Solution processible triphenylphosphine-oxide-cored dendritic hosts featuring thermally activated delayed fluorescence for power-efficient blue electrophosphorescent devices

Article abstract of DOI:10.1039/c9tc03013a

Based on oligocarbazole, a carbazole/acridine hybrid and oligoacridine as the dendron, a series of solution processible triphenylphosphine-oxide-cored dendrimers denoted as 3CzCz-PO, 3AcCz-PO, and 3AcAc-PO have been designed and synthesized, respectively.

Full text of DOI:10.1039/c9tc03013a

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DOI: 10.1039/C9TC03013A
Solution
Dendritic Hosts Featuring Thermally Activated Delayed
Fluorescence for Power-Efficient Blue
Electrophosphorescent Devices
Processible
Triphenylphosphine-Oxide-Cored
Mingming Zhang,a,b Liang Chen,a,b Xiushang Xu,a Lei Zhao,a Shumeng Wang,a Junqiao
Ding,*,a,b Lixiang Wang*,a,b
a
State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of
Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China
b University of Science and Technology of China, Hefei 230026, P. R. China
Keywords: PhOLEDs; dendrimer; TADF; blue; triphenylphosphine oxide

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DOI: 10.1039/C9TC03013A
Abstract
Based on oligocarbazole, carbazole/acridine hybrid and oligoacridine as the dendron,
a series of solution processible triphenylphosphine-oxide-cored dendrimers denoted as
3CzCz-PO, 3AcCz-PO, and 3AcAc-PO have been designed and synthesized,
respectively. With the increasing electron-donating ability from the periphery dendron,
the singlet-triplet splitting (ΔEST) is found to be gradually reduced from 0.35 eV of
3CzCz-PO to 0.22 eV of 3AcCz-PO and 0.20 eV of 3AcAc-PO while remaining high
triplet energy (T1) around 2.9 eV. As a result, 3AcAc-PO exhibits an obvious thermally
activated delayed fluorescence (TADF) with a lifetime of 1.0 µs. When used as the host
for the blue phosphor Ir(mpim)3, the corresponding solution-processed devices reveal
a peak power efficiency of 46.4 lm/W. An about 57% improvement is obtained for
3AcAc-PO compared with the non-TADF host H2 (29.6 lm/W). The results clearly
manifest the superiority of solution processible dendritic TADF hosts in power-efficient
blue electrophosphorescent devices.

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1. Introduction
Solution-processed phosphorescent organic light-emitting diodes (s-PhOLEDs) have
attracted considerable attention because of their possibility to realize a near unity
internal quantum efficiency and compatibility with cost-effective wet methods (e.g.
inkjet printing and spin-coating).1-5 Given the strong concentration quenching caused
by triplet−triplet annihilation (TTA), phosphorescent dyes are usually doped into
solution processible hosts, such as small molecules,6-10 dendrimers8, 11-14 and polymers8,
15, 16. Among them, dendrimers can achieve a trade-off between high triplet energy (T1)
and shallow highest occupied molecular orbital (HOMO) energy level.17 Thus they are
believed to be one of the most promising solution processible hosts especially for
power-efficient blue s-PhOLEDs. For example, with oligocarbazole as the dendron and
N-phenylcarbazole as the core, a dendrimer H2 (Figure 1) was reported to host the
typical blue phosphor FIrpic. The corresponding devices gave a peak power efficiency
(PE) of 15.4 lm/W, which was about 86% higher than that of PVK.17 Moreover, when
another high-energy-level blue phosphor FIr-p-OC8 was developed to match the
dendritic H2 host to avoid the unwanted hole scattering, the PE was almost doubled to
34.2 lm/W.18 And this further led to the successful fabrication of solution-processed
white devices with comparative efficiency to the fluorescent tubes.
On the other hand, thermally activated delayed fluorescence (TADF) featured small
molecules have been widely used as the hosts to sensitize blue phosphors in vacuum-
deposited electrophosphorescent devices.19-28 Because of extremely small singlet-triplet
gap (ΔEST < 0.3 eV), their energy level of the lowest singlet excited state (S1) could be

