Derivatives of 2-phenylindole and carbazole as host materials for phosphorescent organic light emitting diodes

Article abstract of DOI:10.1016/j.dyepig.2016.09.038

Synthesis and thermal, electrochemical, photophysical and charge-transporting properties of the derivatives of carbazole and 2-phenylindole are reported. Compounds with reactive functional groups are emphasized. Two compounds form molecular glasses with t

Full text of DOI:10.1016/j.dyepig.2016.09.038

Accepted Manuscript
Derivatives of 2-phenylindole and carbazole as host materials for phosphorescent
organic light emitting diodes
Rasa Keruckiene, Dmytro Volyniuk, Jolita Ostrauskaite, Laura Peciulyte, Juozas V.
Grazulevicius, Sergei V. Kostjuk, Algirdas Lazauskas
PII:
S0143-7208(16)30738-0
10.1016/j.dyepig.2016.09.038
DYPI 5490
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 27 June 2016
Revised Date: 13 September 2016
Accepted Date: 15 September 2016
Please cite this article as: Keruckiene R, Volyniuk D, Ostrauskaite J, Peciulyte L, Grazulevicius
JV, Kostjuk SV, Lazauskas A, Derivatives of 2-phenylindole and carbazole as host materials
for phosphorescent organic light emitting diodes, Dyes and Pigments (2016), doi: 10.1016/
j.dyepig.2016.09.038.
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to
our customers we are providing this early version of the manuscript. The manuscript will undergo
copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please
note that during the production process errors may be discovered which could affect the content, and all
legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT
DERIVATIVES OF 2-PHENYLINDOLE AND CARBAZOLE AS HOST MATERIALS FOR
PHOSPHORESCENT ORGANIC LIGHT EMITTING DIODES
Rasa Keruckiene1, Dmytro Volyniuk1, Jolita Ostrauskaite1, Laura Peciulyte1, Juozas V.
Grazulevicius1*, Sergei V. Kostjuk2, Algirdas Lazauskas3
1Department of Polymer Chemistry and Technology, Kaunas University of Technology, Radvilenu
pl.19, LT-50254 Kaunas, Lithuania
2Research Institute for Physical Chemical Problems of the Belarusian State University, 14
Leningradskaya st., 220030 Minsk, Belarus.
3Institute of Materials Science, Kaunas University of Technology, Barsausko 59, LT-51423,
Kaunas, Lithuania
Abstract
Synthesis and thermal, electrochemical, photophysical and charge-transporting properties of the
derivatives of carbazole and 2-phenylindole are reported. Compounds with reactive functional
groups are emphasized. Two compounds form molecular glasses with the glass transition
temperatures of 57 οC and 134 οC. The synthesized compounds absorb electromagnetic radiation in
the range of 200–375 nm and emit in the range of 350–550 nm. Their solutions exhibit Stokes shifts
up to 81 nm. Their triplet energy levels were found to be in the range of 2.88–3.04 eV. The
ionization potentials of the synthesized compounds were found to range from 5.45 eV to 5.88 eV.
The ability of photopolymerization in the solid state of the synthesized monomers was
demonstrated by ATR-FTIR spectroscopy. The charge transporting properties were studied by the
space-charge-limited current (SCLC) method. The zero-field hole mobilities reaching 1.97×10-5
cm2V-1s-1 were observed. According to the results of characterization, two compounds were selected
for studying them as the hosts in blue and green phosphorescent organic light emitting diodes. The
* Corresponding author. Tel.: +370 37 300193; fax: +370 37 300152. Email: juozas.grazulevicius@ktu.lt
1

ACCEPTED MANUSCRIPT
best fabricated device consisting of indium tin oxide anode, hole-transporting layer, emitting layer
with 3-((1-(4-vinylbenzyl)-2-phenyl-1H-indol-3-yl)methyl)-9-ethyl-9H-carbazole as a host and the
green emitter, electron transporting layer and calcium layer topped with aluminum layer as cathode,
exhibited the maximum current, power, and external quantum efficiencies of 10.3 cd/A, 7.2 lm/W,
2.9 %, respectively, in the absence of light out-coupling enhancement. The devices based on the
twin derivatives of 2-phenylindolylcarbazolylmethane demonstrated the relatively low values of the
turn-on voltages of 3.7 and 3.1 V as well as the efficiency roll-offs of 12.5 and 22.7 %.
Keywords: indole, carbazole, phosphorescence, PHOLED.
Introduction
Phosphorescent organic light emitting diodes (OLEDs) are a prime focus of organic light
emitting device research due to their high light-emitting efficiency efficiencies [1]. The efficiencies
of light emission from fluorescent OLEDs are limited compared to that from OLEDs with
phosphorescent (triplet) emitters. In a device without triplet emitters only the singlet excitons can be
as high as 100% since the triplet emitters can harvest both singlet and triplet excitons [2]. The
selection of high-triplet-energy host materials is of great importance for the preparation of efficient
phosphorescent OLEDs [3]. Effective charge transport, good thermal stability, high triplet energy,
capability of glass formation are the reasons why electron-rich carbazole moieties are widely used
in the design of electroactive compounds [4,5]. Derivatives of carbazole with electronically isolated
electroactive groups exhibit high triplet energies [6]. They showed good performance as host
materials of blue phosphorescent OLEDs [7,8]. Derivatives of indole were also reported to exhibit
high triplet energies [9]. However, their applicability in phosphorescent OLEDs is much less
explored.
2

ACCEPTED MANUSCRIPT
The strategy of this work was to combine carbazole and indole moieties in the design and
synthesis of host materials for phosphorescent OLEDs exhibiting high triplet energy values, charge-
transporting and glass-forming abilities. A series of new derivatives of 2-phenylindole and
carbazole were synthesized and studied. The materials with the most suitable sets of the properties
were tested as host materials in green phosphorescent light emitting diodes.
Experimental
Materials
2-Phenylindole-3-carbaldehyde, 2-phenylindole, potassium tert-butoxide, iron (III) chloride, zinc,
titanium tetrachloride, sodium carbonate, tetrabutylammonium hexafluorophosphate,
benzyltrimethylamonnium
chloride
(BTMAC),
4-vinylbenzylchloride,
bromoethane,
epichlorohydrin, 9-ethylcarbazole, cyclopentadienyl(fluorene)iron(II)hexafluorophosphate, boron
trifluoride diethyl etherate (all from Aldrich), 9H-carbazole (Reakhim) were used as received. 3-
Hydroxymethyl-2-phenylindole (HMPI) was synthesized as described before [10].
3-((9H-Carbazol-3-yl)(2-phenyl-1H-indol-3-yl)methyl)-9H-carbazole (CPIC). To a mixture of 9H-
carbazole (5 g, 30 mmol) and 2-phenylindole-3-carbaldehyde (3.3 g, 15 mmol) in chloroform
(40mL), hydrochloric acid (conc., 5 ml) was added dropwise and the reaction mixture was stirred at
room temperature for 8 h. Then, water (10 mL) was added to quench the reaction and the mixture
was stirred vigorously for the additional 10 min. The crude product was extracted with chloroform
several times. The chloroform solution was washed with water, dried with anhydrous sodium
sulphate, filtered and the solvent was evaporated. The product was purified by silica gel column
chromatography using hexane:ethylacetate (1:6) as an eluent. Yield of reddish powder was 4.5 g (56
1
%). Mw= 537 g/mol. C39H27N3. H NMR (400 MHz, CDCl3) δ 8.34 (s, 3H, NH), 8.23 – 7.72 (m,
9H, Ar.), 7.60 – 7.48 (m, 14H, Ar.), 5.41 (s, 1H, Ar-CH). 13C NMR (101 MHz, CDCl3) δ 139, 126,
124, 119, 50.
3

