Luminescence of Alizarin and its Metal Complexes
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LUMINESCENCE OF ALIZARIN AND ITS METAL COMPLEXES LONDA J. LARSON AND JEFFREY I. ZINK Department of Chemistry and Biochemistry, University of California, Los Angeles, 405 Hilgard Ave., Los Angeles, CA 90024-1569
ABSTRACT The red/near-infrared emission spectra of alizarin (1,2-dihydroxyanthraquinonone), the potassium salt of alizarin and the aluminum and zinc complexes of alizarin in the solid state are reported. The emission is assigned to an alizarin localized transition from an in-plane nonbonding orbital delocalized over the anthraquinone rings to a predominantly n* C=O acceptor orbital (nt* ligand localized excited state). The cobalt, nickel and copper complexes do not emit from their nn* excited state. In these complexes, metal centered d-d excited states which lie lower in energy than the ligand localized state effectively deactivate the luminescence. INTRODUCFION In this article, an investigation of the absorption and luminescence properties of alizarin and some of its metal complexes is reported. Alizarin, the principle coloring material of the ancient dyestuff madder [1], gives colored compounds with metals (pigments or "lakes"), the color of which depends upon the metal [2, 3]. In addition to its use in dyes and pigments, alizarin is used for the spectrophotometric determination of metals [4-.7]. Both the historical use of madder as a dye and pigment and the more recent use of alizarin in metal analysis have made the chemical characterization of alizarin of great interest. The structural and spectroscopic properties of alizarin, OH 0 its salts and its metal complexes have been investigated by several workers [2, 3, 8-16]. OH The free alizarin molecule is shown in Fig. 1. Metals complex to this ligand through the ionized 1-hydroxyl oxygen and the adjacent carbonyl .N0 oxygen forming a six-member chelate ring structure [2, 3]. The lowest energy visible 0 absorption band of alizarin has been assigned to an intramolecular charge transfer from the Fig. 1. Structure of alizarin. anthraquinone rings out to the C--O acceptor groups [10]. Luminescence spectra of alizarin and of its aluminum, gallium and indium complexes in the solid state as well as in solution have been reported [10, 15, 161. We have extended the spectroscopic studies of alizarin by investigating the electronic absorption and emission properties of uncomplexed alizarin, its potassium salt and its AI(III), Co(H), Ni(U), Cu(lI) and Zn(II) complexes. Absorption spectra were taken of halocarbon grease mulls and N,N-dimethylformamide (DMF) solutions of each compound. Emission is observed from the lowest energy ligand localized excited state for alizarin, potassium alizarinate and the Al(III) and Zn(II) complexes. The Co(II), Ni(II) and Cu(lI) complexes do not emit from their lowest energy ligand based excited state. The lowest lying excited electronic states are assigned and the effect of the metal on the luminescence is discussed. RESULTS Absorption Spectra. Table I lists the position in wavenumbers of the lowest energy resolved transition observed i
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