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easily tuned to locate close to that of T1 of a phosphorescent guest. Due to theDfOaI:v1o0.r1e03d9/C9TC03013A
charge injection, the decreased driving voltage and improved PE are within our
expectation for such TADF hosts compared with the conventional counterparts. For
instance, Liao et al. demonstrated a spiro-based TADF host Tri-o-2PO for a sky-blue
phosphor Ir-817, revealing a low turn-on voltage (Von) (at 1 cd/m2) of 2.94 V and a high
PE of 58.2 lm/W.28
With these ideas in mind, here we report the realization of power-efficient blue s-
PhOLEDs by endowing the dendritic host with a TADF feature. As shown in Figure 1,
three hosts named as 3CzCz-PO, 3AcCz-PO and 3AcAc-PO are developed from H2
according to several considerations: (i) Instead of N-phenylcarbazole, the
triphenylphosphine oxide core is adopted as the acceptor for its weak electron-
withdrawing ability and π-conjugation breaking to ensure high T1;29 (ii) Different kinds
of dendrons including oligocarbazole, carbazole/acridine hybrid and oligoacridine are
selected as the donor to modulate the intramolecular charge transfer (CT) character.
Based on this design, the ΔEST of the resultant dendrimers is gradually reduced
following a sequence of 3CzCz-PO > 3AcCz-PO > 3AcAc-PO. Consequently, 3AcAc-
PO shows an obvious delayed fluorescence with a lifetime of about 1.0 µs. When used
as the host for Ir(mpim)3, the solution-processed blue electrophosphorescent devices
obtain a maximum PE of 46.4 lm/W. The value is much higher than H2-based devices
(29.6 lm/W), clearly indicating the superiority of TADF hosts in blue s-PhOLEDs.
2. Results and discussion
2.1 Synthesis and characterization

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We synthesized the triphenylphosphine-oxide-cored dendrimers accordinDgOIt:o10.t1h03e9/C9TC03013A
synthetic route depicted in Scheme S1. Starting from 1,4-dibromobenzene, the
corresponding mono Grignard reagent was firstly prepared, which then reacted with
POCl3 to afford tris(4-bromophenyl)phosphine oxide). Finally, Pd-catalyzed C–N
cross-coupling was performed between such a key intermediate and previously reported
oligocarbazole (CzCz)30, carbazole/acridine hybrid (AcCz)31 or oligoacridine (AcAc)31
to produce the target dendrimers with an acceptable moderate yield of 46-57%. The
molecular structures of 3CzCz-PO, 3AcCz-PO and 3AcAc-PO were identified by mass
1
13
31
spectrometry, H, C and P NMR spectroscopy and elemental analysis. They show
good thermal stability with a decomposition temperature (Td) above 400 °C (Figure S1).
o
And the glass transition temperature (Tg) is detected to be 276 C for 3CzCz-PO and
257 oC for 3AcCz-PO. Also, their solubility is good in common organic solvents (e.g.
tetrahydrofuran, chloroform, chlorobenzene and toluene), ensuring the high-quality
film generation via solution processing.
2.2 Photophysical properties
The UV-Vis absorption in dichloromethane, fluorescence and phosphorescence
spectra in the neat films of 3CzCz-PO, 3AcCz-PO and 3AcAc-PO are shown in Figure
2. Compared with the donor and acceptor fragments (Figure S2), the absorption bands
in the range of 250 – 400 nm can be reasonably attributed to the π–π* and n-π*
transitions of the peripheral dendrons, while the CT absorption is too weak to be
distinguished. Ongoing from 3CzCz-PO to 3AcCz-PO and 3AcAc-PO, the emission

