3,6-Bis(indol-1-yl)-9-phenylcarbazoles as electroactive materials for electrophosphorescent diodes

Article abstract of DOI:10.1016/j.dyepig.2013.07.020

Four derivatives of 3,6-bis(indol-1-yl)-9-phenylcarbazole were synthesized by Ullmann-type coupling including those having methoxy groups at the different positions. Thermal, optical and photophysical properties of the synthesized materials were studied.

Full text of DOI:10.1016/j.dyepig.2013.07.020

Dyes and Pigments 100 (2014) 66e72
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3,6-Bis(indol-1-yl)-9-phenylcarbazoles as electroactive materials
for electrophosphorescent diodes
,
a
,
,
J. Keruckas a, J.V. Grazulevicius a , D. Volyniuk b, V. Cherpak b, P. Stakhira b
*
**
a Department of Organic Technology, Kaunas University of Technology, Radvilenu Plentas 19, 50254 Kaunas, Lithuania
b Lviv Polytechnic National University, S. Bandery Str. 12, 79013 Lviv, Ukraine
a r t i c l e i n f o
a b s t r a c t
Article history:
Four derivatives of 3,6-bis(indol-1-yl)-9-phenylcarbazole were synthesized by Ullmann-type coupling
including those having methoxy groups at the different positions. Thermal, optical and photophysical
properties of the synthesized materials were studied. All the synthesized 3,6-bis(indol-1-yl)-9-
phenylcarbazoles are capable of glass formation with the glass transition temperatures ranging from
77 to 95 ꢀC. They exhibit high thermal stability with the 5% weight loss temperatures ranging from 393 to
459 ꢀC. Photoluminescence spectra of the dilute solutions of 3,6-bis(indol-1-yl)-9-phenylcarbazoles are
characterized by two well-resolved vibrational peaks at ca. 390 nm and 405 nm. The solid films of the
materials also exhibit well-resolved vibrational peaks, however the wavelengths of the maxima are red-
shifted by ca. 10 nm. 3,6-Bis(indol-1-yl)-9-phenylcarbazoles were tested in the single-layer organic light
emitting diodes as active materials and in phosphorescent light emitting diodes as host materials of
emitting layers in combination with Ir(Fppy)3 as a guest phosphor. Blue single-layer electroluminescent
and electrophosphorescent devices with current efficiencies of 3.7e4.1 cd/A and 10.8e12.6 cd/A
respectively were fabricated.
Received 4 April 2013
Received in revised form
11 June 2013
Accepted 19 July 2013
Available online 30 July 2013
Keywords:
Carbazole
Indole
Molecular glass
Organic light-emitting diodes
Iridium complex
Host-guest films
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
and characterization of glass-forming 3,6-bis(indol-1-yl)-9-
phenylcarbazoles (BIPCs) as well as on their thermal, optical,
Starting from the first publication [1], organic light-emitting
diodes (OLEDs) have attracted a great deal of interest because of
their potential applications in full-color flat-panel displays and
lighting sources [2e4]. In recent years, a lot of efforts have been
concentrated on the development of electrophosphorescent light
emitting devices (PHOLEDs) based on luminescent transition-
metal complexes because of high quantum efficiencies which
can be attained in such devices [5e8]. Developing of highly effi-
cient blue OLEDs attracts a special attention since their charac-
teristics are so far inferior with respect to those of red or green
ones [9]. High energies needed to produce blue light rise the
special requirements for the photostability of the materials used in
blue PHOLEDs [10]. The efforts to develop efficient and reliable
blue PHOLEDs are aggravated by the lack of efficient charge-
transporting materials with a wide energy gap for sufficient car-
rier injection [11]. In this work we report on the synthesis
luminescent and photoelectrical properties. The performance
of the synthesized materials in OLEDs was also studied. Two types
of OLEDs were fabricated: single-layer blue light emitting devices
based on BIPCs and blue-green phosphorescent devices obtained
by simultaneous vacuum deposition from one crucible of
host materials, BIPCs, and of the triplet emitter tris[2-(4,6-
difluorophenyl)pyridinato-C2,N]iridium(III) complex (Ir(Fppy)3).
