Blue Phosphorescence Research Articles

Orthogonal Molecular Structure for Better Host Material in Blue Phosphorescence and Larger OLED White Lighting Panel

High‐efficiency blue phosphorescence devices with external quantum efficiencies above 25% are developed using a new bipolar host material, diphenyl(10‐phenyl‐10H‐spiro[acridine‐9,9′‐fluoren]‐2′‐yl)phosphine oxide (POSTF), which is constructed in orthogonal molecular structure with a spiro‐coree. The separation of bipolarity from effective spiro‐fluorene‐triphenylamine (STF) structure is elucidated and its versatility in device is evaluated by two kinds of sky‐blue phosphors. Noticeably, large‐size white light‐emitting panel (150 mm × 150 mm) is fabricated with max power efficiency of 75.9 l m W−1 using this new host.

Luminescent properties of blue long-lasting phosphorescence phosphors Sr6Al18Si2O37:Eu2+,RE3+

A series of novel blue long-lasting phosphorescence phosphors Sr6Al18Si2O37:Eu2+,RE3+ (RE3+=Ho3+, Gd3+, Dy3+ and Pr3+) were prepared by the conventional high-temperature solid-state reaction in a reductive atmosphere. Their properties were systematically investigated utilizing X-ray diffraction (XRD), photoluminescence, phosphorescence and thermoluminescence (TL) spectra. The phosphors emitted blue light that was related to the emission of Eu2+ due to 5d-4f transition. Bright blue long-lasting phosphorescence (LLP) could be observed after the excitation source was switched off. For the optimized sample, the blue long-lasting phosphorescence could last for nearly 4 h in the light perception of the dark-adapted human eye (0.32 mcd/m2). The effects of RE3+ ions on phosphorescence properties of the phosphors were studied, and the results showed that the co-doping of RE3+ ions greatly enhanced the intensity of the peak around 315 K which was related to the long lasting phosphorescence of the phosphors at room temperature and consequently improved the performance of the blue phosphorescence such as intensity and persistent time.

Effect of interfacial mixed layer in blue phosphorescent organic light-emitting diodes

Blue phosphorescence organic light-emitting diodes (PHOLEDs) with interfacial mixed layer (IML) were fabricated to observe their electrical characteristics. The IML was composed of mCP as host material in the emissive layer (EML) and 3TPYMB as material for the electron transport layer (ETL) with the blue phosphorescent dopant FIrpic. PHOLEDs with IML were improved by 41% of current density and 35% of luminous efficiency compared to PHOLEDs without IML. The electron and energy transition mechanisms between EML and ETL in the presence of IML were confirmed through I–V–L characteristics and blue color coordinates.

A rational design of carbazole-based host materials with suitable spacer group towards highly-efficient blue phosphorescence

Two novel carbazole derivatives, 3,3′-(2,6-dimethyl-1,4-phenylene)bis(9-phenyl-9H-carbazole) (DMCz) and 3,3′-(2,3,5,6-tetramethyl-1,4-phenylene)bis(9-phenyl-9H-carbazole) (TMCz), are designed and synthesized as host materials for blue phosphorescent organic light emitting diodes (PHOLEDs). Both the materials possess sufficiently high glass transition temperatures and triplet energies for the blue dopant Ir-complex iridium(III) bis(4,6-(difluorophenyl)-pyridinato-N,C′)picolinate (FIrpic, 2.65 eV). The device using DMCz as the host material showed a better performance than that of TMCz with a maximum power efficiency of 36.0 ± 0.7 lm per W, a maximum current efficiency of 43.7 ± 0.6 cd per A and a maximum external quantum efficiency (EQE) of 18.5 ± 0.1%.

Theoretical study of new blue iridium complexes comprising a bipyridine derivative and various ancillary ligands.

