Heteroleptic Iridium Complexes Research Articles

Cocatalyst-free Photocatalytic Hydrogen Evolution with Simple Heteroleptic Iridium(III) Complexes

A simple heteroleptic iridium(III) photosensitizer, Ir-1, containing two ligands 5-(trifluoromethyl)-2-phenylpyridine (ĈN–CF3) and bipyridine (NN) has for the first time been studied for cocatalys...

Acenaphthene-imidazole based red-to-NIR Emissive Homoleptic and Heteroleptic Ir(III) complexes for OLEDs: Combined experimental and theoretical approach

A series of homoleptic and heteroleptic Iridium(III) complexes (Ir-1–Ir-4) bearing on acenaphthene-imidazole have been successfully designed and synthesized. The photophysical properties of these complexes were examined with appropriate spectroscopic methods. The results, in combination with theoretical calculations, led us to clearly assign various electronic transition states. All the complexes show remarkably intense red-to-near-infrared (NIR) emission in the range 600–800 nm with moderate quantum yield (0.10–0.17). Singlet and triplet energy levels were calculated by using TD-DFT (Time Dependent-Density functional theory) calculations. In addition, investigations have been performed to explore the optical, electronic, charge transport, and stability properties of Ir(III) complexes as charge transport and emissive materials for OLEDs (Organic light emitting diodes). Furthermore, all the complexes have promising luminescence and hole transporting performance towards OLED application. According to theoretical calculations, it is clearly indicating that the Ir-4 complex has more potential as an emitter for deep red to NIR-OLED applications.

High-Throughput Screening and Automated Data-Driven Analysis of the Triplet Photophysical Properties of Structurally Diverse, Heteroleptic Iridium(III) Complexes.

Steady state emission spectra and excited state lifetimes were measured for 1440 distinct heteroleptic [Ir(C^N)2(N^N)]+ complexes prepared via combinatorial parallelized synthesis; 72% of the complexes were found to be luminescent, and the emission maxima of the library spanned the visible spectrum (652-459 nm). Spectral profiles ranged from broad structureless bands to narrow emissions exhibiting vibrational substructure. Measured excited state lifetimes ranged between ∼0.1-14 μs. Automated emission spectral fitting with successive Gaussian functions revealed four distinct measured classes of excited states; in addition to well understood metal-ligand to ligand-charge transfer (3MLLCT) and ligand-centered (3LC) excited states, our classification also identified photophysical characteristics of less explored mixed 3MLLCT/3LC states. Electronic structure features obtained from DFT calculations performed on a large subset of these Ir(III) chromophores offered clear insights into the excited state properties and allowed the prediction of structure/luminescence relationships in this class of commonly used photocatalysts. Models with high prediction accuracy (R2 = 0.89) for emission color were developed on the basis of experimental data. Furthermore, different degrees of nuclear reorganization in the excited state were shown to significantly impact emission energy and excited state lifetimes.

Highly efficient phosphorescent organic light-emitting diodes based on novel bipolar iridium complexes with easily-tuned emission colors by adjusting fluorine substitution on phenylpyridine ligands

Two novel heteroleptic iridium complexes’ emitting colors can be tuned easily by adjusting F/CF3-substituted cyclometalated ligands, which achieved very high EL efficiency level with peak EQE values of >20% at wide concentration range of 8-25%.

Bis-Tridendate Ir(III) Polymer-Metallocomplexes: Hybrid, Main-Chain Polymer Phosphors for Orange-Red Light Emission.

