Diphenylethenyl- and methylphenylethenyl-substituted triphenylamines as effective hole transporting and emitting materials

Article abstract of DOI:10.1016/j.dyepig.2016.08.007

Diphenylethenyl- and methylphenylethenyl-substituted triphenylamines were prepared by condensation of trimethoxy-substituted triphenylamines with 2,2-diphenylacetaldehyde or 2-phenylpropionaldehyde respectively. The synthesized compounds were found to be

Full text of DOI:10.1016/j.dyepig.2016.08.007

Accepted Manuscript
Diphenylethenyl- and methylphenylethenyl-substituted triphenylamines as effective
hole transporting and emitting materials
M. Cekaviciute, J. Simokaitiene, G. Sych, J.V. Grazulevicius, V. Jankauskas, D.
Volyniuk, P. Stakhira, V. Cherpak, K. Ivanyuk
PII:
S0143-7208(16)30382-5
10.1016/j.dyepig.2016.08.007
DYPI 5387
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 3 June 2016
Revised Date: 2 August 2016
Accepted Date: 3 August 2016
Please cite this article as: Cekaviciute M, Simokaitiene J, Sych G, Grazulevicius JV, Jankauskas V,
Volyniuk D, Stakhira P, Cherpak V, Ivanyuk K, Diphenylethenyl- and methylphenylethenyl-substituted
triphenylamines as effective hole transporting and emitting materials, Dyes and Pigments (2016), doi:
10.1016/j.dyepig.2016.08.007.
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Diphenylethenyl- and methylphenylethenyl-substituted triphenylamines as
effective hole transporting and emitting materials
a
a
a
a*
b
a
M. Cekaviciute , J. Simokaitiene , G. Sych , J.V. Grazulevicius , V. Jankauskas , D. Volyniuk , P.
c
c
c
Stakhira , V. Cherpak , K. Ivanyuk
a
Department of Polymer Chemistry and Technology, Kaunas University of Technology, Radvilenu
pl. 19, LT-50254, Kaunas, Lithuania
b
Department of Solid State Electronics, Vilnius University, Sauletekio al. 9, LT-10222, Vilnius,
Lithuania
c
Lviv Polytechnic National University, S. Bandera 12, 79013 Lviv, Ukraine
*Corresponding author: juozas.grazulevicius@ktu.lt (Juozas V. Grazulevicius)
Abstract
Diphenylethenyl- and methylphenylethenyl-substituted triphenylamines were prepared by
condensation of trimethoxy-substituted triphenylamines with 2,2-diphenylacetaldehyde or
2-phenylpropionaldehyde respectively. The synthesized compounds were found to be capable of
glass formation with glass transition temperatures above 49 ºC for methylphenylethenyl-substituted
derivatives and above 82 ºC for diphenylethenyl-substituted compounds. The ionization potentials
of the layers of the derivatives were found to be in the range of 5.31-5.44 eV. Time-of-flight hole
drift mobility values of the glassy layer of tri(4-diphenylethenyl-2-methoxyphenyl)amine well
-2
2
exceeded 10 cm /Vs at high electric fields at room temperature. The clear dependence of the
charge mobilities on the positions of methoxy groups was observed. Compounds having methoxy
groups at the meta positions of triphenylamine groups exhibited superior hole-transporting
properties compared to the compounds having methoxy groups at the ortho positions. Blue-green
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emitting OLEDs based on triphen e derivatives exhibited relatively high efficiencies
ACCE Pa mT Ei nD MANUSCRIPT
yl
2
(
brightness in order 10000 cd/m and current efficiency from 4 to 10 cd/A) as for the fluorescent
OLEDs.
Keywords: star-shaped triphenylamine derivatives, molecular glasses, hole drift mobility, organic
light emitting diodes
1
. INTRODUCTION
Derivatives of triphenylamine (TPA) represent one of the largest and one of the most widely
studied classes of organic electroactive materials [1].They are successfully used in organic light-
emitting diodes (OLEDs) for hole-transporting layers [2,3], as emitters [4] and as hosts of the
triplet-emitting layers [5,6], in bulk-heterojunction [7,8] and dye-sensitized solar cells [9-11],
electrophotographic photoreceptors [12,13], electrochromic devices [14,15], organic field-effect
transistors (OFETs) [16,17]. The current level of development of OLEDs enables their effective
application in the low-cost flat panel display technology and in lighting [18]. Improvement of the
operation of OLEDs to a great extent depends on the increase of electroluminescence efficiency, in
particular on the decrease of the power consumption while providing a high level of brightness. One
possible way to achieve this task is the design and synthesizes of new organic multifunctional
materials with excellent charge transporting and efficient emitting characteristics [19].
In this work we report on the synthesis and properties of star-shaped methoxy-substituted
triphenylamine derivatives with diphenylethenyl or methylphenylethenyl groups. The later groups
