Highly efficient non-doped deep blue fluorescent emitters with horizontal emitting dipoles using interconnecting units between chromophores

Article abstract of DOI:10.1039/c6cc05076j

New deep blue fluorescent emitters composed of anthracene as an electron rich unit, a diphenyltriazine as a strong electron acceptor unit, and phenyl or xylene as interconnecting units were synthesised. The interconnecting unit between chromophores increased the singlet transition energy and the ratio of horizontal emitting dipoles. As a result, a non-doped blue fluorescent organic light-emitting diode (OLED) using a new emitter was demonstrated, with an external quantum efficiency (EQE) of 6.6% and Commision Internationale de l'Eclairage (CIE) colour coordinates of (0.145, 0.068). This device performance has been the highest EQE observed in deep blue non-doped OLEDs with CIE coordinates less than (0.145, 0.068) to date.

Full text of DOI:10.1039/c6cc05076j

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Highly efficient non-doped deep blue fluorescent emitter with
horizontal emitting dipoles using an interconnecting unit between
chromophores
Received 00th January 20xx,
Accepted 00th January 20xx
Kwon-Hyeon Kim+a, Jang Yeol Baek+b, Chan Woo Cheonc, Chang-Ki Moona, Bomi Sima, Myeong
Yong Choib, Jang-Joo Kima*, and Yun-Hi Kimc*
DOI: 10.1039/x0xx00000x
www.rsc.org/
pi-conjugation of the emitter between the donor and acceptor and
to combine donor and acceptor moieties to prevent bathochromic
New deep blue fluorescent emitters composed of anthracene as
an electron rich unit, a diphenyl triazine unit as a strong electron
acceptor unit, and phenyl or xylene as interconnecting units were
synthesised. The interconnecting unit between chromophores
increased the singlet transition energy and the ratio of horizontal
emitting dipoles. As a result, a non-doped blue fluorescent organic
light-emitting diode (OLED) with new emitter was demonstrated,
with an external quantum efficiency (EQE) of 6.6% and Commision
Internationale de l’Eclairage (CIE) colour coordinates of (0.145,
0.068). This device performance is the highest EQE observed in
deep blue non-doped OLEDs with CIE coordinates less than (0.145,
0.068) to date.
shift. To our knowledge, the non-doped OLED using
a
phenanthroimidazole derivative reported by Lee and co-workers
has exhibited the highest EQE of 5.56%, with a CIE coordinate of
less than (0.15, 0.08).2i
The orientation of emitting dipoles in emitters significantly
influences the outcoupling efficiency of OLEDs.3 A higher proportion
of horizontal dipoles
efficiency, as horizontal emitting dipoles reduce wave-guiding loss
and surface plasmon loss, where and represent the
( ) results in higher EL
Θ = P / (P + P )
⊥
P
P
⊥
proportion of horizontal and vertical dipoles, respectively. Some
group reported horizontal orientation of fluorescent-emitting
dipoles and demonstrated highly efficient OLEDs 3a-f. However, the
colour quality and efficiency of deep blue OLEDs still requires
improvement for practical applications.
In this work, we developed novel anthracene-based blue emitters,
composed of anthracene as an electron rich unit and
diphenyltriazine as a strong electron acceptor unit and discussed
effect of interconnecting unit on emission spectrum and orientation
of emitting dipoles of compounds. Two chromophores are linked
directly, or with phenyl or xylene as an interconnecting unit. The
phenyl or xylene interconnecting unit increases the dihedral angle
between anthracene and the adjacent moiety, resulting in a higher
lowest unoccupied molecular orbital (LUMO) level, by shortening
the conjugation length at the LUMO state. Moreover, the
interconnecting unit lengthens the emitter along the long axis to
Organic light-emitting diodes (OLEDs) are widely used in displays
and lighting. For lighting and full-colour display, it is important to
develop a deep blue OLED exhibiting high efficiency and satisfying
Commission International de l’Eclairage (CIE) coordinates (x, y) =
(0.14, 0.08), referenced by the National Television System
Committee (NTSC). In recent decades, there have been significant
research efforts to develop deep blue phosphorescent emitters to
increase the efficiency of OLEDs. Unfortunately, however, blue
phosphorescent OLEDs are still not sufficiently stable for adoption
into lighting and displays. An alternative approach is the
development of efficient deep blue fluorescent OLEDs.
