Characterization of starburst dendrimers by the EPR technique. 1. Copper complexes in water solution

Abstract

The structure of Cu(I1) complexes formed with anionic starburst dendrimers (n.5 G-SBD) in aqueous solution has been investigated by the electron paramagnetic resonance (EPR) technique. The line shapes of the EPR spectra of the complexes at room temperature show a distinction between earlier (n < 3) and later (n 1 3) generations and are consistent with a change of the dendrimer shape, which supports the results of molecular simulation of the dendrimer morphology as a function of generation. The earlier generations appear to possess a more open structure, which leads to a greater mobility of the copper complexes. Three different complexes of copper with groups composing the dendrimer structure are identified by analyzing the spectra as a function of the dendrimer size (generation), the pH, and the temperature. The magnetic parameters, evaluated at low temperature with the aid of spectral computation, indicate that the copper ions form monomeric carboxylate complexes at low pH (signal C). With an increase of pH, the ions interact with nitrogen centers in the internal porous structure of the dendrimers. The complex formed at intermediate pH is identified as a Cu(II)-N202 complex (signal A). Such a complex, which involves both the carboxylic groups at the dendrimer interface and the internal nitrogen centers, is preferentially formed by Iow-generation dendrimers. This result is consistent with the morphology of the dendrimer structure. The higher generation dendrimers present a wide number of internal sites in which the tightly packed structure appears to facilitate the interaction with more than two nitrogens. This Cu(II)-N30 or Cu(I1)-N4 complex gives a third signal (termed signal B), which increases its intensity at the expense of signal A, both with the increase of generation and with the increase of pH. However, the interaction with nitrogen centers in both cases is not strong enough to give superhyperfine structure in the EPR spectra. For freshly prepared samples, formation of the complex with the larger nitrogen coordination corresponds to enhancement in the EPR room temperature spectra of a signal of a nitrogen-centered radical species, which is also observed in the spectra of pure dendrimers. The spectral features of this radical are identified, by spectral computation, as resulting from an unpaired electron which couples with one nitrogen and four protons from two slightly nonequivalent CHz groups. Aging of the samples leads to an increase in intensity of signal B, simultaneously with the disappearance of the radical signal. This allows the identification of one of the coordinating sites in the internal dendrimer structure. Heating of the 6.5 and 7.5 G-SBD at high pH causes decomposition of the dendrimers, whereas lower generation dendrimers show good thermal stability. The evaluation of the Cu(I1) bonding parameters, a2, d2, and 8lz, indicates substantial covalency of the in-plane bonds, with the covalent character increasing respectively for the species corresponding to signal C, signal A, and signal B.