Luminous butterflies: Efficient exciton harvesting by benzophenone derivatives for full-color delayed fluorescence OLEDs

Article abstract of DOI:10.1002/anie.201402992

Butterfly-shaped luminescent benzophenone derivatives with small energy gaps between their singlet and triplet excited states are used to achieve efficient full-color delayed fluorescence. Organic light-emitting diodes (OLEDs) with these benzophenone deri

Full text of DOI:10.1002/anie.201402992

Angewandte
Chemie
DOI: 10.1002/anie.201402992
Hyperfluorescence
Luminous Butterflies: Efficient Exciton Harvesting by Benzophenone
Derivatives for Full-Color Delayed Fluorescence OLEDs**
Sae Youn Lee, Takuma Yasuda,* Yu Seok Yang, Qisheng Zhang, and Chihaya Adachi*
Abstract: Butterfly-shaped luminescent benzophenone deriv-
atives with small energy gaps between their singlet and triplet
excited states are used to achieve efficient full-color delayed
fluorescence. Organic light-emitting diodes (OLEDs) with
these benzophenone derivatives doped in the emissive layer can
generate electroluminescence ranging from blue to orange–red
and white, with maximum external quantum efficiencies of up
to 14.3%. Triplet excitons are efficiently harvested through
delayed fluorescence channels.
organometallic materials, as the strong spin-orbit coupling
of the heavy-metal center enhances intersystem crossing
(ISC). This results in nearly 100% electron-to-photon con-
version efficiency in phosphorescent OLEDs.[5] However,
these phosphorescent materials suffer from instability in
practical applications, especially for blue and white emission.
The expense and rarity of precious-metal complexes also
limits their cost effectiveness and long-term mass production.
Realizing high-efficiency RGB electroluminescence (EL)
through a simple organic system while aiming to reduce
production costs and simplify manufacturing processes is
pivotal for next-generation OLED technology.
We recently explored an alternative approach to harvest
excited triplet energy through upconversion from the T1 to S1
states by thermal activation, giving rise to thermally activated
delayed fluorescence (TADF),[6–10] without using precious-
metal complexes. For efficient TADF emission, a very small
energy gap (DEST) between the S1 and T1 states is required to
promote reverse intersystem crossing (RISC) and harvest
formally spin-forbidden T1 excitons. We, and others, have
developed various TADF pure organic luminophores[6–10] and
copper(I) complexes,[11] with small DEST values with high
radiative decay rates. However, the basic molecular frame-
works for organic TADF molecules remain limited to
azaheteroaromatics (e.g. triazines[6] and oxadiazoles[7]), cya-
nobenzenes,[8] sulfones,[9] and spirofluorene derivatives.[10]
Thus, the challenge is to rationally design novel families of
TADF luminophores based on simple versatile molecular
scaffolds which exhibit emission across the entire visible
range.
O
rganic molecules exhibiting strong visible-light emission
continue to receive considerable attention because of their
promising application in organic light-emitting diodes
(OLEDs)[1] and as bioimaging probes.[2] The technological
prospects for full-color flat-panel displays and solid-state
lighting sources have driven the development of red, green,
and blue (RGB) light-emitting materials with high lumines-
cence quantum efficiencies.[3] The discovery of phosphores-
cent materials containing heavy transition metals, such as
iridium(III) and platinum(II), was a major breakthrough
towards high-performance OLEDs.[4] According to spin
statistics, 75% triplet (T1) and 25% singlet (S1) excitons are
formed by the recombination of a hole and an electron under
electrical excitation. In conventional fluorescent organic
materials, only up to 25% of the total electro-generated
excitons can be used for light emission. In contrast, both S1
and T1 excitons can be harvested in phosphorescent
[*] S. Y. Lee, Prof. Dr. T. Yasuda, Dr. Y. S. Yang, Dr. Q. Zhang,
Prof. Dr. C. Adachi
Department of Applied Chemistry and Center for Organic Photonics
and Electronics Research (OPERA), Kyushu University
744 Motooka, Nishi-ku, Fukuoka 819-0395 (Japan)
E-mail: yasuda@cstf.kyushu-u.ac.jp
Herein, we report a series of butterfly-shaped TADF-
emitting benzophenones featuring donor–acceptor–donor
(D-A-D) electronic configurations (Figure 1). Benzophenone
itself was previously explored as an emitter in OLEDs,[12]
adachi@cstf.kyushu-u.ac.jp
Homepage: http://www.cstf.kyushu-u.ac.jp/~adachilab/
Prof. Dr. T. Yasuda, Prof. Dr. C. Adachi
International Institute for Carbon Neutral Energy Research
(WPI-I2CNER), Kyushu University
744 Motooka, Nishi-ku, Fukuoka 819-0395 (Japan)
Prof. Dr. T. Yasuda
PRESTO (Japan) Science and Technology Agency (JST)
Chiyoda-ku, Tokyo 102-0076 (Japan)
[**] This work was supported by a Grant-in-Aid from the Funding
Program for World-Leading Innovation R&D on Science and
Technology (FIRST), Precursory Research for Embryonic Science
and Technology (PRESTO) from JST (no. 10111), and from JSPS
Young Scientist (A) (no. 25708032). C.A. and T.Y. acknowledge the
support of WPI-I2CNER, sponsored by MEXT (Japan).
