Twisted intramolecular charge transfer state for long-wavelength thermally activated delayed fluorescence

Article abstract of DOI:10.1021/cm402428a

Emission wavelength tuning of thermally activated delayed fluorescence from green to orange in solid state films is demonstrated. Emission tuning occurs by stabilization of the intramolecular charge transfer state between a phenoxazine (PXZ) donor unit an

Full text of DOI:10.1021/cm402428a

Article
pubs.acs.org/cm
Twisted Intramolecular Charge Transfer State for Long-Wavelength
Thermally Activated Delayed Fluorescence
Hiroyuki Tanaka,† Katsuyuki Shizu,† Hajime Nakanotani,†,‡ and Chihaya Adachi*,†,‡,§
†Center for Organic Photonics and Electronics Research (OPERA), Kyushu University, 744 Motooka, Nishi, Fukuoka 819-0395,
Japan
‡Innovative Organic Device Laboratory, Institute of Systems, Information Technologies and Nano-Technologies (ISIT), 744
Motooka, Nishi, Fukuoka 819-0395, Japan
§International Institute for Carbon Neutral Energy Research (WPI-I2CNER), Kyushu University, 744 Motooka, Nishi, Fukuoka
819-0395, Japan
S
* Supporting Information
ABSTRACT: Emission wavelength tuning of thermally
activated delayed fluorescence from green to orange in solid
state films is demonstrated. Emission tuning occurs by
stabilization of the intramolecular charge transfer state between
a phenoxazine (PXZ) donor unit and 2,4,6-triphenyl-1,3,5-
triazine (TRZ) acceptor unit separated by a large twist angle.
The emission wavelengths of mono-, bis-, and tri-PXZ-
substituted TRZ exhibit a gradual red shift while maintaining
a small energy gap between the singlet and triplet excited states.
An organic light-emitting diode containing a tri-PXZ-TRZ emitter exhibited a maximum external quantum efficiency of 13.3
0.5% with yellow-orange emission.
KEYWORDS: organic light-emitting diodes, thermally activated delayed fluorescence, emission wavelength tuning,
intramolecular charge transfer
transitions. Their emission wavelength has also been tuned by
extending the conjugation of the ligand.4
INTRODUCTION
■
Since our group reported the first observation of electro-
luminescence (EL) based on thermally activated delayed
fluorescence (TADF) from a Sn4+−porphyrin complex,1 the
potential of TADF materials as emitters for organic light-
emitting diodes (OLEDs) has been revealed.2 A remarkable
feature of TADF is up-conversion of excitons from the lowest
triplet excited state (T1) of a compound to its lowest singlet
excited state (S1), which strongly depends on the energy gap
between them (ΔES‑T). Up-conversion from T1 to S1 can be
realized in molecules with donor−acceptor (D−A) moieties
that induce intramolecular charge transfer (ICT). TADF
materials are currently attracting considerable attention as
third-generation OLEDs because of their high EL efficiency and
lack of rare metals such as Ir and Pt. To replace the present
OLEDs based on fluorescent and phosphorescent materials,
methodology for RGB emission wavelength tuning is essential.
The emission process of general fluorescent materials involves
π−π* transitions via singlet excitons, and emission wavelength
tuning is achieved by controlling the length of π-conjugation.
Attaching substituents such as electron-donating or -with-
drawing groups to fluorescent molecules is also an effective way
to tune emission wavelength.3 Conversely, the emission process
of typical phosphorescent materials such as iridium complexes
is based on triplet metal-to-ligand charge transfer (3MLCT)
In the case of TADF materials, the emission process is
categorized as ICT transitions via triplet excitons. To minimize
ΔES‑T for efficient up-conversion, a TADF molecule needs its
highest occupied molecular orbital (HOMO) and lowest
unoccupied molecular orbital (LUMO) to be effectively
separated, which can be achieved using a twisted structure.