In this work, hybrid polymeric bis-tridentate iridium(III) complexes bearing derivatives of terpyridine (tpy) and 2,6-di(phenyl) pyridine as ligands were successfully synthesized and evaluated as red-light emitters. At first, the synthesis of small molecular bis-tridendate Ir(III) complexes bearing alkoxy-, methyl-, or hydroxy-functionalized terpyridines and a dihydroxyphenyl-pyridine moiety was accomplished. Molecular complexes bearing two polymerizable end-hydroxyl groups and methyl- or alkoxy-decorated terpyridines were copolymerized with difluorodiphenyl-sulphone under high temperature polyetherification conditions. Alternatively, the post-polymerization complexation of the terpyridine-iridium(III) monocomplexes onto the biphenyl-pyridine main chain homopolymer was explored. Both cases afforded solution-processable metallocomplex-polymers possessing the advantages of phosphorescent emitters in addition to high molecular weights and excellent film-forming ability via solution casting. The structural, optical, and electrochemical properties of the monomeric and polymeric heteroleptic iridium complexes were thoroughly investigated. The polymeric metallocomplexes were found to emit in the orange–red region (550–600 nm) with appropriate HOMO and LUMO levels to be used in conjunction with blue-emitting hosts. By varying the metal loading on the polymeric backbone, the emitter’s specific emission maxima could be successfully tuned.

Water-Soluble Iridium(III) Complexes Containing Tetraethylene-Glycol-Derivatized Bipyridine Ligands for Electrogenerated Chemiluminescence Detection.

Four cationic heteroleptic iridium(III) complexes containing a 2,2′-bipyridine (bpy) ligand with one or two tetraethylene glycol (TEG) groups attached in the 4 or 4,4′ positions were synthesized to create new water-soluble electrogenerated chemiluminescence (ECL) luminophores bearing a convenient point of attachment for the development of ECL-labels. The novel TEG-derivatized bipyridines were incorporated into [Ir(C∧N)2(R-bpy-R′)]Cl complexes, where C∧N = 2-phenylpyridine anion (ppy) or 2-phenylbenzo[d]thiazole anion (bt), through reaction with commercially available ([Ir(C∧N)2(μ-Cl)]2 dimers. The novel [Ir(C∧N)2(Me-bpy-TEG)]Cl and [Ir(C∧N)2(TEG-bpy-TEG)]Cl complexes in aqueous solution largely retained the redox potentials and emission spectra of the parent [Ir(C∧N)2(Me-bpy-Me)]PF6 (where Me-bpy-Me = 4,4′methyl-2,2′-bipyridine) luminophores in acetonitrile, and exhibited ECL intensities similar to those of [Ru(bpy)3]2+ and the analogous [Ir(C∧N)2(pt-TEG]Cl complexes (where pt-TEG = 1-(TEG)-4-(2-pyridyl)-1,2,3-triazole). These complexes can be readily adapted for bioconjugation and considering the spectral distributions of [Ir(ppy)2(Me-bpy-TEG)]+ and [Ir(ppy)2(pt-TEG)]+, show a viable strategy to create ECL-labels with different emission colors from the same commercial [Ir(ppy)2(μ-Cl)]2 precursor.

Inter-Ligand Energy Transfer Process in an Ir-Complex with Expanding π-Conjugated Ligand.

The inter-ligand energy transfer (ILET) process in heteroleptic iridium complex, [Ir(dfppy)2 (bpy-Im2 )]+ , where dfppy=2-(2,4-difluorophenyl)pyridine and bpy-Im2 =4,4'-bis(1,2-diphenyl-1H-benzo[d]imidazole)-2,2',-bipyridine, was investigated using a femtosecond transient absorption (fs-TA) spectroscopic technique. The photophysical properties of [Ir(dfppy)2 (bpy-Im2 )]+ with significantly expanding π-conjugated ligand are compared to those of [Ir(dfppy)2 (bpy)]+ (bpy=2,2'-bipyridine) and a free bpy-Im2 ligand. The emission spectrum of [Ir(dfppy)2 (bpy-Im2 )]+ shows no shift upon changing the solvent polarity, whereas the free ligand bpy-Im2 showed bathochromic fluorescence shifts with increasing solvent polarity, which is attributed to intramolecular charge transfer (ICT). The unique photophysical properties of [Ir(dfppy)2 (bpy-Im2 )]+ are due to the fast ILET process from 3 MLCTdfppy to 3 MLCT/3 LCbpy-Im2 , resulting in the phosphorescence emission originating from 3 MLCT/3 LCbpy-Im2 . On the other hand, the TA bands of bpy-Im2 are observed at 540 and 480 nm, corresponding to the singlet and triplet manifolds, respectively. In contrast, the TA spectrum of [Ir(dfppy)2 (bpy-Im2 )]+ showes broad bands centered at 420 and 600 nm, attributed to the transitions from 3 MLCTdfppy and 3 MLCT/3 LCbpy-Im2 , respectively. Time-resolved spectroscopic results confirm the efficient ILET dynamics from 3 MLCTdfppy to 3 MLCT/3 LCbpy-Im2 in [Ir(dfppy)2 (bpy-Im2 )]+ . From the relaxation times determined by singular value decomposition analysis and simple sequential kinetic model, we infer that the ILET process from 3 MLCTdfppy to 3 MLCT/3 LCbpy-Im2 occurs with a time constant of ca. 4 ps. The presented results in this study show that the introduction of an expanding π-conjugated ligand can lead to the efficient ILET dynamics for improving the OLED performance.

