Bipolar highly solid-state luminescent phenanthroimidazole derivatives as materials for blue and white organic light emitting diodes exploiting either monomer, exciplex or electroplex emission

Article abstract of DOI:10.1016/j.dyepig.2017.07.029

Four phenanthroimidazole-based bipolar compounds having electron-donating carbazole or diphenylamino moieties were synthesized and characterized. All compounds form glasses and exhibit high glass transition temperatures ranging from 183 to 239 °C. Solid state blue emission was detected for all synthesized compounds and quantum yields in solid state reached 0.55. Room temperature hole and electron mobilities in the layers of phenanthroimidazole derivatives reached 3.14 × 10^?4 and 5.69 × 10^?4 cm²/V·s, respectively, at an electric field of 3.6 × 10⁵ V/cm. Quantum chemical calculations on the molecular level were employed to interpret optical, photophysical and photoelectrical properties of the compounds. Due to the efficient blue solid-state emission and ambipolar charge transport the phenanthroimidazole-based derivatives were used for the preparation of non-doped emitting layers of blue OLEDs. The electroplex forming properties of the compounds were observed in a host:guest emitting layer with a commercial hole-transporting material 4,4′-cyclohexylidenebis [N,N-bis(4-methylphenyl)benzenamine]. Two most promising phenanthroimidazole-based compounds were used for the fabrication of white OLEDs which were based on both fluorescence and either electroplex or exciplex emissions. The best almost blue and white OLEDs were characterized by maximal current efficiency of 1.3 and 5.3 cd/A, power efficiency of 0.8 and 1.7 lm/W, and external quantum efficiency of 0.95 and 2.9 %.

