Novel yellow phosphorescent iridium complexes containing a carbazole–oxadiazole unit used in polymeric light-emitting diodes

Yellow iridium complexes Ir(PPOHC)₃ and (PPOHC)₂Ir(acac) (PPOHC: 3-(5-(4-(pyridin-2-yl)phenyl)-1,3,4-oxadiazol-2-yl)-9-hexyl-9H-carbazole) were synthesized and characterized. The Ir(PPOHC)₃ complex has good thermal stability with 5% weight-reduction occurring at 370 °C and a glass-transition temperature of 201 °C. A polymeric light-emitting diode using the Ir(PPOHC)₃ complex as a phosphorescent dopant showed a luminance efficiency of 16.4 cd/A and the maximum external quantum efficiency of 6.6% with CIE coordinates of (0.50, 0.49). A white polymeric light-emitting diode was fabricated using Ir(PPOHC)₃ which showed a luminance efficiency of 15.3 cd/A, with CIE coordinates of (0.39, 0.44). These results indicate that the iridium complexes containing a linked carbazole–oxadiazole unit are promising candidates in high-efficiency electroluminescent devices.     

Phosphorescent,green-emitting Ir(III) complexes with carbazolyl-substituted 2-phenylpyridine ligands: Effect of binding mode of the carbazole group on photoluminescence and electrophosphorescence

The ligands, 9-((6-phenylpyridin-3-yl)methyl)-9H-carbazole and 9-(4-(pyridin-2-yl)benzyl)-9H-carbazole were synthesized by attaching a carbazolyl group to the pyridine and phenyl rings of 2-phenylpyridine, respectively. Ir(III) complexes were prepared by a simple procedure and the solubility of the novel complexes was significantly better than that of the conventional, green-emitting conventional fac-tris(2-phenylpyridinato-C²,N)iridium(III). The Ir(III) complexes were used to prepare electrophosphorescent polymer light-emitting devices. The device comprising 10% of fac-tris(2-(4′-((9H-carbazol-9-yl)methyl)phenyl)pyridinato-C²,N)iridium(III) exhibited an external quantum efficiency of 7.88%, luminous efficiency of 23.01 cd/A, and maximum brightness of 32,640 cd/m². The color of the emissions of fac-tris(2-(4′-((9H-carbazol-9-yl)methyl)phenyl)pyridinato-C²,N)iridium(III) was similar to that of conventional fac-tris(2-phenylpyridinato-C²,N)iridium(III). This work shows that integration of a rigid hole-transporting carbazole and phosphorescent complex in one molecule provides a new route to highly efficient, solution-processable complexes for electrophosphorescent applications.     

Electroluminescent Performances of Iridium Complexes with Dibenzo-18-crown-6

Three novel iridium complexes with dibenzo-18-crown-6 substituted 2-penylpyridine (ppy) ligand have been synthesized and characterized. In order to investigate the electroluminescent properties of the resulting iridium complexes, polymer light-emitting diodes (PLEDs) with device structure of ITO/PEDOT:PSS/Emissive Layer/LiF/Al are fabricated using soluble poly(N-vinylcarbazole) (PVK) as the host and the resultant iridium complexes as dopant. Consequently, the PLEDs with G1 as dopant exhibited the highest luminous efficiencies of 13.3 cd A^?1 and the maximal brightness of 13523 cd m^?2 at the doping concentration of 8 wt%. Moreover, the iridium complexes G1, G2 and G3 exhibited nearly identical Commission Internationale de L’Eclairage (CIE) coordinates of (0.34?±?0.1, 0.62?±?0.1), which are very close to the CIE coordinates of (0.33, 0.61) for Ir(ppy)₃. This indicates that the CIE coordinates of the iridium complexes would not be influenced as the dibenzo-18-crown-6 groups pended on the meta-position of benzene ring of ppy.     

