Quinoxaline and Pyrido[x,y-b]pyrazine-Based Emitters: Tuning Normal Fluorescence to Thermally Activated Delayed Fluorescence and Emitting Color over the Entire Visible-Light Range

Article abstract of DOI:10.1002/chem.201902116

Quinoxaline (Q), pyrido[2,3-b]pyrazine (PP) and pyrido[3,4-b]pyrazine (iPP) are used as electron acceptors (A) to design a series of D–π–A-type light-emitting materials with different donor (D) groups. By adjusting the molecular torsion angles through changing D from carbazole (Cz) to 10-dimethylacridine (DMAC) or 10H-phenoxazine (PXZ) for a fixed A, the luminescence is tuned from normal fluorescence to thermally activated delayed fluorescence (TADF). By gradually enhancing the intramolecular charge-transfer extent through combining different D and A, the emission color is continuously and regularly tuned from pure blue to orange–red. Organic light-emitting diodes (OLEDs) containing these compounds as doped emitters exhibit bright electroluminescence with emission colors covering the entire visible-light range. An external quantum efficiency (η_ext) of 1.2 % with excellent color coordinates of (0.16, 0.07) is obtained for the pure-blue OLED of Q-Cz. High η_ext values of 12.9 (35.9) to 16.7 % (51.9 cd A^?1) are realized in the green, yellow, and orange–red TADF OLEDs. All PP- and iPP-based TADF emitters exhibit superior efficiency stabilities to that of analogues of Q. This provides a practical strategy to tune the emission color of Q, PP, and iPP derivatives with the same molecular skeletons over the entire visible-light range.

Full text of DOI:10.1002/chem.201902116

A Journal of
Accepted Article
Title: Quinoxaline and Pyrido[x,y-b]pyrazine Based Emitters:
Tuning Normal Fluorescence to Thermally Activated Delayed
Fluorescence and Emitting Color Over Entire Visible Light Range
Authors: Di Liu, Tingting Huang, Jingyang Jiang, and Wenfeng Jiang
This manuscript has been accepted after peer review and appears as an
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To be cited as: Chem. Eur. J. 10.1002/chem.201902116
Link to VoR: http://dx.doi.org/10.1002/chem.201902116
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10.1002/chem.201902116
Chemistry - A European Journal
Quinoxaline and Pyrido[x,y-b]pyrazine Based Emitters: Tuning
Normal Fluorescence to Thermally Activated Delayed Fluorescence
and Emitting Color Over Entire Visible Light Range
Tingting Huang, Di Liu,* Jingyang Jiang, Wenfeng Jiang
State Key Laboratory of Fine Chemicals, College of Chemical Engineering, Dalian
University of Technology, 2 Linggong Road, Dalian, 116024, China. E-mail:
liudi@dlut.edu.cn
Abstract: Quinoxaline (Q), pyrido[2,3-b]pyrazine (PP) and pyrido[3,4-b]pyrazine
(iPP) are used as electron acceptors (A) to design a series of D--A type light-emitting
materials with different donor (D) groups. By adjusting the molecular torsion angles
through changing donor from carbazole (Cz) to 10-dimethyl-acridine (DMAC) or
10H-phenoxazine (PXZ) for a fixed acceptor, the luminescence is tuned from normal
fluorescence to thermally activated delayed fluorescence (TADF). By gradually
enhancing the intramolecular charge transfer extent through combining different D
and A, the emission color is continuously and regularly tuned from pure blue to
orange-red. Organic light-emitting diodes (OLEDs) containing these compounds as
doped emitter exhibit bright electroluminescence with emission color covering the
entire visible light range. An external quantum efficiency (ηext) of 1.2% with excellent
color coordinates of (0.16, 0.07) is obtained for the pure blue OLED of Q-Cz. High
1
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ηexts of 12.9% (35.9 cd A-1) to 16.7% (51.9 cd A-1) are realized in the green, yellow
and orange-red TADF-OLEDs. All the PP and iPP based TADF emitters exhibit
superior efficiency stabilities to the quinoxaline analogues. It provides a practical
strategy to tune the emission color of quinoxaline and pyrido[x,y-b]pyrazine
derivatives with same molecular skeletons over the entire visible light range.
Keywords: thermally activated delayed fluorescence (TADF), quinoxaline,
pyrido[2,3-b]pyrazine, entire visible light range, organic light-emitting diodes
(OLEDs)
Introduction
As the third-generation organic light-emititng diodes (OLED) emitters, thermally
activated delayed fluorescence (TADF) mateirals have attracted more and more
attentions due to advantageous merits of high emission efficiency and low cost.1,2 It is
required that the energy splitting between S1 and T1 (∆EST) should be as small as
possible to decrease the exchange interaction integral of the highest occupied
molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO)
wave functions of a molecule and to guarantee efficient reverse intersystem crossing
(RISC) of T1 with thermal activation into S1 and the subsequent delayed
fluorescence.3,4 Accordingly HOMO and LUMO should be spatially separated. In
most cases highly twisted molecular configuration is necessary to enhance the
HOMO/LUMO separation and facilitate the TADF feature. When TADF materials are
used as the emitting component of OLEDs, all the electrogenerated triplet excitons
formed on TADF molecules (that occupying 75% of all the excitons generated
2
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through charge recombination) are possible to be upconverted into singlet excitons
and emit delayed fluorescence, and all the electrogenerated singlet excitons of TADF
emitter (that occupying 25% of all the electrogenerated excitons) can emit prompt
fluorescence in a certain fluorescence quantum yield. In this way, all the
electrogenerated singlet and triplet excitons can be harvested for light emission and a
theoretical uplimit of unity for internal quantum efficiency (IQE) can be achieved in
TADF-OLEDs.5,6 Obviously the same electroluminescence (EL) performance as
phosphorescent OLEDs can be achieved by using the pure organic TADF emitters
without involving the noble metal phosphors. Since the pioneering report of
TADF-OLEDs by Adachi and coworkers,7 have always been the hotspot in OLED
research field. In recent years, a great many efficient TADF emitters have been
developed based on various type of electron acceptor units such as triazine,8 sulfone,9
benzophenone,10,11 pyrimidines,12 quinoxaline,13,14 and so forth, leading the OLEDs
research field stepping into a new stage of diversity in light-emitting materials.