ACCEPTED MANUSCRIPT
Bis(2-phenyl-1H-indol-3-yl)methane (BPIM). To a mechanically stirred solution of 3-
hydroxymethyl-2-phenylindole (1.3 g 5.8 mmol) and 2-phenylindole (1.12 g, 5.8 mmol) in dry
dichloromethane (40 ml), boron trifluoride diethyl etherate (3.2 ml) was added dropwise and the
reaction mixture was stirred at room temperature for 3 h. The crude product was extracted with
dichloromethane several times. The dichloromethane solution was washed with water, dried with
anhydrous sodium sulphate, filtered and the solvent was evaporated. The product was purified by
silica gel column chromatography using hexane:ethylacetate (1:9) as an eluent. Yield of greenish
1
powder was 1.1 g (62 %). Mw=398 g/mol. C29H22N2. H NMR (400 MHz, CDCl3) δ 8.33 (s, 2H,
NH), 8.00 – 7.74 (m, 18H, Ar.), 5.41 (s, 2H, Ar-CH). 13C NMR (101 MHz, CDCl3) δ 141, 137, 129,
127, 126, 123, 121, 119, 22.
3-((2-Phenyl-1H-indol-3-yl)methyl)-9H-carbazole (PIMC). To a mechanically stirred solution of 3-
hydroxymethyl-2-phenylindole (1.3 g 5.8 mmol) and 9-ethylcarbazole (1.5 g, 5.8 mmol) in dry
dichloromethane (40 ml), boron trifluoride diethyl etherate (3.2 ml) was added dropwise and the
reaction mixture was stirred at room temperature for 3 h. The crude product was extracted with
dichloromethane several times. The dichloromethane solution was washed with water, dried with
anhydrous sodium sulphate, filtered and the solvent was evaporated. The product was purified by
silica gel column chromatography using hexane:ethylacetate (1:9) as an eluent. Yield of greenish
1
powder was 1.3 g (58 %). Mw=400 g/mol. C29H24N2. H NMR (400 MHz, CDCl3) δ 10.01 (s, 1H,
NH), 8.00 – 7.74 (m, 9H), 7.43 – 7.15 (m, 7H), 5.69 (s, 2H, Ar-CH), 4.04 (q, 2H, J = 7.1 Hz, CH2),
13
1.32 (t, 3H, J = 7.1 Hz, CH3). C NMR (101 MHz, CDCl3) δ 141, 137, 133, 128, 121, 118, 112,
111, 108, 37, 30, 14.
9-(4-Vinylbenzyl)-3-((1-(4-vinylbenzyl))-2-phenyl-indol-3-yl)(9-(4-vinylbenzyl)-carbazol-
3-yl)methyl)-carbazole (1) was prepared by the procedure similar to that described elsewhere[11].
To a mechanically stirred mixture of 3-((9H-carbazol-3-yl)(2-phenyl-1H-indol-3-yl)methyl)-9H-
carbazole (CPIC) (0.7 g, 1.3 mmol), dimethyl sulphoxide (DMSO) (10 ml), potassium tert-
4

ACCEPTED MANUSCRIPT
butoxide (0.04 g, 0.4 mmol), BTMAC (0.04 g, 0.01 mmol) and 4-vinylbenzylchloride (1 ml, 7.8
mmol) were added. The resulting mixture was stirred at room temperature for 2 h. The reaction was
stopped by adding water and neutralized with 10% HCl (8 ml) to pH 6–7. The crude product was
extracted with chloroform several times (50 ml×3). The chloroform solution was washed with
water, dried with anhydrous sodium sulphate, filtered and the solvent was evaporated. The product
was purified by silica gel column chromatography using hexane as an eluent. Yield of reddish
1
powder was 0.86 g (75 %). Mw= 886 g/mol. C66H51N3. H NMR (400 MHz, CDCl3) δ 8.23 – 7.72
(m, 9H, Ar.), 7.60 – 7.48 (m, 14H, Ar.), 7.21 – 7.07 (m, 12H), 6.62 (m, 3H, AMX system CH=CH2
proton HA), 5.67 (dd, 3H, AMX system -CH=CH2 proton HM trans JAM=5.75 Hz and gem JMX=5.80
Hz), 5.41 (s, 1H, Ar-CH), 5.18 (d, 3H, AMX system of -CH=CH2 proton HX cis JAX=11 Hz), 3.39
13
(s, 6H, CH2). C NMR (101 MHz, CDCl3) δ 139, 125, 123, 119, 110, 50, 45. FT-IR (KBr) cm–1:
3050 (ν C–H Ar), 2919 (ν C–H aliph.), 1450 (ν C=C Ar), 1363 (ν C–N Ar), 995, 928 (C-H of vinyl
group), 841, 820, 745 (γ C–H Ar). Elemental analysis for C66H51N3. % Calc.: C, 89.46; H, 5.80; N,
4.74; % Found: C, 89.41; H, 5.85; N, 4.69. MS: m/z 886 [M+].
9-((Oxiran-2-yl)methyl)-3-((1-((oxiran-2-yl)methyl)-2-phenyl-indol-3-yl)(9-((oxiran-2-yl)methyl)-
carbazol-3-yl)methyl)-carbazole (2). A solution of 3-((9H-carbazol-3-yl)(2-phenyl-1H-indol-3-
yl)methyl)-9H-carbazole (CPIC) (0.7 g, 1.3 mmol) potassium in epichlorohydrin (4 ml, 51 mmol)
was stirred at 120 °C for 24 h under argon atmosphere. Then epichlorhydrin was removed by
distillation The product was purified by silica gel column chromatography using hexane as an
eluent. It was recrystallized from methanol. Yield of reddish crystals was 0.74 g (81 %). M.p. 110
1
oC (DSC). Mw= 705 g/mol. C48H39N3O3. H NMR (400 MHz, CDCl3) δ 7.88 (d, 9H, J = 7.8 Hz,
Ar.), 7.28 – 7.22 (m, 7H, Ar.), 7.1-7.0 (m, 7H, Ar.), 5.07 (s, 1H, Ar-CH), 4.40 (dd, 6H, J = 15.8, J =
3.4 Hz, CH2), 4.18 (dd, 4H, J = 15.8, J =8.1 Hz, CH2), 3.12 (d, 2H, J = 6.4Hz, CH2), 2.57 (t, 3H, J =
4.1 Hz, CH).13C NMR (101 MHz, CDCl3) δ 140, 125, 123, 119, 109, 50, 45. FT-IR (KBr) cm–1:
3052 (ν C–H Ar), 2920 (ν C–H aliph.), 1453 (ν C=C Ar), 1351 (ν C–N Ar), 1220 (ν C–O–C), 841,
5