2. Experimental
2.1. Instrumentation
NMR spectra of deuterated chloroform (CDCl3) solutions were
recorded using a Varian Unity Inova (300 MHz) apparatus. IR
spectra of KBr pellets were obtained on a PerkineElmer Spectrum
GX II FT-IR System spectrometer. Mass spectra (MS) were obtained
using a Bruker Maxis 4G system. Elemental analysis was performed
with an Exeter Analytical CE-440 Elemental Analyzer. Melting
points were measured with Electrothermal MEL-TEMP apparatus.
Differential scanning calorimetry (DSC) measurements were car-
ried out using a DSC Q100 calorimeter with a heating/cooling rate
* Corresponding author. Tel.: þ370 37 300193; fax: þ370 37 300152.
** Corresponding author. Tel.: þ380 50 2006563.
E-mail addresses: juozas.grazulevicius@ktu.lt (J.V. Grazulevicius), stakhira@
polynet.lviv.ua (P. Stakhira).
0143-7208/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2013.07.020

J. Keruckas et al. / Dyes and Pigments 100 (2014) 66e72
67
of 10 ꢀC/min under nitrogen. Thermogravimetric analysis (TGA)
2.2.1. The general procedure for the synthesis of 3,6-bis(indol-1-yl)-
9-phenylcarbazoles
was performed on a PerkineElmer TGA 4000 apparatus with a
heating rate of 20 ꢀC/min under nitrogen. Ionization potentials (Ip)
of solid films were established by electron photoemission in air
method as described earlier [12]. The samples for the Ip measure-
ments were prepared by casting tetrahydrofuran solutions of the
materials on aluminum plates pre-coated with methylmethacrylate
and methacrylic acid copolymer adhesive layer. UV absorption and
photoluminescence (PL) spectra of dilute tetrahydrofuran solutions
(0.05 mM) were recorded at ambient conditions on a PerkineElmer
Lambda 35 spectrometer and PerkineElmer LS55 fluorescence
spectrometer respectively. UV absorption spectra of vacuum
deposited films were recorded with Cary 5000 UVeVISeNIR and
Specord UVeVis spectrometers. The electroluminescent devices
ITO/CuI (12 nm)/BIPCs (30 nm)/Ca (40 nm)/Al (200 nm) and ITO/
CuI/BIPCs:Ir(Fppy)3 (50 nm)/TCz1 (10 nm)/Ca (40 nm)/Al (200 nm)
were fabricated by means of vacuum co-deposition of BIPCs and
Ir(Fppy)3 as reported earlier [13]. Copper iodide (CuI) was used as
the hole injection layer [14]. 3,6-Di(9-carbazolyl)-9-(2-ethylhexyl)
carbazole (TCz1) served as the electron-transporting layer [15]. The
thickness of the obtained layers was measured by NanoCalc 2000
Reflectometry System. The active area of the obtained devices was
3 ꢁ 2 mm2. To experimentally observe the triplet state emission,
the luminescence at 77 K from vacuum deposited BIPCs films was
5 mmol of 4a (2.48 g) or 4b (2.63 g), 15 mmol of 1H-indole (5a,
1.76 g) or 5-methoxy-1H-indole (5b, 2.21 g) and 0.25 g 18-crown-6
were dissolved in 20 ml of o-dichlorobenzene. The mixture was
heated up to 150 ꢀC under nitrogen and 2.1 g of potassium carbonate
and 0.95 g of copper powder were added. The reaction mixture was
kept under reflux and controlled by TLC. After 4e6 h the complete
consumption of diiodo-derivatives 4aeb was observed and the
heating was ended. The reaction mixture was cooled down to
ambient temperature and filtered washing with o-dichlorobenzene
which was further removed by vacuum distillation. The dark crude
product was thereafter purified by column chromatography on
silica-gel using the mixture of acetone and hexane (volume ratio 1e
5) as an eluent and then recrystallized from the organic solvent
indicated at the description of each compound.