TADF molecules are typically characterized by the donor-π-acceptor (D-π-A) or
D-A architectures, in which the weak intramolecular charge transfer between D and A
determines the S1 and T1 excited state energies of the molecule. Apparently TADF
molecules of different emission colors or even covering the whole visible light range
can be constructed by combinating different D and A and by adjusting the steric
molecular conformations. Wong and coworkers reported a series of TADF emitters
using 9,9’-dimethylacridan (DMAC) as common donor. The emission wavelengths
were widely tunable from deep blue to yellow not only by the general acceptor
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strength due to the characters of heteroarene and CN-substitution pattern but also by
the subtle control of in-acceptor conformation (twist angles). Highly efficient
blue-green to yellow TADF-OLEDs having external quantum efficiencies (EQE, ηext)
of up to 23.1%–31.3% were achieved using these efficient TADF emitters.15 Yasuda
and coworkers built a group of full-color TADF molecules comprising a central
phthalonitrile or 2,3-dicyanopyrazine acceptor core coupled with various donor units
of different electron-donating abilities, which exhibited the moderate to high ηexts of
8.7%, 18.9%, 13.3% in blue, green and orange-red OLEDs, respectively.16 Wang and
coworkers tuned the emission color from yellow to red for a group of
2,3-dicyanopyrazinophenanthrene (DCPP) based TADF emitters by varying the
electron donor from Cz, via diphenylamine (DPA), to DMAC. Moderate ηexts of
10.1% to 16.9% were obtained for those yellow to red TADF-OLEDs.17 The same
acceptors coupled with different donors may result in different luminescence
mechanisms. For example, even based on the identical D-π-A structural strategy, the
molecule incorporating carbazole as a mild electron-donor and sulfone as an
electron-acceptor (CzS2) is a normal fluorescent compound and exhibited a maximum
ηext of 2.7% in OLED,18 while the analogue containing DMAC as donor and one
diphenylsulfone (DPS) acceptor (DMAC–DPS) revealed TADF feature and achieved
a maximum ηext of 19.5% in corresponding OLED.19 The small molecular D–A or
D-π-A chromophores of various colors have achieved good performance so far, and
considerable progresses have been made in ηexts of RGB TADF OLEDs, e.g. nearly
37% for sky-blue,20 over 30% for green21 and 21-29.2% for orange-red.22 As far as we
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know, these values should be the state-of-art efficiency for each primary color TADF
OLEDs reported so far. It is clear that the electroluminescent properties of individual
RGB emitting TADF materials are efficient and stable and have been successfully
utilized in OLEDs. However, the TADF materials of similar molecular skeletons with
emission covering the wide or entire visible light range have been rarely reported so
far. In addition, it can be found that many orange–red TADF emitters have played
positive roles in both sinlge-color and white OLEDs, however, only a few materials
can achieve high efficiency.23 This is partially because the long-wavelength emitters
(orange to red) are generally subjected to the increasing non-radiative internal
conversion process governed by the energy gap law, and then resulting in low
photoluminescence quantum yield (PLQYs, ΦPL).24 Furthermore, efficiency roll-off at
high driving range is also a concerned issue for orange-red TADF-OLEDs. There is a
design regulation that most of the reported TADF emitters adopt a twisted
intramolecular charge-transfer (ICT) configuration, in which the electron donor and
acceptor units are incorporated to be orthogonal to each other.25 Therefore, high
rigidity and strong charge transfer (CT) states in D-A systems are desired for
constructing efficient orange-to-red TADF emitters.
With the development and evolution of D-A or D-π-A type TADF molecules, the
challenge is to rationally design novel emitters based on simple versatile molecular
scaffolds. Quinoxaline (Q) and pyrido[x,y-b]pyrazine have the same double
hexatomic ring aromatic frameworks despite the only difference that
pyrido[x,y-b]pyrazine has one nitrogen atom replacing one carbon atom of
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quinoxaline ring. It can be expected that the introduction of the additional sp2 type N
atom in pyrido[x,y-b]pyrazine will definitely enhance the electron-withdrawing
ability in comparison with quinoxaline. In addition, with the different location of the
N
atom in pyridine ring, pyrido[2,3-b]pyrazine (PP) exhibits stronger
electron-deficient ability than its isomer pyrido[3,4-b]pyrazine (iPP). Quinoxaline has
been explored as electron acceptor to build TADF molecules,26,27 while there is few
report utilizing pyrido[2,3-b]pyrazine or pyrido[3,4-b]pyrazine as acceptor units to
construct TADF emitters. Based on the stepwise enhancement of the
electron-withdrawing ability in the order of Q, iPP, and PP, it can be expected that
continuous color tuning can be realized if these acceptors are used to build
fluorophors by simultaneously adjusting the electron donating ability of the donor
groups. Herein, we designed and prepared a series of novel D–π–A type fluorescent
emitters using Q, iPP, and PP as electron acceptors and central linking bridges, and
carbazole (Cz) 9,10-dimethyl-acridine (DMAC) and 10H-phenoxazine (PXZ) as the
electron donors. Double donor groups were linked to each central acceptor bridge to
form the wedge-shaped molecules Q-Cz, Q-DMAC, iPP-DMAC, PP-DMAC,
iPP-PXZ and PP-PXZ (Scheme 1). The photophysical, electrochemical, and thermal
properties of these compounds were systematically studied. It was observed that,
despite the similar acceptor skeleton and same D–π–A–π–D configuration and
wedgeshape in all these molecules, the carbazole based Q-Cz is the normal
fluorescent compound, while the other five analogues show typical TADF feature,
which is attributed to the less crowded steric feature of carbazole ring than other
6
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donor groups. In virtue of continuous regulation of both acceptor strength and donor
strength, the emission wavelengths of these six analogues were successfully tunable
from pure blue via green to orange-red. These emitters were used to fabricate OLEDs
and electroluminescence with continuously tunable color from blue to orange-red was
obtained. Moderate to high efficiencies of 1.2%, 16.7%, 14.1% and 13.0% were
achieved for the pure blue, green, yellow, and orange-red OLEDs. It provides a
practical strategy using quinoxaline and pyrido[x,y-b]pyrazine to construct simple
molecules to realize continuous tuning of emission color from pure blue to orange red.
This is the first journal report to use pyrido[2,3-b]pyrazine and pyrido[3,4-b]pyrazine
as acceptor units to construct TADF emitters.
