In the Laboratory
Preparation of Two Luminescent Complexes: AlIII(8-hydroxyquinolinolato)3 and EuIII(thenoyltrifluoroacetonato)3(1,10-phenanthroline)
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Qinde Liu and Suning Wang* Department of Chemistry, Queen’s University, Kingston, Ontario K7L 3N6, Canada; *
[email protected] Organic light-emitting diodes (OLEDs) are devices that convert electrical energy to light (electroluminescence). Two classes of materials widely used in OLEDs are organic polymers and small organic or coordination compounds. A compound must be an efficient photoluminescent emitter to be able to function as an emitter in OLEDs. For organic or main group coordination compounds such as AlIIIq3, where q is 8hydroxyquinolinolato, the luminescence originates typically from π to π* transitions of a conjugate system or ligands (1, 2). Most organic and main group emitters in OLEDs are fluorescent with a decay lifetime typically in the nanosecond range. The fluorescent emission band from an organic molecule or an organic ligand is usually fairly broad as a result of the vibronic couplings. For lanthanide compounds, the luminescence typically originates from f to f electronic transitions, resulting in very narrow emission bands (3, 4). Lanthanide emission bands usually have long decay lifetimes (in the microsecond and millisecond range) and low emission
intensities. To enhance the emission intensity of lanthanide ions, a ligand capable of transferring energy to the lanthanide center, and thus functioning as an activator for lanthanide emission, is usually required. AlIIIq3 was used as a green emitter in the first organic light-emitting device, which operated at ∼10 V and was reported by Tang and coworkers in 1987 (1). AlIIIq3 can also function as an electron-transport material in OLEDs. Eu III(tta) 3(phen), where tta is thenoyltrifluoroacetonato and phen is 1,10-phenanthroline, is one of the brightest red emitters known in the literature and has been widely used as a red emitter in OLEDs (4). This experiment demonstrates the syntheses and properties of these two well-known emitters in OLEDs (1, 5). Experimental The synthetic procedures for these two compounds are shown in Scheme I. Alq3 was synthesized in nearly quantitative yield from the reaction of aluminum nitrate with 8-hy-
O
O N
3
+
N
N
N
base
3ⴙ
Al
Al
Al
N
O OH
N
O
O
N
O
8 - hydroxyquinoline Alq3
CF3
S F 3C
O
O
O H
3
CF3
S O
N
H
+
Eu
1, 10 - phenanthroline 3ⴙ
Eu
O
base
S
ttaH
N O
O F3C
O S
Eu(tta)3(phen) Scheme I. Synthetic procedures for the two luminescent compounds.
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Journal of Chemical Education • Vol. 80 No. 12 December 2003 • JChemEd.chem.wisc.edu
In the Laboratory
droxyquinoline in the presence of a base in a solution of methanol and water. The yellow powders of Alq3 precipitate from the solution and can be separated by simple filtration. The entire process typically takes about 1 h to complete. Eu(tta)3(phen) was synthesized by the reaction of europium chloride with thenoyltrifluoroacetone (ttaH) in the presence of a base in methanol, followed by the addition of 1,10phenanthroline (phen). Eu(tta)3(phen) is a colorless solid with a poor solubility in methanol and therefore precipitates from the solution. The typical isolated yield of Eu(tta)3(phen) is about 80%. The entire process for the synthesis of Eu(tta)3(phen) takes no more than 1 h to complete. The luminescence of Alq 3 and Eu(tta) 3(phen) was checked visually by using a UV lamp. Alq3 emits a bright green color while Eu(tta)3(phen) emits a red glow like a fire. The brilliant glow by both compounds has never ceased to impress the students. UV–vis and fluorescent spectra for both compounds were recorded, which showed the color origin due to both absorption and emission of visible light and the difference between ligand-based transitions and lanthanide metal ion-based transitions. The ligands’ role in enhancing EuIII emission of the Eu(tta)3(phen) complex was demonstrated by visually examining the emission intensity difference between EuCl3 and the Eu(tta)3(phen) using a UV lamp. Alq3 displays two geometric isomers (6), mer and fac, which can be demonstrated by NMR. Hazards Methanol is highly toxic and an irritant. Dichloromethane is a suspected carcinogen. The experiments should be performed in a fume hood. Concentrated NaOH is corrosive and irritating to the skin and eyes. Care should be taken to avoid contact during the preparation of the solution. Other chemicals involved in this experiment are also toxic if swallowed. Conclusion In summary, a simple synthetic procedure for two important electroluminescent materials has been demonstrated.
This experiment illustrates the importance of coordination chemistry in luminescent materials and the distinct difference between luminescence of main group compounds and that of lanthanide compounds. W
Supplemental Material
A handout for students and notes for the instructor are available in this issue of JCE Online. Literature Cited 1. (a) Tang, C. W.; Van Slyke, S. A. Appl. Phy. Lett. 1987, 51, 913. (b) Tang, C. W.; Van Slyke, S. A.; Chen, C. H. J. Appl. Phys. 1989, 65, 3610. (c) Sibley, S.; Thompson, M. E.; Burrows, P. E.; Forrest, S. R. In Optoelectronic Properties of Inorganic Compounds; Roundhill, M., Fackler, J. P., Jr., Eds.; Plenum Press: New York, 1999; p 29. 2. (a) Wang, S. Coord. Chem. Rev. 2001, 215, 79. (b) Hu, N.X.; Esteghamatian, M.; Xie, S.; Popovic, Z.; Ong, B.; Hor, A. M.; Wang, S. Adv. Mater. 1999, 11, 1460. 3. (a) Kido, J.; Nagai, K.; Ohashi, Y. Chem. Lett. 1990, 657. (b) Jabbour, G. E.; Wang, J. F.; Kippelen, B.; Peyghambarian, N. Jpn. J. Appl. Phys., Lett. 1999, 38, L1553. (c) Zhu, D.; Liu, Y.; Bai, F. Thin Solid Films 2000, 363, 51. (d) Shipley, C. P.; Salata, O. V.; Capecchi, S.; Christou, V.; Dobson, P. J. Adv. Mater. 1999, 11, 533. (e) Robinson, M. R.; O’Regan, M. B.; Bazan, G. C. Chem. Commun. 2000, 1645. (f ) Wang, J.; Wang, R.; Yang, J.; Zheng, Z.; Carducci, M.; Cayou, T.; Peyghambarian, N., Jabbour, G. E. J. Am. Chem. Soc. 2001, 123, 6179. 4. Sano, T.; Fujita, M.; Fujii, T.; Hamada, Y.; Shibata, K.; Kuroki, K. Jpn. J. Appl. Phys.1 1995, 34, 1883. 5. (a) Kalinowski, J.; Fattori, V.; Di Marco P. Chem. Phys. 2001, 266, 85. (b) Stampor, W.; Kalinowski, J.; DiMarco, P.; Fattori, V. Appl. Phys. Let. 1997, 70, 1935. (c) Wang, K. Z.; Li, L. J.; Liu, W. M.; Xue, Z. Q.; Huang, C. H.; Lin, J. H. Materials Research Bulletin 1996, 31, 993. (d) Adachi, C.; Baldo, M. A.; Forrest, S. R. J. Appl. Phys. 2000, 87, 8049. 6. Schmidbaur, H.; Lettenbauer, J.; Wilkinson, D. L.; Müller, G.; Kumberger, O. Z. Naturforsch 1991, 46b, 901.
JChemEd.chem.wisc.edu • Vol. 80 No. 12 December 2003 • Journal of Chemical Education
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