Unexpected Sole Enol-Form Emission of 2-(2′-Hydroxyphenyl)oxazoles for Highly Efficient Deep-Blue-Emitting Organic Electroluminescent Devices

Article abstract of DOI:10.1002/adfm.201605245

Considerable efforts have been devoted to the development of highly efficient blue light-emitting materials. However, deep-blue fluorescence materials that can satisfy the Commission Internationale de l'Eclairage (CIE) coordinates of (0.14, 0.08) of the N

Full text of DOI:10.1002/adfm.201605245

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Unexpected Sole Enol-Form Emission of
2
-(2′-Hydroxyphenyl)oxazoles for Highly Efficient
Deep-Blue-Emitting Organic Electroluminescent Devices
Bijin Li, Guoqiang Tang, Linsen Zhou, Di Wu, Jingbo Lan,* Liang Zhou,* Zhiyun Lu,
and Jingsong You*
fluorescence probes, optical memory,
Considerable efforts have been devoted to the development of highly efficient
blue light-emitting materials. However, deep-blue fluorescence materials that can
satisfy the Commission Internationale de l’Eclairage (CIE) coordinates of
proton transfer lasers, and organic
light-emitting
diodes
(OLEDs).[
1,2]
2-(2′-Hydroxyphenyl)azoles, a kind of typ-
ical ESIPT molecules, exist exclusively as
an enol form (E) in the ground state. Upon
excitation, the phenyl hydroxyl proton is
transferred from the oxygen atom to the
nitrogen atom of azole via the intramole-
cular hydrogen bond (H-bond), resulting
in an isomerization from the excited enol
(
0.14, 0.08) of the National Television System Committee (NTSC) standard blue
and, moreover, possess a high external quantum efficiency (EQE) over 5%,
remain scarce. Here, the unusual luminescence properties of triphenylamine-
bearing 2-(2′-hydroxyphenyl)oxazoles (3a–3c) and their applications in organic
light-emitting diodes (OLEDs) are reported as highly efficient deep-blue emitters.
The 3a-based device exhibits a high spectral stability and an excellent color purity
with a narrow full-width at half-maximum of 53 nm and the CIE coordinates of
[1b,c]
form (E*) to the excited keto form (K*).
The formation of intramolecular H-bond
is an essential prerequisite for the efficient
ESIPT process, since the proton transfer
takes place along the locus of intramole-
cular H-bond. Polar or protic solvents can
form an intermolecular H-bond with the
proton donor or acceptor of ESIPT mole-
(0.15, 0.08), which is very close to the NTSC standard blue. The exciton
utilization of the device closes to 100%, exceeding the theoretical limit of 25% in
conventional fluorescent OLEDs. Experimental data and theoretical calculations
demonstrate that 3a possesses a highly hybridized local and charge-transfer
excited state character. In OLEDs, 3a exhibits a maximum luminance of
9
054 cd m 2 and an EQE up to 7.1%, which is the first example of highly efficient
−
cules, which could suppress the generation
blue OLEDs based on the sole enol-form emission of 2-(2′-hydroxyphenyl)azoles.
[1b,c]
of the intramolecular H-bond.
There-
fore, 2-(2′-hydroxyphenyl)azoles generally
exhibit only the ESIPT keto-form emission
in hydrocarbon and nonpolar solvents, while the dual emission
of the excited enol and keto forms can be observed in polar and
protic solvents. In the solid solution, such as in poly(methyl meth-
acrylate) and polystyrene (PS) films, the photoluminescence
1
. Introduction
Excited-state intramolecular proton transfer (ESIPT) mate-
rials have received much attention due to their applications in
(PL) spectra of 2-(2′-hydroxyphenyl)azoles as well as their elec-
troluminescence (EL) spectra in OLEDs are usually similar to
those in nonpolar solvent, which typically show a keto-form
Dr. B. Li, Dr. G. Tang, Prof. D. Wu, Prof. J. Lan,
Prof. Z. Lu, Prof. J. You
Key Laboratory of Green Chemistry and Technology
of Ministry of Education, College of Chemistry
Sichuan University
[1b–d]
emission.
accepting groups on the azole could significantly affect the
Although the electron-donating or electron-
[1b]
keto-form emission peak position and its intensity, the exam-
ples of the complete suppression of the keto-form emission of
2-(2′-hydroxyphenyl)azoles in OLEDs have not been described
yet. Recently, we unexpectedly discovered that the introduc-
tion of an electron-donating triphenylamine (TPA) group into
the side of azole can lead to a sole enol-form emission in PL
spectra, and the ESIPT keto-form emission is suppressed com-
pletely. Moreover, due to the lack of keto-form emission at the
long wavelength region, these compounds not only show a blue
emission at the short wavelength region, but also possess a high
fluorescence quantum efficiency in both solution and PS film,
which predicts potential high-efficiency blue OLEDs.
