Synthesis and characterization of phosphorescent iridium complexes containing trifluoromethyl-substituted phenyl pyridine based ligands

Several iridium complexes containing trifluoromethyl-substituted phenyl pyridine based ligands have been synthesized and characterized to try to investigate the effect of trifluoromethyl group and its position on physical properties. The complexes have the general structure of (C-N)₂Ir(LX), where the C-N are 2-phenylpyridine (ppy), 2-(3,5-bis-trifluoromethylphenyl)pyridine (fmppy), 2-(3,5-bis-trifluoromethylphenyl)-4-methylpyridine (fmpmpy), 2-(3,5-bis-trifluoromethylphenyl)-5-trifluoromethylpyridine (tfmppy) and the LX are 2-picolinic acid (pic) and acetylacetonate (acac). The (tfmppy)₂Ir(pic) was characterized using X-ray crystallography. The absorption, emission, and thermostability of the complexes were systematically investigated. Introduction of CF₃ substituents into 2-phenylpyridine in (ppy)₂Ir(pic) lead to some decrease in the sublimation temperature, which is more suitable to devices fabrication. The experimental results revealed that the emissive colors of these complexes could be finely tuned by suitable incorporation of trifluoromethyl substituents on the 2-phenylpyridine ligand, obtaining bright green-blue emission λ_max values from 471 to 489 nm in CH₂Cl₂ solution at room temperature, with high solution quantum efficiencies ranging from 0.37 to 1.89 relative to Ir(ppy)₃.     

Synthesis, crystallography, phosphorescence of platinum complexes coordinated with 2-phenylpyridine and a series of β-diketones

Metallocyclic platinum(II) complexes coordinating with 2-phenylpyridine (ppy) and a series of β-diketone ancillary O^∧O ligands, (ppy)Pt(acac), (ppy)Pt(ba), (ppy)Pt(dbm), and (ppy)Pt(tta) (acac = acetylacetone, ba = benzoylacetone, dbm = dibenzoylmethane, tta = thenoyltrifluoroacetone) were synthesized. The crystal structure, absorption, emission, quantum yield and phosphorescence life time were characterized. As the conjugative π system of the O^∧O ligand increases in the order acac < ba < dbm, or there is a group -CF₃to attract the electron density of the tta ligand, the quantum yield decreases in the order (ppy)Pt(acac) > (ppy)Pt(ba) > (ppy)Pt(dbm) > (ppy)Pt(tta) due to an energy back-flow from ppy to the O^∧O ligand, a trend also in contrast to the phosphorescence emission spectra and time decay (biexponentially, ∼0.7-13 μs).     

Red to near-infrared electrophosphorescence from an iridium complex coordinated with 2-phenylpyridine and 8-hydroxyquinoline

An iridium complex coordinated with 2-phenylpyridine (ppy) and 8-hydroxyquinoline (q), ppy₂Irq, was synthesized and its thermal stability, absorption, photoluminescence, crystal structure and electrophosphorescence were characterized. The melting point of this material reaches as high as 374 °C and does not suffer decomposition upon heating at high vacuum therefore can be well sublimated. When ppy₂Irq was used as a guest emitting material in the electrophosphorescent device, the emission is 100% saturated red light starting at ∼600 nm, extending into the near-infrared region. The bathochromic shift, compared to the fluorescence and phosphorescence from Alq₃, Ptq₂ and Ir(ppy)₃, was analyzed to originate from the triplet excited state of 8-hydroxyquinoline ligand and the crystal structure analysis excludes the origin of π-π intermolecular interactions.     

Synthesis, structure, and photophysical properties of a bis-cyclometalated heteroleptic iridium(III) complex containing 2,2′-dipyridylamine

The synthesis, X-ray crystal structure and luminescence properties of a new bis-cyclometalated heteroleptic complex, [Ir(ppy)₂(HDPA)](PF₆) (1, ppy = 2-phenylpyridine, HDPA = 2,2′-dipyridylamine), are reported. Both at room temperature and 77 K, complex 1 exhibits intense blue emission, assigned as ³MLCT [dπ(Ir)-π^∗(HDPA)] phosphorescence. Analysis of voltammetric data also provides evidence in support of this assignment.     