Effective HOMO−LUMO separation induces the ICT
transition from HOMO to LUMO. Regarding the molecular
design of TADF materials, simple extension of the π-conjugated
system results in a large ΔES‑T and weak ICT transition, which
induces a locally excited state and dual fluorescence.5 While
efficient blue2,6 and green2,7 TADF materials have been
fabricated using various molecular designs that suitably connect
the donor and acceptor units, there are currently few examples
of orange-red TADF materials.1,2 The limitation of molecular
design means that, efficient red TADF is undeveloped
compared with blue and green TADF.
Because TADF is a charge transfer (CT) emission from ICT-
type luminescent materials with small ΔES‑T, control of the ICT
state must affect the emission wavelength of TADF. We
Received: July 18, 2013
Revised: September 1, 2013
© XXXX American Chemical Society
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Figure 1. (a) Molecular structures of branched PXZ-TRZ type TADF molecules. One branch, PXZ-TRZ; two branches, bis-PXZ-TRZ; three
branches, tri-PXZ-TRZ. (b,c) Highest occupied and lowest unoccupied natural transition orbitals (HONTO and LUNTO, respectively) of bis-PXZ-
TRZ and tri-PXZ-TRZ, respectively, calculated at TD-wB97X-D/cc-pVDZ level.
recently reported a green TADF emitter, PXZ-TRZ, with a
7b
small ΔES‑T
.
PXZ-TRZ has a typical twisted ICT (TICT)
state,8 with phenoxazine (PXZ) as a donor unit and triphenyl-
triazine (TRZ) as an acceptor unit. As a model molecular
system to control the ICT state of TADF materials, we
designed a branched TICT-type D−A system, in which PXZ
was attached to the para-position of the terminal phenyl rings
of the TRZ acceptor core unit, as shown in Figure 1. In this
article, we demonstrate that the emission wavelength of TADF
can be tuned in the long-wavelength region using the branched
PXZ-TRZ-type TADF materials bis-PXZ-TRZ (D2-A) and tri-
PXZ-TRZ (D3-A).
Figure 2. ORTEP drawing of bis-PXZ-TRZ. Thermal ellipsoids are
drawn at the 50% probability level. A solvent molecule (toluene) is
omitted for clarity.
RESULTS AND DISCUSSION
■
To estimate the frontier molecular orbitals of the designed
molecules, we performed density functional theory (DFT)
calculations. First, the ground-state geometries were optimized
with Gaussian 09 software9 using the ωB97X-D functional10
and cc-pVDZ basis set.11 The geometrical parameters of the
optimized ground state of bis-PXZ-TRZ agreed well with those
derived from its X-ray structure. The molecular structures and
highest occupied and lowest unoccupied natural transition
orbitals (HONTO and LUNTO, respectively)12 for the
optimized ground-state geometries of the molecules are
shown in Figure 1. The optimized geometries of bis-PXZ-
TRZ and tri-PXZ-TRZ have a large twist angle between the
donor and acceptor planes, similar to PXZ-TRZ. The dihedral
angles between PXZ and TRZ units were calculated to be about
82° for bis-PXZ-TRZ and 81° for tri-PXZ-TRZ. Dihedral
angles estimated from X-ray structure analysis of the
compounds were about 88°, which agreed with the calculated
values. An ORTEP (Oak Ridge Thermal-Ellipsoid Plot
Program) drawing showing the twisted structure of bis-PXZ-
TRZ is depicted in Figure 2 (see Supporting Information for
details). The phenyl rings of the TRZ acceptor core unit were
almost coplanar. The twist between PXZ and TRZ units
suppressed the delocalization of electron density distribution of
the HONTO and LUNTO. The HONTOs are mainly
distributed on the PXZ moieties, while the LUNTO is localized
on the TRZ moiety. The molecular structure of bis-PXZ-TRZ
determined by X-ray analysis almost corresponded to the
optimized structure estimated from the DFT calculation. There
was a small overlap of HONTO and LUNTO at the phenyl
ring connecting PXZ and TRZ. To investigate the low-lying
excited states of the molecules, we calculated the 10 lowest
singlet and triplet excited states for their optimized ground-
state geometries by time-dependent DFT13,14 (TD-DFT)
calculations using the same functional and basis set (denoted
as TD-ωB97X-D/cc-pVDZ). As expected from the electron
density distribution of the frontier molecular orbitals and the
results of X-ray crystal structure analysis, the estimated ΔES‑T
values were quite small (those of bis-PXZ-TRZ and tri-PXZ-
TRZ were 0.054 and 0.065 eV, respectively). These results
indicate that the twist between PXZ and TRZ moieties induced
effective separation of HONTO and LUNTO and TICT
excited states, similar to PXZ-TRZ with a small ΔES‑T of 0.070
eV (see Supporting Information for details).
