Effect of Substitution of Methyl Groups on the Luminescence Performance of IrIII Complexes: Preparation, Structures, Electrochemistry, Photophysical Properties and Their Applications in Organic Light-Emitting Diodes (OLEDs)

Development of Blue Phosphorescent Pt(II) Materials Using Dibenzofuranyl Imidazole Ligands and Their Application in Organic Light-Emitting Diodes

Organic light-emitting diodes (OLEDs) are energy-efficient; however, the coordinating ligand can affect their stability. Sky-blue phosphorescent Pt(II) compounds with a C^N chelate, fluorinated-dbi (dbi = [1-(2,4-diisopropyldibenzo [b,d]furan-3-yl)-2-phenyl-1H-imidazole]), and acetylactonate (acac) (1)/picolinate (pic) (2) ancillary ligands were synthesized. The molecular structures were characterized using various spectroscopic methods. The Pt(II) Compound Two exhibited a distorted square planar geometry, with several intra- and inter-molecular interactions involving C_π⋯H/C_π⋯C_π stacking. Complex One emitted bright sky-blue light (λ_max = 485 nm) with a moderate photoluminescent quantum efficiency (PLQY) of 0.37 and short decay time (6.1 µs) compared to those of 2. Theoretical calculations suggested that the electronic transition of 1 arose from ligand(C^N)-centered π-π* transitions combined with metal-to-ligand charge-transfer (MLCT), whereas that of 2 arose from MLCT and ligand(C^N)-to-ligand(pic) charge-transfer (LLCT), with minimal contribution from C^N chelate to the lowest unoccupied molecular orbital (LUMO). Multi-layered phosphorescent OLEDs using One as a dopant and a mixed host, mCBP/CNmCBPCN, were successfully fabricated. At a 10% doping concentration of 1, a current efficiency of 13.6 cdA^-1 and external quantum efficiency of 8.4% at 100 cdm^-2 were achieved. These results show that the ancillary ligand in phosphorescent Pt(II) complexes must be considered.

Modular Photocatalytic Synthesis of α-Trialkyl-α-Tertiary Amines

Molecules displaying an α-trialkyl-α-tertiary amine motif provide access to an important and versatile area of biologically relevant chemical space but are challenging to access through existing synthetic methods. Here, we report an operationally straightforward, multicomponent protocol for the synthesis of a range of functionally and structurally diverse α-trialkyl-α-tertiary amines, which makes use of three readily available components: dialkyl ketones, benzylamines, and alkenes. The strategy relies on the of use visible-light-mediated photocatalysis with readily available Ir(III) complexes to bring about single-electron reduction of an all-alkyl ketimine species to an α-amino radical intermediate; the α-amino radical undergoes Giese-type addition with a variety of alkenes to forge the α-trialkyl-α-tertiary amine center. The mechanism of this process is believed to proceed through an overall redox neutral pathway that involves photocatalytic redox-relay of the imine, generated from the starting amine-ketone condensation, through to an imine-derived product. This is possible because the presence of a benzylic amine component in the intermediate scaffold drives a 1,5-hydrogen atom transfer step after the Giese addition to form a stable benzylic α-amino radical, which is able to close the photocatalytic cycle. These studies detail the evolution of the reaction platform, an extensive investigation of the substrate scope, and preliminary investigation of some of the mechanistic features of this distinct photocatalytic process. We believe this transformation will provide convenient access to previously unexplored α-trialkyl-α-tertiary amine scaffolds that should be of considerable interest to practitioners of synthetic and medicinal chemistry in academic and industrial institutions.

