Dicarbazolyldicyanobenzenes as thermally activated delayed fluorescence emitters: Effect of substitution position on photoluminescent and electroluminescent properties

Article abstract of DOI:10.1246/cl.130907

We demonstrate two efficient blue-green thermally activated delayed fluorescence (TADF) compounds comprising a dimeric phenylcarbazole and four cyano substituents on the phenyl rings. A comparison of the 2,6-dicyano- substituted derivative (26IPNDCz) with the 3,5-dicyano-substituted derivative (35IPNDCz) shows that 26IPNDCz provides a larger dihedral angle and a lower decrease in the energy difference between the first singlet and triplet excited states (ΔE_ST) and the TADF lifetime. An organic light-emitting diode based on 26IPNDCz afforded an external quantum efficiency of 10% with reduced efficiency roll-off.

Full text of DOI:10.1246/cl.130907

Received: October 8, 2013 | Accepted: November 7, 2013 | Web Released: November 13, 2013
CL-130907
Dicarbazolyldicyanobenzenes as Thermally Activated Delayed Fluorescence Emitters:
Effect of Substitution Position on Photoluminescent and Electroluminescent Properties
Bo Li,1 Hiroko Nomura,1 Hiroshi Miyazaki,1,2 Qisheng Zhang,1 Kou Yoshida,1
Yoshinori Suzuma,3 Akihiro Orita,*3 Junzo Otera,3 and Chihaya Adachi*1
1Center for Organic Photonics and Electronics Research (OPERA), Kyushu University,
744 Motooka, Nishi-ku, Fukuoka 819-0395
2Functional Materials Laboratories, Nippon Steel & Sumikin Chemical Co., Ltd.,
46-80 Nakabaru, Sakinohama, Tobata, Kitakyushu, Fukuoka 804-8503
3Department of Applied Chemistry, Okayama University of Science, 1-1 Rida-cho, Kita-ku, Okayama 700-0005
(E-mail: adachi@opera.kyushu-u.ac.jp)
We demonstrate two efficient blue-green thermally activated
delayed fluorescence (TADF) compounds comprising a dimeric
phenylcarbazole and four cyano substituents on the phenyl rings.
of TADF emitters in detail and further demonstrate molecular
design principles.
According to the Boltzmann statistics, a small ¦EST is
essential for the efficient thermally activated repopulation of
the emissive S1 state; this also governs the TADF lifetime. To
achieve a small ¦EST, a small overlap between the occupied and
unoccupied orbitals involved in the excitation is necessary,8
although the T1 state of such a charge-transfer (CT) compound
A
comparison of the 2,6-dicyano-substituted derivative
(26IPNDCz) with the 3,5-dicyano-substituted derivative
(35IPNDCz) shows that 26IPNDCz provides a larger dihedral
angle and a lower decrease in the energy difference between the
first singlet and triplet excited states (¦EST) and the TADF
lifetime. An organic light-emitting diode based on 26IPNDCz
afforded an external quantum efficiency of 10% with reduced
efficiency roll-off.
3
may not undergo CT in some cases.9,10 A comparison with the
carbazolyldicyanobenzene (CDCB) derivatives mentioned
above shows that the current phenylcarbazole- and triphenyl-
amine-based blue and red TADF emitters have relatively small
dihedral angles between their donor and acceptor moieties,
leading to a less efficient separation of their HOMO and
LUMO.7a,7b To better understand the influence of the twisting
angle in D-A compounds on their TADF lifetime and electro-
luminescent (EL) performance, we designed and synthesized
two related bicarbazolylyl dicyanobenzene derivatives: 9,9¤-
bis(3,5-dicyanophenyl)-3,3¤-bicarbazolyl (35IPNDCz) and 9,9¤-
bis(2,6-dicyanophenyl)-3,3¤-bicarbazolyl (26IPNDCz). For both
Organic light-emitting diodes (OLEDs) have attracted
much interest recently because of their potential application
in flat-panel displays and light sources. In OLEDs containing
only fluorescent dyes, the internal quantum efficiency (IQE) is
limited to 25% because of the exciton branching ratio of singlet
states. Phosphorescent OLEDs containing noble-metal-based
phosphors can harvest the remaining 75% triplet excitons and
can thus achieve 100% internal quantum efficiency.1,2 However,
the drawback of phosphorescent OLED materials is their cost.
Recent progress in OLEDs development has focused on low-
cost solutions to improve the device efficiency. A promising
way is to harvest triplet excitons in fluorescent OLEDs by
triplet-triplet annihilation (TTA) or thermally activated delayed
fluorescence (TADF). TTA comprises intermolecular fusion, in
which one singlet and one ground state can be produced from
two triplet states. Thus, the upper IQE limit of OLEDs made
using TTA can increase to 63% (0.25 + 0.75 © 0.5). Assuming
that the optical outcoupling is 0.20, the maximum external
quantum efficiency (EQE) is estimated to be 13%.3 In 2009, the
EQE of TTA devices was reported to be 11%, which is very
close to the theoretical limit.3 On the other hand, triplets can
also be converted to singlets by reverse intersystem crossing
(RISC) under thermal activation when the energy difference
between the first singlet (S1) and triplet (T1) excited states
(¦EST) is small enough.4,5 In 2012, our group demonstrated that
an OLED with a green TADF emitter composed of a carbazole
donors (D) and a dicyanobenzene acceptor (A) can achieve an
EQE as high as 19%, corresponding to an IQE of nearly
100%.6 At the same time, pure blue and orange-red TADF
OLEDs have also achieved high EQEs of 10% and 17.5%,
respectively.7a,7b However, their EQE roll-off at high current
density is rather significant because of their long TADF
lifetimes. We, thus, investigated the photophysical mechanism
3
these compounds, their T1 states were found to be CT states,
and, therefore, the influence of a locally excited triplet state
(3LE) can be excluded from the following discussion.
The synthesis of 35IPNDCz and 26IPNDCz was accom-
plished by the procedures outlined in Scheme 1. The interaction
between 9H-carbazoles with FeCl3 in a CHCl3 solution was used
to obtain the starting material 3,3¤-bicarbazolyl.11 35IPNDCz
Scheme 1. Synthesis of 35IPNDCz and 26IPNDCz.
Chem. Lett. 2014, 43, 319–321 | doi:10.1246/cl.130907
© 2014 The Chemical Society of Japan | 319

