New low-molecular-weight electroluminescent materials for green organic light emitting diodes

Article abstract of DOI:10.1016/j.mencom.2014.03.007

A new family of low-molecular-weight fluorescent emitters comprising a 1,2,3,4-tetraphenylbenzo[4,5]imidazo[2,1-a]isoindol-11-one core has been designed and applied to the construction of highly efficient organic light emitting diodes with strong green exciplex emission.

Full text of DOI:10.1016/j.mencom.2014.03.007

Mendeleev
Communications
Mendeleev Commun., 2014, 24, 88–90
New low-molecular-weight electroluminescent
materials for green organic light emitting diodes
Diana K. Susarova,a Alexander S. Peregudov,b Svetlana M. Peregudovab and Pavel A. Troshin*a
a Institute of Problems of Chemical Physics, Russian Academy of Sciences, 141432 Chernogolovka,
Moscow Region, Russian Federation. Fax: +7 496 515 5420; e-mail: troshin2003@inbox.ru
b A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, 119991 Moscow,
Russian Federation
DOI: 10.1016/j.mencom.2014.03.007
A new family of low-molecular-weight fluorescent emitters comprising a 1,2,3,4-tetraphenylbenzo[4,5]imidazo[2,1-a]isoindol-11-one
core has been designed and applied to the construction of highly efficient organic light emitting diodes with strong green exciplex emission.
Intensive research on organic light emitting diodes (OLEDs)
resulted in the development of a novel generation of flat screens
which outperform conventional LCD displays in many aspects.1
The industrial production of OLED displays (e.g., AMOLED
displays) by Korean companies affected significantly the market
of all mobile electronic devices.
Another niche for the application of OLEDs is related to
energy-saving lighting and signage. Solution-processible conjugated
polymers are the most appropriate materials for the design of
OLED lighting products.2–6 Particular attention is also paid to
energy-transfer systems comprising the quantum dots of inorganic
semiconductors,7 organic dyes8,9 and phosphorescent metal com-
plexes10 as light emitters.
their molecular frameworks were extensively studied as electro-
luminescent materials for blue OLEDs. Fluorene-benzothiadiazole
hybrid molecules were found very promising fluorescent green
emitters.16 Bright green electroluminescence (up to 15330 cd m–2)
with high efficiency (up to 7.9 cd A–1) was also achieved using
a bisquinoxaline core with appended triphenyl amine units.17
Here we report a new family of small molecular electro-
luminescent materials for OLEDs based on 1,2,3,4-tetraphenyl-
benzo[4,5]imidazo[2,1-a]isoindol-11-one derivatives such as
TPBIIN and bis-TPBIIN. The synthesis of these compounds
requires two steps. The first step is the reaction of tetra-
phenylcyclopentadienone with maleic anhydride, which yields
tetraphenylphthalic anhydride 1, according to a published pro-
cedure.18,19 The condensation of 1 with o-phenylenediamine or
3,3'-diaminobenzidine affords TPBIIN and bis-TPBIIN, respec-
tively, in high yields (Scheme 1).† Note that the synthesis of
TPBIIN (via a different chemical route) and similar compounds
was reported previously.18,20
However, solution processible small molecules are also suitable
for the design of OLED devices for lighting applications. For
example, low-molecular-weight electroluminescent materials
comprising tetraarylsilole11–14 or tetraphenylbenzene15 units in
Ph
O
The samples of TPBIIN and bis-TPBIIN were characterized
1
using ESI mass spectrometry, FTIR, H and 13C NMR spectro-
Ph
N
Ph
Ph
Ph
N
Ph
O
TPBIIN
Ph
Ph
Ph
O
Ph
O
Ph
Ph
Ph
Ph
N
N
Ph
Ph
O
NH2
O
O
i,
N
N
ii, Br2
NH2
TPBIIN
bis-TPBIIN-1
bis-TPBIIN-2
Ph
quinoline, 180 °C, 5 h
O
Ph
Ph
Ph
Ph
Ph
Ph
O
N
O
NH2
NH2
H2N
H2N
quinoline, 180 °C, 5 h
Ph
Ph
Ph
Ph
N
N
O
Ph
bis-TPBIIN
N
O
1
Scheme 1
Ph
O
Ph
Ph
†
Tetraphenylcyclopentadienone (97%), quinoline (98%), chlorobenzene
Ph
O
N
(99%), o-phenylenediamine (98%) and maleic anhydride (99%) were
purchased from Acros Organics and used as received. Optical spectra
(photoluminescence, electroluminescence) were recorded using Avantes
AvaSpec-2048 and Varian 3G UV-VIS spectrometers. NMR spectra were
obtained using Bruker AMX 400 (400 MHz) and Bruker Avance (600 MHz)
instruments.
