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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_5468_Библиотеки_им_академика_М_И_Перельмана
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1,2,4 and 1,3,4-Oxadiazole Scaffolds in Designing Organic Light-Emitting Diodes 23
O
N
N
23
O
N
N
24
N
N
O
25
Structures 23–25
derivatives (Tao et al. 2010). Interesting observations were found in terms of
thermal as well as morphological stability. Around 136–210°C as glass-transition
temperature and 401–480°C as decomposition temperature were noticed for the
fabricated devices. The EL performances of the OLED devices constructed from the
hybrid compounds followed the sequences 25 > 24 > 23. Out of the three devices, the
ortho-linked device (25) displayed the highest external quantum efficiency (9.8%)
and highest current efficiency (7.6 cd/A). Structural characteristics, such as twisted
configuration, high triplet energy, and separation of HOMO-LUMO energy level
favor superior EL performances in the case of hybrid compound 25.
Liou et al. developed aromatic poly(amine-1,3,4-oxadiazole)s 26 and 27 from
substituted dicarboxylic acid and terephthalic dihydrazide and (or) isophthalic
dihydrazide followed by thermal cyclodehydration (Liou et al. 2006). Both thermal
stability and solubility improved significantly due to a bulky naphthalene group in
poly(amine-1,3,4-oxadiazole)s. Polymer 26 displayed blue fluorescence with an
emission maximum at 463 nm in N-methyl-2-pyrrolidinone solution with a high
quantum yield (32%). The polymeric materials also exhibited high glass transition
temperatures (263277°C).
Tao et al. obtained hybrid oxadiazole-carbazole product 28 (with 80%
yield) during a reaction of 2,5-diphenyl-1,3,4-oxadiazole (at ortho position) with
carbazole (of 9-position) through an aromatic nucleophilic substitution reaction (Tao
et al. 2008). The OLED device constructed from this hybrid oxadiazole-carbazole
exhibited a high value of external quantum efficiencies, particularly 18.5% in the
case of deep red and 20.2% in the case of green electrophosphorescence.
Similarly, in 2010, Tao et al. developed another hybrid compound as 29
(89% yield) utilizing reaction at the meta position of a phenyl ring in
triphenylamine (Tao et al. 2010). Tuning the electron transport layer with
1,3,5-tris(N -phenylbenzimidazol-2-yl)benzene, the authors achieved 23% external
quantum efficiency and 94.3 lm/W maximum power efficiency in the case of the
constructed device. Also, the OLED device exhibited the best electroluminescence
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N N
O
N N
O
O
NN
n
O
NN
n
N
N
27
26
Structures 26 and 27
N
28
NN
O
N
N
N
O
N
N
29
NN
O
N
N
30
Structures 28–30
24 Oxadiazole in Material and Medicinal Chemistry
(105 lm/W as maximum power efficiency), while a thin layer of compound 29 was
introduced between the emissive layer and hole transport. Therefore, the device with
29 could be utilized for designing excellent phosphorescent OLEDs.
Tao et al. also studied the isomer of compound 29 as 30 for phosphorescent
OLED applications (Tao et al. 2010). Compound 30 exhibited a high LUMO level
(5.25 eV). The efficiency of phosphorescent OLED improved significantly with
adding 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene as an electron transport layer.
Power efficiency, peak current efficiency, and external quantum efficiency increased
to 97.7 lmW
–1
, 90.0 cdA
–1
, and 23.5%, respectively, on incorporating 10 nm of
compound 30 as an exciton blocking layer.
In order to develop non-doped OLED devices with blue emitting characteristics,
Tao et al. prepared bipolar materials with compounds 31 and 32 utilizing Suzuki
cross-coupling reaction catalyzed by palladium(0) (Tao et al. 2009). Compounds 31
and 32 exhibited 2.35 and 2.46 eV triplet energy, respectively. Better localization
NN
O
N
N
N
O
N
N
N
31
32
Structures 31 and 32
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1,2,4 and 1,3,4-Oxadiazole Scaffolds in Designing Organic Light-Emitting Diodes 25
NN
O
34
O
N
N
N
33
N
N
N
NN
O
N
N
35
Structures 33–35
of charges and a bigger triplet energy gap in compound 32 led to superior device
performance.