D-A-based polyfluorene derivatives end-capped with cyclometalated iridium complexes by unconjugated linkage: Structure-property relationships

A series of donor(D)-acceptor(A)-based polyfluorene derivatives, which contain carrier-transporting units of carbazole and oxadiazole as the substitutes of the C-9 position of ?uorene and are end-capped with the red-emitting iridium bi(phenylisoquilonato)(picolinato) unit by unconjugated linkage, were synthesized and characterized. The molar ratios between the donor of carbazole and the acceptor of oxadiazole moieties were found to significantly influence photoluminescent efficiency, electrochemical and electroluminescent properties of these D-A-based polyfluorene derivatives. While the ratio increased to 3:7, the PFCz3OXD7Ir showed the best device performance in the polymer light-emitting device with a configuration of ITO/PEDOT/polymers/LiF/Al. A turn-on voltage of 6.0 V, a maximum current efficiency of 0.59 cd/A and the highest luminance of 917 cd/m² were presented.     

Deep-Blue Phosphorescent Emitters with Phosphoryl Groups for Organic Light-Emitting Diodes by Solution Processes

The new deep-blue iridium complex, Ir(dppfm)₂pic, consisting of a phosphoryl group at the 3′-position of the phenyl ring and a methyl group at the 4-position of pyridine, was synthesized and characterized for applications in organic light-emitting diodes (OLEDs). Ir(dppfm)₂pic exhibited an emission peak of 455 nm and high photoluminescence quantum yields of 73 %. Phosphorescent OLEDs based on Ir(dppfm)₂pic exhibited a maximum external quantum efficiency of 7.8 % and Commission Internationale de l’Eclairage (CIE) coordinates of (0.15, 0.22).     

Thermally cross-linkable hyperbranched polymers containing triphenylamine moieties: Synthesis, curing and application in light-emitting diodes

This paper demonstrates synthesis of hyperbranched polymers (HTP and HTPOCH₃), containing triphenylamine moieties in main chain and thermally cross-linkable periphery or terminal vinyl groups, and application as hole-transporting layer (HTL) in multilayer light-emitting diodes. Absorption and photoluminescence (PL) spectroscopy, cyclic voltammetry (CV) and differential scanning calorimetry (DSC) were employed to investigate their photophysical, electrochemical properties and thermal curing behaviors, respectively. The hyperbranched HTP and HTPOCH₃ were readily cross-linked by heating scan, with the exothermic peaks being at 221 and 210 °C respectively. The glass-transition temperatures (T_g) of the hyperbranched polymers were higher than 140 °C after thermal cross-linking at 210 °C for 30 min. Multilayer light-emitting diodes (ITO/PEDOT:PSS/HTL/MEH-PPV/Ca/Al), using HTP and HTPOCH₃ as HTL, were readily fabricated by successive spin-coating. The performance of MEH-PPV device (maximum luminance: 9310 cd/m², luminance efficiency: 0.26 cd/A) was effectively enhanced by inserting the thermally cross-linked HTP or HTPOCH₃ as HTL (HTP: 12610 cd/m², 0.32 cd/A; HTPOCH₃: 14060 cd/m², 0.33 cd/A). This indicates that these thermally cross-linkable hyperbranched HTP and HTPOCH₃ are very suitable for the fabrication of multilayer PLEDs using solution processes.     

The synthesis,characterization and electroluminescent properties of zinc(II) complexes for single-layer organic light-emitting diodes

(E)-2-(2-(9-p-tolyl-9H-carbazol-3-yl)vinyl)-8-hydroxyl-quinoline, (E)-2-(2-(9-(4-methoxyphenyl)-9H-carbazol-3-yl)vinyl)-8-hydroxyquinoline and their respective zinc(II) complexes were synthesized and their structures confirmed using UV–vis, FT-IR, ESI-MS, FAB-MS, ¹H NMR and elemental analysis. The two Zn(II) complexes displayed high thermal stability with thermal decomposition temperatures of 422 °C and 410 °C, respectively. Photoluminescence spectra revealed that the complexes possessed maximum emissions at 575 and 570 nm, respectively, in the green region. Single-layer, organic light-emitting diodes built using these complexes emitted yellow light, as a result of a red-shift, with maximum luminances of 489 cd/m² and 402 cd/m² as well as luminance efficiencies of 0.41 cd A^?1 and 1.81 cd A^?1, respectively.     

