Masahisa Osawa

Highly Efficient Green Organic Light-Emitting Diodes Containing Luminescent Three-Coordinate Copper(I) Complexes

A series of highly emissive three-coordinate copper(I) complexes, (dtpb)Cu(I)X [X = Cl (1), Br (2), I (3); dtpb =1,2-bis(o-ditolylphosphino)benzene], were synthesized and investigated in prototype organic light-emitting diodes (OLEDs). 1-3 showed excellent photoluminescent performance in both degassed dichloromethane solutions [quantum yield (Φ) = 0.43-0.60; lifetime (τ) = 4.9-6.5 μs] and amorphous films (Φ = 0.57-0.71; τ = 3.2-6.1 μs). Conventional OLEDs containing 2 in the emitting layer exhibited bright green luminescence with a current efficiency of 65.3 cd/A and a maximum external quantum efficiency of 21.3%.

Highly efficient green organic light-emitting diodes containing luminescent tetrahedral copper(I) complexes

A series of highly emissive sublimable copper(I) complexes with tetrahedral geometries, Cu(dppb)(pz2Bph2) 1, Cu(dppb-F)(pz2Bph2) 2, and Cu(dppb-CF3)(pz2Bph2) 3 [dppb = 1,2-bis(diphenylphosphino)benzene, dppb-F = 1,2-bis[bis(3,5-difluorophenyl)phosphino]benzene, and dppb-CF3 = 1,2-bis[bis[3,5-bis(trifluoromethyl)phenyl]phosphino]benzene, pz2Bph2− = diphenyl-bis(pyrazol-1-yl)borate], were synthesized and investigated as luminescent guest molecules in prototype organic light-emitting diodes. Thermogravimetric analysis of 1–3 under vacuum revealed that introduction of F or CF3 substituents to the meta positions of the four peripheral phenyl groups in the dppb skeleton increased the ability of the copper(I) complexes to be sublimed. 1–3 exhibited strong green emission in amorphous films at 293 K with maximum emission wavelengths of 523–544 nm, quantum yields of 0.50–0.68, and decay times of 3.6–8.2 μs. Molecular orbital calculations indicated that the origin of green phosphorescence from 1–3 was a mixture of σ → π* and π → π* transitions. Conventional bottom-emitting devices with three-layer structures containing 1–3 produced bright green luminescence with maximum external quantum efficiencies of 11.9, 16.0, and 17.7% for 1, 2 and 3, respectively.

Vapochromic and Mechanochromic Tetrahedral Gold(I) Complexes Based on the 1,2-Bis(diphenylphosphino)benzene Ligand

Tetrahedral gold(I) complexes containing the diphosphane ligand (dppb=1,2-bis(diphenylphosphino)benzene), [Au(dppb)(2)]X [X=Cl (1), Br (2), I (3), NO(3) (4), BF(4) (5), PF(6) (6), B(C(6)H(4)F-4)(4) (7)], and the ethanol and methanol adducts of complex 4, 8, and 9, were prepared to analyze their unique photophysical properties. These complexes are classified into two categories on the basis of their crystal structures. In Category I, the complexes (1-5) have relatively-small counter anions and two dppb ligands are symmetrically coordinated to the central Au(I) atom, and display an intense blue phosphorescence. Alternatively, the complexes (6-9) in Category II have large counter anions and two dppb ligands asymmetrically coordinated to Au(I) atom, and display a yellow or yellow orange phosphorescence. The difference in the phosphorescence color of the complexes between the Category I and II is ascribed to the change in the structure of the cationic moiety in the complex. According to DFT calculations, the symmetry reduction caused by the large counter anion of the complex in Category II gives the destabilization of HOMO (σ*) levels, leading to the red-shift of the emission peak. We have demonstrated that the symmetry reductions are responsible for the phosphorescence color alteration caused by external stimuli (volatile organic compounds and mechanical grinding).

Application of neutral d10 coinage metal complexes with an anionic bidentate ligand in delayed fluorescence-type organic light-emitting diodes

A series of heteroleptic coinage metal(I) complexes [Cu(PP)(PS)] 1, [Ag(PP)(PS)] 2, and [Au(PP)(PS)] 3 [PP = 1,2-bis(diphenylphosphino)benzene and PS− = 2-diphenylphosphinobenzenethiolate] were synthesized. X-ray crystallography demonstrated that 1–3 possessed tetrahedral structures containing two types of bidentate ligands, PP and PS−. Photophysical studies and time-dependent density functional theory calculations indicated that the emission from 1–3 in the solid state at room temperature originated from thermally activated delayed fluorescence (TADF). The thiolate ligand with strong electron-donating character (PS−) reduced the contribution from metal orbitals to the highest occupied molecular orbitals of the complexes, decreasing the metal-to-ligand charge-transfer character of the excited states of 1–3. The origin of TADF in 1–3 was attributed to ligand-to-ligand charge-transfer on the basis of molecular orbital calculations. Au(I) complex 3 was unstable in solution because of a rapid ligand exchange reaction, and Ag(I) complex 2 showed limited solubility in organic solvents. Cu(I) complex 1, which exhibited efficient green TADF with a maximum emission wavelength of 521 nm and a quantum yield of 0.52 in the solid state, was used to fabricate TADF-type organic light-emitting diodes via a wet process.

