2-Phenyl-1,2,3-benzotriazole Ir(III) complexes with additional donor fragment for single-layer PhOLED devices

Article abstract of DOI:10.1016/j.dyepig.2012.08.012

Phosphorescent bis-cyclometallated iridium (III) complexes based on 2-phenyl-1,2,3-benzotriazole with additional donoric diphenylamine or carbazole moieties were synthesized, studied and tested as phosphorescent materials in the single-layer phosphorescent organic light emitting diodes. These single-layer devices were fabricated employing a simple technological approach, based upon the simultaneous vacuum deposition from one crucible of host material and the corresponding phosphorescent 1,2,3-triazole-based iridium complexes. Red and orange electrophosphorescent single-layer devices with current efficiencies of 5.3 cd/A and 6.8 cd/A respectively are reported. Results of the impedance spectroscopy suggest that the proposed method of formation of the working layer provides homogeneous distribution of molecular guest in the matrix of host.

Full text of DOI:10.1016/j.dyepig.2012.08.012

Dyes and Pigments 96 (2013) 278e286
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
2-Phenyl-1,2,3-benzotriazole Ir(III) complexes with additional donor fragment
for single-layer PhOLED devices
,
D. Tomkute-Luksiene a, J. Keruckas a, T. Malinauskas a, J. Simokaitiene a, V. Getautis a
**
,
,
b
b
b,
c
c
d
J.V. Grazulevicius a , D. Volyniuk , V. Cherpak , P. Stakhira
, V. Yashchuk , V. Kosach , G. Luka ,
*
***
J. Sidaravicius e
a Faculty of Chemical Technology, Kaunas University of Technology, 50254 Kaunas, Lithuania
b Lviv Polytechnic National University, 79013 Lviv, Ukraine
c Kyiv Taras Shevchenko National University, Faculty of Physics, 03680 Kyiv, Ukraine
d Institute of Physics, Polish Academy of Sciences, 02-668 Warsaw, Poland
e Department of Polygraphic Machines, Vilnius Gediminas Technical University, 03224 Vilnius, Lithuania
a r t i c l e i n f o
a b s t r a c t
Article history:
Phosphorescent bis-cyclometallated iridium (III) complexes based on 2-phenyl-1,2,3-benzotriazole with
additional donoric diphenylamine or carbazole moieties were synthesized, studied and tested as phos-
phorescent materials in the single-layer phosphorescent organic light emitting diodes. These single-layer
devices were fabricated employing a simple technological approach, based upon the simultaneous
vacuum deposition from one crucible of host material and the corresponding phosphorescent 1,2,3-
triazole-based iridium complexes. Red and orange electrophosphorescent single-layer devices with
current efficiencies of 5.3 cd/A and 6.8 cd/A respectively are reported. Results of the impedance spec-
troscopy suggest that the proposed method of formation of the working layer provides homogeneous
distribution of molecular guest in the matrix of host.
Received 28 June 2012
Received in revised form
10 August 2012
Accepted 14 August 2012
Available online 23 August 2012
Keywords:
Iridium complexes
Triphenylamines
Ó 2012 Elsevier Ltd. All rights reserved.
Carbazoles
Single-layer organic light emitting diode
Vacuum deposition
Phosphorescence
1. Introduction
functional layers for charge carrier injection, transport, blocking, as
well as the confinement of exciton diffusion, are required [3]. Such
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 [1]. In recent years a lot of
effort has been concentrated on the development of luminescent
transition-metal complexes, particularly of the second- and third-
row transition metals [2]. As a result of efficient spineorbit
coupling in these complexes, both singlet and triplet excitons can
be harvested, and theoretically up to 100% internal quantum effi-
ciencies can be attained.
sophisticated device configuration inevitably increases the
manufacturing complexity and the production cost. It is therefore
highly desirable to fabricate PhOLEDs with the simplified device
structure. Particularly attractive goal is the single layer device,
which can perform at the comparable efficiency as its multilayer
counterparts. Efficient bi-layered devices consisting of emissive and
buffer layers have been reported by several researchers [4e10].
However, the single-layer approach, adopting direct charge injec-
tion and transport onto the triplet dopants dispersed in an appro-
priate host matrix, is quite rare and only a very limited number of
publications have been reported to date [11e17].
In order to efficiently extract the electro-generated emissive
excitons in typical electrophosphorescent device (PhOLED), various
In the past few years various nitrogen-containing heterocycles
such as quinolines, isoquinolines, quinazolines, and quinoxalines
have been of special interest in accomplishing efficient orange and
red phosphorescent emission from iridium complexes [18].
However, 1,2,3-triazoles have been overlooked and only recently
few examples have been published [19e23]. The aim of the present
* Corresponding author. Tel.: þ370 37 300193; fax: þ370 37 456525.
** Corresponding author. Tel.: þ370 37 300196; fax: þ370 37 300152.
*** Corresponding author. Tel.: þ380 50 2006563.
E-mail addresses: vytautas.getautis@ktu.lt (V. Getautis), juozas.grazulevicius@
ktu.lt (J.V. Grazulevicius), stakhira@polynet.lviv.ua (P. Stakhira).
