White‐Light Emission Strategy of a Single Organic Compound with Aggregation‐Induced Emission and Delayed Fluorescence Properties

A novel white‐light‐emitting organic molecule, which consists of carbazolyl‐ and phenothiazinyl‐substituted benzophenone (OPC) and exhibits aggregation‐induced emission‐delayed fluorescence (AIE‐DF) and mechanofluorochromic properties was synthesized. The CIE color coordinates of OPC were directly measured with a non‐doped powder, which presented white‐emission coordinates (0.33, 0.33) at 244 K to 252 K and (0.35, 0.35) at 298 K. The asymmetric donor–acceptor–donor′ (D‐A‐D′) type of OPC exhibits an accurate inherited relationship from dicarbazolyl‐substituted benzophenone (O2C, D‐A‐D) and diphenothiazinyl‐substituted benzophenone (O2P, D′‐A‐D′). By purposefully selecting the two parent molecules, that is, O2C (blue) and O2P (yellow), the white‐light emission of OPC can be achieved in a single molecule. This finding provides a feasible molecular strategy to design new AIE‐DF white‐light‐emitting organic molecules.     

White‐Light Electroluminescence with Tetraphenylethylene as Emitting Layer of Aggregation‐Induced Emissions Enhancement

Tetraphenylethylene (TPE) based molecules with easy synthesis, good thermal stability, and especially their aggregation‐induced emissions enhancement (AIEE) effect recently become attractive organic emitting materials due to their potentially practical application in OLEDs. Herein, the AIEE behaviors of tetraphenylethylene dyes (TMTPE and TBTPE) were investigated. Fabricated luminesent device using TMTPE dye as emitting layer displays two strong emitting bands: the blue emission coming from the first‐step aggregation and the yellow emission attributed to the second‐step aggregation. Thus, it can be utilized to fabricate the white‐light OLEDs (WOLEDs) of the single‐emitting‐component. A three‐layer device with the brightness of 1200 cd·m^?2 and current ef?ciency of 0.78 cd·A^?1 emits the close to white light with the CIE coordinates of x=0.333 andy=0.358, when applied voltage from 8–13 V, verifying that the TPE‐based dyes of AIEE effect can be effectively applied in single‐emitting‐component WOLEDs fabrication.     

Optical Sensing of Current Dynamics in Organic Light‐Emitting Devices at the Nanometer Scale

Photoluminescence quenching of single dibenzoterrylene (DBT) dye molecules in a polymeric organic light‐emitting diode was utilized to analyze the current dynamics at nanometer resolution. The quenching mechanism of single DBT molecules results from an increase in the triplet‐state population induced by charge carrier recombination on individual guest molecules. As a consequence of the long triplet‐state relaxation time, its population results in a reduced photoluminescence of the dispersed fluorescent dyes. From the decrease in photoluminescence together with photon correlation measurements, we could quantify the local current density and its time‐dependent evolution in the vicinity of the single‐molecule probe. This optical technique establishes a non‐invasive approach to map the time‐resolved current density in organic light‐emitting diodes on the nanometer scale.     

Recent progress in the development of polymers for white light-emitting polymer devices

Much effort has been devoted to the design and synthesis of polymers for use in flat panel display, solid state lighting, transistors, and photovoltaic devices. Especially, development of white light emitting polymeric materials has recently attracted much interest owing to their possible use in lighting application and backlights for flat panel displays. White emission has been obtained from polymeric molecules, small organic molecules, organometallic molecules, and phosphor-based or quantum dot-based inorganic molecules. Among materials used in white light emitting diodes, we summarize the white light emitting polymeric materials synthesized and published till December 2007.     