New blue emitting ligand Iridium(III) complexes with two phosphines trans to each other and two ancillary ligands, such as Ir(dfpypy)(PPh3)2(H)(CI) and Ir(dfpypy)(PPh3)2(H)(CN), [dfpypy = 2,6-difluoro-3-(pyridin-2-yl)pyridine] were designed and studied to tune the phosphorescence wavelength to the deep blue region and to enhance the luminescence efficiencies. To gain insight into the factors responsible for the emission color change and the different luminescence efficiency, we performed the DFT and TD-DFT calculations on the ground and excited states of these phosphors. (1) The fluorine-substituted dfpypy ligand lower the HOMO energy levels because a N of the pyridyl ligand is more electronegative than a C of the nonsubstituted phenyl ligand and also (2) mono-cyclometalated Iridium(III) complexes using two phosphines trans to each other increased HOMO-LUMO gap by strong field effects of ancillary ligands. From these results, we discuss how the dfpypy ligand and the ancillary ligand influences the emission peak as well as the metal to ligand charge transfer (MLCT) transition efficiency. As the maximum emission spectra of FIrpic known as blue phosphorescence material is about 475 nm. The resulting Iridium(III) complexes, Ir(dfpypy)(PPh3)2(H)(CN), would appear pure blue region about 415 nm with more intensified efficiency.

P‐156: Blue Phosphorescence OLED with Interfacial Mixed Layer between EML and ETL

AbstractBlue PHOLEDs with interfacial mixed layer (IML) were fabricated to observe their electrical characteristics. IML was composed of mCP as host material in EML and 3TPYMB as electron transport material with phosphorescent blue dopant of FIrpic. PHOLED with IML were improved 41% of current density and 35% of luminous efficiency comparing to PHOLED without IML.

Long lasting blue phosphorescence and photostimulated luminescence in 12CaO⋅7Al2O3:Eu thin films grown by pulsed laser deposition

Long lasting blue emitting Eu-doped 12CaO⋅7Al2O3 thin films have been prepared on quartz substrates by pulsed laser deposition combined with post-annealing. The 12CaO⋅7Al2O3 phase is formed after annealing the films above 900°C in vacuum ambient. Upon 323nm excitation, the thin films exhibit a blue long lasting emission peaked at 444nm, assigned to the 5d–4f transitions of Eu2+. The films annealed at 950°C have stronger luminescence and longer lasting time than that annealed at 900°C. The phosphorescence intensity of vacuum annealed film is better than that annealed in H2/N2 atmosphere, which might be due to the increase of shallow trap numbers during the vacuum annealing process. Thermoluminescence curves demonstrate that vacuum and H2/N2 atmosphere annealing could introduce a large number of both shallow and deep traps into 12CaO⋅7Al2O3:Eu thin films. After UV irradiation, excellent photostimulated luminescence can be observed under 980nm excitation. Our results suggest that 12CaO⋅7Al2O3:Eu phosphor may serve as a promising blue luminescent materials used in fabrication of optical storage and illumination in dark environments.

Synthesis and Characterization of Heteroatom-Bridged Bis-spirobifluorenes for the Application of Organic Light-Emitting Diodes

Pure 2-iodo-9,9'-spirobifluorene was synthesized by an efficient method without troublesome iodination of 9,9-spirobifluorene (SP) or the Sandmeyer reaction of 2-amino-9,9'-spirobifluorene. A series of main group element-bridged bis-9,9'-spirobifluorene derivatives were synthesized via coupling reactions of 2-iodo-9,9'-spirobifluorene and main group element-containing precursors. These heteroatom-bridged bis-spirobifluorenes show large triplet state energy gaps, high glass transition temperatures, and varied charge-transporting properties advantageous to the host materials for blue phosphorescence organic light-emitting diodes.

Phosphorescence, near-infrared absorption and nonlinear optical property of a new chiral organic crystal

A new enantiomerically pure compound was synthesized by the single step reduced reaction from 2-(imidazo[1,2-a]pyridin-2-yl)-2-oxo- N -(pyridin-2-yl)acetamide via chiral induction with D-tartaric acid in good yield. Single crystal data confirm this compound crystallizes in chiral space group P21. Transmission spectrum reveals that the crystal has low UV cut-off of 372 nm and has a good transmittance in the entire visible and near-infrared (NIR)region to 1100 nm, indicating its optical application. Kurtz powder test shows a good second harmonic generation (SHG) which also demonstrates its chiral structure. Moreover, this material exhibits blue phosphorescence with quantum yield of 3.6% and unusually NIR absorption between 1500 nm and 2500 nm. Therefore, this new chiral crystal is a promising multifunctional material for the blue phosphorescence, NIR absorption and nonlinear optical (NLO) applications.

