Application of heteroleptic iridium complexes with fluorenyl-modified 1-phenylisoquinoline ligand for high-efficiency red polymer light-emitting devices

A series of new heteroleptic iridium complexes bearing fluorenyl-modified 1-phenylisoquinoline as the first ligand and different ancillary ligands has been prepared and characterized. These complexes bis(1-(3-(9,9-dimethyl-fluoren-2-yl)phenyl)isoquinoline-C2,N′)iridium(III)acetylacetonate(Ir(DMFPQ)₂acac)), bis(1-(3-(9,9-dimethyl-fluoren-2-yl)phenyl)isoquinoline-C2,N′)iridium(III)(3-(pyridin-2-yl)-1,2,4-triazolate)(Ir(DMFPQ)₂pt) and bis(1-(3-(9,9-dimethyl-fluoren-2-yl)phenyl)isoquinoline-C2,N′)iridium(III)(2-(2-pyridyl)benzimidazolate)(Ir(DMFPQ)₂pbi) showed red phosphorescent emissions of 615-630 nm in dichloromethane solution. The device fabricated with these complexes doped into a host polyfluorene (PFO) blend with 30% of an electron transport material 2-(4-biphenyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD) showed high device efficiencies. Ir(DMFPQ)₂acac exhibited red emission with an external quantum efficiency(η_ext) of 14.3% and luminous efficiency(η_c) of 7.8 cd/A at 1.2 mA/cm² and the maximum brightness reached 10 006 cd/m² (Commission Internationale de I’Eclairage(CIE) chromaticity coordinates: (0.67, 0.32)) at 412 mA/cm². Ir(DMFPQ)₂pt showed a η_ext of 13.0% and η_c of 9.2 cd/A at 17 mA/cm², 1532 cd/m², and the maximum brightness reached 15085 cd/m² (CIE: 0.64, 0.34) at 360 mA/cm².     

Synthesis and properties of electrophosphorescent chelating polymers with iridium complexes in the conjugated backbone

The synthesis of electrophosphorescent chelating polymers by Suzuki polycondensation of A-A- and B-B-type monomers is described, in which the fluorene-alt-carbazole (PFCz) segment is used as polymer backbone. By using alkyl-substituted ligands of iridium complex monomers, chelating copolymers with higher contents of iridium complex can be synthesized. Chemical and photophysical characterization confirm that the Ir complex is incorporated into the polymer backbone as one of the monomer repeat units by means of two 5-bromotolylpyridine ligands. Chelating polymers with Ir complexes in the conjugated polymer backbone show highly efficient energy transfer of excitons from the PFCz host segment to the Ir complex by an intramolecular trapping mechanism. The external quantum and luminous efficiencies of a device made with PFCzMppyIrhm4 copolymer reach 4.1 % ph/el (photons/electron) and 5.4 cd A(-1), respectively, at a current density of 32.2 mA cm(-2), an emission peak of 577 nm, and a luminance of 1730 cd cm(-2). Most important, the devices made from the chelating copolymers show no notable efficiency decay with increasing current density due to reduced concentration quenching and triplet-triplet (T-T) annihilation. This indicates that incorporation of the phosphorescent complex into the rigid conjugated polymer main chain is a new way to simultaneously realize high efficiency, long-term stability, and simple processing of phosphorescent polymer light-emitting diodes.     

Highly efficient iridium(III) complexes with diphenylquinoline ligands for organic light-emitting diodes: Synthesis and effect of fluorinated substitutes on electrochemistry, photophysics and electroluminescence

A series of novel cyclometalated iridium(III) complexes bearing 2,4-diphenylquinoline ligands with fluorinated substituent were prepared and characterized by elemental analysis, NMR and mass spectroscopy. The cyclic voltammetry, absorption, emission and electroluminescent properties of these complexes were systematically investigated. Electrochemical studies showed that the oxidation of the fluorinated complexes occurred at more positive potentials (in the range 0.57-0.69 V) than the unfluorinated complex 1 (0.42 V). In view of the energy level, the lowering of the LUMO by fluorination is significantly less than that of the HOMO. The weak and low energies absorption bands in the range of 300-600 nm are well resolved, likely associated with MLCT and ³π-π^∗ transitions. These complexes show strong orange red emission both in the solution and solid state. The emission maxima of the fluorinated complexes showed blue shift by 9, 24 and 15 nm for 2, 3 and 4, respectively, with respect to the unfluorinated analogous 1. Multilayered organic light-emitting diodes (OLEDs) were fabricated by using the complexes as dopant materials. Significantly higher performance and lower turn-on voltage were achieved using the fluorinated complexes as the emitter than that using the unfluorinated counterpart 1 under the same doping level. OLED devices using complexes 2 and 3 as the phosphorescent dopant at 3 wt% doping level exhibit very high performance. To complex 2, the maximum luminance is 16 410 cd/m² at a current density of 210 mA/cm², and the maximum luminance efficiency and power efficiency are 9.34 cd/A and 5.20 lm/W, respectively, with the emission of 605 nm. To complex 3, those data are 16 797 cd/m² at a current density of 211 mA/cm², 11.12 cd/A and 4.97 lm/W, respectively, with the emission of 593 nm.     

