Coordination properties of a polyamine cryptand with two different bindingmoieties. A case of a pH-modulated antenna device based on a new Eu(III) cryptate complex

Protonation and alkali- and alkaline-earth-metal coordination by the dipyri dine-containing cryptand L have been studied by means of potentiometric and spectroscopic (UV-vis, H-1 NMR) measurements in aqueous solutions. This li gand is constituted by an aliphatic polyamine chain and a coordinating clef t, delimited by two dipyridine units, where the metal ion is lodged. The re sulting complexes are characterized by an unusually high stability. The pol yamine chain is not involved, or weakly involved, in metal coordination, an d facile protonation can occur on the nitrogen atoms of this moiety. Simila r coordination features are found in the Eu(IH) complex. A fluorescence emi ssion study reveals that the Eu(IH) cryptate shows the characteristic visib le emission of the metal, due to the intramolecular energy transfer to the metal ion mainly from the lower energy triplet state of the cryptand. On th e other hand, the emission intensity is modulated by pH, giving a maximum a t neutral pH and decreasing at both acidic and alkaline pH values.