Anthracene derivatives have been widely used as blue fluorescent
emitters due to their good colour quality, stability, and efficiency.1
In recent years, donor–acceptor-type blue fluorescent emitters
have been reported to enhance charge balance and, as a result, to
improve electroluminescence (EL) efficiency.2 However, donor–
acceptor-type emitters have a large bathochromic shift due to
lengthened pi-conjugation in the emitter and formation of an
intramolecular charge transfer state. It is thus important to disable
increase the aspect ratio of the emitter. As
interconnecting unit increases the singlet transition energy by
increasing the LUMO level and of the molecule. We
a result, the
Θ
demonstrated a maximum external quantum efficiency (EQE) of
6.6% with CIE coordinates of (0.145, 0.068) for the newly
synthesised emitters. To our knowledge, this is the highest EQE
among deep blue non-doped fluorescent OLEDs with
coordinate of less than (0.145, 0.068).
a CIE
The synthetic schemes of anthracene-based blue emitting
materials with diphenyltriazine, 2-(10-(naphthalen-1-yl)anthracen-
9-yl)-4,6-diphenyl-1,3,5-triazine (NAPT), 2-(4-(10-(naphthalen-1-
yl)anthracen-9-yl)phenyl)-4,6-diphenyl-1,3,5-triazine (NAPPT), and
2-(2,5-dimethyl-4-(10-(naphthalen-1-yl)anthracen-9-yl)phenyl)-4,6-
diphenyl-1,3,5-triazine (NAXPT), are shown in Scheme 1. 2-chloro-
4,6-diphenyl-1,3,5-triazine was obtained by nucleophilic addition of
a.WCU Hybrid Materials Program, Department of Materials Science and
Engineering, Seoul National University Seoul 151-744, South Korea
b.School of Materials Science and Engineering, Gyeongsang National University,
Jinju 66-701, South Korea.
c.Department of Chemistry and Engineering Research Institute (ERI), Gyeongsang
National University, Jinju 66-701, South Korea
† Electronic Supplementary Informaꢀon (ESI) available: Experimental procedure,
NMR data, TGA and DSC thermograms. See DOI: 10.1039/x0xx00000x
+ K. –H. Kim and J. Y. Baek made equal contribution to this paper.
This journal is © The Royal Society of Chemistry 20xx
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Scheme 1. Synthesis scheme of NAPT, NAPPT, and NAXPT.
phenylmagnesium bromide and 2,4,6,-trichloro-1,3,5-triazine. NAPT
without a linker group was synthesised by a Suzuki coupling
reaction of 4,4,5,5-tetramethyl-2-(10-(naphthalen-1-yl)anthracen-9-
yl)-1,3,2-dioxaborolane and 2-chloro-4,6-diphenyl-1,3,5-triazine.
NAPPT with a phenyl linker group and NAXPT with a xylene linker
group were also synthesised by Suzuki coupling between 4,4,5,5-
tetramethyl-2-(10-(naphthalen-1-yl)anthracen-9-yl)-1,3,2-
dioxaborolane, and 2-(4-bromophenyl)-4,6-diphenyl-1,3,5- triazine
and
2-(4-bromo-2,5-dimethylphenyl)-4,6-diphenyl-1,3,5-triazine,
respectively. The chemical structures of the compounds were
confirmed using a range of spectroscopic methods such as proton
nuclear magnetic resonance (H-NMR), carbon 13 NMR (13C-NMR)
and mass spectroscopy. Material characterisation details are in the
Supporting Information section.
The thermal stabilities of NAPT, NAPPT, and NAXPT were studied
using thermogravimetric analysis (TGA) and differential scanning
calorimetry (DSC) under a nitrogen atmosphere. NAPT, NAPPT, and
NAXPT exhibited decomposition temperatures (Td: corresponding to
5% weight loss) as high as 420°C, 469°C, and 418°C, and glass
transition temperatures around 137°C, 166°C, and 168°C,
respectively. The introduction of a phenyl or a xylene linker group
may increase the rigidity of the materials, leading to good thermal
stability with a high glass temperature.