Full text of DOI:10.1016/j.dyepig.2017.07.029

Accepted Manuscript
Bipolar highly solid-state luminescent phenanthroimidazole derivatives as materials
for blue and white organic light emitting diodes exploiting either monomer, exciplex or
electroplex emission
Rita Butkutė, Ramūnas Lygaitis, Viktorija Mimaitė, Dalius Gudeika, Dmytro Volyniuk,
Gjergji Sini, Juozas Vidas Grazulevicius
PII:
S0143-7208(17)30101-8
10.1016/j.dyepig.2017.07.029
DYPI 6118
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 17 January 2017
Revised Date: 15 June 2017
Accepted Date: 14 July 2017
Please cite this article as: Butkutė R, Lygaitis Ramū, Mimaitė V, Gudeika D, Volyniuk D, Sini G,
Grazulevicius JV, Bipolar highly solid-state luminescent phenanthroimidazole derivatives as materials for
blue and white organic light emitting diodes exploiting either monomer, exciplex or electroplex emission,
Dyes and Pigments (2017), doi: 10.1016/j.dyepig.2017.07.029.
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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
Bipolar highly solid-state luminescent phenanthroimidazole derivatives as
materials for blue and white organic light emitting diodes exploiting either
monomer, exciplex or electroplex emission
Rita Butkutė1, Ramūnas Lygaitis1, Viktorija Mimaitė1, Dalius Gudeika1, Dmytro Volyniuk1, Gjergji
Sini2, Juozas Vidas Grazulevicius1
1Department of Polymer Chemistry and Technology, Kaunas University of Technology, Radvilėnų pl. 19, LT-50254
Kaunas, Lithuania
2
Laboratoire de Physicochimie des Polymères et des Interfaces, Université de Cergy-Pontoise, EA 2528, 5 mail Gay-
Lussac, Cergy-Pontoise Cedex, 95031, France
Abstract
Four phenanthroimidazole-based bipolar compounds having electron-donating carbazole or
diphenylamino moieties were synthesized and characterized. All compounds form glasses and
exhibit high glass transition temperatures ranging from 183 to 239 oC. Solid state blue emission was
detected for all synthesized compounds and quantum yields in solid state reached 0.55. Room
temperature hole and electron mobilities in the layers of phenanthroimidazole derivatives reached
3.14×10-4 and 5.69×10-4 cm2/V·s, respectively, at an electric field of 3.6×105 V/cm. Quantum
chemical calculations on the molecular level were employed to interpret optical, photophysical and
photoelectrical properties of the compounds. Due to the efficient blue solid-state emission and
ambipolar charge transport the phenanthroimidazole-based derivatives were used for the preparation
of non-doped emitting layers of blue OLEDs. The electroplex forming properties of the compounds
were observed in a host:guest emitting layer with a commercial hole-transporting material 4,4′-
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cyclohexylidenebis[N,N-bis(4-methylphenyl)benzenamine].
Two
most
promising
phenanthroimidazole-based compounds were used for the fabrication of white OLEDs which were
based on both fluorescence and either electroplex or exciplex emissions. The best almost blue and
white OLEDs were characterized by maximal current efficiency of 1.3 and 5.3 cd/A, power
efficiency of 0.8 and 1.7 lm/W, and external quantum efficiency of 0.95 and 2.9 %.
Keywords. Phenanthroimidazole, solid-state emission, ambipolar mobility, exciplex, electroplex
1. Introduction
Intermolecular charge transfer systems enabling thermally activated delayed fluorescence (TADF)
are utilized for the fabrication of organic light-emitting diodes (OLEDs). The possibility of emissive
harvesting of 100 % excited states in the emitting layers based on such systems was discovered [1,
2]. The emissive harvesting of TADF occurs through the lowest singlet energy level to which
energy is additionally transferred from the triplet energy level [1]. Interaction of holes on a highest
occupied molecular orbital (HOMO) level of donor molecules with electrons on a lowest
unoccupied molecular orbital (LUMO) level of acceptor molecules in a donor-acceptor molecular
mixture leads to the formation of the intermolecular charge transfer systems [3,4,5].
Exciplex emission can be obtained from the donor-acceptor molecular mixtures under both photo-
and electro-excitations [4]. The singlet and triplet energy levels are close for exciplex-forming
compounds and this similarity leads to the reverse intersystem crossing (RISC) and, as a result,
TADF can be obtained [6]. However, since the energy levels of exciplexes are always lower than
singlet energies of exciplex-forming donor and acceptor molecules, the exciplex emission is shifted
to lower-energy spectral region compared to the emission of pure donor and acceptor molecules
[7,8]. Despite that, expedient selection of donor and acceptor molecules by carefully analysing their
HOMO and LUMO energies allows to obtain exciplex emission with colours ranging from blue to
red, which was successfully exploited in all-exciplex OLEDs of different colours including white
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devices [3]. Exciplex-forming donor-acceptor mixtures are also exploited as hosts in
phosphorescent OLEDs for energy transfer from an exciplex to an emitter [9]. White OLEDs with
single emitting layer can be designed since interface exciplexes can be formed between compounds
constituting neighbouring layers [9]. A vertical organic light-emitting transistor with adjustable
emission colour was demonstrated using a periodic interrupted middle electrode in the region of an
interface exciplex [10]. Recently, long-range coupling of electron-hole pairs which produce
exciplexes in organic multilayer structures was observed [11]. This observation opens new
possibilities in the development of efficient exciplex-based devices.
Another intermolecular charge transfer system, i.e. electroplex can also be formed between
donor and acceptor molecules [3]. Electroplex emission can be observed only under electrical
excitation through carrier recombination of holes on a HOMO level of donor and of electrons on a
LUMO level of acceptor [12]. Electroplex emission can be utilized as a good approach for the
fabrication of OLEDs similarly to exciplex emission. For example, a white top-emitting OLED with
a single emissive layer based on an interface electroplex between low-molar-mass compounds 4,4’ -
bis(2,2’-diphenylvinyl)-1,1’-biphenyl (DPVBi) and 2,9-dimethyl-4,7- diphenyl-1,1’-phenanthroline
(BCP) was demonstrated [13]. Metal complexes and polymeric electroactive materials were found
to be capable of electroplex formation which were used for the fabrication of white OLEDs [14,15].
However, the number of electroplex-forming molecules is still low which limits diversity of color
of electroplex emission and efficiency of electroplex emitters. To better unclose potential of usage
of electroplex emitters for OLED applications, the search for new electroplex-forming molecules is
still an urgent task.
Phenanthroimidazole–based derivatives called quite big attention of scientific community due
to their attractive bipolar properties and excellent luminescence efficiency. Mainly,
phenanthroimidazole–based materials were exploited in two application areas related to OLED's
relying on their emitting or bipolar properties. Starting with the publication of Wang et al [16]
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phenanthroimidazole–based derivatives were developed and used mainly as deep blue singlet
emitters for organic light emitting diodes [17], nevertheless some of the compounds showed
emissions in UV [18] and red regions of electromagnetic radiation [19].
The aim of this work was synthesis and studies of new solid state emissive bipolar
electroactive compounds having electron-accepting phenanthroimidazole moieties and electron-
donating carbazolyl or diphenylamino units. We studied new phenanthroimidazole derivatives as
blue emitters for non-doped OLEDs. In addition, the electroplex forming properties of such
compounds were studied in a host:guest emitting layer with a commercial hole-transporting material
4,4′-cyclohexylidenebis[N,N-bis(4-methylphenyl)benzenamine] (TAPC). Two most promising
phenanthroimidazole-based derivatives were used for the fabrication of white OLEDs based on both
fluorescence and either electroplex or exciplex emissions.
2. Experimental methods
2.1 Instrumentation
13
Nuclear magnetic resonance (1H NMR and C NMR) spectra were obtained using a Varian Unity
Inova (400 MHz (1H), 101 MHz (13C)). All the data are given as chemical shifts in δ (ppm),
multiplicity, integration down-field from (CH3)4Si. Mass spectra were obtained on a Waters ZQ 2000
(Waters). Infrared (IR) spectra were performed on Perkin Elmer Spectrum BX II FT-IR System,
using KBr pellets. Elemental analysis was performed with an Exeter Analytical CE-440 Elemental
Analyzer. Differential scanning calorimetry (DSC) measurements were performed on Perkin Elmer
DSC 8500 (heating/cooling rate 20 oC/min). Thermogravimetric analysis (TGA) measurements were
performed on a Mettler Toledo TGA/SDTA 851e. Fluorescence emission spectra were recorded
with a Perkin Elmer LS 55 spectrometer. Quantum yields in solutions determined using
comparative method [20]. The solid-state luminescence quantum yields measured by FLS980
spectrometer (Edinburgh Instruments) using integrating sphere supplied by the spectrometer
producer. Absorption spectra were recorded with a Perkin Elmer Lambda 35 spectrophotometer.
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Electrochemical investigations carried out using µ-AUTOLAB III (Metrohm Autolab B.V.)
potentiostat-galvanostat. The solutions of the synthesized compounds with the concentration of 10-3
M were used for cyclic voltamperometry (CV) measurements. The standard ferrocene/ferrocenium
redox system used as an internal standard for calculating ionisation potentials [21]. The ionization
potentials (IPE) were obtained by the electron photoemission method in air as described earlier [22,
23]. The samples for the measurements were prepared by casting THF solutions of the compounds
onto pre-cleaned indium tin oxide (ITO) coated glass substrates. Charge drift mobilities of the
vacuum deposited layers of the compounds were tested by the time-of-flight (ToF) method as
described earlier [24,25]. Diode-like samples with the thicknesses of layers ranging from 1.8 to 7.1
µm and with bottom ITO and top aluminium electrodes were fabricated. The experimental setup
included a Nd:YAG laser EKSPLA NL300, a oscilloscope Tektronix TDS 3032C, and a 6517B
Keithley electrometer. To estimate the charge drift mobilities, the formula µ=d2(U×ttr)-1 was used,
where d is the layer thickness, U is applied voltage to the samples, and ttr is the transit time. OLEDs
containing non-doped and doped emitting layers with the following architectures were fabricated:
ITO/MoO3(3 nm)/m-MTDATA(30 nm)/ the synthetized compound (20 nm)/PBD(10 nm)/TPBi(25
nm)/Ca(10 nm)/Al(60 nm) and ITO/MoO3(3 nm)/m-MTDATA(30 nm)/ the molecular mixture of
the synthesized compounds with TAPC or m-MTDATA (20 nm)/PBD(10 nm)/TPBi(25 nm)/Ca(10
nm)/Al(60 nm). The layers of organic compounds and those of calcium and aluminium were
prepared by vacuum-deposition under the vacuum of 2-5×10−6 mBar using the equipment from Kurt
J. Lesker in-built in an MB EcoVap4G glove box. Before deposition of the layers, the ITO-coated
glass substrates with a sheet resistance of 15 Ω/sq were cleaned in acetone and isopropyl alcohol
ultrasonic bath during ca. 10 min. To record the density-voltage and luminance-voltage
characteristics, a Keithley 6517B electrometer and a Keithley 2400-C source-meter were used in air
without passivation immediately after the formation of OLEDs. A calibrated photodiode and an
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Avaspec-2048XL spectrometer were utilized for the measurements of brightness and
electroluminescence spectra, respectively.
Quantum chemical calculations. The computations were performed in the frame of density
functional theory (DFT) [25] employing the B3LYP [27] functional with Gaussian 09 program [28].
The 6-31G(d,p) basis set was utilized for all calculations. The spectroscopic properties of the
molecules were calculated by mean of time dependent density functional theory method (TDDFT)
[29]. Up to 40 excited states were calculated and the theoretical absorption bands were obtained by
considering a band half-width at half-maximum 0.3 eV. The theoretical ionization potentials (IP)
were calculated at the B3LYP/6-31G(d,p) level as energy difference between neutral and cation
radical species at the neutral state geometry. The intramolecular reorganization energies of holes
and electrons for the isolated molecules were calculated according the method described in
literature [30].
1.1 Materials
9,10-phenanthrenequinone (Aldrich, 80%), trifluoromethanesulfonic acid (Aldrich, 98%),
sodium hydrosulfite (Aldrich, 85%), sodium-tert-butoxide (Aldrich, 97%), bromine (Aldrich, pure),
9-ethyl-3-carbazole-carboxaldehyde (Aldrich, 98%), 18-crown-6 (Aldrich, 99%), copper (Cu)
(Aldrich, 99%), tris(dibenzylideneacetone)dipalladium(0) (Aldrich, 97% ), tri-tert-butylphosphine
(P(t-Bu)3) (Aldrich, 1 mol/l), N-iodosuccinimide (Fluka, 97%), aniline (Fluka, 98%), ammonium
acetate (Reachem, 98%), potassium carbonate (K2CO3) (Reachim, 99 %), bis(4-tert-
butylphenyl)amine (TCI, 90%) were used as received. 2,7-Diiodophenanthrene-9,10-dione (1) and
3,6-dibromophenanthrene-9,10-dione (2) were synthesized using procedures described in literature
[30,31].
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1.2 Synthesis
1.2.1 2-(9-Ethyl-9H-carbazol-3-yl)-5,10-diiodo-1-phenyl-1H-phenanthro[9,10-d]imidazole (3)
To a solution of 2,7-diiodophenanthrene-9,10-dione (1) (0.5 g, 1.09 mmol), 9-ethyl-3-
carbazolecarboxaldehyde (0.24 g, 1.09 mmol), ammonium acetate (0.84 g, 10.9 mmol) in glacial
acetic acid (5 ml) aniline (0.12 g, 1.3 mmol) was added dropwise. The reaction mixture was stirred
o
at 120 C and after 2 h cooled to room temperature. Brown precipitate was filtered, washed with
methanol and dried. The crude product was purified by column chromatography (silica gel, 3:1
toluene:hexane) to give a white solid. Yield: 0.2 g (25%). 1H NMR (400 MHz, d6-DMSO, δ, ppm):
1.10 (t, J=7.0 Hz, 3H), 4.46 (q, J=7.0 Hz, 2H) 7.21-7.28 (m, 1H), 7.38 (d, J=1.8 Hz, 1H), 7.46-7.54
(m, 1H), 7.58-7.68 (m, 2H), 7.73-7.88 (m, 7H) 7.97-8.07 (m, 2H), 8.37 (s, 1H), 8.65-8.74 (m, 2H),
9.07 (d, J=1.8 Hz, 1H). 13C NMR (101 MHz, CDCl3, δ, ppm): 152.34, 140.99, 140.30 (3C), 138.47,
131.86 (2C), 130.39 (2C), 130.16, 129.10 (3C), 127.64, 127.14, 126.88, 126.09, 125.39, 124.58
(2C), 122.89, 122.72, 121.80 (2C), 120.53 (2C), 120.18, 119.37 (2C), 114.07, 108.68, 108.29,
37.66, 13.81. IR, KBr, (cm-1): 3057, v (CHar); 2975, v, (CHaliph); 1591, v (C=C-N); 1559, 1498,
1453, v (C=Car); 1346, v (C-N); 808, 748, 699, γ (C-Har); 474, v (C-I). MS (APCl+, 25 V), m/z: 740
([M+H]+), (C35H23I2N3, MW=739.39 g/mol)
1.2.2 6,9-Dibromo-2-(9-ethyl-9H-carbazol-3-yl)-1-phenyl-1H-phenanthro[9,10-d]imidazole (4)
The compound was prepared from 3,6-dibromophenanthrene-9,10-dione (2) (0.5 g, 1.37
mmol), 9-ethyl-3-carbazolecarboxaldehyde (0.3 g, 1.37 mmol), aniline (0.153 g, 1.64 mmol),
ammonium acetate (1.05 g, 13.7 mmol), and 5 ml glacial acetic acid using a similar synthesis
1
procedure for 3, affording a yellow solid. Yield: 0.6 g (68 %). H NMR (400 MHz, d6-DMSO, δ,
ppm.): 1.0 (t, J=7.1 Hz, 3H), 4.44 (q, J=7.1 Hz, 2H), 7.04 (d, J=8.9 Hz, 1H), 7.14-7.28 (m, 1H),
7.46-7.51 (m, 1H), 7.56-7.61 (m, 2H), 7.63 (d, J=8.2 Hz, 1H), 7.69-7.74 (m, 3H), 7.76-7.80 (m,
2H), 7.95-8.01 (m, 2H), 8.29 (d, J=1.5 Hz, 1H), 8.68 (d, J=8.6 Hz, 1H), 9.20 (d, J=1.7 Hz, 1H),
7