Probing the Effects of the Number and Positions of -OCH3 and -CN Substituents on Color Tuning of Ir(III) Complex Derivatives through a Joint Computational and Experimental Study

We performed a joint theoretical and experimental study on sixteen Ir(III) complexes bearing a similar molecular platform of bis(2-phenylbenzothiozolato-N,C^2' ) iridium(III) (acetylacetonate) by grafting -OCH₃ group and/or -CN group on different positions of the C-related arene moiety of the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>C</mml:mi> <mml:mspace/> <mml:mover><mml:mspace/> <mml:mo>^</mml:mo></mml:mover> <mml:mspace/> <mml:mi>N</mml:mi></mml:mrow> </mml:math> ligand (C-ring). Our results reveal that the introduction of -CN renders an overall drop in the FMO energy levels while a reverse increase is observed for -OCH₃ . The ortho- and para-sites of the C-ring are more effective substitution positions to modulate the HOMO energy level due to the fact that the electronic density of HOMO mainly locates at them while the meta-site would induce a stronger impact on LUMO since the electronic density of LUMO mainly distributes over the position. Utilizing the synergistic effects of the substituents and the substituted positions, a wide color-tuning range from 479 nm to 637 nm was achieved, which covers nearly the whole window of visible spectrum. In particular, the tri-substituted Ir35mo4cn complex (λ_em max =637 nm) may be a potential candidate for high efficiency red OLEDs materials due to its greatly enhanced absorption processes, relatively higher ³ MLCT (%), lower ΔE_S1-T1 , enlarged separation between ³ MLCT/π-π* and ³ MC d-d states, and good hole and particle-transporting performances. Finally, six representative complexes were synthesized and their spectra were determined, which confirm the reliability of our computational strategy.

Polypyridyl ligands as a versatile platform for solid-state light-emitting devices

A comprehensive review of tuneable polypyridine complexes as the emissive components of OLED and LEC devices is presented, with a view to bridging the gap between molecular design and commercialization.

Bipyridine-based iridium(iii) triplet emitters for organic light-emitting diodes (OLEDs): application and impact of phenyl substitution at the 5′-position of the N-coordinating pyridine ring

Three blue phosphorescent homoleptic iridium(iii) complexes based on a bipyridine ligand were synthesized.

Synthesis and structural characterisation of unprecedented primary N-nitrosamines coordinated to iridium(iv)

The one electron oxidation of the N-nitrosamine complexes [Ir^IIICl₅(RN(H)NO)]^2– (R = benzyl or n-butyl) was studied in detail.

Spectroscopic, Electrochemical and DFT Studies of Phosphorescent Homoleptic Cyclometalated Iridium(III) Complexes Based on Substituted 4-Fluorophenylvinyl- and 4-Methoxyphenylvinylquinolines

This study reports the synthesis and comparative investigation of the substituent effects of a new series of highly luminescent homoleptic tris-cyclometalated iridium(III) complexes of the type [Ir(N^˄C)₃]. These are based on two ligand type derivatives comprising of 4-fluorophenylvinylquinolines and 4-methoxyphenylvinylquinolines with electron-donating and/or electron-withdrawing groups as aryl substituents at 2-position. The structures of the ligands and their complexes were characterized by means of FT-IR, UV-Vis and NMR spectrometry complemented with photoluminescence and cyclic voltammetry. The photophysical properties of 2-aryl-4-(4-fluorophenylvinyl)quinoline and its corresponding complex were also studied using the density functional theory method. The photoluminescent properties of the ligands and the corresponding complexes showed high fluorescent intensities and quantum yields in solvents of different polarities. The photoluminescence spectra of the complexes in solid film, showed common transmission curves at longer wavelengths maximum (λ_em = 697 nm) possibly originating from the interference of scattered light of higher-order transmission of monochromators.

Structural Mimics of Phenyl Pyridine (ppy) - Substituted, Phosphorescent Cyclometalated Homo and Heteroleptic Iridium(III) Complexes for Organic Light Emitting Diodes - An Overview

Today organic light emitting diodes are a topic of significant academic and industrial research interest. OLED technology is used in commercially available displays, and efforts have been directed to improve this technology. Design and synthesis of phosphorescent based transition metals are capable of harvesting both singlet and triplet excitons and achieve 100 % internal quantum efficiency is an active area of research. Among all the transition metals, iridium is considered a prime candidate for OLEDs due to its prominent photophysical characteristics. In the present review, we have concentrated on the Iridium based homo and heteroleptic complexes that have dissimilar substitutions on phenylpyridine ligands, different ancillary ligands and the effect of substitution on HOMO/LUMO energies and a brief discussion and correlation on the photophysical, electrochemical and device performances of the different complexes have been reviewed for organic light emitting diodes.

