Hyperbranched luminescent polyfluorenes containing aromatic triazole branching units

Using a new aromatic 1,2,4‐triazole branching monomer (4.8–13.3 mol %), three hyperbranched polyfluorenes ( P2 – P4 ) were synthesized by the Suzuki coupling reaction to investigate the structural effect on optoelectronic properties. Poly(9,9‐dihexylfluorene) ( P1 ) was also prepared for comparative investigation. Their weight‐average molecular weights and polydispersity indices are in the range of 1.16 × 10⁴ to 5.9 × 10⁴ and 1.49–2.25, respectively. Optical properties, both in solution and film state, were investigated using absorption and photoluminescence (PL) spectra. In film state, the absorption and PL spectra peaked at 377–392 and 424–425 nm, respectively, blue‐shift with increasing triazole concentration. Furthermore, a linear relationship between 1/λ_max,abs and 1/(1 ? n_triazole) is correlated (n: molar fraction), indicating a smooth decrease in conjugation length by incorporation of the branch unit. The P4 containing 13.3 mol % triazole reveals stable blue emission even at 150 °C (in air). The HOMO and LUMO levels of P2 – P4 , estimated from cyclic voltammograms, are ?5.69, ?5.73, ?5.78 eV and ?2.63, ?2.64, ?2.63 eV, respectively. The maximal brightness (current efficiency) of the electroluminescent devices (ITO/PEDOT:PSS/ P2 – P4 /Ca/Al) improves from 828 cd/m² (0.19 cd/A) to 2054 cd/m² (0.46 cd/A) with increasing triazole concentration. The results suggest that incorporation of aromatic 1,2,4‐triazole branch units is an effective way to improve annealing stability and EL performance of polyfluorenes. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 4465–4476, 2007     

Synthesis and optoelectronic properties of alternating benzofuran/terfluorene copolymer with stable blue emission

The solution processable alternating benzofuran/terfluorene copolymer bearing side oxadiazole groups ( PBF‐OXD ) was synthesized and its optoelectronic properties and color stability were investigated. Electron‐deficient and stereohindered oxadiazole units were used as pendent groups to compensate for the poor electron‐transporting ability of a p‐type polymer backbone, to depress the intermolecular π‐stacking, and to improve solubility while retaining polymer blue emission. PBF‐OXD showed a glass transition at 135 °C and an onset decomposition temperature of ～345 °C. A simple EL device, with the configuration of ITO/PEDOT:PSS/ PBF‐OXD /Ba/Al, displayed a stable blue emission (λ_max = 434 nm), good color purity (full width half‐maximum = 59 nm), maximum brightness of 1400 cd/m², and a maximum luminance efficiency of 0.95 cd/A. The PL and EL spectra changed slightly on annealing and on increasing the applied voltage. These results show that the as‐synthesized copolymer PBF‐OXD had integrated respective functions of its different building blocks and exhibited good thermal and color stability with improved EL performance. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 5488–5497, 2009     

Deep‐red light‐emitting phosphorescent dendrimer encapsulated tris‐[2‐benzo[b]thiophen‐2‐yl‐pyridyl] iridium (III) core for light‐emitting device applications