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9.22 (d, J=1.9 Hz, 1H). 13C NMR (101 MHz, CDCl3, δ, ppm.): 152.60 (2C), 140.32, 140.14, 138.70
(2C), 130.87, 130.34 (2C), 130.02, 129.90, 129.46, 129.23 (2C), 128.63, 127.69, 127.16, 126.90,
126.05, 125.90, 125.84, 124.75, 122.89, 122.69, 122.27, 121.86, 121.74, 120.46, 119.91, 119.32,
119.04, 108.68, 108.24, 37.65, 13.80. IR KBr, (cm-1): 3050, v (CHar); 2968, v, (CHaliph); 1598, v
(C=C-N); 1508, 1495, 1452, v (C=Car); 1350, v (C-N); 810, 751, 702, γ (C-Har); 539, v (C-Br). MS
(APCl+, 25 V), m/z: 646 ([M+H]+), (C35H23Br2N3, MW=645.39 g/mol)
1.2.3 N-(4-(5,10-diiodo-1-phenyl-1H-phenanthro[9,10-d]imidazol-2-yl)phenyl)-N-phenyl-
benzenamine (5)
To a solution of 2,7-diiodophenanthrene-9,10-dione (1) (0.5 g, 1.09 mmol) in glacial acetic
acid (5 ml) 4-(diphenylamino)benzaldehyde (0.3 g, 1.09 mmol), ammonium acetate (0.83 g, 10.9
mmol) and aniline (0.12 g, 1.3 mmol) were added. The temperature was increased to 120 oC. After
2 h, the reaction mixture was cooled to room temperature. The precipitate was filtered, washed with
methanol and dried. The crude product was purified by column chromatography (silica gel, 3:1
toluene:hexane) to give a yellow solid. Yield: 0.6 g (71 %). 1H NMR (400 MHz, CDCl3, δ, ppm.):
6.94–6.98 (m, 2H), 7.05–7.12 (m, 9H), 7.25–7.31 (m, 6H), 7.37 (d, J = 1.7 Hz, 1H), 7.45–7.48 (m,
2H), 7.74 (dd, J = 6.2 Hz, J = 2.6 Hz, 1H), 7.91 (dd, J = 6.0 Hz, J = 2.0 Hz, 1H), 8.33 (d, J = 8.9
Hz, 1H), 8.38 (d, J = 8.9 Hz, 1H), 9.23 (d, J = 1.9 Hz, 1H). 13C NMR (101 MHz, CDCl3, δ, ppm.):
176.60 (2C), 147.07, 134.42 (2C), 133.37, 130.39 (3C), 130.08 (4C), 129.39 (6C), 128.92 (3C),
126.88, 125.39 (2C), 125.19 (6C), 124.58, 123.70 (3C), 121.69 (2C), 93.85, 92.64. IR, KBr, (cm-1):
3058, v (CHar); 1610, v (C=C-N); 1592, 1524, 1490, v (C=Car); 1331, v (C-N); 797, 742, 695, γ (C-
Har); 436, v (C-I). MS (APCl+, 25 V), m/z: 712.21 ([M-C6H5]+), (C39H25I2N3, MW=789.44 g/mol)
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1.2.4 N-(4-(6,9-dibromo-1-phenyl-1H-phenanthro[9,10-d]imidazol-2-yl)phenyl)-N-
phenylbenzenamine (6)
The compound was prepared from 3,6-dibromophenanthrene-9,10-dione (2) (0.5 g, 1.37
mmol), 4-(diphenylamino)benzaldehyde (0.37 g, 1.37 mmol), aniline (0.15 g, 1.65 mmol),
ammonium acetate (1.05 g, 13.7 mmol), and 5 ml of glacial acetic acid using a similar synthesis
procedure for 5, yellow crystals was finally obtained. Yield: 0.5 g (53 %). 1H NMR (400 MHz, d6-
DMSO, δ, ppm.): 7.08–7.17 (m, 11H), 7.33–7.41 (m, 6H), 7.90 (d, J=8.5 Hz, 2H), 8.15 (d, J=8.8
Hz, 2H), 8.46 (d, J = 8.6 Hz, 2H), 9.11 (d, J=1.4 Hz, 2H). 13C NMR (101 MHz, d6-DMSO, δ,
ppm.): 150.38, 148.87, 147.17 (2C), 131.04 (2C), 130.22 (7C), 128.59, 127.95 (2C), 127.27 (2C),
125.21 (7C), 124.30 (6C), 123.93, 122.50 (2C), 119.20 (2C). IR KBr, (cm-1): 3035, v (CHar); 1612,
v (C=C-N); 1591, 1519, 1477, v (C=Car); 1331, v (C-N); 812, 738, 695, γ (C-Har); 542, v (C-Br).
MS (APCl+, 25 V), m/z: 618.03([M-C6H5]+), (C39H25Br2N3, MW=695.44 g/mol)
1.2.5 5,10-Bis(3,6-di-tert-butyl-9H-carbazol-9-yl)-2-(9-ethyl-9H-carbazol-3-yl)-1-phenyl-1H-
phenanthro[9,10-d]imidazole (7).
To a solution of 2-(9-ethyl-9H-carbazol-3-yl)-5,10-diiodo-1-phenyl-1H-phenanthro[9,10-
d]imidazole (3) (0.05 g, 0.07 mmol), 3,6-di-tert-butylcarbazole (0.06 g, 0.21 mmol) and 18-crown-6
(0.004 g, 0.014 mmol) in o-dichlorobenzene (1 ml) were added K2CO3 (0.027 g, 0.21 mmol) and
o
Cu powder (0.018 g, 0.28 mmol). The reaction was stirred at 190 C for 24 h under argon
atmosphere. Insoluble inorganic salts were removed by filtration and washed with small amount of
toluene. After removal of o-dichlorobenzene brown liquid was added to hexane. The precipitate was
collected by filtration and purified by column chromatography (silica gel, 6:1 hexane:acetone) to
1
give a white solid. Yield: 0.05 g (70 %). H NMR (400 MHz, CDCl3, δ, ppm.): 1.42 (t, J=7.0 Hz,
3H), 1.54 (d, J=3,9 Hz, 36H), 4.34 (q, J=7.0 Hz, 2H), 7.16-7.69 (m, 17H), 7.80-7.95 (m, 5H), 8.15
(s, 1H), 8.25 (s, 3H), 8.97 (d, J=8.9 Hz, 1H), 9.03 (d, J=9,0 Hz, 1H), 7.56-7.64 (m, 2H), 9.23 (d,
J=1.7 Hz, 1H). 13C NMR (101 MHz, CDCl3, δ, ppm.): 152.77, 143.32, 142.97 (2C), 140.30,
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140.03, 139.52, 138.90, 138.55 (2C), 137.97, 137.28, 136.28, 130.55 (3C), 129.77, 128.90 (2C),
128.45, 127.24, 126.88, 126.59, 125.84 (3C), 124.94, 124.29, 124.09, 123.59 (8C), 122.95, 122.64,
121.61, 120.39 (4C), 119.23 (2C), 117.69, 116.12 (3C), 109.62 (4C), 108.61 (2C), 108.27, 34.80
(2C), 32.07 (16C). IR, KBr, (cm-1): 3045, v (CHar); 2959, v, (CHaliph); 1614, v (C=C-N); 1531,
1479, 1452, v (C=Car); 1349, v (C-N); 810, 749, 703, γ (C-Har). MS (APCl+, 25 V), m/z: 1042.6
([M]+), (C75H71N5, MW=1042.4 g/mol). Anal. Calcd. for C75H71N5: C, 86.42; H, 6.87; N, 6.71 %;
found C 86.37, H 6.90, N 6.73%.
1.2.6 6,9-Bis(3,6-di-tert-butyl-9H-carbazol-9-yl)-2-(9-ethyl-9H-carbazol-3-yl)-1-phenyl-1H-
phenanthro[9,10-d]imidazole (8)
To a solution of 6,9-dibromo-2-(9-ethyl-9H-carbazol-3-yl)-1-phenyl-1H-phenanthro[9,10-
d]imidazole (4) (0.2 g, 0.31 mmol), 3,6-di-tert-butylcarbazole (0.2 g, 0.71 mmol) and t-BuONa
(0.18 g, 1.86 mmol) in anhydrous toluene (5 ml) were added Pd2(dba)3 (0.012 g, 0.01 mmol) and
P(t-Bu)3 (0.003 g, 0.001 mmol, 0,1 M solution in toluene). The reaction was stirred at 110 oC for 24
h under argon atmosphere. Solvent was removed using vacuum, the precipitate was purified by
column chromatography (silica gel, toluene) to give a yellow solid. Yield: 0.11 g (34 %). 1H NMR
(400 MHz, d6-DMSO, δ, ppm.): 1.31 (t, J=7.0 Hz, 3H), 1.39 (d, J=5.7 Hz, 36H), 4.46 (q, J=6.9 Hz,
2H), 7.21-7.29 (m, 2H), 7.38 (d, J=8.7 Hz, 1H), 7.34-7.54 (m, 6H), 7.58-7.68 (m, 3H), 7.73-7.79
(m, 3H), 7.81 (dd, J=8.6 Hz, J=1.7 Hz, 1H), 7.88-7.93 (m, 2H), 8.03 (d, J=7.5 Hz, 2H), 8.26 (d,
J=1.7 Hz, 1H), 8.31 (d, J=1.6 Hz, 1H), 8.37 (d, J=1.5 Hz, 1H), 9.03 (d, J=8.5 Hz, 1H), 9.25 (s,
1H), 9.29 (s, 1H). 13C NMR (101 MHz, d6-DMSO, δ, ppm.): 153.04, 149.48, 147.51, 142.95,
139.12, 138.14, 137.32, 136.36, 135.43, 134.78, 133.68, 130.03 (2C), 129.93 (2C), 129.40, 127.66
(2C), 125.80 (2C), 125.75, 125.67 (2C), 125.22, 124.36, 124.33 (2C), 123.27 (2C), 123.09, 122.22
(2C), 121.56, 121.46, 121.30, 121.16, 120.94, 120.01 (2C), 119.35 (2C), 118.75, 117.78, 117.20
(2C), 116.26, 113.63, 113.45, 109.62 (3C), 109.34 (2C), 109.20, 106.69 (3C), 34.97 (2C), 32.30
(16C). IR, KBr, (cm-1): 3053, v (CHar); 2954, v, (CHaliph); 1616, v (C=C-N); 1489, 1475, 1457, v
10