2
9 Wangjiang Road, Chengdu 610064, P. R. China
E-mail: jingbolan@scu.edu.cn; jsyou@scu.edu.cn
Prof. L. Zhou
State Key Laboratory of Rare Earth Resource Utilization
Changchun Institute of Applied Chemistry
Chinese Academy of Sciences
5
625 Renmin Street, Changchun 130022, P. R. China
Dr. L. Zhou
Center of Interface Dynamics for Sustainability
Institute of Materials
China Academy of Engineering Physics
5
96 Yinhe Road, Chengdu 610200, P. R. China
E-mail: zhoul@ciac.ac.cn
Blue OLEDs play a pivotal role in the realization of full-
[
3]
DOI: 10.1002/adfm.201605245
color display. Developing efficient blue EL emitters remains
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a huge challenge due to the intrinsic wide band gap of blue
emitting materials, which makes it difficult to inject charges
into emitters.[ The high-efficiency blue OLEDs mainly uti-
work, we will focus on the investigation of unusual lumines-
cence properties of TPA-bearing 2-(2′-hydroxyphenyl)oxazoles
and their promising applications in highly efficient deep-blue
OLEDs.
3b]
[
4]
lize phosphorescent metal complexes as the emission layer.
However, the use of expensive rare metals, mainly involving
iridium (Ir) and platinum (Pt), limits their practical applica-
tions, let alone difficult synthesis of metal complexes, com-
plicated device fabrication, instability, degradation, and
2
. Results and Discussions
[
5]
serious roll-off in devices at high brightness. Moreover,
the most widely used blue phosphorescent material, FIrpic,
is a sky-blue emitter with the Commission Internationale de
l’Eclairage (CIE) coordinates of (0.16, 0.29).[ Therefore, it is
very necessary to explore highly efficient deep-blue fluores-
cent emitters.
2.1. Synthesis, Characterization, and Photoluminescence
2.1.1. Synthesis of 2-(2′-Hydroxyphenyl)oxazoles
6]
Based on the rhodium-catalyzed internally oxidative CH/CH
cross-coupling reaction between phenols and azoles developed
[
8]
In recent years, considerable efforts have been devoted to
previously by us, the highly efficient and concise synthetic
routes to 2-(2′-hydroxyphenyl)oxazoles 3a–3c are shown in
Scheme 1. First, N-aryloxyamines were obtained by the reac-
tion of potassium phenoxide with O-(mesitylsulfonyl)hydroxy-
lamine as the aminating reagent or by the copper-mediated
cross-coupling of (4-(trifluoromethyl)phenyl)boronic acid
with N-hydroxyphthalimide and followed hydrazinolysis.
Next, the N-acetylation of N-aryloxyamines with acetyl chlo-
ride gave N-aryloxyacetamides 1 in acceptable yields.
palladium-catalyzed Suzuki cross-coupling of arylboronic acid
[
3,7]
developing highly efficient deep-blue fluorescence materials.
However, efficient deep-blue light-emitting materials, which
can satisfy the National Television System Committee (NTSC)
standard blue CIE coordinates of (0.14, 0.08), are rare, and espe-
cially, deep blue emitters with high current efficiency (CE) and
external quantum efficiency (EQE) are more scarce.[ To date,
no reports of efficient deep-blue OLEDs based on 2-(2′-hydroxy-
phenyl)azoles are available, except a 2-(2′-hydroxyphenyl)imi-
dazole derivative with the CIE coordinates of (0.15, 0.11) and
a maximum EQE of 2.94% reported by Park et al.[ In this
7]
[9]
[
9d]
The
2c]
with 5-(4′-bromophenyl)oxazole delivered 2 in an excellent
OH
ONH2
ONHAc
t
KO Bu
acetyl chloride
0 oC, 2 h
MSH
R1
R1
R1
1
1
a, R1 = H 72%
b, R1 = Me 60%
B(OH)2
ONH2
ONHAc
CF3
1
2
) N-hydroxyphthalimide,
CuCl, pyridine, 4 Åsieves
acetyl chloride
0 oC, 2 h
) H2NNH2, MeOH/CHCl3
CF3
CF3
1
c 65%
O
5-(4'-bromophenyl)oxazole
N
B(OH)2
N
N
Pd(OAc)2, XPhos, K3PO4
8
0 oC, 12 h
2
94%
N
O
OH
H
N
ONHAc
H
[
RhCp*Cl2]2,
O
AgSbF6, Ag2CO3
R1
+
PivOH, CsOPiv, DMF,
1
40 oC, 24 h
R1
N
1
1
1
a, R1 = H
N
1
3a, R1 = H
b, R = Me
75%
1
3b, R1 = Me 73%
c, R = CF3
3
c, R1 = CF3 59%
2
Scheme 1. Synthetic routes of 3a–3c. XPhos: 2-(dicyclohexylphosphino)-2′,4′,6′-triisopropylbiphenyl.
