Chiral lanthanide hexaazamacrocycles for circularly polarized luminescence, high relaxivity and magnetic resonance imaging

Abstract

Multifunctional lanthanide complexes have been extensively studied in recent years owing to their widespread applications in physics, chemistry and biology, including quantum information processing, molecular spintronics and theranostics. The multifunctionality includes chirality, luminescence, magnetism, ferroelectricity, fluorescence/magnetic resonance imaging etc. Although various N- and O-donor ligands have shown the ability to synthesize these complexes, Schiff bases are still the most widely used in constructing air-stable species. Herein, we report the facile gram-level synthesis of three pairs of lanthanide macrocycle enantiomers via an in situ [2 + 2] imine condensation with a trivalent lanthanide ion as a template. Eu(III)-based compounds, R/S-Eu and R/S-Eu-Ph₃PO, both show efficient circularly polarized luminescence (CPL) with maximum dissymmetry factors (g_lum) of 0.098 and 0.110, respectively. The latter exhibited stronger emission intensity and a longer luminescence lifetime than the former due to the lower vibrational coordination structures around the Eu(III) ion. Gd(III)-based species, R/S-Gd, possess a relatively high relaxivity of up to 35.04 and 34.09 mM⁻¹ s⁻¹ for R- and S-enantiomers, benefitting from the presence of one coordinated water molecule and abundant intermolecular H-bonds. In addition, the results of the MTT assay and in vitro experiments demonstrated the low toxicity and efficient MRI of Gd(III)-chelates in A549 cells.