Synthesis of an electroluminescent polymer and its non‐doped light‐emitting diodes with stable green emission

An electroluminescent polymer was synthesized by Wittig condensation and characterized by the measurements of ¹H‐NMR, IR, gel permeation chromatography (GPC), UV–Vis, PL, and cyclic voltammetry (CV). The polymer can be dissolved in common organic solvents such as tetrahydrofuran (THF), chloroform, and dichloromethane. The electroluminescent investigation showed that the non‐doped devices with a double‐layer configuration (ITO/PEDOT:PSS/Polymer/Mg:Ag) have a stable green emission property. The maximum luminance of the annealed device reaches 2317 cd/m². The emission maximum and the CIE 1931 coordinate values are respectively stabilized at 552 nm and near (x, y) = (0.43, 0.55) with different voltages. Copyright © 2008 John Wiley & Sons, Ltd.     

Synthesis and characterization of electrophosphorescent jacketed conjugated polymers

A monomer containing bent side chains with oxadiazole unit was synthesized. And it was copolymerized with polyfluorene at different ratios. The photophysical and electrochemical properties of the copolymers were characterized. The results show that the introduction of the oxidiazole‐containing side chains into the polymer reduces the lowest unoccupied molecular orbital level. And the steric hindrance of the side groups can effectively suppress the aggregation of the polymer backbones. Electroluminescent devices were fabricated with a configuration of indium tin oxide (ITO)/poly(3,4‐ethylenedioxythiophene (PEDOT):PSS/Sample/Ca/Al. All of the devices emit blue light. The device of the copolymer PFOXD50 shows the best performance with the maximum luminance of 1033 cd/m² and the maximum current efficiency of 0.29 cd/A. Then a cyclometalated iridium complex monomer (ppy)₂Ir(BrPhPyBr) was copolymerized with PFOXD50 at different ratios. The devices with the same configuration emit orange light. The efficiency generally increases with the increasing Ir content. Among them, the device of the copolymer PFOXDIr7 shows the best performance with the maximum luminance of 846 cd/m² and the maximum current efficiency of 0.61 cd/A. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Synthesis, characterization and photophysics of fluorene-alt-oxadiazole copolymers containing pendent iridium (III) complex moiety

A series of fluorene-alt-oxadiazole copolymers containing a pendent phosphor chromophore of the (piq)₂Ir(pic) complex were synthesized via the palladium-catalyzed Suzuki coupling reaction, where piq is 1-phenylisoquinoline and pic is picolinic acid. These copolymers exhibited a similar absorption spectrum with a peak at about 330 nm and a typical emission peak at 408 nm in CH₂Cl₂ from the fluorene-alt-oxadiazole backbone. However, a significantly red-shifted emission peak at about 625 nm was observed in the neat films of these copolymers, which are attributed to the pendent iridium (III) complex unit. Using these copolymers as single emission layer, the polymer light-emitting devices with a configuration of ITO/PEDOT:PSS/copolymers/LiF/Al exhibited a saturated red emission with a peak at 632 nm. Significant influence of the attached iridium (III) complex ratio on EL performance was presented. A maximum current efficiency of 1.2 cd/A at 63 mA/cm² and a maximum luminance of 1125 cd/m² at 12 V were achieved from the device with the copolymer containing iridium (III) complex in a 3% molar ratio.     