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peak is found to be red-shifted from 420 to 454 and 467 nm. Correspondingly, the full
width at half maxima (FWHM) is up from 69 to 84 and 97 nm. With respect to 3CzCz-
PO, the stronger CT observed in 3AcCz-PO and 3AcAc-PO may be from the increased
electron-donating capability after the replacement of carbazole by acridine. This is
further confirmed by the broader and structureless phosphorescence spectra of 3AcCz-
PO and 3AcAc-PO relative to 3CzCz-PO. Furthermore, they all have a T1 around 2.9
eV, higher than most of blue phosphors. Combined with S1 estimated from the
fluorescence onset, the ∆Est is determined to be down from 0.35 eV of 3CzCz-PO to
0.22 eV of 3AcCz-PO and 0.20 eV of 3AcAc-PO (Table 1). The results mean the great
potential of 3AcAc-PO, which is able to act as the solution processible host to realize
power-efficient blue s-PhOLEDs.
To demonstrate whether they are TADF emitters, the transient photoluminescence
(PL) spectra were also measured at 298 K. It is worth noting that 3CzCz-PO exhibits a
strong prompt but negligible delayed fluorescence even under an O2-free condition
(Figure 3). By contrast, a distinct delayed component is observed for 3AcCz-PO and
3AcAc-PO, giving an excited state lifetime of 1.5 and 1.0 µs, respectively. The different
TADF behaviors are understandable when considering the decreased ∆Est to promote
the T1-to-S1 upconversion. Additionally, the delayed fluorescence of 3AcCz-PO and
3AcAc-PO can be partly or completely quenched by O2 (Figure S3), indicative of their
TADF nature.
2.3 Electrochemical properties

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The electrochemical behavior of dendrimers 3CzCz-PO, 3AcCz-PO and D3OAI: 1c0A.10c3-9/C9TC03013A
PO were investigated with cyclic voltammetry (CV). All of them show quasi-reversible
oxidation waves upon the positive scan, and no reduction signals appear in the negative
scan in dichloromethane (Figure 4). With ferrocene/ferrocenium (Fc/Fc+) as the
reference, the HOMO and LUMO levels are determined to be −5.40/-2.25 eV, −5.18/-
2.29 eV and −5.13/-2.21 eV for 3CzCz-PO, 3AcCz-PO and 3AcAc-PO, respectively.
The values correlate well with the theoretical simulation. As presented in Figure 5, the
LUMO is localized on the triphenylphosphine oxide core, whereas the HOMO mainly
distributes over the outer dendrons but with a different delocalization degree. The
observation can be tentatively ascribed to the different torsion angle θ1 between the
dendron and the phenyl linkage together with θ2 between the carbazole and/or acridine
moieties in the dendron (Figure S4). As a result of the enhanced θ1/θ2 from 52.8o/56.3o
to 90.2o/56.3o and 88.4o/90.2o, the HOMO delocalization exhibits a decreased trend
following a sequence of 3CzCz-PO > 3AcCz-PO > 3AcAc-PO. It should be noted that
all the three dendrimers possess close LUMO levels owing to the same used core.
Meanwhile, with the increasing dendron’s electron donating ability from
oligocarbazole to carbazole/acridine hybrid and oligoacridine, an upshifted trend for
their HOMO levels is observed from 3CzCz-PO to 3AcCz-PO and 3AcAc-PO. This is
beneficial for the hole injection and thus low driving voltage, which will be discussed
later.
2.4 Electroluminescent Properties

