Deep-blue thermally activated delayed fluorescence dendrimers with?reduced singlet-triplet energy gap for low roll-off non-doped solution-processed organic light-emitting diodes

Article abstract of DOI:10.1016/j.dyepig.2017.01.036

Two TADF dendrimers composed of diphenylsulfone core and oligo-carbazole dendrons were developed. Compared with the reported small molecular TADF emitter bis[4-(3,6-di-tert-butylcarbazole)phenyl]sulfone (G1), dendrimers G2 and G3 inherited the blue light

Full text of DOI:10.1016/j.dyepig.2017.01.036

Dyes and Pigments 140 (2017) 79e86
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Deep-blue thermally activated delayed fluorescence dendrimers
with reduced singlet-triplet energy gap for low roll-off non-doped
solution-processed organic light-emitting diodes
Jie Li a
,
b
,
*
, Xiaoqing Liao , Huixia Xu b, Lu Li c , Jie Zhang a b, Hua Wang a b,
c
a
,
,
**
,
,
a, b
Bingshe Xu
a
Key Laboratory of Interface Science and Engineering in Advanced Materials, Taiyuan University of Technology, Taiyuan, 030024, China
Research Center of Advanced Materials Science and Technology, Taiyuan University of Technology, Taiyuan, 030024, China
Co-innovation Center for Micro/Nano Optoelectronic Materials and Devices, Research Institute for New Materials Technology, Chongqing University of Arts
b
c
and Science, Chongqing, 402160, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 5 December 2016
Received in revised form
Two TADF dendrimers composed of diphenylsulfone core and oligo-carbazole dendrons were developed.
Compared with the reported small molecular TADF emitter bis[4-(3,6-di-tert-butylcarbazole)phenyl]
sulfone (G1), dendrimers G2 and G3 inherited the blue light emission but exhibited a reduced singlet-
14 January 2017
triplet energy gap (
1
DEST) with increasing the dendron generation. The efficient triplet-to-singlet (T -
Accepted 15 January 2017
Available online 16 January 2017
to-S ) transition processes, excellent thermal and amorphous stabilities, good solution processability, and
1
appropriate HOMO/LUMO energy levels enabled these dendrimers to serve as non-doped emitting layers
for solution-processed OLEDs. The G2 device achieved deep-blue light with emission peak at 428 nm, a
CIE coordinate at (0.15, 0.12) and relatively low efficiency roll-off at high current density.
Keywords:
TADF
Deep-blue light emitting
Dendrimer
©
2017 Elsevier Ltd. All rights reserved.
Non-doped
1
. Introduction
100% internal quantum efficiency by harvesting triplet excitons
through a reverse intersystem crossing process [9e11]. Very
recently, several highly efficient blue TADF emitters based on
diphenylsulfone acceptor were reported by Adachi and coworkers
[11e13]. In particular, bis[4-(3,6-di-tert-butylcarbazole)phenyl]
sulfone (G1) demonstrated deep-blue emission and the OLED with
this dopant achieved a high EQE of 10% (Scheme 1) [12]. However,
the OLED suffered from efficiency roll-off at high current density,
which was primarily caused by the relatively large energy gap be-
Organic light emitting diodes (OLEDs) have attracted much in-
terest due to their applications in the new generation of flat-panel
display and solid-state lighting. Among the three primary colors,
the development of efficient blue emitters is of high significance for
realizing full color displays and white OLEDs for solid-state lighting
applications [1e3]. However, the efficiency and stability of blue
y
emitting materials (CIE coordinate < 0.15) are still lower than the
red and green counterparts [4,5]. The development of deep blue
emitters is especially challenging because of their intrinsic wide
band gaps, unbalanced carrier transport ability, and poor matching
of the energy levels with other functional layers of the OLEDs [6e8].
Among the various design strategies for novel blue emitting ma-
terials, thermally activated delayed fluorescence (TADF) materials
have becoming promising new generations because of potentially
1 1
tween the S and T states (DEST). On the other hand, the reported
blue TADF emitters are small molecules, which have to be doped
into some host materials to suppress emission quenching in the
devices [14e16]. For the deep-blue emitters, the use of the host
material with a larger energy gap often means inefficient hole and/
or electron injection into the emitting layer [17]. Moreover, such
doped devices require precise control of the doping concentration,
rather complicated structure, and high fabrication costs. As a result,
developing efficient blue TADF emitters with solution processabil-
ity and reduced
significance.
DEST values for non-doped device is of important
*
Corresponding author. Key Laboratory of Interface Science and Engineering in
Advanced Materials, Taiyuan University of Technology, Taiyuan, 030024, China.
