A CRYPTAND CONTAINING 3 DIIMINOPYRIDYL UNITS - AROMATIC-AROMATIC INTERACTION AND THE STABILITY OF THE COMPLEXES

The solution properties of a hexaimino cryptand containing 2,6-diimino pyridyl units have been investigated by NMR, UV-VIS absorption and lum inescence spectroscopy. The difference in the NMR and UV spectra of th e Ba2+ complex and the cryptand is attributed to the conformational ch ange of the diiminopyridyl group. The luminescence spectra of the comp ounds show near-UV bands and unexpected low-energy bands. The former i s attributed to the normal pi-pi transition, while the origin of the low-energy bands is discussed in terms of inter- and intra-molecular a romatic-aromatic interactions. The inter-molecular interaction is impo rtant in the free ligand, however, intra-molecular interaction is sugg ested in the Ba2+ complex. Further, the specific conformational change of the Ba2+ complex in DMSO-acetonitrile solution is investigated by the measurements of the stability constant of the Ba2+ complex in vari ous DMSO concentrations. The large decrease over 4.0 in logK is found only for the Ba2+ complex. The behavior is explained by the factors br ought about the nature of DMSO and the interactions among the diiminop yridyl units, DMSO and the Ba2+ ion.