0143-7208/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2012.08.012

D. Tomkute-Luksiene et al. / Dyes and Pigments 96 (2013) 278e286
279
work was synthesis and characterization of orange and red-
emitting 1,2,3-triazole-based iridium complexes bearing hole-
transporting carbazolyl or diphenylamine moieties, and investiga-
tion of their suitability for the fabrication of the single-layer elec-
trophosphorescent devices. The single-layer PhOLEDs were
fabricated employing a simple technological approach, based upon
the simultaneous vacuum deposition from one crucible of host
material CBM4 (Fig. 1a) and one of the phosphorescent emitting
1,2,3-triazole-based iridium complexes, red IC1 (Fig. 1b) or orange
IC2 (Fig. 1c), at the ratio of 90% (CBM4): 10% (IC1 or IC2).
calorimetry measurements were carried out on TA Instruments Q10
calorimeter at a heating rate of 10 ꢀC/min in the nitrogen atmo-
sphere. The glass transition temperatures (Tg) for the investigated
compounds were determined during the second heating scan.
2.2.1. 5-Bis[(4-methylphenyl)amino]-2-phenyl-1,2,3-benzotriazole
(1)
5-Amino-2-phenyl-1,2,3-benzotriazole (1.0 g, 4.76 mmol), 4-
iodotoluene (2.59 g, 11.9 mmol), copper(I) iodide (0.033 g,
0.17 mmol), 2,20-bipiridyl (0.027 g, 0.17 mmol) and potassium tert e
butoxide (1.602 g,14.28 mmol) were refluxed in 15 ml of dry toluene
under argon atmosphere for 3 h. After termination of the reaction
(TLC, n-hexane/acetone, 4:1 v/v) the mixture was filtered off and the
solvent was evaporated. The residue was purified by column chro-
matography using n-hexane/acetone (24:1 v/v) as an eluent to afford
1 (0.78 g, 42%) as a yellow to orange solid. 1H NMR (300 MHz, CDCl3)
2. Experimental details
2.1. Materials
All chemicals were purchased from Aldrich and used as received
without further purification. The details of the synthesis of 5-
amino-2-phenyl-1,2,3-benzotriazole are described in Ref. [24]. 1,1-
Bis(4-(30,30’-dimethoxy)triphenylamino)cyclohexane (CBM4) was
prepared by the earlier reported procedure [25].
d
, ppm: 8.29 (dd, J ¼ 8.7 Hz, J ¼ 1.2 Hz, 2H), 7.73 (dd, J ¼ 9.3 Hz,
J ¼ 0.9 Hz, 1H), 7.51 (m, 2H), 7.39 (m, 1H), 7.31 (dd, J ¼ 2.1 Hz,
J ¼ 0.9 Hz, 1H), 7.23 (dd, J ¼ 9.3 Hz, J ¼ 2.1 Hz, 1H), 7.14e7.03 (m, 8H),
2.34 (s, 6H). 13C NMR (75 MHz, CDCl3)
d, ppm: 147.90, 146.34, 145.17,
142.03, 140.48, 133.39, 130.20, 129.49, 128.50, 126.51, 125.22, 120.18,
118.46, 106.94. MS (APCIþ, 20 V) m/z: 391 [M þ H]þ. Anal. Found: C,
79.87; H, 5.60; N,14.53. C26H22N4 requires C, 79.97; H, 5.68, N,14.35%.
2.2. General
The 1H and 13C NMR spectra were recorded on a Varian Unity
Inova spectrometer (1H-300 MHz, 13C-75 MHz) at room tempera-
ture. The chemical shifts are expressed in ppm, downfield from
tetramethylsilane (TMS), used as internal standard. The course of
the reactions was monitored by TLC on ALUGRAM SIL G/UV254
plates and developed with UV light. Silica gel (grade 9385, 230e400
2.2.2. Iridium(III)-bis{5-bis[(4-methylphenyl)amino]-2-phenyl-
1,2,3-benzotriazolato-N,C20}acetyl-acetonate (IC1)
Iridium trichloride hydrate (0.47 g, 1.57 mmol) and 1 (1.3 g,
3.33 mmol) were dissolved in the mixture of 2-ethoxyethanol
(20 ml) and water (2.5 ml). The mixture was refluxed under
argon atmosphere for 20 h, and cooled down to ambient temper-
ature. The formed crystals were filtered and washed with water and
then ethanol. The product was dried under vacuum, and used in
ꢀ
mesh, 60 A, Aldrich) was used for column chromatography.
Elemental analysis was performed with an Exeter Analytical CE-440
Elemental Analyzer. MS were recorded on an Agilent 110 (series MS
with VL) apparatus. Cyclic voltammetry (CV) measurements were
carried out by a three-electrode assembly cell from Bio-Logic SAS
next step without further purification. The yield of the m-chloride-
bridged dimer was 1.18 g (37%).