Rational Design and Characterization of Heteroleptic Phosphorescent Complexes for Highly Efficient Deep-Red Organic Light-Emitting Devices

Two new deep-red iridium(III) complexes, (fpiq)₂Ir(dipba) (fIr1) and (f₂piq)₂Ir(dipba) (dfIr2), comprising two cyclometaling ligands of fluorophenyl-isoquinoline derivatives (fpiq and f₂piq) and a N-heterocyclic carbene (NHC)-based ancillary ligand of N,N'-diisopropylbenzamidinate (dipba) are designed, synthesized, and characterized. Given the unique four-membered Ir-N-C-N backbone built by the metal center and the ancillary ligand, both phosphors achieve significant improvement for their comprehensive optoelectronic characteristics. Density function theory (DFT) calculations and electrochemical measurements support the genuine pure red phosphorescent emission of fIr1 and dfIr2 based on their clearly distinct electron density distributions of the HOMO/LUMO orbitals compared with other red-emitting Ir(III) derivatives. Both new phosphors show deep-red emission with λ_max values in the region of 650-660 nm with high PLQYs and short excited-state lifetimes. The phosphorescent organic light emitting diodes (PhOLEDs) based on fIr1 and dfIr2 realize deep-red EL with the stable CIE_x,y coordinates of (0.70, 0.30) and (0.69, 0.31), the peak EQE/PE values of 15.4%/9.3 lm W^-1 and 16.7%/10.4 lm W^-1, respectively, which maintain such high levels as 10.6%/3.5 lm W^-1 and 10.8%/3.6 lm W^-1 at the practical luminance of 1000 cd m^-2. They are the highest EL values reported for the OLEDs with such deep-red CIE coordinates.

Syntheses, Spectroscopic, Electrochemical, and Third-Order Nonlinear Optical Studies of a Hybrid Tris{ruthenium(alkynyl)/(2-phenylpyridine)}iridium Complex

The synthesis of fac-[Ir{N,C1′-(2,2′-NC5H4C6H3-5′-C≡C-1-C6H2-3,5-Et2-4-C≡CC6H4-4-C≡CH)}3] (10), which bears pendant ethynyl groups, and its reaction with [RuCl(dppe)2]PF6 to afford the heterobimetallic complex fac-[Ir{N,C1′-(2,2′-NC5H4C6H3-5′-C≡C-1-C6H2-3,5-Et2-4-C≡CC6H4-4-C≡C-trans-[RuCl(dppe)2])}3] (11) is described. Complex 10 is available from the two-step formation of iodo-functionalized fac-tris[2-(4-iodophenyl)pyridine]iridium(III) (6), followed by ligand-centered palladium-catalyzed coupling and desilylation reactions. Structural studies of tetrakis[2-(4-iodophenyl)pyridine-N,C1′](μ-dichloro)diiridium 5, 6, fac-[Ir{N,C1′-(2,2′-NC5H4C6H3-5′-C≡C-1-C6H2-3,5-Et2-4-C≡CH)}3] (8), and 10 confirm ligand-centered derivatization of the tris(2-phenylpyridine)iridium unit. Electrochemical studies reveal two (5) or one (6–10) Ir-centered oxidations for which the potential is sensitive to functionalization at the phenylpyridine groups but relatively insensitive to more remote derivatization. Compound 11 undergoes sequential Ru-centered and Ir-centered oxidation, with the potential of the latter significantly more positive than that of Ir(N,C′-NC5H4-2-C6H4-2)3. Ligand-centered π–π* transitions characteristic of the Ir(N,C′-NC5H4-2-C6H4-2)3 unit red-shift and gain in intensity following the iodo and alkynyl incorporation. Spectroelectrochemical studies of 6, 7, 9, and 11 reveal the appearance in each case of new low-energy LMCT bands following formal IrIII/IV oxidation preceded, in the case of 11, by the appearance of a low-energy LMCT band associated with the formal RuII/III oxidation process. Emission maxima of 6–10 reveal a red-shift upon alkynyl group introduction and arylalkynyl π-system lengthening; this process is quenched upon incorporation of the ligated ruthenium moiety on proceeding to 11. Third-order nonlinear optical studies of 11 were undertaken at the benchmark wavelengths of 800 nm (fs pulses) and 532 nm (ns pulses), the results from the former suggesting a dominant contribution from two-photon absorption, and results from the latter being consistent with primarily excited-state absorption.