1
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yield.[9c] Finally, 2-(2′-hydroxyphenyl)oxazoles 3a–3c were syn-
thesized using the rhodium-catalyzed internally oxidative
CH/CH cross-coupling reaction of N-aryloxyacetamide 1
with 2 through dual CH activation. Their structures were
confirmed by NMR, high resolution mass spectra (HRMS),
and single crystal X-ray diffraction. Using the similar synthetic
method, 2-(2′-hydroxyphenyl)-5-phenyloxazole (HPO) was also
prepared as a reference compound (Section III, Supporting
Information).
2
.1.2. Absorption and Emission Properties
The UV–vis absorption spectra of compounds 3a–3c and
HPO were recorded in toluene and PS film at room tempera-
ture (Figure S1, Supporting Information), and their absorp-
tion maxima are summarized in Table 1. For HPO, the lower
absorption band at 300 nm corresponds to the π–π* transition
of 5-phenyloxazole, while another intense absorption band
at 330 nm is assigned to the coupling between 5-phenyloxa-
zole and the hydroxyphenyl ring.[
1b,10]
There is an absorption
[
10e]
shoulder at 346 nm, which may be a vibrational structure.
Compared to HPO, the two absorption bands of 3a–3c at the
short wavelength region are red-shifted slightly, and an addi-
tional absorption band at the long wavelength region appears at
around 370 nm (Figure S1a, Supporting Information), which is
assigned to a charge-transfer (CT) transition from TPA to azole.
The CT band of 3a–3c essentially overlaps with the absorption
band of the π–π* transition of 2-(2′-hydroxyphenyl)-5-phenylox-
azole moiety. Thus, the latter as absorption shoulders appear at
around 334 and 339 nm for 3a and 3c, respectively (Figure S1a,
Supporting Information). For 3b, the absorption shoulder is not
observed, which may be due to the electron-donating effect of
the methyl substituent on the phenolic ring. The absorption
spectra of 3a–3c and HPO in PS film are almost consistent with
those in toluene (Figure S1b, Supporting Information).
Figure 1. Normalized emission spectra of 3a–3c and 2-(2′-hydroxy-
Most 2-(2′-hydroxyphenyl)azoles such as 2-(2′-hydroxy-
−5
phenyl)-5-phenyl-oxazole (HPO) in a) toluene (5.0 × 10 m) and b) PS
phenyl)benzimidazole (λem
:
469 nm, in cyclohexane),
film (c = 5.0 wt%).
2
-(2′-hydroxyphenyl)benzothiazole (λem: 525 nm, in hexane),
and 2-(2′-hydroxyphenyl)benzoxazole (λem: 500 nm, in hexane),
only exhibit the ESIPT keto-form emission in nonpolar and
Table 1). The substituent on the phenolic ring, whether it is
electron-donating methyl (3b) or electron-withdrawing trifluo-
romethyl (3c), shows a small effect on the emission spectrum.
The enol-form emissions of 3a–3c are located in the blue spec-
tral range, while the ESIPT keto-tautomer emissions were not
observed in both toluene solution and PS film (Figure 1 and
Table 1), which are distinctly different from HPO as well as
most of the existing 2-(2′-hydroxyphenyl)azoles, and thus are
rather unusual.
low polarity solvents.[
1b,10a,c,11]
The emission spectrum of HPO
is consistent with those of these typical 2-(2′-hydroxyphenyl)-
azoles, which shows a sole keto-form emission at 475 nm in
toluene (Figure 1a and Table 1). However, compounds 3a, 3b,
and 3c in toluene solution only exhibit the sole enol-form
emission at 420, 420, and 432 nm, respectively (Figure 1a and
Table 1. Absorption and emission maxima of 3a–3c and HPO.
Compoundsa)
In toluene
λabs [nm]
In PS films
λabs [nm]
2
.1.3. Crystal Structure
λem [nm]
420
λem [nm]
444
3a
3b
3c
368
370
372
330
370
370
373
331
To confirm whether the absence of intramolecular H-bond
causes the sole enol-form emission, 3a was chosen as a rep-
resentative example, and its single crystals were obtained by
slow diffusion of petroleum ether into methylene dichloride
solution. The crystal structure of 3a is shown in Figure S18
(Supporting Information), and the key crystallographic data
420
447
432
450
HPO
475
489
a)Absorption and emission maxima were measured in toluene (5.0 × 10−5 m) and
PS film (5.0 wt%).
