Lanthanide macrocyclic quinolyl conjugates as luminescent molecular-level devices

The Eu(III) tetraazamacrocyclic complexes [Eu·1] and [Eu·2], and the Tb(III) and Yb(III) complexes [Tb·1] and [Yb·2], have been synthesized as luminescent molecular-level devices. The Eu complexes exhibit unique dual pH switching behavior in water under ambient conditions. The delayed Eu emission is reversibly switched on in acid, with an enhancement factor of several hundred for [Eu·1). These observations are consistent with the protonation of the quinoline aryl nitrogen moiety (pK_a ≈ 5.9 for [Eu·1]). The fluorescence emission spectra of these complexes are unaffected by acid, but pronounced changes occur in alkaline solution due to the deprotonation of the aryl amide nitrogen (pK_a ≈ 9.4 for [Eu·1]). [Tb·1] shows a more intriguing pH dependence; Tb emission is switched "on" only in the presence of H⁺ and in the absence of molecular oxygen, whereas the fluorescence emission properties are similar to those observed with [Eu·1). This behavior can be conveniently described as a molecular-level logic gate, corresponding to a two-input INHIBIT function, A λ B′. The analogous [Yb·2] complex shows no such pH or O₂ dependence.