A high performance deep-blue emitter with an anti-parallel dipole design

Article abstract of DOI:10.1016/j.dyepig.2017.07.008

To increase the fluorescence quantum yield of hybrid local and charge-transfer (HLCT) material, we introduce a symmetric linear D-π-A-π-D structure into molecular design and synthesized a new blue emitter, 2,2'-(2′,3′,5′,6'-tetrafluoro-[1,1':4′,1″-terphen

Full text of DOI:10.1016/j.dyepig.2017.07.008

Dyes and Pigments 146 (2017) 219e225
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journal homepage: www.elsevier.com/locate/dyepig
A high performance deep-blue emitter with an anti-parallel dipole
design
,
, **
Ze-Lin Zhu a b, Shao-Fei Ni c, Wen-Cheng Chen b, Yi Yuan d, Qing-Xiao Tong a
,
Fu-Lung Wong b, Feng Lu a, Chun-Sing Lee b
,
*
a Department of Chemistry, Shantou University, Guangdong 515063, China
b Center of Super-Diamond and Advanced Films (COSDAF) & Department of Chemistry, City University of Hong Kong, Hong Kong SAR, China
c Department of Chemistry, South University of Science and Technology of China, Shenzhen 518055, China
d 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, China
a r t i c l e i n f o
a b s t r a c t
Article history:
To increase the fluorescence quantum yield of hybrid local and charge-transfer (HLCT) material, we
introduce a symmetric linear D-p-A-p-D structure into molecular design and synthesized a new blue
Received 9 May 2017
Received in revised form
5 July 2017
Accepted 6 July 2017
Available online 8 July 2017
emitter, 2,2'-(20,30,50,6'-tetrafluoro-[1,1':40,100-terphenyl]-4,400-diyl)bis(1-(4-(tert-butyl)phenyl)-1H-phe-
nanthro[9,10-d]imidazole) (4FBTPI). Organic light-emitting devices employing 4FBTPI as a dopant
emitter show good performances, with CIE (Commission Internationale de l’Enclairage) coordinates of
(0.15, 0.09), which is very close to NTSC blue light standard (0.14, 0.08), current efficiencies (CE) of
6.03 cd Aꢀ1, external quantum efficiencies of 6.94%. This performance is comparable to those of recently
reported state-of-the-art deep-blue materials with CIEy < 0.10.
Keywords:
Organic light-emitting diodes
Deep blue-emitting material
HLCT
© 2017 Elsevier Ltd. All rights reserved.
D-p-A-p-D
1. Introduction
Normally, g and hout are assumed to be 1 and 0.2 respectively for an
optimized device fabricated on a glass-substrate without out-
coupling enhancement structure [5e9]. The two remaining pa-
rameters 4PL and hr are material-dependent and key parameters for
evaluating the material performance.
Blue emitter has been a hot research topic of organic light-
emitting devices (OLEDs) because it is essential for realizing high
performance full-color displays and solid-state lighting [1,2]. Blue
material with a Commission International de L'Eclairage (CIE) co-
ordinate of y < 0.10 is highly desirable because it can improve both
color gamut and energy efficiency of full-color OLEDs [2]. Making
progress in OLED needs both device optimization and material
development [3,4]. External quantum efficiency (EQE) of an OLED is
determined by the following equation:
Although it is possible for a conventional fluorescent material to
achieve a 4PL higher than 90%, the theoretical maximum of hr of
25% limits its device efficiency [2]. Recently, materials with charge-
transfer (CT) character break the hr limit of conventional fluores-
cent emitters, leading to breakthrough in fluorescent OLED [10,11].
The newly emerged thermally activated delayed fluorescent (TADF)
materials are mostly constructed with linked strong donors (Ds)
and acceptors (As). With spatially separated highest occupied
molecular orbital (HOMO) and lowest unoccupied molecular orbital
(LUMO) distribution, energy offset between the singlet and the
triplet excited states (S1 and T1) becomes small. This enables
reverse intersystem crossing (RISC) from T1 to S1 at room temper-
ature to harvest the energy of T1 state for light emission [12,13].
