THE PREPARATION AND CRYSTAL AND MOLECULAR-STRUCTURES OF NEW LUMINESCENT SCHIFF-BASE COMPLEXES FEATURING COUPLED LANTHANIDE(III) CATIONS

The reaction between salicylaldehyde, diethylenetriamine and lanthanid e(III) (Ln(3+)) nitrates in refluxing methanol produced new acyclic di meric Schiff-base compounds [{LnL(7)(NO3)}(2)] (H(2)L(7) is the 2:1 Sc hiff-base condensation product of salicylaldehyde and diethylenetriami ne) In which the coupled Ln(3+) ions are bridged by phenolate groups a nd separated by a short Ln ... Ln distance of ca. 3.81 Angstrom. In co ntrast with lanthanide(III) dinuclear macrocyclic compounds of Schiff bases studied previously where the ligand negative charge to Ln(3+) ca tion ratio is 1:1, chelate L(7) possess a higher ratio of 2:1. The dim eric compounds of L(7) feature a shorter Ln(3+)... Ln(3+) separation, greater stability and enhanced electronic coupling among Ln(3+)-Ln(3+) ion pairs. For example, [{LnL(7)(NO3)}(2)] complexes readily yielded intact dimeric species such as [{EuL(7)}(2)](+) and [Eu(2)L(2)(7)(NO3) ](+) during fast atom bombardment mass spectrometric analyses and exhi bit Eu3+-Eu3+ and Tb3+-Tb3+ energy migration while macrocyclic analogu es do not. However, like other phenolate Schiff-base chelates, L(7) is a good sensitizer for Tb3+ and Eu3+ emission. Metal-to-ligand back-en ergy transfer is consistent with the temperature evolution of the Eu3 (D-5(0)) and Tb3+ (D-5(4)) luminescence decay rates. Heteropair Eu3+- Dy3+ interactions are observed; assuming a dominant dipole-dipole Eu3 --> Dy3+ energy transfer mechanism the coupling constant is cn. 7 x 1 0(-52) m(6) s(-1).