Angewandte
Communications
Chemie
respectively (Table 1). Furthermore, the lifetime of PrFPCz
measured by monitoring the intensity of the PL peak
shows only a single exponential decay component (27.5 ns) in
toluene, while that of PrFCzP and PrFTPA clearly reveals two
single exponential decay components, consisting of a fast
decay with a lifetime of 31 ns (for PrFCzP) and 63 ns
wavelength at room temperature clearly exhibits a delayed
component (with a lifetime of 4.3 ms, 2.4 ms, and 3.1 ms,
respectively, Table S4) following a prompt component (with
a lifetime of 143 ns, 33 ns, and 40 ns, respectively), confirming
their distinct TADF characteristics in doped films. Intrigu-
ingly, while distinct TADF is not readily observed for PrFPCz
in polar solvents, it exhibits clear TADF characteristics in
doped films. At room temperature, PrFPCz, PrFCzP, and
PrFTPA doped in TCTA exhibit a PLQY of approximately
40%, 38%, and 60%, respectively. Noticeably, PrFTPA
doped in TCTA exhibits a significantly higher PLQY of
about 60% than the others at room temperature.
(
(
PrFTPA ) and a slow decay with a lifetime of 16 ms
PrFCzP) and 4.2 ms (PrFTPA; see Figure S3 and Table 1).
Both the short and long decay components are subject to
oxygen quenching. We thus conclude that the short and long
decay components are ascribed to the prompt fluorescence
and TADF, the TADF originates from thermal equilibrium at
room temperature between the S and the T state, and the
1
1
difference in energy DE (defined as T minus S ) is derived
T-S
1
1
[17]
as follows [Eq. (1)]:
The TADF emitters PrFPCz, PrFCzP, and PrFTPA were
further subjected to electroluminescence (EL) studies. The
devices had the structure: glass substrate/ITO anode/
PEDOT:PSS (30 nm)/TCTA: 8 wt% PrFPCz, PrFCzP, or
ꢀ
ꢁ
kisc
krisc
Àt=t1
Àt=t2
½
I ¼ I
kisc þ krisc e
þ k
þ krisc e
ð1Þ
t
0
isc
PrFTPA
(40 nm)/3TPYMB
(50 nm)/LiF
(0.6 nm)/Al
where I0 is a proportional constant incorporating both
radiative decay rate constant of the CT emission and instru-
ment factor, t and t are the observed lifetime of the fast and
(100 nm). The transparent poly(3,4-ethylenedioxythiophene):
poly(styrenesulfonate) (PEDOT:PSS) served as the hole-
[19]
1
2
injection layer.
employed as the hole-transport and host material. Tris-[3-
TCTA with a large-triplet-energy was
[19]
slow decay components. As a result, the equilibrium constant
K = k /k can be obtained by the ratio of the pre-
[20]
eq
isc risc
(3-pyridyl)mesityl]borane (3TPYMB),
LiF and Al acted
exponential factor (at t = 0; see Table 1) in Equation (1),
which is deduced to be 332 (PrFCzP) and 181 (PrFTPA) in
as the electron-transport layer, electron injection layer and
the cathode, respectively. Figure 3a shows the device con-
figuration and chemical structures of materials used in this
work.
degassed toluene. According to the DET-SÀK relationship
eq
expressed as DET-S = ÀRT ln(K /3) where a factor of 3 stands
eq
for the triplet degenerate states, DE is then deduced to be
Figure 3b–d shows the EL characteristics of all studied
devices, while data are summarized in Table 2. The EL spectra
(Figure 3b) of all the devices are similar to their correspond-
ing PL spectra in doped films (i.e., blue-green to green
emission and more red-shift EL for the PrFPCz and PrFTPA
devices), indicating pure EL from either PrFPCz, PrFCzP, or
PrFTPA. All devices exhibit similar I–V characteristics (Fig-
ure 3c), with a low turn-on voltage of about 2–2.5 V (defined
as the voltage when the luminance becomes detectable) and
low operation voltage (e.g. ca. 3–4 V for a practical brightness
T-S
À1
À2.8 and À2.4 kcalmol for PrFCzP and PrFTPA in toluene,
respectively. TADF was not observed for PrFPCz in toluene
(
see above) and other polar solvents. This is perhaps due to
the larger solvent polarity stabilization of the triplet state (T1)
than the singlet state (S ), resulting in a large negative DET-S
1
for PrFPCz such that T !S reverse intersystem is thermo-
1
1
dynamically unfavorable. Nevertheless, as elaborated below,
TADF for all the new boron complexes is evident in the solid
film, which contributes greatly to OLEDs.
Spectroscopic measurement of the boron com-
plexes in the solid film has also been performed.
Figure S4a depicts the fluorescent spectra (measured at
room temperature) and phosphorescent spectra (mea-
sured at 77 K) of PrFPCz, PrFCzP, and PrFTPA doped
(
with a doping concentration of 8 wt%) in a large-
triplet-energy host TCTA [(4,4’,4’’-tri(N-carbazolyl)tri-
[18]
phenylamine)].
The fluorescence and phosphores-
cence of PrFPCz, PrFCzP, and PrFTPA in doped films
give broad and structureless spectra peaking around
5
22 nm, 518 nm for PrFPCz, 505 nm, 491 nm for
PrFCzP, and 520 nm, 531 nm for PrFTPA , respectively
Table S4). From the difference in the onset wave-
(
lengths of fluorescence and phosphorescence spectra,
relatively small triplet-to-singlet energy gaps DET-S of
À1
approximately À57 meV (À1.3 kcalmol ), À65 meV
À1
À1
(
À1.5 kcalmol ), and À38 meV (À0.9 kcalmol ) are
extracted for PrFPCz, PrFCzP, and PrFTPA, respec-
tively, indicating the possibility of TADF in these boron
complexes doped in solid films as well. Indeed, the
transient photoluminescence (PL; Figure S4b) of
Figure 3. a) The device configuration and chemical structures of materials
used in this work. b) EL spectra, c) current–voltage-luminance (I–V–L) charac-
teristics, and d) external quantum efficiency and power efficiency of PrFPCz,
PrFPCz, PrFCzP, and PrFTPA in the TCTA host PrFCzP, and PrFTPA devices.
Angew. Chem. Int. Ed. 2016, 55, 3017 –3021
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3019