As one of the impressive materials for the non-doped emissive
layer, dendrimers exhibit great advantages owning to their well-
** Corresponding author.
E-mail addresses: lijie01@tyut.edu.cn (J. Li), lli@cqwu.edu.cn (L. Li).
http://dx.doi.org/10.1016/j.dyepig.2017.01.036
143-7208/© 2017 Elsevier Ltd. All rights reserved.
0

80
J. Li et al. / Dyes and Pigments 140 (2017) 79e86
fluorescence spectra and fluorescence quantum yield were
measured using a Horiba FluoroMax-4 spectrophotometer at room
temperature. The phosphorescence spectra were obtained by a
Hitachi F7000 spectrophotometer at 77 K. The transient photo-
luminescence decay profiles of the dendrimers in film state were
recorded using an Edinburgh Instruments FLS980 spectrometer.
The temperature dependence experiment is conducted under low
temperature refrigeration system from Advanced Research Systems
Company. Cyclic voltammetry was performed using a CHI660E
analyzer with a scan rate of 100 mV/s in MeCN solution at room
temperature. A platinum plate as the working electrode, a Pt-wire
counter electrode, and a saturated calomel electrode (SCE) as the
reference electrode were used. The supporting electrolyte was
tetrabutylammonium hexafluorophosphate (0.1 M) and ferrocene
was selected as the internal standard. Thermogravimetric analysis
(
TGA) of the compounds was conducted on a Setaram thermogra-
ꢀ
vimetric analyzer at a heating rate of 10 C/min under nitrogen
atmosphere. Differential scanning calorimetry (DSC) measure-
ments were performed at both heating and cooling rates of 5 C/
Scheme 1. Molecular structures of TADF dendrimers.
ꢀ
min under nitrogen atmosphere, using DSC Q100 V9.4 Build 287
apparatus.
defined structure, controllable molecular size, as well as the
inhabited molecular interactions [18,19]. TADF dendrimers dis-
playing green and greenish-blue emission have been utilized for
the non-doped solution-processed OLEDs and exhibited satisfying
electroluminescent performance [20e23]. However, until now, the
reported blue TADF dendrimers are still rare [21], and the rela-
2.3. OLED fabrication and characterization
Patterned glass substrates coated with indium tin oxide (ITO)
ꢁ1
tionship between the dendron generation and the S
1
-T
1
energy gap
(20 U square ) were cleaned by a surfactant scrub, washed suc-
cessively with deionized water, acetone and isopropanol in an ul-
of the molecule, and further the non-doped device performance are
still unclear.
ꢀ
trasonic bath, and then dried at 120 C in a heating chamber for 8 h.
A
30-nm-thick
poly(3,4-ethylenedioxythiophene):
poly(-
In this paper, we report blue TADF dendrimers G2 and G3 based
on G1 molecular skeleton as non-doped emissive layers for OLEDs.
The density functional theory (DFT) calculation on conformer
optimization reveals that 1) the bulky carbazole dendrons lead to
three-dimensional molecular conformations and 2) the spatially
distribution of the highest occupied molecular orbital (HOMO) and
lowest unoccupied molecular orbital (LUMO) of the molecule are
related to the generation of the dendrons. Thus, inhibited inter-
styrenesulfonic acid) (PEDOT:PSS) hole injection layer was spin-
ꢀ
coated on top of ITO and baked at 120 C for 20 min. Thin films
(45 nm thick) of the dendrimers as the emitting layer were
deposited on top of the PEDOT:PSS layer by spin-coating the
chlorobenzene solution of G2 or G3. Then an electron-transporting
layer of 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene (TPBI,
30 nm) and LiF (1 nm) and Al (100 nm) as the cathode were
deposited by vacuum evaporation under a base pressure of
molecular interactions, adjustable
reduced efficiency roll-off of the non-doped device can be antici-
pated by adjusting the -conjugation length of the carbazole
DEST of the molecule and further
ꢁ
4
5 ꢂ 10 Pa. The EL spectra and CIE coordinates were measured
with a PR-655 spectra colorimeter. The current-voltage-forward
luminance curves were measured using a Keithley 2400 source
meter and a calibrated silicon photodiode. All the measurements
were carried out at room temperature under ambient conditions.
p
dendrons. Here, the thermal, photophysical, and electrochemical
properties of these dendrimers were systematically investigated.
Efficient deep-blue OLEDs with low efficiency roll-off at high cur-
rent density have been achieved.
2.4. DFT calculation
2
. Experimental section
Geometry optimization for the dendrimers was carried out us-
ing Gaussian 03. The results were obtained by DFT calculation using
B3LYP functional and the 6-31G (d, p) basis. The molecular orbitals
were visualized using Gaussview.
2
.1. Materials and reagents
Unless otherwise indicated, all reagents were purchased from
commercial resources and used without further purification.
