Highly efficient deep-blue OLEDs based on hybridized local and charge-transfer emitters bearing pyrene as the structural unit

Article abstract of DOI:10.1039/c9cc02355k

A new family of hybridized local and charge-transfer (HLCT) emitters bearing a pyrene structural unit has been developed. Deep-blue OLEDs based on them achieved high brightness over 10000 cd m^-2, high maximum external quantum efficiency over 10.5%, and high maximum exciton utilization efficiency approaching 100%, with CIE coordinates of (0.152, 0.065).

Full text of DOI:10.1039/c9cc02355k

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc3N@C2n (2n 68 and 80). Synthesis of the
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DOI: 10.1039/C9CC02355K
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Highly Efficient Deep-Blue OLEDs Based on Hybridized Local and
Charge-Transfer Emitters Bearing Pyrene as the Structural Unit
Caixia Fu,†a Shuai Luo,†a Zhenzhen Li,†b Xin Ai,†c Zhenguo Pang,a Chuan Li,a Kuan Chen,a Liang
Received 00th January 20xx,
Accepted 00th January 20xx
Zhou,*b Feng Li,*c Yan Huanga and Zhiyun Lu*a
DOI: 10.1039/x0xx00000x
A new family of hybridized local and charge-transfer (HLCT) can exhibit both high Lmax and insignificant efficiency roll-off,
emitters bearing pyrene structural unit has been developed. Deep- but their EQEmaxs are relatively low (≤ 8%),5b,6,8 despite the high
blue OLEDs based on them achieved high brightness over 10000 cd theoretical EUEmax of 100% of HLCT-OLEDs.5b This should be
m-2, high maximum external quantum efficiency over 10.5%, and ascribed to either the low photoluminescence (PL) efficiency
high maximum exciton utilization efficiency approaching 100%, (φPL) of the emitters,8a,b or the low EUE of the devices.6a,8c-d
with CIE coordinates of (0.152, 0.065).
Owing to its large optical bandgap (Eg), good thermostability
and high TTA efficiency, pyrene has attracted much attention as
the building block for deep-blue fluorophores.9 Yet despite the
fact that pyrene owns a large energy gap between its T2 and T1
states (ΔE(T2-T1) = 1.3 eV) that is good for the suppression of
internal conversion (IC), and a small ΔE(S1-T2) (~0.2 eV) that is
good for the promotion of reverse intersystem crossing (RISC)
of the “hot excitons”,10 so far little effort has been devoted to
the exploitation of pyrene-containing HLCT materials. A
possible reason may lie in the symmetrically forbidden S1→S0
transition nature hence low radiative decay rate constant (kr,
~106 s-1) and low φPL (0.29) of pyrene unit.11 Note that for
traditional HLCT materials, to guarantee a relatively large kr, a
lower-lying highly fluorescent local excited singlet state (1LE) is
indispensable.5b
Herein, we unveil that pyrene is a quite promising building
block for deep-blue HLCT materials. To endow pyrene-based
donor-acceptor-structured (D-A) materials with relatively high
kr, the twisting angle between the D and A units should be
relatively small, so that a CT-featured excited singlet state (1CT)
with relatively high radiative efficiency can be acquired. To
ensure deep-blue PL emission, 9,9-dimethylthioxanthene-S,S-
dioxide (TXO2) with LUMO energy level of ‒2.33 eV is selected
as the A moiety,12 and pyrene is used as the D unit. The
structure of the resultant DP-TXO2 and P-TXO2 is shown in
As deep-blue organic light-emitting diodes (OLEDs) can
promote better color gamut and reduced power consumption
in displays and lightings,1 the development of high-performance
deep-blue emitters, especially those capable of satisfying the
required CIE coordinates (x, y) = (0.15, 0.06) defined by High-
Definition Television (HDTV) ITU-RBT.709, is highly demanded.2
However, despite their theoretical maximum exciton utilization
efficiency (EUEmax) of 100%, highly efficient and stable deep-
blue phosphorescent devices are quite scarce by far.2,3
To address these issues, several alternative strategies for
utilizing triplet excitons via cheaper fluorescent materials, such
as triplet-triplet annihilation (TTA), thermally activated delayed
fluorescence (TADF), and hybridized local and charge-transfer
(HLCT) have been proposed.4,5 Currently, deep-blue TTA-OLEDs
can show relatively high maximum luminance (Lmax) together
with mild efficiency roll-off, but they often suffer from relatively
low maximum external quantum efficiency (EQEmax) due to their
intrinsic low EUEmax of ≤ 62.5%.6 In contrast, deep-blue TADF-
OLEDs can exhibit high EQEmaxs, but their Lmax and efficiency roll-
off are quite unsatisfactory.7 For deep-blue HLCT-OLEDs, they
a. Key Laboratory of Green Chemistry and Technology (Ministry of Education),
College of Chemistry, Sichuan University, Chengdu 610064, China.
