Rare-Earth Quinolinates: Infrared-Emitting Molecular Materials with a Rich Structural Chemistry Rik Van Deun,* Pascal Fias, Peter Nockemann, An Schepers, Tatjana N. Parac-Vogt, Kristof Van Hecke, Luc Van Meervelt, and Koen Binnemans Department of Chemistry, Katholieke UniVersiteit LeuVen, Celestijnenlaan 200F, B-3001 LeuVen, Belgium Received September 9, 2004 Near-infrared-emitting rare-earth chelates based on 8-hydroxyquinoline have appeared frequently in recent literature, because they are promising candidates for active components in near-infrared-luminescent optical devices, such as optical amplifiers, organic light-emitting diodes, .... Unfortunately, the absence of a full structural investigation of these rare-earth quinolinates is hampering the further development of rare-earth quinolinate based materials, because the luminescence output cannot be related to the structural properties. After an elaborate structural elucidation of the rare-earth quinolinate chemistry we can conclude that basically three types of structures can be formed, depending on the reaction conditions: tris complexes, corresponding to a 1:3 metal-to-ligand ratio, tetrakis complexes, corresponding to a 1:4 metal-to-ligand ratio, and trimeric complexes, with a 3:8 metal-to-ligand ratio. The intensity of the emitted near-infrared luminescence of the erbium(III) complexes is highest for the tetrakis complexes of the dihalogenated 8-hydroxyquinolinates. Introduction There is a growing interest to replace the traditional inorganic optoelectronic components in optical amplifiers by organic molecular materials, because of advantages such as solution processing, flexibility, and low cost. Whereas organic materials emitting in the visible region have already taken their place in the display application field, the development of organic materials emitting in the near- infrared region has only started recently, stimulated by innovations in communication network technology. One class of materials that has been studied as near- infrared-emitting organic compounds are the rare-earth quinolinates (REQs). Historically, 8-hydroxyquinoline (8- quinolinol) has been used as a reagent for gravimetric analysis, because of its excellent complexing properties with a wide range of metal ions, including the rare earths, and because of the low solubility of the resulting complexes in aqueous media. 1,2 It has also found applications as an extrac- tion reagent. 3-11 The most intensively investigated metal complex of 8-hydroxyquinoline is tris(8-hydroxyquinolinato)- aluminum(III), 12-19 because this complex exhibits an intense green electroluminescence and performs well as the active component in organic light-emitting diodes (OLEDs) operat- * Author to whom correspondence should be addressed. E-mail: rik.vandeun@chem.kuleuven.ac.be. (1) Pirtea, T. I. Z. Anal. Chem. 1936, 107, 191. (2) Birnbaum, E. R.; Forsberg, J. H. Gmelin Handbook of Inorganic Chemistry, Sc, Y, La-Lu Rare Earth Elements; Springer-Verlag: Berlin, 1982; Part D2, System Nr. 39, pp 5-24 and references therein. (3) Moeller, T.; Jackson, D. E. Anal. Chem. 1950, 22, 1393. (4) Tochiyama, O.; Freiser, H. Anal. Chem. 1981, 53, 874. (5) Freiser, H. SolVent Extr. Ion Exch. 1988, 6, 1093. (6) Czakis-Sulikowska, D.; Malinowska, A.; Pustelnik, N.; Kuznik, B. Acta Phys. Pol., A 1996, 90, 427. (7) Czakis-Sulikowska, D.; Pustelnik, N.; Malinowska, A.; Kuznik, B. Chem. Anal. (Warsaw) 1997, 42, 23. (8) Czakis-Sulikowska, D.; Kuznik, B.; Malinowska, A.; Pustelnik, N. Chem. Anal. (Warsaw) 1999, 44, 925. (9) Czakis-Sulikowska, D.; Pustelnik, N.; Kuznik, B.; Malinowska, A. J. Alloys Compd. 2000, 300-301, 234. (10) Czakis-Sulikowska, D.; Kuznik, B.; Malinowska, A. Chem. Anal. (Warsaw) 2001, 46, 93. (11) Czugler, M.; Neumann, R.; Weber, E. Inorg. Chim. Acta 2001, 313, 100. (12) Ohnesorge, W. E.; Rogers, L. B. Spectrochim. Acta 1959, 15, 27. (13) Lytle, F. E.; Storey, D. R.; Jurich, M. E. Spectrochim. Acta 1973, 29, 1357. (14) Schmidbaur, H.; Lettenbauer, J.; Wilkinson, D. L.; Mu ¨ller, G.; Kumberger, O. Z. Naturforsch., B 1991, B46, 901. (15) Sano, K.; Kawata, Y.; Urano, T. I.; Mori, Y. J. Mater. Chem. 1992, 2, 767. (16) Garbuzov, D. Z.; Bulovic, V.; Burrows, P. E.; Forrest, S. R. Chem. Phys. Lett. 1996, 249, 433. (17) Fujii, I.; Hirayama, N.; Ohtani, J.; Kodama, K. Anal. Sci. 1996, 12, 153. (18) Brinkmann, M.; Gadret, G.; Muccini, M.; Taliani, C.; Masciocchi, N.; Sorini, A. J. Am. Chem. Soc. 2000, 122, 5147. (19) Utz, M.; Chen, C.; Morton, M.; Papadimitrakopoulos, F. J. Am. Chem. Soc. 2003, 125, 1371. Inorg. Chem. 2004, 43, 8461-8469 10.1021/ic048736a CCC: $27.50 © 2004 American Chemical Society Inorganic Chemistry, Vol. 43, No. 26, 2004 8461 Published on Web 11/30/2004