2.2.2. 3,6-Bis(indol-1-yl)-9-phenylcarbazole (BIPC)
Precipitated in methanol; recrystallized from hexane. Small
white crystals. Yield 1.55 g (65%). M.p. 179e180 ꢀC. 1HNMR (CDCl3,
300 MHz) ppm: 6.71 (dd, 2H, aromatics, J1 ¼ 3.3 Hz, J2 ¼ 0.7 Hz),
7.13e7.26 (m, 4H, aromatics), 7.42 (d, 2H, aromatics, J ¼ 3.3 Hz),
7.50e7.74 (m, 13H, aromatics), 8.21 (dd, 2H, aromatics, J1 ¼ 1.8 Hz,
J2 ¼ 0.7 Hz). 13CNMR (CDCl3, 75 MHz) ppm: 103.0,110.3,110.8,116.9,
120.1,121.0, 122.2,123.5,124.0,127.0, 128.1,128.7, 128.9,130.1, 132.8,
measured at the delay time of 100 ms after an excitation pulse and
using a detector with a large gate width of 200 ms. The current
densityevoltageeluminance (JeVeL) characteristics and electro-
luminescence (EL) spectra were recorded using a Programmable
Test Power LED300E spectrometer HAAS-2000 with an integrated
sphere (d ¼ 0.3 m). All characteristics of the fabricated devices were
recorded in ambient air without passivation.
136.6, 140.1. FT-IR (KBr) cmꢂ1: 3101, 3047 (
1580, 1513, 1492, 1475 ( C]C aromatic), 1332, 1312, 1278 (
aromatic), 884, 846, 814, 761, 741 (
n
CeH aromatic), 1594,
CeN
n
n
g
CeH aromatic). FW ¼ 473.57. MS
(APCIþ) m/z: 474.2017 (M þ H). Elemental analysis for C34H23N3:
calc. C 86.23%, H 4.90%, N 8.87%; found C 86.34%, H 4.95%, N 8.96%.
2.2.3. 3,6-Bis(indol-1-yl)-9-(4-methoxyphenyl)carbazole acetone
solvate [BIPC1$(CH3)2CO]
2.2. Materials
Recrystallized from acetone. Colorless rhomboid plates. Yield
1.99 g (79%). M.p. 126e127 ꢀC. 1HNMR (CDCl3, 300 MHz) ppm: 2.17
(s, 6H, CH3, acetone) 3.94 (s, 3H, OCH3), 6.71 (dd, 2H, aromatics,
J1 ¼ 3.3 Hz, J2 ¼ 1.1 Hz), 7.13e7.26 (m, 6H, aromatics), 7.40e7.60 (m,
10H, aromatics), 7.69e7.74 (m, 2H, aromatics), 8.20 (d, 2H, aro-
matics, J ¼ 1.6 Hz). 13CNMR (CDCl3, 75 MHz) ppm: 55.6, 102.9, 110.3,
110.7,115.3,116.9,120.1,121.0,122.2,123.3,123.9,128.5,128.8,128.9,
9H-carbazole, 4-iodoanisole, 1H-indole, 5-methoxy-1H-indole,
9-phenylcarbazole, 18-crown-6, o-dichlorobenzene and copper
powder were purchased either form Aldrich or Fluka. Potassium
carbonate, potassium iodide, potassium iodate and glacial acetic
acid were purchased either from Penta or Lach-Ner, Czech Republic.
The materials were used as received. Solvents for column chro-
matography and crystallization of target compounds were addi-
tionally distilled.
9-(4-Anisyl)carbazole (3b) was prepared by modified Ullmann
condensation of 9H-carbazole (1) with 4-iodoanisole (2) in similar
way as described earlier [16]. 3,6-Diiodo-derivatives of 9-
arylcarbazoles (4aeb) were obtained by the iodination of 9-
phenylcarbazole (3a) or 3b respectively according to Tucker [17].