OLEDs=organic light-emitting diodes.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201402992.
Figure 1. Structural formulae of the butterfly-shaped benzophenone
derivatives 1–5.
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because of its phosphorescence and efficient intersystem
crossing.[13] To our knowledge, however, practical EL from
benzophenone derivatives at ambient temperature has not
been detected. A remarkable feature of our system is that
OLEDs employing benzophenones with a D-A-D structure as
light-emitting centers can generate highly efficient blue to
orange–red TADF by upconversion. The external EL quan-
tum efficiencies of these systems greatly exceed the theoret-
ical limit of conventional fluorescence OLEDs.
The syntheses of the benzophenone derivatives 1–5 were
accomplished using Buchwald–Hartwig aminations of 4,4’-
dibromobenzophenone or bis(4-bromobenzoyl)benzenes
with two equivalents of the corresponding donor units
(carbazole to prepare 1, bicarbazole for 2, and phenoxazine
for 3–5) in high yields (84–96%) by employing a [Pd(tBu3P)2]/
K2CO3 catalytic system. Detailed synthetic procedures and
characterization data for compounds 1–5 are described in the
Supporting Information.
Figure 2. Frontier-molecular-orbital distributions, energy levels and
energy gaps (DEST) between S1 and T1 for compounds 1–3, character-
ized by TD-DFT calculations.
that excitons may be harvested by T1!S1 upconversion. All
investigated lowest-energy excited states correspond to an
intramolecular charge transfer (ICT) with a small exchange
energy and are mainly described by the HOMO!LUMO
transition (Table S1). The S1 states for 1 and 2 have relatively
large oscillator strengths (f = 0.36 and 0.15, respectively). The
oscillator strength for the electronic transition tends to
decrease upon substitution with more sterically-hindered
phenoxazine donor units (f = 0.01–0.02 for compounds 3–5).
Consistent with the quantum chemical calculations, tuning
of emission color can be observed with the naked eye;
compounds 1–5 exhibit bright photoluminescence (PL) in
solution, with emission color varying from blue to green, and
orange–red (Figure 3a). This suggests that changing the
degree of ICT effectively modulates the emission wavelength
of the molecules. Steady-state UV/Vis absorption and PL
spectra of compounds 1–5 in toluene are depicted in Fig-
ure 3b, and the photophysical data are summarized in
Table 1. In oxygen-free toluene solution, compounds 1–5
exhibited broad structureless emission with PL quantum
yields (FPL) of 10–44%; whilst the PL intensity drastically
decreased in the presence of triplet oxygen (3O2) in aerated
solutions. From compound 1, with carbazole donor units, to 3,
with phenoxazine units, the PL emission peak (lPL) is red-
shifted by 71 nm. Depending on the substitution mode of an
additional benzoyl group on the innermost benzene ring,
considerable red shifts of lPL were observed in the PL spectra
In principle, DEST should decrease upon decreasing the
exchange interaction integral of the HOMO and LUMO
wavefunctions of a molecule.[14] Consequently, careful selec-
tion of appropriate donor units is crucial to form full-color
TADF molecules, with their HOMO and LUMO being
localized on different constituents. To gain insight into the
geometric and electronic structures of 1–5, time-dependent
density functional theory (TD-DFT) calculations were per-
formed at the B3LYP/6-31G(d,p) level. The transition ener-
gies, oscillator strengths, and assignments for the most
relevant singlet and triplet excited states for compounds 1–5
are provided in Table S1 (Supporting Information). All
molecules exhibit clear separation of the HOMO and
LUMO distributions. As shown in Figure 2, for compounds
1–3 the LUMO is predominantly located on the central
electron-accepting benzophenone core, whereas the HOMO
is mainly localized on the electron-donating peripheral
carbazole or phenoxazine substituents, because of the highly
twisted geometry between the donor and acceptor constitu-
ents. In their optimized ground-state geometries, the dihedral
angles between the phenyl rings of the benzophenone unit
and adjacent carbazoles are calculated to be 518 for com-
pound 1 and 618 for 2, and for 3 with phenoxazines to be 858.