The
m-chloride-bridged dimer (1.18 g, 0.586 mmol), 2,4-
and
a micro-AUTOLAB Type III potentiostat-galvanostat. The
pentanedione (0.176 g, 1.75 mmol) and sodium carbonate (0.62 g,
5.85 mmol) were refluxed in 20 ml of degassed 2-ethoxyethanol
under argon atmosphere for 2 h. After termination of the reaction
(TLC, n-hexane/acetone, 4:1 v/v) the mixture was cooled down to
ambient temperature and diluted with water (10 ml). The formed
solid was isolated by filtration, washed with water and ethanol. The
product was purified by column chromatography using n-hexane/
acetone (24:1 and 23:2 v/v) as the eluent to obtain IC1 (0.94 g, 78%)
measurements were carried out with a glassy carbon electrode in
dichloromethane solutions containing 0.1 M tetrabutylammonium
hexafluorophosphate as electrolyte, Ag/AgNO3 as the reference
electrode and a Pt wire counter electrode. Thermogravimetric
analysis (TGA) was performed with a Netzch STA 409 PC Luxx at
a scan rate 10 K/min under a nitrogen flow. Differential scanning
as red solid. 1H NMR (300 MHz, CDCl3)
d, ppm: 7.82e7.71 (m, 3H),
7.54e7.46 (m,1H), 7.37e7.32 (m,1H), 7.30e7.00 (m,19H), 6.97e6.88
(m, 2H), 6.78e6.68 (m, 2H), 6.25e6.12 (m, 2H), 5.08 (s, 1H), 2.40e
2.30 (m, 12H), 1.82 (s, 3H), 1.44 (s, 3H); 13C NMR (75 MHz, CDCl3)
d, ppm: 186.61, 186.08, 185.44, 149.58, 148.01, 145.58, 144.96,
144.90,143.56,142.89,142.70,140.63,138.65,134.59,133.98,133.73,
130.40, 130.30, 129.44, 128.14, 127.41, 125.69, 125.46, 122.43, 119.39,
115.65, 114.94, 106.54, 103.26, 103.21, 101.62, 28.36, 27.33, 27.12,
21.12; Anal. Found: C, 63.84; H, 4.55; N,10.57. C57H49IrN8O2 requires
C, 63.97; H, 4.61, N, 10.47%.
2.2.3. 2-Phenyl-5-iodo-1,2,3-benzotriazole (2)
Solution of 5-amino-2-phenyl-1,2,3-benzotriazole (8.4 g,
39.95 mmol) in acetic acid (150 ml) and hydrochloric acid (60 ml)
was heated to 80 ꢀC and the obtained brown suspension was then
cooled down to 0 ꢀC. Sodium nitrite (3.7 g, 53.62 mmol) in water
(20 ml) was added slowly and the temperature was held at 0e5 ꢀC
throughout the addition. The resulting mixture was stirred for
45 min below 5 ꢀC and then potassium iodide (9.6 g, 57.87 mmol)
solution in water (40 ml) was added. The reaction mixture was
warmed up to ambient temperature and then heated at 50 ꢀC for
20 min. The mixture was cooled down to ambient temperature and
Fig. 1. Chemical structures of CBM4, IC1, IC2.

280
D. Tomkute-Luksiene et al. / Dyes and Pigments 96 (2013) 278e286
sodium metabisulfite was added until the red color disappeared.
The solid was isolated by filtration, washed with aqueous sodium
bicarbonate, water and finally hexane. The product was purified by
column chromatography using n-hexane/acetone (24.5:0.5 v/v) as
an eluent to yield 8.0 g (62%) of pale yellow solid. 1H NMR
hexane. The product was dried under vacuum, and used in next
step without further purification. The yield of the
m-chloride-
bridged dimer was 0.88 g (44%).
The
m-chloride-bridged dimer (0.88 g, 0.46 mmol), 2,4-
pentanedione (0.14 g, 1.40 mmol) and sodium carbonate (0.49 g,
4.62 mmol) were refluxed in 20 ml of degassed 2-ethoxyethanol
under argon atmosphere for 3 h. After termination of the reaction
(TLC, n-hexane/acetone, 4:1v/v)mixturewascooleddowntoambient
temperature and diluted with water (10 ml). The formed solids were
isolated by filtration, washed with water and ethanol. The product
was purified by column chromatography using n-hexane/acetone
(24:1 and 23:2 v/v) as the eluent to obtain IC2 (0.61 g, 65%) as orange
(300 MHz, CDCl3)
d
, ppm: 8.36e8.28 (m, 3H), 7.71e7.60 (m, 2H),
, ppm: 145.56, 143.95,
7.58e7.42 (m, 3H). 13C NMR (75 MHz, CDCl3)
