a PE SCIEX LC/MS spectrometer. Elemental analyses were
carried out on a Vario EL Elementar by the Flash EA 1112
method. Modulated differential scanning calorimetry (MDSC)
and thermogravimetric analysis (TGA) were performed under
a nitrogen atmosphere at a heating rate of 10 ꢁC minꢀ1 on a TA
Instruments MDSC 2910 and TGA Q50, respectively. Solution
PL quantum yields (FPL) were determined by using the method
of Demas and Crosby23 with a degassed aqueous solution of
quinine sulfate as a reference. Absorption and PL spectra were
measured with a Perkin Elmer Lambda 2S UV/VIS spectrometer
and a Perkin Elmer LS 50B luminescence spectrometer, respec-
tively. Cyclic voltammetric measurements (CV) were carried out
on a CH Instruments CHI 600A electrochemical analyzer.
OLEDs were fabricated on indium-tin oxide coated glass
substrates with sheet resistance of 30 U squareꢀ1. Prior to
successive film deposition, they were cleaned with Decon 90,
rinsed in de-ionized water, then dried in an oven, and finally
treated in an ultraviolet-ozone cleaner. The organic materials
used have acronyms as follows: NPB: N,N0-diphenyl-1,10-
biphenyl-4,40-diamine; BPhen: 4,7-diphenyl-1,10-phenanthro-
line; and TPBI: 1,3,5-tris(N-phenylbenzimidazol-2-yl) benzene.
Devices with a base structure of either ITO/NPB (70 nm)/PyTPA
or TPyPA (30 nm)/TPBI (20 nm)/LiF (0.5 nm)/Al (100 nm) or
ITO/PyTPA or TPyPA (100 nm)/TPBI (20 nm)/LiF (0.5 nm)/Al
(100 nm) were fabricated by thermal evaporation in a high
vacuum chamber (base pressure ꢂ10ꢀ6 Torr). In the three-layer
devices (i.e. NPB/TPA derivatives/TPBI), the TPA derivatives
were used as host emitters and NPB and TPBI were used as the
hole-transporting layer (HTL) and the electron-transporting
layer (ETL), respectively. The TPA derivatives function as both
the host emitter and the HTL in the two-layer devices (i.e. TPA
derivatives/TPBI) such that their hole-transporting abilities can
be accessed. All films were sequentially deposited at a rate of 0.1–
0.2 nm sꢀ1 without vacuum break. A shadow mask was used to
define the cathode and to make four 0.1 cm2 devices on each
substrate. Current density–voltage–luminance (J–V–L) charac-
teristics and EL spectra were measured simultaneously with
a programmable Keithley model 237 power source and a Pho-
toresearch PR650 spectrometer. All measurements were carried
out at room temperature under ambient atmosphere without any
encapsulation.
endowing them with an important attribute of low production
cost required for large-scale commercial applications.
Apart from the Td, melting and glass transition temperatures
(Tm and Tg) of TPyPA and PyTPA were also measured. As
determined by MDSC, Tm and Tg of TPyPA are 316 and 174 ꢁC;
while PyTPA melts at 160 ꢁC (PyTPA has no obvious glass
transition). The high Tm for TPyPA may be attributed to the two
extra pyrene groups at the C4 position of the TPA backbone,
which effectively enhance the degree of symmetry and increase
the molecular weight. In addition, both PyTPA and TPyPA show
high FPL; whereas solution FPL were estimated to be 74 and 80%
for PyTPA and TPyPA, respectively.
Fig. 1 depicts the absorption and PL spectra of PyTPA and
TPyPA neat films deposited on quartz substrates. It is well
established that the increase in conjugation length may result in
a red-shift of onset absorption edge (labs.edge) and PL peak (lPL).
As expected, the TPyPA film shows slight red-shifts of 10 and
16 nm for labs.edge and lPL compared to the PyTPA film. In
particular, strong blue emission was observed with a single peak
maximum at 452 nm (full spectral width at half maximum,
FWHM ¼ 54 nm) for PyTPA, relative to that for TPyPA, i.e. 468
nm (FWHM ¼ 52 nm). Intense PL peaks with small FWHM of
the two materials suggest that they have good potential to offer
saturated blue emission in OLEDs. All photophysical data of
PyTPA and TPyPA are summarized in Table 1.
Fig. 2 shows normalized EL spectra of the PyTPA- and the
TPyPA-based OLEDs with either TPBI or BPhen as ETL at L ¼
100 cd mꢀ2. While the EL spectra for PyTPA-based devices with
different ETL are nearly identical, EL spectra of TPyPA-based
devices with TPBI and BPhen ETL show obvious differences. We
also observed that the PyTPA/TPBI device performs consider-
ably better than the PyTPA/BPhen device; our efforts are thus
concentrated on the PyTPA/TPBI device hereafter. For the
TPyPA devices, in addition to the main peak at 472 nm which
corresponds well with the PL peak, an additional peak at 570 nm
was observed. The broad and featureless shape of the 570 nm
peak indicates that it might be related to exciplex emission. This
is further confirmed by the same peak appearing in a PL spec-
trum of a blended film of TPyPA and BPhen (Fig. 2). With the
addition of the strong exciplex emission, the TPyPA/BPhen
device gives an efficient white emission. The TPyPA/BPhen
device was thus further explored for its efficient white emission.
It is worth noting that exciplex emission is found to originate
from the delocalization of electronic excitation over the two
molecules and from partial electron transfer between the elec-
tron-donating and electron-accepting molecules. As reported by
Offermans et al.,25 the charge transfer character of the exciplex is
strongly dependent on the strength of the electron-donating
properties of the donor materials and with the electron affinity of
the acceptor. Thus, it is reasonably inferred that the formation of
exciplex emission in TPyPA is due to the increase in electron-
donating ability of TPyPA resulting from an increase in the
number of pyrene arms. This is not the case for the PyTPA-based
device, in which no additional exciplex peak is observed when it
is in contact with the same electron-accepting material (TPBI or
BPhen).
Results and discussion
Scheme 1 shows the chemical structures of PyTPA and TPyPA,
the identities of which have been confirmed by 1H-nuclear
magnetic resonance, elemental analysis, and mass spectrometry.
Incorporation of the TPA with a pyrene group is expected to lead
to efficient hole-injection and transportation in these
compounds, and to form non-coplanar structures which bear
bulky substituents in order to disrupt the intermolecular inter-
action and inhibit the problematic re-crystallization, and thus
improve the thermal and morphological stability of these mate-
rials. From the TGA data, both PyTPA and TPyPA are highly
stable in nitrogen with high decomposition temperatures (Td) of
416 and 565 ꢁC respectively. On the other hand, these
compounds can be obtained in high yields via simple Suzuki
coupling reactions between commercial starting materials,24
Fig. 3 shows J–V–L characteristics of the PyTPA/TPBI and
TPyPA/BPhen devices with and without the NPB layer. It can be
seen that at the same voltage, the bi-layer PyTPA/TPBI device
This journal is ª The Royal Society of Chemistry 2011
J. Mater. Chem., 2011, 21, 1206–1211 | 1207