However, due to strong intramolecular charge transfer (ICT) in
these TADF materials, their emission spectra are typically board and
unfavorable for getting deep-blue emission (e.g. CIEy < 0.10).
Moreover, recent research shows the prototypical blue TADF
EQE ¼ ghout4PLhr
(1)
where, is the recombination efficiency of hole and electron, hout is
g
the light out-coupling efficiency, 4PL is the photoluminescence
quantum yield (PLQY) and hr is the ratio of radiative exciton.
* Corresponding author.
** Corresponding author.
E-mail addresses: qxtong@stu.edu.cn (Q.-X. Tong), apcslee@cityu.edu.hk
(C.-S. Lee).
http://dx.doi.org/10.1016/j.dyepig.2017.07.008
0143-7208/© 2017 Elsevier Ltd. All rights reserved.

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Z.-L. Zhu et al. / Dyes and Pigments 146 (2017) 219e225
emitter DTC-DPS is unstable under excited state because of its weak
C-S bond [14]. Alternatively, Ma et al. [8] have developed a series of
D-A molecules that have special hybridized local and charge-
transfer (HLCT) excited states, in which both the locally excited
(LE) and the charge-transfer (CT) states contribute to the excited
states. The LE state, with significant HOMO-LUMO overlap, often
offers high efficiency fluorescence. On the other hand, while the CT
state exciton often has relatively low luminescent efficiency, its
weak Coulomb bound facilitates electron flip to raise hr in fluo-
rescent OLED [8,15e18]. By mixing these two distinct excited states,
HLCT materials show impressive devices performances (high hr and
EQE). For instances, with a quasi-equivalent hybridized excited
state, TBPMCN [19] achieved a maximum EQE of 7.8% with CIE
coordinates of (0.16, 0.16) and hr of 97% in a non-doped OLED. The
extent of LE and CT mixing is inversely proportional to the differ-
ence of their energy levels. With closely laid LE and CT energy
levels, HLCT material PMSO [20] presents a maximum EQE of 6.8%
with CIEy < 0.08 (hr of 47%) in a doped device. However, as
mentioned above, 4PL is also an important performance parameter.
4PL of TBPMCN and PMSO thin films are only 0.40 and 0.45
respectively. It is thus interesting to explore approaches for
improving the 4PL of HLCT materials.
Increasing the relative contribution of the LE component has
been shown to be an effective way for raising 4PL in HLCT materials
[19,20]. Meanwhile for deep-blue materials, choosing D and A
groups wisely are required in order to control ICT for maintaining
the deep-blue emission. In this work, we report a new deep-blue
material named 2,2'-(20,30,50,6'-tetrafluoro-[1,1':40,100-terphenyl]-
4,400-diyl)bis(1-(4-(tert-butyl)phenyl)-1H-phenanthro[9,10-d]imid-
azole) (4FBTPI) (Fig. 1a) with HLCT character. Phenanthroimidazole
(PI) is an excellent blue-violet fluorophore and bipolar block that
can act either as a weak acceptor or weak donor [11,21e23].
Actually, PI is an ideal donor building block for deep-blue emitter
considering its weak electron-donating abilities and outstanding
photophysical properties. Tetrafluorobenzene with four strong
electron withdrawing fluorine atoms was chosen here as the
acceptor to link with two PI units. It has been reported that tetra-
fluorobenzene could cause hypochromatic shift in molecular
spectrum and PLQY enhancement, reducing the unwanted red shift
and fluorescence quenching from the ICTeffects [24,25]. In thin film
state, fluorine atoms could further benefit the film morphology
[26]. Inserting a benzene ring between the D and the A unit buffers
the push-pull electronic effect and enhances HOMO and LUMO
overlap thus increase LE component and raise PLQY in the target
molecule. Using a linearly linking, the molecule would process a
higher conjugation extent and better electronic communication,
which will benefit to CT-LE mixing. For molecules with asymmetric
pull-push systems (e.g. D-A and D-p-A), they often suffer from se-
vere emission quenching in condensed aggregated state as those
dipole-induced intermolecular interactions inactivate a consider-
able part of their excitons. In 4FBTPI, it is a linear D-p-A-p-D
electronic structure (Fig. 1b) and the anti-parallel dipoles origi-
nated from the two donor ends cancel out. Such a design is aiming
at reducing dipole-induced intermolecular interactions and
enhancing luminescence of the molecule in solid state [27,28].
