Mesoscopic structure of meso-tetrakis(4-sulfonatophenyl)porphine J-Aggregates

Samples containing J-aggregates formed by the porphyrin meso-tetrakis(4-sulfonatophenyl)porphine (H2TPPS44-) were studied by a combination of elastic (ELS) and dynamic (DLS) light scattering techniques. Aggregation was fostered by lowering the pH and increasing the ionic strength (I; selected experimental conditions: (i) pH = 0.7; (ii) pH = 2.8, I = 0.5 M; (iii) pH = 0.7, I = 2 M). The ELS data suggest the presence of self-similar structures, whose fractal dimension are df = 1.7, 2.13, and 2.09 (for case i, ii, and iii, respectively). The DLS experiments indicate the presence of large (1 -1.5 μm)-, medium (100-200 nm)-, and small (3-6 nm)-sized aggregates. An aggregation number (N) ranging between 6 and 32 was calculated for the smaller components, whereas a range of 105 to 106 was found in the case of the large clusters. The aggregation kinetics were followed by the resonance light scattering technique. The average aggregation time and the growth models, as derived from the ELS experiments, are in agreement with the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory for aggregation of colloidal particles. The experimental findings point out the different ability of H+ and Na+ in driving the final mesoscopic structure.