New deep‐red light‐emitting phosphorescent dendrimers with hole‐transporting carbazole dendrons were synthesized by reacting tris(2‐benzo[b]thiophen‐2‐yl‐pyridyl) iridium (III) complex with carbazolyl dendrons by DCC‐catalyzed esterification. The resulting first‐, second‐, and third‐generation dendrimers were found to be highly efficient as solution‐processable emitting materials and for use in host‐free electrophosphorescent light‐emitting diodes. We fabricated a host‐free dendrimer EL device with configuration ITO/PEDOT:PSS (40 nm)/dendrimer (55 nm)/BCP (10 nm)/Alq₃ (40 nm)/LiF (1 nm)/Al (100 nm) and characterized the device performance. The multilayered devices showed luminance of 561 cd/m² at 383.4 mA/cm² (12 V) for 15 , 1302 cd/m² at 321.3 mA/cm² (14 V) for 16 , and 422 cd/m² at 94.4 mA/cm² (18 V) for 17 . The third‐generation dendrimer, 17 (η_ext = 6.12% at 7.5 V), showed the highest external quantum efficiency (EQE) with an increase in the density of the light‐harvesting carbazole dendron. Three dendrimers exhibited considerably pure deep‐red emission with CIE 1931 (Commission International de L'Eclairage) chromaticity coordinates of x = 0.70, y = 0.30. The CIE coordinates remained very stable with the current density. The integration of rigid hole‐transporting dendrons and phosphorescent complexes provides a new route to design highly efficient solution‐processable materials for dendrimer light‐emitting diode (DLED) applications. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 7517–7533, 2008     

Fluorene‐ and benzimidazole‐based blue light‐emitting copolymers: Synthesis,photophysical properties,and PLED applications

Blue light‐emitting materials are receiving considerable academic and industrial interest due to their potential applications in optoelectronic devices. In this study, blue light‐emitting copolymers based on 9,9′ ‐ dioctylfluorene and 2,2′‐(1,4‐phenylene)‐bis(benzimidazole) moieties were synthesized through palladium‐catalyzed Suzuki coupling reaction. While the copolymer consisting of unsubstituted benzimidazoles (PFBI0) is insoluble in common organic solvents, its counterpart with N‐octyl substituted benzimidazoles (PFBI8) enjoys good solubility in toluene, tetrahydrofuran, dichloromethane (DCM), and chloroform. The PFBI8 copolymer shows good thermal stability, whose glass transition temperature and onset decomposition temperature are 103 and 428 °C, respectively. Its solutions emit blue light efficiently, with the quantum yield up to 99% in chloroform. The electroluminescence (EL) device of PFBI8 with the configuration of indium‐tin oxide/poly(ethylenedioxythiophene):poly(styrene sulfonic acid)/PFBI8/1,3,5‐tris(1‐phenyl‐1H‐benzimidazole‐2‐yl)benzene/LiF/Al emits blue light with the maximum at 448 nm. Such unoptimized polymer light‐emitting diode (PLED) exhibits a maximum luminance of 1534 cd/m² with the current efficiency and power efficiency of 0.67 cd/A and 0.20 lm/W, respectively. The efficient blue emission and good EL performance make PFBI8 promising for optoelectronic applications. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Water‐soluble dendritic‐conjugated polyfluorenes: Synthesis,characterization, and interactions with DNA

Three generation of Boc‐protected dendritic‐conjugated polyfluorenes ( Boc‐PFP‐G_0‐2 ) were synthesized by Suzuki coupling 1,4‐phenyldiboronic ester with dendritic monomers that were synthesized through generation‐by‐generation approach. The gel permeation chromatography (GPC) analyses showed that the weight‐average molecular weight (M_w) of Boc‐PFP‐G_0‐2 was in the range of 11,400–20,400 Da with the polydispersity index (PDI) in the range of 1.32–1.96. Treatment of Boc‐protected polymers with 6 M HCl in dioxane yielded cationic dendritic‐conjugated polyfluorenes ( PFP‐G_0‐2 ). They were soluble in common polar solvents such as DMSO, DMF, and water with absorption maxima between 345 and 379 nm. The solutions of PFP‐G_0‐2 in water were highly fluorescent with emission maxima between 416 and 425 nm. Because higher generation dendrons could prevent the formation of π‐stacking aggregates of backbones of conjugated polymer, the fluorescence quantum efficiencies (QEs) of PFP‐G_0‐2 enhance as the dendritic generation grew. The interactions between 25 mer double‐stranded DNA (dsDNA) and PFP‐G_0‐2 were studied using ethidium bromide (EB) as fluorescent probe. The electrostatic bindings of PFP‐G_0‐2 with dsDNA/EB complex result in displacement of EB from DNA double helix to the solution accompanying by a quenching of EB fluorescence. The PFP‐G₂ with highest generation of dendritic side chains possessed a highest charge density and could form most stable complex with dsDNA. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 7462–7472, 2008     