Asymmetric Design of Bipolar Host Materials with Novel 1,2,4-Oxadiazole Unit in Blue Phosphorescent Device

The intrinsic asymmetry of 1,2,4-oxadiazole was utilized to synthesize three isomers, DCzmOXD-1, DCzmOXD-2, and mCzmOXD, and high triplet energies over 2.80 eV made them good candidates for host materials in blue OLEDs. The best efficiencies of 23.0 cd A(-1)/20.5 lm W(1-)/11.2% in CE/PE/EQE were achieved by DCzmOXD-1 with derivation at the β-carbon position of 1,2,4-oxadiazole.

The phosphorescent and magnetic properties of a novel radical and its salt derived from 2,3′-biimidazo[1,2-a]pyridin-2′-one radical

A novel neutral radical was synthesized from 2,3′-biimidazo[1,2-a]pyridin-2′-one radical via Cu(I) catalyzed ring-opening and carbon–carbon coupling reactions. The neutral radical (green-yellow phosphorescence) and its sulfate salt (blue phosphorescence) display obvious different phosphorescent colors, attributable to their different electronic structures, and stacking structures as demonstrated by powder X-ray diffraction patterns. The neutral radical itself exhibits weak antiferromagnetic property. The sulfate salt radical mainly presents diamagnetism between 58K and 380K owing to its dimeric structure, below 58K, it reveals complex magnetic behavior.

Impact of linking arylene units off-axis on the photophysical properties of polyfluorenes

The design of new organic molecules with the first triplet (T1) excitation energy high enough for use as host materials for blue phosphorescence is challenging because the energy transfer from host to guest requires the host to have a T1 energy greater than the already high T1 energy of the guest phosphor. By using density functional theory methods on polyfluorenes as an example, we show that moving the inter-ring linkage on arylene units away from the para position can raise the T1 energy by as much as 0.36eV While the ortho linkage raises the T1 energy by a greater extent than the meta, it is the latter that opens the transport gap wider. With either linkage position, electron density at the transport levels remain fully delocalized over the joined units, a necessary condition for efficient charge transport.

Unichromophoric Platinum-Acetylides That Contain Pentiptycene Scaffolds: Torsion-Induced Dual Emission and Steric Shielding of Dynamic Quenching

The effect of the rigid bulky pentiptycene scaffolds on the photoluminescence, redox properties, and oxygen sensing behavior of unichromophoric Pt-acetylides is reported. When the pentiptycene groups are near the Pt(PBu3)2 center, the Pt-acetylides display both blue fluorescence and green phosphorescence with long phosphorescence lifetimes (90-202 μs) in THF. Their phosphorescence intensity is highly sensitive to molecular oxygen, and the emission color depends on the concentration of not only oxygen but also the complexes, which allows a feasible determination of oxygen in the range of 1-5% air volume. The dynamic quenching rate constants decrease linearly with increasing the number of pentiptycene groups, revealing the steric shielding effect of the peripheral rings of pentiptycene. A dependence of oxidation potential on the number of pentiptycene groups also revealed the steric shielding effect on the electron transfer between the complexes and the electrode. In a PMMA matrix, the dual emissive properties are diminished due to increased phosphorescence and decreased fluorescence intensity, and the phosphorescence lifetimes are significantly increased (up to ∼700 μs), leading to an "on-off" optical response to oxygen concentration. Both the dual emissive properties and long-lived triplet excitons are attributed to diminished spin-orbit couplings caused by twisting and steric shielding of the π-conjugated backbone around the Pt center.

一种基于二-［2′,6′-二氟-2,3′-联吡啶-N,C4′］［3-乙酰基-1,7,7-三甲基-双环［2.2.1］-2-庚酮-O,O］铱(Ⅲ)的非掺杂蓝色磷光发光器件

