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

Aggregation of the tetra-anionic meso-tetrakis(4-sulfonatophenyl)porphine a nd its zinc(II) derivative onto extensively protonated, cationic polylysine leads to relevant fluorescence quenching. While in the absence of polylysi ne the porphyrin emission is independent of pH (no aggregation), in the pre sence of the polymer matrix its emission decreases with pH in a sigmoidal f ashion, indicating that porphyrin self-assembly is modulated by the number of polylysine protonated residues. Aggregation phenomena are also influence d by temperature and ionic strength. The titration curves of the two anioni c porphyrins differ in many aspects, and this has been attributed to the di fferent size of the aggregates they form. In particular, the zinc(II) deriv ative can only form a co-facial dimer due to the presence of a water molecu le 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. (C) 2001 E lsevier Science B.V. All rights reserved.