Scheme 1. Structures of the quinoxaline and pyrido[x,y-b]pyrazine based RGB TADF
emitters.
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Results and Discussion
Synthesis and Thermal Properties. The structure and detailed synthetic
procedures of all compounds are described in Scheme 1 and Scheme S1. The
important
intermediates
2,3-dichloroquinoxaline
acid
(a),28
(b),29
(4-(9H-fluoren-9-yl)phenyl)boronic
1,2-bis(4-bromophenyl)ethane-1,2-dione (c),30 2,3-bis(4-bromophenyl)quinoxaline (d),
2,3-bis (4-bromophenyl) pyrido [3,4-b] pyrazine (e) and
2,3-bis(4-bromophenyl)pyrido[2,3-b]pyrazine (f)31 were synthesized according to
previously reported methods. The carbazole based compound Q-Cz was then prepared
in high yield by Suzuki cross-coupling reaction of intermediates a and b. The other
five analogues were synthesized via Buchwald-Hartwig reaction of d, e, or f with
9,9-dimethylacridan (DMAC-H) or 10H-phenoxazine (PXZ-H) at moderate yields of
40-50%. The details of synthesis and characterization for all these compounds are
illustrated in supporting information.
The thermal and morphological properties of these compounds were studied
using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC)
under a nitrogen atmosphere, the corresponding data are summarized in Table 1. As
shown by TGA thermograms in Figure S1, most of these compounds show high
decomposition temperatures (Td, corresponding to 5% weight loss) over 400 ℃,
indicating excellent thermal stabilities. While PP-PXZ exceptionally starts to
decompose at 164 ℃. During the second heating cycle in the DSC measurement
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(Figure 1), Q-DMAC, PP-DMAC and iPP-PXZ were found to show high glass
transition temperatures (Tg) of 124 ℃, 137 ℃ and 131 ℃, respectively. No glass
transition was observed for Q-Cz and iPP-DMAC under the present measurement
conditions. PP-PXZ was not explored for DSC measurement due to easy
decomposition in low temperature range. The high Td and Tg indicate that emitters
have good thermal and morphological stability, which should be attributed to their
relatively high molecular weights and appropriate molecular conformations and be
favorable for good device stability when used in OLEDs.
124 °C
131 °C
Q-Cz
Q-DMAC
iPP-DMAC
PP-DMAC
iPP-PXZ
137 °C
125
50
75
100
150
Temperature (ºC)
Figure 1. DSC traces (second heating cycle) of compounds recorded under nitrogen
at a heating rate of 10 °C min-1.
Steady-State Photophysical Properties. The steady-state photophysical
properties of these compounds were analyzed using UV-vis absorption and
photoluminescence (PL) spectra in dilute toluene solutions. As shown by the
absorption spectra in Figure 2a, Q-Cz shows different absorption features with other
analogues. The strong and moderate absorption bands below 350 nm for Q-Cz should
be assigned to the characteristic π–π* and n–π* transitions of carbazole and
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quinoxaline rings.32,33 The additional absorption band at about 370 nm should
originate from the intramolecular charge transfer (ICT) transition from
electron-donating carbazole to the electron-deficient quinoxaline moieties. For three
DMAC-based compounds, the strong absorption bands at about 280 nm and moderate
bands at 345 nm should be due to the intrinsic electron transitions of acridine and
acceptor parts (pyrido[x,y-b]pyrazine), while the weak absorption bands at 370-475
nm should be due to intramolecular ICT transitions. Both two PXZ-based analogues
show strong absorption bands centered at about 340 nm and weak bands at 430 nm,
which should be assigned to the π–π* transition of PXZ ring and ICT transition. Upon
photoexcitation, these compounds emit strong fluorescence in dilute toluene solutions
with peak wavelengths at 453, 522, 583, 578, 615, 620 nm, respectively, as illustrated
in Figure 2b.The emitting color of this series of compounds continuously cover the
whole visible light range from blue to red. It can be deduced that the fluorescence
should originate from the ICT excited state of these molecules based on the
structureless spectra profile. The results indicate that acceptor and donor groups play a
leading role in the emission color of luminescent materials. Figure S2 shows the
low-temperature (LT) fluorescence and phosphorescence spectra of these compounds
in frozen dimethyl tetrahydrofuran (2-Me-THF) at 77 K. The energy levels of the S1
and T1 states are calculated from the peak wavelength of the highest-energy
fluorescence and phosphorescent subbands. The S1-T1 energy splittings (ΔEST) are
calculated as 0.53, 0.03, 0.09, 0.06, 0.04 and 0.02 eV for Q-Cz, Q-DMAC,
iPP-DMAC, PP-DMAC, iPP-PXZ and PP-PXZ, respectively. All the pertinent data
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are summarized in Table 1. It is obvious that Q-Cz possess much larger ΔEST than all
other five analogues. In contrary, the extremely low ΔEST values imply that efficient
thermally activated RISC from T1 to S1 and thus delayed fluorescence may be
expected to occur in five DMAC and PXZ based compounds.34
(a)
Q-Cz
1.0
0.5
0.0
Q-Cz
(b)
Q-DMAC
iPP-DMAC
PP-DMAC
iPP-PXZ
PP-PXZ
Q-DMAC
iPP-DMAC
PP-DMAC
iPP-PXZ
PP-PXZ
1.0
0.5
0.0
300
400
500
600
400
600
Wavelength (nm)
800
Wavelength (nm)
Figure 2. UV-vis absorption and fluorescence spectra in dilute toluene solutions (10-5
mol L-1) at room temperature.
