A novel spiro-annulated benzimidazole host for highly efficient blue phosphorescent organic light-emitting devices

Article abstract of DOI:10.1039/c8cc00903a

A novel host material featuring a spiro-annulated benzimidazole configuration is exploited for blue phosphorescent organic light-emitting devices (PhOLEDs). The new material exhibits a high triplet energy (3.07 eV) and a bipolar characteristic and is effective as the host for FIrpic-, FIr6- and FK306-based blue PhOLEDs with high performances.

Full text of DOI:10.1039/c8cc00903a

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc
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3
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DOI: 10.1039/C8CC00903A
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A novel spiro-annulated benzimidazole host for highly efficient
blue phosphorescent organic light-emitting devices
a
b
a
c
b
b
Wen-Cheng Chen, Yi Yuan, Ze-Lin Zhu, Shao-Fei Ni, Zuo-Quan Jiang, Liang-Sheng Liao,* Fu-
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
Lung Wong and Chun-Sing Lee*
DOI: 10.1039/x0xx00000x
www.rsc.org/chemcomm
T
A novel host material featuring spiro-annulated benzimidazole materials with high E s due to their intrinsic non-conjugated
5,6
configuration is exploited for blue phosphorescent organic light- orthogonal configuration. The stereo molecular configuration
emitting devices (PhOLEDs). The new material exhibits a high triplet also endows spiro-based organic semiconductors with good
3
energy (3.07 eV) and a bipolar characteristic, and is effective as the thermal properties. The sp -hybridized spiro carbon atom
host for FIrpic-, FIr6- and FK306-based blue PhOLEDs with high separates the molecule into two moieties, so that electron
performances.
donor and acceptor can be respectively incorporated to the two
molecular halves for bipolar nature without lowering the E
T
Phosphorescent organic light-emitting devices (PhOLEDs)
employing noble metal complexes as emitting cores have
received tremendous attention, because they can fully utilize
both singlet and triplet excitons for radiation.1,2 To date, high-
performance sky-blue, green and red PhOLEDs have been
widely used. Nevertheless, effective pure-blue or even deep-
blue PhOLEDs have not been realized, mainly due to the lack of
reliable host materials.
Generally, in a PhOLED the emissive phosphors are dispersed
in host materials to alleviate triplet exciton quenching via
7
markedly. However, this strategy is still hard to obtain bipolar
host materials with very high E s, which are limited by the
T
inherent conjugation of the incorporated electron donor or
acceptor groups.
In this work, we report a novel bipolar host with a high E
tactfully incorporating a triphenylamine (TPA) moiety to a novel
spiro-configured benzimidazole (BI) scaffold via non-
T
by
a
3
conjugated linking style (Scheme 1). BI was often used as an
efficient electron-transporting building block in host materials
8,9
for blue PhOLEDs. The number 2 carbon atom (C2) of BI is
jointed to the spiro carbon, while benzene ring is
4
triplet-triplet annihilation (TTA). To realize high-performance
a
blue PhOLEDs, the host material is required to have (i) suitable
highest occupied molecular orbital (HOMO) and lowest
unoccupied molecular orbital (LUMO) levels for efficient hole
and electron injection, (ii) bipolar electrical property for
ensuring a wide carrier recombination zone, (iii) a high enough
simultaneously attached to the pyrrole-like nitrogen of BI and
spiro carbon with an sp linkage, forming a rigid non-conjugated
triangular configuration. This special donor-spiro-acceptor
design endows the resulting material SPBI-TPA with a bipolar
3
T
first triplet energy level (E ) to sensitize the blue
phosphorescent dopant and to minimize energy leak and (iv)
high enough thermal stability for thermal evaporation and
against Joule heat during device operation. Molecules with
spiro-core are promising candidates in constructing bipolar host
a.Center of Super-Diamond and Advanced Films (COSDAF) and Department of
Chemistry, City University of Hong Kong, Hong Kong SAR, PR China. E-mail:
apcslee@cityu.edu.hk
b.
Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Institute
of Functional Nano & Soft Materials (FUNSOM) & Collaborative Innovation Center
of Suzhou Nano Science and Technology, Soochow University, Suzhou, 215123, PR
China. E-mail: lsliao@suda.edu.cn
c.