White electroluminescence from a single polymer: A blue‐emitting polyfluorene incorporating orange‐emitting benzoselenadiazole segments on its main chain

We have developed efficient white‐light‐emitting polymers through the incorporation of low‐bandgap orange‐light‐emitting benzoselenadiazole ( BSeD ) moieties into the backbone of a blue‐light‐emitting bipolar polyfluorene (PF) copolymer, which contains hole‐transporting triphenylamine and electron‐transporting oxadiazole pendent groups. By carefully controlling the concentrations of the low‐energy‐emitting species in the resulting copolymers, partial energy transfer from the blue‐fluorescent PF backbone to the orange‐fluorescent segments led to a single polymer emitting white light and exhibiting two balanced blue and orange emissions simultaneously. Efficient polymer light‐emitting devices prepared using this copolymer exhibited luminance efficiencies as high as 4.1 cd/A with color coordinates (0.30, 0.36) located in the white‐light region. Moreover, the color coordinates remained almost unchanged over a range of operating potentials. A mechanistic study revealed that energy transfer from the PF backbone to the low‐bandgap segments, rather than charge trapping, was the main operating process involved in the electroluminescence process. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 2938–2946, 2007     

In situ access to white light‐emitting fluorescent polymer nanocomposites via plasma‐ignited frontal polymerization

We describe a facile fabrication of white light‐emitting cadmium sulfide (CdS)‐poly(HEA‐co‐NVK) nanocomposites [2‐hydroxyethyl acrylate (HEA) and N‐vinylcarbazole (NVK)] via plasma‐ignited frontal polymerization (PIFP), a novel and rapid reaction mode of converting monomers into polymers in minutes. Frontal polymerization was initiated by igniting the upper side of the reactant with plasma. Once initiated, no additional energy was required for the polymerization to occur. The chemical functional groups of the as‐prepared nanocomposites were thoroughly investigated using Fourier transform infrared spectra. The dependence of the front velocity and front temperature on the initiator concentration and weight ratios of HEA/NVK was also investigated in detail. Perhaps more interestingly, the white light‐emitting materials synthesized by ingeniously incorporating the compensating colors of yellow emitting from 3‐(trimethoxysilyl)‐1‐propanethiol‐capped CdS nanocrystals and blue emitting from carbazole‐containing polymer were conveniently applied onto a commercial UV light‐emitting diode (LED) to generate white LEDs. The subtle change in the weight ratios of CdS/NVK can significantly impact the color hue. The white light becomes gradually colder with the increase of NVK, but becomes gradually warmer with the increase concentration of CdS nanocrystals. In a broad perspective, these white light‐emitting materials designed by PIFP approach will open a new pathway to develop “QD‐polymer nanocomposite down‐conversion LED” in a fast and efficient way. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Molecular Triangles: Synthesis,Self‐Assembly,and Blue Emission of Cyclo‐7,10‐tris‐triphenylenyl Macrocycles

A set of cyclo‐7,10‐tris‐triphenylenyl macrocycles have been prepared by a Yamamoto cyclotrimerization protocol. In these novel macrocycles, three triphenylene units are covalently linked to each other, resulting in the formation of triangular‐shaped molecules. The fully planar derivative revealed pronounced self‐assembly behavior. NMR spectroscopy was used to determine the association constant in solution. 2D wide‐angle X‐ray scattering was applied to the study of the liquid crystallinity of this new discotic mesogen in the bulk state. Furthermore, nonplanar, laterally substituted derivatives were successfully tested as blue emitters in organic light‐emitting diodes owing to their unique optoelectronic properties and their high stability. In this case, substitution with sterically demanding phenyl groups was efficiently used to suppress intermolecular packing, thus preventing undesired quenching effects.     

Immobilization of AIEgens into metal‐organic frameworks: Ligand design,emission behavior,and applications

Aggregation‐induced emission (AIE), in which the luminophores are highly emissive in aggregate state, is one of the most unique photophysical phenomena and has shown interesting applications in many areas. The immobilization of AIE luminogens (AIEgens) into metal‐organic frameworks (MOFs), which are inorganic‐organic hybrid porous materials with tunable and predictable structures, has been investigated over the past few years. These well‐defined porous frameworks cannot only provide an ideal platform for studying the mechanism of AIE phenomenon in solid state, but also show potential applications from sensing to white light‐emitting diodes. In this highlight, we will summarize the recent progress of AIEgens‐based MOFs, including ligand design, emission behavior, and applications. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 1809–1817     

Hue‐ and Chromaticity‐Based Exploration of Surface Complexation‐Induced Tunable Emission from Non‐Luminescent Quantum Dots