were introduced by the direct condensation of methoxy-substituted triphenylamines with
appropriately substituted acetaldehydes in a similar way as reported earlier [20]. We also report on
the fabrication and characterization of OLEDs based on the newly synthesized materials.
2. EXPERIMENTAL METHODS
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2.1. Instrumentation
1
13
1
H NMR and C NMR spectra were recorded with Varian Unity Inova [300 MHz ( H),
1
3
7
5.4 MHz ( C)] spectrometer at room temperature. All the data are given as chemical shifts δ
(
ppm) downfield from Si(CH ) . Infrared (IR) spectra were recorded using PerkinElmer Spectrum
3 4
GX II FT-IR System. The samples of the solid compounds were prepared in the form of KBr
pellets. Mass spectra (MS) were obtained on a Waters ZQ. Elemental analysis was performed with
an Exeter Analytical CE-440 Elemental. Differential scanning calorimetry (DSC) measurements
were carried out in a nitrogen atmosphere with a TA Instruments Q10 calorimeter at a heating rate
of 10 ºC/min. Thermogravimetric analysis (TGA) was performed on Mettler
TGA/SDTA851e/LF/1100 in a nitrogen atmosphere at a heating rate of 20 ºC/min. Melting points
were measured with Electrothermal MEL-TEMP melting point apparatus. Absorption spectra of
-5
dilute (10 M) solutions in tetrahydrofuran (THF) were recorded on a UV−vis−NIR
spectrophotometer Lambda 950 (Perkin-Elmer). Fluorescence spectra, fluorescence quantum yields
-5
(
Φ ) and fluorescence transients of dilute solutions in THF (10 M) or solid films of the compounds
F
were recorded with Edinburgh Instruments LS980 spectrometer. Cyclic voltammetry (CV)
measurements were carried out using a micro-Autolab III (Metrohm Autolab) potentiostat-
galvanostat equipped with a standard three-electrode configuration. A three-electrode cell equipped
with a glassy carbon working electrode, an Ag/AgNO (0.01 M in anhydrous acetonitrile) reference
3
electrode and a Pt wire counter electrode were employed. The measurements were done in
anhydrous dichloromethane with tetrabutylammonium hexafluorophosphate (0.1 M) as the
supporting electrolyte under nitrogen atmosphere at a scan rate of 0.1 V/s [21-23].The
measurements were calibrated using an internal standard, ferrocene/ferrocenium (Fc) system. The
oxidation potentials (E vs Fc) for the reversible oxidation were taken as the average values of the
1
/2
ep
anodic and cathodic peak potentials, Epa and Epc, respectively. Ionization potentials (I ) of the
P
films of the synthesized compounds were measured by electron photoemission in air method as
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described before [24,25]. Hole drift mobilities were measured by a xerographic time of-flight
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(XTOF) method [12,26,27]. The layer thickness was in the range of 2.9-6.0 µm.
2.2. OLED fabrication
OLEDs were fabricated by means of vacuum deposition of organic semiconductor layers and
−5
metal electrodes onto pre-cleaned ITO coated glass substrate under vacuum of 10 Torr. The
devices were fabricated by step-by-step deposition of the different organic layers. The layers of
compounds 1 or 2 were used as hole-transporting and turquoise light-emitting layers. CuI was used
as hole-injecting layer [28]. According to that the work function of CuI is lower than HOMO of 1 or
2
compounds; CuI hole-injecting layer completely reduced the energy barer between ITO and 1 or 2
compounds in fabricated OLEDs. Because of high hole mobility of 1 and 2 and their high LUMO
levels (ca. -2 eV), 3,6-di(9-carbazolyl)-9-(2-ethylhexyl)carbazole (TCz1) [29] films were used as
effective hole blocking and electron-transporting layers which promote charge balance in the light
emitting layer and reduce the energy barrier of electron injection. TCz1 is a versatile compound
[
27,30,31]. It can be used as an effective electron-injection material, which helps to provide a
stepwise electron transfer from the Ca:Al cathode to the emissive layer [29]. (8-Hydroxyquinoline)
aluminum (Alq3) was used as electron-transporting and green light-emitting material. Ca layer
topped with thick aluminum (Al) layer was used as the cathode.
The structures of the devices were as follows:
Device (A): ITO/CuI/1/TCz1/Ca:Al
Device (B): ITO/CuI/1/Alq3/Ca:Al
Device (C): ITO/CuI/2/TCz1/Ca:Al
Device (D): ITO/CuI/2/Alq3/Ca:Al
The active area of the obtained devices was 3x6 mm.
The density-voltage, luminance-voltage characteristics of OLEDs were measured using a
semiconductor parameter analyzer (HP 4145A) in air without passivation immediately after
