Blue-emitting Materials Research Articles

An efficient solvent free Amberlite IRA-400 Cl resin mediated multicomponent synthesis and photophysical properties of fluorescent 4H-chromene derivatives

An efficient Amberlite IRA-400 Cl basic anion exchange resin mediated multicomponent reaction of 2-hydroxybenzaldehydes, 1, 3-diketone and nucleophiles under neat condition afforded a number of fluorescent 4H-chromene derivatives in excellent yield. The structure of the synthesized compounds 4k and 4u were confirmed from single crystal XRD studies. 4H-Among the chromene derivatives, compound 4v synthesized from 4-diethylamino substituted 2-hydroxybenzaldehyde with dimedone and 2-hydroxynaphthaquinone has been found best florescent material. Photophysical properties such as solvatochromic effect, absorption, emission, Stocks shift and quantum yield of the selected compounds have been evaluated. The emission properties of the synthesized compounds suggested that these are a class of blue emissive fluorescent materials.

Non-Doped Deep Blue and Doped White Electroluminescence Devices Based on Phenanthroimidazole Derivative.

A novel deep-blue emitter PhImPOTD based on phenathroimidazole was synthesized, which is incorporated by an electron-donating dibenzothiophene unit and electron-withdrawing phenanthroimidazole and diphenylphosphine oxide moieties. Furthermore, the weak π-π stacking and intermolecular aggregation render the photoluminescence quantum yield is as high as 0.34 in the solid state. Non-doped organic light emitting diodes (OLEDs) based on PhImPOTD emitter exhibits a low turn-on voltage of 3.6V, a favorable efficiency of 1.13cd A-1 and a deep blue emission with Commission Internationale de l'Eclairage (CIE) coordinates of (0.15, 0.08). The CIE is very close to the NTSC (National Television Standards Committe) blue standard (CIE: 0.14, 0.08). PhImPOTD is also utilized as blue emitter and the host for a yellow emitter (PO-01) to fabricate white organic light-emitting diodes (WOLEDs). This gives a forward-viewing maximum CE of 4.83cd A-1 and CIE coordinates of (0.32, 0.32) at the luminance of 1000cdm-2. Moreover, the single-carrier devices unambiguously demonstrate that typical bipolar-dominant characteristics of PhImPOTD. This work demonstrates not only that the phenanthroimidazole unit is an excellent building block to construct deep blue emission materials, but also the introduction of a diphenylphosphine oxide deprotonation substituent is an efficient tactic for harvesting deep-blue emitting devices.

Blue AIEgens bearing triphenylethylene peripheral: adjustable intramolecular conjugation and good device performance

The new approaches to construct deep blue aggregation-induced emission (AIE) materials have been explored, which control the conjugation by two different strategies, to make a great step for the commercialization of organic light-emitting diodes. In order to shorten the intramolecular conjugation length, triphenylethylene (tPE) was utilized to construct blue AIEgens as peripheral groups, instead of tetraphenylethylene (TPE), the famous AIE star molecule, to yield three blue AIEgens of 3,4-BtPE-PI, 4,4-BtPE-PI and 4,4-BtPE-PPI. Nondoped electroluminescence devices are fabricated by using these three AIEgens as the emitting material layer, the best performance of 3.8cd/A as the maximum current efficiency achieved at the commission internationale de l’Eclairage coordinates of (0.17, 0.18).

