Journal Article

Alleyne, Bert D.; Hall, Lincoln A.; Kahwa, Ishenkumba A.; White, Andrew J.; Williams, David J.

Department of Chemistry

Comparative study of the luminescence properties of the europium (III) complexes of the squarate, phenylsquarate, and (Diphenylamino)squarate ligands

Inorganic Chemistry

Refereed

1999

38

6278-84

Reaction of EuCl3.6H20 with 3-phenyl-4-hydroxycyclobut-3-ene-1,2-dione (phenylsquarate) in methanol produces the polymeric complex {Eu(µ-C6H5C4O3)2(C6H5C4O3)2(CH3OH)2(H2O)2.(CH3OH)}n (1) which crystallizes in the space group P21/n with a = 13.4049(5) Å, b = 13.7043(7) Å, c = 18.5026(6) Å, ß = 106.351(4)°, and Z = 4. The Eu(III) ion is eight coordinate with two bridging and two pendant phenylsquarate ligands, the coordination polyhedron being completed by two aqua and two methanol ligands. The emission spectra of 1 and the already reported europium (III) squarate, [Eu(H2O)4]2(C4O4)3 (2), and europium (III) (diphenylamino) squarate, {Eu[µ-C6H5)2-NC4O3]2[C6H5)2NC4O3][NO3][H2O]4}2.4H2O (3) show emissions dominated by the Eu3+(5D0 ® 7Fj) transitions which are typical for Eu3+ species in low-symmetry sites. For 1 the emission occurs at room temperature and is apparently sensitized by the phenyl substituents on the phenylsquarate ligand groups; for 2 and 3 the emission is only observed at low temperature (77 K). The broad band (370-575 nm) in the excitation spectrum of 3 at 77 K is consistent with excimer formation, which can be attributed to enhanced p-p interactions between the C4- cycles of neighboring phenylsquarate ligand groups whose centroid...centroid separation decreases from 3.630(7) Å at room temperature (293 K) to 3.565(7) Å at 96 K. The temperature dependence of the luminescence of all three complexes revealed strong Eu...Eu interactions which keep the energy transport process in the dynamic regime.....
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europium; methanol;