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.