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

Samples containing J-aggregates formed by the porphyrin meso-tetrakis(4-sul fonatophenyl)porphine (H2TPPS44-) were studied by a combination of elastic (ELS) and dynamic (DLS) light scattering techniques. Aggregation was foster ed by lowering the pH and increasing the ionic strength (I; selected experi mental 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, who se fractal dimension are d(f) = 1.7, 2.13, and 2.09 (for case i, ii, and ii i, respectively). The DLS experiments indicate the presence of large (1-1.5 mu m)-, medium (100-200 nm)-, and small (3-6 nm)-sized aggregates. An aggr egation number (N) ranging between 6 and 32 was calculated for the smaller components, whereas a range of 10(5) to 10(6) was found in the case of the large clusters. The aggregation kinetics were followed by the resonance lig ht scattering technique. The. average aggregation time and the growth model s, as derived from the ELS experiments, are in agreement with the Derjaguin -Landau-Verwey-Overbeek (DLVO) theory for aggregation of colloidal particle s. The experimental findings point out the different ability of H+ and Nain driving the final mesoscopic structure.