ACCEPTED MANUSCRIPT
(C=Car); 1348, v (C-N); 809, 746, 702, γ (C-Har). MS (APCl+, 25 V), m/z: 1042.6 ([M]+),
(C75H71N5, MW=1042.4 g/mol). Anal. Calcd. for C75H71N5: C, 86.42; H, 6.87; N, 6.71 %; found C
86.39, H 6.89, N 6.72%.
1.2.7 2-(4-(Diphenylamino)phenyl)-5,10-(4-tert-butylphenyl)-1-phenyl-1H-phenanthro[9,10-
d]imidazole-5,10-diamine (9)
The compound was prepared from N-(4-(5,10-diiodo-1-phenyl-1H-phenanthro[9,10-
d]imidazol-2-yl)phenyl)-N-phenyl-benzenamine (5) (0.5 g, 0.6 mmol), bis(4-tert-butylphenyl)amine
(0.51 g, 1.8 mmol), 18-crown-6 (0.032 g, 0.12 mmol), K2CO3 (0.23 g, 1.8 mmol), copper powder
(0.15 g, 2.4 mmol), and 6 ml of o-dichlorobenzene using a similar synthetic procedure as for
compound 7, white solid was finally obtained. Yield: 0.1 g (15 %). 1H NMR (400 MHz, CDCl3, δ,
ppm.): 1.34-1.39 (m, 36H), 6.85-6.90 (m, 8H), 7.00-7.07 (m, 14H), 7.12-7.17 (m, 3H), 7.20-7.27
13
(m, 14H), 7.32-7.37 (m, 2H). C NMR (101 MHz, CDCl3, δ, ppm.): 145.35 (14C), 129.80 (3C),
129.29 (6C), 128.49 (4C), 125.89 (9C), 125.00 (8C), 123.61 (19C), 31.52 (16C). IR, KBr, (cm-1):
3060, v (CHar); 2960, v, (CHaliph); 1599, v (C=C-N); 1510, 1460, v (C=Car); 1317, v (C-N); 807,
754, 695, γ (C-Har). MS (APCl+, 25 V), m/z: 1096.9 ([M]+), (C79H77N5, MW=1096.5 g/mol). Anal.
Calcd. for C79H77N5: C, 86.53; H, 7.08; N, 6.39 %; found C 86.48, H 7.11, N 6.41%.
1.2.8 2-(4-(Diphenylamino)phenyl)-6,9-(4-tert-butylphenyl)-1-phenyl-1H-phenanthro[9,10-
d]imidazole-6,9-diamine (10).
The compound was prepared from N-(4-(6,9-dibromo-1-phenyl-1H-phenanthro[9,10-
d]imidazol-2-yl)phenyl)-N-phenylbenzenamine (6) (0.4
g, 0.58 mmol), bis(4-tert-
butylphenyl)amine (0.37 g, 1.33 mmol), t-BuONa (0.33 g, 3.48 mmol), Pd2(dba)3 (0.024 g, 0.02
mmol), P(t-Bu)3 (0.006 g, 0.002 mmol, 0,1 M solution in toluene), and 5 ml of anhydrous toluene
using a similar synthetic procedure as for 8, affording a white solid. Yield: 0.35 g (56 %). 1H NMR
(400 MHz, d6-DMSO, δ, ppm.): 2.09 (m, 36H), 7.11-7.21 (m, 25H), 7.22-7.30 (m, 16H). 13C NMR
11

