Calixarene-triacids as receptors for lanthanides; synthesis and luminescence of neutral Eu3+ and Tb3+ complexes
Calixarene triacids (3a–d) have been prepared that are able to form neutral complexes with lanthanides. Complexes of 3a–d with Eu3+ and Tb3+ have been studied with respect to their luminescent properties in a protic solvent (methanol). In all cases it was found that the luminescent lifetime of the complexed lanthanide ions is significantly enhanced compared with that of the free ions in the same solvent. Solvent deuterium isotope effects confirm that shielding of the lanthanide ion from the solvent in the calixarene complexes is the main mechanism responsible for the lifetime difference between free and complexed ions, however, the calixarene itself also exerts a moderate lifetime-shortening effect. Excitation spectra show that in the complexes efficient energy transfer to the lanthanide ions occurs both from the calixarene aromatic moieties as well as from aromatic (pyridine) chromophores attached to it.