Table 1. Physical data summary of new compounds.
a
a
b
c
e
f
g
λabs
(nm)
λem
λem
Td /Tg
(℃)
HOMOd LUMOd ΔEST
ΦPL
τPF/τDF
Compound
(nm) film(nm)
(eV)
-5.61
-5.27
-5.25
-5.25
-5.01
-5.01
(eV)
-2.99
-2.99
-3.26
-3.29
-3.26
-3.30
(eV) film(%) (ns)/(µs)
Q-Cz
Q-DMAC
293/341/368 453
284/340/400 522
iPP-DMAC 287/341/414 583
451
501
524
545
578
591
422/NF
367/124
389/NF
425/137
407/131
164/NF
0.53
0.03
0.06
0.09
0.02
0.04
29
38
87
83
65
31
3.0/-
20.4/6.8
19.8/69.7
48.2/21.2
32.1/1.0
48.4/0.8
PP-DMAC 286/345/409 578
iPP-PXZ
PP-PXZ
287/329/437 615
287/335/438 620
a
Absorbance and PL measured in toluene solutions at room temperature, taken from
b
Fig. 2; PL of TADF emitters in CBP films (5 wt%), Q-Cz in mCP film (5 wt%),
taken from Fig. S3f; c Td: decompose temperature corresponding to 5% weight loss; d
HOMO and LUMO levels were calculated by CV data; e ΔEST = ES – ET, both ES and
f
ET were determined from the fluorescence and phosphorescence spectra at 77 K; PL
quantum yield for these compounds doped in CBP (5 wt%), measured by integrating
sphere; g Lifetimes of prompt fluorescence (τPF) and delayed fluorescence (τDF) for the
doped films of these compounds in CBP (5 wt%) at room temperature.
Transient PL decay property. To reveal the different light-emitting property of
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these compounds, the time-resolved transient decay measurements were performed
for their doped films in 4,4′-bis(N-carbazolyl)-1,1′-biphenyl (CBP) host. When these
doped films were excited by a picosecond laser by means of time-correlated single
photon counting (TCSPC) technique, a prompt fluorescence (PF) with the typical
lifetime (τPF) of 3.0, 20.4, 48.2, 19.8, 48.4 and 32.1 ns were detected for each of them
at room temperature, respectively, as shown in Figure 3a and Figure S3. When these
films were excited by a microsecond lamp using multi-channel scanning (MCS)
technique, long lifetime component was not detected for Q-Cz, indicating that Q-Cz
does not possess the delayed fluorescence property. While a long-lifetime emitting
component was detected for all other five compounds with the decay lifetime (τDF) of
6.8, 21.2, 69.7, 0.8 and 1.0 µs (Figure S3 a-e), respectively, which should be assigned
as delayed fluorescence (DF) as judged from the fact the delayed components have
similar emitting wavelength to the PF of each compound. Apparently the delay
fluorescence lifetimes of DMAC-based compounds are all longer than those of
PXZ-based compounds. To explore the generation mechanism of the delayed
fluorescence observed for these five DMAC- and PXZ-based compounds,
temperature-dependent transient PL decay were measured from 100 K to 300 K, and
decay curves are shown in Figure 3. It can be discerned that the fluorescence lifetimes
of these compounds increased with temperature, which is a typical character for
TADF emitters. This indicates that high temperature is favorable for RISC and
emission of delayed fluorescence. Along with the extremely small ΔEST values (Table
1), the observed temperature-dependent feature of PL decay for these DMAC- and
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PXZ-based compounds confirms that their delayed fluorescence is really generated by
TADF mechanism.
The PL quantum yield (ΦPL) of these emitters doped in CBP films (5 wt%) were
measured using an integrating sphere as 29%, 38%, 87%, 83%, 65% and 31%,
respectively. According to the ΦPL and lifetime values, the prompt fluorescence
quantum yield (ΦPF), delayed fluorescence quantum yield (ΦDF), radiative decay rate
constant of S1 state (krS), the intersystem crossing (ISC) rate constant (kISC), and the
reverse ISC rate constant (kRISC) of the five TADF emitters were estimated and listed
in Table S1. Among the four TADF emitter with PP and iPP acceptors, the
DMAC-based PP-DMAC and iPP-DMAC showed much higher delayed fluorescence
quantum yields ΦDF of 46% and 48%, in contrast to 5% and 14.3% for PXZ-based
PP-PXZ and iPP-PXZ. At the same time, the ΦDF/ΦPF ratios of PP-DMAC and
iPP-DMAC are also much higher than those of PP-PXZ and iPP-PXZ. These indicate
that the DMAC donor, relative to PXZ, should be more favorable for high TADF
efficiency when PP and iPP are used as acceptors to design TADF emitters.
104
(b)
100K
200K
300K
(a)
Q-Cz
103
102
101
100
103
102
101
Q-DMAC
0
100
200
Time (us)
300
400
0
50
100
Time (ns)
150
200
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104
103
102
101
100
104
(c)
100K
150K
200K
250K
300K
(d)
100K
150K
200K
250K
103
102
101
100
300K
iPP-DMAC
PP-DMAC
0
200
400
600
800
0
300
Time (us)
600
900
Time (us)
104
103
102
101
100
104
(e)
100K
150K
200K
250K
300K
(f)
100K
150K
200K
250K
300K
103
102
101
100
PP-PXZ
iPP-PXZ
0
100
200
Time (us)
300
400
0
100
200
300
400
Time (us)
Figure 3. Transient PL decay curves of 5 wt% PP/iPP-DMAC and PP/iPP-PXZ doped
CBP films at various temperatures.
Theoretical calculations. The density functional theory (DFT) calculations were
performed to get insight on the different light-emitting feature of these quinoxaline
and pyrido[x,y-b]pyrazine based compounds. As shown in Figure 4, all these
compounds have similar wedgeshaped conformation. The central acceptor Q, PP, or
iPP in each molecule has the almost same dihedral angles (36-38°) with the two
suspended phenylenes bridges. However, the terminal donor rings, i.e. Cz, DMAC, or
PXZ, have different distorsion extents with the phenylene bridges. For all analogues
containing DMAC and PXZ rings, these donor rings are almost perpendicular to the
phenylene bridges with large dihedral angles of 71.86 to 89.35°. It is reasonable since
this highly twisted conformations can effectively avoid the steric repulsion of
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hydrogen atoms of the cross-linked phenylene and DMAC and PXZ rings.35 While for
carbazole based Q-Cz, the carbazole ring has much smaller dihedral angle (50.34°)
with the pheneylene bridges, suggesting less twisted conformation for Q-Cz relative
to other five analogues. The highest occupied molecule orbital (HOMO) of Q-Cz is
mainly located on terminal carbazole rings, with small contribution from the bridging
phenylene rings, while the lowest unoccupied molecule orbital (LUMO) is distributed
on the quinoxaline part. Apparently, the HOMO and LUMO have sufficient overlap
with each other due to the less twisted conformation and result in the efficient
π-conjugation between donor and acceptor. In contrast, the HOMOs of Q-DMAC and
other DMAC- and PXZ-based analogues are exclusively located on the terminal
donor rings, while their LUMOs are mainly located on the Q, PP or iPP acceptor rings
with tiny spreading on the phenylene rings, with almost no spatial overlap between
donors and acceptors owing to the highly twisted donors relative to the bridging
phenylenes. The noticeable spatial overlap of HOMO/LUMO should interpret the
relatively large ΔEST (0.53 eV) of Q-Cz and normal fluorescence behavior observed in
photophysical study. In contrast the highly twisted structures of all DMAC- and
PXZ-based analogues prevent orbital interaction between electron-donating and
withdrawing moieties, leading to the well-separated HOMOs and LUMOs. The
complete spatial separation of HOMOs and LUMOs provides a weak exchange
interaction between HOMOs and LUMOs and results in sufficiently small ΔEST (0.02
– 0.09 eV) to induce efficient S1←T1 RISC and finally generate TADF.36 It should be
noted that the HOMOs of these compounds look like not eaqually distributed on two
15
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10.1002/chem.201902116
Chemistry - A European Journal
donor rings (Figure 4), although the two donor parts are chemically equivalent in
these symmetrical molecules. In fact, as judged by the almost identical energies of
HOMO and HOMO-1 orbitals (Figure S4) for these molecules,37 the two terminal
donor rings should have identical contribution to their HOMOs and they are
energetically equivalent.
