Structures, and luminescence and magnetic properties of Ln(III) complexes bearing dibenzoylmethane ligand (Ln=Eu and Gd)

Abstract

The single crystal structure, luminescence and magnetic susceptibility of Cs[Ln(DBM)4] and Ln(DBM)3(phen) (Ln=Eu and Gd, DBM=dimenzoylmethanto and phen=1,10-phenanthroine) were characterized in detail. A pentanuclear Eu(III) cluster with the formula, Eu5(DBM)10(OH)5, was obtained from Cs[Eu(DBM)4] in ethanol, while from Cs[Gd(DBM)4] in acetonitrile, a one-dimensional polymeric Gd(III) assembly with the formula, {Cs[Gd(DBM)4]}n, was achieved via a heterobimetallic backbone, composed of alternating Cs and Gd atoms bridged by DBM. Exciting the Eu(III) complexes with near-UV light produced sensitized red emission by energy transfer from the triplet excited state of DBM to the Eu(III) ion. Replacing DBM by phen increased the quantum yield from Q=8.3% for Cs[Eu(DBM)4] to Q=16.6% for Eu(DBM)3(phen). This was attributed to a reduction in the site-symmetry of Eu(III) from D4h to C2v. The magnetic susceptibilities of the synthesized Ln(III) complexes in the crystalline or powder states were measured using a SQUID magnetometer. The Eu(III) complexes illustrated magnetic anisotropy at low temperatures and temperature-dependent susceptibilities. The coercive field inducing the magnetic anisotropy of Eu5(DBM)10(OH)5 was more than 40-fold stronger than those of Cs[Eu(DBM)4] and Eu(DBM)3(phen). In particular, the calculation based on the Van Vleck׳s treatment confirmed that the thermal mixing between the excited and ground states played a key role in the temperature-dependent susceptibilities of Eu(III). The magnetic susceptibility of the Gd(III) ion in Gd(DBM)3(phen) was identical to that of the free Gd(III) ion, wheras that of the Gd(III) ion in Cs[Eu(DBM)4] was higher. The phen ligand with the π-conjugate rigid planar structure decreased the magnetic susceptibilities of the Eu(III) and Gd(III) ions.