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are summarized in Table S10 (Supporting Information). The
crystallography data of 3a clearly indicate the presence of
intramolecular H-bond between the phenyl hydroxyl proton
and the oxazole nitrogen atom with a distance of 1.938 Å.
Moreover, the 2-(2′-hydroxyphenyl)oxazole moiety is almost
coplanar due to the fixation of H-bond. The dihedral angle of
the central biphenyl is 31.9° (θ ). As for the twisted TPA moiety
3
resembling propeller blades, the dihedral angles between the
phenyl ring as a linker and the other two phenyl rings are
6
6.65° (θ ) and 60.88° (θ ), respectively. The presence of intra-
4 5
1
molecular H-bond is further confirmed by H NMR data. In
CDCl , the phenyl hydroxyl proton resonance signal appears
3
at δ 11.18 ppm (Figure S19, Supporting Information), which is
consistent with the large downfield shift (δ > 10 ppm) of the
phenyl hydroxyl proton in most ESIPT molecules possessing a
strong intramolecular hydrogen bond.[
2f]
2
.1.4. Solvatochromic Effects
The solvent polarity-dependent emission behavior of 3a was
[
12]
studied using the Lippert–Mataga model.
spectra of 3a shows a slight change within the scope of
0 nm as the solvent polarity increases, indicating a negli-
The absorption
1
gible dipolar change in the ground state with the change of
solvent polarity. Its emission spectra present a remarkable red
shift with the increase of the solvent polarity: from 406 nm
in nonpolar hexane to 481 nm in polar acetonitrile, indi-
cating a typical CT character in the excited state (Figure 2a
and Table S1, Supporting Information).[ In contrast, HPO
exhibits solely the keto-form emission in nonpolar and low
polarity solvents, but in conjunction with a weak emission of
enol form in highly polar solvents (Figure S3 and Table S2,
Supporting Information). As shown in Figure 2c, the keto
emission of HPO seems to be solvent-independent, whereas
its enol emission exhibits a slightly bathochromic shift of
13]
7
nm (from 375 to 382 nm) with increasing solvent polarity,
which is in accordance with the characteristics of the typical
ESIPT molecules.[
1]
The dipole moment change between the excited state and
ground state (Δμ = μ − μ ) can be estimated by the slope of
e
g
the fitted line of the Stokes shift in different solvents versus
the corresponding solvent polarity parameters.[12] Notably, 3a
exhibits only one slope of 10 956 (r = 0.98), and the Δμ value
is calculated to be 14.4 D (Figure 2b, and see Section V,
Supporting Information),[ which is different from a negligible
solvatochromism of HPO. Such a large Δμ value of 3a clearly
illustrates a considerable contribution of intramolecular charge-
transfer (ICT) character in the excited state.[
14]
14]
2
.1.5. ICT/ESIPT Mechanism
Figure 2. a) Normalized emission spectra of 3a in different solvents (con-
centration: 5 × 10 m). Linear fitting of Lippert–Mataga model of b) 3a
and c) HPO.
−
5
The ESIPT and ICT processes of organic dyes are usually cou-
pled together.[
1a–c]
A proposed ICT/ESIPT mechanism of 3a is
shown in Scheme 2. Solvatochromic experiments have shown
that ultrafast ICT reaction takes place upon photoexcitation,
generating the ICT state (I), which can be stabilized by delo-
calization energy of ICT-induced transient push-pull structure.
In contrast, the energy level of conformer (II) is relatively
higher due to the lack of the delocalization energy of push-pull
effects, which leads to a high energy barrier for isomerization
from enol form (I) to keto form (II). Theoretical calculations
1
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O H
N
O H
O H
N
O
ESIPT
N
O
hv
O
ICT
N
N
N
3a
I
II
Scheme 2. The ICT/ESIPT mechanism of 3a.
further provide a supplementary support for this deduction.
The enol-form species of the ground-state HPO and 3a are
dominant because their energies are lower than those of the
Table S4, Supporting Information), indicating only a single
emission species.
