Carbazole derivatives containing one or two tetra-/triphenylethenyl units as efficient hole-transporting OLED emitters

Article abstract of DOI:10.1016/j.dyepig.2019.04.045

New efficient carbazole-based emitters containing tetra-/triphenylethene units were developed for boosting efficiency of non-doped fluorescent organic light-emitting diodes. Comparable study of the properties of derivatives containing one or two tetra-/triphenylethenyl units was performed using various experimental and theoretical techniques. Depending on the substitution pattern, emitters exhibited strong blue or green emission, which was enhanced by aggregation. Compounds with two substituents showed higher glass transition temperatures (up to 120 °C) and lower ionization potentials (of ca. 5.15 eV) comparing to mono substituted derivatives. Time-of-flight hole drift mobility values of the studied compounds with two substituents reached 10^?3 cm²/V at high electric fields. Non-doped fluorescent OLEDs based on carbazole derivative containing two tetraphenylethenyl units demonstrated extremely high external quantum efficiency as for simple fluorescent organic light-emitting devices, which reached 5.32%.

Full text of DOI:10.1016/j.dyepig.2019.04.045

Accepted Manuscript
Carbazole derivatives containing one or two tetra-/triphenylethenyl units as efficient
hole-transporting OLED emitters
Sohrab Nasiri, Monika Cekaviciute, Jurate Simokaitiene, Aina Petrauskaite,
Dmytro Volyniuk, Viktorija Andruleviciene, Oleksandr Bezvikonnyi, Juozas Vidas
Grazulevicius
PII:
S0143-7208(19)30175-5
https://doi.org/10.1016/j.dyepig.2019.04.045
DYPI 7500
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 22 January 2019
Revised Date: 9 April 2019
Accepted Date: 17 April 2019
Please cite this article as: Nasiri S, Cekaviciute M, Simokaitiene J, Petrauskaite A, Volyniuk D,
Andruleviciene V, Bezvikonnyi O, Grazulevicius JV, Carbazole derivatives containing one or two tetra-/
triphenylethenyl units as efficient hole-transporting OLED emitters, Dyes and Pigments (2019), doi:
https://doi.org/10.1016/j.dyepig.2019.04.045.
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Carbazole derivatives containing one or two tetra-/triphenylethenyl units as efficient
hole-transporting OLED emitters
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Sohrab Nasiri, Monika Cekaviciute, Jurate Simokaitiene, Aina Petrauskaite, Dmytro Volyniuk, Viktorija
Andruleviciene, Oleksandr Bezvikonnyi, Juozas Vidas Grazulevicius
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Department of Polymer Chemistry and Technology, Kaunas University of Technology, Radvilenu rd. 19,
LT-50254, Kaunas, Lithuania
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*Corresponding author: juozas.grazulevicius@ktu.lt (Juozas V. Grazulevicius)
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Abstract
New efficient carbazole-based emitters containing tetra-/triphenylethene units were developed for
boosting efficiency of non-doped fluorescent organic light-emitting diodes. Comparable study of the
properties of derivatives containing one or two tetra-/triphenylethenyl units was performed using various
experimental and theoretical techniques. Depending on the substitution pattern, emitters exhibited strong
blue or green emission, which was enhanced by aggregation. Compounds with two substituents showed
higher glass transition temperatures (up to 120 ºC) and lower ionization potentials (of ca. 5.15 eV)
comparing to mono substituted derivatives. Time-of-flight hole drift mobility values of the studied
compounds with two substituents reached 10-3 cm2/Vs at high electric fields. Non-doped fluorescent
OLEDs based on carbazole derivative containing two tetraphenylethenyl units demonstrated extremely
high external quantum efficiency as for simple fluorescent organic light-emitting devices, which reached
5.32 %.
Keywords: Tetra-/triphenylethene, carbazole, aggregation induced emission enhancement, hole mobility,
electroluminescence
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1. INTRODUCTION
Development of organic luminescent materials exhibiting aggregation induced emission enhancement
(AIEE) is hot research topic because of their wide possible applications ranging from biological imaging
including specific cancer cell imaging, photothermal therapy, physical and chemical sensing to
optoelectronics [1-4]. Intensity of emission of such organic luminescent materials dramatically increase in
solid-state in comparison to that of solutions due to either the restriction of intramolecular motions or
intermolecular through-space interaction upon clustering or aggregation [5]. Since high emission
efficiency in solid-state is required for emitters of advance organic light emitting diodes (OLEDs), many
AIEE compounds were utilized as electroluminescent materials [6]. Such emitters typically contain AIEE
active tetra- or triphenylethene units, which may boost emission efficiency of solid films to unity [7].
Most of such emitters are characterized by prompt fluorescence. Because of the spin-statistic limit
efficiencies of OLED based on such emitters are rather low compared to those of devices based on
thermally activated delayed fluorescence or phosphorescent OLEDs. However, they are unquestioningly
important active OLED materials allowing to achieve long-term stability and emission color purity [8-
10]. In addition, simple molecular structure of tetra- or triphenylethenyl moieties allows facile preparation
and easy chemical modification of OLED emitters with AIEE phenomenon [11]. For instance,
photoluminescence quantum yields (PLQY) close to unity were observed for tetraphenylethenyl-
substituted carbazole derivatives [7, 12]. However, no impressive maximum external quantum
efficiencies (EQEs) were observed for non-doped devices based on these such derivatives [7]. EQE
reached only 2.3%. Study of another example of tetraphenylethenyl-substituted carbazole derivatives
showed that hole-transporting properties of these derivatives are very sensitive to the position of
tetraphenylethenyl moieties [13]. As a result, EQEs of non-doped OLEDs based on these derivatives
ranged from 0.4 to 3.2 % [14]. This observation was explained by poor charge-injection properties of the
materials. In such case, additional hole-transporting layers are required. Simple fluorescent OLEDs based
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on triphenylethenyl-substituted carbazole derivatives showed relatively high EQEs up to 3.8 % [15]. It is
expected that further modification of tetra-/triphenylethenyl-substituted carbazole derivatives as AIEE
active OLED materials may lead to improvement of their properties including charge-injection and
charge-transporting characteristics [16, 17].