Besides, both D and A moieties can give help in electron injunction
and transporting hopefully making the target molecule more
charge balance compared to common organic emitter with hole-
dominant character, reducing efficiency in device roll-off at high
current density [11,29].
4FBTPI shows high PLQYs of 0.99, 0.69 and 0.88 in respectively
solution, neat film and doped in a 4,40-Bis(N-carbazolyl)-1,1'-
biphenyl (CBP) film. Using 4FBTPI as a dopant emitter in CPB, an
OLED delivers a deep-blue emission with CIE coordinates of (0.15,
0.09), a max current efficiency (hC) of 6.03 cd Aꢀ1 and a maximum
EQE of 6.94%.
2. Experiment
2.1. Material and methods
The synthesis route of 4FBTPI is shown in Scheme 1. Mass and
1H NMR spectra of 4FBTPI were recorded on a PE SCIEX API-MS
spectrometer and a Varian Gemin-400 spectrometer respectively.
The glass-transition temperature (Tg) of the compound was deter-
mined with differential scanning calorimetry (DSC) under a nitro-
gen atmosphere by using
a TA Instrument DSC2910 and
thermogravimetric analysis (TGA) was performed on a TA Instru-
ment TGAQ50. UVevis absorption and PL spectra were obtained
with a Perkin-Elmer Lambda 950 UVevis Spectrometer and a
Perkin-Elmer LS50 fluorescence spectrometer, respectively. Abso-
lute fPL was measured with a Labsphere™ integrating sphere using
a monochromatized Xe lamp (Newport™) as exciting source (at
365 nm). PL lifetime was recorded on an Edinburgh Instruments
Fig. 1. a) Molecular structure and b) design concept of 4FBTPI.
Scheme 1. Synthesis route of 4FBTPI.

Z.-L. Zhu et al. / Dyes and Pigments 146 (2017) 219e225
221
FLS980 spectrophotometer. Cyclic voltammetry (CV) was carried
out in nitrogen-purged CH2Cl2 (positive scan) at room temperature
using a CHI voltammetric analyzer. Tetrabutylammonium hexa-
fluorophosphate (TBAPF6) (0.1 M) was used as the supporting
electrolyte. The measurements were carried out at a scan rate of
100 mV sꢀ1 with a conventional three-electrode configuration
consisting of a glassy carbon working electrode, a platinum wire
auxiliary electrode, and a saturated calomel reference electrode
calibrated with ferrocene/ferrocenium (Fc/Fcþ). For theoretical
calculation, geometrical properties were optimized at the B3LYP/6-
31 g(d, p) level using the Gaussian 09 program. The excited states
are then calculated at the TD-PBE0/6-31 g(d, p) level. The LUMO
energy level was estimated by subtracting the optical band gap
from the HOMO energy level.
d2)
d
8.78 (dd, J ¼ 21.4, 8.4 Hz, 6H), 7.81 (d, J ¼ 7.5 Hz, 6H), 7.70 (d,
J ¼ 8.3 Hz, 6H), 7.58 (d, J ¼ 9.2 Hz, 2H), 7.52 (dd, J ¼ 11.3, 8.0 Hz, 8H),
7.32 (t, J ¼ 7.7 Hz, 2H), 7.24 (d, J ¼ 8.2 Hz, 2H), 1.47 (s, 18H); MS (ESI)
m/z: [MþH]þ calcd for C68H50F4N4, 999.15; found, 1000.26 and
500.33.