d
140.07, 136.07, 129.59, 129.42, 127.57, 120.75, 119.96, 92.22; MS
(APCIþ, 20 V) m/z: 322 [M þ H]þ; Anal. Found: C, 44.99; H, 2.44; N,
12.97. C12H8IN3 requires C, 44.88; H, 2.51, N, 13.09%.
2.2.4. 5-(9-Carbazolyl)-2-phenyl-1,2,3-benzotriazole (3)
Carbazole (1.15 g, 6.85 mmol), 2 (2.0 g, 6.23 mmol), copper (I)-
tri(triphenylphosphine)bromide (Cu(PPh3)3Br) (0.5 g, 1.25 mmol)
and cesium carbonate (3.04 g, 9.35 mmol) were refluxed in 30 ml of
dimethyl sulfoxide under argon atmosphere for 8 h. After termina-
tion of the reaction (TLC, n-hexane/acetone, 22:3 v/v) the mixture
was treated with dichloromethane and water. The organic layer was
dried over anhydrous sodium sulfate, filtered and solvents were
evaporated. The obtained residue was purified by column chroma-
tography using n-hexane/acetone (24:1 v/v) as an eluent to obtain 3
solid.1H NMR (300 MHz, CDCl3)
d, ppm: 8.25e8.12 (m, 5H), 8.07e7.95
(m, 4H), 7.90e7.82 (m, 1H), 7.77e7.30 (m, 14H), 7.12e6.96 (m, 2H),
6.90e6.75 (m, 2H), 6.37e6.25 (m, 2H), 5.48 (s,1H),1.95 (s, 3H),1.50 (s,
3H); 13C NMR (75 MHz, CDCl3)
d, ppm: 187.11, 186.65, 186.49, 144.75,
144.71, 142.83, 142.79, 142.55, 141.19, 140.87, 140.54, 138.97, 137.00,
136.96, 134.92, 134.82, 130.24, 130.19, 130.13, 129.55, 129.24, 126.96,
126.89, 126.45, 124.11, 123.95, 123.07, 122.99, 122.95, 122.88, 121.16,
120.94, 120.79, 120.72, 120.68, 116.93, 116.90, 116.81, 116.69, 111.85,
110.05, 101.92, 68.66, 28.48, 22.22; Anal. Found: C, 63.10; H, 3.80; N,
10.96. C53H37IrN8O2 requires C, 63.02; H, 3.69, N, 11.09%.
(1.45 g, 65%) as yellowsolid.1H NMR (300 MHz, CDCl3)
8.40 (m, 2H), 8.22e8.13 (m, 4H), 7.70e7.58 (m, 3H), 7.55e7.42 (m,
5H), 7.38e7.27 (m, 2H). 13C NMR (75 MHz, CDCl3)
, ppm: 145.64,
d, ppm: 8.46e
d
2.3. Fabrication and characterization of the films and devices
144.22, 141.00, 140.43, 136.77, 129.76, 129.52, 127.57, 127.51, 126.35,
123.78, 120.86, 120.63, 120.51, 120.24, 116.18, 110.02; MS (APCIþ,
20 V) m/z: 361 [M þ H]þ; Anal. Found: C, 79.95; H, 4.40; N, 15.65.
C24H16N4 requires C, 79.88; H, 4.47, N, 15.54%.
The electroluminescent devices ITO/CBM4:IC1(26 nm)/
Ca(40 nm)/Al(200 nm) (A) and ITO/CBM4:IC2(26 nm)/Ca(40 nm)/
Al(200 nm) (B) were fabricated by means of vacuum co-deposition
of CBM4 and IC1 (IC2) from one source boat (at the ratio of 90%
(CBM4): 10% (IC1 or IC2)) onto ITO coated glass substrate under
vacuum of 10ꢁ5 Torr.
2.2.5. Iridium(III)-bis{5-(9-carbazolyl)-2-phenyl-1,2,3-
benzotriazolato-N,C20}acetyl-acetonate (IC2)
Iridium trichloride hydrate (0.316 g, 1.06 mmol) and 3 (0.8 g,
2.2 mmol) were dissolved in the mixture of 2-ethoxyethanol
(25 ml) and water (3 ml). The mixture was refluxed under argon
atmosphere for 22 h, and cooled down to ambient temperature. The
formed crystals were filtered and washed with water and then
The thickness of the layers of CBM4:IC1 and CBM4:IC2 was
measured by NanoCalc 2000 Reflectometry System. The active area
of the obtained devices A and B was 3 ꢂ 2 mm2.
For the measurements of photoluminescence (PL) and absorption
spectra of CBM4, IC1, IC2, CBM4:IC1 and CBM4:IC2 the films were
Scheme 1. Synthesis of iridium complexes IC1, IC2.

D. Tomkute-Luksiene et al. / Dyes and Pigments 96 (2013) 278e286
281
vacuum deposited on quartz substrates. UV spectra were recorded
with Cary 5000 UVeVISeNIR and Specord UVeVis spectrometers. For
the measurements of photoluminescence spectra of CBM4 the films
werecastedfromtetrahydrofuran(THF)solutiononquartzsubstrates.
Low-temperature (77 K) photoluminescence measurements were
performed using the cryostat Optistat (Oxford Instruments).
