Calix[4]arene podands and barrelands incorporating 2,2'-bipyridine moieties and their lanthanide complexes: Luminescence properties

A new family of cone-shaped podands and barrel-shaped cryptands based on calix[4]arenes incorporating 5,5'-substituted 2,2'-bipyridine subunits were prepared and characterized. The Eu3+ and Tb3+ complexes of the podands bearing two, three, or four bipyridine chromophores could be isolated. High molar absorption coefficients (epsilon(max) = 39 600 M-1 cm(-1) for Eu4 and 26 700 M-1 cm(-1) for Eu 3) and high metal luminescence quantum yields (16% for Eu4 and 15% for Eu3) were obtained. Molecular dynamics simulations on Eu4 showed that the bipyridine arms wrap around the lanthanide cation, efficiently shielding the cation from solvent molecules. In the presence of chloride counterions the fourth bipyridine arms does not coordinate the lanthanide ion. Ligands bearing two bipyridine units and two additional functional groups - ethyl butyrate or N-propylpyrrole - did not give stable lanthanide complexes, The barreland containing two calix[4]arene moieties and four bipyridine groups did not form complexes with lanthanide ions, most probably because of the rigidity of the ligand.