ACCEPTED MANUSCRIPT
779 (γ C–H Ar). Elemental analysis for C48H39N3O3. % Calc.: C, 81.68; H, 5.57; N, 5.95; O, 6.80;
% Found: C, 81.63; H, 5.52; N, 5.90; O, 6.85. MS: m/z 706 [(M+H)+].
9-Ethyl-3-((1-ethyl-2-phenyl-1H-indol-3-yl)(9-ethyl-9H-carbazol-3-yl)methyl)-9H-carbazole (3).
To a mechanically stirred mixture of 3-((9H-carbazol-3-yl)(2-phenyl-1H-indol-3-yl)methyl)-9H-
carbazole (CPIC) (0.7 g, 1.3 mmol), dimethylsulfoxide (10 ml), potassium tert-butoxide (0.44 g,
3.9 mmol), BTMAC (0.04 g, 0.01 mmol) and bromoethane (0.58 ml, 7.8 mmol) were added. The
resulting mixture was stirred at room temperature for 2 h. The reaction was stopped by adding water
and neutralized with 10% HCl (8 ml) to pH 6–7. The crude product was extracted with chloroform
several times (50 ml×3). The chloroform solution was washed with water, dried with anhydrous
sodium sulphate, filtered and the solvent was evaporated. The product was purified by silica gel
column chromatography using hexane as an eluent. Yield of reddish powder was 0.53 g (66 %).
Mw= 621 g/mol. C45H39N3. 1H NMR (400 MHz, CDCl3) δ 7.89 (d, 9H, J = 7.8 Hz, Ar.), 7.38 – 7.29
(m, 7H, Ar.), 7.2 – 7.11 (m, 7H, Ar.), 5.20 (s, 1H, Ar-CH), 4.02 (q, 6H, J = 7.3 Hz, CH2), 1.32 (t,
9H, J = 7.1 Hz, CH3). 13C NMR (101 MHz, CDCl3) δ 139, 125, 123, 119, 110, 50, 45. FT-IR (KBr)
cm–1: 3052 (ν C–H Ar), 2925 (ν C–H aliph.), 1492, 1450 (ν C=C Ar), 1358 (ν C–N Ar), 842, 745 (γ
C–H Ar). Elemental analysis for C45H39N3. % Calc.: C, 86.92; H, 6.32; N, 6.76; % Found: C, 86.87;
H, 6.37; N, 6.71. MS: m/z 635 [(M+Na)+].
1-(4-Vinylbenzyl)-3-((1-(4-vinylbenzyl)-3,7-dihydro-2-phenyl-1H-indol-3-yl)methyl)-2-phenyl-
1H-indole (4). To a mechanically stirred solution of bis(2-phenyl-1H-indol-3-yl)methane (BPIM)
(1.5 g, 3.7 mmol) in dimethylsulfoxide (10 ml), potassium tert-butoxide (1.26 g, 11.3 mmol),
BTMAC (0.04g, 0.01 mmol) and 4-vinylbenzylchloride (2.1 ml, 15.6 mmol) were added. The
resulting mixture was stirred at room temperature for 2 h. The reaction was stopped by adding water
and neutralized with 10% HCl (8 ml) to pH 6–7. The crude product was extracted with chloroform
several times (50 ml×3). The chloroform solution was washed with water, dried with anhydrous
sodium sulphate, filtered and the solvent was evaporated. The product was purified by silica gel
6

ACCEPTED MANUSCRIPT
column chromatography using hexane as an eluent. Yield of greenish crystals was 1.7 g (74 %).
1
M.p. 120oC (DSC). Mw=632 g/mol. C47H40N2. H NMR (400 MHz, CDCl3) δ 8.05 (d, 8H, J = 7.8,
Ar.), 8.00 – 7.74 (m, 18H, Ar.), 6.62 (m, 2H, AMX system CH=CH2 proton HA), 5.61 (dd, 2H
AMX system -CH=CH2 proton HM trans JAM=16.2 Hz and gem JMX=8.40), 5.41 (s, 2H, Ar-CH),
5.26 (d, 2H, AMX system of -CH=CH2 proton HX cis JAX=12.7 Hz), 5.14 – 5.07 (m, 2H, AMX
13
system of -CH=CH2 proton HX), 3.38 (s, 4H, CH2). C NMR (101 MHz, CDCl3) δ 140, 137, 128,
127, 125, 123, 121, 116, 46, 41, 23. FT-IR (KBr) cm–1: 3052 (ν C–H Ar), 2927 (ν C–H aliph.),
1485, 1450 (ν C=C Ar), 1381 (ν C–N Ar), 994, 969, 927 (C-H of vinyl group), 842, 785, 746 (γ C–
H Ar). Elemental analysis for C47H40N2. % Calc.: C, 89.20; H, 6.37; N, 4.43; % Found: C, 89.15; H,
6.41; N, 4.48. MS: m/z 633 [(M+H)+].
3-((1-(4-Vinylbenzyl)-2-phenyl-1H-indol-3-yl)methyl)-9-ethyl-9H-carbazole
(5).
To
a
mechanically stirred solution of 3-((2-phenyl-1H-indol-3-yl)methyl)-9H-carbazole (PIMC) (2 g, 5
mmol) in dimethylsulfoxide (10 ml), potassium tert-butoxide (1.68 g, 15 mmol), BTMAC (0.04g,
0.01 mmol) and 4-vinylbenzylchloride (1.4 ml, 10 mmol) were added. The resulting mixture was
stirred at room temperature for 2 h. The reaction was stopped by adding water and neutralized with
10% HCl (8 ml) to pH 6–7. The crude product was extracted with chloroform several times (50
ml×3). The chloroform solution was washed with water, dried with anhydrous sodium sulphate,
filtered and the solvent was evaporated. The product was purified by silica gel column
chromatography using hexane as an eluent. Yield of greenish powder was 1.7 g (68 %). Mw=516
1
g/mol. C38H32N2. H NMR (400 MHz, CDCl3) δ 8.13 – 8.01 (m, 2H), 8.00 – 7.74 (m, 9H), 7.43 –
7.15 (m, 7H), 6.69 – 6.51 (m, 1H, AMX system CH=CH2 proton HA), 5.69 (s, 2H, Ar-CH), 5.66 –
5.54 (m, 1H, AMX system of -CH=CH2 proton HX cis), 5.22 – 5.05 (m, 1H, AMX system of -
CH=CH2 proton HX), 4.04 (q, 2H, J = 7.1 Hz, CH2), 3.40 (s, 2H), 1.32 (t, 3H, J = 7.1 Hz, CH3). 13C
NMR (101 MHz, CDCl3) δ 141, 137, 130, 129, 125, 121, 120, 116, 111, 38, 22. FT-IR (KBr) cm–1:
3052 (ν C–H Ar), 2925 (ν C–H aliph.), 1480, 1452 (ν C=C Ar), 1359 (ν C–N Ar), 993, 969, 926 (C-
7

