A fluorescent molecular switch driven by the input sequence of metal cations: An azamacrocyclic ligand containing bipolar anthracene fragments

Abstract

An azamacrocyclic ligand (L) containing two anthracene (AN) fragments connected through two triethylenetetramine bridges has been synthesized, in which each of the bridges can coordinate with one metal cation. The effects of pH and metal cations (Zn2+ and Cd2+) on the emission properties of L were studied in water. Without metal cations, L does not show any emission at basic pH values. The addition of Zn2+ leads to the production of excimer emission, which is due to a static excimer formed by direct excitation of the intramolecular ground-state dimer of the bipolar AN fragments that approach each other by Zn2+ binding. In contrast, Cd2+ addition does not result in excimer emission because the Cd 2+-AN π complex, formed by donation of a π electron of the AN fragments to the adjacent Cd2+, suppresses π-stacking interactions of the AN fragments. The most notable feature is the appearance of excimer emission controlled by the input sequence of metal cations: Zn 2+→Cd2+ sequential addition (each one equivalent) allows excimer emission, whereas the reverse sequence (Cd2+→ Zn2+) does not. In the Zn2+→Cd2+ sequence, Cd2+ coordination is structurally restricted by the first Zn 2+ coordination with the other polyamine bridge, leading to the formation of a weak Cd2+-AN π complex. In contrast, for the reverse sequence, the first Cd2+ coordination forms a stable Cd 2+-AN π complex, which is not weakened by sequential Zn 2+ coordination, resulting in no excimer emission. © 2008 Wiley-VCH Verlag GmbH & Co. KGaA.