MULTI-PROTONATION OF A [3]-CATENAND AND A MONO-COPPER [3]-CATENATE - ABSORPTION-SPECTRA AND LUMINESCENCE PROPERTIES

The absorption and luminescence spectra and the excited state lifetimes and quantum yields of a [3]-catenand and its mono-copper catenate in CH2Cl2 solution at room temperature undergo profound, fully reversible changes upon addition of CF3COOH. The [3]-catenand contains four 2,9-dianisyl-1,10-phenanthroline (dap) chelating ligands. In its mono-copper complex, two of the four ligands are engaged in the coordination of a Cu(I) ion. Protonation of the free catenand takes place through four distinct families of isobestic points exhibited by the absorption spectrum on increasing acid concentration. The presence of four successive protonation steps is also indicated by changes in the luminescence spectra. Comparison of the absorption and luminescence spectral changes observed upon the protonation of isolated dap units and of a previously investigated [2]-catenand shows that protonation of the [3]-catenand does not involve independent ligand units, but implies a cooperative action of pairs of ligands which organize themselves around one or two protons to form ''proton catenates''. For the mono-copper complex, the spectral changes indicate the presence of two distinct protonated species which again implies a cooperative action of the two dap units not involved in metal coordination. In the partially protonated forms of the [3]-catenand, the protonated moiety quenches the fluorescence of the non-protonated one, while in the protonated mono-copper catenate the fluorescence of the protonated moiety is quenched by the metal containing one.