AOT/water/decane microemulsions have been used to entrap the water-soluble 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin (TPPS4). Quasi-elastic light scattering technique has confirmed the confinement of the porphyrin and its various aggregates into the inner water pool. Various species have been detected as function of the size of the microemulsions, concentration of the porphyrin, pH, and aging of the solutions by using a combination of UV-vis absorption, steady fluorescence emission, fluorescence lifetime measurements, and time-resolved fluorescence anisotropy. Under neutral pH conditions, the porphyrin is present as the free base monomer (S-414) in the inner water compartment, and it is free to rotate when the size of the droplet is large enough and the porphyrin concentration is low. On increasing the concentration and/or decreasing the microemulsion size, a H-dimer of the free base (S-406) is prevalently formed. Aging both the S-414 and S-406 species leads to the formation of a new species (S-424), which has been postulated as a H-type dimer of the diacid porphyrin. On decreasing the pH, the species S-414 and S-406 almost instantaneously convert into the diacid porphyrin, which is monomeric (S-434). This latter is an intermediate in the eventual formation of J-aggregated TPPS4 (S-490). A marked stability has been observed for the S-424 species, which do not interconvert on changing the pH of the bulk aqueous phase.
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