The performance of OLED devices can be fine-tuned with suitable structural
arrangements to yield better photophysical and optical properties. Tao et al. also
studied oxadiazole-triphenylamine-based compounds 33–35 for designing deep-red
phosphorescent materials (Tao et al. 2010). Compounds 34 and 35 with attached
ortho-triphenylamine exhibited blue-shifted emission, small intramolecular charge
transfer, large energy gap, and elevated triplet energy compared to compound 33,
where triphenylamine is connected through para position. OLED device fabricated
with compound 35 exhibited 21.6% EQE
max
with deep-red phosphorescence
response.
Augmenting molecular size can improve morphological, photophysical, and
electrochemical characteristics. In 2012, Gong et al. tailored a series of hybrid
oxadiazole-arylamine-based compounds 36–39 through a double silicon bridge
(Gong et al. 2012). Higher glass-transition temperature (92–190°C), remarkable
solubility, better solution processability, and high thermal stability are some of the
features that boost the performance of the tailored compounds. Engineering through
tetra-meta-position and incorporation of diphenylamine scaffold led to a higher
HOMO level (5.30 eV) as well as very high triplet energy (2.72 eV) in the case of
compound 38. Blue OLED device developed from compound 38 resulted in 10.7%
EQE , 23.4 cdA
–1
as maximum current efficiency, and 10.2 lmW
–1
as maximum
max
power efficiency. Reddy et al. synthesized a series of compounds 40–45 containing
anthracene emitter and oxadiazole units in five synthetic steps from commercially
available anthracene and tert-Butyl alcohol (Reddy et al. 2009). OLED materials
utilizing compounds 40–45 exhibited a rise in thermal robustness and significant
glass-transition temperature (52–131°C %RWK &+ ± ʌ DQG ʌ±ʌ LQWHUDFWLRQV DUH
mainly responsible for stabilizing the synthesized compounds 40 and 45 in the solid
state. These compounds could be useful as electron transport layers in multifunctional
OLEDs.
Venkatakrishnan et al. developed twisted bimesitylene centered oxadiazole
compounds 46 and 47 in 82% and 87% respectively (Venkatakrishnan et al. 2012).
The fabricated OLED devices obtained from compounds 46 and 47 exhibited
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26 Oxadiazole in Material and Medicinal Chemistry
Structures 36–45
N
N
N
N
R
1
R
2
O
O
R
2
R
1
40.
R
1
=
H,
R
2
=
H
41.
R
1
=
CH
3
,
R
2
=
H
42.
R
1
=
H,
R
2
=
CH
3
43.
R
1
=
OCH
3
,
R
2
=
H
44.
R
1
=
C(CH
3
)
3
,
R
2
=
H
45.
R
1
=
hexyl,
R
2
=
H
Structures 46 and 47
very high glass-transition temperatures (173°C and 182°C) and a high band gap
(ca. 3.71 eV). OLED devices made with these twisted bimesitylenes and Ir(ppy)
3
as
dopant exhibited 19.0 cdA
–1
luminance efficiency (Figure 1).
In an effort to enhance triplet energy, Leung et al. at National Taiwan University
prepared silane containing oxadiazole materials 48–52 (Leung et al. 2012). The
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O
Si
Si
N
N
n
Si
O
4-n
48
.n=
1
84%
NN
49
.n=
2
83%
52
50
.n=
3 60%
51
.n=
4
37%
Structures 48–52
1,2,4 and 1,3,4-Oxadiazole Scaffolds in Designing Organic Light-Emitting Diodes 27
Figure 1. (Top) Plot showing current-voltage-luminance characteristics; (bottom) luminous, power, and
external efficiencies vs. current density of the OLED device made from (2,2’,2’’-(1,3,5-benzenetriyl)-
tris(1-phenyl-1-H-benzimidazole) and the synthesized compounds (left) 46 and (right) 47. Reprinted with
permission from Elsevier (Venkatakrishnan et al. 2012).
introduction of Si atoms inside–aromatic conjugation facilitated high triplet energy
as well as large band gap. Compound 52 with symmetrically positioned oxadiazole
motif exhibited 5,124 cd/m
2
as maximum luminance, 6.9 V as turn-on voltage, 13.1%
as external quantum efficiency, and 39.9 cd/A as current efficiency, indicating novel
host material for phosphorescent OLED.
In order to develop cheaper solution-processable host materials, Gong et al.
isolated oxadiazole compound 53 with a double silicon bridge between arylamine and
oxadiazole as a white solid in 62% yield (Gong et al. 2013). This extended designed
compound 53 demonstrated greater thermal stability and high glass-transition
temperature (149°C). Subsequently, an OLED device with extended molecular size
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N
SiSi
O
N
N
O
N
N
53
Structure 53
O
N
Structures 54 and 55
NN
O
N
O
54
NN
O
N
O
55
28 Oxadiazole in Material and Medicinal Chemistry
exhibited 15.3 cd/A as maximum current efficiency, which is suitable for designing
efficient blue phosphorescent materials.
