Chloro-bridged Iridium Research Articles

Synthesis, spectroscopic characterizations, photophysical properties and DFT studies of a novel iridium(III) complex containing 2-(2-butoxy-4-fluorophenyl)pyridine ligand

The iridium(III) complex, Ir(bFppy)2(pyim) (bFppy = 2-(2-butoxy-4-fluorophenyl)pyridine and pyim = 2-(1H-Imidazol-2-yl)pyridine) was excellently prepared between 2-(1H-Imidazol-2-yl)pyridine and chloro-bridged iridium(III) dimer via reflux reaction. Butoxy functional group and fluorine atom were added to the phenyl ring of 2-phenylpyridine at positions 2 and 4 respectively, as cyclometalating ligand. The Ir(bFppy)2(pyim) complex was characterized by spectroscopic methods: FTIR, NMR, LCMS and UV–Vis absorption. The presence of two sharp peaks at 1256 cm−1 and 1129 cm−1 due to ν(CO) was revealed by IR analysis. The bands appear in the range of 1594–1567 cm−1 owing to the CC and CN aromatic ring stretching vibrations of phenyl and pyridine. The 1H NMR spectrum showed signals in the range of δ 5.50–8.60 ppm corresponding to phenylpyridine and pyridylimidazole protons. In contrast, the proton signals in the range of δ 0.80–4.50 ppm, assignable to butyl substituent. The UV–Vis spectrum displayed weaker and broader bands (333 nm and 376 nm) in the visible region due to the spin-forbidden 3MLCT transitions. The steady-state emission spectrum of the Ir(bFppy)2(pyim) complex in air-equilibrated methylene chloride solution at 298 K exhibited almost blue emission with a λmax at 466 nm. To support the experimental spectroscopic data, DFT and TD-DFT measurements were conducted using B3LYP in combination with the LanL2DZ basis set. The chemical shifts, vibrational modes and maximum absorptions of the Ir(bFppy)2(pyim) complex were relatively well reproduced.

Blue Light-Emitting Electrochemical Cells with Ir(Ⅲ) Complex

Light-emitting electrochemical cells (LECs) are one of promising technology for light emitting devices. LECs have some advantages which are single layer structure with air-stable metal electrodes and to use solution process for the device fabrication. Full color emission which means red, green and blue lights is needed for the light application. Phosphorescent Ir(Ⅲ) complex have potential to emit full color because iridium complex has a wide energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). In particular, archetypal cationic Ir(Ⅲ) complex [Ir(ppy)2(bpy)]+, where ppy and bpy stands for 2-phenylpyridine (Hppy) as the cyclometalating ligand and 2,2’-bipyridine (bpy) as the ancillary ligand, respectively. Green, yellow, orange and red emitting cationic Ir(Ⅲ) complexes have been synthesized so far. In contrast, the number of blue emitting cationic Ir(Ⅲ) complexes are limited. Blue light emission is essential for full color emission. Widening energy gap of Ir(Ⅲ) complexes causes blue shift. For example, Fluorine element which is electron-deficient substituents have been attached cyclometalating ligands to stabilize HOMO. However, fluorine substitution degrades the electrochemical and thermal stability of the complex. It is necessary to find a cyclometalating ligand without fluorine substitution for stable blue light emission.1-(2,6-dimethylphenyl)-3-methyl-5-phenyl-1H-1,2,4-triazole (dphtz) was synthesized as a cyclometalating ligand for blue light emitting cationic Ir(Ⅲ) complex. 2-(1Hpyraol-1- yl)pyridine (pzpy) was used as an ancillary ligand. [Ir(dphtz)2(pzpy)](TFSI) was synthesized in a three-step reaction. First, the chloro-bridged iridium dimers, [Ir(dphtz)2Cl]2 was synthesized from IrCl3・nH2O and dphtz. Second, [Ir(dphtz)2(pzpy)]Cl was synthesized from [Ir(dphtz)2Cl]2 and pzpy. Third, LiTFSI was added to [Ir(dphtz)2(pzpy)]Cl. Cl- was replaced by TFSI-, and [Ir(dphtz)2(pzpy)](TFSI) was synthesized[1].Absorption and photoluminescence (PL) spectrum of [Ir(dphtz)2(pzpy)](TFSI) were measured in CH3CN solution to characterize photophysical property. Absorption spectrum indicates that the complex has absorption under 399 nm, resulting that the bandgap is calculated to be 3.11 eV. PL spectrum indicates blue emission with peak wavelength of 466 nm. The peak wavelength is blue-shifted by nearly 98 nm compared with archetypal cationic Ir(Ⅲ) [Ir(ppy)2(bpy)](TFSI) (λ= 564 nm) of our previous work. Cyclic voltammetry in solution was performed to investigate the electrochemical property of the complex. The voltammogram shows that the oxidation potential is 1.40 V and reduction potential is -1.77 V vs. Ag/Ag+. From these two values, the theoretical drive voltage of LECs using the complex can be calculated to be 3.17 V. In addition, the complex exhibits reversible oxidation process and irreversible reduction process in CH3CN. These properties would cause unbalanced electron and hole injection of LECs device.LECs using the complex was fabricated. The device structure consists of ITO/complex:20 wt％ EMIMTFSI:6.25 wt％ PMMA(polymethylmethacrylate) /LiF (0.5 nm)/Ag (75 nm). EL spectrum shows blue light emission with centered 440 nm. However, the blue emission did not keep for a long time. After a minute, the emission color changed gray EL with centered 463 nm. Under a constant driving current of 2.0 mA, the LECs provide a peak brightness of 9.9 cd/m2. This low luminance may be due to unbalanced electron and hole injection. Under a constant driving current of 2.0 mA, the driving voltage shows 3.95 V, whereas after a minute, it exceeds 4.5 V, indicating that [Ir(dphtz)2(pzpy)](TFSI) was degraded, and LECs does not function optimally.In conclusion, the blue light emitting complex in this study shows poor luminance and short lifetime for LECs. In order to get higher luminance and longer lifetime, we will improve the chemical stability of the Ir(Ⅲ) complex and the balance of the electron and hole injection into the emitting layer of LECs.Reference[1] X.Wang et al., Inorg.Chem., 58, 12132 (2019).