Department of Chemistry, Southern University of Science and Technology,
Shenzhen, 518055, PR China
Scheme 1 Chemical structure of SPBI-TPA.
†
Electronic
Supplementary
Information
(ESI)
available:
See
DOI: 10.1039/x0xx00000x
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T
nature and a high E of 3.07 eV, which is higher than most of the
fluorene-spiro-annulated compounds.5,6,10,11 SPBI-TPA is
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DOI: 10.1039/C8CC00903A
effective as a host for FIrpic-, FIr6- and FK306-based blue
PhOLEDs with high performances, which outperform its
fluorene-based counterpart. This study presents the first
example of spiro-annulated BI compound and provides a
promising molecular template on the evolution of highly
T
efficient host materials with high E s.
SPBI-TPA was prepared by a facile three-step procedure as
shown in Scheme S1, and its detailed synthesis is shown in the
ESI†. The key intermediate 11H-benzo[4,5]imidazo[1,2-a]indol-
12
1
1-one (BIO) was synthesized via a base-catalyzed cyclization
reaction involving 2-fluorobenzaldehyde and 1H-
benzo[d]imidazole under an atmosphere condition. Then BIO
was reacted to lithium activated 2-bromo-N,N-diphenylaniline
to afford an alcohol compound (hard to dissolve in common
solvents, not isolated), followed by an intramolecular
cyclization in an acidic condition to obtain the final product. To
study the effects of using the BI-based spiro ring instead of the
commonly used fluorene-based spiro system, a corresponding
fluorene-based compound SPF-TPA (Scheme S2) was also
synthesized.
Fig.
2
Absorption (Abs), fluorescence (FL) and
phosphorescence (Phos) spectra of SPBI-TPA and SPF-TPA.
phenyl-1H-benzo[d]imidazole segment dominates the LUMO,
implying a potential bipolar characteristic.
Thermal properties of the SPBI-TPA and SPF-TPA were
studied using thermogravimetric analyzer (TGA) and differential
scanning calorimetry (DSC). Benefited from the rigid and bulky
spiro-configured BI scaffold, SPBI-TPA has a considerably higher
Calculated molecular configuration of SPBI-TPA is shown in
Fig. 1. Similar to the SPF-TPA (Fig. S1) and other reported
5
,6
fluorene-based spiro compounds,
the 1-phenyl-1H-
d
decomposition temperature of 315 °C (T , 5% weight loss)
benzo[d]imidazole section and the TPA fragments are
perpendicular to each other, showing a high twisting angle of
comparing to that (285 °C) of SPF-TPA (Fig. S2, ESI†). Due to the
bulky and rigid 3D molecular configuration by virtue of the
orthogonal molecular structure, no evident glass transition
8
7°. This orthogonal geometry is conducive to confining π
13
conjugation and enhancing morphological stability.
g
temperatures (T s) are observed in SPBI-TPA and SPF-TPA (inset
Interestingly, TPA moiety in the molecule also displays a bend
configuration (marked with a red arrow in Fig. 1), possibly
stemming from the intramolecular interaction between the
pyridine-like nitrogen atom of BI (N3) and the hydrogen atom of
phenyl group in TPA (Fig. 1). As expected, the HOMO
distribution is mainly located on the TPA fragment, while the 1-
of Fig. S2, ESI†), demonstrating good thermal properties.
The absorption and fluorescence (FL) spectra in toluene
solution are displayed in Fig. 2. The ~290-nm absorption bands
can be attributed to the n-π* transition of the TPA, which are
14,15
similar to the previous reports,
and this is also in accord with
the S
0
1
→S
1
natural transition orbital (NTO) analyses (Fig. S3,
6
ESI†). In addition, the NTO analyses suggested that the longer-
wavelength absorption bands may involve ICT absorptions
(From TPA to BI or fluorene). From the absorption onset of the
solid thin films (Fig. S4, ESI†), the optical energy gaps (E s) of
g
SPBI-TPA and SPF-TPA are estimated to be 3.52 and 3.41 eV,
respectively. Combined with the HOMO levels measured with
cyclic voltammetry (Fig. S5, ESI†), which are respectively
estimated to be -5.40 and -5.29 eV for SPBI-TPA and SPF-TPA,
LUMO levels of the two compounds are both estimated to be -
1
.88 eV. Both compounds show similar PL profiles with violet-
blue emissions peaking at 361 and 362 nm for SPBI-TPA and SPF-
TPA, respectively. Fig. 2 also shows the phosphorescence (Phos)
spectra in 2-methy-THF matrix at 77 K. It is obvious that SPBI-
TPA shows a much shorter-wavelength Phos comparing to that
of SPF-TPA. E
TPA and SPF-TPA, respectively. Fig. S6 in the ESI† shows the spin
density distribution (SDD) in T states of SPBI-TPA and SPF-TPA.