Herein we report the use of a hue parameter of HSV (Hue, Saturation and Value) color space—in combination with chromaticity color coordinates—for exploring the complexation‐induced luminescence color changes, ranging from blue to green to yellow to white, from a non‐luminescent Fe‐doped ZnS quantum dot (QD). Importantly, the surface complexation reaction helped a presynthesized non‐luminescent Fe‐doped ZnS QD to glow with different luminescence colors (such as blue, cyan, green, greenish‐yellow, yellow) by virtue of the formation of various luminescent inorganic complexes (using different external organic ligands), while the simultaneous blue‐ and yellow‐emitting complex formation on the surface of non‐luminescent Fe‐doped ZnS QD led to the generation of white light emission, with a hue mean value of 85 and a chromaticity of (0.28,0.33). Furthermore, the surface complexation‐assisted incorporation of luminescence properties to a non‐luminescent QD not only overcomes their restricted luminescence‐based applications such as light‐emitting, biological and sensing applications but also bring newer avenues towards unravelling the surface chemistry between QDs and inorganic complexes and the advantage of having an inorganic complex with QD for their aforementioned useful applications.     

Star-shaped Organic Molecules That Comprise a 1,3,5-Trisubs-tituted Benzene Core and Three Oligoaryleneethynylene Arms as Light-emitting Materials

Star‐shaped rigid molecules that comprise a 1,3,5‐trisubstitued benzene core and three oligoaryleneethynylene arms have great potential application in organic light‐emitting devices (OLEDs). Their optical and electronic properties are tuned by the star‐shaped molecular size. To reveal the relationship between the properties and structures, we perform a systemic investigation for these organic molecules. The ground and excited state molecules are studied using density functional theory (DFT), the ab initio HF, and the single excitation configuration interaction (CIS), respectively. And the electronic absorption and emission spectra are investigated with time‐dependent density functional theory (TDDFT) and Zerner's intermediate neglect of differential overlap (ZINDO) methods. The results show that the HOMOs, LUMOs, energy gaps, ionization potentials (IP), electron affinities (EA), absorption and emission spectra are controlled by the star‐shaped molecular size, which favor the hole and electron injection into OLEDs. With increasing the molecular conjugated length, the absorption and emission spectra exhibit red shifts to some extent and are in good agreement with the experimental ones. Also, the calculated emission spectra range from 330 to 440 nm. All the calculated show that the star‐shaped molecules are promising as blue light emitting materials     

Synthesis,Structure, Properties,and Application of a Carbazole‐Based Diaza[7]helicene in a Deep‐Blue‐Emitting OLED

A carbazole‐based diaza[7]helicene, 2,12‐dihexyl‐2,12‐diaza[7]helicene ( 1 ), was synthesized by a photochemical synthesis and its use as a deep‐blue dopant emitter in an organic light‐emitting diode (OLED) was examined. Compound 1 exhibited good solubility and excellent thermal stability with a high decomposition temperature (T_d=372.1 °C) and a high glass‐transition temperature (T_g, up to 203.0 °C). Single‐crystal structural analysis of the crystalline clathrate ( 1 )₂ ? cyclohexane along with a theoretical investigation revealed a non‐planar‐fused structure of compound 1 , which prevented the close‐packing of molecules in the solid state and kept the molecule in a good amorphous state, which allowed the optimization of the properties of the OLED. A device with a structure of ITO/NPB (50 nm)/CBP:5 % 1 (30 nm)/BCP (20 nm)/Mg:Ag (100 nm)/Ag (50 nm) showed saturated blue light with Commission Internationale de L’Eclairage (CIE) coordinates of (0.15, 0.10); the maximum luminance efficiency and brightness were 0.22 cd A^?1 (0.09 Lm W^?1) and 2365 cd m^?2, respectively. This new class of helicenes, based on carbazole frameworks, not only opens new possibilities for utilizing helicene derivatives in deep‐blue‐emitting OLEDs but may also have potential applications in many other fields, such as molecular recognition and organic nonlinear optical materials.     