E-mail: luzhiyun@scu.edu.cn.
b. State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of
Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China.
E-mail: zhoul@ciac.ac.cn.
c. State Key Laboratory of Supramolecular Structure and Materials, College of
Chemistry, Jilin University, Changchun, 130012, China.
E-mail: lifeng01@jlu.edu.cn
† These authors contributed equally to this work.
‡ Electronic Supplementary Information (ESI) available: Experimental procedures,
computational details. CCDC: 1894318 and 1894319. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/x0xx00000x.
Figure 1. Molecular structure of the objective compounds.
Figure 1.
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DOI: 10.1039/C9CC02355K
Figure 2. a) UV-vis absorption and PL spectra (in Tol, 298 K), and phosphorescence spectra (in 2-Me-THF, 77 K) of DP-TXO2, P-TXO2, and TPP-TXO2; b) PL spectra of DP-TXO2 in
Hex, BE, Tol and DCM (298 K); c) Lippert-Mataga plots of the fluorescence maxima of DP-TXO2, P-TXO2, and TPP-TXO2 against the solvent polarity parameters; d) transient PL
decay curves of DP-TXO2 in Hex, BE, and DCM at λem of 390 nm and 460 nm; e) PL spectra of a 1 wt.% DP-TXO2:PMMA film at 77-295 K; f) transient PL decay curves of a 1 wt.%
DP-TXO2:PMMA film at 77-295 K (λem = 400 nm).
Excitingly, crystallographic analysis indicates that DP-TXO2 observed in BE are tentatively assigned to the 1CT and 1LE states
and P-TXO2 both show relatively small D-A twisting angles of 46- of DP-TXO2, respectively. These findings suggest the mixing of
1
54° (Figure S1). This is appropriate to the formation of not only 1LE and CT states of DP-TXO2 in BE. Note that this is the first
HLCT, but also highly radiative 1CT states. As shown in Figure 2a, direct experimental evidence for the state-mixing of 1LE and 1CT
in toluene (Tol), DP-TXO2 and P-TXO2 show similar absorption in HLCT compounds. In the case of P-TXO2, since only one decay
maxima (λabs = 349 nm) with relatively high molar extinction component with τ of 2.9 ns is found in DCM (Table S2), in
coefficients (log ε > 4), and similar PL maxima (λem = 410 nm) combination with its φPL in DCM (0.68), the kr(CT) of P-TXO2 is
with relatively high φPL of 0.59 (Table 1). Analogous to most determined to be 2.3 × 108 s-1, which is similar to that of DP-
reported HLCT compounds,8b-d DP-TXO2 and P-TXO2 show more TXO2. But in both Hex and BE, P-TXO2 shows a bi-exponential PL
distinct positive solvatochromism in more polar solvents, and decay hence state-mixing of its 1LE and 1CT states (Table S2).