Carbazole was recrystallized from acetone before use. Sodium hy-
dride (60% dispersion in mineral oil) was washed by petroleum
ether, dried by ether before use. All air-sensitive reactions were
carried out under nitrogen.
2
2
.5. Synthetic procedures
.5.1. 3,6-Di-tert-butylcarbazole (D1)
Carbazole (5.016 g, 30 mmol) and AlCl
3
(4.0 g, 30 mmol) were
ꢀ
suspended in CH
2
Cl
2
(120 mL). After cooled to 0 C, tert-butyl
chloride (7.5 ml 60 mmol) in CH
2
2
Cl (20 mL) was added dropwise
2.2. Characterization
over 20 min to give a dark yellow solution. The mixture was stirred
at 0 C for 1 h, at room temperature for 9 h, and then poured into ice
ꢀ
The NMR spectra were measured on a Bruker DRX 600 spec-
water and extracted with CH
washed with water, brine, dried over anhydrous MgSO
and concentrated under reduced pressure. The crude product was
purified by recrystallization (CH Cl /petroleum ether) to give D1 as
a white solid (5.03 g, 60%). H NMR (600 MHz, CDCl ): 8.07 (d,
2
Cl
2
. The combined organic phase was
trometer, and chemical shifts were reported in ppm using tetra-
methylsilane as an internal standard. Mass spectra were obtained
on a Bruker autoflex MALDI-TOF mass spectrometer. Elemental
analysis (EA) was performed by determined with an Vario EL
elemental analyzer. The UVevisible absorption spectra were
4
, filtered,
2
2
1
3
d
J ¼ 1.8 Hz, 2H), 7.85 (s, 1H), 7.46 (dd, J ¼ 8.4, 1.8 Hz, 2H), 7.33 (d,
determined on
a
Hitachi U-3900 spectrophotometer. The
J ¼ 8.4 Hz, 2H), 1.45(s, 18H).

J. Li et al. / Dyes and Pigments 140 (2017) 79e86
81
2
.5.2. 3,6-Diiodocarbazole (1)
Carbazole (1.00 g, 5.98 mmol) was dissolved into acetic acid
room temperature, water was added and the mixture was extracted
with CH Cl . The combined organic phase was washed with water,
brine, dried over anhydrous MgSO , filtered, and concentrated.
Purification by column chromatography over silica gel (eluent:
2
2
ꢀ
(
16 mL) and the solution was heated to 50 C. Then finely-crushed
4
KI (1.29 g, 7.77 mmol) was added. After a few minutes of stirring,
KIO (1.279 g, 5.98 mmol) in acetic acid (2 mL) was added dropwise.
The mixture was refluxed for 1 h to give a light red suspension.
After cooling to room temperature, water was added and the
mixture was extracted with CH
1
3
CH
2
Cl
2
/petroleum ether 1:4) gave 3 as a white solid (1.18 g, 45%). H
):
NMR (600 MHz, CDCl
3
d
8.56 (d, J ¼ 8.4 Hz, 2H), 8.14 (d, J ¼ 1.8 Hz,
4H), 8.04 (d, J ¼ 1.8 Hz, 2H), 7.92 (d, J ¼ 8.4 Hz, 2H), 7.73 (dd,
J ¼ 9.0,1.8 Hz, 2H), 7.44 (dd, J ¼ 9.0,1.8 Hz, 4H), 7.29e7.33 (m, 6H),
2.40 (s, 3H), 1.45 (s, 36H).
2 2
Cl . The combined organic phase
was washed with water, brine, dried over anhydrous MgSO
4
,
filtered, and concentrated. Purification by recrystallization (ethyl
acetate/petroleum ether) gave 1 as a red-brown powder (1.61 g,
2.5.5. 3,6-Bis(3,6-di-tert-butylcarbazol-N-yl)carbazole (D2)
To a solution of 3 (1.0 g, 1.14 mmol) in DMSO (6 mL)-THF
(12 mL)-water (2 mL) was added powdered KOH (0.13 g,
1
6
3%). H NMR (600 MHz, CDCl
3
):
d
8.32 (d, J ¼ 1.8 Hz, 2H), 8.10 (s,
1
H), 7.68 (dd, J ¼ 8.4,1.8 Hz, 2H), 7.22 (d, J ¼ 8.4 Hz, 2H).