Design,synthesis, photophysical,and electrochemical properties of DCM‐based conjugated polymers for light‐emitting devices

A series of conjugated hyperbranched polymers, hyperbranched copolymers, and linear polymers containing 2‐pyran‐4‐ylidenemalononitrile (acceptor) and triphenylamine/fluorene (donor) units were synthesized and characterized by FTIR, ¹H NMR, thermogravimetric analyses, differential scanning calorimetry, gel permeation chromatography, UV–visible, photoluminescence, and cyclic voltammetry measurements. All the polymers show red‐light emission in the range of 566–656 nm both in solution and in solid state. The quantum efficiency of the polymers was in the range of 56–82%. Among the six polymers synthesized, only polymers containing fluorene units show T_g and polymers based on triphenylamine not exhibit T_g. The band gap of these polymers were found to be reasonably low; hyperbranched copolymer containing fluorene unit shows lowest band gap of 2.18 eV due to the stabilization of LUMO energy level by the electron withdrawing ? CN groups. The thermal and solubility behavior of the polymers were found to be good. All the EL spectra of the devices (indium‐tin oxide/poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate)/polymer/2,9‐dimethyl‐4,7‐diphenyl‐1,10‐phenanthroline/tris(8‐hydroxyquinoline)aluminum)/LiF/Al) show red‐light emission, and the device fabricated with P3 and P4 shows maximum luminance and luminous efficiency of 4104 cd m^?2 and 0.55 cd Å^?1 and 3696 cd m^?2 and 0.47 cd Å^?1, respectively, indicates that they had the best carrier balance. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Hyperbranched and thermally cross‐linkable oligomer from a new 2,5,7‐tri‐functional fluorene monomer

New hyperbranched (HOFV) and linear oligomers (LOFV) were prepared from 2‐bromo‐5,7‐divinyl‐9,9‐dihexylfluorene (AB₂) and 2‐bromo‐7‐vinyl‐9,9‐dihexylfluorene, respectively, by the Heck reaction to study the effect of hyperbranched structure. The oligomers were readily soluble in common organic solvents. The weight‐average molecular weights (M_w) of HOFV and LOFV, determined by gel permeation chromatography using polystyrene as standard, were 2350 and 3950, respectively. Optical properties, both in solution and film state, were investigated using absorption and photoluminescence (PL) spectra. In film state, the absorption and PL spectra peaked at 416 ～ 425 nm and 473 ～ 503 nm, respectively. The HOFV showed energy funnel effect and enhanced fluorescence efficiency owing to the hyperbranched structure. The HOMO and LUMO levels of HOFV (LOFV), estimated from their cyclic voltammograms, were ?5.25 (?5.34) eV and ?2.66 (?2.75) eV, respectively. Thermal curing of HOFV to form cross‐linked HPFV (hyperbranched poly(fluorenevinylene)) was studied by IR, DSC, UV–visible spectra, NMR, AFM and SEM. The terminal vinyl groups in HOFV film almost disappeared to provide smooth, homogeneous and solvent‐resistant films of HPFV. Two‐layer PLED devices (ITO/PEDOT/HPFV/Ca/Al) exhibited maximal luminance and luminous efficiency of 1480 cd/m² and 0.18 cd/A, respectively, which were superior to its linear counterpart LPFV (352 cd/m², 0.06 cd/A). © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 70–84, 2008     

Synthesis and characterization of hyperbranched polyfluorenes containing triarylpyrazoline cores towards efficient blue light‐emitting diodes