以2’,6’-二氟-2,3’-联吡啶(Hdfpypy)为主配体,空间位阻的3-乙酰基樟脑(Hacam)为辅助配体,合成了二-[2’,6’-二氟-2,3’-联吡啶-N,C4’][3-乙酰基-1,7,7-三甲基-双环[2.2.1]2-庚酮-O,O]铱(Ⅲ)((dfpypy)2Ir(acam))。在四氢呋喃(THF)溶液中,配合物光致发光(PL)光谱最大发射峰值为466 nm,在487nm左右有一个不明显的肩峰,半峰宽为55 nm。配合物在脱气THF溶液中的PL量子效率为0.51。以(dfpypy)2Ir(acam)为发光层,制备了器件结构为ITO/HATCN(1 nm)/TAPC(40 nm)/(dfpypy)2Ir(acam)(10 nm)/BmpypB(40 nm)/LiF(1 nm)/Al(90 nm)的蓝色非掺杂磷光发光器件。电致发光(EL)光谱的最大发射峰值为474 nm。器件的启动电压为3.5 V。在电流密度为20 mA·cm-2时,CIE色坐标值为(0.17,0.29)。在驱动电压为11 V时,器件最大亮度为2 170 cd·m-2。在驱动电压为4.2 V时,最大功率效率为5.25 lm·W-1,最大亮度效率为6.45 cd·A-1。

Enhancing the blue phosphorescence of iridium complexes with a dicyclometalated phosphite ligand via aza-substitution: a density functional theory investigation

The influence of azasubstitution on electronic and photophysical properties of iridium complexes of blue phosphorescent dicyclometalated phosphite has been explored using quantum chemical methods.

Tunable long lasting phosphorescence due to the selective energy transfer from defects to luminescent centres via tunnelling in Mn(2+) and Tm(3+) co-doped zinc pyrophosphate.

A series of new phosphors Zn2(0.97-x)P2O7:0.06Tm(3+),2xMn(2+) (0 ≤ x ≤ 0.05) were synthesized and their luminescence properties were investigated. The results showed that the defects in all the phosphors were related to Tm(3+), and Mn(2+) merely served as the emission centres. Tm(3+) also acted as an emission centre and yielded blue phosphorescence corresponding to its characteristic f-f emissions in the phosphors where the Mn(2+) concentration was low (x ≤ 0.001), while in the phosphors with high concentrations of Mn(2+) it mainly served as a defect by forming Tm. The electrons thermally released from defects selectively transferred to Mn(2+) centres mainly through thermally-assisted tunnelling and this resulted in their red to near-infrared phosphorescence. By adjusting the ratio of Mn(2+) to Tm(3+) to control the spectral distribution, tunable long lasting phosphorescence from blue to near-infrared was achieved.

Theoretical Studies on Phosphorescent Materials: The Conjugation-Extended PtII Complexes

A theoretical study on the PtII complex A based on a dimesitylboron (BMes2)-functionalized [Pt(C^N)(acac)] (C^N = 2-phenyl-pyridyl, acac = acetylaceton) complex, as well as three conjugation-extended analogues of the methylimidazole (C*) ligand BMes2-[Pt(C^C*)(acac)] complexes B–D is performed. Their theoretical geometries, electronic structures, emission properties, and the radiative decay rate constants (kr) were also investigated. The energy differences between the two highest occupied orbitals with dominant Pt d-orbital components (Δddocc) of D both at the ground and excited states are the smallest of all. Compared with B, the charge transfer in D possesses a marked trend towards the extended conjugated group, while C changed inconspicuously. The lowest-lying absorptions and the phosphorescence of them can be described as a mixed metal-to-ligand charge transfer (MLCT)/intra-ligand π→π* charge transfer (ILCT) and 3MLCT/3ILCT, respectively. The variation of charge transfer properties induced by extended conjugation and the radiative decay rate constants (kr) calculated revealed that D is a more efficient blue phosphorescence material with a 497 nm emission transition.

Comparison of light out-coupling enhancements in single-layer blue-phosphorescent organic light emitting diodes using small-molecule or polymer hosts

Single-layer blue phosphorescence organic light emitting diodes (OLEDs) with either small-molecule or polymer hosts are fabricated using solution process and the performances of devices with different hosts are investigated. The small-molecule device exhibits luminous efficiency of 14.7 cd/A and maximum power efficiency of 8.39 lm/W, which is the highest among blue phosphorescence OLEDs with single-layer solution process and small molecular hosts. Using the same solution process for all devices, comparison of light out-coupling enhancement, with brightness enhancement film (BEF), between small-molecule and polymer based OLEDs is realized. Due to different dipole orientation and anisotropic refractive index, polymer-based OLEDs would trap less light than small molecule-based OLEDs internally, about 37% better based simulation results. In spite of better electrical and spectroscopic characteristics, including ambipolar characteristics, higher carrier mobility, higher photoluminescence quantum yield, and larger triplet state energy, the overall light out-coupling efficiency of small molecule-based devices is worse than that of polymer-based devices without BEF. However, with BEF for light out-coupling enhancement, the improved ratio in luminous flux and luminous efficiency for small molecule based device is 1.64 and 1.57, respectively, which are significantly better than those of PVK (poly-9-vinylcarbazole) devices. In addition to the theoretical optical simulation, the experimental data also confirm the origins of differential light-outcoupling enhancement. The maximum luminous efficiency and power efficiency are enhanced from 14.7 cd/A and 8.39 lm/W to 23 cd/A and 13.2 lm/W, respectively, with laminated BEF, which are both the highest so far for single-layer solution-process blue phosphorescence OLEDs with small molecule hosts.