To investigate the effect of attaching different numbers of
PXZ donor units to a TRZ acceptor core unit on emission
wavelength, we measured UV−vis absorption and photo-
luminescence (PL) spectra of the compounds in toluene
solution (Figure 3). The absorption spectra of the three
molecules exhibited CT absorption between 365 and 500 nm
derived from electron transfer from the HOMO to LUMO.
The intensity of the CT absorption gradually increased and the
maximum also red-shifted as the number of PXZ donor units
increased. We estimated the HOMO−LUMO gaps of the three
compounds from the onset positions of their absorption spectra
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The dependence on solvent polarity decreased as the number
of PXZ donor units increased. This result indicates that the
ICT state of tri-PXZ-TRZ is the most stable among the three
compounds and enhancement of the donor property of PXZ-
TRZ stabilizes the ICT state. In addition, the stabilization of
the ICT state induces the broadening of the CT emission (see
Supporting Information).
The small ΔES‑T values of the three compounds allow
efficient up-conversion of excitons from the T1 to the S1 state
through reverse intersystem crossing (RISC) and increase their
emission quantum yield. The up-conversion phenomenon can
be observed as a delayed fluorescence component with a
lifetime of micro- to millisecond order after a nanosecond-order
prompt fluorescence component. To study the effect of an
increased number of PXZ donor units on PL lifetime, we
observed the transient PL decay profiles of the three
compounds in toluene solution. It is known that the triplet
excitons of TADF materials are inactivated by included triplet
oxygen molecules (3O2). Therefore, lifetime profiles were
measured after deoxygenation of the solutions by bubbling
nitrogen through them. The PL quantum yields (ϕPL) of the
three compounds increased from ca. 12−14% (prompt
component only) to ca. 23−29% (prompt and delayed
components) upon deoxygenation. The transient PL decay
profiles of the three compounds are depicted in Figure 5. The
Figure 3. Normalized absorption (dashed lines) and PL (solid lines)
spectra of PXZ-TRZ (green), bis-PXZ-TRZ (yellow), and tri-PXZ-
TRZ (orange) in toluene solution with a concentration of 1.0 × 10−5
M.
(see Supporting Information for details). The HOMO−LUMO
gaps gradually decreased as the number of PXZ donor units
increased. These results indicate that the ICT state is stabilized
by an increased number of PXZ donor units. The emission
maxima and color for mono-, bis-, and tri-PXZ-TRZ were 545
nm (green), 560 nm (yellow), and 568 nm (yellow-orange),
respectively. In particular, the half-width of the emission
spectrum increased considerably with the attachment of PXZ
donor units. Although the onset of the emission spectra of
these three compounds was almost the same, the half-width of
emission increased from 94 nm for PXZ-TRZ to 120 nm for tri-
PXZ-TRZ. This broadening indicates that ICT is enhanced as
the number of PX donor units increases, which was observed as
an increase of the intensity of the CT absorption.
Generally, the photophysical properties of molecular systems
with an ICT state exhibit solvatochromism, in which the
emission wavelength undergoes a large red shift as the polarity
of the solvent increases.15 To verify the stability of the ICT
states of the three compounds, we studied their photophysical
properties in various solvents. The emission maxima exhibited a
gradual red shift as the polarity of the solvent increased. The
emission maxima (νmax, in wavenumber) are plotted against the
solvent polarity parameter ET(30) in Figure 4.15 The
compounds exhibited positive solvatochromism, with emission
maxima depending approximately linearly on solvent polarity.