ACCEPTED MANUSCRIPT
H of vinyl group), 842, 745 (γ C–H Ar). Elemental analysis for C38H32N2. % Calc.: C, 88.34; H,
6.24; N, 5.42; % Found: C, 88.394; H, 6.29; N, 5.38. MS: m/z 540 [(M+Na)+].
Photopolymerization of monomers 1 and 4 was performed using an UV lamp OmniCure®
S2000 (Lumen Dynamics). The solution of a monomer containing 3 mol % of photoinitiator
cyclopentadienyl(fluorene)iron(II)hexafluorophosphate was drop-casted on the surface of ATR
crystal and the decrease of the intensity of a signal of reactive functional group under exposure of
UV radiation source was monitored.
Polymerization monomer 5 was carried out in the solution of dry dichloromethane (0.5
mol/l) under argon blanket. After dissolving the monomer, initiator BF3·(C2H5)2O (0.15 mol/l) was
added dropwise. The reaction was carried out for 3 days and monitored by TLC (eluent:
dichloromethane). The initiator was neutralized with NH3
and the reaction product was
(aq)
precipitated in methanol. The precipitate was washed with methanol several times and dried.
Instrumentation
1H and 13C NMR spectra were obtained using a Bruker Avance III (400 MHz). The data are
given as chemical shifts (δ) in ppm against trimethylsilane (in parenthesis: multiplicity, integration,
coupling constant). IR spectra were recorded using a Vertex 70 Bruker spectrometer. Mass spectra
were obtained on a Waters ZQ 2000 mass spectrometer (electronspray ionization). Elemental
analyses were performed with an Exeter Analytical CE-440 Elemental Analyzer. UV-vis spectra of
the dilute THF solutions (10-4 mol/l) of the synthesized compounds were recorded with a Perkin
Elmer Lambda 35 spectrophotometer. Fluorescence spectra of the dilute tetrahydrofuran solutions
(10-4 mol/l) of the synthesized materials were recorded at room temperature and phosphorescence
spectra of the the dilute tetrahydrofuran solutions (10-4 mol/l) were recorded at 77 K with
luminescencent spectrometer Edinburgh Instruments FLS980 spectrometer (excitation wavelength
λex = 320 nm). Differential scanning calorimetry (DSC) measurements were carried out using a TA
Instruments Q2000 thermosystem. The samples were examined at a heating/cooling rate of 10
8

ACCEPTED MANUSCRIPT
οC/min under nitrogen atmosphere. Thermogravimetric analysis (TGA) was performed on a TA
Instruments Q50. The heating rate was 20 οC/min under nitrogen atmosphere. The melting points of
the compounds were determinated using an Electrothermal MEL-TEMP. Molecular weights of the
polymers were determined by gel chromatography using Waters SEC system with Waters 501 UV
detector and polystyrene standards.
The ionization potentials (Ip) were estimated by the electron photoemission in air method as
reported earlier [12,13].The samples for the measurements were prepared by casting chloroform
solutions of the compounds onto pre-cleaned indium tin oxide (ITO) coated glass substrates.
Cyclic voltammetry measurements were performed using a glassy carbon working electrode
(disk with the diameter of 2 mm) in a three-electrode cell using an Autolab Type potentiostat -
galvanostat. The measurements were carried out for the solutions in dry dichloromethane containing
0.1 M tetrabutylammonium hexafluorophosphate at 25 οC, scan rate 50 mV/s, sample concentration
10-3 M. The potentials were measured against Ag/AgNO3 as reference electrode. Platinium wire was
used as a counter electrode. The potentials were calibrated with the standard ferrocene/ferrocenium
(Fc/Fc+) redox system [14].
DFT calculations employing the B3LYP functional were performed with the Spartan’14
program [15]. Neutral, cationic and triplet state geometries of molecules in vacuum were optimized
from conformational analysis data using 6-31G (d,p) basis set as a starting point. The
HOMO/LUMO orbitals were calculated at the B3LYP/6-31G (d, p) level.
Phosphorescent OLEDs (PhOLEDs) were fabricated by thermal vacuum deposition under
the vacuum higher than 3·10−6 mBar. The host:guest emission layers were deposited by co-
deposition of host (m/m 90%) (compound 2 or 5) and dopant (m/m 10%) (tris[2-phenylpyridinato-
C2,N]iridium(III) (Ir(ppy)3)) from two different sources. The deposition rate of guest was of 0.1
Å/s, while the deposition rate of hosts was set 1.0 Å/s. A Keithley source meter 2400-C was utilized
for recording of the current density–voltage characteristics of the devices. The current density–
9

ACCEPTED MANUSCRIPT
luminance characteristics were estimated using a calibrated silicon photodiode with the 6517B
Keithley electrometer. Electroluminescence (EL) spectra were recorded by an Avantes AvaSpec-
2048XL spectrometer. The current, power and external quantum efficiencies were estimated
utilizing the current density, luminance, and EL spectra as reported earlier. [16]
Hole-only devices were fabricated for the space-charge-limited current (SCLC) experiments.
The indium tin oxide (ITO) coated glass substrates with a sheet resistance of 15 Ω/sq were cleaned
in acetone and isopropyl alcohol ultrasonic baths for ca. 5 min before depositing functional layers.
Organic and metal layers were vacuum-deposited using the same conditions as for preparation of
PhOLEDs. The thickness of thin films was monitored during the deposition using a quartz crystal
microbalance. The sample area was 6 mm2 with 7 devices per substrate.
The surface morphology of light-emitting layers was investigated by atomic force
microscopy (AFM). AFM experiments were carried out in air at room temperature using a
NanoWizardIII atomic force microscope (JPK Instruments), while the data were analyzed using
SurfaceXplorer and JPKSPM Data Processing software. The AFM images were collected using a
V-shaped silicon cantilever (spring constant of 3 N/m, tip curvature radius of 10.0 nm and the cone
angle of 20º) operating in a contact mode.
The X-ray diffraction measurements at grazing incidence (XRDGI) were performed using a
D8 Discover diffractometer (Bruker) with Cu K ( = 1.54 Å) X-ray source. Parallel beam geometry
with 60 mm Göbel mirror (X-ray mirror on a high precision parabolic surface) was used. This
configuration enables transforming the divergent incident X-ray beam from a line focus of the X-
ray tube into a parallel beam that is free of K radiation. Primary side also had a Soller slit with an
axial divergence of 2.5º. The secondary side had a LYNXEYE (1D mode) detector with an opening
angle of 2.16º and slit opening of 6.0 mm. Sample stage was a Centric Eulerian cradle mounted to
horizontal D8 Discover with a vacuum chuck (sample holder) fixed on the top of the stage. X-ray
generator voltage and current was 40.0kV and 40mA, respectively. The XRDGI scans of emitting
layers were performed in the range of 3-134.0º with a step size of 0.065º, time per step of 19.2 s and
10

ACCEPTED MANUSCRIPT
auto-repeat function enabled. The XRDGI scans were performed at incidence angle of 1.0º.
Processing of the resultant diffractograms was performed with DIFFRAC.EVA software.
Results and Discussion
Synthesis, thermal and photophysical properties
The routes of the synthesis of the derivatives of 2-phenylindole and carbazole are shown in
Scheme 1. In the first step, the mixture of 2-phenylindole-3-carbaldehyde and 9H-carbazole was
treated with HCl to obtain the intermediate compound CPIC. 2-Phenylindole-3-carbaldehyde was
reduced to 3-hydroxymethyl-2-phenylindole (HMPI) which was then condensed with 2-
phenylindole and 9-ethylcarbazole to obtain BPIM and PIMC respectively. The last step in the
synthesis of the target compounds was the introduction of the reactive functional groups by the
interaction of the intermediate products with the corresponding halides in the presence of potassium
tert-butoxide. All the target compounds were purified by column chromatography and characterized
by spectroscopic tools and elemental analysis. They were found to be soluble in common organic
solvents.
<Insert Scheme 1.>
Thermal properties of the derivatives were investigated by thermogravimetric (TG) analysis
and differential scanning calorimetry (DSC). The thermal characteristics of compounds 1–5 are
collected in Table 1. TG curves and differential thermogravimteric (DTG) curves are presented in
Fig.1a, b. The temperatures of the onsets of the thermal degradation of most of the compounds were
found to be rather moderate. They were found in the interval from 228 to 284 °C, while that of
compound 5 was considerably higher (404 °C).
<Insert Figure 1>
11

