Ru(II) polypyridine complexes: photophysics, photochemistry, eletrochemistry, and chemiluminescence

The synthesis and photophysical study of a family of cyclometalated iridium(III) complexes are reported. The iridium complexes have two cyclometalated (C∧N) ligands and a single monoanionic, bidentate ancillary ligand (LX), i.e., C∧N2Ir(LX). The C∧N ligands can be any of a wide variety of organometallic ligands. The LX ligands used for this study were all β-diketonates, with the major emphasis placed on acetylacetonate (acac) complexes. The majority of the C∧N2Ir(acac) complexes phosphoresce with high quantum efficiencies (solution quantum yields, 0.1−0.6), and microsecond lifetimes (e.g., 1−14 μs). The strongly allowed phosphorescence in these complexes is the result of significant spin−orbit coupling of the Ir center. The lowest energy (emissive) excited state in these C∧N2Ir(acac) complexes is a mixture of 3MLCT and 3(π−π*) states. By choosing the appropriate C∧N ligand, C∧N2Ir(acac) complexes can be prepared which emit in any color from green to red. Simple, systematic changes in the C∧N ligands, whic...

Highly phosphorescent bis-cyclometalated iridium complexes: synthesis, photophysical characterization, and use in organic light emitting diodes.

The synthesis, electrochemistry, and photophysics of a series of square planar Pt(II) complexes are reported. The complexes have the general structure C∧NPt(O∧O),where C∧N is a monoanionic cyclometalating ligand (e.g., 2-phenylpyridyl, 2-(2‘-thienyl)pyridyl, 2-(4,6-difluorophenyl)pyridyl, etc.) and O∧O is a β-diketonato ligand. Reaction of K2PtCl4 with a HC∧N ligand precursor forms the chloride-bridged dimer, C∧NPt(μ-Cl)2PtC∧N, which is cleaved with β-diketones such as acetyl acetone (acacH) and dipivaloylmethane (dpmH) to give the corresponding monomeric C∧NPt(O∧O) complex. The thpyPt(dpm) (thpy = 2-(2‘-thienyl)pyridyl) complex has been characterized using X-ray crystallography. The bond lengths and angles for this complex are similar to those of related cyclometalated Pt complexes. There are two independent molecular dimers in the asymmetric unit, with intermolecular spacings of 3.45 and 3.56 A, consistent with moderate π−π interactions and no evident Pt−Pt interactions. Most of the C∧NPt(O∧O) complexes...

Synthesis and characterization of phosphorescent cyclometalated platinum complexes.

The potential of palladacycles: more than just precatalysts

One goal of this critical review is to provide advanced methodologies for systematic preparation of transition-metal based phosphors that show latent applications in the field of organic light emitting diodes (OLEDs). We are therefore reviewing various types of cyclometalating chelates for which the favorable metal–chelate bonding interaction, on the one hand, makes the resulting phosphorescent complexes highly emissive in both fluid and solid states at room temperature. On the other hand, fine adjustment of ligand-centered π–π* electronic transitions allows tuning of emission wavelength across the whole visible spectrum. The cyclometalating chelates are then classified according to types of cyclometalating groups, i.e. either aromatic C–H or azolic N–H fragment, and the adjacent donor fragment involved in the formation of metallacycles; the latter is an N-containing heterocycle, N-heterocyclic (NHC) carbene fragment or even diphenylphosphino group. These cyclometalating ligands are capable to react with heavy transition-metal elements, namely: Ru(II), Os(II), Ir(III) and Pt(II), to afford a variety of highly emissive phosphors, for which the photophysical properties as a function of chelate or metal characteristics are systematically discussed. Using Ir(III) complexes as examples, the C^N chelates possessing both C–H site and N-heterocyclic donor group are essential for obtaining phosphors with emission ranging from sky-blue to saturated red, while the N^N chelates such as 2-pyridyl-C-linked azolates are found useful for serving as true-blue chromophores due to their increased ligand-centered π–π* energy gap. Lastly, the remaining NHC carbene and benzyl phosphine chelates are highly desirable to serve as ancillary chelates in localizing the electronic transition between the metal and remaining lower energy chromophoric chelates. As for the potential opto-electronic applications, many of them exhibit remarkable performance data, which are convincing to pave a broad avenue for further development of all types of phosphorescent displays and illumination devices (94 references).