3. Results and discussion
3.1. Theoretical calculations
To analyze ground state and excited state properties of 4FBTPI,
quantum chemistry calculation was carried out using the Gaussian
09 program. Fig. S1 shows the optimized geometry of its ground
state. Across the whole D-p-A-p-D system, twisting angles be-
Before device fabrication, pre-cleaned ITO-coated glass sub-
strates with a sheet resistance of 15
tween building blocks are relatively small. The dihedral angle be-
tween the PI unit and the phenyl bridge is 25.4ꢂ and that between
the acceptor and phenyl bridge is 41.6ꢂ. This gives good conjuga-
tion, electronic communication and state mixing. As shown in
Fig. 2, the “hole” of S1/S0 transition spreads over the whole
skeleton of the molecule, except the benzene ring connected to the
N1 position (nitrogen atom in position 1 of PI unit). On the other
hand, its “particle” mainly locates at the p-tetrafluorophenyl bridge.
Obviously, the transition contains both a LE component at the tet-
rafluorophenyl bridging group and a CT transition from PI units to
the terphenyl group. Besides, the very large oscillator strength of
S1/S0 transition (GS /S ¼2.1897), which is comparable to some LE
U
squareꢀ1 were subjected to
UV-ozone treatment for 20 min. All organic films were deposited by
thermal evaporation in a deposition chamber with a base vacuum
of 5 ꢁ 10ꢀ6 Torr. J-V characteristics were recorded with a Keithley
2400 Sourcemeter. Electroluminescence spectra and CIE color co-
ordinates were measured with a Spectrascan PR650 photometer.
2.2. Preparation of compounds
2.2.1. Synthesis of 2-(4-bromophenyl)-1-(4-(tert-butyl)phenyl)-1H-
phenanthro[9,10-d]imidazole (1) [21]
1
0
The 4-bromobenzaldehyde (1.86 g, 10 mmol), 9,10-
phenanthrenequinone (2.08 g, 10 mmol), 4-(tert-butyl)aniline
(1.49 g, 10 mmol), and ammonium acetate (4.62 g, 60.0 mmol) were
refluxed in an acetic acid solution under an argon atmosphere for
24 h. After cooling to room temperature, an orange-yellow mixture
was obtained and poured into methanol under stirring. The raw
product was separated by filtration and washed with methanol,
and then dried under vacuum. The product was purified by column
chromatography (petroleum ether: CH2Cl2, 1:2) on silica gel to give
a white powder, with a 92.1% yield.
molecules [31,32], suggests the LE component has a larger contri-
bution than the CT component in this HLCT state. Such a HLCT state
is a reasonable result as we insert a
p bridge between D and A
groups to weaken electronic push-pull effect and enlarge LE tran-
sition range. Other key calculation data including energy diagram
and transition of singlet and triplet states are shown in Fig. S2 and
Table S1 in the supporting information.
3.2. Photophysical properties
To acquire a better understanding of its excited state property,
photophysical properties of the material was investigated by
characterizing its UVeVis absorption and photoluminescence (PL)
spectra (Fig. 3) in dilute dichloromethane solution (10ꢀ5 M). In the
2.2.2. Synthesis of 1-(4-(tert-butyl)phenyl)-2-(4-(4,4,5,5-
tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)-1H-phenanthro[9,10-
d]imidazole (2) [30]
A 3-neck 100 mL round bottom flask was charged with a stir bar,
1 (4.04 g, 8 mmol), bis(pinacolato)diboron (B2pin2) (2.12 g,
8.4 mmol), anhydrous potassium acetate (KOAc) (2.14 g,
21.6 mmol), dichloro[1,1'-bis(diphenylphosphino)-ferrocene]palla-
dium(II) dichloromethane adduct (Pd(dppf)Cl2$CH2Cl2) (174 mg,
0.214 mmol) and degassed dioxane (40 mL). After the reaction
mixture was heated at 85 ꢂC for 16 h, the product was extracted
with CH2Cl2 and washed with distilled water. The organic extract
was dried over MgSO4, and solvent was removed under reduced
pressure to obtain a brown viscous oil. Purification with flash
chromatography (petroleum ether: CH2Cl2, 1: 4) yielded 3.38 g
(76.5%) of tacky white solid.