−1
keto-forms by 11.65 and 11.72 kcal mol , respectively (Figure 3
and Table S3, Supporting Information). In the lowest excited
2.1.6. Photoluminescence Efficiencies and CIE Coordinates
state (S ), the energy level of the keto-form of HPO is lower
1
−1
than that of the enol form by 1.09 kcal mol , while it is higher
As seen from Table 2, 3a exhibits high fluorescence quantum
efficiency in various solvents with different polarities (74%–99%)
as well as in PS film (41%) (Table S1, Supporting Informa-
tion). It is presumed that the introduction of propeller-like TPA
into the 2-(2′-hydroxyphenyl)azole skeleton would restrain the
molecular aggregation, which may endow them with a strong
emission. Moreover, an intramolecular H-bond would restrict
the relative rotation of the linkage between phenol and oxazole,
which may reduce the possible energy loss through the non-
radiative rotational relaxation. In general, bridged-model conju-
gated molecules in which the intramolecular rotation is blocked,
such as fluorene and its derivatives, exhibit higher fluorescence
quantum yields than the corresponding unbridged compounds,
−
1
by 6.41 kcal mol for 3a. Therefore, the enol form of HPO
is prone to transferring to the keto form in the excited state,
whereas the transformation of 3a from the excited-state enol to
keto is a significantly endergonic process, which indicates that
the thermodynamic equilibrium favors the enol species, thus
resulting in the lack of keto-form emission. Furthermore, time-
resolved PL experiments revealed a monoexponential decay
of the excited state for 3a in different solvents (Figure S4 and
[15]
such as biphenyl derivatives. The intramolecular H-bond of
-(2′-hydroxyphenyl)azoles may be considered as a special type
2
of noncovalently bridged model, but it has seldom been studied
as a bridged model to lock molecular conformation and thus
enhance emission intensity, because the typical 2-(2′-hydroxy-
[1]
phenyl)azoles usually tend to undergo the ESIPT reaction.
The 2-(2′-hydroxyphenyl)azoles with the ESIPT characteristics
[1b]
have been widely studied as blue emitting materials. How-
ever, most ESIPT molecules exhibit relatively low fluorescence
quantum efficiency due to the presence of various nonradia-
[
1b]
tive deactivation pathways of the keto-form excited state. For
example, HPO shows low fluorescence quantum efficiency
of from 0.45 in toluene to 0.10 in acetonitrile (Table 2 and
Table 2. Fluorescence quantum yields and CIE1931 of 3a–3c and HPO.
Compoundsa)
In toluene
In PS films
Φf [%]
Φf [%]
88
CIE1931
CIE1931
3a
3b
3c
(0.15, 0.05)
(0.15, 0.05)
(0.15, 0.07)
(0.16, 0.31)
41
41
40
<1
(0.15, 0.11)
(0.15, 0.13)
(0.15, 0.11)
(0.19, 0.37)
86
84
HPO
45
a)
were measured in toluene (5.0 × 10−5 m)
1931
Figure 3. Diagrams of the ESIPT processes and calculated energy levels
of HPO and 3a.
Absolute quantum yield (Φ ) and CIE
f
and PS film (5.0 wt%).
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Table S2, Supporting Information). In PS film, its quantum
yield is even lower than 0.01. In contrast, 3a–3c exhibit remark-
ably high fluorescence quantum yields in toluene solution
is ITO/MoO3 (3 nm)/TAPC (50 nm)/3a (x wt%):CBZ -F
(20 nm)/TmPyPB (50 nm)/LiF (1 nm)/Al (100 nm). In the
2
1
devices, MoO and LiF serve as the hole- and electron-injecting
3
(
84%–88%) and in PS films (40%–41%). The molecular pixel
materials, respectively. 3a doped in 3,3′-(9H-fluorene-9,9-
diyl)bis(9-phenyl-9H-carbazole) (CBZ -F ) with an optimized
behaviors of 3a–3c were also evaluated by reproducing colors
in the CIE1931 XYZ color space and the xy chromaticity dia-
gram. All three compounds exhibit bright deep-blue emissions
in both toluene solution and PS film (Table 2 and Figure S5,
Supporting Information), which show great promise for appli-
cations in high-performance blue OLEDs.
2
1
doping level of 13 wt% is used as the emitting layer (For details,
see Section X, Supporting Information). TAPC [1,1-bis{(di-4-tol-
ylamino)phenyl}cyclohexane] and TmPyPB [1,3,5-tri(m-pyrid-
3-ylphenyl)benzene)] act as the hole- and electron-transporting
materials, respectively (Figure S10, Supporting Information).