ACCEPTED MANUSCRIPT
Compounds 2 and 4 were isolated as crystalline substances. Their melting points were found to
be 110 °C and 120 °C, respectively. It was not possible to transform them to the glassy state by
cooling from the melts. Compounds 1, 3 and 5 were isolated as amorphous materials The glass
transition temperatures of compounds 3 and 5 were found to be 134 °C and 85 °C respectively. The
higher glass transition temperature of compound 3 can be explained by its higher molecular weight
which determines stronger intermolecular interaction in the solid state. We did not manage to detect
glass transition temperature of compound 1. In DSC heating scan it showed only sharp endothermic
signal at 246 oC due to sublimation.
<Insert Table 1>
UV-vis spectra of the dilute solutions of compounds 1, 4 and 5 representing two different
families of the derivatives of 2-phenylindole and carbazole are shown in Fig. 2. The wavelengths of
absorption maxima of the compounds (1–5) are summarized in Table 2.
<Insert Figure 2>
The derivatives absorb UV-vis radiation in the range from 200 to 375 nm. The absorption
bands observed in the region of 250–270 nm can be ascribed to the localized aromatic π-π*
transitions of 2-phenylindole moiety while those appeared in the region of 275–330 nm can be
assigned to a localized aromatic π-π* transition of carbazole species. The wavelengths of absorption
bands of the solutions of the compounds increase in the order 1 > 4 > 5. The optical band gap values
estimated from the absorption band edges of the solutions of compounds 1–5 are in the range from
3.29 to 3.46 eV. Compound 5 showed the lowest value of optical band gap (3.29 eV). These values
are lower than the theoretically calculated optical band gap values that range from 4.35 eV to 4.92
eV.
<Insert Table 2>
12

ACCEPTED MANUSCRIPT
Emission spectra recorded at room and at low (77 K) temperatures of the dilute solutions of
compounds 1–5 are shown in Fig.3. The wavelengths of photoluminescence (PL) intensity maxima
are summarized in Table 2.
<Insert Figure 3>
PL spectra are characterized by the single peak at ca. 359 nm for the solutions of compounds
1 and 3, at 363 nm for the solutions of compounds 2 and 4, and at 381 nm for the solution of
compound 5. Similarly, as in the case of absorption spectra, the wavelengths of fluorescence
intensity maxima of the solutions increase in the order of 1 > 4 > 5. The observed fluorescence can
be attributed to the relaxation of the excited states in which the whole molecules are involved. The
triplet energy levels established for the dilute solutions of the monomers from their
phosphorescence spectra at 77 K reach 3.02–3.04 eV for 2-phenylindolylcarbazolylmethane
derivatives 1–3 and 2.91 eV and 2.88 eV for compounds 4 and 5. These values are much higher
than those previously reported for other host materials such as indole-carbazole hybrids (2.62–2.99
eV) [17] and triindolylmethanes (2.97–2.99 eV) [9].
Electrochemical and electron photoemission properties
Electrochemical properties of the compounds were investigated by cyclic voltammetry (CV).
The resulting data are summarized in Table 2. As an example, cyclic voltammograms of compound
1 are shown in Fig. 4.
<Insert Figure 4>
All the studied compounds showed irreversible oxidation suggesting electrochemical
coupling. During the multiple scans new reversible oxidation peaks at around 0.65 V were observed
for all the compounds. It was found that the onset oxidation potential was much lower for the 2-
phenylindolylcarbazolylmethane derivative 5 than for 2-phenylindole twin compound 4 which may
be due to the strong electron donating origin of the carbazole moiety. The solid-state ionization
13

ACCEPTED MANUSCRIPT
potential (IPCV) and electron affinity (EACV) values were estimated from the oxidation onset
potentials (Eox onset vs.Fc) and optical band gaps (Egopt). The ionization potential values of compounds
1-5 established by CV were found to be in the short range of 5.50–5.84. The electron affinity values
ranged from 2.12 to 2.45 eV.
The ionization potentials of the solid samples of compounds 1–5 were estimated by the
electron photoemission spectrometry in air. Electron photoemission spectra are shown in Fig. 5 and
the results are summarized in Table 2.
<Insert Figure 5>
The values of ionization potentials recorded by electron photoemission spectrometry were
found to be close to those estimated by CV and ranged from 5.74 eV to 5.88 eV. Monomer 5
showed the lowest ionization potential (5.45 eV). This result correlates with UV spectrometry data
which showed the lowest value of Egopt for compound 5.
Results of computational study
Quantum chemical calculations were performed for compounds 1–5 using DFT/B3LYP/ 6-31
(d, p) method. The optimized structures revealed that the carbazole and 2-phenylindole moieties in
all the compounds are set in the different planes. Fig. 6 shows the molecular orbital plots of the
optimized structures of 1-5. π-Electrons in the HOMO orbitals of the 2-
phenylindolylcarbazolylmethane derivatives 1–3 and 5 are delocalized over all carbazole-indole
backbone, except phenyl- moiety, whereas the HOMO orbital of the indole twin derivative 4 is
delocalized over both indole moieties with the same exception of the phenyl group. The LUMO
orbitals of 1, 4 and 5 are delocalized over vinylbenzyl fragment, while those of compounds 2 and 3
are delocalized on carbazole moieties (Fig. 6). The smallest energy gap, which was experimentally
observed for compound 5, indicates the most extensive π-electron delocalization among the series.
Although DFT calculations show that delocalization of HOMO orbitals of the compound 5 are
14

ACCEPTED MANUSCRIPT
similar to that of compound 1, the absence of the additional carbazole moiety in 5 has impact on its
photophysical and photoelectrical properties.
<Insert Figure 6>
Polymerization
Photocross-linking of monomers 1 and 4 was studied by ATR-FTIR spectrometry. For the
measurements the solution of a monomer (0.5M) containing 3 mol % of photoinitiator
cyclopentadienyl(fluorene)iron(II)hexafluorophosphate was drop-casted on the surface of ATR-
FTIR analysis crystal, the solvent was vaporized by heating at 40 °C for 15 min and then the
monitoring of the decrease of the intensity of the absorption band of vinyl group under exposure of
UV radiation source was performed.
<Inset Figure 7>
Fig. 7 shows the fragments of FTIR spectra of the films of compounds 1(a) and 4 (b) at the
different stages of photocuring. They illustrate the decrease of the intensity of absorption bands of
vinyl groups at 997 cm-1 with the increase of the time of photocuring (photocurred for 15 min). The
values of the insoluble fractions of the obtained materials were found to be 76 % and 56 %,
respectively (estimated after the extraction of the samples with chloroform for 24h).
Polymerization of monomer 5 was performed in solution, and the product was characterized by
gel chromatography. The weight average relative molecular weight was determined to be 1870 and
number average molecular weight was of 1110. The thermal degradation temperature (474 °C) of
the product was determined by TG (see Fig. S3) and it was 70 °C degrees higher than the monomer
5 onset degradation temperature (404 °C). The glass transition temperature (120 °C) obtained from
the DSC curves is 62 °C degrees higher than the monomer glass transition temperature (58 °C).
Charge-transporting and device properties
15