absorption spectrum, typical absorption of p-p* transition of ben-
zene ring and the PI unit at around 260 and 340e370 nm are
observed, which are similar to other PI derivatives [11,33], indi-
cating that the tetrafluorobenzene acceptor has relatively little in-
fluences on molecular ground state properties. The PL spectrum of
4FBTPI in solution peaks at 448 nm, while the PL spectrum of the
film shows a mildly bathchromatically shifted peak at 457 nm.
Encouragingly, impressive PLQY data were obtained in both solu-
tion and neat film of 4FBTPI (99% and 69%) (Table 1). In order to
gain a deeper blue emission, we doped the material into CBP. With
8
1 wt% of 4FBTPI as dopant, PL emission of the doped film ex-
hibits a significant hypochromatic shift of 29 nm compared to neat
film of 4FBTPI. Correspondingly, PLQY of the doped film is
improved to 88%. Such significant changes may not only origin from
eradicating of aggregation effect in solid state, but also influenced
by sensitive emitting nature toward circumstance of the CT state
and blue shifting effect of tetrafluorobenzene [24].
2.2.3. 2,2'-(20,30,50,6'-tetrafluoro-[1,1':40,100-terphenyl]-4,400-diyl)
bis(1-(4-(tert-butyl)phenyl)-1H-phenanthro[9,10-d]imidazole)
(4FBTPI)
1,2,4,5-tetrafluoro-3,6-diiodobenzene (0.80 g,
2 mmol), 2
(2.66 g, 4.8 mmol), Pd(PPh3)4 (0.23 g, 0.02 mmol), and K2CO3
aqueous (2 M, 6 mL) in toluene (60 mL) and ethanol (12 mL) was
heated to reflux in an argon atmosphere for 48 h. The solution was
cooled to room temperature and extracted with CH2Cl2. The extract
was dried with anhydrous Na2SO4 and concentrated by rotary
evaporation. The residue was purified by column chromatography
(ethyl acetate: CH2Cl2, 1: 100) to obtain the pure product as white
powder yield: 53% (0.74 g). 1H NMR (400 MHz, Methylene Chloride-
3.3. Solvatochromic effects
Systematic solvatochromic experiments using ten different
aprotic solvents were conducted to further study properties of both
ground state and excited state of 4FBTPI. From its UVeVis ab-
sorption spectra, similar absorption peaks and curves were
observed from low polarity hexane to high polarity acetonitrile.

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Z.-L. Zhu et al. / Dyes and Pigments 146 (2017) 219e225
Fig. 2. Natural transition orbitals (NTOs) of S1/S0 of 4FBTPI.
Fig. 4. Solvatochromic PL spectra of 4FBTPI.
Fig. 3. Absorption and PL spectra of 4FBTPI.
Table 1
Summary of the physical data of 4FBTPI.
a
b
e
Compound
Td
Tg
HOMOc
[eV]
LUMOd
[eV]
labs
lem
Ffh
[ꢂC]
[ꢂC]
n.o.
[nm]
[nm]
%
4FBTPI
498
ꢀ5.56
ꢀ2.44
342, 367
448e/457f/428g
99e/69f/88g
a
Temperature for 5% weight loss.
n.o. ¼ not observed.
b
c
Detected via cyclic voltammetry.
d
e
f
Estimated by deducting Eg from HOMO levels.
Measured in dilute CH2Cl2 solution (10ꢀ5 mol Lꢀ1).