Photo- and Vapor-Controlled Luminescence of Rhombic Dicopper(I) Complexes Containing Dimethyl Sulfoxide

Halide-bridged rhombic dicopper(I) complexes, [Cu2(μ-X)2(DMSO)2(PPh3)2] (X = I(-), Br(-); DMSO = dimethyl sulfoxide; PPh3 = triphenylphosphine), were synthesized, the iodide complex of which exhibited interesting photochromic luminescence driven by photoirradiation and by exposure to DMSO vapor in the solid state. Single-crystal X-ray diffraction measurements revealed that the iodo and bromo complexes (abbreviated Cu2I2-[O,O] and Cu2Br2-[O,O]) were isomorphous, and that the two DMSO ligands were coordinated to the Cu(I) ion via the O atom in both complexes. Both complexes exhibited bright blue phosphorescence at room temperature (λ(em) = 435 nm, Φ(em) = 0.19 and 0.14 for Cu2I2-[O,O] and Cu2Br2-[O,O], respectively) with a relatively long emission lifetime (τ(em) ~ 200 μs at 77 K) derived from the mixed halide-to-ligand and metal-to-ligand charge transfer ((3)XLCT and (3)MLCT) excited state. Under UV irradiation, the blue phosphorescence of Cu2Br2-[O,O] disappeared uneventfully and no new emission band appeared, whereas the blue phosphorescence of Cu2I2-[O,O] rapidly disappeared with simultaneous appearance of a new green emission band (λ(em) = 500 nm). On further irradiation, the green emission of the iodide complex gradually changed to bright yellowish-green (λ(em) = 540 nm); however, this change could be completely suppressed by lowering the temperature to 263 K or in the presence of saturated DMSO vapor. The initial blue phosphorescence of Cu2I2-[O,O] was recovered by exposure to DMSO vapor at 90 °C for a few hours. IR spectroscopy and theoretical calculations suggest that the DMSO ligand underwent linkage isomerization from O-coordination to S-coordination, and both the occurrence of linkage isomerization and the removal of DMSO result in contraction of the rhombic Cu2(μ-I)2 core to make the Cu···Cu interaction more effective. In the contracted core, the triplet cluster-centered ((3)CC) emissive state is easily generated by thermal excitation of the (3)XLCT and (3)MLCT mixed transition state, resulting in the green to yellowish-green emission. In contrast, the Cu···Cu distance in Cu2Br2-[O,O] is considerably longer than that of Cu2I2-[O,O], which destabilizes the (3)CC emissive state, resulting in the nonemissive character.

Reversibly photoswitchable dual-color (blue ↔ green) phosphorescence from β-cyclodextrin inclusion complex materials

We report herein two new supramolecular materials based on β-cyclodextrin inclusion complex samples with partially included organic radicals, which emit blue phosphorescence (excitation at 365 nm), and exhibit higher phosphorescent quantum yields (9.5 and 20.3%). However, purely organic radicals emit green phosphorescence, and possess low quantum yields (1.8 and 3.3%). One inclusion complex material exhibits the reversible dual-color (blue ↔ green) switching behavior in the response to ultraviolet and visible light stimuli, respectively: upon excitation with ultraviolet light (365 nm), it emits blue phosphorescence with quantum yield of 9.5%; however, it emits green phosphorescence and possesses higher quantum yield of 29.0%, upon irradiation by visible light (425 nm). The reversibly photoswitchable behavior is probably ascribed to the light-driven guest molecular conformation conversions in the β-cyclodextrin cavities. This light-responsive dual-color phosphorescent supramolecule is unique in terms of molecular conformation conversions.