Characterization of Starburst Dendrimers by EPR. 3. Aggregational Processes of a Positively Charged Nitroxide Surfactant M. Francesca Ottaviani* Department of Chemistry, UniVersity of Florence, 50121 Florence, Italy Nicholas J. Turro and Steffen Jockusch Department of Chemistry, Columbia UniVersity, New York, New York 10027 Donald A. Tomalia Michigan Molecular Institute, Midland, Michigan 48640 ReceiVed: January 29, 1996; In Final Form: April 30, 1996 X The aggregation characteristics of aqueous solutions of a positively charged nitroxide surfactant (CAT16) in the presence and absence of half-generation polyamidoamine starburst dendrimers (n.5-SBDs) have been investigated by electron paramagnetic resonance (EPR). Computer simulation of the EPR spectra allowed the convenient extraction of several parameters that were related to the supramolecular structure of the aggregates formed by CAT16 and SBDs. From examination of the EPR spectra as a function of variation of the concentration of CAT16, the concentration of SBDs, and the ionic strength and application of the EPR parameters available from simulation of the spectra, a paradigm for the structure and dynamics of the aggregates formed by CAT16 in the presence and absence of SBDs under various conditions is deduced. A study of the fluorescence quenching of pyrene in the presence of CAT16 is compared to a previous investigation of the interaction of SBDs with cationic surfactants. It is concluded that at low SBD concentration, for earlier generation SBDs (G < 3.5), whose size is smaller than or comparable to the size of the CAT16 micellar aggregates, the SBDs act as “guests” that bind to the micelles that serve as “hosts”. In contrast, at low SBD concentration of the later generation SBD (G > 3.5), the size of the SBD is now larger than that of the micelles so that the latter can serve as “guests” for the former. A bilayer aggregate of the surfactant on the SBD is proposed. Finally, at high concentration of the later generation SBD, it is proposed that because of the large number of sites compared to the number of surfactants, an aggregate in which two or more SBDs are bridged by bilayers is formed by the surfactant and coexists with CAT16 micelles. Introduction Cascade (dendritic) polymers are a novel class of macromol- ecules possessing branched, repeating units emanating from a central core. The controlled synthesis of dendrimers produces structures with nanoscopic size and well-defined composition and constitution. 1-3 The polyamidoamine family of starburst dendrimers (SBDs) is synthesized by grafting the repeating layers of amidoamine units starting from an ammonia core. 1 Each shell of the amidoamine units constitutes a “full” genera- tion (G) terminating with an external shell of amino groups. This family of starburst dendrimers possesses a certain similarity to biomacromolecules such as proteins and enzymes. Molecular simulation 4 of the SBD structure (in this paper SBD will always refer to polyamidoamine dendrimers) has revealed a rather dramatic change in the dendrimer morphology occurring after the full third generation. The so-called earlier generation dendrimers (G < 3) were found to possess an asymmetric shape and open structure, whereas the later generation dendrimers (G > 3) possess a nearly spherical shape and a densely packed external structure. Branches at the external surface of the SBDs may be terminated at half-generation (G ) n.5) by forming carboxylate groups with sodium contraions (Scheme 1 shows a schematic of representative earlier and later generation n.5-SBD structure). The external structure and shape of these n.5-SBDs also bear a formal resemblance to anionic micelles. Therefore, the n.5-SBDs may be good models for both biomacromolecular and micellar structures. Photophysical studies have been carried out to support the conclusion of molecular simulation that the dendrimer morphol- ogy varies qualitatively from earlier to later generations, 5 and this conclusion has tested the similarities between n.5-SBDs and anionic micelles. 6 Analysis of electron paramagnetic X Abstract published in AdVance ACS Abstracts, July 1, 1996. SCHEME 1 13675 J. Phys. Chem. 1996, 100, 13675-13686 S0022-3654(96)00291-2 CCC: $12.00 © 1996 American Chemical Society