Red–Green–Blue Trichromophoric Nanoparticles with Dual Fluorescence Resonance Energy Transfer: Highly Sensitive Fluorogenic Response Toward Polyanions

A red–green–blue (RGB) trichromophoric fluorescent organic nanoparticle exhibiting multi‐colour emission was constructed; the blue‐emitting cationic oligofluorene nanoparticle acted as an energy‐donor scaffold to undergo fluorescence resonance energy transfer (FRET) to a red‐emitting dye embedded in the nanoparticle (interior FRET) and to a green‐emitting dye adsorbed on the surface through electrostatic interactions (exterior FRET). Each FRET event occurs independently and is free from sequential FRET, thus the resultant dual‐FRET system exhibits multi‐colour emission, including white, in aqueous solution and film state. A characteristic white‐emissive nanoparticle showed visible responses upon perturbation of the exterior FRET efficiency by acceptor displacement, leading to highly sensitive responses toward polyanions in a ratiometric manner. Specifically, our system exhibits high sensitivity toward heparin with an extremely low detection limit.     

Molecular design of efficient white‐light‐emitting fluorene‐based copolymers by mixing singlet and triplet emission

A new strategy to realize efficient white‐light emission from a binary fluorene‐based copolymer (PF‐Phq) with the fluorene segment as a blue emitter and the iridium complex, 9‐iridium(III)bis(2‐(2‐phenyl‐quinoline‐N,C³′)(11,13‐tetradecanedionate))‐3,6‐carbazole (Phq), as a red emitter has been proposed and demonstrated. The photo‐ and electroluminescence properties of the PF‐Phq copolymers were investigated. White‐light emission with two bands of blue and red was achieved from the binary copolymers. The efficiency increased with increasing concentration of iridium complex, which resulted from its efficient phosphorescence emission and the weak phosphorescent quenching due to its lower triplet energy level than that of polyfluorene. In comparison with the binary copolymer, the efficiency and color purity of the ternary copolymers (PF‐Phq‐BT) were improved by introducing fluorescent green benzothiadiazole (BT) unit into polyfluorene backbone. This was ascribed to the exciton confinement of the benzothiadiazole unit, which allowed efficient singlet energy transfer from fluorene segment to BT unit and avoided the triplet quenching resulted from the higher triplet energy levels of phosphorescent green emitters than that of polyfluorene. The phosphorescence quenching is a key factor in the design of white light‐emitting polyfluorene with triplet emitter. It is shown that using singlet green and triplet red emitters is an efficient approach to reduce and even avoid the phosphorescence quenching in the fluorene‐based copolymers. The strategy to incorporate singlet green emitter to polyfluorene backbone and to attach triplet red species to the side chain is promising for white polymer light‐emitting diodes. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 453–463, 2008     

Bipolar Host Materials for Organic Light‐Emitting Diodes

It is important to balance holes and electrons in the emitting layer of organic light‐emitting diodes to maximize recombination efficiency and the accompanying external quantum efficiency. Therefore, the host materials of the emitting layer should transport both holes and electrons for the charge balance. From this perspective, bipolar hosts have been popular as the host materials of thermally activated delayed fluorescent devices and phosphorescent organic light‐emitting diodes. In this review, we have summarized recent developments of bipolar hosts and suggested perspectives of host materials for organic light‐emitting diodes.     

Characteristics of a single‐layered organic electroluminescent device using a carrier‐transporting copolymer and a nonconjugated light‐emitting polymer

We have synthesized a novel carrier‐transporting copolymer and a nonconjugated light‐emitting polymer. The carrier‐transporting copolymer has a triphenylamine moiety as a hole‐transporting unit and a triazine moiety as an electron‐transporting unit, both of which are located in the polymer side chain. The nonconjugated light‐emitting polymer has a perylene moiety, which acts as an emitting unit in the polymer side chain. These polymers are very soluble in most organic solvents, such as monochlorobenzene, tetrahydrofuran, chloroform, and benzene. A single‐layered electroluminescent device consisting of ITO/copolymer and emitting‐material 4‐(dicyanomethylene)‐2‐methyl‐6‐(4‐dimethylaminostyryl)‐4H‐pyran (DCM) or light‐emitting polymer)/Al mixtures exhibits maximum external quantum efficiency when the concentration of the emitting material is 30 wt %. The device emits red or blue light according to the emitting material. When CsF is used as the electron‐injecting material, the drive voltage decreases drastically to 7 V, and the highest quantum efficiency is 0.5%. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 2733–2743, 2003     

Recent progress in the development of polymers for white light-emitting polymer devices