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fabrication of the devices. The measur ts of brightness were done using a calibrated photodiode
ACC Ee mP Te nE D MANUSCRIPT
[
32]. The electroluminescence spectra were recorded with an Ocean Optics USB2000 spectrometer.
2.3. Materials
All the required chemicals, i.e. 2,2-diphenylacetaldehyde, 2-phenylpropionaldehyde,
+/-)-camphor-10-sulfonic acid were obtained from Sigma-Aldrich and used as received.
(
2
3
,2’,2’’-Trimethoxytriphenylamine (Mp = 144-147 ºC, lit. Mp = 139.5-140 ºC [33]) and
,3’,3’’-trimethoxytriphenylamine were obtained under Ullmann conditions [34,35].
General procedure for the synthesis of compounds 1-4. They were prepared by condensation of
the appropriate o- or m- trimethoxy-substituted triphenylamine derivative (1 g, 2.98 mmol) with
diphenylacetaldehyde (2.9 g, 14.8 mmol) or 2-phenylpropionaldehyde (2.0 g, 14.8 mmol) in toluene
at a reflux temperature, in the presence of (+/-)-camphor-10-sulfonic acid (0.7 g, 3.0 mmol). The
reaction mixtures were stirred at the reflux temperature for 24 h. The reactions were monitored by
thin-layer chromatography (TLC). When the reactions were finished the solvent was distilled in
vacuum and the crude product was subjected to purification by silica gel column chromatography.
Tris(4-diphenylethenyl-2-methoxyphenyl)amine (1) was prepared according to the general
procedure from 2,2’,2’’-trimethoxytriphenylamine and 2,2-diphenylacetaldehyde. The resulting
product was purified by column chromatography using the eluent mixture of acetone,
dichloromethane and hexane in a volume ratio of 1:1:15. Yellow amorphous powder was obtained
after precipitation in methanol with the yield of 1.04 g (40 %). IR νmax (KBr): 3076, 3052, 3019
1
(
C-H Ar), 2930 (C-H), 2829 (O-CH ), 1589, 1505 (C=C Ar), 1279 (C-O-C). H NMR (300 MHz,
3
CDCl ) δ (ppm): 3.10 (s, 9H, -OCH ), 6.36 (s, 3H, Ar), 6.54 (s, 6H, Ar), 6.92 (s, 3H, -C=CH),
3
3
13
7
.21-7.32 (m, 30H, Ar). C NMR (75.4 MHz, CDCl ) δ (ppm): 55.00, 112.90, 122.68, 123.58,
3
1
27.31, 128.16, 128.68, 130.45, 132.96, 135.86, 140.89, 143.40, 151.90. Elemental analysis for
C H NO : % calcd. C 86.97, H 5.91, N 1.61, O 5.52; % found C 86.39, H 6.16, N 1.73.
63
51
3
+
MS (APCI+, 20V), m/z(%): 871 ([M+H] , 60).
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ACCEPTED MANUSCRIP Tp repared according to the general
Tri(4-diphenylethenyl-3-methoxyphenyl)amine (2) was
procedure from 3,3’,3’’-trimethoxytriphenylamine and 2,2-diphenylacetaldehyde. The resulting
product was purified by column chromatography using the eluent mixture of THF and hexane in
volume ratio of 1:20. Yellow crystals were obtained after recrystallization from methanol with the
yield of 0.74 g (29 %). Mp = 201-204 ºC. IR νmax (KBr): 3077, 3055, 3021 (C-H Ar), 2932 (C-H),
1
2
832 (O-CH ), 1593, 1494 (C=C Ar), 1224 (C-O-C). H NMR (300 MHz, CDCl ) δ (ppm): 3.65
3
3
(s, 9H, -OCH ), 6.24 (d, 3H, J = 8.0 Hz, Ar), 6.51 (s, 3H, -C=CH), 6.56(d, 3H, J = 8.5 Hz, Ar), 7.10
3
1
3
(
s, 3H, Ar), 7.18-7.31 (m, 30H, Ar). C NMR (75.4 MHz, CDCl ) δ (ppm): 55.73, 106.61, 116.63,
3
1
21.69, 122.78, 127.32, 128.00, 128.57, 130.82, 141.62, 144.09, 147.04, 158.55. Elemental analysis
for C H NO : % calcd. C 86.97, H 5.91, N 1.61, O 5.52; % found C 86.11, H 5.88, N 1.79.
6
3
51
3
+
MS (APCI+, 20V), m/z(%): 871([M+H] , 100).
Tri[4-(2-mehyl-2-phenylethenyl)-2-methoxyphenyl]amine (3) was prepared according to the
general procedure from 2,2’,2’’-trimethoxytriphenylamine and 2-phenylpropionaldehyde. The
resulting product was purified by column chromatography using the eluent mixture of acetone,
dichloromethane and hexane in a volume ratio of 1:1:15. Yellowish amorphous powder was
obtained after precipitation in methanol with the yield of 0.79 g (37 %). IR νmax (KBr): 3078, 3053,
3
024 (C-H Ar), 2954, 2930 (C-H), 2830 (O-CH ), 1594, 1506 (C=C Ar), 1276 (C-O-C).
3
1
H NMR (300 MHz, CDCl ) δ (ppm): 2.35-2.38 (m, 9H, -CH ), 3.61-3.67 (m, 9H, -OCH ), 6.83-
3
3
3
6.87 (m, 3H, Ar), 6.92 (s, 3H, -C=CH), 6.94-6.96 (m, 3H, Ar), 7.32-7.35 (m, 3H, Ar), 7.36-7.44
1
3
(
m, 9H, Ar), 7.52-7.58 (m, 6H, Ar). C NMR (75.4 MHz, CDCl ) δ (ppm): 18.09, 56.11, 113.94,
3
1
22.04, 124.35, 126.24, 127.20, 128.23, 128.55, 128.71, 134.35, 136.50, 144.67, 152.89. Elemental
analysis for C H NO : % calcd. C 84.30, H 6.63, N 2.05, O 7.02; % found C 84.53, H 6.72,
4
8
45
3
+
N 2.01. MS (APCI+, 20V), m/z(%): 684 ([M+H] , 100).
Tri[4-(2-mehyl-2-phenylethenyl)-3-methoxyphenyl]amine (4) was prepared according to the
general procedure from 3,3’,3’’-trimethoxytriphenylamine and 2-phenylpropionaldehyde. The
resulting product was purified by column chromatography using the eluent mixture of THF and