New tetraphenylethylene-containing conjugated polymers: Facile synthesis,aggregation-induced emission enhanced characteristics and application as explosive chemsensors and PLEDs

In this paper, tetraphenylethylene (TPE) units, one of the typical aggregation-induced emission (AIE) moieties, are utilized to construct a new functional polyfluorene (PF) P1, which exhibited the exciting property of the aggregation-induced emission enhancement (AIEE), instead of the aggregation-caused quenching (ACQ) of normal PFs, and could probe the explosive with high sensitivity both in the nanoparticles and solid state. Three other TPE-containing polymers, P2–P4, were also successfully prepared, and demonstrated good performance as explosive chemosensors and light-emitting materials. P3, bearing carbazole as hole-transporting units showed the best performance with a maximum luminance efficiency of 1.17 cd/A and a maximum brightness of 3609 cd/m² at 12.9 V in its light-emitting diode device.     

Novel carbazole/pyridine-based host material for solution-processed blue phosphorescent organic light-emitting devices

3,6-Bis(3,5-di(pyridin-3-yl)phenyl)-9-phenyl-9H-carbazole, a novel host material for solution-processed blue phosphorescent organic light-emitting devices was synthesized by a Suzuki coupling reaction. The optical, electrochemical and thermal properties of this novel crabazole have been characterized. The compound exhibits a high glass-transition temperature (Tg = 161 °C) and high triplet energy (E_T = 2.76 eV). Additionally, atomic force microscopy measurements indicate that high-quality amorphous films of this novel compound can be prepared by spin-coating. Solution-processed blue phosphorescent organic light-emitting devices were obtained using the carbazole as the host material for the phosphorescence emitter iridium(III) bis(4,6-difluorophenylpyridinato)- picolinate and their electroluminescence properties were evaluated.     

Novel white-light-emitting polyfluorenes with benzothiadiazole and Ir complex on the backbone

Novel white-emitting polyfluorenes were synthesized by mixing fluorescence and phosphorescence emission. Benzothiadiazole(BT) and iridium(III)bis(2-(1-naphthalene)pyridine-C^2′,N)-2,2,6,6-tetramethyl-3,5-heptanedione[(1-npy)₂Ir(tmd)] units were incorporated into polyfluorene backbone as green and red chromophores by Suzuki polycondensation. The device from PFG03-IrR07 shows a maximum luminous efficiency (LE) of 5.3 cd/A, a maximum luminance of 9900 cd/m² at a current density of 453 mA/cm² and a CIE coordinate of (0.32, 0.34) with the configuration: ITO/PEDOT:PSS/PVK/emissive layer/CsF/Al. Besides, the EL efficiencies decline slightly with increasing the current density. All emissions located very close to the equi-energy white point (0.33, 0.33) when applied voltage change from 9 to 14 V. Furthermore, the white emission of devices based on these materials shows very good color quality, with high color rendering index range between 84 and 89. Our results indicate that, by incorporation of singlet and triplet species into polymer backbone, the obtained white-emitting materials and devices are promising candidates for display and solid-state-lighting purpose.     

