected on a sensitive gated iCCD camera
2
2
=
15.4 Hz, 1H of CH N), 7.34 (d, 2H, J = 8.6 Hz, Ar), 7.51 (dd,
2
(Stanford Computer Optics) with sub-nanosecond resolution.
2
H, J = 1.9 Hz, J = 5.3 Hz, Ar), 8.10 (d, 2H, J = 1.9 Hz, Ar).
PEDOT:PSS and polymer layer doped with 20% Ir complex
1
3
Complex 1 were spincoated from chlorobenzene solution at
3
1
C NMR (75.4 MHz, CDCl
000 rpm and annealed at 200°C for 5 min and at 120°C for 90 (S-CH-), 32.4 (6×-CH ), 35.0 (2×-C-(CH
0 min, respectively, on top of ITO/glass substrates. 1,3,5-tris(N-
108.4, 116.7, 123.3, 123.8, 139.0, 142.4.
3
), δ (ppm): 24.8 (S-CH
), 48.9 (N-CH
2
2
-), 27.2
)
-),
3
3
3
-
1
IR (KBr, cm ): 3046  (C-H arene); 2960, 2863  (C-H
aliphatic); 1632, 1609, 1491, 1476  (C=C arene); 1363, 1297,
phenylbenzimiazole-2-yl)benzene (TPBi) layer, LiF and Al
cathode were evaporated with Kurt. J. Lesker evaporator at
-
6
1
272  (C=C, C-N arene); 1045  (C-S-C thiirane ring); 876, 804,
5
×10 mBar vacuum at 0.6 Å/s rate.
Under same spincoating conditions layers on SiSiO substrates 95 611 γ (C-H arene).
2
+
+
+
were spincoated for film thickness measurements using
J. A. Woollam Ellipsometer. Current-voltage characteristics ant
emission intensities were measured in a calibrated integrating
sphere.
MS (APCl ), m/z: 296 ([M-C
4
H
9
] ), 240 ([M-C
8
H
18] ).
3
,6-Di-tert-butyl-9-[(3-methyloxetan-3yl)methyl]-9H-
carbazole (OKK).
,6-di-tert-butyl-9H-carbazole (2 g, 10 mmol) was disolved in
3
1
1
1
1
00 ethylmethylketone (8 ml) at 70°C. 3-(Bromomethyl)-3-
methyloxetane (1.64 ml, 20 mmol), sodium sulfate (1.02 g,
Synthesis of monomers
1
8
0
0 mmol), tetrabutylammonium hydrogen sulfate (3.21 g,
0 mmol) and freshly powdered potassium hydroxide (0.05 g,
.2 mmol) were added stepwise and the reaction mixture was
3
3
,6-Di-tert-butyl-9-(oxiran-2-ylmethyl)-9H-carbazole (EPKK).
,6-Di-tert-butyl-9H-carbazole (4 g, 10 mmol) was disolved in
4
5
5
5
0
5
ethylmethylketone (15 ml) at 70°C. Epichlorohydrin (1.12 ml,
0 mmol), potassium carbonate (1.97 g, 10 mmol) and freshly
05 stirred at 70°C for 1.5 h. The reaction was monitored by TLC
eluent: hexane/ethylacetate, 6:1). The reaction mixture was
3
(
powdered potassium hydroxide (6.41 g, 80 mmol) were added
stepwise and stirred at 70°C for 1 h. The end of the reaction was
detected by thin-layer chromatography (TLC) (eluent:
hexane/ethylacetate, 6:1). The reaction mixture was extracted
with ethylacetate. The crude product was purified by column
chromatography using an eluent mixture of hexane and
ethylacetate in volume ratio of 15:1 and crystallized from 2-
propanol. The yield was 60% (2.88 g) of white crystals (FW =
335 g/mol, m.p.: 153-154°C).
extracted with ethylacetate, the crude product was purified by
column chromatography using an eluent mixture of hexane and
ethylacetate in volume ratio of 15:1 and crystallized from the
10 eluent mixture of solvents. The yield was 72% (1.87 g) of white
crystals (FW = 363 g/mol, m.p.: 176-169°C).
