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HOMO/LUMO and ΔEST of these molecules.[15a,19] The TD-DFT
results of singlet (S1) and triplet (T1) states of molecules are
summarized in Tables S1−S3 (Supporting Information). As
presented in Figure 1b, the LUMOs of all compounds were
evenly distributed over the PPC core, while a tiny portion of
the two methyl groups ortho to TPA rings could be seen on the
LUMO+1. The HOMO was mainly located on TPA or TPAm
groups and a small part on the pyridine ring of the PPC core.
Overall, there was a slight overlap between the HOMO and
LUMO over the 2,6-dimethyl-TPA rings and the pyridine ring
of the PPC core. To further investigate the nature of the excited
states (S1, T1–T3) of PPC-based compounds, we calculated their
natural transition orbitals (NTOs) and optimized geometries,
shown in Figures S1 and S2 (Supporting Information). For
the S1 states of the three compounds, the holes were mainly
localized on the triphenylamine donors, and they all exhib-
ited evident CT characters (1CT). In contrast, the particle dis-
tributions and emission characters were strongly affected by
the steric effect of the dimethyl moieties for the T1 state. The
T1 state of TPAPPC showed stronger LE characters due to its
more planar structure, in which both the holes and the parti-
cles were distributed over it almost entirely. As for the other
two compounds, good separations of the holes and particles
indicated the CT characters of their T1 states. Moreover, the
TD-DFT calculation exhibited a small ΔEST of 237, 9, and
10 meV for TPAPPC, TPAmPPC, and tTPAmPPC, respectively.
These results suggested that the role of methyl groups on the
central benzene ring is rather important.[20] Compared to our
previous report,[15f ] PPCs with highly substituted carbazoles
on the phenyl ring only generated the dihedral angles around
70° even with the excess steric hindrance. In this work, PPCs
with TPA having two perfectly matched methyl groups revealed
much smaller ΔEST values than those of the reported pyridine-
carbonitrile-based emitters.[14,15]
in Figure S4 (Supporting Information). The emission spectra
are bathochromic-shifted in polar solvents, suggesting the ICT-
character of those emitters. Additionally, the PLQYs in degassed
toluene are higher than those in (non-degassed) solutions; they
are 11.0 (6.4), 15.5 (8.7), and 7.8 (3.5)% for TPAPPC, TPAmPPC,
and tTPAmPPC, respectively. The decrease in PLQYs is likely
due to the quenching of T1 by the triplet ground state O2.[6]
3. Device Performance
Next, we measured the fluorescence and phosphorescence
spectra of these emitters in 10wt% doped mCPCN thin films,
where mCPCN is 9-(3-(9H-carbazol-9-yl)phenyl)-9H-carba-
zole-3-carbonitrile. The maximum emission wavelengths of
these emitters are recorded in Table 1. The S1 and T1 energy
levels were calculated from the onset of the fluorescence and
phosphorescence emission spectra measured at 300 and 77 K,
respectively (Figure 2a–c). Accordingly, we determined the ΔEST
(from ES − ET) values as 207, 27, and 20 meV for 10wt% doped
films of TPAPPC, TPAmPPC, and tTPAmPPC. The phospho-
rescence spectrum of TPAPPC showed a sharp vibronic struc-
ture with a triplet energy of 2.52 eV, which corroborated the 3LE
nature of its T1 state. In contrast, the phosphorescence spectra
of TPAmPPC and tTPAmPPC exhibited broad 3CT emission
due to enhanced donor strength, which concurred with the
NTO analyses (Figure S1, Supporting Information). TPAmPPC
and tTPAmPPC appear to have much lower ΔEST values than
TPAPPC, which is likely due to the large dihedral angles gener-
ated between donor and acceptor moieties by the two methyl
groups at C(2,6)-positions of the central benzene ring (see
Figure 1). We calculated the energy values using doped films
for simulating the environment in the solid-state OLED devices.
The fluorescence and phosphorescence spectra in solution and
detailed discussion are provided in Figures S5 and S6 (Sup-
porting Information). The PLQYs of these mCPCN thin films
doped with 10wt% of TADF molecules were measured using an
integrating sphere. The values are in the range of 79−100%. To
our surprise, a high PLQY of 90% for TPAmPPC neat film was
observed. Compared with only 60% PLQY of TPAPPC neat film,
this highly twisted conformation of TPAmPPC might suppress
the aggregation-caused quenching process, which could be the
further benefit to use our material design strategy. As shown
by XRD packing analysis (Figure S7, Supporting Information),
the intermolecular pyridine distance of TPAmPPC is 4.5 Å,
while TPAPPC exhibits a relatively shorter distance of 4.1 Å due
to its more coplanar structure. The increased intermolecular
2.2. Photophysical and Thermal Properties
The UV–vis absorption and emission spectra of these dopants
in toluene at 10–5 m are shown in Figure S3 (Supporting Infor-
mation) and summarized in Table S4 (Supporting Informa-
tion). The prominent absorptions at 290 to 302 nm are due
to the π–π* transition, whereas the broad peaks with a much
weaker intensity in the region of 365 to 460 nm are attributed
to the intramolecular charge transfer (ICT) transition from TPA
to PPC units. In addition, the photoluminescence (PL) spectra
of these emitters in various solvents were measured, as shown
Table 1. Summary of photophysical properties of the emitters.
Emittera–h)
PLQY
[%]d)
Td/Tg
HOMO/LUMO
[eV]g)
λ
max, abs
[nm]a)
λ
max, fl
λ
max, fl/phos
ΔEST [meV]c)
Exp/Cal
τ
PF [ns]/τDF
[µs]e)
[°C]f)
[nm]a)
[nm]b)
TPAPPC
290, 402
302, 388
304, 400
503
508/531
518/533
539/549
207/237
27/9
100
100
79
10.0/133.9
33.7/3.4
14.1/2.4
380/195
336/–
−5.76/−3.12
−5.69/−3.08
−5.60/−3.07
TPAmPPC
tTPAmPPC
544
564
20/10
370/–
a)Measured in the toluene solution (10–5 m, 300 K); b)Measured in the doped films (mCPCN: 10wt% emitter) at 300 K/77 K; c)ΔEST = ES − ET; d)Measured in the doped film
(300 K); e)The transient photoluminescence (PL) lifetimes of the doped (mCPCN: 10wt%) film (300 K); f)Measured by thermogravimetric analysis (TGA)/differential scan-
ning calorimetry (DSC) methods; g)The HOMO was measured by photoelectron spectroscopy, and ELUMO = EHOMO + Eg.
2008032 (3 of 8)
© 2021 Wiley-VCH GmbH
Adv. Mater. 2021, 2008032