This clear separation of the frontier orbitals leads to small
calculated DEST values of 0.01–0.32 eV (Table 1), and suggests
Table 1: Photophysical properties of the benzophenone-based TADF luminophores 1–5.
Compound
labs [nm]
lPL [nm]
FPL [%][c]
tp (FF)
td (FTADF
)
HOMO
[eV][e]
LUMO
[eV][f]
ES/ET
DEST
calc. DEST
sol[a]
sol[a]/film[b]
sol[a]/film[b]
[ns (%)][d]
[ms (%)][d]
[eV][g]
[eV][h]
[eV][i]
Cz2BP (1)
CC2BP (2)
Px2BP (3)
m-Px2BBP (4)
p-Px2BBP (5)
340, 353[j]
345
324, 413
324, 416
322, 425
438/444
462/475
509/538
566/541(575)
600/555
21/55
38/73
44/70
36/71(29)
10/36
59(5)
710(50)
460(62)
12(29)
13(27)
2.9(10)
ꢀ5.74
ꢀ5.65
ꢀ5.44
ꢀ5.64
ꢀ5.62
ꢀ2.64
ꢀ2.63
ꢀ2.92
ꢀ3.03
ꢀ3.13
3.10/2.89
3.02/2.88
2.61/2.58
2.62/2.52
2.59/2.53
0.21
0.14
0.03
0.10
0.06
0.32
0.10
0.01
0.02
0.02
62(11)
23(41)
24(44)
26(26)
[a] Measured in oxygen-free toluene solution at room temperature. [b] 6 wt%-doped film in a host matrix (host=DPEPO for 1 and 2; mCP for 3; mCBP
for 4 and 5). The data for a neat film are given in parentheses. [c] Absolute PL quantum yield evaluated using an integrating sphere. [d] PL lifetimes of
prompt (tp) and delayed (td) decay components for the 6 wt%-doped film measured at 300 K under vacuum. The fractional contributions of
fluorescence (FF) and TADF (FTADF) to the total FPL [%] are given in parentheses. [e] Determined by photoelectron yield spectroscopy in thin films.
[f] Deduced from the HOMO and optical energy gap (Eg). [g] Singlet (ES) and triplet (ET) energies estimated from onset wavelengths of the doped film
emission spectra at 300 and 5 K, respectively. [h] DEST =ESꢀET. [i] Calculated by TD-DFT at B3LYP/6-31G(d,p). [j] Shoulder peak.
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Figure 3. a) Luminescence images of compounds 1–5 in toluene
solutions recorded under UV irradiation (lex =365 nm). b) Absorption
(left) and normalized PL (right) spectra of compounds 1–5 in toluene
at room temperature.
Figure 4. a) Streak image and corresponding time-dependent
emission spectra, and b) transient PL decay profile of a
of bis(benzoyl)benzenes 4 (lPL = 566 nm) and 5 (lPL
600 nm) in solution, when compared with 3 (lPL = 509 nm).
This suggests that the additional electron-withdrawing ben-
zoyl unit with the para-linkage (compound 5) stabilizes the
LUMO, resulting in a decreased bandgap energy compared
with meta-linked compound 4. Thus, fine tuning of the
luminescence properties is achieved by simple chemical
modification of benzophenone.