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In order to investigate their electroluminescent (EL) properties, solution-processed
nondoped devices were firstly fabricated with 3CzCz-PO, 3AcCz-PO or 3AcAc-PO as
the emitting layer (EML) independently. The device configuration is ITO/PEDOT:PSS
(40 nm)/EML (40 nm)/TSPO1 (5 nm)/TmPyPB (45 nm)/LiF (1 nm)/Al (100 nm),
where PEDOT:PSS, TmPyPB and TSPO1 serve as the hole-injection layer, electron-
transporting layer and exciton blocking layer, respectively (Figure S5).
The EL characteristics are presented in Figure 6 and summarized in Table 2. Similar
to the PL counterparts, the EL spectra are becoming more and more broad, and the
corresponding Commission Internationale de L’Eclairag (CIE) coordinates are red-
shifted from (0.18, 0.10) of 3CzCz-PO to (0.20, 0.21) of 3AcCz-PO and (0.21, 0.27) of
3AcAc-PO (Figure 6a).. In addition, both the current density–voltage (J-V) and
luminance–voltage (L-V) curves move towards a negative voltage from 3CzCz-PO to
3AcCz-PO and 3AcAc-PO (Figure 6b). As mentioned above, the elevated HOMO
levels are anticipated to facilitate the hole injection, thus leading to the reduced driving
voltage. Consequently, 3AcAc-PO obtains the best device performance among these
three dendrimers (Figure 6c and 6d), revealing a peak EQE of 6.7% (13.7 cd/A, 12.7
lm/W).
We note that the luminescent performance of 3AcAc-PO is not as good as that of
previously-reported TADF emitters32-35 due to its poor PL quantum yield (PLQY) of
0.33. However, it may be a promising host for blue phosphors in terms of its high T1
and low ∆Est. To verify this point, blue s-PhOLEDs were subsequently assembled by
further doping Ir(mpim)3 into 3AcAc-PO as the EML with an optimized content of 20

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wt.% (Table S2 and Figure S6). Under the same condition, a control device was also
prepared based on the non-TADF host H2 instead of 3AcAc-PO. As can be clearly seen,
a bright sky-blue EL only from Ir(mpim)3 is observed with CIE coordinates of (0.21,
0.44). There is no additional emission from 3AcAc-PO (Figure 7a), manifesting
complete energy transfer attributable to the good overlap between the absorption of
Ir(mpim)3 and the PL of 3AcAc-PO (Figure S7). In comparison to H2, most importantly,
the luminance and current density at the same driving voltage for 3AcAc-PO are
significantly enhanced (Figure 7b). Ongoing from H2 to 3AcAc-PO, the Von is
decreased from 3.0 V to 2.6 V. Accordingly, the maximum luminance, CE, PE and
EQE are increased from 13261 cd/m2, 30.7 cd/A, 29.6 lm/W and 11.9% to 25815 cd/m2,
40.7 cd/A, 46.4 lm/W and 15.8%, respectively (Figure 7c and 7d). The low ∆Est as well
as the higher film PLQY (0.66 for 3AcAc-PO : 20 wt.% Ir(mpim)3 Vs 0.53 for H2 : 20
wt.% Ir(mpim)3) may be responsible to the superior device performance of 3AcAc-PO
to H2, which is among the highest ever reported for solution-processed blue PhOLEDs
(Table S3).
3. Conclusion
In conclusion, solution processible triphenylphosphine-oxide-cored dendritic hosts
featuring TADF have been demonstrated for power-efficient blue
electrophosphorescent devices. By tuning the electron-donating ability of the periphery
dendron combined with the triphenylphosphine oxide core, a distinct delayed
fluorescence could be observed for 3AcAc-PO. It shows not only high T1 to confine the

