Conjugates of cyclodextrins with charged and neutral macrocyclic europium, terbium and gadolinium complexes: sensitised luminescence and relaxometric investigations and an example of supramolecular relaxivity enhancement

The synthesis and characterisation of lanthanide complexes of mono- and tetra-amide β-cyclodextrin derivatives of 1,4,7,10-tetraazacyclododecane tetraacetate (DOTA) are reported. Luminescence and relaxivity measurements confirm that the Eu, Tb and Gd complexes of the eight-coordinate mono-amide ligand possess one bound water molecule while the tetra-amide complexes are rare examples of q = 0 systems is aqueous solution. The relaxivity of the host β-CD Gd complex (8.50 mM^-1 s^-1, 20 MHz, 298 K) is enhanced when non-covalently bound to a second gadolinium complex bearing two phenyl moieties with an enhancement that is limited by the slowness of the water exchange rate (τ_m = 0.6 μs, 298 K). Sensitisation of the terbium luminescence in the mono-amide β-CD complex occurs in the absence of oxygen using various substituted naphthalene derivatives (e.g. naphthalene, K = 1.04 × 10⁴ M^-1, 293 K) and methyl p-tert-butylbenzoate. The slowness of the intra-complex energy transfer step severely limits the efficiency of this process and restricts the scope of 'non-covalently triggered luminescence' to a narrow range of guest substrates, as deduced by variable temperature time-resolved luminescence and flash-photolysis studies.