With the aim of boosting efficiency of non-doped fluorescent OLEDs, new AIEE emitters were designed
in this work. The design was based on exploitation two tetra-/triphenylethene substitutuents linked
through the nitrogen atom in C-3 position of carbazole moiety and compared with previously published
carbazole derivatives [12, 18, 19] when substituent is in N-9 carbazole position. The choice of such
substitution strategy was supported by the fact that previously reported tetra-/triphenylethenyl-substituted
derivatives were found as effective OLED materials with very high PLQYs and good hole transporting
properties [12, 15]. Attachment of two donating tetra-/triphenylethene units to carbazole moiety can lead
to decrease of oxidation potential and increase of molar mass. This may lead to improved hole-injection
properties, thermal stability and increased glass transition temperature. To demonstrate efficiency of the
proposed design strategy, comparable study of new carbazole derivatives containing two tetra-
/triphenylethenyl moieties and previously reported carbazole derivatives with single tetra-/triphenylethene
unit was performed [7, 12, 18]. As it was expected, improved hole-injection abilities were observed for
the compounds containing two tetra-/triphenylethenyl moieties in comparison to mono-substituted ones.
All the studied compounds were characterized by relatively high hole mobilities reaching 10-2 cm2/Vs in
high electric fields. Simple non-doped fluorescent OLED based on newly developed carbazole derivative
containing two tetrahenylethenyl moieties as light-emitting material exhibited impressive maximum EQE
of 5.32 %.
2. RESULTS AND DISCUSSION
2.1. Synthesis
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The synthetic routes towards appropriate tetra- or triphenylethenyl-substituted carbazole derivatives are
shown in Scheme 1. The compounds were synthesized by the Buchwald-Hartwig method [20, 21] in the
presence of palladium complex or by the Buchwald procedure [22] in the presence of CuI and 2,2,6,6-
tetramethyl-3,5-heptanedione as ligand. The synthesized compounds were identified by mass-, IR- and
1H, 13C NMR spectrometries and elemental analysis. The details of synthesis and characterization data are
given in Supporting Information.
N
1
yield 8 %
(a)
Br
H
N
N
Br
N
Br
(a)
(a)
2
3
yield 20 %
yield 23 %
Br
N
N
N
Br
(b)
(b)
4
5
N
NH2
N
yield 52 %
yield 26 %
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Scheme 1. Synthetic routes for compounds 1–5. (a) CuI, TMHD, K2CO3, DMF, reflux; (b) t-Bu3P,
Pd2dba3, t-BuOK, anhydrous toluene, 90 °C.
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2.2 Geometries and frontier orbitals
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The geometrical structures of compounds 1–5 were analyzed employing quantum chemical calculations
(Figure 1). Density functional theory (DFT) for 1–5 showed that arylethenyl substituents are rotated with
respect to carbazole moiety. In the case of compound 1, the dihedral angle between carbazole moiety and
ethenyl linkage was of 64º. Dihedral angles between carbazole units and phenyl ring planes in compounds
2 and 3 were found to be of 55º, while dihedral angles between phenyl ring planes and ethenyl linkages
were of 28º and 49º, respectively. For compounds 4 and 5 in which arylethenyl substituents are connected
through the nitrogen atom in C-3 positions of carbazole moiety dihedral angles between carbazole unit
and C-N-C bonds range from 54º to 55º. The dihedral angles between phenyl ring planes and C-N-C
bonds in compounds 4 and 5 were 37º and 36º, respectively, while dihedral angles between phenyl ring
and ethenyl linkages are similar to those observed for compounds 2 and 3.
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The frontier orbitals of the molecules are shown in Figure 1. HOMO is localized throughout all molecule.
HOMO of compounds 1–3 with single arylethenyl substituent is more prominent on carbazole moiety,
while HOMO of compounds 4, 5 with two arylethenyl substituents is more localized on inner
triphenylamine moiety. The LUMO main electron density of compounds 1–5 is essentially on the
arylethenyl fragments, with small contribution from the carbazole moiety. The electronic properties
related to these orbitals consequently were expected to exhibit some similarities.
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Figure 1. Optimized geometries, HOMO and LUMO of compounds 1–5 calculated at B3LYP/6-31G (d,p)
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level
2.3. Photophysical properties
Absorption and photoluminescence (PL) spectra of dilute solutions in toluene and THF as well as of
vacuum-deposited films of the compounds are shown in Figure 2. Their photophysical characteristics are
summarized in Table 1. The low-energy absorption bands for all compounds 1–5 were practically not
sensitive to the solvent used (toluene or THF). The low-energy absorption bands of solid films were red-
shifted in comparison to those of the corresponding diluted solutions. The red-shifts are attributed to the
aggregation effects due to the intermolecular interactions in solid state. Dilute solutions of mono-
substituted carbazoles (1–3) showed absorption maxima at ca. 342 nm, which are in accordance with
previously published data on tetra-/triphenylethenyl-substituted carbazole derivatives [7, 12, 16]. These
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absorption maxima are related to the π–π* transitions. Absorption maxima of dilute solutions of
compounds 4 and 5 containing two tetra-/triphenylethenyl moieties were observed at ca. 404 and 382 nm,
respectively (Table 1). Time dependent (TD) DFT calculations carried out to interpret impact of tetra-
/triphenylethene units to the optical properties of the synthesized compounds. The experimental UV
spectra of compounds 1–5 were found to accord with the theoretical UV spectra. The lowest energy
absorption bands of compounds 4 and 5 having two tetra-/triphenylethenyl moieties are red-shifted
relative to those of compounds 1–3 containing one arylethenyl moiety (Figure S1) indicating the decrease
of optical band gap from 3.80 eV to 3.31 eV as well as increase of extent of conjugation. Corresponding
to the results of quantum chemical calculations, the lowest energy absorption bands of derivatives 1–5 are
characterized as transitions from ground state (S0) to first excited state (S1). In case of compound 5
transition from ground state (S0) to second excited state (S2) also contributes (Figure S1, Table S1). The
transitions from ground state to the first excited state (S0/S1) for compounds 1–5 correspond to the
transitions from HOMO to LUMO.