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triplet excitons on blue phosphor, but also low ∆Est to favor charge injection and
transporting. Consequently, solution processed blue electrophosphorescent devices
based on 3AcAc-PO achieve lower driving voltage and higher PE relative to the non-
TADF host H2. We believe our study can promote further development of dendritic
hosts towards power-efficient blue s-PhOLEDs.
Nomenclatures: The abbreviations used for molecules are as follows. [Bis(4,6-
difluorophenyl)-pyridinato-N,C2]-picolinate (FIrpic), poly(9-vinylcarbazole) (PVK),
tris[1-(3,5-diisopropylbiphenyl-4-yl)-2-phenyl-1H-imidazole] iridium(III) (Ir-817),
tris(mesityl-2-phenyl-1H-imidazole)iridium(III)
[Ir(mpim)3],
poly(3,4-
ethylenedioxythiophene:poly(styrenesulfonate)
(PEDOT:PSS),
diphenyl(4-
(triphenylsilyl)phenyl)phosphine
oxide
(TSPO1),
1,3,5-tri(m-pyrid-3-yl-
phenyl)benzene (1,3,5-tri(m-pyrid-3-yl-phenyl)benzene).
Supporting Information:
Experimental section including general information, device fabrication and testing as
well as synthesis; TGA and DSC plots; transient PL spectra under nitrogen and oxygen;
1
13
31
device configuration; H, C, P NMR and MALDI-TOF spectra. This material is
available free of charge via the Internet at http://pubs.rsc.org.
Author Information:
Corresponding authors: junqiaod@ciac.ac.cn; lixiang@ciac.ac.cn

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Notes:
The authors declare no competing financial interest.
Acknowledgments:
The authors are grateful for the financial support from the National Key Research
and Development Program (2016YFB0401301), the 973 Project (2015CB655001), and
the National Natural Science Foundation of China (Nos. 51873205 and 51573183).
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Figure 1. Molecular structures of the dendrimers 3CzCz-PO, 3AcCz-PO and 3AcAc-
PO.

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2.5
2.0
1.5
1.0
0.5
02.0
3CzCz-PO
Abs
1.0
0.8
0.6
0.4
0.2
Fluo
Phos
0.0
1.0
3AcCz-PO
Abs
Fluo
Phos
1.5
1.0
0.5
0.8
0.6
0.4
0.2
0.0
1.5
0.0
1.0
3AcAc-PO
Abs
Fluo
Phos
0.8
0.6
0.4
0.2
0.0
1.0
0.5
0.0
250
300
350
400
450
500
550
600
Wavelength (nm)
Figure 2. UV-Vis absorption in CH2Cl2, fluorescence (298K) and phosphorescence
(77K) spectra in neat films for 3CzCz-PO, 3AcCz-PO and 3AcAc-PO.

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105
104
103
102
101
3CzCz-PO
3AcCz-PO
3AcAc-PO
0
2000
4000
6000
8000
10000
Time/ ns
Figure 3. PL decay curves of 3CzCz-PO, 3AcCz-PO and 3AcAc-PO in film under
nitrogen at 298 K.

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3CzCz-PO
3AcCz-PO
3AcAc-PO
-0.4
0.0
0.4
0.8
Potential(V vs Fc/Fc+)
Figure 4. CV plots for 3CzCz-PO, 3AcCz-PO and 3AcAc-PO.

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Figure 5. Frontier orbital distributions and energy levels of 3CzCz-PO, 3AcCz-PO and
3AcAc-PO calculated at the B3LYP/6-31G* level by Gaussian 09.

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102
104
(b)
(a)
3CzCz-PO
3AcCz-PO
3AcAc-PO
1.0
0.8
0.6
0.4
0.2
0.0
101
100
103
102
101
100
10-1
10-2
10-3
10-4
10-5
3CzCz-PO
3AcCz-PO
3AcAc-PO
0
2
4
6
8
10
400
500
600
700
Voltage(V)
Wavelength (nm)
103
(d)
3CzCz-PO
3AcCz-PO
3AcAc-PO
(c)
101
100
101
100
10-1
102
101
100
10-1
10-1
3CzCz-PO
3AcCz-PO
3AcAc-PO
10-2
100
101
102
103
100
101
102
Luminance (cd m-2)
Luminance (cd m-2)
Figure 6. Non-doped device performance for 3CzCz-PO, 3AcCz-PO and 3AcAc-PO:
(a) EL spectra at a driving voltage of 8 V; (b) current density-voltage-luminance curves;
(c) current efficiency and EQE as a function of luminance; (d) power efficiency as a
function of luminance.