The current densityevoltageeluminance (JeVeL) characteris-
tics and electroluminescence (EL) spectra were recorded using
a Programmable Test Power LED300E, spectrometer HAAS-2000
and integrated sphere (d ¼ 0.3 m).
Impedance spectroscopy was employed for the characteriza-
tion of the conductive properties of devices A and B. Experiments
were performed in the frequency range of 101 O 106 Hz at
constant bias voltage of 0 V, 1.0 V, 3.0 V and 5 V using “AUTOLAB”
spectrometer (Eco Chemie, The Netherlands) and FRA-2 and GPES
software. Frequency dependencies of complex resistivity Z were
analyzed by graphiceanalytical method using ZView 2.3 (Scribner
Associates) package. Approximation inaccuracy was found not to
exceed 2%.
Fig. 2. DSC second heating curves for IC1 and IC2 (heating rate 10 K/min).
3. Results and discussion
3.1. Synthesis and thermal characterization
Absorption spectra of spin-coated films (Fig. 3a) can be char-
acterized by non-structured absorption band at 310 nm due to
* transitions. The short-wave PL maximum of CBM4 films (at
300 K) is observed at 366 nm with corresponding Stokes shift of
56 nm. In order to evaluate the energy of the first triplet state of the
p
e
Synthesis of the iridium complexes IC1 and IC2 containing 2-
phenyl-1,2,3-benzotriazole moieties was carried out by a synthetic
route given in Scheme 1. 2-Phenyl-5-iodo-1,2,3-benzotriazole (2)
was synthesized using a classical Sandmeyer reaction, while ligands
5-bis[(4-methylphenyl)-amino]-2-phenyl-1,2,3-benzotriazole (1)
and 5-carbazolyl-2-phenyl-1,2,3-benzotriazole (3) were prepared
using modified Ullmann-type reactions. Nonoyama reaction of the
corresponding ligands and IrCl3$nH2O in a mixed solvent system of
p
2-ethoxyethanol and water gave the intermediate cyclometalated
m-
chloride-bridged Ir(III) dimer. Subsequently, chloride of -chloride-
m
bridged Ir(III) dimer was substituted by the acetylacetone ligand in
the presence of potassium carbonate base in a thoroughly degassed
2-ethoxyethanol at 135 ꢀC.
Thermal behavior of the complexes IC1, IC2 was explored by DSC
and TGA methods. Melting points (Tm), glass transition tempera-
tures (Tg) and 5% mass loss temperatures (Tdec) were established,
and the data are summarized in Table 1.
Both complexes are amorphous materials with high glass transi-
tion temperatures (Fig. 2). Replacement of the relatively flexible
diphenylamine moiety with the rigid carbazole unit results in the
increaseof Tg by 31 ꢀC.TGAperformedundernitrogenwiththeheating
rate of 10 K/min revealed good thermal stability of the synthesized
compounds. Their thermal degradation starts above 370 ꢀC.
3.2. Optical, photophysical, and electrochemical properties
Fig. 3 shows absorption and photoluminescence spectra of the
CBM4 films prepared by casting from THF solutions at room
temperature.
Table 1
Thermal characteristics of CBM4 and IC1, IC2.
Compound
Tg, (ꢀC) a
Tm, (ꢀC) b
Tdec, (ꢀC) c
IC1
IC2
CBM4
181
212
48
e
383
370
454
e
123
a
Determined by DSC: scan rate, 10 K/min; N2 atmosphere; second run.
Melting point was only detected during the first heating; the compound vitrified
b
on cooling to room temperature with 10 K/min.
Onset of decomposition determined by TGA: heating rate, 10 K/min; N2
atmosphere.
Fig. 3. Normalized absorption and photoluminescence spectra of the CBM4 films
prepared by casting from THF solutions at room temperature (a); photoluminescence
and phosphorescence spectra of the CBM4 films at 77 K (b).
c

282
D. Tomkute-Luksiene et al. / Dyes and Pigments 96 (2013) 278e286
Table 2
molecule, low-temperature (77 K) emission measurements were
performed. Fluorescence spectrum of the film of CBM4 at 77 K is
more structured compared to the room temperature one (Fig. 3b).
Phosphorescence spectrum of the CBM4 film at 77 K has four main
emission peaks at 435 nm, 463 nm, 481 nm and 500 nm.