On the use of metal purine derivatives (M=Ir, Rh) for the selective labeling of nucleosides and nucleotides

The reactions of neutral or cationic IrIII and RhIII derivatives of phenyl purine nucleobases with unsymmetrical alkynes produce new metallacycles in a predictable manner, which allows for the incorporation of either photoactive (anthracene or pyrene) or electroactive (ferrocene) labels in the nucleotide or nucleoside moiety. The reported methodology (metalation of the purine derivative and subsequent marker insertion) could be used for the postfunctionalization and unambiguous labeling of oligonucleotides.

Ligand functionalization as a deactivation pathway in a fac-Ir(ppy)3-mediated radical addition

Knowledge of the kinetic behavior of catalysts under synthetically relevant conditions is vital for the efficient use of compounds that mediate important transformations regardless of their composition or driving force. In particular, these data are of great importance to add perspective to the growing number of applications of photoactive transition metal complexes. Here we present kinetic, synthetic, and spectroscopic evidence of the mechanistic behavior of fac-Ir(ppy)3 in a visible light-mediated radical addition to 3-methylindole, demonstrating the instability of fac-Ir(ppy)3 under these conditions. During the reaction, rapid in situ functionalization of the photocatalyst occurs, eventually leading to deactivation. These findings demonstrate a conceivable deactivation process for catalytic single electron reactions in the presence of radicophilic ligands. Attempts to inhibit photocatalyst deactivation through structural modification provide further insight into catalyst selection for a given system of interest.

Highly efficient deep-blue phosphorescence from heptafluoropropyl-substituted iridium complexes

(HFP)₂Ir(pic), (HFP)₂Ir(mpic), and (HFP)₂Ir(fptz) with electron-withdrawing heptafluoropropyl groups exhibited wide band gaps with high PL QYs and the devices showed the EQEs of 19.7%, 21.4% and 14.2% and CIE coordinates of (0.147, 0.164), (0.146, 0.165) and (0.152, 0.148), respectively.

High performance blue-green and green phosphorescent OLEDs based on iridium complexes with N^C^N-coordinated terdentate ligands

Two iridium complexes with N^C^N-coordinated terdentate ligands exhibit high PE and EQE values of 35.5 lm W⁻¹ & 15.8% for blue-green emission, 47.4 lm W⁻¹ & 16.7% for green emission via direct charge recombination on both dopant emitters in the bipolar host–dopant systems.

A novel cyclometalated Iridium(iii) complex containing dibenzo-24-crown-8: synthesis, luminescence and application in highly efficient green phosphorescent OLEDs

Several novel dibenzo-24-crown-8 substituted iridium complexes have been obtained, which emit green light with a high Φ_PL ranging from 0.56 to 0.63. The PLEDs with G3 as a dopant display the best EL performances with a LE of 29.6 cd A⁻¹.

Theoretical research on the effect of regulated π-conjugation on the photophysical properties of Ir(iii) complexes

Larger π-skeleton-conjugation in host ligands of Ir(iii) carbene complexes can result in remarkably blue-shifted emission and can effectively improve the quantum efficiency.