1
H NMR (300 MHz, CDCl ), δ (ppm): 1.44 (s, 3H, CH ), 1.45
3
3
(s, 18H, 6×CH ), 4.34-4.37 (m, 4H, 2×CH O), 4.77 (d, 2H, J =
3
2
6
.0 Hz, CH N), 7.28 (d, 2H, J = 8.6 Hz, Ar), 7.49 (dd, 2H, J = 1.9
2
1
15 Hz, J = 8.6 Hz, Ar), 8.10 (d, 2H, J = 1.9 Hz, Ar).
H NMR (300 MHz, CDCl ), δ (ppm): 1.45 (s, 18H, 6×CH ),
3
3
1
3
C NMR (75.4 MHz, CDCl ), δ (ppm): 23.8 (-CH ), 32.3 (6×-
2
.55 (dd, 1H, J = 2.6 Hz, J = 4.8 Hz, 1H of CH O), 2.76 (dd, 1H,
3
3
2
CH ), 34.9 (2×-C-(CH ) ), 43.3 (-C- in oxetane ring), 49.8 (N-
J = 3.9 Hz, J = 4.8 Hz, 1H of CH O), 3.27-3.32 (m, 1H, CHO),
3
3 3
2
1
0
| Journal Name, [year], [vol], 00–00
This journal is © The Royal Society of Chemistry [year]

View Article Online
DOI: 10.1039/C3TC31441C
Page 11 of 12
Journal of Materials Chemistry C
CH -), 81.1 (2×O-CH -), 108.7, 116.6, 123.2, 123.7, 140.1,
aliphatic); 1626, 1596, 1484, 1458  (C=C arene); 1325  (C=C,
60 C-N arene) 1121, 1002  (C-S-C); 748, 722 γ (C-H arene).
Poly[3,6-Di-tert-butyl-9-(oxiran-2-ylmethyl)-9H-carbazole]
(PEPKK) was obtained as white amorphous powder with a yield
2
2
1
42.3.
-
1
IR (KBr, cm ): 3055  (C-H arene); 2961, 2866  (C-H
aliphatic); 1629, 1605, 1491, 1477, 1459  (C=C arene); 1362,
1299, 1260  (C=C, C-N arene); 1240, 1186, 973  (C-O-C
oxetane ring); 876, 808, 613 γ (C-H arene).
5
of 65%.
1
H NMR (300 MHz, CDCl ), δ (ppm): 1.30-1.50 (m, 18H,
3
+
+
+
MS (APCl ), m/z: 308 ([M-C H ] ), 252 ([M-C H ] ).
65 6×CH ), 2.60-4.60 (m, 5H, CH , CH , CH), 7.20-7.64 (m, 4H,
4
9
8
18
3
2
2
3
,6-Di-tert-butyl-9-[2-(vinyloxy)ethyl]-9H-carbazole (VKK)
was synthesized by similar procedure as OKK. 3,6-di-tert-butyl-
9H-carbazole (4 g, 10 mmol), (2-chloroethyl)-vinylether
2.91 ml, 20 mmol), sodium sulfate (2.05 g, 10 mmol),
Ar), 8.00-8.22 (m, 2H, Ar).
IR (KBr, cm ): 3051  (C-H arene); 2960, 2866  (C-H
aliphatic); 1632, 1609, 1491, 1479  (C=C arene); 1363, 1297,
-
1
1
1
2
2
3
0
(
1
273  (C=C, C-N arene); 1107  (C-O-C); 878, 804, 612 γ (C-H
tetrabutylammonium hydrogensulfate (0.09 g, 0.2 mmol), freshly
powdered potassium hydroxide (6.41 g, 80 mmol) and
ethylmethylketone (15 ml) were used. The crude product was
purified by column chromatography using an eluent mixture of
hexane and ethylacetate in volume ratio of 15:1 and crystallized
from 2-propanol. The yield was 72% (3.6 g) of white crystals
7
0
arene).