T
s are determined to be 3.07 and 2.86 eV for SPBI-
1
It is found that the SDD of SPF-TPA mainly localized on fluorene,
the fragment with the highest degree of molecular conjugation.
By contrast, the SDD of SPBI-TPA is mainly concentrated on the
segment of the spiro-annulated TPA with a minor contribution
Fig. 1 Calculated molecular geometry and frontier
molecular orbital distribution of SPBI-TPA.
2
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Table 1 EL performances of the fabricated PhOLEDs
Communication
displays much higher electron current. The better electron-
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a
b
transporting property can be attributed to the azo BI segment,
DOI: 10.1039/C8CC00903A
V
on
Max efficiency
Host
Dopant
CIE (x, y)c
-
1
(
V)
(cd A /%)
which may facilitate intermolecular electron hopping in solid
state. The employment of BI in the spiro system can
simultaneously enhance the electron-transporting property and
T
increase the E , which are beneficial for designing high-
SPBI-TPA
SPBI-TPA
SPBI-TPA
FIrpic
FIr6
2.9
3.1
2.9
55.1 ± 0.7/25.5 ± 0.3
38.9 ± 0.7/24.1 ± 0.4
32.1 ± 0.4/20.9 ± 0.3
(0.17, 0.34)
(0.16, 0.25)
(0.16, 0.20)
18
FK306
performance host for blue PhOLEDs.
To study the electroluminescence (EL) characteristics of SPBI-
TPA as a host, a series of blue PhOLEDs were fabricated with a
SPF-TPA
SPF-TPA
FIrpic
3.0
3.4
33.4 ± 0.8/16.6 ± 0.5
10.5 ± 1.1/6.8 ± 0.7
(0.16, 0.33)
(0.16, 0.20)
FK306
a
-2
b
Turn-on voltage (detected at 1 cd m ). Current efficiency and
external quantum efficiency at maximum. Obtained at 100 cd m .
c
-2
structure
of
ITO/1,4,5,8,9,11-hexaazatriphenylene-
hexacarbonitrile (HAT-CN, 10 nm)/1,1-bis[4-[N,N-di(p-
tolyl)amino]phenyl]cyclohexane (TAPC, 50 nm)/4,4',4"-
tris(carbazol-9-yl)triphenylamine (TCTA, 10 nm)/SPBI-TPA: 10
wt% blue phosphor (20 nm)/1,3,5-tri[(3-pyridyl)-phen-3-
yl]benzene (TmPyPB, 40 nm)/LiF (1 nm)/Al (150 nm). FIrpic, FIr6
from the BI moiety, which may be the reason for its relatively
high E
T T
. It is worth pointing out that the E of SPBI-TPA is among
5,6,10,11
the highest values of the spiro-based host materials.
We
also compare the PL spectra of the present spiro-configured
SPBI-TPA with a reported single bond connecting molecule BI-
TPA1 in different solvents, as shown in Fig. S7 (ESI†). Upon
solvent polarity increases, the PL spectra of SPBI-TPA only show
a mild red shift, and their spectral profiles remain almost
unchanged. On the other hand, BI-TPA demonstrates significant
red shifts in emission as medium polarity raises, showing a
broad and structureless PL spectrum in acetonitrile (λPL = 439
nm versus 385 nm in hexane). This suggests that the spiro-
linking style can effectively suppress intramolecular charge
transfer (ICT) interaction between the electron donor (TPA) and
1
9
T
and FK306 with E s of 2.62, 2.73 and 2.83 eV, respectively, are
7
employed as the emissive dopants. The key device performance
parameters are listed in Table 1. All the SPBI-TPA based devices
-2
show low turn-on voltage (Von, at 1 cd m ) at around 3 V. The
SPBI-TPA hosted FIrpic PhOLED emits a sky-blue EL emission
with Commission Internationale de l’Eclairage (CIE) coordinates
́
of (0.17, 0.34) (Fig. 3a), and has a respectable external quantum
efficiency (EQE) of 25.5% at maximum (Fig. 3b), which
outperform the corresponding device with the SPF-TPA host
T
(EQEmax = 16.6%, Fig. S9 in the ESI†). Beside the high E , the more
T
acceptor (BI), and thus ensuring a high E .