Figure 4. Calculated spatial distributions of HOMO and LUMO and geometry
optimized structures of the investigated molecules in their S0 state level.
Electrochemical properties. The electrochemical redox activity of these
compounds were investigated by using cyclic voltammetry (CV), and the cyclic
voltammograms are shown in Figure 5, and the corresponding data are summarized in
Table 1. These compounds showed quasi-reversible oxidation and reversible reduction
waves, indicating these materials have excellent electrochemical stability. It would be
reasonable to assign the oxidation wave of each compound to the one-electron
donation of the carbazole, DMAC and PXZ part, and the reduction to the electron
withdrawing on quinoxaline, PP and iPP moieties. During the anodic scan, Q-Cz and
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Q-DMAC showed a quasi-reversible oxidation peak with the onset potential of the
first oxidation wave ( Eooxnset ) at 1.210 V and 0.874 V. The other two DMAC-based
onset
ox
compounds iPP-DMAC and PP-DMAC exhibit the
E
of 0.859 V and 0.852 V,
respectively. At the same time the both PXZ-based compounds iPP-PXZ and PP-PXZ
onset
ox
onset
ox
have same
E
of 0.609 V. It is evident that the
E
becomes lower and lower
with the donor going in the order of Cz, DMAC, and PXC, indicating that these
compounds can be oxidized more and more readily due to the reduced oxidation
potential of the donor part. For the isomers containing same donor but different
acceptors, e.g. iPP-DMAC and PP-DMAC or iPP-PXZ and PP-PXZ, the Eooxnset are
almost identical to each other. This is reasonable since the oxidaiton of these
compounds occur on the donor part within these molecules. During cathodic scan, the
quinoxaline based two compounds Q-Cz and Q-DMAC were electrochemically
reduced at much lower potentials with the onset potential of the first reduction wave
(
Eroendset ) at -1.408 V and -1.410 V. With the acceptor going to iPP and PP, the reduction
onset
red
occurred at much lower
E
of -1.139 V, -1.113 V, -1.139 V and -1.101 V for
iPP-DMAC, PP-DMAC, iPP-PXZ and PP-PXZ, respectively. For the isomers with
same donor groups, e.g. iPP-DMAC and PP-DMAC, or iPP-PXZ and PP-PXZ, the
isomers containing PP acceptor has less negative reduction potentials than iPP-based
isomers. This further proves that PP has slightly stronger electron withdrawing ability
than iPP, both which have stronger electron withdrawing ability than quinoxaline (Q).
The HOMOs and LUMOs energy levels of these compounds were determined with
the empirical equations EHOMO = −e(
E
+ 4.4) and ELUMO = −e(
E
+ 4.4).38 The
onset
ox
onset
red
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HOMO levels of these compounds were estimated to be -5.61 eV, -5.27 eV -5.25 eV,
-5.25 eV, -5.01 eV and -5.01 eV, and the LUMOs are -2.99 eV, -2.99 eV, -3.26 eV,
-3.29 eV, -3.26 eV and -3.30 eV, respectively. The electrochemical band gap (Eg) of
onset
ox
onset
red
each compound was calculated as the potential difference between
E
and
E
multiplied by the electron charge (e). The Eg are determined as 2.62, 2.28, 1.99, 1.96,
1.75, and 1.71 eV for Q-Cz, Q-DMAC, iPP-DMAC, PP-DMAC, iPP-PXZ and
PP-PXZ, respectively. Evidently the regular change in electrochemical band gaps of
these compounds is in consistent with and can well account for the trend observed in
their fluorescence wavelength and emitting color. Therefore, by gradually increasing
the electron donating ability of donor unit and simultaneously increasing the electron
withdrawing ability of acceptor, the fluorescence of the constructed D--A type
compounds have successfully been tuned from pure blue via green and yellow to
orange-red, covering the whole visible light range.
Q-Cz
Q-DMAC
iPP-DMAC
PP-DMAC
iPP-PXZ
PP-PXZ
2000
1000
0
-1000
-2000
Potential (mV vs SCE)
Figure 5. Cyclic voltammograms of the compounds measured in CH2Cl2/DMF
solutions. The black dotted lines schematically indicate the onsets of oxidation and
reduction for each compound.