Yellowish-orange phosphorescent iridium(III) complexes of bis-cyclometalated ligand with pyrazolone derivatives: synthesis, characterization, photophysical and thermal properties

Three novel heteroleptic iridium complexes containing 2-phenylbenzothiazole (bt) as the cyclometalated ligands and variety of pyrazolone-based Schiff-based derivatives as ancillary ligands were synthesized and characterized by spectroscopic techniques such as 1H-NMR, 13C-NMR, mass, FT-IR and elemental analysis. The electronic and photonic properties of these complexes were tuned by introducing electron donating groups on the 4-position of phenyl ring attached to nitrogen. All the complexes show good photoluminescence properties in solid state as well as in solution state having λPL in the range of 540–562 nm with CIE coordinates (0.40, 0.59). Electrochemical and photophysical properties were performed to evaluate suitability of designed compounds for organic light-emitting diodes (OLEDs). The designed complexes show bandgap (Eg) and quantum yield (ΦPL) in the range of 2.38 to 2.40 eV and 0.48 to 0.57, respectively. The emission lifetime of complexes I, II and III were measured in solution and solid state and were found to be 0.243 μs, 0.391 μs and 0.280 μs and 0.190 μs, 0.557 μs, 0.414 μs, respectively. All the synthesized complexes display high thermal stability up to 300 °C.

Theoretical investigation on green emitting heteroleptic cyclometalated iridium(III) complexes with fluorinated 2-phenylpyridine ligands

In this work, we studied the geometrical structures, optical properties, UV-vis spectra and phosphorescence properties of three cyclometalated heteroleptic iridium complexes : (ppy-tfm)2Ir(pico) (Ir1), (dFppy- tfm)2Ir(pico) (Ir2), and (tFppy- tfm)2Ir(pico) (Ir3) with tfm = trifluoromethyl and pico = picolinate, dF = difluoro and tF = trifluoro by mean DFT and TD-DFT methods. Di- and tri-fluorination substitutions on phenyl rings of ppy ligands affect the metal-ligand bonds, decrease the HOMO energies of the studied complexes and increase the energy gaps. The weak and intense observed bands have been assigned in term of natural transition orbitals. Phosphorescence wavelengths were calculated according to vertical and adiabatic procedures. Simulated phosphorescence spectra agree finely with the experimental ones. Main normal modes contributing to the intense band were identified in term of shift vector.

Saturated red phosphorescent Iridium(III) complexes containing phenylquinoline ligands for efficient organic light-emitting diodes

