Lanthanide Podates with Programmed Intermolecular Interactions: Luminescence Enhancement through Association with Cyclodextrins and Unusually Large Relaxivity of the Gadolinium Self-Aggregates

Abstract

The synthesis of the phenyl anchored podand H4L1 fitted with four 3-carboxylate pyrazole arms and programmed for intermolecular interactions is reported, and its protonation constants are determined. Interaction with Ln3+ ions (Ln = La, Eu, Lu) in dilute aqueous solutions leads to complexes with 1:1 and 1:2 metal−ligand stoichiometry. The stability constants are in the range log β110 = 12.7−13.5 and log β120 = 22.5−23.8 (pLn values in the range 9−10). The podates display a fair sensitization of the metal-centered luminescence with an absolute quantum yield of 5% in case of TbIII. The average numbers of water molecules coordinated to the LnIII ion amount to 3.8 and 4.9 for the 1:1 Eu and Tb podates, respectively, as determined by lifetime measurements. The addition of β- and γ-cyclodextrins to the 1:1 podates results in a strong association with the host, mainly through the phenyl anchor (Ln = Tb, log K11 = 5−6 depending on the cyclodextrin) and, for the Tb complex, to a large increase in luminescence intensity at physiological pH. Self-aggregation of the podates occurs at concentration larger than 3 × 10-5 M. This process is characterized by luminescence, using a pyrene probe, light-scattering measurements, and transmission electron microscopy. The novelty of the reported systems is their ability to self-aggregate into nanometric, rigid, and spherical particles in a controlled way (mean diameter: 10, 60, and ∼300 nm), opening large perspectives for various applications. In particular, a record-high relaxivity (r1 = 53 mM-1 s-1 at 20 MHz, T = 25 °C) is observed for the aggregates of 1:2 Gd podate, which is not modified upon addition of an equimolar quantity of ZnII.