Toluene
THF
film
1,0
0,5
0,0
1
2
3
4
5
300
300
300
300
300
400
400
400
400
400
500
500
500
500
500
600
600
600
600
600
1,0
0,5
0,0
1,0
0,5
0,0
1,0
0,5
0,0
1,0
0,5
0,0
Wavelength, nm
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Figure 2. Normalized absorption and fluorescence spectra of dilute (10-5 M) toluene, THF solutions and
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solid films of compounds 1–5.
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Dilute toluene and THF solutions of all studied compounds showed weak emission (Figure 2). PL spectra
of toluene and THF solutions of compounds 1–3 were found to be similar. Dilute solutions of 1 and 2
showed blue emission with PL maxima at ca. 450–460 nm, while greenish-blue emission with the
intensity maxima at ca. 500 nm was observed for solutions of 3. Lower PL quantum yields (ΦF) were
observed for dilute THF solutions of 1–3 in comparison to their toluene solutions. This consideration can
be explained by the slight polarity effect (ε=7.5 for THF and ε=2.38 for toluene) (Table 1). In case of
solutions of compounds 4 and 5 with two arylethenyl substituents, solvent polarity affected not only ΦF
values but also emission color. Thus, PL spectra of THF solutions of compounds 4 and 5 were red-shifted
with respect of the spectra of toluene solutions (Figure 2). Compounds 2 and 3 have tetra-
/triphenylethenene units incorporated through heterocyclic nitrogen atom of carbazole moiety, while in
compounds 4 and 5 tetra-/triphenylethenene units are attached through nitrogen atom attached to C-6
position. Therefore, different strength of electron donating ability of nitrogen atoms to which tetra-
/triphenylethenene units are attached can be the reason of different PL behavior of these compounds. In
addition, this deliberation can be explained by charge-transfer (CT) character of emission due to the
twisted molecular structures of compounds 4 and 5. Similarly to that of solutions of compounds with one
arylethenyl substituent, emission spectra of solutions of tetraphenylethenyl-substituted carbazole
derivative 5 were red-shifted in correlation to those of compound 4 containing triphenylethenyl
substituents.
Table 1. Photophysical data for dilute solutions and solid films of compounds 1–5
Toluene solution
THF solution
Film
max
abs
max
abs
max
λFmax
λ
λFmax
λ
abs
λFmax
,
Compound
,
,
,
,
,
λ
ΦF, %
ΦF, %
ΦF, %
nm
nm
462
448
507
484
nm
nm
454
448
499
508
nm
nm
480
458
479
500
342
342
342
404
4
3
342
342
342
404
2
2
366
354
345
420
48
52
86
28
1
2
3
4
3
1
17
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382
517
9
382
λFmax
– wavelength of emission maximum (λex: 350 nm). ΦF – quantum yield.
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517
35
5
max
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159
160
161
162
163
164
165
166
167
168
– wavelength of absorption maximum.
λ
abs
Solid films of compounds 1–5 shown strong emissions stretching from blue to green region of visible
spectra (Figure 2, Table 1). The wavelengths of PL maxima of solid films of the compounds were found
to range in the order 2<1≈3<4<5, reflecting impacts of configuration and amount of arylethenyl moieties.
ΦF values of the solid films of compounds 1–5 were recorded to be much higher, than those of the
corresponding solutions (Table 1). These results are apparently related to AIEE effect which previously
proved for compounds 1 and 3 [12, 16]. It should be noted that solid-state ΦF values of 55.7 and 100 %
previously reported for compounds 1 and 3 are slightly higher than those obtained in this work (48 and 86
%, respectively). These differences can apparently be related to reabsorption in solid films, which we did
not take into account during ΦF measurements. Additionally, different methods of preparation of films in
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two different laboratories can be reason for different ΦF values. Nevertheless, ΦF values (which is close
to unity in case of compound 3) demonstrate high emission efficiency of the studied emitters 1–5 in solid-
state (Table 1). It was expected that on increasing the number of tetra-/triphenylethene units increase the
PLQY induced by AIEE effect. However, PLQY of 4 and 5 in film state is decreased compared to 1, 2
and 3 which are possessing one tetra-/triphenylethene unit. PLQYs of solid films of organic materials are
very sensitive to many factors including molecular structure, intermolecular interactions, film-forming
properties, dipole moments of molecule which may increase with increasing the number of tetra-
/triphenylethene units, etc. Therefore, the increasing number of tetra-/triphenylethene units should not
obligatory increase PLQY induced by AIEE effect. For example, PQLY values in the order of 93, 78, and
99 % of PLQY were obtained for the films of compounds containing one, two and four tetraphenylethene
units, respectively [23]. PLQYs ranging from 33.9 to 73.9 % were observed for the films of compounds
based on triphenylamine core and three triphenylethene units [24]. Despite the very similar chemical
structures of these compounds, big differences in their PLQYs were related to the twisting of the
triphenylethene units relative to the triphenylamine core. The increasing extent of molecular conjugation,
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taking place with increasing number of tetra-/triphenylethene units, may lead to enhancement of PLQY
and charge mobilities of such molecules. However, higher number of substituents induce more steric
hindrance, which subsequently may lead to decrease of PLQYs [25]. Thus, to our opinion, overlapping of
different effects results in lower PLQY of the films of 4 and 5 in comparison to that of the films of
compounds 1, 2 and 3.
