A meta-molecular tailoring strategy towards an efficient violet-blue organic electroluminescent material

Article abstract of DOI:10.1039/c4ra16954a

In this paper, an efficient violet-blue emitter 4,4′′-bis(1-(4-(tert-butyl)phenyl)-1H-phenanthro[9,10-d]imidazol-2-yl)-1,1′:3′,1′′-terphenyl (m-BBTPI) was designed and synthesized by linking two phenanthroimidazole units via the meta position of a freely rotatable phenyl bridge. The present design provides a suitable level of conjugation between the two phenanthroimidazole units such that fluorescence is strengthened over the single unit while a violet-blue emission can be maintained by limiting the amount of redshift. The new emitter m-BBTPI is also found to have good thermal stability, strong violet-blue emission and bipolar charge transporting properties. An electroluminescent device using m-BBTPI as a non-doped emission layer shows a low turn-on voltage (3.2 V), good colour purity (0.16, 0.06) as well as high current and power efficiencies (1.99 cd A^-1, 1.81 lm W^-1). These performance parameters are comparable to the state-of-the-art non-doped violet-blue OLEDs. This journal is

Full text of DOI:10.1039/c4ra16954a

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A Meta-Molecular Tailoring Strategy Towards Efficient
Violet-Blue Organic Electroluminescent Material
WenꢀCheng Chen,a,b GuangꢀFu Wu,a Yi Yuan,a,b HuaiꢀXin Wei,b FuꢀLung Wong,b
QingꢀXiao Tong*a and ChunꢀSing Lee*b
a Department of Chemistry, Shantou University, 243 University Road, Shantou, Guangdong, 515063, P.R. China.
Eꢀmail: qxtong@stu.edu.cn
b
Center of SuperꢀDiamond and Advanced Films (COSDAF) and Department of Physics and Materials Science, City
University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong SAR, P. R. China.
Eꢀmail: apcslee@cityu.edu.hk
Abstract
In this paper, an efficient violetꢀblue emitter 4,4''ꢀbis(1ꢀ(4ꢀ(tertꢀbutyl)phenyl)ꢀ1Hꢀphenanthro
[9,10ꢀd]imidazolꢀ2ꢀyl)ꢀ1,1':3',1''ꢀterphenyl (m-BBTPI) was designed and synthesized by
linking two phenanthroimidazole units via the meta position of a freely rotatable phenyl
bridge. The present design provides a suitable level of conjugation between the two
phenanthroimidazole units such that fluorescence is strengthened over the single unit while a
violetꢀblue emission can be maintained by limiting the amount of redshift. The new emitter
m-BBTPI is also found to have good thermal stability, strong violetꢀblue emission and
bipolar charge transporting properties. An electroluminescent device using m-BBTPI as a
nonꢀdoped emission layer shows a low turnꢀon voltage (3.2 V), good colour purity (0.16,
0.06) as well as high current and power efficiencies (1.99 cd Aꢀ1, 1.81 lm Wꢀ1). These
performance parameters are comparable to the stateꢀofꢀtheꢀart nonꢀdoped violetꢀblue OLEDs.
Keywords
:
phenanthroimidazole, metaꢀtailoring, violetꢀblue emission, organic
electroluminescence.
1. Introduction
Violetꢀblue lightꢀemitting materials are used in multifarious applications, such as
phototherapy,1 photocopying,2 sterilization,3 drinkingꢀwater purification etc.4 Especially,
highꢀenergy violetꢀblue emitters can not only serve as energy donors to excite lowerꢀenergy
dopants for achieving white light emission,5 but also play a key role in reducing power
consumption in fullꢀcolour displays.6 Currently, the most conventional approach for getting
violetꢀblue emission is to convert the 254ꢀnmꢀemission of lowꢀpressure Hg discharge to the
desired colours by using Sr2P2O7:Eu2+ phosphors.7 However, mercury is a strong neurotoxin
and its use is considered to be not ecoꢀfriendly.8 Organic lightꢀemitting devices (OLEDs)
could be a potential alternative to generate violetꢀblue emission. On one hand, by using
suitable emitters, OLEDs can directly generate violetꢀblue emission without using any
additional photoꢀconverter. On other hand, it can serve as Hgꢀfree and relatively ecoꢀfriendly
light sources for the green environment. However, so far, materials for violetꢀblue OLEDs
exhibiting high efficiency and meeting the requirements of standard colour purity (0.15, 0.06)
are rarely reported.6b,9 For an emitter to emit in violetꢀblue, a wide energy gap is a
prerequisite. However, the wide energy gap typically lowers the efficiency of carrier injection
and transportation, leading to higher operation voltage and shorter operation lifetime.10,11

