Comparative Study of the Luminescence Properties of the Europium(III) Complexes of the Squarate, Phenylsquarate, and (Diphenylamino)squarate Ligands

Reaction of EuCl(3).6H(2)O with 3-phenyl-4-hydroxycyclobut-3-ene-1,2-dione (phenylsquarate) in methanol produces the polymeric complex {Eu(-C(6)H(5)C(4)O(3))(2)(C(6)H(5)C(4)O(3))(2)(CH(3)OH)(2)(H(2)O)(2).(CH(3)OH)}(n)() (1) which crystallizes in the space group P2(1)/n with a = 13.4049(5) Å, b = 13.7043(7) Å, c = 18.5026(6) Å, beta = 106.351(4) degrees, 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(H(2)O)(4)](2)(C(4)O(4))(3) (2), and europium(III) (diphenylamino)squarate, {Eu[-(C(6)H(5))(2)NC(4)O(3)](2)[(C(6)H(5))(2)NC(4)O(3)][NO(3)][H(2)O](4)}(2).4H(2)O (3) show emissions dominated by the Eu(3+)((5)D(0) --> (7)F(j)) transitions which are typical for Eu(3+) 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 pi-pi interactions between the C(4)-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.