Fig. 4a and b shows normalized absorption and phosphores-
cence spectra of the vacuum deposited films of IC1, IC2 at 300 K and
77 K, and of dilute solutions of IC1 and IC2 in toluene at 300 K. The
absorption spectrum of the vacuum deposited film of IC1 corre-
sponds to the absorption spectrum of its dilute toluene solution and
Optical and photophysical characteristics of the toluene solutions of IC1, IC2.
ab
Ph
l
max
, (nm)
Log ε, (Mꢁ1 ꢂ cmꢁ1
4.76, 4.39
4.67, 4.54, 4.49,
4.34, 4.09, 3.99
)
l
, (nm)
max
h, (%)
IC1
IC2
300, 450
292, 326, 340,
393, 433, 460
611, 654
564, 597
21
41
Table 3
Photophysical characteristics of the films of IC1, IC2, and films of CBM4, CBM4:IC1,
CBM4:IC2.
has the maximum around 300 nm, which is due to intra-ligand
* transitions, and the peak at 450 nm which can be assigned to
metal-to-ligand charge transfer (MLCT) [26].
pe
FL
Ph
lmaxa b, (nm)
l
, (nm)
l
, (nm)
max
max
p
a
a
IC1
IC2
300, 453
294, 314, 326, 340, 388, 437
313
366, 451
312, 385
e
613
588
e
The character of the absorption spectrum of the film of IC2
corresponds to that of toluene solution and peaks at around 340 nm
are due to the presence of carbazole moieties [24]. In a lower
energy region from 380 to 460 nm (Fig. 4b) weak and broad
absorption bands with the corresponding shoulders are observed,
which can be attributed to spin-allowed and spin-forbidden MLCT
transitions of the Ir(III) complexes [26]. The wavelengths of
absorption and phosphorescence maxima are summarized in
Tables 2 and 3.
The PL spectra of the iridium complexes IC1 and IC2 are little
changed going from solution to the film, although low-temperature
(77 K) emission spectra are somewhat more structured. Due to the
presence of stronger electron donor diphenylamine, IC1 has a red
shifted emission maximum, compared to that of IC2, w610 nm and
w570 nm accordingly. However, due to less rigid structure of IC1
and narrower band gap, more energy is lost in nonradiative path-
ways and overall quantum yield in toluene solutions is lower (41%
for IC2 and 21% for IC1).
CBM4 b
367
e
435, 463, 481, 500
613
566, 591
a
a
CBM4:IC1
CBM4:IC2
e
a
Vacuum deposited films.
Films deposited from THF solution.
b
The absorption spectrum of the film of the host-guest system
CBM4:IC1 (Fig. 5) is characterized by two bands i.e. the short
wavelength band with the maximum at 369 nm and long-
wavelength band peaking at 420e520 nm. The latter is appar-
ently due to MLCT. The absorption spectrum of the film of the hoste
guest system CBM4:IC2 is characterized by two bands i.e. by the
short wavelength band peaking at 314 nm characteristic to CBM4
and by the long-wavelength band peaking at 360e460 nm that
could be assigned to MLCT complex.
Fig. 5. Normalized absorption and PL spectra of the solid films CBM4:IC1 and
CBM4:IC2 at 300 K.
Table 4
EHOMO, ELUMO, Ip, band gap and triplet energies for CBM4, IC1 and IC2.a
Compound E1/2, red
vs Fc, (V) Fc, (V)
ꢁ2.25 0.50
ꢁ2.07 0.76
0.37b
E1/2, ox vs EHOMO, (eV)c ELUMO, (eV) Eg, (eV) ET, (eV)f
IC1
IC2
CBM4
ꢁ5.30
ꢁ5.56
ꢁ5.17
ꢁ2.55
ꢁ2.73
ꢁ1.62d
2.75
2.83
3.55e
2.02
2.11
2.85
e
a
The CV measurements were carried out at
a
glassy carbon electrode in
tetrabutylammonium hexa-
dichloromethane solutions containing 0.1
M
fluorophosphate as electrolyte and Ag/AgNO3 as the reference electrode. Each
measurement was calibrated with ferrocene (Fc).
b
Oxidation not reversible, Eonset calculated.
c
EHOMO, LUMO ¼ e(4.8 þ E1/2 vs Fc).
ELUMO ¼ EHOMO ꢁ Egopt
.
d
The optical band gaps Egopt estimated from the edge of electronic absorption
e
Fig. 4. Normalized absorption and phosphorescence spectra of the toluene solutions
(10ꢁ5 M) and solid films of IC1 (a), IC2 (b) at 300 K and 77 K.
spectra.
f
ET triplet energy measured in films at 77 K.

D. Tomkute-Luksiene et al. / Dyes and Pigments 96 (2013) 278e286
283
Fig. 6. First oxidation and reduction waves of IC1 (scan rate ¼ 200 mV sꢁ1) in argon-
purged dichloromethane solution.
PL spectrum of the film of the mixture CBM4:IC2 (Fig. 5) has
maxima at ca. 562 and 600 nm and is similar to the PL spectrum of
the film and toluene solution of IC2 (Fig. 4a). The similar picture is
observed for the film of CBM4:IC1, but long-wavelength lumines-
cence band is not clearly expressed. No CBM4 emission was
observed in the PL spectra of the films of the host-guest systems.
Fig. 9. CIE 1931 chromaticity diagram and photos of the devices A and B.
PL spectrum of CBM4 partially overlaps with absorption band of
IC1 (Fig. 3b and Fig. 4a) and fully overlaps with the MLCTabsorption
band of IC2 (Fig. 4b). Thus, efficient electronic excitation energy
transfer is expected to take place from CBM4 to IC1 and IC2 [27].
