The imidazole group of histidine deprotonates and bridges the two CuII centers of a dimetallic polyamine macrocyclic complex, displacing the previously bound and quenched fluorescent indicator I. Thus, histidine recognition is signaled by the revival of the fluorescence of I. Selectivity with respect to other natural amino acids is achieved by choosing an indicator of tuned affinity toward the dicopper(II) receptor.

Fluorescent sensors have been designed for a variety of analytes during the last decade by following the classical ™fluorophore-spacer-receptor∫ (FSR) approach in which a light-emitting fragment is covalently linked to a receptor subunit. [1] More recently, a different approach has been introduced, the ™chemosensing ensemble∫ (CE) paradigm, which relies on the use of an indicator (I) bound to a receptor (R) by means of noncovalent interactions. [2] In this approach the highly colored or fluorescent probe I is displaced from R by the competing analyte (S), and this displacement produces a drastic change in the optical properties of released I. All the CE systems described until now were designed for anion sensing and most of them were based on hydrogen-bonding interactions. These interactions are relatively weak and in most cases do not compensate for the rather endothermic desolvation of R and S, which prevents the utilization of such receptors in pure water. We have recently reported the first example of a CE operating through metal±ligand interactions in which the receptor core is a dinuclear Cu II macrobicyclic complex which is able to detect the carbonate ion in water through the revival of visible light emission of the displaced indicator. [3] The availability of optical molecular sensors for anion detection is highly beneficial for the investigation of aqueous media in food science, cell physiology, and environmental chemistry. [4] Transition metal ions, for example, Cu II , offer substantial advantages when designing CEs for anions. First of all, coordinatively unsaturated Cu II complexes display strong binding tendencies towards anionic substrates because of the d 9 electronic configuration of the metal center, which ensures high ligand field stabilization effects: as a consequence, anionic substrates can be bound and effectively recognized even in the strongly solvating medium water. Moreover, the Cu II ion provides an operative control of the signal as it is able to completely quench the emission of a coordinated indicator, either of an electron-transfer or an energy-transfer nature, by means of very efficient intramolecular processes. [5] We now demonstrate how the judicious choice of the indicator can turn macrocyclic complexes of Cu II ions with well-known recognition tendencies towards anions into smart optical sensing devices, whose spectral and COMMUNICATIONS

The synthesis and photophysical evaluation of two enatiomerially pure dimetallic lanthanide luminescent triple-stranded helicates is described. The two systems, formed from the chiral (R,R) ligand 1 and (S,S) ligand 2, were produced as single species in solution, where the excitation of either the naphthalene antennae or the pyridiyl units gave rise to Eu(III) emission in a variety of solvents. Excitation of the antennae also gave rise to circularly polarized Eu(III) luminescence emissions for Eu(2):1(3) and Eu(2):2(3) that were of equal intensity and opposite sign, confirming their enantiomeric nature in solution providing a basis upon which we were able to assign the absolute configurations of Eu(2):1(3) and Eu(2):2(3).