Synthesis of regio- and stereoselective alkoxy-substituted spirobifluorene derivatives for blue light emitting materials

Three new octyloxy substituted spirobifluorenes, 2,7-diphenyl-3′,6′-bis(octyloxy)-9,9′-spirobifluorene (DPBSBF, 1a), 2,7-dibiphenyl-3′,6′-bis(octyloxy)-9,9′-spirobifluorene (DBBSBF, 1b) and 2,7-diterphenyl-3′,6′-bis(octyloxy)-9,9′-spirobifluorene (DTBSBF, 1c) were prepared. All the compounds had been fully characterized by ¹H and ¹³C NMR, UV-Vis, DSC, mass spectrometry and gave satisfactory elemental analyses. They possessed good solubility in common organic solvents and good homogeneous film formation. The optical energy band gap of DBBSBF was 3.27 eV between the HOMO energy level, 5.85 eV, measured by UPS and the LUMO, 2.58 eV, calculated from absorption spectrum. A blue organic light emitting diode (OLED) based on the structure of ITO/TPD (60 nm)/DBBSBF (40 nm)/Alq₃ (20 nm)/LiF (1 nm)/Al (100 nm) showed good performance. The luminance of 3125 cd/m² was observed at a drive voltage of 12.8 V and the colour coordinate in CIE chromaticity was (0.14, 0.12). The external quantum efficiency was obtained to be 2.8% at 100 cd/m².     

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.     

Synthesis and characterization of cyclometalated iridium(III) complexes containing pyrimidine-based ligands

New pyrimidine derivatives (pyr) have been synthesized using palladium-catalyzed Suzuki coupling reaction. These compounds can undergo cyclometalation with iridium trichloride to form bis-cyclometalated iridium complexes, (pyr)₂Ir(acac) (acac = acetylacetonate; pyr = cyclometalated pyr). The substituents at the both cyclometalated phenyl ring and pyrimidine ring were found to affect both electrochemical and photophysical properties of the complexes. Computation results on these complexes are consistent with the electrochemical and photophysical data. The complexes are green-emitting with good solution quantum yields at ∼0.30. Light-emitting devices using these complexes as dopants were fabricated, and the device performance at 100 mA/cm² are moderate: 9 (17 481 cd/m², 4.8%, 18 cd/A, 5.1 lm/W); 10 (18 704 cd/m², 4.9%, 18.9 cd/A, 4.7 lm/W); 13 (20 942 cd/m², 5.4%, 21.0 cd/A, 6.1 lm/W).     

A yellow-emitting iridium complex for use in phosphorescent multiple-emissive-layer white organic light-emitting diodes with high color quality and efficiency

A cyclometalated iridium(III) complex containing 2-(9,9-diethylfluoren-2-yl)pyridine [Ir(Flpy)₃] was prepared and used in the fabrication of both yellow and white organic light-emitting diodes (OLEDs). A hole-blocking material has been used as a hole barrier layer in-between different emission layers, helping the formation of the hole limitation region. With the proper position of a hole barrier layer and the construction of a four-emission-layer structure involving the use of [Ir(Flpy)₃], the resulting WOLED shows sound device performance as well as very stable color even at high luminances. Such WOLEDs have been demonstrated to reveal superior white light color stability/efficiency trade-off optimization. The Commission Internationale de L’Eclairage (CIE) coordinate differences Δx and Δy are confined to ±0.015 when the luminance increases from 13 to 14806 cd/m². The color rendering index (CRI) of the device is also very good, which varies only from 86 to 87 by changing from the normal direction to 80° off-normal at 12 V. The peak electrophosphorescence efficiency can reach as high as 24.6 cd/A at 168 cd/m²and it can still be kept at 17.2 cd/A at 10834 cd/m². Such outstanding performance renders this all-phosphor WOLED very attractive as a white light source for illumination applications, which typically demand high efficiency, high CRI, and stable color in high brightness work conditions.     

Multifunctional zig-zag-shaped D-π-A-π-D emitters with high conjugation extent for blue FOLEDs and host for PHOLEDs