Poly[3,6-Di-tert-butyl-9-(thiiran-2-ylmethyl)-9H-carbazole]
(PTKK) was obtained as white amorphous powder with a yield
5
0
5
0
of 75%.
1
H NMR (300 MHz, CDCl ), δ (ppm): 1.30-1.51 (m, 18H,
3
75
6×CH ), 2.20-2.95 (m, 2H, CH ), 3.10-3.70 (m, 1H, CH), 3.95-
3
2
(
FW = 349 g/mol, m.p.: 89-91°C).
4
.53 (m, 2H, CH N), 6.94-7.56 (m, 4H, Ar), 7.97-8.14 (m, 2H,
2
1
H NMR (300 MHz, CDCl ), δ (ppm): 1.45 (s, 18H, 6×CH ),
Ar).
3
3
-
1
3.98 (dd, 1H, J = 2.3 Hz, J = 6.8 Hz, 1H of CH=CH ), 4.03 (t,
IR (KBr, cm ): 3049  (C-H arene); 2960, 2866  (C-H
aliphatic); 1631, 1609, 1490, 1478  (C=C arene); 1363, 1297,
2
2
H, J = 6.1 Hz, CH O), 4.12 (dd, 1H, J = 2.3 Hz, J = 14.2 Hz, 1H
2
of CH=CH ), 4.51 (t, 2H, J = 6.1 Hz, CH N), 6.42 (dd, 1H, J =
2
2
80 1262  (C=C, C-N arene); 1032  (C-S-C); 878, 803, 611 γ (C-H
arene).
6
.8 Hz, J = 14.4 Hz, CH=CH ), 7.33 (d, 2H, J = 8.6 Hz, Ar), 7.50
2
(
dd, 2H, J = 2.0 Hz, J = 8.6 Hz, Ar), 8.09 (d, 2H, J = 1.9 Hz, Ar).
Poly[3,6-Di-tert-butyl-9-[2-(vinyloxy)ethyl]-9H-carbazole]
(PVKK) was obtained as white amorphous powder with a yield
1
3
C NMR (75.4 MHz, CDCl ), δ (ppm): 32.4 (6×-CH ), 35.0
3
3
(
2×-C-(CH ) ), 42.5 (O-CH -), 66.0 (N-CH -), 87.2 (-CH=CH ),
of 50%.
3
3
2
2
2
1
1
08.4, 116.6, 123.2, 123.7, 139.3, 142.2, 151.6 (O-CH=CH ).
IR (KBr, cm ): 3054  (C-H arene); 2960, 2866  (C-H
85
H NMR (300 MHz, CDCl ), δ (ppm): 0.89-1.58 (m, 20H,
2
3
-
1
6
×CH , CH ), 3.02-3.50 (m, 4H, CH O, CH ), 3.82-4.16 (m, 2H,
3
2
2
2
aliphatic); 1667, 1619, 1482, 1461  (C=C arene and vinyl
group); 1363, 1298, 1260  (C=C, C-N arene); 1196, 985, 959 
CH, CH N), 6.94-7.37 (m, 4H, Ar), 7.91-8.10 (m, 2H, Ar).
IR (KBr, cm ): 3050  (C-H arene); 2960, 2866  (C-H
aliphatic); 1633, 1609, 1490, 1480  (C=C arene); 1363, 1298,
2
-
1
(
C-O-C); 877, 799, 611 γ (C-H arene).
+
+
+
MS (APCl ), m/z: 294 ([M-C H ] ), 238 ([M-C H ] ).
90 1263  (C=C, C-N arene); 1121  (C-O-C); 878, 802, 611 γ (C-H
arene).