2.32 mmol). The mixture was stirred at reflux for 1.5 h. After cooling
2
.5.3. 3,6-Diiodo-N-tosylcarbazole (2)
To a solution of 1 (1.5 g, 3.58 mmol) and KOH (0.92 g, 16.4 mmol)
to room temperature,10% HCl (20 mL) was added followed by water
(10 mL) and methanol (5 mL). The precipitate was collected by
filtration and washed with water. The crude product was recrys-
in acetone (20 mL) was added slowly a solution of p-toluene-
sulfonylchloride (3.127 g, 16.4 mmol) in acetone (2 mL). The reac-
tion mixture was reflux for 15 min, then poured into water and
tallized (CH
H NMR (600 MHz, CDCl
6H), 7.66 (d, J ¼ 8.4 Hz, 2H), 7.60 (dd, J ¼ 8.4,1.8 Hz, 2H), 7.45 (dd,
J ¼ 8.4,1.8 Hz, 4H), 7.13 (d, J ¼ 8.4 Hz, 4H), 1.46 (s, 36H).
2
Cl
2
/methanol) to give D2 as a white solid (0.82 g, 99%).
):
1
3
d
8.40 (s, 1H), 8.17 (dd, J ¼ 4.8,1.8 Hz,
extracted with CH
with water, brine, dried over anhydrous MgSO
concentrated. Purification by recrystallization (CH
2
Cl
2
. The combined organic phase was washed
, filtered, and
Cl /petroleum
ether) gave 2 as a yellow solid (1.44 g, 70%). H NMR (600 MHz,
CDCl ):
8.17 (d, J ¼ 1.2 Hz, 2H), 8.07 (d, J ¼ 9.0 Hz, 2H),7.78 (dd,
4
2
2
1
2.5.6. 3,6-Bis[3,6-bis(3,6-di-tert-butylcarbazol-N-yl)carbazol-N-
yl]-N-tosylcarbazole (4)
3
d
J ¼ 9.0,1.8 Hz, 2H), 7.65 (d, J ¼ 8.4 Hz, 2H), 7.13 (d, J ¼ 7.8 Hz, 2H),
Following the procedure for the synthesis of 3, 2 (1.15 g,
2 mmol), D2 (3.26 g, 4.5 mmol), CuI (191 mg, 1 mmol), K PO
2
.29 (s, 3H).
3
4
(
1.06 g, 5 mmol), (±)-trans-1,2-diaminocyclohexane (0.15 mL,
1.2 mmol) were heated at reflux for 36 h. The mixture was purified
by column chromatography over silica gel (eluent: CH Cl /petro-
leum ether 1:4e1:2) to yield 4 as a light pink solid (1.41 g, 40%). H
NMR (600 MHz, CDCl ):
8.96 (s, 2H), 8.81 (d, J ¼ 12.6 Hz, 6H), 8.29
(s, 8H), 8.10 (m, 4H), 7.71 (d, J ¼ 8.4 Hz, 4H), 7.66 (d, J ¼ 8.4 Hz, 4H),
2
.5.4. 3,6-Bis(3,6-di-tert-butylcarbazol-N-yl)-N-tosylcarbazole (3)
CuI (0.29 g, 1.5 mmol), K
,2-diaminocyclohexane (DACH, 0.2 mL, 1.6 mmol) were added to a
solution of 2 (1.72 g, 3 mmol) and D1 (1.85 g, 6. 6 mmol) in toluene
40 mL). The mixture was stirred at reflux for 24 h. After cooling to
3
PO
4
(1.592 g, 7.5 mmol), and (±)-trans-
2
2
1
1
3
d
(
Scheme 2. Synthesis of the dendrimers.

82
J. Li et al. / Dyes and Pigments 140 (2017) 79e86
7
.50 (d, J ¼ 8.4 Hz, 2H), 7.44 (d, J ¼ 7.8 Hz, 8H), 7.30 (d, J ¼ 9 Hz, 8H),
(600 MHz, CDCl
3
):
d
8.43 (dd, J ¼ 7.2, 1.8 Hz, 4H), 8.24 (d, J ¼ 1.8 Hz,
2
.37 (s, 3H),1.39 (s, 72H).