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To investigate emission nature of compounds 1–5 in solid-state, PL decays were recorded (Figure S2).
Since these PL decays were well-fitted giving PL lifetimes in nanosecond range, it could be concluded
that solid compounds of 1–5 were characterized by fluorescence. For adequate representation of PL decay
curves double exponential fits were required. This observation can evidently be explained by the
formation of excited dimers in solid state.
2.4. Aggregation induced emission enhancement
PL spectra of dispersions of the studied compounds in the mixtures of THF and water with various water
fractions (fw) were recorded (Figures 3, S2–S5). Since the compounds were insoluble in water, their
emissive aggregates were formed at the certain fw displaying AIEE phenomenon. For instance, dispersion
of compound 5 in THF/water mixtures shown weak emission at low water fractions. This emission
intensity essentially did not change within large range of water fraction (up to 80 %). Until aggregates
started to be formed, slight changes in emission intensities of the dispersions in THF/water mixtures were
related to the changes in solution polarity due to the increase of water fraction. However, emission
intensity dramatically increased at fw>80% when formation of aggregates in THF/water mixtures
occurred (Figure 3). As it usually explained [1], such behavior is related to AIEE effect due to the
limitation of intramolecular motions in solid-state. The similar effects were observed for other studied
compounds (Figures S2–S5).
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(b)
2.5x107
(a)2.4x10 Compound 5
FW (vol%)
Compound 5
0%
2.1x107
1.8x107
1.5x107
1.2x107
9.0x106
6.0x106
3.0x106
0.0
10%
20%
30%
40%
50%
2.0x107
1.5x107
60%
70%
1.0x107
80%
90%
95%
5.0x106
0.0
400
450
500
550
600
650
700
750
40
60
80
100
F
(vol%)
Wavelength, nm
W
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Figure 3. Non-normalized PL spectra of the dispersions of compound 5 in the THF/water mixtures (a) and
PL maximum intensities versus water volume fraction in THF/water solutions (b).
2.4. Thermal properties
The thermal characterization of the synthesized compounds was carried out by DSC and TGA under
nitrogen atmosphere. The thermal properties are summarized in Table 2. All the synthesized compounds
(1–5) exhibited moderately high thermal stability. The temperatures of five percent of weight loss (Tdes-
5%) ranged from 275 ºC to 440 ºC, as confirmed by TGA with a heating rate of 20 ºC/min. Carbazole
derivatives usually possess high thermal stability and their thermal decomposition usually starts above
300 °C [26]. Rather low Tdes-5% of compounds 1–3 can be explained by their lower molar mass which is
favorable for sublimation.
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Figure 4. DSC curves of compound 2 (scan rate of 10 ºC/min. N2 atmosphere) (a) and photoelectron
emission spectra of solid films of compounds 1–5 (b).
Compounds 1–3 and 5 were obtained as crystalline substances, while 4 was separated after the synthesis
as amorphous powder. When the melt samples of carbazole derivatives 1, 2 and 5 were cooled down
during DSC experiments, they formed molecular glasses with glass transition temperatures (Tg) of 70, 53
and 120 ºC, respectively. The higher Tg of compound 1, when compared with that of compound 2, can
evidently be explained by its more compact structure which prevents freedom of movement of molecular
fragments. It was impossible to transform compound 3 into the glassy state due to its strong tendency to
crystallize. Tg of compounds 4 and 5 with two arylethenyl moieties were found to be greater than those of
compounds 1 and 2 containing single arylethenyl substituent. Compound 2 showed polymorphism. Two
endothermic melting peaks (Tm) were observed in the first DSC heating scan at 170 and 178 °C (Fig. 4).
Upon second heating, compound 2 showed besides glass transition at 53 °C but also crystallization (Tcr)
at 119 °C and melting at 170 °C.
Table 2. Thermal properties of compounds 1–5
Compound
Tcr, ºC
Tm, ºC
174
Tg, ºC
70
Tdes-5%,ºC
275
1
2
3
4
5
119
136
170, 178
255
53
315
340
108
120
410
205
440
240
241
Tm , Tcr and Tg estimated by DSC at heating rate of 10 ºC/min, N2 atmosphere; a second heating scan.
Tdes-5% estimated by TGA at heating rate of 20 ºC/min; N2 atmosphere.
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2.5. Electrochemical, photoelectrical and charge transporting properties
Ionization potentials, electron affinities and charge mobilities are important characteristics for evaluation
of applicability of compounds in optoelectronic devices. Electrochemical properties of the compounds
were analyzed by cyclic voltammetry (CV). Cyclic voltammograms of compounds 1–5 presented in
Figure 5. The electrochemical properties are resumed in Table 3. Compounds 1–3 underwent irreversible
oxidation. However, compounds 4 and 5, having an open reactive C-6 position, showed reversible
oxidation similarly as previously reported [13, 27]. We assume that the formation of radical cations
leading to the formation of the dimers, which is generally observed in CV experiments of 3-
monosubstituted carbazole derivatives, did not occur in this case. [13, 27, 28]. The ionization potential
cv
values (Ip ) were determined from the values of the onset oxidation potential considering ferrocene (Fc).