New series of hyperbranched polyfluorenes containing triarylpyrazoline cores, PFZ10 , PFZ20 , PFNZ10 , and PFNZ20 , have been synthesized according to the “A₂ + A′₂ + B₃” Suzuki coupling method. The structures and property of the monomers and conjugated polymers were characterized by elemental analysis, gel permeation chromatography, cyclic voltammetry, thermogravimetric analysis, differential scanning calorimetry, and UV–visible absorption, ¹H NMR, ¹³C NMR, and photoluminescence spectroscopies. All these polymers exhibited good solubility in common organic solvents and good thermal stability. The long‐wavelength emission of polyfluorenes had been effectively reduced in these hyperbranched polymers. Standard polymer light emitting devices (PLEDs) from PFZ10 , PFZ20 , PFNZ10 , and PFNZ20 , with the configuration of ITO/PEDOT/polymer/TPBI/Alq₃/Mg:Ag, exhibited good electroluminescence (EL) properties The PLED based on PFNZ10 emitted pure blue light with a low turn‐on voltage of 5.3 V and a high EL efficiency of about 1.93%. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 5296–5307, 2007     

Host copolymers containing pendant carbazole and oxadiazole groups: Synthesis,characterization and optoelectronic applications for efficient green phosphorescent OLEDs

Vinyl copolymers (PCOn), containing pendant carbazole and aromatic 1,3,4‐oxadiazole attached with dodecyloxy group, were prepared from their corresponding precursor poly(9‐vinyl carbazole‐co‐4‐vinylbenzyl chloride) (PCBn) by the Williamson condensation (n: mole% of 4‐vinylbenzyl chloride). These copolymers were used as host materials for green phosphorescent light‐emitting diodes after blending 4 wt % of Ir(ppy)₃. PL spectra of the PCOn films showed the formation of excimer or exciplex. The phosphorescent EL devices were fabricated with a configuration of ITO/PEDOT:PSS/host copolymers:Ir(ppy)₃/BCP/Ca/Al. The PL and EL spectra of the blends [PCOn:Ir(ppy)₃] revealed dominant green emission at 517 nm attributed to Ir(ppy)₃ due to efficient energy transfer from the host to Ir(ppy)₃. Efficient green phosphorescent OLEDs was obtained when employing copolymer PCO16 as the host and Ir(ppy)₃ as the guest. The maximal luminance efficiency and the maximal luminance of this device were 17.9 cd/A and 19,903 cd/m², respectively. After doped with Ir(ppy)₃, the morphology of the films, both controlled PCO20 and PCO20 with attached dodecyloxy groups, were investigated by tapping‐mode AFM and FE‐SEM. The film of PCO20 exhibited uniform, featureless image and showed much better device performance than PCO20, which have been attributed to good compatibility of PCO20 with Ir(ppy)₃. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 5180–5193, 2008     

Synthesis of fluorene‐based hyperbranched polymers for solution‐processable blue,green, red,and white light‐emitting devices

For the purpose of making hyperbranched polymer (Hb‐Ps)‐based red, green, blue, and white polymer light‐emitting diodes (PLEDs), three Hb‐Ps Hb‐ terfluorene ( Hb‐TF ), Hb ‐4,7‐bis(9,9′‐dioctylfluoren‐2‐yl)‐2,1,3‐benzothiodiazole ( Hb‐BFBT ), and Hb‐ 4,7‐bis[(9,9′‐dioctylfluoren‐2‐yl)‐thien‐2‐yl]‐2,1,3‐benzothiodiazole ( Hb‐BFTBT ) were synthesized via [2+2+2] polycyclotrimerization of the corresponding diacetylene‐functionalized monomers. All the synthesized polymers showed excellent thermal stability with degradation temperature higher than 355 °C and glass transition temperatures higher than 50 °C. Photoluminance (PL) and electroluminance (EL) spectra of the polymers indicate that Hb‐TF , Hb‐BFBT , and Hb‐BFTBT are blue‐green, green, and red emitting materials. Maximum brightness of the double‐layer devices of Hb‐TF , Hb‐BFBT , and Hb‐BFTBT with the device configuration of indium tin oxide/poly(3,4‐ethylene dioxythiophene):poly(styrene sulfonate)/light‐emitting polymer/CsF/Al are 48, 42, and 29 cd/m²; the maximum luminance efficiency of the devices are 0.01, 0.02, and 0.01 cd/A. By using host–guest doped system, saturated red electrophosphorescent devices with a maximum luminance efficiency of 1.61 cd/A were obtained when Hb‐TF was used as a host material doped with Os(fptz)₂(PPh₂Me₂)₂ as a guest material. A maximum luminance efficiency of 3.39 cd/A of a red polymer light‐emitting device was also reached when Hb‐BFTBT was used as the guest in the PFO (Poly(9,9‐dioctylfluorene)) host layer. In addition, a series of efficient white devices were, which show low turn‐on voltage (3.5 V) with highest luminance efficiency of 4.98 cd/A, maximum brightness of 1185 cd/m², and the Commission Internationale de l'Eclairage (CIE) coordinates close to ideal white emission (0.33, 0.33), were prepared by using BFBT as auxiliary dopant. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

