Experimental Evidence for Self-Similar Structures in the Aggregation of Porphyrins in Aqueous Solutions

We have studied the aggregation of the porphyrin t-H₂Pagg in aqueous solution by light scattering. The intensity profile of the elastically scattered light, studied in the exchanged wave-vector range 0.2≤q≤31.4μm^-1, indicates that the aggregation produces large monodisperse clusters having a fractal structure, and is driven by diffusion-limited aggregation kinetics. Additional measurements performed at different q values confirm such a picture giving a hydrodynamic radius RH consistent with the radius of gyration Rg measured by elastic scattering. This is explained taking into account the q² dependence observed in the mean decay rate of the intensity-intensity correlation function and the effect on this latter dynamical quantity of anisotropies in the cluster structure.