hexane in volume ratio of 1:50. Yellowish amorphous powder was obtained after precipitation in
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(KBr): 3051, 3022 (C-H Ar), 2931 (C-H), 2832
MANUSCRIPT
max
methanol with the yield of 0.24
A
g
C
(1
C
2
E
%
P T) .
E
IR
D
ν
1
(
-O-CH ), 1594, 1496 (C=C Ar), 1213 (C-O-C). H NMR (300 MHz, CDCl ) δ (ppm): 2.28-2.33
3 3
(
m, 9H, -CH ), 3.72-3.77 (m, 9H, -OCH ), 6.68-6.78 (m, 3H, Ar), 6.80 (s, 3H, -C=CH), 6.97 (s, 3H,
3
3
1
3
Ar), 7.31-7.34 (m, 3H, Ar), 7.36-7.44 (m, 9H, Ar), 7.58-7.63 (m, 6H, Ar). C NMR (75.4 MHz,
CDCl ) δ (ppm): 17.89, 55.88, 107.00, 116.23, 122.36, 123.00, 126.28, 127.20, 128.49, 130.83,
3
1
36.53, 144.27, 147.55, 158.36. Elemental analysis for C H NO : % calcd. C 84.30, H 6.63,
48 45 3
+
N 2.05, O 7.02; % found C 84.45, H 6.98, N 2.08. MS (APCI+, 20V), m/z(%): 684 ([M+H] , 100).
3
3
. RESULTS AND DISCUSSION
.1. Synthesis and characterization
The synthetic route towards the appropriate arylethenyl-substituted triphenylamines is shown in
Scheme 1. Compounds 1–4 were prepared by one-step procedures from the appropriate trimethoxy-
substituted triphenylamines by condensation with 2,2-diphenylacetaldehyde or
-phenylpropionaldehyde in toluene at reflux temperature, in the presence of
+/-)-camphor-10-sulfonic acid. All the compounds were purified by column chromatography. They
2
(
1
13
were identified by elemental analysis, IR-, H NMR-, C NMR- spectroscopies and mass
spectrometry. The synthesized compounds were found to be soluble in common organic solvents
such as THF, toluene, chloroform.
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O
O
O
N
O
N
O
O
2
1
O
O
O
O
O
O
O
+
N
O
N
O
+
O
O
NH2
I
NH
2
I
O
O
O
O
O
O
N
N
O
O
O
3
4
Scheme 1. Synthetic route of compounds 1–4.
3.2. Thermal properties
The thermal characterization of the synthesized compounds was performed by DSC and TGA
under a nitrogen atmosphere. The thermal characteristics are summarized in Table 1. The
synthesized compounds (1–4) showed high thermal stability. The temperatures of the onsets of the
thermal degradation (T ) were found to be lower for compounds 3, 4 having methylphenylethenyl
dec
substituents (354 and 373 ºC respectively) than for compounds 1, 2, containing diphenylethenyl
groups (418 and 439 ºC respectively). Compounds 1, 3, 4 were isolated after the synthesis as
amorphous powders, while compound 2 was isolated as a crystalline substance. All the synthesized
compounds (1–4) were found to be able to form molecular glasses. The glass transition
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temperatures of diphenylethenyl-substituted tripheny , 2) were found to be higher than
those of the corresponding methylphenylethenyl-substituted derivatives (3, 4). This observation can
be explained by the larger conformation lability because of smaller substituents in the samples of
compounds 3 and 4.
<Table 1>
3.3. Optical and photophysical properties
Absorption and fluorescence spectra of the synthesized compounds are shown in Fig. 1. The
absorption spectra of dilute solutions in THF of the derivatives are dominated by two absorption
bands. The wavelengths of absorption maxima of the higher-energy bands (at ca 300 nm) were
found to be close to that of triphenylamine [36]. The lower-energy absorption bands were similar in
character and position. To evaluate these absorption bands density functional theory (DFT)
calculations employing the B3LYP functional were performed for compounds 1-4. The
spectroscopic properties of the molecules were calculated using a time dependent density functional
theory method (TDDFT), in conjunction with 6-31G(d,p) basis set with the Gaussian 09 program
[
37]. Absorption peaks witch appears in the range of 358-395 nm correspond to HOMO→LUMO
and HOMO→LUMO+1 electronic transitions contributing to the S →S and S →S excitations.
0
1
0
2
HOMO orbitals are localized on the triphenylamine core. LUMO and LUMO+1 orbitals are
localized on two of arylethenyl sidearms with contribution from triphenylamine core. The
absorption edges of diphenylethenyl-substituted compounds (1, 2) were red shifted with respect of
the absorption edges of the corresponding methylphenylethenyl-substituted derivatives (3, 4) due to
the increased conjugated systems [38]. The similar regularity was observed for the fluorescence
spectra (Fig. 1). Fluorescence intensity maxima of diphenylethenyl-substituted compounds (1, 2)
exhibited bathochromic shifts with respect of the maxima of methylphenylethenyl-substituted
compounds (3, 4). Moreover, fluorescence intensity maxima of o-methoxytriphenylamine-based
9