New host homopolymers containing pendant triphenylamine derivatives: Synthesis,optical, electrochemical properties and its blend with Ir(ppy)3 for green phosphorescent organic light‐emitting devices

Two vinyl homopolymers poly(N‐(4‐(4‐(4‐vinylbenzyloxy)styryl)phenyl)‐N‐phenylbenzenamine) (PVST ) and poly(4‐vinyltriphenylamine) (PTPA ) containing pendant hole‐transporting triphenylamine and 4‐oxystyryltriphenylamine groups, respectively, were synthesized by radical polymerization and employed as hosts for tris(2‐phenylpyridine) iridium [Ir(ppy)₃] phosphor. Structural influences of the hole‐transporting groups upon optoelectronic properties were investigated by photophysical, electrochemical, and electroluminescent methods. The polymers were readily soluble in common organic solvents and their weight‐average molecular weights (M_w) were 5.68 × 10⁴ and 1.90 × 10⁴, respectively. The emission spectra (both photoluminescence, PL and electroluminescent, EL) of the blends [PTPA with 4 wt % Ir(ppy)₃] showed dominant green emission (517 nm) attributed to Ir(ppy)₃ due to efficient energy transfer from PTPA to Ir(ppy)₃. The HOMO levels of PVST and PTPA, estimated from onset oxidation potentials in their cyclic voltammograms, were ?5.14 and ?5.36 eV, which are much higher than ?5.8 eV of the conventional poly(9‐vinylcarbazole) (PVK) host owing to high hole‐affinity of the triphenylamine groups. The optoelectronic performances of phosphorescent EL devices, using PVST and PTPA as hosts and Ir(ppy)₃ as dopant (indium tin oxide, ITO/poly(3,4‐ethylenedioxythiophene): poly(styrene sulfonate) (PEDOT:PSS)/PVST or PTPA:Ir(ppy)₃(4 wt %):PBD(40 wt %)/BCP/Ca/Al), were investigated. The maximum luminance and luminance efficiency of the PTPA device were 9220 cd/m² and 6.1 cd/A, respectively, which were significantly improved relative to those of PVK and PVST. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 7960–7971, 2008     

Novel blue light-emitting hyperbranched polyfluorenes incorporating carbazole kinked structure

A novel series of soluble hyperbranched polyfluorenes P1−P6 with various branching degrees and contents of kinked carbazole units were successfully synthesized with good yields and high molecular weight via a facile “A2 + B2 + C3 + D2” approach. The thermal, optical, and electrochemical properties as well as thermal spectral stability of the resulting hypberbranched polymers were investigated. All polymers exhibited good thermal stabilities and bright blue emission in both solutions and solid-states. Hyperbranched polyfluorenes (P3 and P6) exhibited improved spectral stability upon annealing at 200 °C in air, in sharp contrast to the linear poly(9,9-dihexylfluorene) (PDHF) that showed significant additional green emission at ca. 530 nm within minutes. In particular, outstanding spectral stability was observed with carbazole-incorporating hyperbranched polyfluorene P6. Electrochemical characterization indicated that the presence of carbazole also effectively raised the HOMO level with respect to that of polyfluorene homopolymer, suggesting better hole-injection properties. Hence, the incorporation of kinked carbazole unit into hyperbranched polyfluorenes could provide a new methodology for preparing blue light-emitting polymers with improved optoelectronic characteristics.     