The absorption and photoluminescence (PL) spectra of the
compounds are shown in Figure 1. The absorption peaks of the
emitters were at 356 nm, 374 nm, and 394 nm for NAPT, and 356
nm, 375 nm, and 396 nm for NAPPT and NAXPT. The three
absorption peaks originate from anthracene absorption, and no
shift was observed in different solvents. The peak wavelengths of
the PL spectra were 460 nm and 497 nm for NAPT, 465 nm for
NAPPT, and 432 nm for NAXPT in tetrahydrofuran. Unlike the trend
observed for absorption properties, the PL spectra for NAPT,
NAPPT, and NAXPT exhibited large spectral shifts in solvents, from
497, 430, and 410 nm in hexane to 515, 500, and 490 nm in
acetone, respectively. This shift is caused by the solvatochromic
effect, indicating that charge transfer (CT) emission is dominant in
the emission mechanism. NAXPT is expected to have the strongest
CT characteristic as it demonstrates the largest spectral shift of the
three compounds. The PL quantum yields of the 50-nm-thick
pristine films were 43 ± 2%, 40 ± 2%, 50 ± 2% and those of 50-nm-
thick CBP films doped with 10 wt% dopants were 48 ± 2%, 38 ± 2%,
45 ± 2% for NAPT, NAPPT, and NAXPT, respectively. These
phenomena are explained in detail below using density functional
theory (DFT). Ground state (S0) geometries were optimised using
DFT calculations and energy levels; frontier orbitals and transition
Figure 1. Absorption and PL spectra of NAPT, NAPPT and NAXPT in
various solvent
energies were calculated using DFT and time-dependent DFT.4 The
calculation methods are
detailed in the methods section and the optimised geometries are
shown in Figure 2. The dihedral angle of the S0 geometry between
anthracene and the adjacent moiety (triazine for NAPT, phenyl for
NAPPT, xylene for NAXPT) was 59.7˚ for NAPT, 78.7˚ for NAPPT, and
89˚ for NAXPT. NAXPT shows the highest dihedral angle among the
three dyes, due to the large steric hinderance of the xylene unit.
The frontier orbitals of the highest occupied molecular orbital
(HOMO) and LUMO in the ground state are shown in Figure 2. The
calculated HOMO levels were 5.20 eV for NAPT, 5.17 eV for NAPPT,
and 5.16 eV for NAXPT, and the calculated LUMO levels were 1.88
eV for NAPT, 1.87 eV for NAPPT, and 1.79 eV for NAXPT. Separation
between the HOMO and LUMO orbital surfaces is facilitated by
phenyl and xylene moiety resulting in CT characteristics. The
electron densities of NAPPT and NAXPT were localised on the
diphenyltriazine unit in the LUMO and on the anthracene unit in the
HOMO. In contrast, the electron density of NAPT was localised on
both anthracene and diphenyltriazine in the LUMO and localised on
anthracene in the HOMO. NAPT exhibits both localised excited (LE)
and CT characteristics. The calculated singlet transition energies
and triplet transition energies were 2.82 eV and 1.70 eV for NAPT,
2.87 eV and 1.73 eV for NAPPT, and 2.99 eV and 1.73 eV for NAXPT,
respectively. As the dihedral angle increases, the singlet transition
energy increases because the linker groups inhibit the extension of
conjugation of the compounds.
The HOMO and LUMO levels of NAPT, NAPPT, and NAXPT were
measured using cyclic voltammetry, and LUMO levels were
calculated using the optical bandgap. The three emitters had similar
HOMO levels, with 5.4 eV for NAPT and 5.35 eV for NAPPT and
NAXPT, because the electron distributions of the three emitters in
the HOMO state are located on anthracene, as shown in Figure 2.
2 | J. Name., 2012, 00, 1-3
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Figure 2. Optimized geometry at ground state of NAPT, NAPPT and
NAXPT and the frontier orbitals of HOMO and LUMO state.
Figure 3. The refractive index and extinction coefficient of (a)
NAPT, (b) NAPPT, (c) NAXPT. (d) Angle-dependent PL intensity of p-
polarized light from the 50-nm-thick film at 480 nm for NAPT, 455
nm for NAPPT, 440 nm for NAXPT.