Saturated red iridium phosphors are strongly desired for application in organic light-emitting diodes (OLEDs). Most of the reported iridium complexes with 2-arylquinoline or its derivatives as cyclometalating ligands are orange-red phosphorescent with peak wavelength below 620 nm. Herein trifluoromethyl (CF3) or Fluorine (F) is introduced into the 6-site of quinoline ring to design and synthesize the cyclometalated ligands 2-phenyl-6-CF3/F-quinoline (CF3-phq or F-phq) and heteroleptic iridium complexes Ir(CF3-phq)2(acac) (Ir-1), Ir(CF3-phq)2(tmd) (Ir-2), Ir(F-phq)2(acac) (Ir-3) and Ir(F-phq)2(tmd) (Ir-4). Owing to the electron-deficient feature of CF3 and F, the electron affinity of these complexes are increased, the corresponding LUMOs are pulled downwards, and the phosphorescence of CF3-containing iridium complexes are saturated red with peaks at 627–632 nm in photoluminescence. At the same time, by varying the ancillary ligand from acetylacetonate (acac) to 2,2,6,6-tetrametylheptane-3,5-dionate (tmd), the emitting performance are dramatically improved. As a result, the Ir(CF3-phq)2(tmd) based red OLED realized pure red electroluminescence with Commision Internationale de L'Eclairage (CIE) coordinates of (0.67, 0.33) with a high brightness of 12695 cd m−2 and a maximum external quantum efficiency of 14.96%, respectively, which maintained as high as 9.62% at a practical luminance of 1000 cd m−2. This is the first example of saturated red iridium phosphors with structure modification at the 6-site of quinoline ring of 2-phenylquinoline ligand.

Heteroleptic Cyclometalated NHC Iridium(III) complex with a bulky acetylacetonate: Photophysics of an unexplored class of compounds

The photophysical properties of heteroleptic cyclometalated NHC iridium(III) complexes with bulky acetylacetonates have previously not been explored. To fill this gap, we prepared a new heteroleptic iridium(III) complex and measured its photophysical properties. We investigated the electronic properties by means of DFT calculations (PBE0/6-311G∗) and propose a possible mechanism for the radiationless deactivation from the excited state.

Theoretical study of geometric, optical, nonlinear optical, UV–Vis spectra and phosphorescence properties of iridium(III) complexes based on 5-nitro-2-(2′,4′-difluorophenyl)pyridyl

In this work, we studied the structural, optical, nonlinear optical properties , absorption spectra and phosphorescence properties of four cyclometalated heteroleptic iridium complexes [Ir(dFNppy)2(PPh3)L] with dFNppy = 5-nitro-2-(2′,4′-difluorophenylpyridyl, PPh3 = triphenylphosphine, L = Cl− (1), NCS− (2), NCO− (3) and N3− (4) using DFT and TD-DFT methods. The electronic and geometrical structures of the S0 and T1 have been studied and compared. Experimental absorption bands were assigned on the basis of natural transition orbitales, and a good agreement with experience has been obtained. Phosphorescence wavelengths of the four complexes were calculated with vertical and adiabatic methods. Further, linear optical properties (mean polarizability $$\left\langle \alpha \right\rangle$$, anisotropy polarizability $$\left| \alpha \right|$$), the static first hyperpolarizabilities electric-field-induced second harmonic generation $$\beta_{//} \left( { - 2\omega ;\omega ,\omega } \right)$$ and the hyper-Rayleigh scattering (HRS) first hyperpolarizability $$\beta_{\text{HRS}} \left( { - 2\omega ;\omega ,\omega } \right)$$), and the depolarization ratio DR of these complexes are calculated by DFT with PBE0 functional in order to understand nonlinear optical properties. Higher polarizability $$(\alpha )$$ and first hyperpolarizabilities $$(\beta )$$ of these complexes are obtained. It is also observed an inverse correlation between the predicted β value and the HOMO–LUMO energy difference (Δe).

Monodentate Benzo[d]imidazole‐Based Iridium(III) Complexes and Their Dual Fluorescent and Phosphorescent Emissions

We herein reported the preparation of three heteroleptic iridium complexes possessing the monodentate benzo[d]imidazole ligand, namely IrBzH (bis[2‐(4,6‐difluorophenyl)pyridinato‐C^N](1‐pyridin‐2‐yl)‐1H‐benzo[d]imidazolate)iridium(III)), IrBzCN (bis[2‐(4,6‐difluorophenyl)pyridinato‐C^N](2‐(1H‐benzo[d]imidazolate‐1‐yl)isonicotinonitrile)iridium(III)), and IrBzBr (bis[2‐(4,6‐difluorophenyl)pyridinato‐C′N](1‐(4‐bromopyridin‐2‐yl)‐1H‐benzo[d]imidazolate)iridium(III)). The successful preparation of these complexes was confirmed by multinuclear NMR spectroscopy and elemental analysis, while the molecular structures were determined by X‐ray diffraction. The photoluminescence spectra of all three complexes in toluene at ambient temperature exhibited a dual‐emissive pattern in the high‐ and low‐energy regions. For IrBzCN and IrBzBr, the emissions were simultaneous in accordance with the excitation energies (λem = 342 and 370 nm), indicating that exhibiting both fluorescent (in the high‐energy region) and phosphorescent (in the low‐energy region) emissions is an inherent property of these complexes. Time‐dependent density functional theory calculations verified that each fluorescent emission of the Ir complexes is associated with a ligand‐to‐ligand charge transfer transition, and the phosphorescent emission can be attributed to typical triplet metal‐to‐ligand charge transfer transitions between Ir3+ and the difluorophenylpyridine ligand.

