Platinum(II) Complexes with π-Conjugated, Naphthyl-Substituted, Cyclometalated Ligands (RC^N^N): Structures and Photo- and Electroluminescence

Recent advances in transition metal complexes and light-management engineering in organic optoelectronic devices

Two of the recent major research topics in optoelectronic devices are discussed: the development of new organic materials (both molecular and polymeric) for the active layer of organic optoelectronic devices (particularly organic light-emitting diodes (OLEDs)), and light management, including light extraction for OLEDs and light trapping for organic solar cells (OSCs). In the first section, recent developments of phosphorescent transition metal complexes for OLEDs in the past 3-4 years are reviewed. The discussion is focused on the development of metal complexes based on iridium, platinum, and a few other transition metals. In the second part, different light-management strategies in the design of OLEDs with improved light extraction, and of OSCs with improved light trapping is discussed.

Luminescent palladium(II) complexes with π-extended cyclometalated [R-C^N^N-R'] and pentafluorophenylacetylide ligands: spectroscopic, photophysical, and photochemical properties

A series of cyclometalated Pd(II) complexes that contain π-extended R-C^N^N-R' (R-C^N^N-R'=3-(6'-aryl-2'-pyridinyl)isoquinoline) and chloride/pentafluorophenylacetylide ligands have been synthesized and their photophysical and photochemical properties examined. The complexes with the chloride ligand are emissive only in the solid state and in glassy solutions at 77 K, whereas the ones with the pentafluorophenylacetylide ligand show phosphorescence in the solid state (λmax =584-632 nm) and in solution (λmax =533-602 nm) at room temperature. Some of the complexes with the pentafluorophenylacetylide ligand show emission with λmax at 585-602 nm upon an increase in the complex concentration in solutions. These Pd(II) complexes can act as photosensitizers for the light-induced aerobic oxidation of amines. In the presence of 0.1 mol% Pd(II) complex, secondary amines can be oxidized to the corresponding imines with substrate conversions and product yields up to 100 and 99%, respectively. In the presence of 0.15 mol% Pd(II) complex, the oxidative cyanation of tertiary amines could be performed with product yields up to 91%. The Pd(II) complexes have also been used to sensitize photochemical hydrogen production with a three-component system that comprises the Pd(II) complex, [Co(dmgH)2 (py)Cl] (dmgH=dimethylglyoxime; py=pyridine), and triethanolamine, and a maximum turnover of hydrogen production of 175 in 4 h was achieved. The excited-state electron-transfer properties of the Pd(II) complexes have been examined.

Synthesis and characterization of luminescent cyclometalated platinum(II) complexes of 1,3-bis-hetero-azolylbenzenes with tunable color for applications in organic light-emitting devices through extension of π conjugation by variation of the heteroatom

A series of luminescent cyclometalated platinum(II) complexes of N^C^N ligands [N^C^N=2,6-bis(benzoxazol-2'-yl)benzene (bzoxb), 2,6-bis(benzothiazol-2'-yl)benzene (bzthb), and 2,6-bis(N-alkylnaphthoimidazol-2'-yl)benzene (naphimb)] has been synthesized and characterized. Two of the platinum(II) complexes have been structurally characterized by X-ray crystallography. Their electrochemical, electronic absorption, and luminescence properties have been investigated. In dichloromethane solution at room temperature, the cyclometalated N^C^N platinum(II) complexes exhibited rich luminescence with well-resolved vibronic-structured emission bands. The emission energies of the complexes are found to be closely related to the electronic properties of the N^C^N ligands. By varying the electronic properties of the cyclometalated ligands, a fine-tuning of the emission energies can be achieved, as supported by computational studies. Multilayer organic light-emitting devices have been prepared by utilizing two of these platinum(II) complexes as phosphorescent dopants, in which a saturated yellow emission with Commission International de I'Eclairage coordinates of (0.50, 0.49) was achieved.

Robust phosphorescent platinum(II) complexes with tetradentate O^N^C^N ligands: high efficiency OLEDs with excellent efficiency stability

The Pt(II) complexes (1-3) bearing tetradentate O^N^C^N ligands display high emission quantum yields (0.76-0.90) and good thermal stability (T(d) > 400 °C). Complex is an excellent green phosphorescence dopant for OLEDs with excellent efficiency and low efficiency roll-off (η(L), η(Ext)(max) = 66.7 cd A(-1), 18.2%; η(L), η(Ext) (1000 cd m(-2)) = 65.1 cd A(-1), 17.7%).

