Dendrimers are highly branched organic macromolecules with successive layers or “generations” of branch units surrounding a central core. Organic inorganic hybrid versions have also been produced, by trapping metal ions or metal clusters within the voids of the dendrimers. Their unusual, tree-like topology endows these nanometre-sized macromolecules with a gradient in branch density from the interior to the exterior. Here we show that tin ions, SnCl₂ and FeCl₃complexes to the imines groups of a spherical polyphenyl-azomethine dendrimer in a stepwise fashion according to an electron gradient, with complexation in a more peripheral generation proceeding only after complexation in generations closer to the core has been completed.

Many metal halides can be assembled inside the phenylazomethine dendrimer with a Zn-porphyrin core(DPA-ZnP) by the strong coordination ability of the imines in DPAs. The 60 equivalents of SnCl₂ should be included in the fourth generation DPA-ZnP (DPAG4-ZnP), which has 60 imine sites, because SnCl₂ is complexed with an imine at the ratio of 1:1. Complexation of DPA G4-ZnP with SnCl₂ was confirmed as a shift in the absorption based on p-p* transition of imines in the UV-vis spectra. During the complexation of DPA G4-ZnP with SnCl₂, four shifts in the isosbestic point were observed in the spectra. If SnCl₂ molecules are complexed with the imines in DPAs at random, the isosbestic point is not shifted during the titration of SnCl₂. However, the isosbestic point was observed at 370, 368, 366, and 362 nm during the addition of 0-4, 5-12, 13-28, and 29-60 equivalents of SnCl₂ to the dichloromethane/ acetonitrile solution of DPA G4-ZnP, respectively. These shifts show that the complexation proceeds in four steps. The amounts of the added SnCl₂ in each step, i.e., 4, 8, 16, and 32 equivalents, agree with the number of imines in the 1^st to 4^th shells. This result shows that SnCl₂ molecules are assembled from the core to the terminals of DPA G4-ZnP in a stepwise fashion. This radial stepwise complexation was also observed for the other dendrimers with phenylazomethine dendron skeletons. This complexation behavior means that the number and the position of the metal ions in the dendrimers are controllable.