Microwave-Accelerated C,N-Cyclometalation as a Route to Chloro-Bridged Iridium(III) Binuclear Precursors of Phosphorescent Materials: Optimization, Synthesis, and Studies of the Iridium(III) Dimer Behavior in Coordinating Solvents.

We present the results of our research on the use of microwaves as an unconventional heat source for the acceleration of iridium(III) chloro-bridged dimer preparation. The results enabled us to revise and improve known guidelines for the very quick and highly efficient synthesis of iridium(III) dimeric complexes in a very simple isolation manner. According to the developed methodology, the already known dimers containing ligands based on the 2-phenylpyridinato motif, as well as new ones stabilized with functionalized benzo[h]quinolinato and 2-phenoxypyridinato-based ligands, were efficiently synthesized. The scope of the incorporated ligands included compounds equipped with electron-donating (-Me, -OMe, -OPh, -NMe2), electron-withdrawing (-F, -Br, -CF3, -C6F5), and hole-transporting (-NPh2, -C6H4NPh2) groups. The obtained complexes were characterized by NMR, X-ray diffraction, and electrospray ionization mass spectrometry methods, and their behavior was examined in the presence of coordinating solvents such as dimethyl sulfoxide and acetonitrile. Investigation of the interactions between the above-mentioned solvents and dimers enabled us to confirm the ability of the former to cleave μ-chloride bridges, which enriches the knowledge in the field of organometallic chemistry. This knowledge can be particularly useful for the scientists working in the field of iridium-based materials, helping to avoid misinterpretation of the spectroscopic data.

Phosphorescent heteroleptic iridium(III) cyclometallates: Improved syntheses of acetylacetonate complexes and quantum chemical studies of their excited state properties

We have investigated methods to prepare cyclometallated iridium(III) complexes with efficient photoluminescence spanning a broad color palette. In particular, we find that addition of ancillary ligands to chloro-bridged iridium dimers proceeds cleanly in refluxing 1,2-dimethoxyethane (DME) without the need for additional product purification. This represents an improvement over the conventional use of 2-ethoxyethanol which requires column chromatographic separation. Our efforts in this work have focused on acetylacetonate complexes such as (F2ppy)2Ir(acac), where F2ppy = 2-(4′,6′-diflurophenyl)pyridinato. We have prepared fifteen compounds by the route, eight of which are newly reported; in four cases we were able to prepare complexes which were inaccessible via the conventional route. Nine of the complexes were characterized by single crystal x-ray diffraction and possess the same distorted octahedral geometry around the iridium with two bidentate phenylpyridine ligands and one bidentate acetylacetonate ligand. Seven of the complexes exhibited efficient photoluminescence with colors ranging from yellow to blue and quantum yields of 0.51–0.74. All of the compounds with trifluoromethyl or phenyl substituents on the acetylacetone displayed emission in the orange with low quantum efficiency. The use of TD-DFT calculations, along with natural transition orbitals (NTOs), permitted a detailed interpretation of the electronic structures for the complexes. The nature of the acceptor orbitals for the low energy triplet state NTOs proved to be an important predictor for the emission spectra of the compounds.

Formylated chloro-bridged iridium(III) dimers as OLED materials: opening up new possibilities.