Much effort has been devoted to the design and synthesis of polymers for use in flat panel display, solid state lighting, transistors, and photovoltaic devices. Especially, development of white light emitting polymeric materials has recently attracted much interest owing to their possible use in lighting application and backlights for flat panel displays. White emission has been obtained from polymeric molecules, small organic molecules, organometallic molecules, and phosphor-based or quantum dot-based inorganic molecules. Among materials used in white light emitting diodes, we summarize the white light emitting polymeric materials synthesized and published till December 2007. Correspondence: In Tae Kim, Department of Chemistry, Kwangwoon University, 447-1 Wolgye-Dong, Nowon-Ku, Seoul 139-701, Korea, So Ha Lee, Life Sciences Research Division, Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul 130-650, Korea.     

Molecular and Device Design Strategies for Ideal Performance White Organic Light‐Emitting Diodes

The progress of white organic light‐emitting diodes (WOLEDs) via adopting fluorescent and phosphorescent organic materials have attracted commercial interest for their broad range of visible spectrum and potential of 100 % internal quantum efficiency. In this account, smart molecular designs for developing efficient phosphorescent host and good color purity blue fluorescent emitters are prepared to be discussed, especially donor‐acceptor modification to regulate their triplet states and bipolar transport properties. Rational device configuration design strategies were also introduced by cooperating with efficient conventional fluorescent and thermally activated delayed fluorescent emitting molecules to achieve full exciton utilization and simplified device structures, further suggesting perspectives of potentially low‐cost, ideal performance and promoted operational lifetime in WOLED devices.     

Rhodium(III)‐Catalyzed ortho‐Heteroarylation of Phenols through Internal Oxidative CH Activation: Rapid Screening of Single‐Molecular White‐Light‐Emitting Materials

Reported herein is the first example of a transition‐metal‐catalyzed internal oxidative C? H/C? H cross‐coupling between two (hetero)arenes through a traceless oxidation directing strategy. Without the requirement of an external metal oxidant, a wide range of phenols, including phenol‐containing natural products, can undergo the coupling with azoles to assemble a large library of highly functionalized 2‐(2‐hydroxyphenyl)azoles. The route provides an opportunity to rapidly screen white‐light‐emitting materials. As illustrative examples, two bis(triphenylamine)‐bearing 2‐(2‐hydroxyphenyl)oxazoles, which are difficult to access otherwise, exhibit bright white‐light emission, high quantum yield, and thermal stability. Also presented is the first example of the white‐light emission, in a single excited‐state intramolecular proton transfer system, of 2‐(2‐hydroxyphenyl)azoles, thus highlighting the charm of C? H activation in the discovery of new organic optoelectronic materials.     

Synthesis of Unsymmetric Bipyridine–PtII–Alkynyl Complexes through Post‐Click Reaction with Emission Enhancement Characteristics and Their Applications as Phosphorescent Organic Light‐Emitting Diodes

Two unsymmetric bipyridine–platinum(II)–alkynyl complexes have been synthesised by a post‐click reaction. These metal complexes are found to exhibit emission enhancement properties. The photoluminescence quantum yield can be significantly increased from 0.03 in solution to 0.72 in solid‐state thin films. Efficient solution‐processable organic light‐emitting diodes have been fabricated by utilizing these complexes as phosphorescent dopants. A high external quantum efficiency of up to 5.8 % has been achieved.     

A Unique Blend of 2‐Fluorenyl‐2‐anthracene and 2‐Anthryl‐2‐anthracence Showing White Emission and High Charge Mobility

White‐light‐emitting materials with high mobility are necessary for organic white‐light‐emitting transistors, which can be used for self‐driven OLED displays or OLED lighting. In this study, we combined two materials with similar structures—2‐fluorenyl‐2‐anthracene (FlAnt) with blue emission and 2‐anthryl‐2‐anthracence (2A) with greenish‐yellow emission—to fabricate OLED devices, which showed unusual solid‐state white‐light emission with the CIE coordinates (0.33, 0.34) at 10 V. The similar crystal structures ensured that the OTFTs based on mixed FlAnt and 2A showed high mobility of 1.56 cm² V⁻¹ s⁻¹. This simple method provides new insight into the design of high‐performance white‐emitting transistor materials and structures.