Synthesis and photo physical study of iridium complex of new pentafluorophenyl-substituted ligands

New iridium complexes, [Ir(dpq)₂(acac), Ir(PF-dpq)₂(acac) and Ir(PF-dpq-5F)₂(acac)] (dpq = 2,4-diphenylquinoline, dpq-5F = 2-(3′-fluorophenyl)-4-phenylquinoline), PF-dpq-5F = 2-(3-fluoro-phenyl)-6-pentafluorophenyl-4-phenylquinoline and acac = acetylacetonate) have been synthesized and characterized for efficient red organic light-emitting diodes (OLEDs). In order to improve the luminescence efficiency by preventing self-quenching and to tune photoluminescence (PL) and electroluminescence (EL) spectra to a longer wavelength, dpq ligand was fluorinated by -PF and -F moieties. However, the iridium complex of PF-dpq-5F underwent a weak MLCT transition because of the weak coupling between the 5d orbital of the iridium atom and HOMO of the substituted ligand. Thus, the maximum luminous efficiencies of the device using Ir(dpq)2(acac), Ir(PF-dpq)₂(acac) and Ir(PF-dpq-5F)₂(acac) are 4.36 cd/A, 6.04 cd/A and 4.35 cd/A, respectively.     

Introduction of the benzoquinoline ancillary ligand to the iridium complexes for organic light-emitting diodes

The new iridium complexes, Ir(C^N)2(bq), (C^N = ppy, F2-ppy, 2,3-dpqx-F2 or 4-Me-2,3-dpq) were prepared and their luminescence properties were investigated, where ppy, F2-ppy, 2,3-dpqx-F2, 4-Me-2,3-dpq and bq represent 2-phenylpyridine, 2-(4',6'-difluorophenyl)-pyridine, 2,3-bis (4'-fluorophenyl)quinoxaline, 4-methyl-2,3-diphenylquinoline and 10-hydroxybenzoquinoline ligands, respectively. We expected that the relative energy levels of the main ligands (C^N) and ancillary ligand, bq, in the complexes could determine the possibility of interligand energy transfer (ILET) in the complexes and thereby luminescence properties. The main ligands, F2-ppy and 2,3-dpqx-F2, which have drastically different energy gaps between the HOMO and LUMO energy levels were chosen and their complexes were synthesized. The photoabsorption, photoluminescence and electroluminescence of the complexes were studied. Ir(ppy)2(bq), Ir(F2-ppy)2(bq) Ir(2,3-dpqx-F2)2(bq) and Ir(4-Me-2,3-dpq)2(bq) exhibited the luminescence maxima between 600-694 nm and their efficiencies were affected by the main ligands. While Ir(ppy)2(bq) and Ir(F2-ppy)2(bq) showed relatively high luminous efficiencies (> 10 cd/A), Ir(2,3-dpqx-F2)(bq) had poor luminous efficiency (0.30 cd/A). The electrochemical properties were studied to support ILET in the ppy-based iridium complexes. Their luminescence performances were compared with those of the complexes containing acetylacetonate (acac) ancillary ligand which are not allowed to have ILET.     

Study on energy relation between blue and red emissive layer of organic light-emitting diodes by inserting spacer layer

In this paper, energy relation between blue emissive layer (blue-EML) and red emissive layer (red-EML) in organic light-emitting diodes based on blue-emitting and red-emitting phosphorescent dopants, bis(3,5-difluoro-2-(2-pyridyl)phenyl-(2-carboxypyridyl) iridium III (Firpic) and bis(2-(2′-benzo[4,5-α]thienyl)pyridinato-N,C3′)iridium(acetylacetonate) (Btp₂Ir(acac)), was studied. Two phosphorescent dopants, Firpic and Btp₂Ir(acac), were co-doped in the single emissive layer, and the results exhibit complete energy transfer from Firpic to Btp₂Ir(acac). Then, Firpic and Btp₂Ir(acac) were doped into blue-EML and red-EML, separately. By inserting 4,4′-bis(N-carbazolyl)biphenyl (CBP) spacer between blue- and red-EML, energy relation between blue- and red-EML was researched. The results of this work reveal that, CBP spacer may strengthen energy transfer between blue- and red-EML. The reason is that CBP triplets at blue-/red-EML interface can transfer their energies to both CBP molecules of red-EML and Firpic molecules of blue-EML in spacer-without devices, while CBP triplets in the spacer can transfer their energies only to CBP molecules of red-EML. Therefore, energy flow from blue- to red-EML is strengthened because of the avoidance of energy transfer from CBP triplets in the spacer to Firpic molecules of blue-EML, leading to the relative enhancement of red emission in CBP-spacer devices.     

