pH Modulation of Porphyrins Self-Assembly onto Polylysine

The tetraanionic meso-tetrakis(4-sulfonatophenyl)porphine (H2TPPS) in the pH range 5−12 exists in a monomeric form, and its fluorescence is not pH-dependent. However, in the presence of polylysine, absorption, circular dichroism, and resonance light scattering data indicate extensive polymer-induced self-aggregation of the porphyrins. In addition, the fluorescence intensity vs pH behavior is deeply modified, showing a sigmoidal profile. In particular, at low pHs (≤7), the protonated polylysine promotes porphyrins binding and self-aggregation with consequent strong quenching of their fluorescence, while at pHs higher than 9−10, the porphyrins exist in solution essentially as free monomers which retain their fluorescence. Interestingly, the molecular recognition processes leading to the formation of these aggregates can be modulated by using porphyrins containing different central metal ions with particular coordination geometry.