To better understand the nature of the emitting states, the
transient PL characteristics and temperature dependence
were analyzed using a streak camera (Figure 4). We focused
on host:guest solid thin films containing low concentrations of
the guest emitters 1–5, to avoid concentration quenching of
the emitter and complications by solvent phase changes. For
=
2(6 wt%):DPEPO codeposited film recorded at 300 K. Prompt fluores-
cence (black) and TADF (red) components are shown. Each dot in the
streak image represents a PL photon count. c) Temperature depend-
ence of transient PL decay. Inset: the fractional contribution of
fluorescence and TADF to the total PL quantum yield (FPL). d) Sche-
S
matic representation of PL decay of 2. krS and knr are the radiative and
non-radiative decay rate constants of the S1 state, respectively, kISC and
k
RISC are the ISC (S1!T1) and RISC (T1!S1) rate constants, respec-
T
tively, and knr is the non-radiative rate constant of the T1 state.
doped film reaches 73 ꢁ 2% at room temperature, which
includes quantum efficiencies of 11 ꢁ 2% for fluorescence
(FF) and 62 ꢁ 2% for TADF (FTADF), as listed in Table 1.
Using the PL efficiency and lifetime data measured at 300 K,
the radiative decay rate constant of the S1 state (krS), the ISC
rate constant (kISC), and the RISC rate constant (kRISC) of 2 in
the doped film are estimated to be approximately 1–2 ꢀ 106, 1–
2 ꢀ 107, and 1–2 ꢀ 104 sꢀ1, respectively (Figure 4d, see Sup-
porting Information for details).
The photophysical properties of the D-A-D-structured
benzophenones with high FTADF values of approximately 36–
73% in their doped films are favorable for developing highly
efficient OLEDs. To evaluate the performance of 1–5 as
emitters, multilayer OLED devices A–E were fabricated (see
Figure 5 for device configurations). Devices A and B had
compounds 1 and 2, respectively, as emitters, and devices C–E
had compounds 3–5 as emitters. The compound 4,4’-bis[N-(1-
the blue emitters
1 and 2, bis[2-(diphenylphosphino)
phenyl]ether oxide (DPEPO)[15] with higher S1 and T1
energies (ES/ET = 3.5/3.1 eV) was selected as a host to prevent
the backward excited energy transfer from the guest to host.
The FPL for compounds 1–5 in suitable amorphous host
matrices are approximately two- to three-times larger than
those measured in dilute solutions (Table 1). This result is
because of reduced non-radiative relaxation as intramolecu-
lar torsional/vibrational motions are restricted in the solid
state.[16]
The results displayed in Figure 4 for a 2(6 wt%):DPEPO
codeposited film indicate two-component emission decays,
consisting of a nanosecond-scale prompt component (lifetime
tp = 62 ns) and a microsecond-scale delayed component (td =
460 ms) at 300 K. As the delayed emission has the same
spectral profile as normal fluorescence from the S1 state but
with a longer decay time, the long-lived emission can be
assigned to TADF. Furthermore, the TADF emission inten-
sity is found to increase with increasing temperature from 50
to 300 K (Figure 4c). At ambient temperature (300 K), the
upper-lying S1 state is significantly populated by thermally
activated upconversion from the T1 state, and in turn emits
efficient TADF. Hence, the overall FPL of the 2:DPEPO-
naphthyl)-N-phenylamino]-1,1’-biphenyl
(a-NPD)
was
employed as a hole-transporting layer (HTL) and 1,3,5-
tris(N-phenylbenzimidazol-2-yl)benzene (TPBi) as an elec-
tron-transporting layer (ETL). For devices A and B, thin
layers of 1,3-bis(carbazol-9-yl)benzene (mCP) and DPEPO
with sufficiently high ET were inserted at the emitting layer/
HTL and ETL interfaces, respectively, to confine T1 excitons
in the blue-emitting molecules, and suppress T1 exciton
quenching. For the orange–red-emitting device E, compound
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similar to the corresponding PL spectra of their doped (or
neat) films (Figure 5a), confirming that EL emission was
generated solely from the emitters themselves, through the
same radiative decay process. As illustrated in a Commission
Internationale de lꢁꢂclairage (CIE) chromaticity diagram in
Figure 5c, the EL emission color can be tuned from blue
(0.16, 0.14; device A) to orange–red (0.58, 0.36; device E)
using the TADF benzophenones 1–5.