Efficient phosphorescent white organic light-emitting diodes using Ir(4-Me-2,3-dpq)2(przl-C6H4F) as a red emitter

We demonstrated white organic light-emitting diodes (WOLED) using the iridium bis(4-methyl-2,3-phenylquinolinato-N,C2) fluorophenylpyrazolonate complex (Ir(4-Me-2,3-dpq)2(przl-C6H4F)) as a phosphorescent red dopant and iridium bis[(4,6-difluorophenyl)-pyridinato-N,C2] picolinate (Flrpic) as a phosphorescent blue dopant. The WOLED with Ir(4-Me-2,3-dpq)2(przl-C6H4F) had better exciton confinement in emitting layer and indicated smaller movement of exciton than the WOLED with iridium bis(2-phenylquinoline) acetylacetonate (Ir(2-pq)2(acac)) as phosphorescent red dopant. The optimized WOLED had a peak external quantum efficiency of 7.16%, current efficiency of 11.84 cd/A, and Commission Internationale de l'Eclairage (CIE(x,y)) coordinates of (0.35, 0.32). The WOLED also exhibited the minimal change with delta CIE(x,y) coordinates of +/- (0.01, 0.00) from 100 to 4000 cd/m2.     

Fill factor enhancement of organic solar cells based on a wide bandgap phosphorescent material and C60

Planar heterojunction organic solar cells using wide bandgap phosphorescent material bis[2-(4-tertbutylphenyl)benzothiazolato-N,C^2'] iridium (acetylacetonate) [(t-bt)₂Ir(acac)] as electron donor and fullerene (C₆₀) as electron acceptor were fabricated. A large open circuit voltage of 0.94 V was achieved due to low highest occupied molecular orbital level of (t-bt)₂Ir(acac). The effect of different hole transport layers and substrate heating were investigated to improve fill factor. It is shown that the open circuit voltage is strongly influenced by the interface energy barrier, whereas the fill factor is mainly limited by the charge carrier transport properties in active materials. The fill factor was significantly improved by either using hole transport layer with high carrier mobility or increasing the hole mobility of (t-bt)₂Ir(acac). A power conversion efficiency of 2.10% under AM 1.5 solar illumination at an intensity of 100 mW/cm² was achieved by heating the substrate during the deposition of active materials.     

Efficient red electrophosphorescent organic light-emitting diodes based on the new sensitized heteroleptic tris-cyclometalated Ir(III) complexes

Organic light-emitting device (OLED) was fabricated using the novel red phosphorescent heteroleptic tris-cyclometalated iridium complex, bis(2-phenylpyridine)iridium(III)[2(5′-methylphenyl)-4-diphenylquinoline] [Ir(ppy)₂(dpq-5CH₃)], based on 2-phenylpyridine (ppy) and 2(5′-methylphenyl)-4-diphenylquinoline (dpq-5CH₃) ligand. Generally, the ppy ligand in heteroleptic iridium complexes plays an important role as “sensitizer” in the efficient energy transfer from the host (CBP; 4,4,N,N′-dicarbazolebiphenyl) to the luminescent ligand (dpq-5CH₃). We demonstrated that high efficiency through the “sensitizer” can be obtained, when the T₁ of the emitting ligand is close to T₁ of the sensitizing ligand. The device containing Ir(ppy)₂(dpq-5CH₃) produced red light emission of 614 nm with maximum luminescence efficiency and power efficiency of 8.29 cd/A (at 0.09 mA/cm²) and 5.79 lm/W (at 0.09 mA/cm²), respectively.     