129.6, 132.6, 140.6. FT-IR (KBr) cmꢂ1: 3056, 3042 (
2962, 2933, 2838 ( CeH aliphatic), 1709 ( C]O), 1609, 1582, 1513,
1491, 1457 ( C]C aromatic), 1333, 1314, 1292, 1280 ( CeN aro-
matic), 1247, 1216, 1028 ( CeOeC), 884, 833, 762, 747 ( CeH ar-
n CeH aromatic),
n
n
n
n
n
g
omatic). FW (BIPC1) ¼ 503.59. MS (APCIþ) m/z: 504.2119 (M þ H).
Elemental analysis for C35H25N3O$(CH3)2CO: calc. C 81.26%, H
5.56%, N 7.48%, O 5.70%; found C 81.05% H 5.62% N 7.54%.
Scheme 1. The synthetic route to 3,6-bis(indol-1-yl)-9-phenylcarbazoles.

68
J. Keruckas et al. / Dyes and Pigments 100 (2014) 66e72
Fig. 1. DSC thermograms of BIPC2.
2.2.4. 3,6-Bis(5-methoxyindol-1-yl)-9-phenylcarbazole (BIPC2)
Recrystallized from acetone. Small white crystals. Yield 2.02 g
(76%). M.p. 200e201 ꢀC. 1HNMR (CDCl3, 300 MHz) ppm: 3.88 (s, 6H,
OCH3), 6.62 (dd, 2H, aromatics, J1 ¼ 3.3 Hz, J2 ¼ 0.7 Hz), 6.88 (dd,
2H, aromatics, J1 ¼ 8.8 Hz, J2 ¼ 2.2 Hz), 7.16 (d, 2H, aromatics,
J ¼ 2.2 Hz), 7.93 (d, 2H, aromatics, J ¼ 3.3 Hz), 7.44 (d, 2H, aromatics,
J ¼ 8.8 Hz), 7.49e7.59 (m, 5H, aromatics), 7.60e7.71 (m, 4H, aro-
matics), 8.19 (dd, 2H, aromatics, J1 ¼ 1.8 Hz, J2 ¼ 0.7 Hz). 13CNMR
(CDCl3, 75 MHz) ppm: 55.8, 102.6, 110.8, 111.1, 112.4, 116.6, 123.7,
127.0, 128.0, 129.1, 129.4, 130.1, 131.9, 132.9, 140.0, 154.4. FT-IR
(KBr) cmꢂ1: 3099, 3046, 3012 (
2940, 2906, 2831 ( CeH aliphatic), 1622, 1598, 1579, 1491, 1465 (
C]C aromatic), 1353, 1334, 1289 ( CeN aromatic), 1258, 1229,
1212, 1070, 1034 ( CeOeC), 873, 863, 850, 825, 789, 763 ( CeH
aromatic). FW ¼ 533.62. MS (APCIþ) m/z: 534.2221 (M þ H).
Elemental analysis for C36H27N3O2: calc. C 81.03%, H 5.10%, N 7.87%,
O 6.00%; found C 80.94%, H 5.22%, N 7.96%.
n CeH aromatic), 2986, 2957,
n
n
n
Fig. 2. Normalized absorption and PL spectra (a) of solutions (square) and vacuum
deposited (circle) BIPCs films, and low temperature fluorescence and phosphorescence
spectra (b) of vacuum deposited BIPCs films.