Multimetallic compounds containing cyclometalated Ir(III) units: Synthesis, structure, electrochemistry and photophysical properties

The reaction of the cyclometalated Ir^III dimer [{(ppy)₂Ir}₂(μ-Cl)₂] (ppyH = 2-phenylpyridine) with silver triflate followed by a multidentate ligand [1,4-bis[3-(2-pyridyl)pyrazolylmethyl]benzene (bppb), 1,3,5-tri[3-(2-pyridyl)pyrazolylmethyl]-2,4,6-trimethylbenzene (tppb), 2,4,6-tris(2-pyridyl)-1,3,5-triazine (tptz), 2-chloro-4,6-bis(dipyridin-2-ylamino)-1,3,5-triazine (cddt) or 2,4,6-tris(dipyridin-2-ylamino)-1,3,5-triazine (tdat)] afforded di- or trinuclear compounds: [{Ir(ppy)₂}₂(μ-bppb)](OTf)₂ (1), [{Ir(ppy)₂}₃(μ-tppb)](OTf)₃ (2), [{Ir(ppy)₂}₂(μ-tptz-OH)](OTf) (3), [{Ir(ppy)₂}₂(μ-cddt)](OTf)₂ (4) and [{Ir(ppy)₂}₂(μ-tdat)](OTf)₂ (5). All of these compounds contain cationic metal cores with corresponding triflate counter anions. The molecular structures of 1-4 reveal that the structural feature of the Ir(ppy)₂ center of the starting precursor is conserved in the products. Also, because of the nature of the ligands, there is virtually no electronic communication between the Ir^III centers except in 3 where a ring hydroxylation at the triazine carbon atom is effected upon metalation. Compounds 1-5 are robust in solution where they retain their structural integrity. The UV-Vis and emission spectra of 1-5 compounds are very similar to each other with the exception of 3 which seems to possess a different electronic structure. All the compounds are luminescent at room temperature. The emission bands indicate significant contribution from ³LC. Increase in the number of ‘Ir(ppy)₂’ units does not have any effect on emission color.     

Novel solid state emitting iridium complexes with reduced phosphorescence-self-quenching triggered by steric blocking phosphine ligands

In this paper, we report two easily-synthesized Ir(III) complexes equipped with steric blocking phosphine ligands of bis(2-(diphenylphosphanyl)phenyl) (POP) and triphenylphosphane (PPh₃), Ir-POP and Ir-PPh₃. Their crystal structures, electronic nature, photophysical properties, and phosphorescence self-quenching mechanism are discussed in detail. It is found that the thermally activated intermolecular interaction is responsible for the phosphorescence self-quenching. The introduction of steric blocking ligands into Ir(III) complexes can efficiently suppress the phosphorescence self-quenching in solid state. We successfully realize a solid state emission peaking at 494 nm, with a long excited state lifetime of 15 μs and a high photoluminescence quantum yield of 0.17.     

Electrogenerated chemiluminescence of Pb(II)-bromide complexes

The electrochemistry and electrogenerated chemiluminescence (ECL) of Pb₄Br₁₁ ³⁻ in acetonitrile solution is reported. Pb₄Br₁₁ ³⁻ is formed in situ by the reaction of lead(II) and bromide ions with ECL generated upon sweep to positive potentials using tri-n-propylamine (TPrA) as an oxidative-reductive coreactant. An ECL efficiency (φ_ecl) of 0.0079 was obtained compared to Ir(ppy)₃ (ppy=2-phenylpyridine; φ_ecl=1). The ECL intensity peaks at a potential corresponding to oxidation of TPrA and Pb₄Br₁₁ ³⁻ indicating that emission is from the lead-bromide cluster.     