High-efficiency blue fluorescent devices with simple structure

Thermally activated delayed fluorescence (TADF) materials can harvest both singlet and triplet excitons for light emission without the utilization of heavy metal elements such as iridium and platinum and become the third generation organic optoelectronic materials after fluorescent and phosphorescent materials. TADF material bis[4-(9,9-dimethyl-9, 10-dihydroacridine)phenyl]sulfone (DMAC-DPS) shows high photoluminescence quantum yield in solid films, rendering it as a suitable emitter for non-doped organic light emitting diodes. We optimize the hole transport layer, electron transport layer and emission layer thickness for the none-doped DMAC-DPS devices to pursuit high luminance efficiency. We compare the properties of the devices using indium tin oxide (ITO)/MoO3 hole injection contact with either a 40 nm mCP or 30 nm NPB/10 nm mCP hole transport layer and the results indicate (1) the former devices show larger current than the latter devices for there is a large energy barrier of ca. 0.7 eV for hole injection from NPB into mCP in the latter devices; (2) Both of the devices show similar luminance efficiency and the underlying reason is that the small modulation of the dominant hole current slightly affects the device efficiency. In order to investigate the influence of different electron transport layers on the device performance, we have prepared the devices with either 10 nm DPEPO/40 nm TmPyPB, 50 nm TmPyPB or 50 nm SPPO13 electron transport layer and find out (1) the devices with DPEPO electron transport layer show the highest EQE of 16.2% due to the fact that DPEPO with triplet energy of 3.3 eV can confine excitons inside the emisson layer effectively; (2) The devices with SPPO13 possess the highest current density at certain voltage among the devices and the maximum external quantum efficiency and power efficiency for the SPPO13 based devices are 14.3% and 26.8 lm W - 1, which are higher than 13.8% and 22.2 lm W - 1 for the TmPyPB based devices. The possible reason is that highly polarized phosphorus oxygen groups of SPPO13 greatly improve the electron injection and as a result improve the carrier balance in the devices. Moreover, we compare the properties of the devices with a 15 and 30 nm emission layer and find that the devices with a 30 nm emission layer have slightly better luminance efficiency. High efficiency of the DMAC-DPS devices can be attributed to the effective conversion of triplet excited states to singlet excited states via reverse intersystem crossing (RISC) process. TADF sensitized fluorescence devices can achieve high luminance efficiency by using the well-established fluorescent materials with the advantages such as the superior stability, versatility and abundance. We report highly efficient blue TADF sensitized devices using DMAC-DPS as the host material and the traditional fluorescence blue-emitting material 2,5,8,11-tetra-tert-butylperylene (TBPe) as the guest material. The devices with 1.0% TBPe show the maximum external quantum efficiency and power efficiency of 12.7% and 22.9 lm W - 1, which are about 3 times higher than those of the conventional 2-methyl-9,10-bis(naphthalen- 2-yl)anthracene (MADN): TPBe devices. This can be attributed to the effective use of triplet excited states for light emission via RISC process and highly efficient Forster energy transfer process from DMAC-DPS to TBPe. Utilization of a TADF material as host material and sensitizer and a traditional fluorescence material as emitting dopant serves as a novel approach to achieve high-efficiency organic light emitting diodes.

Blue light emitting P-Hydroxy DPQ phosphor for OLEDs

A blue light emitting novel P-Hydroxy DPQ organic phosphor based on pyrazoloquinoline derivative was synthesized by Friedlander condensation reaction of P-Hydroxyacetophenone and 2-aminobenzophenone in the presence of diphenylphosphate and m-cresol at 140°C. Structural, thermal and optical properties of the synthesized organic phosphor were characterised by recording X-ray diffraction (XRD), Fourier Transform Infrared (FT-IR) spectra, Thermo gravimetric analysis/Differential Thermal analysis (TGA/DTA), UV-absorption spectra, Photoluminescence (PL) spectra, respectively. XRD reveals its crystalline nature, while FTIR spectra reveal the formation of the quinoline complex. TGA/DTA analysis reveals significant loss in the weight of sample above room temperature, while the DTA curve of the complex shows two endothermic peaks at 99°C and 355°C and two exothermic peaks at 200°C and 380°C. The UV–vis absorption spectra of P-Hydroxy DPQ in acetic acid solution (10−3 concentration) shows a strong absorption peak λmax at 376nm with a weak shoulder at 281nm, which may be attributed due to n→π* and π→π* transitions of quinoline moieties, respectively. Optical energy band gap Eg of P-Hydroxy DPQ was measured by optical process absorption method and was found to be 2.78eV. When P-Hydroxy DPQ powder is excited at 385nm, it displays emission peak at 468nm, which falls in the blue visible region of electromagnetic radiation with (0.1391, 0.1887) as CIE coordinates. Thus this polymeric compound exhibits bright emission in blue region, indicating its potential as blue emissive material for organic light-emitting diodes (OLEDs).