4H), 8.15 (d, J ¼ 2.4 Hz, 8H), 8.04 (dd, J ¼ 7.2, 1.8 Hz, 4H), 7.74 (d,
J ¼ 9.0 Hz, 4H), 7.45(dd, J ¼ 8.4,1.8 Hz, 4H), 7.35 (dd, J ¼ 8.4,1.8 Hz,
13
8
H), 7.32 (d, J ¼ 9.0 Hz, 8H),1.45 (s, 72H); C NMR (600 MHz, CDCl
3
)
2.5.7. 3,6-Bis[3,6-bis(3,6-di-tert-butylcarbazol-N-yl)carbazol-N-yl]
d
146.7, 146.3, 143.1, 142.9, 142.4, 134.9, 133.1, 130.4, 129.2, 127.6,
26.6, 126.2, 122.4, 121.2, 113.9, 111.9, 37.7, 35.0; MALDI TOF-MS: m/
carbazole (D3)
1
Following the procedure for the synthesis of D2, 4 (881 mg,
þ
4 120 6 2
] ; Anal. Calcd for C118H N O S: C, 84.04, H,
z 1674.91 [MþNH
0
.5 mmol) and KOH (68 mg, 1.2 mmol) were treated at reflux for
.5 h. D was obtained by recrystallization (CH Cl /methanol) as a
white solid (788 mg, 99%). H NMR (600 MHz, DMSO-d 10.03 (s,
H), 8.81 (d, J ¼ 1.2 Hz, 2H), 8.68 (s, 4H), 8.29 (dd, J ¼ 8.4,1.8 Hz, 8H),
.97 (d, J ¼ 8.4 Hz, 2H), 7.92 (dd, J ¼ 7.8, 1.2 Hz, 2H), 7.88 (dd, J ¼ 8.4,
.8 Hz, 2H), 7.70 (d, J ¼ 7.8 Hz, 2H), 7.67 (d, J ¼ 1.9 Hz, 4H), 7.46 (dd,
J ¼ 8.4, 1.8 Hz, 8H), 7.32 (d, J ¼ 8.4 Hz, 8H), 1.40 (s, 72H).
7.17, N, 4.98; Found: C, 82.70, H, 7.26, N, 4.64.
1
3
2
2
1
6
) d
1
7
1
2
.5.8.3. Bis(4-(3,6-bis(3,6-bis(3,6-di-tert-butylcarbazol-N-yl)carba-
zol-N-yl)carbazole) phenyl) sulfone (G3). G3 was obtained from D3
3.535 g) and recrystallized from methanol followed by column
chromatography over silica gel (eluent: CH Cl /petroleum ether
:2) as white crystals (1.575 g). H NMR (600 MHz, CDCl 8.56 (q,
4H), 8.26 (d, J ¼ 1.2 Hz, 8H), 8.68 (d, J ¼ 9.0 Hz, 20H), 7.61 (q, 16H),
(
2
2
1
1
3
) d
2.5.8. General synthetic procedure for the dendrimers
13
To a solution of the dendron (2.2 mmol) in dry dime-
7.43 (m, 20H), 7.32 (m, 16H), 1.44 (s, 144H). C NMR (600 MHz,
CDCl 142.6, 141.2, 140.2, 131.0, 126.1, 123.9, 123.5, 123.1, 119.5,
thylformamide (DMF) (5 mL) was added a solution of sodium hy-
dride (96 mg, 4 mmol) in dry DMF (6 mL). After the solution was
stirred at room temperature for 30 min, bis(p-fluorophenyl) sulfone
3
) d
116.2, 110.9, 109.0, 34.706, 32.021; MALDI TOF-MS: m/z 3451.25
þ
[MþNa] ; Anal. Calcd for C246
240 14 2
H N O S: C, 85.48, H, 7.00, N, 5.67;
(
254 mg, 1 mmol) in dry DMF (5 mL) was added, and the mixture
Found: C, 83.53, H, 6.88, N, 5.14.
ꢀ
was stirred at 100 C for an additional 7 h. After cooling to room
temperature, the mixture was poured into ice water (100 mL) and
filtered. The crude product was purified by recrystallization or by
column chromatography to give the dendrimers.
3. Results and discussion
3.1. Synthesis of the dendrimers
2
.5.8.1. Bis(4-(3,6-di-tert-butylcarbazole)phenyl)
sulfone
(G1).
The synthetic routes of the dendrimers were shown in Scheme
. The carbazole-based dendrons were synthesized according to the
G1 was obtained from D1 (615 mg) and recrystallized from ethanol
as white crystals (400 mg, 52%). H NMR (600 MHz, CDCl ): d 8.24
3
2
1
literature using tosyl groups to protect the carbazole nitrogen
atoms [24]. Dendrimers G1, G2 and G3 with diphenyl sulfone core
were synthesized through a catalyst-free aromatic nucleophilic
substitution reaction between the corresponding dendrons and
bis(4-fluorophenyl)sulfone at moderate yields of 40e52%, respec-
tively. All of the new compounds were characterized by NMR
spectroscopy, MALDI-TOF mass spectroscopy, and elemental
(
1
dd, J ¼ 7.2, 1.8 Hz, 4H), 8.13 (d, J ¼ 2.4 Hz, 4H), 7.82 (dd, J ¼ 6.6,
.8 Hz, 4H), 7.49e7.44 (m, 8H), 1.46 (s, 36H).
2
.5.8.2. Bis(4-(3,6-bis(3,6-di-tert-butylcarbazol-N-yl)carbazole)
phenyl) sulfone (G2). G2 was obtained from D2 (1.589 g) and
recrystallized from ethanol as white crystals 794 mg, 48%). H NMR
1
Fig. 1. The spatial distributions of (bottom) HOMO and (top) LUMO energy densities of G2 and G3 by DFT B3LYP/6-31G(d) calculation.