Tuning iridium(III) complexes containing 2-benzo[b]thiophen-2-yl-pyridine based ligands in the red region

Synthesis and characterization of four iridium(III) complexes containing 2-benzo[b]thiophen-2-yl-pyridine based ligands are reported. The absorption, emission, electrochemistry, and thermostability of the complexes were systematically investigated. The (btmp)₂Ir(acac) (btmp = 2-benzo[b]thiophen-2-yl-4-methyl-pyridyl, acac = acetyl acetone) was characterized using X-ray crystallography. Calculation on the electronic ground state for (btmp)₂Ir(acac) was carried out using B3LYP density functional theory, HOMO levels are a mixture of Ir and btmp ligand orbitals, while the LUMO is predominantly btmp ligand based. Introduction of substituents (CH₃, CF₃) into pyridyl ring in a typical red emitter (btp)₂Ir(acac) leads to a marked decrease in the sublimation temperature, which is more suitable for OLEDs process. Electrochemical studies showed that (btmp)₂Ir(acac) has a slightly lower oxidation potential, but (btfmp)₂Ir(acac), (btfmp)₂Ir(dbm), and (btfmp)₂Ir(pic) (btfmp = 2-benzo[b]thiophen-2-yl-5-trifluoromethyl-pyridine, dbm = dibenzoylmethane, pic = 2-picolinic acid) containing CF₃ group are much difficult to oxidate than (btp)₂Ir(acac). The emission characteristics of these complexes can be tuned by either changing the substituents and their position on 2-benzo[b]thiophen-2-yl-pyridine or using different monoanionic ligands, showing emission λ_max values from 604 to 638 nm in CH₂Cl₂ solution at room temperature.     

Blue phosphorescent iridium complexes based on 2-(fluoro substituted phenyl)-4-methylpyridines: Synthesis, crystal structure, and photophysics

A series blue phosphorescent emitting materials based on 2-(fluoro substituted phenyl)-4-methylpyridine as the cyclometalated ligands have been synthesized and characterized. The complexes have the general structure (C^{^}N)₂Ir(pic), where C^{^}N is a monoanionic cyclometalating ligand (e.g., 2-(2,4-difluorophenyl)-4-methylpyridine (24f₂pmpyH), 2-(3,4-difluorophenyl)-4-methylpyridine (34f₂pmpyH), 2-(3,5-difluorophenyl)-4-methylpyridine (35f₂pmpyH), and 2-(3,4,5-trifluorophenyl)-4-methyl-pyridine (345f₃pmpyH)), pic is 2-picolinic acid. The absorption, emission, cyclic voltammetry and thermostability of the complexes were systematically investigated. The (46f₂pmpy)₂Ir(pic) has been characterized using X-ray crystallography and the electronic ground state calculated using B3LYP density functional theory. HOMO levels are a mixture of Ir and 2-(fluoro phenyl)-4-methylpyridine ligand orbitals, while the LUMO is predominantly pic ligand based. Introduction of fluorine atoms and methyl group into ppy ligand and changing in position of F substituents in phenyl ring can finely tune emission of the complexes, showing bright blue-to-green luminescence at a wavelength of 463-501 nm at room temperature in CH₂Cl₂.     

Eu3+‐tetrakis β‐diketonate complexes for solid‐state lighting application

Eu³⁺–β‐diketonate complexes are used, for example, in solid‐state lighting (SSL) or light‐converting molecular devices. However, their low emission quantum efficiency due to water molecules coordinated to Eu³⁺ and low photostability are still problems to be addressed. To overcome such challenges, we synthesized Eu³⁺ tetrakis complexes based on [Q][Eu(tfaa)₄] and [Q][Eu(dbm)₄] (Q1 = C₂₆H₅₆N⁺, Q2 = C₁₉H₄₂N⁺, and Q3 = C₁₇H₃₈N⁺), replacing the water molecules in the tris stoichiometry. The tetrakis β‐diketonates showed desirable thermal stability for SSL and, under excitation at 390 nm, they displayed the characteristic Eu³⁺ emission in the red spectral region. The quantum efficiencies of the dbm complexes achieved values as high as 51%, while the tfaa complexes exhibited lower quantum efficiencies (28–33%), but which were superior to those reported for the tris complexes. The structures were evaluated using the Sparkle/PM7 model and comparing the theoretical and the experimental Judd–Ofelt parameters. [Q1][Eu(dbm)₄] was used to coat a near‐UV light‐emitting diode (LED), producing a red‐emitting LED prototype that featured the characteristic emission spectrum of [Q1][Eu(dbm)₄]. The emission intensity of this prototype decreased only 7% after 30 h, confirming its high photostability, which is a notable result considering Eu³⁺ complexes, making it a potential candidate for SSL.     

