Comparative study of the luminescence properties of the Europium(III) complexes of the squarate, phenylsquarate, and (diphenylamino)squarate ligands

Reaction of EuCl3. 6H(2)O with 3-phenyl-4-hydroxycyclobut-3-ene-1,2-dione (phenylsquarate) in methanol produces the polymeric complex ( Eu(mu-C6H5C4O3)(2)(C6H5C4O3)(2)(CH3OH)(2)(H2O)(2).(CH3OH)}(n) (1) which crystallizes iri the space group P2(1)/n with a = 13.4049(5) Angstrom, b = 13.7043(7) Angstrom, c = 18.5026(6) Angstrom, 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(I) squarate, [Eu(H2O)(4)](2)(C4O4)(3) (2), and europium(III) (diphenylamino)squarate, {Eu[mu-(C6H5)(2)NC4O3](2)[(C6H5)(2)NC4O3][NO3][H2O](4)}(2). 4N(2)O (3) show emissions dominated by the Eu3+(D-5(0) --> F-7(j)) 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 pi-pi interactions between the C-4-cycles of neighboring phenylsquarate ligand groups whose centroid...centroid separation decreases from 3.630(7) at room temperature (293 K) to 3,565(7) Angstrom 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.