THE PREPARATION AND LUMINESCENCE DECAY DYNAMICS OF COUPLED HETEROLANTHANIDE(III) CATIONS IN DINUCLEAR SCHIFF-BASE COMPLEXES

Molecular recognition events in which lanthanide(III) (Ln3+) cation pa irs are formed have been studied using the template condensation cryst alline products (Ln1-xEux)2L(NO3)4.H2O and (Ln1-xTbx)2L(NO3)4.H2O (whe re H2L is the [2 + 2] macrocyclic Schiff base obtained from 2,6-diform yl-p-cresol and 3,6-dioxaoctane-1,8-diamine). The observed relationshi p 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 Ln TbL(NO3)4.H2O. In both cases the cation discrimination index, computed as the ratio of probabilities of Ln3+ incorporation into the crystall ine heterolanthanide compounds, is in favour of the larger Ln3+ ion. H owever indiscriminate complexation of Nd3+ and Eu3+ in the Nd1-x-Eux)2 L(NO3)4.H2O system is unusual and reflects the importance of co-operat ive heteropair effects. Luminescence decay dynamics of the (Sm1-xEux)2 L(NO3)4.H2O and (Pr1-xTbx)2L(NO3)4.H2O systems (0 < x < 1) reveal two microscopic environments for Eu3+ and Tb3+ which were attributed to ho modinuclear molecules, Ln2L(NO3)4.H2O (Ln = Eu or Tb) (slow component) and heterodinuclear molecules SmEuL(NO3)4.H2O and PrTbL(NO3)4.H2O (fa st component). The luminescence decay rate constants for intramolecula rly coupled Eu-Sm and Pr-Tb pairs are 8200 and 12 500 s-1, which yield coupling constants (alpha) of 2.9 x 10(-53) and 4.7 x 10(-53) m6 s-1 respectively when dominant dipolar interactions are assumed. No exchan ge interactions are evident despite the presence of a phenolate linkag e shared by the heteroatoms only almost-equal-to 4 angstrom 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 intervent ion of molecular recognition processes favouring the homo- to hetero-p aired species in the ion pairing events leading to (Sm1-xEux)2L(NO3)4. H2O compounds.