The sky-blue-emitting device B containing emitter 2
achieved a high maximum hext of 14.3% and current efficiency
(hc) of 25.5 cdAꢀ1 at low current densities. With increasing
current density, the hext of device B and blue-emitting
device A decrease more rapidly than for the other devices.
This efficiency roll-off at high current density is mainly
attributed to excess T1 excitons in the emitting layer, which
cause exciton quenching by triplet–triplet and/or singlet–
triplet annihilation.[17] The green-emitting device C incorpo-
rating emitter 3 also displayed notable EL efficiencies, with
a hext measuring 10.7%, a hc of 35.9 cdAꢀ1, and a high
maximum luminance (Lmax) of 86100 cdmꢀ2. At a high
current density of 100 mAcmꢀ2, the hext value of device C
was still as high as 5%, indicating a reduced efficiency roll-off,
presumably because of the relatively short TADF lifetime
(12 ms) for the doped film of 3. Although the performances of
the blue-emitting device A and yellow-emitting device D
were lower than that of device C, maximum hext values of
8.1% and 6.9% were obtained, respectively, which are much
higher than the 5% theoretical limit for the hext of conven-
tional fluorescence OLEDs.
The combination of these TADF benzophenones with
emission wavelengths spanning the whole visible region
should allow TADF-based white OLEDs to be fabricated.
There are two approaches for generating white light from
OLEDs: three primary-color (RGB) and two-color (blue/
yellow) emitting systems. Because of the simpler device
fabrication process, we adopted a two-color-based device
configuration (device F) with ITO/a-NPD (35 nm)/ 5-
Figure 5. a) External EL quantum efficiency (hext) versus current-density
plots (inset: normalized EL spectra measured at 10 mAcmꢀ2, l/nm)
and b) current-density–voltage (J–V) curves of the OLEDs A–F contain-
ing the TADF emitters 1–5. c) EL emission color coordinates in the CIE
1931 chromaticity diagram and photographs of devices A–F. Device
configurations: for A and B, ITO/a-NPD (35 nm)/mCP (5 nm)/ 1 or
2(6 wt%):host (20 nm)/DPEPO (10 nm)/TPBi (30 nm)/LiF (0.8 nm)/Al
(80 nm); for C and D, ITO/a-NPD (40 nm)/ 3 or 5(6 wt%):host
(20 nm)/TPBi (40 nm)/LiF (0.8 nm)/Al (80 nm); for E, ITO/a-NPD
(40 nm)/4 (neat, 20 nm)/TPBi (40 nm)/LiF (0.8 nm)/Al (80 nm); for F,
ITO/a-NPD (35 nm)/5(18 wt%):mCBP (4 nm)/2(6 wt%):PPF (14 nm)/
PPF (40 nm)/LiF (0.8 nm)/Al (80 nm).
(18 wt%):mCBP
(4 nm)/2(6 wt%):PPF
(14 nm)/PPF
(40 nm)/LiF (0.8 nm)/Al (80 nm), in which 2 and 5 function
as sky-blue and yellow emitters, respectively. As expected,
device F emitted white EL with CIE coordinates of (0.32,
0.39), which are close to those of pure white light CIE (0.33,
0.33), and achieved a hext of 6.7% (Table 2). The resulting EL
Table 2: TADF-OLED performance of the devices A–F[a].
4 was used as a non-doped light-
emitting layer. The energy-level
diagrams of the devices and struc-
tural formulae of their materials are
provided in the Supporting Infor-
mation.
