Spectroscopic Investigations of Poly(Propyleneimine) Dendrimers Using the Solvatochromic Probe Phenol Blue and Comparisons to Poly(Amidoamine) Dendrimers

The physical and chemical properties of PPI dendrimersʹ interior were investigated using the fluorescent, solvatochromic probe phenol blue. In aqueous solutions of each generation studied, two discrete dye populations were clearly observed. PPI dendrimers were shown to form a tight, nonpolar association with the vast majority of available dye, within the dendrimer interior, near the core. In the steady-state fluorescence emission spectra, a microenvironment of decreasing polarity in increasingly larger-generation PPI dendrimers (up to G3) was seen for the associated probe. Each of the remaining larger-generation dendrimers provided a microenvironment of essentially equal polarity. Fluorescence anisotropy values for phenol blue in the PPI dendrimers demonstrated the dyeʹs sensitivity to the changing molecular volumes of the dendrimer generations. Model compounds that mimicked PPIʹs surface groups and branching moieties were used to better define the associated dyeʹs location. The mimics further confirmed that phenol blue was associated inside the dendrimer, where it did not interact with the dendrimer surface groups. The comparison of amine-terminated PPI and PAMAM dendrimers clearly demonstrated the effects of their structural differences and the ability of phenol blue to have sensed those differences, including the initiator core length, branching unit length, and branching unit chemical composition.