Characterization of starburst dendrimers by EPR .4. Mn(II) as a probe of interphase properties

Mn(II) has been used as a probe to investigate the interacting abilities of the various ligand sites at the surface of starburst dendrimers (SBDs) at different protonation conditions, and as a function of generation (G), both for half- and full-generation SBDs (n.5-SBDs and n-SBDs, respectively). The computer-aided analysis of the electron paramagnetic resonance (EPR) spectra of Mn(II) provided information on the location of Mn(II) in the hydration layers of the internal and external SBD surface and on the physical status of water inside the dendrimer structure and at the surface/solution interface. Mn(II) did not show any interaction with the full-generation dendrimers and only interacted, at the second-third solvation shells, with the surface carboxylate groups (SBD-COO-) of n.5-SBDs. Two main components were identified in the EPR spectra: (1) a minor component due to Mn(II) that is localized in the solvation layers of the internal and external SBD surface and is in fast exchange with the bulk solution. This Mn(II) fraction underwent a freezing transition at about 255 K; (2) a major component arising from complexation of Mn(II) with SBD-COO- groups at the second solvation shell of the ions. The portion of solution that contained the Mn(II) complexes underwent a glass transition with a decrease in temperature. The complexation of Mn(II) was favored for later generation dendrimers (G > 4.5), at high pH and low Mn(II) concentrations.