The preparation and crystal and molecular structures of new luminescent Schiff-base complexes featuring coupled lanthanide(III) cations

Abstract

The reaction between salicylaldehyde, diethylenetriamine and lanthanide(III)(Ln³⁺) nitrates in refluxing methanol produced new acyclic dimeric Schiff-base compounds [{LnL⁷(NO₃)}₂](H₂L⁷ is the 2 : 1 Schiff-base condensation product of salicylaldehyde and diethylenetriamine) in which the coupled Ln³⁺ ions are bridged by phenolate groups and separated by a short Ln ⋯ Ln distance of ca. 3.81 Å. In contrast with lanthanide(III) dinuclear macrocyclic compounds of Schiff bases studied previously where the ligand negative charge to Ln³⁺ cation ratio is 1 : 1, chelate L⁷ possess a higher ratio of 2 : 1. The dimeric compounds of L⁷ feature a shorter Ln³⁺⋯ Ln³⁺ separation, greater stability and enhanced electronic coupling among Ln³⁺–Ln³⁺ ion pairs. For example, [{LnL⁷(NO₃)}₂] complexes readily yielded intact dimeric species such as [{EuL⁷}₂]⁺ and [Eu₂L⁷₂(NO₃)]⁺ during fast atom bombardment mass spectrometric analyses and exhibit Eu³⁺–Eu³⁺ and Tb³⁺–Tb³⁺ energy migration while macrocyclic analogues do not. However, like other phenolate Schiff-base chelates, L⁷ is a good sensitizer for Tb³⁺ and Eu³⁺ emission. Metal-to-ligand back-energy transfer is consistent with the temperature evolution of the Eu³⁺(⁵D₀) and Tb³⁺(⁵D₄) luminescence decay rates. Heteropair Eu³⁺–Dy³⁺ interactions are observed; assuming a dominant dipole–dipole Eu³⁺→ Dy³⁺ energy transfer mechanism the coupling constant is ca. 7 × 10^–52 m⁶ s^–1.