Fig. 7. The general structure of the single-layer PhOLED.
Fig. 10. Current densityevoltage and luminanceevoltage characteristics (a) and
current efficiencyecurrent density characteristics (b) for electroluminescent device A.
Fig. 8. Electroluminescence spectra of the devices A and B.

284
D. Tomkute-Luksiene et al. / Dyes and Pigments 96 (2013) 278e286
3.3. Characterization of PhOLED
Two types of single-layer electroluminescent devices of the
configurations ITO/CBM4:IC1(26 nm)/Ca (40 nm)/Al (200 nm) (A)
and ITO/CBM4:IC2(26 nm)/Ca (40 nm)/Al (200 nm) (B) were
fabricated by means of vacuum co-deposition of CBM4 and IC1 or
IC2 from one source boat (at the ratio of 90% (CBM4): 10% (IC1 or
IC2)) onto ITO coated glass substrate (Fig. 7). Electroluminescence
maxima are observed at 610 nm and 650 nm for the device A and at
562 and 600 nm for the device B (Fig. 8). The shapes of the spectra
of electroluminescence of the devices A and B are similar to those of
PL spectra of the solid films of the mixtures of CBM4:IC1 and
CBM4:IC2 respectively (Fig. 5). This observation confirms the
electrophosphorescent emitting nature of the devices.
Commission Internationale de l’Eclairage (CIE 1931) chroma-
ticity coordinates (x, y) of the devices A and B were found to be
(0.44, 0.53) and (0.28, 0.56) corresponding to the red and orange
color respectively (Fig. 9, Table 4).
The device A reaches the current efficiency of ca. 5.0 cd/A
(Fig. 10b) and this value is almost independent on the current
density in the range from 20 to 100 mA/cm2.
Lower turn on voltage value (Table 5) of the device B can
apparently be explained by the better match of energy levels of the
host and guest and of the work function of cathode (Fig. 12). The
difference in brightness of the devices A and B is apparently due to
the higher sensitivity of eye to the luminescence spectral range of
the device B.
Energy-band diagrams of OLEDs A and B are shown in Fig. 12a
and b. EHOMO and ELUMO values of the iridium complexes were
determined using cyclic voltammetry (CV). The ELUMO of the CBM4
was calculated from its optical band gap (Eogpt) and EHOMO obtained
from the CV measurements ðELUMO ¼ EHOMO ꢁ EgoptÞ.
Fig. 11. Current densityevoltage and luminanceevoltage characteristics (a) and
current efficiencyecurrent density characteristics (b) for electroluminescent device B.
This observation indicates that PL emission originates from the
triplet excited states of the IC1 and IC2 complexes [28] and suggests
that energy transfer from CBM4 host to Ir(III) complexes is efficient.
Moreover, a sufficiently higher triplet level of the CBM4 (ca.
2.8 eV, Fig. 3b) allows efficient exciton confinement on the guest
IC1 and IC2 complexes. The triplet energy of CBM4 is higher than
the T1-energy of IC1 and IC2 by more than 0.8 eV and 0.7 eV
respectively (Table 4).
To elucidate the energetic conditions for energy and electron
transfer in dilute solutions, the EHOMO and ELUMO values were also
determined using cyclic voltammetry (CV) (Table 4). The cyclic
voltammograms of the synthesized compounds in dichloro-
methane show quasi-reversible oxidation and reduction couples
(Fig. 6).
IC1, with diphenylamine moiety, exhibits the first oxidation
wave half-wave potential corresponding to 0.5 V vs Fc, which
results in a EHOMO value of ꢁ5.30 eV (on the basis of the EHOMO
energy level of ferrocene as 4.8 eV) and the first reduction half-
wave potential of ꢁ2.25 V vs Fc, resulting in ELUMO ¼ ꢁ2.55 eV.
Substitution of the diphenylamine moiety with that of carbazole
in IC2 results in the decrease of both EHOMO and ELUMO values, as
well as in the slight increase of band gap Eg. These changes in
energy levels of IC2 result in the blue shift of the PL emission
maxima by w50 nm (from 611 nm for IC1 to 564 nm for IC2).
3.4. Characterization of the devices A and B by impedance
spectroscopy
Fig. 13a,b show experimental data of the impedance Z of the
device, measured at three different bias voltages, i.e. 0 V, 1 V, 3 V
and 5 V (symbols correspond to experimental data and the solid
lines represent the fitting data). The dependence is represented by
one semicircle, thus making possible to model the current flow in
the structure using the equivalent circuit characteristic of the diode
structure, consisting of one R1C1 circuits in serial with Rs (Fig. 13c).
The data presented in Tables 5 and 6 show that C1 remains
rather constant versus bias voltage for the frequency range
measured and R1 drops significantly as the bias voltage increases,
especially at voltages higher than the threshold voltage of the
structures (Figs. 10 and 11). The elements C1 and R1 represent bulk
components and are determined by the electrical properties of the
host-guest organic electroluminescent layers (CBM4:IC1 and
CBM4:IC2) [29]. Rs corresponds to overall contact resistance of the
structures.