In this study, a series of four formyl-substituted chloro-bridged iridium(iii) dimers were prepared. Their absorption, photophysical and electrochemical properties were studied in dichloromethane solution. It was found that as the formyl content increased on the cyclometalating ligands, emission unexpectedly became brighter. Organic light-emitting diodes (OLEDs) were fabricated using each of these iridium dimers as the emitter. The OLED fabricated using the brightest of the series, 2b, as the dopant afforded a decent external quantum efficiency (EQE) of 2.6%. This suggests that chloro-bridged iridium dimers are potential candidates as solid-state emitters.

A Cyclometalated Ir(III) Complex Containing N-naphthyl Picolinamide Ancillary Ligand

The reaction of the cyclometalated chloro-bridged iridium(III) dimer, [(ppy)2 Ir(μ-Cl)]2 (ppy = 2-phenyl pyridine) with N-naphthyl picolinamide (LH) in the presence of sodium methoxide afforded a neutral heteroleptic monomer [Ir(ppy)2L] (1). The crystal structure of 1 reveals that the ligand, in its deprotonated form, binds to the iridium center as a N,O donor. The photophysical properties reveal that 1 is a weakly emissive complex (515, 538 nm) when excited at 450 nm. DFT calculations were carried out to understand the photophysical properties. A cyclometalated iridium(III) complex [Ir(ppy)2L] (1) (LH = N-naphthyl picolinamide) where L functions as N,O chelating donor has been synthesized and structurally characterized. 1 is weakly emissive (515, 538 nm) when excited at 450 nm.

Direct observation of reversible electronic energy transfer involving an iridium center.

A cyclometalated iridium complex is reported where the core complex comprises naphthylpyridine as the main ligand and the ancillary 2,2′-bipyridine ligand is attached to a pyrene unit by a short alkyl bridge. To obtain the complex with satisfactory purity, it was necessary to modify the standard synthesis (direct reaction of the ancillary ligand with the chloro-bridged iridium dimer) to a method harnessing an intermediate tetramethylheptanolate-based complex, which was subjected to acid-promoted removal of the ancillary ligand and subsequent complexation. The photophysical behavior of the bichromophoric complex and a model complex without the pendant pyrene were studied using steady-state and time-resolved spectroscopies. Reversible electronic energy transfer (REET) is demonstrated, uniquely with an emissive cyclometalated iridium center and an adjacent organic chromophore. After excited-state equilibration is established (5 ns) as a result of REET, extremely long luminescence lifetimes of up to 225 μs result, compared to 8.3 μs for the model complex, without diminishing the emission quantum yield. As a result, remarkably high oxygen sensitivity is observed in both solution and polymeric matrices.

Homoleptic vs. Heteroleptic Orange Light-Emitting Iridium Complexes Chelated with Benzothiazole Derivatives

The homoleptic and heteroleptic iridium(III) complexes exhibiting orange phosphorescence were investigated to compare their emission colors, luminance efficiency, and stability. The homoleptic iridium complexes, Ir(pbt-R)3, were prepared from the reaction of the iridium(III) precursor and phenylbenzothiazole derivatives. The heteroleptic ones containing the anions of 4-methyl-2,3-diphenylquinoline (4-Me-dpq) and 2-phenylbenzothiazole (pbt) as ligands, Ir(pbt-OMe)2(4-Me-2,3-dpq) and Ir(4-Me-2,3-dpq)2(pbt-OMe), were synthesized via the chloro-bridged iridium dimer. We investigated photoabsorption and photoluminescence (PL) properties of the iridium complexes and studied their bandgaps with cyclic voltammetry (CV). Orange phosphorescence with the PL maxima of 530–620 nm was observed with these complexes, and the bandgaps between their highest occupied molecular orbitals (HOMOs) and the lowest unoccupied molecular orbitals (LUMOs) were correlated with CV data. The electro-luminescence (EL) study of these complexes was not possible due to lack of sublimability. The light-emitting mechanisms regarding the phosphorescence colors were discussed.

A heteroleptic cyclometalated iridium(iii) fluorophenylpyridine complex from partial defluorohydrogenation reaction: synthesis, photophysical properties and mechanistic insights

In this paper, a C-F bond activation reaction of a chloro-bridged iridium(III) dimer (dfppy)(2)Ir(μ-Cl)(2)Ir(dfppy)(2) (1) (dfppy denotes 2-(4,6-difluorophenyl)pyridyl) in the presence of sodium methoxide has been reported, leading to the formation of a heteroleptic cyclometalated iridium(III) fluorophenylpyridine complex 2. HPLC-mass analysis confirmed the release of formaldehyde in the product mixture. When sodium benzyloxide was used as the base, complex 2 was also generated with the release of a benzaldehyde derivative. Complex 2 has been fully characterized by (1)H-NMR, (19)F-NMR and X-ray crystallographic methods, confirming the partial loss of one of the fluorine atoms on one of the cyclometalated phenylpyridyl ligands. Photophysical studies of complex 2 show that it has a similar absorption spectrum to that of Ir(III)(dfppy)(3). However, the emission spectrum shows a red shift maximum emission band at 478 nm due to the loss of a single fluorine atom, highlighting the critical effect of fluorine on the photoluminescence of these Ir(III) complexes. Finally, intensive mechanistic studies including HPLC-mass analysis, (1)H-NMR, and (19)F-NMR studies demonstrate that the formation of complex 2 should involve a critical β-hydride elimination of Ir(III)-alkoxide intermediate and the participation of Ir-hydride and/or Ir-fluoride intermediates.