From the EL spectrum of 3a, as shown in Figure 4b, one can
see an obvious blue emission with λmax at 443 nm, which is
consistent with its PL emission in thin film (Figure S13, Sup-
porting Information), indicating that the EL emission is still the
enol-form emission rather than the ESIPT keto-tautomer emis-
sion. This result also demonstrates that the EL emission comes
from the emissive layer, and moreover, excimer or exciplex spe-
cies are effectively suppressed. It is worth noting that the full-
width at half-maximum (FWHM) of the EL spectrum is only
about 53 nm, which is narrower than those of most previously
2
.1.7. Thermal Properties
The thermal properties of 3a were studied by thermogravimetric
analyzer (TGA) and differential scanning calorimeter (DSC).
TGA measurement demonstrates that 3a is a highly thermally
stable material. Its thermal-decomposition temperature (T ) at
d
5
% weight loss is 373 °C. DSC measurement, performed in the
temperature range from 20 to 200 °C, shows that 3a melts at
92 °C. Before the sample completely melts, a rather smooth
[
2c,3,7]
1
reported deep-blue-emitting materials.
The OLED based
DSC curve was observed without any glass transition peak or
other phase transformation, demonstrating a good morpholog-
ical stability (Figures S6 and S7, Supporting Information).
on 3a exhibits a deep-blue electroluminescence with the CIE
coordinates of (0.15, 0.08) very close to the NTSC coordinates of
(0.14, 0.08) (Figure 4c). The EL spectra show little change under
different current densities, indicating that the 3a-based device
possesses excellent spectral stability (Figure 4b). Furthermore,
the device exhibits a low turn-on voltage of 3.1 V and excellent
2
.1.8. Electrochemical Properties
−2
performances with a maximum luminance of 9054 cd m , CE
−
1
−1
Cyclic voltammetry (CV) was carried out to evaluate the elec-
trochemical properties and energy levels of the 3a–3c and
HPO. Compounds 3a–3c show reversible oxidation waves, but
HPO only shows an irreversible oxidation wave (Figure S8,
Supporting Information). The optical bandgaps are calculated
from the absorption onset of their UV–vis spectra in dichlo-
romethane, which are 3.00, 2.99, 2.96, and 3.52 eV for 3a, 3b, 3c,
and HPO, respectively. The highest occupied molecular orbital
of 5.04 cd A , power efficiency (PE) of 4.89 lm W , and EQE
of 7.1% (Figure 4 and Table 3). In addition, under the lumi-
−
2
nance of 1000 cd m , the device still has outstanding perfor-
−
1
mances with CE of 3.43 cd A and EQE of 4.8%, which further
demonstrates the robust stability of the device.
Given that the singlet exciton ratio is limited to a maximum
of 25% due to the spin statistics, the EQE upper limit for clas-
[
3]
sical fluorescent materials is about 5%. The device with 3a as
the emitter exhibits the highest EQE value of 7.1%, indicating
a breakthrough in exciton utilization efficiency. Therefore, the
(
HOMO) and lowest unoccupied molecular orbital (LUMO)
levels are estimated by CV and optical bandgaps, which are
5.32/−2.32, −5.33/−2.32, −5.30/−2.34, and −5.63/−2.11 eV for
a, 3b, 3c, and HPO in CH Cl , respectively (Table S5, Sup-
[
3a]
−
3
radiative exciton yield (η ) is calculated by Equation (1):
r
2
2
η = EQEmax /(γ ×ηPL ×ηout )
r
porting Information). In comparison with HPO, the HOMO
levels of 3a–3c are raised, while the LUMO levels are lowered.
Therefore, the absorption bands of 3a–3c are significantly red-
shifted compared to HPO. The HOMO levels of 3a–3c are also
evaluated by ultraviolet photoelectron spectroscopy spectra,
which is in good agreement with the corresponding values
determined by CV. The LUMO levels are estimated by sub-
tracting the optical bandgap of the solid films from the HOMO
levels. The HOMOs and LUMOs of 3a–3c in solid states were
calculated to be −5.35/−2.47, −5.36/−2.50, −5.31/−2.46 eV,
respectively (Table S6 and Figure S9, Supporting Information).
(
1)
( )
= 7.1%/ 100% × 37.6% × 20% = 94%
where EQEmax is the maximum external quantum efficiency;
ηout is the light-out-coupling efficiency, which is about 20%
in absence of out-coupling enhancement layer in the OLED;
ηPL is the PL quantum efficiency of 3a (13 wt% in CBZ -F ),
2
1
which was measured to be 37.6%; and γ is the recombination
probability of injected holes and electrons in the emission layer,
which is ideally 100%. Thus, the η value of the device with
r
3a as the emitter was calculated to be 94%, which exceeds the
upper limit of the singlet exciton ratio of 25% for conventional
fluorescent OLEDs.
2
.2. Electroluminescence
2
.2.1. Electroluminescence Performances
2.2.2. Electroluminescence Mechanism
The EL properties of 3a in OLED devices were further surveyed.