Efficient blue-green organic light-emitting diodes based on heteroleptic tris-cyclometalated iridium(III) complexes

The blue-green organic light-emitting diodes based on heteroleptic tris-cyclometalated iridium(III) complexes containing the F₂-ppy (2,4-difluorophenylpyridine) and ppy (2-phenylpyridine) ligands were fabricated. Ir(ppy)₃ has been known to have a high phosphorescence efficiency in electroluminescence owing to its strong metal-to-ligand-charge transfer (MLCT) excited state, whereas the luminous efficiency of Ir(F₂-ppy)₃ was found to be low due to weak MLCT. Herein, we report two heteroleptic phosphorescent blue-green emitters, Ir(ppy)₂(F₂-ppy) and Ir(ppy)(F₂-ppy)₂, that exhibit emission peaks at 502 nm and 495 nm, respectively. The maximum luminous efficiencies of the devices with Ir(ppy)₂(F₂-ppy) and Ir(ppy)(F₂-ppy)₂ were 8.93 cd/A and 13.80 cd/A, respectively. The quantum efficiency of the device containing Ir(ppy)(F₂-ppy)₂ was 3.63% at J = 10 mA/cm².     

Efficient red-emitting phosphorescent iridium(III) complexes of fluorinated 2,4-diphenylquinolines

Ir(β) complexes of fluorinated dpqs(dpq-3-F, dpq-4-CF₃) as a cyclometallated ligand were prepared and their photonic properties were investigated, where dpq-3-F and dpq-4-CF₃ represent 2-(3-fluoro-phenyl)-4-phenylquinoline and 4-phenyl-2-(4-trifluoromethylphenyl)quinoline, respectively. Fluorinated dpq derivatives were introduced to the iridium complexes to increase the efficiency compared to Ir(dpq)₂(acac) which was recently reported to have emission wavelength of 614 nm with quantum efficiency of 0.14. These fluorinated ligands and their Ir(III) complexes were computationally calculated by ab initio methods to support our experimental results. It was found that the Ir complex containing dpq-3-F ligands exhibits the largest emission efficiency with maximum emission peak at 593.5 nm. The result of ab initio calculation using the time-dependent density functional theory (TD-DFT) showed that the strong ³MLCT transition of the complex occurs due to the strong coupling between the 5d orbital of the Ir atom and the highest occupied molecular orbitals (HOMOs) of these ligands.     

High efficiency red organic light-emitting diodes using a phosphorescent iridium complex doped into a hole-blocking material

We demonstrate high-efficiency red electrophosphorescent organic light-emitting devices (OLEDs) by doping a red-emitting iridium complex, Bis[7-methyl-1-p- tolyisoquinolinato-N,C²′]-iridium(III)(acetylacetonate) [(7-mtiq)₂Ir(acac)], into a hole-blocking material, 4-biphenyloxolato aluminum(III)bis(2-methyl-8- quinolinato)4-phenylphenolate. Both the phosphorescent characteristics of (7-mtiq)₂Ir(acac) and the electroluminescence mechanisms of OLEDs are investigated in this study. The Commission Internationale de L'Eclairage coordinates of (0.66, 0.34) is very close to the National Television System Committee standard red point (0.66, 0.33). With a dopant concentration of about 4%, a maximum luminance of 31317 cd/m² and a luminous efficiency of 21.6 cd/A have been obtained.     

Synthesis and phosphorescent properties of the copolymers of N-vinylcarbazole,methyl methacrylate and iridium complex

The copolymers containing carbazole unit and iridium complexes, such as (Ir(bpy)₂Cl, Ir(mbpy)₂Cl and Ir(Brbpy)₂Cl, were synthesized via radical copolymerization of N-vinylcarbazole, methyl methacrylate and iridium complex. The synthesized copolymers were characterized by FT-IR, UV-Vis absorption spectroscopy and photoluminescence (PL) spectroscopy, respectively. According to the results, the copolymers (Ir(Brbpy)₂Cl/PVK and Ir(mbpy)₂Cl/PVK) exhibit yellow phosphorescence with an emission peak at around 553 nm under UV-visible light in the solid state. The results also reveal almost complete energy transfer from the host carbazole segments to the guest Ir complex in the copolymer film when the Ir content reaches 1.0 wt.%. The synthesized copolymers are good candidates as blue or yellow phosphorescent materials for PLED applications.     