ACCEPTED MANUSCRIPT
(101 MHz, CDCl3, δ, ppm.): 144.57 (6C), 130.19 (9C), 129.26 (14C), 128.15 (3C), 125.65 (25C),
123.51 (6C), 31.47 (16C). IR, KBr, (cm-1): 3034, v (CHar); 2958, v, (CHaliph); 1591, v (C=C-N);
1510, 1473, v (C=Car); 1317, v (C-N); 828, 753, 695, γ (C-Har). MS (APCl+, 25 V), m/z: 1096.64
([M]+), (C79H77N5, MW=1096.5 g/mol). Anal. Calcd. for C79H77N5: C, 86.53; H, 7.08; N, 6.39 %;
found C 86.50, H 7.12, N 6.38%.
3. Results and Discussion
Synthetic routes for the donor-substituted phenanthroimidazole derivatives 7-10 are outlined
in Scheme 1. The starting compounds 1 and 2 were prepared according to the procedures earlier
reported in literature [31,32]. The key intermediate phenanthroimidazole derivatives 3-6 were
obtained by the reactions of compounds 1 or 2 with the corresponding aldehydes. The target
compounds 7 and 9 were prepared by Ullmann coupling reactions [33]. Derivatives 8 and 10 were
synthesized using Buchwald-Hartwig conditions [34]. All the synthesized compounds were purified
by column chromatography and characterized by IR, NMR spectroscopies and mass spectrometry,
elemental analyses as described in the experimental section.
12

ACCEPTED MANUSCRIPT
R1
R1
R2
R2
N
N
N
N
e,f
c
N
N
R2
R2
R1
R1
R1
R2
7: R1=
*, R2=H;
N
3: R1=I, R2=H;
4: R1=H, R2=Br.
O
O
O
a,b
*
.
O
N
8: R1=H, R2=
R2
R1
R1
R1
R2
R2
1: R1=I, R2=H;
2: R1=H, R2=Br.
N
N
N
e,f
d
N
N
N
R2
2
R2
1
N
3
R1
R1
N
N
11
10
4
6
9: R1=
*, R2=H;
5: R1=I, R2=H;
6: R1=H, R2=Br.
5
*
N
10: R1=H, R2=
.
9
8
7
Scheme 1. Synthetic routes for phenanthroimidazole derivatives. a. N-iodosuccinimide, CF3SO3H, 6 h, 20 oC; b.
Br2, NaHSO3, nitrobenzene, 22 h, 120 oC; c. 9-ethyl-3-carbazolecarboxaldehyde, CH3CO2NH4, aniline, CH3COOH, 2h,
120 oC; d. 4-(diphenylamino)benzaldehyde, ammonium acetate, aniline, CH3COOH, 2h, 120 oC; e. 3,6-di-tert-
butylcarbazole (bis(4-tert-butylphenyl)amine), 18-crown-6, Cu, K2CO3, o-dichlorobenzene, 24h (48h), 190 oC; f. 3,6-
di-tert-butylcarbazole (bis(4-tert-butylphenyl)amine), t-BuONa, Pd2(dba)3, P(t-Bu)3, toluene, 24 h, 110 oC.
3.1 Thermal properties
The thermal properties of phenanthroimidazole derivatives 7-10 were investigated by TGA
and DSC under a nitrogen atmosphere. Their thermal characteristics are summarized in Table 1. All
compounds exhibit high thermal stability and glass-forming ability. Their 5% weight-loss
temperatures (Td) range from 491 to 524 °C. The DSC curves of the second heating of the
compounds 7-10 demonstrate that compounds could be transformed into amorphous state (Figure
o
1). The glass-transition temperatures (Tg) of 7-10 were observed in the range of 183-239 C.
Compounds 7 and 8 containing carbazole moieties exhibited considerably higher Tg than
phenanthroimidazole derivatives 9 and 10 containing diphenylamino substituents. This observation
13

ACCEPTED MANUSCRIPT
can apparently be explained by the higher rigidity of carbazole moiety. Such high Tg and Td values
of compounds 7-10 indicate that the amorphous layers of these compounds can be fabricated by
vacuum thermal evaporation technology. In addition, materials are expected to exhibit high
morphological stability which is desirable for the materials used in OLEDs [35].
Table 1. Thermal characteristics of compounds 7-10
Compound
Tma (°C)
Tga (°C)
206
Tdb (°C)
524
-
-
7
8
239
511
394
-
192
491
9
183
503
10
a Determined by DSC, scan rate 20 oC/min, N2 atmosphere. b Td is 5% weight loss temperature; scan rate 20 oC/min, N2 atmosphere.
8
7
ο
T =239
C
g
ο
T =206
g
C
9
ο
T =192
g
C
10
ο
T =183
C
g
160
180
200
220
240
260
ο
Temperature ( C)
Figure 1. DSC curves (2nd heating) of the phenanthroimidazole derivatives 7-10, at the heating rate
of 20 °C/min, N2 atmosphere
14

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3.2 Geometries and frontier orbitals
The theoretical geometries of synthesized compounds 7-10 are presented in Figure 2. The
fused phenanthroimidazole core is planar and substituents attached to it are out of plane with
dihedral angles in the range of 22-76°. In the case of 7 and 8, the dihedral angles between carbazole
moieties with tert-butyl groups and phenyl rings of central plane are ranging from 51° to 58°. The
replacement of carbazole substituents by diphenylamino moieties in 9 and 10 leads to decreased
dihedral angles (38°- 49°). It was found, that substituents at 10th position of phenanthroimidazole
are weakly affected by phenyl ring attached at imidazolyl moiety. Therefore, carbazole and
diphenylamine at this position are more twisted than their analogues fragments attached at 5th
position (7° for 7 and 11° for 9).
Figure 2. Optimized geometries of compounds 7-10 obtained at B3LYP/6-31G(d, p) level
The HOMO for the molecules with substituents at the same positions were found to be very
similar (Figure 3). In the case of 7 and 9, HOMO is localized on phenyl rings of the central core
and substituents with tert-butyl groups. However, due to the larger inter-fragment dihedral angles in
compound 7 as compared to 9 by roughly 10 ° (Figure 2), the bridging between the two lateral
substituents through the core is less efficient in the case of compound 7 (Figure 3). The above
explanation is also consistent with the larger contributions in the HOMO of compounds 7 and 9
from the substituents at 5th position (dihedral angles of 38-41° and 51° respectively) as compared to
those at 10th position (dihedral angles of 46-49° and 58°). In the case of compounds 8 and 10,
15

ACCEPTED MANUSCRIPT
HOMO is essentially distributed over the entire phenanthroimidazole core, with less contribution
from the 9-ethylcarbazole or triphenylamine units.
The LUMOs of compounds 7-10 are mainly localized on the central core, exhibiting some
additional distribution on the substituents attached to the imidazole ring (Figure 3), in turn being
more important in the case of triphenylamine substituents (compounds 9 and 10). The energies of
the frontier orbitals (Figure 3, Table S1) translate the above observations, indicating higher HOMO
energies for 9 and 10 due to better conjugation between the central core and the substituents. The
range of the LUMO energies is narrower, which is due to enhanced LUMO similarity across the
series as compared to HOMOs.
Figure 3. HOMO and LUMO of compounds 7-10 calculated at B3LYP/6-31G(d,p) level
3.3 Optical and photophysical properties
The experimental absorption spectra of the dilute solutions of 7-10 in tetrahydrofuran (THF)
are in agreement with the theoretical spectra (Figure 4). The optical characteristics are summarized
in Table 2 and Table S1. The lowest energy absorption bands of compounds with diphenylamino
16

