5,10,15,20-Tetrasulfonatophenylporphyrin regioisomers: How the location of the sulfonato groups determines the formation of their supramolecular aggregates

An improved method to obtain and purify the tetrasodium salts of 5,10,15,20-tetrakis(4-sulfonatophenyl) porphyrin (TPP-pS4) and 5,10,15,20-tetrakis(3-sulfonatophenyl) porphyrin (TPP-mS4) is reported. With these substances as standards, the analysis of a typical reaction crude obtained in the sulfonation of tetraphenylporphyrin with hot sulfuric acid shows that the para/meta selectivity of sulfonation at each individual phenyl ring is 95: 5. The pKa values of both TPP-pS4 and TPP-mS4 were spectrophotometrically measured (4.73 +/- 0.01 and 4.85 +/- 0.01 respectively). These results show that the sulfonato group has a stronger mesomeric electronic effect than the inductive one. The all-meta isomer of meso-tetrasulfonatophenylporphyrin in acidic aqueous solutions (10(-3)-10(-4) M) shows a total absence of aggregation under the same experimental conditions in which its all-para counterpart is fully aggregated, as inferred from the typical Soret J- and H-aggregation bands characteristic of the supramolecular homoassociates. The effect of the number of meta-sulfonato groups on the structure of the aggregates at different length-scales is discussed.