Polysiloxane-Based Autonomic Self-Healing Elastomers Obtained Through Dynamic Boronic Ester Bonds Prepared by Thiol-Ene "Click" Chemistry.

Macromol. Rapid Commun. 2016, 37, 1052–1059 After publication, the authors realized that proper credit to the work of Sumerlin et al. (Macromolecules, 2015, 48, 2098) has only been given in the Supporting Information. Considering the importance of this research work, the authors wish to add a sentence to the Introduction relating to the work as follows: “The reversible nature of boronic esters has been exploited in the design of self-healing materials, blue-emissive materials, covalent organic frameworks, and sensors.[14] Sumerlin et al. fabricated cross-linked polymers using dynamic-covalent boronic esters by a photo-initiated thiol−ene “click” reaction (J. J. Cash, T. Kubo, A. P. Bapat, B. S. Sumerlin, Macromolecules, 2015, 48, 2098). The resulting bulk materials exhibited highly efficient self-healing properties.” The authors apologize for the inconvenience this may have caused.

White OLED in Hybrid Structure for Enhancing Color Purity.

We synthesized the red emission material, bis(1,4-bis(5-phenyloxazol-2-yl)phenyl) iridium(picolate) [Ir-complexes] and the blue emission material, bis (2-(2-hydroxyphenyl) benzoxazolate)zinc [Zn(HPB)2]. White Organic Light Emitting Diodes were fabricated by using Zn(HPB)2 for a blue emitting layer, Ir-complexes for a red emitting layer and a tris (8-hydroxy quinoline)aluminum [Alq3] for a green emitting layer. The important experimental results obtained, white OLED was fabricated by using double emitting layers of Zn(HPB)2 and Alq3:Ir-complexes, and hole blocking layer of 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline[BCP]. We also varied the thickness of BCP. When the thickness of BCP layer was 5 nm, white emission was achieved. We obtained a maximum luminance of 5400 cd/m2 at a current density of 650 mA/cm2. The CIE coordinates was (0.339, 0.323) at voltage of 10 V.

Hydrothermal synthesis of blue-emitting YPO4:Yb3+ nanophosphor

The blue-emitting YPO4 phosphors doped with Yb3+ were prepared by a simple hydrothermal method. All the products were characterized by XRD and TEM, which revealed that they were zircon structure with leaf-like morphology. According to the analysis of photoluminescence spectra, upon ultraviolet (275 nm) excitation, the Yb3+ doped YPO4 phosphor showed an intense blue emission composed of two main bands at 420 and 620 nm assigned to charge transfer state (CTS) → 2F5/2 and CTS → 2F7/2, respectively. Moreover, the optimum doping concentration of Yb3+ in YPO4 phosphor was 1%, which exhibited the maximum emission intensity. The possible physical mechanism of concentration quenching was discussed, and the critical transfer distance determined to be 23.889 A. In particular, the color purity of the as-synthesized Yb3+ doped YPO4 phosphor was as high as 83%, which made it an excellent candidate for blue-emitting materials.

Highly Efficient White Organic Light-Emitting Diodes with Ultrathin Emissive Layers and a Spacer-Free Structure.

Ultrathin emissive layers (UEMLs) of phosphorescent materials with a layer thickness of less than 0.3 nm were introduced for high-efficiency organic light-emitting diodes (OLEDs). All the UEMLs for white OLEDs can be prepared without the use of interlayers or spacers. Compared with devices fabricated with interlayers inserted in-between the UEMLs, our spacer-free structure not only significantly improves device efficiency, but also simplifies the fabrication process, thus it has a great potential in lowering the cost of OLED panels. In addition, its spacer-free structure decreases the number of interfaces which often introduce unnecessary energy barriers in these devices. In the present work, UEMLs of red, green and blue-emitting phosphorescent materials and yellow and blue phosphorescent emitters are utilized for the demonstration of spacer-free white OLEDs. Upon optimization of the device structure, we demonstrated spacer-free and simple-structured white-emitting OLEDs with a good device performance. The current and power efficiencies of our white-emitting devices are as high as 56.0 cd/A and 55.5 lm/W, respectively. These efficiencies are the highest ever reported for OLEDs fabricated with the UEML approach.