Figure 1. The HOMO and LUMO of 35IPNDCz and
26IPNDCz calculated at the B3LYP/6-31G* level.
was obtained in excellent yield by a nucleophilic substitution
reaction of commercially available 5-chloroisophthalonitrile and
3,3¤-bicarbazolyl using potassium carbonate as a catalyst and
DMF as the solvent. 26IPNDCz was obtained by a similar
reaction between 2-chloroisophthalonitrile12 and 3,3¤-bicarba-
zolyl. Their crude products were purified by column chroma-
tography and then by multiple sublimations to afford the final
products as white powders. The complete synthesis and
structural characterization of 35IPNDCz and 26IPNDCz are
reported in the Supporting Information.13
Figure 2. Absorption and emission spectra of 35IPNDCz and
26IPNDCz in toluene.
To better understand the nature of the electronic excited
states of 35IPNDCz and 26IPNDCz, their ground-state geo-
metries were optimized using density functional theory (DFT) at
the B3LYP/6-31G* level. As shown in Figure 1, their HOMO
and LUMO are efficiently separated. The electronic spatial
distribution of the HOMO is mainly distributed over the
bicarbazolyl moieties, while that of the LUMO is completely
localized over the dicyanobenzene moieties. This indicates a
charge-transfer character for these A-D-A-type compounds. As
mentioned previously, the large separation between HOMO and
LUMO minimizes the singlet-triplet splitting energy of the
HOMO-LUMO transition, providing a promising way for an
Figure 3. Decay spectra of 35IPNDCz and 26IPNDCz in
degassed toluene at room temperature.
process. The transient PL characteristics of 35IPNDCz and
26IPNDCz in oxygen-free toluene at RT are shown in Figure 3.
Both compounds show two-component decays. A fast decay
with a lifetime of 24 ns for 35IPNDCz and 28 ns for 26IPNDCz
can be assigned to normal fluorescence, and a slow component
with a lifetime of 145 ¯s for 35IPNDCz and 9.2 ¯s for
26IPNDCz corresponds with the TADF decay, which cannot
be observed in air-equilibrated solution. By comparing the
integral of each emission components in the transient decay
spectra, the PLQY of fluorescence and TADF were determined
to be 0.26 and 0.24 for 35IPNDCz, and 0.24 and 0.48 for
26IPNDCz, respectively, in oxygen-free toluene at RT. The
relatively low PLQY and lifetime (14 ns for both) of the
fluorescence component in air-equilibrated toluene can be
interpreted as singlet excited-state quenching by dissolved
oxygen owing to the long fluorescent lifetime of these two
charge-transfer compounds.16-18
efficient energy up-conversion from the CT to CT states.8,14
The calculated results also demonstrate that the twisting angle
between the carbazole and dicyanobenzene planes in 26IPNDCz
(ca. 69°) is significantly larger than that in 35IPNDCz (ca. 50°).
This leads to a more adequate HOMO-LUMO separation. The
amount of CT (q) used to quantify the degree of HOMO-LUMO
separation was calculated to be 0.85 and 0.88 for 35IPNDCz and
26IPNDCz, respectively, by an orbital composition analysis
method reported previously.9,10
3
1
The photophysical properties of 35IPNDCz and 26IPNDCz
were investigated in toluene at room temperature (RT). As
shown in Figure 2, the broad and structureless emission
spectrum was centered at 470 nm for 35IPNDCz and 488 nm
for 26IPNDCz; this can be attributed to intramolecular CT from
the bicarbazolyl donor to the dicyanobenzene acceptors, as
1
revealed by the DFT results. The zero-zero energy of the CT
transition was approximately 3.0 eV for 35IPNDCz and 2.9 eV
For device-application purposes, the photophysical proper-
ties of 35IPNDCz and 26IPNDCz doped into bis[2-(diphenyl-
phosphino)phenyl]ether oxide (DPEPO) films at a concentration
of 10 wt % was also investigated. DPEPO was selected as a host
because of its excellent triplet energy confining ability for both
the phosphors and the TADF emitters.7a,15 The emission spectra
of these two compounds in a DPEPO film (300 K) are very close
for 26IPNDCz, as determined from their emission onset.9,15 The
1
corresponding CT bands in the absorption spectra appear as a
shoulder around 350-400 nm (Figure 2). The photolumines-
cence quantum yield (PLQY) of 35IPNDCz increases from 0.15
to 0.50 after oxygen degassing by nitrogen bubbling. The PLQY
of 26IPNDCz changes dramatically from 0.12 to 0.72 during this
320 | Chem. Lett. 2014, 43, 319–321 | doi:10.1246/cl.130907
© 2014 The Chemical Society of Japan

