Lanthanide complexes with a p-tert-butylcalix[4]arene fitted with phosphinoyl pendant arms

The lower-rim substituted calix[4]arene, 5,11,17,23-tetra-tert-butyl-25,26, 27,28-tetrakis(dimethylphosphinoylmethoxy)calix[4]arene (L) has been synthe sized and characterised by the single-crystal structure of its acetonitrile adduct, L . 2MeCN (monoclinic, space group P2(1)/c). The ligand adopts a c one conformation and a Delta enantiomeric form with the four phosphinoyl ar ms in a delta delta delta delta configuration. The cone conformation is mai ntained in organic solution where L displays a time-averaged C-4 symmetry. Trivalent lanthanum ions interact with L in acetonitrile to yield both 1:1 (log beta (1);=11.4 +/- 1.5) and 1:2 (log beta(2)=19.6 +/- 1.8) complexes; two forms of the 1:1 complex are identified depending on the water content of the solutions. A photophysical study of both the ligand- and metal-centr ed luminescence of complexes of La, Eu and Tb points to L having a moderate quantum yield (ca. 10%, ligand-centred luminescence) and being a poor sens itiser of europium and terbium ions. It confirms the presence of differentl y solvated 1:1 (and 1:2) complexes, depending on the solvent composition. T he lifetimes of the metal-centred luminescence of the unhydrated 1:1 and 1: 2 complexes are long, in the range 1.4-2.4 ms for Eu and 1.7-7.3 ms for Tb, which indicates that the lanthanide(III) ions are well encapsulated in the cavity formed by the donor groups of the calix[4]arene molecule(s).