Device
A
B
C
D
E
F
Emitter
Host
lEL [nm]
Von [V]
1
2
3
mCP
539
3.2
86100
35.9
5
4
2/5
DPEPO
446
4.3
510
9.3
DPEPO
484
4.4
3900
25.5
14.3
mCBP
548
3.6
none
586
2.8
PPF/mCPB
489/548
5.0
L
max [cdmꢀ2
]
57120
20.1
6.9
50820
11.1
4.2
9800
16.4
6.7
Figure 5 presents external EL
hc [cdAꢀ1
hext [%]
]
quantum efficiency (hext) versus
8.1
10.7
current-density plots, current-den-
sity–voltage (J–V) characteristics,
and EL spectra of the fabricated
devices. The key characteristics are
summarized in Table 2. All devices
based on 1–5 exhibited EL spectra
CIE [x, y]
0.16, 0.14
0.17, 0.27
0.37, 0.58
0.49, 0.51
0.58, 0.36
0.32, 0.39
[a] Abbreviations: lEL =EL emission maximum, Von =turn-on voltage at 1 cdmꢀ2, Lmax =maximum
luminance, hc =maximum current efficiency, hext =maximum external EL quantum efficiency (at
L>1 cdmꢀ2), CIE=Commission Internationale de l’ꢀclairage color coordinates measured at
10 mAcmꢀ2, DPEPO=bis[2-(diphenylphosphino)phenyl]ether oxide, mCP=1,3-bis(carbazol-9-yl)ben-
zene, mCBP=3,3’-bis(carbazol-9-yl)biphenyl, PPF=2,8-bis(diphenylphosphoryl)dibenzo[b,d]furan.
4
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[4] a) M. A. Baldo, D. F. OꢁBrien, Y. You, A. Shoustikov, S. Sibley,
spectrum exhibited component peaks at l = 489 and 548 nm,
corresponding to emission from 2 and 5, respectively (see
Figure 5a inset).
The hext for OLEDs is generally expressed by Equa-
tion (1):
M. E. Thompson, S. R. Forrest, Nature 1998, 395, 151 – 154;
b) M. A. Baldo, S. Lamansky, P. E. Burrows, M. E. Thompson,
S. R. Forrest, Appl. Phys. Lett. 1999, 75, 4 – 6; c) S. Lamansky, P.
Djurovich, D. Murphy, F. Abdel-Razzaq, H.-E. Lee, C. Adachi,
P. E. Burrows, S. R. Forrest, M. E. Thompson, J. Am. Chem. Soc.
2001, 123, 4304 – 4312; d) J. Brooks, Y. Babayan, S. Lamansky,
P. I. Djurovich, I. Tsyba, R. Bau, M. E. Thompson, Inorg. Chem.
2002, 41, 3055 – 3066.
hext ¼ hint hout ¼ g hST FPL hout
ð1Þ
[5] a) C. Adachi, M. A. Baldo, M. E. Thompson, S. R. Forrest, J.
Appl. Phys. 2001, 90, 5048 – 5051; b) C. Adachi, M. A. Baldo,
S. R. Forrest, M. E. Thompson, Appl. Phys. Lett. 2000, 77, 904 –
906; c) L. Xiao, S.-J. Su, Y. Agata, H. Lan, J. Kido, Adv. Mater.
2009, 21, 1271 – 1274.
[6] a) S. Y. Lee, T. Yasuda, H. Nomura, C. Adachi, Appl. Phys. Lett.
2012, 101, 093306; b) A. Endo, K. Sato, K. Yoshimura, T. Kai, A.
Kawada, H. Miyazaki, C. Adachi, Appl Phys. Lett. 2011, 98,
083302; c) K. Sato, K. Shizu, K. Yoshimura, A. Kawada, H.
Miyazaki, C. Adachi, Phys. Rev. Lett. 2013, 110, 247401; d) H.
Tanaka, K. Shizu, H. Miyazaki, C. Adachi, Chem. Commun.
2012, 48, 11392 – 11394.
where, hint is the internal EL quantum efficiency, hout is the
light out-coupling efficiency (typically around 20%, which is
derived from 1ꢀ(1ꢀ1/n2)1/2, where n is the refractive index of
the organic layers),[18] g is the charge balance factor (ideally g
ꢂ 1 if holes and electrons are fully balanced and recombine to
generate excitons), hST is the fraction of radiative excitons
(hST = 0.25 for conventional fluorescent emitters, according to
the classical spin degeneracy statistics of 1:3 for singlet-to-
triplet excitons), and FPL is the quantum yield of the emitting
layer. Considering the measured FPL values of 1–5 (see
Table 1), greatly enhanced hST efficiencies of 72–98% are
estimated for devices A–E. Note, the maximum hST of 2
(98%) is nearly four times higher than the 25% limit of spin
statistical ratio for conventional fluorescent materials.