We have synthesized zig-zag shaped, meta- and para-linked D-π-A-π-D blue emitters, m-BTPAPy and p-BTPAPy based on a non-symmetrical connection strategy of two identical π-conjugated groups. The phenanthrimidazole moiety coupled to pyridine via naphthyl spacer by para- and meta-linking modes. Both m-BTPAPy (T_d/T_g, °C: 564/281) and p-BTPAPy (T_d/T_g, °C: 502/246) exhibit excellent thermal stability and can form a stable amorphous film. Changing the connection strategy from para to meta mode, m-BTPAPy shows deep blue emission with CIE (0.15, 0.07). The highly twisted m-BTPAPy exhibit higher Photoluminescence quantum yield (PLQY)_s/f of 0.98/0.85 than p-BTPAPy (0.95/0.80) owing to the suppression of intermolecular stacking. The non-doped blue device (BOLEDs) with multifunctional m-BTPAPy/p-BTPAPy show external quantum efficiency (EQE) of 7.12/5.12% with small roll-off efficiency of 1.68/2.14%, power efficiency (PE) of 5.92/5.42 lm/W, the luminance of 58675/76234 cd/m², and current efficiency (CE) of 6.12/5.86 cd/A. The non-doped device using m-BTPAPy/p-BTPAPy as both emitting and electron-transporting material exhibit luminance of 40671/49539 cd/m², CE of 5.01/5.08 cd/A, PE of 4.68/4.76 lm/W, EQE of 6.12/4.81%, roll-off efficiency of 1.63/1.87%, and CIE (0.15, 0.10)/(0.15, 0.11). These bipolar materials with high triplet energy were employed as hosts in green and red PhOLEDs. The green (m-BTPAPy: Ir(ppy)₃)/red device (m-BTPAPy: Ir(MDQ)₂(acac)) exhibit maximum EQE of 29.85/20.09%, luminance of 79523/42412 cd/m², CE of 78.62/27.56 cd/A, and PE of 72.36/23.86 lm/W, and CIE (0.33, 0.60)/(0.65,0.33).     

Adsorption effects of poly(hydroxybutyric acid) depolymerase on chain-folding surface of polyester single crystals revealed by mutant enzyme and frictional force microscopy

Adsorption effects of poly(hydroxybutyric acid) (PHB) depolymerase from Ralstonia pickettii T1 on various polymer single crystals were studied using a catalytically inactive mutant of PHB depolymerase by means of transmission electron microscopy (TEM), atomic force microscopy (AFM), and frictional force microscopy (FFM). Six types of polymer single crystals, poly[(R)-3-hydroxybutyric acid] (P(3HB)), poly[(R)-3-hydroxybutyric acid-co-6 mol% (R)-3-hydroxyvaleric acid] (P(3HB-co-6 mol% 3HV)), poly[(R)-3-hydroxybutyric acid-co-8 mol% (R)-3-hydroxyhexanoic acid] (P(3HB-co-8 mol% 3HH)), poly(l-lactic acid) (PLLA), poly(d-lactic acid) (PDLA), and polyethylene (PE), were prepared to examine the influence of an ester bond and stereoregularity of a polymer on the enzymatic adsorption. The numbers of PHB depolymerase enzymes adsorbed on P(3HB) and P(3HB-co-6 mol% 3HV) single crystals were determined as 171 and 183 enzymes/μm² by AFM, respectively. AFM observation revealed that the concentration of PHB depolymerase enzymes adsorbed onto PLLA and PDLA single crystals is much higher compared to those on a P(3HB) single crystal, whereas the concentration of enzyme adsorbed onto PE and P(3HB-co-8 mol% 3HH) single crystals is much less. In addition, the single crystals of each polymer were characterized by TEM and FFM before and after enzymatic treatment by mutant for 1 h at 37 °C. The surface properties of P(3HB), P(3HB-co-6 mol% 3HV), and P(3HB-co-8 mol% 3HH) single crystals were changed by the enzymatic adsorption, whereas the internal structures were not affected. On the basis of these results, the properties of the binding domain of PHB depolymerase to polymer chain-folding surfaces have been discussed.     

Synthesis of copolyfluorenes containing green chromophores based on triphenylamine unit and their application in light‐emitting diodes

Two series of new copolyfluorenes ( PFTP, PFTT ) were prepared by the Suzuki coupling reaction from two green‐emitting dibromo monomers (TP‐Br, TT‐Br) based on triphenylamine unit to be applied in white light electroluminescent devices. They were characterized by molecular weight determination, elemental analysis, DSC, TGA, absorption and photoluminescence spectra, and cyclic voltammetry. The estimated actual contents of the TP and TT chromophores were lower than 7.8 mol % and 1.9 mol % for PFTP and PFTT , respectively. In film state both copolyfluorenes showed photoluminescence at 400–470 and 470–600 nm originated from fluorene segments and the chromophores, respectively, due to incomplete energy transfer. Light‐emitting diodes with a structure of ITO/PEDOT:PSS/copolymer/Ca(50 nm)/Al(100 nm) showed major emission at 493–525 nm, plus minor emission at 400–470 nm when chromophore contents were low. The maximum brightness and maximum current efficiency of PFTP2 device were 8370 cd/m² and 1.47 cd/A, whereas those of PFTT1 device were 9440 cd/m² and 1.77 cd/A, respectively. Tri‐wavelength white‐light emission was realized through blending PFTT1 with poly(9,9‐dihexylfluorene) and a red‐emitting iridium complex, in which the maximum brightness and CIE coordinates were 6880 cd/m² and (0.31, 0.33), respectively. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 1553–1566, 2009     