In 2013, researchers at Kyushu University developed donor-acceptor-donor-type
efficient TADF materials with oxadiazole analogs 54 and 55 (Lee et al. 2013).
Compounds 54 and 55 displayed green emission response. Compound 55
exhibited a very high photoluminescence quantum yield (87%) when doped with
bis(2-(diphenylphosphino)phenyl)ether oxide. OLED devices prepared from
compound 55 yielded 14.9% EQE .
max
In 2013, Chang et al. observed that introducing CsF/Al while developing
oxadiazole (56–57) based phosphorescent OLED enhanced electron transport and
simultaneously decreased interface resistance (Chang et al. 2013). The authors
observed very high current efficiency (17.3 cd/A) and 8.86 lm/W maximum luminous
efficiency due to effective n-type doping. By introducing oxadiazole at the C
3
and
C
6
positions of carbazole moiety, Chen et al. synthesized a bipolar host material 58
O
NN
56
N
O
N
N
N
N
O
58
O
N
N
O
N
N
O
N
N
57
Structures 56–58
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1,2,4 and 1,3,4-Oxadiazole Scaffolds in Designing Organic Light-Emitting Diodes 29
Structures 59–62
N
N
N
O
t
Bu
N
N
N
O
t
Bu
N
t
Bu
N
N
O
N
O
N
N
t
Bu
59
60
61
62
NN
O
in three synthetic steps (Chen et al. 2012). Both HOMO and LUMO orbitals were
observed to be delocalized over carbazole and oxadiazole groups in compound 58.
Introducing a green phosphor increased EQE to 17.7%, whereas adding a red
max
phosphor yielded 20.6% EQE , owing to better delocalization and small triplet
max
energy (2.65 eV).
In order to develop robust bipolar host materials, Cheng et al. tailored
carbazole-oxadiazole (donor-acceptor) connected compounds 59–61 by varying the
D/A ratio (Cheng et al. 2013). Tuning the carbazole-oxadiazole ratio influenced several
parameters such as energy levels, hole mobility, and thermal as well as morphological
stability. Interestingly, the addition of bis(2, N-diphenylbenzimidazolito)iridium(III)
acetylacetonate, i.e., (PBi)
2
Ir(acac) as emitter while preparing green phosphorescent
OLED materials enhanced their performance. Around 20.7%, 20.4%, and 17.3%
EQE were observed from materials containing compounds 59–61, respectively.
max
Carrier transport properties of the compounds 59–61 are shown in Figure 2.
Shih et al. developed a centrally positioned oxadiazole-based material 62 with
a symmetrical structure in three synthetic steps (70% yield) (Shih et al. 2015).
The material displayed a high T
g
(thermal stability) value (220°C) as well as high
triplet energy (2.76 eV). Several advantages of the material were observed, such as
successful enhancement of charge balance, electron transport, blocking holes and
excitons, etc. It was successfully utilized as a universal electron transporter for green,
blue, and red phosphorescent tools. The green phosphorescent device displayed
97.6 cd A
–1
as a high value of current efficiency.
In 2016, Chidirala et al. synthesized pyrene-oxadiazole scaffolds 63–65 for
green-emitting application materials (Chidirala et al. 2016). Incorporation of
2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane as a hole-injecting layer and
bathocuproine as a well-known electron transport material during OLED device
fabrication yielded 9.56 lm/W power efficiency, 13.56 cdA
–1
current efficiency along
with 5.34% quantum yield. The study might be useful for designing efficient electron
transporters using hybrid chemical entities.
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30 Oxadiazole in Material and Medicinal Chemistry
Figure 2. *UDSKVVKRZLQJKROHWUDQVLHQWSKRWRFXUUHQW VLJQDOV IRU D FRPSRXQG ȝP WKLFNDW
E = 8.3 × 10
5
9FP E FRPSRXQG ȝP WKLFN DW ( î
5
V/cm, and (c) compound 61
ȝPWKLFNDW( î
5
V/cm. Insets are the double logarithmic plots of (a–c). (d) Hole mobilities of
compounds 59–61 plotted concerning E
1/2
. Reprinted with permission from Elsevier (Cheng et al. 2013).