J. Li et al. / Dyes and Pigments 140 (2017) 79e86
83
Fig. 2. TGA thermograms and DSC traces (inset) of the dendrimers (at a heating rate of
ꢀ
10
C/min).
Fig. 3. UVevis absorption and PL spectra of dendrimers G1-G3 in dilution solution of
ꢁ
5
toluene (10 M) and in solid film, and their phosphorescence spectra (delayed by
0 ms) in toluene at 77 K.
1
analysis. The target compounds G1, G2 and G3 exhibited good
solubility in common solvents such as toluene, chloroform and
tetrahydrofuran, thus they can be purified by column chromatog-
raphy or recrystallization, and can be applied in OLEDs through
solution-process.
morphology stabilities of G2 and G3 were confirmed by DSC, from
which no obvious phase transition peaks were observed, and only
an endothermic step due to the glass transition was detected at 242
ꢀ
and 325 C for G2 and G3, respectively. The increased glass tran-
g
sition temperature (T ) from G2 to G3 can be attributable to their
3
.2. DFT calculation
increased size of the bulky carbazole dendrons. The good thermal
and morphological stability of G2 and G3 suggested that homoge-
neous and amorphous thin films could be prepared through solu-
tion processing, which is expected for application in OLEDs.
The calculation results revealed that the molecules adopted
nonplanar three-dimensional structures with carbazolyl dendrons
arranging twisted around the diphenyl sulfone core (Fig. 1). For
dendrimers G2 and G3, the HOMOs were mainly distributed over
the outer carbazolyl units, whereas the LUMOs were delocalized on
the electron-withdrawing diphenyl sulfone core. The twisted ge-
ometry, together with the effective spatial separation between the
HOMO and LUMO, implied a small singlet-triplet splitting energy
3
3
.4. Photophysical properties
.4.1. Steady-state photoluminescence
The UVevis absorption and photoluminescene (PL) spectra of
the dendrimers in toluene were shown in Fig. 3. All the dendrimers
showed an intramolecular charge-transfer (ICT) absorption band
around 347 nm, indicating that the ICT character of the molecules
remained with the increase of the dendrimer generation. The ICT
characters of the dendrimers were further demonstrated by the PL
spectra in various solvents from nonpolar solvent toluene to polar
solvent dichlorobenzene (Fig. S1, Supplementary material). Broad
and unstructured emission bands with the emission peaks shifted
distinctly by changing the polarity of the solvent could be observed.
The effect of solvent polarity on the excited state was analyzed
using the Lippert-Mataga relationship (Eq. S(1), Supplementary
material), and all of the dendrimers exhibited positive sol-
vatochromism with emission maxima depending approximately
linearly on the solvent polarity, indicating the ICT behavior of the
dendrimers.
1 1
for the T -to-S transition of the molecules, which was considered
to be beneficial for efficient reverse intersystem crossing (RISC) and
as a result for TADF behavior.
3.3. Thermal stabilities
The thermal properties of the dendrimers were investigated by
means of thermal gravimetric analyses (TGA) and differential
scanning calorimetry (DSC) (Fig. 2), and the data was listed in
Table 1. The dendrimers exhibited high thermal stability with
decomposition temperatures (T
d
, corresponding to 5% weight loss)
ꢀ
in the range of 311e425 C. Their thermal stability increased with
the increase of the generation, probably owning to the increased
number of the thermal-stabilized carbazole groups. The
Table 1
Physical properties of the dendrimers.
a
b
HOMO/LUMOc
(eV)
Compound
Solution
PL (nm)
Film
E
(
g
S
(eV)
1
/T
1
D
(eV)
E
ST
t
(ns)
p
t
d
g d
T /T
ꢀ
( C)
eV)
(m
s)
l
UV
/
l
F
Air
/F
N2 (%)
l
UV
/l
PL (nm)
Air
F /FN2 (%)
G1
G2
G3
346/406
348/426
349/439
53/60
53/64
44/52
347/421
347/438
349/451
23/42
34/58
15/22
3.2
3.1
3.2
3.34/3.01
3.25/3.00
3.15/2.98
0.33
0.25
0.17
7.6
6.5
8.7
540/2600
347/2530
378/2860
ꢁ5.80/ꢁ2.60
ꢁ5.74/ꢁ2.64
ꢁ5.70/ꢁ2.50
e/311
242/372
325/425
a
Calculated from the absorption spectra threshold, E
g
¼ 1240/
) and phosphorescene (for T
Estimated with reference to ferrocene (4.8 eV); LUMO ¼ HOMO þ E
l.
b
c
Calculated from the onset of the fluorescene (for S
1
1
) spectra.
g
.