The Ip values of the carbazole derivatives ranged from 4.98 to 5.62 eV. Electron affinities (EAcv) were
cv
found from optical band gaps and ionization energy values [29]. They ranged from 2.16 to 2.45 eV.
Table 3. Ip , EAcv, band gap energies, electrochemical characteristics and hole-transporting parameters of
cv
compounds 1–5
opt
ep
cv
cv
Eox
,
Eg
,
IP
,
Ip
,
EA
,
µh0
,
µh,
α,
onset
Compound
(cm/V)1/2
cm2/Vs
1.3×10-3
1.93×10-4
2.95×10-4
2×10-4
cm2/Vs
2.1×10-2
6×10-3
6×10-3
3×10-3
1.9×10-3
V
eV
eV
eV
eV
0.82
0.82
0.81
0.18
0.18
3.17
3.27
3.28
2.75
2.82
5.85
5.70
5.68
5.14
5.15
5.62
5.62
5.61
4.98
4.98
2.45
2.35
2.33
2.23
2.16
5.23×10-3
5.43×10-3
6.45×10-3
4.9×10-3
1
2
3
4
5
2.45×10-4
3.84×10-3
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The onset oxidation potentials (Eoxonset) versus Fc measured by CV from the first redox cycle. The optical band gap (Egopt) estimated from the
opt
ep
edges of absorption spectra of THF solutions of compounds (Eg = 1240/λonset.). Ionization potentials (Ip ) measured by photoelectron
emission in air method. Ionization potentials (Ipcv) measured by CV.: Ipcv=4.8+Eox
[29]. Electron affinities calculated using equation
onset
cv
EAcv= Ip - Egopt. Hole mobility (µh) values at electric field of 2.9×105 V/cm. Zero-field hole mobility (µh0) values and field dependence
parameter (α) are indicated as well.
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Ionization potentials of vacuum deposited layers (Ip ) of compounds 1–5 also determined by electron
ep
photoemission method in air (Figure 4b). The Ip values of 1–5 ranged from 5.14 to 5.85 eV. The
ionization potential values of carbazole derivatives with two arylethenyl moieties (4, 5) were found to be
lower than, those of carbazole derivatives with single arylethenyl substituent (1–3). Both methods of
estimation of ionization potentials as well as HOMO energy values (see Figure 1) revealed the same
trend. As it is mentioned above, the ionization potential values (5.14 and 5.15 eV) of carbazole
derivatives with two arylethenyl moieties (4, 5) were found to be much lower than, those of carbazole
derivatives with single arylethenyl substituent (1–3) (5.62, 5,62 and 5,61 eV), respectively. Such
improvement in hole-injection properties of compounds 4 and 5 derives from nitrogen attached to C-3
atom of carbazole moiety because of its donating ability. Most probably, such improvement in hole-
injection properties of compounds 4 and 5 (ionization potential values of 5.14 and 5.15 eV) could not be
obtained by incorporating only tetra-/triphenylethene units to carbazole moiety. However, we believe that
the achieved hole-injection properties of compounds 4 and 5 are related to both the C-3 nitrogen atom and
tetra-/triphenylethene units because of relatively big differences between ionization potential values of
carbazole derivatives with two arylethenyl moieties (4, 5) and carbazole derivatives with single
arylethenyl substituent (1–3).
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Figure 5. Cyclic voltammograms of dilute solutions of compounds 1–3 (a) and 4, 5 (b) in
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Time of flight (TOF) technique was used to estimate charge transporting properties of the compounds.
Transit times (ttr) were well observed for holes in the corresponding low-dispersive photocurrent
transients of the layers of compounds 1–5 (Figures 6a and S9). Transit times were not observed for
electrons indicating absence of electron transport in the films of compounds 1–5. Electric field
dependencies of hole drift mobilities (µh) for the layers of compounds 1–5 are exhibited in Figure 6. All
the compounds displayed linear dependencies of hole mobilities to the square root of electric field. Thus,
µh for the layers of compounds 1–5 verify 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 [30]. Organic
semiconductors typically confirmed this dependence (Figure 6) [31, 32]. Hole-drift mobility values are
collected in Table 3. Hole mobilities of compounds 4 and 5 exceeded 10-3 cm2/Vs at high electric fields.
µh values of the compounds were found to range in order 1>2~3>4>5. According to the previously
published crystal structure of compound 3 multiple C-H···π hydrogen bonds with distances of 2.719–
3.090 Å are formed between hydrogen atoms of phenyl rings of tetraphenylethenyl unit in one molecule
and the π cloud of planar aromatic ring in another molecule [7]. Such molecular packing may provide
good HOMO-HOMO overlapping of neighboring molecules, which results in good hole-transporting
properties of compound 3. Apparently, distances between neighboring molecules of compound 1 are even
shorter, multiple C-H···π hydrogen bonds with distances of 2.620 and 3.075 Å are formed between
carbazole protons of one molecule and the π cloud of the carbazole of the neighboring molecule [21].
Since distances between molecules are shorter, the higher hole mobilities were observed for compound 1
relative to those of other studied compounds.
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(a)
130V
120V
110V
100V
90V
5
101
100
80V
70V
60V
50V
40V
30V
20V
6
4
2
0
10-1
10-2
30V
3.0x10-6
6.0x10-6
9.0x10-6
0.0
Time, s
10-6
10-5
Time, s
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Figure 6. TOF pulses for holes in the layers of compound 5 at different electric fields (a) and hole
mobility dependences on applied electric field for the layers of compounds 1–5 (b).