[7] J. Lee, K. Shizu, H. Tanaka, H. Nomura, T. Yasuda, C. Adachi, J.
Mater. Chem. C 2013, 1, 4599 – 4604.
[8] a) H. Uoyama, K. Goushi, K. Shizu, H. Nomura, C. Adachi,
Nature 2012, 492, 234 – 238; b) T. Nishimoto, T. Yasuda, S. Y.
Lee, R. Kondo, C. Adachi, Mater. Horiz. 2014, 1, 264 – 269.
[9] a) Q. Zhang, J. Li, K. Shizu, S. Huang, S. Hirata, H. Miyazaki, C.
Adachi, J. Am. Chem. Soc. 2012, 134, 14706 – 14709; b) F. B.
Dias, K. N. Bourdakos, V. Jankus, K. C. Moss, K. T. Kamtekar, V.
Bhalla, J. Santos, M. R. Bryce, A. P. Monkman, Adv. Mater. 2013,
25, 3707 – 3714.
[10] a) G. Mꢄhes, H. Nomura, Q. Zhang, T. Nakagawa, C. Adachi,
Angew. Chem. 2012, 124, 11473 – 11477; Angew. Chem. Int. Ed.
2012, 51, 11311 – 11315; b) K. Nasu, T. Nakagawa, H. Nomura,
C.-J. Lin, C.-H. Cheng, M.-R. Tseng, T. Yasuda, C. Adachi,
Chem. Commun. 2013, 49, 10385 – 10387.
[11] a) J. C. Deaton, S. C. Switalski, D. Y. Kondakov, R. H. Young,
T. D. Pawlik, D. J. Giesen, S. B. Harkins, A. J. M. Miller, S. F.
Mickenberg, J. C. Peters, J. Am. Chem. Soc. 2010, 132, 9499 –
9508; b) R. Czerwieniec, J. Yu, H. Yersin, Inorg. Chem. 2011, 50,
8293 – 8301; c) H. Yersin, A. F. Rausch, R. Czerwieniec, T.
Hofbeck, T. Fischer, Coord. Chem. Rev. 2011, 255, 2622 – 2652;
d) R. Czerwieniec, K. Kowalski, H. Yersin, Dalton Trans. 2013,
42, 9826 – 9830.
In conclusion, efficient exciton-harvesting through spin
upconversion from non-radiative T1 to radiative S1 states has
been realized, using butterfly-shaped benzophenone deriva-
tives. Our results demonstrate that the judicious molecular
design of benzophenone based on D-A-D frameworks is valid
for the production of organic luminophores exhibiting highly
efficient full-color TADF emissions. OLEDs employing the
benzophenone derivatives as emitters have achieved max-
imum external quantum efficiencies of up to 14.3%, which far
exceed the theoretical limit for conventional fluorescence
OLEDs. These purely organic luminophores do not require
expensive precious metals, in contrast to phosphorescent
emitters, and will therefore benefit large-area applications.
Continued exploration of similar materials offers a viable
route for producing efficient and stable full-color TADF
luminophores for future display and lighting applications.
[12] a) S. Hoshino, H. Suzuki, Appl. Phys. Lett. 1996, 69, 224 – 226,
and references therein; b) T. Tsutsui, C. Adachi, S. Saito in
Photochemical Processes in Organized Molecular Systems (Ed.:
K. Honda), Elsevier, Amsterdam, 1991, pp. 437 – 450.
[13] a) M. W. Wolf, K. D. Legg, R. E. Brown, L. A. Singer, J. H.
Parks, J. Am. Chem. Soc. 1975, 97, 4490 – 4497; b) Y.-P. Sun, D. F.
Sears, Jr., J. Saltiel, J. Am. Chem. Soc. 1989, 111, 706 – 711; c) S.
Aloꢅse, C. Ruckebusch, L. Blanchet, J. Rꢄhaut, G. Buntinx, J.-P.
Huvenne, J. Phys. Chem. A 2008, 112, 224 – 231.