their Lippert-Mataga plots are nonlinear with their crossover
At 295 K, the doped film of 1 wt.% DP-TXO2 in poly(methyl
points near the solvent polarity of butyl ether (BE) (Figures 2b, methacrylate) (PMMA) exhibits an unstructured PL band with
c, S2, S3). In hexane (Hex) and dichloromethane (DCM), DP- λem of 414 nm (Figure 2e), and its PL decay curve shows a tri-
TXO2 shows a single-exponential PL decay with lifetime (τ) of exponential function with τ1 = 3.8 ns, τ2 = 10.4 ns, and τ3 = 33.2
15.7 and 2.4 ns, respectively (Figures 2d, S4). Judging from the ns (Figure 2f, Table S3). According to its transient PL properties
presence and absence of vibronic bands in its PL spectra in Hex in BE, the τ1 and τ2 species are assigned to the 1CT and 1LE states
and DCM, the S1 state of DP-TXO2 should be of purely LE- and of DP-TXO2, respectively. For the longest τ3 component, as the
CT-feature in Hex and DCM, respectively. Based on the τ and φPL content of DP-TXO2 is as low as 1 wt.%, and the monitored λem
data (φPL: 0.63 in Hex; 0.57 in DCM), the kr(LE) and kr(CT) of DP- (400 nm) is in the high-energy spectral region, the interference
TXO2 is calculated to be 4.0 × 107 and 2.4 × 108 s-1, respectively. from the excimer should be suppressed effectively. Hence the
The much larger kr(CT) than kr(LE) verifies the higher radiative τ3 component with lifetime of 33.2 ns is tentatively assigned to
1
1
efficiency of the CT than LE state of DP-TXO2. But different the delayed fluorescence (DF) of DP-TXO2.
from most HLCT fluorophores,5b,8 DP-TXO2 displays a bi- rather
With decreasing temperature to 77 K, the PL intensity hence
than single-exponential PL decay behavior in BE (τ1 ~2.0 ns; τ2 the overall φPL of the film sample depresses significantly (Figure
1
~7.2 ns). As the radiative transition of LE state is less-allowed 2e).13 Since the φPL(LE) and φPL(CT) of DP-TXO2 in Hex and DCM are
1
than that of CT state, the shorter and longer τ components comparable, the markedly lowered φPL of the DP-TXO2-based
Table 1. Photophysical characteristics, thermal stability, and frontier molecular orbital energy level of the three compounds.
Compd.
DP-TXO2
P-TXO2
λabsa) [nm]
349
349
logεa)
4.58
4.50
4.66
λem a)/b)/c)/d)/e)/f) [nm]
φPL a)/b)/c)/d)/e)/f)[%]
Td/Tg/Tm [ºC]
Egg) [eV]
3.21
3.25
HOMOh)[eV] LUMOi)[eV]
410/473/447/444/462/453 59/35/48/41/15/27 378/177/299
410/470/423/426/452/443 59/46/48/45/19/27 388/ ‒ /278
418/463/433/436/443/437 59/48/47/52/28/39 490/164/263
‒5.76
‒5.70
‒5.68
‒2.55
‒2.45
‒2.56
TPP-TXO2
365
3.12
a) In 2 × 10-6 M Tol solutions at 295 K; b) in neat thin films; c) 12 wt.%-doped thin films in PMMA matrix; d) 12 wt.%-doped thin films in 26DCzPPy host matrix; e) 12
wt.%-doped thin films in TcTa host matrix; f) 12 wt.%-doped thin films in TcTa:26DCzPPy (1:1) mixed host matrix; g) estimated from the onset of the absorption edge
in DCM solution; h) calculated from the formula HOMO = ‒ [Eox - E(Fc/Fc+) + 4.8] eV (in DCM solution); i) calculated from the formula LUMO = HOMO + Eg.
2 | J. Name., 2012, 00, 1-3
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Therefore, as depicted in Figure 3, our pyrene-containing
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objective compounds show a quite diffeDreOnI:t10H.1L0C3T9/mC9eCcCh0a2n3i5s5mK
from that of traditional HLCT materials: 1) for traditional HLCT
1
emitters, their LE states are responsible for the large kr,5b but
for pyrene-containing ones, relatively large kr is realized by an
efficiently radiative 1CT state; 2) for traditional HLCT materials,
their RISC processes of the “hot exciton” occur from the 3CT to
1CT states, and are facilitated by hyperfine coupling (HFC),7b,17
but for pyrene-containing ones, SOC is also an important driving
force facilitating the RISC processes from the 3LETn to 1CT states.
To trigger small charge injection barriers and balanced charge
injection and transportation, double-emissive layer OLEDs with
structure of ITO/HAT-CN (6 nm)/HAT-CN (0.2 wt.%): TAPC (50
nm)/emitter: TcTa (12 wt.%, 10 nm)/emitter:26DCzPPy (12
wt.%, 10 nm)/Tm3PyP26PyB (50 nm)/LiF (1 nm)/Al (100 nm) are
fabricated, in which DP-TXO2 or P-TXO2 acts as the emitter,
4,4′,4′′-tri(N-carbazolyl)tripheylamine (TcTa) and 2,6-bis(3-(9H-
carbazol-9-yl)phenyl)pyridine (26DCzPPy) serve as the hole- and
electron-transporting hosts, 1,1-bis(4-(N,N-di(p-tolyl)amino)
phenyl)cyclohexane (TAPC) and 1,3,5-tris(6-(3-(pyridine-3-yl)
phenyl)pyridin-2-yl)benzene (Tm3PyP26PyB) act as the hole-
and electron-transporting materials, respectively (Figure S15).