ACCEPTED MANUSCRIPT
moieties (9 and 10) are red-shifted with respect to those of phenanthroimidazole derivatives 7 and 8
containing carbazole substituents. This is due to more effective conjugation of π-electrons in 9 and
10 molecules over all π-backbone (Figure 3) which lead to the smaller values of band gap for those
compounds. The band gaps decrease in the order 7 > 8 > 9 > 10 (3.82 eV > 3.75 eV > 3.49 eV >
3.39 eV). According to the results of quantum chemical calculations, the lowest energy absorption
bands of derivatives 7-10 can be characterized as a mixture of electronic transitions towards several
excited states (Figure S1). The transition from ground state to first excited state (S0→S1) in
compounds 7-10 correspond to transition from HOMO to LUMO, LUMO+1. Absorption spectra of
the films of compounds 7-10 are similar to those of dilute solutions (Figure S2). The lowest energy
absorption bands of the films of compound 7-9 are slightly red-shifted with respect of those of the
solutions, due to enhanced intermolecular interactions in the solid state, whereas no shift is
observed in the case of compound 10. The HOMO distributions (Figure 3) indeed indicate larger
contribution of the peripheral molecular entities in the case of compounds 7-9, hence larger
sensitivity of HOMO energies to intermolecular interactions and geometrical deformations, as
compared to compound 10.
2,0
1,8
1,6
1,4
1,2
1,0
0,8
0,6
0,4
0,2
0,0
100000
90000
80000
70000
60000
50000
40000
30000
20000
10000
0
7
8
9
10
7
8
9
10
a)
b)
250
300
350
400
450
500
250
300
350
400
450
500
550
Wavelength (nm)
Wavelength (nm)
Figure 4. Normalized experimental (a) and theoretical in gas phase (b) absorption spectra of compounds 7-10
Fluorescence and phosphorescence spectra and photophysical properties of the dilute
solutions of 7-10 in THF are presented in Figure 5 and Table 2. The spectra of the solutions of
compounds exhibit emission peaks in the range from 414 to 467 nm. In the solid state, the emission
17

ACCEPTED MANUSCRIPT
of these four materials show negligible red shift with emission intensity maxima still localized in
the blue spectral region (421 - 468 nm) (Figure S2), which is coherent with the similar red-shifts of
the corresponding absorption spectra. Fluorescence spectra presented in Figure 5 translate
consequently the effective conjugation of π-electrons over all π-backbone (or the substituent-core π-
donor efficiency) in the molecules as it was mentioned previosly. The similar observations were
recently reported for triazol-containing derivatives [36,37]. In the case of carbazolyl substituted
compounds (7, 8) for instance, the emission maxima are blue-shifted compared to the spectra of the
derivatives containing diphenylamino moieties (9, 10), which is coherent with the same trend
observed for the low energy absorption bands and the theoretical results (Figure 3 and Table 2). On
the other hand, the position of emission bands also depends on substitution pattern of
phenanthroimidazole chromophore. The emission is blue-shifted in the case of phenanthroimidazole
moiety substituted at 5th and 10th positions (7, 9) compared to the spectra of the derivatives having
substituents at 6th and 9th positions (8, 10), in agreement with the smaller degree of HOMO
delocalizations in compounds 7, 9 as compared to 8 and 10. Both the nature of the substituent and
the linking position are important for tuning the emission colour of the synthesised
phenanthroimidazoles.
Fluorescence quantum yields (Φf) of the solid films of compounds 7-9 were found to be
considerably higher than those of dilute THF solutions and reach 0.55 (Table 2). Quantum chemical
calculations revealed that optimized geometries show quite large dihedral angles between
phenanthroimidazole moiety and bulky donors - tert-butylated carbazolyl and diphenylamino units.
The increase of the Φf values in solid state could be due to freezing of rotational degrees of freedom
between the substituents and the phenanthroimidazole core in the case of compounds 7-9, thus
suggesting an effect similar to aggregation induced enhanced emission (AIEE) [38]. This
hypothesis seems to be supported by the coherence between the geometrical characteristics of the
compounds and their Φf. Indeed, in the case of compounds 7 and 9, the phenyl group of the
18

ACCEPTED MANUSCRIPT
imidazole ring prevents from rotations of neighbour substituents in 10th position, whereas the
rotations of the substituents in 5th position are free from steric hindrance. The corresponding
contribution to the self-quenching in solution can thus be reduced (eliminated) in solid films,
resulting in increased Φf. In the case of compound 8 both groups in 6th and 9th positions seem to
undergo free rotations in solution (no steric hindrance from the phenyl ring), hence resulting in
almost total fluorescence self-quenching (0.09 in THF, Table 2). Obviously, this effect is absent in
the case of compound 10 (no AEIEE effect, Table 2), due to the larger volume of diphenylamino
groups as compared to carbazoles, mutually preventing from their free rotations. Indeed, Figure 2
indicates larger space separation between the two substituents in 6th and 9th positions in compound 8
as compared to 10.
Commission Internationale de l’Eclairage (CIE) chromaticity coordinates (x,y) for the
solution of compounds 7, 8, 9, and 10 were calculated from emission spectra and were found to be
(0.16, 0.03), (0.15, 0.03), (0.15, 0.09), and (0.14, 0.07) respectively, which correspond to the blue
colour. The lowest energy phosphorescence peaks of 7-10 were observed in the range from 471 to
535 nm. Based on these values the calculated triplet energies of 7, 8, 9, and 10 arrange in the order
2.63 eV > 2.53 eV > 2.46 eV > 2.32 eV, respectively. In the all cases, experimental triplet energies
compare well with theoretical vertical values (Table 2).
Table 2. Optical and photophysical characteristics of compounds 7-10
d
λabs(soln/film)a
(nm)
λFL(soln/film)a
(nm)
λPh
ET(exp/theor)e
(eV)
Compound
Φf(soln/film)b
CIEc (x;y)
(nm)
471
349/353
414/421
0.35/0.55
0.09/0.49
0.22/0.51
0.26/0.27
(0.16;0.03)
(0.15;0.03)
(0.15;0.09)
(0.14;0.07)
2.63/2.65
7
8
348/350
365/374
364/364
417;434/424;444
435/443
491
505
535
2.53/2.61
2.46/2.48
2.32/2.39
9
444;467/443;468
10
a Maxima bands of absorption and fluorescence recorded at room temperature. b The fluorescence quantum yield of the solution using a 0.5 M H2SO4
solution of quinine as reference (0.51); the solid state quantum yield estimated using an integrating sphere. c Estimated from the fluorescence spectra
19

ACCEPTED MANUSCRIPT
of the solutions in THF. d Lowest energy maxima bands of phosphorescence measured in THF at 77 K. e Experimental and theoretical vertical values
of triplet energy.
Fluorescence
Phosphorescence
7
8
9
10
400
450
500
550
600
650
700
Wavelength (nm)
Figure 5. Normalized room temperature fluorescence and phosphorescence recorded at 77K spectra of dilute solutions
of 7-10 in THF (10-5).
3.4 Electrochemical and photoelectrical properties
To evaluate applicability of the materials in optoelectronic devices, the electrochemical
properties of compounds 7-10 were further studied by CV measurements using their in 10-3 M
solutions. The results of the CV measurements are shown in Table 3. The CV voltamperogram of
10 is given in Figure 6. During the positive potential sweep compound 10 showed three reversible
oxidation peaks located at 0.42, 0.65 and 1.06 V which indicate the generation of a stable tricationic
species. The three-electron process probably could be assigned to the formation of cations on each
diphenylamino and triphenylamino moieties.
The other compounds under investigation showed similar behaviour in CV experiments (see
Figure S11). Has been noticed, that the same values of electrochemical peaks were observed for
derivatives 7-10 after repeated cycles. Compounds form stable cations, because active para
positions of diphenylamino and 3rd, 6th positions of carbazolyl moieties are blocked by tert-butyl
20

