From p-dimethoxybenzene to crown-benzenophanes, 4: Cation-complexing properties of Bis(p-phenylene-34-crown-10) - A structural and spectrophotometric study

Article abstract of DOI:10.1002/jlac.199719970115

The title compound 2, an electron-rich macrocyclic paracyclophane of the coronand type, known to form a charge-transfer complex with paraquat, 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 overlap, whereas in the single crystals grown from Mg(ClO₄)₂, the metal cation prefers to lie outside the coronand (1:1 stoichiometry). In acetonitrile solution, cations were observed to trigger an hypsochromic shift of the UV absorption spectra, proportional to their size and charge density. The stoichiometries and binding constants were also determined by UV absorption titration in acetonitrile using the LETAGROP-SPEFO 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 ligand, a weak interaction between the benzene rings was detected by fluorescence decay kinetic analysis, indicating the presence of two conformer 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 heavy atom quenching with Sr²⁺and Ba²⁺, in contrast to the effect observed in absorption. However, Sr²⁺ and Mg²⁺induce a clear hypsochromic shift in the single crystal fluorescence spectra. Compound 2 was prepared by a new and efficient route which is compared to the previous procedures. VCH Verlagsgesellschaft mbH, 1997.