Synthesis and properties of a solution-processable truxene derivative for OLED devices.

A soluble truxene derivative (TR1) attached with triphenylamine at the peripheral position was designed and synthesized. The structure and purity of TR1 were carefully characterized by 1H NMR, UV/vis and photoluminescent spectroscopy, mass spectroscopy, and thermal analyses. It exhibited good solubility in common organic solvents and good film forming properties. The maximum absorption and emission peaks in THF solution were shown at 358 nm and 415 nm, respectively. Bright blue emission was observed in both solution and solid states under UV excitation. The fluorescent quantum efficiency was 0.46. The best luminous efficiency was found to be 3.65 cd/A with CIE coordinates of (0.163, 0.260) in electroluminescence devices.

Spin-assembled nanolayer of a hyperbranched polymer on the anode in organic light-emitting diodes: the mechanism of hole injection and electron blocking.

We introduced a spin-assembled nanolayer of hyperbranched poly(ether sulfone) with sulfonic acid terminal on top of an indium-tin oxide anode in organic light-emitting diodes. This results in great improvement in luminous efficiency, better than that of devices using a commercially available conducting polymer composition as a hole-injection layer. The effect of the nanolayer was investigated by impedance spectroscopy, photovoltaic measurement for built-in-potential, and transient electroluminescence. We concluded that the high luminous efficiency resulted from the efficient electron-blocking by the nanolayer and hole-injection assisted by the accumulation of electrons at the interface. This result implies that, for an efficient hole-injection layer, the electron-blocking capability should be incorporated in addition to the hole-injection and -transport capability.

Study on a set of bis-cyclometalated Ir(III) complexes with a common ancillary ligand.

Bis-cyclometalated iridium(III) complexes [Ir(F2ppy)2ZN] (FZN), [Ir(F2CNppy)2ZN] (FCZN), [Ir(DMAF2ppy)2ZN] (FDZN) and [Ir(MeOF2ppy)2ZN] (MeOFZN) (F2ppy = 4',6'-difluoro-2-phenylpyridinate, F2CNppy = 5-cyano-4',6'-difluoro-2-phenylpyridinate, DMAF2ppy = 4',6'-difluoro-4-dimethylamino-2-phenylpyridinate, MeOF2ppy = 4',6'-difluoro-4-methyl-2-phenylpyridinate and ZN = 3,5-dimethylpyrazole-N-carboxamide) emitting in the sky blue region were synthesized. We studied the effect of the ancillary ligand ZN and the substituents on the cyclometalating ligands on the crystal structures, photophysical and electrochemical properties and the frontier orbitals. Density functional theory (DFT) calculation results indicate that in FCZN and FDZN the cyclometalating ligands show negligible participation in the HOMO, the ancillary ligand ZN being the main participant along with the Ir(III) d-orbitals. MeOFZN exhibits the maximum photoluminescence quantum efficiency and radiative emission rates along with the dominant low frequency metal-ligand vibrations and maximum reorganization energy in the excited state. All the substituted complexes show more polar characteristics than FZN, FCZN possessing the highest dipole moment among the complexes. The performances of the solution-synthesised organic light emitting devices (OLEDs) of FZN, FCZN and FDZN doped in a blend of mCP (m-bis(N-carbazolylbenzene)) and polystyrene are studied.

Solution-processed high-efficiency organic phosphorescent devices utilizing a blue Ir(III) complex.

We report high-efficiency blue phosphorescence organic light-emitting devices by solution process utilizing a blue Ir(III) complex [(F2ppy)2Ir(ph-imz)CN] (F2ppy = 2',6' -difluoro-2-phenyl pyridine and ph-imz = N-phenyl imidazole) blended with the host mCP (N, N'-dicarbazolyl-3,5-benzene), and the inert polymers polystyrene (PS) and polymethylmethacrylate (PMMA). The effects of the dopant confinement in the PS and PMMA matrix on the device performance are studied by field emission transmission electron microscopy (FE-TEM) and atomic force microscopy (AFM). The complex shows photoluminescence peaked at 458 nm with CIE color coordinates (0.14, 0.21) in solution and (0.14, 0.18) in doped PMMA film. The PS based device showed better device performance than the PMMA based device with a maximum luminous efficiency of (etaL) 5.11 cd/A with CIE color coordinates (0.17, 0.29) (at 10 mA/cm2) and a maximum luminance of 9765 cd/m2.

Substituent effect on the luminescent properties of a series of deep blue emitting mixed ligand Ir(III) complexes.

The syntheses of the bright deep blue emitting mixed ligand Ir(III) complexes comprising two cyclometalating, one phosphine and one cyano, ligands are reported. In this study, a firm connection between the nature of the excited states and the physicochemical behavior of the complexes with different ligand systems is elucidated by correlating the observed crystal structures, spectroscopic properties, and electrochemical properties with the theoretical results obtained by the density functional theory (DFT) methods. The cyclometalating ligands used here are the anions of 2-(4',6'-difluorophenyl)-pyridine (F2ppy), 2-(4',6'-difluorophenyl)-4-methyl pyridine (F2ppyM), and 4-amino-2-(4',6'-difluorophenyl)-pyridine (DMAF2ppy). The phosphine ligands are PhP(O-(CH2CH2O)3-CH3)2 and Ph2P(O-(CH2CH2O)n-CH3), where Ph = phenyl and n = 1 (P1), 3 (P3), or 8 (P350). The thermal stabilities of the complexes were enhanced upon increasing the "n" value. The crystal structures of the complexes, [(DMAF2ppy)2Ir(P1)CN], (P1)DMA, and [(F2ppyM)2Ir(P3)CN], (P3)F2M, show the cyano and phosphine groups being in a cis configuration to each other and in a trans configuration to the coordinating Cring atoms. The long Ir-Cring bond lengths are ascribed to the trans effect of the strong phosphine and cyano ligands. DFT calculations indicate that the highest occupied molecular orbital (HOMO) is mainly contributed from the d-orbitals of the iridium atom and the pi-orbitals of cyclometalating and cyano ligands, whereas the lowest unoccupied molecular orbital (LUMO) spreads over only one of the cyclometalating ligands, with no contribution from phosphine ligands to both frontier orbitals. Dimethylamino substitution increases the energy of the emitting state that has more metal-to-ligand-charge-transfer (MLCT) character evidenced by the smaller vibronic progressions, smaller difference in the 1MLCT and 3MLCT absorption wavelengths, and higher extinction coefficients (epsilon) than the F2ppy and F2ppyM complexes. However, the increase in the basicity of the dimethylamino group in the DMAF2ppy complexes in the excited states leads to distortions and consequent nonradiative depopulation of the excited states, decreasing their lower photoluminescence (PL) efficiency. The effect of the substituents in the phosphine ligand is more pronounced in the electroluminescence (EL) than in the PL properties. Multilayer organic light emitting devices (OLEDs) are fabricated by doping the Ir(III) complexes in a blend of mCP (m-bis(N-carbazolyl benzene)) and polystyrene, and their device characteristics are studied. The (P3)F2M complex shows a maximum external quantum efficiency (etaex) of 2%, a maximum luminance efficiency (etaL) of 4.13 cd/A at 0.04 mA/cm2, and a maximum brightness of 7200 cd/m2 with a shift of the Commission Internationale de L'Eclairage (CIE) coordinates from (0.14, 0.15) in film PL to (0.19, 0.34) in EL.