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2.6. Electroluminescent devices
Non-doped OLEDs were fabricated to explore the effect of substitution pattern of the studied carbazole
derivatives on their electroluminescent properties and to demonstrate potential of the compounds as
electroactive functional materials (mainly emitters) in organic electronics. The structures of the fabricated
OLEDs were ITO/MoO3 (1 nm)/NPB (60 nm)/TCTA (5 nm)/non-doped light-emitting layer (30
nm)/TSPO1 (3 nm)/TPBi (40 nm)/LiF (0.5nm)/Al, which were named as devices D1–D5 depending on
compounds 1–5 used as emitters. In these OLED structures, molybdenum trioxide (MoO3) and lithium
fluoride (LiF) were utilized for the deposition of hole and electron injecting layers, respectively. N,N′-
di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (NPB) / tris(4-carbazoyl-9-ylphenyl)amine
(TCTA) and 2,2′,2″-(1,3,5-benzinetriyl)-tris(1-phenyl-1H-benzimidazole) (TPBi)
/
diphenyl-4-
triphenylsilyl-phenylphosphineoxide (TSPO1) were exploited as hole and electron transporting layers,
respectively. Tandemly, TCTA and TSPO1 played roles of electron and hole blocking layers,
respectively. As their usual roles, indium-tin-oxide (ITO) was optically transparent anode and LiF/Al was
cathode. Using these device structures, good charge-injection and charge-transport were ensured for
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devices D1–D5, despite the different HOMO/LUMO energy levels of compounds 1–5 (Figure 7).
ep
Moreover, HOMO (Ip ) values of solid films were used in the design of the devices.
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Figure 7. Equilibrium energy diagrams for devices D1–D5.
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The fabricated OLEDs were characterized by electroluminescence with colors falling in spectral range
from blue to green with corresponding CIE coordinates, which are presented in Table 4 (Figure S10). EL
spectra of devices D1–D5 were in agreement with corresponding PL spectra of solid films of compounds
1–5 (Figure S11). Small differences between them are related to the different optical and electrical
excitation sources used. EL spectra were stable at different applied voltages showing that hole-electron
recombination occurred only within light-emitting layer and electroluminescence of devices D1–D5 was
related to emission of solid films of compounds 1–5, respectively (Figure 8a).
Comparable turn-on voltages ranging from 3.7 to 4.5 V were noticed for all devices (Figure 8b). The
small differences of turn-on voltages and voltage-current density characteristics were apparently related
to the differences in charge transporting properties and HOMO/LUMO values of light-emitting layers
(Table 3). The highest maximum brightness exceeding 15000 cd/m2 was noted for device D5 (Figure 8c,
Table 4). The highest values of the maximum current, power and external quantum efficiencies (EQE) of
5.88 cd/A, 3.15 lm/W, and 2.02 % respectively were also obtained for the device D5 (Figure 8c, Table 4).
The studied devices exhibited very low efficiency roll-off apparently because of good stability of the
fluorescent emitters 1-5 under electrical excitation. In case of some devices, higher EQE values were
observed at high brightness of 1000 cd/m2 in comparison to those recorded at lower brightness of 100
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cd/m2 (Table 4). This observation is apparently related to the balance of hole and electron transport in the
non-doped light-emitting layers at higher current densities.
(a)
4V
5V
6V
7V
8V
9V
10V
1.0
0.8
0.6
0.4
0.2
0.0
Device D1
Device D2
Device D3
Device D1a
Device D2a
1.0
0.8
0.6
0.4
0.2
0.0
Device D3a
Device D4a
Device D4
Device D5
1.0
0.8
0.6
0.4
0.2
0.0
Device D5a
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400
450
500
550
600
650
700
Wavelength, nm
347
Device D1
Device D2
Device D3
Device D4
Device D5
(c)
10
(b)
D1
D2
D3
D4
400
300
200
100
0
10000
1
D5
D1a
D2a
D3a
D4a
D5a
1000
100
10
0.1
10
10
100
1000
10000
1
Device D1a
Device D2a
Device D3a
Device D4a
Device D5a
0.1
0
1
2
3
4
5
6
7
8
9
10
11
10
100
1000
10000
-2
Voltage(V)
Brightness (cd m )
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Figure 8. EL spectra recorded at the different applied voltages (a), current density-voltage and brightness-
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voltage (b) and external quantum efficiency- brightness characteristics (c) of devices D1–D5 and D5a
The device structure was further modified and optimized in order to make it more appropriate for
compounds 4 and 5. The modified device structure was ITO/MoO3 (1 nm)/mMTDATA (50 nm)/light-
emitting layer (30 nm)/TSPO1 (5 nm)/TPBi (65 nm)/LiF (0.5 nm)/Al in which hole-transporting layers
were skipped since HOMO of compounds 4 and 5 was appropriate for hole injection (Figure 7). In these
devices, compounds 1-5 were tested, and the corresponding devices were named as D1a-D5a,
respectively. EL spectra of devices D4a and D5a were practically the same as those of devices D4 and
D5. Both corresponded to PL spectra of compounds 4 or 5 (Figure 8a, S12). However, EL spectra of
devices D1a, D2a and D3a were different from those of devices D1, D2 and D3. Because of the high
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barrier for holes at the interfaces m-MTDATA/compound 1, m-MTDATA/compound 2 and m-
MTDATA/compound 3, radiative recombination of hole-electron pairs in devices D1a, D2a and D3a
apparently occurs not only in the light-emitting layer but also in m-MTDATA layer. Device D5a
exhibited very high maximum brightness exceeding of 50000 cd/m2 at 9.3 V (Figire 8b, Table 4). As a
result, device D5a showed improved maximum EQE of 5.32 % (Figure 8c). This value is very high as for
simple non-doped fluorescent devices. The theoretical maximum of such devices is of 5-7.5% taking out-
coupling efficiency of 20-30% [33]. It worth of noting that maximum EQEs of device D5a based on
carbazole derivative 5 with two tetraphenylethenyl moieties is considerably higher than those of
previously reported OLEDs based on carbazole derivatives, containing single tetra-/triphenylethenyl
substituent (Table S2). Since PLQY values are below unity for the films of the fluorescent compounds 4
and 5, which harvest only singlet excitons to light under electrical excitation, it is not clear how such high
EQE values of devices D4a and D5a were achieved even if the charge-balance factor was close to unity.