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Electroluminescence property. To evaluate the performance of these
compounds as light emitters, multilayer OLED devices were fabricated. Based on the
high excited state energy of the blue fluorescent Q-Cz, after a careful exploration of
different hosts and different device configurations,
a
high-energy host
1,3-di(9H-carbazol-9-yl)benzene (mCP) was selected for it to fabricate blue OLED
with the device structure of ITO/PEDOT:PSS (40 nm)/NPB (20 nm)/mCP:Q-Cz (2
wt%, 20 nm)/TPBI (30 nm)/LiF (1 nm)/Al (200 nm). In this device, poly(3,
4-ethylenedioxythiophene) (PEDOT:PSS) was used as the hole-injecting layer,
N,N'-bis(1-naphthyl)-N,N'-diphenyl-1,1'-biphenyl-4,4'-diamine
(NPB)
as
the
hole-transporting layer (HTL), 1,3,5-tri(1-phenyl-1H-benzo[d]imidazol-2-yl)phenyl
(TPBI) as electron-transporting layer (ETL) and hole-blocking layer (HBL), LiF and
Al as the electron-injecting material and the cathode, respectively. While for other
five TADF emitters with emitting color from sky-blue to orange-red, a relatively
low-energy host, 4,4′-bis(N-carbazolyl)-1,1′-biphenyl (CBP, ET=2.6 eV) was selected
to fabricate the OLEDs with configuration of ITO/PEDOT:PSS (40 mn)/TAPC (20
nm)/CBP:TADF emitter (5 wt% or 10 wt%, 20 nm)/TmPyPB (40 nm)/LiF (1 nm)/Al
(200 nm). In these devices, 1,1-bis[4-[N,N-di(p-tolyl)-amino] phenyl]cyclohexane
(TAPC) and 1,3,5-tri[(3-pyridyl)-phen-3-yl]benzene (TmPyPB) were employed as the
HTL and ETL. In order to confine light-emiting excitons in the emitting layers, the
adjacent layers including NPB, TPBI, TAPC and TmPyPB were selected to possess
higher singlet and tripet energies than the dopant in the light-emitting layers in both
types of OLEDs. The energy level diagrams and the chemical structures of the related
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materials used in OLEDs are shown in Figure S5. The HOMOs and LUMOs of these
TADF emitters are sandwiched between those of the CBP host, relatively equilibrium
hole and electron capture abilities can be obtained.39 For the blue device of Q-Cz, the
doping concentration was set as 2 wt% due to its traditional fluorescence feature.
While all other TADF emitters based OLEDs, the doping concentration was explored
from 5 wt% to 10 wt% for performance optimization. And it was observed that 5 wt%
devices exhibited much higher efficiencies than higher dopong levels. As a special
case, additional concern was raised on the chemical stability of PP-PXZ and the
reliability of its OLED data since a weight loss was detected at a low temperature of
164 °C in TGA measurement (Table 1 and Figure S1). No new product or obvious
impurity was found in the residue after thermal evaporation of PP-PXZ during the
device fabrication of 5 wt% device, as confirmed by the thin layer chromotagraphy
(TLC) detection on the residue. And the fluorescence spectra of the evaporated
PP-PXZ films and 5 wt% OLED did not show discrepancy in terms of spectral profile
to its inherent PL spectra in dilute solution. These imply that the thermal deposited
PP-PXZ did not encounter decomposition if slowly and carefully heated and its EL
data are reliable in the case of 5 wt% OLED. However, slight thermal decomposition
was observed for PP-PXZ during device fabrication of 10 wt% device, as verified by
the darkening of the residue after thermal evaporation and simutaneous detection of a
1
new strong polar impurity in the residue by TLC and a weak H NMR signal at a
chemical shift of about 6.1. Thermal decomposition was possible for PP-PXZ when it
was heated and evaporated more rapidly to reach a higher doping concentration.
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Actually a slightly higher heating temperature was applied on the sample to get higher
doping concentration, which along with the long-time heating operation result in
thermal decomposition of PP-PXZ. The EL spectra, the current
density-voltage-brightness (J-V-B) characteristics, and the efficiency curves for the
TADF OLEDs are depicted in Figure 6, and those for blue device of Q-Cz are
provided in Figure S6 in supporting information. All the electroluminescence (EL)
data are summarized in Table 2.
The Q-Cz based OLED exhibited pure blue electroluminescence (EL) with
emission peak at 436 nm, the full width at half maximum (FWHM) of 50 nm, and
Commission Internationale de l'Éclairage (CIE) color coordinates of (0.16, 0.07), as
indicated by the EL spectra in Figure 6a and data in Table 2. Obviously the CIE
coordinates of Q-Cz device are quite close to (0.14, 0.08) for standard blue light. This
blue device turned on (to delievered a brightness of 1 cd m-2) at a voltage of 4.8 V
(Von), and exhibited a maximum brightness (Lmax) of 1480 cd m-2, a maximum current
efficiency (ηc) of 0.7 cd A-1, corresponding to a peak forward viewing external
quantum efficiency (EQE, ηext) of 1.2% and a power efficiency (ηp) of 0.5 lm W-1
(Figure S6).
At a doping level of 5 wt%, the five TADF OLEDs turned on at 3.1-3.5 V and
exhibited bright sky-blue via green and yellow to orange-red EL, as illustrated by the
EL spectra in Figure 6a. The EL of all these six OLEDs successfully cover almost the
whole visible light range from pure blue to orange-red, which should benefit from the
appropriate molecular designing by adjusting ICT extent through varying the donor
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and acceptor groups. The EL peak wavelengths of all these TADF emitters at the
doping level of 5 wt% (Table 2) are identical to or slightly red-shifted relative to the
PL peak wavelengths of their doped films in CBP host (Table 1 and Figure S3 f). For
example, the Q-DMAC device shows a EL peak at 512 nm, which is red-shfted by 11
nm relative to its PL in CBP film (501 nm), while iPP-DMAC exhibits identical EL
and PL peak wavelength at 524 nm. The red shift of EL spectra in comparison with
the corresponding PL should be caused by the influence of the electrical field in
OLEDs on the light-emitting molecules, as frequenly observed in various types of
OLEDs. These five TADF OLEDs have much lower Vons (3.1-3.5 V) than the Q-Cz
device (4.8 V), which may be ascribed to the enhanced hole and electron injections
into the emitting layer due to both reduced electron barriers and hole barrier at the
emitting layer interfaces in comparison with Q-Cz device. As shown by the energy
level diagram in Figure S5, the LUMOs and HOMOs of all TADF emitters are
sandwiched within those of CBP host and even the adjacent layers, which must result
in direct charge traps by the dopant molecules as an important excitation way in
addition to the host-dopant energy transfer. For example, in Q-DMAC based TADF
device (Figure S5 b), the hole injecting barrier from TAPC to CBP host is 0.19 eV,
from TAPC to Q-DMAC is -0.23 eV, while the electron injecting barrier from
TmPyPB to CBP is 0.50 eV, from TmPyPB to Q-DMAC is -0.26 eV. This indicates
that both hole injection and electron injection from adjacent layers direct onto
Q-DMAC are sponteneous processes, which must facilitate the charge injection into
emitting layer and help to reduce driving voltage of the devices. However, in Q-Cz
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device (Figure S5 a), the hole injecting barrier from NPB to mCP is 0.22 eV, from
NPB direct to Q-Cz is 0.11 eV. While the electron injecting barrier from TPBi to mCP
is 0.51 eV, and from TPBi direct to Q-Cz is -0.29 eV. Apparently in both TADF
devices and Q-Cz device, the hole barriers and electron barriers from adjacent layers
into the host are almost comparable to each other. However, direct charge trapping by
the dopant seems be much easier in TADF devices than in Q-Cz device, which should
contribute to the relatively lower turn-on voltages for these TADF devices. These five
TADF OLEDs all exhibited much higher EL efficiencies than the Q-Cz device, this is
reasonable since the TADF nature of these dopant emitters must have extra
contribution to the brightness and efficiencies of OLEDs due to highly efficient
harvesting of both electrogenerated singlet and triplets excitons for light emission by
TADF emitters. The Q-DMAC and iPP-DMAC based devices exhibited green
emission with EL peaks at 512 and 524 nm, and maximum ηc of 35.9 cd A-1 and ηext
of 12.9% for Q-DMAC device, and ηc of 51.9 cd A-1 and ηext of 16.7% for iPP-DMAC
device. Under identical conditions, the isomer PP-DMAC based device exhibited
yellow EL with peak at 563 nm and maximum efficiencies of 42.4 cd A-1 and 14.1%.