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Furthermore, the wide energy gap is often achieved by confining the extent of πꢀconjugation
and this would typically reduce the fluorescent quantum yield.12
In the exploration for highꢀperformance violetꢀblue emitters, phenanthroimidazole (PI)
derivatives have drawn much attention due to their facile synthesis,6b,9d,9e,13 high fluorescent
quantum yields,9d,13a,13c,13e appropriate πꢀconjugation for violetꢀblue emissions6b,9d,9e,13a,13d,13e
and good thermal stabilities.6b,9d,9e,13 Additional, PI compounds also serve as efficient hosts for
phosphors due to their relatively high triplet energy level.13hꢀm Moreover, the different
electronic properties of the two nitrogen atoms14 in PI endow them with both hole13d and
electron transporting capabilities.13aꢀc For example, Ma’s group has successfully developed a
highꢀefficiency
bipolar
blue
emitter
by
linking
two
1,2ꢀdiphenylꢀ1Hꢀphenanthro[9,10ꢀd]imidazole (PPI) units.15 However, while the use of two
emitting units does substantially enhance the fluorescence and device efficiencies, the
extended molecular conjugation can lead to considerable emissive redshifts.
In order to obtain efficient violetꢀblue emitters, it has been suggested that by changing
the molecular linkage from paraꢀ to orthoꢀ or metaꢀmodes, the extent of conjugation could be
well confined.10a Nevertheless, there are few works using this strategy to achieve violetꢀblue
emission and high device performance simultaneously. For instance, Huang et al. reported a
series of 4,4’ꢀbis(1,2,2ꢀtriphenylvinyl)biphenyl (BTPE) derivatives via orthoꢀ or
metaꢀlinkages, achieving deeper blue electroluminescence (EL). However, comparing to the
paraꢀcounterpart compound BTPE, the ortho/metaꢀlinked compounds failed to maintain the
high device performances, with maximum external quantum efficiency (EQE) ranging from
only 1.4 to 1.9%. Recently, taking advantages of the PI derivatives and the nonꢀpara linkage,
Wang et al. designed and synthesized two dimeric PI compounds, LꢀBPPI and ZꢀBPPI, so as
to
blueshift
the
skyꢀblue
emission
of
4,4’ꢀbis(1ꢀphenylꢀphenanthro[9,10ꢀd]imidazolꢀ2ꢀyl)biphenyl (BPPI).16 However, the colour
purities and efficiencies of the LꢀBPPIꢀ and ZꢀBPPIꢀbased devices still have room for
improvement. Suitable molecular tailoring for the emitters and device structure are needed to
fundamentally boost the device performances.
Scheme 1 Molecular structure and design strategy of m-BBTPI.