Figure 5. Transient PL decay profiles of PXZ-TRZ (green), bis-PXZ-
TRZ (yellow), and tri-PXZ-TRZ (orange) in toluene solution at a
concentration of 1.0 × 10−5 M.
transient decay profiles obviously had two components. The
first was assigned to the prompt component derived from the
direct S1 → S0 transition, which has a lifetime (τ1) of ca. 17 ns.
The second can be assigned to the delayed component
resulting from the recursive S1 → S0 transition via successive
RISC and up-conversion of the excitons from the T1 state,
which exhibited a lifetime (τ2) that lengthened from 0.68 μs for
PXZ-TRZ to over 1 μs for bis- and tri-PXZ-TRZ. This behavior
can be explained by considering the half-width of CT emission
from the compounds in toluene solutions. The half-width of
PXZ-TRZ emission was the narrowest among the three
compounds. The narrow width of the CT emission indicates
the ICT excited state was localized on the single D−A
structure, which results in the shortest lifetime (τ2 is 0.68 μs)
among the three compounds. The half-widths of bis- and tri-
PXZ-TRZ were wider than that of PXZ-TRZ (Figure 3),
indicating that the ICT excited state was delocalized over the
PXZ donor units and TRZ acceptor core unit in these
Figure 4. Dependence of the emission maxima for PXZ-TRZ (green),
bis-PXZ-TRZ (yellow), and tri-PXZ-TRZ (orange) on the solvent
polarity parameter ET(30).
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compounds. Such delocalization of the ICT state over the
multiple D−A structures stabilizes it.16 The energy of the
delocalized ICT excited state is changed by the torsional
disorder between the PXZ donor units and the TRZ acceptor
unit, which lengthen the lifetime (τ2 were 1.33 and 1.10 μs for
bis- and tri-PXZ-TRZ, respectively). However, τ2 of tri-PXZ-
TRZ was shorter than that of bis-PXZ-TRZ. This is attributed
to increased vibrational nonradiative deactivation of the
excitons of tri-PXZ-TRZ via solvent molecules compared to
those of bis-PXZ-TRZ. To verify the thermal activation of the
observed transient PL decay profiles, we investigated the
temperature dependence of the PL spectrum of 6 wt % doped
films of bis- and tri-PXZ-TRZ in 3,3′-bis(N-carbazolyl)-1,1′-
biphenyl (mCBP) as a host using a streak camera (Figures S4−
S9, Supporting Information). As the temperature was increased
from 10 K, the ϕPL of the delayed components of bis- and tri-
PXZ-TRZ gradually increased and reached 50% and 40% at 300
K, respectively. These results confirm that bis- and tri-PXZ-
TRZ are TADF materials.
OLED characteristics (see Supporting Information for details).
Although the ϕPL of 6 wt % bis- and tri-PXZ-TRZ doped films
in mCBP were 64 and 58%, respectively, the EQE values
showed a large difference of near 4%. From the estimations, the
reason can be attributed to a difference of γηout, where γ is the
ratio of the charge injection to the electron and hole
transportation and ηout is the out-coupling constant. The γηout
values of bis- and tri-PXZ-TRZ were 0.153 and 0.260,
respectively. These estimated values indicate a morphological
difference of the 6 wt % bis- and tri-PXZ-TRZ doped films in
mCBP. However, these results substantially exceed the
theoretical limit of general fluorescent materials (5−7%
assuming an outcoupling efficiency of 0.2−0.3). These high
EQEs are attributed to the efficient up-conversion of triplet
excitons to the S1 state, as for PXZ-TRZ.7b Figure 7 shows the
We fabricated OLEDs containing bis- or tri-PXZ-TRZ as
TADF emitters with structures of indium tin oxide (ITO)/
N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,10-biphenyl-4,4′-dia-
mine (α-NPD) (35 nm)/6 wt % bis- or tri-PXZ-TRZ:mCBP
(15 nm)/1,3,5-tris(2-N-phenylbenzimidazolyl) benzene (TPBi)
(65 nm)/LiF (0.8 nm)/Al (80 nm). The HOMO and LUMO
levels of bis- and tri-PXZ-TRZ were estimated to be 5.7 and 3.4
eV, respectively, from the UV−vis absorption spectra of neat
films and work function measurements. To confine generated
excitons in the emitting layer, mCBP was selected as the host
material because it has a wider HOMO−LUMO gap than bis-
and tri-PXZ-TRZ (Figures S10 and S11, Supporting
Information). The OLED characteristics of bis- and tri-PXZ-
TRZ are shown as yellow and orange plots in Figure 6,
Figure 7. Normalized EL spectra from the OLEDs containing PXZ-
TRZ (green), bis-PXZ-TRZ (yellow), or tri-PXZ-TRZ (orange).
