Porphyrin binding and self-aggregation onto polymeric matrix: a combined spectroscopic and modelling approach

Aggregation of the tetra-anionic meso-tetrakis(4-sulfonatophenyl)porphine and its zinc(II) derivative onto extensively protonated, cationic polylysine leads to relevant fluorescence quenching. While in the absence of polylysine the porphyrin emission is independent of pH (no aggregation), in the presence of the polymer matrix its emission decreases with pH in a sigmoidal fashion, indicating that porphyrin self-assembly is modulated by the number of polylysine protonated residues. Aggregation phenomena are also influenced by temperature and ionic strength. The titration curves of the two anionic porphyrins differ in many aspects, and this has been attributed to the different size of the aggregates they form. In particular, the zinc(II) derivative can only form a co-facial dimer due to the presence of a water molecule axially coordinated to the central metal ion. Here, we show by means of an experimental and theoretical approach that the size of the aggregates is critical in determining the behaviour of these complex species.