Heteroleptic tris-cyclometalated iridium(III) complexes with phenylpridine and diphenylquinoline derivative ligands

The synthesis and photophysical study of efficient phosphorescent heteroleptic tris-cyclometalated iridium(III) complexes having two different (C^N) ligands are reported. In order to improve the luminescence efficiency by avoiding triplet-triplet (T-T) annihilation, new heteroleptic tris-cyclometalated iridium complexes, Ir(ppy)₂(dpq), Ir(ppy)₂(dpq-3-F) and Ir(ppy)₂(dpq-CF₃), are designed and prepared where ppy, dpq, dpq-3-F and dpq-CF₃ represent 2-phenylpyridine, 2,4-diphenylquinoline, 2-(3-fluorophenyl)-4-phenylquinoline, and 4-phenyl-2-(4-(trifluoromethyl)phenyl)quinoline, respectively. Ppy ligands and dpq derivatives can act as a source of energy supply. When new heteroleptic tris-cyclometalated iridium complex, Ir(ppy)₂(dpq-3-F) is placed in the lowest excited state, the excitation energy is neither quenched nor deactivated but quickly intermolecularly transferred from two ppy ligands to one luminescent dpq-3-F ligand. Such transfer can occur because the triplet energy level of Ir(ppy)₃ is higher than that of Ir(dpq-3-F)₃ and because Ir(dpq-3-F)₃ was known to have a shorter lifetime than that of Ir(ppy)₃. As a result, Ir(ppy)₂(dpq-3-F) shows strong emission band at 620 nm from dpq-3-F ligand in the end. Thus it allows more reddish luminescent color and improves the luminescence by the decrease of quenching or energy deactivation by decreasing the number of the luminescent ligand. To analyze luminescent mechanism, we calculated these complexes theoretically by using computational method.     

New MOCVD precursor for iridium thin films deposition

Thin films of metallic iridium were grown by metal organic chemical vapor deposition in a vertical hot-wall reactor. The new solid compound Ir(thd)₃ (thd = 2,2,6,6-tetramethyl-3,5-heptadione) was used as the iridium source. The iridium precursor was analyzed by elemental analysis, infrared spectroscopy, ¹H NMR spectroscopy and thermogravimetry (TG). The results of TG showed that the iridium β-diketonate was found to vary with the nature of the β-diketonate group and the use of the thd led to a precursor with higher volatilities than the Ir(acac)₃ (acac = acetylacetonate) source. Deposited iridium films were characterized by X-ray diffraction (XRD) and atomic force microscopy (AFM) in order to determine crystallinity and surface morphology.     

Synthesis and characterization of diiridium complexes with a pi-conjugated nanowire chain

The reaction of the iridium dimer [Ir(ppy)2(micro-Cl)]2 (ppy = 2-pyridiylphenyl) with bis(2,2'-bipyridin-5-yl) ethyne (bpy-C2-bpy), bis(2,2'-bipyridine)-butadiyne (bpy-C4-bpy), and bis(2,2'-bipyridin-5-yl-(Z)-hexa-3-ene-1,5-diyl) (bpy-C6H2-bpy) affords the following diiridium complexes: [ClO4]2 [(ppy)2Ir(bpy-C2-bpy)Ir(ppy)2] (1), [ClO4]2 [(ppy)2Ir(bpy-C4-bpy)Ir(ppy)2 (2), and [ClO4]2 [(ppy)2Ir(bpy-C6H6-bpy)Ir (ppy)2 (3), respectively. Herein, we describe the synthesis, characterization, and physical and electrochemical properties of the diiridium complexes in which the two iridium units are connected by a pi-conjugated nanowire bridge.     