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derivatives (1, 3) exhibited bathochromic shifts compared to those of m-methoxytriphenylamine-
based derivatives (2, 4). The emission spectra of the solid films of compounds 1–4 were found to be
without vibronic features and were situated in between those of the dilute solutions in toluene (less
polar) and in THF (more polar). The emission intensity maxima of the solutions of these four
compounds in THF showed red-shifts with respect to those of the solutions in toluene. The
solvatochromic effect was larger (ca. 24 nm) in the case of ortho-methoxy-substituted
compounds 1 and 3, compared with that observed for meta-methoxy-substituted compounds 2 and 4
(8-16 nm).
<Table 2>
<Figure 1>
Fluorescence quantum yields (Φ ) of the dilute solutions in THF, toluene and of the solid films
F
of compounds 1–4 are given in Table 2. The dilute solutions of o-methoxytriphenylamine-based
derivatives 1 and 3 exhibited higher emission quantum yields than those of their
m-methoxytriphenylamine-based counterparts 2 and 4. Φ values observed for the dilute solutions
F
of methylphenylethenyl-substituted compounds (3, 4) were found to be higher than those of
diphenylethenyl-substituted compounds (1, 2). Φ values for the amorphous films of the
F
synthesized compounds were found to be close and ranged from 2 to 3%. Fluorescence decay
curves of the dilute solutions compounds 1–4 demonstrated mono-exponential decay profiles
2
(
Fig. 2). χ values and weighted residuals were used as the goodness-of-fit criteria. Fluorescence life
times (τ) of the dilute solutions of methylphenyl-substituted compounds (3, 4) were found slightly
longer than those of the corresponding diphenyl-substituted compounds (1, 2).
<Figure 2>
10

