2
Zhang et al. Sci China Chem
However, the operational stability of the OLEDs based on
peripheral groups in two TADF emitters with a stable
o-phthalodinitrile (PN) as the acceptor (named 2tBuCzPN
and 2PhCzPN, respectively). The photo-physical and elec-
trochemical properties of those emitters were systematically
studied. Compared with their analogue 2CzPN that employs
carbazole as the electron donor, the electrochemical stability
of 2tBuCzPN and 2PhCzPN was dramatically improved, and
the shortened exciton lifetimes were observed derived from
the reduced ΔESTs. Furthermore, the steric hindrance of the
peripheral phenyl group can effectively restrain the inter-
molecular interaction. The OLEDs based on 2tBuCzPN and
2PhCzPN achieved improved maximum EQEs of 17.0% and
14.0%, respectively (8.5% for 2CzPN), and enhanced life-
times with T50 of 7.6 and 13.4 h at 500 cd/m2, respectively
(1.7 h for 2CzPN), as well as reduced roll-off characteristic
for the 2PhCzPN based OLED. These results have proved
the rationality of carbazole modification with peripheral
groups and potential advantages of TADF materials in
OLEDs.
TADF emitters (TADF-OLEDs) is still far from satisfaction,
which impedes their prospective commercialization [13–
15,25,26]. Thus, methods to increase the operational stability
of TADF-OLEDs have attracted increasing attention in the
last several years [27–31]. Although the intrinsic cause for
the instability of OLEDs is complex, it is generally accepted
that the electrochemical deterioration and the severe triplet-
triplet annihilation (TTA), triplet-polaron annihilation
(TPA), and singlet-triplet annihilation (STA) of the mole-
cules in the vicinity of the recombination zone are in-
trinsically associated with the operational stability of the
device [32–38]. By utilizing a stable dicarbazolylbenzene
donor and a stable triazine acceptor, Lee et al. [28] designed
and synthesized two stable blue TADF emitters, DCzTrz and
DDCzTrz, and accomplished a long T80 lifetime (time to 80%
of initial luminance) of 52 h at an initial luminance of
500 cd/m2. Adachi and co-workers [29] have developed four
donor-acceptor (D-A) type TADF emitters consisting of a
xanthone (XT) acceptor unit coupled with different donor
units, and it is observed that adopting the donor units with
non-planar peripheral substitutions can drastically restrain
the concentration quenching behavior of TADF molecules by
increasing their intermolecular distance in thin films to in-
hibit the electron-exchange interactions of triplet excitons. In
2016, our group [30] proposed a molecular structure with the
luminance cores of TADF molecules sterically shielded by
the tert-butyl units to simultaneously improve the efficiency
and the operational stability of TADF-OLEDs, achieving a
5TCzBN based OLED with a maximum EQE as high as
21.2% and a record long T50 of 770 h at an initial luminance
of 500 cd/m2. Albeit these considerable efforts that have
been made, to date only a few TADF emitters can realize
good operational stability in OLEDs. Therefore, further de-
sign strategies for highly efficient and stable TADF emitters
are still highly desired.
To realize a small ΔEST, most TADF emitters possess
spatially separated electron-donors and acceptors. As
aforementioned, to prepare stable TADF emitters, the
adopted electron donor and acceptor moieties should possess
excellent stability [28]. As one of the most commonly used
electron-donors in TADF molecules, the carbazole unit fea-
tures relatively good stability because it is made up of only
aromatic moieties with high bond dissociation energies.
However, the hydrogen atoms in the 3,6-positions of the
carbazole are comparatively electrochemically active, lead-
ing to the dimerization [39,40], which hinders the long-term
stability of the corresponding TADF emitters.
In the present study, we show that by simply modifying the
carbazole units with peripheral groups, simultaneous en-
hancement of the efficiency and the operational stability of
OLEDs can be realized. The tert-butyl and phenyl groups
were attached to the 3,6-positions of carbazole units as
2 Experimental
2.1 Syntheses of 2CzPN, 2tBuCzPN and 2PhCzPN
The syntheses of the three TADF emitters: 4,5-di(9H-car-
bazol-9-yl)phthalonitrile (2CzPN), 4,5-bis(3,6-di-tert-butyl-
9H-carbazol-9-yl)phthalonitrile (2tBuCzPN), and 4,5-bis
(3,6-diphenyl-9H-carbazol-9-yl)phthalonitrile (2PhCzPN),
were easily achieved in a nitrogen atmosphere through aro-
matic nucleophilic substitution reactions by using carbazolyl
units as nitrogen nucleophiles and the phthalonitrile unit as
the electrophile, as outlined in Scheme 1. The syntheses were
performed according to the synthetic methods reported pre-
viously [7,41]. The reagents and all the solvents were used as
received without any further purification. 9H-carbazole
(2.04 g, 12.2 mmol), 3,6-di-tert-butyl-9H-carbazole (3.41 g,
12.2 mmol), or 3,6-di-phenyl-9H-carbazole (3.89 g,
12.2 mmol) was dissolved in dry N,N-dimethylformamide
(DMF) (20 mL) and then added dropwise to a stirred solution
of potassium tert-butoxide (1.36 g, 12.2 mmol) in dry DMF
(10 mL) under a nitrogen atmosphere at room temperature.
After stirring at 40 °C for 30 min, 4,5-difluorophthalonitrile
(1.0 g, 6.1 mmol) dissolved in dry DMF (10 mL) was added
dropwise. The reaction mixture was stirred at 80 °C for 12 h.
After it was cooled to room temperature, the reaction mixture
was quenched with saturated potassium carbonate aqueous
solution (100 mL), and the resulting precipitate was col-
lected by filtration. The residue was dried in a vacuum drying
oven at 100 °C for 5 h, and then purified by the column
chromatography on the silica gel (petroleum ether:di-
chloromethane, 5:1, v/v). The three compounds were further
purified by vacuum sublimation, yielding green or yellowish
1
green powder. 2CzPN (1.95 g, 70%), H NMR (400 MHz,