N
N
O
N
N
N
N
O
O
O
O
O
63
64
65
Structures 63–65
Structures 66–68
N
Si
Me
Me
N
N
O
N
N
N
O
66
67
O
N
N
O
N
N
68
Introducing tetrahedral silicon structural motifs in OLED materials can
significantly improve band gap and triplet energy levels (Han et al. 2009). Silicon
atoms enhance device performance by breaking conjugation character while
maintaining thermal stability (Yi et al. 2015). Lee et al. synthesized compounds
66 and 67 with or without tetrahedral silicon scaffold (Lee et al. 2016). Around
15.83% EQE and 54.55 lm/W maximum power efficiency were observed for
max
OLED devices containing compound 66. The incorporation of tetrahedral silicon
moiety also improved the HOMO-LUMO band gap. In another study in 2007, Leung
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1,2,4 and 1,3,4-Oxadiazole Scaffolds in Designing Organic Light-Emitting Diodes 31
69
NN
O
N
N
71
N
N
O
N
N
70
N
N
N
N
O
N
N
72
N
N
N
N
O
N
N
73
N
N
O
N
N
74
N
N
N
N
O
N
N
Structures 69–74
et al. observed 26 cd/A efficiency and 43,000 cd/m
2
maximum brightness of the
phosphorescent OLED device while incorporating compound 68 as host material
(for electron transport) with Ir(ppy)
3
[Tris(2-phenylpyridine)iridium] (Leung
et al. 2007).
Zhao et al. studied the influence of the carbazole-oxadiazole (donor-acceptor)
ratio on phosphorescent OLED utilizing compounds 69–74 that are made up of
1,3-Bis(carbazole-9-yl)benzene as hole transport host and oxadiazole derivatives
through a benzene linkage (Zhao et al. 2017). The authors prepared different types
(blue, green, and red) of phosphorescent OLEDs and observed interesting patterns
concerning ortho/para linkage. Compounds such as 69 and 70 fixed through ortho
linkage structurally produced maximum triplet energy value (2.8 eV) and exhibited
blue-emitting phosphorescent behavior (Figure 3). Similarly, compounds such as 73
and 74 that are connected through para linkage displayed maximum current densities
with short triplet energy values.
Wong et al. developed a series of four TADF emitters 75–78 by fixing
two carbazole units at the ortho position of the benzene derivative (Wong
et al. 2018). The addition of hosts such as 1,3-bis(carbazol-9-yl)benzene and
2,8-bis(diphenylphosphoryl) dibenzo[b,d] thiophene during device construction
resulted in 14–55% photoluminescence quantum yields. Devices prepared from
compound 77 yielded 11.2% EQE with sky-blue emission, whereas devices
max
constructed from compound 78 produced 6.6% EQE with deep-blue emission.
max
Similarly, CIE color coordinates for OLED devices made from compounds 77 and
78 were (0.17, 0.25) and (0.15, 0.11), respectively.
https://t.me/med1917
32 Oxadiazole in Material and Medicinal Chemistry
Figure 3. (Left) Comparison of power efficiency and external quantum efficiency versus current density
characteristics; (right) electroluminescence spectra of red phosphorescent OLEDs. The inset shows an
enlargement of emission from 400–500 nm. Reprinted with permission from Elsevier (Zhao et al. 2017).
OMe
F
3
C CF
3
MeO
O
N
N
N
N
O
N
N
75
O
N
N
N
N
O
N
N
76
O
N
N
N
N
O
N
N
77
O
N
N
N
N
O
N
N
78
Structures 75–78
NN
O
N
N
NN
O
N
N
79
80
NN
O
N
N
81
Structures 79–81
Tan et al. developed donor-acceptor-donor type compounds 79–81 in 2019 (Tan
et al. 2019). Compounds with substituted acridine, such as 80 and 81, exhibited
distinct TADF properties due to small energy splitting (E
ST
) between singlet
and triplet excited states. Acridine-based compounds also displayed very high
photoluminescent quantum yields of up to 93% and are potential blue TADF emitters.
Similarly, compounds 80 and 81 displayed 14.4% and 22.3% EQE , respectively.
max
Li et al. developed a series of oxadiazole-based TADF compounds 82–85 from
commercially available 1-bromo-2,4-difluorobenzene precursor (Li et al. 2019).
Their emissive color could be easily fine-tuned from sky blue to blue. A small value
of singlet-triplet splitting was experienced from the highly twisted conformation
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