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3.4. Electrochemical, photoelectrical and charge transporting properties
Electrochemical properties of the synthesized compounds were studied by cyclic voltammetry
(CV). The cyclic voltammograms of compounds 1–4 show the reversible oxidation steps with the
peak height varying linearly with sweep rate and the same values of anodic and cathodic peak
currents [39] (Fig. 3). The shapes of cyclic voltammograms were found to be similar for all the
studied compounds. The electrochemical data are summarized in Table 3. The ionization potential
cv
values (I ) were determined from the values of the first oxidation potential with respect to
p
ferrocene (Fc). The ionization potential values of the synthesized compounds were found to be very
cv
close and ranged from 4.88 to 4.94 eV. The electron affinities (EA ) were determined from the
opt
optical energy band gaps (Eg ) and ionization energy values. They ranged from -2.13 to -1.83 eV.
ep
The ionization potentials (I ) of the layers of compounds 1–4 were established by electron
p
ep
photoemission method in air (Fig. 4). The Ip values of the compounds ranged from
5.31 to 5.44 eV. Both methods of estimation of ionization potentials revealed the same trend. The
ionization potentials (I ) values of the dilute solutions of methylphenyl-substituted compounds
p
(3, 4) were found slightly lower than those of the corresponding diphenyl-substituted compounds
(
1, 2). I values of o-methoxytriphenylamine-based compounds (1, 3) were found to be lower than
p
those of m-methoxytriphenylamine-based compounds (2, 4).
<Table 3>
<
Figure 3>
Figure 4>
<
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ACCEP Te dE Df orMANUSCRIP To f charge-transporting properties of
Time of flight measurements were us the estimation
the synthesized compounds. Fig. 5 shows electric field dependencies of hole drift mobilities (µ) for
the layers of compounds 1–4. All the studied compounds showed linear dependencies of hole
mobilities (µ) on the square root of the electric field. The hole-drift mobility values are summarized
in Table 3. Diphenylethenyl-substituted compounds (1, 2) showed higher charge mobilities than the
corresponding methylphenylethenyl-substituted derivatives (3, 4). Moreover, meta-methoxy-
substituted compounds 2 and 4 exhibited higher charge mobilities than ortho-methoxy-substituted
compounds 1 and 3. The highest hole mobility was observed for the layer of compound 2.
-2
2
It exceeded 10 cm /Vs at high electric fields.
<Figure 5>
3.5. Electroluminescent devices
Compounds 1 and 2 were tested in the structures of electroluminescent devices (OLEDs). Fig. 6
shows electroluminescence spectra of the OLEDs, the structures of which are described in the
experimental part. The shapes of the electroluminescence spectra of the devices A and C are similar
to those of fluorescence spectra of the solid films of 1 and 2, respectively. Taking into account the
values of HOMO and LUMO energy levels (Fig. 7 a), TCz1 was used as an electron-injection as
well as hole blocking material in devices A and C. The devices B and D containing layers of Alq3
were characterized by broad emission bands with bathochromic shifts compared with the
fluorescence spectra of vacuum deposited films of 1 and 2. The electroluminescence of the devices
B and D originate from the superposition of the emission bands 1 or 2 with those of Alq3. We
assume that the electron−hole recombination takes place at the volume of the layers of 1 or 2 and
Alq3 which provides excited states in these species under application of the external electric field.
<Figure 6>
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<Figure 7>
It is evident from energy-band diagrams of the devices shown in Figure 7 that there are almost
no energy barriers for injection of holes from CuI into HOMO level of 1 or 2. The similar situation
is observed for electron injection from Ca into the LUMO level of Alq3 or TCz1. Thus it can be
assumed that effective double-carrier injection was possible. Characteristics of the devices A-D are
shown in Figure 8. All the devices exhibited relatively high efficiencies as for the fluorescent
OLEDs. High current efficiency of devices A and B is mainly due to excellent electron blocking
properties of 1 and 2 as well as hole blocking properties of Alq3 [40] and TCz1. The recombination
zone is located at the organic-organic interface, away from ITO and Ca contacts. In such a way,
quenching of electroluminescence is reduced [41]. The turn-on voltages of devices B and D as well
as of devices A and C are close due to similarity of their structures (Fig. 8 a). The turn-on voltages
of the devices are low (ca. 3.0-3.5 V) showing the absence of energy barriers for both holes and
electrons in the devices. The applied voltages were of the same order reaching of 15 V for all the
devises (Fig. 8 a). The current density of device B was lower than that observed for the other
devices (Fig. 8 a). This observation can be explained either by the difference in hole mobilities of
-3
2
-3
2
compounds 1 (4.2×10 cm /Vs) and compound 2 (7.9×10 cm /Vs) or by the different balance of
charges in the structure of device B.
<Figure 8>
Conclusions
We synthesized and characterized new derivatives of methoxytriphenylamine having
diphenylethenyl or methylphenylethenyl moieties as substituents and studied their thermal, optical
13