Ph
Ph
N
N
Ph
N
Ph
bis-TPBIIN-3
© 2014 Mendeleev Communications. All rights reserved.
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Mendeleev Commun., 2014, 24, 88–90
scopy, H-H COSY and H-C HSQC two-dimensional NMR spectra
2100
1800
1500
1200
900
600
300
0
4
3
and chemical analysis. TPBIIN was formed as an individual
compound, while bis-TPBIIN was isolated as a mixture of three
isomers in a molar ratio of 0.6:0.25:0.15 (NMR data). The most
abundant component of the isomer mixture was unsymmetrical
bis-TPBIIN-3; this can be due to its superior thermodynamic
stability.
Taking into account strong similarity of the structures of
bis-TPBIIN and the steric hindrance of polar carbonyl groups by
phenyl rings, it was not surprising that column chromatography
did not allow us to achieve even partial separation of the isomeric
species. Therefore, the mixture of bis-TPBIIN isomers was utilized
as an electroluminescent material in this work.
2
1
300
400
500
600
700
800
900
Wavelength/nm
We studied the photoluminescence (PL) properties of TPBIIN
and bis-TPBIIN in solution and in a solid state. The luminescent
properties of TPBIIN were published.21–26 The solution state
PL spectrum of TPBIIN is characterized by a sharp maximum at
485 nm with a shoulder at ~540 nm (Figure 1, curves 1,2). The
solid state spectrum exhibited two distinct maxima at 500 and
550 nm. The observed change of the PL spectrum going from the
solution to the solid state suggests the formation of exciplex
states by aggregated TPBIIN molecules. The difference between
the solution and solid state PL spectra becomes even more
pronounced for bis-TPBIIN (Figure 1, curves 3,4). Indeed, the
maximum of the emission band shifted from 495 (in solution)
to 560 nm (in a solid state). The observed spectral changes are
remarkable for organic materials, especially for small nonplanar
molecules such as bis-TPBIIN, and indicate the formation of
strongly bound exciplex states in thin films. These observations
are very important for OLED applications since exciplex emission
might also dominate in the electroluminescence spectra of TPBIIN
and bis-TPBIIN. Similar strong exciplex emission bands were
Figure 1 (1,3) Solution (in CHCl3) and (2,4) solid-state PL spectra of
(1,2) TPBIIN and (3,4) bis-TPBIIN.
revealed previously for zinc complexes with sulfanylamino-sub-
stituted ligands.27
Note that both TPBIIN and bis-TPBIIN are readily soluble in
low-polarity organic solvents such as chloroform, toluene, chloro-
benzene and 1,2-dichlorobenzene, which allow one to prepare
thin films of these compounds using solution processing techniques
such as spin coating.
Simple OLED devices were fabricated by coating ITO glass
with hole injecting layer of PEDOT:PSS, drying it at 200°C and
subsequent deposition of TPBIIN or bis-TPBIIN from solutions
in a mixed solvent (chloroform–chlorobenzene, 2:1, 10 mg ml–1).