Supramolecular Phosphorescent Polymer Based on Cationic Iridium Complexes for Polymer Light-Emitting Diodes

Three novel orange emission supramolecular phosphorescent polymers (SPPs) with cationic iridium complex have been developed for polymer light-emitting diodes (PLEDs) through efficient self-assembly. The supramolecular assembly process was monitored by ¹H nuclear magnetic resonance (¹H NMR) and viscosity measurement. These SPPs give orange phosphorescence with a peak at about 595 nm and display good thermal properties with a glass-transition temperature (T_g) about 90 °C. The single-emissive-layer PLEDs with charged SPPs exhibited the highest device efficiency of 2.81 cd A^?1 with the Commission Internationale de L’Eclairage coordinates of (0.58, 0.40). The present work reported the charged SPPs self-assembled by the cationic iridium complex for the first time and provided a new guide to develop orange emitters for solution-processable optoelectronic devices.

     

Efficient white polymer-light-emitting-diodes based on polyfluorene end-capped with yellowish-green fluorescent dye and blended with a red phosphorescent iridium complex

A novel series of blue and yellowish-green light-emitting single polymers were prepared by end-capping of low contents of 4-bromo-7H-benzo [de]naphtha[2′,3′:4,5]imidazo[2,1-a]isoquinolin-7-one (M1) into polyfluorene. Electroluminescence (EL) spectra of these polymers exhibit blue emission (λ_max = 430/460 nm) from the fluorene segments and yellowish-green emission (λ_max = 510/530 nm) from the M1 units. For the polymer (PFNAP-0.06) with the M1 unit content of 0.06 mol-%, its EL spectrum shows balanced intensities of blue emission and yellowish-green emission with Commission Internationale de l'Eclairage (CIE) coordinates of (0.25, 0.34). The maximum brightness of the device prepared from the polymer (PFNAP-0.06) is 6704 cd/m² at 10 V with a structure of indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene):poly(styrene sulfonic acid) [PEDOT:PSS]/PVK/emission layer/Ca/Ag. A new white polymer-light-emitting-diode (WPLED) can be developed from the single polymer (PFNAP-0.06) system blended with a red phosphorescent iridium complex [Bis(2-[2′-benzothienyl)-pyridinato-N,C^3′] iridium (acetylacetonate) (BtpIr)]. We were able to obtain a white-light-emission device by adjusting the molar ratio of BtpIr to PFNAP-0.06 with a structure of indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene):poly(styrene sulfonic acid) [PEDOT:PSS]/PVK/emission layer/Ca/Ag. The brightness in such a device configuration is 4030 cd/m² at 9 V with CIE coordinates of (0.32, 0.34).     

Four new cyclometalated phenylisoquinoline-based Ir(III) complexes: Syntheses,structures, properties and DFT calculations

Four new cyclometalated phenylisoquinoline-based iridium(III) complexes [Ir(C^∧N)₂(bipy)]PF₆ (5a–5d) (bipy = 2,2′-bipyridine) have been synthesized and fully characterized, where the C^∧N ligands are 1-(4-(trifluoromethyl)phenyl)isoquinoline, 4-(isoquinolin-1-yl)benzaldehyde, 4-(isoquinolin-1-yl)benzonitrile and 1-(3-fluoro-4-methylphenyl)isoquinoline, respectively. The crystal structures of 5a and 5c have been determined. The photophysical and electrochemical properties of these new complexes 5a–5d have been studied. All Ir(III) complexes exhibit orange phosphorescence in dichloromethane solution at room temperature with a maximum at 593–618 nm, quantum yield of 0.046–0.16. The frontier molecular orbital diagrams and the lowest-energy electronic transitions of 5a–5d have been calculated with density functional theory (DFT) and time-dependent DFT (TD-DFT).     