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In
this
work,
a
blue
emitter
4,4''ꢀbis(1ꢀ(4ꢀ(tꢀbutyl)phenyl)ꢀ1Hꢀphenanthro[9,10ꢀd]imidazolꢀ2ꢀyl)ꢀ1,1':4',1''ꢀterphenyl
(BBTPI)13e is used as a prototype for designing the present violet blue emitter
4,4''ꢀbis(1ꢀ(4ꢀ(tertꢀbutyl)phenyl)ꢀ1Hꢀphenanthro[9,10ꢀd]imidazolꢀ2ꢀyl)ꢀ1,1':3',1''ꢀterphenyl
(m-BBTPI,
Scheme
1).
Here,
we
joint
two
1ꢀ(4ꢀ(tertꢀbutyl)phenyl)ꢀ2ꢀphenylꢀ1Hꢀphenanthro[9,10ꢀd]imidazole (TPI) units via a phenyl
ring at its metaꢀposition. In this design the freely rotatable benzene ring in the molecular
center plays a key role in confining πꢀconjugation due to its tetrahedral electronic
conformation.9c It is found that this strategy successfully leads to efficient violetꢀblue
emission and also maintains the bipolar transporting properties of the PI unit. On the other
hand, the pꢀ(tertꢀbutyl)phenyl group in each PI core not only provides convenience for the
facile synthesis (improve solubility in common solvents), but also serves as an
intermolecularꢀdistance controller against aggregationꢀinduced redshifts in solid state. By
using m-BBTPI as a nonꢀdoped emitter, we obtained a highꢀperformance (1.99 cd Aꢀ1)
violetꢀblue OLED (colour purity: (0.16, 0.06)) based on m-BBTPI, which is comparable to
the reported highꢀefficiency OLEDs in similar colour gamut.
2. Experimental
2.1 Synthesis
All raw materials and solvents were used directly as received from commercial suppliers
without further treatments. Synthesis routes of the new compound are depicted in Scheme S1
in the Supplementary Information.
2.2 Characterization
Absorption and photoluminescence spectra of the new compounds were determined with a
PerkinꢀElmer Lambda 2S UVꢀVis spectrophotometer and a PerkinꢀElmer LS50B
Luminescence spectrophotometer respectively. 1H NMR was recorded with a Varian
Geminꢀ400 spectrometer. Mass spectra were performed with a PE SCIEX APIꢀMS
spectrometer. Elemental analysis (C, H, N) was measured using a Vario EL III CHNS
elemental analyzer. Ionization potential (IP) of the material was measured on an ITO
substrate in a thinꢀfilm state via ultraviolet photoelectron spectroscopy (UPS) in a VG
ESCALAB 220iꢀXL surface analysis system, while electron affinity (EA) was estimated via
subtracting from IP with optical band gap. Thermal gravity analysis (TGA) and differential
scanning calorimetry (DSC) measurements were recorded with a TA Instrument TGAQ50
and a TA Instrument DSC2910 respectively. Frontier orbital distributions and molecular
geometries were calculated through density functional theory with Gaussian 09 program at
B3LPY/6ꢀ31G(d) level.
2.3 Device Fabrication and Measurement
Preꢀpatterned indium tin oxide (ITO) glass substrates with a sheet resistance of 15 ꢁ squareꢀ1
were cleaned with isopropyl alcohol and Decon 90 solution, then rinsed in deionized water
and dried in an oven. After a 15min UVꢀozone treatment, the ITO substrates were
immediately transferred into a deposition chamber with a base pressure of 5 × 10ꢀ7 torr for

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organic and cathode depositions. Organic materials were deposited via thermal deposition at a
rate of 1 Å sꢀ1, while the cathodes were completed via deposition of LiF (0.1ꢀ0.2 Å sꢀ1) and Al
(5ꢀ6 Å sꢀ1) successively. Electroluminescent spectra and the corresponding Commission
Internationale de l’Enclairage (CIE) coordinates were measured with a Spectrascan PR650
photometer. Currentꢀvoltageꢀluminance (JꢀVꢀL) characteristics were recorded with a Keithley
2400 Sourcemeter under ambient atmosphere without device encapsulation.
3. Results and Discussion
3.1 Synthesis and characterization
Synthetic routes for m-BBTPI are shown in Scheme S1 in the Supplementary Information.
Two steps are involved in the synthesis of the new compound. First,
[1,1':3',1''ꢀterphenyl]ꢀ4,4''ꢀdicarbaldehyde (m-BBCHO) was synthesized by reaction of
1,3ꢀdibromobenzene and 4ꢀformylphenylboronic acid via a Suzukiꢀcoupling reaction in a mild
condition.17 Second, a “oneꢀpot” reaction13 was adopted to introduce the PI moieties to
construct m-BBTPI. In this facile twoꢀstep process, all the raw chemicals are commercially
available and used directly without further purification, and the products were isolated with
considerable yields (see Supplementary Information). The molecular structure was
determined by 1H NMR, mass spectrometry and elemental analysis.
3.2 Thermal Property
Thermal property of m-BBTPI was investigated by TGA and DSC measurements under a N2
atmosphere. Key parameters are listed in Table 1. Generally, materials with high thermal
properties can play a key role in combating joule heat, which is often generated during device
operation deriving from current injection. Additionally, thermally stable materials are in
favour of the process of vacuum deposition. Therefore, OLED materials should be equiped
with enough high thermal stabilities. The bulky and rigid structure of m-BBTPI is beneficial
to the thermal stability, as supported by its high decomposition temperature of 512 °C (Td, 5%
weight loss, Fig. 1a). DSC measurements were performed from 50 to 420 °C. During the first
heating process, m-BBTPI only shows an endothermic peak at its melting point (Tm, 360 °C).
After heating again, a glassꢀtransition state appears at 190 °C (Tg). Comparing to its
paraꢀcounterpart BBTPI (Table 1),13e m-BBTPI exhibits a higher Td and a comparative Tg,
which should be attributed its more bulky and asymmetric molecular configuration. Such a
good thermal stability is high enough for general device fabrication and operation.