Luminescent platinum(II) complexes of 1,3-bis(N-alkylbenzimidazol- 2'-yl)benzene-type ligands with potential applications in efficient organic light-emitting diodes

A series of luminescent platinum(II) complexes of tridentate 1,3-bis(N-alkylbenzimidazol-2'-yl)benzene (bzimb) ligands has been synthesized and characterized. One of these platinum(II) complexes has been structurally characterized by X-ray crystallography. Their electrochemical, electronic absorption, and luminescence properties have been investigated. Computational studies have been performed on this class of complexes to elucidate the origin of their photophysical properties. Some of these complexes have been utilized in the fabrication of organic light-emitting diodes (OLEDs) by using either vapor deposition or spin-coating techniques. Chloroplatinum(II)-bzimb complexes that are functionalized at the 5-position of the aryl ring, [Pt(R-bzimb)Cl], not only show tunable emission color but also exhibit high current and external quantum efficiencies in OLEDs. Concentration-dependent dual-emissive behavior was observed in multilayer OLEDs upon the incorporation of pyrenyl ligand into the Pt(bzimb) system. Devices doped with low concentrations of the complexes gave rise to white-light emission, thereby representing a unique class of small-molecule, platinum(II)-based white OLEDs.

Improvement in phosphorescence efficiency through tuning of coordination geometry of tridentate cyclometalated platinum(II) complexes

A series of tridentate cyclometalated platinum(II) complexes (C(∧)N*N)PtL (L = Cl or acetylide) featuring a fused five-six-membered metallacycle were synthesized. The structure of the complexes was confirmed by X-ray crystallography. In contrast to the C(∧)N(∧)N platinum complexes with a fused five-five-membered metallacycle, the platinum coordination in C(∧)N*N complexes is much closer to a square planar geometry. The photophysical properties of the complexes were studied. The geometrical change from C(∧)N(∧)N to C(∧)N*N led to a substantial improvement in phosphorescence efficiency of the complexes with an acetylide ligand in solution at room temperature. For example, the quantum yield of (C(∧)N*N)PtCCPh was measured to be 56%, demonstrating a big jump from 4% reported for (C(∧)N(∧)N)PtCCPh.

Mesomorphism and luminescence properties of platinum(II) complexes with tris(alkoxy)phenyl-functionalized pyridyl pyrazolate chelates

A series of new mesomorphic platinum(II) complexes 1-4 bearing pyridyl pyrazolate chelates are reported herein. In this approach, pyridyl azolate ligands have been strategically functionalized with tris(alkoxy)phenyl groups with various alkyl chain lengths. As a result, they are ascribed to a class of luminescent metallomesogens that possess distinctive morphological properties, such as their intermolecular packing arrangement and their associated photophysical behavior. In CH(2) Cl(2), independent of the applied concentration in the range 10(-6)-10(-3)  M, all Pt(II) complexes exhibit bright phosphorescence centered at around 520 nm, which is characteristic for monomeric Pt(II) complexes. In stark contrast, the single-crystal X-ray structure determination of [Pt(C4pz)(2)] (1) shows the formation of a dimeric aggregate with a notable Pt⋅⋅⋅Pt contact of 3.258 Å. Upon heating, all Pt(II) complexes 1-4 melted to form columnar suprastructures, for which similar intracolumnar Pt⋅⋅⋅Pt distances of approx. 3.4-3.5 Å are observed within an exceptionally wide temperature range (>250 °C), according to the powder XRD data. Upon casting into a neat thin film at RT, the luminescence of 1-4 is dominated by a red emission that spans 630-660 nm, which originates from the one-dimensional, chainlike structure with Pt-Pt interaction in the ground state. Taking complex 4 as a representative, the emission intensity and wavelength were significantly decreased and blueshifted, respectively, on heating from RT to 250 °C. Further heating to liquefy the sample alters the red emission back to the green phosphorescence of the monomer. The results highlight the pivotal role of tris(alkoxy)phenyl groups in the structural versus luminescence behavior of these Pt(II) complexes.

