Sensitization of Europium(III) Luminescence by Benzophenone-Containing Ligands: Regioisomers, Rearrangements and Chelate Ring Size, and Their Influence on Quantum Yields

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• 作者：Andrew J. Wilkinson ; Davide Maffeo ; Andrew Beeby ; Clive E. Foster ; J. A. Gareth Williams
• 刊名：Inorganic Chemistry
• 出版年：2007
• 出版时间：October 29, 2007
• 年：2007
• 卷：46
• 期：22
• 页码：9438 - 9449
• 全文大小：180K
• 年卷期：v.46,no.22(October 29, 2007)
• ISSN：1520-510X

文摘

A series of europium(III) complexes based on the macrocyclic azacarboxylate structure, DO3A, have been investigated,incorporating benzophenone appended at N10 of the macrocycle via linkers containing amide bonds (H₃DO3A =1,4,7,10-tetraazacyclododecane-1,4,7-tris-acetic acid). Complexes [EuL^1-3] incorporate N¹⁰-CH₂CONH-BP linkers(BP = benzophenone), which allow formation of a five-membered chelate ring containing the metal ion uponchelation of the amide oxygen; these three isomeric complexes differ from one another in the substitution positionof the BP unit, namely para, meta, and ortho for L¹, L², and L³ respectively. The quantum yields of europiumluminescence sensitized via the chromophore are found to be highly dependent upon the position of substitution,being 20 times smaller for the ortho compared to the para-substituted complex. A related para-substituted BPcomplex [EuL⁴], prepared by an unusual Michael reaction of the azamacrocycle with a BP-containing acrylamide,incorporates an additional methylene unit in the linker, namely N¹⁰-CH₂CH₂CONH-BP. Despite the longer linker,this complex equals the luminescence quantum yield achieved with [EuL¹] (_lum = 0.097 and 0.095, respectively,in H₂O at 298 K). Analysis of the pertinent kinetics reveals that the decreased energy transfer efficiency in thiscomplex, arising from the longer donor-acceptor distance, is compensated by an increased radiative rate constant.Under basic conditions, the ortho-substituted complex [EuL³] undergoes an intramolecular rearrangement to generatean unprecedented complex [EuL⁵] incorporating a 4-phenyl-2-hydroxyquinoline unit directly bound to the ring nitrogen.Although this complex is a poor emitter, an analogous complex obtained from 2-amino-acetophenone, which generates4-methyl-2-hydroxyquinoline during the corresponding rearrangement, is an order of magnitude more emissivewhile still benefiting from relatively long-wavelength absorption. The emission from this complex is pH sensitive,being dramatically quenched under mildly basic conditions.