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Fig. 1 (a) TGA and (b) DSC measurements of m-BBTPI.
3.2 Photophysical properties
Fig. 2 presents the absorption and photoluminescence (PL) spectra of m-BBTPI and BBTPI
in dilute CH2Cl2 solution (10ꢀ5 mol Lꢀ1) and as thin solid film on a quartz plate. Key optical
parameters are listed in Table 1. Both of the bisphenanthroimidazole derivatives show
analogous photophysical properties. The absorption spectrum in CH2Cl2 exhibits a narrow
band peaked at ~260 nm with high absorption intensity, which is attributed to the πꢀπ*
transition of benzene.13b On other hand, a wide band with maxima at ~330 nm are originated
from πꢀπ* transition of the PI group.9d The new compound m-BBTPI gives a violetꢀblue
emission peaked at 406 nm in solution shows an evident blueshift in comparison with that of
BBTPI (λem = 427 nm). The PL results are consistent with our design idea for violetꢀblue
emitters. Electron delocalization in m-BBTPI is effectively confined via the metaꢀlinkage
strategy. Noticeably, the PL spectrum of the m-BBTPI’s thin film still locates at violetꢀblue
region, and its profile is similar to that of the dilute solution, with only ~20 nm redshift. This
suggests that there is little aggregationꢀinduced redshifts via πꢀπ stacking in m-BBTPI solid.
We find that the photoluminescent efficiency of m-BBTPI in solid state is lower than that of
BBTPI (0.59 versus 0.82). As the two molecules are composed of the same PI emitting units
with only small differences in their linking mode, the difference in their photoluminescent
efficiency is more likely to be originated from the differences in their overall molecular

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conjugation instead of their emitting moieties. It has be pointed out that photoluminescent
efficiency of a molecule decreases with the conjugation length given other factors
unchanged.18 Evidently, the conjugation extent of m-BBTPI is smaller than that of BBTPI,
which is attributed to the conjugation disruption stemming from m-BBTPI’s
metaꢀarchitecture,18a thus leading to relatively low ΦPL of m-BBTPI. The optical energy gap
(Eg), as determined from the optical absorption threshold, is 3.21 eV. The highest occupied
molecular orbital (HOMO) level measured by UPS is ꢀ5.34 eV. On other hand, the lowest
unoccupied molecular orbital (LUMO) energy, as determined by subtracting the energy gap
from the HOMO energy, is ꢀ2.13 eV.
Table 1 Summary of the physical data of m-BBTPI and BBTPI.
a
emitter
Td /Tgb (°C)
λabs/λemc (nm)
λemd (nm)
Φc/Φd (%)
Eg/HOMO/LUMO (eV)
512/190
481/192
263, 334/406
261, 367/427
432
448
70/59
93/82
3.21/ꢀ5.34/ꢀ2.13
3.08/ꢀ5.56/ꢀ2.48
m-BBTPI
BBTPI
a 5% weight loss temperature. b Glass transition temperature. c Measured in dilute CH2Cl2 solution. d Measured in film.
Fig. 2 Absorption and PL spectra of
m-BBTPI and BBTPI in dilute CH2Cl2 (solid
line), and in vacuumꢀdeposited film (dashed line).
3.3 Theoretical calculation
To better understand the correlation between molecular structures and physical
properties, theoretical molecular orbital calculations for
m-BBTPI and BBTPI were
performed with Gaussian 09 at B3LYP/6ꢀ31G (d) level. The optimized geometries and
calculated frontier molecular orbital electron density distributions are shown
schematically in Fig. 3. Both of the two compounds show a similar twisting
configuration. Generally, due to the different electrical structures, the steric effect of
fiveꢀmembered rings is intrinsically smaller than that of sixꢀmembered counterparts.
Therefore, the dihedral angle between imidazole ring and the 2ꢀposition linked benzene
(around 25°, see Fig. 3) is smaller than that of the two sixꢀmembered benzene rings at
the center of molecule (~36°). Comparing to BPPI and its isomers (
ꢀBPPI),16 the additional phenyl in the bridging section of BBTPI
-BBTPI can
efficiently limit π conjugation via molecular rotation. Like most PI derivatives,13a,b the
LꢀBPPI and
Z
/m

