Synthesis, photophysical properties, and complexation behavior of three new luminescent tetraaza-tetraoxamacrobicyclic receptors

Three luminescent receptors 1-3 have been synthesized by linking photo-active groups to a tetraaza-tetraoxamacrobicyclic subunit. These multicomponent species constitute a homogeneous series with identical binding subunits, where changes are made to the chromophore and to the spacers. The new compounds Exhibit absorption spectra and luminescence properties in acetonitrile dominated by the photo-active components, perturbed by the presence of the nitrogens of the azacrown subunits. Although structurally similar, compounds 1-3 exhibit three qualitatively different photophysical properties: 2 emits from a pure pi-pi* (L-1(a)) level, 3 from both a charge-transfer (CT) level and from an excimeric species, and 1 exhibits emission from CT and L-1(a) levels. The luminescent quantum yields of 1-3 are significantly enhanced (up to two orders of magnitude) upon protonation, mainly as a consequence of the removal of the CT excited states from the compounds. Whereas Na+ and K+ are not complexed by the luminescent receptors, Ca2+ and Mg2+ are strongly complexed in a 1:1 fashion and significantly affect the luminescence properties. In particular, selectivity for Ca2+ is found. The results suggest that adduct formation is dominated by electrostatic interactions and that the spacers connecting the photo-active and the receptor subunits also play active roles in the stabilization of the adducts.