To check the possible pathway of triplet harvesting, interface exciplex emission was identified between
the layer of m-MTDATA and light-emitting layer of devices D1a-D5a (Figure S13). The shapes of PL
spectra and PL decays of the solid mixtures of m-MTDATA(50%) and compounds 1-5 (50%) were
obtained such as those typically observed for exciplexes [34, 35]. It was recently shown that device
performance of OLEDs based on emitting layers of materials exhibiting AIEE can be enhanced by triplet
harvesting due to utilization of interface exciplex-host [36]. Apparently, relatively high EQE values of
devices D4a and D5a were achieved due to the utilization of interface exciplexes m-MTDATA:4 and m-
MTDATA:5. Since PL spectra of solid films 4 (5) and solid mixtures m-MTDATA:4 (m-MTDATA:5)
were observed in the same region, enhancement of EQE of devices D4a and D5a was apparently achieved
due to the triplet energy transfer from the exciplex-host to emitter 4 (5). In contract, the corresponding PL
spectra of exciplexes m-MTDATA:1, m-MTDATA:2, and m-MTDATA:3 were red-shifted in
comparison to PL spectra of solid films of 1-3. As a result, lower EQE values and red-shifted EL spectra
of devices D1a, D2a and D3a in comparison to those of devices D1, D2 and D3 were observed due to the
exciplex-formation. Thus, formation of exciplexes m-MTDATA:1-3 resulted in poor performances of
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devices D1a, D2a and D3a. On the other hand, EQE efficiencies of devices D4a and D5a were enhanced
owing to exciplex-hosts m-MTDATA:4 and m-MTDATA:5 (Table 4).
Table 4. Non-doped OLED characteristics
CIE1931
coordinates
(X,Y)
Devic
e
Von
(V)
λmax
(nm)
Lmax
(cd/m2)
CEmax
(cd/A)
PEmax
(lm/W)
EQEmax
(%)
EQE (%)
at 100/1000 cd/m2
Device structure ITO/MoO3/NPB/TCTA/light-emitting layer/TSPO1/TPBi/LiF/Al
1.29/0.86
1.04/1.09
1.7/1.3
(0.168,0.209)
(0.159,0.126)
(0.174,0.244)
(0.194,0.463)
(0.248,0.514)
3.81
3.99
3.99
4.19
3.79
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499
518
4700
4300
3
2.35
0.82
2.6
2.2
3
1.62
1.08
1.95
1.6
D1
D2
D3
D4
D5
1.4
3.8
4.26
5.8
2000
12100
15500
0.99/1.52
1.3/1.97
2
Device structure ITO/MoO3/m-MTDATA/light-emitting layer/TSPO1/TPBi/LiF/Al
4.1
5.3
4.5
5.1
3.8
497
521
496
500
518
6600
10300
7700
28000
52000
2.1
3.9
2.76
9.6
17
1.17
1.7
0.85
1.3
0.66/0.84
1.06/1.28
0.9/1.09
3.1/3.25
3.1/5.22
(0.202,0.34)
(0.252,0.456)
(0.19,0.318)
(0.21, 0.464)
(0.242,0.52)
D1a
D2a
D3a
D4a
D5a
1.38
4.6
1.16
3.3
9.2
5.32
Device structure ITO/MoO3/light-emitting layer/TSPO1/TPBi/LiF/Al
5.0
5.9
500
521
25300
37600
8.0
4.3
5.3
2.7
4.0
2.24/2.63
2.43/3.99
(0.205,0.483)
(0.24,0.52)
D4b
D5b
12.6
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393
394
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396
397
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399
400
401
402
403
In addition, the designed compounds are very good candidates to be used as multifunctional OLED
materials. Thus, compounds 4 and 5 may be used for both hole-transporting (HTM) and emissive (EM)
layers due to their relatively high PLQYs, relatively high hole mobilities and high HOMO values for good
hole injection from either anode or hole-injection layers. To prove this concept, devices D4b and D5b
having a structure ITO/MoO3 (1 nm)/light-emitting layer (60 nm)/TSPO1 (5 nm)/TPBi (60 nm)/LiF (0.5
nm)/Al without m-MTADATA layer were fabricated exploiting compounds 4 or 5 for the preparation of
the light-emitting and hole-transporting layers, respectively. Stable at different voltages, EL spectra of
devices D4b and D5b were practically the same as those observed for other 4 or 5-based devices D4, D5,
D4a and D5a (Figure S14, Table 4). Displaying potential of compounds 4 or 5 as multifunctional OLED
materials, relatively high OLED performances were observed for fluorescent devices D4b and D5b
(Table 4). Their maximum EQE values reached 2.7 and 4.0 % (Figure S15, S16). However, these EQE
values were lower than maximum EQE values of devices D4a and D5a with m-MTADATA layer. In the
case of devices D4a and D5a, there are energy barriers of ca. 0.68-0.28 eV for electrons between m-
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MTADATA and 4 or 5 (Figure 7). Thus, no leakage of electrons from the light-emitting layer to anode
take place because of electron-blocking properties of m-MTADATA layer. Since electron-blocking layer
was not used in case of devices D4b and D5b slightly lower EQE values in were observed relative to
those recorded for devices D4a and D5a (Table 4).