Heteroleptic κ(N,C)-2-phenylpyridine platinum complexes: The use of bis(pyrazolyl)borates as ancillary ligands

Luminescent platinum(II) bis(pyrazolyl)borate complexes bearing orthometalated phenylpyridines (based on 2-phenylpyridine, 3-hexyloxy-2-phenylpyridine and (4-(pyridine-2-yl)-phenyl)methanol) and bis(pyrazolyl)borate ligands (employed as potassium dihydridobis(pyrazolyl)borate and potassium dihydridobis(3,5-dimethylpyrazolyl)borate) have been synthesized and characterized. By reaction of K₂PtCl₄ with phenylpyridine ligands a precursor has been obtained which was further converted to the corresponding heteroleptic complexes. All platinum(II) bis(pyrazolyl)borate complexes have been investigated by nuclear magnetic resonance, Fourier transform infrared spectroscopy, thermal analysis and UV-Vis absorption spectroscopy. Luminescence as well as lifetime measurements in solution and in the solid state have been performed to establish a qualitative relationship between structure and luminescence properties. The absorption and emission properties of these complexes are governed by the nature of the orthometalating ligand with emission maxima ranging from 488 to 551 nm. Emission spectra at room temperature show well-resolved vibronic fine structures and a strong spin-orbit coupling. For the compounds under investigation the mixing of the excited states leads to apparent lifetimes in the microsecond regime (5.7-8.6 μs), rendering these materials phosphorescent.     

Electrogenerated chemiluminescence from polymer-bound ortho-metallated iridium(III) systems.

Three ortho-metallated iridium complexes whose emission maxima fall in different regions of the electromagnetic spectrum were bound in either Nafion or poly(9-vinylcarbazole) and their electrogenerated chemiluminescence (ECL) reported. The reaction of F(Ir)pic [bis(3,5-difluoro-2-(2-pyridyl)phenyl-(2-carboxypyridyl)-iridium III] with the oxidative-reductive co-reactant tri-n-propylamine (TPrA) resulted in ECL when the iridium complex was bound in Nafion. No significant ECL was observed for (btp)(2)Ir(acac) (bis[2,(2'-benzothienyl)-pyridinato-N,C3'](acetylacetonate)Ir(III)), and Ir(ppy)(3) (where ppy = 2-phenylpyridine) under these conditions. However, all three compounds displayed ECL with TPrA when bound in poly(9-vinylcarbazole).     

Ionic luminescent cyclometalated Ir(III) complexes with polypyridine co-ligands

Two novel ligands containing a functionalized N ∧ N chelating moiety (pbpy-OBut and tpy-COOH, respectively) were treated with [Ir(ppy)₂(μ-Cl)]₂ (ppy = 2-(2-pyridyl)phenyl), leading to the cationic cyclometalated complexes [Ir(ppy)₂(pbpy-OBut)]⁺ (2) (pbpy-OBut = 4-{4′-(4-phenyloxy)-6′-phenyl-2,2′-bipyridyl}butene) and [Ir(ppy)₂(tpy-COOH)]⁺ (3) (tpy-COOH = 4′-(4-carboxyphenyl)-2,2′:6′,2″-terpyridine). Complexes 2 and 3 exhibit intense room temperature luminescence both in solution and as solid films. Assignment of the emissive behavior to a ³LLCT (ppy-to-N ∧ N) excited state is proposed.     

