The preparation and luminescence decay dynamics of coupled heterolanthanide(III) cations in dinuclear Schiff-base complexes

Abstract

Molecular recognition events in which lanthanide(III)(Ln³⁺) cation pairs are formed have been studied using the template condensation crystalline products (Ln_1–xEux)₂L(NO₃)₄·H₂O and (Ln_1–xTb_x)₂L(NO₃)₄·H₂O (where H₂L is the [2 + 2] macrocyclic Schiff base obtained from 2,6-diformyl-p-cresol and 3,6-dioxaoctane-1,8-diamine). The observed relationship between the concentrations of Eu³⁺(or Tb³⁺) in the reaction mixture X_Eu or X_Tb(from neutron activation analyses) with corresponding concentrations in the crystalline products (x) suggests that formation of LnEuL(NO₃)₄·H₂O heteromolecules is more favourable than that of LnTbL(NO₃)₄·H₂O. In both cases the cation discrimination index, computed as the ratio of probabilities of Ln³⁺ incorporation into the crystalline heterolanthanide compounds, is in favour of the larger Ln³⁺ ion. However indiscriminate complexation of Nd³⁺ and Eu³⁺ in the (Nd_1–xEu_x)₂L(NO₃)₄·H₂O system is unusual and reflects the importance of co-operative heteropair effects. Luminescence decay dynamics of the (Sm_1–xEu_x)₂L(NO₃)₄·H₂O and (Pr_1–xTb_x)₂L(NO₃)₄·H₂O systems (0 < x < 1) reveal two microscopic environments for Eu³⁺ and Tb³⁺ which were attributed to homodinuclear molecules, Ln₂L(NO₃)₄·H₂O (Ln = Eu or Tb)(slow component) and heterodinuclear molecules SmEuL(NO₃)₄·H₂O and PrTbL(NO₃)₄·H₂O (fast component). The luminescence decay rate constants for intramolecularly coupled Eu–Sm and Pr–Tb pairs are 8200 and 12 500 s^–1, which yield coupling constants (α) of 2.9 × 10^–53 and 4.7 × 10^–53 m⁶ s^–1 respectively when dominant dipolar interactions are assumed. No exchange interactions are evident despite the presence of a phenolate linkage shared by the heteroatoms only ≈4 Å apart. The ratio of Eu–Eu to Eu–Sm ‘cation pairing selectivity’ constants of 1:1.5 (expected ratio for random pairing is 1:2) supports the intervention of molecular recognition processes favouring the homo- to hetero-paired species in the ion pairing events leading to (Sm_1–xEu_x)₂L(NO₃)₄·H₂O compounds.