The new iridium complexes involving pyridylpyridine derivatives for the saturated blue emission

To obtain a saturated blue phosphorescent material with a good color purity, we have synthesized the new blue emitting iridium complexes with 2, 6-difluoro-3-(4-methylpyridin-2-yl)pyridine (4-Me-dfpypy) as a main ligand. We expected that the LUMO energy levels of the complex might increase upon introduction of an electron donating group such as a methyl group to the pyridyl moieties of the ligand, leading to a wide energy gap of the complex to give the saturated blue emission. We have also introduced a variety of the ancillary ligands to the iridium center to compare the effect of the ancillary ligards on the emission of their complexes. The resulting iridium complexes, Ir(4-Me-dfpypy)3, Ir(4-Me-dfpypy)2(acac), Ir(4-Me-dfpypy)2(pic) and Ir(4-Me-dfpypy)2(trzl-CH3) where acac, pic, and trzl-CH3 represent acetylacetonate, picolinate, and 2-(5-methyl-2H-1,2,4-triazol-3-yl) pyridinate, respectively exhibited the blue emission at 451, 447, 440 and 425 nm in CH2Cl2 solution. The organic light emitting device (OLED) employing homoleptic Ir(4-Me-dfpypy), as the blue dopant was prepared and their electroluminescence was investigated. Ir(4-Me-dfpypy)3 exhibited the blue emission of CIE coordinates (0.22, 0.32).     

Strong ligand field effects of blue phosphorescent mono-cyclometalated iridium(III) complexes

A series of mono-cyclometalated blue phosphorescent iridium(III) complexes with two phosphines trans to each other and two cis-ancillary ligands, such as Ir(F₂Meppy)(PPhMe₂)₂(H)(Cl), [Ir(F₂Meppy)(PPhMe₂)₂(H)(NCMe)]⁺ and Ir(F₂Meppy)(PPhMe₂)₂-(H)(CN), [F₂Meppy = 2-(2′,4′-difluorophenyl)-4-methyl-pyridine] were synthesized and studied to tune the phosphorescence wavelength to the deep blue region and to enhance the luminescence efficiencies. We investigate the electron-withdrawing capabilities of ancillary ligands using the DFT and TD-DFT calculations on the ground and excited states of the three complexes to gain insight into the factors responsible for the emission color change and the different luminescence efficiency. Reducing the molecular weight of phosphine ligand with PPhMe₂ leads to a strategy of the efficient deep blue organic light-emitting devices (OLED) by thermal processing instead of the solution processing. The electron-withdrawing difluoro group substituted on the phenyl ring and the cyano strong field ancillary ligand in the trans position to the carbon atom of phenyl ring increased HOMO-LUMO gap and achieved the hypsochromic shift in emission color. As a result, the maximum emission spectra of Ir(F₂Meppy)(PPhMe₂)₂(H)(Cl), [Ir(F₂Meppy)(PPhMe₂)₂(H)-(NCMe)]⁺ and Ir(F₂Meppy)(PPh-Me₂)₂ (H)(CN) were in the ranges of 446, 440, 439 nm, respectively.     

Observation of phosphorescence from fluorescent organic material Bebq2 using phosphorescent sensitizer

Absorption, emission and excitation spectra of bis(10-hydroxybenzo [h] quinolinato)-beryllium (Bebq₂) were studied using polystyrene film doped with 5 wt% Bebq₂, N,N-di(naphthalene-1-yl)-N,N-diphenyl-benzidene (NPB) film doped with 60 wt% Bebq₂, and neat film. The monomer and aggregate of Bebq₂ give fluorescence at 492 and 511 nm at 12 K, respectively. A strong T₁ emission with a vibronic structure was observed from Bebq₂ below 70 K by heavily doping with phosphorescent tris(2-phenylpyridine) iridium [Ir(ppy)₃]. The T₁ energy of Bebq₂ was estimated to be 2.26 eV from the onset of the 573 nm 0–0 vibronic emission band. The energy transfer mechanism from Ir(ppy)₃ to the T₁ state of Bebq₂ is discussed.     