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photophysical and photoelectri
found to constitute glass-forming materials with glass transition temperatures being in the range of
9-102 °C as characterized by differential scanning calorimetry. Electron photoemission spectra of
the amorphous films of the materials revealed ionization potentials of 5.25–5.55 eV. Time-of-flight
cal properties. The synthesized triphenylamine derivatives were
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4
-2
2
hole mobilities in the solid amorphous layers of synthesized arylethenes reached 10 cm /V·s at
high electric fields. All blue-green emitting OLEDs based on triphenylamine derivatives exhibited
2
relatively high efficiencies (brightness in order 10000 cd/m and current efficiency from 4 to 10
cd/A) as for the fluorescent OLEDs.
Acknowledgements
This work was supported by the Horizon 2020 ICT29-2014 project PHEBE (grant No 641725).
References
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ACCEPTE DT A
BL
MA NE SU SCRIPT
Table 1. Thermal characteristics of compounds 1–4.
Compound
Tm., ºC
Tg, ºC
82
T ,ºC
dec
1
2
3
4
-
200
-
418
439
354
373
102
55
-
49
T estimated by DSC at heating rate of 10 ºC/min; N atmosphere; second heating scan.
g
2
T estimated by TGA at a heating rate of 20 ºC/min; N atmosphere.
dec
2
19