However, the LUMO levels of NAPT, NAPPT, and NAXPT were 2.48
eV, 2.39 eV and 2.35 eV, respectively due to different electron
distributions in the LUMO state. As the dihedral angle between
anthracene and the adjacent moiety increases, the conjugation
length of LUMO state shortens, resulting in a higher LUMO level.
Thus, NAXPT, with the highest dihedral angle, exhibits the highest
LUMO level, and NAPT with the lowest dihedral angle, exhibits the
lowest LUMO level. As a result, the higher LUMO level of NAXPT
results in a higher singlet transition energy. Transient PL was
measured using a 337-nm N2 sub-picosecond laser and a streak
camera (Figure. S1). NAPT, NAPPT, and NAXPT had mono-
exponential decay times of 5.91 ns, 1.41 ns, and 2.77 ns,
respectively. Delayed fluorescence in transient PL was not observed
for any of the compounds, because the low triplet energy of the
anthracene moiety (1.7–1.8 eV) prohibited reverse intersystem
crossing from the triplet to the singlet state. Photophysical and
electrochemical properties are summarised in Table 1.
The
Θ
was analysed using angle-dependent PL emission profiles
measured with 50-nm-thick organic films on fused silica (Figure 3d–
3f). Birefringence was considered in the theoretical fittings (solid
lines). Details of the measurements and analysis have been
reported previously.3h,6The experimental data fit to the theoretical
calculations with
Θ values of 74%, 89%, and 90% for NAPT, NAPPT,
and NAXPT, respectively, and showed similar trends with VASE
analysis. But variations of θs and Θs of each molecules are
different, implying that the direction of absorption dipoles and
emitting dipoles in molecules are different. The higher preferred
orientation along the horizontal direction of NAXPT and NAPPT
compared with NAPT can be understood by the higher aspect ratio
of the first two molecules compared with NAPT, because their long
axis is more likely to be juxtaposed with the substrate than that of
NAPT.
We fabricated OLEDs to investigate the potential of the three
emitters for use in OLEDs. The devices had the following structure;
Table 1. Photophysical and electrochemical properties of emitters
Band
gap
[eV]
Absorptiona
[nm]
HOMO LUMO
PLb
[nm]
PLQYc
[%]
τd
[ns]
indium
tin
oxide
(ITO)
(70
nm)/
1,1-bis(4-p-
[eV]
5.40
5.35
5.35
[eV]
2.48
2.39
2.35
toluylaminophenyl)cyclohexane (TAPC) (40 nm)/ Tris(4-carbazoyl-9-
ylphenyl)amine (TCTA) (10 nm)/ non doped EML (NAPT, NAPPT, and
NAXPT) (20nm)/ 4,6-(3,5-di-3-pyridylphenyl)-2-methylpyrimi-dine
(B3PYMPM) (35 nm)/ LiF (0.7 nm)/ Al (100 nm). The current
density–voltage–luminance (J–V–L) characteristics of the OLEDs are
shown in Figure 4a. The turn-on voltages of the OLEDs were the
same, at 2.7 V, which is almost identical to the voltage
corresponding to the emitted photons. The emission peak
wavelengths of the NAPT-, NAPPT-, and NAXPT-based OLEDs were
located at 468 nm, 456 nm, and 436 nm, respectively (Figure 4b).
The CIE coordinates (x, y) of NAPT, NAPPT, and NAXPT at 10
mA/cm2 were (0.158, 0.250), (0.149, 0.142), and (0.145, 0.068),
respectively. The NAXPT-based OLED showed the deepest blue
NAPT
NAPPT
NAXPT
357, 375,
396
357, 377,
397
357, 397,
398
2.92
2.96
3.0
497
465
432
43
40
50
5.91
1.41
2.77
aMeasured in a CHCl3 solution at room temperature. PL maximum
of thin film. cPL quantum yields from pristine films. dPL decay
lifetime of thin film. PLQY: photoluminescence quantum yield.