Highly twisted pyrene derivatives for non-doped blue OLEDs

New highly twisted rigid blue light-emitting materials were designed, composed of pyrene with a xylene core unit and either naphthalene or phenyl end units. These blue-emitting materials were synthesized via the Suzuki cross-coupling reaction and their structures were confirmed using FT-IR, 1H NMR, 13C NMR, and mass spectroscopy. The optical, electrochemical and thermal properties of the materials were investigated. The non-coplanar structure introduced by highly twisted xylene units provides steric hindrance, resulting in very deep blue emission. The fabricated devices exhibited a maximum external quantum efficiency (EQE) of 3.69% with CIE color coordinates (x, y: 0.15, 0.06).

Computational insights into the photophysical and electroluminescence properties of homoleptic fac-Ir(C^N)3 complexes employing different phenyl-derivative-featuring phenylimidazole-based ligands for promising phosphors in OLEDs.

The electronic structures and photophysical properties of three homoleptic iridium(iii) complexes IrL3 with C^N ligands, including 2a (L = 1-(2,6-diisopropylphenyl)-2-phenyl-1H-imidazole), 5a (L = 1-(2,6-dimethylphenyl)-2-phenyl-1H-imidazole), and 6a (L = 1-(3,5-diisopropylbiphenyl-4-yl)-2-phenyl-1H-imidazole), are investigated by means of the density functional method. Furthermore, seven new complexes are theoretically designed, including 1a (L = 1,2-diphenyl-1H-imidazole), 3a (L = 1-(2,6-dimethoxyphenyl)-2-phenyl-1H-imidazol), 4a (L = 2-(2-phenyl-1H-imidazol-1-yl)isophthalaldehyde), 1b (L = 2-(biphenyl-3-yl)-1H-imidazole), 2b (L = 2-(2',6'-diisopropylbiphenyl-3-yl)-1H-imidazole), 3b (L = 2-(2',6'-dimethoxybiphenyl-3-yl)-1H-imidazole), and 4b (L = 3'-(1H-imidazol-2-yl)biphenyl-2,6-dicarbaldehyde), to explore the influence of different substituents and different substituted positions on the electronic structures, phosphorescence properties, and organic light-emitting diode (OLED) performance. The HOMO-LUMO energy gap is greatly decreased by introduction of the -CHO group into the phenyl ring (4a and 4b see -sketched structures for all the investigated Ir(iii) complexes). As a result, their absorption and emission spectra present red-shifting leading them to be potential red-emitting phosphors. Other complexes are all blue-emitting materials, indicating that the effect of the substituted position on the emitting color is negligible. However, the addition of the substituent on the para-position of the phenyl ring in the phenylimidazole ligand would increase the quantum yield and electroluminescence (EL) performance compared with that on the imidazole ring.

Efficient blue organic light-emitting diodes using N(2) ,N(2) ,N(11) ,N(11) ,5,6,7,8-octaphenyltriphenylene-2,11-diamine derivatives.

In this study, we have synthesized phenyl-substituted triphenylene derivatives, using the Diels-Alder reaction and the Buchwald-Hartwig reaction. To investigate electroluminescence properties of these materials, multilayer organic light-emitting diode (OLED) devices were fabricated with a structure of indium-tin-oxide (ITO) (180 nm)/4,4'-bis(N-(1-naphthyl)-N-phenylamino)biphenyl (NPB) (50 nm)/blue-emitting materials (1-3) (30 nm)/bathophenanthroline (Bphen) (35 nm)/lithium quinolate (Liq) (2 nm)/Al (100 nm). A device using N(2) ,N(2) ,N(11) ,N(11) ,5,6,7-heptaphenyltriphenylene-2,11-diamine (2) exhibited efficient blue emission with luminous, power, and external quantum efficiencies of 0.92 cd/A, 0.67 lm/W, and 1.17% at 20 mA/cm(2) , respectively. The Commission International de L'Éclairage coordinates of this device were (x = 0.15, y = 0.09) at 6.0 V. Copyright © 2015 John Wiley & Sons, Ltd.