Measured in neat film.
g
h
Measured in doped film (8 1 wt% in CBP).
The fluorescence quantum yield in solution was determined with a calibration standard of 10ꢀ5 mol Lꢀ1 cyclohexane solution of 9,10-Diphenylanthracene (0.90); and the
solid state quantum yield on quartz plate was determined with an integrating sphere.
While its PL emission (Fig. 4) shows a red-shift of 56 nm from
409 nm in hexane to 464 nm in acetonitrile with widening of full
width at half maximum (FWHM) from 53 nm to 83 nm. The PL
behavior clearly shows CT character in the excited stated of 4FBTPI.
Moreover, it is interesting that the PL spectrum in hexane only
shows a single peak at 409 nm with a shoulder at around 425 nm,
which is not a typical LE emission with vibrational structure
spectrum nor a structureless CT character spectrum. When the
polarity of solvent (in butyl ether) increases slightly, the emission
spectrum becomes non-vibrational and keeps this feature until
using acetonitrile. The result may indicate that the CT and the LE
energy levels are laid very close in 4FBTI and their mixing state
(HLCT) is highly hybridized as the hybrid extent is inversely pro-
portional to energy difference of CT and LE state. As shown in Fig. 5,
a linear correlation between Stokes shift and solvent polarity was
found, which is in accordance with the PL spectra behavior of
4FBPTI and other reported highly mixed HLCT materials [19,20]. In
these materials, LE and CT energy levels are close and changes in
solvent may decrease the CT energy much more obviously than LE
energy, so the mixed state will be dominated by the CT ingredient
at lower polarity solvent resulting in the observed linear relation-
ship. With the fitting slope, excited state dipole moment (me) of
4FBTPI is calculated to be 27 D. This result is so close to a CT
molecule instead of an HLCT one. However, as we mentioned above,
the energy level of CT and LE state lay so close that slight change of
the environmental polarity makes the CT character dominate the
excited state. Therefore, the LE component cannot be detected and
only CT character shown. Similar result is also found in reference
20. 4FBTBI show slightly increased PL lifetimes, as the solvent po-
larity increases (Table S4), similar effects were also observed in

Z.-L. Zhu et al. / Dyes and Pigments 146 (2017) 219e225
223
bis(naphthalen-1-yl)-N,N’-bis(phenyl)benzidine (NPB) (70 nm)/
4,40,400-Tris(carbazol-9-yl)triphenylamine (TCTA) (5 nm)/4FBTPI
(20 nm)/1,3,5-tris(1-phenyl-1H-benzimidazol-2-yl)benzene (TPBI)
(30 nm)/LiF (1 nm)/Al (100 nm). In the device, NPB is the hole-
transporting layer, TCTA is the exciton-blocking layer, TPBI is the
electron-transporting layer and LiF is the electron injection layer. As
show in Fig. S5, blue emission peaking at 460 nm was obtained
from the device with a corresponding CIE coordinates of (0.16, 0.18).
Turn-on voltage (at 1 cd mꢀ2) was as low as 2.9 V (Fig. 6b). The EL
spectrum was well-matched with the PL spectrum of the 4FBTPI
neat film and kept almost the same at 10, 100 and 1000 cd mꢀ2
.
Moreover, 4FBTPI gives impressive performance in the device with
a maximum current efficiency (CE) of 6.86 cd Aꢀ1 and a maximum
EQE of 4.86% (Fig. 6b). It is worth noting that the device shows only
mild efficiency roll-off as luminescence increases and delivers ef-
ficiencies of 6.42 cd Aꢀ1 (CE) and 4.62% (EQE) at 1000 cd mꢀ2
.
Considering equation (1), hr was calculated to be 35% at the
maximum EQE, which clearly breaks the 25% limit of conventional
fluorescent materials.
Fig. 5. Linear correlation fitting with Stokes shifts as a function of solvent polarity for
4FBTPI in different solvents.
