Both magnetic relaxation and luminescence of Zn2Dy2 cluster complexes regulated by the bis-imine chain in Schiff base ligands

Abstract

Two novel Zn₂Dy₂ Schiff base cluster complexes composed of [DyZn₂(salen)₂Cl₂]⁺ and [Dy(bpdc)₂]⁻ complex-fragment building blocks (bpdc²⁻ = 2,2′-bipyridine-6,6′-dicarboxylate) have been prepared, which exhibit interesting T-shaped metal cores and ferromagnetic interactions. Two-step magnetic relaxation was observed under a 1500 Oe dc field for both cluster complexes, and their effective energy barrier can be regulated by the bis-imine chain in the Schiff base. Complex 1, [Dy₂Zn₂(L^a)₂(bpdc)₂Cl₂)]·3MeOH [H₂L^a = N,N′- bis(3-methoxysalicylidene)-1,3-diaminopropane], has the U_eff/k value of 20.2(0.3) K and the τ₀ value of 6.7(0.2) × 10⁻⁸ s for the fast relaxation (FR) phase as well as the U_eff/k value of 243.2(4.9) K and the τ₀ value of 6.0(0.3) × 10⁻¹⁰ s for the slow relaxation (SR) phase; while complex 2, [Dy₂Zn₂(L^b)₂(bpdc)₂Cl₂)]·MeOH·H₂O [H₂L^b = N,N′-bis(3-methoxysalicylidene)-1,2-diaminoethane], possesses the U_eff/k value of 22.0(0.4) K and the τ₀ value of 4.6(0.1) × 10⁻⁸ s for the FR phase as well as the U_eff/k value of 205.7(4.4) K and the τ₀ value of 1.7(0.2) × 10⁻⁹ s for the SR phase. Furthermore, the two complexes exhibit dual emission centered on not only the lanthanide(III) ion but also the ligand in the visible region, of which the metal-centered emission is closely related to the length of the bis-imine chain on the salen ligand. The average fluorescence lifetime (τ) of 1 (10.6 ns) is an order of magnitude longer than that of 2 (0.86 ns).