n
g
3. Results and discussion
2.2.5. 3,6-Bis(5-methoxyindol-1-yl)-9-(4-methoxyphenyl)
carbazole (BIPC3)
3.1. Synthesis and characterization
Recrystallized from tetrahydrofuran. Small white crystals. Yield
2.09 g (74%). M.p. 205e206 ꢀC. 1HNMR (CDCl3, 300 MHz) ppm: 3.87
(s, 6H, OCH3), 3.93 (s, 3H, OCH3), 6.62 (d, 2H, aromatics, J ¼ 2.9 Hz),
6.87 (dd, 2H, aromatics, J1 ¼ 8.8 Hz, J2 ¼ 2.6 Hz), 7.12e7.19 (m, 4H,
aromatics), 7.38 (d, 2H, aromatics, J ¼ 3.3 Hz), 7.40e7.58 (m, 8H,
aromatics), 8.18 (d, 2H, aromatics, J ¼ 2.2 Hz). 13CNMR (CDCl3,
75 MHz) ppm: 55.6, 55.9, 102.6, 110.7, 111.1, 112.4, 115.3, 116.6, 123.3,
123.7, 128.5, 129.2, 129.4, 129.6, 132.0, 132.8, 140.5, 154.4, 159.3. FT-
Heterogenous Ullmann-type coupling reaction using powdered
copper as a catalyst [18] was found to be a convenient method for
the synthesis of N-indolyl-substituted carbazoles from N-aryl-3,6-
diiodocarbazoles. The target compounds were obtained in good
IR (KBr) cmꢂ1: 3082, 2995 (
aliphatic),1618,1580,1514,1477 (
N aromatic), 1253, 1043, 1024 ( CeOeC), 866, 829, 817, 795, 758 (
n
CeH aromatic), 2929, 2831 (
n
CeH
Ce
Table 2
Optical and photophysical characteristics of BIPCs and Ir(Fppy)3.
n
C]C aromatic), 1340,1298 (
n
Material
BIPC
BIPC1
BIPC2
BIPC3
Ir(Fppy)3
n
g
abs
CeH aromatic). FW ¼ 563.64. MS (APCIþ) m/z: 564.2327 (M þ H).
Elemental analysis for C37H29N3O3: calc. C 78.84%, H 5.19%, N 7.46%,
O 8.52%; found C 79.44%, H 5.31%, N 7.59%.
l
(nm)
(nm)
251, 270,
296, 349
312, 375
251, 271,
292*, 352 306, 352
312, 376
253, 278, 251, 278,
e
sol:
308, 353
312, 367
abs
film
l
315, 377
276, 357,
390
abs
l
l
l
l
l
(nm)
(nm)
(nm)
397
387, 404
396*, 411 397, 415
398
391, 406
395
396
e
on
PL
Table 1
394, 408
398, 416
397, 421
454, 494
5.52
396, 409
404, 418
403, 426
453
e
sol:
PL
Thermal characteristics of BIPCs.
e
film
PL
(77 K) (nm) 393, 415
(nm)
396, 421
454, 492
5.66
e
a
Tg (ꢀC)
b
b
film
Material
m.p.a (ꢀC)
TD1% (ꢀC)
TD5% (ꢀC)
phos
film
442
471, 500
BIPC
175
133c
197
202
89
77
95
92
300
384
402
405
393
441
451
459
Ip (eV)
Eg (eV)
ELUMO (eV)
5.68
3.12
ꢂ2.56
5.45
e
e
e
BIPC1
BIPC2
BIPC3
3.12
3.14
3.13
ꢂ2.54
ꢂ2.38
ꢂ2.32
abs
PL
phos
e absorption, e photoluminescence,
e phosphorescence,
e dilute so-
sol.
a
Established by DSC.
Established by TGA.
Decomposition of acetone solvate.
lutions, film e films, on e onset of absorption of films, Ip e ionization potential of solid
state (attributed to EHOMO), Eg e optical band-gap (evaluated according to laonbs), * e
b
c
shoulder. LUMO levels were calculated on assuming that Eg ¼ EHOMO ꢂ ELUMO
.

J. Keruckas et al. / Dyes and Pigments 100 (2014) 66e72
69
3.2. Thermal properties
The ability of glass formation of BIPCs was explored by DSC. All
the synthesized 3,6-bis(indol-1-yl)-9-phenylcarbazoles were iso-
lated after the synthesis and purification as crystalline materials
and exhibited well-defined melting signals in the first DSC heating
scans. When the melted samples were cooled down during DSC
experiments, only the glass transitions were observed with no
signals of crystallization of the substances. When the obtained
glassy samples were heated again, only glass-to-liquid transitions
were observed. For the illustration of the above stated, DSC curves
of BIPC2 are shown in Fig. 1. The values of glass transition tem-
peratures (Tg) are given in Table 1.