Correction: Tetrafluorinated phenylpyridine based heteroleptic iridium(iii) complexes for efficient sky blue phosphorescent organic light-emitting diodes

Correction for ‘Tetrafluorinated phenylpyridine based heteroleptic iridium(iii) complexes for efficient sky blue phosphorescent organic light-emitting diodes’ by Zhao Chen et al., J. Mater. Chem. C, 2020, 8, 2551–2557, DOI: 10.1039/C9TC05779J.

Unusual dual-emissive heteroleptic iridium complexes incorporating TADF cyclometalating ligands.

Five new neutral heteroleptic iridium(iii) complexes IrL2(pic) (2-6) based on the archetypical blue emitter FIrpic have been synthesised. The cyclometallating ligands L are derived from 2-(2,6-F2-3-pyridyl)-4-mesitylpyridine (7), 2-(3-cyano-2,6-F2-phenyl)-4-mesitylpyridine (8), 2-(2,6-F2-phenyl)-4-[2,7-(HexO)2-9H-carbazol-9-yl]pyridine (9), 2-(2,6-F2-3-pyridyl)-4-[2,7-(HexO)2-9H-carbazol-9-yl]pyridine (10) and 2-(3-cyano-2,6-F2-phenyl)-4-[2,7-(HexO)2-9H-carbazol-9-yl]pyridine (11) for complexes 2, 3, 4, 5 and 6, respectively. The carbazole-functionalised ligands 9-11 show weak thermally activated delayed fluorescence (TADF) in solution. Complexes 5 and 6 reveal dual emission in polar solvents. A broad charge transfer (CT) band appears and increases in intensity relative to the higher energy emission band as solvent polarity is increased. The dual emission occurs when the energy of the ligand 3CT state is comparable to that of the 3MLCT state of the complex, resulting in fast interconversion between the two. Assignment of the ligand TADF and dual emission properties is supported by hybrid density functional theory (DFT) and time dependent DFT (TD-DFT) calculations. Phosphorescent organic light emitting devices (PhOLEDs) have been fabricated using these complexes as sky-blue emitters, and their performance is compared to devices using FIrpic and the previously reported complex IrL2(pic) 1 (L from the 2-(2,6-F2-phenyl)-4-mesitylpyridine ligand). For identical device structures, the device containing the carbazole complex 4 performs best out of the seven complexes. The dual emission observed in solution for complexes 5 and 6 is not observed in their devices.

Tetrafluorinated phenylpyridine based heteroleptic iridium(iii) complexes for efficient sky blue phosphorescent organic light-emitting diodes

Tetrafluorinated phenylpyridine has been used to achieve sky blue-emitting iridium(iii) phosphors and their organic light-emitting diodes exhibit high external quantum efficiencies (∼20%) with cyan emission colours.

