Xiaodong Liu

Theoretical Studies of Blue-Emitting Iridium Complexes with Different Ancillary Ligands

The structural and electronic properties of two heteroleptic iridium complexes Ir(dfppy)2(pic) (FIrpic) and Ir(dfppy)2(acac) (FIracac) have been investigated theoretically, where dfppy = 2-(2,4-difluorophenyl) pyridine, pic = picolinic acid, and acac = acetoylacetonate. The geometries of ground and excited states are optimized at PBE0/LANL2DZ and CIS/LANL2DZ levels, respectively. Time-dependent density functional theory (TDDFT) method is employed to explore the absorption and emission properties. In the ground state, the highest-occupied molecular orbital has a significant mixture of metal Ir(d) and dfppy(pi), the lowest-unoccupied orbital locates primarily on pi* of pic for FIrpic and pi* of dfppy for FIracac. The luminescence of each complex originates from the lowest triplet excited state, which is assigned to the mixing of metal-to-ligand charge transfer and intraligand charge transfer characters. The effects of ancillary ligands pic and acac on absorption and emission spectra are observed by analysis of TDDFT results. The connection between the nature of excited states and the behavior of the complexes with different ancillary ligands is elucidated.

The Origin of the Improved Efficiency and Stability of Triphenylamine-Substituted Anthracene Derivatives for OLEDs: A Theoretical Investigation†

Herein, we describe the molecular electronic structure, optical, and charge-transport properties of anthracene derivatives computationally using density functional theory to understand the factors responsible for the improved efficiency and stability of organic light-emitting diodes (OLEDs) with triphenylamine (TPA)-substituted anthracene derivatives. The high performance of OLEDs with TPA-substituted anthracene is revealed to derive from three original features in comparison with aryl-substituted anthracene derivatives: 1) the HOMO and LUMO are localized separately on TPA and anthracene moieties, respectively, which leads to better stability of the OLEDs due to the more stable cation of TPA under a hole majority-carrier environment; 2) the more balanceable hole and electron transport together with the easier hole injection leads to a larger rate of hole-electron recombination, which corresponds to the higher electroluminescence efficiency; and 3) the increasing reorganization energy for both hole and electron transport and the higher HOMO energy level provide a stable potential well for hole trapping, and then trapped holes induce a built-in electric field to prompt the balance of charge-carrier injection.

Bipolar host molecules for efficient blue electrophosphorescence: a quantum chemical design.

On the basis of density functional theory (DFT) calculations, a new series of bipolar host molecules for efficient blue electrophosphorescence devices are designed by linkage of hole-transporting moiety carbazole (CZ) and electron-transporting unit diphenylphosphoryl (ph(2)P horizontal lineO) to the core molecules with high triplet energies. The electronic structures in the ground states, cationic and anionic states, and lowest triplet states of the designed molecules have been studied with emphasis on triplet energies, spin density distributions, ionization potentials, electron affinities, and the influence of molecular topology. Designed bipolar host molecules possess the following features: (1) relatively higher highest occupied molecular orbital (HOMO) for hole injection and, relatively lower lowest unoccupied molecular orbital (LUMO) for electron injection; (2) HOMO and LUMO separation and localization in the respective hole- and electron-transporting moieties; (3) dramatic bond length changes in ionic states occurring at different parts of the bipolar molecules with respect to their neutral states; (4) keeping higher triplet energy. The DFT results provide deep insight into the nature of bipolar molecules and show that the designed molecules are feasible to meet the requirements of the host materials for blue triplet emissions.

Fluorinated Alq3 derivatives with tunable optical properties

This communication reports that not only the emission colour but also the photoluminescence quantum yield of Alq3 can be tuned by introducing fluorine atoms at different positions; with fluorination at C-5 the emission is red-shifted with a tremendously decreased intensity, fluorination at C-6 causes a blue-shift with a significantly increased intensity, and fluorination at C-7 has a minor effect on both the colour and intensity of Alq3s emission.

Spectral investigation for phosphorescent polymer light-emitting devices with doubly doped phosphorescent dyes

The photoluminescence and electroluminescence (EL) spectral characteristics of two phosphorescent dyes, green fac-tris(2-phenylpyridine) iridium(III) [Ir(ppy)3] and red tris-(4,7-diphenyl-1,10-phenanthroline)rhenium dications [Ru(4,7-Ph2-phen)3]2+, doubly doped poly(N-vinylcarbazol) (PVK) films were investigated. The efficient energy transfers from PVK host to both Ir(ppy)3 and [Ru(4,7-Ph2-phen)3]2+ dopants were observed in photoexcitation and EL devices. The intensity ratio (Ri) of red emission versus green emission as function of doping ratio (Rc) of [Ru(4,7-Ph2-phen)3]2+ and Ir(ppy)3 in PVK host is found according to a power-law function with the slopes of 2.28±0.2 for PL which obey the power-law with Ri–Rc2 predicated by Forster energy transfer model. EL spectra show a driving voltage depentent power law of Ri–Rc1.53±0.14 at low voltage and Ri–Rc1.16±0.08 at high voltage, respectively. The shift in the power law of Forster energy transfer is likely due to additional emission mechanisms such as carrier...

Efficient blue light-emission from new soluble, PPV-analogous with meta-linkage biphenyl moieties

Two poly(p-phenylenevinylene) (PPV) analogous, combining with meta-linkage 2,2-dibutyloxy biphenyl and benzene as aromatic units, which have good solubility in organic solvents and emit bright blue fluorescence in solutions and blue green fluorescence in films, have been synthesized by a Wittig-Horner reaction.

Low-temperature annealing to enhance efficiency in organic small-molecule solution-processable OLEDs

Single-layer solution-processable organic light-emitting diodes (OLEDs) based on a green emissive molecule of 4,7-bis(9,9-bis(6-9H-carbazol-9-yl)hexyl)-9H-fluoren-2-yl)benzo[c][1,2,5]thiadiazole (TCBzC) are fabricated. The luminous efficiency reaches 6.9 cd A−1 after annealing at low temperature of 40 °C, which is double that of the un-annealing device, and this performance is among the highest efficiency of single-layer solution-processable OLEDs reported so far. This enhancement may be due to rearrangement of the peripheral alkyl-linked carbazole units in TCBzC after annealing, which helps the hole and electron transport to be more balanced.

Polymer light-emitting devices with phosphorescence and fluorescence dye double-doped polymer as emitting layer

The electroluminescence(EL) properties of a green phosphorescence dye, (dmbpy)Re(CO) 3 Cl, and a red fluorescence dye, DCM, co-doped polyvinylcarbazole(PVK) thin film were studied. The clear and pure red emission from DCM were observed from PL and EL spectra of (dmbpy)Re(CO) 3 Cl(>2.0 wt. %):DCM(> 0.5 wt. %)-doped PVK films. The optimized device ITO/ (dmbpy)Re(CO) 3 Cl (10 wt. %):DCM(1.0 wt.%):PVK/Al shows the highest EL efficiency of 0.42cd / A at current density of 10 mA/cm 2 , which is more than ten times that of the DCM(1.0 wt %): PVK devices.