The devices were finalized by the vacuum deposition of Ca
(20 nm)/Ag (100 nm) cathodes [Figure 2(a)]. The prepared
OLEDs showed weak yellowish green electroluminescence with
a maximum brightness of ~200 cd m–2 achieved at 13 V for a
bis-TPBIIN device.
The observed poor performance of TPBIIN and bis-TPBIIN
as electroluminescent materials in single-layer devices suggests
the unbalanced injection of holes and electrons. If one type
of charge carriers is not injected efficiently, the excitons are
generated in the close proximity to one of the electrodes, which
leads to massive recombination losses.
Synthesis of TPBIIN. Tetraphenylphthalic anhydride (3 g, 6.64 mmol)
was mixed with 3 equiv. of o-phenylenediamine (2.15 g, 19.91 mmol)
and 50 ml of quinoline. The resulting mixture was deaerated and heated
in an argon atmosphere at reflux for 7 h. The reaction mixture was cooled
and poured into 600 ml of 10% aqueous HCl. The precipitated product
was separated by filtration, washed with methanol (3×50 ml) and dried
in air. The yield of product was 1.5 g (43%). 1H NMR (600 MHz, CDCl3)
d: 6.78 (m, 4H), 6.90 (m, 6H), 7.17 (m, 3H), 7.25 (m, 9H), 7.54 (d, 1H),
7.64 (d, 1H). 13C NMR (150 MHz, CDCl3) d: 112.35, 121.85, 124.68,
126.19, 126.25, 126.36, 127.01, 127.06, 127.43, 127.5, 127.65, 127.72,
129.65, 129.74, 130.00, 130.32, 130.75, 130.86, 135.42, 135.94, 137.81,
137.97, 138.27, 141.97, 145.64, 148.03, 149.42, 155.84, 160.36.
(a)
(b)
Ag (100 nm)/
Ca (20 nm)
Ag (100 nm)/
Ca (20 nm)
EL material
EL material
EL material
EL material
PEDOT:PSS
Synthesis of bis-TPBIIN. Tetraphenylphthalic anhydride (4 g, 8.85 mmol)
was mixed with 3 equiv. of 3,3'-diaminobenzidine (0.74 g, 2.95 mmol)
and 75 ml of quinoline. The resulting mixture was deaerated and heated
in an argon atmosphere at reflux for 7 h. The reaction mixture was then
cooled down, poured into 600 ml of 10% aqueous HCl. The precipitated
product was separated by filtration, washed with methanol (3×50 ml)
and dried in air. The crude product was dissolved in toluene and poured
on the top of a silica gel column (silica 40–60 mm, 60Å). Elution with a
mixture of toluene with methanol (99.6:0.4, v/v) produced bright yellow
fluorescent solution of bis-TPBIIN. The yield of the product was 700 mg
EL maCteBriPal
PEDOT:PSS
ITO
ITO
PEDOT:PSS
PEDOT:PSS
ITO
ITO
Glass
Glas
Glass
Glas
O
O
O
O
S
S
N
1
(23%). H NMR (600 MHz, CDCl3) d: 6.83 (m, 8H), 6.95 (m, 12H),
S
S
7.21 (m, 6H), 7.28 (m, 14H), 7.44–7.55 (m, 2H), 7.60 (m, 1.2H), 7.70
(m, 0.8H), 7.78 (s, 0.3H), 7.82 (s, 0.5H), 7.93 (s, 0.6H), 7.96 (s, 0.6H).
