Spectroscopic Characterization of Azo Dye Aggregation on Dendrimer Surfaces

Abstract

The spectral properties of azo dye aggregates have been used to determine the influence of the dendritic surface of Starburst poly(amidoamine) dendrimers on the alignment of electrostatically bound azo dyes. The surface structure and morphology of dendrimers provide enhanced opportunities for the ordered assembly of azo dyes. In particular, the spacing of dendrimer surface groups is influenced by dendrimer generation and dictates the proximity of attached azo dyes and the ability of intermolecular forces between dyes to operate. Absorption spectra reveal the formation of highly ordered, spectrally distinct dye aggregates. Multivariate spectral analysis techniques enable a more complete characterization of the dye aggregation on the dendrimer surface and demonstrate the influential control of the dendrimer surface on the orientation of anchored azo dyes. Spectra are attributed to both H-aggregation (face-to-face arrangements of the aromatic rings attached to the azo functionality yielding blue-shifted spectra) and J-aggregation (side-by-side orientations of the aromatic rings inducing red-shifted spectra). In particular, J-aggregation was observed with azo dyes with (1) unsubstituted phenyl linking rings and hydrophilic, hydrogen-bonding substituents in the 4-position on the terminal ring or (2) hydroxy substituents on a linking phenyl or naphthyl ring and sulfonate and azo groups oriented at 1,4- or 1,3-positions on the linking ring. Azo dyes with unsubstituted naphthyl linking rings or hydrophobic substituents on a terminal phenyl ring most effectively promoted H-aggregation.