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DOI: 10.1039/C9TC03013A
700
(a)
(b)
3AcAc-PO
H2
1.0
0.8
0.6
0.4
0.2
0.0
104
103
102
101
100
3AcAc-PO
H2
600
500
400
300
200
100
0
0
2
4
6
8
10
12
400
500
600
700
Voltage (V)
Wavelength (nm)
100
10
1
(d)
(c)
100
10
1
3AcAc-PO
H2
10
3AcAc-PO
H2
100
101
102
103
100
101
102
103
Luminance (cd m-2)
Luminance (cd m-2)
Figure 7. Device performance of 3AcAc-PO and H2 as host for 20wt% Ir(mpim)3: (a)
EL spectra at a driving voltage of 4 V; (b) current density-voltage-luminance curves;
(c) current efficiency and EQE as a function of luminance; (d) power efficiency as a
function of luminance.

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DOI: 10.1039/C9TC03013A
Table 1. Photophysical, electrochemical and thermal properties for 3CzCz-PO, 3AcCz-
PO and 3AcAc-PO.
[b]
[c]
[d]
[d]
[e]
[g]
[g]
[g]
[i]
λabs(logε) [a]
λem
ФPL
τp
τd
Eg
HOMO[f]
[eV]
LUMO[f]
[eV]
S1
T1
∆Est
Td[h]/Tg
[oC]
[nm]
[nm]
[%]
[ns]
[μs]
[eV]
[eV]
[eV]
[eV]
3CzCz-PO
3AcCz-PO
3AcAc-PO
333 (4.84), 299 (5.17), 265 (5.21)
314 (4.97), 299 (5.18), 265 (5.05)
296 (5.17)
420
454
467
31
22
33
23
31
31
-
3.15
2.89
2.92
-5.40
-5.18
-5.13
-2.25
-2.29
-2.21
3.25
3.08
3.12
2.90
2.87
2.92
0.35
0.22
0.20
468/276
431/257
428/-
1.5
1.0
aMeasured in 10-5 M CH2Cl2 solution. bMeasured in film. cMeasured in film with integrating sphere
under N2. dLifetimes of prompt emission (τp) and delayed emission (τd) measured in film at 298 K
e
f
onset
in N2. The optical band gap estimated from the absorption onset. HOMO=-e(Eox
+ 4.8 V),
LUMO= HOMO + Eg , where Eoxonset is the onset value of the first oxidation wave. gS1 and T1 of the
dendrimers were calculated from the onset of the fluorescence and the phosphorescence spectra in
films, and ΔEST = S1 - T1. hDecomposition temperatures corresponding to a 5% weight loss. iGlass
transition temperatures determined by DSC in N2.

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DOI: 10.1039/C9TC03013A
Table 2. EL performance of non-doped device and blue PhOLEDs.
Von
[V]
Lmax
[cd m-2]
CE b
PE b
EQE b
[%]
CIE c
[x, y]
a
EML
[cd A-1] [lm W-1]
100% wt 3CzCz-PO
100% wt 3AcCz-PO
100% wt 3AcAc-PO
3AcAc-PO: 20 wt.% Ir(mpim)3
H2: 20 wt.% Ir(mpim)3
4.8
3.8
3.2
2.6
3.0
241
1197
1782
25815
13261
1.0
7.7
0.6
5.4
1.4
4.6
(0.18, 0.10)
(0.20, 0.21)
(0.21, 0.27)
(0.21, 0.44)
(0.21, 0.44)
13.7
40.7
30.7
12.7
46.4
29.6
6.7
15.8
11.9
aTurn-on voltage at a brightness of 1 cd m-2; bMaximum values for current efficiency (CE), power
efficiency (PE) and EQE, respectively; cCIE at 1000 cd m-2.

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DOI: 10.1039/C9TC03013A
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