The established Tg values for BIPCs are situated around 90 ꢀC
except that of BIPC1, the Tg of which was found to be lower by
more than 10 ꢀC in comparison to those of the rest of the mate-
rials. The presence of methoxy substituents at the C-5 positions of
indole moieties slightly increases glass-transition temperatures of
the derivatives. The introduction of methoxy group apparently
elongates indole moiety in the direction across its spinning axis
(CeN bond) and the rotation of this moiety becomes slightly
hindered in bulk material which results in the higher Tg of BIPC2
(95 ꢀC) among these derivatives. In the case when methoxy sub-
stituent is present in the C-4 position of phenyl ring, i.e. along its
spinning axis (CeN bond), it apparently makes this structure more
flexible and decreases the Tg of BIPC1 down to 77 ꢀC. As the both of
these effects take place in the molecule of BIPC3, the Tg of this
material is close to the Tg of non-substituted BIPC (92 and 89 ꢀC
respectively).
Fig. 3. Electron photoemission spectra of BIPCs films.
yields of 65e79% after all the purification procedures. A three-step
synthetic route which is presented in Scheme 1 included N-aryla-
tion of 9H-carbazole (1) followed by the iodination of N-arylcar-
bazoles 3aeb. Diiodo-derivatives 4aeb were obtained in good
yields and they were further used for CeN coupling reaction with
indoles 5aeb to synthesize four new 3,6-bis(indol-1-yl)-9-
phenylcarbazoles, referred as BIPC, BIPC1, BIPC2 and BIPC3 (the
number was added to the abbreviation according to the amount of
methoxy substituents in the molecule).
The obtained materials were found to be well-soluble in com-
mon organic solvents such as tetrahydrofuran and chloroform.
Thermal stability of 3,6-bis(indol-1-yl)-9-phenylcarbazoles was
examined by TGA. Their 5% weight loss temperatures (TD5%) range
Fig. 4. Electroluminescence spectra of single-layer devices (a); current densityevoltage and luminance-voltage characteristics (b) and current efficiencyecurrent density char-
acteristics and electroluminescent quantum yields (c) of single layered devices.

70
J. Keruckas et al. / Dyes and Pigments 100 (2014) 66e72
from 393 to 459 ꢀC (Table 1). The thermal stability of BIPC having
no methoxy groups is inferior with respect of that of methoxy-
substituted derivatives. The introduction of methoxy substituents
increases 5% weight loss temperatures by ca. 50e60 ꢀC. To our
knowledge, there are few reports considering low-molar-mass
methoxy-substituted tertiary aromatic amines and non-
substituted their analogs [19e22]. Yet, the thermal stability of the
materials was discussed in neither of the referred papers. We
suppose that the initial loss of weight is more determined by
evaporation of a substance than by chemical decomposition. A
sudden loss of weight occurs only above 350 ꢀC or even 400 ꢀC,
when the samples are well above their melting points (Table 1). A
faster loss of weight in the initial phase (up to 300e350 ꢀC) is
characteristic of non-substituted derivative (BIPC), while the
methoxy-substituted counterparts of BIPC lose only less than 1% of
their weight up to 350 ꢀC (compare TD1% and TD5% values in Table 1).
An introduction of methoxy substituents apparently enhances
intermolecular interactions due to enhanced polarity of the mole-
cules, which results into lower evaporation rate of the melts as well
as of decomposition products.