EL spectra from the OLEDs containing mono-, bis-, or tri-PXZ-
TRZ. The EL spectra of bis- and tri-PXZ-TRZ almost
overlapped and yellow-orange EL spectra were observed at
around 552 nm. For the EL spectra, the half-width slightly
increased from 92 (PXZ-TRZ) to 98 nm (tri-PXZ-TRZ) and
the maximum red-shifted from 529 (PXZ-TRZ) to 553 nm (tri-
PXZ-TRZ) as the number of PXZ donor units increased.
CONCLUSIONS
■
We tuned the emission wavelength of TADF materials from
green to orange using branched PXZ-TRZ-type molecules with
small ΔES‑T. Importantly, the branched PXZ-TRZ molecules
maintained high EQE of around 9−13%. Enhancement of the
donor property by increasing the number of PXZ donor units
induces the delocalization of the ICT state over the multiple
D−A structures. This stabilizes the ICT state, causing the CT
emission to exhibit a red shift and broaden. These results offer a
powerful methodology to realize efficient red TADF emitters,
which differs from the tuning strategy of fluorescent and
phosphorescent materials of extending the conjugated system.
Effective modification of the frontier orbitals to achieve red
TADF is in progress.
Figure 6. Dependence of EQE on current density for PXZ-TRZ
(green), bis-PXZ-TRZ (yellow), and tri-PXZ-TRZ (orange).
respectively. For the OLED containing tri-PXZ-TRZ, the
external quantum efficiency (EQE) vs. current density plots
exhibited a maximum EQE of 13.3 0.5%, and the dependence
of EQE on the current density behaved similar to that of PXZ-
TRZ as shown by green and orange plots. In contrast to mono-
and tri-PXZ-TRZ, the EQE vs. current density plots of bis-
PXZ-TRZ exhibited a maximum EQE of 9.1 0.5%, and the
value was lower than those of mono- and tri-PXZ-TRZ. On the
basis of the results of the temperature-dependence of the
transient PL spectra of bis- and tri-PXZ-TRZ, we estimated the
EXPERIMENTAL SECTION
■
General Procedures. Starting materials were purchased from
Wako Pure Chemical Industries, Ltd., Tokyo Chemical Industry Co.,
Ltd., and Sigma-Aldrich Co. and used without further purification,
unless otherwise stated. UV−vis spectra were recorded on a Shimadzu
UV−vis absorption spectrometer UV2550 for the solution states and
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the solid states. Fluorescence spectra were recorded on a Horiba Jovan
Yvon FluoroMax-4 spectrometer for the solution states and the solid
states. PL quantum yield (ϕPL) in the solution states were measured
with a Hamamatsu absolute PL quantum yield spectrometer C11347
(Quantaurus-QY). Transient PL decay profiles in the solution states at
ambient temperature were measured with Hamamatsu compact
fluorescence lifetime spectrometer C11367 (Quantaurus-Tau). The
temperature dependence of the transient PL decay profiles was
measured with a Hamamatsu streak camera system C4334 equipped
with an IWATANI cryostat GASESCRT-006-2000. A Lasertechnik
Berlin nitrogen gas laser MNL200 with an excitation wavelength of
337 nm was used. The codeposited films were fabricated by a thermal
deposition. The OLED characteristics were evaluated at ambient
temperature using an Agilent semiconductor parameter analyzer
E5273A. NMR spectra used in the characterization of products were
recorded on a JEOL-ECP400 400 MHz spectrometer. All NMR
spectra were referenced to solvent. Matrix-assisted laser desorption
ionization time-of-flight mass spectrometries (MALDI-TOF-MS) were
recorded on a Bruker Daltonics Autoflex III spectrometer using
positive mode.