Bis‐Tridentate Ir(III) Metal Phosphors for Efficient Deep‐Blue Organic Light‐Emitting Diodes

Emissive Ir(III) metal complexes possessing two tridentate chelates (bis‐tridentate) are known to be more robust compared to those with three bidentate chelates (tris‐bidentate). Here, the deep‐blue‐emitting, bis‐tridentate Ir(III) metal phosphors bearing both the dicarbene pincer ancillary such as 2,6‐diimidazolylidene benzene and the 6‐pyrazolyl‐2‐phenoxylpyridine chromophoric chelate are synthesized. A deep‐blue organic light‐emitting diode from one phosphor exhibits Commission Internationale de l'Eclairage (CIE_{(x ,y )}) coordinates of (0.15, 0.17) with maximum external quantum efficiency (max. EQE) of 20.7% and EQE = 14.6% at the practical brightness of 100 cd m^?2.     

Visible to near-infrared organic light-emitting diodes using phosphorescent materials by solution process

The characteristics of visible to near-infrared OLEDs with co-doping three phosphorescent dyes, iridium (III) bis(2-(4,6-difluorephenyl)pyridinato-N,C²′) (FIrpic), tris(1-phenylisoquinoline)iridium(III) (Ir(piq)₃), Pt-tetraphenyltetrabenzoporphyrin (Pt(tpbp)) in poly(N-vinylcarbazole) host as blue, red and near-infrared emitters are investigated. Visible to near-infrared OLEDs covering the wavelength range from 450 to 850 nm were achieved. The device with 11.7 wt.% FIrpic, 0.3 wt.% Ir(piq)₃ and 0.1 wt.% Pt(tpbp) showed white light emission of CIE (0.34, 0.39). The co-doping results in efficient cascade energy transfer from host through Ir complexes. For 0.1 wt.% Pt(tpbp), the optimal device exhibited the maximum output power of 3 mW/cm², maximum luminance of 2900 cd/m² and the maximum efficiency of 7 cd/A.     

Synthesis and luminescence of a new phosphorescent iridium(III) pyrazine complex

The synthesis and luminescent study of a new iridium pyrazine complex are reported. The iridium complex [Ir(MDPP)2(acac)] (MDPP=5-methyl-2,3-diphenylpyrazine, acac=acetylacetone) shows strong ¹MLCT (singlet metal-to-ligand charge-transfer) and ³MLCT (triplet metal to ligand charge-transfer) absorption at 386 and 507 nm, respectively. Organic light emitting device (OLED) with a configuration of ITO / NPB (30 nm) / NPB: 7% (wt.) Ir(MDPP)₂(acac) (25 nm) / BCP (10 nm) / Alq₃(30 nm) / Mg : Ag (mass ratio 10 : 1)120 nm / Ag(10 nm) exhibits an external quantum efficiency of 6.02% (power efficiency 9.89 lm W^− 1 ) and a maximum brightness of 78,924 cd m^− 2. The device also shows high color purity with a maximum peak at 576 nm without any shoulder.     

High-efficiency white organic light-emitting devices with a non-doped yellow phosphorescent emissive layer

Highly efficient phosphorescent white organic light-emitting devices (PHWOLEDs) with a simple structure of ITO/TAPC (40 nm)/mCP:FIrpic (20 nm, x wt.%)/bis[2-(4-tertbutylphenyl)benzothiazolato-N,C²′] iridium (acetylacetonate) (tbt)₂Ir(acac) (y nm)/Bphen (30 nm)/Mg:Ag (200 nm) have been developed, by inserting a thin layer of non-doped yellow phosphorescent (tbt)₂Ir(acac) between doped blue emitting layer (EML) and electron transporting layer. By changing the doping concentration of the blue EML and the thickness of the non-doped yellow EML, a PHWOLED comprised of higher blue doping concentration and thinner yellow EML achieves a high current efficiency of 31.7 cd/A and Commission Internationale de l'Eclairage coordinates of (0.33, 0.41) at a luminance of 3000 cd/m² could be observed.