Synthesis and optoelectronic properties of thermally cross-linkable hole-transporting poly(fluorene-co-triphenylamine)

This paper describes the synthesis of a new thermally cross-linkable hole-transporting poly(fluorene-co-triphenylamine) (PFTV) by Suzuki coupling reaction and its application in polymer light-emitting diodes (PLEDs). The characteristics of PFTV were analyzed by ¹H NMR, differential scanning calorimetry, optical spectroscopy, cyclic voltammetry, and atomic force microscopy. Its HOMO level lies between those of PEDOT:PSS and poly(9,9-dioctylfluorene), forming a stepwise energy ladder to facilitate hole-injection. Multilayer device with thermally cross-linked PFTV as hole-transporting layer (ITO/PEDOT:PSS/HTL/PFO/LiF/Ca/Al) was readily fabricated by successive spin-coating processes, its maximum luminance efficiency (2.27 cd/A) was significantly higher than that without PFTV layer (0.50 cd/A). In addition, the PFTV was successfully applied as host for red-emitting Ir(piq)₂acac to obtain a device with moderate performance (5300 cd/m² and 2.64 cd/A). The PFTV is a promising hole-transporting material for the fabrication of multilayer PLEDs by wet processes as well as a potential host for phosphorescent PLEDs.     

A highly selective turn-on sensor for Hg2 + based on a phosphorescent iridium (III) complex

A highly selective phosphorescent chemosensor for Hg^2 + based on the iridium (III) complex Ir(DTBT)₂(acac) (DTBT = 2-(5-(1,2 dihydroacenaphthylen-5-yl)thiophen-2-yl)benzothiazole, acac = acetylacetone) was synthesized and characterized. Ir(DTBT)₂(acac) exhibited relatively weak fluorescenceat at about 700 nm. Ir(DTBT)₂(acac) displayed a dramatic color change from near-infrared to yellow-green with the addition of Hg^2 +. More significantly, the chemosensor performed “turn-on” phosphorescent responses toward Hg^2 +.     

Synthesis, crystal structure and photophysical properties of a series of new neutral iridium(III) complexes with 2-phenylpyridine

A series of new neutral iridium(III) complexes containing strong-field ancillary ligands, [Ir(ppy)₂(PPh₃)L] (ppy = 2-phenylpyridine, PPh₃ = triphenylphosphine, L = NCS⁻, 1; , 2; NCO⁻, 3), have been synthesized and fully characterized by ¹H NMR, IR, ESI mass spectral and elemental analysis. The crystal structure of 1 has been determined by X-ray analysis. The photoluminescence (PL) spectra of 1–3 show emission maxima at 477, 489 and 485 nm, respectively, corresponding to blue light-emitting of 1 and blue-green light-emitting of 2 and 3. PL quantum yields (PLQYs) of 1–3 are 0.39, 0.13 and 0.43, respectively.     

High-performance poly(2,3-diphenyl-1,4-phenylene vinylene)-based polymer light-emitting diodes by blade coating method

A new series of super high brightness and luminance efficient poly(2,3-diphenyl-1,4-phenylene vinylene) (DP-PPV)-based electroluminescent (EL) polymers containing methoxy or long branched alkoxy chains were synthesized via Gilch polymerization. The branched alkoxy groups were introduced to enhance solubility for blade and spin-coating processes. Monomers of DMeO-PPV and m-Ph-PPV were used to increase steric hindrance and prevent close packing of the main chain. By controlling the feeding ratio of different monomers during polymerization, DP-PPV derivatives with high molecular weight were obtained. All synthesized polymers possess high glass transition temperatures and thermal stabilities. The maximum photoluminescent emissions of the thin films are located between 544 and 547 nm. Cyclic voltammetry analysis reveals that the band gaps of these light-emitting materials are in the range of 2.75-2.84 eV. Blade coating was used to fabricate multilayer polymer light-emitting diodes. A multilayer electroluminescent device with the configuration of ITO/PEDOT:PSS/TFB/P1/TPBi/LiF/Al exhibited a very high luminescence efficiency (10.96 cd A⁻¹). The maximum brightness of the multilayer EL device ITO/PEDOT:PSS/TFB/P3/CsF/Al reached up to 78,050 cd m⁻² with a low turn-on voltage (4.0 V). For further investigation, polymer P3 was blended with DPPFBNA to achieve white light-emitting device; the multilayer devices generated a maximum brightness of 1085 cd m⁻² and a luminance efficiency of 0.75 cd A⁻¹, with CIE coordinates (0.28, 0.33) at 11 V.     

