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

Reaction of EuCl3·6H2O 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, β = 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)2NC4O3]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 π−π 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.