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Non-covalent dendrimer-porphyrin interactions: the intermediacy of H-aggregates?

The interaction of anionic meso-tetrakis(4-sulfonatophenyl)porphyrin (TSPP) with poly(amidoamine) (PAMAM) dendrimers of generations 2 and 4 in aqueous solution was investigated with steady-state absorption and fluorescence techniques. At low dendrimer concentrations, the formation of low absorptive/emissive species occurs in these systems. With an increase of dendrimer concentration, the porphyrins rearrange at the dendritic outer shell and the absorption and emission spectra suggest the presence of H-aggregates of TSPP. These aggregates dissociate to yield monomeric complexed species in the limit of high dendrimer concentration, which presents very similar spectra for both generations studied. On the other hand, the emission of the same systems at pH 2 shows significant differences between dendrimer generations. While for generation 2 the fluorescence spectra practically coincide with that of the diacid TSPP and its J-aggregate, for generation 4 the spectra obtained are similar to that at high pH. This was interpreted according to a morphologic transition in PAMAM dendrimers around generation 3, to give a more compact structure, which provides a hydrophobic environment for the associated TSPP. At low pH, an increase in J-aggregation is observed in the dendrimer's presence. Aging effects were observed, in particular for the systems where different aggregated forms of TSPP coexist, showing that for intermediate dendrimer concentrations these are thermodynamically labile systems.