With donor going to PXZ, two PXZ-based TADF emitters exhibited yellow and
orange-red EL in their OLEDs, respectively. The iPP-PXZ based OLED exhibited
yellow emission with peak at 584 nm and maximum efficiencies of 30.6 cd A-1 and
12.3%. While its isomer PP-PXZ based OLED exhibited orange-red emission with
peak at 594 nm and efficiencies of 30.6 cd A-1 and 13.0%.
In comparison with the quinoxaline based Q-DMAC, the other four TADF
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emitters based on iPP and PP acceptors show more stable EL efficiencies with
increasing brightness, as illustrated by the efficiency curves in Figures 6c and 6d. For
example, at the high brightness of 100, 1000 and 5000 cd m-2, the ηexts of Q-DMAC
device are at 5.86%, 2.15% and 1.20%, which represent the efficiency roll-off by
54.6%, 83.3%, and 90.7% relative to the maximum ηext (12.9%). In contrary, at the
high brightness of 100, 1000 and 5000 cd m-2, the PP-PXZ (as an example) device
still remain high ηext at 12.98%, 11.10% and 4.63%, representing the efficiency
roll-off by 0.2%, 14.6% and 64.4% relative to the maximum ηext value (13.0%). It is
supposed that the more stable efficiencies of these four iPP and PP based TADF
emitters in comparison with another TADF analogue Q-DMAC should benefit from
the incorporation of iPP and PP acceptors in these TADF molecules. It can also be
discerned that two PP-based analogues show higher EL efficiencies under high
current density region than their iPP-isomers, although the iPP-based isomers may
exhibit higher or at least comparable efficiencies with the PP-based isomers at low
brightness region. This implies that the PP-based analogues may possess better
stability to withstand high driving current density or high brightness. Furthermore,
two DMAC-based TADF emitters (iPP-DMAC and PP-DMAC) exhibited high
efficiencies than their PXZ analogues (iPP-PXZ and PP-PXZ), as shown in Table 2
and Figure 6. This can be mainly attributed to the higher PLQYs of DMAC-based
emitters than the PXZ-based analogues.
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1.0
0.8
0.6
0.4
0.2
0.0
Q-Cz (2 wt%)
Q-DMAC
iPP-DMAC
PP-DMAC
iPP-PXZ
(a)
(b)
5 wt%
Q-DMAC
400
200
0
104
103
102
101
iPP-DMAC
PP-DMAC
iPP-PXZ
PP-PXZ
PP-PXZ
5 wt%
100
14
-2
0
2
4
6
8
10
12
400
500
600
700
800
Wavelength (nm)
Voltage (V)
(d)
(c)
102
101
100
10-1
10
100
5 wt%
Q-DMAC
1
Q-DMAC
10-2
10-4
iPP-DMAC
PP-DMAC
iPP-PXZ
iPP-DMAC
PP-DMAC
iPP-PXZ
5 wt%
PP-PXZ
PP-PXZ
0.1
100
101
102
103
104
100
101
102
103
104
Brightness (cd m-2)
Brightness (cd m-2)
Figure 6 EL spectra (a) for all six OLEDs, and J-V-B characteristics (b) and the
efficiency curves (c and d) for five TADF OLEDs at 5 wt% doping concentration.
Table 2. Summary of EL data for all OLEDs based on blue fluorescent Q-Cz and
TADF materials at various doping concentrations.
Von
Lmax
(cd m-2)
ηc
(cd A-1)
ηP
(lm W-1)
ηext
λem
Concent.
2 wt%
Emitter
CIE(x,y)
(V)
(%) (nm)
Q-Cz
4.8
3.5
3.5
3.5
3.1
3.4
4.0
4.1
3.9
3.6
3.4
1480
6850
0.7
0.5
1.2
12.9
16.7
14.1
12.3
13.0
9.8
436
512
524
563
584
594
518
550
573
598
595
0.16,0.07
0.27,0.49
0.32,0.56
0.44,0.53
0.49,0.50
0.52,0.46
0.30,0.52
0.47,0.54
0.47,0.52
0.54,0.46
0.53,0.46
Q-DMAC
iPP-DMAC
PP-DMAC
iPP-PXZ
35.9
51.9
42.4
30.6
30.6
28.3
18.3
21.4
20.0
20.6
32.2
43.5
31.3
30.5
24.6
22.2
18.7
13.5
17.2
19.0
13900
16500
13400
19500
4790
5 wt%
PP-PXZ
Q-DMAC
iPP-DMAC
PP-DMAC
iPP-PXZ
6140
7.8
10 wt%
4710
7.7
15000
7160
9.6
PP-PXZ
9.3
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The doping concentration of TADF emitters was increased to 10 wt% with same
device structure as above 5 wt% devices to explore the possibility of performance
optimization. The J-V-B characteristics, EL spectra, and the efficiency curves for these
10 wt% TADF OLEDs are described in Figure S7, and EL data are summarized in
Table 2. In comparison with the 5 wt% OLEDs, the EL spetra of these 10 wt% devices
all red shifted by 10-24 nm. At the same time, the EL efficiencies also decreased to
some extent with increasing doping level. For example, the Q-DMAC device at 10
wt% exhibited a maximum ηext of 9.8% (vs 12.9% at 5 wt%), and the iPP-PXZ device
at 10 wt% exhibited a maximum ηext of 9.6% (vs 12.3% at 5 wt%). The spectral
red-shift along with efficiency decrease with increasing doping concentration has
been frequently observed in TADF OLEDs mainly due to concentration quenching
effect.40,41 Similar to the observation in 5 wt% TADF OLEDs, in 10 wt% devices, the
PP-DMAC device showed higher efficiencies and enhanced efficiency stability at
high brightness region than isomer iPP-DMAC, and all these PP- and iPP-based
TADF emitters exhibited higher efficiencies than the quinoxaline based Q-DMAC.