Photoluminescence properties, molecular structures, and theoretical study of heteroleptic silver(I) complexes containing diphosphine ligands.

The homoleptic complex [Ag(L)(2)]PF(6) (1) and heteroleptic complexes [Ag(L)(L(Me))]BF(4) (2) and [Ag(L)(L(Et))]BF(4) (3) [L = 1,2-bis(diphenylphosphino)benzene, L(Me) = 1,2-bis[bis(2-methylphenyl)phosphino]benzene, and L(Et) = 1,2-bis[bis(2-ethylphenyl)phosphino]benzene] were synthesized and characterized. X-ray crystallography demonstrated that 1-3 possess tetrahedral structures. Photophysical studies and time-dependent density functional theory calculations of 1-3 revealed that alkyl substituents at the ortho positions of peripheral phenyl groups in the diphosphine ligands have a significant influence on the energy and intensity of phosphorescence of the complex in solution at room temperature. The results can be interpreted in terms of the geometric preferences of each complex in the ground and excited states. The homoleptic complex 1 exhibits weak orange phosphorescence in solution arising from its flat structure in the triplet state, while heteroleptic complexes 2 and 3 show strong green phosphorescence from triplet states with tetrahedral structure. Larger interligand steric interactions in 2 and 3 caused by their bulkier ligands probably inhibit geometric relaxation within the excited-state lifetimes, leading to higher energy phosphorescence than that observed for 1. NMR experiments revealed that 2 and 3 in solution possess structures that are much more immobilized than that of 1; fluxional motion is completely suppressed in 2 and 3. Accordingly, conformational changes of 2 and 3 are expected to be suppressed by the alkyl substituents not only in the ground state but also in excited states. Consequently, nonradiative decay of the excited states of 2 and 3 occurs less efficiently than in 1. As a result, the quantum yields of phosphorescence for 2 and 3 are 6 times larger than that for the homoleptic complex 1.

Photochemistry and photophysics of the tetrahedral silver(I) complex with diphosphine ligands: [Ag(dppb)2]PF6 (dppb = 1, 2-bis[diphenylphosphino]benzene)

The photophysical and photochemical properties of [Ag(dppb)2]PF6 (1) having a tetrahedral coordination geometry have been described.

Intra-Complex Energy Transfer of Europium(III) Complexes Containing Anthracene and Phenanthrene Moieties

Laser excitation and luminescence studies were carried out for an ethanol solution of tris(hexafluoroacetylacetonato)europium(III), Eu(hfac)(3)(H(2)O)(2) (1), and n-hexane solutions of tris(hexafluoroacetylacetonato)europium(III) bis(9-diisopropylphosphorylanthracene), Eu(hfac)(3)(DiPAnO)(2) (2), and tris(hexafluoroacetylacetonato)europium(III) bis(2-diisopropylphosphorylphenanthrene), Eu(hfac)(3)(DiPPheO)(2) (3). The absorption spectra of 2 and 3 in n-hexane are interpreted by assuming that the central Eu(III) ion weakly interacts with the DiPAnO and DiPPheO moieties. The emission spectroscopic studies revealed that (1) Eu(hfac)(3)(DiPAnO)(2) emits only fluorescence from the DiPAnO moiety and (2) Eu(hfac)(3)(DiPPheO)(2) gives luminescence from the central Eu(III) ion, and (3) Eu(hfa)(3)(H(2)O)(2) and Eu(hfac)(3)(DiPPheO)(2) afford emission from both the (5)D(1) and (5)D(0) states of the Eu(III) ion upon 355 nm laser excitation. The intracomplex energy transfer processes are presented on the basis of absorption, emission and laser excitation studies.

Synthesis and molecular structure of an unsymmetric dimanganese(II) carboxylato complex

A dimanganese(II) complex [Mn{HB(3,5-Pri2pz)3}(µ-OBz)3Mn(3,5-Pri2pzH)2]1 has been synthesized from a mixture of [Mn(Cl){HB(3,5-Pri2pz)3}], 3,5-Pri2pzH, MnCl2·4H2O and NaOBz;X-ray analysis establishes the unsymmetric coordination environment of each manganese ion with a structurally unique bridging unit consisting of three carboxylate groups.