3. EXPERIMENTAL
3.1. Methods
1H NMR and 13C NMR spectra were registered with Varian Unity Inova [300 MHz (1H), 75.4 MHz (13C)]
and Bruker Avance III [400 MHz (1H), 100 MHz (13C)] spectrometers at room temperature. All the data
are provided as chemical shifts δ (ppm) downfield from Si(CH3)4. Infrared (IR) spectra were registered
using PerkinElmer Spectrum GX II FT-IR System. The samples of the solid compounds were prepared in
form of KBr pellets. Mass spectra (MS) were obtained on the Waters SQ Detector 2. Elemental analysis
was performed with an Exeter Analytical CE-440 Elemental Analyzer. Differential scanning calorimetry
(DSC) measurements were recorded in nitrogen atmosphere with a Perkin Elmer at DSC 8500 equipment
at heating rate of 10 ºC/min. Thermogravimetric analysis (TGA) was executed on a Perkin Elmer TGA
4000 apparatus in a nitrogen atmosphere at heating rate of 20°C/min. Melting points were measured with
Electrothermal MEL-TEMP melting point apparatus. Absorption spectra of dilute (10-5 M) solutions in
tetrahydrofuran (THF) were registered on an UV-vis-NIR spectrophotometer Lambda 950 (Perkin-
Elmer). Fluorescence spectra, fluorescence quantum yields (ΦF) and fluorescence transients of dilute
solutions in THF (10-5 M) or solid films of the compounds were registered with Edinburgh Instruments
LS980 spectrometer. Cyclic voltammetry (CV) measurements were recorded using a micro-Autolab III
(Metrohm Autolab) potentiostat equipped with a standard three-electrode configuration. A three-electrode
cell equipped with a glassy carbon-working electrode, an Ag/AgNO3 (0.01 M in anhydrous acetonitrile)
reference electrode and a Pt wire counter electrode were employed. The measurements were carried out in
anhydrous dichloromethane with tetrabutylammonium hexafluorophosphate (0.1 M) as supporting
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electrolyte under nitrogen atmosphere at scan rate of 0.05 V/s [37]. The measurements were calibrated
using an internal standard, ferrocene/ferrocenium (Fc) system. Oxidation potentials (E1/2 vs Fc) for
reversible oxidation were collected as average values of the anodic and cathodic peak potentials, Epa and
ep
Epc, respectively. Ionization potentials (IP ) of the vacuum-deposited films of the synthesized compounds
were measured by electron photoemission method in air as described before [38, 39]. Hole drift mobilities
were measured by a time of-flight (TOF) method as reported earlier [40]. The pulsed Nd:YAG laser
(EKSPLA NL300, 355 nm, 3-6 ns), Keithley 6517B electrometer, Tektronix TDS 3052C oscilloscope
were exploited in the TOF experimental setup. Hole mobility was calculated as µ=d2/U·tt using transit
time (tt) with the applied bias (U) and the entire thickness (d) of the samples.
Thermo-vacuum deposition technique (Kurt J. Lesker in-built in an MB EcoVap4G glove box)
was used for fabrication of electroluminescent devices under pressure lower than 2×10−6 mBar.
Density-voltage and luminance-voltage characteristics were registered using Keithley 2400C
sourcemeter and certificated photodiode PH100-Si-HA-D0 together with the PC-Based Power and
Energy Monitor 11S-LINK as it was previously described [41, 42]. External quantum efficiency
was estimated using the luminance, current density, and EL spectrum. Electroluminescence (EL)
spectra were recorded by an Aventes AvaSpec-2048XL spectrometer. EL spectra were used for
calculations of chromaticity coordinates (x, y) of the devices.
The computations were carried out in the frame of density functional theory (DFT) engaging the B3LYP
functional with Gaussian 16 program [43-46]. The functional was exploited in conjunction with the 6-
31G(d,p) basis set. The spectroscopic properties and of the molecules was calculated by mean of time
dependent density functional theory method (TDDFT) [47-51]. Up to 40 excited states were calculated
and the theoretical absorption bands were gained by considering a band half-width at half-maximum 0.3
eV.
Conclusion
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We synthesized and characterized carbazole derivatives containing on or two tri/tetraphenylethenyl
moieties as fluorescent OLED emitters showing high photoluminescence quantum yields in solid-state
due to the aggregation induced emission enhancement. The synthesized compounds formed glasses with
glass transition temperatures in the range of 53–120 ºC. Ionization potentials of ca. 5.15 eV and hole
mobilities reaching 10-3 cm2/Vs were observed for the compounds with two tri/tetraphenylethenyl
substituents. They showed both good hole-injecting and hole-transporting properties. OLED, based on
carbazole derivative with two tetraphenylethenyl substituents linked through the nitrogen atom in C-3
position of carbazole moiety, showed the highest maximum brightness exceeding of 50000 cd/m2 as well
as high maximum current, power and external quantum efficiencies of 17 cd/A, 9.2 lm/W, and 5.32 %
respectively.
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Acknowledgements
This research was funded by the European Regional Development Fund according to the supported
activity ‘Research Projects Implemented by World-class Researcher Groups’ under Measure No. 01.2.2-
LMT-K-718. Nataliya Kostiv is acknowledged for the initial tests of photophysical properties of the
compounds.
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