4
9
8
18
Synthesis of polymers
Poly[3,6-Di-tert-butyl-9-[(3-methyloxetan-3yl)methyl]-9H-
carbazole] (POKK) was obtained as white amorphous powder
Polymers PEPK, PTPK, PEPKK, PTKK, POKK, PVKK were
prepared from EPK, TPK, EPKK, TKK, OKK, VKK
3
4
4
5
5
5
0
5
0
5
with a yield of 60%.
5
3,54
1
respectively by the similar procedure as reported earlier.
Cationic polymerization of the appropriate monomers initiated
with BF ·O(C H ) (0.1 mol% with respect to monomer) was
95
H NMR (300 MHz, CDCl ), δ (ppm): 1.10-1.24 (m, 3H,
3
CH ), 1.29-1.49 (m, 18H, 6×CH ), 3.39-3.59 (m, 4H, CH , CH ),
3
3
2
2
3
2
5 2
4.30-4.46 (m, 2H, CH N), 7.31-7.45 (m, 4H, Ar), 8.02-8.09 (m,
2
2
carried out in dry 1,2-dichloromethane solution at room
temperature for 20 h. The initial concentration of a monomer was
H, Ar).
IR (KBr, cm ): 3051  (C-H arene); 2961, 2867  (C-H
00 aliphatic); 1632, 1608, 1490, 1479  (C=C arene); 1363, 1297,
259  (C=C, C-N arene); 1106  (C-O-C); 878, 805, 612 γ (C-H
-1
1
mol/l. Polymers were isolated by precipitation in methanol
1
containing aqua ammonia which was added for neutralization of
the initiator. The low molecular weight fractions of polymers
were removed by Soxhlet extraction with methanol for 70 h.
Then polymers were re-precipitated.
1
arene).
Acknowledgements
Poly[9-(oxiran-2-ylmethyl)-9H-carbazole]
(PEPK)
was
We thank Professor Martin Bryce for providing the blue dopant
05 Complex 1. This research was partially funded by the European
Social Fund under the Global Grant measure. This work was also
supported by EPSRC (grant ref. EP/H051902/1).
obtained as white amorphous powder with a yield of 80%.
1
1
H NMR (300 MHz, CDCl ), δ (ppm): 2.27-4.53 (m, 5H, CH ,
3
2
CH , CH), 6.76-7.56 (m, 6H, Ar), 7.81-8.09 (m, 2H, Ar).
2
-
1
IR (KBr, cm ): 3048  (C-H arene); 2928, 2870  (C-H
aliphatic); 1626, 1597, 1484, 1453  (C=C arene); 1325  (C=C,
C-N arene); 1130, 1121  (C-O-C); 749, 722 γ (C-H arene).
Notes and references
Poly[9-(thiiran-2-ylmethyl)-9H-carbazole]
(PTPK)
was
a
Department of Organic Technology, Kaunas University of Technology,
obtained as white amorphous powder with a yield 75%.
1
10 Radvilenu pl. 19, Kaunas LT-50254, Lithuania
1
b
H NMR (300 MHz, CDCl ), δ (ppm): 1.70-2.60 (m, 2H, CH ),
Institute of Applied Research, Vilnius University, Sauletekio 9-III,
3
2
Vilnius LT-10222, Lithuania. E-mail: steponas.raisys@tmi.vu.lt.
2
6
.70-3.30 (m, 1H, CH), 3.60-4.50 (m, 2H, CH N), 6.80-7.60 (m,
2
c
Physics Department, University of Durham, South Road, Durham DH1
H, Ar), 7.70-8.20 (m, 2H, Ar).
3
LE, United Kingdom. E-mail: vygintas.jankus@durham.ac.uk.
-
1
IR (KBr, cm ): 3049  (C-H arene); 2922, 2862  (C-H
This journal is © The Royal Society of Chemistry [year]
Journal Name, [year], [vol], 00–00 | 11

View Article Online
DOI: 10.1039/C3TC31441C
Journal of Materials Chemistry C
Page 12 of 12
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