similar. They are characterized by four peaks at around 250, 275, 300
and350 nm. Thelowest-energyabsorption bands and theabsorption
edges of the films exhibit small red-shifts with respect of those of the
dilute solutions which can be attributed to the enhanced intermo-
lecular interactions in condensed material. The influence of methoxy
substituents on the position and intensity of the lowest-energy ab-
sorption band (ca. 350 nm) as well as to thewavelengthof absorption
edge is negligible. The onset absorption (which is determined by
S0 / S1 transitions) of all the derivatives is observed at around
385 nm for dilute solutions and near 400 nm for the films. The latter
value corresponds to the optical band-gap Eg of ca. 3.1 eV. PL spectra
of the dilute solutions of 3,6-bis(indol-1-yl)-9-phenylcarbazoles are
characterized by two well-resolved vibrational peaks at ca. 390 nm
and 405 nm. Since molecular fragments of carbazole and indole
exhibit emission below 360 nm [23,24] the observed fluorescence
band at 390 nm can be attributed to the excitation of the whole
molecules. The solid films of 3,6-bis(indol-1-yl)-9-phenylcarbazoles
also exhibit well-resolved vibrational peaks, however the wave-
lengthofmaximaareatca. 400nmand415nm. Thesered-shiftsofup
to 10 nm in the fluorescence of films in respect to those of the solu-
tions are apparently caused both by the negligible reabsorption of
fluorescence and by the slight reciprocal influence of neighboring
molecules in the films [25]. The above presented analysis of UV and
fluorescence spectra of 3,6-bis(indol-1-yl)-9-phenylcarbazoles show
that absorption and fluorescence centers of these compounds are
mainly the same both in the solutions and in the films.
3.3. Optical and photophysical properties
Absorption and photoluminescence (PL) spectra of the solu
tions and vacuum deposited films of 3,6-bis(indol-1-yl)-9-
phenylcarbazoles are shown in Fig. 2. The wavelengths of absorp-
tion and PL maxima are summarized inTable 2. Absorption spectra of
the dilute solutions of 3,6-bis(indol-1-yl)-9-phenylcarbazoles are
The phosphorescence spectra of the films of 3,6-bis(indol-1-yl)-
9-phenylcarbazoles recorded at 77 K exhibit emission peaks that
Fig. 5. Electroluminescence spectra of electrophosphorescent devices ITO/CuI/BIPCs:Ir(Fppy)3/TCz1/Ca/Al (a), chemical structure of Ir(Fppy)3 (b), CIE chromaticity diagram and
photo of the electrophosphorescent device based on BIPC3:Ir(Fppy)3 (c).

J. Keruckas et al. / Dyes and Pigments 100 (2014) 66e72
71
can be attributed to a sandwich excimer of adjacent carbazole
moieties [26]. High triplet-levels (ca. 2.7 eV) of BIPCs give a pos-
sibility of using these materials as hosts for blue PHOLEDs [27].
Ionization potentials (Ip) of the solid films of BIPC and its
methoxy-substituted derivatives were estimated from electron
photoemission spectra, which are presented in Fig. 3. The values of
Ip are given in Table 2. The effect of methoxy-substitution at the
phenyl ring on the energy levels of BIPC derivatives seems to be
negligible as the Ip values of BIPC and BIPC1 are, in essence, the
same. The values of Ip of BIPC2 and BIPC3 are also close, however
somewhat lower (by ca. 0.2 eV) than those of BIPC and BIPC1
apparently because of methoxy-substitution at indole moieties.
3.4. Electroluminescent single-layer devices
The electroluminescence (EL) spectra of the single-layer devices
(Fig. 4a) based on BIPC1 and BIPC2 are identical to the PL spectra of
the films, while EL spectra of the devices based on BIPC and BIPC3
are broadened as compared to the PL spectra of the films of the
corresponding compounds. EL spectra of the devices based on BIPC
and BIPC3 are characterized by tree well-defined narrow peaks.
Two high energy peaks correspond to those of fluorescence spectra
of the films of BIPC and BIPC3 (Fig. 2b, Table 2) while the third one
is due to excimer emission. Excimers are formed due to resonance
interactions of a molecular exciton with the neighboring non-
excited molecules [28]. Fig. 4b and c shows characteristics of the
single-layer electroluminescent structures ITO/CuI/BIPCs/Ca/Al.
The current densityevoltage curves of the devices show turn-on
voltage Von of 4.0e6.2 V and the devices exhibit current effi-
ciency values of 3.7e4.1 cd/A, which are comparable to those of the
similar devices [29].