To obtain further insight into the high exciton utilization
The device configuration fabricated by vacuum deposition
efficiency exceeding 25% of the 3a-based OLED, the EL
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mechanism of 3a-based device was investigated. Recently,
triplet–triplet annihilation (TTA), thermally activated delayed-
fluorescence (TADF), and hybridized local and charge-transfer
(
HLCT) excited-state, as three main theoretical models that
break through the singlet exciton statistics limit, have received
[
7e,g,h,16,17,18]
considerable attention.
The TTA process can provide
a theoretically maximized exciton utilization efficiency of 62.5%
[
7g,16]
through triplet–triplet fusion to singlet excitons.
For 3a,
the possibility of the TTA mechanism can be obviated, because
its exciton utilization in OLED device is far more than 62.5%.
TADF can realize a nearly 100% exciton utilization through
thermally assisted reverse intersystem crossing (RISC) from
[
7e,17]
the lowest triplet state (T ) to the lowest singlet state (S ).
1
1
In order to reach thermal equilibrium of the relaxed excited
states, TADF materials generally possess long fluorescence
[
17c]
lifetime (τ > 1 μs) at room temperature.
The time-resolved
F
PL measurement shows that the excited-state lifetime of 3a is
only on the order of nanoseconds (ns) in both low- and high-
polarity solvents (1.23 ns in hexane and 2.81 ns in acetonitrile)
(
Figure S4 and Table S4, Supporting Information). Moreover,
the temperature-dependent transient PL measurement for 3a
in the CBZ -F film also shows no long-lifetime fluorescence
2
1
component, as the temperature increases from 77 to 307 K
Figure S14 and Table S8, Supporting Information), indicating
(
an essential difference between 3a and typical TADF materials
in luminous mechanism.
The HLCT excited state is considered to be the coexistence
[7h,18]
or hybridization of locally excited (LE) and CT states.
The
LE state is beneficial to improving PL efficiency. The CT state
offers the possibility for a small singlet–triplet energy level
[18]
splitting (ΔE ), which can trigger a potential RISC (T→S ).
ST
1
Therefore, HLCT materials can provide high PL efficiency and
exciton utilization of up to 100% through efficient RISC. As
shown in Figures 1 and 2, the PL spectra of 3a present vibra-
tional fine structure in low polar solvents including n-hexane,
trimethylamine, toluene, n-butyl ether, and benzene, which
reveals the existence of LE state. With the increase of solvent
polarity, the fine structure of emission spectrum disappears,
and 3a shows a gradually red-shifted solvatochromic behavior,
demonstrating the CT feature in the excited state. In both low
polarity and high polarity solvents, 3a exhibits only one slope
of 10 956 (r = 0.98) in linear fitting of Lippert–Mataga model
(Figure 2b and Section V, Supporting Information). The excited-
state dipole moment (μ ) is calculated to be 16.3 D, which is
e
slightly smaller than that of the typical CT molecule 4-(N,N-
dimethylamino)-benzonitrile (μ = 23 D), and larger than that
e
[
15,18]
of usual LE emitters (≈8 D).
Moreover, 3a also possesses a
monoexponential decay of the excited state in various solvents
Figure S4 and Table S4, Supporting Information). These results
indicate that the excited state of 3a consists of a sufficient or
(
[7h,18d]
quasi-equivalent hybridization of LE and CT states.
Furthermore, the natural transition orbitals (NTOs) of S → S
1
0
calculated by TD-M062X/6-31G(d, p) method also demonstrate
that 3a may possess the HLCT excited states in the EL process.
As shown in Figure S15 (Supporting Information), the hole is
mostly spread over the whole molecule skeleton, while the par-
ticle is mainly distributed on the conjugated backbone, except
for the two phenyl rings resembling propeller blades. The hole
and particle distributions are almost completely overlapped
Figure 4. a) Luminance–current density–voltage (L–J–V) characteristics
of the 3a-based device (13 wt% 3a in CBZ -F ). b) The EL efficiency–cur-
rent density and EQE–current density curve of the 3a-based device. Inset:
normalized EL spectra of the 3a-based device operating at different cur-
rent densities. c) CIE1931 coordinates of the device.
2
1
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Table 3. The device performances with 3a as the emitter.
