Bireactive calamitic liquid crystals derived from fluorene ester linkage

New thermotropic liquid crystalline monomer esters, containing fluorene in their mesogenic core with luminescent properties, were synthesized and characterized. The monomers studied exhibited a broad mesomorphic range and a nematic phase was identified by differential scanning calorimetry and polarized optical microscopy. Absorption and photoluminescence spectra studied in solution indicated that these compounds possess good optical stability. Linearly polarized light emission was observed in oriented thin films prepared by an in‐situ photopolymerization technique. The preparation of oriented thin films shows a preferential emission direction necessary for application in linearly polarized blue light‐emitting diodes.     

Electroluminescent properties of anthracene‐containing polyimides

Optical and electroluminescent properties of a new soluble anthracene‐containing polyimide (ACPI) was studied. Solubility of ACPI in organic solvents allows direct spin casting of the polymer films exhibiting intense photo‐ and electroluminescence (EL) in the visible range. This non‐conjugated polymer was used as emitting and electron‐hole transporting layers in polymer light‐emitting devices (LEDs). EL properties of the uni‐ and bilayer LEDs are discussed in terms of the band structure, bipolar transport and electron donor‐acceptor interactions. Copyright © 2000 John Wiley & Sons, Ltd.     

Simple synthesis,photophysics, and electroluminescent properties of poly[2,7‐bis(4‐tert‐butylstyryl)fluorene‐9, 9‐diyl‐alt‐alkane‐α,ω‐diyl]

A simple synthetic route was used for the synthesis of a novel series of alternating copolymers based on substituted 2,7‐distyrylfluorene bridged through alkylene chains. First, 2,7‐dibromofluorene was reacted with 2 equiv of butyllithium, and this was followed by a treatment with 1 equiv of α,ω‐dibromoalkane to yield the intermediate, poly(2,7‐dibromofluorene‐9,9‐diyl‐alt‐alkane‐α,ω‐diyl). ( 1 ) Heck coupling of the latter with 1‐tert‐butyl‐4‐vinylbenzene afforded the target, poly[2,7‐bis(4‐tert‐butylstyryl)fluorene‐9,9‐diyl‐alt‐alkane‐α,ω‐diyl] ( 2 ). The two versions of 2 ( 2a and 2b which have hexane and decane, respectively, as alkane groups) were readily soluble in common organic solvents. Their glass‐transition temperature was relatively low (52 and 87 °C). An intense blue photoluminescence emission with maxima at about 408 and 409 nm was observed in tetrahydrofuran solutions, whereas thin films exhibited an orange emission with maxima at 569 and 588 nm. Very large redshifts of the photoluminescence maxima and Stokes shifts in thin films indicated strong aggregation in the solid state. Both polymers oxidized and reduced irreversibly. Single‐layer light‐emitting diodes with hole‐injecting indium tin oxide and electron‐injecting aluminum electrodes were fabricated. They emitted orange light with external electroluminescence efficiencies of 0.52 and 0.36% photon/electron, as determined in light‐emitting diodes made of 2a and 2b , with alkylenes of (CH₂)₆ and (CH₂)₁₀, respectively. An increase in the external electroluminescence efficiency up to 1.5% was reached in light‐emitting diodes made of polymer blends consisting of 2a and poly(9,9‐dihexadecylfluorene‐2,7‐diyl), which emitted blue‐white light. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 809–821, 2007.     

Polarized electroluminescence of a discotic mesogenic compound

Polarized electroluminescence is observed for a uniaxially aligned sample of a discotic compound in its crystalline state. The alignment was achieved by coating the ITO anode with an aligned film of poly(tetrafluoroethylene) (PTFE) deposited using the friction method. Organic light emitting devices with the layer sequence glass/ITO/PTFE/tetraethyl perylene-3,4,9,10-tetracarboxylate (55 nm)/aluminium (100 nm) show linearly polarized emission of light with a dichroic ratio up to 3.2.     

Novel blue‐greenish electroluminescent poly(fluorenevinylene‐alt‐dibenzothiophenevinylene)s and their model compounds