84
J. Li et al. / Dyes and Pigments 140 (2017) 79e86
Fig. 4. Temperature-dependence of the transient PL spectra for (a) 5 wt% G2 and (b) 5 wt% G3 doped in PMMA.
In toluene solution, the maximum PL wavelength showed a
bathochromic shift from G1 at 406 nm to G3 at 439 nm with the
increase of the dendrimer generation. The lowered S energy may
dendrimers in both solution and neat film exhibited enhancement
under N atmosphere with respect to those in air condition, which
suggested that an effective up-conversion of the triplet excitons did
occur when oxygen was excluded.
2
1
be derived from the increased distance between the donor (outer
carbazole) and the acceptor (diphenyl sulfone) from G1 to G3
[20,25]. In films, no obvious bathochromic shifts were detected in
3.4.2. Time-resolved fluorescence decays
the UVevis absorption spectra of the dendrimers, and a bath-
ochromic shift of 15, 15 and 12 nm in the PL spectra with respect to
that in toluene solution was observed for G1, G2 and G3, respec-
tively. The small bathochromic effect implied that the intermolec-
ular interactions could be effectively prevented in the solid state
due to the bulky carbazole dendrons.
To further demonstrate that TADF occurred in the solid state,
G2-and G3-doped PMMA films (5 ± 1 wt %) were investigated. The
transient PL decay characteristics and temperature dependence
were measured under vacuum (Fig. 4). The signals displayed a fast
decay and a slower one with their spectra showing good overlap
from each other, which can be assigned to the prompt and delayed
fluorescence, respectively (Table 1, and Fig. S2, Supplementary
material). Moreover, the intensity of the delayed component
increased with the temperature from 100 K to 200 K, implying the
TADF mechanism of the delayed fluorescence. Interestingly, the
intensity of the delayed component slightly dropped with
increasing the temperature from 200 to 300 K, suggesting a slight
increase in non-radiative processes through vibration or molecular
interactions over 200 K [26]. At 300 K, the lifetimes were estimated
In contrast to their fluorescence spectra with strong CT charac-
ters, the phosphorescence spectra of the dendrimers measured at
7
7 K with 10 ms decay showed well-resolved vibrational structures,
3
indicating their T
12]. The onset of the fluorescence spectrum and that of the highest
energy peak of the phosphorescence spectrum were used to
1
states were locally excited triplet states ( LE)
[
1
3
determine the zero-zero energies (E0-0) of the CT and LE states,
respectively, and the energy levels E0-0 were listed in Table 1. It is
noted that the T
1
levels of the dendrimers are quite similar to each
to be 464
ms for G2 and 814
ms for G3, respectively. It is noted that
other (~3.0 eV), implying that by attaching higher generation car-
bazoles via their N-position to the 3,6-positions of the inner
carbazole moiety does not influence on the triplet energy of the
G3 has a smaller
attributable to the slower radioactive rate constant (k and kTADF) of
G3 because of the lower fluorescent quantum yield (Table S1,
D
EST but a longer lifetime, which might be
F
whole molecule [24]. With the decreased S
the ST values of the molecules decreased with the increase of the
dendron generation from 0.33 eV for G1 to 0.17 eV for G3. In
addition, the fluorescence quantum yield ( ) values of the
1
energies from G1 to G3,
Supplementary material) [27].
DE
3.5. Electrochemical properties
F
f
The electrochemical characteristics of the dendrimers were
measured by cyclic voltammetry. Dendrimers G1 to G3 exhibited
reversible oxidation processes upon the anodic sweep. The first
oxidation waves of the dendrimers shifted to lower potentials with
increasing the dendrimer generation from G1 at 1.43 V to G3 at
1.31 V, owning to the extended conjugation of the dendrons and the
increased electron-donating ability with increasing the dendron
generation. The HOMO levels of dendrimers G1 to G3 determined
from the onsets of the oxidation potentials were ꢁ5.80, ꢁ5.73
þ
and ꢁ5.68 eV, respectively, by assuming the energy level of Fc/Fc
to be 4.8 eV below the vacuum level. No reduction signals were
detected for any of the compounds. Thus the LUMO levels of the
dendrimers were calculated from the HOMO levels and the corre-
sponding optical bandgaps (E
.6. Electroluminescent properties
Considering the neat films of G2 and G3 exhibited favorable
g
s) (Table 1) (See Fig. 5).
3
properties, to further investigate their electroluminescent (EL)
performance, double-layer solution-processed OLEDs were
Fig. 5. CV curves of dendrimers G1-G3 in CH
Bu NPF at a scan rate of 100 mV/s.