ACCEPTED MANUSCRIPT
Charge-transporting properties of the solid layers of 2 and 5 were studied by the method of
space-charge-limited current (SCLC) [18]. The hole-only devices with the following architectures
were fabricated: ITO/m-MTDATA(20 nm)/ 2 or 5 (80 nm)/m-MTDATA(20 nm)/Al(60 nm). The
layer of m-MTDATA (4,4′,4′′-tris[3-methylphenyl(phenyl)amino]triphenylamine) was used as the
hole-injecting and electron-blocking layer. The current density vs. voltage characteristics of the
hole-only devices are plotted in Fig. 1. The values of hole mobility were estimated using the Mott-
Gurney law [18]:
9
V 2
d 3
JSCLC
=
µSCLC εε0
,
(1)
8
where JSCLC is the steady-state current density (SCLC), µSCLC is the SCLC mobility; V is applied
voltage; d is the film thickness,
ε
is the permittivity of the film (~3); ε0 is the absolute permittivity
of the free space.
<Insert Figure 8>
The similar current density vs. voltage dependencies were observed for compounds 2 and 5 (Fig.8).
Values of the zero-field hole mobility of 1.97×10-5 cm2V-1s-1 and 1.78×10-5 cm2V-1s-1 for 2 and 5,
respectively, were obtained by fitting the region (III), in which the SCLC occurred, of the
experimental current density vs. voltage curve with Eq. 1 (Fig. 8). The disagreement between the
experimental and fit curves can be explained by taking in account the existence of hole traps in 2
and 5 which can occur due to volume or morphology effects in solid state layers [19]. Additional
explanation of that can be the absent Ohmic contact, which is required for SCLC method, in the
hole-only and electron-only devices [18]. The region I is the Ohmic region while the region II can
be caused by injection-limited behavior, trap filling, or built-in voltage [18].
To study the performance of 2 and 5 as hosts, the simple PhOLEDs were fabricated. In the
devices with the structures ITO/m-MTDATA (25 nm)/2 or 5:Ir(ppy)3(15 nm)/Bphen (30 nm)/Ca:Al
(devices A and B, respectively), 4,4′,4′′-tris[phenyl(m-tolyl)amino]triphenylamine (m-MTDATA)
was used as the hole-transporting material and 4,7-diphenyl-1,10-phenanthroline (Bphen) was
16

ACCEPTED MANUSCRIPT
utilized as the electron-transporting material. To prepare the emitting layers 2:Ir(ppy)3 and
5:Ir(ppy)3 for the devices A and B, the hosts 2 and 5 were doped with tris[2-phenylpyridinato-
C2,N]iridium(III) (Ir(ppy)3) as a green phosphorescent emitter. Efficient exciton confinement on the
dopant Ir(ppy)3 was expected due to relatively high triplet levels of 3.02 eV and 2.88 for 2 and 5
(Table 2) [20].
The electroluminescence (EL) spectra of the devices A and B are shown in Figure 8. The
fabricated PhOLEDs were characterized by green emissions with the intensity maxima at ca. 510
nm, confirming the radiative recombination of excitons on Ir(ppy)3. However, low intensity blue
emissions with the intensity maxima at ca. 430 nm were observed in EL spectra of both PhOLEDs.
The wavelengths of these emissions were close to that of photoluminescence of m-MTDATA
showing that the recombination of excitons occurred not only in the emitting layers but also in the
hole-transporting layer of devices A and B. To avoid the recombination of excitons in the hole-
transporting layer, either additional electron-blocking layer or exciton-blocking layer are required
[21]. The similar shapes of EL spectra at the different applied voltages for both the devices were
observed. However, intensity of the blue emissions increased in the EL spectra of the PhOLEDs
with the increase of the applied voltage. The CIE coordinates were (0.29; 0.59) and (0.35; 0.64)
which were calculated for the EL at 8 V of the devices A and B, respectively. The better colour
purity was observed for device A.
<Insert Figure 9>
The luminance and efficiency characteristics of the devices A and B are plotted in Figure 10.
Both the devices were characterized by the relatively low values of the turn-on voltage (Von=3.7 for
device A and Von=3.1 V for device B), confirming the very efficient injection from the electrodes
and transport of holes and electrons to the emission layers (Figure 11). The driving voltage of 4.9 V
at 1000 cd/m2 was observed for device B and it was by 1 V lower than that observed for device A
(Figure 10 a). This observation indicates the better charge balance in device B than in device A,
17

ACCEPTED MANUSCRIPT
owing the better charge injection due to the more appropriate HOMO and LUMO levels (Figure
11). The higher maximum brightness of ca. 11900 cd/m2 was recorded for device B than that
observed for device A (5700 cd/m2) (Figure 10). The higher brightness and lower driving voltages
observed for the device B compared to those recorded for the device A shows more effective
exciton recombination and radiative transition in the emitting layer with the host 5.[22]
<Insert Figure 10>
The efficiency curves of the PhOLEDs are shown in Figure 10 b-d. The devices A and B based
on hosts 2 and 5 exhibited the maximum current efficiency, power efficiency, and external quantum
efficiency of 4.2 and 10.3 cd/A, 2.5 and 7.2 lm/W, 1.2 and 2.9 % respectively in the absence of light
out-coupling enhancement. The external quantum efficiencies of 1.05 and 2.24 % at 1000 cd/m2
were recoded (Figure 10 b-d) showing the efficiency roll-offs of 12.5 and 22.7 % for the devices A
and B respectively. The external quantum efficiencies of PhOLEDs decreased at high current
density and high exciton concentration apparently due to the unbalanced charges and quenching
effects. [22] We noted that the simple and unoptimized PhOLEDs based on hosts 2 and 5 were
fabricated, therefore, the maximum efficiencies of devices A and B were lower compared to those
reported for PhOLEDs containing Ir(ppy)3 [23,21,24]. The better performance of the devices based
on the hosts 2 and 5 can apparently be reached either after introduction of the additional layers, after
the additional optimization of the concentration of dopant Ir(ppy)3 in hosts 2 and 5, or after the
additional optimization of the thicknesses of the layers of the devices.
<Insert Figure 11>
Since the triplet energy levels of 2 and 5 were found to be high, they were tested as the host
materials for the blue phosphorescent emitter bis[2-(4,6- difluorophenyl) pyridinato -C2,N]
(picolinato)iridium(III) (FIrpic). We fabricated blue PhOLEDs with structures ITO/m-MTDATA
(25 nm)/2 (5nm)/2 or 5:FIrpic (15 nm)/DPEPO(5 nm)/Bphen (30 nm)/Ca:Al (devices C and D,
respectively). DPEPO is an abbreviation of Bis[2-(diphenylphosphino)phenyl] ether oxide which
18