However, the efficiencies of PP-PXZ device at 10 wt% are exceptionally reduced in
comparison with its isomer iPP-PXZ, which is in contrary to the case at 5 wt% doping
level. This should be because the PP-PXZ was partially decomposed when it was
rapidly heated for a higher doping concentration, as proved by the detection of
impurity in residue of PP-PXZ after device fabrication.
Conclusions
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In summary, by introducing one sp2-hybridized N atom into different sites of the
quinoxaline (Q) ring, the novel acceptors pyrido[2,3-b]pyrazine (PP) and
pyrido[3,4-b]pyrazine (iPP) were generated with enhanced electron withdrawing
abilities relative to the parent Q to different extent. A series of wedge shaped
compounds were designed and synthesized by attaching two electron donors (D), i.e.,
carbazole (Cz), 10-dimethyl-acridine (DMAC) or 10H-phenoxazine (PXZ),
symmetrially on the central acceptor (A) unit such as Q, PP or iPP. The luminescent
behavior of these compounds greatly depends on the combination style of donor and
acceptor and the resultant steric configuration. By increasing the molecular distorsion
angles due to enhanced steric hindrance with the donor going from Cz to DMAC, the
light emission was tuned from the normal fluorescence of Q-Cz to the TADF of
Q-DMAC. And all the DMAC- and PXZ-based compounds in present study have
TADF feature mainly due to the inherent three six-membered ring structures of
DMAC and PXZ moieties and thus the highly twisted molecular structures. It has
been confirmed by electrochemical measurements and theoretical calculations that the
electron donating ability stepwisely increases in the order of Cz, DMAC and PXZ,
while the electron withdrawing ability increases gradually in the order of Q, iPP, and
PP. Therefore, by gradually enhancing the intramolecular charge transfer extent by
combining appropriate D and A, the emission color of this series of compounds were
regularly tuned from pure blue via green and yellow to orange-red. The OLEDs
containing these compounds as doped emitters exhibited full-color
electroluminescence covering the whole visible light range. The Q-Cz based OLEDs
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exhibited an external quantum efficiency (ηext) of 1.2% with pure blue color
coordinates of (0.16, 0.07). All other TADF-OLEDs showed green to orange-red EL
with moderate to high ηexts from 12.9% (35.9 cd A-1) to 16.7% (51.9 cd A-1). All the
PP and iPP based TADF emitters have improved efficiency stabilities than the
quinoxaline based analogues, suggesting that PP and iPP should be ideal acceptor
units to construct novel light-emitting materials, especially those low-cost and
high-performance TADF emitters for OLED applications.
Experimental Section
1
General Information. The H NMR, 13C NMR and mass spectra were
characterized by a Bruker Avance Ⅲ 400 and 101 MHz and Bruker Avance Ⅲ 500
and 126 MHz spectrophotometer and HP1100LC/MSD MS spectrometer. A
PerkinElmer Lambandda 35 spectrophotometer PerkinElmer LS55 spectrometer were
used to measure UV–vis absorption, fluorescence and phosphorescence spectra,
respectively. Photoluminescence quantum yields (PLQYs) of films were measured on
a HAMAMATSU absolute PL quantum yield spectrometer C11347. Differential
scanning calorimetry (DSC) and Thermogravimetric analysis (TGA) were measured
by using a Netzsch DSC 201 and PerkinElmer thermogravimeter (Model TGA7)
instrument. Transient fluorescence decays were detected with an Edinburgh
Instruments FLS1000 spectrometer. Electrochemical workstation (BAS100B, USA)
was used to performe the cyclic voltammetry study. Density functional theory (DFT)
calculations using B3LYP functional were performed with Gaussian 09.
28
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OLED fabrication and measurements. Before evaporating organic film and
preparation device, the ITO glass substrates were precleaned with acetone,
isopropanol detergents and deionized water successively, and treated by oxygen
plasma for 30 min. The sample then was transferred into a deposition chamber.
Devices were prepared by evaporating organic layers at a rate of 1.0 Å s−1 onto the
ITO substrate sequentially at a pressure below 1 × 10−4 Pa. The emitting layers of
OLEDs were fabricated by thermal co-evaposition of the dopant and host materials
onto the hole transporting layer. The doped films for PL measurements were prepared
on the cleaned quartz substrates in the same way, and have the thickness of 50 nm.
The doping concentration was carefully controlled by adjusting the different
evaporation rate of the host and the dopant. The thin layer of LiF (1 nm) was
thermally deposited onto organic layers to facilitate electron injection and then an Al
film (200 nm) was deposited as the cathode. The current-voltage-brightness
characteristics and EL spectra of OLEDs were measured using Keithley 2400 SMU,
Chroma Meter 200, and PR705 photometer.
Compound syntheses. Important intermediates a, b, and c were synthesized
according to literature methods.28-31
General procedure for synthesis of 2,3-bis(4-bromophenyl)quinoxaline（d）,
2,3-bis(4-bromophenyl)pyrido[3,4-b]pyrazine
2,3-bis(4-bromophenyl)pyrido[2,3-b]pyrazine (f):A mixture of pyridine-3,4-diamine
or pyridine-2,3-diamine (400 mg, 3.66 mmol) and
(e)
and
1,2-bis(4-bromophenyl)ethane-1,2-dione (1340 mg, 3.66 mmol) in glacial acetic acid
29
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