Received: March 4, 2014
Published online: && &&, &&&&
Keywords: benzophenone · donor–acceptor systems ·
.
organic light-emitting diodes · organic semiconductors ·
photochemistry
[14] N. J. Turro, V. Ramamurthy, J. C. Scaiano, Modern Molecular
Photochemistry of Organic Molecules, University Science Books,
Sausalito, 2010.
[15] a) Q. Zhang, T. Komino, S. Huang, S. Matsunami, K. Goushi, C.
Adachi, Adv. Funct. Mater. 2012, 22, 2327 – 2336; b) C. Han, Y.
Zhao, H. Xu, J. Chen, Z. Deng, D. Ma, Q. Li, P. Yan, Chem. Eur.
J. 2011, 17, 5800 – 5803.
[16] a) Y. Liu, S. Chen, J. W. Y. Lam, P. Lu, R. T. K. Kwok, F. Mahtab,
H. S. Kwok, B. Z. Tang, Chem. Mater. 2011, 23, 2536 – 2544;
b) C. Y. K. Chan, Z. Zhao, J. W. Y. Lam, J. Liu, S. Chen, P. Lu, F.
Mahtab, X. Chen, H. H. Y. Sung, H. S. Kwok, Y. Ma, I. D.
Williams, K. S. Wong, B. Z. Tang, Adv. Funct. Mater. 2012, 22,
378 – 389; c) B.-K. An, S.-K. Kwon, S.-D. Jung, S. Y. Park, J. Am.
Chem. Soc. 2002, 124, 14410 – 14415; d) J. Y. Kim, T. Yasuda,
[1] a) Organic Light-Emitting Devices: Synthesis Properties and
Applications (Eds.: K. Mꢃllen, U. Scherf), Wiley-VCH, Wein-
heim, 2006; b) Highly Efficient OLEDs with Phosphorescent
Materials (Ed.: H. Yersin), Wiley-VCH, Weinheim, 2008.
[2] For recent Reviews, see: a) J. Chan, S. C. Dodani, C. J. Chang,
Nat. Chem. 2012, 4, 973 – 984; b) L. Yuan, W. Lin, K. Zhang, S.
Zhu, Acc. Chem. Res. 2013, 46, 1462 – 1473; c) Z. Guo, S. Park, J.
Yoon, I. Shin, Chem. Soc. Rev. 2014, 43, 16 – 29.
[3] a) B. W. DꢁAndrade, S. R. Forrest, Adv. Mater. 2004, 16, 1585 –
1595; b) M. C. Gather, A. Kçhnen, K. Meerholz, Adv. Mater.
2011, 23, 233 – 248; c) K. T. Kamtekar, A. P. Monkman, M. R.
Bryce, Adv. Mater. 2010, 22, 572 – 582; d) H. Sasabe, J. Kido,
Chem. Mater. 2011, 23, 621 – 630.
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Communications
Y. S. Yang, C. Adachi, Adv. Mater. 2013, 25, 2666 – 2671; e) R.
Kondo, T. Yasuda, Y. S. Yang, J. Y. Kim, C. Adachi, J. Mater.
Chem. 2012, 22, 20689 – 20695.
[17] a) Y. Zhang, S. R. Forrest, Phys. Rev. Lett. 2012, 108, 267404;
b) C. Adachi, M. A. Baldo, S. R. Forrest, J. Appl. Phys. 2000, 87,
8049.
[18] B. E. A. Saleh, M. C. Teich, Fundamentals of Photonics, 2nd ed.,
Wiley, New York, 2007.
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Angew. Chem. Int. Ed. 2014, 53, 1 – 7
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Angewandte
Chemie
Communications
Hyperfluorescence
Lighting up: Butterfly-shaped benzophe-
none derivatives with small excited sin-
glet–triplet energy gaps are demonstrated
to exhibit efficient full-color delayed fluo-
rescence. Organic light-emitting diodes
employing these benzophenones as
emitters can generate electrolumines-
cence across most of the color gamut
range, including white.
S. Y. Lee, T. Yasuda,* Y. S. Yang, Q. Zhang,
C. Adachi*
&&&& — &&&&
Luminous Butterflies: Efficient Exciton
Harvesting by Benzophenone Derivatives
for Full-Color Delayed Fluorescence
OLEDs
Angew. Chem. Int. Ed. 2014, 53, 1 – 7
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
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These are not the final page numbers!