Copolyfluorenes containing pendant bipolar carbazole and 1,2,4‐triazole groups: Synthesis,characterization, and optoelectronic applications

Three novel copolyfluorenes ( P1 ‐ P3 ) containing pendant bipolar groups (2.5–7.7 mol %), directly linked hole‐transporting carbazole and electron‐transporting aromatic 1,2,4‐triazole, were synthesized by the Suzuki coupling reaction and applied to enhance emission efficiency of polymer light‐emitting diodes based on conventional MEH‐PPV. The bipolar groups not only suppress undesirable green emission of polyfluorene under thermal annealing, but also promote electron‐ and hole‐affinity of the resulting copolyfluorenes. Blending the bipolar copolyfluorenes with MEH‐PPV results in significant enhancement of device performance [ITO/PEDOT:PSS/MEH‐PPV+ P1 , P2 or P3 /Ca(50 nm)/Al(100 nm)]. The maximum luminance and luminance efficiency were enhanced from 3230 cd/m² and 0.29 cd/A of MEH‐PPV‐only device to 15,690 cd/m² and 0.81 cd/A (blend device with MEH‐PPV/ P3 = 94/6 containing about 0.46 wt % of pendant bipolar residues), respectively. Our results demonstrate the efficacy of the bipolar copolyfluorenes in enhancing emission efficiency of MEH‐PPV. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Synthesis and optoelectronic properties of luminescent poly(p‐phenylenevinylene) derivatives containing electron‐transporting 1,3,4‐oxadiazole groups

Three random copolymers ( P1–P3 ) comprising phenylenevinylene and electron‐transporting aromatic 1,3,4‐oxadiazole segments (11, 18, 28 mol %, respectively) were prepared by Gilch polymerization to investigate the influence of oxadiazole content on their photophysical, electrochemical, and electroluminescent properties. For comparative study, homopolymer poly[2‐methoxy‐5‐(2′‐ethylhexyloxy)‐1,4‐p‐phenylenevinylene] ( P0 ) was also prepared by the same process. The polymers ( P0–P3 ) are soluble in common organic solvents and thermally stable up to 410 °C under a nitrogen atmosphere. Their optical properties were investigated by absorption and photoluminescence spectroscopy. The optical results reveal that the aromatic 1,3,4‐oxadiazole chromophores in P1–P3 suppress the intermolecular interactions. The HOMO and LUMO levels of these polymers were estimated from their cyclic voltammograms. The HOMO levels of P0–P3 are very similar (?5.02 to ?5.03 eV), whereas their LUMO levels decrease readily with increasing oxadiazole content (?2.7, ?3.08, ?3.11, and ?3.19 eV, respectively). Therefore, the electron affinity of the poly(p‐phenylenevinylene) chain can be gradually enhanced by incorporating 1,3,4‐oxadiazole segments. Among the polymers, P1 (11 mol % 1,3,4‐oxadiazole) shows the best EL performance (maximal luminance: 3490 cd/m², maximal current efficiency: 0.1 cd/A). Further increase in oxadiazole content results in micro‐phase separation that leads to performance deterioration. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 4377–4388, 2007     

Synthesis,characterization, and electroluminescent performance of a novel copolyfluorene containing pendant crown ether groups