Highly luminescent tetradentate bis-cyclometalated platinum complexes: design, synthesis, structure, photophysics, and electroluminescence application

N,N-Di(6-phenylpyridin-2-yl)aniline (L1), N,N-di(6-(2,4-difluorophenyl)pyridin-2-yl)aniline (L2), N,N-di(3-(pyridin-2-yl)phenyl)aniline (L3), N,N-di(3-(1H-pyrazol-1-yl)phenyl)aniline (L4), N,N-di(3-(3-methyl-1H-pyrazol-1-yl)phenyl)aniline (L5), and N,N-di(3-(4-methyl-1H-pyrazol-1-yl)phenyl)aniline (L6) undergo cyclometalation to produce two types of tetradentate bis-cyclometalated platinum(II) complexes: C--N*N(wedge)C platinum complexes 1 and 2 and N--C*C--N platinum complexes 3-6, respectively, where an "X--Y" (X, Y = C or N) denotes a bidentate coordination to the platinum to form a five-membered metallacycle and "X*Y" denotes a coordination to form a six-membered metallacycle. The crystal structures of 1, 3, and 5 were determined by the single-crystal X-ray diffraction analysis, showing distorted square-planar geometry, that is, two C--N coordination moieties are twisted. Complex 5 showed much greater distortion with largest deviation of 0.193 A from the mean NCCNPt coordination plane, which is attributed to the steric interaction between the two 3-methyl groups on the pyrazolyl rings. Density functional theory (DFT) calculations were carried out on the ground states of 1 and 3-6. The optimized geometries are consistent with the crystal structures. The highest occupied molecular orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMOs) of the molecules displayed a localized characteristic with the contribution (18-45%) of the platinum metal to the HOMOs. All complexes are emissive at ambient temperature in fluid with quantum yields of 0.14 to 0.76 in 2-methyltetrahydrofuran. The emission of the complexes covers from blue to red region with lambda(max) ranging from 474 to 613 nm. Excimer emission was observed for 1 and 2 at high concentration of the complexes. The emission lifetime at infinite dilution for 1 and 2 was determined to be 7.8 and 11.4 micros, respectively. Concentration quenching was observed for 3 and 4, but the excimer emission was not observed. The life times for 3-6 were determined to be in the range of micro seconds, but those of 4-6 (3.4-5.7 micros) were somewhat shorter than that of 3 (7.6 micros). The highly structured emission spectra, long life times, and DFT calculations suggested that the emissive state is primarily a (3)LC state with metal-to-ligand charge-transfer (MLCT) admixture. The ZFS of 23 cm(-1) for the emissive triplet state was observed directly by high resolution spectroscopy for 1 in a Shpol'skii matrix, which also suggested an emission from a triplet ligand centered ((3)LC) state with admixture of MLCT character. Complex 1 was incorporated into an organic light-emitting diode (OLED) device as an emitter at 4 wt % in the mixed host of 4,4',4''-tris(N-carbazolyl)triphenylamine (TCTA) and 2,2',2''-(1,3,5-benzenetriyl)tris(1-phenyl-1-H-benzimidazole) (TPBI) and demonstrated excellent performance with maximum external quantum efficiency of 14.7% at the current density of 0.01 mA/cm(-1).

Ion-molecule reactions of "rollover" cyclometalated [Pt(bipy-H)](+) (bipy=2,2'-bipyridine) with dimethyl ether in comparison with dimethyl sulfide: an experimental/computational study

The ion-molecule reactions of dimethyl ether with cyclometalated [Pt(bipy-H)](+) were investigated in gas-phase experiments, complemented by DFT methods, and compared with the previously reported ion-molecule reactions with its sulfur analogue. The initial step corresponds in both cases to a platinum-mediated transfer of a hydrogen atom from the ether to the (bipy-H) ligand, and three-membered oxygen- and sulfur-containing metallacycles serve as key intermediates. Oxidative C--C bond coupling ("dehydrosulfurization"), which dominates the gas-phase ion chemistry of the [Pt(bipy-H)](+) ion with dimethyl sulfide, is practically absent for dimethyl ether. The competition in the formation of C(2)H(4) and CH(2)X (X=O, S) in the reactions of [Pt(bipy-H)](+) with (CH(3))(2)X (X=O, S) as well as the extensive H/D exchange observed in the [Pt(bipy-H)](+)/(CH(3))(2)O system are explained in terms of the corresponding potential-energy surfaces.