A highly efficient blue-light-emitting copolymer with bulky hole-transporting triphenylamine (TPA) and electron-transporting oxadiazole (OXD) pendant groups at the C-9 position of fluorene was synthesized. The results from photoluminescence and electrochemical measurements reveal that both the side chains and the polyfluorene main chain retain their own electronic characteristics in the copolymer. It shows a pure blue emission with no aggregates or excimers formed even after being annealed at 150 °C under nitrogen for 20 h. In addition, it also demonstrates improved charge injection and balanced charge transport in electroluminescence. The maximum external quantum efficiency of a single-layer device using this copolymer as the emitting layer is 1.21% (at a brightness of 354 cd/m 2 with driving voltage of 7.6 V). The maximum luminance of the device reaches 4080 cd/m 2 at a bias of 12.0 V and a current density of 640 mA/cm 2 .
TFTPA (tris[4‐(9‐phenylfluoren‐9‐yl)phenyl]amine), a novel host material that contains a triphenylamine core and three 9‐phenyl‐9‐fluorenyl peripheries, was effectively synthesized through a Friedel‐Crafts‐type substitution reaction. Owing to the presence of its sterically bulky 9‐phenyl‐9‐fluorenyl groups, TFTPA exhibits a high glass transition temperature (186 °C) and is morphologically and electrochemically stable. In addition, as demonstrated from atomic force microscopy measurements, the aggregation of the triplet iridium dopant is significantly diminished in the TFTPA host, resulting in a highly efficient full‐color phosphorescence. The performance of TFTPA‐based devices is far superior to those of the corresponding mCP‐ or CBP‐based devices, particularly in blue‐ and red‐emitting electrophosphorescent device systems. The efficiency of the FIrpic‐based blue‐emitting device reached 12 % (26 cd A–1) and 18 lm W–1 at a practical brightness of 100 cd m–2; the Ir(piq)2acac‐based red‐emitting device exhibited an extremely low turn‐on voltage (2.6 V) and a threefold enhancement in device efficiency (9.0 lm W–1) relative to those of reference devices based on the CBP host material.
Three blue‐light emitting dipyrenylbenzene derivatives, 1‐(4‐(1‐pyrenyl)phenyl)pyrene (PPP), 1‐(2,5‐dimethoxy‐4‐(1‐pyrenyl)phenyl)pyrene (DOPPP), and 1‐(2,5‐dimethyl‐4‐(1‐pyrenyl)phenyl)pyrene (DMPPP), have been prepared by the Suzuki coupling reaction of aryl dibromides with pyreneboronic acid in high yields. These compounds exhibit high glass‐transition temperatures of 97–137 °C and good film‐forming ability. As revealed from single‐crystal X‐ray analysis, these dipyrenylbenzenes adopt a twisted conformation with inter‐ring torsion angles of 44.5°–63.2° in the solid state. The twisted structure is responsible for the low degree of aggregation in the thin films that leads to fluorescence emission of the neat films at 446–463 nm, which is shorter than that of the typical pyrene excimer emission. The low degree of aggregation is also conducive for the observed high fluorescence quantum yields of 63–75%. In organic light‐emitting diode (OLED) applications, these dipyrenylbenzenes can be used as either the charge transporter or host emitter. The non‐doped blue OLEDs that employ these compounds as the emissive layer can achieve a very high external quantum efficiency (ηext) of 4.3–5.2%. In particular, the most efficient DMPPP‐based device can reach a maximum ηext of 5.2% and a very high luminescence of 40 400 cd m–2 in the deep‐blue region with Commission Internationale d'Énclairage (CIE) coordinates of (0.15, 0.11).
A novel oxdiazole-containing polyfluorene copolymer (PF−OXD) was synthesized by
attaching two electron-deficient, 4-tert-butylphenyl-1,3,4-oxadiazole groups onto the C-9
position of the alternating fluorene unit to form a 3-D cardo-structure. This polymer possesses
a very high glass transition temperature (T
g: 213 °C) and very good thermal stability due
to its rigid cardo-linkage. The results from the photoluminescence measurements of the
isothermally heated PF−OXD thin film (150 °C for 20 h) show that the commonly observed
aggregate/excimer formation in polyfluorenes is very effectively suppressed in this polymer
due to its 3-D structure and high T
g. A double-layer LED device using this polymer as the
emitting layer shows a bright blue emission with a low turn-on voltage at 5.3 V and a high
brightness of 2770 cd/m2 at a drive voltage of 10.8 V. The maximum external quantum
efficiency is 0.52% at 537 cd/m2 with a bias of 7.4 V. The improved device performance over
that of poly(9,9-dioctylfluorene) (POF) may be due to better electron injection and transport
in PF−OXD and the efficient energy transfer from the OXD side chain to the polyfluorene
main chains.
The nonionic, red-emitting complexes [Os(fppz)2L2] (L = PPh2Me (1), PPhMe2 (2)) and [Os(bptz)2L2] (L = PPh2Me (3)) were synthesized, showing
highly intense red phosphorescent emission in CH2Cl2
solution at λmax 617, 632, and 649 nm, respectively. The
electroluminescent properties of these compounds on
OLEDs showed promising device efficiencies required for
future OLED applications.
Rational design and syntheses of four iridium complexes (1-4) bearing two substituted quinoxalines and an additional 5-(2-pyridyl) pyrazolate or triazolate as the third coordinating ligand are reported. Single-crystal X-ray diffraction studies of 1 reveal a distorted octahedral geometry, in which two dpqx ligands adopt an eclipse configuration, for which the quinoxaline N atoms and the C atoms of orthometalated phenyl groups are located at the mutual trans- and cis-positions, respectively. The lowest absorption band for all complexes consists of a mixture of heavy-atom Ir(III)-enhanced 3MLCT and 3pipi* transitions, and the phosphorescent peak wavelength can be fine-tuned to cover the spectral range of 622-649 nm with high quantum efficiencies. The cyclic voltammetry was measured, showing a reversible, metal-centered oxidation with potentials at 0.76-1.03 V, as well as two reversible reduction waves with potentials ranging from -1.61 to -2.06 V, attributed to the sequential addition of two electrons to the more electron-accepting heterocyclic portion of two distinctive cyclometalated C/N ligands. Complex 1 was used as the representative example to fabricate the red-emitting PLEDs by blending it into a PVK-PBD polymer mixture. The devices exhibited the characteristic emission profile of 1 with peak maxima located at 640 nm. The maximum external quantum efficiency was 3.15% ph/el with a brightness of 1751 cd/m2 at a current density of 67.4 mA/cm2, and the maximum brightness of 7750 cd/m2 was achieved at the applied voltage of 21 V and with CIE coordinates of (0.64, 0.31).
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