Surface Reconstructions in Organic Crystals: Simulations of the Effect of Temperature and Defectivity on Bulk and (001) Surfaces of 2,2':6',2″-Ternaphthalene.

2,2′:6′,2″-Ternaphthalene (NNN) is a novel, blue-emitting material, suitable for preparation of organic light-emitting diodes. Its crystal structure has been solved recently, but its thermal behavior and surface properties have not yet been explored, partly due to the difficulty in obtaining high quality crystals. In the present study we use classical molecular dynamics to investigate the thermal behavior of bulk and (001) surfaces of NNN. Our bulk simulations indicate the occurrence of a phase transition at about 600 K. The transition is facilitated by the presence of a free (001) surface, since a reconstruction leading to a very similar structure occurs around 550 K in our surface models. This holds for both ideal and defective surface models, containing a small number of vacancies (one or two missing molecules in the outermost layer). In all cases, the process is triggered by thermal motion and involves the reorientation of the molecules with respect to the (001) plane. Both the bulk and surface phases share the monoclinic space group P21/a with a herringbone disposition of molecules. These findings and their implications for the use of NNN in organic electronics are discussed.

Luminescence Properties of a Novel Blue-Emitting Phosphor NaBaBO<sub>3</sub>:Tm<sup>3+</sup>,K<sup>+</sup>

A novel blue-emitting phosphor NaBaBO3doped with Tm3+was prepared using a conventional high temperature solid-state reaction method. Its crystal structure and luminescence properties were studied. Photoluminescence measurements indicate that the phosphor features a satisfactory blue performance due to the1D2→3F4transition of Tm3+with the highest photoluminescence intensity located at 460 nm excited by 359 nm near-ultraviolet (NUV) light. In addition, the concentration of Tm3+was adjusted in order to obtain the optimum emission intensity. When the Tm3+concentration in NaBaBO3is 6.0 mol% the maximum intensity can be obtained. The concentration quenching occurs when Tm3+concentration is beyond 6.0 mol% and the concentration quenching mechanism can be explained by the dipole–dipole interaction. The measured chromaticity coordinate for the NaBaBO3:Tm3+phosphor under 359 nm excitation is determined to be (0.1470, 0.1090). The present work suggests that the NaBaBO3:Tm3+,K+phosphor is a promising blue-emitting material for NUV white light-emitting diodes.

Synthesis and photoluminescence of Eu2+ doped Lu2CaMg2Si3O12 garnet phosphors

A series of Lu2Ca1−xMg2Si3O12:xEu2+ phosphors were synthesized by the solid state method and the garnet structure can be clearly observed from X-ray diffraction patterns. These phosphors can be excited efficiently by near-UV light in the wavelength range of 300–420nm, and emit strong and wide blue light peaking at 450nm. The photoluminescence intensity reaches maxima when Eu2+ concentration (x) is 0.03 and the emission intensity of Lu2CaMg2Si3O12:0.03Eu2+ phosphor is stronger than commonly used BaMgAl10O17:Eu2+ blue phosphor. The thermal quenching of photoluminescence property of Lu2CaMg2Si3O12:0.03Eu2+ phosphor at 25–220°C was also investigated and the quenching temperature (Tq) of this phosphor was measured to be 120°C. All these results indicate that Lu2CaMg2Si3O12:0.03Eu2+ phosphor is a promising blue-emitting luminescence material for near-UV based white light-emitting diodes.

Deep blue phosphorescent organic light-emitting diodes with very high brightness and efficiency.

The combination of both very high brightness and deep blue emission from phosphorescent organic light-emitting diodes (PHOLED) is required for both display and lighting applications, yet so far has not been reported. A source of this difficulty is the absence of electron/exciton blocking layers (EBL) that are compatible with the high triplet energy of the deep blue dopant and the high frontier orbital energies of hosts needed to transport charge. Here, we show that N-heterocyclic carbene (NHC) Ir(III) complexes can serve as both deep blue emitters and efficient hole-conducting EBLs. The NHC EBLs enable very high brightness (>7,800 cd m(-2)) operation, while achieving deep blue emission with colour coordinates of [0.16, 0.09], suitable for most demanding display applications. We find that both the facial and the meridional isomers of the dopant have high efficiencies that arise from the unusual properties of the NHC ligand-that is, the complexes possess a strong metal-ligand bond that destabilizes the non-radiative metal-centred ligand-field states. Our results represent an advance in blue-emitting PHOLED architectures and materials combinations that meet the requirements of many critical illumination applications.