13C NMR (150 MHz, CDCl3) d: 110.85, 110.95, 112.38, 112.41, 120.38,
120.44, 121.89, 121.93, 124.03, 124.06, 125.32, 125.59, 125.67, 125.80,
126.26, 126.39, 126.44, 126.75, 126.86, 126.90, 127.02, 127.07, 127.16,
127.22, 127.35, 127.42, 127.44, 127.48, 127.51, 127.59, 127.67, 127.73,
127.86, 128.25, 128.69, 128.91, 129.06, 129.11, 129.76, 129.79, 129.89,
129.96, 130.02, 130.30, 130.34, 130.56, 130.77, 130.88, 130.90, 130.96,
131.87, 132.59, 132.65, 135.32, 135.35, 135.39, 135.40, 135.48, 135.51,
135.54, 135.83, 135.87, 135.95, 137.61, 137.62, 137.68, 137.79, 137.84,
137.90, 137.94, 137.98, 138.00, 138.04, 138.08, 138.27, 138.30, 139.10,
139.41, 139.68, 139.92, 142.01, 142.08, 142.10, 145.60, 145.73, 147.71,
148.04, 148.06, 148.16, 148.78, 148.98, 150.12, 150.17, 156.10, 156.22,
156.46, 156.51, 160.22, 160.26.
n
O
O
O
O
SO3H SO3H SO3H SO3H
N
n
PEDOT : PSS
CBP
Figure 2 OLED architectures (a) without and (b) with hole transport layer
CBP (EL material states for TPBIIN or bis-TPBIIN).
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Mendeleev Commun., 2014, 24, 88–90
Considering the molecular structures of TPBIIN and bis-
molecules. Efficient self-assembling of bis-TPBIIN in a solid state
might be responsible for the observed strong exciplex emission.
A combination of bis-TPBIIN with an appropriate hole-transport-
ing material allowed us to fabricate highly efficient green OLEDs
outperforming reference devices comprising Alq3 as an electro-
luminescent material. Thus, the results demonstrate that TPBIIN,
bis-TPBIIN and related compounds form a promising family of
fluorescent emitters.
TPBIIN, one could assume that they would form anions more
readily than cations. This hypothesis was proved by cyclic volt-
ammetry measurements which revealed that both compounds
undergo reversible one-electron reduction with the half-wave
potentials E1/2 = –1.13 (TPBIIN) and –1.20 V (bis-TPBIIN) vs.
SCE. At the same time, no reversible oxidation processes were
observed in the DMF solutions of test materials. This means that
injection of holes in TPBIIN and bis-TPBIIN thin films might
be hindered compared to the injection of electrons.
To improve hole injection in OLEDs, we applied an additional
hole-transport layer. To fabricate such devices, the coating of
PEDOT:PSS films on cleaned ITO slides and their drying at
190°C for 15 min was followed by the vacuum deposition of
CBP (30 nm) and bis-TPBIIN (80 nm) layers. Finally, a metal
cathode was deposited on the top of organic layers by thermal
evaporation. The architecture of the produced device is shown in
Figure 2(b).
The fabricated OLED devices showed a turn-on voltage of
3–4 V, a maximal brightness of 14000 cd m–2 at 14 V and an
efficiency of 5–6 cd A–1. The reference OLEDs comprising
conventional electroluminescent material Alq3 [tris(8-hydroxy-
quinolinato)aluminum] instead of bis-TPBIIN in the same device
configuration exhibited inferior characteristics: a turn-on voltage
of 6.0–6.5 V, a maximal brightness of 7500 cd m–2 at 14 V and an
efficiency of 1.5–2.2 cd A–1. The EL spectrum of the OLED
based on bis-TPBIIN and a photograph of the operating device
are shown in Figure 3. It is notable that EL spectrum fits quite
well to the solid state PL spectrum of bis-TPBIIN represented by
exciplex emission band. Therefore, the emission coming from
the light-emitting device should also be attributed to the exciplex
states. It is intriguing that the exciplex emission is so strong in
the case of bis-TPBIIN that it allows for the fabrication of highly
efficient OLED devices.
This work was supported by the Russian Ministry of Educa-
tion and Science (contract no. 14.513.11.0020) and the Russian
Foundation for Basic Research (grant no. 13-03-01170).
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state and (b) a photograph of the operating device.
Received: 12th August 2013; Com. 13/4180
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