Noting a bluer emission and a higher PLQY of 4FBTPI when
doped in CBP, we fabricated a doped OLED with configuration of
ITO/MoO3 (3 nm)/NPB (55 nm)/TCTA (10 nm)/4FBTPI: CBP (8 1 wt
other HLCT molecules [8,19].
%
30 nm)/1,3,5-tri[(3-pyridyl)-phen-3-yl]benzene (TmPyPB)
3.4. Electroluminescence (EL)
(40 nm)/LiF (1 nm)/Al (100 nm). It is noted that NPB, TCTA and CBP
are all arylamine derivatives possessing good hole-transporting
An OLED was fabricated with a configuration of ITO/N,N’-
Fig. 6. Performance of non-doped device: a) Current density-voltage-luminescence curves, b) Current efficiencyeLuminanceeExternal quantum efficiency curves.
Fig. 7. Performance of doped device: a) current density-voltage-luminescence curves, b) the Current efficiencyeLuminanceeExtenal quantum efficiency curves.

224
Z.-L. Zhu et al. / Dyes and Pigments 146 (2017) 219e225
abilities. To better balance electron and hole currents, TPBI was
replaced with TmPyPB [34e36] for increasing the electron mobility
of the device and blocking hole more effectively. The EL spectra at
different brightness values are shown in Fig. S6. Deep-blue emis-
sion with peak at 440 nm and CIE coordinates of (0.15, 0.09) were
obtained and the peak position shows little change from 10 to
1000 cd mꢀ2. Turn-on voltage was determined to be 3.3 V. As
shown in Fig. 7 and Table S5, maximum CE and EQE of 6.03 cd Aꢀ1
and 6.94% were achieved in the doped device. Performance of
4FBTPI is comparable to those of the recently reported state-of-art
deep-blue materials with CIEy < 0.10 [20,29,33,37e41]. hr of the
doped device was calculated to be 39%, which is similar to that of
the non-doped device indicating the improvement of device per-
formance should be mainly attributed to the increase in PLQYof the
doped film.
high-lying CT exciton, “hot” CT channel [5,19].
4. Conclusions
In conclusion, by using PI as
a weak donor and tetra-
fluorobenzene as an acceptor and applying an anti-parallel dipole
design strategy, a novel blue-emitting molecule named 4FBTPI has
been designed, synthesized and applied in deep-blue OLEDs. High
PLQYs in both solution and solid state were achieved. A non-doped
OLED with 4FBTPI as emitter shows high CEmax of 6.86 cd Aꢀ1 and
EQEmax of 4.86% with CIE of (0.16, 0.18). A doped OLED using 4FBTPI
as dopant exhibits deep-blue emission with CIE coordinates of
(0.15, 0.09) and high efficiency with a maximum CE of 6.03 cd Aꢀ1
EQE of 6.94% and hr of 39%. The study provides an example of an
anti-parallel dipole design of HLCT molecule to endow the material
with high PLQY and hr simultaneously.
,
To better explain the high device efficiency, time-dependent PL
and low temperature (77 K) PL measurements were carried out. As
illustrated by Fig. 8, only nanosecond scale decay was observed and
Acknowledgments
the larger difference of S1 and T1 (DEST was determined to be
0.68 eV, Fig. 9) eliminated the possible contribution of TADF
[12,42]. The linear correlation between current density (Fig. S3 and
S4) and luminescence reveals long-live T1 exciton should be not
involved in the EL process [20,43]. According to the above discus-
sion on the excited state properties, the high hr in these two devices
is most likely due to the contribution of the ultrafast spin flip of
This work was supported by the National Natural Science
Foundation of China (Project No. 51473138 and 51673113) and the
National Basic Research Program of China (973 Program No.
2013CB834803).
Appendix A. Supplementary data
Supplementary data related to this chapter can be found at
http://dx.doi.org/10.1016/j.dyepig.2017.07.008.
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