CATION-COMPLEXING PROPERTIES OF BIS(P-PHENYLENE-34-CROWN-10) - A STRUCTURAL AND SPECTROPHOTOMETRIC STUDY

The title compound 2, an electron-rich macrocyclic paracyclophane of t he coronand type, known to form a charge-transfer complex with paraqua t, was found to encapsulate strontium cations and to bind to magnesium cations. X-ray analysis revealed that 2 forms a 2:1 (metal/substrate) complex with Sr(ClO4)(2), in which the two benzene rings weakly overl ap, whereas in the single crystals grown from Mg(ClO4)(2), the metal c ation prefers to lie outside the coronand (1:1 stoichiometry). In acet onitrile solution, cations were observed to trigger an hypsochromic sh ift of the UV absorption spectra, proportional to their size and charg e density. The stoichiometries and binding constants were also determi ned by UV absorption titration in acetonitrile using the LETAGROP-SPEF O program for several monovalent and divalent cations. For Na+, Ca2+, and Sr2+, 1:1 and 2:1 complexes were shown to be formed. In the free L igand, a weak interaction between the benzene rings was detected by fl uorescence decay kinetic analysis, indicating the presence of two conf ormer populations within the nanosecond time scale. In solution, metal cations neither induce detectable excimer formation nor seem to have a strong influence on the fluorescence emission spectra, except a heav y atom quenching with Sr2+ and Ba2+, in contrast to the effect observe d in absorption. However, Sr2+ and Mg2+ induce a clear hypsochromic sh ift in the single crystal fluorescence spectra. Compound 2 was prepare d by a new and efficient route which is compared to the previous proce dures.