Sandwich-type complexes of alkaline-earth metal cations with a bisstyryl dye containing two crown ether units

The formation of complexes between a bisstyryl dye containing two benzo-15-crown-5 ether groups, 2, and Mg2+, Ca2+, Sr2+, and Ba2+ ions was studied in acetonitrile solution by using a spectrophotometric approach. In contrast to its monostyryl analogue 1, which shows a rather low cation-binding selectivity, the bisstyryl dye 2 selectively binds the Sr2+ and Ba2+ ions because of formation of intramolecular sandwich complexes. Alkaline-earth metal cations induce substantial blue shifts of the absorption maximum of 1 and 2, particularly in the case of sandwich complex formation. The emission behavior of 1 and 2 and their Ba2+ complexes in acetonitrile solution has also been studied. In both cases, complex formation leads to a blue shift of the fluorescence maximum and a decrease in the emission quantum yield. The fluorescence decay is monoexponential for the complex of 1 and biexponential for that of 2. The longer-lived fluorescence component, which corresponds to the lower-energy band in the steady-state spectrum, is assigned to an intramolecular excimer state.