balanced hole/electron-transporting properties are also
responsible for the higher efficiency in the SPBI-TPA based
PhOLED. Because the use of bipolar SPBI-TPA as a host can avoid
recombination zone shifting towards the neighboring layers for
a better charge utilization. Upon changing the dopant to FIr6
and FK306, the EL spectra shift towards shorter wavelength
region (Fig. 3a), demonstrating deeper blue color indexes of
Hole-only and electron-only devices (HODs and EODs) using
SPBI-TPA and SPF-TPA were fabricated and their current
density-voltage (J-V) characteristics are shown in Fig. S8 (ESI†).
Both compounds show high hole conductivity, which are
original from the TPA donor. Because the HOMO offset between
SPBI-TPA and SPF-TPA is not large (~0.1 eV), and both materials
have negligible hole-injection barrier to NPB (~5.40 eV). Thus,
the much higher hole current in the HOD based on SPF-TPA
indicates that SPF-TPA has a better hole-transporting property
than that of SPBI-TPA, mainly due to the better π conjugation of
fluorene. On the other hand, even though SPBI-TPA and SPF-
TPA have the same LUMO levels, the SPBI-TPA based EOD
(
0.16, 0.25) and (0.16, 0.20), respectively, which are similar to
1
9,20
those in literature.
It is worth noting that the FIr6 and the
FK306 based PhOLEDs employing SPBI-TPA as host exhibit
decent performances with maximum EQEs higher than 20%. By
contrast, the FK306 based PhOLED using SPF-TPA as the host
has a low performance, with an EQEmax of only 6.8% (Fig. S9).
The inferior efficiency of the SPF-TPA hosted FK306-based
T
PhOLED is due to the comparable E of the host (2.86 eV) and
the dopant (2.83 V); the triplet excitons cannot be effectively
confined within the dopant molecules. To the best of our
knowledge, the performances of the devices employing SPBI-
TPA as host are comparable to those of corresponding state-of-
21
22
19
the-art PhOLEDs with respectively FIrpic, FIr6 and FK306
dopants.
In summary, a spiro-configured BI compound is designed and
synthesized for the first time. The unique triangular
configuration among BI, benzene (N1 substituted to BI) and TPA
fragments is established via a spiro carbon atom, resulting in a
short-conjugation compound SPBI-TPA with a high E
SPBI-TPA is demonstrated to have better thermal properties, a
higher E and more balanced charge transportation than its
T
of 3.07 eV.
T
fluorene counterpart. SPBI-TPA is effectively used as host for
FIrpic, FIr6 and FK306 based PhOLEDs, showing respectable
EQEs over 20%. This work provides a novel host design strategy
for high-performance blue PhOLEDs. In fact, the present BI-
Fig. 3 (a) EL spectra and (b) EQE-current density curves of the
PhOLEDs using SPBI-TPA as host.
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configured spiro system offers an isolated phenyl group (N1 23 J.-G. Yang, L.-Z. Xu, L. Huang, J.-R. Gao, M.-C. Liu, F.-Y. Pan and
View Article Online
D.-B. Chen, Chin. Chem. Lett., 2016, 27, 3D4O0I–: 1304.410.39/C8CC00903A
substituted to BI) for modifying. We also believe that
substituting electron-withdrawing moiety to this phenyl group
2
3
via a facile approach can further deepen the LUMO level
without greatly lowering E , which should be a promising host
T
system for thermally activated delayed fluorescence dopants
with generally deep HOMO and LUMO levels.
We would like to acknowledge the financial support from the
National Key R&D Program of China (Project No.:
2
016YFB0401002), and Natural Science Foundation of China
(Project Nos. 61575136 and 21572152).
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