Figure 5. EQE-current density characteristics of the OLEDs
based on 35IPNDCz and 26IPNDCz; (inset) their electro-
luminescence spectra.
Figure 4. Transient photoluminescence spectra of 35IPNDCz
and 26IPNDCz doped in DPEPO at a concentration of 10 wt %
at 5 K as measured by a streak camera under a vacuum of
approximately 3 Pa.
phenyl rings, the twisting angle between the D-A moieties could
be adjusted. Ortho-substitution was found to lead to a larger
twisting angle because of steric hindrance leading to a more
efficient separation of the HOMO and LUMO (i.e., 26IPNDCz).
Although the TADF phenomenon was observed for both bipolar
compounds, the compound with the larger twisting angle
afforded a smaller ¦EST, shorter TADF lifetime, and reduced
EL efficiency roll-off at high current density.
to those in toluene solution. The PLQY of 35IPNDCz and
26IPNDCz in the DPEPO films are 0.58 and 0.72, respectively,
and these are also close to the values obtained in toluene
solution. The low-temperature (5 K) transient spectra of the
doped films are shown in Figure 4. The shapes of their
phosphorescence spectra are similar to their fluorescence spectra,
indicating the 3CT nature of their T1 state. The ¦EST value
determined from the difference between the fluorescence and
phosphorescence onset are 0.14 and 0.06 eV for 35IPNDCz and
26IPNDCz, respectively. This difference in ¦EST is in accord-
ance with their different CT behaviors as revealed by DFT. This
can be used to explain their different TADF lifetimes as
mentioned above.
Using 10 wt % 35IPNDCz- or 26IPNDCz-doped DPEPO
films as emitting layers (EML), two multilayer OLEDs were
fabricated with a configuration of ITO/α-NPD (30 nm)/mCP
(10 nm)/EML (15 nm)/TPBi (60 nm)/LiF (0.8 nm)/Al (100
nm), where α-NPD, mCP, and TPBI represent N,N¤-diphenyl-
N,N¤-bis(1-naphthyl)-1,1¤-biphenyl-4,4¤-diamine, 1,3-bis(9-car-
bazolyl)benzene, and 1,3,5-tris(N-phenylbenzimidazol-2-yl)ben-
zene, respectively. Their EQE versus current density character-
istics and EL spectra are shown in Figure 5.
The EL maxima of the 35IPNDCz- and 26IPNDCz-based
devices were at 487 and 501 nm, respectively. These are slightly
red-shifted compared with the PL maxima of the ETLs. At a low
current density of 0.05 mA cm¹2, the EQEs of the 35IPNDCz-
and 26IPNDCz-based devices were 9.2% and 9.6%, respective-
ly, and these values are significantly higher than the theoretical
limitation (5%) for fluorescent OLEDs. With an increase in
current density, the EQE of the 35IPNDCz-based device
decreased dramatically and was 1.4% at 100 mA cm¹2. This
occurs because the long TADF lifetime contributes to the
saturation of the emissive sites as well as triplet-triplet
annihilation at high current density. Because the TADF lifetime
of 26IPNDCz is only one-sixteenth that of 35IPNDCz, the
This work was partly supported by the Funding Program for
World-Leading Innovative R&D on Science and Technology
(FIRST).
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Chem. Lett. 2014, 43, 319–321 | doi:10.1246/cl.130907
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