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sideꢀcapping tertꢀbutylphenyl groups in
m-BBTPI molecule are high twisted (~78°)
with the PI units in the molecules, and have little contribution to frontier orbital
electron density distributions. This can play a key role in suppressing emissive
redshifts by hindering close molecular packing in solid state, consequently leading to
good colour purity as well as stable amorphous thin film.
As shown in Fig. 3, electron density distributions are spatially different in these
two molecules, originating from their diverse conjugation linkages. However, the
electron cloud distributions of HOMO are mainly located at electronꢀrich PI
moieties,9d,16 while the LUMO electron clouds are dispersed at the bridging sections.
This characteristics of frontier orbital distribution indicate that the PI moieties in the
two bisphenanthroimidazole derivatives act as weak electron donors, which is
beneficial to hole transporting.
Fig. 3 Optimized geometries, calculated HOMO and LUMO electron density
distributions of BBTPI and
m-BBTPI.
3.3 Electrical properties
To study the relationship between the molecular structures and charge transporting properties,
holeꢀ and electronꢀonly devices were fabricated with respectively configurations of indium tin
oxide (ITO)/4,4'ꢀbis[Nꢀ(1ꢀnaphthyl)ꢀNꢀphenylamino]ꢀbiphenyl (NPB) (20 nm)/BBTPI or
m-BBTPI
(50
nm)/NPB
(20
nm)/Al
(100
nm)
and
ITO/1,3,5ꢀtris(1ꢀphenylꢀ1Hꢀbenzimidazolꢀ2ꢀyl)benzene (TPBI) (20 nm)/BBTPI or m-BBTPI
(50 nm)/TPBI (20 nm)/LiF (1 nm)/Al (100 nm). Herein, NPB and TPBI are respectively used
for blocking electrons and holes.19

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Fig. 4 Electron and hole current densities versus applied voltage of the single carrier devices
based on BBTPI and m-BBTPI.
As shown in Fig. 4, the current densityꢀvoltage (JꢀV) characteristics of the holeꢀ and
electronꢀonly devices illustrate that both of the two emitters have bipolar charge transporting
capacity as evidenced by the considerable hole/electron current density with similar
magnitudes comparing to a complete OLED. Similar to most organic materials,20 we can find
that hole currents are also higher than those of electron in the two bisphenanthroimidazole
derivatives. According to the chargeꢀhopping theory of organic solids21 and analyzing with
the theoretical results, this may be due to the dominant contribution of the two peripheral PI
groups to HOMO, which benefits the HOMO intermolecular overlaps in solid state. On the
other hand, the LUMO electron clouds are mainly located at the central bridging sections.
Based on the features of molecular geometries, it is different to achieve efficient
intermolecular wave function overlaps in LUMOs, which would lead to relatively low
electron transporting properties. It can be seen from Fig. 4 that the charge transporting
property of BBTPI is superior to that of m-BBTPI. It can be explained by its symmetrical
and linear molecular configuration with relatively ordered and congested solid packing,
leading to higher electrical properties.13e
3.4 Electroluminescent properties
A nonꢀdoped blueꢀemitting device was fabricated with a configuration of ITO/NPB (70
nm)/4,4',4''ꢀtris(carbazolꢀ9ꢀyl)ꢀtriphenylamine (TCTA) (5 nm)/m-BBTPI (30 nm)/TPBI (30
nm)/LiF (1 nm)/Al (150 nm), in which m-BBTPI was used as nonꢀdoped emission layer
(EML). Herein, NPB and TPBI served as holeꢀtransporting and electronꢀtransporting
materials respectively, while a thin TCTA layer was used to confine excitons within EML.9a
Fig. S1 presents the device configuration and the energy levels of the used materials.
Wellꢀmatched energy level alignment was achieved among the contiguous functional layers in
asꢀfabricated device. The key device performances are listed in Table 2. For comparison, the
EL properties of BBTPI with the same device structure are also presented.