A red emissive poly(dendrimer) for solution processed organic light-emitting diodes

Abstract A red-emissive poly(dendrimer) comprised of a norbornenyl-derived polymer backbone with a phosphorescent dendrimer side-chain has been prepared by ring-opening methathesis polymerization with a number average molecular weight of 164 kDa and a dispersity of 2.4. The dendrimer side-chain had a heteroleptic iridium(III) complex core containing two 2-phenylpyridyl ligands and one 2-(thiophen-2-yl)quinoline ligand. The 2-phenylpyridyl ligands had first generation fluorenylcarbazole dendrons with n-propyl surface groups attached and the 2-(thiophen-2-yl)quinoline ligand was linked to the polymer backbone. In spite of the presence of the two 2-phenylpyridine ligands that would normally give rise to green emission when complexed with iridium(III), the 2-(thiophen-2-yl)quinoline was responsible for the observed saturated red emission with CIE coordinates of (0.67, 0.33). The poly(dendrimer) had a high photoluminescence quantum yield (PLQY) for a red emissive species at 72 ± 7%, which was maintained when blended with 4,4′-bis(carbazol-9-yl)biphenyl (CBP) at a concentration of 10 wt % (PLQY = 74 ± 8%). Simple two-layer devices containing the blend emissive layer and an electron transporting hole blocking layer had a maximum external quantum efficiency of almost 11%.

Organometallic Iridium(III) Complex Sensitized Ternary Hybrid Photocatalyst for CO2 to CO Conversion.

A series of heteroleptic iridium(III) complexes functionalized with two phosphonic acid (-PO3 H2 ) groups (dfppy IrP, ppy IrP, btp IrP, and piq IrP) were prepared and anchored onto rhenium(I) catalyst (ReP)-loaded TiO2 particles (TiO2 /ReP) to build up a new IrP-sensitized TiO2 photocatalyst system (IrP/TiO2 /ReP). The photosensitizing behavior of the IrP series was examined within the IrP/TiO2 /ReP platform for the photocatalytic conversion of CO2 into CO. The four IrP-based ternary hybrids showed increased conversion activity and durability than that of the corresponding homo- (IrP+ReP) and heterogeneous (IrP+TiO2 /ReP) mixed systems. Among the four IrP/TiO2 /ReP photocatalysts, the low-energy-light (>500 nm) activated piq IrP immobilized ternary system (piq IrP/TiO2 /ReP) exhibited the most durable conversion activity, giving a turnover number of ≥730 for 170 h. A similar kinetic feature observed through time-resolved photoluminescence measurements of both btp IrP/TiO2 and TiO2 -free btp IrP films suggests that the net electron flow in the ternary hybrid proceeds dominantly through a reductive quenching mechanism, unlike the oxidative quenching route of typical dye/TiO2 -based photolysis.

Theoretical insight into the photophysical properties of five phosphorescent heteroleptic iridium(iii) complexes bearing oxadiazol-substituted amide ligands.

A DFT/TDDFT (density functional theory/time-dependent density functional theory) investigation on the geometries in the ground and lowest triplet excited states, the frontier molecular orbitals, the absorption spectra and the phosphorescent emission properties of five heteroleptic iridium(iii) complexes have been performed to obtain a better understanding of structure-property relationships. The key aim was to investigate the effect of π-conjugation on the photophysical properties of these studied complexes. The lowest energy absorption wavelengths are located at 461 nm for 1, 418 nm for 2, 422 nm for 3, 426 nm for 4 and 417 nm for 5. The lowest energy emissions of complexes 1-5 are localized at 572, 463, 583, 707 and 553 nm, respectively, simulated in acetonitrile medium at M052X level. These findings could be useful to provide good guidance for the further design of new potential phosphorescent organic light-emitting diode (OLED) materials.

Ancillary Ligand Effects on Red-Emitting Cyclometalated Iridium Complexes.

In this work, a series of ten new red-emitting heteroleptic iridium(III) complexes of the type Ir(C^N)2 (L^X) (C^N=cyclometalating ligand, L^X=monoanionic chelating ancillary ligand) is introduced. The suite of new complexes includes two different cyclometalating ligands and five different ancillary ligands, with the primary goal of investigating the effect of the ancillary ligand structure on the excited-state dynamics. The structural variety of the ancillary ligands permitted investigations of the effects of donor atom identity, chelate ring size, and substituents on the electronic structure and excited state properties. Electrochemical analysis showed that the ancillary ligand has a substantial effect on the energy of the HOMO, whereas the LUMO is left unperturbed. Photoluminescence spectra showed that the ancillary ligand can sometimes strongly influence the emission wavelength, but in all cases is an important determinant of the excited-state dynamics.