Poly(9,9‐dihexylfluorene‐2,7‐vinylene‐alt‐dibenzothiophene‐2,8‐vinylene) (PS) and poly(9,9‐dihexylfluorene‐2,7‐vinylene‐alt‐dibenzothiophene‐5,5‐dioxide‐2,8‐ vinylene) (PSO) as well as corresponding model compounds were synthesized by Heck coupling. Both the polymers and model compounds were readily soluble in common organic solvents such as tetrahydrofuran, dichloromethane, chloroform, and toluene. The polymers showed a decomposition temperature at ～430 °C and a char yield of about 65% at 800 °C in N₂. The glass‐transition temperatures of the polymers were almost identical (75–77 °C) and higher than those of the model compounds (26–45 °C). All samples absorbed around 390 nm, and their optical band gaps were 2.69–2.85 eV. They behaved as blue‐greenish light emitting materials in both solutions and thin films, with photoluminescence emission maxima at 450–483 nm and photoluminescence quantum yields of 0.52–0.72 in solution. Organic light‐emitting diodes with an indium tin oxide/poly(ethylene dioxythiophene):poly(styrene sulfonic acid)/polymer/Mg:Ag/Ag configuration with polymers PS and PSO as emitting layers showed green electroluminescence with maxima at 530 and 540 nm, respectively. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 6790–6800, 2006     

Red‐Emitting Dendritic Iridium(III) Complexes for Solution Processable Phosphorescent Organic Light‐Emitting Diodes

Functionalization of a red phosphorescent iridium(III) complex core surrounded by rigid polyphenylene dendrons with a hole‐transporting triphenylamine surface allows to prevent the intermolecular aggregation‐induced emission quenching, improves charge recombination, and therefore enhances photo‐ and electroluminescence efficiencies of dendrimer in solid state. These multifunctional shape‐persistent dendrimers provide a new pathway to design highly efficient solution processable materials for phosphorescent organic light‐emitting diodes (PhOLEDs).     

Donor–acceptor copolymers for red‐ and near‐infrared‐emitting polymer light‐emitting diodes

We report a comparative study of two organic soluble, vinylene‐based, alternating donor–acceptor copolymers with 1,4‐(2,5‐dihexadecyloxyphenylene) as the donor; the acceptor is either a 2,5‐linked pyridine or a 5,8‐linked 2,3‐diphenylpyrido[3,4‐b]pyrazine. The polymers are synthesized via a Heck coupling methodology from a dihalo monomer and a divinyl monomer to yield number‐average molecular weights of 16,000 g/mol for the pyridine polymer (PPyrPV) and 6500 g/mol for the pyridopyrazine polymer (PPyrPyrPV), with high solubility in common chlorinated solvents and lower solubility in less polar solvents (e.g., tetrahydrofuran). Thin‐film measurements show band gaps of 2.2 and 1.8 eV for PPyrPV and PPyrPyrPV, respectively. Both polymers exhibit photoluminescence in solution and in the solid state and exhibit electroluminescence when incorporated into light‐emitting diodes. In this case, a broad red emission centered at 690 nm for PPyrPV and a near‐infrared emission centered at 800 nm for PPyrPyrPV have been observed. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 1417–1431, 2005     

Chromophore‐labeled dendrimers for use in single‐layer light‐emitting diodes

Two novel chromophore‐labeled dendrimers are presented and their incorporation into two‐component, single layer organic light emitting diodes (OLEDs) is described. The photoluminescence (PL) spectra, both in solution and in the solid state, demonstrate that Forster energy transfer from the donor chromophores on the dendrimers periphery to the acceptors located at the core is highly efficient, and affords emission exclusively from the core dyes, either coumarin 343 or a benzene‐capped pentathiophene. When these dendrimers are doped into single layer OLEDs, the electroluminescence (EL) is nearly identical to the photoluminescence described above, indicating once again exclusive emission from the core chromophores.     

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     

PL and EL behavior of near‐red luminescent metallopolymer easily prepared from phosphine‐containing copolymer and chloro{bis(1‐phenylisoquinolinato‐N,C2′)}iridium(III), and its monomeric analog

We successfully developed phosphorescent cyclometallated iridium‐containing metallopolymers, which are near‐red luminescent iridium complexes bearing phosphine‐containing copolymers used as polymer ligands, and investigated their photoluminescence and electroluminescence behavior. The phosphine copolymer ligand made from methyl methacrylate and 4‐styryldiphenylphosphine can be used as an anchor, which coordinates luminescent iridium units to form the metallopolymer easily. Organic light‐emitting diodes were fabricated from the metallopolymer and its nonpolymer analog, [IrCl(piq)₂PPh₃]. These complexes exhibited quite similar luminescence behavior, except for emission from the free‐phosphine‐units in the polymer side chain and their energy‐transferring properties from host to guest materials. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 4366–4378, 2009     

Aggregation‐Induced Delayed Fluorescence Based on Donor/Acceptor‐Tethered Janus Carborane Triads: Unique Photophysical Properties of Nondoped OLEDs

Luminescent materials consisting of boron clusters, such as carboranes, have attracted immense interest in recent years. In this study, luminescent organic–inorganic conjugated systems based on o‐carboranes directly bonded to electron‐donating and electron‐accepting π‐conjugated units were elaborated as novel optoelectronic materials. These o‐carborane derivatives simultaneously possessed aggregation‐induced emission (AIE) and thermally activated delayed fluorescence (TADF) capabilities, and showed strong yellow‐to‐red emissions with high photoluminescence quantum efficiencies of up to 97 % in their aggregated states or in solid neat films. Organic light‐emitting diodes utilizing these o‐carborane derivatives as a nondoped emission layer exhibited maximum external electroluminescence quantum efficiencies as high as 11 %, originating from TADF.     