Homoleptic tris-cyclometalated platinum(iv) complexes: a new class of long-lived, highly efficient 3LC emitters

Tris-cyclometalated Pt(IV) complexes are reported for the first time. The facial isomers exhibit long-lived ³LC emissions with quantum yields up to 0.49, the highest ever found for Pt(IV) complexes, combined with a strong oxidizing character in the excited state.

Synthesis and photochemical properties of pH responsive tris-cyclometalated iridium(III) complexes that contain a pyridine ring on the 2-phenylpyridine ligand

In our previous publication, it was reported that fac-Ir(atpy)3 3 (atpy = 2-(5'-amino-4'-tolyl)pyridine), which contains three amino groups at the 5'-position of the atpy ligands, exhibits a pH-dependent change in the color of the emitted radiation. Aqueous solution of 3 shows a weak red emission (at around 613 nm) under neutral or basic conditions, but the emission color changes to green (at around 530 nm) under acidic conditions, where the NH2 group is protonated to become an electron-withdrawing (NH3)(+) group. In this manuscript, we report on the preparation of some new pH-responsive Ir(III) complexes; fac-Ir(4Pyppy)3 5 and fac-Ir(3Pyppy)3 6 that contain three pyridyl groups at the 5'-position of the 2-phenylpyridine (ppy) ligand, and Ir(4Pyppym)3 7 and Ir(3Pyppym)3 8 that contain a pyridyl group at the same position of the 2-phenylpyrimidine (ppym) ligand. The introduction of three pyridyl groups on iodinated Ir(ppy)3 and Ir(ppym)3 was achieved via Suzuki-Miyaura cross-coupling reaction assisted by microwave irradiation. Solutions of the acid-free Ir(III) complexes 5, 6, 7, and 8 showed a strong green emission (at around 500 nm) in dimethylsulfoxide (DMSO). Protonation of three pyridyl groups of 5 and 7 causes a significant red-shift in the emission wavelength (at around 600 nm) with a decrease in emission intensity. The pH-dependent emission change of these complexes is also discussed. The generation of singlet oxygen ((1)O2) by the photoirradiation of the Ir complexes 5 and 6 was evidenced by the decomposition of 1,3-diphenylisobenzofuran (DPBF), the oxidation of thioanisole, and the oxidation of 2,2,5,5-tetramethyl-3-pyrroline-3-carboxamide (TPC). The induction of necrosis-like cell death of HeLa-S3 cells upon photoirradiation of 5 at 465 nm is also reported.

Synthesis, photophysical, electrochemical and electroluminescent properties of a novel iridium(III) complex based on 2-phenylbenzo[d]oxazole derivative

A new phosphorescent iridium (III) complex based on 2-(4-tert-butylphenyl)-5-methylbenzo[d]oxazole as main ligand, i.e. bis(2-(4-tert-butylphenyl)-5-methylbenzo[d]oxazole-N,C(2'))iridium(acetylacetonate) [(tmbo)2Ir(acac)], was synthesized for organic light-emitting diodes (OLEDs), and its photophysical, electrochemical and electroluminescent properties were investigated. The complex displayed strong phosphorescence emission, high decomposition temperature, short phosphorescent lifetime and reversible redox electrochemical behavior. The OLEDs based on (tmbo)2Ir(acac) as dopant emitter exhibited maximum luminance efficiency of 26.1cdA(-1) and high luminance of 16,445 cd m(-2). Interestingly, highly doped device based on (tmbo)2Ir(acac) showed high efficiency with negligible roll-off under a wide range of driving current density, which was mainly attributed to the effect of bulky steric hindrance of multi-methyl groups on this complex and its short phosphorescent lifetime.