2,4-Bis(4-bromophenyl)-6-phenyl-1,3,5-triazine. A mixture of
1.55 g of benzoyl chloride (11.0 mmol) and 4.00 g of 4-
bromobenzonitrile (22.0 mmol) in 15 mL of dichloromethane was
cooled at 0−5 °C by ice-bath and stirred for 30 min. Then, 3.30 g of
antimony chloride (11.0 mmol) was added dropwise to the above
solution. The mixture was stirred at room temperature for one hour
and further stirred and refluxed overnight. The cooled mixture was
filtrated, and the collected yellow solid was washed by dichloro-
methane. The solid was slowly added to 75 mL of 28% ammonia
solution cooled at 0−5 °C by ice-bath and stirred for 30 min at same
temperature. Then the mixture was stirred for 3 h at room
temperature. Subsequently, the mixture was filtrated, and the collected
white solid was washed by water. The solid was added to 30 mL of
N,N′-dimethylformamide and stirred at 155 °C for 30 min. The
insoluble solid was separated by the filtration. The filtration was
repeated 3 times. Then the solvent was removed under vacuum, and
2.55 g of 2,4-bis(4-bromophenyl)-6-phenyl-1,3,5-triazine was obtained
as a white solid. The yield was over 50%. This material was used at
next step without a further purification. [NMR] 1H NMR (CDCl3, 300
MHz) δ = 7.47 (m, 3H), 7.70 (d, 4H), 8.62 (d, 4H), 8.73 (d, 2H); 13C
NMR (CDCl3, 300 MHz) δ = 113.3, 115.7, 121.9, 123.3, 127.0, 129.2,
129.7, 131.8, 131.9, 133.7, 142.4, 144.0. [MS] MALDI-TOF-MS m/z
calcd for C21H13Br2N3, 467; found, 467.
2,4-Bis(4-(10H-phenoxazin-10H-yl)phenyl)-6-phenyl-1,3,5-
triazine (bis-PXZ-TRZ). To a solution of 1.40 g of 2,4-bis(4-
bromophenyl)-6-phenyl-1,3,5-triazine (3.00 mmol), 1.21 g of phenox-
azine (6.60 mmol), and 2.74 g of potassium carbonate (19.8 mmol) in
50 mL of toluene was added, with stirring, a solution of 44.9 mg of
palladium (II) acetate (0.20 mmol) and 148 mg of tri-tert-
butylphosphine (0.73 mmol) in 50 mL of toluene. Subsequently, the
mixture was stirred and refluxed for one day. The cooled mixture was
partitioned between chloroform and water. The organic layer was
separated, and the aqueous layer was extracted with chloroform. The
combined organic layers were washed with brine, dried over Mg2SO4,
and concentrated in vacuo. Column chromatography of the residue
solid (eluent, chloroform) afforded 1.69 g of bis-PXZ-TRZ as a yellow
solid. The yield was over 84%. This material was further purified by
sublimation under reduced pressure for OLED fabrication. [NMR] 1H
NMR (CDCl3, 300 MHz) δ = 6.05 (d, 4H), 6.62 (t, 4H), 6.68 (t, 4H),
6.72 (d, 4H), 7.58 (m, 7H), 8.81 (d, 2H), 9.00 (d, 4H); 13C NMR
(CDCl3, 300 MHz) δ = 113.3, 115.7, 121.7, 123.3, 131.2, 131.8, 133.9,
143.4, 144.0, 171.3. [MS] MALDI-MS m/z Calcd for C45H29N5O2,
671; found, 671. [Element analysis] Calcd for C45H29N5O2: C, 80.46;
H, 4.35; N, 10.43. Found: C, 80.17; H, 4.20; N, 10.37.
hydroxide. Then, 50 mL of mixed solvent of chloroform/acetone =
50/50 was added to the above mixture, the organic layer was
separated, and the aqueous layer was extracted with the mixed solvent.