Novel Emission Color‐Tuning Strategies in Heteroleptic Phosphorescent Ir(III) and Pt(II) Complexes

Color‐tuning for phosphorescent emitters in organic light‐emitting diodes (OLEDs) across the entire visible spectrum is prerequisite to fulfil flexible full‐color displays and white solid‐state lighting. Heteroleptic 2‐phenylpyridine‐type (ppy‐type) Ir(III) and Pt(II) complexes as phosphorescent emitters have been well exploited in the electroluminescence (EL) field due to their outstanding EL performance. Furthermore, the photophysical characters of these heteroleptic Ir(III) and Pt(II) complexes are generally dominated by the nature of cyclometalating ppy‐type ligands. Accordingly, either sophisticated modification or judicious combination of different cyclometalating ppy‐type ligands will provide a wonderful platform to tune their emission color. In this personal account, we put a special emphasis on our contributions to the novel color‐tuning strategies in these heteroleptic ppy‐type Ir(III) and Pt(II) complexes. In addition, afforded by our novel color‐tuning strategies, ambipolar character or enhanced electron injection/transport (EI/ET) features can be furnished to bring high EL performances.     

Luminescent copolymers for applications in multicolor‐light‐emitting devices

Light‐emitting diodes based on organic materials [organic light‐emitting diodes (OLEDs)] have attracted much interest over the past decade. Several different attempts have been made to realize multicolor OLEDs. This article describes a new approach based on energy transfer in a donor/acceptor system. A copolymer containing both donor and acceptor compounds as comonomer units is prepared. The polymer consists of a derivative of a luminescent dye [4‐dicyanmethylene‐2‐methyl‐6‐4H‐pyran (DCM); acceptor compound], which is copolymerized with fluorene (donor compound) to combine the properties of an electroactive polymer with a highly luminescent dye. Photochemical processing is achieved by UV irradiation of this copolymer in the presence of gaseous trialkylsilanes. This reagent selectively saturates the C?C bonds in the DCM comonomer units while leaving the fluorene units essentially unaffected. As a result of the photochemical process, the red electroluminescence of the acceptor compound vanishes, and the blue‐green electroluminescence from the polyfluorene units is recovered. Compared with previous approaches based on polymer blends, this copolymer approach avoids problems associated with phase‐separation phenomena in the active layer of OLEDs. © 2006Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 4317–4327, 2006     

New blue electroluminescent n‐type polyfluorene copolymers with a 1,3,4‐oxadiazole unit in the main chain

Novel polyfluorene copolymers alternately having an 1,3,4‐oxadiazole unit in the main chain were prepared by both one‐step and two‐step methods for polyoxadiazole synthesis. They displayed highly efficient blue photoluminescence, the properties of which were affected by the extent of conjugation and the changes in the electron density by a side chain. An electrochemical analysis of the polymers using cyclic voltammetry suggested that they could be used as electron‐transport/hole‐blocking materials as well as blue emission materials for polymer light‐emitting diodes. A simple double‐layer device consisting of poly(N‐vinylcarbazole) as a hole‐transport layer and poly[(9,9′‐didodecylfluorene‐2,7‐diyl)‐alt‐((1,4‐bis(1,3,4‐oxadiazole)‐2,5‐di(2‐ethylhexyloxy)phenylene)‐5,5′‐diyl)] as an emission layer exhibited narrow blue electroluminescence with a maximum at 430 nm. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 1058–1068, 2004     

Synthesis and characterization of novel germanium‐containing poly(p‐phenylenevinylene) derivatives

Novel blue‐emitting germanium‐containing poly(p‐phenylenevinylene) (PPV) derivatives with well‐defined conjugation lengths were synthesized via Wittig‐condensation polymerizations. The polymers can be color‐tuned by the introduction of various chromophores into the PPV‐based polymer backbones. The photoluminescence (PL) spectra of the polymers, GePVK (containing carbazole moieties), GeMEH (containing dialkoxybenzene moieties), and GePTH (containing phenothiazine moieties), were found to exhibit blue, greenish blue, and green emissions, respectively. GePTH produces more red‐shifted emission than GeMEH and GEPVK, resulting in green emission, and the solution and solid state PL spectra of GePVK consist of almost blue emission. The electroluminescence spectra of GeMEH and GePTH contain yellowy green and yellow colors, respectively. Interestingly, GePVK exhibits white emission with CIE coordinates of (0.33, 0.37) due to electroplex emission in the light‐emitting diodes. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 979–988, 2008     

