Nan-Xing Hu

Bio: Nan-Xing Hu is an academic researcher from Xerox. The author has contributed to research in topics: Layer (electronics) & Coating. The author has an hindex of 38, co-authored 306 publications receiving 6733 citations. Previous affiliations of Nan-Xing Hu include LG Display & Philips.

Degradation Mechanism of Small Molecule-Based Organic Light-Emitting Devices

Abstract: Studies on the long-term degradation of organic light-emitting devices (OLEDs) based on tris(8-hydroxyquinoline) aluminum (AlQ 3 ), the most widely used electroluminescent molecule, reveal that injection of holes in AlQ 3 is the main cause of device degradation. The transport of holes into AlQ 3 caused a decrease in its fluorescence quantum efficiency, thus showing that cationic AlQ 3 species are unstable and that their degradation products are fluorescence quenchers. These findings explain the success of different approaches to stabilizing OLEDs, such as doping of the hole transport layer, introducing a buffer layer at the hole-injecting contact, and using mixed emitting layers of hole and electron transporting molecules.

Ligand-Accelerated Catalysis of the Ullmann Condensation: Application to Hole Conducting Triarylamines

Abstract: High-purity triarylamines find employment in xerographic photoreceptors where, as concentrated solid solutions in polymeric transport layers, they function as efficient hole conductors.1,2 Under the influence of an applied electric field, injected positive charge migrates through such layers by a hopping mechanism to create latent electrostatic images.3 This process occurs readily since triarylamines possess an easily accessible oxidation potential and on hole injection give up a nonbonding electron to generate amine cation radicals, the stable spin centers responsible for transport. Triarylamines are also important to a number of emerging technologies. Transport layers are equally fundamental to electroluminescent devices where, in essentially the reverse of electrophotography, electrons and holes are separately injected and transported to an emitting species where their recombination produces singlet excitons whose radiative decay results in visible light.4 They are also constituents of nonlinear optical chromophores useful in the design of integrated electrooptic switches and modulators.5 In each of these applications, achievement of an electronic grade purity level is required for optimal device performance. Thus, clean and efficient syntheses which can be readily scaled to multi-kilogram lot sizes are desirable. Despite their structural simplicity, synthetic protocols which satisfy these criteria are uncommon and the heightened importance of triarylamines has led to extensive recent research attention. The classic technique for the construction of triarylamines has been the venerable Ullmann condensation.6 As usually practiced, the reaction entails the condensation of a diphenylamine and an unactivated aryl halide with catalysis by some form of copper (metal, alloy, copper(I) or -(II) salt) in the presence of added base. The reaction is noted for its capricious nature and sensitivity to catalyst type. Strongly aggressive conditions involving high temperature and extended reaction times are generally needed to secure at best moderate yields. Although halide reduction and homocoupling often negatively impact yields, control of the substitution pattern of all three rings is afforded. In a variant, bis(arylation) of a substituted aniline with 2 equiv of an aromatic halide allows access, depending on reactants, to products in which two or perhaps all three of the aromatic ring are identically substituted. Yields tend to be substantially poorer in these cases. Attempts have been made to moderate the harshness of the reaction conditions. Frechet and Gauthier reported that crown ethers induce rate accelerations and improve yields in certain Ullmann condensations.7 The procedure, however, does not obviate the requirement for high temperature, and the long reaction times (15 h in the best case) make it unattractive for large scale industrial applications. Catalyst effects still persist, as is common when potassium carbonate is used as base, and the high cost of the crown would necessitate its recovery and reuse. An area of research showing rapid progress is the application of transition metal catalysis to