Synthesis and optical properties of fluorene and p-phenylenevinylene copolymers containing non-conjugated spacer

A series of alternating fluorene and p-phenylenevinylene copolymers containing non-conjugated spacer have been synthesized through the Wittig polycondensation reaction. These amorphous copolymers are highly soluble in common organic solvents and can be spin-cast to obtain transparent films. The effects of non-conjugated spacers in the main chain and the methoxyl groups on the side chain on the thermal behavior, photoluminescence (PL) and electroluminescence (EL) properties of these copolymers have been investigated in detail. Single-layered light-emitting diodes (LEDs) have been fabricated in the configuration of ITO/PEDOT/copolymer/Ca/Al and emitted blue light in the range of 456-492 nm. The measurements of current vs voltage show turn-on voltages at 6.2-12.4 V. Among the LEDs based on the six copolymers, the maximum EL brightness and efficiency of the LED based on P1 containing 4CH₂ aliphatic segment length in the main chain and without methoxyl groups on side chain are reached 3936 cd/m² and 0.70 cd/A, respectively.     

Novel cyclopenta[def]phenanthrene based blue emitting oligomers for OLEDs

Novel blue emitters, oligo-MCPPs (tri-MCPP, tetra-MCPP, and penta-MCPP), have been synthesized and characterized. The introduction of cyclopenta[def]phenanthrene (CPP) units into the structure of oligo-MCPPs gave LEDs with high efficiency and pure blue emission. UV-visible absorption spectra of the thin films of these compounds appear at 333-354 nm, and their maximum PL emission at 416-447 nm. Multilayer organic EL devices with oligo-MCPPs as an emitting layer showed the turn-on voltage of about 4.8 V, the maximum brightness of 1076 cd/m² (at 8.2 V), the maximum luminescence efficiency of 0.81 cd/A, and the CIE coordinates of (0.17, 0.14) with blue color.     

Emission color tuning of copolymers containing polyfluorene,benzothiadiazole, porphyrin derivatives

Copolymer containing benzothiadiazole (BT) and porphyrin (POR) derivatives as dopants (<0.3 mol%) was synthesized to polyfluorene (PF) backbone using Suzuki coupling reaction. The synthesized polymer was thermally stable and soluble in general organic solvents. UV–vis spectra of the polymer showed the similar behaviors in solution and on film. However, PL spectra was similar to PF in solution, but its peak increased around 520 and 612 nm as BT and POR, the dopants, went up in casting film. The more POR increased, the more effective Forster energy transfer was observed by POR than BT in PF. The device was made in ITO/PEDOT:PSS/polymer/BaF₂/Ba/Al structure. For PFB02P05 polymer, the luminous efficiency was 0.66 cd/A, the power efficiency 0.29 lm/W and the maximum brightness 936 cd/m². CIE coordinate (0.36, 0.34) was closer to pure white. For PFB15P20, the luminous efficiency was 1.40 cd/A, the power efficiency 0.32 lm/W, the maximum brightness 5997 cd/m². PFB15P20 demonstrated the best performance as green emission.     

Two novel triphenylamine-substituted poly(p-phenylenevinylene) derivatives: synthesis, photo- and electroluminescent properties

Two novel triphenylamine-substituted poly(p-phenylenevinylene) derivatives, P1 and P2, have been successfully synthesized through the Witting-Horner reaction. The structures and properties of the monomers and the resulting polymers were characterized by using ¹H NMR, FT-IR, GPC, TGA, UV-vis absorption spectroscopy, cyclic voltammetry (CV) and electroluminescence (EL) spectroscopy. The obtained polymers exhibited good thermal stability and high photoluminescence quantum yield (0.42-0.90). The polymer light-emitting diodes (PLEDs) with the configuration of ITO/PEDOT/polymers/Ca/Al were fabricated. The single-layer device based on P1 and P2 emitted stable blue and yellow light with the turn-on voltage of 4 and 6 V, respectively. The maximum luminance of 3003 cd/m² at 10 V was obtained for device P2.     