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Table 2. Optical and photophysical characteristics of the dilute (10 M) solutions in THF and neat
-5
films of compounds 1–4.
Solution
Film
in THF
, %
in toluene
Compound
max
abs
max
2
max
max
λ
, nm λF , nm
Φ
F
τ
, ns
χ
λF , nm
Φ
, %
F
λF , nm
Φ , %
F
1
2
3
4
291, 395
286, 394
262, 358
272, 362
511
486
469
446
12
2
0.57
0.18
1.51
0.58
1.256
1.301
1.127
1.111
488
478
444
430
14
2
497
484
466
445
3
3
2
3
30
14
33
16
max
abs
λ
– absorption maximum.
max
λF – emission maximum (
λ
ex = 310 nm).
Φ – quantum yield.
F
max
τ
– fluorescence life time measured at λF
(λex = 310 nm).
2
χ – least squares fitting of experimental fluorescence decay data.
20

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Table 3. I , EA, E energies and
A
ho
C
le
C
m
E
o
P
b
T
il
E
ity
D
d
M
ata
A
f
N
or
U
co
S
m
C
p
R
o
I
u
P
n
T
ds 1–4.
p
g
e
E vs
µ0
,
µh,
1
/2
opt a
cv b
cv c
ep d
Compound
Eg , eV I , eV EA , eV I , eV
p P
2
2
Fc, V
0.08
0.14
0.09
0.18
cm /Vs
cm /Vs
-
-
-
4
4
5
-3
1
2
3
4
2.80
2.81
3.04
3.06
4.88
4.94
4.89
4.98
2.09
2.13
1.86
1.83
5.31
5.39
5.33
5.44
2.6⋅10
7.1⋅10
1.6⋅10
4.2⋅10
-3
7.9⋅10
-4
6.5⋅10
-
4
-3
3⋅10
5.8⋅10
a
The optical band gap estimated from the edges of electronic absorption spectra.
b
c
d
e
cv
The ionization potentials measured by electrochemical studies I =4.8+E vsFc [21].
p
1/2
cv
cv
opt
The electron affinities calculated using equation EA = Ip - Eg
.
The ionization potentials measured by electron photoemission in air method;
5
Mobility value at 6.4·10 V/cm field strength.
21

ACCEPTED MANUSCRIPT
SCRIPT
URES
ACCEPTE DF I GM ANU
8
6
4
2
000
sol THF
sol Tol
film
Sol. Tol.
4
3
000
000
000
Φ =16%
F
Film
Φ =3%
F
Sol. THF
Φ =14%
F
0
8
6
4
2
000
000
000
000
Sol. Tol.
Φ =33%
F
Film
Sol. THF
Φ =30%
Φ
F
=2%
F
0
8
6
4
2
000
Sol. Tol.
2
1
Φ =2%
000
000
000
F
Sol. THF
Φ =2%
F
Film
Φ =3%
F
0
8
6
4
2
000
Sol. Tol.
Φ =14%
Sol. THF
F
000
000
000
0
Φ =12%
F
Film
Φ =3%
F
2
50
300
350
400
450
500
550
600
650
Wavelength, nm
-5
Figure 1. Absorption spectra of dilute (10 M) solutions in THF (solid lines) and fluorescence
spectra of dilute solutions in THF and toluene and of the solid films of compounds 1-4
(λex = 310 nm).
22

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-5
Figure 2. Fluorescence decay curves of dilute (10 M) solutions in THF (grey points) of
compounds 1–4 (λex = 310 nm). Lines mark exponential fits to the experimental data. Fluorescence
2
lifetimes (
τ
) and least squares fitting of experimental data (χ ) are indicated.
23

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Figure 3. Cyclic voltammograms of dilute solutions of compounds 1–4 in dichloromethane
o
recorded at 25 C at the sweep rate of 0.1 V/s
24

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Figure 4. Electron photoemission spectra of the films of compounds 1–4 recorded in air at 25 °C.
25

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Figure 5. Electric field dependencies of hole drift mobilities for the layers of compounds 1–4.
26

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Figure 6. Electroluminescence spectra of OLEDs.
27

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Figure 7. Energy-band diagrams of the devices A, C (a) and B, D (b).
28

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Figure 8. Current density-voltage and luminance-voltage characteristics (a) and current
efficiency-current density characteristics (b) of OLEDs
29