Vturn-ona) (V)
CIE1931b)
EQEmaxc)
[%]
CEmax
d)
PEmaxe)
[lm W−1]
Lmaxf)
[cd m−2]
Device performances at 1000 cd m−2
Emitter
[
x, y]
[cd A−1]
CE [cd A−1]
3.43
PE [lm W−1
]
EQE [%]
4.8
3
a
3.1
(0.15, 0.08)
7.1
5.04
4.89
9054
1.89
a)Turn-on voltage at a brightness of 1 cd m−2; b)0.1 mA cm−2; c)External quantum efficiency; d)Maximum current efficiency; e)Maximum power efficiency; f)Maximum luminance.
from the 2-(2-hydroxyphenyl)azole to the adjacent biphenyl,
example of highly efficient deep-blue EL materials employing
the oxazole as an acceptor, which shows good CIE and high
EQE over 5%.
implying a LE featured transition of S → S . The hole-particle
1
0
distributions are separated on the two outstretched phenyl
rings of TPA, which demonstrates a CT featured transition. In
addition, the energies of singlet and triplet excited states were
also calculated (Table S9, Supporting Information). The energy
3
. Conclusions
levels of the lowest singlet state (S ) is 3.567 eV, while the triplet
1
state energies (T , T , and T ) are 2.778, 3.234, and 3.570 eV,
In summary, the unusual luminescence properties of TPA-
bearing 2-(2′-hydroxyphenyl)oxazoles and their application in
highly efficient deep-blue OLEDs have been investigated. The
introduction of TPA into the side of oxazole of 2-(2′-hydroxy-
phenyl)oxazoles leads to a single enol-form emission and
complete suppression of the ESIPT keto-form emission. Thus,
the resulting compound only shows a deep-blue enol-form
emission in both PL and EL spectra due to the absence of the
keto-form emission at the long wavelength region. The doped
device of 3a with CBZ -F host exhibits a high spectral stability
1
2
3
respectively. The energy gap of S and T amounts to 0.789 eV,
1
1
indicating that the TADF process by RISC from T to S is less
1
1
likely to occur (Figure 5a and Table S9, Supporting Informa-
tion).[ The energy levels of S and T are almost identical,
17]
1
3
implying a potential RISC process from T to S . Moreover, the
3
1
energy gap between T and T (0.336 eV) is much larger than
3
2
that between T and S (0.003 eV), which demonstrates that the
3
1
internal conversion rate from T to T may be lower than the
3
2
[7h,18]
RISC rate from T to S .
3
1
2
1
In the molecular structure of 3a, the oxazole and TPA
moieties act as a weak acceptor (A) and a moderate donor
D), respectively, and the biphenyl unit between oxazole and
and an excellent color purity with a narrow FWHM of 53 nm
and the CIE coordinates of (0.15, 0.08), which are very close to
the CIE (0.14, 0.08) of the NTSC standard blue. The maximum
(
−2
TPA nitrogen-atom plays the role of π-bridge (Figure 5b).
The slightly twisted biphenyl with the dihedral angle of 31.9°
enables both the hole and the particle to partially delocalize to
the whole molecule, rather than fully separate or localize, and
therefore, is not only beneficial to the formation of the LE state,
but also favors the generation of the CT state.[ In addition,
the significantly twisted conformation between the biphenyl
and the other two benzene rings of TPA with the dihedral
angles of 66.65° and 60.88°, respectively, also may be respon-
sible for the CT state. The intramolecular H-bond between
phenol hydroxyl and oxazole plays an important role in main-
taining the planarity of the 2-(2′-hydroxyphenyl)oxazole as
well as suppressing the nonradiative rotational relaxation. It is
worthy of note that the oxazole skeleton is seldom used as the
acceptor in D–A type EL materials. This work presents the first
luminance, CE, PE, and EQE of device are 9054 cd m ,
−
1
−1
5.04 cd A , 4.89 lm W , and 7.1%, respectively. The exciton
utilization of the device is calculated to be 94%, which exceeds
the theoretical limit of the singlet exciton ratio of 25% for con-
ventional fluorescent OLEDs. Experimental data and theoretical
calculations demonstrate that 3a possesses a HLCT excited
state character, which consists of a sufficient or quasi-equiv-
alent hybridization of LE and CT states. The slightly twisted
π-conjugated backbone of 3a is beneficial to both the forma-
tion of LE state and the generation of CT state. The intramo-
lecular H-bond between phenol hydroxyl and oxazole plays an
important role in maintaining the molecular planarity and sup-
pressing the nonradiative rotational relaxation. As a result, 3a
features a high PL efficiency and exhibits a high exciton utiliza-
tion efficiency of nearly 100% with EQE up to 7.1% in OLEDs.
18]
Figure 5. a) The simple model for exciton relaxation in the EL process and calculated singlet–triplet energy of 3a. b) Molecular conformation of 3a.
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of China (Grant Nos. 21672154, 21432005, 21372164, and 21321061)
and Youth Innovation Promotion Association of Chinese Academy of
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