ACCEPTED MANUSCRIPT
groups. Additionally, the para positions of fragments attached to the imidazole ring lack of HOMO
electron density (Figure 3).
The ionization potential (IPCV) and electron affinity (EACV) values of the compounds were
calculated from their onset potentials of oxidation and their Eoptg. IPCV values of the four
compounds varied in the range from 4.96 to 5.52 eV and EACV values ranged from 2.01 to 2.38 eV.
IPCV, calculated theoretical IP and HOMO energies have the same trend and decrease in the order 7
> 8 > 9 > 10. This trend could be explained by chromophore's nature and substitution pattern of
phenanthroimidazole moiety. Derivative 7 with carbazolyl units attached at 5th and 10th positions of
phenanthroimidazole moiety shows the highest IP and on the contrary the lowest IP was observed
for the derivative 10 which contains diphenylamino units attached at 6th and 9th positions of
phenanthroimidazole moiety. The difference between ionization potential values estimated by
electron photoemission in air method (IPEP) and IPCV is due to the different environments in
solutions and solid layers of compounds.
1.5x10-5
1.0x10-5
5.0x10-6
0.0
-5.0x10-6
-1.0x10-5
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
Potential, (V vs Ag/Ag+)
Figure 6. CV of compound 10 (scan rate of 50 mV s-1 vs Ag/Ag+).
Table 3. Electrochemical characteristics of compounds 7-10.
Compound
Eoxa (V)
1.02
IPCVb (eV) Eoptg c (eV) EACVd (eV) IPEPe (eV)
IPf, eV
5.74
5.52
5.38
3.14
3.04
2.38
2.34
5.29
5.31
7
8
0.85
5.61
21

ACCEPTED MANUSCRIPT
0.51
0.42
5.03
4.96
3.02
2.81
2.01
2.15
5.27
5.15
5.26
5.10
9
10
a Onset oxidation potential vs Ag/Ag+. b IPCV = (Eox - EFc/Fc+) + 4.8 (eV). c Estimated from the onset wavelength of optical absorption according to the
g
g e
empirical formula: Eopt = 1240/ edge, in which the edge is the onset value of absorption spectrum in long wave direction. d EACV = IPCV− Eopt
.
Established from electron photoemission in air spectra. f Theoretical adiabatic IP values.
To examine the charge-transporting properties of compounds 7-10, the charge-drift mobilities
were measured by the ToF technique (Figure 7, Figure 8, Table 4). The transit times (ttr) for both
holes and electrons on transient curves in log-log scales were well seen showing the ambipolar
charge transport for the layers of all the compounds studied (Figure 8a, b, Figure S1). Hole (µh)
and electron (µe) mobilities for the layers of 7-10 were estimated using the formula µ=d2/(U×ttr),
where d is the thickness of the layer and U is voltage. Well balanced hole and electron transport was
observed for the layers of 7-10. The vales of µe were only slight higher than the values of µh (Figure
7). Both µh and µe for the layers of 7-10 confirm Poole–Frenkel type electric field dependence
µ=µ0×exp(αE)1/2, where µ0 is the zero electric field charge mobility, and α is the field dependence
parameter [39]. This dependence is typically confirmed by organic ambipolar semiconductors
(Figure 7, Table 4) [40]. Close values of charge mobilities in the range from ca. 10-5 to 10-3
cm2/(Vs) at electric fields of (0.4-8.1)×105 V/cm were found for compounds 7-10. The differences
in shapes of the current transients for holes and electrons for 7-10 were observed indicating that the
hole transport is less dispersive than that of electrons (Figure 8, Figure S3).
7_holes
7_electrons
8_holes
8_electrons
10-3
9_holes
9_electrons
10_holes
10_electrons
10-4
10-5
10-6
0
200
400
600
800
1000
1/2
Electric field1/2 (V1/2/cm
)
22

ACCEPTED MANUSCRIPT
Figure 7. Hole-drift and electron-drift mobilities for the layers of compounds 7-10 at room temperature
300V
280V
300V
a)
280V
260V
240V
220V
200V
180V
160V
140V
120V
100V
80V
0.1
0.01
1E-3
260V
240V
220V
200V
180V
160V
140V
120V
100V
80V
b)
1
0.1
60V
40V
0.027
0.018
0.009
0.01
1E-3
0.006
0.003
0.000
80V
140V
0.0000
0.0003
0.0006
0.0009
0.00000
0.00004
0.00008
0.00012
1E-6
1E-5
1E-4
1E-3
1E-6
1E-5
1E-4
Time s
Time(s)
Figure 8. Hole (a) and electron (b) transient curves at the different electric fields for 10 and electric field dependencies
To get insight into the charge mobilities in the layers of synthesized compounds,
intramolecular reorganization energies of holes (λh) and electrons (λe) for the isolated molecules of
7-10 were calculated. According Marcus theory [41,42,43], the smaller values of intramolecular
reorganization energies will enhance charge mobilities. The λh and λe values are presented in the
Table 4 and are estimated to increase in the order 7 < 8 ≈ 9 < 10 and 7 < 8 < 9 < 10, respectively.
These values are consistent with the increase of charge mobilities in the order 7 > 8 > 10, at an
electric field of 3.6×105 cm2/Vs (Table 4). However, the much larger λe values as compared to λh
ones are inconsistent with the roughly similar (even slightly larger) electron mobilities as compared
to hole ones. Additionally, the trend of λe values is opposite to the trend of µe. These observations
indicate that the hole and electron transport in these compounds are dominated by disorder
phenomena. Support to this conclusion seems to come from the comparison between HOMO-
versus LUMO distributions: LUMOs are more concentrated on the rigid phenanthroimidazole core
preventing from large distribution of LUMO energies (density of states). An opposite situation
occurs for the HOMO energies, being more sensitive to the deformation of the
phenanthroimidazole-ligand dihedral angles.
23

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Table 4. Charge-drift mobilities and reorganization energies of compounds 7-10.
Compound
d (µm)
3.1
µh (cm2/V·s)a
3.14×10-4
1.91×10-4
3.02×10-5
1.60×10-4
λh (eV)
0.12
µe, (cm2/V·s)a
5.69×10-4
2.14×10-4
8.52×10-5
1.91×10-4
λe (eV)
0.30
7
8
3.9
0.21
0.36
1.8
0.21
0.50
9
7.1
0.28
0.52
10
a Hole- and electron-drift mobilities at an electric field of 3.6·105 cm2/Vs.
3.5 Electroluminescent properties
Since derivatives 7-10 showed strongly enhanced blue emission in the solid state (Φf=0.27-
0.55) and ambipolar charge transport with well-balanced hole and electron mobilities, it was of
interest to study their electroluminescent properties. Electroluminescent devices based on non-
doped light-emitting layers 7-10 with the following architectures were fabricated: ITO/MoO3(3
nm)/m-MTDATA(30 nm)/7,8,9,10 (20 nm)/PBD(10 nm)/TPBi(25 nm)/Ca(10 nm)/Al(60 nm)
(marked as devices A,B,C, and D, respectively). To achieve the high charge-balance the multilayer
architectures of OLEDs were chosen (Figure S4). In such devices, molybdenum trioxide (MoO3)
was
used
for
the
preparation
of
hole-injection
layers;
4,4′,4′′-tris[3-
methylphenyl(phenyl)amino]triphenylamine (m-MTDATA) was employed for the fabrication of
hole-transporting and electron-blocking layers, 2-(4-biphenylyl)-5-phenyl-1,3,4-oxadiazole (PBD)
was used for the preparation of electron-transporting and hole-blocking layers and 1,3,5-tris(N-
phenylbenzimidazol-2-yl)benzene (TPBi) was utilized for the preparation of electron-injection
layers. Owing to the additional layers, the charge carrier barriers between work functions of the
electrodes (ITO, Ca) and the molecular orbitals of light-emitting layers (7-10) were reduced (Figure
S4).
24