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Fig. 5 (a) Current densityꢀvoltageꢀluminescence characteristics, (b) EL spectra at different
luminescence and luminescenceꢀcurrent density plot (inset), (c) CIE coordinates and (d) plots
of current efficiency and external quantum efficiency versus current density of the
m-BBTPIꢀbased nonꢀdoped OLEDs.
Current densityꢀluminance characteristics of the m-BBTPIꢀbased device are shown in
Fig. 5a. A low turnꢀon voltage (3.2 V, voltage at 1 cd mꢀ2) was obtained. This value is higher
than that of the BBTPIꢀbased device (2.7 V) and is mainly attributed to m-BBTPI’s larger Eg
(Table 1), giving rise to higher barrier for charge injection. Significantly, the m-BBTPIꢀbased
device emits violetꢀblue light peaking at 428 nm (Fig. 5b), which shows an evident blueshift
compared to that of the BBTPIꢀbased device (λem = 448 nm).13e Moreover, EL spectrum of
this device shows little change over a wide brightness range from 10 to 1,000 cd mꢀ2 (Fig. 5b)
and a linear increase of luminescence with respect to current density (inset of Fig. 5b),
indicating little currentꢀinduced fluorescence quenching in the device. Noticeably, the CIE
coordinates of the m-BBTPIꢀbased device is (0.16, 0.06), which is very close to the standard
blue coordinates of HighꢀDefinition Television (HDTV) (0.15, 0.06),22 as shown in Fig. 5c.
Maximum current efficiency (CE) of the m-BBTPIꢀbased device is 1.99 cd Aꢀ1 (Fig. 5d).
Comparing to the performances of the BBTPIꢀbased device (Table 2),13e this photometric
efficiency appears to be lowered by the molecular tailoring via metaꢀlinkage. However, it has
to be pointed out that the human eye sensitivity drops dramatically over the short wavelength
region,23 leading to difficulty in achieving high CE in violetꢀblue OLEDs. For instance, the
human eye sensitivity decreases by ~70% from 450 to 430 nm. Therefore, the performance of

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the m-BBTPIꢀbased device is still compared favorably when the sensitivity of human eye is
taken into consideration.
On the other hand, the EQE of the BBTPIꢀbased device is also higher than that of the
m-BBTPIꢀbased device (3.63% versus 5.77%, Table 2). This can be explained as follows.
The EQE of an OLED can describes as,24
EQE = γ Φpl ηoc ηexc
(1);
where γ is the recombination efficiency of injected holes and electrons, which is assumed to
be 1;25 Φpl is the intrinsic PL efficiency; ηoc is the optical outꢀcoupling factor; ηexc is the
exciton utilizing efficiency. It is expected that the values of ηexc are similar in the BBTPIꢀ and
the m-BBTPIꢀbased devices with similar molecular structure for the emitter and the same
device architecture. As we discussed, BBTPI has a linear planar molecular geometry.13e Thus,
it is expected that the ηoc of the BBTPIꢀbased device is larger than the m-BBTPIꢀbased
device. Meanwhile, the Φpl of m-BBTPI is smaller than that of BBTPI due to the conjugation
decrease. Therefore, the EQE the BBTPIꢀbased device is superior to the m-BBTPIꢀbased
device.
Table 2 Summary of the device performances of the
m-BBTPIꢀ and the BBTPIꢀbased
nonꢀdoped OLEDs.
emitter
m-BBTPI
BBTPI
Von (V)
3.2
λEL (nm)
428
CEa (cd Aꢀ1)
1.99, 1.97, 1.77
5.48, 5.35, 5.10
PEb (lm Wꢀ1)
EQEc (%)
CIE (x, y)
0.16, 0.06
0.15, 0.10
1.81, 1.42, 0.82
4.77, 4.41, 2.90
3.63, 3.61, 3.36
5.77, 5.60, 5.41
2.7
448
a Current efficiency, b power efficiency, c external quantum efficiency corresponding to the value at the maximum, 100
cd mꢀ2 and 1000 cd mꢀ2, respectively.
Fig. 6 Comparison between the PL spectrum of m-BBTPI in solid state and the EL spectrum
of the m-BBTPIꢀbased device.
Although performance of the violetꢀblue m-BBTPIꢀbased device is lower than its blue
counterpart, it is markedly improved in comparison with those of LꢀBPPIꢀ and ZꢀBPPIꢀbased
devices.16 Analyzing the device structures carefully, the TCTA layer also plays a key role in
achieving higher efficiency in this work. In fact, the bisphenanthroimidazole derivatives are
highꢀenergyꢀgap compounds, with Eg larger than 3.00 eV (3.21, 3.15, 3.30 eV for m-BBTPI,