Acid-Induced Degradation of Phosphorescent Dopants for OLEDs and Its Application to the Synthesis of Tris-heteroleptic Iridium(III) Bis-cyclometalated Complexes

Investigations of blue phosphorescent organic light emitting diodes (OLEDs) based on [Ir(2-(2,4-difluorophenyl)pyridine)(2)(picolinate)] (FIrPic) have pointed to the cleavage of the picolinate as a possible reason for device instability. We reproduced the loss of picolinate and acetylacetonate ancillary ligands in solution by the addition of Brønsted or Lewis acids. When hydrochloric acid is added to a solution of a [Ir(C^N)(2)(X^O)] complex (C^N = 2-phenylpyridine (ppy) or 2-(2,4-difluorophenyl)pyridine (diFppy) and X^O = picolinate (pic) or acetylacetonate (acac)), the cleavage of the ancillary ligand results in the direct formation of the chloro-bridged iridium(III) dimer [{Ir(C^N)(2)(μ-Cl)}(2)]. When triflic acid or boron trifluoride are used, a source of chloride (here tetrabutylammonium chloride) is added to obtain the same chloro-bridged iridium(III) dimer. Then, we advantageously used this degradation reaction for the efficient synthesis of tris-heteroleptic cyclometalated iridium(III) complexes [Ir(C^N(1))(C^N(2))(L)], a family of cyclometalated complexes otherwise challenging to prepare. We used an iridium(I) complex, [{Ir(COD)(μ-Cl)}(2)], and a stoichiometric amount of two different C^N ligands (C^N(1) = ppy; C^N(2) = diFppy) as starting materials for the swift preparation of the chloro-bridged iridium(III) dimers. After reacting the mixture with acetylacetonate and subsequent purification, the tris-heteroleptic complex [Ir(ppy)(diFppy)(acac)] could be isolated with good yield from the crude containing as well the bis-heteroleptic complexes [Ir(ppy)(2)(acac)] and [Ir(diFppy)(2)(acac)]. Reaction of the tris-heteroleptic acac complex with hydrochloric acid gives pure heteroleptic chloro-bridged iridium dimer [{Ir(ppy)(diFppy)(μ-Cl)}(2)], which can be used as starting material for the preparation of a new tris-heteroleptic iridium(III) complex based on these two C^N ligands. Finally, we use DFT/LR-TDDFT to rationalize the impact of the two different C^N ligands on the observed photophysical and electrochemical properties.

Cyclometalated Ir(III) complexes containing N-aryl picolinamide ancillary ligands

The reaction of the cyclometalated chloro-bridged iridium(III) dimer, [(ppy) 2 Ir(μ-Cl)] 2 (ppy – 2-phenyl pyridine) with N-aryl picolinamides (LH, LH–NO 2, LH–CH 3, LH–l, LH–F) resulted in the formation of neutral heteroleptic complexes [Ir(ppy) 2L] ( 1), [Ir(ppy) 2L–NO 2]( 2), [Ir(ppy) 2L–CH 3]( 3), [Ir(ppy) 2L–Cl]( 4) and [Ir(ppy) 2L–F] ( 5). These complexes contain a six-coordinate iridium with a 2C, 4N coordination environment. The N-aryl picolinamide ligands are deprotonated during complexation and the resulting amidates bind to iridium in a chelating manner ( N, N). Optical spectroscopic studies revealed that the complexes 1– 5 exhibited intense π→π ∗ absorptions in the ultraviolet region. In addition low energy transitions due to 1MLCT, 1LLCT and 3MLCT are also seen. The emission spectra of 1– 5, upon excitation at 450 nm, show a single emission with a λ max around 513 nm. The lifetimes of this emission are in between 7.4 and 9.6 μs while the quantum yields are quite high and range from 0.2 to 0.5. Based on density functional theory (DFT) calculations on 1 and 3, the three highest occupied orbitals are composed of ligand π orbitals mixed with Ir-d orbitals while the three lowest unoccupied orbitals are mostly π orbitals of the ligands. From the time dependent DFT calculations it is revealed that the lowest energy electronic singlet and triplet excitations are a mixture of MLCT and LLCT.