Synthesis and Property of New Blue Emitting Materials with Bulky Side Group

Two emitting compounds, 9,10-bis-[1,1′;3′,1′′]terphenyl-5′-yl-1,5-di-o-tolyl-anthracene [TP-DTA-TP] and 9,10-bis-phenyl[1,1′;3′,1′′]triphenyl-5′-yl-1,5-di-o-tolyl-anthracene [TPB-DTA-TPB] based on new twisted core moiety were synthesized through boration, Suzuki reaction, and Sandmeyer reactions. EL performance was improved by varying the chemical structure of the side group. Physical properties such as optical, electrochemical, and electroluminescent properties were investigated. Synthesized compounds were used as an EML in OLED device: ITO / 2-TNATA (60nm) / NPB (15nm)/ TP-DTA-TP or TPB-DTA-TPB (35 nm) / Alq3 (20nm) / LiF (1nm) / Al (200nm). It was found that TPB-DTA-TPB showed higher luminance efficiency and better C.I.E. value than TP-DTA-TP device.

Shedding light on the photophysical properties of newly designed platinum(II) complexes by adding substituents on functionalized ligands as highly efficient OLED emitters from a theoretical viewpoint

The phosphorescent properties of three synthesized and three new designed platinum(II) complexes are focused on in this work. To reveal their structure–property relationships, a density functional theory/time-dependent density functional theory (DFT/TDDFT) investigation is performed on the geometric and electronic structures, absorption and emission spectra. The electroluminescent (EL) properties are evaluated by the ionization potential (IP), electron affinity (EA), and reorganization energy (λ). Furthermore, the radiative rate constant (kr) is qualitatively elucidated by various factors including the strength of the SOC interaction between the higher-lying singlet excited states (Sn) and the T1 state, the oscillator strength (f) of the Sn states that can couple with the T1 state, and the energy separation between the coupled states. A combined analysis of various elements that could affect the phosphorescent efficiency is beneficial to exploring efficient triplet phosphors in OLEDs. Consequently, complexes Pt-1 and 1 would be more suitable blue-emitting phosphorescent materials with balance of EL properties and acceptable quantum yields.

Synthesis and photophysical properties of 1,2-diphenylindolizine derivatives: fluorescent blue-emitting materials for organic light-emitting device.

New blue-emitting materials based on 1,2-diphenylindolizine were designed and synthesized through a microwave-assisted Suzuki coupling reaction. The photophysical, electrochemical, and thermal properties of the 1,2-diphenylindolizine derivatives were investigated using UV-visible and fluorescence spectroscopy, cyclic voltammetry, thermogravimetric analysis, and differential scanning calorimetry. The 1,2-diphenylindolizine derivatives had band gaps of 3.1-3.4 eV and indicated proper emission of around 450 nm without significant difference between in solution and thin solid film. The indolizine derivatives show an enhanced thermal stability (∆Tm > 100 °C), compared with 1,2-diphenylindolizine. These results suggest the 1,2-diphenylindolizine derivatives are suitable for blue-emitting materials in organic light-emitting devices.

DFT/TDDFT investigation on the electronic structures and photophysical properties of phosphorescent platinum(II) complexes with triarylboron/triarylnitrogen-functionalized N-heterocyclic carbene chelate ligands

Two series of platinum(II) complexes with triarylboron/triarylnitrogen-functionalized N-heterocyclic carbene chelate ligands have been theoretically investigated by using the density functional theory (DFT) and time-dependent DFT (TDDFT) methods. The influence of different substituents (triarylboron-functionalized or triarylnitrogen-functionalized) on the electronic structure and optical properties of Pt(II) complexes was also explored. The results reveal that the different substituents can influence the electron density distributions of frontier molecular orbitals and their energies, resulting in the change of transition character and emission color. The designed complex 1b and 1d are considered to be a potential candidate as deep blue-emitting material.