Synthesis,Photophysical Properties and Near Infrared Electroluminescence of 1(4),8(11),15(18),22(25)‐Tetra‐(methoxy‐phenoxy)phthalocyanine

A new soluble phthalocyaine 1(4),8(11),15(18),22(25)‐tetra‐(methoxy‐phenoxy)phthalocyanine (MPPc) was synthesized and verified by mass spectrum (MS), ¹H NMR, IR and elemental analysis. The methoxy‐phenoxy groups were introduced in order to enhance the solubility of the phthalocyanine. The photophysical and electroluminescent properties were investigated. The organic light‐emitting diodes (OLEDs) with the structure of ITO/PVK:MPPc(40 nm)/BCP(20 nm)/Alq₃(30 nm)/Al were fabricated. Room‐temperature near infrared (NIR) electroluminescence (EL) was observed near 891 nm that effectively covered the ?rst optical communication window near 850 nm.     

Synthesis and properties of new poly(terfluorene) derivatives containing spirobifluorene and electron transport groups for stable blue electroluminescence

We have synthesized three new blue‐emitting poly(terfluorene) derivatives containing spirobifluorene and electron transport groups (quinoline or oxadiazole). The strategy is to connect the monomers via positions 2 and 2′ of spirobifluorene, which significantly restricts the interchain interaction and effectively adjusts the conjugation length. The incorporation of electron‐deficient units (quinoline or oxadiazole) into the positions 7 and 7′ of spirobifluorene tunes the emission band and reduces the lowest unoccupied molecular orbital energy level. Blue electroluminescence with narrow emission was achieved in the devices of ITO/PEDOT/polymer/Ca/Al. The maximum luminances are in the range of 102–235 cd/m² and the maximum photometric efficiencies are in the range of 0.17–0.21 cd/A. All the polymers show good spectral stability, and are promising for use as stable blue‐emitting or electron‐transport/injection materials in polymeric light‐emitting diodes. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 4517–4529, 2005     

Thermally Activated Delayed Fluorescence in a Y3N@C80 Endohedral Fullerene: Time‐Resolved Luminescence and EPR Studies

The endohedral fullerene Y₃N@C₈₀ exhibits luminescence with reasonable quantum yield and extraordinary long lifetime. By variable‐temperature steady‐state and time‐resolved luminescence spectroscopy, it is demonstrated that above 60 K the Y₃N@C₈₀ exhibits thermally activated delayed fluorescence with maximum emission at 120 K and a negligible prompt fluorescence. Below 60 K, a phosphorescence with a lifetime of 192±1 ms is observed. Spin distribution and dynamics in the triplet excited state is investigated with X‐ and W‐band EPR and ENDOR spectroscopies and DFT computations. Finally, electroluminescence of the Y₃N@C₈₀/PFO film is demonstrated opening the possibility for red‐emitting fullerene‐based organic light‐emitting diodes (OLEDs).     

The Preparation of (8‐Hydroxyquinolinato)Bis(2‐Phenylpyridyl)Iridium Complexes and Their Photophysical Properties

Four derivatives of the titled compounds, (8‐hydroxyquinoline)bis(2‐phenylpyridyl)iridium ( IrQ(ppy)₂ ), were prepared. Two of them were confirmed by single crystal X‐ray diffraction analyses, in which solvent molecules were found to be incorporated in the crystal lattices. Their emission spectra display separated dual bands in de‐aerated solutions at about 515 and 645 nm upon excitation. These green and red emissions are attributed to the triplet metal‐to‐ligand charge transfer (³MLCT) and triplet ligand centered (³LC) transitions in Ir(ppy)₂ and IrQ, respectively. It is suggested that such a multiple emission is feasible by nearly orthogonal orientation between the ppy and quinoline ligands in the mixed‐ligand Ir‐compounds which prohibits energy transfer between the two different ligands. The electroluminescence (EL) of these compounds was examined by the fabrication of light‐emitting diodes (LEDs). Unlike the spectra in solutions, their EL spectra displayed only the red emission band. Devices displaying white light can be obtained by mixing the red emission of IrQ(ppy)₂ with a compatible blue emitter (NPB) in separated layers.