New multinuclear europium(III) complexes as phosphors applied in fabrication of near UV-based light-emitting diodes

Two new multinuclear europium(III) complexes, [Eu(AFTFBD)₃]₂(1,4-bmb) and [Eu(AFTFBD)₃]₃(tmb) were designed and synthesized, where HAFTFBD was 2-acetylfluorene-4,4,4-trifluorobutane-1,3-dione, 1,4-bmb was 1,4-bis[2-(2′-pyridyl)benzimidazolyl]benzene and tmb was 1,3,5-tris[2-(2′-pyridyl)benzimidazolyl]benzene. The complexes were characterized by IR, UV–visible, photoluminescence (PL) spectroscopy and thermogravimetric analysis (TGA). The complexes emit the characteristic red luminescence of Eu³⁺ ion due to the ⁵D₀–⁷F_J (J = 0–4) transitions under 395 nm-light excitation. Bright red light-emitting diodes were fabricated by coating the selected complex (Eu(AFTFBD)₃)₃(tmb) onto ～395 nm-emitting InGaN chips, and the LEDs show appropriate CIE chromaticity coordinates in red area. All these results suggest that the Eu(III) complexes are promising candidates as red component for application in fabrication of near UV-based white light-emitting diodes.     

Synthesis and characterization of ortho-twisted asymmetric anthracene derivatives for blue organic light emitting diodes (OLEDs)

New ortho-twisted asymmetric anthracene derivatives have been synthesized and characterized. The anthracene derivatives show good thermal stability with high glass transition temperatures and a pure blue emission with a narrow full width at half maximum in a film state (λ_max = 454 nm with 71 nm for 2-(2-methylnaphtathalene-1-yl)-9,10-di(naphthalene-2-yl)anthracene and λ_max = 445 nm with 60 nm for 2-(biphenyl-2-yl)-9,10-di(naphthalene-2-yl)anthracene). A multi-layered device using 2-(2-methylnaphtathalene-1-yl)-9,10-di(naphthalene-2-yl)anthracene as an emitting material exhibits a maximum quantum efficiency of 3.61% (power efficiency of 2.15 lm/W, current efficiency of 3.55 cd/A) and blue Commission Internationale de l’Eclairage chromaticity coordinates (x = 0.15, y = 0.13). A fabricated device using 2-(biphenyl-2-yl)-9,10-di(naphthalene-2-yl)anthracene as an emitting material exhibits a maximum quantum efficiency of 3.7% (power efficiency of 2.11 lm/W, current efficiency of 3.55 cd/A) and a blue Commission Internationale de l’Eclairage chromaticity coordinates (x = 0.15, y = 0.12).     

Single-layer white polymer light-emitting diodes based on an iridium (III) complex containing alkyltrifluorene picolinic acid

To explore the relationship between the electronic properties of a host/dopant system and obtain a high-efficiency single-dopant white polymer light-emitting device two novel blue-emitting cyclometalated iridium (III) complexes of (dfppy)₂Ir(Tfl-pic) and (dfppy)₂Ir(Brfl-pic) have been synthesized and characterized, where dfppy is 2-(2,4-difluorophenyl)pyridine, Tfl-pic and Brfl-pic are picolinic acid derivatives containing trialkylfluorene and dibromoalkylfluorene units bridged with an alkoxy chain, respectively. Both iridium (III) complexes exhibited blue emission in dichloromethane solution and their neat films, and possessed good dispersibility and thermal properties. Two different devices using (dfppy)₂Ir(Tfl-pic) as a single component emitter and a blend of poly(N-vinylcarbazole) and 2-(4-biphenyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole as the host matrix were fabricated. Improved white emission was obtained by adjusting the electron injection layer leading to efficient exciplex emission.