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LꢀBPPI and ZꢀBPPI respectively). In the device structures of this work and previous work by
Wang et al.,16 NPB (Eg = ~3.00 eV) layer was used as holeꢀtransporting layer (HTL).
Provided that NBP HTL was deposited in proximity to bisphenanthroimidazole
derivativesꢀbased EML, the excitons generated by the recombination of the injected carriers
in the EML would leak into HTL due to the relatively small Eg of NPB. This can be supported
by the evidence that EL emissions of the LꢀBPPIꢀ and ZꢀBPPIꢀbased devices mismatch their
corresponding intrinsic PL emissions. In this work, we applied a TCTA (Eg = ~3.40 eV) layer
as buffer layer,9a so as to confine the excitons within the EML. As shown in Fig. 6, the EL
spectrum of m-BBTPIꢀbased device is similar to the PL emission of m-BBTPI, implying
efficient confinement of excitons. To obtain a comprehensive comparison, key device
performance parameters of m-BBTPI and recently reported nonꢀdoped violetꢀblue OLEDs
are listed in Table S1.6b,9bꢀe,13e It can be seen that the device performances of the
m-BBTPIꢀbased device are comparable to the highly efficient nonꢀdoped violetꢀblue OLEDs.
All the above evidences indicate that the strategies used in this work for violetꢀblue OLEDs is
effective.
Conclusions
In summary, a novel violetꢀblue fluorophor m-BBTPI has been designed with the strategy
jointing two TPI units using a freely rotatable benzene ring via the metaꢀlinkage. In this work,
the relationships between modification in molecular structure and various materials’
properties, including thermal stabilities, photophysical characteristics, theoretical frontier
orbital distributions as well as charge transporting properties, are comprehensively discussed.
We find that the new compound m-BBTPI exhibits good thermal stability, efficient
violetꢀblue emission and bipolar charge transporting characteristics. Finally, the differences of
device performances originating from chemical structure adjustment for emitter are analyzed.
By using a suitable device structure, OLED based on m-BBTPI as a nonꢀdoped EML exhibits
decent performance with CE (PE) up to 1.99 cd Aꢀ1 (1.81 lm Wꢀ1) and a colour index of (0.16,
0.06), which is comparable to the reported efficient nonꢀdoped violetꢀblue devices.
Acknowledgements
This research was supported by the Research Grants Council of the Hong Kong (Project No.
T23ꢀ713/11), the National Natural Science Foundation of China (Project Nos. 51273108 and
91027041) and the National Basic Research Program of China (973 Program No.
2013CB834803).
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Graphical Abstract
In this study, a novel violetꢀblue emitter
metaꢀmolecular tailoring. Using ꢀBBTPI as nonꢀdoped emitter, we obtained a
highꢀefficiency violetꢀblue organic electroluminescent device.
mꢀBBTPI was designed and synthesized via
m