Abstract
The aggregation behavior of protoporphyrin IX in aqueous solution, as a function of pH and ionic strength, has been studied by means of UV/vis, fluorescence emission spectroscopy, and resonant light scattering (RLS) techniques. Our experimental results agree with previous literature assignments: (i) protoporphyrin IX is a monomer in the pH range 0−3, (ii) a dimer is present for pH > 8, and (iii) higher aggregates are present in the pH range 3−7. Addition of sodium chloride up to 0.3 M to a porphyrin solution at pH 12 gives a process resembling a phase transition, whereas it has little effect on acidic or neutral solutions. The apparent split Soret band observed in the intermediate pH range has been explained using a model in which dimers of porphyrins (with a slip angle α = 38° or 52°, as derived from depolarized RLS measurements) are the basic units and they interact axially through π−π stacking and laterally by edge-to-edge hydrophobic contacts. The half neutralization of the carboxylic acid side chain...
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