Characterization of a modified poly(propylene imine) dendrimer host system

Abstract

The research in this dissertation focuses on the design of a novel redox-responsive host system that has the ability to encapsulate guest molecules. The host at hand is a tri(ethylene oxide), TEO, pyrrole-terminated PPI dendrimer that possesses water-solubility as a result of the presence of the TEO chains on the pyrrole groups and redox-responsive activity from the pyrrole groups after their oligomerization. The pyrrole moieties at the periphery of the dendrimer exhibit the ability to be oligomerized by oxidation chemistry, and this is shown to aid in the retention and egress of guest molecules located within the dendrimer interior. These studies were followed by Visible spectroscopy, in which various solvents and oxidizing agents were employed for the oligomerization of the pyrrole end groups. The synthesis of this dendrimer system employed many reaction types, including but not limited to, bromination, SN2, deprotection and protection, and coupling reactions. Three substitution of the pyrrole moiety with the TEO chain was necessary to allow the 2- and 5-positions to remain available for oligomerization. Three new three-substituted pyrrole molecules were synthesized in this work, along with 4 new pyrrole-modified PPI dendrimers, referred to as TPDx. Characterization of these TPD host systems consisted of light scattering studies of their aqueous and non-aqueous solutions, encapsulation/release of a hydrophobic guest by TPDx monitored with Vis and fluorescence spectroscopies, and TPDx oligomerization followed with UV-Vis-NIR and FT-IR spectroscopies. The light scattering studies revealed aggregation of the host molecules, which was dependent on many variables, including solvent system, TPDx concentration, and temperature. The ability of the pyrrole moieties on the dendrimer to remain redox-active was clearly demonstrated. The TPDx were treated with different oxidizing agents in various solvent systems, and as a result they were able to form intramolecular oligo(pyrrole) units around the periphery of the dendrimer. The TPDs also have the ability to trap guest molecules, namely Nile Red, in aqueous media within their interior regions through static and dynamic trapping mechanisms. The trapping of guests within the TPDs was determined to be time dependent and largely a function of the steric constraints at the periphery of the TPDs.