Although the two devices emit deep-blue electroluminescent
(EL) with similar EL maxima (λEL) of 443 nm and EQEmax of 4%,
their CIE coordinates are (0.154, 0.098) for DP-TXO2-based, and
(0.160, 0.085) for P-TXO2-based devices (Table 2). Inferred from
the more red-shifted PL spectrum and the lower φPL of the 12
wt.% DP-TXO2:TcTa film than those of the P-TXO2- based one
(Figure S17, Table 1), the inferior chromaticity of the DP-TXO2-
based device is ascribed to the more serious exciplex-forming
intermolecular interactions between the TcTa host and the
pyrene subunit in DP-TXO2 than that in P-TXO2.8a Nevertheless,
taking into account that the φPLs of the 26DCzPPy-hosted and
TcTa-hosted emitting layers are ≤ 0.45 and ≤ 0.19, respectively,
the high EQEmax of 4% of these devices implies the relatively
high EUEmax of these devices (> 40%, Table S10), indicative of
the effective harvesting of triplet excitons in these devices.
To alleviate the intermolecular interactions between the
pyrene subunit of the objective compounds and the TcTa host,
a bulky 1,1':3',1''-terphenyl (TP)8a group is grafted at the pyrene
moiety of P-TXO2 to render TPP-TXO2 (Figure 1). As depicted in
Table 1, Figures 2a and S2, while the presence of the TP
substituent will lead to slightly red-shifted absorption and PL
Figure 3. The mechanism diagram of EL and molecular design. a) Traditional HLCT
materials; and b) HLCT materials bearing pyrene subunits.
film should be mainly ascribed to the decreased contribution
from the DF rather than the alteration in contribution from the
1
1CT and LE states of DP-TXO2. Meanwhile, the lowered φPL of
DP-TXO2 and the prolonged lifetime of the τ3 species (from 33.2
to 60.3 ns, vide Table S4) upon cooling down verify the
thermally activated nature of the DF species of DP-TXO2.14 It is
noteworthy that the quite short lifetime of the TADF species of
DP-TXO2 indicates the presence of very fast RISC processes in
DP-TXO2. Actually, as shown in Table S5, the RISC rate constant
(kRISC) of DP-TXO2 is calculated to be as high as 7.3 × 106 s-1,4a,13-15
which is not only good for the efficient harvesting of triplet
excitons, but also favorable for the suppression of efficiency
roll-off in OLED applications.7a
1
1
Nevertheless, the energy levels of LE and CT states of DP-
TXO2 are both ~ 3.2 eV, but that of its LE-featured T1 state is only
2.0 eV (Figure 2a). Accordingly, the ΔEST between 1LE/1CT and T1
is 1.2 eV, which is too large to promote fast RISC process
between them. Consequently, the TADF in DP-TXO2 should be
attributed to the RISC of the “hot exciton”. To gain insight into
the TADF mechanism of DP-TXO2, density functional theory
(DFT) calculations are performed. As shown in Figure S9, DP-
TXO2 owns pseudo-degenerated S1 and S2, T1 and T2, and T3 and
T4 states due to its symmetrical D-A-D structure. Both the S1 and
S2 states show a HLCT transition character; both the T1 and T2
states show a LE-dominated feature mainly confined to the
pyrene moiety; while both T3 and T4 states possess a LE-
dominated feature. Because of the quite small ΔEST (0.02 eV)
and marked difference in the transition nature, relatively large
spin-orbital coupling (SOC) elements hence large kRISC between
the S1/S2 and T3/T4 states of DP-TXO2 can be expected.7b,15,16
Figure 4. Electroluminescence (EL) characteristics of OLEDs based on DP-TXO2, P-TXO2, and TPP-TXO2. a) EL spectra at current density of 100 mA cm-2; b) current density-
voltage-luminance (J-V-L) characteristics; c) EQE–luminance characteristics.