Transition-metal phosphors with cyclometalating ligands: fundamentals and applications

Temperature dependence of the luminescence of cyclometalated palladium(II), rhodium(III), platinum(II), and platinum(IV) complexes

Luminescence of ortho-metallated platinum(II) complexes

The palladium(II) and platinum(II) bis-homoleptic complexes M(C⌒N)2, where C⌒N is benzo[h]quinoline (H-bhq), 2-phenylpyridine (H-phpy), 2-(2‘-benzothienyl)pyridine (H-bthpy), 2-(2‘-thienyl)quinoline (H-thq), and 2-(2‘-thienyl)pyridine (H-thpy), were prepared by metal exchange of the lithiated ligands C⌒N with M(Et2S)2Cl2. The palladium(II) bis-heteroleptic complexes, Pd(C⌒N)(C‘⌒N‘), were synthesized from Pd(C⌒N)2 bis-homoleptic complexes, which were converted by HCl into the dichloro-bridged compounds [Pd(C⌒N)Cl]2. By addition of Et2S, the Pd(C⌒N)(Et2S)Cl complexes were formed, which were allowed to react with Li(C‘⌒N‘), yielding M(C⌒N)(C‘⌒N‘) compounds. An alternative way for obtaining the bis-heteroleptic molecules is by ligand exchange, according to the equilibrium M(C⌒N)2 + M(C‘⌒N‘)2 = 2M(C⌒N)(C‘⌒N‘). The crystal structures of Pt(bhq)2 (1) and Pt(thq)2 (3) present an important distortion of the square planar (SP-4) geometry toward a two-bladed helix. Bis-homoleptic and some bis-heteroleptic complexes ...

Cyclometalated Complexes of Palladium(II) and Platinum(II): cis -Configured Homoleptic and Heteroleptic Compounds with Aromatic C⌒N Ligands

The absorption spectra, luminescence spectra, and luminescence lifetimes of the isomeric [M(bph)(bpy)] and [M(phpy)2] complexes M = Pt(II) or Pd(II), bph2− = 1,1′-biphenyl-2,2′-diyl dianion, phpy− = 2-phenylpyridine-2′-yl anion, and bpy = 2,2′-bipyridine have been investigated and compared with those of [M(bpy)2]2+ complexes and of the free protonated ligands H2bph, Hbpy+, and Hphpy. In the absorption spectra, the region below 320 mm is dominated by ligand-centered (LC) transitions, whereas metal-to-ligand charge transfer (MLCT) transitions are responsible for the bands present in the near UV/VIS region. The MLCT bands move to higher energies on replacing Pt with Pd and in going from [M(bph)(bpy)] to the [M(phpy)2] isomer. For the mixed-ligand complexes, evidence for both M bph2− (at higher energies) and M bpy bands is found. The structured luminescence observed at 77 K shows lifetimes of 4.0 and 1.1 μs for [Pt(phpy)2] and [Pt(bph)(bpy)], respectively, and 480 and 250 μs for the analogous Pd complexes. On the basis of the energy and lifetime data, the luminescence of the Pt(II) complexes is assigned to the lowest triplet MLCT excited state, whereas for the Pd complexes the luminescent state is thought to result from a mixture of MLCT and LC triplet levels.

Absorption Spectra and Luminescence Properties of Isomeric Platinum (II) and Palladium (II) Complexes containing 1,1′-biphenyldiyl, 2-phenylpyridine, and 2,2′-bijpyridine as ligands

The absorption and emission spectra, emission lifetimes, luminescence quantum yields, and electrochemical behavior of the complexes Pd(Phpy)2, Pd(Thpy)2, and Pd(bhq)2 (Phpy−, Thpy−, and bhq−, and bhq− are the deprotonated forms of 2-phenylpyridine, 2-(2-thienyl)pyridine, and benzo[h] quinoline, respectively) have been studied, and the results obtained have been compared with those available for Pt(II) and Pt(IV) complexes containing the same ligands. The intense ligand-centered absorption bands below 340 nm are strongly perturbed by matalation, and the absorption features in the 340–450-nm region are likely to include contributions from formally metal-to-ligand charge-transfer transitions. The structured luminescence spectra observed at 77 K (lifetimes are 0.48, 0.28 and 2.6 ms for Pd(Phpy)2, Pd(Thpy)2, and Pd(bhq)2, respectively) have been assigned to transitions having mainly ligand-centered character, with an increasing metal-to-ligand charge-transfer contribution in going from Pd(bhq)2 to Pd(Phpy)2 and to Pd(Thpy)2. The complexes Pd(phpy)2 and Pd(thpy)2 show two reversible one-electron reduction waves, whereas reduction of Pd(bhq)2 is irreversible, as is the oxidation of the three complexes.

Philippe Jolliet

Papers

Temperature dependence of the luminescence of cyclometalated palladium(II), rhodium(III), platinum(II), and platinum(IV) complexes

Luminescence of ortho-metallated platinum(II) complexes

Cyclometalated Complexes of Palladium(II) and Platinum(II): cis -Configured Homoleptic and Heteroleptic Compounds with Aromatic C⌒N Ligands

Absorption Spectra and Luminescence Properties of Isomeric Platinum (II) and Palladium (II) Complexes containing 1,1′-biphenyldiyl, 2-phenylpyridine, and 2,2′-bijpyridine as ligands

Spectroscopic and Electrochemical Behavior of Cyclometalated Pd(II) Complexes