the formation of the aromatic carbon-nitrogen bond. Palladiumcatalyzed aromatic amination reactions, as described by both Buchwald and Hartwig,8 have created interesting new possibilities. Aromatic amines with a wide structural variation have been successfully synthesized under exceptionally mild reaction conditions. It is as yet premature to judge whether this chemistry will supplant the Ullmann condensation as the robustness and economics of a method requiring noble metal catalysis in a large scale industrial application requires demonstration, particularly in the commodity chemicals arena.9 Background. Important information on the kinetics of amine Ullmann condensations is available. The reaction is zero order in amine with the rate-determining step being the loss of halide from the substrate with the reactivity order being I > Br > Cl . F.6a,10 Detailed kinetics have proven difficult to extract under the heterogeneous conditions normally employed. Paine, by studying a homogeneous but synthetically nonviable reaction surrogate, concluded that only copper(I) states actively participate as catalysts regardless of the oxidation level of the added copper. Additionally, despite the apparent heterogeneity of the reaction, the nucleophilic species is posited to be a soluble amine cuprate.11 Rate accelerations have been reported in the related, industrially important Ullmann condensation of phenols leading to diphenyl ethers. Careful work by Weingarten clearly demonstrated that impurities present in his reaction solvent, diglyme, imparted enhanced catalytic activity. Such activity was destroyed by LAH treatment † Tel. (905) 823-7091. Fax: (905) 822-7022. E-mail: Bruce.Goodbrand@crt.xerox.com. (1) Borsenberger, P. M.; Weiss, D. S. Organic Photoreceptors for Imaging Systems; Marcel Dekker: New York, 1993. (2) Law, K.-Y. Chem. Rev. 1993, 93, 449. (3) (a) Facci, J. S.; Stolka, M. Philos. Mag. B 1986, 54, 1. (b) Abkowitz, M.; Bassler, H.; Stolka, M. Philos. Mag. B 1991, 63, 201. (4) (a) Thelekkat, M.; Fink, R.; Haubner, F.; Schmidt, H.-W. Macromol. Symp. 1997, 125, 157-164. (b) Tamato, N.; Adachi, C.; Nagai, K. Chem. Mater. 1997, 9, 1077-1085. (c) Tanaka, H.; Tokito, S.; Taga, Y.; Okada A. Chem. Commun. 1996, 2175-2176. (d) Chen, C.; Shi, J.; Tang, C. W. Macromol. Symp. 1997, 125, 1-48. (e) Tsutsui, T. MRS Bull. 1997, 39. (f) Bellmann, E.; Shaheen, S.; Thayumanavan, S.; Barlow, S.; Grubbs, R.; Marder, S.; Kippelen, B.; Peyghambarian, N. Chem. Mater. 1998, 10, 1668-1676. (5) Miller, R. D.; Lee, V. Y.; Twieg, R. J. Chem. Commun. 1995, 245246. (6) For general reviews of Ullmann condensation chemistry, see: (a) Lindley, J. Tetrahedron 1984, 40, 1433-1456. (b) Fanta, P. E. Synthesis 1974, 1-21. (c) Bacon, R. G. R.; Hill, H. A. O. Q. Rev. 1965, 19, 95125. (7) Gauthier, S.; Frechet, J. M. J. Synthesis 1987, 383-385. (8) For leading references, see: (a) Hartwig, J. F. Angew. Chem., Int. Ed. Engl. 1998, 37, 2046-2067. (9) (a) Beller, M. Angew. Chem., Int. Ed. Engl. 1995, 34, 1316-1317. (b) Reaction modifications with a view to accommodating large scale work have recently been described, see: Wullner, G.; Jansch, H.; Kannenberg, S.; Schubert, F.; Boche, G. Chem. Commun. 1998, 15091510. (10) Weingarten, H. J. Org. Chem. 1964, 29, 977-978. (11) Paine, A. J. J. Am. Chem. Soc. 1987, 109, 1496-1502. 670 J. Org. Chem. 1999, 64, 670-674

Synthesis, Structure, and Electroluminescence of BR2q (R = Et, Ph, 2-Naphthyl and q = 8-Hydroxyquinolato)

Abstract: Three 8-hydroxyquinolato (q) boron compounds B(C2H5)2q (1), BPh2q (2), and B(2-naph)2q (3) have been synthesized by the reaction of 8-hydroxyquinoline with an appropriate BR3 compound. Compounds 1−3 have a tetrahedral geometry as demonstrated by the structure of 1 determined by a single-crystal X-ray diffraction analysis. Compounds 1−3 emit a green-blue color at λmax = 495−500 nm when irradiated by UV light. The electroluminescent (EL) properties of 2 and 3 were examined by fabricating EL devices using 2 and 3 as the light-emitting layer, respectively. The devices of 2 produce a yellow-green light with broad emission spectra, attributed to the formation of an exciplex of 2 with the N,N‘-di-1-naphthyl-N,N‘-diphenylbenzidine (NPB) in the hole transport layer while the intrinsic EL emission of compound 3 was observed. Both 2 and 3 were found to be good electron transport materials in EL devices.