ACCEPTED MANUSCRIPT
was used from a site of electron injection as the exciton-blocking material with a high triplet level
(3.3 eV). An additional layer 2 was also used as the exciton-blocking layer from the other site of the
emitting layer. Such multilayer structure did not allow to obtain effective blue PhOLEDs since the
additional emission band was observed in the low energy region of the EL spectra (Figure 12). The
emission with the maxima at 470 and 503 nm is related to pure FIrpic emission while the low
energy maximum apparently results from intermolecular system (an exciplex or an electroplex)
formed in the emitting layer. Taking into account this observation for the blue PhOLED, the
shoulders in the EL spectra of the green PhOLEDs can be identified only after the careful analyzis.
Those shoulders can also be assigned to exciplex or electroplex emission. Recently the
phosphorescence exciplex between tri(9-hexylcarbazol-3-yl)amine and FIrpic was reported [25].
<Insert Figure 12>
Morphology of the layers
AFM measurements were performed for vacuum deposited light-emitting layers 2:Ir(ppy)3 and
5:Ir(ppy)3 on glass substrates. The AFM topographical images are shown in Fig. 13. The
topography of 2:Ir(ppy)3 shows relatively homogeneous surface with morphological features having
a mean height of 1.18 nm and the root mean square roughness (Rq) of 0.36 nm. The surface of
2:Ir(ppy)3 is dominated by the peaks with skeweness (Rsk) value of 0.79 and has a leptokurtoic
distribution of the morphological features with kurtosis (Rku) value of 4.67 indicating relatively
many high peaks and low valleys. In contrast to 2:Ir(ppy)3, the 5:Ir(ppy)3 surface was found to be
more rough with randomly oriented surface mounds having a mean height of 3.60 and Rq value of
1.24 nm. The surface of 5:Ir(ppy)3 exhibited relatively similar symmetry and leptokurtoic
distribution of the morphological features with Rsk and Rku values of 0.58 and 4.42, respectively.
Both, light-emitting layers 2:Ir(ppy)3 and 5:Ir(ppy)3 exhibited quite low roughness values which are
acceptable for structuring OLEDs as the low roughness values of host:guest emission layers
contribute to the higher brightness and efficiency of PhOLEDs [26].
19

ACCEPTED MANUSCRIPT
<Insert Figure 13>
The X-ray diffraction patterns at grazing incidence angle of 1.0º of 2:Ir(ppy)3 and 5:Ir(ppy)3 thin
films are shown in Fig. S2. The broad reflex at about 2θ = 23.6º in both diffractograms indicates
amorphous character of thin films. Since crystallization and aggregation significantly affect the
device stability and lifetime, amorphous layers as those of 2:Ir(ppy)3 and 5:Ir(ppy)3 are favourable
for OLEDs [27]. In addition, the amorphous materials exhibit the excellent processability,
homogeneity and isotropic properties, which are also required for the highly efficient OLEDs [28].
Conclusions
2-Phenylindole and carbazole derivatives were synthesized and studied as highly-efficient
hosts for phosphorescent OLEDs. Photopolymerization of the compounds having vinylbenzyl
reactive functional groups was demonstrated in the solid films and in solutions. The compounds
with high triplet energies of 3.02 and 2.88 eV as well as with favourable for charge injection
HOMO/LUMO values were tested as the hosts in blue and green phosphorescent OLEDs. The
green devices demonstrated practically pure emission of the used dopant, the relatively low values
of the turn-on voltages of 3.7 and 3.1 V as well as the efficiency roll-offs of 12.5 and 22.7 %. Light-
emitting layers of green PhOLEDs based on the synthesized hosts exhibited low roughness values
and amorphous character. The best green device exhibited the maximum current, power, and
external quantum efficiencies of 10.3 cd/A, 7.2 lm/W, 2.9 %, respectively, in the absence of light
outcoupling enhancement. Electroluminescence of the blue device was related not only to pure
emission of the dopant but also to exciplex and/or electroplex emission formed between host and
guest.
Acknowledgments
Financial support from the Research Council of Lithuania (project No. TAP LB-03/2015) is
gratefully acknowledged.
20

ACCEPTED MANUSCRIPT
References
[1] Minaev B., Baryshnikov G., Agren H. Principles of phosphorescent organic light emitting
devices. Phys. Chem. Chem. Phys. 2014;16:1719–58. Doi: 10.1039/c3cp53806k
[2] Adachi C., Baldo M., Thompson M.E., Forrest S.R.J. Nearly 100% internal phosphorescence
efficiency in an organic light-emitting device. Appl. Phys. 2001;90:5048–51. Doi:
dx.doi.org/10.1063/1.1409582
[3] Huang J.H., Su J. H., Tian H. The development of anthracene derivatives for organic light-
emitting diodes, J. Mater. Chem., 2012;22: 10977-10989. Doi: 10.1039/c2jm16855c
[4] Lia J., Grimsdale A. C. Carbazole-based polymers for organic photovoltaic devices. Chem. Soc.
Rev. 2010;39:2399–10. Doi:10.1039/B915995A
[5] Huang J. H. Su J.H., Li X., Lam M. K., Fung K.M., Fan H.H., Cheah K.W., Chen C.H., Tian H.
Bipolar anthracene derivatives containing hole- and electron-transporting moieties for highly
efficient blue electroluminescence devices, J. Mater. Chem. 2011;21: 2957-2964
[6] Dumur F. Carbazole-based polymers as hosts for solution-processed organic light emitting
diodes: simplicity, efficacy. Org. Electron. 2015;25:345–61. Doi:0.1016/j.orgel.2015.07.007
[7] Stanislovaityte E., Simokaitiene J., Raisys S., Al-Attar H., Grazulevicius J. V., Monkman A. P.,
Jankus V. Carbazole based polymers as hosts for blue iridium emitters: synthesis, photophysics
and high efficiency PLEDs. J. Mater. Chem. C. 2013;1:8209-21. Doi:10.1039/C3TC31441C
[8] Dumur F., Beouch L., Peralta S., Wantz G., Goubard F., Gigmes D. Solution-processed blue
phosphorescent OLEDs with carbazole-based polymeric host materials. Org. Electron.
2015;25:21–30. Doi: 10.1016/j.orgel.2015.06.013
[9] Kirkus M., Lygaitis R., Tsai M.H., Grazulevicius J.V., Wu C.C. Triindolylmethane-based high
triplet energy glass-forming electroactive molecular materials. Synthetic Met. 2008;158:226–32.
Doi:10.1016/j.synthmet.2008.01.006
21

ACCEPTED MANUSCRIPT
[10] Leete E. 3-Hydroxymethylindoles. J. Am. Chem. Soc. 1959;81:6023–26.
Doi:10.1021/ja01531a042
[11] Kruzinauskiene A., Matoliukstyte A., Michaleviciute A., Grazulevicius J.V., Musnickas J.,
Gaidelis V., Jankauskas V. Carbazolyl- and diphenylamino substituted fluorenes as hole
transport
materials.
Synthetic
Met.
2007;157:401–06.
Doi:dx.doi.org/10.1016/j.synthmet.2007.04.007
[12] Miyamoto E., Yamaguchi Y., Yokoyama M. Ionization potential of organic pigment film by
atmospheric photoelectron emission analysis. Denshishashin Gakkai-shi (Electrophotography)
1989;28:364–70. Doi: 10.11370/isjepj.28.364
[13] Kukhta N.A., Volyniuk D., Peciulyte L., Ostrauskaite J., Juska G., Grazulevicius J.V..
Structure property relationships of star-shaped blue-emitting charge-transporting 1,3,5-
triphenylbenzene derivatives. Dyes Pigm. 2015;117:122–32. Doi:10.1016/j.dyepig.2015.02.013
[14] Gritzner G., Kuta J. Recommendations on reporting electrode potentials in nonaqueous
solvents. Pure Appl. Chem. 1984;56:461–66. Doi:dx.doi.org/10.1351/pac19845604046
[15] SPARTAN’14 for Windows Version 1.1.2. 1840 Von Karman Avenue, Suite 370, Irvine, CA
92612: Wavefunction, Inc.; 2013.
[16] Okamoto, S.; Tanaka, K.; Izumi, Y.; Adachi, H.; Yamaji, T.; Suzuki, T. Simple Measurement
of Quantum Efficiency in Organic Electroluminescent Devices. JPN J A P 2 2001;40:L783–84.
Doi:10.1143/JJAP.40.L783
[17] Kirkus M., Tsai M.H., Grazulevicius J.V., Wu C.C., Chi L.C., Wong K.T. New indole-
carbazole hybrids as glass-forming high-triplet-energy materials. Synthetic Met. 2009;159:729–
34. Doi:10.1016/j.synthmet.2008.12.027
[18] Blakesley J.C., Castro F.A., Kylberg W., Dibb G.F., Arantes C., Valaski R., Cremona M., Kim
J.S., Kim J.-S. Towards reliable charge-mobility benchmark measurements for organic
semiconductors. Org Electron 2014:15:1263–72. Doi: 10.1016/j.orgel.2014.02.008
22