Copolyfluorene PFC containing pendant crown ether moieties was prepared by the palladium‐catalyzed Suzuki coupling reaction. The photo‐physical and electrochemical properties were investigated by absorption, photoluminescence (PL) spectroscopy, and cyclic voltammetry to elucidate the influence of the crown ether groups. In film state, its PL spectra (peaked at 430 and 452 nm) show noticeable red‐shift relative to 423 and 448 nm of poly(9,9‐dihexylfluorene) ( PF ). Thermal annealing leads to appearance of new emission at about 520 nm which has been attributed to formation of excimer. The highest occupied molecular orbital and lowest unoccupied molecular orbital levels of PFC were estimated to be ?5.68 and ?2.65 eV which contributed to balanced charges injection. Double‐layer electroluminescent device using PFC as emitting layer (ITO/PEDOT:PSS/ PFC /Ca/Al) revealed maximum luminance (7910 cd/m²) and maximum luminance efficiency (2.3 cd/A) superior to those of PF device (860 cd/m², 0.29 cd/A). Moreover, inserting a PFC layer between the PF emitting layer and calcium cathode led to reduced turn‐on voltage (4.1 V), much lower than 7.1 and 6.6 V of the double‐layer PFC and PF devices, respectively, and enhanced device performance (2800 cd/m² and 0.53 cd/A). © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 2985–2995, 2009     

Synthesis, photophysics and electroluminescence of new vinylene-copolymers with 2,4,6-triphenylpyridine kinked segments along the main chain

Two new vinylene alternating copolymers F and C that contained 2,4,6-triphenylpyridine as a common moiety and fluorene or carbazole, respectively, as an alternating moiety were prepared by Heck coupling. They showed an outstanding thermal stability being stable up to approximately 350 °C and had relatively high glass transition temperatures (140 and 111 °C). The existence of the 2,4,6-triphenylpyridine kinked units along the polymer backbone caused a partial interruption of the π-conjugation. The copolymers emitted blue-green light with emission maximum at 446-464 nm and quantum yields of 0.52 and 0.28 in THF solution. The electrochemical properties of copolymers F and C, including HOMO and LUMO levels, were estimated from their cyclic voltammograms. Their electroluminescence (EL) emission maxima (greater than 500 nm) showed significant red-shifts relative to the PL maxima, which has been explained by the direct cross recombination transition between electrons and holes trapped on carbazole or triphenylpyridine subunits. Moreover, the emission colors transform gradually with increasing bias and approach to white color at about 30 ∼ 35 V. The maximal luminance (maximal luminance efficiency) of the EL devices (ITO/PEDOT:PSS/F or C/Ca/Al) were 647 cd/m² (0.13 cd/A) or 615 cd/m² (0.10 cd/A), respectively.     

Poly(p‐phenylenevinylene) presenting pendent pentaphenylene dendron groups for light‐emitting diodes

We have synthesized, using the Gilch method, a novel poly(p‐phenylenevinylene) derivative (PPV‐PP) containing two pendent pentaphenylene dendritic wedges, and have characterized its structure and properties. The incorporated side chain pentaphenylene dendrons serve as solubilizing groups, prevent π‐stacking interactions from occurring between the polymer main chains, and suppress the formation of excimers in the solid state. Photoluminescence studies indicate that efficient intramolecular energy transfer occurred from the photoexcited pentaphenylene groups to the poly(p‐phenylenevinylene) backbone. The polymer film exhibits a maximum emission at 510 nm and had a photoluminescence efficiency of 46%, which is similar to that measured in dilute solution. The photoluminescence spectra remained almost unchanged after thermal annealing at 150 °C for 20 h, and displayed inhibited excimer formation. Polymer light‐emitting diodes that we fabricated in the configuration ITO/PEDOT/PPV‐PP/Mg:Ag/Ag exhibited a maximum emission peak at 513 nm, corresponding to the green region [x = 0.30 and y = 0.62 in the Commission Internationale de L'Eclairage (CIE) chromaticity coordinates]. The maximum brightness and maximum luminance efficiency were 1562 cd/m² and 1.93 cd/A, respectively. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 5147–5155, 2005     