The combined organic layers were washed with brine, dried over
Mg2SO4, and concentrated in vacuo. Then, 3.34 g of 2,4,6-tri(4-
bromophenyl)-1,3,5-triazine was obtained as a white solid. The yield
was over 94%. This material was used in the next step without further
1
purification. [NMR] H NMR (CDCl3, 300 MHz) δ = 7.70 (d, 6H),
8.60 (d, 6H); 13C NMR (CDCl3, 300 MHz) δ = 120.0, 128.3, 128.7,
129.0, 130.9, 132.4, 135.2, 136.1. [MS] MALDI-MS m/z Calcd for
C21H12Br3N3, 546; found, 546.
2,4,6-Tri(4-(10H-phenoxazin-10H-yl)phenyl)-1,3,5-triazine
(tri-PXZ-TRZ). To a solution of 1.09 g of 2,4,6-tri(4-bromophenyl)-
1,3,5-triazine (2.00 mmol), 1.22 g of phenoxazine (6.60 mmol), and
2.74 g of potassium carbonate (19.8 mmol) in 60 mL of toluene was
added, with stirring, a solution of 45.0 mg of palladium (II) acetate
(0.20 mmol) and 148 mg of tri-tert-butylphosphine (0.73 mmol) in 60
mL of toluene. Subsequently, the mixture was stirred and refluxed for
one day. The cooled mixture was partitioned between chloroform and
water. The organic layer was separated, and the aqueous layer was
extracted with chloroform. The combined organic layers were washed
with brine, dried over Mg2SO4, and concentrated in vacuo. Column
chromatography of the residue solid (eluent, chloroform/hexane =
1:1) afforded 1.65 g of tri-PXZ-TRZ. The yield is over 97%. This
material was further purified by sublimation under reduced pressure
for OLED fabrication. [NMR] 1H NMR (CDCl3, 300 MHz) δ = 6.06
(d, 6H), 6.63 (t, 6H), 6.69 (t, 6H), 6.73 (d, 6H), 7.60 (d, 6H), 9.01 (d,
6H); 13C NMR (CDCl3, 300 MHz) δ = 113.3, 115.6, 121.7, 123.3,
128.8, 129.1, 131.1, 131.7, 132.9, 133.9, 136.1, 143.2, 144.0, 171.1.
[MS] MALDI-MS m/z Calcd for C57H36N6O3, 853; found, 853.
[Element analysis] Calcd for C57H36N6O3: C, 80.27; H, 4.25; N, 9.85.
Found: C, 80.34; H, 4.07; N, 9.82.
ASSOCIATED CONTENT
* Supporting Information
■
S
Synthetic routes and additional photophysical properties of bis-
PXZ-TRZ and tri-PXZ-TRZ discussed in the text. CCDC
937721 for bis-PXZ-TRZ. This material is available free of
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*(C.A.) E-mail: adachi@opera.kyushu-u.ac.jp.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research was supported by the Funding Program for
World-Leading Innovative R&D on Science and Technology
(FIRST Program) and the International Institute for Carbon
Neutral Energy Research (WPI-I2CNER), sponsored by the
Japanese Ministry of Education, Culture, Sports, Science and
Technology. We acknowledge Ms. H. Nomura and Ms. N.
Nakamura for performing TG-DTA measurements and
sublimation and Mr. T. Matsumoto for X-ray crystal structure
analysis. Computations were partly carried out using the
computer facilities at the Research Institute for Information
Technology, Kyushu University.
REFERENCES
■
(1) Endo, A.; Ogasawara, M.; Takahashi, A.; Yokoyama, D.; Kato, Y.;
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