3.5. 3,6-Bis(indol-1-yl)-9-phenylcarbazoles as hosts for
phosphorescent light-emitting diodes
Fig. 6. Current density-voltage and luminance-voltage characteristics (a) and current
efficiency e current density characteristics and electroluminescent quantum yields (b)
for electrophosphorescent devices ITO/CuI/BIPCs:Ir(Fppy)3/TCz1/Ca/Al.
In organic phosphorescent light emitting devices (PHOLEDs),
host materials of the emitting layers are of great importance [11].
The host materials used for the transition metal-ligand complexes
have to show good charge-transporting properties and high triplet
energy that allows efficient exciton confinement on the guest
molecules [10]. Because of favorable optical and electroluminescent
properties of BIPCs we have used these materials as hosts for
PHOLEDs with the commercial Ir(Fppy)3 (Fig. 5b) guest phosphor.
The triplet energy of 3,6-bis(indol-1-yl)-9-phenylcarbazoles is
more than by 0.1 eV larger than that of Ir(Fppy)3 [30]. The fluo-
rescence of BIPCs partially overlap with the absorption band of
Ir(Fppy)3 (Fig. 2, Table 1). Thus the efficient Förster energy transfer
is expected to take place from BIPCs to Ir(Fppy)3 [31]. Electrolu-
minescence maxima of the devices based on BIPCs:Ir(Fppy)3
(Fig. 5a) are observed at 476 nm and 500 nm The shape of the
spectra of electroluminescence is similar to that of PL spectra of the
solid films of Ir(Fppy)3 [30,32]. This observation confirms that
electrophosphorescence is naturally the emission of the devices.
The current densityevoltage curves of the devices show turn-on
voltage Von of 6.8e8.0 V and the devices exhibit current effi-
ciency values of 10.8e12.6 cd/A (Fig. 6). CIE chromaticity co-
ordinates (x, y) of the devices ITO/CuI/BIPCs:Ir(Fppy)3/TCz1/Ca/Al
exhibiting blue-to-green color were found to be (0.23, 0.44)
(Fig. 5c). Though the emitting color of the device was not deep-
blue, it is expected to be useful in other applications such as
blue-green displays and lighting [33]. Lower turn-on voltage of
devices based on BIPC2 and BIPC3 can be explained by lower
ionization potentials (Ip) of the solid films and respectively lower
potential barrier for holes. However the lower current efficiency
was observed for the devices based on BIPC2 and BIPC3 apparently
due to higher current density caused by non-radiative recombina-
tion [34].
4. Summary
In summary, we have synthesized 3,6-bis(indol-1-yl)-9-
phenylcarbazole and three its methoxy-substituted derivatives
and studied the properties of the synthesized compounds. They
exhibit high thermal stability with the 5% weight loss temperatures
ranging from 393 to 459 ꢀC. All the compounds prepared form
glasses with the glass transition temperatures ranging from 77 to
95 ꢀC. 3,6-Bis(indol-1-yl)-9-phenylcarbazoles were tested in the
single-layer organic light emitting diodes as active materials and in
phosphorescent light emitting diodes as host materials of emitting
layers in combination with Ir(Fppy)3 as a guest phosphor. The
maximum brightness of the fabricated single-layer electrolumi-
nescent devices was ca. 1000 cd/m2 with current efficiency of
4.0 cd/A and that of electrophosphorescent devices was ca.
11,000 cd/m2 with current efficiency of 12.2 cd/A.
Acknowledgments
This research was funded by the European Social Fund under the
Global Grant measure. We thank habil. dr. V. Gaidelis and dr. V.
Jankauskas (Department of Solid State Electronics, Vilnius Univer-
sity, Lithuania) for the help in estimation of the photoelectrical
properties.

72
J. Keruckas et al. / Dyes and Pigments 100 (2014) 66e72
[18] Gauthier S, Frechet JMJ. Phase-transfer catalysis in the Ullmann synthesis of
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