As the applied bias increases the resonance frequency up shifts
towards the higher frequency indicating a decrease in relaxation
time (Tables 6 and 7). This observation can be explained by the
decrease in the resistance of the bulk as the number of injected
charge carriers increases [30].
Table 5
EL characteristics of the devices A and B.
Lmax, (cd/m2)
LEmax, (cd/A)
EQEmax, (%)
l
, (nm)
CIE 1931 XYZ coordinates, (x, y)
CIE 1960 UCS coordinates, (u, v)
Tc, (K)
el
Devices
Von, (V)
max
A
B
4.0
3.6
5945
10,060
5.3
6.8
2.29
1.45
610, 652
566, 592
(0.65, 0.35)
(0.53, 0.47)
(0.44, 0.53)
(0.28, 0.56)
2436
1054

D. Tomkute-Luksiene et al. / Dyes and Pigments 96 (2013) 278e286
285
Table 6
Fitting parameters of impedance data for device A at various bias voltages.
Bias voltage,
(V)
Value element
Serial
resistor
Parallel
resistor
Parallel
capacitor
C1, (nF)
Peak
frequency,
(kHz)
Dielectric
relaxation
time, (ms)
R1, (
U
)
R2, (k
U
)
0
1
3
5
470.2
471.1
470
397.5
129.7
76.2
1.9
6.3
6.2
6.2
4.6
2
3.2
5
500
312.5
200
25
448.6
40
Table 7
Fitting parameters of impedance data for device B at various bias voltages.
Bias voltage,
(V)
Value element
Fig. 12. Energy-band diagrams of the devices A(a) and B(b).
Serial
resistor
Parallel
resistor
Parallel
capacitor
C1, (nF)
Peak
frequency,
(kHz)
Dielectric
relaxation
time, (ms)
R1, (
U
)
R2, (k
U
)
From the good agreement between the experimental data and
the model for all the applied bias voltages and for all the
frequency range, one can conclude about the absence of addi-
tional potential barriers throughout the structures that indicate
effective energy transfer from the host to guest and the minimum
of energy barrier between the electrodes and organic materials
(Fig. 12).
0
1
3
5
344
361
342
17.5
11.2
1.2
5.7
5.6
5.2
5.2
10
100
79
20
12.6
50.1
63
238.5
1.2
16
4. Conclusions
Phosphorescent bis-cyclometallated iridium(III) complexes
based on the 2-phenyl-1,2,3-benzotriazole with additional donoric
units were synthesized. The isolated amorphous compounds were
found to express good solubility in common organic solvents, and
high glass transition temperatures. The obtained iridium complexes
were found to exhibit red and orange phosphorescence at 610 and
570 nm respectively with the CIE chromaticity coordinates of (0.65,
0.35) for IC1 and (0.53, 0.47) for IC2. Stronger diphenylamine
donoric unit in IC1 has more influence on the energy levels of the
molecule, which results in a bathochromic shift of the PL emission
maxima compared with carbazole-containing IC2.
It was shown that the phosphorescence of the CBM4 films
occurs in the region of 2.5e2.8 eV. According to evaluation the
position of triplet level of CBM4 is by more than 0.7 eV higher than
the first excited triplet states of IC1 and IC2 and allows efficient
exciton confinement on the guest complexes.
Single-layer PhOLEDs were fabricated employing a simple
technological approach, based upon the simultaneous vacuum
deposition from one crucible of host material CBM4 and a phos-
phorescent 1,2,3-triazole-based iridium complex. The maximum
brightness of the obtained PhOLEDs is about 6000 and 10,000 cd/
m2 with current efficiency 5.3 cd/A and 6.8 cd/A for the devices
with IC1 and IC2 respectively. These characteristics are quite high,
taking into the account the absence of any additional injection and
transport layers in the constructed PhOLEDs.
The results of impedance spectroscopy suggest that the
proposed method of formation of emitting layer provides homo-
geneous distribution of molecular guest in the matrix of host and
the PhOLED is characterized by low energy barrier between the
electrodes and host-guest layer.
Acknowledgments
The support from the programme of collaboration in the field of
science and technologies between Lithuania and Ukraine is grate-
fully acknowledged (project No TAP LU 01/2012). This research was
funded by the European Social Fund under the Global Grant
ꢁ
measure (VP1-3.1-SMM-07-K 01-078). We would also want to
thank Dr. Karolis Kazlauskas (Vilnius University, Lithuania) for
photoluminescence quantum yield measurements.
Fig. 13. Impedance spectra of the device A (a), B (b) and equivalent circuits (c).

286
D. Tomkute-Luksiene et al. / Dyes and Pigments 96 (2013) 278e286
[14] Qiao X, Tao Y, Wang Q, Ma D, Yang C, Wang L, et al. Controlling charge balance
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