b
We analysed orientation of transition dipoles for the emitters using
two methods: variable angle spectroscopy ellipsometry (VASE) and
angle-dependent PL emission profiles. The ordinary and
extraordinary refractive indices (no, ne) and extinction coefficients colour as expected. Figure 4c shows the EQEs and current
(ko, ke) of 50-nm-thick organic films on a Si substrate are shown in
Figures 3a, b, and c. The no and ne of NAPPT and NAXPT exhibited angle-dependent emission profiles. The maximum EQEs were 6.0%,
larger differences than that of NAPT, indicating that NAPPT and
NAXPT have a larger preferred orientation along the horizontal and 3.9 cd/A for the NAPT, NAPPT, and NAXPT devices, respectively.
efficiencies (CE) of the devices. The EQEs were calibrated with
4.7%, and 6.6%, and the maximum CEs were 11.3 cd/A, 6.0 cd/A,
direction than NAPT. The angle (θ) between the direction of
transition dipole moment and the surface normal direction were
obtained from orientation order parameter of rod-like molecules
(Srod) using ko, ke values of films.5 The θ of NAPT, NAPPT, and NAXPT
are 46.0°, 62.8°, 69.1°, respectively.
To our knowledge, the NAXPT device exhibited the highest EQE in
deep blue non-doped OLEDs with a CIE coordinate of less than
(0.145, 0.068). The larger difference between CE compared with
EQE originates from spectral differences. The EL emission with the
shortest wavelength from the NAXPT-based OLED had lower eye
sensitivity than other compounds; thus, the NAXPT device exhibited
the lowest CE even though it had the highest EQE among the three
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Notes and references
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Figure 4. (a) The current density–voltage–luminance curves of the
OLEDs. (b) The normalized EL spectra of the OLEDs. (c) EQEs and
current efficiencies of OLEDs against current density The EQEs were
calibrated using the angle-dependent emission intensity profiles. (d)
Transient ELs of OLED for NAPT, NAPPT and NAXPT.
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compounds. The maximum EQEs of the OLEDs were calculated
6b
using the classical dipole model based on measured PLQY and
Θ .
The calculated maximum achievable EQEs of devices were 2.9% for
NAPT, 3.7% for NAPPT, and 4.9% for NAXPT assuming that the
fraction of radiative excitons ( rad ) was 25%. These EQE values are
η
much lower than experimental EQEs, indicating that triplet excitons
participate in a radiative process via triplet–triplet annihilation
(TTA) rather than reverse intersystem crossing (RISC). This is
because the large exchange energy of anthracene compounds
prohibits reverse intersystem crossing from the triplet to the singlet
state. The ηrad values were 51.7% for NAPT, 33.0% for NAPPT, and
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periodically applied to the OLED with a frequency of 100 Hz (Figure
4d). All of the transient ELs showed delayed fluorescence caused by
TTA, increasing the ηrad ratio above 25%. Decay lifetimes of delayed
fluorescence of transient ELs are 9.37 μs for NAPT, 8.97 μs for
NAPPT, 8.16 μs for NAXPT. The combined effects of the PLQY,
emitting dipole orientation, and the degree of TTA resulted in
different EQEs for the three devices.
3
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In conclusion, new anthracene-based blue emitters were
developed, composed of anthracene as an electron rich unit and
diphenyltriazine as a strong electron acceptor unit. Interconnecting
units were introduced to the emitters to increase the singlet
transition energy by increasing the LUMO level and enhance the
ratio of horizontal emitting dipoles by lengthening the emitters.
Deep blue emission spectra and high
Θ
of 90% were realised using
the interconnecting units. As a result, we demonstrated a maximum
EQE of 6.6% with CIE coordinates of (0.145, 0.068) with a new
emitter.
This work was supported by the Industrial Strategic Technology
Development Program [10048317] funded by MKE/KEIT and NRF
grant by MSIP (2015R1A2A1A10055620)
Commun., 2014,
J.-H. Lee, J. Y. Baek, Y.-H. Kim, J.-J. Kim, Adv. Optical Mater.,
2015, , 1191-1196.
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4 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/C6CC05076J
Journal Name
(a) J. Frischeisen, D. Yokoyama, C. Adachi, W. Brütting, Appl.
COMMUNICATION
6
Phys. Lett., 2010, 96, 073302. (b) R. R. Chance, A. Prock, R.
Silbey in Advances in Chemical Physics, Vol. 37, Wiley, New
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