The role of 5-R-1,10-phenanthroline (R = CH3, NO2) on the emission properties and second-order NLO response of cationic Ir(III) organometallic chromophores

[Ir(cyclometallated 4,5-diphenyl-2-methyl-thiazole)₂(5-R-1,10-phenanthroline)][PF₆] (R = CH₃, NO₂) complexes were prepared and fully characterized, the structure of the complex with 5-CH₃-1,10-phenanthroline being also determined by X-ray diffraction. The emission properties of both complexes have been investigated and their second-order nonlinear optical (NLO) response has been determined experimentally by the EFISH technique and found to be similar but slightly lower than that of related [Ir(ppy)₂(5-R-1,10-phenanthroline)][PF₆] (ppy = cyclometallated 2-phenylpyridine), characterized by one of the highest second-order NLO response ever reported for a metal complex. In the complexes, SOS/TDDFT calculations show that the large and negative sign of the measured hyperpolarizability is mainly due to the significant contribution of rather intense MLCT transitions involving the phenanthroline as acceptor ligand.     

Synthesis of a novel tris-cyclometalated iridium(III) complex containing triarylamine unit and its application in OLEDs

The synthesis of the tris-cyclometalated iridium(III) complex [Ir(DCP)₃] (HDCP = 1-(N,N-diphenyl-amino)-4-(4-chlorophenyl)-phthalazine) from hydrated iridium(III) chloride and the ligand HDCP under mild reaction conditions was described. The photophysical, electrochemical and electrophosphorescent properties of this complex were investigated. Organic light-emitting diodes (OLEDs) using the complex as a dopant and a blend of poly(vinylcarbazole) (PVK) with 2-tert-butylphenyl-5-biphenyl-1,3,4-oxadiazol (PBD) as a host exhibited bright red emission at 620 nm with the Commission Internationale de l’Eclairage (CIE) coordinate of (0.67, 0.32). A maximum external quantum efficiency of 13.6% photos/electron with a luminous efficiency of 7.4 cd/A at a current density of 0.73 mA/cm², and a maximum luminance of 2941 cd/m² at 99 mA/cm² were obtained in the device at 4 wt% doping concentration.     

Tuning electronic structure and photophysical properties of [Ir(ppy)2(py)2]+ by substituents binding in pyridyl ligand: a computational study

Iridium (III) 2-phenylpyridine (ppy) complexes with two suitable monodentate L ligands [Ir(ppy)(2)(L)(2)](+) (ppy = 2-phenylpyridine, py = pyridine, L = 4-pyCN 1, 4-pyCHO 2, 4-pyCl 3, py 4, 4-pyNH(2) 5) were studied by density functional theory (DFT) and time-dependent DFT methods. The influences of ligands L on the electronic structure and photophysical properties were investigated in detail. The compositions and energy levels of the lowest unoccupied molecular orbital (LUMO) are changed more significantly than those of the highest occupied molecular (HOMO) by tuning L ligands. With the electronegativity decrease of L ligands 4-pyCN > 4-pyCHO > 4-pyCl > py > 4-pyNH(2), the LUMO distributing changes from py to ppy, and the absorptions have an obvious red shift. The calculated results showed that the transition character of the absorption and emission can be changed by adjusting the electronegativity of the L ligands. In addition, no solvent effect was observed in the absorptions and emissions.     

Electrogenerated chemiluminescence of tris(2‐phenylpyridine)iridium(III) in water,acetonitrile and trifluorethanol

The spectroscopic, electrochemical and coreactant electrogenerated chemiluminescence (ECL) properties of Ir(ppy)3 (where ppy = 2‐phenylpyridine) have been obtained in aqueous buffered (KH2PO4), 50 : 50 (v/v) acetonitrile–aqueous buffered (MeCN–KH2PO4) and 30% trifluoroethanol (TFE) solutions. Tri‐n‐propylamine was used as the oxidative‐reductive ECL coreactant. The photoluminescence (PL) efficiency (ϕem) of Ir(ppy)3 in TFE (ϕem ≈ 0.029) was slightly higher than in 50 : 50 MeCN–KH2PO4 (ϕem ≈ 0.0021) and water (ϕem ≈ 0.00016) compared to a Ru(bpy)32+ standard solution in water (Φem ≈ 0.042). PL and ECL emission spectra were nearly identical in all three solvents, with dual emission maxima at 510 and 530 nm. The similarity between the ECL and PL spectra indicate that the same excited state is probably formed in both experiments. ECL efficiencies (ϕecl) in 30% TFE solution (ϕecl = 0.0098) were higher than aqueous solution (ϕecl = 0.00092) system yet lower than a 50% MeCN–KH2PO4 solution (ϕecl = 0.0091). Copyright © 2014 John Wiley & Sons, Ltd.     