Self-host yellow iridium dendrimers based on carbazole dendrons: synthesis,characterization and application in solution-processed organic light-emitting diodes

On the basis of different generation carbazole dendrons, a series of self-host yellow Ir dendrimers (Y-G0, Y-G1 and Y-G2) have been successfully synthesized and characterized in detail. It is found that the peripheral dendrons can effectively reduce the intermolecular interactions between emissive Ir cores, as verified by the increased photoluminescence quantum yields and film lifetimes. Among these dendrimers, Y-G2 bearing the second generation dendrons shows the best non-doped device performance, revealing a peak luminous efficiency of 20.2 cd/A. The value is nearly twice that of Y-G0 without any dendrons, which could be further improved to 32.1 cd/A by dispersing Y-G2 into a host matrix. We believe that this work will shed light on the development of highly efficient yellow phosphorescent dendrimers with a self-host strategy.     

Synthesis,characterization, and application of light‐emitting copolyfluorenes slightly doped with distyrylbenzene derivatives

Copolyfluorenes PFG1～PFG4 slightly doped with 0.006–0.5 mol % of 2,5‐dihexyloxy‐1,4‐bis(2‐phenyl‐2‐cyanovinyl)benzene (green chromophore) were synthesized by the Suzuki coupling reaction to be evaluated as hosts for white‐light electroluminescent (EL) devices. Their optical, thermal and electrochemical properties were almost identical to those of polyfluorene ( PF ) due to minimal chromophore content. However, the electroluminescent (EL) spectra of the PFGn were very different from photoluminescence spectra in film state. Relative intensity of green emission (ca. 521 nm) in EL spectra are much stronger than those in PL spectra, which can be attributed to charges trapping in the chromophores due to its narrow band gap (E_g ≈ 2.56 eV). The performance of EL devices [ITO/PEDOT:PSS/polymer/Ca (50 nm)/Al (100 nm)] were improved with an increase in chromophore content. The PFG4 device revealed the best performance (6790 cd/m², 1.69 cd/A), and the PFG1 and PFG2 devices exhibited comparable intensity in blue and green emissions. Blend EL devices were fabricated by using the PFGn as the hosts and a red iridium complex [Ir(piq)₂(acac)] as dopant. By controlling the amount of the iridium complex, the white‐light emitting device was achieved with PFG2 , with maximum brightness and CIE coordinate being 4120 cd/m² and (0.31, 0.28), respectively. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 149–160, 2009     

Copoly(p‐phenylene)s containing bipolar triphenylamine and 1,2,4‐triazole groups: Synthesis,optoelectronic properties,and applications

Two novel copoly(p‐phenylene)s ( P1 – P2 ) containing bipolar groups (12.8 and 6.8 mol %, respectively), directly linked hole transporting triphenylamine and electron transporting aromatic 1,2,4‐triazole, were synthesized to enhance electroluminescence (EL) of poly(p‐phenylene vinylene) (PPV) derivatives. The bipolar groups not only enhance thermal stability but also promote electron affinity and hole affinity of the resulting copoly(p‐phenylene)s. Blending the bipolar copoly‐(p‐phenylene)s ( P1 – P2 ) with PPV derivatives ( d6‐PPV ) as an emitting layer effectively improve the emission efficiency of its electroluminescent devices [indium tin oxide (ITO)/poly(3,4‐ethylenedioxythiophene) (PEDOT):poly(styrenesulfonate) (PSS)/polymer blend/Ca (50 nm)/Al (100 nm)]. The maximum luminance and maximum luminance efficiency were significantly enhanced from 310 cd m^?2 and 0.03 cd A^?1 ( d6‐PPV ‐based device) to 1450 cd m^?2 and 0.20 cd A^?1 (blend device with d6‐PPV / P1 = 96/4 containing ～0.5 wt % of bipolar groups), respectively. Our results demonstrate the efficacy of the copoly(p‐phenylene)s with bipolar groups in enhancing EL of PPV derivatives. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2010