2 2
Cl or MeCN solution using 0.1 M
4
6

J. Li et al. / Dyes and Pigments 140 (2017) 79e86
85
Fig. 6. (a) Energy level diagrams, (b) EL spectra, (c) Current densityevoltagee luminance and (d) Current efficiencyecurrent densitye power efficiency characteristics of the OLEDs.
Table 2
EL performance of the OLEDs.
a
CEmax (cd A )b
ꢁ1
PEmax (lm W )c
ꢁ1
PEJmax/PEmax (%)d
CIE (x,y)e
Device
Von (V)
G2
G3
4.8
5.2
4.1
1.07
1.6
0.49
14
56
(0.15, 0.12)
(0.19, 0.15)
a
ꢁ2
Turn-on voltage (at 1 cd m ).
The maximum current efficiency.
The maximum power efficiency.
b
c
d
e
The PE value at the maximum current density vs the peak value.
At the driving voltage of 12 V.
fabricated with the structure of ITO/PEDOT:PSS (30 nm)/EML
45 nm)/TPBI (30 nm)/LiF (1 nm)/Al (100 nm), in which G2 and G3
property of G2 well demonstrated our design on TADF dendrimers
for molecular modification, i.e., smaller ST, weak intermolecular
(
DE
were employed as the nondoped emitting layers, respectively. The
energy levels and the electroluminescent characteristics of the
devices were shown in Fig. 6 and Table 2. For comparison, G1 was
also applied as the non-doped emitter for the control device (Fig S3,
Supplementary material). The OLEDs exhibited deep-blue and true
interactions, good film forming ability (Fig. S5, Supplementary
material), as well as balanced charge transport ability.
blue emission with
lmaxs at 428 and 440 nm, and CIE coordinates at
4. Conclusion
(
0.15, 0.12) and (0.19, 0.15), respectively (Fig. 6b). The EL spectra of
both two devices were almost identical to their corresponding PL
spectra in toluene solutions (Fig. S4, Supplementary material). Both
of the two devices displayed low turn-on voltages at about 5 V,
which could be attributable to the high-lying HOMO levels and as a
result a small energy barrier at the PEDOT:PSS/dendrimer interface.
The G2 device achieved a better performance with a maximum
current efficiency (CEmax) of 4.1 cd/A and a maximum power effi-
ciency (PEmax) of 1.6 lm/W. As shown in Fig. 6d and Fig. S3b,
compared with the control device, the efficiencies of both devices
decreased quite slowly with increasing current density. Remark-
ably, the power efficiency of G3 device at the maximum current
density was as high as 56% respects to the PEmax, suggesting the
triplet-singlet and triplet-triplet annihilation rate could be reduced
In conclusion, two TADF dendrimers G2 and G3 based on bis[4-
3,6-di-tert-butylcarbazole)phenyl]sulfone (G1) core incorporating
carbazole dendrons were developed. These dendrimer emitters
inherited blue emitting and similar triplet energy level to the
parent G1, but superior in smaller DEST to facilitate the RISC process,
inhibited intermolecular interactions, higher thermal and amor-
phous stabilities and good solution processability. Solution-
processed OLED by employing dendrimer G2 as the non-doped
emissive layer achieved deep blue with a maximum current effi-
ciency of 4.1 cd/A and a CIE coordinate at (0.15, 0.12). Similar device
based on G3 emitting layer displayed negligible efficiency roll-off at
high current density. These results pave a way for the molecular
modification on adjusting DEST for the TADF emitters, and the deep-
blue TADF dendrimers could be promising candidates for applica-
tions in solution-processable OLEDs.
(
with the reduction of
DEST and the inhibited intermolecular in-
teractions by introducing carbazole dendrons. The satisfying EL

8
6
J. Li et al. / Dyes and Pigments 140 (2017) 79e86
[11] Zhang Q, Li B, Huang S, Nomura H, Tanaka H, Adachi C. Efficient blue organic
Acknowledgements
light-emitting diodes employing thermally activated delayed fluorescence.
Nat Phot 2014;8:326e32.
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This work was financially supported by the National Natural
[
[
[
Science Foundation of China (61605138, 61205179, 61307030,
6
Talents (NCET) in University” (NCET-13-0927), the Qualified
Personnel Foundation of Taiyuan University of Technology (QPFT)
1307029, 51503022), the “Program for New Century Excellent
(
No:tyutrc-201255a), and the Shanxi Provincial Key Innovative
Research Team in Science and Technology (201513002-10), Open
Foundation of State Key Laboratory of Electronic Thin Films and
Integrated Devices (KFJJ201507), Basic and Frontier Research Pro-
gram of Chongqing Municipality (cstc2015jcyjA50036), Natural
Science Foundation of Yongchuan District (Ycstc,2015nc4001).
[15] Huang B, Qi Q, Jiang W, Tang J, Liu Y, Fan W, et al. Thermally activated delayed
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