A Six–Armed Phenhomazine Ligand with a Potential “Turn–Off” Copper(II) Sensing Capability through Terbium(III) Luminescence Quenching

Herein we describe the design, synthesis and characterization of a phenhomazine ligand with six pendant acetate arms designed for the combined coordination of copper(II) and lanthanide(III) ions, with the perspective of the development of a "turn-off" copper sensor. The key step for the ligand preparation was the one-step endomethylene bridge fission of a diamino Troger's base with a concomitant alkylation. Fluorescence and absorption spectroscopies as well as nuclear magnetic resonance (NMR) experiments were performed in order to analyze and understand the coordination properties of the ligand. Transition metal coordination was driven by the synergistic effect of the free nitrogen atoms of the diazocinic core and the two central acetate arms attached to those nitrogens, whereas lanthanide coordination is performed by external acetate arms, presumably forming a self-assembled 2:2 metallosupramolecular structure. The terbium complex shows the typical green emission with narrow bands and long luminescence lifetimes. The luminescence quenching produced by the presence of copper(II) ions was analyzed. This work sets therefore a starting point for the development of a phenhomazine-based "turn-off" copper(II) sensor.