Noncovalent Binding of Sensitizers for Lanthanide(III) Luminescence in an EDTA-bis(β-cyclodextrin) Ligand

EDTA-linked beta-cyclodextrin dimer 3 was synthesized from EDTA bis(anhydride) 1 and mono(propylamino)-appended beta-cyclodextrin 2. p-tert-butylbenzoate 5, bound by the beta-cyclodextrin cavities of 3 with an association constant of 10(4) M(-1) in water, acts as a sensitizer for the Eu(III) and Tb(III) complexes of 3. Luminescence spectroscopy, microcalorimetry, and Gd(III)-induced NMR relaxation rate measurements prove that 3 forms a 1:2 complex with 5 and that one of the beta-cyclodextrin-bound sensitizers coordinates to the EDTA-encapsulated Ln(III) ion. The Eu(III) complex of 3 forms strong 1:1 complexes (K approximately 10(7) M(-1)) with bis(propylamido adamantyl)-functionalized biphenyl sensitizers 7 and 8 in water. Both beta-cyclodextrins of 3 are involved in the binding of these guests. The amide functionality adjacent to the biphenyl unit in 7 and 8 coordinates to the EDTA-encapsulated Ln(III) ion. For these biphenyl-based antennae both binding to beta-cyclodextrin and coordination to the Ln(III) center are crucial for efficient sensitization.