Structural Manipulation of Pyrrole-Terminated Dendrimers to Create a More Efficient Encapsulation/Release System

Abstract

This work demonstrates the ability to manipulate the structure of pyrrole-terminated dendrimers by modifying solution conditions which directly affects their encapsulation properties. Investigations were conducted using FTIR and 2D NMR to determine the location of end groups. NMR relaxation studies were conducted at various temperatures and different solution pH to gain an understanding of the relative mobility and rigidity of different areas of the dendrimer. It was found that that the release properties were affected by the solution pH and the presence of salt. Further the effectiveness of retaining molecular guests by oligo-pyrrole terminated dendrimers with a range of oligomer lengths was investigated. Hydrogen-Bonding studies, conducted on an FTIR, and 2D NMR data suggest the end groups of similarly structured PPI dendrimers are not backfolded into the interior of the dendrimer. Results from H-Bonding studies, as well as NMR relaxation studies, demonstrate an increase in steric crowding at the periphery of dendrimers as the dendrimer generation is increased. NMR relaxation measurements revealed that the mobility along the periphery of the dendrimer was dependant upon the solution pH. Longer oligomers were formed upon chemical oxidation of the pyrrole termini at higher pH, when the dendrimer possessed a more rigid structure. Nile Red was encapsulated by the dendrimer host and its release was measured by visible spectroscopy. It was found that increasing the solution pH caused the dendrimer to retain encapsulated guests at a higher efficiency than when the pH was decreased. It is also shown that the addition of salt causes the dendrimer to quickly expel any encapsulated guests. Further, the effectiveness of the oligo-pyrroles in retaining encapsulated guests was investigated. As it turns out, longer oligomers about the periphery of the dendrimer were less successful in retaining incarcerated guests. Shorter oligomers obtained from oligomerization at pH 2 proved to be more efficient in retaining encapsulated guests than longer oligomers formed at pH 7. This work demonstrates that the encapsulation and release properties can be controlled by altering simple solution parameters.