Hyperbranched polymers with aggregation-induced emission property for solution-processed white organic light-emitting diodes

In this work, a new series of hyperbranched polymers of PFTPE-Ir(piq)₃-X(X?=?1, 5, 10) were designed and synthesized, in which tris(1-phenylisoquinoline)iridium(Ш) (Ir(piq)₃) acts as red emission core and PFTPE acts as branches. The photophysical study reveals that these hyperbranched polymers exhibit aggregation-induced emission (AIE) characteristic, inducing in much higher photoluminescent quantum yield (Φ_Y) in neat film than that in dilute tetrahydrofuran (THF) solution. The white-light OLEDs using PFTPE-Ir(piq)₃-X as emission layer show rather weaker efficiency roll-off. Especially, the white-light OLED based on PFTPE-Ir(piq)₃-5 as emission layer shows a maximum luminance of 4686?cd/m², a maximum luminous efficiency of 2.43?cd/A, a maximum external quantum efficiency of 1.08% and the Commission Internationale de l’Eclairage coordinate of (0.26, 0.36).     

Oligofluorene-based push-pull type functional materials for blue light-emitting diodes

A series of new oligofluorene-based push-pull type blue light-emitting functional materials, namely, 2-(9H-carbazole-9-yl)-7-(4-cyanophenyl)-9,9-dihexylfluorene (F1), 7-(9H-carbazol-9-yl)-7′-(4-cyanophenyl)-2,2′-bi(9,9-dihexylfluorene) (F2), 7-(9H-carbazole-9-yl)-7″-(4-cyanophenyl)-2,2′:7′,2″-ter(9,9-dihexylfluorene) (F3), and 7-(9H-carbazole-9-yl)-7″′-(4-cyanophenyl)-2,2′:7′,2″:7″,2″′-quarter(9,9-dihexylfluorene) (F4) were synthesized and characterized. Their onset decomposition temperatures for the thermal bond cleavage and the glass-transition temperatures were in general increased with increasing number of fluorene units. In dilute toluene solution, the oligofluorenes exhibited main absorption peaks in the range of 343-370 nm, photoluminescence maxima from 403 to 410 nm, and absolute quantum yields (Φ_PLs) of higher than 87%. In contrast, the absorption spectra of these compounds in the thin films had no large differences from those in the solutions except for the slight peak red-shifts (2-8 nm). The main emission maxima of F1, F2, and F3 in the thin films were located at 418-420 nm, while the main emission of F4 was found to be shifted to 446 nm, followed by a shoulder peak at 421 nm. The Φ_PLs of these thin films were estimated in the range of 59.2-68.7%. The existence of the electron-pull and -push end groups could effectively tune the energy levels of the oligofluorenes. By using the organic light emitting device (OLED) configuration of ITO/PEDOT:PSS/oligofluorenes/TPBi/LiF/Al by solution-process, F4 displayed the best performance: the lowest turn-on voltage (4.1 V) and highest maximum luminance (2180 cd/m²) with maximal current efficiency of 1.17 cd/A. When F4 was fabricated into the optimized device of ITO/MoO₃/NPB/CBP:F4(1:4)/TPBi/LiF/Al by vapor deposition, highest brightness of 5135 cd/m² and current efficiency of 1.76 cd/A were achieved with the Commission Internationale de l’Eclairage (CIE) coordinates of (0.16, 0.09).     

Synthesis and electroluminescence properties of carbazole-containing 2,6-naphthalene-based conjugated polymers

Three types of carbazole containing 1,5-disubstituted poly(2,6-naphthalene) derivatives, i.e., 2,6-naphthalene homopolymer that has a carbazolyl side chain at 1,5-positions, random copolymers and alternating copolymers consisting of 1,5-dialkoxynaphthalene-2,6-diyl and N-phenylcarbazole-2,7-diyl were newly synthesized by Ni-mediated Yamamoto polycondensation and Pd-catalyzed Suzuki coupling reaction. The number-average molecular weights (M_n) of the polymers and their polydispersity indices (M_w/M_n) were 5.4-8.2 × 10³ and 1.4-1.7, respectively. These polymers exhibited blue photoluminescence in the film states and high fluorescence quantum efficiencies in CHCl₃ (?_fl = 0.70-1.00). The electroluminescence properties of these polymers were investigated by fabricating a PLED device that has a configuration of ITO/PEDOT(PSS)/polymer/CsF/Al. The device fabricated with the random copolymer exhibited highest performances showing a maximum brightness of 8370 cd/m² at 13 V and a maximum efficiency of 2.16 cd/A at 7 V.