ACCEPTED MANUSCRIPT
[19] Angioni E., Chapran M., Ivaniuk K., Kostiv N., Cherpak V., Stakhira P., Lazauskas A.,
Tamulevicius S., Volyniuk D., Findlay N.J., Tuttle T., Grazulevicius J.V., Skabara P.J. A single
emitting layer white OLED based on exciplex interface emission. J. Mater. Chem. C
2016;4:3851–56. Doi: 10.1039/C6TC00750C
[20] Tomkute-Luksiene D., Keruckas J., Malinauskas T., Simokaitiene J., Getautis V.,
Grazulevicius J.V., Volyniuk D., Cherpak V., Stakhira P., Yashchuk V., Kosach V., Luka G.,
Sidaravicius J.. 2-Phenyl-1,2,3-benzotriazole Ir(III) complexes with additional donor fragment
for
single-layer
PhOLED
devices.
Dyes
Pigm.
2013;96:278–86.
Doi:
10.1016/j.dyepig.2012.08.012
[21] He G., Pfeiffer M., Leo K., Hofmann M., Birnstock J., Pudzich R., Salbeck J. High-efficiency
and low-voltage p-i-n electrophosphorescent organic light-emitting diodes with double-emission
layers. Appl. Phys. Lett. 2004;85:3911–13. Doi: 10.1063/1.1812378
[22] Zhang Z., Zhang Z., Ding D., Wei Y., Xu H, Jia J., Zhao Y., Pan K., Huang W. Selectively
Investigating Molecular Configuration Effect on Blue Electrophosphorescent Host Performance
through a Series of Hydrocarbon Oligomers. J. Phys. Chem. C 2014;118:20559−70. Doi:
10.1021/jp506513x
[23] Baldo M. A., Thompson M. E., Forrest S. R. High-efficiency fluorescent organic light-emitting
devices using a phosphorescent sensitizer. Nat. 2000;403:750−53. Doi:10.1038/35001541
[24] Keruckas J., Grazulevicius J.V., Volyniuk D., Cherpak V., Stakhira P.. 3,6-Bis(indol-1-yl)-9-
phenylcarbazoles as electroactive materials for electrophosphorescent diodes. Dyes Pigm
2014;100:66-72. Doi.org/10.1016/j.dyepig.2013.07.020
[25] Cherpak V., Stakhira P., Minaev B., Baryshnikov G., Stromylo E., Helzhynskyy I., Chapran
M., Volyniuk D., Hotra Z., Dabuliene A., Tomkeviciene A., Voznyak L., Grazulevicius J.V.
Mixing of Phosphorescent and Exciplex Emission in Efficient Organic Electroluminescent
Devices. ACS Appl. Mater. Interfaces 2015;7:1219−1225. Doi: 10.3116/16091833/16/2/95/2015
23

ACCEPTED MANUSCRIPT
[26]Bagdziunas G., Grybauskaite G., Kostiv N., Ivaniuk K., Volyniuk D., Lazauskas A. Green and
red phosphorescent organic lightemitting diodes with ambipolar hosts based on phenothiazine
and carbazole moieties: photoelectrical properties, morphology and efficiency. RSC Adv., 2016,
6, 61544–61554. Doi: 10.1039/c6ra12692h
[27] Sun Y.-H., Zhu X.-H., Chen Z., Zhang Y., Cao, Y. Potential Solution Processible
Phosphorescent Iridium Complexes toward Applications in Doped Light-Emitting Diodes:ꢀ
Rapid Syntheses and Optical and Morphological Characterizations. J. Org. Chem.,
2006;71:6281–6284. Doi: 10.1021/jo060840h.
[28] Shirota Y., Kageyama H. Charge Carrier Transporting Molecular Materials and Their
Applications in Devices. Chem. Rev., 2007;107:953–1010. Doi: 10.1021/cr050143+.
24

ACCEPTED MANUSCRIPT
SCHEMES
Scheme 1.
FIGURES
Figure Error! Main Document Only..
25

ACCEPTED MANUSCRIPT
Figure Error! Main Document Only..
Fluorescence
1
Phosphorescence
1
2
3
4
5
1.0
0.8
0.6
0.4
0.2
0.0
3.04 eV
1.0
0.8
0.6
0.4
0.2
0.0
2
3
4
5
350
375
400
425
450
300
350
400
450
500
550
600
Wavelength (nm)
Wavelength (nm)
a)
b)
Figure Error! Main Document Only..
Figure Error! Main Document Only..
26

ACCEPTED MANUSCRIPT
Figure Error! Main Document Only..
-0.5
-1.0
-1.5
-2.0
-2.5
-3.0
-3.5
-4.0
-4.5
-5.0
-0.66
-0.66
-0.92
-0.9
-0.91
-4.93
2
-4.95
3
-4.99
1
-5.01
5
-5.08
4
Figure Error! Main Document Only..
Figure Error! Main Document Only..
27

ACCEPTED MANUSCRIPT
5 (holes)
2 (holes)
102
fitting
III
101
II
100
10-1
I
10-2
1
10
Voltage, V
Figure Error! Main Document Only..
1.0
4V
6V
8V
10V
Device A
0.8
0.6
0.4
0.2
0.0
400
500
600
700
1.0
0.8
0.6
0.4
0.2
0.0
4V
6V
8V
10V
Device B
400
500
600
700
Wavelength (nm)
Figure 9.
10000
Device A
Device B
Device A
Device B
10
8
400
300
200
100
0
1000
100
10
6
4
2
0
1
10
100
0
2
4
6
8
10
Current density, mA/cm2
Voltage, V
a)
b)
28

ACCEPTED MANUSCRIPT
8
7
6
5
4
3
2
1
Device A
Device B
3.0
2.5
2.0
1.5
1.0
0.5
0.0
Device A
Device B
1
10
100
10
100
Current density, mA/cm2
Current density, mA/cm2
c)
d)
Figure 10.
Figure 11.
1.0
8V
10V
12V
8V
10V
12V
1.0
0.8
0.6
0.4
0.2
0.8
0.6
0.4
0.2
0.0
400
450
500
550
600
650
700
750
800
350
400
450
500
550
600
650
700
750
800
Wavelength (nm)
Wavelength (nm)
a)
b)
Figure 12.
29