ACCEPTED MANUSCRIPT
The electroluminescence (EL) spectra of the devices A-D coincided with the
photoluminescence (PL) spectra of solid films of emitters 7-10 (Figure 5 and 9). The
electroluminescence characteristics of the non-doped electroluminescent devices ITO/MoO3/m-
MTDATA/7-10/PBD/TPBi/Ca/Al are shown in Figure S5 a-d and are summarized in Table 5. The
turn-on voltages (Von) of the devices were found to be in the range from 4.0 to 5.5 V at the
brightness of 1 cd/m2, while the maximum brightness was recorded in the range from 130 to 750
cd/m2 at the driving voltage of 10 V. These parameters are comparable with those of the fluorescent
deep blue OLEDs [44,45], however, the efficiencies of the devices were rather low (the maximal
current, power, and external quantum efficiencies were lower than 1.3 cd/A, 0.8 lm/W, 0.95 %,
respectively (Figure S5 b-d)). Commission Internationale de l’Eclairage (CIE) chromaticity
coordinates (x,y) for the devices based on non-doped emitting layers 7, 8, 9, and 10, were found to
be (0.183, 0.162), (0.265, 0.25), (0.19, 0.184), and (0.166, 0.133) respectively, which correspond to
the blue colour. We note that the potential of the studied compounds was not fully disclosed in the
non-doped blue electroluminescent devices. The efficiencies of non-doped blue electroluminescent
devices will apparently be increased when appropriate auxiliary materials are available.
1.0
7
8
9
10
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
350
400
450
500
550
600
Wavelength (nm)
Figure 9. Electroluminescence spectra of the devices based on the non-doped emitting layers of 7-10.
25

ACCEPTED MANUSCRIPT
Table 5. EL characteristics of the fabricated devices.
Current
efficiency, efficiency,
Power
External
quantum
efficiency,
%
CIE 1931
UCS
Von at
Max
cd/A
lm/W
Device
1 Cd/m2, brightness,
coordinates,
(x,y)
V
cd/m2
max
0.8
5.5
750
270
1.3
0.06
1.15
0.97
4
0.95
0.03
0.75
0.7
(0.183;0.162)
(0.265;0.250)
(0.190;0.184)
(0.166;0.133)
(0.275;0.467) a
(0.468;0.381) a
(0.324;0.289) a
(0.207;0.268) a
A
B
4.5
4.0
4.0
4.8
5.1
2.6
2.5
0.03
0.8
140
C
130
0.64
1.5
D
1970
6130
870
1.75
2.9
AI
AII
DI
DII
5.3
1.7
0.58
0.56
0.53
0.53
0.24
0.24
1300
a – the values were taken at applied voltage of 8 V.
To increase the efficiencies of devices, doped light-emitting layers were used with the most
promising emitters 7 and 10 which allowed to obtain the blue colour devices with higher
efficiencies,
while
m-MTDATA
and
4,4′-cyclohexylidenebis[N,N-bis(4-
methylphenyl)benzenamine] (TAPC) were additionally utilized as the hosts. As CIE chromaticity
coordinates for the devices based on non-doped emitting layers from 7 and 10 ((0.183, 0.162) and
(0.166, 0.133), respectively) were related to the deeper blue colour than those devices based on non-
doped emitting layers from 8 and 9 ((0.265, 0.25) and (0.19, 0.184), respectively), the compounds 7
and 10 were chosen for the fabrication of the white OLEDs. Electroluminescent devices based on
doped light-emitting layers with the following architectures were fabricated: ITO/MoO3(3 nm)/m-
MTDATA(30 nm)/ TAPC:7, m-MTDATA:7, TAPC:10 or m-MTDATA:10 (20 nm)/PBD(10
nm)/TPBi(25 nm)/Ca(10 nm)/Al(60 nm) (marked as devices AI, AII, DI and DII). Low-energy
bands with the maxima at 585 nm for the devices AI and DI and at 540 nm for the devices AII and
26

ACCEPTED MANUSCRIPT
DII were additionally observed in the electroluminescence spectra of the doped OLEDs as
compared to the spectra of the non-doped OLEDs (Figure 10 a,b and Figure S6 a,b). Appearance
of the additional band can apparently be explained by the formation of the dimeric excited states
(e.g. exciplexes or electroplexes) either in the doped light-emitting layers or at the interfaces
between the layers of OLEDs due to the cross-interaction between the holes of donor and the
electrons of acceptor [3]. To prove this assumption, PL spectra of the film of the molecular mixtures
of the hosts and emitters were recorded (Figure S7). The PL spectra of the mixtures TAPC:7, m-
MTDATA:7, TAPC:10 and m-MTDATA:10 were very similar to the PL spectra of pure 7 or 10 and
did not contain bands at the wavelengths close to those of the low energy bands observed in EL
spectra of the doped OLEDs (Figure S7). The similarity of shapes and positions of the PL spectra
of the mixtures to the PL spectra of the pure emitters 7 or 10 indicates effective energy transfer
from hosts to emitters. In addition, the PL spectra of the mixtures m-MTDATA:PBD and
TAPC:PBD were found to be different as compared with the PL spectra of the pure m-MTDATA,
TAPC, or PBD. The maximum at 540 nm of the PL spectrum of m-MTDATA:PBD film was found
to be at the similar wavelength as the maximum of the additional band in the EL spectra of the
devices AII and DII with m-MTDATA as the host. The PL decay curves of the film of the mixture
m-MTDATA:PBD mixture recorded at the wavelength of maximum were well fitted by double-
exponential function (Figure S8 a). The shorter lived component is most probably related to the
monomeric fluorescence, while the longer lived component is apparently related to the exciplex
emission which occurred due to the reverse intersystem crossing (RISC) from the singlet to the
triplet state [46]. Therefore, the low-energy emission band in EL spectra of the doped OLEDs with
the m-MTDATA host can be ascribed to the interface exciplex between m-MTDATA and PBD.
Despite the exciplex emission with the maximum at 490 nm between TAPC and PBD observed in
PL spectrum of the molecular mixture (Figure S7 and Figure S8 b), the exciplex TAPC:PBD was
not observed in the EL spectra of the devices AI and DI. The possible origin of electroluminescence
27

ACCEPTED MANUSCRIPT
in low-energy region (585 nm) of the devices AI and DI is apparently the formation of electroplexes
between TAPC and 7 (10) which only occurs under the electric fields. The similar electroplex
emission between TAPC and a bipolar transporting material 2,6-bis(3-(carbazol-9-yl)
phenyl)pyridine was observed previously [47].
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
Device:
ITO/Mo2O3/
Device:
ITO/Mo2O3/
m-MTDATA/
TAPC:10
/PBD/TPBi/
Ca:Al
4V
m-MTDATA/
m-MTDATA:10
/PBD/TPBi/
Ca:Al
3V
4V
5V
6V
7V
8V
9V
10V
5V
6V
7V
8V
9V
10V
450
500
550
600
650
700
400
450
500
550
600
650
700
750
Wavelength (nm)
Wavelength (nm)
a)
b)
Figure 10. Electroluminescence spectra of the devices based on the doped emission layers TAPC:10 (a) and m-
MTDATA:10 (b).
Commission Internationale de l’Eclairage (CIE) chromaticity coordinates (x,y) of the TAPC:10
based device at voltages from 8 V to 9 V was found to be very close to the white point (0.33, 0.33)
(Figure S9) due to the mixed blue monomer emission and orange electroplex emission in the
TAPC:10 emitting-layer. The current density–voltage–luminance and efficiencies versus current
density characteristics for the doped devices are plotted in Figure 11 and Figure S10. Improved
maximal current of 4 and 5.3 cd/A, power of 1.5 and 1.7 lm/W, and external quantum of 1.75 and
2.9 % efficiencies were achieved for the devices AI and DI respectively (Table 5).
28