Electroluminescent (EL) devices

Abstract: An electroluminescent device containing an anode, an organic electroluminescent element, and a cathode wherein the electroluminescent element contains, for example, a fluorescent hydrocarbon component of Formula (I) wherein R1 and R2 are substituents, which are selected from the group consisting of hydrogen, an alkyl, an alicyclic alkyl, an alkoxy, a halogen, and a cyano; Ar1 and Ar2 are each independently an aromatic component or an aryl group comprised of a from about 4 to about 15 conjugate-bonded or fused benzene rings

Humidity-induced crystallization of tris (8-hydroxyquinoline) aluminum layers in organic light-emitting devices

Abstract: We report electroluminescence degradation studies of tris (8-hydroxyquinoline) aluminum (Alq3) organic light-emitting devices (OLEDs) under ambient conditions. Alq3 films and organic bilayer anode/naphthyl-substituted benzidine derivative/Alq3/cathode devices are studied via electroluminescence, photoluminescence, polarization microscopy and atomic force microscopy, and via microscopic infrared spectroscopy. Results reveal that humidity induces the formation of crystalline Alq3 structures in originally amorphous films. The same phenomenon is found to occur in OLEDs and causes cathode delamination at the Alq3/cathode interface that results in the formation of black (nonemissive) spots in the devices.

The path to ubiquitous and low-cost organic electronic appliances on plastic

Abstract: Organic electronics are beginning to make significant inroads into the commercial world, and if the field continues to progress at its current, rapid pace, electronics based on organic thin-film materials will soon become a mainstay of our technological existence. Already products based on active thin-film organic devices are in the market place, most notably the displays of several mobile electronic appliances. Yet the future holds even greater promise for this technology, with an entirely new generation of ultralow-cost, lightweight and even flexible electronic devices in the offing, which will perform functions traditionally accomplished using much more expensive components based on conventional semiconductor materials such as silicon.

Copper-mediated coupling reactions and their applications in natural products and designed biomolecules synthesis.

Abstract: 6.4. Polyynes 3123 6.5. Using R-Hydroxy Stannanes 3124 6.6. Using the Hurtley Reaction 3124 6.7. Using a Methylenation Reaction 3125 7. Conclusions and Future Prospects 3125 8. Uncommon Abbreviations 3125 9. Acknowledgments 3125 10. Note Added in Proof 3125 11. References 3126 * Authorstowhomcorrespondenceshouldbeaddressed(evano@chimie.uvsq.fr, nicolas.blanchard@uha.fr). † Université de Versailles Saint Quentin en Yvelines. ‡ Université de Haute-Alsace. Chem. Rev. 2008, 108, 3054–3131 3054

The electroluminescence of organic materials

Abstract: This article provides a review about electroluminescence from organic materials and deals in detail with organic light-emitting diodes (OLEDs), light-emitting electro-chemical cells (LECs) and electrogenerated chemilumi-nescence (ECL) reflecting different electrooptical appli-cations of conjugated materials. It is written from an organic chemist's point of view and pays particular attention to the development of organic materials involved in corresponding devices. In recent years a substantial amount of both academic and industrial research has been directed to organic electroluminescence in an effort to improve the processability and tunability of organic materials and the longevity of OLEDs and LECs. On the eve of the commercialization of organic electrolumi-nescence this review provides an overview of lifetimes and efficiencies attained and reflects materials and device concepts developed over the last decade. In this context electrogenerated chemiluminescence is discussed with respect to its importance as a versatile tool to simulate the fundamental electrochemical processes in OLEDs.

Catalytic C-C, C-N, and C-O Ullmann-type coupling reactions.

Abstract: Copper-catalyzed Ullmann condensations are key reactions for the formation of carbon-heteroatom and carbon-carbon bonds in organic synthesis. These reactions can lead to structural moieties that are prevalent in building blocks of active molecules in the life sciences and in many material precursors. An increasing number of publications have appeared concerning Ullmann-type intermolecular reactions for the coupling of aryl and vinyl halides with N, O, and C nucleophiles, and this Minireview highlights recent and major developments in this topic since 2004.