William C. Lenhart

Operational degradation of organic light-emitting diodes: Mechanism and identification of chemical products

Despite the importance of the operational lifetime of organic light-emitting diodes (OLEDs) in practical applications, little is known about the nature of chemical reactions associated with efficiency losses during operation. To gain an insight into a chemical mechanism of operational degradation, we studied operation-induced changes in chemical compositions of fluorescent and phosphorescent OLEDs utilizing carbazole derivatives in emissive layers. We detected substantial losses of the emissive components, including the carbazole-derived host 4,4′-bis(N-carbazolyl)biphenyl (CBP) and, if present, phosphorescent dopant. Several different materials were found only in the degraded OLEDs, and some of them were isolated and identified by nuclear magnetic resonance and mass spectrometry. A similar set of products was found upon UV irradiation of CBP films. Structures of degradation products suggest that the key step in operational degradation of OLEDs is homolytic cleavage of weaker bonds, e.g., an exocyclic C–N...

Effect of polyvinylpyrrolidone on the self-assembly of model hydrocarbon amphiphiles

The arrangement of molecules in the polymer-surfactant complex and the standard free energy of formation of the complex in an aqueous solution of anionic surfactant and polyvinylpyrrolidone (PVP) are investigated using 13C NMR spectroscopy and surface tension measurement at the air/water interface. The behavior of the single-chained surfactant, sodium dodecyl sulfate (SDS), is compared with that of the double-chained surfactant, sodium bis(2-ethylhexyl) sulfosuccinate (Aerosol OT). It was found that the basic structure of the complex is the same in both cases; the surfactant molecules are assembled in a micelle-like aggregate; and the polymer molecules are wrapped around the aggregate, shielding hydrocarbon groups on the surface of the micelle from contact with water. For SDS the standard free energy of micellization is lowered by 1.5 RT per mol in the presence of PVP. However, for Aerosol OT the decrease in the free energy of micellization is only 0.7 RT per mol. The lesser influence of PVP on the monomer-micelle equilibrium in solutions of Aerosol OT is attributed to the presence of the highly polar carbonyl groups in the shell of the Aerosol OT micelle which are believed to act as a barrier to the attachment of the backbone of the polymer to the surface of the micelle.

Coumarin-Based, Electron-Trapping Iridium Complexes as Highly Efficient and Stable Phosphorescent Emitters for Organic Light-Emitting Diodes

A new class of coumarin-based iridium tris-cyclometalated complexes has been developed. These complexes are highly emissive, with emission colors ranging from green to orange-red. Besides modification of ligand structures, color tuning was realized by incorporation of ligands with different electrochemical properties in a heteroleptic structure. The organic light-emitting diodes (OLEDs) using these compounds as emissive dopants are highly efficient and stable. Unlike other Ir(III) phosphorescent dopants, these coumarin-based Ir(III) dopants can effectively trap and transport electrons in the emissive layer.

Free‐radical pathways in operational degradation of OLEDs

Abstract— Based on the observed changes in chemical compositions of fluorescent and phosphorescent carbazole-based OLEDs during operation, a free-radical mechanism of operational degradation is proposed. Chemical analysis and identification of low molecular weight and oligomeric products, device physics, photochemistry, and electron paramagnetic resonance (EPR) studies point to the excited-state homolytic-bond dissociation followed by radical additions as key mechanism steps. Comparable bond dissociation energy and singlet excited-state energy result in a relatively fast degradation process of carbazole-based OLEDs. OLED operation leads to the accumulation of solid-matrix-trapped long-lived π-radical species in their charged or neutral forms, acting as non-radiative recombination centers and luminescence quenchers. The proposed free-radicals-mediated degradation mechanism could be a common degradation mechanism affecting a wide range of OLED compositions and structures. In the framework of this mechanism, the relationship between the excited-state energy and the weakest bond dissociation energy of OLED materials is of the fundamental importance for the operational stability of OLED devices.

Structural Determination of Silicon-Containing Oligonucleotides by 1H−29Si Long-Range Heteronuclear Multiple Quantum Correlation NMR Spectroscopy

Abstract The technique of 1H−29Si Long-Range Heteronuclear Multiple Quantum Correlation NMR Spectroscopy was used to determine the structure of silicon-containing oligonucleotides. Trimers which contained silicon instead of phosphorus as part of the oligonucleotide link were synthesized through a synthetic route that required minimal hydroxyl protection. The resulting trimer could have one of two possible structures. Through the use of 1H−29Si HMQC NMR spectroscopy, it was possible to link the 3′-hydroxymethine proton of one sugar to the 5′-hydroxymethylene proton of an adjacent sugar by correlation to the same silicon atom, thus elucidating the final structure.

47.2: Distinguished Paper: Structural Identification of Chemical Products and Mechanism of Operational Degradation of OLEDs

Based on the observed changes in chemical compositions of fluorescent and phosphorescent carbazole-derived OLEDs during operation, the operational degradation mechanism is proposed. Substantial losses of chemical components, identification of degradation products, and photochemical studies point to the excited state homolytic bond dissociation followed by radical additions as key mechanism steps.

Bis(2‐methyl­quinolin‐8‐olato‐κ2N,O)(6‐phenyl‐2‐naphtholato‐κO)aluminium(III)

The five-coordinate geometry of the Al atom in the title compound, [Al(C10H8NO)2(C16H11O)], is trigonal–bipyramidal, in which the O-donor atoms of the naphtholate and the two quinolinolate ligands are in the trigonal equatorial plane and the N atoms in the axial positions.

(2,6-Diphenyl­phenolato-κO)bis­(2-methyl­quinolin-8-olato-κ2N,O)gallium(III)

The title compound, [Ga(C10H8NO)2(C18H13O)], is a mononuclear five-coordinate Ga complex having a trigonal–bipyramidal geometry in which the O-donor atoms of the phenolate and of the two quinolinolate ligands form the trigonal equatorial plane, and the N atoms are in axial positions.

Bis(2,6-diphenyl­phenolato)(2-methylquinolin-8-olato)aluminium(III)

The title compound, [Al(C10H8NO)2(C18H13O)], is representative of the class of compounds commonly known as the blue aluminium chelates (BAlq) that are useful in organic electroluminescent devices. The geometry of this compound is approximately trigonal–bipyramidal, with the trigonal base formed by the O donor atoms of the phenolate and two quinolinolate ligands.