Structural rearrangements in 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin J-aggregates under strongly acidic conditions

Recently, we showed that J-aggregates, formed by the zwitterionic diacid form of the porphyrin 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin (H2TPPS44-) under acidic conditions, can be described in terms of fractal geometry. We have extended the investigation of the system under strongly acidic conditions through a combination of UV/vis and fluorescence emission spectroscopy, together with static (SLS), quasi-elastic (QELS), and resonant (RLS) light-scattering techniques. The experimental findings suggest that porphyrin J-aggregates begin to form at pH I and are stable up to 4 M HCl. On further increasing the acid concentration, protonation of the sulfonate end groups occurs, leading to a disruption of the electrostatic interactions between these anionic groups and the charged protonated nitrogen atoms in the porphyrin core. At [HCl] > 8 M, the spectroscopic features suggest the presence of the monomeric dicationic porphyrin, resulting from a full protonation of the four sulfonate groups. Under the acid concentration range in which J-aggregates exist, light-scattering data indicate the formation of clusters having a fractal internal structure. At [HCl] = 0.1 M, the aggregation is driven by the interaction between small clusters leading to a loose diffusion-limited cluster-cluster aggregation (DLCCA) structure (d(f) = 1.75 +/- 0.05). On increasing the acid concentration up to 2 M HCl, a structural crossover occurs. The reduction of the net charge on the monomeric unit leads to an increased sticking probability between monomers which is responsible for the observed compact diffusion-limited aggregation (DLA) structure (d(f) = 2.5 +/- 0.03). A further reduction of the net charge of the porphyrin ([HCl] = 4 M) determines the formation of nucleating clusters no longer having a fractal structure. An important role is played by the mixing order of the reagents both at the level of kinetics of growth and for the final mesoscopic structures. Our findings suggest that this effect should be related to the large volumetric ratio between the reagent solutions to be mixed, which causes very different spatial concentrations of both reagents.