Deep-blue phosphorescence from perfluoro carbonyl-substituted iridium complexes

The new deep-blue iridium(III) complexes, (TF)2Ir(pic), (TF)2Ir(fptz), (HF)2Ir(pic), and (HF)2Ir(fptz), consisting of 2',4″-difluororphenyl-3-methylpyridine with trifluoromethyl carbonyl or heptafluoropropyl carbonyl at the 3' position as the main ligand and a picolinate or a trifluoromethylated-triazole as the ancillary ligand, were synthesized and characterized for applications in organic light-emitting diodes (OLEDs). Density function theory (DFT) calculations showed that these iridium complexes had a wide band gap, owing to the introduction of the strong electron withdrawing perfluoro carbonyl group. Time-dependent DFT (TD-DFT) calculations suggested that their lowest triplet excited state was dominated by a HOMO → LUMO transition and that the contribution of the metal-to-ligand charge transfer (MLCT) was higher than 34% for all four complexes, indicating that strong spin-orbit coupling exists in the complexes. The 10 wt % (TF)2Ir(pic) doped 9-(3-(9H-carbazole-9-yl)phenyl)-3-(dibromophenylphosphoryl)-9H-carbazole (mCPPO1) film exhibited the highest photoluminescence quantum yield of 74 ± 3% among the films based on the four complexes. Phosphorescent OLEDs based on (TF)2Ir(pic) and (TF)2Ir(fptz) exhibited maximum external quantum efficiencies of 17.1% and 8.4% and Commission Internationale de l'Eclairage (CIE) coordinates of (0.141, 0.158) and (0.147, 0.116), respectively. These CIE coordinates represent some of the deepest blue emissions ever achieved from phosphorescent OLEDs with considerably high EQEs.

Synthesis and electrophosphorescence of iridium complexes containing benzothiazole-based ligands

Four heteroleptic bis-cyclometalated iridium(III) complexes containing 2-aryl-benzothiazole ligands, in which the aryl is dibenzofuran-2-yl [Ir(O-bt)2(acac)], dibenzothiophene-2-yl [Ir(S-bt)2(acac)], dibenzothiophene-S,S-dioxide-2-yl [Ir(SO2-bt)2(acac)] and 4-(diphenylphosphoryl)phenyl [Ir(PO-bt)2(acac)], have been synthesized and characterized for use in organic light-emitting diodes (OLEDs). These complexes emit bright yellow (551 nm) to orange-red (598 nm) phosphorescence at room temperature, the peak wavelengths of which can be finely tuned depending upon the electronic properties of the aryl group in the 2-position of benzothiazole. The strong electron-withdrawing aryls such as dibenzothiophene-S,S-dioxide2-yl and 4-(diphenylphosphoryl)phenyl caused bathochromatic shift of the iridium complex phosphorescence. These iridium complexes were used as doped emitters to fabricate yellow to orange-red OLEDs and good performance was obtained. In particular, a maximum luminance efficiency of 58.4 cd A(-1) (corresponding to 30.6 lm W(-1) and 19%) with CIE coordinates of (0.45, 0.52) was achieved for Ir(O-bt)2(acac)-based yellow device. Furthermore, the yellow emitting Ir(S-bt)2(acac) was used to fabricate two-element white OLED that exhibited a high efficiency of 32.4 cd A(-1) with CIE coordinates of (0.28, 0.44).

Synthesis and photophysical properties of yellow-emitting iridium complexes. Effect of the temperature on the character of triplet emission

The parent study includes synthesis end photophysical characterization of four new phosphorescent cyclometalated iridium (III) complexes. The cyclometaled ligands used here are 2-(4-chlorophenyl)benzo[d]thiazole, 2(4isopropylphenyl)benzo[d] thiazole, 2-(4-methoxyphenyl)benzo[d]thiazole, 4-(benzo[d]thiazol-2-yl)-N,N-dimethylaniline. The auxiliary ligand is acetylacetone (Hacac). The photometric analyses show temperature dependence on phosphorescent spectra and the nature of the emitting state of the synthesized complexes. Using fluorescent and absorption spectroscopy we suggest an explanation of the observed effect points out the transition from ligand centered (LC) to MLCT main character of the emitting state. The quantitative photometric analysis states high phosphorescent quantum yields of the complexes in solution.