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Table 2. Summary of EL characterisation results of the objective compounds.
Acknowledgements
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DOI: 10.1039/C9CC02355K
EQEa)/b)
[%]
Lmax CEmax
[cd m-2] [cd A-1]
CIE 1931
(x, y)
We acknowledge the support from the National Natural Science
Foundation of China (Nos. 21672156, 21771172, 51573108,
51673080, 21432005). We thank Dr. P. Wu and Dr. D. Luo of
Analytical & Testing Center, Sichuan University for photophysical
and crystallographic measurements.
Von
[V]
Device
DP-TXO2 3.3
P-TXO2 3.6
TPP-TXO2 3.1 10.5/4.6
4.0/3.5
4.0/3.4
9120
5730
10480
4.6
3.0
11.1
(0.154, 0.098)
(0.160, 0.085)
(0.152, 0.065)
a) Maximum external quantum efficiency; b) EQE at a luminance of 1000 cd m-2.
Conflicts of interest
bands than those of DP-TXO2 and P-TXO2 in solution, in both
neat and doped films, TPP-TXO2 shows not only the best deep-
blue color purity, but also the highest φPL, verifying the lessened
intermolecular interactions. More importantly, TPP-TXO2 has a
much higher kRISC of 1.4 × 107 s-1 than that of DP-TXO2 or P-TXO2
(Table S5). According to the DFT calculation results, the ultra-
high kRISC of TPP-TXO2 should originate from its quite small ΔE(S1-
There are no conflicts to declare.
Notes and references
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2
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markedly different nature of its LE-dominated T2 state (confined
to the TP segment) and HLCT-featured S1 state (localized in the
TXO2 and pyrene units) (Figure S11). Excitingly, the TPP-TXO2-
based OLED shows much enhanced EL performances than those
based on DP-TXO2 and P-TXO2, with more desirable color purity
of CIE (0.152, 0.065) approaching the deep-blue standard
defined by the European Broadcasting Union (EBU), together
with much higher EQEmax of 10.5% and Lmax of 10480 cd m-2.
Note that the high EQEmax of 10.5% indicates that the ultra-high
EUEmax even can approach 100% in this device (Table S10). More
fascinatingly, this device exhibits just moderate effciency roll-
off, with EQE values of 4.6% at 1000 cd m-2 and 2.8% at 10000
cd m-2. This should be attributeded to the quite fast RISC
process of the emitter material TPP-TXO2.
Since pyrene is also a widely-used building block of TTA materials,
to probe if TTA process contributes significantly to the total EL in
these devices, magneto-electroluminescence (MEL) response of a
DP-TXO2-based OLED is measured, because DP-TXO2 owns the
highest content of pyrene unit among these compounds. The MEL of
the device increases sharply within the low-field regime (< 50 mT),
then saturate in a higher B-field with increasing external magnetic
field at varied applied bias (Figure S20). Accordingly, the emitting
mechanism of DP-TXO2 should not be TTA-dominant.18
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In summary, we demonstrate that pyrene is a quite promising
structural unit for the construction of high-performance HLCT
emitters. Different from traditional HLCT emitters, in these pyrene-
containing HLCT materials, an efficiently radiative 1CT state is
responsible for the high kr of these compounds, while the effective
1
3
SOC between their CT and LETn states may be an important factor
responsible for the high kRISC of these compounds. This represents a
new molecular design strategy for HLCT emitters capable of showing
both high kr and high kRISC, through which ultra-high EUE could be
realized. OLEDs based on one objective molecule of TPP-TXO2 can 16 X. K. Chen, D. Kim and J. L. Bredas, Acc. Chem. Res., 2018, 51,
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emit deep-blue light with decent color purity of CIE (0.152, 0.065)
and EQEmax of 10.5%, together with high luminance of 10480 cd m-2.
This facile constructive strategy for HLCT OLED emitters can greatly
extend the design rationales, which will light up the enthusiasm of
scientists to develop more promising HLCT materials showing other
emission colors.
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ChemComm
View Article Online
DOI: 10.1039/C9CC02355K
Pyrene is a quite promising structural unit for HLCT emitters, and deep-blue OLED
showing quite high brightness over 10000 cd m-2 and EQEmax over 10.5% has been
achieved.