[Ir(acac)(η-C8H14)2]: A precursor in the synthesis of cyclometalated iridium(III) complexes

A new convenient synthesis and the crystallographic characterization of [Ir(acac)(coe)₂] (2, acac = acetylacetonato; coe = cis-cyclooctene) are described. The title compound crystallized from THF/ethanol in two modifications (monoclinic P2₁/c, 2a, and triclinic , 2b). Complex 2 represents an efficient starting material in the synthesis of mononuclear iridium(III) complexes containing cyclometalated 2-phenylpyridinato ligands using oxidative addition reactions of the corresponding ligands towards 2. Thus [Ir(acac)(ppy)₂] (3, ppy = 2-phenylpyridinato) and [Ir(ppy)₃] (4) (mer, 4a; fac, 4b) were prepared in excellent yields and short reaction times in a kind of one-pot procedure starting from [{Ir(μ-Cl)(coe)₂}₂] (1). Furthermore a convenient synthesis of [{Ir(μ-Cl)(ppy)₂}₂] (5) from 1 and Hppy is described.     

Remarkable chiral and luminescent properties of novel Yb(III) and Eu(III) complexes containing BINAPO ligand

Lanthanide complexes are of great importance for their prospective applications in wide range of science and technology. Chiral lanthanide complexes can constitute stereo-discriminating probes in biological media, owing to the luminescent properties of the rare-earth ions. Sensitized emission with narrow bandwidth, having fast radiation rate and high emission quantum efficiency are the main perspective for synthesizing the complexes. Attention has been given on remarkable chirality with high dissymmetry factors (g = Δε_ext/ε_max) of the complexes. For this purpose, beta-diketonato ligands with chiral BINAPO (1,1′-binapthyl phosphine oxide) ligand were chosen to achieve the goal. The complexes [Ln(TFN)₃(S-BINAPO)](TFN = 4,4,4-trifluoro-1(2-napthyl)-1,3-butanedione), [Ln(HFT)₃(S-BINAPO)] (HFT = 4,4,5,5,6,6,6-heptafluoro-1-(2-thienyl)-1,3-hexanedione) and [Ln(HFA)₃(S-BINAPO)](hfa = hexafluoroacetylacetonate) (where Ln = Yb, Eu) were synthesized. The complex, [Eu(TFN)₃(S-BINAPO)] gives strong red emission at 615 nm with narrow emission band (<10 nm) when excited by 465 nm light with quantum efficiency 86%. The dissymmetry factors (g = Δε_ext/ε_max) corresponding to the ⁷F₁ → ⁵D₀ transition at 590 nm is 0.091 for [Eu(TFN)₃(S-BINAPO)] and for [Yb(hfa)₃(S-BINAPO)](hfa = hexafluoroacetylacetonate) corresponding to the ²F_7/2 → ²F_5/2 transitions is 0.12, are among the largest values for both Eu and Yb complexes to date, respectively. The Eu complexes, [Eu(HFT)₃(S-BINAPO)] and [Eu(TFN)₃(S-BINAPO)] are found to be spontaneously emissive, showing bright red emission, when placed in sunlight or even in the laboratory when light is switched on.     

A selective phosphorescent chemodosimeter for mercury ion

The synthesis and crystal structure of an anionic phosphorescent iridium complex TBA[Ir(dfppy)₂(NCS)₂] (1) were reported. 1 can selectively detect Hg²⁺ with the help of UV-Vis absorption and emission spectra titration. In the presence of Hg²⁺, the obvious decrease of the luminescence intensity at 475 nm was investigated, which could be observed by the naked eyes. The phosphorescence quantum efficiency in CH₃CN solution changed from 0.07 to 0.00085. No obvious spectra changes were observed upon addition of a large excess of other transition metals. Due to its strong thiophilic affinity, the special chemical reaction induced by Hg²⁺ is responsible for the significant change of absorption and luminescence spectra, which is confirmed by ESI-MS.